Conduction Prevails Research Articles

Thermal and hydrogeological aquifers characterization by coupling depth-resolved thermal response test with moving line source analysis

In this paper for the first time the Moving Line Source (MLS) model is combined with a depth-resolved Thermal Response Test (TRT). The latter was performed in a heterogeneous and groundwater rich subsoil, composed by a layering of silty sand, medium-fine sand and coarse-medium sand, with a layer of clayey silt separating a shallow aquifer from a deep one.The temperature evolution in the ground along the vertical axis was analysed with both the standard Infinite Line Source (ILS) and the MLS. The two models lead to similar estimates of the thermal conductivity in those regions of the subsoil where conduction prevails, while the MLS performs better where a significant groundwater velocity is expected. In these layers, the MLS analysis allows to derive both the thermal conductivity and the Darcy velocity. The MLS results were validated by comparison with a numerical simulation on a multiple-layers ground model developed in MODFLOW/MT3DMS using a constant energy boundary condition. The combined depth-resolved TRT/MLS approach represents an important method for an accurate design of the Ground Heat Exchangers under the presence of groundwater flow.

Approaching the high intrinsic electrical resistivity of NbO2 in epitaxially grown films

NbO2 is a promising candidate for resistive switching devices due to an insulator-metal transition above room temperature, which is related to a phase transition from a distorted rutile structure to an undistorted one. However, the electrical resistivity of the NbO2 thin films produced so far has been too low to achieve high on-off switching ratios. Here, we report on the structural, electrical, and optical characterization of single-crystalline NbO2 (001) thin films grown by pulsed laser deposition on MgF2 (001) substrates. An annealing step reduced the full width at half maximum of the NbO2 (004) x-ray Bragg reflection by one order of magnitude, while the electrical resistivity of the films increased by two orders of magnitude to about 1 kΩcm at room temperature. Temperature-dependent resistivity measurements of an annealed sample revealed that below 650 K, two deep-level defects with activation energies of 0.25 eV and 0.37 eV dominate the conduction, while above 650 K, intrinsic conduction prevails. Optical characterization by spectroscopic ellipsometry and by absorption measurements with the electric field vector of the incident light perpendicular to the c-axis of the distorted rutile structure indicates the onset of fundamental absorption at about 0.76 eV at room temperature, while at 4 K, the onset shifts to 0.85 eV. These optical transitions are interpreted to take place across the theoretically predicted indirect bandgap of distorted rutile NbO2.

Electrical properties and positron annihilation studies for LaxCoFe2−xO4

The electrical measurements and positron annihilation lifetime spectroscopy (PALS) of LaxCoFe2−xO4 (x = 0.05, 0.1, 0.15, 0.2, 0.25, and 0.3) synthesized by two different techniques (citrate-gel autocombustion and autocombustion flash techniques) were carried out. The dielectric constant (eʹ) and dielectric loss (tan δ) were reduced with La concentration at low La value samples. On contrary, La enhanced the values of eʹ and tan δ at high La content samples. AC-resistance measurement recommended that hopping conduction prevails in the studied samples. PALS indicated that defects present at grains’ boundaries and vacancy-type defects in the structure are not changed with different La3+ contents. PALS also indicated the presence of large free volume regions between grains. Finally, it is recommended that these prepared samples are preferable for use in applications like microwave devices, actuators, isolators, and phase shifters.

Effect of Y-doping on the phase relation and electrical properties of Fe-doped BaZrO3

The effects of the co-doping of Y in Fe-doped BaZrO3 (BaZr0.7-yFe0.3YyO3-δ) on the phase relation and electrical properties are investigated in this study. While a single phase is formed up to y=0.1 by the substitution of Y for Zr-sites (phase-1), the amphoteric substitution of Y for both Zr-sites and Ba-site (phase-2) is observed above y=0.1. Electrical conductivity measurements suggest that p-type conduction prevails under an oxidizing atmosphere. The hole conductivity increases with an increase of Y concentration up to y=0.1, but a further increase of Y concentration causes a decrease in the conductivity. The maximum conductivities of both bulk and grain boundary are obtained at y=0.1, and a further increase of Y concentration leads to a decrease of the conductivity due to the possible formation of phase-2 with the partial replacement of Y for Ba-sites (YBa•). Partial proton conductivity is also improved by Y-doping up to y=0.1, but a further doping cannot contribute to an increase in the proton conductivity. The results indicate that the mixed proton and hole conductivity of Fe-doped BaZrO3 can be improved by the co-doping of a rare-earth acceptor (Y3+), but the doping level is limited to y=0.1 to avoid the formation of phase-2.

