Pyroelectricity

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Internals of a pyroelectric sensor

Pyroelectricity (from Greek: pyr (πυρ), "fire" and electricity) is a property of certain crystals which are naturally electrically polarized and as a result contain large electric fields.<ref>Ashcroft, N. W. & Mermin, N. D. Solid State Physics. (Cengage Learning, 1976).</ref> Pyroelectricity can be described as the ability of certain materials to generate a temporary voltage when they are heated or cooled.<ref>Charles Kittel-8th Edition. 2016. Introduction to Solid State Physics.</ref><ref name=Webster/> The change in temperature modifies the positions of the atoms slightly within the crystal structure, so that the polarization of the material changes. This polarization change gives rise to a voltage across the crystal. If the temperature stays constant at its new value, the pyroelectric voltage gradually disappears due to leakage current. The leakage can be due to electrons moving through the crystal, ions moving through the air, or current leaking through a voltmeter attached across the crystal.<ref name=Webster>Template:Cite book</ref><ref>In this article, the term "voltage" is used in the everyday sense, i.e. what a voltmeter measures. This is actually the electrochemical potential, not the electrostatic potential (Galvani potential).</ref>

ExplanationEdit

Pyroelectric charge in minerals develops on the opposite faces of asymmetric crystals. The direction in which the propagation of the charge tends is usually constant throughout a pyroelectric material, but, in some materials, this direction can be changed by a nearby electric field. These materials are said to exhibit ferroelectricity.

All known pyroelectric materials are also piezoelectric. Despite being pyroelectric, novel materials such as boron aluminum nitride (BAlN) and boron gallium nitride (BGaN) have zero piezoelectric response for strain along the c-axis at certain compositions,<ref>Template:Cite journal</ref> the two properties being closely related. However, note that some piezoelectric materials have a crystal symmetry that does not allow pyroelectricity.

Pyroelectric materials are mostly hard and crystals; however, soft pyroelectricity can be achieved by using electrets.<ref>Template:Cite journal</ref>

Pyroelectricity is measured as the change in net polarization (a vector) proportional to a change in temperature. The total pyroelectric coefficient measured at constant stress is the sum of the pyroelectric coefficients at constant strain (primary pyroelectric effect) and the piezoelectric contribution from thermal expansion (secondary pyroelectric effect). Under normal circumstances, even polar materials do not display a net dipole moment. As a consequence, there are no electric dipole equivalents of bar magnets because the intrinsic dipole moment is neutralized by "free" electric charge that builds up on the surface by internal conduction or from the ambient atmosphere. Polar crystals only reveal their nature when perturbed in some fashion that momentarily upsets the balance with the compensating surface charge.

Spontaneous polarization is temperature dependent, so a good perturbation probe is a change in temperature which induces a flow of charge to and from the surfaces. This is the pyroelectric effect. All polar crystals are pyroelectric, so the 10 polar crystal classes are sometimes referred to as the pyroelectric classes. Pyroelectric materials can be used as infrared and millimeter wavelength radiation detectors.

An electret is the electrical equivalent of a permanent magnet.

Mathematical descriptionEdit

The pyroelectric coefficient may be described as the change in the spontaneous polarization vector with temperature:<ref>Template:Cite journal</ref> <math display="block"> p_i = \frac{\partial P_{S,i}} {\partial T} </math> where p_i (Cm⁻²K⁻¹) is the vector for the pyroelectric coefficient.

