US20230417492A1

US20230417492A1 - Heat pipe, system and method for switching and/or programming a transport of heat

Info

Publication number: US20230417492A1
Application number: US18/011,247
Authority: US
Inventors: Martin Kluge; Jürgen Clade; Christoph Eberl; Markus Winkler; Erik Wischerhoff; Kilian Bartholomé; Olaf Schäfer-Welsen; Murat TUTUS; Martin Krus; Christian Teicht
Original assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Current assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Priority date: 2020-06-18
Filing date: 2021-06-17
Publication date: 2023-12-28
Also published as: CN115968437A; EP3926285A1; JP2023531430A; WO2021255176A1; KR20230037570A

Abstract

A heat pipe having a working chamber having an evaporator region operatively connected to a heat source, and a condenser region operatively connected to a heat sink. A working fluid is provided in the working chamber, and heat is transferred from the heat source to the heat sink by the working fluid in a first state of operation. The heat pipe is a switchable and/or programmable thermal diode or a switchable and/or programmable heat switch, at least one activatable functional material is provided, which is arranged and configured to keep the evaporator region free of the working fluid and/or to prevent the working fluid from evaporating in a second state of operation, in order to reduce and/or hinder heat transfer and/or to alter the preferential direction of heat conduction. A system and method for switching and/or programming heat transfer in a heat pipe are also provided.

Description

TECHNICAL FIELD

The invention relates to a heat pipe, to a system comprising a heat pipe, and to a method of switching and/or programming heat transfer in a heat pipe.

BACKGROUND

As is well known, heat pipes enable a high heat flow density by virtue of heat transfer via heat of evaporation. Typically, heat pipes have a hot side, the heat source, and a cold side, the heat sink. A working fluid is provided in the heat pipe, which is evaporated in the region of the heat source and condensed in the region of the heat sink. The heat transfer takes place via the transport of the working fluid and transfer by means of latent heat of condensation and evaporation.
Known heat pipes have a preferential direction for heat flow, meaning that they are designed as thermal diodes. This means that the diode conducts heat very well in one direction and very poorly in the opposite direction.
Such a thermal diode is described, for example, in Boreyko et al. 2011, Applied Physics Letter 99 (23), and in document U.S. Pat. No. 8,716,689 B2. The use of superhydrophobic coatings in the region of the heat sink and of superhydrophilic coatings in the region of the heat source gives rise to a preferential direction of the thermal diode described for heat: by virtue of the superhydrophobic coating, the surface in the region of the heat sink repels the working fluid, such that it is transported back into the superhydrophilic region of the heat source, where it can evaporate again.
A disadvantage of the known thermal diodes from the prior art is that the preferential direction for heat transfer is defined and diodicity is fixed by the design, i.e. cannot be varied or altered during operation.

