Carbides layered compounds

Silicon, like carbon, is relatively inactive at ordinary temperatures. But, when heated, it reacts vigorously with the halogens (fluorine, chlorine, bromine, cmd iodine) to form halides and with certain metals to form silicides. It is unaffected by all acids except hydrofluoric. At red heat, silicon is attacked by water vapor or by oxygen, forming a surface layer of silicon dioxide. When silicon and carbon are combined at electric furnace temperatures of 2,000 to 2,600 °C (3,600 to 4700 °F), they form silicon carbide (Carborundum = SiC), which is an Importeint abrasive. When reacted with hydrogen, silicon forms a series of hydrides, the silanes. Silicon also forms a series of organic silicon compounds called silicones, when reacted with various organic compounds. 

Primary glide occurs on the (111) planes. Shear of a carbon layer over a metal layer (or vice versa), when the core of a dislocation moves, severely disturbs the symmetry, thereby locally dissociating the compound. Therefore, the barrier to dislocation motion is the heat of formation, AHf (Gilman, 1970). The shear work is the applied shear stress, x times the molecular (bond) volume, V or xV. Thus, the shear stress is proportional to AHf/V, and the hardness number is expected to be proportional to the shear stress. Figure 10.2 shows that this is indeed the case for the six prototype carbides. 

The major use of sodium cyanamide is in the production of sodium cyanide, a compound that is used extensively in preparing solutions for the electroplating of metals. Another use for NaCN is in extraction processes employed to separate gold and silver from ores as a result of their forming complexes with CN . Sodium cyanide, an extremely toxic compound, is also used in the process known as case-hardening of steel. In this process, the object to be hardened is heated and allowed to react with the cyanide to form a layer of metal carbide on the surface. 

Most chemical properties of technetium are similar to those of rhenium. The metal exhibits several oxidation states, the most stable being the hep-tavalent, Tc +. The metal forms two oxides the black dioxide Tc02 and the heptoxide TC2O7. At ambient temperature in the presence of moisture, a thin layer of dioxide, Tc02, covers the metal surface. The metal burns in fluorine to form two fluorides, the penta- and hexafluorides, TcFs and TcFe. Binary compounds also are obtained with other nonmetaUic elements. It combines with sulfur and carbon at high temperatures forming technetium disulfide and carbide, TcS2 and TcC, respectively. 

It should be noted that in the limit of very large heats of chemisorption one may form surface compounds, oxides or carbides, for example. In this circumstance ordering of the new surface phase may require the relocation of the substrate atoms as well as the adsorbate atoms. Such chemisorption-induced reconstructions have been observed for several systems and its presence makes the conditions necessary for ordering in the surface layer very difficult to analyze indeed. Some of these systems will be discussed later in this paper. 

It should be pointed out that in many cases it seems uncertain which substance it is that constitutes the veritable catalyst. Particularly for metallic and oxidic catalysts each separate case must be investigated for formation of a monomolecular layer of compounds or adsorbates, for instance, sulfides, carbides, hydrides, etc., which constitutes the real catalyst after an individual activation period of the metal or the oxide. In such cases the electron exchange between the film and the substrate will, of course, be the decisive factor 13). 

Theoretically, since these are layered homologous compounds, a numer-ous/infinite number of compounds are possible in the family However, realistically, we have been able to synthesize pure phases of only the three compounds. Compounds which contained more than four layers of the B12 icosahedral and C-B-C chain layers (which is the case for RB28.5C4) always contained a mixture of other number layers also. In the limit of the boron icosahedra and C-B-C chain layers separating the metal layers reaching infinity (i.e. no rare earth layers) the compound is actually analogous to boron carbide. In the opposite limit, a compound with just one boron icosahedra layer is imaginable. And in actuality, such a MgB9N compound was independently discovered by Mironov et al. (2002). However, such a compound with rare earth atoms has not yet been synthesized. 

As noted in Section 9, the structures of the R-B-C(N) compounds (Figure 21) are homologous to that of boron carbide which exhibits typical p-type characteristics. Boron carbide is the limit where the number of boron icosahedra and C-B-C chain layers separating the metal layers reaches infinity (i.e. no rare earth layers). It has been speculated that the 2 dimensional metal layers of these rare earth R-B-C(N) compounds are playing a role for the unusual n-type behavior, but the mechanism is not yet clear. 

Pyrolysis at 200°C of Os3(CO)j2 in a scaled, evacuated tube afforded a mixture of at least seven different carbonyl clusters which could be separated by thin-layer chromatography. In addition to some unreacted Os3(CO)i2, the new compounds, Os.i(CO)i3, Os5(CO)i6, Os6(CO)i8, Os8(CO)23, and Os8(CO)2iC, were identified by mass spectroscopy (58) the last compound was originally formulated as Oss(CO)i5C4 (61). Further pyrolysis of Os6(CO)i8 at 255°C gives the pentanuclear carbide derivative, Os5(CO)I5C, in 40% yield (59). 

Note that the silicide layer may grow not only between silicon and a transition metal, but also between a silicon-containing phase and a transition metal or an intermetallic compound. Such layers are known to occur in the process of brazing the transition metals by their own melts with Si3N4-base ceramics239 and also during the interaction of transition metals with silicon carbide.238 240 245... 

Boron forms a binary carbide, often written B4C but actually non-stoichiometry, and compounds with most metals. The stoichiometries and structures of these solids mostly defy simple interpretation. Many types of chains, layers and polyhedra of boron atoms are found. Simple examples are CaB6 and UB12, containing linked octahedra and icosahedra, respectively. 

Normally, dislocation-based plastic deformation is irreversible, that is, it is not possible to return the material to its original microstructural state. Remarkably, fully reversible dislocation-based compressive deformation was recently observed at room temperature in the layered ternary carbide Ti3SiC2 (Barsoum and El-Raghy, 1996). This compound has a hexagonal stmcture with a large cja ratio and it is believed that the dominant deformation mechanism involves dislocation movement in the basal plane. 

Interpolation or intercalation (see Intercalation Chemistry) is said to occur when additional species are placed into a host stmcture to change either composition or properties. At one extreme, intercalation can refer to the insertion of gnest molecnles into cage stmctures such as that of the zeolites (see Zeolites), or between the layers of laminated compounds snch as the clays (see Silicon Inorganic Chemistry). At the other extreme, the insertion of small atoms snch as C or N into metal phases to form interstitial alloys (see Alloys Carbides Transition Metal Solid-state Chemistry Nitrides Transition Metal Solid-state Chemistry), is inclnded in the category. A large variety of stmctures can be found in snch materials, and... 

Ternary phases with structures different from those of the phases of the binary boundary systems are more the exception than the rule. Such phases have been reported in the systems Nb-Mo-N, Ta-Mo-N, Nb-Ta-N, Zr-V-N, Nb-Cr-N, and Ta-Cr-N. Information about ternary transition metal-nitrogen systems is often available for specific temperatmes only. This is even more the case for quaternary nitride systems, which play a role in the production of carbonitride cermets where quaternary compounds of the types (Ti,Mo)(C,N) and (Ti,W)(C,N) are of interest (see Carbides Transition Metal Solid-state Chemistry), as well as in layer technology where titanium nitride-based coatings of the type Ti(C,B,N) are prepared by magnetron sputtering. Layers consisting of ternary compounds of the type (Ti,Al)N and (Ti,V)N also have favorable properties with respect to abrasion resistance. 

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