The Seebeck effect is the generation of an electric voltage across a conducting material when its two ends are held at different temperatures. Discovered in 1821 by Thomas Johann Seebeck, it’s the basic principle behind thermocouples (the most common way to measure temperature in industrial settings) and thermoelectric generators that power NASA spacecraft billions of miles from the sun.
How a Temperature Difference Creates Voltage
Every conductor contains charge carriers, either electrons or “holes” (spots where an electron is missing). When one end of the material is hot and the other is cold, those charge carriers at the hot end have more energy and move faster. They naturally diffuse toward the cold end, much like warm air spreading into a cool room.
As charge carriers pile up at the cold end, they leave behind an opposite charge at the hot end. This separation creates a voltage that opposes further movement. Eventually the system reaches equilibrium: the electrical force pushing carriers back exactly balances the thermal push driving them forward. The voltage that builds up at equilibrium is the Seebeck voltage, and you can measure it with a simple voltmeter connected across the two ends.
The size of this voltage relative to the temperature difference is called the Seebeck coefficient, defined as the voltage divided by the temperature difference. It’s measured in microvolts per degree (µV/K). Metals typically produce small Seebeck coefficients, on the order of 1 to 30 µV/K. Copper sits at about 1.83 µV/K, nickel at roughly −19.5 µV/K, and cobalt at −30.8 µV/K. Semiconductors can reach hundreds of microvolts per degree, which is why they’re the material of choice for thermoelectric power generation.
Why the Sign Matters
The Seebeck coefficient can be positive or negative, and the sign tells you which type of charge carrier dominates the material. In n-type semiconductors, where electrons are the majority carriers, the coefficient is negative. In p-type semiconductors, where holes dominate, it’s positive. Metals follow the same logic depending on their band structure: gold and silver have small positive values, while chromium (21.8 µV/K) is strongly positive and potassium (−13.7 µV/K) is strongly negative.
This polarity distinction is essential for building practical devices. Thermoelectric modules pair an n-type leg with a p-type leg so their voltages add together rather than cancel out. The larger the difference in Seebeck coefficients between the two materials, the more useful voltage you get from a given temperature difference.
Thermocouples: The Most Common Application
A thermocouple is simply a junction of two different metals or alloys. Because the two metals have different Seebeck coefficients, a temperature difference between the junction and the measurement end produces a predictable voltage. By measuring that voltage, you can calculate the temperature at the junction with high accuracy.
Different metal pairings cover different temperature ranges. A Type K thermocouple pairs nickel-chromium with nickel-aluminum and works from −200°C up to +1,200°C, making it the workhorse of industrial temperature measurement. A Type J thermocouple uses iron paired with constantan (a copper-nickel alloy) and covers a similar range, from −210°C to +1,200°C. These devices are cheap, durable, and reliable, which is why they show up everywhere from furnaces to food processing.
Powering Spacecraft With Heat
The Seebeck effect does more than measure temperature. Stringing many thermocouples together in series creates a thermoelectric generator that converts heat directly into electricity with no moving parts. NASA has relied on this approach for decades in radioisotope thermoelectric generators, or RTGs. Inside an RTG, the natural decay of a radioactive element produces heat on one side of the thermocouple array, while the cold of space cools the other side. The temperature difference drives a steady current.
NASA’s two Voyager probes, launched in 1977, are still operating on RTG power more than 45 years later. That kind of longevity is possible precisely because there are no turbines, no pumps, nothing mechanical to wear out. The same technology powers rovers and orbiters exploring some of the darkest and dustiest places in the solar system, where solar panels aren’t practical.
Waste Heat Recovery
Closer to home, researchers are working to capture waste heat from car exhaust and industrial processes using thermoelectric generators built on the Seebeck effect. The appeal is obvious: roughly two-thirds of the energy in gasoline leaves through the tailpipe as heat. If even a fraction of that could be converted to electricity, it would improve fuel efficiency without any redesign of the engine itself.
Current results are modest. In vehicle exhaust experiments, thermoelectric generators have achieved overall conversion efficiencies around 1.2%, with heat collection efficiencies near 46.5% and a thermoelectric conversion step around 2.9%. Those numbers sound small, but they represent free energy from heat that would otherwise be wasted. The bottleneck is materials: the best widely available thermoelectric material, bismuth telluride, has a figure of merit (called ZT) of about 1 near room temperature. A ZT of 1 is roughly the threshold for practical use, and researchers have reported nanostructured versions reaching as high as 2.4, though that value hasn’t been independently confirmed. Reaching a ZT of 4 would require a Seebeck coefficient approaching ±400 µV/K, paired with other favorable material properties.
Three Related Thermoelectric Effects
The Seebeck effect is one of three thermoelectric phenomena discovered in the 1800s that turned out to be different faces of the same physics. The Peltier effect is essentially the reverse: instead of a temperature difference producing voltage, an electric current flowing through a junction of two materials causes one side to absorb heat and the other to release it. This is how solid-state coolers work, including the small cooling plates inside portable drink coolers and some CPU cooling systems.
The Thomson effect is subtler. When current flows through a single uniform material that has a temperature gradient along its length, additional heating or cooling occurs beyond normal resistive heating. All three effects are linked by thermodynamic relationships. The Seebeck and Peltier coefficients are connected through temperature, and the Thomson coefficient is the temperature derivative of the Seebeck coefficient. William Thomson (Lord Kelvin) was the first to work out the mathematical connections between all three, showing they arise from the same underlying interaction between heat flow and electrical current in conductors.
What Determines a Material’s Seebeck Coefficient
The Seebeck coefficient isn’t just a fixed number for a given substance. It depends on temperature, crystal structure, and, for semiconductors, doping level. In semiconductors, adding impurities (doping) changes the number of free charge carriers. At low doping concentrations the Seebeck coefficient grows rapidly, reaches a peak, then decreases as doping increases further. This tradeoff exists because a higher carrier concentration improves electrical conductivity but reduces the voltage each carrier contributes.
Beyond the traditional explanation involving the density of available energy states near the Fermi level, researchers have found that a charge mobility that changes rapidly with temperature can also produce a significant Seebeck coefficient. In other words, if charge carriers move much more easily at one temperature than another, that alone can drive a voltage, even without the usual asymmetry in electronic structure. This insight opens new strategies for designing better thermoelectric materials by engineering how freely carriers move, not just how many energy states are available to them.