Volume 17, Issue 6
November 1994, pages 1-1196
pp 1- November 1994
pp 563-575 November 1994
Scanning tunneling microscopy of C70 films deposited on HOPG and gold substrates has been carried out to investigate the 2D packing, defects and disorder. Besides providing direct evidence for orientational disorder, high resolution images showing the carbon skeleton as well as the molecular arrangement in a solid solution of C70 and C60 are presented. Tunneling conductance measurements indicate a small gap in the C70 film deposited on HOPG substrate.
pp 577-584 November 1994
YBa2SnO5·5 has been synthesized and sintered as single phase material for its use as substrate for both YBCO and BiSCCO superconductors. YBa2SnO5·5 has a complex cubic perovskite (A2BB’O6) structure with the lattice constanta = 8·430 Å. The dielectric constant and loss factor of YBa2SnO5·5 are in a range suitable for its use as substrate for microwave applications. YBa2SnO5·5 is found to be chemically compatible with both YBCO and BiSCCO superconductors. The thick film of YBCO screen printed on polycrystalline YBa2SnO5·5 substrate gave aTc(0) of 92 K and a critical current density (Jc) of 4 × 104 A/cm2 at 77 K. A screen printed BiSCCO thick film on YBa2SnO5·5 substrate gaveTc(0) = 110 K and current density 3 × 103 A/cm2 at 77 K.
pp 585-594 November 1994
Critical exponent of the electrical conductivity in the paracoherence region (γ) of the high temperature superconductor YBa2Cu3O7−x (YBCO) has been estimated for high quality thin film on ZrO2 substrate prepared by high pressure oxygen sputtering. High energy ion irradiation was carried out using 100 MeV16O7+ ions at liquid nitrogen temperature to see the effects of disorder on the value of the exponent. The critical exponent (γ) changes from a value of about 2 to 1·62 upon irradiation. Studies were also carried out on this film to see the effect of ageing and annealing.
pp 595-606 November 1994
Experimental data on the real (x′) and imaginary (x″) parts of AC magnetic susceptibility as a function of DC bias field (H) shows the effect of trapped flux in granular highTc superconductors. The aim was to substantiate our recent theoretical findings on the basis of a two-component critical state model suitable for granular highTc superconductors. Stress has been given to understanding the origin of hysteresis inx′(H) andx″(H). It was seen in the experimental data that above a certain value of DC field range irreversibility appears inx′(H) andx″(H) creating hysteresis like loops. Comparison of data with calculated loops shows good agreement.x′(H) andx″(H) curves show considerable asymmetry in presence of trapped flux.
pp 607-613 November 1994
Low-field (H<40 G) magnetoresistance measurements have been made on Bi1·6Pb0·4Sr2Ca2Cu3O10 polycrystals at several temperatures between 80 and 105 K. Considerable hysteresis in ρ(H) is found in a zero-field-cooled sample when the applied field is increased from 0 to a maximum value and then lowered back to 0 at all temperatures. The observation of hysteresis is taken as an evidence for field trapping in the grains. We show that the hysteresis in ρ(H) occurs for applied fields much lower than that at whichdρ(H)/dH exhibits a discontinuity. In addition, we find that when the applied magnetic field (Ha) is lowered from a maximum field, the effective intergranular field,Heff, becomes zero forHc>0, which gives rise to a minimum in ρ(H).
pp 615-624 November 1994
We have carried out studies on the effect of oxide additives (Ca0·85Sr0·15CuO2, BaBiO3 and BaPbO3) on Bi-2212. Compositions up to 6 mole% of all the additives have been prepared and characterized by XRD, resistivity and ac susceptibility techniques. XRD studies indicate that all the materials are single phase. Resistivity and ac susceptibility studies indicate enhanced granular behaviour with the oxide additives. Low temperature (400°) sintering results in degradation of the Bi-2212 phase in the presence of the additives.
pp 625-632 November 1994
Microstructural and superconducting properties of YBa2Cu3O7−x thin films grownin situ on bare sapphire by pulsed laser deposition using YBa2Cu3O7−x targets doped with 7 and 10 wt% Ag have been studied. Ag-doped films grown at 730°C on sapphire have shown very significant improvement over the undoped YBa2Cu3O7−x films grown under identical condition. A zero resistance temperature of 90 K and a critical current density of 1·2×106 A/cm2 at 77 K have been achieved on bare sapphire for the first time. Improved connectivity among grains and reduced reaction rate between the substrate and the film caused due to Ag in the film are suggested to be responsible for this greatly improved transport properties.
