Showing papers on "Conductivity published in 2002"

POLYANILINE. PREPARATION OF A CONDUCTING POLYMER (IUPAC Technical Report)

Abstract: Eight persons from five institutions in different countries carried out polymerizations of aniline following the same preparation protocol. In a "standard" procedure, aniline hydrochloride was oxidized with ammonium peroxydisulfate in aqueous medium at ambient temperature. The yield of polyaniline was higher than 90% in all cases. The electrical conductivity of polyaniline hydrochloride thus prepared was 4.4 1.7 S cm(-1) (average of 59 samples), measured at room temperature. A product with defined electrical properties could be obtained in various laboratories by following the same synthetic procedure. The influence of reduced reaction temperature and increased acidity of the polymerization medium on polyaniline conductivity were also addressed. The conductivity changes occurring during the storage of polyaniline were monitored. The density of polyaniline hydrochloride was 1.329 g cm(-3). The average conductivity of corresponding polyaniline bases was 1.4 x 10(-8) S cm(-1), the density being 1.245 cm(-3). Additional changes in the conductivity take place during storage. Aging is more pronounced in powders than in compressed samples. As far as aging effects are concerned, their assessment is relative. The observed reduction in the conductivity by similar to10% after more than one-year storage is large but, compared with the low conductivity of corresponding polyaniline (PANI) base, such a change is negligible. For most applications, an acceptable level of conductivity may be maintained throughout the expected lifetime.

Experimental observation of scaling laws for alternating current and direct current conductivity in polymer-carbon nanotube composite thin films

Abstract: Alternating current (ac) and direct current (dc) conductivities have been measured in polymer-nanotube composite thin films. This was carried out for a range of concentrations of multiwall nanotubes in two polymer hosts, poly(m-phenylenevinylene-co-2,5-dioctyloxyp-phenylenevinylene) (PmPV) and polyvinylalcohol (PVA). In all cases the dc conductivity σDC was ohmic in the voltage range studied. In general the ac conductivity displayed two distinct regions, a frequency independent region of magnitude σ0 at low frequency and a frequency dependent region at higher frequency. Both σDC and σ0 followed a percolation scaling law of the form σ∝(p−pc)t with pc=0.055% by mass and t=1.36. This extrapolates to a conductivity of 1×10−3 S/m for 100% nanotube content. Such a low value reflects the presence of a thick polymer coating, resulting in poor electrical connection between tubes. This leads to the suggestion that charge transport is controlled by fluctuation induced tunneling. In the high frequency regime the cond...

Ionic Conductivity of an Extruded Nafion 1100 EW Series of Membranes

Abstract: The proton conductivity of a series of extruded Nafion membranes @of equivalent weight ~EW! of 1100 and nominal dry thickness of 51, 89, 127, and 178 mm# has been studied. Measurements were made in 1 M H2SO4 at 298 K using a four-electrode, dc technique. The membrane area resistance increases with thickness, as expected, from 0.07 to 0.16 V cm2 for Nafion 112 and Nafion 117, respectively. However, in contrast to the published literature, after correcting for the membrane thickness, the conductivity of the membranes decreases with decreasing membrane thickness. For example, values of 0.083 and 0.16 S cm21 were obtained for Nafion 112 and 117 membranes, respectively. In situ current-interrupt measurements in a proton exchange membrane fuel cell confirmed the relatively poor conductivity of the membrane electrode assemblies ~MEAs! based on the thinner membranes. While a high contact resistance to the electrodes may have contributed to the in situ MEA resistance, water balance measurements over the MEA showed that the high resistance was not due to a low water content or to an uneven water distribution in the MEAs. The implications of the findings for the understanding of the membrane properties are discussed.

Electrical conductivity and dielectric behaviour of nanocrystalline NiFe2O4 spinel

Abstract: Electrical conductivity and dielectric measurements have been performed for nanocrystalline NiFe2O4 spinel for four different average grain sizes, ranging from 8 to 97 nm. The activation energy for the grain and grain boundary conduction and its variation with grain size have been reported in this paper. The conduction mechanism is found to be due to the hopping of both electrons and holes. The high-temperature conductivity shows a change of slope at about 500 K for grain sizes of 8 and 12 nm and this is attributed to the hole hopping in tetrahedral sites of NiFe2O4. Since the activation energy for the dielectric relaxation is found to be almost equal to that of the dc conductivity, the mechanism of electrical conduction must be the same as that of the dielectric polarization. The real part e' of the dielectric constant and the dielectric loss tanδ for the 8 and 12 nm grain size samples are about two orders of magnitude smaller than those of the bulk NiFe2O4. The anomalous frequency dependence of e' has been explained on the basis of hopping of both electrons and holes. The electrical modulus analysis shows the non-Debye nature of the nanocrystalline nickel ferrite.

