Total Resistivity Research Articles

In-plane gas permeability and thought-plane resistivity of the gas diffusion layer influenced by homogenization technique and its effect on the proton exchange membrane fuel cell cathode performance

In this study, in-plane gas permeability and through-plane resistivity of gas diffusion layer (GDL) made by different microporous layers (MPLs) were characterized by a new combined measurement setup under different compression ratios. Applied MPL ink-homogenizing techniques were pulse sonication, continuous sonication, bath ultrasonic and mechanical stirring. Results showed that 20% GDL compression produced optimized in-plane gas permeability and through plane resistivity, which is suitable for application in proton exchange membrane fuel cell (PEMFC). However, bath ultrasonic technique is more desirable technique for MPL ink homogenization in term of obtained gas permeability and total resistivity values; 0.45 (×10−12 m2) and 2.63 (mΩ cm2) respectively.Morphological investigation was also presented a uniformly distributed MPL microstructure without large cracks for the MPL which was prepared by bath ultrasonic technique (MPL-C).The Obtained results in H2/air PEMFC under various cathode relative humidity (RHC), 33–100%, revealed that the performance of the MEA made by MPL-C is 20% higher than others; 504 mWcm−2 versus 385 mWcm−2at 33% RHC. More analysis by electrochemical impedance spectroscopy demonstrated that MPL ink preparation by bath ultrasonic techniques resulted a lower ohmic and charge transfer resistance in PEMFC at least in small scale MEA.

Evaluation of the Different Test Methods of the Concrete Durability for the Persian Gulf Environment

Environmental conditions of the Persian Gulf in terms of the durability of the reinforced concrete structures are highly aggressive and corrosive. Hence, in the Persian Gulf and Oman sea areas, based on the national code of practice, concrete durability is controlled by water absorption, water permeability and rapid chloride ion penetration tests. In this research, the above three mentioned tests plus total water absorption test, electrical resistivity, and capillary sorption tests were performed on 216 samples from six concrete mixtures with different water to binder ratios and pozzolan content (fly ash and silica fume). Based on the experimental results, the total water absorption and the water permeability tests were the most and the less accurate tests respectively. Results also showed that the capillary sorption test can be used as a suitable method to evaluate the concrete durability in the marine areas. In this study also a relationship between the specific limits of these tests for different environmental conditions was proposed.

Thermal Expansion and Electrical Resistivity Studies of Nickel and ARMCO Iron at High Temperatures

The electrical resistance, \(\rho (T)\), and thermal expansion coefficient, \(\beta (T)\), of nickel and ARMCO iron have been simultaneously measured over a wide temperature range from (300 to 1100) K. The well-known standard four-probe potentiometric method was used for measurements of the electrical resistance. The thermal expansion coefficient was measured using the quartz dilatometer technique. Both techniques were combined in the same apparatus for simultaneous measurements of the electrical resistance and TEC for the same specimen. The combined expanded uncertainty of the electrical resistance and thermal expansion coefficient measurements at the 95 % confidence level with a coverage factor of \(k = 2\) is estimated to be 0.5 % and (1.5 to 4.0) %, respectively. The distinct \(\rho (T)\) scattering contribution (phonon \(\rho _{\mathrm{ph}}\), magnetic \(\rho _{\mathrm{m}}\), and residual \(\rho _{\mathrm{S}}\)) terms were separated and extracted from the measured total resistivity. The physical nature and details of the temperature dependence of the electrical resistance of solid materials and correct estimations of the contributions of various scattering mechanisms to the measured total resistivity were discussed in terms of the anharmonic effect. We experimentally found simple, universal, physically based, semiempirical linear correlations between the kinetic coefficient (electrical resistance) and a thermodynamic (equilibrium) property, the thermal expansion coefficient, of solid materials. The developed, physically based, correlation model has been successfully applied for nanoscale materials (ferromagnetic nickel nanowire). A new s-d-exchange interaction energy determination technique has been proposed.

