What Is the Seebeck Effect?

The Seebeck effect is a thermoelectric phenomenon in which a temperature difference across a conductor or semiconductor generates an electromotive force, producing a measurable voltage. When the two junctions of a circuit made from two dissimilar materials are held at different temperatures, charge carriers migrate from the hotter end toward the cooler end, creating a potential difference proportional to the temperature gradient. The proportionality constant is the Seebeck coefficient, expressed in microvolts per kelvin, and its sign and magnitude depend on the material's electronic structure and carrier type.

Thomas Johann Seebeck discovered the effect in 1821, observing that a compass needle deflected when a circuit of dissimilar metals was exposed to a temperature difference. The effect is the physical basis for thermocouples, which have been used as temperature sensors for nearly two centuries, and for thermoelectric generators, which convert waste heat into electrical power without moving parts. NIST's thermoelectric measurements program develops standard reference materials and measurement protocols for the Seebeck coefficient, addressing the lack of standardized methods that previously made interlaboratory comparison difficult.

Thermoelectric Voltage and the Seebeck Coefficient

The Seebeck coefficient S is defined as the open-circuit voltage generated per unit temperature difference across a material: S = ΔV / ΔT. In metals, S is typically small, on the order of a few microvolts per kelvin, because the high carrier density limits the asymmetry in energy transport. In doped semiconductors, S can reach hundreds of microvolts per kelvin, because the narrower energy distribution of carriers near the band edge produces a stronger energy-filtering effect. In n-type semiconductors, electrons are the majority carriers and the Seebeck coefficient is negative; in p-type material, holes dominate and S is positive. Thermocouples pair an n-type and a p-type leg in a circuit so that the voltages from both legs add constructively, increasing the total output for a given temperature differential.

Thermoelectric Materials and the Figure of Merit

The performance of a thermoelectric material is characterized by the dimensionless figure of merit ZT, defined as S²σ/κ, where σ is the electrical conductivity and κ is the thermal conductivity. A high ZT requires a large Seebeck coefficient, high electrical conductivity, and low thermal conductivity, properties that compete with one another in most materials because good electrical conductors tend also to be good thermal conductors. Bismuth telluride (Bi₂Te₃) and its alloys remain the leading materials for near-room-temperature applications, achieving ZT values approaching or exceeding 1.0. Research published in PMC on bismuth telluride thermoelectric generation covers the electronic and structural properties that make this material uniquely suited to thermoelectric conversion. Newer compound families including lead telluride, half-Heusler alloys, and skutterudites extend viable operating ranges to higher temperatures.

Thermoelectric Devices and Energy Conversion

Thermoelectric generators (TEGs) arrange multiple thermoelectric couples electrically in series and thermally in parallel, allowing small per-couple voltages to sum to useful output levels. They produce power silently, with no working fluid and no mechanically moving parts, making them attractive for remote monitoring, space power systems, and waste-heat recovery from industrial processes. Wearable TEGs exploit the Seebeck effect to harvest energy from human body heat, supplying low-power electronics such as wireless sensors. The NIST computational Seebeck coefficient measurement study characterizes measurement uncertainty rigorously, which is important for evaluating TEG performance claims across different research groups.

Applications

The Seebeck effect has applications in a wide range of disciplines, including:

• Industrial temperature measurement using Type K, Type J, and Type S thermocouples
• Waste heat recovery from automotive exhaust, industrial furnaces, and power plant flue gas
• Spacecraft power generation using radioisotope thermoelectric generators (RTGs)
• Wearable electronics powered by body heat gradients
• Remote sensor nodes in oil, gas, and environmental monitoring installations
• Precision calorimetry and thermal flux measurement in laboratory instrumentation