U.S. patent application number 09/800923 was filed with the patent office on 2001-09-13 for conductive filler.
Invention is credited to Kaneyoshi, Masami.
Application Number | 20010021730 09/800923 |
Document ID | / |
Family ID | 18583003 |
Filed Date | 2001-09-13 |
United States Patent
Application |
20010021730 |
Kind Code |
A1 |
Kaneyoshi, Masami |
September 13, 2001 |
Conductive filler
Abstract
A conductive filler in the form of non-conductive particles
coated with plural layers of metal plating is provided wherein the
lower layer is of copper or copper alloy plating, and the uppermost
layer is of gold plating. The conductive powder has a high
conductivity, improved durability, especially oxidation resistance,
and a relatively low specific gravity.
Inventors: |
Kaneyoshi, Masami;
(Takefu-shi, JP) |
Correspondence
Address: |
BIRCH STEWART KOLASCH & BIRCH
PO BOX 747
FALLS CHURCH
VA
22040-0747
US
|
Family ID: |
18583003 |
Appl. No.: |
09/800923 |
Filed: |
March 8, 2001 |
Current U.S.
Class: |
523/207 ;
523/204 |
Current CPC
Class: |
C08K 2201/014 20130101;
C08K 9/02 20130101 |
Class at
Publication: |
523/207 ;
523/204 |
International
Class: |
C08K 009/02; C08K
003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 8, 2000 |
JP |
2000-063085 |
Claims
1. A conductive filler comprising non-conductive particles which
are coated on their surface with plural layers of metal plating,
wherein a lower layer of metal plating is formed of copper or
copper alloy plating, and the uppermost layer of metal plating is
formed of gold plating.
2. The conductive filler of claim 1 wherein a layer of nickel or
nickel alloy plating intervenes between the layer of copper or
copper alloy plating and the layer of gold plating.
3. The conductive filler of claim 1 wherein the non-conductive
particles are selected from the group consisting of silicon oxide,
aluminum oxide, titanium oxide, zirconia, rare earth oxides,
yttrium oxide, mica, diatomaceous earth, sodium silicate glass,
polyurethane, polystyrene, polycarbonate, phenolic resin,
polyamide, polyimide, silicone resin and epoxy resin.
4. The conductive filler of claim 1 wherein the non-conductive
particles are of silicon oxide.
5. The conductive filler of claim 1 wherein the layer of copper or
copper alloy plating is formed by electroless plating.
6. The conductive filler of claim 1 wherein the non-conductive
particles have a diameter of up to 150 .mu.m, and the layer of
copper or copper alloy plating has a thickness of 50 to 500 nm.
7. The conductive filler of claim 6 wherein the layer of gold
plating has a thickness of at least 7 nm.
8. The conductive filler of claim 6 wherein the layer of gold
plating has a thickness of at least 7 nm, and a layer of nickel or
nickel alloy plating having a thickness of 15 to 200 nm is
interposed as an intermediate layer between the layer of copper or
copper alloy plating and the layer of gold plating.
Description
This invention relates to a conductive filler which is blended in
rubber and resin compositions for imparting electrical
conductivity.
BACKGROUND OF THE INVENTION
[0001] It is known in the art that molded rubber parts as a whole
can be made electrically conductive by blending a powder of
conductive particles in rubber compositions, typically silicone
rubber compositions and molding. Such conductive rubber parts are
used in antistatic applications. The traditional conductive powder
is carbon black. In recent years, conductive molded rubber parts
are sometimes used for electrical connection on circuit boards in
electronic equipment. In this application, high conductivity is
required from the antistatic standpoint. More conductive additives,
typically metal powders are then used as the conductive agent. The
metal powders, however, have the problems that they are susceptible
to ignition during handling and are readily oxidized to detract
from conductivity, and most of them have a high specific
gravity.
[0002] To overcome these shortcomings, it was recently developed to
metallize core particles of resin or ceramic material. A typical
powder takes the form of core particles which are coated with
nickel by electroless plating and further on the outermost surface
with gold by displacement plating. The coating of gold on the
outermost surface cooperates with the underlying nickel to provide
conductivity and oxidation resistance. The specific gravity is low
since the metallization is limited to the surface. The cost is
acceptable because of the thin buildup of gold.
