Glossar Peltier Elements

Glossary of Peltier technology / thermoelectrics

The key technical terms around Peltier elements and thermoelectric modules – briefly explained.

Al₂O₃ (aluminium oxide): Standard ceramic for the substrates of Peltier elements; thermal conductivity approx. 24–30 W/(m·K).
AlN (aluminium nitride): High-grade substrate ceramic at approx. 150–180 W/(m·K); surcharge variant for high heat-flux densities.
Aspect ratio: Ratio of leg height to cross-sectional area of a semiconductor leg. Determines the current-voltage behaviour and the achievable ΔT.
Bismuth telluride (Bi₂Te₃): Thermoelectric semiconductor material with optimal performance around room temperature; standard material for cooling modules.
Cascade (multi-stage module): Stacking of several modules thermally in series to achieve very large temperature differences (two-stage approx. 91–106 K, three-stage approx. 111 K).
Cold side: Module face that absorbs heat in cooling operation and sits against the object to be cooled.
Conduction: Heat conduction within a material due to a temperature gradient.
COP (coefficient of performance): Ratio of pumped heat to electrical power input: COP = Q_c/P_el.
CTE: Coefficient of thermal expansion. Differences between the module and adjacent parts create shear stresses.
Cycler module: Peltier element optimised for frequent and large temperature swings, with increased cycling endurance.
Dew point: Temperature at which air is saturated with water vapour. If the cold side falls below the dew point, moisture condenses on the module.
Die (pellet): Individual semiconductor block (leg) of thermoelectric material from which the module is built.
Emissivity (ε): Ratio of the energy radiated by a surface to that of a black body at the same temperature; determines the radiative heat load.
Energy harvesting: Generation of electrical energy from waste heat via the Seebeck effect. Economical only in niches (space, very large ΔT).
Figure of merit Z / ZT: Z = S²·σ/λ. Metric for the quality of a thermoelectric material; good Bi₂Te₃ modules reach ZT ≈ 0.8–1.0.
Forced convection: Heat removal assisted by a fan or blower; lowers a heat sink's thermal resistance many times over compared with free convection.
Four-wire measurement (Kelvin): Measurement method in which the current and voltage paths are routed separately to eliminate the lead resistance from the result.
Free (natural) convection: Heat removal solely via natural air flow without a fan.
Gap filler: Very soft, highly thermally conductive interface material (10–50 W/(m·K)) to compensate large mechanical tolerances; thickness 0.2–2.0 mm.
Graphite foil: Anisotropic interface material of over 98 % pure graphite; up to 200 W/(m·K) in-plane, silicone-free, stable above 300 °C.
Heat of fusion: Energy required for the solid/liquid phase transition; to be included in the design for applications involving icing.
Heat sink: Component that transfers the hot side's waste heat to the surroundings. Its thermal resistance largely determines the achievable cold-side temperature.
Height tolerance: Permissible deviation of the module height; standard ±0.3 mm, lapped version ±0.03 mm.
Hot side: Module face that releases heat in cooling operation and sits against the heat sink – recognisable by the soldered connecting wires.
I_max: Current at which the module reaches its maximum temperature difference.
Interstage temperature: Temperature between two stages of a cascade module.
Joule heat: Loss heat generated by the electrical resistance in the module; occurs on both sides and limits performance at high currents.
Latent heat: Heat required for a phase change of a substance without the temperature changing.
Lead telluride (PbTe): Thermoelectric material for approx. 250–450 °C, mainly used in power generation.
Metallisation: Solderable coating on both ceramic faces; enables direct soldering of small modules (suffix M).
Miniature module / micro-TEC: Peltier element with a very small base area and height, often in TO packages for cooling laser diodes and detectors.
Module (TEC): Thermoelectric cooler; complete component made of many thermocouples between two ceramic substrates.
Mounting pressure: Surface pressure with which the module is clamped between heat sink and cold plate. Recommended 3–8 kg/cm², design upper limit 15 kg/cm².
MTBF: Mean time between failures. For Peltier elements in steady-state operation over 200,000 hours; considerably lower in cycling operation.
n-type: Semiconductor material with an excess of electrons.
Optimal current: Current at which the best COP is achieved for a given task; usually below 50 % of I_max.
Parylene: Very thin protective coating deposited in vacuum (suffix P); good moisture barrier with minimal heat feedback.
Peltier effect: Heat transport from one junction to the other when current flows through two dissimilar materials; the physical reverse of the Seebeck effect.
Phase change material (PCM): Interface material that softens on heating, wets roughness and displaces trapped air; thermal resistances down to 0.2 K·cm²/W, silicone-free, migration-free.
p-type: Semiconductor material with a deficit of electrons (hole conduction).
PWM: Pulse-width modulation. Low-cost drive, but causes additional Joule heat and reduces ΔT and COP compared with direct current.
Q_c: Heat power absorbed (pumped) at the cold side.
Q_h: Heat power to be dissipated at the hot side: Q_h = Q_c + P_el.
Q_max: Maximum heat-pumping power at ΔT = 0 K and I_max.
Radiation (thermal): Heat transfer by electromagnetic waves between bodies of different temperature; a relevant parasitic load at low cold-side temperatures.
Ripple: AC component of the supply voltage. Recommended < 5 %, maximum 10 %, for cascades < 2 %.
R_th (thermal resistance): Temperature rise per unit of applied heat power, given in K/W. A measure of the performance of a heat sink or a joint.
Seebeck effect: Generation of an electrical voltage when two junctions of dissimilar materials are held at different temperatures.
Silicon-germanium (SiGe): High-temperature thermoelectric material for approx. 500–1,000 °C; used in radioisotope generators.
Specific heat capacity (c_p): Energy required to warm 1 kg of a substance by 1 K; determines the transient load when cooling a mass.
Substrate: Ceramic plate on which the module is built – thermally conductive and electrically insulating.
Thermal cycle: Complete temperature swing of the module. The dominant wear mechanism – number of cycles, swing, upper limit and rate of change determine the service life.
Thermal grease: Pasty interface material that fills microscopic unevenness between two surfaces. For Peltier applications preferably without a silicone carrier fluid.
Thermal shock: Very rapid temperature change; test method per MIL-STD-202 Method 107.
Thermistor: Temperature-dependent resistor (usually NTC) for temperature measurement; optionally integrated directly on the module.
Thermocouple (couple): One p-doped and one n-doped leg, connected electrically in series and thermally in parallel. Voltage drop around 0.12 V per couple.
Thermoelectric generator (TEG): Module for generating electricity from a temperature gradient via the Seebeck effect; typical efficiency 2–3 %.
Thin-film module: Peltier element whose thermoelectric material is deposited as a layer instead of being sawn from blocks; very flat and fast, but low-power and expensive.
Thomson effect: Reversible absorption or release of heat in a current-carrying conductor with a temperature gradient.
U_max: Voltage present at I_max and maximum temperature difference.
ΔT (delta-T): Temperature difference between the hot and cold side of the module.
ΔT_max: Largest achievable temperature difference at Q_c = 0 W and I_max; measured in vacuum at a defined hot-side temperature.