A. Gidwani

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.

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.

Aerodynamic Focusing of Nanoparticles: II. Numerical Simulation of Particle Motion Through Aerodynamic Lenses

Abstract We have developed a numerical simulation methodology that is able to accurately characterize the focusing performance of aerodynamic lens systems. The commercial computational fluid dynamics (CFD) software FLUENT was used to simulate the gas flow field. Particle trajectories were tracked using the Lagrangian approach. Brownian motion of nanoparticles was successfully incorporated in our numerical simulations. This simulation tool was then used to evaluate the performance of an aerodynamic lens assembly that was designed to focus 3 nm spherical unit density particles following the guidelines described in Paper I (Wang et al. 2005). Our simulations show that the performance of this lens assembly is close to what is predicted by the design guidelines. The simulations also demonstrate the ability of aerodynamic lenses to focus sub-30 nm spherical unit density particles.

Thermal plasma deposition of nanophase hard coatings

Thermal plasmas offer several specific advantages for the generation of hard coatings. In particular, the high energy density of the thermal plasma allows higher precursor flow rates and a wider choice of precursors. Expansion of the plasma into a low pressure chamber offers the additional advantages that improved control over the chemistry can be achieved or that nanosize particles can be generated. In this contribution, two experiments are described and the results reviewed in which supersonic plasma jets have been used to deposit nanophase hard coatings. In one of them, hard boron carbide coatings have been deposited using a supersonic plasma jet and a secondary discharge between the nozzle and the substrate. Spectroscopic analysis has been used to determine the reaction processes responsible for the deposition. In the other experiment, a plasma-containing silicon or titanium and carbon vapor has been expanded through a supersonic nozzle to form nanosize silicon or titanium carbide particles which are subsequently deposited on a substrate. Addition of a system of aerodynamic lenses allows the formation of a beam of nanosized particles which are deposited with high spatial definition. Narrow lines of a hard coating can thus be produced. In both processes, the deposition occurs rapidly, a fact which makes the processes attractive for a variety of potential applications.

EHD-Enhanced Condensation of Alternative Refrigerants in Smooth and Corrugated Tubes

An experimental study of EHD-enhanced in-tube condensation of alternative refrigerants is presented. The refrigerants tested were the single-component refrigerant R-134a, the zeotropic mixture R-407c, and the near-azeotrope R-404a. Tests for R-404a and R-407c were performed in smooth and corrugated tubes, whereas R-134a tests were performed only in corrugated tubes, with smooth tube data extracted from Singh (1995). The tests were performed with internally mounted cylindrical electrodes. It was found that, in general, all three refrigerants respond remarkably well to the EHD enhancement. The heat transfer performance of near-azeotrope R-404a is enhanced 18.8-fold in the smooth tube at the highest applied voltage of 18 kV, with a corresponding pressure drop penalty of 11.8-fold, and the maximum enhancement inside corrugated tube is 5.8-fold for the range of conditions tested in this study. R-134a has a maximum heat transfer enhancement of 8.3-fold, with a corresponding pressure drop penalty of 20.8-fold at an applied voltage of 18 kV inside the corrugated tube. Among the three refrigerants tested, R-407c shows the lowest heat transfer enhancement, with a 3.9-fold maximum enhancement at an applied EHD voltage of 18 kV inside the smooth tube, and a maximum enhancement of 2.9-fold inside the corrugated tube.

Modeling Oxidation of Soot Particles Within a Laminar Aerosol Flow Reactor Using Computational Fluid Dynamics

This work examines the measurement of surface specific soot oxidation rates with the High Temperature Oxidation-Tandem Differential Mobility Analyzer (HTO-TDMA) method. The Computational Fluid Dynamics package CFD-ACE+ is used to understand particle flow, oxidation and size dependent particle losses in the laminar aerosol flow reactor using an Eulerian-Lagrangian framework. Decrease of DMA selected mono-disperse particle size distribution due to oxidation within the aerosol tube is modeled using fitted kinetic soot oxidation parameters. The effects of Brownian diffusion and thermophoresis on particle flow and loss to the reactor walls are evaluated. The position of peak particle diameter, which is used as an indicator to determine oxidation rate, is found to be independent of diffusion, thermophoresis and secondary flow effects, thus validating its use in deriving kinetic soot oxidation parameters. Diffusion does not affect the evolution of particle size distribution within the reactor. However, thermophoresis is found to be the dominant mechanism influencing both shape of particle size distribution and particle loss to the walls of the aerosol reactor. Simulations show reduced effects of secondary recirculating flows on the particle flow trajectories in a vertical furnace as compared to horizontal furnace orientation. This work highlights the importance of making accurate measurements of temperature within the modeling domain. Since gas temperature within the flow tube could not be measured with high radial resolution using radiation shielded thermocouple, the derived soot oxidation rate may be uncertain by a factor of 2. Importantly, CFD simulations suggest that a distribution of temperature and size-dependent particle reactivities may be present in the reactor, requiring further theoretical and experimental investigation.

Characterization of Nanoparticle Films and Structures Produced by Hypersonic Plasma Particle Deposition

Abstract : There is great potential for the use of nanostructures in numerous applications. Investigation of nanoparticle films and structures is an important area of research for the production of nanoengineered devices. However for these devices to become a reality, a production method that can yield high-rate synthesis of nanostructured powders is necessary. The hypersonic plasma particle deposition (HPPD) process has been shown to be capable of such high-rate production of nanoparticle films and structures. Versatile in its ability to manufacture nanoparticles of different chemistries HPPD also has the capability of in situ particle consolidation and assembly. In this study, chemically diverse films and structures have been produced by HPPD on a variety of substrates. Using novel specimen preparation techniques, these nanoparticles have been characterized by TEM. Fundamental issues of importance have been investigated for both the nanoparticle structure and the constituent nanoparticles. These issues include nanoparticle crystallinity and defect structure. The chemical homogeneity and structural characteristics of the deposition are also investigated. This application of microscopy to aid process development has resulted in insights into the nanoparticle formation process and the dynamics of the HPPD process.