Electrochemical Studies on the passive behaviour of tin in different buffer solutions

The effect of pH on the passivity of Sn has been investigated in buffered (phthalate-borate, and citrate phosphate buffer series), and in unbuffered phosphate solutions, using the cyclic voltammetric technique. Two anodic peaks; the first being more pronounced than the second, and on cathodic peak have been observed in the two buffer series at different scan rates and pH values. The anodic and cathodic peak current densities (I_p) and potentials (E_p) are functions of the scan rate (v) and the pH value. Plots of I_p against v^1/2 yield straight lines at each pH value. The effect of pH on I_p for the first anodic peak shows a shallow minimum in the near neutral and slightly alkaline range at all scan rates. Straight lines are also observed between E_p and v^1/2, the extrapolation of which to v = 0 gives the spontaneous (no polarization) oxidation or reduction potentials (E′ and E″). The absence of polarization effects in E′ and E″ make them the most suitable values for comparison with thermodynamic data. Therefore, the two straight lines obtained between E′ and E″ on one hand and the pH on the other hand to give an estimate of (i) the slope (dE°/dpH), and (ii) the value of E° at pH - 0 (from intercept). Comparison of the two experimental values with all available thermodynamic data for Sn(II) and Sn(IV) oxides and hydroxides shows that: (i) the first anodic peak represents the formation of Sn(OH)₂ from Sn and OH^?, (ii) the second anodic peak represents the formation of Sn(OH)₄ directly from Sn and OH^?, and (iii) the cathodic reduction peak correponds to the reduction of Sn(IV) oxidized species to Sn. The results in citratephosphate buffer have been treated in the same manner. However, the result of (dE°/dpH) and E° at pH = 0 deviate from thermodynamic data, because of the possible participation of other anodic reactions such as the formation of soluble Sn compounds, Sn complexes, and incorporation of anions in the anodic film.     

Temperature dependence of the anodic oxidation of titanium below the oxygen evolution potential

The growth of thin anodic oxide films on Ti in the temperature range between 20 and 70° C can be expressed in purely coulometric terms. In diluted and aerated borate solutions the potential increases linearly with time; the slope of the curves increases with increasing current intensity; after 10 to 20 min all the curves show a transition to a flattened shape. Temperature changes have but an insignificant influence on the general character of the curves; the transition to the flatter portion occurs after about 20 min, too, but potentials are the lower, the higher the temperature. The values of the activation energy calculated from the results show, that the layer formation is not only controlled by the bound are face reactions, but also by certain factors coming from the bulk of the solution.     

Cavity growth in neutron-irradiated magnesium oxide

The growth of cuboidal cavities in neutron-irradiated magnesium oxide after annealing in the temperature range 1475 to 1775° C has been followed using the techniques of transmission electron microscopy and electron spin resonance. Microscopic examination has shown that, while cavities are nucleated on annealing at about 1500° C, most of their growth does not occur until a temperature approaching 1625° C is exceeded. Electron spin resonance spectra from the same samples annealed in the temperature range 1475 to 1575° C indicate that some of the vacancies, which are released to the lattice when cavities are nucleated, are used in the formation, from iron present at impurity level in the crystals, of Fe³⁺ in octahedral symmetry at magnesium sites. This corresponds to the region of negligible cavity growth. For annealing treatments at and above 1625° C however, the fractional volume of crystal occupied by cavities increases by a factor of ten and simultaneously the octahedral symmetry Fe³⁺ transitions disappear. The role of iron in controlling vacancy movement and cavity growth is discussed.     

Anodic polarization of tungsten in neutral and alkaline solutions under conditions of anode film growth

Anodic polarization of W has been studied in neutral (Na₂SO₄, NaCl) and alkaline solutions (ammonium borate, Na₂CO₃, Na₃PO₄, and NaOH) by the galvanostatic method under conditions which lead to anode film growth. The rate of oxide growth has been calculated as a function of current density, nature and concentration of electrolyte. Film growth to potentials above 100 V is achieved in alkaline solutions, provided the solution is dilute and the current density is high. The applicability of the high field ionic conduction has been verified from the dependence of both reciprocal capacity and potential on the logarithm of current density. Data are recorded for the coulometric increase of oxide thickness. The constants of the exponential law, and the electric field.     

Galvanostatic formation of barrier-type Anodic oxides

Experimental Factors Affecting Oxide Growth Results of galvanostatic anodization of various metals with varying valve-metal behaviour including Nb, Ta, Zr, Ti, Al, W, Te, Sb, and Bi are compiled and compared with respect to the effect of various experimental factors such as surface pretreatment, stirring, solution composition, temperature of the bath, on the efficiency of oxide growth relative to that for other anodic processes. It is concluded that all of the above factors play an important role in the anodization of the non-typical valve metals, particularly Sb, Bi and W. Typical anodic charging curves associated with the process of oxide growth are given for several valve metals. Experimental data relevant to the phenomena of oxide breakdown are recorded and compared. The results indicate that the mechanism of breakdown varies for the same metal with variations in the experimental conditions.     

Cylindrical projection improved formulation for sunshine assessment in buildings

A method which extends the use of the cylindrical projection tool for the assessment of sunshine was developed. This method is a computer-based mathematical analysis using trigonometric functions. It enables to project building components and sun’s position in the sky on a two-dimensional cylindrical projection chart for a wide range of simple and complex plane shapes, namely; inclined and non inclined vertical planes, segment of circular planes, semi-circular planes and circular planes. In this method, cylindrical projection will be more handled and manageable and will give more sensitive results. It uses Matlab programming language and its graphical potentialities. It will provide to designers and architects a quick and reasonable assessment of sunshine availability for complex building configurations at any time on any date of a given locality and consequently saving time and achieving a successful project. However, this projection method, may exhibit some limitations of its use in low latitudes (<20°) if the sun is high in the sky.     

Design,modeling, and optimization of a dual reflector parabolic trough concentration system

The aim of the present work is to enhance the thermal management avoiding the high-thermal stress on the outer surface of the parabolic trough receiver (PTR) derived from nonuniform concentrated solar flux distribution. A parabolic trough concentrating (PTC) system with second homogenizing reflector (HR) is numerically designed and optimized to ensure a uniform concentrated solar flux on the PTR walls. For this purpose, a three-dimensional optical model has been developed to analyze quantitatively the improvement made by the HR using the optical efficiency and qualitatively basing on the uniformity of the solar flux density distribution over the entire surface of the PTR. The validation of the numerical tool is presented, and the algorithm of the design process has been proposed and detailed. As a preliminary trait, it was revealed that the peak of the designed system performance is achieved with a rim angle of 68° avoiding simultaneously the aberration and the blocking effects. Despite the optical efficiency decrease by about 7% compared with the conventional PTC design, the uniformity of the solar flux distribution has been strongly improved such that the maximum local solar flux density gradient is decreased from 80 to 11 kW/m² equivalent to a decrease of 86.25% with respect to the conventional PTC and the average local density is about 25.5 kW/m².