ALBERT

Notes: A falling head permeameter is described in which pairs of infrared emitters and detectors on a sight tube are used to measure the flow rate associated with the passage of water through a granular solid under the action of a diminishing pressure head. An equation relating pressure head to elapsed time is derived from which permeability may be calculated. In order to verify the accuracy and sensitivity of the instrument, permeability measurements carried out on a graded quartz sand are compared to those obtained by the more conventional constant head measurement. Excellent agreement is obtained between the permeability values obtained using both measurement methods. Experimental results are also reported for the measurement of the permeability of a range of sieved sand fractions. The falling head permeameter described here is particularly suitable for the measurement of the hydraulic conductivity of granular solids such as sands and soils through which a high flow rate may be expected. © 2000 American Institute of Physics.

Notes: Well-sintered snow examined in a scanning electron microscope revealed a newly observed morphological structure that protrudes into the pore space along ice grain boundaries. We have termed this a "grain boundary ridge." Grain boundary diffusion is a sintering process that occurs at the interface of two crystals, whereby mass migrates from the center of the contact to the surface of the bond. Since mass tends to sublimate from sharp features toward smaller curvature surfaces through vapor diffusion, a ridge developed by grain boundary diffusion will readily sacrifice mass to the surrounding ice surfaces. A mass balance between vapor and grain boundary diffusion based on the observed geometry is considered. This analysis indicates grain boundary diffusion may play a far more significant role than generally acknowledged. While this study was restricted to ice, it may have implications for other crystalline materials. © 2001 American Institute of Physics.

Notes: An analysis of the heat capacity of deuterated and undeuterated NH4ReO4 has been carried out in which the effects of the anisotropy of the thermal expansion have been considered, an approach hitherto not used for ammonium compounds. In the ammonium scheelites, the axial thermal expansion coefficients are very large, but of opposite sign, and as a result the volume of the scheelite lattice is nearly independent of temperature. It is shown that the correction from constant stress to constant strain results in a major contribution to the heat capacity of this highly anisotropic lattice. The difference between the experimental and calculated values of heat capacity, referred to as ΔCp, is expressed as the sum of the contributions from the anisotropy and the rotational heat capacity. The results of the analysis show that the rotational contribution is much smaller then previously thought. However, the exact contribution of the anisotropy cannot be calculated at this time because the elastic constants are not known. In calculating the heat capacity, maximum use has been made of external optical mode frequencies derived from spectroscopic measurements.

Notes: We have performed intra- and extra-cavity microwave frequency (1–100 GHz) measurements on high quality Y1Ba2Cu3O7−x superconducting thin films on (100) LaAlO3 substrates. The ∼0.3 μm thin films fabricated by the pulsed laser deposition technique exhibit superconducting transition temperatures 〉90 K, as determined by resistivity and ac susceptibility measurements, and critical current densities of 5×106 A/cm2 at 77 K. Moreover, ion beam channeling minimum yields of ∼3% were measured, indicating the extremely high crystalline quality of films grown on the LaAlO3 substrate. Microwave surface resistance values at 77 K for these films are found to be more than one to two orders of magnitude lower than for copper at 77 K for almost the entire frequency range explored. We postulate that the reason we observe such low surface resistances in these films is the virtual absence of grain and phase boundaries coupled with the high degree of crystallinity. Furthermore, we believe that the residual resistance measured below Tc is at present dominated by losses occurring in the substrate and the cavities rather than by losses intrinsic to the Y-Ba-Cu oxide superconductor.

Notes: Very low resistance nonalloyed ohmic contacts of Pt/Ti to 1.5×1019 cm−3 Zn-doped In0.53Ga0.47As have been formed by rapid thermal processing. These contacts were ohmic as deposited with a specific contact resistance value of 3.0×10−4 Ω cm2. Cross-sectional transmission electron microscopy showed a very limited interfacial reacted layer (20 nm thick) between the Ti and the InGaAs as a result of heating at 450 °C for 30 s. The interfacial layer contained mostly InAs and a small portion of other five binary phases. Heating at 500 °C or higher temperatures resulted in an extensive interaction and degradation of the contact. The contact formed at 450 °C, 30 s exhibited tensile stress of 5.6×109 dyne cm−2 at the Ti/Pt bilayer, but the metal adhesion remained strong. Rapid thermal processing at 450 °C for 30 s decreased the specific contact resistance to a minimum with an extremely low value of 3.4×10−8 Ω cm2 (0.08 Ω mm), which is very close to the theoretical prediction.

