Carbothermal Synthesis of Silicon Nitride Xiaohan Wan A thesis in fulfilment of the requirements for the degree of Doctor of Philosophy School of Materials Science and Engineering Faculty of Science March 2013 THE UNIVERSITY OF NEW SOUTH WALES Thesis/Dissertation Sheet Surname or Family name: Wan First name: Xiaohan Other name/s: Abbreviation for degree as given in the University calendar: PhD School: Materials Science and Engineering Faculty: Science Title: Carbothermal synthesis of silicon nitride Abstract 350 words maximum Carbothermal synthesis of silicon nitride Si3N4 followed by decomposition of Si3N4 is a novel approach to production of solar-grade silicon. The aim of the project was to study reduction/nitridation of silica under different conditions and to establish mechanism of silicon nitride formation. Carbothermal reduction of quartz and amorphous silica was investigated in a fixed bed reactor at 1300-1650 °C in nitrogen at 1-11 atm pressure and in hydrogen-nitrogen mixtures at atmospheric pressure. Samples were prepared from silica-graphite mixtures in the form of pellets. Carbon monoxide evolution in the reduction process was monitored using an infrared sensor; oxygen, nitrogen and carbon contents in reduced samples were determined by LECO analyses. Phases formed in the reduction process were analysed by XRD. Silica was reduced to silicon nitride and silicon carbide; their ratio was dependent on reduction time, temperature and nitrogen pressure. Reduction products also included SiO gas which was removed from the pellet with the flowing gas. In the experiments, reduction of silica started below 1300 °C; the reduction rate increased with increasing temperature. Silicon carbide was the major reduction product at the early stage of reduction; the fraction of silicon nitride increased with increasing reaction time. Maximum silicon nitride to carbide ratio (Si3N4/SiC) in the reduction of silica in nitrogen at atmospheric pressure was observed at 1450 °C. Further increase in temperature decreased Si3N4/SiC ratio. Increase in hydrogen content to 10 vol% favoured SiO2 reduction. Further increase in H2 content led to decreased N2 partial pressure, which had a negative effect on the nitridation process. Elevated nitrogen pressure increased silicon nitride yield and stability.When nitrogen pressure was 11 atm, maximum Si3N4/SiC ratio was observed at 1550-1600 °C. Increasing nitrogen pressure increased reduction and nitridation rates and suppressed SiO loss under otherwise the same conditions. Synthesis of silicon nitride proceeded through silicon carbide. In the beginning of the reduction/nitridation process, silica was predominantly reduced to silicon carbide which was converted to silicon nitride. This mechanism of silicon nitride synthesis was supported by kinetic modeling of reduction/nitridation process. Feasibility of production of solar silicon was demonstrated in a study of decomposition of Si3N4 The decomposition rate increased with increased temperature. Declaration relating to disposition of project thesis/dissertation I hereby grant to the University of New South Wales or its agents the right to archive and to make available my thesis or dissertation in whole or in part in the University libraries in all forms of media, now or here after known, subject to the provisions of the Copyright Act 1968. I retain all property rights, such as patent rights. I also retain the right to use in future works (such as articles or books) all or part of this thesis or dissertation. I also authorise University Microfilms to use the 350 word abstract of my thesis in Dissertation Abstracts International (this is applicable to doctoral theses only). …………………………………………………… ……………………………………..…… ……….……………………... Signature Witness Date The University recognises that there may be exceptional circumstances requiring restrictions on copying or conditions on use. Requests for restriction for a period of up to 2 years must be made in writing. Requests for a longer period of restriction may be considered in exceptional circumstances and require the approval of the Dean of Graduate Research. FOR OFFICE USE ONLY Date of completion of requirements for Award: THIS SHEET IS TO BE GLUED TO THE INSIDE FRONT COVER OF THE THESIS I ACKNOWLEDEMENTS I would like to thank Professor Oleg Ostrovski and Dr Guangqing Zhang for their quality supervision. I am appreciative of their constant encouragement and generous support. The comprehensive advices on various aspects of the project and the critical reading of the proof are acknowledged. The scholarship provided by them is one of the important supports to the completion of this project. It is grateful that Associate Professor Hal Aral supports on sample characterizations. His valuable advices on the