Defect chemistry and transport properties of Nd-doped Pb(ZrxTi1−x)O3

The oxygen partial pressure (pO2) dependent conductivity of Nd3+ doped PZT was investigated at different temperatures in order to get information on the nature of the conducting charge carriers and on the chemical oxygen diffusion coefficient. From a positive slope close to 0.25 in log (conductivity) vs. log (pO2) plots it is concluded that PZT with donor-type Nd3+ shows predominant hole conduction already at moderately high oxygen partial pressures, while under reducing conditions electron conduction prevails. This unexpected occurrence of hole conduction in a nominally donor doped system is interpreted in terms of the only partially controllable loss of lead oxide during preparation, which results in the formation of lead vacancies. Those act as acceptors at the temperatures employed in this study (560–615 °C) and turn the nominally donor doped material into a slightly acceptor doped one. From the conductivity relaxation after partial pressure changes, chemical oxygen diffusion coefficients could be extracted and values of about 6.5·10−9 cm2/s at 560 °C and 2·10−8 cm2/s at 611 °C are found in air.

Localized tail state distribution in amorphous oxide transistors deduced from low temperature measurements

In this paper, we extract density of localized tail states from measurements of low temperature conductance in amorphous oxide transistors. At low temperatures, trap-limited conduction prevails, allowing extraction of the trapped carrier distribution with energy. Using a test device with a-InGaZnO channel layer, the extracted tail state energy and density at the conduction band minima are 20 meV and 2 × 1019 cm−3 eV−1, respectively, which are consistent with values reported in the literature. Also, the field-effect mobility as a function of temperature from 77 K to 300 K is retrieved for different gate voltages, yielding the activation energy and the percolation threshold.

Hopping Conduction Observed in Thermal Admittance Spectroscopy

We observe variable-range hopping conduction in thermal admittance spectroscopy and develop a method to evaluate the signal under this condition. As a relevant example of demonstration we employ Cu(In,Ga)(Se,S)2 thin-film solar cells and show that the fundamental N1 signal, which has been discussed for more than a decade in terms of minority carrier traps, does not display trap parameters, but is generated by the freezing-out of carrier mobility with decreasing temperature when hopping conduction prevails. This effect offers a new approach to carrier hopping and to semiconductors suffering from small mobility.

High-temperature phase transitions in the improper ferroelectric KIO3

The ac conductivity (σac) and dielectric permittivity (ϵ) are determined in the temperature range 300 K < T< 520 K at some selected frequencies (0.5–10 kHz) for the polycrystalline samples of KIO3 compound. The results indicated that the compound behaves as an improper ferroelectric and undergoes a ferroelectric phase transition from a high temperature rhombohedral phase I to a low temperature monoclinic phase II at T c = (486 ± 1) K. A second structural phase transition was observed around 345 K. The conductivity varies with temperature range and for T > 428 K intrinsic conduction prevails. Different activation energies in the different temperature regions were calculated. The frequency dependence of σ(ω) was found to follow the universal dynamic response [σ(ω)∝(ω) s(T)]. The thermal behaviour of the frequency exponent s(T) suggests the hopping over the barrier model rather than the quantum mechanical tunneling model for the conduction mechanism.

Catalytic and electrocatalytic activity of Pt-Ru/C electrode for hydrogen oxidation in alkaline and acid aqueous electrolytes: EMSI and EPOC effects

The kinetics of the oxidation of H2 on PtRu/C gas-diffusion electrode was studied by interfacing the electrode with aqueous electrolytes at different pH values. The conducting electrolytes were KOH and HClO4 aqueous solutions with different concentrations. It is shown that the nature of the aqueous electrolyte plays the role of an active catalyst support for the PtRu/C electrode which drastically affects its catalytic properties. During the aforementioned interaction, termed electrochemical metal support interaction (EMSI), the electrochemical potential of the electrons at the catalyst Fermi level is equalized with the electrochemical potential of the solvated electron in the aqueous electrolyte. The electrochemical experiments carried out at various pH values showed that the electrochemical promotion catalysis (EPOC) is more intense when the catalyst-electrode is interfaced with electrolytes with high pH values where the OH ionic conduction prevails. It was concluded that similar to the solid state electrochemical systems EPOC proceeds through the formation of a polar adsorbed promoting layer of OH ? -, electrochemically supplied by the OH species, at the three phase boundaries of the gas exposed gas diffusion catalyst-electrode surface.

