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Conductors and Resistors

Conductors and Resistors. Chapter 14. Imperfections solutes , vacancies , etc. dislocations grain boundaries act as scattering centres and thereby decrease the mean free path and thus decrease  .

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Conductors and Resistors

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  1. Conductors and Resistors Chapter 14

  2. Imperfectionssolutes, vacancies, etc. dislocations grain boundaries act as scattering centres and thereby decrease the mean free path and thus decrease . Of all theimperfections, dissolved impurities (solutes)aremore effectivethan the others as scattering centres.

  3. Phonons: elastic waves produced by the random vibrations of atoms Random nature destroys the ideal periodicity and interferes with the electron motion. Conductivity thus decreases with increasing temperature.

  4. Fig. 14.6 Dependence of resistivity on temperature and composition  Cu-3%Ni  = T + r = resitivity T = thermal part of the resitivity r =residual resitivity due to impurity and imperfections Cu-2%Ni Pure Cu Mattheissen’s Rule : T and r are independent of each other; i.e., T depends only on tempearture and r depends only on compositon T Experiment 9

  5. Applications Conductors:Requirements 1. Low I2R loss (High Conductivity) 2. Fabricability 3. Cost 4. Strength

  6. Candidate Materials Long distance transmission lines - Al - ACSR: Al conductor steel reinforced

  7. (Cu is more expensive) Distribution lines, Bus bars, Energy Conversion Applications - OFHC copper Use of Cd as solute in improving the strength

  8. ElectricalRequirements Contacts:1. High  switches2. High Thermal brushes Conductivity relays3. High m.p. 4. Good Oxidation Resistance

  9. Candidate Materials - Cu and Ag Cu is cheaper Ag, which is expensive, is preferred for critical contacts. Strength of Ag is increased by dispersed CdO (Dispersion Strengthening) Absorbs heat by decomposing

  10. Resistors:Requirements 1. Uniform resistivity 2. Stable resistance 3. Small temp. Coefficient of resistivity 4. Low thermoelectric pot. w.r.t. copper 5. Good resistance to atmospheric corrosion

  11. Candidates: Manganin (87% Cu, 13% Mn)  = 20 × 10-6 K-1 low as compare to that for Cu, which is 4000 × 10-6 K-1 . Constantan (60% Cu, 40% Ni) Ballast Resistors are used in circuits to maintain constant current – these must have high . 71% Fe, 29% Ni alloy is used  = 4500 × 10-6 K-1

  12. HeatingRequirements Elements:1. High m.p. 2. High resistivity 3. Good Oxidation Resistance 4. Good Creep Strength 5. Resistance to thermal fatigue - low elastic modulus - low therm. expansion

  13. Candidates Nichrome (80% Ni, 20% Cr) Kanthal (69% Fe, 23% Cr, 6% Al, 2% Co) SiC MoSi2 Graphite in inert atmosphere Mo, Ta Poor oxidation resistance W (filaments) – ThO2 dispersion to improve creep resistance

  14. Resistance Thermometers: Requirement -High  Candidate - Platinum (pure metal)

  15. Superconductors Section 14.5

  16. Resistivity of silver 1. Phenomenon  (10-11 ohm m) Fig. 14.7 a T, K

  17. Resistivity of tin Can be used for producing large permanent magnetic field Fig. 14.7 b  (10-11 ohm m) T, K

  18. Fig. 14.8 Loss of superconductivity Normal 0 Hc, Wb m-2 Superconductor Tc T, K

  19. The maximum current that a superconductor carries at a given temperature below Tc is limited by the magnetic field it produces at the surface of the superconductor Normal Jc, A m-2 Superconductor Tc Fig not in book T, K

  20. Meissner Effect Fig. 14.9 T>Tc T<Tc Normal Superconductor

  21. BCSTheory (Bardeen, Cooper, Schreiffer) Three way interaction between two electrons and a phonon Electron pair (cooper pair): The attractive interaction energy The repulsive energy Attraction is disrupted at T  Tc

  22. -M 2. Two types I and II of superconductors Type I Type II -M Hc1 Hc2 Hc H H

  23. Type II Great practical interest because of high Jc. This state is determined by the microstructural conditions of the material

  24. Nb-40% Ti at 4.2 K, 0.9 Hc2

  25. 3. Potential Applications • Strong Magnets (50 Tesla) MHD power generation • Logic and Storage functions in computers switching times  10 ps • Levitation transportation

  26. Transmission No I2R loss

  27. Magnetic Levitation (Maglev) is a system in which the vehicle runs levitated from the tracks by using electromagnetic forces between superconducting magnets on board the vehicle and coils on the ground. Yamanashi Maglev Test Line December 2, 2003, maximum speed 581 km/h (manned run). Max speed of Rajdhani Express 140 km/h

  28. Magnetic Resonance Imaging

  29. 4. New Developments Nb3Ge 23 K1976 La-Ba-Cu-O 34 K 1986, Bednorz and Muller YBa2Cu3O7-x 90 K 1988

  30. Recipe: Y2O3, BaCO3, CuO compacted powder in right proportion is heated (900 - 1100°C) BaCO3 BaO +CO2 Annealing at 800 °C in O2 atmosphere The super conducting properties appear to be sensitive function of the oxygen content and, therefore, of the partial pressure of oxygen during heat treatment

  31. YBa2Cu3O7-x Cu Y Ba O

  32. Engineering aspects remain Elusive Reactive and Brittle • Unable to support any significant stress • Cannot be easily formed or joined Superconducting properties deteriorate during heating for forming purposes Or even in humid room Attempts Explosive forming 50 000 atm (100°C) Isostatic Pressing

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