• Volume 22, Issue 3

May 1999,   pages  139-739

• Foreword

• Porous materials: A case study of supramolecular organization in materials design

Supramolecular synthesis is being increasingly employed in materials design. In this article, the design of porous solids is discussed as a case study. Examples from recent work in the areas of open-framework inorganic materials, mesoporous solids and organic porous solids are presented to illustrate this important aspect of materials chemistry.

• Materials response to high pressures

There are many fascinating areas of research related to the response of materials at high static and dynamic pressures. The experimental range of compression achievable in the condensed state under pressure is much larger than the range of expansion achievable before melting by variation of temperature. The advances in the experimental techniques have been matched by the developments in the first principles theories and in computational resources. Studies of equation of state and of phase transitions in materials have helped to increase our basic understanding of the condensed state of matter, with possible applications in many fields, including nuclear technology. The current status of high pressure research is briefly reviewed, taking examples mainly from the work of our group at Trombay.

• Novel synthesis methods for new materials in solid state chemistry

The most common method used for synthesizing inorganic solids is the so-called ‘ceramic method’ which involves synthesizing at elevated temperatures and leads to thermodynamically stable phases. This method has a serious limitation because many inorganic solids of current interest are rather metastable and consequently their preparation requires low temperature methods. In this article, we have reported some methods of synthesis which also include methods that belong to soft-chemistry routes. The selected examples presented in this paper are: (i) the intercalation and or deintercalation of oxygen in oxides by electrochemical oxidation or reduction in aqueous or nonaqueous solutions, (ii) the use of chemical agents such as NO2 or borohydrides for obtaining oxides with tunneled and lamellar structures, (iii) a multistage synthesis processing for intercalating, in layered oxygenated materials, anionic species and acrylic monomers and polymers, (iv) the supercritical fluid processing for preparing nitrides, carbon-nitrides, lamellar oxides and oxy-(hydroxy-) fluorides, and (v) the mechanical alloying for synthesizing new Mg-based binary intermetallics.

• Disordering and reordering of mechanically milled superlattice alloys

High-energy mechanical working of ordered alloys has the twin effects of refining the grain size to the nano-range, and reducing (or even eliminating) the long-range order (LRO). In this way, states of order can be achieved that cannot be obtained by heat-teatment. Subsequent annealing will re-establish the order. Some experiments in this field, notably with Ni3Al, will be outlined.

• In situ TEM observation of spontaneous alloying in nanometer-sized particles

The alloying behaviour in nanometer (nm)-sized particles was studied by transmission electron microscopy (TEM). When solute atoms are vapour-deposited onto nm-sized particles at room temperature, rapid dissolution of solute atoms into particles occur, and solid solution or compound particles are successfully formed. Such spontaneous alloying occurs even between nm-sized particles of different elements. Our results can be summarized as: (i) spontaneous alloying takes place via a solid-state process, (ii) spontaneous alloying becomes more difficult with increasing particle size, (iii) spontaneous alloying is not an artifact originating from the temperature rise in particles which might be induced by heat of condensation and (iv) remarkable enhancement of solubility is observed in nm-sized compound particles.

• Friction and wear properties of quasi-periodic material coatings

Quasicrystalline (QC) materials are of interest not only because of their unique structure, but also because they exhibit unusual properties which are potentially useful in industrial applications. In the present study a range of aluminium-based QC alloys, in the form of plasma-sprayed coatings, has been produced and evaluated. For the first time the influence of heat treatment on the structure and properties of these coatings has been systematically investigated. Of the nine QC coatings investigated, those of the Al-Cu-Fe-Cr system and the Al-Pd-Mn system showed the most promising combination of good hardness with low coefficient of friction.

• ‘Soft solution processing’in situ fabrication of morphology-controlled advanced ceramic materials in low temperature solutions without firing

A process called ‘soft solution processing’ has been introduced to fabricate advanced solid state materials in an economical, environmental friendly, and energy and material efficient way. A concise discussion of how to improve the synthetic conditions and how to extend the applied system is given. The successful examples show that soft solution processing is capable of preparing advanced materials with planned properties through the easy control of reaction conditions in a suitable aqueous solution in a single synthetic step without huge energy consumption for sintering or melting and without any sophisticated equipment.

• Sol-gel synthesis of hybrid materials

Sol-gel chemistry allows the synthesis of hybrid organic-inorganic materials. Organic molecules can be used as complexing ligands to provide a chemical control over hydrolysis and condensation reactions, leading to the formation of stable suspensions of nanoparticles. They can also be trapped within the sol-gel matrix in order to provide some optical properties such as photochromism. When chemically bound to the oxide network via Si-C bonds they lead to hybrid nanocomposites that offer new possibilities in the field of materials science. Even biospecies, such as enzymes or antibodies, can be trapped within sol-gel matrices in order to make bio-sensors or bio-catalysts.

• A high potential material—zirconia

This is a review as well as an overview of zirconia. Although there are several methods of preparation of zirconia powder, we have described some of the methods of its preparation in this paper, and their application in ceramic bodies.

• Mixed plutonium-uranium carbide fuel in fast breeder test reactor

This paper describes the development of the indigenous plutonium-uranium mixed carbide nuclear fuel for the fast breeder test reactor. The fuel has performed satisfactorily and produced, for the first time in our country, nuclear electricity from a fast reactor. The experience and knowledge gained in the fuel development has provided great confidence for undertaking a programme on utilization of fast reactor technology for power production.

• Sol-emulsion-gel synthesis of ceramic particles

The basic characteristics of the sol-emulsion-gel (SEG) process are described as derived from water-in-oil type emulsions when ‘water’ is replaced by an aqueous sol and ‘oil’ indicates water-immiscible organic liquids. The main roles of a surfactant in emulsion formation are discussed. Steps in the generation of ceramic particles from the SEG process through a variety of experimental options are explained. Relevant examples are described where control of surfactant contents (below and above the working range of critical micelle concentration) in a sol-emulsion can lead to oxide particles of different sizes and shapes. Attempts are made to correlate the products of high-surfactant emulsions with micelles and mesophase structures known to form by surfactant molecules in large concentrations.

• Direct observation of the interaction between vortices and dislocations in superconducting crystals by a cryo-Lorentz EM (interaction between vortices and dislocations)

Quantized magnetic flux lines (vortices) in a Nb foil were directly observed in different magnetic fields up to 200 G by a cryo-Lorentz electron microscope. The interaction of vortices with dislocations in the specimen was examined and clarified; edge-on dislocations weakly pin individual vortices at magnetic fields below 100 G. In higher magnetic fields the formation of a regular hexagonal vortex lattice started preferentially at in-plane dislocations. At 200 G the Abrikosov vortex lattice was formed with small domains, whose centre included the dislocations, showing their important role on the formation of the vortex lattice. For a NbTi foil no clear image of vortices could be seen, because the surface was rough due to the formation of fine grains and precipitates.

• Microstructure controls and their effects on the properties of Bi2Sr2Ca1Cu2Ox superconductors

This paper reports our recent progresses in the development of Bi2Sr2Ca1Cu2Ox/Ag tape conductors for the applications of magnetic field generation in liquid helium or around 20 K, using a refrigerator. We have carried out extensive work to optimize the processing parameters, investigating the relationship between the microstructure and transportJc. We have found that the partial melting in oxygen atmosphere is effective to have large transportJc with good reproducibility. The pre-annealing and intermediate rolling (PAIR) process has been successfully applied to the multilayer conductors to improve the grain alignment and transportJc. TheJc of 5×105A/cm2 at 4·2 K and 10 T has been achieved, which is the highest value reported so far. Two magnets fabricated by using different types of Bi-2212/Ag conductors were tested. One is a magnet designed as an insert magnet for a 18 T-class large bore Nb-Ti/Nb3Sn superconducting magnet. The conductor of this magnet was multifilamentary tape processed by powder-in-tube method. TheIc was 98 A in the backup field of 18 T, which generated the self field of 1·79 T. A large pancake coil was fabricated with multilayer conductor and tested under the operation of cryocooler system. The coil was stably operated up to theJc of the coil at the temperatures below 30 K.

