U.S. patent application number 14/539595 was filed with the patent office on 2015-09-10 for heat-dissipating sheet having high thermal conductivity and its production method.
The applicant listed for this patent is Seiji KAGAWA. Invention is credited to Seiji KAGAWA.
Application Number | 20150257251 14/539595 |
Document ID | / |
Family ID | 51417061 |
Filed Date | 2015-09-10 |
United States Patent
Application |
20150257251 |
Kind Code |
A1 |
KAGAWA; Seiji |
September 10, 2015 |
HEAT-DISSIPATING SHEET HAVING HIGH THERMAL CONDUCTIVITY AND ITS
PRODUCTION METHOD
Abstract
A heat-dissipating sheet having a density of 1.9 g/cm.sup.3 or
more and an in-plane thermal conductivity of 570 W/mK or more,
which comprises carbon black uniformly dispersed among fine
graphite particles, a mass ratio of fine graphite particles to
carbon black being 75/25 to 95/5, is obtained by repeating plural
times a cycle of applying a dispersion of fine graphite particles,
carbon black and an organic binder in an organic solvent to a
surface of a support plate, and then drying it, to form a
resin-containing composite sheet; burning the resin-containing
composite sheet to remove the organic binder; and pressing the
resultant composite sheet of fine graphite particles and carbon
black for densification.
Inventors: |
KAGAWA; Seiji;
(Koshigaya-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KAGAWA; Seiji |
Koshigaya-shi |
|
JP |
|
|
Family ID: |
51417061 |
Appl. No.: |
14/539595 |
Filed: |
November 12, 2014 |
Current U.S.
Class: |
428/220 ; 252/79;
264/294; 428/408 |
Current CPC
Class: |
B05D 5/00 20130101; C09K
5/14 20130101; B05D 2507/005 20130101; B05D 1/36 20130101; B05D
3/007 20130101; H01L 2924/0002 20130101; B05D 3/02 20130101; H01L
23/373 20130101; B05D 1/02 20130101; B05D 3/0254 20130101; H01L
23/3737 20130101; H05K 1/0203 20130101; B05D 7/52 20130101; B05D
2401/10 20130101; B05D 2508/00 20130101; B05D 3/0272 20130101; H01L
2924/0002 20130101; Y10T 428/30 20150115; B05D 2502/00 20130101;
H01L 2924/00 20130101 |
International
Class: |
H05K 1/02 20060101
H05K001/02; C09K 5/14 20060101 C09K005/14 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 5, 2014 |
JP |
2014-43196 |
Claims
1. A heat-dissipating sheet having a structure in which carbon
black is uniformly dispersed among fine graphite particles, a mass
ratio of fine graphite particles to carbon black being 75/25 to
95/5; and said heat-dissipating sheet being obtained by burning and
pressing a composite sheet of fine graphite particles, carbon black
and an organic binder, so that it has a density of 1.9 g/cm.sup.3
or more and an in-plane thermal conductivity of 570 W/mK or
more.
2. The heat-dissipating sheet according to claim 1, which has
thickness of 25-150 .mu.m.
3. The heat-dissipating sheet according to claim 1, wherein said
fine graphite particles have an average diameter of 3-150 .mu.m and
average thickness of 200 nm or more.
4. The heat-dissipating sheet according to claim 1, which is coated
with insulating resin layers or insulating plastic films.
5. A method for producing the heat-dissipating sheet according to
claim 1, comprising the steps of (1) preparing a dispersion
comprising 5-25% by mass in total of fine graphite particles and
carbon black, and 0.05-2.5% by mass of an organic binder, in an
organic solvent, a mass ratio of said fine graphite particles to
said carbon black being 75/25 to 95/5; (2) repeating plural times a
cycle of applying said dispersion to a surface of a support plate
and then drying it, to form a resin-containing composite sheet
comprising said fine graphite particles, said carbon black and said
organic binder; (3) burning said resin-containing composite sheet
to remove said organic binder; and (4) pressing the resultant
composite sheet of fine graphite particles and carbon black for
densification.
6. The method for producing a heat-dissipating sheet according to
claim 5, wherein a mass ratio of said organic binder to the total
amount of said fine graphite particles and said carbon black is
0.01-0.5.
7. The method for producing a heat-dissipating sheet according to
claim 5, wherein the amount of said dispersion applied by one
operation is 5-15 g/m.sup.2 (expressed by the total weight of fine
graphite particles and carbon black per 1 m.sup.2).
8. The method for producing a heat-dissipating sheet according to
claim 5, wherein said organic binder is an acrylic resin, a
polystyrene resin or polyvinyl alcohol.
9. The method for producing a heat-dissipating sheet according to
claim 5, wherein said organic solvent is at least one selected from
the group consisting of ketones, aromatic hydrocarbons and
alcohols.
10. The method for producing a heat-dissipating sheet according to
claim 5, wherein said dispersion is applied by a spraying
method.
11. The method for producing a heat-dissipating sheet according to
claim 5, wherein said burning step is conducted at a temperature of
550-700.degree. C.
12. The method for producing a heat-dissipating sheet according to
claim 5, wherein cooling after burning is gradually conducted to
room temperature over 1 hour or more.
13. The method for producing a heat-dissipating sheet according to
claim 5, wherein said pressing step is conducted at pressure of 20
MPa or more.
14. The method for producing a heat-dissipating sheet according to
claim 5, wherein said resin-containing composite sheet is pressed
in a state sandwiched by a pair of planar die plates in a die plate
apparatus.
15. The method for producing a heat-dissipating sheet according to
claim 14, wherein said die plate apparatus comprises a lower die
plate and an upper die plate; and wherein using said lower die
plate as said support plate, said resin-containing composite sheet
is formed in a cavity of said lower die plate, burned without being
peeled from said lower die plate, and then pressed with said lower
die plate combined with said upper die plate.
16. The method for producing a heat-dissipating sheet according to
claim 5, wherein said composite sheet of fine graphite particles
and carbon black is cooled to a temperature equal to or lower than
the freezing point of water, and then pressed.
17. The method for producing a heat-dissipating sheet according to
claim 5, wherein said pressing step is conducted at a temperature
of room temperature to 200.degree. C.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a heat-dissipating sheet
having high thermal conductivity for efficiently dissipating heat
generated from electronic parts, etc. in small electronic
appliances such as note-type personal computers, smartphones,
mobile phones, etc., and its production method.
