WO2018071734A1 - Chemical modification of inorganic phase change materials - Google Patents

Abstract

Methods and apparatus provide for encapsulation of phase change materials (PCM) that exhibit superior qualities while offering reduced production costs over conventional techniques. The resulting phase change materials (PCM) are suited for incorporation into a variety of industrial, consumer and other products.

Description

CHEMICAL MODIFICATION OF INORGANIC PHASE CHANGE MATERIALS CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is filed under 35 U.S.C. §111(a), and claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application No. 62/406,986, filed October 12, 2016, the entire disclosure of which is incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0002] The invention disclosed herein relates to phase change materials, and in particular to phase change materials useful for building materials and other consumer products.

2. Description of the Related Art

[0003] Industry is in constant need of energy storage systems to improve energy efficiency. In particular, incorporation of thermal storage systems into other technologies has proven useful for reducing costs associated with thermal management. One type of thermal storage system includes Phase Change Materials (PCMs).

[0004] PCMs make use of the latent heat of a phase change, usually between a solid state and a liquid state. Since a phase change involves a large amount of latent heat absorbed or released over a small temperature range, PCMs are useful for temperature stabilization and for storing heat with high energy density.

[0005] Successful use of PCMs requires a balance of having materials with high latent heat, with a capability to thermally charge and discharge the PCM in a manner suited for the particular application. One drawback of state of the art latent thermal energy storage is the low thermal conductivity of some materials used as PCMs, which limits the power that can be extracted from the thermal energy storage. Other major drawbacks for building applications are the flammability and cost of some types of PCMs. Two types of PCM generally dominate.

[0006] Popular PCMs include organic and inorganic PCMs. Organic PCMs include hydrocarbon materials that may be obtained from petrochemicals or from biological and organic feedstocks and generally exhibit good thermal reversibility, but with relatively low thermal conductivity. Organic PCMs are typically more flammable, typically produced from more expensive feedstocks, and typically exhibit lower volumetric latent heat than inorganic PCMs. Examples of these materials include metal salt solvates. Despite some superior properties of inorganic PCMs, progress in processing them has lagged behind processes for producing organic phase change materials (PCM).

[0007] Incorporating PCMs into useful forms is a continuing challenge. Many chemical processing routes for the modification of organic PCM properties have been established, and adequate routes for encapsulating and shape stabilizing organic PCMs are known. While some additives have been identified to adjust the transition temperature, enthalpy and cyclability of inorganic PCMs, facile routes for encapsulation and shape stabilization of these materials other than macroencapsulation are lacking. Existing routes for encapsulation of these inorganic PCMs require, for example, energy intensive and time consuming evaporation of water during encapsulation, and some known encapsulation routes are compatible with high-temperature inorganic PCMs but not compatible with PCMs transforming near room temperature. Such encapsulation routes are therefore of limited use. Particularly, facile and simultaneous control over particle size, encapsulation, composition and transition temperature is difficult to achieve.

[0008] An encapsulation method for inorganic PCMs recently published in the academic literature (Journal of Materials Chemistry A 4.43 (2016): 16906-16912.) evinces the interest in and need for improved encapsulation strategies. It is shown that to use this method, excess water on the order of tens of weight percent must be added to the PCM in order to successfully produce encapsulated PCM particles. This excess water is removed during encapsulation and drying. This method is thus applicable to inorganic PCMs such as Mg(N0₃)2*6H20 that are known to form spontaneously in ambient air and which can be dried from solution to result in the desired stoichiometry. However, because this method relies on excess water it cannot be used to encapsulate PCMs which cannot be dried from solution to consistently form the desired stoichiometry. Relevant PCMs which therefore cannot be encapsulated using this method include Calcium Chloride Hexahydrate, CaC^et^O, a PCM of great interest due to its transition temperature close to room temperature. The stoichiometric ratio of metal salt to water allowing thermally cyclable material is sufficiently narrow to make processes involving water evaporation impractical for encapsulating this and other metal salt hydrates. The drying steps used in this method to remove water also add time and energy to the production process, which is inefficient for commercial production. Similar limitations apply to other published solvent evaporation-precipitation processes.

[0009] Thus, what are needed are methods and apparatus to improve processes for encapsulation of inorganic phase change materials (PCMs). Preferably, the methods and apparatus provide for encapsulation of phase change materials (PCMs) that exhibit superior qualities, while being inexpensive. The resulting phase change materials (PCMs) should be suited for incorporation into a variety of products and useful for enhanced thermal energy storage systems.

SUMMARY OF THE INVENTION

[0010] In one embodiment, a composition of matter is disclosed. The composition of matter includes a plurality of particles of phase change material (PCM), the particles including an inner core and an outer shell, in which the inner core includes an inorganic phase change material in combination with a nucleator and is able to change phase at a desired temperature and the outer shell provides shape stabilization and acts as a barrier between the inner core and a surrounding environment.

[0011] The PCM may further include a melting point modifier. The PCM may include at least one of a metal salt, a metal salt hydrate, and a eutectic mixture. The outer shell may include a polymer that is selected from the group including: a polyacrylate, a polycyanoacrylate, a polyurethane or a copolymer thereof. The PCM may be selected from the group including: CaCl₂-6H₂0, Na₂SO₄ 10H₂O, Mn(N0₃)₂-6H₂0, Na₂Si0₃-5H₂0. The PCM may include calcium chloride hexahydrate, the nucleator includes strontium chloride hexahydrate, the melting point modifier includes sodium hydroxide.

