U.S. patent number 3,816,097 [Application Number 05/261,079] was granted by the patent office on 1974-06-11 for powders of metal, silver and gold.
This patent grant is currently assigned to Owens-Illinois, Inc.. Invention is credited to Valdis R. Daiga.
United States Patent |
3,816,097 |
Daiga |
June 11, 1974 |
POWDERS OF METAL, SILVER AND GOLD
Abstract
A substantially homogeneous, finely divided powder,
substantially free of silver chloride or cyanide comprising at
least one metal other than silver or gold, and silver and gold, and
an anti-agglomerating agent substantially homogeneously dispersed
throughout said powder. The powders are particularly useful in the
electronic arts.
Inventors: |
Daiga; Valdis R. (Toledo,
OH) |
Assignee: |
Owens-Illinois, Inc. (Toledo,
OH)
|
Family
ID: |
26838700 |
Appl.
No.: |
05/261,079 |
Filed: |
June 8, 1972 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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141006 |
May 6, 1971 |
3717453 |
|
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Current U.S.
Class: |
75/252 |
Current CPC
Class: |
B22F
9/24 (20130101) |
Current International
Class: |
B22F
9/16 (20060101); B22F 9/24 (20060101); B22f
009/00 () |
Field of
Search: |
;75/.5B,.5A,.5R,165,172,173R |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Stallard; W. W.
Attorney, Agent or Firm: Dence; Richard B. Holler; E. J.
Parent Case Text
This is a division, of application Ser. No. 141,006, filed May 6,
1971 now U.S. Pat. No. 3,717,453.
Claims
I claim:
1. A substantially homogeneous, finely divided powder,
substantially free of silver chloride or cyanaide comprising at
least one metal other than silver or gold, and silver and gold, and
an anti-agglomerating agent substantially homogeneously dispersed
throughout said powder.
2. A powder according to claim 1 wherein said powder has a particle
size less than about 5 microns.
3. A powder according to claim 1 wherein said at least one metal is
selected from palladium, rhodium, iridium, ruthenium, indium, and
mixtures thereof.
4. A powder according to claim 1 wherein said at least one metal is
the single metal palladium.
5. A powder according to claim 1 wherein said anti-agglomerating
agent is particulate silica submicron in size.
6. A powder according to claim 1 consisting essentially of about:
5-95% by weight palladium, 5-95% by weight of an admixture of gold
and silver in a weight ratio of about 4:1 - 1:4, and about 0.5-15%
by weight anti-agglomerating agent.
7. A powder according to claim 1 which consists of about 20-65% by
weight palladium, about 80-35% by weight of said silver, gold
admixture, and about 5% of submicron particle size silica as said
anti-agglomerating agent.
8. A powder according to claim 1 consisting of by weight about: 52%
Ag, 35% Pd, and 13% Au on a metal basis, the total composition
including about 5% by weight of said silica.
9. A powder according to claim 1 wherein said palladium is
indicated as alloyed with said silver by X-ray diffraction
techniques.
Description
This invention relates to processes for making metal-silver-gold
powder systems and the resulting powders made therefrom. More
particularly, this invention relates to processes for making
metal-silver-gold powders and the powders resulting therefrom which
are extremely homogeneous in nature and thus find a wide variety of
uses particularly in the electronic arts, such as those uses
disclosed in concurrently filed, commonly owned copending
application Ser. No. 140,988, filed May 6, 1971 in the name of
Bernard Greenstein and entitled RESISTOR COMPOSITIONS AND
METHODS.
The value of metal oxides such as palladium oxide and the like as
electronically active materials has long been known. For example,
as set forth in the above-cited commonly owned copending
application, it is known, that in the microelectronic circuitry
art, a resistive metal such as palladium may be admixed with a
glass binder and an organic vehicle to form a printing paste. The
paste is then printed onto a dielectric substrate such as aluminum
oxide or the like by the use of a screen or mask of the desired
mesh and formed to provide the desired pattern. The desired pattern
is then fired in air to oxidize the palladium to palladium oxide
and form an electronically active device (e.g. resistor).
Unfortunately, many resistive metal oxides (including palladium
oxide) have relatively large and negative temperature coefficients
of resistivity (hereinafter referred to as TCR) which must usually
be adjusted if the resistor is to be operative for its intended
purposes. In order to regulate this TCR, (and in effect to increase
it to a tolerable level somewhere about .+-.0 ppm/.degree.C)
certain metals such as silver or gold have been alloyed with the
resistive metal. In addition to regulating TCR, these additive
metals have also been found to dilute the system and thereby
control resistivity and increase the "stability" of the
resistor.
