Diffusion refractory metal compounds

Although the transition metal ehaleogenides usually are quite refractory, direct reaction is feasible in many cases. In some cases (e.g., W and Mo), the oxide is volatile, making the surface at least accessible to reaction. In addition, the metals often have high rates of diffusion in the compounds, thus reducing the surface passivation effect of compound formation. This is probably because diffusion jumps are more probable in the presence of elements that can change their charge states. This property can be helpful in conversion of an oxide to a sulfide via H2S or CS2. 

Mi) Hot Pressing of Boron-Rich Carbides (2273-2373 K, 20- 0 MPa, vacuum or inert atmosphere, 30-60 min). The hot pressing of these compounds is a delicate operation. Extensive carbon diffusion takes place from the mold toward the sample, and the sintering of pure boron produces only boron carbide . The diffusion diagram in the B-C system is established . Refractory metallic foils (Ta, Mo, W, etc.) can be used for protection " . Diffusion of the Mo barrier is noted (1.3-1.9 wt %) . 

Two fuel forms have the potential to satisfy the GFR requirements (1) a ceramic plate-type fuel element and (2) a ceramic pin-type fuel element. The reference material for the structure is reinforced ceramic comprised of a sUicon carbide composite matrix ceramic. The fuel compound is made of pellets of mixed uranium-plutonium-minor actinide carbide. A leak-tight barrier made of a refractory metal or of Si-based multilayer ceramics is added to prevent fission products from diffusing through the clad [1]. 

Sintering can be brought about by a variety of reactions. Of these, material transport by viscous flow is important in glasses but less so in metals or ceramics. Evaporation and condensation is important in rather volatile compounds such halides and some oxides. Diffusion - bulk, grain boundary and surface diffusion - is important for refractory materials and, for these materials, the presence of traces... 

The table "Parameters of the Diffusion of Nonmetals into Metals with the formation of Refractory Compounds" mainly gives data on the parameters of reaction diffusion, i.e., diffusion occurring with the formation of the corresponding compounds in a number of cases, however, data have been utilized on the ordinary heterodiffusion of metalloids into transition metals. The majority of the data have been obtained by the classical method, and the minority by the use of radioactive isotopes. 

PARAMETERS OF THE DIFFUSION OF NONMETALS INTO METALS AND REACTIONS WITH THE FORMATION OF REFRACTORY COMPOUNDS... 

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