Lowering of relaxor transition temperature in Lead-based perovskites under pressure

The dielectric constants of lead iron niobate (PFN) and 40% lead zinc niobate (PZN) added to lead iron niobate (PFN0.6-PZN(0.4)) have been measured as a function of pressure up to 6 GPa under isothermal conditions between room temperature and 348 K. The relaxer transition temperature measured at 1 kHz excitation frequency varies at a rate -24.5 K/GPa for PFN and at a rate of - 28.8 K/GPa for the PFN0.6-PZN(0.4) composition.

Development of temperature-tolerant strains of Thiobacillus ferrooxidans to improve bioleaching kinetics

An isolate of Thiobacillus ferrooxidans derived from gold mine water samples was repeatedly subcultured at increasing temperatures (from 30 degrees to 42 degrees C) in 9K medium. The temperature-adapted strain was found to be more efficient in the bioleaching of pyrite mineral than the wild type. When temperature-tolerant strains were cultured repeatedly in 9K medium at 30 degrees C, the temperature tolerance was completely lost, These results indicate that the temperature tolerance was stress-dependent and not a permanent trait of the adapted strain, The potential utility of such temperature-tolerant strains of Thiobacillus ferrooxidans in sulphide mineral dissolution is demonstrated.

Dynamical Transition of Water in the Grooves of DNA Duplex at Low Temperature

At low temperature (below its freezing/melting temperature), liquid water under confinement is known to exhibit anomalous dynamical features. Here we study structure and dynamics of water in the grooves of a long DNA duplex using molecular dynamics simulations with TIP5P potential at low temperature. We find signatures of a dynamical transition in both translational and orientational dynamics of water molecules in both the major and the minor grooves of a DNA duplex. The transition occurs at a slightly higher temperature (TGL ≈ 255 K) than the temperature at which the bulk water is found to undergo a dynamical transition, which for the TIP5P potential is at 247 K. Groove water, however, exhibits markedly different temperature dependence of its properties from the bulk. Entropy calculations reveal that the minor groove water is ordered even at room temperature, and the transition at T ≈ 255 K can be characterized as a strong-to-strong dynamical transition. Confinement of water in the grooves of DNA favors the formation of a low density four-coordinated state (as a consequence of enthalpy−entropy balance) that makes the liquid−liquid transition stronger. The low temperature water is characterized by pronounced tetrahedral order, as manifested in the sharp rise near 109° in the O−O−O angle distribution. We find that the Adams−Gibbs relation between configurational entropy and translational diffusion holds quite well when the two quantities are plotted together in a master plot for different region of aqueous DNA duplex (bulk, major, and minor grooves) at different temperatures. The activation energy for the transfer of water molecules between different regions of DNA is found to be weakly dependent on temperature.

Temperature measurements in the boron carbide manufacturing process - A hot model study

Boron carbide is produced in a heat resistance furnace using boric oxide and petroleum coke as the raw materials. The product yield is very low. Heat transfer plays an important role in the formation of boron carbide. Temperature at the core reaches up to 2600 K. No experimental study is available in the open literature for this high temperature process particularly in terms of temperature measurement and heat transfer. Therefore, a laboratory scale hot model of the process has been setup to measure the temperatures in harsh conditions at different locations in the furnace using various temperature measurement devices such as pyrometer and various types of thermocouple. Particular attention was paid towards the accuracy and reliability of the measured data. The recorded data were analysed to understand the heat transfer process inside the reactor and the effect of it on the formation of boron carbide.

Unsteady Natural Convection Flow in a Square Cavity Filled with a Porous Medium Due to Impulsive Change in Wall Temperature

Unsteady natural convection flow in a two- dimensional square cavity filled with a porous material has been studied. The flow is initially steady where the left- hand vertical wall has temperature T-h and the right- hand vertical wall is maintained at temperature T-c ( T-h > T-c) and the horizontal walls are insulated. At time t > 0, the left- hand vertical wall temperature is suddenly raised to (T-h) over bar ((T-h) over bar > T-h) which introduces unsteadiness in the flow field. The partial differential equations governing the unsteady natural convection flow have been solved numerically using a finite control volume method. The computation has been carried out until the final steady state is reached. It is found that the average Nusselt number attains a minimum during the transient period and that the time required to reach the final steady state is longer for low Rayleigh number and shorter for high Rayleigh number.

