U.S. patent application number 13/948989 was filed with the patent office on 2015-01-29 for cooling device including an electrocaloric composite.
The applicant listed for this patent is Ailan CHENG, Qiming ZHANG. Invention is credited to Ailan CHENG, Qiming ZHANG.
Application Number | 20150027132 13/948989 |
Document ID | / |
Family ID | 51877749 |
Filed Date | 2015-01-29 |
United States Patent
Application |
20150027132 |
Kind Code |
A1 |
ZHANG; Qiming ; et
al. |
January 29, 2015 |
COOLING DEVICE INCLUDING AN ELECTROCALORIC COMPOSITE
Abstract
Cooling devices, heat pumps, and climate controlling devices
employing an electrocaloric composite of high thermal conductivity
and significant electrocaloric effect are disclosed. The
electrocaloric composites include a combination of one or more
EC-fluoropolymers and their blends with one or more
electric-insulating fillers of high thermal conductivity.
Inventors: |
ZHANG; Qiming; (State
College, PA) ; CHENG; Ailan; (State College,
PA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ZHANG; Qiming
CHENG; Ailan |
State College
State College |
PA
PA |
US
US |
|
|
Family ID: |
51877749 |
Appl. No.: |
13/948989 |
Filed: |
July 23, 2013 |
Current U.S.
Class: |
62/3.1 |
Current CPC
Class: |
Y02B 30/66 20130101;
Y02B 30/00 20130101; F25B 2321/001 20130101; F25B 21/00
20130101 |
Class at
Publication: |
62/3.1 |
International
Class: |
F25B 21/00 20060101
F25B021/00 |
Claims
1. A cooling device comprising a high thermal conductivity
electrocaloric (EC) composite as the refrigerant, wherein the high
thermal conductivity electrocaloric composite can cycle through a
temperature increase and decrease.
2. The device of claim 1, wherein the high thermal conductivity EC
composite has a thermal conductivity, along one direction, higher
than 0.5 W/mK in the temperature range from 0.degree. C. to
50.degree. C.
3. The device of claim 1, wherein the high thermal conductivity EC
composite has a thermal conductivity, along one direction, higher
than 1 W/mK in the temperature range from 0.degree. C. to
50.degree. C.
4. The device of claim 1, wherein the high thermal conductivity EC
composite has a thermal conductivity, along one direction, higher
than 1 W/mK in the temperature range from -20.degree. C. to
70.degree. C.
5. The device of claim 1, wherein the high thermal conductivity EC
composite comprises one or more EC fluoropolymers having a
significant electrocaloric effect in combination with one or more
fillers, wherein the one or more fillers are electrically
insulating and have high thermal conductivity.
6. The device of claim 5, wherein the one or more EC fluoropolymers
include a fluoropolymer made from vinylidene fluoride (VDF) based
polymers which contain at least one additional fluoro-monomer
including trifluoroethylene (TrFE), chlorofluoroethylene (CFE),
chlorodifluoroethylene (CDFE), chlorotrifluoroethylene (CTFE),
tetrafluoroethylene (TFE), hexafluoropropylene (HFP),
hexafluoroethylene (HFE), vinylidene chloride (VDC), vinyl fluoride
(VF) or mixture thereof.
7. The device of claim 5, wherein the high thermal conductivity EC
composite includes a blend of EC fluoropolymers.
8. The device of claim 7, wherein the blend of EC fluoropolymers is
a blend of a fluoropolymer that is a terpolymer with a
fluoropolymer that is a copolymer.
9. The device of claim 5, wherein the filler includes one or more
of inorganic or organic high thermal conductivity fillers of
oxides, nitrides, silicon carbide, certain carbons, polyethylene
highly oriented fibers (PE) or mixtures thereof.
10. The device of claim 5, wherein the filler volume fraction in
the high thermal conductivity EC composite is less than 10 volume
percent but higher than 0.1 volume percent.
11. The device of claim 5, wherein the one or more fillers have
thermal conductivity higher than 30 W/mK.
12. The device of claim 5, wherein the one or more fillers are in a
shape of nano-tube, nano-fiber, or nano-sheet.
13. The device of claim 5, wherein the high thermal conductivity EC
composite has a thermal conductivity, along one direction, higher
than 0.5 W/mK in the temperature range from 0.degree. C. to
50.degree. C.
14. The device of claim 5, wherein the high thermal conductivity EC
composite has a thermal conductivity, along one direction, higher
than 1 W/mK in the temperature range from 0.degree. C. to
50.degree. C.
15. The device of claim 5, wherein the high thermal conductivity EC
composite has a thermal conductivity, along one direction, higher
than 1 W/mK in the temperature range from -20.degree. C. to
70.degree. C.
16. The device of claim 5, wherein the one or more fillers are in
the shape of a fiber of diameter larger than 1 micron and aligned
which enhance the thermal conductivity of the composite along the
fiber length direction while do not affect the thermal conductivity
of the composite in the direction perpendicular to the aligned
filler fibers.
17. The device of claim 16, wherein the high thermal conductivity
EC composite has a thermal conductivity, along one direction,
higher than 1 W/mK in the temperature range from 0.degree. C. to
50.degree. C.
18. The device of claim 16, wherein the high thermal conductivity
EC composite has a thermal conductivity, along one direction,
larger than 2 W/mK in the temperature range from -10.degree. C. to
60.degree. C.
19. The device of claim 16, wherein the high thermal conductivity
EC composite has a thermal conductivity, along one direction,
larger than 4 W/mK in the temperature range from -20.degree. C. to
70.degree. C.