Microwave synthesis & sintering of Sm and Ca co-doped ceria ceramics

In this work we report on the combined use of microwave (MW) heating sources for the powder synthesis and the ceramic sintering of Sm and Ca co-doped ceria Ce0.8Sm0.18Ca0.02O1.9 polycrystalline materials for potential application as an electrolyte in intermediate-temperature solid oxide fuel cells (IT-SOFCs). We investigate the crystal structure, ceramic microstructure and the oxygen ion conductivity in detail and compare the latter to conventionally sintered ceramics. MW sintering of ceramic pellets leads to only slightly increased resistivity as compared to conventional sintering, but offers massive energy and time savings for potential industrial production processes. Exceptionally high oxygen ion conductivity without any significant electronic contribution was found in MW synthesized and MW sintered pellets, where the total resistivity, composed of grain boundary and bulk contributions, was found to be in the range of 0.5–1 kΩ at 500 °C. Sm- and Ca co-doped ceria may be well-suited for electrolyte materials in IT-SOFCs.

Effects of final annealing in oxygen on characteristics of BaTiO3 thin films for resistance random access memory

We have prepared BaTiO3 thin films by metal–organic decomposition, focusing on the effects of a final annealing in oxygen. In particular, we investigated the relation between the resistive switching and ferroelectric properties and the behaviors of the resultant resistive hysteresis and switching in the films. The films annealed in oxygen showed the hysteresis of bipolar-type switching with a current on/off ratio from 100 to 1000 for both bias polarities. It was particularly noted that abrupt peaks of the leakage current density occurred in the hysteretic resistivity curve. We consider that the abrupt increase in current density is related to the polarization reversal due to ferroelectricity in the BaTiO3 film. However, the main resistive switching mode was not affected by this behavior, thus suggesting that polarization reversal is insufficient to control the hysteresis in the total resistivity.

Impedance analysis and dielectric properties of Ce modified bismuth titanate lead free ceramics synthesized using solution combustion route

Cerium modified bismuth titanate ceramics were studied as potential lead-free ferroelectric materials over a broad temperature range. Polycrystalline samples of Bi4−xCexTi3O12 (x = 0.2, 0.4, 0.6, 0.8) (BCeT) were prepared using the solution combustion technique. The effect of Ce doping on their crystalline structure, ferroelectric properties and electrical conduction characteristics were explored. An increase in the dielectric constant and decrease in loss tangent is observed due to addition of Ce in bismuth titanate lattice up to a certain threshold doping concentration. The presence of pyrochlore phase decreases the density of ceramics with composition x = 0.8 which may be due to grain inhibitor property of Ce. The value of remnant polarization increases with increasing doping content up to x = 0.6 due to decrease in defect sites in the samples, further increase in cerium content induces leakage current which makes the sample lossy. The real–imaginary impedance plots are interpreted using RC model, this explains the contribution of the grain-bulk and the grain boundary resistivities to the total resistivity of the materials. Activation energy increases with increasing Ce concentration indicating a decrease in defect sites due to which the conductivity of the doped ceramics reduces.

Multiferroic (NiZn) Fe2O4–BaTiO3 composites prepared from nanopowders by auto-combustion method

Nickel zinc ferrite (NZF) and barium titanate (BT) were prepared by auto-combustion synthesis as an effective, simple and rapid method. Multiferroic composites with the general formula yNi1−xZnxFe2O4−(1−y)BT (x=0.3, 0.5, 0.7, y=0.5) were prepared by mixing NZF and BT powders in a liquid medium in the ball mill. The FEG micrographs indicated the primary particle size less than 100nm for both, barium titanate and nickel zinc ferrite phases. X-ray analysis and Raman spectroscopy indicated the formation of well crystallized structure of NZF and BT phase in the composite powders and ceramics, with a small contribution of the secondary phase. The homogenous phase distribution in obtained composites was also confirmed. Impedance spectroscopy measurements were carried out in order to investigate the electrical resistivity of materials, showing that grain boundaries have greater impact on the total resistivity than grains. Saturation magnetization and remnant magnetization continuously decrease with barium titanate phase increase.

