Thermochromism
Thermochromism is the property of substances to change color due to a change in temperature. A mood ring 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 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.
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
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
Some such materials are cholesteryl nonanoate 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 nonanoate, and cholesteryl benzoate. 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.[1]
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 rings.
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
The dyes most commonly used are
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 (23 °F) and 60 °C (140 °F), 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,
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 200–230 °C (392–446 °F) typically cause irreversible damage to leuco dyes; a time-limited exposure of some types to about 250 °C (482 °F) is allowed during manufacturing.
Thermochromic paints use
Papers
Thermochromic papers are used for
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.[2] The application of thermochromic polymers for adaptive solar protection is of great interest.[3] For instance, polymer films with tunable thermochromic nanoparticles, reflective or transparent to sunlight depending on the temperature, have been used to create windows that optimize to the weather.[4] A function by design strategy,[5] e.g. applied for the development of non-toxic thermochromic polymers has come into the focus in the last decade.[6]
Inks
Thermochromic inks or dyes are temperature sensitive
Inorganic materials
Virtually all inorganic compounds are thermochromic to some extent. Most examples however involve only subtle changes in color. For example, titanium dioxide, zinc sulfide and zinc oxide 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 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 or exhibit charge-transfer bands near the visible region. Examples include
- Cuprous mercury iodide (Cu2[HgI4]) undergoes a phase transition 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 complexes.[7]
- Silver mercury iodide (Ag2[HgI4]) 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 complexes.[7]
- Mercury(II) iodide is a crystalline material which at 126 °C undergoes reversible phase transition from red alpha phase to pale yellow beta phase.
- Bis(dimethylammonium) tetrachloronickelate(II) ([(CH3)2NH2]2NiCl4) 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.[8] Many other tetrachloronickelates are also thermochromic.
- Bis(diethylammonium) tetrachlorocuprate(II) ([(CH3CH2)2NH2]2CuCl4) 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.[7]
- crystal field.[9]
- lowers the transition temperature to 29 °C.
Other thermochromic solid semiconductor materials include
- CdxZn1−xSySe1−y (x = 0.5–1, y = 0.5–1),
- ZnxCdyHg1−x−yOaSbSecTe1−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).[13]
Many
Some minerals are thermochromic as well; for example some chromium-rich pyropes, normally reddish-purplish, become green when heated to about 80 °C.[15]
Irreversible inorganic thermochromes
Some materials change color irreversibly. These can be used for e.g. laser marking of materials.[16]
- Copper(I) iodide is a solid pale tan material transforming at 60–62 °C to orange color.[17]
- Ammonium metavanadate is a white material, turning to brown at 150 °C and then to black at 170 °C.[17]
- Manganese violet (Mn(NH4)2P2O7) is a violet material, a popular pigment, turning to white at 400 °C.[17]
References
- ^ "Preparation of Cholesteryl Ester Liquid Crystals".
- ISBN 9789814411035
- PMID 28883374.
- ^ "Solutions | AMERICAN ELEMENTS®". American Elements. Retrieved 2024-02-08.
- PMID 24479772.
- .
- ^ a b c Amberger, Brent & Savji, Nazir (2008). "Thermochromism of Transition Metal Compounds". Amherst College. Archived from the original on 2009-05-31.
- doi:10.1021/ed086p30.
- ISBN 978-1-84755-868-8.
- . Retrieved 15 July 2022.
- ^ "Sol-Gel Vanadium oxide". Solgel.com. Archived from the original on 2018-03-25. Retrieved 2010-07-12.
- ^ "Intelligent Window Coatings that Allow Light In but Keep Heat Out – News Item". Azom.com. 12 August 2004. Retrieved 2010-07-12.
- ^ US 5499597, Kronberg, James W., "Optical temperature indicator using thermochromic semiconductors", issued 1996
- ISBN 047193044X.
- ^ "Thermochromic Garnets". Minerals.gps.caltech.edu. Retrieved 2010-07-12.
- ^ US 4861620, "Method of laser marking"
- ^ ISBN 9789814411035.