pp 633-641 November 1994
The effect of alloying additions viz. cobalt, molybdenum, cerium and a combination of cobalt and molybdenum, on theKISCC of NiSiCr steel in 3·5% NaCl aqueous solution was studied. Addition of cobalt to NiSiCr steel resulted in an increase in theKISCC whereas molybdenum addition decreased theKISCC. Cerium addition did not affect theKISCC while the combination of cobalt and molybdenum resulted in an increase in theKISCC although not as much as in the case of cobalt addition. The effect of alloying elements onKISCC could be attributed to their effect on the critical fracture stress and yield strength.
pp 643-662 November 1994
In recent times, efforts have been directed towards a better understanding of the mechanisms associated with deformation and restoration of austenite during continuous multipass hot rolling of microalloyed steels. The correspondence between the condition of austenite before transformation and the resultant microstructure upon cooling holds the key to the attainment of interesting properties.
In the present paper, some results obtained on the deformation of austenite in a microalloyed steel, using a hot compression machine, are presented. Here, the idea was to simulate actual plate rolling or hot strip rolling conditions and study the evolution of microstructure at different stages of the hot deformation process i.e. after precise reductions at given strain rate and temperature of deformation.
The paper further discusses recent results obtained by us on the influence of hot deformation parameters (strain, finish rolling temperature, temperature of deformation) and cooling rates (air cooling, spray water cooling) on the microstructure of microalloyed steel. The precise conditions leading to the evolution of acicular ferrite and bainitic microstructures have been identified. New information on the influence of short tempering treatments (15 min at 550, 600 and 650°C) on the microstructure and properties of a microalloyed steel are also outlined.
pp 663-670 November 1994
The paper examines the potential of a model based approach for predicting creep behaviour of 2 1/4 Cr 1Mo steel. Effects of prior strain and thermal aging on the shape of the creep curve have been experimentally established. Particle coarsening has been identified as the dominant mechanism of creep. Creep curves theoretically simulated using particle coarsening model have been compared with those obtained for a variety of initial microstructural conditions.
pp 671-684 November 1994
Metallurgical properties have strong effects on corrosion. The paper discusses and reviews the work done at CECRI on the metallurgical aspects of corrosion of some industrially important alloys like steel and aluminium alloy weldments, stainless maraging steel and prestressing steel. The corrosion control methods for the above materials are also reviewed.
pp 685-698 November 1994
In this paper, experimental results on stress corrosion cracking in austenitic stainless steels are described. Crack growth data in sodium chloride solution for AISI 304 steel obtained for different metallurgical conditions, acoustic emission data recorded during crack growth and fractographic observations have been discussed with a view to identifying the operating mechanism. Some of the experimental observations such as crack propagation occurring in discontinuous jumps of the order of a few microns, lowering of the threshold stress intensity andJ-integral values on sensitization and cold working, typical transgranular fractographic features, transition in mode of fracture from transgranular to intergranular in sensitized conditions and activation energies of the order of 50 to 65 kJ/mol can all be accounted by hydrogen embrittlement mechanism. Hydrogen generated at the crack tip by corrosion reaction diffuses ahead of the crack tip under hydrostatic stress and influences the deformation process at the crack tip and also leads to the brittle component of the crack advance in jumps.
pp 699-715 November 1994
Acoustic emission (AE) behaviour during fatigue crack growth (FCG) in a ductile AISI type 316 austenitic stainless steel is reported. The two substages in the stage II Paris regime of FCG could be distinguished by a change in the rate of acoustic activity with increase in crack growth rate. The transition point in the cumulative ringdown count plot coincides with that in the da/dn plot. The AE activity increases with increase in ΔK during stage IIa and decreases during stage IIb. The major source of AE during stage IIa is found to be the plastic deformation within the cyclic plastic zone (CPZ) as compared to the phenomena such as monotonic plastic zone (MPZ) expansion, ductile crack growth, crack closure, etc. The increase in AE activity with increase in ΔK during stage IIa is attributed to the increase in the size of the CPZ which is generated and developed only under plane strain conditions. The decrease in AE activity during stage IIb is attributed to the decrease in the size of the CPZ under plane stress condition. The high acoustic activity during the substage IIa is attributed to irreversible cyclic plasticity with extensive multiplication and rearrangement of dislocations taking place within the CPZ. The AE activity is found to strongly depend on the optimum combination of the volume of the CPZ, average plastic strain range and the number of cycles before each crack extension. Based on this, an empirical relationship between the cumulative RDC and ΔK has been proposed and is found to agree well with experimentally observed values.