• MOCVD-grown homoepitaxial YBa2Cu3Ox thin films and its multilayers

YBa2Cu3Ox (YBCO) films, Zn-doped YBCO (YBCO : Zn) films, and their bilayers have been epitaxially grown on SrTiO3(100) and single-crystal YBCO(001) substrates by metalorganic chemical vapour deposition. The YBCO(001) films homoepitaxially grown on YBCO(001) substrates have flat surfaces on an atomic scale, and interfaces free from crystalline defects. We can systematically reduce the superconducting transition temperature (Tc) of YBCO : Zn films from 90 K to 37 K by increasing Zn concentration. The bilayers have a sharp distribution of Zn as evaluated fromTc measurements of the upper YBCO films and depth profiles of secondary ion mass spectrometer, suggesting the possibility to form the homoepitaxial SNS (S, superconductor; N, normal metal) junction operatable between 40 K and 90 K.

• Novel ceramic substrates for high Tc superconductors

A group of complex perovskite oxides REBa2NbO6 (RE=La and Dy) have been synthesized and developed for their use as substrates for both YBa2Cu3O7-δ and Bi(2223) superconductors. These materials have a complex cubic perovskite (A2BB′O6) structure with lattice constants,a=8·48−8·60 Å. REBa2NbO6 did not show any phase transition in the temperature range 30–1300°C. The thermal expansion coefficient, thermal diffusivity and thermal conductivity values of REBa2NbO6 are favourable for their use as substrates for highTc superconductors. The dielectric constant and loss factor of REBa2NbO6 are in a range suitable for their use as substrates for microwave applications. Both YBa2Cu3O7-δ and Bi(2223) superconductors did not show any detectable chemical reaction with REBa2NbO6 even under extreme processing conditions. Dip coated YBa2Cu3O7-δ thick films on polycrystalline REBa2NbO6 substrate gave aTc(0) of 92 K and a current density of ∼1·1×104 A/cm2 and Bi(2223) thick film on polycrystalline REBa2NbO6 substrate gave aTc(0) of 110 K and a current density of ∼ 4×103 A/cm2 at 77 K and zero magnetic field. A laser ablated YBa2Cu3O7-δ thin film deposited on polycrystalline REBa2NbO6 substrate gave aTc(0) of 90 K and a current density of ∼5×105 A/cm2.

• 1/f Noise properties of swift heavy ion irradiated epitaxial thin films of YBCO

Effect of 250 MeV107Ag ion irradiation induced columnar defects on the noise properties of the YBCO superconductor in the normal and superconducting state have been investigated. Magnitude of the spectral density of the noise is found to scale inversely with the frequency and exhibit a quadratic dependence on the bias current confirming that the noise arises due to the resistance fluctuations. The magnitude ofSv has been found to decrease with decrease in temperature and shows a noise peak in the transition region. The noise performance of these materials in the vicinity of the superconducting transition as well as in the normal state is found to improve by an order of magnitude after irradiation with 250 MeV107Ag ions. The decrease in the magnitude of 1/f noise peak is due the irradiation induced enhanced flux pinning of the material which suppresses the flux motion induced noise in the vicinity ofTc.

• Superconducting and non-superconducting PrBa2Cu3O7

Superconducting properties of Pr123 and microscopic structural differences between the superconducting and non-superconducting Pr123 crystals are reviewed. It is shown that the variety of physical properties of Pr123 is closely related to Cu deficiency at the chain site and our experimental results are consistently explained with a model that the Pr 4f-O 2p hybridization forms a narrow band and carriers are localized by a kind of atomic disorder in non-superconducting crystals.

• Study of the impact of HgO addition and low-field magnetic relaxation behaviour in granular high-Tc superconductors

A series of high-Tc superconductors have been prepared with the HgO addition/substitution. Significant improvement in theTconset as well asTc0 was observed in all the cases. Substitution of Hg at the Sr site and Ba site in the case of (Bi, Pb)-Sr-Ca-Cu-O and Y-Ba-Cu-O systems, respectively over a range 0·01–0·6 at% helps in constructing an entire spectrum: improvement ofTc0 up to 0·4 at % in the case of Bi-system and up to 0·03 at % in the case of Y123 system and slight drop inTc thereafter. Such improvement is the result of abundant supply of highly reactive nascent oxygen all through the bulk. HgO decomposes and provides oxygen which helps in maintaining proper oxygen stoichiometry throughout the bulk. No Hg or Hg-based impure phases were observed in the X-ray diffraction spectra. Low-field (10–100 Oe) magnetic relaxation studies reveal faster relaxation of the intergranular critical state in the case of silver added samples as the grain boundary coupling energyEJ becomes quite uniform across the entire bulk which leads to smaller flux pinning energy. The distribution of the pinning energy is evaluated from the observed relaxation pattern and is found to be narrower in the case of silver added samples. It was also observed that the transportJc ∼ exp[−ΔTc/Tc0] and the flux pinning energyU ∼ ΔTc, where ΔTc is the transition width and is a measure of the inhomogeneity within the sample. Such relationships may help in devising a strategy for achieving highJc, highU yet low ΔTc. Silver addition turns out to be an effective tool in tailoring the sample properties depending on requirement.

• Evidence for supercooling across a vortex-matter phase transition: studies on CeRU2 and Bi2Sr2CaCu2O8

We present DC magnetization data indicating a first-order phase transition in the vortex state of CeRu2, with the higher entropy phase exhibiting enhanced pinning. Minor hysteresis loops show evidence of supercooling of the higher entropy phase as the phase boundary is crossed both isothermally as well as at constant field. These features are shown to be absent across the Bragg-glass to vortex-glass transition in Bi2Sr2CaCu2O8. The supercooling is more persistent in the constant field case.

• Synthesis and properties of novel materials at high pressure and temperature—molecular dynamics simulation studies

Molecular dynamics simulations, in combination with lattice dynamics studies, based on semiempirical interatomic potentials, have been very useful in the study of properties of complex novel materials at high temperature and pressure. Various properties such as the equation of state, elastic and thermodynamic properties, phase transitions and melting have been studied. These studies help in understanding the synthesis of important new and novel materials, especially the amorphous materials, compounds with unusually coordinated atoms, (e.g. with five-coordinated silicon atoms), materials with controlled thermal expansion, etc. A few examples will be discussed from our recent studies.

• High-pressure high-temperature transitions in nanocrystallineγ Al2O3,γ Fe2O3 and TiO2

We report first observation of new polymorphs of Al2O3 and Fe2O3 in specimens of xerogelγ Al2O3 andγ Fe2O3 quenched from high pressures and temperatures. At about 5 GPa and 1400°C, xerogel gamma alumina (XGA) transformed into a polymorphic mixture of phasesα Al2O3, B Al2O3 and C Al2O3, while XGA containing 1 wt% Cr2O3 transformed into a mixture of phasesαAl2O3, H Al2O3 and k′ Al2O3. The phases B Al2O3, C Al2O3 and H Al2O3 have the monoclinic-, cubic- and hexagonal-rare earth sequioxide (Ln2O3) type structure, respectively. At 5·2 GPa and 1450°C, XGA yielded a mixture ofα Al2O3 and hexagonalμ Al2O3. At STP, the phaseμ Al2O3 was found to transform to another hexagonal phaseλAl2O3 over a 10 week period.

At 5·2 GPa and 900°C,γ Fe2O3 showed transition to a new phase H Fe2O3 which probably has an 8 layer close packed structure. In nanocrystalline TiO2, only the anatase to rutile transition was found. The results are discussed using the free energy vs temperature diagram for xerogel and nanocrystalline materials.

• Materials synthesis and phase transitions under shock waves

Shock wave compression of materials is accompanied by high pressure, high-strain-rate loading, elevated temperatures, large shear stresses and excessive plastic deformation. All these features create an unusual state in materials that is not possible in any processing method. Thus, in multi-component system of powders, these can lead to enhanced reactivity because of defect promoted diffusional flow of the reactants in a solid-solid chemical reaction for materials synthesis. On the other hand, in a one component system, these shock compression features may significantly influence the onset pressure, the kinetics, the mechanism and the reversibility of a phase transformation. We shall illustrate these using examples on the synthesis ofβ-C3N4 and our work on the crystal to amorphous phase transitions inq-GeO2 andα-FePO4.

• Properties of solids under high pressure—An electronic band structure approach

The predicting capability of various parameters related to solids by performing their high pressure electronic band structures is discussed in detail. The studies on ground state crystal structure, magnetic structure and magnetic phase transitions under pressures,s-d electron transitions, bulk modulus, Debye temperature, elastic constants, insulator to metal transitions and the phenomenon of pressure induced superconductivity will be discussed with examples. Especially the method of calculating the Debye temperature for a large number of ternary charlcopyrite systems using their recently evaluated band structure will be presented. The limitations of the band theory when used with the local density approximation with respect to the determination of band gaps and magnetic properties will also be discussed.

