Showing papers on "Conductivity published in 1994"

Impedance Spectroscopy of Hydrating Cement‐Based Materials: Measurement, Interpretation, and Application

Abstract: This work concerns the state of the art for use of impedance spectroscopy for studying the evolving microstructure of cement-based materials during hydration. Features of the spectra are discussed and related to components of the microstructure with the assistance of pixel-based computer modeling techniques. It is proposed that the enormously high relative dielectric constants (∼105) observed just after set are the result of dielectric amplification and are related to the distribution of pore sizes and the thickness of product C─S─H layers separating the pores. The conductivity is related to the volume fraction of porosity, the conductivity of the pore solution, and the interconnectivity of the porosity. The conductivity, when normalized by that of the pore solution, i.e., inverse formation factor, is a measure of this interconnectivity and can be used to predict such engineering properties as ionic diffusivity and water permeability. Composite mixing laws are employed to aid in explaining the behavior of the conductivity and to obtain a qualitative measure of the pore shape with hydration. Procedures for predicting the conductivity of the pore solution and for subtracting out electrode lead effects at high frequency are discussed.

Thermal properties of organic and inorganic aerogels

Abstract: Aerogels are open-cell foams that have already been shown to be among the best thermal insulating solid materials known. This paper examines the three major contributions to thermal transport through porous materials, solid, gaseous, and radiative, to identify how to reduce the thermal conductivity of air-filled aerogels. We found that significant improvements in the thermal insulation property of aerogels are possible by (i) employing materials with a low intrinsic solid conductivity, (ii) reducing the average pore size within aerogels, and (iii) affecting an increase of the infrared extinction in aerogels. Theoretically, polystyrene is the best of the organic materials and zirconia is the best inorganic material to use for the lowest achievable conductivity. Significant reduction of the thermal conductivity for all aerogel varieties is predicted with only a modest decrease of the average pore size. This might be achieved by modifying the sol-gel chemistry leading to aerogels. For example, a thermal resistance value of R = 20 per inch would be possible for an air-filled resorcinol-formaldehyde aerogel at a density of 156 kg/m3, if the average pore size was less than 35 nm. An equation is included which facilitates the calculation of the optimum density for the minimum total thermal conductivity, for all varieties of aerogels.

Electrochemical Properties of Organic Liquid Electrolytes Based on Quaternary Onium Salts for Electrical Double‐Layer Capacitors

Abstract: The electrolytic conductivities and limiting reduction and oxidation potentials for various organic liquid electrolytes based on quaternary onium salts have been measured to find better electrolytes for electrical double‐layer capacitors. An electrolyte composed of tetraethylammonium cation, tetrafluoroborate anion, and propylene carbonate solvent showed well‐balanced performance of high electrolytic conductivity, a wide stable potential window and resistance to hydrolysis. Among quaternary onium salts, triethylmethylammonium, ethylmethylpyrrolidinium, and tetramethylenepyrrolidinium tetrafluoroborate salts exhibited higher electrolytic conductivity than the conventional tetraethylammonium salt due to their much greater solubility.

Predicting the effect of temperature on soil thermal conductivity

Abstract: Simulation of soil temperatures under forest and range fires requires reliable estimates of soil thermal properties over a range of temperatures from ambient to about 600°C. We measured thermal conductivity of soil samples differing in texture, bulk density, water content, and temperature and then fit the results with a modification of the de Vries equation. Thermal conductivity increases dramatically with temperature in moist soil, reaching values 3 to 5 times the ambient value at 90°C. The modified de Vries theory agreed well with data at low temperature, and provided an upper boundary for measurements at higher temperatures. Only four parameters are required to specify thermal conductivity as a function of bulk density, temperature, and water content: thermal conductivity of the mineral fraction, water content at which liquid flow becomes limiting, a power for the liquid flow function, and a shape factor. Adequate estimates of the conductivity of the mineral fraction can be obtained from handbooks if the soil mineralogy is known, and the water content for liquid return flow can be predicted from soil texture. The other two parameters show a fairly narrow range of variation and can probably be estimated with sufficient accuracy for most simulation purposes.

