Electronic Transport Properties of Speer Carbon Resistors—Experimental Results and Semi-empirical Fits

Speer carbon composition resistors, in particular the 470 and 220 1/2 W grade 1002 resistors, have been used as secondary thermometers at temperatures below 4 K for many years. Their zero field resistances have been measured between 300 K and 4 K using a dip probe. Above 10 K, the resistance behavior can be explained using a simple temperature power law, R(T) R₀/T^0.16. The resistance measurements have been extended to 0.02 K using dilution refrigerators. Between 4 K and 0.3 K, the resistances exhibited activated laws having hopping exponents y 0.5. Below 0.3 K, the 470 resistors exhibit a crossover to a weaker activated law. Crossover resistance expressions suggest that the resistances follow a Mott variable-range hopping (VRH) law below 0.05 K. The low temperature magnetoresistance (MR) data showed changes of less than ±12 % of the zero field resistance values in fields up to 10 T. Fits using the wave function shrinkage and the forward interference models gave only fair agreement with the MR data.     

Relations Between Transport Coefficients in Lennard–Jones Fluids and in Liquid Metals

Several simple approximate hard-sphere relations for transport coefficients are compared with the results of molecular dynamics (MD) simulations performed on Lennard–Jones (LJ) fluids. Typically the individual transport coefficients: self-diffusion coefficients, D, shear viscosity, _s, bulk viscosity, _B, and thermal conductivity, , agree within a factor of two of the exact results over the fluid and liquid parts of the phase diagram, which seems reasonable in view of the approximations involved in the models. We have also considered the ratio, /_s, and the product, D_s, for which simple analytic expressions exist in the hardsphere models. These two quantities also agree within a factor of two of the simulation values and hard sphere analytic expressions. Using time correlation functions, Tankeshwar has recently related the ratio /D to thermodynamic quantities, in particular, to the differences in specific heats, C _p – C _V, and to the isothermal compressibility, T. Using D and thermodynamic values taken solely from LJ MD simulations, his relation was tested and found to give typically better than ~20% agreement at liquid densities, deteriorating somewhat as density decreases into the gas phase. Finally liquid metals are considered. In this case, is dominated by its electronic contribution, which is related approximately to the electrical conductivity by the Wiedemann–Franz Law. Some theoretical results for the electrical conductivity of Na are referenced, which allow a semiquantitative understanding of the measured thermal conductivity of the liquid metal. Shear viscosity is also discussed and, following the work of Tosi, is found to be dominated by ionic contributions; Nevertheless, at the melting temperature of Na, a relation emerges between thermal conductivity, electrical resistivity and shear viscosity.     

Experimental Study of Gas–Liquid Two-Phase Flow in Glass Micromodels

To estimate the most important flow variables in reservoir engineering, such as the relative permeability, it is required to know with high precision, other variables such as saturation, pressure drop of each phase, and porous media data such as porosity and absolute permeability. In this study, experimental tests were performed inside a glass micromodel using gas–liquid two-phase flow in steady-state conditions. The liquid-phase flow and the pressure drop of the porous media were determined. Additionally, the flow development inside the porous media was visualized using a high-speed video camera system. These pictures were recorded at 500 fps, and they were used to compute the phase saturation and the gas velocity in the glass micromodel. The visualization was performed in three regions of the glass micromodel demonstrating that saturation gradients were not present. The effect of the capillary number was studied over the gas–liquid relative permeability curves and on the flow mechanisms. It was concluded that high flow rates minimize edge effects, that the capillary number modifies the relative permeability values and the flow patterns inside the micromodel, and that the high-speed visualization is an efficient and accurate technique to determine saturation values and to study the flow patterns in transparent porous media such as glass micromodels.     

