Miscibility Gap Alloys Research Articles

Role of microstructure in the exploitation of self-healing potential in form-stable composite phase change materials based on immiscible alloys

Metallic Phase Change Materials (PCMs), based on solid-liquid transitions, represents one of the most promising technologies for efficient Thermal Energy Storage (TES), due to their superior thermal conductivity and energy storability per unit volume, but suffer of limited solutions for their handling at the molten state. The use of Miscibility Gap Alloys (MGAs) allows to manage PCM volume expansion and keep it confined when molten, preventing interaction with the environment. A relevant example is provided by the Al-Sn system, where Al covers the role of the high-temperature stable and highly thermal-conductive passive matrix and Sn the active PCM. The alloy can thus be considered a Composite PCM (C-PCM). The response fastness of these systems depends on their thermal diffusivity, subjected to abrupt variations under the presence of discontinuities and damages. In this sense, the authors investigated the possibility to employ molten Sn mobility in a potentially damaged C-PCM for self-healing purposes, aimed to restore, at least partially, the material continuity and thus its thermal diffusivity. Exudation heat treatments above the melting temperature of Sn were performed on sets of Al-40%wt. Sn metallic composites, produced either with powder metallurgy or liquid metal routes, in order to quantify and assess the mobility of the Sn under simulated operating conditions. Exudation tests assess Simple Mixed powders and liquid metal routes sample as the ones with the highest healing potential. Al dissolution and re-deposition was established by EDS analyses as one of the principal Sn mobility mechanisms. Laser Flash Analysis tests, as well as microstructural investigations, were performed on the samples before and after both healing-focused and simulated service heat treatments to evaluate the changes of thermal diffusivity. Healing-focused treatment at 250°C for 1 hour generally displayed a moderate thermal diffusivity recovery and simulated service by shorter cycles between 170°C and 270°C slightly reduce it. The beneficial role of healing focused heat treatments at 250°C for 1 hour suggests that the presence of fully molten Sn phase during service for relatively long time could be beneficial for functional healing. The requirements of suitable Al-Sn microstructures for self-healing purposes, granting at the same time the C-PCM functionalities, i.e., thermal energy storage and form-stability, were set.

Critical assessment of thermal conductivity models for Miscibility Gap Alloy-based composite Phase Change Materials for high temperature Thermal Energy Storage

Miscibility Gap Alloys (MGAs), such as Al-Sn-based systems, provide a viable solution for the development of composite Phase Change Materials (PCMs) for Thermal Energy Storage (TES) purposes. Their successful production depends on the cooling rate imposed to the melt. Finite Element Analyses (FEA), which relies also on thermal conductivity values, represent a powerful tool for the design of the production process. Thermal conductivity, which depends on the arrangement of the phases in the system, also affects the thermal response of the alloy. In the view of evaluating the impact of the phase morphology, the authors adapted some of the models developed for composites and solutions to Al-Sn and Al-Sn-Si-Mg alloys, characterized by broad solidification ranges in terms of composition and temperature and by significantly different phases thermal conductivity. In the fully-liquid range, Filippov and Novoselova model was selected for the description of both alloys. Models that consider sphere-like dispersions give values quite close to the theoretical upper Wiener bound when the high-melting phase is solid. The phase morphology impact is relevant when the solidification range is considered. The resulting arrangement-related thermal conductivity curves are compared to those supplied by CALPHAD-based software and to available literature data.

Effect of Process Parameters on Laser Powder Bed Fusion of Al-Sn Miscibility Gap Alloy

Al-Sn binary system is a miscibility gap alloy consisting of an Al-rich phase and a Sn-rich phase. This system is traditionally applied in bearings and more recently found application as form-stable phase change material (PCM) exploiting solid-liquid phase transition of Sn. A careful choice of production process is required to avoid macro-segregation of the two phases, which have different densities and melting temperatures. In the present study, the additive manufacturing process known as laser powder bed fusion (LPBF) was applied to an Al-Sn alloy with 20% volume of Sn, as a rapid solidification process. The effect of process parameters on microstructure and hardness was evaluated. Moreover, feasibility and stability with thermal cycles of a lattice structure of the same alloy were experimentally investigated. An Al-Sn lattice structure could be used as container for a lower melting organic PCM (e.g., a paraffin or a fatty acid), providing high thermal diffusivity thanks to the metallic network and a “safety system” reducing thermal diffusivity if the system temperature overcomes Sn melting temperature. Even if focused on Al-Sn to be applied in thermal management systems, the study offers a contribution in view of the optimization of manufacturing processes locally involving high solidification rates and reheat cycles in other miscibility gap alloys (e.g., Fe-Cu) with similar thermal or structural applications.

