A new combustion route to γ-Fe2O3 synthesis

A new combustion route for the synthesis of γ-Fe ₂ O ₃ is reported by employing purified a-Fe ₂ O ₃ as a precursor in the present investigation. This synthesis which is similar to a self propagation combustion reaction, involves fewer steps, a shorter overall processing time, is a low energy reaction without the need of any explosives, and also the reaction is completed in a single step yielding magnetic iron oxide i.e. γ-Fe ₂ O ₃ .The as synthesized γ-Fe ₂ O ₃ is characterized employing thermal, XRD, SEM, magnetic hysteresis, and density measurements. The effect of ball-milling on magnetic properties is also presented.     

Rings, chains and planes: Variation ofT g with composition in chalcogenide glasses

We propose a microscopic, phenomenological model for the decrease in the viscosity observed at glass transition. Our model is primarily applicable to chalcogenide glasses. According to this model, the decrease in the viscosity at glass transition is mainly due to the breaking of the Van der Waals bonds in the chalcogenides. Using this model, we derive a relationship between the glass transition temperature,T _g ,and the molar volume V _m.The validity of this relation is checked using experimental data available in the literature for two binary systems (Ge-Se and As-S) and a pseudo-binary system (As ₄₀ Se _x Te _60-x .     

Preparation of Sm-Ru bimetallic alloy films on Ru(0001) surface by vapour-deposition and annealing

Sm-Ru intermetallic surface alloy films were prepared by vacuum deposition and annealing of rare earth Sm on single crystal Ru(0001) surface. The Ru 3 d and Sm 3 d core level spectra clearly show the formation of surface alloy layers. XPS measurements on surface alloy film revealed an induced peak in the Ru 3 d region at lower binding energy by 1 eV compared to the bulk Ru (elemental) suggesting an electronic effect of alloying and Sm-Ru bond formation. The Sm 3 d _5/2 photoemission peak of Sm film consists of strong features characteristic of Sm(II) with electron configuration 4 f ⁶ (5 d 6 s ) ² and Sm(III) with electron configuration 4 f ⁵ (5 d 6 s ) ³ .It is observed that the Sm(II) feature decreases in intensity upon alloy formation with surface Ru atoms. Oxidation of these films with carbon monoxide indicates alloy breakdown due to the oxidation of Sm atoms selectively. Alloy oxidation also shows a clear shift of Sm 3 d _5/2 feature.     

Preparation and characterization of Ag-added Bi1.84Pb0.4Sr2Ca2.2Cu3O10+x bulk tube conductors for cryogen free superconducting magnet

Bulk tube conductors of Bi _1.84 Pb _0.4 Sr ₂ Ca _2.2 Cu ₃ O _10+x with addition of silver varying from 0 to 25 wt% (not reported earlier) were systematically studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), electrical transport and a.c. susceptibility techniques. The tube conductors formed by cold isostatic pressing (CIP) of the powders obtained from spray drying method have been made successfully. It was found that Ag addition has not only affected the formation of the desired Bi-2223 phase and the microstructure of these large bulk tube samples thereby influencing on the critical current ( I _c ),it also reduces the contact resistance to minimize the cryogen losses. These variations have been found to be Ag content dependent. An optimized value of 10 wt% Ag has been found to produce the best quality tubes showing reproducible I _c value > 120 Amp at 77 K which is in general a requirement to energies of the cryogen free conventional/HTSC superconducting magnets below 20 K.     

Effect of zirconium addition on the recrystallization behaviour of a commercial Al-Cu-Mg alloy

It is well known that the second phase particles have an effect on recrystallization and grain growth behaviour of an alloy. Particularly the bimodal distribution of second phase particles has an effect which is opposite in sense where coarse second phase particles (> 1 μm) stimulate nucleation while fine particles exhibit Zener drag. In the literature, the effect of zirconium addition to aluminium alloys has been well documented in order to produce superplasticity by giving ultra fine grain size to the alloy. Addition of zirconium produces Al ₃ Zr particles which pin the grain boundaries during recrystallization and grain growth. In the present work, zirconium was added to a commercial Al-Cu-Mg alloy and by heat treatment Al ₃ Zr particles were precipitated and after forging, the grain size was an order of magnitude lower than the alloy without zirconium. Transmission electron microscopy was employed to characterize the second phase particles, i.e. Al ₃ Zr particles and found to be rod shaped and identified to be cubic ordered L 1 ₂ phase with a lattice parameter of 0.408 nm. Further, it was observed that fine (100 nm) Al ₃ Zr particles promote only continuous recrystallization which is polygonization of subgrains and subgrain growth. It was found that the fine dispersion of Al ₃ Zr particles inhibits both recrystallization and grain growth in the commercial Al-Cu-Mg alloy.     