Notes: The simplest model for the contribution of pore surfaces to nuclear magnetic resonance (NMR) relaxation of a pore fluid gives R, the average relaxation rate minus the bulk rate, equal to a constant ρ, the velocity at which nuclear magnetization flows out of the pore fluid at the surfaces, times the pore-space surface-to-volume ratio S/V. Although ρ can vary widely, a great variety of porous media exhibit ρ values of the order of a few μm/s for longitudinal relaxation when S/V is measured by gas adsorption by the Brunauer, Emmett, and Teller (BET) method or high pressure mercury injection. For samples with wide distributions of relaxation rates it is of interest to find what functions of the relaxation data correlate best with S/V measurements and how different relaxation parameters relate to each other. Longitudinal relaxation data were taken for 77 sandstone samples of different origin, which had been cleaned and saturated with brine. After the NMR measurements the samples were dried and surface areas measured by BET. The samples have S/V from 1.5 to 150 (μm)−1, porosity from 3% to 28%, and permeability from less than 0.1 mD to more than 1 D. Longitudinal relaxation data were taken from 400 μs to 6 s and analyzed in many different ways, including stretched-exponential fits and multiexponential fits up to five components. S/V and ln(S/V) were correlated with various relaxation rates derived from these computed parameters.In principle, the relaxation parameter to use with a ρ value is the average rate, which is initial slope divided by initial amplitude, namely, R(0), where R(t)=(d/dt)ln S(t) at t=0 and S(t) is the relaxing signal. One can extrapolate an n component fit to t=0 to get Rn(0), but very good signal quality is required even to get small short components reliably for t well within the times covered by the data. Over half of the points have ρ's within a factor of 2 of the minimum value 0.9 μm/s when the average rate of a five-component fit to the data is used. There are numerous points with ρ up to 7 μm/s, but none of the high-ρ points are for samples with high S/V. All samples with high S/V have wide distributions of relaxation rates, but not vice versa. The best simple correlation with ln(S/V) was ln(S/V)≈1.81 ln(R33)−5.73, where R33 is the highest rate of a three-component fit without regard to the corresponding amplitude, and where S/V is in (μm)−1 and rate in s−1. This result was unexpected. This fit does not represent proportionality to a velocity ρ and does not correspond to any obvious physical model, but it can be of practical interest to estimate in a very simple and noninvasive manner S/V at the BET scale in sandstones. © 1996 American Institute of Physics.

Notes: Impurities in MeV-implanted and annealed silicon may be trapped at interstitial defects near the projected ion range, Rp, and also at vacancy-related defects at approximately Rp/2. We have investigated the temperature dependence of impurity trapping at these secondary defects, which were preformed by annealing at 900 °C. The binding energies of Fe, Ni, and Cu are greater at the vacancy-related defects than at extrinsic dislocation loops. During subsequent processing at temperatures up to 900 °C, the amount of these impurities trapped at Rp/2 increases with decreasing temperature while the amount trapped at Rp decreases, with most of the trapped metals located at Rp/2 in samples processed at temperatures (approximately-less-than) 700 °C. However, intrinsic oxygen is trapped at both types of defects; this appears to have little effect on the trapping of metallic impurities at extrinsic dislocations, but may inhibit or completely suppress the trapping at vacancy-related defects. © 1998 American Institute of Physics.

Notes: An investigation was made of the defect characteristics of single-crystal Bi12SiO20 (BSO) grown by both Czochralski (Cz) and hydrothermal transport methods. Only Cz-grown BSO is photorefracting using a coherent pumping source centered around 500 nm, as undoped hydrothermal BSO is transparent throughout the visible spectrum. Thermally stimulated current (TSC) studies in conjunction with temperature-dependent optical-absorption measurements and room-temperature photoconductivity data all indicate that the hydrothermal material is near intrinsic in terms of its low defect content. TSC measurements made below room temperature indicate that concentrations of traps of activation energy 〈0.7 eV are a factor of 103 smaller in hydrothermal than in Cz BSO. At least six different defects were identified in the TSC measurements. Temperature-dependent optical-absorption measurements indicate two Urbach band tails for Cz-grown materials that are not observed in the hydrothermal materials. Cz material of lower purity also possesses an impurity band tail which can be observed through temperature dependent optical absorption measurements. Comparison of Cz material with hydrothermal BSO of similar impurity content suggests that the BiSi defect responsible for the 500 nm absorption may be complexed with an impurity such as Fe or V. In the absence of this defect, however, these impurities have no effect on the absorption. An additional defect in the TSC data is also related to a transition metal impurity. The results indicate that the photoconductivity associated with the photorefractive effect in Cz material must proceed via a trap-hopping conduction mechanism that is missing in the intrinsic material.

Notes: The effect of applying an electric field across a quasi-phase-matched frequency doubling lithium tantalate waveguide has been investigated. The waveguide was fabricated by a two-stage ion exchange process in pyrophosphoric acid. An electric field of 2 kV/mm was found to shift the phase matching wavelength by 0.05 nm. It is estimated that more optimized waveguides could produce wavelength shifts of ±4 nm for an applied electric field of ±20 kV/mm and could compensate for temperature variations of ±67 °C. © 1996 American Institute of Physics.

Notes: The fabrication, performance characteristics, and design rules of buried-facet optical amplifiers are described. Chip gain of 25 dB, gain ripple of 〈1 dB, and gain difference of ≤1 dB for TE- and TM-polarized light are observed. The gain ripple and polarization dependence of gain correlate well with the ripple and polarization dependence of the amplified spontaneous emission spectrum. The performance of buried-facet amplifiers is comparable to that of cleaved-facet amplifiers with very good antireflection (R〈10−4) coatings. The buried-facet design reduces the requirement on antireflection coatings and makes the fabrication process more reproducible.