MgF2—CaF2 binary system sintered body for radiation moderator and method for producing the same

Abstract

A MgF.sub.2—CaF.sub.2 binary system sintered body for a radiation moderator having a compact polycrystalline structure excellent in radiation moderation performance, especially neutron moderation performance, comprises MgF.sub.2 containing CaF.sub.2 from 0.2% by weight to 90% by weight inclusive, having a bulk density of 2.96 g/cm.sup.3 or more, and a bending strength of 15 MPa or more and a Vickers hardness of 90 or more as regards mechanical strengths.

Claims

1. A MgF.sub.2—CaF.sub.2 binary system sintered body for a radiation moderator, comprising MgF.sub.2 containing CaF.sub.2 from 1.5% by weight to 80% by weight inclusive, having a compact polycrystalline structure with a bulk density of 2.96 g/cm.sup.3 or more and having radiation moderation performance.

2. The MgF.sub.2—CaF.sub.2 binary system sintered body for a radiation moderator according to claim 1, wherein the radiation moderation performance is neutron moderation performance.

3. The MgF.sub.2—CaF.sub.2 binary system sintered body for a radiation moderator according to claim 1, having a bending strength of 15 MPa or more and a Vickers hardness of 90 or more as regards mechanical strengths.

4. A method for producing the MgF.sub.2—CaF.sub.2 binary system sintered body for a radiation moderator according to claim 1, comprising the steps of: mixing a MgF.sub.2 powder with a CaF.sub.2 powder of 1.5-80% by weight and further adding 0.02-1% by weight of a sintering aid thereto to mix; molding the raw material powder compounded in the preceding step at a molding pressure of 5 MPa or more using a press molding device; molding the press molded article at a molding pressure of 5 MPa or more using a cold isostatic pressing (CIP) device; conducting preliminary sintering by heating the CIP molded article in a temperature range of 600° C.-700° C. in an air atmosphere; conducting sintering by heating in a temperature range from (Tn-100)° C. to (Tn)° C. when the starting temperature of foaming of the preliminary sintered body is (Tn)° C., in an air atmosphere or in an inert gas atmosphere; and forming a sintered body having a compact structure by heating in a temperature range of 900° C.-1150° C. in the same atmosphere as the preceding step.

5. The method for producing a MgF.sub.2—CaF.sub.2 binary system sintered body for a radiation moderator according to claim 4, wherein the shape of a particle size distribution curve of the compound shows a sub-1-peak-type or 1-peak-type particle size distribution, the maximum particle diameter is 50 μm or less and the median diameter of the particles is 6 μm or less.

6. The method for producing a MgF.sub.2—CaF.sub.2 binary system sintered body for a radiation moderator according to claim 4, wherein the inert gas atmosphere in the sintering step comprises one kind of gas or a mixture of plural kinds of gases, selected from among nitrogen, helium, argon and neon.

7. The method for producing a MgF.sub.2—CaF.sub.2 binary system sintered body for a radiation moderator according to claim 5, wherein the inert gas atmosphere in the sintering step comprises one kind of gas or a mixture of plural kinds of gases, selected from among nitrogen, helium, argon and neon.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a diagram showing an example of a particle size distribution when a raw material of MgF.sub.2 simple is pulverized;

(2) FIG. 2 is a phase diagram of the MgF.sub.2—CaF.sub.2 binary system;

(3) FIG. 3 is a diagram showing a flow of steps for producing a MgF.sub.2—CaF.sub.2 binary system sintered body;

(4) FIG. 4 is a diagram showing the relationship between the secondary sintering temperatures and the relative densities of MgF.sub.2—CaF.sub.2 binary system sintered bodies and sintered bodies of MgF.sub.2 simple;

(5) FIG. 5 is a diagram showing the relationship between the secondary sintering temperatures and the relative densities of MgF.sub.2—CaF.sub.2 binary system sintered bodies in the case of the atmospheric gas in the sintering step switched to nitrogen gas and to helium gas;

(6) FIG. 6 is a table (Table 1) showing the measurement results of the neutron moderation performance of MgF.sub.2—CaF.sub.2 binary system sintered bodies having varied raw material mix proportions; and

(7) FIG. 7 is a table (Table 2) showing measured data of Examples and Comparative Examples.

DESCRIPTION OF EMBODIMENTS

(8) The preferred embodiments of the MgF.sub.2—CaF.sub.2 binary system sintered body for a radiation moderator having a compact polycrystalline structure excellent in radiation moderation performance, especially neutron moderation performance, and the method for producing the same according to the present invention are described below by reference to the Figures.

(9) In the method for producing a MgF.sub.2—CaF.sub.2 binary system sintered body according to the preferred embodiment, as shown in FIG. 3, a high-purity (purity of 99.9% by weight or more) MgF.sub.2 powder was mixed with a high-purity (purity of 99.9% by weight or more) CaF.sub.2 powder in the proportion of 0.2-90% by weight (included in a total of 100), and as a sintering aid, for example, a carboxymethyl cellulose (CMC) solution was added thereto in the proportion of 0.02-1% by weight (not included in 100) to 100 of the mixture and mixed. The mixture was used as a starting raw material (raw material mixing step).

(10) The starting raw material was molded at a molding pressure of 5 MPa or more using a uniaxial press device (uniaxial press molding step), and this press molded body was molded at a molding pressure of 5 MPa or more using a cold isostatic pressing (CIP) device (CIP molding step).

(11) Preliminary sintering was conducted by heating this CIP molded body in a temperature range of 600° C.-700° C. in an air atmosphere (preliminary sintering step).

(12) This preliminary sintered body was heated in a temperature range just below the starting temperature of foaming Tn, that is, in a temperature range from (Tn-100° C.) to Tn for a relatively long period of time (specifically, 3-12 hours) in an air atmosphere or in an inert gas atmosphere so as to allow sintering to make progress more uniformly (primary sintering step).

(13) The temperature range just below the starting temperature of foaming Tn was defined through the measurement using a differential thermal analyzer, and the temperature range varies in a range of about 750° C.-900° C. depending on the mix proportion of the raw materials of MgF.sub.2 and CaF.sub.2. As described above, it varies in a temperature range of 750° C.-850° C. in the case of a composition mainly comprising MgF.sub.2, in that of 800° C.-900° C. in the case of a composition mainly comprising CaF.sub.2, and in that of 775° C.-875° C. in the case of an intermediate composition of the both.

(14) Thereafter, in the same atmosphere, the same was heated in the vicinity of the temperature limits in which a solid solution starts to be formed (the temperature limits in the vicinity of 980° C., being a temperature at which a solid solution starts to be formed in the phase diagram of the MgF.sub.2—CaF.sub.2 binary system in FIG. 2), that is, in a temperature range of 900° C.-1150° C. for a relatively short period of time (0.5-8 hours), and then cooled so as to produce a MgF.sub.2—CaF.sub.2 binary system sintered body having a compact structure (secondary sintering step).

(15) The reason why the sintering step was divided into two steps, primary and secondary, is in order to suppress foaming as much as possible, and make the difference of the degree of sintering progress in every part (such as a periphery portion and a center portion) of the sintered body as small as possible.

(16) Particularly, in order to produce a large-size compact sintered body, the technique is important. The large size here is applied to press molded bodies in the below-described Examples having the size of about 220 mm×220 mm×H85 mm, while the small size is applied to the below-mentioned press molded bodies having the size of dia. 80 mm×H50 mm.

(17) In a test conducted in order to roughly grab proper heating conditions of the sintering step, the starting raw materials comprising MgF.sub.2—CaF.sub.2 binary system and MgF.sub.2 simple, respectively, were used, the sample size was the above large size, and both of the two stages of sintering were conducted in a nitrogen gas atmosphere. In the primary sintering, the temperature was held at 840° C. for 6 hours and in the subsequent secondary sintering, the heating time was set to be 2 hours with varied heating temperatures so as to measure the relative densities of the sintered bodies.

(18) As a result, as shown in FIG. 4, the relative densities of 95% or more could be secured in a wide range of heating conditions when two-stage sintering step was conducted, in either case of MgF.sub.2—CaF.sub.2 binary system and MgF.sub.2 simple, and particularly, in the case of MgF.sub.2—CaF.sub.2 binary system, those of 96%-97% could be obtained in the good condition range (heating at 950° C.-1050° C.).

(19) On the other hand, as shown in the below-described Comparative Examples 11 and 12, when only one-stage sintering step was conducted, the relative densities were 94% or less.

(20) The aim of mixing a CaF.sub.2 powder being a secondary raw material into a MgF.sub.2 powder being a main raw material is to cause the sintering reaction which allows the region of the formation of a solid solution on the phase diagram shown in FIG. 2 to become clearer, since MgF.sub.2 simple has a high melting point of 1252° C. and the temperature region of the formation of a solid solution is partially unclear, shown with dot lines.

(21) By mixing the right quantity of CaF.sub.2, being a fluoride of Ca which is presumed to have similar characteristics to Mg since Ca belongs to the same group as Mg on the periodic table of elements and its period is next to Mg, the melting point can be lowered and the temperature conditions of the formation of a solid solution can be clarified. By mixing CaF.sub.2, the melting point can be moved from the dot line region on the left end portion of the line indicating the temperature region of starting of the formation of a solid solution in FIG. 2 toward the solid line region of the intermediate mix proportions positioned on the right hand. As a result, it becomes easy to make the sintering temperature conditions proper. As a material to be mixed into MgF.sub.2 other than CaF.sub.2 being a fluoride of Ca, LiF being a fluoride of Li can be exemplified.

(22) As the sintering aid, two kinds, the CMC and the calcium stearate (SAC), were selected. With various adding proportions of each of them, the effects of addition thereof were confirmed. For comparison, a test with no sintering aid was also conducted.

(23) The main raw material MgF.sub.2 were mixed with the secondary raw material CaF.sub.2 in various mix proportions in a range of 0-97.5% by weight (included in a total of 100). After mixing using a ball mill for half a day, the two kinds of sintering aids were added in the proportion of 0-2% by weight (not included in the total), respectively. And using a pot mill, the same was mixed a whole day and night so as to obtain a starting raw material.

(24) The ball mill made of alumina having an inside diameter of 280 mm and a length of 400 mm was used, and balls of φ5: 1800 g, φ10: 1700 g, φ20: 3000 g and φ30: 2800 g, made of alumina were filled therein. The pot mill made of alumina having an inside diameter of 200 mm and a length of 250 mm was used.

