The Thermopile

The thermopile is one of the oldest infrared detectors, being first described by Melloni [3.4] but it is still widely used and in its latest form (the thin film thermopile) is used in space instrumentation. 

The basic element in a thermopile is a junction between two dissimilar conductors having a large Seebeck coefficient 0. To perform efficiently a large electrical conductivity a is required to minimize Joulean heat loss and a small thermal conductivity K to minimize heat conduction loss between the hot and cold junctions of the thermopile. These requirements are incompatible and we find that in common with other thermoelectric devices (Goldsmid [3.12]) the best choice of thermoelectric material is that for which a0 K is a maximum and that this occurs for certain heavily doped semiconductors, for example BijTcj and related compounds. To make an efficient thermal infrared detector the device must also be an efficient absorber o f the incident radiation and must have a small thermal mass to give as short a response time as possible. 

1 Mullard research sample using alanine doped TGS10 xm thick and area 1.5 x 1.5 mm. Mounted in space qualified encapsulation and performance independently verified 

2 Spectroscopic thermopile. Based on Fellgett [3.17], Brown et al. [3.18], and Schwarz [3.10]. Typical receiver area 0.4 mm, time constant 40 ms 

3 Golay cell after Hickey and Daniels [3.20], Stafsudd and Stevens [3.21], and Gill [3.22] 

4 TRIAS cell. Space qualified encapsulation Chatenier and Gauffre [3.16]) 

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Another application of the Seebeck effect is to be found ill detectors of small quantities of heat radiation. These sensitive detectors comprise a thermopile, a pile of thermocoup)les (small pieces of two different metals connected in V form and put into series). Half of the junctions of the thermopile are shielded within the detector, whereas the other half are exposed to... 

A thermopile sensor generates an output voltage that depends on the temperature difference between its hot and cold contacts. For infrared temperature measurement, the hot contacts are normally thermally insulated and placed on a thin membrane, whereas the cold contacts are thermally connected to the metal housing. Infrared radiation, which is absorbed by the hot contacts of the thermopile, causes a temperature difference between hot and cold contacts. The resulting output voltage is a measure for the temperature difference between radiation source and cold contacts of the thermopile sensor. It is therefore necessary to measure also the temperature of the cold contacts by an additional ambient temperature sensor in order to determine the temperature of the radiation source. 

Using Planck s law, the output voltage of the thermopile sensor can be written as... 

The thermopile output signal is converted to a digital value Zv. To take into account the temperature coefficient of the thermopile, the value U/S is written as ... 

For the calibration of most infrared ear thermometers the sensitivities S0 and R0 and the temperature coefficients Sj and a for both sensors have to be determined. Typically a two-step calibration is performed. In the first step the ambient sensor is calibrated by immersing it into two different temperature controlled baths. In the second step the thermopile sensor is calibrated by measuring the output signal while placing it before two different blackbody radiation sources. 

The measurement of an enthalpy change is based either on the law of conservation of energy or on the Newton and Stefan-Boltzmann laws for the rate of heat transfer. In the latter case, the heat flow between a sample and a heat sink maintained at isothermal conditions is measured. Most of these isoperibol heat flux calorimeters are of the twin type with two sample chambers, each surrounded by a thermopile linking it to a constant temperature metal block or another type of heat reservoir. A reaction is initiated in one sample chamber after obtaining a stable stationary state defining the baseline from the thermopiles. The other sample chamber acts as a reference. As the reaction proceeds, the thermopile measures the temperature difference between the sample chamber and the reference cell. The rate of heat flow between the calorimeter and its surroundings is proportional to the temperature difference between the sample and the heat sink and the total heat effect is proportional to the integrated area under the calorimetric peak. A calibration is thus... 

During operation the voltage developed at the thermopile output is proportional to the thermoelectric power of each of the two different materials and to the temperature difference between the warm and cold junction (Seebeck effect). 

A thermopile can also be used as a chemical sensor if one of the two materials is a catalytic metal for a given volatile compound. In this case it is necessary to keep the warm and cold junctions at constant temperature. During absorption of the volatile compound on behalf of the catalytic material the thermoelectric power may change, giving rise to an output voltage which can be related to the concentration of the volatile compound. A typical example is the thermopile as hydrogen sensor, where one of the two materials is palladium, a standard hydrogen catalyzer. 

