Showing papers in "Bulletin of Materials Science in 1995"

Use of microwaves for the synthesis and processing of materials

Abstract: An overview of the synthesis of materials under microwave irradiation has been presented based on the work performed recently. A variety of reactions such as direct combination, carbothermal reduction, carbidation and nitridation have been described. Examples of microwave preparation of glasses are also presented. Great advantages of fast, clean and reduced reaction temperature of microwave methods are emphasized. The example of ZrO2-CeO2 ceramics has been used to show the extraordinarily fast and effective sintering which occurs in microwave irradiation.

Interfaces in discontinuously reinforced metal matrix composites: An overview

Abstract: The fundamental and engineering aspects pertaining to the matrix-reinforcement interfaces in discontinuously reinforced metal matrix composites are presented in this overview. The interfaces play a key role in determining mechanical properties, namely Young’s modulus, yield strength, elongation, creep and fracture behaviour, as well as physical properties like coefficient of thermal expansion, thermal conductivity and damping characteristics of metal matrix composites; these are discussed in detail. The ratio of the experimental value of the Young’s modulus to that predicted from the rule of mixtures has been used as a measure of interfacial bond strength. Various issues such as the nature of the interfacial bond, chemical reaction at the interfaces, and effect of alloying and processing on the structure of the interfaces and the properties of the composite are examined. In order to exploit the full potential of reinforcing the metallic matrix, the suggested strategies include creation of metallic bonding at the interface, use ofin situ processing, choice of right type of alloying elements, and heat treatments and engineering of interfaces.

Synthesis of nano particle of inorganic oxides by polymer matrix

Abstract: Ultrafine (⩽ 150 nm) powders of spinels [MFe2O4 where M = Ni(II), Co(II) and Zn(II)]; rare-earth orthoferrites [RFeO3 where R = Sm, Nd and Gd], and rare-earth garnets [R3Fe3O12 where R = Sm, Nd and Gd] with good purity and chemical homogeneity were prepared through two new versatile chemical routes. The first route involved the coprecipitation of the desired metal nitrates from their aqueous solution, in presence of a water soluble polymer-polyvinyl alcohol (PVA), by triethyl ammonium carbonate solution. The other process involved complete evaporation of a mixture of optimum amounts of PVA and the desired aqueous metal nitrate solutions, with and without the addition of optimum amounts of urea when the mixture was evaporated to a pasty mass. In addition, detailed study on the reported potassium ferricyanide route was also carried out for the production of the rare-earth orthoferrite powders. The various precursor as well as the heat-treated mixed-oxide powders, prepared through each of the routes, were compared by the physical characterization studies involving thermal gravimetry and differential scanning calorimetry, infrared spectroscopy, X-ray powder diffraction, transmission electron microscopy, and room temperature magnetic measurements.

Electrically conducting polymers: from fundamental to applied research

Abstract: A review of the state of the art of electrically conducting polymers is presented. Special emphasis is laid on the nature of the doping processes which induce high electrical conductivity in conjugated organic polymers and on the nature of charge carriers such as solitons, polarons and bipolarons produced in them as a result of such processes. Various factors affecting the electrical conductivity of these polymers and the various models proposed to explain charge transport in them are also discussed. The present status of the structure-property relationship in these conducting polymers and the various routes currently pursued to tailor novel conducting polymers are also briefly mentioned. Finally the various applications of these novel electronic materials are described.

Thermal stability of electrochemically prepared polythiophene and polypyrrole

Abstract: The degradation behaviour of electrochemically prepared polythiophene and polypyrrole has been studied by thermal gravimetric analysis technique. Studies on both the polymers show that they are more stable than polyacetylene but still undergo degradation reactions which involve two steps, viz. loss of dopant and then degradation of polymer backbone. The general features of degradation mechanisms are discussed.

Equivalent circuit models for electronic ceramics

Abstract: Complex immittance spectra of model equivalent circuits involving resistive and capacitive elements are calculated. A comparison of experimentally obtained complex immittance plots with these diagrams greatly facilitates the search for the most appropriate equivalent circuit representing the electrical properties of electronic ceramics.

