| Solar energy is a renewable resource of non-pollution and inexhaustible in quantity and can be converted to various forms of energy to be used. One of the most widely application is solar thermal power generation. Solar collector is the key component of the thermal power generation system, its efficiency influenced by the selective absorbing coating directly. Majority of the coatings only have medium-low temperature application due to the influence of materials, structures and processing methods, which can not meet the service requirements of the thermal power generation system. Therefore, it is emergent to seek a new material system to develop a new high-temperature selective absorbing coating which could be stable under high temperature and have high solar absorptance and cheaply prepared, in order to promote the development of core technology of solar thermal power generation system.Composite SiC-cordierite ceramics coating materials were prepared by using silicon carbide, Kaolin, α-Al2O3and talc as starting materials. The apparent porosity, water absorption, bulk density, bending strength, thermal shock resistance, thermal expansion coefficient, thermal conductivity, absorptance and emittance of SiC-cordierite composite ceramics were measured. The mechanism of thermal shock resistance was also studied by XRD and SEM. When the theoretical cordierite was1Owt%, the sample A1fired at1300℃has the best properties, the water absorption, porosity, and bulk density were10.20%,21.81%,2.14g·cm-3, respectively. The bending strength was up to102.36MPa, the thermal expansion coefficient was5.4x10-6℃-1, the thermal conductivity was7.50W·(m·K)-1(RT), the absorptance was0.788. The crystalline phases determined by the X-ray diffraction analysis (XRD) were a-SiC, β-cordierite and a-cristobalite. The crystalline phase species did not change after30thermal cycles(RT-1000℃, air cooling), but the diffraction patterns of samples were dispersed, and the strength of sample Al increased37.87%. The bending strength increased after thermal shock, the reasons for this phenomenon may be listed as follows. Firstly, Suzhou Kaolin and talc contain many impurities such as TiO2. Repeated thermal cycling could produce aluminosilicate glass phases, the glassy phases could be crystallised by thermal treatment after refiring or by the incorporation of nucleating agents, therefore, more cordierite were formed. Secondly, the initial strengthening during thermal cycling may be due to the effect of tempering. Thirdly, when the thermal cycling excesses a certain value, as the thermal cycles increasing, the cause of strength enhancement was more close to the effect of quenching strengthen. Lastly, The thermal stress cracks stability factor Rst of the samples increased due to the microcracks, decrease of the diameter of pores, generation of cordierite and mullite crystal in pores, so that the sample has good thermal shock resistance.In order to further reduce the thermal emissivity of this material, on the basis of series formula A, BN and H3BO3were added to modify the substrate material of coating. Series formula B and C were designed, the SiC of B series samples were325meshes, the SiC of C series samples were240meshes. The performance and microstructure of the samples were characterized by XRD, SEM and other means. The results indicated that sample B3(325meshes SiC90wt%,talc3.97wt%, Suzhou Kaolin4.82wt%,α-Al2O31.21wt%, H3BO31wt%) fired at1300℃had the bending strength of50.17MPa and, sample C3(240meshes SiC90wt%,talc3.97wt%, Suzhou Kaolin4.82wt%,a-Al2O31.21wt%, α-Al2O31.21wt%, H3BO3l wt%) fired at1300℃had the bending strength of74.31MPa and the thermal conductivity of18.27W·(m·K)-1(RT). After30thermal cycles (RT-1000℃, air cooled), the strength loss of sample C4(boric acid content of2wt%) were negative, the strength didn’t reduce but increase, the increase rate was7.42%. Combined with XRD and SEM analysis, for sample C4, the characteristic peak of silicon carbide did not change, and the characteristic peak of cristobalite decreased, and created new phase (kyanite), those factors improved the strength of the samples effectively. The thermal conductivity of samples significantly improved because more and more heat conduction pathways and networks appeared after adding BN and H3BO3. With the the particle size decrease of SiC, namely, surface area of SiC particles per unit volume increased, thermal conductivity of composite materials would decreased.The research of the performance of oxidation resistance showed that passive oxidation rate of silicon carbide base material not completely satisfied the parabolic rule while oxidated at1000℃, the order of the oxidation resistance: C3>C1>C4>B3>C2>B1>B4>B2. From the phase composition analysis of the sample surfaces after oxidation, it can be concluded that the primary phases of the surface ofthe B series samples were SiC, cristobalite and sodium borate after oxidized at1000℃for50h. The primary phases of the surface of C series samples were SiC and cristobalite after oxidized at1000℃for50h. Microstructure analysis showed that, the liquid film pack large crystal particles surface come from the oxidation of the glass phase, which not only bridged the material surface defects, reduced the fine cracks of surface, but also protected the material internal from the continuous oxidation, moreover, the self-healing properties of the surface of materials were enhanced obviously, as well as improved the oxidation resistance of the sample.When the content of SiC was90wt%, the absorptivity of the samples was highest (0.788), when the content of SiC was70wt%, the emissivity of the samples was lowest (0.639). With the addition of SiC reducing, the absorptivity of samples decreased gradually. With the addition of BN and boric acid, the boric acid salt glass phase formed on the surface of the samples plays the role of antireflective coating film, reduced the emissivity of the samples, and improved the absorptivity of the samples. With the increase of BN and boric acid, the solar absorption of samples gradually raised, thermal emissivity gradually reduced, when the addition of BN and boric acid exceeded a certain content, the absorptivity of the samples no longer increase, emissivity increased inversely. When the addition of H3BO3was1wt%, the comprehensive properties of samples were best, the solar absorption of the samples reached up to0.910simultaneousy; When the addition of H3BO3was1wt%, the thermal emissivity of the samples low to0.5767. The solar absorption of samples were increased after burnished, the thermal emissivity of the sample was reduced. The roughness of the surface of the samples has a major influence on the solar absorption and thermal emissivity. |
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