Intergranular Conductivity Research Articles

Microplotter Printing of Co3O4 Films as Receptor Component of Hydrogen Sulfide-Sensitive Gas Sensors

A hierarchically organized Co3O4 nanopowder was obtained via programmed chemical precipitation, exhibiting several levels of microstructural self-organization: the initial particles are 40 ± 5 nm in size (average CSR size is 32 ± 3 nm), have a somewhat distorted rounded shape and are combined into curved chains, which, in turn, form flat agglomerates of approximately 350 ± 50 nm in diameter. The thermal behavior of the semiproduct (β-Co(OH)2) was studied by means of a synchronous thermal analysis (TGA/DSC). The obtained powders were examined by X-ray diffraction analysis (XRD) and Fourier-transform infrared spectroscopy (FTIR). Nanopowder of cobalt(II,III) oxide was employed as a functional ink component for the microplotter printing of the corresponding film on the chip surface, and the preservation of the material’s crystal structure was confirmed by XRD and Raman spectroscopy (RS). The microstructural features of the resulting film were analyzed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Kelvin probe force microscopy (KPFM) was utilized to estimate the work function of the material surface, and the scanning capacitance microscopy (SCM) data indicated the intergranular conductivity type. The results of the conducted chemosensor measurements demonstrate that the printed Co3O4 film exhibits hydrogen sulfide selectivity and a rather high sensory response (S = 131% for 100 ppm) to this analyte at an operating temperature of 250 °C. The dependence of the sensor response value and time when detecting H2S in the concentration range of 4–200 ppm was determined and the high reproducibility of the signal was demonstrated.

Physical properties and superconductivity of SrTa2S5

SrTa2S5 is revealed to be a BCS superconductor with bulk superconducting transition temperature Tc = 2.27(3) K, as determined from specific heat. The superconducting transition from electrical resistivity ρ measurements is 2.40(16) K; the slightly higher value is associated with filamentary superconductivity. Intergranular conductivity dominates ρ, as surmised from its large magnitude (ρ(3 K) = 454 μΩ cm). The magnetic susceptibility is paramagnetic and temperature independent with a large Van Vleck contribution (estimated at χVV = 1.93(2)x10−4 cm3/mol); below Tc the Meissner effect is observed. Hall effect measurements show that the majority charge carriers are holes with charge-carrier concentration n(5 K) = 2.75(15) × 1020 cm−3. The electronic specific heat coefficient is observed to be γ = 7.62(14) mJ/mol K2. The energy gap associated with the superconducting state is found to be Eg = 0.575(15) meV. Measurements of ρ(T) in magnetic field allow an estimate of the superconducting coherence length ξ ∼ 3.3 Å. SrTa2S5 is classified as a Type II BCS superconductor.

The effect of organic additives on the intergranular conductivity of Al-doped ZnO

The development of ink-based solutions as alternative routes to vacuum-based deposition techniques requires methods to improve their intergranular conductivity without thermal sintering.

High-Jc YBa2Cu3O7−x-Ag superconducting thin films synthesized through a fluorine-free MOD method

Obtaining a high critical current density (Jc) remains the main challenge in developing fluorine-free metal organic deposition (MOD) methods to fabricate YBCO superconducting thin films. Silver addition was used to raise the Jc values in this research work. By reacting with propionic acid and ammonia, AgNO3 was initially mixed with YBCO carboxylate precursors dissolved in methanol. High-temperature in situ XRD measurements on the YBCO-Ag powders revealed that silver addition lowers the incongruent melting temperature of YBCO to 760°C and resulted in a smooth surface morphology of the YBCO films at a temperature as low as 760°C. Grain growth and intergranular conductivity were also found to be improved by silver doping. After annealing under optimized conditions, a high Jc of 4.6MA/cm2 was obtained in a YBCO-Ag thin film with 10wt% Ag.

