Note: Descriptions are shown in the official language in which they were submitted.
12~
SPECIFICATI_
The present invention relates to flowable
conductive compositions, such as molding compositions,
conductive paste~, conductive paints, and to electro-
conductive ele~ents fabricated from such composi~ions, to
the making of electroconductive elements from the flowable
conductive compositions, and to mica flakes having a
conductive metal coating and suitable for making the
above-mentioned conductive compositions and electro-
conductive elements.
Backqround of the lnvention
Prior to this invention, conductive paste has beenfabricated from silver particles, an inorganic bonding
component, and an organic binding component. Typically,
such a paste contains, by weight, 60 to 70% silver, 5 to 10%
glass frit and 20 to 35% of a mixture of various solvents,
plasticizers and resins. This paste has been applied to a
substrate, for example, a ceramic capacitor, and the
substrate and paste fired to form a component comprising the
ubstrate and a fired-on electroconductive body which
provided an electrically conductive connection. Frequently,
these components have been coated with solder for ready
integration into a circuit at a later date.
A disadvantage of these precursor paste
compositions and electroconductive bodies is that
fabrication of the electroconductive body must be performed
by heating the paste at high temperatures which may damage
the substrate to which a paste has been applied.
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Attempts have been made to find satisfactory
substitutes for the above-mentioned pastes and
electroconductive bodies. It has been ~uggested that a
conductive paste may be fired to produce an
electroconductive body containing micron-sized glass spheres
coated with a ncsle metal, such as palladium cr platln~m, or
~n alloy, for example of palladium, gold and silver,
embedded in a matrix of glassy dielectric material having a
fusion temperature lower than the softening temperature of
the glass spheres. An electroconductive body consisting of
particles of alumina coated by palladium, particles of
alumina coated by palladium oxide and particles of silver
embedded in a glassy matrix has also been suggested. The
use of metals such as palladium and gold entails substantial
expense in the production of the electroconductive body, and
its precursor conductive paste. Also, disadvantages
attendant to fabrication at high firing temperatures are not
avoided with these substitutes.
It has also been suggested that a conductive paste
comprising an organic resin binder and a particulated
electrically conductive metal-containing material, for
example, either silver-coated glass spheres or silver
flakes, may be treated to form a conductive body. Other
electrically conductive metals are also suggested.
Another proposal involves forming a conductive
body from a paste comprising inorganic non-metallic
particles coated with silver, silver particles and an
organic binder formable into a matrix, or comprising
inorganic non-metallic particles coated with silver, silver
~, ~12s~30
particles, particles of a glassy material fusible into a
m~trix and an oryanic vehicle. In the conductive body, the
silver particles ana silver-coated inorganic non-metallic
particles are in effective contacting relationship within
the matrix. One illustration of such a system ;s described
in ~.S. Patént 4,419,279 g~ranted December 6, 1983~
.. . . .
~,, ~ ,, . , , ; . , .
Summary
~he present invention provides an electro-
conductive element with advanta~eous electrical and otherproperties, and materials from which it is made, at low
expense in comparison to typical prior art embodiments.
One general object of this invention is to provide
a new and improved flowable conductive composition or
electroconductive element.
Another general object of this invention is to
provide a new and improved conductive filler for converting
a normally non-electrically conductive thermoplastic or
thermosetting plastic into an electroconductive element.
More specifically, it is an object of this
invention to provide a conductive composition and an
electroconductive element at a relatively low cost.
It is also an object of this invention to provide
a conductive composition, for example molding composition~
which is well-suited to injection, compression and/or
extrusion molding applications without degradation, such as
breaking up, of the conductive filler in the composi;tion.
~t is another object of this invention to provide,
as a flowable conductive composition, conductive paste or
1~5~330
paint which has a consistency appropriate for screening on,
or other application to, a sub~trat~.
It is yet another object of this invention to
provide, as a flowable conductive composition, conductive
paste which is formable into an electroconductive body at
conditions not destructive to an attached substra_e.
It i8 still another object of this invention to
provide an electroconductive element which is durable and
exhibits acceptable conductivity for long periods of time
during storage and operation.
It is a further object of this invention to
provide an electroconductive element which, when attached to
a substrate, exhibits acceptable adhesion to such substrate
for long periods of time in storage and operation.
It is a still further object of this invention to
provide a method for producing the foregoing
electroconductive element.
It is also an object of this invention to provide
a mica flake coated with a conductive metal, for example, a
noble metal, copper or nickel, which is suited to the
production of the foregoing flowable conductive composition
and electroconductive element.
In accordance with a feature of the present
invention, a flowable conductive composition consists
essentially of a mixture of mica flakes coated with a
conductive metal and of an organic component formable into a
matrix. The metal-coated mica flakes are in the organic
component thereby forming the composition. The composition,
if a molding composition, is suitable for injection,
12S4330
compression or extrusion molding into an electroconductive
element appropriately shaped for - illustratively -
electromagnetic shielding applications, and if a conductive
paste or paint i5 suitable for application to a substrate to
form an electroconductive element such as a coating, body,
etc. on the substrate.
It will be understood that for purposes of this
invention a "flowable conductive composition" is one
containing conductive metal-coated mica flakes (as well as
other conductive filler material in some embodiments of the
invention) and an organic binder as set forth above, and
which flows at room temperature and atmospheric pressure,
such as conductive paint and conductive paste, or can be
made to flow by application of conventionally increased
temperature and/or pressure conditions, such as a molding
composition which flows under conditions conventionally
imposed during injection, compression or extrusion molding.
In accordance with another feature of the
invention, in several particularly advantageous embodiments,
an electroconductive element consists essentially of mica
flakes coated with a conductive metal, embedded in a matrix
of organic material. The metal-coated mica flakes are in
effective contacting relationship within said matrix.
For purposes of this invention an "electro
conductive element" is an article of manufacture having a
non-flowable organic matrix in which are embedded conductive
metal-coated mica flakes (and, in some embodiments, other
conductive filler material) such that the element will
conduct electricity. The element is shaped in any suitable
12S~33V
form, and in various embodiments is either a termination
element for capacitors, an internal conductive element in
capacitors of the type used in thick-film technology
applications, an element for dissipation of electrostatic
charge, or electromagnetic shielding.
In an~ther aspect, the present invention relates
to a method of making an electroconductive element, which
comprises combining, to form a flowable composition, mica
flakes coated with a conductive metal, and an organic binder
formable into a matrix, and subjecting the flowable
composition to conditions effective to form the organic
binder into a matrix in which the metal-coated mica flakes
are embedded.
A further aspect of the present invention relates
to an article of manufacture useful in practicing the
invention. This article comprises a mica flake which is
coated on substantially its entire surface with a conductive
metal, the metal preferably constituting at least 4~ by
weight of the article.
Electroconductive elements in accordance with the
claimed invention are characterized in that they have volume
resistivities of less than 106 ohm-cm (a volume resistivity
of 106 ohm-cm. or greater is generally viewed in the art as
characterizing an insulating material). By way of giving a
reference point, the volume resistivity of pure silver is
10 8 ohm-cm. It will be appreciated, however, that within
the above-mentioned ranqe, electroconductive elements of the
present invention exhibit volume resistivities which vary
according to the intended use.
