Cutoff frequencies of a parabolic-core W-type fibre

The cutoff frequencies of guided modes in a weakly guiding W-type fibre with parabolic core profile have been obtained. It is found that the cutoff frequency of the TE01-mode in such a fibre is much higher than for a homogeneous-core W-type fibre. This should allow greater ease in splicing.     

Simulation of complex molecules—Computational requirements for the 1980''s

Improving the understanding of complex molecular systems is an important and interesting use of supercomputers. Computer simulation currently plays, and will continue to play, a vital role in research in this area by acting as a guide to site-specific mutagenesis. This study explains simulations of complex molecules.     

Occurrence of Superconductivity and Magnetism in Nominally Undoped LaOFeAs

Superconductivity in a LaOFeAs system is known to get introduced by F-doping (LaO_1−x F _x FeAs) even under ambient conditions and oxygen deficiency (LaO_1−x FeAs) under high pressure conditions. Hitherto unreported, superconductivity in F-free undoped LaOFeAs samples is observed and confirmed for the first time by various characterization tools–resistive ρ(T,B), magnetic M(T,B) and modulated microwave absorption (MMA) measurements. The ρ(T) at B=0 shows a clear superconducting transition with an onset at T _con∼17 K and a tail-like behavior when R goes to zero at T _c0∼8 K. In the presence of B, the superconducting transition shifts to lower T with a rate ∼−5.5 and −1.65 T/K, depending on whether the ρ(T) has dropped to 90% or 5% of its normal state value, respectively. M(T) in zero field cooling at B=10 mT shows diamagnetic downturn below at ∼12 K. At T<T _c0 the change in MMA with B shows a low field (B∼1 mT) peak, which vanishes at T>T _c0, indicating the presence of weak link superconducting networks in the sample. The sample shows a complex electrical and magnetic behavior in the normal state. For instance, ρ(T) reveals a weak SDW-like anomaly at T∼132 K along with a resistivity minimum at T _min∼78 K. M(T) also shows the presence of a magnetic anomaly at T∼130 K. Both below and above T _con, presence of an additional ferromagnetic component is observed in the isothermal M(B) loop measurements. The superconducting and normal state features of our sample are compared with other undoped and doped LaOFeAs systems reported in the literature.     

Effect of chromium on the corrosion behaviour of powder-processed Fe-0.45wt% P alloys

The corrosion behaviour of Fe-0.45P with/without addition of chromium, prepared by powder forging route was studied in different environments. The corrosion studies in acidic (0.25M H₂SO₄ solution of pH 0.6) and neutral/marine (3.5% NaCl solution of pH 6.8) solutions were conducted using Tafel Extrapolation method. The rate of corrosion in alkaline medium (0.5M Na₂CO₃ + 1.0M NaHCO₃ solution of pH 9.4) was measured using linear polarization technique. The studies compare electrolytic Armco iron with Fe-P alloys. It was observed that, chromium improved the resistance to corrosion in acidic and marine environments. The corrosion rates were minimal in alkaline medium and low in neutral solution.     

Growth of highly (1 1 1) oriented CuIn0.75Al0.25Se2 thin films

CuIn_0.75Al_0.25Se₂ thin films prepared onto glass substrates at T_S=573 K were single phase, nearly stoichiometric and polycrystalline with a strong (1 1 1) preferred orientation showing sphalerite structure. The results of X-ray diffraction and electron diffraction studies are compared, interpreted and correlated with micro-Raman spectra. The optical absorption studies indicated a direct band gap of 1.16 eV with high absorption coefficient (>10⁴ cm^?1) near the fundamental absorption edge.     

Charge transport and structural morphology of HCl-doped polyaniline

The d.c. and a.c. (100 Hz–1 MHz) conductivities of HCl-doped polyaniline have been measured in the temperature range 77–300 K. At 77 K, the a.c. conductivity data, (), can be described by the relation ()=As, where the parameter s lies close to unity and decreases with increase in the doping level. The ratio of measured a.c. to d.c. conductivity shows dispersion at 77 K, which decreases with increase in the doping level. This decrease is found to be sharp around pH3.0. In the temperature range 77–150 K, the observed d.c. conductivity data can be described by Mott's three dimensional variable range hopping (VRH) model. Scanning electron microscopy studies reveal a sharp change in structural morphology of HCl-doped polyaniline at a pH3.0. A strikingly remarkable structural morphology has been observed in the formc of a channel at this pH value. This change is accompanied by a rapid increase in d.c. conductivity, dielectric constant, along with sharp changes in structural morphology, which indicates the existence of a doping-induced structural conductivity correlation in this system. © 1998 Chapman & Hall     

Optimizing information efficiency in a direct-sequence mobile packet radio network

We consider a direct-sequence multihop packet radio network using slotted ALOHA in a Rayleigh fading environment. We are interested in optimally choosing the transmission range, code rate, and slotted ALOHA transmission probability to be used by each node. We use a new performance measure, information efficiency, to analyze the network and show that the information efficiency of the low-cost mobile packet radio network can be improved approximately 20 fold by using the optimum parameters. We suggest a practical trellis-coded modulation scheme which comes close to realizing the theoretic limits found.     