BACKGROUND OF THE INVENTION
[0002] In small electronic appliances such as note-type personal
computers, smartphones, mobile phones, etc., which have been
provided with increasingly higher performance and more functions,
electronic devices such as microprocessors, imaging chips,
memories, etc. should be mounted densely. Accordingly, to prevent
malfunction due to heat generated by them, the dissipation of heat
generated from such electronic devices has become increasingly
important.
[0003] As a heat-dissipating sheet for electronic devices, JP
2006-306068 A discloses a heat-conductive sheet comprising at least
a graphite film and an adhesive resin composition, which is a
reaction-curable vinyl polymer. The graphite film is (a) expanded
graphite formed by an expanding method, or (b) obtained by
heat-treating a polyimide film, etc., at a temperature of
2400.degree. C. or higher. The expanded graphite film is obtained
by immersing graphite in acid such as sulfuric acid, etc. to form a
graphite interlayer compound, heat-treating the graphite interlayer
compound to foam it, thereby separating graphite layers, washing
the resultant graphite powder to remove acid, and rolling the
resultant thin-film graphite powder. However, the expanded graphite
film has insufficient strength. Also, the graphite film obtained by
the heat treatment of a polyimide film, etc. is disadvantageously
expensive despite high heat dissipation.
[0004] JP 2012-211259 A discloses a heat-conductive sheet
comprising graphite pieces, which comprise pluralities of first
graphite pieces obtained by thinly cutting a thermally decomposed
graphite sheet, and second graphite pieces smaller than the widths
of the first graphite pieces, at least the first graphite pieces
connecting both surfaces of the heat-conductive sheet. This
heat-conductive sheet is obtained, for example, by blending the
first and second graphite pieces with a mixture of an acrylic
polymer and a solvent, and extruding the resultant blend. However,
the extruded heat-conductive sheet does not have sufficient heat
dissipation, because of a high volume fraction of the resin.
[0005] JP 2006-86271 A discloses a heat-dissipating sheet as thick
as 50-150 .mu.m comprising graphite bonded by an organic binder
having a glass transition temperature of -50.degree. C. to
+50.degree. C., such as an amorphous copolyester, a mass ratio of
graphite/organic binder being 66.7/33.3 to 95/5. This
heat-dissipating sheet is produced by applying a slurry of graphite
and an organic binder in an organic solvent to a
parting-agent-coated film on the side of a parting layer, drying
the slurry by hot air to remove the organic solvent, and then
pressing it, for example, at 30 kg/cm.sup.2. JP 2006-86271 A
describes that the pressing of a graphite/organic binder sheet
improves its thermal conductivity. In JP 2006-86271 A, a slurry of
graphite and an organic binder in an organic solvent is applied by
one operation. It has been found, however, that the application of
an entire slurry by one operation provides non-uniform distribution
of graphite. In addition, because a mass ratio of graphite to an
organic binder is not so high in Examples (80/20 in Example 1, and
89/11 in Example 2), sufficiently high thermal conductivity
inherent in graphite is not exhibited.
[0006] JP 11-1621 A discloses a high-thermal-conductivity, solid
composite material for a heat dissipater comprising highly oriented
graphite flakes and a binder polymer polymerized under pressure.
This solid composite material is produced by mixing graphite flakes
with a thermosetting monomer such as an epoxy resin to prepare a
composition comprising at least 40% by volume of graphite, and
polymerizing the monomer while compressing the composition under
sufficient pressure to align graphite substantially in parallel. JP
11-1621 A describes that a volume fraction of graphite in the
composite material is preferably 55-85%, though it may be from 40%
to 95%. However, the distribution of graphite flakes is non-uniform
in an epoxy resin containing graphite flakes in a high
concentration of 95%. Thus, JP 11-1621 A describes only
experimental results at a graphite flake volume fraction of
60%.
[0007] JP 2012-136575 A discloses a conductive, heat-dissipating
sheet comprising organic particles made of polyamides, acrylic
resins, etc. and having an average particle size of about 0.1-100
.mu.m, conductive inorganic fillers having an average particle size
of about 10 nm to about 10 .mu.m, and a cured resin such as an
epoxy resin, etc., organic particles/inorganic fillers being 1000/1
to 10/1, and the percentage of inorganic fillers being 5-30% by
weight based on the total amount. JP 2012-136575 A illustrates
graphite, coke, carbon black, etc. as inorganic fillers, though
only carbon black is used in Examples. However, because the
percentage of conductive carbon black is as small as 5-30% by
weight, the conductive heat-dissipating sheet of JP 2012-136575 A
does not have sufficient heat dissipation.
[0008] As described above, conventional heat-dissipating sheets
containing a small percentage of graphite or carbon black do not
have sufficient heat dissipation despite uniform distribution of
graphite or carbon black. Increase in the percentage of graphite or
carbon black results in lower sheet strength despite improved
thermal conductivity, particularly causing the problem of easy
detachment of graphite or carbon black from the heat-dissipating
sheet.
[0009] In addition, it has been found that a high percentage of
graphite particularly causes non-uniform distribution. Because
heat-dissipating sheets produced industrially are usually cut to
predetermined shapes and sizes and then disposed in small
electronic appliances, the non-uniform distribution of graphite
provides cut heat-dissipating sheets with unevenness in
performance.
[0010] Accordingly, inexpensive heat-dissipating sheets having
uniform, high heat dissipation as well as mechanical properties
necessary for handling are desired.
OBJECT OF THE INVENTION
[0011] Accordingly, an object of the present invention is to
provide an inexpensive heat-dissipating sheet having uniform, high
heat dissipation as well as mechanical properties necessary for
handling, and its production method.
DISCLOSURE OF THE INVENTION
[0012] As a result of intensive research in view of the above
object, the inventor has found that (a) a heat-dissipating sheet
comprising a small amount of carbon black uniformly dispersed among
fine graphite particles has high thermal conductivity, as well as
sufficient mechanical properties for handling, with substantially
no detachment of fine graphite particles and carbon black; and (b)
such heat-dissipating sheet is obtained by forming a sheet
comprising fine graphite particles and carbon black dispersed in a
small amount of an organic binder, burning the sheet to remove the
organic binder, and pressing the resultant composite sheet of
graphite and carbon black for densification. The present invention
has been completed based on such findings.
[0013] Thus, the heat-dissipating sheet of the present invention
has a structure in which carbon black is uniformly dispersed among
fine graphite particles, a mass ratio of fine graphite particles to
carbon black being 75/25 to 95/5; and the heat-dissipating sheet
being obtained by burning and pressing a composite sheet of fine
graphite particles, carbon black and an organic binder, so that it
has a density of 1.9 g/cm.sup.3 or more and an in-plane thermal
conductivity of 570 W/mK or more.
