Transition carbide systems

In the preparation of standards, diffusional techniqnes are of special importance. Using snch techniqnes diffusion layers are formed. With a special modification by use of wedge-type samples, these layers are broadened and are thus better accessible for the microprobe.The samples consist of several diffusion bands because of the various phases in the respective carbide system. Usually, however, the samples should be single phase and homogeneous in order to make chemical gross compositional analysis applicable. Only with a comparison of different techniques a real standardization is possible. Such transition metal carbide standards have been made from the phases VC, NbC, TaC and Cr3C2. ... 

Ternary systems between two different metal atoms and carbon containing mostly Fe with minor amounts of transition metals are used as structural materials alloys of the transition carbides, mainly WC, containing Co, Fe or Ni as a binder, are used as cutting tools and wear-resistant surfaces and alloys between the various actinide carbides or the transition metals are used as nuclear fuel. 

In the absence of experimental thermochemical evidence about the strength of the metal-carbon bonds in metal carbonyl carbide systems, we can turn to the binary compounds formed between transition metals and carbon for information about the last point, the strength of metal-carbon bonds to core carbon atoms. Transition metal carbides are important. They include, in substances such as tungsten carbide, WC, some of the hardest substances known, and the capacity of added carbon to toughen metals has been known since the earliest days of steel-making. Information about them is, however, patchy. They are difficult to prepare in stoichiometric compositions of established structure and thermochemistry the metals we are most interested in here (osmium, rhenium, and rhodium) are not known to form thermodynamically stable binary phases MC and the carbides of some other metals adopt very complicated structures. Enough is, however, known about the simple structures of the carbides of the early transition metals to provide some useful pointers. 

Although the electronic requirements of these transition metal systems differ from those of the carbides, the clusters are also trigonally compressed, but to a much greater extent, cf. sect. 2.1. This is consistent with the analysis of overlap populations for the two inequivalent edges of the octahedron in the trigonal field of the complete Y71,2Fe structure, and originates from Y-Y interactions due to a negligible contribution of the Fe 4p orbitals in the HOMO. 

Several of the transition metal-nitrogen systems were established only recently and there is still some lack of knowledge for systems for the investigation of which a high nitrogen partial pressure is necessary. As for the carbides systems only recent results will be presented here (see also [10]). 

Holleck, H., Ternary Carbide Systems of Actinoids wifli fire Transitions Metals of 4 to 8 Groups , J. Nucl Mater, 124,129-146 (1984) (Assessment, Crys. Structure, Phase Diagram, Phase Relations, 78)... 

The application of ly transition metal carbides as effective substitutes for the more expensive noble metals in a variety of reactions has hem demonstrated in several studies [ 1 -2]. Conventional pr aration route via high temperature (>1200K) oxide carburization using methane is, however, poorly understood. This study deals with the synthesis of supported tungsten carbide nanoparticles via the relatively low-tempoatine propane carburization of the precursor metal sulphide, hi order to optimize the carbide catalyst propertira at the molecular level, we have undertaken a detailed examination of hotii solid-state carburization conditions and gas phase kinetics so as to understand the connectivity between plmse kinetic parametera and catalytically-important intrinsic attributes of the nanoparticle catalyst system. 

Perspectives for fabrication of improved oxygen electrodes at a low cost have been offered by non-noble, transition metal catalysts, although their intrinsic catalytic activity and stability are lower in comparison with those of Pt and Pt-alloys. The vast majority of these materials comprise (1) macrocyclic metal transition complexes of the N4-type having Fe or Co as the central metal ion, i.e., porphyrins, phthalocyanines, and tetraazaannulenes [6-8] (2) transition metal carbides, nitrides, and oxides (e.g., FeCjc, TaOjcNy, MnOx) and (3) transition metal chalcogenide cluster compounds based on Chevrel phases, and Ru-based cluster/amorphous systems that contain chalcogen elements, mostly selenium. 

Line compounds. These are phases where sublattice occupation is restricted by particular combinations of atomic size, electronegativity, etc., and there is a well-defined stoichiometry with respect to the components. Many examples occur in transition metal borides and silicides, III-V compounds and a number of carbides. Although such phases are considered to be stoichiometric in the relevant binary systems, they can have partial or complete solubility of other components with preferential substitution for one of the binary elements. This can be demonstrated for the case of a compound such as the orthorhombic Cr2B-type boride which exists in a number or refractory metal-boride phase diagrams. Mixing then occurs by substitution on the metal sublattice. 