Decoupling of Transport, Charge Storage, and Interfacial Charge Transfer in the Nanocrystalline TiO2/Electrolyte System by Impedance Methods

Abstract: Processes in the dark of electron transport and recombination in several nanoporous titanium dioxide films have been studied as a function of the applied potential, using the electrochemical impedance technique. Contact and bulk characteristics have been identified, decoupled, and interpreted, applying a transmission line model that identifies the following elements: (i) the capacitance of the interface between the exposed surface of the substrate and the electrolyte, (ii) the electron transport resistance, (iii) the charge-transfer resistance distributed in the TiO2/electrolyte interface, and (iv) a distributed capacitive element related to charging the porous matrix. The model provides a satisfactory description of the spectra in widely different conditions of conductivity of the TiO2 phase. The electron conductivity has been determined as a function of applied potential and coincides for the different samples under study. Classical electrochemical frameworks of transport and interfacial charge transfe...

Electrical relaxations in polymeric particulate composites of epoxy resin and metal particles

Abstract: Composites of epoxy resins and nickel particles in various amounts were prepared and their dielectric spectra were measured in the frequency range 5 Hz–13 MHz and temperature interval from ambient to 140°C. The formalism of electric modulus proved to be efficient in analysing and interpreting obtained data. For these composites two relaxation processes are revealed in the frequency range and temperature interval of the measurements. One is an interfacial dielectric relaxation (Maxwell–Wagner–Sillars), MWS and the other is a conductivity relaxation. They both follow the Cole–Davidson approach with the exponent γ reflecting a distribution of relaxation times with the characteristics of each process. AC conductivity of these composites is frequency and temperature dependant, it generally follows the exponential law σ ac ∼ ω s and reveals a conductivity relaxation process, in the low frequencies.

Control of p- and n-type conductivity in sputter deposition of undoped ZnO

Abstract: Recent theoretical studies have concluded that the low formation enthalpies of intrinsic donor defects should preclude achievement of p-type conductivity in undoped ZnO grown in thermal equilibrium with a molecular oxygen reservoir. This letter demonstrates that reactive sputtering can produce intrinsic p-type ZnO, controlled by adjusting the oxygen partial pressure in the sputtering plasma. We report the properties of p–n homojunctions fabricated in this way, and characterize transport in the films by Hall measurements. Our finding of p-type conductivity in undoped ZnO grown with dissociated oxygen is qualitatively consistent with the effect of higher chemical potential of atomic oxygen reactant on defect formation enthalpies. This parallels to some degree the recent attention to nitrogen acceptor incorporation by means of dissociating nitrogen source gases.

Electrical Properties of Yttrium-Doped Strontium Titanate under Reducing Conditions

Abstract: The electrical conductivity of SrTiO 3 containing rare earth dopants (Y,La,Pr,Sm,Nd,Gd,orYb)was measured ar 600-900 C in reducing atmospheres. An unusually high conductivity was observed for the yttrium-doped samples compared to those with rare earth dopants. particularly at the composition of the solubility limit Sr 0.88 Y 0.08 TiO 3-δ where the conductivity at 800°C was 64 S/cm. The conductivity was confirmed to be n-type by thermopower measurements. The oxygen deficiency was determined to be 1% by thermogravimetric analysis. Increasing the strontium vacancy concentration to Sr 0.86 Y 0.08 TiO 3-δ effected a further improvement in conductivity to 82S/cm. This material has high structural stability over a broad range of temperature (up to 1400°C) an oxygen partial pressure (1-10 20 atm). A simple defect model was developed to explain the change of electrical conductivity as a function of yttrium content and oxygen deficiency.

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

Abstract: Yttria-stabilized zirconia (YSZ) electrolytes with diverse microstructures were prepared by using nano-size (Y 2 O 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.

Evaluation of yttrium-doped SrTiO3 as an anode for solid oxide fuel cells

Abstract: Yttrium-doped SrTiO3 (SYT) was assessed as an anode material for solid oxide fuel cells in terms of electrical conductivity, phase stability, redox behavior, chemical compatibility with yttria-stabilized zirconia (YSZ) and La0.8Sr0.2Ga0.8Mg0.2O2.8 (LSGM), thermal expansion coefficient, and fuel cell performance. With the optimized composition Sr0.86Y0.08TiO3−δ, the electrical conductivity was as high as 82 S/cm at 800 °C and oxygen partial pressure of 10−19 atm. A reversible change of conductivity was observed upon oxidation and reduction. The resistance to oxidation was enhanced by partially replacing Ti with transition metals such as cobalt. This material has high structural stability over a broad range of temperature (up to 1400 °C) and oxygen partial pressure (1–10−20 atm). No phase change was found for mixtures of SYT with YSZ or LSGM sintered at 1400 °C for 10 h. The thermal expansion of doped-SrTiO3 was determined to be compatible with that of YSZ and LSGM. A maximum power density of 58 mW/cm2 at 900 °C was obtained for single cells with the new anode.

