Steven L. Girshick

Kinetic nucleation theory: A new expression for the rate of homogeneous nucleation from an ideal supersaturated vapor

The ‘‘kinetic theory’’ of homogeneous nucleation developed by Katz and Wiedersich is extended to derive a new expression for the rate of nucleation from an ideal supersaturated vapor. Compared to the classical expression for the nucleation rate, the new expression has a slightly different dependence on supersaturation, and a substantially different dependence on temperature. A comparison of the new expression with experimental data on nucleation rates of several organic liquids indicates that in some but not all cases the new expression gives much closer agreement with the data than does the classical expression. Discrepancies between the theory and the data are ascribed mainly to the physical assumptions of the theory presented, which are the same as in the classical theory—particularly, that the physical properties of microscopic clusters are the same as those of the bulk liquid.

Superhard silicon nanospheres

Abstract Successful deposition and mechanical probing of nearly spherical, defect-free silicon nanospheres has been accomplished. The results show silicon at this length scale to be up to four times harder than bulk silicon. Detailed measurements of plasticity evolution and the corresponding hardening response in normally brittle silicon is possible in these small volumes. Based upon a proposed length scale related to the size of nanospheres in the 20– 50 nm radii range, a prediction of observed hardnesses in the range of 20– 50 GPa is made. The ramifications of this to computational materials science studies on identical volumes are discussed.

Nanoparticle formation using a plasma expansion process

We describe a process in which nanosize particles with u narrow size distribution are generated by expanding a thermal plasma carrying vapor-phase precursors through a nozzle. The plasma temperature and velocity profiles are characterized by enthalpy probe measurements. by calorimetric energy balances. and by a model of the nozzle flow. Aerosol samples are extracted from the flow downstream of the nozzle by means of a capillary probe interfaced to a two-stage ejection diluter. The diluted aerosol is directed to a scanning electrical mobility spectrometer (SEMS) which provides on-line size distributions down to particle diameters of 4 nmt. We have generated silicon, carbon, and silicon carbide particles with number mean diameters of about 10 not or less, and we have obtained some correlations between the product and the operating conditions. Inspection of the size distributions obtained in the experiments, together with the modeling results, suggests that under our conditions silicon carbide formation is initiated by nucleation of extremely small silicon particles from supersaturated silicon vapor, followed by chemical reactions at the particle surfaces involving carbon-containing species from the gas phase.

Focused nanoparticle-beam deposition of patterned microstructures

A method was developed for fabricating nanocrystalline microstructures. This method involves synthesizing nanoparticles in a thermal plasma expanded through a nozzle, and then focusing the nanoparticles to a collimated beam by means of aerodynamic lenses. High-aspect-ratio structures of silicon carbide and titanium were deposited on stationary substrates, and lines and two-dimensional patterns were deposited on translated substrates. Linewidths equalled approximately 50 μm. This approach allows the use of much larger nozzles than in previously developed micronozzle methods, and also allows size selection of the particles that are deposited.

Thermal plasma synthesis of ultrafine iron particles

Abstract Ultrafine iron powder was synthesized in an atmospheric-pressure radio-frequency plasma reactor by injecting relatively course iron powder into the plasma, where it evaporated. The renucleated iron particles were characterized by means of a sampling capillary and dilution system interfaced to an electrical aerosol analyzer, a condensation nucleus counter and an electrostatic aerosol sampler. Volume-mean particle diameters for samples obtained near the downstream end of the reactor ranged from about 20 to 70 nm, with particle size increasing as the feed rate of injected iron powder was increased. A two-dimensional numerical model was developed, which solved the plasma conservation equations to predict temperature and velocity distributions, heating and evaporation of the injected iron powder, nucleation and growth of iron particles, and particle transport by convection, diffusion and thermophoresis. Mean particle diameters predicted by the model were in good agreement with the experimental data, although the data indicated broader size distributions and flatter radial profiles of particle concentration than predicted by the model.

Time-dependent aerosol models and homogeneous nucleation rates

Two types of numerical models for homogeneous nucleation and particle growth are compared: models in which the time rate of change in the stable aerosol population is given by an analytical expression for the nucleation rate, and discrete models, in which an expression for the nucleation rate is not required nor explicitly calculated. The classical expression for the homogeneous nucleation rate, coupled to a moment model, is found to produce poor agreement with a discrete-sectional model except for very low values of dimensionless surface tension. A new expression for the homogeneous nucleation rate is proposed, which when coupled to a moment model produces excellent agreement with a discrete-sectional model over a wide range of dimensionless surface tensions. The new expression is also consistent with experimental data on homogeneous nucleation of dibutylphthalate.

Homogeneous nucleation of particles from the vapor phase in thermal plasma synthesis

Particle nucleation and growth are simulated for iron vapor in a thermal plasma reactor with an assumed one-dimensional flow field and decoupled chemistry and aerosol dynamics. Including both evaporation and coagulation terms in the set of cluster-balance rate equations, a sharply defined homogeneous nucleation event is calculated. Following nucleation the vapor phase is rapidly depleted by condensation, and thereafter particle growth occurs purely by Browntan coagulation. The size and number of nucleated particles are found to be affected strongly by the cooling rate and by the initial monomer concentration. An explanation is presented in terms of the response time of the aerosol to changing thermodynamic conditions.

Atomic carbon vapor as a diamond growth precursor in thermal plasmas

A detailed surface chemistry mechanism is proposed for chemical vapor deposition of diamond films, which extends the growth‐by‐methyl mechanism proposed by Harris to treat any CHm radical, m=0–3, as a growth monomer. Numerical computations were performed in which the mechanism was coupled to a model for the boundary layer above the substrate, for conditions typical of diamond deposition in an atmospheric‐pressure thermal plasma. The predicted linear growth rate increases strongly as the boundary layer thickness δ is decreased, and the results indicate a strong dependence of the diamond growth chemistry on δ. For relatively thick boundary layers (modest velocities of the reactant jet) growth is dominated by CH3. For very thin boundary layers (high velocities) the model predicts that growth is dominated by C. For the transition region where C and CH3 each contribute about 40% to growth, CH2 also contributes about 17%. The carbon conversion efficiency is also predicted to peak in the transition region, and d...

Superhard nanocrystalline silicon carbide films

Nanocrystalline silicon carbide films were deposited by thermal plasma chemical vapor deposition, with film growth rates on the order of 10μm∕min. Films were deposited on molybdenum substrates, with substrate temperature ranging from 750-1250 °C. The films are composed primarily of β-SiC nanocrystallites. Film mechanical properties were investigated by nanoindentation. As substrate temperature increased the average grain size, the crystalline fraction in the film, and the hardness all increased. For substrate temperatures above 1200 °C the average grain size equaled 10-20 nm, the crystalline fraction equaled 80-85 %, and the film hardness equaled approximately 50 GPa.

Modelling of silicon hydride clustering in a low-pressure silane plasma

A new silicon hydride clustering model was developed to study the nucleation of particles in a low-temperature silane plasma. The model contains neutral silanes, silylenes, silenes and silyl radicals as well as silyl and silylene anions. Reaction rates were estimated from available data. Simulations were carried out for typical discharge parameters in a capacitive plasma. It was shown that the main pathway leading to silicon hydride clustering was governed by anion-neutral reactions. SiH2 radical insertion was found to be important only in the initial stages of clustering, whereas electron-induced dissociations were seen to lead to dehydrogenation. Increased ion density (radiofrequency power density) leads to faster clustering due to increased formation of reactive radicals.