[0014] The heat-dissipating sheet has thickness of 25-150 .mu.m
preferably.
[0015] The fine graphite particles preferably have an average
diameter of 3-150 .mu.m and average thickness of 200 nm or
more.
[0016] The carbon black preferably has an average primary particle
size of 20-200 nm.
[0017] The heat-dissipating sheet is coated with insulating resin
layers or insulating plastic films preferably.
[0018] The method of the present invention for producing the above
heat-dissipating sheet comprises the steps of (1) preparing a
dispersion comprising 5-25% by mass in total of fine graphite
particles and carbon black, and 0.05-2.5% by mass of an organic
binder, in an organic solvent, a mass ratio of the fine graphite
particles to the carbon black being 75/25 to 95/5; (2) repeating
plural times a cycle of applying the dispersion to a surface of a
support plate and then drying it, thereby forming a
resin-containing composite sheet comprising the fine graphite
particles, the carbon black and the organic binder; (3) burning the
resin-containing composite sheet to remove the organic binder; and
(4) pressing the resultant composite sheet of fine graphite
particles and carbon black for densification.
[0019] The mass ratio of the organic binder to the total amount of
fine graphite particles and carbon black is preferably
0.01-0.5.
[0020] The amount of the dispersion applied by one operation is
preferably 5-15 g/m.sup.2 (expressed by the total weight of fine
graphite particles and carbon black per 1 m.sup.2).
[0021] The organic binder is preferably an acrylic resin, a
polystyrene resin or polyvinyl alcohol.
[0022] The organic solvent is preferably at least one selected from
the group consisting of ketones, aromatic hydrocarbons and
alcohols.
[0023] The application of the dispersion is preferably conducted by
a spraying method.
[0024] The burning step is conducted preferably at a temperature of
550-700.degree. C.
[0025] The pressing step is conducted preferably at pressure of 20
MPa or more.
[0026] The resin-containing composite sheet is preferably pressed
in a state sandwiched by a pair of planar die apparatuses.
[0027] The die plate apparatus preferably comprises a lower die
plate and an upper die plate. Using the lower die plate as the
support plate, the resin-containing composite sheet is formed in a
cavity of the lower die plate, burned without being peeled from the
lower die plate, and then pressed with the lower die plate combined
with the upper die plate.
[0028] The after being burnt is preferably cooled gradually to room
temperature over 1 hour or more.
[0029] The composite sheet of fine graphite particles and carbon
black is preferably cooled to a temperature equal to or lower than
the freezing point of water, and then pressed.
[0030] The pressing step is preferably conducted at a temperature
of room temperature to 200.degree. C.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] FIG. 1 is a schematic cross-sectional view showing the
structure of a heat-dissipating sheet composed of fine graphite
particles and carbon black.
[0032] FIG. 2 is a cross-sectional view showing a method for
determining the particle size of a fine graphite particle.
[0033] FIG. 3 is a perspective view showing a die plate apparatus
capable of conducting the application of a dispersion of fine
graphite particles, carbon black and an organic binder, the burning
of a resin-containing composite sheet, and the pressing of a
composite sheet of fine graphite particles and carbon black.
[0034] FIG. 4 is a cross-sectional view schematically showing a
thick coating of a dispersion of fine graphite particles, carbon
black and an organic binder on a support (lower die plate), in
which fine graphite particles are agglomerated.
[0035] FIG. 5 is a cross-sectional view schematically showing a
thin coating of the dispersion on a support (lower die plate), in
which fine graphite particles and carbon black are uniform
dispersed.
[0036] FIG. 6 is a cross-sectional view schematically showing a
thin coating of a dispersion formed on a dried dispersion on a
support (lower die plate).
[0037] FIG. 7 is a perspective view showing the application of a
dispersion to a cavity of a lower die plate in a planar die
apparatus.
[0038] FIG. 8 is a perspective view showing the pressing of a
composite sheet of fine graphite particles and carbon black in a
state sandwiched by a planar die apparatus.
[0039] FIG. 9 is a partially cross-sectional side view showing the
roll-pressing of a composite sheet of fine graphite particles and
carbon black in a state sandwiched by a planar die apparatus.
[0040] FIG. 10 is a perspective view showing the peeling of a
heat-dissipating sheet obtained by pressing from a lower die
plate.
[0041] FIG. 11 is a schematic cross-sectional view showing a heat
dissipation test apparatus of a heat-dissipating sheet.
[0042] FIG. 12 is an exploded view of FIG. 11.
[0043] FIG. 13 is a plan view showing temperature-measuring points
on a heat-dissipating sheet test piece set in a heat dissipation
test apparatus.
[0044] FIG. 14 is a graph showing the relation between the
concentration of carbon black and in-plane thermal conductivity in
the heat-dissipating sheets of Examples 1, 2, 6, 7, 11 and 12.
[0045] FIG. 15 is a graph showing the relation between the
concentration of carbon black and in-plane thermal conductivity in
the heat-dissipating sheets of Examples 1 and 3-5 and Comparative
Example 2.
[0046] FIG. 16 is a graph showing the relation between the
concentration of carbon black and in-plane thermal conductivity in
the heat-dissipating sheets of Examples 1, 2 and 8-10 and
Comparative Example 2.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0047] The embodiments of the present invention will be explained
in detail below referring to the attached drawings. Explanations of
each embodiment are applicable to other embodiments unless
otherwise mentioned. Explanations below are not restrictive, but
various modifications may be made within the scope of the present
invention.
[0048] [1] Heat-Dissipating Sheet
[0049] As shown in FIG. 1, the heat-dissipating sheet 1 of the
present invention is substantially composed of fine graphite
particles 2, and carbon black 3 uniformly dispersed among fine
graphite particles 2. Though gaps between fine graphite particles 2
and carbon black 3 are exaggerated in FIG. 1 for clarity, the fine
graphite particles 2 and the carbon black 3 are actually bonded
closely with substantially no gaps.
[0050] (1) Fine Graphite Particles
[0051] A fine graphene particle has a flake- or plate-like,
multi-layer structure, in which benzene rings are two-dimensionally
connected. Because the fine graphene particle has a hexagonal
lattice structure, each carbon atom is bonded to three carbon
atoms, one of four peripheral electrons used for chemical bonding
being in a free state (free electron). Because free electrons can
move along the crystal lattice, fine graphene particles have high
thermal conductivity.