Self-Consistent Theory of Transport in Quasi–One-Dimensional Superconducting Wires

We study superconducting transport in quasi–one-dimensional homogeneous wires in the cases of both equilibrium and nonequilibrium quasiparticle populations, using the quasiclassical Green's function technique. We consider superconductors with arbitrary current densities and impurity concentrations ranging from the clean to the dirty limit. Local current conservation is guaranteed by ensuring that the order parameter satisfies the self-consistency equation at each point. For equilibrium transport, we compute the current, the order parameter amplitude, and the quasiparticle density of states as a function of the superfluid velocity, temperature, and disorder strength. Nonequilibrium is characterized by incoming quasiparticles with different chemical potentials at each -end of the superconductor. We calculate the profiles of the electrostratic potential, order parameter, and effective quasiparticle gap. We find that a transport regime of current-induced gapless superconductivity can be achieved in clean superconductors, the stability of this state being enhanced by nonequilibrium.     

Pyroceram 9606, A Certified Ceramic Reference Material for High-Temperature Thermal Transport Properties: Part 1—Material Selection and Characterization

A three-year program of work supported by the European Commission (EC) involving eleven partners from six countries has been carried out to select, procure, characterize, and certify the thermal transport properties (thermal conductivity, λ, and thermal diffusivity, a) of Pyroceram 9606, a polycrystalline glass ceramic identified as the most suitable reference material for use up to at least 1000 °C. For the initial calibration, six blocks were chosen from a batch of 30 blocks purchased for the project. Measurements of the following properties were undertaken: chemical analysis and microstructure, density and porosity, homogeneity, anisotropy, thermal cycling between 20 °C and 1000 °C, and long-term stability and reproducibility. In addition, specific heat capacity, linear thermal expansivity, and thermal transmissivity measurements were carried out so that the thermal conductivity could also be determined from thermal-diffusivity values obtained from the certification measurements. As a result of a successful characterization program, the partners had no hesitation in recommending to the European Commission that full certification of the thermal conductivity and thermal diffusivity of Pyroceram 9606 should be undertaken.     

E-Glass/Vinylester Composites in Aqueous Environments – I: Experimental Results

2- and 4-layered specimens of E-Glass/Vinylester fabricated from uniaxial, biaxial, and triaxial, non-woven fabrics processed using the resin infusion process are immersed in deionized water at 23°C (73°F) and 60°C (140°F), and a potassium based pH 10 buffer at 73°F, for a period of 57 weeks in order to investigate durability in aqueous environments. It is shown that the coefficients of apparent diffusion and levels of moisture gain are the highest for the deionized water immersed samples at 60°C (140°F), and this results in the highest levels of tensile strength and modulus degradation. Tensile tests show the presence of an aqueous medium based post-cure that competes with the conventionally recognized mechanisms of deterioration in the resin, at the level of the fiber-matrix interface, and in the fiber, resulting in a retardation of absolute level of effects. It is also shown that effects of the immersion are different in the warp and fill directions and can in fact be affected by intricacies of the fabric architecture and thickness. It is shown that damage takes place through interface debonding and degradation as well as fiber pitting, and cracking, each of which serve as the means for renewed absorption of water resulting in moisture uptake at levels above the initial plateau. Effects of immersion on short-beam-shear strength and glass transition temperature are also elucidated.     

Experimental Results on Tungsten-Wire Explosions in Air at Atmospheric Pressure—Comparison with a One-Dimensional Numerical Model

Experimental results on exploding tungsten wires in air at atmospheric pressure at current densities 10⁷ A·cm^–2 and a current rise 10¹⁰ A·s^–1 are presented. Besides the current through the probe and the voltage across it, the diameter of the wire material and its surface temperature have been measured. The final aim of this investigation is the determination of the thermophysical properties of a high-melting liquid metal up to its critical point. Here a first step should be made to demonstrate the reliability of the method and to justify the crucial assumptions. To determine the limits for the applicability of a homogeneous approach used so far, a one-dimensional numerical model in Z-pinch geometry has been used which gives the time evolution of the profiles of temperature, density, and pressure across the wire. The model describes well the main features observed in these experiments. A physical explanation for the maximum in the time dependences of the surface temperature is proposed. This behavior is related to special thermodynamic properties of a two-phase (liquid–gas) mixture forming in a peripheral layer around the liquid metal. The temperature limit is determined for which there are no remarkable gradients of temperature and density across the wire. The specific heat, the thermal expansion coefficient, and the electrical as well as thermal conductivity of liquid tungsten can now, in principle, be obtained. The parameters of the critical point of the liquid-vapor phase transition can also be estimated.     