Electrical studies on sputtered CuCl thin films

CuCl is a wide-direct band gap semiconductor, lattice matched to Si and it possesses excellent ultra violet (UV) emission properties. It is thus a promising candidate for the next generation Si based UV optoelectronics. CuCl films were deposited using RF magnetron sputtering technique. X-ray diffraction analysis reveals that the grains are strongly oriented. Triangular crystallites of CuCl were observed in the AFM surface topograph. Au–CuCl–Si–Au structures were fabricated and field dependent electrical studies were carried out in the electric field range of 1.25 × 106 to 2.5 × 107 V/m. I–V characteristics show that ohmic conduction prevails in low electric fields up to 2.5 × 106 V/m. In the higher field range, from 2.5 × 106 to 2.5 × 107 V/m, the conduction mechanism was Schottky emission controlled. There was no trap related charge transport observed at higher electric fields. Preliminary electrical studies are reported in this article.

Promotional effects on a PtRu/C catalyst-electrode interfaced with aqueous electrolytes: electrochemical metal support interaction (EMSI) and electrochemical promotion of catalysis (EPOC)

The kinetics of the catalytic oxidation of H2 on PtRu/C gas diffusion electrode (GDE) was studied by interfacing the electrode with aqueous electrolytes at different pH values. The conducting electrolytes were aqueous solutions of varying concentrations of KOH and HClO4 so that the pH was ranging between 2 and 13. The open circuit catalytic reaction rates exhibit the lowest value at pH = 13, while the catalytic activity is progressively increasing with decreasing pH values. The enhancement of the open circuit catalytic reaction rate can be even an order of magnitude higher in the acidic solution with respect to the alkaline electrolyte. It is shown that the nature of the aqueous electrolyte plays the role of an active catalyst support for the PtRu/C electrode, which drastically affects its catalytic properties. This is substantiated through the electrochemical equilibrium charge transfer reactions at the catalyst-electrode/electrolyte interface: $${ \eqalign{ \hbox{H}_{3}\hbox{O}^{+}+\hbox{e}^{-}+\hbox{S}\leftrightarrow\hbox{H}_ {\rm ad}+\hbox{H}_{2}\hbox{O}\ (\hbox{acidic})\cr \hbox{OH}^{-}+\hbox{S}\leftrightarrow\hbox{OH}_{\rm ad}+ \hbox{e}^{-}\ (\hbox{alkaline}) }}$$ According to the aforementioned interaction, termed electrochemical metal support interaction (EMSI), the electrochemical potential of the electrons at the catalyst Fermi level is equalised with the electrochemical potential of the solvated electron in the aqueous electrolyte. The electrochemical promotion experiments carried out at various pH values showed that the non Faradaic modification of the catalytic activity is more intense when the catalyst-electrode is interfaced with electrolytes with high pH values where the OH− ionic conduction prevails. It was concluded that similar to the solid state electrochemical systems the non Faradaic electrochemical modification of the catalytic activity proceeds through the formation of a polar adsorbed promoting layer of $${\hbox{OH}_{\rm ad}^{\delta-}}$$ , electrochemically supplied by the OH− species, at the three phase boundaries of the gas exposed gas diffusion catalyst-electrode surface.

Dielectric-spectroscopic and a.c. conductivity investigations on copper doped layered Na 1.8K 0.2Ti 3O 7 ceramics

Dielectric studies on copper doped derivatives of polycrystalline layered mixed alkali trititanate Na 1.8K 0.2Ti 3O 7 ceramics indicate that the losses are of mixed type and decrease on copper doping. However, the temperature dependent permittivity plots are characteristic of the diffuse nature of a possible ferroelectric phase transition and hence give indication of relaxor ferroelectric behaviour. From the EPR spectra, recorded at room temperature, it can be seen that the substitution of copper occurs at Ti 4+ as preferred site with a divalent oxidation state (Cu 2+) for all compositions. Also, copper doping enhances the transition temperature, which is indicative of the stabilization of the existing ferroelectric phase up higher temperatures. Besides bolstering electron hopping conduction, acceptor doping restrains the interlayer ionic conduction. Moreover, electron hopping (polaron) conduction is dominant over the lower temperature region, while interlayer ionic conduction prevails in the higher temperature region.

High-temperature proton conductivity in acceptor-doped LaNbO 4

The conductivity of acceptor-doped LaNbO 4 has been investigated in the temperature range 300 to 1200 °C as a function of the oxygen pressure and water vapor pressure by means of impedance spectroscopy and EMF measurements. The conductivity is predominantly ionic below 800 °C in air and for higher temperatures under reducing conditions. Protons are the major ionic charge carrier in the presence of water vapor. A maximum in proton conductivity of ∼ 0.001 S/cm was obtained at 950 °C in atmospheres containing ca 2% H 2O. At high temperatures (> 1000 °C) under oxidizing conditions, electron hole conduction prevails. The conductivity has been modeled assuming that oxygen vacancies and protons compensate the acceptor doping. Transport coefficients describing mobility of defects and thermodynamic constants for the incorporation of protons have been derived.

