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Temperature-dependent dielectric relaxation spectra of cross-linked styrene-butyl acrylate copolymers were measured at frequencies between 10 mHz and 1 MHz. The results were analyzed using the empirical Kohlrausch-Williams-Watts (KWW) and Havriliak-Negami (HN) formalisms. While the KWW and the HN formalisms were equally well able to fit the experimental data for the uncross-linked copolymer, the HN formalism provided superior fits at high levels of cross-linking. The fitting parameters obtained from the HN routine were used to interpret the data in terms of the model proposed by Schónhals and Schlosser that relates the molecular motion of the polymer with the HN parameters. In this model, = ß is correlated with the local intramolecular dynamics of the polymer and is not influenced by the level of cross-linking. On the other hand, the parameter m-a, which is related to the intermolecular correlations of the polymer chain segments, decreases with increasing level of cross-linking.However, in recent years there have been a number of papers published wherein researchers correlate the fitting parameters with molecular motions of the polymer chains.3-5The first successful attempt to treat permittivity was developed by Debye.6 However, the treatment developed by Debye predicted that a molecule would exhibit only a single relaxation time. For such a single relaxation process, the dielectric response can be described by eq 2 with a = 10=1 and as the Debye relaxation time.

An industrially important class of passively smart materials is electrically nonlinear polymer composites. The transition of conducting composites from low to high resistivity can be utilized for current limitation. Due to Joule losses the material is heated by a fault or short-circuit current. With increasing temperature the polymer matrix expands and the current paths over the conducting filler particles are interrupted. Within milliseconds, the material responds to the fault current by an increase in resistivity up to eight orders of magnitude. Due to the strong nonlinear resistivity - temperature relation, a narrow hot-zone is formed even for long samples. The length of the hot-zone limits the maximum switching voltage. By adding a second filler material of varistor- type, however, the maximum voltage can be considerably increased. When a hot spot is formed in one of the current paths over the conducting particles, a small voltage increase allows already a commutation of the current to neighboring varistor particles. Consequently, the current can still flow to a certain degree and allows to heat also the rest of the material around its path. This leads finally to a very broad hot area, which can resist much higher voltages. By the development of a smart material with two strong non-linearities, a dramatic improvement has been achieved for the application of thermistor composites in current limitation.

Great advances have been achieved in the development of silver pastes. The use of smaller silver particles, higher silver content, and, thus, less glass frit allow modern silver pastes to contact high resistive emitters without the necessity of a selective emitter or subsequent plating. To identify the microscopic key reasons behind the improvement of silver paste, it is essential to understand the current transport mechanism from the silicon emitter into the bulk of the silver finger. Two current transport theories predominate: i) The current flows through the Ag crystallites grown into the Si emitter, which are separated by a thin glass layer or possibly in direct contact with the silver finger.ii) The current is transported by means of multistep tunneling into the silver finger across nano-Ag colloids in the glass layer, which are formed at optimal firing conditions; the formation of Ag crystallites into the Si surface is synonymous with over-firing. In this study, we contact Si solar cell emitters with different silver pastes on textured and flat silicon surfaces. A sequential selective silver-glass etching process is employed to expose and isolate the different contact components for current transport. The surface configurations after the etching sequences are observed with scanning electron microscopy. Liquid conductive silver is then applied to each sample and the contact resistivity is measured to determine the dominant microscopic conduction path system. We observe glass-free emitter areas at the tops of the pyramidal-textured Si that lead to the formation of direct contacts between the Ag crystallites grown into the Si emitter and the bulk of the silver finger. We present experimental evidence that the major current flow into the silver finger is through these direct contacts.

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