pp 717-732 November 1994
Recent developments in the complex and controversial, but fertile and fascinating field of aluminium-transition metal alloys are reviewed with particular reference to the so-called quasicrystalline phases and their rational approximants. Pauling’s several approaches to icosahedral phases on the basis of twinning in cubic crystals with large unit cells are described and examined along with the difficulties in checking the structures proposed by him. A new unified crystallographic approach is presented by the author, starting with icosahedral atomic clusters in concerned alloy melts, and it is shown that this integral approach leads to a reasonably good understanding of the interrelationship between the numerous known solid phases in aluminium alloys, including icosahedral phases, decagonal phases and their rational approximants.
pp 733-745 November 1994
Magnesium-lithium alloys are among the lowest density metallic materials. Addition of lithium, with a relative density of 0·53, in magnesium reduces the density of the alloy significantly. Furthermore, addition of nearly 11 wt.% lithium converts hexagonal close packed structure of pure magnesium to a body centered cubic lattice, markedly improving formability of the alloy. The development of these alloys, however, had been hampered due to the high reactivity of lithium and magnesium in the molten state and also, due to poor creep resistance and instability of mechanical properties at room temperature. In an attempt to indigenize these ultra light alloys for possible applications in Indian satellite programme, detailed research work was initiated in DMRL. The difficulties associated with producing sound cast ingots have been overcome by controlling melting and casting parameters of these alloys. Extensive work has been done on structure-property correlation of alloys with varying lithium content and minor alloying additions. Based on these work, advanced magnesium-lithium alloys have been developed with improved tensile properties, room temperature stability and creep resistance. Wrought products (plates/sheets) of magnesium-lithium alloy have been supplied to ISAC, Bangalore and are being used in their INSAT-2 programme. This paper describes the systematic studies carried out in the laboratory to indigenize these ultra light alloys.
pp 747-770 November 1994
Hardness is a measure of the resistance of a material to indentation and a wide variety of indentation tests have been devised to measure the hardness of materials. In the case of hardness tests which utilize spherical balls as the indentor, it is also possible to derive flow stress-strain relationships from hardness tests carried out either over a range of loads (static test) or over a range of impact velocities (dynamic test). This paper first describes the experimental procedure for obtaining stress-strain curves from hardness tests. In addition, the paper also analyzes in detail, the indentation test conditions under which the conversion of the hardness-average strain data to flow stress-strain data is simple and straightforward in the sense that the constraint factor which is the correlating parameter for the above conversion is not only independent of strain but also easily computable on the basis of known mechanical property data of the test material.
pp 771-781 November 1994
There has been revival of interest in Jerky flow from the point of view of dynamical systems. The earliest attempt in this direction was from our group. One of the predictions of the theory is that Jerky flow could be chaotic. This has been recently verified by us. We have recently extended the earlier model to account for the spatial aspect as well. Both these models are in the form of coupled set of nonlinear differential equations and hence, they are complicated in their structure. For this reason we wish to devise a model based on the results of these two theories in the form of coupled lattice map for the description of the formation and propagation of dislocation bands. We report here one such model and its results.
pp 783-794 November 1994
Statistically averaged lattices provide a common basis to understand the diffraction properties of structures displaying deviations from regular crystal structures. An average lattice is defined and examples are given in one and two dimensions along with their diffraction patterns. The absence of periodicity in reciprocal space corresponding to aperiodic structures is shown to arise out of different projected spacings that are irrationally related, when the grid points are projected along the chosen coordinate axes. It is shown that the projected length scales are important factors which determine the existence or absence of observable periodicity in the diffraction pattern more than the sequence of arrangement.
pp 795-810 November 1994
Electron diffraction studies were carried out to establish the icosahedral phase formation in rapidly quenched Ti-37 at% Mn and Ti-24 at% Mn-13 at% Fe alloys. Distortions in the diffraction spots and diffuse intensities in the diffraction patterns were investigated. The existence of a rational approximant structure and a decagonal like phase are also reported.
