Showing papers in "Bulletin of Materials Science in 1997"

Correlation between electronic structure, mechanical properties and phase stability in intermetallic compounds

Abstract: From the detailed electronic structure studies on intermetallic compounds, it has been found that these materials have low heat of formation and large glass-forming ability, if the Fermi level falls on the peak in the density of states (DOS) curve. On the other hand, if theE F falls on the pseudogap in the DOS curve, the ordering energy will be larger and the system prefers to form an ordered alloy. The first principles electronic structure calculations performed on Ni3Al show that it is possible to vary the filling of bonding, nonbonding and antibonding states in the DOS curve and this in turn shows gradual structural transformation as well as formation of multiphases by ternary alloying. Possibilities of tailoring the properties of materials by tuning the Fermi level is discussed in this paper.

Effect of heavy-ion irradiation on dielectric constant and electrical conductivity of doped and undoped nonlinear substance

Abstract: Implantations were carried out on gel-grown potassium dihydrogen orthophosphate (KDP) and those doped with magnesium oxide (MgO) single crystals using 100 MeV Ag+ heavy-ion beam of 15 UD 16 MV pelletron accelerator. To conduct a comparative study, measurements were carried out in the temperature range 243 K-403 K at frequencies ranging from 1 kHz-1 MHz on irradiated and nonirradiated nonlinear samples. It was observed that the mechanism of dielectric behaviour varied with frequency, temperature and ion irradiation. Further, implantation produced erratic variation in the conductivity both in the intrinsic and extrinsic regions, and also in the dielectric behaviour of the substance. The property of sensitive dependence on initial conditions, namely, chaos had set in after ion irradiation. However, the doping effect had not completely terminated the above transition, leading to chaos in the nonlinear medium.

Alloy design with the aid of molecular orbital method

Abstract: A molecular orbital approach to alloy design has recently made great progress This is applicable not only to structural alloys, but also to functional alloys In this paper we have focussed on two materials as examples: high Cr ferritic steels and hydrogen storage alloys

An overview of fullerene chemistry

Abstract: Fullerenes which were thought to be ‘superaromatics’ are actually ‘superalkenes’. Reactions in fullerenes are varied, ranging from the addition of simple molecules like H2 to large molecules like dendrimers. The synthesis, structure, characterization, along with simple reactions like halogenation, oxygenation, metalation, cycloaddition, polymerization and dendrimer addition are discussed.

Some structural aspects of neodymium praseodymium oxalate single crystals grown in hydro silica gels

Abstract: The mixed crystals of neodymium praseodymium oxalate are grown by the diffusion of a mixture of aqueous solutions of neodymium nitrate and praseodymium nitrate (as an upper reactant) into the set gel embedded with oxalic acid. By varying the concentration (by volume) of rare earth nitrates in the upper reactant, the incorporation of Nd and Pr in the mixed crystals has been studied. Tabular crystals with the well defined hexagonal basal planes are observed in the mixed crystals of varying concentrations. X-ray diffraction patterns of these powdered samples reveal that these mixed crystals are ‘isostructural’, while IR and FTIR spectra establish the presence of oxalate groups. TGA and DSC analyses show the correctness of the chemical formula for the mixed crystals, by the release of water molecules (endothermic) and of CO and CO2 (exothermic), with the rare earth oxides as the stable residue. X-ray fluorescence (XRF) and energy dispersive X-ray analyses (EDAX) establish the presence of heavy rare earth elements qualitatively and to a good extent quantitatively. X-ray photoelectron spectroscopic (XPS) studies confirm the presence of rare earth elements (Nd and Pr) as their respective oxides. The findings of these techniques of characterization are in excellent agreement with the proposed empirical structure for the mixed rare earth oxalates. The implications are discussed.

Mass spectrometry based evolved gas analysis system for thermal decomposition studies

Abstract: An experimental facility for evolved gas analysis by mass spectrometry (EGA-MS) has been built in-house and extensively used to study the temperature programmed decomposition (TPD) of a number of inorganic solids. Fractional extent of reactionα acquired from real time multiple ion detection trend analysis mass spectra of gases released from thermally impressed specimen has been used to obtain functional transformf(α) of non-isothermal solid state kinetic rate expressions. The corresponding model integral functionsg(α) based on mechanisms like random nucleation, diffusion and interface motion have been used to establish kinetics control regimes for specific decomposition sequences. From ln[g(α)/T 2] vs 1/T plots Arrhenius parameters like activation energy and pre-exponential factor could be determined. Signature of the rate controlling mechanism governing the gas release behaviour was found in the crystallographic transformation brought about by the temperature programme. This paper describes the scope and capabilities of our EGA-MS facility with typical results on temperature programmed decomposition of CuSO4·5H2O and AlNH4(SO4)2·12H2O.

