The Crystallization of Diamond

Section 1.2 of this Chapter reviews the crystallization of diamond and cubic boron nitride using high pressure, high temperature techniques. Chapter 4 of Part II deals with the subject of chemical vapor deposition of diamond at low pressure. 

The high pressure sintering of diamond and cubic boron nitride to form super-hard composites is described in Section 1.3, a review of attempts to produce other hard materials is given in Section 1.4. 

Section 1.5 summarizes the many and varied applications of diamond and cBN materials. The applications of diamond grown using the CVD technique are discussed in Chapter 2 of Part III. 

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Let us begin by considering the elements of the second and third periods (Li-Ne and Na-A). In each of these the orbitals available for bond formation are limited to four in number, namely, one s and three p orbitals, so that if four or more electrons are present it will be possible for all the orbitals to be filled by stable groups of electrons, as in the crystals of diamond and silicon or in the molecules of nitrogen, oxygen, fluorine, etc. Metallic properties in these two short periods will therefore be confined to the elements of the first three groups. 

Proi>erties.—Diamond.—The crystals of diamond, which is almost pure carbon, are usually colorless or yellowish, but may be blue, green, pink, brown or black. It is the hardest substance known, and the one which refracts light the most strongly. Its index of refraction is 2.47 to 2.75. It is very brittle a bad conductor of heat and of electricity sp. gr. 3.50 to 3.55. When very strongly heated in vacuo, it swells up, and is converted into a black mass, resembling coke. 

A related advantage of studying crystalline matter is that one can have synnnetry-related operations that greatly expedite the discussion of a chemical bond. For example, in an elemental crystal of diamond, all the chemical bonds are equivalent. There are no tenninating bonds and the characterization of one bond is sufficient to understand die entire system. If one were to know the binding energy or polarizability associated with one bond, then properties of the diamond crystal associated with all the bonds could be extracted. In contrast, molecular systems often contain different bonds and always have atoms at the boundary between the molecule and the vacuum. 

Other Industrial Applications. High pressures are used industrially for many other specialized appHcations. Apart from mechanical uses in which hydrauhc pressure is used to supply power or to generate Hquid jets for mining minerals or cutting metal sheets and fabrics, most of these other operations are batch processes. Eor example, metallurgical appHcations include isostatic compaction, hot isostatic compaction (HIP), and the hydrostatic extmsion of metals. Other appHcations such as the hydrothermal synthesis of quartz (see Silica, synthetic quartz crystals), or the synthesis of industrial diamonds involve changing the phase of a substance under pressure. In the case of the synthesis of diamonds, conditions of 6 GPa (870,000 psi) and 1500°C are used (see Carbon, diamond, synthetic). 

Electronic. Diamonds have been used as thermistors and radiation detectors, but inhomogeneities within the crystals have seriously limited these appHcations where diamond is an active device. This situation is rapidly changing with the availabiHty of mote perfect stones of controUed chemistry from modem synthesis methods. The defect stmcture also affects thermal conductivity, but cost and size are more serious limitations on the use of diamond as a heat sink material for electronic devices. 

In addition to diamond and amorphous films, nanostructural forms of carbon may also be formed from the vapour phase. Here, stabilisation is achieved by the formation of closed shell structures that obviate the need for surface heteroatoms to stabilise danghng bonds, as is the case for bulk crystals of diamond and graphite. The now-classical example of closed-shell stabilisation of carbon nanostructures is the formation of C o molecules and other Fullerenes by electric arc evaporation of graphite [38] (Section 2.4). 

There is considerable evidence in the thermoset literature that the fracture energy decreases with increasing crosslink density, consistent with the intuitive result that crosslinking inhibits flow. In the limit of very high crosslink density, where for example we approach the structure of diamond, fracture can occur on a single crystal plane such that... 

SoUd ice forms a crystal of diamond structure, in which one water molecule is hydrogen-bonded with four adjacent water molecules. Most (85%) of the hydrogen bonds remain even after solid ice melts into liquid water. The structure of electron energy bands of liquid water (hydrogen oxide) is basically similar to that of metal oxides, 6dthough the band edges are indefinite due to its amorphous structure. 

The synthesis of diamond is the most famous high-pressure and high-temperature industrial process, and vast quantities of this material are produced using modem industrial technology. The small synthetic crystals obtained are principally used for cutting tools and abrasives. 

Cubic Phase of Boron Nitride c-BN. The cubic phase of boron nitride (c-BN) is one of the hardest materials, second only to diamond and with similar crystal structure. It is the first example of a new material theoretically predicted and then synthesized in laboratory. From automated synthesis a microcrystalline phase of cubic boron nitride is recovered at ambient conditions in a metastable state, providing the basic material for a wide range of cutting and grinding applications. Synthetic polycrystalline diamonds and nitrides are principally used as abrasives but in spite of the greater hardness of diamond, its employment as a superabrasive is limited by a relatively low chemical and thermal stability. Cubic boron nitride, on the contrary, has only half the hardness of diamond but an extremely high thermal stability and inertness. 

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