SUMMARY

It is thus an object of the invention to propose a heat pipe or a method of heat transfer, which is more variable and overcomes the limits of the methods and devices known from the prior art.
This object is achieved by a heat pipe and by a method of switching and/or programming heat transfer in a heat pipe having one or more of the features described herein. Preferred configurations of the heat pipe of the invention can be found below and in the claims. Configurations of a system comprising a heat pipe of the invention are also provided. Preferred configurations of the method of the invention can be found below and in the claims.
The heat pipe of the invention comprises, as is known per se, at least one working chamber having at least one evaporator region and at least one condenser region. The evaporator region is operatively connected to a heat source, and the condenser region is connected to a heat sink. A working fluid is provided in the working chamber. In a first state of operation, the working fluid transfers heat from the heat source to the heat sink.
The essential feature is that the heat pipe takes the form of a switchable and/or programmable thermal diode or heat switch in which at least one activatable functional material is provided, which is arranged and configured in order to keep the evaporator region free of the working fluid and/or to stop the working fluid from evaporating in a second state of operation, in order to reduce and/or to prevent heat transfer and/or to alter the preferential direction of heat conduction of the heat transfer.
The working fluid fills the working chamber and is in either liquid or gaseous form depending on pressure and temperature. The wording “to keep the evaporator region free of the working fluid” relates to the working fluid in the liquid phase in direct contact and/or direct interaction with the surface of the evaporator region. It is likewise within the scope of the invention that working fluid is in the gaseous phase in the evaporator region since the working fluid in the gaseous phase fills the entire volume of the working chamber of a heat pipe.
The invention is founded in the finding of the applicant that it is possible by means of appropriate configuration of the conditions in the working chamber to control and even reverse heat transfer.
The heat pipe of the invention thus differs in essential aspects from known heat pipes:
An activatable functional material provided in the heat pipe is capable of switching from a first state (first state of operation of the heat pipe) to a second state (second state of operation of the heat pipe). In the first state, the activatable functional material enables heat transfer in the preferential direction of heat conduction of the first state of operation, or does not affect the function of the heat pipe. In the second state, the activatable functional material keeps the evaporator region free of the working fluid, or stops the working fluid from evaporating. Since heat transfer in the heat pipe functions to a crucial degree via the evaporation of the working fluid in the evaporator region and the transport of the evaporated working fluid into the condenser region, this reduces or prevents heat transfer in the heat pipe. It is likewise within the scope of the invention that the activatable functional material is configured such that the preferential direction of heat conduction through the activatable functional material is altered in the second state of operation.
In a preferred embodiment of the invention, the heat pipe is configured as a switchable thermal diode or heat switch in which the at least one activatable functional material is configured to alter its properties at least in part in an external field. Possible properties of the activatable functional material that are alterable by the external field are surface wetting properties, swelling capacity, fluid-binding properties and volume.
In an alternative embodiment of the invention, the heat pipe is configured as a programmable thermal diode or heat switch, in which the at least one activatable functional material is configured to alter its properties at least in part depending on conditions within the working chamber. Possible properties of the activatable functional material that are alterable via the external field are surface wetting properties, swelling capacity, fluid-binding properties and volume.
The activatable functional material is thus preferably switchable or programmable via external or internal effects. What is meant by “switchable” in this connection is that the state of operation can be switched by the active application of an external field. What is meant by “programmable” in this connection is that the heat pipe independently changes state by virtue of internal factors inherent to the material when there is a change in ambient conditions, especially the conditions in the working chamber.
This results in the benefit that heat transfer can be controlled in a targeted manner in the heat pipe of the invention.
In a preferred embodiment of the invention, the heat pipe is configured as a programmable thermal diode or heat switch in which the at least one activatable functional material is configured to change its properties depending on the conditions within the working chamber, especially temperature, pH of the working fluid and/or ionic strength of the working fluid. It is thus advantageous that no external fields are needed; instead, heat transfer can be controlled in the heat pipe solely via the working fluid or direct properties of the heat pipe.
The working chamber preferably takes the form of a closed volume, especially in such a way that heat is transferred by means of convection of the evaporated fluid and transport of the condensed fluid from the condenser region back into the evaporator region. In particular, the closed volume of the working chamber takes the form of a pressure-tight system. In particular, essentially all extraneous gases except for the working fluid are removed from the pressure-tight system. Various designs are useful for this purpose, which differ in the way in which the working fluid is transported back. Known configurations here are as a heat pipe or as a 2-phase thermosiphon.
The activatable functional material is preferably provided within the working chamber. It is likewise possible here that the activatable functional material is provided as part of the working chamber, for example as the base and lid of the working chamber.
