Capacitive properties

The dielectric constant of a polymer (K) (which ve also refer to as relative electric permittivity or electric inductive capacity) is a measure of its interaction with an electrical field in which it is placed. It is inversely related to volume resistivity. The dielectric constant depends strongly on the polarizability of molecules within the polymer. In polymers with negligible dipole moments, the dielectric constant is low and it is essentially independent of temperature and the frequency of an alternating electric field. Polymers with polar constituents have higher dielectric constants. When we place such polymers in an electrical field, their dipoles attempt 

The dissipation factor of a polymer (which we also refer to as tan 5) is the ratio of energy lost to the energy stored when it is placed in an alternating field. The dissipation factor is analogous to a mechanical tan 5 describing rheological behavior. The dissipation factor at a specific frequency is defined according to Eq. 8.14. 

We measure the capacitive properties of a polymer in a capacitor that is constructed so that we can compare the properties of the test material relative to a vacuum. 

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Figure 5. Cyclic voltammograms of (a) 2,5"" -di-methyl-a-hexathiophene and (b) poly(2,2 -bithio-phene) films in acetonitrile containing 0.1 M E NCIO 103 (Reprinted from G. Zotti, G. Schia-von, A. Berlin, and G. Pagani, Electrochemistry of end-ca )ed oligothienyls-new insights into the polymerization mechanism and the charge storage, conduction and capacitive properties of polythiophene, Synth. Met. 61 (1-2) 81-87, 1993, with kind permission from Elsevier Science S.A.)...

We can define the principal electrical properties of polymers in terms of four characteristics electrical resistance, capacitive properties, dielectric strength, and arc resistance. We can change the surface characteristics of a polymer by subjecting it to a corona discharge generated by a strong electrical field. Lastly, we must also consider the influence of other physical properties on the application of polymers in electrical applications. 

Ganesan R, Shanmugam S, Gedanken A (2008) Pulsed sonoelectrochemical synthesis of polyaniline nanoparticles and their capacitance properties. Synt Met 158 848-853... 

Due to their moderate specific surface area, carbon nanotubes alone demonstrate small capacitance values. However, the presence of heteroatoms can be a source of pseudocapacitance effects. It has been already proven that oxygenated functional groups can significantly enhance the capacitance values through redox reactions [11]. Lately, it was discovered that nitrogen, which is present in carbon affects also the capacitance properties [12]. 

In this work, MWNTs will be mixed with polyacrylonitrile (PAN) and the MWNTs/PAN composite will be carbonized, giving rise to a new C/C composite with interesting capacitance properties. It is striking that the composite components alone give negligible capacitance values below 20... 

The high purity carbon nanotubes (CNTs) used in this study were obtained by decomposition of acetylene over a powdered CoxMgi xO solid solution catalyst [19]. Different proportions of CNTs from 15 to 70% and polyacrylonitrile (PAN, Aldrich) have been mixed in an excess of acetone to obtain a slurry. After evaporation of acetone, precursor electrodes were formed by pressing the CNTs/PAN mixture at 1-2 tons/cm2. The C/C composites were formed by carbonisation of the pellets at 700-900°C for 30-420 min under nitrogen flow [20], The optimal capacitance properties of the composite were obtained for a mixture CNTs/PAN (30/70 wt%) treated at 700°C. Such C/C composite remains still quite rich in nitrogen (9 at% of N) demonstrating that PAN is an efficient nitrogen carrier. On the other hand,... 

Jurewicz K., Vix C., Frackowiak E., Saadallach S., Reda M., Parmentier J., Patarin J., Beguin F. Capacitance properties of ordered porous carbon materials prepared by a templating procedure. J Phys Chem Solids (2004) in press. 

The observed small and interconnected pores are expected to perform as electrodes of supercapacitors. Cyclic voltammetry (CV) and galvanostatic charge/discharge curves were used to characterize the capacitive properties the resulting data in simple acid (1 mol L 1 II2S0/() are shown in Fig. 7.11. 

