Sythesis And Thermophysical Properties Of La₂Ce₂O₇ Doped With M²⁺/M³⁺

With the development of advanced aero engines, thermal barrier coating (TBC) ceramics with higher thermal expansion coefficients and lower thermal conductivity is urgently needed. La2Ce2O7 has been regarded as a prospective TBC ceramic because it has high thermal expansion coefficient (TEC), low thermal conductivity (TC) and good phase stability at high temperature. In order to explore potential TBC ceramics, the influences of the doping of ions with lower value into La2Ce207 (La-site?Ce-site) on its thermophysical properties have been studied in this dissertation. La2Ce207 and its doping solid solution of (La1-xMx)2Ce2O7-x and La2(Ce1-xMx)2O7-x were synthesized by by Sol-gel method.Their structures were analysed by powder XRD, and their TEC and TC were measured with a high-temperature dilatometer and laser thermal conductivity tester respectively.A series of (La1-xMx)2Ce2O7-x (M=Mg and Sr) solid solutions were prepared by sol-gel method. Powder XRD data show that the solubility of Mg2+ and Sr2+ in the La-site in La2Ce207 lattice are 40% and 10% respectively; All the solid solutions exhibit identical structure type with La2Ce207 (defect fluorite structure), and the lattice parameters of (La1-xMx)2Ce2O7-x samples decreases gradually with the increase of doping amount. The TECs of Mg2+ and Sr2+-doped sample are higher than La2Ce207, and increase with the increasing of doping amount. At the same temperature, the TCs of (La1-xMgx)2Ce2O7-x sample increase when Mg2+-doping amount varies from 0-20%, then decrease when Mg2+-doping amount varies from 20%-40%. Whereas, the TCs of (La1-xSrx)2Ce2O7-x sample decrease when Sr2+-doping amount varies from 0-7.5%, then increase when Sr2+-doping amount varies from 7.5%-10%.A series of La2(Ce1-xMx)2O7-x (M=Gd,Y) solid solutions were also prepared by sol-gel method. Powder XRD data show that the solubility of both Gd3+ and Y3+ in the Ce-site in La2Ce2O7 lattice are about 10%, and their structures transfer from defect fluorite (La2Ce207) to pyrochlore. The TECs of La2(Ce1-xMx)2O7-x (M=Gd, Y) ceramics increase with the temperature, and is higher than that of undoped sample at 1000?. The TCs of La2(Ce1-xGdx)2O7-x decrease first and then increase with the increasing of x value with x=0.075 nearby as turning point. Whereas, the TCs of La2(Ce1-xYx)2O7-x decrease with Y3+-doping amount.In summary, the doping of ions with lower value into La2Ce207 (La-site? Ce-site) can improve its TEC and decrease its TCs, which provides experimental and theoretical basis for developing novel TBC ceramic materials.