Thermochromism

As thermochromism is the ability of certain substances to change the color when heated. This process is reversible, ie after cooling they take back to their original color. Reason for these color changes are changes in the molecular or crystal structure. This behavior is known, inter alia, the inorganic compounds of rutile, and zinc oxide, which change their color when heated strongly from white to yellow.

Thermochromism is more common in organic compounds. Such chromophoric organic compounds that also exhibit thermochromic behavior change when the temperature changes its molecular state. For the first time such behavior has been 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 first found by the work of Harnik & Schmidt (1954) and Mills & Nyburg (1963).

Examples

In addition to the above-mentioned inorganic compounds of rutile, and zinc oxide have also some gems thermochromic behavior, as well as some compounds of mercury, in particular mercury ( II) iodide.

9,9 '- Bixanthyliden and 10,10'- Bianthronyliden are typical representatives of organic compounds that change under the influence of color temperature. The color change is here connected with the change in the molecular structure. The halves of the molecule at the central double bond in these compounds change when moving from a pyramidalization to a twist.

The indicator dye bromothymol blue, embedded in a pH-dependent polymer matrix may have under certain circumstances thermochromic properties. The matrix changes when the temperature changes the pH and thereby causes a color change of the indicator. Such a system was developed at the Fraunhofer Institute for Applied Polymer Research. The lithium chloride-containing polyether is green in the temperature range from -5 ° to 33 C and is also yellow.

Bis ( diethylammonium ) Tetrachlorido cuprate (II ) shows at about 53 ° C, a solid-solid phase transition associated with a color change from green to yellow. This is due to the change in tetrachloro - cuprate complex geometry.

All solvatochromic dyes (the solution color is dependent on the solvent ) change color when heated or cooled, the solvent. The special feature of this phenomenon is that the color change takes place continuously and reversibly.

Application

Technically, this effect is used for temperature measurement and thermal color paints. May be estimated using a color scale, due to the discoloration of the coating, the surface temperature of an object. Common are those safety paintings in the chemical industry.

Another area of ​​application is the packaging technology. Food packaging can for example show a break in the cold chain or storage at the wrong temperatures on the label and thus inform the consumer about the quality of the product.

A new promising field of application for thermochromic are adaptive thermochromic sunglass lenses. These are laminated glass in which two glass sheets are laminated together with a film. The film comprises thermochromic substances. Heats up the thermochromic composite glass, the film turns a. It comes less light through the laminated glass to the inside. The rooms do not heat up so much. In addition, the glare of the sun is reduced. A free view through the thermochromic composite glass is always possible. Typically, the light transmittance of a thermochromic composite disc varies as a function of wafer temperature anywhere between about 80% and 15%. Thermochromic lenses are among the so-called intelligent glasses.

There are also known examples from the budget. These include:

• Magic cups, change an applied motif depending on the level of the ( hot ) drink
• Aquariums thermometer, which are outside the thermal foil glued onto the glass pane of the basin

Thermal wax-based crayons are available with specified transition intervals, such as to indicate warming qualitatively and spatially on metal or electronic parts. To locate the overheating of a bearing to narrow down the heating by welding or brazing to protect a component from overheating or to obtain appropriate temperatures for hardening and tempering or hardening of adhesive.

Marks by ASFINAG in Austria - are applied to highway pavements by colored danger of black ice View - tentatively for the first time in autumn 2013 at two locations in Styria. From each colorless gray snow crystal logo is blue and the red triangular frame, with the first changes already at 4 ° C to be recognizable.

Other

Digital thermometer with color scale

Around 1975 strip thermometer came on, at least about 2 cm wide and 15 cm high, with superimposed fields, eg 15-25 ° C reaching, all fields were black but one was transparent and unpainted, the measure of the prevailing temperature. You may also 1-2 neighboring fields of the rigid plastic film were partially lit. The colors of the fields ranged from blue for the lowest and rainbow similar to red and purple for the highest temperature. Not Thermochromism but a number of special liquid crystals, which were transparent in each case at a certain degree Celsius in a narrow temperature interval were behind the functioning mostly in reflected light display.

Heat traces

Burning of skin, burn in wood and paper behind, depending on the temperature and duration of specific tracks that document the causes visible and legible. Thermoplastics and metals begin to live - even triggered by thermal expansion alone - or deform the dead weight to flow, thermosets discolor, blister on, jump, because decomposition products outgas. Tire rubber and asphalt melt under heat and friction show dark spots. Not tempered (bottles or window ) glass jumps. Crude protein denatures: cloudy egg white and meat and solidify. Snowflakes pack together to form a layer of ice. Smooth material surfaces can be matt, painted glass or enamel powder melts together into a glossy layer. Shrink tubing shrinks, a hot deep drawn yogurt containers made of polystyrene is flat, because he has the initial shape "noted". Dosimeterkristalle discharge flashing.