Temperature-modulated differential scanning calorimetry studies of the structure of bulk and film GexAsyS60 chalcogenide glasses

The recent novel temperature-modulated differential scanning calorimetry (TA Instruments MDSCTM) technique has been applied to the measurement of thermal properties of GexAsyS60 chalcogenide glasses in the glass transition region in bulk glasses and in their thin films. The reversing and non-reversing heat flows through the glass transformation region during both heating and cooling schedules were measured and the values of the parameters, Tg, ΔH, Cp and ΔCp, which characterize the thermal events in the glass transition region, were determined. The structurally determined parameters, Tg, ΔH, Cp and ΔCp, reveal significant changes with composition because in the GexAsyS60 glasses the average coordination number, 〈r〉, increases from 2.4 to 2.8 with increasing x from 0 to 40 at% Ge. A maximum in Tg, ΔH and Cp and a minimum in the heat capacity change, ΔCp, at Tg occur near the composition for which x≈30 at% Ge. These extrema which appear in both films and bulk glasses are ascribed to a change in the network function of Ge atoms replacing As atoms in a covalent network. Recent structural models for chalcogenide glasses have been considered to explain the observed thermal properties. This revised version was published online in November 2006 with corrections to the Cover Date.     

Kinematical transformations in amorphous selenium alloys used in xerography

Differential scanning calorimetric (DSC) studies were carried out on xerographic photoreceptor materials of amorphous selenium (a-Se), a-Se_1&#x2203-xTe_x and a-Se:0.5 wt% As alloys with various amounts of chlorine doping. Glass transformation kinetics of bulk and vapour-deposited film samples were essentially identical indicating a similar amorphous structure for both forms. The glass transition temperature,T _g, in the a-Se_1&#x2203-xTe_xsystem was found to increase monotonically with the tellurium content up to 13 wt% which was accompanied by a fall in the activation energy,E _gl, for the structural relaxation. 0.5wt% As increasedT _g, whereas chlorine doping in the amounts used (100 ppm) had no effect onT _g. Crystallization exotherms were analysed using the Kissinger method which was shown to remain a valid analysis by integrating the fundamental Kolmogoroff equation describing crystallization transformations by nucleation and growth. The signifiance of the activation energy,E _A, and the frequency factor,C ₀, in the Kissinger analysis is discussed and used in interpreting the different crystallization behaviour exhibited by the various alloys. The apparent activation energy,E _A, of the crystallization kinetics was found to fall with the temperature range accessed in the DSC experiment. When other authors' previous crystallization studies by other methods were also considered, then 1/E _A was found to fall nearly linearly with the mean reciprocal temperature, 1/T, used in the experiment. It is argued that this behaviour arises if the crystal growth rate scales with the inverse viscosity of the melt and the latter is described by a Vogel-Tammann-Fulcher type of behaviour. The additives tellurium and arsenic retard the crystallization essentially by increasing the viscosity. Chlorine doping was also found to have an inhibitory effect but due probably to a change in the nucleation kinetics, rather than a viscosity rise.     

Using heuristic algorithms for capacity leasing and task allocation issues in telecommunication networks under fuzzy quality of service constraints

Nowadays, every firm uses telecommunication networks in different amounts and ways in order to complete their daily operations. In this article, we investigate an optimisation problem that a firm faces when acquiring network capacity from a market in which there exist several network providers offering different pricing and quality of service (QoS) schemes. The QoS level guaranteed by network providers and the minimum quality level of service, which is needed for accomplishing the operations are denoted as fuzzy numbers in order to handle the non-deterministic nature of the telecommunication network environment. Interestingly, the mathematical formulation of the aforementioned problem leads to the special case of a well-known two-dimensional bin packing problem, which is famous for its computational complexity. We propose two different heuristic solution procedures that have the capability of solving the resulting nonlinear mixed integer programming model with fuzzy constraints. In conclusion, the efficiency of each algorithm is tested in several test instances to demonstrate the applicability of the methodology.     

A fuzzy long-term investment planning model for a GenCo in a hybrid electricity market considering climate change impacts

We study the long-term generation capacity investment problem of an independent power generation company (GenCo) that functions in an environment where GenCos perform business with both bilateral contracts (BC) and transactions in the day-ahead market (DAM). A fuzzy mixed integer linear programming model with a fuzzy objective and fuzzy constraints is developed to incorporate the impacts of imprecision/uncertainty in the economic environment on the calculation of the optimal value of the GenCo’s objective function. In formulating the fuzzy objective function we also include the potential impacts of climate change on the energy output of hydroelectric power plants. In addition to formulating and solving the capacity planning/investment problem, we also performed scenario-based (sensitivity) analysis to explore how investment decisions of the GenCos change when fuzziness (tolerance) in the maximum energy output of hydroelectric units and/or drought expectation increases. The proposed model is novel and investigates the effects of factors like drought expectations of climate changes, hydroelectric power plant investments, and other power generation technology investment options.     

