Patent Application
20230407159
The various embodiments of the present disclosure relate generally to systems with enhanced thermal conductivity, and more particularly to graphene aerogels impregnated with phase change materials.
Phase change materials (PCMs) show great potential for use in thermal load management and energy storage applications. One particularly appealing class of PCM materials is that of organic PCMs like paraffins and polyethylene glycol (PEG). These materials are chemically stable, exhibit a congruent phase change, and are non-corrosive. However, they have one particularly significant drawback: low thermal conductivity (0.1-0.3 W/m·K). The low thermal conductivity causes the phase transition to primarily occur at the surface, making it difficult to rapidly dump energy into (or retrieve stored energy from) the bulk of the material. To combat this shortcoming, researchers have been exploring additives like particulates to enhance the thermal conductivity of the material without significantly decreasing capacity; however, particulate additives like graphene nanoplatelets are prone to aggregation, clumping, and settling. PCMs with particulate additives suffer from inhomogeneous distribution after thermal cycling. Thus, there is a need for systems and methods to create PCMs with increased effective thermal conductivity while maintaining their desirable energy storage properties during thermal cycling.
The present disclosure relates to phase change materials with enhanced thermal conductivity. An exemplary embodiment of the present disclosure provides a system comprising an aerogel and a first phase change material (“PCM”) imbedded in the aerogel.
In any of the embodiments disclosed herein, the aerogel can comprise graphene.
In any of the embodiments disclosed herein, a thermal conductivity of the system can increase by at least approximately 10 percent compared to a PCM thermal conductivity.
In any of the embodiments disclosed herein, the first PCM can comprise eicosane, docosane, octadecanoic acid, tetradecanol, a polymer, or combinations thereof.
In any of the embodiments disclosed herein, the first PCM can comprise a polymer configured to reversibly crosslink with respective monomers of the polymer based on a phase transition temperature.
In any of the embodiments disclosed herein, the polymer can comprise polyethylene glycol (PEG), polylactide (PLA), polyglycolide (PGA), polydioxanone (PDO), polylactide-co-glycolide (PLGA), or combinations thereof.
In any of the embodiments disclosed herein, the system can comprise from approximately 0.01 wt. % to approximately 3 wt. % of graphene, based on a total weight of the system.
In any of the embodiments disclosed herein, the system can comprise a ratio of graphene to first PCM of between approximately 1:1000 to approximately 1:33.
In any of the embodiments disclosed herein, the system can further comprise a second PCM imbedded in the aerogel. The second PCM can be different than the first PCM.
In any of the embodiments disclosed herein, the aerogel can comprise an oxygen content between approximately 0.1 mol. % and 25 mol. %.
In any of the embodiments disclosed herein, the aerogel can comprise an oxygen content of at least approximately 6 mol. %, and wherein the aerogel is hydrophilic.
In any of the embodiments disclosed herein, the aerogel can comprise an oxygen content equal to or less than approximately 5 mol. %. The aerogel can be hydrophobic.
An exemplary embodiment of the present disclosure provides a method of forming a system comprising phase change material (“PCM”). The method can comprise forming an aerogel and adding a first PCM within the aerogel. The aerogel can comprise graphene. The aerogel can comprise an oxygen content ranging from between approximately 0.1 mol. % to approximately 25 mol. %.
In any of the embodiments disclosed herein, the method can further comprise increasing a thermal conductivity of the system by at least approximately 10 percent compared to a PCM thermal conductivity.
In any of the embodiments disclosed herein, the method can comprise reducing graphene oxide to form graphene.
In any of the embodiments disclosed herein, forming an aerogel can further comprise freeze-drying the graphene to form pores.
In any of the embodiments disclosed herein, adding the first PCM in the aerogel can further comprise exposing the aerogel and the first PCM to a vacuum.
In any of the embodiments disclosed herein, the system can comprise from approximately 0.01 wt. % to approximately 3 wt. % of graphene, based on a total weight of the system.
In any of the embodiments disclosed herein, the first PCM can comprise eicosane, docosane, octadecanoic acid, tetradecanol, polyethylene glycol (PEG), polylactide (PLA), polyglycolide (PGA), polydioxanone (PDO), polylactide-co-glycolide (PLGA), or combinations thereof.
