Transcript irreversible thermoelectric effects

THERMOELECTRIC MATERIALS
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THERMOELECTRIC EFFECT:
Any phenomenon involving an inter-conversion of heat and
electrical energy may be termed a thermoelectric effect.
IRREVERSIBLE THERMOELECTRIC EFFECTS
The best known irreversible thermoelectric effect is the
Joule effect, where an electric current, I, is transformed
irreversibly into heat P according to P=I2R , where R is the
electrical resistance of the conductor.
REVERSIBLE THERMOELECTRIC EFFECTS
The Seebeck, Peltier and Thomson effects are three
related reversible thermoelectric effects.
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SEEBECK EFFECT (1826):
A temperature difference between two points
in a conductor or semiconductor results in a
voltage difference between these two points.
This phenomenon is called SEEBECK
EFFECT.
E  aT  bT 2
Neutral temp
Here, a and b are called
thermoelectric constants
and T is the temperature
difference between hot
and the cold junction.
Inversion temp
Tn is fixed for a given pair of thermocouple while Ti (Inversion
temperature) depends on the temperature of the cold junction. 3
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CAUSE OF THERMOELECTRIC EFFECTS (Refer to the figure in the
previous slide)
Two different metals have different free electron densities while they are
both at the same temperature. When two metals are joined, the most
energetic electrons from one metal will migrate to the other metal in order
to establish a new equilibrium of the junctions and balance the charge
difference.
This move disturbs the individual equilibrium of each of the metals.
This causes an electric field to be formed across the junctions.
Since the temperature determines how energetic the free electrons will be
and since their migration determines how many exposed positive and
excess negative charges are on the two sides of the junction, it follows
that the magnitude of the electric field is a function of temperature.
In a closed circuit, a SEEBECK CURRENT forms from the electric field
and circulates in the loop. At one junction where the electric field has the
same direction as the generated current, the current flows easily; at the
other junction where the electric field and the generated current have
opposite directions, the current must travel against the electric field. 5
 eV  Eav
Now, from “Free” electron theory of metals.
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5 2 kT 2
E av  E FO [1 
(
) ]
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12 E FO
 eV 
 k TT
2
2
2 E FO
Since S 
S 
V
T
 2 k 2T
2eE FO
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Seebeck investigated the thermoelectric effect for a large
number of metals and arranged them in a series called
SEEBECK SERIES. Some of the familiar metals in the
series are:
Selenium (78.1), Antimony (43.9), Iron (11.9), Zinc (3.4), Copper
(3.3), Gold (3.2), Silver (3.0), Platinum Rhodium (2.3), Tin (0),
Lead (0), Brass (-0.1), Aluminum (-0.3), Mercury (-4.1), Platinum
(-4.1), Nickel (-20.5), Constantan (-38.5), Bismuth (-69.1)
Two important characteristics of the series are:
1. Greater the distance between the metals, higher the thermoemf developed for given temperature difference in the
thermocouple made of them.
2. The current will flow through the cold junction from the one
metal occurring earlier to the one occurring later in the series.
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The numbers in bracket indicate the approximate EMF in
micro-volt per degree above zero.
The EMF for antimony-bismuth couple is 43.9-(-69.1) = 113
micro volt/0C
The EMF for copper-constantan couple is 3.3-(-38.5) = 41.8
micro volt/0C
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Peltier coefficient () is given as:
E
 T
 TS
T
Here, E = Thermo-EMF &
S = Seebeck coefficient
Distinction between Peltier effect and Joule’s effect:
1. Joule’s effect (H = i2Rt) depends on i2 and irreversible, and
reversal of direction of current has no effect on it. The Peltier
effect (H =  It), on the other hand, depends on the direction of
current. The reversal of current reverses the effect from cooling
to heating.
2. Peltier effect takes place at the junction only while Joule’s
effect is observed throughout the conductor.
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THOMSON EFFECT
When a current flows through a conductor maintained at a
temperature gradient, heat is either evolved or absorbed along
length of the conductor. This is known as Thomson effect.
Direction of current determines whether heat is absorbed or
evolved.
Q
Tcold
I
Thot
Since density of electrons in a conductor depends upon
temperature, a potential gradient is developed in it maintained at
a gradient. When current flows through such a conductor, heat is
absorbed when current flows from lower to higher potential and
released in the contrary situation.
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In Thomson effect:
QαI
Qαt
Q α dT
Q = .I.t.dT
Here,  is Thomson’s coeff.
2E
 T 2
T
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Numerical
If Emf produced is V=aT+bT2, Determine
Seeback coefficient, Neutral temperature, Peltier’s
coefficient and Thomson’s coefficient.
Sol: Seeback coeff= dV/dT = a+2bT
• Neutral temperature Tn, put dV/dT=0. T=-a/2b
• Peltier’s coeff =T(dV/dT) = aT+2bT2
• Thomson’s coeff =T(d2V/dT2) = 2bT
What material properties make a good
thermoelectric?
TE FoM
S 2
Z   

k
S2
S 2
Optomized FoM ( ZT ) 
T
k
Where;
S is Seebeck Coefficient (large)
ρ is resistivity (small) or  Is
conductivity (large)
k is Thermal conductivity (small)

Optimized Figure of Merit ZT
minimize the
thermal conductivity
up-to-date best bulk thermoelectrics:
Bi2Te3/Sb2Te3, PbTe, Si-Ge
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Thermoelectric
Sub-Mount
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Seebeck Effect
Peltier Effect
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Transportation energy use in India from
2.0 quadrillion Btu in 2008
to 8.7 quadrillion Btu in 2035.
Radiator :TE
1 Btu = 1055 Joules
1drillion = 10^15
1 quadrillion = 4 x10^15
Parasitic
heat loss
Coolant
Exhaust
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BMW patented design
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