[0052] Because a fine graphene particle has a flake- or plate-like
shape, its size is represented by the diameter of its planar
surface. Because a flake-like, fine graphene particle 2 has a
planar contour of an irregular shape as shown in FIG. 2, the size
(diameter) of each fine graphene particle 2 is defined as a
diameter d of a circle having the same area S. Because the size of
each fine graphene particle 2 is expressed by a diameter d and a
thickness t, the average diameter of fine graphene particles 2 used
is expressed by (.SIGMA.d)/n, wherein n represents the number of
fine graphene particles 2 measured, and the average thickness of
fine graphene particles 2 is expressed by (.SIGMA.t)/n. The
diameters d and thickness t of fine graphene particles 2 can be
determined by the image treatment of photomicrographs of fine
graphene particles 2.
[0053] The average diameter of fine graphene particles 2 used in
the present invention is preferably in a range of 3-150 .mu.m. When
the average diameter of fine graphene particles 2 is less than 3
.mu.m, bonded carbon atoms are not sufficiently long, providing a
heat-dissipating sheet 1 with too small thermal conductivity. On
the other hand, fine graphene particles 2 having an average
diameter of more than 150 .mu.m would make spray coating difficult.
The average diameter of fine graphene particles 2 is more
preferably 5-100 .mu.m, further preferably 5-50 .mu.m, most
preferably 10-30 .mu.m. The average thickness of fine graphene
particles 2 is preferably 200 nm or more, more preferably 200 nm to
5 .mu.m, most preferably 200 nm to 1 .mu.m.
[0054] (2) Carbon Black
[0055] Carbon black 3 usable in the present invention includes
furnace black, channel black, acetylene black, arc black, ketjen
black, etc. Carbon black 3 preferably has an average primary
particle size of 20-200 nm. With carbon black 3 having an average
primary particle size of less than 20 nm, agglomeration is likely
to occur, making difficult the uniform dispersion of carbon black 3
among fine graphite particles 2. Carbon black 3 having an average
primary particle size of more than 200 nm is too large to be
uniformly dispersed among fine graphite particles 2. The average
primary particle size of carbon black 3 is more preferably 30-100
nm, most preferably 40-80 nm.
[0056] (3) Mass Ratio
[0057] The mass ratio of fine graphite particles to carbon black is
75/25 to 95/5. Within the above mass ratio range of fine graphite
particles to carbon black, a heat-dissipating sheet having as high
in-plane thermal conductivity as 570 W/mK or more and sufficient
mechanical properties (tensile strength, bendability and
cuttability) for handling can be obtained. When fine graphite
particles is more than 95% by mass (carbon black is less than 5% by
mass), their total amount being 100% by mass, a sufficient effect
of adding carbon black cannot be obtained. On the other hand, when
fine graphite particles are less than 75% by mass (carbon black is
more than 25% by mass), a heat-dissipating sheet having an in-plane
thermal conductivity of 570 W/mK or more cannot be obtained. The
mass ratio of fine graphite particles to carbon black is preferably
80/20 to 95/5, more preferably 82.5/17.5 to 90/10.
[0058] (4) Thickness
[0059] To secure sufficient cooling power, the heat-dissipating
sheet is preferably as thick as 25-150 .mu.m. When it is thinner
than 25 .mu.m, the heat-dissipating sheet has insufficient cooling
power despite high thermal conductivity. Even if the
heat-dissipating sheet were thicker than 150 .mu.m, further
improvement in the cooling power would not be expected. The
preferred thickness of the heat-dissipating sheet is 40-100 .mu.m
for practical purposes.
[0060] (5) Density
[0061] The heat-dissipating sheet of the present invention has a
density of 1.9 g/cm.sup.3 or more. Because fine graphite particles
have a density of 2.25.+-.0.05 g/cm.sup.3, the heat-dissipating
sheet of the present invention has a density extremely close to
that of fine graphite particles, thereby having thermal
conductivity close to the inherent thermal conductivity of
graphite. The density of the heat-dissipating sheet of the present
invention is preferably 1.9-2.2 g/cm.sup.3.
[0062] (6) Thermal Conductivity
[0063] As described above, because the heat-dissipating sheet of
the present invention has a structure in which carbon black is
uniformly dispersed among fine graphite particles, and has a
density of 1.9 g/cm.sup.3 or more, it has thermal conductivity of
570 W/mK or more in an in-plane direction. The thermal conductivity
in an in-plane direction may be called simply "in-plane thermal
conductivity." The "in-plane direction" is an XY direction in
parallel with a surface (XY plane) of the heat-dissipating sheet,
and the "thickness direction" is a Z direction perpendicular to the
XY plane. The heat-dissipating sheet of the present invention
preferably has thermal conductivity of 600 W/mK or more in an
in-plane direction, and about 10 W/mK or more in a thickness
direction.
[0064] [2] Production Method of Heat-Dissipating Sheet
[0065] (1) Preparation of Dispersion
[0066] A dispersion of fine graphite particles, carbon black and an
organic binder in an organic solvent is first prepared. Because
fine graphite particles are easily agglomerated, it is preferable
to mix a dispersion of fine graphite particles in an organic
solvent with a dispersion of carbon black in an organic solvent and
a solution of an organic binder in an organic solvent. With the
entire dispersion as 100% by mass, the total amount of fine
graphite particles and carbon black is 5-25% by mass. When the
total amount of fine graphite particles and carbon black is less
than 5% by mass, too thin a resin-containing composite sheet is
obtained by one operation, resulting in too many steps of applying
the dispersion, and thus too low a production efficiency of a
heat-dissipating sheet. On the other hand, when the total amount of
fine graphite particles and carbon black is more than 25% by mass,
the concentrations of fine graphite particles and carbon black are
too high in the dispersion, likely causing agglomeration. The
preferred total amount of fine graphite particles and carbon black
is 8-20% by mass, as long as the mass ratio of fine graphite
particles to carbon black is in a range of 75/25 to 95/5 as
described above.
[0067] The mass ratio of the organic binder to the total amount of
fine graphite particles and carbon black is 0.01-0.5. When the mass
ratio of organic binder/(fine graphite particles+carbon black) is
less than 0.01, the dispersion does not have enough viscosity for
efficient spraying, and the resultant resin-containing composite
sheet is not sufficiently integral, making its handling difficult.
When the above mass ratio is more than 0.5, it takes too much time
to burn off the organic binder in a subsequent burning step,
resulting in low production efficiency of a heat-dissipating sheet.
