electrical properties

We shall complete this chapter on the physical properties of Diamond with brief references to electrical properties. These play an important part in certain mining techniques and also play a slight part in some recent technological applications of Diamond, which we just indicate here. From the view­point of electrical behaviour solids can be divided into three classes. First are the insulators, e.g. glass, porcelain, wax, which have extremely high electric resistance. At the other extreme are the good conductors of electricity, the metals. In between is a class of solids of growing techno­logical importance, the semi-conductors. These are slightly conducting but have very special valuable properties in certain cases. In some semi-conductors the conductivity changes when light falls on them, i.e. they are called photo-conductive. By combining some of these we get what is called a photo­voltaic cell, an arrangement which gives current when illumin­ated. Most readers will have met such a cell in the light-meter used in photography. On a more massive scale such cells have been used to produce current to feed the telemetering systems in orbiting satellites.

Other semi-conductors exhibit marked changes in property when slightly heated. They are then called thermistors and are extremely sensitive instruments for recording slight changes of temperature.

The familiar transistor is a combination of small pieces of semi-conductor material which has the faculty of acting as (he equivalent of a radio valve, whether it be for rectification, amplification or oscillation, and a whole radio technology has been built round this component. Finally, some semi-conduc­tors have the ability to record fast atomic particles; they are what are called particle-counters. When a fast atomic particle passes through such a semi-conductor, a small pulse of electric current is generated and, if amplified, the passage of the atomic particle is thus recorded.

Now we are ready to consider where Diamonds fit into these schemes of insulator, conductor and semi-conductor. By far the majority of good-quality Diamonds are effective insulators (this is why their good heat conductivity is an anomaly). A few very unusual Diamonds can conduct electricity; they are almost invariably clear and of blue colour. Some can pass even heavy currents, enough to make them red hot! As yet, little is known about these rarities. A small proportion of Diamonds are semi-conductors and some of these have already found use as atomic particle-counters. They have the great advantage in this connection of robustness and indifference to shock and high temperature. They can be used as counters in difficult situations where temperature is high or where violent pressure changes happen, as in rocketry. So far, only a beginning has been made in the use of Diamonds for this purpose, but several studies of these properties have already been reported.

The insulating character of the vast majority of Diamonds is exploited in one method of separating Diamond from its ores, the method called electrostatic separation, a technique to be discussed briefly later when we describe the way in which Diamond is now recovered from the earth.