Numerical simulation of heat transfer during growth of single vapor bubbles in nucleate boiling

In the model presented in this paper the region around a single growing vapor bubble in nucleate boiling is subdivided into three parts: a small, ring-shaped zone between heating wall and bubble, called micro-region, the bubble itself, and its surrounding liquid, referred to as macro-region. As the micro-region is most important for heat transfer, a recently developed model put special emphasis on this region, and predicted heat transfer, bubble growth, and departure diameters of vapor bubbles for low to moderate heat fluxes fairly well. The current paper aims at modeling the macro-region in more detail than done before. For this purpose, Navier–Stokes equations for both vapor and liquid phase were solved with the aid of a finite element method. It turned out that the flow pattern in the liquid around a growing vapor bubble is determined by the movement of the bubble surface, but also by the vapor flow inside the bubble. The fluid in the macro-region transports cooler liquid towards the wall, thus increasing the heat transfer significantly. In regions farther away from the bubble, heat conduction prevails. Buoyancy was found to be of small influence compared to forced convection. Velocity and temperature fields, heat fluxes, bubble contours, and departure diameters were calculated for different fluids. The apparent contact angle is decisive for growth rate and departure diameter. Disturbances, caused by departing and ascending bubbles, are included into the calculations.

Influence of electrode on electrical conduction of ZincPhthalocyanine (ZnPc) thin films

The electrical conduction properties of ZincPhthalocyanine (ZnPc) thin films have been studied using copper, silver and aluminium electrodes. The sandwich structures were prepared by the thermal evaporation method. The I–V characteristics were investigated to identify the dominant charge transport mechanism in the films. Among all possible mechanisms, it was observed that the data fits well to the SCLC type of conduction in the Al/ZnPc/Al and Schottky type of conduction prevails in the Ag/ZnPc/Ag and Cu/ZnPc/Cu devices. The trap levels and its dependence of structure have been studied and results are discussed. The charge transport phenomenon in the ZnPc films seems to depend highly on the electrode material and temperature. The carrier mobility increases with increasing temperature whereas the density of trapped holes decreases with increasing temperature. The barrier height also decreases with increase in temperature. The influence of the temperature on the electrical parameters such as saturation current density (Js), barrier height (Φb), density of states in the valence band edge Nd (m−3), the position of the Fermi level EF (eV), ionized acceptor atom density Ne (m−3), activation energy ΔΦ (eV), mobilities of hole (μ0) and the concentration of free holes in the valence band (n0) have been discussed in detail.

Magnetic and Electric Studies of a New Cu(II) Perovskite-like Material

Thermal analysis of a lipid-like bilayer of [(CH2)9 (NH3)2]CuCl4 indicates one minor transition at 303 K and the following five major ones (with entropies in J/K-mole): The initial magnetic susceptibility at 320 Hz and a magnetic field of 160 A/m in the temperature range 75 - 290 K revealed a Curie-Weiss behavior of phase (V) and possible ferromagnetic ordering of phases (VI) and (IV) at low temperatures. The variation of the dielectric permittivity with temperature at 60.0 Hz - 100 kHz indicates large changes of the dipole moment at the transition temperatures. The conductivity is thermally activated and frequency dependent, following the universal power law: σ (ω) =σdc +A(T)ωs(T). Values of s being phase dependent. For phase (I), T &gt; 383 K, band type conduction prevails. Hopping conduction is found in phases(II) and (III). Comparison with other Cuand Cd-containing materials is made. -PACS: 76, 77

Conductivity and Permittivity Studies in the Diluted Perovskite System [(NH3)(CH2)6(NH3)]FexZn1-xCl4, x=1, 0.8, 0.5, and 0

