Self encapsulated phase-change alloys and process for the production thereof

Abstract

There is described a method which allows producing, in a single operation, composite particles formed from at least two distinct metal phases, one having a higher melting temperature which forms the shell of the particle, and one having a lower melting temperature in the core of the particle. These particles are usable as phase-change materials for storing and releasing energy.

Description

[0001] “SELF ENCAPSULATED PHASE-CHANGE ALLOYS AND PROCESS FOR THE PRODUCTION THEREOF”

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to the field of materials for storing and releasing energy, in particular using phase-change materials.

[0004] BACKGROUND ART

[0005] Phase-change materials are materials capable of absorbing or releasing latent heat, with small temperature variations, during a phase transformation. Due to such properties, they are of practical interest in the industrial and energy fields as thermal storage systems, temperature regulation systems, etc.

[0006] Phase-change materials are commonly referred to in the industry by the acronym PCMs, which will also be used in the remainder of the description.

[0007] These materials can be of the organic, inorganic (salts) or metal (single metals or alloys) type.

[0008] PCMs of the organic type are described, for example, in patent applications EP 3760689 Al and KR 20120031765, while systems using salts as PCMs are described, for example, in patent applications WO 2014 / 100096 Al and US 2014 / 0197355 Al.

[0009] However, metallic PCMs are preferable because they allow for use at higher temperatures than organic ones and in many cases also than inorganic ones; however, for this reason they require containers having higher chemical and mechanical resistance, which are not corroded and / or degraded during use.

[0010] In early practical applications, the containers were relatively large in size, and a large amount of PCMs was present therein; however, this approach has been abandoned in particular due to the slow heat absorption and release kinetics, due to the decrease in heat transfer efficiency from the center to the periphery of the PCM mass as the size thereof increases.

[0011] In order to overcome the problem of poor heat transfer efficiency in large-sized PCM elements, patent application WO 2023 / 115098 Al describes a system consisting of blocks of composite material, in the form of miscibility gap alloys, in which particles of a low-melting alloy are embedded in a higher melting matrix, in which channels for fluids used for heat transfer are obtained. However, this system still has some practical problems, such as the need for spacers to avoid that thermal expansions of the blocks (the expansions being greater the larger is the size of the blocks) lead to deformations and eventually to the breakage of the system; this requires, in turn, the use of tubes for connecting the channels between adjacent blocks. The system also requires containment walls of the set of blocks, thus ultimately resulting in a system of complex construction.

[0012] In view of the difficulties in operating with elements made of PCMs of relatively large size, research in the industry has been directed towards materials in the form of granules, particles, or microparticles, commonly of substantially spherical shape. These granules, particles, or microparticles have a shell / core structure, in which the shell material has a higher melting temperature than that at which the phase change of the core material occurs, thus allowing for the confinement of the latter when it is in the liquid phase.

[0013] Obviously, the shell material also has a melting temperature, and thus in principle it could also be defined as a phase-change material; therefore, the definition of phase-change material is linked to the operating temperature at which the specific system is used, which must be intermediate between the melting temperature of the core and that of the shell. For simplicity, in the following description, the core material is defined as an “active material”, while the definition “PCM particles” is intended to mean elements in divided form with a volume of less than 10 cm3, thus comprising all sizes which are indicated in the field by the terms granules, particles, and microparticles.

[0014] An example of such a system is disclosed in patent application CN 112226208 A, which describes a hybrid system in which the active material consists of low melting alloys based on gallium or bismuth, possibly in the form of powders mixed with paraffins, and the shell is formed from a synthetic or natural polymer or resin; gelatin, sodium alginate, or formaldehyde- based or poly(meth)acrylic resins are given as examples of materials for the shell. This system has the limitation that the produced particles can be used only up to temperatures compatible with the polymer shell, typically not above about 250 °C.

[0015] In order to take advantage of the potentials of higher melting alloys, the shell of the particle must entirely consist of materials with higher melting temperatures than those of organic materials, and typically above 500 °C. The solutions adopted so far suggest the confinement of metal active materials in capsules consisting of different metals from the active one, or of oxides or other ceramic materials, by means of processes of thermal and / or chemical type which are performed downstream of the production of the active material spherules.

