Non-diamond Material Research Articles

Diamond Nucleation from an Activated Vapor Phase

Nucleation of thermodynamically metastable diamond in both bulk of an activated vapor phase and at the surface of nondiamond materials is considered. We discuss the key factors leading to a reduction in the surface energy of the diamond phase being formed and in the activation energy associated with incorporation of new particles into diamond nuclei being formed. We remark on the importance of experimentally established patterns of diamond nucleation, in particular, for the synthesis of submicron-thick nanocrystalline diamond films.

Зарождение алмаза из активируемой газовой фазы

The thermodynamically metastable diamond nucleation in the volume of activated vapor phase and on nondiamond materials surfaces, as well, have been considered. Basic factors influencing for emerging diamond phase surface energy reducing and for the activation energy for addition to a building blocks nucleus lowering are considered. The importance of experimentally revealed regularities for instance for nanocrystalline submicrometerr diamond film synthesis was stressed.

Engineering Ultra-Low Work Function of Graphene.

Low work function materials are critical for energy conversion and electron emission applications. Here, we demonstrate for the first time that an ultralow work function graphene is achieved by combining electrostatic gating with a Cs/O surface coating. A simple device is built from large-area monolayer graphene grown by chemical vapor deposition, transferred onto 20 nm HfO2 on Si, enabling high electric fields capacitive charge accumulation in the graphene. We first observed over 0.7 eV work function change due to electrostatic gating as measured by scanning Kelvin probe force microscopy and confirmed by conductivity measurements. The deposition of Cs/O further reduced the work function, as measured by photoemission in an ultrahigh vacuum environment, which reaches nearly 1 eV, the lowest reported to date for a conductive, nondiamond material.

Initial damage processes for diamond film exposure to hydrogen plasma

Diamond is considered to be a possible alternative to other carbon based materials as a plasma facing material in nuclear fusion devices due to its high thermal conductivity and resistance to chemical erosion. In this work CVD diamond films were exposed to hydrogen plasma in the MAGnetized Plasma Interaction Experiment (MAGPIE): a linear plasma device at the Australian National University which simulates plasma conditions relevant to nuclear fusion. Various negative sample stage biases of magnitude less than 500V were applied to control the energies of impinging ions. Characterisation results from SEM, Raman spectroscopy and ERDA are presented. No measureable quantity of hydrogen retention was observed, this is either due to no incorporation of hydrogen into the diamond structure or due to initial incorporation as a hydrocarbon followed by subsequent etching back into the plasma. A model is presented for the initial stages of diamond erosion in fusion relevant hydrogen plasma that involves chemical erosion of non-diamond material from the surface by hydrogen radicals and damage to the subsurface region from energetic hydrogen ions. These results show that the initial damage processes in this plasma regime are comparable to previous studies of the fundamental processes as reported for less extreme plasma such as in the development of diamond films.

Experimental study of electric discharge treatment of nanodiamond particles in water

A novel type of high voltage pulsed electric discharge in water flow in a Venturi tube is proposed. The influence of the novel discharge on sizes and properties of nanodiamond particles has been studied. Experiments were carried out in water media with purified detonation nanodiamonds made impure by non-diamond carbon material. The ability of high voltage pulsed electric discharge in water to modify nanoparticle conglomerates in water solution and to relieve spherically shaped nanodiamond conglomerates from the initial mixture with non-diamond material can be seen. Prolonged treatment of the suspension made it possible to relieve primary nanodiamond crystals from conglomerates. Formation of ordered and unordered structures from primary (3–5 nm) nanodiamond crystals has been observed. Study of the electric discharge in water was carried out at the pressure region from atmospheric down to 0.02 atm to reproduce conditions which are typical for the discharge in the Venturi tube in liquid flow and different gap lengths. Two ‘types’ of discharge behavior depending on the geometry of the discharge system and other external parameters have been observed. Characteristics that are critical for understanding the behavior of the discharge in the Venturi tube in water flow have been investigated.

