• Volume 26, Issue 2

February 2003,   pages  207-278

• The promise of fuel cell-based automobiles

Fuel cell-based automobiles have gained attention in the last few years due to growing public concern about urban air pollution and consequent environmental problems. From an analysis of the power and energy requirements of a modern car, it is estimated that a base sustainable power of $ca$. 50 kW supplemented with short bursts up to 80 kW will suffice in most driving requirements. The energy demand depends greatly on driving characteristics but under normal usage is expected to be 200 Wh/km. The advantages and disadvantages of candidate fuel-cell systems and various fuels are considered together with the issue of whether the fuel should be converted directly in the fuel cell or should be reformed to hydrogen onboard the vehicle. For fuel cell vehicles to compete successfully with conventional internal-combustion engine vehicles, it appears that direct conversion fuel cells using probably hydrogen, but possibly methanol, are the only realistic contenders for road transportation applications. Among the available fuel cell technologies, polymer–electrolyte fuel cells directly fueled with hydrogen appear to be the best option for powering fuel cell vehicles as there is every prospect that these will exceed the performance of the internal-combustion engine vehicles but for their first cost. A target cost of $50/kW would be mandatory to make polymer–electrolyte fuel cells competitive with the internal combustion engines and can only be achieved with design changes that would substantially reduce the quantity of materials used. At present, prominent car manufacturers are deploying important research and development efforts to develop fuel cell vehicles and are projecting to start production by 2005. • Preparation of microwave dielectric, Sn0.2Zr0.8TiO4 A simple coprecipitation technique is described for the preparation of tin substituted zirconium titanate ceramic powders. • Effect of sillimanite beach sand composition on mullitization and properties of Al2O3–SiO2 system Mullite was developed by reaction sintering of sillimanite beach sand and calcined alumina. Two varieties of sillimanite beach sand viz. S and Z having different compositions were selected. Synthesis and properties of mullite were very much dependent on the sillimanite beach sand composition. Presence of higher amount of impurities in the Z-variety of sillimanite sand favours the densification by liquid phase formation. Presence of zircon in Z-variety increases the hardness and fracture toughness. Alumina addition improves the mechanical/thermomechanical properties of the samples. Mullite retains the usual orthorhombic habit of sillimanite. Rounded to sub rounded zirconia dispersed within the mullite matrix of the sample ZA is noticed. • Role of binder in the synthesis of titania membrane The synthesis of titania membrane through sol–gel route involves hydrolysis of alkoxide, peptization of hydrous oxide of titanium to obtain a sol, adjustment of the sol viscosity by including a binder and filtration of the viscous sol through a microporous support, gelation and sintering to desired temperature. The binder plays an important role in that it not only helps in adjustment of sol viscosity but also helps in binding the sol particle with porous support. Here a comparative study on the role of different binders, viz. polyvinyl alcohol, polyethyleneimine, polyacrylamide, effect of their viscosity and surface tension effect on the morphology of the titania membrane is presented. The results show that among the three binders studied polyvinyl alcohol gave rise to membranes of desired characteristics when the sol viscosity was 0.08 pa.s. • Effect of silica sol of different routes on the properties of low cement castables Silica sols synthesized through different routes viz. inorganic, organic and ion-exchange routes, have been incorporated in the low cement alumina castable composition in various proportions and cast samples in the form of 2 inch cube briquettes were prepared. The cast briquettes after curing were subjected to heat treatment at various temperatures. The effect of different sources of such sols on the important physico-mechanical properties of the castables such as bulk density, apparent porosity, compressive strength, volume shrinkage and residual strength after spalling have been studied. Some of the selected samples were also subjected to microstructural evaluation. It is observed that silica sol synthesized through cation exchange routes has significant beneficial role on the properties of castable with 3% addition. • Deposition of TiN/CrN hard superlattices by reactive d.c. magnetron sputtering Multilayer superlattice coatings of TiN/CrN were deposited on silicon substrates using a reactive d.c. magnetron sputtering process. Superlattice period, also known as modulation wavelength (𝛬), was controlled by controlling the dwell time of the substrate underneath Ti and Cr targets. X-ray diffraction (XRD), nanoindentation and atomic force microscopy (AFM) were used to characterize the films. The XRD data showed 1st and 2nd order satellite reflections along the principal reflection for films having 132 Å$\geq \Lambda \geq$84 Å, thus confirming the formation of superlattice. The multilayer coatings exhibited hardness (𝐻) as high as 3200 kg/mm2, which is 2 times the rule-of-mixtures value (i.e.