Some Observations of Graphite-Diamond Transformation by Electro-Thermal Analysis

Abstract

The transformation process of graphite to diamond without any intensive addition of metal was observed dynamically by following the change in the applied voltage-current relation directly passed through graphite sample under pressure (by so-called the electro-thermal analysis).
Spectroscopic graphite bar and graphite bar for electrode of 80 to 250 mg were exposed to the static pressure of 80 to 140 kb (14 GPa) at 2100 to 2900 K.
By applying the a. c. voltage stepwise, the current throu gh the graphite sample increased stepwise. When the voltage was attained to about 1.8 to 1.9 V, the current drastically decreased, indicating the conversion of graphite to diamond. It was found that the graphite sample must be machined into the optimum size in order to avoid the reconversion of diamond to graphite by the superheating of diamond. Under the pressure below 120 kb, the induction period prior to the conversion to diamond and the, apparent crystallization time increased with the decrease of the applied pressure. The pressure of 120 kb was essential to convert graprhite to diamond without any induction period. It was found that the phenomena accompanied by the diamond formation could be observed reproducibly by controlling the dimension and the charge density of the starting graphite sample. with the same graphitizability. The crystal morphology of diamond formed depends remarkably on the applied pressure. Above 120 kb, diamond was crystallized in dendritic form indicating the rapid nucleation of diamond. Below 120 kb, the crystal growth becomes dominant to develop the (111) face.
The analysis of the relation of the applied voltage to current, that is to say, the electrothermal analysis was found useful for the efficient diamond production under a controlled cOndition, since the behavior of the starting graphite sample can be followed dynamically in the process of the transformation.