Joint inversions of three types of electromagnetic data explicitly constrained by seismic observations: results from the central Okavango Delta, Botswana

The Okavango Delta of northern Botswana is one of the world's largest inland deltas or megafans. To obtain information on the character of sediments and basement depths, audiomagnetotelluric (AMT), controlled-source audiomagnetotelluric (CSAMT) and central-loop transient electromagnetic (TEM) data were collected on the largest island within the delta. The data were inverted individually and jointly for 1-D models of electric resistivity. Distortion effects in the AMT and CSAMT data were accounted for by including galvanic distortion tensors as free parameters in the inversions. By employing Marquardt–Levenberg inversion, we found that a 3-layer model comprising a resistive layer overlying sequentially a conductive layer and a deeper resistive layer was sufficient to explain all of the electromagnetic data. However, the top of the basal resistive layer from electromagnetic-only inversions was much shallower than the well-determined basement depth observed in high-quality seismic reflection images and seismic refraction velocity tomograms. To resolve this discrepancy, we jointly inverted the electromagnetic data for 4-layer models by including seismic depths to an interface between sedimentary units and to basement as explicit a priori constraints. We have also estimated the interconnected porosities, clay contents and pore-fluid resistivities of the sedimentary units from their electrical resistivities and seismic P-wave velocities using appropriate petrophysical models. In the interpretation of our preferred model, a shallow ∼40 m thick freshwater sandy aquifer with 85–100 Ωm resistivity, 10–32 per cent interconnected porosity and <13 per cent clay content overlies a 105–115 m thick conductive sequence of clay and intercalated salt-water-saturated sands with 15–20 Ωm total resistivity, 1−27 per cent interconnected porosity and 15–60 per cent clay content. A third ∼60 m thick sandy layer with 40–50 Ωm resistivity, 10–33 per cent interconnected porosity and <15 per cent clay content is underlain by the basement with 3200–4000 Ωm total resistivity. According to an interpretation of helicopter TEM data that cover the entire Okavango Delta and borehole logs, the second and third layers may represent lacustrine sediments from Paleo Lake Makgadikgadi and a moderately resistive freshwater aquifer comprising sediments of the recently proposed Paleo Okavango Megafan, respectively.

Analysis of the degradation of BaTiO3 resistivity due to hydrogen ion incorporation: Impedance spectroscopy and diffusion analysis

The resistivity of insulator oxide materials can be compromised when they are exposed to hydrogen gas at high temperatures, as insulation resistance can significantly decrease. In this study, we differentiate between electrode, grain boundary, and grain contributions to total resistivity by impedance spectroscopy (IS) tests and a 3RC model. It turns out that the largest contribution to total resistivity comes from electrode Schottky barriers, which control the major part of the degradation. Based on the IS analysis, the hydrogen diffusion coefficients of those three components were successfully calculated and compared with the diffusion coefficient in other systems. Determination of the hydrogen diffusion in grains and grain boundaries is important in understanding how hydrogen penetrates to capacitors and can also be useful for applications that involve extreme environments. In this study, we also considered the kinetics and role of the metal electrode chemistry (Ag, Au, and Pt) and the thickness of active layers on the hydrogen degradation.

Influence of Substrate on Conductivity of Sr and Mg Doped Lanthanum Gallate Thin Film Electrolyte for Solid Oxide Fuel Cell

