| Since solar energy is huge reserves, broad distribution, it is generally considered as one of the clean and renewable energy. In recent years, it has been researched by many scholars at home and abroad. Solar thermal power generation is an important way for utilization of solar energy. It has been known as the most potential method for the development and application of solar energy. This topic is based on the national "973Program", according to the present needs of heat-absorber materials in the solar thermal power generation system, researching the heat-absorber materials with high performance.In this paper,β-Sialon and β-Sialon/Si3N4composite ceramics used for solar thermal power generation were designed and prepared with the method of pressureless sintering and the main raw materials of α-Si.3N4, AN and different aluminum-containing compounds. The effects of different metal oxides and rare earth oxides as sintering aids on the sintering process and integrated performance were discussed. The water absorption, porosity, bulk density, firing shrinkage, flexural strength, thermal shock resistance and oxidation resistance were tested. Phase composition and microscopic morphology were tested and analysised with modern techniques such as XRD and SEM. Relationships between sample formula composition, processing technology, microstructure and properties were discussed. The mechanism of thermal shock resistance and oxidation resistance properties of samples were studied. The main results of the research are as follows:(1) Using a-Si3N4, calcined bauxite, AlN as main raw materials and MgO, Y2O3as sintering aids, C series formula was designed. The experiment results show that the effect of MgO and Y2O3is remarkable. β-Sialon with Z values range from1to4are synthesized as expected in C1～C4samples. C2sample (62.17wt%Si3N4,28.71wt%calcined bauxite,9.12wt%AN, plus5wt%MgO and3wt%Y2O3) has better performance after firing at1560℃. Water absorption is7.38%, porosity19.22%, bulk density2.61g·cm-3, the flexural strength88.43MPa. The thermal expansion coefficient is6.64×10-6℃-1(RT～900℃), thermal conductivity is5.56W/m·K at room temperature. The phase compositions of C2sample are MgAl2O4, β-Sialon, α-Si3N4 and β-Si3N4. Research shows that with the increase of Z value, the size of β-Sialon increase, structure loose. When Z=2, β-Sialon has high strength and good thermal stability, the higher content of β-Sialon is, the better strength and thermal shock resistance of the sample have.(2) In order to achieve higher strength and density, D series was designed on based of C2formula with Y2O3, La2O3and borax as sintering aids. The results show that, D2sample (62.17wt%Si3N4,28.71wt%calcined bauxite,9.12wt%AN, plus3wt%Y2O3and3wt%La2O3) has better performance after firing at1580℃. Water absorption is3.33%, porosity9.45%, bulk density2.84g·cm-3, the160.73MPa. The thermal expansion coefficient (RT～900℃) is6.06×10-6℃-1,thermal conductivity is7.8W/m-K at room temperature. After100h oxidation experiment at1300℃, the weight gain rate of the sample is25.56mg·cm-2. The crystal phases of D2sample are β-Sialon, β-Si3N4and corundum. D2has dense microstructure, less and evenly distributed porosity, so it has more comprehensive performance than C2.(3) Mechanism investigation of thermal shock resistance shows that the sample has high thermal conductivity, density and strength because of the generation of β-Sialon and its connection with Si3N4, decreasing the tensile stress on the surface of the sample during the process of thermal shock cycles, which may damage the sample. The sample has excellent thermal shock resistance, after30times’ thermal shock (RT～1100℃), the bending strength of C2and D2samples increased comparied with the initial samples. Mechanism of oxidation resistance shows that β-Sialon/Si3N4composite ceramic has good oxidation resistance performance. The process of oxidation belongs to the passivation oxidation. The diffusion rate of oxygen into the internal is slower because of its high density. There exists dense surface layer of quartz and mullite because of the oxidation of β-Sialon and Si3N4, and the thicker the surface layer is, the lower oxidation rate the sample has. It is complying with the parabolic law.(4) β-Sialon/Si3N4composite foam ceramic was prepared by precursor impregnation method, using D2as the powder formula, adding adhesive, rheological agent, defoaming agent and so on. After firing at1540℃, the porosity of sample is82.55%, compressive strength0.89MPa. After30times’ thermal shock (RT～1100℃), the appearance of sample is intact, meetting the requirements of the national standard GB/T25139-2010. XRD results show that the phase composition of E2sample is the same as D2. SEM results show that E2sample has connected and thick pores. It guarantees the smooth circulation of air and three-dimensional connecting structure, which ensure the higher efficiency of heat exchange. It is suitable for the absorber material of solar tower thermal power generation system. |
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