Phase change material and method for manufacturing the same

Abstract

A method for producing a form-stable phase change material includes freezing a slurry of a solid and a solvent to provide a frozen slurry, exposing the frozen slurry to conditions that cause the frozen slurry to sublimate to remove the solvent and provide a body having voids therein, sintering the body to provide a scaffold, and adding molten phase change material to the scaffold to provide a form-stable phase change material, the same scaffold when milled acting as a phase change nucleation agent.

Description

Technical Field

[0001] Technical Field The present disclosure relates to, for example, phase change materials for use in insulation, thermal energy storage, or thermal management.

Background Art

[0002] Background Phase change materials absorb or release energy in the form of heat when the phase change material changes phase. As a result, these materials are suitable for storing and releasing thermal energy. Such materials may be utilized in a variety of applications, for example, in electronic applications, for insulation or thermal energy storage, or for thermal management.

[0003] Phase change materials have the disadvantage that packaging or encapsulation of such materials is expensive. Although the potential applications for such materials are extensive, the use of phase change materials is limited by cost. Phase change materials also have the drawback of low thermal conductivity. The low thermal conductivity limits the rate at which heat can be brought into or taken out of the phase change material.

[0004] Improvements in the encapsulation and thermal conductivity of phase change materials are desirable.

Summary of the Invention

Means for Solving the Problems

[0005] Summary According to one aspect of the present invention, a method for manufacturing a form-stable phase change material includes freezing a slurry of a solid and a solvent to provide a frozen slurry, exposing the frozen slurry to conditions that cause sublimation of the solvent from the frozen slurry to provide a body having voids therein, sintering the body to provide a scaffold, and adding a molten phase change material to the scaffold to provide a form-stable phase change material.

[0006] The frozen slurry may be freeze-dried to cause sublimation of the frozen solvent.

[0007] The binder may be added to the slurry before freezing. The solvent used may be water.

[0008] Freezing is performed directionally, for example, by pouring the slurry into a mold, placing the mold on a cooling plate, and applying a temperature gradient during freezing.

[0009] The body may include ceramics, carbon, metal, or a combination thereof.

[0010] The slurry includes one or more of a binder, a surfactant, a dispersant, a freezing point depressant, and a structure modifier.

[0011] Optionally, the surface of the scaffold is functionalized or activated.

[0012] The scaffold may be heated, while when the molten phase change material is added to the scaffold, the molten phase change material may be added such that the temperature of the scaffold is greater than the melting point of the phase change material.

[0013] Optionally, the molten phase change material is added to the scaffold by adding drops of the molten phase change material to the scaffold to provide a form-stable phase change material. The phase change material may be added, for example, up to saturation of the scaffold, at atmospheric pressure.

[0014] According to another aspect, the form-stable phase change material includes a scaffold including a generally regular solid structure having voids therein containing the phase change material.

[0015] Optionally, the scaffold is pulverized and added directly to the phase change material.

[0016] Drawings Embodiments of the present invention are described by way of example with respect to the drawings and the following description.

Brief Description of the Drawings

[0017]

Figure 1

Figure 2

Figure 3A

Figure 3B

Figure 3C

Figure 3D

Figure 3E

Figure 4

Mode for Carrying Out the Invention

[0018] Detailed Description For the sake of brevity and clarity of illustration, reference numerals may be repeated between figures to indicate corresponding or similar elements. A great many details are shown to provide an understanding of the embodiments described herein. The embodiments may be practiced without these details. In other instances, well-known methods, procedures, and components are not described in detail so as not to obscure the described embodiments. This description should not be considered as limited to the scope of the embodiments described herein.

[0019] The present disclosure generally relates to a method for manufacturing a shape-stable phase change material. The method includes freezing a slurry of a solid and a solvent to provide a frozen slurry, exposing the frozen slurry to conditions that cause sublimation of the frozen solvent to remove the solvent and provide a body having voids therein, sintering the body to provide a more rigid scaffold, and adding a molten phase change material to the scaffold to provide a shape-stable phase change material.

[0020] Referring to FIG. 1, a flowchart illustrating a method for manufacturing a shape-stable phase change material is shown. The method may include additional or fewer processes than those shown and described, and portions of the method may be performed in a different order.

[0021] The freeze-formed scaffold is prepared from the slurry. The slurry is first prepared at 102, and the base of the slurry is a solvent, such as distilled water. Additives may be added to the solvent and dissolved by agitation with the application of heat, including one or more of a binder, surfactant or dispersant, freezing point depressant, and structure modifier. Solid particles of a solid material, such as ceramic, carbon, metal, polymer, oxide, or other material, or a combination of materials, are added to the aqueous mix. The solid material is utilized to form the scaffold. The additives utilized and the amounts depend on factors such as the desired structure and the material of the solid particles.

