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Presents a study that investigated the phase stability and atomic-level compression mechanisms for both silicon oxide cristobalite and for cristobalite partially stabilized by calcium/aluminum doping. Details of the experiment; Results and discussion; Conclusion.

Presents the results of single-crystal electron microprobe analysis of a lanthanum (La) lutetium (Lu) gallium (Ga) garnet (G). Importance of compound in laser-diode-pumped solid-state laser materials; Use of electron microprobe; Information on the atomic coordinates and thermal parameters of LaLuGaG.

Magnesium hydroxyfluoride, Mg(OH)F, has been synthesized by a subcritical hydrothermal route from a 1:1 molar mixture of brucite, Mg(OH)2, and sellaite, MgF2 with a rutile type structure, in excess water. Using a combination of synchrotron X-ray and time-of-flight neutron powder diffraction, the structure of Mg(OH)F has been solved in the diaspore space group Pnma with a = 10.116(3), b = 4.6888(10) and c = 3.0794(7) Å at ambient conditions. The most intense diffraction lines are [dobs (hkl) Iobs]: 2.291 (211) 10, 4.253 (101) 7, 1.747 (212) 7, 2.229 (401) 6 and 1.480 (610) (4) Å, with the largest d-spacing at 5.058 Å. Sharp infrared stretching bands are located at 3679 and 3645 cm-1, with a broader band at 3535 cm-1. The topology of the structure is intermediate between that of the OH and F endmembers, being derived through notional shearing nearly normal to the sheets of octahedra of the CdI2/Mg(OH)2-type structure. Further similar shearing at an interval ½a would lead to a Cd(OH)F-type structure, which is also related to the rutile structure type. The observations and model presented here indicate a close correlation between the structural properties of the endmembers and Mg(OH)F. [ABSTRACT FROM AUTHOR]

Presents information about a simulation of an ethanol?water distillation column assisted by a vapor-compression heat pump. Development of a simulation model to study working fluids R-11 and R-114; Findings that considerable reduction in energy consumption can be achieved with the installation of a heat pump.

The present work introduces a transient endoreversible model of a heat-driven refrigeration plant, which is driven by a fuel-burning heater. The model consists of a combustion chamber with negligible heat loss to the ambient, a refrigerator with three finite-size heat exchangers, namely, the evaporator between the refrigeration load and refrigerant, the condenser between the refrigerant and the ambient, and the generator between the combustion chamber and the refrigerant, and finally the refrigerated space. The total thermal conductance of the three heat exchangers is fixed. A thermodynamic optimization of the absorption cycle is then performed, reporting the operating conditions for minimum time to reach a prescribed cold-space temperature, thus maximum refrigeration rate, specifically, the optimal mass fuel flow rate and the optimal way of allocating the thermal conductance inventory. Half of the total supply of thermal conductance has to be divided equally between the generator and evaporator and the other half allocated to the condenser, for optimal operation. A narrow range of fuel flow rates lead to the minimum time to achieve a prescribed cold-space temperature, thus stressing the importance of the transient analysis. Appropriate dimensionless groups were identified and the generalized results are reported in dimensionless charts. [ABSTRACT FROM AUTHOR]

Explains the use of the pressure generated in a multi-anvil apparatus to affect changes in crystal structure, elastic properties and the stress state in ceramics. Crystallographic studies; Discussions on the strength and rheological properties of ceramic materials; Simultaneous measurements of ultrasonic and X-ray diffraction.

X-ray diffraction measurements of liquid water are reported at pressures up to 360 MPa corresponding to a density of 0.0373 molecules per Å3. The measurements were conducted at a spatial resolution corresponding to Qmax = 16 Å-1. The method of data analysis and measurement in this study follows the earlier benchmark results reported for water under ambient conditions having a density of 0.0333 molecules per Å3 and Qmax = 20 Å-1 [J. Chem. Phys. 138, 074506 (2013)] and at 70°C having a density of 0.0327 molecules per Å3 and Qmax = 20 Å-1 [J. Chem. Phys. 141, 214507 (2014)]. The structure of water is very different at these three different T and P state points and thus they provide the basis for evaluating the fidelity of molecular simulation. Measurements show that at 360 MPa, the 4 waters residing in the region between 2.3 and 3 Å are nearly unchanged: the peak position, shape, and coordination number are nearly identical to their values under ambient conditions. However, in the region above 3 Å, large structural changes occur with the collapse of the well-defined 2nd shell and shifting of higher shells to shorter distances. The measured structure is compared to simulated structure using intermolecular potentials described by both first-principles methods (revPBE-D3) and classical potentials (TIP4P/2005, MB-pol, and mW). The DFT-based, revPBE-D3, method and the many-body empirical potential model, MB-pol, provide the best overall representation of the ambient, high-temperature, and high-pressure data. The revPBE-D3, MB-pol, and the TIP4P/2005 models capture the densification mechanism, whereby the non-bonded 5th nearest neighbor molecule, which partially encroaches the 1st shell at ambient pressure, is pushed further into the local tetrahedral arrangement at higher pressures by the more distant molecules filling the void space in the network between the 1st and 2nd shells. [ABSTRACT FROM AUTHOR]

