[0001] The present invention relates to a thermally conductive material and, more particularly,
to a thermally conductive grease composition which comprises liquid silicone, liquid
hydrocarbon or/and fluorohydrocarbon oil as base oil and has thermal conductivity
adequate to the use for reduction of the heat from electronic parts.
[0002] Most electric and electronic appliances generate heat while they are used, so that
the removal of the generated heat therefrom is necessary for their normal operation.
With the intention of removing the generated heat from those appliances, many means
have been proposed. For instance, in miniature electronic parts, especially electronic
devices provided with integrated circuits, thermally conductive materials, such as
thermally conductive grease and a thermally ccnductive sheet [Japanese Tokko Sho 57-36302
(U.S. Patent No. 4,265,775), wherein the term "Tokko" means an "examined patent publication",
and Japanese Tokkai Sho 61-157587, wherein the term "Tokkai" means an "unexamined
published patent application"], have been used as such means.
[0003] In general, an electronic device comprises integrated circuits and cap parts for
protecting them, and a thermally conductive material is applied so as to contact directly
with both the circuit element and the heat reducing part, or indirectly therewith
via certain materials. Thus, the heat generated from integrated circuit chips during
operation is transmitted in the thermally conductive material to be transferred directly
or indirectly to the heat reducing part, and further radiated therefrom. A rough sectional
view of such an electronic device, wherein a thermally conductive material is used,
is shown in Fig. 1.
[0004] As the thermally conductive material mentioned above, there has already been known
a heat-reducing grease of the type which uses a silicone oil as a base material and
a zinc oxide or alumina powder as a thickener (Japanese Tokko Sho 52-33272 and Japanese
Tokko Sho 59-52195). In recent years, aluminum nitride has been developed as a thickener
which enables further improvement of thermal conductivity (as disclosed, e.g., in
Japanese Tokkai Sho 52-125506).
[0005] Therein, however, the grease compositions undergo no great improvement in thermal
conductivity by containing aluminum nitride instead of other known thickeners though
the aluminum nitride itself has high thermal conductivity. This is because the permissible
content of aluminum nitride in silicone oil is restricted within very narrow limits,
or to the range of about 50 to about 95 weight parts per 100 weight parts of silicone
oil used as a base oil, due to its insufficient oil-keeping power.
[0006] In Japanese Tokko Sho 57-36302 (U.S. Patent No. 4,265,775) is disclosed the thixotropic
thermally conductive material comprising an oily organosilicone carrier, silica fiber
in an amount effective for prevention of oily carrier exudation and a thermal conductivity-providing
powder selected from a group consisting of dendrite-form zinc oxide, thin-leaf aluminum
nitride, thin-leaf boron nitride and a mixture of two or more thereof. From this material
also, sufficient improvement of thermal conductivity cannot be expected because it
is inevitable to reduce the aluminum nitride powder content due to the incorporation
of spherical silica fiber as an essential component for enhancement of oil keeping
power.
[0007] This drawback can be mitigated by using a particular organopolysiloxane and a spherical
aluminum nitride powder having a hexagonal crystal form and grain sizes in a specified
range in combination to enable a very large amount of aluminum nitride to be incorporated
in the silicone oil (Japanese Tokkai Hei 2-153995).
[0008] However, the effect produced by such an increase of the aluminum nitride powder content
upon improvement of thermal conductivity is smaller than expected, and the thermal
conductivity attained is of the order of 2.3 W/m°K which is still unsatisfactory.
The reason therefor is that aluminum nitride is a very hard material having Mohs'
hardness of from 7 to 9, and so there are spaces between aluminum nitride grains when
they are coarse.
[0009] As a measure to solve this problem, the combined use of fine and coarse aluminum
nitride powders is known (Japanese Tokkai Hei 3-14873). While the thermal conductivity
is elevated in this case, the resulting composition is too small in consistency (too
hard) as grease and has poor dispensation suitability; as a result, it is unsuitable
for practical use.
[0010] Further, there are proposals such that organopolysiloxanes of the kind which can
hold inorganic fillers in large amounts are employed as base oil and they are combined
with at least one inorganic filler selected from the group consisting of ZnO, Al
2O
3, AlN and Si
3N
4 [e.g., Japanese Tokkai Hei 2-212556 (U.S. Patent No. 5,221,339) and Japanese Tokkai
Hei 3-162493 (U.S. Patent No. 5,100,568)]. However, those combinations as heat-reducing
grease are still unsatisfactory.
[0011] As a result of our intensive studies to further improve the dispensation suitability
and the thermal conductivity of heat-reducing grease, it has been found that, when
at least one base oil selected from the group consisting of liquid silicones, liquid
hydrocarbons and fluorohydrocarbon oils is used in combination with a thermally conductive
inorganic filler having Mohs' hardness of 6 or above and thermal conductivity of at
least 100 W/m°K and a thermally conductive inorganic filler having Mohs' hardness
of 5 or below and thermal conductivity of at least 20 W/m°K, the resulting grease
composition can acquire excellent dispensation suitability as well as high thermal
conductivity, thereby achieving the present invention.
[0012] Therefore, an object of the present invention is to provide a thermally conductive
grease having both high thermal conductivity and excellent dispensation suitability.
[0013] The above-described object of the present invention is attained with a thermally
conductive grease composition comprising:
(A) 100 weight parts of at least one base oil selected from the group consisting of
liquid silicones, liquid hydrocarbons and fluorohydrocarbon oils, and
500-1,000 weight parts of a thermally conductive filler mixture constituted of (B)
an inorganic filler having Mohs' hardness of at least 6 and thermal conductivity of
at least 100 W/m°K and (C) an inorganic filler having Mohs' hardness of at most 5
and thermal conductivity of at least 20 W/m°K;
wherein the ratio of Component (C) to the sum total of Component (B) and Component
(C) is from 0.05 to 0.5 by weight.
[0014] In a thermally conductive grease composition according to the present invention,
the gaps among hard particles of thermal conductive inorganic filler having Mohs'
hardness of at least 6 and thermal conductivity of at least 100 W/m°K are filled up
with soft particles of thermal conductive inorganic filler having Mohs' hardness of
at most 5 and thermal conductivity of at least 20 W/m°K; as a result, high thermal
conductivity is secured and dispensation suitability is improved. Thus, it is easy
for the present composition to acquire the oil separation degree reduced to 0.01 %
or below and the thermal conductivity higher than 2.5 W/m°k.
