U.S. patent number 3,655,440 [Application Number 04/803,688] was granted by the patent office on 1972-04-11 for electrical resistance elements, their composition and method of manufacture.
This patent grant is currently assigned to CTS Corporation. Invention is credited to Lynn J. Brady.
United States Patent |
3,655,440 |
Brady |
April 11, 1972 |
**Please see images for:
( Certificate of Correction ) ** |
ELECTRICAL RESISTANCE ELEMENTS, THEIR COMPOSITION AND METHOD OF
MANUFACTURE
Abstract
An electrically nonconductive crystal growth controlling agent
comprising submicron inert particles is mixed with a crystalline
conductive phase comprising an oxide of Ruthenium or Iridium, a
vehicle, and a moisture impervious binder with which the inert
particles will not react and in which the inert particles will not
dissolve to any appreciable extent at elevated temperatures. After
being applied to a high temperature-resistant, electrically
nonconductive substrate, the composition is fired at elevated
temperatures for a sufficient period of time to permit the crystals
of the conductive phase to grow until an equilibrium condition is
reached. This condition is determined in part by the crystal growth
controlling agent. Upon cooling, the binder bonds together a
composite mass comprising an inert intersticed matrix made up of
the crystal growth controlling agent and the crystalline conductive
phase which forms an interstitial mass within the interstices of
the matrix. The method comprises the steps of thoroughly mixing the
above-identified materials, applying a layer of the mixture to the
substrate, and firing the substrate and layer of material for 45 to
60 minutes to a preferred peak temperature in the range of
975.degree.-1025.degree. C. During the firing cycle the crystals of
the conductive phase increase in size until further growth is
limited by the crystal growth controlling agent.
Inventors: |
Brady; Lynn J. (Edwardsburg,
MI) |
Assignee: |
CTS Corporation (Elkhart,
IN)
|
Family
ID: |
25187184 |
Appl.
No.: |
04/803,688 |
Filed: |
March 3, 1969 |
Current U.S.
Class: |
427/101;
106/1.21; 148/DIG.51; 252/514; 428/338; 252/518.1; 106/1.28;
338/308; 428/697 |
Current CPC
Class: |
H01C
17/0654 (20130101); Y10S 148/051 (20130101); Y10T
428/268 (20150115) |
Current International
Class: |
H01C
17/06 (20060101); H01C 17/065 (20060101); H01b
001/02 (); H01b 001/08 () |
Field of
Search: |
;117/227,201 ;106/1
;252/514 ;338/308 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Jarvis; William L.
Claims
What is claimed as new and desired to be secured by Letters Patent
of the United States is:
1. A resistance composition for making resistance elements having a
resistance greater than 100 ohms and comprising by weight of
solids, from 2 to 60 percent of a conductive material selected from
the group consisting of Ru, Ir and their compounds, a glass
vitreous binder, and from 11/2 to 30 percent of inert noncolloidal
chemically unreactive particles having an average particle size of
less than 20 microns.
2. A resistance composition for making resistance elements having a
resistance greater than 100 ohms and comprising a glass vitreous
binder, at least 2 percent, by weight of solids of the composition,
of conductive crystals selected from the group consisting of Ru
compounds and Ir compounds that undergo crystalline growth and
increase in size in a molten binder environment, and means for
controlling the crystalline binder environment, and means for
controlling the crystalline growth of the conductive crystals in a
molten binder environment, said means comprising at least 11/2
percent by weight of solids of the composition.
3. The composition of claim 2 wherein the conductive crystals
comprise a compound selected from the group consisting of PdO,
RuO.sub.2 and IrO.sub.2.
4. The composition of claim 2 wherein the means for controlling the
growth of the conductive crystals comprises particles of inert
noncolloidal chemically unreactive material having an average
particle size of less than 10 microns.
5. The composition of claim 4 wherein the inert material comprises
a refractory material selected from the group consisting of alumina
and barium ferrite.
6. The composition of claim 4 wherein the particles of inert
material comprise from 3 to 16 percent by weight of solids of said
composition.
7. The composition of claim 6 wherein the conductive crystals
comprise a compound selected from the group consisting of PdO,
RuO.sub.2 and IrO.sub.2.
8. A resistance element having a resistance greater than 100 ohms
and comprising an intersticed inert matrix, an interstitial mass of
a conductive phase comprising conductive crystals selected from the
group consisting of Ru compounds and Ir compounds disposed within
said matrix, said matrix comprising inert noncolloidal chemically
unreactive crystal growth controlling particles bounding the
conductive crystals and spacing adjacent ones of said crystals, and
a glass vitreous binder bonding together the composite mass.
9. The resistance element of claim 8 wherein the conductive phase
comprises crystals having a size of from 40 to 80 microns.
10. The resistance element of claim 8 wherein the inert particles
comprise from 11/2 to 30 percent by weight of the element, and the
conductive crystals comprise from 2 to 60 percent by weight of the
element.
11. The resistance element of claim 8 wherein the crystal growth
controlling particles have an average particle size of less than 20
microns.
12. The resistance element of claim 8 wherein the conductive
crystals comprise a compound selected from the group consisting of
PdO, RuO.sub.2 and IrO.sub.2.
13. The resistance element of claim 8 wherein the conductive
crystals comprises a Pd compound.
