Pressure Dependence of Glass Transition in As2Te3 Glass

Amorphous solids prepared from their melt state exhibit glass transition phenomenon upon heating. Viscosity, specific heat, and thermal expansion coefficient of the amorphous solids show rapid changes at the glass transition temperature (T-g). Generally, application of high pressure increases the T-g and this increase (a positive dT(g)/dP) has been understood adequately with free volume and entropy models which are purely thermodynamic in origin. In this study, the electrical resistivity of semiconducting As2Te3 glass at high pressures as a function of temperature has been measured in a Bridgman anvil apparatus. Electrical resistivity showed a pronounced change at T-g. The T-g estimated from the slope change in the resistivity-temperature plot shows a decreasing trend (negative dT(g)/dP). The dT(g)/dP was found to be -2.36 degrees C/kbar for a linear fit and -2.99 degrees C/kbar for a polynomial fit in the pressure range 1 bar to 9 kbar. Chalcogenide glasses like Se, As2Se3, and As30Se30Te40 show a positive dT(g)/dP which is very well understood in terms of the thermodynamic models. The negative dT(g)/dP (which is generally uncommon in liquids) observed for As2Te3 glass is against the predictions of the thermodynamic models. The Adam-Gibbs model of viscosity suggests a direct relationship between the isothermal pressure derivative of viscosity and the relaxational expansion coefficient. When the sign of the thermal expansion coefficient is negative, dT(g)/dP = Delta k/Delta alpha will be less than zero, which can result in a negative dT(g)/dP. In general, chalcogenides rich in tellurium show a negative thermal expansion coefficient (NTE) in the supercooled and stable liquid states. Hence, the negative dT(g)/dP observed in this study can be understood on the basis of the Adams-Gibbs model. An electronic model proposed by deNeufville and Rockstad finds a linear relation between T-g and the optical band gap (E-g for covalent semiconducting glasses when they are grouped according to their average coordination number. The electrical band gap (Delta E) of As2Te3 glass decreases with pressure. The optical and electrical band gaps are related as Delta E-g = 2 Delta E; thus, a negative dT(g)/dP is expected when As2Te3 glass is subjected to high pressures. In this sense, As2Te3 is a unique glass where its variation of T-g with pressure can be understood by both electronic and thermodynamic models.

Pressure-induced structural phase transitions and phonon anomalies in ReO3: Raman and first-principles study

We report high-pressure Raman-scattering studies on single-crystal ReO3 up to 26.9 GPa at room temperature, complemented by first-principles density functional calculations to assign the modes and to develop understanding of the subtle features of the low-pressure phase transition. The pressure (P) dependence of phonon frequencies (omega) reveals three phase transitions at 0.6, 3, and 12.5 GPa with characteristic splitting and changes in the slope of omega(P). Our first-principles theoretical analysis confirms the role of the rotational modes of ReO6, M-3, to the lowest pressure structural transition, and shows that the transition from the Pm3m to the Im3 structure is a weak first-order transition, originating from the strong anharmonic coupling of the M-3 modes with the acoustic modes (strain).

Simple Method of Preparing Graphene Flakes by an Arc-Discharge Method

Arc discharge between graphite electrodes under a relatively high pressure of hydrogen yields graphene flakes generally containing 2-4 layers in the inner wall region of the arc chamber. The graphene flakes so obtained have been characterized by X-ray diffraction, atomic force microscopy, transmission electron microscopy, and Raman spectroscopy. The method is eminently suited to dope graphene with boron and nitrogen by carrying out arc discharge in the presence of diborane and pyridine respectively.

Size-dependent elasticity of nanocrystalline titania

Synchrotron-based high-pressure x-ray diffraction measurements indicate that compressibility, a fundamental materials property, can have a size-specific minimum value. The bulk modulus of nanocrystalline titania has a maximum at particle size of 15 nm. This can be explained by dislocation behavior because very high dislocation contents can be achieved when shear stress induced within nanoparticles counters the repulsion between dislocations. As particle size decreases, compression increasingly generates dislocation networks hardened by overlap of strain fields that shield intervening regions from external pressure. However, when particles become too small to sustain high dislocation concentrations, elastic stiffening declines. The compressibility has a minimum at intermediate sizes.

