Solvothermal Approach Synthesis Of Silicon Nitride And Other High-temperature Endurant Nanomaterials

This dissertation is dedicated to explore the low-temperature solvothermal synthesis of high-temperature resistant materials such as silicon nitride and tungsten carbide,and the formation mechanism of the final product were also investigated.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℃.These methods often require high temperature and the as-obtained products are usually 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 is still a puzzle.Up to now,the starting temperature in the solvothermal synthesis of silicon nitride nanomaterials has been decreased to 400-600℃by University of Science and Technology of China,and Shandong University researchers have reduced the temperature to 100℃by using the same method but the pressure of the reaction system was relatively high.By introducing the metal reducing agents we have successfully preparedα,βphase silicon nitride nanowires and nanorods in relatively low temperature and the relevant formation mechanism is also discussed.Compared with other literatures,not only reaturction temperature but also system pressure was reduced via solvothermal method after adding metals.Moreover,noncrystalline silicon nitride hollow spheres were also obtained at 250℃.The industrial production of tungsten carbide normally uses powder metallurgy technology in which the carbonization treatment temperature reaches 1500-2000℃.Other methods such as thermal decomposition method of metal complexes,self-propagating high-temperature reaction,sol-gel method,CVD and solid phase exchange reaction are also used to prepare tungsten carbide powders in which the temperatures are generally above 1000℃.So it has a practical significance to explore a simple low-temperature synthetic route of tungsten carbide. We prepared tungsten carbide cubes at 600℃and small amount of tungsten carbide nanorods in the presence of surfactant(hexadecyl-trimethyl-ammonium bromide)at 200℃.By using the similar method carbon nanobelts were also synthesized.The main research contents are as follows:1.Low-temperature metal reducing solvent methods were.developed for the preparation of high-temperature endurant nanomaterials such asα-,βsilicon nitride nanowires or nanorods and tangusten carbide cubes.The roles of metals and the organic precursors act as carbon sources were investigated in detail.Metal reductives played a crucial role in the process of preparingα-,βsilicon nitride nanowires or nanorods at low temperature.They could not only reduce the reaction temperature,but also decrease the system pressure in the autoclave. Silicon was yielded after the reaction between metals and silicon tetrachloride, which solicitation the reaction to continue in low temperature(200-300℃).This reaction temperature was close to the critical temperature(234℃)of silicon tetrachloride and the pressure in an autoclave was low.Compared with the roles of different metals in this experiment,the results showed that the adding of magnesium powder,aluminium powder and calcium conduce to the formation ofβ-Si₃N₄;On the contrary,iron powder,nickel powder and sodium were benefit to the formation ofα-Si₃N₄.Among these metals,magnesium powder,aluminium powder,calcium and sodium were both in favor of strengthening the sample crystaUinity in low temperature.In addition,the reason about the formation of silicon nitride with different phases and obtaining silicon nitride nanomaterials with difference shapes after adding different metals were discussed.The crystal structure of silicon nitride crystal structure illustrated that Si-Si and Si-N bonds existed inα-Si₃N₄ and only Si-N bond was inβ-Si₃N₄.The introduction of magnesium powder urged to produce magnesium silicide and went against the formation of Si-Si,soβ-Si₃N₄ was the final main products.Howerever,the metallicity and reducibility of iron powder were lower than magnesium powder, therefore,α-Si₃N₄ was the main product.The experimental results proved that the metal that was added not only could reduce the reaction temperature but also increase the product crystallinity.The lowest reaction temperature that required for crystalline Si3N4 formation was 200℃when adding magnesium powder,but 250℃while ferrous powder was introduced.The detailed high-resolution transmission electron microscopy and field scanning electron miscropy were used to observate the structure and morphology.The results showed thatβ-Si₃N₄ nanowires with diameters of 30-125nm,length to several micrometers and silicon nitride nanorods with diameters ranged from 250nm to 800nm(changed the volume of solvents)when adding magnesium powder;α-Si₃N₄ nanowires with diameters of 50-165nm,length to several micrometers when ferrous powder was introduced.Compared with other literatures that described the preparation of silicon nitride nanomaterials under similar conditions,the reaction conditions used here was milder and it is more likely for the industrial production of silicon nitride in autoclaves.In another experiment of preparing tungsten carbide,carbon with high activity and strong reducing property were obtained via the reaction between magnesium powder and ethanol,which reacted with tungsten oxide to yield the tungten carbide cubic particles.The magnesium powder that was added and the chose of tungsten sources were the crucial factors to decide the reaction temperature.Reaction temperature could not be set lower than 600℃if tungsten oxide was selected as tungsten sources,however,if sodiumtungstatedihydrate as tungsten sourses,the lowest reaction temperature could be 200℃.We found that the reaction temperature,system pressure,carbon sources and reaction time were important influencing factors for the formation of tungsten carbide.The higher the system pressure,the easier the solicitation of reaction.The proper reaction time was about 15 hours.Only alcohols were chosen as the carbon source can contribute to the formation of the tungsten carbide.The raw materials used in this synthesis route is cheap and with low toxicity,and the experiment equipments used here is very simple.They were all beneficial for industrial production of tungsten carbide in autoclaves.This research received the recongization of Thapar Uniyersity Patiala,Republic of INDIA.2.Low-temperature solvothermal method was developed to prepare amorphous silicon nitride nanomaterials.To conform the effect of the experimental parameters on the morphology,a series of comparative experiments was carried out,such as running the reaction temperature,reaction time,quantity of the reagent,kinds of nitrogen sources,and amorphous silicon nitride hollow spheres were successfully synthesized.The experimental results proved that the morphology of the final products was silicon nitride nanorods and its crystallinity was raised along with the increasing reaction temperature(from 250℃to 600℃); The morphology of final products was irregular silicon nitride nanocrystals when the quantity of sodium azide was increased.The systematic pressure,reaction time and kinds of reagent sources were found have little influence on the morphology of final products,but affect the reaction results.The detailed high-resolution transmission electron microscopy observation displayed that hollow spheres were amorphous.The enerny dispersive X-ray detector(EDX)confirmed the products were consisted of silicon and nitrogen element.The ratio of silicon atom and nitrogen atom was1:1.09(close to the theoretical value),which proved that the components of amorphous hollow spheres were silicon nitride.3.Solvothermal method was developped.At a relatively low temperature(200℃), curvous carbon nanobelts and a few tungsten carbide nanorods were produced at the presence of surfactants.Curvious carbon nanobelts were obtained via controlling reaction temperature,systematic pressure and the kinds and mount of surfactants.The possible formation mechanism of curvious carbon nanobelts and the roles of surfactants during its growth was discussed according to the experimental results and analysis data.Some routes such as hard-template and surfactant-assisted were adopted to control the shape in preparing tungsten carbide nanomaterials with irregular morphology.The experimental results proved that there was few tungsten carbide nanorods and nanocrystals with smaller size at the presence of surfactants.Detailed experiments on these aspects still need to be further carried out.