Structural and phase dependent thermo and photoluminescent properties of Dy(OH)(3) and Dy2O3 nanorods

Hexagonal Dy(OH)(3) and cubic Dy2O3 nanorods were prepared by hydrothermal method. Dy(OH)(3) nanorods was directly obtained at 180 degrees C for 20 h after hydrothermal treatment whereas subsequently heat treatment at 750 degrees C for 2 h gives pure cubic Dy2O3. SEM micrographs reveal that needle shaped rods with different sizes were observed in both the phases. TEM results also confirm this. The TL response of hexagonal Dy(OH)(3) and cubic Dy2O3 nanorods have been analyzed for gamma-irradiation over a wide range of exposures (1-5 kGy). TL glow peak intensity increases with gamma dose in both the phases. The activation energy (E), order of kinetics (6), and frequency factor (s) for both the phases have been determined using Chen's peak shape method. The simple glow curve shape, structure and linear response to gamma-irradiation over a large span of exposures makes the cubic Dy2O3 as a useful dosimetric material to estimate high exposures of gamma-rays. (c) 2012 Elsevier Ltd. All rights reserved.

Full Chemical Kinetic Simulation of Biogas Early Phase Combustion in SI Engines

This paper presents computational work on the biogas early phase combustion in spark ignition (SI) engines using detailed chemical kinetics. Specifically, the early phase combustion is studied to assess the effect of various ignition parameters such as spark plug location, spark energy, and number of spark plugs. An integrated version of the KIVA-3V and CHEMKIN codes was developed and used for the simulations utilizing detailed kinetics involving 325 reactions and 53 species The results show that location of the spark plug and local flow field play an important role. A central plug configuration, which is associated with higher local flow velocities in the vicinity of the spark plug, showed faster initial combustion. Although a dual plug configuration shows the highest rate of fuel consumption, it is comparable to the rate exhibited by the central plug case. The radical species important in the initiation of combustion are identified, and their concentrations are monitored during the early phase of combustion. The concentration of these radicals is also observed to correlate very well with the above-mentioned trend.Thus, the role of these radicals in promoting faster combustion has been clearly established. It is also observed that the minimum ignition energy required to initiate a self-sustained flame depends on the flow field condition in the vicinity of the spark plug.Increasing the methane content in the biogas has shown improved combustion.

``On-the-fly'' kinetics of enzymatic racemization using deuterium NMR in DNA-based chiral oriented media

We report the in situ and real-time monitoring of the interconversion of L- and D-alanine-d(3) by alanine racemase from Bacillus stearothermophilus directly observed by H-2 NMR spectroscopy in anisotropic phase. The enantiomers are distinguished by the difference of their H-2 quadrupolar splittings in a chiral liquid crystal containing short DNA fragments. The proof-of-principle, the reliability, and the robustness of this new method is demonstrated by the determination of the turnover rates of the enzyme using the Michaelis Menten model.

Freezing of the octahedral tilt near ferromagnetic transition and appearance of a glassy phase at low temperature driven by the tilt instabilities in SrRuO3

The emergence of low temperature glassy phase in widely known itinerant ferromagnet SrRuO3 is remotely understood. In order to understand this aspect, we have undertaken a detailed temperature dependent (5-250 K) neutron diffraction study. We observe a freezing of the octahedral tilt near the ferromagnetic transition and an unusual deviation in the octahedral tilt near the onset of low temperature spin glass like phase. A reduction of the ordered magnetic moment and a decline in the total integrated magnetic intensity is observed around the temperature where the glassy behaviour starts to appear. The magnetotransport study also reveals the possibility for an additional magnetic ordering by demonstrating a peak in magnetoresistance at the low temperature side as well. The neutron diffraction study presented here provides useful information to understand the observed unusual low temperature magnetic phenomena in SrRuO3.

Spark plasma sintering may lead to phase instability and inferior mechanical properties: A case study with TiB2

In contrast to the widely reported beneficial aspects of spark plasma sintering in developing materials with better properties, we report here two interesting aspects recorded with difficult-to-sinter titanium diboride: (a) in situ formation of second phase (TiB) and (b) inferior hardness (by similar to 30%) and elastic modulus (by similar to 20%) for spark plasma sintered TiB2, with respect to hot pressed TiB2. The formation of TiB is discussed with reference to the enhanced reaction kinetics in the presence of pulsed electric field. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Study on important diffusion parameters of binary Ni3Sn4 phase

Important diffusion parameters, such as-parabolic growth constant, integrated diffusivity, ratio of intrinsic diffusivities of species Ni and Sn, Kirkendall marker velocity and the activation energy for diffusion kinetics of binary Ni3Sn4 phase have been investigated with the help of incremental diffusion couple technique (Sn/Ni0.57Sn0.43) in the temperature range 200-150 degrees C. Low activation energy extracted from Arrhenius plot indicates grain boundary controlled diffusion process. The species Sn is three times faster than Ni at 200 degrees C. Further, the activation energy of Sn tracer diffusivity is greater than that of Ni.

