Study On Si₃N₄ Heat Absorber Ceramic Materials For Solar Thermal Power Generation

The air heat absorber in tower solar thermal power generation plays an important role in receiving solar radiation.The heat absorber materials used in high temperature air for a long time are easy to occur thermal shock damage and oxidation failure,limiting it’s work efficiency and useful life.Based on the requirements of improving the thermal shock resistance and oxidation resistance of heat absorber materials,black corundum was firstly used as starting material combined with Si₃N₄ to prepare O’-Sialon/Si3N4 composite heat absorber ceramics in the study.The effects of black corundum content and sintering temperature on the structure and properties of materials and the synthesis mechanism of O’-Sialon were investigated,and then the mechanisms of thermal shock damage and oxidation failure in high temperature were revealed.The effects of Yb₂O₃ and Gd₂O₃ on the structures and properties of O’-Sialon/Si₃N₄composite ceramics were investigated by using rare earth oxides Yb₂O₃ and Gd₂O₃ as additives.The mechanism of promoting sintering and improving the properties of samples were revealed.Co₂O₃ and MnO were used as additives to improve the solar absorption rate of O’-Sialon/Si₃N₄ composite heat absorber ceramics.O’-Sialon/Si₃N₄ composite heat absorber ceramics were prepared by pressureless sintering method using Si₃N₄ and black corundum as starting materials.The results show that adding black corundum can improve the sintering performance of Si₃N₄ceramics,and promote the crystal transformation ofα-Si3N4 and the growth of rod-likeβ-Si₃N₄ grains,which is beneficial to enhance the density and performances of O’-Sialon/Si₃N₄ composite ceramics.The O’-Sialon(Si_2-xAl_xO_1+xN_2-x)phase in the sample is generated from the solid solution reaction of Si₃N₄,SiO₂ and Al₂O₃,and solid solubility（x）is affected by the black corundum content.When the amount of black corundum above 20 wt.%,the solid solubility of Al₂O₃ in O’-Sialon phase is 0.38,namely the Si_1.62Al_0.38O_1.38N_1.62 is generated.When the sintering temperature above1580oC,the rod-likeβ-Si3N4 grains grow in a interwoven structure with an aspect ratio of 6.43±1.43.The sample（Si3N4:80 wt.%,black corundum:20 wt.%）sintered at 1600oC achieved the optimum performances which are listed as follows:water absorption of 14.06%,the apparent porosity of 29.64%,the bulk density of 2.11 g·cm^-3,and the bending strength of 71 MPa.The bending strength of sample after 30 thermal shock cycles（room temperature¹100oC,air cooled）increased to 78.24 MPa with the increasing rate of 11.65%.The weight gain of sample after oxidation at 1300oC for100 h is 27.7316 mg·cm^-2 with the oxidation rate of 6.4876 mg²·cm^-4·h^-1.Its excellent thermal shock resistance is attributed to uniform pore distribution and low thermal expansion coefficient of the sample.The oxidation behavior of sample in high temperature follows the parabolic law,and the oxidation mechanism changes from passive oxidation to active oxidation in the late stage with weight loss.The cracks and holes provide a passage for oxygen to enter the interior of sample,allowing oxidizing reaction to continue.In order to improve the density and oxidation resistance of O’-Sialon/Si₃N₄composite heat absorber ceramics,Yb₂O₃ and Gd₂O₃ were used as additives.The results show that the addition of Yb₂O₃,Gd₂O₃ or Yb₂O₃-Gd₂O₃ can significantly enhance the density,mechanical properties,thermal shock resistance and oxidation resistance of O’-Sialon/Si₃N₄ composite ceramics.The function mechanism of adding Yb₂O₃ or Gd₂O₃ is as follows:first,the generation of rare-earth silicate phase is promoted during sintering in the Si3N4-Al2O3-SiO2 system,which accelerates the dissolution-precipitation process ofα-Si3N4 and is beneficial to the crystal transformation ofα-Si₃N₄ toβ-Si₃N₄.Second,the growth ofβ-Si₃N₄ grains in the longitudinal direction is promoted,namely the aspect ratio of the rod-likeβ-Si3N4grains increases.Third,the solid solution reaction of Si3N4,Al2O3 and SiO2 to generate O’-Sialon phase is promoted in the ranges from 1560 to 1580oC.The sample（Si₃N₄:80 wt.%,black corundum:20 wt.%,Yb₂O₃:6 wt.%,Gd₂O₃:6 wt.%）sintered at 1600oC achieved the optimum performances which are listed as follows:water absorption of 8.13%,the apparent porosity of 19.57%,the bulk density of 2.41 g·cm^-3,and the bending strength of 103.10 MPa.The bending strength of sample after 30 thermal shock cycles（room temperature¹100oC,air cooled）increased to 113.45 MPa with the increasing rate of 10.04%.The weight gain of sample after oxidation at 1300oC for100 h is 4.1158 mg·cm^-2 with the oxidation rate of 0.2169 mg²·cm^-4·h^-1.The O’-Sialon/Si₃N₄ composite heat absorber ceramics after adding Yb₂O₃-Gd₂O₃ formed dense glassy protective film on the surface after oxidation at 1300oC for 100 h,which hinders the external oxygen from entering the sample and slows down the rate of oxidizing reaction.In addition,due to the stable structure in high-temperature and low thermal expansion coefficient(3.5⁴.5×10^-6/oC)of Yb₂SiO₅,the thermal stress of sample formed during the thermal shock cycle is low,which is beneficial for the sample to resist thermal shock damage.In order to improve the solar absorption rate of O’-Sialon/Si₃N₄ composite heat absorber ceramics,Co₂O₃ and MnO were used as additives.The results show that the effect of Co₂O₃ is better than MnO.Co₂O₃ has high absorption rate in the range of 0.3to 2.5μm,which can significantly improve the solar absorption rate of O’-Sialon/Si3N4ceramics.The absorption rate of sample（Si₃N₄:80 wt.%,black corundum:20 wt.%,Yb₂O₃:6 wt.%,Gd₂O₃:6 wt.%,Co₂O₃:3 wt.%）sintered at 1600°C in the range of 0.3to 2.5μm is 87.41%,besides the absorption rate in the ultraviolet region,visible region and near-infrared region are 80.48%,81.30%and 86.09%,respectively.