[0022] The solution is agitated, for example, with a magnetic stir bar to suspend the particles and produce a slurry. Depending on the composition, rather than a magnetic stir bar, a planetary mill may be utilized to assist in suspending the particles to produce a slurry. The addition of a surfactant or other dispersant may also be utilized to assist in suspending the particles, and a binder or other modifier may be added.

[0023] The slurry is frozen at 104. A schematic diagram of an apparatus for freezing in a method of manufacturing a shape-stabilized phase change material is shown in FIG. 2. The slurry may be directionally frozen, for example, by pouring the slurry into a mold having a thermally conductive bottom and freezing it directionally from the bottom up on a cooling plate or other device that provides a temperature gradient. Freezing in this way at a rate suitable for the actual composition and conditions produces unidirectional columns of frozen solvent, which push suspended solid particles into the gaps between them. A freezing point depression additive may be utilized to regulate the morphology of the solid solvent growth.

[0024] The frozen composite is then subjected to sublimation at 106 to remove the solid solvent, e.g., water in the form of ice. The frozen composite may be freeze-dried to remove the frozen solvent, leaving a green body (proto-template). Vacancies remain in place of where the solid solvent, e.g., ice, was previously present. Any optional structure-modifying additive included in the slurry may affect the geometry of the solid solvent, e.g., planar, hexagonal, etc., and thus may modify the resulting pore geometry.

[0025] The green template is then sintered at 108. The template may be sintered in air or an inert atmosphere, depending on the material, to strengthen and densify the solid, thereby forming a more rigid template. If the template is not sufficiently sintered, the template may collapse under thermal cycling. In this way, appropriate preparation and sintering processes and conditions, including time, temperature, binding agent, etc., are determined for each template material.

[0026] The shape-stabilized phase change structure is prepared at 110 by adding a phase change material. To prepare the shape-stabilized phase change material, also referred to herein as a shape-stabilized phase change material (PCM) composite, the scaffold is heated to a temperature above the melting point of the phase change material. The phase change material may include fatty acids, sugar alcohols, fatty alcohols, esters, polymers, paraffin waxes, salt hydrates, and others, as well as combinations thereof. The molten PCM is dropped onto the surface of the scaffold, and the molten PCM is drawn into the scaffold by capillary action. The PCM may be added at atmospheric pressure. If the surface of the scaffold is not at a temperature higher than the melting point of the PCM, the PCM may crystallize on the surface of the scaffold and thus not be fully absorbed. The addition of the phase change material is complete when the scaffold reaches saturation, and no further phase change material is absorbed.

[0027] Referring now to FIGS. 3A-3E, they illustrate the schematic manufacture of a shape-stabilized phase change material.

[0028] The slurry is illustrated in FIG. 3A. As illustrated, the solid particles are suspended in the slurry. The slurry is initiated at the bottom and at a controlled rate and frozen to produce a regular frozen solvent structure. An example of such a structure is shown in FIG. 3B. The frozen slurry is lyophilized to remove all of the frozen solvent and leave all of the other solid particles, as illustrated in FIG. 3C. The scaffold is sintered to increase strength and density and to remove any organic additives, as illustrated in FIG. 3D. The scaffold is penetrated with the PCM to give a shape-stabilized phase change material as illustrated in FIG. 3E.

[0029] Referring now to FIG. 4, a method of manufacturing a phase change material according to another embodiment will be described. This method may include additional, or fewer, processes than those shown and described, and portions of this method may be performed in a different order. Rather than a shape-stabilized phase change material, a nucleation aid, also referred to as a nucleation aid, is manufactured and utilized to nucleate the phase change material.

[0030] Many of the processes of the method of FIG. 4 are similar to those described above in connection with FIG. 1 and, accordingly, will not be described in detail in this case either.

[0031] The freeze-forming scaffold is prepared from a slurry. The slurry is first prepared at 102. The slurry is frozen at 104, and the frozen composite is subjected to sublimation at 106 to remove the solid solvent, for example, water in the form of ice. The green scaffold is sintered at 108. Next, the sintered scaffold is ground at 410 into powder. The powder is utilized as a nucleation aid in the phase change material by mixing the powder into the molten phase change material at 412. The powder is mixed with the molten phase change material, for example, with at least about 5 wt % of the powder.

Examples

[0032] Examples The following examples are presented to further illustrate various embodiments of the present invention. These examples are intended to be merely illustrative and are not intended to limit the scope of the present invention.

[0033] The form-stable (FS) PCM, including a porous solid scaffold infiltrated with a phase change material (PCM), was manufactured by freeze-forming. The scaffolds were post-processed using alumina, alumina and carbon, titania, carbon black, chitosan, and graphitized chitosan. Other ceramic materials, metals, polymers, and oxides are also suitable for such scaffolds. Laboratory experiments were conducted to confirm that polymers and other oxides are suitable for fabricating such scaffolds. Metal scaffolds are also suitable since metals are also susceptible to freeze-forming.

