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E45%20Lab%201%20Conductivity

E45%20Lab%201%20Conductivity - Engineering 45 Properties of...

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E E n n g g i i n n e e e e r r i i n n g g 4 4 5 5 P P r r o o p p e e r r t t i i e e s s o o f f M M a a t t e e r r i i a a l l s s L L a a b b o o r r a a t t o o r r y y © Copyright 2001 Professor Ronald Gronsky the Arthur C. and Phyllis G. Oppenheimer Chair in Advanced Materials Analysis Department of Materials Science & Engineering University of California Berkeley, California 94720-1760
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E 45 2 Lab 5 Electrical Conductivity Objectives To understand the origins of conductivity in different classes of materials To understand the change of resistivity with temperature in metals, semiconductors, and insulators To determine the relationship between resistivity and the presence of impurities in these materials Overview This lab is designed to help the student understand how conductivity is related to microstructure, temperature, and impurity concentration. The student will: (1) determine the concentration of impurities in metallic samples by measuring resistivity; (2) determine the energy gap of a germanium semiconductor; and (3) determine both the activation energy for migration and the formation energy of charged vacancies in an ionic solid (sodium chloride). Equipment Part A Kelvin double bridge apparatus for measuring electrical resistance Series of five samples of binary copper alloys containing the same solute in a single-phase solid solution Constant temperature baths: 77K (boiling N 2 ); 200K (crushed CO 2 ; 273K (ice-water); approximately 305K (room temperature); and 373K (boiling water). Part B 1. Sample of germanium, in holder with leads attached, hotplate, with beaker 2. Mercury-in-glass thermometer (0°C - 100°C) 3. Dewar flask for crushed CO 2 4. Volt-ohm-milliammeter Part C 1. Furnace 2. Potentiometer 3. Temperature controller 4. Resistance measuring bridge with an a.c. signal source and null detector (range approximately 10 3 to 10 7 ohms) 5. Specimen holder
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E 45 3 Background When an electric field is applied to a solid, free electrons are accelerated. They lose or decrease their kinetic energy by collisions with the atoms of the lattice. The current that results is proportional to the average electron velocity, which is determined by the intensity of the applied electric field and the collision frequency. Only electrons with energy near the Fermi level may be accelerated, as the other electrons inhabit states that are surrounded by full (occupied) states, and are consequently forbidden to accelerate (change state) by the Pauli Exclusion Principle. If the valance band is full and does not overlap empty bands, the lack of adjacent empty states severely limits conduction. This is the accepted model for insulators and semiconductors. Electrons may move through an ideal crystal without resistance, but in real crystals electrons collide with phonons, dislocations, vacancies, impurities, and any other lattice imperfections. The resistivity due to impurities and imperfections is called the residual resistivity and is usually independent of temperature. The total resistivity is the sum of the residual resistivity and thermal contributions.
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