U.S. patent application number 14/128266 was filed with the patent office on 2014-06-12 for thermal management system with variable-volume material.
This patent application is currently assigned to Commissariat a l'energie atomique et aux ene alt. The applicant listed for this patent is Philippe Coronel, Olivier Dellea, Jerome Gavillet, Emmanuel Ollier, Helga Szambolics. Invention is credited to Philippe Coronel, Olivier Dellea, Jerome Gavillet, Emmanuel Ollier, Helga Szambolics.
Application Number | 20140158334 14/128266 |
Document ID | / |
Family ID | 46321044 |
Filed Date | 2014-06-12 |
United States Patent
Application |
20140158334 |
Kind Code |
A1 |
Dellea; Olivier ; et
al. |
June 12, 2014 |
THERMAL MANAGEMENT SYSTEM WITH VARIABLE-VOLUME MATERIAL
Abstract
A thermal management system configured to be installed between a
heat source and a heat sink, including a first heat conductor and a
second heat conductor, a thermal switch configured to allow or
prevent thermal connection between the first and second heat
conductors, the thermal switch including at least one thermally
conductive material that can connect the first and second
conductors by a change in its volume, and the thermal switch
including a controller configured to transfer thermal energy to the
phase-change material to change a connection state. The connection
is made when the heat source goes above a critical temperature,
since the connection enables a heat flux to be established between
the heat source and the heat sink.
Inventors: |
Dellea; Olivier; (La
Talaudiere, FR) ; Coronel; Philippe; (Barraux,
FR) ; Gavillet; Jerome; (Saint-Egreve, FR) ;
Ollier; Emmanuel; (Grenoble, FR) ; Szambolics;
Helga; (Grenoble, FR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Dellea; Olivier
Coronel; Philippe
Gavillet; Jerome
Ollier; Emmanuel
Szambolics; Helga |
La Talaudiere
Barraux
Saint-Egreve
Grenoble
Grenoble |
|
FR
FR
FR
FR
FR |
|
|
Assignee: |
Commissariat a l'energie atomique
et aux ene alt
Paris
FR
|
Family ID: |
46321044 |
Appl. No.: |
14/128266 |
Filed: |
June 21, 2012 |
PCT Filed: |
June 21, 2012 |
PCT NO: |
PCT/EP2012/062001 |
371 Date: |
February 25, 2014 |
Current U.S.
Class: |
165/185 |
Current CPC
Class: |
H01L 2924/0002 20130101;
G05D 23/028 20130101; F28D 15/00 20130101; F28D 20/02 20130101;
F28F 2013/008 20130101; F28F 13/00 20130101; H01L 2924/0002
20130101; H01L 2924/00 20130101; H01L 23/4275 20130101; F28F 27/00
20130101; F28F 2013/005 20130101 |
Class at
Publication: |
165/185 |
International
Class: |
F28F 27/00 20060101
F28F027/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 22, 2011 |
FR |
11 55511 |
Claims
1-28. (canceled)
29. A thermal management system configured to be installed between
at least one heat source and at least one heat sink, comprising: a
first heat conductor and a second heat conductor; a thermal switch
configured to allow or prevent thermal connection between the first
and second heat conductors, the thermal switch including at least
one thermally conductive material, a volume of which varies
according to a thermal energy input, the material configured to
connect the first and second conductor by a change in its volume,
and the thermal switch including a controller configured to provide
thermal energy to the material to change a connection state.
30. A thermal management system according to claim 29, comprising
at least three heat conductors, the switch being configured to put
the at least three heat conductors into thermal connection with one
another.
31. A thermal management system according to claim 29, distributed
in plural planes.
32. A thermal management system according to claim 29, in which the
controller is formed by the first conductor, the first conductor
being in permanent contact with the material, and is configured to
be connected to the at least one heat source.
33. A thermal management system according to claim 29, in which the
controller includes an additional heat conductor, the first and the
second conductors not being in contact with the material in an
unconnected state.
34. A thermal management system according to claim 29, in which the
controller is formed directly by an external environment, and the
thermal energy is provided to the material by convection.
35. A thermal management system according to claim 29, in which the
first and second conductors have ends configured to be in contact
with the material, the end of the first and/or second conductor
being shaped to provide gradual contact between the end and the
material when the volume of the material changes.
36. A thermal management system according to claim 29, configured
to facilitate return of the material to a predetermined area in
which the first and second conductors are thermally unconnected in
a disconnection area.
37. A thermal management system according to claim 29, further
comprising a substrate in or on which the heat conductors are
formed, and in which the switch comprises a cavity formed in the
substrate and containing the material, the heat conductors
penetrating into the cavity.
38. A thermal management system according to claim 37, in which the
cavity comprises at least one inclined side wall facilitating
return of the material into a disconnection area.
39. A thermal management system according to claim 37, in which the
cavity has a flared shape.
40. A thermal management system according to claim 37, in which the
two heat conductors are formed on the surface of the substrate, the
end of the first conductor penetrating into the cavity more deeply
than the end of the second conductor, and in which the material is
in contact only with the end of the first conductor in an
unconnected state.