The mass ratio of organic binder/(fine graphite particles+carbon
black) is preferably 0.02-0.3, more preferably 0.03-0.2.
[0068] The organic binder used in the present invention is not
particularly restricted, as long as it can be dissolved in an
organic solvent to uniformly disperse fine graphite particles and
carbon black, and easily removed by burning. Such organic binders
include, for example, acrylic resins such as polymethylacrylate and
polymethylmethacrylate, polystyrenes, polycarbonates, polyvinyl
chloride, ABS resins, etc. Among them, polymethylmethacrylate and
polystyrenes are preferable.
[0069] The organic solvent used in the dispersion is preferably an
organic solvent capable of well dispersing fine graphite particles
and carbon black and dissolving an organic binder, and volatile
enough to shorten the drying time. Examples of such organic
solvents include ketones such as methyl ethyl ketone, aliphatic
hydrocarbons such as hexane, aromatic hydrocarbons such as xylene,
alcohols such as isopropyl alcohol, etc. Among them, methyl ethyl
ketone, xylene, etc. are preferable. They may be used alone or in
combination.
[0070] (2) Application and Drying of Dispersion
[0071] The dispersion is applied to a surface of a support plate.
When a resin-containing composite sheet formed on a surface of the
support plate is burned in a subsequent step, and then pressed, the
support plate is conveniently a planar die apparatus 10 shown in
FIG. 3. The planar die apparatus 10 comprises a lower die plate 11
having a flat cavity 11a, and an upper die plate 12 having a
projection 12a having a complementary shape to the cavity 11a. In
the depicted example, the cavity 11a is open at both ends, though
not restrictive.
[0072] When a necessary amount of a dispersion is applied by one
operation as schematically shown in FIG. 4, fine graphite particles
2 and carbon black 3 in the dispersion 5 are agglomerated in a
drying process (region 2a). Intensive research has revealed that
when the dispersion 5 is divided to as small amounts of lots as
possible to carry out plural application operations, the
agglomeration of fine graphite particles 2 and carbon black 3 can
be prevented. In the first application shown in. FIG. 5, a
dispersion layer 101 is formed by a small amount of the dispersion,
and its thickness is sufficiently small to the average diameter of
fine graphite particles 2. Accordingly, when the dispersion layer
101 is dried, the dispersed state of fine graphite particles 2 and
carbon black 3 is kept without agglomeration. Accordingly, fine
graphite particles 2 and carbon black 3 bonded with an extremely
small amount of the organic binder are substantially uniformly
distributed in a coating layer (resin-containing composite sheet
layer) 101' obtained by drying the dispersion layer 101.
[0073] The amount of the dispersion 5 applied by one operation is
preferably 5-15 g/m.sup.2, more preferably 7-12 g/m.sup.2, as the
total amount of fine graphite particles 2 and carbon black 3 per a
unit area. When the amount of the dispersion 5 applied is less than
5 g/m.sup.2, it takes too much time to form a resin-containing
composite sheet. On the other hand, when it is more than 15
g/m.sup.2, fine graphite particles 2 and carbon black 3 are easily
agglomerated. To apply such a small amount of the dispersion 5
uniformly, a spraying method using a nozzle 15 as shown in FIG. 7
is preferable. With the nozzle 15 moved in horizontal directions (X
and Y directions), it is preferable to spray the dispersion 5 for
uniform thickness.
[0074] After the dispersion layer 101 is dried, the next
application operation is conducted. The dispersion layer 101 may be
spontaneously dried, or by heating to shorten the drying time. The
heating temperature may be determined depending on the boiling
point of an organic solvent used. For example, when a mixed solvent
of xylene and isopropyl alcohol, or methyl ethyl ketone is used,
the heating temperature is preferably 30-100.degree. C., more
preferably 40-80.degree. C. The drying need not be conducted until
an organic solvent in the applied dispersion layer 101 is
completely evaporated, but may be conducted to such an extent that
fine graphite particles 2 and carbon black 3 are not diffused from
the dispersion layer 101 in the next application operation.
[0075] When the second application of the dispersion 5 is conducted
onto the dried coating layer (resin-containing composite sheet
layer) 101', a new dispersion layer 102 is formed substantially
without dissolving the dried coating layer 101' as schematically
shown in FIG. 6. The number of cycles of applying and drying the
dispersion 5 may be determined depending on the thickness of a
resin-containing composite sheet to be formed. Applying such a
small amount of a dispersion 5 plural times provides a
resin-containing composite sheet 20 (FIG. 7), in which fine
graphite particles 2 and carbon black 3 are sufficiently uniformly
distributed.
[0076] (3) Burning
[0077] The resin-containing composite sheet 20 is burned to remove
the organic binder. When the resin-containing composite sheet 20 is
formed in a cavity 11a of a lower die plate 11 in a planar die
apparatus 10, the resin-containing composite sheet 20 is preferably
charged into a furnace (not shown) together with the lower die
plate 11. The furnace may be an electric furnace, a gas furnace, or
a continuous furnace in which the resin-containing composite sheet
20 in the lower die plate 11 is conveyed on a belt conveyor. In the
case of a continuous furnace, a gradually-cooling furnace is
preferably positioned at the end of the continuous furnace, to
secure gradual cooling described later.
[0078] The burning temperature is preferably 550-750.degree. C.
When the burning temperature is lower than 550.degree. C., the
removal of the organic binder takes too much time, and the
resultant heat-dissipating sheet cannot have sufficiently high
thermal conductivity. On the other hand, when the burning
temperature is higher than 750.degree. C., carbon black may be
burned out at least partially, resulting in a heat-dissipating
sheet with insufficient thermal conductivity. The preferred burning
temperature is 600-700.degree. C.
[0079] The resin-containing composite sheet 20 is burned preferably
in an atmosphere sufficiently containing oxygen, for example, in
the air. In an oxygen-containing atmosphere (air), the organic
binder is rapidly burned out without leaving a carbonized binder.
However, burning in an inert gas such as a nitrogen gas tends to
carbonize the organic binder, providing a heat-dissipating sheet
with low thermal conductivity. The oxygen content in the atmosphere
is preferably 10% or more, more preferably 15% or more.
