Background of Thermopiles
General information about thermopiles and thermopile sensors is provided below. Some terms are linked to the the free internet encyclopedia www.wikipedia.org, where more cross-referenced information can be found.
Thermopiles convert thermal energy (temperature differences) to electrical energy. Thermopiles are built from a series of so called thermocouples, i.e. two leads (also referred to as legs) of different/disparate materials (typically metal/alloy or semiconductor materials) attached to each other at two junctions. When the junctions are held at different temperatures, an electrical voltage is produced in the circuit. The physical principle of this is called the thermoelectric effect , or the Seebeck effect. This voltage is proportional to the applied temperature difference. Hence, the thermoelectric circuit can be used as a heat flux sensor, a differential temperature sensor, or a thermal generator. They are also commonly used as thermal radiation sensors for infrared light. Depending on the application, other thermal radiation sensors, such as bolometers and pyrometers, can be used.
History
In 1821 Thomas Johan Seebeck found out that a magnetic field was produced when a thermal gradient was applied to a circuit built from two metals. Later, he found out that the reason for this was that an electrical current was generated in the circuit. The Italian physicist Leopoldo Nobili invented the first thermopile device in 1835. The purpose was to measure radient heat, and his thermocouple leads were built from antimony and bismuth metals. Find out more here . Several further constuctions soon followed. Of the many different motivations, one interest purpose was to be able to convert heat (from fire or gas) into electricity to obtain a simle electrical energy source.
More information about thermopiles can be found here.
The JonDeTech Difference: Vertical VS. Horizontal Configuration
The thermopile leads can be arranged in different manners; for most commercial sensors of today, typically, horizontal or "in-the-plane" architecture is used. The reason for this is strictly process related. Microelectronic processes have historically been developed to make lateral structures on silicon wafers. However, this is not always the best sensor solution since the hot and the cold junctions are placed next to each other on the same surface. To use this as a infrared sensor an aperture must be integrated with the thermopile so that the heat radiation strikes only a limited area, i.e. the hot junction of the sensor. In other words, the full surface of horizontally configured thermopiles cannot be used, neither can true "contact mode" be utilized.
The advantage of the vertical thermocouple lead arrangement (perpendicular to plane configuration) is that the hot and cold junctions are separated by the thickness of the substrate--the heat gradient is through the substrate. The object measured can be close to the sensor or actually come in contact with the sensor.
Vertical or "out-of-the-plane" configuration requires that the leads of the thermocouples are created through the substrate material, i.e. "via" interconnections are required. These vias have to be very small, which has been difficult to achieve previously. However, thanks to creative thinking and research at Uppsala University, an innovative idea with thermocouple lead structures based on nanotechnology has been realized, which is what JonDeTech AB offers.
The figure above illustrates the principle of a thermocouple. Two different thermoelectric materials (could be metals, alloys or semiconductor materials) are combined into a circuit. |
The Figure above illustrates the vertical heat flow model of JonDeTech's thermopiles. The figure shows the foil from a cross section view. Radiant heat is heating the top absorber layer and produces a temperature gradient over the sensor. |