41. A thermal management system according to claim 37, in which the
first conductor is formed in the substrate and its end emerges in a
base of the cavity and the second conductor is formed on the
substrate.
42. A thermal management system according to claim 37, in which the
cavity is closed in a sealed fashion by a cover.
43. A thermal management system according to claim 29, in which the
variable volume material is a phase change material.
44. A thermal management system according to claim 43, in which the
material has a solid-liquid phase change in a temperature range
which is to be managed by the system.
45. A thermal management system according to claim 43, in which the
material subject to a phase change comprises particles increasing
its thermal conductivity.
46. A thermal management system according to claim 29, in which the
variable volume material is a monophasic material.
47. A thermal management system according to claim 29, in which the
variable-volume material is functionalized such that it has a given
electrical conductivity, sensitivity to magnetic fields, or is
subject to a photoluminescence phenomenon.
48. A thermal management system according to claim 29, in which the
substrate is a material of low thermal conductivity.
49. A thermal management system according to claim 29, in which the
substrate is a thermally insulating material.
50. A thermal management system according to claim 29, in which the
substrate is surface-insulated thermally conducting.
51. A thermal management system according to claim 29, in which the
heat conductors are metal.
52. A thermal management assembly comprising at least two systems
according to claim 29, in which the switch of one of the systems is
controlled by the first system.
53. A thermal management assembly according to claim 52, in which
control is provided by a temperature of the material of the switch
of the first system, or that of a heat source or that of a heat
sink.
54. A thermal management assembly according to claim 52, in which
the material of the first system is different than that of the
second system.
55. An electronic device comprising at least one thermal management
system according to claim 29 and/or at least one thermal management
assembly.
56. An electronic device according to claim 55, in which the heat
source is formed by at least one electronic component, and at least
one heat sink is formed by a heat exchanger.
57. A thermal management system according to claim 29, in which the
material is a liquid monophasic material.
58. A thermal management system according to claim 50, in which the
substrate is a silicon substrate with an oxide layer on its face,
on which the first and second heat conductors are formed.
Description
TECHNICAL FIELD AND PRIOR ART
[0001] The present invention relates to a thermal management system
for controlling the transfer of a heat flux from a heat source to a
heat sink.
[0002] In the electronics field, rapid changes, relating in
particular to increased power densities and operating speeds, are
making thermal management through dissipation a dimensioning
parameter in the design of electronics systems. Thermal stress has
a direct influence on performance, reliability and the cost of an
electronic system.
[0003] In addition to small-signal transistors and integrated
circuits, electronic systems also now contain medium-power
transistors in units of the CMS (Surface-Mounted Components) type,
the power dissipations of which for an isolated transistor are
approaching one watt. In terms of heat fluxes, a thyristor may
generate a flux of the order of 100 to 200 W/cm.sup.2. Some power
electronics components for military applications may generate a
heat flux of the order of 300 W/cm.sup.2 and systems using laser
diodes up to 500 W/cm.sup.2. Non-uniform distribution of these heat
fluxes in a printed circuit board may result in the presence of
areas having a heat flux of up to five times higher than the
average heat flux found in a printed circuit board (.about.30
W/cm.sup.2).
[0004] A recent study by the US Air Force concluded that nearly 55%
of failures of electronic systems are due to heat problems.
[0005] Management and dissipation of heat are therefore crucial
problems for maintaining the temperature of each element at its
nominal operating temperature, which varies depending on the field
of application.
[0006] Several techniques exist to dissipate heat from electronics
systems.
[0007] Passive radiators exist which consist of fins made of
thermally conductive material which transfer the heat emitted by
the active components to the ambient air.
[0008] Active radiators also exist; these are radiators fitted with
ventilators, the ventilator being positioned above the radiator to
facilitate heat extraction.
[0009] Ventilators are also commonly used, for example, for the
whole of the casing of the electronic device. The air is then
subject to forced convection, substantially increasing the heat
exchange coefficients.
[0010] Internal circulation of a fluid, for example by means of a
pump, may also be envisaged. Phase-change materials have also been
envisaged for passive cooling of electrical and electronic
components. Heat pipes and micro heat pipes are also effective.
Microchannels have also been envisaged to extract the heat from
electronic components. However, a pump is used which may cause
disruption and noise disturbance.
[0011] Thermal management in the electronics field is therefore
particularly critical, especially since systems are providing an
increasing number of functions within an increasingly small
volume.
DESCRIPTION OF THE INVENTION
[0012] The aim of the present invention is to provide a simple and
robust system for thermal management of the operating heat flux,
which is able to guide the heat flux, for example with a view to
its dissipation.
[0013] The aim stated above is attained through a thermal
management system comprising means able to put two heat conductors
in thermal communication, or to prevent such communication these
means comprising a material the volume of which varies according to
temperature changes. The variable-volume material may therefore be
heated for example either by one of the conductors or by an
additional heat conductor forming thermal control means.
[0014] Management of the heat transfer between the two conductors
can then be accomplished automatically and very safely.