pp 811-827 November 1994
Diamond films were deposited by magnetron sputtering of vitreous carbon disc and also by plasma CVD technique using C2H2 + H2 or CO2 + H2 gas mixtures. The films were characterized by measuring the electrical, optical and microstructural properties. FTIR and Raman studies were carried out to study the effect ofsp2 andsp3 bonds present in the films. The films had a high mechanical stress which was determined from the broadening of the optical absorption tail in the films.
pp 829-839 November 1994
Diamond like carbon films and C-N films were prepared using ion beam assisted deposition technique (IBAD). Tribological properties were studied by subjecting DLC coated films to the accelerated wear tests. These tests indicated a significant improvement in the mechanical surface properties of glass by DLC coating. Better wear features were obtained for thinner DLC coating as compared to the thicker ones. We also studied the optical properties and obtained a band gap of 1·4 eV for these films. An attempt was made to prepare C3N4 films by using IBAD. We observed variation in the nitrogen incorporation in the film with the substrate temperature.
pp 841-848 November 1994
A scanning tunneling microscope (STM) has been used to create nanostructures on nickel thin films. A systematic procedure for the creation of such structures has been developed. Various possible mechanisms involved in such creations have been discussed. Based on our observations, a field-induced suction of the plastically deformed surface has been proposed.
pp 849-853 November 1994
Using nanocrystalline particles of mullite and zirconia respectively with diameters in the range 20–30 nm prepared by a sol-gel route, aluminium metal-matrix composites have been synthesized. A hot pressing technique has been used with temperatures varying from 450–610°C. The vickers hardness values for the composites are found to be substantially higher than that of pure aluminium. An order of magnitude increase in hardness is achieved when Al5Mo phase is grown in the composite.
pp 855-862 November 1994
Thin films of zirconia have been synthesized using reactive DC magnetron sputtering. It has been found that films with good optical constants, high refractive index (1·9 at 600 nm) and low extinction coefficient can be prepared at ambient temperatures. The optical constants and band gap and hence the composition are dependent on the deposition parameters such as target power, rate of deposition and oxygen background pressure. Thermal annealing of the films revealed that the films showed optical and crystalline inhomogeneity and also large variations in optical constants.
pp 863-873 November 1994
Blocks of 3Y-TZP were indented with conical diamond indenters. Indentation caused tetragonal to monoclinic phase transformation in a subsurface. Of the cracks generated in the subsurface, radial and lateral cracks can be accounted for by a continuum model of the indented subsurface, built using a combination of the Boussinesq and blister stress fields. Additional ring, median and cone cracks were also observed. It is hypothesized that the latter are motivated by the reduction in blister strength or residual energy brought about by the material damage caused by the phase transformation. This damage reduces the load bearing capacity of the material progressively with increasing normal load.
pp 875-881 November 1994
The present work reports the results on the deformation behaviour of ZrO2-3 mol% Y2O3 (3Y-TZP) ceramics which were prepared by pressureless sintering at 1400°C. Dense, cylindrical samples were subjected to uniaxial compression tests under a constant stress of 15 MPa in the temperature range of 1200–1400°C. The ceramics exhibit considerable ductility, attaining over 60% true strain without any edge cracking. Microstructural changes due to interaction of grain boundary viscous phase with the ultrafine and equiaxed grains were analyzed by transmission electron microscopy. Results show the grain boundary sliding accompanied by a diffusion accommodation process as the predominant deformation mechanism in these ceramics.
pp 883-891 November 1994
Ferroelectrics undergo one or more crystallographic phase transitions, which involve lattice distortions. The direction of spontaneous polarization in ferroelectrics can be reoriented by an applied electric field (or mechanical stress). There is a spontaneous strain accompanying spontaneous polarization. Phase transitions and domain reorientations thus result in microdeformations. Many devices such as actuators and transducers are based on this behaviour. The origin of microdeformations in ferroelectrics and their consequences are discussed here.
pp 893-910 November 1994
Fatigue cracked and fast fractured regions in four-point bend specimens prepared from 25 wt% silicon carbide whisker reinforced alumina composite were examined by scanning electron microscopy. This composite was found to be susceptible to a fatigue crack growth phenomenon similar to that in the case of metallic materials, but with a higher crack growth exponent. In the fatigue region, the alumina matrix failed mainly in a transgranular mode and the whiskers mainly failed with a flat fracture surface but without their pullout. On the other hand, in the fast fracture region, the whiskers failed predominantly by pullout and the alumina matrix failed in a mixed mode with about half in transgranular and the other half in intergranular mode. Thus, to improve the fracture toughness of the material, the grain boundary strength of alumina and the matrix whisker interfacial bonding should be improved. To increase the resistance to fatigue, the fracture strength of the alumina grains should be improved by using finer α-alumina particles and the fatigue strength of the whisker have to be increased by improving the uniformity in distribution of β-SiC whiskers during hot pressing.
