Hisao Kanda

Synthesis of diamond from graphite-carbonate system under very high temperature and pressure

Abstract Although transition metals such as Fe, Co, Ni and their alloys have been used as diamond-producing solvent-catalysts under very high temperature and pressure, non-metallic compounds such as carbonates and oxides have also been claimed in the patents as the catalysts. In the present study, to make clear the catalytic effect of carbonates on the formation of diamond, high pressure experiments were carried out in the mixture of graphite and the carbonates of Li, Na, Mg, Ca and Sr. Diamond could reproducibly be synthesized from graphite in the presence of these carbonates at high pressure and temperature of 7.7 GPa and 2150°C. Although starting graphite was completely transformed to diamond in the presence of the carbonates, no transformation to diamond could be detected from graphite only at the same pressure and temperature condition. Therefore, it can be concluded that the carbonates have strong solvent-catalytic effect on the transformation of graphite to diamond.

Colour changes produced in natural brown diamonds by high-pressure, high-temperature treatment

Abstract Absorption and luminescence spectra have been measured for natural brown diamonds before and after high-pressure, high-temperature (HPHT) treatment at 1700–1800°C, and after HPHT treatment at 2025°C. The reduction of the brown colour noted in diamonds with low nitrogen concentration is attributed to the annealing of plastic deformation. In nitrogen-containing diamonds the vacancies released during this annealing are trapped to form N–V–N centres. In addition, the photoluminescence band with a zero-phonon line (ZPL) at 2.526 eV (490.7 nm), characteristic of decorated slip traces, is reduced in intensity during this process, confirming that changes are taking place at the slip traces which have resulted from the plastic deformation. At the lower annealing temperatures the N–V–N centres are in the neutral charge state, giving rise to the H3 absorption band, with a ZPL at 2.463 eV (503.2 nm) and resulting in a yellow colour. At the higher annealing temperature some nitrogen aggregates decompose, producing single substitutional nitrogen which is an electrical donor. Consequently some N–V–N centres are in the negative charge state, and give rise to the H2 absorption band with a ZPL at 1.257 eV (986.9 nm). The combination of absorption in the H2 and H3 bands gives the diamonds a yellow-green or green colour. Annealing at the highest temperature, at pressures very close to the diamond/graphite transition, seems to be effective at reducing the non-radiative recombination channels in the diamonds, and, in white light, many specimens exhibit bright green luminescence, produced by absorbed energy being re-emitted in the H3 band. To the eye these processed diamonds appear similar to the very rare, naturally-occurring, ‘green transmitters’. However, because the latter have resulted from much lower geological temperatures, acting over much longer periods of time, they contain negligible concentrations of single substitutional nitrogen and consequently have low H2 absorption.

Lightly phosphorus-doped homoepitaxial diamond films grown by chemical vapor deposition

Lightly phosphorus-doped {111} homoepitaxial diamond films have been grown by microwave plasma-assisted chemical vapor deposition under optimized growth conditions. The Phosphorus concentration in the film can be controlled at a low doping level of the order of 1016cm−3. N-type conductivity of the films with phosphorus concentrations above 1×1016cm−3 is reproducibly confirmed by Hall-effect measurements in the temperature range from 300to873K. The highest value of the Hall mobility at room temperature is 660cm2∕Vs obtained for a film with a phosphorus concentration of 7×1016cm−3.

Diffusion-controlled kinetics of carbon nanotube forest growth by chemical vapor deposition

A detailed theoretical study of carbon nanotube (NT) forest growth by chemical vapor deposition is given, including (i) ballistic mode of carbon species impingement into the NT surface, (ii) the carbon diffusion over NT surface and through the metal nanoparticle, and (iii) the temperature drop at the NT tip occurring with increase in NT length. For typical NT forest growth parameters the ballistic flux of carbon species impinging into the NT surface decays quasiexponentially within several microns from the top. A variety of feasible growth modes, ranging from linear to exponential versus time, is predicted agreeing well with reported experiments. The presence of a metal nanoparticle is shown to shift NT growth from being surface diffusion controlled to being controlled by bulk diffusion through the nanoparticle. For typical growth conditions the growth rate is shown to be controlled simultaneously by surface diffusion over NT surface and bulk diffusion of carbon through metal nanoparticle. However, even i...

