ALBERT

Notes: A four-probe technique for electrical and photoelectrical measurements in a gasketed diamond anvil cell is presented. The special anvil profile and the method for the gasket preparation used in these experiments are described. Insulation and continuity of the electrical leads are shown to be satisfactory at least up to 18 GPa. As an example we report photoconductivity measurements in GaAs beyond crossover, under pressure, between direct and indirect band-gap configurations.

Notes: Resistance heating of the gasket strip in a gem-anvil high pressure cell was successful in obtaining sample temperatures up to 1100 °C, under pressures up to 4.0 GPa. The heating capabilities, as well as the mechanical and chemical stability, of several different gasket strips (two Ni-based alloys, Ta, Pt/Rh, and a Re/Mo alloy) with different design shapes, and two different single-crystal anvil materials (diamond and cubic zirconia) were investigated. Two gasket-strip designs were found to provide optimum uniform heating conditions while decreasing the required current needed to achieve 1100 °C. Two anvil systems were investigated to reduce the temperature increase of the pressure cell body. Cubic zirconia anvils reduced the cell-body temperature to 100 °C at sample temperatures up to 1100 °C. However, zirconia anvils often failed during heating and almost always failed during cooling. Diamond anvils with cubic zirconia mounting plates also permitted temperatures up to 1100 °C to be reached without anvil failure. However, the cell-body temperature increased to 300 °C. A sealed vacuum-type chamber was employed to eliminate the problem with gasket and anvil oxidation. The optimized operating parameters reported here provide a routine method for high temperature-high pressure studies. The method was used to densify and sinter nanosize amorphous silicon nitride and γ-alumina powders at high temperatures and high pressures.

Notes: This work describes the characteristics of a bench station designed for both the control of the operational parameters of a unitary, low temperature fuel cell and the data acquisition for performance evaluation. The controlled parameters are the current through the cell, the pressure and the flow of the reactants, and the temperature of the cell and of the gaseous reactants. The system measures the potential drop across the cell and the potential of each electrode with respect to a reference, as a function of the current, which is varied on a predetermined interval. The program plots the results and fits the experimental curve to a theoretical expression, which allows the determination of the cell resistance and the Tafel kinetic parameters of the cell reactions. The test station provides a rapid route to the work with single/half cells in the evaluation of materials and fuel cell performance. The computer controlled procedures, data acquisition, and analyses proposed here substitute with advantages time consuming manual procedures. © 1997 American Institute of Physics.

Notes: The conversion of a commercial dye laser into a narrow-linewidth laser system, tunable over a large frequency range, is described. Its experimental details as well as its application to high resolution laser spectroscopy in molecular beams are reported. The laser linewidth in different configurations was determined. The major advantages of the system described include its low cost and simplicity in construction and performance. © 1996 American Institute of Physics.

Notes: Crystals of germanium or silicon provide monochromatic radiation on synchrotron radiation beamlines. By using asymmetrically cut crystals, it is possible to alter the properties of the beam in position, angle, wavelength space. The results obtained from using two types of asymmetrically cut monochromators are compared with theory. For single crystal focusing monochromators, the asymmetry results in a dispersion term and a resultant broadening of the focal spot. The expression for the broadening given here differs from that given previously for these type of monochromators and has implications for the design of beamlines where the x-ray source has a small size. Asymmetric channel cut monochromators are not commonly used for x-ray diffraction on a synchrotron. However, we show here that, compared with a symmetric monochromator, intensity gains of three or more can be obtained with a simple compact design. © 1995 American Institute of Physics.

Notes: A focused white beam from existing synchrotrons has an intensity comparable with the monochromatic beams available from future undulator beamlines. The beam from station 9.5 on the SRS has therefore been used to examine the radiation damage caused within a protein crystal, cooled to near liquid nitrogen temperature. It was found that observable radiation damage occurred at doses comparable with those which have been observed in cryo-electron microscopy. It would be possible to collect many data sets from the test lysozyme crystal before radiation damage occurred under these conditions. However, it would not be possible to collect a good quality data set from a single protein crystal of a few tens of microns in size, even at liquid nitrogen temperature.

