Study The Thermal Expansion Properties Of Dy_1-xSr_xMnO_3-δ And CoFe₂O₄/ZrMgMo₃O₁₂ Composites

Most common materials in nature are present thermal expansion and contraction phenomenon, but with the development of science and technology, the material requirements of each field is becoming more and more strict, and the thermal expansion and contraction of the material is always a common problem in urgent need to solve in aerospace, precision machinery, communications, electronics and fuel cells and other areas. Such as in aerospace, spacecraft in high-altitude and high-speed flight with air friction will produce large amounts of heat. If the thermal coefficient expansion of materials of thermal protection shell and bearing shell mismatch, it is easy to make the shell micro cracks, even make the parts damaged and destroyed, while negative thermal expansion materials resistant to high temperature can make up for these deficiencies. The optical properties of the surface of high precision optical mirrors decreased due to the change in temperature. It requires zero thermal expansion material surface as a coating. It is precisely because the thermal expansion and contraction affects the performance of various devices, so the research and development of new negative thermal expansion material or zero thermal expansion material is particularly meaningful and value.The research of this paper mainly introuce two aspects:First part: the preparation of sample Dy_1-xSr_xMnO_3-δ and performance characterization and analysis of experimental results are as follows:Using conventional solid state reaction method to get sample Dy_1-xSr_xMnO_3-δ, its expansion coefficient determined by high temperature thermal expansion instrument. It is discovered that in 0≤x≤0.2, the thermal expansion coefficient has been in reducing, in 0.1≤x≤0.17, it becomes negative thermal expansion materials, 0.2≤x expansion coefficient is positive. In x=0.15 the average linear expansion coefficient is the smallest and negative expansion has the widest range.The results of XRD diffraction test determine the sample phase is the orthogonal perovskite structure. By scanning electron microscope（SEM） to observe the microstructures and morphologies of samples Dy_1-xSr_xMnO_3-δ. It is seen that the sample with overall degree of crystallinity is well, and doesn’t exist particularly significant void, and particles distributed evenly, also density is high. The grain size mostly between 1³ μm. EDS were employed to determine elements included in the sample and the elements for percentage is consistent with the expected test. With differential thermal analyzer and thermal gravimetric analyzer,the test results shows that the samples Dy_1-xSr_xMnO_3-δ in the process of increasing temperature exists different degree of oxygen deficiency.With increasing amount of Sr2+, the thermal expansion coefficient of Dy_1-xSr_xMnO_3-δ sample decreases and then becomes negative, and when Sr2+ doping amount reaches a certain value and becomes positive. According to the analysis, the thermal expansion mechanism is caused by lattice vibrations and ion electron transfer（Mn3+ and mutual transformation Mn4+ between）. This is because the Sr2+（1.18 ?） ionic radius partly replace Dy3+（0.912 ?） ionic radius, the lattice distortion caused by ionic radius changes. Such as sample Dy0.85Sr0.15MnO_3-δ, at room temperature to 473 K, which has low thermal expansion properties, and temperature higher than 473 K, which shows a negative thermal expansion properties at the macro level. Because the Mn4+ transformation of Mn3+ dominant, resulting in a axis elongation, c axis, and b axis direction of contraction, but the amount is greater than the amount of shrinkage and elongation, the cell volume decreases.The second part: the preparation of sample CoFe₂O₄-ZrMgMo₃O₁₂ composite material and performance characterization and analysis of experimental results are as follows:The ZrMgMo₃O₁₂ powders prepared by solid phase method, according to the different molecular weight than CoFe₂O₄-ZrMgMo₃O₁₂ composites preparation. And the XRD phase, microstructure morphology and thermal expansion test is analyzed. CoFe₂O₄-ZrMgMo₃O₁₂ composites after sintering, through XRD phase analysis, the original characteristics of their diffraction peak were better respective preserved, no other impurity phase is generated in the process of reaction.From the microstructure morphology under different proportion, with the increase of ZrMgMo₃O₁₂ molar mass ratio, the grain size decreased, the overall density is better, ZrMgMo₃O₁₂ centered around big grain CoFe₂O₄, no other impurities generated.Through to the thermal expansion performance test, we can see ZrMgMo₃O₁₂ can better regulate CoFe₂O₄ thermal expansion coefficient.