Metal rich

Figure 1 shows the particulate loading of a pipe containing gas and particulates where the nonuniformity induced by a disturbance, ie, a 90° bend, is obvious (2). A profile of concentration gradients in a long, straight, horizontal pipe containing suspended soHds is shown in Figure 2. Segregation occurs as a result of particle mass. Certain impurities, eg, metal-rich particulates, however, occur near the bottom of the pipe others, eg, oily flocculates, occur near the top (3). Moreover, the distribution may be affected by Hquid-velocity disturbances and pipe roughness.

In tire transition-metal monocarbides, such as TiCi j , the metal-rich compound has a large fraction of vacairt octahedral interstitial sites and the diffusion jump for carbon atoms is tlrerefore similar to tlrat for the dilute solution of carbon in the metal. The diffusion coefficient of carbon in the monocarbide shows a relatively constairt activation energy but a decreasing value of the pre-exponential... 

Non-stoichiometric oxides near the metal-rich border ... 

FIGURE 6.35 Examples of the (a) cUsulfide-rich and (b) metal-rich proteins. (Jane... 

The various stoichiometries are not equally common, as can be seen from Fig. 6.5 the most frequently occurring are M2B, MB, MB2, MB4 and MBfi, and these five classes account for 75% of the compounds. At the other extreme RunBg is the only known example of this stoichiometry. Metal-rich borides tend to be formed by the transition elements whereas the boron-rich borides are characteristic of the more electropositive elements in Groups 1-3, the lanthanides and the actinides. Only the diborides MB2 are common to both classes. 

The structures of metal-rich borides can be systematized by the schematic arrangements shown in Fig. 6.6, which illustrates the increasing tendency of B atoms to catenate as their concentration in the boride phase increases the B atoms are often at the centres of trigonal prisms of metal atoms (Fig. 6.7) and the various stoichiometries are accommodated as follows ... 

Figure 6.6 Idealized patterns of boron catenation in metal-rich borides. Examples of the structures (a)-(f) are given in the text. Boron atoms are often surrounded by trigonal prisms of M atoms as shown in Fig. 6.7.

Figure 6.7 Idealized boron environment in metal-rich borides (see text) (a) isolated B atoms in M3B and M7B3 ...

The lanthanoids also form metal-rich carbides of stoichiometry M3C in which individual C atoms occupy at random one-third of the octahedral Cl sites in a NaCl-like structure. Several of the actinoids (e.g. Th, U, Pu) form monocarbides, MC, in which all the octahedral Cl sites in the NaCl structure are occupied and this stoichiometry is also observed for several other carbides of the early transition elements, e.g. M = Ti, Zr, Hf V, Nb, Ta Mo, W. These... 

The predominant reaction associated with the sublimation of each intermetallic studied was assumed to involve gaseous Pu and the associated noble metal rich adjacent phase as products. The equilibrium sublimation reactions are as follows ... 

Composition HfCg so to HfCg 99. Composition rarely reaches stoichiometry and HfC is normally metal rich. 

Borides, in contrast to carbides and nitrides, are characterized by an unusual structural complexity for both metal-rich and B-rich compositions. This complexity has its origin in the tendency of B atoms to form one- two-, or three-dimensional covalent arrangements and to show uncommon coordination numbers because of their large size (rg = 0.88 10 pm) and their electronic structure (deficiency in valence electrons). The structures of the transition-element borides are well established " . 

In contrast to the carbides and nitrides, there are a large variety of formulas and structure types for borides (from M4B to MB 5). Although it is possible to establish a comparison between metal-rich borides and carbides, B-rich borides have no counterpart in the carbides. 

Interstitial metal-rich borides are found among typical elemental or binary eompound types, stabilized by filling the voids of the metal host lattice with isolated B atoms. 

The formation of higher transition-metal borides depends on the competition and the statistical weight of the d and states of the metal atoms. Consequently, the acceptor metals Ni, Pd and Pt are expected to form metal-rich borides only i.e., besides the known PtBo.7 (anti-NiAs), Pt forms two borides, Pt 2B and Pt 4B, which... 

Only Gd2B5 and Nd2Bg have the SmjBj-type structure . These borides melt peritectieally to give the tetraboride plus a metal-rich liquid. 

This method is used extensively in the laboratory because it is particularly suitable for preparing borides of rare or expensive metals, e.g., the transition-metal-rich borides CrB, Cr3B4, CrB2 (except Cr3B2 and Cr4B), the diborides ScB2, TiB2 the rare-earth hexaborides, dodecaborides and MB -type borides. 

Typically, Be-containing alloys and intermetallic phases have been prepared in beryllia or alumina crucibles Mg-containing products have been synthesized in graphite, magnesia or alumina crucibles. Alloys and compounds containing Ca, Sr and Ba have been synthesized in alumina , boron nitride, zircon, molybdenum, iron , or steel crucibles. Both zircon and molybdenum are satisfactory only for alloys with low group-IIA metal content and are replaced by boron nitride and iron, respectively, for group-IIA metal-rich systems . Crucibles are sealed in silica, quartz, iron or steel vessels, usually under either vacuum or purified inert cover gas in a few cases, the samples were melted under a halide flux . 

Metal-rich compounds are readily obtained when a metallothermic reduction is included into the metathesis reaction by using an electropositive metal (10). In addition, metal-rich and nitrogen-rich compounds are obtained when a metal and a metal nitride are employed in reactions (11, 12) ... 

In summary, the new Ln5 xCaxGe4 alloys show that the formation of the metal-rich Zintl compounds RsTt4 can be extended to an electron-deficient region... 

Regardless of their possible metallic properties, metal-rich Zintl system or phases are defined here as cation-rich compounds exhibiting anionic moieties of metal or metalloid elements whose structures can be generally understood by applying the classical or modern electron counting rules for molecules. 

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