14. The resistance element of claim 8 wherein the crystal growth
controlling particles have an average particle size of from
one-tenth micron to 10 microns and comprise from 3 to 16 percent,
by weight, of the element, and wherein the conductive phase
comprises crystals having a size of from 40 to 80 microns and
comprises from 10 to 50 percent, by weight, of the element.
15. A resistance element having a resistance greater than 100 ohms
and comprising an intersticed inert matrix, an interstitial mass of
a conductive phase comprising conductive crystals comprising a
compound selected from the group consisting of RuO.sub.2,
IrO.sub.2, and PdO disposed within said matrix, said matrix
comprising inert noncolloidal chemically unreactive crystal growth
controlling particles comprising a refractory material selected
from the group consisting of alumina and barium ferrite bounding
the conductive crystal and spacing adjacent ones of said crystals,
the crystal growth controlling particles having an average particle
size of from one-tenth micron to 10 microns and comprising from 3
to 16 percent, by weight, of the element, and wherein the
conductive phase comprises crystals having a size of from 40 to 80
microns and comprises from 10 to 50 percent, by weight, of the
element, and a glass vitreous binder bonding together the composite
mass.
16. A method of forming an electrical resistance element comprising
the steps of: mixing together a binder, a conductive phase
comprising crystals of a predetermined average size selected from
the group consisting of Ru compounds and Ir compounds, particles of
inert material having a predetermined average size, and a vehicle
to provide a resistance composition, forming an unfired resistance
element from the composition, firing the resistance element to a
temperature of at least 760.degree. C. to increase the size of
crystals in the conductive phase, controlling the amount of size
increase of said crystals in the conductive phase, and cooling the
resistance element.
17. A method of forming an electrical resistance element comprising
the steps of: mixing together a binder, a conductive phase
comprising crystals of a predetermined average size selected from
the group consisting of Ru compounds and Ir compounds, particles of
inert material having a predetermined average size, and a vehicle
to provide a resistance composition, forming an unfired resistance
element from the composition, applying a layer of the composition
to a high temperature resistant electrically nonconductive
substrate, firing the resistance element for a period of time in
excess of 45 minutes to a temperature of at least 760.degree. C. to
increase the size of crystals in the conductive phase, controlling
the amount of size increase of said crystals in the conductive
phase, and cooling the resistance element.
18. A method of forming an electrical resistance element comprising
an intersticed matrix of inert particles, an interstitial mass of
conductive crystals selected from the group consisting of Ru
compounds and Ir compounds disposed within said matrix, and a
binder bonding together the composite mass, said method comprising
the steps of mixing together 2 to 60 parts by weight of conductive
crystals that have a predeterminable average size and that increase
in size in a molten binder environment, 11/2 to 30 parts by weight
of particles of an inert material, a moisture impervious binder,
and a vehicle to provide a resistance composition, forming a
portion of the composition into an unfired resistance element,
liquefying the binder in said resistance element and promoting
growth of conductive crystals in said resistance element, limiting
the growth of said conductive crystals with the particles of inert
material, and solidifying the binder to bond together the particles
of inert material and conductive crystals.
19. A method of forming an electrical resistance element comprising
an intersticed matrix of inert particles, an interstitial mass of
conductive crystals disposed within said matrix, and a binder
bonding together the composite mass, said method comprising the
steps of mixing together 2 to 60 parts by weight of conductive
crystals that have a predeterminable average size and that increase
in size in a molten binder environment, 11/2 to 30 parts by weight
of particles of an inert material, a moisture impervious binder,
and a vehicle to provide a resistance composition, forming a
portion of the composition into an unfired resistance element,
applying said portion to a high temperature resistance substrate,
liquefying the binder in said resistance element by firing the
resistance element for an extended period of time and at an
elevated temperature and promoting growth of said conductive
crystals in said resistance element, limiting the growth of said
conductive crystals with the particles of inert material, and
cooling the binder to bond together the particles of inert material
and conductive crystals.
20. A method of forming an electrical resistance element comprising
an intersticed matrix of inert particles, an interstitial mass of
conductive crystals disposed within said matrix, and a binder
bonding together the composite mass, said method comprising the
steps of mixing together 2 to 60 parts by weight of conductive
crystals that have a predeterminable average size and that increase
in size in a molten binder environment, 11/2 to 30 parts by weight
of particles of an inert material, a moisture impervious binder,
and a vehicle to provide a resistance composition, forming a
portion of the composition into an unfired resistance element,
applying said portion to a high temperature resistance substrate,
liquefying the binder in said resistance element by firing the
resistance element to a temperature of at least 975.degree. C. and
for at least 45 minutes promoting growth of conductive crystals in
said resistance element, limiting the growth of said conductive
crystals with the particles of inert material, and cooling the
resistance element.
21. A method of making an electrical resistance element comprising
conductive crystals, inert particles, and a binder and
characterized by a high sheet resistance, a Sheet VCR of less than
400 parts per million per volt, and a voltage withstanding ability
in excess of 1,000 volts per inch, said method comprising the steps
of making a resistance composition by mixing together conductive
crystals selected from the group consisting of Ru compounds and Ir
compounds, inert particles and binder, forming an unfired
resistance element from the composition, firing the resistance
element to a temperature of at least 760.degree. C. to provide a
molten binder environment for the inert particles and conductive
crystals in order to attain growth of the conductive crystals until
an equilibrium condition is approached, and thereafter cooling the
resistance element.
Description
This invention relates to improved cermet resistance elements and
to their composition and method of manufacture.
