Interfacial electrical properties of diamond-like carbon/gallium arsenide heterostructures

Deposition onto GaAs of dielectric film with good interfacial properties is difficult owing to the high surface state densities at the GaAs surface. Study of diamond-like carbon (DLC)/GaAs heterostructures is worthwhile because of the advantageous properties of insulating DLC film and the low temperatures involved in its preparation. In this paper, we compare the electrical interfacial properties of DLC/GaAs with those of DLC/Si and DLC/Ge structures. The DLC films were prepared by r.f. plasma-assisted chemical vapour deposition using a mixture of propane and n-butane. Capacitance measurements were taken in the frequency range 400 Hz to 1 MHz. The interface trap density distribution N_ss(E) was estimated for the DLC---Si interface using Terman's method. Compared with the DLC---Si and DLC---Ge interfaces, the DLC---GaAs interface shows qualitatively different behaviour. There is an indication that the interfacial layer at the DLC---GaAs interface is more conducting than the DLC film, giving rise to Maxwell-Wagner type relaxation. As a result, the effective permittivity of the DLC film is one order of magnitude higher. The formation of this interfacial layer could be related to pre-etching of the GaAs substrate using argon plasma.     

Ultrasonic Ply-by-Ply Detection of Matrix Cracks in Laminated Composites

In the design of cryogenic fuel tanks for the next generation Reusable Launch Vehicles (RLVs), permeability of cryogenic fuels across the thickness of the tank is a critical issue. In this context, the detection of matrix cracks and their connectivity takes on an unprecedented significance. In this work we develop an ultrasonic backscattering technique for the detection of matrix cracks in each of the plies of a damaged graphite/epoxy laminated composite.     

Initial design strategies for iterative design

This paper discusses methods to retrieve designs from a design library in order to achieve a better initial starting point for the iterative model of the design process. The motivation for doing this is to reduce the extensive analysis time required for many iterative design problems. Starting a design at a favorable initial point should help reduce the number of iterations. Four initial design methods have been investigated, varying from a simple nonlibrary method to methods that use designs from a library. The iterative design scheme used is conventional hill-climbing. To evaluate the effectiveness of these methods, the initial design methods were tested on four example problems. They are, a cantilever beam, a gear-pair, a V-belt, and an extruder-die. It was found that the number of iterations reduced approximately to the order of 1/n [referred in the rest of the article as O (1/n)], wheren is the number of stored values in the design library.     

Buckling of functionally graded and elastically restrained non-uniform columns

Columns with non-uniform distribution of geometrical or material parameters i.e. functionally graded material distribution, varying cross-sectional area and flexural stiffness provide an economical solution to carry the desired higher compressive loads in engineering structures. In this paper, a low-dimensional mathematical model is presented, which is capable of computing the buckling loads of uniform and non-uniform functionally graded columns in the axial direction. The columns with spatial variation of flexural stiffness, due to material grading and/or non-uniform shape, are approximated by an equivalent column with piecewise constant geometrical and material properties. Such a formulation leads to certain transcendental eigenvalue problems where the matrix elements are transcendental functions. This model is further extended in analyzing some uniform and non-uniform elastically restrained or braced axially graded columns with equal or unequal spans. The mathematical modeling, closed-form transcendental functions and numerical solution technique are described and several examples of estimating buckling loads for various boundary configurations are presented. Some of the results are also validated against available solutions, representing the convergence, effectiveness, accuracy and versatility of the proposed modeling and numerical method. Formulation of such low-dimensional eigenvalue problems can also be extended for analyzing, designing and optimizing the static and dynamic behavior of structural components that are made of functionally graded materials.     

Grain boundary effects on the electrical and magnetic properties of Pr2/3Ba1/3MnO3 and La2/3Ca1/3MnO3 manganites

Electrical and magnetic properties of orthorhombic Pr_2/3Ba_1/3MnO₃ (PBMO) and La_2/3Ca_1/3MnO₃ (LCMO) manganites with considerable difference in variance factors (σ²) are reported here. PBMO with higher variance exhibits distinct intrinsic (due to grains) and extrinsic (due to grain boundaries) transitions in the resistivity behaviour. Extrinsic effects, however, are not observed in the lower σ² LCMO system. Low field magnetoresistivity (LFMR) data also substantiate these results. Increase in the density of states obtained through Mott's 3-D variable range hopping mechanism in the paramagnetic insulating regime indicates the suppression of magnetic domain scattering with applied magnetic field. Ferromagnetic metallic regime below the extrinsic transition in PBMO seems to emanate from the electron-magnon scattering process. LFMR at 77 K also points towards the higher canting of spins in the vicinity of grain boundary regions in PBMO compared to that in LCMO.     

