Current versus voltage (I versus V) and complex impedance, z*(f) = z′(f) − iz″(f), measurements for polyaniline (PANI) films were carried out in samples with different doping levels and gold and/or aluminium as electrodes. The complex impedance of Au/PANI/Au presents the typical behaviour of a somewhat solid disordered material with negligible electrode influence. DC measurements confirm this evidence. However, some additional influence of the interface was observed to occur when Al was used as the electrode. A phenomenological model employing the Cole–Cole dielectric function for generating the conduction process is developed and the calculated z′(f) and z″(f) as functions of the polymer doping level and the bias polarization are found to be in good agreement with experimental data, thereby separating bulk and interface contributions to the complex impedance, as well as providing approximate values of the PANI–Al interface thickness and resistivity, around 10 nm and 1013 Ω cm, respectively.
The mathematical equivalence between a non-Debye and a frequency distributed process, both having a single activation energy, under non-isothermal and time varying poling, is established using the concept of intrinsic time. The well known time-temperature superposition principle is derived and it is concluded that the vast class of processes obeying this principle might indicate that they are indeed non-Debye processes rather than processes distributed in frequency with a single activation energy.
Complex conductance, σ * (f ) = σ (f ) + iσ"(f ), measurements of polyaniline/carnauba wax composites were carried out at room temperature in samples containing different amounts of PAni. While σ"(f ) ever grows as a function of the frequency f, σ (f ) was observed to vary as f n (n ≈ 1) in the high frequency domain, and to be frequency-independent for lower frequencies. This is the quasi-universal behavior characteristic of conduction in a disordered medium. Superposed on this, another signal was observed to occur, and the whole impedance response was then analysed in an Argand Diagram, disclosing an electrode process in series with the bulk one. An equivalent conductance expression was used to explain such electrical behavior, in which the bulk properties of the composite were represented through the Random Free Energy Barrier model, while the composite-electrode interface by a simple parallel combination of a capacitor and a resistor. From the theoretical fittings it was concluded that the increase of PAni concentration acts in the sense of increasing not only the sample conductance, but also the bulk capacitance. The separation of the bulk and the interface contributions was then achieved. I IntroductionIn recent years, many authors have pointed out the possibility of producing organic composites that associate the low cost and the mechanical properties of natural materials, as rubber or waxes, with the electronic properties of conducting polymers [1,2]. Among such materials, carnauba wax (cw) and polyaniline (PAni) appears as potential candidates to be used in this class of composites. While PAni presents high electric conductivity when doped by acid solutions [3,4], cw -a natural product of the carnauba palm (Copernicia cerifera) [5,6] -is a dielectric material produced in abundance in northeast of Brazil. In this context, the preparation of materials based on the mixture of PAni and cw appears as an alternative to obtain organic composites with high electrical conductance at low cost.In this work we carried out a study of the electrical conductance of cw/PAni composites, at room temperature, prepared with different weights of PAni. It was observed that even small amounts of PAni cause a significant increase of the dc conductance. The present study is mainly concerned with this 'percolation' regime. The results show typical features of conduction in a disordered medium [7], usually displayed in a log-log plot of the complex conductivity as function of the frequency: for the real component, a plateau followed by an almost linear rise, while the imaginary component steadily rises from the lowest frequency on. Superposed on this process, another one at lower frequencies was detected and disclosed in an Argand Impedance diagram, as a series process with the bulk one. It is attributed to the metal/composite interface. The results were analyzed by an equivalent conductance expression, which represents the bulk properties of the composite by the Random Free Energy Barrier model (RFEB model) [7][8][9][10], and the...
Recebido em 18/5/04; aceito em 29/9/04; publicado na web em 17/2/05 ADSORBING ACTION OF BAUXITE ON NEW INSULATING OIL. Samples of new insulating mineral oil, after contact with bauxite, were analyzed by visible spectrophotometry, impedance spectroscopy and their total acidity index was measured. The results of these analyses were compared to samples of new insulating mineral oil, which had not been in contact with bauxite. The comparison demonstrated that the bauxite didn't reduce the insulating capacity of the mineral oil and thus could be used to treat the oil in situ during the operation of an electric transformer.
Short-circuit electrical currents were observed in 60:40 mol % P(VDF/TrFFJ copolymers above 80 O C , temperature in which this polymer undergoes a structural ferro-to-paraelectric phase transition. The signal of this current was inverted when the sample is rotated so that the electrodes had their positions exchanged. Different combinations of metalic materials (aluminium, copper, nickel, and gold) were used as electrodes in metal( l)-polymer-metal(2) structures. Voltage signals were also detected in paraelelctric samples, whose? signal was equally dependent on the electrode combinations.. This phenomenon is probably due to an electromotive force arising from different interfacial potential on both metal-sample interfaces.Electrochemical effects in metal( l)-polymer-metal(2) (MI -P-M2) structures have been discussed since spontaneous currents in PET films [1,2], and voltages in M1-P-M2 structures in different polymers [3], were observed during their heating process. To explain these phenomena, anode potentials due to the reaction of water with metal which would produce a metal oxidation and proton generation, was firstly proposed [41. An alternative explanation takes into account the probable ionization of residual acid groups (COOH -+ COO-+ €I+) which liberates protons at 80 "C, temperature in which the spontaneous current usually appears. Voltages in such kind of structures have been measured: 1 .O V for Au-PE-AI, 1.4 V for CuNylon 6-A1, 0.7 V for Au-ethylvinyl acetate-AI, 0.6 V for AI-polycarbonate-Au, and 0.27 V for Ag-Kapton-Au. No vestige of spontaneous voltage was detected in Teflon films. In this article we present spontaneous short-circuit current in P(VDF/TrFE) copolymer of 60:40 molar ratio, which increases substantially above its Curie temperature (at about 80 "C). In order to investigate the origin of this 0-7803-1939-7/94/$4.000 1994 IEEE
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