Hygroscopicity And Negative Thermal Expansion Of Y_2-2x（ZrM）_xMo₃O₁₂(M=Ca²⁺,Sr²⁺)

Among the negative thermal expansion materials with the general formula A2M3O12,where "A" is A1,Y or lanthanum group element and M is W or M. The materials of the A2M3O12family with larger A3+Ionic radius give rise to larger molecular spacing and lattice constant. The openings in the network become large enough to admit water molecules easily. The negative thermal expansion at room temperature disappears due to the presence of water molecules. After the release of water molecules at high temperature, the negative thermal expansion property can be observed.Y2MO3O12presents NTE properties over a large temperature range. Because of larger Y3+ionic radius, the Y2MO3O12can absorb water easily and forms a stable trihydrate structure at room temperature and positive thermal expansion in this material is observed over room temperature to130℃. This is because the water molecules inhibit the flexibility as well as the internal vibration of the polyhedral which are necessary for negative thermal expansion. NTE in this material is complete release of water molecules.In this paper, some elements are selected to replace Y element to decrease the hygroscopicity and adjust the NTE coefficient of Y2MO3O12. Firstly, ZrCaMo3O12and ZrSrMo3O12complex materials have been synthesized by solidstate method and sol-gel method and their hygroscopicity, thermal expansion have been investigated with TG, Raman and coefficient of thermal expansion. Then materials with the formula Y2-2x(ZrCa)xMo3O12and Y2-2x(ZrSr)xMo3O12have been synthesized and the change of hygroscopicity, negative thermal expansion have been also studied.It is found that the result of decreasing hygroscopicity and controllable thermal expansion of Y2-2x(ZrCa)xMo3O12are the same as those of Y2-2x(ZrSr)xMo3O12. The content of water molecules decrease obviously for x＞0.3, the number of water molecules is less than one for x＞0.5. The water molecules are inside crystal for x≤0.5and are on the surface of materials for x＞0.5. So the hygroscopicity is reduced perfectly.The removal of water molecules have two processes for;c=0,0.1,0.2. The first process from350to425K is slow and the losing water molecules are on the surface of materials. The second process from425to450K is very fast and the losing water molecules are inside the crystalloid.The negative thermal expansion coefficient is the smallest and almost zero for x=0.5,larger for x=0,0.1.The NTE coefficients are almost equal for x=0,0.1,0.2.Through replaced Y element by Zr and Ca or Sr, nearly zero expansion material can be obtained.