Cermet resistance elements and compositions known in the art are
exemplified by U.S. Pat. No. 3,271,193 entitled "Electrical
Resistance Element And Method Of Making The Same," issued to O. F.
Boykin; and U.S. Pat. No. 3,304,199 entitled "Electrical Resistance
element," issued to W. M. Faber, Sr., et al. Some of the heretofore
known cermet resistance compositions and resistance elements made
therefrom have exhibited characteristics suitable for many special
applications. For example, in applications requiring exceptional
thermal stability and low sheet resistances, resistance
compositions such as those discussed in Boyd et al. U.S. Patent No.
3,372,058 have been very useful. However, general and widespread
use of cermet compositions has still been limited because of
objectionable amounts of electrical noise associated with
resistance elements made from such materials and particularly so in
the higher sheet resistance ranges currently attainable with such
compositions. Usage of these compositions has also been limited
because very high resistivity or sheet resistances have not been
attainable therewith. Although the prior art discusses resistance
materials having sheet resistances of up to about 500,000 ohms per
square after firing, when these materials have been used to form
the resistance elements in variable resistance controls the
equivalent noise resistance of such controls has been of such great
magnitude that it has far exceeded acceptable levels and in fact
has not been measurable when following standard test
procedures.
The prior art materials have not been usable in very high voltage
applications as fixed or variable resistors because they have not
had satisfactory voltage stability characteristics. More
specifically, such materials have not been characterized by an
acceptable sheet voltage coefficient of resistance when subjected
to voltage gradients in excess of about 1,000 volts per linear inch
of resistive path. Accordingly, it would be desirable to provide
resistance elements and compositions that are stable at high
voltages, i.e., that have a relatively low sheet voltage
coefficient of resistance when subjected to high voltage gradients
per inch of resistive path. As used herein, sheet voltage
coefficient of resistance (Sheet VCR) means the number of parts
change per million that one square of a resistive path exhibits
when there is a change of 1 volt in the potential applied
thereacross. Sheet voltage coefficient of resistance differs from
the voltage coefficient of resistance (VCR) of a given resistance
element in that the former quantity is not dependent on the
geometry of the resistive path and can be readily used to compare
one resistance element to another, whereas the latter quantity is a
function of the geometry of the resistive path under consideration
and can be meaningfully used for comparison only when the exact
dimensions of the resistance element are specified. The sheet
voltage coefficient of resistance and the voltage coefficient of
resistance of a given resistance element are similar in that they
both indicate essentially reversible or temporary changes due to
changes in voltage. For example, if an increase of 1 volt will
cause a given increase in resistivity, a decrease of 1 volt will
cause a corresponding decrease. Thus, voltage coefficient of
resistance and sheet voltage coefficient of resistance are not
descriptive of the permanent change in resistive value that may
occur during overload testing or "initial power stressing" as
discussed for example in Kelly et al. U.S. Pat. No. 3,416,960 in
connection with the problem of "power instability."
One type of electrical noise associated with cermet resistance
elements is generated within the body of each element, is referred
to as "current noise" in technical literature, and is measured as
an increase in noise above the thermal noise of the element while
current is being passed therethrough. This "current noise" is most
often measured in decibels on Quan-Tech testing equipment of the
type referred to in Kim U.S. Pat. No. 3,352,797 and manufactured by
Quan-Tech Laboratories of Boonton, New Jersey; is normally referred
to as "Quan-Tech" noise in the electronic components industry; and,
to avoid confusion, will be referred to herein as Quan-Tech noise.
Another type of electrical noise is encountered in variable
resistance control application, is normally identified as
"Equivalent Noise Resistance" (ENR), is associated with the
movement of a contactor or wiper across the surface of a resistance
element, and is measurable in ohms by following standardized test
procedures as will be hereinafter more fully described. With the
advent of solid-state devices such as transistors and integrated
circuits, which are extremely "quiet" in comparison with vacuum
tubes, it has become increasingly important to reduce the Quan-Tech
noise levels in passive circuit elements such as cermet resistance
elements in order to attain the reduced circuit noise levels that
are possible when using such devices. It has also become
increasingly important to reduce the ENR of variable resistance
controls and particularly so when such controls include cermet
resistance elements having relatively high sheet resistances.
Previously known resistance compositions have also had less than
desirable voltage withstanding abilities. More specifically, the
critical voltage gradient of resistors made from such compositions
has been relatively low and when the applied voltage, per inch of
resistive path, has been in excess of a critical level of about 800
volts, the prior art resistors have exhibited a drastic and
permanent change in ohmic values. Since such resistors have been
able to withstand only about 800 volts per inch of resistive path,
it has been necessary to use extremely long resistive paths for
high voltage applications. For example, such resistive paths would
have to be at least 30 inches long in order to withstand a
potential of 24,000 volts across the ends thereof. Accordingly, it
would be desirable to provide improved resistance compositions and
resistance elements having improved voltage withstanding
abilities.
In addition to the problems and limitations of the prior art
related to equivalent noise resistance, Quan-Tech noise, relatively
low sheet resistances, voltage stability, and voltage withstanding
abilities, another problem has long existed in connection with
obtaining satisfactory production yields and particularly so when
higher sheet resistances have been desired. Heretofore, relatively
low production yields frequently have been accepted as unavoidable.