[0015] In one embodiment, the thermal management system according
to the invention may then form a thermal switch made of a
volume-expansion material, this switch allowing, in its closed
state, heat conduction between the two conductors. The heat flux
transmitted from one conductor to the other may vary gradually. In
another embodiment the heat system may "manage" the thermal
conduction between 2 conductors by a heat flux deriving from a
third conductor; in this case it may be considered equivalent to a
"thermal transistor". In one variant, the volume-expansion material
may have other physical properties such as, for example, electrical
conduction, the management system then having a dual function, both
thermal and electrical.
[0016] For example, the material used is a solid-liquid
phase-change material.
[0017] The subject-matter of the present invention is then a
thermal management system intended to be installed between at least
one heat source and at least one heat sink, comprising a first and
second heat conductor, a thermal switch able to allow or prevent
thermal connection between the first and second heat conductors,
said thermal switch comprising at least one thermally conductive
material, the volume of which varies according to a thermal energy
input, said material being able to connect the first and second
conductor by means of a change in its volume, and said thermal
switch comprising control means able to provide thermal energy to
the variable-volume material to change the connection state.
[0018] The thermal management system may comprise at least three
heat conductors, the switch being able to put the at least three
heat conductors into thermal connection with one another.
[0019] In one example embodiment the thermal management system may
be distributed in several planes.
[0020] In one embodiment the control means are formed by the first
conductor, said first conductor being in permanent contact with the
variable-volume material, and is intended to be connected to the at
least one heat source.
[0021] In another embodiment, the control means are formed by an
additional heat conductor, the first and second conductors being
not in contact with the variable-volume material in an unconnected
state.
[0022] In another embodiment the control means are formed directly
by the external environment, where the heat energy is provided to
the material by convection.
[0023] For example, the first and second conductors have ends
intended to be in contact with the variable-volume material, said
end of the first and/or second conductor being shaped so as to
provide gradual contact between said end and the variable-volume
material when the volume of the variable-volume material
changes.
[0024] The thermal management system preferably comprises means to
facilitate the variable-volume material's return to a predetermined
area in which the first and second conductors are thermally
unconnected, called the disconnection area.
[0025] The thermal management system may comprise a substrate in or
on which the heat conductors are formed, and in which the switch
comprises a cavity formed in the substrate and containing the
variable-volume material, the heat conductors penetrating into said
cavity.
[0026] For example, the cavity comprises at least one inclined side
wall facilitating the return of the variable-volume material into
an area called the disconnection area, said at least one inclined
surface forming the means facilitating the return of the
variable-volume material to the disconnection area.
[0027] The cavity may have a flared shape, for example the shape of
an inverted pyramid.
[0028] In one example embodiment the two heat conductors are formed
on the surface of the substrate, the end of the first conductor
penetrating into the cavity more deeply than does the end of the
second conductor (and in which the variable-volume material is in
contact only with the end of the first conductor in a unconnected
state.
[0029] In another example embodiment the first conductor is located
in the substrate and its end emerges in a base of the cavity, and
the second conductor is formed on the substrate.
[0030] The cavity may be closed in sealed fashion by a cover.
[0031] In a preferred example the variable-volume material is a
material which is subject to a phase change. The material may have
a solid-liquid phase change in the temperature range which is to be
managed by the system. The material subject to a phase change
preferably comprises particles improving its thermal
conductivity.
[0032] The variable-volume material may be a monophasic material,
for example a liquid material, for example mercury.
[0033] In one variant the variable-volume material is
functionalised such that it has a given electrical conductivity,
sensitivity to magnetic fields, or is subject to a
photoluminescence phenomenon.
[0034] The substrate may be made from a material of low thermal
conductivity. For example, it may be a thermally insulating
material such as a polymer, glass or a ceramic.
[0035] Alternatively, the substrate may be a surface-insulated
thermally conducting material such as, for example, a silicon
substrate with an oxide layer on its face, on which the first and
second heat conductors are formed.
[0036] For, the heat conductors are metallic, such as for example
gold, copper or aluminium.
[0037] Another object of the present invention is a thermal
management assembly comprising at least two systems according to
the present invention, in which the switch of one of the systems is
controlled by the first system.
[0038] Control may be provided by the temperature of the
variable-volume material of the switch of the first system, or that
of the heat source or that of the heat sink.
[0039] The variable-volume material of the first system may be
different to that of the second system.
[0040] Another subject-matter of the present invention is an
electronic device comprising at least one thermal management system
according to the present invention and/or at least one thermal
management assembly according to the present invention.
[0041] The heat source is, for example, formed by at least one
electronic component, and the at least one heat sink is, for
example, formed by a heat exchanger, for example a radiator and/or
an air flow, and/or at least one micro heat pipe and/or a
phase-change material and/or microchannels and/or means
implementing convective boiling and/or thermoelectric
materials.