Photoconductivity of Ultrathin Zinc Oxide Films

Abstract: Electrical and photoelectrical properties of nondoped and doped zinc oxide films coated on glass plates by the dip-coating method are investigated at room temperature in various ambient atmospheres. The dark conductivity of the nondoped films exponentially decreased with decreasing film thickness while the conductivity under illumination of 350 nm light was almost constant at 100 Scm-1 irrespective of the film thickness. Consequently thinner films showed larger photoresponse than thicker films. This thickness dependence is explained by the variation of ZnO particle size with the film thickness (fine particle model) and the additional effect of the Schottky barrier generated between the film and gold electrodes.

Physical Properties of Mixed Conductor Solid Oxide Fuel Cell Anodes of Doped CeO2

Abstract: Samples of CeO2 doped with oxides such as CaO and Gd203 were prepared. Their conductivities and expansions on reduction were measured at 1000~ and the thermal expansion coefficients in the range 50 to 1000~ were determined. The ionic and electronic conductivity were derived from curves of total conductivity vs. oxygen partial pressure. For both types of conductivity a dependence on dopant valency was observed. The electronic conductivity was independent of dopant radius in contrast to the ionic which was highly dependent. These measured physical properties are compared with the ideal requirements for solid oxide fuel cell anodes. Not all requirements are fulfilled. Measures to compensate for this are discussed.

Toward real applications of conductive polymers

Abstract: The chemical polymerization of pyrrole in the presence of textile substrates results in the formation of electrically conducting, polypyrrole coated fabrics. Applications of these materials include microwave attenuation, static charge dissipation, and EMI shielding. The degradation kinetics of the electrical conductivity of these materials has been investigated and found to proceed by multiple rate laws. In the initial stages of conductivity loss, a diffusion controlled mechanism predominates, where oxygen diffusion into the polypyrrole film is rate limiting. At longer times, the degradation follows a simple first-order decay. The dopant anion employed during polymerization has a profound influence on the stability of the resulting film, and SEM analysis indicate that film morphology plays an important role in this effect. In the absence of oxygen, conductivity loss is significantly slower and displays a strong dependence on chloride content.

High Li+ Conducting Ceramics

Abstract: In this Account, the authors describe one of the most suitable ways for preparing a high Li+ conducting polycrystalline material An excellent conductivity was realized by detailed studies of both the lattice size in the Li+ migrating bulk (intragrain) and the condition of the grain boundaries (integrain) 55 refs, 8 figs, 1 tab

Adsorbate effects on photoluminescence and electrical conductivity of porous silicon

Abstract: Porous silicon (p type) has been exposed to several chemical vapors at various partial pressures. The quenching of the photoluminescence by the adsorbates has been quantified and correlation to the electrical conductivity of the porous silicon sample has been studied. Some gases, e.g., water and benzene, have a small effect on the photoluminescence and on the conductivity, while others, e.g., methanol, reduce the photoluminescence by a factor of 2 and increase the conductivity by four orders of magnitude. This is accompanied with a qualitative change in the current‐voltage characteristics. These changes have been found to be reversible and the temporal behavior of the system has been investigated.

Thermal conductivity of sputtered and evaporated SiO2 and TiO2 optical coatings

Abstract: The thermal conductivity of SiO2 and TiO2 optical coatings are measured in the temperature range 80–400 K. For SiO2, the thermal conductivity of sputtered and evaporated films are 77% and 55% of the bulk value, respectively, independent of temperature. Similarly, the thermal conductivity of evaporated TiO2 is 63% of the conductivity of sputtered TiO2.

Conversion of chemically deposited photosensitive CdS thin films to n‐type by air annealing and ion exchange reaction

Abstract: A method is presented for the deposition of CdS thin films of 005–07 μm thickness from solutions at 50–70 °C containing citratocadmium(II) complex ions and thiourea The films show an optical band gap Eg≳26 eV Optical transmittance is about 80% for photon energy

Heat transfer in foams

Abstract: One of the primary applications of polymeric foams is for thermal insulations Because of the modest proportion of solid in a foam and the consequent large volume fraction of gas which has a much lower thermal conductivity the resultant conductivity of the foam is much less than that of a solid body made of the same material In this chapter the foam conductivity refers to the effective value exhibited by the foam It is the ratio of the rate of heat transfer per unit cross-sectional area to the applied temperature difference

Frequency domain analysis of time domain reflectometry waveforms: 2. A four‐component complex dielectric mixing model for soils