On the ballistic performance of metallic materials

Abstract: The paper presents a ballistic performance index for metallic armour materials in terms of the commonly determined mechanical properties such as strength and modulus. The index is derived using an energy-balance approach, where the kinetic energy of the projectile is assumed to be absorbed by the elastic and the plastic deformation involved in the penetration process as well as the kinetic energy imparted to the target material during deformation. The derivation assumes two distinct stages to exist during the penetration of the projectile. At the striking face of the armour, the material is assumed to flow radially in a constrained deformation region but longitudinally at the rear surface leading to typically observed bulging of the armour without constraint. The index is validated using the available experimental and empirical data obtained in the case of small arm projectiles for an impact velocity of about 800 m/sec. This index is expected to facilitate the development of metallic armour, since the number of the ballistic experiments can be reduced significantly and only the promising materials need to be considered.

Chemical bath deposition of CdS thin films and their partial conversion to CdO on annealing

Abstract: CdS thin films prepared by chemical bath deposition technique are characterized using X-ray diffraction, optical absorption spectrometry and scanning electron microscopy. The results of the annealing studies on the films in flowing argon and air atmospheres are also presented in this paper. The resistivity has drastically reduced on annealing in flowing air which is attributed to the partial conversion of CdS to CdO phase.

Application of MO calculation to plasma-enhanced CVD using organosilicon compounds

Abstract: Ultra water-repellent films for which contact angles of water drops exceed 150° were prepared by microwave plasma-enhanced CVD using two kinds of organosilicon compound and fluoro-alkyl silane (FAS) at low substrate temperature. Hexamethyldisilane (HMDS) and hexamethyldisiloxane (HMDSO) were used as starting materials. Molecular orbital (MO) calculation suggested that HMDS was more easily decomposable than HMDSO. The films prepared with HMDS and FAS had ultra water repellency. On the other hand, water repellency of the films prepared with HMDSO and FAS was similar to that of polytetrafluoroethylene.

Magnetic properties of magnesium-cobalt ferrites synthesized by co-precipitation method

Abstract: The alternating current (a.c.) low field susceptibility vs temperature, magnetization and57Fe Mossbauer effect measurements are reported for the spinel solid solution series Mg x Co1−x Fe2O4 synthesized by a wet-chemical method before and after high temperature annealing. The observed features for the wet samples, such as the coexistence of paramagnetic doublet and magnetic sextets in Mossbauer spectra and lower saturation magnetization values confirm small particle ferrite behaviour. Especially, Mossbauer spectra of wet samples reveal the presence of superparamagnetic particles which exist simultaneously with ferrimagnetic regions in the materials well supported by a.c. susceptibility data. The high temperature annealing changes the wet-prepared ferrites into the ordered magnetic structure of ceramic ferrites.

Crystallization and microstructural behaviour of strontium titanate borosilicate glass ceramics with Bi2O3 addition

Abstract: Glasses in the system (65 −x) [SrO·TiO2] − (35) [2SiO2·B2O3] − (x) [Bi2O3] wherex = 1, 5, 10 (wt%) prepared by melting in alumina crucible (1375–1575 K), were subjected to different heat treatment schedules followed by DTA studies. Crystallization study showed the formation of Sr2B2O5 as major phase at low temperature (≈950°C) heat treatment. At high temperatures, TiO2 and SrTiO3 with or without Sr2B2O5 crystallize out depending on heat treatment. In this paper, the influence of variation in composition, thermal treatment on the nature of crystallizing phases as well as on the resulting microstructures are investigated through XRD, IR and SEM. Uniform crystallization was achieved by suitable addition of Bi2O3 and proper heat treatment.