[0080] The burning time of the resin-containing composite sheet 20
in the above temperature range in an oxygen-containing atmosphere
is generally 5-30 minutes, though variable depending on the burning
temperature. The burning time is a time period in which the
resin-containing composite sheet 20 is kept at the burning
temperature, without including the temperature elevation time and
the cooling time. When the burning time is less than 5 minutes, the
organic binder is not completely burned out. When the burning time
is more than 30 minutes, carbon black is excessively exposed to
high temperatures, so that carbon black may be burned out at least
partially, resulting in a heat-dissipating sheet with insufficient
thermal conductivity. The preferred burning time is 7-15
minutes.
[0081] (4) Cooling
[0082] A composite sheet 21 of fine graphite particles and carbon
black obtained by burning the resin-containing composite sheet 20
is preferably gradually cooled in the furnace. It has been found
that when the composite sheet 21 of fine graphite particles and
carbon black is left to cool outside the furnace, the resultant
heat-dissipating sheet tends to have low thermal conductivity. The
composite sheet 21 of fine graphite particles and carbon black is
preferably gradually cooled over 1 hour or more in the furnace. The
cooling speed is preferably 15.degree. C./minute or less, more
preferably 10.degree. C./minute or less.
[0083] It has been found that when the composite sheet 21 of fine
graphite particles and carbon black is cooled to a temperature
equal to or lower than the freezing point of water before pressing,
the heat-dissipating sheet exhibits high thermal conductivity in a
wide range of the carbon black content. The cooling temperature may
be 0.degree. C. or lower, and is preferably -5.degree. C. or lower.
When cooled to a temperature equal to or lower than the freezing
point of water, moisture in the air is likely frozen on the
composite sheet 21. Accordingly, cooling is conducted preferably in
a dry atmosphere. The cooling time is not particularly restricted,
but may be 10 minutes or more.
[0084] (5) Pressing
[0085] As shown in FIG. 8, a composite sheet 21 of fine graphite
particles and carbon black obtained by burning the resin-containing
composite sheet 20 on the lower die plate 11 is pressed by
combining the lower die plate 11 with the upper die plate 12, such
that a projection 12a of the upper die plate 12 is pressed onto the
composite sheet 21 in the cavity 11a of the lower die plate 11. The
lower die plate 11 and the upper die plate 12 may be pressed by a
pressing apparatus, or by a pair of rolls 30, 30 with the composite
sheet 21 sandwiched by the lower die plate 11 and the upper die
plate 12 as shown in FIG. 9. Pressure applied to the lower die
plate 11 and the upper die plate 12 is preferably 20 MPa or more.
Pressing is not limited to once, but may be conducted plural times.
Pressing may be conducted at room temperature, or at high
temperature up to 200.degree. C. to increase the pressing
efficiency.
[0086] During pressing, the lower die plate 11 and the upper die
plate 12 are preferably vibrated via rolls 30. Vibration promotes
the densification of the composite sheet 21 of fine graphite
particles and carbon black even under the same pressure. The
vibration frequency may be about 100-500 Hz. Vibration may be added
by a vibration motor.
[0087] The heat-dissipating sheet 1 obtained by pressing the
composite sheet 21 of fine graphite particles and carbon black is
peeled from the lower die plate 11 as shown in FIG. 10. Because of
uniform dispersion of carbon black 3 among fine graphite particles
2 and densification by pressing, the heat-dissipating sheet 1 is
neither broken nor cracked when peeled from the lower die plate 11.
The heat-dissipating sheet 1 thus obtained has sufficient
bendability, so that it is not broken or fractured even when bent,
for example, to 90.degree. with a radius of curvature of 2 cm.
[0088] (6) Cutting of Heat-Dissipating Sheet
[0089] When a large heat-dissipating sheet 1 is formed by the above
process, it should be cut to a proper size so that it can be
attached to a small electronic appliance. Because of uniform
dispersion of carbon black 3 among fine graphite particles 2, the
heat-dissipating sheet 1 of the present invention cut by an
ordinary cutter has a sharp cut surface without raggedness.
[0090] (7) Surface Coating of Heat-Dissipating Sheet
[0091] The heat-dissipating sheet 1 of the present invention
comprising fine graphite particles and carbon black is preferably
coated with an insulating resin or a plastic film, to prevent the
detachment of fine graphite particles and carbon black and to
achieve surface insulation. The insulating resins are preferably
thermoplastic resins soluble in organic solvents, for example,
acrylic resins such as polymethylmethacrylate, polystyrenes,
polycarbonates, polyvinyl chloride, polyurethanes, etc. The
insulating plastic films may be made of polyolefins such as
polyethylene and polypropylene, polyesters such as polyethylene
terephthalate, polyamides such as nylons, polyimides, etc. The
insulating plastic film preferably has a heat-sealing layer. As
long as the functions of preventing the detachment of fine graphite
particles and carbon black and adding insulation are exhibited, the
thickness of the insulating resin coating and the insulating
plastic film may be several micrometers to about 20 .mu.m. Surface
coating may be preferably conducted after cutting the
heat-dissipating sheet 1 to a desired size, to surely prevent the
detachment of fine graphite particles and carbon black from the cut
surface of the heat-dissipating sheet 1.
[0092] [4] Heat Dissipation Test
[0093] The heat dissipation test of the heat-dissipating sheet of
the present invention may be conducted by an apparatus 50 shown in
FIGS. 11-13. This heat dissipation test apparatus 50 comprises a
heat-insulating, electric-insulating table 51 having an annular
recess 52, a circular plate heater 53 received in the annular
recess 52, temperature-measuring thermocouples 54 attached to a
lower surface of the heater 53, a temperature controller 55
connected to the heater 53 and the temperature-measuring
thermocouples 54, and a 1-mm-thick acrylic plate (100 mm.times.100
mm) 57 covering a test piece 56 of 50 mm.times.100 mm of the
heat-dissipating sheet 1 placed on the table 51, at such a position
that the heater 53 is located at a center of the acrylic plate 57.
The test piece 56 has nine temperature-measuring points
t.sub.0-t.sub.8 at positions shown in FIG. 13, a temperature
measured at the point t.sub.0 being the highest temperature (Tmax),
an average of temperatures measured at the points t.sub.1-t.sub.4
being an intermediate temperature (Tm), an average of temperatures
measured at the points t.sub.5-t.sub.8 being the lowest temperature
(Tmin), and an average of Tm and Tmin being an average temperature
(Tav).
[0094] The present invention will be explained in more detail with
Examples below without intention of restricting the present
invention thereto.
