# X-ray Diffraction Study of Metallized Polyethylene for Creating Heat Storage Systems
## 1 Introduction
The influence of the number of fins and the thickness and nature of the heat transfer surface material on the melting process was studied and modeled for a composite phase transition material consisting of polyethylene wax with expanded graphite particles [18]. Based on the results obtained, the authors state that an increase in the number and thickness of fins allows reducing the time of melting and crystallization. The use of materials with high thermal conductivity, low density, and low specific heat capacity for fins provides a higher heat storage capacity and a reduced melting time. The effect of fin geometry on the solid-liquid phase transition when using heat pipes for heat transfer enhancement was also studied [19]. Similar studies using fins and metal foam to improve heat transfer efficiency were also conducted in subsequent works [20; 21; 22; 23; 24]. Thus, it can be concluded that the main research on the development of heat storage systems using phase transition materials is focused on increasing the efficiency of heat transfer surfaces and is aimed, to a lesser extent, at developing new phase transition materials. The main attention of researchers in this area is focused on modifying the existing materials to improve their thermophysical properties and, as a rule, involves the use of nanosized fillers [25; 26; 27]. We have proposed a different approach to increase the thermal conductivity of the phase transition material: 1. to use granules of a highly crystalline organic polymer as a phase transition material, and 2. To form a continuous metal (copper) coating on the surface of polymer granules. Such a solution, in our opinion, will make it possible to avoid the complex design of heat transfer surfaces, which will result in their reduced metal capacity, weight, and cost. The use of high-tonnage industrial crystalline polymer (polyethylene) as a phase transition material may also be promising [9; 28]. Another advantage of using polyethylene is its high degree of crystallinity, which can reach 95%. Polyethylene is a polymer that crystallizes with the formation of an orthorhombic unit cell consisting of parallel methylene groups that are entirely in trans conformations. The dimensions of polyethylene unit cell: a = 7.406 A; b = 4.939 A; c = 2.547 A [29]. It should be kept in mind that if we are talking about crystalline polymers, we must understand that they are only partially crystalline in structure and consist of crystalline and amorphous regions. The crystalline regions in such polymers are characterized by an ordered arrangement of macromolecules, while amorphous regions consist of randomly distributed macromolecules. This structural feature of crystalline polymers significantly complicates their study, in particular, by X-ray diffraction analysis. The obtained X-ray diffraction results include peaks from the crystalline regions and halos from the amorphous phase, which are superimposed. In addition, the diffraction curves of crystalline polymers depend on the ratio between the crystalline and amorphous phases in the bulk of the solid polymer. Thus, wide-angle X-ray diffraction can be used to study the crystal structure of a polymer. The position of peaks of the crystalline phase as well as their height, width, and shape provide information about the unit cell parameters, crystallite size, degree of crystallinity, and other characteristics [30]. Wide-angle X-ray diffraction can be used to study both the structure of crystalline polymers and the changes in this structure under the influence of various external factors. The main task is to choose the best optimization method, which will allow decomposing the obtained experimental curve into crystalline peaks and an amorphous component. The analysis of the wide-angle X-ray diffraction curves of crystalline polymers involves the correct decomposition of these curves into crystalline peaks and amorphous components. Such reliable and unambiguous decomposition is a critical step for calculating the crystallinity of polymers [30; 31].