In order to reveal the influence of substrate on the conductivity change of La0.9Sr0.1Ga0.8Mg0.2O3-δ (LSGM) thin film from the bulk, we prepared LSGM thin films on the sapphire and the quartz substrates by radio frequency magnetron sputtering, and evaluated the crystallinity, the microstructure, and the conductivity. The lattice constant of 0.6 μm thick film was different between the substrates. Compared with the bulk, the lattice constant of the thin film on the sapphire substrate was decreased 1.3%, and that on the quartz substrate was increased 0.7%. The grains in the thin film on the quartz substrate were approximately 0.2 μm in size, and those on the sapphire substrate were approximately 0.1 μm in size. The conductivity was measured at 710 °C by electrochemical impedance spectroscopy. The conductivity of the thin film on the quartz substrate was σ = 2.2 × 10-3 Scm-1 and that on the sapphire substrate was 9.0 × 10-4 Scm-1. The reason why the thin film on the quartz substrate had higher ion conductivity than that on the sapphire substrate can be attributed to the higher density of the grain boundaries which govern the total resistivity of the film on the quartz substrate than that on the sapphire substrate.

A Resistivity Model for Ultrathin Films and Sensors

Gas sensors have been demonstrated based on the conductivity changes in ultrathin films. These sensors operate in a regime where three different physical phenomena determine the total resistivity of the film; quantum mechanical coupling between metallic islands, bulk material conductivity of the islands, and network resistivity. We present a lumped parameter model that simulates thin-film growth and calculates the total film resistance during the growth process accounting for these three phenomena. The model contains four free parameters and yields a good agreement with experimental data presented for palladium, titanium, and gold. The primary benefit of this model is that it shows the relative contribution of each source of conductivity during the growth process providing insight into the operation of ultrathin films as gas sensors. We then model an ultrathin-film palladium-based hydrogen sensor and show that the sensing mechanism is primarily due to variations in quantum tunneling.

Shielding technique for deposition of Au electrical contacts on graphene by sputtering

Here, the authors report on a novel shielding technique for the fabrication of electrical contacts on exfoliated graphene by sputtering and lift-off process. The technique solves this problem by removing unwanted gold film in patterning contacts and reduces the high contact resistivity typically found in sputtered devices ranging from 260 to 940 kΩ μm induced by sputtered Au on graphene. By using a shielding tube integrated into our sputtering machine and optimizing the sputtering parameters, contact resistivity as low as 1.04 kΩ μm has been achieved. Consequently, the total device resistivity is significantly reduced, and the yield rate of the devices fabrication has also increased from 17% to 90%.

Nonlinear properties of antimony-doped BaTiO3 ceramics

Barium titanate nanopowders doped with various concentrations of antimony were synthesized by polymeric precursors method based on modified Pechini process. Obtained powders were pressed and sintered at 1,573 K for 8 h. The temperature dependence of dielectric permittivity pointed on the significant influence of added antimony on dielectric properties of obtained materials. The analysis of impedance spectrum in the temperature range 250–445 K has shown the presence of one semicircle that corresponds to the grain contribution but impedance spectrum at 773 K showed possible overlapping of two semicircles indicating also grain boundary resistivity. The total specific resistivity obtained from impedance analysis showed modest PTCR jump in the heavily doped barium titanate ceramics. The electric field dependence of permittivity of the ceramics was investigated in detail. The experimental data were analyzed using Johnson model.

Measurement of the displacement cross-section of copper irradiated with 125 MeV protons at 12 K

To validate Monte Carlo codes for the prediction of radiation damage in metals irradiated by >100MeV protons, the defect-induced electrical resistivity changes related to the displacement cross-section of copper were measured with 125MeV proton irradiation at 12K. The cryogenic irradiation system was developed with a Gifford–McMahon cryocooler to cool the sample via an oxygen-free high-conductivity copper plate by conduction cooling. The sample was a copper wire with a 250-μm diameter and 99.999% purity sandwiched between two aluminum nitride ceramic sheets. The electrical resistivity changes of the copper wire were measured using the four-probe technique. After 125MeV proton irradiation with 1.45×1018 protons/m2 at 12K, the total resistivity increase was 4.94×10−13Ωm (resistance increase: 1.53μΩ), while the resistivity of copper before irradiation was 9.44×10−12Ωm (resistance: 29.41μΩ). The resistivity increase did not change during annealing after irradiation below 15K. The experimental displacement cross-section for 125MeV irradiation shows similar results to the experimental data for 1.1 and 1.94GeV. Comparison with the calculated results indicated that the defect production efficiency in Monte Carlo codes gives a good quantitative description of the displacement cross-section in the energy region >100MeV.