In a preferred embodiment of the invention, the heat pipe is formed with a fluid circuit for the working fluid. The fluid circuit preferably comprises a fluid return conduit for transport of the condensed working fluid from the condenser region back to the evaporator region. In this way, it is possible to guide the working fluid back into the evaporator region in a controlled and metered manner and hence to prevent the evaporator region from drying out.
In a preferred embodiment of the invention, the closed volume has a fluid-phobic coating in the evaporator region and/or a fluid-philic coating in the condenser region. It is likewise within the scope of the invention that the closed volume, i.e. the working chamber, has additional structuring both in the evaporator region and/or in the condenser region. In this way, it is possible, for example, to optimize the wetting properties of the surfaces.
Preferably, the at least one activatable functional material takes the form of a switchable coating of the evaporator region and/or the condenser region of the working chamber, in that at least the surface properties of the coating of the evaporator region are variable from fluid-philic to fluid-phobic. Preferably, both the coating of the evaporator region and of the condenser region are designed such that the surface property of the coating of the evaporator region is variable from fluid-philic to fluid-phobic, whereas the surface property of the coating of the condenser region is variable from fluid-phobic to fluid-philic. The heat pipe in this case takes the form of a switchable thermal diode. The application of an external field can change the heat pipe from the first state of operation to the second state of operation.
If the hot side, i.e. the evaporator region, is heated by the heat source in the first state of operation, the working fluid that has collected on the fluid-philic coating of the evaporator region evaporates and enables heat transfer from the evaporator region to the condenser region. The working fluid condenses here on the fluid-phobic coating of the condenser region. On account of the fluid-phobic surface property in the condenser region, there is formation of droplets of the working fluid. In the case of a strongly fluid-phobic configuration of the surface, the working fluid “jumps” back to the evaporator region. Alternatively, fluid recycling of the droplets via capillary forces may also be envisaged, for example in the form of a hydrophilic wick structure, as known from the prior art for heat pipes. In this state, the thermal diode is thermally conductive.
If the surface properties of the coating in the evaporator region and/or condenser region are altered in the second state of operation (also called blocking state hereinafter), for example by application of an external electrical field, the hot side, i.e. the evaporator region, on the heat source then has hydrophobic properties. There is insufficient collection of working fluid on this coating, and the working fluid that collects there evaporates quickly and condenses on the fluid-philic coating of the condenser region. The working fluid remains there and is not transported back into the evaporator region since the abovementioned recycling mechanisms are inactive. Thus, the hot side of the working chamber dries out, and no heat transfer takes place by the working fluid. The thermal diode is blocked.
The switchable coating preferably takes the form of ORMOCER® and/or comprises ORMOCER®. ORMOCER®s are inorganic-organic hybrid polymers that can have an advantageous effect on the surface properties of many substrates; cf., for example, Sanchez et al., Chem. Soc. Rev. 40, 2011, 696-753. ORMOCER®s can also switch from hydrophilic hydrophobic and back as switchable coatings, exploiting mechanisms known from the technical literature; see B. Xin, J. Hao, Chem. Soc. Rev. 39, 2010, 769-782. ORMOCER®s of the invention therefore contain, for example, imidazoliumalkyl end groups for electrically switchable surface properties or fluoroalkylazobenzene or spiropyran end groups for photochemically switchable surface properties.
In a particularly preferred embodiment, the ORMOCER® coatings have micro-, meso- or nanostructuring, which enhances the fluid-philic/fluid-phobic properties thereof by exploitation of capillary or lotus effects.
In a preferred embodiment of the invention, the activatable functional material is configured such that evaporator region and condenser region switch properties in the second state of operation. The second state of operation is thus not a blocking state, but enables heat transfer in the opposite direction from the first state of operation. In this case, in the second state of operation, the working fluid in the original condenser region, which now acts as evaporator region, can evaporate and absorb heat from a heat source and transport it to the original evaporator region, now acting as condenser region. The working fluid condenses in the new condenser region and releases the heat to a heat sink. This turns round the preferential direction of heat conduction of the thermal diode.
Preferably, the activatable functional material takes the form of an ORMOCER® coating both in the evaporator region and in the condenser region. The coatings are chosen such that the application of an external field, preferably an electrical field or a radiation field, i.e. (UV) light radiation, can exchange the surface wetting properties of evaporator region and condenser region.
ORMOCER®s used for achievement of electrical switchability of the invention in fluid-philic/fluid-phobic properties are, for example, those wherein the functional end groups consist of an ionic group (trialkylammonium, imidazolium, sulfonate etc.), covalently bonded to the ORMOCER® network via a spacer, i.e. a linear alkyl chain having 2-20 carbon atoms, preferably 3-12 carbon atoms. Application of an electrical field (cf. Langer et al., Science 299, 2003, 371-374) results in repulsion of the ionic end groups by a substrate of the same nominal electrical charge, and projection into the interior of the thermal diode, which leads to a hydrophilic property of the surface. The ionic groups, by contrast, are attracted by a substrate of opposite electrical charge, such that the nonpolar spacer chains project into the interior of the thermal diode, which leads to a hydrophobic property of the surface. Thus, if the two opposite surfaces of the thermal diode are coated with the same functional ORMOCER®, application of an electrical field will result in a hydrophilic side and a hydrophobic side, likewise with reversal of the properties through reversal of the field direction. The electrical voltage applied to the thermal diode is preferably <50 V, more preferably <5 V.
