Synthesis, magnetic and electrical properties of Fe-containing SiO2 nanocomposite

Nanocrystalline Fe powders were synthesized by transmetallation reaction and embedded in silica to form Fe-SiO2 nanocomposite. Thermomagnetic study of the as-prepared Fe sample indicates the presence of Fe3O4 and Fe particles. Oxidation studies of Fe and Fe-SiO2 show an increased thermal stability of Fe-SiO2 nanocomposite over pure Fe. The Fe-SiO2 shows an enhanced oxidation temperature (i.e., 780 K) and a maximum saturation magnetization value of (135 emu/g) with 64 wt.% of Fe content in silica. Electrical and dielectric behaviour of the Fe-SiO2 nanocomposite has been investigated as a function of temperature and frequency. Low frequency ac conductivity and dielectric constants were found to be influenced by desorptions of chemisorbed moisture. High saturation magnetization, thermal stability, frequency-dependent conductivity and low power loss make Fe-silica a promising material for high frequency applications. (C) 2010 Elsevier B.V. All rights reserved.

Crystallization Studies of ZrO2-SiO2 Composite Gels

Composite ZrO2-SiO2 powders were prepared using a gel route. Morphological and crystallographic features of ZrO2 particles formed during the heat treatment, and the particle sizes of the composites have been investigated. The following polymorphic changes have been observed during the heat treatment: amorphous -> metastable-cubic/tetragonal ZrO2 -> tetragonal ZrO2 -> monoclinic ZrO2. SiO2 crystallizes above 1273 K. The martensitic transformation of ZrO2 (t -> m) was observant in situ, when exposed to a high-energy electron beam. These results are important in the production of ZrO2-toughened ceramics of controlled microstructure.

An electron paramagnetic resonance study of Pr0.6Ca0.4MnO3 across the charge-ordering transition

We report the first electron paramagnetic resonance studies of single crystals and powders of Pr0.6Ca0.4MnO3 in the 300-4.2 K range, covering the charge-ordering transition (Tco) at ~240 K and antiferromagnetic transition (TN) at ~170 K. The asymmetry parameter for the Dysonian single-crystal spectra shows an anomalous increase at Tco. Below Tco the g-value increases continuously, suggesting a gradual strengthening of the orbital ordering. The linewidth undergoes a sudden increase at Tco and continues to increase down to TN. The intensity increases as the temperature is decreased until Tco is reached, due to the renormalization of the magnetic susceptibility arising from the build-up of ferromagnetic correlations.

Dielectric properties of barium magnesiotitanate ceramics, BaMg6Ti6O19

Phase-pure samples of barium magnesiotitanate, BaMg6Ti6O19 (BMT) are prepared by the wet chemical `gel-carbonate' method wherein the formation of BMT is complete below 950 degrees C as a result of the reaction between nanoparticles of BaCO3, MgO and TiO2. BMT powders are sintered at 1350-1450 C to dense ceramics. Extensive melting is noticed when the bulk composition falls between 0.4MgTiO(3)+0.6BaTiO(3)) and (0.6MgTiO(3)+0.4BaTiO(3)) along the MgTiO3-BaTiO3 tie-line in BaO-MgO-TiO2, phase diagram. Dielectric properties of sintered (BMT) ceramics have been investigated which showed epsilon similar or equal to 39 at 2 GHz, quality factor Q >= 10,000 and positive temperature coefficient of dielectric constant around 370 ppm degrees C-1.

Structural, dielectric and ferroelectric properties of four-layer Aurivillius phase Na0.5La0.5Bi4Ti4O15

Monophasic Na0.5La0.5Bi4Ti4O15 powders were synthesized via the conventional solid-state reaction route. The X-ray powder diffraction (XRD), selected area electron diffraction (SAED) and high resolution transmission electron microscopy (HRTEM) studies carried out on the as synthesized powdered samples confirmed the phase to be a four-layer Aurivillius that crystallizes in an orthorhombic A2(1)am space group. The microstructure and the chemical composition of the sintered sample were examined by scanning electron microscope (SEM) equipped with an energy dispersive X-ray analyzer (EDX). The dielectric properties of the ceramics have been studied in the 27-700 degrees C temperature range at various frequencies (100 Hz to 1 MHz). A sharp dielectric anomaly was observed at 580 degrees C for all the frequencies corresponding to the ferroelectric to paraelectric phase transition. Saturated ferroelectric hysteresis loops were observed at 200 degrees C and the associated remnant polarization (P-r) and coercive field (E-c) were found to be 7.4 mu C/cm(2) and 34.8 kV/cm, respectively. AC conductivity analysis confirmed the existence of two different conduction mechanisms in the ferroelectric region. Activation energies calculated from the Arrhenius plots were similar to 0.24 eV and similar to 0.84 eV in the 300-450 degrees C and 450-580 degrees C temperature ranges, respectively. (C) 2010 Elsevier B.V. All rights reserved.