Contact Conductivity Detection in Poly(methyl methacylate)-Based Microfluidic Devices for Analysis of Mono- and Polyanionic Molecules

Abstract: An on-column contact conductivity detector was developed for the analysis of various mono- and polyanionic compounds separated by electrophoresis chips fabricated in poly(methyl methacrylate) (PMMA) using hot embossing techniques from Ni electroforms. The detector consisted of a pair of Pt wires (127 μm diameter) with an end-to-end spacing of approximately 20 μm and situated within the fluidic channel. The waveform applied to the electrode pair was a bipolar pulse with a frequency of 5.0 kHz and was used to reduce the charging current from measurement so that the current recorded at the end of one pulse is more representative of the solution conductivity. Using the detector, separations of amino acids, peptides, proteins, and oligonucleotides were demonstrated. For the amino acids and peptides, free-solution zone electrophoresis was performed. A calibration plot for the amino acid alanine was found to be linear from approximately 10 to 100 nM in a carrier electrolyte consisting of 10 mM triethylamonium ac...

Dependence of Ionic Conductivity on Composition of Fast Ionic Conductors Li1+xTi2-xAlx(PO4)3, 0 ≤ x ≤ 0.7. A Parallel NMR and Electric Impedance Study

Abstract: Materials with nominal compositions Li1+xTi2-xAlx(PO4)3, 0 ≤ x ≤ 0.7, have been prepared and studied with X-ray diffraction (XRD), nuclear magnetic resonance (NMR), and electrical impedance techniques. Substitution of Ti4+ by Al3+ reduces the unit cell dimensions of the NASICON framework but enhances the grain interior conductivity. The overall conductivity is dominated by the grain boundary composition. Detection of new broad peaks in the 31P and 27Al MAS NMR spectra supports the formation of an amorphous Li-conducting phase that accounts for the increase observed in the overall conductivity. In samples with x > 0.5, crystallization of Li4P2O7 as a segregated phase is accompanied by the appearance of a new electrical response. In these samples, a decrease in overall conductivity, associated with a modification of the amorphous phase, is also observed.

Low-conductivity buffers for high-sensitivity NMR measurements.

Abstract: The sensitivity of nuclear magnetic resonance (NMR) probes, especially the recently introduced cryogenic probes, can be substantially reduced by the electrical noise generated by conductive samples. In particular, samples of biological macromolecules, which usually contain salts to keep the pH constant and to prevent aggregation, can experience a significant reduction in sensitivity. So far this dependence has forced researchers to minimize the salt concentrations in their samples. Here we demonstrate that the decisive factor is not the salt concentration itself but the conductivity which is a function of both the concentration and the mobility of the ions in solution. We show that by choosing buffers with low ionic mobility, the sample conductivity can be dramatically reduced and the sensitivity substantially enhanced compared to the same measurement with an equal concentration of a standard NMR buffer such as phosphate. We further show that the highest sensitivity gain of one buffer over another buffer is equal to the square root of the ratio of their ion mobilities and describe a simple method to evaluate the effect of a certain buffer on the sensitivity.

Semiconductor device and method for fabricating the same

Abstract: A semiconductor device has an n channel conductivity type field effect transistor having a channel formation region formed in a first region on one main surface of a semiconductor substrate and a p channel conductivity type field effect transistor having a channel formation region formed in a second region on the main surface of the semiconductor substrate, which second region is different from the first region. An internal stress generated in the channel formation region of the n channel conductivity type field effect transistor is different from an internal stress generated in the channel formation region of the p channel conductivity type field effect transistor. The internal stress generated in the channel formation region of the n channel conductivity type field effect transistor is a tensile stress, while the internal stress generated in the channel formation region of the p channel conductivity type field effect transistor is a compressive stress.

Contacting scheme for large and small area semiconductor light emitting flip chip devices

Abstract: A light emitting device includes a layer of first conductivity type, a layer of second conductivity type, and a light emitting layer disposed between the layer of first conductivity type and the layer of second conductivity type. A via is formed in the layer of second conductivity type, down to the layer of first conductivity type. The vias may be formed by, for example, etching, ion implantation, diffusion, or selective growth of at least one layer of second conductivity type. A first contact electrically contacts the layer of first conductivity type through the via. A second contact electrically contacts the layer of second conductivity type. A ring that surrounds the light emitting layer and is electrically connected to the first contact electrically contacts the layer of first conductivity type.