A <sub> 2 </ Sub> (wo <sub> 4 </ Sub>) <sub> 3 </ Sub> Tungstate Laser Sintering, High Temperature Rapid Synthesis Of Research

Negative thermal expansion (NTE) materials have a number of importantpotential uses in optical, electronic, medicinal and mechanical applications. Tungstateof A₂ (WO₄)₃ series is a very important kind of negative thermal expansion materials.The composition of such compounds is flexible and A³⁺ can be any trivalent cationwhose radius is from 0.0675 nm (Al³⁺) to 0.1075 nm (Gd³⁺). So, we can adjust thethermal expansion coefficient through the differert element substitution andadulteration. But so far, all the systhesis processes for the production of the tungstatesare either tedious or environment unfriendly or unsuitable for industrial massproduction. Exploring new rapid systhetic routes is demanded imminently. So, themethods of laser sintering and high-heat and fast way in the synthesis of tungstateNTE materials were adopted in this thesis. Sc₂ (WO₄)₃ and Eu₂ (WO₄)₃ weresynthesized by using high-heat and fast way and Al₂ (WO₄)₃ was produced with ahigh power CO₂ laser. Then, X-ray diffraction,Raman spectroscopy,Scanningelectron microscopy, thermal analysis and other analytical methods were used for thesystematic analysis of the samples.The main results and conclusions are as the following:1,Negative thermal expansion materials Al₂ (WO₄)₃ is successfully synthesized by the laser sintering method for the first time. This is a new fast and convenient method for synthesis in which a sample can be produced in few seconds and large-scale industrial production is promising.2,When the defocus length is set to 120 mm, the laser synthesis power between 700 W and 800 W, laser scanning speed from 2 mm/s to 4 mm/s are the appropriate parametersfor the synthesis of Al₂ (WO₄)₃.3,The grains are compact and distribute uniformly, about 10 nm in size, and show orientated growth in Al₂ (WO₄)₃ ceramic material synthesized by laser sintering method.4,The rapidness characteristic in laser sintering mothod can inhibit the volatilization of the raw material WO₃ However, the samples still contain a small amount of non-equilibrium phases Al_XWO₃ (x<1) and WO_3X (x <1) because of the non-equilibrium synthesis processes.5,Sc₂(WO₄)₃ and Eu₂(WO₄)₃ are successfully synthesized by using high-heat and fast way for the first time. This approach not only greatly increases the speed of sintering, but also inhibits the volatilization of the raw material WO₃ effectively, and eventually makes compound very pure.6,The synthesis temperature from 1360°C to 1400°C, synthesis time betweem 0.5 h to 1 h are the appropriate parameters for Sc₂ (WO₄)₃ synthesis by high-heat and fast way, and the temperature from 1280°C to 1320°C, time betweem 0.5 h to 2 h are the appropriate parameters for Eu₂ (WO₄)₃.7,Sc₂ (WO₄)₃ synthesized by high-heat and fast way has an orthogonality structure, belongs to the Pnca space group, may possesses negative thermal expansion property.8,Eu₂ (WO₄)₃ synthesized by high-heat and fast way has a monoclinic structure , belongs to the C12/cl space group, is impossible to possess negative thermal expansion property.