[0012] In another embodiment, a method for encapsulating a plurality of particles of phase change material (PCM) is provided. The method includes: preparing a PCM melt at a controlled temperature, the PCM melt containing phase change material and a nucleator; adding the PCM melt to an encapsulation bath to realize a mixture, the encapsulation bath containing a shell precursor; controlling mixing of the mixture to result in particles of the phase change material (PCM) encapsulated in a shell formed from the shell precursor.

[0013] The PCM particles may be encapsulated by interfacial polymerization at the interface between the PCM particles and the encapsulation bath. The interfacial polymerization may be induced by the addition of an initiator to the PCM melt. The interfacial polymerization may be an anionic polymerization and the initiator may be an ionic compound. The shell precursor may include at least one of: a cyanoacrylate, an acrylate, a urethane, a mixture thereof, and a mixture of a monomer with solid particles. The PCM may include calcium chloride hexahydrate, the nucleator includes strontium chloride hexahydrate, the melting point modifier includes sodium hydroxide.

[0014] In another embodiment, an article of manufacture is provided. The article of manufacture includes a material including a matrix included as a part of the article; and a plurality of particles of phase change material (PCM), the particles including an inner core and an outer shell, in which the inner core includes an inorganic phase change material in combination with a nucleator and is able to change phase at a desired temperature and the outer shell provides shape stabilization and acts as a barrier between the inner core and a surrounding environment dispersed in the matrix. The matrix material may be a foamed insulation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The features and advantages of the invention are apparent from the following description taken in conjunction with the accompanying drawings in which:

[0016] FIG. 1 depicts aspects of an exemplary embodiment of a particle according to the teachings herein;

[0017] FIG. 2 depicts aspects of a process for producing quantities of particles such as the embodiment depicted in FIG. 1 ;

[0018] FIG. 3 depicts an example of a finished product that contains a matrix that includes particles such as the embodiment depicted in FIG. 1 ;

[0019] FIG. 4 is a photograph of an exemplary embodiment of a particle such as the one depicted in FIG. 1 ;

[0020] FIG. 5 shows exemplary raw differential scanning calorimetry test results of an encapsulated PCM particle produced according to the teachings herein [0021] FIGS. 6A and 6B, collectively referred to herein as FIG. 6, two optical micrographs of PCM particles produced with (6A) and without (6B) encapsulating monomer;

[0022] FIG. 7 shows exemplary processed differential scanning calorimetry test results of an encapsulated PCM sample produced according to the teachings herein; and

[0023] FIGS. 8A, 8B, 8C, 8D, 8E, 8F, 8G and 8H, collectively referred to herein as FIG. 8, are photographs of raw and encapsulated PCM powder under various conditions.

DETAILED DESCRIPTION OF THE INVENTION

[0024] Disclosed herein are methods and apparatus for encapsulation of phase change materials (PCM). The methods and apparatus provide for encapsulation of phase change materials (PCM) that exhibit superior qualities while offering reduced production costs over conventional techniques. The resulting phase change materials (PCM) are suited for incorporation into a variety of industrial, consumer and other products.

[0025] Referring now to FIG. 1, an exemplary embodiment of an encapsulated phase change material (PCM) particle 10 (hereafter, "PCM particle") is shown. Generally, the PCM particle 10 includes a phase change material (PCM) that has been modified through additives and through an interfacial polymerization process to encapsulate the phase change material (PCM). Once appropriately encapsulated, PCM particles 10 may be dispersed within other media useful for a variety of applications.

[0026] In the embodiment shown in FIG. 1, the PCM particle 10 includes a volume of phase change materials (PCM) 1 with nucleator particles 2 dispersed therein. The combination of phase change materials (PCM) 1 with nucleator particles 2 is surrounded by a shell 3. The PCM particle 10 is generally spherical in shape, and exhibiting an outer diameter 4.

[0027] Referring now to FIG. 2, aspects of an exemplary fabrication process 20 for fabricating a quantity of the PCM particles 10 is shown. The fabrication process 20 may be carried out in batches or continuously.

[0028] FIG. 2 depicts aspects of an overview of the fabrication process 20, for continuous processing. In this example, the fabrication process 20 includes mixing a volume of phase change material (PCM) in a container to create a PCM melt 21. Generally, the PCM melt 21 is maintained at a controlled temperature. At the same time, an encapsulation bath is maintained in a container 22. The encapsulation bath may be maintained at a controlled temperature. Mixing 23 occurs under controlled conditions and provides for encapsulation steps 24. The resulting PCM particles 10 are then subjected to a washing step 25. The washed PCM particles 10 are then available for production. Some additional detail on each of these steps of the fabrication process 20 is now provided.

[0029] In some embodiments, the fabrication process 20 begins with preparation of the PCM melt 21.

