BiTs: Bispectral Camouflage System Based on Switchable Phase Change Materials (sPCM) and Thermochromic Coatings

Traditional infrared (IR) signature management systems for camouflage are based on materials and systems
with low emissivity or a high thermal convection coefficient. Combining these systems with phase change
materials (PCM) opens new technical possibilities for camouflage systems in stationary and mobile
applications. PCM are materials that store or release heat due to their melting or solidification enthalpy.
Because of this phase change, PCM respond to heat input within a certain temperature range with minimal
temperature change. This temperature range, usually only a few °C, is called the ‘latent range.’ The location
of the latent range depends on the PCM used, and the amount of heat stored is in the order of 150 to 300 kJ/kg.
When the melting and solidification behavior of PCM is only dependent on the ambient temperature and
cannot be actively controlled, they are referred to as passive PCM. In camouflage applications, PCM can be used to absorb heat to prevent unwanted heating of the surface of the object. This can be used to influence and reduce the IR signature of an object. The heat can originate from internal sources, such as electronic components, or external sources, such as solar radiation. The advantage of PCM technology over pure insulation materials is that it prevents the interior from overheating if there are internal heat sources. The disadvantages of the passive PCM systems outlined are mainly that they cannot be actively controlled and that they are only effective in the IR wavelength. For this reason, this work focuses on actively controllable bispectral camouflage systems that are currently under development. These systems use switchable PCM
(sPCM) instead of passive PCM. During melting, sPCM behave like passive PCM (i.e., they melt when their
melting temperature is exceeded and absorb heat). However, when the temperature of PCM falls below the
normal phase transition temperature, sPCM do not solidify but enter a supercooled state. Much of the
absorbed heat is still stored in the material, but this is not apparent from the material temperature. The
advantage: the camouflage system can adopt the temperature of the environment and is difficult to detect in
an IR image. Since the temperature of the PCM is the same as the ambient temperature, there is no further
heat loss. Crystallization of the supercooled PCM can then be triggered by an actuation system. When
activated, the supercooled PCM heats up to its melting temperature and releases the heat of fusion to the
environment. This can be useful, for example, if a weapon system or electrical device is generating waste heat
during operation, which is stored in the PCM, and the mission is at a critical stage. The PCM can then be
cooled to ambient temperature and the heat can be dissipated later when the mission is less critical. Another
option is to use this technology in an active deception system.