Evidence for Permo-Triassic collision in Far East Asia: The Korean collisional orogen

Eclogites from paragneiss in the Korean Peninsula are characterized by a peak pressure assemblage of garnet + omphacite + quartz + rutile, that is overprinted by multiphase symplectites involving augite, amphibole, orthopyroxene, ilmenite and plagioclase and by a similar high-pressure assemblage with a pronounced absence of the omphacite component in clinopyroxene formed during the peak and orthopyroxene in the retrograde stage. Eclogites were metamorphosed at a minimum pressures of not, vert, similar 20–23 kbar at temperatures of not, vert, similar 840–1000 °C, equivalent to a crustal depth of not, vert, similar 70–75 km, whereas high-pressure granulite in Late Paleozoic rocks underwent metamorphic conditions of not, vert, similar 18–19 kbar at not, vert, similar 950 °C with a minimum crustal depth of not, vert, similar 60–65 km. The presence of the eclogites and high-pressure granulite suggests deep-seated subduction of crustal complexes with metamorphism at different crustal levels. The eclogites were exhumed quickly resulting in near- isothermal decompression. On the other hand, the multistage exhumation of the high-pressure granulites suggests retrograde overprinting after initial decompression. The similarity of these petrological characteristics, metamorphic conditions and also the regional structural styles with those of the Sulu belt (China) strongly suggests the existence of a Permo-Triassic Alpine-type “Korean collision belt” in Far East Asia. This model provides a better understanding of the paleogeograpic evolution of Permo-Triassic East Asia, including a robust tectonic correlation of the Korean collision belt with the Qinling–Dabie–Sulu collision belt.

Micro-Raman imaging of Te precipitates in CdZnTe (Zn similar to 4%) crystals

Micro-Raman imaging of the distribution of Te precipitates in CdZnTe crystals in different phases is reported. For the normal phase of Te precipitates, the Raman modes appear centered around 121(A1), 141(E)/TO(CdTe) cm−1 and a weak mode around 92(E) cm−1 in CdZnTe indicating the presence of trigonal lattice of Te. Under high pressure phase, the volume of Te precipitates collapses, giving more bond energy resulting in the blueshift of the corresponding Raman bands. Also, the spatial distribution of the area ratio of 121 to 141 cm−1 Raman modes is used to quantify Te precipitates. Further, near-infrared microscopy images support these results.

Bismuth-doped amorphous-germanium sulfidesemiconductors

A study of the effect of bismuth dopant on the electronic transport properties of the amorphous semiconductors Ge20S80-xBix under high pressure (up to 140 kbar) has been carried out down to liquid-nitrogen temperature. The experiments reveal that the electronic conduction is strongly composition dependent and is thermally activated with a single activation energy at all pressures and for all compositions. A remarkable resemblance between the electronic conduction process, x-ray diffraction studies, and differential thermal analysis results is revealed. It is proposed that the n-type conduction in germanium chalcogenides doped with a large Bi concentration is due to the effect of Bi dopants on the positive correlation energy defects present in germanium chalcogenides. The impurity-induced chemical modification of the network creates a favorable environment for such an interaction.

Impurity states in Sb-doped amorphous semiconductors

A systematic investigation of the effects of antimony dopant on the electronic transport properties of amorphous (GeSe3.5)100−xSbx under high pressure (up to 120 kbar) has been carried out down to liquid-nitrogen temperature for the first time. Differential thermal analysis and x-ray diffraction methods were used for the characterization of freshly prepared and pressure-quenched materials which indicated the presence of structural phase transition in both GeSe3.5 and (GeSe3.5)100−xSbx around 105 kbar pressure. Electrical transport data revealed the strong compositional dependence of the electronic conduction process. A distinct kink in the conductivity temperature plot at pressures>15 kbar was observed in the Sb-doped compositions indicating the presence of different conduction processes. An attempt has been made to interpret the pressure-induced effect in the transport properties of these glasses considering the possible presence of both thermally activated conduction in the extended states and hopping process in the localized tail states. However, the interpretation of the transport data is not straightforward and the pressure dependence of the thermoelectric power will be needed to complete the picture. Journal of Applied Physics is copyrighted by The American Institute of Physics.