[0016] Patent application US 2012 / 018116 Al describes a method for producing PCM particles, in which active material spherules are suspended in a gas stream and coated with a first material which acts as a binder for powders of a second material deposited later; the first material is then thermally decomposed, leaving a sintered shell consisting of the second material on the surface of the active material spherule. This method is clearly very complex to be implemented.

[0017] The articles “Micro- and nano-encapsulated metal and alloy-based phase-change materials for thermal energy storage”, Zhu S. etal., Nanoscale Advances, vol. 3 no. 13 (2021), and “A brief strategy for designing self-encapsulated Al-Si base phase change materials with high thermal energy storage performance” Wang C. etal., Journal of Energy Storage 62 (2023), describe methods in which an oxide layer is grown by means of thermal oxidation on the surface of a spherule consisting of the active material alone. These techniques involve a two-step production process, and the oxide layer has a low thermal conductivity, slowing the heat exchange between the core of the particle and the outer environment.

[0018] Patent application CN 115873562 A describes a method for producing PCM particles, which consists in depositing a layer of silazanes on spherules of Al-Si alloys by operating in an organic solvent solution; by means of a subsequent heat treatment, silazane decomposes forming a SiCN shell. In this case, in addition to requiring again a two-step production procedure, the entire process must be conducted under an inert atmosphere due to the instability to air and humidity of silazanes.

[0019] Patent US 4657067 describes PCM particles obtained by solidification of a liquid of Al- Si composition; the resulting particles consist of a binary-alloy core and a silicon shell, that however is brittle and not capable of withstanding the thermal cycles to which these particles are subjected.

[0020] Finally, the article “Microstructural features and thermal response of granulated Al and A356 alloy with relevant Sn additions”, P. Bassani et al., Materials & Design 229 (2023) 111879, describes and compares the properties of PCM particles formed from the commercial A356 alloy (having the approximate composition Al 92%-Si 7.4% by weight, with minor amounts or traces of other metals), particles formed from a binary alloy essentially of 40% Sn- 60% Al composition (with traces of other metals), and particles of a ternary composition obtained from 40% Sn and 60% A356 alloy. The Sn percentage of the alloys shown in the article corresponds to 20% by volume, which volume fraction is not optimal for using the material as a PCM, but is due to the need to try to limit or avoid percolations of liquid Sn from the PCM particles.

[0021] Therefore, the need is still present in the industry to have available PCM particles that can be obtained by simpler procedures and offer performance advantages compared to those currently known.

[0022] SUMMARY OF THE INVENTION

[0023] This object is achieved by the present invention, which relates, in a first aspect thereof, to a method for obtaining PCM particles consisting of a core formed from at least a first alloy and a shell formed from at least a second alloy having a different composition from that of the first alloy and a higher melting temperature than that of the first alloy, wherein said first and second alloys spontaneously form through solidification from a single liquid-phase mixture by cooling at a speed between 50 and 3000 °C / s, where said liquid-phase mixture is obtained by melting between 42% and 65% by weight of Sn and between 35% and 58% by weight of an Al alloy, where said Al alloy contains between 3% and 12% by weight of Si, other elements in a total amount by weight less than 10%, and Al as balance to 100%.

[0024] In the second aspect thereof, the invention relates to the PCM particles obtained by means of the method described above.

[0025] BRIEF DESCRIPTION OF THE FIGURES Fig. 1 shows a section micrograph of a particle of the invention;

[0026] Figs. 1 A and IB show two enlargements of the shell and the core of the micrograph in Fig. 1, respectively.

[0027] DETAILED DESCRIPTION OF THE INVENTION

[0028] The method of the invention allows obtaining the confinement of the active material within PCM particles directly during the production step thereof, without resorting to a subsequent step of making the capsule for the containment thereof. The principle can be applied to the production of PCM particles having various geometries and sizes, so as to adapt it to the desired industrial use.

