| With the continual reducing of oil, coal, natural gas and other traditional fossil fuels, human development encountered unprecedented energy shortage problem. Countries around the world thereby adopted a series of measures to utilize renewable energy as the alternatives of traditional fossil fuels. Solar thermal power generation technology was developed in this background as an effective way to solve the energy shortage problem. This paper relies on973program. Aiming at solving the poor oxidation-resistance and thermal shock resistance of materials used in solar heat absorber, this paper carried out the following research.Formulations of mullite-SiC and Sialon-Si3N4-SiC composite ceramics were designed using Danjiangkou SiC, Korla andalusite and Gaoyi Si3N4as the main raw material. The heat absorber materials of two different systems were obtained by buried sintering method and non-contact buried sintering method, respectively. Afterwards, sol-gel method was used to prepare oxidation-resistant coatings which had good matching of thermal expansion coefficient for the solar heat absorbing substrate. The physical properties were tested by modern test technologies, especially analyzing the influencing factors for oxidation-resistance and thermal resistance; the phase compositions and microstructure of the samples were analyzed using the XRD, SEM, etc. Mechanisms on improving oxidation-resistance and thermal shock resistance were explored.Formulations of mullite-SiC composite ceramics were designed using Danjiangkou SiC, Korla andalusite as the main raw material. The heat absorber material was obtained by using buried sintering method. The results reveal that the firing atmosphere and the type of alumina source are the main influencing factors for the performance of in-situ synthetic mullite-SiC composite. In reducing atmosphere, D4is the best formulation of series D and α-Al2O3is the best type of alumina source for mullite-SiC composite. As for oxidizing atmosphere, γ-Al2O3seems to be the best type of alumina source for mullite-SiC composite. But oxidizing atmosphere will promote the production of excess cristobalite phase which lead to unexpected phase composition.To further improve oxidation-resistance and thermal shock resistance of mullite-SiC composite, Danjiangkou SiC, Korla andalusite powder and α-Al2O3as the main raw material and MnO2as additive were selected to modify the performance of sample D4. The results show that reducing the particle size of raw material and the introduction of MnO2could enhance the performance of composite and adding0.2wt%MnO2will improve the oxidation-resistance of samples. D4-1is the best formula(SiC56wt%,α-Al2O320wt%, andalusite24wt%, Y2O33wt%, MnO20.2wt%) and firing temperature range seems to be1480～1520℃.The performance of formula D4-1fired at1520℃is optimum and the properties of D4-1are listed as followings:water absorption of7.64%, porosity of18.04%, bulk density of2.23g/cm3, bending strength of116.91MPa, the weight gain after oxidation at1300℃for10h is7.0972mg/cm2; thermal expansion coefficient of7.205×10-6/℃, bending strength after30times thermal shock cycles (RT～1100℃, air cooling) of100.87MPa; thermal conductivity of8.32W/(m·K) at31.5℃, heat capacity of0.64J/g-K, thermal diffusivity of5.05×10-2cm2/s. The results from the research on the oxidation weight gain behavior of samples show that improving density and decreasing porosity of samples are effective ways to improve the oxidation-resistance.Formulations of series G were designed using Danjiangkou SiC, Gaoyi Si3N4as the main raw material. For avoiding the reaction between graphite and Si3N4, the non-contact buried sintering method was selected to prepare Sialon/Si3N4bonded SiC composite materials. The results show that formula G2achieve the best overall performance of series G. Formulation of G2is listed as follow:Danjiangkou SiC60wt%, Gaoyi Si3N428wt%, synthesised mullite14.04wt%, Silicon oxide8.91wt%, Y2O33wt%. The performance of formula G2is:water absorption of11.45%, porosity of27.10%, bulk density of2.23g/cm3, bending strength of62.66MPa, thermal expansion coefficient of5.441×10-6/℃, bending strength after30times thermal shock cycles (RT～1100℃, air cooling) of68.36MPa. The weight gain after oxidation at1100℃for100h is20.9659mg/cm2; thermal conductivity of9.61W/(m·K) at28.4℃, heat capacity of0.64J/g-K, thermal diffusivity of6.304×10-2cm2/s. Analysis on oxidation-resistance differences of series G reveals that the oxidation weight gain rate of sample contains more flux is greater at the beginning but reduce faster with the oxidizing time increasing.Sample of formula G2has been chosen for oxidation-resistant coating-modification. With the application of AICl3·6H2O and Al (NO3)3·9H2O as the main raw material, stable alumina sol and SiO2-Al2O3composite sol were prepared through sol-gel method. Composite sol stabilizing mechanism was discovered and prepared SiO2-Al2O3composite sol had good chemical stability and could be preserved stably in sealed condition for a month or more. The reaction temperature, pH, gel time and reaction solvent added are the main influencing factors to prepare transparent and stable composite sol. The temperatures TEOS solution hydrolyzed at and the Al(OH)3suspension stirred at should be50℃and85℃, respectively; the hydrolysis of TEOS solution was prepared in the condition of pH value controlled between2and3; concentration of HNO3should be lmol/L and the added amount should be controlled in n(H+):n(Al3+)=0.28(molar ratio). After the addition of TEOS hydrolysis to Al(OH)3suspension, stirring time of the mixture when heated should be2h. Samples impregnating twice in the composite sol of0.25mol/L and of1.5wt%for PVA addition could obtained mullite coating whose thickness was of2μm. After modification on the surface of the samples, oxidation-resistance and porosity of the samples had been tested. The results show that the modified samples obtained good oxidation resistance, comparing to the test results of the unmodified samples. The weight gain of modified samples after oxidation at1100℃for100h reduced to8.908mg/cm2; modified samples have lower porosity and water absorption than unmodified samples. With the impregnating times increasing, degree of the porosity reduction decrease.The results from the research on oxidation-resistant mechanism suggest that densification and decreasing porosity are the effective ways to improve the oxidation-resistance of the samples. Reducing the particle size of raw materials and addition of flux will promote the densification of samples. Moreover, the flux will also help the porosity of samples reduce when serving. The oxidation-resistant coating formed on the sample surface is also an effective way to inhibit infiltration of oxygen diffusion, which will decrease the oxidation rate of weight gain of the modified samples. |
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