pp 911-920 November 1994
Three different ceramic powders, viz. alumina, zirconia and silicon nitride were dispersed using two polyelectrolytes, one cationic (Betz 1190) and the other anionic (Darvan-C). All powders examined during the study could be well dispersed only under conditions of polymer dosage and pH such that the working pH is at least 2 pH units away from the pHIEP of the powder-dispersant combination. The shift in the isoelectric point (IEP) of the powders were determined through electro-acoustic measurements on 1% volume suspensions. Specific free energy of interaction were also computed using a model based on the electrical double layer theory of surfactant absorption.
pp 921-934 November 1994
The paper reviews the role of electronic configuration model of condensed state in explaining the sintering behaviour of various alloys. The systems are copper base alloys, ferrous alloys containing phosphorus, tungsten based heavy alloys, Al-refractory carbide composites, 6061 Al-alloy composites, high speed steel composites and tungsten carbide based cemented carbides. These studies cover the research activities of the Powder Metallurgy Laboratory at IIT, Kanpur.
pp 935-950 November 1994
This paper describes some of the theoretical and experimental studies being carried out at IGCAR on the equation of state of refractory materials like reactor fuels at high temperatures (>3000 K). The equation of state is primarily calculated by the principle of corresponding states. The influence of these equations of state on energy release in a hypothetical core disruptive accident in a fast breeder reactor is indicated. Details of an experimental facility based on laser induced vapourization mass spectrometry, which is being developed to measure the vapour pressures of materials at high temperatures is presented. Possible applications of this facility in other fields of materials research are indicated.
pp 951-958 November 1994
The ferroelectric Bi4SrTi4O15 has been synthesized and a study of the electrical (AC) conductivity was made on both poled and unpoled samples in the frequency range from 100 Hz to 1 MHz and from room temperature to 550°C. In the case of unpoled samples the activation energy was found to be 0·54 eV and subsequent to poling it was lowered to 0·39 eV indicating an increased conductivity after poling. Further the conductivity increased with increasing frequency and temperature. DC conductivity measurements were also carried out. Dielectric measurements indicate a peak in the dielectric constant at 530°C.
pp 959-965 November 1994
The photoelectrochemical behaviour of oxide bronzes of the type A6Nb8Ti2O30 (A = Ba. Sr) has been studied. They show good photoactivity with photopotentials of the order of 700–800 mV. The flat band potential has been estimated to be − 0·9 V vs SCE. The band gap estimated from the spectral response is 3·1 eV.
pp 967-975 November 1994
Precursors to MFe2O4 [spinels ferrites; where M = Ni(II), Co(II) and Zn(II)] have been prepared by the evaporation of polyvinyl alcohol added mixed metal nitratesolution, in presence and absence of urea. Theprecursor materials have low ignition temperature and are spontaneously combustible at low temperatures (250°C to 400°C). The heat liberated through the process is sufficient for the crystallization of the desired ferrite phase. The urea added process resulted in finer, superparamagnetic particles (12–17 nm) compared to the process without urea (particle size 25–30 nm). The ultrafine ferrite powders obtained have been characterized by X-ray powder diffraction (XRD), thermal gravimetry (TG), differential scanning calorimetry (DSC), infrared spectroscopy (IR), transmission electron microscopy (TEM) and room temperature magnetic measurement studies.
pp 977-987 November 1994
Oxide materials like perovskite, zirconolite, hollandite, pyrochlore, NASICON and sphene which are used for nuclear waste immobilization have been prepared by a solution combustion process. The process involves the combustion of stoichiometric amount of corresponding metal nitrates and carbohydrazide/tetraformyl trisazine/diformyl hydrazide at 450°C. The combustion products have been characterized using powder X-ray diffraction, infrared spectroscopy, and29Si MAS-NMR. The fine particle nature of the combustion derived powders has been studied using density, particle size, BET surface area measurements and scanning electron microscopy. Sintering of combustion derived powder yields 85–95% dense ceramics in the temperature range 1000°–1300°C.
