A mood ring shown face front. Note the band of color change.

Thermochromism is the ability of substance to change color due to a change in temperature. A mood ring is an excellent example of this, but it has many other uses. Thermochromism is one of several types of chromism. Image File history File linksMetadata No higher resolution available. ... Image File history File linksMetadata No higher resolution available. ... Look up substance in Wiktionary, the free dictionary. ... Color is an important part of the visual arts. ... For other uses, see Temperature (disambiguation). ... For the Mýa album, see Moodring. ... In chemistry, chromism is a process that induces a reversible change in the colors of compounds. ...

The two basic approaches are based on liquid crystals 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. Schlieren texture of Liquid Crystal nematic phase Liquid crystals are substances that exhibit a phase of matter that has properties between those of a conventional liquid, and those of a solid crystal. ... A leuco dye is a dye whose molecules can acquire two forms, one of which is colorless. ...

Principles

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 anisotropic 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. Look up anisotropy in Wiktionary, the free dictionary. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... Isotropic means independent of direction. Isotropic radiation has the same intensity regardless of the direction of measurement, and an isotropic field exerts the same action regardless of how the test particle is oriented. ...

The twisted nematic phase has the molecules oriented in layers with regularly changing orientation, which gives them periodic spacing. The light passing 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. As the crystal undergoes changes in temperature, thermal expansion occurs, resulting in change of spacing between the layers, and therefore in the reflected wavelength. The color of the thermochromic liquid crystal can therefore continuously range from black through the spectral colors to black again, depending on the temperature. The Bragg formulation of X-ray diffraction (also referred to as Bragg diffraction) was first proposed by William Lawrence Bragg and William Henry Bragg in 1913 in response to their discovery that crystalline solids produced surprising patterns of reflected X-rays (in contrast to that of, say, a liquid). ... For other uses, see Interference (disambiguation). ... In physics, thermal expansion is the tendency of matter to increase in volume or pressure when heated. ... A spectral color is a color that is evoked by the optical spectrum; every wavelength of light yields a different spectral color, in a continuous spectrum. ...

Some such materials are cholesteryl nonanoate or cyanobiphenyls. 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. ...

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.

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 Hypercolor fashion, where microcapsules with crystal violet lactone, weak acid, and a dissociable salt dissolved in dodecanol 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. Micro-encapsulation is a process in which tiny particles or droplets are surrounded by a coating to give small capsules with many useful properties. ... One of the many Generra Hypercolor t-shirt variations. ... Crystal violet lactone, leuco form Crystal violet lactone, protonated colored form 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. ... A weak acid is an acid that does not fully ionize in solution; that is, if the acid was represented by the general formula HA, then in aqueous solution a significant amount of undissolved HA still remains. ... 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. ... Along with smart materials that have the ability to change colour in response with heat and light there are a similar set of materials which are responsive to pH. The pH of a substance is how acidic or basic the substance is. ...

The dyes most commonly used are spirolactones, fluorans, spiropyrans, and fulgides. The weak acids include bisphenol A, parabens, 1,2,3-triazole 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. This article needs to be wikified. ... General chemical structure of a paraben (a para-hydroxybenzoate) where R = an alkyl group Parabens are a group of chemicals widely used as preservatives in the cosmetic and pharmaceutical industries. ... 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. ...

Leuco dyes have less accurate temperature response than liquid crystals, and are used in applications where accuracy is not required. 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: eg. 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. Frisbee redirects here. ...

An interesting 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. A pair of Duracell AAA batteries. ...

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.

Materials

Inks

Thermochromic inks or dyes are temperature sensitive compounds, developed in the 1970s, that temporarily change color with exposure to heat. They come in two forms, liquid crystals and leuco dyes. Liquid crystals are used in mood rings. Leuco dyes are easier to work with and allow for a greater range of applications. These applications include: flat thermometers, battery testers, clothing, and the indicator on bottles of maple syrup that change color when the syrup is warm. The most well-known line of clothing utilizing thermochromics was Hypercolor. The thermometers are often used on the exterior of aquariums, or to obtain a body temperature via the forehead. A chemical compound is a chemical substance of two or more different chemically bonded chemical elements, with a fixed ratio determining the composition. ... Year 1970 (MCMLXX) was a common year starting on Thursday (link shows full calendar) of the Gregorian calendar. ... Color is an important part of the visual arts. ... For other uses, see Heat (disambiguation) In physics, heat, symbolized by Q, is energy transferred from one body or system to another due to a difference in temperature. ... Schlieren texture of Liquid Crystal nematic phase Liquid crystals are substances that exhibit a phase of matter that has properties between those of a conventional liquid, and those of a solid crystal. ... A leuco dye is a dye whose molecules can acquire two forms, one of which is colorless. ... For the Mýa album, see Moodring. ... A common mercury thermometer A thermometer is a device that measures temperature or temperature gradient, using a variety of different principles. ... This article does not adequately cite its references or sources. ... In cooking, a syrup (from Arabic شراب sharab, beverage, via Latin siropus) is a thick, viscous liquid, containing a large amount of dissolved sugars, but showing little tendency to deposit crystals. ... Clothing protects the vulnerable nude human body from the extremes of weather, other features of our environment, and for safety reasons. ... One of the many Generra Hypercolor t-shirt variations. ... “Aquaria” redirects here. ... Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when temperature surrounding is very different. ... In human anatomy, the forehead or brow is the bony part of the head above the eyes. ...

