Utilization Study Of Coal Series Kaolin In In-situ Synthesis Of ?-Al₂O₃-SiC_w Composite Ceramics For Solar Thermal Storage

Ceramics as the thermal storage medium for the 3^rd generation solar tower concentrated power plant,are required to possess good thermal shock resistance,good heat conduction,and high thermal capacity,while the development direction for thermal storage system in the 3^rd generation solar power has turned to sensible-latent composite ones.Coal series kaolin?CSK?,the particular kaolin resource in China,is characterized by its high purity,large reserves and low-cost,which can be used for preparing sensible thermal storage materials?TSMs?with low-cost.The phase compositions of CSK produced in China are mainly kaolinite and quartz,and it is found during research that the excess Si O₂ in CSK will cause the fluctuation in bending strength as well as exert negative effects on the heat conduction and heat capacity of TSMs.In this study,in order to utilize CSK effectively,a new reaction system of CSK-Al was designed to in-situ form?-Al₂O₃ matrix under high-temperature reducing atmosphere,whilst the excess SiO₂ in CSK reacted with carbon supplied by the buried graphite to in-situ synthesizing SiC whiskers?SiC_w?,in-situ producing self-assembly?-Al₂O₃-SiC_w composites with high heat conduction and high heat capacity.In this research,firstly,the phase transformation,microstructural evolution,and the relevant variations in the physical properties of CSK during firing were investigated for evaluating its advantages and disadvantages in the uses of preparing ceramics and obtaining the rules existed in the phase transformation and microstructural evolution during heating.Then through carbothermal reduction of CSK,?-Al₂O₃-SiC_w composite ceramics were in-situ synthesized self-assembly.Furthermore,the methods to improve the output of SiC_w and the thermal shock resistance of the composite ceramics were studied.Finally,to further improve the thermal storage capacity of thermal storage materials,a concept of using?-Al₂O₃-SiC_w composite ceramics in honeycomb form in-situ synthesized self-assembly to encapsulate the latent Al-Si alloys for preparing sensible-latent composite thermal storage materials had been put forward.For selecting the suitable honeycomb structures for?-Al₂O₃-SiC_w composite ceramics,simulation studies of flow and heat transfer in different geometric structure of honeycomb ceramic were implemented,and the ceramic honeycombs with suitable geometric structure encapsulated Al-Si alloys used for sensible-latent composite thermal storage were prepared basing on the simulation results.The relationship between the chemical composition,preparation techniques,structure and properties have been investigated by XRF,XRD,SEM,EPMA,etc.in detail,whilst the interaction of PCMs and ceramic matrix during thermal cycles of storage and retrieval was studied.The main research results are listed as follows:?1?The chemical composition,phase transformation,microstructural evolution and relevant variations in physical properties of fired CSK during firing have been studied,and the relationship between crystalline phases,structure and properties of the fired products was analyzed.Results indicated that the SiO₂ content in CSK was48.99%and the content of iron and titanium oxides was as low as 0.58%.The main crystalline phase in CSK was kaolinite and the microstructure reveals flake-shaped nature.The main crystalline phase of fired CSK was mullite,whose formation temperature was low to 1002.8°C.At 1500°C,columnar mullite particles were formed on the plane perpendicular to die-pressing direction to construct an interlocking texture,endowing the products with relatively high strength and densification.It is concluded that CSK is a kind of raw materials with high quality,which is featured of high purity,low mullitization temperature,good firing properties and high refractoriness.The feasibility about using CSK as raw materials for preparing thermal storage ceramics indicated that there were excess SiO₂ in CSK,which would reduce the thermal conductivity and thermal storage capacity of thermal storage ceramics.?2?Two different carbothermal reaction mechanisms which used carbon black and Al metal as reducing agents,respectively,were designed to self-assembly in-situ synthesized sensible?-Al₂O₃-SiC_w composite ceramic samples of series C by carbothermal reduction of CSK using pressureless graphite-buried firing method.The principle of in-situ synthesis of SiC whiskers and the relationship between phase composition,microstructure and properties had been studied.Results indicated that the reduction of CSK-Al system was found to be more superior in preparation of monolithic?-Al₂O₃-SiC_w composites under CO-rich condition comparing with the conventional CSK-carbon system.The carbothermal reactions in the former were completed at 1550°C,which was much lower than the later of 1640°C.The growth of SiC whiskers depended on the known Vapor-Solid mechanism.SiC output was as high as 14.8%and the Si C whiskers shew long and straight morphology,which revealed a high length-diameter ratio of 30.The SiC whiskers were found to distributed between?