Electrical conductivity

Transcription

1 Electrical conductivity Ohm's Law: voltage drop (volts = J/C) C = Coulomb A (cross sect. area) ΔV = I R Resistivity, ρ and Conductivity, σ: -- geometry-independent forms of Ohm's Law resistance (Ohms) current (amps = C/s) -- Resistivity is a material property & is independent of sample e - I ΔV L E: electric field intensity " V L = I A! resistivity (Ohm-m) J: current density Resistance: R = " L A = L A! conductivity " = 1 # 1

2 Conductivity: comparison Room T values (Ohm-m) -1 = (Ω - m) -1 METALS conductors Silver 6.8 x 10 7 Copper 6.0 x 10 7 Iron 1.0 x 10 7 CERAMICS Soda-lime glass 10 Concrete Aluminum oxide <10-13 SEMICONDUCTORS POLYMERS Silicon 4 x 10-4 Polystyrene <10-14 Germanium 2 x 10 0 Polyethylene GaAs 10-6 semiconductors insulators Selected values from Tables 18.1, 18.3, and 18.4, Callister 7e. 2

3 Electronic structure Adapted from Fig. 18.2, Callister 7e. 3

4 Electronic structure Valence filled highest occupied energy levels Conduction empty lowest unoccupied energy levels Conduction valence Adapted from Fig. 18.3, Callister 7e. 4

5 Conduction and electron transport Metals (Conductors): -- Thermal energy puts many electrons into a higher energy state Energy States: -- for metals nearby energy states are accessible by thermal fluctuations. Energy empty partly filled valence filled GAP filled states Energy filled states empty filled valence filled 5

6 Energy states: Insulators & semiconductors Insulators: -- Higher energy states not accessible due to gap (> 2 ev). Semiconductors: -- Higher energy states separated by smaller gap (< 2 ev). Energy empty GAP Energy? empty GAP filled states filled valence filled filled states filled valence filled 6

7 Charge carriers Adapted from Fig (b), Callister 7e. Two charge carrying mechanisms Electron negative charge Hole equal & opposite positive charge Move at different speeds - drift velocity Higher temp. promotes more electrons into the conduction σ as T Electrons scattered by impurities, grain boundaries, etc. 7

8 Charge carriers Imperfections increase resistivity -- grain boundaries These act to scatter -- dislocations electrons so that they -- impurity atoms take a less direct path. -- vacancies Resistivity, ρ (10-8 Ohm-m) Cu at%ni Cu at%ni deformed Cu at%ni Cu at%ni Pure Cu T ( C) Resistivity increases with: -- temperature -- wt% impurity -- %CW ρ = ρ thermal + ρ impurity + ρ deformation Adapted from Fig. 18.8, Callister 7e. (Fig adapted from J.O. Linde, Ann. Physik 5, p. 219 (1932); and C.A. Wert and R.M. Thomson, Physics of Solids, 2nd ed., McGraw-Hill Book Company, New York, 1970.) 8

9 Pure semiconductors: Conductivity vs T Data for Pure Silicon: -- σ increases with T -- opposite to metals electrical conductivity, σ (Ohm-m) pure (undoped) T(K) Adapted from Fig , Callister 5e. (Fig adapted from G.L. Pearson and J. Bardeen, Phys. Rev. 75, p. 865, 1949.) # " e undoped Energy? filled states material Si Ge GaP CdS empty GAP filled valence filled!e gap Selected values from Table 18.3, Callister 7e. kt electrons can cross gap at higher T gap (ev) / 9

10 Conduction in terms of electron and hole migration Concept of electrons and holes: valence electron Si atom electron hole pair creation electron hole pair migration no applied applied applied electric field electric field electric field Electrical Conductivity given by:! = n e µ + p e µ # holes/m 3 # electrons/m 3 electron mobility e h hole mobility Adapted from Fig , Callister 7e. 10

11 Intrinsic vs extrinsic conduction Intrinsic: # electrons = # holes (n = p) --case for pure Si Extrinsic: --n p --occurs when impurities are added with a different # valence electrons than the host (e.g., Si atoms) n-type Extrinsic: (n >> p) p-type Extrinsic: (p >> n) "! n e µ Phosphorus atom Adapted from Figs (a) & 18.14(a), Callister 7e. e no applied electric field hole conduction electron valence electron Si atom Boron atom h no applied electric field 4+ "! p e µ 11

12 Doped semiconductor: conductivity vs. T Data for Doped Silicon: -- σ increases with doping -- reason: imperfection sites lower the activation energy to produce mobile electrons. electrical conductivity, σ (Ohm-m) at%B doped at%B pure (undoped) T(K) Adapted from Fig , Callister 5e. (Fig adapted from G.L. Pearson and J. Bardeen, Phys. Rev. 75, p. 865, 1949.) Comparison: intrinsic vs extrinsic conduction extrinsic doping level: /m 3 of a n-type donor impurity (such as P). -- for T < 100 K: "freeze-out, thermal energy insufficient to excite electrons. -- for 150 K < T < 450 K: "extrinsic" -- for T >> 450 K: "intrinsic" conduction electron concentration (10 21 /m 3 ) freeze-out doped undoped 200 extrinsic 400 intrinsic 600 Adapted from Fig , Callister 7e. (Fig from S.M. Sze, Semiconductor Devices, Physics, and Technology, Bell Telephone Laboratories, Inc., 1985.) T(K) 12

