This invention relates generally to electrically conductive materials
and, more particularly, to high-temperature electrically conductive materials that
are curable at low temperatures.
Conductive adhesives and pastes are well known. Generally, these
materials include an organic polymer binder material filled with varying amounts
of different types of conductive metals. U.S. Patent No. 6,156,237 discloses an
example of one such material. Specifically, this patent describes a conductive
paste that includes an organic binder, an inorganic glass or ceramic powder, a
conductive metal, and a mono-ol compound.
One problem of organic-based conductive materials is their tendency
to exhibit fairly high curing temperatures and relatively low operating temperatures.
The material of the '237 patent, like other materials of this type, requires a
high curing temperature of about 850 °C. Even the best performing materials of
this type, however, generally have operating temperatures that do not exceed 250
°C. Also, materials of this type, while electrically conductive, generally have
fairly high resistance values in the range of about 30 Ω/cm to about 50 Ω/cm.
Furthermore, these materials often lack good adhesion to various surfaces, especially
ceramics, and many lose their electrically conductive and adhesive properties over
It should be noted that the curing temperature for a material is a
temperature necessary to transform a green material, or uncured material, into
a material having a desired set of characteristics and properties. The operating
temperature refers to an upper temperature limit below which a given material
maintains a particular property or characteristic. For example, an operating temperature
may be marked by a temperature where a material melts or begins to soften to a
point where desired structural characteristics of the material are degraded below
a predetermined level. In addition to structural properties, the operating temperature
may be related to any temperature-dependent characteristic of a material.
Another category of electrically conductive materials, namely aluminum-filled
phosphate coatings, is also known. These materials, however, are available only
as thin coatings. Further, they require high temperature curing (about 650°C -
about 850°C) and need bead peaning to develop electrical conductivity.
The present invention solves one or more of the problems associated
with the methods and materials of the prior art and provides a high-temperature,
electrically conductive material having a low curing temperature and low resistance.
Summary of the Invention
One aspect of the present invention includes a method of making an
electrically conductive material. This method includes supplying a phosphate binder
and adding Ag particles to the binder to obtain a mixture including Ag in an amount
of between about 8% to about 70% by volume. The mixture is dried for a predetermined
length of time. The dried mixture is then cured.
A second aspect of the present invention includes an electrically
conductive material. This material includes a phosphate glass having a generic
chemical formula of AB(PO4), where A is selected from Al, Fe, and oxides
thereof, and where B is selected from Cr, Mo, and oxides thereof. Ag particles
are dispersed within the phosphate glass in an amount of between about 8% to about
70% by volume.
Brief Description of the Drawings
The accompanying figure, which is incorporated in and constitutes
a part of this specification, illustrates data relating to exemplary embodiments
of the invention and, together with the written description, serves to explain
the principles of the invention. In the drawings:
The sole figure illustrates resistance and conductance data, for an
exemplary embodiment of the invention, as a function of percent Ag by volume.
In the following description, exemplary embodiments in which the
invention may be practiced are described in sufficient detail to enable those skilled
in the art to practice the invention. It is to be understood that other embodiments
may be utilized and that changes may be made without departing from the scope of
the present invention. The following description is, therefore, not to be taken
in a limited sense.
As a general overview, the process for making an electrically conductive
material according to an exemplary embodiment of the present invention includes
first preparing a phosphate binder. Next, Ag particles are added to the phosphate
binder. At this stage, the consistency of the green material, which is an uncured
mixture including the Ag particles dispersed in the phosphate binder, may be adjusted
to suit a desired application of the material. Details relating to varying the
consistency of the green material are provided below. Further, if desired, the
green material may also be formed into a predetermined shape. The green material
is then dried and subsequently cured. During the curing step, the temperature of
the green material is slowly raised. The rising temperature forces the release
of any water remaining in the mixture after the drying step. Ultimately, the temperature
reaches a "false melt" temperature. At this temperature, an irreversible structural
change occurs in the green material. Tightly held water is released from the green
material, which allows a reconfiguration of the chemical bonds between the constituents
of the mixture. Subsequent to achieving the false melt temperature, the green material
hardens into a stable, electrically conductive material.
Returning to the details of preparing the phosphate binder, preparation
of the phosphate binder may begin with a solution of phosphoric acid and water.