Raman spectroscopy of nanocrystalline diamond: Anab initioapproach

The use of Raman spectroscopy to detect nano-sized diamond crystals is controversial; the origins of peaks at $\ensuremath{\sim}1150\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ in chemical vapor deposition nanodiamond films and $\ensuremath{\sim}500\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ in nanodiamond particles, which have both been suggested as evidence for nanophase material, remain uncertain. Many studies have produced evidence showing that the $\ensuremath{\sim}1150\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ peak is in fact due to polyacetylenelike structures at grain boundaries and interfaces, but little work has been done to confirm the assignment of the $\ensuremath{\sim}500\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ peak. In this paper we approach the problem from the molecular level, using Hartree-Fock theory to calculate the Raman spectra of diamond hydrocarbons, and observe the variation of the spectra with molecular size. Molecules with ${T}_{\mathrm{d}}$ symmetry are studied, varying in size from adamantane to ${\mathrm{C}}_{84}{\mathrm{H}}_{64}$, an octahedral $1\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$-sized diamond crystallite. For comparison with nanodiamond thin films, the mass of the terminal hydrogen atoms were artificially increased to $100\phantom{\rule{0.3em}{0ex}}\mathrm{amu}$, approximating the effects of matrix isolation. The calculated spectra are discussed in terms of the signals commonly observed in the Raman spectra of nanocrystalline diamond samples. This study finds no evidence for Raman active vibrations of diamond nanocrystals at either $\ensuremath{\sim}1150\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ or $\ensuremath{\sim}500\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, whether hydrogen terminated or confined in a matrix. Further, it appears that the only signals produced by a nanodiamond crystal are the broadened zone-center $(1332\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1})$ mode and low frequency $(<100\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1})$ deformations/Lamb-type vibrations. This suggests any other peaks observed in the Raman spectra of nanocrystalline diamond are due to defects, surface structures, amorphous material, or any other nondiamond material in the sample, and should not be taken as definitive evidence of nanocrystalline diamond.

Electronic properties of plasma hydrogenated diamond surfaces: A microscopic study

The quality of hydrogen termination of undoped diamond surfaces is investigated on a microscopic level by applying atomic force microscopy, Kelvin force microscopy, and scanning electron microscopy. The results show that high temperature hydrogenation in a microwave plasma can lead to microscopic inhomogeneities at the surface, both in morphology and electron emission properties. These inhomogeneities are due to a very thin (1 nm) layer of non-diamond material on the surface, which appears to be inherently generated by the hydrogen plasma process. By scanning in contact AFM an underlying diamond surface can be exposed, which retains properties typical for hydrogenated diamond and exhibits significantly improved electron emission properties.

Report on a second round robin measurement of the thermal conductivity of CVD diamond

Abstract The results of a round robin series of measurements of the thermal conductivity and thermal diffusivity of chemical-vapor-deposited (CVD) diamond are reported. The present round robin (RR2) is an outgrowth of an earlier series (RR1) aimed at understanding which measurement techniques are best suited to measuring the high conductivity of CVD diamond. RR2 includes diamond specimens of higher homogeneity, non-diamond specimens of lower conductivity as controls, and measurements by 14 laboratories using five techniques, with all techniques but one used by more than one laboratory. The data are converted to thermal conductivity at 25 °C to facilitate comparisons among specimens and laboratories. The statistical analysis excludes outliers using Chauvenet's criterion, resulting in data from typically two or three laboratories being excluded for each specimen. The analysis arrives at mean values in the range 1300–2000 W m −1 K −1 for the diamond samples, as expected because of the conditions of preparation, with uncertainties in the range 1.5–4%. For the non-diamond materials, SiC and AlN, the results are 268 W m −1 K −1 ±2.2% and 178 W m −1 K −1 ±2.5%, respectively. Comparing techniques, the d.c.-heated bar is found to be the most accurate, typically ±5% or better. Under favorable conditions, Angstrom's thermal wave method can apparently yield relative uncertainties of ±5–10%, and the mirage effect ±5–15%. Too few laboratories used the transient thermal grating and laser flash methods to make general comments on their accuracy, but one expects a comparable accuracy if used on specimens that are fine-grained and thermally isotropic. Deviations, sometimes large, from these optimal accuracies are examined, and it is suggested that certain experimental details are important for achieving accuracy. It is also found that the estimates of experimental uncertainty provided by most laboratories seriously underestimate the actual deviations of their data from the mean conductivity.

