Ch-17 Compatibility Mode

7/28/2019 Ch-17 Compatibility Mode

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CHAPTER 17

Chapter 18-


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CHAPTER 17

Learning Objective: Electrical Properties

Topic: ELECTRICAL RESISTIVITY OF METALS

Chapter 18-

ISSUES TO ADDRESS...ISSUES TO ADDRESS...ISSUES TO ADDRESS...ISSUES TO ADDRESS...

1

For metals, how is conductivity affected by

imperfections, T, and deformation?


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Ohm's Law:V = I R

voltage drop (volts) resistance (Ohms)current (amps)

V

Ie-A

(crosssect.area)

L

ELECTRICAL CONDUCTION

Chapter 18- 3

Resistivity, and Conductivity, :--geometry-independent forms of Ohm's Law

V

L

=I

A

E: electric

field

intensity

resistivity

(Ohm-m)

J: current density

=

I

conductivity

Resistance:

R =L

A=

L

A


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Silver 6.8 x 107

Copper 6.0 x 107

Iron 1.0 x 107

METALS

Soda-lime glass 10-10

Concrete 10-9

Aluminum oxide


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Factors affecting electrical resistivity of metals

Resistivityincreases with:

--temperature

--wt% impurity--%CW

Electrons do not scatter from a perfectlattice. They scatter by defects, which

can be:

atoms displaced by latticevibrations

vacancies and interstitials

dislocations, grain boundaries

Chapter 18- 5

tot of a metal depends on:

temperature

Impurities plastic deformation

Mathiessens rule

tot = T + i + d

n uence o empera ure

For pure metal and alloys (like

Cu-Ni), rises linearly with T

above about -200o

C.

Thus T = 0 +aT


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versus T for Cu and Cu-Ni alloys

Chapter 18- 6


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Solid solution alloys always has a higher resistivity than do its

pure component metals.

1. Atoms ofdifferent sizes cause a variation in the lattice parameter

in electron scattering.2. Atoms ofdifferent valences introduces a local charge difference

which increases the scattering..

3. For dilute sin le hase allo s : Z 2 Lindes Rule.

Influence of Impurities

Chapter 18-

4. For concentrated single phase alloys: i = AA+ BB+AABA+ B =1 and A is material constant. Nordheims Rule.

5. For a two phase alloy: i = V+ V,

8

Influence of Plastic deformation

Plastic deformation also raises the as a result of increased

numbers of electron-scattering dislocations.


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Question:--Estimate the electrical conductivity of a Cu-Ni alloy

that has a yield strength of 125MPa.

sistivity,

-8Ohm-m

)

30

4050

ength(MPa)

120

140

160

180

EX: ESTIMATING CONDUCTIVITY

Chapter 18- 9

wt. %Ni, (Concentration C)

R

e

(1

10 20 30 40 500

10

0Yields

t

wt. %Ni, (Concentration C)0 10 20 30 40 50

60

80 21 wt%Ni

= 30x108 Ohm m

=

1

= 3.3x106 (Ohm m)1

Adapted from Fig.18.9, Callister 6e.

Adapted from Fig.7.14(b), Callister 6e.


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Conductivity of most materials is expressed by

n = no. of free electrons per unit volume

= electrical charge

= electron mobilit

een =

e

e

Chapter 18-


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Electrical Characteristics of Commercial Alloys

Silver: High electrical conduction (lower ) but very

expensive

Good mechanical strength- the alloy CuBe is used.

Chapter 18- 10

When weight is important one uses Al, which is half as good

as Cu. Al is also more resistant to corrosion.

high resistivity materials -nichrome (NiCr) or graphite areused.

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