Thermal expansion studies on ferroelectric materials

Abstract: Thermal expansion data is reported over a wide temperature range (80-800 K) for $BaTiO_3 (BT)$ and $Pb(Fe_{1/2}Nb_{1/2})O_3 (PFN)$, belonging to different classes of ferroelectric materials. The sharp structural phase transitions of BT result in lowering of the thermal expansion coefficient (α) at the transitions which is proportional to the change in polarization at the transitions. In PFN, a relaxor ferroelectric, lowering of α is spread over a wider temperature range across the dielectric maxima

Elastic properties of ZnF2-PbO-TeO2 glasses doped with certain rare earth ions

Abstract: The elastic moduli (Y, η, σ andH) and some thermodynamical parameters, such as Debye temperatureθ D, diffusion constantD i and latent heat of melting ΔH m, of ZnF2-PbO-TeO2 glasses doped with some rare earth (Pr3+, Nd3+, Eu3+ and Tb3+) ions are determined as functions of temperature using piezo electric composite oscillator technique. All the parameters are found to decrease with increase in the atomic number Z of the rare earth ions and with increase in the temperature of measurement. The results are explained on the basis of the density of localized bonding defect states within the band gap of the material.

Hardening and properties of cement-based materials incorporating heavy metal oxides

Abstract: The subject of the study was the influence of oxides PbO, ZrO2 and Cr2O3 immobilized in cement matrix. The obtained results show the positive effect of the present oxides in hydrating cement on compressive strength development and the quality of the formating pore structure of the resulting material. A surprising effect showing the paralysis of the positive effect of heavy metal oxide admixture has been shown by the increasing of the added quantity of PbO (from 1 to 4%) and the used w/c ratio (0·7 instead of 0·4). The increase of the quantity overcoming the optimum of the reaction product of the interaction between hydrating cement and PbO added for the cause of the paralysis is supposed. The explanation of the adverse effect and the identification of the reaction product in question needs more detailed study.

Hydrogen diffusivity in FeAl

Abstract: The room temperature diffusivity of hydrogen in a fully B2 ordered iron aluminide of composition Fe-35·8 Al was estimated from the experimental hydrogen depth profile to be 2·38×10−15m2/s. The mathematical procedure utilized for data analysis has been described. The estimated diffusivity is a lower bound value due to surface trapping effects.

Evidence of two ferroelectric PTCR components in valence-compensated ceramic system Ba1−xLaxTi1−xCoxO3

Abstract: The electrical behaviour of valence-compensated ceramic system Ba1−x La x Ti1−x Co x O3 has been studied as a function of temperature (300–600 K) and composition (x ⩽ 0·20), using the method of impedance spectroscopy. The necessary equivalent circuit models that represent the data best have been obtained using impedance and modulus formalisms and grain and grain boundary contributions have been separated out. The compositionsx = 0·20 and 0·10 show a negative temperature coefficient of resistance (NTCR) behaviour whereas a very small variation of the grain and grain boundary resistance with temperature is observed forx = 0·05. A positive temperature coefficient of resistance (PTCR) behaviour having two ferroelectric components is observed forx = 0·01. These results reveal limitations in current theories of the PTCR effect.

Nanocrystallization in Si:H and quantum size effect on optical gap

Abstract: Interruption of growth and H-plasma exposure on stacking layers of Si:H film resulted in a remarkable change in material properties. Widening of optical gap and increase in dark conductivity were simultaneous with the reduction in photoconductivity, bonded hydrogen content and optical absorption. An associated change in the network structure from amorphous towards crystalline was observed. Enhanced dose of plasma exposure resulted in the gradual lowering in the size of nanograins and increase in their number density. Systematic widening in optical gap during dehydrogenation of the network appears to be a unique feature related to amorphous semiconductors, which suggests nanocrystallization and quantum size effect in hydrogenated binary alloy.

A comparative study on the processing, dielectric and electrical properties of PMN based solid solutions

Abstract: Lead magnesium niobate (PMN) based binary with lead titanate (PT), lead iron niobate (PFN) and lead zinc niobate (PZN) and ternary with PZN-PT and PFN-PT solid solutions have been synthesized. In depth characterization using X-ray diffraction and SEM techniques have revealed the major perovskite phase formation. Amongst all the solid solutions, PMN-PZN-PT has given the highest values of permittivity as 19,740 and 23,700 withT c as 34°C when sintered at 1080°C and 1180°C respectively on the one hand and on the other, PFN containing solid solutions exhibited distinct deviation from the relaxor behaviour.

