Mumbai University > Electronics Engineering > Sem 8 > MEMS Technology

Marks: 4M

• In the absence of a magnetic field, there are three distinct thermoelectric effects: the Seebeck, the Peltier, and the Thomson effects [25].
• The Seebeck effect is the most frequently used (e.g., in thermocouples for the measurement of temperature differences).
• The Peltier effect is used to make thermoelectric coolers (TECs) and refrigerators.
• The Thomson effect is less known and uncommon in daily applications. In the Peltier effect, current flow across a junction of two dissimilar materials causes a heat flux, thus cooling one side and heating the other.
• Today, Peltier devices are made of n-type and p-type bismuth telluride elements and are used to cool high-performance microprocessors, laser diodes, and infrared sensors.
• Peltier devices have proven to be difficult to implement as micromachined thin-film structures.
• In the Seebeck effect, named after the scientist who made the discovery in 1822 ,a temperature gradient across an element gives rise to a measurable electric field that tends to oppose the charge flow (or electric current) resulting from the temperature imbalance.
• The measured voltage is, to first order, proportional to the temperature difference with the proportionality constant known as the Seebeck coefficient.
• While, in theory, a single material is sufficient to measure temperature, in practice, thermocouples employ a junction of two dissimilar materials.
• The measurable voltage at the leads, ΔV, is the sum of voltages across both legs of the thermocouple. Therefore,

$$\Delta = a_1 . (T_{cold} - T_{hot}) + a_2 . (T_{hot} - T_{cold}) = (a_2 - a_1) . (T_{hot} - T_{cold})$$

• where α1 and α2 are the Seebeck coefficients of materials 1 and 2, and, Thot and Tcold are the temperatures of the hot and cold sides of the thermocouple, respectively (see Figure 2.8).
• Alternately, one may use this effect to generate electrical power by maintaining a temperature difference across a junction.