• Non-covalent chiral fibres in aqueous gels and their functionalization

The fully reversible synthesis of noncovalent assemblies which are held together by directed bonds is called synkinesis. Micellar fibres with a distinct stereochemistry and gels are formed by amphiphiles forming strong hydrogen bonds between chiral head groups. The 3D-crystallization of such molecular assemblies, without dominating repulsive hydration forces, is prevented by the curvature of the fibres. Curvature might perhaps be directly connected with chirality. Addition of enantiomers often leads to an irreversible destruction of gels. Recent developments relating to ‘hydrophobic water’ in Ångström-wide membrane gaps are also shortly discussed.

• Forces between colloidal droplets in the presence of a weak polyelectrolyte

In this paper, we present the results on the forces between individual colloidal liquid droplets in the presence of a weak polyelectrolyte, poly(acrylic acid), using magnetic chaining technique. The effect of the repulsive forces have been investigated under different experimental conditions such as polyelectrolyte concentration, adsorption time, salt concentration etc. At a PAA concentration of 0·01% (weight), a long range repulsive force profile is observed due to the adsorption of polyelectrolyte on the droplet, without any irreversible aggregates even at very small inter-droplet spacing. Above a concentration of 0·01 wt% of PAA, formation of irreversible chaining of droplets is observed at short inter-droplet separations due to polyelectrolyte bridging. The onset of binding is also independently confirmed by microscopic observation. Compared to the slow adsorption on mica surfaces, the PAA adsorption on the colloidal droplets is found to be rapid. Up to 0·1 M NaCl, the range of repulsion and the hydrodynamic radius of the droplet is found to be increasing.

• Alloying behaviour in nanocrystalline materials during mechanical alloying

The alloying behaviour in a number of systems such as Cu-Ni, Cu-Zn, Cu-Al, Ni-Al, Nb-Al has been studied to understand the mechanism as well as the kinetics of alloying during mechanical alloying (MA). The results show that nanocrystallization is a prerequisite for alloying in all the systems during MA. The mechanism of alloying appears to be a strong function of the enthalpy of formation of the phase and the energy of ordering in case of intermetallic compounds. Solid solutions (Cu-Ni), intermetallic compounds with low ordering energies (such as Ni3Al which forms in a disordered state during MA) and compounds with low enthalpy of formation (Cu-Zn, Al3Nb) form by continuous diffusive mixing. Compounds with high enthalpy of formation and high ordering energies form by a new mechanism christened as discontinuous additive mixing. When the intermetallic gets disordered, its formation mechanism changes from discontinuous additive mixing to continuous diffusive one. A rigorous mathematical model, based on iso-concentration contour migration method, has been developed to predict the kinetics of diffusive intermixing in binary systems during MA. Based on the results of Cu-Ni, Cu-Zn and Cu-Al systems, an effective temperature (Teff) has been proposed that can simulate the observed alloying kinetics. TheTeff for the systems studied is found to lie between 0·42–0·52T1.

• Mono-dispersed single-wall carbon nanotubes formed in channels of zeolite crystal: Production, optical and electrical transport properties

Carbon nanotube materials can now be produced in macroscopic quantities. However, the raw material has a disordered structure and unsorted size, which restrict investigations of both the properties and applications of the nanotubes. In this paper, an alternative approach to the synthesis of mono-sized and parallel-aligned single wall carbon nanotubes (SWCNs) is reported. The SWCNs are formed in 1 nm-sized channels of aluminophosphate zeolite crystallites by pyrolysis of tripropylamine molecules. As verified by tunnel electron microscopy and micro-Raman scattering, the SWNT is of zigzag structure. Electrical transport properties of the SWNT are measured in the temperature range of 0·3K∼300K. The temperature-dependent dc conductivity shows that the SWNT is an intrinsic semiconductor with a narrow band-gap of 52 meV. The well-aligned and mono-sized SWCNs allow us to make more controlled characterization as well as open a door to potential nano-technological application for the novel electronic nanotube system.

• A novel chemical route for the preparation of nanosized oxides, phosphates, vanadates, molybdates and tungstates using polymer precursors

A variety of nanosized (particle diameter ranging between 10–90 nm) ceramic oxide powders have been prepared from a versatile, efficient and technically simple polymer matrix based precursor solution. The precursor solution constituted of the metal nitrates mixed with the polymer-PVA/PAA/carboxylated starch in presence of mono-/disaccharides or, poly hydroxy compound. Thermolysis/flame pyrolysis of the precursor mass at external temperatures of around 300–500°C resulted in the oxide phase.

• Size quantization effects in optical and electrical properties of II–VI semiconductor films in nanocrystalline form

The electrical properties of CdTe and optical properties of ZnS in nanocrystalline thin film form are studied with a view to have a clearer understanding of the optical processes and the carrier transport mechanisms in nanocrystalline II–VI semiconductors, in general. Nanocrystalline ZnS and CdTe films were deposited by magnetron sputtering of respective targets in argon plasma. The optical absorption data of nanocrystalline ZnS films (thickness 10–40 nm) could be explained by the combined effects of phonon and inhomogeneity broadening along with optical loss due to light scattering at the nanocrystallites. The conductivity of CdTe (grain size within 4–4·7 nm) showed (T0/T)p dependence withp ∼ 0·5 indicating the presence of a Coulomb gap near the Fermi level. The width of the Coulomb gap varied within 0·02–0·04 eV depending on the deposition condition. The existing theoretical models were used for estimating hopping energy (0·02–0·04 eV) and hopping distance (2·8–5·1 nm) in nano CdTe films.

• Modeling of diffusion and oxidation in two dimensions during silicon device processing

A process simulator named “2D-DIFFUSE” has been developed where the coupled diffusion equation of dopant impurity and point defects: interstitials and vacancies, has been solved numerically in two-dimension. The interaction of point defects has been modeled assuming quasi (i.e. local) equilibrium,C1Cv=C1*C*V and constant vacancy,Cv=Cv*, conditions. Indeed, these two assumptions decouple the two point defects diffusion equations. The processes modeled in the present version of the simulator include pre-deposition, diffusion and oxidation. The simulator is quite successful at modeling each process individually as well as integrating various processes and models. The program has also been applied to the simulation of phenomena as the dopant diffusion under various ambients, oxidation enhanced and retarded diffusion, emitter push effect etc. Comparisons between simulation based on point defect parameters from various sources have been made.

• Heteroepitaxial growth of III–V compound semiconductors for optoelectronic devices

An AlGaAs/GaAs multi-quantum well vertical-cavity surface-emitting laser diode (VCSELD) has been grown on a Si substrate using metalorganic chemical vapour deposition (MOCVD). The VCSELD with a 23-pair of AlAs/Al0·1Ga0·9As distributed Bragg reflector on a Si substrate exhibited a threshold current of 223 mA under continuous-wave condition at 220 K. Electroluminescence observation showed that an optical degradation was caused by generation and growth of dark-line defects. An MOCVD-grown InGaN/AlGaN double-heterostructure light-emitting diode on a sapphire substrate exhibited an optical output power of 0·17 mW, an external quantum efficiency of 0·2%, a peak emission wavelength at 440 nm with a full width at half-maximum of 63 nm and a stable operation up to 3000 h under 30 mA DC operation at 30°C. A high current level of 281 mA/mm and a large transconductance (gm) of 33 mS/mm have been achieved for a GaN metal semiconductor field-effect transistor (MESFET) with a gate length of 2 µm and a width of 200 µm at 25°C. The GaN MESFET at 400°C showed degraded characteristics: a lowgm of 13·4 mS/mm, a gate leakage and a poor pinch-off.

• Isotope energy shift of luminescence in hydrogen-and deuterium-terminated porous silicon

We review our recent results on the comparative studies between hydrogen-(H-PS) and deuterium-(D-PS) terminated porous Si nanostructures. The photoluminescence (PL) spectrum of D-PS was different from that of H-PS despite both H-PS and D-PS showing the same structure and the same absorption spectrum. We quantitatively discuss these results based on the trapping of conduction electrons at a surface Si-H or Si-D bond supported by a tight binding calculation. In addition to these basic results, we also show that the replacement of hydrogen with deuterium reduces the degradation not only of PL but also of electroluminescence. We also discuss the luminescence degradation mechanism from the point of view of a photoinduced oxidation model.