Thermal behaviour of gel-grown pure and mixed rare earth tartrates of yttrium and samarium

Abstract: Thermal behaviour of gel-grown pure and mixed rare earth tartrates of yttrium and samarium is investigated using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The thermal behaviour suggests that the materials are unstable at lower energies and pass through various stages of decomposition, decomposing to respective rare earth oxides which remain stable on further heating. It is estimated that both pure yttrium and pure samarium tartrate crystals carry eight waters of hydration, while mixed yttrium samarium tartrate crystals carry six waters of hydration. Critical examination of TG and DSC curves shows that the initial decompositions are endothermic and the latter are exothermic. Thermal kinetics of these materials has been worked out using Horowitz-Metzger, Piloyan-Novikova and Coats-Redfern equations. Application of these equations to these materials yields values of activation energy, order of reaction and frequency factor which are in reasonably good agreement.

Monte Carlo and molecular dynamics simulation of argon clusters andn-alkanes in the confined regions of zeolites

Abstract: Geometry and energy of argon clusters confined in zeolite NaCaA are compared with those of free clusters. Results indicate the possible existence of magic numbers among the confined clusters. Spectra obtained from instantaneous normal mode analysis of free and confined clusters give a larger percentage of imaginary frequencies for the latter indicating that the confined cluster atoms populate the saddle points of the potential energy surface significantly. The variation of the percentage of imaginary frequencies with temperature during melting is akin to the variation of other properties. It is shown that confined clusters might exhibit inverse surface melting, unlike medium-to-large-sized free clusters that exhibit surface melting. Configurational-bias Monte Carlo (CBMC) simulations ofn-alkanes in zeolites Y and A are reported. CBMC method gives reliable estimates of the properties relating to the conformation of molecules. Changes in the conformational properties ofn-butane and other longern-alkanes such asn-hexane andn-heptane when they are confined in different zeolites are presented. The changes in the conformational properties ofn-butane andn-hexane with temperature and concentration is discussed. In general, in zeolite Y as well as A, there is significant enhancement of thegauche population as compared to the pure unconfined fluid.

Measurement of residual stresses in CFRP laminates by X-ray diffraction method

Abstract: The fabrication of CFRP laminates from prepregs involves curing at elevated temperatures. Residual stresses are set up due to the difference in thermal expansion coefficient between the matrix and the fibre. In this investigation, the X-ray diffraction method is used to measure the curing stresses in CFRP laminates by incorporating a very fine layer of aluminium particles during the lay up of the laminate. A calibration procedure is followed to correlate the strain in the crystalline particles, as measured by X-rays, with the composite strain and stress. Curing stresses measured by this technique are quite close to the value calculated from the differential coefficient of thermal expansion.

Preparation and characterization of indium oxide (In2O3) films by activated reactive evaporation

Abstract: The electrical and optical properties of In2O3 films prepared at room temperature by activated reactive evaporation have been studied. Hall effect measurements at room temperature show that the films have a relatively high mobility 15 cm2v−1s−1, high carrier concentration 2·97 × 1020/cm3, with a low resistivityρ = 1·35 × 10−3 ohm cm. As-prepared film is polycrystalline. It shows both direct and indirect allowed transitions with band gaps of 3·52eV and 2·94eV respectively.

Effect of deformation and oxygen content on mechanical properties of different copper wires

Abstract: Investigations were done at dip-forming, contirod and SCR copper wire rods. These continuously cast and hot rolled copper wire rods (8 mm in diameter) were cold drawn in two drawing programmes (soft and hard drawn) to various diameters without any intermediate annealing. The results of influence of deformation and oxygen content on the flow stress, tensile elongation and number of twist to failure are given. Strength and strain hardening rates of hard-drawn wires always were higher than the soft-drawn wires. Oxygen effect on the ductility of copper wires measured with the tensile elongation and the number of twist to failure is not clear at large drawing prestrains.

DC electrical conduction in heat-treated poly nickel phthalocyanine

Abstract: Heat treatment of poly nickel phthalocyanine leads to a charge transfer and hence there is an improvement in conductivity by 5 orders of magnitude. DC electrical conductivity measurements have been carried out in the heat-treated samples in the temperature range 270 K-10 K. Mott’s variable range hopping conduction in one dimension is observed in the temperature range 270K-77K and the conductivity remains constant in the temperature range 77K-10K.