Absence of the ordinary and extraordinary Hall effects scaling in granular ferromagnets at metal-insulator transition

Universality of the extraordinary Hall effect scaling was tested in granular three-dimensional Ni-SiO2 films across the metal-insulator transition. Three types of magnetotransport behavior have been identified: metallic, weakly insulating and strongly insulating. Scaling between both the ordinary and extraordinary Hall effects and material resistivity is absent in the weakly insulating range characterized by logarithmic temperature dependence of conductivity. The results provide compelling experimental confirmation to recent models of granular metals predicting transition from logarithmic to exponential conductivity temperature dependence when inter-granular conductance drops below the quantum conductance value and loss of Hall effect scaling when inter-granular conductance is higher than the quantum one. The effect was found at high temperatures and reflects the granular structure of material rather than low temperature quantum corrections.

Effect of pressure on capacitor electrodes formed with oxide nanoparticles

The effect of compactness and pressure on the conductivity of supercapacitor electrodes and their resulting electrochemical properties is studied. Mixtures of cobalt oxides MxCoO2 (M = H or Na) with carbon and polymer additives are studied under pressures up to 20 MPa. A power function links the compactness and the resistivity of the powder formulations in this pressure range, coherent with powder rearrangement and grain packing. Although the cobalt oxide grains have a metallic character, the powders remain quite resistive because of weak connections between the grains. Carbon additives enhance the intergranular conductivity in dry formulations and the resistivity mainly depends on the carbon volume content. Electrochemical measurements on a symmetric cell using formulated electrodes in a Na2SO4 electrolyte clearly show a large positive effect of the pressure on the electrochemical performances of the cell: After release of the pressure, the electrode retains a part of its optimum performance.

Microestrutura e condutividade elétrica do eletrólito sólido de céria-20% mol gadolínia com adições de SrO, TiO2 e SrTiO3

Os efeitos da adição de SrO, TiO2 e SrTiO3 na sinterização, na microestrutura e na condutividade elétrica do eletrólito sólido céria-gadolínia foram sistematicamente investigados. Foram preparadas pelo método convencional de mistura dos reagentes de partida seguida de reação em estado sólido composições contendo 1, 2,5 e 5% mol dos aditivos. Diversas técnicas de caracterização foram utilizadas. Os resultados mostraram que os aditivos exercem influência em todas as propriedades estudadas, mas de forma diferente dependendo do tipo e do teor. De forma geral, o SrO tornou desprezível o bloqueio aos portadores de carga nos contornos de grão, mas prejudicou a densificação. O TiO2 promoveu aumento substancial na densificação da céria-gadolínia, mas também no bloqueio exercido pelos contornos de grão, além de ser responsável pela exsolução do gadolínio e pela formação da fase pirocloro Gd2Ti2O7, quando adicionado em teores acima do limite de solubilidade. O SrTiO3 não produziu alterações significativas na densificação, e resultou em aumento da condutividade intergranular pela diminuição da energia de ativação para o processo de condução.

Granular and intergranular conduction in La1.32Sr1.68Mn2O7 layered manganite system

We report a comprehensive study of the electrical and magneto-transport properties of La1.32Sr1.68Mn2O7 layered manganite system under different pressures and in the temperature range of 4.2K to 300K. An increase in the resistivity at low temperature has been observed and explained in terms of Coulomb Blockade effect. Magnetoresistance data are explained by assuming that the transport occurs through the mechanism of intergranular as well as granular paths with a pronounced manifestation of Spin Polarization Tunneling phenomenon. By qualitatively comparing the relative fractions of these channels, we explain the resistivity behavior in each temperature range.

Electrical properties of cerium fluoride thin films

Thin films of ionic conductors have low internal resistance. Hence, it could be used as an electrolyte material in sensors to operate at ambient temperatures. Cerium fluoride, a unipolar fluoride ion conductor, has got a different application in electrochemical sensor. In the present work, cerium fluoride thin films have been prepared by physical vapor deposition method and their electrical properties are studied. X-ray diffraction studies reveal the polycrystalline nature of the prepared thin films and the structure of the material. Scanning electron microscopy (SEM) images show grain-like structures. Conductivity analysis of the thin films has been studied by ac impedance analysis and the maximum conductivity value is found to be 1.04 × 10−6 S cm−1. The impedance spectra emphasize intergranular conduction in the prepared thin films.

