In situ Raman spectroscopic study of the dissociation of methane hydrate at high pressure

The dissociation of methane hydrate at high pressure is studied by in situ Raman spectroscopy in a diamond-anvil cell. As for the Raman spectrum of sI methane hydrate, the v(1) band of CH(4) is split into two peaks v'(1) and v'(1), indicating the partitioning of CH(4) between the large (5(12)6(2)) and small (5(12)) cavities, respectively. With increasing temperature, the intensity ratio of Iv'(1)/Iv'(1) decreases obviously, and the d(Iv'(1)/Iv'(1))/dT is -0.079 K(-1). Additionally, the v(1) band of the dissolved CH(4) is close to v'(1) of the CH(4) molecule engaged in the small 5(12) cavity. This implies that, in the initial stage of hydrate formation, the abundance of small 5(12) cavities is greater than that of large 5(12)6(2) cavities.     

Raman studies of selenium nanowires under high pressure

The selenium nanowires with diameter of 70 nm and length of 40 μm were synthesized by a facile solution method. High-pressure behavior of Se nanowires has been investigated by in situ Raman scattering up to 20.2 GPa at room temperature. A reversible phase transition from hexagonal to monoclinic occurs at 18.1 GPa. This transition pressure is higher than that of 14.0 GPa for bulk Se. The intrinsic geometry and/or the increasing energy band gap of Se nanowires are considered to contribute to the increase of transition pressure.     

Raman spectroscopic studies of the phase transitions in hexane at high pressure

Raman spectroscopic study of n-hexane was carried out in a cubic zirconia anvil cell up to approximately 2.0 GPa. Under high pressure, the C-H stretching region of the spectrum at 2850-3000 cm(-1) shows measurable changes in frequency, bandwidth, and intensity. These Raman bands shift towards higher frequencies with increasing pressure. At about 1.4 GPa, phase transition from liquid to solid was induced by compression, as was simultaneously observed with the built-in microscope.     

In situ deposition of metal coatings

Ceramic powder particles were metal coated by in situ deposition from decomposable compounds. The ceramic powder and a decomposable metal compound were mixed by tumbling and then heat treated in argon or in hydrogen to deposit the metal. Cermets with continuous metal matrices were obtained by hot pressing metal-coated powders or by in situ decomposition to form the metal during hot pressing. Tungsten-coated Eu₂O₃ powders were obtained by thermal decomposition of W(CO)₆ or by hydrogen reduction of WO₃. Europia-tantalum cermets were made by decomposition of tantalum hydride during hot pressing. Particles of Eu₂O₃ were coated with rhenium by decomposition of ReCl₃ or ReO_x and with molybdenum from the oxide. Cobalt and platinum coatings were deposited on ZrO₂-Y₂O₃ particles by decomposition of the chlorides. The uniformity of metal deposition depends on the mixing and on the particle size of the decomposable compound. The concurrent chemical reaction appears to enhance distribution of the metal by surface diffusion during the heat treatment that results in deposition. The extent of adherence or chemical bonding can be varied through the rate of deposition.     

In situ surface-enhanced Raman scattering analysis of biofilm

Biofilms represent the predominant form of microbial life on Earth. They are aggregates of microorganisms embedded in a matrix formed by extracellular polymeric substances (EPS). Detailed information about chemical composition and structure of the EPS matrix is relevant e.g. for the optimization of biocides, of antifouling strategies and for biological wastewater treatment. Raman microscopy (RM) is a capable tool that can provide detailed chemical information about biofilm constituents with spatial resolution of optical microscope. However, the sensitivity of RM is limited. Surface-enhanced Raman scattering (SERS), which enables investigations of biomolecules at very low concentration levels, allows overcoming this drawback. To our knowledge, this paper is the first report on reproducible SERS spectra from different constituents of a multispecies biofilm. We believe that the reproducibility is partly owed to the in situ measurement of the biofilm, while up to now SERS measurements of microbiological samples by RM were carried out after sample drying. We employed colloidal silver nanoparticles for in situ SERS measurements by RM. The achieved enhancement factor of up to 2 orders of magnitude illustrates a high potential of SERS for ultrasensitive chemical analysis of biofilms, including the detection of different components and the determination of their relative abundance in the complex biofilm matrix.     

