In-Situ Reactive Synthesis of Si 2 N 2 O Ceramics and Its Properties
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INTRODUCTION
SILICON oxynitride (Si2N2O) is the only compound in the SiO2-Si3N4 system.[1] It has many good properties such as good thermal shock resistance, high thermodynamic stability temperature (about 2073 K (1800 C)), excellent oxidation resistance in air up to 1873 K (1600 C), and high flexural strength up to 1673 K (1400 C) without degradation.[2–8] In addition, Si2N2O possesses a lower dielectric constant than that of Si3N4.[9] Therefore, Si2N2O is considered as a new great potential structural/functional material in place of Si3N4 in some fields. Sintering of Si2N2O is similar to that of Si3N4, as both materials have strongly covalent bonds and low diffusion coefficient, requiring high sintering temperatures. Oxide additives are usually used to form a liquid phase with a eutectic melting point low enough to permit sintering without excessive dissociation;[2,5,6] Si2N2O is synthesized by reactive sintering of equimolar mixtures of Si3N4 and SiO2. For example, when sintering aids, such as Li2O, ZrO, Al2O3, MgO, and Ln2O3 (where Ln = Yb, Er, Ce, Nb, Gd, Dy), were added, almost all Si3N4 and SiO2 reacted with each other to produce Si2N2O, and the Si2N2O ceramics possessed good mechanical properties.[2,6–9] When the Si2N2O was fabricated by a nitridizing powder mixture of Si and SiO2,[10–12] the strong exothermic reaction propagates spontaneously through the reactant mixture, converting it into product, and leads to rapid product fabrication and energy efficiency. However, there was much Si3N4 produced during the fabrication process, as well as residual Si and SiO2 due to the fact that they could not react with each other completely. SHOUJUN WU and XIANGMING LI, Associate Professors, are with the College of Water Resources and Architectural Engineering, Northwest A&F University, Yangling 712100, Shaanxi, People’s Republic of China. Contact e-mail: [email protected] Manuscript submitted August 29, 2011. Article published online August 30, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
The properties, especially thermodynamic stability and dielectric properties of Si2N2O ceramics, were badly influenced by a minor amount of the other phase, such as Si3N4, Si, SiO2, and oxide additives. Therefore, it is necessary to fabricate pure Si2N2O ceramic material without other phases in it. Recently, Si2N2O ceramic was fabricated by a combining approach of selfpropagating high-temperature synthesis with spark plasma sintering,[13] but the fabrication methods are costly and not suitable for project application. In the present work, the following steps took place. First, in order to propose an optimized sintering process, a thermodynamic calculation based on Gibbs free energy was applied to analyze all possible reactions of nitridizing powder mixtures of Si and SiO2. Second, Si2N2O ceramics were fabricated according to the proposed sintering process, and then the effect of final sintering temperatures on composition, microstructure, and mechanical properties of the produced Si2N2O ceramics were studied in detail. Moreover, the re
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