Low-temperature transport in highly boron-doped nanocrystalline diamond

We studied the transport properties of highly boron-doped nanocrystalline diamond thin films at temperatures down to 50 mK. The system undergoes a doping-induced metal-insulator transition with an interplay between intergranular conductance $g$ and intragranular conductance ${g}_{0}$, as expected for a granular system. The conduction mechanism in the case of the low-conductivity films close to the metal-insulator transition has a temperature dependence similar to Efros-Shklovskii type of hopping. On the metallic side of the transition, in the normal state, a logarithmic temperature dependence of the conductivity is observed, as expected for a metallic granular system. Metallic samples far away from the transition show similarities to heavily boron-doped single-crystal diamond. Close to the transition, the behavior is richer. Global phase coherence leads in both cases to superconductivity (also checked by ac susceptibility), but a peak in the low-temperature magnetoresistance measurements occurs for samples close to the transition. Corrections to the conductance according to superconducting fluctuations account for this negative magnetoresistance.

Fluctuation-induced tunneling conductance and enhanced magnetoresistance in polycrystallineCrO2and its composites

Intergranular conduction in half metallic ${\mathrm{CrO}}_{2}$ is known to occur through a combination of spin dependent tunneling [driven by Coulomb blockade (CB) effects] together with certain spin independent (SI) hopping processes. We present evidence that in polycrystalline ${\mathrm{CrO}}_{2}$ with enhanced grain size, both these process (CB effect and SI hopping) are suppressed and the functional form of conductance is best described by fluctuation-induced tunneling (FIT) below $200\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. In this temperature range, FIT is seen to be retained when pure ${\mathrm{CrO}}_{2}$ is diluted by two different insulators ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ and ${\mathrm{Cr}}_{2}{\mathrm{O}}_{5}$. Above $200\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, a step in conductance is observed in pure ${\mathrm{CrO}}_{2}$ which is seen to be modulated by the magnetic state of the grain boundary as well as applied magnetic field. In the vicinity of this step, a significant enhancement of magnetoresistance is observed. Overall, magnetotransport studies in these compounds reveal that a systematic variation in crystallographic microstructure in granular ${\mathrm{CrO}}_{2}$ leads to a different functional form of conductance which is intimately coupled to the enhanced magnetoresistive properties as observed here.

Consolidation and properties of Gd0.1Ce0.9O1.95 nanoparticles for solid-oxide fuel cell electrolytes

Gd-doped ceria solid solutions have been recognized to be leading electrolytes for use in intermediate-temperature fuel cells. In this paper, the preparation, solubility, and densification of Gd0.1Ce0.9O1.95 ceramics derived from carbonate co-precipitation are reported. The dissolution of Gd2O3 in CeO2 lattice was identified to be completed during the co-precipitation process by studying the lattice parameter as a function of temperature. After calcination at 800 °C for 2 h, the nano-sized Gd0.1Ce0.9O1.95 powder (∼33 nm) with a nearly spherical shape and a narrow particle-size distribution was obtained. This calcined powder has high sinterability and maximum densification rate at ∼1000 °C. Sintering at 1300 °C for 4 h yielded over 97% relative density with near maximum. The grain size increased with increases in sintering temperature. The ionic conductivity of these pellets was tested by alternating current impedance spectroscopy to elucidate the contribution of intragranular and intergranular conductivity to the total ionic conductivity. It was found that sintering temperature does not affect intragranular conductivity, though intergranular conductivity was strongly influenced by grain size, grain boundary area, and relativity density. This pellet sintered at 1500 °C for 4 h showed a high ionic conductivity of 5.90 × 10−2 s/cm when measured at 750 °C. The characterization and structural evaluation of the as-received powders were carried out using x-ray diffraction, transmission electron microscopy, Brunauer–Emmett–Teller, and dilatometer and impedance analysis.