The term "stability" is well-understood in the art and is used
herein in accordance with this well-known meaning. That is to say,
stability defines that characteristic of an electronic system which
enables it to maintain its resistivity value within tolerable
limits over extended periods of time and use.
As further disclosed in the aforementioned copending application
and as disclosed in commonly owned, copending application Ser. No.
58,740 filed July 26, 1970, now U.S. Pat. No. 3,681,261 one of the
major problems attendant resistive metal systems is their great
sensitivity to the firing process used to formulate the ultimate
device from its printing paste. Slight fluctuations and/or
variations in temperature during the firing cycle, for example,
greatly change the resistivity of the resulting product. Air flow
and firing times are further variables to which the characteristics
of the final product are extremely sensitive. Such sensitivity, of
course, renders these oxide systems extremely difficult to
reproduce. Not only is reproducibility low, but for some reason,
not entirely understood, stability is also very low.
While the additive metals used in admixture with the resistive
metal oxide generally provide commercially tolerable stability to
the system, they are generally found to detrimentally increase TCR,
usually far above the desirable .+-. 0 ppm/.degree.C, when used in
amounts sufficient to obtain tolerable stability. In this respect,
and especially when silver is used as the stabilizing metal,
stability must be sacrificed for acceptable TCR; while, on the
other hand, TCR must be sacrificed for acceptable stability. In
almost all instances, reproducibility, regardless of the metal
stabilizers used, is detrimentally low.
The aforementioned commonly owned, copending application, Ser. No.
58,740, filed July 27, 1970, discloses a unique and valuable
solution to the above-described problems attendant these resistive
metal oxide systems, especially when these systems are used to form
microelectronic resistors. The entire disclosure of this copending
application is incorporated herein by reference.
In this copending application, the problems of stability,
sensitivity, and reproducibility are eliminated by a unique process
which, when effected, produces extremely homogeneous alloys of
palladium or other resistive metals with at least one stabilizing
metal. By achieving high homogeneity, reproducibility is increased
because sensitivity of the system to later processing such as
firing and the like is materially decreased. In this respect, while
one stabilizing metal alone can be employed, it was found to be
preferred to use at least two of these metals together in amounts
which were found to synergistically reduce their affect upon TCR.
Thus, by using two metals in combination, excellent stability is
attained without unduly increasing TCR. One of the most important
of these combinations is a combination of silver and gold, usually
in alloy form.
Generally speaking, this aforementioned copending application
obtained the necessary degree of homogeneity for nonsensitivity of
the palladium (or other resistive metal) with the stabilizing metal
systems, by initially forming an admixture of the resistive metals
and stabilizer metals in the form of organometallic compounds and
adding thereto an anti-agglomerating agent. Upon heating of the
admixture of a sufficient temperature and for a sufficient period
of time, the organo constituents are driven off and the admixture
is concentrated to an extremely homogeneous powder. Such powders
were found to be extremely useful when alloyed and employed with
glass binders as resistor compositions for microelectronic
circuitry.
While the technique hereinabove described relative to this
copending application is extremely valuable, it has a few drawbacks
attendant with it which make the finding of an alternative method
of forming an extremely homogeneous powder of a resistive metal
with a silver and gold system most desirable. Examples of such
drawbacks include the relatively high expense of the organometallic
compounds used as starting materials and the pollution and danger
caused by the volatilization of the organo constituents during
heating to powder form. It has also been found that many of the
commercially available organometallics useful in the practice of
the invention disclosed in the copending application contain
chlorine-bearing solvents. During the concentration step, the
chloride ions have a tendency to react with the silver metal and
thereby form silver chloride precipitate which contaminates the
resulting powder. As discussed more fully hereinafter, silver
chloride cannot be tolerated in any substantial amounts in resistor
compositions.
It is a purpose of this invention to provide the art with an
alternative process for forming extremely homogeneous, finely
divided powders of resistive metals in combination with silver and
gold, which process does not employ organometallic compounds and
thereby overcomes the above-described problems relative thereto. In
this respect, it is a further purpose of this invention to provide
the art with a starting material powder and a process for forming
this starting material powder from which the unique resistors of
the above-cited concurrently filed copending application Ser. No.