Sputtered metal oxide broken gap junctions for tandem solar cells

Broken gap metal oxide tunnel junctions have been created for the first time by sputtering. Using a ceramic ZnO–SnO2 target and a reactively sputtered copper target we deposited ZnSnO3 and Cu2O for the n-type and p-type layers, respectively. The band edges and work functions of these materials are suitable for favorable alignment with the bands of copper indium gallium selenide (CIGS) for a tandem CIGS-based solar cell applications. Total junction specific resistivities under 1Ω-cm2 have been achieved with Ohmic current–voltage (I–V) characteristics pointing to a broken gap band alignment. Low temperature I–V measurements confirmed the lack of traps at the interface despite other measurements pointing to an interface where bands overlap. Cu2O films contained copper inclusions, but they were shown, by conductive atomic force microscopy, not to be the dominant paths for conduction across the junction. Post-deposition annealing of junctions demonstrated thermal stability up to 300°C, and the ability to improve conduction and influence device material’s electron affinity by rapid thermal anneal (RTA). Optical transmission over 78% below a band gap of 2.4eV was attained for as-deposited films.

Structure and properties of Ni–Zn ferrite obtained by auto-combustion method

Abstract Nanosized nickel zinc ferrite powders with general formula Ni1−xZnxFe2O4 (x=0.0, 0.3, 0.5, 0.7, 1.0) were synthesized by an auto-combustion method starting from metal nitrates. X-ray analysis indicated the formation of well crystallized Ni–Zn ferrite phases, and Raman spectroscopy enabled a precise phases identification. The change of the grain size and density with increasing Zn content was confirmed by scanning electron microscopy. Magnetization of saturation and remnant magnetization, both, continuously increase up to x=0.3 of Zn, and then decrease for more Zn. The influence of the grain size of investigated systems on the saturation magnetization was studied. Impedance spectroscopy measurements were carried out in order to investigate the electrical resistivity of materials, showing that grain boundaries have greater impact on total resistivity that grains.

Thermal resistance of indium coated sapphire–copper contacts below 0.1 K

High thermal resistances exist at ultra-low temperatures for solid–solid interfaces. This is especially true for pressed metal–sapphire joints, where the heat is transferred by phonons only. For such pressed joints it is difficult to achieve good physical, i.e. thermal contacts due to surface irregularities in the microscopic or larger scale. Applying ductile indium as an intermediate layer reduces the thermal resistance of such contacts. This could be proven by measurements of several researchers. However, the majority of the measurements were performed at temperatures higher than 1K. Consequently, it is difficult to predict the thermal resistance of pressed metal–sapphire joints at temperatures below 1K.In this paper the thermal resistances across four different copper–sapphire–copper sandwiches are presented in a temperature range between 30mK and 100mK. The investigated sandwiches feature either rough or polished sapphire discs (Ø 20mm×1.5mm) to investigate the phonon scattering at the boundaries. All sandwiches apply indium foils as intermediate layers on both sides of the sapphire. Additionally to the indium foils, thin indium films are vapour deposited onto both sides of one rough and one polished sapphire in order to improve the contact to the sapphire.Significantly different thermal resistances have been found amongst the investigated sandwiches. The lowest total thermal resistivity (roughly 26cm2K4/W at 30mK helium temperature) is achieved across a sandwich consisting of a polished sapphire with indium vapour deposition. The thermal boundary resistance between indium and sapphire is estimated from the total thermal resistivity by assuming the scattering at only one boundary, which is the warm sapphire boundary where phonons impinge, and taking the scattering in the sapphire bulk into account. The so derived thermal boundary resistance agrees at low temperatures very well with the acoustic mismatch theory.

