Electret effects

There have been a number of investigations of the electret behavior of anodic oxides formed on various valve metals in different electrolytes and anodization regimes. The purpose of these 

Various mechanisms for electret effect formation in anodic oxides have been proposed. Lobushkin and co-workers241,242 assumed that it is caused by electrons captured at deep trap levels in oxides. This point of view was supported by Zudov and Zudova.244,250 Mikho and Koleboshin272 postulated that the surface charge of anodic oxides is caused by dissociation of water molecules at the oxide-electrolyte interface and absorption of OH groups. This mechanism was put forward to explain the restoration of the electret effect by UV irradiation of depolarized samples. Parkhutik and Shershulskii62 assumed that the electret effect is caused by the accumulation of incorporated anions into the growing oxide. They based their conclusions on measurements of the kinetics of Us accumulation in anodic oxides and comparative analyses of the kinetics of chemical composition variation of growing oxides. 

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The negative resistance effect is observed when anodic oxides are subjected to so-called electroforming (i.e., annealing in vacuum).93 Such a treatment removes the special features of the anodic oxides (asymmetry of conduction and electric strength, electret effect, etc.), and the negative resistance effect may be explained using the general approach developed for amorphous dielectrics.5... 

Qrou and Gultinger [486] applied the method of ionk thermal currents to a polarized electret which is discharged by the thermally stimulated discharge current technique and showed a method of calculation of dipok relaxation times and their activation energies. Perimsn and Meunkr [487], van Tumhout [488], and several others (489,490) have used it to analyze the electret effect quantitatively. 

According to some workers [142-145], becomes comparable to Pm. The distinction between the two processes may be made by using different electrodes because S-R emission is electrode dependent whereas the P-F effect is not (155). These two effects arc important for understanding the electret effect in pure and doped polymeric systems (136-142). 

Ihbic 1 shows some of the important polymeric materials that have been used to study the electret effect and their applicatioM in devices [110). 

The electronic charge carriers injected into organic solids from ohmic contacts can build up a stable space charge by trapping in deep traps thus some dyes assume the properties of electrets on application of an electric field in the dark. According to Euler et al. 140>, this effect can be utilized for the storage of electrical energy. 

Piezoelectricity in polymer films is greatly increased by polarizing the film under a high d.c. field at high temperatures. This effect, the piezoelectricity of a polymer electret, has attracted the attention of many investigators from the scientific as well as industrial point of view, since the early work by Kocharyan and Pachadzhyan (1963,1966) on PMMA and PVC. 

Murayama.N. Piezoelectric and pyroelectric effects of polymer electrets. Microsymposium on Electrical Properties of Polymers, Tokyo (Jan. 1972). 

Elastic effects in polymer melts, 578 Elastic moduli of some materials, 732 Elastic parameters, 383,386,391 Elastic shear deformation, 500, 531 Elastic shear quantities, 556 Electret, 329,331... 

A relatively recent development in radon gas monitoring is the electret system (Kotrappa et al, 1988). In effect this is an ionization chamber with an electret foil in the chamber, into which radon gas diffuses through a filter. The electret is a plastic foil which carries a permanent electric charge and therefore an electric potential. The decay of Rn and RnD in the chamber ionises the air, and the electric field caused by the potential on the electret attracts these ions to the electret. The electret loses its charge proportionally to the number of ions collected. The charge, or voltage on the electret is... 

The electret ion chamber for radon measurement or E-PERM (commercial name) can be used to determine the long-term average Rn concentration over periods of one week to months, depending on the levels. The electret system, as any other ionization chamber, is sensitive to pressure variations, and corrections for this effect should be made. A major disadvantage is that the electret will also respond to radiation from external sources. When used in gamma radiation fields (in many situations) a duplicate set of monitors have to be exposed to be able to correct for the effect of the external gamma radiation on the radon measurement. 

To study cooling rate effects which might affect the supramolecular self-assembly of trisamides in i-PP, we investigated the influence of the processing conditions on the electret performance. Isotactic polypropylene containing 0.09 wt% of compound 1 was chosen for these experiments. After compression molding, the films... 

A comparative study on the influence of chemical structure and solubility of a series of low-molecular-weight 1,4-phenylene-bisamides in i-PP was conducted to explore their performance as electret additives [30]. Bisamides are also capable of forming well-defined nano-aggregates via self-assembly from the i-PP melt upon cooling. Here, a comparison of a dicyclohexyl-substituted 1,4-phenylene-bisamide 5 and the asymmetrically substituted cyclohexyl/ -alkyl-l,4-phenylenebisamide 6 will be presented the chemical structures are included in Fig. 24. The symmetry of the molecule, the type of substitution, and the length of the alkyl chain have an influence on the solubility in the i-PP melt while the nucleation efficiency and the ratio of the a- to [3-crystal modification will have an effect on the charge storage properties. 

To investigate the potential of these blends for electret applications at elevated temperatures, the annealing temperature for the ITPD experiment was increased to 120°C and 150°C the effect on the surface potential decay is presented in Fig. 29. The complete potential loss at 120 °C of the 25/75 PPE/PS blend and PS films is due to the fact their glass transition temperatures are below this temperature. At 120°C (Fig. 29 top) the two PPE/PS blends 75/25 and 50/50 are still in the glassy solid state and show similar ITPD trends compared to the curves obtained at 90 °C, but with accelerated decay. The 75/25 PPE/PS blend films still possess a remaining surface potential of 74% after 24h at 120°C. 

In this chapter we have presented concepts and methods to improve polymer electret materials. Corona charging was employed for film charging since this method possesses the advantages of being cost-effective, easy to perform, and applicable in short periods of time. 

I.M. Vertyachikh, V.A. Goldade, A.S. Neverov and L.S. Pinchuk. Effect of electrical field of a polymer electret on vapor sorption of organic solvent, Vysoko-molec. Soedinenia, 1982, Vol. B24, No. 9, pp. 683-687. 

The presence of dispersed fillers in the polymer material in low amounts may intensify electrization, increase the residual charge and change the friction coefficient. Introduction of the filler in the electret state exerts a still stronger effect on polymer electrization on frictional interaction with metals. Depending on the direction of the field intensity vector formed by the filler particles, the field generated by triboelectrization can be attenuated or intensified. This means that the principle of the electret-triboelectrization superposition is realized [49], which can be used to regulate the parameters of frictional interactions. Thus, by the introduction of the electret filler, e.g. mechanically activated F-3 powder, it is possible to decrease the friction force (Fig. 4.9). 

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