(25) This compound of a prescribed quantity was filled into a wooden mold form, and using a uniaxial press device, compressed and molded at a uniaxial press pressure of 5 MPa or more. The inside size of the mold form used in the Examples was 220 mm×220 mm×H150 mm, and the inside size of the mold form used in a small-size test was 80 mm in diameter and 100 mm in height.

(26) This press molded body was put into a thick vinyl bag, which was then deaired and sealed, and it was put through a cold isostatic pressing (CIP) device. The press molded body was put into a molding part having a two-split structure (inside diameter 350 mm×H120 mm), which was sealed. The space between the vinyl bag with the press molded body inside and the molding part was filled with clean water, and then, isostatic pressing was conducted at a hydraulic pressure of 5 MPa or more so as to form a CIP molded body.

(27) The preliminary sintering step was conducted on the CIP molded bodies in an air atmosphere with various kinds of conditions in a heating temperature range of 500° C. to 750° C. and in a heating time range of 3 to 18 hours.

(28) After observing the appearance of the preliminary sintered bodies, the preliminary sintered bodies were sintered with the conditions which were regarded as good sintering conditions in the preceding preliminary test. The sintering step was conducted with the conditions wherein, in a nitrogen gas atmosphere, the temperature was raised from room temperature to 600° C. at a fixed rate for 6 hours, and held there for 8 hours, and then, it was raised to 1000° C. at a fixed rate for 2 hours and held there for 1 hour. And thereafter, it was lowered to 100° C. for 20 hours.

(29) By observing the appearance of the taken-out sintered bodies, the state of compactness of the inside thereof and the like, proper raw material mix proportions, raw material processing conditions, preliminary sintering conditions and the like were investigated.

(30) As a result, in cases where the mix proportion of the secondary raw material CaF.sub.2 to the main raw material MgF.sub.2 was less than 0.2% by weight, the sintering performance did not become much better due to mixing of CaF.sub.2. The difference in compactness between the inside portion and the periphery portion of the sintered body was likely to be large as is the case with MgF.sub.2 simple. Therefore, in order to improve the sintering performance by mixing thereof, it was judged that CaF.sub.2 of 0.2% by weight or more was required.

(31) On the other hand, in the case of 90.1% by weight or more, a larger number of large bubbles were left in the inside portion of the sintered body, compared with the periphery portion thereof, resulting in insufficient compactness.

(32) Judging from these situations, the mix proportions of CaF.sub.2 to MgF.sub.2, in which the difference in compactness between the inside portion and the periphery portion of the sintered body was small, that is, the sintering performance was in a good state, were 0.2-90% by weight. It was confirmed that the more desirable mix proportions thereof in which the difference in compactness between the inside portion and the periphery portion of the sintered body was smaller, resulting in an excellent degree of uniformity, were 1.5-80% by weight. Hence, the proper range of mix proportions of CaF.sub.2 was judged to be 0.2-90% by weight, more desirably 1.5-80% by weight.

(33) There was no big difference between the effects of the two kinds of sintering aids, but when the mix proportion of the sintering aid was less than 0.02% by weight, the shape keeping performance of the molded body was poor. And when the mix proportion thereof exceeded 1.1% by weight, coloring which appeared to be a residual of the sintering aid was noticed on the preliminary sintered body or the sintered body in some cases. Hence, the proper range of mix proportions of the sintering aid was judged to be 0.02-1% by weight.

(34) In a uniaxial press test using the above wooden mold form for a small-size test, when the molding pressure of the uniaxial press device was less than 5 MPa, the press molded body easily lost its shape in handling. As the molding pressure was gradually increased from 5 MPa, the bulk density of the press molded body gradually increased, and it was recognized that the bulk densities of the preliminary sintered body and the sintered body also tended to increase though slightly. The test was conducted with the molding pressure gradually increased to 100 MPa. However, even if the molding pressure was raised to 20 MPa or more, no improvement of performance of the preliminary sintered body or the sintered body was recognized. Hence, the proper value of the molding pressure of the uniaxial press device was decided to be 5 MPa or more, desirably 20 MPa.

(35) When the molding pressure of the CIP device was less than 5 MPa, the CIP molded body easily lost its shape in handling. As the molding pressure was gradually increased from 5 MPa, the bulk density of the CIP molded body gradually increased, and it was recognized that the bulk densities of the preliminary sintered body and the sintered body also tended to increase though slightly. The test was conducted with the CIP molding pressure gradually increased to 60 MPa. However, even if the molding pressure was raised to 20 MPa or more, no great improvement of performance of the preliminary sintered body or the sintered body was recognized. Hence, the proper value of the molding pressure of the CIP device was decided to be 5 MPa or more, desirably 20 MPa.

(36) The research of preliminary sintering conditions of the CIP molded body in an air atmosphere was conducted under the below-described conditions. By mixing MgF.sub.2 with CaF.sub.2 of 3% by weight, and adding CMC of 0.1% by weight as a sintering aid thereto, a starting raw material was prepared. Using the wooden mold form for a small-size test, by setting the molding pressure of a uniaxial press device to be 20 MPa and setting the molding pressure of a CIP device to be 20 MPa, CIP molded bodies were formed. Using the CIP molded bodies formed under such conditions, the preliminary sintering conditions were researched.

(37) At heating temperatures of less than 600° C., shrinkage was small compared with the size of the molded body, while at heating temperatures of 710° C. or more, the shrinkage rate was too high and therefore, shrinkage was difficult to control. Hence, the proper range of the preliminary sintering temperatures was decided to be 600° C.-700° C.

(38) Concerning the heating time, at 600° C., it was judged that 8-9 hours were optimal, and that 4-10 hours were proper, judging from the evaluation of the shrinkage rate. At 700° C., it was judged that 6-8 hours were optimal, and that 4-10 hours were proper. From these results, the heating conditions in the preliminary sintering step were decided to be at 600° C.-700° C. for 4-10 hours in an air atmosphere.

(39) What is likely to give most influence on the performance of a sintered body in producing the MgF.sub.2—CaF.sub.2 binary system sintered body for a radiation moderator is the sintering step. From the above researches and tests, the proper conditions until just before the sintering step were clarified.

(40) The sintering step and the sintering mechanism which appear to be desirable to a MgF.sub.2—CaF.sub.2 binary system sintered body for a radiation moderator are put in order.

(41) The terms “primary flocculation process” and “secondary flocculation process” which express the degrees of progress of the sintering step, are described below. The “primary flocculation process” refers to an event in the first half of the stage of sintering, and in the initial stage thereof, the intervals between particles gradually become narrower and the voids among particles also become smaller. With further progress of sintering, the particle-to-particle contact portions become thick and the voids among them become further smaller. Here, the majority of the voids are open pores connecting to the surrounding atmosphere. Up to this stage is called “primary flocculation process”.

(42) On the other hand, after the end of the primary flocculation process, with further progress of sintering, the open pores gradually decrease and turn into closed pores. Roughly, the stage of turning into closed pores and the subsequent stage of defoaming and compacting are generically called “secondary flocculation process”.

(43) In the producing method according to the preferred embodiment, due to raw material mixing, particle size control, mixing, two-stage molding (uniaxial press molding and CIP molding), preliminary sintering and the like, it was noticed that the voids among particles of the preliminary sintered body were small, and that the voids almost uniformly scattered without gathering (the first half stage of the primary flocculation process).

(44) In the heating process of the next sintering step, the heating temperature is gradually raised. Around the temperature limits (500° C.-550° C.) slightly lower than the preliminary sintering temperatures (600° C.-700° C.), particles start to gather, and thereafter, solid phase reaction starts in the temperature limits far lower than 980° C. at which a solid solution starts to be formed. With that, flocculation of particles makes progress, leading to shorter particle-to-particle distances and smaller voids.

(45) It is generally said that the solid phase reaction starts in the temperature limits lower by the order of 10% or further lower than the temperature at which a solid solution starts to be formed. From the observation results in the preliminary test by the present inventors, it was considered that the solid phase reaction started in far lower temperature limits than the above generally said temperature limits, in the order of 500° C.-550° C.

(46) It can be said on the ground that at 600° C., the lowest limit of the preliminary sintering temperature, sintering by the solid phase reaction has already made progress considerably so that the preliminary sintered body considerably shrinks compared with the CIP molded body.

(47) It is considered that the solid phase reaction makes progress at a low reaction rate in the temperature limits and that it makes progress at a quite high reaction rate in the temperature limits in the vicinity of 750° C., or more up to 980° C. Here, in the case of heating at relatively low temperatures (600° C.-700° C.) like assumed preliminary sintering for a short period of time, most of the voids remain in a state of open pore (which is the first half stage of the primary flocculation process).

(48) What attention should be paid to here is behavior of fine bubbles (foaming bubbles) generated through vaporization of part of the raw material in the temperature limits of about 850° C.-900° C. or more, as mentioned above. In the case of heating at about 1000° C. or more, the heating time should be as short as possible, since this formation of foaming bubbles comes to be noticeable.

(49) In the producing method according to the preferred embodiment, the sintering step is divided into two. In the primary sintering step, by heating in the relatively low temperature limits in which no foaming bubbles are formed for a long period of time, sintering of the whole body is allowed to make progress almost uniformly. The micro structure of the sintered body comprises mainly open pores, but part of them is turned into closed pores (after finishing the second half stage of the primary flocculation process, partially in the secondary flocculation process).

(50) In the secondary sintering step, heating is conducted in the relatively high temperature limits in the vicinity of 980° C. at which a solid solution starts to be formed for a minimum required period of time. As the micro structure of the sintered body, the formation of foaming bubbles is suppressed as much as possible, while the sintering reaction is allowed to make progress so as to turn almost all the open pores into closed pores, that is, the secondary flocculation process is finished, resulting in obtaining a high-density sintered body.

(51) Micro behavior of raw material particles is described here. It is presumed that particles of CaF.sub.2 are present around particles of the main raw material MgF.sub.2 and promote interface reaction with the particles of MgF.sub.2. Around a heating temperature exceeding 980° C. at which a solid solution starts to be formed, melting starts in the vicinity of a particle interface where the particles of CaF.sub.2 are present, and a solid solution of a MgF.sub.2—CaF.sub.2 binary system compound starts to be formed. It is presumed that this solid solution fills the voids among particles and that in some part, finer voids are also filled therewith through capillary phenomenon.

(52) On the other hand, even if the heating temperature is lower than 980° C., by heating and holding at about 750° C. or more for a relatively long period of time as described above, the solid phase reaction easily makes progress, the voids gradually decrease with the elapse of time so as to be closed pores. Parallel with that, a gas component within the closed pores diffuses within the bulk (parent) of the sintered body, leading to the progress of defoaming so as to make the sintered body compact with few bubbles (which is the secondary flocculation process).

