1838
H. M. STRONG AND R. M. CHRENKO
Further Studies on Diamond Growth Rates and Physical Properties of Laboratory-Made Diamond by H. M. Strong and R. M. Chrenko General Electric Research and Development Center, Schenectady, New York ld3Oi
(Received November 30, 1970)
Publication costs assisted by the General Electric Go.
Diamond crystals up to 1 carat size were grown in the Fe-C and Fe-Ni-C systems at -57 kbars. In the Fe-C binary systems, Fed2 is stable to 1688'K, preventing diamond growth below this temperature. Additions of Ni lower the melting temperature of Fe3C and widen the temperature range for diamond growth. Carbon diffusivcm2/sec,and diamond growth rates of 1to 2.5 mg/hr were obtained. ity at 57 kbars in molten Fe is about 4 X Depending on the nature of the bath metal used, crystal shapes obtained included cuboctahedra or crystaIs with { 1131, { 110} facesin addition to { I l l }and { l o o }faces. N andB, whichoccupy substitutional positions in diamond, are preferentially absorbed in certain directions producing interesting color patterns. Laboratorymade diamonds have comparable mechanical and electrical properties with natural diamond. The thermal conductivities of laboratory diamonds were -5 times the conductivity of Cu at room temperature.
I. Introduction Thousands of carats of industrial abrasive diamond are produced every day by the catalytic process in which graphite is transformed to diamond through a molten layer of metal such as iron or nickel. I n this method, the driving force for the formation of diamond is derived from the difference in thermodynamic stability between the two phases of carbon. Other methods were discovered for the direct conversion of graphite to diamond at pressures exceeding 140 kbars. 1--3 These methods have produced considerable quantities of diamond in both the cubic and the more recently recognized hexagonal modification of diamonda3g4 Due to the rapidity of the direct conversion processes, the crystals are no larger than a few hundred angstroms. Even the melt freeze process, which requires the extreme conditions of 40OO0K a t 140 kbars, has been used successfully to produce a few milligrams of tiny diamond crystal^.^ None of these laboratory methods has yet successfully demonstrated a feasibility for sustained growth of diamond over sufficiently long time periods t o form large diamonds. The process by which nature has produced large gem diamonds has been speculated about but it is not assuredly understood. Experiments on transporting carbon in molten silicates, in which natural diamonds were assumed to have grown, were failures. Laboratory experiments may sometimes reproduce natural conditions in respect to the thermodynamic variables but can be a t a disadvantage in respect to the time variable. Wentorfa has described in the preceding paper a process which sustains stable diamond growth over longer time periods than in previously reported laboratory experiments to produce larger diamonds in a reasonable time span. This process uses molten group VI11 697
The Journal of Physical Chemistry, Val. 76, No. 28, 1971
metal baths in which to transport carbon in a temperature gradient. These are complex systems to control satisfactorily for the growth of large perfect diamond crystals. In this report, the nature of some of these systems and how they were used to obtain good quality single diamond crystals in the 1 carat size range will be described. Some of the interesting physical properties of these diamonds will be described also.
11. Transition Metal-Carbon Systems for Diamond Growth Long experience on the part of crystal growers has shown that the best media for forming large well faceted perfect crystals in the shortest times and without excessive nucleation of interfering crystals are those in which the nutrient dissolves to the extent of 10 to 60%. Crystallization will take place at temperatures a little above the eutectic for the mixture. There are rough guidelines for finding such media and the locations of their eutectic^.^ It is usually true that the most favorable conditions are achieved when the melting points of nutrient and growth media are as nearly alike as possible in order that the eutectic compositions will be in the desired range. These guidelines applied to carbon-metal systems (1) F. P. Bundy, Science, 137, 1057 (1962). (2) P. 5.DeCarli and J. C. Jamieson, ibid., 133, 1821 (1961). (3) E. I. du Pont de Nemours & Co., Netherlands patent release 6506395 (1965). (4) R. E. Hanneman, H. M. Strong, and F. P. Bundy, Science, 155, 995 (1967). (5) F. P. Bundy, J . Chem. Phys., 38, 631 (1963). (6) R. H. Wentorf, Jr., and H. P. Bovenker, Astrophys. J., 134,995 (1961). (7) G. C. Kennedy and B. E. Nordlie, Econ. Geol., 63, 495 (1968). (8) R . H. Wentorf, Jr., J . Phys. Chem., 75, 1833 (1971). (9) G. T. Kohman and D. H. Andrews, ibid., 29, 1317 (1925).
PHYSICAL PROPERTIES OF LABORATORY-MADE DIAMOND indicate that carbon should be used with one of the refractory metals. Systems of this type have very high melting ranges (stable carbides are a problem also) which are difficult to support at diamond stable pressuresl0for prolonged periods. It is not presently feasible to use these metals in the gradient diamond growth process. The choice of bath metals is thusnarrowed to the loiver melting ferrous metals and their alloys, whose eutectic compositions were < 10% carbon. The performance in diamond growth of these ferrous metal alloys has not matched, by a wide margin, that of nature's system which has produced a long list of fabulous gem diamonds. In using the lower melting point metals, the problems which arise are those of excessive nucleation and development of flaws from entrapment of veils of metal. The growth rates must be kept low to avoid these difficulties. The method of using these alloys NBS described by Wentorfin the preceding paper.* The iron-carbon system forms a base for mixbures of metals used in diamond growth. At 57 kbars this system is quite different from the system a t 1 atm. Melting points have shifted upward; carbon appears in two stable phases and Fe& has a region of stability. The resulting system, Figure 1, has three eutectics, only one of which (the Fe-FerC eutectic) is stable. The Fe-d and Fe-g eutectics fall a t lower temperatures and are unstable with respect to FeaC (experimental details to be published.) Diamond growth is restricted to the region between the diamondsraphite equilibrium a t 1830°K and the melting of Fe8C a t 1688°K. Note that in the presence of Fe, the diamond-graphite equilibrium is shifted downward in temperature from the pure carbon equilibrium." At temperatures lower than 1688'K, diamond gives way to the greater stability of FerC which rapidly consumes the diamond. Crystals grown in the Fe-C system develop slowly and often with flaws. In the liquid range where diamond growth is possible, the gradients of temperature and carbon concentration g/cms cm, respecare -50"/cm and -6 X tively. An equilibrium rarely seen in nature occurs in the Fe-C system. At 52.5 kbars and 1680"1
