Variable Electrical Resistivity Research Articles

The effect of pressure and temperature on the structure and electrical transport properties of MoO2

The structure and electrical transport properties of molybdenum dioxide (MoO2) under high pressure were studied by X-ray diffraction, in situ Hall effect and variable temperature electrical resistivity measurements. The X-ray diffraction measurements revealed two structural phase transitions at 14.3 GPa and 22.6 GPa, and unusual changes in the electrical parameters with pressure were attributed to these transitions. The internal conduction mechanism of MoO2 under compression was explained by the Hall mobility and charge carrier concentration. The abnormal changes in the temperature coefficient reflected changes in the lattice thermal vibration trend. MoO2 keeps a low temperature coefficient up to 34.4 GPa, showing good stable electrical transport performance. The results of this study will help guide the future development of pressure-modulated devices based on MoO2.

Large entropy change accompanying two successive magnetic phase transitions in TbMn2Si2 for magnetic refrigeration

Structural and magnetic properties in TbMn2Si2 are studied by variable temperature X-ray diffraction, magnetization, electrical resistivity, and heat capacity measurements. TbMn2Si2 undergoes two successive magnetic transitions at around Tc1 = 50 K and Tc2 = 64 K. Tc1 remains almost constant with increasing magnetic field, but Tc2 shifts significantly to higher temperature. Thus, there are two partially overlapping peaks in the temperature dependence of magnetic entropy change, i.e., −ΔSM (T). The different responses of Tc1 and Tc2 to external magnetic field, and the overlapping of −ΔSM (T) around Tc1 and Tc2 induce a large refrigerant capacity (RC) within a large temperature range. The large reversible magnetocaloric effect (−ΔSMpeak ∼ 16 J/kg K for a field change of 0–5 T) and RC (=396 J/kg) indicate that TbMn2Si2 could be a promising candidate for low temperature magnetic refrigeration.

Effect of Material Composition Distribution on the Electro-thermal Response of a Non-uniform Functionally Graded Metal Line under a Direct Current Field

The effect of material composition distribution on the electro-thermal response of a thin, non-uniform functionally graded metal (FGM) line is investigated under a direct current field. The distribution of material composition along the FGM line is expressed in terms of a general power-law function of space variable, which, in turn, suggests the corresponding distributions of material properties involved. The governing differential equations associated with the electro-thermal problem are derived in terms of variable electrical resistivity and thermal conductivity of the material. The solution of the coupled boundary-value problem is then obtained numerically using finite-difference technique. The electro-thermal response of the FGM line is obtained for different composition profiles and environmental conditions, which are discussed in light of comparison with those of individual constituent metal lines.

Prediction of Electro-Thermal Responses of Non-Uniform Functionally Graded Metal Lines under a Direct Current Field

The electro-thermal response of thin, non-uniform functionally-graded metal lines subjected to a direct current flow is determined. The material properties associated with the problem are assumed to vary following a given functional relationship. The corresponding governing differential equations are derived in terms of variable electrical resistivity and thermal conductivity of the material. The solution of the boundary-value problem is then obtained numerically by using finite-difference technique. The electrical and thermal responses of a functionally graded metal line are presented in comparison with those of individual constituent metals. The results are claimed to be highly accurate and reliable, and are considered to be a valuable guide to thermal analysis of modern electronic devices.

High-cycle fatigue damage measurement based on electrical resistance change considering variable electrical resistivity and uneven damage

The definition of damage variable based on electrical resistance changes was investigated. Under the hypothesis of a current-conducting section equivalent to a load-bearing section, the fatigue damage evolution may be represented effectively. For rotary bending high-cycle fatigue test, the formula of fatigue damage measurement considering uneven damage distribution and variable electrical resistivity was developed, which is the hypergeometric function. The analysis shows that in the range of damage 0–0.6, the formula is approximate to damage measurement based on electrical resistance changes due to uniform damage and constant electrical resistivity. High-cycle fatigue damage measurement for common structural steels was performed, and the comparison of experimental data with the predictions shows agreement.

Three-dimensional magnetotelluric modelling of the Vilariça depression (NE Portugal)

The Vilariça depression is located astride the northern branch of a Late Variscan (NNE–SSW) left-handed strike–slip fault known as the Vilariça Fault Zone (NE Portugal), whose reactivation in Pliocene–Quaternary times is well documented. The magnetotelluric method was applied in order to evaluate the electrical structure of the northern part of this tectonic depression. Thirty magnetotelluric soundings were carried out and a three-dimensional model was constructed. Different segments of the Vilariça Fault Zone, as well as the earlier Variscan thrusts preserved in the studied area, are well displayed in the model. Low-resistivity (30–200 Ω m) structures were revealed in the central part of the depression up to a depth of 1 km. These shallow conductors are mainly associated with the sedimentary infillings of the basin and are bordered and/or interrupted by strongly weathered and fractured blocks of metasediments displaying variable electrical resistivity. Deeper domains show a relatively homogeneous high resistivity (3500 Ω m), being difficult to distinguish the nonfractured and nonweathered metamorphosed autochthonous metasediments (at depths above 2 km) from the gneissic basement. A conductive layer (500 Ω m) was detected at a depth ranging from 8 to 12 km; this feature is tentatively associated with an important crustal detachment that may correspond to the root domain of the major active strike–slip faults in NW Iberia.