20. The device of claim 1, wherein the high thermal conductivity EC
composite has an electric field induce temperature change of more
than 5.degree. C. and an isothermal entropy change of larger than
22 Jkg.sup.-1K.sup.-1 under an electric field not higher than 100
MV/m.
21. The device of claim 1, wherein the high thermal conductivity EC
composite has an electric field induce temperature changes, in the
adiabatic condition, of more than 5.degree. C. and an isothermal
entropy change of larger than 22 Jkg.sup.-1K.sup.-1 in the
temperature range from 0.degree. C. to 50.degree. C., under an
electric field not higher than 100 MV/m.
22. The device of claim 1, wherein the high thermal conductivity EC
composite has an electric field induce temperature changes, in the
adiabatic condition, of more than 5.degree. C. and an isothermal
entropy change of larger than 22 Jkg.sup.-1K.sup.-1 in the
temperature range from -10.degree. C. to 60.degree. C., under an
electric field not higher than 100 MV/m.
23. The device of claim 1, wherein the high thermal conductivity EC
composite has a dielectric breakdown field higher than 200
MV/m.
24. The device of claim 1, wherein the high thermal conductivity EC
composite comprises one or more EC fluoropolymers and one or more
EC ceramics and one or more high thermal conductivity fillers.
Description
TECHNICAL FIELD
[0001] The present disclosure is directed to cooling devices,
including heat pumps, refrigerators, air conditioning, and climate
control devices, employing high thermal conductivity electrocaloric
composites. The composite comprises an electrocaloric polymer and
filler that has the characteristics of electrical insulation and
high thermal conductivity. Such composites exhibit a high thermal
conductivity and a sufficient electrocaloric effect to act as
refrigerants in cooling devices.
BACKGROUND
[0002] Most conventional air conditioners and refrigerators achieve
cooling through a mechanical vapor compression cycle (VCC). These
systems suffer from low efficiency and there does not appear to be
any economically viable avenue to significantly improve the
efficiency of these VCC systems. Further, air conditioning is a
major contributor to electric utility peak loads. The peak load
electricity production is generally characterized by high
generation costs and is provided by relatively inefficient and
peak-poor polluting plants. Peak loads are also a major factor
contributing to poor grid reliability. A related problem with
today's VCC cooling technology is the adverse environmental impact
of the refrigerant gases employed. Even though the
hydrofluorocarbon (HFC) refrigerants in the current cooling systems
are much safer for the ozone layer than previously used
chlorofluorocarbons (CFC) refrigerants, they remain strong
greenhouse gases. These factors necessitate a search for new
cooling technologies for air-conditioning and refrigeration that
possess improved energy efficiency, low cost and are
environmentally friendly.
[0003] Cooling devices based on the electrocaloric effect (ECE)
have been considered as an alternative to conventional VCC
conventional heat pumps. Polymeric materials that exhibit an
electrocaloric effect have been disclosed for use in cooling
devices. See, e.g., U.S. Patent Application Publication
2011/0016885; Gu et al. "Simulation of chip-size eletrocaloric
refrigerator with high cooling-power density", Applied Physics
Letters, 2013:102:112901-5; and Gu et al., "A chip scale
electrocaloric effect based cooling device", Applied Physics
Letters, 2013:102:122904-4. The electrocaloric (EC) effect is a
reversible temperature change that occurs in a polar material upon
application of an electric field. The EC effect is a result of
direct coupling between the thermal properties (such as entropy)
and electric properties (such as electric field and polarization)
in a dielectric material. In this type of material, a change in the
applied electric field induces a corresponding change in
polarization, which in turn causes a change in the dipolar entropy
S.sub.p as measured by the isothermal entropy change .DELTA.S in
the dielectrics. If the field change is carried out in an adiabatic
condition, the dielectric will experience an adiabatic temperature
change .DELTA.T. Recently, large electrocaloric effect has been
discovered and developed in modified polar-fluoropolymers such as
poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene)
(P(VDF-TrFE-CFE)) terpolymer and polymer blends. Such
polar-fluoropolymers have also been used as composites that exhibit
enhanced polarization response. See, e.g., Chu et al., "Large
Enhancement in Polarization Response and Energy Density of
Poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) by
Interface Effect in Nanocomposites", Appl. Phys. Lett. 91, 122909
(2007). However, the art does not appear to have recognized certain
deficiencies with EC materials and improvements in such materials
are still needed to make them practical for use in cooling
devices.
SUMMARY OF THE DISCLOSURE
[0004] Advantages of the present disclosure include a cooling
device comprising at least one high thermal conductivity
electrocaloric composite. The high thermal conductivity
electrocaloric composite (HiThCd EC composite) includes one or more
electrocaloric polymers in combination with one or more fillers.
The fillers are advantageously electrically insulating so as to
avoid substantial interference with the operation of the
electrocaloric effect but have high thermal conductivity to improve
the performance of the EC composite in the cooling device.
[0005] These and other advantages are satisfied, in part, by a
cooling device comprising at least one high thermal conductivity
electrocaloric composite. The high thermal conductivity
electrocaloric composite includes (1) one or more EC
fluoropolymers, such as those made from vinylidene fluoride (VDF)
based polymers which contain at least one additional fluoro-monomer
including trifluoroethylene (TrFE), chlorofluoroethylene (CFE),
chlorodifluoroethylene (CDFE), chlorotrifluoroethylene (CTFE),
tetrafluoroethylene (TFE), hexafluoropropylene (HFP),
hexafluoroethylene (HFE), vinylidene chloride (VDC), vinyl fluoride
(VF); and (2) one or more fillers which have high thermal
conductivity but electrical insulating characteristics. Examples of
such fillers include inorganic or organic high thermal conductivity
fillers of oxides (such as Al.sub.2O.sub.3, MgO), nitrides (such as
Si.sub.3N.sub.4, Aluminum nitride AlN, boron nitride (BN)), silicon
carbide (SiC), certain carbons such as diamond, polyethylene highly
oriented fibers (PE), and other similar fillers which possess high
thermal conductivity. The fillers of the present disclosure can be
in the size and shape of nano-fillers such as nano-tubes,
nano-fibers, and nano-sheets and micron-sized fibers (fibers whose
diameter are one or more microns).
