Thermochromism is the property of
substances to change
color
Color (American English) or colour (British English) is the visual perceptual property deriving from the spectrum of light interacting with the photoreceptor cells of the eyes. Color categories and physical specifications of color are assoc ...
due to a change in
temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied on ...
. A
mood ring
A mood ring is a finger ring that contains a thermochromic element, or "mood stone", that changes colors based on the temperature of the finger of the wearer. Finger temperature, as long as the ambient temperature is relatively constant, is ...
is an excellent example of this phenomenon, but thermochromism also has more practical uses, such as baby bottles which change to a different color when cool enough to drink, or kettles which change color when water is at or near boiling point. Thermochromism is one of several types of
chromism.
Organic materials
Thermochromatic liquid crystals
The two common approaches are based on
liquid crystal
Liquid crystal (LC) is a state of matter whose properties are between those of conventional liquids and those of solid crystals. For example, a liquid crystal may flow like a liquid, but its molecules may be oriented in a crystal-like way. The ...
s and
leuco dyes. Liquid crystals are used in precision applications, as their responses can be engineered to accurate temperatures, but their color range is limited by their principle of operation. Leuco dyes allow wider range of colors to be used, but their response temperatures are more difficult to set with accuracy.
Some liquid crystals are capable of displaying different colors at different temperatures. This change is dependent on selective reflection of certain wavelengths by the crystallic structure of the material, as it changes between the low-temperature crystallic phase, through
anisotropic
Anisotropy () is the property of a material which allows it to change or assume different properties in different directions, as opposed to isotropy. It can be defined as a difference, when measured along different axes, in a material's phys ...
chiral or twisted
nematic phase, to the high-temperature
isotropic liquid phase. Only the nematic mesophase has thermochromic properties; this restricts the effective temperature range of the material.
The twisted nematic phase has the molecules oriented in layers with regularly changing orientation, which gives them periodic spacing. The light passing through the crystal undergoes
Bragg diffraction on these layers, and the wavelength with the greatest constructive
interference is reflected back, which is perceived as a spectral color. A change in the crystal temperature can result in a change of spacing between the layers and therefore in the reflected wavelength. The color of the thermochromic liquid crystal can therefore continuously range from non-reflective (black) through the
spectral color
A spectral color is a color that is evoked by '' monochromatic light'', i.e. either a single wavelength of light in the visible spectrum, or by a relatively narrow band of wavelengths (e.g. lasers). Every wavelength of visible light is perce ...
s to black again, depending on the temperature. Typically, the high temperature state will reflect blue-violet, while the low-temperature state will reflect red-orange. Since blue is a shorter wavelength than red, this indicates that the distance of layer spacing is reduced by heating through the liquid-crystal state.
Some such materials are
cholesteryl nonanoate
Cholesteryl nonanoate, also called cholesteryl pelargonate, 3β-cholest-5-en-3-ol nonaoate or cholest-5-ene-3-β-yl nonanoate, is an ester of cholesterol and nonanoic acid. It is a liquid crystal material forming cholesteric liquid crystals with ...
or cyanobiphenyls.
Mixtures with 3–5 °C span of temperatures and ranges from about 17–23 °C to about 37–40 °C can be composed from varying proportions of
cholesteryl oleyl carbonate
Cholesteryl oleyl carbonate (COC) is an organic chemical, a carbonate ester of cholesterol and oleyl alcohol with carbonic acid. It is a liquid crystal material forming cholesteric liquid crystals with helical structure. It is a transparent liqu ...
,
cholesteryl nonanoate
Cholesteryl nonanoate, also called cholesteryl pelargonate, 3β-cholest-5-en-3-ol nonaoate or cholest-5-ene-3-β-yl nonanoate, is an ester of cholesterol and nonanoic acid. It is a liquid crystal material forming cholesteric liquid crystals with ...
, and
cholesteryl benzoate
Cholesteryl benzoate, also called 5-cholesten-3-yl benzoate, is an organic chemical, an ester of cholesterol and benzoic acid. It is a liquid crystal material forming cholesteric liquid crystals with helical structure.
It can be used with choles ...
. For example, the mass ratio of 65:25:10 yields range of 17–23 °C, and 30:60:10 yields range of 37–40 °C.
Liquid crystals used in dyes and inks often come microencapsulated, in the form of suspension.
