Thermochromism
Encyclopedia
Chemical substance
In chemistry, a chemical substance is a form of matter that has constant chemical composition and characteristic properties. It cannot be separated into components by physical separation methods, i.e. without breaking chemical bonds. They can be solids, liquids or gases.Chemical substances are...
to change color
Color
Color or colour is the visual perceptual property corresponding in humans to the categories called red, green, blue and others. Color derives from the spectrum of light interacting in the eye with the spectral sensitivities of the light receptors...
due to a change in temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
. A mood ring
Mood ring
A mood ring is a ring which contains a thermochromic element, such as liquid crystal. The ring changes color in response to the body temperature of its wearer...
is an excellent example of this, but it has many other uses such as baby bottles (changes to a different color when cool enough to drink) and kettles. Thermochromism is one of several types of chromism
Chromism
In chemistry, chromism is a process that induces a change, often reversible, in the colors of compounds. In most cases, chromism is based on a change in the electron states of molecules, especially the π- or d-electron state, so this phenomenon is induced by various external stimuli which can alter...
.
The two basic approaches are based on liquid crystal
Liquid crystal
Liquid crystals are a state of matter that have properties between those of a conventional liquid and those of a solid crystal. For instance, an LC may flow like a liquid, but its molecules may be oriented in a crystal-like way. There are many different types of LC phases, which can be...
s and leuco dye
Leuco dye
A leuco dye is a dye whose molecules can acquire two forms, one of which is colorless.For example, the spiro form of an oxazine is a colorless leuco dye; the conjugated system of the oxazine and another aromatic part of the molecule is separated by an sp3-hybridized "spiro" carbon...
s. 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.
Thermochromatic liquid crystals
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 anisotropicAnisotropy
Anisotropy is the property of being directionally dependent, as opposed to isotropy, which implies identical properties in all directions. It can be defined as a difference, when measured along different axes, in a material's physical or mechanical properties An example of anisotropy is the light...
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
Spectral color
A spectral color is a color that is evoked by a single wavelength of light in the visible spectrum, or by a relatively narrow band of wavelengths...
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
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 helical structure...
or cyanobiphenyls.
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 thermochromid liquid crystals are the mood ring
Mood ring
A mood ring is a ring which contains a thermochromic element, such as liquid crystal. The ring changes color in response to the body temperature of its wearer...
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 suitable other chemicals, displaying a color change (usually between the colorless leuco form and the colored form) in dependence on 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 HypercolorHypercolor
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 and outside the US as Global Hypercolor...
fashion, where microcapsules with crystal violet lactone
Crystal violet lactone
Crystal violet lactone 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....
, weak acid
Weak acid
A weak acid is an acid that dissociates incompletely. It does not release all of its hydrogens in a solution, donating only a partial amount of its protons to the solution...
, and a dissociable salt dissolved in dodecanol
Dodecanol
Dodecanol, also known by its IUPAC name 1-dodecanol or dodecan-1-ol, and by its trivial name dodecyl alcohol and lauryl alcohol, is a fatty alcohol. Dodecanol is a colourless, water insoluble solid of melting point 24 °C and boiling point 259 °C. It has a floral odor...
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
Halochromism
A halochromic material is a material which changes colour when pH changes occur. The term ‘chromic’ is defined as materials that can change colour reversibly with the presence of a factor. In this case, the factor is pH...
.
The dyes most commonly used are spirolactones, fluoran
Fluoran
Fluoran is a triarylmethane dye. It is the structural core of a variety of other dyes....
s, spiropyrans, and fulgides. The weak acids include bisphenol A
Bisphenol A
Bisphenol A is an organic compound with two phenol functional groups. It is used to make polycarbonate plastic and epoxy resins, along with other applications....
, parabens, 1,2,3-triazole
1,2,3-Triazole
1,2,3-Triazole is one of a pair of isomeric chemical compounds with molecular formula C2H3N3, called triazoles, which have a five-membered ring of two carbon atoms and three nitrogen atoms...
derivates, and 4-hydroxycoumarin 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 −5 °C and 60 °C, in wide range of colors. The color change usually happens in a 3 °C interval.
Leuco dyes are used in applications where temperature response accuracy is not critical: e.g. novelties, bath toys, flying discs, 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
Duracell
Duracell is a brand of batteries manufactured by Procter & Gamble.Additionally, Duracell owns the Procell professional-use brand.-Products:Duracell manufactures alkaline batteries in many common sizes, such as AAA, AA, C, D, and 9V...
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 200–230 °C typically cause irreversible damage to leuco dyes; a time-limited exposure of some types to about 250 °C is allowed during manufacturing.
Inks
Thermochromic inks or dyes are temperature sensitive compoundChemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
s, developed in the 1970s, that temporarily change color
Color
Color or colour is the visual perceptual property corresponding in humans to the categories called red, green, blue and others. Color derives from the spectrum of light interacting in the eye with the spectral sensitivities of the light receptors...
with exposure to heat
Heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...
