Synthesis of Si2N2O Powder from Desert Sand by Nitriding Combustion Method

Near single-phase silicon oxynitride, Si 2N2O, powder has been successfully prepared by the combustion synthesis method from a mixture of pulverized desert sa nd and reclaimed silicon under pressurized nitrogen gas. In this highly exothermic combustion reac tion, elemental silicon is first converted to β-silicon nitride phase which once formed reacts with the pulverized s an to produce silicon oxynitride. The X-ray diffractogram of the produced powder shows s trong peaks for silicon oxynitride of an orthorhombic structure with very low or negli gib e peaks corresponding to β-silicon nitride. A series of heating experiments for cold-isostat ic-pressed compacts was carried out at high temperatures to investigate the thermal stability of silicon oxynitride in air and under vacuum. Above 1400 °C, silicon oxynitride is slightly oxidized in air and a cristobali te phase is formed. However, silicon oxynitride seems not to be so stable under vac uum onditions and experienced decomposition at temperatures above 1250 °C to silicon and both αand β-silicon nitride phases. The synthesized silicon oxynitride powder is suitable to used in the form of dense or porous components for refractories and filters applications. Introduction During the preliminary stage of experiments performed to investigat e the potential materials that can be obtained from a natural desert sand, an unexpected question came to mind: why is silicon oxynitride not such a popular ceramic as the well-known ceramic, silicon nitride ? An intensive review of the published data on the silicon oxynitride ce ramic material within the past 35 years shows that silicon oxynitride has superior chemical stability and good oxidation resistance in many environments at high temperatures in compari son to the more well-known ceramics, silicon nitride and silicon carbide [1]. Silicon oxynitride has been also reported to have interesting and promising potential in refractories and ceramics applications either in dense or porous form [24]. But, there is still a clear lack of accurat e data for some of its physical properties because of the difficulty in obtaining the silicon oxynitride in pure form [5, 6]. Among the considerable number of synthesizing and sintering methods that has been disclosed [7], to date it seems difficult to state a suitable, economic and reproducible proce ss to synthesize the pure material. This factor reflects the limited popularity of sili con oxynitride ceramic in spite of its excellent properties, which may reveal the necessity to find a different proc essing route. The use of the self-propagating high temperature synthesis, SHS, or combustion method in synthesizing advanced ceramics has proved to be successful since t he process was discovered by Russian scientists in late 1967 [8]. Its great advantages are hig h exothermicity and the selfand fast-propagation. So, it can be used to produce highly pure ceramics rapi dly nd inexpensively in simple apparatus. In this work, we applied the SHS method to prepare silicon oxynitride by selecting cheap starting materials, desert sand and reclaimed silicon which is a byproduct of zinc smelting industry. The thermal stability of the resultant material in both air and u er vacuum conditions is also investigated. Key Engineering Materials Online: 2003-04-15 ISSN: 1662-9795, Vol. 237, pp 111-116 doi:10.4028/www.scientific.net/KEM.237.111 © 2003 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific.net. (Semanticscholar.org-13/03/20,21:55:25) Experimental Procedure Natural desert sand obtained from Egypt and reclaimed silicon (8 μm in size) from Toho Zinc Co. Ltd., Japan, were used in this study. The purity of each was 99.7% and 94%, respectively. Sand was pulverized at first to –40 μm size by using a vibration mill. The reaction mixture was pre par d from 53.68 wt% pulverized sand, 36.32 wt% reclaimed silicon and 10 wt% pre-synthes ized silicon oxynitride powder as a diluent. The nitriding combustion experiment was performed in a combustion furnace u der 3 MPa nitrogen gas. 50 g of the reaction mixture was placed in a porous car bon crucible. The reaction was initiated at the bottom from burning 5g thermite mixture placed bene ath the reaction burden by passing 50-60A current through a carbon ribbon (heater) as schematical ly illustrated in Fig. 1. The temperature profile and the average combustion temperature, T max, of the reaction were recorded by using W-5%Re / W-26%Re thermocouple placed in the centre of the re action burden. After combustion, the ignited product was easily removed from the ignition age nt nd ground prior to a phase identification by X-ray powder diffraction analysis with CuKα radiation. A series of heating experiments was performed to investigate the thermal stability of the silicon oxynitride both in air and under vacuum conditions in the temperat ur s range of 1000-1600 C. In this series, 2.5 g of the ground material was cold-isostatic ally-pressed, placed in an alumina crucible lined with carbon sheet and then heated at the desired tempe rature in a muffle furnace with a controlled heating program. The heating rate in these experiments was const nt at 20 C/min. Figure 1: Schematic diagram of SHS apparatus (combustion furnace): (1) burden, (2) ignition agent, (3) porous container, (4) thermocouple, (5) carbon ribbon, (6) electrodes, (7) high-pressure chamber, (8) nitrogen gas atmosphere, and (9) glass window for observation. Results and Discussion The general synthesis reaction of the silicon oxynitride from a mixture of sand a nd silicon under pressurized nitrogen can be described as follows: 3Si(s) + 2N2(g) + SiO2(s) → 2Si2N2O(s) ∆Hr ° = -984.46 kJ/2moles (1) This reaction can be considered to be a combination of two reactions: 3Si(s) + 2N2(g) → β-Si3N4(s) ∆Hr ° = -787.80 kJ/mole (2) Pressure Gauge