[0015] Fig. 1 is a schematic view showing the removal of heat generated from electronic
parts by the use of thermally conductive grease. Therein, the figures 1, 2 and 3 represent
a heat-releasing member, thermally conductive grease and heat-generating electronic
parts respectively.
[0016] The following are characteristics which the base oil used as Component (A) in the
present invention is required to have:
1) Appropriate viscosity characteristics, including a slight change of viscosity with
temperature and solidification at a low temperature (a low pour point),
2) Low volatility at high temperatures and a high flash point,
3) High stability against oxidation and satisfactory thermal stability, more specifically
good receptiveness to the benefit from antioxidants and no changes in color and properties
upon heating up to about 200°C,
4) Good oiliness,
5) Slight aggravating influences upon surrounding materials, such as sealing materials
and resinous or ceramic cover, and
6) Good affinity for fillers (thickeners).
[0017] As examples of a base oil having the foregoing characteristics, mention may be made
of base oils generally used for lubricating oil, including liquid silicones, such
as silicone oils containing methyl groups or/and phenyl groups, and mineral oils of
naphthene and paraffin types. Besides these oils, a wide variety of synthetic oils
as recited below are suitable for the base oil of grease or the like to be used under
temperatures covering a wide range because of their excellent fluidity, viscosity
index and thermal stability.
[0018] The liquid silicone used as base oil in the present invention can be properly selected
from known silicones which are liquid at room temperature, such as organopolysiloxanes,
polyorganosilalkylenes, polyorganosilanes and copolymers thereof. From the viewpoint
of ensuring heat resistance, stability and electric insulation, however, it is desirable
to use organopolysiloxanes, particularly an organopolysiloxane represented by compositional
formula R
aSiO
(4-a)/2. Each R in this formula is a group selected from monovalent organic groups, and all
R groups may be the same or different.
[0019] Examples of a monovalent organic group as R include monovalent unsubstituted or substituted
hydrocarbon groups having 1 to 30 carbon atoms, such as alkyl groups (e.g., methyl,
ethyl, propyl, butyl, pentyl, hexyl, octyl, decyl, dodecyl, tetradecyl, hexadecyl,
octadecyl, etc.), cycloalkyl groups (e.g., cyclohexyl, etc.), alkenyl groups (e.g.,
vinyl, allyl, etc.), aryl groups (e.g., phenyl, naphthyl, tolyl, etc.), and groups
formed by substituting halogen atom(s), cyano group(s), hydroxyl group(s) or/and so
on for part or all of the hydrogen atoms attached to carbon atoms present in the above-recited
groups (e.g., chloromethyl, 3,3,3-trifluoropropyl, cyanopropyl, phenol, hindered phenol,
etc.); and organic functional groups, such as an organic group having an amino group,
an organic group having a polyether group and an organic group having an epoxy group.
Of these organic groups, a methyl group, a phenyl and an alkyl groups having 6 to
14 carbon atoms are preferred over the others.
[0020] Further, "a" in the above formula is a number of 1.8-2.3.
[0021] For acquiring satisfactory grease characteristics, it is desirable that the viscosity
of the foregoing organopolysiloxane be from 50 to 500,000 cs, particularly from 50
to 300,000 cs, at 25°C. When the viscosity is below 50 cs at 25°C, the grease obtained
shows a strong tendency to oil separation; while, when it is above 500,000 cs at °C,
the grease prepared is so high in viscosity that it cannot possibly be dispensed to
a substrate in a satisfactory condition.
[0022] The organopolysiloxane used in the present invention can have any of linear, branched
and cyclic structures, and it is not necessarily a single compound, but it can be
a mixture of two or more of organopolysiloxanes different in structure. Although "a"
may be from 1.8 to 2.3, it is desirable for the organopolysiloxane to have "a" in
the range of 1.9 to 2.1, because this range enables the organopolysiloxane to have
a linear structure or a structure close thereto.
[0023] Suitable examples of such an organopolysiloxane include dimethylpolysiloxane, diethylpolysiloxane,
methylphenylpolysiloxane, dimethylsiloxane-diphenylsiloxane copolymer, and alkyl-modified
methylpolysiloxanes. Of these polysiloxanes, homopolymers and copolymers produced
from dimethylsiloxane, alkylmethylsiloxane, methylphenylsiloxane or/and diphenylsiloxane
and blocked at their molecular-chain ends with trimethylsilyl or dimethylhydrosilyl
groups are preferred in particular.
[0024] More specifically, such organopolysiloxanes are represented by, e.g., the following
formula (I):

wherein each of R
1 groups is a group selected from monovalent unsubstituted or substituted hydrocarbon
groups having 1 to 30 carbon atoms, such as alkyl groups (e.g., methyl, ethyl, propyl,
butyl, amyl, octyl, etc. ), alkenyl groups (e.g., vinyl, allyl, etc.), aryl groups
(e.g., phenyl, tolyl, etc.) and groups formed by substituting halogen atom(s), cyano
group(s), hydroxyl group(s) or/and so on for part or all of the hydrogen atoms attached
to carbon atoms present in the above-recited groups (e.g., chloromethyl, 3,3,3-trifluoropropyl,
cyano-propyl, phenol, hindered phenol, etc.); R
2 and R
3 groups are the same or different, and each of them is the same monovalent hydrocarbon
group as R
1 represents, an amino group-containing organic group, a polyether group-containing
organic group or an epoxy group-containing organic group; R
4 is a hydrogen atom, the same monovalent hydrocarbon group as R
1 represents or the same monovalent organic group as R
2 or R
3 represents; and ℓ is a positive number to ensure the viscosity of from 50 to 500,000
cs at 25°C in the organopolysiloxane.
[0025] It is desirable for the organopolysiloxane used in the present invention to be blocked
with trimethylsilyl groups at the molecular-chain ends thereof. As R
1, R
2 and R
3 each, an alkyl group such as methyl or ethyl, an aryl group such as phenyl or tolyl,
or a group formed by substituting hydroxyl group(s) for a part of the hydrogen atoms
of the group as recited above, particularly a methyl group, a phenyl group or an alkyl
group having 6 to 14 carbon atoms, is preferable with respect to easiness of synthesis
and thermal resistance and electric insulation of the oil obtained.
