U.S. patent application number 11/886738 was filed with the patent office on 2009-12-10 for heat-transfer sheet, heat transfer system, and method of using heat-transfer sheet.
This patent application is currently assigned to Toyo Tanso Co., Ltd.. Invention is credited to Yoshiaki Hirose.
Application Number | 20090301697 11/886738 |
Document ID | / |
Family ID | 37114943 |
Filed Date | 2009-12-10 |
United States Patent
Application |
20090301697 |
Kind Code |
A1 |
Hirose; Yoshiaki |
December 10, 2009 |
Heat-Transfer Sheet, Heat Transfer System, and Method of Using
Heat-Transfer Sheet
Abstract
This invention provides (i) a heat-transfer sheet which comes
into close contact with a heat generator and a radiator when caught
there between and can be used repeatedly, (ii) a method of using
the heat-transfer sheet, and (iii) a heat transfer system
comprising the heat-transfer sheet. The heat-transfer sheet is made
of expanded graphite and disposed between a heat generator and a
radiator, and its bulk density is less than 1.0 Mg/m.sup.3. Because
its bulk density is low, it is easily compressed between the heat
generator and the radiator even under low pressure to be put into
close contact with them. Thus, the thermal resistance between the
heat generator and the heat sink is low and, hence, the heat
generator is efficiently cooled down.
Inventors: |
Hirose; Yoshiaki; (Kagawa,
JP) |
Correspondence
Address: |
Cheng Law Group, PLLC
1100 17th Street, N.W., Suite 503
Washington
DC
20036
US
|
Assignee: |
Toyo Tanso Co., Ltd.
Osaka-shi
JP
|
Family ID: |
37114943 |
Appl. No.: |
11/886738 |
Filed: |
March 13, 2006 |
PCT Filed: |
March 13, 2006 |
PCT NO: |
PCT/JP2006/304865 |
371 Date: |
June 17, 2009 |
Current U.S.
Class: |
165/148 ;
165/185 |
Current CPC
Class: |
H01L 2924/01019
20130101; H01L 2924/01012 20130101; H01L 23/373 20130101; H01L
2224/73253 20130101; C01B 32/225 20170801; H01L 2924/01004
20130101 |
Class at
Publication: |
165/148 ;
165/185 |
International
Class: |
F28D 1/00 20060101
F28D001/00; F28F 7/00 20060101 F28F007/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 31, 2005 |
JP |
2005-100622 |
Claims
1. A heat-transfer sheet which is made of expanded graphite and
disposed between a heat generator and a radiator and whose bulk
density is less than 1.0 Mg/m.sup.3.
2. The heat-transfer sheet according to claim 1 whose
compressibility is 50% or more and whose recovery is 5% or more
when it is thicknesswise compressed by the pressure of 34.3
MPa.
3. A heat transfer system comprising a radiator and the
heat-transfer sheet of claim 1 disposed between the radiator and a
heat generator.
4. A method of using a heat-transfer sheet which is made of
expanded graphite, whose bulk density is less than 1.0 Mg/m.sup.3,
and which is disposed between a heat generator and a radiator to be
thicknesswise compressed therebetween by the pressure of 2.0 MPa or
less.
5. A method of using a heat-transfer sheet which is made of
expanded graphite, whose bulk density is less than 0.9 Mg/m.sup.3,
and which is disposed between a heat generator and a radiator to be
thicknesswise compressed therebetween by the pressure of 1.5 MPa or
less.
6. A heat transfer system comprising a radiator and the
heat-transfer sheet of claim 2 disposed between the radiator and a
heat generator.
Description
TECHNICAL FIELD
[0001] This invention relates to a heat-transfer sheet. Radiator
such as heat sinks is used to cool heat generators such as CPU's of
computers, etc. If a radiator does not come into close contact with
a heat generator, thermal conductivity from the heat generator to
the radiator is low and the heat generator is not cooled
sufficiently. Accordingly, usually disposed between such a heat
generator and such a radiator is a sheet which has high thermal
conductivity and comes into close contact with the heat generator
and the radiator.
[0002] This invention relates to (i) a heat-transfer sheet which is
caught between a heat generator and a radiator, (ii) a method of
using the heat-transfer sheet, and (iii) a heat transfer system
comprising the heat-transfer sheet.