Counterclockwise exhumation of a hot orogen: The Paleoproterozoic ultrahigh-temperature granulites in the North China Craton

Regional metamorphic belts provide important constraints on the plate tectonic architecture of orogens. We report here a detailed petrologic examination of the sapphirine-bearing ultra-high temperature (UHT) granulites from the Jining Complex within the Khondalite Belt of the North China Craton (NCC). These granulites carry diagnostic UHT assemblages and their microstructures provide robust evidence to trace the prograde, peak and retrograde metamorphic evolution. The P–T conditions of the granulites estimated from XMgGrt(Mg/Fe + Mg) − XMgSpr isopleth calculations indicate temperature above 970 °C and pressures close to 7 kbar. We present phase diagrams based on thermodynamic computations to evaluate the mineral assemblages and microstructures and trace the metamorphic trajectory of the rocks. The evolution from Spl–Qtz–Ilm–Crd–Grt–Sil to Spr–Qtz–Crd–Opx–Ilm marks the prograde stage. The Spl–Qtz assemblage appears on the low-pressure side of the P–T space with Spr–Qtz stable at the high-pressure side, possibly representing an increase in pressure corresponding to compression. The spectacular development of sapphirine rims around spinel enclosed in quartz supports this inference. An evaluation of the key UHT assemblages based on model proportion calculation suggests a counterclockwise P–T path. With few exceptions, granulite-facies rocks developed along collisional metamorphic zones have generally been characterized by clockwise exhumation trajectories. Recent evaluation of the P–T paths of metamorphic rocks developed within collisional orogens indicates that in many cases the exhumation trajectories follow the model subduction geotherm, in accordance with a tectonic model in which the metamorphic rocks are subducted and exhumed along a plate boundary. The timing of UHT metamorphism in the NCC (c. 1.92 Ga) coincides with the assembly of the NCC within the Paleoproterozoic Columbia supercontinent, a process that would have involved subduction of passive margins sediments and closure of the intervening ocean. Thus, the counterclockwise P–T path obtained in this study correlates well with a tectonic model involving subduction and final collisional suturing, with the UHT granulites representing the core of the hot or ultra-hot orogen developed during Columbia amalgamation.

Evidence for a new metastable crystalline compound in Ge---Te system

Pressure dependence of the electrical resistivity of bulk, melt quenched GexTe100−x glasses (15 less-than-or-equals, slant x less-than-or-equals, slant 28) has been studied up to 8GPa pressure. All the glasses exhibit a sharp, discontinuous glass to crystal transition under pressure. The high pressure crystalline phases are identified to have a face centered cubic structure. The value of the cell constant is 0.779nm for 15 less-than-or-equals, slant x less-than-or-equals, slant 17, 0.642nm for x=20 and 0.55lnm for 22 ≤ x ≤ 28 samples respectively. The cell constants of the high pressure crystalline phases suggest the possible existance of a new metastable crystalline compound in the Ge---Te system with F.C.C. structure and cell constant equal to 1.109nm as reported by Moore et al.

Crystal growth and characterization of two-leg spin ladder compounds: Sr14Cu24O41 and Sr2Ca12Cu24O41

Single crystals of Sr14−xCaxCu24O41 (x=0 and 12) are grown by the travelling solvent floating zone technique using an image furnace. The grown crystals are characterized for their single crystallinity by the X-ray and Neutron Laue method. The magnetic susceptibility measurements in Sr14Cu24O41 show considerable anisotropy along the main crystallographic axes. Low-temperature specific heat measurement and DC susceptibility measurement in Ca-doped crystal showed antiferromagnetic ordering at 2.8 K at ambient pressure. High-pressure AC susceptibility measurement on Ca-doped crystal showed a sharp superconducting transition at 2 K under 40 kbars. Tc onset reached a maximum value of 9.9 K at 54 kbars. The bulk superconductivity of the sample is confirmed by the high-pressure AC calorimetry with Tc max=9.4 K and TN=5 K at 56 kbars.

Shock tunnel testing of a Mach 6 hypersonic waverider

A waverider is a lifting body configuration whose upper surface is parallel to the free stream, and the lower surface aerodynamically so designed, that the resulting shock at the design Mach number, is always attached with the leading edge of the vehicle. This prevents spillage from high pressure (lower) surface to the low pressure (upper) surface.In the present study a conical waverider has been designed, fabricated and tested at Mach 6 in the IISc hypersonic shock tunnel HST2. The measurements show that the waverider has a lift to drag ratio of 4.28 at the designed Mach number. Exhaustive FEM and CFD studies are also carried out to complement the force measurements in the tunnel.