project conductions and paper preparation are appreciative. Technical support from Mr John W. Sharp is one of the critical factors for the completion of the project. His creative and efficient work outcomes overcome lots of problems in the labs. Without his help it is impossible to accomplish the complex experimental works. The assistance of Dr C. H. Kong on the electron microscope analysis and paper revision, Dr Yu Wang on the X-ray diffraction analysis and the school staff for technical and administration supports are acknowledged. Thanks are due to Dr Xing Xing and Mr Le Yu for providing plenty of supports on my research life as friends and colleagues. Finally, my special acknowledgements are for all of my family members for their disinterested and constant support and encouragement. II PUBLICATIONS ORIGINATED FROM THIS PROJECT (1) Xiaohan Wan, Guangqing Zhang, Hal Aral and Oleg Ostrovski (2011): “Reduction Mechanism of Carbothermal Synthesis of Silicon Nitride” . High Temperature Processing Symposium (2) Xiaohan Wan, Guangqing Zhang, O. Ostrovski and Hal Aral (2013): “Carbothermal Reduction of Silica in Nitrogen and Nitrogen-Hydrogen Mixture”. INFACON XIII Congress III ABSTRACT Carbothermal synthesis of silicon nitride (Si3N4) followed by its decomposition is a novel approach to production of solar-grade silicon. The aim of this project was to study reduction/nitridation of silica (SiO2) under different conditions and to establish mechanism of Si3N4 formation. Carbothermal reduction of crystallised and amorphous SiO2 was investigated in a fixed bed reactor at 1300-1650 °C in nitrogen at 1-11 atm pressure and in hydrogen-nitrogen mixtures at atmospheric pressure. Samples were prepared from silica-graphite mixtures in the form of pellets. Carbon monoxide evolution in the reduction process was monitored using an infrared sensor; oxygen, nitrogen and carbon contents in reduced samples were determined by LECO analyses. Phases formed in the reduction process were analysed by XRD. Silica was reduced to Si3N4 and silicon carbide (SiC); their ratio was dependent on reduction time, temperature and nitrogen pressure. Reduction products also included SiO gas which was removed from the pellet with the flowing gas. In the experiments, reduction of SiO2 started below 1300 °C; the reduction rate increased with increasing temperature. SiC was the major reduction product at the early stage of reduction; the fraction of Si3N4 increased with increasing reaction time. Maximum Si3N4/SiC ratio in the reduction of SiO2 at atmospheric pressure was observed at 1450 °C. Further increasing temperature decreased Si3N4/SiC ratio. Addition of hydrogen into nitrogen promoted silica conversion. Addition of 5 vol% of H2 significantly increased the rate of reduction of silica. The effect of hydrogen on the kinetics of silica reduction was attributed to formation of methane (CH4) by reacting with carbon, which transferred carbon from graphite particles to the silica particles. The maximum Si3N4 to SiC ratio was obtained with addition of 10 vol% H2 at 1450 °C after 12-hour reduction. Higher H2 addition led to lower N2 partial pressure with a negative effect on Si3N4 formation. IV Increasing nitrogen pressure increased reduction and nitridation rates, and the stability of nitride at higher temperatures. The maximum Si3N4/SiC ratio was observed at 1550- 1600 °C under a N2 pressure of 11 atm. 71.4% of silicon was converted to Si3N4; the rest was mostly SiC with trace residual SiO2 after 1 hour reaction. Increasing carbon to silica molar ration increased the rate of reduction of silica. With stoichiometric amount of carbon, the reduction was very slow. When C/SiO2 was equal to 4.5, the reduction completed in about 300-minute. Addition of silicon nitride to the graphite-silica mixture promoted reduction of silica and formation of silicon nitride. This indicates that nucleation of silicon nitride is one of the controlling factors in the reaction process. The reduction rate of silica was slightly increased by increasing gas flow rate to a peak, and decreased with further increase in gas flow rare. It was due to the CO, played the key role on SiO2 reduction, loss by higher gas flow rate. There was no obvious effect on SiO2 reduction rate by the type of silica used. Synthesis of Si3N4 proceeded through SiC. In the beginning of the reduction/nitridation process, SiO2 was predominantly reduced to SiC which was further converted to Si3N4. TEM analysis of a silicon carbide sample subjected to partial nitridation showed that silicon diffused out of silicon carbide lattice onto the surface and reacted with nitrogen to form silicon nitride. This mechanism of Si3N4 synthesis was supported by kinetic modelling of reduction/nitridation process. The reaction model