[0030] In preparation of the PCM melt 21, the PCM material to be encapsulated is prepared as a liquid, at a temperature exceeding the melting point of the PCM material. In this example, the PCM material used is an inorganic PCM, such as inorganic salt solvates including ionic salt hydrates. Additives may be added to this mixture. In some embodiments, these additives may include: excess solvent in order to promote in encapsulation, improve cyclability of the phase change materials (PCM) 1, modify the freezing point or enthalpy, or as sacrificial solvent lost during subsequent steps. An example of a solvent that may be added is water. Melting point modifiers that modify the PCM transition temperature may be included as an additive. These melting point modifiers may use thermodynamic freezing point depression as a mode of action. The melting point modifiers may include inorganic compounds such as sodium chloride, inorganic salt solvates such as ionic salt hydrates, acids, bases, or organic compounds. Nucleators may be included as an additive to improve cyclability of the phase change materials (PCM) 1. Thickening agents may be included as an additive in order to improve cyclability of the phase change materials (PCM) 1. Polymerization initiators may be included as an additive in order to control formation of the shell 3 during encapsulation of the phase change materials (PCM) 1. It should be noted that a single additive may serve multiple functions. For example, sodium hydroxide may serve both as a melting point modifier and as a polymerization initiator.

[0031] Preparation of the encapsulation bath 22 includes preparation of a solution used to encapsulate the phase change materials (PCM) 1. Temperature of encapsulation bath 22 is controlled and may be greater or lower than the temperature of the PCM melt 21. The encapsulation bath 22 may include, without limitation: a solvent. For example, for phase change materials (PCM) one such as inorganic salt hydrates, the encapsulation bath 22 may employ an organic solvent. The encapsulation bath 22 may include a shell precursor. An example of a shell precursor includes a solvent monomer, used to form the encapsulating shell. Specific examples of the solvent monomer may include a cyanoacrylate, such as methyl-2 cyanoacrylate, ethyl-2 cyanoacrylate, n-butyl cyanoacrylate, or octyl cyanoacrylate. The shell precursor may be an isocyanate such as methylene diphenyl isocyanate. Alternatively, the shell precursor may be another monomer or an inorganic compound. The encapsulation bath may include a mixture of shell precursors. The encapsulation bath 22 may include a surfactant in order to control particle size. The encapsulation bath 22 may include solid particles or dissolved polymers used to have a surfactant effect (such as a pickering emulsion) or to modify the shell properties. The encapsulation bath 22 may include modifiers, such as polymers, to adjust the viscosity of the encapsulation bath. For example, modifying viscosity of the encapsulation bath 22 may be used to suspend or stabilize the phase change material (PCM) during addition. It should be noted that a single component may serve multiple functions. For example, some solvents may also polymerize to form a shell during subsequent steps. Furthermore, mixtures of any component may be used. A mixture of solvents, monomers or surfactants may be employed in an optimized system. The container for the encapsulation bath may be chosen to have energetically unfavorable interactions with the phase change materials (PCM), to prevent adhesion of the particles to the container during subsequent processing.

[0032] Phase change material (PCM) from the PCM melt 21 is mixed with the encapsulation bath 22 during a mixing step 23. The mixing process can be used to control the particle size and properties of the PCM particles 10 produced. The mixing processes may involve pouring the PCM melt 21 directly into the encapsulation bath 22. The mixing processes may involve pouring the PCM melt 21 and the encapsulation bath 22 together into a new container. The mixing processes may involve adding the PCM melt 21 dropwise into the encapsulation bath 22. The mixing processes may involve spraying the PCM melt 21 into the encapsulation bath 22. The mixing processes may involve sonication, stirring, or homogenization of the PCM melt 21 and the encapsulation bath 22 to disperse the solutions. The mixing processes may involve combining the PCM melt 21 and the encapsulation bath 22 in a pipe, tube, or microfluidic device, for example under controlled flow.

[0033] Generally, the PCM melt 21 and encapsulation bath 22 mixture are controlled in order to promote encapsulation of the particles. The conditions may be optimized for a system and for desired properties of the resulting PCM particles 10. Examples of control processes that may be used to achieve the desired encapsulation properties include, without limitation, control of temperature, such as heating or cooling for set amounts of time in order to initiate, speed or slow deposition and shell growth. Control processes may include time control in order to control shell thickness, mechanical properties and permeability. Control processes may include control of agitation, for example in order to bring additional shell precursor to the particle surface. Control processes may include addition of an initiator to promote shell deposition. Control processes may include addition of additional precursor(s) or other shell components.

[0034] In some embodiments, control over the encapsulation process and encapsulated shell properties may affect the compatibility of the phase change material (PCM) with subsequent processing steps, cyclability of the phase change material (PCM), and the latent heat of the encapsulated particles. Accordingly, the encapsulation process may be tailored and controlled for the particular applications destined to use the PCM particles 10.

[0035] Once encapsulated, PCM particles 10 may be removed from the encapsulation bath 22, for example by filtering, decanting, drying, or centrifugation. The PCM particles 10 may be washed with solvent in order to remove excess encapsulation bath 22. The wash solvent may be the same solvent as used in the encapsulation bath 22, a different solvent or a combination. Following washing, the encapsulated PCM particles may be dried through evaporation of solvent under ambient conditions, heating, vacuum drying, or any other drying procedure.

[0036] In some embodiments, the PCM particles 10 may be added to the encapsulation bath 22 again. Introducing encapsulated PCM particles 10 into the encapsulation bath 22 an additional time provides for further encapsulation of the PCM particles 10. The encapsulation bath 22 used for the additional encapsulation may be the same composition as initially used for the first shell 3, or may be of a different composition. As a result, repetitive encapsulation may provide for increased thickness of the shell 3, a multilayer shell 3 and for various other properties. Additional processing may be undertaken to provide for the deposition of additional shells 3. For example, intermediate treatment of the PCM particles 10 with an initiator may be performed.