140,988, filed May 6, 1971 in the name of Bernard Greenstein
entitled RESISTOR COMPOSITIONS AND METHODS, can be formed and the
other teachings of the invention disclosed therein carried forth.
The entire disclosure of this copending application is incorporated
herein by reference.
Generally speaking the subject invention accomplishes these
purposes by, in part, relying upon a coprecipitation technique
wherein at least some of the metals employed are precipitated by
the use of a reducing agent from solution to thereby aid in the
formation of the homogeneous, finely divided powders of this
invention. In this respect, the homogeneity achieved in this
invention is to an extent greater than that achievable by mere
mechanical comminution or comminution and metal alloying. Thus, the
term "homogeneous" is used herein to define a dispersion of
materials which goes beyond that achievable by these mechanical or
partially mechanical techniques.
As exemplified by U.S. Pat. Nos. 3,390,981 and 3,385,799 and
British Pat. No. 1,004,652, the basic concept of coprecipitating
two electronically active noble metals to thereby form a finely
divided alloy of the two metals is known. Gnerally speaking, such a
known process comprises dissolving each of the metals in a common
inorganic solvent such as, for example, nitric acid, so as to form
a solution of soluble salts (e.g. nitrates) of the two metals.
Thereafter, the solution is contacted with a reducing agent which
simultaneously reduces the dissolved metal salts and precipitates
them as pure metal alloy powders.
The prior art coprecipitation techniques, as represented by the
patents cited above, have several serious drawbacks. Firstly, they
never contemplate the use of a system having more than two metals
therein. Secondly, although they do make mention of the use of
silver and gold as a coprecipitation, the application of the
disclosed techniques to the formation of such a coprecipitate has
many problems and detriments attached thereto. For example, there
are only a very limited number of inorganic media in which both
silver and gold are soluble. Of these, aqua regia (e.g. 3 parts HCl
to 1 part HNO.sub.3) and the various inorganic cyanide solutions
are, generally speaking, the only ones which feasibly could be
employed. While aqua regia may generally be safely employed,
insoluble silver chloride is formed simultaneously with the silver
and gold soluble salts, thus forming a contaminant in the
ultimately precipitated metal alloy powder. Silver cyanide is
somewhat more soluble than silver chloride. However, cyanide salt
contamination often does occur to a limited extent and may become a
material problem especially when a high degree of silver is
required to be present in the system. In addition, the use of
cyanides is dangerous and to be avoided. In summary, then, the art,
by its prior techniques is unable to safely form a noncontaminated
coprecipitate of silver and gold.
The problem of contamination of coprecipitated powders,
particularly of silver and gold, is an especially acute problem in
many arts such as where the powder is to be used, for example, in
microelectronic devices such as conductors, resistors and the like.
In microelectronic resistors, for example, substantially no
contaminating silver chloride or cyanide can be tolerated. Since,
therefore, the prior art coprecipitation techniques cannot provide
a safely formed, noncontaminated powder comprised of silver and
gold, they are incapable of satisfying a specific but important
need in the art. Therefore, in addition to the above-explained need
relative to a new technique for forming homogeneous, finely divided
powders which do not employ organometallic compounds, there also
exists a definite need in the art for a new technique for forming
homogeneous, finely divided metal powders which include both silver
and gold in their composition makeup, which is safe to conduct and
which eliminates or substantially reduces the contamination
problem.
It is the purpose of this invention not only to fulfill the first
above-described need, but also to fulfill this latter need relative
to a new technique wherein silver and gold may both be present in
the powder composition but without the degree of metal salt
contamination experienced when employing the prior art
techniques.
In fulfilling the above-described needs, this invention generally
envisions two alternative techniques for forming homogeneous,
finely divided powders of one or more metals in combination with
silver and gold (or two other metals which give rise to similar
problems of contaminated coprecipitation as silver and gold).
The first alternative technique of this invention generally
comprises forming a soluble salt solution of a metal other than
silver or gold, and silver and thereafter reducing the solution in
accordance with prior art techniques similar to those described in
the aforementioned patents. Such a reduction reaction with
agitation forms a metal-silver slurry. To this metal-silver slurry
is then added gold in the form of a gold salt solution generally of
the chloride type. The slurry solution now containing gold is then
contacted with a reducing agent either already present or
additionally added, to precipitate the gold into the metal-silver
slurry and thereby form the trimembered powder of metal, silver and
gold.
