Effect of gas pressure on electron field emission from carbon nanotube forests

This article quantifies the effect of the operating pressure of the H 2 + C 2H 4 gas mixture on the current density and threshold voltage of the electron emission from dense forests of multiwalled carbon nanotubes synthesized using thermal catalytic Chemical Vapor Deposition under near atmospheric pressure process conditions. The results suggest that in the pressure range of interest 400-700 Torr the field emission properties can be substantially improved by operating the process at lower gas pressures when the nanostructure aspect ratios are higher. The obtained threshold voltage ∼1.75 V/μm and the emission current densities ∼10 mA/cm 2 offer competitive advantages compared with the results reported by other authors. Copyright

Highly effective fungal inactivation in He+O2 atmospheric-pressure nonequilibrium plasmas

Highly effective (more than 99.9%) inactivation of a pathogenic fungus Candida albicans commonly found in oral, respiratory, digestive, and reproduction systems of a human body using atmospheric-pressure plasma jets sustained in He+ O2 gas mixtures is reported. The inactivation is demonstrated in two fungal culture configurations with open (Petri dish without a cover) and restricted access to the atmosphere (Petri dish with a cover) under specific experimental conditions. It is shown that the fungal inactivation is remarkably more effective in the second configuration. This observation is supported by the scanning and transmission electron microscopy of the fungi before and after the plasma treatment. The inactivation mechanism explains the experimental observations under different experimental conditions and is consistent with the reports by other authors. The results are promising for the development of advanced health care applications.

Effect of input power and gas pressure on the roughening and selective etching of SiO2/Si surfaces in reactive plasmas

We report on the application low-temperature plasmas for roughening Si surfaces which is becoming increasingly important for a number of applications ranging from Si quantum dots to cell and protein attachment for devices such as "laboratory on a chip" and sensors. It is a requirement that Si surface roughening is scalable and is a single-step process. It is shown that the removal of naturally forming SiO2 can be used to assist in the roughening of the surface using a low-temperature plasma-based etching approach, similar to the commonly used in semiconductor micromanufacturing. It is demonstrated that the selectivity of SiO2 /Si etching can be easily controlled by tuning the plasma power, working gas pressure, and other discharge parameters. The achieved selectivity ranges from 0.4 to 25.2 thus providing an effective means for the control of surface roughness of Si during the oxide layer removal, which is required for many advance applications in bio- and nanotechnology.

Self-organized carbon connections between catalyst particles on a silicon surface exposed to atmospheric-pressure Ar + CH4 microplasmas

Ag nanoparticles and Fe-coated Si micrograins were separately deposited onto Si(1 0 0) surfaces and then exposed to an Ar + CH4 microplasma at atmospheric pressure. For the Ag nanoparticles, self-organized carbon nanowires, up to 400 nm in length were produced, whereas for the Fe-coated Si micrograins carbon connections with the length up to 100 μm were synthesized on the plasma-exposed surface area of about 0.5 mm2. The experiment has revealed that long carbon connections and short nanowires demonstrate quite similar behavior and structure. While most connections/nanowires tended to link the nearest particles, some wires were found to 'dissolve' into the substrate without terminating at the second particle. Both connections and nanowires are mostly linear, but long carbon connections can form kinks which were not observed in the carbon nanowire networks. A growth scenario explaining the carbon structure nucleation and growth is proposed. Multiscale numerical simulations reveal that the electric field pattern around the growing connections/nanowires strongly affects the surface diffusion of carbon adatoms, the main driving force for the observed self-organization in the system. The results suggest that the microplasma-generated surface charges can be used as effective controls for the self-organized formation of complex carbon-based nano-networks for integrated nanodevices.

Temperature-dependent properties of nc-Si thin films synthesized in low-pressure, thermally nonequilibrium, high-density inductively coupled plasmas

Silicon thin films were synthesized simultaneously on single-crystal silicon and glass substrates by lowpressure, thermally nonequilibrium, high-density inductively coupled plasma-assisted chemical vapor deposition from the silane precursor gas without any additional hydrogen dilution in a broad range of substrate temperatures from 100 to 500 °C. The effect of the substrate temperature on the morphological, structural and optical properties of the synthesized silicon thin films is systematically studied by X-ray diffractometry, Raman spectroscopy, UV-vis spectroscopy, and scanning electron microscopy. It is shown that the formation of nanocrystalline silicon (nc-Si) occurs when the substrate temperature is higher than 200 °C and that all the deposited nc-Si films have a preferential growth along the (111) direction. However, the mean grain size of the (111) orientation slightly and gradually decreases while the mean grain size of the (220) orientation shows a monotonous increase with the increased substrate temperature from 200 to 500 °C. It is also found that the crystal volume fraction of the synthesized nc-Si thin films has a maximum value of ∼69.1% at a substrate temperature of 300 rather than 500 °C. This rather unexpected result is interpreted through the interplay of thermokinetic surface diffusion and hydrogen termination effects. Furthermore, we have also shown that with the increased substrate temperature from 100 to 500 °C, the optical bandgap is reduced while the growth rates tend to increase. The maximum rates of change of the optical bandgap and the growth rates occur when the substrate temperature is increased from 400 to 500 °C. These results are highly relevant to the development of photovoltaic thin-film solar cells, thin-film transistors, and flat-panel displays.