Liquid crystals are used in applications where the color change has to be accurately defined. They find applications in thermometers for room, refrigerator, aquarium, and medical use, and in indicators of level of propane in tanks. A popular application for thermochromic liquid crystals are the
mood ring
A mood ring is a finger ring that contains a thermochromic element, or "mood stone", that changes colors based on the temperature of the finger of the wearer. Finger temperature, as long as the ambient temperature is relatively constant, is ...
s.
Liquid crystals are difficult to work with and require specialized printing equipment. The material itself is also typically more expensive than alternative technologies. High temperatures, ultraviolet radiation, some chemicals and/or solvents have a negative impact on their lifespan.
Leuco dyes
Thermochromic dyes are based on mixtures of leuco dyes with other suitable chemicals, displaying a color change (usually between the colorless leuco form and the colored form) that depends upon temperature. The dyes are rarely applied on materials directly; they are usually in the form of
microcapsules with the mixture sealed inside. An illustrative example is the
Hypercolor
Hypercolor was a line of clothing, mainly T-shirts and shorts, that changed color with heat.
They were manufactured by Generra Sportswear Company of Seattle and marketed in the United States as Generra Hypercolor or Generra Hypergrafix a ...
fashion, where microcapsules with
crystal violet lactone
Crystal violet lactone (CVL) is a leuco dye, a lactone derivate of crystal violet 10B. In pure state it is a slightly yellowish crystalline powder, soluble in nonpolar or slightly polar organic solvents.
The central carbon in the leuco form i ...
,
weak acid
Acid strength is the tendency of an acid, symbolised by the chemical formula HA, to dissociate into a hydron (chemistry), proton, H+, and an anion, A-. The Dissociation (chemistry), dissociation of a strong acid in solution is effectively comple ...
, and a dissociable salt dissolved in
dodecanol
Dodecanol , or lauryl alcohol, is an organic compound produced industrially from palm kernel oil or coconut oil. It is a fatty alcohol. Sulfate esters of lauryl alcohol, especially sodium lauryl sulfate, are very widely used as surfactants. ...
are applied to the fabric; when the solvent is solid, the dye exists in its lactone leuco form, while when the solvent melts, the salt dissociates, the pH inside the microcapsule lowers, the dye becomes protonated, its lactone ring opens, and its absorption spectrum shifts drastically, therefore it becomes deeply violet. In this case the apparent thermochromism is in fact
halochromism
A halochromic material or pH indicator is a material which changes colour when pH changes occur. The term ‘ chromic’ is defined for materials that can change colour reversibly with the presence of an external factor. In this case, the factor ...
.
The dyes most commonly used are
spirolactone
Spirolactones are a class of functional group in organic chemistry featuring a cyclic ester attached spiro to another ring system. The name is also used to refer to a class of synthetic steroids, called steroid-17α-spirolactones, 17α-spirol ...
s,
fluoran
Fluoran is a triarylmethane dye. It is the structural core of a variety of other dyes.
These dyes have a variety of applications such as chemical stains (for example eosin
Eosin is the name of several fluorescent acidic compounds which bi ...
s,
spiropyran A spiropyran is a type of organic chemical compound, known for photochromic properties that provide this molecule with the ability of being used in medical and technological areas. Spiropyrans were discovered in the early twentieth century. However ...
s, and fulgides. The acids include
bisphenol A
Bisphenol A (BPA) is a chemical compound primarily used in the manufacturing of various plastics. It is a colourless solid which is soluble in most common organic solvents, but has very poor solubility in water. BPA is produced on an industrial ...
,
paraben
Parabens are a class of widely used preservatives in cosmetic and pharmaceutical products. Chemically, they are a series of parahydroxybenzoates or esters of parahydroxybenzoic acid (also known as 4-hydroxybenzoic acid). Parabens are effectiv ...
s,
1,2,3-triazole derivates, and
4-hydroxycoumarin
4-Hydroxycoumarin is a coumarin derivative with a hydroxy group at the 4-position.
Occurrence
4-Hydroxycoumarin is an important fungal metabolite from the precursor coumarin, and its production leads to further fermentative production of the nat ...
and act as proton donors, changing the dye molecule between its leuco form and its protonated colored form; stronger acids would make the change irreversible.
Leuco dyes have less accurate temperature response than liquid crystals. They are suitable for general indicators of approximate temperature ("too cool", "too hot", "about OK"), or for various novelty items. They are usually used in combination with some other pigment, producing a color change between the color of the base pigment and the color of the pigment combined with the color of the non-leuco form of the leuco dye. Organic leuco dyes are available for temperature ranges between about and , in wide range of colors. The color change usually happens in a 3 °C (5.4 °F) interval.