(53) Also here, in heating at temperatures not lower than the starting temperature of foaming Tn (the starting temperature of foaming differs depending on the mix proportion of the raw materials MgF.sub.2 and CaF.sub.2), that is, temperatures exceeding 850° C.-900° C., attention should be paid to the formation of fine bubbles (foaming bubbles) generated through vaporization of the raw material. That is because it is presumed that the foaming bubbles contain fluorine gas, and it is considered that this gas is a relatively heavy element and difficult to diffuse in the bulk of the sintered body. As measures for that, to avoid heating in the temperature limits of vaporization as much as possible, and if necessary, to heat at a temperature as low as possible or to heat for a short period of time are considered.

(54) The difference in appearance between such foaming bubbles and bubbles left after pores became closed but could not be defoamed in the sintering step (hereinafter, referred to as residual bubbles) is described below. The sizes of the foaming bubbles generated by general heating for a relatively short period of time are approximately several μm in diameter, and the shapes thereof are almost perfect spheres.

(55) On the other hand, the shapes of the residual bubbles are not perfect spheres but irregular, and the sizes thereof are all mixed up, large, medium and small. Therefore, it is possible to distinguish the both according to the difference in shape. Here, in the case of high-temperature heating at temperatures far exceeding 1160° C., or heating at temperatures exceeding 1160° C. for a long period of time, a foaming bubble and a foaming bubble, or a residual bubble and a foaming bubble gather and grow to a large irregular bubble in some cases, resulting in difficulty in judging its origin.

(56) With the progress of the secondary flocculation process, the voids among particles become smaller, and all or most of the voids are surrounded by particles or a bridge portion of the sintered body so as to be closed pores (bubbles). Or depending on the conditions, gases are released through the voids (open pores), or gases within the bubbles permeate into the bulk (parent) such as the particles or the bridge portion of the sintered body to degas, resulting in no bubbles (referred to as a defoaming phenomenon).

(57) Whether the voids among particles are left as closed pores, that is, bubbles, or by degassing, no bubbles are formed, is a significant element for deciding the degree of achievement of compactness of the sintered body, leading to deciding the characteristics of the sintered body.

(58) Particularly in the case of sintering in a light element gas atmosphere such as He or Ne among inert gases, it is considered that the lighter element more easily diffuses within the pores or the bulk of the sintered body, leading to promoting the capillary phenomenon and defoaming phenomenon, so that bubbles are difficult to remain, leading to easy compacting.

(59) Thus, in order to make the whole compact, it is important to advance the primary flocculation process (in detail, it is presumed that the primary flocculation process is divided into the first half stage and the second half stage) and the secondary flocculation process almost simultaneously and almost uniformly on the whole in each process.

(60) In the invention according to the preferred embodiment, the preliminary sintering step chiefly equivalent to the first half stage of the primary flocculation process, the primary sintering step chiefly equivalent to the second half stage of the primary flocculation process, and the secondary sintering step chiefly equivalent to the secondary flocculation process are separately conducted, so as to make the two flocculation processes easy to make progress almost uniformly throughout the sintered body.

(61) However, Even if the sintering step is divided into two steps of preliminary sintering and sintering like this, a noticeable difference in degree of compactness is caused without proper heating conditions. For example, in the case of heating at high temperatures exceeding the proper limits in the preliminary sintering step, in the case of rapidly heating at the temperature raising stage of the sintering step, or in cases where the holding temperature in the sintering step is a high temperature exceeding the proper limits, a remarkable difference in degree of compactness is caused between the periphery portion and the inside portion of the sintered body. By improper heating, degassing becomes difficult in the process of compacting of the inside portion of the sintered body, and the compactness of the inside portion thereof is likely to be insufficient.

(62) It means that it is important to make the heating temperature pattern in the sintering step proper according to the size. Particularly, when producing a large-size sintered body, it is necessary to strictly control the heating conditions since a difference in degree of compactness between the periphery portion and the inside portion of such sintered body is easily caused.

(63) In order to clarify the relationship between the sample size and the sintering state, the present inventors conducted a small-size test using samples molded in a mold form of a uniaxial press device the inside size of 80 mm in diameter and 100 mm in height, and a large-size test using samples molded in a mold form thereof the inside size of 220 mm×220 mm×H150 mm.

(64) As a result, in the small-size test, there were cases where a high-density sintered body having a relative density exceeding 95% was obtained depending on the heating conditions even if one sintering step was conducted. On the other hand, in the large-size test, with one sintering step, any of the sintered bodies had a low density of less than 94% under the same sintering conditions as the small-size test.

(65) What is important here is that the whole of the preliminary sintered body has already advanced almost uniformly to the first half stage of the primary flocculation. Only preliminary sintered bodies in a state in which the whole body has already advanced to the first half stage of the primary flocculation were provided to these tests of the sintering step.

(66) The description of the sintering step test

(67) A mixture of a main raw material MgF.sub.2 with CaF.sub.2 of 3% by weight, and a raw material of MgF.sub.2 simple as a comparative material were used as starting materials. CMC of 0.1% by weight was added thereto as a sintering aid. And using the above mold form for a large-size test, the compounds were molded at a molding pressure of 20 MPa of a uniaxial press device and at a molding pressure of 20 MPa of a CIP device.

(68) The CIP molded bodies were preliminary sintered at 650° C. for 6 hours in an air atmosphere so as to obtain preliminary sintered bodies.

(69) In a nitrogen atmosphere, as primary sintering step, the preliminary sintered bodies were heated to 840° C. and the temperature was held there for 6 hours and then raised to a secondary sintering temperature for 2 hours.

(70) The secondary sintering temperature was varied from 700° C. to 1250° C., at an interval of every 50° C., and the temperatures each were held for 2 hours.

(71) Thereafter, the heating was stopped and the temperature was lowered by self-cooling (so-called furnace cooling) for about 20 hours, and when reaching 100° C. or lower at which time it was previously set to take out the sintered body, it was taken out.

(72) As a result of the sintering test with such two-stage sintering step, as shown in FIG. 3, in the case of a range from 900° C. to 1150° C., most of the bulk densities of the sintered bodies exceeded 2.96 g/cm.sup.3, which were high. The true density of the binary system compound was 3.15 g/cm.sup.3 and the relative density thereof was 94.0%, while the true density of the raw material of MgF.sub.2 simple was also 3.15 g/cm.sup.3 and the relative density thereof was also 94.0%.

(73) In either case of sintering temperatures of less than 900° C., and those of 1160° C. or more, the relative densities were lower than 94.0% (the bulk density of 2.96 g/cm.sup.3). The sintered bodies of the MgF.sub.2—CaF.sub.2 binary system raw material tended to have a higher relative density by the order of 0.5%-1.5% than those of MgF.sub.2 simple in a range of good sintering conditions.

(74) When observing the sections of those sintered bodies, in the case of sintered bodies sintered at temperatures lower than 900° C., not many but some open pores were noticed in some of them, wherein the bridge width of the sintered portion was narrow, so that it could be regarded as absolutely insufficient progress of sintering.

(75) In the case of sintered bodies sintered at temperatures of 1160° C. or more, especially 1200° C. or more, those had a porous pumiceous structure as if bubbles were innumerably formed inside. Fine bubbles which were almost perfect spheres of several to dozen μm in diameter were observed all over the sintered body and innumerable irregular bubbles (foaming bubbles and aggregates thereof) of 10 μm or more in diameter were found all over the sections.

(76) From another examination using a differential thermal analyzer by the present inventors, it was found out that when heating the compound of MgF.sub.2—CaF.sub.2 binary system, the weight clearly started to decrease at a temperature of about 800° C.-850° C. (the temperature becomes gradually higher within the temperature range as the mix proportion of CaF.sub.2 to MgF.sub.2 increases), and that the weight started to drastically decrease at about 850° C.-900° C. This means that a sublimation phenomenon in which MgF.sub.2 or CaF.sub.2 starts to dissolve/vaporize to generate fluorine starts due to heating at about 800° C.-850° C. or more.

(77) A foaming phenomenon through this fluorine sublimation is noticeably caused by heating at about 850° C.-900° C. or more, and fine bubbles are formed all over the sintered body. The behavior of the foaming bubbles such as defoaming or remaining as bubbles is decided according to the degree of progress of the sintering step, in which portion of the sintered body they were formed and the like. In the primary flocculation process, for example, since the whole sintered body contains mainly open pores, the majority of foaming bubbles can be defoamed through the open pores, leading to few bubbles left. In the secondary flocculation process, since the sintered body contains mainly closed pores, a large number of foaming bubbles cannot be defoamed, leading to remaining as bubbles. To swiftly complete the sintering in the secondary flocculation process leads to suppressing foaming and reducing residual bubbles.

(78) Hence, it is preferable that the transition from the primary flocculation process to the secondary flocculation process should be advanced in the whole sintered body with as small a difference of the degree of progress as possible among the portions thereof. However, it is not easy to undergo the transition from the primary flocculation process to the secondary flocculation process in the whole sintered body without a difference of the degree of progress among the portions thereof.

(79) Then, the present inventors considered the below-described method.

(80) Heating at a relatively low temperature in the temperature limits just below the starting temperature of foaming Tn (850° C.-900° C.), specifically in the temperature limits between (Tn-100° C.) and Tn for a relatively long period of time was conducted, so that the primary flocculation process and the first half of the secondary flocculation process were completed. And then, by heating at a temperature in the vicinity of the temperature (980° C.) at which a solid solution starts to be formed for a relatively short period of time, the second half of the secondary flocculation process was completed. By such sintering, the degree of progress of sintering could be made uniform in the whole sintered body, and the formation of bubbles could be suppressed as much as possible.

(81) How the proper sintering conditions were decided is described below.

(82) In the same manner as the above sintering condition change test, a main raw material MgF.sub.2 was mixed with CaF.sub.2 of 3% by weight. CMC of 0.1% by weight was added thereto as a sintering aid. The same was molded using a mold form for a large-size test at a molding pressure of 20 MPa of a uniaxial press device and a molding pressure of 20 MPa of a CIP device. Preliminary sintering was conducted on this CIP molded body at 650° C. which was held for 6 hours in an air atmosphere.

(83) As the conditions of the sintering step, the atmosphere was set to be a nitrogen gas atmosphere. Preliminary tests concerning each of heating and cooling conditions in the heating pattern were conducted in three cases of the required time of 4, 6 and 8 hours. As a result, in the case of 4 hours, small cracks occurred in the sintered body, while in the other cases, the results were good. Therefore, the required time was set to be 6 hours, shorter one selected from 6 and 8 hours.