In an alternative embodiment of the invention, the at least one activatable functional material takes the form of a reservoir for the working fluid, especially in the form of a liquid reservoir. The reservoir controls the uptake and release of the working fluid needed for heat transfer. This means that the available amount of working fluid can be made variable. In the first state of operation of the heat pipe, the working fluid is available for heat conduction. The heat pipe conducts heat. In the second state of operation, the blocking state, the working fluid is bound in the reservoir, especially in the form of a liquid reservoir. In this bound form, the working fluid is no longer available for heat transfer. The heat pipe no longer conducts heat.
In the context of this description, the wording “the heat pipe no longer conducts heat” means the blocking state of the diode. This means that heat transfer is considerably reduced compared to the other switching state. Nevertheless, a small flow of heat may take place, for example including via the thermal conduction of components.
The reservoir for the working fluid preferably takes the form of a gel, especially of a polymer gel, adsorbent or mesoscopically structured surface.
Especially preferably, the reservoir takes the form of a chemically crosslinked polymer gel. The crosslinked polymer gel is configured such that it swells by virtue of the working fluid and then has a volume phase transition, preferably between a swollen and collapsed state of the hydrogel.
Especially if the working fluid is water, the reservoir preferably takes the form of a water-binding hydrogel. The polymer gel has a water-binding and a non-water-binding state. The transition from the first state of operation to the blocking state of the heat pipe, i.e. from a non-fluid-binding state of the polymer gel to the fluid-binding state of the polymer gel, is preferably induced by a temperature transition. The polymer gel may take the form of a polymer gel having a volume phase transition of the UCST (Upper Critical Solution Temperature) type or of the LCST (Lower Critical Solution Temperature) type. In the case of a volume phase transition of the UCST type, the crosslinked polymer gel is only swollen by the working fluid on exceedance of the critical temperature (limiting temperature). In the case of a volume phase transition of the LCST type, the working fluid is displaced from the crosslinked polymer gel on exceedance of the critical temperature (limiting temperature). Accordingly, the heat pipe in the case of a UCST transition is blocked above the critical temperature. In the case of an LCST transition, the heat pipe is blocked below the critical temperature. The limiting temperature can thus define a switching temperature for the transition from the first state of operation to the blocking state of the heat pipe.
Known polymers having a UCST volume phase transition are described, for example, in Macromol. Rapid Commun. 33, 1898-1920, 2012. Known polymers having an LCST volume phase transition are described, for example, in Adv. Polym. Sci. 242, 29-89, 2011. The polymer gels mentioned interact with water and are therefore especially suitable for a heat pipe in which water is used as working fluid. However, there are also a number of polymers that have the properties described and the behavior described with organic fluids, for example mineral oils; for example, J. Polym. Sci. A46, 5724-5733, 2008. In this case, it is also possible to use a different fluid than water as working fluid.
In a preferred embodiment of the invention, the reservoir takes the form of an adsorbent. An adsorbent binds fluid. The amount of fluid bound in the adsorbent is also referred to as loading. With rising temperature (and the associated rising vapor pressure of the bound fluid), the loading of an adsorbent decreases, and the fluid is released again.
The adsorbent preferably has a limiting temperature, such that, on exceedance of this limiting temperature, or a particular vapor pressure of the fluid, the fluid is released again quite abruptly by the adsorbent. The limiting temperature thus defines a switching temperature for the transition from the blocking state to the first state of operation of the heat pipe.
An example of material for an adsorbent with a defined limiting temperature or the associated vapor pressure of the fluid is the AQSOA™-Z05 adsorbent from Mitsubishi™.
It is likewise within the scope of the invention that the properties of the liquid reservoir are not affected by temperature, but by another physical or chemical stimulus. Examples of these are UV light or microwave radiation, and pH, ionic strength or the presence of particular organic molecules. Examples of these are described in Angew. Chem. Int. Ed. 55, 6641-6644, 2015. The switching of the thermal diode is thus possible by virtue of a wide variety of different factors and can be adjusted correspondingly to the field of use and the ambient conditions.
The object of the invention is likewise achieved by a system having a heat pipe with the above-described properties of the invention, and means of applying a field in order to alter the properties of the activatable functional material.
Means of field application envisaged are preferably field generators for an electrical field, a magnetic field, a stress-strain field, for generation of light, especially UV light, for generation of heat and/or for generation of refrigeration. It is possible to provide either only one of the field generators mentioned or a combination of two or more of the field generators mentioned. Examples of these are a condenser, a coil, an eccentric, a (UV) light source or a heating and cooling device. In this way, it is possible to adjust the control means individually to the working fluid used and to the activatable functional material used.
The system of the invention likewise has the above-described advantages and properties of the heat pipe of the invention and/or a preferred embodiment thereof.
