Superheated Rubber for Cold Storage

Abstract

Supercooled liquids are still considered to be magic materials that can store heat, which is easily to be released with a suited trigger. Storage of cold is much harder to achieve, e.g., by using superheated liquids or liquifi ed gases, which are naturally hard to handle and require laborious equipment. Storing cold with solid materials is not yet realized because superheated solids are generally assumed to be impossible. [ 1 ] Here we report on designed natural rubber (NR) networks that can store cold and release it on a trigger. The unique base of this effect in the NR networks is the ability to form stable crystals upon strain and to stabilize them at the stretching temperature after the stress is released, also retaining up to 1000% elongation. This is a 5-10 times greater strain storage capacity compared to recently described high-tech shape memory materials. [2][3][4] Increasing the temperature to a certain point, called the trigger temperature, results in a spontaneous collapse of the crystals that affords absorption of a signifi cant amount of heat originating from the disruption of the crystals and the relaxation of the rubber itself. This is the fi rst example of a solid material that is capable of storing a signifi cant amount of cold by bringing rubber in a superheated state. Furthermore, we found that the trigger temperature is tunable for one sample in a broad range by varying the stretching conditions.Energy storage is one of the greatest current scientifi c issues. Most systems store energy in the form of electron transfer processes by the heating of matter or pressurizing of gases. Only a few examples are known that use the enthalpy of phase transition (latent heat) for this purpose. The most popular example is the supercooled liquid sodium acetate trihydrate, which does not crystallize until a nucleus is added as trigger. The resulting crystallization releases heat into the environment. In order to have a cold storage system, one would need a material that conserves a superheated state, i.e., the material stores cold that can be released spontaneously via an external trigger. Up to now, such solid materials have been considered impossible. [ 1 ] An interesting way to cool a material is to release the strain of an elastomer. Because the internal energy Δ U of a rubber remains nearly unchanged during deformation ( Δ U = Δ Q -Δ W = 0), the performed work, Δ W , is directly released as heat, Δ Q , during the stretching process and is taken up during its relaxation. This behavior is well-known for NR, which shows a supercooling of up to 10 K by releasing an elongation of λ = 5 (400% strain). [ 5 ] In contrast to common entropy-elastic materials, NR forms crystals upon strain (strain-induced crystallization (SIC)), which additionally contribute to the cooling of the rubber during relaxation because the necessary heat of fusion Δ h f is taken up from the surrounding. [6][7][8][9][10][11] Altogether the stored cold corresponds to the sum of Δ h f and Δ W .So far only a few materials are known that can s...

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