(84) The atmosphere was set to be a nitrogen gas atmosphere, and the heating temperature was varied in a range of 700° C. to 1250° C. In eleven cases of the holding time of 2, 3, 4, 5, 6, 8, 10, 12, 14, 16 and 18 hours, the tests were conducted.

(85) As a result, in the case of less than 750° C., the compactness was insufficient, regardless of the holding time. In the case of heating at 750° C., the compactness was insufficient with a holding time of 4 hours or less. On the other hand, in the case of heating at 1160° C. or more, a large number of bubbles were generated due to too fast sintering speed, regardless of the holding time. In the case of a holding time of 18 hours, in some cases, foaming occurred in part of the periphery of the sintered body, leading to getting out of shape in appearance.

(86) Reviewing the results, in the case of heating at 750° C., the sintering state was good with a holding time of 14 and 16 hours.

(87) In the case of heating at 800° C., the sintering state was good with a holding time of 10 and 12 hours, while slightly insufficient with 6 and 8 hours, and beyond decision of quality with 14 hours or more.

(88) In the case of 830° C., the sintering state was good with a holding time of 10 and 12 hours.

(89) In the case of 850° C., the sintering state was good with a holding time of 8, 10 and 12 hours, while slightly insufficient with 5 hours, and beyond decision of quality with 14 hours or more.

(90) In the case of 900° C., the sintering state was good with a holding time of 5 to 12 hours, while slightly insufficient with 4 hours, and beyond decision of quality with 14 hours or more.

(91) In the case of 1000° C., the sintering state was good with a holding time of 5 to 12 hours, while slightly insufficient with 4 hours, and beyond decision of quality with 14 hours or more.

(92) In the case of 1050° C., the sintering state was good with a holding time of 5 to 10 hours, while slightly insufficient with 4 hours, and much foaming was seen with 12 hours or more.

(93) In the case of 1100° C., the sintering state was good with a holding time of 4 to 8 hours, while slightly insufficient with 3 hours or less, and much foaming was seen with 10 hours or more.

(94) In the case of 1150° C., the sintering state was good with a holding time of 2 and 3 hours, while much foaming was seen with 4 hours or more.

(95) In the case of 1160° C. or more, much foaming was seen with any holding time, and the results were beyond decision of quality or poor because of too much melting.

(96) Here, when the heating temperature was a comparatively low temperature of 750° C. to 850° C., the sintering state was good with a holding time of 6 to 12 hours, while that was slightly insufficient with a holding time of 3 to 5 hours. Since the method according to the preferred embodiment has the subsequent secondary sintering step, with the evaluation in this step (equivalent to the primary sintering step), the holding time of 3-12 hours was regarded as a good heating condition.

(97) In order to examine the relationship between the heating temperature and the bulk density of the sintered body, using the same preliminary sintered bodies as the above, the heating temperature was varied within a range of 600° C. to 1300° C. (with a holding time of 6 hours in any case).

(98) As a result, in the case of a heating temperature of 850° C., the bulk density was approximately 2.96 g/cm.sup.3 (the relative density of 94.0%). The sintered body having a bulk density of that value or more was judged to have sufficient compactness without troubles such as losing its shape in the treatment of the second step. On the other hand, in the case of heating temperatures of 1160° C. or more, in some cases, foaming occurred in part of the periphery of the sintered body, resulting in a trouble such as getting out of shape in appearance.

(99) From the above examination results of the sintering conditions and the relationship between the heating temperature and the bulk density, it was judged that, if the sintering step was one heating step, the heating temperature of 850° C. to 1150° C. and the holding time of 3 to 12 hours were proper.

(100) What was clarified here is, when relatively long time heating, such as at 900° C. for 14 hours or more, at 1000° C. for 14 hours or more, at 1100° C. for 10 hours or more, or at 1150° C. for 8 hours or more, was conducted, a large number of foaming bubbles were generated and part of those gathered and grew to large bubbles. Such sintered body involved defects which would cause cracks to occur from a large bubble portion or cause splitting in processing of the next mechanical processing step.

(101) From these situations, as a fundamental plan of the sintering step, it was decided that foaming should be suppressed as much as possible, and the sintering reaction should be allowed to sufficiently make progress, leading to producing a sintered body having a good processability in the subsequent mechanical processing step.

(102) At the first stage of the sintering step (the primary sintering step), it was aimed to suppress foaming to a minimum, to allow the sintering to make slow progress, and to minimize a difference of the degree of progress between the inside portion and the periphery portion of the sintered body.

(103) Therefore, the heating temperature was decided to be within the above range of 700° C. to 1150° C. Since the starting temperature of foaming Tn is 850° C. in the case of a raw material mainly comprising MgF.sub.2, it was judged that the heating temperature should be 850° C. or less, not exceeding the temperature. On the other hand, since the sintering state was insufficient in the case of heating at temperatures lower than the Tn by 100° C. or more, it was judged that the heating temperature at the first stage of the sintering step should be between (Tn-100° C.) and Tn, between 750° C. and 850° C. in the case of a raw material mainly comprising MgF.sub.2.

(104) The proper heating conditions in the primary sintering step were the heating temperature between (Tn-100° C.) and Tn, and the holding time of 3-12 hours. The same tendency was found even in cases where the mix proportion of CaF.sub.2 to MgF.sub.2 varied between 0.5-90% by weight.

(105) Heating at the stage of enhancing the sintering reaction of the sintered body, that is, heating in the secondary sintering step, was decided to be conducted properly in the temperature limits in the vicinity of 980° C. at which a solid solution starts to be formed, that is, 900° C. to 1150° C. It was aimed to make the holding time as short as possible in order to enhance the sintering reaction and suppress foaming. The proper holding time was decided to be 0.5 to 8 hours, since the enhancement of the sintering reaction was poor in the case of less than 0.5 hour, and too many bubbles were formed in the case of 9 hours or more.

(106) The examination of the proper conditions of the heating temperature and the holding time in the secondary sintering process when the atmospheric gas was changed from nitrogen gas to helium gas is described below.

(107) A mixture of a main raw material MgF.sub.2 with CaF.sub.2 of 3% by weight was used as a starting material, to which CMC of 0.1% by weight was added as a sintering aid.

(108) Using a mold form of press molding for a large-size test, the material was molded at a molding pressure of 20 MPa of a uniaxial press device and at a molding pressure of 20 MPa of a CIP device. This CIP molded body was preliminary sintered at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body.

(109) Using helium gas as the atmospheric gas in the primary and secondary sintering processes, the preliminary sintered body was heated to 840° C. which was held for 6 hours as primary sintering. Then, it was raised to each of secondary sintering temperatures varying in a range of 700° C. to 1250° C., at an interval of every 50° C. for 2 hours, and the target temperature was held for 2 hours. And then, the heating was stopped and the temperature was lowered by self-cooling (so-called furnace cooling) for about 20 hours, and when reached a predetermined taking-out temperature of 100° C. or lower, the sintered body was taken out.

(110) As a result of the sintering test with the above two-stage sintering step, as shown in FIG. 5, in the case of a temperature range of 900° C. to 1150° C., most of the sintered bodies had a high bulk density exceeding 2.96 g/cm.sup.3. The true density of the binary system compound was 3.15 g/cm.sup.3 and the relative density thereof was 94.0%, while the true density of the raw material of MgF.sub.2 simple was also 3.15 g/cm.sup.3 and the relative density thereof was also 94.0%.

(111) In either case of sintering temperatures lower than 900° C., and those of 1160° C. or more, the relative density was lower than 94.0% (the bulk density of 2.96 g/cm.sup.3). The sintered bodies sintered in a helium gas atmosphere tended to have a higher relative density within a range of good sintering conditions by the order of 0.5%-1% than in a nitrogen gas atmosphere.

(112) It is considered that the reason why the bulk density becomes high in a helium gas atmosphere is because the diffusion velocity of helium gas within the bulk (parent) of the sintered body is higher than that of nitrogen gas. It is presumed that, since helium gas more easily diffuses within the bulk than nitrogen gas, when voids become closed pores with the progress of sintering in the sintering process, part of the closed pores disappear without becoming bubbles, or the sizes of the closed pores become smaller.

(113) However, helium gas showed better effects within a range of the above proper sintering conditions, while the effects were not all-around, being not noticeably seen in the region other than the proper sintering conditions.

(114) As the reasons of such result, it was considered that under the sintering conditions outside the proper range, for example, there was a limit in improving too slow sintering speed due to an insufficient heating condition, or in the case of an excessive heating condition, the ununiformity of the sintering speed of every part of the sintered body could not be improved by enhancing the diffusivity of helium gas in the bulk.

(115) In the case of helium gas, when the heating temperature in the sintering step was less than 900° C., regardless of the holding time, or in the case of a holding time of 4 hours or less, the compactness was insufficient. When the heating temperature was 1160° C. or more, the sintering speed was too high, regardless of the holding time, as is the case with nitrogen gas, resulting in occurrence of a large number of bubbles, and in the case of a holding time of 16 hours or more, because of foaming, the appearance got out of shape in some cases.

(116) Accordingly, in the case of a starting raw material made by mixing mainly MgF.sub.2 with CaF.sub.2, it was judged that the proper range of sintering temperatures was 900° C.-1150° C., regardless of the kind of inert atmospheric gas in the sintering step. Furthermore, in the case of sintering temperatures of 930° C.-1100° C., even when the sintered body was provided to the mechanical processing, structural defects such as cracks were difficult to occur, leading to good mechanical processability. As a result, it was judged that the sintering temperature was more preferably in a temperature range of 930° C.-1100° C.

(117) Therefore, as proper heating conditions of the sintering step in a helium gas atmosphere, as is the case with the above nitrogen gas atmosphere, the proper condition of the primary sintering step was in a range of 750° C. or more and less than the starting temperature of foaming, while that of the secondary sintering step was in a temperature range of 900° C.-1150° C.

(118) The inert gas is not limited to nitrogen and helium. In the case of argon or neon, the same effects can be obtained. Moreover, since neon is expected to have high solubility or high diffusivity in the parent of the sintered body, like helium, the defoaming phenomenon can be more promoted and effects equal to those of helium can be expected.

(119) When the heating conditions of the sintering step were within the proper range, the state of the completed sintered body was wholly compact in any case, and no clearly defective portion such as a locally-found large void or a crack seen in a general ceramic sintered body could be found in this sintered body.