pp 989-997 November 1994
Cd2SnO4 powder has been synthesized through acrylamide sol-gel route for the first time. The gelation-calcination steps have been studied. Morphological examination of the network matrix and the calcined products has been done through SEM. XRD shows the orthorhombic structure of the Cd2SnO4 powder obtained. Arrhenius plots yield activation energies of 0·01 and 0·61 eV for the low and high slope regions, respectively. A semiconductor to metal transition is observed in the range 210–225°C.
pp 999-1004 November 1994
Gelation times of tetraethyl orthosilicate-derived sols containing selected di- to tetravalent cations as dopants were noted at different temperatures (15°–50°C). An analysis of these data, in conjunction with relevant published information, led to the tentative conclusions that (i) unhydrolyzed cations in bare form linked negatively charged silicate polymers in sols, thereby accelerating gelation and (ii) hydrolyzed cations retarded the process by offering molecular size-related hindrance and competition for the available water for hydrolysis.
pp 1005-1014 November 1994
An RBS study has been carried out to study the relative extent of Pb diffusion in the substrate from PZT films. It is found that extensive diffusion of Pb occurs into the quartz substrate making the film severely deficient in Pb. No such diffusion occurs in the case of PZT films on sapphire and the concentration of Pb is near stoichiometric, except for a small loss due to volatilization. Excessive Pb deficiency, rather than any epitaxial effect, has earlier been proposed by us to be the crucial factor responsible for the existence of the pyrochlore phase in PZT thin films on substrates such as Si, glass, quartz etc. The present results confirm this. The effects of other process variables such as thickness and chemical composition (Zr/Ti ratio) of the film can also be understood in terms of the same phenomenon.
pp 1015-1028 November 1994
A novel chemical passivation route is established to obtain microcrystalline solid solutions of binary oxidé system Mn1−xNixO (0·01 <x < 0·30). During the passivation process, controlled thermal decomposition of manganous oxalate is carried out to obtain pure MnO and its subsequent reaction with NiO in oxygen-free nitrogen resulted in microcrystalline powder of these solid solutions. The powder is thoroughly characterized by various physicochemical techniques such as XRD, DTA/TG/DTG, diffused reflectance spectra, magnetic susceptibility, TEM, XPS etc. The observed processing-structure-property correlations confirmed the improved thermal stability of these powders (relative to pure MnO) in air. The important role of dopant paramagnetic Ni2+ ions in enhancing the passivation of the bulk Mn2+ species is explained on the basis of the formation of mixed oxide complex species on the surface of these microcrystalline powders.
pp 1029-1037 November 1994
The local order around molybdenum and tungsten atoms in various sodium molybdophosphate and sodium tungstophosphate glasses has been investigated using extended X-ray absorption fine structure (EXAFS). Both molybdenum and tungsten atoms are present in six-coordinated environment in these glasses. Magic angle spinning nuclear magnetic resonance (MAS NMR) of31P suggests that metaphosphate or neutral [POO3/2] groups are present in these glasses.
pp 1039-1047 November 1994
GaTe is a III–VI semiconductor which has layered structure with large anisotropy in electrical properties. Growth of single crystals by the Bridgman technique permitted the measurement of thermoelectric power in orthogonal directions from which the anisotropy of hole effective masses were determined for the first time. From resistivity and Hall effect measurements the carrier activation energies and scattering mechanisms between 10–300°K were found.
Study of the temperature dependence of conductivity revealed a variety of conduction mechanisms including weak localization below 20°K, hopping conduction between 20–50 K and band conduction in and across the layer planes atT>70 K. Weak localization was confirmed through observation of negative magnetoresistance. TheI–V characteristics showed quantized behaviour due to tunneling across potential barriers, which may be due to stacking faults between layer planes as observed by TEM studies.
pp 1049-1055 November 1994
In successive pulse plating, the pulse deposition is repeated after a definite duration of plating preceded by a mild cleaning of the electrode and with a fresh deposition bath for the same duration. In this study, CdSe films were deposited on Ti substrates by successive pulse plating from a bath containing CdSO4 and SeO2 at a current density of 80 mA cm−2 and a duty cycle of 3·3% for a duration of 30 min. The films heat-treated to 550°C for 20 min in argon atmosphere, were polycrystalline with a hexagonal structure. At an illumination of 60 mW cm−2, a conversion efficiency of 4·5% for the photoetched film and 1·7% for the chemically etched one were determined.
pp 1057-1064 November 1994
Low temperature photoluminescence of vacuum and cadmium annealed CdTe:In is reported here. A new peak at ∼ 1·14 eV related to transitions from the conduction band to an acceptor involving a tellurium vacancy has been observed.