[0006] Embodiments of the present disclosure include wherein the
one or more fillers have thermal conductivity higher than 10 W/mK,
e.g., higher than 20, 30 W/mK; the filler volume fraction in the EC
composite is less than about 20 volume percent, e.g., less than
about 10 volume percent, but higher than about 0.1 volume percent.
The EC composite can advantageously have an electric field induce
temperature changes, in the adiabatic condition, of more than
5.degree. C. and an isothermal entropy change of larger than 22
Jkg.sup.-1K.sup.-1 in the temperature range from 0.degree. C. to
50.degree. C., under an electric field not higher than 100 MV/m
under certain embodiments.
[0007] Additional advantages of the present invention will become
readily apparent to those skilled in this art from the following
detailed description, wherein only the preferred embodiment of the
invention is shown and described, simply by way of illustration of
the best mode contemplated of carrying out the invention. As will
be realized, the invention is capable of other and different
embodiments, and its several details are capable of modifications
in various obvious respects, all without departing from the
invention. Accordingly, the drawings and description are to be
regarded as illustrative in nature, and not as restrictive.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] Reference is made to the attached drawings, wherein elements
having the same reference numeral designations represent similar
elements throughout and wherein:
[0009] FIG. 1 is a schematic illustration of an idealized
thermodynamic refrigeration cycle (Carnot cycle) employing an
electrocaloric material.
[0010] FIG. 2 is a schematic illustration of a thermodynamic
refrigeration cycle employing an electrocaloric (EC) material which
takes into consideration of the temperature gradient in the heat
exchange between the EC material and hot end and between the EC
material and cold end.
[0011] FIG. 3 is an illustration of the cooling cycles in FIGS. 1
and 2 in a device configuration to show the heat exchange process
between the EC material acting as a refrigerant and a hot and/or
cold load. The four processes are: (A) The temperature of the ECE
material is increased as the electric field is applied on; (B) The
ECE material is in thermal contact with the hot end (heat sink) to
eject heat; (C) The temperature of the ECE material is decreased as
the applied electric field is removed; (D) The ECE material is in
thermal contact with the cold load to absorbed heat, thus cools the
load.
[0012] FIGS. 4A and 4B show mixing models of a HiThCd EC composite
of the present disclosure. FIG. 4A illustrates a HiThCd EC
composite having an EC polymer to filler arrangement in parallel
and FIG. 4B illustrates a HiThCd EC composite having an EC polymer
to filler arrangement in series.
[0013] FIG. 5 illustrates modeling results for the thermal
conductivity for the HiThCd EC composite arrangements parallel
(crosses) and series (circles) shown in FIG. 4.
[0014] FIG. 6 is a chart showing the comparative results of the
induced polarization of an EC composite comprised of EC polymer (a
PVDF based terpolymer) with boron nitride nano-fillers (5 vol %) to
that of the EC polymer alone.
[0015] FIG. 7 is a schematic of a HiThCd EC composite with high
thermal conductivity micron-diameter fibers and EC polymer.
DETAILED DESCRIPTION OF THE DISCLOSURE
[0016] The present disclosure is directed to cooling devices,
including but not limited to heat pumps, refrigerators, air
conditioning, climate control systems, etc. that include a high
thermal conductivity electrocaloric composite to remove heat during
the operation of the device. Advantageously, the electrocaloric
composite exhibits a significant temperature/entropy change upon
the application and removal of electric field or voltage and a
cyclic temperature change (i.e. an increase and decrease in
temperature upon a cyclic voltage applied to the composite).
[0017] In addition to optimizing the electrocaloric effect (ECE) of
the material, the present disclosure describes electrocaloric
composites that have improved thermal conductivity. By
advantageously improving the thermal conductivity of the composite,
optimized device design, operation frequency, and cooling power can
be implemented.
[0018] For example, a key component of a cooling device is the
transportation of entropy from the cold end to the hot end. The
objective is to transport entropy from one temperature level to
another temperature level in a reversible manner, that is, to
transport the entropy without generating any additional entropy in
the process. This requires a substance whose entropy depends on
properties other than temperature. In the cooling devices of
present disclosure, this substance is the electrocaloric material,
whose entropy can be changed by external electric fields.
[0019] All steady state converters must be cyclic since the entropy
carrying substance is not consumed. FIG. 1 illustrates an ideal
cooling cycle (Carnot cycle) which includes two adiabatic and two
isothermal processes. In the figure, S is entropy, T is the
temperature, E is applied electric field. The subscripts h and c
refer to high and low temperatures. The arrows show the dipole
ordered and less ordered states of EC material.
[0020] For the Carnot cycle, the heat absorbed from the cold source
is Q.sub.c=T.sub.c(S.sub.c-S.sub.h) and the coefficient of
performance, COP=Q.sub.c/W (where W is the total external work in
the cooling cycle) can be expressed as
COP=T.sub.c/(T.sub.h-T.sub.c) Eq. (1)
[0021] For a typical ECE based cooling cycle, there must exist a
temperature gradient between the EC material and the hot-sink or
cold load.
