RARE EARTH OXYSULFIDE COLD STORAGE MEDIUM

Abstract

A cold storage material having a large thermal capacity in a ultra-low temperature range of 10 K or less and being highly durable against thermal shock and mechanical vibration. The cold storage material contains a rare earth oxysulfide ceramic represented by the general formula R.sub.2O.sub.2S in which R is one or more kinds of rare earth elements selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y, and Al.sub.2O.sub.3 having a specific surface area of 0.3 m.sup.2/g to 11 m.sup.2/g is added to the cold storage material.

Claims

1. A method for estimating a composition ratio of a rare earth oxysulfide-based cold storage material, wherein Al.sub.2O.sub.3 reacts with Gd.sub.2O.sub.2S to obtain Gd.sub.2O.sub.2S, Al.sub.2O.sub.3 and GdAlO.sub.3 as the composition, and taking the X-ray intensity of Gd.sub.2O.sub.2S at a diffraction angle (2θ) of 29.909° as 100, the proportion (%) of the X-ray intensity of GdAlO.sub.3 at a diffraction angle (2θ) of 33.989° relative to the X-ray intensity of Gd.sub.2O.sub.2S is a first relative X-ray intensity and the proportion (%) of the X-ray intensity of Al.sub.2O.sub.3 at a diffraction angle (2θ) of 35.152° relative to the X-ray intensity of Gd.sub.2O.sub.2S is a second relative X-ray intensity, and the first relative X-ray intensity is within a prescribed range and the second relative X-ray intensity is within another prescribed range.

2. The method according to claim 1, wherein Al.sub.2O.sub.3 in an amount of 3 to 12% by weight in terms of aluminum reacts with Gd.sub.2O.sub.2S.

3. The method according to claim 2, wherein the first relative X-ray intensity is within a range of 0.6 to 38.0, and the second relative X-ray intensity is within a range of 0.5 to 5.9.

4. The method according to claim 3, wherein the first relative X-ray intensity is 0.6 to 4.2 and the second relative X-ray intensity is 0.5 to 1.7 when Al.sub.2O.sub.3 is in an amount of 3% by weight in terms of aluminum, the first relative X-ray intensity is 1.1 to 6.6 and the second relative X-ray intensity is 0.8 to 2.3 when Al.sub.2O.sub.3 is in an amount of 5% by weight in terms of aluminum, the first relative X-ray intensity is 1.5 to 21.0 and the second relative X-ray intensity is 0.5 to 3.3 when Al.sub.2O.sub.3 is in an amount of 7% by weight in terms of aluminum, the first relative X-ray intensity is 2.0 to 32.0 and the second relative X-ray intensity is 1.1 to 4.9 when Al.sub.2O.sub.3 is in an amount of 10% by weight in terms of aluminum, the first relative X-ray intensity is 2.4 to 38.0 and the second relative X-ray intensity is 1.2 to 5.9 when Al.sub.2O.sub.3 is in an amount of 12% by weight in terms of aluminum.

Description

BRIEF DESCRIPTION OF DRAWING

[0023] FIG. 1 shows a scanning electron microscope (SEM) image (×1000) of a fractured surface of an Al-doped Gd.sub.2O.sub.2S cold storage material.

DESCRIPTION OF EMBODIMENTS

[0024] Hereinafter, preferred embodiments of the present invention will be described based on specific examples, but it is needless to say that the present invention is not limited to the examples described below and various alterations and modifications may be made without departing from the technical scope of the invention.

EXAMPLES

Test Example 1

[0025] Into a quartz boat was charged 10 g of gadolinium oxide (Gd.sub.2O.sub.3) having an average grain size of 0.46 μm (with a specific surface area of 4.2 m.sup.2/g) measured by the Fisher method, which was then subjected to a reaction with hydrogen sulfide (H.sub.2S) gas passed through a quartz reaction tube at a flow rate of 0.2 L/min at 650° C. for 4 hours. The reaction product was measured by X-ray diffraction, in which peaks of gadolinium oxysulfide (Gd.sub.2O.sub.2S) were solely observed, showing that the yield of the reaction with the rare earth oxide was 100%. The obtained Gd.sub.2O.sub.2S powder was formed into a disk-like shape 12 mm in diameter at a pressure of 30 MPa. The formed product was subjected to isostatic pressing at 200 MPa, followed by pressureless sintering at 1500° C. for 6 hours under argon atmosphere. The heating rate was 200° C./h.