Methodology of materials discovery in complex metal hydrides using experimental and computational tools

We present a review of the experimental and theoretical methods used in the discovery of new metal–hydrogen materials systems for hydrogen storage applications. Rather than a comprehensive review of all new materials and methods used in the metal hydride community, we focus on a specific subset of successful methods utilizing theoretical crystal structure prediction methods, computational approaches for screening large numbers of compound classes, and medium-throughput experimental methods for the preparation of such materials. Monte Carlo techniques paired with a simplified empirical Hamiltonian provide crystal structure candidates that are refined using density functional theory. First-principle methods using high-quality structural candidates are further screened for an estimate of reaction energetics, decomposition enthalpies, and determination of reaction pathways. Experimental synthesis utilizes a compacted-pellet sintering technique under high-pressure hydrogen at elevated temperatures. Crystal structure determination follows from a combination of Rietveld refinements of diffraction patterns and first-principles computation of total energies and dynamical stability of competing structures. The methods presented within are general and applicable to a wide class of materials for energy storage.     

In situ synthesis of Al3Niw/Al composites under high pressure

Abstracts are not published in this journal     

Probing oxygen ordering in YBCO byin situ high temperature resistivity and X-ray diffraction studies

In the high temperature superconducting material yttrium barium copper oxide (YBCO) the basal plane oxygen plays a dominant role in determining the superconducting properties of the material. We have carried outin situ high temperature measurements of (a) electrical resistivity and (b) X-ray diffraction in vacuum as well as in helium atmospheres to probe the nature of ordering of oxygen atoms. While published phase diagram of temperature vs oxygen concentration in different partial pressures allow only a tetragonal phase for the annealing temperatures we have investigated, the experiments point to the coexistence of orthorhombic (OII) and tetragonal phases.     

A high temperature high purity source for metal beam epitaxy

We report the first use of modulated beam mass spectrometry to assess the purity of metal beams generated from two types of Knudsen effusion oven used in metal beam epitaxy. One of these types contained sapphire components in contact with tantalum heater windings. In this case considerable contamination of gold and silver beams by Al₂O and aluminum was observed over the temperature range 1000–1400°C. Thermodynamic considerations support the view that these contaminants originated from contact reduction of the sapphire by the tantalum metal. The condensation of these impurities on an (001) indium phosphide surface at not greater than 40°C was detected by Auger electron spectroscopy.In the second type of oven all oxide components were replaced by pyrolytic boron nitride and the tantalum windings by molybdenum windings. Gold and silver beams generated from this oven were of high purity with no detectable oxide species or other beam contaminants. Aluminium beams contained Al₂O as a transitory beam impurity due to atmospheric oxidation of the aluminium charge prior to loading.The implications of the beam impurities from the first type of source are discussed, and the performance of the second type of source in generating epitaxial silver, aluminium and gold films on indium phosphide at growth temperatures not greater than 40°C is reviewed.     

In situ high temperature X-ray diffraction study of UO2 nanoparticles

Nanocrystallites of UO₂ with a size of 3–5 nm were studied in situ with high temperature X-ray diffraction (HT-XRD), thermogravimetry (TGA), and differential thermal analysis. The evolution of the crystallite size, the lattice parameter, and the strain were determined from ambient temperature up to 1200 °C. Below 700 °C, a weak effect on the crystallite size occurs and it remains below 10 nm, while a strong expansion of the lattice parameter is measured. The strain decreases with temperature and is completely released at 700 °C. Above this temperature, begins the sintering of the nanocrystallites reaching a size of about 80 nm at 1200 °C. The weight loss curve observed in TGA is assigned to the desorption of water molecules and is correlated with the strain evolution observed by HT-XRD. The linear thermal expansion and the thermal expansion coefficient at 800 °C are 1.3% and 16.9 × 10⁻⁶ °C⁻¹, respectively.     

Behaviour of B-N-Si under high pressure and high temperature

The phase relations of the B-N-Si system have been studied using a quenching method up to 10GPa and 2000 °C using a high-pressure apparatus of the octahedral anvil type. Pressure-temperature conditions for obtaining z-BN (diamond analogue of boron nitride) were delineated for turbostratic BN (t-BN), t-BN/amorphous Si₃N₄ and t-BN/-Si₃N₄. These conditions shift toward higher regimes of temperature as amorphous Si₃N₄ or -Si₃N₄ is incorporated into t-BN. Spontaneous sintering occurringin situ at high pressure yields z-BN-based composite compacts.     