[0022] FIG. 2 is a schematic illustration of a more practical
thermodynamic refrigeration cycle employing an electrocaloric
material. This figure shows the temperature gradient required for
heat transfer between the EC material and the hot end (at C) and
cold end (at A). A high thermal conductivity EC material will
reduce the temperature gradient and hence improve the efficiency
and cooling power. The process A-B occurs during polarization that
the EC material becomes hot with its temperature increasing from
T.sub.A to T.sub.B. In the process B-C the heat in EC material is
transferred to a hot-sink and eventually reaches the hot-sink
temperature. Then the depolarization occurs in the process C-D and
the EC material temperature drops to T.sub.D, and finally the EC
material absorbs heat from a cold-source and reaches T.sub.A in the
process D-A.
[0023] The cooling cycles in FIGS. 1 and 2 are presented for
illustration. There are other types of cooling cycles, which employ
regenerative processes and can generate temperature changes
T.sub.h-T.sub.c larger than the temperature changes in FIG. 1 and
also much larger than the adiabatic temperature change .DELTA.T of
the EC material.
[0024] In these devices, the EC refrigerant (i.e., ECE (or EC, for
simplicity) material acting as a refrigerant) will exchange heat
with the environment and thermal loads (such as the hot and cold
ends) in order to achieve cooling, refrigeration, pumping heat, and
climate control. FIG. 3 is an illustration of the cooling cycles in
FIGS. 1 and 2 in a device configuration to show the heat exchange
process between the EC material and a hot and/or cold load. As
shown in FIG. 3, heat sink (3010) has a temperature of T.sub.h and
cold load (3030) has a temperature of T.sub.c. The cooling device
also includes EC material 3020 and low thermal conductivity
enclosure 3040 separating heat sink 3010 at T.sub.h and cold load
3030 at T.sub.c. Arrows indicate the heat flow direction. In the
heat pumping process as illustrated in FIG. 3, the heat exchanges
between the EC material (3020) and the hot (3010) end and between
the EC material (3020) and cold (3030) load. The heat exchange
occurs in the EC material within a distance .delta., the thermal
diffusion length, at the interfaces between the EC material and the
hot or cold elements. Here .delta. is the thermal diffusion length,
.delta.= {square root over (2k/(c.omega.))} where .omega. is the
angular frequency (=2.pi..times.operation frequency) and k is the
thermal conductivity, and c is the specific heat. For EC polymers,
the thermal conductivity k is very small, e.g., k=0.2 W/mK to 0.25
W/mK, compared with metals, which typically have thermal
conductivities of greater than 100 W/mK. Hence when operating the
cooling device at 10 Hz (operation frequency=10 Hz), the thermal
diffusion length is only 73 .mu.m, which means to maintain a high
COP of the EC cooling device, the EC material thickness should not
be larger than 73 .mu.m, is too thin for many practical devices.
For thicker EC polymers in the cooling device, this means a low
operation frequency. Since for each cooling cycle in FIGS. 1 and 2,
the heat removed from the cold end is fixed
Q.sub.c=T.sub.c(S.sub.h-S.sub.c), where (S.sub.h-S.sub.c) is the
entropy change in the EC material induced electrically, the cooling
power of a heat pump is directly proportional to the operation
frequency.
W.sub.c=fQ.sub.c Eq. (2)
[0025] By enhancing the thermal conductivity by one order of
magnitude, the thermal diffusion length can be increased by a
factor of more than 3 or the operation frequency of the device can
be raised by 3 fold, which will lead to enhanced cooling power and
cooling device efficiency because .delta.= {square root over
(2k/(c.omega.))}.
[0026] The inventors of the present disclosure discovered that by
adding high thermal conductivity fillers to EC polymers to form
HiThCd EC composites, the composites could be produced with high
thermal conductivity while not significantly adversely affecting
(reducing) the EC response. That is, the EC response of the
resulting HiThCd EC composite is comparable or even better than
that of the EC polymer without the filler.
[0027] In one aspect of the present disclosure a HiThCd EC
composite includes one or more EC fluoropolymers in combination
with one or more fillers. In an embodiment of the present
disclosure, the HiThCd EC composite including one or more
fluoropolymers and one or more filler has an adiabatic temperature
change of higher than 4.degree. C. under 100 MV/m or lower electric
field, e.g., greater than about 6, 8 or even an adiabatic
temperature change of higher than 10.degree. C. under 100 MV/m or
lower electric field. The HiThCd EC composite including one or more
fluoropolymers and one or more filler can also have an isothermal
entropy change of larger than 22 Jkg.sup.-1K.sup.-1 in the
temperature range from 10.degree. C. to 60.degree. C., e.g., from
0.degree. C. to 50.degree. C., under an electric field not higher
than 100 MV/m. The HiThCd EC composite of the present disclosure
can also have a dielectric breakdown field higher than 200 MV/m,
preferably higher than 300 MV/m, and more preferably higher than
400 MV/m
[0028] Fluoropolymers that are useful for the HiThCd EC composites
of the present disclosure preferably exhibit a significant
electrocaloric effect, e.g. an adiabatic temperature change of
higher than 4.degree. C. under 100 MV/m or lower electric field,
e.g., greater than about 6, 8 or even an adiabatic temperature
change of higher than 10.degree. C. under 100 MV/m or lower
electric field. In an embodiment of the present disclosure, the EC
fluoropolymers useful for the EC composite have a dielectric
constant higher than 7, preferably higher than 8 or 9, at room
temperature
[0029] The EC polymers useful for the present EC composite include
but are not limited to EC polymers made from vinylidene fluoride
(VDF) based polymers which contain at least one additional
fluoro-monomer including trifluoroethylene (TrFE),
chlorofluoroethylene (CFE), chlorodifluoroethylene (CDFE),
chlorotrifluoroethylene (CTFE), tetrafluoroethylene (TFE),
hexafluoropropylene (HFP), hexafluoroethylene (HFE), vinylidene
chloride (VDC), vinyl fluoride (VF), etc. These polymers can be
copolymers or terpolymers, for example. The HiThCd EC composite can
include one (i.e., neat) EC polymer or a blend of EC polymers.