[0026] The organopolysiloxane oil as mentioned above can be produced in accordance with
known methods. For instance, dimethylpolysiloxane oil can be produced by subjecting
a low molecular cyclic siloxane, such as octamethylcyclotetrasiloxane or decamethylcyclopentasiloxane,
to a ring-opening reaction in the presence of an acid catalyst, such as sulfuric acid,
chlorosulfonic acid, nitric acid, phosphoric acid, activated clay, acid clay or trifluoroacetic
acid, or an alkaline catalyst, such as potassium hydroxide, sodium hydroxide, rubidium
hydroxide, caesium hydroxide, potassium oxide, potassium acetate or calcium silanolate,
and then polymerizing the reaction product.
[0027] In order to produce a dimethylpolysiloxane oil having the intended viscosity by controlling
the polymerization degree in the foregoing method, a low molecular weight siloxane
having a terminal blocking group, such as hexamethyldisiloxane, octamethyltrisiloxane
or decamethyltetrasiloxane, can be added properly at the stage of polymerization.
[0028] As for the production of organopolysiloxanes having carbon functional groups, on
the other hand, an amino group-containing organopolysiloxane can be produced by the
dealcoholating condensation reaction between an organopolysiloxane having at least
one silanol group and an amino group-containing alkoxysilane, and an epoxy group-
or polyether group-containing organopolysiloxane can be produced by subjecting a compound
having both epoxy or polyether group and an unsaturated group, such as vinyl group,
and an organohydrogenpolysiloxane having hydrogen-attached silicon atom(s) to an addition
reaction in the presence of a platinum catalyst.
[0029] However, the organopolysiloxane oils produced in accordance with the foregoing methods
generally contain low molecular weight siloxanes having at most 12 siloxane units
in a proportion of about 10 %, because they are obtained as equilibrated mixtures
of polysiloxanes produced with the progress of polymerization which are various in
their polymerization degrees.
[0030] After the syntheses using the foregoing methods, therefore, the products obtained
generally undergo a stripping treatment at a temperature of 120-250°C under a reduced
pressure to remove the low molecular weight siloxanes therefrom. Even after the stripping
treatment, however, the low molecular weight siloxanes still remain in a quantity
of 500-20,000 ppm. These low molecular weight siloxanes have a strong adsorbing power,
compared with nonpolar combustible gases, so that their vapors are adsorbed strongly
by various electrical contact parts and so on.
[0031] The low molecular weight siloxanes adsorbed to electrical contact parts are converted
into SiO
2· nH
2O by undergoing oxidation, and further accumulated in the form of αSiO
2 on the surface of the contact parts to cause a contact point disturbance. Therefore,
the presence of low molecular weight siloxanes is already known to be undesirable.
[0032] In addition, it is also known that such a trouble can be prevented by reducing each
of the contents of low molecular weight siloxanes having no more than 12 siloxane
units to at most 50 ppm.
[0033] The removal of the foregoing low molecular weight siloxanes can be effected by subjecting
an organopolysiloxane oil produced by the foregoing conventional method to a stripping
treatment at a high temperature of 150-300°C under a reduced pressure of 50 mmHg or
below in an atmosphere of dried nitrogen gas, or by extracting the low molecular weight
siloxanes contained in the foregoing organopolysiloxane oil with an alcohol or ketone
solvent. Thus, each of the contents of low molecular weight siloxanes in the organopolysiloxane
oil produced in the foregoing manner can be reduced to less than 50 ppm, and the total
content of the low molecular weight siloxanes having from 2 to 12 siloxane units can
be reduced to less than 500 ppm.
[0034] From the standpoint of improving, e.g., the thermal resistance, R
1 in formula (I) may be a monovalent substituted hydrocarbon group having the hindered
phenol structure as described in Japanese Tokko Hei 3-131692.
[0035] Examples of a liquid silicone suitable for the present invention include those represented
by the following formula (II), but these examples should not be construed as limiting
on the scope of the present invention anyway:

wherein R
5 is -C
4H
9, -C
6H
13, -C
8H
17, -C
10H
21, -C
12H
25, -C
15H
31 or -C
18H
37; R
6 is -(CH
2)
s-Q; s is an integer of 1 to 6; Q is a group selected from the following monovalent
organic groups having hindered phenol structures,

R
7 is a 2-phenylethyl group or a 2-phenylpropyl group; and m, n, p, q and r are each
a number satisfying the following equations: 0≤ m ≤ 1,000, 0 ≤ n ≤ 100, 0 ≤ p ≤1,000,
0 ≤ q ≤ 1,000, 0 ≤ r ≤ 2,000 and

.
[0036] From the viewpoint of consistency and dispensation property required for a silicone
grease composition, it is desirable that the liquid silicone used in the present invention
have its viscosity in the range of 50 to 500,000 cs, particularly 100 to 100,000 cs,
at 25°C.
[0037] Examples of mineral oil and synthetic oil usable as base oil in the present invention
include paraffin oil, naphthene oil, α-olefin oligomers (poly-α-olefins), polybutenes
(polyisobutylenes), substituted aromatic compounds, polyalkylene glycols (polyglycol,
polyether, polyalkylene oxides), diesters (dibasic acid esters), polyol esters (neopentylpolyol
esters and hindered esters), phosphoric acid esters (phosphate esters), fluorinated
compounds, such as chlorofluorocarbons, fluoroesters and perfluoroalkyl ethers (fluoropolyglycols,
perfluoropolyethers, polyperfluoroalkylethers), and polyphenylether.
[0038] With respect to the synthetic oils recited above, the α-olefin oligomers include
those represented by the following formula (III), the polybutenes include those represented
by the following formula (IV), the substituted aromatic compounds include those represented
by the following formula (V) and the polyalkylene glycols include those represented
by the following formula (VI):

[0039] In the above formula (VI), R, and R'' each are generally H or CH
3, R' is H, CH
3 or C
2H
5, and n is from 1 to 200. In general, the polyalkylene glycol of formula (VI) is polyethylene
glycol or polypropylene glycol. Further, it may be a copolymer of these two glycols.