Al-Sn Miscibility Gap Alloy produced by Power Bed Laser Melting for application as Phase Change Material

Alloys based on the Al-Sn system are Miscibility Gap Alloys consisting of two phases: a low-melting Sn phase and a high-melting Al phase. The two phases are almost immiscible in solid state, with minimal miscibility of Al in liquid phase just above the Sn-phase melting temperature. This situation allows the alloy to be used as composite Phase Change Material, where the low-melting (active) phase (Sn) acts as the actual Phase Change Material, storing/releasing latent heat at each thermal cycle across its activation temperature, and the high-melting (passive) phase (Al) plays its main role in fast driving heat within the composite material, thanks to its high thermal conductivity. Thermal response of PCM composites is expected to be faster if the active phase is finely combined to the matrix. Moreover, if the active phase is embedded in the passive phase, leakage phenomena can be prevented and the continuity of the passive phase, acting as matrix, also allows residual structural properties to the composite above the temperature at which the low-melting phase is activated. In these conditions, the composite PCM can be considered as a form-stable composite material. In this paper, the effect of rapid solidification has been investigated as a tool to get suitable microstructure of an Al-Sn alloy in view of its application as a composite PCM. The rapid solidification has been obtained by a Power Bed Laser Melting (also known as Selective Laser Melting) process. In the case of positive results for microstructure, the process will allow to overcome critical points in the manufacturing of composite PCM parts by powder metallurgy, mainly consisting in limitations on size and geometrical features of the parts. Data availabilityThe 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.

Numerical and Experimental Evaluation of Thermal Conductivity: An Application to Al-Sn Alloys

Evaluation of thermal conductivity of composite materials is extremely important to control material performance and stability in thermal applications as well as to study transport phenomena. In this paper, numerical simulation of effective thermal conductivity of Al-Sn miscibility gap alloys is validated with experimental results. Lattice Monte-Carlo (LMC) method is applied to two-phase and three-phase materials, allowing to estimate effective thermal conductivity from micrographs and individual phase properties. Numerical results are compared with literature data for cast Al-Sn alloys for the two-phase model and with a specifically produced powder metallurgy Al-10vol%Sn, tested using laser flash analysis, for a three-phase simulation. A good agreement between numerical and experimental data was observed. Moreover, LMC simulations confirmed the effect of phase morphology as well as actual phase composition on thermal conductivity of composite materials.

Effect of Different Production Processes on Metallic Composite Phase Change Materials for Thermal Energy Storage

Phase Change Materials (PCMs) can be applied in Thermal Energy Storage and Thermal Management systems, exploiting the storage and release of latent heat associated to a phase transition. Among them, metallic PCMs can be used at medium and high temperatures (i.e. above 150°C), storing higher heat per unit volume at higher temperatures with respect to the most widely investigated polymeric and salt-based PCMs. Miscibility Gap Alloys (MGAs) can be used to obtain multiple-phase mixtures in which the active phase (the actual PCM) is mixed to a second, high-melting temperature phase, with negligible interaction between them. These can actually be considered as fully metallic composite materials specifically developed for thermal management. Suitable microstructures can prevent leakage of active phase when the solid-liquid transition occurs, resulting in a form-stable PCM (FS-PCM). However, obtaining these microstructures it is not trivial. The present study focuses on a solid-liquid FS-PCM consisting of a ‘classical’ fully metallic FS-PCM, an Al-Sn based MGAs produced by powder metallurgy. The goal was to evaluate the effect of different production processes on thermal and mechanical behaviour of the PCM. Particularly, powder metallurgy routes including both simple mixing and ball milling were compared and further combined. Moreover, several compression and sintering conditions were considered, also substituting Al powders with Al-alloy powders, in order to optimize the material microstructures in view of suitable thermal and mechanical properties. Finally, the casting route with a rapid solidification approach was investigated for the same alloy.