• Optical properties of tetragonal germanium nanocrystals deposited by the cluster-beam evaporation technique: New light emitting material for future

The germanium (Ge) nanocrystals were deposited on substrates whose temperature was kept at room or liquid nitrogen (LN2) temperature by the cluster-beam evaporation technique. The deposited films are found to consist of the tetragonal crystalline structure rather than the diamond structure of bulk Ge. Such a phase-transition has been theoretically predicted for sizes smaller than 4 nm, which agrees with the size measured by the transmission electron microscopy (TEM). The tetragonal Ge is expected to have a direct band gap of 1·47 eV. Furthermore, the Ge film deposited at LN2 temperature exhibits unique properties, such as photo-oxidation and blue-light emission. The Ge-nanocrystal films deposited by the cluster-beam evaporation technique are attractive materials for application to light emitting devices in future.

• Rapid processing of low-cost, high-efficiency silicon solar cells

Rapid and potentially low-cost processing techniques are analyzed and applied toward the fabrication of high-efficiency Si solar cells. (i) A technology that can simultaneously form the phosphorus emitter, boron BSF, andin situ oxide in a single high-temperature furnace step or: simultaneously diffused, textured, and AR coated process (STAR) is presented. (ii) A high quality screen-printed (SP) contact methodology is developed that results in fill factors of 0·785–0·790 on monocrystalline Si. (iii) Aluminum back surface field (Al-BSF) formation is studied in detail to establish the process conditions that result in optimal BSF action. (iv) Screen-printing of Al conductor paste and rapid thermal processing (RTP) are integrated into the BSF procedure, and effective recombination velocities (Seff) as low as 200 cm/s are demonstrated on 2·3 Ω-cm Si with this rapid thermal processing of screen-printed contacts, Al alloyed BSF processes. (v) A novel passivation scheme consisting of a dielectric stack (plasma silicon nitride on top of a rapid thermal oxide) is developed to reduce the surface recombination velocity (S) to ≈ 10 cm/s at the 1·3 Ω-cm Si surface. The important feature of this stack passivation scheme is its ability to withstand a high-temperature anneal (700–850°C) without degradation in surface recombination velocity. This feature is critical for most current commercial processes that utilize SP contact firing. (vi) Finally, the individual processes are integrated to form high-efficiency, manufacturable devices. Solar cell efficiencies of 17% and &gt;19% are achieved on FZ Si with SP and evaporated (photolithography) contacts, respectively.

• Enhanced formation of low-resistivity TiSi2 contacts for deep submicron devices

Low resistivity C54-TiSi2 is currently the most commonly used silicide for metal contacts in ultralarge scale integrated circuits devices. In the present paper, we review recent results of investigations on the effects of stress and high temperature sputtering on the formation of C54-TiSi2. Enhanced formation of C54-TiSi2 on (001)Si by tensile stress and high temperature sputtering is correlated to the growth of thicker amorphous interlayer at the Ti/(001)Si interface. The enhanced transformation is attributed to the presence of higher density of silicide crystallites, which serve as the nucleation sites for the C49-TiSi2, in the amorphous layer. As a result, the average grain size of C49-TiSi2 is smaller which leads to lower C49- to C54-TiSi2 transformation temperature.

• Development of new NLO borate crystals

Recently new borate crystals, CsLiB6O10 (CLBO), YCa4O(BO3)3 (YCOB) and GdxY1−xCa4O(BO3)3 (GdxY1−xCOB) have been developed by the present authors. Here, the growth and nonlinear optical properties of CLBO, YCOB and GdxY1−xCOB crystals are reviewed and their properties are discussed in relation to those of other nonlinear optical crystals, such asβ-BaB2O4 (BBO), and LiB3O5 (LBO).

• Quasi-phase-matched second harmonic generation device in Mg-doped LiNbO3 and its application to high-density optical disk system

The resistance to photorefractive damage is investigated for several nonlinear crystals. A quasi-phase-matched (QPM) second harmonic generation (SHG) waveguide device is fabricated in 5 mol% Mg-doped LiNbO3 which has high resistance to optical damage. The SHG blue laser of the QPM-SHG waveguide device and a tunable distributed Bragg reflector (DBR) laser diode is demonstrated, wherein output stability of continuous blue light is measured. The SHG blue laser, using the QPM-SHG waveguide device with broadened flat matching response, shows good modulation characteristics. The pulsed peak power of 23 mW with rectangular modulated waveform is generated. The SHG blue laser is installed to an optical head, and good recording and readout characteristics are demonstrated. Moreover, butt-coupled SHG blue laser is examined to gain a miniature module with volume of 0·8 cc.

• Periodically domain-inverted LiNbO3 for waveguide quasi-phase matched nonlinear optic wavelength conversion devices

The authors’ work on LiNbO3 waveguide quasi-phase matched nonlinear optic wavelength conversion devices is reviewed. The requirements of domain-inverted gratings for efficient wavelength conversions are shown by theoretical analysis of the device performances. The fabrication apparatus of the domain-inverted gratings is described, and it is shown that the fabricated gratings satisfy the requirements. A variety of LiNbO3 waveguide quasi-phase matched (QPM) wavelength conversion devices were fabricated and examined. Second-harmonic green/blue/ultraviolet light generation with normalized conversion efficiencies of ∼ 100%/W, third-harmonic generation by cascading second-harmonic generation and sum-frequency generation, and difference-frequency generation for wavelength conversion in infrared wavelength range are demonstrated.

• Linear and nonlinear optical properties of organic crystals: MBANP and its derivatives

In addition to possessing strong optical nonlinearity, the (−)-2-(α-methylbenzylamino)-5-nitropyridine (MBANP) crystal has been shown to have unusual linear optical properties. Two methyl derivatives of MBANP have now been synthesized and grown as large single crystals; their refractive indices have been measured as a function of frequency and second-harmonic generation, including the determination of the phase matching loci has been investigated. Data is also available on the racemic form of crystalline MBANP. The properties of this family of crystals have been reviewed and an attempt is made to relate the experimental responses of the crystals to their molecular structure and the calculated properties of the molecules.

• Growth and characterization of new oxide and fluoride crystals for optical applications

High quality oxide and fluoride single crystals for optical applications have been grown by the Czochralski technique. Lattice parameter investigation of grown Gd3YbxGa5−xO12 suggested that this crystal will be a superior material as substrate for optical isolators with large Faraday effect. Growth conditions of (La,Sr)(Al,Ta)O3 single crystals are discussed. These crystals have excellent lattice matching with GaN, a promising material for optoelectronic devices. Ce-doped fluoride single crystals—LiCaAIF6, LiYF4 and BaLiF3—have been grown for solid state UV laser applications. Growth results and optical characterization are discussed.

• Second-harmonic generation in distributed feedback ferroelectric liquid crystal cells

The helicoidal structures in ferroelectric liquid crystals can be utilized to realize a special phase matching for the second-harmonic generation (SHG) when two counter-propagating fundamental waves are normally incident and the wavelength of the second-harmonic wave is near the selective reflection band edge. In this paper, we report the thickness and polarization dependences of the SHG thus observed. By comparing the experimental results with the theoretical ones developed, good agreement was confirmed.

• Photoinduced fabrication of complex surface relief structures on azobenzene functionalized polymers

Light induced fabrication of complex surface relief structures on azobenzene functionalized polymers is reported. Large class of side chain and main chain polymers can be utilized to record these relief structures. The recording and erasure process are strongly dependent on the polarization. Possible transport mechanism of polymer chains well below the glass transition temperature due to photoinduced effect is discussed.

• Influence of counter ion on polyaniline and polypyrrole

The electronic, magnetic, optical and redox properties of conjugated polymers are greatly influenced by structure, electronegativity, solvation and orientation of a counter ion. The doping by counter ion not only creates a band structure but also stabilizes localized bound states to impart unusual optical and magnetic properties to macromolecular system. When the polymer is in contact with solution of an electrolyte, counter ion tends to be solvated and mobile in the polymer phase, imparting it a property of ion exchange membrane. Here we briefly summarize the influence of anion on properties and electrochemical behaviour of polyaniline and polypyrrole.