Here we present diffraction data that yield the oxygen-oxygen pair distribution function, gOO(r) over the range 254.2-365.9 K. The running O-O coordination number, which represents the integral of the pair distribution function as a function of radial distance, is found to exhibit an isosbestic point at 3.30(5) Â. The probability of finding an oxygen atom surrounding another oxygen at this distance is therefore shown to be independent of temperature and corresponds to an O-O coordination number of 4.3(2). Moreover, the experimental data also show a continuous transition associated with the second peak position in gOO(r) concomitant with the compressibility minimum at 319 K. [ABSTRACT FROM AUTHOR]

We present the in situ neutron diffraction data of a water and silica mixture at high pressure and temperature. We show initially the formation of ice VI at 1.5 GPa at 290 K in the presence of crystalline SiO2, upon heating we observe its melting at ∼400 K. Upon further warming to 1200 K, we observe melting of the crystalline silica. Upon cooling to 290 K and recovery to ambient pressure, we obtain a mixture of silica in the coesite structure and liquid water. These results have implications for the phase diagram of the coesite–water solidus and hence the behaviour of fluids at mantle conditions. [ABSTRACT FROM AUTHOR]

The pressure induced [formula] phase transition in BaTiO3 perovskite was investigated by x-ray total scattering. The evolution of the structure was analyzed by fitting pair distribution functions over a pressure range from ambient pressure up to 6.85(7) GPa. Evidence for the existence of tetragonal ferroelectric nanodomains at high pressure was found. The average size of the nanodomains in the high-pressure phase decreases with increasing pressure. Extrapolation of the domain size to pressures higher than studied experimentally suggests a disappearance of the ferroelectric domains at about 9.3(5) GPa and a cubic symmetry of BaTiO3 high-pressure phase. [ABSTRACT FROM AUTHOR]

The relationship between local structure and dynamics is explored for molten sodium carbonate. A flexible fluctuating-charge model, which allows for changes in the shape and charge distribution of the carbonate molecular anion, is developed. The system shows the evolution of highly temperature-dependent complex low-dimensional structures which control the dynamics (and hence the liquid fragility). By varying the molecular anion charge distribution, the key interactions responsible for the formation of these structures can be identified and rationalized. An increase in the mean charge separation within the carbonate ions increases the connectivity of the emerging structures and leads to an increase in the system fragility. [ABSTRACT FROM AUTHOR]

Uranium dioxide (UO2) is the major nuclear fuel component of fission power reactors. A key concern during severe accidents is the melting and leakage of radioactive UO2 as it corrodes through its zirconium cladding and steel containment. Yet, the very high temperatures (>3140 kelvin) and chemical reactivity of molten UO2 have prevented structural studies. In this work, we combine laser heating, sample levitation, and synchrotron x-rays to obtain pair distribution function measurements of hot solid and molten UO2. The hot solid shows a substantial increase in oxygen disorder around the lambda transition (2670 K) but negligible U-O coordination change. On melting, the average U-O coordination drops from 8 to 6.7 ± 0.5. Molecular dynamics models refined to this structure predict higher U-U mobility than 8-coordinated melts. [ABSTRACT FROM AUTHOR]

