NEGATIVE THERMAL EXPANSION MATERIAL AND PRODUCTION METHOD THEREOF

Abstract

A negative thermal expansion material, being formed from M.sub.xSr.sub.yBa.sub.zZn.sub.2Si.sub.2O.sub.7 (wherein M is one or more types of Na and Ca, and x+y+z=1, 0<x0.5, 0.3<z<1.0), and an XRD peak intensity I.sub.NTR and a background intensity I.sub.BG of a primary phase of an orthorhombic crystal structure which exhibits negative expansion characteristics satisfy a relation of I.sub.NTE/I.sub.BG>15. An object of the present invention is to provide a negative thermal expansion material having a low specific gravity, and a negative thermal expansion material having a low Ba content.

Claims

1. A negative thermal expansion material, being formed from M.sub.xSr.sub.yBa.sub.zZn.sub.2Si.sub.2O.sub.7 (wherein M is one or more types of Na and Ca, and x+y+z=1, 0<x0.5, 0.3<z<1.0), and an XRD peak intensity I.sub.NTE and a background intensity I.sub.BG of a primary phase of an orthorhombic crystal structure which exhibits negative expansion characteristics satisfy a relation of I.sub.NTE/I.sub.BG>15.

2. The negative thermal expansion material according to claim 1, wherein the negative thermal expansion material has a coefficient of thermal expansion of 5 ppm/K or less.

3. The negative thermal expansion material according to claim 2, wherein the negative thermal expansion material has a specific gravity of less than 4.2.

4. The negative thermal expansion material according to claim 3, wherein the negative thermal expansion material has a median diameter D50 of 5 m or more and 90 m or less.

5. A method of producing the negative thermal expansion material according to claim 4, wherein a BaCO.sub.3 powder, a SrCO.sub.3 powder, a CaCO.sub.3 powder, a Na.sub.2CO.sub.3 powder, a ZnO.sub.2 powder, and a SiO.sub.2 powder are used as raw material powders, weighed, mixed, and the mixed powder is thereafter subject to calcination in an atmosphere at 1050 C. or higher and 1250 C. or less.

6. The negative thermal expansion material according to claim 1, wherein the negative thermal expansion material has a specific gravity of less than 4.2.

7. The negative thermal expansion material according to claim 1, wherein the negative thermal expansion material has a median diameter D50 of 5 m or more and 90 m or less.

8. A method of producing the negative thermal expansion material according to claim 1, wherein a BaCO.sub.3 powder, a SrCO.sub.3 powder, a CaCO.sub.3 powder, a Na.sub.2CO.sub.3 powder, a ZnO.sub.2 powder, and a SiO.sub.2 powder are used as raw material powders, weighed, mixed, and the mixed powder is thereafter subject to calcination in an atmosphere at 1050 C. or higher and 1250 C. or less.

Description

DESCRIPTION OF EMBODIMENTS

[0009] The negative thermal expansion material according to an embodiment of the present invention is characterized in being formed from M.sub.xSr.sub.yBa.sub.zZn.sub.2Si.sub.2O.sub.7 (wherein M is one or more types of Na and Ca, and x+y+z=1, 0<x0.5, 0.3<z<1.0). The ionic radiuses of Na and Ca are respectively 116 and 114 m and smaller than Ba (149 m) and Sr (132 m), but it is possible to obtain a crystal phase (orthorhombic crystal) having negative thermal expansion characteristics by substituting them for Ba and Sr at predetermined ratios. The specific gravity can thereby be reduced as well. Moreover, as the elements to be substituted, it is considered that the same effects can also be obtained with K (potassium) and Mg (magnesium) in addition to Ca and Na.

[0010] Moreover, with the negative expansion material according to an embodiment of the present invention, an XRD peak intensity I.sub.NTR and a background intensity I.sub.BG of a primary phase of an orthorhombic crystal structure which exhibits negative expansion characteristics satisfy a relation of I.sub.NTE/I.sub.BG>15. Here, the XRD peak intensity I.sub.NTE (NTE: Negative Thermal Expansion) and the background intensity I.sub.BG (BG: Back Ground) of the primary phase of the orthorhombic crystal structure which exhibits negative expansion characteristics are defined as follows.

I.sub.NTE: XRD peak intensity in a range of 26.0228.0
I.sub.BG: XRD mean intensity in a range of 10.02<12.0

[0011] In the foregoing composition, 0<x0.5, x+y+z=1, 0.3<z<1.0. With regard to x as the content ratio of Na and Ca, when x is 0; that is, when only using Ba and Sr as conventionally, the specific gravity will increase. Meanwhile, when x exceeds 0.5, it will not be possible to maintain a crystal phase (orthorhombic crystal) having negative thermal expansion characteristics. Moreover, with regard to z as the content ratio of Ba, when z is less than 0.3 or exceeds 0.9, it is not possible to maintain a crystal phase (orthorhombic crystal) having negative thermal expansion characteristics.

