| The solar thermal power tower system requires heat transport pipe enduring more than1000℃and has good thermal shock resistance. According to the material structure and property of heat transport piping material of the solar thermal power generation, many works were done in this paper. The mullite ceramics were prepared by situ synthesis method at low-temperature and the high temperature cordierite ceramics prepared by situ synthesis method together, then the mullite-cordierite composite ceramic were prepared by situ synthesis method and its thermal shock resistance were characterized by different mix design the low-temperature optimal mullite and high temperature optimal cordierite. In order to further improve the thermal shock resistance of mullite-cordierite composite ceramic, another modified mullite-cordierite composite ceramic was prepared for solar thermal power modified mullite-cordierite composite heat transfer pipe material. And many properties of solar heat power generation pipeline material were characterized such as sintering properties, mechanical properties, oxidation resistance, corrosion resistance, thermal shock resistance and bonding property. Using modern testing techniques such as XRD, SEM, EDS, EPMA, DSC to study the material composition, preparation process, microstructure and properties, and the mechanism of thermal shock and bonding mechanism were explored. The major achievements and innovations were made in this paper as follows:(1) The low-temperature mullite and high-temperature cordierite were prepared by situ synthesis method. The mullite ceramic was designed by adding different content of α-Al2O3to soft porcelain and the high temperature cordierite formula composition was screened by different aluminum sources.The mullite was prepared by the pressureless sintering conditions. The factors of adding different content of α-Al2O3to the mullite ceramic, the thermal shock resistance property and the mechanism were explored. The study has shown that the typical mullite sample A3which sintered at1340℃had apparent porosity rate of0.12%, water absorption of0.04%, bulk density of2.71g/cm3, bending strength of94.82MPa. The composite ceramic has better thermal shock resistance property. The bending strength loss rate of sample is60%(from room temperature to1100℃by air quenching-strength test) after30times thermal shock cycle. The main crystal phase is mullite and the second phases are corundum and quartz. And the high-temperature cordierite was prepared by situ synthesized method by screening the various sources of aluminum according to the theory cordierite compostion, the study results show that the optimal cordierite sample A3which sintered at1420℃had the apparent porosity of5.04%,water absorption of2.35%, bulk density of2.14g/cm3, bending strength of82.44MPa, bending strength of sample increase rather than decrease to5%.The properties of thermal shock show that the thermal shock resistance mechanism of mullite ceramic is microcrack-nanorod composite toughening mechanism and the hermal shock resistance mechanism of cordierite is microcrack toughening mechanism.(2) The C series mullite-cordierite composite ceramics were prepared using natural mineral material by means of different ratio of low-temperature mullite A3and high-temperature cordierite B1. The relationship between the structure and property of the system was discussed. The study shows that the optimal sample C5which sintered at1300℃had the apparent porosity of0.364%, water absorption of0.142%, bulk density of2.56g/cm3, bending strength of121.08MPa, bending strength loss rate of sample is8%(from room temperature to1100℃by air quenching-strength test) after30times thermal shock cycle. The results of the strength and the thermal shock loss are between A3and B1. The thermal shock resistance study shows that the thermal shock resistance mechanism of composite ceramic is the microcrack-nanowhiskers composite toughening mechanism.(3) According to the C series of study, a modified D series mullite-cordierite composite ceramics were prepared by situ synthesis method and by designing different proportion of mullite and cordierite. The relationship among material composition, structure and properties was discussed. The study shows that the optimal sample D3which sintered at1300℃had the apparent porosity of0.44%, water absorption of0.14%, bulk density of2.55g/cm3, bending strength of124.09MPa, bending strength loss rate of sample is2%(from room temperature to1100℃by air quenching-strength test) after30times thermal shock cycle and the composite ceramic has a excellent corrosion resistance. The thermal shock resistance property of modified ceramic study shows that the thermal shock resistance mechanism is the microcrack-particles dispersed composite toughening mechanism.(4) The D3was heat treated for different holding time in order to enhance the composite ceramic thermal shock resistance, reduce the glass phase and promote the growth of cordierite crystal. The important properties of D3sample such as apparent porosity, mechanical properties, corrosion resistance, oxidation resistance, thermal shock resistance and related mechanisms for the solar thermal power were discussed. The results show that the D3holding for9hours has excellent performance, the water absorption of the sample is0.09%, apparent porosity of0.22%, bulk density2.56g/cm3, bending strength of121.97MPa, bending strength of sample increase rather than decrease to1%(from room temperature to1100℃by air quenching-strength test)after30times thermal shock cycle, indicating that the thermal shock resistance property of heat pipe material increased significantly by heat treated method. The weight loss rate is <0.4%after100h oxidation at1100℃. The corrosion weight loss percentages are0.06%in20wt%H2SO4and0.05%in10wt%NaOH respectively. The oxidation resistance, corrosion resistance and thermal shock resistance study show that the oxidation weight loss is due to the volatile alkali metal oxide and the ceramic has better alkali resistance than acidic resistance because of the impact of the cordierite crystal structure, the thermal shock can improve property of thermal shock resistance because the thermal shock can release glass phase and form vacancy. The microstructure analysis shows that prolonging holding time can promote the grain growth, however, the bending strength decreases because the cordierite can decompose to glass phase after5h holding time.(5) The bonding properties of the D3formula sample were studied by using D3sample as substrate and serious inorganic binders. The study results show that the refractory cement has the best binder performance. Its bonding strength is3.052MPa at room temperature and the bond strength is6.62MPa at1100℃. The EPMA analysis shows that the ceramic adhesive bond mechanism is adsorption-mechanical binding complex mechanism. |
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