| Thermal barrier coating (TBC) systems have attracted increasing attention over the past few years, driven by the demands for substantially high thrust-weight ratio, high-flow ratio, higher operating temperatures and enhanced reliability envisaged for the next generations of gas turbine engines. The current state-of-the-art of these TBCs is zirconia partially stabilized with 7 wt.%-8wt.% yttria (YSZ), which will undergo detrimental microstructural and phase changes resulting in deteriorating thermo-physical properties and decreasing service life of TBCs, respectively. Thus it is inevitable to develop novel promising candidates for the next generation TBCs to overcome above problems associated with the use of YSZ at higher temperatures. In this dissertation, magnetoplumbite-type LnMA11O19 (Ln=lanthanide; M=Mg, Mn to Zn; A=Al, Fe, Zr) series ceramics were identified.This is explored in this work by partial substitution of the La3+, Mg2+ and Al3+ ions with Sr2+, Zn2+ and Ti4+ ions, respectively. The La1-xSrxMg1-xZnxAl11-xTixO19 (LSMZATO, x=0.0,0.1,0.2,0.3,0.4,0.5) solid solution series ceramics with two dimensional crystal grouth steps were synthesized by citric acid sol-gel technique. By the aid of Debye’s phonon gas theory, the calculated relaxation times due to point defects, stepped surfaces and grain boundaries in LSMZATO were calculate. Multi-enhanced-phonon scattering modes by phonon umklapp and point defects were estiblished.Rare-earth hexaaluminates LnMgAl11O19 (Ln:La; Pr; Nd; Sm; Eu; Gd) with magnetoplumbite-type crystal structure were synthesized by a citric acid sol-gel technique. The mechanism behind the lowered thermal conductivity was mainly due to the multi-enhanced-phonon scattering modes, namely, the intrinsic scattering in the complex crystal cell and the distorted AlO6 octahedron influenced by the interaction with RE3+ ions, as well as nano-platelet-like structure to incorporate additional grain boundary scattering.The anti deliquescence properties and deliquescence mechanism of LaMeAl11O19 Me:Mg;Mn;Ni;Cu;Zn) series were investigated systemically.The kinetics of reaction between LaMgAl11O19 and H2O should be dominated by the Mg2+/H+ ionic exchange, leading to a structural transition from LaMgAl11O19 to a partially protonated magnetoplumbite phase LaMg1-xHxAl11O19-0.5x. An effective approach was proposed to improve its anti-deliquescent property by completely substituting divalent alkaline earth ions Mg2+ with Zn2+. Due to the larger molecular weight, the LaZnAl11O19 specimen showed a slight reduction in thermal conductivity comparing with the LaMgAl11O19 simple.The phase stability and sintering pesistance of LaZnAl11O19 ceramics at high temperature(>1500℃) were investigated. After heat-treated at 1600℃,1650℃ and 1700℃ 12h, the thickness of plate-like crystals along c-axis was changeless, and all the LaZnAl11O19 samples exhibited uniform magnetoplumbite-type hexagonal crystal structure. As the operating temperature up to 1200℃, the contribution to thermal conductivity from photon radiation was a value of 0.17 W·m-1·K-1 (a 7.5% contribution at 1200℃)The lattice thermal conductivity of Nd3+/Ni2+ co-doped LaZnAl11O19 ceramics was lowered by the point defects, and the photon radiation was also decreased by color centers rare earth Nd3+ to increase La0.5Nd0.5Zno.sNi0.5Al11O19 ceramics opaqueness to visible and near infrared radiation. Finnally, the thermal protective performance of Nd3+/Ni2+ co-doped LaZnAl11O19 ceramics at high temperature was enhanced by lowering lattice and photon thermal conductivity simultaneously. |
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