| Renewable energy utilization is an important way to solve the current energy crisis. Solar Thermal Tower Plant(STTP) technology has attracted much attention due to its clean and efficient. As one of the most important component, solar absorbing materials are designed to receive concentrated solar energy and heat transfer/absorb, which determine the power generation efficiency of STTP system. At present, solar absorbing materials still has shortage in working temperature, thermal shock resistance and high temperature oxidation resistance. For the sake of those problems, SiC-Sialon composite ceramics used for solar absorbing materials were synthesized from SiC, ?-Si3N4, AlN, calcined bauxite, quartz and rare earth oxide(RE2O3) via dual-layer buried sintering. The relationships between composition, preparation technique, microstructure and performance were discussed by using XRF, XRD, SEM and EPMA, etc. The synthesis mechanism of Sialon ceramics, the influence of SiC and sintering temperature on microstructures and properties, the role of RE2O3 in SiC/O'-Sialon ceramics were studied. The optimal addition of Y2O3 and Sm2O3 was determined. The improving mechanism of Y2O3 and Sm2O3 on thermal shock and high temperature oxidation resistance were researched. The main results are as follows:(1) The ?-Sialon(sample B1), O'-Sialon(sample B2) and ?/O'-Sialon(sample B3) ceramics were prepared via dual-layer buried sintering. The associated performance and synthesis mechanism were discussed. Compared comprehensively, ?-Sialon has denser structure, higher bending strength and good thermal shock resistance. The apparent porosity and bending strength of sample B1 sintered at 1600 °C are 3.90% and 195.36 MPa, respectively. The bending strength of material after 30 thermal shock cycles(room temperature~1100 °C, windcooling) is 130.54 MPa, which is decreased by 33.18%. The oxidation resistance of O'-Sialon is better. The specific weight gain and the kinetic constant of oxidation of sample B1 sintered at 1600 °C is 13.9893 mg·cm-2 and 0.6958 mg2·cm-4·h-1 after oxidized at 1300 °C for 100 h. It is suggested that the synthesis of Sialon should go through a process including dissolution, phase change, deposition, nucleation and grain growth. The ?-Si3N4 would dissolve in the liquid phase and turns into ?-Si3N4 firstly at high temperature. When the concentration of Si-Al-O-N in this system is supersaturated, ?-Si3N4 could react with Al2O3 and AlN, resulting Sialon crystal nucleus deposited on the surface of heterogeneous grains, and then those grains grow up. The results indicate that the optimal sintering temperature of Sialon ceramics is 1600 °C. The synthesis rate of O'-Sialon is relatively high at 1540 °C~1640 °C. There is some x-Sialon generated in ?/O'-Sialon composite ceramics.(2) The in-situ synthesized Sialon bonded SiC composite ceramics used for Solar Thermal Power Generation Receiver was prepared, providing a new approach to low-cost and modular manufacturing of solar absorbing materials. The results indicate that the solar absorptance, working temperature and thermal shock resistance could be enhanced by adding SiC. In the first thermal shock cycles, the SiC and Si3N4 is oxidized to form SiO2 liquid phase and then fill pores, promoting the densification of materials. At the same time, a small amount of Sialon is decomposed into mullite, which could reinforce the materials and resulting higher bending strength. Excessive quartz and glassy phase is generated during the repeated thermal shock cycles, leading to more serious thermal shock damage and lower bending strength. The SiC/?-Sialon(sample C1)ceramics sintered at 1600 °C possess best thermal shock resistance due to its higher bending strength(70.04 MPa), lower thermal expansion coefficient(5.86×10-6 °C-1) and evenly distributed aperture. The bending strength of material after 30 thermal shock cycles(room temperature~1100 °C, windcooling) is 66.68 MPa, which is decreased by 4.79%. It is believed that O'-Sialon is oxygen-enriched. The oxygen diffusion could be prevented by the oxidation film generated on the sample surface. Therefore, SiC/O'-Sialon(sample C2) have good oxidation resistance. The specific weight gain and the kinetic constant of oxidation of sample C2 sintered at 1600 °C is 23.6445 mg·cm-2 and 2.0119 mg2·cm-4·h-1 after oxidized at 1300 °C for 100 h.