BRIEF DESCRIPTION OF THE ILLUSTRATIONS
[0042] The present invention will be better understood using the
description which follows and the illustrations, in which:
[0043] FIG. 1A is a schematic representation of an embodiment of a
thermal management system according to the present invention in a
thermally unconnected state,
[0044] FIG. 1B is a schematic representation of the thermal
management system of FIG. 1A in a thermally connected state,
[0045] FIG. 2A is a schematic representation of a thermal
management system forming a gradual thermal connection,
[0046] FIGS. 2B to 2E represent schematically variant embodiments
of systems providing a variable heat transfer,
[0047] FIG. 3A is a schematic representation of an embodiment of a
thermal management system comprising separate control means, in a
thermally unconnected state,
[0048] FIG. 3B is a schematic representation of the thermal
management system of FIG. 3A in a thermally connected state,
[0049] FIG. 4A is a schematic representation of an embodiment of a
system for thermal management by the external environment, in a
thermally unconnected state,
[0050] FIG. 4B is a schematic representation of the thermal
management system of FIG. 4A in a thermally connected state,
[0051] FIGS. 5A and 5B are schematic representations of embodiments
of the thermal management system according to the invention,
[0052] FIGS. 6A and 6B are schematic representations of a thermal
system in several planes in a thermally unconnected state, and in a
thermally connected state, respectively,
[0053] FIG. 7 is a schematic representation of an architecture
comprising several interconnected thermal management systems,
[0054] FIG. 8A is a perspective view of a practical example
embodiment of a thermal management system according to the
invention,
[0055] FIG. 8B is a section of the system of FIG. 8A comprising the
thermally conductive fluid,
[0056] FIG. 8C is a view similar to that of FIG. 8B fitted with
encapsulation means,
[0057] FIG. 9 is a graphical representation of the variation in
height in .mu.m of a phase-change material, and of the temperature
difference in .degree. C. applied according to the % volume
increase of the phase-change material in the system of FIG. 8A,
[0058] FIG. 10 is a perspective view of a variant embodiment of the
system of FIG. 8A.
DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS
[0059] In all the figures the heat fluxes are represented
symbolically by arrows in the form of chevrons.
[0060] In FIGS. 1A and 1B a first embodiment of a thermal
management system according to the invention may be seen comprising
a first heat conductor 2 and a second heat conductor 4 able to be
connected by a thermal switch 6. First conductor 2 is connected by
a first end 2.1 to a heat source SC and by a second end 2.2 to
switch 6, and second heat conductor 4 is connected by a first end
4.1 to switch 6 and by a second end 4.2 to a heat sink SF.
[0061] Heat source SC is, for example, formed by a power
transistor, a laser diode, an integrated circuit, etc., and a heat
sink SF is, for example, formed by a passive radiator, an active
radiator, etc.
[0062] The system is produced in a substrate 8.
[0063] Heat conductors 2, 4 are, for example, produced in the form
of metal tracks on the surface of substrate 8.
[0064] As a variant, the conductors may be formed by wires, and may
be buried.
[0065] The substrate preferably has a low thermal conductivity,
typically less than 5 W/.degree. Km, being for example made of
glass or polymer. As a variant, it may also be envisaged to
insulate conductors 2, 4 thermally relative to the substrate, by
producing a barrier layer between the conductors and the substrate.
For example, in the case of a silicon substrate, the barrier layer
may be generated by surface oxidation, in order to obtain a layer
of silicon dioxide.
[0066] Switch 6 comprises a chamber 10 containing a material 12,
the volume of which increases when it is heated. Above a given
temperature, called critical temperature T.sub.C, the material
provides a thermal connection between the first and second
conductors. The heat-transfer material partially fills chamber 10,
at least when it does not thermally connect the two heat
conductors.
[0067] The variable-volume material will be called the
"heat-transfer material" in the remainder of the description. This
may be a monophasic material, for example a material which is
liquid whatever the operating temperature of the system, such as
mercury. In this case its volume increases with the temperature and
becomes sufficient, when the temperature reaches T.sub.C, to be in
contact with both conductors.
[0068] It may be a diphasic material which is in the solid state
below the critical temperature, and liquid above critical
temperature T.sub.C, for example a phase-change material. In this
case the volume of this material in the liquid state is greater
than that in the solid state. When the temperature reaches T.sub.C
the material becomes molten and its volume increases; the thermal
connection between the conductors is then made when the volume of
the material is sufficient. These may be materials which are
generally designated "phase-change materials", but any materials
with a solid-liquid transition may also be used.
[0069] Second end 2.2 of first conductor 2 penetrates into chamber
10, and first end 4.1 of second conductor 4 also penetrates into
chamber 10, such that conductors 2, 4 can come into contact with
heat-transfer material 12, and effectively be thermally connected
to one another through the latter above the critical
temperature.
[0070] Examples of materials which may be used will be described in
detail below.
[0071] Switch 6 preferably comprises means to ensure that the
heat-transfer material is once again only in contact with the
second end of the first conductor when the temperature descends
below the critical temperature. These means will be described
below.
[0072] In FIG. 1B, in which neither of heat conductors 2, 4 are
connected, heat-transfer material is in contact with first
conductor 2 connected to heat source SC; heat-transfer material 12
is thus connected to heat source SC and experiences all temperature
variations of heat source SC. When the temperature of heat source
SC goes above critical temperature T.sub.C, the volume of
heat-transfer material 12 is sufficient to come into contact with
end 4.1 of second conductor 4. The thermal connection between the
two conductors 2, 4 is then made (FIG. 1B), and the heat flux is
transferred from heat source SC to heat sink SF. The heat is then
dissipated, and heat source SC is cooled. The volume variation may,
for example, be of the order of 10%. The required volume variation
depends on the properties of the variable-volume material. The size
of the chamber and the dimensions of the conductors are chosen
accordingly.