The system is preferably designed to be flexible in relation to hot side and cold side. If the heat pipe takes the form of a heat pipe with a reversible preferential direction of heat conduction, preference is given to providing means of assigning the function of evaporator region and condenser region via contact with a hot side or correspondingly with a cold side. Preference is given to providing good thermal contact between evaporator region and condenser region, and hot side and cold side. Good thermal contact of heat sink and heat source to the heat pipe is provided.
In a preferred embodiment, the system is formed with a heat pipe having a combination of two functional materials, with one of the two functional materials in the form of an above-described liquid storage means, especially in the form of a polymer gel. The other functional material preferably takes the form of an ORMOCER® having variable fluid-philic/fluid-phobic properties, preferably under the influence of light, especially UV light.
The object of the invention is likewise achieved by a method having one or more the features described herein. As is likewise known per se, the method of switching and/or programming heat transfer is conducted with a heat pipe having at least one working chamber having at least one evaporator region and at least one condenser region and a working fluid. This comprises the following method steps:
A) evaporating the working fluid in the evaporator region, wherein heat is transferred by the gaseous working fluid from the evaporator region to the condenser region, and
B) condensing the working fluid in the condenser region, wherein the heat is removed to a heat sink.
The essential feature is that the heat pipe is operated as a thermal diode or thermal switch, in that the thermal conductivity is altered by the applying of an external field and/or depending on conditions within the working chamber.
The method of the invention is preferably designed for performance by means of the heat pipe of the invention and/or a preferred embodiment of the heat pipe of the invention. The heat pipe of the invention, by contrast, is preferably designed to perform the method of the invention and/or a preferred embodiment of the method of the invention.
The method of the invention likewise shows the above-described advantages and features of the heat pipe of the invention and/or the system of the invention.
The thermal conductivity of the heat pipe is preferably altered by keeping the evaporator region free of the working fluid and/or stopping the working fluid from evaporating.
In a preferred embodiment of the invention, in the heat pipe, heat is transferred from a hot side disposed on the evaporator region (heat source) to a cold side disposed on the condenser region (heat sink) in a first state of operation. The applying of an external field in a method step C converts the heat pipe to a second state of operation. Preferably, for this purpose, an electrical field, a magnetic field, a stress-strain field is generated, or the activatable functional material is exposed to light, especially UV light, or to heat and/or refrigeration. In the second state of operation, no working fluid, or at least insufficient working fluid, is available in the evaporator region. The evaporator region dries out, and the heat pipe no longer conducts heat in the direction of the preferential direction of heat conduction in the first state of operation.
Alternatively, the working fluid, depending on conditions within the working chamber, can switch from the first state of operation to the second state of operation. Parameters that can initiate change from the first state of operation to the second state of operation are temperature, pH of the working fluid and/or ionic strength of the working fluid. This gives rise to the advantage that the heat pipe can be “programmed” to change state of operation under particular conditions without any need for an external influence.
The working fluid is preferably displaced by means of a switchable surface coating from the evaporator region of the working chamber, as already described above.
Alternatively, the working fluid may be bound by means of an activatable functional material. For this purpose, the at least one activatable functional material preferably takes the form of a reservoir for the working fluid, especially the form of a liquid reservoir. The reservoir controls accommodation and release of the working fluid needed for heat transfer. This means that the available amount of working fluid can be varied. In the first state of operation of the heat pipe, the working fluid is available for heat conduction. The heat pipe conducts heat. In the second state of operation, the blocking state, the working fluid is bound in the reservoir, especially in the form of a liquid reservoir. In this bound form, the working fluid is no longer available for heat transfer. The heat pipe no longer conducts heat.
In a preferred embodiment of the invention, the preferential direction of heat conduction of the thermal diode is reversed, in that the surface properties of evaporator region and condenser region are exchanged by the applying of an external field and/or depending on conditions within the working chamber. In this case, in a second state of operation, the working fluid can evaporate in the original condenser region, now acting as evaporator region, and absorb heat from a heat source and transport it to the original evaporator region, now acting as condenser region. The working fluid condenses in the new condenser region and releases the heat to a heat sink. This turns round the preferential direction of heat conduction by comparison with state of operation 1.
The heat pipe of the invention, the system of the invention and the method of the invention are especially suitable for being able to effectively control switching-on and -off and control or regulation of heat flows. Heat switches or thermal diodes based on heat pipes are especially suitable since these can achieve high switching factors and have only very low heat resistance by virtue of high heat transfer in the conductive state. Furthermore, they can be implemented as very compact designs and are therefore easily integratable. Depending on configuration, the heat pipes are of simple construction, consist of few individual parts and need not include any moving parts.

