Electrochemical behaviour of Ti–6Al–4V alloy and Ti in azide and halide solutions

Electrochemical techniques including open circuit potential measurement, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were used to evaluate the corrosion and passivation behaviour of Ti–6Al–4V alloy in sodium azide (NaN₃) solutions compared to the behaviour of its pure base metal Ti. The results showed that increasing azide concentration increases the rate of corrosion (i_corr) and shifts negatively the rest potential (E_f), as well as decreases the spontaneous thickening rates of the inner and outer layers constituting the passive oxide film on each sample. These effects are more accentuated for the alloy than for the metal. Moreover, the electrical resistance (R_ox) and the relative thickness (1/C_ox) of the oxide films on the two samples exhibit an almost linear decrease vs. NaN₃ concentration. The results suggested that addition of Al and V to Ti, although improves its machinability, yet it decreases the performance of its surface oxide film to protect the degradation of the metal. The alloy was found to be more susceptible to corrosion than its base metal, since Ti expresses higher apparent activation energy (E_a) for the corrosion process than Ti–6Al–4V. Electrochemical behaviour of Ti in azide medium was also compared with that in various halide solutions. It was found that Ti has a stronger propensity to form spontaneous passivating oxide layers in bromide more than in azide and other halide media, where the positive shift in the value of E_f and the simultaneous increase in the oxide film resistance (R_ox) follow the sequence: Br⁻ > > Cl⁻ > I⁻ > F⁻.     

Analysis of collector-storage building walls using phase-change materials

The use of thermal storage walls that serve both as solar collector and thermal storage is well known. The wall is usually composed of masonry or containers filled with water to provide sensible heat storage, i.e., storage resulting from the specific heat capacity of a material as it increases in temperature. An interesting alternative to the standard materials are phase-change materials (PCMs) which employ latent heat storage. Latent heat storage utilizes the energy associated with a change of state of a material such as the transition from a solid-to-liquid, or liquid-to-gas. The solid-to-liquid phase change is preferred for many applications because of the much smaller volume change resulting in this transition for a given amount of energy storage. This paper summarizes the results of a simulation study of the use of PCMs as a collector-storage wall.     

Corrosion behaviour of zirconium electrode in sodium azide electrolyte as compared to the alkali halide solutions

The kinetics of open circuit growth of an oxide film on zirconium electrode in NaN₃ solution of various concentrations was investigated using potential and capacitance measurements. The protective oxide film formed in azide solutions was found to thicken with time in two steps, the rate of oxide growth was found to decrease as the azide concentration increased. The corrosion behaviour of the electrode was characterized by ac impedance measurements to obtain detailed information about the effect of azide ion concentration on the electrical properties of the Zr electrode surface. The impedance response for the metal‐metal oxide‐electrolyte system was modeled with a transfer function. The potentiodynamic oxidation of Zr was also studied as a function of the azide ion concentration. The polarization curves showed the peak‐shaped active‐to‐passive transition and the corrosion rate was found to increase with increase of azide ion concentration. The activation energy of corrosion was calculated according to Arrhenius plot and found to be 14.5 kJ mol^?1. The polarization results in azide solutions were compared to those obtained in solutions of different halide ions, namely, F^?, Cl^?, Br^? and I^? ions. The rate of corrosion was found to decrease in the order Br^? > Cl^? > F^?, I^? > azide. Opposite to the behaviour in azide solutions, the halide ions do not show the active‐passive transition.     

Results of treatment in carcinoma of the bilharzial bladder

Radical cystectomy with some form of urinary diversion still remains the most commonly used method to treat patients with carcinoma of the bilharzial bladder. The reported 5-year survival rate following this procedure ranges from 27 to 38 per cent. Research efforts currently are directed to 2 areas. One area is screening the population at high risk by urine cytology for detection of early cases. This procedure may either improve the end results of existing methods of treatment or allow the adoption of techniques of lesser magnitude than radical cystectomy with its attendant relatively high mortality and inevitable physiologic and functional losses. Studies also are in progress in an attempt to enhance the therapeutic yield of the current treatment modalities. Preoperative radiation and cystectomy are examples. The possibility of promotion of the radioresponsiveness and improvement of the therapeutic response also is being explored by administration of chemical hypoxic cell sensitizers and/or the application of more optimized fractionation techniques.     

Thermal Conductivity of Lead Borate Glasses

Thermal conductivity of glasses in the system PbO-B₂O₃ at 30°C were determined by steady-state methods. The values decrease progressively with increasing PbO content. The experimental thermal conductivities and those calculated from factors representing the contributions of individual oxides (on a weight percent basis) agree fairly well.     