[0030] In an embodiment of the present disclosure, HiThCd EC
composite includes one or more terpolymers of
P(VDF.sub.1-x-y-R.sup.1.sub.x-R.sup.2.sub.y), where R.sup.1 is
selected from the group consisting of TrFE and TFE, and R.sup.2 is
selected from the group consisting of CFE, CTFE, CDFE, HFP, HFE,
VDC, VF, and mixtures thereof. The variable x is in the range 0.01
to 0.49, and y is in the range from 0.01 to 0.15. Preferred
terpolymers include P(VDF.sub.1-x-y-TrFE.sub.x-CFE.sub.y),
P(VDF.sub.1-x-y-TrFE.sub.x-CTFE.sub.y),
P(VDF.sub.1-x-y-TrFE.sub.x-HFP.sub.y),
P(VDF.sub.1-x-y-TFE.sub.x-CTFE.sub.y), and
P(VDF.sub.1-x-y-TFE.sub.x-CFE.sub.y) (0.01<y<0.15 and
0.10<x<0.49) which exhibit significant EC responses.
Preferred terpolymers include polyvinylidene
fluoride-trifluoroethylene-chlorofluoroethylene (P(VDF-TrFE-CFE)),
polyvinylidene fluoride-tri fluoroethylene-chlorodifluoroethylene
(P(VDF-TrFE-CDFE)), polyvinylidene
fluoride-trifluoroethylene-chlorotrifluoroethylene
(P(VDF-TrFE-CTFE)), polyvinylidene
fluoride-trifluoroethylene-hexafluoropropylene (P(VDF-TrFE-HFP)),
polyvinylidene fluoride-trifluoroethylene-tetrafluoroethylene
(P(VDF-TrFE-TFE)), polyvinylidene
fluoride-trifluoroethylene-vinylidene chloride P(VDF-TrFE-VDC),
polyvinylidene fluoride-trifluoroethylene-vinyl fluoride
P(VDF-TrFE-VF), polyvinylidene
fluoride-trifluoroethylene-hexafluoroethylene P(VDF-TrFE-HFE),
polyvinylidene fluoride-tetrafluoroethylene-chlorofluoroethylene
(P(VDF-TFE-CFE)), polyvinylidene
fluoride-tetrafluoroethylene-chlorodifluoroethylene
(P(VDF-TFE-CDFE)), polyvinylidene
fluoride-tetrafluoroethylene-chlorotrifluoroethylene
(P(VDF-TFE-CTFE)), polyvinylidene
fluoride-tetrafluoroethylene-hexafluoropropylene (P(VDF-TFE-HFP)),
polyvinylidene fluoride-tetrafluoroethylene-hexafluoroethylene
P(VDF-TFE-HFE), polyvinylidene
fluoride-tetrafluoroethylene-vinylidene chloride P (VDF-TFE-VDC),
polyvinylidene fluoride-tetrafluoroethylene-vinyl fluoride
P(VDF-TFE-VF), etc.
[0031] In another embodiment of the present disclosure, HiThCd EC
composite includes one or more copolymers of
P(VDF.sub.z-CTFE.sub.1-z), P(VDF.sub.z-CFE.sub.1-z),
P(VDF.sub.z-TrFE.sub.1-z), P(VDF.sub.z-TFE.sub.1-z),
P(VF.sub.z-CTFE.sub.1-z), P(VF.sub.z-CFE.sub.1-z),
P(VF.sub.z-HFP.sub.1-z), P(VF.sub.z-TrFE.sub.1-z), and
P(VF.sub.z-TFE.sub.1-z), the variable .sub.z is not limited but can
range from .sub.z of 0.7 to 0.98. Preferred copolymers include
P(VDF-CFE), P(VDF-CTFE), P(VDF-TFE), P(VDF-TrFE), and mixture
thereof.
[0032] The HiThCd EC composite of the present disclosure can also
include high energy (>1 MeV) electron irradiated copolymers of
P(VDF.sub.1-x-TrFE.sub.x) and P(VDF.sub.1-x-TFE.sub.x),
(0.2<x<0.5) and/or P(VDF.sub.1-x-CTFE.sub.x) (x<0.08). It
has been shown that irradiation increases cross-linking of such
copolymers and turns such copolymers into relaxors and enhances the
ECE.
[0033] In another embodiment, the HiThCd EC composite includes
blends of EC polymers. Such blends can include, for example, one or
more terpolymers, such as those referenced above, with one or more
copolymers, such as those referenced above. A preferred terpolymer
and copolymer blend includes (1) one or more terpolymers of
P(VDF.sub.1-x-y-TrFE.sub.x-R.sup.3.sub.y) (0.03<y<0.09,
0.5>x>0.25),
P(VDF.sub.1-x-y-TFE.sub.x-R.sup.3.sub.y)(0.03<y<0.1,
0.1<x>0.3) or a mixture thereof, where R.sup.3 is CTFE or CFE
or HFP, in combination with (2) one or more copolymers of
P(VDF.sub.1-z-TrFE.sub.z) (0.5>z>0.25) or
P(VDF.sub.1-z-TFE.sub.z) (0.25<z<0.5),
P(VDF.sub.1-z-CTFE.sub.z) (z<0.1), P(VDF.sub.1-z-HFP.sub.z)
(z<0.05) or a mixture thereof.