In any of the embodiments disclosed herein, the method can further comprise increasing an oxygen content in the aerogel to at least approximately 6 mol. % to form a hydrophilic aerogel.
In any of the embodiments disclosed herein, the method can further comprise decreasing an oxygen content in the aerogel equal to or less than approximately 5 mol. % to form a hydrophobic aerogel.
These and other aspects of the present disclosure are described in the Detailed Description below and the accompanying drawings. Other aspects and features of embodiments will become apparent to those of ordinary skill in the art upon reviewing the following description of specific, exemplary embodiments in concert with the drawings. While features of the present disclosure may be discussed relative to certain embodiments and figures, all embodiments of the present disclosure can include one or more of the features discussed herein. Further, while one or more embodiments may be discussed as having certain advantageous features, one or more of such features may also be used with the various embodiments discussed herein. In similar fashion, while exemplary embodiments may be discussed below as device, system, or method embodiments, it is to be understood that such exemplary embodiments can be implemented in various devices, systems, and methods of the present disclosure.
The following detailed description of specific embodiments of the disclosure will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the disclosure, specific embodiments are shown in the drawings. It should be understood, however, that the disclosure is not limited to the precise arrangements and instrumentalities of the embodiments shown in the drawings.
To facilitate an understanding of the principles and features of the present disclosure, various illustrative embodiments are explained below. The components, steps, and materials described hereinafter as making up various elements of the embodiments disclosed herein are intended to be illustrative and not restrictive. Many suitable components, steps, and materials that would perform the same or similar functions as the components, steps, and materials described herein are intended to be embraced within the scope of the disclosure. Such other components, steps, and materials not described herein can include, but are not limited to, similar components or steps that are developed after development of the embodiments disclosed herein.
As used herein, the terms “about” or “approximately” for any numerical values or ranges indicate a suitable dimensional tolerance that allows the part or collection of components to function for its intended purpose as described herein. More specifically, “about” or “approximately” may refer to the range of values ±20% of the recited value, e.g. “about 90%” may refer to the range of values from 71% to 99%.
The large phase transition energies of this class of materials allow them to store and release significant amounts of energy through latent heat storage. This latent heat storage can be used in a number of ways. For example, solar energy can be stored in a PCM material during the day and then gradually released at night to heat a building. PCM materials can also be used as a heat sink for high power electronics to keep operating temperatures near a target temperature. Another use is in satellite systems, which can experience significant temperature fluctuations as they pass in and out of the Earth's shadow or due to the heat generated from onboard electronics during peak calculation periods. The PCM materials described herein can absorb excess heat generated by the onboard electronics or from the sunlight and then release it when the system is in the Earth's shadow and calculations are not ongoing, greatly reducing temperature fluctuations and the stress of thermal expansion and contraction.
As shown in
Aerogels are a class of synthetic porous ultralight material which the liquid component within a pore has been replaced with a gas. In general, aerogels can have open porosities as high as 99.9% (v/v). In some embodiments, aerogel 102 can function as a macroscopic support for first PCM 110. In some embodiments, aerogel 102 or macroscopic support can be made of carbon-based materials such as carbon nanotubes, carbon nanofibers, expanded graphite, graphene, graphene oxide, and the like. In some embodiments, the carbon-based material can be pure or doped with foreign elements such as metals (Au, Pd, Ru, Ni, Cu, etc.,), metal oxides (e.g., TiO2, Al2O3, SiO2, etc.,), organic components, and combinations thereof.
In some embodiments, the macroscopic support can also be made of silicon-based materials such as silicon carbide (α- or β-SiC or related SiC-based supports, either pure or doped with foreign elements such as TiO2, Al2O3, SiO2), silica, etc. It also can be made of aluminum-based materials such as alumina (α- or β-Al2O3 or related alumina-based supports, either pure or doped with foreign elements such as TiO2, SiO2, ZnO2, Fe2O3, etc.,). The macroscopic material can also have a binary composition, such as SiC—Al2O3, SiC-silica, SiC-carbon, etc., and it can also be doped with different metal or metal-oxide dopants, such as TiO2. Aerogel 102 can be made in any known form available in the art, for example grains, pellets, rings, foams, and the like.