It is quite true that, in this technique, some small amount of
insoluble silver chloride may form. However, the amount of silver
chloride formed is substantially below that which would form if the
prior art concepts were carried out by dissolving gold, silver and
metal in, for example, an aqua regia medium which would serve as a
medium for all three of these metals. This is because the initial
reduction of the metal and silver into a powdered slurry renders
the metal and silver less sensitive to chlorination.
The second alternative method contemplated by this invention
provides for the total elimination of the formation of any
insoluble silver chloride or cyanide in the system and thus is
preferred where noncontaminated powders are required to be used
such as in the microelectronics art, particularly for making
resistors and the like. This second alternative method generally
comprises initially forming a soluble salt solution of the metal or
metals other than silver or gold, and silver in a manner similar to
the first alternative method. To this silver-metal salt solution
there is then added a finely divided gold powder usually having a
particle size less than about 5 microns, preferably less than about
2 microns, and most preferably substantially submicron in size. The
solution containing the gold powder, which is not soluble in the
salt solution, is then thoroughly mixed as by agitation, to form a
slurry of the gold and there is then added thereto a reducing agent
for the metal and silver. While some small amount of homogeneity is
sacrificed because the gold powder is not precipitated,
precipitation of the metal and silver into the slurried gold powder
effects a substantial amount of homogeneity to the extent that
excellent products can be made therefrom for the purposes of
microelectronic circuitry, particularly in the resistor area.
In view of this second alternative technique, this invention for
the first time provides the art with a unique powder also
considered a part of this invention. Such a powder generally
comprises a substantially homogeneous admixture of at least one
metal other than silver or gold, with silver and gold, which
admixture is substantially free of silver chloride or cyanide. As
stated above, the homogeneity achieved and contemplated is beyond
that achievable by known mechanical comminution and/or alloying
techniques and is usually to the point where the silver and other
metal are actually alloyed together according to X-ray diffraction
indications with the gold highly dispersed therethroughout. The
particle size of the powder, without comminution is generally less
than about 5 microns and usually submicron in size and thus the
powder is said to be "finely divided."
Regardless of which of the above two alternative methods is
employed to make the finely divided homogeneous powders of this
invention, this invention relates generally to all systems wherein
at least one finely divided metallic powder other than silver and
gold must be formulated in admixture with both silver and gold. In
this respect silver and gold will be referred to hereinafter since
they are the preferred combination with which this invention deals.
However, it is understood, as stated, above that this invention, as
an equivalent, contemplates other metal combinations which
experience the difficulty of contamination by coprecipitation
similar to that of silver and gold.
The metal used in admixture with the silver and gold must be
capable of forming a soluble salt in a liquid reaction medium in
which a silver salt is also soluble. In addition, the soluble salt
of the metal as well as that of the silver must be capable of being
coprecipitated in metallic form by the addition of an appropriate
reducing agent thereto.
The preferred metals for the purposes of this invention are the
known resistive metals such as, for example, palladium, rhodium,
iridium, ruthenium, indium, and mixtures thereof. Of these,
palladium is the most preferred for the preferred environmental use
of microelectronic resistors because of its excellent resistive
properties. Examples of other metals useful in combination with
silver and gold include platinum, copper, nickel, and mixtures
thereof.
The metal or metals employed may be formed into any of their known
salts which are soluble in a liquid medium in which a silver salt
is also soluble. Therefore, the formation of the metal salt and
silver salt may be independent or simultaneous. In addition, the
metal and silver anion may be the same or different, the only
criteria being that both salts are soluble in a common liquid
medium and capable of being precipitated in metallic form from the
medium by the addition of a reducing agent thereto, preferably
without the formation of any insoluble salt occurring which cannot
easily be removed from the powdered solution.
In the preferred embodiments of this invention, the liquid reaction
medium employed is water. In still more preferred embodiments, at
least one of the metals is palladium, and the soluble salts formed
are, at least in part, the soluble nitrates of palladium and
silver. Nitrate salts, of palladium and silver are simply formed by
reacting the silver and palladium with nitric acid. In this
respect, and because of the ready commercial availability of silver
nitrate solutions, it is preferred to form the metal-silver nitrate
salt solutions of this invention by separately forming a nitrate
solution of the metal and thereafter add a commercially prepared
silver nitrate solution thereto.
