【作者】 刘孟；

【导师】 吴建锋；

【作者基本信息】 武汉理工大学 ， 材料学， 2013， 博士

【摘要】 在塔式太阳能热发电用空气吸热器中,吸热体材料是吸热器核心部件。由于塔式吸热器聚光能流密度不均匀性和不稳定性形成的吸热体局部热斑造成材料热应力破坏、空气流动稳定性差以及耐久性不高等问题,因而需迫切的开发具有抗高温氧化性好、抗热震性好、具有三维或者二维的连通结构、高比表面以及高热导率的新型吸热体材料。基于太阳能热发电吸热器的特点及对吸热体材料的要求,本文设计了用于塔式太阳能热发电系统的吸热体材料配方组成。实验中以Si3N4和SiC为基材,以红柱石、α-Al2O3、Y2O3以及Cr203等为添加剂,合成耐高温结合相,制备了可用于塔式太阳能热发电吸热体陶瓷材料。采用现代测试技术研究了用于塔式太阳能热发电系统的陶瓷吸热体材料料的组成、制备工艺对微观结构与性能的影响规律,探讨了提高陶瓷吸热体材料料抗氧化性及抗热震性的机理、途径。研制了适合塔式太阳能热发电用吸热体泡沫陶瓷。设计了高效容积式吸热器结构,以空气作为传热介质,利用本文所制备泡沫陶瓷为吸热体,采用Ansys Workbench对吸热体及吸热器温度场、压强场以及速度矢量场进行了模拟分析。本文的主要研究成果如下：(1)本文首先研究了以Si3N4和SiC为主要原料,红柱石和α-Al2O3为添加剂,采用无压烧成工艺,制备了莫来石结合Si3N4-SiC陶瓷。实验设计了A系列配方组成,测试和分析了烧成样品抗热震性、抗氧化性、相组成以及微观结构等性能。结果显示,在空气中进行烧成时,Si3N4和SiC复相陶瓷含有大量的石英玻璃相,导致烧成样品抗热震性能不佳。(2)研究了提高复相陶瓷材料抗热震性能途径。实验在A系列实验基础上,通过外加煅烧铝矾土提供铝源,使得高温反应过程中,更多游离Si02能够与Al203反应生成莫来石,从而降低样品中玻璃相量,提高样品抗热震性能。实验设计了B系列配方组成,分别研究了空气中无压烧成和埋粉烧成(用青岛石墨微粉将样品覆盖)两种烧成方式对烧成样品性能的影响规律。对埋烧样品研究表明,最佳样品为煅烧铝矾土添加量为15%的B3,其最佳烧成温度为1480℃,抗折强度为53.20MPa,1100℃至室温30次热震后样品抗折强度为87.86MPa,强度增加率为65.15%。相组成分析表明,样品热震前晶相为碳化硅、氮化硅以及莫来石,样品热震后晶相为碳化硅、氮化硅、方石英以及莫来石。热震后产生少量石英相,填充于气孔中,覆盖于Si3N4和SiC晶粒表面,阻止它们的进步氧化,从而提高了样品抗热震性。采用埋粉烧成方法,可以减少烧成样品中石英玻璃相的含量,可显著提高样品抗热震性。(3)研究了提高吸热体复相陶瓷材料致密度的方法。在B3配方基础上,通过添加各种添加剂,设计了F系列配方组成。研究结果显示,在B3配方中添加Y2O3,并采用埋烧工艺,可以极大提高烧成样品致密度,其致密度从2.06g.cm-3提升至2.57g.cm-3。最佳样品为外加9%Y2O3的F3,其最佳烧成温度为1500℃,抗折强度为100.02MPa。相组成分析表明,样品晶相为碳化硅、氮化硅、O’-塞隆及莫来石。样品断面SEM图显示样品结构紧密,样品晶界结合相以O’-塞隆为主,这种结合方式有助于烧成样品致密度的提高。(4)研究了提高吸热体陶瓷抗氧化性能方法。在F1和F3配方基础上,设计了G系列配方组成,研制结果表明,Cr2O3和Y2O3复合添加有利于提高复相陶瓷材料抗氧化性能。实验最佳配方为G1,其最佳烧成温度为1500℃,抗折强度为157.04MPa。在1300℃氧化100h后其氧化速率常数为1.7389mg2·cm-4·h-1,抗氧化性能优于改性前A、B、及F系列样品。样品的相组成以碳化硅、氮化硅、莫来石以及O’-塞隆为主。复合添加Cr2O3和Y2O3可以在样品表面形成一层保护膜,使得O在样品表面的扩散速率降低,从而提高烧成样品抗氧化性能。(5)研究了提高复相陶瓷材料抗热震性能的途径。实验中,通过在样品中引入ZrO2,利用ZrO2随温度变化时产生的微裂纹来增韧复相陶瓷材料,达到提高复合材料抗热震性能的目的。设计了H系列配方组成。研制结果表明,在G1配方基础上外加8%ZrO2及7.26%Y2O3时的H3配方,样品抗热震性能显著提高。经1480℃烧成的H3样品,抗折强度为100.26MPa,1100℃至室温经30次热震后,样品抗折强度不减反增,增加率为10.34%。样品的相组成均为碳化硅、氮化硅、莫来石以及O’-塞隆,同时还存在少量石英相。显微结构及EPMA结果表明添加ZrO2、Cr2O3以及Y2O3对样品形核作用明显,三种氧化物复合添加有利于形成花簇状和网络状结构固溶体,这种结构有助于样品抗热震性能的改善和提高。(6)制备和研究了适合塔式太阳能热发电用吸热体泡沫陶瓷。实验分别以最佳配方G1、H3以及流变剂为原料制备料浆,以聚氨酯泡沫为前驱体,采用有机泡沫浸渍工艺,制备了泡沫陶瓷吸热体材料。研究结果显示,利用G1配方浆料所制备泡沫陶瓷性能较优。最佳烧成温度为1500℃,气孔率为93.7%,抗压强度为0.27MPa,30次热震后抗压强度为0.30MPa。烧成后泡沫陶瓷主晶相为碳化硅、氮化硅、莫来石以及O’-塞隆相。G1泡沫陶瓷宏观结构及显微结构研究表明,泡沫陶瓷气孔均匀,孔径在1～3mm之间,孔肋骨架较粗壮,样品骨架较致密,有利于泡沫陶瓷强度提高。可望用作塔式太阳能热发电系统的陶瓷吸热体材料,解决目前吸热体材料抗高温氧化差以及抗热震性能差的不足。(7)设计了高效容积式吸热器结构,以空气作为传热介质,利用本文制备泡沫陶瓷为吸热体,采用Ansys Workbench对吸热体及吸热器温度场、压强场以及速度矢量场进行了模拟分析。设计吸热器开口角度为500,吸热器进口直径为180cm,吸热器出口直径为50cmm,吸热体总长为110cm。对不同孔隙率Si3N4-SiC复合材料吸热体温度场模拟结果显示,孔隙率越大,吸热体出口处空气温度越高。对不同孔隙率吸热体对吸热器压强分布分析显示,孔隙率越高,吸热器进出口压降越小,越有利于吸热器的强化换热,有利于吸热器换热效率的提高。同时,对孔隙率为0.95时,进口空气速度与吸热器压强分布的模拟结果显示,当进口空气速度在5-8m/s时,有利于吸热器的稳定工作。更多还原

【Abstract】 The absorber material is the core components of the receiver in solar thermal power plant (STPP). Due to the energy flux density inhomogeneity and the instability which can easy lead to local hot spot in the absorber, and result to a series of proplems such as thermal stress damage, low air movement stability and durability. Therefore, it is urgent to develop new absorber material, which should have good oxidation resistance, good thermal shock resistance, with three or two dimensional connected structure, high specific surface area and high heat conductivity. Based on the requirement of the absorber material and the characteristic of the receiver, this paper have studied the batch formula composition which would use as absorber material in STPP system.In the experiment, Si3N4and SiC were used as substrate material, andalusite,α-Al2O3, Y2O3, Cr2O3and etc. were choosed as additives to synthesise high-temperature resistance binder phase, than form the ceramic composite absorber material which meet the demand of absorber material in STPP system.Modern testing techniques were taken to study the influence regularity of composition and