• Some new structural and electronic characteristics of quasicrystals

The quasicrystals being based on quasiperiodic order other than crystal like periodic translational order and embodying self similarity, present unique condensed matter phases. In addition to their curious structural characteristics the paucity of translational periodicity leads to drastic deviations in their electronic behaviour as compared to crystalline counterparts. This paper describes and discusses some new developments in regard to structural and electronic aspects of quasicrystalline materials. In regard to the structural aspects, two comparatively newer features will be described. One of them relates to the observation of variable strain approximants (VSA) first found in Ti68Fe26NiSi5, qc alloys; the other relates to the structure of decagonal phases. The variable strain approximants correspond to qc phases exhibiting variable strain for the different diffraction spots for the same reciprocal lattice row (possessing linear shifts). The VSA is thought to result from variable phason strain mode locking; this in contrast to RAS which results due to linear phason mode locking. The results obtained in our laboratory on VSA will be described and discussed. Another interesting structural feature emanating in the last few years relate to the development of structural models for the decagonal phases which have nearly answered the question ‘Where are the atoms?’ for this qc variant. High resolution electron microscopy has revealed the existence of two types of atom cluster columns with a diameter of 20Å; a pentagonal cluster column and a decagonal one. The decagonal quasicrystals can be classified into three types of structures according to the space groups and symmetries and arrangements of the cluster columns. These three deca structures have been typified by: deca Al-Co-Cu, deca Al-Mn and deca Al-Fe-Pd types. Some basic features on the structures of decagonal phases as obtained recently will be outlined.

Since materials owe their practical importance due to their physical behaviours, assessment of qc materials from this point of view is of imperative importance. However, the real physics/science governing properties for qc is not well understood as yet. For example, the fundamental property relating to electronic conductivity and its temperature variation has been attempted to become explicable based on (a) taking qc as disordered materials, (b) assuming qc as hierarchy of clusters and (c) bringing in new concepts governing the wave functions of electrons (critical wave functions) and some other models. However, the same results are not universally reported/reproduced by various workers. Thus the variation of electronic conductivities with temperature of Al65Cu20Ru15, Al70Pd20Re10 and other stable quasicrystals have found varied interpretations e.g. based on QIEs and power law temperature variation. Some results onσ-T and related characteristics for stable qc crystals obtained by us and also by other workers will be analysed in terms of feasible transport mechanisms.

• Structure and tribological property of B2-based approximants

The present paper is concerned with a special group of approximants with B2 superstructures. In the first part, recent work on structural features of the B2 superstructure approximants is summarized. Experimental results obtained in Al-Cu-Mn and Al-Cu systems are presented, where a series of B2-based approximants are observed. These phases all have similar valence electron concentrations, in full support of thee/a-constant definition of approximants. Special emphasis is laid on the chemical twinning modes of the B2 basic structure in relation to the Al-Cu approximants. It is revealed that the B2 twinning mode responsible for the formation of local pentagonal atomic arrangements is of 180°/[111] type. This is also the origin of 5-fold twinning of the B2 phase on quasicrystal surfaces. Crystallographic features of phases B2, τ2, τ3,γ, and other newly discovered phases are also discussed. In all these phases, local pentagonal configurations are revealed.

In the second part, dry tribological properties of some AlCuFe samples containing the B2-type phases are presented. The results indicated that the B2 phase having their valence ratio near that of the quasicrystal possesses low friction coefficient under various loads, comparable with the annealed quasicrystalline ingot. Such a result indicates that the B2-type phase withe/a near that of quasicrystal is indeed an approximant, which is in full support of the valence electron criterion for approximants.

• Electron microscopy study of striation contrast in Al-Cu-Co-Si decagonal quasicrystals

Detailed transmission electron microscopy study was carried out in single crystals of a decagonal phase in the Al-Cu-Co-Si quaternary system. X-ray diffraction and convergent beam electron diffraction patterns of the powder samples confirmed the structures to be decagonal quasicrystals. No microcrystalline nor crystalline phases could be identified. Thin slices normal to the 10-fold directions were prepared for transmission electron microscopy. Diffuse streaks along symmetric directions around the fundamental spots were observed in the diffraction patterns. Bright field images and dark field images showed discontinuous lines or striations lying perpendicular to the direction of diffuse streaking. The striation contrast appears to be originating from anti-phase boundary (APB) in the decagonal superstructures. The diffuse streaks seem to be a characteristic feature of a partially ordered decagonal superlattice structure. The atomic rearrangement or phasonic movement in certain symmetric directions along the pentagrids or Ammann lines in the structure has obviously caused the type of contrast observed in the images. The evolution of rhombic domains consisting of APBs in localized regions can be understood as one of the signature of an intermediate structural state formed prior to a superstructure formation.

• Interfaces in icosahedrally related structures: problems and prospects

Icosahedral quasicrystals are frequently observed to coexist with their related phases like pentagonal/decagonal quasicrystals and rational approximant structures. Owing to this, they have common interfaces. The crystallography of these interfaces needs consideration for providing an aid for their characterization through experiments. The purpose of this communication is to present examples of heterophase and homophase interfaces in quasicrystalline systems and to discuss their structural details in terms of higher dimensional crystallography. Some of the uncommon aspects of these interfacesvis-a-vis their crystalline counterparts will be highlighted. We shall conclude by identifying the problems and prospects of further research in this area.

• Contributions of muon spin rotation/relaxation/resonance to the understanding of problems in magnetism — Current and future

During these 25 years following the start of muon spin rotation/relaxation/resonance (µSR) studies on condensed matter, various new aspects of magnetic materials have been explored by using high sensitivity to the microscopic dynamical behaviour of the host magnetism, specifically, the studies on magnetic ordering in strongly-correlated electron systems, elementary excitations in conducting polymers and protein, etc. Intense (more than 106/cm·s) and ultra-slow (below keV) muon beams to be available at future accelerator facilities may open new areas such as spin dynamics studies of surface magnetism, cluster particles, etc.

• Nanocrystalline materials for high temperature soft magnetic applications: A current prospectus

Conventional physical metallurgy approaches to improve soft ferromagnetic properties involve tailoring chemistry and optimizing microstructure. Alloy design involves consideration of induction and Curie temperatures. Significant in the tailoring of microstructure is the recognition that the coercivity, (Hc) is roughly inversely proportional to the grain size (Dg) for grain sizes exceeding ∼0·1−1 µm (where the grain size exceeds the Bloch wall thickness,δ). In such cases grain boundaries act as impediments to domain wall motion, and thus fine-grained materials are usually harder than large-grained materials. Significant recent development in the understanding of magnetic coercivity mechanisms have led to the realization that for very small grain sizesDg&lt;∼100 nm,Hc decreases sharply with decreasing grain size. This can be rationalized by the extension of random anisotropy models that were first suggested to explain the magnetic softness of transition-metal-based amorphous alloys. This important concept suggests that nanocrystalline and amorphous alloys have significant potential as soft magnetic materials. In this paper we have discussed routes to produce interesting nanocrystalline magnets. These include plasma (arc) production followed by compaction and primary crystallization of metallic glasses. A new class of nanocrystalline magnetic materials, HITPERM, having high permeabilities at high temperatures have also been discussed.

• Evidence for selective resputtering as the growth mechanism of pair-order anisotropy in amorphous TbFe films

Processing conditions for rf magnetron sputter-deposited amorphous TbFe films in which an atomic scale structural anisotropy (ASSA) results as a natural consequence of selective resputtering at the film surface during growth are presented. The ASSA, measured using polarization-dependent X-ray absorption fine structure with quantitative modelling via a parametrized nonlinear least squares fitting, is described as a pair order anisotropy (POA) where a statistical preference exists for like atom pairs in-plane and unlike atom pairs perpendicular to the film plane. The perpendicular magnetic anisotropy (PMA) energy increases with increasing POA for samples grown using decreasing rf power and increasing working gas pressure. The POA directly reflects the anisotropic electrostatic environment at the rare earth site which is required for PMA to result from a single ion anisotropy mechanism.

• Recent advances in 2 : 17 and 3 : 29 permanent magnet materials

The structural and magnetic properties of 2 : 17 compounds, their nitrides and carbides and the recent development of the novel 3 : 29 compounds are discussed.

• Self-organized growth of In(Ga)As/GaAs quantum dots and their opto-electronic device applications

The self-assembly technique of forming three-dimensional islands in the growth of highly strained semiconductor heterostructures has emerged as a powerful technique for the realization of an ordered array of quantum dots. Such quantum dots have been incorporated into the active region of optoelectronic and microelectronic devices in the hope of improving device performance or engineering new ones. Here we present the growth, optical characterization, and device applications for self-assembled InGaAs/GaAs quantum dots.

