pp 175-180 June 2002 Nanomaterials
Synthesis and characterization of undoped and Mn2+ doped ZnS nanocrystallites (radius 2–3 nm) embedded in a partially densified silica gel matrix are presented. Optical transmittance, photoluminescence (PL), ellipsometric and electron spin resonance measurements revealed manifestation of quantum size effect. PL spectra recorded at room temperature revealed broad blue emission signal centred at ∼ 420 nm and Mn2+ related yellow–orange band centred at ∼ 590 nm while ESR indicated that Mn in ZnS was present as dispersed impurity rather than Mn cluster.
pp 181-189 June 2002 Nanomaterials
Sinterable nano silicon carbide powders of mean particle size (37 nm) were prepared by attrition milling and chemical processing of an acheson type alpha silicon carbide having mean particle size of 0.39 𝜇m (390 nm). Pressureless sintering of these powders was achieved by addition of boron carbide of 0.5 wt% together with carbon of 1 wt% at 2050°C at vacuum (3 mbar) for 15 min. Nearly 99% sintered density was obtained. The mechanism of sintering was studied by scanning electron microscopy and transmission electron microscopy. This study shows that the mechanism is a solid-state sintering process. Polytype transformation from 6H to 4H was observed.
pp 191-196 June 2002 Thermal Properties
The low thermal expansion ceramic system, Ca1-𝑥Sr𝑥Zr4P6O24, for the compositions with 𝑥 = 0, 0.25, 0.50, 0.75 and 1 was synthesized by solid-state reaction. The sintering characteristics were ascertained by bulk density measurements. The fracture surface microstructure examined by scanning electron microscopy showed the average grain size of 2.47 𝜇m for all the compositions. The thermal expansion data for these ceramic systems over the temperature range 25–800°C is reported. The sinterability of various solid solutions and the hysteresis in dilatometric behaviour are shown to be related to the crystallographic thermal expansion anisotropy. A steady increase in the amount of porosity and critical grain size with increase in 𝑥 is suggested to explain the observed decrease in the hysteresis.
pp 197-201 June 2002 Electrical Properties
Proton conducting composites of heteropolyacid hydrates (phosphomolybdic acid H3PMo12O40.𝑛H2O,PMA; phosphotungstic acid H3PW12O40.𝑛H2O, PTA) and salt hydrate like NiCl2.6H2O were prepared with insulating Al2O3 as dispersoid. The ionic conductivity peaks at two concentrations of Al2O3 indicating two percolation thresholds for proton conduction. Two separate experiments were carried out to check the existence of such percolation thresholds viz. the volta battery experiment involving the measurement of e.m.f. of an electrochemical cell with composites of different compositions used as electrolyte and the composition vs conductivity measured by the complex impedance spectroscopy. The presence of two maxima has been attributed to two different percolation thresholds for the two possible mobile protonic species 𝐻+ (H3O+) and OH– arising from the hydrates.
pp 203-207 June 2002 Mechanical Properties
Vickers and knoop hardness measurements were carried out on CsBr and CsI single crystals. Polycrystalline blanks of CsCl, CsBr and CsI were prepared by melting and characterized by X-ray diffraction. Vickers hardness measurements were carried out on these blanks. The hardness values were correlated with the lattice constant and the Schottky defect formation energy.
pp 209-211 June 2002 Mechanical Properties
The effect of hydrogen on the mechanical behaviour of two carbon-alloyed iron aluminides was studied. Weakening of some carbide–metal interfaces in the presence of hydrogen was indicated. The effect of cathodic hydrogen charging on the microstructure has also been addressed.
pp 213-217 June 2002 Alloys and Steels
Under the condition of tungsten inert gas shielded welding (TIG) + shielded metal arc welding (SMAW) technology, the microstructure in the welding zone of 9Cr–1Mo–V–Nb (P91) heat-resisting steel is studied by means of X-ray diffractometry (XRD) and transmission electron microscopy (TEM). The test results indicate that when the weld heat input (𝐸) of TIG is 8.5 ∼ 11.7 kJ/cm and the weld heat input of SMAW is 13.3 ∼ 21.0 kJ/cm, the microstructure in the weld metal is composed of austenite and a little amount of 𝛿 ferrite. The substructure of austenite is crypto–crystal martensite, which included angle. There are some spot precipitates in the martensite base. TEM analysis indicates that the fine structure in the heat-affected zone is lath martensite. There are some carbides (lattice constant, 1.064 nm) at the boundary of grain as well as inside the grain, most of which are Cr23C6 and a little amount of (Fe, Me)23C6.