The complete set of partial pair distribution functions for a rare earth oxide liquid are measured by combining aerodynamic levitation, neutron and x-ray diffraction on Y2O3, and Ho2O3 melts at 2870 K. The average Y-O (or Ho-O) coordination of these isomorphic melts is measured to be 5.5(2), which is significantly less than the octahedral coordination of crystalline Y2O3 (or Ho2O3). Investigation of La2O3, ZrO2, and Al2O3 melts by x-ray diffraction and molecular dynamics simulations also show lower-than- crystal cation-oxygen coordination. These measurements suggest a general trend towards lower coordination compared to their crystalline counterparts. It is found that the coordination drop is larger for lower field strength, larger radius cations and is negligible for high field strength (network forming) cations, such as SiO2. These findings have broad implications for predicting the local structure and related physical properties of metal-oxide melts and oxide glasses. [ABSTRACT FROM AUTHOR]

We have performed neutron diffraction isotopic substitutionexperimentson aerodynamically levitated droplets of CaSiO3, to directlyextract intermediate and local structural information on the Ca environment.The results show a substantial broadening of the first Ca–Opeak in the pair distribution function of the melt compared to theglass, which comprises primarily of 6- and 7-fold coordinated Ca-polyhedra.The broadening can be explained by a redistribution of Ca–Obond lengths, especially toward longer distances in the liquid. Thefirst order neutron difference function provides a test of recentmolecular dynamics simulations and supports the MD model which containsshort chains or channels of edge shared Ca-octahedra in the liquidstate. It is suggested that the polymerization of Ca-polyhedra isresponsible for the fragile viscosity behavior of the melt and theglass forming ability in CaSiO3. [ABSTRACT FROM AUTHOR]

The high-pressure behavior of nanocrystalline mackinawite (FeS) with particle sizes of 6, 7 and 8 nm has been investigated by high-energy X-ray total scattering and pair distribution function analysis. An irreversible first-order structural phase transition from tetragonal mackinawite to orthorhombic FeS-II was observed at about 3 GPa. The transition is induced by the closure of the van der Waals gap in the layered mackinawite structure. A grain size effect on the transition pressure and the compressibility was observed. [ABSTRACT FROM AUTHOR]

This paper presents a contribution to understanding the behavior of solar-powered air conditioning and refrigeration systems with a view to determining the manner in which refrigeration rate, mass flows, heat transfer areas, and internal architecture are related. A cogeneration system consisting of a solar concentrator, a cavity-type receiver, a gas burner, and a thermal storage reservoir is devised to simultaneously produce heat (hot water) and cooling (absorption refrigerator system). A simplified mathematical model, which combines fundamental and empirical correlations, and principles of classical thermodynamics, mass and heat transfer, is developed. The proposed model is then utilized to simulate numerically the system transient and steady-state response under different operating and design conditions. A system global optimization for maximum performance (or minimum exergy destruction) in the search for minimum pull-down and pull-up times, and maximum system second law efficiency is performed with low computational time. Appropriate dimensionless groups are identified and the results are presented in normalized charts for general application. The numerical results show that the three-way maximized system second law efficiency, ηII,max,max,max, occurs when three system characteristic mass flow rates are optimally selected in general terms as dimensionless heat capacity rates, i.e. (ψss, ψwxwx, ψHs)opt=(0.335, 0.28, 0.2). The minimum pull-down and pull-up times, and maximum second law efficiencies found with respect to the optimized operating parameters are sharp and, therefore, important to be considered in actual design. As a result, the model is expected to be a useful tool for simulation, design, and optimization of solar energy systems in the context of distributed power generation. Copyright © 2008 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The crystal structure of deuterated topaz [topaz-OD; Al2SiO4(OD)2], synthesized at 10 GPa and 800 °C, has been determined using neutron powder diffraction at pressures up to 7.5 GPa. The linear axial compressibilities obtained from regressions of the lattice constants vs. pressure are βa = 1.87(1) x 10-3 GPa-1, βb = 1.71(1) x 10-3 GPa-1, and βc = 2.73(1) x 10-3 GPa-1. The occupancy of the D1 site was found to be greater than that of D2, as shown independently using neutron diffraction and infrared spectra at ambient conditions. A bifurcated hydrogen bond involving the D1 site, O4-D1···O2 and O4-D1···O3, and a trifurcated hydrogen bond involving D2 site, O4-D2···O1, O4-D2···O2, and O4-D2···O4 are proposed for hydrogen-bond donor and acceptor pairs in addition to those reported previously. The observed pressure dependences of the hydrogen-bonding geometry show that these donor and acceptor pairs are classifiable into two types of interaction: (1) those that strengthen as a function of pressure (O4-D1···O3, O4-D2···O2, and O4-D2···O4) and (2) those that weaken (O4-D1···O1 and O4-D2···O1). These results also demonstrate that the reason for the contrasting behavior of the ν(OH) between F-rich natural topaz and topaz-OH are both the cooperative effect, O4-D2···O4-D1···O3, and the increasing Al-O4 distance. [ABSTRACT FROM AUTHOR]