• Quantum dots fabricated by selective area MOVPE and their application to single electron devices

A novel method of formation of uniform GaAs quantum dot (QD) structures, using selective area metalorganic vapour phase epitaxy (SA-MOVPE), and their application to single electron transistors (SETs) are demonstrated. The SiNx-coated substrates having a wire-like opening with three prominences are used. The wire-like opening is aligned in the [110] direction, which corresponds to channel region of SET. AlGaAs/GaAs modulation-doped heterostructures are grown on these substrates. Due to three prominences on the wire, the quasi-one-dimensional electron gas (Q-1DEG) channel, having a periodic variation in its width, are naturally formed. This leads to the formation of a quantum dot near the central prominence and two tunneling barriers beside the dot, which are connected to quantum wires.IDVG characteristics under constant source-drain bias condition show clear conductance oscilations near the pinch-off, and oscillations are observed up to 65 K.IDVDS characteristics measured at 2·1 K show clear Coulomb blockade. The results indicate the formation of SET by SA-MOVPE. Using similar method, resistance-load single electron inverter circuit is also fabricated.

• Self-assembled quantum dots: The route to novel optical, electronic, magnetic and superconducting properties

We report recent results pertaining to the magnetic, optical, electronic, superconducting, and topographic properties of electrochemically self-assembled quantum dots. These dots self-order into two-dimensional hexagonal-close-packed arrays that are among the most periodic reported so far. They have revealed interesting properties with potential applications in magnetics, electronics, non-linear optics and novel neural architectures for ultrafast computation and signal processing.

• InGaAs quantum disk: Fabrication via self-organization and spectroscopies

We have examined the spontaneous rearrangement of a strained InGaAs/AlGaAs heterostructure, on a (311)B, substrate into naturally ordered array of InGaAs disks automatically buried in Al-rich alloy. Unlike nano-islands formed via Stranski-Krastanov mechanism, it serves as a strong tendency to align themselves. We have stressed important interplay of islanding of the material with lattice mismatch, atomic diffusion across the interface between unstrained and strained materials, lateral mass transport, and development of the surface into low index surfaces with low surface energies: all these seek to lower the total energy. Because of the damageless fabrication, these quantum disks showed excellent optical properties, which facilitated single-dot spectroscopy. Such spectroscopy revealed that lateral together with vertical confinement of exciton motion discretizes the exciton density-of-states resulting in sharp and distinct photo emission/absorption spectra despite their mesoscopic confinement. These characterize optical properties which are specific only to zero-dimensional system, thereby proving quantum dot characteristic. As for future device application, we undertook an attempt to artificially position the self-organized structure more accurately by periodic seeding on the virgin (311)B substrate. The results show that it is feasible to further improve the ordering of the array of quantum disks.

• Chemical bond manipulation for nanostructure integration on wafer scale

In this paper, we have briefly summarized our activity in the area of chemical bond manipulation for the integration of nanostructures on a full wafer scale. Chemical bond manipulation involves a judicious combination of surface phenomena: reactions or diffusion, and growth process such as molecular beam epitaxy (MBE). Here, we present our results on oxidation, metallization and nitridation and their role in the formation of nanostructures. We find that oxygen changes the bonding partner from Ge to Si and this phenomenon can be controlled by controlling the annealing temperature. We have employed this phenomenon for the fabrication of novel, multiperiod Si/SiO2/Ge layered structure which exhibits interesting light emitting properties. Further, by making use of selective diffusion of cobalt atoms through Ge layers it is possible to incorporate metallic features into Ge quantum dots. Moreover, it is possible to fabricate Si nanopillars through high temperature reaction of nitric oxide. NO molecules dissociate on the surface and nitrogen atoms thus produced form nitride islands. These islands act as protective masks for the etching of Si by the oxygen atoms, through the desorption of SiO species. Occurrence of these two simultaneous processes result in the formation of nanometre-sized Si pillars capped by silicon nitride. All these results emphasize the fact that we can extend information obtained through traditional surface science experiments for the fabrication of novel structures on a full wafer scale.

• Semiconductor quantum dots: Theory and phenomenology

Research in semiconductor quantum dots (q-dots) has burgeoned in the past decade. The size (R) of these q-dots ranges from 1 to 100 nm. Based on the theoretical calculations, we propose energy and length scales which help in clarifying the physics of this mesoscopic system. Some of these length scales are: the Bohr exciton radius (αB*), the carrier de Broglie and diffusion length (λD andlD), the polaron radius (αp), and the reduction factor modulating the optical matrix element (Mx).R&lt;αB is an individual particle confinement regime, whereas the larger ones are exciton confinement regime wherein Coulomb interaction play an important role. Similarly a size-dependent dielectric constantε(R) should be used forR&lt;αp&lt;αB. An examination ofMx reveals that an indirect gap material q-dot behaves as a direct gap material in the limit of very small dot size. We have carried out effective mass theory (EMT) calculations to estimate the charge density on the surface of the quantum dot. We present tight binding (TB) calculation to show that the energy upshift scales as 1/Rx, wherex is less than 2 and the exponent depends on the orientation of the crystallite.

• Environmental effects in iron aluminides

Iron aluminides based on the stoichiometric compositions of Fe3Al and FeAl exhibit poor room temperature ductilities due to hydrogen embrittlement (HE). The presence of surface passive films reduces HE. The reduction is due to the lower rate of hydrogen liberation on the surface of iron aluminides with a passive layer. Theoretical and experimental verification for this idea are provided. The effect of addition of passivity-inducing elements Ti, Zr, V, Nb, Ta, Cr, Mo, W, Si and Ni to Fe3Al on the thermomechanical and electrochemical behaviour has been outlined. The Cr- and Ti-alloyed intermetallics exhibited significant room temperature ductilities. Microstructural studies of the alloyed intermetallics revealed that when the addition of passivity-inducing element results in the precipitation of brittle phases with Fe and Al, they crack during the processing operation. The addition of oxygen-active elements on the embrittlement behaviour is also discussed. The effect of these additions on the potentiodynamic polarization behaviour and high temperature oxidation behaviour is also briefly addressed. Methods to minimize HE by the addition of elements that irreversibly trap hydrogen and that prevent recrystallization have also been discussed.

• Evolution and stability of texture during thermomechanical processing of Ti-24Al-11Nb alloy

The evolution of basal texture during thermomechanical processing of Ti-24Al-11Nb alloy has been studied as a function of different processing variables like hot rolling temperature, amount of deformation, cooling conditions etc. The stability of the deformation texture during post-rolling annealing and during theα2βα2 phase transformation cycle was also investigated. Unrestricted rolling of primaryα2 to maximum thickness reduction at the lowest rolling temperature has been found to be most favourable for obtaining a good basal texture. Texture of transformed (secondary)α2 is generally non basal when the transformation takes place from deformedβ. Rolling texture does not seem to change during annealing leading to recrystallization. Theα2βα2 phase transformation cycle does not change the starting basal texture and a starting non basal texture also does not give rise to basal texture due to this treatment.

• Microstructures in Al-richγ-TiAl strained in the domain of temperature of flow stress anomalies

The plastic behaviour of single-phaseγ-TiAl poses a certain number of fundamental problems as to the origin of the flow stress anomaly and to how this is related to dislocation properties. Understanding these questions is of significant interest in the investigations of flow stress anomalies in intermetallics and in other materials. Also, it may reveal useful in the study of theγ-based lamellar alloys. The present contribution accounts for an investigation of mechanical properties and microstructural organization conducted in Al-richγ-TiAl single crystals oriented to activate single slip. Three separate topics are addressed.

1. 1/2 〈110] ordinary dislocations after deformation between room temperature and 800°C. The magnitude of the cusping appears to depend on the chronology of dislocation immobilization. The distribution of pinning points is analysed and the temperature dependence of their density confronted to that of the flow stress.

2. The core structure of 〈011] dislocations. After deformation at room temperature, 〈011] dislocations are dissociated into two partials separated by a stacking fault. The role of temperature and possible transitions of the dissociation are addressed.

3. The observation and analysis of background striations which cover the thin foils entirely. The striations are shown to be a deformation by-product resulting from the passage of perfect dislocations of the L10 structure, and from the interaction of these with a short-range ordered (SRO) Ti3Al5 phase. Striations can serve to discuss the absence of slip localization and the properties of cross-slip.