BRIEF DESCRIPTION OF THE DRAWINGS

Further preferred features and embodiments of the heat pipe of the invention and of the methods of the invention are elucidated hereinafter with reference to working examples and the figures. The figures show:

FIG. 1 : a schematic diagram of a first working example of a heat pipe of the invention,

FIG. 2 : a schematic diagram of a second working example of a heat pipe of the invention.

DETAILED DESCRIPTION

FIG. 1 shows a schematic diagram of a thermal diode with an activatable functional material in the form of a switchable coating in the evaporator region and the condenser region with part-images a) in the conductive state and b) in the blocking state.
The heat pipe 1 has a working chamber 2 with at least one evaporator region 3 and at least one condenser region 4. The evaporator region 3 is operatively connected to a heat source (not shown), in the present case with temperature T₁=100° C., and the condenser region 4 conjunction with a heat sink (not shown) with temperature T₂=10° C. A working fluid 5 is provided in the working chamber 2.
The working chamber 2 in the present context is defined as a closed, pressure-tight volume which is configured such that heat is transferred by means of convection of the evaporated working fluid 5, and transport of the condensed working fluid 5 in the reverse direction.
The working fluid 5 in the present context is water.
The evaporator region 3 and the condenser region 4 are formed with a coating 6 of activatable functional material. The coating 6 both of the evaporator region 3 and of the condenser region 4 is designed such that the surface property of the coating 6 a of the evaporator region 3 is alterable from hydrophilic to hydrophobic and back again, while the surface property of the coating 6 b of the condenser region 4 is alterable from hydrophobic to hydrophilic and back again. The coatings 6 are designed such that evaporator region 3 and condenser region 4 have exactly the opposite surface wetting properties.
In the present context, the coating 6 of activatable functional material takes the form of a switchable coating 6 composed of ORMOCER® and/or comprising ORMOCER®. ORMOCERS®, as already described, are organic-inorganic hybrid polymers that can have an advantageous effect on the surface properties of many substrates. ORMOCERS® may also be formed as switchable coatings 6 from hydrophilic to hydrophobic and back, exploiting mechanisms known from the technical literature; see B. Xin, J. Hao, Chem. Soc. Rev. 39, 2010, 769-782.
In the present context, the coatings 6 a in the evaporator region 3 and 6 b in the condenser region 4 are formed with an electrically switchable ORMOCER®, as described above. In the present context, the coatings consist of an ORMOCER® with functional end groups in the form of methylimidazolium-dodecylsilyl groups. Through application of an electrical field (cf. Langer et al., Science 299, 2003, 371-374), these ionic end groups are repelled by a substrate of the same nominal electrical charge and project through “stretching” of the dodecyl chain into the interior of the thermal diode. In the present context, the substrate in the evaporator region has the same nominal electrical charge. This leads to a hydrophilic property of the surface 6 a in the evaporator region 3. A substrate of opposite electrical charge is provided in the condenser region 4. This, by contrast, attracts the ionic groups, such that the nonpolar dodecyl chains project into the interior of the thermal diode, which leads to a hydrophobic property of the surface 6 b in the condenser region.
Application of an electrical field gives rise to a hydrophilic side and a hydrophobic side, likewise with reversal of the properties by reversal of the field direction.
The heat pipe 1 in the present context thus takes the form of a switchable thermal diode: in a first state of operation, by means of evaporation of the working fluid 5, heat is transported from the heat source to the heat sink, in that heat is transferred by the gaseous working fluid 5 from the evaporator region 3 to the condenser region 4. The evaporator region 3 is heated by the heat source, and the working fluid 5 that has collected on the hydrophilic coating 6 a in the evaporator region 3 evaporates and enables heat transfer from the evaporator region 3 to the condenser region 4. In the condenser region 4, the working fluid 5 condenses on the hydrophobic coating 6 b of the condenser region 4, and the heat is removed to a heat sink. On account of the hydrophobic surface property in the condenser region 4, droplets of the working fluid 5 are formed. On account of the highly hydrophobic configuration of the surface, the working fluid 5 “jumps” back into the evaporator region 3.
The application of an external field, in the present context with a voltage of 5 V, allows the heat pipe 1 to be switched from the first, heat-conducting state of operation to the second, non-heat-conducting state of operation.
The applying of the external field, as described, alters the surface properties of the coating 6 in the evaporator region 3 and in the condenser region 4. The evaporator region 3 on the heat source now has hydrophobic properties. Insufficient working fluid 5 collects on the coating 6 a of the evaporator region 3, and the working fluid 5 that collects there evaporates quickly and condenses on the hydrophilic coating 6 b of the condenser region 4. The working fluid 5 remains there and is not transported back into the evaporator region 3 since the working fluid 5 is not repelled by the now hydrophilic surface. Thus, the hot side of the working chamber 2 dries out, and no heat transfer takes place by the working fluid 5. The thermal diode is in the blocking state.
FIG. 2 shows a schematic diagram of a thermal switch with an activatable functional material in the form of a liquid reservoir with part-images a) in the conductive state and b) in the blocking state.
For avoidance of repetition, merely the differences from FIG. 1 will be discussed hereinafter.
In the present context, the at least one activatable functional material takes the form of a reservoir for the working fluid 5, namely the form of a water-binding hydrogel 7. The water-binding hydrogel 7 in the present context takes the following form:
Hydrogels having a volume phase transition of the LCST type may be produced, for example, by free-radical polymerization using the following monomers. The compositions specified should not be considered to be exhaustive:


Compo-		mol
sition	Monomer
1	%

1		50-80

2		50-85

3		50-85

4		30-80

5		30-80

6		30-80

7		80-98

8		20-80

9		30-90

9		20-80

Compo-		mol
sition	Monomer
2	%

1		0-30

2		2-30

3		2-30

4		10-45

5		10-45

6		10-45

7	—

8		10-50

9		10-40

9		10-50

Compo-		mol
sition	Crosslinker	%

1		2-20

2		2-20

3		2-20

4		2-25

5		2-25

6		2-25

7		2-20

8		2-20

9		2-20

9		2-20

Hydrogels having a volume phase transition of the UCST type may be produced, for example, by free-radical polymerization using the monomers that follow. The compositions mentioned should not be considered to be exhaustive:


Com-
posi-		mol
tion	Monomer
1	%

1		80-98

2		80-98

3		60-90

4		60-90

Com-
posi-		mol
tion	Monomer
2	%

1	—
2	—

3		10-30

4		10-30

Com-
posi-
tion	Crosslinker	mol %

1		2-20

2		2-20

3		2-20

4		2-20

In addition, it is also possible by subsequent crosslinking of soluble polymers to produce suitable hydrogels having a volume phase transition. In order in this way to obtain a hydrogel having a volume phase transition of the LCST type, it is possible, for example, to crosslink partially hydrolyzed poly(vinyl acetate) with butane-1,4-diol diglycidyl ether, poly(ethylene glycol) diglycidyl ether or other di- or multifunctional epoxides.
By virtue of the water-binding hydrogel 7, the amount of working fluid 5 available is made variable. The water-binding hydrogel 7 has a water-binding state and a distinctly less water-binding state. In the present context, the transition from the first operating state to the blocking state of the heat pipe 1, i.e. from a distinctly less water-binding state of the hydrogen 7 to the water-binding state of the hydrogel 7, is induced by a change in temperature, presently within a temperature range from room temperature to about 150° C. This heating is effected by heating of the hot side on the evaporator side, i.e. without an external field.
In the first state of operation of the heat pipe 1, the working fluid 5 is available for heat conduction. The heat pipe 1 conducts heat. In the second state of operation, the blocking state, the working fluid 5 is bound in the water-binding hydrogel 7. In this bound form, the working fluid is no longer available for heat transfer. The heat pipe 1 no longer conducts heat.
By contrast with FIG. 1 , no coating that ensures transport of the working fluid 5 from the condenser region 4 back to the evaporator region 3 is provided. The heat pipe 1 in the present context is therefore formed with fluid recycling in the form of a wick structure (not shown).