Stability of pargasite during ultrahigh-temperature metamorphism:A consequence of titanium and REE partitioning

Orthopyroxene-clinopyroxene-plagioclase needles and symplectite along the cleavage planes and grain boundaries of fluorine-bearing titanian-ferroan pargasite from the Highland Complex, Sri Lanka, are interpreted as evidence for dehydration melting at ultrahigh-temperature conditions. High Ti (up to 0.4 pfu) and F (XF up to 0.56) content in pargasite extends its stability to higher temperatures, and the composition indicates the dehydration melting reaction may take place at ultrahigh-temperatures (~950 °C) at a pressure around 10 kbar, close to peak metamorphic conditions. The increase of Ti content close to the grain boundaries and cleavage planes in pargasite indicates titanium partitioning from the melt during dehydration melting enhanced the stability of the mineral toward ultrahigh-temperature conditions. The REE content in the pargasite shows a similar behavior to that of titanium. The cores with no breakdown assemblage consist of low and flat REE concentrations with respect to the high and Eu-depleted rim. Clinopyroxene in symplectite and needle-shaped lamellae within the pargasite porphyroblasts have similar REE patterns with slightly low-concentrations relative to that of pargasite. In the breakdown assemblage, LREEs are partitioned mainly into plagioclase while the HREEs are partitioned into orthopyroxene. The REE enrichment in the pargasite rims signals their relative partitioning between pargasite rims and melt. Modeling of the partitioning of Ti and REEs associated with pargasite breakdown demonstrates that its stability is greatly enhanced at UHT conditions. This investigation implies that the stability of hydrous minerals such as amphibole can be extended to UHT conditions, and expands our knowledge of metamorphism in the lower crust.

Rotating crystal ESR cavity for angular variation studies at room temperature

We describe an X-band ESR cavity for angular variation studies on single crystals at room temperature. The cavity was found to have a high Q over wide rotation angles. Review of Scientific Instruments is copyrighted by The American Institute of Physics.

Crystal data for high-temperature dicalcium lead propionate, Ca2Pb(CH3CH2COO)6

The high-temperature paraelectric phase of dicalcium lead propionate, DCLP, at 363 ± 5 K is tetragonal, with a = 12.574 (6), c = 17.403 (9) Å, V = 2751.4 Å3, Z = 4 and corresponds to the space group P41212 (or P43212). The thermal expansion curve shows the transition somewhere between 328 and 343 K.

High-Pressure Low-Temperature Locknut Cell For Both Electron-Paramagnetic-Res And Nmr-Studies To 10 Kilobars And 77-K

A locked high-pressure cell with working pressure range up to 10 kbars suitable for low-temperature studies to 77 K has been described. It can be used for both EPR and NMR studies of single crystals (and other solid samples). The high-pressure seal and all other aspects of the cell remain the same for either application. Only a change of the bottom plug is required for a switch from a nuclear-magnetic-resonance (NMR) to an electron-paramagnetic-resonance (EPR) experiment. Details of the procedure for the calibration of pressure inside the cell at various temperatures are discussed. The performance of the cell in EPR (Cr3+ion) and NMR (27Al nucleus) studies is reported.

On the short-range order in Al-Mn quasicrystals during low-temperature ageing

We report the evolution of diffuse intensity during the low-temperature ageing of Al-Mn quasicrystals. This is taken as evidence of short-range order in the icosahedral phase prior to its decomposition. The implication of these diffuse intensities is discussed.

