Carbides precursors

In general, CDC is synthesized by high-temperature extraction of metals and metalloids from carbide precursors. This can be accomplished using halogens [24,25], vacuum decomposition [26], or etching in supercritical water [27], High-temperature chlorination (Equation 3.1) and vacuum decomposition (Equation 3.2) are the most widely used methods for CDC production ... 

As first shown by Boehm in 1975 [41] on CDC synthesized from TaC and SiC, and later for most carbide precursors [33], the resulting carbons have type I isotherms in the Brunauer classification, which are indicative of microporous carbon having pore sizes less than 2nm and relatively high surface areas up to 2000m2/g [37,39,42], The pore size of CDC can be tailored by the selection of carbide precursors with different spatial distributions of carbon atoms in the initial carbide lattice, changing the nanotextural ordering in the CDC by varying the synthesis temperature, and posttreatment in a... 

R. West, X.-H. Zhang, I. P. Djurovich, andH. Stuger, "Crosslinking of Poly silanes as Silicon Carbide Precursors," in L. L. Henck andD. R. 

Other more complex polymers have been employed as silicon carbide precursors. For instance, the mixture of methylchlorodisilanes obtained as a by-product in the direct synthesis of Me2SiCl2 can be redistributed with catalysts to give a polycyclic, partially cross-Uuked polymer with the approximate composition shown in equation (44). Pyrolysis of this precursor produces silicon carbide in good yield. Partially cross-linked polymers made by condensing vinylmethyldichlorosilane and other methylchlorosilanes with sodium also are efficient precursors for silicon carbide. 

Only recently was a new caibothermal reduction process developed in which the WC is synthesized by a rapid carbothermal reduction of tungsten oxides in a vertical graphite transport reactor (RCR entrainment process) [3.49]. Rapid heating of the WO3/C mixture driven by thermal radiation allows conversion of the mixture into a carbide precursor (WCi. c) within very short reaction times (a matter of seconds). In a second step, additional carbon is added to the carbide precursor to form a mixture, which then imdergoes a second heat treatment to convert the precursor into substantially pure WC. 

The Rapid Carbothermal Reduction (RCR) Process (The Dow Chemical Company) [9.19,9.20], In principle, the patented RCR process is another type of direct carburization. The tungsten carbide powder is continuously synthesized by an extremely rapid carbothermal reduction of WO3 in a graphite transport reactor. Calculated heating rates are in the range of 100 million K per second by thermal radiation. The WO3/C mixture during this process is converted to a tungsten carbide precursor (WCi c) within seconds. 

Several silicon carbide precursors were tested on the AFOSR/DARPA study, e.g., polycarbosilanes (PCSs) with backbones... 

The copolymer (PhMeSi) (Mc2Si) is also used as a silicon carbide precursor, especially for monolithic formed objects. 

Figure 29 Reaction pathway to synthesize siiicon carbide precursors.

The potential of silicon carbide pre-ceramic compounds was recognized especially after Yajima et al. had prepared silicon carbide-based ceramic fibers [1]. The development of tailor-made silicon carbide precursor molecules has to be seen in close relation to the tremendously fast growth of the field of... 

Scheme 1 presents important synthesis routes of silicon carbide precursor molecules. 

Figure 5.4 Different morphologies of carbide-derived carbons (CDC) inherited from their metal carbide precursors.

One can also define the shape of CDC material on the nanoscale by changing the morphology of the carbide precursor (MCx) (Fig. 5.4). 

The boranes nido-B and m do-BioHi4 and carbaboranes n/do-2,3-Et2C2B4H6 and c/oro-l,2-C2BioHi2 have been used as precursors for boron and boron carbide thin films. ISEELS has been used to characterise the molecular precursws at solid surfaces a comparison has been made between the bond lengths determined for the molecular clusters by MNDO calculations and results from X-ray or electron diffraction and from extended EELS fine structure. A study of the deposition of boron carbide films on Si(lll) surfaces from the synchrotron radiation induced decomposition of nido-2,3-diethyldicarbahexaborane has been described. The dissociation of the cluster is the rate determining step in the process. As with the deposition of boron from nido-BioHi4, it is found that thoe is an activation barrier to the dissociation of the boron carbide precursor on Si(l 11) surfaces.43... 

TD Diffusion Coating. Carbides are coated on to iron and steel by dipping in a molten salt bath for 1-10 hours. The bath is mainly borax added to low cost carbide precursors. Coatings 5 to 15mm thick are deposited, and cooled in air or by quenching in oil or water to obtain optimum mechanical properties. (Toyota R D Laboratories, Japan. Engrs. Dig. 36, (7), 1975, p27). 

Applications silicon carbide precursor, wear coating and sealer coating for ceramic composites and precursor for porous composites for high-temperature filters and catalysts ... 

The advances in nitride and boron carbide precursor development have not been mirrored in the development of next generation SiC precursors even though SiC precursors are the most well studied of all the preceramics known. The extensive effects of Yajima et al [6-14] continue to serve as a scientific baseline for developing new SiC fiber precursors. 

With the screening results obtained with the polycarbosilane it had become obvious that a polymer or a series of polymers that had bodi controllable carbon levels in their ceramic chars and respectable char yields (ca. greater than 40 %) was needed. By combining portions of two previous studies (vida infra) we began to investigate the use of silazanes as silicon carbide precursors and their use as binders for SiC powders. 

Since the development of the first useful organosilicon compound, a silicon carbide precursor, by Yajima [11], there has been extensive research on the design of polymers which transform to silicon—containing ceramics upon pyrolysis. In recent years, progress has been made in the synthesis of polymeric precursors of silicon nitride. Seyferth and coworkers [12] developed a methylsilazane compound, basically an ammonolysis product of methyldichlorosilane, and reported the... 

The SSA of CDC is controlled by various parameters. This includes the type of carbide precursors, their particle size, initial pore size distribution, chlorine treatment temperature, halogen flow rate, cooldown gas, post synthesis treatment, and cleaning/purification procedures post synthesis, to name a few. Many carbides show a pseudo-bell-shaped variation in surface area as a function of the chlorine treatment temperature, where at lower temperatures, an increase in surface area is seen and at a certain maximum, a decrease in surface area is seen with further increasing tanperatures. The decrease in the surface area at elevated temperatures can be linked to the formation of graphene ribbon structures and the structural closing of internal pores. There are cases where this trend is not seen within the range of temperatures studied, " but it is likely... 

In general, addition of catalysts to the carbide precursors has been shown to significantly impact the resulting CDC structure (Figure 11.7) even at concentrations as low as 0.1 wt.%. Addition of catalysts often results in increased ordering, formation of ordered crystallites such as nanobarrels, MWCNT and OLCs, reduced SSA, and increased mesoporosity. 

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