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Hence, conductive paste of this invention is
useful as an intermediate in the manufacture of an
electroconductive element, and more specifically as a
vehicle by which the components of an electroconductive body
or electroconductive coating are conveniently applied to
substrates, such as capacitors, die~ectric compenents, and
the like. Electroconductive bodies and coatings of this
invention are, in turn, useful to provide an electrically
conductive connection or film on a substrate. For instance,
the electroconductive bodies find application as termination
elements for ceramic capacitors, such as those of the
multi-layer variety. Such electroconductive bodies may also
be useful as internal conductive elements employed in
combination with nonconductive elements in, for example, a
multi-layer capacitor or a capacitor of the type employed in
thick-film technology applications.
Additionally, various embodiments of the
electroconductive element of the claimed invention are
useful in the dissipation of an electrostatic charge, or as
electromagnetic shielding. In the former, the electro-
conductive element serves as a medium through which the
electrostatic charge can be moved at a controlled rate
(depending on the volume resistivity, typically at least 103
ohm-cm), while in the latter the element is to at least a
substantial extent reflective of electromagnetic energy
(volume resis~ivity typically being in the range of 1
ohm-cm).
The metal-coated mica flake of this invention is
useful as a component of the flowable conductive composition
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12S~330
and electroconductive element of this invention,
contributing to the favorable properties thereof and,
senerally, d~creasing production cost.
~ he present invention affords the advantages of
providing a flowable conductive composition wh$ch is
relatively inexpensive and comprises easily obtcina~le
materials and which is well-suited for conversion to an
electroconductive element in a variety of applications. In
this connection, the flowability of the conductive
composition is especially favorable. The electroconductive
elements, themselves, are additionally advantageous because
of their versatility and desirable performance
characteristics, particularly in res~ect of their wide range
of attainable conductivities and their ability to adhere to
substrates. A further advantage of the present invention is
that the metal-coated mica flake, preferably containing at
least 4~ by weight conductive metal, provides a convenient,
relatively low-cost starting material for making an
electroconductive element and precursor flowable conductive
composition. Through employment of this coated flake a
normally non-electrically conductive thermoplastic or
thermosetting plastic is converted into an electroconductive
element.
The present invention, as well as further objects
and features thereof, will be more fully understood from the
following description of certain preferred embodiments, when
read with reference to the accompanying drawings.
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Brief Descri~tion of Drawinqs
Figure 1 i8 an enlarged fragmentary sectional view
of flowable conductive composition in accordance with the
invention.
Figure 2 is an enlarged fragmentary ~ectional view
of an electroconductive element made from a flowzble
conductive composition, all in accordance with the
invention.
Figure 3 is an enlarged fragmentary sectional view
of an alternative embodiment of flowable conductive
composition in accordance with the invention.
Figure 4 is an enlarged fragmentary sectional view
of an alternative embodiment of an electroconductive element
made from a flowable conductive composition, all in
accordance with the invention.
Figure 5 is a plot of volume resistivity of
various electroconductive elements against the volume % of
conductive filler therein.
Figure 6 is a plot of volume resistivity of
various electroconductive elements against the weight % of
conductive filler therein.
It will be understood that the views shown in the
drawings are not to scale, but that certain aspects, such as
amount of organic component, amount of matrix, distances
between particles and the like, have been emphasized for
purposes of clarity.
Description of Preferred Embodiments
Referring to Figure 1 of the drawing, there is
shown a flowable conductive composition comprising an
~ZS~33~)
organic material 10, illustratively, an acrylic resin, such
as Rohm 6 ~aas B-66 acryloid resin, in which are suspended
mica flakes 12 having a 6ilver coating 14. A silver coating
14 on a mica flake 12 constitutes, illustratively, 20 to 70%
by weight of the flake and coating.
This ~ilver coating on a mica flake s~-ves aisG to
illustrate a silver-coated mica flake containing at
least 44, and preferably at least 124, by weight silver. As
typical, the conductive composition is deposited on an
appropriate substrate 16.
In Figure 2, there is shown an electroconductive
element comprising a matrix of an organic material 42, in
this embodiment comprising a hardened acrylic resin, in this
instance Rohm 6 Haas B-66 acryloid resin, in which are
embedded mica flakes 20, having a silver coating 22.
Again, as typical, the electroconductive element is
deposited on and adheres to a substrate 44. A silver
coating 22 on a mica flake 20 constitutes, illustratively,
20 to 704 by weight of the flake and coating. Adjacent
silver-coated mica flakes are in intersurface contact with
one another at locations 24, or are close enough together,
at locations 26, 28, 30 and 32, so that electrons can pass
between them. It will be noted that the flakes overlap,
thus providing an advantageously large conductive area. At
location 34 the coated flake forms part of the surface of
the electroconductive element. Also, for instance at
locations 36, 38 and 40, silver-coated mica flakes are
sufficiently close to the surface of the electroconductive
element or substrate 44 to allow passage of the electrons
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125~330
between the surface and the silver-coated flakes. Thus,
conductive paths through the electroconductive element are
established.
Alternative embodiments of a flowable conductive
composition of the present invention also contain particles
of pure conductive metal and/or inorganic non-metallic
particles coated with same. Referring to Figure 3, an
embodiment of the conductive composition wherein both are
present is illustrated. The composition comprises an
organic binder S0, in this embodiment containing an acrylic
resin, again Rohm & Haas B-66 acryloid resin, in which are
suspended mica flakes 52 having a gold coating 54, alumina
granules 56 havinq a gold coating 58 and pure gold particles
in the form of flake~ 60. Gold coatings 54 and 58 on a mica
flake 52 and alumina granule 56, respectively, constitute,
illustratively, 20 to 70~ by weight of the total weight of
the coated flake and coated granule. The composition also
contains carbon black particles 62. As typical, the
conductive paste is deposited on an appropriate
substrate 63.