[0029] The active material corresponds to at least a first alloy defined above; i.e., it can consist of a single phase or of multiple phases having a lower melting temperature than that of the shell alloy. The shell could be formed, in turn, from two or more alloys, but typically it consists of a single alloy.

[0030] In principle, the method can be applied from any liquid composition which, upon solidifying, gives rise to at least two solid phases of different composition having melting temperatures differing by at least 50 °C. Examples of systems exhibiting this type of solidification are Al-Sn, Al-Si-Sn, Al-Si-Mg-Sn, Al-Si-Cu-Sn, Al-In-Sn, Cu-Bi and Fe-Cu. However, the invention only relates to PCM particles obtained from ternary alloys of Al-Si-Sn composition or multicomponent alloys derived therefrom.

[0031] The starting liquid compositions to carry out the process of the invention are mixtures obtained by melting from 65% to 42% by weight of tin and from 35% to 58% by weight of an Al alloy containing Si between 3% and 12% by weight and other minor elements with total content by weight less than 10% (e.g., one or more of Cu, Mg, Fe and Mn).

[0032] With the alloys obtained from the above-described liquid mixtures, the spontaneous formation of a high-melting shell that allows for the self-encapsulation of at least one lower- melting phase is observed during the process of the invention.

[0033] Therefore, the metal material obtained upon completion of solidification is comparable to a composite material, formed by two portions corresponding to the at least two solid phases (one at higher melting T and one at lower melting T) indicated above.

[0034] The method of the invention consists in three main steps.

[0035] In the first step, the composition to be melted is formed, i.e., the starting solid components are weighed in such amounts that the sum thereof corresponds to the desired composition. The initial solid components can be substantially pure metals or metalloids, or alternatively can themselves be in the form of an alloy comprising two or more of the desired elements, in different ratios from those of the desired average composition. In order to promote the melting of the composition, the starting components are preferably used in pieces of small size.

[0036] The mixture of the starting solid components is then melted, obtaining a liquid mixture of uniform composition; in order to promote the homogeneity of the mixture composition, it can be maintained under stirring and preferably at a temperature of at least 30 °C above that of complete melting.

[0037] The liquid mixture is then divided into portions of desired weight or volume, useful for obtaining the final PCM particles. The division of the liquid into portions can occur, for example, by means of a dripper or by pouring controlled amounts thereof into the cavities of a mold. In order to facilitate the outflow of the liquid mixture from the orifices of the dripper or dispenser, a gas overpressure can also be applied to the mixture container.

[0038] The process of dripping or pouring into the cavities of a mold can occur in air or other atmosphere; in the latter case, the entire system must be maintained in a confined space where the gaseous environment can be controlled. Unlike other known processes, indeed, in the present invention the production of a surface shell enclosing the active material is not due to the formation of a surface oxide layer, but as said above, to the separation of a high-melting solid phase from the initial liquid mixture.

[0039] Commonly, solidification is obtained by dripping portions of the liquid metal mixture into a different liquid that is liquid at the final temperature reached by the cooling process; in the simplest embodiment, this final temperature is the ambient temperature and the cooling liquid is water or a water-based mixture, but it can also be another cooling fluid, e.g., quenching oils or the like. The cooling can also occur in two steps and possibly in two different media. For example, there can be a first cooling in a liquid medium up to an intermediate temperature between the melting temperature and ambient temperature, in the case of a plant operating in semicontinuous / continuous mode, in which particles are continuously extracted from the liquid before the complete cooling thereof, and the transfer of the particles to a second zone of the plant in which the cooling is completed, e.g., in a gaseous atmosphere.

[0040] The essential feature of the method of the invention is the cooling rate of the portion of liquid mixture formed by dripping or casting, or, in the case of two-step cooling, the cooling rate in the first step. The inventors have observed that if the cooling rate in the temperature range between T liquidus of the liquid mixture and an intermediate temperature between such a temperature and the solidus of the active phase is lower than 50 °C / s, the possible phases, depending on the composition of the starting liquid mixture, solidify under equilibrium or near equilibrium conditions, and form a single body with a homogeneous distribution of the phases, without the formation of the shell. Instead, at cooling rates above 3000 °C / s, the cooling occurs under strong non-equilibrium conditions with reduced or no phase demixing, which does not allow for the localization on the surface of the alloy corresponding to the shell. The cooling rate referred to is that obtainable close to the surface, understood as the outer layer of the particle being formed, with an indicative thickness of 2,5% of diameter thereof, since at the indicated rates there is certainly a gradient between particle surface and core, and the phenomena of interest occur on the surface.