In addition, in the case of many known cermet compositions having a
sheet resistance in excess of 250,000 ohms per square, it has been
necessary to sequentially deposit and fire two or more layers of
such composition in order to obtain tolerable production yields.
The problems of obtaining increased sheet resistances and
acceptable production yields are considered at length in the
aforementioned Daily et al. U.S. Pat. No. 3,329,526. Accordingly,
it would be desirable to provide resistance elements and
compositions therefore that are characterized by improved
production yields, and by high sheet resistances.
Therefore, it is a general object of the present invention to
provide an improved resistance element, composition, and method of
making the same. Another object of the present invention is to
provide an improved method of making a resistance composition and
method of making an improved resistance element. A further object
of the present invention is to provide an improved resistance
element having an acceptable voltage coefficient of resistance per
square when subjected to high voltage gradients. An additional
object of the present invention is to provide an improved
resistance element having a measurable low equivalent noise
resistance. Yet another object of the invention is to provide an
improved resistance element having a reduced Quan-Tech noise level.
Yet a further object of the present invention is to provide cermet
resistance elements having very high sheet resistances. Yet an
additional object is to provide cermet resistance elements having a
very high sheet resistance and comprised of a single fired layer of
resistance composition. Still another object of the present
invention is to provide a resistance element having an improved
voltage withstanding ability. A still additional object of the
present invention is to provide improved compositions useful in
making resistance elements that accomplish the above stated
objects. A more specific object of the present invention is to
provide compositions that provide improved production yields of
electrical resistance elements. Another specific object of the
present invention is to provide a cermet resistance composition
wherein means are provided for controlling the growth of crystals
of a conductive phase during a firing process. A still more
specific object of the present invention is to provide a method of
making a resistance element that includes the steps of firing a
resistance composition and creating a molten glass environment for
a crystalline conductive phase in the presence of means for
controlling the growth of crystals of the conductive phase, and
maintaining the composition at an elevated temperature until
crystal growth is impeded by such means. Further objects and
advantages of the present invention will become apparent as the
following description proceeds, and the features of novelty
characterizing the invention will be pointed out with particularity
in the claims annexed to and forming a part of this
specification.
Briefly, the present invention is concerned with improved
resistance elements and with their composition and methods of
manufacture. One specific composition embodying the invention
comprises a crystal growth controlling agent in the form of
electrically nonconductive, inert particles that are submicron in
size. The composition further comprises a moisture impervious
binder with which the inert particles will not react and in which
the inert particles will not dissolve to any appreciable extent at
elevated temperatures; a conductive phase; and a thermally
decomposable or evaporable vehicle that facilitates the application
of the resistance composition to a suitable substrate. In a
specific embodiment of the invention, the composition includes a
conductive phase comprising an oxide of ruthenium; a binder
comprising glass particles; means for controlling the growth of the
crystals of the oxide of ruthenium in the form of particles of
alumina having an average particle size of 0.3 microns; and a
conventional vehicle such as ethylcellulose. The composition is
thoroughly mixed, applied to an alumina substrate by conventional
silk screen methods, and fired to a peak temperature in the range
of 975.degree.-1025.degree. C. in a 45 minute to 1 hour firing
cycle. Firing the composition decomposes and drives off the organic
ingredients and causes crystallographic changes to occur in the
composition. The resistance elements are fired at elevated
temperatures for extended periods of time so that the binder will
become molten and the crystals of the conductive phase will
increase in size until further increases are limited by the crystal
growth controlling agent. After cooling, the crystal growth
controlling agent forms an intersticed inert matrix, the conductive
phase forms an interstitial mass within the interstices of the
matrix, and the binder bonds together the matrix and conductive
phase. Resistance elements embodying the present invention fulfill
one or more of the above stated objects by exhibiting one or more
of the following characteristics: a measurable low ENR of less than
1 percent in variable resistor applications; a very high sheet
resistance; a Sheet VCR of 400 or less ppm per volt at voltage
gradients of up to 3,000 volts per inch of resistive path; a
surprisingly improved voltage withstanding ability; and
consistently improved production yields.
For a better understanding of the present invention, reference may
be had to the accompanying drawing wherein:
FIG. 1 is a fragmentary isometric view of an electrical resistance
element embodying the present invention;
FIG. 2 is a very greatly enlarged cross-sectional view of a portion
of the resistance element of FIG. 1 and is a two-dimensional
representation of the microstructure of such resistance element;
and
FIG. 3 is a flow chart illustrating the steps in the inventive
method.
Recent research efforts have been directed to the development of a
variable resistance control comprised of a cermet resistance
element having a sheet resistance of 500,000 ohms or more per
square. During the course of this work, it was discovered that the
equivalent noise resistance between a movable contactor and the
resistance element greatly exceeded acceptable limits when measured
with a Model 400 X-Y recorder made by "Electro-Instruments" of San
Diego, California. When using this recorder to measure the ENR of a
resistor, standard procedures were followed by calibrating the
equipment for the nominal overall ohmic value of the resistor to be
tested and then continuously recording, as a percent of the overall
ohmic value, the changes in resistance that occurred as the movable
contactor was moved across the resistance element. By definition,
such changes are the equivalent noise resistance of the resistor
for each position of the contactor and are a measurement of the
noise at the interface of the contactor and the resistance element.
Following the above procedure, resistors normally are considered
acceptable if the observed changes do no exceed 1 percent of the
nominal overall ohmic value. When prior art type cermet resistance
elements with a sheet resistance of about 500,000 ohms per square
were treated, the observed ENR greatly exceeded 10 percent of the
overall ohmic value of such elements.
