# X-ray Diffraction Study of Metallized Polyethylene for Creating Heat Storage Systems
## 1 Introduction  
The shortage of traditional energy sources as well as restrictions on their use in order to protect the environment have already become the present-day reality. This necessitates an increase in the share of renewable energy sources in the general energy balance of many countries. Renewable energy sources can offer actual practical solutions to solve a number of problems. However, they have a number of significant disadvantages. One such drawback is the impossibility of coordinating energy consumption and production schedules, which prevents their rapid implementation in the energy supply structure. This makes it necessary to look for additional technological and arrangement solutions that allow using the renewable energy sources effectively. Technologies for thermal energy accumulation and storage using phase transition materials make it possible to effectively distribute heat consumption over time, which is necessary to solve the main problem of renewable energy (solar, wind), namely the discrepancy between the demand and supply of available energy. In addition to renewable energy, such technologies can also be used in other industries, such as energy efficient construction, waste heat recovery systems, heat pumps and temperature management of lithium-ion batteries [1, 2, 3]. Systems using phase transition materials will store and release heat energy as a result of the phase transition of melting and crystallization of the material with a high latent heat capacity. There are a number of known materials that can be used to accumulate heat during a phase transition. It was proposed to use sugar alcohols as a phase transition material [4, 5]. In [5], the cyclic stability of alcohols was assessed and it was shown that the heat of the phase transition decreased during successive cycles of melting and crystallization, and it also depended on the degree of overheating of the alcohol above the melting point. It is also proposed to use multicomponent systems that have an increased thermal conductivityand heat capacity compared to individual substances. Moreover, such multicomponent systems can be obtained not only via simple mixing [6]. Better results can be achieved using, for example, surface polymerization methods [7]. An example of obtaining such phase transition material based on poly(methyl methacrylate)/expanded graphite/n-octanoic acid is given in [8]. From our point of view, the use of highly crystalline polymers, in particular polyethylene, as phase transition materials is promising [9]. A considerable interest in highly crystalline polymers as a basis for creating heat storage systems is also due to the possibility of using secondary raw materials, which will enhance the positive impact of such heat storage systems on the environmental aspects of their implementation. Highly crystalline polyethylene terephthalate and polypropylene can be pointed out among the recycled polymers, which, together with polyethylene, polyvinyl chloride, and polystyrene derivatives, make up the bulk of polymers used in everyday life and subsequently pollute the environment [10; 11; 12; 13]. As we can see, the vast majority of materials used for heat storage have low thermal conductivity, which significantly limits the implementation and development of technologies based on them [14; 15; 16]. Numerous studies are being conducted to improve the efficiency of using phase transition materials in heat storage systems. The main attention is focused on the design features of heat exchange surfaces, as well as on increasing the thermal conductivity of the phase transition material. In [17], two approaches are simultaneously used: adding longitudinal fins to heat transfer surfaces and using Al\({}_{2}\)O\({}_{3}\) nanoparticles to increase the efficiency of heat transfer. It is shown that the proposed solutions improve the thermophysical properties of the phase transition material and reduce its melting time.