Variation Of Transport Properties Research Articles

Switching phenomena and nonlinear conduction in (YbS) 1.25CrS 2

This paper presents electric field-induced variations in transport properties of the mixed-valence misfit-layer compound (Y bS) 1.25CrS 2. Nonlinear conduction with negative differential resistivity and a switching phenomenon is found at high electric fields. I – V characteristics are accompanied with large voltage fluctuation. Switching or voltage fluctuation arises from a transition between low and high conductivity states. It has been suggested that these transport phenomena are derived from the collective nature of charge dynamics similar to strongly correlated carriers in Mott insulators, charge-ordered insulators and organic charge-transfer complexes. Our experimental results are well represented by proposed nonlinear conductivity.

Convective and absolute instabilities in non-Boussinesq mixed convection

The problem of non-Boussinesq mixed convection in a vertical channel formed by two differentially heated infinite plates is investigated and the complete convective/absolute instability boundary is computed for a wide range of physical parameters. A physical insight into the mechanisms causing instabilities is given. In particular, it is shown that the appearance of absolute instability is always dictated by a flow reversal within a channel; however, existence of the flow reversal does not exclude the possibility of convective instability. It is also shown that fluid’s non-linear transport property variations have a dramatic effect on the structure and complexity of spatio-temporal instabilities of the co-existing buoyancy and shear modes as the temperature difference across the channel increases. The validity of the stability results obtained using the procedure described in Suslov (J Comp Phys 212, 188–217, 2006) is assessed using the method of steepest descent.

Effect of silver doping on the electrical properties of a-Sb2Se3

This paper reports the effect of Ag-doping on electrical properties of a-Sb2Se3 in the temperature range 230–340K and frequency range 5–100kHz. The variation of transport properties with thermal doping has been studied. Ag-doping produces two homogeneous phases in the sample, which are found to be voltage dependent in the temperature range studied and frequency dependent in lower frequency region (0.1–10kHz). Activation energy Eg and C′ [=σ0 exp (γ/k), where γ, is the temperature coefficient of the band gap] calculated from dc conductivity has been found to vary from (0.42±0.01)eV to (0.26±0.01)eV and (4.11±0.01)×10−5 to (2.90±0.02)×10−6Ω−1cm−1 respectively. Ag-doping can be used to make the sample useful in device applications.

Study of anodization parameters effects on photoconductivity of porous silicon

We have prepared porous silicon by etching p-type crystalline silicon in different conditions such as: varying electrolyte concentration, current density, and etching time. The primary objective of this research is to develop a scientifically based technique for the measurement of photosensitivity. One such technique involves measuring the photoconductivity of the porous silicon under halogen lamp irradiation. Our photoconductivity measurements agree with pho- toluminescence measurements in previous work, and demonstrate the direct transition of porous silicon. Varied etching conditions change the peak of photo- conductivity from 600 to 520 nm (from 2.13 eV to 2.4 eV) as the porosity of the layer gradually increases, and the photoconductivity band also be- comes slightly more intense. The photoconductivity peak shift toward shorter wavelength was interpreted to be the result of band gap widening. We observe two distinct regimes in the time decay of photoconductiv- ity, fast decay and steady state, that arise from the recombination process and electron-hole asymmetry near the Fermi surface. Experimental measurements of photoconductivity give useful information about the band gap, band structure, and variation of transport properties due to the micro-structural porosity created during the etching process.

Variability in Transport Properties Regarding Drying Behavior for Blackbutt Timber in New South Wales

