Artificial Diamonds

F. P. Bundy, H. T. Hall,
H. M. Strong, and R. H. Wentorf, Jr.*

In the 1920s it was generally accepted that diamond could be obtained from carbon under conditions of high temperature and great pressure. The history of this question was indeed fascinating. It involved famous chemists, an engaging subject, and remarkable experimental technique. Recently, however, informed scientists have unanimously come to the opinion that there is not a single reliable example of obtaining diamond in the laboratory.

The fact that for more than a hundred years claims and refutations concerning the synthesis of diamond have appeared characterizes both the attractiveness of the problem and the extraordinary difficulty of its experimental solution.

Henri Moissan⁴ dissolved charcoal in molten iron and rapidly cooled the solution with cold water in order to crystallize carbon under high internal pressure (it was assumed that this was created by compression, since the mass cooled from the surface). When the metal was removed from the solidified alloy by dissolution, traces remained of a transparent substance possessing optical properties similar to those of diamond and giving a certain amount of CO₂ on combustion. Moissan was therefore convinced that he had obtained diamond.

In 1880 J. B. Hannay¹ reported that he had made diamonds by heating a mixture of hydrocarbons, bone oil, and lithium to red heat in sealed iron tubes. This method involved great difficulties, since the tubes exploded; only three out of eight withstood the process. The identification of the diamonds obtained by Hannay seemed very convincing, since a density of 3.5 and a carbon content of 97.85% were reported.

* Nature, July 9, 1955, p. 51. Translated by A. A. Ilyina.

C. Parsons^8 spent thirty years attempting to synthesize diamonds, while also trying to reproduce the work of Moissan and Hannay. At first Parsons thought that he had achieved success, but later, having certain doubts, he again scrupulously reviewed all his work in this direction. Parsons’ new work clearly showed that he had been mistaken in regarding as diamonds various transparent minerals (spinels) that did not possess double refraction, were very resistant to chemical reagents, and were noncombustible. Finally he concluded that neither he nor anyone else had ever succeeded in producing diamonds in the laboratory.

Fig. 1. Phase diagram of carbon.

Early reports on the synthesis of diamonds were again reviewed by Prof. N. V. Sedgwick^21 (Oxford, 1950) and H. Eyring^23 (Utah, 1952). These authors came to the conclusion that the synthesis of diamond under laboratory conditions had never been accomplished and that thermodynamic considerations indicate the impossibility of such synthesis under the conditions of the experiments described.

Thermodynamic considerations. The phase diagram of carbon as it is presently understood is shown in Fig. 1. From experiment, only the boundary between graphite and its vapor at low pressure has been established with certainty. The boundary between graphite, vapor, and liquid at higher pressures is based on Basset’s work and has not been fully established.

The position of the boundary between graphite and the region of stability of diamond is at present based on very scant direct experimental data. The position of the low-temperature part of this boundary was calculated thermodynamically by Rossini and Jessup^25 at the U.S. Bureau of Standards in 1938. The high-temperature part of this boundary

(indicated by the dotted line) represents a pure extrapolation of the remaining part, since no physical data have yet been obtained in this region.

The dash-dotted line running through the entire diagram shows the upper limits of pressure and temperature reached in controlled experiments (excluding atomic bombs), according to published data. Prof. P. W. Bridgman (Harvard University) is an outstanding investigator in this field, having attained pressures above 400,000 atmospheres at room temperature. He was also able to reach temperatures of about \(3000^\circ\mathrm{K}\) at a pressure of \(30\,000\ \mathrm{kg}/\mathrm{cm}^2\) for short intervals of time.

The diagram shows that Bridgman worked in the diamond-stable region, and that he attempted many times to synthesize diamonds, but without success. In several papers he established that the reason for his failures was connected with the fact that, at the relatively low temperatures of his experiments, the reaction rate was negligibly small.

He recognized the need to move to higher temperatures, but did not develop an apparatus capable of operating simultaneously at such high pressures and temperatures. The maximum conditions he reported were \(\sim 30\,000\ \mathrm{kg}/\mathrm{cm}^2\) and \(2200\text{--}3000^\circ\mathrm{K}\) for \(1\text{--}2\) seconds.

Bridgman attempted to establish a point on the diamond—graphite equilibrium curve at which diamond ceases to graphitize. In this series of experiments, diamond was heated for several seconds to a temperature of \(2500^\circ\mathrm{K}\), and the pressure was increased until graphitization of the diamond ceased. Such a pressure was reached at \(30\,000\ \mathrm{kg}/\mathrm{cm}^2\). Bridgman expressed the supposition in his paper that this point (\(30\,000\ \mathrm{kg}/\mathrm{cm}^2\) and \(2500^\circ\mathrm{K}\)) lies near the diamond—graphite equilibrium curve.

In Bridgman’s experiments, the possibility cannot be excluded that pressure reduces the rate of transformation of diamond into graphite. If this is so, then the diamonds should have ceased graphitizing within several seconds at \(30\,000\ \mathrm{kg}/\mathrm{cm}^2\) and at sufficiently high temperatures. But in that case the point \(30\,000\ \mathrm{kg}/\mathrm{cm}^2\) and \(2500^\circ\mathrm{K}\) still lies in the region of stability of graphite.