Returning to
As shown in
In some embodiments, aerogel 102 has an oxygen content that ranges between approximately 0.01 mol. % to 25 mol. %. When aerogel 102 has an oxygen content equal to or greater than 5.01 mol. %, the aerogel 102 is hydrophilic. An oxygen content between about 13 mol. % and 19 mol. % will result in a stably hydrophilic graphene. In some embodiments, an oxygen content greater than 20 mol. % may allow the graphene to completely disperse in water, which may be ideal in some applications of phase change materials. Alternatively, when aerogel 102 has an oxygen content equal to or less than 5 mol. %, the aerogel 102 is hydrophobic. In some embodiments, to make aerogel 102 more hydrophobic, additional conditioning is required to have such a low level of oxygen. For instance, aerogel 102 may undergo additional chemical treatment to reduce excess oxygen. In some embodiments, heat-treatment may be used to reduce oxygen content to below 5 mol. %. As would be understood by a person of skill in the relevant art, hydrophilicity is a spectrum such that the hydrophilicity of aerogel 102 may be adjusted by various techniques known in the field.
In some embodiments, the hydrophilicity of aerogel 102 is selected and altered based on the type of PCM 110 imbedded in system 100. For instance, when imbedding a PEG PCM, a hydrophilic aerogel 102 having an oxygen content equal to or greater than 6 mol. % is preferred.
In some embodiments, graphene 104 can be substantially planar in a single layer of graphene or can contain a related sp2 graphite-like allotrope with multiple layers of graphene. In one embodiment, graphene 104 is sp in a hexagonal arrangement and has a thickness of one atom of a two-bonded carbon atom. In another embodiment, graphene 104 is a one-atom-thick planar sheet of sp2-bonded carbon atoms in a hexagonal arrangement in a honeycomb crystal lattice. In another embodiment, graphene 104 has a carbon-carbon bond with a length of about 0.142 nm.
In some embodiments, first PCM 110 is imbedded throughout the pores of aerogel 102. The first PCM 110 can undergo the phase transition within the porous structure of aerogel 102 that forms a shape-stabilizing composite. First PCM 110 is restricted in the pores of aerogel 102 owing to the weak interaction of capillary force or surface tension effect at the interfacial regions of graphene 104.
In certain embodiments, first PCM 110 comprises organic hydrocarbons or sugar alcohols that freeze without much supercooling and can melt congruently. Some example, first PCMs 100 can include, but are not limited to, eicosane, docosane, octadecanoic acid, tetradecanol, or combinations thereof. In some embodiments, first PCM 110 can be a polymer. In some embodiments, first PCM 110 as a polymer can be configured to reversibly crosslink with respective monomers 112 based on a transition temperature of first PCM 110. Polymers can include, without limitation, polyethylene glycol (PEG), polylactic acid (PLA), polyglycolide (PGA), polydioxanone (PDO), polylactide-co-glycolide (PLGA), polycaprolactone (PCL), polyethylenimine (PEI), hyaluronic acid, PEGylated hyaluronic acid, polyamino acid, perfluorocarbon (PFC), poloxamer (PPO), or combinations thereof.
In some embodiments, aerogel 102 may also include a second PCM 120 imbedded in the pores of aerogel 102. Second PCM 120 can be a different structure than first PCM 110. Second PCM 120 can react with first PCM 110. For instance, second PCM 120 may be any polymer identified above or may crosslink with another polymer and form co-polymers such as, for example, PEG-PCL, PEG-PEI, PEG-PLA, PEG-PLGA, PFC-PEG, PEG-PPO-PEG, stearyl methacrylate (SMA) and methyl methacrylate (MMA), and the like. As another example, second PCM 120 may be any polymer and first PCM 110 may be a sugar alcohol that fits within smaller pores of aerogel 102.
In some embodiments, first PCM 110 can comprise a polymer having a phase transition temperature suited for the application of the phase change material. The molecular weight of first PCM 110 and second PCM 120 can be increased or decreased to change the melting behaviors of the system 100. The molecular weight of first PCM 110 and/or second PCM 120 can range from between approximately 100 to approximately 40000. The transition temperature for example PCMs having a range in molecular weight are listed in Table 1 below.