ABSTRACT Variability is a key issue in the processing of many biological materials, in this case the drying of hardwood timber. This article reports the measurements of variability of the diffusion coefficient (a transport property), the initial moisture content, and the basic density that are relevant to the drying of blackbutt, Eucalyptus pilularis Sm, from northern New South Wales in Australia. The diffusion coefficient was quantified using a mathematical model solving Fick's second law of diffusion for mass transfer, and Fourier's law for heat transfer. The initial moisture content and the basic density were measured using experimental procedures. Specifically, within-tree and between-tree variations are reported. The coefficients of variation of the initial moisture contents and final moisture contents are 0.24 and 0.19, respectively, for within-tree variability. A similar result was found for the amount of between-tree variability. Compensating differences in the diffusion coefficients of the timber boards were a significant reason for the small dispersion of final moisture contents, despite the large variation in initial moisture contents. An analysis of variance showed that some timber properties were affected by the board positions within trees and between trees. Circumferential and radial effects were significant for the within-tree variability of most transport properties. Moreover, principal components analysis suggested that timber boards with low densities have high initial moisture contents and high diffusion coefficients. A potential reason is that if there is less wood material per unit volume (lower density), then there is more space to be occupied by water (higher initial moisture content), and there is also less resistance to the diffusive transport of moisture (higher diffusion coefficients).

Variability in Transport Properties for Blackbutt Timber in New South Wales: Within and Between‐Tree Variability

AbstractVariability is a key issue in the processing of biological materials, in this case the hying of hardwood timber. This paper reports the measurements of variability of transport properties. which are relevant to the drying of blackbutt, Eucalyptus pilularis Sm, from northern New South Wales. Specifically, within‐tree and between‐tree variations are reported for two blackbutt regrowth logs. An analysis of variance showed that some timber properties were affected by the board positions within‐trees and between‐trees. Circumferential and radial efects were significant for the within‐ tree variability of most transport properties. Similarly, radial and circumferential effects were signlficant for most of the transport parameters between trees, but can be tentatively stated because only two regrowth logs were assessed. Timber boards with high initial moisture contents had higher rates of diffirsion and low basic densities using principal components analysis. A possible reason is that if there is less wood material per unit volume, these vacant spaces may be occupied by water, and there is also less resistance for diffusive transport of moisture.

Effect of Conjugate Heat Transfer on MEMS-Based Thermal Shear Stress Sensor

ABSTRACT The effect of conjugate heat transfer resulting from a microelectromechanical systems (MEMS)-based thermal shear stress is investigated. Due to the length-scale disparity and large solid–fluid thermal conductivity ratio, a two-level computation is used to examine the relevant physical mechanisms and their influences on wall shear stress. The substantial variations in transport properties between the fluid and solid phases and their interplay with regard to heat transfer and near-wall fluid flow structures are investigated. It is demonstrated that for state-of-the-art sensor design, the buoyancy effect can noticeably affect the accuracy of the shear stress measurement.

Analysis of coupling between hydrodynamic and thermal instabilities in non-Boussinesq convection

High-temperature convection arises in many technical applications such as thermal insulation systems, chemical vapour deposition reactors, etc. Under high-temperature conditions, fluid density and transport property variations can reach up to 30% of the average values across the flow region. Associated symmetry breaking nonlinearities are responsible for a wide spectrum of flow instabilities not found in low-temperature flows that are typically described by the Boussinesq approximation of the Navier–Stokes equations. In this work we use a set of low-Mach-number equations suggested by Paolucci in the early 1980s to describe a high-temperature mixed convection flow between two vertical plates. We find that non-Boussinesq instabilities have either a hydrodynamic (shear, common to both low- and high-temperature flows) or thermal (buoyancy, purely non-Boussinesq) character and they can occur simultaneously at certain values of the governing physical parameters (the so-called codimension-2 points). We use a weakly nonlinear analysis to show that such situations can be successfully modelled by two coupled cubic complex Landau equations. Subsequently the unfoldings of the double Hopf bifurcations involving shear modes detected in weakly non-Boussinesq mixed convection are investigated, and the complete set of resulting flow patterns is then studied as functions of the governing parameters. The spatio-temporal competition between shear and buoyancy disturbances in a strongly non-Boussinesq regime is also modelled by two coupled complex Ginzburg–Landau equations. The results obtained for these model equations are then interpreted from the physical point of view and the nature and asymptotic outcomes of instability mode competition at large times are discussed.