[0003] Nevertheless, the conductivity of nickel/gold plated
particles is insufficient in some applications or purposes. This is
partly because the underlying nickel layer is usually a
nickel-phosphorus alloy which has a high resistivity. It is thus
desired to improve the conductivity of conductive particles
(resulting from electroless plating) without a significant increase
of material cost.
SUMMARY OF THE INVENTION
[0004] An object of the invention is to provide a conductive filler
having a high conductivity, improved durability, especially
oxidation resistance, and a relatively low specific gravity.
[0005] According to the invention, a layer of copper or copper
alloy plating is formed on surfaces of non-conductive particles, a
layer of nickel or nickel alloy plating is optionally formed
thereon, and a layer of gold plating is formed as the uppermost
layer. The resulting conductive particulate filler has a low
resistivity, improved durability, and a lower specific gravity than
wholly metal particles.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0006] The electrically conductive filler of the invention is in
the form of non-conductive particles or core particles which are
coated on their surface with plural layers of metal plating. A
lower layer of metal plating is formed of copper or copper alloy
plating, and the uppermost layer of metal plating is formed of gold
plating. Preferably a layer of nickel or nickel alloy plating
intervenes between the lower layer of copper or copper alloy
plating and the uppermost layer of gold plating. The intermediate
layer prevents the gold plating layer from diffusing into the
copper or copper alloy plating layer to form an alloy when
heated.
[0007] The core material used herein to construct the
non-conductive particles to be coated with metallizing layers is
selected from a wide variety of materials including oxides such as
silicon oxide, aluminum oxide, titanium oxide, zirconia, rare earth
oxides, and yttrium oxide, naturally occurring compounds such as
mica and diatomaceous earth, glasses such as sodium silicate glass,
resins such as polyurethane, polycarbonate, phenolic resin,
polyamide, polyimide, silicone resin, epoxy resin and polystyrene,
and other electrically insulating materials. In a typical
application, 100 parts by weight of a rubber composition, typically
a silicone rubber composition or a resin composition, typically an
epoxy resin composition is loaded with about 80 to 500 parts by
weight of the inventive conductive filler, and the blend is milled
prior to use. In order to prevent the plating from being stripped
from the core particles during the milling step, the core particles
should preferably have a certain rigidity. In this sense, an
inorganic core material, especially silicon oxide is preferred.
Core particles in excess of 150 .mu..mu.m should desirably be
excluded because they are likely to separate from the rubber or
resin matrix even after milling. More desirably core particles in
excess of 100 .mu.m are excluded. It is preferable to use core
particles having a diameter of up to 150 .mu.m, more preferably up
to 100 .mu.m, and most preferably 5 to 50 .mu.m. A particle shape
more approximate to sphere is generally preferable. Particles of
nearly spherical shape are most preferred since they are uniformly
dispersed during milling.
[0008] Formed on surfaces of core particles is a layer of copper or
copper alloy plating. The layer of copper or copper alloy plating
is preferably formed by electroless or chemical plating.
[0009] Since the core particles are of insulating material, a
catalyst must be applied thereto for initiating electroless
plating. Catalyzing is carried out by prior art well-known methods,
for example, a method of immersing in a tin (II) chloride solution
and then in a palladium (II) chloride solution, and a method of
immersing in a mixed solution of tin chloride and palladium
chloride. To facilitate the application of catalyst, the core
particles can be subjected to suitable treatments, for example,
brief etching with suitable chemical agents, such as strong
alkalis, mineral acids or chromic acid, treatment with chemical
agents possessing both a functional group having affinity to the
catalytic metal and a functional group having affinity to the core
particles, such as silane coupling agents having an amino group,
and mechanical treatments such as plasma treatment.
[0010] The electroless copper plating solution used for forming the
copper or copper alloy plating layer may be any of well-known
compositions, and commercially available compositions are
acceptable. The plating conditions may be well-known ones. The
electroless copper plating solution generally uses formaldehyde as
a reducing agent although the use of hypophosphites and borides as
the reducing agent is acceptable.
[0011] The copper or copper alloy plating layer is preferably
formed of substantially pure copper. By the term "substantially
pure," it is meant that copper may contain a minor amount of other
elements as impurities. Preferably the copper or copper alloy
plating layer has a thickness of 50 to 500 nm, and more preferably
75 to 400 nm. With a thickness below 50 nm, the metallized particle
powder may become less conductive. A thickness in excess of 500 nm
may not be cost effective since it brings about little further
advantages and adds to the expense of metal material.
