Abstract

Thermoelectricity, also called Peltier-Seebeck effect, direct conversion of heat into electricity or electricity into heat through two related mechanisms, the Seebeck effect and the Peltier effect.

When two metals are placed in electric contact, electrons flow out of the one in which the electrons are less bound and into the other. The binding is measured by the location of the so-called Fermi level of electrons in the metal; the higher the level, the lower is the binding. The Fermi level represents the demarcation in energy within the conduction band of a metal between the energy levels occupied by electrons and those that are unoccupied. The energy of an electron at the Fermi level is W relative to a free electron outside the metal. The flow of electrons between the two conductors in contact continues until the change in electrostatic potential brings the Fermi levels of the two metals (W1 and W2) to the same value. This electrostatic potential is called the contact potential 12 and is given by e12 = W1 W2, where e is 1.6 10^19 coulomb.

Seebeck effect, production of an electromotive force (emf) and consequently an electric current in a loop of material consisting of at least two dissimilar conductors when two junctions are maintained at different temperatures. The conductors are commonly metals, though they need not even be solids. The German physicist Thomas Johann Seebeck discovered (1821) the effect. The Seebeck effect is used to measure temperature with great sensitivity and accuracy and to generate electric power for special applications.

Introduction

Thermoelectricity

Thermoelectricity is a two-way process. It can refer either to the way a temperature difference between one side of a material and the other can produce electricity, or to the reverse: the way applying an electric current through a material can create a temperature difference between its two sides, which can be used to heat or cool things without combustion or moving parts. The first part of the thermoelectric effect, the conversion of heat to electricity, was discovered in 1821 by the Estonian physicist Thomas Seebeck and was explored in more detail by French physicist Jean Peltier, and it is sometimes referred to as the Peltier-Seebeck effect.

The reverse phenomenon, where heating or cooling can be produced by

running an electric current through a material, was discovered in 1851 by

William Thomson, also known as Lord Kelvin (for whom the absolute Kelvin

temperature scale is named), and is called the Thomson effect. The effect is

caused by charge carriers within the material (either electrons, or places

where an electron is missing, known as holes) diffusing from the hotter side

to the cooler side, similarly to the way gas expands when it is heated. The

thermoelectric property of a material is measured in volts per Kelvin.

Thermocouple

Thermocouples are very simple and durable temperature sensors. They are comprised of two different materials joined at one end and separated at the other. The separated ends are considered the output, and they generate voltage which is proportional to the heat they are measuring or monitoring. That is, the hotter the temperature, the higher the voltage. The fact that two metals generate voltage is known as the Seebeck effect.

Principle Of Operation & Seebeck Effect

A thermocouple is a device made by two different wires joined at one end, end or measuring end. The two wires are called thermoelements or legs of the thermocouple: the two thermoelements are distinguished as positive and negative ones. The other end of the thermocouple is called tail end or reference end. The junction end is immersed in the environment whose temperature T2has to be measured, which can be for instance the temperature of a furnace at about 500C, while the tail end is held at a different temperature T1, e.g. at ambient temperature.

The temperature vs voltage relationship is given by:

1.A null voltage is measured if the two thermoelements are made of the same materials: different materials are needed to make a temperature sensing device.

2.A null voltage is measured if no temperature difference exists between the tail end and the junction end: a temperature difference is needed to operate the thermocouple.

3.The Seebeck coefficient is temperature dependent.

In order to clarify the first point let us consider the following example: when a temperature difference is applied between the two ends of a single Ni wire a voltage drop is developed across the wire itself. The end of the wire at the highest temperature, T2, is called hot end, while the end at the lowest temperature, T1, is called cold end.

When a voltmeter, with Cu connection wires, is used to measure the voltage drop across the Ni wire, two junctions need to be made at the hot and cold ends between the Cu wire and the Ni wire; assuming that the voltmeter is at room temperature T1, one of the Cu wires of the voltmeter will experience along it the same temperature drop from T2 to T1 the Ni wire is experiencing. In the attempt to measure the voltage drop on the Ni wire a Ni-Cu thermocouple has been made and so the measured voltage is in reality the voltage drop along the Ni wire plus the voltage drop along the Cu wire.

