Reinforced insulation properties of epoxy resin/SiO2 nanocomposites by atmospheric pressure plasma modification

Nanocomposite dielectrics hold a promising future for the next generation of insulation materials because of their excellent physical, chemical, and dielectric properties. In the presented study, we investigate the use of plasma processing technology to further enhance the dielectric performance of epoxy resin/SiO2 nanocomposite materials. The SiO2 nanoparticles are treated with atmospheric-pressure non-equilibrium plasma prior to being added into the epoxy resin host. Fourier transform infrared spectroscopy (FTIR) results reveal the effects of the plasma process on the surface functional groups of the treated nanoparticles. Scanning electron microscopy (SEM) results show that the plasma treatment appreciably improves the dispersion uniformity of nanoparticles in the host polymer. With respect to insulation performance, the epoxy/plasma-treated SiO2 specimen shows a 29% longer endurance time than the epoxy/untreated SiO2 nanocomposite under electrical aging. The Weibull plots of the dielectric breakdown field intensity suggest that the breakdown strength of the nanocomposite with the plasma pre-treatment on the nanoparticles is improved by 23.3%.

Interface control of surface photochemical reactivity in ultrathin epitaxial ferroelectric films

Asymmetrical electrical boundary conditions in (001)-oriented Pb(Zr 0.2TiO0.8)O3 (PZT) epitaxial ultrathin ferroelectric films are exploited to control surface photochemical reactivity determined by the sign of the surface polarization charge. It is shown that the preferential orientation of polarization in the as-grown PZT layer can be manipulated by choosing an appropriate type of bottom electrode material. PZT films deposited on the SrRuO3 electrodes exhibit preferential upward polarization (C) whilst the same films grown on the (La,Sr)CoO 3-electrodes are polarized downward (C-). Photochemical activity of the PZT surfaces with different surface polarization charges has been tested by studying deposition of silver nanoparticles from AgNO3 solution under UV irradiation. PZT surfaces with preferential C orientation possess a more active surface for metal reduction than their C- counterparts, evidenced by large differences in the concentration of deposited silver nanoparticles. This effect is attributed to band bending at the bottom interface which varies depending on the difference in work functions of PZT and electrode materials.

Silica nanoparticles treated by cold atmospheric-pressure plasmas improve the dielectric performance of organic–inorganic nanocomposites

We report on the application of cold atmospheric-pressure plasmas to modify silica nanoparticles to enhance their compatibility with polymer matrices. Thermally nonequilibrium atmospheric-pressure plasma is generated by a high-voltage radio frequency power source operated in the capacitively coupled mode with helium as the working gas. Compared to the pure polymer and the polymer nanocomposites with untreated SiO2, the plasma-treated SiO2–polymer nanocomposites show higher dielectric breakdown strength and extended endurance under a constant electrical stress. These improvements are attributed to the stronger interactions between the SiO2 nanoparticles and the surrounding polymer matrix after the plasma treatment. Our method is generic and can be used in the production of high-performance organic–inorganic functional nanocomposites.

Kinetics of low-pressure, low-temperature graphene growth : toward single-layer, single-crystalline structure

Graphene grown on metal catalysts with low carbon solubility is a highly competitive alternative to exfoliated and other forms of graphene, yet a single-layer, single-crystal structure remains a challenge because of the large number of randomly oriented nuclei that form grain boundaries when stitched together. A kinetic model of graphene nucleation and growth is developed to elucidate the effective controls of the graphene island density and surface coverage from the onset of nucleation to the full monolayer formation in low-pressure, low-temperature CVD. The model unprecedentedly involves the complete cycle of the elementary gas-phase and surface processes and shows a precise quantitative agreement with the recent low-energy electron diffraction measurements and also explains numerous parameter trends from a host of experimental reports. These agreements are demonstrated for a broad pressure range as well as different combinations of precursor gases and supporting catalysts. The critical role of hydrogen in controlling the graphene nucleation and monolayer formation is revealed and quantified. The model is generic and can be extended to even broader ranges of catalysts and precursor gases/pressures to enable the as yet elusive effective control of the crystalline structure and number of layers of graphene using the minimum amounts of matter and energy.

Discussion on full-scale internal pressure measurements

The wind loading on most structural elements is made up of both an external and internal pressure. Internal pressures are also important for the design of naturally ventilated buildings. The internal pressure is the interaction between the external pressure propagating through the building envelope and any internal plant causing building pressurization. Although the external pressure field can be well defined through a series of wind tunnel tests, modeling complexities makes accurate prediction of the internal pressure difficult. For commercial testing for the determination of design cladding pressures, an internal pressure coefficient is generally assumed from wind loading standards. Several theories regarding the propagation of internal pressures through single and multiple dominant openings have been proposed for small and large flexible buildings (Harris (1990), Holmes, (1979), Liu & Saathoff (1981 ), Vickery (1986, 1994), Vickery & Bloxham (1992), Vickery & Georgiou (1991))...

Solitary filamentary structures and nanosecond dynamics in atmospheric-pressure plasmas driven by tailored dc pulses

Nanosecond dynamics of two separated discharge cycles in an asymmetric dielectric barrier discharge is studied using time-resolved current and voltage measurements synchronized with high-speed (∼5 ns) optical imaging. Nanosecond dc pulses with tailored raise and fall times are used to generate solitary filamentary structures (SFSs) during the first cycle and a uniform glow during the second. The SFSs feature ∼1.5 mm thickness, ∼1.9 A peak current, and a lifetime of several hundred nanoseconds, at least an order of magnitude larger than in common microdischarges. This can be used in alternating localized and uniform high-current plasma treatments in various applications.