Synthesis and luminescence properties of Eu2+-activated Ca4Mg5(PO4)6 for blue-emitting phosphor

Ca ₄ Mg ₅ (PO ₄ ) ₆ :Eu²⁺^{\boldsymbol{2+}} blue-emitting phosphor was synthesized by the combustion-assisted synthesis method under reductive atmosphere. The products were characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM) and photoluminescence (PL) spectrum. XRD analysis confirmed the formation of Ca ₄ Mg ₅ (PO ₄ ) ₆ pure phase. Photoluminescence results showed that the phosphor can be excited efficiently by UV light range from 230–400 nm, and then exhibited bright blue light with peak wavelength at 431 nm. It is a very promising candidate as a blue-emitting phosphor for potential applications in display devices.     

Longitudinal and transverse waves in anisotropic elastic materials

Summary It is shown that a necessary and sufficient condition for a longitudinal wave to propagate in the direction n in an anisotropic elastic material is that the elastic stiffness C ₁₁ (n) is a stationary value (maximum, minimum or saddle point) at n. Explicit expressions of all n and the corresponding elastic stiffness C ₁₁ (n) for which a longitudinal wave can propagate are presented for orthotropic, tetragonal, trigonal, hexagonal and cubic materials. As to longitudinal waves in triclinic and monoclinic materials, only few explicit expressions are possible. We also present necessary and sufficient conditions for a transverse wave to propagate in the direction n. As an illustration, explicit expressions of all n, the polarization vector a and the wave speed c for which a transverse wave can propagate in cubic and hexagonal materials are given. The search for n in hexagonal materials confirms the known fact that a transverse wave can propagate in any direction. A longitudinal wave is necessarily accompanied by two transverse waves. However, a transverse wave can propagate without being accompanied by a longitudinal wave.     

Observation of curious spiral growth features in Tl doped Hg bearing high temperature superconducting tapes

Synthesis of Hg(Tl)Ba ₂ Ca ₂ Cu ₃ O _8+δ superconducting tapes have been accomplished by annealing the precursor tape, Ba ₂ Ca ₂ Cu ₃ O _y (fabricated by doctor blade tape casting technique) in an environment of Hg(Tl) vapour. Characterization of superconducting HTSC tape sample was carried out through XRD, TEM, SEM and R-T measurements. Surface morphological investigations of the as-synthesized Hg(Tl)Ba ₂ Ca ₂ Cu ₃ O _8+δ HTSC tapes by scanning electron microscope have shown the occurrence of curious growth characteristics resembling spiral like features. These growth spirals encompass nearly the whole grain suggesting that spiral growth led to the formation of small crystal like grains of superconducting material Hg(Tl)Ba ₂ Ca ₂ Cu ₂ O _8+δ .The likely mechanism for the generation of these screw dislocations has been elucidated in terms of incoherent coalescence of growth fronts formed from Hg(Tl):1223 and Hg(Tl):1234 nuclei.     

Environmentally benign novel green pigments: Pr1-xCaxPO4 (x = 0-0.4)

Rare earth based materials have recently attracted considerable attention as potential ecofriendly colourants for low temperature as well as high temperature applications. In the present study, we have synthesized a series of Ca-doped PrPO₄ compounds with the general formula, Pr_1-xCa_xPO₄ (x = 0-0.4 in steps of 0.1) and characterized the compounds by powder X-ray diffraction. All the compositions show a monoclinic monazite structure. The optical properties of the brilliantly coloured pigments [ L (brightness),a* (+ red- green),b* (+ yellow - blue) have been examined. These materials can find application as potential green colourants.     

Photoelectric properties of Se-doped Cd1.23Zn1.77As2 crystals

The spectral response of the photocurrent through Se-dopedCd _1.23 Zn _1.77 As ₂ single crystals was studied. The results were used to determine the band gap at absolute zero (0.540 eV) and the temperature coefficient of band gap (-0.39 meV/K). The photocurrent spectra were found to contain a photovoltaic peak attributable to Schottky barriers at the interface between the semiconductor and the metallic contact. The heights of the barriers responsible for residual conductivity were evaluated using a two-barrier model. Both surface and bulk inhomogeneities were shown to play a significant part in residual conductivity.     

