Selective area epitaxy of CdTe

We have performed selective area epitaxy (SAE) of CdTe layers grown by molecular beam epitaxy using a shadow mask technique. This technique was chosen over other SAE techniques due to its simplicity and its compatibility with multiple SAE patterning steps. Features as small as 50 microns × 50 microns were obtained with sharp, abrupt side walls and flat mesa tops. Separations between mesas as small as 20 microns were also obtained. Shadowing effects due to the finite thickness of the mask were reduced by placing the CdTe source in a near normal incidence position. Intimate contact between the mask and the substrate was essential in order to achieve good pattern definition.     

Kinetic investigation of the carbothermal reduction of an Iranian clay

The kinetics of the carbothermal reduction of clay under argon atmosphere has been investigated by the X-ray diffraction method. The clay-carbon (excess) mixture was formed into 2 cm diameter disks of different thicknesses. Experimental data evidences the significant effect of sample thickness on the reaction rate at 1400 °C. Decreasing thickness promotes mullite dissociation and formation of SiC and alumina powders. Mathematical modeling of the reaction system showed the gas diffusion in the Knudsen regime through the pellet to be the rate controlling step. Diffusivity of CO and reacted core tortuosity factor have been calculated.     

Removal of aqueous Fe2+ using MnO2–clinoptilolite in a batch slurry reactor: Catalyst synthesis,characterization and modeling of catalytic behavior

An Iranian clinoptilolite has been modified with MnO₂ for the catalytic removal of Fe²⁺ cations from water in a batch slurry reactor. The modified zeolite was subjected to FESEM, XRD, WDX, XRF and specific surface area analysis. A correlation for the intrinsic catalytic reaction rate incorporating both Fe²⁺ and dissolved oxygen concentration as a function of reaction temperature has been presented. The effect of the modified zeolite aggregate particle size on the iron removal kinetics has been investigated. It was shown that for particles larger than 150 μm, diffusion through the mesopores of the zeolite aggregate is rate controlling. The effective diffusion coefficient through the particles at RT has been calculated as 2.3 × 10^?6 cm² s^?1. It is shown that liquid phase molecular diffusion within the mesopores is the dominating mass transfer mechanism.     

Low BER Performance Estimation of LDPC Codes via Application of Importance Sampling to Trapping Sets

We introduce an importance sampling (IS) method that successfully simulates the performance of Low density Parity Check (LDPC) Codes in an AWGN channel at very low bit error rates (BERs). By effectively finding and biasing bit node combinations that are the dominant sources of error events, called trapping sets, the developed technique provokes more frequent decoder failures. Consequently, fewer simulation runs and higher simulation gains are achieved.     

Reconciliation between experimental and Monte Carlo-based simulation of the pore size distribution in mesoporous silicon

It is demonstrated for the first time that mesoporous PS structures obtained by the electrochemical etching of p(+)(100) oriented silicon wafers might assume the peculiarity of invariance of the first peak positions in their pore size distribution curves, albeit for current densities far from the electropolishing region and at constant electrolyte composition. A new Monte Carlo-based simulation model is presented that predicts reasonably the pore size distribution of the PS layers and the observed invariance of peak position with respect to changes in current density. The main highlight of the new model is the introduction of a 'light avalanche breakdown' process in a mathematical fashion. The model is also able to predict an absolute value of 4.23?? for the smallest pore created experimentally. It is discussed that the latter value has an exact physical meaning: it corresponds with great accuracy to the width of a void created on the surface due to the exclusion of one Si atom.     

Taguchi design of experiments approach to the manufacture of one-step alumina microfilter/membrane supports by the centrifugal casting technique

The Taguchi design of experiments method was implemented for the optimization of the manufacture of sintered one-step alumina microfilter/membrane supports by the centrifugal casting technique for the first time. A 10 wt.% alumina aqueous slip containing Tiron (0.001 g/g alumina) as dispersant and PVA as binder were used. Acceleration (3 levels), slip volume (3 levels), binder content (3 levels) and pH (2 levels) were selected as controlling parameters (saturated L-9 array). The use of three different target functions has been discussed: (1) the product of top-layer surface porosity times the reciprocal of top-layer surface pore diameter; (2) the product of permeability times thickness; and (3) membrane curvature. It is deduced that the first target function is the most appropriate as far as the membrane characteristics of the sintered compact are concerned. Using this target function a distinct optimum configuration for the controlling parameter levels could be obtained.     

New Insights of the Glycine-Nitrate Process For the Synthesis of Nano-Crystalline 8YSZ

Nano-crystalline 8YSZ was synthesized by the glycine-nitrate process in a muffle furnace using zirconyl nitrate and yttrium nitrate as sources of oxidants. The effect of changing the elemental stoichiometric coefficient over a wide range (0.756–1.605) on the product characteristics (crystallite size, percent unreacted, surface area) has been investigated. It is demonstrated that the combustion reaction proceeds most efficiently in the fuel-lean region. A novel and simple mathematical model is presented that allows to evaluate the thickness of the sheet-like particles of the foamy final product based on BET data. The calculated thickness values are in good agreement with the experimental data. For the first time, the effect of using ammonium nitrate as a combustion aid for the stoichiometric precursor composition has been studied. The optimum amount of combustion aid addition has been determined.     

Multideterminant role of calcium in hippocampal synaptic plasticity

Hippocampal CA1 cells possess several varieties of long-lasting synaptic plasticity: two different forms of long-term potentiation (LTP) and at least one form of long-term depression (LTD). All forms of synaptic plasticity are induced by afferent activation, all involve Ca2+ influx, all can be blocked by Ca2+ chelators, and all activate Ca(2+)-dependent mechanisms. The question arises as how different physiological responses can be initiated by activation of the same second messenger. We consider two hypotheses which could account for these phenomena: voltage-dependent differences in cytosolic Ca2+ concentration acting upon Ca2+ substrates of differing Ca2+ affinities and compartmentalization of the Ca2+ and its substrates.     

Demonstration of 184 and 255-GHz Amplifiers Using InP HBT Technology

In this letter, 184 and 255 GHz single-stage heterojunction bipolar transistor (HBT) amplifiers are reported. Each amplifier uses a single-emitter 0.4 mum 15 mum InP HBT device with maximum frequency of oscillation (f_max) greater than 500 GHz and of 200 GHz. The 183 GHz single-stage amplifier has demonstrated gain of 4.3 plusmn 0.4 dB for all sites on the wafer. The 255 GHz amplifier has measured gain of 3.5d B and demonstrates the highest frequency measured HBT amplifier gain reported to date. Both amplifiers show excellent agreement with original simulation.