In Figure 4 is illustrated an electroconductive
body comprising a matrix of an organic material 108, in this
embodiment containing a hardened acrylic resin,
illustratively Rohm 6 ~aas B-66 acryloid resin, in which are
embedded mica flakes 64 having a gold coating 66, alumina
granules 68 having a gold coating 70, and pure gold
particles in the form of flakes 72. The incorporation of
gold (or other conductive metal) particles of other shapes
is also within the scope of this invention; however, flakes
lZS~330
of pure material are especially advantageous. The
electroconductive element also contains carbon black
particles 73. Again, ~s typical, the electroconductive body
is deposited on and adheres to a ~ubstrate 106. Gold
coatings 66 and 70 on a mica flake 64 and alumina granules
68, respectively, constitute, illustratively, 20 to 70~ ~;
weight of the total weight of the coated flake and coated
granule. Adjacent qold-coated mica flakes are in
intersurface contact with one another, at location 69, or
are close enough together, at location 71, so that electrons
can pass between them. Likewise, for adjacent gold-coated
alumina granules at locations 74 and 76, respectively, as
well as for adjacent gold flakes at locations 78 and 80,
respectively. Also, an adjacent gold-coated mica flake and
gold-coated alumina granule are in direct intersurface
contact at location 82, a gold-coated alumina granule and
gold flake at location 84; and, a gold-coated mica flake and
pure gold flake at location 86. (It will be understood
that, in comparison to overlapping of metal-coated flakes
which greatly enhances the conductive capacity of the
electroconductive element, the coated granules provide only
tangential or point-to-point contact, and are in that
respect somewhat less advantageous.) At location 88 a
gold-coated flake and gold-coated granule are sufficiently
close to allow passage of electrons. Such condition exists
between a gold-coated flake and pure gold flake at location
90 and between a gold-coated granule and pure gold flake at
- location 92. At locations 91 and 93 gold-coated mica flakes
form part of the element's surface. Additionally, for
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instance at locations 94, 96 and 98, a gold-coated mica
flake and qold-coated alumina granule and gold flake,
respectively, are sufficiently close to the surface of the
electroconductive body to allow passage of the electrons
between the surface and themselves. And, a gold particle, a
gold-coated mica flake and a gold-coated granule are
sufficiently close to the substrate, at locations 100, 102
and 104, respectively, to allow passage of electrons
therebetween. Accordingly, conductive paths through the
electronconductive element are established.
The illustrations of Figures 1 to 4 serve to
depict other electroconductive elements and precursor
conductive compositions, such as electromagnetic shielding
articles and precursor molding compositions. It will be
understood that in most instances such articles and molding
compositions are not attached to a substrate, and also are
on the order of about 1/8 inch thick (whereas the layers
illustrated in Figures 1 to 4 are typically about 30 microns
thick); otherwise Figures 1 to 4 accurately represent an
aforementioned shielding article and precursor composition.
The conductive metal incorporated in a flowable
conductive composition in accordance with this invention, as
its designation indicates, provides a conductive component
in an electroconductive element ultimately fabricated from
the conductive composition. Therefore, the amount and form
of the conductive metal in the composition are in large part
dependent on the properties desired for such an
electroconductive element.
lZS~330
The conductive metal i~ suitably copper or nickel,
or silver, gold, palladium, platinum or any other of the
noble metals. In some embodiments, silver, gold and/or
copper are preferred due to their advantageous conductivity
and/or relatively low cost. While the deposition of metal
pigments, such as titanium dioxide and iron oxides, on mica
and their employment in coloring has been mentioned in the
prior art, no use of conductive metal-coated mica flakes in
conductive applications has been found.
The coating of conductive metal on mica flakes of
the flowable conductive composition is a layer which covers
su~stantially the entire surface of each such flake. It is
preferred that this layer be of uniform thickness, but
thickness may vary from point to point on the flake surface
without departing from the present invention. The layer of
conductive metal need only be of sufficient thickness to
ensure the conductivity (at the level desired) of an
electroconductive element produced from the conductive
composition of the invention; however, thickness of the
layer may be increased above this minimum, for instance, to
the extent that cost considerations permit. Typically, the
thickness of this layer of conductive metal is a minimum of
100 angstroms.
In accordance with the invention, a conductive
metal-coated mica flake contains at least 4~, and up to 70~,
by weight conductive metal coating. It is preferable that
the conductive metal-coated mica flake contain at least 12%,
and especially at least 16~, by weight metal coating.
Incorporation in the flowable conductive composition of such
1254330
metal-coated flakes maintains good conductivity of a product
electroconductive element without appreciable impairment of
its electrical and other properties. It will be under~tood
that cost advantages afforded by the employment Of
conductive metal-coated mica flakes, as opposed to
employment of pure metal (such as silver) alone, are also
attendant to this embodiment.
The amount of conductive metal incorporated in an
electroconductive element in accordance with the present
invention ranges from 2 to 90% by weight. Depending on the
amount of organic binder and other materials which are
included in the flowable composition and survive formation
of the electroconductive element (a parameter easily
ascertainable by one of ordinary skill in the art), the
amount of conductive metal incorporated in the precursor
flowable conductive composition is up to 90~, but more
typically about 15% or less.
The conductive metal coating, when deposited on a
mica flake generally conforms to the contours of the flake,
and, therefore, the shape of the metal-coated flake
corresponds qenerally to the shape of the uncoated flake.
Including the metal layer, these coated flakes are of a size
which is compatible with the attainment of the desired
properties of the flowable conductive composition and
electroconductive element of this invention. That is, the
metal-coated flakes should be sized, for example, so that
incorporation of same in the flowable conductive composition
does not appreciably interfere with the ease of its flow
properties in connection with application to a substrate or
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12S4330
with injection, compression or extrusion molding, as
appropriate. Typically, the metal-coated flakes are of a
size from 0.1 to 200, preferably 0.5 to 44, microns in
maximum dimension.
In accordance with the foregoing, the metal-coated
flakes are pre~erably silver-coated mica flakes. Again,
typically, the silver-coated flakec, including the silver
layer, are preferably of a size from 0.5 to 44 microns in
maximum dimension. Further examples of appropriately sized
silver-coated mica flakes are those of a size from 44 to 74
microns and 74 to 200 microns.
The mica flakes, themselves, may be composed of
either natural or synthetic mica. Micas are a group of
laminated silica materials. The structures of micas have
been rather extensively treated in the art, as for instance
in L. Pauling, Proc. Nat'l. Acad. Sci., 16, 123 (1930); W.W.
Jackson and J. West, Z. Xrist., 76, 211 (1930); and J.W.
McCauley, R.E. Newnham and G.V. Gibbs, Am. Mineral., 58, 249
(1973). While having somewhat varying chemical composition,
all contain hydroxyl and/or fluoride radicals, a silicate or
germate group and an alkali or alkaline earth component.
Examples are biotite, muscovite, phlogopite, lepidolite,
fluorophlogopite, barium disilicate and lead disilicate.
Typically, mica has the following properties: specific
gravity 2.6-3.2; Mohs hardness 2.B-3.2; refractive index
1.56-1.60; dielectric constant 6.5-8.7; noncombustible; heat
resistant to about 600C.
Natural mica occurs in the Vnited States, Canada,
Madagascar, India, South Africa and South America.
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Synthetic mica is produced by any known or common technique,
for example, by qrowing a single crystal electrothermally.
An interesting example of a synthetic mica is
fluorophlogopite (i.e., a fluorine derivative of phlogopite)
which, illustratively, is made by a technique involving
melting raw materials co~.prising potassium silica fluoride,
alumina, silica, potassium feldspar and magnesia in an
internal resistance furnace and cooling the melt slowly down
through the crystallization temperature during which
crystals of the synthetic mica form. This product has a
higher temperature stability than natural mica, and its
dielectric properties and machinability are about the same.
