Optical cell for in situ vibrational spectroscopic measurements at high pressures and shear

An optical cell is described for performing simultaneous static high-pressure and shear experiments. This cell design is a modification of the previously designed megabar diamond anvil cell used by Mao and Bell that allows for controlled, remote shear. With this diamond anvil cell, it is possible to use a wide range of existing experimental techniques and pressure media. The cell was validated on a sample of calcite at 5 kbar. Raman measurements show the onset of the phase transformation from calcite to aragonite at 10° of rotation.     

Development of a nuclear magnetic resonance system for in situ analysis of hydrogen storage materials under high pressures and temperatures

A NMR system for in situ analysis of hydrogen storage materials under high pressure and temperature conditions was developed. The system consists of a gas pressure and flow rate controlling unit, a temperature controller, a high temperature NMR probe tunable for both (1)H and other nuclei, and a sample tube holder. Sample temperature can be controlled up to 623 K by heated N(2) gas flow. Sample tube atmosphere can be substituted by either H(2) or Ar and can be pressurized up to 1 MPa under constant flow rate up to 100 ml/min. During the NMR measurement, the pressure can be adjusted easily by just handle a back pressure valve. On the blank NMR measurement, (1)H background noise was confirmed to be very low. (1)H and (11)B NMR spectrum of LiBH(4) were successfully observed at high temperature for the demonstration of the system. The intensity of the (1)H NMR spectra of H(2) gas was also confirmed to be proportional to the applied pressure.     

Laser-Doppler vibrating tube densimeter for measurements at high temperatures and pressures

A laser-Doppler vibrometer was used to measure the vibration of a vibrating tube densimeter for measuring P-V-T data at high temperatures and pressures. The apparatus developed allowed the control of the residence time of the sample so that decomposition at high temperatures could be minimized. A function generator and piezoelectric crystal was used to excite the U-shaped tube in one of its normal modes of vibration. Densities of methanol-water mixtures are reported for at 673 K and 40 MPa with an uncertainty of 0.009 g/cm3.     

Sample cell for EXAFS measurements on molten materials at elevated temperatures

We describe the design, fabrication, and utilization of a simple sample cell for extended x-ray absorption fine structure (EXAFS) measurements at elevated temperature. The cell is rigid, inert, easy to fabricate, cheap and effective in maintaining the critical specimen dimensions of both solid and liquid samples up to 1000 degrees C.     

High efficiency multichannel collimator for structural studies of liquids and low-Z materials at high pressures and temperatures

A high efficiency multichannel collimator (MCC) device has been developed at the high pressure beamline ID27 of the European Synchrotron Radiation Facility to drastically reduce the x-ray background from the sample environment in the Paris-Edinburgh press. The main technical difficulty, which resides in the minimum slits size achievable using the classical mono-bloc design, has been resolved using an original concept based on a set of independent slits. Then, a very small slit size of 50 μm was manufactured resulting in a great improvement of the signal to background ratio. In addition, the transfer function of the MCC has been measured using the x-ray diffusion signal of a metal doped glass and efficiently applied to correct the raw data. The potential of this new device is illustrated in two challenging examples: iron-sulfur liquid structures and C(60) polymerization process at high pressure and high temperature.     

High pressure cell for Raman scattering at low temperatures

An optical cell suitable for Raman studies and for absorption studies of gases, liquids, and solids is described. The cell has been used at temperatures down to 4.2 K and pressures up to 10 kilobars. It contains a sample chamber with special scattering geometry and a ruby manometer for in situ pressure measurements. Typical results are given for the impurity induced one phonon Raman spectra of doped alkali halides and for the fundamental vibrational Raman spectra of matrix-isolated molecules under pressure.     