• Novel oxygen sensor using hot spot on ceramic rod

A novel oxygen sensor using hot spot on ceramic rod of high-Tc superconductor RBa2Cu3O7−δ (R: rare earth element) has been developed. The hot spot appears by the self-heating of the local part on the RBa2Cu3O7−δ ceramic rod when a voltage above threshold is applied at room temperature. This sensor operates without any separate heater by taking advantage of the high temperature of the hot spot wherein oxide ions can diffuse easily. The oxygen concentration is determined from the value of the current flowing through the rod by utilizing the change in the resistivity of the hot spot depending on oxygen partial pressure in atmosphere. Oxygen concentration of 0∼100% can be detected with high sensitivity and the response time is several seconds. The response performance of this oxygen sensor is almost the same as that of limiting-current-type zirconia sensor operating at 500°C.

• Characterization of thin films using heavy ion beams

Thin films are used in many technological applications. The characterization of thin films requires compositional information as a function of sample depth. Ion beam analysis techniques, such as Rutherford backscattering spectrometry and elastic recoil detection (ERD), can provide this information in a uniform way for all elements and as absolute concentrations without relying on standards. These techniques can fully be exploited when projectile beams of heavy ions such as Si or Au are used. This improves the elemental resolution and the depth resolution when compared with standard He ion beam analysis. The use of gas ionization detectors increases detection efficiency and minimizes the beam exposure of the samples, so that the analysis is essentially nondestructive. The sampling depth of a few micrometers makes these techniques ideal for the stoichiometric analysis of the surface region of homogeneous materials and, in particular, thin surface films.

• X-ray photoelectron spectroscopy: A powerful tool for a better characterization of thin film materials

X-ray photoelectron spectroscopy (XPS) is one of the most powerful tools to characterize thin films materials. To illustrate the use of XPS, some examples will be given on materials used as positive electrode in microbatteries.

Further analyses of the film to understand the redox process are quite difficult with conventional methods due to the amorphous nature of the cathode. Here surface methods like XPS are very useful. Two main kinds of information can be obtained from XPS analysis: the oxidation states, and the determination of atomic environments.

Different kinds of positive electrode materials were studied, titanium and molybdenum oxysulfides (MOySz, M=Ti, Mo) and lithium cobalt oxide (LixCoO2+y) and have been illustrated in the present work. In light of the binding energies obtained for the reference compounds, several types of environments and different formal oxidation states have been found for the transition elements.

XPS is also very useful for folllowing the oxydo-reduction mechanisms occurring during the intercalation and the de-intercalation of lithium, corresponding respectively to the discharge and the charge of the battery. After strict identification of each species, the evolution of their binding energies could be followed very easily. The XPS analyses of oxysulfides thin films at different stages of their cycling process have shown apparently good efficiency of the oxygen-rich compositions.

During the redox process, the results obtained have clearly shown the important contribution of the sulfur atoms beside the transition metal atom.

• Effect of etchant concentration and defects on pyramid formation in TMAH etched silicon

An investigation on the effect of TMAH concentration on the etch rate of silicon, and the influence of etchant concentration, ambient temperature and wafer thermal history on the formation of pyramids at the surface of TMAH etched silicon has been carried out. From the results obtained from this study, we are able to explain the influence of TMAH concentration and ambient temperature on the silicon etch rate and the changes occurring at the silicon surface satisfactorily using the pH theory. However, the results from wafers with different thermal history seem to favour the defects theory. We suggest that in order to explain the etching mechanism of TMAH of silicon satisfactorily, a combination of pH and defects theories is necessary.

• Synthesis and characterization of surface-modified and organic-functionalized MCM-41 type ordered mesoporous materials

A new and efficient method for the preparation of MCM-41 type ordered mesoporous phases using phosphate as promoter under reflux conditions is reported. The various mesoporous materials studied were: silica (Si-MCM-41), alumino-silicate (Al-MCM-41), and titanium-silicate (Ti-MCM-41). Our concept of promoter-assisted synthesis of zeolites and related microporous materials was found to be applicable in the synthesis of ordered mesoporous materials as well. The addition of small catalytic quantity of phosphate ions (PO43−), used as promoters, significantly reduced the synthesis time (by a factor of 3–4) of all these mesoporous materials. The synthesis of new MCM-41 type organic-inorganic composite materials with unique properties is also reported.

• Key factors in tissue engineering

Tissue engineering is an emerging biomedical form of engineering aiming at regeneration of natural tissues or creation of biological substitutes for defective or lost tissues and organs using cells of different species. It is essential for the tissue self-regeneration, for instance, to provide a space necessary for regeneration with certain materials and technologies. This article overviews such key factors required for tissue engineering.

• Role of platelets in blood-biomaterial interactions

Over 2 million cardiovascular procedures are performed annually in the United States. Every one of these procedures requires some period of contact with blood with several different biomaterials used in the manufacture of assist devices or implant devices. In view of the increasing importance of the biomaterials in clinical practice, it would be timely to review briefly physicochemical characterization as well as biological evaluations. Blood compatibility encompasses a variety of events associated with blood interaction with the biomaterials used in various procedures. Two separate coagulation mechanisms are involved (arterial and venous) depending upon the flow characteristics. At least three interacting factors modulate normal hemostasis and the pathogenesis of thromboembolic events. They are the state of activation of coagulation cascade, circulating levels of thrombin and fibrinogen and relative activity of platelets. In this overview we discuss the current concepts on the role of platelets in blood-biomaterial interactions.

When blood contacts a biomaterial surface a variety of blood components interact with the surface. Some of the key players in platelet activation on biomaterial surface include fibrinogen and von Willebrand factor. Currently available antiplatelet drugs effectively block aggregation and secretion induced by physiological agonists such as epinephrine, adenosine diphosphate and thromboxane in suspension. However, they do not prevent platelet interaction on biomaterial surfaces. Mechanisms involved in platelet activation in suspension and on surfaces as well as the pharmacology of newer antiplatelet drugs will be discussed.

• Hybrid bioceramics with metals and polymers for better biomaterials

The number of people needing artificial bones and teeth will rapidly increase in the world. Hydroxyapaptite (HAp) is known to be a suitable ceramic owing to its biological affinity. However, its use in artificial joints and dental roots is difficult due to its poor mechanical properties. A new method to implant HAp granules into superplastic titanium alloy has been developed. We also succeeded to enhance the biomimetic growth of calcium phosphate on biological fibres (cellulose and chitin) through their surface modification by phosphorylation or silane coupling. This hybrid material may be used as virus filters, artificial trachea etc. The future of hybrid bioceramic materials appears to be promising.

• On the relevance and requirements of biomaterials

The technological orientation of current medical practice is reflected in the effective utilization of a number of diagnostic and therapeutic devices. Synthetic and natural biomaterials alone or in combination form the basis for development of such devices that are in contact with different tissues. The effect of materials on tissues andvice versa needs to be understood to ensure safety and effectiveness of the devices. This calls for biological evaluation of materials and devices, in addition to a spectrum of recommended toxicological testing of materials depending upon the duration and the type of tissue in contact with. Besides being non-toxic, the material is required to meet the functional requirements with the appropriate host response which is termed as biocompatibility.

This paper reviews the evolution and the requirements of biomaterials. Thein vitro andin vivo evolution methodologies are highlighted based on our experience in developing blood bag, dental composite, hydroxyapatite and fibrin glue. The requirements of biomaterials in the current context of advances in the fields of tissue engineering and biomimetics is outlined.

• Composite materials for aerospace applications

Fibre-reinforced polymer composite materials are fast gaining ground as preferred materials for construction of aircraft and spacecraft. In particular, their use as primary structural materials in recent years in several technology-demonstrator front-line aerospace projects world-wide has provided confidence leading to their acceptance as prime materials for aerospace vehicles. This paper gives a review of some of these developments with a discussion of the problems with the present generation composites and prospects for further developments. Although several applications in the aerospace sector are mentioned, the emphasis of the review is on applications of composites as structural materials where they have seen a significant growth in usage. The focus of the paper is especially on the developments on the Indian aerospace scene.

A brief review of composites usage in aerospace sector is first given. The nature of composite materials behaviour and special problems in designing and working with them are then highlighted. The issues discussed relate to the impact damage and damage tolerance in general, environmental degradation and long-term durability. Current solutions are briefly described and the scope for new developments is outlined. In the end, some directions for future work are given.

