High pressure–high temperature growth of diamond crystals using split sphere apparatus

Abstract

An overview of the application of crystal growth fundamentals in the high pressure–high temperature production of diamond by solvent/catalyst technique is presented. The process, also called temperature gradient process, makes use of a molten catalyst to dissolve carbon from a source (graphite or diamond powder) and transport the dissolved carbon to a growth site where they precipitate on a diamond seed. The pressure and temperature requirements for the process are generally around 5.0–6.5 GPa and 1300–1700 °C, depending on the chemistry of the solvent used and the desired crystal geometry. In spite of major progress in the science and technology of diamond growth, large scale commercial production of diamonds single crystals for jewelry or electronic applications has not been feasible until recently. This has been mainly due to the substantial cost associated with the presses needed, and the difficulties in controlling the growth parameters and catalyst chemistry. The recent developments in the commercial production of diamond single crystals utilizing the Split Sphere pressurization apparatus are discussed.

Introduction

Diamond possesses many unique physical and chemical properties. It is the hardest, least compressible and stiffest substance. It also has high dispersion, reflectance and index of dispersion of any transparent materials. In addition, diamond is chemically inert to most acids and alkalis. While diamond has remarkable thermal conductivity, it is also an excellent electrical insulator. Moreover, with a band gap of 5.49 eí, diamond has better semiconducting properties than silicon for many electronic applications, particularly for high temperature and high power electronics. These unique properties make diamond material of choice for a variety of applications from Jewelry to surgical blades, grinding and polishing to wire drawing dies, and electronic heat sinks to infrared windows. The demand for producing man-made diamonds with tailored properties has been increasing throughout the years. The main challenge in wider production, however, remains the high cost of manufacturing, particularly for large monocrystals.

Since it was first developed by Bundy et al. at GE in 1955 [1], high pressure–high temperature (HPHT) technology via a temperature gradient process has been widely used to produce type Ib and IIa an IIb single crystals. The pressure and temperature required for the process, generally around 5–6.5 GPa and 1300–1700 °C, depend on the chemistry of the solvent used and the desired crystal geometry. The temperature gradient process has been used to grow type Ib and IIa an IIb single crystals. The most commonly used machines for producing diamond have been Belt, cubic, tetrahedral or toroidal machines [2], [3], [4]. The catalysts used for growing monocrystals involve alloys of Fe, Ni, Co and Mn–C, with other elements such as Ti, Al, B, Cu, and Ge added to getter nitrogen impurities for producing colorless or blue diamonds [4], [5], [6], [7], [8], [9], [10]. Non-metallic solvents have also been used for a limited extent as catalysts [11], [12]. However, the growth rates and crystal sizes seem to be more limited. The growth rates using metallic catalysts are in the range of 2–15 mg/h, with the largest single crystal grown reported as 34.80 carats [5].

Regardless of the composition of the catalyst used, the most critical requirement for producing quality crystals is the precise control of the temperature and pressure through the entire growth process. Moreover, the temperature difference between the source materials and growth location must also be controlled properly to allow for an optimum growth rate. Excessive rates cause entrapment of inclusions and morphological instability at the solid–liquid interface. Faster growth rates also cause strains and excessive point or line defects.