Patent Description:
Accordingly, there has been interest in developing cooling technologies as alternatives to vapor compression refrigerant loops. Various technologies have been proposed such as field-active heat or electric current-responsive heat transfer systems relying on materials such as electrocaloric materials, magnetocaloric materials, or thermoelectric materials. However, many proposals have been configured as bench-scale demonstrations with limited capabilities for scalability or mass production.

Electrocaloric polymers such as polyvinylidene fluoride (PVDF) have been proposed for use in heat transfer systems. However, they are subject to a number of potential problems when fabricated in very thin film systems in an effort to achieve performance parameters, such as temperature lift comparable to conventional vapor compression heat transfer systems. Conventional polymer film fabrication techniques such as thermoplastic film extrusion often do not produce a polymer crystal structure that is optimal for electrocaloric performance. Solvent casting is available as an alternate film fabrication technique. However, solvent casting can produce porous films that can lead to electrical arcing when exposed to an electric field to induce an electrocaloric effect in the film, which in turn can lead to premature film breakdown.

Disclosed is a method of making an electrocaloric element comprising dissolving or dispersing an electrocaloric polymer in an organic solvent having a boiling point of less than <NUM> at <NUM> atmosphere to form a liquid composition comprising the electrocaloric polymer, wherein the liquid composition comprises from <NUM> wt. % to <NUM> wt. % of the electrocaloric polymer, based on the total weight of the liquid composition. A film of the liquid composition is cast on a substrate, and the organic solvent is evaporated to form a film comprising the electrocaloric polymer. The film is removed from the substrate and disposed between electrical conductors to form an electrocaloric element.

Also disclosed is a method of making a heat transfer system comprising dissolving or dispersing an electrocaloric polymer in an organic solvent having a boiling point of less than <NUM> at <NUM> atmosphere to form a liquid composition comprising the electrocaloric polymer, wherein the liquid composition comprises from <NUM> wt. % to <NUM> wt. % of the electrocaloric polymer, based on the total weight of the liquid composition. A film of the liquid composition is cast on a substrate, and the organic solvent is evaporated to form a film comprising the electrocaloric polymer. The film is removed from the substrate and disposed between electrical conductors to form an electrocaloric element. A fluid flow path is provided between the electrocaloric element and a heat source or a heat sink for controlled heat transfer between the electrocaloric element and the heat source or heat sink.

The solvent may have a boiling point of less than <NUM> at <NUM> atmosphere.

The solvent may have a boiling point of at least <NUM> at <NUM> atmosphere.

The electrocaloric polymer may be polyvinylidene fluoride or a liquid crystal polymer.

The organic solvent may be evaporated at a temperature of <NUM> to <NUM> at <=<NUM> atm but more specifically <=<NUM> atm but more specifically <=<NUM> atm.

The film may be annealed for <NUM> minute to <NUM> hours following evaporation of the organic solvent.

The film may be annealed at a temperature of <NUM> to <NUM> following evaporation of the organic solvent.

The film may have a breakdown strength of at least <NUM> V/µm.

The film may have an electrocaloric adiabatic temperature lift of at least <NUM> when measured at <NUM> V/um and an external temperature of <NUM>.

The film may have a electrocaloric adiabatic temperature lift of at least <NUM> when measured at <NUM> V/um and an external temperature of <NUM>.

The film may have a resistivity of at least <NUM><NUM> Ω·cm.

The film may have a dielectric loss tangent of less than or equal to <NUM>.

The film may have a dielectric loss tangent of at least <NUM>.

Also disclosed is an electrocaloric element is formed by any of the above methods.

Also disclosed is a heat transfer system comprising an electrocaloric element formed by any of the above methods, a first thermal flow path between the electrocaloric element and a heat sink, a second thermal flow path between the electrocaloric element and a heat source, and a controller configured to control electrical current to the conductive layers and to selectively direct transfer of heat energy from the electrocaloric element to the heat sink along the first thermal flow path or from the heat source to the electrocaloric element along the second thermal flow path.

Subject matter of this disclosure is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification.

