Volume 17, Issue 4
August 1994, pages 307-451
pp 307-346 August 1994
The present article reviews the current status of research and development on the structure and properties of nanocrystalline materials. Nanocrystalline materials are polycrystalline materials with grain sizes of up to about 100 nm. Because of the extremely small dimensions, a large fraction of the atoms in these materials is located at the grain boundaries, and this confers special attributes. Nanocrystalline materials can be prepared by inert gas-condensation, mechanical alloying, plasma deposition, spray conversion processing, and many other methods. These have been briefly reviewed.
A clear picture of the structure of nanocrystalline materials is emerging only now. Whereas the earlier studies reasoned out that the structure of grain boundaries in nanocrystalline materials was quite different from that in coarse-grained materials, recent studies using spectroscopy, high-resolution electron microscopy, and computer simulation techniques showed unambiguously that the structure of the grain boundaries is the same in both nanocrystalline and coarse-grained materials. A critical analysis of this aspect and grain growth is presented.
The properties of nanocrystalline materials are very often superior to those of conventional polycrystalline coarse-grained materials. Nanocrystalline materials exhibit increased strength/hardness, enhanced diffusivity, improved ductility/toughness, reduced density, reduced elastic modulus, higher electrical resistivity, increased specific heat, higher thermal expansion coefficient, lower thermal conductivity, and superior soft magnetic properties in comparison to conventional coarse-grained materials. Recent results on these properties, with special emphasis on mechanical properties, have been discussed.
New concepts of nanocomposites and nanoglasses are also being investigated with special emphasis on ceramic composites to increase their strength and toughness. Even though no components made of nanocrystalline materials are in use in any application now, there appears to be a great potential for applications in the near future. The extensive investigations in recent years on structure-property correlations in nanocrystalline materials have begun to unravel the complexities of these materials, and paved the way for successful exploitation of the alloy design principles to synthesize better materials than hitherto available.
pp 347-354 August 1994
Deep level transient spectroscopy (DLTS) was carried out on single crystals of the layered chalcogenide p-GaTe using Schottky barriers parallel and perpendicular to the layer planes to study the possible anisotropy of the defect levels. Deep levels with the same energies (0·28 eV and 0·42–0·45eV) have been found in both directions with concentrations ranging from 1013cm−3 to 1014 cm−3 and capture cross-sections from 10−15cm2 to 10−17cm2. The difference in the spectra obtained from the two planes and the possible reason for the deep level energies being independent of crystal orientation are discussed.
pp 355-360 August 1994
X-ray photoelectron spectroscopic (XPS) study of the valence band and the core levels of amorphous Fe40Ni40B20 are presented. The oxides which formed at the surface of as-received sample are due to oxidation of iron and boron. For etched samples, the presence of oxide is not discernible, and the chemical environment is predominantly iron-boron-like, while nickel remains unassociated. The valence band has a high density of states at Fermi levels of amorphous Fe40Ni40B20 are presented. The oxides which formed at the surface of B2s and Fe3d states, and metalloids-states respectively.
pp 361-365 August 1994
A novel matrix resin system, viz. oligoimide-epoxy resin, has been developed to prepare glass-fibre-reinforced composites. Diaminodiphenylmethanebismaleimide-diaminodiphenylmethane (DDMBM-DDM) andp-phenylenebismaleimide-diaminodiphenylmethane (PBM-DDM) oligomers having more-NH2 groups were prepared through Michael addition reaction. These oligoimides were used for curing commercial epoxy resin (i.e. diglycidyl ether of bisphenol A) at 120–140°C to fabricate crosslinked oligoimide-epoxy resin glass-fibre-reinforced composites without evolution of byproduct. The fabricated composites (i.e. laminates) were characterized by their chemical resistance and mechanical properties.
pp 367-387 August 1994
The present study aims at investigating MOCVD technique for the deposition of magnetic oxide thin films using volatile metal-organic compounds as source material. A three-step scheme has been described to form γ-Fe2O3 phase starting from α-Fe2O3 films as-deposited in optically heated atmospheric cold wall CVD reactor. Growth of γ-Fe2O3 in a two-step process has been performed by depositing Fe3O4 phase directly by resistively heated low-pressure CVD (LPCVD) technique. Role of substrate temperature in controlling the oxidation leading to direct formation of metastable γ-Fe2O3 phase (single-step scheme) by atmospheric CVD technique has been described. A new mode of introduction of cobalt in the film, namely heterogeneous dispersion of cobalt in the γ-Fe2O3 matrix, has also been described. Crystallographic structure, microstructure and magnetic properties of the films have been studied in detail. Biaxial vector coil and high-temperature magnetic studies were carried out for determining the nature of anisotropy in the γ-Fe2O3 film. Growth of γ-Fe2O3 films in different schemes have been discussed from the studies of growth kinetics in a cold-and hot-wall-type reactor chambers.
