Glossary Of Thermoelectric Terms
The amount of heat (in Watts) being generated by the device that is on top of the TE Cooler. Typically, this is the input power of this device (Voltage x current).
Ceramics made of aluminum oxide (Al2O3 ). These ceramics are used on most of our standard TECs. A positive of Al2O3 is that it is inexpensive and can be designed for snap states instead of dice, which considerably reduces production costs. Negative aspects of this material are its lower thermal conductivity and it is difficult to use in 3 to 6 stage coolers.
Al2O3 also a very popular ceramic for it’s high thermal conductivity properties . Typically in the range of 70-200 watts/mk
Temperature of the air or environment surrounding a thermoelectric cooling system; sometimes called room temperature.
The numerical ratio of the length (height) to cross-sectional area of a thermoelectric element. An element’s L/A aspect ratio is inversely proportional to its optimum current.
Ceramics made of beryllium oxide thermal conductivity of 260 watts/mk. Typically used in multistage coolers due to its higher thermal conductivity. The advantages to this material are that it enhances the thermal performance of the TE Cooler as well as makes it easier to assemble because of the high heat conductance. Disadvantages are that it is more expensive and BeO22 is toxic when its dust is inhaled. The dust comes from dicing and sanding of the material, both of which are performed on a TE Cooler in its final condition. However, the risks of BeO2 sometimes prohibit it as an option
for commercial use.
BISMUTH TELLURIDE ******
A thermoelectric semiconductor Bi2Te3 material that exhibits optimum performance in a “room temperature” range. An alloy of Bismuth Telluride most often is used for thermoelectric cooling applications and also power generation. It is by far the most efficient thermoelectric material presently used for power generation in the 250°C hot side temperature range.
LEAD TELLURIDE ******
A Thermo electric material used for high temperature (hot side 500°C Thermal Electric power generation). This material although is well known for it’s power generation ability is very difficult to find as a module. Sources of this material can only be found in completed thermoelectric power generation appliances.
A thermoelectric semiconductor material that exhibits optimum performance characteristics at relatively low temperatures.
BTU (British Thermal Unit)
The amount of thermal energy required to raise one pound of water by one° Celsius at a standard temperature of 15¼C.
Bonded Heat Sink
A heat sink which has Fins which have been bonded to the base plate. Heat sinks constructed in this manner typically have much greater heat dissipation characteristics and their much lower thermal resistance is far superior to that of an extruded heat sink.
A power cycling test performed by repeatedly powering on and off the TE Cooler for short intervals of time. The test is designed to detect latent manufacturing or material defects that would cause premature failure of the TE Cooler.
CASCADE MODULE (MULTISTAGE MODULE)
A thermoelectric cooler configuration whereby one cooler is stacked on top of another so as to be thermally in series. This arrangement makes it possible to reach lower temperatures than can be achieved with a single-stage cooler.
Cascade Seebeck Module A Cascade Seebeck module takes advantage of large temperature gradients, by exploiting each temperature zone with a material that peck efficiency equals that zones temperature. Typically, up to 2 different materials are thermally stacked (Hot Side to Cold side). The Seebeck effect module is constructed using material that is most efficient in that temperature range. Semi-conductors are most efficient when they are exposed to specific temperatures that exploit their targeted peak efficiency.
A patterned substrate, the finished part which goes into a TE Cooler. This material conducts heat and insulates electric current. Typically comprised of Al2O3 , BeO2 or AIN. At least one side of the ceramic has a metal pattern required for the operation of the TE Cooler. Al2O3 , AIN, BeO2 Thermal Conductivity (W/in C) .051 4.0 6.5 CTE (10-6/C) 7.0 4.0 9.0
CFM (Cubic Feet per Minute)
The volumetric flow rate of a gas, typically air, expressed in the English system of units. For thermoelectric applications, this generally refers to the amount of air passing through the fins of a forced convection heat sink.