$H_{TiN}$= 2200 kg/mm2 and$H_{CrN}$= 1000 kg/mm2). Detailed investigations on the effects of various process parameters indicated that hardness of the superlattice coatings was affected not only by modulation wavelength but also by nitrogen partial pressure and ion bombardment during deposition. • Studies on thin film materials on acrylics for optical applications Deposition of durable thin film coatings by vacuum evaporation on acrylic substrates for optical applications is a challenging job. Films crack upon deposition due to internal stresses and leads to performance degradation. In this investigation, we report the preparation and characterization of single and multi-layer films of TiO2, CeO2, Substance2 (E Merck, Germany), Al2O3, SiO2 and MgF2 by electron beam evaporation on both glass and PMMA substrates. Optical micrographs taken on single layer films deposited on PMMA substrates did not reveal any cracks. Cracks in films were observed on PMMA substrates when the substrate temperature exceeded 80°C. Antireflection coatings of 3 and 4 layers have been deposited and characterized. Antireflection coatings made on PMMA substrate using Substance2 (H2) and SiO2 combination showed very fine cracks when observed under microscope. Optical performance of the coatings has been explained with the help of optical micrographs. • Impedance spectroscopy study of polycrystalline Bi6Fe2Ti3O18 The electrical properties of polycrystalline Bi6Fe2Ti3O18 are investigated by impedance spectroscopy in the temperature range 30–550°C. The imaginary part of impedance as a function of frequency shows Debye like relaxation. Impedance data are presented in the Nyquist plot which is used to identify an equivalent circuit and the fundamental circuit parameters are determined at different temperatures. The grain and grain-boundary contributions are estimated. The results of bulk a.c. conductivity as a function of temperature and frequency are presented. The activation energies for the a.c. conductivity are calculated. The polaron hopping frequencies are estimated from the a.c. conductivity data. • Ion transport in Au+ doped/undoped KDP crystals with KI/NaI as additives Undoped KDP and KDP crystals containing KI/NaI with/without gold doping were grown by slow evaporation technique. All the grown crystals were 𝛾 -irradiated using 60Co source. Electrical conductivity measurements were carried out on all these crystals perpendicular to the unique direction before and after 𝛾-irradiation. The present results show that the conductivity of KDP crystals increases with the addition of KI/NaI and with gold doping as well as upon rise in temperature. Computed values of activation energies from the conductivity measurements are given. For all the grown crystals, dielectric constant is measured as a function of frequency. • Systematic hardness measurements on mixed and doped crystals of rubidium halides Efforts are made to improve the hardness of rubidium halide crystals by 1. solid solution hardening and 2. impurity hardening. Systematic microhardness measurements have been made on rubidium halide mixed crystals (RbBr–RbI and KI–RbI) and rubidium halide crystals doped with Sr2+ ions. The composition dependence of the hardness of mixed crystals follows the law$\Delta H_V$=$K\ x$(1–𝑥), where$\Delta H_V$is the enhancement in hardness, 𝐾 a constant and 𝑥 and (1 – 𝑥) the concentrations of the first and second component of the mixed crystals, respectively. The hardness of doped crystals increases with the concentration 𝐶 of the dopant according to the law,$\Delta H_V$=$k\ C^m$, where 𝑘 and 𝑚 are constants. The relative efficacy of the two methods of hardening is discussed. • Thermally stimulated discharge current (TSDC) and dielectric constant of semiconducting glasses In this paper the results of thermally stimulated discharge current (TSDC) and dielectric constant for 40PbO–60Bi2O3 glass thermoelectrets are presented. Measurements of TSDC and dielectric constant,$\varepsilon'\$, have been carried out in the temperature range 30–300°C. The thermoelectrets were prepared at different polarizing fields. The various observed peaks in the thermograms are discussed on the basis of space charge polarization. The trap energy is evaluated from the Garlick–Gibson plot of initial rise method. Similarly other parameters such as relaxation time, charge release etc are evaluated.

• Fine structure in the inter-critical heat-affected zone of HQ130 super-high strength steel

The microstructure in the inter-critical heat-affected zone (ICHAZ) of HQ130 steel, has been investigated by thermo-simulation test, SEM and TEM. The problem of toughness decrease in the ICHAZ (𝑇p = 800°C) as well as the effect of M–A constituent and carbide precipitation on brittleness was analysed. The test results indicated that the microstructure in the ICHAZ of HQ130 steel was mostly a mixture of lath martensite (ML) and granular bainite (Bg) with a fine but nonuniform grain structure. The cause of brittleness in the ICHAZ was related to production of the M–A constituent in the local region and carbide precipitation. By controlling the welding heat input carbide precipitation and the formation of the M–A constituent can be avoided or decreased.

• # Bulletin of Materials Science

Volume 43, 2020
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• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019