pp 389-393 August 1994
We have investigated doping effects on the transition temperature (Tc) of La2 −xMxCuO4 (M=Ca, Na and K) by incorporating the effects of two-dimensional (2D) acoustic plasmons in the framework of the strong coupling theory. The contributions from 2D acoustic plasmon mechanism toTc have been obtained from a Fourier-transformed effective potential, which has been earlier found to be successful in predicting the composition dependence ofTc in La2 −x(Ba, Sr)xCuO4. The results obtained by us on the variation of transition temperature with composition (x) in La2−xMxCuO4 superconductors are in reasonably good agreement with the available experimental data. This success has led to the conclusion that 2D acoustic plasmons are adequate to explain the pairing mechanism and the variation ofTc with composition (x) in cuprate superconductors.
pp 395-398 August 1994
The gist of the paper is to introduce the other compelling factors for the boundary displacement which are dominant in the lower temperature regime such as non equilibrium vacancy concentration and anisotropy and to prove that the driving force due to the above factors is sufficient to induce grain boundary migration.
pp 399-403 August 1994
The electrical resistivity and Seebeck coefficient for Ni-Cd ferrites have been studied as a function of temperature. The lattice constant of the phases have been evaluated from X-ray powder data. The thermoelectric power measurements indicate that the samples aren-type semiconductors and the conduction mechanism is interpreted on the basis of localized model of polarons.
pp 405-409 August 1994
Composites of BaTiO3 and CoFe2O4 have been prepared with various compositions by double sintering method. The presence of the two phases has been confirmed by XRD. Variation of dielectric constant with temperature in these samples has been studied. All the samples have shown linear magnetoelectric conversion in the presence of static magnetic field. The magnetoelectric effect (dE/dH) has been studied as a function of intensity of magnetic field. The maximum value of the conversion factor (dH/dH)max was found to be 0·16 mV/cm/Oe.
pp 411-420 August 1994
The dielectric modulus spectra of glasses in the system V2O5-TeO2 have been studied as a function of frequency in the temperature range 230 to 330K. A heterogeneous conductor model developed recently with the assumption of a sinusoidally varying local conductivity in the conducting phase has been successfully applied to analyse the data in this glass system. The Kohlrausch-Williams-Watts (KWW) stretched exponential function has also been used to fit the measured modulus spectra. The exponentβ is found to be correlated to the conductivity fluctuation in the conducting phase as assumed in the heterogeneous model.
pp 421-427 August 1994
Analysis of changes in surface roughness of CdS thin films with preparation temperature was carried out using variable angle spectroscopic ellipsometry (VASE). The films studied were prepared by spray pyrolysis technique, in the substrate temperature range 200–360°C. The VASE measurements were carried out in the visible region below the band gap (Eg=2·4eV) of CdS so as to reduce absorption by the film. The thickness of the films was in the range 500–600 nm. Bruggeman’s effective medium theory was used for analysis of the surface roughness of the film. The roughness of the film had a high value (∼ 65 nm) for films prepared at low temperature (200°C) and decreased with increase in substrate temperature. This reached minimum value (∼ 27 nm) in the temperature range 280–300°C. Thereafter roughness increased slowly with temperature. The growth rate of the films was calculated for different temperature ranges. It was found that the deposition rate decreases with the increase in substrate temperature and have an optimum value at 300°C. Above this temperature deposition rate decreased sharply. The scanning electron micrograph (SEM) of the film also showed that the film prepared at 280–300°C had very smooth surface texture.
pp 429-437 August 1994
Photovoltaic performance of semiconductor-liquid junction solar cell using electro-codeposited thin filmn-CdSe is found to improve significantly by proper surface treatments. The solid state parameters of annealed films are calculated and compared with those of unannealed film-based cells. Chemical etching is found to improve short circuit current and fill factor whereas photoelectrochemical etching technique improves the stability of photoanode in polysulphide electrolyte. Annealing promotes incipient fusion of small crystallites, thus reducing the grain boundaries which are known to act as recombination centres for minority carriers and trapping centre for majority carriers. The conversion efficiency and stability are found to improve by chemical etching of the semiconductor layer because chemical etching pins Fermi level of CdSe photoanode and promotes exchange current density.
pp 439-451 August 1994
The tetrahedron approximation of the cluster variation method (CVM) has been employed to investigate phase diagrams having fcc-based ordered and disordered phases. This approximation is also applicable to the binary hcp ordered structures with ideal axial ratio. The CVM developed by Kikuchi consists of calculating approximate expressions for the number of configurations and hence entropy of a crystal lattice having definite distribution of clusters (points, pairs, triangles, tetrahedra, etc.) of lattice points which in general may be occupied by one of a given set of atomic species. Tetrahedral multi-atom interactions denoted by α and β are utilized for expressing the configurational energy. The equilibrium cluster distribution is then found by minimizing the free energy by utilizing the natural iteration method developed by Kikuchi. The effect of α and β parameters on the topology of the resulting phase diagrams is observed by assigning several negative and positive values to them. The invariant reactions were also determined in each case. Finally a study was made on the Cd-Mg diagram.
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