A T-history method allows for testing of the melting temperature, degree of supercooling, heat of fusion, specific heat, and thermal conductivity of several PCMs simultaneously.
As used herein, the “k-value” means the comparison of thermal conductivity of a material and specifies the rate of heat transfer. As a non-limiting example, a 1 m3 cube of material with a k-value of 1 will transfer heat at a rate of 1 watt for every degree of temperature difference between opposite faces. The k-value is expressed as 1 W/mK. In some embodiments, the lower the k-value, the less heat the material will transfer. In some embodiments, system 100 can have a k-value that is at least approximately 10% greater compared to a k-value of pure PCM. For instance, an example system described herein has a thermal conductivity equal to about 0.482 Watts per meter-Kelvin (W/mK), while pure PCM has a thermal conductivity of approximately 0.369 W/mK, an increase by approximately 24%. When first PCM 110 is embedded in aerogel 102, the thermal conductivity of system 100 increases by at least about 10%. In some embodiments, the thermal conductivity of phase change material 100 increases by up to 50% compared to the thermal conductivity of pure PCM.
Modified transient plane source (MTPS) sensor is a method that employs a single-sided sensor to directly measure thermal conductivity, effusivity, and other thermophysical properties of materials such as those described herein. The single-sided sensor comprises a guard ring that can accommodate solids, liquids, powders, and pastes. The MTPS sensor has a wide measurement range of 0-500 W/mK, and a temperature range of about −50 to about 500° C.
In some embodiments, graphene 104 is formed into porous aerogel 104 such that the density of the system 100 before PCM impregnation ranges between about 0.001 g/cm3 to about 0.05 g/cm3. After impregnation of first PCM 110, the density of the system 100 increases significantly, as shown in Table 3 below.
In some embodiments, the ratio of first PCM 110 to graphene 104 comprises from approximately 0.01 wt. % to approximately 3 wt. % of graphene, based on a total weight of the system 100. The system 100 can have a ratio of graphene 104 to first PCM 110 of between approximately 1:1000 to approximately 1:33.
As would be appreciated by one of skill in the art, the type of PCM and density of PCM impregnated in aerogel 102 can be adjusted based on the application of system 100. Some nonlimiting example applications can include thermal energy storage, such as the FlexTherm Eco by Flamco; solar cooking; cold energy battery; conditioning of buildings, such as ‘ice-storage’; cooling of heat and electrical engines; cooling of foods, beverages, coffee, wine, milk products, green houses; delaying ice and frost formation on surfaces; medical applications: transportation of blood, operating tables, hot-cold therapies; human body cooling under bulky clothing or costumes; waste heat recovery; off-peak power utilization such as heating hot water; heat pump systems; passive storage in bioclimatic building/architecture (high-density polyethylene, paraffin); smoothing exothermic temperature peaks in chemical reactions; solar power plants; spacecraft thermal systems; thermal comfort in vehicles; thermal protection of electronic devices; thermal protection of food; textiles used in clothing; computer cooling; turbine inlet chilling with thermal energy storage; telecom shelters in tropical regions; and the like.
The following examples further illustrate aspects of the present disclosure. However, they are in no way a limitation of the teachings or disclosure of the present disclosure as set forth herein.
Three-dimensional (3D) graphene hydrogel was first synthesized from a commercial graphene oxide (GO) water suspension (MSE Supplies). L-ascorbic acid (L-AA, 99%, Sigma-Aldrich) was added to a GO suspension (4 mg/mL) as a reducing agent for cross-linking the GO sheets. After dissolution of the L-AA, the reaction solution was placed in an oven at 60° C. for 16 hours to obtain a graphene hydrogel. Freeze-drying was performed to obtain 3D graphene aerogels by removing water from the 3D graphene hydrogel samples, maintaining their original porous structures. A vacuum-impregnation process was then employed to prepare 3D graphene aerogel/PEG composite samples (Gr/PEG). The solid-phase polyethylene glycol (PEG, MW1000, Sigma-Aldrich) was transferred into a reactor and heated at 70° C. for conversion to a liquid phase. The 3D graphene aerogel was then added into the reactor followed by evacuation for effectively impregnating the PEG into the pores of 3D graphene. The Gr/PEG samples were dried in air at room temperature and cut into a specific dimension for follow-on characterization. The initial preparation used for the T-history test samples comprised a 3:1 ratio of L-ascorbic acid to graphene by weight. In some examples, the ratio was varied in order to control the porosity and density of the graphene aerogels. For the modified transient plane source (MTPS) testing, three example batches were prepared: Gr/PEG-1, Gr/PEG-2, and Gr/PEG-3, which had ratios of 3:1, 3:2, and 3:4 L-AA to GO powder by weight, respectively.
