Solvothermal Approach Synthesis Of Silicon Carbide And Other Nanomaterials

Silicon carbide is an important wide band gap semiconducting material,and has attracted extensively attention due to its excellent properties,including high breakdown electric field strength,high saturated drift velocity of electrons,wide band gap,high thermal conductivity,high mechanical strength,high chemical stability,and low induced activity;therefore,SiC-based devices could be used at higher temperature and in harsh conditions as a functional ceramic or as a high temperature semiconductor.SiC with different nanostructures including nanowires,nanotubes, nanorods,nanobelts,solid nanospheres,nanocages,etc.have been synthesized,and many indications show that their electrical and optical properties exhibit strong dependences on shape-structure and size.Moreover 3C-SiC nanowires,because of their high strength,low density,high stiffness and high temperature stability,are of considerable interest as reinforcements for composites.Heretofore,a lot of efforts have been devoted to the synthesis of silicon carbide nanocrystals.Various techniques have been used for the synthesis of silicon carbide nanocrystals,especially one-dimensional(1D) SiC nanomaterials.Such as carbon nanotube confined growth, chemical vapor deposition,and arc discharge et al.These methods have been successfully used to synthesize pure SiC nanorods or nanowires.However,there are several important questions still exists,for example,high temperature thermal methods in the temperature of generally above 1000℃are successfully used to synthesize SiC nanocrystals but with a relative higher industrial cost.Therefore, low-temperature synthetic route for the high yield synthesis of SiC nanocrystals is an effective approach to reduce the industrial cost;most works were focused on the 3C-SiC nanowires,there were few reports about the 2D and 3D SiC nanostructures; Compared with 3C-SiC,there are few reports about the synthesis ofα-SiC nanocrystals,such as 2H-SiC,4H-SiC.On the basis of comprehensive and thorough literature investigations,in this dissertation,3C-SiC hollow nanospheres with a high yield(～80%) were prepared by using SICl₄,CBr₃H,and Na-K alloy at 130℃for 15 h and a subsequent HClO₄ treatment process at 180℃.When Na-K alloy was substituted by Na(or K) and in the mean time the temperature was set at 240℃while keeping other conditions unchanged,a large quantity of randomly distributed and highly crystalline SiC nanowires also can be produced.The possible formation mechanisms of the products with distinct dimensions were briefly discussed.Another novel aluminum catalyzed reduction route was developed for the large-scale synthesis of SiC/Al₂O₃ nano-composite powder.When aluminum was substituted by Magnesium,4H-SiC prisms and 2H-SiC triangle flakes were synthesized by using diatomite(TEOS) and Silicone oil,respectively.Magnesium silicon nitride(MgSiN₂) has an orthorhombic structure,and it has been widely studied owning to its attractive properties.MgSiN₂ powders have been synthesized by the reaction of SICl₄,N₂H₄-HCl,and Mg in an autoclave at 450℃for 5 h.The effects of different synthesis conditions on the final formation of MgSiN₂ powder,such as the different ratios of the precursors,reaction temperature,and nitrogen sources were also investigated.The main research contents are listed as follows:(1) A novel metal reduction route was developed for the large-scale synthesis of 3C-SiC hollow nano-spheres at relative low temperature by using SiCl₄,CBr₃H, and Na-K alloy at 130℃for 15 h in 12ml autoclave and a subsequent HClO₄ treatment process at 180℃.TEM Analysis results of a number of products produced by the same process indicate that the proportion of the nanospheres (most of them have diameters ranging from 80 to 120 nm) in the product is approximately 80%.The HRTEM image of a randomly selected nanosphere shows that the nanosphere has a rough surface,and its corresponding HRTEM image reveals that the nanostructure has an average interplanar spacing of 0.25 nm,which corresponds to(111) spacing of 3C-SiC.When metallic Na(or K) was used instead of Na-K alloy as the reductant.It is found that the final morphology of the as-obtained crystalline SiC product at 240℃was distinct from those that using the Na-K alloy.TEM image shows that the sample is composed of a large quantity of randomly distributed nanowires with uniform diameters.Analysis of a number of the nanowires shows that most of them have diameters