A viscoelastic-viscoplastic constitutive equation and its finite element implementation

A viscoelastic-viscoplastic constitutive model for isotropic materials undergoing isothermal infinitesimal deformation is proposed. The model is based on the assumption that the total strain rate is decomposable into a viscoelastic and a viscoplastic portion. Consequently, the model consists of a linear viscoelastic model in series with a modified plasticity model. This modified plasticity model adopts the classical Drucker-Prager yield surface with isotropic hardening and the associative flow rule of the invicid theory of plasticity. However, hardening is assumed to be a function of both the viscoplastic strain as well as the total strain rate. In this manner, the proposed model acquires the advantage of having both the initial and the subsequent yield surfaces to be a function of the strain rate, a property which has its experimental supportive evidence for viscoplastic materials such as polymers and some metals at highly elevated temperatures.A finite-element algorithm is developed to implement the constitutive equation derived in Part I. This algorithm adopts a combination of the tangent stiffness matrix and the initial load approach. The method of treating the transitional region between viscoelastic and viscoelastic-viscoplastic behavior is given. The details of implementation is described. Convergence of the computation scheme is discussed.Two examples are calculated numerically to demonstrate the strain rate and the pressure effects on the mechanical behavior of some viscoelastic-viscoplastic material. Results show that essential features in the stress-strain diagram obtained experimentally are exhibited by the model.     

A NUMERICAL METHOD IN SOLVING A COUPLED THERMOELASTICITY EQUATION AND SOME RESULTS

This paper discusses a numerical method for solving a coupled dynamical ther-moelasticity problem in a long hollow cylinder. The inner boundary conditions are the traction and the fluid temperature imposed on the inner surface of the cylinder, whereas the outer boundary is subjected to the ambient fluid temperature and is free of traction. This problem is solved by a finite element method in which the spatial and the time variables are discretized by several schemes. The study shows that the central explicit scheme for space discretization combined with a time marching scheme, which incorporates moderate damping, is suitable for this particular problem.

Numerical results of the temperature and stress responses due to a modified step or single pulse are presented and discussed. One interesting observation is that, under high rate of stress loading, the coupling in the energy equation could generate temperature variations of significant magnitude.     

Assesment of grid-connected photovoltaic systems in the Kuwaiti climate

Grid-connected photovoltaic (PV) systems is one of the most promising applications of PV systems. Till now, no detailed studies have been carried out to assess the potential of grid-connected systems in Kuwait. This work investigates the feasibility of implementing grid-connected PV systems in the Kuwaiti climate. The proposed system consists of crystalline solar modules mounted on the building roof and an inverter to convert PV dc output to ac voltage. The building receives electricity from both the PV array and the utility grid. In this system, the load is the total electrical energy consumption in the building.The objective of this work is to examine the performance as well as the economic feasibility of grid-connected PV systems in the Kuwaiti climate. A program is written to evaluate the performance as well as the economic feasibility of such systems in Kuwait. The input to the program is the weather data for Kuwait, time dependent building loads, as well as the utility rates for Kuwait. Weather data generator subroutine included in the program is used to generate hourly weather conditions from the monthly average values of daily radiation on horizontal surface, and ambient temperature available for Kuwait. The five-parameter PV model, which is applicable to both crystalline and amorphous PV modules, is used to determine the performance of the solar modules used in this study.The transient simulation program ( ) is used to link the components of the grid-connected PV system together. The inverter efficiency is represented as a linear function of input power. In this case, it is assumed that the AC output from the system will never be greater than the building load. Electricity tariffs will have an important impact on the cost-effectiveness of the system studied. The tariff used for electric utility is a flat rate per unit kWh of electrical energy. Simulations of the proposed system were carried out over the academic year.The building examined in this study is a flat roof building with a single story. The building roof area is large enough so that the PV arrays can be spaced widely to minimize shading losses. Different array slopes, and azimuth angles were studied to maximize the annual energy generated by the PV modules. Finally, the economic feasibility of grid-connected PV systems in Kuwait are examined.     

Solid-state diffusion as an efficient doping method for silicon nanowires and nanowire field effect transistors

In this work we investigate doping by solid-state diffusion from a doped oxide layer, obtained by plasma-enhanced chemical vapor deposition (PECVD), as a means for selectively doping silicon nanowires (NWs). We demonstrate both n-type (phosphorous) and p-type (boron) doping up to concentrations of 10(20) cm(-3), and find that this doping mechanism is more efficient for NWs as opposed to planar substrates. We observe no diameter dependence in the range of 25 to 80 nm, which signifies that the NWs are uniformly doped. The drive-in temperature (800-950?°C) can be used to adjust the actual doping concentration in the range 2 × 10(18) to 10(20) cm(-3). Furthermore, we have fabricated NMOS and PMOS devices to show the versatility of this approach and the possibility of achieving segmented doping of NWs. The devices show high I(on)/I(off) ratios of around 10(7) and, especially for the PMOS, good saturation behavior and low hysteresis.