BACKGROUND ART
[0003] Graphite sheets are usually used as heat-transfer sheets
which are caught between such heat generators and radiators. A
graphite sheet is caught and compressed between a heat generator
and a radiator; therefore, the graphite sheet comes into close
contact with the heat generator and the radiator, reducing the
thermal resistance of contact surfaces and, thereby, raising the
cooling efficiency. The pressure applied to the graphite sheet
depends on the force securing the heat sink to the heat generator.
If large force is applied to a CPU, it may be deformed.
Accordingly, force securing heat sinks to heat generators has been
reduced from 5 MPa or so to as small as 2 MPa. If force securing a
heat sink to a heat generator is small, the graphite sheet
therebetween does not come into close contact with the heat
generator or the heat sink, raising the thermal resistance and,
thereby, reducing the cooling efficiency.
[0004] To solve the above problem, the Japanese Unexamined Patent
Publication No. 2004-363432 discloses a heat-transfer sheet
comprising (i) a substance which is liquid at normal temperature
and whose phase does not change at the temperature range wherein it
is used and (ii) a graphite sheet. Because the liquid in the
graphite sheet can move freely, the graphite sheet is disposed in
minute concave areas, or recesses, and the liquid accumulates in
relatively large concave areas of 5-100 .mu.m in the surfaces of
the heat generator and the heat sink if the force securing a heat
sink to a heat generator is small. Thus, gaps are prevented from
being formed between the heat-transfer sheet and the heat generator
and between the heat-transfer sheet and the heat sink and, hence,
the thermal resistance is minimized and the heat generator is well
cooled.
[0005] However, because the liquid exists in the graphite sheet,
the heat-transfer sheet is not compressed very well, namely, it
does not come into close contact with the heat generator or the
heat sink; therefore, the thermal resistance is not low enough.
[0006] Besides, the production of the heat-transfer sheet requires
a step of impregnating the graphite sheet with the liquid.
Accordingly, it cannot be produced relatively efficiently, its
production cost is relatively high, the liquid may deteriorate, and
nearby components may be polluted by liquid coming out of the
graphite sheet.
DISCLOSURE OF INVENTION
[0007] Problems that the Invention is to Solve
[0008] Accordingly, the object of the present invention is to
provide (i) a heat-transfer sheet which comes into close contact
with a heat generator and a radiator when caught therebetween and
can be used repeatedly, (ii) a method of using the heat-transfer
sheet, and (iii) a heat transfer system comprising the
heat-transfer sheet.
Means of Solving the Problems
[0009] According to the first feature of the present invention,
there is provided a heat-transfer sheet which is made of expanded
graphite and disposed between a heat generator and a radiator and
whose bulk density is less than 1.0 Mg/m.sup.3.
[0010] According to the second feature of the present invention,
there is provided the heat-transfer sheet according to the first
feature. Its compressibility is 50% or more and its recovery is 5%
or more when it is thicknesswise compressed by the pressure of 34.3
MPa.
[0011] According to the third feature of the present invention,
there is provided a heat transfer system comprising (i) a radiator
and (ii) the heat-transfer sheet according to the first or second
feature which is disposed between the radiator and a heat
generator.
[0012] According to the fourth feature of the present invention,
there is provided a method of using a heat-transfer sheet which is
made of expanded graphite, whose bulk density is less than 1.0
Mg/m.sup.3, and which is disposed between a heat generator and a
radiator to be thicknesswise compressed therebetween by the
pressure of 2.0 MPa or less.
[0013] According to the fifth feature of the present invention,
there is provided a method of using a heat-transfer sheet which is
made of expanded graphite, whose bulk density is less than 0.9
Mg/m.sup.3, and which is disposed between a heat generator and a
radiator to be thicknesswise compressed therebetween by the
pressure of 1.5 MPa or less.
[0014] The radiator according to the present invention includes
devices with the function of dissipating the heat of heat-transfer
sheets to gas, liquid, and other members by radiation, convection,
heat transfer, etc., devices with the function of absorbing the
heat of heat-transfer sheets, and devices with both the
heat-dissipating function and the heat-absorbing function.