Paints

Thermochromic paint is a relatively recent development in the area of color-changing pigments. It involves the use of liquid crystal 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. Natural Ultramarine pigment in powdered form. ... Schlieren texture of Liquid Crystal nematic phase Liquid crystals are substances that exhibit a phase of matter that has properties between those of a conventional liquid, and those of a solid crystal. ... A leuco dye is a dye whose molecules can acquire two forms, one of which is colorless. ... Transparent glass ball In optics, transparency is the property of allowing light to pass. ...

Papers

Thermochromic papers are used for thermal printers. One example is the paper impregnated with the solid mixture of a fluoran 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. A thermal printer (or direct 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. ...

Others

Another good example of this is the color indicators on batteries. The indicator turns green if the battery still possesses a charge. This works by passing the charge of the battery through a small resistor 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. Symbols representing a single Cell (top) and Battery (bottom), used in circuit diagrams. ... Resistor symbols (non-European) Resistor symbols (Europe, IEC) Axial-lead resistors on tape. ...

A simple-to-make thermochromic compound is zinc oxide, which is white at room temperature but when heated changes to yellow due to various types of crystal lattice defects. On cooling the zinc oxide reverts to white. Also lead(II) oxide has a similar color change on heating. These solids are technically semiconductors, and the color change is linked to their electronic properties. Zinc oxide is a chemical compound with formula ZnO. It is nearly insoluble in water but soluble in acids or alkalis. ... For other uses, see Crystal (disambiguation). ... Lead(II) oxide or litharge is a yellow oxide of lead of formula PbO, created by heating lead in air. ... A semiconductor is a material that is an insulator at very low temperature, but which has a sizable electrical conductivity at room temperature. ...

Copper mercury iodide undergoes a phase transition at 55 °C, reversibly changing from a solid material at low temperature to a dark brown solid at high temperature. Other such material is mercury(II) iodide, a crystalline material which at 126 °C undergoes reversible phase transition from red alpha phase to pale yellow beta phase. Yet another example is nickel sulfate, green at room temperature but becoming yellow at 155 °C. In physics, a phase transition, (or phase change) is the transformation of a thermodynamic system from one phase to another. ... Mercury(II) iodide (HgI2) is a chemical compound with an appearance of red-orange crystals. ... In physics, a phase transition, (or phase change) is the transformation of a thermodynamic system from one phase to another. ... Nickel(II) sulfate or just nickel sulfate, usually refers to the chemical compound with the formula NiSO4. ...

Vanadium dioxide has been investigated for use as a "spectrally-selective" window coating to block infrared transmission and reduce the loss of building interior heat through windows. This material behaves like a semiconductor at lower temperatures, allowing more transmission, and like a conductor at higher temperatures, providing much greater reflectivity. [1] [2] The phase change between transparent semiconductive and reflective conductive phase occurs at 68 °C; doping the material with 1.9% of tungsten lowers the transition temperature to 29 °C. Vanadium(IV) oxide is a solid Chemical symbol: VO2. ... For other uses, see Infrared (disambiguation). ... A semiconductor is a solid whose electrical conductivity is in between that of a conductor and that of an insulator, and can be controlled over a wide range, either permanently or dynamically. ... In optics, reflectivity is the reflectance (the ratio of reflected power to incident power, generally expressed in decibels or percentage) at the surface of a material so thick that the reflectance does not change with increasing thickness; , the intrinsic reflectance of the surface, irrespective of other parameters such as the... For other uses, see Tungsten (disambiguation). ...

Other thermochromic solid semiconductor materials investigated for commercial use are Cd_xZn_1-xS_ySe_1-y (x=0.5...1, y=0.5...1), Zn_xCd_yHg_1-x-yO_aS_bSe_cTe_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_xCd_yZn_1-x-yS_bSe_1-b (x=0...1, y=0...1, b=0.5...1). [3] General Name, Symbol, Number cadmium, Cd, 48 Chemical series transition metals Group, Period, Block 12, 5, d Appearance silvery gray metallic Standard atomic weight 112. ... General Name, symbol, number zinc, Zn, 30 Chemical series transition metals Group, period, block 12, 4, d Appearance bluish pale gray Standard atomic weight 65. ... This article is about the chemical element. ... For other uses, see Selenium (disambiguation). ... General Name, Symbol, Number mercury, Hg, 80 Chemical series transition metals Group, Period, Block 12, 6, d Appearance silvery Standard atomic weight 200. ... General Name, symbol, number oxygen, O, 8 Chemical series nonmetals, chalcogens Group, period, block 16, 2, p Appearance colorless (gas) very pale blue (liquid) Standard atomic weight 15. ... General Name, Symbol, Number tellurium, Te, 52 Chemical series metalloids Group, Period, Block 16, 5, p Appearance silvery lustrous gray Standard atomic weight 127. ...

Some minerals are thermochromic as well; for example some chromium-rich pyropes, normally reddish-purplish, become green when heated. [4] General Name, symbol, number chromium, Cr, 24 Chemical series transition metals Group, period, block 6, 4, d Appearance silvery metallic Standard atomic weight 51. ... The pyrope is a garnet. ...

See Also

• Zero-Ink Printing

External links

• Fraunhofer IAP - Department Chromogenic Polymers
• Thermographic Measurements Ltd
• B&H Colour Change Ltd
• GEM'INNOV thermochromic microcapsules
• Thermochromic pigments
• Color Change Corporation
• Liquid Crystal Resources LLC
• ALSA Corporation Exotic Paint
• Thermochromism in commercial products, PDF