Conductivity, viscosity and volumetric behavior of molten hydrates of M3 + and M2+ ions

Conductivity, density and viscosity of molten hydrated salts consisting of M 3 + (Al 3 + and Fe 3 + ) and M 2 + (Ca 2 + , Cd 2 + , Mg 2 + and Zn 2 + ) ions were measured over a wide composition range and at temperatures ranging between 273.2 to 363.2 K. Representative data for FeCl 3 .6H 2 O + MgCl 2 .6H 2 O is taken for the discussion purposes. A change in hydration envelope of the cations and the competition of anions for hydration are considered to be the main factors for deviations from the ideal behavior in volumetric properties-composition isotherms. Temperature dependence of conductivity (A) and viscosity (η) was found to be non-Arrhenian; curvature in In (A, η) vs T - 1 curves indicate an increase in activation energy with decrease in temperature. The data were expressed using VTF equation, Free Volume Model (FVM) and Configurational Entropy Model (CEM) of liquid transport. The calculated values of zero mobility temperature, T o , exhibited a random variation with composition. The plot of In (Λ, η) vs (V - V o ) - 1 were linear indicating the applicability of Doolittle equation. A parallelism between V o and (V) T o vs composition indicate that free volume theory is applicable to describe the behavior of liquid transport in these system over the available temperature range. Composition dependence of the coefficients of VTF and Doolittle equations do not exhibit a systematic trend as expected from FVM and CEM. However, the standard deviations from the computer data fittings reveal that CEM formulations represent the data more closely to experimental values. Applicability of environmental relaxation model (ERM) of liquid transport was also examined. Constancy of C o , the limiting activation energy, for conductivity and viscosity is attributed for small temperature range available in this study. The e o values obtained in this study matches closely to the reported values for the similar systems. Though, ERM successfully predicts the temperature variations of transport properties, it is general observation that the parameters of ERM evaluated from a short range temperature data may not be useful to predict the behavior over wide range of temperatures.

Effect of discrete impurities on electron transport in ultrashort MOSFET using 3D MC simulation

This paper discusses the influence of the channel impurity distribution on the transport and the drive current in short-gate MOSFET. In this purpose, a careful description of electron-ion interaction suitable for the case of discrete impurities has been implemented in a 3D particle Monte Carlo simulator. This transport model is applied to the investigation of 50 nm MOSFET operation. The results show that a small change in the number of doping impurities or in the position of a single discrete impurity in the inversion layer may significantly influence the drain current. This effect is not only related to threshold voltage fluctuations but also to variations in transport properties in the inversion layer, especially at high drain voltage. The results are analyzed in terms of local fluctuations of electron velocity and current density. In a set of fifteen simulated devices the drive current Ion, determined at VGS = VDS = 0.6 V, is found to vary in a range of 23% according to the position of channel impurities.

Influence of discrete impurity position in the channel of an ultra-small MOSFET

This paper discusses the influence of the channel impurity distribution on the transport and the drive current in a short-gate MOSFET. For this purpose, a careful description of electron–ion interaction suitable for the case of discrete impurities has been implemented in a 3D (three-dimensional) particle Monte Carlo simulator. This transport model is applied to the investigation of a 50 nm MOSFET operation. The paper shows that a small change in the position of a single discrete impurity may significantly influence the drain current. This effect is not only related to threshold voltage fluctuations but rather to variations in transport properties in the inversion layer.

Structures of gramicidins A, B, and C incorporated into sodium dodecyl sulfate micelles.

Gramicidins A, B, and C are the three most abundant, naturally occurring analogues of this family of channel-forming antibiotic. GB and GC differ from the parent pentadecapeptide, GA, by single residue mutations, W11F and W11Y, respectively. Although these mutations occur in the cation binding region of the channel, they do not affect monovalent cation specificity, but are known to alter cation-binding affinities, thermodynamic parameters of cation binding, conductance and the activation energy for ion transport. The structures of all three analogues incorporated into deuterated sodium dodecyl sulfate micelles have been obtained using solution state 2D-NMR spectroscopy and molecular modeling. For the first time, a rigorous comparison of the 3D structures of these analogues reveals that the amino acid substitutions do not have a significant effect on backbone conformation, thus eliminating channel differences as the cause of variations in transport properties. Variable positions of methyl groups in valine and leucine residues have been linked to molecular motions and are not likely to affect ion flow through the channel. Thus, it is concluded that changes in the magnitude and orientation of the dipole moment at residue 11 are responsible for altering monovalent cation transport.