[0026] The density of the resulting Gd.sub.2O.sub.2S ceramic determined by the Archimedes method was 99.9% relative to the theoretical density, and the average crystal size calculated using the following formula was 3.2 μm.

d=1.56 C/(MN)

(d: average grain size, C: the length of an arbitrarily placed line on a high-resolution image by SEM or the like, N: the number of grains intercepted by the line, M: magnification of the image)

[0027] The obtained Gd.sub.2O.sub.2S ceramic had a magnetic phase transition temperature of about 5 K and a heat capacity of 1.2 J/cc.Math.K at the magnetic phase transition temperature. The heat capacity around the liquid helium temperature was 3 to 5 times as compared to that of conventional cold storage materials of HoCu.sub.2, ErNi, etc., and thus the usability of the Gd.sub.2O.sub.2S ceramic as a cold storage material at about 4.2 K was confirmed.

Example 1

[0028] In a ball mill, placed were the gadolinium oxide used in Test Example 1 and an alumina having a specific surface area of 0.1 to 14 m.sup.2/g measured by the BET single point adsorption method using N.sub.2 gas in accordance with JIS Z 8830:2013, which were mixed using ethanol as a solvent for 24 hours. The resulting slurry was dried, and calcined (900° C. for 3 hours). The resulting product was reacted with hydrogen sulfide gas, and through a procedure as in Test Example 1 (formed into a disk-like shape 12 mm in diameter at a pressure of 30 MPa, the formed product was subjected to isostatic pressing at 200 MPa, followed by pressureless sintering at 1500° C. for 6 hours under argon atmosphere), a Gd.sub.2O.sub.2S ceramic comprising Al (Al-doped Gd.sub.2O.sub.2S) was prepared. The density of the resulting Al-doped Gd.sub.2O.sub.2S ceramic determined by the Archimedes method was 99.9% relative to the theoretical density. To determine the composition ratio of Gd.sub.2O.sub.2S, GdAlO.sub.3, and Al.sub.2O.sub.3 in the thus prepared Al-doped Gd.sub.2O.sub.2S, ground and polished samples were subjected to X-ray diffraction to measure the X-ray intensity at each angle of diffraction (2θ). Taking the X-ray intensity of the principal component Gd.sub.2O.sub.2S at a diffraction angle (2θ) of 29.909° as 100, the proportion (%) of the X-ray intensity of GdAlO.sub.3 at a diffraction angle (2θ) of 33.989° relative to the X-ray intensity of Gd.sub.2O.sub.2S was determined. Also, the proportion (%) of the X-ray intensity of Al.sub.2O.sub.3 at a diffraction angle (2θ) of =35.152° relative to the X-ray intensity of Gd.sub.2O.sub.2S was determined. The results are shown in Tables 1 to 5. A total of 5 samples were prepared by varying the amount of alumina added, i.e., 3% by weight, 5% by weight, 7% by weight, 10% by weight, and 12% by weight in terms of aluminum, relative to the amount of Gd.sub.2O.sub.2S. Table 1 shows the cases where the amount of the added alumina was 3% by weight in terms of aluminum. Table 2 shows the cases where the amount of the added alumina was 5% by weight in terms of aluminum. Table 3 shows the cases where the amount of the added alumina was 7% by weight in terms of aluminum. Table 4 shows the cases where the amount of the added alumina was 10% by weight in terms of aluminum. Table 5 shows the cases where the amount of the added alumina was 12% by weight in terms of aluminum.