[0012] Moreover, the negative thermal expansion material according to an embodiment of the present invention preferably has a coefficient of thermal expansion of 5 ppm/K or less. While most materials have positive thermal expansion, since a negative thermal expansion material has a unique property of its volume contracting pursuant to a rise in temperature, strain associated with a change in temperature can be suppressed by compositing the two and controlling the coefficient of thermal expansion. Problems caused by such thermal expansion can be adequately avoided if the coefficient of thermal expansion is 5 ppm/K or less.

[0013] Moreover, the negative thermal expansion material according to an embodiment of the present invention preferably has a specific gravity that is less than 4.2 g/cm.sup.3. While a negative thermal expansion material is composited by being kneaded into resin or the like, here, there is a problem in that the negative thermal expansion material tends to become sedimented in the resin. Nevertheless, by substituting a part of Ba or Sr having a large specific gravity with one or more types of Na and Ca, the specific gravity can be reduced to be less than 4.2 g/cm.sup.3, and the foregoing problem of sedimentation can be mitigated.

[0014] Moreover, the negative thermal expansion material according to an embodiment of the present invention preferably has a median diameter D50 that is 5 m or more and 90 m or less. As described above, while a negative thermal expansion material is composited by being kneaded into resin or the like, when the median diameter D50 is too small, the handling of the powder becomes difficult. Meanwhile, when the median diameter D50 is too large, the powder becomes aggregated in the resin and it becomes difficult to uniformly disperse the powder. Accordingly, the median diameter D50 is preferably 5 m or more and 90 m or less. The term median diameter D50 means the particle size at an integrated value of 50% regarding the particle size distribution obtained with a laser diffraction scattering particle size measuring device. For example, the median diameter D50 can be measured with the laser diffraction/scattering particle size distribution analyzer LA-960 manufactured by HORIBA, Ltd.

[0015] The method of producing the negative thermal expansion material according to an embodiment of the present invention is now explained.

[0016] Foremost, a BaCO.sub.3 powder, a SrCO.sub.3 powder, a CaCO.sub.3 powder, a Na.sub.2CO.sub.3 powder, a ZnO.sub.2 powder, and a SiO.sub.2 powder may be used as the raw material powders. Preferably, the mean particle size of these raw material powders is 0.3 m or more and 10 m or less. Moreover, since the characteristics may become deteriorated when numerous impurities are contained in the raw material powders, it is preferable to use raw material powders having a purity of 3N or higher.

[0017] Next, the raw material powders are weighed to attain a predetermined ratio, and then mixed. As the mixing method, mortar mixing may be used, and the mixing time may be set to be approximately 15 minutes. Next, the mixed powder is filled in a die, and reacted at 1050 to 1250 C. based on the reactive sintering method. When the calcination temperature is less than 1050 C., the reaction will be insufficient, and there are cases where the intended crystal phase (orthorhombic crystal) cannot be obtained. Moreover, when the calcination temperature is 1250 C. or higher, a part of the oxides will evaporate, and there is a possibility that a compositional deviation will occur.

[0018] A furnace cooling is conducted after sintering to obtain a M.sub.xSr.sub.yBa.sub.zZn.sub.2Si.sub.2O.sub.7 sintered body, and by pulverizing the obtained M.sub.xSr.sub.yBa.sub.zZn.sub.2Si.sub.2O.sub.7 sintered body with a mortar or the like, a M.sub.xSr.sub.yBa.sub.zZn.sub.2Si.sub.2O.sub.7 powder having an intended particle size can be produced. Note that, by using an X-ray diffraction analysis method (XRD) to measure the peak intensity ratio of a primary phase of an orthorhombic crystal structure which exhibits negative thermal expansion characteristics, and using thermomechanical analysis method (TMA) to measure the coefficient of thermal expansion, it is possible to confirm whether the powder has negative thermal expansion characteristics.

[0019] In an embodiment of the present invention, the measurement method and other conditions, including the Examples and Comparative Examples, may be as follows.

(XRD Analysis)

[0020] Device: Ultima IV manufactured by Rigaku Corporation
X-ray source: Cu-K rays
Tube voltage: 40 kV

Current: 30 mA

[0021] Measurement method: 2- diffraction method
Scan speed: 4.0/min
Sampling interval: 0.02
Measurement range: 10 to 90

[0022] (Measurement (TMA Measurement) of Coefficient of Thermal Expansion)

Device:

[0023] Rate of temperature rise: 5 C./min
Measured temperature range: 50 C. to 500 C.

Atmosphere: Ar

[0024] Standard sample: Al.sub.2O.sub.3

[0025] The coefficient of thermal expansion of a molded body of powder was measured based on the foregoing conditions. The coefficient of thermal expansion was the mean value of 100 C. to 130 C.

[0026] (Measurement of Specific Gravity of Powder)

[0027] A dry-type automatic densimeter (AccuPyc II 1340 manufactured by SHIMADZU CORPORATION) was used to measure the specific gravity of dry type powder based on He gas replacement. The measurement was performed 10 times, and the mean value thereof was used as the specific gravity of the powder.

