A novel fuel cell design foroperandoenergy-dispersive x-ray absorption measurements.

A polymer electrolyte fuel cell has been designed to allowoperandox-ray absorption spectroscopy (XAS) measurements of catalysts. The cell has been developed to operate under standard fuel cell conditions, with elevated temperatures and humidification of the gas-phase reactants, both of which greatly impact the catalyst utilisation. X-ray windows in the endplates of the cell facilitate collection of XAS spectra during fuel cell operation while maintaining good compression in the area of measurement. Results of polarisation curves and cyclic voltammograms showed that theoperandocell performs well as a fuel cell, while also providing XAS data of suitable quality for robust XANES analysis. The cell has produced comparable XAS results when performing a cyclic voltammogram to an establishedin situcell when measuring the Pt LIII edge. Similar trends of Pt oxidation, and reduction of the formed Pt oxide, have been presented with a time resolution of 5 s for each spectrum, paving the way for time-resolved spectral measurements of fuel cell catalysts in a fully-operating fuel cell.

Carbon nitrides: synthesis and characterization of a new class of functional materials.

Carbon nitride compounds with high N : C ratios and graphitic to polymeric structures are being investigated as potential next-generation materials for incorporation in devices for energy conversion and storage as well as for optoelectronic and catalysis applications. The materials are built from C- and N-containing heterocycles with heptazine or triazine rings linked via sp2-bonded N atoms (N(C)3 units) or -NH- groups. The electronic, chemical and optical functionalities are determined by the nature of the local to extended structures as well as the chemical composition of the materials. Because of their typically amorphous to nanocrystalline nature and variable composition, significant challenges remain to fully assess and calibrate the structure-functionality relationships among carbon nitride materials. It is also important to devise a useful and consistent approach to naming the different classes of carbon nitride compounds that accurately describes their chemical and structural characteristics related to their functional performance. Here we evaluate the current state of understanding to highlight key issues in these areas and point out new directions in their development as advanced technological materials.

High pressure polymorphism of ß-TaON.

The high pressure behavior of TaON was studied using a combination of Raman scattering, synchrotron X-ray diffraction, and X-ray absorption spectroscopy in diamond anvil cells to 70 GPa at ambient temperature. A Birch-Murnaghan equation of state fit for baddeleyite structured ß-TaON indicates a high bulk modulus value Ko = 328 ± 4 GPa with K = 4.3. EXAFS analysis of the high pressure XAS data provides additional information on changes in the Ta-(O,N) and Ta-Ta distances. Changes in the X-ray diffraction patterns and Raman spectra indicate onset of a pressure induced phase transition near 33 GPa. Our analysis indicates that the new phase has an orthorhombic cotunnite-type structure but that the phase transition may not be complete even by 70 GPa. Similar sluggish transformation kinetics are observed for the isostructural ZrO2 phase. Analysis of compressibility data for the new cotunnite-type TaON phase indicate a very high bulk modulus Ko ∼ 370 GPa, close to the theoretically predicted value.

Voltage sensitive dye imaging of transient neuronal assemblies in brain slices under hyperbaric conditions.

We have developed a high pressure optical cell to study large-scale transient neuronal coalitions--"assemblies" using voltage sensitive dye (VSD) fluorescence combined with fast CCD imaging of brain slices under hyperbaric conditions. The new cell has been tested at pressures up to P = 150-200 atm, corresponding to the range over which effects such as "pressure-reversal" of anaesthesia have been described previously. Brain slices were maintained in a flow of artificial cerebrospinal fluid (aCSF) and hyperbaric conditions were controlled to within +/- 0.2 atm using a back-pressure regulator placed in the pumping system. Preliminary VSD imaging experiments were carried out on rat hippocampal slices at pressures up to P approximately 50 atm. An electrode placed in the CA3 region was used to stimulate a signal along the Schaffer collateral towards CA1. First results indicate that good VSD data can be obtained that can be analysed to provide a new view on how hydrostatic pressurisation affects the dynamic propagation of neuronal assemblies.

X-ray and neutron diffraction studies and MD simulation of atomic configurations in polyamorphic Y2O3-Al2O3 systems.

Supercooled liquids in the Y(2)O(3)-Al(2)O(3) system undergo a liquid-liquid phase transition between a high-temperature, high-density amorphous (HDA) polymorph form and one with lower density (LDA form) that is stable at lower temperature. The two amorphous polymorphs have the same chemical composition, but they differ in their density (ca. 4% difference) and in their heat content (enthalpy) and entropy determined by calorimetry. Here we present new results of structural studies using neutron and high-energy X-ray diffraction to study the structural differences between HDA and LDA polyamorphs. The combined datasets show no large differences in the average nearest-neighbour Al-O or Y-O bond lengths or coordination numbers between the low- and high-density liquids. However, the data indicate that substantial changes occur among the packing geometries and clustering of the Al-O and Y-O coordination polyhedra, i.e. within the second-nearest-neighbour shell defined by the metal-metal (i.e. Y-Y, Y-Al, Al-Al) interactions. Polarizable ion model molecular dynamics simulations of Y(2)O(3)-Al(2)O(3) liquids are used to help interpret the pair-correlation functions obtained from X-ray and neutron scattering data. Unexpectedly large density fluctuations are observed to occur during the simulation of nominally equilibrated systems. These are interpreted as being due to dynamic sampling of high- and low-density configurations within the single-phase liquid at temperatures above the critical point or phase transition line. Calculated partial radial distribution functions indicate that the primary differences between HDA and LDA configurations occur among the Y-Y correlations.