Example 2

[0029] The Al-doped Gd.sub.2O.sub.2S powder (after sulfuration and before sintering) shown in Example 1 was formed into a ball shape by tumbling granulation, and the obtained grains were sieved using two kinds of mesh filters, i.e., Mesh A (aperture: 597 μm) and Mesh B (aperture: 435 μm). The sieved grains were rolled on a mirror-polished iron plate tilted at an angle of about 25°. The grains that rolled and slid were collected, i.e., shape separation was performed. The average grain size of 100 grains was 0.5 mm. The average grain size of the Al-doped Gd.sub.2O.sub.2S grains was determined based on images taken using a microscope video system.

[0030] The obtained Al-doped Gd.sub.2O.sub.2S grains were charged into a crucible made of alumina, and the crucible was placed in a firing furnace. After thorough vacuum deaeration, argon gas was introduced into the furnace, and pressureless sintering was performed in argon atmosphere. Sintering at 1500° C. for 6 hours gave an Al-doped Gd.sub.2O.sub.2S cold storage material in the form of grains having an average grain size of 0.4 mm and an average aspect ratio of 1.1. The average grain size and the average aspect ratio of the Al-doped Gd.sub.2O.sub.2S grains were determined based on microscope video images.

[0031] Observation of the degree of destruction of the Al-doped Gd.sub.2O.sub.2S cold storage material obtained as above was carried out as follows. First, a nylon-based medium and an alumina slurry at a concentration of 10% by weight were put into a work tank. To this, the Al-doped Gd.sub.2O.sub.2S cold storage material was added, and surface treatment was performed by rotary barrel processing. The thus-obtained Al-doped Gd.sub.2O.sub.2S cold storage material was charged into a vessel for vibration test (a cylindrical body 20 mm in diameter and 14 mm in height). To the vessel, simple harmonic motion in which the maximum acceleration was 300 m/s.sup.2 was applied 1×10.sup.6 times, and then the degree of destruction of the Al-doped Gd.sub.2O.sub.2S cold storage material was observed. In the above vibration test, if the maximum acceleration is less than 300 m/s.sup.2, most Al-doped Gd.sub.2O.sub.2S cold storage materials are not broken, and the degree of destruction cannot be evaluated. Also, if the number of times the simple harmonic motion in which the maximum acceleration is 300 m/s.sup.2 is applied is less than 1×10.sup.6 times, the load is supposed to be less than that on the Al-doped Gd.sub.2O.sub.2S cold storage material used in a regenerator of an actual refrigerator in operation, and therefore, reliable observation results of the degree of destruction cannot be obtained. The results of observation by the naked eye regarding the degree of destruction of the Al-doped Gd.sub.2O.sub.2S cold storage material in the above vibration test are shown in the tables below. Table 1 shows the cases where the amount of the added alumina was 3% by weight in terms of aluminum. Table 2 shows the cases where the amount of the added alumina was 5% by weight in terms of aluminum. Table 3 shows the cases where the amount of the added alumina was 7% by weight in terms of aluminum. Table 4 shows the cases where the amount of the added alumina was 10% by weight in terms of aluminum. Table 5 shows the cases where the amount of the added alumina was 12% by weight in terms of aluminum.

TABLE-US-00001 TABLE 1 Specific surface Relative X-ray Relative X-ray area of alumina intensity of GdAlO.sub.3 intensity of Al.sub.2O.sub.3 Degree of (m.sup.2/g) (%) (%) destruction 0.1 0 2.1 about 10% was broken 0.3 0.6 1.7 not broken 0.5 0.8 1.5 not broken 1.0 1.4 1.2 not broken 4.5 2.1 1.0 not broken 6.1 3.2 0.7 not broken 7.2 4.2 0.5 not broken 11.0 9.8 0 about 5% was broken 14.0 22.0 0 about 5% was broken

TABLE-US-00002 TABLE 2 Specific surface Relative X-ray Relative X-ray area of alumina intensity of GdAlO.sub.3 intensity of Al.sub.2O.sub.3 Degree of (m.sup.2/g) (%) (%) destruction 0.1 0 2.7 about 10% was broken 0.3 1.1 2.3 not broken 0.5 1.3 2.1 not broken 1.0 2.7 1.5 not broken 4.5 3.3 1.2 not broken 6.1 4.7 1.0 not broken 7.2 6.6 0.8 not broken 11.0 14.0 0 about 5% was broken 14.0 39.0 0 about 5% was broken

