23 Thermoelectric Effect

Introduction :-

World’s present demand for energy is causing a dramatic escalation of social and political unrest. Likewise, the environmental impact of global climate change due to the combustion of fossil fuels and depletion of conventional energy sources is becoming increasingly alarming. One way to improve the sustainability of our energy sources is through the scavenging of waste heat with thermoelectric generators. Home heating, automotive exhaust, and industrial processes all generate an enormous amount of unused waste heat that could be converted to electricity by using thermoelectric properties of some specific materials. Thermoelectric materials have gained interest of various scientists worldwide, specifically in the field of converting waste heat energy into electrical energy which can be utilized further for various applications. Thus, by using a thermoelectric material, waste terrestrial heat can be harvested which can be further converted to electrical power.

Thin film based Thermoelectrics :

The exploitation of these materials has led to a novel idea of developing self-powering devices where these devices are driven by the heat released as a waste from their working environment. The thermoelectric materials are usually in the form of bulk, thin film or low-dimensional structures such as Skutterudite type alloys. Since thin films are expected to have lower thermal conductivity than the bulk materials, due to the presence of strong phonon scattering at their interfaces, thin films based thermoelectric materials have opened the possibility for improvement of thermoelectric efficiency and their applications in flexible devices. Moreover, some thermoelectric materials, such as Bi2S3 and Bi2Te3 in the form of thin films, have found applications in cooling at room temperature however their efficiencies are not still enough for commercial applications.

Thermoelectric Principle :

Thermoelectricity means the direct conversion of heat into electric energy, or vice versa. The term is generally restricted to the irreversible conversion of electricity into heat described by the English physicist James P. Joule and to three reversible effects named for Seebeck, Peltier, and Thomson, their respective discoverers. Thermoelectricity, thus 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.

According to Joule’s law, a conductor carrying a current generates heat at a rate proportional to the product of the resistance (R) of the conductor and the square of the current (I). The German physicist Thomas J. Seeback discovered in the 1820s that if a closed loop is formed by joining the ends of two strips of dissimilar metals and the two junctions of the metals are at different temperatures, an electromotive force, or voltage, arises that is proportional to the temperature difference between the junctions. A circuit of this type is called a thermocouple; a number of thermocouples connected in series is called a thermopile.

In 1834 the French physicist Jean C. A. Peltier discovered an effect inverse to the Seeback effect: If a current passes through a thermocouple, the temperature of one junction increases and the temperature of the other decreases, so that heat is transferred from one junction to the other. The rate of heat transfer is proportional to the current and the direction of transfer is reversed if the current is reversed.

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 ϕ12 and is given by eϕ12 = W1 − W2, where e is 1.6 × 10−19 coulomb.

If a closed circuit is made of two different metals, there will be no net electromotive force in the circuit because the two contact potentials oppose each other and no current will flow. There will be a current if the temperature of one of the junctions is raised with respect to that of the second. There is a net electromotive force generated in the circuit, as it is unlikely that the two metals will have Fermi levels with identical temperature dependence. To maintain the temperature difference, heat must enter the hot junction and leave the cold junction; this is consistent with the fact that the current can be used to do mechanical work. The generation of a thermal electromotive force at a junction is called the Seebeck effect (after the Estonian-born German physicist Thomas Johann Seebeck). The electromotive force is approximately linear with the temperature difference between two junctions of dissimilar metals, which are called a thermocouple. For a thermocouple made of iron and constantan (an alloy of 60 percent copper and 40 percent nickel), the electromotive force is about five millivolts when the cold junction is at 0 °C and the hot junction at 100 °C. One of the principal applications of the Seebeck effect is the measurement of temperature. The chemical properties of the medium, the temperature of which is measured, and the sensitivity required dictate the choice of components of a thermocouple.