Effect of Coulomb blockade, gold resistance, and thermal expansion on the electrical resistance of ultrathin gold films

Controlling the electrical resistance of granular thin films is of great importance for many applications, yet a full understanding of electron transport in such films remains a major challenge. We have studied experimentally and by model calculations the temperature dependence of the electrical resistance of ultrathin gold films at temperatures between 2 K and 300 K. Using sputter deposition, the film morphology was varied from a discontinuous film of weakly coupled meandering islands to a continuous film of strongly coupled coalesced islands. In the weak-coupling regime, we compare the regular island array model, the cotunneling model, and the conduction percolation model with our experimental data. We show that the tunnel barriers and the Coulomb blockade energies are important at low temperatures and that the thermal expansion of the substrate and the island resistance affect the resistance at high temperatures. At low temperatures our experimental data show evidence for a transition from electron cotunneling to sequential tunneling but the data can also be interpreted in terms of conduction percolation. The resistivity and temperature coefficient of resistance of the meandering gold islands are found to resemble those of gold nanowires. We derive a simple expression for the temperature at which the resistance changes from non-metal-like behavior into metal-like behavior. In the case of strong island coupling, the total resistance is solely determined by the Ohmic island resistance.

Rapid, simultaneous activation of thin nanowire growth in low-temperature, low-pressure chemically active plasmas

Multiscale numerical modeling of the species balance and transport in the ionized gas phase and on the nanostructured solid surface complemented by the heat exchange model is used to demonstrate the possibility of minimizing the Gibbs-Thompson effect in low-temperature, low-pressure chemically active plasma-assisted growth of uniform arrays of very thin Si nanowires, impossible otherwise. It is shown that plasma-specific effects drastically shorten and decrease the dispersion of the incubation times for the nucleation of nanowires on non-uniform Au catalyst nanoparticle arrays. The fast nucleation makes it possible to avoid a common problem of small catalyst nanoparticle burying by amorphous silicon. These results explain a multitude of experimental observations on chemically active plasma-assisted Si nanowire growth and can be used for the synthesis of a range of inorganic nanowires for environmental, biomedical, energy conversion, and optoelectronic applications.

Atmospheric-pressure discharges for the fabrication of surface-based metal nanostructures

Various reactor configurations for generating atmospheric-pressure discharges were tested, and several types of nanostructures, including Mo nanoflakes, were successfully synthesized. Here, we present photographs of the discharges, as well as SEM images of representative nanostructures.

Deterministic plasma-aided synthesis of high-quality nanoislanded nc-SiC films

Despite major advances in the fabrication and characterization of SiC and related materials, there has been no convincing evidence of the synthesis of nanodevice-quality nanoislanded SiC films at low, ultralarge scale integration technology-compatible process temperatures. The authors report on a low-temperature (400 °C) plasma-assisted rf magnetron sputtering deposition of high-quality nanocrystalline SiC films made of uniform-size nanoislands that almost completely cover the Si(100) surface. These nanoislands are chemically pure, highly stoichiometric, have a typical size of 20-35 nm, and contain small (∼5 nm) nanocrystalline inclusions. The properties of nanocrystalline SiC films can be effectively controlled by the plasma parameters.

Low-pressure planar magnetron discharge for surface deposition and nanofabrication

Current-voltage characteristics of the planar magnetron are studied experimentally and by numerical simulation. Based on the measured current-voltage characteristics, a model of the planar magnetron discharge is developed with the background gas pressure and magnetic field used as parameters. The discharge pressure was varied in a range of 0.7-1.7 Pa, the magnetic field of the magnetron was of 0.033-0.12 T near the cathode surface, the discharge current was from 1 to 25 A, and the magnetic field lines were tangential to the substrate surface in the region of the magnetron discharge ignition. The discharge model describes the motion of energetic secondary electrons that gain energy by passing the cathode sheath across the magnetic field, and the power required to sustain the plasma generation in the bulk. The plasma electrons, in turn, are accelerated in the electric field and ionize effectively the background gas species. The model is based on the assumption about the prevailing Bohm mechanism of electron conductivity across the magnetic field. A criterion of the self-sustained discharge ignition is used to establish the dependence of the discharge voltage on the discharge current. The dependence of the background gas density on the current is also observed from the experiment. The model is consistent with the experimental results. © 2010 American Institute of Physics.

Structural, electronic, and optical properties of wurtzite and rocksalt InN under pressure

Structural stability, electronic, and optical properties of InN under high pressure are studied using the first-principles calculations. The lattice constants and electronic band structure are found consistent with the available experimental and theoretical values. The pressure of the wurtzite-to-rocksalt structural transition is 13.4 GPa, which is in an excellent agreement with the most recent experimental values. The optical characteristics reproduce the experimental data thus justifying the feasibility of our theoretical predictions of the optical properties of InN at high pressures.

Inductively coupled plasma-assisted RF magnetron sputtering deposition of boron-doped microcrystalline Si films

An innovative custom-designed inductively coupled plasma-assisted RF magnetron sputtering deposition system has been developed to synthesize B-doped microcrystalline silicon thin films using a pure boron sputtering target in a reactive silane and argon gas mixture. Films were deposited using different boron target powers ranging from 0 to 350 W at a substrate temperature of 250 °C. The effect of the boron target power on the structural and electrical properties of the synthesized films was extensively investigated using X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and Hall-effect system. It is shown that, with an initial increase of the boron target power from 0 to 300 W, the structural and electrical properties of the B-doped microcrystalline films are improved. However, when the target power is increased too much (e.g. to 350 W), these properties become slightly worse. The variation of the structural and electrical properties of the synthesized B-doped microcrystalline thin films is related to the incorporation of boron atoms during the crystallization and doping of silicon in the inductively coupled plasma-based process. This work is particularly relevant to the microcrystalline silicon-based p-i-n junction solar cells.

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.