Download ALUMINUM AND ITS ALLOYS - redemat

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).