Abstract: Although time domain reflectometry (TDR) is becoming accepted as an important tool for the measurement of soil water content and bulk soil electrical conductivity, a major part of the method is based on empirical relationships. An improved understanding of dielectric measurements on soils may give more insight into soil properties other than soil water content and bulk soil electrical conductivity. Frequency domain analysis of TDR waveforms enables the measurement of the frequency dependent complex dielectric permittivity of soils. The frequency dependent complex dielectric permittivity of soils can be described with a four-component complex dielectric mixing model based on the volumetric mixing of the refractive indices of the soil components. The four soil components in the model are air, solids, bound water, and free water. Results indicate that the apparent dielectric permittivity obtained from the travel time of the TDR pulse in the soil is the dielectric permittivity at the highest measurement frequency of the cable tester, probe, and soil system. The model based on the volumetric mixing of real permittivities underestimates the measurements in situations with high values of the imaginary part of the dielectric permittivity. Because the model based on the mixing of the complex dielectric permittivities can describe the data, we conclude that the apparent dielectric permittivity is influenced by the imaginary parts in the dielectric, permittivities of the soil components. Combination of the four-component complex dielectric mixing model with the complex dielectric permittivity obtained from the frequency domain analysis of TDR waveforms gives a tool for modeling the bulk soil electrical conductivity by separating the conductivity of the soil water into a bound water conductivity and a free water conductivity.

High efficiency Cu(In,Ga)Se/sub 2/-based solar cells: processing of novel absorber structures

Abstract: Our effort towards the attainment of high performance devices has yielded several devices with total-area conversion efficiencies above 16%, the highest measuring 16.8% under standard reporting conditions (ASTM E892-87, Global 1000 W/m/sup 2/). The first attempts to translate this development to larger areas resulted in an efficiency of 12.5% for a 16.8-cm/sup 2/ monolithically interconnected submodule test structure, and 15.3% for a 4.85-cm/sup 2/ single cell. Achievement of a 17.2% device efficiency fabricated for operation under concentration (22-sun) is also reported. All high efficiency devices reported here are made from graded bandgap absorbers. Bandgap grading is achieved by compositional Ga/(In+Ga) profiling as a function of depth. The fabrication schemes to achieve the graded absorbers, the window materials and contacting are described.

The influence of pressure and conductivity on the pulsed breakdown of water

Abstract: The influence of hydrostatic pressure and liquid conductivity on the dielectric breakdown of water solutions subjected to high amplitude electric fields of sub-microsecond duration has been investigated. Well-defined pulses (80 kV, 3 ns risetime, 100 ns duration) have been applied to a gap (0.4 to 2.1 mm), between Rogowski profile electrodes (thus ensuring a uniform electric field), containing de-ionized water (non-distilled, and distilled and ultrasonically treated), sodium chloride solutions (0.001 to 1.0 molar), or magnesium sulfate solutions (0.01 to 0.1 molar). Breakdown in these liquids has been studied at pressures from atmospheric up to 40 MPa. The inter-electrode potential and the current response were measured indicating the time lag to breakdown, breakdown voltage, and temporal characteristics of the breakdown process. The breakdown time lag increases with increasing pressure, and is insensitive to the liquid conductivity. These findings have relevance to the ongoing discussion concerning 'thermal' vs. 'electronic' mechanisms for dielectric breakdown in liquids. In particular, the results suggest that breakdown evolves via 'bubble' formation by field emitted currents near asperities on the cathode, and that the time for the change in liquid conductivity as a result of breakdown is limited by processes other than ionization growth (due to electron impact ionization of molecules in the 'bubble') of prebreakdown electron currents. >

Effect of intergranular glass films on the electrical conductivity of 3Y-TZP

Abstract: The electrical conductivity of 3Y-TZP ceramics containing SiO2 and Al2O3 has been investigated by complex impedance spectroscopy between 500 and 1270 K. At low temperatures, the total electrical conductivity is suppressed by the grain boundary glass films. The equilibrium thickness of intergranular films is 1-2 nm, as derived using the “brick-layer” model and measured by HRTEM. A change in the slope of the conductivity Arrhenius plots occurs at the characteristic temperature Tb at which the macroscopic grain boundary resistivity has the same value as the resistivity of the grains. The temperature dependence of the conductivity is discussed in terms of a series combination of RC elements.