preparation process on the microstructure and properties.The mechanism and methods to improve the oxidation resistance and thermal shock resistance of the ceramic absorber material were also discussed.The foam ceramic used as absorber which suitable for STPP was developed. A high efficient receiver was designed to fit the prepared absorber material by using the air as heat transfer medium.What’s more, simulation analysis of the receiver and absorber material on temperature field, pressure field and velocity vector field was catched out by Ansys workbench. The main results of the research are as follows:(1) At first, the composite ceramic of mullite bonded SiC and Si3N4was conduct-ed by pressureless firing with SiC and Si3N4as the main materials, andalusite and α-Al2O3as additives. The batch formula of series A have designed, the thermal shock resistance, oxidation resistance, phase composition, microstructure and performances have been tested and analyzed. Results showed that while fired in the air, the composite ceramic of Si3N4-SiC ceramic contains a large number of quartz phase, which result the decrease of the thermal shock resistance.(2) Based on the experiments of series A, ways for improving thermal shock performance of the composite materials are researched. Through additional bauxite source of aluminum in the experiments, which enabled more free SiO2can react with Al2O3to generate mullite in high temperature reaction process, reducing glass phase in the samples and improving thermal shock resistance. Batch formula of series B is designed.The properties effects of the fired samples by pressureless firing in air and burying powder firing (using Qingdao graphite powder to cover sample in firing process) have been studied respectively. Study on the buried fired samples showed that B3is the optimal formula after firing, the addition of bauxite is15%, the optimum firing temperature is1480℃, the bending strength is53.20MPa, while the bending strength changed to87.86MP after thermal shocks from1100℃to room temperature for30times, which increased by65.15%. Phase compositions analysis showed that the phases are a-SiC, Si3N4and mullite before thermal shock, while it is α-SiC, Si3N4, quartz, and mullite after thermal shock. A small amount of quartz phase is generated after thermal shock, filling in the pores, covering on the crystal surface, thus improving the thermal shock resistance of samples. Using burying powder firing method can reduce the content of quartz phase, and improve the thermal shock resistance of fired samples.(3) On the basis of batch formula of B3, methods for improving the density of the absorber ceramic have been researched. By adding different additives in the experiments, the batch formula of series F is designed. Results showed that adding Y2O3in batch formula of B3with the burying firing process, can greatly improve the density of the fired samples, it has changed from2.06g.cm-3up to2.57g.cm-3.F3is the optimal formula, that is plusing9%of Y2O3in the B3formula to fire, and the best firing temperature is1500℃, the bending strength is100.02MPa. Phase compositions analysis showed that the main phases of the samples are α-SiC, Si3N4, O’-Sialon and mullite.The microstructure of the samples by using SEM showed a compact structure, meanwhile, massive O’-Sialon solid solution can be observed in the grain interface which is useful for improving of the density.