TABLE-US-00003 TABLE 3 Specific surface Relative X-ray Relative X-ray area of alumina intensity of GdAlO.sub.3 intensity of Al.sub.2O.sub.3 Degree of (m.sup.2/g) (%) (%) destruction 0.1 0 3.8 about 10% was broken 0.3 1.5 3.3 not broken 0.5 1.9 3.0 not broken 1.0 4.0 2.2 not broken 4.5 5.3 1.8 not broken 6.1 6.8 1.5 not broken 7.2 11.0 1.2 not broken 11.0 21.0 0.5 not broken 14.0 56.0 0 about 5% was broken

TABLE-US-00004 TABLE 4 Specific surface Relative X-ray Relative X-ray area of alumina intensity of GdAlO.sub.3 intensity of Al.sub.2O.sub.3 Degree of (m.sup.2/g) (%) (%) destruction 0.1 0.5 5.5 about 10% was broken 0.3 2.0 4.9 not broken 0.5 2.8 4.3 not broken 1.0 5.9 3.3 not broken 4.5 8.3 2.7 not broken 6.1 10.0 2.1 not broken 7.2 17.0 1.9 not broken 11.0 32.0 1.1 not broken 14.0 82.0 0 about 5% was broken

TABLE-US-00005 TABLE 5 Specific surface Relative X-ray Relative X-ray area of alumina intensity of GdAlO.sub.3 intensity of Al.sub.2O.sub.3 Degree of (m.sup.2/g) (%) (%) destruction 0.1 0.9 6.6 about 10% was broken 0.3 2.4 5.9 not broken 0.5 3.4 5.2 not broken 1.0 7.2 4.0 not broken 4.5 10.0 3.3 not broken 6.1 12.0 2.7 not broken 7.2 21.0 2.4 not broken 11.0 38.0 1.2 not broken 14.0 91.0 0 about 5% was broken

[0032] As shown in Tables 1 to 5, in cases where the specific surface area of the alumina was 0.1 m.sup.2/g, about 10% of the Al-doped Gd.sub.2O.sub.2S cold storage material was destructed. A fractured surface of the cold storage material comprising 10% by weight of the alumina having a specific surface area of 0.1 m.sup.2/g added thereto was observed using a scanning electron microscope (SEM). As shown in FIG. 1, many gaps are present between the alumina grains and the Gd.sub.2O.sub.2S phase, and therefore, the destruction of the Al-doped Gd.sub.2O.sub.2S cold storage material is considered to be attributable to sintering inhibition due to the alumina grains. To prevent such sintering inhibition, the specific surface area of the alumina is preferably not too small, and considering from the results shown in Tables 1 to 5, the specific surface area of the alumina is preferably 0.3 m.sup.2/g or more.

[0033] In cases where the amount of the added alumina having a specific surface area of 11 m.sup.2/g was 3% by weight and 5% by weight in terms of aluminum, about 5% of the Al-doped Gd.sub.2O.sub.2S cold storage material was destructed, and in cases where the amount of the added alumina having a specific surface area of 14 m.sup.2/g was 7% by weight, 10% by weight, and 12% by weight in terms of aluminum, about 5% of the Al-doped Gd.sub.2O.sub.2S cold storage material was destructed. Considering that the relative X-ray intensity of Al.sub.2O.sub.3 at the specific surface area of alumina of 11.0 m.sup.2/g is 0 in Tables 1 and 2, and the relative X-ray intensity of Al.sub.2O.sub.3 at the specific surface area of alumina of 14.0 m.sup.2/g is 0 in Tables 3, 4, and 5, the destruction is considered to be attributable to the insufficient amount of alumina as a strength-imparting material in the Al-doped Gd.sub.2O.sub.2S cold storage material.

[0034] Also, comparison of Tables 1 and 2 with Tables 3, 4, and 5 shows the following: in cases where an increased amount of alumina is added, the amount of alumina remaining after the reaction with Gd.sub.2O.sub.2S is also increased as compared to the cases where less alumina is added, and destruction is less likely to occur even when the specific surface area of the alumina is increased.