One of the most vexing problems facing the nuclear industry and countries with nuclear weapons is the safe disposal of the generated nuclear waste. Huge quantities of nuclear waste arising from weapons manufacture are stored at the Hanford and Savannah River sites in the USA. The general method of remediation involves the removal of Cs-137, Sr-90 and actinides from a huge quantity of salts, principally NaNO3, organics and complexing agents. It has been found that a sodium silicotitanate is able to remove Cs+ selectively from the waste and certain sodium titanates remove Sr2+ and actinides. These compounds have been prepared by ex-situ hydrothermal methods. We have studied the in situ growth of these materials at the National Synchrotron Light Source, Brookhaven National Laboratory. In addition we will describe the mechanism of ion exchange in the titanosilicate as observed by in situ methods and how the combination of these techniques coupled with an intimate knowledge of the structure of the solids is helping to solve the remediation process. In general, the in situ method allows the investigator to follow the nucleation and crystal growth or phase transformations occurring in hydrothermal reactions. [ABSTRACT FROM AUTHOR]

OH stretching vibration modes for F-rich natural topaz (F-topaz) and for fully hydrated topaz (topaz-OH) synthesized at high pressure, were observed using IR and Raman spectroscopies at pressures up to 30.4 GPa and 17.3 GPa, respectively. In F-topaz, the pressure derivative of the frequency of the OH stretching band observed at 3650 cm-1 at ambient pressure was 0.91(3) cm-1/GPa, which was consistent with the value recently reported by Bradbury and Williams (2003). On the other hand, in topaz-OH, the pressure derivatives of the bands initially at 3599 and 3522 cm-1 were-5.2(2) and -2.56(6) cm-1/GPa, respectively. This contrasting behavior between the two forms of topaz at high pressures suggests that the OH substitution for F in topaz affects the hydrogen-bonding behavior under high pressure. [ABSTRACT FROM AUTHOR]

The present paper is concerned with the numerical simulation of an ethylene re-liquefaction plant. The plant is part of a petrochemical complex, set up in the city of Aratu, state of Bahia, Brazil. The cooling effect is obtained through a two-stage cascade cycle, involving two circuits with different working fluids: ammonia in the high pressure side, and ethylene is the low pressure. The main components of thesystem include: two-stage double acting reciprocating compressors, shell-and-tube heat exchanger, store and flash tanks and control valves. The simulation model was developed by considering the energy and mass conversion laws, the empirical heat transfer coefficient relations, heat exchanger and compressor theories and the thermophysical properties of the working fluids. The result was a set of nonlinear equations solved by a sequential modular technique. Predicted thermodynamic parameters (temperature, pressure and mass flow rate), at differentpoints of the system, were compared with the available plant operational data, with good agreement being obtained. A parametric analysis of the plant was also carried out. [ABSTRACT FROM AUTHOR]

Examines the linear and nonlinear optical properties of the solid solution potassium titanyl phosphate isomorphs. Growth of domain crystals by high temperature solution growth technique; Implication of compositional parameter for the optical birefringence of crystals; Analysis on the non-Vegard dependence of the crystal birefringence and cell constants.

Monoclinic and triclinic high-pressure phases of KHCO3 were identified using in situ high-pressure single-crystal X-ray analysis. These monoclinic and triclinic phases, designated as phases IV and V, respectively, differ from three previously identified phases: I-III. The lattices of the two phases are superimposed along a*IV = 2 x a*IV and hk0 and h0l nets of both phases lie on the same plane, i.e., the (100) plane is common in both lattices in real space. The space group of phase IV is P21/b11 with lattice constants of a = 10.024(3) Å, b = 6.912(5) Å, c = 4.1868(11) Å, α = 115.92(4)°, and V = 260.9(2) Å3. The crystal structure of phase IV, excluding the hydrogen atoms, was successfully determined by direct methods and is isostructural with a cesium hydrogen carbonate CsHCO3. [ABSTRACT FROM AUTHOR]