-Al₂O₃ particles,endowing the composites with relatively high thermal conductivity,good thermal shock resistance and oxidation resistance.Samples C6?CSK:67.84%,Al:32.16%?fired at 1550°C shew the optimum comprehensive properties,in which the linear firing shrinkage,water absorption,open porosity,bulk density,bending strength,room-temperature thermal conductivity and room-temperatureheatcapacity,theoreticalthermalstoragedensity?room temperature-1000°C?were 0.14%,13.23%,30.68%,2.31g/cm³,43.91MPa,8.96W/·?m·K?^-1,0.82 J·?g·K?^-1,998kJ/kg,respectively.No cracks formed in samples after thermal shock of 30 cycles?room temperature-1000°C?and the weight gain of samples after oxidation tests of 100 h was 11.54mg/cm².?3?For improving the thermal conductivity,the method to increase the output of SiC_w was employed and basing on the formula composition of C6,the formula compositions of series D and F were designed by adding Si and SiO₂,respectively.The effects of the added Si and SiO₂ on the output and morphology of SiC_w,microstructure and properties of the composites were studied.Results indicated that the effect of Si on improving the output of SiC_w was more notable than SiO₂.When the addition content of Si was?5.6%,the SiC_w output increased with the SiC addition and samples D3?CSK:67.84%,Al:9.76%,Al-20Si alloys:28%,nano-SiC in addition:3%?fired at 1640°C obtained the highest SiC content of 21.6%with an increasing ratio of 45.9%as compared with samples C6.The reaction to determine generation of the intermediate SiO gases was SiO₂?s?+Si?s??2SiO?g?.Under the graphite-buried firing condition,the densification could be promoted by adding Si and samples D3fired at 1600°C possessed the optimum properties:13.14%of water absorption,32.2%of open porosity,2.45g/cm³of bulk density,53.15MPa of bending strength,7.38×10^-6·°C^-1 of thermal expansion coefficient,9.36 W·?m·K?^-1 of room-temperature thermal conductivity,0.85 J·?g·K?^-1 of heat capacity,theoretical thermal storage density?room temperature-1000°C?of 1016 kJ/kg.Due to the larger SiC_w content,the thermal conductivity and the heat capacity had been improved by 4.4%and 3.7%,respectively.?4?Aiming at enhancing the thermal shock resistance of?-Al₂O₃-SiC_w composites by improving the density,the strength and the microstructure,the formula composition of series H had been designed by adding rare earth oxide?REO?compounds.The effect of REO compounds and their content on phase composition,structure and thermal shock resistance were studied.Results indicated that the addition of REO could improve the thermal shock resistance of the?-Al₂O₃-SiC_w composites by improving the densification,bending strength and modifying the microstructure.The addition of Sm₂O₃ could promote densification by refining?-Al₂O₃ grains and increasing the liquid content.Y₂O₃ could enhance the strength of samples by forming yttrium aluminium garnet?YAG?,not increasing the density notably.Samples H3?7%Sm₂O₃-7%Y₂O₃?fired at 1620°C possessed the densest structure,the highest strength and the optimum thermal shock resistance,and their water absorption,open porosity,bulk density and bending strength were 11.7%,29.71%,2.5g/cm³,57.5MPa.The interlocking texture formed by the flake-like corundum grains after thermal shock of 30 cycles could improve the bending strength with an increasing ration of 19.59%.?5?In order to select suitable honeycomb structure for?-Al₂O₃-Si C_w composites,the flow and heat transfer of four kinds of different honeycomb structures?square,hexagonal,triangle,round?had been studied by Ansys-workbench software simulation.Results indicated that when cross sectional areas?CSA?of different honeycomb structures were equal,the CSA of through-hole and the porosity of the hexagonal one were the largest,which resulted in the smallest pressure lost,but along with the lowest heat transfer efficiency.The square one possessed the highest specific surface area,the largest contact area of fluid and solid and the most notable heat transfer efficiency.In terms of the comprehensive properties,the hexagonal one was chosen for the preparation of honeycomb ceramics.By improving the thermal conductivity of thermal storage materials could enhance the heat transfer between fluid and solid,thus increasing the heat transfer efficiency.?6?In this study?-Al₂O₃-Si C_w composite ceramic honeycomb had been prepared self-assembly by carbothermal reduction of CSK and the Al-Si alloys were encapsulated in the through-hole?hexagonal?of honeycomb to increase the heat capacity.The related properties and the effect of encapsulating alloys for the ceramic honeycomb were characterized.The physical and chemical compatibility between Al-Si alloys and ceramic matrix was also investigated as well.Results indicated that the composite ceramic honeycombs were advantageous because of their good volume stability during firing,good thermal shock resistance and the good effect of encapsulating Al-Si alloys.The diameter-shrinkage of sample fired at 1620°C was3.06%.No cracks formed after thermal shock of 5 cycles and no alloy leakage was found after thermal cycles of 100 times?400-900°C?.The theoretical thermal storage density of the sample was larger than 1500 kJ/kg?room temperature-1000°C?.Alloys and the composite ceramics shew good physical and chemical compatibility as no cracks formed after thermal cycles of 100 times and no chemical reaction between them.