On the algebraic characterization of a Mueller matrix in polarization optics. II. Necessary and sufficient conditions for Jones-derived Mueller matrices

Abstract

We show that every Mueller matrix, that is a real 4 × 4 matrix M which transforms Stokes vectors into Stokes vectors, may be factored as M = L ₂ KL ₁ where L ₁ and L ₂ are orthochronous proper Lorentz matrices and K is a canonical Mueller matrix having only two different forms, namely a diagonal form for type-I Mueller matrices and a non-diagonal form (with only one non-zero off-diagonal element) for type-II Mueller matrices. Using the general forms of Mueller matrices so derived, we then obtain the necessary and sufficient conditions for a Mueller matrix M to be Jones derived. These conditions for Jones derivability, unlike the Cloude conditions which are expressed in terms of the eigenvalues of the Hermitian coherency matrix T associated with M, characterize a Jones-derived matrix M through the G eigenvalues and G eigenvectors of the real symmetric N matrix N = [Mtilde]GM associated with M. Appending the passivity conditions for a Mueller matrix onto these Jones-derivability conditions, we then arrive at an algebraic identification of the physically important class of passive Jones-derived Mueller matrices.     

Homogenization of zinc distribution in vertical Bridgman grown Cd0.96Zn0.04Te crystals

One of the most pressing issues in the growth of high quality single crystal Cd _0.96 Zn _0.04 Te material, is to achieve homogenization of the high axial variation of Zn concentration, caused by the larger than unity segregation coefficient of Zn in CdTe. This is achieved in our crystals (i) by thermal annealing of the CdZnTe crystal, which redistributes the as grown Zn distribution by solid state diffusion of Zn (this solid state diffusion of Zn occurs at three stages (a) during the growth when the solidified crystal is near to the melting point temperature, (b) during the post growth annealing of the crystal at a high temperature and (c) during the cooldown to room temperature) and (ii) by the reduction of Zn segregation during the growth stage by enhanced convective mixing of the melt, through a proper choice of ampoule and furnace dimensions. By adopting suitable growth parameters and sufficient post growth annealing it has been possible to grow Cd _0.96 Zn _0.04 Te crystals, which have nearly 75% of their fraction within 1% Zn concentration variation.     

Process monitoring research with various estimator-based MEWMA control charts

Multivariate exponentially weighted moving average (MEWMA) control chart with five different estimators as population covariance matrix is rarely applied to monitor small fluctuations in the statistical process control. In this article, mathematical models of the five estimators (S₁, S₂, S₃, S₄, S₅) are established, with which the relevant MEWMA control charts are obtained, respectively. Thereafter, the process monitoring performance of the five control charts is simulated. And the simulation results show that the S₄ estimator-based MEWMA control chart is of the best performance both in step offset failure mode and ramp offset failure mode. Since the inline process monitoring of photovoltaic manufacturing is intended to be a problem of multivariate statistics process analysis, the feasibility and effectiveness of the proposed model are elaborated in the case study during the cell testing and sorting process control for the fabrication of multicrystalline silicon solar cells.     

X-ray and magnetic studies of Zn2+ substituted Ni-Pb ferrites

Seven samples of the polycrystalline, Ni_1.25-xZn_xPb_0.25Fe_1.5O₄ (x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6) ferrites, were prepared by usual double sintering ceramic method. X-ray diffraction patterns of the samples revealed single-phase cubic spinel structure. The magnetic properties were investigated by means of magnetization and a.c. susceptibility (χ) measurements. A.C. susceptibility was measured from room temperature to the Curie temperatureT _C) of the samples. Variation of a.c. susceptibility (χ) with temperature showed that the samples withx = 0.3 and 0.5 contain single-domain (SD) particles whereas the samples with x = 0.4 and 0.6 showed multi-domain (MD) nature. Values of Curie temperature ( T _C)as obtained from the study of variation of a.c. susceptibility with temperature were found to decrease with increase in the Zn ²⁺ concentration (x bd. Magnetic measurements showed increase in magnetization as Zn²⁺ content was increased from 00 to 0.5. Further increase in Zn²⁺ contentx bd reduces the magnetization.     

Combustion synthesis and photoluminescence study of silicate biomaterials

Silicate based bioceramics are promising candidates as biomaterials for tissue engineering. The combustion synthesis method provides control on the morphology and particle size of the synthesised material. This paper discusses the combustion synthesis of akermanite (Ca ₂ MgSi ₂ O ₇ and Sr ₂ MgSi ₂ O ₇ ), which has been shown to have good in vitro and in vivo bioactivities by earlier studies. Both Ca ₂ MgSi ₂ O ₇ and Sr ₂ MgSi ₂ O ₇ have akermanite structure. Ca ₂ MgSi ₂ O ₇ and Sr ₂ MgSi ₂ O ₇ were prepared using urea and ammonium nitrate. The combustion synthesis using urea and ammonium nitrate was found to be cost effective and efficient method of synthesis. The photoluminescence study of Ca ₂ MgSi ₂ O ₇ : Eu²⁺^{\boldsymbol{2+}} and Sr ₂ MgSi ₂ O ₇ : Eu²⁺^{\boldsymbol{2+}} shows host specific intense emission of Eu²⁺^{\boldsymbol{2+}}.     

