## 1 Introduction
### Long-Term Performance  
Besides the use of water/ice for cooling, specifically of food and beverages, the first use of PCM in a technical application was probably in the 1930s when [PERSON] investigated the use of PCM to store solar heat available during summer and use it for space heating in winter [3]. To store heat, containers with a large amount of Na\({}_{2}\)SO\({}_{4}\).10H\({}_{2}\)O as PCM, a salt hydrate called Glauber salt, were used. Already in the first year, a significant decrease in the performance of the heating system was observed. It was attributed to a decrease in the heat storage capability of the PCM with repeated phase change. Besides that, the metallic containers used showed leakage due to corrosion by the PCM contained. Not surprising, investigations on long-term performance have a long history after this memorable start. Leakage of PCM from containers due to corrosion is a matter of material compatibility, thus not a PCM-specific problem but rather a general topic of materials research--specifically, development and testing. Testing a combination of a PCM with another material is usually done by immersing a sample of the material in the PCM or by containing the PCM in a container with walls made of the other material. Testing is done at application-typical conditions--specifically regarding temperature and atmosphere--and sometimes including change of phase of the PCM but more commonly with the PCM in the liquid phase all the time. The understanding of mechanisms behind compatibility problems has increased in the past decades, as well as options to avoid them. Still, the variety of PCMs and materials used in contact with them for containment or heat exchange continues to make this a field with demand for more R&D. Investigations covering compatibility problems with salt hydrates and metals, or paraffins and plastics, are, e.g., described in [4; 5; 6], with the last one including a list of different studies. Compatibility is often tightly connected to leaking, such that leaking is avoided if a suitable material combination is found. For most cases, optionsexist and just have to be identified. In contrast, the long-term performance of the PCM itself is more difficult. Testing the long-term performance of a PCM, specifically on full phase change, commonly called cycling, is usually done at application-typical conditions with regard to temperature and time, or just number of cycles. Typical causes of the poor cycling stability of a PCM are phase separation or the deactivation of nucleators. While the understanding of the mechanisms how nucleators work is still low, and also of their deactivation, a basic understanding of the mechanisms behind phase separation has existed for several decades already. While pure materials generally change phase between a liquid and a solid phase of same composition, mixtures of chemical compounds can form phases with different mixing ratio, so different composition. Consequently, the amount of material with correct composition and heat storage capability is reduced. If phases have different density, gravitation can lead to macroscopic separation, which is worsening the problem. Shallow containers, addition of materials for gelling or thickening, or mixing, can limit phase separation. While phase diagrams can be used to show the possible degree of phase separation [8], they cannot show the dynamics. So, preferred is to develop PCMs that have no risk of phase separation, which means PCMs that change phase between one single phase and another, just like pure materials. But phase separation and deactivation of nucleators are not the only causes of a poor cycling stability. A high phase change enthalpy in a narrow temperature range is typical if the solid phase is crystalline, however, not if it is amorphous. For example, water in small amounts or if cooled fast can stay amorphous. For polymers, a volume fraction being generally amorphous is typical; it increases with the cooling rate.