HistoryEdit

Template:See also The first record of the pyroelectric effect was made in 1707 by Johann Georg Schmidt, who noted that the "[hot] tourmaline could attract the ashes from the warm or burning coals, as the magnet does iron, but also repelling them again [after the contact]".<ref>Johann Georg Schmidt, Curiöse Speculationes bey Schalflosen Nächten [Curious Speculations During Sleepless Nights] (Chemnitz and Leipzig (Germany): Conrad Stössen, 1707), pages 269-270. An English translation of the relevant passage appears in: Sidney B. Lang, Sourcebook of Pyroelectricity, vol. 2 (New York, New York: Gordon and Breach, 1974), page 96.</ref> In 1717 Louis Lemery noticed, as Schmidt had, that small scraps of non-conducting material were first attracted to tourmaline, but then repelled by it once they contacted the stone.<ref>"Diverse observations de la physique generale," Histoire de l'Académie des Sciences (1717); see pages 7-8.</ref> In 1747 Linnaeus first related the phenomenon to electricity (he called tourmaline Lapidem Electricum, "the electric stone"),<ref>Carl von Linné ("Linnaeus"), Flora Zeylanica: Sistens Plantas Indicas Zeylonae Insulae [The Flora of Ceylon: consisting of Indian plants of the island of Ceylon] (Stockholm ("Holmiae"), Sweden: Laurentii Salvii, 1747), page 8. A translation of the relevant passage appears in Lang (1974), page 103.</ref> although this was not proven until 1756 by Franz Ulrich Theodor Aepinus.<ref>Aepinus (1756) "Memoire concernant quelques nouvelles experiences électriques remarquables" [Memoir concerning some remarkable new electrical experiments], Histoire de l'Académie royale des sciences et des belles lettres (Berlin), vol. 12, pages 105-121.</ref>

Research into pyroelectricity became more sophisticated in the 19th century. In 1824 Sir David Brewster gave the effect the name it has today.<ref>Template:Cite journal</ref> Both William Thomson in 1878<ref>William Thomson (1878) "On the thermoelastic, thermomagnetic and pyroelectric properties of matter," Philosophical Magazine, series 5, vol. 5, pages 4 - 26.</ref> and Woldemar Voigt in 1897<ref>W. Voigt (1897) "Versuch zur Bestimmung des wahren specifischen electrischen Momentes eines Turmalins" (Experiment to determine the true specific electric moment of a tourmaline), Annalen der Physik, vol. 60, pages 368 - 375.</ref> helped develop a theory for the processes behind pyroelectricity. Pierre Curie and his brother, Jacques Curie, studied pyroelectricity in the 1880s, leading to their discovery of some of the mechanisms behind piezoelectricity.<ref>Jacques Curie & Pierre Curie, "Développement par compression de l'électricité polaire dans les cristaux hémièdres à faces inclinées", Bulletin de la Société Minéralogique de France, vol. 3 (4), 90-93, 1880.</ref>

It is mistakenly attributed to Theophrastus (c. 314 BC) the first record of pyroelectricity. The misconception arose soon after the discovery of the pyroelectric properties of tourmaline, which made mineralogists of the time associate the legendary stone Lyngurium with it.<ref>Earle R. Caley and John F.C. Richards, Theophrastus: On Stones (Columbus, Ohio: Ohio State University, 1956), page 110, line 12 of the commentary: "Watson identifies the lyngounon of Theophrastus with tourmaline, but evidently his opinion is partly based on the attractive properties of heated tourmaline which had recently been discovered. This identification is repeated by various later writers. For example, Dana states that lyncurium is supposed to be the ancient name for common tourmaline. However, the absence of tourmaline among surviving examples of ancient gems is clearly against this view."</ref> Lyngurium is described in the work of Theophrastus as being similar to amber, without specifying any pyroelectric properties.<ref>Earle R. Caley and John F.C. Richards, Theophrastus: On Stones (Columbus, Ohio: Ohio State University, 1956), page 51, paragraph 28 of the original text: "It [smaragdos] is remarkable in its powers, and so is the lyngourion [i.e., lynx-urine stone] ... . It has the power of attraction, just as amber has, and some say that it not only attracts straws and bits of wood, but also copper and iron, if the pieces are thin, as Diokles used to explain."</ref>

