Size-Dependent Phase Transition of Diamond to Graphite at High Pressures

Abstract

Size-dependent phase transition of nanosize diamond particles (NDPs) to graphite was found in the course of studies of high pressure high temperature (HPHT) induced transformations of NDPs at 8 GPa and in the temperature range of 873−1623 K. Analysis of the products of HPHT treatment of powder NDPs (with the mean size of 4.5 nm) by X-ray diffraction (XRD), scanning (SEM) and transmission electron (TEM) microscopies, and Raman spectroscopy have shown that at 8 GPa the increase of the treatment temperature results in a gradual enlargement of NDPs. According to XRD data, the temperature increase from 873 to 1473 K at the treatment time of 60 s gives rise to NDP mean size enlargement from initial 4.5 to 8.8 nm. However, the largest NDPs can reach the sizes of 13.5−14.0 nm at 1473 K, as measured directly from TEM images. At 1623 K, size-dependent phase transition of diamond to graphite has been observed. By applying the Arrenius equation to the particle mean size versus treatment temperature plot, activation energy for the diamond solid phase growth at 8 GPa was calculated to be 112 ± 8 kJ/mol. The critical size of NDP corresponding to the point of phase transition at 8 GPa and 1623 K was estimated to be ∼18 nm. The likely effect of the size-dependent diamond-to-graphite phase transition on the process of diamond synthesis and, in particular, the problem of formation of critical seeds for crystal growth of macroscale diamond are discussed in the paper.

Characterization of the High-Pressure Structural Transition and Thermodynamic Properties in Sodium Chloride: A Computational Investigation on the Basis of the Density Functional Theory

“…With the rapid development of high-pressure techniques, there has been considerable interest in recent years in the investigations of the structural phase transition and high-pressure behavior in material because the knowledge of physical and chemical properties of material at high pressure is important for a variety of scientific and technological applications. − For instance, the optical properties, the thermodynamic properties, and the equation of state (EOS) of mineral material at high pressure is essential in the investigations of the interior of the earth in geophysics and for the investigations of the evolutions of stars in astrophysics. NaCl is one of the most widely used nonmetallic mineral materials.…”

Luminescent nanodiamonds for biomedical applications

“…The final product is always rich in graphite and other nondiamond carbon forms, which then require extensive mechanical and chemical processing to remove. The advantage of DND is the uniformly small size (4-5 nm) of the diamond grains, which is comparable to conventional fluorophores and suitable for biological applications (Danilenko 2004;Davydov et al 2007).…”

Direct Observation of Curved Surface Enhanced Disordering in Ag2S Nanoparticles

“…Being one of the most fundamental physicochemical phenomena in nature, first order solid–solid phase transition have been widely observed and extensively studied, − yet a detailed understanding of its microscopic mechanism is still evolving. − For a homogeneous system, once the nucleation barrier is crossed, the system will eventually evolve to a daughter phase that corresponds to the global minimization of the free energy . However, for a system with free surfaces, the first order phase transition becomes complicated and the surface contribution must be seriously considered.…”