Notes: In this article we present as an experiment in single-photon-decay spectroscopy an alternative method to that based on the measurement of the probability distribution function, Pf(t), of the time intervals for the time of arrival of the first photon after the excitation pulse. We propose the measurement of the sine transform of Pf(t), which gives a maximum when the fluorescence signal is of the single exponential decay type. The existence of this maximum allows us to determine very accurately the value of the decay constant using a number of measurements which is lower than that required to establish a Pf(t) that is accurate enough to draw the same conclusions. A theoretical model of the error is studied and is compared by means of a computer simulation with the usual method of measuring Pf(t). An experiment is performed using a TMMC crystal to verify the validity of the theoretical predictions and of the simulation results.

Notes: An electronic circuit has been designed to sustain the oscillations of a vibrating element. It has been applied to density measurements of fluids with a vibrating tube densimeter. The electronic part of the apparatus has been designed to avoid the presence of elements attached to the tube (such as wires or magnets), since the tube itself performs the excitation and the detection, making possible an easy manufacture. The mechanical part permits the best temperature control and a fast thermal response, since it can be totally immersed in a thermostat. The precision has been tested by calibration with pure substances and applied to density measurements of a colloidal solution (sodium decanoate in water). Relative measurements are obtained with a precision of better than 1×10−6 g cm −3, in the temperature range studied (between 293 and 313 K). © 1997 American Institute of Physics.

Notes: Crystal structure solution by anomalous dispersion methods has been greatly facilitated using the rapidly tunable station 9.5 at the Daresbury SRS. Both SIROAS and MAD techniques, with IP data, have been used in the phasing of a brominated nucleotide and a seleno deaminase, respectively. The electron density maps in each case are interpretable. Throughput of projects could be improved upon with a better duty cycle detector. Another category of data collection is that at very high resolution. Detailed structure refinement pushes the limits of resolution and data quality. Station 9.5 has been used to collect high resolution (1.4 A(ring)) native data for the protein concanavalin A. This utilized very short wavelengths (0.7 A(ring)), the image plate, and crystal freezing. A total of 155 407 measurements from two crystals benefited from the on-line nature of the IP detector device, but a slow and quick pass are required to capture the full dynamic range of the data. There are data seen to 1.2 A(ring) and beyond for a pure Mn substituted form of the protein, but a higher intensity still is required to actually record these data. By comparison, trials at CHESS, on a multipole wiggler (station A1) with a CCD (without image intensifier) system, yield native concanavalin A data to 0.98 A(ring) and beyond. This demonstrates that the combination of yet higher intensity and the ease of use of a CCD offers worthwhile improvements; in this case an increase in the data by a factor of (1.4/0.98)3, thus at least doubling the data to parameter ratio for protein structure model refinement and potentially opening up direct structure determination of proteins of the size of concanavalin A (25 kDa).Finally, possibilities at ESRF and further detector developments, such as mosaic CCDs and scintillator coatings, offer further impetus for the field. These include more intense rapidly tunable beams for anomalous dispersion-based structure solution and "ideal'' higher resolution data collection and reactivity studies. ESRF BL19 is described; facilities on BL19 will include a system for freezing and storing crystals at cryogenic temperatures, so that data can be recorded from the same crystal on different runs. Overall, there have been tremendous strides made in this field in the last 15 years, and yet further improvements are to come. © 1995 American Institute of Physics.

Notes: An automatic high-precision ultrasonic system with the corresponding ultrasonic cell is described to measure simultaneously the speed of sound and the attenuation as a function of frequency. The technique is based on the pulse-echo method and is designed to detect small structural changes in liquids, when an external variable is changed. The precision of the technique is limited by the stability of the temperature, which was kept constant within ±0.5 mK. The method, which permits one to obtain the absolute value of the velocity as well as the attenuation with respect to the solvent, is fully automated for the whole variation of the external variable, which added to its fast response permits easy use in dynamic processes.