[0034] The scaffolds were successfully infiltrated with PCMs such as dodecanoic acid, which is solid at room temperature, octanoic acid, which is liquid at room temperature, erythritol, paraffin wax, sodium acetate trihydrate, and polyethylene glycol. Water was used as the solvent. However, other solvents may also be successfully utilized.

[0035] Based on the PCM used, other PCMs that are not limited to, but include, other long-chain fatty acids, other sugar alcohols, long-chain alkanes, long-chain esters, long-chain fatty alcohols, long-chain fatty amides, various waxes, hydrates, and mono-, di-, and tri-glycerides are possible.

[0036] Table 1 summarizes the scaffolds prepared to date. Table 2 summarizes the prepared FS PCM compositions and their respective properties. Further, the Vickers hardness of the alumina / dodecanoic acid FS PCM was determined to be 40.

[0037]

Table 1

[0038]

Table 2

[0039] Preparation of scaffolds: Alumina: The listed percentages in the prepared slurry are reported as mass % of the total slurry. Other percentages and compositions may also be used successfully. Deionized water (37.8%), zirconium acetate (14.1%, based on a 16% solution in dilute acetic acid, Aldrich) and sucrose (1.2%; >99%, BDH Chemicals) were mixed with a magnetic stir bar. Zirconium acetate was used to induce the hexagonal-columnar growth of ice. Sucrose was added to lower the melting point of ice. The solution was then heated to 40 °C and polymethylmethacrylate (PMMA) (0.66%; 100 mesh, Aldrich) was added. PMMA was used to assist in the suspension of alumina particles in the slurry. Then, two particle sizes of alumina, 0.25 - 0.45 μm (44.25%; 99.95%, Alfa Aesar) and 40 - 50 nm (1.9%; 99.5%, Alfa Aesar) were added and the slurry was stirred until the alumina powder was completely suspended. The slurry was poured into copper-bottom plastic molds. Two molds with diameters of 11 mm and 7 mm were utilized, the molds were filled to a depth of 6 - 20 mm and directionally frozen from the bottom up on a Peltier-cooled cold plate. The frozen alumina bodies were then freeze-dried in an Edwards Modulyo freeze dryer at approximately -40 °C and approximately 2 × 10 -3 mbar for 24 h. The green alumina pieces were heated in air at 5 °C for -1 500 °C for 1 h to burn out the organic compounds and then sintered at 1500 °C for 2 h.

[0040] Alumina + carbon: The slurry was prepared as for alumina, except that the amount of 0.5 - 0.45 μm alumina (1 or 5 mass %) was replaced with an equivalent mass of carbon black (Black Pearls 2000). A different sintering process was also used: the green alumina + carbon pieces were heated to 1500 °C under argon at 5 °C for -1 2 h and sintered.

[0041] Carbon black: The carbon slurry was prepared with an aqueous PVA solution (24 g L -1)(75.6%) was prepared by mixing with zirconium acetate (14.1%) and sucrose (1.2%). Then, this solution was heated to 40 °C and polymethyl methacrylate (PMMA) (0.8%; 100 mesh, Aldrich) was added. Then, carbon black (8.3%) was added and the solution was stirred until all the carbon was suspended. The slurry was poured into a copper-bottom plastic mold (11 mm or 7 mm in diameter, filled to a depth of 6 - 20 mm) and frozen from the bottom up on a Peltier-cooled cold plate. Then, the frozen body was freeze-dried in an Edwards Modulyo freeze dryer at about -40 °C and about 2×10 -3 mbar for 24 hours.

[0042] Chitosan: The xanthone gel was prepared by vigorously mixing deionized water (95.7%), chitosan (2.4%; Aldrich), and glacial acetic acid (1.9%). This gel was poured into a copper-bottom plastic mold (11 mm or 7 mm in diameter, filled to a depth of 6 - 20 mm) and frozen from the bottom up on a Peltier-cooled cold plate. Then, the frozen chitosan body was freeze-dried in an Edwards Modulyo freeze dryer at about -40 °C and about 2×10 -3 mbar for 24 hours.

[0043] Graphitized chitosan: The prepared chitosan scaffold was heated to 800 °C under argon for 5 minutes -1 and held at that temperature for 3 hours to be graphitized.

[0044] A part of the sample of the graphitized chitosan scaffold was saturated with a 1 M aqueous sodium hydroxide solution and held at 100 °C for 24 hours to functionalize the surface. Then, the scaffold was rinsed 5 times with deionized water and dried at 100 °C for 24 hours.

[0045] Another functionalized, graphitized chitosan scaffold sample was ground into powder and added to the PCM as a nucleation aid.

