(位移传感器专业英语thermoelectriceffect

The Seebeck effect is the conversion of temperature differences directly into electricity and is named for German-Estonian physicist Thomas Johann Seebeck, who, in 1821 discovered that a compass needle would be deflected by a closed loop formed by two metals joined in two places, with a temperature difference between the junctions. This was because the metals responded differently to the temperature difference, creating a current loop and a magnetic field. Seebeck did not recognize there was an electric current involved, so he called the phenomenon the thermomagnetic effect. Danish physicist Hans Christian Ørsted rectified the mistake and coined the term "thermoelectricity". The voltage created by this effect is of the order of several microvolts per kelvin difference. One such combination, copper-constantan, has a Seebeck coefficient of 41 microvolts per kelvin at room temperature.[2]
The voltage V developed can be derived from:
where S A and S B are the thermopowers (Seebeck coefficient) of metals A and B as a function of temperature and T1 and T2 are the temperatures of the two junctions. The
Seebeck coefficients are non-linear as a function of temperature, and depend on the
conductors' absolute temperature, material, and molecular structure. If the Seebeck
coefficients are effectively constant for the measured temperature range, the above
formula can be approximated as:
The Seebeck effect is used in the thermocouple to measure a temperature difference;
absolute temperature may be found by setting one end to a known temperature. A
metal of unknown composition can be classified by its thermoelectric effect if a
metallic probe of known composition, kept at a constant temperature, and is held in
contact with it. Industrial quality control instruments use this as thermoelectric alloy
sorting to identify metal alloys. Thermocouples in series form a thermopile,
sometimes constructed in order to increase the output voltage, since the voltage
induced over each individual couple is small. Thermoelectric generators are used for
creating power from heat differentials and exploit this effect.
Peltier effect
The Peltier effect is the presence of heat at an electrified junction of two different metals and is named for French physicist Jean-Charles Peltier, who discovered it in 1834. When a current is made to flow through a junction composed of materials A and B, heat is generated at the upper
junction at T2, and absorbed at the lower junction at T1. The Peltier heat absorbed by the lower junction per unit time is equal to
where ΠAB is the Peltier coefficient for the thermocouple composed of materials A and B and ΠA(ΠB) is the Peltier coefficient of material A (B). Π varies with the material's temperature and its specific composition: p-type silicon typically has a positive Peltier coefficient below ~550 K, but n-type silicon is typically negative.
The Peltier coefficients represent how much heat current is carried per unit charge through a given material. Since charge current must be continuous across a junction, the associated heat flow will develop a discontinuity if ΠA and ΠB are different. Depending on the magnitude of the current, heat must accumulate or deplete at the junction due to a non-zero divergence there caused by the carriers attempting to return to the equilibrium that existed before the current was applied by transferring energy from one connector to another. Individual couples can be connected in series to enhance the effect. Thermoelectric heat pumps exploit this phenomenon, as do thermoelectric cooling devices found in refrigerators
Thomson effect
The Thomson effect was predicted and subsequently observed by Lord Kelvin in 1851. It describes the heating or cooling of a current-carrying conductor with a temperature gradient.
Any current-carrying conductor (except for a superconductor) with a temperature difference between two points either absorbs or emits heat, depending on the material. If a current density J is passed through a homogeneous conductor, the heat production q per unit volume is:
where ρ is the resistivity of the material, dT/dx is the temperature gradient along the wire
and μ is the Thomson coefficient. The first term is the Joule heating, which does not change in sign; the second term is the Thomson heating, which follows J changing sign.
In metals such as zinc and copper, whose temperature is directly proportional to their potential, when current moves from the hotter end to the colder end, there is a generation of heat and the positive Thomson effect occurs.[citation needed] Conversely, in metals such as cobalt, nickel, and iron, whose temperature is inversely proportional to their potential, when current moves from the hotter end to the colder end, there is an absorption of heat and the negative Thomson effect occurs.
If the Thomson coefficient of a material is measured over a wide temperature range, it can be integrated using the Thomson relations to determine the absolute values for the Peltier and Seebeck coefficients. This needs to be done only for one material, since the other values can be determined by measuring pairwise Seebeck coefficients in thermocouples containing the reference material and then adding back the absolute thermopower of the reference material.
Lead is commonly stated to have a Thomson coefficient of zero; in fact, it is non-zero, albeit being very small.[5] In contrast, the thermoelectric coefficients of all known superconductors are zero
Figure of merit
The figure of merit Z for thermoelectric devices is defined as
where σ is the electrical conductivity, κ is the thermal conductivity, and S is the Seebeck coefficient. The dimensionless figure of merit ZT is formed by multiplying Z with the average temperature.
A greater ZT indicates a greater thermodynamic efficiency, subject to certain provisions, particularly that the two materials in the couple have similar Z. ZT is therefore a method for comparing the potential efficiency of devices using different materials. Values of 1 are considered good; values in the 3–4 range are essential for thermoelectrics to compete with mechanical devices in efficiency. To date, the best reported ZT values are in the 2–3 range. Currently this goal of high ZT values is referred to as: "high-figure-of-merit" [6][7][8] Much of the research in thermoelectric materials has focused on increasing S and reducing κ by manipulating the nanostructure of the materials.[citation needed]
[edit]Device efficiency
The efficiency of a thermoelectric device for electricity generation is given by η, defined as
The maximum efficiency ηmax is defined as
where T H is the temperature at the hot junction and T C is the temperature at the surface being
cooled. is the modified dimensionless figure of merit, which takes into consideration the thermoelectric capacity of both thermoelectric materials being used in the device and is defined as
where ρ is the electrical resistivity, is the average temperature between the hot and cold surfaces and the subscripts n and p denote properties related to the n- and p-type
semiconducting thermoelectric materials, respectively. Since thermoelectric devices are heat engines, their efficiency is limited by the Carnot efficiency, hence the T H and T C terms in . Regardless, the coefficient of performance of current commercial thermoelectric refrigerators ranges from 0.3 to 0.6, one-sixth the value of traditional vapor-compression refrigerators.[9]
[edit]Applications
See also: Thermoelectric materials
[edit]Seebeck effect
Main article: Thermoelectric generator
The Seebeck effect is used in the thermoelectric generator, which functions like a heat engine, but is less bulky, has no moving parts, and is typically more expensive and less efficient. These have a use in power plants for converting waste heat into additional power (a form
of energy recycling), and in automobiles as automotive thermoelectric generators(ATGs) for increasing fuel efficiency. Space probes often use radioisotope thermoelectric generators with the same mechanism but using radioisotopes to generate the required heat difference.
[edit]Peltier effect
Main article: Thermoelectric cooling
The Peltier effect can be used to create a refrigerator which is compact and has no circulating fluid or moving parts; such refrigerators are useful in applications where their advantages out weigh the disadvantage of their very low efficiency.
[edit]Temperature measurement
Thermocouples and thermopiles are devices that use the Seebeck effect to measure the temperature difference between two objects, one connected to a voltmeter and the other to the probe. The temperature of the voltmeter, and hence that of the material being measured by the probe, can be measured separately using cold junction compensation techniques。