Synthesis, characterization and photoluminescence properties of Gd2O3:Eu3+ nanophosphors prepared by solution combustion method

Gd2O3:Eu3+ (0.5-8.0 mol%) nanophosphors have been prepared by low temperature solution combustion method using metal nitrates as oxidizers and oxalyl dihydrazide (ODH) as a fuel. The phosphors are well characterized by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR) and photoluminescence (PL) techniques. PXRD patterns of as-formed and calcined (800 degrees C, 3 h) Gd2O3 powders exhibit monoclinic phase with mean crystallite sizes ranging from 20 to 50 nm. Eu3+ doping changes the structure from monoclinic to mixed phase of monoclinic and cubic. SEM micrographs shows the products are foamy, agglomerated and fluffy in nature due to the large amount of gases liberated during combustion reaction. Upon 254 nm excitation the photoluminescence of the Gd2O3:Eu3+ particles show red emission at 611 nm corresponding to D-5(0)-> F-7(2) transition. It is observed that PL intensity increases with calcination temperature. This might be attributed to better crystallization and eliminates the defects, which serve as centers of non-radiative relaxation for nanomaterials. It is observed that the optical energy gap (E-g) is widened with increase Eu3+ content. (C) 2010 Elsevier B.V. All rights reserved.

Magnetic, electric, and dielectric properties of FeCo alloy nanoparticles dispersed in amorphous matrix

Nanocrystalline Fe53Co47 alloy was synthesized by a single-step transmetallation chemical method at room temperature. The Fe53Co47 alloy nanoparticles of 77 and 47 wt% were dispersed in silica matrix by the sol-gel process using tetraethyl orthosilcate. Structural studies reveal that the as-prepared alloy powders are in bcc phase and silica is in an amorphous state. The phase-transition temperature and Mossbauer spectra analysis of the Fe-Co alloy establishes the homogeneous alloy formation. A saturation magnetization of 218 emu/g was obtained for pure FeCo alloy at room temperature. Scanning electron microscopic analysis demonstrates the hollow-sphere morphology for FeCo alloy particles. Magnetic nanocomposite consisting of 47 wt% FeCo-silica shows enhanced thermal stability over the native FeCo alloy. Electrical and dielectric properties of 47 wt% FeCo-silica nanocomposites were investigated as a function of frequency and temperature. It was found that the dielectric constants and dielectric loss were stable throughout the measured temperature (310-373 K). Our results indicate that FeCo-silica nanocomposite is a promising candidate for high-frequency applications. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Combustion synthesis of rare earth cuprates

Rare earth cuprates, La2CuO4 Nd2CuO4, La1.8M0.2CuO4 (M=Ca.Sr) and Nd1.85Ce0.15CuO4 have been prepared by the combustion of redox mixtures containing corresponding metal nitrates and maleic hydrazide, C4H4N2O2, at 350°C. The solid combustion products are submicron size amorphous powders which on heat treatment (700°C, 30 minutes) yield crystalline single phase cuprates. Strontium doped lanthanum cuprate, La1.8Sr0.2CuO4, shows an onset of superconductivity at 36K.

Mechanical alloying—a novel synthesis route for amorphous phases

Mechanical alloying (MA) pioneered by Benjamin is a technique for the extension of solid solubility in systems where the equilibrium solid solubility is limited. This technique has, in recent years, emerged as a novel alternate route for rapid solidification processing (RSP) for the production of metastable crystalline, quasicrystalline, amorphous phases and nanocrystalline materials. The glass-forming composition range (GFR), in general, is found to be much wider in case of MA in comparison with RSP. The amorphous powders produced by MA can be compacted to bulk shapes and sizes and can be used as precursors to obtain high strength materials. This paper reports the work done on solid state amorphization by MA in Ti-Ni-Cu and Al-Ti systems where a wide GFR has been obtained. Al-Ti is a classic case where no glass formation has been observed by RSP, while a GFR of 25–90 at.% Ti has been obtained in this system, thus demonstrating the superiority of MA over RSP. The free energy calculations made to explain GFR are also presented.

