Study Of MgAl₂O₄-Si₃N₄ Composite Ceramics Used For Heat Receiver In Solar Thermal Power Generation

The only way to solve the problems of global energy crisis and environmental pollution is to develop and utilize the renewable energy,for example the solar energy.Heat absorbing material is a core component of tower solar thermal power generation,whose performance would affect the working efficiency of the whole system directly.However,usual absorbing materials have a series of disadvantages,such as single types,low solar absorptance,poor thermal shock resistance and oxidation resistance,which restrict the operating temperature and thermal efficiency of the system seriously.Consequently,MgAl₂O₄-Si₃N₄ composite ceramics which exhibit excellent properties and can meet the requirement of solar thermal power generation were prepared as heat absorbing material via in-situ synthesized and pressureless sintering technique in this study.Relationships between the compositions,preparation process,microstructure and performance of the composites were researched,and the synthesis mechanism was analyzed.Effect of Sm₂O₃,TiO₂ and their composite additives on the microstructure and performance of the material were studied.The ways and mechanisms to improve the physical properties,thermal shock resistance,oxidation resistance and solar absorptance of the composites were revealed also.ANSYS workbench was used to study the effect of different factors on the temperature field distribution and outlet temperature of the absorber when the air medium flows across it.The influence laws were obtained.The main results are as follows:?1?MgAl₂O₄ ceramics were synthesized using calcined bauxite,talc,?-Al₂O₃ and MgO as raw materials.Results show that sintering temperature of sample with minerals as aluminum and magnesium resources is lower.MgAl₂O₄ comes from the transformation of cordierite which is synthesized at a lower temperature.At the same time,a large number of liquid is produced,resulting in the narrow sintering temperature range?1380¹400°C?.Reaction of?-Al₂O₃ and MgO begins below1000°C.The amount of synthesized MgAl₂O₄ and crystal degree increase with rising temperature,as a result performance of the samples is improved.After sintered at1540°C,apparent porosity of sample is 0.36%and the bending strength is 117.80MPa.The synthesis temperature range of MgAl₂O₄ is wide?1200¹560°C?,and the sample has high refractoriness which is more suitable to be used as the combination phase for Si₃N₄.?2?MgAl₂O₄-Si₃N₄ composites were prepared based on the?1?,which had better performance than Si₃N₄ ceramic prepared under the same condition.In-situ synthesized MgAl₂O₄ inhibits the grain growth of Si₃N₄,ensuring dense microstructure of the sample.The best MgAl₂O₄ content is about 30 wt%.After sintered at 1620°C,bending strength is 198.85 MPa,which increases after 30 thermal shock cycles?1100°C^RT,wind cooling?with a growth rate of 24.48%.After 100 h oxidation at 1300°C,weight gain rate of the sample is 2.87 mg·cm^-2 and the oxidation rate constant is 3.40×10^-3mg²·cm^-4·h^-1.Solar absorptance of the sample in the wavelength range of 0.3².5?m is 89.50%.Oxidation kinetics of MgAl₂O₄-Si₃N₄ceramics transform from passivation oxidation into activation oxidation with increasing MgAl₂O₄ content.Too much MgAl₂O₄?exceeds 30 wt%?is harmful to the oxidation resistance.?3?Bulk density,strength and thermal shock resistance of MgAl₂O₄-Si₃N₄composites were improved via adding Sm₂O₃.Sm₂O₃ can contribute to the synthesis of MgAl₂O₄,crystal transformation and diameter ratio of Si₃N₄,resulting in the denser microstructure and higher strength.The best Sm₂O₃ content is 1 wt%.After sintered at 1620°C,apparent porosity of the sample is as low as 0.50%,bending strength is 339.42 MPa and solar absorptance is 90.50%After 30 thermal shock cycles the bending strength experiences an improvement of 2.60%,which reaches to348.23 MPa.After 100 h oxidation the weight gain rate is 2.68 mg·cm^-2,and oxidation rate constant is 1.75×10^-2 mg²·cm^-4·h^-1.The excellent thermal shock resistance benefits from the liquid phase formed during the process and the mechanism of micro crack toughening.?4?Oxidation resistance and solar absorptance of MgAl₂O₄-Si₃N₄ composites were improved via adding TiO₂.The best TiO₂ content is 2 wt%.After sintered at 1620°C,bulk density of the sample reaches 2.95 g·cm^-3,bending strength is 319.73 MPa,solar absorptance is 91.40%,and thermal conductivity is 13.27 W/?m·K?.After 30 thermal shock cycles the bending strength increases with an improvement of 1.46%.After 100h oxidation the weight gain rate is as low as 0.89 mg·cm^-2,oxidation rate constant is only 2.00×10^-3 mg²·cm^-4·h^-1.Amounts of Ti⁴⁺migration to surface under the same condition is less than Sm³⁺because it has lower radius,resulting in the increasing oxidation resistance.After Ti⁴⁺replace Al³⁺in Al₂O₃,large amount of cationic space would be formed,which contributes to the improvement of solar absorptance.Although Sm₂O₃ and TiO₂ composite additives can reduce the sintering temperature of MgAl₂O₄-Si₃N₄ composites and improve the solar absorptance,they are harmful to the oxidation resistance of the ceramics.?5?MgAl₂O₄-Si₃N₄ composite honeycomb ceramic which is suitable for heat absorbing material in tower solar thermal generation is prepared with 2 wt%TiO₂ and other additives.The best water content is 23²6 wt%.After sintered at 1620°C,apparent porosity of the sample is 2.85%,compression strength is 19.37 MPa.After30 thermal shock cycles,the sample has intact morphology,increasing bulk density and compression strength.The weight gain rate after 500 h thermal cycles?1300°C^RT?is only 6.12%,and the properties keep stable.?6?ANSYS workbench software was used to simulate and calculate the temperature field distribution and outlet temperature of the honeycomb ceramic heat absorber when the air flows across it.Results show that the temperature field distribution of the honeycomb ceramic absorbers with different type is uniform.The air resistance of the hole decreases with the raising edge number under the condition of the same air flow,resulting in the decreasing outlet temperature.Influence of the air velocity on outlet temperature of the absorber decreases with the increasing length or decreasing aperture.The most cost-effective length and velocity are 300 mm and 3m/s respectively,the side length within 5 mm,and then the outlet temperature of the absorber exceeds 1200 K.The results can provide important theory evidence for choosing and confirming working parameters for heat absorbing materials in solar thermal power generation system.