Phosphorus: An Elemental Catalyst for Diamond Synthesis and Growth

As diamond-producing catalysts, 12 transition metals such as iron, cobalt, and nickel were first reported by General Electric researchers more than 30 years ago. Since then, no additional elemental catalyst has been reported. An investigation of the catalytic action of group V elements is of great interest from the viewpoint of producing an n-type semiconducting diamond crystal. In the present study, diamond was synthesized from graphite in the presence of elemental phosphorus at high pressure and temperature (7.7 gigapascals and 1800�C). Furthermore, single-crystal diamond was grown on a diamond seed crystal.

An annealing study of nickel point defects in high‐pressure synthetic diamond

Results of an annealing study, carried out in the temperature range 1500–1900 °C, of nickel‐related optical centers in high‐pressure synthetic diamond are presented. It is established that the well‐known 1.883 and 2.51 eV systems anneal out during the temperature regime of nitrogen aggregation and the concurrent growth of an array of structure, which extends throughout the visible region of the absorption spectrum, and gives the previously bright‐yellow‐colored diamonds a rich golden‐yellow color, is observed. By carrying out the annealing sequence on diamonds grown using various solvent catalysts a correlation is found between preanneal 1.883 eV absorption and maximized absorption of some of the annealed structure. From the results it is proposed that centers associated with nickel and nitrogen are produced and their possible natures are speculated on. It is found that the defect‐induced one‐phonon spectra of the diamonds examined may be satisfactorily decomposed into three components. To account for certain changes in the infrared spectra during the annealing sequences, and using previously reported results, it is proposed that one of these components may result from nitrogen in a positive charge state.

Growth mechanism of carbon nanotube forests by chemical vapor deposition

Analysis of kinetics processes involved in carbon nanotube (NT) forest growth during chemical vapor deposition suggests that: (i) carbon species are unable to penetrate to the forest bottom whenever the mean free path in gas is much larger than the typical distance between NTs; instead they collide with NT surfaces, chemisorbing within the top few microns, diffuse along the surface, and feed the growth at nanotube tips, (ii) wherever a catalyst nanoparticle is present, at the substrate or on the nanotube tip, in the postnucleation stage its role in feeding NT growth by C dissolution and bulk diffusion is negligibly small in comparison with the surface diffusion of C species adsorbing on the lateral surface of nanotubes, and (iii) bulk diffusion of C through the catalyst nanoparticle, defining the characteristic times of C penetration to nanoparticle base and surface saturation with C, is shown to play a major role in selection of the initial mode of nanotube nucleation and growth.

New catalysts for diamond growth under high pressure and high temperature

Diamond has been found to grow from copper, zinc, and germanium when temperatures and pressures in excess of those usually used for growth via conventional catalysts are used. Around their melting temperatures these metals are inert with respect to graphite. However, under the conditions used in this study, namely temperatures of 1600 °C and pressures of 6 GPa, they exhibit catalytic action. The conventional catalysts, which were first discovered by General Electric, act as catalysts immediately after melting in the presence of graphite, and this distinguishes them from the catalysts used in this study which should therefore be placed in a different category. A new model of diamond growth is proposed in order to explain the behavior of these new catalysts.

Spectroscopic study of cobalt‐related optical centers in synthetic diamond

This article presents evidence that cobalt forms a series of optically active defect centers in diamond grown by high‐temperature, high‐pressure synthesis. Photoluminescence (PL) studies reveal that the newly observed vibronic systems with zero‐phonon energies at 1.989, 2.135, 2.207, 2.277, 2.367, and 2.590 eV appear only in samples grown using a cobalt‐containing solvent–catalyst. Results of an annealing study, carried out in the temperature range 1500 to 1800 °C, establish that many of the new bands appear during the temperature regime of nitrogen aggregation. It is therefore proposed that nitrogen forms complexes with cobalt to produce optically active centers, in a manner analogous to that of nickel point defects in diamond. Detailed radiative decay time measurements and temperature dependence measurements show that all but one of the bands which are here associated with nitrogen–cobalt complexes have long radiative decay times (∼100 μs), and this again is a characteristic of the PL centers arising fr...

Crystallization of diamond from a silicate melt of kimberlite composition in high-pressure and high-temperature experiments

In high-pressure and high-temperature experiments (1800- 2200 °C and 7.0-7.7 GPa), diamond crystallized and grew in a volatile-rich silicate melt of kimberlite composition. This diamond has well- developed {111} faces, and its morphologic characteristics resemble those of natural diamond but differ from those of synthetic diamond grown from metallic solvent-catalysts. The kimberlite melt has a strong solvent-catalytic effect on diamond formation, supporting the view that some natural diamonds crystallized from volatile-rich melts in the upper mantle.