EFFECTS OF THE INVENTION
[0015] The advantage offered by the first feature of the present
invention is as follows. Because the bulk density of the
heat-transfer sheet is low, it is easily compressed between a heat
generator and a radiator even under relatively low pressure and,
hence, comes into close contact with the heat generator and the
radiator. Therefore, the thermal resistance between the heat
generator and the radiator is small and, hence, the cooling
efficiency is high.
[0016] The advantage offered by the second feature of the present
invention is as follows. Because the recovery of the heat-transfer
sheet is high, its bulk density remains below a certain level even
after it is used more than once. Therefore, the heat-transfer sheet
comes into close contact with a heat generator and a radiator even
after it is used more than once, the thermal resistance between the
heat generator and the radiator remaining low. Thus, the
heat-transfer sheet can be used repeatedly. Because the
heat-transfer sheet can be used repeatedly, it contributes toward
resources saving.
[0017] The advantage offered by the third feature of the present
invention is as follows. Because the bulk density of the
heat-transfer sheet is low, it is easily compressed between a heat
generator and a radiator even under relatively low pressure and,
hence, comes into close contact with the heat generator and the
radiator. Therefore, the thermal resistance between the heat
generator and the radiator is small and, hence, the cooling
efficiency is high.
[0018] The advantage offered by the fourth feature of the present
invention is as follows. Because its bulk density remains below a
certain level even after it is used more than once, it comes into
close contact with a heat generator and a radiator even after it is
used more than once and, hence, the thermal resistance between the
heat generator and the radiator remains low. Thus, the
heat-transfer sheet can be used repeatedly. Besides, the force
securing a radiator to a heat generator, or the pressure applied to
a heat-transfer sheet therebetween, can be reduced compared with
the pressure which has been applied to heat-transfer sheets so far;
therefore, the load stress on the heat generator is reduced and,
hence, damage to the heat generator is prevented.
[0019] The advantage offered by the fifth feature of the present
invention is as follows. The heat-transfer sheet comes into close
contact with a heat generator and a radiator and can be used
repeatedly.
BEST MODE FOR CARRYING OUT THE INVENTION
[0020] Preferred embodiments of the present invention will be
described below by referring to drawings.
[0021] The heat-transfer sheet of the present invention is used to
cool such heat generators as CPUs of computers, printed circuit
boards of mobile phones, DVD recorders, and servers. The
heat-transfer sheet is put between a heat generator and a radiator
such as a head sink or a cooling fan. The thermal resistance of the
heat-transfer sheet is small when it is caught and compressed
between a heat generator and a radiator. Namely, the thermal
conductivity of the heat-transfer sheet is high when it is caught
and compressed between a heat generator and a radiator.
[0022] Thermal resistance is the temperature difference between two
points separate from each other of a member which is fed with heat
from a heat generator divided by the heat-generation rate of the
heat generator. In FIG. 1 (B), the thermal resistance is as
follows.
[0023] Thermal resistance=(temperature at point "B"-temperature at
point "A")/amount of heat-generation of heat generator
[0024] The heat-transfer sheet of the present invention is a sheet
of expanded graphite which is made by soaking natural or kish
graphite in a liquid such as sulfuric or nitric acid and then
heat-treating the graphite over 400.degree. C. Its thickness is
0.05-5.0 mm and its bulk density is less than 1.0 Mg/m.sup.3.
[0025] The expanded graphite is fibrous or caterpillar-like. The
expanded-graphite fibers are, for example, 1.0 mm or so in length
and 300 .mu.m or so in diameter and entangled.
[0026] The heat-transfer sheet of the present invention may be made
of expanded graphite containing a certain amount (for example, 5%
or so) of binder such as phenol resin or rubber.
[0027] Any processes for making the heat-transfer sheet of the
present invention of the above expanded graphite may be
adopted.
[0028] As the bulk density of expanded-graphite sheets made of such
expanded graphite as described above increases, their thermal
conductivity of parallel to surface increases and their flexibility
decreases. Accordingly, their bulk density is adjusted in
accordance with their uses. Expanded-graphite sheets used as
heat-transfer sheets are usually given high bulk density (for
example, 1.3 Mg/m.sup.3 or more) for high thermal conductivity.