Xerographic properties of a-Se:Te photoconductors

It is shown that the dark decay of the electrostatic surface potential on a corona-charged a-Se:Te alloy photoreceptor occurs via electric field-enhanced xerographic depletion discharge (FEXDD) in which Poole-Frenkel-assisted thermal emission of holes from deep mobility-gap states and their subsequent sweep out generates a negative bulk space charge. The theoretical model development is applied to explain the observed experimental dark discharge data over a wide range of charging (initial) voltages. It is shown that although the time required for the surface potential to decay to its half value t_1/2 initially increases with the charging voltage V₀ at the highest charging voltage, t_1/2 actually decreases with V₀. Results obtained from cycled-up xerographic experiments on single and double-layer photoreceptors are also reviewed and discussed in conjunction with transient photoconductivity experiments     

Vacuum deposited amorphous Se-Te photoconductor films: fractionation effects and charge transport in homogeneous films

A series of a-Se:Te photoconductor films of thickness 60 m were prepared by conventional vacuum deposition onto heated Al substrates to study fractionation effects. There is a Te-rich surface region, a plateau of relatively uniform bulk region, and a Se-rich zone near the substrate. For a source material of Se:12.5% Te (all compositions in wt %), the substrate surface of the film was about 5.5–6.2% Te and the top surface was Te-rich up to about 25% Te content. The plateau region is between approximately 10 m to 40 m from the substrate and has about 10% Te. The existence of the plateau region provides a means of preparing relatively homogeneous films by suitably shuttering the substrate during the evaporation to capture this plateau region. A first-order fractionation model of the evaporation process was developed to account for the observed compositional variation across the films. The fractionation model applies concepts from mass transport to the melt by considering a surface region (SR) of a fixed number of atoms in which rapid diffusional changes maintain a relatively uniform Te composition that changes over the course of the evaporation process. The model is able to explain the general features of the experimental results and is in reasonable quantitative agreement with the experiments. A series of nearly homogeneous a-Se_1-xTe_t films (composition homogeneous to within 10%) were prepared by suitably shuttering the vacuum deposition process to capture the plateau region. Charge transport in both homogeneous and fractionated films was studied using the conventional time-of-flight (TOF) and interrupted time-of-flight (IFTOF) transient photoconductivity experiments. As a-Se is alloyed with Te, the hole drift mobility _h decreases rapidly with the Te content up to about 5% Te beyond which the decrease in the mobility becomes small. Both hole and electron drift mobility has been found to depend on the electric field E as E ⁿ where n is a small index that increases with the Te content in agreement with earlier work on probably fractionated samples. Consequently, the power law dependence is not an artifact of the fractionation effect. Hole lifetime in Cl-doped a-Se:Te films from IFTOF experiments was found to increase with the Te composition, nearly in proportion to the decrease in the drift mobility, in agreement with the shallow-trap-controlled model proposed for hole transport in a-SeTe films.     

Electrical properties of thick film resistors from noise measurements

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Thermal and photoluminescence properties of Er2S3-doped (As2Se3)90(GaSe)5Ge5 glasses

Thermal and photoluminescence characteristics of (As₂Se₃)₉₀(GaSe)₅Ge₅ glasses doped with different amounts of Er₂S₃ (Er amount from 0.1 to 3 at %) have been studied. Temperature-modulated differential scanning calorimetry measurements were used to study the glass transformation and thermal stability of these glasses. A 980-nm laser diode was used to pump the Er³⁺-doped glass samples, inducing a 1550-nm photoluminescence output signal. By appropriately pulsing the excitation, we measured the photoluminescence lifetime of the Er³⁺ state in these chalcogenide glasses.     

Study on growth and characterizations of GaxIn1−xP/GaAs solar cell structure

Ga_xIn_1?xP/GaAs solar cell (SC) structure was grown on p-type (100)-oriented GaAs substrate by using solid-source molecular beam epitaxy technique. The structural, optical and morphological properties of Ga_xIn_1?xP/GaAs SC structure have been evaluated by means of high resolution X-ray diffraction, photoluminescence, spectroscopic ellipsometry and atomic force microscopy measurements at room temperature. In addition, Ga_xIn_1?xP/GaAs SC structure was fabricated for obtaining the cell’s electrical output parameters. For this purpose, the current–voltage characteristics of Ga_xIn_1?xP/GaAs SC structure were performed and analyzed at the room temperature under both dark and illuminations by using air mass global 1.5 (AM1.5) solar simulator. The device parameters such as the open-circuit voltage, the short-circuit current (I _sc ), the fill factor and the energy convertion efficiency (η) of Ga_xIn_1?xP/GaAs SC structure were extracted from the current–voltage characteristics.     

Selective separation of nickel from cobalt in ammoniacal solutions by emulsion type liquid membranes using 8-hydroxyquinoline (8-HQ) as mobile carrier

The selective separation and concentration of nickel from ammoniacal solutions containing nickel and cobalt by an emulsion liquid membrane (ELM) technique using 8-hydroxyquinoline (8-HQ) as carrier has been examined. The emulsion liquid membrane consists of a diluent (kerosene), a surfactant (ECA 4360J), a carrier (8-HQ), and a stripping solution (0.025 M EDTA solution, buffered at pH 4.0). Cobalt (II) in the 6 M ammonia feed solution was oxidised to cobalt (III) by adding H₂O₂ and the pH adjusted to 10 with hydrochloric acid (HCl). The important variables were found to be membrane composition, ammonia concentration, diluent type, surfactant concentration, extractant concentration, EDTA concentration in the stripping solution, pH of the feed and the stripping solutions, phase ratio, and treatment ratio. It was possible to selectively extract 96.5 to 99.0% of nickel from a mixture of nickel and cobalt.