Capacitance — Property of a circuit element characterized by an insulating medium contained between two conductive parts. The unit of capacitance is a farad (F), named for the English scientist Michael Faraday. Capacitance values are more commonly expressed in microfarad ( iF), which is 10 of a farad. Capacitance is one means by which energy or electrical noise can couple from one electrical circuit to another. Capacitance between two conductive parts can be made infinitesimally small but may not be completely eliminated. 

To prepare a working electrode, carbon powder (PFA-P7-H or PFA-AN8-H) was mixed with polytetrafluoroethylene (PTFE) (5 wt%) to form a pellet and it was sandwiched in Ni mesh as a current collector. The EDL capacitance properties were measured by CV and galvanostatic charge/ discharge in a three-electrode cell vs. Ag/AgCl reference electrode. 1 M-(C2H5)4NBF4 in PC was used as a nonaqueous electrolyte. 

Raymundo-Pinero E, Kierzek K, Machnikowski J, and Beguin F. Relationship between the nanoporous texture of activated carbons and their capacitance properties in different electrolytes. Carbon, 2006 44(12) 2498-2507. 

Similar issues were raised by Beguin et al. [97] regarding the capacitance properties of carbons in general and nanotubes in particular, because the higher the... oxygen content of the nanotubes, the higher the capacitance value, whereas after a few cycles, the capacitance of nanotubular materials rich in surface groups noticeably decreases the authors did not discuss the reasons for such behavior. 

Figure 6 The effect of pore polarization, caused by the capacitance properties of the skin, on the transport of monovalent solute cations.

The capacitive properties of nanotubes obtained with Cho et al. s method have been studied by Liu et al. from the same research group.208 Tubular structures were obtained with electrodeposition in acetonitrile solution containing 20 mM EDOT under potentio-static condition at 1.6 V vs. Ag/AgCl while nanofibers were synthesized in lOOmM EDOT and at 1.4 V applied potential. Thin wall nanotubes of PEDOT exhibited a stable specific capacitance of 140 Fg 1, while the specific capacitance for the solid nanofibers was 50Fg 1, under the identical test conditions. Figure 9 compares the... 

Lota, K., Khomenko, V. and Frackowiak, E. (2004). Capacitance properties of poly(3,4-ethylenedioxythiophene)/carbon nanotubes composites J. Phys. Chem. Solids, 65, 295-301. 

Capacitance-based chemical sensors are in the class of devices that transduce analytes into electrical currents. Such sensors are typically comprised of a dielectric, chemically-sensitive film coated onto a substrate electrode these films pass low conduction current, making amperometric or conductimetric measurements less sensitive or attractive for signal transduction. To detect an analyte, changes in the chemically-sensitive film s capacitive properties (associated with its dielectric constant, charge uptake, or formation of interface dipole layers) are measured when an active species is present or generated. 

This section deals solely with the response of ceramics to the application of a constant electric field and the nature and magnitude of the steady-state current that results. As discussed below, the ratio of this current to the applied electric field is proportional to a material property known as conductivity, which is the the focus of this section. The displacement currents or non-steady-state response of solids which gives rise to capacitive properties is dealt with separately in Chaps. 14 and 15 which treat the linear and nonlinear dielectric properties, respectively. 

In conclusion, while being environment friendly, this construction offers electrochemical characteristics comparable with EDLCs in organic electrolytes. It demonstrates perfectly that combining materials with pseudo-capacitance properties in an asymmetric cell is a very promising issue for developing a new generation of higji performance supercapacitors. 

This chapter shows that all the electrochemical parameters related with capacitance properties and hydrogen storage are strongly correlated with the nanotexture and surface... 

The cavity so formed has both inductive and capacitive properties which, coupled together make the circuit resonant at a particular frequency or frequencies. The cavity also has resistive loss that must be overcome before the circuit can oscillate. In low-frequency circuits this can be achieved with an amplifier whose gain is greater than the resistive attenuation of the circuit. In a negative resistance oscillator the attenuation is overcome by the negative resistance of the diode. 

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