The known data on the thermodynamic stability of diamond and graphite allowed us to hope that the formation of diamonds should occur in the pressure range \(30\,000\text{--}100\,000\ \mathrm{kg}/\mathrm{cm}^2\) at temperatures of about \(1000\text{--}3000^\circ\mathrm{K}\).

Compression chambers

Practically all known apparatuses for obtaining ultrahigh pressures are based on the fact that a piston is introduced into a cylinder containing the substance subjected to pressure. The principal factor limiting the maximum pressure attained in the chamber is the

strength of materials. The strongest steels, in the most favorable shape and dimensions (for example, a piano wire), have tensile strengths on the order of \(14\,000—21\,000\ \mathrm{kg/cm^2}\). Synthetic carbides, such as, for example, “carboloy,” possess compressive strengths on the order of \(50\,000\ \mathrm{kg/cm^2}\) or more. Our group believed that the pressures required for sufficiently rapid diamond synthesis would have to exceed the compressive resistance of “carboloy,” since they lie in the range \(50\,000—100\,000\ \mathrm{kg/cm^2}\).

A simple thickening of the chamber walls has little effect on its ability to withstand pressure once a certain wall thickness has been exceeded. By using multiple supporting rings enclosing the cylindrical part (a method long used in the construction of barrels for large guns) and special gaskets between the piston and the cylinder, Bridgman designed compression chambers suitable for producing pressures on the order of \(50\,000\ \mathrm{kg/cm^2}\) at room temperature. At high temperatures, however, materials become less strong and, in general, the limit of attainable pressures decreases.

By introducing certain new methods of stress distribution and strengthening the especially critical parts of the apparatus, our research group was able to realize compression chambers operating at pressures of at least \(100\,000\ \mathrm{kg/cm^2}\) and at temperatures exceeding \(2300^\circ\mathrm{K}\), for hours of continuous operation.

Calibration of Pressures and Temperatures

Pressures of this order are calibrated by means of four electrical-resistance transitions discovered by P. W. Bridgman^26 in the pressure region up to \(80\,000\ \mathrm{kg/cm^2}\). These transitions are observed in bismuth at \(25\,400\), thallium at \(45\,000\), cesium at \(55\,000\), and barium at \(80\,000\ \mathrm{kg/cm^2}\). For pressures exceeding \(80\,000\ \mathrm{kg/cm^2}\), the variation of the melting point of germanium as a function of pressure can be used. It was found that it decreases linearly with pressure up to \(100\,000\ \mathrm{kg/cm^2}\).

The temperature inside the compression chamber was measured by means of thermocouples, the melting points of various substances, determination of the electrical resistance of wires, Curie points of magnetic materials, the use of paints that change color on heating, etc.

The Region Now Available for Research

Figure 2 presents a pressure–temperature diagram with linear scales along both axes, showing the new region of research that becomes accessible through the use of

our new apparatus for obtaining high pressures and temperatures. From the diagram it is evident that this region occupies almost the same area as all the regions reached up to the present time.

Synthesis of diamond

The new region of attainable pressures and temperatures also extends into the region of diamond stability, as is seen from a comparison of Figs. 1 and 2. Under these conditions, corresponding to the presumed stability of diamond, processes were found as a result of which diamonds were obtained with linear dimensions from \(l < 100 \ \mu\) to 1 mm and larger.

Fig. 2. New region of pressures and temperatures accessible for investigations.

Various variants of these processes were repeated independently of us by other workers of the General Electric Company in more than a hundred cases; in each case crystals grew, and they were subjected to critical tests for their identification with diamonds. The dimensions of our present chamber permit the synthesis of diamonds up to \(\sim \frac{1}{4}\) carat.

Fig. 3. Artificial diamonds: \(a\)—a 1-mm diamond shown by the needle of a phonograph, \(b\)—0.2–0.5-mm octahedra.

At the same time there is no need to introduce diamond grains. The formation of nuclei and growth proceed spontaneously and in great numbers when “diamond-stable” conditions are reached. In some,

In earlier experiments diamond powder was added; however, crystal growth was observed independently of additions and in other places. In a small number of cases, under special conditions, some growth was also observed on the added crystals.

Artificial diamonds form the same shapes as various natural diamonds, i.e., octahedra, tetrahedra, and dodecahedra. Some typical forms can be seen in Fig. 3.

Identification of diamonds

The conclusion that precisely diamonds had been produced was based on the following convincing tests:

1) The identity of the crystal structure of the artificial crystals with natural diamonds, as is evident from a comparison of X-ray diffraction patterns (Fig. 4).

Fig. 4. X-ray diffraction on artificial and natural diamonds (photographs in a powder camera).

2) Chemical analysis, showing that the crystals obtained consist of carbon. Their analysis gave 86% C and 14% inorganic ash, identical with the original medium.

3) Hardness tests. The artificial diamonds proved sufficiently hard to scratch the hardest surface (111) of a natural diamond (Fig. 5).

