Tables nitrides, borides

According to literature sources mainly oxides, nitrides, borides and carbides are used as ceramic raw materials of chemical and structural applications and the most common elements in these compounds are Be, Mg, Ca, Ti, V, Cr, Y, Zr, La, Hf, W, B, Al, Si and Sn. These elements can be found in rather small area of the periodic table, i.e. in the groups 2 up to and including 6, 13 and 14. So apparently relatively few ingredients are used in this branch of ceramics to produce a wide range of products. The tricks of the trade are in the preparation. The properties are determined by a number of factors, such as the nature of the building blocks, the kind of bonds, the strength of the bonds, the crystal structure and the reactivity of the material. 

Some selected superconducting carbides, nitrides, borides, and sulfides are presented in Table 1 with their values. Among these superconducting ceramics, the most influential factor was the crystal structure. Many of the important superconductors were based on the NaCl-type structure (also referred to as the B1 structure by metallurgists) and the... 

Table 1. Selected Superconducting Carbides, Nitrides, Borides, and Sulfides with Their T- Values...

From a gas or ionized phase, we synthesize a compound (carbide, nitride, boride) in situ on the part to be protected or to modify the surface properties. The energies brought into play (nucleation, growth) to form the solid are weaker than in the traditional method, which makes it possible to manufacture dense materials at much lower temperatures. Table 7.1 shows the various compotmds with the temperature range for the production of coatings by CVD. 

Table 3. Physical Properties of Titanium Borides, Carbides, and Nitrides ...

Table 3 summarizes the properties of the so-called nonmetallic hard materials, including diamond and the diamondlike carbides B C, SiC, and Be2C. Also iacluded ia this category are comadum, AI2O2, cubic boroa nitride, BN, aluminum nitride, AIN, siUcon nitride, Si N, and siUcon boride, SiB (12). 

The most extensive group of nitrides are the metallic nitrides of general formulae MN, M2N, and M4N in which N atoms occupy some or all of the interstices in cubic or hep metal lattices (examples are in Table 11.1, p. 413). These compounds are usually opaque, very hard, chemically inert, refractory materials with metallic lustre and conductivity and sometimes having variable composition. Similarities with borides (p. 145) and carbides (p. 297) are notable. Typical mps (°C) are ... 

The binary borides (p. 145), carbides (p. 299), and nitrides (p. 418) have already been discussed. Suffice it to note here that the chromium atom is too small to allow the ready insertion of carbon into its lattice, and its carbide is consequently more reactive than those of its predecessors. As for the hydrides, only CrH is known which is consistent with the general trend in this part of the periodic table that hydrides become less stable across the d block and down each group. 

The CVD coating materials for wear and corrosion resistance consist mostly of carbides and nitrides and, to a lesser degree, borides. Table 17.1 compares the relative properties of these materials. 

Compounds isotypic with the k phases arc found among intcrmetallics, borides, carbides and oxides and also with silicides, germanides, arsenides, sulfides and sclcnides no nitrides, however, are found. The mode of filling the various voids in the metal host lattice of the k phases follows the schemein Ref. 4 and is presented in Table 1 for all those compounds for which the atom distribution is well known from x-ray or neutron diffraction. Accordingly, B atoms in tc-borides, Zr, Mo, W, Re)4B and Hfy(Mo, W, Re, Os)4B , occupy the trigonal prismatic interstices within the parent metal framework of a Mn, Aln,-type structure (see Table 1 see also ref. 48). Extended solid solutions are found for (Hf, Al)[Pg.140]

The small atoms at the center of the first row of the Periodic Table (B, C, N, O, and to a lesser extent Al, Si, and P) can fit into the interstices of aggregates of larger transition metal atoms to form boride, carbide, and nitride compounds. These compounds are both hard and moderately good electronic conductors. Therefore, they are commonly known as hard metals (Schwarzkopf and Kieffer, 1953). 

TABLE 4 Selected Binary Carbides, Nitrides, and Borides with Transition Temperature. 

In earlier work, it was found for borides, silicides and nitrides that specific activity, expressed as total rate of methane consumption per unit surface area, plummeted with increasing surface area of the catalyst samples.1718 The same relationship was also found for transition metals carbides (Figure 16.4). It should be noted the dependence of specific activity on surface area rather than catalyst composition is unusual for heterogeneous catalytic reactions. In addition, it can be found that the reaction order in the oxidant is perceptibly in excess of 1 (Tables 16.8 and 16.9). Such an order is hard to explain in terms of common mechanism schemes for heterogeneous catalytic oxidative reactions. 