Expanded-graphite sheets used as heat-insulating sheets and
electromagnetic-wave shielding sheets for walls, etc. are usually
given low bulk density (for example, 1.0 Mg/m.sup.3 or less).
[0029] In the case of the heat-transfer sheet of the present
invention, more importance is given to flexibility than to thermal
conductivity. The heat-transfer sheet of the present invention is
an expended-graphite sheet which has bulk density less than 1.0
Mg/m.sup.3 and is used as heat-insulating sheets and
electromagnetic-wave shielding sheets. If an expanded-graphite
sheet is given a bulk density of 1.0 Mg/m.sup.3 or more, its
flexibility is low and it does not come into close contact with a
heat generator or a radiator. Accordingly, the expanded-graphite
sheet of the present invention is given a bulk density less than
1.0 Mg/m.sup.3 so that it will come into close contact with a heat
generator and a radiator as described later. It is preferable
particularly if the expanded-graphite sheet is given a bulk density
of 0.9 Mg/M.sup.3 or less for the reason to be described later.
[0030] Next, how to use the heat-transfer sheet of the present
invention will be described.
[0031] FIG. 1 (A) is an illustration of an example of how to use
the heat-transfer sheet of the present invention. FIG. 1 (B) shows
temperature-measuring points "A" and "B." The reference sign "H" is
a heat generator such as a CPU of a computer; reference sign 2, a
radiator such as a heat sink; reference sign "F." a cooling fan
attached to the heat sink 2.
[0032] As shown in FIG. 1, the heat-transfer sheet 1 of this
invention is caught between the heat generator "H" and the heat
sink 2. When the heat sink 2 is secured to the heat generator "H"
with a fixing part "S" such as a clamp, the heat-transfer sheet 1
is squeezed between the heat generator "H" and the heat sink 2.
[0033] Because the bulk density of the heat-transfer sheet 1 is
less than 1.0 Mg/M.sup.3, it is compressed between the heat
generator "H" and the heat sink 2 when pressure is applied to it.
Accordingly, as the heat-transfer sheet 1 is compressed, it comes
into close contact with the heat generator "H" and the heat sink 2.
The reason for it is that because the bulk density of the
heat-transfer sheet 1 is low and there are spaces between graphite
fibers, graphite fibers at the surfaces of the heat-transfer sheet
1 enter concave areas, or recesses, in the surfaces of the heat
generator "H" and the heat sink 2.
[0034] Accordingly, both the thermal resistance between the heat
generator "H" and the heat-transfer sheet 1 and that between the
heat-transfer sheet 1 and the heat sink 2 decrease. Because the
bulk density of the heat-transfer sheet 1 is less than 1.0
Mg/M.sup.3 and its thermal conductivity of perpendicular to surface
is about 5 W/mk, the thermal resistance between the heat generator
"H" and the heat sink 2 is small and, thus, the thermal
conductivity therebetween is high. Thus, the heat generator "H" is
efficiently cooled down by the heat-transfer sheet 1 and the heat
sink 2.
[0035] Besides, because thermal conductivity of parallel to surface
of the heat-transfer sheet 1 is about 50-200 W/mk which is larger
than its thermal conductivity of perpendicular to surface, its
temperature distribution of parallel to surface is even; therefore,
heat spots do not formed on the surfaces of the heat-transfer sheet
1, heat generator "H," or heat sink 2.
[0036] Moreover, because the heat-transfer sheet 1 is simply caught
between the heat generator "H" and the heat sink 2, it can be
replaced with a new one 1 easily.
[0037] The heat-transfer sheet 1 may be affixed to the heat sink 2
with adhesive or the like so long as the heat-transfer sheet 1 is
disposed between the heat generator "H" and the heat sink 2.
[0038] The heat-transfer sheet 1 and the heat sink 2 constitute the
radiator described in the claims of the present invention.
[0039] The heat generator "H" is fitted with the radiator as
follows.
[0040] The heat-transfer sheet 1 is first put on the heat generator
"H" and, then, the heat sink 2 is put on the heat-transfer sheet 1.