Dielectric and microstructural behaviour of strontium titanate borosilicate glass ceramic system

Abstract: 65(SrO·TiO2)−35(2SiO2·B2O3) wt% glass was synthesized. Differential thermal analysis study shows one exothermic peak which shifts towards higher temperature with increasing heating rate. Glass ceramics prepared by controlled crystallization of strontium titanate borosilicate glass produce uniform distribution of crystallites in a glassy matrix. Attempt was made to crystallize strontium titanate phase in this glass ceramic. Different phases precipitated out during ceramization have been identified by X-ray diffraction. It appears that due to high reactivity of SrO with B2O3, strontium borate crystallizes as principal phase followed by TiO2 (rutile) and Sr3Ti2O7 phases. Dielectric constant of these glass ceramics was observed to be more or less temperature independent over wide range of temperatures with low values of dielectric constant and dissipation factor.

Synthesis and properties of rare earth doped lamp phosphors

Abstract: Red, blue and green emitting lamp phosphors such as Eu3+ doped Y2O3 (red phosphor), Eu2+ doped Ba0·64Al12O18·64, BaMgAl10O17 and BaMg2Al16O27 (blue phosphors) and Ce0·67Tb0·33MgAl11O19 and Eu2+, Mn2+ doped BaMgAl10O17 (green phosphors) have been prepared by the combustion of the corresponding metal nitrates (oxidizer) and oxalyl dihydrazide/urea/carbohydrazide (fuel) mixtures at 400°–500°C within 5 min. The formation of these phosphors has been confirmed by their characteristic powder X-ray diffraction patterns and fluorescence spectra. The phosphors showed characteristic emission bands at 611 nm (red emission), 430–450 nm (blue emission) and 515–540 nm (green emission). The fine-particle nature of the combustion derived phosphors has been investigated using powder density, particle size and BET surface area measurements.

A new materials processing—hydrothermal processing

Abstract: The hydrothermal processing has emerged as the new materials processing for powder preparation. This describes definition, history, advantages, characteristics of the powder, ideal and hydrothermal powder.

Short fibre elastomer composites: effect of fibre length, orientation, loading and bonding agent

Abstract: A series of short fibre reinforced styrene butadiene rubber (SBR) composites were prepared by incorporating sisal fibres of different lengths and different concentrations. The vulcanization behaviour and the properties of the vulcanizates were measured by the relevant ASTM procedures. The processing characteristics such as green strength and mill shrinkage were improved by the addition of fibres. The properties like tensile strength, tear strength, moduli at 10% and 20% elongations were found to be maximum for composites containing longitudinally oriented fibres of length 6 mm. Mixes containing 35 phr loading of fibres and bonding agent showed better mechanical performance than others. The failure behaviour of tensile and tear specimens have been analysed by scanning electron microscopy techniques.

X-ray analysis of different starch granules

Abstract: Crystal sizes and lattice distortion parameters for root, pulse and cereal starch granules have been determined using observed X-ray diffraction reflections by Fourier method. Enthalpy for the formation of the lattice in root, pulse and cereal starches has been estimated and compared. It is found that the crystal size is normally high in root starch compared to pulse and cereal starches.

Acoustical parameters of polyethylene glycol/water mixtures

Abstract: Ultrasonic velocity (v) and density (d) have been measured for polyethylene glycol/water mixtures at 30°C. The adiabatic compressibility (β ad), molar compressibility (β), specific acoustic impendance (Z), Rao number (R) and van der Waals constant (b) have been computed. The variations ofv, d, β ad,β, Z, R andb with mole ratio of water/ether group oxygen have been studied. The association between the components and the formation of tetrahydrate have been reported.

A comparative study on CdSe synthesized at low temperature

Abstract: CdSe is a well-known semiconductor which finds applications as photoconductors, photovoltaic cells and other optoelectronic devices. In this paper structural studies and carrier concentration measurements of vacuum evaporated CdSe films obtained by using CdSe synthesized by simple precipitation technique are presented.