Interrupted creep behaviour of Mg alloys developed for powertrain applications

A conventional magnesium alloy, AZ91D, and two creep resistant magnesium alloys, developed for powertrain applications, MRI 153M and MRI 230D, are prepared by high pressure die casting. These alloys are tested for their creep behaviour in the continuous manner, as is the Current practice, and in the interrupted manner, which represents the real life Situation more closely. It is observed that the interrupted creep tests give rise to a primary creep appearing at the beginning of each cycle resulting in a higher average strain rate than that encountered in the continuous creep tests. Further, the shorter the cycle time, higher is the average strain rate in the interrupted creep tests. A higher average strain rate will give rise to a higher strain over the same period. This is attributed to the recovery taking place during the cooling and heating between two cycles. The effect of additional precipitation during interrupted creep tests depends on the nature of the precipitates. The additional precipitation of beta phase during the cooling and heating between two cycles increases the steady state strain rate in the AZ91D and MRI 153M alloys. whereas the additional precipitation of C36 phase during the cooling and heating between two cycles decreases the steady state strain rate in the MRI 230D alloy. (C) 2009 Elsevier B.V. All rights reserved.

Chemical potentials of oxygen for fayalite-quartz-lron and fayalite-quartz-magnetite equilibria

The oxygen potentials corresponding to fayalite-quartz-iron (FQI) and fayalite-quartz-magnetite (FQM) equilibria have been determined using solid-state galvanic cells: Pt,Fe + Fe2SiO4 + SiO2/(Y2O3)ZrO2/Fe + \r"FeO,\l"Pt and Pt, Fe3O4 + Fe2SiO4 + SiO2/(Y2O3)ZrO2/Ni + NiO, Pt in the temperature ranges 900 to 1400 K and 1080 to 1340 K, respectively. The cells are written such that the right-hand electrodes are positive. Silica used in this study had the quartz structure. The emf of both cells was found to be reversible and to vary linearly with temperature. From the emf, Gibbs energy changes were deduced for the reactions: 0.106Fe (s) + 2Fe0.947O (r.s.) + SiO2 (qz) → Fe2SiO4 (ol) δG‡= -39,140+ 15.59T(± 150) J mol-1 and 3Fe2SiO4 (ol) + O2 (g) → 2Fe3O4 (sp) + 3SiO2 (qz) δG‡ = -471,750 + 160.06 T±} 1100) J mol-1 The “third-law≓ analysis of fayalite-quartz-wustite and fayalite-quartz-magnetite equilibria gives value for δH‡298 as -35.22 (±0.1) and -528.10 (±0.1) kJ mol-1, respectively, independent of temperature. The Gibbs energy of formation of the spinel form of Fe2SiO4 is derived by com-bining the present results on FQI equilibrium with the high-pressure data on olivine to spinel transformation of Fe2SiO4.

Gas turbine diagnostics using a soft computing approach

A fuzzy system is developed using a linearized performance model of the gas turbine engine for performing gas turbine fault isolation from noisy measurements. By using a priori information about measurement uncertainties and through design variable linking, the design of the fuzzy system is posed as an optimization problem with low number of design variables which can be solved using the genetic algorithm in considerably low amount of computer time. The faults modeled are module faults in five modules: fan, low pressure compressor, high pressure compressor, high pressure turbine and low pressure turbine. The measurements used are deviations in exhaust gas temperature, low rotor speed, high rotor speed and fuel flow from a base line 'good engine'. The genetic fuzzy system (GFS) allows rapid development of the rule base if the fault signatures and measurement uncertainties change which happens for different engines and airlines. In addition, the genetic fuzzy system reduces the human effort needed in the trial and error process used to design the fuzzy system and makes the development of such a system easier and faster. A radial basis function neural network (RBFNN) is also used to preprocess the measurements before fault isolation. The RBFNN shows significant noise reduction and when combined with the GFS leads to a diagnostic system that is highly robust to the presence of noise in data. Showing the advantage of using a soft computing approach for gas turbine diagnostics.