Thermochromism
As Thermochromism is defined as the change in temperature the property of certain substances, color change. This process is reversible, i. H. after cooling, they return to their original color. The reason for these color changes are changes in the molecular or crystal structure. This behavior is known, among other things, for the inorganic compounds rutile and zinc oxide , which change their color from white to yellow when heated strongly.
Thermochromism is more common in organic compounds . Such chromophoric organic compounds, which also show thermochromic behavior, change their molecular state when the temperature changes . Such behavior was first observed in 1909 by the Prague chemist Hans Leopold Meyer . A plausible explanation of this phenomenon on the example of Bixanthyliden - and Bianthronyliden - derivatives but was found only by the work of Harnik & Schmidt (1954) and Mills & Nyburg (1963).
Examples
In addition to the above-mentioned inorganic compounds rutile and zinc oxide, some gemstones also exhibit thermochromic behavior, as do some mercury compounds , in particular mercury (II) iodide and silver tetraiodomercurate (II) .
9,9'-Bixanthyliden and 10,10'-Bianthronyliden are typical representatives of organic compounds that change their color under the influence of temperature. The change in color is linked to the change in the molecular structure. The halves of the molecule on the central double bond in these compounds change in the transition from pyramidalization to torsion .
The indicator dye bromothymol blue , embedded in a pH-dependent polymer matrix , can under certain circumstances have thermochromic properties. The matrix changes the pH value when the temperature changes , thereby causing the indicator to change color. Such a system was developed at the Fraunhofer Institute for Applied Polymer Research. The lithium chloride-containing polyether matrix is green in the temperature range from −5 to +33 ° C and turns yellow above this.
Bis (diethylammonium) tetrachloridocuprate (II) shows a solid-solid phase transition at approx. 53 ° C combined with a color change from green to yellow. The reason for this is the modified tetrachloro-cuprate complex geometry.
All solvatochromic dyes (the color of the solution depends on the solvent) change color when the solvent is heated or cooled. The special thing about this phenomenon is that the color change is continuous and reversible.
application
This effect is used technically for temperature measuring paints and thermal lacquers . The surface temperature of an object can be estimated using a color scale due to the discoloration of the paint. Such safety coatings are common in the chemical industry .
Thermochromatic colored dots applied to heat-shrinking parts such as shrink tubing indicate whether the minimum required temperature has been reached (shrink indicator).
Another area of application is packaging technology. For example, food packaging can indicate a break in the cold chain or storage at incorrect temperatures on the label and thus inform the consumer about the quality of the product.
A new, promising area of application for thermochromism is adaptive thermochromic sun protection glasses . These are laminated glasses in which two panes of glass are laminated together with a film. The film contains thermochromic substances. If the thermochromic laminated glass warms up, the film becomes colored. Less light comes in through the laminated glass. The rooms do not heat up as much. It also reduces the glare from the sun. A clear view through the thermochromic laminated glass is always possible. The light transmission of a thermochromic composite pane typically changes continuously between about 80% and 15% as a function of the pane temperature. Thermochromic glasses belong to the so-called intelligent glasses .
There are also well-known examples from the household. This includes:
- Magic mugs that change an applied motif depending on the level of the (hot) drink
- Aquarium thermometers , which are stuck to the outside of the glass pane of the pool as thermal foil
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Thermal crayons based on wax are available with specified turnover intervals, for example to display heat on metal or electronic parts qualitatively and spatially. To locate the overheating of a bearing, to limit the heating by welding or soldering, to protect a component from overheating or to reach suitable temperatures for hardening and tempering or setting of adhesive.
Markings from ASFINAG in Austria - on a trial basis for the first time in autumn 2013 on two locations in Styria - applied to motorway lanes are to be colored to indicate the risk of black ice . From each colorless gray, the snow crystal logo should become blue and the triangular frame red, whereby the first changes should already be recognizable at 4 ° C.