. They come in two forms, liquid crystal
Liquid crystal
Liquid crystals are a state of matter that have properties between those of a conventional liquid and those of a solid crystal. For instance, an LC may flow like a liquid, but its molecules may be oriented in a crystal-like way. There are many different types of LC phases, which can be...
s and leuco dye
Leuco dye
A leuco dye is a dye whose molecules can acquire two forms, one of which is colorless.For example, the spiro form of an oxazine is a colorless leuco dye; the conjugated system of the oxazine and another aromatic part of the molecule is separated by an sp3-hybridized "spiro" carbon...
s. Leuco dyes are easier to work with and allow for a greater range of applications. These applications include: flat thermometer
Thermometer
Developed during the 16th and 17th centuries, a thermometer is a device that measures temperature or temperature gradient using a variety of different principles. A thermometer has two important elements: the temperature sensor Developed during the 16th and 17th centuries, a thermometer (from the...
s, battery testers, clothing, and the indicator on bottles of maple syrup
Maple syrup
Maple syrup is a syrup usually made from the xylem sap of sugar maple, red maple, or black maple trees, although it can also be made from other maple species such as the bigleaf maple. In cold climates, these trees store starch in their trunks and roots before the winter; the starch is then...
that change color when the syrup
Syrup
In cooking, a syrup is a thick, viscous liquid consisting primarily of a solution of sugar in water, containing a large amount of dissolved sugars but showing little tendency to deposit crystals...
is warm. The most well-known line of clothing
Clothing
Clothing refers to any covering for the human body that is worn. The wearing of clothing is exclusively a human characteristic and is a feature of nearly all human societies...
utilizing thermochromics was Hypercolor
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 and outside the US as Global Hypercolor...
. The thermometers are often used on the exterior of aquarium
Aquarium
An aquarium is a vivarium consisting of at least one transparent side in which water-dwelling plants or animals are kept. Fishkeepers use aquaria to keep fish, invertebrates, amphibians, marine mammals, turtles, and aquatic plants...
s, or to obtain a body temperature via the forehead
Forehead
For the Arsenal striker see GervinhoIn human anatomy, the forehead is the fore part of the head. It is, formally, 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...
. Coors light uses thermochromic ink on its cans now, changing from white to blue to indicate the can is cold.
Paints
Thermochromic paint is a relatively recent development in the area of color-changing pigmentPigment
A pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.Many materials selectively absorb...
s. It involves the use of liquid crystals or leuco dye
Leuco dye
A leuco dye is a dye whose molecules can acquire two forms, one of which is colorless.For example, the spiro form of an oxazine is a colorless leuco dye; the conjugated system of the oxazine and another aromatic part of the molecule is separated by an sp3-hybridized "spiro" carbon...
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.
Papers
Thermochromic papers are used for thermal printerThermal printer
A thermal printer produces a printed image by selectively heating coated thermochromic paper, or thermal paper as it is commonly known, when the paper passes over the thermal print head. The coating turns black in the areas where it is heated, producing an image...
s. One example is the paper impregnated with the solid mixture of a fluoran
Fluoran
Fluoran is a triarylmethane dye. It is the structural core of a variety of other dyes....
dye with octadecylphosphonic acid. This mixture is stable in solid phase; however, when the octadecylphosphonic acid is melted, the dye undergoes 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; this may lead to interesting effects in combination with accounting records, receipts from a thermal printer, and a tax audit.
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 properties with temperature. In addition, the application of thermochromic polymers for adaptive solar protection is of great interest in the last decade.Others
Another good example of this is the color indicators on batteriesBattery (electricity)
An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...
. The indicator turns green if the battery still possesses a charge. This works by passing the charge of the battery through a small resistor
Resistor
A linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...
on the battery, and causes the pigment to absorb heat. Once the paint has absorbed enough heat from the current of the battery, it changes from black to green (usually), thus indicating that the battery still has a fair amount of charge left in it. Another approach is using a resistor in the shape of a thin triangular layer, under a thermochromic pigment. The variable width of the resistor causes it to be heated unevenly, with the position of transition threshold temperature varying depending on the current the battery is providing.
- Titanium dioxideTitanium dioxideTitanium dioxide, also known as titanium oxide or titania, is the naturally occurring oxide of titanium, chemical formula . When used as a pigment, it is called titanium white, Pigment White 6, or CI 77891. Generally it comes in two different forms, rutile and anatase. It has a wide range of...
and Zinc oxideZinc oxideZinc oxide is an inorganic compound with the formula ZnO. It is a white powder that is insoluble in water. The powder is widely used as an additive into numerous materials and products including plastics, ceramics, glass, cement, rubber , lubricants, paints, ointments, adhesives, sealants,...