On the exothermic peak during annealing of electrodeposited nanocrystalline nickel

Nanocrystalline metals frequently exhibit poor thermal stability, and the exothermic peak in differential scanning calorimetry is usually attributed to grain growth. We show from experiments on electrodeposited nano-Ni with varying levels of S, and tests with microcrystalline Ni and S powders, that the exothermic peak is associated with the formation of a nickel sulfide phase and concurrent grain growth. Analysis suggests that segregation plays a more important role in limiting grain growth than second-phase particles in nano-Ni. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Crystal structure and electrical properties of CaNaBi2Nb3O12 ceramics

Monophasic CaNaBi2Nb3O12 powders were synthesized via the conventional solid-state reaction route. Rietveld refinement of the X-ray powder diffraction (XRD) data and selected area electron diffraction (SAED) studies confirmed the phase to be a three-layer Aurivillius oxide associated with an orthorhombic B2cb space group. The dielectric properties of the ceramics have been studied in the 300-800 K temperature range at various frequencies (1 kHz to 1 MHz). A dielectric anomaly was observed at 676 K for all the frequencies corresponding to the ferroelectric to paraelectric phase transition as it was also corroborated by the high temperature X-ray diffraction studies. The incidence of the polarization-electric field (P vs. E) hysteresis loop demonstrated CaNaBi2Nb3O12 to be ferroelectric.

Structure evolution and phase change in Ag-5.1 at.% Bi alloy during mechanical alloying

In this paper we explore the enhancement of solubility in a mechanically driven immiscible system experimentally using a mixture of Ag and Bi powders corresponding to a composition of Ag-5.1 at.% Bi. Increase in solubility can be correlated with the combination of sizes of both Ag and Bi at the nanometric scale. It is shown that complete solid solution of Ag-5.1 at.% Bi forms when the respective sizes of :Bi and Ag exceed 13 and 8 nm respectively. We have carried out a thermodynamic analysis of the size- and strain-dependent free energy landscape and compared the results to the initial mixture of microsized particles to rationalize the evolution of Ag solid solution. The agreement indicates that the emerging driving force for the formation of solid solution is primarily due to size reduction rather than the enhanced kinetics of mass transport due to mechanical driving. (c) 2011 Published by Elsevier Ltd. on behalf of Acta Materialia Inc.

Characterization of coprecipitated ferroelectric Bi2 VO5·5

Fine powders of orthorhombic bismuth vandate (Bi2VO5.5) have been synthesized by coprecipitation method. Powder X-ray diffraction and electron microscopic techniques have been used to characterize these samples. The formation of the monophasic Bi2VO5.5 was confirmed. The compacted powders sintered at 1070 K have been characterized for their dielectric properties as a function of both temperature (300-900 K) and frequency (100 Hz-10 MHz) and found to be superior to those obtained by the conventional solid-state reaction route.

Combustion synthesis and properties of Ln3Fe5O12 and yttrium aluminium garnets

The rare earth iron garnets Ln3Fe5O12 and Y3AlxFe5-xO12, where x=1.0-5.0, and Y1.5Gd1.5Al0.2Fe4.8O12 have been prepared by the combustion of redox mixtures containing corresponding metal nitrates and oxalyl dihydrazide, i.e. C2H6N4O2 at 350-degrees-C. The solid combustion products are amorphous, submicrometre-sized powders which, on heating at 750-degrees-C for 3 h, yield crystalline single-phase garnets. The particle size of the garnets is below 1 mum and the surface area ranges from 16 to 90 m2 g-1. Yttrium iron garnet could be sintered to a density of more than 95% at 1200-degrees-C for 3 h, giving an average grain size of 3-5 mum.

Combustion synthesis of oxide materials for nuclear waste immobilization

Oxide materials like perovskite, zirconolite, hollandite, pyrochlore, NASICON and sphene which are used for nuclear waste immobilization have been prepared by a solution combustion process. The process involves the combustion of stoichiometric amount of corresponding metal nitrates and carbohydrazide/tetraformyl trisazine/diformyl hydrazide at 450 degrees C. The combustion products have been characterized using powder X-ray diffraction, infrared spectroscopy, and Si-29 MAS-NMR. The fine particle nature of the combustion derived powders has been studied using density, particle size, BET surface area measurements and scanning electron microscopy. Sintering of combustion derived powder yields 85-95% dense ceramics in the temperature range 1000 degrees-1300 degrees C.