I have found that the ENR of materials having a high sheet
resistance can be reduced and maintained at acceptable levels by
including a crystalline growth controlling agent, such as inert
particles, in the resistance composition prior to firing. Somewhat
surprisingly, the inclusion of such material in resistance
compositions also provides resistance elements having improved
voltage stability and voltage withstanding ability. The inert
particles also may be used to provide resistance elements having
significantly increased sheet resistances, i.e., in excess of 1
megohm per square, and, when such particles are included in prior
art cermet resistance compositions, the production yield of such
compositions can be significantly increased and significant
reductions in Quan-Tech noise levels can be attained.
The aforementioned U.S. Pat. No. 3,304,199 teaches a preferred
method of preparing cermet resistance compositions and elements.
That preferred method comprises mixing a vitreous binder such as
finely divided glass particles with a conductive material such as
ruthenium dioxide or iridium dioxide and a liquid such as water or
an organic vehicle to form a slurry. The liquid is then evaporated
and the dry mixture of vitreous binder and conductive material is
combined with a vehicle so that the resistive composition can be
applied to a supporting substrate. The composition and substrate is
then fired to fuse the vitreous binder to the surface of the
substrate. Other patents, such as Place, Sr., et al. U.S. Patent
No. 3,149,002 teach an alternative method of preparing cermet
resistance compositions and elements wherein finely divided glass
particles are mixed or milled with a resinate or other solution
containing the constituents that will form the conductive phase of
the resistor after firing. When resinates are used, the binder and
resinate mixture is heated and stirred to remove the volatiles and
organic materials from the mixture while decomposing the compounds
in the resinate. After heating, the dry mixture is ground to a fine
powder and calcined to assure removal of all organic materials.
Thereafter the dry material is mixed with a suitable vehicle,
applied to a desired substrate, and fired to fuse the mixture.
According to the present invention, either of the aforementioned
methods may be followed provided that the conductive material be
capable of attaining a desired amount of crystal growth, that
crystal growth controlling means be used to control such crystals
growth, and that special firing procedures be followed, all as will
be hereinafter more fully explained.
Although the exact reason for the significantly improved results
obtained from the practice of the present invention are not fully
understood, it is preferred that the crystal growth controlling
means be very finely divided particles. Such particles must
comprise a material with which the binder will not react at
elevated temperatures. In addition, the inert particles must not
dissolve to any appreciable extent in the binder at elevated firing
temperatures. As previously mentioned, the conductive phase must be
capable of achieving crystal growth and, preferably, the conductive
phase is a finely divided crystalline material, the crystals of
which grow while being fired. The structural units of suitable
crystalline material may be idiomorphic crystals, i.e., a group of
space lattices of the same orientation that show symmetry by the
development of regular faces, or allotrimorphic crystals that do
not have regular faces. An allotrimorphic crystal is also referred
to as an imperfect crystal, a xenomorphic crystal, or a grain. The
term "crystal" is used generically herein to designate a
crystalline structure, whether it be idiomorphic or allotrimorphic,
since materials suitable for use in the present invention may be
comprised of either one or both of these forms of crystals. During
the firing step of the process the binder becomes molten and the
crystals of the conductive phase increase in size in such molten
environment until the inert particles control crystal growth by
substantially impeding further crystal growth. After an equilibrium
condition is approached, i.e., when the growth rate of the crystals
has, for practical purposes, approached zero, the composition is
cooled, and the inert particles form an intersticed matrix. The
crystals of the conductive phase form an interstitial mass within
the interstices of the matrix, and the binder bonds together the
composite mass. The time required to approach the equilibrium
condition depends of course on the firing temperature and on the
nature of the materials being fired. Because of this, the firing
conditions necessary to attain near equilibrium conditions for any
given resistance composition can be most readily ascertained by
firing each one of a plurality of substantially identical samples
embodying the present invention at a selected high temperature for
different periods of time and then graphically plotting the sheet
resistance of each sample as a function of the corresponding firing
time for each sample. When this is done, it will be observed that
for an initial period of time the sheet resistance will vary as a
function of time but that after such initial period, the sheet
resistance will stabilize and very little additional changes in
sheet resistance will be observable. Such stabilization of sheet
resistance then indicates that an equilibrium condition has been
approached. In addition, microscopic examination of the samples
will reveal that the conductive crystals increase in size as the
firing time is increased until an apparent maximum size is
attained. The period of time required to attain such apparent
maximum size corresponds quire closely to the initial period of
time referred to above.
I have found that the withstanding voltage of the resistance
element may be controlled by controlling the average particle size
of the crystal growth controlling agent. Thus, if increased
withstanding voltages and improved voltage stability are not
desired, the crystal growth controlling agent may be formed of
inert particles that have an average particle size as large as 20
microns. However, in order to obtain significantly improved
withstanding voltages, i.e., withstanding voltages of about 3,000
volts or more per inch of resistive path, the inert particles are
submicron in size and, preferably, have an average particle size of
about 0.3 micron. If even higher withstanding voltages are desired,
i.e., withstanding voltages in the neighborhood of 6,000 volts per
inch of resistive path, the inert particles should have an average
particle size of approximately 0.1 micron. The average particle
size of the crystal growth controlling agent significantly affects
the voltage stability and sheet resistance as well as the voltage
withstanding ability of the fired resistance element. For example,
when 0.3 micron particles are used, the Sheet VCR of the fired
resistance material is 400 or less ppm per volt at voltage
gradients of 3,000 volts per inch and higher and a sheet resistance
of 600 megohms or higher per square may be obtained.
