Study On SiC Thermal Storage Ceramic Materials For Solar Thermal Power Generation

The third generation of solar thermal energy storage ceramics,with excellent thermal shock resistance and and high thermal conductivity,could improve operating efficiency of thermal energy storage system.SiC ceramics suit the above property requirements.In order to further reduce the preparation cost and improve the thermal properties,this study designed in-situ synthesized SiC whiskers（SiC_w）bonded SiC ceramics via reaction bonded silicon carbide.Firstly,the effects of Si addition and firing temperature on the physical properties,microstructure and phase composition of SiC_w/SiC composite ceramics were analyzed.The synthesis mechanism of SiC_w was also discussed.Secondly,Fe₂O₃ was introduced as SiC_w catalyst,the effect of Fe₂O₃ on the properties of the sample and the catalytic mechanism were invidtigated.Thirdly,the sintering aid Y2O3 was added to achieve the densification modification of SiCw/SiC composite ceramics.The capability of PCM bonded SiC based ceramics were researched finally.The relationships between phase composition,microstructure,and properties of samples was analyzed by XRD,SEM,FE-SEM and other modern characterization methods.The main results are as follows:（1）Formulations of the series A were designed with graded SiC,AlN and Si powders as the starting materials.The SiC_w/SiC composite ceramics were successfully prepared by the method of burying powders.The results showed that the partial liquid phase formed by Si powders at high temperature promoted mass transfer,and the bulk density and bending strength of the sample increased with the Si addition.The microstructure indicated that a large amount of in-situ grown SiC_w were densely packed in the grain voids and the surface.However,the structure was loose due to abundant gas were formed in SiC_w synthesization process.The SiC_w morphology mainly controlled by the Si addition and firing temperature.SiC_w with a long straight of sample A4（10.39 wt%Si）fired at 1550℃ When the firing temperature was greater than 1550℃,the beaded SiC_w were synthesized.SiC_w nucleation was controlled by the reaction between intermediates SiO and CO,and then SiC crystral nucleus grown by the vapor-solid mechanism.As soon as the reaction came into the adjustment stage,excess SiO deposited on the surface of SiC_w formed a protective layer.The comprehensive performance of A4 samples fired at 1550℃ is optimal.The Wa,Pa,D andσ_b are:14.69%,31.54%,2.15 g/cm³,29.68 MPa,respectively.The thermal conductivity of the sample was improved through in-situ self-assembled network SiC_w forming a high thermal conductivity channel.Thermal conductivity at room temperature and specific heat capacity are 6.74 W/（m·K）and 0.81 J/（g·K）,respectively,and heat storage density is 994.50 kJ/kg（ΔT=975℃）.SiC_w still stably existed in the sample after 30 times thermal shock（from room temperature to 1000℃）,and therefore the strength increased by 34.91%to reach 40.04 MPa.After oxidation at 1300℃ for 100 h,the SiO₂ protective film was broken due to the volume effect caused by intragranular oxidation,and the specific mass change is 61.43 mg/cm².（2）Fe2O3 was introduced as a SiCw growth catalyst on the basis of formula A4designed the series CF.Results revealed that the introduction of Fe₂O₃ successfully improved the morphology of SiC_w and properties of the SiC_w/SiC composite ceramics.Optimum sintering temperature was reduced from 1550℃ to 1500℃ after Fe₂O₃introduced.Fe₂O₃ particles firstly reduced to iron in the reducing atmosphere,then formed Fe-Si alloy droplets.The SiC_w morphology has been optimized when the growth mechanism transformed into the vapor-liquid-solid mechanism.As the firing temperature and Fe2O3 addition increased,SiCw morphology became worse,and the whiskers exhibited bifurcation growth.The sample CF3（1.5 wt%Fe2O3）fired at1500℃ has the best comprehensive performance,and its Wa,Pa,D,andσ_b are 12.87%,28.32%,2.20 g/cm³,and 42.38 MPa,respectively.The whiskers with small defects and large aspect ratio significantly reduced the matrix-whisker barrier and the pore barrier.Consequently,the thermal conductivity of the sample increased by 103%to 13.75W/（m·K）,and heat storage density is 1101.75 kJ/kg.Fe₂O₃ improved the density of sample CF4（2.0 wt%Fe₂O₃）,thus the thermal conductivity reaches 18.74 W/（m·K）.However,the high-temperature performance was unfavorable compared with sample CF3.The iron and Fe²⁺were gradually oxidized into Fe³⁺and transfered to the surface of sample,which resulted in forming an oxidation film in the process of high heat treatment.After the thermal shock test,the bending strength of sample CF3 increased by 22.08%to 51.83 MPa.The specific mass change is 43.18 mg/cm² after oxidation at1300℃ for 100 h.（3）In order to promote the densification of SiC_w/SiC composite ceramics to achieve the modification,Y₂O₃ was added as a sintering aid on the basis of CF3formulation designed the series DS.The results indicated that the addition of Y₂O₃reduced the synthesis temperature ofβ-Sialon and significantly improved the physical properties of the sample.XRD analysis illustrated that YAG phase was synthesized at1550℃.The microstructure revealed that the morphology of SiC_w was differentiated into fiber and short column,and the amount of whisker synthesis was significantly reduced.Meaning that Y2O3 consumed a large amount of Si source in the formation of liquid phase and suppressed SiC_w grew in one dimension.Considering its high-temperature performance and service environment,the sample DS3（9 wt%Y2O3）fired at 1525℃ was selected as the optimal sample.The Wa,Pa,andσ_b are 11.19%,26.51%,2.37 g/cm³,and 75.98 MPa,respectively.The increase of density reduced pore-phonon scattering,thermal conductivity at room temperature increase to 19.45 W/（m·K）,and the specific heat capacity is 0.96 J/（g·K）,heat storage density is 1121.25 kJ/kg.Theβ-Sialon grains were combined with glass phase.Different heat treatment systems formed different precipitation phases,which accelerated the oxidative decomposition ofβ-Sialon and have poor high temperature stability.Hence,thermal shock resistance and oxidation resistance of the samples were inconsistent.YAG was precipitated after 30thermal shock resistance cycles,accordingly,residual strength of the sample decreased by 8.70%to 69.30 MPa.Y₂Si₂O₇ was precipitated after oxidation treatment,the sample DS2 had the best oxidation resistance and the specific mass change is 50.19 mg/cm².（4）Al-Si-Cu alloy combined with SiC-based ceramics heat storage ceramics showed preferable compatibility,and the heat storage density is 1498.25 kJ/kg.The Al-Si-Cu alloy mainly penetrated under the action of capillary force along the pores in the matrix.The NaCl-bonded SiC-based heat storage ceramics also presented good chemical compatibility.However,the operation temperature was greater than the melting point of NaCl（800℃）,which caused the molten salt leaked along the pores and the sealing cover.