[0034] In a preferred embodiment, the HiThCd EC composite includes
EC fluoropolymers of P(VDF.sub.1-x-y-R.sup.1.sub.x-R.sup.2.sub.y),
where R.sup.1 is selected from the group consisting of TrFE and
TFE, and R.sup.2 is selected from the group consisting of CFE,
CTFE, CDFE, HFP, HFE, VDC, VF, and mixtures thereof. The variable x
is in the range 0.01 to 0.49, and y is in the range from 0.01 to
0.15. Preferred terpolymers include
P(VDF.sub.1-x-y-TrFE.sub.x-CFE.sub.y),
P(VDF.sub.1-x-y-TrFE.sub.x-CTFE.sub.y),
P(VDF.sub.1-x-y-TrFE.sub.x-HFP.sub.y),
P(VDF.sub.1-x-y-TFE.sub.x-CTFE.sub.y), and
P(VDF.sub.1-x-y-TFE.sub.x-CFE.sub.y) (0.01<y<0.15 and
0.10<x<0.49) which exhibit significant EC responses, and
blends of terpolymer and copolymer blend includes (1) one or more
terpolymers of P(VDF.sub.1-x-y-TrFE.sub.x-R.sup.3.sub.y)
(0.03<y<0.09, 0.2<x<0.49),
P(VDF.sub.1-x-y-TFE.sub.x-R.sup.3.sub.y) (0.03<y<0.1,
0.1<x<0.4) or a mixture thereof, where R.sup.3 is CTFE or CFE
or HFP, in combination with (2) one or more copolymers of
P(VDF.sub.1-z-TrFE.sub.z) (z<0.5) or P(VDF.sub.1-z-TFE.sub.z)
(z<0.3), P(VDF.sub.1-z-CTFE.sub.z) (z<0.1),
P(VDF.sub.1-z-HFP.sub.z) (z<0.05) or a mixture thereof.
[0035] The weight ratio of any fluoropolymer in a blend can range
from about 70 to 97 weight % of one polymer over others in the
blend based on the total weight of the EC fluoropolymers in the
blend. For example, for the terpolymer and copolymer blends, the
blends can range from 70 wt % of the terpolymer and 30 wt %
copolymer to 97 wt % of terpolymer and 3 wt % copolymer.
Preferably, the copolymer in any blend containing copolymers is
less than 15 wt % based on the total weight of the terpolymer and
copolymer in the blend. Preferably any blend used with the EC
composite exhibits a temperature change (adiabatic temperature
change) of more than 5.degree. C., induced under an electric field
of 100 MV/m or lower, e.g., greater than about 6, 8 or even an
adiabatic temperature change of higher than about 9.degree. C.
under 100 MV/m or lower electric field.
[0036] The fillers that are useful with the HiThCd EC composites of
the present disclosure include one or more fillers that thermally
conducting but electrically insulating. For the HiThCd EC
composite, electrically conductive fillers in the composites can be
detrimental since these fillers will cause electrical conduction
and also reduce the operation field due to lowered dielectric
strength (<150 MV/m). Thus, one consideration in selecting a
filler is that the filler should not significantly affect the
electric insulation property of the EC polymer since any electric
conduction loss will lower the dielectric strength (reliability)
and cause heating of the EC composite which will reduce the
efficiency and cooling power of the cooling devices. An
electrically insulating filler as used herein means a filler that
can maintain the dielectric breakdown field of the EC polymers.
Such fillers have electrical resistivity higher than about 10.sup.6
ohm meter (.OMEGA.m), preferably higher than 10.sup.8 .OMEGA.m and
even higher than 10.sup.9 .OMEGA.m.
[0037] Since the filler can enhance the performance of the EC
composite in a cooling device due to thermal conductivity, it is
preferred that the filler has high thermal conductivity, e.g.,
greater than 10 W/Km, preferably greater than about 20, 30 or 50
W/Km. In an embodiment of the present disclosure, it is preferred
that the one or more fillers have very high thermal conductivity,
e.g., greater than about 100 W/mK, 200, 300, and even greater than
about 500 W/mK. Preferably, the one or more fillers in the HiThCd
EC composite will act to enhance the thermal conductivity of the
HiThCd EC composite to more than about 2 and up to about 20 times
that compared to the same EC fluoropolymer without the filler while
having a minimal effect on the EC response.
[0038] Such fillers can include one or more organic high thermal
conductivity fillers or inorganic fillers, such as those of oxides
(such as Al.sub.2O.sub.3, MgO), nitrides Si.sub.3N.sub.4, Aluminum
nitride AlN, boron nitride (BN)), carbides (silicon carbide (SiC)),
certain carbons such as diamond, polyethylene highly oriented
fibers (PE), and other similar fillers which possess high thermal
conductivity with low electrical conductivity and mixtures
thereof.
[0039] In an embodiment of the present disclosure, the filler in
the EC composite is higher than about 0.1 volume percent but no
more than about 30% by volume, e.g., no more than about 20%, 10%,
and 5% by volume.