Next, the heat generator "H." heat-transfer sheet 1, and heat sink
2 are caught between a clamp "S" and a printed circuit board if the
heat generator "H" is a CPU. Thus, the heat generator "H" is fitted
with the radiator.
[0041] The heat sink 2 may be fitted with a cooling fan "F" for
larger cooling capacity, the heat transfer system constituted by
the heat-transfer sheet 1, heat sink 2, and cooling fan "F."
[0042] Alternatively, the radiator may be constituted by a
heat-transfer sheet 1 and a heat-dissipating component such as a
cooling fan "F" or a heat absorbing component such as a
cooling-water jacket.
[0043] If the heat-transfer sheet 1 designed to have
compressibility of 50% or more and recovery of 5% or more when it
is thicknesswise compressed by pressure of 34.3 MPa for the first
time, its bulk density remains less than 1.0 Mg/m.sup.3 after it is
compressed more than one time and the last pressure is removed.
Accordingly, even after the heat-transfer sheet 1 is used more than
one time, it comes into close contact with the heat generator "H"
and the heat sink 2 when it is compressed therebetween. Therefore,
the thermal resistance between the heat generator "H" and the heat
sink 2 remains low even after the heat-transfer sheet 1 is used
more than one time. Thus, the heat-transfer sheet 1 can be used
repeatedly.
[0044] Particularly if the heat-transfer sheet 1 designed to have
compressibility of 55% or more and recovery of 6% or more when it
is thicknesswise compressed by pressure of 34.3 MPa for the first
time, its bulk density remains more certainly less than 1.0
Mg/m.sup.3, for example less than 0.9 Mg/m.sup.3, after it is
compressed more than one time and the last pressure is removed.
Accordingly, the heat-transfer sheet 1 can be used more
repeatedly.
[0045] If the compressibility of the heat-transfer sheet 1 is less
than 50% when it is thicknesswise compressed by pressure of 34.3
MPa for the first time, it is not desirable because the
heat-transfer sheet 1 does not come into close contact with the
heat generator "H" or the heat sink 2. If the recovery of the
heat-transfer sheet 1 is less than 5% when it is thicknesswise
compressed by pressure of 34.3 MPa for the first time, it is not
desirable because the heat-transfer sheet 1 does not come into
close contact with the heat generator "H" or the heat sink 2 when
it is used for the second time and further.
[0046] Even though the bulk density of the heat-transfer sheet 1 is
less than 1.0 Mg/m.sup.3, the bulk density of the heat-transfer
sheet 1 after the removal of the pressure may be 1.0 Mg/m.sup.3 or
more if the pressure is too large when the heat-transfer sheet 1
and the heat sink 2 is secured to the heat generator "H" by a clamp
"S." If the heat-transfer sheet 1 is compressed by pressure over
2.0 MPa, the heat generator "H" is exposed to large load stress and
the repetitive usability of the heat-transfer sheet 1 is
reduced.
[0047] Therefore, if the heat generator "H" is fitted with the
heat-transfer sheet 1 and the heat sink 2 so as to apply pressure
of 2.0 MPa or less, preferably 1.5 MPa or less, to the
heat-transfer sheet 1, the bulk density of the heat-transfer sheet
1 is less than 1.0 Mg/m.sup.3 after the removal of the pressure.
Therefore, the heat-transfer sheet 1 can be used repeatedly and no
damage is done to the heat generator "H."
[0048] If the heat-transfer sheet 1 is designed to have bulk
density 0.9 Mg/m.sup.3 or less and the heat generator "H" is fitted
with the heat-transfer sheet 1 and the heat sink 2 so as to apply
pressure of 1.5 MPa or less to the heat-transfer sheet 1, the bulk
density of the heat-transfer sheet 1 is 0.9 Mg/m.sup.3 of less
after the removal of the pressure. Therefore, the heat-transfer
sheet 1 can be used repeatedly and comes into closer contact with
the heat generator "H" and the heat sink 2, and the thermal
resistance between the heat generator "H" and the heat sink 2 is
small.
