Structural Regulation Of La₂Zr₂O₇and Fabrication Of Bulk La₂Zr₂O₇Ceramic

Compared with the conventional ZrO2, Al2O3insulation ceramics, the rare-earth zirconate is widely utilized in many fields because of its complex crystal structure, high melting temperature (>2000℃), excellent chemical stability, excellent thermal stability and low thermal conductivity. In order to meet the requirements of thermal protective system (TPS) to the insulation ceramics, we prepared various La2Zr2O7ceramics employing cerium (Ce) as doping element, and BaTiO3and SiC as the second phases. La2Zr2O7ceramics were characterizated by XRD, SEM and TG-DTA, and their thermal conductivity and mechanical strength were studied in detail. Based on the above studies, we fabricated the large-sized La2Zr2O7ceramics to meet the needs of TPS by hot pressed sintering method.Firstly, we study the influence of cerium (Ce) element on the structure and properties of (LaGd)(Zr2-mCem)O7ceramics. The results show that, with the increase of Ce content, the crystal structures of (LaGd)(Zr2-mCem)O7ceramics undergo the order-disorder phase transition from ordered pyrochlore to disordered fluorite, and (LaGd)(Zr1.4Ceo.6)O7ceramic has low thermal conductivity and excellent compressive strength in the series of (LaGd)(Zr2-mCem)O7ceramics. Compared with ceramic, thermal conductivity of (LaGd)(Zr1.4Ceo.6)O7ceramic is1.20W·m-1·K-1, decreased by20%, and its compressive strength can reach680MPa, a two-fold increase.Secondly, BaTiO3as the second phase was added into La2Zr2O7ceramic. The structure and properties of xBaTiO3/LZO composite ceramics were studied. The results show that BaTiO3has a chemical stability and doesn’t react with the La2Zr2O7ceramic matrix. BaTiO3particles are uniformly located at the grain boundary of La2Zr2O7. When the additive amount of BaTiO3is15vol%, the (15vol%) BaTiO3/LZO composite ceramic simultaneously has the low thermal conductivity and excellent mechanical strength. Compared with the La2Zr2O7ceramic, its thermal conductivity is1.04W·m-1·K-1, decreased by30%, its compressive strength and flexural strength are480MPa and150MPa, increased by110%and100%respectively.Thirdly, we also employ SiC nanoparticles as the second phase to synthesize the ySiC/LZO composite ceramics. The results show that SiC can also form the second phase locating at the grain boundary of La2Zr2O7, but it would partly react with the La2Zr2O7ceramic matrix and form La28/3Si6O26.When the additive amount of SiC is10vol%, the (10vol%) SiC/LZO composite ceramic simultaneously has the low thermal conductivity and excellent mechanical strength. Compared with the ceramic, its thermal conductivity is0.85W·m-1·K-1, decreased by40%, its compressive strength and flexural strength are360MPa and170MPa, increased by60%and130%respectively.Finally, we use the hot pressed sintering method to fabricate the large-sized La2Zr2O7ceramic, and solve the problem of ceramic cracking by regulating the sintering parameters including heating rate, cooling rate, sintering temperature and dwell time to reduce stress in the ceramics. The samples prepared by hot pressed sintering method have a higher relative density and compressive strength, compared with the La2Zr2O7ceramic prepared by the pressureless method, its value can reach98%and700MPa, which are increased by10%and210%respectively.