As mentioned above, a method of making an electrocaloric element comprises dissolving or dispersing an electrocaloric polymer in an organic solvent having a boiling point of less than <NUM>. As used herein, unless otherwise explicitly stated boiling point means boiling point at <NUM> atmosphere pressure. The electrocaloric polymer can be any type of polymer that exhibits an electrocaloric effect when subjected to an electric field. Examples of electrocaloric polymers include, but are not limited to ferroelectric polymers, liquid crystal polymers, and liquid crystal elastomers.

Ferroelectric polymers are crystalline polymers, or polymers with a high degree of crystallinity, where the crystalline alignment of polymer chains into lamellae and/or spherulite structures can be modified by application of an electric field. Such characteristics can be provided by polar structures integrated into the polymer backbone or appended to the polymer backbone with a fixed orientation to the backbone. Examples of ferroelectric polymers include polyvinylidene fluoride (PVDF), polytriethylene fluoride, odd-numbered nylon, copolymers containing repeat units derived from vinylidene fluoride, and copolymers containing repeat units derived from triethylene fluoride. Polyvinylidene fluoride and copolymers containing repeat units derived from vinylidene fluoride have been widely studied for their ferroelectric and electrocaloric properties. Examples of vinylidene fluoride-containing copolymers include copolymers with methyl methacrylate, and copolymers with one or more halogenated co-monomers including but not limited to trifluoroethylene, tetrafluoroethylene, chlorotrifluoroethylene, trichloroethylene, vinylidene chloride, vinyl chloride, and other halogenated unsaturated monomers.

Liquid crystal polymers, or polymer liquid crystals comprise polymer molecules that include mesogenic groups. Mesogenic molecular structures are well-known, and are often described as rod-like or disk-like molecular structures having electron density orientations that produce a dipole moment in response to an external field such as an external electric field. Liquid crystal polymers typically comprise numerous mesogenic groups connected by non-mesogenic molecular structures. The non-mesogenic connecting structures and their connection, placement and spacing in the polymer molecule along with mesogenic structures are important in providing the fluid deformable response to the external field. Typically, the connecting structures provide stiffness low enough so that molecular realignment is induced by application of the external field, and high enough to provide the characteristics of a polymer when the external field is not applied.

In some exemples, a liquid crystal polymer can have rod-like mesogenic structures in the polymer backbone separated by non-mesogenic spacer groups having flexibility to allow for re-ordering of the mesogenic groups in response to an external field. Such polymers are also known as main-chain liquid crystal polymers. In some exemples, a liquid crystal polymer can have rod-like mesogenic structures attached as side groups attached to the polymer backbone. Such polymers are also known as side-chain liquid crystal polymers.

Examples of main-chain liquid crystal polymers include those having the repeating structures shown with C<NUM> and C<NUM> polyethylene spacer groups, respectively:
<CHM>
<CHM>.

Examples of side-chain liquid crystal polymers include those having the repeating structures shown with C<NUM> and C<NUM> polyethylene spacer groups, respectively:
<CHM>
<CHM>.

Of course, the above structures are exemplary. Many other liquid crystal polymers are known, and can be readily utilized by the skilled person.

In some examples liquid crystal elastomers can be based on liquid crystal polymers that have been modified with crosslinking. The crosslink density can be adjusted to be low enough so that local molecular flexibility is retained to allow nematic or smectic ordering of the mesogenic groups in response to an external field. However, the crosslink density can be set high enough to produce a macro elastic deformation response of the polymer to the external field instead of the Brownian molecular motion that results in a fluid, non-elastic macro response to the external field. The crosslinking reaction can rely on any type of crosslinking mechanism such as including tri- or higher-functional monomer in the monomer reactants during polymerization or by including functional side groups such as hydroxyl attached to the polymer chain, which can be reacted with a crosslinking agent such as a diisocyanate. The functional side groups can be selected to result in a mesogenic group integrated in the crosslink chain, or the mesogenic groups can be attached as side groups on the polymer chain separate from crosslink chains that are non-mesogenic. Many liquid crystal elastomers are known, and can be readily utilized by the skilled person.