After compositions embodying the present invention have been
applied by brushing, spraying, stenciling, transfer wheel, or
screening onto the substrate, the composition is fired to a peak
temperature sufficiently high for the binder to form a molten
environment surrounding the crystals of the conductive phase and
the crystal growth controlling agent. The peak firing temperature
must be sufficiently high for growth of the crystals to occur at a
reasonable rate. The maximum permissible peak temperature must be
less than the volatilization temperature of the various portions of
the resistance composition in the firing atmosphere and also less
than the temperature at which either the conductive phase or
crystal growth controlling agent become appreciably soluble in the
binder or at which chemical reactions, pernicious to the desired
characteristics of the fired resistance element, would take place
between the various portions of the resistance composition or
between such portions and the firing atmosphere. Each of the
specific exemplifications mentioned herein were fired for 45 to 60
minutes in a conventional tunnel kiln to a peak temperature in the
range of 975.degree.-1025.degree. C. Equally acceptable resistance
elements were also obtained by firing the exemplary compositions
for two to three hours to a peak temperature of 850.degree. C. The
former conditions are preferred because of the obviously reduced
production or process time involved. Both of these firing
conditions are more extensive than those used in the prior art.
As indicated by the flow chart of FIG. 3, the resistance
composition is maintained at a sufficiently high temperature for a
sufficient period of time to permit the crystals of the conductive
material to grow or increase in size until the equilibrium
condition is at least approximately attained. As this equilibrium
condition is approached, the growth of the conductive phase
crystals can be observed to diminish and apparently stop. As
previously mentioned, the inert particles impede further crystal
growth as the equilibrium condition is attained. Although the
precise inter-action between the conductive crystals and inert
particles is not precisely understood, I have found that when
crystals of conductive material having an average size of about 1
micron are fired in the presence of 0.3 micron crystal growth
controlling particles in a molten binder environment, the
conductive crystals will grow to an equilibrium size of about 50
microns. In addition, even though it is difficult to determine the
changes in final conductive crystal size that result from changing
the size of the crystal growth controlling particles, it is
possible to determine a preferred range of sizes of growth
controlling particles that should be used to attain desirable
electrical characteristics. For example, if crystal growth
controlling particles having an average size greater than about 20
microns is used, significantly improved voltage stability and
voltage withstanding ability characteristics will not be attained.
On the other hand, if the crystal growth controlling particles used
in the present exemplifications of the invention are colliodal in
size, the fired resistance element is nonconductive.
The primary criteria for the specific inert material used as the
crystal growth controlling means is the relationship between the
inert material and binder. These materials must be selected so that
the inert material will not chemically react with the vitreous
binder (particularly during the firing cycle) and so that the inert
material will not dissolve in the molten binder to any appreciable
extent during the firing cycle. The problem of solubility is
accentuated when the crystal growth controlling agent comprises
particles in the micron and submicron size range because many
materials that are relatively insoluble in a molten binder when the
particle size is from 40 to 70 microns are objectionably soluble in
the same molten binder when the particle size is in the micron and
submicron range. As will be understood by persons skilled in the
art, the determination of the relative solubility of two or more
materials is best determined empirically, and "not soluble to an
appreciable extent" is meant herein to refer to the observable
characteristics of submicron alumina particles when subjected to a
molten environment of lead-aluminia-silicate glass at from
850.degree.-1,000.degree. C. For the purpose of discussion herein,
other materials will be considered to be not soluble to an
appreciable extend in a given molten binder if the observable
characteristics of micron and submicron particles in such binder
are similar to the just mentioned characteristics of alumina.
With reference to the illustrated embodiment of FIG. 1, a substrate
10 formed of high alumina supports on a surface thereof a fired
resistance element 11. FIG. 2 illustrates the characteristic
microstructure of the resistance element 11 which comprises
crystals 12 of a conductive phase, crystal growth controlling means
in the form of inert submicron alumina particles 14, and a binder
15. It will be appreciated from an inspection of FIG. 2 that the
particles 14 are not dissolved in the binder and that the particles
14 comprise an intersticed matrix, the crystals 12 form an
interstitial mass within such matrix, and the binder 15 bonds the
composite mass together. It will of course also be understood that
FIG. 2 is a two-dimensional representation of a three-dimensional
microstructure and that the conductive crystals 12 actually occupy
random relative orientations and positions within the resistance
element 11. The inert particles, selected conductive material,
binder, and any well known vehicle are first thoroughly mixed
together and then used to form an unfired resistance element.
Although the unfired resistance elements could be of the volumetric
type, in the exemplifications described hereinafter the unfired
resistance elements were applied as a film to a substrate such as
the substrate 10. The film and substrate were then fired so that
the conductive phase crystals approached an equilibrium
condition.