Electrical, thermal and infrared studies of cerium(III) orthovanadate

Abstract: Cerium(III) orthovanadate with a small deviation from stoichiometric composition is ap-type semiconductor between 30 and 800°C. The electrical conduction in cerium(III) orthovanadate is due to thermally activated hopping of holes on equivalent Ce3+-Ce4+ lattice sites. The DTA result of CeVO4 indicated a possible phase transition at about 70°C. The IR spectrum of the sample showed bands at 865 and 810 cm−1, typical of VO4 group of orthovanadates.

Electrical and magnetic studies of iron (III) vanadate

Abstract: Iron(III) vanadate (FeVO4) is an n-type semiconductor between 300 and 800 K. Electrical conduction in this phase occurs due to small deviation from oxygen stoichiometric composition. The mechanism of electrical transport is of a thermally activated hopping of charge carriers (electrons) on equivalent iron lattice sites. The FeVO4 obeys Curie-Weiss law between 80 and 300 K. The measured magnetic moment (μ eff) of Fe3+ ion in FeVO4 is 5·270 BM at 298 K, which is lower than theμ spin only value. The predominant exchange interactions are the weak 90° M-O-M superexchange and M-O-O-M super-super-exchange. The negative Weiss constantϑ=− 30 K of the phase indicated the possibility of an antiferromagnetic ordering of the iron(III) vanadate lattice. The IR absorption spectrum of FeVO4 gave bands at 990, 900, 825 and 725 cm−1 due to the presence of distorted VO4 polyhedra of the lattice.

Cast metal matrix composites—challenges in processing and design

Abstract: Development of metal matrix composites has extended the choice of materials particularly for the space, aero-space and automobile industries, with a view to reduce fuel consumption and operating cost by weight saving. Tailoring of a composite to suit a given application requires choice of constituents and promotion of desirable interfaces. Solidifcation processing of composites demands particular attention to wetting of dispersoids by alloys, defects like porosities and evolution of microstructures particularly in respect of changing the nature of interfaces in a composite.

Studies on spray pyrolyzed molybdenum trioxide thin films

Abstract: Semiconducting molybdenum trioxide thin films have been prepared by employing simple and inexpensive spray pyrolysis technique. Films are found to be polycrystalline in nature with hexagonal phase. Optical band gap energy (direct) and room temperature electrical resistivity are of the order of 2·9 eV and 108 ohm-cm, respectively. These films exhibit cathodic electrochromism.

Thermoelectric power in Gd3+-substituted Cu-Cd ferrites

Abstract: Polycrystalline Cd x Cu1−x Fe2−y Gd y O4 ferrites fory=0·0 and 0·1 were prepared by ceramic technique. X-ray diffractograms of powder samples show cubic symmetry withx⩾0·2 fory=0·0 and 0·1, while compositions withx=0·0 fory = 0·0 and 0·1 are tetragonal. The thermopower measurements for Gd3+-undoped ferrites in the temperature range 300 K to 788 K shown-type conductivity forx⩾0·2. The substitution of Gd3+ changedn-type conductivity of the compositions top-type. The mobilities calculated show decreasing trend on Gd3+ substitution. The values of activation energy ΔE and drift mobilityE d suggest polaron formation in substituted samples. The conduction mechanism is explained on the basis of localized model and formation of Gd3++Fe2+ stable pairs at B site and Cu1++Fe3+ at A site.

Piezoelectric properties, hysteresis behaviour and dielectric properties of PMN-PZT ceramics

Abstract: Ferroelectric and piezoelectric properties ofxPb(Mg1/3Nb2/3)O3−(1−x)Pb (Zr0·55Ti0·45)O3 system have been investigated. X-ray diffraction patterns indicate rhombohedral and cubic structures. Maximum dielectric constant and piezoelectric properties are exhibited by 0·5–0·5 PMN-PZT composition.P r is high in 0·6–0·4 PMN-PZT composition.

Photoconductivity of iodine-doped single crystals of phthalocyanine

Abstract: Single crystals of metal-free phthalocyanine (H2Pc) and of copper phthalocyanine (CuPc) were grown in the presence of iodine vapour. The presence of iodine enhances the spectral dependence of photoconductivity of H2Pc in the visible region but of CuPc in the near-IR region. The dark current is decreased but the photocurrent is increased by one order of magnitude in iodine-doped H2Pc but in the case of iodine-doped CuPc both currents are increased by nearly three orders of magnitude. Introduction of iodine results in about one order of magnitude decrease in response time for both modifications. Thus the introduction of iodine into Pc crystals decreases the energy barrier for conduction and increases the drift mobility of charge carriers thereby enhancing the conductivity of the material.