: Simple-to-make thermochromic compounds are titanium dioxideTitanium dioxideTitanium dioxide, also known as titanium oxide or titania, is the naturally occurring oxide of titanium, chemical formula . When used as a pigment, it is called titanium white, Pigment White 6, or CI 77891. Generally it comes in two different forms, rutile and anatase. It has a wide range of...
and zinc oxideZinc oxideZinc oxide is an inorganic compound with the formula ZnO. It is a white powder that is insoluble in water. The powder is widely used as an additive into numerous materials and products including plastics, ceramics, glass, cement, rubber , lubricants, paints, ointments, adhesives, sealants,...
, which are white at room temperature but when heated change to yellow due to various types of crystalCrystalA crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
lattice defects and the accompanying decrease of its band gapBand gapIn solid state physics, a band gap, also called an energy gap or bandgap, is an energy range in a solid where no electron states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference between the top of the valence band and the...
. On cooling the oxides revert to white. - Indium(III) oxideIndium(III) oxideIndium oxide is a chemical compound, an amphoteric oxide of indium.-Crystal structure:Amorphous indium oxide is insoluble in water but soluble in acids, whereas crystalline indium oxide is insoluble in both water and acids. The crystalline form exist in two phases, cubic and rhombohedral . Both...
is yellow and changes to yellow-brown when heated, due to decrease of its band gap. - Lead(II) oxideLead(II) oxideLead oxide is the inorganic compound with the formula PbO. Lead oxide occurs in two polymorphs, red, having a tetragonal crystal structure and yellow, having an orthorhombic crystal structure...
has a similar color change on heating. These solids are technically semiconductors, and the color change is linked to their electronic properties. - Cuprous mercury iodide (Cu2HgI4) undergoes a phase transitionPhase transitionA phase transition is the transformation of a thermodynamic system from one phase or state of matter to another.A phase of a thermodynamic system and the states of matter have uniform physical properties....
at 55 °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 complexes. - Silver mercury iodide (Ag2HgI4) 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 Cu(I)-Hg(II) charge-transfer complexes.
- Mercury(II) iodideMercury(II) iodideMercury iodide is a chemical compound with an appearance of red-orange crystals. Unlike mercury chloride it is hardly soluble in water Mercury(II) iodide (HgI2) is a chemical compound with an appearance of red-orange crystals. Unlike mercury(II) chloride it is hardly soluble in water Mercury(II)...
is a crystalline material which at 126 °C undergoes reversible phase transitionPhase transitionA phase transition is the transformation of a thermodynamic system from one phase or state of matter to another.A phase of a thermodynamic system and the states of matter have uniform physical properties....
from red alpha phase to pale yellow beta phase. - Bis(diethylammonium) tetrachloronickelate is a light yellow-brown color solid material, which at 72–73 °C reversibly changes color to brilliant blue. The yellow-brown phase forms octahedral Ni-Cl complexes with sheets of [NiCl6] units sharing chlorine atoms with neighboring units. At higher temperature the bridging bonds break and the sheets separate into individual tetrahedral NiCl4 units, which changes the configuration of d-orbitals and the energy levels between them, which changes the compound's color. The color change is continuous.
- Bis(dimethylammonium) tetrachloronickelate 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.
- Bis(diethylammonium) tetrachlorocuprate 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.
- Nickel sulfate is green at room temperature but becoming yellow at 155 °C.
- Chromium(III) oxideChromium(III) oxideChromium oxide is the inorganic compound of the formula Cr2O3. It is one of principal oxides of chromium and is used as a pigment. In nature, it occurs as the rare mineral eskolaite.-Structure and properties:...
:aluminium(III) oxide in 1:9 ratio is red at room temperature and grey at 400 °C, due to changes in its crystal field. - Vanadium dioxide has been investigated for use as a "spectrally-selective" window coating to block infraredInfraredInfrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
transmission and reduce the loss of building interior heat through windows. This material behaves like a semiconductorSemiconductorA semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
at lower temperatures, allowing more transmission, and like a conductor at higher temperatures, providing much greater reflectivityReflectivityIn optics and photometry, reflectivity is the fraction of incident radiation reflected by a surface. In general it must be treated as a directional property that is a function of the reflected direction, the incident direction, and the incident wavelength...
. The phase change between transparent semiconductive and reflective conductive phase occurs at 68 °C; doping the material with 1.9% of tungstenTungstenTungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...
lowers the transition temperature to 29 °C.
Other thermochromic solid semiconductor materials investigated for commercial use are CdxZn1–xSySe1–y (x=0.5...1, y=0.5...1), ZnxCdyHg1–x–yOaSbSec1–a–b–c (x=0...0.5, y=0.5...1, a=0...0.5, b=0.5...1, c=0...0.5), HgxCdyZn1–x–ySbSe1–b (x=0...1, y=0...1, b=0.5...1).
Some minerals are thermochromic as well; for example some chromium
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...
-rich pyrope
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 eye. Despite being less common than most garnets, it is a...
s, normally reddish-purplish, become green when heated to about 80 °C.