Finite element modeling of cyclic thermal response of shape memory alloy wires with variable material properties

This paper considers the influence of variable thermal conductivity and electrical resistivity on the cyclic thermal response of a shape memory alloy (SMA) wire. The specific boundary value problem of an SMA wire under zero-stress conditions is considered, wherein the wire, in an initially martensitic state, is heated by electrical current and cooled by convection. An isothermal boundary condition is considered at the ends of the wire. The heating is continued until the transformation from martensite to austenite has essentially taken place. A Galerkin finite element method (FEM)-based numerical approach is used to solve the boundary value problem. The two key parameters of interest are the maximum possible transformation achievable (i.e. extent of actuation) and the time period for one cycle. It is seen that for short wires, the assumption of constant thermal conductivity underestimates the actuation and overestimates the cycle time significantly. The assumption of a constant electrical resistivity does not affect the actuation but significantly overestimates the cycle time. For long wires, the thermal conductivity effect is diminished whereas the electrical resistivity effect is very similar to that for the short wires.

Magnetic and electrical properties of Eu 2VO 4

Eu 2VO 4 with the K 2NiF 4-type structure was prepared by heating Eu 2O 3, V 2O 5, and V metal at 1473 K in an evacuated silica ampoule. Rietveld refinement of the powder X-ray diffraction (XRD) data confirmed the presence of layers of corner-shared VO 6 octahedra with V–O bond distances of 1.928(1) Å (× 4) and 2.04(2) Å (× 2) perpendicular to the tetragonal c axis. X-ray absorption near-edge spectroscopy (XANES) indicated that the Eu is mixed-valent [Eu(II)/Eu(III)], while vanadium exists exclusively in the trivalent state. These valence assignments were supported by the results of temperature-dependent magnetic susceptibility and Mössbauer spectroscopy data. Variable temperature electrical resistivity measurements indicated that the sample was insulating (ρ RT ≈ 10 4 Ω-cm). The compound undergoes a charge-ordering transition associated with the Eu sublattice at 445 K, as evidenced by Mössbauer spectroscopy, electrical resistivity, and in situ high temperature X-ray diffraction studies.

Heal: Transfer through a high temperature superconducting current lead at cryogenic temperatures

A study of heat transfer through a high temperature superconducting (HTSC) current lead in both, resistive and superconducting mode, has been performed. The set of dimensionless governing parameters was defined, and the physical meaning of different parameters elaborated. An optimal selection of thermal design parameters was suggested. The selection of parameters has been based on: (i) the numerical solution of a set of nonlinear governing differential equations; and (ii) an estimation of the irreversibility induced by heat transfer and Joule heating phenomena. The results were discussed for a YBa 2Cu 3O 7-δ (YBCO) compound system. The case study has been based on a cryogenic current lead model that includes the nonlinearities as follows: (i) variable thermal conductivity; (ii) variable electrical resistivity (in resistive mode); and (iii) variable heat transfer coefficient. The results obtained correspond to the temperature range between 4.2 and 77 K. It was shown that a local variation in heat transfer coefficient does not influence the selection of optimal design parameters.

Mechanical and Electrical Properties of Glass and Carbon Fiber-Reinforced Composites

The design of composite rotors for high-energy density pulsed power supplies demands accurate characterization of both the mechanical and electrical properties of fiber-reinforced epoxy. The mechanical properties of S-2 glass-epoxy, IM6 graphite-epoxy, and hybrid graphite-glass epoxy composites were measured in tension and torsion tests, providing strength and stiffness parameters for rotor dynamics modeling. Variable frequency electrical resistivity tests were conducted to allow estimation of eddy current losses arising in carbon-reinforced materials. Volume fraction measurements using electron microscopy and analysis by digestion allow for normalization of the test results with respect to composite fiber content. The experimental results were used to evaluate the micromechanical rule of mixtures and Halpin-Tsai correlations.

