Chapter 14: HOW TO MAKE SYNTHETIC DIAMONDS
CARBON IS ONE OF THE MOST INTERESTING of all the ninety-two elements, because of the many forms in which the pure element is found and the infinite number of compounds of which it is a component part. One of the most interesting forms of carbon is the diamond, not only because of its high value but also because of the amount of work which has been done to obtain it by means of synthesis.
A great deal of work still remains to be done before synthetic diamonds can be produced on a commercial basis. Soon after Lavoisier demonstrated in his remarkable work that diamond was crystalline carbon, attempts at its artificial preparation were made. Many attempts have been made to manufacture synthetic diamonds, that is, to enhance the commercial value of carbon by transforming it from its ordinary opaque black condition to limpid crystals of diamond.
The attempts made in this direction have been numerous, but little has been done in a methodical and systematic manner. If we except some important works on the subject, we see from the historical side of the question, how much contradictory or doubtful matter has been published. However, while the number of experiments has been large, the publications have not been as extended as one would at first expect.
It appears, without doubt, that many of the workers have considered more the synthetic preparation of carbon for the diamond than the study of the different allotropic forms of carbon. Among the first attempts to make synthetic diamonds may be mentioned that of J. B. Hannay of Glasgow, who commenced his experiments in 1879. After many trials, some of which resulted in violent explosions, he is said to have succeeded. The method adopted by Mr. Hannay is described as follows:
A tube twenty inches long by four inches in diameter was bored so as to have an internal diameter of half an inch. In the tube was placed a mixture of ninety per cent of rectified bone oil, and ten per cent of paraffin spirit, together with four grams of the metal lithium. The open end of the tube was welded air-tight, and the whole mass was heated to redness for fourteen hours; on opening it a great volume of gas rushed from the tube, and within was a hard, smooth mass adhering to the sides of the tube.
It was quite black, and appeared to be composed of iron and lithium, but on closer inspection small transparent pieces were found imbedded in it. The mass was dissolved, and the small transparent pieces proved to be ‘crystalline carbon’, exactly like diamonds but almost microscopic.
Out of eighty complex and extensive experiments only three succeeded. Violent explosions were frequent, steel tubes burst, scattering their fragments around, and furnaces were blown up. ‘The continued strain on the nerves’, writes Mr. Hannay, ‘watching the temperature of the furnace, and in a state of tension in case of an explosion, induces a nervous state which is extremely weakening, and when the explosion occurs it sometimes shakes one so severely that sickness supervenes’. (1)
Sir William Crookes attributes the possibility of making artificial diamonds to the facilities afforded by the enormously high temperatures which have been obtainable only in recent years by the use of electricity. While electricity has, no doubt, played an important part in the scientific researches during the last decades of the nineteenth century, Mr. Hannay’s experiments would indicate that it is not absolutely essential to have extremely high temperatures or pressures in order to produce artificial diamonds.
Still Sir William Crookes shows that by means of these high temperatures substances such as carbon obey the common laws which govern other substances, and can be made volatile and fusible under certain conditions. He has demonstrated that the temperature necessary to volatilize pure carbon is about 3,600° C, and that it passes into the gaseous state without liquefying.
He infers that, if, however, sufficient pressure were applied with the high temperature, liquid carbon would be produced which upon cooling would crystallize in diamonds. In making this product the absence of oxygen is absolutely necessary since carbon at high temperatures is chemically most energetic, and if it can possibly get at oxygen from the atmosphere or from any compound containing oxygen it will combine with it and fly off in the form of carbon dioxide.
Heat and pressure, therefore, are of no value unless the carbon can be kept inert. Sir William Crookes went through the process of producing diamonds before the eyes of his audience, but was able to show them the result of his experiment only by producing a lantern slide of microscopic diamonds which he had made in the same way previously, for it takes a fortnight to separate them from the iron and other substances in which they are imbedded.
The scientific principle upon which this experiment rests, according to Sir William Crookes, is that iron dissolves carbon, and it increases in volume as it passes from the liquid to the solid state. Authorities differ somewhat as to the exact moment when molten iron expands in cooling, but it is the generally accepted theory that expansion takes place at the moment of solidification. It is also a well known fact that shrinkage or contraction takes place as the solidified metal cools.
It is therefore possible to obtain enormous pressure in the molten center of a casting by the contraction of the outer shell which has been rapidly cooled and the expansion of the inner mass just as it begins to solidify. This process supplies the two factors necessary for the crystallization of the diamond—heat and pressure.
Of the early attempts to make synthetic diamonds the most successful was that of Henri Moissan, a Frenchman, who, after patient and careful experimenting succeeded in 1896 in obtaining minute particles of diamonds. He very carefully investigated the scheme that Nature employed in making her diamonds and he found that she did so by heating carbon to a very high temperature and cooling it suddenly under enormous pressure.
Swedish iron was melted in the presence of sugar carbon in an electric furnace at a temperature which, seldom exceeded two thousand degrees Centigrade. When the iron was saturated with the carbon at the temperature of the electric furnace the crucible containing the fused metal was removed from the furnace and plunged into ice cold water.
The pressure produced by the quick chilling of the outer crust with, an internal expansion changed the black sugar carbon to crystal diamonds. The iron of the solidified mass was dissolved in strong acids. From this residue Moissan obtained tiny crystals which had the properties of diamonds. His largest synthetic diamonds were three-fourths of a millimeter in diameter.