Preparation of Artificial Diamond

The preparation of artificial diamonds from carbon is a problem which has long fascinated the minds of scientific men and others.

Despretz in 1849 tried to make diamonds by means of an electric arc formed between a carbon rod and a bunch of platinum wire, and obtained a crystalline dust which scratched ruby. Hannay professed to have prepared diamond in 1880, and Marsden, in 1880 - 81, attempted to prepare it by dissolving carbon in molten silver, and obtained small crystals which may have been diamond.

Moissan, who determined to repeat the experiments of Hannay, made a careful study of the allotropic forms of carbon, of their modes of formation and transformation, and of their physical and chemical distinctions.

The three following criteria must be obeyed before a substance can be pronounced to be diamond:

(i) Hardness; (ii) density; (iii) combustion figures.

1. Other substances prepared in the electric furnace, e.g. boride of carbon, carbosilicide of titanium, are almost or quite as hard as diamond.
2. Density (3.5) is insufficient to characterise diamond; some substances prepared in the electric furnace, e.g. metallic borides and silicides, are as dense as or denser than diamond.
3. Carbon boride and some metallic carboborides burn producing carbon dioxide, but only carbon itself can produce 3.6 times its weight of carbon dioxide.

These three criteria taken together prove a substance to be diamond.

Moissan recognised that diamonds seldom, if ever, are found adhering to a matrix; and a study of "blue ground" from the Cape and of diamantiferous sand from Brazil proved that native microscopic diamonds are both black and transparent, and always accompanied by graphite. From "blue ground" Moissan isolated bort, carbonado, transparent diamond, and graphite. A study of a meteorite found in the Diablo Canon, Arizona, furnished, however, the key to the problem. In the middle of a metallic mass were found "two small, transparent diamonds, of rough and grained surface, surrounded by amorphous carbon in distinctly compressed strips." This discovery impelled Moissan to experiment on the solution of carbon in metals at high temperature; and for this purpose he first adapted the electric furnace. In this furnace he dissolved carbon in silver, iron, aluminium, glucinum, chromium, manganese, nickel, cobalt, tungsten, molybdenum, uranium, zirconium, vanadium, thorium, titanium, platinum, silicon. He obtained carborundum and many metallic carbides, but no diamond; carbon always separated from solution in metals as graphite.

Since graphite is the form of carbon stable at high temperature under ordinary pressure, Moissan next tried the effect of great pressure. Geological considerations favoured the view that diamond is formed by crystallisation from molten iron under high pressure. The occurrence of the diamonds in the centre of the Diablo Canon meteorite, and in alluvial deposits as perfect crystals, suggested that terrestrial diamond may have been formed by crystallisation from molten iron under great pressure in the depths of the earth. In order to test this opinion Moissan carried out the following experiments:

1. Two hundred grammes of Swedish iron were covered with sugar-charcoal in a graphite crucible, heated to 3000° C. in the electric furnace, and then plunged beneath water to cool the molten metal suddenly. The iron of the regulus was dissolved in hydrochloric acid, and there remained three kinds of carbon: (a) graphite, (b) convoluted strips, as in Diablo Canon meteoric iron, and (c) several greyish-black particles which were proved to be diamond.
2. Better results were obtained by cooling the molten iron, saturated with carbon, in molten lead, because no layer of steam retarded cooling and external solidification.
3. A further improvement consisted in packing a cylinder of soft iron with sugar-charcoal, strongly compressing the charcoal by means of a screw stopper of the same metal, and then immersing the cylinder in molten iron which was contained in a crucible and had been heated in the electric furnace for a few minutes. After the introduction of the cylinder the crucible was at once removed from the furnace and rapidly cooled.

The success of the experiment depends on rapid cooling, because iron, like water, expands on solidifying, and so the external crust exerts an enormous pressure inwards upon the core, which is rich in carbon.

By this means partially or wholly transparent diamonds were obtained which satisfied the most rigorous criteria. The largest, however, were not more than 0.6 mm. in diameter.

In his researches Moissan obtained transparent diamond crystallised in regular octahedra and cubes, irregularly crystallised fragments, crystals which split into fragments on keeping, like Prince Rupert's drops, and carbonado.

That diamond has been formed by crystallisation of carbon from a solvent, probably metallic, at high temperature and under great pressure, thus appeared to be established by Moissan.

Von Bolton, however, has shown that crystalline particles of diamond dust act as nuclei for the deposition of carbon in the form of diamond from the hydrocarbons in coal-gas.

The observation that diamonds occur in fissures associated with nodules containing sulphur, silicon, and phosphorus led Moissan to try the effect of adding these elements to the molten iron from which the diamond was to crystallise. Sulphur improved the yield; silicon also improved the yield, but the quality of the diamonds was poorer; phosphorus had an altogether unfavourable influence.

Moissan's final conclusions may thus be summarised: diamond is carbon which has been liquefied under high pressure; carbon heated to high temperature under atmospheric pressure vaporises without liquefying and yields a sublimate of graphite.

Moissan's conclusion as to the influence of high pressure in the formation of diamond has been substantiated by an observation of W. Crookes. Crookes has examined the residues from the experiments of A. Noble on the explosion of cordite in closed cylinders. These residues, produced under high temperature and high pressure occurring simultaneously, were found to contain diamonds.

C. Y. Burton, however, maintains that if carbon can be crystallised at comparatively low temperatures, the minimum pressure required to produce diamond will be lower than that employed by Moissan; and in some tentative experiments in which carbon was dissolved in a molten alloy of lead with about 1 per cent, of calcium, diamond appears to have been produced between 550° C. and 700° C. under atmospheric pressure when steam was passed over the alloy so as to react with and eliminate the calcium. Burton also seems to have produced diamond by heating benzene or toluene with carbon tetrachloride or chloroform in a bomb to 200°-300° C. In this reaction carbon is separated, whilst hydrogen chloride accumulates under great pressure.

Further, as a result of attempts to melt carbon by electrical resistance heating under high pressure, which failed to yield more than a suspicion of diamond, C. A. Parsons concludes that mechanical pressure is not the cause of the production of diamond in rapidly cooled iron containing carbon. Van Deventer has criticised Moissan's views from a theoretical standpoint.