[0073] Conversely, when the temperature of heat source DC drops and
becomes less than critical temperature T.sub.C, the volume of
heat-transfer material 12 is reduced sufficiently that it is no
longer in contact with second conductor 4. The latter regains its
initial position of being in contact only with first conductor 2,
which interrupts the thermal connection between the two conductors
2, 4.
[0074] Switch 6 is therefore directly controlled by the heat
emitted by heat source SC.
[0075] In FIG. 2A a schematic representation is shown of the
different steps of the variation of volume of the heat-transfer
material, these steps being represented as dashed lines 12.1. FIG.
2A shows the gradual increase of the volume of heat-transfer
material. The covering of first end 4.1 of second conductor 2 by
heat-transfer material 12 is therefore gradual. And since the heat
transfer is directly proportional to the contact area, the
transmitted heat flux therefore also increases gradually. Switch 6
consequently forms a thermal transfer variation. This gradualness
of the transfer depends in particular on the dimension of first end
4.1 of second conductor 4 penetrating into chamber 10, and which
may therefore be surrounded by heat-transfer material 12. Indeed,
the shorter this length, the more rapidly the first end is fully
covered. The gradualness of the thermal connection may thus be
adjusted. This gradualness may allow a certain degree of regulation
within the system.
[0076] The geometry and volume of the chamber and the containment
in which it results, the quantity of variable-volume material, the
distance between the conductors, the choice of material and the
surface conditions to define the wettability of the surface and to
define the migration process of the liquid front enable the
reactivity of the switch to be adjusted. For example, by preferring
a small chamber volume, a short distance between the conductors, a
wetting surface condition to facilitate thermal transfer and a
rapid phase transition of the phase-change material, a reactive
device may be obtained.
[0077] In FIGS. 2B to 2E various example embodiments of a first end
4.1 of second conductor 4 may be seen enabling the gradualness of
the thermal connection to be improved.
[0078] In FIG. 2B first end 4.1 of second conductor 4 comprises a
face 14 which is skew relative to the front of heat-transfer
material 12. The contact between heat-transfer material 12 and
first end 4.1 thus occurs gradually, as may be seen in FIG. 2C.
[0079] In FIG. 2D, in addition to the skew face, end 4.1 of
conductor 4 is fitted with fingers 16; by this means the contact
between heat-transfer material and first end 4.1 is even more
gradual. Heat-transfer material 12 first comes into contact with
longest finger 16.1, and then with second finger 16.2;
simultaneously it covers finger 16.1 to a greater extent; and it
continues in this fashion until it covers all fingers 16, and
attains a maximum thermal connection area.
[0080] In the example represented in FIGS. 2B to 2E, the first end
of the second conductor has a larger section than the remainder of
the conductor, but this is in no sense restrictive.
[0081] In FIGS. 3A and 3B another embodiment of a thermal
management system may be seen in which the switch is controlled by
a source of heat other than the heat source.
[0082] The system comprises a third heat conductor 18, one end of
which penetrates into chamber 10, and another end of which is
connected to a source of heat. In addition, when the temperature is
less than the critical temperature, heat-transfer material 12 is in
contact only with third conductor 18. In the represented example,
third conductor 18 is positioned between the two ends 2.2, 4.1 of
first conductor 2 and second conductor 4.
[0083] When critical temperature T.sub.C is applied to the third
conductor, the volume of heat-transfer material 12 is sufficient to
bring ends 2.2, 4.1 of first conductor 2 and second conductor 4
into contact, thus providing the thermal connection between the two
conductors 2, 4. When the temperature of third conductor 18 is less
than critical temperature T.sub.C, the volume of heat-transfer
material 12 is such that it is no longer in contact with either of
the first and second conductors. It returns to its initial
position, and the thermal connection is interrupted.
[0084] In this case the change in volume of the heat-transfer
material occurs in two opposite directions.
[0085] The temperature of third conductor 18 may be imposed by
another thermal management system, as will be seen below.
[0086] As a variant, the third conductor may be positioned opposite
the two heat conductors.
[0087] In FIGS. 4A and 4B, another embodiment may be seen in which
the thermal connection is controlled by the overall temperature of
the device in which the system is installed. Chamber 10 is subject
to the external temperature imposed by the entire device which it
is desired to cool, and the thermal exchanges occur by natural
convection through the wall of chamber 10. When the temperature is
below a critical temperature T.sub.C the heat-transfer material is
contained between the ends of the two conductors 2, 4 and there is
no contact with them. When this temperature goes above critical
temperature T.sub.C, the volume of heat-transfer material 12 is
sufficient to connect the two conductors 2, 4 thermally. In this
example the material of the wall of chamber 10 is such that it
facilitates natural convection.
[0088] In FIGS. 5A and 5B variant embodiments of the thermal switch
according to the invention may be seen. In FIG. 5A the switch
enables four conductors 2, 4, 4', 4'' to be connected; in this case
the connection is controlled by the heat source as in the system of
FIG. 1A.
