SELF-ENCAPSULATED PHASE CHANGE ALLOYS AND THE PROCESS FOR THEIR PRODUCTION

Description

P024085IT-01 Notarbartolo & Gervasi SpA “SELF-ENCAPSULATED PHASE CHANGE ALLOYS AND PROCESS FOR THE THEIR PRODUCTION” ***** ***** ***** FIELD OF INVENTION 5 The present invention relates to the field of materials for the storage and release of energy, particularly through the use of phase change materials. STATE OF THE ART Phase change materials are materials that can absorb or release heat. latent, with small temperature variations, during a phase transformation. For such 10 properties, are of application interest in the industrial and energy fields as systems of thermal storage, temperature regulation, etc. Phase change materials are commonly referred to in the industry with the acronym PCM (from the English “Phase Change Materials”), which will also be adopted in the rest of the description. These materials can be organic, inorganic (salts) or metallic (single 15 metals or alloys). Organic PCMs are described for example in patent applications EP 3760689 A1 and KR 20120031765, while systems using salts as PCM are described for example in patent applications WO 2014 / 100096 A1 and US 2014 / 0197355 A1. However, metallic PCMs are preferable because they allow use at temperatures 20 higher than organic ones and in many cases even than inorganic ones; for this reason However, this requires containers with higher chemical and mechanical resistance, which do not become corroded and / or degraded during use. In early practical applications, the containers were relatively large in size, and inside them there was a high amount of PCM; this approach was however 25 abandoned in particular due to the slow kinetics of absorption and release of heat, due to the decrease in heat transfer efficiency from the center to the periphery of the PCM mass when their size increases. To overcome the problem of poor heat transfer efficiency in heat transfer elements P024085IT-01 Notarbartolo & Gervasi SpA large-sized PCM material, patent application WO 2023 / 115098 A1 describes a system consisting of blocks of composite material, in the form of miscibility gap alloys, in in which particles of a low-melting alloy are immersed in a higher-melting matrix, in which channels are obtained for fluids used for heat transfer. This system features 5 but still some practical problems, such as the need for spacers to prevent the expansions thermal expansion of the blocks (the greater the expansion the larger the size of the blocks) blocks) lead to deformations and ultimately to failure of the system; this in turn requires the use of pipes to connect the channels between adjacent blocks. The system also requires retaining walls of the set of blocks, resulting, ultimately, in a system of 10 complex construction. Given the difficulties of working with elements made of PCM of relatively large dimensions large, research in the sector has been directed towards materials in the form of granules, particles or microparticles, commonly essentially spherical in shape. These granules, particles or microparticles have a shell / core structure, where the shell material has a core temperature 15 melting point higher than that at which the phase change of the core material occurs, allowing the confinement of the latter when it is in liquid phase. Of course the shell material also has a melting point, and therefore in line in principle it could also be defined as a phase change material; consequently, the definition of phase change material is linked to the temperature of 20 exercise in which the specific system is used, which must be intermediate between that of fusion of the core and that of the shell. For simplicity, in the following description the material of the core is defined as “active material”, while with the definition “PCM particles” it is They mean elements in subdivided form with a volume of less than 10 cm, therefore including all the dimensions that are indicated in the sector with the terms granules, particles and 25 microparticles. An example of such a system is reported in patent application CN 112226208 A which describes a hybrid system, where the active material consists of low-melting alloys gallium or bismuth base, possibly in the form of powders mixed with paraffins, and the shell P024085IT-01 Notarbartolo & Gervasi SpA It is made of a polymer or resin, synthetic or natural; as examples of shell materials Gelatin, sodium alginate, or formaldehyde-based or poly(meth)acrylic resins are indicated. This system has the limitation that the particles produced can only be used up to temperatures compatible with the polymer shell, typically no higher than approximately 250°C. 5 In order to exploit the potential of higher melting alloys, it is necessary that the shell of the particle is entirely made up of materials with melting temperatures higher than those of organic materials, and typically above 500 °C. The solutions adopted so far propose the confinement of metallic active materials in capsules made of metals other than the active, or from oxides or other ceramic materials, through thermal and / or chemical processes 10 which are made downstream of the production of the active material spherules. US Patent Application 2012 / 018116 A1 describes a method for producing PCM particles, in which spherules of active material are suspended in a gas flow and coated with a first material acting as a binder for powders of a second deposited material subsequently; the first material is then thermally decomposed leaving a shell 15 sintered consisting of the second material on the surface of the active material sphere. This method is obviously very complex to implement. The articles “Micro- and nano-encapsulated metal and alloy-based phase-change materials for thermal energy storage”, Zhu S. et al., Nanoscale Advances, vol. 3 no. 13 (2021), and “A brief strategy for designing self-encapsulated Al-Si base phase change materials with high thermal 20 energy storage performance” Wang C. et al., Journal of Energy Storage 62 (2023), describe methods in which a chemical reaction is made on the surface of a spherule consisting of only the active material grow, through thermal oxidation, an oxide layer. These techniques involve a two-step production process, and the oxide layer has poor thermal conductivity, slowing down the heat exchange between the core of the particle and the exterior. 