As a group, micas are characterized by excellent
cleavage properties. ~ence, they can be split into very
thin flexible elastic sheets. This property affords control
over the thickness of the mica which is to be employed.
Cleavage is suitably effected by grinding a mica, but other
common or known methods are also acceptable. In such
manner, mica flakes of desired thickness can be obtained for
practice of the present invention. The aspect ratio of
these flakes is generally greater than 20.
Conductive metal-coating of the mica flakes is
suitably carried out by numerous means known in the art.
For example, the sil~er-coating is applied by fluidization
by dry or wet methods, by electroless plating, and the like.
See, for instance, U.S. Patent No. 3,635,824, granted
January 18, 1972 to Raymond G. Brandes et al.
A flowable conductive composition, such as a paste
or molding composition, containing conductive metal-coated
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lZ54330
mica flakes is advantageous in that it has favorable flow
and distribution properties. That i6 to ~ay, the paste or
molding composition of the invention flows sufficiently well
so that it is conveniently applicable to substrates, i6
suitable for injection molding without undue resi6tance,
etc. Likewise, assuming conventional techniques are
followed, this conductive composition is such that the
distribution of metal-coated flakes after application is
quite uniform, thereby minimizing local variations in
properties of an electroconductive element made from the
flowable conductive composition. Furthermore, the mica
flakes are quite resistant to breakdown, that is, breaking
apart into smaller particles during operations such as
injection, compression and extrusion molding.
And, due to the aforementioned overlapping, or
areal instead of point-to-point contact, of conductive
metal-coated mica flakes and their high surface area, the
total metal-coated-flake loading of an electroconductive
element made from the flowable conductive composition of the
invention is typically only about 20% by volume. Indeed,
this loading of the electroconductive element is, in some
embodiments, as low as 5~ by volume, on a cured basis.
However, in other embodiments it is cuitably as high as 90~,
preferably as high as 50%, especially as high as 40%, by
volume. Depending on the amount of organic binder and other
materials which are included in the flowable conductive
composition and survive formation of the electroconductive
element, the amount of these metal-coated flakes which is
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- l~S4330
incorporated in the precursor conductive composition is up
to 90~ by volume, typically about 30~ by volume or leEs.
While a metal-coated-mica-flake-containing
conductive composition is highly advantageou8, ~n some
embodiments of the invention inorganic non-metallic
particles coated with a conductive metal, conductive metal
particles, fi~er~ coated with a conductive metal, or a
combinàtion of two or more of the foregoing, are also
incorporated in the conductive paste.
The coating of conductive metal on inorganic
non-metallic particles or on fiber material is also a layer
which covers substantially the entire surface of each such
particle or fiber. Again, it is preferred that this layer
be of uniform thickness, but thickness may vary from
point-to-point on the particle or fiber surface without
departing from the present invention. The layer of
conductive metal need only be of sufficient thickness to
maintain conductivity ~at the desired level) in an
electroconductive element; however, thickness of the layer
may be increased above this minimum, for instance, to the
extent that cost consiâerations permit. Typically, the
thickness of this layer of conductive metal ranges up to 10
of the minimum dimension of an inorganic non-metallic
particle, and/or up to 10~ of the diameter of the fiber
material. It will be appreciated that even when conductive
fill material other than conductive metal-coated mica flakes
is incorporated in the electroconductive element and
precursor flowable conductive composition the total volume
loading of the element generally remains in the range of
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lZ5433U
from 5 to 90%, preferably 5 to 50%. Thus, if the element
and its precursor conductive composition incorporate
conductive metal-coated ~norganic non-metallic particles
and/or fibers in addition to conductive metal-coated mica
flakes, the volume ~ loading of the element ~nd precursor
composition with the coated mica flakes decreases
correspondingly. Likewise, since the total weight ~ loading
of the element with conductive metal generally is in the
range of from 2 to 90~ even when conductive filler material
other than conductive metal-coated mica flakes is
incorporated, the weight % loading of the element and
precursor composition with metal from the coated mica flakes
correspondingly decreases. It will be appreciated that with
the above-mentioned metal-coated inorganic non-metallic
particles and metal-coated fibers, the metal coating makes
up from 4 to 70% of the total weight.
Turning now particularly to the above-mentioned
conductive metal-coated inorganic non-metallic particles,
these preferably contain at least 8%, especially at least
12%, by weight metal coating. It is also especially
preferable that the metal be gold or copper. In some
embodiments, incorporation of these metal-coated particles
aids in attaining and maintaining good rheological
properties of the flowable conductive composition and good
conductivity of an ultimately formed electroconductive
element without appreciable impairment of its electrical
or physical properties, such as adhesion. It will
be understood that cost advantages afforded by the
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12S4330
employment of conductive metal-coated inorganic non-metallic
particles as opposed to employment of pure conductive metal
alone, are also attend~nt to these embodiments. It is al~o
within the scope of the present invention to ~ncorporate in
flowable conductive composition conductive metal-coated
inorganic non-m~tallic parti~les cont~ining up to 60~ by
weight metal coating, for example, of from 25% to 60~ by
weight thereof. And metal-coated particles containing
somewhat less than 25% by weight metal are suitable in
~everal advantageous embodiments. In some other especially
preferable embodiments, the metal-coated particles contain
from 4 to 16% by weight metal coating. Examples of these
are particles containing approximately 4%, 8%, 12~ and 16%
by weight metal coating.
The inorganic non-metallic particles, themselves,
are suitably irregular in shape, or, alternatively,
substantially regular in shape. Thus, these particles are,
for example, granules, spheres or spheroids. Since the
conductive metal coating generally conforms to the contours
of the particle, the shape of the coated particle
corresponds to the shape of the uncoated particle. The
coated particles are sized to be compatible with the
attainment of the desired properties of the conductive paste
and electroconductive body of this invention, as previously
and hereinafter described. Typically, the metal-coated
particles are of a size from 1 to 200 microns in maximum
dimension, on average.
The inorganic non-metallic particles are suitably
composed of any of a wide range of materials which exhibit
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lZS'~3;~0
properties and physical characteristics consistent with
attainment of the objectives of thi8 invention. In this
connection, it will be understood that ~non-metallic~ refers
to the properties And physical characteristic6 of these
materials, and does not preclude the presence of metal atoms
or ions as long as "non-metallic" properties and physical
characteristics are exhibited. Suitable materials typically
display non-electroconductive properties. Accordingly,
these materials are, typically, glasses, ceramic substances
and naturally occurring mineral substances. The following
are other examples of suitable materials: oxides, such as
bauxite, corundum, ilmenite, brookite, anatase, rutile and
magnetite, and hydroxides such as brucite; sulfides, such as
galena, pyrite, chalcopyrite and sphalerite: halides, such
as sodium chloride, sylvite and fluorite; carbonates such as
calcite, magnesite and siderite, nitrates, such as sodium
nitrate, and borates, such as borax and kernite; sulfates,
chromates and molybdates, examples being celestite,
anhydrite and gypsum; and phosphates, such as bivianite,
apatite and pyromorphite, arsenates such as erythrite, and
vanadates, such as bavanadinite. Additional examples of
suitable materials are conveniently classified into
categories as follows: the tectosilicates, including the
silica group, the feldspar group, the feldspathoid group,
the zeolite group; various philosilicates, including
kaolinite, talc and vermiculite; the inosilicates, including
the amphibole group, for instance the cummingtonite series,
the pyroxene group, including the hypersthene series, for
instance ~podumene, and the pyroxenoid group; the
-22-
12S~30
cyclosilicates including beryl and tourmaline; the
sorosilicate group, ~or instance, idocrase; the
neosilicates, including the olivine series, such AS
magnesium iron silicate, and also including willemite; the
aluminum silicate group; the garnet group; and silicates of
indeterminate struct~re such as prehnite, chrys~colla ar.~
dumortierite. It will be understood that synthetic, as well
as naturally occurring, inorganic non-metallic materials are
suitable for practicing this invention.
