Karl E. Spear

Growth mechanism of vapor-deposited diamond

An elementary-reaction mechanism of diamond growth by a vapor deposition process is proposed. The central postulate is that the main monomer growth species is acetylene. The mechanism basically consists of two alternating steps: surface activation by H abstraction of a hydrogen atom from a surface carbon and the addition of one or two acetylene molecules. During the addition reaction cycle a number of solid C–C bonds is formed and hydrogen atoms migrate from a lower to an upper surface layer. The mechanism is in general agreement with the macroscopic views of the Russian researchers and is consistent with the numerous experimental observations reported in the literature.

Crystallization of diamond crystals and films by microwave assisted CVD (Part II)

Results of experiments on crystallization of diamond micro single crystals (∼10μm) and polycrystalline diamond films by microwave plasma assisted chemical vapor deposition are presented. Discussed are problems related to growth mechanisms from CH4H2 plasma: nucleation on different substrates (diamond, graphite, Si, βSiC, SiO2 and Ni), catalytic growth, Raman scattering from deposits and planar defects of diamond structure. Optimal conditions for diamond growth were found in relation to maximum growth rate and exceptional surface phenomena appearing at temperatures close to 1000°C.

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.

From diamond-like carbon to diamond coatings

Abstract Carbon films deposited by pyrolytic and plasma-activated deposition from hydrocarbon-containing gases encompass a virtual infinity of compositions (hydrogen concentrations) and structures (from amorphous to single crystal and with variable amounts of sp 1 , sp 2 and sp 3 bonding). Coatings which have a high degree of sp 3 bonding generally have properties, especially hardness, close to those of single- crystal diamond and are often referred to as diamond-like carbon. Recently, large grain size diamond crystals and continuous diamond coatings have been prepared by plasma chemical vapor deposition methods. Although such materials are different from the diamond-like carbon class of materials, there is clearly a continuum of materials which is expected to lead to vagueness and confusion in nomenclature. In this paper, such issues are dealt with and a working definition of “diamond” coatings is offered.

Diamond polytypes and their vibrational spectra

A series of diamond polytype structures are described and their IR and Raman vibrational modes predicted. The diamond polytypes are analogous to the well-known silicon carbide polytypes. The intermediate 6H diamond polytype was recently identified by single crystal electron diffraction of vapor precipitated diamond powder. In addition, end member polytypes of 3C (cubic diamond) and 2H diamond (hexagonal lonsdaleite) have been previously established, and polytypes such as 4H, 8H, 15R, and 21R diamond are predicted, but may be difficult to isolate and identify. The various diamond polytype structures differ only in the stacking sequences of identical puckered hexagonal carbon layers. These identical carbon layers lie parallel to the cubic 3C {111} and the hexagonal 2H {001} planes. A new method for uniquely labeling the structural layers in the polytype stacking sequences is presented. Factor group analysis was used to determine the IR and Raman selection rules for five diamond polytypes with structures intermediate between those of end members diamond and lonsdaleite. Brillouin zone folding techniques were used to determine band positions, in analogy with analyses of SiC polytypes discussed in the literature. The results predict that (i) all diamond polytypes are Raman active, (ii) limiting polytypes 3C and 2H are not IR active, and (iii) polytypes 4H, 6H, 8H, 15R, and 21R have IR active modes.

Induced nucleation of diamond powder

The effects of heteroatom addition on the nucleation of solid carbon in a low‐pressure plasma reactor were investigated. Silane or diborane were added to acetylene mixed in hydrogen or argon. Oxygen was added to some of the diborane containing gas mixtures. Silane containing mixtures resulted in powder comprised of weakly bonded amorphous hydrogenated carbon‐silicon material. The addition of diborane resulted in substantial production of diamond particles, 5 to 450 nm in diameter, under the conditions that show no diamond formation without diborane present. The observed yield of the oxidation‐resistant powder produced in boron‐containing mixtures reached 1.3 mg/h with the linear growth rates of diamond particles on the order of 102–104 μm/h. Implication of these results to interstellar dust formation is discussed.

Thermodynamic Analysis of Silica Refractory Corrosion in Glass-Melting Furnaces

Corrosion of refractory silica brick used to line the roof or crown of many glass-melting furnaces is a serious problem in furnaces using oxygen-fuel rather than air-fuel mixtures. In this work, we report equilibrium calculations that support a corrosion mechanism in which alkali hydroxide gas (NaOH or KOH), produced by reaction of water vapor in the combustion gas with the molten glass reacts with the silica brick in the furnace crown to produce an alkali silicate liquid with a composition that depends on the temperature of the crown. Our reported calculations predict the variable-composition liquid-solution corrosion product phase as a function of key furnace variables. Critical thermodynamic data needed for the liquid corrosion product were generated using a modified associate species solution model and critical analysis of thermochemical information found in the literature for the Na 2 O-SiO 2 and K 2 O-SiO 2 systems. Excellent agreement with reported Na 2 O-SiO 2 and K 2 O-SiO 2 phase diagrams and with experimentally measured activities for Na 2 O and K 2 O is achieved. The results of our current calculations are for temperatures between 1273 and 1973 K (1000-1700°C) under either air-fired or oxy-fired conditions, and are used to define a critical temperature, above which corrosion is not expected to occur for a given NaOH(g) or KOH(g) partial pressure.

Synthesis of diamond powder in acetylene oxygen plasma

Diamond particles 10–500 nm in diameter were produced by microwave‐assisted combustion of acetylene in oxygen. Both premixed and diffusion flame configurations were investigated. A mixture of cubic and hexagonal polytypes of diamond were identified. Larger particle sizes were observed at lower reactor pressure and higher C to O atomic ratios. C to O atomic ratios between 0.83 and 1.0 produced crystalline diamond powder while other ratios produced graphite, soot, and amorphous carbon phases. Diamond formation was not observed when reaction pressures were above 150 Torr.

Chemical bonding in AlB2-type borides☆

Abstract The chemical bonding in AlB2-type metal diboride phases is considered from a crystal chemical viewpoint. The ability of metals to deform from a spherical shape appears to be very important in explaining why the diboride is formed by such a wide variety of metals. Crystal chemical parameters are used to compare relative bond strengths.

Morphology of chemical vapor deposited titanium diboride

Titanium diboride has been chemical vapor deposited from TiCl4, BCl3, and H3 gases on a heated graphite substrate. The morphologies of the deposits were correlated with the deposition parameters of substrate temperature, total pressure, B: Ti atomic ratio, Cl: H atomic ratio, and total flow rate. The diboride deposits exhibited a variety of morphologies which include coherent coatings with nodular or faceted surfaces, plate-like crystallites, dendritic crystallites, and irregularly shaped, faceted crystallites. The deposit morphology was found to be most sensitive to deposition temperature, B: Ti atomic ratio, and Cl: H atomic ratio. Variations in both the total pressure and total flow rate did not significantly affect the diboride morphology. All the TiB2 deposits except those produced under a large excess of H2 had a preferred orientation such that the hexagonal c-direction was parallel to the substrate surface.