25 Patent application CN 115873562 A describes a method of producing particles PCM which consists in depositing a layer of silazanes on Al-Si alloy spherules by operating in organic solvent solution; with subsequent heat treatment, the silazane decomposes forming a SiCN shell. In addition to requiring a special procedure in this case too P024085IT-01 Notarbartolo & Gervasi SpA two-phase production, due to the instability of silazanes to air and humidity, all the process must be carried out under an inert atmosphere. There is therefore still a need in the sector to have PCM particles available. obtainable with simpler procedures than those currently known. 5 SUMMARY OF THE INVENTION This aim is achieved with the present invention, which in a first aspect concerns a method for obtaining PCM particles consisting of a core formed by at least one first alloy and a shell formed by at least one second alloy having a different composition from that of the first alloy and a melting temperature higher than that of the first alloy, in 10 where the said first and second alloys are formed spontaneously through solidification from a single mixture in liquid phase by cooling at a rate between 50 and 3000 °C / s. In its second aspect the invention relates to PCM particles obtained by the method described above. BRIEF DESCRIPTION OF THE FIGURES 15 Fig. 1 shows, at two different magnification levels, two cross-sectional micrographs of a particle of the invention, showing the shell and core thereof. DETAILED DESCRIPTION OF THE INVENTION The method of the invention allows to obtain the confinement of the active material inside PCM particles directly during the production phase of the same, without 20 having to resort to a subsequent phase of construction of the capsule for its containment. The principle can be applied to the production of PCM particles having multiple geometries and dimensions, in order to adapt it to the desired industrial use. The active material corresponds to at least one first alloy defined above: that is, it can consist of a single phase or of several phases having a melting point lower than that of the 25 shell alloy. The shell may in turn be made of two or more alloys, but typically It is made of a single alloy. In principle, the method can be applied starting from any composition liquid which gives rise, upon solidification, to at least two solid phases of different composition having P024085IT-01 Notarbartolo & Gervasi SpA melting temperatures differing by at least 50 °C. Examples of systems that exhibit 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. The preferred compositions for the purposes of the invention are a liquid mixture 5 comprising by weight from 35% to 70% of Al and in complement, from 65% to 30% of Sn, and a liquid mixture obtained by melting 35% to 70% by weight of an Al alloy containing Si between 3% and 10% by weight and other minority elements with a total weight content of less than 10% (for example but not limited to Cu, Mg, Fe, Mn) and the complement to 100% by weight of Sn (i.e. from 65% to 30% by weight of tin). 10 Some of these alloys are described in 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, which studies the phases produced following solidification of the molten compositions, but the spontaneous production of a high melting shell that allows the self-encapsulation of a lower melting phase. 15 The metallic material obtained at the end of solidification is therefore similar to a composite material, formed by two portions corresponding to at least two solid phases (one (T upper melting point and T lower melting point) indicated above. The method of the invention consists of three main stages. In the first stage, the composition to be melted is created, that is, the 20 starting solid components in quantities such that their sum corresponds to the composition desired. The initial solid components may be essentially pure metals or metalloids, or alternatively they may themselves be in the form of an alloy comprising two or more of the desired elements, in ratios different from those of the desired average composition. to facilitate the creation of the composition, the fusion, the starting components are 25 preferably small pieces used. The mixture of starting solid components is then melted, obtaining a liquid mixture of uniform composition; to promote the homogeneity of the composition of the mixture, this can be kept under stirring and preferably at a temperature of P024085IT-01 Notarbartolo & Gervasi SpA at least 30°C higher than the temperature of complete melting. The liquid mixture is then divided into portions of desired weight or volume, useful to obtain the final PCM particles. The liquid can be divided into portions by for example by means of a dripper or by pouring controlled quantities of the same into cavities of a 5 mold. To facilitate the release of the liquid mixture from the orifices of the dripper or the dispenser, it is also possible to apply an overpressure of gas to the mixture container. The process of dripping or pouring into a mold cavity can occur in air or in another atmosphere; in the latter case the entire system must be kept in an environment circumscribed area in which it is possible to control the gaseous environment. Unlike other known processes, 10 in fact, in the present invention the production of a surface shell that encloses the active material is not due to the formation of a surface oxide layer, but as mentioned previously, to the separation of a high-melting solid phase from the initial liquid mixture. Solidification is usually achieved by dripping portions of the mixture metallic liquid into a different liquid, which is liquid at the final temperature reached by the 15 cooling process; in the simplest embodiment, this temperature final is the ambient temperature and the coolant is water or a mixture of water, but it can also be another cooling fluid such as, for example, quenching oils or equivalent. Cooling can also take place in two stages and possibly in two different media. 20 For example, one can have an initial cooling in a liquid medium up to a temperature intermediate between that of the melt and the ambient temperature, in the case of a system that works in semi-continuous / continuous where there is a continuous extraction of particles from the liquid before the their complete cooling, and the transfer of the particles to a second zone of the system in which the cooling is completed, for example in a gaseous atmosphere. 