[0089] In FIG. 5B the switch enables three heat conductors 2, 4, 4'
to be connected; connection is controlled by a conductor 18 which
is independent of heat source SC as in the example of FIG. 5B.
[0090] In FIGS. 6A and 6B an example of a thermal management system
in several planes may be seen.
[0091] The system comprises a first system similar to that of FIG.
1A comprising a heat source SC, a first conductor 402 connected to
heat source SC, a second conductor 404 connected to a first heat
sink SF1 and a switch 406 intended to connect the two conductors
402, 404, on which is superposed a substrate 408 containing a heat
conductor 422 traversing substrate 408 forming a thermal via, a
conductor 424 deposited on substrate 408 and a second heat sink
SF2. Thermal via 422 emerging above switch 406 in the upper wall
contains an aperture to allow the heat-transfer material to be
brought into contact with thermal via 422.
[0092] Switch 406 is directly controlled by heat source SC. When
the temperature goes above critical temperature T.sub.C, the volume
of the heat-transfer material makes the thermal connection between
first conductor 402 and second conductor 404, and thus heat source
SC with first heat sink SF1. When the heat accumulated in the
heat-transfer material increases, its volume increases until it
reaches the upper wall of switch 406 and comes into contact with
thermal via 422. Heat source SC is then connected to first heat
sink SF1 and to second heat sink SF2.
[0093] The volume variation occurs in two orthogonal
directions.
[0094] In this example embodiment, if variable-volume material 12
is a solid-liquid phase-change material, the thermal connection
between heat source SC and first heat sink SF1 is made when only a
portion of material 12 has changed to the liquid state. The
connection with second heat source SC2 is made after an additional
portion of the material has changed to the liquid state.
[0095] This system advantageously enables cooling safety to be
improved. If the heat flux between the heat source and the first
heat sink proves to be insufficient to lower the temperature of the
heat source below the critical temperature, switch 406 detects
this, and provides a connection with a second heat sink, in order
to increase the heat flux extracted from the heat source.
[0096] It may be envisaged that one or more heat sources are
connected to several heat sinks by means of systems according to
the present invention.
[0097] In FIG. 7 an example architecture of an assembly of
interconnected management systems according to the intervention may
be seen.
[0098] The architecture of FIG. 7 comprises a system S1 which is
similar to that of FIG. 1A, a management system S2 similar to that
of FIG. 3A, and a third management system S3 combining management
systems of FIGS. 1A and 3A.
[0099] First system S1 comprises a first conductor 102 connected to
a first heat source SC1, a second conductor 104 connected to a
first heat sink SF1, and a switch 106 directly controlled by the
temperature of heat source SC1.
[0100] Second system S2 comprises a first conductor 202 connected
to a second heat source SC2, a second conductor 204 connected to a
second heat sink SF2, and a switch 206. A third conductor 218
controls the switching of second conductor 206, where third
conductor 218 is at the temperature of first switch 106 and
therefore at the critical temperature of first system S1.
[0101] Third system S3 comprises a first conductor 302 connected to
a third heat source SC3 and a second conductor 304 connected to a
third heat sink SF3 through switch 306', where conductor 302 and
second conductor 304 are able to be connected by a third switch 306
directly controlled by the temperature of heat source SC3. Third
system S3 also comprises a fourth switch 306' controlled by a
conductor 318 connected to second heat sink SF2. The third system
also comprises a conductor 320 connecting fourth switch 306' to
third heat sink SF3.
[0102] The switches have different critical temperatures such
that:
T.sub.C106<T.sub.C206<T.sub.C306<T.sub.C306'.
[0103] These variations of critical temperature may be obtained
either by altering the design of the device, or by altering the
composition and nature of the material.
[0104] We shall now explain the operation of this assembly.
[0105] When first heat source SC1 reaches critical temperature
T.sub.C, the heat-transfer material of first switch 206 brings into
contact first and second conductors 102, 104, and the thermal
connection between the first heat source and first heat sink is
made.
[0106] Third conductor 218 of second system S2 is covered by the
heat-transfer material of first switch 106 of first system S1; the
latter is then at critical temperature T.sub.C. The volume of the
heat-transfer material of second switch 206 increases, making the
thermal connection between second heat source SC2 and heat sink
SF2. In the represented configuration the material of first switch
106 will reach third conductor 218 before reaching second conductor
104.
[0107] As regards third system S3, the thermal connection between
third heat source SC3 and third heat sink SF3 is controlled by
switch 306, which is directly controlled by third heat source SC3
and switch 306', which is controlled by second heat sink SF2. Third
heat source SC3 will initially be in contact with second heat sink
SF2, and then in contact with second and third heat sinks SF2 and
SF3, if the heat flux is sufficient.
[0108] It should be noted that in order to activate a switch the
substantial heat before the change of phase and then the latent
heat causing the change of phase must first be accumulated. The
material then has a buffer effect in the heat transfer. This buffer
effect means that this substantial heat absorption before the
change of phase is able to allow very short temperature rises,
requiring no connection to the heat sink, to be absorbed. In the
case of a high and persistent temperature the connection is indeed
made after absorption of the phase-change heat.