Transition metal carbide catalysts have also been explored as methane partial oxidation catalysts [110] promising results were obtained over M02C systems and enhancements were reported with the addition of transition metal promoters. 

Here again certain trends were observed, and the most influential factor was the crystal structure which the superconducting material adopted. The most fruitful system was the NaCl-type structure (also referred to as the B1 structure by metallurgists). Many of the important superconductors in this ceramic class are based on this common structure, or one derived from it. Other crystal structures of importance for these ceramic materials include the Pu2C3 and MoB2 (or ThSi2) prototypes. A plot of transition temperature versus the number of valence electrons for binary and ternary carbides shows a broad maximum at 5 electrons per atom, with a Tc maximum at 13 K. 

We need to develop methods to understand trends for complex reactions with many reaction steps. This should preferentially be done by developing models to understand trends, since it will be extremely difficult to perform experiments or DFT calculations for all systems of interest. Many catalysts are not metallic, and we need to develop the concepts that have allowed us to understand and develop models for trends in reactions on transition metal surfaces to other classes of surfaces oxides, carbides, nitrides, and sulfides. It would also be extremely interesting to develop the concepts that would allow us to understand the relationships between heterogeneous catalysis and homogeneous catalysis or enzyme catalysis. Finally, the theoretical methods need further development. The level of accuracy is now so that we can describe some trends in reactivity for transition metals, but a higher accuracy is needed to describe the finer details including possibly catalyst selectivity. The reliable description of some oxides and other insulators may also not be possible unless the theoretical methods to treat exchange and correlation effects are further improved. 

The transition metal carbides do have a notable drawback relative to engineering applications low ductility at room temperature. Below 1070 K, these materials fail in a brittle manner, while above this temperature they become ductile and deform plastically on multiple slip systems much like fee (face-centered-cubic) metals. This transition from brittle to ductile behavior is analogous to that of bee (body-centered-cubic) metals such as iron, and arises from the combination of the bee metals strongly temperature-dependent yield stress (oy) and relatively temperature-insensitive fracture stress.1 Brittle fracture is promoted below the ductile-to-brittle transition temperature because the stress required to fracture is lower than that required to move dislocations, oy. The opposite is true, however, above the transition temperature. 

The present study made use of diffusion couples where metals were reacted with carbon or nitrogen with the purpose of determining both phase equilibria and nonmetal diffusivities for Group 4 and 5 transition metal carbide and nitride systems. 

Group 5 transition metal-carbon systems. The diffusion bands of the phases of the transition metal carbides are generally rather narrow so that an unambiguous proof of their presence or absence is difficult to obtain. Therefore wedge-type or thin plane-sheet samples were... 

A detailed understanding of the properties of early transition metal carbides is of practical importance because they often demonstrate unique catalytic advantages over their parent metals in activity, selectivity and resistance to poisoning.1,2 We have chosen vanadium carbide films, produced on a vanadium (110) single crystal surface, as model systems to understand the fundamental aspects related to the electronic and catalytic properties of carbide materials. One of the main advantages of using the single crystal surface is that it enables one to apply powerful surface science spectroscopies to determine the electronic and catalytic modifications... 

While DFT may or may not be more accurate than MP2 for absolute shielding calculations is debatable, the strength of the DFT method in calculations of shieldings is in the ability of DFT to provide a consistent picture over a wide range of chemical systems, since calculations can be done at a very modest computational cost compared to MP2. Among the successes of the method is in ligand chemical shifts in transition metal complexes. For example, 13C, 170,31P and H chemical shifts for oxo (12,14,15), carbonyl (16-19), interstitial carbide (20), phosphine (21,22), hydride (23), and other ligands have been successfully reproduced to within tens of ppm in... 

The variation of hardness with multilayer wavelength in a range of different types of structures. These include multilayers of (a) isostructural transition metal nitrides and carbides, which show the greatest hardening (b) nonisostructural multilayer materials, where slip cannot occur by the movement of dislocations across the planes of the composition modulation, because the slip systems are different in the two materials and (c) materials where different crystal structures are stabilized at small layer thicknesses, such as AIN deposited onto TiN. 

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