Permeability versus depth in the upper oceanic crust: In situ measurements in DSDP hole 504B, eastern equatorial Pacific

In situ permeabilities measured within the upper kilometer of oceanic crust in hole 504B on the south flank of the Costa Rica Rift decrease exponentially from 10−13 to 10−14 m2 in layer 2A (the upper 150 m) to 10−15 to 10−17 m2 in layer 2B (150–550 m into basement), and to 10−17 m2 and lower in layer 2C (deeper than 550 m). We estimate the permeability (k) versus depth (z) to vary as k(z) = 0.11e(−z/50) × 10−12 m2. If this permeability versus depth function is representative of the oceanic crust in general, then hydrothermal convection would be vertically stratified, with the most vigorous circulation confined to the shallowest pillow basalt layers of the crust. The use of a full suite of geophysical logs (including borehole televiewer imagery; multichannel, full waveform sonic; nuclear; and variable spacing electrical resistivity logs) allows the characterization of the scale and interconnectedness of fractures, and the degree of infilling of these fractures by alteration minerals. At hole 504B, we observe a relationship between permeability and fracture porosity, determined from the geophysical logs. If fractures with similar aspect ratios are encountered at other oceanic sites, then prediction of the permeability of the oceanic crust to within an order of magnitude is possible from geophysical logs. At hole 395A in the Atlantic, for example, in situ permeability measurements yielded values similar to those predicted by the logging relationships established at hole 504B.

Ionosphere‐magnetosphere coupling: 2. Electric fields

The past 4 years represents an unprecedented period of scientific achievement with regard to advances made in our knowledge of electrodynamic processes acting within the magnetosphere and ionosphere. Perhaps the most impressive accomplishment has been the emergence of conceptual models that include dynamical links between the terrestrial upper atmosphere, the ionosphere, and the magnetosphere. It is now clear that the flow of the magnetized solar wind plasma past the magnetosphere establishes a permanent internal pattern of electric currents connecting the magnetopause with ionospheric regions at the inner peripheries of the auroral ovals. These currents, aligned in narrow longitudinal sheets, close through the ionosphere to create a global pattern of electric fields and currents. It should not be thought that the ionosphere and the upper atmosphere are simply passive receptors of these external influences, however, since they both exert important influences upon the magnetosphere through energy dissipation, variable electrical resistivity, and other processes. As a consequence it is no longer reasonable to consider separately the electrodynamic phenomena of the ionosphere or the magnetosphere. Instead, one must adopt a unified view based upon consideration of the full implications of magnetosphere‐ionosphere‐atmosphere (MIA) couplings and dynamics.

The influence of variable thermal conductivity and variable electrical resistivity on thermoelectric generator performance : Jerry S. Lee, Assistant Professor of Mechanical Engineering, North Carolina State University, Raleigh, North Carolina. Energy Conversion9, 91–97 (1969)

The influence of variable thermal conductivity and variable electrical resistivity on thermoelectric generator performance : Jerry S. Lee, Assistant Professor of Mechanical Engineering, North Carolina State University, Raleigh, North Carolina. Energy Conversion9, 91–97 (1969)

The influence of variable thermal conductivity and variable electrical resistivity on thermoelectric generator performance

cross-sectional area of thermoelement (cm z) defined in Section 6 (dimensionless) defined in Section 3 (dimensionless) electric current (A) thermal conductivity (W/cm-°K) length of thermoelement (em) defined in Section 6 (dimensionless) defined in Section 6 (dimensionless) electrical power output (W) electrical power output, PL/kmAT,~ (dimensionless) heat transferred from cold junction (W) heat transferred from cold junction, Q~L/k,nAT,~ (dimensionless) heat transferred from hot junction (W) heat transferred to hot junction, QnL/kr~ATn, (dimensionless) load resistance (sq) Seebeek coefficient (V/°K) absolute temperature (°K) mean temperature, (To + Tn)/2 (°K) position variable, x/L (dimensionless) position variable (cm) electric current, 1SmL/kraA (dimensionless) figure of merit, S2/pk (°K-X)

The influence of variable thermal conductivity and variable electrical resistivity on thermoelectric generator performance : Jerry S. Lee, Assistant Professor of Mechanical Engineering, North Carolina State University, Raleigh, North Carolina. Energy Conversion9, 91–97 (1969)

The influence of variable thermal conductivity and variable electrical resistivity on thermoelectric generator performance : Jerry S. Lee, Assistant Professor of Mechanical Engineering, North Carolina State University, Raleigh, North Carolina. Energy Conversion9, 91–97 (1969)

The influence of variable thermal conductivity and variable electrical resistivity on thermoelectric generator performance : Jerry S. Lee, Assistant Professor of Mechanical Engineering, North Carolina State University, Raleigh, North Carolina. Energy Conversion9, 91–97 (1969)

The influence of variable thermal conductivity and variable electrical resistivity on thermoelectric generator performance : Jerry S. Lee, Assistant Professor of Mechanical Engineering, North Carolina State University, Raleigh, North Carolina. Energy Conversion9, 91–97 (1969)