Materials Today Energy
Ball-milled Al–Sn alloy as composite Phase Change Material
Graphical abstract
Introduction
Phase Change Materials (PCMs) are materials in which a phase transition occurs under specific conditions and causes a significant change in at least one material property. Considering thermal properties, they can be applied as Thermal Energy Storage (TES) systems, storing the latent heat associated to the phase transition. This approach is referred as latent heat storage (LH TES) and Kuta et al. [1] consider it as the most efficient way to store thermal energy; in facts, according to Fiedler et al. [2], this approach allows to store a higher energy density using less material. PCMs can be applied in several different fields. Kuta et al. [1] reported PCM-based building envelopes, which can keep proper internal conditions storing or releasing energy depending on external conditions, thus reducing the use of air conditioning or heating. Navarrete et al. [3] proposed to use self-nanoencapsulated metal/metal alloys to enhance thermal property of a Heat Transfer Fluid, obtaining a nanofluid. Considering solar energy sector, Reed et al. [4] developed a C-(Al–Si) system to be applied in concentrated solar power plants. In addition, Nazir et al. [5] reviewed many other applications, like thermal management in photovoltaic cells, smart textiles and cooling systems in electronic devices.
According to Sun et al. [6], the most important properties of PCMs for TES are the transition temperature and the latent heat associated to the transition; at the same time, good thermal stability and reliability of thermal performance. Moreover, Wei et al. [7] mentioned as desired properties also high thermal conductivity, high specific heat and high density. For some applications, also good mechanical properties could be required.
Although every phase transition (gas-liquid, solid-gas, solid-liquid, solid-solid) could be exploited to store energy, usually only solid-solid and solid-liquid transition are applied, due to the large volume changes associated to gas transitions. Among them, solid-liquid transitions have generally higher latent heat with respect to solid-solid ones; however, the liquid phase needs to be enclosed in some way to avoid leakage. The main properties of potential PCMs based on solid-liquid transition are shown in Table 1. Among the wide range of materials that have been considered as PCMs for TES and, more in general, for thermal management of systems, metallic PCMs are so far the less developed class; Mohamed et al. [8] ascribed this fact to their low heat of fusion per unit weight. Nevertheless, as highlighted by Zhou and Wu [9], they have higher operative temperature ranges (including phase transition) which makes them suitable for high-temperature applications. Moreover, they have higher latent heat per unit volume, which makes them attractive for compact devices.
In addition to the ‘classic’ goals of maximizing thermal response and stability, the design of metallic PCM can be based on the possibility to exploiting the structural properties typical of metals. Metallic solid-liquid PCMs are usually produced by encapsulation of the active phase in a passive-phase capsule, whose size ranges from millimeters to nanometers. However, Zhou and Wu [9] highlighted that encapsulated PCMs may oxidize or deteriorate particularly at high temperatures, reducing durability and energy storage performance; moreover, Pielichowska and Pielichowski [13] observed that encapsulation process can be complex and expensive.
To overcome these issues, Zhou and Wu [9] suggest to use of solid-solid PCMs. Another viable alternative is the choice of alloys which behave as form-stable PCMs (FS-PCMs), i.e. materials in which the active phase (the actual PCM) is embedded in a higher-melting passive matrix, which remains solid at all stages preventing leakage and keeping structural properties, as defined by Pielichowska and Pielichowski [13]. According to Sugo et al. [14], thermal energy storage at high temperature is most efficient and compact using two-phase mixtures in which phases are completely immiscible at solid state: in this way, it is possible to prevent the formation of solid solutions or intermetallics, keeping the composition of the two phases stable over time or with thermal cycles. So, Sugo et al. [14] suggested that alloys with these features can be obtained exploiting miscibility gaps in phase diagrams and so they are called Miscibility Gap Alloys (MGAs); examples of MGAs are Al–Sn, Fe–Cu and Fe–Mg alloys. Among them, Sugo et al. [14] recommended Al–Sn based alloys as one of the metallic systems to be applied as PCMs for high temperature energy storage, characterized by activation temperatures close to 230 °C (232 °C for pure Sn).
Al–Sn based alloys are often used as bearing alloys, thanks to their excellent tribological properties; the target microstructure for this application [15] is similar to the one desired for metallic PCMs. A previous study on PCMs by Gariboldi and Perrin [16] focused on an Al alloy with 20% volume content of Sn, which corresponds to about 40% mass content; the same alloy composition was considered in this paper, as shown on the Al–Sn phase diagram in Fig. 1. This Sn content is higher than in bearing alloys, which is usually lower than 30 mass%; for example, Liu et al. [15] used 20% mass of Sn and Noskova et al. [17] used 30% mass. The goal of the study was to verify if it is possible to use well-known and proven industrial processes for bearing alloys also to produce FS-PCMs.