CLOSED-LOOP TEMPERATURE CONTROLLER
A temperature controlling device having some type of temperature sensor (thermocouple, thermistor, RTD, etc.) that will transmit or “feed back” temperature data to the controller. Based on the returned information, the controller will automatically adjust its output to maintain the desired temperature.
COEFFICIENT OF PERFORMANCE (COP)
A measures of the efficiency of a thermoelectric cooler, device or system. Mathematically, COP is the total heat transferred through the thermo electric device divided by the electric input power (COP=Qc/W). COP sometimes is stated as COPR (Coefficient of Performance as a Refrigerator) or as COPH (Coefficient of Performance as a Heater).
COLD SIDE OF A THERMOELECTRIC MODULE
The side of a cooler that normally is placed in contact with the object being cooled. When the positive and negative cooler leads are connected to the respective positive and negative terminals of a DC power source, the cooler’s cold side will absorb heat. Typically, the leads of a TE cooler are attached to the hot side.
The transfer of heat within a material caused by a temperature difference through the material. The actual material may be a solid, liquid or gas (or a combination) where heat will flow by means of direct contact from a high temperature region to a lower temperature region.
The transfer of heat by air (gas) movement over a surface. Convection actually is a combined heat transfer process that involves elements of conduction, mixing action, and energy storage.
A pair of thermoelectric elements consisting of one N-type and one P-type connected electrically in series and thermally in parallel. Because the input voltage to a single couple is quite low, a number of couples normally are joined together to form a “cooler.”
Direct Current is the electricity that comes from a battery or electronics power supply. DC powers TEC but can be stress tested using AC.
The temperature difference between the cold and hot sides of a thermoelectric power generation module. Delta T may also be expressed as “DT” or “DT.” The larger the differential there is the higher output of power achieved !
DELTA-T Test For cooling application
Test performed in which the TE Cooler is placed on a temperature controlled base plate (typically 27°C) and powered at Imax. A thermocouple is pressed onto the top ceramic using a spring plunger and the cold side temperature as well as voltage is measured.
The mass of a material per unit volume, often expressed as pounds per cubic foot or grams per cubic centimeter.
A general term for blocks of the thermoelectric semiconductor material or “elements” prepared for use in a thermoelectric cooler.
For thermoelectric coolers, mathematical efficiency is the heat pumped by a cooler divided by the electrical input power; for thermoelectric generators, efficiency is the electrical output power from the cooler divided by the heat input (Qc/ Qh). To convert to percent, multiply by 100. See definition of Coefficient of Performance.
An individual block of thermo electric semiconductor material. Slicing an ingot of TE material into wafers, then dicing these wafers into very tiny, very precise, and accurately sized blocks, which are placed inside a TE Cooler, makes elements. Each TE Cooler has P elements and N elements. Elements are sometimes referred to as columns or TE material. See definition of DIE.
FIGURE-OF-MERIT (Z factor)
A measure of the overall performance of a thermo electric device or material. Material having the highest figure-of-merit also has the highest thermoelectric performance. A good thermoelectric material will have a high Z, high Seebeck coefficient and low thermal conductivity and resistively. Unfortunately the testing of figure of merit is not standardized
so many claims of High-Z cannot be validated.
FORCED CONVECTION HEAT SINK
A heat sink that incorporates a fan or blower to actively move air over the heat sink’s fins. Greatly improved cooling performance may be realized with a forced convection system when compared to a natural convection heat sink.
The quantity of heat presented to a thermoelectric device that must be absorbed by the device’s cold side. The term heat load, when used by itself, tends to be somewhat ambiguous and it is preferable to be more specific. Terms such as active heat load, passive heat load or total heat load are more descriptive and less uncertain as to meaning.
A general term describing a thermoelectric cooling device, often being used as a synonym for a thermoelectric cooler. In somewhat less common usage, the term heat pump has been applied to a thermoelectric device operating in the heating mode.
