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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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.
Random nature destroys the ideal periodicity and interferes with the electron motion.
Conductivity thus decreases with increasing temperature.
Dependence of resistivity on temperature and composition
= T + r
T = thermal part of the resitivity
r =residual resitivity due to impurity
Mattheissen’s Rule : T and r
are independent of each other; i.e.,
T depends only on tempearture and
r depends only on compositon
1. Low I2R loss
Long distance transmission lines
- ACSR: Al conductor steel reinforced
Distribution lines, Bus bars, Energy Conversion Applications
- OFHC copper
Use of Cd as solute in improving the strength
switches2. High Thermal brushes Conductivity
relays3. High m.p.
4. Good Oxidation Resistance
Cu is cheaper
Ag, which is expensive, is preferred for critical contacts.
Strength of Ag is increased by dispersed CdO
Absorbs heat by decomposing
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
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
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
Nichrome (80% Ni, 20% Cr)
Kanthal (69% Fe, 23% Cr, 6% Al, 2% Co)
Graphite in inert atmosphere
Mo, Ta Poor oxidation resistance
W (filaments) – ThO2 dispersion to improve creep resistance
Candidate - Platinum (pure metal)
Can be used for producing large permanent magnetic field
Fig. 14.7 b
(10-11 ohm m)
Loss of superconductivity
0 Hc, Wb m-2
Jc, A m-2
Fig not in book
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
Great practical interest because of high Jc.
This state is determined by the microstructural conditions of the material
MHD power generation
switching times 10 ps
No I2R loss
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
Nb3Ge 23 K1976
La-Ba-Cu-O 34 K 1986,
Bednorz and Muller
YBa2Cu3O7-x 90 K 1988
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
Reactive and Brittle
Superconducting properties deteriorate during heating for forming purposes
Or even in humid room
Explosive forming 50 000 atm (100°C)