Fabrication of Al-Bi frozen emulsion alloys due to high-intense ultrasound irradiation

The aim of this study is to develop an ultrasound-assisted process for fabrication of frozen emulsion alloys, which are a type of miscibility gap alloys (MGAs). Al-Bi system was used to produce 2-kg ingots of Al-Bi frozen emulsion alloys in the following two steps: (1) emulsification treatment of Al-Bi melt by introducing high-intense ultrasound vibrations into it in a temperature range of Al-Bi binodal decomposition, and (2) casting of the melt into a book mold. The ingot samples were analyzed with SEM/EDX for the alloy microstructure and bismuth particle distribution. Besides, cold model experiments, thermodynamic and CFD numerical simulations were performed to elucidate the underlying mechanisms of emulsification including droplet fragmentation and acoustic streaming.The results reveal that the alloys with Bi concentration of 4%~9.4% can be fabricated by the proposed technique. Within this range, 90% of Bi particles have a diameter under 10 µm, and the majority of them are incorporated into aluminum grains. Especially, a uniform microstructure is obtained when concentration of Bi is less than 6%. Emulsification was found to occur at the interface between two liquids when cavitation bubbles collapse in the interface vicinity followed by high-velocity micro-jetting which is responsible for the emulsion droplet formation. Another positive effect of ultrasound is acoustic streaming which is responsible for the effective agitation of molten bath preventing the sedimentation of Bi droplets during the ultrasound treatment and casting operation.

Miscibility gap alloys with a ceramic matrix for thermal energy storage

New miscibility gap alloys with a ceramic matrix have been explored in the ZrO2–Al, AlN–Al, AlN-(Al–Si), Al2O3–Al and MgO–Al systems with a view to creating oxidation-resistant macroscopically solid, phase change-enhanced, thermal energy storage materials. Materials were manufactured by mixing the components, pressing and firing at 700 °C under argon. Oxidation tests at 700 °C in air for up to 72 h showed a mix of responses. Despite favourable thermodynamic properties of some published Ellingham diagrams, samples in the ZrO2–Al system underwent a strong exchange reaction, which did not result in a useful material. In the AlN–Al, AlN–(Al–Si) systems, viable MGA-type materials with encapsulated Al particles providing latent heat storage within a solid matrix were formed. There was no degradation of the materials after 24 h of oxidation testing; however, severe oxidation of the Al or Al–Si was observed after 72 h. The Al2O3–Al and MgO–Al systems not only formed a viable ceramic matrix material after firing, but also showed no signs of Al oxidation after 72 h in air at 700 °C. These results open up a promising new series of thermal energy storage materials, some of which appear to have very good oxidation resistance under the test conditions.

Combined powder metallurgy routes to improve thermal and mechanical response of Al−Sn composite phase change materials

Abstract Powder metallurgy processes are suitable to produce form-stable solid−liquid phase change materials from miscibility gap alloys. They allow to obtain a composite metallic material with good dispersion of low-melting active phase particles in a high-melting passive matrix, preventing leakage of the particles during phase transition and, therefore, increasing the stability of thermal response. Also, the matrix provides structural properties. The aim of this work is to combine conventional powder mixing techniques (simple mixing and ball milling) to improve active phase isolation and mechanical properties of an Al−Sn alloy. As matter of fact, ball milling of Sn powder allows to reduce hardness difference with Al powder; moreover, ball milling of the two powders together results in fine microstructure with improved mechanical properties. In addition, different routes applied showed that thermal response depends on the microstructure and, in particular, on the particle size of the active phase. In more detail, coarse active phase particles provide a fast heat release with small undercooling, while small particles solidify more slowly in a wide range of temperature. On the other hand, melting and, consequently, heat storage are independent of the particle size of the active phase. This potentially allows to “tailor” the thermal response by producing alloys with suitable microstructure.

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Powder metallurgy processes are suitable to produce form-stable solid−liquid phase change materials from miscibility gap alloys. They allow to obtain a composite metallic material with good dispersion of low-melting active phase particles in a high-melting passive matrix, preventing leakage of the particles during phase transition and, therefore, increasing the stability of thermal response. Also, the matrix provides structural properties. The aim of this work is to combine conventional powder mixing techniques (simple mixing and ball milling) to improve active phase isolation and mechanical properties of an Al−Sn alloy. As matter of fact, ball milling of Sn powder allows to reduce hardness difference with Al powder; moreover, ball milling of the two powders together results in fine microstructure with improved mechanical properties. In addition, different routes applied showed that thermal response depends on the microstructure and, in particular, on the particle size of the active phase. In more detail, coarse active phase particles provide a fast heat release with small undercooling, while small particles solidify more slowly in a wide range of temperature. On the other hand, melting and, consequently, heat storage are independent of the particle size of the active phase. This potentially allows to “tailor” the thermal response by producing alloys with suitable microstructure.

