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Rede Temática em Engenharia de Materiais UFOP - CETEC – UEMG Pós-Graduação em Engenharia de Materiais UFOP - CETEC - UEMG Ouro Preto, 07 de fevereiro de 2011 Exame de Seleção para ingresso no Mestrado em Engenharia de Materiais Primeiro Semestre de 2011 Prova de Inglês Instruções ao candidato: 1. Leia o texto abaixo e redija, em português, separadamente e na mesma ordem, cada um dos parágrafos presentes no original em inglês. Todas as informações importantes devem estar presentes na tradução e será considerada a qualidade do texto em português. 2. O exame terá uma duração de 1:30h (uma hora e trinta minutos). Adapted from "Materials Science and Engineering: An Introduction, Fifth Edition; W.D. Callister" CONDUCTING POLYMERS Within the past several years, polymeric materials have been synthesized that have electrical conductivities close to that of metallic conductors; they are appropriately termed conducting polymers. Conductivities as high as 1.5 x 107 (.m)-1 have been achieved in these materials; on a volume basis, this value corresponds to one fourth of the conductivity of copper, or twice its conductivity on the basis of weight. This phenomenon is observed in a dozen or so polymers, including polyacetylene, polyparaphenylene, polypyrrole, and polyaniline that have been doped with appropriate impurities. As is the case with semiconductors, these polymers may be made either n-type (i.e., free-electron charge carriers) or p-type (i.e., electron-hole charge carriers) depending on the dopant. However, unlike semiconductors, the dopant atoms or molecules do not substitute for or replace any of the polymer atoms. High-purity polymers have electron band structures characteristic of electrical insulators. The mechanism by which large numbers of free electrons and holes are generated in these conducting polymers is complex and not well understood. In very simple terms, it appears that the dopant atoms lead to the formation of new energy bands that overlap the valence and conduction bands of the intrinsic polymer, giving rise to a partially filled band, and the production at room temperature of a high concentration of free electrons or electron-holes. Orienting the polymer chains, either mechanically or magnetically, during synthesis results in a highly anisotropic material having a maximum conductivity along the direction of orientation. These conducting polymers have the potential to be used in a host of applications as they have low densities, are highly flexible, and are easy to produce. Rechargeable batteries are currently being manufactured that employ polymer electrodes; in many respects these are superior to their metallic counterpart batteries. Other possible applications include wiring in aircraft and aerospace components, antistatic coatings for clothing, electromagnetic screening materials, and electronic devices (e.g., transistors and diodes).