Formation Of Metal Droplets Research Articles

Molecular beam epitaxial growth and thermodynamic analysis of InGaAs and InAlAs lattice matched to InP

Reflection high-energy electron diffraction (RHEED) intensity oscillations have been used to detect the onset of metal droplet formation on the surface of InAs, InGaAs and InAlAs during molecular beam epitaxy. The growth conditions which produce such droplets are shown to be in good agreement with predictions based on thermodynamic arguments.

Etching of InP(100) 4×2 and molecular-beam epitaxially grown GaAs(100)-c(4×4) with atomic hydrogen

Phonon, plasmon, and vibrational modes of clean and hydrogenated molecular-beam epitaxially grown As-rich GaAs(100)-c(4×4) and ion-bombarded and annealed InP(100) 4×2 have been studied with high-resolution electron energy-loss spectroscopy. The plasmon energy at the interface of depletion layer and bulk enabled us to determine the carrier concentration for Si-doped GaAs samples. With increasing hydrogen exposure characteristic changes in plasmon energy, its intensity and the intensity of the surface optical phonons have been observed. This is explained by changes in the space charge regime. Using hydrogen as a local probe provides sensitivity to the stoichiometry and to the chemical bonding situation at the uppermost surface layer. It is shown that the As-terminated GaAs(100) surface can be converted finally into a Ga-terminated surface by etching with atomic hydrogen. For InP, for the very first time an interface plasmon was observed at 90 meV, due to high S doping. In contrast to GaAs, sequential hydrogenation results in the formation of metallic droplets (In), which increase in size and concentration with increasing hydrogen exposure. Consequently, the phonon and plasmon intensities are reduced drastically. Models of the atomic surface structures for different stages of hydrogenation are discussed.

Photo-excitation of single crystals of La 2CuO 4+δ near the metal-insulator transition

Transient photoconductivity (σ ph) experiments have been carried out in single crystals of insulating La 2CuO 4+δ near the metal-insulator transition ( δ≈ 1–2%). The time evolution of the σ ph changes dramatically with light intensity ( I L). At low I L, σ ph is characterized by power law time decay, t − α , in the nanosecond time regime, and the exponent, α, decreases significantly with increasing I L. As I L increases to 5 × 10 15 photons/cm 2, σ ph reaches ∼ 15 S/cm, and the lifetime of the conducting state is enhanced by more than two orders of magnitude: σ ph exhibits a delayed peak centered at approximately 30 ns. followed by exponential decay with time constant of ∼ 360 ns. The data reveal a continuous distribution of localized electronic states above the (hole) mobility edge; the M-I transition is achieved when the Fermi level is shifted away from the localized states and across the mobility edge either by chemical dopiing or by photo-excitation. The time of σ ph at high I L implies the formation of metastable metallic droplets after photo-excitation.

Influence of alloying elements and metalloids on the emulsion and separation behaviour of steel and slag

With the aid of surface and interfacial tension data it has been possible to clarify the effect of the elements nickel, titanium and sulphur in steel and titanium oxide and sulphur in slag on the emulsifying or separating tendencies of liquid steel/molten slag phases. This has been related to the tendency of (scattered) liquid steel films to adhere to gas bubbles crossing the interface, which in turn leads to the formation of metal droplets in the slag phase. Nickel containing steels tend to form emulsions, whereas titanium, both dissolved in the steel and as titanium oxide in the slag, tends to suppress adherence of the steel to the bubbles and promotes steel/slag separation. Sulphur containing steels show also an emulsifying tendency in contact with both sulphur containing and sulphur free slags. With sulphur present in the slag, however, the opposite effect is observed, so that, as sulphur transfer proceeds between the steel and the slag during desulphurization, the emulsifying tendency is markedly reduced, leading to steel/slag separation.

Growth mechanism of In 1− xGa xP and In 1− xAl xP in metalorganic molecular beam epitaxy

Crystal growth of InGaP and InAlP was carried out on GaAs(100) by MOMBE using TEIn, TEGa, TEAl and PH 3. Three distinct stages were observed in the temperature dependence of the growth rate and the Ga composition x of In 1− x Ga x P epilayers. The growth rate and the Ga composition increased with temperature up to 420°C, while they were constant at 420–520°C. By dividing the growth rate γ of In 1− x Ga x P into the two growth rates of quasi-binaries ( xγ for GaP and (1− x)γ for InP), the pyrolysis of TEGa was found to limit the growth below 420°C. Beyond 520°C, the growth rate decreased rapidly while the Ga composition increased. The formation of metal droplets on the epilayer surface above 520°C suggests that phosphorus evaporation governs the growth at higher temperatures. A quite similar temperature dependence was observed in In 1− x Al x P growth. The small activation energy in TEA1 decomposition is discussed.

Process control during high pressure atomization

High pressure inert gas atomization (HPIGA) has been studied using various metal alloy systems. The high yield of ultrafine (particles less than 45 μm in size) powder produced using HPIGA makes it an ideal system for producing rapidly solidified metal powder. High speed photography and laser scattering techniques have been applied to study droplet formation and measure powder size with the intent of future feedback and control of particle size during atomization. Liquid metal droplet formation will be discussed as well as on-line particle size measurement and control.