The growth of diamond crystals under ultra-high-pressure high-temperature conditions is a rather complex physical-chemical process. It is closely related with pressure, temperature, and solvent-catalyst and starting carbonaceous material.

By summing up the experience gained in the past on the basis of intensified practice and marking thermodynamic analyses of the pressure-temperature dependence of synthetic diamond crystal growth, we have discovered the primary principles guiding the growth of larger synthetic diamonds of high strength and given a process for producing larger diamond crystals with outstanding strength nearly as high as that of natural diamond, suitable for geological drills.

â… . INTRODUCTION

Since the discovery by Lavoisier and Tennant that both graphite and diamond are composed of the same element carbon, much interest has been aroused in finding means for converting graphite into diamond. In the early 1950’s, after a long time search, the graphite-to-diamond conversion was eventually accomplished under high-pressure high-temperature conditions in the presence of molten metals. Along with the process in the techniques of growing synthetic diamond crystals, various proposals have been made to define the graphite-to-diamond transformation mechanism, such as solid-to-solid transformation, conversion by the catalytic action, the crystallization from supersaturated solution and so forth.

At present, diamonds are made mainly by two methods: the static high-pressure high-temperature method and the dynamic high-pressure high-temperature (or dynamic shock) method. The former is more widely used.

Adhering to Chairman Mao’s teachings of“Maintaining independence and the initiative in our own hands”and “Relying on our own efforts”, we have carried out fruitful researches in the filed of techniques of growing synthetic diamond crystals. Experiments were performed on the DS-023 type ultra-high-pressure high-temperature press of our own design and make.

Pressure calibration of the reaction vessel is achieved by measurements with reference to the phase changes accompanied by abrupt electrical resistance changes of such metals at particular pressures as Biâ… -â…¡ (at 25.50 kb), TLâ…¡-â…¢(at 36.7±0.3 kb) and Biâ…¢-â…¤(at 77±3 kb). Reaction temperature is determined by means of a Pt/Pt-10 R h thermocouple of 0.3 mm wire diameter. The starting carbonaceous material used was spectroscopically pure artificial graphite or spectroscopically pure electrode graphite. The solvent-catalysts employed were binary or multi-component alloys containing group VIII metals such as nickel, cobalt and iron (as the base-metals) and alloying elements such as silicon, chromium, manganese and copper.

â…¡ EXPERIMENTS AND RESULTS

It is generally held that the crystal growth is determined by two factors: the internal structure of the crystal and the external growth conditions. Pressure, temperature and solvent-catalyst are three important external conditions affecting the growth of synthetic diamond, each of which in specific way has influences on the formation and growth of crystal grains. Practice has demonstrated that any change in these factors will lead to remarkable variations in crystal habit, color and defects. In this paper, our chief concern is the pressure-temperature effects on the formation and growth of diamond crystal grains.

1. Pressure Effect Experiments

Pressure is one of the important variables affecting the characteristics of substance. The pressure effects are of much greater importance to such a process as the graphite-to-diamond transformation which is accompanied by diminution of volume and substantial changes in structure. Comparative experiments have been performed at preassigned temperatures to observe the dependence of diamond crystal grain formation and growth a different pressure values applied. The experimental results show that as pressure increases within a certain range the number of grains formed and the growth rate gradually increases, and the crystals produced range from regular to irregular shapes, from lower to higher defect levels, and from light to dark colors. Figs. 1, 2 and 3 show the dependence of the crystallization amount, the coarse grain percentage and the compressive strength, respectively, on pressure.