In nonoxide ceramics, nitrogen (N) or carbon (C) takes the place of oxygen in combination with silicon or boron. Specific substances are boron nitride (BN), boron carbide (B4C), the silicon borides (SiB4 and SiBg), silicon nitride (SisN4), and silicon carbide (SiC). All of these compounds possess strong, short covalent bonds. They are hard and strong, but brittle. Table 22.5 lists the enthalpies of the chemical bonds in these compounds. 

In the so-called interstitial nitrides the metal atoms are approximately, or in some cases exactly, close-packed (as in ScN, YN, TiN, ZrN, VN, and the rare-earth nitrides with the NaCl structure), but the arrangement of metal atoms in these compounds is generally nor the same as in the pure metal (see Table 29.13, p. 1054), Since these interstitial nitrides have much in common with carbides, and to a smaller extent with borides, both as regards physical properties and structure, it is convenient to deal with all these compounds in Chapter 29. 

U.S. Bureau of Mines Bull. 672, 674, and 677. Bulletins 672 and 674 cover the elements, binary oxides and binary halides in a very complete fashion. Bulletin 677, summarizes the values from Bulletins 672 and 674, and adds a modest selection of tables for arsenides, antimonides, borides, carbides, carbonates, hydrides, nitrides, phosphides, selenides, silicates, silicides, sulfates, sulfides and tellurides. The coverage of these added compound types, however, is far from complete for example, there are no tables for PbS04, SnS04, GaS and Li2S. The only ternary compounds included are the carbonates, sulfates and silicates, and no quaternary compounds are listed except for a limited number of hydrated compounds. Only brief references are given to the data sources, without attempt to explain the choice between conflicting values. 

A variety of oxide, carbide, nitride, and boride films and coatings can be readily prepared by CVD techniques as shown in Table 28.2. The reacting compounds must exist in a volatile form and must be sufficiently reactive in the gas phase. 

Following the previous description of the atomic structure of boride, carbide and nitride ceramics, Table 2 lists physical crystalline stmctural differences of a variety of UHTCs along with respective density and melting point. Note that density increases with increasing mass of the metal atom. Note also the differences in melting points between materials whereby the carbides typically have the highest melting points, above borides or nitrides of the same metal constituent. 

Bulk moduli for ambient conditions (with index o ) are listed in Tables 10.7 for elements, SlO.l for compounds MX, S10.2 for MX2, S10.3 for MX3, S10.4 for binary oxides, S10.5 for binary nitrides, S10.6 for binary borides, S10.7 for binary carbides and silicides, S10.8 for binary phosphides and arsenides, S10.9 for ternary oxides and coordination compounds, SIO.IO for molecular substances and polymers, SlO.l 1 for characteristics of polymorphous modifications of elements and the MX compounds, S10.12 for various phases ofMX2 crystals. 

As can be seen in Table 4.2, the thermal conductivities of the Group rv carbides, nitrides, and borides are relatively close. They are also similar to those of the host metals and, from this standpoint, reflect the metallic character of these compounds. However, their conductivities are much lower than that of the best conductors such as Type II diamond (2000 W/m-K), silver (420 W/m-K), copper (385 W/m K), beryllium oxide (260 W/m-K), and aluminum nitride (220 W/m-K). 

As shown in the above table, the Group IV carbides (and Groups V and VI carbides as well) are good electrical conductors and have an electrical resistivity only slightly higher than that of the parent metals, reflecting the metallic character of these compounds. The nitrides and especially the borides have even lower resistivity. The large spread in the reported values may be attributed to differences in composition and the presence of defects and impurities. 

Hardness. Table 5.6 shows that carbides are die hardest, followed by the borides and the nitrides. The Group V carbides have higher hardness than those of Group VI but are not quite as hard as those of Group IV (see Ch. 4, Sec. 4.4 and Ch. 6, Sec. 4.0). This reflects the intermediate strength of M-C bonds found in these carbides. 

In this section and the next three, the properties and characteristics of the covalent carbides are reviewed and compared whenever appropriate with those of the parent elements and of the refi tory compounds of titanium. For comparison with other carbides, nitrides, or borides, see the appropriate tables in Chs. 4-6. Reported property values often vary considerably and the values given here are a general average. 

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