pp 1065-1070 November 1994
Poly(vinyl alcohol) is a hydrogel which is extensively studied for a variety of biomedical applications. Membranes developed from crosslinked poly(vinyl alcohol) (PVA) is having excellent permeability to solutes. However its wet breaking strength is low. Polyetherurethaneurea (PEUU), having an excellent mechanical strength is blended with PVA as a reinforcement, and membranes developed are studied for its permeability and mechanical strength. The optimum membrane selected, is having permeability and wet breaking strength almost equal to the commercially available cellulose acetate membrane.
pp 1071-1077 November 1994
This study reports the physical properties and gas permeation characteristics of polyarylates prepared by disubstitution of the bisphenolA bridge carbon. The results are compared with data from previously reported unsubstituted and mono-substituted polyarylates. Disubstitution is an effective method of changing the packing density, as measured by the d-spacing, and also increases the chain stiffness. The intersegmental spacing can be either decreased (by phenyl disubstitution) or significantly increased (by -CF3 disubstitution). Increased d-spacing in the case of -CF3 disubstitution increases permeabilities and reduces selectivities compared to the unmodified bisphenolA based polymer. The increased permeability and selectivity of the phenyl disubstituted polyarylates, in spite of the reduced d-spacing, supports an earlier finding that decreased bisphenol phenyl ring mobility tends to increase permeability.
pp 1079-1089 November 1994
The properties of molded PPS parts are dependent on the crystalline morphology developed during processing. Even though processed under identical conditions, the crystalline morphology may differ owing to the differences in the crystallization process. The crystallization behaviour of a polymer is known to depend on its molecular architecture which in turn depends upon polymerization process. Thus the study of the crystallization behaviour of polymer with reference to its molecular architecture is essential for obtaining product with desirable properties. In the present paper, the crystallization behaviour of two grades of polyphenylene sulfide was investigated using differential scanning calorimetry (DSC). An attempt has been made to explain the differences in the crystallization behaviour of PPS samples on the basis of the differences in their molecular architecture. The structural differences of PPS manifest themselves in terms of the depression in the equilibrium melting point, retardation of nucleation and overall crystallization rate and coarsening of spherulitic texture.
pp 1091-1094 November 1994
Thermal expansion of irradiated nylon-6 has been studied in the temperature range 10 to 340 K using a three-terminal capacitance bridge technique. Irradiation is carried out using cobalt-60 γ-rays up to 500 Mrad dosage. Radiation enhances chain scission over crosslinking. α increases from 0 to 250 Mrad between 10 to 340 K and not much variation is observed between 250 to 500 Mrad for samples from 10 to 250 K.
pp 1095-1101 November 1994
The paper describes the synthesis of vinyl ester resins based on diglycidyl ether of bisphenol-A (epoxy equivalent = 450–465 g/eq) (VR resin) and tetrabromobisphenol-A (epoxy equivalent = 380–420 g/eq) (VR-1 resin). The viscosity of styrenated VR resin was higher than VR-1 resin. The effect of styrene andα-methyl styrene on curing of VR resins was studied. An increase in styrene from 30 to 50 wt% resulted in an increase in gel time and a decrease in exothermic peak. Addition ofα-methyl styrene delayed and depressed the exotherm. The mechanical properties of VR resin sheets and glass fabric reinforced laminates were better than VR-1 resins; whereas LOI of VR-1 was higher. A resin formulation containing 20–30 wt% of VR: VR-1 showed optimum mechanical properties and LOI.
pp 1103-1108 November 1994
It is conjectured that anionic-cationic surfactant combination can be regarded as equivalent to a double chain surfactant and using molecular packing considerations it is shown that vesicles, viscoelastic solutions and liquid crystals can be designed by the proper choice of chain lengths of the pair. Using these concepts new systems are designed, from mixtures of cetyltrimethyl ammonium bromide and sodium alkyl sulfonates, to produce both viscoelastic gels and vesicles.
pp 1109-1117 November 1994
We highlight our recent experimental work on an efficient molecular nonlinear optical crystal, 3-methoxy 4-hydroxy benzaldehyde (MHBA). Optical quality single crystals of MHBA were grown from mixtures of solvents and from melt. The overall absorption and transparency window were improved by growing them in a mixture of chloroform and acetone. The grown crystals were characterized for their optical transmission, mechanical hardness and laser damage. We have observed a strong correlation between mechanical properties and laser induced damage.