Stabilization of metastable tetragonal phase in a rhombohedral magnetoelectric multiferroic BiFeO3-PbTiO3

The phase formation behaviour of the magnetoelectric multiferroic 0.8BiFeO(3)-0.2PbTiO(3) was studied as a function of heat treatment at different temperatures of a sol-gel derived powder. While under ordinary synthesis conditions this composition exhibits antiferromagnetic ordering and a rhombohedral structure; the sol-gel-enabled low-temperature synthesis could stabilize a tetragonal metastable phase along with the stable rhombohedral phase, mimicking a morphotropic phase boundary state. The phase coexistence state exhibits relatively enhanced ferromagnetic correlation. The same system with a relatively higher PbTiO3 concentration, 0.65BiFeO(3)-0.35PbTiO(3), on the other hand, exhibits a rhombohedral metastable phase. These results suggest that the occurrence of metastable phases is a very common feature in the BiFeO3-PbTiO3 magnetoelectric ferroelectric system and that it affects the ferroelectric and magnetic properties of system quite remarkably.

Temperature dependent magnetic ordering and electrical transport behavior of nano zinc ferrite from 20 to 800 K

The nano ZnFe2O4 compound was prepared by eco-friendly hydrothermal method. The characterization of the sample for its structure, morphology and composition were done by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), dynamic light scattering, Fourier transform infrared spectroscopy, zeta surface profiler and UV-Visible spectroscopy studies. The PXRD measurement reveals that the compound shows spinel cubic phase belong Fd (3) over barm (227) space group. Morphology of the compound from SEM and surface profile shows nearly spherical agglomerated particles with well defined grains and grain boundaries. The material shows the semiconducting behavior with E-g of 2.3 eV at room temperature (RT). The variation in the magnetic ordering was observed for wide range of temperature. The compound behaves like a soft magnetic material with ferrimagnetic at various temperatures except at RT. Both magnetic and EPR studies supports the superparamagnetic behavior of the the sample. The DC conductivity, dielectric and AC conductivity behavior of the 1000 degrees C pellets sintered for 2 h shows good frequency dependent transport properties. The present study facilitate in selecting the suitable materials for the nanoelectronics and spintronic applications. (C) 2013 Elsevier B.V. All rights reserved.

Beyond the ``Coffee Ring'': Re-entrant Ordering in an Evaporation-Driven Self-Assembly in a Colloidal Suspension on a Substrate

We study the phenomenon of evaporation-driven self-assembly of a colloid suspension of silica microspheres in the interior region and away from the rim of the droplet on a glass plate. In view of the importance of achieving a large-area, monolayer assembly, we first realize a suitable choice of experimental conditions, minimizing the influence of many other competing phenomena that usually complicate the understanding of fundamental concepts of such self-assembly processes in the interior region of a drying droplet. Under these simplifying conditions to bring out essential aspects, our experiments unveil an interesting competition between ordering and compaction in such drying systems in analogy to an impending glass transition. We establish a re-entrant behavior in the order disorder phase diagram as a function of the particle density, such that there is an optimal range of the particle density to realize the long-range ordering. The results are explained with the help of simulations and phenomenological theory.

Reentrant Superspin Glass Phase in a La0.82Ca0.18MnO3 Ferromagnetic Insulator

We report results of the magnetization and ac susceptibility measurements down to very low fields on a single crystal of the perovskite manganite, La-0.82 Ca-0.18 MnO3. This composition falls in the intriguing ferromagnetic insulator region of the manganite phase diagram. In contrast to earlier beliefs, our investigations reveal that magnetically (and in every other sense), this is a single- phase system with a ferromagnetic ordering temperature of around 170 K. However, this ferromagnetic state is magnetically frustrated, and the system exhibits pronounced glassy dynamics below 90 K. Based on measured dynamical properties, we propose that this quasi-long-ranged ferromagnetic phase, and the associated superspin glass behavior, is the true magnetic state of the system, rather than being a macroscopic mixture of ferromagnetic and antiferromagnetic phases, as often suggested. Our results provide an understanding of the quantum phase transition from an antiferromagnetic insulator to a ferromagnetic metal via this ferromagnetic state as a function of x in La1-xCaxMnO3, in terms of the possible formation of magnetic polarons.

Equilibrium phases in the multiferroic BiFeO3-PbTiO3 system - a revisit

The(1-x) BiFeO3-(x) PbTiO3 solid solution exhibiting a Morphotropic Phase Boundary (MPB) has attracted considerable attention recently because of its unique features such as multiferroic, high Curie point (T-C similar to 700 degrees C) and giant tetragonality (c/a -1 similar to 0.19). Different research groups have reported different composition range of MPB for this system. In this work we have conclusively proved that the wide composition range of MPB reported in the literature is due to kinetic arrest of the metastable rhombohedral phase and that if sufficient temperature and time is allowed the metastable phase disappears. The genuine MPB was found to be x=0.27 for which the tetragonal and the rhombohedral phases are in thermodynamic equilibrium. In-situ high temperature structural study of x=0.27 revealed the sluggish kinetics associated with the temperature induced structural transformation. Neutron powder diffraction study revealed that themagnetic ordering at room temperature occurs in the rhombohedral phase. The magnetic structure was found to be commensurate G-type antiferromagnetic with magnetic moments parallel to the c-direction (of the hexagonal cell). The present study suggests that the equilibrium properties in this solid solution series should be sought for x=0.27.

