Seebeck Coefficient in Bulk and Quantum Wells of Non-Parabolic Kane-Type Materials

We present a simplified and quantitative analysis of the Seebeck coefficient in degenerate bulk and quantum well materials whose conduction band electrons obey Kane's non-parabolic energy dispersion relation. We use k.p formalism to include the effect of the overlap function due to the band non-parabolicity in the Seebeck coefficient. We also address the key issues and the conditions in which the Seebeck coefficient in quantum wells should exhibit oscillatory dependency with the film thickness under the acoustic phonon and ionized impurity scattering. The effect of screening length in degenerate bulk and quantum wells has also been generalized for the determination of ionization scattering. The well-known expressions of the Seebeck coefficient in non-degenerate wide band gap materials for both bulk and quantum wells has been obtained as a special case and this provides an indirect proof of our generalized theoretical analysis.

Hydrogen-Bond-Directed Nonlinear-Optical Organic Materials - Evidence For The Control Of 2nd-Harmonic Generation Activities From The X-Ray Crystal-Structures Of The Complexes Of L-Tartaric Acid With M-Anisidine and P-Toluidine

Several substituted anilines were converted to binary salts with L-tartaric acid. Second harmonic generation (SHG) activities of these salts were determined. The crystal packing in two structures, (i) m-anisidinium-L-tartrate monohydrate (i) and (ii) p-toluidinium-L-tartrate (2), studied using X-ray diffraction demonstrates that extensive hydrogen bonding steers the components into a framework which has a direct bearing on the SHG activity

Novel materials synthesis by mechanical alloying/milling

An account is given of the research that has been carried out on mechanical alloying/milling (MA/MM) during the past 25 years. Mechanical alloying, a high energy ball milling process, has established itself as a viable solid state processing route for the synthesis of a variety of equilibrium and non-equilibrium phases and phase mixtures. The process was initially invented for the production of oxide dispersion strengthened (ODS) Ni-base superalloys and later extended to other ODS alloys. The success of MA in producing ODS alloys with better high temperature capabilities in comparison with other processing routes is highlighted. Mechanical alloying has also been successfully used for extending terminal solid solubilities in many commercially important metallic systems. Many high melting intermetallics that are difficult to prepare by conventional processing techniques could be easily synthesised with homogeneous structure and composition by MA. It has also, over the years, proved itself to be superior to rapid solidification processing as a non-equilibrium processing tool. The considerable literature on the synthesis of amorphous, quasicrystalline, and nanocrystalline materials by MA is critically reviewed. The possibility of achieving solid solubility in liquid immiscible systems has made MA a unique process. Reactive milling has opened new avenues for the solid state metallothermic reduction and for the synthesis of nanocrystalline intermetallics and intermetallic matrix composites. Despite numerous efforts, understanding of the process of MA, being far from equilibrium, is far from complete, leaving large scope for further research in this exciting field.

ChemInform Abstract: Chemical Design of Solid Inorganic Materials

Newer strategies for the synthesis of inorganic solids have made a great impact on present-day materials chemistry. In this article, typical case studies of synthesis involving new methods and soft chemical routes are discussed besides recent results from nebulized spray pyrolysis and synthesis of nanoscale metal and alloy particles.

Synthesis and properties of nanodispersed granular materials

Nanometric granular materials represent a new class of materials with significant promise. We shall discuss in this paper two phase granular materials where one of the phases having nanometric dimension is embedded in a matrix of larger dimension. These materials show many interesting properties which include structural, magnetic and transport properties, The phase transformation of the embedded particles shows distinctive behavior and yields new insight. We shall first highlight the strategy of synthesis of these materials through rapid solidification. This will be followed by three examples where the nanoscale dimension of the embedded particles play a unique role. These are melting and solidification of the nanodispersed embedded particles and the superconducting transition. (C) 1997 Elsevier Science S.A.

The effect of aluminosilicate on the delayed onset of upturn voltages of zinc oxide varistor ceramics

The effect of aluminosilicate (Al2SiO5) on the upturn characteristics of ZnO varistor ceramics has been investigated. Addition of Al2SiO5 shifts the point of upturn above 10(4) A cm(-2). The extended nonlinearity in the high current density region is better correlatable to the presence of higher density of trap stales and changing pattern of trap depths at the grain boundary interface as much as the grain interior conductivity. Microstructure studies show the formation and involvement of a liquid phase during sintering. The secondary phases, predominantly are antimony spinel, Zn7Sb2O12, zinc silicate, Zn2SiO4 and magnesium aluminium silicate. MgAl2Si3O10. Energy dispersive X-ray analyses (EDAX) show that Al and Si are distributed more in the grain boundaries and within the secondary phases than in the grain interiors. Capacitance-voltage analyses and dielectric dispersion studies indicate the presence of negative capacitance and associated resonance, indicative of the oscillatory charge redistribution involving increased trapping at the interface states. The admittance spectroscopy data show that the type of trap slates remains unaltered whereas the addition of Al2SiO5 increases the density of low energy traps. (C) 1997 Published by Elsevier Science S.A.