The determination of the heat flow relies on the so-called Seebeck effect. An electric potential, known as thermoelectric force and represented by E, is observed when two wires of different metals are joined at both ends and these junctions are subjected to dilferent temperatures, 7j and T2 (figure 9.1a). Several thermocouples can be associated, forming a thermopile (figure 9.1b). For small temperature differences, the thermoelectric force generated by the thermopile is proportional to 7j - T2 and to the number of thermocouples of the pile (>/) ... 

The first heat flow calorimeter based on Seebeck, Peltier, and Joule effects was built by Tian at Marseille, France, and reported in 1923 [156-158]. The set-up included two thermopiles, one to detect the temperature difference 7) — 7) and the other to compensate for that difference by using Peltier or Joule effects in the case of exothermic or endothermic phenomena, respectively. This compensation (aiming to keep 7) = T2 during an experiment) was required because, as the thermopiles had a low heat conductivity, a significant fraction of the heat transfer would otherwise not be made through the thermopile wires and hence would not be detected. 

Another problem related to the validity of equation 9.9 is that equation 9.6 applies only to heat conduction. If T — 12 is large, some significant fraction of heat will be transferred by convection and radiation and thus will not be monitored by the thermopile. Consequently, the use of partial compensating Peltier or Joule effects was essential in the experiments involving Calvet s calorimeter, whose thermopiles had a fairly low thermal conductivity. 

Building a heat flow microcalorimeter is not trivial. Fortunately, a variety of modern commercial instruments are available. Some of these differ significantly from those just described, but the basic principles prevail. The main difference concerns the thermopiles, which are now semiconducting thermocouple plates instead of a series of wire thermocouples. This important modification was introduced by Wadso in 1968 [161], The thermocouple plates have a high thermal conductivity and a low electrical resistance and are sensitive to temperature differences of about 10-6 K. Their high thermal conductivity ensures that the heat transfer occurs fast enough to avoid the need for the Peltier or Joule effects. 

All modern heat flow calorimeters have twin cells thus, they operate in the differential mode. As mentioned earlier, this means that the thermopiles from the sample and the reference cell are connected in opposition, so that the measured output is the difference between the respective thermoelectric forces. Because the differential voltage is the only quantity to be measured, the auxiliary electronics of a heat flux instrument are fairly simple, as shown in the block diagram of figure 9.3. The main device is a nanovoltmeter interfaced to a computer for instrument control and data acquisition and handling. The remaining electronics of a microcalorimeter (not shown in figure 9.3) are related to the very accurate temperature control of the thermostat and, in some cases, with the... 

Figure 12.2 Scheme of a cylinder-type heat flux differential scanning calorimeter. A cell B furnace C thermopile D differential connection of the thermopiles S sample R reference. 

The first attempt in our laboratory to apply flow techniques to high temperature operation was the construction by Dr. E.E. Messikomer of a flow, heat-of-mixing calorimeter(lO). Unfortunately, because the thermopiles used in this instrument did not work above 100°C the instrument was limited to this temperature. However, the results were encouraging because they showed that very rapid and accurate thermodynamic data could be obtained and that the operation of the calorimeter was as easy at 100°C as it was at room temperature. 

The thermopile depends on basic physical principles with the inherent ability to measure temperature differences directly with high common-mode rejection [13]. Furthermore the compatibility with silicon technology enables size reduction and direct immobilization of the biochemical sensing part onto the chip. 

If a heat conduction calorimeter is left for some time and no process takes place in the reaction vessel, there will, ideally, be no temperature gradients in the system made up by vessel, thermopile, and heat sink. The thermopile potential, U, which is proportional to the temperature difference between vessel and heat sink will thus be zero. If a reaction takes place in the vessel and heat is produced (or absorbed), the temperature of the vessel will increase (decrease) leading to 17 0 (see Figure 4). The temperature gradient will cause the heat evolved in the vessel to flow through the thermopile to the heat sink or, in case of an endothermic process, in the... 

For the ideal case where no significant thermal gradients exist in the vessel, the heat production rate will be related to the heat flow through the thermopile, dQ/dt, and to the change of the vessel temperature, dT/dt ... 

Thus, in the ideal case and for a given type of thermopile, the sensitivity of the calorimeter is independent of the number of thermocouples in the thermopile wall. Furthermore, and in contrast to adiabatic instruments, the sensitivity of a thermopile heat conduction calorimeter is independent of the heat capacity of the reaction vessel and its content. 

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