[0037] In one embodiment, particles containing CaCl₂»6H₂0 PCM with 6% SrCl₂»6H₂0 nucleator, five weight percent (5wt%) NaOH initiator and a microscale shell made of poly ethyl-2-cyanoacrylate was produced. These particles had a diameter in the millimeter size range. The PCM melt was mixed at 120 degrees Celsius, and included the nucleator and sodium hydroxide additive. The encapsulation bath was made using 1-octadecene as a solvent, ethyl-2-cyanoacrylate as a monomer, and no surfactant. While the encapsulation bath was maintained at 70 degrees Celsius, the PCM melt was added dropwise. The particles were allowed to settle at the bottom of the container. Interfacial anionic polymerization of the ethyl-2-cyanoacrylate was initiated by the PCM solution at the PCM/organic solution interface. Polymerization was allowed to proceed for four minutes, then the temperature was lowered using a freezer to -20 degrees Celsius in order to prevent further polymerization. The particles were decanted and washed with isopropanol. In order to increase the shell thickness, some particles with isopropanol residue as an initiator on the surface were added back into the encapsulation bath and polymerization was allowed to progress. In order to demonstrate the success of this test at producing thermally cyclable, chemically modified, encapsulated PCMs, the particles were cycled in a differential scanning calorimeter. These test results are shown in FIG. 5. The melting point was found to have been successfully modified through addition of the NaOH initiator and melting point modifier additives, and the PCM was found to be thermally cyclable.

[0038] In one embodiment, particles containing CaCl₂»6H₂0 PCM with 6% SrCl₂»6H₂0 nucleator, five weight percent (5wt%) NaOH and a microscale shell made of pol(ethyl-2- cyanoacrylate) and poly(octyl cyanoacrylate) was produced. These particles had a diameter in the thirty (30) micrometer size range. The PCM melt was mixed at 120 degrees Celsius, containing the nucleator and sodium hydroxide additive. The encapsulation bath was made using 1-octadecene as a solvent, ethyl-2-cyanoacrylate as a monomer, and no surfactant. Octyl cyanoacrylate monomer was added to the encapsulation bath in order to control the rigidity of the shell. Span- 80 surfactant was used to promote emulsification of the PCM melt in the encapsulation bath. The PCM melt was mixed into the encapsulation bath and stirred in order to form an emulsion. The bath temperature was maintained at 70 degrees Celsius for four minutes, then frozen in order to control particle size. The particles were filtered and characterized with a microscope and with a differential scanning calorimeter, demonstrating encapsulated and thermally cyclable particles. Over 200 cycles of enthalpy and supercooling data from this batch of particles are shown in FIG. 7.

[0039] FIG. 3 depicts a finished product 30 comprising an encapsulated PCM 10 dispersed in a matrix 31. The matrix 31 may be a polyurethane foam insulation. The learnings described herein enable the production of encapsulated PCM particles which may be dispersed into materials, because the shell is able to prevent undesired reactions during production of the product. These particles 10 afford temperature regulation to the PCM-enhanced product 30.

[0040] FIG. 4 shows a photograph of a PCM particle 10, according to one or more embodiments, in which the encapsulated inorganic PCM particle sits on a quarter. The PCM comprises calcium chloride hexahydrate, strontium chloride hexahydrate as a nucleator, and sodium hydroxide as an initiator and a melting point modifier. The shell comprises poly(ethyl cyanoacrylate).

[0041] FIG. 5 shows a differential scanning calorimetry result of an encapsulated PCM particle, produced according to one embodiment of the invention. It is shown that the particle is thermally cyclable with limited supercooling.

[0042] FIG 6 shows microscope images of PCM particles fabricated by one or more embodiments. A comparison is made between particles synthesized in an encapsulation bath with no monomer (6a) and particles made using an encapsulation bath containing monomer but otherwise under identical conditions (6b). The presence of a visible shell 3 in (b) and not (1) indicates that the observed shell 3 is due to interfacial polymerization of the monomer to encapsulate the PCM 1.

[0043] FIG 7 shows processed differential scanning calorimetry results of an encapsulated PCM sample thermally cycled for more than 200 cycles. These data show that the encapsulated particles are cyclable.

[0044] FIG. 8 demonstrates the benefits of PCM encapsulation using one embodiment of the method described. The images compare samples of encapsulated PCM to non-encapsulated PCM. The encapsulated PCM samples were prepared using a mixture of CaCl₂»6H₂0 PCM with 6% SrCl₂»6H₂0 nucleator, five weight percent (5wt%) NaOH and a microscale shell made of pol(ethyl-2-cyanoacrylate). The PCM melt was mixed at 120 degrees Celsius, containing the PCM, the nucleator and sodium hydroxide additive. The encapsulation bath was produced using toluene as a solvent, ethyl-2-cyanoacrylate as a monomer and shell precursor, and Span-60 as a surfactant. The PCM melt was mixed into the encapsulation bath and stirred in order to form an emulsion. The bath temperature was maintained at 70 degrees Celsius for ten minutes, then the particles were filtered from the bath and washed with additional toluene. The particles were further dried using a rotary evaporator. Illustrations of the resulting product are provided in FIG. 8.