(120) As the particle size control of a MgF.sub.2 powder and a CaF.sub.2 powder each, using a container of a pot mill made of alumina the size of an inside diameter of 200 mm and a length of 250 mm as a ball mill, balls made of alumina, φ20 mm: 3000 g and φ30 mm: 2800 g, were filled therein. And about 3000 g of each of the raw material powders was filled therein and rotated for a prescribed period of time. The rotation was stopped every two or three days so as to take powder samples and measure the same.

(121) The particle size distribution were measured using ‘a laser diffraction particle size analyzer (type number: SALD-2000)’ made by Shimadzu Corporation according to JIS R 1629 ‘Determination of particle size distributions for fine ceramic raw powders by laser diffraction method’. The sample preparation at that time was conducted according to JIS R1622 ‘General rules for the sample preparation of particle size analysis of fine ceramic raw powder’.

(122) As the light source of the SALD-2000, a semiconductor laser of a wavelength of 680 nm is used. The sensitivity to particles having a diameter larger than this wavelength (about 1 μm or more) was good and the measurement accuracy was high. On the other hand, as for the sensitivity to fine particles of the order of submicron, it was considered that the measurement accuracy was low compared with the particles having a large diameter though some way to improve the measurement accuracy was devised.

(123) Therefore, it is considered that the actual number of fine particles of the order of submicron may be larger than the analysis result. In other words, ‘there is a high possibility that the ratio of fine particles may be larger than the analysis result in the actual particle size distribution, and that the mean particle diameter may be smaller than the shown value thereof.’ However, in the present application, the values of the particle sizes measured according to the above measurement method are shown as they are.

(124) FIG. 1 shows a particle size distribution in the case of the above pulverization of a MgF.sub.2 raw material. It was found that the median diameter was about 10 μm after three-day pulverization, about 8 μm after five-day pulverization, about 6 μm after one-week pulverization, about 5 μm after two-week pulverization, and about 3 μm after four-week pulverization. Even if a MgF.sub.2 raw material and a CaF.sub.2 raw material were mixed, the particle size distribution similar to that of the raw material of MgF.sub.2 simple could be obtained.

(125) Concerning the original raw material powder and the above particles whose mean particle diameter became about 4 μm after three-week pulverization, the shapes in appearance of the particles of them, respectively, were observed using an electron microscope. In the original raw material powder, some irregular-shaped particles, mainly angular particles were seen, while most of the particles after three-week pulverization were rounded. It was found that most of the angular portions of the particles of the original raw material powder were worn by pulverization so as to be approximated to sphere shapes.

(126) The shape of the particle size distribution curve of the powder after this particle size control can be expressed by being likened to the shape of a mountain range. When the shape of the curve looks like as if “two peaks” or “three peaks” run in a line, it is called ‘2-peak type’ or ‘3-peak type’. The curve of three-day pulverization and that of five-day pulverization obviously showed a high ratio of coarse particle portions, respectively, which were regarded as ‘2-peak type’ or ‘3-peak type’. On the other hand, in the case of one-week pulverization and two-week pulverization, respectively, the ratio of the coarse particle portions substantially decreased, the coarse particle portion of 30 μm or more remained several % by weight, but that of 50 μm or more almost disappeared, and the shape of the particle size distribution curve was reaching almost the 1-peak type having a small gently inclined portion around the particle diameter of 10 μm-15 μm (this state is called ‘sub-1-peak type’). And in the case of four-week pulverization, the coarse particle portion of 30 μm or more almost disappeared and the shape of the particle size distribution curve could be approximately similar to a normal distribution (this state is called ‘1-peak type’).

(127) Thus, the particle shapes were rounded and approached sphere shapes by pulverization of the raw material powder, and the ratio of coarse particles decreased, resulting in a great change of the shape of the particle size distribution curve from ‘2-peak type’ or ‘3-peak type’ to ‘sub-1-peak type’, and further to ‘1-peak type’. This change exerted a noticeably good influence on sintering reaction in the sintering process.

EXAMPLES

(128) Examples according to the present invention are described below by reference to the Figures, but the present invention is not limited to the below-described Examples.

(129) Here, a characteristic evaluation test conducted on sintered bodies is described. Samples for evaluation were prepared by prototyping large-size sintered bodies (rough size of the sintered body: about 205 mm×about 205 mm×H about 70 mm) and conducting mechanical processing such as cut-out in the shape of a required sample thereon.

(130) In order to evaluate the neutron moderation performance, as shown in the above Non-Patent Documents 1 and 2, a beam emitted from an accelerator was allowed to collide with a plate made of Be being a target, and by nuclear reaction, high-energy neutrons (fast neutrons) were mainly generated.

(131) Then, using Pb and Fe each having a large inelastic scattering cross section as a moderator in the first half of moderation, the fast neutrons were moderated to some extent (approximately, up to 1 MeV) while suppressing the attenuation of the number of neutrons.

(132) The moderated neutrons were irradiated to a moderator to be evaluated (a moderator in the second half of moderation), and by examining the neutrons after moderation, the moderator was evaluated.

(133) The examination of the neutrons was conducted according to the method described in the above ‘Non-Patent Document 3’.

(134) The moderators to be evaluated were made of raw materials MgF.sub.2 and CaF.sub.2 in some varied mix proportions. Through the mixing step of each kind of raw materials, molding step and sintering step, a high-density MgF.sub.2—CaF.sub.2 binary system sintered body having a relative density in a fixed range (95.0±0.5%) was produced. The total thickness of a moderator in the second half was set to be 600 mm in any case.

(135) What was evaluated here is the dose of epithermal neutrons having intermediate-level energy which is effective for therapy, and how many fast neutrons and gamma-rays having high-level energy which has a high possibility of adversely influencing a patient (side effects), remained in the neutrons moderated by the moderator. The results are shown in FIG. 6 (Table 1).

(136) The dose of epithermal neutrons effective for therapy slightly varied, as the quantity of CaF.sub.2 mixed into MgF.sub.2 was increased, but the digit of the neutron flux (dose) of epithermal neutrons was the ninth power in any case, so that regardless of the mix proportion, the dose thereof sufficient for therapy was secured.

(137) On the other hand, the mix rate of fast neutrons having a high possibility of adversely influencing a patient (the ratio of fast neutron dose in the total neutron dose after passing through a moderator) was the lowest in the case of mixing CaF.sub.2 of several to 10% by weight. It gradually increased as the mix proportion thereof far exceeded these mix proportions and increased to 20% by weight, and to 40% by weight. It was the highest when CaF.sub.2 was 100% by weight.

(138) The mix rate of gamma-rays having the next highest possibility of adversely influencing a patient after fast neutrons (the ratio of gamma-ray dose in the total neutron dose after passing through a moderator) was a low digit of E.sup.−14 (the minus 14th power), regardless of the mix proportion of CaF.sub.2 to MgF.sub.2. The influence of gamma-rays was small, regardless of the mix proportion of CaF.sub.2.

(139) From these results, it was proved that when the main raw material MgF.sub.2 was mixed with CaF.sub.2 of 2-10% by weight, it had the most excellent performance as a moderator. Even if the mix proportion was other than such mix proportions, for example, 0.2% by weight or more and less than 2% by weight, or 10.1% by weight or more and 90% by weight or less, the neutrons were on the level usable for therapy.

(140) The evaluation results are limited to the cases where the relative density of the sintered body is roughly within a fixed range (95.0±0.5%). The higher relative density the sintered body has, the lower the residual dose of fast neutrons is. Conversely, the lower relative density the sintered body has, the higher the residual dose of fast neutrons is. Accordingly, the importance of improvement of the density of the sintered body is the same.

(141) Concerning the moderation performance of a moderator to neutrons, it was sufficient only that a MgF.sub.2—CaF.sub.2 binary system sintered body having a compact structure should have a bulk density of 2.96 g/cm.sup.3 or more.

(142) The moderator to neutrons is required to have mechanical strength other than moderation performance. It was proved by the below-described examination of mechanical strength that the sintered body for a radiation moderator according to the present invention had sufficient mechanical strength, with which it could be used without problems in processing and molding such as cutting-off, grinding, polishing, cleaning and drying as a moderating member in a moderation system device for BNCT, and further in handling such as the installation thereof in the moderation system device. Even if it was irradiated with neutrons, it was capable of resisting their irradiation impacts, being extremely excellent.

(143) As mechanical strengths, bending strength and Vickers hardness were examined. The samples for bending strength, having a size of 4 mm×46 mm×t3 mm with the upper and lower surfaces optically polished were prepared according to JIS C2141, and tested according to the three-point bending test JIS R1601.

(144) To obtain the Vickers hardness, according to JIS Z2251-1992, using ‘Micro Hardness Tester’ made by Shimadzu Corporation, an indenter having a load of 100 g was pressed for 5 seconds of loading time so as to measure the diagonal length of the impression, which was converted into hardness in the following manner.
Vickers hardness=0.18909×P/(d).sup.2

(145) Here, P: load (N) and d: diagonal length of impression (mm)

Example 1

(146) A high-purity MgF.sub.2 powder being a main raw material (mean particle diameter of 4 μm and purity of 99.9% by weight or more) was mixed with a CaF.sub.2 powder (mean particle diameter of 4 μm and purity of 99.9% by weight or more) of 1.5% by weight, and mixed using a ball mill for 12 hours. Thereafter, a carboxymethyl cellulose (CMC) solution was added thereto as a sintering aid in the proportion of 0.1% by weight to 100 of the mixture, which was mixed in a pot mill for 12 hours so as to obtain a starting raw material.

(147) This starting raw material was filled into a mold form (inside size of 220 mm×220 mm×H150 mm) of a uniaxial press device and compressed at a uniaxial press pressure of 20 MPa to be molded. This press molded body (size of about 220 mm×220 mm×t85 mm), which was put into a thick vinyl bag and sealed after deairing, was put into a molding part (inside size: dia. 350 mm×H120 mm) of a cold isostatic pressing (CIP) device. Clean water was filled into the space between the vinyl bag with the press molded body therein and the CIP molding part, and by isostatic pressing at a molding pressure of 20 MPa at room temperature, CIP molding was conducted.

(148) The preliminary sintering step was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body.

(149) This preliminary sintered body was heated from room temperature to 800° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere, and the temperature was held there for 8 hours. It was then raised to 1050° C. at a fixed rate for 2 hours and held there for 1.5 hours. Heating was then stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 20 hours to 100° C. or less at which time it was previously set to take out the sintered body, after which it was taken out.

(150) The bulk density of the sintered body was calculated at 3.02 g/cm.sup.3 (the true density of this compound is 3.15 g/cm.sup.3 and the relative density thereof is 95.9%, and hereinafter, referred to as “true density of 3.15 g/cm.sup.3 and relative density of 95.9%”) from the bulk volume of the appearance thereof and the weight thereof. The sintering state thereof was good.

