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Thermochromism

Perovskite materials

Rare-earth perovskites with the generic formula RENiO3 where RE (rare earth): La, Pr, Sm, Nd, etc. and M: Co, Ni, Mn, etc. are distorted perovskites.

These compounds, which exhibit metal to insulator (MI) transition versus temperature are infrared (IR) transparent below the MI transition temperature (TMI), and become IR reflecting above. TMI rises systematically as the rare-earth size becomes smaller, i.e., as the distortion of the perovskite with respect to the ideal structure increases.

Our researches concern the simplification of the synthesis conditions of these materials (which are sometimes drastic) as well as the electrical and optical characterizations. Concerning the applications, TMI can be easily modulated giving this type of system attractive for developing the thermal regulation. For example, it is possible to passively regulate the temperature of a satellite as a function of its exposure to the solar radiation:

 

 

 

A large portion of the heat exchange between an object in space and the environment is performed through radiation, which is in turn determined solely by the object surface properties.

The modulating device is attached to the object surface and thus provides a thermal window that can adapt to the changing conditions in orbit.

The smart radiator device (SRD) based on perovskite films is one of the most important structures of the functional thermal control surfaces, being lighter, more advanced and without moving devices.

Vanadium dioxide (VO2) films

 Vanadium dioxide (VO2) is a thermochromic material capable of changing its optical properties as a function of temperature.

Thus, at room temperature it shows a semi-conductor state and crystallises in a monoclinic type structure. Above the temperature of 68°C the material adopts a quadratic structure, the electronic gap is closed, VO2 then becomes metallic and its emissivity increases.
Since 2009, the Institut Jean Lamour has been working closely with the solar thermal industrial company Viessmann Faulquemont SAS (France leader in solar thermal panels) in order to improve durability of its solar thermal panels while maintaining their performance. The change in optical properties of VO2-based flat panel is shown in the following figure:

<font size="1"><i>Thermal images of the sensor prototype “Sol Pro select” (marketed in autumn 2015). A solar thermal collector presents a high absorption in the solar spectrum (95%) but also a very low emissivity around 5% in the mid-infrared, which is the case when the meander of the panel is crossed by water at 18 °C (image in blue on left side). When the meander is crossed by hot water at
88 °C (image in orange) emissivity increases from 5 to 40%, avoiding the overheating of the solar thermal panel which rejects the waste heat at high temperature.</font></i>

Alliages à mémoire de forme

Les alliages à mémoire de forme (AMF) sont des matériaux pouvant subir une transformation de phase martensitique sous l’effet d’une variation de température. Cette transformation martensitique, caractérisée par l’apparition de variantes, est réversible et dépend de nombreux paramètres aussi bien externes (température, contrainte), qu’internes (structure cristalline, longueurs internes). Les études que nous avons menées ont montré le potentiel des couches minces AMF car l’effet de la rugosité, qui affecte la dispersion de la lumière, permet de moduler la réflectance du film mince.

<font size="1"><i>A chaud la surface est très douce et la lumière réfléchie est concentrée autour de l'angle spéculaire de réflectance, et la surface apparaît brillante. Lorsque le film refroidi les variantes apparaissent et à mesure que la rugosité augmente, les réflexions deviennent de plus en plus diffuses, et la surface apparaît mate/terne.</font></i>

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