Colonisation and community structure of benthic diatoms on artificial substrates following a major flood event: A case of the Kowie River (Eastern Cape, South Africa)

A major flooding event that occurred during October–November 2012 caused major changes in the Kowie River hydromorphology and aquatic communities. The aim of our study was to identify the environmental variables that structure riverine benthic diatom communities at upstream and downstream locations 25 km apart on the Kowie River, South Africa. This was undertaken using tiles as artificial substrates so that we could study how the communities developed after the flood disturbance. The diatom community structure was assessed over a 28-day period following a flood event in October 2012. The Mann Whitney test indicated that there was a statistically significant difference (p &lt; 0.05) in total dissolved solids, salinity, pH and oxygen reduction potential between the two sites. In total, 58 diatom species belonging to 30 genera were identified over the 28-day study. Achnanthidium minutissimum, Fragilaria biceps, F. ulna var. acus, Pinnularia borealis and P. acrosphaeria were the most numerically dominant on Day 7 and were considered as early colonisers, while on Day 28, Achnathidium minutissimum, F. capucina, Craticula buderi, C. vixnegligenda, Diploneis subovalis and Gomphonema venusta, the late colonisers, were dominant. The species richness increased from 13 (upstream location) on Day 7 to 22 (both locations) by Day 21. A redundancy analysis showed that total suspended solids, salinity, resistivity, pH and oxygen reduction potential were the most significant physico-chemical variables explaining diatom composition. The results from this relatively small-scale tile experiment indicate the complexity of freshwater benthic diatom community structure and development.Keywords: water flow, total dissolved solids, Fragilaria sp., tile substrate, resistivity

Influence of crystal field excitations on thermal and electrical resistivity of normal rare‐earth metals

A simple formula describing the influence of the crystalline electric field free‐ion excitations on the temperature dependence of the contribution of the s–f scattering to the thermal resistivity of normal rare‐earth metals is presented. The corresponding formula for the electrical resistivity is also given and compared to the one being currently used. Theoretical electron–phonon scattering contributions derived in earlier papers and constant impurity scattering contributions are added to the derived s–f contribution formulae in order to fit the total electrical and thermal resistivity represented as functions of the temperature to experimental dependences on the temperature for DyIn and in this way to manifest applicability of the derived formulae to real materials.

Cooperative constrained inversion of multiple electromagnetic data sets

We evaluated a method for cooperatively inverting multiple electromagnetic (EM) data sets with bound constraints to produce a consistent 3D resistivity model with improved resolution. Field data from the Antonio gold deposit in Peru and synthetic data were used to demonstrate this technique. We first separately inverted field airborne time-domain EM (AEM), controlled-source audio-frequency magnetotellurics (CSAMT), and direct current resistivity measurements. Each individual inversion recovered a resistor related to gold-hosted silica alteration within a relatively conductive background. The outline of the resistor in each inversion was in reasonable agreement with the mapped extent of known near-surface silica alteration. Variations between resistor recoveries in each 3D inversion model motivated a subsequent cooperative method, in which AEM data were inverted sequentially with a combined CSAMT and DC data set. This cooperative approach was first applied to a synthetic inversion over an Antonio-like simulated resistivity model, and the inversion result was both qualitatively and quantitatively closer to the true synthetic model compared to individual inversions. Using the same cooperative method, field data were inverted to produce a model that defined the target resistor while agreeing with all data sets. To test the benefit of borehole constraints, synthetic boreholes were added to the inversion as upper and lower bounds at locations of existing boreholes. The ensuing cooperative constrained synthetic inversion model had the closest match to the true simulated resistivity distribution. Bound constraints from field boreholes were then calculated by a regression relationship among the total sulfur content, alteration type, and resistivity measurements from rock samples and incorporated into the inversion. The resulting cooperative constrained field inversion model clearly imaged the resistive silica zone, extended the area of interpreted alteration, and also highlighted conductive zones within the resistive region potentially linked to sulfide and gold mineralization.