Pyroceram 9606, A Certified Ceramic Reference Material For High-Temperature Thermal Transport Properties: Part 2—Certification Measurements

Following the characterization of the batch of Pyroceram 9606 material, a number of the partners in the European Commission (EC) supported program carried out certification measurements of thermal conductivity and thermal diffusivity. Six laboratories undertook thermal-diffusivity measurements using either the flash or the modulated beam methods. Eight laboratories measured the thermal conductivity, using either the steady-state guarded-hot-plate method or one of the transient hot-wire methods. Results from each series of measurements were provided in a standard format as an aid to simplify the statistical analysis of the data. The results were corrected to the nominal measured temperature and for change in dimension, analyzed separately, and presented in a standard format. Outliers were identified and rejected where appropriate, based on both statistical and technical evidences. The individual data sets were combined, and the grand mean data for each property analyzed further to provide the certified values together with their uncertainty limits. Finally, using the specific heat capacity and density values obtained from the characterization tests, values of thermal conductivity were calculated from the measured thermal diffusivity. The difference between the calculated and certified values is less than 2.7 %, which is well within the uncertainty limit assigned for the certified thermal property values.     

Transport and Quantum Coherence in Graphene Rings: Aharonov–Bohm Oscillations,Klein Tunneling,and Particle Localization

Simulating quantum transport through mesoscopic, ring-shaped graphene structures, we address various quantum coherence and interference phenomena. First, a perpendicular magnetic field, penetrating the graphene ring, gives rise to Aharonov–Bohm oscillations in the conductance as a function of the magnetic flux, on top of the universal conductance fluctuations. At very high fluxes, the interference gets suppressed and quantum Hall edge channels develop. Second, applying an electrostatic potential to one of the ring arms, \(nn'n\)- or npn-junctions can be realized with particle transmission due to normal tunneling or Klein tunneling. In the latter case, the Aharonov–Bohm oscillations weaken for smooth barriers. Third, if potential disorder comes in to play, both Aharonov–Bohm and Klein tunneling effects rate down, up to the point where particle localization sets in.     

Experimental Investigation of the Thermal Conductivity of Dibutyl‐ and Diisobutyl Sebacates at High Temperatures and Pressures

Results of experimental investigation of the thermal conductivity of dibutyl and diisobutyl sebacates in the interval of temperatures 308.1–641.3 K and pressures (0.1–40.0) MPa are given.     

Thermal decomposition of ZSM—5 and silicalite precursors

The combined technique of thermogravimetric analysis and mass spectrometry was used to observe the thermal decomposition of the following silicalite and zeolite precursors: tetraethylammonium-silicalite, tetrapropylammonium-silicalite, tetraethylammonium-ZSM—5, tetrapropylammonium-ZSM—5, methylamine-ZSM—5 and propylamine-ZSM—5. It was possible to distinguish the organic base molecules associated with acid sites from those not associated with acid sites by the composition of the evolved volatiles. Reaction sequences for the decomposition of the precursors have been proposed which involve a Hofmann elimination reaction followed by β-elimination reactions for tetraalkylammonium ions not associated with acid sites, and sequential Hofmann reactions for organic ions at acid sites. For the amine-ZSM—5 decompositions the physisorbed amines are evolved, unreacted, at ≈180°C while the amines at the acid sites undergo elimination reactions between 350° and 500°C.     

On Electronic and Transport “Anomalies” in Layered Oxides

We briefly discuss some electronic and transport anomalies observed in superconducting perovskites. In particular we consider: (i) the complex electronic (and crystallographic) phase diagram, (ii) the symmetry of the gap with a special emphasis on the evidence for s-wave component in various experiments, and (iii) non-Fermi-liquid transport in superconducting Sr₂RuO_4– perovskite (T _c<1 K) that, like LSCO cuprate, exhibits linear resistivity up to 1050 K.     