Effect of Li doping on the charge ordering state of La0.5Ca0.5MnO3

The effect of Li doping on the charge ordering (CO) state in perovskite-type manganates La0.5Ca0.5−xLixMnO3 with x=0, 0.05, 0.1, 0.15, 0.2 is investigated by transport and magnetic property measurements. The resistivity of the samples decreases with the increase of the Li-doped level. As x⩾0.1, the insulator–metal (IM) transition has been induced. Accompanying the variation of transport properties, the sample undergoes the variation from antiferromagnetic order for x<0.05 to ferromagnetic (FM) order for x⩾0.1. The experimental results indicate that the Li doping can destroy the CO state and induce the FM metallic (FMM) phase. These phenomena can be interpreted in terms of the strong competition between the CO state and FM phase induced by the Li+ ions substitution for the Ca2+ ions.

Ionic conductivity of fluorite type crystals CaF 2, SrF 2, BaF 2, and SrCl 2 at high temperatures

The ionic conductivity of fluorite-type crystals (CaF 2, SrF 2, BaF 2, and SrCl 2) has been studied at high temperatures including the melting points. Represented in detail, the technique of measurements is based on the use of a capillary-type cell made of hexagonal boron nitride. Measurements were carried out on polycrystalline specimens obtained by the solidification of corresponding melts. A common feature of all fluorites is that the temperature dependences of conductivity in the superionic region are described by sigmoid-like curves in the Arrhenius plot. The behaviour of the apparent Arrhenius energy and molar conductivity in the superionic region as well as on melting crystals has been characterized. From comparative analysis of these data, it is suggested that the conductivity anomaly is mainly due to a non-uniform course of anion sublattice disordering in crystals up to melting points. The variation of transport properties is well correlated with the relative sizes of cations and anions in the fluorite lattice, that assumes a regularity in the degree of sublattice disorder in these solids near the melting points.

Dielectric relaxation in flux grown KTiOPO4 and isomorphic crystals

Dielectric spectroscopy and dc conductivity have been studied in KTiOPO4 and isomorphic crystals in a temperature range 150–400 K. The experimental data indicate two temperature regions related to different types of relaxation processes. Alternation of the activation energy, suppression of the dielectric dispersion, and decreasing dissipation factor by three orders of magnitude has shown that the superionic state in KTiOPO4 and its isomorphs transit gradually to the dielectric state with temperature lowering. The transition leads to variation of transport properties from ionic to electronic conductivity. It was found that the parameters of the relaxation process (activation energy in the superionic state, temperature range of transition from ionic to electron conductivity) are determined by the sort of mobile cations. The temperature dependence of conductivity relaxation time was obtained from the electric modulus plots.

Blood-Perilymph Barrier and Ototoxicity: An In Vivo Study in the Rat

Cisplatin and gentamicin are two ototoxicants that are supposed to be transported by the paracellar route, i.e. via cellular junctions, to the perilymphatic compartment. This study was initiated to test the hypothesis that susceptive variation of individuals to ototoxic drugs may be explained by variability in transport properties. The transport of radioactive mannitol through the blood-perilymph barrier was correlated in vivo with the acute effect of cisplatin and gentamicin on auditory function. Transport of radioactive mannitol across the blood-perilymph barrier was monitored by sampling of scala vestibuli perilymph at 60 and 120 min after an intravenous infusion of the tracer to nephrectomized Long Evans rats. Counting of hair cell loss was performed in the animals receiving 16 mg/kg body weight cisplatin. The transport of radioactive mannitol across the blood-perilymph barrier did not correlate with the ototoxic effect of cisplatin, evaluated as changes in the auditory-evoked brainstem response thresholds or loss of outer hair cells. The results provide evidence that the barrier function is not involved in the interindividual variability of the ototoxic effect of cisplatin. Furthermore, it can be postulated that neither cisplatin nor gentamicin induce a disruption of endothelial cell junction stability in the inner ear.