• Interfacial energy issues in ceramic particulate reinforced metal matrix composites

One major scientific issue that needs to be resolved and understood in order to design ceramic particle reinforced metal matrix composites is the interfacial energy state between the matrix and the reinforcement. Solid-solid interfacial energy between the particle and the matrix effects the final interface characteristics and also significantly influences the particle redistribution due to its effect on particle pushing engulfment by the melt interface. The paper analyses the physics behind the particle pushing and engulfment by the solidifying interface considering models utilizing interfacial force as energy difference between the particle in the solid and particle in the liquid melt. Various methods of evaluating solid-solid interfacial energy have been discussed. Velocity of melt interface movement at which the particles are engulfed by the matrix referred to as critical velocity of the system under given conditions has been shown to be directly related to the interfacial energy. Critical appraisal of experiments to determine the critical velocity have been presented for aluminium matrix dispersed with zirconia particles. Advantages of carrying out experiments under µg environment have been pointed out.

• Transient thermal process induced by swift heavy ions: Defect annealing and defect creation in Fe and Ni

In the present paper, a method is proposed to quantitatively estimate the nuclear defect annealing by the electronic stopping powerSe. The spatial distribution of defects created in metals by nuclear collisions is known from numerical calculation based on the binary collisions with screened Coulomb potential. In the framework of the thermal spike model,Se induced annealing of nuclear defects is simulated without considering athermal recombination. These calculations are applied to iron and nickel. The agreement between experiment and simulation in iron allows to determine the electron-phonon coupling value for iron. The defect creation bySe arises when the energy deposited on the atoms overcomes the energy necessary to melt the matierial.

• Modifications induced by swift heavy ions

On their way through matter, energetic heavy ions induce a continuous trail of ionization and excitation. A narrow path of irreversible physical, chemical and structural changes, the latent track, is formed. In this report, some of the most important techniques (transmission electron and atomic force microscopy, small-angle X-ray scattering, chemical etching) to study and to characterize ion induced modifications will be presented. Furthermore, selected examples for application oriented projects will be given.

• Analytical techniques with a cyclotron on polymers

Nuclear analytical techniques (PIXE, RBS and ERDA) have been applied to investigate diffusion processes in polymer light-emitting diodes (PLEDs), employing a 30 MeV AVF cyclotron. The techniques prove to be useful in identifying the origin of the diffusion process and the nature of molecules transported from the hole injecting ITO electrode into the polymer layer.

• Structural investigation of polydiacetylenes: A variable temperature solid state13C CP/MAS NMR study

The spectral features in the solid state13C CP/MAS NMR of poly (1,1,6,6-tetraphenylhexadiyn diamine) (poly(THD)) are compared with X-ray diffraction. The NMR data suggests that acetylenic carbons are nonequivalent in the unit cell resulting in two resonances. On the other hand, a single acetylenic carbon resonance was observed for poly (1,6-di-N-carbozolyl-2,4-hexadiyne) (poly(DCH)). The orientation of side groups in the crystal structures of poly(DCH) and poly(THD) are responsible for equivalence and nonequivalence of acetylenic carbons. The structural changes due to thermochromic transitions are monitored by variable temperature solid state NMR. The solid state VT NMR data suggest that the backbone and side chain conformations of blue phase and red phases of poly(bis-n-propyl urethane of 5,7-dodecadin-1,12-diol) (poly(PUDO)) are distinctly different. For the first time, we present a solid state NMR spectrum showing the presence of both the blue and red phases of this polymer at the thermochromic transition temperature (ca. 115–125°C).

• In situ TEM observation of long range ordering via short range order in Cu3Pt

The thermoequilibrium state of short range order (SRO) and the ordering process to the long range order (LRO) phase in Cu3Pt have been studied by thein situ transmission electron microscope (TEM) observation method. It is confirmed that the alloy has the SRO state in thermoequilibrium within the temperature region for the disordered fcc phase (designated as A1). The SRO state can be interpreted in the ‘microdomain model’. The degree of order fluctuates temporally as well as spatially at higher temperature, but freezes at a lower temperature in the A1 region. In the temperature region of (A1+LRO), some of the microdomains in the SRO grow up to large domains of LRO. This indicates that the transformation process to LRO via SRO proceeds without a process of nucleation even though the transformation is of a first order. Division of A1 into subregions and physical meaning of the phase boundary between (A1+LRO) and A1 are discussed.

• High resolution electron microscopy of alloys

High resolution electron microscopy (HREM) has emerged as a very powerful tool for probing the structure of metals and alloys. It has not only helped in unravelling the structure of materials which have been at the forefront of novel materials development such as quasicrystalline phases and high temperature superconducting compounds, but also is fast becoming a technique for solving some outstanding issues in the case of the commercial alloys thereby helping alloy development. In addition to the determination of the structures of phases, this tool is used for obtaining a first hand information of the arrangement of atoms around the various types of crystallographic defects and interphase interfaces. This mode of microscopy allows direct observation of orientation relationships between two phases across interfaces. HREM can be used for the direct examination of the prenucleation process. Initial stages of nucleation can also be studied readily in amorphous alloys, precipitation hardening alloys like maraging steels and in those systems where the formation of the omega phase occurs.

This presentation describes some results of HREM studies on various alloys, commercial as well as alloys of scientific interest, where some of the aforementioned aspects have been examined. The specific examples cited pertain to metallic glasses, NiTi shape memory alloys, Ni-Mo, Zr-Nb and Ti-Al alloys.

• Study of diffusion and related phenomena by electron probe microanalyser

Interaction of various materials at high temperatures leads to the formation of reaction zones of differing nature ranging from simple solid solutions to multiphase structures. Understanding the diffusion processes leading to the formation of reaction zone and its nature requires an accurate estimation of composition and the distribution of the phases formed. Electron probe microanalyser (EPMA) is an indispensable technique for this purpose, where the quantitative analysis of micron sized layers formed in the diffusion zone is required. Application of EPMA to study the diffusion processes in several metallic systems is dealt in detail. Utility of this technique to study problems related to contact metallurgy in semiconductors and practical problems of joining, particularly to that of diffusion bonding, is also described in this paper.

• Micro-parameters and micro-characteristics related to the formation ofε-martensite in Fe-based shape memory alloys

The influence of some microstructure parameters and characteristics ofγ-phase e.g. Stacking fault probability, grain size and texture etc onε-martensitic transformation, shape memory effect and other macro-behaviours in Fe-Mn-Si based alloys are reviewed.

• Creation and motion of dislocations and fracture in metal and silicon crystals

By making a step on one surface ($$\left( {11\bar 2} \right)$$) of a rectangular small paralellepiped copper crystal, dislocations could be created by the molecular dynamic method. The dislocation created was not a complete edge dislocation but a pair of Heidenreich-Shockley partial dislocations. Each time a dislocation was created, the stress on the surface was released. Small copper crystals having a notch were pulled (until fracture), compressed and buckled by use of the molecular dynamic method. An embedded atom potential was used to represent the interaction between atoms. Dislocations were created near the tip of the notch. A very sharp yield stress was observed.

The results of high speed deformations of pure silicon small crystals using the molecular dynamics are presented. The results suggest that plastic deformation may be possible for the silicon with a high speed deformation even at room temperature. Another small size single crystal, the same size and the same surfaces, was compressed using molecular dynamic method. The surfaces are {110}, {112} and {111}. The compressed direction was [111]. It was found that silicon crystals are possible to be compressed with a high speed deformation. This may suggest that silicon may be plastically deformed with high speed deformation.

• Device simulation and fabrication of field effect solar cells

The performance of a novel hydrogenated amorphous silicon (a-Si : H) solar cell which utilizes the field effect solar cell (FESC) has been investigated both theoretically and experimentally. The theoretical analysis has been done for bothp- andn-channel FESCs by employing a two-dimensional device simulator which is based on current continuity and Poisson equations. The calculated performance is compared with that of conventional (p-i-n)a-Si : H solar cells. The calculation demonstrated that bothn-channel andp-channel FESCs could improve the conversion efficiency by as much as 50%.

In order to check the reliability of simulation, the transport properties of intrinsica-Si : H film and thin film transistor (TFT) have also been calculated and compared with the experimentally obtained characteristics. Experimental verification of TFT and FESC has been attempted by using MgO anda-SiN : H as dielectric layer materials. Preliminary results are presented.

• Canab initio simulation really predict properties of materials prior to actually carrying out the experiments?

In the present paper, all-electron full-potentialab initio simulation method with mixed-basis is introduced and several typical examples are indicated which successfully show the possibility of predicting properties of materials prior to actually carrying out the experiments. We have used theab initio calculation to extract energy parameters, and apply them to cluster variation and direct methods, which bridge the limited space ofab initio treatment to real complex materials. To overcome the limited computer power, we have also developed parallel processing codes and tested their efficiencies.

• # Bulletin of Materials Science

Current Issue
Volume 42 | Issue 5
October 2019

• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019