Polymorphism in Opto-Electronic Materials with a Benzothiazole-fluorene Core: A Consequence of High Conformational Flexibility of pi-Conjugated Backbone and Alkyl Side Chains

Fluorene and its derivatives are well-known organic semiconducting materials in the field of opto-electronic devices because of their charge transport properties. Three new organic semiconducting materials, namely, 2,2'-((9,9-butyl-9H-fluorene-2,7-diyl)bis(4,1 phenylene))bisbenzod]thiazole, C4; 2,2'-((octyl-9H-fluorene-2,7-diyl)bis(4,1 phenylene))bisbenzod]thiazole, C8; and 2,2'-((9,9-dodecayl-9H-fluorene-2,7-diyl)bis(4,1 phenylene))bisbenzod]thiazole, C12 with a benzothiazole-fluorene backbone, were synthesized and characterized for their photophysical properties. A phenomenon of concomitant polymorphism has been investigated in the first two derivatives (C4 and C8) and has been analyzed systematically in terms of the packing characteristics involving pi ... pi interactions. The conformational flexibility of the pi-conjugated 2,2'-(fluorene-2,7-diyl)bis(4,1 phenylene)bisbenzod]thiazole backbone coupled with orientational freedom of the terminal alkyl chains were found to be the key factors responsible for these polymorphic modifications. Attempts to grow suitable crystals for single crystal X-ray diffraction of compound C12 were unsuccessful.

Micromechanical modeling of granular materials: effect of confining pressure on mechanical behavior

This paper reports the effect of confining pressure on the mechanical behavior of granular materials from micromechanical considerations starting from the grain scale level, based on the results of numerically simulated tests on disc assemblages using discrete element modeling (DEM). The two macro parameters which are influenced by the increase in confining pressure are stiffness (increases) and volume change (decreases). The lateral strain coefficient (Poisson's ratio) at the beginning of the test is more or less constant. The angle of internal friction slightly decreases with increase in confining pressure. The numerical results of disc assemblages indicate very clearly a non-linear Mohr-Coulomb failure envelope with increase in confining pressure. The increase in average coordination number and accompanying decrease of fabric anisotropy reduce the shear strength at higher confining pressures. Micromechanical explanations of the macroscopic behavior are presented in terms of the force and fabric anisotropy coefficients. (C) 1999 Elsevier Science Ltd. AII rights reserved.

Novel materials, materials design and synthetic strategies: recent advances and new directions

There have been major advances in solid state and materials chemistry in the last two decades and the subject is growing rapidly. In this account, a few of the important aspects of materials chemistry of interest to the author are presented. Accordingly, transition metal oxides, which constitute the most fascinating class of inorganic materials, receive greater attention, Metal-insulator transitions in oxides, high temperature superconductivity in cuprates and colossal magnetoresistance in manganates are discussed at some length and the outstanding problems indicated, We then discuss certain other important classes of materials which include molecular materials, biomolecular materials and porous solids. Recent developments in synthetic strategies for inorganic materials are reviewed. Some results on metal nanoparticles and nanotubes are briefly presented. The overview, which is essentially intended to provide a flavour of the subject and show how it works, lists references to many crucial reviews in the recent literature.

Rational design of solid materials: A case study of lithium-ion conductors

In the absence of a reliable method for a priori prediction of structure and properties of inorganic solid materials, an experimental approach involving a systematic study of composition, structure and properties combined with chemical intuition based on previous experience is likely to be a viable alternative to the problem of rational design of inorganic materials. The approach is illustrated by taking perovskite lithium-ion conductors as an example.

Studies on strontium substituted rare earth manganites

Sintering, electrical conductivity and thermal expansion behaviour of combustion synthesised strontium substituted rare earth manganites with the general formula Ln(1-x)Sr(x)MnO(3) (Ln = Pr, Nd and Sm; x = 0, 0.16 and 0.25) have been investigated as solid oxide fuel cell cathode materials. The combustion derived rare earth manganites have surface area in the range of 13-40 m(2)/g. Strontium substitution increases the electrical conductivity values in all the rare earth manganites. With the decreasing ionic radii of rare earth ions, the conductivity value decreases. Among the rare earth manganites studied, (Pr/Nd)(0.75)Sr0.25MnO3 show high electrical conductivity ( > 100 S/cm). The thermal expansion coefficients of Pr0.75Sr0.25MnO3 and Nd0.75Sr0.25MnO3 were found to be 10.2 x 10(-6) and 10.7 x 10(-6) K-1 respectively, which is very close to that of the electrolyte (YSZ) used in solid oxide fuel cells. (C) 1999 Elsevier Science B.V. All rights reserved.

