Black diamond powder: On the thermal oxidation and surface graphitization

Abstract

Oxidation and graphitization are among the most interesting thermochemical characteristics of diamond to be explored. This is due to the influence of these phenomena on either degradation or promotion of the critical properties of diamonds. This article investigates the oxidation and graphitization of diamond powder using thermal analysis combined with X-ray diffraction, Raman spectroscopy, electron microscopy and bulk-electrical conductivity measurements. To this end, the thermokinetic characteristics of micrometre-sized diamond crystals with dominant (1 1 1) facet are investigated at various heating rates from 1 to 100 °C min−1, and the oxidation activation energy is determined to be 135.7 kJ mol−1. In contrast with thermal oxidation, the facile surface graphitization of diamond crystals is also studied as a possible way to promote the electrical conductivity of diamonds without affecting their bulk properties. This surface phase transition involves minimum atomic diffusion, with the crystalline relationship between the interfacial phases described as (0 0 2) graphite || (1 1 1) diamond. The surface-graphitized diamonds, which are black in appearance, exhibit a high value of electrical conductivity of 2100 S m−1. The effect of rapid heating is also confirmed using nanodiamond precursor. This article suggests an ultra-fast and economical method for surface graphitization of diamond crystals, enhancing their electrical performance.