[0045] FIGS. 8 A and 8B compare the raw, unencapsulated PCM CaCl₂»6H₂0 powder before and after exposure to ambient air. In FIG. 8 A, raw PCM powder is shown. In FIG. 8B, the same sample is shown following 10 hours of exposure to ambient air. As shown, the particles have become a liquid as a result of spontaneous absorption of ambient moisture. By contrast, FIG. 8C and 8D show encapsulated particles exposed to the same conditions. FIG. 8C shows encapsulated PCM following the encapsulated PCM synthesis embodiment described in this paragraph. The particles are a powder. FIG. 8D shows the same sample following ten hours of exposure to ambient air under the same conditions as the raw PCM sample shown in FIGS. 8A and FIG. 8B. As shown by comparing FIGS. 8B to 8D, the encapsulation prevents the PCM from becoming a liquid upon air exposure. FIGS. 8E - 8H show encapsulated and non- encapsulated PCM before and after heating to 40 degrees Celsius in a sealed container. FIG. 8E shows encapsulated PCM prepared according to the encapsulated PCM synthesis embodiment described in this paragraph. FIG. 8F shows this same sample after being heated to 40 degrees Celsius in a sealed container for twenty minutes. FIG. 8G shows raw, unencapsulated PCM CaC^et^O powder. FIG. 8H shows the same sample as shown in FIG. 8G, after being heated to 40 degrees Celsius in a sealed container for twenty minutes using the same procedure as was performed on the encapsulated PCM sample in FIG. 8F. As is shown by comparing FIG. 8F and FIG. 8H, the encapsulated PCM continues to exist as a powder after heating but the non-encapsulated PCM becomes a flowing liquid. This demonstrates that the encapsulation method described is able to shape stabilize the PCM.

[0046] A variety of materials may be used in the processes for providing the phase change materials (PCM). Some additional examples are now set forth.

[0047] A variety of phase change materials (PCM) may be used. Examples of phase change materials (PCM) that may be suited for practice of the teachings herein (depending on intended application, other materials used, and other factors) include, without limitation: salt hydrates, with composition deriving from Μ_ηΑ_χ·ζΗ2θ (where more than one metal M and more than one anion A can be included), such as CaCl₂-6H₂0, Na₂SO₄-10H₂O, Μη(Νθ₃)₂-6Η₂θ, Na₂Si0₃-5H₂0, etc; other inorganic salt solvates; inorganic salts, such as NaN0₃, NaOH, NaN0₂, KN0₃, KOH, Na₂C0₃, LiCl, NaCl, etc; molecular or elemental solids, such as water, lead, lithium, aluminum, ammonia, ethylene glycol; or eutectic and non-eutectic mixtures of inorganic salts. Other materials that exhibit phase transformations under temperature variation may also be used.

[0048] A variety of nucleators may be used. Examples of nucleators that may be suited for practice of the teachings herein (depending on intended application, other materials used, and other factors) include, without limitation: metal shavings, sand, salt, S1O₂ particles or particles of other glass derivatives, substrates or side-walls, such as copper, aluminum, iron, manganese, etc; or their alloys and mixtures, also including non-metal substrates such as polymers, functionalized polymers, oxides, rare-earth metals. Other nucleators include chemical modifiers such as those that form alloys or solid-solutions with the PCM, and solid- liquid regions under transformation. These may include metal salt solvates isostructural to the PCM. Other materials that reduce supercooling (also known as subcooling), or other kinetic barriers to phase transformation, may be used.

[0049] A variety of solvents may be used. Examples of solvents that may be suited for practice of the teachings herein (depending on intended application, other materials used, and other factors) include, without limitation: water, ammonia, sulfur dioxide, acetone, toluene, benzene, t-butyl alcohol, cyclohexane, dioxane, glycerin, propanol, xylene, octadencene. Other materials that dissolve the phase change material, or the encapsulant, or the nucleator, for use in helping to facilitate the processing encapsulated phase change materials with nucleator, may be used.

[0050] A variety of melting point modifiers for the PCM may be used. Examples of melting point modifiers that may be suited for practice of the teachings herein (depending on intended application, other materials used, and other factors) include, without limitation: excess water or water deficiency, salt additives, polymer additives, small-molecule additives, chemical composition changes to form a solid-solution or eutectic with the phase change material. Other materials that influence the melting point may be used.

[0051] A variety of materials for the PCM encapsulant may be used. Examples of encapsulation materials that may be suited for practice of the teachings herein (depending on intended application, other materials used, and other factors) include, without limitation: cyanoacrylates such as methyl-2 cyanoacrylate, ethyl-2 cyanoacrylate, n-butyl cyanoacrylate, or octyl cyanoacrylate; isocyanates such as methylene diphenyl isocyanate. Alternatively, the encapsulation material may be another organic monomer or an inorganic monomer. Other materials that encapsulate the PCM, improving chemical and mechanical stability, may be used.

[0052] Examples of the benefits of the disclosed process relative to state of the art processes may include the following: the ability to perform encapsulation over a short timescale due to the addition of an initiator to the PCM melt; the ability to encapsulate PCMs using an initiator additive instead of water evaporation so as to encapsulate PCM stoichiometries that cannot be precisely accessed through water evaporation from solution; protection of the PCM during the production of a composite material; protection of the PCM during extended cycling; use of an inorganic PCM over an organic PCM, conferring the well-documented benefits of inorganic PCM systems including lower feedstock cost, higher volumetric latent heat, and flame retardance; a tunable transition temperature due to melting point additives introduced into the PCM melt; thermally cyclable inorganic PCM particles due to additives introduced into the melt such as nucleators and thickening agents; control over particle size in order to prevent degradation mechanisms such as gravity induced separation; and, control over particle size in order to facilitate particle incorporation into other materials.

