Preparation And Properties Of A (2-x) B (x) M3O12 (A = Er, In; B = La, Sc; M = W, Mo) Negative Thermal Expansion Materials

The most of the materials expand with the increase of temperature due to the non-harmonic vibration of chemical bonds. Thermal expansion is one of the properties which must be considered in the application of highly functional materials because the mismatch of thermal expansion between component materials can cause problems,such as mechanical destruction and positional deviation. So the negative thermal expansion (NTE) materials,which is one of the new research focus in materials science in recent years,because we can solve these problems by preparing materials with controllable or near-zero expansion coefficients. The preparation of materials with low or zero thermal expansion would increase the mechanical reliability and long-term duration.So it has lots of potential applications in electrical,optical and high-temperature devices. Tungstate and molybdate family of A2M3O12 have been extensively studied owing to strong negative thermal expansion in a broad temperature range,and the thermal expansion coefficient can be changed easily by partial chemical substitution of the A cation by another trivalent action.In this work,the negative thermal expansion materials Er2-xLaxW3O12,In2-xScxW3O12, Er2-xLaxMo3O12 and In2-xScxMo3O12 were prepared, and we control the thermal expansion coefficient to be negative, positive and even zero by careful adjustment of the Er/La or In/Sc ration.Carried out a systematic analysis of the sample by XRD、TMA、TG、SEM and TEM. The main results as follow:1. By using solid phase method, we use Er2O3, La2O3 and WO3 as raw materials. Then mill the materials according to certain stoichiometric mixture. We successfully obtained the series of Er2-x:LaxW3O12(0≤x≤2) ceramic materials after sintering 6h at 950℃. XRD analysis showed that, when 0≤x≤0.5, the sample XRD atlas is a single phase of Er2W3O12; when 0.5<x<1.5,it was a composite phase, Er2W3O212 and La2W3O12 phase coexistence; when 1.5Sx<2, they appear entirely in La2W3O12 phase. TG displays that Er2W3O12 absorpt moisture easily at room temperature. With the increase of La incorporation, Er2-xLaxW3O12(0≤x≤2) absorption phenomenon gradually decreases. TMA curve showed that, in the negative thermal expansion temperature interval, with the incorporation of La, its thermal expansion coefficient absolute value showed a rise and then decline. We reach the purpose of controling the thermal expansion coefficient. The thermal expansion coefficient of Er0.25La1.75 W3O12 is nearly zero-1.12×10-6/K-1 at 216-600℃C temperature range. Through experimental analysis, we found that it can control the thermal expansion coefficient of Er2.xLaxW3O122(0<x:<2) by regulating the ratio of Er/La.2. We will composite La2Mo3O122 which shows positive thermal expansion and ET2MO3O12 which shows negative thermal expansion. By using solid phase method, we successfully obtained the series of Er2-xLaxMo3O12(0≤x≤2)) ceramic materials after sintering 6h at 750℃. It can be seen on XRD map, when 0<x<0.5, the map fully reflect the phase of Er2Mo3O12, when x=1,1.5, it is a transitional phase, neither the Er2Mo3O12 phase, no La2M03O12 phase, when x=1.75,2, it appears as the La2Mo3O12 phase. With the incorporation of La, it can be seen from the SEM image of the sample, the grain size of the sample becomes larger, the pore reduced, and density gradually improve. TG shows that its hygroscopicity is similar to the series of Er2-xLaxMo3O12(0<x<2). The bigger the incorporation amount of La, the smaller the hygroscopicity. Thermal expansion analysis shows that, with increasing the La content, the thermal expansion coefficient absolute value of the sample decreases. Whenx=1.75, the thermal expansion coefficient of Ero.25La1.75Mo3012 is-0.64302×10-6℃-1, it shows excellent thermal expansion nearly zero.These materials have very important practical significance, but also that the method of finding near zero thermal expansion materials through combining the positive thermal expansion materials with negative thermal expansion material is feasible.3. We prepared the series of In2-xScxW3O12{x=0,0.1,0.3,0.5,1,2) solid solution by using the traditional solid-state reaction method. The XRD results show that when 0≤x≤1, their map appear In2W3O12 phase. It indicates that they are monoclinic structure at room temperature. Whenx=2, namely SC2W3O12, is orthogonal structure. It can be found from the XRD of the In2W3O12 at diffident temperature, In2W3O12 sample is monoclinic structure in the low temperature range of 25-250℃, is orthorhombic structure in the high temperature range of 300～600℃. SEM shows that, the sample particle is uniform, and there are some holes, particles with similar size and shape.When x=0.5, the sample becomes more dense. Thermal expansion analysis shows, using part of Sc3+ instead of In3+, we can adjust the temperature point, which the composite material transforms from the monoclinic phase to orthorhombic phase, to a lower temperature. The thermal expansion coefficient of this series is between -7.13×10-6℃-1 to-1.08×10-6℃-1, when x=03,0.5, we achieve a thermal expansion coefficient of nearly zero -1.08×10-6℃-1 and -1.28×10-6℃-1.For InScW3O12, the phase transition temperature is at 47℃. It has a stable negative thermal expansion coefficient from temperature range 47℃ to 700℃, which suggesting that it has more potential applications in many fields.4. By using stepwise solid phase method, we successfully obtained the series ofIn2-xScxMo3O12 (0<x<2) ceramic materials after sintering 20h at 780℃. When x=0,0.5, their XRD pattern performance is the In2MO3O12 phase, and x=1,1.5,2, fully reflect the SC2Mo3O12 phase. By the SEM image of the sample we can draw the conclusion that, compared to the tungstate, the molybdate series sample has the poor compactness, and there are a large number of small particles and holes in each sample. With the incorporation of Sc samples, although there are small particles and s, the size of small grain slightly become smaller. The number of the pores and the diameter of grains decreased, the density gradually increased. The TMA curves of the sample display that, In2Mo3O12 sample phase transition temperature occurs at the temperature range 310-360℃. With the increase of Sc doping amount, we successfully adjust the phase transition temperature of In2-xScxMo3O12(0≤x≤2) to the low temperature direction. When x=1, the phase transition temperature of the solid solution InSCMO3O12 decreased to 155.6℃, when x=2, the thermal expansion curves of Sc2Mo3O12 samples is an inclined straight line, but also the thermal expansion coefficient absolute value of In2-xScxMo3O12 (x=0,0.5,1,1.5,2) ceramic constantly decreases, and it remained negative thermal expansion.