Prediction and Measurement of the Thermal Conductivity of Amorphous Dielectric Layers

Abstract: Thermal conduction in amorphous dielectric layers affects the performance and reliability of electronic circuits. This work analyzes the influence of boundary scattering on the effective thermal conductivity for conduction normal to amorphous silicon dioxide layers, k n,eff . At 10 K, the predictions agree well with previously reported data for deposited layers, which show a strong reduction of k n,eff compared to the bulk conductivity, k bulk

Ionic conductivity of a polymer electrolyte with modified carbonate as a plasticizer for poly(ethylene oxide)

Abstract: A new type of plasticizer, a modified carbonate (MC3) made by attaching three ethylene oxide units to the 4-position of ethylene carbonate, has been synthesized. The ionic conductivity of a polymer electrolyte using this type of plasticizer in a poly(ethylene oxide) (PEO)-LiCF[sub 3]SO[sub 3] complex has been studied. Adding 50% of this plasticizer by weight of PEO to the PEO-LiCF[sub 3]SO[sub 3] complex, yielded an ionic conductivity of 5 [times] 10[sup [minus]5] S/cm at 25 C, which is two orders of magnitude higher than that found for a PEO-LiCF[sub 3]SO[sub 3] electrolyte without a plasticizer, and one order of magnitude higher than that found when using propylene carbonate (PC) as a plasticizer. The temperature dependence of the conductivity and thermal analysis results indicate that this new plasticizer increases the ionic conductivity throughout the entire complex system, whereas conventional plasticizers, like PC, simply create a high conductivity pathway through the plasticizer itself. The new plasticizer also enhances the ion pair dissociation which in turn improves the ionic conductivity.

Charge transport in heavily B‐doped polycrystalline diamond films

Abstract: Temperature‐dependent conductivity and Hall measurements have been carried out on heavily in situ B‐doped polycrystalline diamond films in a temperature range from ∼100 to 750 K. The slope of the conductivity is clearly non‐Arrhenius leading to a pronounced tail at low temperatures. Carrier transport at low temperatures is dominated by variable range hopping. The activation energy decreases with increasing doping concentration and the most heavily doped diamond films show metallic behavior above room temperature. Hole carrier concentrations up to 1.8×1021 cm−3 were measured in agreement with secondary‐ion‐mass spectroscopy investigations.

The Development of a new plasticizer for poly(ethylene oxide) based polymer electrolyte and the investigation of their ion-pair dissociation effect

Abstract: One approach to increasing the ionic conductivity of polymer electrolyte is to add plasticizers to the polymer-salt complexes . Recently, we have synthesized a new plasticizer, modified carbonate (MC3), by attaching three ethylene oxide units to the 4-position of ethylene carbonate. AC impedance studies showed that an ionic conductivity of 5{times}10{sup {minus}5} S cm{sup {minus}1} could be achieved at room temperature, by adding 50 wt % of MC3 into PEO-LiCF{sub 3}SO{sub 3} complex. This is two orders of magnitude higher than that found in PEO-LiCF{sub 3}SO{sub 3} electrolyte without a plasticizer, and one order of magnitude higher than that found when using same amount of propylene carbonate (PC) as plasticizer. Temperature dependent conductivity measurement and thermal analysis show that this new plasticizer increased the ionic conductivity throughout the entire complex system, while the conventional plasticizers only create a high conductivity path way through the plasticizer itself. The samples are free standing films with good mechanical properties. When MC3 is used as a plasticizer, the ionic conductivity increase is much higher than using PC as a plasticizer at high temperature (65{degrees}C), implying an increase in the number of charge carriers. Therefore, we believe that MC3 has a stronger ion pair dissociationmore » effect than PC, when used as a plasticizer. The ion pair dissociation effect was studied by Raman, FTIR, and near edge x-ray absorption fine structure (NEXAFS) spectroscopy.« less

Enhanced conductivity of zinc oxide thin films by ion implantation of hydrogen atoms

Abstract: Enhancement of the conductivity of zinc oxide through doping with hydrogen atoms was examined by using ion implantation of highly resistive thin films deposited by rf magnetron sputtering at room temperature. With a doping of 1×1017 atoms cm−2, the conductivity after annealing at 200 °C in an N2 atmosphere at 1 atm rose from the initial 1×10−7 Ω−1 cm−1 to 5.5×102 Ω−1 cm−1.