Iodine-assisted Synthesis Of Silicon Nitride, Aluminum Nitride At Low Temperature

This dissertation is dedicated to explore the low-temperature synthesis of high-temperature resistant materials such as silicon nitride and aluminum nitride in autoclaves. In industry, silicon nitride powders are prepared usually by directly nitriding the silicon powders which is carried out in high temperature of 1200-1400℃, and even by using the self-propagating high-temperature synthesis the ignition temperature is still as high as 1600℃. Industrial synthesis techniques for A1N powders is direct nitridation of Al at temperatures above 1150℃or the carbo-thermal reduction and nitridation of alumina at temperatures around 1700-1900℃. These methods not only require high temperature but also produce products of granular silicon nitride rather than high-purity one-dimensional silicon nitrides. Therefore, how to develop a low-temperature synthesis method of high-purity one-dimensional silicon nitride and aluminum nitride is still a puzzle.By introducing the iodine as an assistant reactant, we have successfully prepared a,β-phase silicon nitride nano-dendrites and nanorods, and aluminum nitride nanorods in relatively low temperature and the reaction mechanism is also discussed. Compared with other literatures, not only reaction temperature was reduced but also the reaction between the raw materials tends easier after adding iodine. In addition, the raw materials used in this synthesis route are cheap and with low toxicity, and the experiment equipments used here are very simple. They are all beneficial for industrial production of silicon nitride and aluminum nitride in autoclaves. The main research contents are as follows:1. The preparation method of high-temperature endurant nanomaterials such asα-silicon nitride nanodendrites coexisting with littleβ-nanorods orα,β-nanorods in antoclaves at low-temperature was developed. Si₃N₄ nanopowders with mixedα-andβphases were produced by using silicon powder, sodium azide and iodine in the temperature range of 190-300℃. X-ray powder diffraction pattern results indicated that the products obtained at 190 and 300℃were both composed ofα-Si₃N₄ (JCPDS card no. 09-0250) andβ-Si₃N₄ (JCPDS card no. 33-1160). Transmission electron microscopy photographs showed the product obtained at 190℃mainly composed of nanodendrites with rod-like branch which have the average diameters in the range of 50-150nm. However, the product obtained at 300℃are silicon nitride nanorods with the diameters in the range of 50-250nm and length up to several microns. According to the experimental analysis results and the calculated thermodynamic factor values, the possible reaction mechanism between the reactants has been discussed. Iodine was found to be crucial for the formation of the silicon nitride nanomaterials at a relative low temperature.2. Developed the preparation method of aluminum nitride nanomaterials in antoclaves at low-temperature. A1N nanomaterials were produced by using aluminum powder, sodium azide and iodine in the temperature range of 200℃. X-ray powder diffraction pattern results indicated that the product was hexagonal A1N with lattice constants a = 3.098 and c = 4.963 A, which are near the reported values (a = 3.111, c = 4.979 (A|°), JCPDS, card No. 25-1133). Transmission electron microscopy photographs showed the sample was mainly consists of nanorods with diameters ranging from 50-100 nm and lengths up to several micrometers. Selected area electron diffraction patterns showed that the samples are hexagonal A1N with single crystal nature. The thermal stability and photoluminescence properties of the sample were tested and the results have been analyzed.