HEAT PUMPING CAPACITY
The amount of heat that a thermoelectric device is capable of pumping at a given set of operating parameters. Frequently, this term will be used interchangeably with the expression maximum heat pumping capacity. The two terms are not strictly synonymous, however, because maximum heat pumping capacity specifically defines the maximum amount of heat that a cooler will pump at the maximum rated input current and at a zero temperature differential.
A body that is in contact with a hotter object and that expedites the removal of heat from the object. Heat sinks typically are intermediate stages in the heat removal process whereby heat flows into a heat sink and then is transferred to an external medium. Common heat sinks include natural (free) convection, forced convection and fluid cooled.
HEAT SINK RESISTANCE
Also referred to as HSR. The thermal path from the hot side of the TE cooler to the ambient, including mounting interfaces, fans, etc., is measure of the effectiveness of a heat sink. In other words, how well does the heat sink remove the heat from the TE cooler? Its units are °C/W and is used to determine the number of degrees the hot side will rise in temperature for a given amount of heat that is dumped into it. For example, a heat sink resistance of 0.1°C/W will result in a hot side temperature rise of 1°C when 10 Watts is applied. The effectiveness of the heat sink greatly affects the performance of the TE cooler. Therefore, the better the heat sink (better is a lower °C/W) results in less input power to the TE cooler or a colder cold side temperature.
HEAT TRANSFER COEFFICIENT
A numerical value that describes the degree of coupling that exists between an object and a cooling or heating fluid. The heat transfer coefficient actually is an extremely complex value that encompasses many physical factors.
HOT SIDE OF A THERMOELECTRIC MODULE
The face of a thermoelectric cooler that usually is placed in contact with the heat sink. When the positive and negative cooler leads are connected to the respective positive and negative terminals of a DC power source, the cooler’s hot side will reject heat. Normally, the wire leads are attached to the hot side ceramic substrate.
Current which, the maximum delta T is produced. Generally, it is not a good to operate a TE cooler at Imax because the amount of input power increases significantly without a significant change in delta T. 70 – 80 % of Imax is usually an optimal operating condition.
A cast alloy of thermoelectric material. The ingot is sawed into wafers that are then diced into elements.
The temperature between specific stages or levels of a multistage or cascade cooler.
The passage of an electrical current through a conductor or material due to the internal resistance, resulting in Heat
A thermoelectric semiconductor that exhibits its optimum performance within a temperature range of 250-450°C. Lead Telluride is used most often for thermoelectric power generation applications.
A heat sink method involving the use of water or other fluids to carry away unwanted heat. When comparing alternative heat-sinking methods, liquid cooled heat sinks normally provide the highest thermal performance per unit volume.
MAXIMUM HEAT PUMPING CAPACITY (MAXIMUM Qc)
The maximum quantity of heat that can be absorbed at the cold face of a thermoelectric cooler when the temperature differential between the cold and hot cooler faces is zero and when the cooler is being operated at its rated optimum current. Qmax is one of the significant thermoelectric cooler/device specifications.
MAXIMUM TEMPERATURE DIFFERENTIAL (MAXIMUM Delta T)
The largest difference that can be obtained between the hot and cold faces of a thermoelectric cooler when heat applied to the cold face is effectively zero. DTmax or Dmax is one of the significant thermoelectric cooler/device specifications.
The conductive copper pattern printed on the ceramics.
A thermoelectric cooling component or device fabricated with multiple thermoelectric couples that are connected thermally in parallel and electrically in series.
MULTISTAGE MODULE (CASCADE MODULE)
A thermoelectric configuration whereby one TEC is mechanically stacked on top of another in series. This arrangement makes it possible to reach lower temperatures than can be achieved with a single-stage cooler.
NATURAL CONVECTION HEAT SINK
A heat sink from which heat is transferred to the surrounding air by means of natural air currents within the environment. No external fan, blower or other appliance is used to facilitate air movement around the heat sink.
The doping of semiconductor material creating an excess of electrons.
The specific level of electrical current that will produce the greatest heat absorption by the cold side of a thermal electric cooler. At the optimum current, a thermoelectric cooler will be capable of pumping the maximum quantity of heat; maximum temperature differential (Delta Tmax) typically occurs at a somewhat lower current level.