[0041] In the second aspect thereof, the invention relates to the PCM particles obtained by means of the method described above.

[0042] The PCM particles of the invention have a size typically between 1 and 15 mm, although other sizes are possible.

[0043] In the case of particles formed by casting the initial liquid mixture into a mold, it is possible to choose the desired shape for the particles, which will be that of the mold cavities. In the most common case of dripping of the liquid mixture into a cooling liquid, the particles have a spherical or near spherical geometry, determined by the surface tension of the metal mixture when it is still liquid, which tends to determine the geometry with the smallest surface for an equal volume.

[0044] The ratio between the shell thickness and the core radius of the particles depends on the composition of the starting mixture and on the cooling conditions of the shell.

[0045] As said above, the PCM particles of the invention have an Al-Si-Sn composition or one derived therefrom, e.g., the Al-Si-Mg-Sn and Al-Si-Cu-Sn systems, and preferably have an average composition by weight that is obtained by melting between 42 and 65% by weight of Sn and between 35 and 58% by weight of an Al alloy containing Si between 3% and 12% by weight and other minor elements with a total content by weight of less than 10% (for example, but not limited to, Cu, Mg, Fe, Mn); examples of such a composition are those obtained by melting a mixture of solids (powders, granules, etc.) consisting of 54% by weight of Sn and 46% by weight of an Al-Si-Mg alloy comprising 7% by weight of Si and 0.4% by weight of Mg, plus traces of other elements and the balance to 100% Al, also referred to in the industry as AlSi7Mg0.4; or a mixture of solids consisting of 54% by weight of Sn and 46% by weight of an Al-Si-Mg alloy comprising 10% by weight of Si and 0.5% by weight of Mg, plus traces of other elements and the balance to 100% Al, also referred to in the industry as AlSilOMg (in formulas AlSi7Mg0.4 and AlSilOMg the numerical values are not stoichiometric indices); obviously, all these compositions are averages, because during the solidification of the particles there are selective solidifications, at different times, of phases of different composition.

[0046] The invention will be further described by means of the following experimental part.

[0047] EXAMPLE 1

[0048] This example relates to the production of the PCM particles of the invention.

[0049] For the production of the PCM particles of the invention, 55% by weight of A356 alloy (standard aluminum alloy, with a composition generally indicated as AlSi7Mg0.4) and 45% by weight of 99.85% purity tin were used, both divided into pieces with a maximum size of about 2 cm, reaching an overall weight of 250 grams.

[0050] The two materials were placed in the graphite crucible of an induction melting furnace, and in the presence of an Ar flow. The crucible was contained in a non-sealed chamber in which Ar can be flowed, with a modest overpressure (not exceeding 0.2 bar). The presence of the gas is not crucial for the purposes of the particle production but limits the material loss by oxidation, once in the molten state. The set of materials was brought to melting, up to reaching the temperature of 700 °C, higher than the melting of both the individual materials and the alloy with the obtained average composition, so as to speed up the melting operations.

[0051] The crucible was provided with a casting hole at the bottom end thereof, kept closed by a rod also made of graphite.

[0052] The alloy was maintained at the temperature of 700 °C for 30 seconds, in order to promote the mixing typical of the induction melting process, and then the temperature was lowered to 630 °C.

[0053] Afterwards, the material was dripped through the casting hole present at the bottom of the crucible, having a diameter of 2 mm, and was dropped into a container placed below the crucible itself, containing a mixture of water and ethyl alcohol (10% by volume), at a temperature of 20 °C, with an overall volume of 12 liters, kept under stirring such as to obtain the movement of the free surface without formation of splashes or bubbles, by means of a recirculation pump.