In situ measurements of Raman scattering in clear ocean water

We have further developed and improved the prototype oceanic Fraunhofer line discriminator by using a well-protected fiber-optic-wire cable and in-water electronic housing. We conducted a series of in situ measurements in clear ocean water in the Florida Straits. By comparing the reduced data with the Monte Carlo simulation results, we verify the Raman scattering coefficient B(r) with an excitation wavelength at 488 nm to be 2.6 x 10(-4)m(-1) [Appl. Opt. 29, 71-84 (1990)], as opposed to 14.4 x 10(-4) m(-1) [Appl. Opt.14, 2116-2120 (1975)]. The wavelength dependence of the Raman scattering coefficient is found to have an insignificant effect on the in-water light field. We also discuss factors that lead to errors. This study can be used as a basis for inelastic light scattering in the radiative transfer theory and will allow other inelastic light, e.g., fluorescence, to be detected with in situ measurements.     

In situ Raman analysis of gas formation in NiMH batteries

In this paper Raman spectrometry is introduced in the field of sealed battery research for in situ gas-phase analysis and for longterm measurements. For this purpose, a new method was successfully applied in order to model battery behavior without interfering with operation. It is shown that oxygen, hydrogen, and nitrogen are responsible for the pressure increase that occurs during overcharging. The relative contribution of the different gases depends on the current imposed on the battery as well as the operating temperature. Reproducible and stable signals could be obtained even under severe conditions such as high pressure and elevated temperature. Oxygen and hydrogen are produced in side reactions taking place during battery operation. However, as nitrogen is unlikely to be a reacting gas inside the battery, the change in its partial pressure can be attributed to electrode expansion and a change in the electrolyte volume.     

Patterning polymer metal precursors for high temperature superconducting Bi-Sr-Ca-Cu-O-films

Bi-Sr-Ca-Cu-O HTSC thin films were prepared from polymer metal precursors in a special annealing procedure. The precursor films can be patterned before the annealing process in which the superconducting phase is generated. We have investigated the patterning of precursor films by UV radiation, and the thermal degradation process by thermogravimetry and mass spectroscopy. The HTSC films were characterized by X-ray diffraction and electrical resistivity measurements.     

Synthesis and properties of novel materials at high pressure and temperature—molecular dynamics simulation studies

Molecular dynamics simulations, in combination with lattice dynamics studies, based on semiempirical interatomic potentials, have been very useful in the study of properties of complex novel materials at high temperature and pressure. Various properties such as the equation of state, elastic and thermodynamic properties, phase transitions and melting have been studied. These studies help in understanding the synthesis of important new and novel materials, especially the amorphous materials, compounds with unusually coordinated atoms, (e.g. with five-coordinated silicon atoms), materials with controlled thermal expansion, etc. A few examples will be discussed from our recent studies.     

In situ raman spectroscopy studies of metal ion complexation by 8-hydroxyquinoline covalently bound to silica surfaces

Raman spectroscopy is applied to an investigation of the interfacial chemistry of silica-immobilized 8-hydroxyquinoline (8HQ) for binding of metal ions over a wide range of solution conditions. Since the derivatized silica has a high specific binding capacity, the mass of silica equilibrated with solution needs to be small for studies of reactions with trace-level (microM) metal ions; otherwise, the solution volume required to reach equilibrium becomes excessive. To address this problem, a small-volume flow cell is designed for this work using a fiber-optic Raman probe inserted directly into the packed end of a microcolumn, allowing excitation and collection of Raman scattering from less than 10 mg of derivatized silica. This cell is attached to a flow system that allows control of solution conditions while the response of the 8HQ-silica material is acquired by continuous monitoring of Raman scattering from the sample. Raman spectra of the deprotonated, neutral, protonated, and copper-complexed forms of the ligand can be distinguished, allowing proton-transfer and metal ion binding reactions of the ligand to be investigated. To account for the effects of changing surface potential on these reactions, zeta-potential measurements are made on the 8HQ-silica particles under the same solution conditions that are employed in the Raman scattering measurements. The observed pH dependence of metal ion binding was corrected for the effect of surface potential using the Boltzmann equation, and the resulting equilibrium constant for binding of Cu2+ was independent of metal ion concentration over a 100-fold range from 30 microM to 5 mM.