The constituents of two common glasses or vitreous binders that may
be used as a moisture impervious binder in the practice of the
present invention are as follows:
GLASS I
Percent PbO 65 SiO.sub.2 34 Al.sub.2 O.sub.3 1 Total: 100
GLASS II
Percent PbO 72.15 SiO.sub.2 13.41 B.sub.2 O.sub.3 9.04 ZnO 5.40
Total: 100.00
The following examples are exemplary cermet type resistance
compositions formed in accord with the present invention. Some of
the improved characteristics of fired resistance elements formed
from such compositions are stated for each example and the
percentage of composition is stated without regard to the presence
of ethylcellulose or other well known vehicles since the amount of
vehicle may be varied in manners well known in the art and depends
primarily on the mode selected for depositing the resistance
composition on the supporting substrate. In any case, the vehicle
is decomposed and volatilized upon firing. Each of the resistance
elements used to obtain the characteristics stated in the following
examples was screened onto a high alumina substrate.
EXAMPLE A
Percent by weight Glass I 63.10 RuO.sub.2 26.83 Al.sub.2 O.sub.3
(0.3 micron average particle size) 10.07 Total: 100.00
Sheet Resistance 405,200 ohms per square Withstanding Voltage 3,000
volts per inch Sheet VCR 400 or less ppm per volt EXAMPLE B Percent
by weight Glass I 50.6 IrO.sub.2 41.4 Al.sub.2 O.sub.3 (0.3 micron
average particle size) 8.0 Total: 100.00
Sheet Resistance 600,000 ohms per square Withstanding Voltage 3,000
volts per inch Sheet VCR 400 or less ppm per volt EXAMPLE C Percent
by weight Glass I 68.5 RuO.sub.2 21.8 Al.sub.2 O.sub.3 (0.3 micron
average particle size) 9.7 Total: 100.0
Sheet Resistance 20,000 ohms per square Withstanding Voltage 3,000
volts per inch Sheet VCR 400 or less ppm per volt EXAMPLE D Percent
by weight Glass I 64.1 RuO.sub.2 27.3 Al.sub.2 O.sub.3 (7 micron
average particle size) 8.6 Total: 100.0
Sheet Resistance 100 ohms per square Withstanding Voltage (not
meaningful for Sheet VCR (sheet resistances this low EXAMPLE E
Percent by weight Glass II 67.5 RuO.sub.2 23.1 Al.sub.2 O.sub.3
(0.3 micron average particle size) 9.4 Total: 100.0
Sheet Resistance 25,000 ohms per square Withstanding Voltage 3,000
volts per inch Sheet VCR 400 or less ppm per volt EXAMPLE F Percent
by weight Glass I 70.1 RuO.sub.2 19.9 Al.sub.2 O.sub.3 (0.3 micron
average particle size) 10.0 Total: 100.0
Sheet Resistance 55,000,000 ohms per square Withstanding Voltage
3,000 volts per inch Sheet VCR 400 or less ppm per volt EXAMPLE G
Percent by weight Glass I 74.2 RuO.sub.2 15.3 BaO.6Fe.sub.2 O.sub.3
(0.3 micron average particle size) 10.5 Total: 100.0
Sheet Resistance 15,000 ohms per square Withstanding Voltage (Not
meaningful for Sheet VCR (sheet resistances this low EXAMPLE H
Percent by weight Glass I 71.0 IrO.sub.2 18.8 Al.sub.2 O.sub.3 (0.3
micron average particle size) 10.2 Total: 100.0
Sheet Resistance 600,000,000 ohms per square Withstanding Voltage
3,000 volts per inch Sheet VCR 400 or less ppm per volt Each of the
above examples had an ENR of less than 1 percent of the nominal
overall ohmic value of the resistance element. Resistance elements
according to the present invention may comprise, on a weight basis,
from 11/2 to 30 percent of a crystal growth controlling agent and
from 2 to 60 percent of conductive material. The preferred
proportions of these materials are from 3 to 16 percent growth
controlling agent and from 10 to 50 percent conductive material. By
adjusting the proportions of the conductive material, binder, and
growth controlling agent the sheet resistance of the fired
resistance element can be controlled. The binder comprises the
remainder of the resistance element and bonds together the fired
matrix and conductive phase when the resistance element is a film
type element and also when the resistor is a volumetric type as
disclosed for example in co-pending Holmes et al. application Ser.
No. 506,449 filed on Oct. 24, 1965.
The crystalline conductive materials used in the examples above
were in powdered form and had an average particle size, prior to
firing, of 0.7 to 1 micron. For the most part, the particles
comprised only a single crystal and thus the average crystal size
was also 0.7 to 1 micron. The average particle size was determined
with standard sub-sieve size measuring apparatus, whereas the
average crystal size was determined with X-ray diffraction
techniques. Using the same X-ray diffraction techniques, it was
determined that after firing, crystals of the conductive phase had
increased in size to between 40 and 80 microns and were about 50
microns, on the average. When control samples of cermet type
resistance elements which did not include a crystal growth
controlling agent were fired under the same conditions as Examples
A-H above, the conductive crystals were several times larger in
size than 50 microns and such elements, after firing, were no
longer conductive.
The significantly improved production yields and Quan-Tech noise
characteristics that may be obtained with the present invention
will be appreciated from a comparison of the characteristics of the
following Example I with the characteristics of Example A above.
Example I typifies a prior art cermet composition and was fired in
accordance with the prior art teachings for ten minutes in a tunnel
kiln to a peak temperature of 830.degree. C. It should be noted
that the silicon dioxide used in Example I went into solution in
the binder during the firing step and did not function as a crystal
growth controlling agent.