(4) On the basis of burying firing of series F, methods for improving oxidation resistance performance of the absorber ceramic material are studied. Based on the formula F1and F3, batch formula of series G is designed, and results showed that adding Cr2O3and Y2O3is beneficial to improve the oxidation resistance of the composite ceramics. The optimum sample is formula Gl fired at1500℃, the bending strength is157.04MPa. The oxidation reaction rate constant is1.7389mg2·cm-4·h-1after oxidation for100h at1300℃, and oxidation resistance is better than the sample of batch formula series A, B and F. The main phases are a-SiC, Si3N4, mullite and O’-Sialon. The compound addition of Y2O3and Cr2O3can form a protective layer which could slow down the spread velocity of O, thus will mprove the oxidation resistance of the sample.(5) Methods to improve the thermal shock resistance performance of composite ceramics have been studied. By adding ZrO2in the samples, which can create small crack with temperature changing to toughening composites, thus improving the thermal shock resistance of composite ceramic materials. Batch formula of series H is designed in the experiment, and results showed that samples exposed good thermal shock resistance by adding8%ZrO2,7.26%Y2O3on the basis of Gl formula. The formula of H3fired at1480℃is the optimum, which have a bending strength of100.26MPa. The bending strength of samples increased after thermal shock for30times from1100℃to room temperature, the increase rate is10.34%. The phases of the fired samples are α-SiC, Si3N4, mullite and O’-Sialon; also a small amount of quartz (SiO2) existed. The microstructure and EPMA results indicated that adding Y2O3, Cr2O3and ZrO2had obvious effects on formatting nucleus, than generate flowers and reticular structure solid solution, which are useful for improving thermal shock resistance.(6) The foam ceramic used as the absorber for the STPP have been prepared and studied. The absorbers of foam ceramic have been prepared from polymeric sponge replication method by using the optimum formula of G1and H3as slurry matrix, rheological agent as solvent, and polyurethane foam as impregnation. Results showed that the foam ceramics that prepared by G1formula slurry had the best performance. The optimal firing temperature is1500℃, the porosity rate is93.7%, the compressive strength is0.27MPa, the compressive strength is0.30MPa after thermal shock for30times.The main phases of the optimum foam ceramic after firing are a-SiC, S13N4, mullite and O’-Sialon.The macrostructure and microstructure analysis of foam ceramics prepared by Gl formula showed an uniform aperture, ranging from1to3mm, and the sample frame is relatively sturdy.Microscopic image of the sample showed that the sample’s skeleton is relatively dense. It has an expectation to use the foam ceramic as absorber in STPP system, which could solve the defects of high temperature oxidation and thermal shock performance to some degree.(7) Efficient volumetric receiver structure by using air as heat transfer medium and the prepared foam ceramic as absorber have been designed. The heat absorption temperature field, pressure field and velocity vector field have been simulated and analysed by using Ansys Workbench. In the design of the paper, the opening angle of the receiver is50°, inlet diameter is180cm, the outlet diameter of the receiver is50cm, and the total length of the absorber is110cm. Simulation results on heat absorption temperature field with different porosity of Si3N4-SiC foam ceramic showed that with the increasing of the porosity,the outlet air temperature increase as well.The pressure distribution analysis with different porosity showed that the higher of the foam ceramic porosity, the smaller of pressure drop in the receiver is, which is more conducive to strengthen heat transfer in the receiver, and benefit to the improvement of the thermal efficiency. At the same time, the inlet air velocity and receiver pressure distribution have simulated by adding the porosity of0.95, and the results showed that when the inlet air velocity is5-8m/s, it is advantageous to keep the receiver working steadily.更多还原