[0040] The diesters (dibasic acid esters) are generally produced by the esterification reaction
between alcohols and dibasic acids as shown below;

wherein R is H or a C
4-18 alkyl group and R' is a C
4-18 alkylene group or an arylene group.
[0041] The dibasic acids used as starting material are generally those represented by formula,
HOOC(CH
2)
nCOOH, with examples including adipic acid (n=4), azelaic acid (n=7), sebacic acid
(n=8) and dodecane diacid (n=10). The alcohols used in combination with those acids
are 7-13C primary alcohols having a side chain, with examples including 2-ethylhexanol
(C
8), isodecanol (C
10) and tridecanol (C
13).
[0042] By using those acids and alcohols in different combinations, various diesters can
be obtained. Examples thereof include diisodecyl phthalate, di-2-ethylhexyl phthalate,
dibutyl phthalate, diisodecyl adipate, diisononyl adipate, diisobutyl adipate, mixed
acid esters of 2-ethylhexanol, di-2-hexyl sebacate, dibutyl sebacate, di-2-ethylhexyl
azelate, di-n-hexyl azelate, di-2-hexyl dodecanoate and dibutoxyethoxyethyl adipate.
[0043] The polyol esters, including neopentylpolyol esters and hindered esters, are monobasic
fatty acid esters of polyhydric alcohols, such as neopentylpolyols.
[0044] In producing polyol esters, neopentylpolyols which are mass-produced as the starting
material for syntheses of alkyd resin and surfactants can be employed as raw materials
of alcohols. Specifically, neopentyl glycol (NPG), trimethylolpropane (TMP), trimethylolethane
(TME), pentaerithritol (PE) and dipentaerithritol (DPE) can be used as polyhydric
alcohol.
[0045] The monobasic fatty acids usable as the other starting material in the polyol ester
synthesis include straight-chain and branched C
3-13 carboxylic acids. For instance, as C
9 carboxylic acids are exemplified the following acids having a straight-chain structure,
a branched structure and a structure having a neopentyl type branch respectively:

[0046] By variously combining the above-recited compounds as starting materials, a wide
variety of polyol esters can be synthesized. In particular, the esters produced by
the reaction of an acid having a neopentyl type branch with an alcohol having a neopentyl
type branch have the advantage of high thermal stability.
[0047] Additionally, by the comparative experiment on stability against pyrolysis between
di(isooctyl)azelate as an ester produced from an alcohol having no neopentyl type
branch and an acid having no neopentyl type branch, bis(2,2-dimethyloctyl)azelate
as an ester produced from an alcohol having a neopentyl type branch and an acid having
no neopentyl type branch, and bis(2,2-dimethylpentyl)-2,2,8,8-tetraethylazelate as
an ester produced from an alcohol having a neopentyl type branch and an acid having
neopentyl type branches, it is known that the last ester as the neopentyl type-neopentyl
type combination has the highest thermal stability.
[0048] The phosphoric acid esters include esters prepared from phosphoric acid as an inorganic
acid and phenols or alcohols. With respect to the phenyl phosphate, as triphenyl phosphate
is in a solid state at ordinary temperature, the phenyl phosphates in a liquid state
can be generally prepared by using phenols substituted by alkyl group(s). Examples
of such a liquid phenyl phosphate include tricresyl phosphate (TCP), trixylenyl phosphate,
tripropylphenyl phosphate and tributylphenyl phosphate.
[0049] Suitable examples of an alkyl phosphate include tributyl phosphate (TBP) and tri-2-ethylhexyl
phosphate (TOP).
[0050] The chlorofluorocarbons have a structure such that hydrogen atoms of n-paraffin are
replaced by fluorine atoms and chlorine atoms, and can be produced by polymerizing
chlorotrifluoroethylene in a low polymerization degree as shown in the following reaction
scheme:

[0051] The chlorofluorocarbon produced has a viscosity depending on the polymerization degree,
and the viscosity can be varied over a wide range.
[0052] Examples of a fluoroester usable in the present invention include sebacic acid esters
of C
7 perfluoroalcohols, pyromellitic acid esters of perfluoroalcohols and camphoric acid
esters of perfluoroalcohols.
[0053] The perfluoroalkyl ethers are generally represented by the following formula (VII)
or (VIII):

[0054] It is possible to produce perfluoroalkyl ethers having from low to high viscosities
by changing the polymerization degree.
[0055] From the viewpoints of consistency and dispensation suitability required for thermally
conductive grease, it is desirable that the liquid hydrocarbons and/or fluorinated
hydrocarbon oil used in the present invention have their viscosity in the range of
50 to 500,000 cs, especially 100 to 100,000 cs, at 25°C.
[0056] The Component (B) of the present grease composition is a thermally conductive inorganic
filler having Mohs' hardness of at least 6 and thermal conductivity of at least 100
W/m°K (in theory). As the Component (B) is used for conferring high thermal conductivity
on the grease composition, it is the more advantageous to the composition to use a
filler having higher thermal conductivity.
[0057] In order to provide a thermally conductive material useful for the heat removal from
heat-generating electronic parts, which is an object of the present invention, it
is required for the filler to have theoretical thermal conductivity of at least 100
W/m°K, preferably at least 300 W/m°K. Further, it is required for the filler as Component
(B) to have Mohs' hardness of at least 6, though inorganic fillers having such high
thermal conductivity are generally high in Mohs' hardness. When the filler used as
Component (B) has Mohs' hardness lower than 6, it cannot have good compatibility with
an inorganic filler having low Mohs' hardness used as Component (C). Examples of a
filler suitable for Component (B) include an aluminum nitride powder, a diamond powder
and a silicon carbide powder.
[0058] The aluminum nitride powder used as a thermal conductivity providing filler in the
present invention is a nitride of Group III-V metal which generally has a crystal
structure of hexagonal system or wurtzite type, and colored white or grayish white
in appearance. The particle shape of the powder is polygonal or spherical depending
on the preparation method adopted.
[0059] Such an aluminum nitride powder is prepared using, e.g., a direct nitriding method
in which a metallic aluminum powder is allowed to react directly with nitrogen or
ammonia, an alumina reduction method in which a mixture of alumina and carbon powders
is heated in an atmosphere of nitrogen or ammonia to undergo reduction and nitriding
reactions at the same time, a method of reacting aluminum vapor directly with nitrogen,
or the pyrolysis of AlCl
3 · NH
3.