(151) Since the appearance of the sintered body was a square form, the ‘bulk density’ here was obtained by a method wherein the bulk volume was calculated from the measured two sides of the square and thickness, and the weight separately measured was divided by the bulk volume. This also applied to the following.

(152) Using a sample taken from this sintered body, evaluation tests of neutron moderation performance and characteristics of every kind were conducted. The results are shown in FIG. 7 (Table 2).

(153) This also applied to the following Examples and Comparative Examples. Here, concerning a sintered body of MgF.sub.2 simple and a sintered body of CaF.sub.2 simple, being comparative materials, the neutron moderation performance and mechanical strengths were measured like the Examples and Comparative Examples.

(154) The sintered body in Example 1 showed excellent neutron moderation performance, and the mechanical strengths thereof were also good enough not to cause problems in handling in the next step.

Example 2

(155) A MgF.sub.2 powder being a main raw material (mean particle diameter of 6 μm and purity of 99.9% by weight) was mixed with a CaF.sub.2 powder (mean particle diameter of 6 μm and purity of 99.9% by weight) of 0.2% by weight, and mixed using a ball mill for 12 hours. Thereafter, with the same molding conditions as in the above Example 1, a CIP molded body was produced, and the preliminary sintering step was conducted on this CIP molded body at 640° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body.

(156) This preliminary sintered body was heated from room temperature to 800° C. at a fixed rate for 6 hours in a helium gas atmosphere, and the temperature was held there for 5 hours. It was then raised to 920° C. at a fixed rate for 4 hours and held there for 1 hour. Then, the temperature was lowered by furnace cooling to a predetermined taking-out temperature of 100° C., and the sintered body was taken out. The bulk density of the sintered body was 2.97 g/cm.sup.3 (true density of 3.15 g/cm.sup.3 and relative density of 94.3%). It was rather light, but the sintering state thereof was not unusual in appearance.

(157) Any of the evaluation results of the neutron moderation performance and mechanical strengths were good as shown in Table 2.

Example 3

(158) The same MgF.sub.2 powder was mixed with the same CaF.sub.2 powder of 2% by weight as those in the above Example 1 and mixed using a ball mill for 12 hours. Thereafter, calcium stearate (SAC) of 1.0% by weight was added thereto as a sintering aid. The compound was mixed in a pot mill for 12 hours so as to obtain a starting raw material.

(159) Using a uniaxial press device, the press molding was conducted at a press pressure of 30 MPa, and then, using a cold isostatic pressing (CIP) device, the CIP molding was conducted at a CIP pressure of 30 MPa. The preliminary sintering step was conducted on this CIP molded body at 700° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body.

(160) This preliminary sintered body was heated from room temperature to 840° C. at a fixed rate for 6 hours in an air atmosphere, and the temperature was held there for 8 hours. It was then raised to 1150° C. at a fixed rate for 2 hours and held there for 0.75 hour. The temperature was then lowered by furnace cooling to a predetermined taking-out temperature of 100° C., and the sintered body was taken out. The bulk density of the sintered body was 3.06 g/cm.sup.3 (true density of 3.15 g/cm.sup.3 and relative density of 97.1%), and the sintering state thereof was good.

(161) Any of the evaluation results of the neutron moderation performance and mechanical strengths were good as shown in Table 2.

Example 4

(162) The same MgF.sub.2 powder was mixed with the same CaF.sub.2 powder of 3% by weight as those in the above Example 1 and mixed using a ball mill for 12 hours. Thereafter, a CMC solution of 0.03% by weight was added thereto as a sintering aid. The compound was mixed in a pot mill for 12 hours so as to obtain a starting raw material.

(163) Using a uniaxial press device, the press molding was conducted at a press pressure of 30 MPa, and then, using a cold isostatic pressing (CIP) device, the CIP molding was conducted at a CIP pressure of 30 MPa. The preliminary sintering step was conducted on this CIP molded body at 660° C. for 8 hours in an air atmosphere so as to obtain a preliminary sintered body.

(164) This preliminary sintered body was heated from room temperature to 830° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere, and the temperature was held there for 6 hours. It was then raised to 1080° C. at a fixed rate for 2 hours and held there for 2 hours. The temperature was then lowered by furnace cooling to a predetermined taking-out temperature of 100° C., and the sintered body was taken out. The bulk density of the sintered body was 3.07 g/cm.sup.3 (true density of 3.15 g/cm.sup.3 and relative density of 97.5%), and the sintering state thereof was good.

(165) Any of the evaluation results of the neutron moderation performance and mechanical strengths were good as shown in Table 2.

Example 5

(166) The same MgF.sub.2 powder was mixed with the same CaF.sub.2 powder of 7.5% by weight as those in the above Example 1 and mixed using a ball mill for 12 hours. Thereafter, calcium stearate (SAC) of 0.07% by weight was added thereto as a sintering aid. The compound was mixed in a pot mill for 12 hours so as to obtain a starting raw material.

(167) Using a uniaxial press device, the press molding was conducted at a press pressure of 40 MPa, and then, using a cold isostatic pressing (CIP) device, the CIP molding was conducted at a CIP pressure of 40 MPa. The preliminary sintering step was conducted on this CIP molded body at 690° C. for 8 hours in an air atmosphere so as to obtain a preliminary sintered body.

(168) This preliminary sintered body was heated from room temperature to 830° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere, and the temperature was held there for 9 hours. It was then raised to 1080° C. at a fixed rate for 2 hours and held there for 2 hours. The temperature was then lowered by furnace cooling to a predetermined taking-out temperature of 100° C., and the sintered body was taken out. The bulk density of the sintered body was 3.06 g/cm.sup.3 (true density of 3.15 g/cm.sup.3 and relative density of 97.1%), and the sintering state thereof was good.

(169) Any of the evaluation results of the neutron moderation performance and mechanical strengths were good as shown in Table 2.

Example 6

(170) A MgF.sub.2 powder being a main raw material (mean particle diameter of 5 μm and purity of 99.9% by weight) was mixed with a CaF.sub.2 powder (mean particle diameter of 5 μm and purity of 99.9% by weight) of 18% by weight, and mixed using a ball mill for 12 hours. Thereafter, a CMC solution of 0.3% by weight was added thereto as a sintering aid. The compound was mixed in a pot mill for 12 hours so as to obtain a starting raw material.

(171) Using a uniaxial press device, the press molding was conducted at a press pressure of 6 MPa, and then, using a cold isostatic pressing (CIP) device, the CIP molding was conducted at a CIP pressure of 15 MPa. The preliminary sintering step was conducted on this CIP molded body at 630° C. for 8 hours in an air atmosphere so as to obtain a preliminary sintered body.

(172) This preliminary sintered body was heated from room temperature to 820° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere, and the temperature was held there for 6 hours. It was then raised to 930° C. at a fixed rate for 2 hours and held there for 4 hours. The temperature was then lowered by furnace cooling to a predetermined taking-out temperature of 100° C., and the sintered body was taken out. The bulk density of the sintered body was 2.98 g/cm.sup.3 (true density of 3.15 g/cm.sup.3 and relative density of 94.6%), and the sintering state thereof was good.

(173) Any of the evaluation results of the neutron moderation performance and mechanical strengths were good as shown in Table 2.

Example 7

(174) The same MgF.sub.2 powder was mixed with the same CaF.sub.2 powder of 2.5% by weight as those in the above Example 6 and mixed using a ball mill for 12 hours. Thereafter, a CMC solution of 0.1% by weight was added thereto as a sintering aid. The compound was mixed in a pot mill for 12 hours so as to obtain a starting raw material. Using a uniaxial press device, the press molding was conducted at a press pressure of 30 MPa, and then, using a cold isostatic pressing (CIP) device, the CIP molding was conducted at a CIP pressure of 30 MPa. The preliminary sintering step was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body.

(175) This preliminary sintered body was heated from room temperature to 840° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere, and the temperature was held there for 5 hours. It was then raised to 1150° C. at a fixed rate for 2 hours and held there for 0.5 hours. The temperature was then lowered by furnace cooling to a predetermined taking-out temperature of 100° C., and the sintered body was taken out. The bulk density of the sintered body was 3.01 g/cm.sup.3 (true density of 3.15 g/cm.sup.3 and relative density of 95.6%), and the sintering state thereof was good.

(176) Any of the evaluation results of the neutron moderation performance and mechanical strengths were good as shown in Table 2.

Example 8

(177) The same MgF.sub.2 powder was mixed with the same CaF.sub.2 powder of 50% by weight as those in the above Example 1 and mixed using a ball mill for 12 hours. Thereafter, a CMC solution of 1% by weight was added thereto as a sintering aid. The compound was mixed in a pot mill for 12 hours so as to obtain a starting raw material. Using a uniaxial press device, the press molding was conducted at a press pressure of 7 MPa, and then, using a cold isostatic pressing (CIP) device, the CIP molding was conducted at a CIP pressure of 12 MPa. The preliminary sintering step was conducted on this CIP molded body at 600° C. for 5 hours in an air atmosphere so as to obtain a preliminary sintered body.

(178) This preliminary sintered body was heated from room temperature to 860° C. at a fixed rate for 6 hours in a helium gas atmosphere, and the temperature was held there for 8 hours. It was then raised to 1080° C. at a fixed rate for 2 hours and held there for 2 hours. The temperature was then lowered by furnace cooling to a predetermined taking-out temperature of 100° C., and the sintered body was taken out. The bulk density of the sintered body was 3.02 g/cm.sup.3 (true density of 3.16 g/cm.sup.3 and relative density of 95.6%). It was rather light, but the sintering state thereof was not unusual in appearance.

(179) Any of the evaluation results of the neutron moderation performance and mechanical strengths were good as shown in Table 2.

Example 9

(180) The same MgF.sub.2 powder was mixed with the same CaF.sub.2 powder of 50% by weight as those in the above Example 2 and mixed using a ball mill for 12 hours. Thereafter, a CMC solution of 1% by weight was added thereto as a sintering aid. The compound was mixed in a pot mill for 12 hours so as to obtain a starting raw material. Using a uniaxial press device, the press molding was conducted at a press pressure of 30 MPa, and then, using a cold isostatic pressing (CIP) device, the CIP molding was conducted at a CIP pressure of 30 MPa. The preliminary sintering step was conducted on this CIP molded body at 610° C. for 7 hours in an air atmosphere so as to obtain a preliminary sintered body.