Low temperature fluorination of sulphur compounds with elemental fluorine

Low temperature fluorination technique is adopted for fluorination of the following sulphur compounds in freon-11 medium (1) Sulphur dioxide (2) Thionyl chloride (3) Sulphuryl chloride (4) Tetrasulphur tetra nitride and (5) Sulphur bromide. All the compounds undergo oxidative fluorination to give rise to sulphur-fluorine compounds except sulphuryl chloride which resists fluorination. Sulphuryl chloride thus behaves as a good solvent medium for fluorination of other reactive compounds like elemental sulphur. Details of the experimental procedures adopted and the identification of the products will be presented.

The sputtering temperature of a cooling cylindrical rod with an insulated core

By a standard application of Jones's method associated with the Wiener-Hopf technique an explicit solution is obtained for the temperature distribution inside a cylindrical rod with an insulated inner core when the rod is allowed to enter into a fluid of large extent with a uniform speed, and a simple integral expression is derived for the value of the sputtering temperature of the rod at the points of entry. Numerical results under certain special circumstances are also obtained and presented in the form of a table.

Low-temperature and high-pressure Raman and x-ray studies of pyrochlore Tb2Ti2O7: Phonon anomalies and possible phase transition

We have carried out temperature- and pressure-dependent Raman and x-ray measurements on single crystals of Tb2Ti2O7. We attribute the observed anomalous temperature dependence of phonons to phonon-phonon anharmonic interactions. The quasiharmonic and anharmonic contributions to the temperature-dependent changes in phonon frequencies are estimated quantitatively using mode Grüneisen parameters derived from pressure-dependent Raman experiments and bulk modulus from high-pressure x-ray measurements. Further, our Raman and x-ray data suggest a subtle structural deformation of the pyrochlore lattice at ~9 GPa. We discuss possible implications of our results on the spin-liquid behavior of Tb2Ti2O7.

Simplified Mathematical Model for Effects of Freezing on the Low-Temperature Performance of the Lead-Acid Battery

Discharge periods of lead-acid batteries are significantly reduced at subzero centigrade temperatures. The reduction is more than what can he expected due to decreased rates of various processes caused by a lowering of temperature and occurs despite the fact that active materials are available for discharge. It is proposed that the major cause for this is the freezing of the electrolyte. The concentration of acid decreases during battery discharge with a consequent increase in the freezing temperature. A battery freezes when the discharge temperature falls below the freezing temperature. A mathematical model is developed for conditions where charge-transfer reaction is the rate-limiting step. and Tafel kinetics are applicable. It is argued that freezing begins from the midplanes of electrodes and proceeds toward the reservoir in-between. Ionic conduction stops when one of the electrodes freezes fully and the time taken to reach that point, namely the discharge period, is calculated. The predictions of the model compare well to observations made at low current density (C/5) and at -20 and -40 degrees C. At higher current densities, however, diffusional resistances become important and a more complicated moving boundary problem needs to be solved to predict the discharge periods. (C) 2009 The Electrochemical Society.

Highly Monodisperse Colloidal Magnesium Nanoparticles by Room Temperature Digestive Ripening

Nanoclusters of 25 nm sized Mg-THF have been prepared by the solvated metal atom dispersion method. Room-temperature digestive ripening of these nanoclusters in the presence of hexadecylamine (HDA) resulted in highly monodisperse colloidal Mg-HDA nanoparticles of 2.8 ± 0.2 nm. An insight into the room-temperature digestive ripening process was obtained by studying the disintegration of clusters for various Mg:HDA ratios. The Mg colloids are quite stable with respect to precipitation of particles under Ar atmosphere. Using this procedure, pure Mg(0) nanopowders were obtained in gram scale quantities. The Mg powder precipitated from the colloid was fully hydrided at 33 bar and 118 °C. Initial desorption of H2 from samples of MgH2 was achieved at a remarkably low temperature, 115 °C compared to >350 °C in bulk Mg, demonstrating the importance of the size on the desorption temperatures.