Fabrication Of SiO₂Composite Ceramic And The Study Of Its Related Properties

SiO2ceramic is a kind of important engineering materials, and has been used in many fields because of its superior dielectric properties and better chemical stability, such as random and missile receiver, and so on. However, the inherent poor mechanical properties limit application scopes of SiO2ceramic. Therefore, it is challenging to fabricate a SiO2ceramic not only with excellent mechanical properties but also with outstanding dielectric properties in present days.In the thesis, micro-and nano-SiO2powders were used as raw materials. The SiO2and additives powders were ball milled in alcohol, and then was sintered by hot pressing in Ar atmosphere. Four different material systems fabricated with the processing technology were that SiO2-Al2O3-BNNPs (S-A-BNNPs), SiO2-Al2O3-BNNTs (S-A-BNNTs), SiO2-BNNPs and SiO2-BNNTs, and we studied the influence of parameters, such as, the content of additives and sintering temperature, on their mechanical and dielectric properties, and also discussed the reinforcing and toughening mechanism of additives with the help of analysis and measurement technologies, for instance, XRD, FESEM, and HRTEM, and finally to analyze how the additives and sintering temperature to influence the dielectric properties of SiO2ceramic. The detailed content of the paper as follows:(1) The material system of S-A-BNNTs. In the chapter,10wt%Al2O3and5wt%BNNTs were determined as additives to fabricate the SiO2ceramic with outstanding mechanical properties when the sintering temperature is1350℃. Comparing with the monolithic SiO2, flexure strength and fracture toughness of S-A-BNNTs increase from27.3MPa to103.5MPa, and from0.58MPa·m1/2to1.39MPa·m1/2, and are379.1%and239.7%higher than those of the monolith, respectively. By means of phase and microstructure analysis, we can ascribe the huge increase to the following points:①adding the Al2O3material possessing better mechanical properties into SiO2, can improve mechanical properties of the monolith.②SiO2,Al2O3and BN can form a typical glass system, so in the grain boundary, the diffusion layers can be generated, which can increase the interface bonding strength and eventually to improve mechanical properties of SiO2.③the effect of Al2O3stopping the propagation of crack and the existence of pull out and bridging of BNNTs can make fracture toughness of SiO2improved obviously.(2) The material system of S-A-BNNPs. In the chapter, we used the same processing technology as that of S-A-BNNTs, and the fabricated composite ceramic also have the same superior mechanical properties. The flexure strength and fracture toughness of S-A-BNNPs were86.3MPa and1.23MPa·m1/2, and were316%and212%higher than those of the monolith, respectively. We consider that, in the system, the effect of Al2O3is the same as that in the S-A-BNNTs composites. As for the effect of BNNPs, we believe that the special layer structure can make the crack deflected and ended in the process of propagation leading to improvement of mechanical properties.(3) The material system of SiO2-BNNPs. In the previous investigation of S-A-BNNTs and S-A-BNNPs, owing to employ the relative higher dielectric constant of Al2O3as an additive, the excellent dielectric properties of SiO2were influenced inevitably. In the chapter, we removed the Al2O3and only added BNNTs which also have superior dielectric properties into SiO2, and fabricated the low dielectric constant and high mechanical properties of SiO2composite ceramic. When the weight content of BNNPs was5wt%, the fracture toughness of SiO2-BNNPs composite ceramic was1.22MPa·m1/2, and was217%higher than that of the monolithic SiO2, and the dielectric constant is4.1. When the weight content of BNNPs was7wt%, the flexure strength was117MPa, and increase230%comparing with SiO2.(4) The material system of SiO2-BNNTs. In the chapter, in order to fabricate the low dielectric constant and high flexure strength of SiO2composite ceramic, we employ BNNTs as additive and SiO2as matrix using methodology of hot pressing to process the SiO2-BNNTs composite ceramic. By means of phase and microstructure analysis, we measured and studied the influence of BNNTs on mechanical and dielectric properties of monolith. The experimental results demonstrated that, when the sintering temperature is1400℃, the flexure strength and fracture toughness of composite reached to the maximum value, and was120.5MPa and1.21MPa·m1/2, respectively. In addition, when the weight content of BNNTs was5wt%, the dielectric constant of SiO2-BNNTs was changed obviously with sintering temperature variation. Specially, when the sintering temperature was1300℃,1350℃and1400℃, the measuring dielectric constant was3.25,3.55and4.05, respectively, and also kept stability in the scope of testing frequency.In this paper, SiO2composite ceramics with high flexure strength and fracture toughness and low dielectric constant were fabricated by hot pressing under different sintering technology. In addition, the micro structure and improved mechanism was analysed by means of XRD, FESEM and HRTEM testing instruments.This work was supported by the National Natural Science Foundation of China (Nos.50872072and51042005), Science and Technology Development Project of Shandong Province (Nos.2011GGX10205and2009GG10003003), and Independent Innovation Foundation of Shandong University (2009TS001and2012TS213).