(181) This preliminary sintered body was heated from room temperature to 860° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere, and the temperature was held there for 8 hours. It was then raised to 970° C. at a fixed rate for 2 hours and held there for 4 hours. The temperature was then lowered by furnace cooling to a predetermined taking-out temperature of 100° C., and the sintered body was taken out. The bulk density of the sintered body was 3.00 g/cm.sup.3 (true density of 3.16 g/cm.sup.3 and relative density of 94.9%). It was somewhat light, but the sintering state thereof was not unusual in appearance.

(182) Any of the evaluation results of the neutron moderation performance and mechanical strengths were good as shown in Table 2.

Example 10

(183) The same MgF.sub.2 powder was mixed with the same CaF.sub.2 powder of 75% by weight as those in the above Example 1 and mixed using a ball mill for 12 hours. Thereafter, SAC of 0.07% by weight was added thereto as a sintering aid. The compound was mixed in a pot mill for 12 hours so as to obtain a starting raw material. Using a uniaxial press device, the press molding was conducted at a press pressure of 8 MPa, and then, using a cold isostatic pressing (CIP) device, the CIP molding was conducted at a CIP pressure of 10 MPa. The preliminary sintering step was conducted on this CIP molded body at 650° C. for 5 hours in an air atmosphere so as to obtain a preliminary sintered body.

(184) This preliminary sintered body was heated from room temperature to 880° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere, and the temperature was held there for 8 hours. It was then raised to 1060° C. at a fixed rate for 2 hours and held there for 3 hours. The temperature was then lowered by furnace cooling to a predetermined taking-out temperature of 100° C., and the sintered body was taken out. The bulk density of the sintered body was 3.02 g/cm.sup.3 (true density of 3.17 g/cm.sup.3 and relative density of 95.3%). It was somewhat light, but the sintering state thereof was not unusual in appearance.

(185) Any of the evaluation results of the neutron moderation performance and mechanical strengths were good as shown in Table 2.

Example 11

(186) The same MgF.sub.2 powder was mixed with the same CaF.sub.2 powder of 88% by weight as those in the above Example 6 and mixed using a ball mill for 12 hours. Thereafter, a CMC solution of 1% by weight was added thereto as a sintering aid. The compound was mixed in a pot mill for 12 hours so as to obtain a starting raw material. Using a uniaxial press device, the press molding was conducted at a press pressure of 30 MPa, and then, using a cold isostatic pressing (CIP) device, the CIP molding was conducted at a CIP pressure of 30 MPa. The preliminary sintering step was conducted on this CIP molded body at 650° C. for 5 hours in an air atmosphere so as to obtain a preliminary sintered body.

(187) This preliminary sintered body was heated from room temperature to 880° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere, and the temperature was held there for 8 hours. It was then raised to 950° C. at a fixed rate for 2 hours and held there for 4 hours. The temperature was then lowered by furnace cooling to a predetermined taking-out temperature of 100° C., and the sintered body was taken out. The bulk density of the sintered body was 3.01 g/cm.sup.3 (true density of 3.17 g/cm.sup.3 and relative density of 95.0%). It was somewhat light, but the sintering state thereof was not unusual in appearance.

(188) Any of the evaluation results of the neutron moderation performance and mechanical strengths were good as shown in Table 2.

Example 12

(189) The same MgF.sub.2 powder was mixed with the same CaF.sub.2 powder of 88% by weight as those in the above Example 1 and mixed using a ball mill for 12 hours. Thereafter, a CMC solution of 1% by weight was added thereto as a sintering aid. The compound was mixed in a pot mill for 12 hours so as to obtain a starting raw material. Using a uniaxial press device, the press molding was conducted at a press pressure of 8 MPa, and then, using a cold isostatic pressing (CIP) device, the CIP molding was conducted at a CIP pressure of 10 MPa. The preliminary sintering step was conducted on this CIP molded body at 650° C. for 5 hours in an air atmosphere so as to obtain a preliminary sintered body.

(190) This preliminary sintered body was heated from room temperature to 880° C. at a fixed rate for 6 hours in a helium gas atmosphere, and the temperature was held there for 8 hours. It was then raised to 1120° C. at a fixed rate for 2 hours and held there for 2 hours. The temperature was then lowered by furnace cooling to a predetermined taking-out temperature of 100° C., and the sintered body was taken out. The bulk density of the sintered body was 3.04 g/cm.sup.3 (true density of 3.17 g/cm.sup.3 and relative density of 95.9%), and the sintering state thereof was not unusual in appearance.

(191) Any of the evaluation results of the neutron moderation performance and mechanical strengths were good as shown in Table 2.

Comparative Example 1

(192) A MgF.sub.2 powder being a main raw material (mean particle diameter of 8 μm and purity of 99.9% by weight) was mixed with a CaF.sub.2 powder (mean particle diameter of 8 μm and purity of 99.9% by weight) of 1.5% by weight, and mixed using a ball mill for 12 hours. Thereafter, SAC of 0.07% by weight was added thereto as a sintering aid. The compound was mixed in a pot mill for 12 hours so as to obtain a starting raw material.

(193) In a similar manner to the above Example 1, using a uniaxial press device, the press molding was conducted at a press pressure of 20 MPa, and then, using a cold isostatic pressing (CIP) device, the CIP molding was conducted at a CIP pressure of 20 MPa. The preliminary sintering step was conducted on this CIP molded body at 550° C. for 8 hours in an air atmosphere so as to obtain a preliminary sintered body.

(194) This preliminary sintered body was heated from room temperature to 670° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere, and the temperature was held there for 6 hours. It was then raised to 1200° C. at a fixed rate for 2 hours and held there for 2 hours. The temperature was then lowered by furnace cooling to a predetermined taking-out temperature of 100° C., and the sintered body was taken out.

(195) The bulk density of the sintered body was 2.93 g/cm.sup.3 (true density of 3.15 g/cm.sup.3 and relative density of 93.0%), which was light. When observing the inside of the sintered body, there were a myriad of large bubbles of 0.1 mm or more in diameter. It was considered that these large bubbles were aggregates of fine foaming bubbles, or those of foaming bubbles and residual bubbles since the high mix proportion of 12% by weight of the MgF.sub.2 powder having a low melting point and heating at a high temperature of 1200° C. in the last sintering step allowed foaming to easily occur.

(196) Some insufficient levels of neutron moderation performance and mechanical strengths were noticed.

Comparative Example 2

(197) A MgF.sub.2 powder being a main raw material (mean particle diameter of 10 μm and purity of 99.9% by weight) was mixed with a CaF.sub.2 powder (mean particle diameter of 10 μm and purity of 99.9% by weight) of 0.2% by weight, and a starting raw material was prepared in a similar manner to the Comparative Example 1.

(198) Using a uniaxial press device, the press molding was conducted at a press pressure of 4 MPa, and then, the CIP molding was conducted on this press molded body at a molding pressure of 4 MPa so as to obtain a CIP molded body in a similar manner to the Example 1. The preliminary sintering step was conducted on this CIP molded body at 600° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body.

(199) This preliminary sintered body was heated from room temperature to 830° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere, and the temperature was held there for 5 hours. It was then raised to 950° C. at a fixed rate for 2 hours and held there for 4 hours. The temperature was then lowered by furnace cooling to a predetermined taking-out temperature of 100° C., and the sintered body was taken out.

(200) The bulk density of the sintered body was 2.90 g/cm.sup.3 (true density of 3.15 g/cm.sup.3 and relative density of 92.1%), which was relatively light. This sintered body was soaked in pure water colored with a small quantity of ink solution for about 1 hour, and after raising it therefrom, the broken-cross section thereof was observed. The periphery portion thereof was wholly colored with this ink solution. It was considered that due to insufficient sintering, a large number of open pores were left.

(201) Some insufficient levels of neutron moderation performance and mechanical strengths were noticed.

Comparative Example 3

(202) A MgF.sub.2 powder being a main raw material (mean particle diameter of 12 μm and purity of 99.9% by weight) was mixed with a CaF.sub.2 powder (mean particle diameter of 12 μm and purity of 99.9% by weight) of 5% by weight, and mixed using a ball mill for 12 hours. Thereafter, a CMC solution of 1.0% by weight was added thereto as a sintering aid. The compound was mixed in a pot mill for 12 hours so as to obtain a starting raw material.

(203) In a similar manner to the Example 1, using a uniaxial press device, the press molding was conducted at a press pressure of 20 MPa, and then, the CIP molding was conducted on this press molded body at a molding pressure of 20 MPa so as to obtain a CIP molded body. The preliminary sintering step was conducted on this CIP molded body at 700° C. for 10 hours in an air atmosphere so as to obtain a preliminary sintered body.

(204) This preliminary sintered body was heated from room temperature to 900° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere, and the temperature was held there for 10 hours. It was then raised to 1200° C. at a fixed rate for 2 hours and held there for 4 hours. The temperature was then lowered by furnace cooling to a predetermined taking-out temperature of 100° C., and the sintered body was taken out.

(205) In part of the periphery portion of the sintered body, peeling was noticed. It was considered that this peeling was caused since foaming bubbles and residual bubbles gathered in the periphery portion and part of the periphery portion was cracked by the internal pressure of the bubbles. Here, since some part of the sintered body lost its shape, the bulk density thereof could not be measured.

Comparative Example 4

(206) The same MgF.sub.2 powder was mixed with the same CaF.sub.2 powder of 5% by weight as those in the above Comparative Example 1, and a starting raw material was prepared in a similar manner thereto. Using a uniaxial press device, the press molding was conducted at a press pressure of 3 MPa, and then, this press molded body was CIP molded at a molding pressure of 3 MPa so as to obtain a CIP molded body in a similar manner to the above Example 1. The preliminary sintering step was conducted on this CIP molded body at 600° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body.

(207) This preliminary sintered body was heated from room temperature to 900° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere, and the temperature was held there for 5 hours. It was then raised to 1200° C. at a fixed rate for 2 hours and held there for 2 hours. The temperature was then lowered by furnace cooling to a predetermined taking-out temperature of 100° C., and the sintered body was taken out.

(208) Since there was a broken part in the periphery edge portion of the sintered body, the obtained bulk density thereof was an approximate value of about 2.92 g/cm.sup.3 (true density of 3.15 g/cm.sup.3 and relative density of 92.7%).

(209) Some insufficient levels of neutron moderation performance and mechanical strengths were noticed.