25 The essential feature of the method of the invention is the cooling speed of the portion of liquid mixture formed by dripping or pouring. The inventors have observed that, if the cooling rate is less than 50 °C / s, the possible phases depending on of the composition of the starting liquid mixture solidify under equilibrium conditions or P024085IT-01 Notarbartolo & Gervasi SpA almost equilibrium, and form a single body with homogeneous distribution of the phases, without the shell formation. At cooling rates above 3000 °C / s, however, a cooling under highly non-equilibrium conditions with reduced or absent phase mixing which does not allow the localization on the surface of the alloy corresponding to the shell. 5 In its second aspect the invention relates to PCM particles obtained by the method described above. The PCM particles of the invention typically have sizes between 1 and 15 mm, although other sizes are possible. In the case of particles formed by pouring the initial liquid mixture into a mold, it is 10 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 coolant, the particles have spherical or nearly spherical geometry, determined by the surface tension of the metallic mixture when it is still liquid, which tends to determine the geometry of minor surface area with the same volume. 15 The ratio of the shell thickness to the core radius of the particles depends on the composition of the starting mixture and the cooling conditions of the shell. As mentioned above, the PCM particles of the invention have a composition derived from Al-Sn, Al-Si-Sn, Al-Si-Mg-Sn, Al-Si-Cu-Sn, Al-In-Sn, Cu-Bi and Fe-Cu systems, and preferably have an average composition, by weight, of Al from 35 to 70% – Sn from 30 to 65%, or the 20 average composition obtained by melting between 35% and 70% by weight of an Al alloy containing Yes between 3% and 10% by weight and other minority elements with a lower total weight content to 10% (e.g. but not limited to Cu, Mg, Fe, Mn) and between 30 and 65% by weight of Sn; an example of This type of composition is obtained by melting a mixture of solids (powders, granules, …) comprising 60% by weight of AlSi Mg alloy and 40% by weight of Sn; all these 7 0.4 25 compositions are of course average because during the solidification of the particles they have selective solidifications at different times of phases of different composition. The invention will be further described through the following experimental part. EXAMPLE 1 P024085IT-01 Notarbartolo & Gervasi SpA This example refers to the production of PCM particles of the invention. For the production of the PCM particles of the invention, 60% by weight were used of alloy A356 (standard aluminum alloy, of composition generally indicated as AlSi Mg ) 7 0.4 and 40% by weight of tin of purity 99.85%, both divided into pieces of approximately 2 cm in diameter 5 size, reaching a total weight of 250 grams. The two materials were placed in the graphite crucible of a casting machine operating with the induction principle, and in the presence of Ar flow. The crucible was in fact contained in a non-watertight chamber in which it is possible to let Ar flow, with a modest overpressure (not higher than 0.2 bar). The presence of gas is not influential in the creation of the 10 particles but limits the loss of material through oxidation, once in the molten state. The set of the materials was brought to fusion, until reaching the temperature of 700 °C, higher both to the fusion of the individual materials and of the alloy with the average composition obtained. The crucible was equipped with a pouring hole at its lower end, held closed by a rod also in graphite. 15 The alloy was kept at a temperature of 700 °C for 30 seconds, to promote the mixing typical of the induction melting process. After that the material was dripped through the pouring hole on the bottom of the crucible, with a diameter of 3.6 mm, and was dropped into a container placed at the underneath the crucible itself, containing a mixture of water and ethyl alcohol (10% by volume), 20 at a temperature of 20 °C, with a total volume of 12 litres, stirred to obtain free surface movement without the formation of splashes or bubbles, by means of a recirculation pump. In normal operation of the casting machine the crucible is placed "sealed" with respect to the chamber in which it is inserted and at the moment of opening the casting hole a significant overpressure to favor the exit of the metal as a continuous vein. 25 The dripping was achieved by partially sealing the crucible with respect to the chamber, allowing to apply limited overpressure and obtain the division of the vein liquid in portions. The free surface of the coolant was placed approximately 50 mm from the end P024085IT-01 Notarbartolo & Gervasi SpA bottom of the casting hole. The cooling of the particles is completed in the liquid cooling. Once the liquid has dripped from the crucible, the container containing the coolant was moved to proceed with the recovery of the particles, now solidified. 5 The overall material produced was then subjected to drying and sorting for eliminate defective particles (incomplete shell formation or unwanted shape). The same procedure was applied to an alloy of different chemical composition, obtained from a mixture of A360 alloy (60% by weight) and tin (40% by weight). Samples of particles thus obtained, from both compositions, were embedded in 10 resin for metallographic preparation, sectioned and metallographically prepared for observation under the electron microscope (Hitachi SU70, FEG-SEM). The micrographs obtained from the composition derived from the A360 alloy are reproduced in Fig. 1: the upper part of the figure highlights the presence of the shell (lower part of the micrograph, which shows an area with a finer grain) while the lower part of the figure is the micrograph obtained at higher 15 magnification of the core of the particle presented in the first micrograph, and shows the presence of different phases in the core. A completely similar result was obtained with the particles obtained with the A356 alloy. Subsequent heating / cooling tests up to a temperature higher than 50°C with respect to the activation temperature they did not reveal any phenomena of loss of active phase, 20 demonstrating the effectiveness of the shell.