[0040] The fillers of the present disclosure can be in the size and
shape of nano-fillers such as nano-tubes, nano-fibers, and
nano-sheets and micron-sized fibers (fibers whose diameter are one
or more microns). Nano-sized fillers with high aspect ratios are
preferred, e.g., a high aspect ratio is when the length vs.
diameter for nano-tubes or the length vs. thickness for nano-sheets
is greater than about 50 or greater than about 100. Further by
using anisotropic fillers such as fillers in the shape of fibers
and sheets, the thermal conductivity of the EC composite can also
be enhanced anisotropically. For example a EC composite including
one or more fillers in the shape of a fiber can be fabricated to
align the fibers to enhance the thermal conductivity of the
composite along the fiber length direction while do not affect the
thermal conductivity of the composite in the direction
perpendicular. In an embodiment of the present disclosure, the EC
composite has a thermal conductivity along one direction higher
than 1 W/mK, e.g., higher than 2, 4, 6 W/mK, in the temperature
range from -20.degree. C. to 70.degree. C., e.g., -10.degree. C. to
60.degree. C. and from 0.degree. C. to 50.degree. C.
[0041] The HiThCd EC composites of the present disclosure can be
fabricated by mixing one or more EC fluoropolymers with one or more
high thermal conductivity fillers by a variety of processes, such
as using a standard melt extrusion process. Alternatively, the EC
composites can be fabricated using a solution casting method in
which the EC polymers are dissolved in a solvent and high thermal
conductivity fillers dispersed in a solvent. The two solutions are
mixed with a proper ratio, determined by the desired composite
composition and then cast on a substrate to form a composite film.
In the solution casting method, the surfaces of the fillers can be
modified to enhance the uniform dispersion of the fillers in the
fluoropolymer matrix. The surface modification can use, for
example, 3-phosphonopropionic acid. Additional fabrication methods
can include aligning micron-diameter fibers of high thermal
conductivity and casting an EC polymer or polymer solution onto the
aligned fibers.
[0042] In one aspect of the present disclosure, the fillers can be
arranged in the HiThCd EC composite randomly or orderly. The two
basic models representing the upper bound and lower bound of the
thermal conductivity of EC composites k.sub.c are when (1) the EC
polymer and filler are arranged in parallel (the EC polymer and
high thermal conductivity filler are arranged in parallel along the
thermal conduction path) and (2) when the EC polymer and filler are
arranged in series (the EC polymer and high thermal conductivity
filler are arranged in series along the thermal conduction path of
k.sub.c). The parallel and series arrangements are illustrated in
FIG. 4. As shown in the figure, light regions (4010) represent EC
polymer and dark regions (4020) represent filler having high
thermal conductivity and low electrical conductivity. FIG. 4a
illustrates a HiThCd EC composite having an EC polymer to filler
arrangement in parallel and FIG. 4b illustrates a HiThCd EC
composite having an EC polymer to filler arrangement in series. The
thermal conductivity of the two models can be expressed by the
following equations:
k.sub.c=f.sub.pk.sub.p+f.sub.mk.sub.m Eq. (3)
and
k.sub.c=1/((f.sub.p/k.sub.p)+(f.sub.m/k.sub.m)) Eq. (4)
Where k.sub.p and k.sub.m are the thermal conductivity of the high
thermal conductivity particle fillers and polymer matrix, and
f.sub.p and f.sub.m are the volume fraction of the two
constituents. Equation (3) is the thermal conductivity for parallel
composite model (FIG. 4A) and equation (4) is that for a series
model (FIG. 4B).
[0043] Presented in FIG. 5 is the results based on Eqs. (3) and (4)
for a hypothetic composite with an EC polymer (k.sub.m=0.2 W/mK)
and with a filler having a thermal conductivity of k.sub.p=250
W/mK. As shown in FIG. 5, the composites with the parallel
structure (morphology) will yield a high thermal conductivity. With
even 5 vol % of high thermal conductivity fillers, the thermal
conductivity of an EC composite can be 12.5 W/mK, which is
significantly larger than the thermal conductivity of an EC polymer
without such a filler, e.g., a difference of more than about 12.3
W/mK. However, the thermal conductivity of the composite in the
series model is not improved significantly.
[0044] In the design presented in FIG. 4, the two models actually
represent the thermal conductivity of a composite along two
perpendicular directions. Thus the composite has anisotropic
thermal conductivity. That is a high k along one direction and a
smaller k along the two perpendicular directions (or a
perpendicular plane). This is acceptable for many EC based cooling
devices, for example, the cooling cycles in FIGS. 1 and 2.
[0045] In addition to an orderly arrangement of the HiThCd EC
composite, it is believed that a randomly oriented EC polymer
filler composite can also achieve very high thermal conductivity.
This can be achieved by combining one or more EC polymers with one
or more nano-fillers such as nano-tubes (boron nitride nano-tubes,
or nano-sheets of these materials). Such nano-tubes have a large
shape aspect ratio that can form a thermal percolation path.
[0046] Carbon nano-tubes and carbon fibers, because of their very
high thermal conductivity (>1,000 W/mK), have been used and
investigated widely to enhance the thermal conductivity of
polymers. However, carbon nano-tubes and fibers cannot be used for
enhancing the thermal conductivity of EC composites here because of
their low electrical resistivity (<10.sup.5 .OMEGA.m).
[0047] There are several highly electric insulation materials which
possess high thermal conductivity that can be used as fillers for
the EC composites of the present disclosure. These fillers include,
for example, diamond, Si.sub.3N.sub.4, Al.sub.2O.sub.3, Boron
Nitride (BN), MgO, Al.sub.2O.sub.3, Aluminum nitride AlN,
polyethylene highly oriented fibers (PE), and SiC.
