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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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slide4
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.

slide5
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.

slide6

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

applications
Applications

Conductors:Requirements

1. Low I2R loss

(High Conductivity)

2. Fabricability

3. Cost

4. Strength

slide8

Candidate Materials

Long distance transmission lines

- Al

- ACSR: Al conductor steel reinforced

slide9
(Cu is more expensive)

Distribution lines, Bus bars, Energy Conversion Applications

- OFHC copper

Use of Cd as solute in improving the strength

slide10
ElectricalRequirements

Contacts:1. High 

switches2. High Thermal brushes Conductivity

relays3. High m.p.

4. Good Oxidation Resistance

slide11
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

slide12
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

slide13
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

slide14
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

slide15
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

slide16
Resistance Thermometers:

Requirement -High 

Candidate - Platinum (pure metal)

superconductors

Superconductors

Section 14.5

1 phenomenon
Resistivity of silver1. Phenomenon

 (10-11 ohm m)

Fig. 14.7 a

T, K

slide19
Resistivity of tin

Can be used for producing large permanent magnetic field

Fig. 14.7 b

 (10-11 ohm m)

T, K

slide20

Fig. 14.8

Loss of superconductivity

Normal

0 Hc, Wb m-2

Superconductor

Tc

T, K

slide21
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

meissner effect
Meissner Effect

Fig. 14.9

T>Tc

T<Tc

Normal

Superconductor

b c s theory bardeen cooper schreiffer
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

type ii
Type II

Great practical interest because of high Jc.

This state is determined by the microstructural conditions of the material

3 potential applications
3. Potential Applications
  • Strong Magnets (50 Tesla)

MHD power generation

  • Logic and Storage functions in computers

switching times  10 ps

  • Levitation

transportation

slide29
Transmission

No I2R loss

slide30

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

4 new developments
4. New Developments

Nb3Ge 23 K1976

La-Ba-Cu-O 34 K 1986,

Bednorz and Muller

YBa2Cu3O7-x 90 K 1988

slide33
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

slide35
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