EXAMPLE 1
[0095] 100 parts by mass in total of 85% by mass of fine graphite
particles (UP-35N available from Nippon Graphite Industries Ltd.,
ash: less than 1.0%, average size: 25 .mu.m), and 15% by mass of
carbon black (acetylene black, average primary particle size: 42
nm) were mixed with 10 parts by mass of polymethylmethacrylate
(PMMA) as an organic binder, and a mixed solvent of
xylene/isopropyl alcohol (mass ratio: 6/4) as an organic solvent,
to prepare a dispersion of fine graphite particles, carbon black
and an organic binder in an organic solvent. The composition of the
dispersion comprised 12.00% by mass of fine graphite particles,
2.12% by mass of carbon black, 1.41% by mass of the organic binder,
and 84.47% by mass of the organic solvent.
[0096] A small portion of this dispersion was cast into a cavity
11a of a lower die plate 11 in the SUS-made planar die apparatus 10
shown in FIG. 3, and dried at 40.degree. C. for 3 minutes to form a
resin-containing composite sheet layer 101' of fine graphite
particles, carbon black and PMMA having a thickness of 10 g/m.sup.2
(expressed by the total amount of fine graphite particles and
carbon black per 1 m.sup.2). The next dispersion as thick as 10
g/m.sup.2 was applied to the dried resin-containing composite sheet
layer 101', and then dried. Such application of the dispersion was
repeated 10 times in total, to produce a resin-containing composite
sheet 20 having a final thickness.
[0097] The resin-containing composite sheet 20 kept in the lower
die plate 11 was introduced into an electric furnace, and burned at
650.degree. C. for 10 minutes in an air atmosphere to remove the
organic binder. The resultant composite sheet 21 of fine graphite
particles and carbon black was gradually cooled over about 3 hours
in the electric furnace.
[0098] The lower die plate 11 containing the composite sheet 21 of
fine graphite particles and carbon black in the cavity 11a was
combined with an upper die plate 12 having a projection 12a having
a complementary shape to the lower die plate 11, such that the
projection 12a of the upper die plate 12 came into contact with the
composite sheet 21 of fine graphite particles and carbon black, and
caused to pass through a gap between a pair of rolls 30, 30
rotating at a peripheral speed of 30 cm/minute 4 times, to press
the composite sheet 21 of fine graphite particles and carbon black
at linear pressure of 20 MPa or more each time, as shown in FIG. 9.
During pressing, vibration at a frequency of 200 Hz was applied by
one roll 30.
[0099] After pressing, a heat-dissipating sheet 1 could be taken
out of the lower die plate 11 without breakage. The
heat-dissipating sheet 1 thus obtained had a thickness of 111 .mu.m
and a density of 2.13 g/cm.sup.3. A test piece of 50 mm.times.100
mm was cut out of this heat-dissipating sheet 1, and set in the
apparatus shown in FIGS. 11-13 to conduct a heat dissipation test
at room temperature (23.6.degree. C.). The test piece was heated at
72.degree. C. (hot spot) by the heater 53. After reaching an
equilibrium state, the temperature at each point of the
heat-dissipating sheet test piece was as follows:
[0100] t.sub.0: 47.4.degree. C.,
[0101] t.sub.1: 42.7.degree. C.,
[0102] t.sub.2: 42.2.degree. C.,
[0103] t.sub.3: 41.9.degree. C.,
[0104] t.sub.4: 42.3.degree. C.,
[0105] t.sub.5: 39.3.degree. C.,
[0106] t.sub.6: 38.3.degree. C.,
[0107] t.sub.7: 37.1.degree. C., and
[0108] t.sub.8: 37.2.degree. C.
[0109] Thus, the highest temperature Tmax was 47.4.degree. C. (hot
spot), the intermediate temperature Tm was (42.7.degree.
C.+42.2.degree. C.+41.9.degree. C.+42.3.degree. C.)/4=42.3.degree.
C., the lowest temperature Tmin was (39.3.degree. C.+38.3.degree.
C.+37.1.degree. C.+37.2.degree. C.)/4=38.0.degree. C., and the
average temperature Tav was (Tm+Tmin)/2=40.2.degree. C.
[0110] The thermal conductivity (W/mK) of the heat-dissipating
sheet 1 was calculated as a product of thermal diffusivity
(m.sup.2/s) measured by a laser flash method and heat capacity
(density.times.specific heat). The specific heat was regarded as
750. As a result, the thermal conductivity of the heat-dissipating
sheet 1 was 625 W/mK in an in-plane direction and 10 W/mK in a
thickness direction.
[0111] When this heat-dissipating sheet 1 was bent to 90.degree.
with a radius of curvature of 2 cm, no breakage occurred. The
heat-dissipating sheet 1 cut by scissors had a clear-cut surface
with substantially no fine graphite particles and carbon black
detached.
COMPARATIVE EXAMPLE 1
[0112] The same heat dissipation test as in Example 1 was conducted
on a graphite sheet PGS as thick as 70 .mu.m (available from
Panasonic Corporation). As a result, the temperature at each point
of a test piece of the heat-dissipating sheet was as follows:
[0113] t.sub.0: 48.7.degree. C.,
[0114] t.sub.1: 42.7.degree. C.,
[0115] t.sub.2: 43.3.degree. C.,
[0116] t.sub.3: 42.4.degree. C.,
[0117] t.sub.4: 42.1.degree. C.,
[0118] t.sub.5: 38.4.degree. C.,
[0119] t.sub.6: 38.1.degree. C.,
[0120] t.sub.7: 38.1.degree. C., and
[0121] t.sub.8: 38.0.degree. C.
[0122] Thus, the highest temperature Tmax was 48.7.degree. C. (hot
spot), the intermediate temperature Tm was (42.7.degree.
C.+43.3.degree. C.+42.4.degree. C.+42.1.degree. C.)/4=42.6.degree.
C., the lowest temperature Tmin was (38.4.degree. C.+38.1.degree.
C.+38.1.degree. C.+38.0.degree. C.)/4=38.2.degree. C., and the
average temperature Tav was (Tm+Tmin)/2=40.4.degree. C. Comparison
with Example 1 revealed that the graphite sheet of Comparative
Example 1 was poorer than the heat-dissipating sheet of Example 1
in any of the highest temperature Tmax, the lowest temperature Tmin
and the average temperature Tav.
EXAMPLE 2
[0123] A dispersion of fine graphite particles, carbon black and an
organic binder in an organic solvent was prepared in the same
manner as in Example 1, except for changing the mass ratio of fine
graphite particles to carbon black to 87.5/12.5. The dispersion
comprised 12.0% by mass of fine graphite particles, 1.71% by mass
of carbon black, 1.37% by mass of an organic binder, and 84.92% by
mass of an organic solvent. Using this dispersion, a
heat-dissipating sheet 1 was produced in the same manner as in
Example 1. This heat-dissipating sheet 1 had thermal conductivity
of 645 W/mK in an in-plane direction and 10 W/mK in a thickness
direction.