With respect to the solvent, any organic solvent within the specified boiling point range can be used. In some examples, a polar organic solvent is used, and can in some aspects provide compatibility with the polarity of the electrocaloric polymer. In other examples, a non-polar organic solvent can be used. Mixtures of polar and non-polar organic solvents can also be used. Examples of solvents include but are not limited to tetrahydrofuran (THF), butanone (i.e., methylethyl ketone or MEK), chlorobenzene, supercritical CO<NUM>. In some examples, the solvent has a boiling point of less than or equal to <NUM>. In some examples, the solvent has a boiling point of less than or equal to <NUM>. In some examples, the solvent has a boiling point of less than or equal to <NUM>. In some examples, the solvent has a boiling point of less than or equal to <NUM>. In some examples, the solvent has a boiling point of greater than <NUM>. In some examples, the solvent has a boiling point of greater than <NUM>. In some examples, the solvent has a boiling point of greater than <NUM>. It is understood that these upper and lower range limits can be independently combined to disclose a number of different possible ranges.

The concentration of the electrocaloric polymer in the coating composition can vary widely, depending on the type of coating technique and the desired dried thickness of the coating. The electrocaloric polymer content ranges from a lower endpoint of <NUM> wt. % to an upper end point of <NUM> wt. % based on the total weight of the coating composition. In some examples, the electrocaloric polymer content may range from a lower endpoint of <NUM> wt. % based on the total weight of the coating composition. In some examples, the electrocaloric polymer content may range up to an upper endpoint of <NUM> wt. %, based on the total weight of the coating composition. The above endpoints can be independently combined to yield a number of different ranges. Additives and coating aids such as surfactants, lubricants, substrate release agents, stabilizers, antioxidants, and others can be included. Various substrates can be used, including but not limited to glass, polished stainless steel, or solvent-resistant polymers (polytetrafluoroethylene) or polymers that are dissolvable in solvents other than the solvent used for coating the electrocaloric polymer (e.g., polypropylene). Release of the electrocaloric film can be facilitated by functionalization of the substrate using low surface-energy liquids such as polyorganosiloxanes or by dissolving a polymer substrate in a solvent that doesn't dissolve the coated electrocaloric film (e.g., a non-polar organic solvent). Any of a variety of coating techniques can be used. Examples of coating techniques include, doctor blade coating, spray coating, roll coating, brush coating, spin coating, and other known coating techniques. In some aspects, doctor blade coating is used with constant speed of <NUM>/second to <NUM>/second. After coating, the cast film can be dried (e.g., for <NUM> to <NUM> hours) and removed from the substrate. In some examples, the organic solvent is evaporated at a temperature of <NUM> to <NUM>. In some examples, the solvent is evaporated under a pressure of less than or equal to <NUM> atmosphere, although pressure can in some examples be up to <NUM> atmospheres or even as high as <NUM> atmospheres. In some examples, the cast film on the substrate can be soaked in water or an organic liquid that will not adversely impact the film properties for a period of time (e.g., <NUM> minute to <NUM> hours, more specifically <NUM> minutes to <NUM> hours) to facilitate separation of the film from the substrate. After removal from the substrate, the film can be subjected to additional drying if needed to remove any liquid that was used to facilitate separation from the substrate. In some examples, the film can optionally be subjected to annealing. Annealing can be carried out at temperatures in a range having a lower endpoint of <NUM>, more specifically <NUM>, and an upper endpoint of <NUM>, more specifically <NUM>, it being understood that these endpoints can be independently combined to produce a number of different ranges.

In some examples, the methods described herein can produce electrocaloric films with various beneficial properties. In some examples, the film has a breakdown strength of at least <NUM> V/µm as determined according to ASTM D149 <NUM>(<NUM>). In some examples, the film has a breakdown strength of at least <NUM> V/µm. In some examples, the film has a breakdown strength of at least <NUM> V/µm. In some examples, the film has an electrocaloric adiabatic temperature lift of at least <NUM> when measured at <NUM> V/um and an external temperature of <NUM>. In some examples, the film has an electrocaloric adiabatic temperature lift of at least <NUM> when measured at <NUM> V/um and an external temperature of <NUM>. In some examples, the film has an electrocaloric adiabatic temperature lift of at least <NUM> when measured at <NUM> V/um and an external temperature of <NUM>. The method of claim <NUM>, wherein the film has an electrocaloric adiabatic temperature lift of at least <NUM> when measured at <NUM> V/um and an external temperature of <NUM>. In some examples, the film has a resistivity of at least <NUM><NUM> Ω·cm. In some examples, the film has a dielectric loss tangent of less than or equal to <NUM>. In some examples, the film has a dielectric loss tangent of at least <NUM>. Dielectric loss tangent can be measured, for example according to known standards such as IS <NUM> or equivalent.

