Duk Yong Yoon

Charged cluster model in the low pressure synthesis of diamond

Gas activation makes possible the synthesis of diamond without codeposition of graphite in the chemical vapor deposition (CVD) diamond process, though its exact role is not clear. Charged carbon nuclei clusters, which are expected to form in the gas phase, are suggested to be responsible for the formation of diamond. When the carbon cluster is sufficiently small, the capillary pressure built up inside the cluster can be high enough to make diamond more stable than graphite. The number of carbon atoms in the cluster that reverses the stability between diamond and graphite increases sensitively with increasing surface energy ratio of graphite to diamond. The gas activation process produces charges such as electrons and ions, which are energetically strong heterogeneous nucleation sites for the supersaturated carbon vapor, leading to the formation of charged nuclei clusters. Once the carbon clusters are charged, the surface energy of diamond can be reduced by electrocapillarity while that of graphite cannot because diamond is dielectric and graphite is conducting. The evolution of graphitic soot and diamond on the iron and the silicon substrates, respectively, can be approached based on the charged cluster model. The charged cluster model is further supported by the result that the insulating quartz block beneath the iron substrate enhances the initiation of diamond on soot.

Effects of donor concentration and oxygen partial pressure on interface morphology and grain growth behavior in SrTiO3

Abstract Change in interface morphology with ionic vacancy concentration and the correlation between interface structure and grain growth behavior in strontium titanate (SrTiO 3 ) have been investigated using SrTiO 3 single crystals and powder compacts. Under experimental conditions where SrTiO 3 contains a negligible amount of ionic vacancies, the shape of the single crystal embedded in matrix grains was well faceted, showing a strong anisotropy in interfacial energy. However, as strontium or oxygen vacancies increased with the addition of an Nb 2 O 5 donor dopant or reduction of oxygen partial pressure, the faceted shape changed to a smoothly curved rough one indicating that an interface roughening transition occurred and, as a result, the anisotropy in interfacial energy was considerably reduced. Grain growth behavior was also strongly dependent on the interface structures; while normal grain growth occurred when the interfaces were rough, abnormal grain growth behavior was observed in the samples with faceted interfaces. It appears, therefore, that the ionic vacancies in SrTiO 3 can cause the interface roughening transition and change the resultant grain growth behavior.

Chemical potential of carbon in the low pressure synthesis of diamond

Abstract The low pressure synthesis of diamond is analyzed with the activity and the chemical potential of carbon, which are the criteria for the transfer of carbon from one phase to another. The atomic hydrogen hypothesis for preferential etching of graphite over diamond is equivalent to the assumption that the chemical potential of carbon in the gas phase is larger than that in diamond and smaller than that in graphite, which is contradictory to the well-established stability of graphite with respect to diamond. The chemical potential of carbon between diamond and graphite is shown to be reversed when the size of the carbon cluster is sufficiently small. The number of atoms in the carbon clusters that make diamond more stable than graphite is estimated to be ∼ 104 at 1200 K and 2700 Pa from the reported values of the surface energy data. This number is much smaller than those for the other systems where the formation of metastable phases is the rule rather than an exception. The role of gas activation is probably to decrease the surface energy ratio of graphite to diamond and thus to increase this number, thus inducing the dominant formation of diamond.

Thermodynamic approach to the paradox of diamond formation with simultaneous graphite etching in the low pressure synthesis of diamond

Abstract In spite of the critical handicap from the thermodynamic point of view, the atomic hydrogen hypothesis is strongly supported by experimental observations of diamond deposition with simultaneous graphite etching. Thermodynamic analysis of the CH system showed that at ∼ 1500 K, carbon solubility in the gas phase is minimal and thus, the equilibrium fraction of solid carbon is maximal. Depending on whether gas phase nucleation takes place or not, the driving force is for deposition or for etching of solid carbon below ∼ 1500 K for the input gas of the typical mixture of 1% CH 4 99% H 2 . The previous observation of etching of the graphite substrate is not expected unless solid carbon precipitated in the gas phase. By rigorous thermodynamic analysis of the previous experimental observations of diamond deposition with simultaneous graphite etching, we suggested that the previous implicit assumption that diamond deposits by an atomic unit should be the weakest point leading to the thermodynamic paradox. The experimental observations could be successfully explained without violating thermodynamics by assuming that the diamond phase had nucleated in the gas phase as fine clusters.

Chemically induced migration of liquid films and grain boundaries in MoNi(Fe) alloy

Abstract The liquid films and grain boundaries in liquid phase sintered 90 Mo-10 Ni alloy (wt%) migrate when Fe is added to the molten matrix. The growth of MoNiFe ternary solid solution at the expense of the MoNi solid initially formed during the liquid phase sintering is clearly revealed by a strong etching. The alloy formed behind the migrating liquid films is shown to have a uniform distribution of Fe atoms by an electron microprobe analysis. The increase of the migration rate with the amount of Fe added and with the estimated change of the lattice parameter from MoNi to MoNi-Fe solid solutions is parabolic in agreement with the prediction of the coherency strain model. An estimate of the driving force from its balance against the interface curvature also agrees in order of magnitude with the coherency strain energy. The observed grain boundary migration is identical to the diffusion induced grain boundary migration (DIGM) with a liquid solute source. The results demonstrate that it is possible to vary the migration rate systematically by manipulating the coherency strain through the amount of solute added to the liquid matrix in this type of experiments.

