A _(2-X) B _(X) M ₃ O ₁₂ (A = Y, Yb; B = La, Ce) Department Of Preparation And Properties Of Negative Thermal Expansion Materials

With the development of microelectronics, optical and micromechanical devices miniaturization and aerospace technology, the precise size of the material is playing an important role in a device. Sometimes a device may be destroyed by the material size change cased by the environment temperature. The discovery of the negative thermal expansion material approves a possible way to slove these problems. We can use a negative thermal expansion coefficient material and a positive thermal expansion coefficient material to complex in order to prepared a variety of controllable thermal expansion coefficient and even zero-expansion coefficient composite material.In the past years, we have already found AM2O7series (A=Zr, Hf, etc.; M=V, P, etc.); AM2O8series (A=Zr, Hf, etc.; M=W, Moisturizer, etc.); A2M3O12series (A=Sc, Y, Yb, Lu; M=W, Mo, etc.). which A2M3O12Department of A-site ion can be from Al3+(radius0.0675nm) to Gd3+(radius0.1075nm) cation, at the same time, we found that A-site ion plays an improtant role in the performance of NTE material. With further research, we found A2M3O12based materials containing monoclinic and tetragonal phase. The monoclinic phase structure A2M3O12material presented positive thermal expansion, typical representative of La2Mo3O12and Nd2Mo3O12, the tetragonal structure of A2M3O12material exhibits a negative thermal expansion, as the typical representative Yb2Mo3O12and Y2Mo3O12etc.As every coin two sides, there are still some problems in NTE materials to solve. For example:the NTE materials can absorb water molecules at room temperature and which greatly restrict its application. With the purpose of finding new negative thermal expansion materials, further investigating the mechanism of negative thermal expansion, improving its water absorption and preparing controllable and zero expansion material. This article has prepared of the Y2-xLaxMo3O12, Yb2-xLaxMo3O12, Y2-xCexW3O12and Yb2-xCexW3O12. by adjusting the amount of Y/La, Yb/La, Y/Ce and Yb/Ce to control the expansion coefficient. The use of XRD, TMA, TG, Raman, SEM, TEM and other means of samples were analyzed. The research results and the main conclusions are as follows:1. We have successfully prepared the Y2-xLaxMo3O12(0≤x≤2) series of ceramic material by solid state methed which sintered at750℃for10H. The TG showed that Y2-xLaxMo3O12(0 ≤x≤2) series of ceramic material can absorb water molecules at room temperature. With La incorporating, the absorbent phenomenon disappeared. At the same time, with the incorporation of La, its thermal expansion coefficient is generally showed a downward trend in temperature range of178to600℃. Where in the composite of Y1.5La0.5Mo3O12, the thermal expansion coefficient can be controled to nearly zero expansion coefficient:0.8788x10-6/K. We found that we can control the thermal expansion coefficient by regulating the ratio of Y/La. This kind of controllable even nearly zero expansion ceramic material has a very broad application prospects.2. In this chapter, we change Y with Yb to compose A-site ion in order to discover the Yb2-xLaxMo3O12(0≤x≤2) series of ceramic material’s thermal expansion coefficient. The experimental results show that:solid phase sintered at750℃for10h high purity was obtained. The TG showed that Yb2-xLaxMo3O12(0≤x≤2) series of ceramic material can absorb water molecules at room temperature which is the same to the Y2-xLaxMo3O12(0≤x≤2) series of ceramic material. At the same time, with the incorporation of La, its thermal expansion coefficient is generally showed a downward trend.3. We have successfully prepared the Yb2-xCexW3O12(0≤x≤2) series of ceramic material by solid state methed which sintered at950℃for12H.We have successfully reduced the sintering temperature to950℃.And at the same time, we explored a new A2M3O12the family members. Successfully reached the original goal of the regulation of the thermal expansion coefficient of thermal expansion. With the accumulation of experimental data, we found that: with the incorporation of Ce, the absorbent phenomenon disappeared. The number of crystal water per molecule is reduced from2.85to0.05. At the same time, with the incorporation of Ce, its thermal expansion coefficient is generally showed a downward trend.4. Like in the previous section, we have successfully prepared the Y2-xCexW3O12(0≤x≤2) series of ceramic material by solid state methed which sintered at950℃for12H. This chapter verify a general law of negative thermal expansion of tungstate series:with the incorporation of Ce, the composite became denser and denser, at the same time, the hygroscopicity also been improved. Generally, we explored a new A2M3O12family members. The number of crystal water per molecule is reduced from2.87to0. Where in the composite of2, the thermal expansion coefficient can be controled to nearly zero expansion coefficient:-0.8199x10-6/K.