In practice, it is not so trivial to obtain the target microstructure, since natural cooling results in an opposite structure (matrix surrounded by active-phase particles). Among the possible solutions listed by Liu et al. [18], powder metallurgy is a group of manufacturing techniques which allows to obtain MGAs with ‘inverse microstructure’. The process involves mixing, compression and finally sintering of powders; in this way, the matrix material can form a continuous body which completely encapsulates the active phase, preventing leakage in operative conditions. As a matter of fact, the resultant morphology of the active phase depends on the initial shape and size distribution of the powdered components, as well as on process parameters. In the abovementioned previous study by the research group [16], relatively fine Sn powders were simply mixed. Ball milling (BM) is the most common comminution technique to obtain fine particles from solids and it is often used in the production of bearing alloys. When a mixture of metal powders is subjected to ball milling, powder particles are mixed and reduced in size at the same time, resulting in a homogeneous alloy; therefore, the whole process is called Mechanical Alloying [19]. The expected difference between simple mixed and ball milled microstructures was a finer structure. Nonetheless, ball milling is more complex and expensive than simple mixing process. A preliminary check on ball-milling production route by Confalonieri et al. [20] suggested promising microstructures, independently on the size-distribution of Sn powders adopted. The innovative feature of the research presented in this paper is the insertion of ball milling of Sn powders into an overall production cycle to obtain FS-PCMs. Compression as well as sintering temperatures were selected as parameters of these process stages. Their effect on microstructures, thermal response and mechanical properties of the FS-PCMs was investigated before and after simulated service, to check the material properties stability.
Section snippets
Materials and experimental procedures
Two Sn powders and an Al powder were selected for the present investigation. The Al powder was an atomized high purity Al (>99.7 mass percentage) powder with diameters smaller than 45 μm (ECKA Granules GmbH, Germany). The two types of Sn powders (Metalpolveri S.r.l) having different particle size and distribution were considered: powder SN (Sn > 99.9 mass percentage), characterized by very fine particle-size distribution and good homogeneity, and powder 106 (Sn > 99.7 mass percentage),
Microstructure
SEM-BSE images of samples obtained using the same process conditions, i.e. compression at 220 °C, but containing different Sn-powder type (106 or SN) are shown in Fig. 3. At low magnification, both samples show coarse homogeneous Sn particles ranging from a few microns to about 20 μm (bright), coarse Al-rich particles (darker) and regions of intermediate color; in high magnification micrographs (Fig. 3), the intermediate regions can be resolved as nanometric particles of Sn and Al. Stationary
Discussion
As far as the microstructure is concerned, also in the case of hot compression, the effect of Sn-powder type is limited to the residual presence of some slightly coarser Sn particles in samples produced using the most heterogeneous Sn powder referred as 106, while most of the volume of the form-stable PCMs produced was characterized by the phase refinement and homogenization brought about by ball milling.
The results from XRD allow to state that the high pressures arisen during ball milling did
Conclusions
- 1.
Al–40Sn mass percentage samples produced by hot compression at 220 and 240 °C of ball-milled Al–Sn mixed powders show minor effects of the initial size distribution of Sn powders.
- 2.
Hot compressed specimens were characterized by fine microstructural features and by the presence of only pure Al and pure β-Sn phase. A phase with almost eutectic composition starts forming during the hot compression. Microstructural features become finer both after sintering processes at 250 °C and 500 °C and/or after
Author contributions
Chiara Confalonieri: Investigation, Formal analysis, Writing – Original Draft, Writing – Review and Editing. Aldo Tommaso Grimaldi: Investigation, Writing – Review and Editing. Elisabetta Gariboldi: Conceptualization, Methodology, Investigation, Writing – Review and Editing.
Data availability
The raw data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. The processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors would like to thank for their help in characterization tests Paola Bassani, Enrico Bassani and Maxime Perrin.
The Italian Ministry of Education, University and Research is acknowledged for the support provided through the Project “Department of Excellence LIS4.0 - Lightweight and Smart Structures for Industry 4.0”.
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Present address: Istituto Nazionale di Fisica Nucleare (INFN) – Laboratorio Acceleratori e Superconduttività Applicata (LASA), Via Fratelli Cervi 201, 20090 Segrate (Milan, Italy).