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.     

Mass Transfer Mechanisms in the Fixed‐Bed Ion‐exchange Process for Dilute Colloidal Silica Manufacture

The ion exchange behavior of the H⁺ form C‐100 Purolite resin for the production of colloidal silica from dilute sodium silicate solutions has been investigated. The exchange isotherm has been found to be almost irreversible. The effective resin diffusion coefficient has been found to be 2.84 × 10^–10 m² s^–1 using a shrinking core model for the batch uptake experiments. Fixed bed experiments for different column heights and feed flow rates were performed. Numerical solution of the governing equations showed that the process is initially controlled by film diffusion and consequently by resin diffusion. Axial dispersion had to be taken into account. A simple power law correlation has been determined that relates the fluid Peclet number to the Reynolds number. It is presumed that some sort of resin ‘deactivation’ due to intraparticle microgel formation is responsible for the sluggish end part of the breakthrough curves.     

Effect of electric arc and ladle furnace slags on the strength and swelling behavior of cement-stabilized expansive clay

Bulletin of Engineering Geology and the Environment - The current research evaluated the effect of expansive clay stabilization using a combination of cement (0, 2, 5, 8%) with electric arc furnace...     

Strength Properties of Hexametaphosphate Treated Soils

The present technical note focuses on the potential application of sodium hexametaphosphate (HMP) in geo-environmental engineering. HMP may have many applications in geotechnical and geo-environmental engineering such as: improvement and stabilization of tailing dams, rehabilitation of old landfills, construction of new clay liners, containment and permeable barriers, etc. In this technical note, it is observed that the strength of silty-clayey sand containing a high concentration of metals (like tailings) is increased by mixing of HMP. On the other hand the strength of the kaolinite clay that may be used during the construction of a liner has been reduced by mixing with HMP. In order to study the influence of HMP, soil mixtures with various amount of HMP are prepared to conduct unconfined compression tests. The strength of the HMP-treated kaolinite specimens is always less than the kaolinite specimens for four weeks. HMP has decreased the strength of silty sand during the first days. However, the cations in the soil (specially Ca and Fe) may lead to precipitation and sorption mechanism to govern and, therefore, the strength increases noticeably up to 740%.     

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.