[0053] In order to provide some context for the teachings herein, some further aspects of terminology used herein are set forth.

[0054] As discussed herein, the term "phase change material (PCM)," generally refers to a material which undergoes a first-order or second-order phase transition, which includes some change in the atomic structure which is accompanied by a change in the enthalpy of the structure. The phase transition is commonly caused by a change in temperature of the PCM, but can also be a result of other forms of applied thermodynamic work, including pressure, electromagnetic fields, light, mechanical stress, strain, or other such influencing factor. As discussed herein, the term "phase change materials" is generally synonymous with "PCM," or "phase change materials," or "PCMs," and other such forms or acronyms.

[0055] As discussed herein, a "PCM particle" generally refers to a discrete volume of PCM. In some embodiments, the PCM particle includes an inner core and an outer shell. The PCM particle may or may not be encapsulated, and may or may not contain additives such as a nucleator.

[0056] As discussed herein, the term "encapsulation" generally refers to the process of forming a barrier around the PCM particle. A "shell" or an "encapsulant" may be provided as a barrier around the PCM particle. Forming the barrier (i.e., shell) around the PCM particle may be referred to as "encapsulation" and by other similar terms.

[0057] Generally, the shell is provided to increase the mechanical and/or chemical stability of the PCM, and/or to reduce PCM exchange with the environment. For example, encapsulation of the PCM particle within the shell protects the PCM under conditions where the PCM would otherwise change composition, thereby decreasing performance, or where the PCM would decrease chemical stability of the host material. Thus, encapsulation of the PCM within the shell can greatly extend usability of the PCM.

[0058] The encapsulation process results in an "encapsulated PCM particle" or simply the "PCM particle." In some of the embodiments disclosed herein, the PCM particle includes the inner core and the outer shell. A plurality of encapsulated PCM particles may be referred to as "encapsulated PCM," "macroencapsulated PCM" or using other similar terms. Generally, the term "encapsulated PCM synthesis" refers to a process wherein raw materials are chemically processed resulting in encapsulated PCM. The term "microencapsulation" refers to an encapsulation method which results in PCM particles exhibiting at least one dimension in the microscale range, between approximately 100 nanometers and one millimeter. An example of a microencapsulation process is an interfacial polymerization process that encapsulates microscale PCM particles.

[0059] As discussed herein, the term "macroencapsulation" refers to any encapsulation method that produces PCM particles with substantially all dimensions greater than about one millimeter. Macroencapsulation methods may include chemical processes such as interfacial polymerization of particles greater than one millimeter in diameter, and physical processes such as placing a PCM sample in a sealed container.

[0060] As discussed herein, the term "nucleator" generally refers to an additive chosen to minimize the kinetic or nucleation barrier associated with a first-order phase transformation. For example, when the phase change material is solidifying from a liquid phase in response to reducing in temperature, there may be a kinetic barrier that impedes the solidification, resulting in greater subcooling or supercooling required to trigger the phase transition, and a longer duration of the PCM in a metastable form. A nucleator reduces the kinetic barrier to phase transformation by reducing the surface energy of the resulting phase, or by increasing the stability of the desired phase. Nucleators can be chemical in origin, and may be used as additives to the PCM composition to induce nucleation. Nucleators may be mechanical, such as particles or substrates that preference the solidification or melting of the PCM at the transition temperature. One class of effective nucleators are chemical substances that are crystallographically isostructural to the phase desired to be nucleated. A nucleator may serve multiple functions, such as those of surfactant, initiator, melting point modifier, or thickening agent.

[0061] As discussed herein, the term "monomer" generally refers to a chemical which can undergo a chemical reaction known as "polymerization." Polymerization involves other chemically similar or distinct monomers in order to produce a "polymer." A polymer is a chemical including a plurality of monomer subunits. In one or more embodiments, polymerization initiated by the PCM melt at the interface between the PCM melt and a monomer-containing solution result in a polymer encapsulating the PCM. A monomer may be added to the PCM melt, and/or to the encapsulation bath prior to the addition of the PCM melt, and/or to the encapsulation bath subsequently to the addition of the PCM melt.

[0062] As discussed herein, the term "initiator" refers to a chemical that causes polymerization of a monomer, causes a monomer to polymerize at a more rapid rate than would occur in the absence of the initiator, or begins a polymerization process that then proceeds independently of the initiator. The initiator may be added to the PCM melt and/or to the encapsulation bath. The initiator may also be coated onto the surface of a solid PCM particle and the particle be subsequently added to the encapsulation bath. An initiator may serve multiple functions, such as those of surfactant, melting point modifier, nucleator or thickening agent. A chemically pure initiator may be solid, liquid or gas under standard conditions.