[0046] Preparation of form-stable PCM: Substrates were prepared in two sizes: 11 mm in diameter and 7 mm in diameter. The length of the substrates ranged from 6 mm to 20 mm depending on the height to which the mold was filled for freeze forming. To generate the shape-stable PCM, the selected substrate was heated to about 10 °C above the melting point of the PCM utilized, and the molten PCM was dropped onto the upper surface of the substrate until it could no longer absorb any more. Then, the FS PCM composite was placed on its side and held at 10 °C above the PCM melting point for 30 minutes to remove any PCM that had precipitated on the surface of the substrate without being absorbed.

[0047] The manufactured freeze-formed substrates accept the phase change materials and maintain their forms against many thermal cycles. In this way, the resulting phase change materials are useful for reversible thermal energy storage.

[0048] The substrate material was able to accept different phase change materials. Thus, different phase change materials may be selected as the substrate material. The phase change material is selected based on the application to ensure that the melting point and crystallization temperature of the phase change material fall within the temperature range of the application. Among those phase change materials suitable based on the application, the phase change material with the highest thermal energy storage density can be selected by choosing a phase change material with a high enthalpy change compared to other phase change materials, or by selecting a substrate that achieves a high phase change material loading, which may be selected by resulting in a high enthalpy change compared to a substrate with a relatively lower ability to load phase change material.

[0049] Among the PCMs tested, erythritol had the highest melting enthalpy change, and chitosan and carbonized chitosan scaffolds had the highest load. The alumina scaffold provided the highest increase in thermal conductivity, which is beneficial in applications where thermal conductivity is important. The alumina scaffold was also the most robust, which is beneficial in applications where strength may be important. In applications where hysteresis at the melting temperature of the PCM is important, the NaOH-treated carbonized chitosan scaffold successfully reduced the hysteresis in some PCMs. The freeze-cast carbon scaffold had relatively poor mechanical and thermal properties.

[0050] The above examples are intended to be merely illustrative. Changes, modifications, and variations may be made by those skilled in the art to a particular example. Accordingly, the scope of the claims should not be limited by the embodiments shown in the examples, but the broadest interpretation consistent with the entire description should be given.

Claims

1. A method for manufacturing a shape-stable phase change material, comprising: directionally freezing a slurry of a solid and a solvent to provide a frozen composite comprising unidirectional columns of the frozen solvent and suspended solid particles in the gaps between the frozen solvent, said freezing comprising pouring the slurry into a mold and placing the mold on a cooling plate to provide a temperature gradient during freezing; exposing the frozen composite to conditions that cause sublimation of the frozen solvent of the frozen composite to remove the frozen solvent and provide a body having columnar voids therein, said exposing the frozen slurry to the conditions that cause sublimation including freeze-drying; sintering the body to provide a scaffold containing the columnar voids therein; adding the molten phase change material to the scaffold while heating the scaffold such that the molten phase change material is drawn into the columnar voids by capillary action to provide the shape-stable phase change material having an increased thermal conductivity compared to the molten phase change material .

2. The method according to claim 1, wherein the slurry comprises a binder.

3. The method according to claim 1, wherein the slurry comprises a structure modifier that affects the geometry of the frozen solvent in the frozen composite and modifies the resulting pore geometry.

4. The method according to claim 1, wherein the temperature of the scaffold is higher than the melting point of the molten phase change material when the molten phase change material is added to the scaffold.

5. The method according to claim 1, wherein adding the molten phase change material to the scaffold to provide the shape-stable phase change material comprises adding drops of the molten phase change material to the scaffold.

6. The method according to claim 1, wherein adding the molten phase change material comprises adding the molten phase change material until saturation of the scaffold.

7. The method according to claim 1, wherein the solid comprises at least one of ceramic, pyrolytic polymer, polymer (excluding those corresponding to the pyrolytic polymer), carbon, metal, oxide (excluding those corresponding to the ceramic, the pyrolytic polymer, or the polymer), or combinations thereof.

8. The method according to claim 1, comprising forming the slurry before freezing.

9. The method according to claim 8, wherein forming the slurry comprises at least one of stirring and planetary milling.

10. The method according to claim 9, wherein forming the slurry comprises adding at least one of a binder, a surfactant, a dispersant, a cryoprotectant, or a structure modifier, and the structure modifier affects the geometry of the frozen solvent in the frozen composite and modifies the resulting pore geometry.

11. The method according to claim 1, wherein the phase change material comprises at least one of a fatty acid, a sugar alcohol, an alkane, a fatty alcohol, a fatty amide, a fatty ester, a wax, a polyethylene glycol, a hydrate, and a mono-, di-, or tri-glyceride.

12. The method according to claim 1, wherein adding the molten phase change material comprises adding the molten phase change material at atmospheric pressure.

13. A form-stable phase change material produced by the method according to claim 1.

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