Ferroelectric phase coexistence by crystallite size reduction in BiFeO3-PbTiO3

Size effect in ferroelectrics has been extensively investigated in the past with the general consensus that the long-range ferroelectric ordering gradually disappears with decreasing crystallite size, eventually leading to the paraelectric state. In this paper, we show that the compositions exhibiting giant tetragonality (c/a similar to 1.18) of the ferroelectric alloy system BiFeO3-PbTiO3 transform from a pure tetragonal phase to a state comprising tetragonal and rhombohedral phases as the average crystallite size is reduced from similar to 10 to similar to 1 mu m. It is argued that the increased surface energy in the smaller sized crystallites creates an equivalent compressive stress that drives the system towards tetragonal-rhombohedral criticality.

Phase relations in the system Ca-Ta-O and thermodynamics of calcium tantalates in relation to calciothermic reduction of Ta2O5

Phase equilibria of the system Ca-Ta-O is established by equilibrating eleven samples at 1200 K for prolonged periods and phase identification in quenched samples by optical and scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. Four ternary oxides are identified: CaTa4O11, CaTa2O6, Ca2Ta2O7 and Ca4Ta2O9. Isothermal section of the phase diagram is composed using the results. Thermodynamic properties of the ternary oxides are measured in the temperature range from 975 to 1275 K employing solid-state galvanic cells incorporating single crystal CaF2 as the solid electrolyte. The cells essentially measure the chemical potentials of CaO in two-phase fields (Ta2O5 + CaTa4O11), (CaTa4O11 + CaTa2O6), (CaTa2O6 + Ca2Ta2O7), and (Ca2Ta2O7 + Ca4Ta2O9) of the pseudo-binary system CaO-Ta2O5. The standard Gibbs energies of formation of the four ternary oxides from their component binary oxides Ta2O5 and CaO are given by: Delta G(f)((ox))(o) (CaTa4O11) (+/- 482)/J mol(-1) = -58644+21.497 (T/K) Delta G(f)((ox))(o) (CaTa2O6) (+/- 618)/J mol(-1) = -55122+21.893 (T/K) Delta G(f)((ox))(o) (Ca2Ta2O7) (+/- 729)/J mol(-1) = -82562+31.843 (T/K) Delta G(f)((ox))(o) (Ca4Ta2O9) (+/- 955)/J mol(-1) = -126598+48.859 (T/K) The Gibbs energy of formation of the four ternary compounds obtained in this study differs significantly from that reported in the literature. The thermodynamic data and phase diagram are used for understanding the mechanism and kinetics of calciothermic and electrochemical reduction of Ta2O5 to metal. (C) 2014 Elsevier B.V. All rights reserved.

Kinetics of dispersion of nanoparticles in thin polymer films at high temperature

We report the first detailed study of the kinetics of dispersion of nanoparticles in thin polymer films using temperature dependent in situ X-ray scattering measurements. We show a comparably enhanced dispersion at higher temperatures for systems which are otherwise phase segregated at room temperature. Detailed analysis of the time dependent X-ray reflectivity and diffuse scattering data allows us to explore the out-of-plane and in-plane mobility of the nanoparticles in the polymer films. While the out-of-plane motion is diffusive with a diffusion coefficient almost two orders of magnitude lower than that expected in bulk polymer, the in-plane one is found to be super-diffusive resulting in significantly larger in-plane displacement at similar time scales. We discuss the origin of the observed highly anisotropic motion of nanoparticles due to their slaved motion with respect to the anisotropic chain orientation and consequent diffusivity anisotropy of matrix chains. We also suggest strategies to utilize these observations to kinetically improve dispersion in otherwise thermodynamically segregated polymer nanocomposite films.

An alluaudite Na-2+Fe-2x(2-x)(SO4)(3)(x=0.2) derivative phase as insertion host for lithium battery

A new desodiated derivative compound, Na0.89Fe1.8(SO4)(3), was prepared by the chemical oxidation of alluaudite Na2.4Fe1.8(SO4)(3) Phase using NOBF4 as oxidant. The structure and valency of Fe were characterized by X-ray diffraction (XRD) and Fe-57 Mossbauer spectroscopy. Intercalation behavior of lithium ions in the structure of Na0.89Fe1.8(SO4)(3) was gauged by electrochemical analyses and ex-situ X-ray diffraction. A high capacity of 110 mAh g(-1) at 0.1 C was obtained with a good rate kinetics within a range of 0.1-10 C(1 C = 118 mAh g-1) involving a high Fe3+/Fe2+ redox potential of 3.75 V (vs. Li/Li+). These results confirmed that the Na2.4-delta Fe1.8(SO4)(3) framework was stable even after oxidation and forms a new competitive cathode for the reversible intercalation of lithium ions. (C) 2014 Elsevier B.V. All rights reserved.