Adjusting the pH of this solution will affect the physical characteristics of the
phosphate binder, which directly influences the physical characteristics of the
green material. In general, as the pH is decreased, the resulting green material
will be softer due to the retention of additional water within the structure. For
example, a pH of approximately 0.85 will yield a green material that remains flexible
and pliable even after drying. As the pH is increased, however, the resulting green
material becomes denser, and upon drying, the green material eventually becomes
hard and non-pliable.
Once the desired pH of the phosphoric acid-based solution has been
obtained, a first metal oxide is dissolved into the solution. In one exemplary
embodiment, this first metal oxide may include chromium oxide. In yet another embodiment,
molybdenum oxide may be substituted for chromium oxide. Next, a second metal oxide
is added to the solution. In an exemplary embodiment, this second metal oxide may
include aluminum oxide. In yet another embodiment, the second metal oxide may include
iron oxide. The second metal oxide may be added to the solution in forms ranging
from a solid block of material to nanometer-scale particles.
The second metal oxide slowly dissolves into the solution. As it
dissolves, hydrogen atoms of the phosphoric acid are replaced with metal ions from
both the first and second metal oxides, thus liberating hydrogen atoms. Over time,
the mixture develops an amorphous, glass-like structure through substitution of
the hydrogen atoms in the acid. The presence of the first metal oxide encourages
the growth of the glass structure by interrupting crystal formation that may otherwise
occur. The reaction is suitably complete when no further gas is evolved from the
mixture and a skin forms over the solution upon exposure to air. Any unreacted
solids are centrifuged out, and the resultant syrup-like liquid represents the
phosphate binder. In general, the phosphate binder has a chemical formula AB(PO4),
where A is selected from one of Al, Fe, and oxides thereof, and B is selected from
one of Cr, Mo, and oxides thereof.
As the next step of forming the electrically conductive material of
the present invention, Ag particles are added to the phosphate binder. The Ag
particles have a size of less than about 5 µm. While particle sizes of between
about 1 µm to about 5 µm distribute easily within the phosphate binder and are
suitable for use in various embodiments of the present invention, particle sizes
of less than 1 µm are also useful. The Ag particles are added to the phosphate
binder in an amount of between about 8% to about 70% by volume.
At this stage, the phosphate binder and Ag particle mixture may take
on the consistency of a thick paste. Optionally, the consistency of the mixture
may be adjusted by adding acidified water (e.g., a solution of water and phosphoric
acid) to the mixture. Through addition of the acidified water, the viscosity of
the mixture may be reduced. The reduced viscosity is useful, for example, in coating
various surfaces with the phosphate binder and Ag particle mixture. The additional
acid present in the mixture may even aid in producing a stronger false melt during
curing. It is possible, however, that too much acidified water at this stage can
actually hinder the occurrence of the false melt transition. In general, an addition
of acidified water in an amount of up to about 10-15% by volume will not impede
the false melt process.
In addition to the possibility of adding acidified water to the mixture
of Ag particles in the phosphate binder, other additives may optionally be included
in the mixture. For example, in one exemplary embodiment, BN may be added to the
phosphate binder along with the Ag particles. Once the material has been cured,
the presence of BN aids in the reduction of the coefficient of friction at the
surface of the material. Thus, parts formed of the electrically conductive material
including BN exhibit superior wear properties and are useful in many applications
requiring low-friction, conductive materials. BN may be added to the phosphate
binder/Ag particle mixture in an amount of up to about 5% by volume. Depending
on a given application, other materials that reduce the coefficient of friction
of the of the material may be either substituted for or added to the phosphate
binder/Ag particle mixture.
Once all of the desired additives have been incorporated into the
phosphate binder and the resulting mixture has the desired consistency, the mixture
may be formed into a desired shape. For thinner consistencies, the mixture may
be painted onto a surface to serve as an electrically conductive coating on, for
example, an electrode. It is also possible to construct an electrode or other elements
by molding and/or forming the mixture into a desired shape. As yet another possibility,
thicker pastes may be spread into cracks on various surfaces to serve as a surface
repair material. Additionally, the material may be lumped onto a surface, and a
mold of a desired shape may be pressed down upon the material such that the resulting
structure, having the desired shape, is formed directly on and adheres to the surface.
Once the material has been configured according to the desired application,
the material is dried at a temperature of up to about 100 °C for a predetermined
length of time. For example, in an exemplary embodiment, the material could be
dried for one or more weeks. Drying times, however, vary depending on the accuracy
of the binder mixture, the amount of water lost from the binder during processing,
the dimensions of the part being formed, and oven configuration, etc. Therefore,
drying times of significantly less or significantly more than two weeks may be
possible. By drying the material, a sufficient amount of water is removed from
the element to form a stable unitary mass. For example, after drying, the material
may include a moisture content of about 0.5% to about 1% water by volume. It is
even possible to re-hydrate the material after drying by placing the material into
a humidity chamber.