In general, the composition of the inorganic
non-metallic particles selected must be such that the
particles do not soften or appreciably distort in shape
under processing conditions to which the flowable conductive
composition of this invention is subjected in making an
electroconductive element therefrom.
The particles of inorganic non-metallic material
are produced in any known, or common, manner.
As with the aforementioned coated mica flakes,
conductive metal-coating of the inorganic non-metallic
particles is suitably effected by any of a range of means
known in the art. Silver-coating is, illustratively,
applied by fluidization by dry or wet methods, by
electroless plating, and the like. See previously cited
U.S. Patent No. 3,635,824.
Incorporation of conductive metal-coated inorganic
non-metallic particles, such as spherical or spheroidal
particles, can be advantageous in embodiments whPre enhanced
flow and distribution characteristics are especially
desired, since the coated particles can be adapted to be
-23-
i2si~330
particularly well-suited to impart such characteristics to a
flowable conductive composition. It i6 also possible that
the conductivity of an electroconductive element per unit
~ilver content can be increased by incorporating such
conductive metal-coated spherical particles. However, as
will be appreciated, the amount of such coated sphe i.al
particles incorporated should be carefully controlled. That
is to say, despite indications that certain conductive
bodies containing only ~ilver-coated spheres as conductive
filler material are more conductive per unit silver content
than a conductive element containing only silver-coated mica
flakes, the former conductive body constitutes a system
which is much too rigid for feasible application in the
fabrication of membrane switches; the material constituting
the button for the switch, if made from the coated-sphere
containing material is too rigid to be deformed so as to
make contact with a counterpart component of the switch. In
contrast, a conductive element in accordance with the
invention can provide a system which is amply flexible while
still being much more conductive than a system containing
only pure silver conductive filler.
As indicated previously, in some embodiments of
the invention a flowable conductive composition also
contains conductive metal-coated fibers, either in
combination with the conductive metal-coated mica flakes
solely or along with conductive metal-coated inorganic
non-metallic particles. Incorporation of such coated fibers
in some respects offers significant advantages. For
example, it provides a filler in the conductive element
-24-
-
lZS433(3
which has a high aspect ratio ~nd high ~urface area.
~owever, such incorporation also involves certain drawbacks:
as previously indicated these fibers can break thereby
altering the properties ~f an electrocondu~tive element, and
further they at best afford line-to-line contact with other
filler materials thereby decreasing potential conductive
efficiency. With the claimed invention, these drawbacks can
be effectively minimized. Incorporation of conductive
metal-coated mica flakes along with these coated fibers
increases conductivity because the coated flakes have an
areal overlap instead of just line-to-line contact; thus,
improved conductivity is obtained. Also, coated flakes are
more resistant to breakdowr. than appropriately coated
fibers. Thus, coated fiber-containing flowable conductive
compositions and electroconductive elements are rendered
highly advantageous in certain applications with the present
invention.
Conductive metal coated fibers suitable for
practicing the claimed invention are commercially available
and well-known to those skilled in the art.
As with mica flakes, the conductive metal coating,
when deposited on a fiber, generally conforms to the fiber
contours, and hence the shape of the coated fiber
corresponds to the shape of the uncoated fiber. It will be
~ppreciated that these coated fibers are of a ~ize and shape
which is compatible with the attainment of the desired
properties of the flowable conductive composition and
-25-
12S~330
electroconductive element of this invention, for instance, so as
not to interfere appreciably with the ease of application of the
composition to a substrate, or with its flow properties during
injection, compression or extrusion molding.
The fibers are suitably composed of any of a wide range
of materials which exhibit properties and physical
characteristics consistent with attainment of the objectives of
this invention. Such materials are, typically, glasses, such as
fiberglass, ceramic substances and naturally occurring mineral
substances. The following are examples of suitable materials:
glass, asbestos, amphibole and wollastonite. It will be
understood that synthetic, as well as naturally occurring,
inorganic materials are suitable for practicing this invention.
Also suitable are various metals such as aluminum, copper,
nickel.
Conductive metal coating of the fibers -is suitably
effected by means known in the art, as previously discussed.
As mentioned above, in various embodiments of the
invention a portion of conductive metal, such as gold, platinum,
palladium or copper is incorporated in the flowable conductive
composition as particles of substantially pure metal.
Preferably, the coating metal is the same as that which
constitutes the particles, and the particles are in the form of
flakes. However, it is within the scope of this invention for
the particles to be of other shapes. Particularly in cases in
which the conductive metal coating is of relatively low amount,
the particles typically make up 0.5% to 40% by weight of the
flowable conductive composition. In some embodiments, the
particles constitute at least 10% by weight of the paste.
-26-
lZS~3;~0
For purposes of some embodiments of this invention the
conductive metal particle suitably is a composite of more than
one metal, in which a conductive metal coats a core of another
metal or metals. Thus, suitable conductive metal particles
comprise, for example, iron coated with copper or with a noble
metal, such as silver or gold. The entire particle, its outer
layer or its core is sometimes an alloy; brass and stainless
steel are examples, although other common alloys such as those of
gold, platinum, palladium and or copper, can be employed. In
some embodiments of the invention, employment of this type of
conductive metal particle affords opportunity for cost economies
in that a large amount of conductive metal is replaced by a more
readily available and/or less expensive core.
The total amount of conductive metal incorporated in the
flowable conductive composition is at least 2% by weight of the
composition. This amount of conductive metal is incorporated in
the form of coating(s) on the mica flakes (and inorganic
non-metallic particles and fibers, if any) and conductive metal
particles - again, if any. In some embodiments, more than one
elemental metal and or alloy is suitably incorporated to
constitute the total amount. It is preferable to incorporate
enough conductive metal in the composition to constitute at least
5%, especially at least 10%, by weight of the composition.
