Carbothermal reduction and nitridation

Chung S-L, Chang C-W (2009) Carbothermal reduction and nitridation S5mthesis of silicon nitride by using solution combustion synthesized precursors. J Mater Sci 44 3784-3792... 

Suri J, Shaw LL, Zawrah ME (2012) Tailoring the relative SiaN4 and SiC contents in Si3N4/ SiC nanopowders through carbothermic reduction and nitridation of silica fume. Int J Appl Ceram Technol 9 291-303... 

Carbothermal reduction and nitridation SrjSijNsiEu -" SrC03, Si3N4, EU2O3, C 1773 Ambient pressure [75]... 

SiAION (silicon aluminium oxynitride) ceramics for engineering applications can be synthesised from aluminosilicates by carbothermal reduction and nitridation (CRN), in which a mixture of the aluminosilicate with carbon is fired in nitrogen at 1400 C. Composites of potassium aluminosilicate geopolymers with graphite, when fired for lOhr under these conditions, have been found to produce predominantly p-SiAlON (Figure 10) by a reaction such as ... 

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... 

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. 

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). 

In conclusion, a kind of rod-Iike P-Sialon (P-SijAl303N5) powder was prepared through a carbothermal reduction-nitridation method using mesoporous silca as the starting material, combined with a carbon and aluminum nanocasting procedure. The resultant P-Sialon powder can be expected to find its use in preparing high performance Sialon ceramics. 

F. J. Li, T. Wakihara, J. Tatami, K. Komeya, and T. Meguro, Synthesis of p-SiAION Powder by Carbothermal Reduction-Nitridation of Zeolites with Different Compositions, J. Eur. Ceram. Soc., 27, 2535 0 (2007). 

A. W. Weimer, Carbothermal reduction synthesis processes, in Carbide, Nitride and Boride Materials Synthesis and Processing, A. W. Weimer (Ed.), Chapman Hall, London, 1997, pp. 75-180. 

Silicon carbide whiskers can also be synthesized by carbothermal reduction of silicon nitride [10]. Silicon nitride decomposes >1300°C, silicon melts at 1410°C, and reacts with graphite. Whisker formation in this process is initiated >1400°C and can be completed between 1550... 

H. Wang, Y. Berta and G. S. Fishman, Microstructure of silicon carbide whiskers synthesized by carbothermal reduction of silicon nitride, J. Am. Ceram. Soc., 75,0,1080-1084 (1992). 

The nitride phosphors prepared by the CRN method always contain some amount of residual carbon, which significandy decreases the absorption and luminescence of the phosphor powder itself. Therefore, it is necessary to remove the excess carbon after the carbothermal-reduction process. The common way for the removal of carbon is to fire the powder in an oxidizing atmosphere at temperatures >600 °C. However, this process may also lead to the oxidation of the phosphor powders, which decreases the luminescence of phosphors. 

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