THERMAL AND OTHER PROPERTIES

Diamond has an unusual power of conducting heat, in which respect it is an exceptional material. There exists a well-known relation between electrical conductivity and heat conductivity.

Experiment shows that good conductors of electricity are also good conductors of heat. A metal conducts electricity because it has a sort of floating population of electrons. These same electrons also contribute to heat conduction. The result is that good conductors of electricity are good thermal conductors. The converse is also true in that an electric insulator (i.e. a very, very poor conductor of electricity), such as glass or porcelain, is at the same time a poor conductor of heat (we can pick up a hot cup of tea in a porcelain cup, but would quickly let go if the hot liquid was in an all-metal cup, with a metal handle).

Diamonds (apart from one or two exceptionally rare blue Diamonds) are very good electric insulators. By analogy one would expect that they would also be good heat insulators. But the contrary is true. In an anomalous fashion, Diamond conducts heat extraordinarily well, as well as the good thermal metal conductor aluminium. The reason for this is a subtle one. The heat is not conducted by electrons but by types of crystal vibration called phonons, a sort of wave propagating through the crystal, and this transmits the heat. This ability to conduct heat well leads to a curious professional test of whether a crystal offered is either a genuine Diamond, another crystal, or a fake. It is not unusual for unscrupulous dealers to attempt to pass off fakes. These are often of one of two kinds: another type of crystal, such as zircon or quartz, is shaped like a Diamond, mounted as a jewel and then passed off as the real article; or, alternatively, a specially dense kind of glass is used (it is called 'paste' in the trade). All three, zircon, quartz and paste, are poor thermal conductors.

The test to distinguish the real Diamond from the fake is simple yet decisive. The object is placed on the table to acquire room temperature. It should not be touched; otherwise it may acquire body temperature. The object is then picked up with a handkerchief and applied to the tip of the tongue. The dia­mond feels cold like a metal, because it conducts away heat. The fakes, poor conductors, remain warm to the feel of the tongue. There can be no mistaking the icy metallic coldness of the real Diamond; but, of course, it warms up rapidly, so

that a repeat of the test is less conclusive unless there be a sufficient interval between tests. This high thermal conductivity plays a basically important part in technological usage. More often than not, the Diamond in technological use does so much work that it gets very hot. Cooling is not always possible. Fortunately, the heated Diamond conducts away the heat it generates in doing its work. This enables it to be driven hard. It is often held in a metal matrix and the excellent conductivity to the matrix preserves the life and prolongs considerably the use of the Diamond tool.

Linked closely to conductivity is an even more remarkable thermal property of Diamond, its exceptionally low expansion. All bodies expand when heated, yet amongst the lowest expansions of any material known to science is that of Diamond. It is less even than the unusually low-expanding metal invar.

This combination of high thermal conductivity with low thermal expansion is of exceptional value technically. When Diamonds are used in a matrix as a tool, a grinder or an abra­sive they conduct away most of the heat and at the same time hardly dilate with the heat they retain. Thus it is that a Diamond bonded in a wheel is retained and not lost. These two thermal properties make Diamond an ideal material in practice for machinery uses. It is an exceptional combination which has much to do with the long life which Diamond tools give in machine-shop practice and which makes it highly economic to use such tools despite relatively high initial cost. We shall see later that sharpened Diamonds are much used as lathe-cutting tools. Their good thermal properties have much to do with their economic excellence.

A physical property closely related to hardness is compressi­bility. The Diamond is phenomenally strong in this respect. Indeed, small high-pressure experimental chambers can be made by drilling a cylindrical hole in a good-quality Diamond and applying pressure from two close-fitting pistons to enclosed liquid, as in Fig. 7. Crystals and solids can then be enclosed in the liquid within the Diamond, and so strong is the resistance of the Diamond that with simple screw devices pressures can be pushed up to 100 000 atm or more. This good compres­sion strength also clearly plays a big part in the use of Diamonds for drilling and for cutting hard alloys and hard materials generally, a widespread use for Diamond tools.

Diamond has a further mechanical property of considerable value. It has a low friction when rubbed either against other surfaces or against itself. One valued consequence is that when a Diamond is used as a grinding or cutting tool, it produces a minimal amount of frictional heat. A second consequence is associated with the widespread use of Diamond styli as record player needles. The low friction reduces the wear of both, record and needle considerably, compared with that of other materials. The friction of Diamond against steel, for example, is only one-fifth of the friction of tungsten carbide on steel, or even of machine-hardened steel on steel. For a hard material the friction of Diamond is surprisingly low.