Thermoelectricity

Thermoelectricity is the family of reversible couplings between heat flow and electrical behavior in conductors and semiconductors. The Seebeck effect converts a temperature difference into an electromotive force. In a thermocouple, two dissimilar materials form a circuit with junctions at different temperatures; the resulting voltage can be used to measure temperature. The Peltier effect is the reverse coupling at a junction: current flow absorbs heat on one side and releases it on the other, enabling solid-state heating or cooling. The Thomson effect describes heating or cooling distributed along a conductor when current flows through a temperature gradient. These are related phenomena, not simply different names for one mechanism.

When the two ends of a material are at different temperatures, mobile charge carriers respond to the gradient, producing an electrical potential difference. In a circuit with two different materials, the measured thermoelectric voltage depends on their respective Seebeck coefficients and on the temperature difference between the junctions. A thermocouple uses this voltage as a temperature signal. Thermoelectric voltage depends on the materials and temperature range, so instruments are calibrated for the specific thermocouple type and measurement conditions.

In 1821, physicist Thomas Johann Seebeck reported that a temperature difference in a circuit made from dissimilar materials could produce an electrical effect. Jean Charles Athanase Peltier later investigated the heating and cooling associated with current at junctions between materials. In 1851, William Thomson, later Lord Kelvin, described the related effect along a conductor with a temperature gradient. The effects became known collectively as thermoelectric phenomena, and their practical applications developed as materials and engineering improved.

Thermocouples are widely used as temperature sensors because they are simple and can operate across broad temperature ranges. Peltier modules can provide localized cooling or heating without moving parts, for example in small electronics or portable temperature-control equipment. Thermoelectric generators produce direct-current electricity when a temperature difference is maintained across a converter. They have been used where reliability and long service life matter, including some spacecraft power systems and remote monitoring equipment, and are also studied for recovering part of otherwise wasted heat from industrial processes and engines.

A thermoelectric device must sustain a temperature difference while allowing electrical charge to flow. Good electrical conductivity helps reduce resistive losses, while relatively low thermal conductivity helps prevent heat from quickly flowing from the hot side to the cold side. These material goals can compete, so thermoelectric research seeks materials and structures that balance them. A generator's useful output also depends on the heat source and sink, thermal contacts, electrical connections, geometry and operating conditions. As a result, high material performance alone does not guarantee high efficiency at the full-system level.