Sintering of a fine-grained BaCeO3 powder obtained from a co-precipitation method

The formation of BaCeO₃ by a co-precipitation method is described herein. The co-precipitation route leads to an orange (BaCe)-precursor powder (1). To improve the sintering behaviour, a small amount of Ge⁴⁺ was incorporated, leading to a (BaCe_0.95/Ge_0.05)-precursor (2). Both precursor powders results in fine-grained preceramic powders (1A, 2A) after calcination. The shrinkage and sintering behaviour of resulting powder compacts were studied in comparison to a coarse-grained mixed-oxide BaCeO₃ powder (3). Compacts of 2A reach a relative density of 90% after sintering at 1350 °C with grain sizes between 0.9 and 3.2 μm. On the other hand ceramics of 1A and 3 have, after sintering at 1500 °C (10 h), relative densities of 85 and 76%, respectively. Ceramic bodies of 1A consisted of phase-pure orthorhombic BaCeO₃, whereas bodies of 2A show reflections of BaCeO₃ and a Ba₂GeO₄ phase. DTA investigations of samples 1A and 2A reveal three phase transitions at 255 °C (1A) and 256 °C (2A) as well as 383 °C (1A) and 380 °C (2A). A very weak one can be obtained in the range 880–910 °C.     

Multiple strengthening mechanisms in nanoparticle-reinforced copper matrix composites

The multiple hardening mechanisms of a copper matrix have been presented and discussed. The pre-alloyed ball milled Cu–3 wt.%Al and the atomized Cu–0·6 wt.%Ti–2·5 wt.%TiB ₂ powders have been used as starting materials. Dispersoid particles Al ₂ O ₃ and TiB _2\thinspace _{{\bf 2}\thinspace }were formed in situ. The powders have been hot consolidated. Optical microscopy, SEM, TEM, and X-ray diffraction analysis were performed for microstructural characterization. Increase in microhardness of Cu–3 wt.%Al compacts is a consequence of the crystallite size refinement and the presence of Al ₂ O ₃ particles. High hardening of Cu–0·6 wt.%Ti–2·5 wt.%TiB ₂ is a consequence of the presence of modular structure, Cu ₄ Ti_(m), and TiB ₂ particles.     

Improved Multiple Comparisons With a Control in Response Surface Analysis

Quadratic response surface methodology often focuses on finding the levels of some (coded) predictor variables x = (x ₁, x ₂,…, x _k) that optimize the expected value of a response variable y. Typically the experimenter starts from some best guess or “control” combination of the predictors (usually coded to x = 0) and performs an experiment varying them in a region around this center point. The question of interest addressed here is whether any x in the experimental region provides a long-run mean response E(y) preferable to that of the control, and if so, by what amount? This article approaches this question via simultaneous confidence intervals for δ(x) = E(y|x) = E(y|0) for all x within a specified distance of 0. A new method for two or more predictors is introduced that gives sharper intervals than the Scheffé method and also the Sa and Edwards adaptation of the Casella and Strawderman method. The new method does not require a rotatable design and allows for one-sided simultaneous bounds for δ(x). Approximate sample-size savings of the improved method over the Sa and Edwards adaptation of the Casella and Strawderman method ranged from 12%–45% for two-sided intervals and 19%–40% for one-sided intervals for designs with two or three predictors. Approximate sample-size savings of the improved method over the Scheffe method ranged from 14%–47% for two-sided intervals and 22%–62% for one-sided intervals.     

IR and Raman spectroscopic studies of sol–gel derived alkaline-earth silicate glasses

IR and Raman spectroscopies have been utilized to study the structure and vibrational modes of sol–gel-derived binary silicate glasses. The present study is motivated by the immense geological significance and focuses on the MO–SiO ₂ (M = Ca, Mg) binary systems in an effort to unveil the role of the CaO and MgO modifiers when incorporated to the 3D silica structure. Glasses in the composition range x =0, 0·1, 0·2, 0·3 and 0·4 prepared by the sol–gel method were compared with the corresponding glasses formed by appropriate mixing of SiO ₂ and MO powders through melting and fast cooling. The vibrational spectra of the sol–gel-derived glasses have revealed considerable changes in relative intensities as a function of the MO mole fraction. These changes signify structural modifications on the silica network. The population of the Q ³ species was found to increase for both modified silicate systems. The rate of increase is more pronounced in the CaO–SiO ₂ glasses. The extent of network depolymerization in the porous glass is higher at the same content of alkaline earth oxide compared to the bulk glass. The results are indicative of a more ‘defective’ nature of the sol–gel glasses compared to the corresponding melt-quenched ones.