Near ideal electrical switching in fast ion conducting glasses: evidence for an electronic process with chemical origin

Abstract: Bulk AgI based fast ion conducting (FIC) glasses have been prepared by a novel microwave technique. Electrical switching characteristics of these glasses have been investigated for the first time. It has been found that AgI based FIC glasses exhibit a current-controlled high speed memory electrical switching behaviour. SEM, EDAX and ESR investigations have been performed on the virgin and switched sampies to understand the nature of the conducting state. A chemical model is proposed to explain the switching behaviour of these glasses, which is consistent with the observed results.

Basic studies leading to the development of an ultrahigh strength, high fracture toughness low-alloy steel

Abstract: Ultrahigh strength steels have been used increasingly in recent years for critical aircraft and aerospace structural applications. In such applications, though materials performance is of prime consideration, cost and availability makes the low-alloy steels an attractive option. This paper describes the development of an ultrahigh strength NiSiCrCoMo low-alloy steel, supported by significant findings obtained from the basic studies that were aimed at understanding how solute additions influence fracture resistance of iron, with and without the presence of carbon. The results of the basic studies, in combination with the work of Garrison (1986) on a NiSiCr steel, have profitably been employed in the development of a NiSiCrCoMo low-alloy steel possessing a strength-toughness combination quite comparable to the highly alloyed 250-grade maraging steel. Reproducibility of attractive strength and toughness properties has been established in tonnage scale melts. This steel, in the softened condition, has good formability and machinability. Weld parameters have also been established. The NiSiCrCoMo low-alloy steel thus meets the requirements of performance and cost rendering it an attractive option for advanced structural applications.

Material science aspects of phase change optical recording

Abstract: Chalcogenide thin films are used as the recording medium for phase change-type optical memory discs. The films are switched between amorphous and crystalline states using the heat of a focussed laser beam. Large reflectivity differences between amorphous and crystalline states are then used to store and retrieve the information. An active chalcogenide layer for this purpose should have a high optical absorption coefficient (α), and good structural and thermal stability. It should be possible to switch the chalcogenide layer between amorphous and crystalline states repeatedly within a short duration, the optical contrast should be high, and the material must have large cycling capability. Keeping the above requirements in mind, we have carried out systematic investigation of structural, optical and crystallization behaviour of thin films of various compositions of GaGeTe, Sb2Te3 and BiSe. These studies have shown that these materials can be good candidates for use as recording media in erasable phase-change optical recording.

Surface treatment of red mud and its influence on the properties of particulate-filled polyester composites

Abstract: The red mud surface was treated with organosilane, organotitanate and organozirconate coupling agents respectively. The change in the surface was determined by IR spectroscopy and SEM. The effect of coupling agents concentration on the tensile and flexural strength of red mud-filled polyester resin has been studied. The efficacy of these coupling agents was discussed in relation to dispersion of red mud in unsaturated polyester resin and also by measuring performance of finished castings under both dry and wet conditions. It was found that organotitanate treated red mud-filled polyester composite gives better physico-mechanical properties than the control and other coupling agents treated red mud. Fractography of failed samples was also discussed in terms of their mechanical properties.

Materials engineering for a better global environment

Abstract: The role of materials engineering including ceramics technology for a better global environment is discussed. Present global environmental issues will be solved by resourceful energy technology and waste management under a minimum pollution of environment. The materials technology will play an important role to mitigate the global environmental issues. Research program on future energy technology and waste management should be considered according to a condition of domestic and/or international regulation. Energy saving and domestic waste management including pollution prevention of atmosphere, water and soil are near term research areas. Medium and long term research areas are non-fossil energy technology and global waste management including removal and/or reuse of greenhouse gas CO2 and nuclear waste management. To mitigate future global environmental issues, traditional materials technology should be reconstructed to build environment benign materials technology which could provide minimum environmental load.