pp 1119-1129 November 1994
The electroclinic effect in the smectic A phase of ferroelectric liquid crystals is a sensitive probe for measuring the soft mode response near the A-C* transition point. It is possible to evaluate several coefficients of the Landau theory describing the A-C* transition by simultaneously measuring the frequency dependence of both the optical signal and the current through the sample. We have made such measurements on a couple of homologous series [2S,3S]-4′-(2-chloro-3-methyl pentanoyloxy) phenyl-trans-4″-n alkoxy cinnamates synthesized in our labortory. The results show that the Landau meanfield theory is adequate to describe the A-C* phase transition in these compounds.
pp 1131-1141 November 1994
Single crystals of calcium hydrazine carboxylate, monohydrate have been studied by ESR of Mn2+ doped in the calcium sites. X-band ESR indicated a large crystal field splitting necessitating experiments at Q band. The analysis shows two magnetically inequivalent (but chemically equivalent) sites withgxx = 2.0042±0.0038,gyy=2.0076 ±0.0029,gzz=2.0314±0.001,Azz=0.0099±0.0002 cm−1,Axx=0.0092±0.0002 cm−1,Ayy=0.0082±0.0002 cm−1,D=3/2Dzz=0.0558±0.0006 cm−1, andE=1/2 (Dyy−Dyy)=0.0127±0.0002 cm−1.
One of the principal components of the crystal field, (Dzz), is found to be along the Ca ↔ Ca direction in the structure and a second one, (Dxx), along the perpendicular to the plane of the triangle formed by three neighbouring calciums. TheA tensor is found to have an orientation different from that of theg andD tensors reflecting the low symmetry of the Ca2+ sites.
pp 1143-1153 November 1994
ETS-10 is a novel large pore (12 membered ring) molecular sieve containing Ti4+ in Oh environment and Si4+ in Td environment sharing common O2− ions. As a result, ETS-10 possesses large ion-exchange and adsorption properties, besides being acidic in the H-form. We report our studies on its synthesis, characterization using SEM, IR, TGA/DTA, UV-VIS, MAS-NMR techniques, adsorption and catalytic properties.
pp 1155-1166 November 1994
New galvanic cell designs, incorporating one or two buffer electrodes, are developed to minimize the electrode polarization caused by electrochemical permeability of the electrolyte at high temperature. When a nonpolarizable reference electrode is employed, a cell with three-electrode compartments can be used to measure the chemical potential of oxygen in two-phase fields of ternary systems, associated with one degree of freedom at constant temperature. A buffer electrode is placed between the reference and measuring electrodes. The buffer electrode, maintained at approximately the same oxygen chemical potential as the measuring electrode, absorbs the semipermeability flux of oxygen between reference and measuring electrodes.
When the reference electrode is polarizable, two buffer electrodes are required between the reference and measuring electrodes. The reference and reference-buffer electrodes have the same chemical potential of the active species. Similarly the measuring electrode and its buffer are of approximately the same chemical potential. A significant chemical potential difference exists only between the two buffers, which may become polarized due to coupled transport of ions and electronic defects through the electrolyte. Since the reference and measuring electrodes are insulated, the emf of the solid state cell is unaffected. The use of the buffer electrode designs permit more accurate thermodynamic measurements on metal and ceramic systems at high temperature.
pp 1167-1179 November 1994
Titanium has emerged as a major structural metal for a wide range of industrial applications due to its attractive engineering properties. India has a large and rich reserve base for this metal in the beach sands of the eastern and southern regions with well established production facilities for their separation into individual minerals. Research and Development activities for establishing the metal production technology have been underway in the country for over two decades. The Defence Metallurgical Research Laboratory, Hyderabad, has already demonstrated the metal production technology by the conventional Kroll process on 2000 kg/batch scale and is now all set for demonstrating the same by the more advanced, energy efficient combined process route on 4000 kg/batch scale. The paper reviews the R & D efforts undertaken so far in the field of metal extraction with emphasis on the current status of this developmental activity at DMRL.
pp 1181-1196 November 1994
Composite materials for aerospace applications through in-house R & D and through collaboration with overseas aerospace organizations and National Laboratories covering a wide spectrum including glass/carbon/kevlar fibre reinforced plastics, metal and Nomex honeycomb sandwich structures, laminated composites, metal matrix composites and metallo-ceramic composites.
Volume 42 | Issue 5
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