Metal-Metalloids Hard Metals

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). 

The prototype hard metals are the compounds of six of the transition metals Ti, Zr, and Hf, as well as V, Nb, and Ta. Their carbides all have the NaCl crystal structure, as do their nitrides except for Ta. The NaCi structure consists of close-packed planes of metal atoms stacked in the fee pattern with the metalloids (C, N) located in the octahedral holes. The borides have the A1B2 structure in which close-packed planes of metal atoms are stacked in the simple hexagonal pattern with all of the trigonal prismatic holes occupied by boron atoms. Thus the structures are based on the highest possible atomic packing densities consistent with the atomic sizes. 

The structures of the prototype borides, carbides, and nitrides yield high values for the valence electron densities of these compounds. This accounts for their high elastic stiffnesses, and hardnesses. As a first approximation, they may be considered to be metals with extra valence electrons (from the metalloids) that increase their average valence electron densities. The evidence for this is that their bulk modili fall on the same correlation line (B versus VED) as the simple metals. This correlation line is given in Gilman (2003). 

Chemistry and Physics of Mechanical Hardness, by John J. Gilman Copyright 2009 John Wiley Sons, Inc. 

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Also metal-metalloid compounds tend to retain their hardnesses as temper-atures become elevated. These compounds have been discussed in Chapter 10. 

The transition metal carbides, nitrides, and diborides have aroused theoretical and practical interest in such unique properties as extremely high melting point, extreme hardness, metallic properties, and superconductivity. These unique common properties are closely connected with electronic structures and the band structures resemble each other among the materials with the same crystal structure. The refractory metalloids, described in Chapter 2, have become highly clarified materials. This chapter is an introduction to Chapters 3-14. 

Boron is a hard metalloid with pronounced nonmetallic properties. Aluminum is a light, strong, amphoteric, reactive metallic element with a surface that becomes passivated when exposed to air. 

Boron is a semimetal, sometimes classed as a metallic or metalloid or even as a nonmetal. It resembles carbon more closely than aluminum, the latter of which is located just below boron in group 13. Although it is extremely hard in its purified form—almost as hard as diamonds—it is more brittle than diamonds, thus limiting its usefulness. It is an excellent conductor of electricity at high temperatures, but acts as an insulator at lower temperatures. It is less reactive than the elements below it in group 13... 

Detailed studies have been carried out with metal-boron (Ni-B, Pd-B, Pt-B) and metal-phosphorous (Ni-P, Pd-P) films prepared by radiofrequency sputtering269-272. Pt hardly interacts with boron and shows the low selectivity of the pure metal269. Interaction between Ni or Pd and the metalloids results in a change in the electron density of... 

With respect to their physical properties, boron is a black, hard, very high-melting, network covalent metalloid, but the other 3 A members are shiny, relatively soft, low-melting metals. Aluminum s low density and three valence electrons make it an exceptional conductor for a given mass, it conducts a current twice as effectively as copper. Gallium has the largest liquid temperature range of any element it melts in your hand but does not boil until 2403°C. 

Aluminum is a congener of boron, thus the two elements display similar chemical characteristics. One important difference is that boron is a metalloid, whereas aluminum is a bona fide metal. As a consequence, the boron center shows soft or hard characteristics depending on the ligands it carries the aluminum center is hard most of the time. 

The crystal structure of silicon is similar to that of diamond however, the Si—Si bonds (226 kJ/mol) are weaker than the C—C bonds (356 kJ/mol), and silicon is not nearly as hard. There is no graphitic allotrope of silicon. Crystalline silicon is a blue-gray, somewhat shiny, brittle element that certainly appears metallic however, it is classified as a nonmetal or metalloid because it is a semiconductor that is, at low temperatures it is an insulator. However, when heated sufficiently, its electrical conductivity increases markedly. Very pure silicon for transistors is produced by reducing silicon tetrachloride prepared by... 

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