[0060] In the present invention can also be used a highly pure aluminum nitride ceramic
prepared by using as a raw material an aluminum nitride powder prepared by the method
as mentioned above and sintering the raw material. In preparing such a highly pure
aluminum nitride ceramic, the aluminum nitride powder used as a raw material is required
to be susceptible to sintering by having high purity and being a fine powder having
a uniform primary particle size of the order of 0.5 µm.
[0061] The aluminum nitride powders prepared by any methods can be used in the present invention,
although they differ in characteristics, including the chemical composition (impurities),
the particle shape and the particle size distribution, depending on the preparation
method adopted. Also, the powders prepared by different methods may be used as a mixture.
[0062] The thus obtained aluminum nitride powder is a very hard material, and has an excellent
thermal conductivity, electric insulation and mechanical strength.
[0063] The aluminum nitride powders having their average particle sizes in a wide range
of 0.5 to 5 µm can be used in the present invention. In view of the dispersibility
in a base oil, however, it is preferable for the aluminum nitride powder to have an
average particle size in the range of 1 to 4 µm, particularly 2 to 4 µm.
[0064] When the average particle size of an aluminum nitride powder used is smaller than
0.5 µm, the powder is undesirable because of its too great viscosity-increasing effect.
In other words, the grease obtained using such a powder has low consistency (or high
hardness and poor dispensation suitability). When the average particle size is larger
than 5 µm, on the other hand, the thermally conductive material obtained is poor in
uniformity and stability and, what is worse, the base oil separates therefrom to a
considerable extent (namely, the material obtained has a high oil-separation degree).
Therefore, it is a matter of course that good grease cannot be obtained in the foregoing
cases.
[0065] Further, it is desirable for such a powder to have a specific surface area of from
1 to 5 m
2/g. In particular, the specific surface area ranging from 2 to 4 m
2/g is preferred from the viewpoint of compatibility with a base oil.
[0066] In general, aluminum nitride is very hard, and the Mohs' hardness thereof is within
the range of 7 to 9. Any aluminum nitride can be used in the present invention as
far as the Mohs' hardness thereof is in the foregoing range. In particular, the aluminum
nitride having Mohs' hardness of from 8 to 9 is used to advantage.
[0067] The thermal conductivity of aluminum nitride is 320 W/m°K in theory, but the actually
measured value is lower than the theoretical value, specifically 250 W/m°K or below,
because the aluminum nitride powder obtained in practice is more or less contaminated
with impurities and contains voids and bubbles. It is required for the aluminum nitride
powder used in the present invention to have a thermal conductivity of at least 100
W/m°K at room temperature. In particular, it is desirable that the thermal conductivity
thereof be at least 150 W/m°K, especially at least 200 W/m°K, at room temperature.
When the thermal conductivity of an aluminum nitride powder used is below 100 W/m°K,
the thermal conductivity of the grease or sheet obtained cannot reach such a high
value as to be aimed at by the present invention.
[0068] Examples of aluminum nitride which can be used in the present invention include US,
UF and UM, trade names, produced by Toyo Aluminum Co., Ltd., XUS-55548, trade name,
produced by Dow Chemical Co., Ltd., H-grade and F-grade, trade names, produced by
K.K. Tokuyama, FA and ES-10, trade names, produced by Nippon Light Metal Co., Ltd.,
and A-100WR, A-100 and AG-SD, trade names, produced by Advanced Refractory Technologies
Inc.
[0069] The diamond powder used as a thermal conductivity providing filler in the present
invention is a synthetic diamond powder which is generally produced on an industrial
scale. The synthetic diamond powders have a density of about 3.5 and a hexahedral
or octahedral crystal shape originating in diamond structure, and produced according
to the ultra-high pressure synthesis method or low pressure synthesis method using
graphite as a starting material.
[0070] The synthetic diamond changes its crystal shape depending on the synthesis condition
including the combination of pressure and temperature, and the crushing characteristic
value thereof depends on the crystal shape. Accordingly, the synthesis condition can
be chosen so as to achieve the grain form and the grain size distribution adapted
for the use intended. Thus, synthetic diamond powders various in grain form and grain
size distribution are procurable. In the present invention, it is possible to employ
diamond powders in micron sizes which are generally used for slurry, paste and tape.
As for the diamond powders of micron sizes, there are known diamond powders the grain
size of which ranges from 0.1 to 60 µm and the bulk density of which ranges from 1.4
to 2.1.
[0071] The synthetic diamond powders usable in the present invention are those various in
average grain size. Specifically, their average grain sizes are in the range of 0.2
to 5 µm. From the viewpoint of dispersibility in base oils, it is desirable for their
average grain sizes to range from 0.5 to 4 µm, particularly from 1 to 3 µm. Moreover,
it is favorable to prepare such a diamond powder by properly mixing a relatively coarse
diamond powder having a grain size of from 8 to 20 µm, a diamond powder having a medium
grain size of from 1 to 8 µm and a relatively fine diamond powder having a grain size
of from 0.1 to 1 µm, because the mixture obtained can have a broad grain size distribution
to ensure excellent dispensation suitability in the heat-reducing grease as the dispersion
thereof in a base oil. Additionally, the theoretical thermal conductivity of synthetic
diamond, though depends on its crystal form, is very high, specifically in the range
of 900 to 2,000 W/m°K.
[0072] Silicon carbide powders usable as a thermal conductivity providing filler in the
present invention are generally obtained by producing high-purity α-SiC ingot from
silica and coke as the main raw materials by means of an electric resistance furnace
(Acheson furnace) and subjecting the thus produced ingot to pulverizing, decarburizing,
iron-removing and sieving steps in succession. According to this process, silicon
carbide powders various in grain size distribution can be produced depending on the
intended uses. Further, an ultra fine silicon carbide powder can be prepared by choosing
a powder having a moderate particle size distribution as starting material, thoroughly
grinding the powder into fine particles of sub-micron order in size, sieving them,
and further purifying by a chemical treatment.