In situ neutron diffraction studies of operating MGA thermal storage materials

Miscibility Gap Alloys (MGA) store energy as the latent heat of fusion of a discrete component within an engineered multicomponent microstructure. The thermal energy density is augmented by the associated sensible heat of the whole alloy. MGA microstructures are organised so that the low melting point component, acting as a Phase Change Material (PCM), is present as dispersed, non-intersecting particles within a continuous 3-dimensional solid matrix of a higher melting point component. The primary advantages of such an arrangement are the high thermal conductivity, 60–200 W/m K and high energy density (0.25–1.26 MJ/L). Long service life, low maintenance and the resulting simplified infrastructure offered by the ability of the MGA storage materials to exchange heat via conduction alone are further notable benefits. Unlike sensible heat storage materials, temperature measurement alone cannot indicate the state-of-charge of PCM storage since the temperature remains relatively constant throughout the melting or freezing transition. The work reported here addresses this issue through the use of in situ neutron diffraction to determine the relative volume fractions of solid (crystalline) and liquid PCM within the MGA material as a function of varying temperature conditions. The aim is to provide calibration data for state-of-charge thermal models of PCM storage systems. Preliminary analysis of the neutron diffraction results and associated thermal measurements from a series of experiments carried out using the KOWARI diffractometer at the Australian Nuclear Science and Technology Organisation are presented in this work.

Price estimation for miscibility gap alloy thermal storage systems

Miscibility gap alloys (MGAs) are an emerging thermal energy storage material with unique thermal properties that may be of particular interest to the renewable energy industry. In this study, they are compared to state of the art thermal storage technologies on an economic basis, with consideration given to material prices, manufacturing costs, specific deployment infrastructure costs, maintenance schedule cost and the potential for material salvage. Cost estimates are provided for seven different MGAs deployed in three different thermal storage implementations.

Effective conductivity of Cu–Fe and Sn–Al miscibility gap alloys

The effective thermal conductivity of Cu–Fe and Sn–Al miscibility gap alloys over a range of temperatures and volume fractions was determined using the Lattice Monte Carlo method. The Cu–Fe system was found to have an effective conductivity predictable by the Maxwell–Eucken model. The Sn–Al system was not consistent with any empirical model analysed. The microstructures of physical samples were approximated using a random growth algorithm calibrated to electron or optical microscope images. Charts of effective conductivity against temperature for a number of volume fractions are presented for the two alloys. It was determined that the Cu–Fe alloy would benefit from an interstice type microstructure and the Sn–Al would be more efficient with a hard spheres type microstructure. More general conclusions are drawn about the efficiency of the two observed microstructures.

Miscibility gap alloys with inverse microstructures and high thermal conductivity for high energy density thermal storage applications

New high energy-density thermal storage materials are proposed which use miscibility gap binary alloy systems to operate through the latent heat of fusion of one component dispersed in a thermodynamically stable matrix. Using trial systems Al–Sn and Fe–Cu, we demonstrate the development of the required inverse microstructure (low melting point phase embedded in high melting point matrix) and excellent thermal storage potential. Several other candidate systems are discussed. It is argued that such systems offer enhancement over conventional phase change thermal storage by using high thermal conductivity microstructures (50–400 W/m K); minimum volume of storage systems due to high energy density latent heat of fusion materials (0.2–2.2 MJ/L); and technical utility through adaptability to a great variety of end uses. Low (<300 °C), mid (300–400 °C) and high (600–1400 °C) temperature options exist for applications ranging from space heating and process drying to concentrated solar thermal energy conversion and waste heat recovery.

Effects of Crucible Wetting During Solidification of Immiscible Pb-Zn Alloys

Many industrial uses for liquid phase miscibility gap alloys are proposed. However, the commercial production of these alloys into useful ingots with a reasonable amount of homogeneity is arduous because of their immiscibility in the liquid state. In the low-g environment of space gravitational settling forces are abated, thus solidification of an immiscible alloy with a uniform distribution of phases becomes feasible. Elimination of gravitational settling and coalescence processes in low-g also makes possible the study of other separation and coarsening mechanisms. Even with gravitational separation forces reduced, many low-g experiments have resulted in severely segregated structures. The segregation in many cases was due to preferential wetting of the crucible by one of the immiscible liquids. The objective was to analyze the wetting behavior of Pb-Zn alloys on various crucible materials in an effort to identify a crucible in which the fluid flow induced by preferential wetting is minimized. It is proposed that by choosing the crucible for a particular alloy so that the difference in surface energy between the solid and two liquid phases is minimized, the effects of preferential wetting can be diminished and possibly avoided. Qualitative experiments were conducted and have shown the competitive wetting behavior of the immiscible Pb-Zn system and 13 different crucible materials.