Leuco dyes are used in applications where temperature response accuracy is not critical: e.g. novelties, bath toys,
flying disc
A frisbee (pronounced ), also called a flying disc or simply a disc, is a gliding toy or sporting item that is generally made of injection-molded plastic and roughly in diameter with a pronounced lip. It is used recreationally and competitiv ...
s, and approximate temperature indicators for microwave-heated foods. Microencapsulation allows their use in wide range of materials and products. The size of the microcapsules typically ranges between 3–5 µm (over 10 times larger than regular pigment particles), which requires some adjustments to printing and manufacturing processes.
An application of leuco dyes is in the
Duracell battery state indicators. A layer of a leuco dye is applied on a resistive strip to indicate its heating, thus gauging the amount of current the battery is able to supply. The strip is triangular-shaped, changing its resistance along its length, therefore heating up a proportionally long segment with the amount of current flowing through it. The length of the segment above the threshold temperature for the leuco dye then becomes colored.
Exposure to ultraviolet radiation, solvents and high temperatures reduce the lifespan of leuco dyes. Temperatures above about typically cause irreversible damage to leuco dyes; a time-limited exposure of some types to about is allowed during manufacturing.
Thermochromic paints use
liquid crystals or
leuco dye technology. After absorbing a certain amount of light or heat, the crystallic or molecular structure of the pigment reversibly changes in such a way that it absorbs and emits light at a different wavelength than at lower temperatures. Thermochromic paints are seen quite often as a coating on coffee mugs, whereby once hot coffee is poured into the mugs, the thermochromic paint absorbs the heat and becomes colored or
transparent, therefore changing the appearance of the mug. These are known as
magic mugs or heat changing mugs. Another common example is the use of leuco dye in spoons used in ice cream parlors and frozen yogurt shops. Once dipped into the cold desserts, part of the spoon appears to change color.
Papers
Thermochromic papers are used for
thermal printer
Thermal printing (or direct thermal printing) is a digital printing process which produces a printed image by passing paper with a thermochromic coating, commonly known as thermal paper, over a print head consisting of tiny electrically heat ...
s. One example is the paper impregnated with the solid mixture of a
fluoran
Fluoran is a triarylmethane dye. It is the structural core of a variety of other dyes.
These dyes have a variety of applications such as chemical stains (for example eosin
Eosin is the name of several fluorescent acidic compounds which bi ...
dye with
octadecylphosphonic acid
Octadecylphosphonic acid is a chemical compound most commonly used in thermal paper for receipts, adding machines, tickets, etc.
References
Phosphonic acids
{{organic-compound-stub ...
. This mixture is stable in solid phase; however, when the octadecylphosphonic acid is melted, the dye undergoes a chemical reaction in the liquid phase, and assumes the protonated colored form. This state is then conserved when the matrix solidifies again, if the cooling process is fast enough. As the leuco form is more stable in lower temperatures and solid phase, the records on thermochromic papers slowly fade out over years.
Polymers
Thermochromism can appear in thermoplastics, duroplastics, gels or any kind of coatings. The polymer itself, an embedded thermochromic additive or a high ordered structure built by the interaction of the polymer with an incorporated non-thermochromic additive can be the origin of the thermochromic effect. Furthermore, from the physical point of view, the origin of the thermochromic effect can be multifarious. So it can come from changes of light
reflection,
absorption and/or
scattering
Scattering is a term used in physics to describe a wide range of physical processes where moving particles or radiation of some form, such as light or sound, are forced to deviate from a straight trajectory by localized non-uniformities (including ...
properties with temperature. The application of thermochromic polymers for adaptive solar protection is of great interest. A function by design strategy, e.g. applied for the development of non-toxic thermochromic polymers has come into the focus in the last decade.
Inks
Thermochromic inks or dyes are temperature sensitive
compounds, developed in the 1970s, that temporarily change
color
Color (American English) or colour (British English) is the visual perceptual property deriving from the spectrum of light interacting with the photoreceptor cells of the eyes. Color categories and physical specifications of color are assoc ...
with exposure to
heat
In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is ...