During drying, pressure may be applied to the material, through a
die of a mold for example, to densify the material to a predetermined porosity
level and to deform the material to predetermined final dimensions. As discussed
previously, subsequent to drying, the material may exhibit a range of structural
properties depending on the conditions of the initial preparation of the phosphate
binder, as well as whether or not any additional acidified water was added after
forming the phosphate binder/Ag particle mixture. For example, the material may
be flexible and pliable, or it may be more rigid.
Once the material has been dried, it is ready for curing. The curing
process proceeds by ramping the temperature of the material upward such that the
mixture is ultimately subjected to a curing temperature of greater than about 180
°C, which is the approximate temperature where the false melt transition occurs,
but less than about 230 °C. In the exemplary embodiment, the temperature is increased
to the false melt transition temperature, or moderately above, over approximately
one hour. Of course, this time will vary according to shape and configuration of
the material being cured. For example, the temperature of thin films may be increased
more quickly than for thicker parts having complex shapes. By slowly increasing
the temperature of the material over, for example, one hour, water that is trapped
within the structure of the material is allowed sufficient time to diffuse through
the material as molecular water without damaging the material. For example, if
the temperature of the material is increased too quickly, steam could form and
could lead to fracture or rupture of the material. Once the false melt transition
has occurred and the material is cooled, it is ready for use.
The term "false melt" refers to a change in the material upon heating
to a specific transition temperature. At or about this transition temperature,
the material temporarily takes on plastic properties and mimics a melt. Unlike
a true melt, which occurs at a much higher temperature and where the composition
of the material is unchanged, some material is lost during the "false melt". In
theory, at the false melt transition temperature, sufficient energy has been introduced
into the system to release chemical bonds and/or tightly held water that are not
affected by drying at lower temperatures. The phosphate binder, which is still
partly hydrated after drying, is momentarily dissolved in the newly released water
and the mixture softens. Once the released water has escaped from the material,
the material hardens into a stable form. The false melt transition is irreversible
(i.e., the material cannot be re-hydrated). Because very little water is actually
involved, a minimal amount of porosity due to water loss results.
Once cured, the resulting material is a dense, hard, and electrically
conductive phosphate glass. While the material is cured at a relatively low temperature
of, for example, about 180 °C, the material has an operating temperature of up
to about 900 °C. The material also has excellent electrical conductivity and displays
low resistance values. For example, resistance values of 0.1 Ω/cm or less
have been achieved with as little as about 17% Ag by volume. Even with only about
8% by volume of Ag, the material still exhibits a resistance of less than about
6 Ω/cm. It is an unusual feature of this material, in fact, that it displays
such low resistance values upon addition of such a small amount of electrically
conductive material into the phosphate binder.
The electrically conductive material of the present invention, by
being curable in any configuration and by exhibiting high conductivity, is useful
in making high voltage contacts for use in, for example, ceramic sensors, piezoelectric
devices, and timing mechanisms. With discrete particles dispersed in a phosphate
glass binder, the electrically conductive material of the present invention offers
an attractive alternative to ordinary metal contacts that tend to weld together
upon repeated application of high voltage. In contrast to ordinary metal contacts,
contacts made from the electrically conductive material of the present invention
do not suffer from a transfer of material from one contact to the other upon application
of high voltage to the contacts. As a result, contacts made from the electrically
conductive material of the present invention exhibit a much longer service life
versus those fabricated from ordinary metal.
Additionally, the electrically conductive material of the present
invention can be configured to provide a low-friction solution for high voltage
contacts. Specifically, through the addition of BN, the coefficient of friction
for the material is reduced while the electrical properties of the material are
preserved. As a result, such a material is suitable for use in applications including
sliding, high voltage contacts. The lower coefficient of friction provided by the
addition of BN serves to prolong the life of the contacts by decreasing frictional
In yet another application, the electrically conductive material of
the present invention is useful as a surface repair material. The material can
be applied as a liquid, paste, putty, or compactable powder, and it has excellent
adhesion to most ceramic materials and some metals, including aluminum. The material
also finds application as an adhesive, as a coating, or as a substitute for solder
in bonding electrodes to coated surfaces, for example.
Other aspects and features of the present invention can be obtained
from a study of the drawings, the disclosure, and the appended claims.