[0109] By virtue of the invention, architectures may thus be
produced with a number of inputs and outputs which are adjustable
and configurable in terms of dimension, material and shape.
[0110] Each of the systems may comprise a heat-transfer material
which is different to that of the other systems, and which is
suitable, for example, for the critical temperature of each
system.
[0111] We shall now describe a practical example embodiment of a
thermal management system according to the present invention.
[0112] In FIGS. 8A to 8C, a perspective view of an example
embodiment of a thermal management system according to the present
system such as that of FIG. 1A may be seen. A cavity 26 forming
chamber 10 of switch 6 is made in substrate 8, and first conductor
2 and second conductor 4 are produced in upper face 8.1 of
substrate 8.
[0113] First conductor 2 comprises one end 2.2 penetrating into
cavity 26 more deeply than end 4.1 of second conductor 4, such that
first conductor 2 is in contact with the heat-transfer material
when this material is not expanded.
[0114] Cavity 26 is shaped such that it facilitates the return of
the heat-transfer material to its unconnected position. Cavity 26
has a deeper first portion 26.1 in the shape of a parallelepipedic
rectangle, and a second portion 26.2 emerging at the surface of
substrate 8; the second portion has two inclined opposite faces 28
opening towards the upper surface.
[0115] The surfaces of the cavity may advantageously be subjected
to a treatment facilitating "dewetting" of the material when the
volume of the material is reduced, allowing its return to a state
of disconnection of the system to be improved.
[0116] The parallelepipedic shape of the base of the cavity is in
no sense restrictive, and it could have an inclined shape,
cylindrical shape, rounded shape, etc.
[0117] First conductor 2 extends along the entire length of side 28
of second portion 26.2, while second conductor 4 extends only over
a portion of a side 28 opposite that on which first conductor 2 is
located.
[0118] The heat-transfer material fills first portion 26.1 and the
base of second portion 26.2 such that it is in contact with first
conductor 2, as may be seen in FIG. 1C.
[0119] The switch is directly controlled by the heat source. When
the temperature goes above critical temperature T.sub.C, the
heat-transfer material comes into contact with the second
conductor, and provides the thermal connection.
[0120] When the temperature is again below critical temperature
T.sub.C, the volume of the heat-transfer material has decreased
such that it is no longer in contact with second conductor 4. The
flared shape of second portion 26.1 of cavity 26 ensures that the
heat-transfer material returns to its unconnected position.
[0121] It could be arranged for the cavity to have inclined walls
over its entire height.
[0122] In addition, cavity 26 could have a tapered shape. The shape
of FIG. 8A has the advantage that flat conductors can be produced
on the sides of the second portion of the cavity.
[0123] In FIG. 10 another example embodiment of a switch may be
seen in which the second portion of the cavity has an inverted
pyramid shape. In this example, first conductor 2 is produced
within substrate 8 and emerges in the base of cavity 26, and second
conductor 4 is similar to that of FIG. 8A. However, it extends more
deeply than second conductor 4 of FIG. 8A, since the quantity of
heat-transfer material may be smaller, and the contact with first
conductor 2 occurs in the base of cavity 26.
[0124] Cavity 26 preferably comprises at least one inclined
side.
[0125] The system may also being encapsulated, as is represented in
FIG. 8C. The system of FIG. 8C has a cover 30 closing cavity 26 in
sealed fashion, thereby making the device very practical and easy
to use. Cover 30, made for example of glass or metal, is for
example attached in sealed fashion on to substrate 8 by a bead of
cement.
[0126] We shall explain the operation of the switch of FIG. 8A. We
consider the case in which a diphasic phase-change (solid/liquid)
material is chosen as the heat-transfer material.
[0127] In the example represented in FIG. 8A, the surface of the
phase-change material is in contact with the ambient air, with
which it exchanges heat only by natural convection.
[0128] When the first conductor is at a temperature above critical
temperature T.sub.C the phase-change material begins to melt. This
melting absorbs energy in latent form. Natural convection currents
gradually occur in the liquid phase-change material. To remedy the
low thermal conductivity of the phase-change materials, we can
consider a phase-change material formed from a pure paraffin wax
and nanoparticles of highly conductive graphite. Certain
compositions bring the thermal conductivity of the phase-change
material to a value of around one (SI) [0]. After the start of the
natural convection in the liquid phase-change material the melting
process is driven by natural convection.
[0129] When it changes to the liquid phase the phase-change
material increases in volume and comes into contact with second
conductor 4.
[0130] As an example, the change in height of the surface of the
phase-change material and also the increase of volume of the
phase-change material, required for making the thermal connection,
may be calculated.
[0131] For example, considering a phase-change material filled with
graphite nanoparticles the characteristics of which are: [0132]
Dynamic viscosity (.mu.): 5.times.10.sup.-3 Pas, [0133] Density
(.rho.): 800 kgm.sup.-3, [0134] Thermal conductivity (.lamda.): 1
Wm.sup.-1 K.sup.-1, [0135] Specific heat (cp): 2500 Jkg.sup.-1
K.sup.-1, [0136] Latent heat (L): 200 kJkg.sup.-1, [0137]
Coefficient of thermal expansion (.beta.): 10-3 K.sup.-1, [0138]
Prandtl number: 12.5, [0139] Phase-change temperature (T.sub.pc):
20.degree. C.