[0073] The grain size and the grain size distribution of silicon carbide are determined
by the methods defined in JIS R6001, JIS R6002 and JIS R6124. The average particle
size of a silicon carbide powder usable in the present invention, though it may be
in a wide range of 0.4 to 10 µm, is desirably in the range of 0.4 to 5 µm from the
viewpoints of securing high dispersibility in base oil and preventing oil separation.
The silicon carbide powders are bluish black in appearance, have a crystal form of
trigonal prism, and are generally hard. The theoretical thermal conductivity of silicon
carbide is 100 W/m°K. The silicon carbide powders having Mohs' hardness in the range
of 8 to 9 are usable in the present invention.
[0074] As a thermally conductive inorganic filler having Mohs' hardness of at least 6 and
a theoretical thermal conductivity of at least 100 W/m°K, which is referred to as
Component (B) of the present invention, the above-recited fillers are exemplified.
However, these exemplified fillers should not be construed as limiting the scope of
the present invention in any way. Additionally, the fillers as recited above may be
used alone or in combination to constitute the Component (B).
[0075] In particular, as it is required for the Component (B) to have a thermal conductivity
of at least 100 W/m°K, especially at least 200 W/m°K in order to impart excellent
thermal properties upon the grease composition, it is desirable that the aluminum
nitride and diamond powders having high thermal conductivity be used alone as Component
(B) or the Component (B) comprises such powders in appropriate proportions.
[0076] The Component (C) of the present invention is a thermally conductive inorganic filler
having Mohs' hardness of no higher than 5 and theoretical thermal conductivity of
at least 20 W/m°K. This Component (C) enables the Component (B) to have a high filling
factor when used in combination with the Component (B). More specifically, when the
present thermally conductive composition is used as heat-reducing grease, the Component
(C) functions so as to secure satisfactory dispensation suitability and, at the same
time, attain a high filling factor of thermally conductive inorganic filler.
[0077] In order to fulfil such a function, the Component (C) is required to have Mohs' hardness
of no higher than 5, desirably in the range of 1 to 5. When the Mohs' hardness thereof
is higher than 5, the Component (C) cannot have good compatibility with the Component
(B) having high hardness. In addition, the Component (C) is required to have theoretical
thermal conductivity of at least 20 W/m°K. When the Component (C) has theoretical
thermal conductivity lower than 20 W/m°K, it is hard to obtain a composition having
the high thermal conductivity aimed at by the present invention. Examples of an inorganic
filler as Component (C) which can meet the foregoing requirements include a boron
nitride powder and a zinc oxide powder.
[0078] The zinc oxide usable in the present invention is a white zinc oxide powder having
a hexagonal or wurtzite crystal structure, generally referred to as "Zinc White".
Such a zinc oxide powder can be prepared using known methods. For instance, one of
the known methods is an indirect method in which the zinc vapor generally produced
by heating metallic zinc to 1,000°C is oxidized with hot air, and another thereof
is a direct method wherein the zinc oxide obtained by roasting zinc ore is reduced
by coal or the like and the zinc vapor produced is oxidized with hot air, or wherein
the slag obtained by the leaching of zinc ore with sulfuric acid is admixed with coke
and then heated in an electric furnace, and further the zinc vapor produced thereby
is oxidized with hot air.
[0079] The zinc oxide produced using any of the foregoing methods is cooled by passing through
an air condenser equipped with a blower, and fractionated according to the grain size.
As still another production method of zinc oxide, there is known a wet method in which
a zinc salt solution is admixed with an alkali carbonate solution to precipitate zinc
hydroxycarbonate and the zinc hydroxycarbonate obtained is roasted.
[0080] The thus obtained zinc oxide powders are defined in accordance with the Japanese
Industrial Standards, JIS K1410 and K5102, or American standards, ASTM-D79.
[0081] In the present invention, the zinc oxide powders produced by any of the aforementioned
methods can be used alone, or a mixture of zinc oxide powders produced by different
methods may be used.
[0082] In general the zinc oxide powder is used not only as a vulcanization accelerator
for rubber but also in the fields of coating color, ceramics, enameled ware, glass,
ferrite, cosmetics and medicines. Further, it is known to use a zinc oxide powder
as a thermal conductivity providing filler in a thermally conductive grease [Japanese
Tokkai Sho 51-55870, Sho 54-116055, Sho 55-45770, Sho 56-28264, Sho 61-157587, Hei
2-212556 (U.S. Patent No. 5,221,339), Hei 3-162493 (U.S. Patent No. 5,100,568) and
Hei 4-202496].
[0083] The average grain size of a zinc oxide powder which can be used in the present invention
is in a wide range of 0.2 to 5 µm. In the view of the dispersibility in base oil and
the relation with the powders used in combination, such as an aluminum nitride powder,
it is desirable to use a zinc oxide powder having a grain size in the range of 0.3
to 4 µm, particularly 0.3 to 3 µm. By using the zinc oxide powder having such a grain
size, the oil separation degree of the thermal conductive material obtained can be
reduced to 0.01 % or below. Further, it is desirable for the zinc oxide used to have
Mohs' hardness of from 4 to 5.
[0084] The boron nitride powders usable in the present invention are boron nitride powders
having a hexagonal crystal structure similar to that of graphite, or a hexagonal network
laminate which are produced by heating boric acid or a borate in combination with
a nitrogen-containing organic compound or ammonia. The boron nitride of hexagonal
system has characteristics such that it retains high lubricity even in a high temperature
range, has high thermal conductivity as well as high electrical insulating capacity,
and further is chemically stable and hardly wetted with fused metal or glass. Accordingly,
it is used as an electrical insulating filler having high thermal conductivity, a
solid lubricant, a filler for modification of resins, or the like.
[0085] These boron nitride powders having a crystal structure ox hexagonal system are white
in appearance, the average grain size thereof is from 1 to 10 µm, and they are generally
soft. In the present invention, the boron nitride powders having Mohs' hardness in
the range of 1 to 3 are usable. In particular, the boron nitride powders having Mohs'
hardness of the order of 2 are used to advantage.
[0086] The boron nitride powders usable in the present invention have their average grain
sizes in a wide range of 1 to 10 µm. In viewing the dispersibility in base oil and
the prevention of oil separation, however, it is desirable for the powder used in
the present invention to have an average particle size in the range of 1 to 5 µm.