[0049] Particularly if the heat-transfer sheet 1 is designed to
have bulk density 0.8 Mg/m.sup.3 or less and the heat generator "H"
is fitted with the heat-transfer sheet 1 and the heat sink 2 so as
to apply pressure of 1.0 MPa or less to the heat-transfer sheet 1,
the bulk density of the heat-transfer sheet 1 is 0.8 Mg/M.sup.3 of
less after the removal of the pressure. Therefore, the
heat-transfer sheet 1 comes into even closer contact with the heat
generator "H" and the heat sink 2 and has enough recovery.
[0050] If the heat-transfer sheet 1 is treated so as to reduce the
impurities in it, such as sulfur and iron, to 10 ppm or less,
particularly reduce sulfur to 1 ppm or less, components and devices
fitted with the heat-transfer sheet 1 are prevented from
deteriorating due to impurities.
[0051] If film of resin such as polyethylene terephthalate is
laminated between the heat-transfer sheet 1 and the heat generator
"H" and/or the heat sink 2, fragments of expanded-graphite fibers
of the heat-transfer sheet 1 are prevented from scattering. Any
resin films may be used so long as their thicknesswise thermal
conductivity is about the same as that of the heat-transfer sheet 1
and they are resistant to the temperature of about 100.degree.
C.
FIRST EMBODIMENT
[0052] The compressibility and recovery of the heat-transfer sheet
1 were measured when it was thicknesswise compressed by pressure of
34.3 MPa.
[0053] The bulk density-compressibility relation and the bulk
density-recovery relation were ascertained by changing the bulk
density of the 0.5-millimeter thick heat-transfer sheet 1 from 0.1
Mg/m.sup.3 to 0.5, 0.8, 1.0, 1.2, 1.5 and 1.8 Mg/m.sup.3. The
compressibility of the heat-transfer sheet 1 was calculated by
dividing its thickness under the pressure by its thickness before
the compression. The recovery of the heat-transfer sheet 1 was
calculated by dividing its thickness after the removal of the
pressure by its thickness before the compression.
[0054] As shown in FIG. 2 (A), as the bulk density increases, the
compressibility decreases and the recovery increases.
[0055] By and large, as the compressibility increases, the recovery
decreases. In the compressibility range over 50%, the slope of
recovery is gentle. Particularly in the compressibility range from
55% to 75%, the recovery changes little.
[0056] Thus, if the bulk density of the heat-transfer sheet 1 is
such that the compressibility is 50% or more, particularly 55-75%,
namely, the bulk density is less than 1.0 Mg/m.sup.3 [see FIG. 2
(A)], the compressibility is high and the recovery is kept in a
certain range.
SECOND EMBODIMENT
[0057] To ascertain the relation between the thermal conductivity
of the heat-transfer sheet 1 and the thicknesswise pressure applied
to it, it was put between a CPU (Intel's Celeron Processor 2 GHz)
and a heat sink (Intel's aluminum heat sink for Celeron) and the
CPU was run at a constant processing speed. And the difference
between the temperature in the CPU and the temperature of the heat
sink was measured.
[0058] As shown in FIG. 1 (B), the distance between the measuring
point "A" on the heat sink and the measuring point "B" in the CPU
was 20 mm.
[0059] Four heat-transfer sheets 1 of the same thickness of 0.5 mm
and different bulk densities of 0.1, 0.5, 0.8, and 1.0 Mg/m.sup.3
were prepared, and the pressure (pressure pressing down the heat
sink to the CPU) was varied from 0.1 MPa to 0.5, 1.0, 2.0, and 5.0
MPa.
[0060] Small temperature difference means high thermal
conductivity, or small thermal resistance, of the heat-transfer
sheet 1. Large temperature difference means low thermal
conductivity, or large thermal resistance, of the heat-transfer
sheet 1.
[0061] As shown in FIG. 3, the temperature difference shows a
tendency to decrease as the pressure increases in any of the four
cases of different bulk densities. It is also shown in FIG. 3 that
temperature difference becomes constant when the pressure goes
beyond a certain level in any cases. Namely, raising the pressure
beyond such a certain level does not reduce the temperature
difference any more.
[0062] It is also shown in FIG. 3 that as the bulk density lowers,
such a certain level lowers and that temperature difference is
almost constant in the pressure range over 2.0 MPa in any cases of
different bulk densities.