Comparative Example 5

(210) The same MgF.sub.2 powder was mixed with the same CaF.sub.2 powder of 25% by weight as those in the above Comparative Example 1, and a starting raw material was prepared in a similar manner thereto. Using a uniaxial press device, the press molding was conducted at a press pressure of 30 MPa, and then, this press molded body was CIP molded at a molding pressure of 30 MPa so as to obtain a CIP molded body in a similar manner to the Example 1. The preliminary sintering step was conducted on this CIP molded body at 550° C. for 8 hours in an air atmosphere so as to obtain a preliminary sintered body.

(211) This preliminary sintered body was heated from room temperature to 870° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere, and the temperature was held there for 6 hours. It was then raised to 1160° C. at a fixed rate for 2 hours and held there for 3 hours. The temperature was then lowered by furnace cooling to a predetermined taking-out temperature of 100° C., and the sintered body was taken out. The bulk density of the sintered body was 2.93 g/cm.sup.3 (true density of 3.15 g/cm.sup.3 and relative density of 93.0%).

(212) Some insufficient levels of neutron moderation performance and mechanical strengths were noticed.

Comparative Example 6

(213) The same MgF.sub.2 powder was mixed with the same CaF.sub.2 powder of 25% by weight as those in the above Comparative Example 1, and a starting raw material was prepared in a similar manner thereto. Using a uniaxial press device, the press molding was conducted at a press pressure of 4 MPa, and then, this press molded body was CIP molded at a molding pressure of 4 MPa so as to obtain a CIP molded body in a similar manner to the above Example 1. The preliminary sintering step was conducted on this CIP molded body at 600° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body.

(214) This preliminary sintered body was heated from room temperature to 830° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere, and the temperature was held there for 5 hours. It was then raised to 950° C. at a fixed rate for 2 hours and held there for 4 hours. The temperature was then lowered by furnace cooling to a predetermined taking-out temperature of 100° C., and the sintered body was taken out. The bulk density of the sintered body was 2.91 g/cm.sup.3 (true density of 3.15 g/cm.sup.3 and relative density of 92.4%).

(215) Some insufficient levels of neutron moderation performance and mechanical strengths were noticed.

Comparative Example 7

(216) The same MgF.sub.2 powder was mixed with the same CaF.sub.2 powder of 50% by weight as those in the above Comparative Example 1, and a starting raw material was prepared in a similar manner thereto. Using a uniaxial press device, the press molding was conducted at a press pressure of 20 MPa, and then, this press molded body was CIP molded at a molding pressure of 20 MPa so as to obtain a CIP molded body in a similar manner to the Example 1. The preliminary sintering step was conducted on this CIP molded body at 550° C. for 8 hours in an air atmosphere so as to obtain a preliminary sintered body.

(217) This preliminary sintered body was heated from room temperature to 880° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere, and the temperature was held there for 5 hours. It was then raised to 1200° C. at a fixed rate for 2 hours and held there for 2 hours. The temperature was then lowered by furnace cooling to a predetermined taking-out temperature of 100° C., and the sintered body was taken out.

(218) The bulk density of the sintered body was 2.91 g/cm.sup.3 (true density of 3.16 g/cm.sup.3 and relative density of 92.1%).

(219) Some insufficient levels of neutron moderation performance and mechanical strengths were noticed.

Comparative Example 8

(220) The same MgF.sub.2 powder was mixed with the same CaF.sub.2 powder of 50% by weight as those in the above Comparative Example 1, and a starting raw material was prepared in a similar manner thereto. Using a uniaxial press device, the press molding was conducted at a press pressure of 4 MPa, and then, this press molded body was CIP molded at a molding pressure of 4 MPa so as to obtain a CIP molded body in a similar manner to the Example 1. The preliminary sintering step was conducted on this CIP molded body at 600° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body.

(221) This preliminary sintered body was heated from room temperature to 850° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere, and the temperature was held there for 5 hours. It was then raised to 960° C. at a fixed rate for 2 hours and held there for 4 hours. The temperature was then lowered by furnace cooling to a predetermined taking-out temperature of 100° C., and the sintered body was taken out.

(222) The bulk density of the sintered body was 2.92 g/cm.sup.3 (true density of 3.16 g/cm.sup.3 and relative density of 92.4%).

(223) Some insufficient levels of neutron moderation performance and mechanical strengths were noticed.

Comparative Example 9

(224) The same MgF.sub.2 powder was mixed with the same CaF.sub.2 powder of 88% by weight as those in the above Comparative Example 1, and a starting raw material was prepared in a similar manner thereto. Using a uniaxial press device, the press molding was conducted at a press pressure of 20 MPa, and then, this press molded body was CIP molded at a molding pressure of 20 MPa so as to obtain a CIP molded body in a similar manner to the Example 1. The preliminary sintering step was conducted on this CIP molded body at 530° C. for 8 hours in an air atmosphere so as to obtain a preliminary sintered body.

(225) This preliminary sintered body was heated from room temperature to 900° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere, and the temperature was held there for 6 hours. It was then raised to 1160° C. at a fixed rate for 2 hours and held there for 4 hours. The temperature was then lowered by furnace cooling to a predetermined taking-out temperature of 100° C., and the sintered body was taken out.

(226) The bulk density of the sintered body was 2.90 g/cm.sup.3 (true density of 3.17 g/cm.sup.3 and relative density of 91.5%).

(227) Some insufficient levels of neutron moderation performance and mechanical strengths were recognized.

Comparative Example 10

(228) The same MgF.sub.2 powder was mixed with the same CaF.sub.2 powder of 88% by weight as those in the above Comparative Example 1, and a starting raw material was prepared in a similar manner thereto. Using a uniaxial press device, the press molding was conducted at a press pressure of 4 MPa, and then, this press molded body was CIP molded at a molding pressure of 4 MPa so as to obtain a CIP molded body in a similar manner to the Example 1. The preliminary sintering step was conducted on this CIP molded body at 600° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body.

(229) This preliminary sintered body was heated from room temperature to 860° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere, and the temperature was held there for 6 hours. It was then raised to 970° C. at a fixed rate for 2 hours and held there for 5 hours. The temperature was then lowered by furnace cooling to a predetermined taking-out temperature of 100° C., and the sintered body was taken out.

(230) The bulk density of the sintered body was 2.93 g/cm.sup.3 (true density of 3.17 g/cm.sup.3 and relative density of 92.4%).

(231) Some insufficient levels of neutron moderation performance and mechanical strengths were noticed.

Comparative Example 11

(232) The same MgF.sub.2 powder was mixed with the same CaF.sub.2 powder of 3% by weight as those in the above Comparative Example 1, and a starting raw material was prepared in a similar manner thereto. Using a uniaxial press device, the press molding was conducted at a press pressure of 30 MPa, and then, this press molded body was CIP molded at a molding pressure of 30 MPa so as to obtain a CIP molded body. The preliminary sintering step was conducted on this CIP molded body at 660° C. for 8 hours in an air atmosphere so as to obtain a preliminary sintered body.

(233) This preliminary sintered body was heated from room temperature to 1060° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere, and the temperature was held there for 2 hours. The temperature was then lowered by furnace cooling to a predetermined taking-out temperature of 100° C., and the sintered body was taken out.

(234) The sintering step was conducted only in one stage (only the secondary sintering of the primary and secondary sintering was conducted). The bulk density of the sintered body was 2.93 g/cm.sup.3 (true density of 3.15 g/cm.sup.3 and relative density of 93.0%).

(235) Some insufficient levels of neutron moderation performance and mechanical strengths were noticed.

Comparative Example 12

(236) The same MgF.sub.2 powder was mixed with the same CaF.sub.2 powder of 25% by weight as those in the above Comparative Example 1, and a starting raw material was prepared in a similar manner thereto. Using a uniaxial press device, the press molding was conducted at a press pressure of 30 MPa, and then, this press molded body was CIP molded at a molding pressure of 30 MPa so as to obtain a CIP molded body. The preliminary sintering step was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body.

(237) This preliminary sintered body was heated from room temperature to 1150° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere, and the temperature was held there for 1.5 hours. The temperature was then lowered by furnace cooling to a predetermined taking-out temperature of 100° C., and the sintered body was taken out.

(238) The sintering step was conducted only in one stage (only the secondary sintering of the primary and secondary sintering was conducted). The bulk density of the sintered body was 2.90 g/cm.sup.3 (true density of 3.15 g/cm.sup.3 and relative density of 92.1%).

(239) Some insufficient levels of neutron moderation performance and mechanical strengths were noticed.

(240) [Comparative Material 1]

(241) Using the same powder of MgF.sub.2 simple as that in the above Example 6, a starting raw material was prepared in a similar manner to the Example 1. In a similar manner to the Example 1, using a uniaxial press device, the press molding was conducted at a press pressure of 20 MPa, and then, this press molded body was CIP molded at a molding pressure of 20 MPa so as to obtain a CIP molded body. The preliminary sintering step was conducted on this CIP molded body at 600° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body.

(242) This preliminary sintered body was heated from room temperature to 840° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere, and the temperature was held there for 6 hours. It was then raised to 1100° C. at a fixed rate for 2 hours and held there for 2 hours. The temperature was then lowered by furnace cooling to a predetermined taking-out temperature of 100° C., and the sintered body was taken out.

(243) The bulk density of the sintered body was 2.97 g/cm.sup.3 (true density of 3.15 g/cm.sup.3 and relative density of 94.3%).

(244) The neutron moderation performance of the sintered body was good enough to compare favorably with the Examples as shown in Table 2. On the other hand, the mechanical strengths thereof were within a range of good levels as shown in Table 2, but equivalent to the lower levels of strengths in the Examples. For information, this Comparative Material 1 is equivalent to a sintered body according to the prior application.

(245) [Comparative Material 2]

(246) Using the same CaF.sub.2 powder being a secondary raw material as that in the Example 6, a starting raw material was prepared in a similar manner to the Example 6. In a similar manner to the Example 1, using a uniaxial press device, the press molding was conducted at a press pressure of 20 MPa, and then, this press molded body was CIP molded at a molding pressure of 20 MPa so as to obtain a CIP molded body. The preliminary sintering step was conducted on this CIP molded body at 600° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body.

(247) This preliminary sintered body was heated from room temperature to 880° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere, and the temperature was held there for 6 hours. It was then raised to 1130° C. at a fixed rate for 2 hours and held there for 2 hours. The temperature was then lowered by furnace cooling to a predetermined taking-out temperature of 100° C., and the sintered body was taken out.

(248) The bulk density of the sintered body was 3.00 g/cm.sup.3 (true density of 3.18 g/cm.sup.3 and relative density of 94.3%).

(249) The mechanical strengths of the sintered body were good, while some insufficient levels of neutron moderation performance were noticed.