Morphologies of the 3D graphene aerogels and their pore structure were observed by a scanning electron microscope (SEM, SU8230, Hitachi). Optical microscopy (VHX-7000, Keyence) was carried out for observing sections of both graphene aerogels (Gr) and Gr/PEG samples. Thermophysical property measurement approaches were used as next described.
The T-history method is a technique that can be used for the determination of multiple thermophysical properties of various phase change materials. The T-history technique is known to be suitable for extracting critical PCM properties including thermal conductivity, enthalpy of fusion, and specific heat. The experimental setup includes logging temperature as a PCM sample is cooled through its transition temperature. A cylindrical sample having a length to diameter ratio of over 15 was used such that the heat transfer was measured as one-dimensional in the radial direction. The temperature vs time curve of the PCM as it is heated or cooled is compared to that of a reference sample (typically water) and the critical parameters are extracted from a set of equations described herein.
In one example, borosilicate glass tubes with an ID of 0.38″ and OD of 0.5″ were used to contain the PCM materials. The samples prepared were 10″ long to attain the necessary aspect ratio for the T-history technique. A 6″ type-K thermocouple probe was inserted into the center of the cylindrical sample to measure the core temperature as it was heated and cooled between 0° C. and 50° C. The thermal conductivity, ks, of the PCM in the solid state can then be calculated from equation (1):
Where cp is the specific heat of the material of the tube, Tm is the melting temperature of the PCM, T∞ is the temperature of the atmosphere (which can be time dependent), R is the radius of the tube, tf is the time of full solidification of the molten PCM, ρp is the density of the PCM, hw is the coefficient for convective heat transfer from the tube to the stirred cool water, and Hm is the heat of fusion of the PCM and is defined by equation (2) below:
Where mw is the mass of the atmosphere, cp,w is the mean specific heat of the atmosphere, mt is the mass of the tube, cp,t is the mean specific heat of the material of the tube, mp is the mass of the PCM, A2 is ∫t
The modified transient plane source (MTPS) method directly measures thermal conductivity and effusivity of materials using a one-sided heat reflectance sensor. The device applies small momentary heat pulses to the sample while monitoring the change in interface temperature via a small voltage sensor. The thermal conductivity is inversely proportional to the rate of temperature increase and thus by monitoring the change in interface temperature versus time the thermal conductivity can be determined. For this work a C-Therm TCi thermal conductivity analyzer with MTPS sensor was used.
Preparation of Gr/PEG samples was done in a method like that of
The freeze drying effectively removes water and other hydrates from the pores of the hydrogel, generating the graphene aerogel in step 4, without any changes in overall morphology as well as volume, which indicates the porous structure of the graphene hydrogel were well maintained during the freeze-drying process. The pore volumes of the graphene aerogels were controlled by varying the weight ratio of L-AA to GO for Gr-1, Gr-2, and Gr-3 at 3:1, 3:2, and 3:4, respectively. The volumetric densities of Gr-1, Gr-2, and Gr-3 were 0.0025, and 0.0017 g/cm3, respectively. These results indicate the more L-AA added into the reaction solution could lead to more reduction of GO due to higher reducing power, resulting in more π-π stacking of the sheets and thus reducing the pore size and volumes.
