Intermetallic nitrides

Although the distinction is not always clear, ternary nitrides (and nitrides in general) often are classified into two groups (1) intermetallic-type and (2) ionic/covalent-type. Intermetallic nitrides are those in which metal-metal (M-M) interactions are dominant and where the nitrogen atoms are interstitial within the metal array.3 Because these phases are stabilized by M-M interactions, the structure and physical properties are similar to those of many other metallic systems, such as alloys, metals, and... 

Finally, we would like to note that considerable progress has been made recently in the study of nitrides, in general, and that is especially true in the case of rare-earth multinary nitride-type compounds. Many of the papers have appeared in recent years, often describing novel types of materials. The best illustration is provided by the R-T-N (T=transition metal) intermetallic nitrides in which several new families have been discovered since 1990. The challenge has given rise to a lot of papers which continue to appear simultaneously, motivated by the potential offered by these phases for development as permanent-magnet materials. 

Whereas finely divided cobalt is pyrophoric, the metal in massive form is not readily attacked by air or water or temperatures below approximately 300°C. Above 300°C, cobalt is oxidized by air. Cobalt combines readily with the halogens to form haUdes and with most of the other nonmetals when heated or in the molten state. Although it does not combine direcdy with nitrogen, cobalt decomposes ammonia at elevated temperatures to form a nitride, and reacts with carbon monoxide above 225°C to form the carbide C02C. Cobalt forms intermetallic compounds with many metals, such as Al, Cr, Mo,... 

M X the explosive black nitride T1 3N is known, and the azides T1 N3 and T1 [T1 (N3)4] the phosphides TI3P, TIP3 and TIP5 have been reported but are not well characterized. With As, Sb and Bi thallium forms alloys and intermetallic compounds TI3X, Tl7Bi2 and TlBi2. 

Chemical vapor deposition (C VD) is a versatile process suitable for the manufacturing of coatings, powders, fibers, and monolithic components. With CVD, it is possible to produce most metals, many nonmetallic elements such as carbon and silicon as well as a large number of compounds including carbides, nitrides, oxides, intermetallics, and many others. This technology is now an essential factor in the manufacture of semiconductors and other electronic components, in the coating of tools, bearings, and other wear-resistant parts and in many optical, optoelectronic and corrosion applications. The market for CVD products in the U.S. and abroad is expected to reach several billions dollars by the end of the century. 

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 . 

A thorough review of the intermetallic compounds of mechanical interest has been written by Westbrook (1993). The borides, carbides, and nitrides will be considered separately in Chapter 10. 

One of the simplest calorimetric methods is combustion bomb calorimetry . In essence this involves the direct reaction of a sample material and a gas, such as O or F, within a sealed container and the measurement of the heat which is produced by the reaction. As the heat involved can be very large, and the rate of reaction very fast, the reaction may be explosive, hence the term combustion bomb . The calorimeter must be calibrated so that heat absorbed by the calorimeter is well characterised and the heat necessary to initiate reaction taken into account. The technique has no constraints concerning adiabatic or isothermal conditions hut is severely limited if the amount of reactants are small and/or the heat evolved is small. It is also not particularly suitable for intermetallic compounds where combustion is not part of the process during its formation. Its main use is in materials thermochemistry where it has been used in the determination of enthalpies of formation of carbides, borides, nitrides, etc. 

These, therefore, constitute the guidelines for finding superconductors or how to raise the superconducting temperature. Since Covalon conduction is a nucleus to superconductivity and covalent bond is a poor conductor at room temperature, a good conductor at room temperature implies a poor covalent bond and therefore will not be a superconductor or will be a poor superconductor at best at low temperature. Inasmuch as a good covalent bond can come from compound formation, good superconductors, particularly Type-II, shall be expected to come from intermetallic compounds or special type of ceramic oxides and nitrides. 

Metals and ceramics (claylike materials) are also used as matrices in advanced composites. In most cases, metal matrix composites consist of aluminum, magnesium, copper, or titanium alloys of these metals or intermetallic compounds, such as TiAl and NiAl. The reinforcement is usually a ceramic material such as boron carbide (B4C), silicon carbide (SiC), aluminum oxide (A1203), aluminum nitride (AlN), or boron nitride (BN). Metals have also been used as reinforcements in metal matrices. For example, the physical characteristics of some types of steel have been improved by the addition of aluminum fibers. The reinforcement is usually added in the form of particles, whiskers, plates, or fibers. 

The important ceramic matrix materials are glass, silicon carbide, silicon nitride, alumina, glass-ceramics, sialons, intermetallics and some elemental materials. A list of some ceramic matrix materials is given in Table 3.5. 

Slip-casting of technical ceramics has been steadily introduced over the past 60 years or so, and now it is standard practice to cast alumina crucibles and large tubes. The process has been successfully extended to include silica, beryllia, magnesia, zirconia, silicon (to make the preforms for reaction-bonded silicon nitride articles) and mixtures of silicon carbide and carbon (to make the preforms for a variety of self-bonded silicon carbide articles). Many metallics and intermetallics, including tungsten, molybdenum, chromium, WC, ZrC and MoSi2, have also been successfully slip-cast. 

Because intermetallic systems undoubtedly display certain special features that follow from their metallic binding forces, considerable importance attached to the growing evidence that the chalcogenides, the essentially ionic oxides, the nitrides, and other representative binary compounds of the transition metals were, not infrequently, both variable and irrational in composition. Schenck and Ding-mann s equilibrium study of the iron-oxygen system (39) was notable in this connection They showed that stoichiometric ferrous oxide, FeOi 000, the oxide of an important and typical valence state, did not exist. It lay outside the broad existence field of a nonstoichiometric phase. It is, perhaps, still not certain... 

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