Isotropic materials with low or negative thermal expansion
- Oregon State Univ., Corvallis, OR (United States); and others
Most of the materials which are of interest due to their very low or negative thermal expansion are not isotropic. They are instead anisotropic crystalline materials. On a unit cell basis, there is thermal expansion in one or two dimensions coupled with thermal contraction in the other two or one dimensions. Cordierite is an example where there is positive thermal expansion in two directions coupled with negative thermal expansion in the third dimension. In the NZP family, one commonly finds positive thermal expansion in one direction coupled negative thermal expansion in the other two directions. In these cases of mixed positive and negative thermal expansions, the net thermal volume expansion may be very low and can be slightly negative. Negative thermal expansion is known in some isotropic materials. The thermal expansion of cristobalite (the cubic crystallite form of SiO{sub 2}) is negative at very high temperatures. The thermal expansion of amorphous SiO{sub 2}, on the other hand, is negative at very low temperatures. No form of SiO{sub 2} shows negative thermal expansion close to room temperature. Some cubic or psuedocubic perovskites show negative thermal expansion behavior close to their phase transitions. We sought isotropic materials which would show very low or negative thermal expansion over a broad temperature range, a range which would include room temperature. Compounds with the cubic ZrP{sub 2}O{sub 7} structure were considered good candidates. Negative thermal expansion for the cubic forms of ThP{sub 2}O{sub 7} and UP{sub 2}O{sub 7} at high temperatures is reported. Furthermore, ZrV{sub 2}O{sub 7} is known to have this structure type and it shows very strong negative thermal expansion from about 150{degrees} to 750{degrees}C. Although we have investigated many compositions with this structure, the most promising results to date have been obtained in solid solutions of ZrP{sub 2}O{sub 7} and ZrV{sub 2}O{sub 7}.
- OSTI ID:
- 86592
- Report Number(s):
- CONF-940416--
- Country of Publication:
- United States
- Language:
- English
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