Mobility of ionic impurities

Nevertheless, it would seem reasonable that, in the absence of any liquid plasticizer medium at all, mobility of ionic impurities would be reduced to such a low level that volume resistivity would remain high. For example, it is well known that polyvinyl chloride can be blended with nitrile rubber, such as Goodrich Hycar 1032 butadiene/acrylonitrile copolymer, and such polyblends are quite soft and flexible without the use of any liquid plasticizer at all (Table VII). 

Electrical Properties. Addition of plasticizer increases mobility of ionic impurities, which lowers electrical resistance (increases conductivity). Plasticizer increases the mobility of polar groups in the polymer, which increases dielectric constant and shifts the loss peak to higher frequencies. 

The absorption of water by the plastic causes two changes which have a direct bearing on reliability it increases the mobility of ionic impurities and, in making the plastic expand, it increases the likelihood of the formation of a gap between the plastic and the semiconductor die. The importance of these two factors will be discussed in the next section. 

Metals are the classical conductors. Most organic solids, and notably polymers, are insulators by nature. In the usual polymers the electrical resistance is very high their conductivity probably results partially from the presence of ionic impurities, whose mobility is limited by the very high viscosity of the medium. 

Another stringent demand is that the polymer have a very low concentration of ionic impurities because such impurities, when mobile, could interfere with the electronic function of the device. For example, the Na ion concentration has to be less than A ppm. This requirement means that the Na ion impurity content of the solvents for the polymer also has to be very low. In general, special grade materials are needed to fulfill the requirements for electronic applications. 

Many polymeric materials consist of dipoles (chemical bonds which have an unbalanced distribution of charge in a molecule) and traces of ionic impurities. If a polymer containing polar groups is heated so that an immobile dipole becomes mobile, an increase in permittivity is observed as the dipole starts to oscillate in the alternating electric field. This effect is referred to as a dipole transition and has a characteristic relaxation time (t) associated with it (76). When exposed to an electric field, the dipoles tend to orient parallel to the field direction and the ions move toward the electrodes, where they form layers. The dipole relaxation time... 

In principle, all of the parameters in this equation are measurable so it should be possible to determine the mobility from the voltage dependence of the current. However, the actual behaviour of cells of this type is critically dependent on the nature of the interface. The currents obtained can also depend on the history of the cell. One example of this and of another way that ppm levels of ionic impurities can cause problems is provided by the field anneal phenomenon [25, 26]. If the sample is heated into the isotropic phase and a DC current applied to the cell for a period of time ionic impurities become adsorbed on the electrode surface where they help to facilitate charge injection. Thereafter the I/V characteristics of the Col phase are found to be totally different and, in extreme cases, the measured conductivity can (apparently) be orders of magnitude higher Because of the extreme sensitivity of sandwich cell measurements to small difference in the electrode surface and, because experimentally the voltage dependence of the current often deviates quite markedly from the expected (V-Vc) behaviour, reliable determination of mobilities from the I/V curves is difficult and this approach is rarely used [27]. 

There are three broad categories of materials that have been utilized in this endeavor. In the first, even in fully stoichiometric compounds, the ionic conductivity is high enough to be useful in devices because the cation or anion substructure is mobile and behaves rather like a liquid phase trapped in the solid matrix. A second group have structural features such as open channels that allow easy ion transport. In the third group the ionic conductivity is low and must be increased by the addition of defects, typically impurities. These defects are responsible for the enhancement of ionic transport. 

In summary, the volume resistivity of polyvinyl chloride plasticized by liquid or elastomeric plasticizers, or internally plasticized by copolymerization, was intermediate between the inherent volume resistivities of the pure components and combined the contributions of each of them. The presence of ionic soluble impurities in liquid plasticizers provided mobile ions which conducted electricity and thus lowered volume resistivity. Copolymerization with 2-ethylhexyl acrylate provided an excellent balance of softness and flexibility with high volume resistivity further studies of internal plasticization by copolymerization are therefore recommended. 

Reitemting, there are generally two effects considered in curing systems, namely conduction of ionic resin impurities and molecular-dipole orientation. The ionic conductivity unit volume and qi is the charge, for species i. Qualitative relationships between conductivity and properties of... 

The behavior of such a system is easy to predict in the absence of mobile ions. Ohmic contacts and a high resistivity bulk give rise to a bulk-controlled or space-charge limited currents (L3). A complete analysis of the space-charge limited transient current in insulating crystals has been given (14)- The analysis in polymers is modified by the presence of mobile ionic impurities in two ways. First, in thermal equilibrium the bulk ion and electron concentrations must be... 

Implications. These results have an important implication concerning the use of Fourier analysis of DC transients in polymeric materials to extract the frequency-dependence of the dielectric response (12)- In order for the principle of superposition to apply the electric field inside the material being measured must be time- and space-invariant. This critical condition may not be met in polymers which contain mobile ionic impurities or injected electrons. Experimentally, we can fix only the average of the electric field. Moreover, our calculations demonstrate that the bulk field is not constant in either time or space. Thus, the technique of extracting the dielectric response from the Fourier components of the transient response is fundamentally flawed because the contribution due to the formation of ionic and electronic space-charge to the apparent frequency-dependent dielectric response can not generally be separated from the dipole contribution. 

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