EXAMPLE I
Percent by weight Glass II 95.00 RuO.sub.2 1.67 SiO.sub.2 3.33
Total: 100.0
Sheet Resistance 399,100 ohms per square
When compositions according to Example I were applied as a single
layer to form resistance elements, 65 percent of such elements had
a sheet resistance of 399,100 ohms per square .+-.15.1 percent, and
95 percent of such elements had a sheet resistance of 399,100 ohms
per square .+-.30.2 percent. The noise level of these elements was
measured with a Quan-Tech Model No. 315 test set and observed to be
+9 db. By comparison, when compositions embodying the present
invention according to Example A above were applied to form single
layer resistance elements, 65 percent of such elements had a sheet
resistance of 405,200 ohms per square .+-.5.9 percent, and 95
percent of the resistance elements had a sheet resistance of
405,200 ohms per square .+-.11.8 percent. The Quan-Tech noise level
of these elements was -17 db. From this comparison, it will be
appreciated that the present invention results in production yield
increases of more than 30 percent. In addition, Quan-Tech noise
levels can be reduced by at least 26 decibels.
Various ones of Examples A through H above exhibit the greatly
increased sheet resistances that are obtainable by practicing the
present invention, and each of those examples also accomplish all
of the stated objects of the invention. Although the above stated
examples illustrate the use of RuO.sub.2 and IrO.sub.2 as the
preferred materials to be used to form the conductive crystals in
an inert matrix, it will be expressly understood that other
crystalline materials may be used. The essential requirements of
such materials are that they be electrically conductive and be
capable of achieveing crystal growth in a molten binder
environment. In addition, such crystal growth must be controllable
by a crystal growth controlling means that is compatible, i.e.,
inert and substantially insoluble in the selected binder. Persons
skilled in the art will understand that even though the oxidation
state of all or part of the crystalline conductive phase may change
during firing, the metallic constituents present in the resistance
composition prior to firing are also present in the resistance
element after firing. Thus, if a crystalline material such as
elemental ruthenium or a ruthenium compound such as an oxide of
ruthenium is present in the composition, ruthenium will also be a
constituent of the crystalline conductive phase whether it appears
in elemental form or in a compound, such as ruthenium dioxide.
A prior art cermet composition that uses palladium oxide-silver
(PdO-Ag) as a conductive phase is sold commercially by E. I. DuPont
de Nemours Co. as Resistive Formulation No. 7860. This formulation
also includes a conventional vehicle and vitreous binder. Example J
below consisted of 100 percent of the purchased formulation and was
fired for 1 hour and 15 minutes to a peak temperature of
760.degree. C. as specified by the manufacturer. Example K below
was fired in the same manner (this provided a sound basis for
comparison) and comprised 94 percent by weight of the above
formulation No. 7860 and 6 percent by weight of 0.3 micron
alumina.
EXAMPLE J
Percent by weight Formulation No. 7860 100
Sheet Resistance 75,000 ohms per square EXAMPLE K Percent by weight
Formulation No. 7860 94 Al.sub.2 O.sub.3 (0.3 micron average
particle size) 6
Sheet Resistance 1,600,000 ohms per square
A comparison of Examples J and K reveals that the present invention
clearly teaches how to attain one object of the invention, i.e.,
significantly greater sheet resistances than were heretofore
contemplated, even though resistance elements according to Example
K do not exhibit improved voltage stability or voltage withstanding
ability. While the differences in characteristics exhibited by
Example K and Examples A through H above are not fully understood,
it is believed probable that various alloys are formed involving
the non-oxidized noble metal constituent of Example K and that this
alloying process interferes with crystal growth and crystal growth
control. In addition, Example K compositions could not be fired at
extremely high temperatures without becoming unstable, and this
difference might also explain why improvements in stability, noise
levels, and voltage withstanding abilities were not attained with
such compositions.
Although alumina or barrium ferrite particles were used as the
growth controlling agent in the above examples, substantially
similar results can be obtained when other refractory materials are
used. However, it must be emphasized that such other materials must
satisfy the selection criteria set out hereinabove and can be
neither colloidal in size nor have an average particle size in
excess of about 20 microns. Steatite is one example of a material
that is apparently too soluble in Glasses I and II to consistently
yield the results obtainable in the above exemplary compositions.
Although steatite was somewhat useful in compositions containing
lesser relative amounts of binder, steatite was not useful on a
substitute basis for the growth controlling agent in Examples A
through H.
It will be appreciated by those skilled in the art that the
materials used in the foregoing exemplifications were materials
that are commercially available and that undoubtedly include
unknown percentages of impurities. Also, production type equipment
was used during processing and impurities may have been added to
the exemplifications during such processing. In addition, other
materials may be added by persons skilled in the art in order to
attain special physical or electrical properties in addition to the
desirable characteristics discussed herein. Accordingly, it will be
understood that resistance compositions and elements embodying the
present invention may comprise such impurities and additives so
long as they are not pernicious to the attainment of the stated
objects of the invention.
From the foregoing, it will be seen that the present invention
provides resistance elements having improved Quan-Tech and
equivalent noise resistance characteristics, withstanding voltages
and voltage stability, and greatly increased sheet resistances and
production yields.
While there has been illustrated and described what are at present
considered to be preferred embodiments of the present invention and
a preferred method of making the same, it will be appreciated that
numerous changes and modifications will occur to those skilled in
the art, and it is intended in the appended claims to cover all
those changes and modifications which fall within the spirit and
scope of the present invention.
* * * * *