ALBERT

Notizen: The long-term compositional stability of hydrogenated and deuterated amorphous germanium was studied by infrared absorption measurements. The results reveal for substrate temperatures below about 200 °C the growth of a void-rich material which is oxidized rapidly by ambient oxygen and water.

Notizen: Single cyrstalline ZnTe layers have been successfully grown on (001) GaAs substrates by hot wall epitaxy at substrate temperatures between 280–370 °C. The vapor phase near the substrate surface was investigated by a quadrupole mass spectrometer. Transmission electron microscopy, reflectivity, and photoluminescence were used for growth optimization, impurity identification, and strain determination. The biaxial inplane strain is about ε(parallel)=−0.5% at the interface and ε(parallel) (320 °C)=−0.06% at the surface for thicknesses of 1–5 μm. Two luminescence peaks can be assigned to As and N.

Notizen: We have studied the strain in ZnTe epilayers grown by atmospheric-pressure metalorganic vapor-phase epitaxy on (001) GaAs and GaSb substrates. Reflectivity and absorption measurements are performed at 2 K using single-crystalline layers with thicknesses of 0.2–2 μm. The biaxial strain in the samples caused by the lattice mismatch of layer and substrate is deduced from the splitting of the degenerate heavy- and light-hole exciton. A polariton model is used to describe the reflectivity structure at the E0 gap and to determine the transverse exciton energies. The deformation potentials obtained from an analysis of the absorption structures are a=−5.5 eV and b=−1.4 eV. The critical thickness for ZnTe/GaSb is lower than 0.8 μm near thermodynamic equilibrium. It also slightly depends on growth temperature which has its optimum at 345 °C.

Notizen: A brief reply to the Comment is provided. Both the procedure I suggested and the tables presented by Carroll [Textile Res. J. 47, 56 (1977)] allow easy and fast estimation of contact angles.

Notizen: The effect of particulate volume fraction vp and diameter dp on the composite Young's modulus Ec is studied both experimentally, using a silica bead/epoxy system, as well as with the help of computer simulations. The experimental and simulation results show that for a given particulate size, the overall Ec vs vp curve displays a concave upward shape and not a linear shape. This superlinear trend of the data implies that the average strain normalized to the applied strain λ=ε¯p/εc transferred to the particulates increases with volume fraction. The above finding is explained in terms of a mean-field picture, where a single particle interacts with an effective medium consisting of the remaining particles embedded in the matrix. As the modulus of the effective medium surrounding a reference particle increases with vp, the modulus mismatch between the reference particulate and the medium is consequently reduced. This leads to an overall increase in the normalized average strain λ transferred to each particulate as vp is increased. The experimental results using silica particulates with various sizes dp, as well as the simulation results, show that smaller particulates provide an increased composite modulus as compared to larger particulates, at constant vp. General equations are developed, which relate the composite modulus to the average particle stress or strain, given only information about the volume fraction and the Young's modulus of each of the phases present.Through the application of these relations, it is found that smaller particulates display a greater amount of normalized average strain λ transferred than larger particulates. The effect of particulate Young's modulus Ep in combination with particulate size on the resulting Ec is also studied using simulations only. It is found that for a low particulate to matrix modulus ratio Ep/Em, the particulate size has very little influence on Ec. Moreover, the shape of the Ec vs vp curve can be well approximated by a straight line up to large values of vp. On the other hand, as the ratio Ep/Em is increased, the superlinear trend of the composite modulus Ec vs vp data is more apparent. This results in a smaller range of the Ec vs vp curve, which can be approximated by a linear function. It is also found that the extent of this linear region also decreases with particle size.

Notizen: A new approach is proposed for assessing the stress concentration factors in two-dimensional undirectional composite materials containing broken fibers. A closed-form expression is derived for the stress concentration factor profiles as a function of materials and geometrical parameters. The model differs significantly from earlier schemes as the local effect of a fiber break on nearest neighbors is much milder than previously assumed, both as a function of the interfiber distance (or fiber content) and of the number of adjacent broken fibers. Comparison with experimental results for SiC/epoxy composites demonstrates the validity of the proposed scheme. The model may help shed light on the (multifilament) fragmentation process [H. D. Wagner and A. Eitan, Appl. Phys. Lett. 56, 1965 (1990)] and on fracture nucleation and growth in composites.

Notizen: The fiber fragmentation phenomenon in the single-filament composite test is currently among the most intensively researched aspects of composite micromechanics. By this method the interfacial shear strength, a physical parameter which reflects the quality of the fiber/matrix bonding, is calculated. In most studies various statistical models for the distribution of fragment lengths have been assumed without any justification other than a good fit to experimental data. Here we argue that if the flaws are assumed to be distributed along the fiber length according to a spatial Poisson process, then far from the saturation point the resulting fragment lengths must exactly follow a shifted exponential distribution. At the saturation limit, the distribution of fragment lengths is still approximately exponential. This is confirmed by single-filament composite experiments with high strength carbon fibers embedded in epoxy. Cumulative distributions of interdefect spacings at various levels of stress (or strain) are made available by means of an experimental technique described here. It is shown that the resulting data may be used to unfold the average strength versus length dependence in the single fiber, and to calculate the relevant Weibull shape parameter.

Notizen: The influence of the plasma excitation frequency on the growth conditions and the material properties of microcrystalline silicon prepared by plasma enhanced chemical vapor deposition at low deposition temperature is investigated. It is found that an increase of the plasma excitation frequency leads to a simultaneous increase of the growth rate, the grain size, and the Hall mobility of microcrystalline silicon. This is attributed to an effective selective etching of disordered material creating more space to develop crystalline grains, while also more species for faster growth of the crystallites are available. © 1994 American Institute of Physics.

Notizen: Far infrared laser magnetic resonance (FIR-LMR) spectra of the CHD radical in its X˜3A‘ electronic ground state were observed and assigned. The radicals were generated in the reaction of Na atoms with CHDBr2. LMR spectra were observed using seven laser lines at wavelengths around 100 to 200 μm. The spectra were assigned to six different rotational transitions and the molecular parameters of CHD were determined by a least squares fit. A number of additional transitions, observed using a laser line at λ=184.3 μm but not yet assigned in detail, were attributed to a coupling between the accidentally almost degenerate NKaKc=505 and 413 rotational levels induced by the εab term in the spin–rotation Hamiltonian and by the off-diagonal components of the hyperfine coupling tensor.