[0087] On the other hand, the hexagonal boron nitride as described above is converted to
cubic boron nitride based on the same structural principle of diamond when it undergoes
a high temperature-ultrahigh pressure processing. The boron nitride having a crystal
structure of cubic system has the hardness second to that of diamond, and its powdered
products available on the market are from liver brown to black in appearance and the
average particle size thereof is in the range of several µm to 800 µm.
[0088] Such cubic boron nitride powders also are usable in the present invention, but they
are not favorable because their thermal conductivity is in a low range of 0.5 to 3.6
W/m °K; as a result, even if they are incorporated in grease or sheet, the achievement
of the thermal conductivity satisfactory to the present invention is difficult. Another
reason for this difficulty is that the thermal conductivity of the present thermal
conductive material depends also on the filling rate of thermal conductivity providing
fillers in the base oil used.
[0089] In order to achieve high thermal conductivity which is an object of the present invention,
it is required that the filling rate of fillers, especially an aluminum nitride or
diamond powder as Component (B), be heightened. To the attainment of a high filling
rate without causing damage to grease characteristics, the shape and size of filler
particles are of great importance. In the case of preparing grease, apart from sheet,
increasing a filling rate renders the resulting composition highly viscous to tend
to fair dispensation suitability.
[0090] The term "dispensation suitability" as used herein indicates the ease of the work
in coating a grease on a substrate. When the grease has an inferior dispensation suitability,
the ease of the coating work using a cylinder-form apparatus equipped with a grease
extruding means is reduced and it becomes difficult to form a thin coating of the
grease on a substrate. In the case where a thermally conductive material is used as
grease, therefore, the shape as well as the size of filler particles constitutes a
very important factor for achieving a high filling rate while securing a dispensation
suitability.
[0091] However, each of aluminum nitride and diamond powders consists of square- or flake-shaped
particles rather than spherical particles due to the production process and crystal
structure thereof, so that the powder tends to increase the viscosity of a thermally
conductive material with a rise in the filling rate. In other words, the rise in the
filling rate causes an increase in the viscosity, or a drop in consistency, of the
thermally conductive material to impair the dispensation suitability required for
a thermally conductive grease.
[0092] On the other hand, the theoretical thermal conductivity of zinc oxide or boron nitride
as Component (C) is in the range of 20 to 60 W/m°K. In other words, this range is
much lower than the thermal conductivity required for the filler as Component (B),
or about 1/(5-15) as high as the thermal conductivity 300 W/m°K which an aluminum
nitride or diamond powder as Component (B) can have. Therefore, zinc oxide and boron
nitride powders have rarely been used in the fields where high thermal conductivity
is needed. As for the hardness, however, these powders are softer than the powders
used as Component (B). Accordingly, if the Component (C) is used in combination with
the Component (B), every individual soft particle of Component (C) can be arranged
among hard particles of Component (B) to function so as to confer a mobility on the
close-packed structure to enable an improvement in dispensation suitability.
[0093] In mixing a filler as Component (B) with a filler as Component (C), it is desirable
that the ratio of the filler as Component (C) to the total fillers, or the Component
(C)/(Component (B) + Component (C)) ratio, be from 0.05 to 0.5 by weight, particularly
from 0.1 to 0.3. When the filler as Component (C) is mixed in a proportion lower than
5 weight %, it cannot fill up sufficiently the gaps among hard filler particles as
Component (B) to fail in not only efficiently improving the thermal conductivity but
also imparting satisfactory dispensation suitability to a thermally conductive material
intended for grease.
[0094] When the filler as Component (C) is mixed in a proportion higher than 50 weight %,
on the other hand, it becomes difficult to produce an improvement in thermal conductivity,
because the thermal conductivity of zinc oxide or boron nitride used as Component
(C), specifically 20-60 W/m °K in theory, is considerably low, compared with the theoretical
thermal conductivity of 300 W/m°K or above the filler used as Component (B), such
as aluminum nitride or diamond, has.
[0095] By mixing a filler as Component (B) with a filler as Component (C) in a proper ratio
according to the present invention, the fillers can be most appropriately dispersed
into a base oil used as component (A) to enable the heat-reducing grease obtained
to have a moderate consistency and to avoid deterioration in the dispensation suitability.
As a result, the present thermal conductive grease can acquire thermal conductivity
of at least 2.5 W/m°K which is in the optimum range for the removal of the heat from
highly heat-generating electronic parts.
[0096] It is required that the proportion of a filler mixture of Component (B) with Component
(C) be from 500 to 1,000 parts by weight, preferably from 800 to 1,000 parts by weight,
to 100 parts by weight of base oil as Component (A). When the content of a filler
mixture in the present composition is less than 500 parts by weight, the thermal conductivity
of the composition obtained is in almost the same level as those of conventional grease
compositions; while, when it is increased beyond 1,000 parts by weight, the resulting
composition becomes hard and poor in dispensation suitability although high in thermal
conductivity, so that it is unfit for grease.
[0097] To the present thermally conductive grease compositions each may further be added
various agents, such as a thixotropy providing agent, an antioxidant, metallic powders,
metallic fibers, a flame retardant, heat-resistant additives, pigments, a blowing
agent, a cross-linking agent, a curing agent, a vulcanizing agent and a mold-releasing
agent, if desired. Specific examples of such additives include reinforcing fillers
(such as aerosol silica, precipitated silica, diatomaceous earth and non-conductive
carbon black), aluminum oxide, mica, clay, zinc carbonate, glass beads, polydimethylsiloxane,
alkenyl group-containing polysiloxanes, and polymethylsilsesquioxane. These additives
can be properly chosen according to the necessity or usefulness, and mixed under heat
or reduced pressure. In kneading the present components with those additives, a closed
kneader, a two-rod roll, a three-rod roll, or a colloid mill can be used to disperse
them homogeneously.
[0098] The present thermally conductive material can be prepared with each in the following
manner: The foregoing Components (A), (B) and (C), e.g., liquid silicone, aluminum
nitride powder and zinc oxide powder, are weighed out in proper amounts, admixed with
additives such as an antioxidant, if needed, and then kneaded together using a planetary
mixer or the like so that the gaps among aluminum nitride particles as Component (B)
are filled up with zinc oxide particles as Component (C) to ensure the consistency
of from 200 to 400 in the composition prepared. In view of dispensation suitability,
it is desirable for the grease to have the consistency of from 200 to 400, particularly
from 250 to 350.
