Carbothermal nitridation

The carbothermal nitridation of oxides is a common method of preparation of nitrides. It is generally applicable to nitrides of high thermal stability like TiN, VN and CrN. Its main drawback is that it produces impure phases. 

Jin XH, Gao L, Sun J, Liu YQ, Gui LH (2012) Highly transparent AlON pressurelessly sintered from powder synthesized by a novel carbothermal nitridation method. J Am Ceram Soc 95 2801-2807... 

Carbothermal nitridation of silica, including rice huUs Diimide synthesis Vapor-phase synthesis Plasmachemical synthesis Laser-induced reactions Pyrolysis of organosilicon compounds... 

One of the oldest industrial methods used to synthesize sialons (see Section 11.3.4.3) involves the carbothermal nitriding of alumosihcates (e.g., kaoHnite) according to ... 

Uranium and mixed uranium—plutonium nitrides have a potential use as nuclear fuels for lead cooled fast reactors (136—139). Reactors of this type have been proposed for use ia deep-sea research vehicles (136). However, similar to the oxides, ia order for these materials to be useful as fuels, the nitrides must have an appropriate size and shape, ie, spheres. Microspheres of uranium nitrides have been fabricated by internal gelation and carbothermic reduction (140,141). Another use for uranium nitrides is as a catalyst for the cracking of NH at 550°C, which results ia high yields of H2 (142). 

As previously stated, uranium carbides are used as nuclear fuel (145). Two of the typical reactors fueled by uranium and mixed metal carbides are thermionic, which are continually being developed for space power and propulsion systems, and high temperature gas-cooled reactors (83,146,147). In order to be used as nuclear fuel, carbide microspheres are required. These microspheres have been fabricated by a carbothermic reduction of UO and elemental carbon to form UC (148,149). In addition to these uses, the carbides are also precursors for uranium nitride based fuels. 

Vapor—sohd reactions (13—17) are also commonly used ia the synthesis of specialty ceramic powders. Carbothermic reduction of oxides, ia which carbon (qv) black mixed with the appropriate reactant oxide is heated ia nitrogen or an iaert atmosphere, is a popular means of produciag commercial SiC, Si N, aluminum nitride [24304-00-3], AIN, and sialon, ie, siUcon aluminum oxynitride, powders. 

Aluminum nitride UFPs have been synthesized by thermal decomposition from many kinds of precursor such as polyminoalanef l/ ) AIH(NR)] (50), aluminum polynuclear complexes of basic aluminum chloride (BAC) or basic aluminum lactate (BAL) (51), and (hydroxo)(succinato) aluminum(lll) complex, A1(0H)(C4H404) jfLO (52). These precursors were calcined under N2 or NH, gas flow. The calcination temperatures, which depend on the individual precursor, can be lower by 600-200°C than the 1700°C in ihe conventional carbothermal reduction method. The XRD measurements at intermediate stages of the calcination process showed the phase change from an amorphous state to a trace of y-alumina with very fine grains and finally to wurtzite-type AIN (51,52). Lowering the calcination... 

Cochran GA, Conner CL, Eismann GA, Weimer AW, Carroll DF, Dunmead SD, Hwang CJ (1994) The Synthesis of a High Quality, Low Cost Silicon Nitride Powder by the Carbothermal Reduction of Silica. In Hoffmann MJ, Becher PF, Petzow G (eds) Silicon Nitride 93. Key Eng Mater 89-91. Trans Tech Publications Ltd, Switzerland, p 3... 

Ishii T, Sano A, Imai I (1987) ot-Si3N4 powder produced by nitriding silica using carbothermal reduction. In Somiya S, Mitomo M, Yoshimura M (eds) Silicon Nitride I. Elsevier, London, p 59... 

Hnatko, M., Galusek, D., Sajgalik, P., Low-cost preparation of Si3N4-SiC micro/ nano composites by in-situ carbothermal reduction of silica in silicon nitride matrix, J. Eur. Ceram. Soc., (2), 24 2004, 189-195. 

Other Metal Nitrides Many other metal nitrides are used in ceramic formulations. These include AIN, TiN, VN, and BN. These metal nitride powders are produced by carbothermal reduction of the relevant metal oxide in a nitrogen-containing atmosphere or reaction of the relevant metal with a nitrogen-containing reducing atmosphere. These metal nitrides are used as abrasives and in hi -temperature wear applications. 

In order to be used as nuclear fuel, carbide microspheres are required. These microspheres have been fabricated by a carbothermic reduction of UO3 and elemental carbon to form UC. In addition to these uses, the carbides are also precursors for uranium nitride based fuels. 

Carbothermal Reactions It is noteworthy that a variety of oxides, nitrides and elemental nanowires can be synthesized by carbothermal reactions. For example, carbon (activated carbon or carbon nanotubes) in mixture with an oxide produces oxide or suboxide vapor species, which react with other reactants (O2, N2 or NH3) to produce the desired nanowires. Thus GaN nanowires are produced by heating a mixture of Ga203 and carbon in N2 or NH3. Silicon nanowires can also be made by heating Si02 with carbon in a suitable atmosphere. 

Hence, by using the correct carbon source with the silica gel under carbothermal conditions it is possible to obtain nitride or carbide nanowires. With the carbon nanotubes the reaction follows one step (Step III shown below) to produce Si3N4 nanowires. 

Molten-Tin Process for Reactor Fuels (16). Liquid tin is being evaluated as a reaction medium for the processing of thorium- and uranium-based oxide, carbide, and metal fuels. The process is based on the carbothermic reduction of UO2 > nitriding of uranium and fission product elements, and a mechanical separation of the actinide nitrides from the molten tin. Volatile fission products can be removed during the head-end steps and by distilling off a small portion of the tin. The heavier actinide nitrides are expected to sink to the bottom of the tin bath. Lighter fission product nitrides should float to the top. Other fission products may remain in solution or form compounds with... 

Initial experiments have demonstrated the feasibility of car-bothermic reduction of UO2 and nitriding of uranium in molten tin. Nitriding of the product of carbothermic reduction of mixed U02 Pu02 and added fission product elements is one of the steps to be confirmed for this process to be deemed potentially useful. 

The formation of silicon nitride whiskers was observed in several different reactions, including vapor deposition, CVD, and growth from a melt. However, only the following techniques are considered to have commercial significance nitriding of metallic silicon or silicon-silica mixture, carbothermal reduction of silica with simultaneous nitridation, and thermal decomposition of silicon halides. 

Other synthesis methods have also been developed but may not be practiced on a large commercial scale as compared to those indicated above. Examples of syntheses that begin from liquid phase precursors include the sol-gel, hydrothermal, and Pechini methods. In the course of these reaction schemes, polycondensation or precipitation occurs, and the volatile components are removed, often by thermal methods. Other synthesis routes to ceramic materials include the nitridation of metals to form metal nitrides and the carbothermal reduction of oxides to form carbides and borides. 

V.G Gilev, IR Spectra and Structure of Si-AI-O-N Phases Prepared by Carbothermal Reduction of Kaolin in Nitriding Atmosphere, Inorg. Mater., 37, 1224-1229(2001). 

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