Pressure-induced amorphization and an amorphous-amorphous transition in densified porous silicon.

Crystalline and amorphous forms of silicon are the principal materials used for solid-state electronics and photovoltaics technologies. Silicon is therefore a well-studied material, although new structures and properties are still being discovered. Compression of bulk silicon, which is tetrahedrally coordinated at atmospheric pressure, results in a transition to octahedrally coordinated metallic phases. In compressed nanocrystalline Si particles, the initial diamond structure persists to higher pressure than for bulk material, before transforming to high-density crystals. Here we report compression experiments on films of porous Si, which contains nanometre-sized domains of diamond-structured material. At pressures larger than 10 GPa we observed pressure-induced amorphization. Furthermore, we find from Raman spectroscopy measurements that the high-density amorphous form obtained by this process transforms to low-density amorphous silicon upon decompression. This amorphous-amorphous transition is remarkably similar to that reported previously for water, which suggests an underlying transition between a high-density and a low-density liquid phase in supercooled Si (refs 10, 14, 15). The Si melting temperature decreases with increasing pressure, and the crystalline semiconductor melts to a metallic liquid with average coordination approximately 5 (ref. 16).

Novel broken symmetry phase from N(2)O at high pressures and high temperatures.

Simple molecular solids become unstable at high pressures, typically transforming to dense framework and/or metallic structures. We report formation of an unusual ionic solid NO(+)NO(3)(-) (nitrosonium nitrate) from N(2)O at pressures above 20 GPa and temperatures above 1000 K. Synchrotron x-ray diffraction indicates that the compound crystallizes with a structure related to the aragonite form of CaCO(3) and NaNO(3). Raman and infrared spectroscopic data indicate that the structure is noncentrosymmetric and exhibits a strong pressure dependent charge transfer and orientational order.

High-pressure bulk synthesis of crystalline C(6)N(9)H(3).HCl: a novel c(3)n(4) graphitic derivative.

A novel carbon nitride compound, structurally related to the proposed graphitic phase of C(3)N(4), has been synthesized in a bulk well-crystallized form. The new material, with stoichiometry C(6)N(9)H(4)Cl, was prepared through a solid-state reaction of 2,4,6-triamino-1,3,5-triazine with 2,4,6-trichloro-1,3,5-triazine at 1.0-1.5 GPa and 500-550 degrees C and also through a self-reaction of 2-amino-4,6-dichloro-1,3,5-triazine at similar conditions. X-ray and electron diffraction measurements on the yellowish compound indicate a hexagonal space group (P6(3)/m) with cell parameters of a = 8.4379(10) A and c = 6.4296(2) A. This new compound possesses a two-dimensional C(6)N(9)H(3) framework that is structurally related to the hypothetical P6m2 graphitic phase of C(3)N(4), but with an ordered arrangement of C(3)N(3) voids. The large voids in the graphene sheets are occupied by chloride ions with an equivalent number of nitrogen atoms on the framework protonated for charge balance. The composition of the sample was determined by bulk chemical analysis and confirmed by electron energy loss (EELS) spectroscopy. The chemical and structural model is consistent with bulk density measurements and with the infrared and (13)C NMR spectra. This work represents the first bulk synthesis of a well-characterized and highly crystalline material containing a continuous network of alternating carbon and nitrogen atoms.

Hydrogen in stishovite, with implications for mantle water content.

Stishovite, the highest pressure polymorph of silicon dioxide, may be an important mineral in some regions of the Earth's mantle. Fourier transform infrared spectroscopy has been used to determine the hydrogen content of synthetic stishovite. The concentration of hydrogen depends on the aluminum content of the sample and reaches a maximum of 549 +/- 23 hydrogen atoms per 10(6) silicon atoms for an Al(2)O(3) content of 1.51 percent by weight. Stishovite could be a storage site for water in deep subducting slabs and in regions of the mantle that are too hot for hydrous minerals to be stable.

Magnesium and Calcium Aluminate Liquids: In Situ High-Temperature 27Al NMR Spectroscopy.

The use of high-temperature nuclear magnetic resonance (NMR) spectroscopy provides a means of investigating the structure of refractory aluminate liquids at temperatures up to 2500 K. Time-averaged structural information indicates that the average aluminum coordination for magnesium aluminate (MgAl(2)O(4)) liquid is slightly greater than for calcium aluminate (CaAl(2)O(4)) liquid and that in both liquids it is close to four. Ion dynamics simulations for these liquids suggest the presence of four-, five-, and six-coordinated aluminate species, in agreement with NMR experiments on fast-quenched glasses. These species undergo rapid chemical exchange in the high-temperature liquids, which is evidenced by a single Lorentzian NMR line.