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Ouro Preto, 07 de fevereiro de 2011

Exame de Seleo para ingresso no Mestrado em Engenharia de Materiais

Primeiro Semestre de 2011Prova de Ingls

Instrues ao candidato:1. Leia o texto abaixo e redija, em portugus, separadamente e na mesma ordem, cada

um dos pargrafos presentes no original em ingls. Todas as informaesimportantes devem estar presentes na traduo e ser considerada a qualidade dotexto em portugus.

2. O exame ter uma durao 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 10 7 (.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 ahighly 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).

Rede Temtica em Engenharia de MateriaisUFOP - CETEC UEMG

Ps-Graduao em Engenharia de MateriaisU F O P - C E T E C - U E M G