Influence of Hg pressure on diffusion coefficient of As in HgCdTe

Arsenic diffusion coefficients were measured in HgCdTe at 350°C within the single phase field. The diffusion coefficients displayed a strong dependence on Hg pressure, increasing by more than 1×10³ with decreasing Hg pressure. These measurements were performed by growing As doped HgCdTe films by Hg-rich liquid phase epitaxy on undoped or In-doped base layers, where the growth temperature ranged between 330 and 350°C. Use of these low growth temperatures under Hg-rich conditions permitted attainment of virtual step profiles in As, with negligible diffusion into the base layers. These provided ideal starting points for subsequent diffusion anneals. Diffusion of arsenic under selected low Hg pressures was then employed to tune the positioning of the p/n junction for double layer heterojunction films, by locating it ahead of the heterointerface. Formation of valence band barriers to the photogenerated minority carriers across the junction could thus be avoided. When on the other hand, diffusion experiments were performed under Hg saturated conditions, the heterointerface moved at a faster rate than the p/n junction, leading to the formation of valence band barriers.     

Investigation of RF sputtered PSG films for MEMS and semiconductor devices

In this work, we report the preparation of phospho-silicate-glass (PSG) films using RF magnetron sputtering process and its application as a sacrificial layer in surface micromachining technology. For this purpose, a 76 mm diameter target of phosphorus-doped silicon dioxide was prepared by conventional solid-state reaction route using P₂O₅ and SiO₂ powders. The PSG films were deposited in a RF (13.56 MHz) magnetron sputtering system at 200-300 W RF power, 10-20 mTorr pressure and 45 mm target-to-substrate spacing without external substrate heating. To confirm the presence of phosphorus in the deposited films, hot-probe test and sheet resistance measurements were performed on silicon wafers following deposition of PSG film and a drive-in step. As a final confirmatory test, a p-n diode was fabricated in a p-type Si wafer using the deposited film as a source of phosphorus diffusion. The phosphorus concentration in the target and the deposited film were analyzed using energy dispersive X-rays (EDAX) tool. The etch rate of the PSG film in buffered HF was measured to be about 30 times higher as compared to that of thermally grown SiO₂ films. The application of RF sputtered PSG film as sacrificial layer in surface micromachining technology has been explored. To demonstrate the compatibility with MEMS process, micro-cantilevers and micro-bridges of silicon nitride were fabricated using RF sputtered PSG as a sacrificial layer in surface micromachining. It is envisaged that the lower deposition temperature in RF sputtering (<150 °C) compared to CVD process for PSG film preparation is advantageous, particularly for making MEMS on temperature sensitive substrates.     

Experimental studies of the force to extrude a rate sensitive material

Distinctions are drawn between three mean strain rates in extrusion: those averaged over the accumulated strain, the distance travelled through the die, and the elapsed time. It is shown that when the strain rate varies, the mean strain rate based on strain is always the greatest, and the mean strain rate based on time the smallest. Three further mean strain rates are introduced, which are root mean power (RMP) values of the above strain rates. Conventional estimates of the extrusion pressure for frictionless, homogeneous flow are based on rate insensitive analyses and lead to pressures proportional to the mean strain rate based on time. When the rate sensitivity during flow is taken into account, the calculated pressure is shown to be a function of the RMP strain rate based on accumulated strain, which is considerably greater than the mean strain rate based on time. Experiments are described in which ice was extruded by the indirect process, leading to nearly frictionless, homogeneous flow. Extrusion ratios of 4, 9, 25, 81, and 144 were used. In the first series of experiments, the ram velocity was adjusted to give the samerate insensitive (i.e. time-based) mean strain rate for all the dies. In the second series of experiments, the ram velocity was adjusted to give the samerate sensitive (i.e. RMP) mean strain rate for all the dies. In all cases the extrusion pressure was proportional to the RMP strain rate and not to the time-based strain rate. The rate sensitivity correction factor is also described.