Other
Digital thermometer with color scale
Around 1975, strip-shaped thermometers appeared, at least about 2 cm wide and 15 cm high, with fields on top of one another, e.g. B. ranging from 15 to 25 ° C, all fields showed black but one showed transparent and monochrome the measure of the prevailing temperature. It is possible that 1 to 2 neighboring fields of the stiff plastic film were partially illuminated. The colors of the fields ranged from blue for the lowest and rainbow-like to red and purple for the highest temperature. Not thermochromism, but a series of special liquid crystals, each of which became transparent at a certain temperature within a narrow temperature range, were behind the display, which usually works in incident light.
Heat traces
Burning of skin, burning into wood and paper leave specific traces depending on the temperature and duration, which document the causes visibly and legibly. Thermoplastics and metals begin to deform and flow under tension - triggered by thermal expansion alone - or under their own weight, thermosets change color, form bubbles, crack because decomposition products outgas. Tire rubber and asphalt melt under frictional heat and show dark spots. Glass (bottle or window) that is not resistant to temperature changes cracks. Raw egg white denatures: egg white and meat become cloudy and solidify. Snowflakes pack up to form a layer of ice. Smooth material surfaces can become matt, decorative glass or enamel powder melts together to form a glossy layer. Shrink tubing shrinks, a hot, deep-drawn polystyrene yogurt cup becomes flat because it has "memorized" the original shape. Dosimeter crystals discharge flashing.
literature
- Hans Meyer: About new reduction products of anthraquinone. In: Reports of the German Chemical Society. Volume 42, 1909, pp. 143-145.
- Hans Meyer: About new derivatives of anthraquinone (first communication about two-ring quinones). In: Monthly magazine for chemistry. Volume 30, 1909, pp. 165-177.
- A. Schönberg, A. Mustafa, ME El-Din Sobhy: Thermochromism of Dixanthylenes. In: Journal of the American Chemical Society . Volume 75, 1953, pp. 3377-3378.
- E. Harnik, GMJ Schmidt: The structure of overcrowded aromatic compounds. Part I. A preliminary survey. Journal of the Chemical Society . (London). 1954, pp. 3288-3294.
- E. Harnik, GMJ Schmidt: The structure of overcrowded aromatic compounds. Part II. The crystal structure of Dianthronylidene. In: Journal of the Chemical Society. (London). 1954, pp. 3295-3302.
- FH Herbstein, GMJ Schmidt: The structure of overcrowded aromatic compounds. Part III. The crystal structure of 3,4-benzophenanthrenes. In: Journal of the Chemical Society. (London). 1954, pp. 3302-3313.
- JFD Mills, SC Nyburg: Thermochromism and related effects in Bixanthenylidenes and Bianthronylidenes. I. Crystal structure analyzes. In: Journal of the Chemical Society. (London). 1963, pp. 308-321.
- JFD Mills, SC Nyburg: Thermochromism and related effects in Bixanthylidenes and Bianthronylidenes. II. General observations. In: Journal of the Chemical Society. (London). 1963, pp. 927-935.
- KA Muszkat, R. Korenstein, S. Sharafy-Ozeri: Photochromism and thermochromism through partial torsion about an essential double bond. Structure of the B colored isomers of bianthrones. In: Journal of the American Chemical Society. Volume 95, 1973, pp. 6177-6181.
- A. Seeboth, J. Kriwanek, R. Vetter: Novel chromogenic polymer gel networks for hybrid transparency and color control with temperature. In: Advanced Materials . Volume 12, No. 19, 2000, pp. 1424-1426.
- R. Lemke: Thermochromism in arylidene isophorone derivatives. In: Chemical Reports. Volume 103, 1970, pp. 3003-3005
Web links
Individual evidence
- ↑ Patent application DE102006049633A1: Flat system with adjustable color. Registered on October 20, 2006, published on April 24, 2008, applicant: Fraunhofer Society for the Promotion of Applied Research e. V., inventors: Florian Rothfuss, Wiltraud Wischmann, Matthias Rapp.
- ↑ hellermanntyton.com: Catalog 2018/19 Kabelisolierung 2.1, page 245 [PDF], accessed on July 13, 2020
- ↑ Gesimat GmbH Germany, manufacturer of thermochromic glass
- ↑ ORF.at: Pilot experiment: The autobahn becomes colorful in the cold , from November 23, 2013.