Hydrostratigraphic characterization of intergranular and secondary porosity in part of the Cambrian sandstone aquifer system of the cratonic interior of North America: Improving predictability of hydrogeologic properties

The Upper Cambrian interval of strata in the cratonic interior of North America has a long history of inconsistent hydrogeologic classification and a reputation for marked and unpredictable variability in hydraulic properties. We employed a hydrostratigraphic approach that requires hydraulic data to be interpreted within the context of a detailed characterization of the distribution of porosity and permeability to arrive at a better understanding of these rocks. As a first step, we constructed a framework of hydrostratigraphic attributes that is a depiction of the spatial distribution of both rock matrix and secondary porosity, independent of hydraulic data such as pumping-test results. The locations of hundreds of borehole geophysical logs and laboratory measurements of rock sample matrix porosity and permeability were mapped on detailed (mostly 1:100,000 or greater), conventional, lithostratigraphic maps. Stratigraphic cross-sections, based on hundreds of natural gamma logs and thousands of water-well records, have provided a markedly improved depiction of the regional distribution of rock matrix hydrostratigraphic components. Borehole, core and outcrop observations of secondary porosity were also tied to detailed stratigraphic sections and interpolated regionally. As a second step, we compiled and conducted a large number of hydraulic tests (e.g., packer tests and borehole flowmeter logs) and analyzed thousands of specific capacity tests (converted to hydraulic conductivity). Interpretation of these data within the context of the hydrostratigraphic attributes allowed us to produce a new hydrogeologic characterization for this stratigraphic interval and gain important insights into geologic controls on hydraulic variability. There are a number of assumptions inherent in most previous hydrogeologic investigations of these strata, such as equivalency of lithostratigraphic and hydrogeologic units and the dominance of intergranular flow in sandstone, that are not consistent with our results. A particularly important outcome of our study is recognition of regionally extensive bedding-plane fracture clusters. Such exceptionally high hydraulic conductivity features dominate the hydraulics of aquifers and confining units in these siliciclastic-dominated strata, including within intervals consisting largely of friable sandstone with high intergranular conductivity. Furthermore, our results provide some measure of fracture predictability, by correlating their abundance and hydraulic importance to specific stratigraphic positions and particular depths of burial beneath younger bedrock. A discrete, consistent stratigraphic interval of fine-grained siliciclastic beds also is apparently resistant to the development of vertically interconnected fractures, making the location of this regionally extensive confining unit predictable. Our more rigorous approach of interpreting typical hydraulic tests as well as relatively new techniques of borehole flowmeter logging, within the context of a hydrostratigraphic framework, results in improved definition of individual aquifers and confining units. It also enables quantification of their hydraulic properties, which leads to improved prediction of groundwater flow paths and time-of-travel.

Effect of solute dispersion on microstructure and electrical conductivity of Ce 0.85Y 0.13Pr 0.02O 2 − δ solid electrolyte

Solid solutions of Ce 0.85Y 0.13Pr 0.02O 2 − δ were prepared by the co-precipitation and mixing of powder techniques and characterized by several techniques. The main purpose of this work was to evaluate the effect of the minority solute dispersion on the microstructure and electrical conductivity of this solid electrolyte. Praseodymium oxide co-doping of ceria–yttria produces changes in the crystallite size and in the average grain size of sintered pellets. Microstructural characterization shows that solid electrolytes with high homogeneity were obtained by the co-precipitation technique. Energy dispersive analysis and Raman spectroscopy results showed the formation of a Pr-rich solid solution of cerium oxide and praseodymium oxide in specimens prepared by the mixing of powder technique. Impedance spectroscopy results are dependent on the microstructure and show significant differences in the intergranular conductivity of sintered pellets prepared by different methods.

Effect of mechanical deformation on the critical current in YBa2Cu3O7−b superconducting films

It is shown that local mechanical bending of YBa2Cu3O7−b Ag-coated superconducting films deposited on flexible metal substrates in the temperature interval 77–300 K may increase the critical density Jc of the transport current to values as high as 106 A/cm2 or even higher at 77 K. Also, bending decreases the voltage criterion. This means a rise in the intergranular conductivity and, accordingly, a reduction of Joule losses.

