Rosario A. Gerhardt

Impedance and dielectric spectroscopy revisited: Distinguishing localized relaxation from long-range conductivity

Abstract The advantages of plotting a.c. data in terms of impedance, electric modulus and dissipation factor simultaneously are illustrated. Complex impedance is generally employed for ionic conductors because it can easily distinguish between bulk and grain boundary effects. However, comparison with the modulus and dissipation factor data allows easier interpretation of the microscopic processes responsible for the measured a.c. response. In particular, the difference between localized (i.e. dielectric relaxation) and non-localized conduction (i.e. long range conductivity) processes within the bulk of the material may be discerned by the presence or the absence of a peak in the imaginary modulus versus frequency plot. Similarly, the absence or presence of a peak in the imaginary impedance versus frequency plot can be correlated to space charge effects and non-localized conductivity. Long-range conductivity results in nearly complete impedance semicircles but no frequency dispersion in the permittivity while localized conductivity is reflected in a frequency dependent permittivity but no measurable conductance. The degree to which these assignments may be made is related to the dielectric relaxation ratio ( r = ϵ s ϵ ∞ ) and the differences between the time constants of the different relaxation processes present in the material being examined.

Calculation of various relaxation times and conductivity for a single dielectric relaxation process

Abstract Various dielectric functions, i.e., ϵ∗, tanδ, M∗, Z∗ and Y∗, have been used to calculate the corresponding time constants for three relaxation models: (1) Debye, (2) Cole-Cole and (3) ideal conduction. It has been proved that each dielectric function gives rise to a different relaxation time which follows the general order: τϵ≥τy>τtanδ>τz≥τm. The difference between them depends largely on the dielectric relaxation ratio, ϵ s ϵ ∞ , as well as the distribution parameter, α. Moreover, on a logarithmic scale, the loss tangent relaxation time is equal to the average relaxation time for any pair of inverse complex functions, e.g., ϵ∗ and M∗, Z∗ and Y∗. The relationships between the relaxation times illustrate the advantages of using different dielectric functions to analyze experimental data. The relaxation times have also been related to the dielectric conductivity in the Debye and ideal conduction cases.

Conductive paper fabricated by layer-by-layer assembly of polyelectrolytes and ITO nanoparticles.

A new salt-free approach was developed for fabricating conductive paper by layer-by-layer (LBL) assembly of conductive indium tin oxide (ITO) nanoparticles and polyelectrolytes onto wood fibers. Subsequent to the coating procedure, the fibers were manufactured into conductive paper using traditional paper making methods. The wood fibers were first coated with polyethyleneimine (PEI) and then LBL assembled with poly(sodium 4-styrenesulfonate) (PSS) and ITO for several bilayers. The surface charge intensity of both the ITO nanoparticles and the coated wood fibers were evaluated by measuring the zeta-potential of the nanoparticles and short fibers, respectively. The ITO nanoparticles were found to preferentially aggregate on defects on the fiber surfaces and formed interconnected paths, which led to the formation of conductive percolation paths throughout the whole paper. With ten bilayer coatings, the as-made paper was made DC conductive, and its sigma(dc) was measured to be 5.2 x 10(-6) S cm(-1) in the in-plane (IP) direction, while the conductivity was 1.9 x 10(-8) S cm(-1) in the through-the-thickness (TT) direction. The percolation phenomena in these LBL-assembled ITO-coated paper fibers was evaluated using scanning electron microscopy (SEM), current atomic force microscopy (I-AFM), and impedance measurements. The AC electrical properties are reported for frequencies ranging from 0.01 Hz to 1 MHz. A clear transition from insulating to conducting behavior is observed in the AC conductivity.

Separation of junction and bundle resistance in single wall carbon nanotube percolation networks by impedance spectroscopy

Single wall carbon nanotube (SWNT) networks of different loadings were measured by impedance spectroscopy. The resistances of the junctions and bundles have been separated by modeling ac impedance spectroscopy data to an equivalent circuit of two parallel resistance-capacitance elements in series. The junction resistance was found to be 3–3.5 times higher than the bundle resistance. The dc and ac properties of the SWNT networks were found to obey a percolation scaling law, with parameters determined by dispersant type and SWNT purity. The values of the critical exponent in all cases were higher than the expected value of 1.3, which is related to widely distributed bundle and junction conductivities.