In some embodiments of the invention the conductive fill
material in the flowable conductive composition co~prises, in
addition to conductive metal-coated mica flakes and optionally
one or more of the other conductive fill materials described in
preceding paragraphs, particles of carbon black. The carbon
black need not be specially treated, and may illustratively be
-27-
~ 125~33()
used directly as it is produced "in the furnacen. Virtually
any type of carb~n black is ~uitable, as long as the size of
the particles thereof do not interfere with the desired flow
characteristics of the conductive composition. Suitably
sized particles are typically of from 20 to 30 microns in
ma>;imum dimen5ior.. Examp~-s Gf suitable carbon biacks are
those sold under the trade marks Ketjen BlacX EC,
~ulcan XC-72 and Acetylene Black.
The flowa~le conductive composition also contains
an organic binder from which the matrix of organic material
of an electroconductive element made from the composition is
formed. The organic binder suitably comprises an inert
organic material or materials formable into the matrix; the
binder imparts to the composition the proper rheology, for
instance, an appropriate consistency for application on a
substrate by screening, painting (e.g., electrostatically or
with a brushl, dipping (followin~ rack loading), continuous
machine dipping, and the like, or for injection, c~mpression
or extrusion molding operations.
In many embodiments the organic binder contains
one or more resins and one or more solvents to give the
conductive composition the desired consistency, but in some
embodiments, for instance in molding compositions, the
organic binder is typically solventless. Examples of
suitable substances are low molecular wei~ht aliphatically
unsaturated organic polymers, or a mixture of an
aliphatically unsaturated organic polymer and a
copolymerizable aliphatically unsaturated organic monomer,
such as styrene. These substances, illustratively, have a
.~;. . .~.
125~330
viscosity of from about 50 to 10,000 centipoises at 25C.
Particularly ~s to ~molding composition~ - embodiments of
the invention, examples of suitable organic binders are
polypropylene, polystyrene, high density polyetbylene,
polyvinyl chloride and nylon. Additional examples ~re: low
molecular weight polyim~des containing acrylamide
unsaturation, for instance as described in U.S. Patent No.
3,535,148, granted October 20, 1970 to Abraham Ravve; low
molecular weight polyesters containin~ acrylic unsaturation,
such as shown in U.S. Patent No. 3,567,494, granted March 2,
1971, to Chester W. Fitko; acrylate esters, and methacrylic
esters of polyhydric alcohols, for instance as set forth in
U.S. Patent Nos. 3,551,246 and 3,551,235, granted December
29, 1970 to Robert W. ~assemir et al. (see also U.S. Patent
No. 3,551,311, granted December 29, 1970 to Gerald I. Nass
et al.); acrylate and methacrylate esters of silicone
resins; malamine; epoxy resins; allyl ethers of polyhydric
alcohols; allyl esters of polyfunctional aliphatic and
aromatic acids; low molecular weight maleimido substituted
aromatic compounds; cinnamic esters of polyfunctional
alcohols; mixtures of two or more of the foregoing: and the
like. Further examples are unsaturated polymers, such as
polyesters from glycols and ~-, g-saturated dicarboxylic
acids, for instance maleic and fumaric acids, either with or
without other dicarboxylic acids free of~~,B-unsaturation,
for instance phthalic, isophthalic and succinic acids,
dissolved in a copolymerizable aliphatically unsaturated
organic solvent, such as styrene, vinyl toluene, divinyl
benzene, methyl methacrylate, or mixtures of such solvents;
-29-
`` 1~5~330
~uch systems are set ~orth in U.S. Patent No. 2,673,151,
granted March 23, 1954 to Howard L. Gerhart and U.S. Patent
No. 3,326,710, granted June 20, 1967 to Mary G. Brodie.
Some other examples ~re unsaturated organosiloxanes of from
5 to 18 ~ilicon atoms, and such siloxanes in combination
with a vinylic organic monomer. Illustratively, the org2nic
binder is an acrylic resin or an epoxy resin. Examples of
suitable acrylic resins are methacrylate polymers. Examples
of suitable epoxy resins are any monomeric, dimeric,
oligomeric or polymeric epoxy material containing one or a
plurality of epoxy functional groups, for instance
bisphenol-A and diglycidyl ether. In a particularly
preferred embodiment, the organic binder is an acryloid
resin, i.e., a synthetic polymer of acrylic acid ester.
Suitable solvents are coal tar hydrocarbons,
chlorinated hydrocarbons, ketones, esters, ether alcohols
and ether esters. Examples are xylene, toluene, methylethyl
ketone and alcohols, such as aliphatic alcohols of up to 20
carbon atoms, for instance ethanol and propanol. The
organic binder may also contain various common additives
such as catalysts and substances which sensitize the binder
to radiation, for example, ultraviolet radiation. The
sensitizers, for example, are suitably incorporated in small
amounts, such as 0.5 to 5~ by weight of the binder.
Examples are ketones, such as benzophenone, acetophenone,
and the like, benzoins and substituted benzoins, thiourea
and aromatic disulfides: also examples are azides,
thioketones and mixtures thereof. The binder is
incorporated in the paste in an amount suitable to impart
t:
-30-
125~330
the above-discussed desired flowability or rheology, for
instance in an amount up to 35 to ~0~ by weight of the
paste, but sometimes as low as 15~, and occasionally even
down to from 5 to 10~, by weight of the paste.
The flowable conductive composition i6 made, for
example, by co~bining condu_tive m~tal-coated mica flakes,
as well as conductive metal-coated inorganic non-metallic
particles, conductive metal-coated fibers, conductive metal
particles and/or carbon black in some embodiments, and an
organic binder formable into a matrix. For example, if
conductive metal particles are incorporated in a conductive
paste, they - in admixture with the organic binder - can be
wetted in a three-roll mill; the coated mica flakes, along
with the coated inorganic non-metallic particles, fibers
and/or carbon black (if employed), can be incorporated and
appropriately mixed into the organic binder (or such binder
containing conductive metal particles) in a suitable
apparatus, for example, a SPEX mixer or a common paint
shaker. Alternatively, in forming a molding composition
containing such conductive metal particles, these particles
along with an organic binder and mica flakes coated with
conductive metal are introduced into a ~compounder~
apparatus to effect mixing. The resulting flowable
conductive composition, as such, is then molded into a
desired shape, or applied to a substrate, for example, a
capacitor or a resistor or other dielectric component, etc.,
in connection with the fabrication of a further circuit
component, or is packaged, and stored or shipped for
subsequent use.
-31-
12S~330
In accordance with this invention, ~n
electroconductive element i6 fabricated from a flowable
conductive composition by subjecting the compositions to
conditions sufficient to form the binder into a matrix in
which the metal-coated flakes, etc. are embedded. During
formation of the matrix, any solvent component present in
the organic constituent is for the most part, preferably
completely, eliminated.
Typical techniques and conditions for forming the
matrix from the organic binder are: air-drying of the
flowable conductive composition at room or elevated
temperature; heating of the composition up to a temperature
of about 350C for a time sufficient for matrix formation:
ultraviolet irradiation of the composition; catalyzed curing
of the composition at a temperature within a range suitable
for operation with the selected catalyst. Other commonly
practiced methods for forming the organic binder into a
matrix, for instance curing, are also suitable. The
resultant matrix is an organic material produced by the
action of the selected forming technique and/or co~ditions.