Conduction mechanism in doped polymethyl methacrylate (PMMA) films

Abstract: The electrical transport behaviour of ferrocene mixed poly (methyl methacrylate) (PMMA) films (≈ 20 µm in thickness) deposited by the isothermal immersion technique has been studied in the temperature range of 333–373 K and field from (2·0–4·0)×104 V/cm. It has been found that at higher fields and temperatures, the observed conduction behaviour could be consistently described by the Richardson-Schottky emission. The increase in current due to doping has been attributed to the formation of charge transfer complexes. The dopant molecules act as an additional trapping centre and provide a link between polymer molecules in amorphous region leading to the formation of charge transfer complex.

Synthesis of spodumene and spodumene — zirconia composite powders using aqueous sol-gel method

Abstract: Pure spodumene and spodumene-zirconia (5, 10, 15 mol%) composite powders were prepared using aqueous sol-gel method employing lithium formate, aluminium formate, zirconium formate and tetraethoxy silane (TEOS) as starting materials in aqueous medium. The gels prepared by this method were dried at 100°C for 24h and then calcined for 2h at different temperatures ranging from 500°C to 800°C. X-ray powder diffraction (XRD), differential thermal analysis (DTA), thermogravimetric analysis (TGA) and infra-red spectroscopy analysis (IR) were utilized to characterize the gel powders and calcined powders. Transmission electron microscope (TEM) was used to measure the average particle size of the calcined powders.

n-CuInS2/polysulfide photoelectrochemical solar cells prepared by spray pyrolysis

Abstract: n-CuInS2 photoanode has been prepared by spray pyrolysis onto SnO2 deposited glass substrate at 350°C. The conductivity type of the photoanode was tested by hot-probe method and was ofn type. The conductivity of the photoanode was of the order of 2–4Θ−1cm−1 and was measured by using four-probe method. The effect of etching (HCl:HNO3 = 5:1 by volume) on photoanode properties has been studied. The best cell had the following parameters:V oc = 0·29V,I sc = 5·33 mA/cm2,ff = 0·571 and η = 1·275%.

Lanthanum samarium oxalate — its growth and structural characterization

Abstract: Lanthanum samarium oxalate (LSO) single crystals are grown in silica gels by the diffusion of a mixture of aqueous solutions of lanthanum nitrate and samarium nitrate into the test tube having the set gel impregnated with oxalic acid. Tabular crystals of LSO having well defined hexagonal basal planes are observed at different depths inside the gel. LSO crystals grown by this method are colourless and transparent. Laue transmission X-ray diffraction pattern of LSO reveals well defined spots with two-fold symmetry along the horizontal axis. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) support that LSO loses water around 120°C, and CO and CO2 around 350–450°C. The infrared (IR) absorption spectrum of LSO establishes the presence of oxalate (C2 O4)2− ions. Energy dispersive X-ray analysis (EDAX) confirms the presence of La and Sm in the sample. X-ray photoelectron spectroscopic (XPS) studies of LSO confirm the presence of La and Sm in their respective oxide states. An empirical structure for LSO has been proposed on the basis of these findings.

Structural properties of amorphous hydrogenated silicon using ab initio molecular dynamics simulations

Abstract: We have studied structural properties of amorphous hydrogenated silicon usingab initio molecular dynamics simulations. A sample was generated by simulated annealing using periodic boundary conditions with a supercell containing 64 silicon and 8 hydrogen atoms. The radial pair distribution functions for Si-Si, Si-H and H-H have been studied at 300 K and are found to be in good agreement with experimental data. Our results show that hydrogen saturates the dangling bonds and reduces bond strain. We also report existence of Si-H-Si bridge sites which are likely to play an important role in understanding the light induced metastability in this material.

Anomalous d.c. conductance in Nd+3-doped ferroelectric lead germanate single crystal

Abstract: Pure lead germanate (LG), 0·1 wt% Nd+3-doped lead germanate (LG.1Nd+3), 0·2wt% Nd+3-doped lead germanate (LG.2Nd+3) were grown by controlled cooling of melt. Variation of d.c. conductivity was studied in the temperature range 293°K–523°K. Conductivity increases with increasing temperature, showing semiconducting behaviour for all the compositions under study. The activation energy in ferroelectric phase is larger than in paraelectric phase for all the compositions. The result suggests polaron hopping to be a significant contributor to conduction mechanism.