[0012] On core particles of a particular type, the adhesion of
electroless copper plating is weak, as compared with electroless
nickel plating, so that it may be strippable. In such a situation,
another electroless plating layer other than copper or copper alloy
may be formed as a primer layer underlying the copper or copper
alloy plating layer.
[0013] The conductive filler of the invention is characterized in
that a layer of copper or copper alloy plating is formed as the
lower layer of metal plating (as mentioned just above) and a layer
of gold plating is formed as the uppermost layer. To augment the
adhesion of the gold plating layer to the copper or copper alloy
plating layer, the copper or copper alloy plating layer is
preferably surface processed. The surface processing may be done by
any of plating treatment, blasting and plasma treatment although it
is preferred to form an intermediate plating layer because
replacement gold plating on copper proceeds at a very slow reaction
rate and often ceases to proceed. In the event of copper being in
direct contact with gold, there is a likelihood of interdiffusion
between copper and gold to form an alloy when heat is applied
during the step of molding the rubber or resin compound after
milling. The diffusion and alloying can adversely affect the
desired oxidation resistance. To prevent the undesired
interdiffusion as well, it is recommended that a layer of nickel or
nickel alloy plating intervene as an intermediate layer.
[0014] As the intermediate layer, a layer of nickel or nickel base
alloy such as pure nickel, nickel-boron, nickel-phosphorus or
nickel-boron-phosphorus is formed. Of these, a nickel-phosphorus
alloy (phosphorus content 2 to 14% by weight) is most preferable
because of ease of electroless nickel plating itself and uniform
progress of subsequent replacement gold plating. The nickel salt
for the electroless nickel plating may be any of well-known ones
such as nickel sulfate, nickel chloride and nickel acetate, and
used in a concentration of 0.01 to 0.5 mol/l of the plating bath.
Too high a nickel concentration may allow a hydroxide to form due
to pH changes and changes in the concentration of complexing agent,
leading to a shortened bath life. At too low a nickel
concentration, a more amount of solution must be replenished to
bring about a substantial change of the plating bath volume. The
phosphorus reducing agent used is hypophosphorous acid or an alkali
metal or ammonium salt thereof, preferably in an amount of 0.1 to 5
mol per mol of the nickel salt.
[0015] Preferably the intermediate layer has a thickness of 15 to
200 nm, and more preferably 25 to 150 nm. With a thickness below 15
nm, the effects of the intermediate layer to facilitate replacement
gold plating and prevent interdiffusion between gold and copper may
become insufficient. Since the intermediate layer has little
contribution to conductivity, a thickness in excess of 200 nm may
merely increase the specific gravity of the power and the raw
material cost.
[0016] The uppermost layer of gold plating should preferably have a
thickness of at least 7 nm when the size and shape of particles,
the gold content by chemical analysis, the constituents of the
remaining plating layers and the specific gravity of core particles
are taken into account. Below 7 nm, a continuous and dense gold
film enough to provide oxidation resistance may not be obtained.
The more preferred thickness is from 10 nm to 30 nm. A gold layer
of more than 30 nm results in an increased specific gravity and an
increased cost. It is not critical how to form the gold layer.
Either electroless plating or electroplating may be used although
the formation of the gold layer by replacement gold plating is
preferred.
[0017] With respect to replacement gold plating, since the object
to be plated is a powder rather than a large size molded part, it
is preferred from the safety and hygienic standpoint to use gold
(I) sulfite complex ions instead of gold cyanide complex ions which
are customarily used in the prior art. The solvent for the gold
salt is selected from water, ketones, and alcohols. The gold salt
is preferably added in a concentration of 0.01 to 20% by
weight.
[0018] It is not critical how to form the respective plating
layers. The plating method is selected from several methods, for
example, a method of premixing a metal ion, reducing agent,
complexing agent, buffer agent, etc. to form a plating solution,
adjusting the pH and temperature thereof, and directly admitting a
powder into the plating solution; a method of admitting a slurry of
a powder in water into the plating solution; and a method of
dispersing a powder in a solution containing some plating solution
components, and adding the remaining plating solution components
thereto. The composition of plating solution may be selected from
conventional bath compositions known for electroless nickel alloy
plating, electroless copper plating and replacement gold
plating.