(3) The SiC/O'-Sialon solar absorbing ceramics were prepared by using 3 wt% Sm2O3, Ho2O3, Er2O3, Tm2O3 and Lu2O3 as additives. The influence of those rare earth oxides on the properties at room temperature, thermal shock resistance and oxidation resistance were investigated. The highest bending strength of each formulation smples sinterd at different temperatures increased first and then decreased with the decrease of RE3+ ionic radius, while the thermal shock resistance and oxidation resistance of materials show the reverse change trend. Sample D1(added 3 wt% Sm2O3) sintered at 1640 °C has the optimal performance. The apparent porosity and bending strength are 1.23% and 68.70 MPa, respectively. The bending strength after 30 times thermal shock cycle test(room temperature~1100 °C, windcooling) is 63.90 MPa, which is decreased by 6.98%. The specific weight gain and the kinetic constant of oxidation of sample D1 oxidized at 1300 °C for 100 h sintered at 1640 °C is 20.8763 mg·cm-2 and 1.1929 mg2·cm-4·h-1. By forming liquid phase with lower viscosity during thermal shock cycles tests, Sm2O3 can prompt the grain growth and densification, resulting the best thermal shock resistance of sample D1. In the process of cyclical oxidation, the oxidation film can rapidly formed and restrain the interfacial reaction of samples by introducing Sm2O3, it makes sample D1 have the optimal oxidation resistance. In the solid solubility limit of O'-Sialon, the grain size can be refined by adding Er2O3, making sample D3(added 3 wt% Er2O3) sintered at 1640 °C received the best bending strength(79.19 MPa).(4) The thermal shock resistance of SiC/O'-Sialon ceramics was significantly enhanced by increasing Y2O3 content. The improving mechanism of thermal shock resistance was studied. It is suggested that the best additive quantity of Y2O3 is 9 wt%. Sample E4(61 ?m SiC 48 wt%, 20 ?m SiC 12 wt%, ?-Si3N4 24.78 wt%?calcined bauxite 5.04 wt%, quartz 10.18 wt%, Y2O3 9 wt%) sintered at 1640 °C has the optimal performance. The apparent porosity and bending strength is 26.54 % and 89.11 MPa, respectively. After 30 times thermal shock cycles, the bending strength is 102.98 MPa, which increased by 15.57%. The specific weight gain and the kinetic constant of oxidation of sample E4 oxidized at 1300 °C for 100 h sintered at 1640 °C is 9.5568 mg·cm-2 and 0.3625 mg2·cm-4·h-1. When the additive quantity of Y2O3 is 5 wt%~9 wt%, the oxidation of SiC and the decomposition of Sialon during thermal shock cycles can be restrained, the Y2Si2O7 synthesized in thermal shock cycles can resume excessive quartz, reducing the rate of cracking effevtively, and then improve the thermal shock resistance of materials.(5) The high temperature oxidation resistance of SiC/O'-Sialon ceramics was significantly enhanced by increasing Sm2O3 content. The improving mechanism of oxidation resistance was studied. It is suggested that sample F5(61 ?m Si C 48 wt%, 20 ?m SiC 12 wt%, ?-Si3N4 24.78 wt%?calcined bauxite 5.04 wt%, quartz 10.18 wt%, Sm2O3 11 wt%) sintered at 1640 °C has the best oxidation resistance. The specific weight gain and the kinetic constant of oxidation of sample F5 oxidized at 1300 °C is 5.5043 mg·cm-2 and 0.2813 mg2·cm-4·h-1. It is believed that the oxidation film can be rapidly formed and restrains the interfacial reaction of samples due to liquid phase with lower viscosity generated from Sm2O3. Therefore, the SiC/O'-Sialon with Sm2O3 added would possess better oxidation resistance when Y2O3 and Sm2O3 added the same quantity, respectively. In the interfacial reaction stage, the weight gain of is reduced by increasing the content of Sm2O3. During the process of diffusion control stage, the viscosity of oxidation film can be raised by forming Sm2Si2O7 grains, resulting lower weight gain rate in diffusion stage. Therefore, the oxidation resistance of series is improved with the content of Sm2O3 increasing. |
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