Crystal classesEdit

All crystal structures belong to one of thirty-two crystal classes based on the number of rotational axes and reflection planes they possess that leave the crystal structure unchanged (point groups). Of the thirty-two crystal classes, twenty-one are non-centrosymmetric (not having a centre of symmetry). Of these twenty-one, twenty exhibit direct piezoelectricity, the remaining one being the cubic class 432. Ten of these twenty piezoelectric classes are polar, i.e., they possess a spontaneous polarization, having a dipole in their unit cell, and exhibit pyroelectricity. If this dipole can be reversed by the application of an electric field, the material is said to be ferroelectric. Any dielectric material develops a dielectric polarization (electrostatics) when an electric field is applied, but a substance which has such a natural charge separation even in the absence of a field is called a polar material. Whether or not a material is polar is determined solely by its crystal structure. Only 10 of the 32 point groups are polar. All polar crystals are pyroelectric, so the ten polar crystal classes are sometimes referred to as the pyroelectric classes.

Piezoelectric crystal classes: 1, 2, m, 222, mm2, 4, -4, 422, 4mm, -42m, 3, 32, 3m, 6, -6, 622, 6mm, -62m, 23, -43m

Pyroelectric: 1, 2, m, mm2, 3, 3m, 4, 4mm, 6, 6mm

Related effectsEdit

Two effects which are closely related to pyroelectricity are ferroelectricity and piezoelectricity. Normally materials are very nearly electrically neutral on the macroscopic level. However, the positive and negative charges which make up the material are not necessarily distributed in a symmetric manner. If the sum of charge times distance for all elements of the basic cell does not equal zero the cell will have an electric dipole moment (a vector quantity). The dipole moment per unit volume is defined as the dielectric polarization. If this dipole moment changes with the effect of applied temperature changes, applied electric field, or applied pressure, the material is pyroelectric, ferroelectric, or piezoelectric, respectively.

The ferroelectric effect is exhibited by materials which possess an electric polarization in the absence of an externally applied electric field such that the polarization can be reversed if the electric field is reversed. Since all ferroelectric materials exhibit a spontaneous polarization, all ferroelectric materials are also pyroelectric (but not all pyroelectric materials are ferroelectric).

The piezoelectric effect is exhibited by crystals (such as quartz or ceramic) for which an electric voltage across the material appears when pressure is applied. Similar to pyroelectric effect, the phenomenon is due to the asymmetric structure of the crystals that allows ions to move more easily along one axis than the others. As pressure is applied, each side of the crystal takes on an opposite charge, resulting in a voltage drop across the crystal.

Pyroelectricity should not be confused with thermoelectricity: In a typical demonstration of pyroelectricity, the whole crystal is changed from one temperature to another, and the result is a temporary voltage across the crystal. In a typical demonstration of thermoelectricity, one part of the device is kept at one temperature and the other part at a different temperature, and the result is a permanent voltage across the device as long as there is a temperature difference. Both effects convert temperature change to electrical potential, but the pyroelectric effect converts temperature change over time into electrical potential, while the thermoelectric effect converts temperature change with position into electrical potential.

Pyroelectric materialsEdit

Although artificial pyroelectric materials have been engineered, the effect was first discovered in minerals such as tourmaline. The pyroelectric effect is also present in bone and tendon.<ref>Template:Cite journal</ref>

The most important example is gallium nitride, a semiconductor.<ref>Gallium Nitride (GaN): Physics, Devices, and Technology." 2015. CRC Press. October 16</ref> The large electric fields in this material are detrimental in light emitting diodes (LEDs), but useful for the production of power transistors.Template:Citation needed

Progress has been made in creating artificial pyroelectric materials, usually in the form of a thin film, using gallium nitride (Ga N), caesium nitrate (Cs N O₃), polyvinyl fluorides, derivatives of phenylpyridine, and cobalt phthalocyanine. Lithium tantalate (Li Ta O₃) is a crystal exhibiting both piezoelectric and pyroelectric properties, which has been used to create small-scale nuclear fusion ("pyroelectric fusion").<ref name="NaranjoGimzewski2005">Template:Cite journal</ref> Recently, pyroelectric and piezoelectric properties have been discovered in doped hafnium oxide (Hf O₂), which is a standard material in CMOS manufacturing.<ref>Template:Cite journal</ref>