プラズマジェットＣＶＤダイヤモンド膜形成におよぼす非ダイヤモンド質の影響

プラズマジェットＣＶＤダイヤモンド膜形成におよぼす非ダイヤモンド質の影響

プラズマジェットＣＶＤダイヤモンド膜形成におよぼすＣＨ４濃度の影響

Diamond films were prepared on SiC substrates with four kinds of CH4-H2 reactant gases (3, 5, 7 and 10% CH4 concentration) using plasma jet CVD. The growth rate of films was a maximum of 3μm/min at 5% CH4 and was approximated to 2μm/min at other CH4 concentrations. Properties of the deposited films were investigated by SEY, TEX, micro Vickers hardness test, XRD analysis and Raman spectra analysis. The films deposited at lower CHI concentration had an agglomerate structure of idiomorphic grains and the morphology of diamond films changed to a predominating {100}face from a {111}face with an increase of CH4 concentration from 3% to 5%. The films deposited at high CH4 concentration were composed of fine diamond crystals and non-diamond materials. The best Raman spectrum indicating the diamond films of crystalline perfection and of the lowest density of non-diamond materials was obtained at 5% CH4 with the maximum growth of {100} face.

Study of a platelet-free infilling of a crack in natural diamond: evidence for a late growth event

In the cores of coated diamonds, healed cracks may be detected electron-microscopically. At these, the parted crack surfaces have reunited coherently over a substantial fraction of the crack surface, which now contains dislocations and sometimes trapped non-diamond material as well. A healed crack has been discovered that is accompanied on either side by an extremely sharply-defined zone of absence of the electron-microscopically observable {100} platelet defects that populated the rest of the diamond core. (Such platelets are characteristic of nitrogen-impurity-containing natural diamonds that have undergone long storage at high pressure and temperature.) The maximum overall width of the platelet-free zone along the observable trajectory of the crack was about 0.5 μm. It narrowed as the crack was followed into the core. Experiments are described that led to interpretation of the platelet-free zone as consisting of a sheet of coherent diamond regrowth upon incompletely closed crack surfaces, filling the gap between them, and representing a late event in the diamond's growth history.

Friction and wear properties of diamonds and diamond coatings

Abstract The recent development of chemical vapor deposition techniques for diamond growth enables bearings to be designed which exploit diamond's low friction and extreme resistance to wear. However, currently produced diamond coatings differ from natural diamond surfaces in that they are polycrystalline and faceted, and often contain appreciable amounts of non-diamond material ( i.e. graphitic or amorphous carbon). Roughness, in particular, influences the friction and wear properties; rough coatings severely abrade softer materials, and can even wear natural diamond sliders. Nevertheless, the best available coatings exhibit friction coefficients as low as those of natural diamond and are highly resistant to wear. This paper reviews the tribological properties of natural diamond, and compares them with those of chemical vapor deposited diamond coatings. Emphasis is placed on the roles played by roughness and material transfer in controlling frictional behavior.

Factors influencing the adhesion of diamond coatings on cutting tools

CVD diamond coatings can greatly extend the tool life of cemented carbide cutting tools when machining abrasive non-ferrous materials. However, reliable tool performance is difficult to obtain due to the problem of controlling coating adhesion. In this work, diamond coatings have been deposited on cemented carbide tool inserts with variation of substrate chemical composition and surface pre-treatment. The coatings have been characterized with respect to nucleation density, crystallinity, non-diamond material content and residual stresses. The adhesion and wear resistance of the coatings were investigated by machining tests in Al-18%Si alloy. The diamond deposition process and the resulting coating adhesion are both strongly influenced by the surface characteristics of the substrate surface. Formation of voids and non-diamond material at the coating-substrate interface in combination with high compressive stresses in the coating are believed to be the main reasons for poor adhesion.

Homogeneous nucleation of diamond powder in the gas phase

Homogeneous nucleation of diamond powder is reported. The experiments were performed in a low-pressure microwave-plasma reactor. The deposits were collected downstream of the reaction zone and subjected to wet oxidation to remove nondiamond carbons. The residues were analyzed by optical and electron microscopy, electron diffraction, and Raman spectroscopy. A variety of hydrocarbons diluted in argon, hydrogen, or oxygen gas mixtures were tested. In most cases only nondiamond materials, like graphite and carbyne, were obtained. Homogeneous nucleation of diamond was clearly observed in dichloromethane- and trichloroethylene-oxygen mixtures. The particles formed had crystalline shapes, mostly hexagonal. The largest particles were about 0.2 μm, although most of the particles were on the order of 50 nm in diameter. The powder was identified to be a mixture of polytypes of diamond.