Influence of the precursor purity and the precipitating agent on impedance spectroscopy of CeO 2:Y 2O 3 ceramics

In this work, yttria-doped ceria solid solutions were prepared to produce dense ceramic specimens with high chemical and microstructural homogeneities, and with high ionic conductivity. Cerium nitrate with different degrees of purity and yttrium oxide were used as starting materials. The co-precipitation technique was performed using either ammonium hydroxide or oxalix acid as precipitating agent. The main results show that the preparation of nanosized powders of yttria-doped ceria does not depend on the precipitating agent used during synthesis. The average grain sizes are found to be different in sintered specimens prepared from powders following the two routes, thereby affecting the electrical resistivity measured by impedance spectroscopy. The intergranular conductivity is also dependent on the purity of the precursor material.

Influence of microstructure on the ionic conductivity of yttria-stabilized zirconia electrolyte

Yttria-stabilized zirconia (YSZ) electrolytes with diverse microstructures were prepared by using nano-size (Y 2O 3) 0.08(ZrO 2) 0.92 powders as precursors through conventional sintering in air. The electrolytes were tested by AC impedance spectroscopy to elucidate the contribution of intragranular and intergranular conductivity to the total ionic conductivity. The intragranular conductivity and intergranular conductivity were correlated with the microstructures of the electrolyte to interpret the transportation of oxygen ions through the electrolyte. The intragranular conductivity was found to be dominated mainly by the relative density while the intergranular conductivity strongly depended on the grain size and grain boundary area of the electrolyte. The sintering temperature and isothermal time dependence of ionic conductivity reached a maximum value of 0.105 S/cm at a sintering temperature of 1350 °C for 4 h and 0.112 S/cm at a holding time of 8 h at 1250 °C when measured at 1000 °C, respectively. Concepts for improving the ionic conductivity of YSZ electrolyte were reviewed.

The calculated structure, stability and composition of the low index surfaces of La 2CuO 4 and Nd 2CuO 4

The {100} and {001} surfaces of La 2CuO 4 and Nd 2CuO 4 are perpendicular and parallel to their (super)conducting CuO 2 planes. Surface and intergranular conductivity and weak links will be influenced by the integrity of these surfaces. This integrity will evolve during the processing of the semi-conducting precursor compounds and will be “frozen-in” when the newly formed lanthanum copper oxide is cooled below the superconducting transition temperature. Therefore, the structures and stabilities of the La 2CuO 4 surfaces and the effect of impurity segregation (Ca 2+, Sr 2+, Ba 2+, Ce 4+ and Th 4+) at these surfaces have been investigated by static lattice simulation. The greatest perturbation of the CuO 2 planes in the pure materials is predicted at the {100} surface of La 2CuO 4, the depth of which is comparable to the coherence length of the superconducting phases. Impurity segregation at {100} surface copper sites of La 2CuO 4 is also predicted, in contrast to the other surfaces.

Prebreakdown conduction in zinc oxide varistors: Thermionic or tunnel currents and one-step or two-step conduction processes

This letter is aimed at understanding the intergranular conduction in zinc oxide varistors. In particular, we have studied current-voltage and Arrhenius plots of the resistive current for the prebreakdown conduction. A current-temperature (I-T) relation has been found to present two breaks at ∼370 and ∼240 K. Although this result can be interpreted as a consequence of distinct regimes, we found that experimental I-T curves can be theoretically reproduced by calculating currents for a double Schottky barrier. Experimental results were fitted with one- and two-step transport process models. It was found that a two-step model better reproduces the electrical properties of these devices.

Energy-dispersive x-ray mapping of grain boundaries in high-Tc superconductors

Superconductors are materials which carry electrical current without resistive loss below a certain critical temperature (Tc). There are many potential uses for materials that are superconducting, but until recently the Tc's of known materials were too low to be useful in large-scale applications such as power transmission. However, with the discovery of high Tc oxide superconductors, the feasibility of such projects are now being considered. The problem with the oxide superconductors is not that their critical temperature is too low (the Tc's of the oxides are almost an order of magnitude better than the metal superconductors), but rather that in bulk form their current carrying capacity (Jc) is too low. It is known that the bulk Jc is determined by intergranular conductivity. Low values for Jc may occur because of: (a) a change of stoichiometry at the grain boundaries, or (b) because of misorientation of adjacent grains. High Jc's can be achieved in thin films by texturing the material so that there are few grain boundaries across the direction of current flow but many grain boundaries perpendicular to the current flow.