13 Doped semiconductor: conductivity vs. T Intrinsic Conductivity σ = n e µ e + p e µ e for intrinsic semiconductor n = p σ = n e (µ e + µ n ) Ex: GaAs #6 " n = e ( µ e + µ n ) = 10 ($%m) #1 (1.6x10 #19 C)( m 2 /V% s) For GaAs n = 4.8 x m -3 For Si n = 1.3 x m -3 13

14 Corrosion of zinc in acid Two reactions are necessary: -- oxidation reaction: -- reduction reaction: Zn " Zn e # 2H + + 2e " # H 2 (gas) Zinc flow of e - in the metal 2e - H + Oxidation reaction Zn Zn 2+ H + H + H + H + H2(gas) H + reduction reaction H + Acid solution Adapted from Fig. 17.1, Callister 7e. (Fig is from M.G. Fontana, Corrosion Engineering, 3rd ed., McGraw-Hill Book Company, 1986.) Other reduction reactions: -- in an acid solution -- in a neutral or base solution O 2 + 4H + + 4e " # 2H 2 O O 2 + 2H 2 O + 4e" # 4(OH) " 14

15 Standard hydrogen (EMF) test Two outcomes: --Metal sample mass --Metal sample mass e - e - e - e - ne - metal, M M n+ ions H 2 (gas) H + H + 2e - Platinum ne - metal, M M n+ ions H + H + 2e - Platinum 25 C 1M M n+ sol n 1M H + sol n --Metal is the anode (-) V o metal < 0 (relative to Pt) 25 C 1M M n+ sol n 1M H + sol n --Metal is the cathode (+) V o metal > 0 (relative to Pt) Standard Electrode Potential Adapted from Fig. 17.2, Callister 7e. 15

16 Standard EMF series EMF series metal Au Cu Pb Sn Ni Co Cd Fe Cr Zn Al Mg Na K more anodic more cathodic V o metal V ΔV o = 0.153V Data based on Table 17.1, Callister 7e. Metal with smaller V corrodes. o metal Ex: Cd-Ni cell - Cd 1.0 M Cd 2+ solution 1.0 M + 25 C Ni Ni 2+ solution Adapted from Fig. 17.2, Callister 7e. 16

17 Effect of solution concentration Ex: Cd-Ni cell with standard 1 M solutions o o V Ni! VCd = Ex: Cd-Ni cell with non-standard solutions o o RT V Ni! VCd = VNi! VCd! nf - + ln X Y Cd 1.0 M 25 C Ni 1.0 M Cd 2+ solution Ni 2+ solution Cd X M Cd 2+ solution T Y M Ni Ni 2+ solution Reduce VNi - VCd by --increasing X --decreasing Y n = #e - per unit oxid/red reaction (= 2 here) F = Faraday's constant = 96,500 C/mol. 17

18 Galvanic series Ranks the reactivity of metals/alloys in seawater more cathodic (inert) more anodic (active) Platinum Gold Graphite Titanium Silver 316 Stainless Steel Nickel (passive) Copper Nickel (active) Tin Lead 316 Stainless Steel Iron/Steel Aluminum Alloys Cadmium Zinc Magnesium Based on Table 17.2, Callister 7e. (Source of Table 17.2 is M.G. Fontana, Corrosion Engineering, 3rd ed., McGraw- Hill Book Company, 1986.) 18

19 Forms of corrosion Stress corrosion Uniform Attack Oxidation & reduction occur uniformly over surface. Stress & corrosion work together at crack tips. Selective Leaching Preferred corrosion of one element/constituent (e.g., Zn from brass (Cu-Zn)). Intergranular Corrosion along grain boundaries, often where special phases exist. g.b. prec. attacked zones Fig , Callister 7e. Forms of corrosion Galvanic Dissimilar metals are physically joined. The more anodic one corrodes.(see Table 17.2) Zn & Mg very anodic. Erosion-corrosion Break down of passivating layer by erosion (pipe elbows). Pitting Downward propagation of small pits & holes. Fig , Callister 7e. (Fig from M.G. Fontana, Corrosion Engineering, 3rd ed., McGraw-Hill Book Company, 1986.) Crevice Between two pieces of the same metal. Rivet holes Fig , Callister 7e. (Fig is courtesy LaQue Center for Corrosion 19 Technology, Inc.)

20 Controlling corrosion Self-protecting metals! Metal oxide -- Metal ions combine with O Metal (e.g., Al, to form a thin, adhering oxide layer that slows corrosion. stainless steel) Reduce T (slows kinetics of oxidation and reduction) Add inhibitors -- Slow oxidation/reduction reactions by removing reactants (e.g., remove O2 gas by reacting it w/an inhibitor). -- Slow oxidation reaction by attaching species to the surface (e.g., paint it!). Cathodic (or sacrificial) protection -- Attach a more anodic material to the one to be protected. Adapted from Fig , Callister e.g., zinc-coated Zn 2+ nail zinc zinc 2e - 2e - 7e. steel steel pipe e.g., Mg Anode Cu wire e - Mg Mg 2+ anode Earth Adapted from Fig (a), Callister 7e. (Fig (a) is from M.G. Fontana, Corrosion Engineering, 3rd ed., McGraw- Hill Book Co., 1986.) 20