[0124] When this heat-dissipating sheet 1 was bent to 90.degree.
with a radius of curvature of 2 cm, no breakage occurred as in
Example 1. The heat-dissipating sheet 1 cut by scissors had a
clear-cut surface with substantially no fine graphite particles and
carbon black detached as in Example 1.
[0125] FIG. 14 shows the relation between in-plane thermal
conductivity and the concentration of carbon black in the
heat-dissipating sheet 1 of Examples 1 and 2. It is clear from FIG.
14 that a higher percentage of fine graphite particles provide
higher thermal conductivity.
EXAMPLES 3-5 AND COMPARATIVE EXAMPLE 2
[0126] Heat-dissipating sheets were produced in the same manner as
in Example 1, except for changing the amount of carbon black in the
total amount (100% by mass) of fine graphite particles and carbon
black to 0% by mass (Comparative Example 2), 10% by mass (Example
3), 20% by mass (Example 4) and 25% by mass (Example 5), and their
thickness, densities and in-plane thermal conductivities were
measured. The thickness, densities and in-plane thermal
conductivities of the heat-dissipating sheets of Examples 3-5 and
Comparative Example 2 are shown in FIG. 15 together with those of
Example 1. It is clear from FIG. 15 that the heat-dissipating
sheets of Examples 1 and 3-5 containing carbon black have higher
in-plane thermal conductivities than that of the heat-dissipating
sheet of Comparative Example 2 containing no carbon black.
[0127] When the heat-dissipating sheets 1 of Examples 3-5 were bent
to 90.degree. with a radius of curvature of 2 cm, no breakage
occurred as in Example 1. The heat-dissipating sheets 1 cut by
scissors had clear-cut surfaces with substantially no fine graphite
particles detached as in Example 1. On the other hand, the
heat-dissipating sheet 1 of Comparative Example 2 was broken by the
above bending test, and fine graphite particles were extremely
detached from the cut surface in a cutting test by scissors.
EXAMPLES 6 AND 7
[0128] The composite sheets 21 of fine graphite particles and
carbon black produced in Examples 1 and 2 were frozen at -5.degree.
C. for 30 minutes, and then pressed under the same conditions as in
Example 1. The resultant heat-dissipating sheets were measured with
respect to in-plane thermal conductivity. The results are shown in
FIG. 14. As is clear from FIG. 14, the heat-dissipating sheet of
Example 6 pressed after the freezing treatment had higher thermal
conductivity than that of the heat-dissipating sheet of Example
1.
[0129] When the heat-dissipating sheets 1 of Examples 6 and 7 were
bent to 90.degree. with a radius of curvature of 2 cm, no breakage
occurred as in Example 1. The heat-dissipating sheets 1 cut by
scissors had clear-cut surfaces with substantially no fine graphite
particles detached as in Example 1.
EXAMPLES 8-10
[0130] Heat-dissipating sheets were produced in the same manner as
in Example 1, except for using carbon black having as large an
average primary particle size as 85 nm, and changing the amount of
carbon black per the total amount (100% by mass) of fine graphite
particles and carbon black to 10% by mass (Example 9), 15% by mass
(Example 8), and 20% by mass (Example 10), respectively, and their
thermal conductivities were measured. The results are shown in FIG.
16 together with those of Examples 1 and 2 and Comparative Example
2. It is clear from FIG. 16 that a larger average primary particle
size provides the heat-dissipating sheet with a lower in-plane
thermal conductivity, and that a higher concentration of carbon
black provides the heat-dissipating sheet with a larger in-plane
thermal conductivity.
[0131] When the heat-dissipating sheets 1 of Examples 8-10 were
bent to 90.degree. with a radius of curvature of 2 cm, no breakage
occurred as in Example 1. The heat-dissipating sheets 1 cut by
scissors had clear-cut surfaces with substantially no fine graphite
particles detached as in Example 1.
EXAMPLES 11 AND 12
[0132] Heat-dissipating sheets 1 were produced and their thermal
conductivities were measured in the same manner as in Example 1,
except that the resin-containing composite sheet 20 was taken out
of an electric furnace after burning and left to cool in the air.
The results are shown in FIG. 14. It is clear from FIG. 14 that the
heat-dissipating sheets 1 of Examples 11 and 12 taken out of the
furnace after being burnt and then left to cool had lower thermal
conductivities than those of the heat-dissipating sheets 1 of
Examples 1 and 2 gradually cooled in the furnace after being burnt,
while meeting the requirement of 570 W/mK or more, even though they
had the same composition.
[0133] When the heat-dissipating sheets 1 of Examples 11 and 12
were bent to 90.degree. with a radius of curvature of 2 cm, no
breakage occurred as in Example 1. The heat-dissipating sheets 1
cut by scissors had clear-cut surfaces with substantially no fine
graphite particles detached as in Example 1.
Effects of the Invention
[0134] Because the heat-dissipating sheet of the present invention
has a structure in which carbon black is uniformly dispersed among
fine graphite particles, a mass ratio of fine graphite particles to
carbon black being 75/25 to 95/5, and has a density of 1.9
g/cm.sup.3 or more, it has as high in-plane thermal conductivity as
570 W/mK or more. Also, because fine carbon black is uniformly
dispersed among fine graphite particles, the heat-dissipating sheet
of the present invention has uniform thermal conductivity as well
as sufficient mechanical properties for handling. Such a uniform,
high-density heat-dissipating sheet is obtained by applying a small
amount of a dispersion comprising fine graphite particles, carbon
black and an organic binder plural times to form a resin-containing
composite sheet comprising uniformly dispersed fine graphite
particles and carbon black, burning the resin-containing composite
sheet to remove the organic binder, and then pressing it for
densification.
[0135] Because the heat-dissipating sheet of the present invention
is produced by a low-cost process of applying, burning and pressing
a relatively inexpensive material comprising fine graphite
particles and carbon black, it is advantageously inexpensive, with
as high in-plane thermal conductivity as 570 W/mK or more and
sufficient mechanical properties for handling. The heat-dissipating
sheet of the present invention having such feature is suitable for
small electronic appliances such as note-type personal computers,
smartphones, mobile phones, etc.
* * * * *