Examples of anions other than the nitrates which may be employed to
form, in a known way, the soluble salts of one or more of the
metals and/or of the silver include: sulfates, sulfites, bromates,
chlorites, fluorides, permanganates, nitrites, and the like.
Salts formed of the nitrate anion are generally soluble in water of
room temperature or below as are some of the salts of the other
exemplary anions listed above. In other instances, the anion salt
may be of limited solubility and thus the liquid medium may have to
be heated or cooled in order to form and/or maintain a truly
soluble solution prior to and during formation of the powders
according to this invention if contamination with the anion or salt
thereof is to be avoided or limited.
The reducing agents which cause the precipitation of metal and
silver from their medium may be any of the well-known reducing
agents or combinations thereof commonly employed in the art.
Examples of such reducing agents include hypophosphorous acid
(H.sub.3 PO.sub.2), a mixture of sodium formate and sodium
borohydride, a mixture of hydroxylamine and sodium borohydride, a
mixture of formic acid and hypophosphorous acid, a mixture or
hydrazine sulfate and hypophosphorous acid, a mixture of formic
acid and hydroquinone or a mixture of tartaric acid and
hydroquinone, or sodium bisulfite. H.sub.3 PO.sub.2 is preferred
especially with silver and palladium since minimal side reactions
and substantially no insoluble contaminating salts are formed when
it is used.
The reducing agents added to effect gold precipitation in the first
alternative technique may be any of the well-known reducing agents
for gold such as sodium sulfite, hydroquinone, hydrazine sulfate,
sulfurous acid, zinc dust, ferrous sulfate, and the like. Sodium
sulfate is preferred because it is readily available and causes the
formation of an excellent particle of gold upon its
precipitation.
In order to insure homogeneity and obtain powders in their best
possible form, especially for use in the microelectronic circuitry
art, an anti-agglomerating agent is preferably added to the salt
solution prior to any coprecipitation so as to be a part of the
ultimately formed, homogeneous, finely divided powder, regardless
of the alternative technique employed. Such anti-agglomerating
agents are inert to the system and form a slurry therein. The
anti-agglomerating agents usually employed are of a fine particle
size, i.e., less than about 5 microns and usually submicron in
size. Examples of these agents include ultra-fine alumina, an
ultra-fine TiO.sub.2, and other ultra-fine refractories. Preferred
for the purposes of this invention is ultra-fine silica (submicron
in size) which is purchasable under the trademark CAB-O-SIL.
The percentages of the various ingredients employed will vary over
a wide range depending upon the ultimate use to which the powder
may be put. It is an aspect of this invention that the percentages
attainable are quite flexible in nature. Thus, for example,
homogeneous, finely divided powders are attainable which contain
less than 1% of any given metal and more than 99% of another metal.
In the preferred uses, as disclosed in the aforementioned copending
Greenstein application, the metal other than silver and gold is a
resistive metal such as palladium or the like which may constitute
from about 5-95% by weight of the powder. Preferably the resistive
metal constitutes 15-75% by weight of the powder, and most
preferably 20-65% by weight thereof.
In preferred embodiments the remainder, i.e. 95-5% by weight, of
the metal content is a combination of silver and gold in a weight
ratio of 4:1 to 1:4 with respect to each other. In those instances
where an anti-agglomerating agent is employed, such usually need
only be employed in amounts of about 0.5-15% by weight of the total
powder, preferably 2-10% by weight and as where ultra-fine silica
is employed, usually about 5% by weight.
The concentrations employed for making the soluble salt solutions
of the metal, silver and gold, and the amount of gold powder
employed in the second alternative technique are matters generally
of convenience and may vary widely in order to insure the requisite
amounts of the various ingredients in the final powder and at the
same time be workable in the manufacturing system devised.
Generally speaking, the upper limit of concentration is the
saturation point of the solution for the salt while the lower limit
is a practical volume consideration. In this respect,
concentrations of the salt solutions lower than about 10% are
normally not employed. In the preferred instances where palladium
and silver are first formed into a soluble nitrate solution by
reaction with nitric acid, the solution containing the two salts
may have a concentration on the order of about 20-60% of the
combined salts. Similar concentrations for the gold solutions in
the first alternative technique may also be employed. The relative
concentrations of each metal within these solutions are then, of
course, adjusted in a known manner to achieve the desired powder
upon reduction and precipitation.