Thus, the organic matrix is suitably a material formed from
a resin or resins, as previously described, in the organic
binder, such resin or resins being polymerized,
cross-linked, or the like to make up the matrix. It will be
understood that the technique and conditions selected for
forming of the matrix are dependent on the type of organic
binder employed and that such technique and conditions
should cause formation of a suitable matrix without
deforming the conductive metal-coated mica flakes and any
-32-
l~S4330
other such filler material present, or otherwise sltering
the components of the ~ystem, 80 as to impede performance,
especially conductivity, o~ the electroconductive element.
Regarding those embodiments of the invention in
which the flowable conductive composition is a conductive
paste or paint or the like suita~le for application to a
substrate, such as a ceramic multi-layer capacitor, it is
within the scope of the invention to form an
electroconductive body from the paste, etc. directly on a
substrate to which the paste has been applied. For example,
a paste is deposited on a substrate and suitably air-dried,
heated, irradiated, catalytically cured or fired alGng with
the substrate to which it has been applied. It will be
understood that conditions for forming the electroconductive
body directly on the substrate are the same as those set
forth previously for such formation, with the additional
consideration that forming techniques or conditions should
not damage or deform the substrate, or, for that matter the
conductive metal-coated mica flakes, inorganic non-metallic
particles or fibers, and conductive metal particles. Thus,
the conductive paste, etc. of the invention is typically
deposited on a substrate made of insulatin~ material to form
a conductive circuit thereon, or on a capacitor as
"termination paste" or sandwiched around a dielectric
material to provide terminal or internal conductive members
of various capacitor components. Application is suitably
effected by any known or common technique, for instance,
screen-printing, doctor-blading or spraying. In this
manner, conveniently sized and used capacitors, often termed
-33-
lZS4330
"chips~, can be obtained for packaging for later use, for
encapsulation in a hermetic package of glass Iwhich,
generally, itself requires firing) or an organic system, for
dipping in solder to provide components which are readily
integrated into circuits as desired, or for leading lusually
by solder-dipping~.
In some other embodiments, such as forming
electromagnetic shielding, the flowable conductive
composition is a molding composition and is formed into a
separate integral body, shaped as desired, by injection,
compression or extrusion molding or the like. The
considerations for formation are the same as mentioned
above, except of course that there is no substrate. As to
these embodiments especially, an aforementioned significant
advantage of incorporating coated flakes over fibers or
coated fibers in the paste is brought out; the flakes are
more resistant to breakdown, that is to say breaking up into
smaller particles, than are fibers. Since breakdown changes
the aspect ratio of the filler flakes and fibers, thereby
modifying performance of the final electroconductive
element, minimization of breakdown is desirable.
As previously indicated, another aspect of this
invention is an electroconductive element which comprises a
matrix of organic material with conductive metal-coated mica
flakes embedded therein. In this embodiment, the conductive
metal-coated mica flakes are in effective contacting
relationship to define one or more electroconductive paths
through the matrix. For the purpose of this invention,
~effective contacting relationship~ means that coated flakes
-34-
iZS~330
adjacent one another are in direct intersurface, generally
areal contact, or close enough ~o that electrons can pass
from one to the next. This is illustrated in Figure 2. For
those embodiments in which conductive metal-coated inorganic
non-metallic particles, conductive metal particles,
conductive metal-c~ated fibers and/~r carbon black are
additionally i~corporated as fill material, then effective
contacting relationship exists for a sufficient number of
adjacent fill material bodies to establish one or more
conductive paths through the matrix. It is not necessary
that such relationship be between filler material of the
same type, e.g., coated flake/coated flake, but is suitably
also between filler materials of different types, e.g.,
coated flake/coated fiber, coated flake/conductive metal
particle, coated flake/carbon black particle, etc. This is
illustrated in Figure 4.
The size of the conductive metal-coated mica
flakes, conductive metal-coated inorganic non-metallic
particles, conductive metal-coated fibers and conductive
metal particles and carbon black particles does not change
appreciably during the fabrication of the electroconductive
body from the flowable conductive composition.
Illustratively, in an electroconductive element the coated
mica flakes are of from 0.1 to 200 microns in maximum
dimension, inorganic non-metal~ic particles are typically of
a si~e from 1 to 200 microns in maximum dimension.
Preferred sizes are as previously discussed.
Similarly, the distribution and thickness of the
metal layer on the mica flakes, inorganic non-metallic
-35-
lZS'~330
particles and/or fibers, as well as the amount of metal in
the layer, is not appreciably altered in this fabrication.
~herefore, as previously indicated, the thickness of the
metal layer is preferably substantially uniform; but it is
consistent with practicing of this invention that the
thickness of such layer vary over a surface, as long as
conductivity of the electroconductive element is not
appreciably impaired. Typical and preferred thicknesses are
as set forth in the foregoing discussion.
In many embodiments, about 5 to 90%, preferably 5
to 50~, of the total volume of the electroconductive element
is made up of conductive metal-coated mica flakes or such
coated flakes in combination with coated inorganic
non-metallic particles, fibers and/or carbon black
particles. ~he total amount of conductive metal in the
electroconductive element suitably ranqes from at least 2~,
preferably at least 10%, by weight, up to 90~ by weight. In
some embodiments it is desirable that the total amount of
conductive metal be at least 25~, and even at least 40% by
weight of the element. Suitable for many applications is an
electroconductive element incorporating conductive
metal-coated mica flakes, optionally with the aforementioned
metal-coated inorganic non-metallic particles, fibers and/or
carbon black particles, wherein the metal constitutes less
than 25%, illustratively of from 4 to 16~, by weight.
Examples are electroconductive elements containing
conductive metal-coated flakes, coated inorganic
non-metallic particles, coated fibers and/or carbon black
-36-
lZS4330
particles wherein conductive metal constitutes 4~, 8~, 12%
and 16~ by weight.
As to embodiments involving inclusion of
conductive metal particles, it will be undertstood that the
less the amount of conductive metal incorporated as flake-,
particle- or fiber-coG~ins, the grea er the amou-t cf su^h
metal which i8 incorporated as substantially pure particles,
preferably flakes, to achieve the desired electrical and
other properties. In this connection, particularly where
the amount of conductive metal in the coatings is relatively
low, the amount of conductive metal in the conductive metal
particles in the electroconductive element advantageously
constitutes at least 5% by weight of the element; in some
embodiments it is preferable that conductive metal particles
constitute at least 10%, and even at least 25%, by weight of
the electroconductive element. However, it is generally
preferred that as much conductive metal as feasible be
incorporated in the form of coatings on mica flakes,
inorganic non-metallic particles and/or fibers, so as to
maximize the amount of such metal available for conducting
electrons in the element.