[0054] In normal operation of the casting machine, the crucible is placed “sealed” with respect to the chamber in which it is inserted, and when opening the casting hole, a significant overpressure is applied to promote the outflow of the metal as a continuous stream.

[0055] Dripping was achieved by partializing the seal of the crucible with respect to the chamber, thus allowing a limited overpressure to be applied and the division of the liquid stream into portions to be obtained.

[0056] The free surface of the cooling liquid was placed about 50 mm from the lower end of the casting hole. The cooling of the particles is completed in the cooling liquid. Upon completion of the step of dripping the liquid from the crucible, the container containing the cooling liquid was displaced to proceed with the recovery of the particles, by now solidified.

[0057] The overall produced material was then subjected to drying and sorting to eliminate defective particles (incomplete formation of the shell or undesired shape). The same method was applied to an alloy of different chemical composition, obtained from the mixture of A360 alloy (46% by weight) and tin (54% by weight).

[0058] Samples of particles thus obtained, from both compositions, were embedded in resin for metallographic preparation, sectioned and metallographically prepared for observation under electron microscope (Hitachi SU70, FEG-SEM). The micrographs obtained from the composition derived from the A360 alloy are reproduced in Figs. 1, 1 A, and IB: Fig. 1 shows the section of the entire particle, with the presence of the shell (gray peripheral part), while Figs. 1 A and IB show, at higher magnification, the zones of the surface skin with the presence of shell (bottom part of the micrograph, showing an zone with uniform gray color) and of the interior of the particle with the presence of different phases in the core, respectively. A completely similar result was obtained with the particles obtained from the alloy A356.

[0059] Subsequent heating / cooling tests up to a temperature 50 °C higher than the activation temperature did not reveal phenomena of active phase loss, demonstrating the effectiveness of the shell.

Claims

1. CLAIMS1. A method for producing particles of phase-change material, consisting of a core formed from at least a first alloy and a shell formed from at least a second alloy having a different composition from that of the first alloy and a higher melting temperature than that of the first alloy, wherein said first and second alloys spontaneously form through solidification from a single liquid-phase mixture by cooling at a speed between 50 and 3000 °C / s, wherein said liquid-phase mixture is obtained by melting between 42% and 65% by weight of Sn and between 35% and 58% by weight of an Al alloy, wherein said Al alloy contains between 3% and 12% by weight of Si, other elements in a total amount by weight less than 10%, and Al as balance to 100%.

2. The method according to claim 1, wherein said elements present in the Al alloy in an overall amount by weight less than 10% comprise Cu, Mg, Fe, Mn.

3. The method according to claim 2, wherein said liquid phase mixture is obtained by melting 58% by weight of A360 alloy and 42% by weight of Sn, said A360 alloy containing 88% by weight of Al, 9-10% by weight of Si, 1.3% by weight of Fe, and percentages less than 1% by weight of Cu, Mn, Mg, Ni, Zn, Sn and Ti.

4. The method according to any one of the preceding claims, wherein said liquid phase mixture is maintained under stirring at a temperature of at least 30 °C higher than that of complete melting.

5. The method according to any one of the preceding claims, wherein said liquid phase mixture is divided into portions of desired weight or volume before cooling.

6. The method according to claim 5, wherein the division of the liquid phase mixture into portions of desired weight or volume is performed by means of a dripper or by pouring controlled amounts thereof into the cavities of a mold.

7. The method according to claim 6, wherein the solidification of the liquid phase mixture is performed by dripping portions thereof into a second liquid phase at the final temperature of the cooling process.

8. The method according to claim 7, wherein said second liquid phase is water, a waterbased mixture, or a quenching oil.

9. The method according to claim 7, wherein the solidification is performed in two steps, from the initial liquid mixture temperature to an intermediate temperature and from the intermediate temperature to ambient temperature, using two different cooling means.

10. Particles of phase-change material, consisting of a core formed from at least a first alloy and a shell formed from at least a second alloy having a different composition from that of the first alloy and a higher melting temperature than that of the first alloy, obtained according to the method of any one of the preceding claims.

11. Particles of phase-change material according to claim 10, having a size between 1 and 15 mm.

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