碳化硅超细粉末的制备及热压烧结机理和强化增韧的研究

【作者】 肖汉宁；

【导师】 杜海清；

【作者基本信息】 湖南大学 ， 无机非金属材料， 1991， 博士

【摘要】 本文首次采用工业硅溶胶和炭黑为原料，用溶胶—凝胶法成功地合成了β-SiC超细粉末。其SiC含量在98％以上，颗粒近于球形，粒径分布范围窄，一般在0.1～0.2μm左右，基本不团聚或呈弱团聚状态，所得粉末不需要任何物理加工或化学提纯。在原料中引入硼酸或硼粉后，还获得了不需要任何外加烧结助剂就能致密烧结的β-SiC／B复合粉末，达到了先进水平。 本文从理论上对SiC的合成机理进行了热力学和动力学探讨。指出了反应自由焓理论在本文研究的Si-O-C系统中应用的局限性。用气相平衡分压理论成功地解释了系统中SiC合成反应的热力学过程。通过对合成过程中SiC转化率的测定和颗粒形貌的观察，探讨了反应的动力学过程，说明了系统中的晶核形成和晶体长大机制。论文提出了SiC的合成反应机理，主要影响因素和适宜的合成工艺条件。 在β-SiC／B复合超细粉末的合成研究中发现，加入硼源能降低系统的反应活化能，使SiC的起始合成温度下降，并且有促进β-SiC晶粒长大和抑制α-SiC相生成的作用。用XRD和IR分析研究了粉末中B的存在形态。结果表明B主要是替代Si进入SiC晶格中，形成SiC-B₄C固溶体。 本文系统地研究了Al、B、C系统烧结助剂和热压温度对α-SiC、β-SiC和β-SiC／B三种不同粉末的热压烧结机理和烧结体的结构和性能的影响。发现了不同SiC粉末表现出不同的烧结机理和微观结构。其中β-SiC／B粉末的热压烧结机理和微观结构在国内尚属首次研究，并获得了弯曲强度和断裂韧性分别达590MPa和5.8MPa.m^1/2的烧结体，达到或超过了目前文献报导的水平。 为了提高SiC陶瓷的强度和韧性，本文在。α-SiC和β-SiC粉末中分别添加不同体积分数的TiC、WC和Al₂O₃作为第二相物质，进行了粒子强化增韧的理论分析和实验研究，获得了弯曲强度和断裂韧性分别达到700MPa和8.6MPa.m（1/2）的SiC-23.6vol％WC的热压烧结材料。用由粒子相和基体相的固有特性决定的应力场理论，对本文和过去由不同学者提出的陶瓷材料中弥散和粒子强化增韧的四种模式及微裂纹增韧的机理进行了本质上的说明，揭示了它们的内在联系与区别。 本文最后对SiC-Al₂O₃陶瓷的抗氧化性进行了初步探讨，指出了加入Al₂O₃能显著提高SiC的抗氧化能力。更多还原

【Abstract】 Ultrafine β-SiC powders have sucessfully synthesized from industrial silica sol and carbonblack by sol-gel method.The powders contained SiC up to 98%, showed spheric particles and narrow size distribution,had a mean size about 0.1⁰.2μm and were not or soft agglomerate. When boronic acid or boron powder was introduced into silica sol, a β-SiC/B composite powder was obtained by the same method.This powder is sinterable without additional additives.The thermodynamic and kinetic investigations about the mechanism of SiC synthesis have carried out. Results showed that free energy enthalpy （ΔG_T^°） was limited to explain the synthetic process of SiC powders in the system of Si-O-C, as the real temperature of SiC yielded was lower than that calculated from （ΔG_T^°）. Author has explained the thermodynamic process in the view of vapour equilibu-lium pressure in the systeym.Kinetic process was discussed according to the determination of SiC yield and observation of TEM micrographs of powders synthesized at differert temperatures. Mechanism was suggested about the synthesis of SiC powders. The main factors affecting the properties of SiC powders and idea technique have been reported.When boron resource was introduced into the synthetic system,the starting temperature of SiC yielded decreased resulting from the decrease of the active energy in the system. It has been found that boron developed the growth of β-SiC particles and limited the formation of α-SiC phase.The state of B existed in β-SiC powders has inverstigated with the help of X-ray diffraction and IR spectra analysis.Results showed that B entered into the lattice of SiC by means of exchanging with Si. In other words, a SiC-B₄C solid solution was formed in β-SiC/B powder.Al, B and C were used as sintering adds for hot-pressing of SiC powders in the present paper. Sintering mechanism, properties and microstructures of HP-SiC bodies prepared from different SiC powders have been widely researched by variying additives and temperatures.更多还原