Adaptive learning algorithms for nernst potential and I-V curves in nerve cell membrane ion channels modeled as hidden Markov models

We present discrete stochastic optimization algorithms that adaptively learn the Nernst potential in membrane ion channels. The proposed algorithms dynamically control both the ion channel experiment and the resulting hidden Markov model signal processor and can adapt to time-varying behavior of ion channels. One of the most important properties of the proposed algorithms is their its self-learning capability-they spend most of the computational effort at the global optimizer (Nernst potential). Numerical examples illustrate the performance of the algorithms on computer-generated synthetic data.     

Organic photoconductors: Dark and photoconduction studies in two p-dimethylamino styryl dyes derived from pyridine-2 and pyridine-4

Results of our experiments on the dark and photoconduction studies in two p-dimethylamino styryl dyes derived from pyridine-2 (PDMS-P2) and pyridine-4 (PDMS-P4) in their pure form without any dopant or additive are reported. Measurements on surface-type (raster pattern) cells show that the dark and photocurrents are dependent on the applied potential, temperature, and the photocurrent on the intensity of the incident radiation and the wavelength. Action spectra of the samples could not be recorded as the intensity of the monochromatic radiation from the monochromator reaching the sample was too low to induce any noticeable photocurrent. The compounds show a low dark conductivity. The dark and photocurrents show a perfect ohmic behaviour in the temperature range studied (288–328 K). Since photoconduction could only be observed in a vacuum, this clearly indicates that the compounds are n-type semiconductors. The observed rise and decay kinetics of the photocurrents indicate the presence of traps in the forbidden zone. The dyes show an enhanced photoconduction on illumination with visible radiation only. The marked open circuit voltage and the short-circuit current observed in these compounds indicate a possible application in solar photovoltaics.     

Test Wrapper and Test Access Mechanism Co-Optimization for System-on-Chip

Test access mechanisms (TAMs) and test wrappers are integral parts of a system-on-chip (SOC) test architecture. Prior research has concentrated on only one aspect of the TAM/wrapper design problem at a time, i.e., either optimizing the TAMs for a set of pre-designed wrappers, or optimizing the wrapper for a given TAM width. In this paper, we address a more general problem, that of carrying out TAM design and wrapper optimization in conjunction. We present an efficient algorithm to construct wrappers that reduce the testing time for cores. Our wrapper design algorithm improves on earlier approaches by also reducing the TAM width required to achieve these lower testing times. We present new mathematical models for TAM optimization that use the core testing time values calculated by our wrapper design algorithm. We further present a new enumerative method for TAM optimization that reduces execution time significantly when the number of TAMs being designed is small. Experimental results are presented for an academic SOC as well as an industrial SOC.     

Kinetics of thermal degradation and thermo-oxidative degradation of conductive styrene-butadiene rubber-carbon black composites

The kinetics of the thermal degradation and thermo-oxidative degradation of conductive styrene-butadiene rubber (SBR)-carbon black composites were investigated using thermogravimetric analysis both in nitrogen and oxygen atmospheres. Experiments were carried out at heating rates of 5, 10, 15 and 20 °C/min in both the atmospheres. Friedman method, Kissinger method, Flynn–Wall–Ozawa method and Coats–Redfern method have been used to determine the activation energies of degradation. The invariant kinetic parameters using the IKP method were also determined. The results showed that the thermal stability of the composites in pure nitrogen is higher than that in air atmosphere and the increase in filler loading was found to increase the thermal stability in nitrogen atmosphere. The probable degradation mechanisms of the polymer in both the atmospheres were evaluated based on Fourier Transform Infra Red Spectroscopy (FT-IR) studies.