[0019] The thus obtained conductive filler preferably has a
resistivity of up to 15 m.OMEGA.-cm, more preferably 0.1 to 10
m.OMEGA.-cm, and most preferably 0.1 to 5 m.OMEGA.-cm. For the
measurement of resistivity (or conductivity), specifically the
measurement of resistance of a sample having a standardized volume
and shape, constant current potentiometric measurement is conducted
by the so-called four terminal method. Since the resistance to be
measured is very low, a contact resistance and a thermally induced
potential difference between contacts can be non-negligible error
factors. It is thus desirable to minimize such error factors and
compensate therefor by alternately inverting the current flow.
[0020] The conductive filler is advantageously used in various
rubber and resin compositions, typically silicone rubber
compositions and epoxy resin compositions.
EXAMPLE
[0021] Examples of the invention are given below by way of
illustration and not by way of limitation.
[0022] Example
[0023] After 30 g of a spherical silicon oxide powder having a mean
particle size of about 10 .mu.m (Silica Ace US-10 by Mitsubishi
Rayon Co., Ltd.) was weighed, it was added to 180 cm.sup.3 of an
aqueous solution of 0.3 g aminoalkylsilane coupling agent (KBM603
by Shin-Etsu Chemical Co., Ltd.). After 30 minutes of agitation at
room temperature, the powder was filtered on a Buchner funnel, and
washed by spraying a small amount of water.
[0024] The silane coupling agent-treated powder was added to 150
cm.sup.3 of an aqueous solution containing 3 mmol/dm.sup.3 of
palladium chloride, 0.05 mol/dm.sup.3 of tin (II) chloride and 2.5
mol/dm.sup.3 of hydrogen chloride, followed by 10 minutes of
agitation. The powder was separated from the mixture by filtration
on a Buchner funnel. The powder was washed by spraying 150 cm.sup.3
of dilute hydrochloric acid having a concentration of 1
mol/dm.sup.3 and further with 100 cm.sup.3 of water.
[0025] Next the catalyzed powder was dispersed in 135 cm.sup.3 of
water by agitation, forming a slurry. Separately, 4 dm.sup.3 of a
plating solution was furnished by dissolving 0.039 mol/dm.sup.3 of
copper (II) sulfate, 0.024 mol/dm.sup.3 of disodium ethylenediamine
tetraacetate and 0.096 mol/dm.sup.3 of formaldehyde in water,
adding an aqueous sodium hydroxide solution thereto for adjusting
to pH 12.9 and heating at a temperature of 42.degree. C. With
stirring, the slurry was added to this plating solution. While
stirring was continued, reaction took place for 15 minutes,
depositing an electroless copper plating film as the lower layer.
At the end of reaction, the powder was separated by filtration on a
Buchner funnel and washed by spraying about 1 dm.sup.3 of water.
Immediately thereafter, the powder was dispersed in 135 cm.sup.3 of
water by agitation, forming a slurry.
[0026] There was furnished 3 dm.sup.3 of a plating solution by
dissolving 0.042 mol/dm.sup.3 of nickel sulfate, 0.084 mol/dm.sup.3
of sodium hypophosphite and 0.05 mol/dm.sup.3 of citric acid in
water, adding aqueous ammonia thereto for adjusting to pH 8.8 and
heating at a temperature of 45.degree. C. The slurry of the copper
plated powder was added to this plating solution. While stirring
was continued, reaction took place for 15 minutes, depositing an
electroless nickel-phosphorus alloy plating film as the
intermediate layer. At the end of reaction, the powder was
separated by filtration on a Buchner funnel and washed by spraying
about 1 dm.sup.3 of hydrochloric acid having a concentration of 0.6
mol/dm.sup.3 and then about 1 dm.sup.3 of water. Immediately
thereafter, the powder was dispersed in 135 cm.sup.3 of water by
agitation, forming a slurry.
