How to mimic the thermo-induced red to green transition of ruby with control of the temperature via the use of an inorganic materials blend?

The design of new smart pigments with a strong colour change in a given temperature (T) range turns out to be often very time-consuming and costly. Based on this observation, we propose here a cheap solution consisting in the mixing in appropriate amounts of a semiconductor and a d-d transition based pigment. The former exhibits a strong temperature dependence of its optical gap while the second presents a null or moderate evolution of the positioning of its absorption band with T. This results in the generation of various reversible thermochromic effects with control of both the temperature transition and the hue in the low and high temperature stages. An example will be devoted to the reproduction of the Al2−xCrxO3 corundum behaviour in the 20–200 °C domain with a Bi2O3–LiCoPO4 blend. The proposed recipe, easily extrapolated to a wide range of colour changes, is only based on the well known subtractive colouring model, and the evolution of the chromatic coefficients of each ingredient with T. This opens up the door to the development of a new generation of thermochromic pigments with unsaturated colours for potential applications as temperature indicators.

► Thermochromic pigment mixtures.
► Charge transfer in semiconductors versus d-d transitions.
► Recipe to increase the intensity of the colour change with temperature.