Diagnostics and two-dimensional simulation of low-frequency inductively coupled plasmas with neutral gas heating and electron heat fluxes

This article presents the results on the diagnostics and numerical modeling of low-frequency (∼460 KHz) inductively coupled plasmas generated in a cylindrical metal chamber by an external flat spiral coil. Experimental data on the electron number densities and temperatures, electron energy distribution functions, and optical emission intensities of the abundant plasma species in low/intermediate pressure argon discharges are included. The spatial profiles of the plasma density, electron temperature, and excited argon species are computed, for different rf powers and working gas pressures, using the two-dimensional fluid approach. The model allows one to achieve a reasonable agreement between the computed and experimental data. The effect of the neutral gas temperature on the plasma parameters is also investigated. It is shown that neutral gas heating (at rf powers≥0.55kW) is one of the key factors that control the electron number density and temperature. The dependence of the average rf power loss, per electron-ion pair created, on the working gas pressure shows that the electron heat flux to the walls appears to be a critical factor in the total power loss in the discharge.

Discharge parameters and dominant electron conductivity mechanism in a low-pressure planar magnetron discharge

Parameters of a discharge sustained in a planar magnetron configuration with crossed electric and magnetic fields are studied experimentally and numerically. By comparing the data obtained in the experiment with the results of calculations made using the proposed theoretical model, conclusion was made about the leading role of the turbulence-driven Bohm electron conductivity in the low-pressure operation mode (up to 1 Pa) of the discharge in crossed electric and magnetic fields. A strong dependence of the width of the cathode sputter trench, associated with the ionization region of the magnetron discharge, on the discharge parameters was observed in the experiments. The experimental data were used as input parameters in the discharge model that describes the motion of secondary electrons across the magnetic field in the ionization region and takes into account the classical, near-wall, and Bohm mechanisms of electron conductivity.

Effect of ambipolar fluxes on nanoparticle charging in low-pressure glow discharges

The effect of ambipolar fluxes on nanoparticle charging in a typical low-pressure parallel-plate glow discharge is considered. It is shown that the equilibrium values of the nanoparticle charge in the plasma bulk and near-electrode areas are strongly affected by the ratio S ath i of the ambipolar flux and the ion thermal velocities. Under typical experimental conditions the above ratio is neither S ath i≪ 1 nor S ath i≫1, which often renders the commonly used approximations of the purely thermal or "ion wind" ion charging currents inaccurate. By using the general approximation for the ambipolar drift-affected ion flux on the nanoparticle surface, it appears possible to obtain more accurate values of the nanoparticle charge that usually deviate within 10-25 % from the values obtained without a proper accounting for the ambipolar ion fluxes. The implications of the results obtained for glow discharge modeling and nanoparticle manipulation in low-pressure plasmas are discussed.

Low-pressure diffusion equilibrium of electronegative complex plasmas

A model for electronegative plasmas containing charged dust or colloidal grains was used. Numerical solutions based on the model demonstrate how a low-pressure diffusion equilibrium of the complex electronegative plasma system is dynamically sustained through plasma particle sources.

Electron energy distribution function in low-pressure complex plasmas

The effect of density and size of dust grains on the electron energy distribution function (EEDF) in low-temperature complex plasmas is studied. It is found that the EEDF depends strongly on the dust density and size. The behavior of the electron temperature can differ significantly from that of a pristine plasma. For low-pressure argon glow discharge, the Druyvesteyn-like EEDF often found in pristine plasmas can become nearly Maxwellian if the dust density and/or sizes are large. One can thus control the plasma parameters by the dust grains.