ApplicationsEdit

Pyroelectric materials, which generate electrical charges in response to temperature fluctuations, have diverse applications due to their ability to convert thermal energy into electricity or detect thermal changes. Key applications include:

Heat sensorsEdit

Very small changes in temperature can produce a pyroelectric potential. Passive infrared sensors are often designed around pyroelectric materials, as the heat of a human or animal from several feet away is enough to generate a voltage.<ref>Template:Cite journal</ref>

Thermal Sensors: Infrared detectors, fire alarms, gas sensors, and motion sensors utilize high voltage/current responsivity. Lead-based materials (e.g., PMN-PT) excel here due to superior figures of merit (FoMs).<ref>Template:Cite journal</ref><ref name=":0">Template:Cite journal</ref>
Environmental Monitoring: Detecting temperature changes in chemical processes or respiratory systems (e.g., self-powered breathing sensors).<ref name=":0" />

Energy Harvesting and Power GenerationEdit

A pyroelectric can be repeatedly heated and cooled (analogously to a heat engine) to generate usable electrical power. An example of a heat engine is the movement of the pistons in an internal combustion engine like that found in a gasoline powered automobile.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name=":0" />

One group calculated that a pyroelectric in an Ericsson cycle could reach 50% of Carnot efficiency,<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> while a different study found a material that could, in theory, reach 84-92% of Carnot efficiency<ref>Template:Cite journal</ref> (these efficiency values are for the pyroelectric itself, ignoring losses from heating and cooling the substrate, other heat-transfer losses, and all other losses elsewhere in the system)

Possible advantages of pyroelectric generators for generating electricity (as compared to the conventional heat engine plus electrical generator) include:

Harvesting energy from waste-heat:<ref>Template:Cite journal</ref><ref name=":1">Template:Cite journal</ref><ref name=":0" />
- Waste Heat Recovery: Harvesting low-grade thermal energy from industrial processes, automotive systems, and electrical appliances using lead-based ceramics (e.g., PZT, PMN-PT), lead-free ceramics (e.g., BNT-BT, KNN), and polymers (e.g., PVDF-TrFE). The Olsen cycle is a prominent thermodynamic method for efficient energy conversion.
Less bulky equipment:<ref name=":0" />
- Flexible and Wearable Devices: Flexible polymers (e.g., PVDF) and composites power wearable/implantable electronics by leveraging body heat or ambient temperature changes. Examples include self-powered sensors and nanogenerators producing μW to mW/cm³ power densities.
Fewer moving parts.<ref>Template:Cite journal</ref>

Although a few patents have been filed for such a device,<ref>For example: US Patent 4647836, US Patent 6528898, US Patent 5644184</ref> such generators do not appear to be anywhere close to commercialization.

Nuclear fusionEdit

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Pyroelectric materials have been used to generate large electric fields necessary to steer deuterium ions in a nuclear fusion process. This is known as pyroelectric fusion.

Challenges and Future DirectionsEdit

Despite their promising applications, pyroelectric materials face several challenges that must be addressed for broader adoption. One key limitation is the trade-off between pyroelectric coefficients, dielectric properties, and thermal stability, which affects overall performance and efficiency. Additionally, the efficiency of pyroelectric energy harvesting is highly dependent on rapid temperature fluctuations, making it challenging to achieve consistent power output in practical applications. Integration into flexible and biocompatible designs for wearable and miniaturized devices also remains a significant hurdle. Ongoing research aims to enhance figures of merit (FoMs), optimize phase transitions near morphotropic boundaries, and develop hybrid systems that combine pyroelectricity with other energy-harvesting mechanisms for multifunctional applications. Despite these challenges, the versatility of pyroelectric materials positions them as critical components for sustainable energy solutions and next-generation sensor technologies.<ref name=":1" /><ref name=":0" />

ReferencesEdit

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Gautschi, Gustav, 2002, Piezoelectric Sensorics, Springer, Template:ISBN Piezoelectric Sensorics: Force Strain Pressure Acceleration and Acoustic Emission Sensors Materials and Amplifiers

External linksEdit

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