The total amount of conductive metal present in
the electroconductive element of this invention is selected
based on the performance characteristics desired for the
component in which the element is to be used. Thus, the
conductivity, adhesion to a selected substrate,
solderability, ~older-leaching resistance, dissipation
factor and the like, which are required of the
electroconductive element to perform compatibly with other
-37-
12S~330
materials, should be considered in selecting the total
amount of conductive metal. In general, all of the
foregoing characteristic6 will be enhanced by increasing the
total amount of conductive metal in the electroconductive
element. It will be understood that, even with increased
conductive metal content to enhance one or more properties
of an electroconductive element, the electroconductive
element of the claimed invention generally exhibits
comparable or superior properties at lower cost than with an
element in which the conductive component is solely silver,
or other conductive metal, particles.
Another factor to be taken into account in
selecting materials for some embodiments of this invention
is formation conditions which the flowable conductive
compositions will be required to undergo in connection with
the fabrication of the electroconductive element. It is
sometimes the case that a relatively high firing temperature
will be necessary, for instance, to effect adhesion of an
electroconductive body to a substrate with which it is used.
Generally, the temperature at which the precursor conductive
composition can be fired is increased by employing, in
addition to mica, refractory materials capable of
withstanding higher temperatures, for example silica,
feldspar or bauxite.
It is a distinct advantage that flowable
conductive compositions and electroconductive elements of
this invention comprise, in significant part, conductive
metal-coated units having cores of mica, inorganic
non-metallic material or appropriate fiber material. This
-38-
l~S~330
is in direct contrast to those prior conductive pastes
wherein the conductive metal component was present entirely
in the form of pure particles. In 6uch prior conductive
pastes containing silver, that metal was typically
incorporated in amounts of from 60 to 85~ by weight. Due to
employment of the above-mentioned cores a significant amount
of the volume previously occupied by conductive metal is
now taken up by much less expensive material, thereby
affording a significant cost saving. Nevertheless, despite
the substitution of other material for a significant amount
of the conductive metal employed in prior paste
compositions, the conductivity of an electroconductive
element made with the flowable conductive composition of the
invention is at least as ~reat as that of an
electroconductive element containing 90~ by weight silver,
and made from a prior paste. Furthermore, this
electroconductive element exhibits favorable electrical and
other properties, such as conductivity, adhesion to a
substrate and durability. Additionally, the invention
affords convenience and cost advantages since materials
employable in the flowable conductive composition and
electroconductive element are easily obtainable and often
relatively inexpensive. Thus, the objects of the invention
are fulfilled in the provision of an electroconductive
element exhibiting favorable performance characteristics,
which is conveniently fabricated from available materials at
comparatively low cost.
A greater understanding of the invention may be
gained from the following examples.
-39-
l~S~330
An electroconductive element containing 23.59
weight % silver and 14.67 volume % silver-coated mica flakes
was fabricated in accordance with the invention as follows.
27.21 g. of acrylic resin, specifically ~ohm ~ Haas type
B-66 ACRYLOID Resin (50% solids by weight) and 10.11 g. of
silver-coated mica flakes (the mica flakes were of size -325
U.S. mesh plus pan, and were water-ground natural mica) in
which the silver constitutes 57.2% by weight of the coated
~lake were each weighed on a SARTORIUS digital top-loading
balance (Model #1202MP) to the nearest 0.01 gram. ACRYLOID
and SARTORIUS are trade marks. The acrylic resin and
silver-coated mica flakes were mixed together by hand.
Toluene was added as necessary to adjust the viscosity to
approximately 6500 centipose. The mixture was poured onto a
glass plate which had been coated with a silicone release
agent, namely Silicone Spray Mold Release from Mark V
Laboratory, Inc. The mixture was doctor-bladed at a blade
height of from 0.02 inches to 0.040 inches using a Gardener
Knife of about 3-inch width from Paul N. Gardener Company.
The mixture was dried overnight at room temperature, and
then cured in an air over for at least 48 hours at 75C to
remove all solvent. The dried mixture was then removed from
the glass plate and cut into 4-inch long by 1-inch wide test
strips. The resistance of a sample strip was tested using
either a Hewlett-Packard Model #4328A Milliohmmeter or a
Keithley Instruments Model #610C Electrometer. The exact
thickness, length and width of a sample was measured wi h
micrometer, and the volume resistivity calculated accordingly.
The average volume resistivity was 0.015 ohm-cm.
-40-
~`
.~
lZS4330
The data relating to the foregoing experiment are
set forth in the following t~ble, to the right of thP
notation Example 5A. Additional experiments were performed
wherein one or more of the weight ~ ~ilver in the
silver-coated mica flakes, weight ~ silver in the co~ductive
element and volume ~ conductive filler in the conductive
element were varied. Data relating to these experiments are
also set forth in the table, following the designations 5B,
5C, 6A-6D and 7A-7D. The electroconductive elements
evaluated in these experiments were produced in the same
manner as set forth above, save for appropriate adjustment
of the amount of silver in the silver-coated mica flakes,
and the respective amounts of silver-coated mica flakes and
B-66 Acryloid Resin. Resùlting volume resistivities (which
are, in each case, an average of volume resistivities
obtained with 4 or 5 different strips) are noted in the
right-most column of the table.
Additionally, several test samples were obtained
in the foregoing fashion, with the exceptions that pure
silver flakes (these flakes were obtained from Metz
Metallurgical Laboratories and are generally considered
suitable for thick film paste uses) were substituted for
silver-coated mica flakes and the amounts of silver flakes
and acrylic resin were adjusted appropriately to obtain the
data set forth in the table following the designations
Examples 8A, 8B and 8C.
-41-
li~S4330
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~ ¦ ~ ~ X ~ a ~ ~ ~ a ~
l~S4330
Figures ~ and 6 illustrate graphically the results
of the above-described experiments. Figure 5 is a plot of
volume resistivity of the electroconductive elements
against the corresponding volume ~ of conductive filler in
those elements. Figure 6 is a plc~ of volume resistivity of
each electroconductive element against the correspondlng
weight 4 of silver in the element. As can be seen, with the
present invention a wide range of volume resistivities can
be obtained by varying the amount of silver in the
silver-coated flakes, the weight of silver in the
electroconductive element and/or the volume of the
conductive filler in the electroconductive element; in
contrast, when pure silver flake is used as the conductive
filler the volume resistivity drops precipitously over a
rather narrow range of weight % of silver and volume 4 of
conductive filler in the electroconductive element. This
illustrates the relative ease with which electroconductive
elements in accordance with the present invention can be
fabricated to exhibit a specific volume resistivity - that
is, the volume resistivity of such an electroconductive
element is not disadvantageously sensitive to minor
variations in weight ~ of silver or volume ~ of conductive
filler in the electroconductive element. Furthermore, it
can be seen that comparably low volume resistivities to
those obtained with electroconductive elements containing
pure silver flake alone can be obtained with the present
invention, in many instances using less than half the amount
of silver in the electroconductive element. This is a major
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lZS4330
factor in reducing cost through practice of the present
invention.
~ he terms and expressions which have been employed
are used as terms of description and not of limit~tion, and
there is no intention in the use of such terms and
expressions of excluding any equivalents of the features
shown and described or portions thereof, it being recognized
that various modifications are possible within the scope of
the invention.
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