. They come in two forms,
liquid crystal
Liquid crystal (LC) is a state of matter whose properties are between those of conventional liquids and those of solid crystals. For example, a liquid crystal may flow like a liquid, but its molecules may be oriented in a crystal-like way. The ...
s and
leuco dyes. Leuco dyes are easier to work with and allow for a greater range of applications. These applications include: flat
thermometer
A thermometer is a device that measures temperature or a temperature gradient (the degree of hotness or coldness of an object). A thermometer has two important elements: (1) a temperature sensor (e.g. the bulb of a mercury-in-glass thermomete ...
s,
battery testers, clothing, and the indicator on bottles of
maple syrup
Maple syrup is a syrup made from the sap of maple trees. In cold climates, these trees store starch in their trunks and roots before winter; the starch is then converted to sugar that rises in the sap in late winter and early spring. Maple t ...
that change color when the
syrup
In cooking, a syrup (less commonly sirup; from ar, شراب; , beverage, wine and la, sirupus) is a condiment that is a thick, viscous liquid consisting primarily of a solution of sugar in water, containing a large amount of dissolved suga ...
is warm. The thermometers are often used on the exterior of
aquariums, or to obtain a
body temperature
Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different. A thermoconforming organism, by contrast, simply adopts the surrounding temperatur ...
via the
forehead
In human anatomy, the forehead is an area of the head bounded by three features, two of the skull and one of the scalp. The top of the forehead is marked by the hairline, the edge of the area where hair on the scalp grows. The bottom of the fore ...
. Coors Light uses thermochromic ink on its cans now, changing from white to blue to indicate the can is cold.
Inorganic materials
Virtually all inorganic compounds are thermochromic to some extent. Most examples however involve only subtle changes in color. For example,
titanium dioxide
Titanium dioxide, also known as titanium(IV) oxide or titania , is the inorganic compound with the chemical formula . When used as a pigment, it is called titanium white, Pigment White 6 (PW6), or CI 77891. It is a white solid that is insoluble ...
,
zinc sulfide
Zinc sulfide (or zinc sulphide) is an inorganic compound with the chemical formula of ZnS. This is the main form of zinc found in nature, where it mainly occurs as the mineral sphalerite. Although this mineral is usually black because of various i ...
and
zinc oxide
Zinc oxide is an inorganic compound with the Chemical formula, formula . It is a white powder that is insoluble in water. ZnO is used as an additive in numerous materials and products including cosmetics, food supplements, rubbers, plastics, ceram ...
are white at room temperature but when heated change to yellow. Similarly
indium(III) oxide is yellow and darkens to yellow-brown when heated.
Lead(II) oxide
Lead(II) oxide, also called lead monoxide, is the inorganic compound with the molecular formula lead, Pboxide, O. PbO occurs in two Polymorphism (materials science), polymorphs: litharge having a tetragonal crystal system, tetragonal crystal struc ...
exhibits a similar color change on heating. The color change is linked to changes in the electronic properties (energy levels, populations) of these materials.
More dramatic examples of thermochromism are found in materials that undergo
phase transition
In chemistry, thermodynamics, and other related fields, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic states ...
or exhibit charge-transfer bands near the visible region. Examples include
* Cuprous mercury iodide (Cu
2 4">gI4 undergoes a
phase transition
In chemistry, thermodynamics, and other related fields, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic states ...
at 67 °C, reversibly changing from a bright red solid material at low temperature to a dark brown solid at high temperature, with intermediate red-purple states. The colors are intense and seem to be caused by Cu(I)–Hg(II)
charge-transfer complex
In chemistry, a charge-transfer (CT) complex or electron-donor-acceptor complex describes a type of supramolecular assembly of two or more molecules or ions. The assembly consists of two molecules that self-attract through electrostatic forc ...
es.
* Silver mercury iodide (Ag
2 4">gI4 is yellow at low temperatures and orange above 47–51 °C, with intermediate yellow-orange states. The colors are intense and seem to be caused by Ag(I)–Hg(II)
charge-transfer complex
In chemistry, a charge-transfer (CT) complex or electron-donor-acceptor complex describes a type of supramolecular assembly of two or more molecules or ions. The assembly consists of two molecules that self-attract through electrostatic forc ...
es.
*
Mercury(II) iodide is a crystalline material which at 126 °C undergoes reversible
phase transition
In chemistry, thermodynamics, and other related fields, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic states ...
from red alpha phase to pale yellow beta phase.
* Bis(dimethylammonium) tetrachloronickelate(II) (
3)2NH2">CH3)2NH2sub>2NiCl
4) is a raspberry-red compound, which becomes blue at about 110 °C. On cooling, the compound becomes a light yellow metastable phase, which over 2–3 weeks turns back into original red. Many other
tetrachloronickelates are also thermochromic.