pp 219-225 June 2002 Alloys and Steels
This work deals with the structural modifications of FeSi and FeSiAl foils when subjected to a thermochemical nitriding treatment (TNT) performed in a fluidized-bed laboratory furnace. The investigations on the nitrided samples were carried out by optical and SEM microscopic observations, X-ray diffraction and Mössbauer spectroscopy. Both the compound and diffusion layers were investigated.
pp 227-230 June 2002 Thin Films
Highly conducting and transparent ZnO : Al thin films were grown by off-axis rf magnetron sputtering on amorphous silica substrates without any post-deposition annealing. The electrical and optical properties of the films deposited at various substrate temperatures and target to substrate distances were investigated in detail. Optimized ZnO : Al films have conductivity of 2200 S cm–1 and average transmission in the visible range is higher than 85%. The conductivity and mobility show very little temperature dependence.
pp 231-234 June 2002 Thin Films
The present paper describes the alcohol sensing characteristics of spin coated SnO2 thin film deposited by using sol–gel process. The sensitivity of the film was measured at different temperatures and different concentrations of alcohol at ppm level. Alcohol detection result shows peak sensitivity at 623 K. The variation of sensitivity and ethanol concentration has shown a linear relationship up to 1150 ppm and after that it saturates. The response time measurement of the sensor was also observed and it was found that the response time is 30 sec. The results obtained favour the sol–gel process as a low cost method for the preparation of thin films with a high sensing characteristic.
pp 235-239 June 2002 Polymers
The effect of different compositions of monomers on the transport properties of copolymers by various techniques such as optical, electrical and magnetic has been investigated and compared with the homopolymers. The UV-visible absorption spectra show a hypsochromic shift with an increase in the o-anisidine content in copolymers indicating a decrease in the extent for conjugation (i.e. an increase in the bandgap). From temperature dependence of electrical conductivity the transport parameters such as charge localization length and average hopping distance are calculated and also the effect of the monomeric composition on the coherence length has been discussed. The magnetic studies show the paramagnetic and diamagnetic nature of homopolymers and copolymers. The X-ray diffraction pattern indicates that the copolymers are of amorphous nature.
pp 241-245 June 2002 Glasses
Measurements of thermal conductivity and thermal diffusivity of twin pellets of Se80Te20–𝑥In𝑥 (𝑥 = 2, 4, 6 and 10) glasses, prepared under a load of 5 tons were carried out at room temperature using transient plane source (TPS) technique. The measured values of both thermal conductivity and diffusivity were used to determine the specific heat per unit volume of the said materials in the composition range of investigation. Results indicated that both the values of thermal conductivity and thermal diffusivity increased with the addition of indium at the cost of tellurium whereas the specific heat remained almost constant. This compositional dependence behaviour of the thermal conductivity and diffusivity has been explained in terms of the iono-covalent type of bond which In makes with Se as it is incorporated in the Se–Te glass.
pp 247-249 June 2002 Experimental Techniques
𝐼–𝑧 spectroscopy measurements using a scanning tunnelling microscope (STM) were carried out to determine the change in the work function of a W tip following one monolayer (1 ML) deposition of Ni and subsequent annealing at 700 K. The variation in the actual gap voltage obtained from the 𝐼–𝑧 data of the clean tip was used in the calculation. The estimated values of the change in work function, 0.16 eV and 0.59 eV, for as-deposited and annealed tips, respectively match closely with the reported values. The method is generally applicable to chemically modified metal tips.
pp 251-257 June 2002 Experimental Techniques
A comprehensive analysis of size and strain broadened profile shapes in X-ray diffraction line broadening analysis is presented. Both size and strain broadened profiles were assumed to be Voigtian and the derived microstructural parameters (size and strain) were found to be in close agreement with those calculated from model independent Warren–Averbach method. The method is applied to three different alumina samples viz. micron size 𝛼-alumina (𝛼-Al2O3) prepared by the combustion of aluminium nitrate and urea mixture, annealed samples and commercial 𝛼-Al2O3 sample. It is likely from the present analysis that a significant Gaussian size contribution is related to narrow size distribution observed from the analysis. It has been concluded that present Voigtian analysis is more reliable and may largely replace the earlier simplified integral breadth methods of analysis often used in line broadening analysis.
Volume 42 | Issue 4