Wire-mesh sensors for high-resolving two-phase flow studies at high pressures and temperatures

Wire-mesh sensors are used for a high-speed visualization of a gas–liquid flow as well as for the measurement of void fraction profiles, bubble size distributions and gas velocity distributions. Recent progress was made in designing and constructing such sensors for an application in a hot steam–water mixture. Two types are presented: (1) a sensor with an inner diameter of 52.3 mm with a measuring matrix of 16×16 and (2) a sensor of 195 mm inner diameter with 64×64 measuring points. Both devices can be operated at 7 MPa and a temperature of max. 286 ^∘C. The spatial and temporal resolutions are equal to earlier used sensors for air–water flow at ambient conditions (3 mm, 2500 fps). In the paper, the function of the sensors is illustrated by presenting flow visualizations obtained at two vertical test sections of the Rossendorf TOPFLOW facility. The pipes are approximately 9 m long and have inner diameters equal to the diameters of the measuring cross sections mentioned above. The results show how the flow structure depends on the thermodynamic parameters by comparing measurements performed at 1, 2, 4 and 6.5 MPa and 180, 212, 250 and 280 ^∘C, correspondingly, under adiabatic conditions with earlier air–water tests.     

Instrument for x-ray magnetic circular dichroism measurements at high pressures

An instrument has been developed for x-ray magnetic circular dichroism (XMCD) measurements at high pressures and low temperatures. This instrument couples a nonmagnetic copper-beryllium diamond anvil cell featuring perforated diamonds with a helium flow cryostat and an electromagnet. The applied pressure can be controlled in situ using a gas membrane and calibrated using Cu K-edge x-ray absorption fine structure measurements. The performance of this instrument was tested by measuring the XMCD spectra of the Gd(5)Si(2)Ge(2) giant magnetocaloric material.     

Simultaneous structure and elastic wave velocity measurement of SiO2 glass at high pressures and high temperatures in a Paris-Edinburgh cell

An integration of multi-angle energy-dispersive x-ray diffraction and ultrasonic elastic wave velocity measurements in a Paris-Edinburgh cell enabled us to simultaneously investigate the structures and elastic wave velocities of amorphous materials at high pressure and high temperature conditions. We report the first simultaneous structure and elastic wave velocity measurement for SiO(2) glass at pressures up to 6.8 GPa at around 500°C. The first sharp diffraction peak (FSDP) in the structure factor S(Q) evidently shifted to higher Q with increasing pressure, reflecting the shrinking of intermediate-range order, while the Si-O bond distance was almost unchanged up to 6.8 GPa. In correlation with the shift of FSDP position, compressional wave velocity (Vp) and Poisson's ratio increased markedly with increasing pressure. In contrast, shear wave velocity (Vs) changed only at pressures below 4 GPa, and then remained unchanged at ~4.0-6.8 GPa. These observations indicate a strong correlation between the intermediate range order variations and Vp or Poisson's ratio, but a complicated behavior for Vs. The result demonstrates a new capability of simultaneous measurement of structures and elastic wave velocities at high pressure and high temperature conditions to provide direct link between microscopic structure and macroscopic elastic properties of amorphous materials.     

Density measurements of noncrystalline materials at high pressure with diamond anvil cell

We describe an x-ray absorption method for in situ density measurement of non-crystalline materials in the diamond anvil cell using a monochromatic synchrotron x-ray microbeam. Sample thickness, which is indispensable in the absorption method, can be determined precisely by extrapolating the thickness profile of the gasket obtained by x-ray absorption and diffraction measurements. Diamond deformation across the sample chamber becomes noticeable at high pressures above 10 GPa, which can be monitored with a precision better than 1%, as demonstrated by measurements on crystalline Ag. We have applied the developed method to measure densities of the classic network-forming GeO(2) glass in octahedral form at pressures up to 56 GPa. The fit to the pressure-volume data with the Birch-Murnaghan equation from 13 to 56 GPa gives parameters of V(0)=23.2+/-0.4 cm(3)mol and K=35.8+/-3.0 GPa, assuming that K(')=4. This method could be applicable for in situ determination of the density of liquids and other noncrystalline materials using a diamond anvil cell up to ultrahigh pressures.     

