Rare carbides

In organometallic chemistry, stable methylene-bridged bimetallic complexes and methyne-capped trimetallic clusters are common, and rare carbide complexes have occasionally been formed by reaction of coordinated CO in clusters (for instance, Hubei s nido cluster shown below) ... 

Several components are required in the practical appHcation of nuclear reactors (1 5). The first and most vital component of a nuclear reactor is the fuel, which is usually uranium slightly enriched in uranium-235 [15117-96-1] to approximately 3%, in contrast to natural uranium which has 0.72% Less commonly, reactors are fueled with plutonium produced by neutron absorption in uranium-238 [24678-82-8]. Even more rare are reactors fueled with uranium-233 [13968-55-3] produced by neutron absorption in thorium-232 (see Nuclear reactors, nuclear fuel reserves). The chemical form of the reactor fuel typically is uranium dioxide, UO2, but uranium metal and other compounds have been used, including sulfates, siUcides, nitrates, carbides, and molten salts. 

These theoretical predictions have been verified experimentally for numerous target materials (Fig. 3.53 [3.139]). Note that in Fig. 3.53 there is a pronounced difference between the neutralization of carbon atoms in a carbide and in graphite, respectively. This is one of the rare examples where matrix effects are observed. 

Structure and morphology. Most of the rare-earth elements were encapsulated in multilayered graphitic cages, being in the form of single-domain carbides. The carbides encapsulated were in the phase of RC2 (R stands for rare-earth elements) except for Sc, for which Sc3C4(20] was encapsulated[21]. 

Fig. 5. A growth model of a nanocapsule partially filled with a crystallite of rare-earth carbide (RCj for R = Y, La,. . . , Lu R,C4 for R = Sc) (a) R-C alloy particles, which may be in a liquid or quasi-liquid phase, are formed on the surface of a cathode (b) solidification (graphitizalion) begins from the surface of a particle, and R-enriched liquid is left inside (c) graphite cage outside equilibrates with RCj (or R3C4 for R = Sc) inside.

In this form of corrosion, the iron carbide (ferric carbide) component of steel is decomposed to graphite and pure iron. The external physical structure of the steel is rarely changed, but the internal structure is significantly weakened. 

Limitations of Plasma CVD. With plasma CVD, it is difficult to obtain a deposit of pure material. In most cases, desorption of by-products and other gases is incomplete because of the low temperature and these gases, particularly hydrogen, remain as inclusions in the deposit. Moreover, in the case of compounds, such as nitrides, oxides, carbides, or silicides, stoichiometry is rarely achieved. This is generally detrimental since it alters the physical properties and reduces the resistance to chemical etching and radiation attack. However in some cases, it is advantageous for instance, amorphous silicon used in solar cells has improved optoelectronic properties if hydrogen is present (see Ch. 15). 

Boron carbide is a non-metallic covalent material with the theoretical stoichiometric formula, B4C. Stoichiometry, however, is rarely achieved and the compound is usually boron rich. It has a rhombohedral structure with a low density and a high melting point. It is extremely hard and has excellent nuclear properties. Its characteristics are summarized in Table 9.2. 

Chromium Carbide. Cr C3 has excellent corrosion and oxidation resistance. It is rarely used alone but mostly in combination with TiC and TiN as a base layer. 

Binary rare-earth compounds such as carbides, sulfides, nitrides, and hydrides have been used to prepare anhydrous trihalides, but they offer no special advantage. Treating these compounds at a high temperature with a halogen (98) or hydrogen halide (115) produces the trihalide, e.g.,... 

Other interferences which may occur in flame AAS are ionization of the analyte, formation of a thermally stable compound e.g., a refractory oxide or spectral overlap (very rare). Non-flame atomizers are subject to formation of refractory oxides or stable carbides, and to physical phenomena such as occlusion of the analyte in the matrix crystals. Depending on the atomizer size and shape, other phenomena such as gas phase reactions and dimerization have been reported. 

When rare earth Y-zeolite (REY) was added to the PILC or to the parent clay, the dried PILC or "as received" clay was reslurried in water and the calcined REY added to the 10 wt % level. This slurry was then mixed, filtered, dried, and calcined at 500 C for 2 hours in air. The calcined REY was obtained from Union Carbide and contained 14.1 wt % rare earth elements, primarily lanthanum and cerium. Portions of the PILC were pretreated by one of the methods listed below prior to the microactivity testing. 

Ramsey theory, 22 201-204 Random-fragmentation model, Szilard-Chalmers reaction and, 1 270 Random-walk process, correlated pair recombination, post-recoil annealing effects and, 1 288-290 Rare-earth carbides, neutron diffraction studies on, 8 234-236 Rare-earth ions energy transfer, 35 383 hydration shell, 34 212-213 Rare gases... 

The modern foundry process for producing nodular iron can be oversimplified by describing it as the treatment of a base iron (3% to 4% carbon, 1% to 2% silicon) having low (0,005% to 0.05%) sulfur levels and containing little (<0,05%) phosphorus. The treatment is carried out by means of the introduction of the appropriate nodulizer into this base iron. Inadequate addition of nodulizer results in incomplete spheroidization. Excessive concentrations of nodulizers promote the formation of unwanted iron carbides. The nodulizing elements include the rare earths, magnesium, yttrium and calcium. The latter two elements find little or no use today because of economical and technical problems. 

These same researchers also explored the efficacy of the individual rare earths as nodulizers (17). They concluded, by their ability to produce nodular iron having adequate physical properties without excessive iron carbides present, that cerium was the most effective of the four rare earth elements (lanthanum-neodymium) evaluated as nodulizers. They reported that it required 1.5 times as much neodymiun or praseodymium and three times as much lanthanum as cerium to yield equivalent results. 

Further, it was demonstrated that the introduction of cerium, as mischmetal, in proper amounts was effective in eliminating iron carbides which cause deterioration in physical properties (21). The elimination of iron carbides in thin sections by proper use of the rare earths represents a major contribution to the industry. Different researchers have agreed that there is an optimum percentage for this rare earths addition, which they reported as cerium only, from 0.01% to 0.02% cerium (from about 0.02% to 0.04% total rare earths) that provides this increase in nodule count and control of iron carbides when used in conjunction with magnesium nodulizers (see Figure 9). 

It should be recalled that the final step in the nodular iron treatment process is termed "post inoculation." The purpose of this procedure is to aid in the elimination of iron carbides and promote enhanced nucleation and proper growth of graphite spheroids. This is accomplished by the introduction of the element silicon (usually a ferrosilicon alloy) along with calcium and maybe some magnesium or rare earth. It has been demonstrated that the benefits of rare earth additions are not affected as a function of the time in the process that they are added (23). For example, the elimination of iron carbides by use of the rare earths is possible if the rare earths are introduced along with the primary nodulizer or with the post inocu-lant. In passing, it should be remarked that both the primary nodulizers and ferrosilicon inoculants contain about 1% calcium. 

It is also interesting to note that the deleterious elements, with the exception of titanium, are elements that can be used to stabilize the iron-carbide phase. Titanium is also the only so-called deleterious element that appears to be somewhat controllable by additions of more magnesium, instead of one of the rare earths in common use (34). However, the use of rare earths... 

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