Effect of Al3+-substitution on the electrical and magnetic properties of Ni1·05Sn0·05Fe1·9O4 ferrites

Abstract: The effect of Al3+-substitution on d.c. resistivity, dielectric constant, initial permeability, saturation magnetization and Curie temperature has been studied for Ni1·05Sn0·05Al x Fe1·9−x O4 ferrites, whereinx varies from 0 to 0·5 in steps of 0·1. The d.c. resistivity increases slowly forx⩽0·3 followed by a rapid increase forx>0·3. The variation is explained on the basis of Verwey hopping as well as polaron hopping mechanisms, and the dilution effect of Al3+ ions. The behaviour of dielectric constant is attributed to interfacial polarization and follows the Koops model. The saturation magnetization and the Curie temperature decrease continuously with increasing Al3+ content and are explained using the concepts of sublattice magnetization and exchange interactions. The decrease in initial permeability is attributed primarily to decrease in saturation magnetization. The dispersions in initial permeability and magnetic loss tangent are also discussed.

Preparation and characterization of gold doped (Zn, Cd)S mixed phosphors for mechano-optical transducers

Abstract: When mechanically excited, initially the ML intensity increases, attains a maximum value and then decreases with time. The total ML intensityIT initially increases with the impact velocityVo of piston and attains a saturation value for higher values ofVo and follows the relationIT=IToexp(−Vc/Vo) whereITo andVc are constants. The ML intensity is maximum for 20% CdS contents in the (Zn, Cd)S phosphors due to increase in hardness which may in turn increase the fracture stress and subsequently the piezoelectric field strength. The wavelength corresponding to the peak of both the ML and PL spectra shift towards longer wavelength with increasing CdS contents. Some models are discussed and it is concluded that the impulsive deformation of these phosphors may be due to piezoelectrification of newly created surfaces. The similarity of ML spectra with EL and PL spectra suggests that although the excitation processes are different, emission process is governed by the states of similar nature.

Photoconductivity in crystalline phthalocyanines

Abstract: The wavelength, temperature, time and intensity dependence of photocurrent of metal-free phthalocyanine (H2Pc) and copper phthalocyanine (CuPc) single crystals were investigated. The thermal activation energies in the dark are 0·5 and 0·6 eV for H2Pc and CuPc respectively and the corresponding photo-thermal activation energies are 0·3 and 0·2 eV. An energy level scheme for single crystals of H2Pc and CuPc is proposed which consists of two trapping levels and five narrow optically active valence bands. In H2Pc (CuPc), one trapping level at 0·5 eV (0·6 eV) above the valence band edge to which the charge carriers are thermally excited in the dark; and the other trapping level is at 0·3 eV (0·2 eV) below the conduction band edge where all the optical transitions terminate. In H2Pc(CuPc), the forbidden gap is 1·44 eV (1·34 eV) wide; the five valence bands are at the band edge, and 0·09 (0·22), 0·42 (0·63), 0·69 (0·90), 1·32 (2·17) eV below the band edge.

Processing of materials—monolithic to composites

Abstract: A multimillion rupee 500-ton hydraulic extrusion/forging facility established at NPL, New Delhi, has been used to undertake extensive studies in forming, the process of plastically deforming, which is the most important way of shaping materials Wrought materials are used extensively for making useful products employing extrusion and forging, the two important secondary processing techniques used to convert materials into useful shapes and sizes and also to improve the mechanical and metallurgical properties The success of these processes depends on the proper control and eventual optimization of different process variables

Static and impact fatigue behaviour of borosilicate glass

Abstract: Failure of a borosilicate glass as a result of repeated impact has been studied. Impact fatigue study was conducted in an improved pendulum type repeated impact apparatus specially designed and fabricated for determining single and repeated impact strength. For elimination of the effect of humidity, repeated impact tests were carried out under liquid nitrogen. Quasi-static measurements were determined under four-point bending. Using a square waveform as applicable to the present impact tests and fracture mechanics interpretation, the number of cycles to failure during impact fatigue tests were predicted from quasi-static fatigue measurements. It has been shown that repeated impact loading has a deleterious effect on the failure cycles compared to slow stressing. The role of an added mechanical effect during repeated impacts has been suggested in controlling the cyclic fatigue behaviour.