Assessment of percolation and homogeneity in ABS/carbon black composites by electrical measurements

In this study, composites consisting of an insulating poly(acrylonitrile-co-butadiene-co-styrene) polymer matrix and a conducting carbon black (CB) additive were produced by twin-screw extrusion. Both direct current and alternating current electrical measurements were used to evaluate the electrical properties of the composite and to assess whether sufficient mixing was achieved. Electrical measurement results and scanning electron micrographs show that once-extruded composites had a porous structure and poor conductivity while twice-extruded composites were much more homogeneous and had higher conductivity. The percolation threshold of the twice-extruded poly(acrylonitrile-co-butadiene-co-styrene)/CB composites was found to be between 8 and 10% CB. Electrical measurements provided a feedback loop for improving processing of the composite material.

Effect of the fabrication method on the electrical properties of poly(acrylonitrile-co-butadiene-co-styrene)/carbon black composites

Poly(acrylonitrile-co-butadiene-co-styrene) (ABS), an engineering plastic, was combined with carbon black (CB) to increase its conductivity. The ABS/CB composites were prepared using two different methods: dissolution of ABS in Butan-2-one and manual mixing of the constituent materials. These fabrication methods led to different microstructures, which led to vastly different electrical properties. The microstructures were acquired using scanning electron microscopy (SEM) and optical microscopy, while the electrical conductivity was obtained using impedance spectroscopy. The percolation threshold of the composites fabricated using the manual mixing method was found to be much lower (0.0054 vol.% CB) than that of the composites fabricated using the dissolution method (2.7 vol.% CB).

Anelastic and dielectric relaxation of scandia-doped ceria

Abstract Of all the trivalent dopants which may be added to ceria (CeO2) to make it an oxygen-ion conductor, Sc3+ appears to be unique in producing unusually low conductivity due to a relatively high vacancy-dopant association energy. In order to better understand the defect structure of Sc3+-doped ceria, the present work was undertaken using both anelastic and dielectric relaxation. A number of relaxation peaks were found. Most strikingly, a new low temperature relaxation with activation enthalpy near 0.2 eV was observed; it was shown to be due to a low-symmetry off-center configuration of an isolated ScCe defect. At higher temperatures the relaxation due to Sc-Vo pairs (where vo = oxygen vacancy) was found to have an activation enthalpy of 0.41 eV (much lower than that of larger dopant ions, e.g. Y3+ and Gd3+. In addition, complex relaxation spectra are observed even for concentrations as low as 0.3mol% Sc2O3, showing that higher clustered defects readily form in this system.

Thermal processing and properties of BaTi4O9 and Ba2Ti9O20 dielectric resonators

Sn-doped and undoped barium titanates pressed powder pellets were sintered at 1360 and 1390°C for 5 h. Though batched to form Ba2Ti9O20, a two-phase microstructure of BaTi4O9 and TiO2 formed from the undoped system. The dielectric constant at 6 GHz was 40 without SnO2 additions. Doping with 1.64 mol % SnO2 stabilized Ba2Ti9O20 and formed a single-phase microstructure but resulted in higher porosity. Dilatometry studies implied that SnO2 additions facilitated a greater fraction of reaction to occur in the solid state, causing a lower quantity of pore-filling fluid to form above 1317°C. As a result of the higher porosity, the dielectric constants and quality factors were also reduced.

Magnetic, electrical, and microstructural properties of YBa 2 Cu 3 O 7 : A comparison of sol-gel, co-precipitated, and solid state processing routes

Samples of YBa{sub 2}Cu{sub 3}O{sub 7} were prepared by sol-gel, co-precipitation, and solid state processes. Sol-gel samples were prepared from a solution of yttrium, barium, and copper nitrates dissolved in ethylene glycol, co-precipitated samples were made by the amorphous citrate method, and solid state samples were prepared by conventional high temperature reaction of the appropriate metal oxides and carbonates. The sol-gel process was shown to yield superconducting samples of superior Meissner effect, critical current, and critical field. The co-precipitated samples contain impurities that affect the critical properties. The sol-gel and co-precipitated processes yield materials with well-formed, plate-like particles with a fairly uniform size of about 10 {mu}m. The grains in the solid state sample are smaller but have a much wider distribution of sizes than the samples prepared by solution methods.

Small-angle-scattering determination of the microstructure of porous silica precursor bodies

Small-angle X-ray and small-angle neutron scattering measurements were carried out on a series of porous silica precursor (unsintered) bodies with different starting chemistries. The samples were prepared from mixtures containing 10 to 30 wt% colloidal silica sol and 90 to 70 wt% potassium silicate. Particle-size distributions were derived from the data using a maximum-entropy technique. Scattering data from the porous silica samples are especially suitable for such an analysis because the colloidal particles and clusters and aggregates of these particles are verified in detail to be spherical, and the scattering instrument use for this study covered the entire range of sizes in this material and was very well calibrated. It was found that the lower the amount of colloidal silica, the broader the size distribution of the silica aggregates.