Study On Growth And Properties Of Large-sized Al₂O₃/YAG:Ce³⁺ Eutectic Ceramics By Horizontal Directional Solidification Method

The rapid development of aerospace technology urgently requires an aviation turbofan engine with high efficiency,high thrust-to-weight ratio and high reliability.The key to improving thrust-to-weight ratio is to increase the turbine front inlet temperature of the turbofan engine,but the increasingly harsh service environment?high temperature,high load,etc.?also puts forward higher requirements for the structural materials of the engine.Al2O3/YAG eutectic ceramic exhibits high melting point,excellent oxidation resistance,superior thermal stability,and high-temperature mechanical properties,which could decrease the quality and bear the long-term work in high temperature oxidizing environment of engine turbine blades processed using the eutectic ceramic.Therefore,Al2O3/YAG eutectic ceramic,as one of the most potential structural materials used in high thrust-to-weight ratio engines,has been widely investigated over recent decades.In addition,the addition of a small amount of Ce O2 can tailor the microstructure and mechanical properties of Al2O3/YAG eutectic ceramics,and the obtained Al₂O₃/YAG:Ce³⁺ eutectic ceramics have characteristic excitation and emission spectra,which can be used as a new solid fluorescent material in the field of WLED.However,due to the small size dimension and low utilization rate for eutectic ceramics prepared by the reported methods,the samples are limited to meet the practical requirements.In order to break through the technical bottleneck,the horizontal directional solidification method?HDS method?used for the growth of large-sized single crystals was introduced into the preparation of large-sized Al₂O₃/YAG:Ce³⁺ eutectic ceramics.The hundred-millimeter plate-like Al₂O₃/YAG:Ce³⁺ eutectic ceramics were successfully obtained by HDS method,via the optimized design and control of the temperature field.The effect of doping of Ce on the microstructure,thermal and mechanical properties of the eutectic ceramics,and the luminous characteristics of the WLEDs with the etuectic pieces were investigated in this thesis,which laid the foundation for its subsequent multi-field application.The 170 mm × 90 mm × 20 mm and 170 mm × 160 mm × 25 mm large-sized platelike Al₂O₃/YAG:Ce³⁺ eutectic ceramics with different doping amounts?molar ratio?Ce/Y = 0%,0.25%,0.50%,1.00% were successfully fabricated by HDS method for the first time.The Al₂O₃/YAG:Ce³⁺ eutectic ceramics with doping amounts Ce/Y = 0.50% or less showed micron grade two-phase three-dimensional interpenetrating network structure,which was visually observed by ?CT.Due to the constitutional supercooling with an interface instability caused by Ce ions accumulation,the connected network-type coarse microstructure and the unconnected layer-type coarse microstructure were displayed on the perpendicular cross-section?? Z?and the parallel cross-section?? X?to the solidification direction,respectively.With the increase of Ce doping amount,the size of coarse microstructure regions increased significantly,and the “Ce-rich phase” formed by the combination of Ce O2 and Al2O3 was also detected when the Ce/Y ratio approached to 1.00%.Moreover,the residual compressive stresses in the Al2O3 phase of the Al₂O₃/YAG:Ce³⁺ eutectic ceramics were gradually increased as the doping amount of Ce increased by analyzing the peak shifts of Raman spectrum.According to the research on the mechanical properties of Al₂O₃/YAG:Ce³⁺ eutectic ceramics,the flexural strength of the eutectic ceramic with Ce/Y = 0.25% was reduced by ?5.8% and the compressive strength?1946±88.0 MPa?of the sample was increased by ?51.7%,which were compared with those of the eutectic ceramic with Ce/Y = 0%.The decrease in flexural strength of eutectic ceramics with Ce/Y = 0.25% was attributed to the increased formation probability of the micropores and microcracks triggered by coarse microstructure caused by Ce doping.The enhancement in compressive strength of this sample was attributed to the appropriate three-dimensional cross-distribution of coarse and fine microstructure regions.Moreover,the Al₂O₃/YAG:Ce³⁺ eutectic ceramics had very small variation in values of flexural and compressive strength under different test temperatures,indicating that the eutectic ceramics exhibited relatively stable high temperature mechanical properties.In addition,the Vickers hardness?16.50?17.50 GPa?,Young's modulus?340.00?350.00 GPa?and fracture toughness?2.80?3.10 MPa·m0.5?of the Al₂O₃/YAG:Ce³⁺ eutectic ceramics only decreased slightly with the increase of Ce doping amount,while these values were comparable to these of Al2O3/YAG eutectic ceramics prepared by other reported methods.According to the research on the thermal properties of Al₂O₃/YAG:Ce³⁺ eutectic ceramics,the average coefficient of thermal expansion?473-1273 K?,high temperature radiation spectra,and thermal stability of the eutectic ceramics with different Ce doping amounts had not changed significantly.The excitation peaks at 340 nm and 460 nm and emission peaks at 535 nm of the Al₂O₃/YAG:Ce³⁺ eutectic ceramics were detected through testing.By testing the performance of the WLEDs with different transparency Al₂O₃/YAG:Ce³⁺ eutectic ceramics,it was found that the luminous flux and efficacy of the WLED with finepolished eutectic were better than that of the WLED with coarse-polished eutectic.And the mechanism of that the luminous efficacy of WLED with Al₂O₃/YAG:Ce³⁺ eutectic ceramic was improved by interfacial scattering of the eutectic ceramic was also clarified.In addition,the WLED?light efficiency was as high as 104.6 lm/W?encapsulated by the transparent YAG:Ce3+ ceramics with controlled porosity?the residual pore as the second phase increased the conversion of blue light?were designed and prepared in this thesis,which not only verified the correctness of the mechanism of that the luminous efficacy of WLED with Al₂O₃/YAG:Ce³⁺ eutectic ceramic was improved by interfacial scattering of the eutectic ceramic,but also provided a more efficient and economical new solid fluorescent material for the field of WLED.