The Emf along a single thermoelement cannot be measured: the Emf measured at the tail end is the sum of the voltage drop along each of the thermoelements. As two thermoelements are needed, the temperature measurement with thermocouples is a differential measurement.

The temperature measurement with thermocouples is also a differential measurement because two different temperatures, T1 and T2, are involved. The desired temperature is the one at the junction end, T2. In order to have a useful transducer for measurement, a monotonic Emf versus junction end temperature T2 relationship is needed, so that for each temperature at the junction end a unique voltage is produced at the tail end.

As explained before, a thermocouple consists of two different metals. This implies that the Seebeck coefficient contains information for both metals. Therefore, the Seebeck coefficient we are actually measuring is the relative Seebeck coefficient.

S= V/T. Here, S is the relative Seebeck coefficient.

To find the absolute Seebeck coefficient the metal/alloy, should be paired with a material whose absolute Sc is 0. The only materials whose absolute Sc is 0 are superconductors. Or one can also use copper whose absolute Sc is already known.Therefore, the equation becomes

S(sample)-S(copper)= V/T

Experiment

Overview

Two thermocouples were made, Brass-Iron and Brass-Copper. The thermocouples were made from commercially available wires of brass, copper and iron by gas welding the tips of the wires to create two junctions. No other substance was used during gas welding. The contact of the two metals is just by fusing the tips of two wires.The setup was designed for measuring the Seebeck effect. Initially, for obtaining pilot readings one end of the thermocouple was kept at room temperature and the other was exposed to direct heating from a bunsen burner and voltage was measured across the ends. The direct heating resulted in breaking of the junction and the setup was improvised by accommodating a water bath in which the junction would be immersed.

Setup 1

To characterize a thermocouple, its response is to be measured over a wide range of temperatures. The obtained values of temperature and the voltage produced can be used for calculating the Seebeck coefficient. Each thermocouple was characterized by heating and cooling processes both.

One end of the thermocouple was placed in an ice bath and the other end was placed in a water bath. The junctions were completely immersed in the ice/water bath and weren't touching the surface of the enamel mug used. An ice bath is used instead of exposing the other end to room temperature because the junction is in proximity to the bunsen burner and thus its temperature varies very rapidly due to convection of heat.

The temperature of the heat bath(water bath) is measured using a thermometer and the temperature of the ice bath is measured using a digital thermometer, which is again a j-k type thermocouple. A digital voltemeter is connected across the two exposed 'ends' of the thermocouple across which the voltage is to be measured.

Brass-Iron thermocouple

Hot Process

Hot Junction(Celsius

)

Cold Junction(Celsius

)

Voltage Difference(V)

S=V/T (V K^(-1))X10^(-3)

21 -6 0.255 9.44 25 -5.7 0.294 9.57 30 -5.5 0.339 9.54 35 -5.5 0.387 9.55

40 -5.5 0.430 9.4545 -5.6 0.488 9.6450 -5.6 0.540 9.7155 -5.6 0.590 9.7360 -5.6 0.621 9.4665 -5.6 0.670 9.4970 -5.6 0.713 9.4375 -5.6 0.746 9.2580 -5.6 0.812 9.48 85 -5.6 0.864 9.53 90 -5.6 0.900 9.41

Cold Process

Hot Junction(Celsius

)

Cold Junction(Celsius

)

Voltage Difference(V)

S'=V/T (V K^(-1))

X10^(-3)

85 -5.8 0.887 9.7680 -5.8 0.849 9.8975 -5.8 0.802 9.9270 -5.8 0.749 9.8865 -5.7 0.706 9.9860 -5.7 0.652 9.9255 -5.7 0.597 9.8350 -5.7 0.556 9.9845 -5.7 0.502 9.9040 -5.7 0.446 9.7535 -5.8 0.391 9.5830 -5.7 0.339 9.4925 -5.7 0.294 9.5720 -5.8 0.246 9.53

30.7 40.8 50.7 60.7 70.7 80.8 90.82730.7 40.5 50.6 60.6 70.6 80.6 90.6

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

HeatingCooling

Temperature Diff

Vol

tage

Calculations

Mean S form hot process= 9.512mv/C= 34.69v/K

Mean S' from cold process= 9.78mv/C= 35.67v/K

S(Iron)= S(Sample)-S(Brass)= 5.83v/K

*S(Brass) is obtained from Brass-Copper thermocouple.