[0099] In applying the grease prepared in the foregoing manner to an electronic apparatus
which generates heat during the operation, it is desirable to use an injector-like
coating device which is fitted up with a means to push the grease charged therein
towards the outlet. In particular, the grease can be easily applied to an electronic
apparatus by users if the foregoing coating device is in advance charged with the
present grease.
[0100] The present invention will now be illustrated in greater detail by reference to the
following examples.
[0101] The entire disclosure of all applications, patents and publications, cited above
and below, and of corresponding Japanese applications No. Hei 10-64581, filed February
27, 1998, and No. Hei 10-64582, filed February 27, 1998, is hereby incorporated by
reference.
[0102] Additionally, all "pars" and "%" in the following examples and comparative examples
are by weight unless otherwise noted.
EXAMPLE 1-14
[0103] The liquid silicones represented by the following formula (III), having the viscosities
set forth in Table 1, were each used as base oil (Component (A)) in the amount of
100 parts.
Table 1
Symbol of Liquid Silicone |
Viscosity (cs) at 25°C |
Average |
Polymerization |
Degree |
|
|
m' |
q' |
r' |
A - 1 |
450 |
20 |
0 |
25 |
A - 2 |
1,000 |
80 |
0 |
100 |
A - 3 |
500 |
0 |
0 |
240 |
A - 4 |
400 |
0 |
3 |
9 |
A - 5 |
10,000 |
0 |
0 |
800 |
[0104] Thermally conductive silicone composition samples according to the present invention
were each prepared as follows; A filler as Component (B), the species and average
particle size of which are set forth in Table 2, and a filler as Component (C), the
species and average particle size of which are also set forth in Table 2, were weighed
out in their respective amounts as set forth in Table 2, and added to a base oil as
specified above. Then, these three components were thoroughly mixed for 20 minutes
by means of a planetary mixer, and further subjected to a kneading process for three
times by means of a three-rod roll.
[0105] The silicone composition samples thus prepared were each examined for consistency
and oil separation degree in accordance with JIS-K-2220 with the intention of using
them as grease, and further the thermal conductivities thereof were measured with
a hot-wire instrument for measuring thermal conductivity, Model TCW-1000, made by
Shinku Riko Co., Ltd. The results obtained are shown in Table 2.
[0106] Additionally, the symbols B-1 and B-2 used in Table 2 to represent Component (B)
are pulverized AlN (average particle size: 0.1-15 µm) and a synthetic diamond powder
(average particle size: 0.1-5 µm) respectively; while the symbols C-1 and C-2 used
therein to represent Component (C) are a zinc oxide powder (average particle size:
0.05-5 µm) and a hexagonal boron nitride powder (average particle size: 0.1-5 µm).

COMPARATIVE EXAMPLES 1-14
[0107] Other thermally conductive silicone composition samples were prepared in the same
manner as in the aforementioned Examples, except that the amounts of fillers used
as Components (B) and (C) were each changed variously. The consistency, oil separation
degree and thermal conductivity of each composition prepared were measured in the
same ways as in each Example, and the measurement results obtained are shown in Table
3.
[0108] Additionally, the symbols A-1 to A-5, B-1, B-2, C-1 and C-2 used in Table 3 represent
the same ingredients as those in each Example, respectively.

[0109] As can be seen from the data shown in Tables 2 and 3, the present thermally conductive
silicone grease compositions had their thermal conductivity in the range of 2.72 to
3.97 W/m°K. In other words, the present compositions underwent considerable improvement
in thermal conductivity over conventional grease compositions and the comparative
compositions. Further, the present compositions had the consistency on the practically
optimum level for the use as grease and satisfactory dispensation suitability.
EXAMPLE 15-27
[0110] The liquid hydrocarbons and fluorohydrocarbon oils having the structural formulae
illustrated below and the viscosities set forth in Table 4 were each used as base
oil (Component (A)) in the amount of 100 parts.

[0111] Thermally conductive grease compositions according to the present invention were
each prepared as follows; A filler as Component (B), the species and average particle
size of which are set forth in Table 5, and a filler as Component (C), the species
and average particle size of which are also set forth in Table 5, were weighed out
in their respective amounts as set forth in Table 5, and added to a base oil as specified
above. Then, these three components were thoroughly mixed for 20 minutes by means
of a planetary mixer, and further subjected to a kneading process for three times
by means of a three-rod roll.
[0112] The thermally conductive composition samples thus prepared were each examined for
consistency and oil separation degree in accordance with JIS-K-2220 with the intention
of using them as grease, and further the thermal conductivities thereof were measured
with a hot-wire instrument for measuring thermal conductivity, Model TCW-1000, made
by Shinku Riko Co., Ltd. The results obtained are shown in Table 5.
[0113] Additionally, the symbols B-1 and B-2 used in Table 5 to represent Component (B)
are pulverized AlN (average particle size: 0.1-5 µm) and a synthetic diamond powder
(average particle size: 0.1-5 µm) respectively; while the symbols C-1 and C-2 used
therein to represent Component (C) are a zinc oxide powder (average particle size:
0.05-5 µm) and a hexagonal boron nitride powder (average particle size: 0.1-5 µm).

COMPARATIVE EXAMPLES 15-25
[0114] Other thermally conductive grease composition samples were prepared in the same manner
as in the aforementioned Examples 15-27, except that the amounts of fillers used as
Components (B) and (C) were each changed variously. The consistency, oil separation
degree and thermal conductivity of each composition prepared were measured in the
same ways as in each Example, and the measurement results obtained are shown in Table
6.
[0115] Additionally, the symbols A-6 to A-9, B-1, B-2, C-1 and C-2 used in Table 6 represent
the same ingredients as those in Examples 15-27, respectively.

[0116] As can be seen from the data shown in Tables 5 and 6, the present thermally conductive
silicone grease compositions had their thermal conductivity in the range of 2.59 to
4.02 W/m°K. In other words, the present compositions underwent considerable improvement
in thermal conductivity over conventional grease compositions and the comparative
compositions. Further, the present compositions had the consistency on the practically
optimum level for the use as grease and satisfactory dispensation suitability.