Controlled multibubble surface cavitation

Heterogeneous bubble nucleation at surfaces has been notorious because of its irreproducibility. Here controlled multibubble surface cavitation is achieved by using a hydrophobic surface patterned with microcavities. The expansion of the nuclei in the microcavities is triggered by a fast lowering of the liquid pressure. The procedure allows us to control and fix the bubble distance within the bubble cluster. We observe a perfect quantitative reproducibility of the cavitation events where the inner bubbles in the two-dimensional cluster are shielded by the outer ones, reflected by their later expansion and their delayed collapse. Apart from the final bubble collapse phase (when jetting flows directed towards the cluster's center develop), the bubble dynamics can be quantitatively described by an extended Rayleigh-Plesset equation, taking pressure modification through the surrounding bubbles into account.     

Angular dependence and vectorial effect of the photoemission from EuO

An extremely strong dependence of the photoemission on incident angle and polarization of the exciting light is observed for EuO. At an incident angle of 45° the vector ratio reaches values as high as 50. A strong spectral dependence is detected. The effects can be explained by a simple model which is based on the volume photoeffect.     

Negative ion mediated electron stimulated disordering of c(8 × 2) benzoate-Cu(110): extension of the displaced harmonic oscillator model

We have extended the displaced harmonic oscillator model for negative ion resonance (NIR) scattering in high-resolution electron energy loss spectroscopy (HREELS), to interpret the energy transfer processes leading to disordering in STM and LEED, by allowing the curvature of the negative ion PES to be different to that of the ground PES. For the benzoate-Cu(110) system, the results of the extended model are significantly better if the ground and negative ion PESs have frequencies hω = 0.40 and 0.48 eV, respectively, rather than the same frequency. The large resonance width and asymmetry, but not the small overtone to fundamental excitation probability ratio, observed in HREELS can be accounted for with values of the coupling constant, β = 22.2 and inverse lifetime of Г = 0.6 eV (1.1 fs). These parameters, with n_D = 7 are simultaneously able to account for the ratio of the STM:LEED disordering cross-sections and, most significantly, predict the large slope observed in the STM disordering cross-section as a function of bias. The properties of the probability distribution, P_n(ε_i, for the NIR process, involving two Franck-Condon transitions, is contrasted with the DIET process and some non-classical behaviour for the case ω₀ ω₁ is noted.     

New stable microwave plasma passivated a-Si MIS cells with high open circuit voltage

A new microwave plasma surface passivation technique has been applied to a-SiH type solar cells at various low pressure O₂ and H₂O atmospheres. Open circuit voltageV _OC close to 900 mV and in excess of 900 mV are obtained with Ir, and with Pt, respectively, as continuous barrier metals. The corresponding unpassivated cells (Schottky type solar cells) show considerably lower values ofV _OC=650 mV. In contrast to published results involving Ni, Pd, Zr, and other metals as Schottky barriers, we have observed over a period of several months a remarkably good stability of the passivated devices.     

Cathode erosion on copper electrodes in steam,hydrogen, and oxygen plasmas

Experimental data are presented for cathode erosion rates on copper cathodes (or magnetically rotated arcs in steam, and mixtures of steam, hydrogen, and oxygen with argon. Measurements were also made of the arc voltage and velocity. The erosion rates for steam and oxygen plasmas were significantly lower than those .16r argon and hydrogen. Pure steam and 10% oxygen in argon gave erosion rates of 2.3 and 6.1 g/ C respectively while pure argon and 70% hydrogen in argon gave rates of 14.8 and 13.0 g/C respectively. Erosion rates decreased with increasing are velocities. The variation of arc velocity with operating conditions is described in terms of both aerodynamic and surface drag on the arc and arc root respectively.     

New In-Situ Sampling and Analysis of the Production of CeO<Subscript>2</Subscript> Powders from Liquid Precursors in a rf Thermal Plasma Reactor. Part 2: Analysis of CeO<Subscript>2</Subscript> Powders,Fuel Addition,and Image Analysis

A novel reactor design, sampling probe and wet collection system were used to investigate the combined effects of plasma operating parameters and particle collection mechanisms on the synthesis of CeO₂ particles from liquid precursors. The sampling of particles in-flight and the collection of particles at several reactor regions were used to provide experimental evidence of particle size at different reactor locations at various plasma operating conditions, i.e., power and plasma gas flow rates. This information provided a picture of how CeO₂ particles were formed and how these particles were collected in various locations. The effect of adding water-soluble fuels (alanine and glycine) to the original cerium nitrate solutions was also investigated. Fuel addition decreased the temperature of CeO₂ formation by acting as a local heat source as a result of fuel auto-ignition. Photographs of the particles in-flight were taken using a fast speed CCD camera.     

The behavior of titanium,stainless steel,and copper-nickel alloys as plasma torch cathodes

Cathode erosion continues to be a problem hindering the widespread application of plasma technology. In this work, cathode erosion was studied on titanium, stainless steel 314, copper-nickel 10% and 30%, and copper 122 for magnetically rotated arcs operating in argon, nitrogen, and argon/hydrogen mixtures at a constant magnetic flux density of 0.1 T Titanium and stainless steel gave very low erosion rates in argon (0.2 and 0.3, g/C respectively). Cupronickels were shown to be suitable for nitrogen and hydrogen plasmas. The slope of hydrogen solubility versus temperature in the cathode material was found to be important in determining hydrogen plasma erosion characteristics. When the plasma gas has a high solubility in the cathode material, or can react with the cathode, a negative erosion rate may result. When gas solubility in the cathode is low, oxide stability and mode of electron emission may govern the erosion rate. A high gas solubility in the cathode material, as with hydrogen, can result in mechanical erosion due to micro-explosions near the cathode surface.     

Characteristics and electrode erosion rates of a D.C. plasma torch operating with TiCl4 plasma gas

A stable TiCl₄ plasma was produced using a simplified d.c. torch. The arc was struck between two concentric ring electrodes, d mm apart, and rotated using a magnetic coil. The electrode material was a new TaC composite. The arc was operated at 100 A for up to I h using plasma gas containing 0–30% TiCl₄. The voltage was 24–60 V, depending on the TiCl₄ concentration. The electrode erosion rate was of the order of 20g/C. It is shown that the erosion rate is a function of the aerodynamic drag of the are column.