EXAMPLES

[0028] The present invention is now explained based on the Examples and Comparative Examples. The Examples are merely illustrative examples, and the present invention is not limited to such Examples. In other words, the present invention includes other modes or modified examples included herein.

Examples

Examples 1-20

[0029] As the raw material powders, a BaCO.sub.3 powder, a SrCO.sub.3 powder, a CaCO.sub.3 powder, a Na.sub.2CO.sub.3 powder, a ZnO powder, and a SiO.sub.2 powder were prepared, and the powders were weight to attain the compositions indicated in Table 1. Next, these powders were mixed with a mortar for 15 minutes. Subsequently, the obtained mixed powder was calcined in an air atmosphere at 1050 to 1200 C. for 10 hours. After calcination, the mixed powder was cooled in a furnace, subsequently pulverized with a mortar, and the median diameter D50 was adjusted to be roughly 10 m.

[0030] As a result of analyzing the thus obtained powders using XRD, it was confirmed that all of the powders are of a crystal phase (orthorhombic crystal) having negative thermal expansion characteristics. Next, as a result of measuring the TMA of the thus obtained M.sub.xSr.sub.yBa.sub.zZn.sub.2Si.sub.2O.sub.7 powders, and it was confirmed that all of the M.sub.xSr.sub.yBa.sub.zZn.sub.2Si.sub.2O.sub.7 powders have a coefficient of thermal expansion of 5 ppm/K or less, and have a coefficient of negative thermal expansion. Moreover, in all cases the specific gravity was less than 4.2 g/cm.sup.3.

TABLE-US-00001 TABLE 1 XRD M.sub.xSr.sub.yBa.sub.zZnSi.sub.2O.sub.7 powder means that negative Specific gravity x y z thermal expansion TMA (powder density) Na Ca Sr Ba phase is primary phase I.sub.NTE/I.sub.BG ppm/K g/cm.sup.3 Example 1 0.2 0 0.5 0.3 26 5.5 4.02 Example 2 0.3 0 0.4 0.3 26 5.8 3.97 Example 3 0.4 0 0.3 0.3 37 7.1 3.93 Example 4 0.1 0 0.5 0.4 37 6.0 4.11 Example 5 0.2 0 0.4 0.4 26 6.0 4.06 Example 6 0.3 0 0.3 0.4 30 6.5 4.00 Example 7 0.4 0 0.2 0.4 25 7.0 3.96 Example 8 0.5 0 0.1 0.4 26 6.2 3.87 Example 9 0.2 0 0.3 0.5 30 .5.9 3.84 Example 10 0.3 0 0.2 0.5 25 6.0 3.87 Example 11 0.2 0 0.2 0.6 26 6.1 3.80 Example 12 0 0.2 0.5 0.3 26 6.5 4.06 Example 13 0 0.3 0.4 0.3 26 6.4 4.03 Example 14 0 0.1 0.5 0.4 37 5.5 4.10 Example 15 0 0.2 0.4 0.4 37 5 4.08 Example 16 0 0.3 0.3 0.4 26 5.2 4.10 Example 17 0 0.1 0.4 0.5 30 6.0 4.15 Example 18 0 0.2 0.3 0.5 25 6.2 4.13 Example 19 0 0.1 0.3 0.6 20 5.8 4.12 Example 20 0.1 0.1 0.5 0.3 29 5.4 4.06 Comparative 0 0 0.5 0.5 26 6.0 4.25 Example 1 Comparative 0.6 0 0 0.4 x 1.7 1.0 Example 2

Comparative Example 1

[0031] A negative thermal expansion material of Ba.sub.xSr.sub.yZnSi.sub.2O.sub.7 to which Ca and Na have not been added was prepared. The negative thermal expansion material was prepared under the same conditions as the foregoing Examples other than not adding the CaCO.sub.3 powder and the Na.sub.2CO.sub.3 powder. As a result of measuring the TMA of the thus obtained Ba.sub.0.5Sr.sub.0.5ZnSi.sub.2O.sub.7 powder, the result was 6 ppm/K, but the specific gravity was 4.25 g/cm.sup.3, and the specific gravity was greater as per conventional negative thermal expansion materials.

Comparative Example 2

[0032] The respective powders were weighed so that the composition ratio of Na deviates from the range of the embodiment of the present invention. Otherwise, the negative thermal expansion material was prepared under the same conditions as the foregoing Examples. As a result of analyzing the thus obtained powder using XRD, the I.sub.NTE/I.sub.BG was less than 15 and the powder was not a crystal phase (orthorhombic crystal) having negative expansion characteristics.

INDUSTRIAL APPLICABILITY

[0033] With the negative thermal expansion material of the present invention, in which its volume contracts pursuant to a rise in temperature, the coefficient of thermal expansion of the composite material can be controlled by being composited with a material having a positive thermal expansion which is common among most materials. The present invention is useful for precision machinery components, optical components, electronic material packages and sensors which require measures to avoid the generation of strain associated with a change in temperature caused by the external environment.