Claims

1. A heat pipe (1), comprising:

at least one working chamber (2) having at least one evaporator region (3) operatively connected to a heat source, and at least one condenser region (4) operatively connected to a heat sink;

a working fluid (5) in the working chamber (2) by which heat is transferred from the heat source to the heat sink in a first state of operation;

wherein the heat pipe (1) is configured as at least one of a switchable or programmable thermal diode or as at least one of a switchable or programmable heat switch;

at least one activatable functional material is arranged and configured to at least one of keep the evaporator region (3) free of the working fluid (5) or prevent the working fluid (5) from evaporating in a second state of operation, in order to at least one of reduce or hinder heat transfer alter a preferential direction of heat conduction.

2. The heat pipe as claimed in claim 1, wherein

the heat pipe (1) is configured as a switchable thermal diode or heat switch, and the at least one activatable functional material is configured to at least partly change properties in an external field.

3. The heat pipe as claimed in claim 1, wherein

the heat pipe (1) is configured as a programmable thermal diode or heat switch, and the at least one activatable functional material is configured to change properties depending on conditions within the working chamber (2).

4. The heat pipe as claimed in claim 1, wherein

the working chamber (2) has a closed volume configured for heat transfer by convection of the working fluid (5) that is evaporated and for reverse transfer of the working fluid (5) that is condensed.

5. The heat pipe as claimed in claim 4, wherein

the heat pipe (1) is formed with a fluid circuit for the working fluid (5), and the fluid circuit comprises a fluid recycling conduit for transport of the condensed working fluid (5) from the condenser region (4) back to the evaporator region (3).

6. The heat pipe as claimed in claim 4, wherein

the closed volume has at least one: of a) at least one of a fluid-phobic coating (6) or structuring in the evaporator region (3) or b) at least one of a fluid-philic coating (6) or structuring in the condenser region (4).

7. The heat pipe as claimed in claim 1, wherein

the at least one activatable functional material comprises a switchable coating of at least one of the evaporator region (3) or the condenser region (4), and

at least a surface property of the coating of the evaporator region (3) is variable from fluid-philic to fluid-phobic.

8. The heat pipe as claimed in claim 7, wherein

the switchable coating (6) at least one of is or comprises ORMOCER®.

9. The heat pipe as claimed in claim 1, wherein

the at least one functional material comprises a reservoir for the working fluid (5).

10. The heat pipe as claimed in claim 9, wherein

the reservoir for the working fluid (5) comprises of a gel as an adsorbent or as mesoscopically structured surface.

11. The heat pipe as claimed in claim 9, wherein

the reservoir for the working fluid (5) comprises a polymer gel having a temperature-induced volume phase transition.

12. A system comprising:

the heat pipe as claimed in claim 1; and any of the preceding claims

means of applying a field in order to alter properties of the activatable functional material.

13. The system as claimed in claim 12, wherein

the means of applying a field comprise field generators for at least one of an electrical field, a magnetic field, a stress-strain field, generation of light, light, generation of heat, or generation of refrigeration.

14. The system as claimed in claim 12, wherein

the system includes a combination of two of the activatable functional materials, with one of the two activatable functional materials comprising a liquid reservoir fir the working fluid and the other of the functional material s comprising an ORMOCER® of variable fluid-philic/fluid-phobic properties.

15. A method of at least one of switching or programming heat transfer in a heat pipe having at least one working chamber (2) having at least one evaporator region (3) and at least one condenser region (4), and a working fluid (5), the method comprising the following method steps:

A) evaporating the working fluid (5) in the evaporator region (3), and transferring heat by the gaseous working fluid (5) from the evaporator region (3) to the condenser region (4), and

B) condensing the working fluid (5) in the condenser region (4), and removing the heat to a heat sink,

operating the heat pipe (1) as a thermal diode or thermal switch, and altering a thermal conductivity by the applying of an external field and/or depending on conditions within the working chamber (2).

16. The method as claimed in claim 15, wherein

the thermal conductivity of the thermal diode or of the heat switch is altered by at least one of keeping the evaporator region (3) free of the working fluid (5) or stopping the working fluid (5) from evaporating.

17. The method as claimed in claim 15, further comprising reversing

a preferential direction of thermal conduction of the thermal diode by exchanging is surface properties of an evaporator region (3) and a condenser region (4) by the applying of an external field and/or depending on conditions within the working chamber (2).