[0063] As discussed herein, the term "surfactant" refers to a chemical that lowers the surface energy between multiple phases or promotes the solubility of one chemical in another, or promotes the miscibility of two chemicals. A surfactant or multiple surfactants may be used to allow the dispersion of one phase in another phase, and/or to stabilize the interface between two phases, and/or to control the particle size of a dispersion or emulsion. One or more surfactants may be added to the PCM melt and/or to the encapsulation bath before or after the addition of the PCM melt. A surfactant may serve multiple functions, such as the role of initiator, nucleator, thickening agent or melting point modifier. [0064] As discussed herein, the term "solvent" refers to any chemical that composes the principle component of a solution by mass. Other components of the solution are referred to as "solute." A common example of a solvent is water. Other common solvents are toluene, isopropanol, and acetone. A solvent need not necessarily be chemically pure. A "solvate" refers to a substance in which a solvent is physically or chemically bound to another compound, chemical, or substance. Examples of solvates include metal salt solvates, such as metal salt hydrates. In a solvate, the solvent does not necessarily compose the majority of the solvate mass. For example, Calcium Chloride Dihydrate is a solvate in which the solvent is water and composes less than 50% of the mass of the solvate.

[0065] As discussed herein, the term "melting point modifier" or "modifier" refers to any additive intended to change the melting point of a PCM, for example through the colligative properties of a solution of the PCM and the modifier. A melting point modifier may be added to the PCM melt. A melting point modifier may serve multiple functions, such as those of surfactant, initiator, nucleator or thickening agent.

[0066] As discussed herein, the term "thickening agent" refers to any additive used to increase the viscosity of a PCM in the liquid state. A thickening agent may be added to the PCM melt. A thickening agent may serve multiple functions, such as those of surfactant, initiator, nucleator or melting point modifier.

[0067] As discussed herein, a "PCM property" is a property of a PCM sample or a property of a PCM type that can be measured and is characteristic of a PCM type. A "PCM type" refers to a group of PCMs that are similar in their chemistry and/or processing. Non-limiting examples of PCM properties include transition temperature, subcooling, density, cyclability, and flame retardant properties.

[0068] As discussed herein, the term "transition temperature" or "melting temperature" or "melting point" is the temperature at which the onset of phase transition occurs as the temperature of the PCM is increased from a temperature below the transition temperature to a temperature above the transition temperature. A measurement of the transition temperature may be made by using differential scanning calorimetry (DSC), in which heat absorbed or released by the sample is measured as the temperature is varied continuously. This typically results in a plot with temperature on the horizontal axis and enthalpy absorbed on the vertical axis. To extract the transition temperature using DSC test data, the baseline heat absorbed during temperature increase may be fit with a line. The low-temperature portion of the melting enthalpy peak would be fit with a second line, and the melting onset would be determined as the temperature corresponding to the intersection of these two fit lines. Software used to operate a DSC device and analyze the data collected typically is able to automatically determine the transition temperature of a PCM using methods similar to those described here. A determination of the transition temperature of a PCM may alternatively be made using other techniques, such as by using a Dynamic Heat Flow Meter Apparatus.

[0069] As discussed herein, the term "supercooling" or "subcooling" refers to the difference obtained by subtracting the freezing temperature from the transition temperature. The "freezing temperature" or "refreezing temperature" or "recrystallization temperature" or "freezing point" or "refreezing point" or "recrystallization point" refers to the temperature at which the onset of phase transition occurs upon bringing the PCM from a temperature greater than the freezing point to a temperature less than the freezing point. The freezing temperature can be measured using methods analogous to those used to measure melting temperature. Typically, performance of a PCM is improved as the supercooling approaches zero degrees and it is a goal of PCM processing to reduce supercooling from positive values to approach zero degrees.

[0070] As discussed herein, the term "latent heat" or "PCM enthalpy" refers to the thermal energy absorbed upon PCM melting or released upon PCM freezing. The term "gravimetric latent heat" or "gravimetric enthalpy" refers to the latent heat of a sample of PCM divided by the mass of that sample. The term "volumetric latent heat" or "volumetric enthalpy" refers to the latent heat of a sample of PCM divided by the volume of that sample. Gravimetric latent heat and volumetric latent heat are PCM properties. Typically, high values of gravimetric latent heat and high values of volumetric latent heat are desired.

[0071] As discussed herein, the term "cyclability" refers to the ability of a PCM to be thermally "cycled" with degradation below a threshold. One "cycle" or "thermal cycle" refers to heating a PCM sample from a starting temperature below its transition temperature to a temperature above its transition temperature, then cooling the PCM sample to a temperature below its freezing temperature, followed by bringing the PCM sample back to the starting temperature. A PCM sample that has undergone one or multiple thermal cycles is said to have been "cycled." The process of a PCM sample undergoing multiple thermal cycles is referred to as "cycling" or "repeated cycling." "Degradation" refers to one or more properties of the PCM becoming less desirable. A PCM sample that has undergone degradation is said to be "degraded." For example, degradation may refer to a decrease in gravimetric latent heat of a PCM sample with cycling. Alternatively, degradation may refer to an increase in supercooling of a PCM sample with cycling, or to an increase of the average supercooling averaged over multiple thermal cycles. A "degradation mode" refers to a chemical or physical mechanism that causes degradation. A "figure of merit" is a PCM property that becomes less desirable as the result of degradation, or a PCM property that is optimized through engineering of the PCM. Cyclability does not connote a specific degradation mode, figure of merit, or threshold for degradation of that figure of merit below which the PCM is said to be "degraded." Generally, a PCM with "improved cyclability" compared to another PCM is a sample or PCM type that is able to undergo more thermal cycles or thermal cycles over a wider temperature range before exhibiting a level of degradation that would impair its performance in a relevant application. A PCM sample is said to be "cyclable" if it is able to undergo multiple thermal cycles with supercooling low enough to be sufficient for a relevant application, and with low enough degradation to be sufficient for a relevant application.