SEM analysis confirmed that the pore size increased while the wt. ratio of L-AA to GO decreased from 3:1 (Gr-1) to 3:2 (Gr-2) and 3:4 (Gr-3). For all three graphene aerogel samples, well-defined porous structures were observed. The graphene aerogel prepared at lower L-AA to GO ratio were composed of larger-sized pores. Optical microscope images in
The aerogels prepared for the T-history were found to have an effective average density of 13.7 mg/cm3 and accounted for 1.3% of the total weight of the final Gr/PEG composites as shown in
with Tm=37° C. and Ts=30° C. the thermal conductivity of the PEG and Gr/PEG is determined from the cooldown runs. Based on a total of four measurements, two using an ice bath and two using room temperature water bath, an average thermal conductivity of 0.42 W/m·K for the Gr/PEG vs 0.37 W/m·K for the pure PEG was calculated, which is approximately 13% increase. It is important to note that the values for thermal conductivity were quite sensitive to the selection of Ts and Tm, which is why confirmation of the thermal conductivity via the MTPS method was critical. However, the effect of the thermal conductivity increase was clear and repeatable in the T-History data throughout repeated thermal cycling.
As shown in
For the MTPS testing, four samples were fabricated using each of the three different graphene aerogel preparations and infused with PEG. As discussed previously, the ratio of L-AA reducing agent to graphene oxide was varied in order to change the graphene aerogel densities. Reducing the amount of L-AA resulted in lowering volumetric density of the graphene aerogels due to differences in remaining oxygen functional groups, which decrease interlayer attraction and stacking. To quantify the ratio of oxygen to carbon for each aerogel preparation, X-ray photoelectron spectroscopy (XPS) can be used. Increasing oxygen functional groups during a secondary thermal reduction of the aerogel could improve performance of the system because oxygen functionalization is known to reduce the thermal conductivity of graphene. The graphene aerogel density was lower for all 3 of the preparations used in MTPS testing than for the previous T-history samples due to the container shape. The increased compaction during sample drying may be due to T-history test samples formed within long vertical tubes.
All three batches showed an increase in average thermal conductivity relative to the pure PEG control samples, although there was perceptible variation within each batch. Gr/PEG-2 showed the greatest increase in thermal conductivity, with a roughly 30% increase relative to the PEG control sample. The expectation was that the highest density graphene samples, Gr/PEG-1, would show the greatest increase in thermal conductivity so it was surprising to find that Gr/PEG-2 resulted in the largest thermal conductivity increase.
An aerogel presenting less stacking and clumping due to the additional oxygen functional groups may lead to a more uniform graphene distribution, as is shown in sample Gr/PEG-2. Additional oxygen functional groups may have also made for a better interface with the hydrophilic PEG.
The results of the disclosed invention demonstrated the potential for significant and stable PCM thermal conductivity enhancement through the addition of a small amount of graphene aerogel. Furthermore, the graphene aerogel's ability to maintain its structure and bulk thermal conductivity enhancement during thermal cycling was also confirmed. This promising hybrid system warrants further research as more precise control of the graphene aerogel properties and uniformity could lead to a robust graphene/PCM hybrid well-suited to meet the energy storage and thermal management needs of the future.
It is to be understood that the embodiments and claims disclosed herein are not limited in their application to the details of construction and arrangement of the components set forth in the description and illustrated in the drawings. Rather, the description and the drawings provide examples of the embodiments envisioned. The embodiments and claims disclosed herein are further capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purposes of description and should not be regarded as limiting the claims.
Accordingly, those skilled in the art will appreciate that the conception upon which the application and claims are based may be readily utilized as a basis for the design of other structures, methods, and systems for carrying out the several purposes of the embodiments and claims presented in this application. It is important, therefore, that the claims be regarded as including such equivalent constructions.
Furthermore, the purpose of the foregoing Abstract is to enable the United States Patent and Trademark Office and the public generally, and especially including the practitioners in the art who are not familiar with patent and legal terms or phraseology, to determine quickly from a cursory inspection the nature and essence of the technical disclosure of the application. The Abstract is neither intended to define the claims of the application, nor is it intended to be limiting to the scope of the claims in any way.
This application claims the benefit of U.S. Provisional Application Ser. No. 63/366,419, filed on 15 Jun. 2022, which is incorporated herein by reference in its entirety as if fully set forth below.
| Number | Date | Country | |
|---|---|---|---|
| 63366419 | Jun 2022 | US |