Brass-Copper thermocouple

Hot Process

Hot Junction(Celsius

)

Cold Junction(Celsius

)

Voltage Difference(mV)

S=V/T (V K^(-1))X10^(-3)

25 -5.8 0.064 2.0730 -5.5 0.073 2.0535 -5.6 0.082 2.0140 -5.5 0.1 2.1945 -5.7 0.11 2.1650 -5.7 0.128 2.2955 -5.7 0.138 2.2760 -5.7 0.154 2.3465 -5.6 0.157 2.2270 -5.6 0.18 2.3875 -5.7 0.194 2.4080 -5.7 0.204 2.3885 -5.7 0.221 2.4390 -5.7 0.230 2.4095 -5.7 0.253 2.51

Cold Process

Hot Junction(Celsi

us)

Cold Junction(Celsi

us)

Voltage Difference(mV)

S'=V/T (V K^(-1))

X10^(-3)-5.6 88 0.228 2.43-5.6 85 0.221 2.43-5.6 80 0.209 2.44-5.6 75 0.197 2.44-5.5 70 0.175 2.31-5.5 65 0.166 2.35-5.5 60 0.155 2.36-5.6 55 0.145 2.39-5.5 50 0.135 2.43-5.4 45 0.127 2.57-4.7 40 0.109 2.43-4.6 35 0.095 2.39-4.6 30 0.080 2.31-4.6 25 0.062 2.09

85.6 70.5 55.5 40.730.8 45.6 55.7 65.7 75.6 85.6 95.7 100.7

0

0.05

0.1

0.15

0.2

0.25

Hot ProcessCooling

Temperature Diff (C)

Vol

tage

(mv)

Calculations

Mean S from hot process= 2.30 mv/C = 8.38v/K

Mean S' from cold process= 2.38mv/C= 8.68v/K

Seebeck Coefficient of Copper= 2.7v/K

Seebeck(brass)= S(mean)-S(copper)= 5.83v/K

Effect Of Magnetic Fields On Thermocouples

Overview

The experimental setup was almost the same as the one used in the previous part; here, one of the junctions is kept in a water bath(junction to be heated) and the other junction is kept at room temperature. The latter junction, the one kept at room temperature is kept between two coils in which the direction of the currents is the same. The coils are kept one above the other and connected to constant voltage-current sources and the junction tip is in the gap between the coils so as to experience the maximum field strength.

Experiment

Each of the coils were connected to different constant voltage-current sources, disregarding the minor fluctuations, it is assumed that the strength of the magnetic fields was constant in each of the coils and they were acting the same direction.The water bath was gradually heated and temperature at regular intervals was measured.

Observations

In the presence of magnetic field

Temperature Of Hot Junction (C)

Temperature Of Junction In Magnetic Field (C)

Voltage(V)

29.7 31 -0.0230.2 35 -0.05831.4 40 -0.0431.3 45 -0.02231.8 50 -0.01131.6 55 -0.00333 60 0.007

32.6 65 0.02533 70 0.03333 75 0.033

34.3 80 0.03333.6 85 0.03333.6 90 0.033

In the absence of magnetic fields

Temperature Of Hot Junction (C)

Temperature Of Junction In Magnetic Field (C)

Voltage(V)

30 26.5 0.00435 27.6 0.01140 27.8 0.01945 29.4 0.00152 33.3 0.03758 35 0.05060 35.5 0.05865 34 0.06070 33.5 0.06077 34.2 0.07080 34.8 0.078

It is seen that, in the presence of magnetic fields the initial voltage developed is negative as compared to the initial developed voltage in the absence of the field. This can be attributed to magnetic force acting on the thermal electrons which is greater than the thermoelectric force the electrons experience. Later on, when the temperature difference increases more number of electrons gain energy and move towards the colder junction but the number is comparatively less as in the absence of magnetic fields. Also, when the magnetic field was switched off the voltage difference increased suddenly to 0.045mV.

Bibliography

Thermocouple reference tables- University of Arizona

Oregon State University Report

Theory Of Thermometry- Robin. E. Bentley

www.its.org (International Thermoelectric Society)