Transport Properties of Hydrogen

etTransport cross sections and collision integrals have been calculated for interactions of hydrogen atoms and diatomic molecules. These results were determined using accurate potential energies and quantum mechanical formulations of the scattering. Collision integrals have been tabulated for H-H and H 2 -H 2 , and have been applied to determine diffusion and viscosity. The variation of transport properties with temperature and relative composition is examined for a gas formed from H atoms and H 2 molecules. The inversion temperature is found to be about a factor 3 lower than that of earlier estimates.

Turbulent developing convective heat transfer in a tube for fluids near the critical point

Numerical simulation is performed to study turbulent convective heat transfer to fluids near the critical point in the entrance region of a vertical tube. This modeling includes the effects of large thermodynamic and transport property variations of the fluid. The effect of proximity to the critical point is also considered as well as the buoyancy force in the vertical tube. Due to the property variation effects and buoyancy force in the tube, there is slow approach to fully developed conditions. The effects of large property variations are considered with a modified mixing length model including density fluctuation. Predictions from the mixing length model are in good agreement with experimental data from the literature and other analyses. The results show a little higher value in turbulent diffusivity than the standard mixing length model. The buoyancy force in the vertical tube depends on fluid temperature and pressure. The buoyancy force parameters increase as the pressure approaches critical pressure due to the steep density gradient with temperature which affects the characteristics of fluid flow and heat transfer in the developing region. The distributions of heat transfer coefficient and friction factor along the tube are quite different from those of the constant property case.

Effect of semibrittle deformation on transport properties of calcite rocks

We investigated the effect of mechanical deformation on transport properties by deforming synthetic calcite/quartz aggregates to strains (ε) up to 5% at confining pressures (Pe) up to 300 MPa, and at temperatures (T) from 300 K to 873 K. Subsequently, we measured permeability (k) using a wide‐range permeameter at effective pressures (Pc) of 15–155 MPa and room temperature. We then measured electrical conductivity (σ) in an impedance‐measuring assembly over the same pressure range at room temperature. The values of permeability and conductivity of the undeformed material at Pe=100 MPa were 4×10−18 m2 and 0.07 S/m. Samples deformed at 673 K and Pc=200 MPa, or at room temperature and Pc = 50–300 MPa, show small variations in permeability and conductivity: k changed only by up to a factor of 3 and σ increased by up to 10%. But, when a sample was deformed at 873 K and Pc=200 MPa, electrical conductivity dropped by 1 order of magnitude and permeability dropped by 2 orders of magnitude. To assess whether changes in length scales of the pore structure owing to deformation may account for large variation in transport properties, we counted cracks and pores, measured their lengths and widths, defined a damage parameter (ξ), and computed effective hydraulic and electrical conductivity using renormalization group methods. The undeformed rocks and the samples deformed at low confining pressure have severe damage, appear to be close to failure, and hence have high transport coefficients. Materials deformed at high pressures and temperatures have lower flaw densities, connectivities, and transport coefficients after deformation. We found that renormalization methods are suitable to model connectivity loss and large changes in transport properties owing to changes in flaw density and length scales. Pore connectivity and transport properties vary strongly during semibrittle deformation.

Variation of transport properties along the channel of a high electron mobility transistor: a quantum influence

In the theoretical modelling of a high electron mobility transistor (HEMT), it is inherently assumed that there is little variation in quantum confinement along the channel from source to drain and therefore the transport properties are independent of the position and only dependent on the electric field along the channel. In this study, the scattering rates for bulk polar optical phonons, bulk acoustic phonons (through deformation potential) and ionized impurity scattering have been studied theoretically based on Fermi's Golden Rule as a function of position along the channel for a As/GaAs HEMT. For operation in the sub-ohmic regime, it is observed that all three mechanisms exhibit a wide variation in the rates from source to drain due to a varying degree of quantum confinement. Some of the intersubband scattering processes involving a lower and higher subband, which are present at the drain end, are absent at the source end due to the variation of energy eigenvalue differences. A quantitative variation of 10 - 50% was observed in the scattering rates at room temperature such as 1 to 1 adsorption and emission of polar optical phonons. It is important to take into account such variation in scattering rates in a complete device model.