The amount of non-active heat (in Watts) being applied on the TE cooler. This includes conductance through wires that extend from the cold side of the TE cooler to the ambient, the convective loads from the surrounding atmosphere (note: Convective loads are present in Nitrogen, Argon, and Xenon, but are not present in a vacuum).
The phenomenon whereby the passage of an electrical current through a junction consisting of two dissimilar metals results in a cooling effect; when the direction of current flow is reversed heating will occur.
Semiconductor material that is doped so as to have a deficiency of electrons.
The maximum amount of heat (in Watts) that a TE cooler can pump. This occurs when the delta T is zero. Only for multistage coolers operating near a Delta Tmax condition.
Resistively is a bulk or inherent property of a material that is unrelated to the physical dimensions of the material. Electrical resistance, on the other hand, is an absolute value dependent upon the cross-sectional area (A) and Length (L) of the material.
SEEBECK EFFECT or SEEBECK Coefficient
The phenomenon whereby an electrical current will flow in a closed circuit made up of two dissimilar metals when the junctions of the metals are maintained at two different temperatures. A common thermocouple used for temperature measurement utilizes this principle.
A high temperature thermoelectric semiconductor material that exhibits its optimum performance within a temperature range of 500-1000°C. Silicon-Germanium material most often is used for special thermoelectric power generation applications that utilize a radioisotope/nuclear heat source.
The most common type of thermoelectric cooling module using a single layer of thermoelectric couples connected electrically in series and thermally in parallel. Single-stage coolers will produce a maximum temperature differential of approximately 70°C under a no-load condition.
The process of cutting the ingots into wafers.
Metals or alloys that melt below 425°C. Common solders used are: 118°C 52 In/48 Sn (mounting); 138°C 42Sn/58 Bi (TEC assembly); 183°C 63 Sn/37 Pd (TEC assembly); 232°C 95 Sn/5 Sb (TEC assembly).
THERMAL COEFFICIENT OF EXPANSION
A measure of the dimensional change of a material due to a change in temperature. Common measurement units include centimeter per centimeter per degree Celsius and inch per inch per degree Fahrenheit.
The amount of heat a given object will transmit per unit of temperature. Thermal conductance is independent of the physical dimensions, i.e., cross-sectional area and length of the object. Typical units include watts per degree Celsius and BTU per hour per degree Fahrenheit.
The amount of heat a material will transmit per unit of temperature based on the material’s cross-sectional area and thickness.
A grease-like material used to enhance heat transfer between two surfaces by filling in the microscopic voids caused by surface roughness. Most thermal greases, also known as Transistor Heat Sink Compound or Thermal Joint Compound, are made from silicone grease loaded with zinc oxide. Non-silicone based compounds are also available which in most cases are superior but more expensive than silicone-based alternatives.
THERMAL RESISTANCE (HEAT SINK)
A measure of a heat sink’s performance based on the temperature rise per unit of applied heat. The best heat sinks have the lowest thermal resistance.
Thermal Shock also is referred to as temperature cycling in some MIL specs. In a thermal shock test, the TE cooler (not powered throughout test) is placed in a hot chamber (for example, 85°C) for a set time (for example, 30 minutes). The part is then transferred to the cold chamber (for example, -40°C) for the same time. This cycle is repeated several times depending on the requirement.
A term used to denote not only the products produced but also the basic scientific principle upon which products are designed.
A device that directly converts energy into electrical energy based on the Seebeck Effect. Bismuth telluride-based thermoelectric generators have very low efficiencies (generally not exceeding two or three percent) but may provide useful electrical power in certain applications.
An alloy of materials that produce thermoelectric properties.
Applying solder paste over the copper or tabbed pattern. The bottom ceramic can also be tinned enabling the ability to mount the TEC on a header or heat sink.
The optimum voltage the maximum delta T is produced for a thermal electric cooling module.