Synthesis of mesoporous materials based on titanium(IV)oxide and titanium nitride

Nanoparticles of titania were obtained by the controlled hydrolysis of Ti(i-OC3H7)(4) in the reverse micelles of dodecylamine derived from dodecylamine-isopropanol-water solution (water/oil microemulsion). The mesolamellar phase based on titanium nitride (TiN) was obtained by first decomposing TiN atleast partially using the 1:1 solution of acid mixture (HF and HNO3 in the ratio of 9:1) in water and then templating onto the cationic surfactant namely, cetyltrimethylammaniumbromide (abbreviated as CTAB) at 80 degrees C. The synthesis of mesolamellar phase based on TiN involves the charge matched templating approach following the counter-ion mediated pathway. The samples thus obtained were characterized by small angle x-ray diffraction using Cuk(a) radiation, scanning electron microscopy and transmission electron microscopy, which indicated some satisfactory results. (C) 1999 Acta Metallurgica Inc.

The effect of boundaries on the plane Couette flow of granular materials: a bifurcation analysis

The tendency of granular materials in rapid shear flow to form non-uniform structures is well documented in the literature. Through a linear stability analysis of the solution of continuum equations for rapid shear flow of a uniform granular material, performed by Savage (1992) and others subsequently, it has been shown that an infinite plane shearing motion may be unstable in the Lyapunov sense, provided the mean volume fraction of particles is above a critical value. This instability leads to the formation of alternating layers of high and low particle concentrations oriented parallel to the plane of shear. Computer simulations, on the other hand, reveal that non-uniform structures are possible even when the mean volume fraction of particles is small. In the present study, we have examined the structure of fully developed layered solutions, by making use of numerical continuation techniques and bifurcation theory. It is shown that the continuum equations do predict the existence of layered solutions of high amplitude even when the uniform state is linearly stable. An analysis of the effect of bounding walls on the bifurcation structure reveals that the nature of the wall boundary conditions plays a pivotal role in selecting that branch of non-uniform solutions which emerges as the primary branch. This demonstrates unequivocally that the results on the stability of bounded shear how of granular materials presented previously by Wang et al. (1996) are, in general, based on erroneous base states.

2-Halooxyethylene ethers of cholesterol as novel single component, room temperature cholesteric LC materials

A series of 2-haloethoxyethyl cholesteryl ethers has been synthesized. Each material shows attractive liquid-crystalline properties as revealed by differential scanning calorimetry, polarizing microscopy, and temperature-dependence of selective reflection characteristic of the cholesteric mesophase. These are interesting examples of simple, nonpolymeric, single component systems that show the cholesteric mesophase at room temperature.

A template-free, combustion-chemical route to macroporous nickel monoliths displaying a hierarchy of pore sizes

Recently, we demonstrated a very general route to monolithic macroporous materials prepared without the use of templates (Rajamathi et al. J. Mater. Chem. 2001, 11, 2489). The route involves finding a precursor containing two metals, A and B, whose oxides are largely immiscible. Firing of the precursor followed by suitable sintering results in a monolith from which one of the oxide phases can be chemically leached out to yield a macroporous mass of the other oxide phase. The metals A and B that we employed in the demonstration were Ni and Zn. From the NiO-ZnO monolith that was obtained by decomposing the precursor, ZnO could be leached out at high pH to yield macroporous NiO. In the present work, we show that combustion-chemical (also called self-propagating) decomposition of a mixture of Ni and Zn nitrates with urea as a fuel yields an intimate mixture of the oxides that can be sintered and leached with alkali to form a macroporous NiO monolith. The new process that we present here thereby avoids the need for a crystalline single-source precursor. A novel and unanticipated aspect of the present work is that the combination of high temperatures and rapid quenching associated with combustion synthesis results in an intimate mixture of wurtzite ZnO and the metastable rock-salt Ni1-xZnxO where x is about 0.3. Leaching this monolith with alkali gives a macroporous mass of rock-salt Ni1-xZnxO, which upon reduction in H-2/Ar forms macroporous Ni and ZnO. There are thus two stages in the process that lead to two modes of pore formation. The first is associated with leaching of ZnO by alkali. The second is associated with the reduction of porous Ni1-xZnxO to give porous Ni and ZnO.