[0072] As discussed herein, the term "shape stabilization" refers to any processing method that leads to a PCM sample retaining its physical shape upon cycling, within tolerances required for a given application. A PCM sample that has undergone a shape stabilization process is said to be "shape stabilized" or said to be a "shape stabilized PCM." For PCMs that undergo a solid to liquid phase transition, shape stabilization prevents the sample from flowing once it transforms into a liquid. Encapsulation and macroencapsulation are non- limiting examples of shape stabilization methods.

[0073] As discussed herein, the term "PCM melt" refers to a flowing solution of PCM that may include additives. These additives may be dissolved or suspended in the PCM liquid. The PCM melt may include solids. Non-limiting examples of additives may include solvent, initiator, nucleator, surfactant, melting point modifier, monomer and thickening agent. A PCM melt will not necessarily contain any or all of these additives. The PCM melt may be maintained below, at, or above room temperature.

[0074] As discussed herein, the term "encapsulation bath" refers to a solution or suspension in which encapsulation occurs or is intended to occur. "Encapsulation bath" may refer to a solution prepared in advance of PCM melt addition, or to a mixture or suspension including the PCM melt. An encapsulation bath typically contains a solvent in which the PCM melt is not soluble. For inorganic PCM processing, this may be an organic solvent. In addition to solvent, an encapsulation bath typically contains additives. Non-limiting examples of additives include initiator, surfactant, and monomer. An encapsulation bath will not necessarily contain any or all of these additives. Following encapsulation of a batch of PCM, the encapsulation bath may or may not be collected and re-used.

[0075] Having introduced aspects of exemplary embodiments, some additional features and aspects are now introduced.

[0076] The shell may be used to protect the PCM during subsequent processing. The shell may also or alternatively improve the subsequent cyclability or stability of the PCM in the finished product.

[0077] Various other components may be included and called upon for providing for aspects of the teachings herein. For example, additional materials, combinations of materials and/or omission of materials may be used to provide for added embodiments that are within the scope of the teachings herein.

[0078] When introducing elements of the present invention or the embodiment(s) thereof, the articles "a," "an," and "the" are intended to mean that there are one or more of the elements. Similarly, the adjective "another," when used to introduce an element, is intended to mean one or more elements. The terms "including," "containing" and "having" are intended to be non-limiting or inclusive such that there may be additional elements other than the listed elements. The term "exemplary" is used merely to imply one of many possible examples. Usage of the term "exemplary" is not meant to necessarily imply a superlative embodiment. Where terms may be considered to be in conflict, interpretation of the terminology shall be the most favorable to the teachings herein.

[0079] While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications will be appreciated by those skilled in the art to adapt a particular instrument, situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.

Claims

CLAIMS What is claimed is:

1. A composition of matter comprising:

a plurality of particles of phase change material (PCM), the particles comprising an inner core and an outer shell, in which the inner core comprises an inorganic phase change material in combination with a nucleator and is able to change phase at a desired temperature and the outer shell provides shape stabilization and acts as a barrier between the inner core and a surrounding environment.

2. The composition of matter as in claim 1, wherein the PCM further comprises a melting point modifier.

3. The composition of matter as in claim 1, wherein the PCM comprises at least one of a metal salt, a metal salt hydrate, and a eutectic mixture.

4. The composition of matter as in claim 1, wherein the outer shell comprises a polymer that is selected from the group comprising: a polyacrylate, a polycyanoacrylate, a polyurethane or a copolymer thereof.

5. The composition of matter as in claim 1, wherein the PCM is selected from the group comprising: CaCl₂-6H₂0, Na₂SO₄ 10H₂O, Mn(N0₃)₂ 6H₂0, Na₂Si0₃ 5H₂0.

6. The composition of matter as in claim 1, in which the PCM comprises calcium chloride hexahydrate, the nucleator comprises strontium chloride hexahydrate, the melting point modifier comprises sodium hydroxide.

7. The composition of matter as in claim 1, wherein the plurality of particles are dispersed in a matrix.

8. A method for encapsulating a plurality of particles of phase change material (PCM), the method comprising:

preparing a PCM melt at a controlled temperature, the PCM melt containing phase change material and a nucleator;

adding the PCM melt to an encapsulation bath to realize a mixture, the encapsulation bath containing a shell precursor;

controlling mixing of the mixture to result in particles of the phase change material (PCM) encapsulated in a shell formed from the shell precursor.

9. The method as in claim 8, wherein the PCM particles are encapsulated by interfacial polymerization at the interface between the PCM particles and the encapsulation bath.

10. The method as in claim 9, wherein the interfacial polymerization is induced by the addition of an initiator to the PCM melt.

11. The method as in claim 10, wherein the interfacial polymerization is an anionic polymerization and the initiator is an ionic compound.

12. The method as in claim 11, wherein the shell precursor comprises at least one of: a cyanoacrylate, an acrylate, a urethane, a mixture thereof, and a mixture of a monomer with solid particles.

13. The method as in claim 8, in which the PCM comprises calcium chloride hexahydrate, the nucleator comprises strontium chloride hexahydrate, the melting point modifier comprises sodium hydroxide.