The dielectric permittivity and AC conductivity of the perovskite-like system [(NH3)(CH2)6(NH3)]FexZn1-xCl4 (HDAFxZ1-x), where x=1, 0.8, 0.5 and 0, were measured at different frequencies in the temperature range 100 K&lt;T&lt;430 K. At temperatures below 273 K, for x=1 the material exhibits a transition at (245±1) K, while for x=0 transitions at (155±5) K, (220±4) K and (255±2) K were found. A rotational-type transition in the range 295 - 305 K was found for the Fe-containing materials. Ferroelectric transitions were observed in the high temperature region for all four concentrations. Differential thermal scanning confirmed the existence of the phase transitions above room temperature. The conductivity decreases with Zn addition, an the conduction mechanism varies with the temperature and concentration. Extrinsic conduction prevails for T&lt;150 K for all concentrations. At intermediate temperatures an Arrhenius relation with frequency dependent activation energy (ΔE=0.46 - 0.06 eV) is observed for Fecontaining materials. The frequency dependent conductivity for all materials has a linear response following the power law: σac=A(T) ·ωS(T) with the exponent s varying with temperature and composition. At high temperatures, for Zn-rich materials series type conduction with s ∼ 0.6±0.1 is identified, while for Fe-rich materials band type conduction prevails. In the low temperature region ionic hopping prevails.

Raman and magneto transport studies of MBE grown β-FeSi2, β-(Fe1-xCrx)Si2, and β-(Fe1-xCox)Si2

ABSTRACTIron disilicide shows great promise as a silicon based light emitter operating in the 1.3 to 1.5 μm wavelength range. However, there exists a number of questions related to the band structure and the ability to alloy and controllably dope the material both n and p type. In this paper we present Raman and magneto transport studies on β-FeSi2, β-(Fe1-xCrx)Si2, and β-(Fe1-xCox)Si2 grown by MBE. By comparing the spectra obtained for undoped and doped samples we provide a general overview of the effects of doping on the crystallinity of the material. The temperature dependent (4K&lt;T&lt;300K) magneto transport illustrates that Cr is a p-type dopant and Co is an n-type dopant in β-FeSi2.The temperature dependence of the resistivity indicates that the transport properties at higher temperatures are determined by free carriers whereas at lower temperatures impurity band conduction prevails.

Electric and magnetic study of the new biodegradable complexes bis-(cyclopentanonyl-2,5-DI(p-aminobenzylidine)MCl4, M = Mn+2, Co+2 and Cu+2

The permittivity and AC conductivity in the temperature range 290 K- 460 K, in the frequency range 5.0 Hz-4.0 kHz, magnetic susceptibility over the temperature range 80K-460K, at different magnetic fields as well as x-ray diffraction and IR spectra have been measured for the biodegradable materials [A]2MCl4, in which A is cyclopentan-onyl-2,5-di(p-aminobenzylidine) and M = Mn+2, Co +2 and Cu +2. X-ray analysis indicated that the materials crystallize in orthorhombic unit cell of dimensions a = 15.41(1) Å, b= 16.77(1) Å, c= 19.38(1) Å, a= 15.75(1) Å, b= 16.95(1) Å, c= 19.56(1) Å and a = 15.89(l)Å, b = 17.53(l)Å, c = 20.01(2)Å for the Mn +2, Co+2 and Cu +2 materials, respectively. Electric and magnetic results indicate the following structural phase transitions: The conductivity follows Arrhenius relation. Near room temperature the activation energies are small and have nearly the same value (ΔEa∼170meV) for the three materials. At the high temperature phase, ΔE1 is frequency dependent and follows the relationship ΔE = ΔE0 [1 - exp - (f0/f)]α with Eo varies from 1.2eV (Mn) to 2.88 eV (Cu). The frequency dependence of conductivity follows the universal power law [sgrave]∼ B(T)ωs(T) Variation of s(T) and In B(T) with temperature shows discontinuities at the phase transitions. At high temperatures, phase (I), band type conduction prevails

Permittivity and ac conductivity study of the layered perovskite [(CH 3)(C 6H 5) 3P] 2MnCl 4 showing evidence of phase transition

The permittivity and ac conductivity of the compound bis(methyltriphenylphosphonium) 2tetrachloro-manganate(II) have been measured in the temperature range 300–400 K and in the frequency range 0.1–20 kHz . Differential thermal analysis reveals structural phase transition in the temperature range 365 K< T<375 K. This is reflected as a plateau in the permittivity (at f>1 kHz) and the conductivity results. The conductivity follows the Arrhenius relation with different activation energies and conduction mechanisms: region (I) → 345±3 K region (II) → 365±3 K region (III) → 372±2 K region (IV) with activation energies Δ E 1∼10 meV, Δ E 2∼1–0.2 eV and Δ E 4∼2–1 eV for temperature regions (I), (II) and (IV), respectively. In region (I) extrinsic type conduction prevails. Analysis of the experimental data based on the jump relaxation model, showed that translation motion of the proton and reorientation hopping between equivalent sites of the metal chloride anion and the phenyl groups are responsible for the observed ac conductivity in the temperature region (II). In region (IV) overlap large polarons predominates .