Seebeck Effect

In our previous session, we learned about the thermocouple. The thermocouple is a device used to measure temperature. It consists of two wires of different metals joined at each end. One of the junctions is kept at the location where the temperature is to be measured, and the other junction is kept at a constant lower temperature. The difference in the temperature creates an EMF known as the Seebeck effect. Let us know more about the Seebeck effect along with its applications.

What is Seebeck Effect

A voltage is created when the temperature difference is seen in a thermoelectric device on each side. When the voltage is applied to the device, we can observe that transfer of heat takes place from one region to another, establishing a difference in the temperature. The straight-forward conversion of temperature variations to electric voltage and vice versa takes place through a thermocouple.

The thermoelectric effect is used to produce electricity and is used to compute temperature changes of objects. Due to the applied voltage, the direction of cooling as well as heating is affected. These thermoelectric devices are excellent temperature controllers.

The thermoelectric effect includes three effects:

• Seebeck effect
• Peltier effect
• Thomson effect

Let us know in detail about the Seebeck effect.

The process where the temperature difference between two distinct electrical semiconductors or conductors generates a voltage difference between the two substances is known as the Seebeck effect.

The Seebeck effect explains the generation of electromotive force and the electric current in a loop featuring at least two distinct conductors which are at two different temperatures, known as thermocouples. It can be termed as the Seebeck effect thermocouple.

The Seebeck effect can be reversed too. For example, when the cold and hot junctions of the circuit are interchanged, then the direction of the current will also change. Therefore, the thermoelectric effect is a reversible process. The magnitude and sign of thermocouple EMF depend on the metals (materials) used and the temperature of the hot and cold junction.

Thermoelectric series is observed when the metal pairs of different thermal properties are arranged in series. This is an ideal example for electromotive force and results in measurable voltages or currents like any other EMF. The density of current is given by:

Where,

S = Seebeck coefficient or thermopower

The Seebeck coefficient generally varies as the function of temperature. It relies on the composition of the conductor. The Seebeck coefficient ranges from −100 μV/K to +1,000 μV/K at room temperature for ordinary materials.

When J=0, the system comes to a steady-state and the voltage gradient is given by:

\triangledown V=-S \triangledown T

Invention of Seebeck Effect

German Physicist Thomas Seebeck in the year 1821 observed various properties of the thermoelectric effect. It was observed that in a circuit which had two different metals having different temperatures created an EMF at their junctions. He observed the deflection in the compass needle when a closed-loop was formed between two different metals or semiconductors. These different metals form a thermocouple. The current that passes through this circuit is known as thermoelectric current.

It was believed earlier that the deflection of the compass needle was due to the magnetic field induced by the temperature differences, and he named the phenomenon a thermo-magnetic effect. Later, Hans Christian Orsted realized that deflection was due to the current induced.

See the video below and learn how an EMF is induced due to the motion of a conductor in a magnetic field:

Explanation of Seebeck Effect

We know that dissimilar metals are connected in the circuit which experiences the Seebeck effect. The main reason for this phenomenon is valence electrons present in the warmer part of the metal. When these electrons move from warmer regions to colder regions, kinetic energy is produced. On the hot side of the metal, we can observe that the Fermi distribution is soft and on the cold side the Fermi distribution is sharp. When the electrons migrate from hotter regions to colder it equilibrates temperature eventually.

The electrons which are placed on a warmer side display higher average momentum as compared to the colder side. This creates the electrical potential since more negative charges are seen on the colder side. Direct current is generated when this pair is connected through an electrical circuit.

Read more : Fermi-energy

The voltage produced is of a few microvolts per Kelvin temperature difference. Now that we are aware of the fact that when the voltage increases in series the current increases in parallel. The Seebeck effect is very feeble and is only a few microvolts per kelvin of temperature. This difference is measured at the junction. When the difference in temperature is high, few Seebeck-effect devices produce a few millivolts. Many such devices which experience the Seebeck-effect can be connected in series to increase the output voltage or in parallel to increase the maximum deliverable current. When the constant temperature difference continues, the energy distribution at both ends will be different, and hence it leads to the successful operation of the circuit.

Consider the below circuit diagram,

The temperature of the colder region is given as T_{c}and the temperature of the hotter region is represented as T_{h}.

T_{c}<T_{h}

The coefficient of proportionality is known as the thermoelectric power or the Seebeck coefficient. The sign of Seebeck coefficients represents the potential of the cold end with respect to the hot end.

If S_{AB} is positive: current flows from A to B at the hot junction

Consider for a thermocouple with platinum as one of the metals and an alloy of Pt-Rh (90:10) the open-circuit voltage is given as:

V=c+aT+bT^{2}

\frac{dV}{dT}=a+2bT

The relationship between voltage and temperature results in a parabola.

T_{n}=-\frac{a}{2b}

Here the thermoelectric power is maximum and is known as the neutral temperature.

T_{i}=T_{0}

T_{i}=T_{0}-\frac{a}{b}

Here, a small change in the difference of the junction temperatures results in a change in the sign of emf known as the inversion temperature.

The Seebeck coefficient depends upon:

• Work functions of the metals in the circuit
• Electron densities are seen on the metal
• Scattering mechanism

To summarize, when two points of the conductor are maintained at different temperatures, electrons or holes in the hotter region will move towards the colder region and diffuse. This diffusion stops when the electric field generated due to the migration of charges has established a strong field. For a metal, carriers being negatively charged electrons, the colder end would become negative so that the Seebeck coefficient is negative. In a p-type semiconductor holes diffuse towards the lower temperature resulting in a positive Seebeck coefficient.

Metals’ Seebeck coefficients are generally given with regard to platinum as a standard whose Seebeck coefficient is taken as a null value. Seebeck coefficients of various metals (mV/K) at zero degrees celsius are given below.

Applications of Seebeck Effect

• This is commonly used in thermoelectric generators. They are used in industries and power plants since they do not let the remaining heat go to waste.
• It is used in thermocouples to calculate the differences in the temperature or to operate the electronic switches that control powering of the system.
• It is used in automobile industries to employ a thermoelectric generator for improving the efficiency of fuel.
• Seebeck effect is used to measure the potential difference between two semiconductors.