An example of a heat transfer system and its operation are further described with respect to <FIG>. As shown in <FIG>, a heat transfer system <NUM> comprises an electrocaloric element comprising an electrocaloric polymer film <NUM> having electrodes <NUM> and <NUM> on opposite sides thereof. Multiple electrocaloric elements configured in a stack can also be used. The electrocaloric element is in thermal communication with a heat sink <NUM> through a first thermal flow path <NUM>, and in thermal communication with a heat source <NUM> through a second thermal flow path <NUM>. The thermal flow paths are described below with respect thermal transfer through flow of a heat transfer fluid through control valves <NUM> and <NUM> between the electrocaloric element and the heat sink and heat source, but can also be through conductive heat transfer through solid state heat thermoelectric switches in thermally conductive contact with the electrocaloric element and the heat source or heat sink, or thermomechanical switches in movable to establish thermally conductive contact between the electrocaloric element and the heat source or heat sink. A controller <NUM> is configured to control electrical current to through a power source (not shown) to selectively activate the electrodes <NUM>, <NUM>. The controller <NUM> is also configured to open and close control valves <NUM> and <NUM> to selectively direct the heat transfer fluid along the first and second flow paths <NUM> and <NUM>.

In operation, the system <NUM> can be operated by the controller <NUM> applying an electric field as a voltage differential across the electrocaloric element to cause a decrease in entropy and a release of heat energy by the electrocaloric elements. The controller <NUM> opens the control valve <NUM> to transfer at least a portion of the released heat energy along flow path <NUM> to heat sink <NUM>. This transfer of heat can occur after the temperature of the electrocaloric elements has risen to a threshold temperature. In some examples, heat transfer to the heat sink <NUM> is begun as soon as the temperature of the electrocaloric elements increases to be about equal to the temperature of the heat sink <NUM>. After application of the electric field for a time to induce a desired release and transfer of heat energy from the electrocaloric elements to the heat sink <NUM>, the electric field can be removed. Removal of the electric field causes an increase in entropy and a decrease in heat energy of the electrocaloric elements. This decrease in heat energy manifests as a reduction in temperature of the electrocaloric elements to a temperature below that of the heat source <NUM>. The controller <NUM> closes control valve <NUM> to terminate flow along flow path <NUM>, and opens control device <NUM> to transfer heat energy from the heat source <NUM> to the colder electrocaloric elements in order to regenerate the electrocaloric elements for another cycle.

In some examples, for example where a heat transfer system is utilized to maintain a temperature in a conditioned space or thermal target, the electric field can be applied to the electrocaloric elements to increase its temperature until the temperature of the electrocaloric element reaches a first threshold. After the first temperature threshold, the controller <NUM> opens control valve <NUM> to transfer heat from the electrocaloric elements to the heat sink <NUM> until a second temperature threshold is reached. The electric field can continue to be applied during all or a portion of the time period between the first and second temperature thresholds, and is then removed to reduce the temperature of the electrocaloric elements until a third temperature threshold is reached. The controller <NUM> then closes control valve <NUM> to terminate heat flow transfer along heat flow path <NUM>, and opens control valve <NUM> to transfer heat from the heat source <NUM> to the electrocaloric elements. The above steps can be optionally repeated until a target temperature of the conditioned space or thermal target (which can be either the heat source or the heat sink) is reached.

Further disclosure is provided in the following Examples.

Claim 1:
A method of making an electrocaloric element, comprising
dissolving or dispersing an electrocaloric polymer in an organic solvent having a boiling point of less than <NUM> at <NUM> atmosphere (<NUM> kPa) to form a liquid composition comprising the electrocaloric polymer, wherein the liquid composition comprises from <NUM> wt.% to <NUM> wt.% of the electrocaloric polymer, based on the total weight of the liquid composition;
casting a film of the liquid composition on a substrate;
evaporating the organic solvent to form a film (<NUM>) comprising the electrocaloric polymer;
removing the film (<NUM>) from the substrate; and
disposing the film (<NUM>) between electrical conductors (<NUM>, <NUM>).