[0140] On this basis, the variation of height of the surface of the
phase-change material (h) may be calculated as a function of the
increase of the volume of the phase-change material (expressed as a
%). The correspondence between the increase of the phase-change
material's volume and the temperature difference applied to the
material (.DELTA.T) may also be established by the following
relationship:
.DELTA.V=.beta..times.V.sub.0.times..DELTA.T
[0141] where .DELTA.V is the increase in volume and V.sub.0 is the
initial volume.
[0142] In FIG. 8C the variation of height of heat-transfer material
in cavity 26 can be seen represented symbolically.
[0143] In FIG. 9 the variation in height (in .mu.m) of the
phase-change material and in temperature difference (in .degree.
C.) applied may be seen as a function of the increase of volume (in
%) of the phase-change material.
[0144] These curves indicate, for example, that the surface of the
phase-change material will reach second conductor 4 connected to
the heat sink when its volume has increased by approximately 6.20,
the height then being h.sub.c=2100 .mu.m. Bearing in mind that the
material has a coefficient of thermal expansion equal to 10.sup.-3
K.sup.-1, this increase of volume corresponds to a temperature
difference of approximately 62.degree. C.
[0145] As described above, the heat-transfer material is, for
example, a phase-change material. Phase-change materials have the
advantage that they are available in wide temperature ranges.
[0146] Phase-change materials have the characteristic that they can
store energy in the form of latent heat. The heat is absorbed or
emitted during the change from the solid state to the liquid state,
and vice versa.
[0147] A phase-change material having a solid-liquid transition in
the temperature range in question is preferably chosen.
Phase-change materials having a solid-liquid transition in the
temperature range considered may be envisaged, however the
transformation is generally slow, which may be disadvantages for
the reactivity of the system.
[0148] Above a certain temperature characteristic of each
phase-change material, phase-change materials liquefy by absorbing
heat from the ambient atmosphere, and emit it when the temperature
drops.
[0149] This property is related to its substantial melt energy per
unit of volume; the higher this is the more advantageous will be
the heat storage/emission properties.
[0150] Paraffins may for example be chosen, such as eicosane,
docosane and tricosane, or other inorganic materials such as
hydrates of salts or metal hydrides. Paraffins have the advantage
that they are thermally stable and inexpensive. Conversely, they
have relatively low thermal conduction. Paraffins may also be used
in association with heat conductive elements capable of
transferring heat within and outside the material efficiently.
These elements may be thermal dissipators, partitions, fins,
graphite nanofibres, metal foams, dispersed conductive particles,
micro-encapsulations of phase-change material, or carbon nanotubes
the presumed thermal conductivity of which is very high.
[0151] As regards substrate 8, as mentioned above, it preferably
has a low thermal conductivity in order to limit heat loss from
thermal conductors 2, 4. The substrate may be made of a polymer
material such as epoxy, or of a ceramic. Silicon may also be
envisaged, which is used for the manufacture of electronic
components. Bearing in mind its relatively high thermal
conductivity, a thermal barrier layer, for example an SiO2 oxide
layer, will be formed on the upper face of the substrate where the
conductors are positioned.
[0152] Epoxy has a thermal conductivity of 0.25 Wm.sup.-1K.sup.-1.
Ceramics have a thermal conductivity of the order of 0.49
Wm.sup.-1K.sup.-1. Silicon has a thermal conductivity of 149
Wm.sup.-1K.sup.-1 and silicon oxide has a thermal conductivity of
several Wattm.sup.-1K.sup.-1.
[0153] The substrate is structured using techniques widely known to
those skilled in the art: casting, machining; as regards silicon,
microelectronic techniques are used: wet etching, dry etching,
electrochemical etching, etc.
[0154] As regards the production of the conductors, they may be
produced by photolithography.
[0155] The conductors are, for example, made of aluminium, of
thermal conductivity equal to 237 Wm.sup.-1K.sup.-1, of gold, of
thermal conductivity equal to 317 Wm.sup.-1K.sup.-1, or of copper,
of thermal conductivity equal to 390 Wm.sup.-1K.sup.-1.
[0156] It may be envisaged to functionalise the heat-transfer
material such that it has a certain electrical conductivity,
sensitivity to magnetic fields, or is subject to a
photoluminescence phenomenon. This functionalisation may be
obtained, for example, through the addition of nanomaterials.
[0157] The system may then form an electrical and/or optical and/or
magnetic switch.
[0158] The thermal management system according to the invention
forms a thermal switch determining whether or not a heat flux
flows. Switching may occur automatically, and provide thermal
regulation between the heat source and the heat sink in an
autonomous manner.
[0159] It may also form a thermal regulator, since the gradual
change in volume of the heat-transfer material enables the heat
flux transfers to be regulated.
[0160] By combining several systems according to the invention,
heat fluxes may be ordered, directed and distributed. In addition,
variable-volume heat-transfer elements store heat; they therefore
form a buffer to the advance of the heat flux. By this means they
form heat sinks.
* * * * *