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Unformatted text preview: UC Davis – EMS 107L Electronic, Magnetic, and Optical Properties Laboratory – Prof. Ricardo Castro 1 UNIVERSITY OF CALIFORNIA, DAVIS Department of Chemical Engineering and Materials Science EMS 172L: Electronic, Magnetic, and Optical Properties Laboratory Laboratory Class I – Conductivity in Metals and Thermoelectricity – Conductivity in Metals – One of the most important properties of materials is their capability or not to conduct electricity. This is the source of the classification of materials as conductors, semiconductors and dielectrics, and also the basis for the vast variety of technologies. Conductivity, σ , is the inverse of the resistivity, ρ , therefore: ρ = σ 1 However, resistivity cannot be directly measured. The resistance, R, can. R of a piece of material is proportional to its resistivity, to its length (L), and also its area (A), such that: A L R ρ ⋅ = To measure R, one may apply a current to a piece of material and measure the current, I, and the voltage, V, between two points. R may be calculated using Ohm’s equation: V = R.I In metals (conductors), the resistivity is not constant with temperature, it usually decreases as a function of temperature in a relatively simple way: ( ) [ ] 1 2 1 2 T T 1- α + ρ = ρ Where α is the temperature resistivity coefficient, and T 1 and T 2 are two different temperatures. But how to explain this phenomenon? Electrons can be accelerated inside a metal using an electric field generated, for example, by a battery. In a classic view, the electrons flow in the metal and may eventually collide with some atoms in the crystalline structure, losing speed (energy). This is the source of the resistivity of the metal. If the electron is colliding in the periodic structure, we Figure 1. Schematic representation of a “classic” electron in a crystal structure. UC Davis – EMS 107L Electronic, Magnetic, and Optical Properties Laboratory – Prof. Ricardo Castro 2 may argue that it flows in a zig-zag way from the cathode to the anode (as shown in Figure 1). At high temperatures, the atoms in the crystal structure begin to oscillate due to thermal energy. This increases the probability of collision with the electrons. As a consequence, there will be an increase in the conductivity as a function of temperature. At 0K, the electrical resistance does not vanish. This is because that, even if the electrons could be aligned to the crystalline structure (avoiding zig-zag), there are usually many imperfections in the crystals, such as impurities, vacancies, grain boundaries and dislocations). This classical view is very reasonable. However, one may question if this would still be true if one considers that an electron has a particle-wave characteristic. Indeed, the modern explanation is very similar to the classic one. However, in this case, the crystalline structure scatters the waves in all directions. That is, the atoms absorb the energy from the electron waves and become oscillators. These oscillators re-emit the energy as spherical waves. Since the and become oscillators....
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- Spring '10
- Thermodynamics, Thermoelectric effect, Optical Properties Laboratory, Prof. Ricardo Castro, EMS 107L Electronic