ranging from 30 to 50 nm and lengths of several tens of micrometers. To substantially study the formation process of the hollow nanospheres under the present synthesis route,we have systematically surveyed their growth process by changing the reaction time.It is found that in the first 7 h(at 130℃),nearly all the CBr₃H reacts and converts into black solids.Meanwhile,most SiCl₄ and part of the Na-K alloy remain unreacted(which were removed carefully before the dilute acid treatment process).The TEM image shows that the product is composed of aggregated and uniform particles with diameters of 50-150 nm. Some dark spots dispersed in the light particles with size of several nanometers also can be seen,and these particles are sheathed by a thin shell on the surface. The result of the XRD pattern confirms that these particles are mainly composed of amorphous graphite.The XRD pattern of another sample(obtained after heating at 130℃for 10 h process) reveals a little Si coexisted with SiC.Their TEM image shows a solid sphere with a residual core.So it is reasonable to speculate that the SiC hollow nanospheres hold the shape and diameter of carbon nanoparticles,which implies that the final shape of spherical SiC partilces maintain the original carbon sphere skeleton and reflects an in situ shape memory.The formation process of the nanowires was also investigated.Similar results were obtained when the autoclave was heated at 240℃for 7h except the sample is composed of amorphous morphology.And a large quantity of nanoparticles were formated when the reaction time was 9h.However,most of them vanished when the reaction time was 12h,and nanowires became the main products of the sample.It is considered that a similar self-catalyzed vapor-liquid-solid(VLS) growth process was largely responsible for the formation of SiC nanowires.Studies show that the low reaction temperature and appropriate metal reductants are the key factors for the formation of SiC hollow nanospheres or nanowires.(2) SiC-Al₂O₃ nano-composite powders have been prepared by the reaction of Al, CH₂Cl₂ and diatomite in an autoclave at 700℃.According to X-ray powder diffraction,the powders are composed of SiC and Al₂O₃.Electron microscopy investigations have revealed that the products are usually complexed of nanoparticles of 50-300 nm and nanowires of 20-40 nm wild and up to tens of micrometers long.HRTEM.EDS and ED shows that the nanoparticles and nanowires are Al₂O₃ and SiC,respectively.(3) A magnesium reduction route was developed for the synthesis of 4H- and 2H-SiC.Crystalline SiC hexagonal prisms were obtained through an in situ magnesium reduction route by using Mg,diatomite,and CH₂Cl₂ as reactants in an autoclave at 600℃.According to XRD and HRTEM analysis,the as-obtained hexagonal prisms were 4H-SiC.Scanning electron microscopy(SEM) images show that the hexagonal prisms with smooth surface,they have typical diameters in the range of 300-800 nm and thickness of 200-500 nm.And a magnesium reduction organic precursor(TEOS or Silicone oil) was also developed to synthesize 4H-SiC flakes and 2H-SiC triangle flakes,respectively.The effects of different synthesis conditions on the final formation 4H- and 2H-SiC,such as the different precursors,reaction temperature,and time were also investigated.(4) Low-temperature solvothermal method was developed to prepare ternary nitride. Recently,Magnesium silicon nitride(MgSiN₂) has been widely studied owing to its attractive properties such as high hardness,reasonable strength,fracture toughness,and high electrical resistance at room temperature.In this study, MgSiN₂ powders have been synthesized by the reaction of SiCl₄,N₂H₄-HCl,and Mg in an autoclave at 450℃for 5 h.The yield of the product is calculated to be about 90%according to the amount of SiCl₄.X-ray powder diffraction patterns indicated that the products are orthorhombic MgSiN₂(cell parameters:a=5.252 A,b=6.426 A,and c=4.979 A) together with little amount of Si.The results of scanning electron microscopy and transmission electron microscopy(TEM) observations indicate that the particles have rough surfaces,and have diameters in the range of 0.5-3 um.The high-resolution TEM image shows clearly resolved fringes separated by 0.248 nm,which corresponds to the(002) d-spacing of the orthorhombic MgSiN₂.The effects of different synthesis conditions on the final formation of MgSiN₂ powder,such as the different ratios of the precursors,reaction temperature,and nitrogen sources were also investigated.