[0027] There was furnished 1.4 dm.sup.3 of a plating solution by
dissolving 0.011 mol/dm.sup.3 of sodium gold (I) sulfite (chemical
formula: Na.sub.3Au(SO.sub.3).sub.2), 0.1 mol/dm.sup.3 of sodium
sulfite and 0.1 mol/dm.sup.3 of malonic acid in water, adjusting to
pH 7.2 and heating at a temperature of 65.degree. C. The slurry of
the nickel plated powder was added to this plating solution. While
stirring was continued, reaction took place for 10 minutes,
depositing a replacement gold plating film as the uppermost layer.
At the end of reaction, the powder was separated by filtration on a
Buchner funnel and washed by spraying about 1 dm.sup.3 of water.
The powder was recoated and dried for 3 hours in air at 60.degree.
C. in a blowing dryer.
[0028] A sample of the powder was completely decomposed using
hydrofluoric acid and aqua regia. Chemical analysis demonstrated a
composition (% by weight): 59.3% of SiO.sub.2, 21.2% of Cu, 11.6%
of Ni, 6.94% of Au, and 0.92% of P. A resistivity of 1.7
m.OMEGA.-cm was computed from the resistance measured by the four
terminal method (using SMU-257 current source by Keithley, 1 to 10
mA, and Model 2000 Nanovolt Meter by Keithley). The true density
was 3.06 g/cm.sup.3. A sample of the powder was heat treated in air
at 250.degree. C. for one hour before its resistance was measured,
finding a resistivity of 1.8 m.OMEGA.-cm, which was substantially
unchanged from the initial. An x-ray diffraction pattern of the
heat treated powder demonstrated that gold had not diffused.
[0029] Comparative Example
[0030] The same spherical silicon oxide powder as used in Example
was catalyzed by the same procedure as in Example. The catalyzed
powder was dispersed in 135 cm.sup.3 of water by agitation, forming
a slurry. There was furnished 4 dm.sup.3 of a plating solution by
dissolving 0.072 mol/dm.sup.3 of nickel sulfate, 0.144 mol/dm.sup.3
of sodium hypophosphite and 0.08 mol/dm.sup.3 of citric acid in
water, adding aqueous ammonia thereto for adjusting to pH 8.8 and
heating at a temperature of 45.degree. C. The slurry of the
catalyzed powder was added to this plating solution. While stirring
was continued, reaction took place for 20 minutes, depositing an
electroless nickel-phosphorus alloy plating film. At the end of
reaction, the powder was separated by filtration on a Buchner
funnel and washed by spraying about 1 dm.sup.3 of hydrochloric acid
having a concentration of 0.6 mol/dm.sup.3 and then about 1
dm.sup.3 of water. Immediately thereafter, the powder was dispersed
in 135 cm.sup.3 of water by agitation, forming a slurry.
Thereafter, replacement gold plating was carried out on the powder
as in Example, followed by filtration, water washing and
drying.
[0031] A sample of the powder was completely decomposed using
hydrofluoric acid and aqua regia. Chemical analysis demonstrated a
composition (% by weight): 58.5% of SiO.sub.2, 32.4% of Ni, 6.84%
of Au, and 2.26% of P. A resistivity of 7.5 m.OMEGA.-cm was
computed from the resistance measured by the four terminal method.
The true density was 3.08 g/cm.sup.3.
[0032] Between Example and Comparative Example, the content and
thickness of gold and the total metal buildup are approximately
equal, and the specific gravity is approximately equal. The
resistivity of Example is significantly lower than that of
Comparative Example.
[0033] In order to inspect the effect of the intermediate layer,
gold was deposited on a copper plate to a thickness of about 20 nm
by magnetron sputtering. This sample was heat treated in air at
250.degree. C. for one hour as in Example. An x-ray diffraction
pattern of the heat treated sample demonstrated that gold and
copper had diffused to form an alloy. It is understood that the
presence of the intermediate layer in Example restrains the
diffusion of gold.
[0034] There has been described a conductive particle powder having
a high conductivity, improved durability, especially oxidation
resistance, and a relatively low specific gravity, which is useful
as a filler in the industry.
[0035] Japanese Patent Application No. 2000-063085 is incorporated
herein by reference.
[0036] Although some preferred embodiments have been described,
many modifications and variations may be made thereto in light of
the above teachings. It is therefore to be understood that the
invention may be practiced otherwise than as specifically described
without departing from the scope of the appended claims.
* * * * *