Combined ultrasonic elastic wave velocity and microtomography measurements at high pressures

Combined ultrasonic and microtomographic measurements were conducted for simultaneous determination of elastic property and density of noncrystalline materials at high pressures. A Paris-Edinburgh anvil cell was placed in a rotation apparatus, which enabled us to take a series of x-ray radiography images under pressure over a 180° angle range and construct accurately the three-dimensional sample volume using microtomography. In addition, ultrasonic elastic wave velocity measurements were carried out simultaneously using the pulse reflection method with a 10° Y-cut LiNbO(3) transducer attached to the end of the lower anvil. Combined ultrasonic and microtomographic measurements were carried out for SiO(2) glass up to 2.6 GPa and room temperature. A decrease in elastic wave velocities of the SiO(2) glass was observed with increasing pressure, in agreement with previous studies. The simultaneous measurements on elastic wave velocities and density allowed us to derive bulk (K(s)) and shear (G) moduli as a function of pressure. K(s) and G of the SiO(2) glass also decreased with increasing pressure. The negative pressure dependence of K(s) is stronger than that of G, and as a result the value of K(s) became similar to G at 2.0-2.6 GPa. There is no reason why we cannot apply this new technique to high temperatures as well. Hence the results demonstrate that the combined ultrasonic and microtomography technique is a powerful tool to derive advanced (accurate) P-V-K(s)-G-(T) equations of state for noncrystalline materials.     

Dynamic properties of outwardly propagating spherical hydrogen-air flames at high temperatures and pressures

Computational experiments on fundamental unstretched laminar burning velocities and flame response to stretch (represented by the Markstein number) of hydrogen-air flames at high temperatures and pressures were conducted in order to understand the dynamics of the flames including hydrogen as an attractive energy carrier in conditions encountered in practical applications such as internal combustion engines. Outwardly propagating spherical premixed flames were considered for a fuel-equivalence ratio of 0.6, pressures of 5 to 50 atm, and temperatures of 298 to 1000 K. For these conditions, ratios of unstretched-to-stretched laminar burning velocities varied linearly with flame stretch (represented by the Karlovitz number), similar to the flames at normal temperature and normal to moderately elevated pressures, implying that the “local conditions” hypothesis can be extended to the practical conditions. Increasing temperatures tended to reduce tendencies toward preferential-diffusion instability behavior (increasing the Markstein number) whereas increasing pressures tended to increase tendencies toward preferential-diffusion instability behavior (decreasing the Markstein number).     

Glass capillary for high resolution NMR measurements at pressures up to 400 MPa

A glass capillary for use in high resolution NMR spectroscopy at pressures up to 400 MPa is described together with the method of construction and use.     

An induction heating diamond anvil cell for high pressure and temperature micro-Raman spectroscopic measurements

A new external heating configuration is presented for high-temperature diamond anvil cell instruments. The supporting rockers are thermally excited by induction from an externally mounted copper coil passing a 30 kHz alternating current. The inductive heating configuration therefore avoids the use of breakable wires, yet is capable of cell temperatures of 1100 K or higher. The diamond anvil cell has no resistive heaters, but uses a single-turn induction coil for elevating the temperature. The induction coil is placed near the diamonds and directly heats the tungsten carbide rockers that support the diamond. The temperature in the cell is determined from a temperature-power curve calibrated by the ratio between the intensities of the Stokes and anti-Stokes Raman lines of silicon. The high-pressure transformation of quartz to coesite is successfully observed by micro-Raman spectroscopy using this apparatus. The induction heating diamond anvil cell is thus a useful alternative to resistively heated diamond anvil cells.     

A robust volumetric apparatus and method for measuring high pressure hydrogen storage properties of nanostructured materials

A volumetric apparatus to measure hydrogen adsorption and desorption at room temperature and up to 100 atm has been constructed and studied for accuracy, reproducibility, and stability. The design principles are presented and considerable attention to detail is given to examine the effects of diurnal temperature changes in the manifold and helium adsorption by carbon-based adsorbents during free volume measurement. A heuristic for helium correction is derived from a model with a basis in literature and verified through calculation of adsorbent density. Several materials with well-known hydrogen capacities are studied to examine reproducibility. The microporous carbon AX-21 is studied to examine the effects of pressure step size and approach to equilibrium caused by gas mixing and the Joule-Thomson effect. Hydrogen spillover on a hybrid material, Pt on templated carbon, is examined for several loadings of metal. Kinetics of both physisorption and spillover are compared via the diffusion time constant (D/R(2)) estimated by fitting models for pore and surface diffusion to time-dependent adsorption profiles. No concentration dependence was found for pore diffusion; however, the surface diffusion time constant was shown to decrease with respect to increasing hydrogen concentration.     