Effect of entrapped inert gas on pore filling during liquid phase sintering

The effect of an inert gas entrapped in isolated pores on liquid flow into them during liquid phase sintering has been studied. An analysis of the balance between the capillary pressure of the liquid menisci and the gas pressure shows that the entrapped gas delays the pore filling and produces bubbles. If the gas pressure exceeds a critical level, the pores remain intact and the critical point for their filling will never be reached. These predictions are confirmed by experimental observations on large spherical pores produced artificially in an Fe-Cu alloy. Argon gas is trapped in the pore by first sintering in Ar-H2 mixture gas and then in H2 after the isolated pores are formed. The entrapped inert gas of even low pressure is thus shown to cause a substantial porosity in liquid phase sintered specimens.

The morphological changes ofγ′ precipitates in a Ni-8Al (wt pct) alloy during their coarsening

When a Ni-8A1 (wt pct) alloy is aged at 1020 °C (30 °C below the solvus temperature of about 1050 °C) by cooling directly from the solution-treatment temperature, theγ′ precipitates undergo Ostwald ripening from the early stage of the aging treatment because the nucleation rate is relatively high. Theγ′ precipitates are initially spheres and, as they coarsen, become cubes which then split into eight smaller cubes (octets) or clusters with square and rectangular faces. The observed critical sizes for these shape transformations are fairly close to those predicted by Khachaturyanet al. on the basis of the elastic strain effect arising from lattice misfit if the lattice parameters measured by Hornbogen and Roth are used. As the octets coarsen, they appear to transform into doublets of plates. Because shapes other than those considered by Khachaturyanet al. are also observed, only the sphere-cube and cube-octet transformations can be compared with the theoretical prediction without ambiguity. At temperatures above 1020 °C, dendritic growth occurs, and at low temperatures, the precipitate number density is so high that apparently the elastic fields of the precipitates overlap, rearranging the precipitates into aligned cubes or plates. These observations show that the shape evolution of the isolated precipitates can be observed only in a narrow aging temperature range where the nucleation rate is moderately high but where the precipitate volume fraction is relatively low.

Deposition behavior of Si on insulating and conducting substrates in the CVD process: approach by charged cluster model

Abstract Deposition behaviors of silicon on dielectric and conducting substrates were studied focussing on the deposition mechanism in the Si–Cl–H system. On the dielectric substrate, silicon particles were deposited after some incubation time and then etched away at a later stage under the same processing conditions. On the conducting substrate, silicon particles were deposited without an incubation time with no observed etching during the experimental time period. In the early stages of etching on the dielectric substrate, the number of silicon particles decreased but the remaining unetched particles became larger, indicating that growth and etching took place simultaneously. This paradoxical phenomenon can be explained on a sound thermodynamic basis if we assume that charged clusters are formed in the gas phase and these clusters are the growth unit. The selective deposition on the conducting material as opposed to the dielectric material can also be explained by the electrostatic interaction between the charged cluster and the conducting or dielectric material.

TEM observations of singular grain boundaries and their roughening transition in TiO2-excess BaTiO3

Abstract When BaTiO3 powder compacts with 0.2 mol% excess TiO2 are sintered in air at 1250°C, below the eutectic temperature, some grains grow abnormally to very large sizes with double twins in the 〈111〉 directions at their centers. The abnormal grains, elongated in the 〈111〉 directions, form mostly straight grain boundaries, lying on their {111} planes with the fine matrix grains. Some grain boundaries between the nearly (111) face of an abnormal grain and the matrix grains are faceted with some facet planes lying on either the (111) plane of the abnormal grain or (210) plane of the matrix grain. These flat grain boundaries, as observed by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM), must be singular, corresponding to cusps in the polar plots of the grain boundary energy against the boundary normal. A schematic analysis with the capillarity vectors shows that the straight {111} grain boundaries across the triple junctions with the matrix grains are...

Thermodynamic approach to the chemical vapor deposition process

Abstract The portion of the free energy change which is solely dissipated for deposition in the chemical vapor deposition (CVD) process is examined. The partial pressure of the species to be deposited in the supersaturated gas phase is determined by minimizing the Gibbs free energy without the condensed phase. The supersaturation ratio for deposition is defined as the ratio of this partial pressure to the equilibrium vapor pressure of its solid. The supersaturation ratio for deposition is the same as the activity when the solid phase is chosen as the standard state. It can be expressed in terms of the partial pressures of the other species which satisfy the law of mass action, allowing the estimation of the supersaturation ratio of the compound whose stoichiometry is not maintained in the vapor phase. This supersaturation ratio is consistent with the CVD phase diagram, being unity along the phase boundary and less than unity in the gas phase region. Based on this scheme, the line of the iso-supersaturation ratio for deposition or etching can be introduced to the conventional CVD phase diagram and the dependence of the CVD processing variables on the supersaturation can be assessed. By applying this scheme to the Si-Cl-H system, the supersaturations for deposition of silicon are evaluated with respect to temperature, pressure and composition.