The absorption or release of heat at a junction in which there is an electric current is called the Peltier effect (after the French physicist Jean-Charles Peltier). Both the Seebeck and Peltier effects also occur at the junction between a metal and a semiconductor and at the junction between two semiconductors. The development of semiconductor thermocouples (e.g., those consisting of n-type and p-type bismuth telluride) has made the use of the Peltier effect practical for refrigeration. Sets of such thermocouples are connected electrically in series and thermally in parallel. When an electric current is made to flow, a temperature difference, which depends on the current, develops between the two junctions. If the temperature of the hotter junction is kept low by removing heat, the second junction can be tens of degrees colder and act as a refrigerator. Peltier refrigerators are used to cool small bodies; they are compact, have no moving mechanical parts, and can be regulated to maintain precise and stable temperatures. They are employed in numerous applications, as, for example, to keep the temperature of a sample constant while it is on a microscope stage.

Thermoelectric materials are of interest for applications as heat pumps and power generators. The performance of thermoelectric devices is quantified by a figure of merit, ZT, where Z is a measure of a material’s thermoelectric properties and T is the absolute temperature. A material with a figure of merit of around unity was first reported over four decades ago, but until recently, there has been only modest progress in finding materials with enhanced ZT values at room temperature.

Development of metal oxide thin films for self power generating integrated devices :-

In the present module, two thermoelectric materials i.e. aluminum-doped ZnO (Al:ZnO) and NiO thin films have been studied. Al doped ZnO (Al:ZnO) and NiO thin films were deposited over corning glass substrates by Pulsed Laser Deposition (PLD) technique using ceramic Al doped ZnO target and Ni metal target respectively. The thermoelectric studies have been carried out using an indigenously developed thermoelectric measurement set-up. Low vacuum of order ~10-2 Torr was created in a cavity where sample is mounted using a rotary pump in order to avoid condensation on the sample under low temperature conditions. The two ends of the sample were kept at different temperatures and the temperature was measured using thermocouples. After maintaining the temperature gradient at ends of the sample the voltage generated was measured using the multimeter as shown in Fig. 2.

Figure 3 (a) and (b) show the variation in thermoelectric voltage generated as a function of temperature gradient at the ends of the Al:ZnO and NiO thin films respectively. It can be observed from fig. 3 (a) that with the increase in the temperature gradient from 5K to 30K, the film. The value of induced thermoelectric voltage is found to be increasing from 0.3 mV to 1 mV with increase in the temperature gradient from 25K to 55K. The Seebeck coefficient is found to be ~31 µV/K and ~20 µV/K for Al:ZnO and NiO thin films respectively. The power factor and figure of merit of the thermoelectric material are estimated from the equations 1 and 2.

Where, zT is figure of merit at 300 K, S is Seeback coefficient, is electrical conductivity, T is temperature and k is thermal conductivity.

Figure 3 : Variation in Induced thermoelectric voltage as function of temperature gradient maintained at the two ends of (a) Al:ZnO and (b) NiO thin films.

Hall measurements were carried out to calculate the electrical conductivity of the Al:ZnO and NiO thin films at room temperature using Van Der Pau method. The value of was found to be 902.87 S/cm and 0.096 S/cm for Al;ZnO and NiO thin films respectively. Since the power factor depends on the square of the Seebeck Coefficient, thus even with the decrease in electrical conductivity there is a significant increase in the power factor which is suitable for a good thermoelectric material. Thus, Al:ZnO and NiO thin films are found to yield the maximum power factor of 8.67×10-5 W/mK² and 38.4×10- 10 W/mK² respectively. Also, the figure of merit of Al:ZnO thin film is 0.023 which is higher than the obtained for NiO thin film as 640×10-10. The obtained value of power factor is slightly lower than the value reported in literature on Al:ZnO thin film but higher than other metal oxides. The obtained higher value of power factor and figure of merit in Al:ZnO thin film may be attributed to the higher electrical conductivity of the film despite of the same order of Seebeck coefficient of both the thin films. Thus, Al:ZnO is proved to be better thermoelectric material as compared to NiO thin film.

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