[0063] It is further shown in FIG. 3 that temperature difference is
almost constant in the pressure range over 1.0 MPa, particularly in
the range over 1.5 MPa, if the bulk density is 0.8 Mg/m.sup.3 or
less.
THIRD EMBODIMENT
[0064] The relation between the bulk density and the thermal
conductivity of the heat-transfer sheet 1 of the present invention
under a certain level of pressure was ascertained. FIG. 1 (B) shows
the temperature measuring points "A" on a heat think and "B" in a
CPU, the distance between the two points being 20 mm.
[0065] Seven 0.5-mm thick heat-transfer sheets 1 were prepared so
that their respective bulk densities would be 0.1, 0.5, 0.8, 1.0,
1.2, 1.8, and 2.0 Mg/M.sup.3 when they were put between the CPU and
the heat sink and pressure of 1.0 MPa was applied to them. Under
the conditions, the temperature difference between the two
measuring points "A" and "B" was measured.
[0066] As shown in FIG. 4 (A), as the bulk density increases from
0.1 Mg/m.sup.3 to 0.8 Mg/m.sup.3, the temperature difference
increases very little, but as the bulk density increases from 0.8
Mg/m.sup.3 to 1.0 Mg/m.sup.3, the temperature difference increases
very much. Namely, it is shown in FIG. 3 that if the pressure is
about 1.0 MPa, the thermal conductivity of the heat-transfer sheet
1 changes greatly in the bulk-density range from 0.8 to 1.0
Mg/m.sup.3.
[0067] The comparison between the result of this embodiment and
that of the second embodiment suggests that the bulk-density range
where the thermal conductivity changes greatly lowers if the
pressure lowers below 1.0 MPa, while the range does not change very
much if the pressure rises beyond 1.0 MPa.
FOURTH EMBODIMENT
[0068] It was ascertained whether the relation between the bulk
density and the thermal conductivity of the heat-transfer sheet 1
of the present invention changed or not depending on the number of
times of compression under constant pressure. FIG. 1 (B) shows the
temperature measuring points "A" on a heat think and "B" in a CPU,
the distance between the two points being 20 mm.
[0069] Four heat-transfer sheets 1 of the same thickness of 0.5 mm
and different bulk densities of 0.1, 0.5, 0.8, and 1.0 Mg/m.sup.3
were prepared. Each heat-transfer sheet 1 was compressed between
the CPU and the heat sink four times.
[0070] As shown in FIG. 4 (B), the temperature difference is
constant regardless of the number of times of compression in any
case of the four heat-transfer sheets 1 of different bulk
densities. Thus, it was ascertained that the thermal conductivity
of the heat-transfer sheet 1 was determined by (i) its original
bulk density before it was first caught between a CPU and a heat
sink and (ii) the pressure applied to it therebetween.
INDUSTRIAL APPLICABILITY
[0071] The heat-transfer sheet of the present invention is suitable
for use as a sheet to transfer heat from CPU's of computers, mobile
phone, etc., DVD recorders, and so on to radiator such as heat
sinks and cooling fans.
BRIEF DESCRIPTION OF DRAWINGS
[0072] FIG. 1 (A) is an illustration of an example of how to use
the heat-transfer sheet of the present invention. FIG. 1 (B) shows
temperature-measuring points "A" and "B."
[0073] FIG. 2 (A) shows the relations between the bulk density and
the compressibility/recovery of the heat-transfer sheet 1 of FIG. 1
when it was thicknesswise compressed by the pressure of 34.3
MPa.
[0074] FIG. 2 (B) shows the relation between the compressibility
and the recovery.
[0075] FIG. 3 shows the relation between the pressure thicknesswise
applied to the heat-transfer sheet of FIG. 1 and its thermal
conductivity. The bulk density of the heat-transfer sheet was
varied.
[0076] FIG. 4 (A) shows the relation between the bulk density and
the thermal conductivity of the heat-transfer sheet of FIG. 1 under
constant pressure. FIG. 4 (B) shows the relation between the number
of times of compression and the thermal conductivity.
EXPLANATIONS OF LETTERS OR NUMERALS
[0077] 1 heat-transfer sheet
[0078] 2 heat sink
[0079] H heat generator
* * * * *