Method for the determination of the specific heat of metals at low temperatures under high pressures

Efforts are made to determine the absolute specific heat of metals as a function of pressure with an accuracy, which, for the first time, will permit a direct evaluation of the pressure variation of the electronic and phonon parameters gamma and vartheta(D'), respectively. This is achieved by employing the ac method of Sullivan and Seidel, and choosing a suitable sample configuration within a piston-cylinder pressure cell. Essential is the use of diamond powder as a pressure transmitting medium, which because of its low heat capacity and high thermal resistance, couples the sample loosely to the temperature bath, represented by the pressure cylinder. Thus, the thermal requirements of the ac method are met, and corrections--if necessary--are fully tractable. High sensitivity even at the lowest temperatures is obtained by the use of thin slices, prepared from standard carbon resistors, for thermometers. Although delicate, these sensors withstand pressures of more than 20 kbars and remain sensitive. A thorough comparison of results on indium with the literature data at zero pressure is undertaken. It shows that the accuracy of the present results is comparable with that from literature data. Preliminary results at pressures up to 8 kbar are shown; they will be discussed in a separate paper.     

Friction and wear behavior of refractory materials at high sliding velocities and temperatures

Various ceramic and cermet materials have been evaluated for unlubricated wear resistance at high sliding speed (100 to 200 ft./sec) and low unit load (5 to 50 p.s.i.) in 1000 to 1800°F air. A statistical correlation was obtained between the measured wear rates under these conditions and the coefficient of friction, the thermal stress resistance, and the thermal diffusivity of the mated materials on which wear predominated. A mechanism of wear has been evolved based on the above correlation and on the experimental study of friction and wear surface-temperature fluctuations using special transducers and color-motion-picture photography. During high-speed sliding, wear appears to be induced by the inability of ceramic and cermet materials to resist thermal stresses produced by temperature gradients within each rubbing surface between small asperities or hot spots in frictional contact and the body of the respective materials. In this situation, the wear rate is influenced both by the configuration of the rubbing parts and by the thermal-stress-resistance properties of the materials. Promising materials for high-temperature high-speed sliding bearings and seals include Al₂O₃—Cr—Mo cermets, SiC ceramics, and TiC—Ni—Mo cermets.     

The interfacial transfer of aluminium to steels and ceramic materials under loaded contact at high temperatures II: Both surfaces at high temperatures

The rolling contact between aluminium strips and rolls of different materials, in which the rolls were heated in situ with an auxiliary furnace to a temperature similar to that of the strips, was studied. Rolls of solid alumina failed during the rolling of the aluminium strip; all the other rolls, including plasma spray-coated rolls, remained undamaged. Examination of the structure and composition of the coatings which developed on the rolls showed that there was more transfer for all the different rolls but that the variations between the rolls were much less than they were when the rolls were at ambient temperatures. The thick coatings formed on the rolls were predominantly aluminium metal and the characteristic structure was independent of the original roll surface structures.     

A remote scanning Raman spectrometer for in situ measurements of works of art

In conservation science, one of the main concerns is to extract information from an artistic surface without damaging it. Raman spectroscopy has emerged in recent years as a reliable tool for the non-destructive analysis of a wide range of inorganic and organic materials in works of art and archaeological objects. Nevertheless, the technique is still mainly limited to the analysis of micro-samples taken from artistic surfaces. The development of an instrument able to perform non-contact analysis of an area of a few square centimeters aims to further increase the employment of this technique. This paper describes the development of a prototype Raman scanning spectrometer based on a diode laser, a 2D scanning mirror stage and a custom optical system, which can map a surface of 6 cm in diameter at a working distance of 20 cm. The device exhibits collecting optics with a depth of field close to 6 cm, which makes the Raman system suitable for the analysis of non-flat surfaces and three-dimensional objects. In addition, the overall dimensions and weight of the instrument have been limited in order to make the device transportable and, in principle, usable for in situ measurements. Details on the design of the device, with particular emphasis on the collecting optical system, and on results of the characterization tests carried out to assess its performances are reported. Finally, an example of an application involving the identification of pigments from a model painting is presented.