* Bis(diethylammonium) tetrachlorocuprate(II) (
3CH2)2NH2">CH3CH2)2NH2sub>2CuCl
4) is a bright green solid material, which at 52–53 °C reversibly changes color to yellow. The color change is caused by relaxation of the hydrogen bonds and subsequent change of geometry of the copper-chlorine complex from planar to deformed tetrahedral, with appropriate change of arrangement of the copper atom's d-orbitals. There is no stable intermediate, the crystals are either green or yellow.
*
Chromium(III) oxide
Chromium(III) oxide (or chromia) is an inorganic compound with the formula . It is one of the principal oxides of chromium and is used as a pigment. In nature, it occurs as the rare mineral eskolaite.
Structure and properties
has the corundum ...
and
aluminium(III) oxide in a 1:9 ratio is red at room temperature and grey at 400 °C, due to changes in its
crystal field Crystal field theory (CFT) describes the breaking of degeneracies of electron orbital states, usually ''d'' or ''f'' orbitals, due to a static electric field produced by a surrounding charge distribution (anion neighbors). This theory has been used ...
.
*
Vanadium dioxide
Vanadium(IV) oxide or vanadium dioxide is an inorganic compound with the formula VO2. It is a dark blue solid. Vanadium(IV) dioxide is amphoteric, dissolving in non-oxidising acids to give the blue vanadyl ion, Osup>2+ and in alkali to give the ...
has been investigated for use as a "spectrally-selective" window coating to block
infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from aroun ...
transmission and reduce the loss of building interior heat through windows.
This material behaves like a
semiconductor
A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. ...
at lower temperatures, allowing more transmission, and like a conductor at higher temperatures, providing much greater
reflectivity
The reflectance of the surface of a material is its effectiveness in reflecting radiant energy. It is the fraction of incident electromagnetic power that is reflected at the boundary. Reflectance is a component of the response of the electroni ...
. The phase change between transparent semiconductive and reflective conductive phase occurs at 68 °C; doping the material with 1.9% of
tungsten
Tungsten, or wolfram, is a chemical element with the symbol W and atomic number 74. Tungsten is a rare metal found naturally on Earth almost exclusively as compounds with other elements. It was identified as a new element in 1781 and first isol ...
lowers the transition temperature to 29 °C.
Other thermochromic solid semiconductor materials include
* Cd
''x''Zn
1−''x''S
''y''Se
1−''y'' (''x'' = 0.5–1, ''y'' = 0.5–1),
* Zn
''x''Cd
''y''Hg
1−''x''−''y''O
''a''S
''b''Se
''c''Te
1−''a''−''b''−''c'' (''x'' = 0–0.5, ''y'' = 0.5–1, ''a'' = 0–0.5, ''b'' = 0.5–1, ''c'' = 0–0.5),
* Hg
''x''Cd
''y''Zn
1−''x''−''y''S
''b''Se
1−''b'' (''x'' = 0–1, ''y'' = 0–1, ''b'' = 0.5–1).
Some minerals are thermochromic as well; for example some
chromium
Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal.
Chromium metal is valued for its high corrosion resistance and h ...
-rich
pyrope
The mineral pyrope is a member of the garnet group. Pyrope is the only member of the garnet family to always display red colouration in natural samples, and it is from this characteristic that it gets its name: from the Greek for ''fire'' and ''e ...
s, normally reddish-purplish, become green when heated to about 80 °C.
Irreversible inorganic thermochromes
Some materials change color irreversibly. These can be used for e.g. laser marking of materials.
*
Copper(I) iodide
Copper(I) iodide is the inorganic compound with the formula CuI. It is also known as cuprous iodide. It is useful in a variety of applications ranging from organic synthesis to cloud seeding.
Copper(I) iodide is white, but samples often appear ...
is a solid pale tan material transforming at 60–62 °C to orange color.
*
Ammonium metavanadate
Ammonium metavanadate is the inorganic compound with the formula NH4VO3. It is a white salt, although samples are often yellow owing to impurities of V2O5. It is an important intermediate in the purification of vanadium.Günter Bauer, Volker Gü ...
is a white material, turning to brown at 150 °C and then to black at 170 °C.
*
Manganese violet
Manganese violet is the common name for ammonium manganese(III) pyrophosphate, an inorganic compound composed of ammonium (), manganese in the +3 oxidation state, and the pyrophosphate anion (). It is prepared by heating a mixture of manganese(II ...
(Mn(NH
4)
2P
2O
7) is a violet material, a popular pigment, turning to white at 400 °C.
References
{{reflist, 30em
Inks
Chromism