[0048] Since these high thermal conductivity fillers do not possess
ECE, the volume fraction of such fillers in the EC composite should
preferably be low as, for example, below 10%, so that the ECE of
the EC composite will not be reduced significantly. For example, in
the case of a parallel arrangement of EC polymer and filler as
shown in FIG. 4a, the reduction of ECE will be 10% when the filler
volume fraction is 10%. The electrocaloric effect (isothermal
entropy change .DELTA.S.sub.comp) of the composite can be expressed
as:
.DELTA.S.sub.comp=.DELTA.S.sub.poly(1-f) Eq. (5)
where f is the volume fraction of the high thermal conductivity
fillers and .DELTA.S is for the EC polymer
[0049] Although it is counter intuitive, it was observed that if
the high thermal conductivity fillers are of a nano-size, such as
nano-fibers with fiber diameter below 100 nm, the mixing of the EC
polymer with nano-fillers such as BN nano-fibers will in fact
enhance the polarization response, i.e., enhanced ECE, if the
volume fraction of the nano-filler is low (for example, below 10%).
FIG. 6 shows an enhanced polarization response for an EC polymer,
e.g., a terpolymer made from vinylidene fluoride-based polymer,
with boron nitride nano-sheets at about 5 vol. % compared to the
same polymer without the boron nitride nano-sheets. The
electrocaloric response of the EC polymer is proportional to the
polarization. Hence, if the volume fraction of the high thermal
conductivity nano-fillers is low (<10 vol %), the composites can
exhibit both higher thermal conductivity and high EC response
compared with the neat EC polymer.
[0050] BN nano-tubes (BNNT) and nano-sheets (BNNS) have been
developed in recent years. These materials are known to have a very
high aspect ratio, e.g., the length vs. diameter for nano-tubes or
the length vs. thickness for nano-sheets is greater than 1,000. The
large aspect ratio of these BNNTs or BNNSs promotes the formation
of thermally connected networks even when the volume fraction of
the nano-fillers is not high. Thus, BNNTs and BNNSs, or in general,
nanotubes and nano-rods of the high thermal conductivity fillers
are advantageous compared with nano-fillers of near sphere or very
low aspect ratio (<10).
[0051] In addition to, or in place of nano-sized fillers, the
HiThCd EC composites of the present disclosure can also include
micron-diameter fibers of high thermal conductivity as illustrated
in FIG. 7. FIG. 7 is a schematic of an EC composite with high
thermal conductivity micron-diameter fibers 7020 and EC polymer
7010. For example, when the fibers with diameter less than 30
microns (such as Al.sub.2O.sub.3, BN fibers) are used, the
composites will exhibit a significantly enhanced thermal
conductivity along the fiber direction (see FIG. 7). For these
composites, the inclusion of these high thermal conductivity fibers
will reduce the EC response. Consequently, the volume fraction of
the high thermal conductivity fillers should be low. The reduction
of the EC response, for example, the isothermal entropy change
.DELTA.S.sub.comp will be reduced from the .DELTA.S.sub.poly as
shown in equation (5).
[0052] On the other hand, in order to have an effective heat
exchange between the high thermal fibers and EC polymer matrix, the
separation between the two neighboring high thermal fibers should
be less than the thermal diffusion length 6. For 10 Hz operation,
the separation should be less than 73 .mu.m. Assuming 5% volume of
high thermal conductivity fillers (thermal conductivity of the
composite is >10 W/mK as shown in FIG. 5), the diameter of the
fibers should be 17 .mu.m or less so that the separation between
the micro-fibers is 73 .mu.m or less. In general, fibers of smaller
diameter will allow for a closer distance between the fibers and
hence improving the thermal transport between the high thermal
conductivity fillers and EC polymers.
[0053] In an embodiment of the present disclosure, EC composites
include EC fluoropolymers with high thermal conductivity polymer
fibers, such as highly oriented polyethylene fibers and other
highly oriented polymer fibers. Based on the modeling studies shown
in FIG. 5, it is expected that such EC composites will have very
high thermal conductivity while maintenance of the electrocaloric
effect.
[0054] Additional fillers that are useful for the HiThCd EC
composites of the present disclosure include insulating
nano-particles of ferroelectric ceramics (EC ceramics) such as
BaTiO.sub.3, Ba(Ti.sub.1-xZr.sub.x)O.sub.3,
Ba(Ti.sub.1-xSn.sub.x)O.sub.3 (x<0.3),
(Ba.sub.1-xSr.sub.x)(Ti.sub.1-yZr.sub.y)O.sub.3 (x<0.3,
y<0.3), (Ba.sub.1-xSr.sub.x)TiO.sub.3 (x<0.3)
(Ba.sub.1-xSr.sub.x)(Ti.sub.1-ySn.sub.y)O.sub.3 (0.05<x<0.3,
0.01<Sn<0.15), SrBiTa.sub.2O.sub.9,
(Ba.sub.0.3Na.sub.0.7)(Ti0.sub..3Nb.sub.0.7)O.sub.3,
Na.sub.0.5Bi.sub.0.5TiO.sub.3, (PbLa)(Zr.sub.1-xTi.sub.x)O.sub.3
(0.4<x<0.6), and
(Pb(MgNb)O.sub.3).sub.1-x--(PbTiO.sub.3).sub.x (x<0.4), and
these EC ceramics with additives (<5 weight %). Although these
fillers possess relatively low thermal conductivity k<10 W/mK,
because these fillers also exhibit significant ECE, their addition
to the HiThCd may enhance the ECE of the HiThCd EC composite.
[0055] Only the preferred embodiment of the present invention and
examples of its versatility are shown and described in the present
disclosure. It is to be understood that the present invention is
capable of use in various other combinations and environments and
is capable of changes or modifications within the scope of the
inventive concept as expressed herein. Thus, for example, those
skilled in the art will recognize, or be able to ascertain, using
no more than routine experimentation, numerous equivalents to the
specific substances, procedures and arrangements described herein.
Such equivalents are considered to be within the scope of this
invention, and are covered by the following claims.
* * * * *