Bipolaron Jahn–Teller Pairing and Charge Transport in Cuprates

Recently the mechanism for an intersite pairing was proposed for cuprates, where the coupling of two fold electronic degenerated levels to local lattice models at finite wave vector was introduced. The model is able to describe the stripe phase and offers a possible explanation of the pseudogap. Moreover, we argue that the single particle and pair conductivity may differ. For a better analysis on this issue, we compute the charge mobility arising from single particle using a variable range hopping model. The general trend is that the mobility has a crossover from 1/T temperature behavior at high temperature to a strong reduction of the mobility at low temperatures.     

Thermal pressure coefficients of ethanenitrile,propanenitrile, and butanenitrile in the region 295–395 K

The thermal pressure coefficient (p/T) _v has been measured for ethanenitrile from 299 to 364 K, for propanenitrile from 295 to 377 K, and for butanenitrile from 297 to 398 K. The results are discussed in terms of the diminishing role of polarity in the alkanenitrile series and of a corresponding-states approach using gas-liquid critical properties as reduction factors. Although (p/T) _v varies unevenly with chain length, the reduced quantity shows a more regular behavior similar to that of the related quantity the cohesive energy density.     

Impact Response Characterization in Composite Plates—Experimental Validation

This paper presents an experimental study for the normalized low-velocity impact response of composite plates. It is demonstrated that a characterization diagram that shows the relationship of three non-dimensional parameters with the normalized maximum impact force can be used to fully characterize the response. With the governing non-dimensional parameters obtained experimentally, it is shown that impact tests having the same non-dimensional parameters, have dynamic similarity and the same non-dimensional response. Furthermore, the experiments can be placed in appropriate dynamic regions in which simplified dynamic models can be used to predict the response.     

Predictions of Multiband s?? Strong-Coupling Eliashberg Theory Compared to Experimental Data in Iron Pnictides

The experimental critical temperatures and the gap values of the superconducting materials LaFeAsO_1?x F _x , SmFeAsO_1?x F _x and Ba_1?x K _x Fe₂As₂, which are exponent of the most important families (1111 and 122) of iron pnictides, can be reproduced by an effective s-wave three-band Eliashberg model in which the dominant coupling is in the interband channel and the order parameter is nodeless but undergoes a sign reversal between hole and electron bands (s?? symmetry). In particular, high values of the electron-boson coupling constants and small typical boson energies (in agreement with neutron diffraction experiments) are necessary to reproduce the exact T _c, the experimental gap values and their temperature dependence. The same parameters allow fitting the experimental temperature dependence of the upper critical field and predicting the presence of characteristic structures related to the electron?Cboson coupling in the Andreev-reflection spectra.     

Finite Element Simulation of Temperature and Current Distribution in a Superconductor,and a Cell Model for Flux Flow Resistivity—Interim Results

A critical problem arises when current distribution in a high-temperature superconductor and its stability against quench shall be predicted: is it correct to assume homogeneous temperature distribution in superconductors, in general or only in LHe-cooled devices? The finite element analysis presented in this paper shows that during the very first instants following a disturbance, like single Dirac or periodic heat pulses, or large fault currents, temperature distribution in a BSCCO 2223 conductor is highly inhomogeneous. This is because disturbances, of transient or continuous, isolated or extended types in conductor volumes, create hot spots of comparatively long life cycle. As a consequence, separation between Ohmic and flux flow current limiter types, or decisions on the mechanism that initialises current sharing, cannot be made definitely. A semi-empirical cell model is presented in this paper to estimate flux flow resistivity in multi-filamentary superconductors in a successive approximation approach. Weak links are modelled, as nano- and microscopic surface irregularities and corresponding resistances, in analogy to thermal transport. Though the model requests input of a large amount of data (dimensions, porosities, field-dependent quantities) that still have to be verified experimentally, it is by its flexibility superior to ideas relying on, for example, imagination of separate, non-interacting chains of strong and weak links switched in parallel. In particular, and in contrast to the standard expression to calculate flux flow resistivity, the cell model suggests to replace solid conduction by an effective resistivity, a method that is more appropriate for multi-filamentary conductors. The paper also discusses integration time steps in numerical simulations that have to be selected in conformity with several characteristic times of current and thermal transport.