Atmospheric-pressure plasma jets : effect of gas flow, active species, and snake-like bullet propagation

Cold atmospheric-pressure plasma jets have recently attracted enormous interest owing to numerous applications in plasma biology, health care, medicine, and nanotechnology. A dedicated study of the interaction between the upstream and downstream plasma plumes revealed that the active species (electrons, ions, excited OH, metastable Ar, and nitrogen-related species) generated by the upstream plasma plume enhance the propagation of the downstream plasma plume. At gas flows exceeding 2 l/min, the downstream plasma plume is longer than the upstream plasma plume. Detailed plasma diagnostics and discharge species analysis suggest that this effect is due to the electrons and ions that are generated by the upstream plasma and flow into the downstream plume. This in turn leads to the relatively higher electron density in the downstream plasma. Moreover, high-speed photography reveals a highly unusual behavior of the plasma bullets, which propagate in snake-like motions, very differently from the previous reports. This behavior is related to the hydrodynamic instability of the gas flow, which results in non-uniform distributions of long-lifetime active species in the discharge tube and of surface charges on the inner surface of the tube.

Computational fluid dynamics simulation and turbomachinery code validation of a high pressure ratio radial-inflow turbine

The present study explores reproducing the closest geometry of a high pressure ratio single stage radial-inflow turbine applied in the Sundstrans Power Systems T-100 Multipurpose Small Power Unit. The commercial software ANSYS-Vista RTD along with a built in module, BladeGen, is used to conduct a meanline design and create 3D geometry of one flow passage. Carefully examining the proposed design against the geometrical and experimental data, ANSYS-TurboGrid is applied to generate computational mesh. CFD simulations are performed with ANSYS-CFX in which three-dimensional Reynolds-Averaged Navier-Stokes equations are solved subject to appropriate boundary conditions. Results are compared with numerical and experimental data published in the literature in order to generate the exact geometry of the existing turbine and validate the numerical results against the experimental ones.

Drawing out critical thinking : testing the methodological value of drawing collaboratively

Early childhood research has long established that drawing is a central, and important activity for young children. Less common are investigations into the drawing activity of adults involved in early childhood. A team of adult early childhood researchers, with differing exposures and familiarities with drawing, experimented with intergenerational collaborative drawing with colleagues, students, family members and others, to explore the effectiveness of drawing as a research process and as an arts-based methodology. This testing prompted critical thinking into how drawing might facilitate research that involves young children, to operate in more communicable ways, and how research-focused drawings might occur in reference to a research project.

High-yield atmospheric-pressure CVD of highly-uniform carbon nanocoils using Co–P catalyst nanoparticles prepared by electroless plating

A simple, fast and low-cost atmospheric-pressure chemical vapor deposition technique is developed to synthesize high-yield carbon nanocoils (CNCs) using amorphous Co–P alloy as catalyst and thiophene as nucleation agent. The uniform catalyst pattern with the mean particle size of 350 nm was synthesized using a simple electroless plating process. This uniformity of the Co–P nanoparticles results in a high yield, very uniform size/shape distribution and regular structure of CNCs at the optimum growth temperature of 800 ◦C. The yield of CNCs reaches ∼76%; 70% of the CNCs have fiber diameters approximately 250 nm. The CNC coil diameters and lengths are 450–550nm and 0.5–2mm, respectively. The CNC nucleation and growth mechanism are also discussed.

Measuring, modelling and understanding the mechanical behavior of bagasse

In the Australian sugar industry, sugar cane is smashed into a straw like material by hammers before being squeezed between large rollers to extract the sugar juice. The straw like material is initially called prepared cane and then bagasse as it passes through successive roller milling units. The sugar cane materials are highly compressible, have high moisture content, are fibrous, and they resemble some peat soils in both appearance and mechanical behaviour. A promising avenue to improve the performance of milling units for increased throughput and juice extraction, and to reduce costs is by modelling of the crushing process. To achieve this, it is believed necessary that milling models should be able to reproduce measured bagasse behaviour. This investigation sought to measure the mechanical (compression, shear, and volume) behaviour of prepared cane and bagasse, to identify limitations in currently used material models, and to progress towards a material model that can predict bagasse behaviour adequately. Tests were carried out using a modified direct shear test equipment and procedure at most of the large range of pressures occurring in the crushing process. The investigation included an assessment of the performance of the direct shear test for measuring bagasse behaviour. The assessment was carried out using finite element modelling. It was shown that prepared cane and bagasse exhibited critical state behavior similar to that of soils and the magnitudes of material parameters were determined. The measurements were used to identify desirable features for a bagasse material model. It was shown that currently used material models had major limitations for reproducing bagasse behaviour. A model from the soil mechanics literature was modified and shown to achieve improved reproduction while using magnitudes of material parameters that better reflected the measured values. Finally, a typical three roller mill pressure feeder configuration was modelled. The predictions and limitations were assessed by comparison to measured data from a sugar factory.