Claims

P024085IT-01 Notarbartolo & Gervasi SpA 1. Method for producing phase change material particles, consisting of a core formed from at least a first alloy and a shell formed from at least a second alloy alloy having a composition different from that of the first alloy and a temperature of melting point higher than that of the first alloy, in which the said first and second alloys are formed spontaneously by solidification from a single liquid phase mixture having a composition chosen between Al-Sn, Al-Si-Sn, Al-Si-Mg-Sn, Al-Si-Cu-Sn, Al-In-Sn, Cu-Bi and Fe-Cu for cooling at a rate between 50 and 3000 °C / s.

2. Method according to claim 1, wherein said liquid phase mixture has composition by weight Al 35-70% - Sn 30-65%.

3. Method according to claim 1, wherein said liquid phase mixture is obtained melting between 35% and 70% by weight of an alloy of Al and between 30 and 65% by weight of Sn, in which said Al alloy contains between 3% and 10% by weight of Si and other elements with total content less than 10% by weight.

4. Method according to claim 1, wherein said elements present in the Al alloy in total weight quantities less than 10% are Cu, Mg, Fe, Mn.

5. Method according to claim 4, wherein said liquid phase mixture is obtained for casting of 60 wt% AlSi Mg alloy and 40 wt% Sn. 7 0.4 6. Method according to claim 3, wherein said liquid phase mixture is obtained by melting 60% by weight of A360 alloy and 40% 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% in weight of Cu, Mn, Mg, Ni, Zn, Sn and Ti.

7. A method according to any of the preceding claims, wherein said mixture in liquid phase is kept under stirring at a temperature of at least 30 °C higher than that of complete fusion.

8. A method according to any of the preceding claims, wherein said mixture in liquid phase is divided into portions of desired weight or volume before P024085IT-01 Notarbartolo & Gervasi SpA cooling.

9. The method according to claim 8, wherein the partitioning of the mixture into liquid phase in portions of desired weight or volume is made by means of a dripper or by pouring controlled quantities of said mixture in liquid phase into the cavities of a die.

10. Method according to claim 9, wherein the solidification of the mixture in liquid phase It is made by dripping portions of the same into a second liquid phase maintained at the final temperature of the cooling process.

11. The method of claim 10, wherein said second liquid phase is water, a water-based mixture, or a tempering oil.

12. Method according to claim 10, wherein the solidification is carried out in two steps, from the temperature of the initial liquid mixture to an intermediate temperature and from the intermediate temperature to the room temperature, using two means of different cooling.

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

14. Phase change material particles according to claim 13, having dimensions between 1 and 15 mm.

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