4) Reproducibility. The synthesis of diamonds was reproduced completely independently of us by other staff members of the General Electric Company. The diamonds obtained by them passed all the above-mentioned final tests for identity with diamonds.

In addition, it was shown that the refractive index of the artificial crystals had a value of 2.4–2.5. In reference books on

mineralogy^27 one can find five isotropic minerals having refractive indices close to these figures. Natural diamond

Fig. 5. Scratches made by synthetic diamonds on the surface of the (111) face of a natural diamond. A large number of scratches is seen chiefly in the vertical direction through the center of the figure. The horizontal strokes correspond to the boundaries of growth surfaces on the crystal face. The small triangles in the lower right corner are also typical growth figures found on diamond surfaces.

is one of these minerals, having a refractive index of 2.419. The remaining four are as follows:

Data from other tests show that these four possibilities are eliminated.

More than four years of work in our laboratory were spent on designing equipment capable of withstanding pressures and temperatures corresponding to the diamond-stable region for hours. The synthesis of diamonds was achieved in more than a hundred cases. Several researchers have successfully repeated various processes for making diamonds independently of us. The artificial diamonds obtained can scratch natural diamonds and have a crystalline structure identical with that of natural diamonds.

TRANSLATOR’S NOTE

In the Soviet press, as early as 1939, an article on the same subject by O. I. Leipunsky was published (Uspekhi khimii, vol. 8, No. 10, 1939). In this article, on the basis of analysis of the data of Rossini and Jessup and taking account of some other data, O. I. Leipunsky calculates the “graphite—diamond” diagram, from which he concludes that for the production of diamond the minimum temperatures required are \(> 2000^\circ\) at pressures of the order of \(60\,000\) atm. The author further states that “the pressure required for the crystallization of diamond in the region of stability may be reduced if the temperature at which crystallization is possible can be successfully lowered.” In addition, O. I. Leipunsky considers the possibility of crystallization of diamonds in the region where graphite is more stable than diamond.

The article contains a review of previous work and a bibliography.

A. I.

LITERATURE

Note. References to the works of Hannay and Moissan, which are the principal ones, are arranged in chronological order. References to earlier works (before 1828) may be found in Mellor’s book Comprehensive Treatise on Inorganic and Theoretical Chemistry, vol. 5. Longmans Green and Co., London, 1924.

1. J. B. Hannay, Proc. Roy. Soc. 30, 188 (1880), or Chem. News 41, 106 (1880).
2. N. Story-Maskelyne, The Times, Feb. 20 (1880), or Chem. News 41, 97 (1880).
3. J. B. Hannay, Proc. Roy. Soc. 30, 450 (1880), or Nature 22, 255 (1880).
4. H. Moissan, C. R. Acad. Sci., Paris 118, 320 (1894) and 123, 206, 210 (1896).
5. J. B. Hannay, Chem. News 86, 173 (1902).
6. W. Crookes, Diamonds (London, 1909).
7. O. Ruff, Zeits. anorg. allgem. Chem. 99, 73 (1917).
8. C. A. Parsons, Proc. Roy. Soc. 79, 532 (1907); J. Inst. Metals 20, 5 (1918); Phil. Trans. A 220, 67 (1920).
9. H. Le Chatelier, Leçons sur le carbon, p. 24 (Paris, 1926).
10. C. H. Desch, Nature 121, 799 (1928).
11. J. W. Hershey, Trans. Kansas. Acad. Sci. 31, 52 (1929) and 40, 109 (1937); “The Book of Diamonds” (Hearthside Press, 1940).
12. F. A. Bannister and K. Lonsdale, Nature 151, 334 (1943); Mineral Mag. 26, 309 (1943).

1. C. H. Desch, Nature 152, 148 (1943).
2. Rayleigh, Lord, Nature 152, 597 (1943).
3. M. W. Travers, Nature 152, 726 (1943).
4. J. W. French, Nature 153, 112 (1944).
5. U. Lonsdale, Nature 153, 669 (1944).
6. D. P. Mellor, J. Chem. Phys. 15, 525 (1947); Research 2, 314 (1949).
7. P. W. Bridgman, J. Chem. Phys. 15, 92 (1947).
8. F. D. Rossini, Chemical Thermodynamics, 453 (Wiley, 1950).
9. N. V. Sidgwick, Chemical Elements and their Compounds 1, 491–3 (Clarendon Press, Oxford, 1950).
10. T. Moeller, Inorganic Chemistry, 669 (Wiley, 1952).
11. H. Eyring and F. W. Cagle, Jr., Zeits. Elektrochem. 56, 480 (1952).
12. A. Neuhaus, Angew. Chem. 66, 525 (1954).
13. F. D. Rossini and R. S. Jessup, J. Res. Nat. Bur. Stand. 21, 491 (1938).
14. P. W. Bridgman, Proc. Amer. Acad. Arts and Sci. 81, 165 (1952).
15. Palache, Berman and Frondel, Dana’s System of Mineralogy (J. Wiley, 1951).