The gold solutions employed in the first alternative technique may
be any well-known soluble salt solution of gold. Generally
speaking, such soluble salt solutions include the various chloride
and cyanide salts of gold. A particularly preferred gold solution
for the purposes of this invention is a solution of
hydrochloroauric acid formed in a conventional manner by dissolving
the requisite amount of gold in aqua-regia (e.g. 3-4 parts HCl to 1
part HNO.sub.3).
The gold powder employed in the second alternative technique may be
any commercially available gold powder preferably having a particle
size of less than about 5 microns. A preferred gold powder, because
of its uniform and ultra-fine nature, for the purposes of this
invention, is pure gold powder produced in accordance with my
copending application Ser. No. 124,558 filed Mar. 15, 1971 and
entitled GOLD POWDER, but washed free of emulsifier or particle
size inhibitor prior to addition to the soluble salt solution. The
entire disclosure of this copending application is incorporated
herein by reference.
Generally speaking, this copending application discloses a method
for forming a gold powder, usually having a bulk density greater
than about 5.0 gms./cc and a particle size of less than about 5
microns, which comprises initially dissolving a gold-bearing
material in a HCl-HNO.sub.3 acid. Thereafter, there is added to the
solution an effective amount of an emulsifying agent capable of
coating freshly formed gold particles and preventing coalescence
and cold-welding thereof and keeping the particle size of said gold
particles less than about 5 microns. To this solution there is then
added a precipitating agent, which precipitates gold powder from
solution substantially free of contaminating chloride or other
salts. The so-formed gold particles are encapsulated with the
emulsifier which is removed, usually by washing the powder in a
solvent such as acetone or water and then drying the powder prior
to using it in this invention. The powder so formed is
substantially pure, uncontaminated, gold powder of the indicated
particle size.
A typical operating procedure for conducting the first alternative
technique described hereinabove is to initially prepare a
metal-soluble, salt solution by dissolving the metal in an
inorganic acid. For example, palladium in commercial sponge, powder
or other form may be dissolved in a 70% solution of HNO.sub.3 to
form a solution preferably of about 300-400 gms/l. of Pd. A
separate silver nitrate solution may be prepared with nitric acid
or commercially obtained. Alternatively, silver sponge, powder,
etc. may be added with the palladium in the requisite amounts to
the 70% HNO.sub.3 solution. The gold solution is prepared by
dissolving gold sponge, powder, etc. in aqua regia and removing
excess HNO.sub.3 by heating the solution. A typical concentration
of the resulting hydrochloroauric acid is about 20-60%.
The nitrate solutions of silver and metal (e.g. palladium) are then
admixed if not simultaneously formed and an antiagglomerating agent
is added thereto. The solution is agitated to slurry the
antiagglomerating agent and a reducing agent such as H.sub.3
PO.sub.2 or sodium bisulfite is added to precipitate the metal and
silver in their metallic form. Generally speaking, it is desirable
to precipitate substantially all metal and silver from the
solution, the ratio of metal to silver being controlled by the
concentrations and amounts of solutions formed. For this reason, at
least a stoichiometric amount of reducing agent and preferably
about 2 or 3 times in excess of this amount, is employed to insure
substantially complete precipitation of the metal and silver.
After this reduction and precipitation, the resulting mixture is
vigorously agitated to slurry the particles of anti-agglomerating
agent, silver, and metal in a highly dispersed fashion throughout
the liquid reaction medium. To this slurried solution is then added
the hydrochloroauric acid solution. In those instances where the
reducing agent initially employed for the metal and silver is also
a reducing agent for gold, gold precipitate will form upon mere
addition of the gold solution to the slurry. In such an event,
sufficient reducing agent is added initially to insure that
substantially all of the gold is precipitated from solution. In
those instances where a different reducing agent is employed or the
reducing agent initially added is insufficient to precipitate gold,
a reducing agent is added with continued vigorous agitation until
the precipitation of the gold is substantially complete. The
reduction reaction of gold does result in some reaction occurring
between the slurried silver and the chloride ions in the decomposed
hydrochloroauric acid. However, as stated above, because the silver
is previously precipitated, a reduction in the amount of insoluble
AgCl normally formed by common-coprecipitation of gold and silver
from a common aqua-regia solution is obtained.
The homogeneously dispersed finely divided powder of metal, silver,
gold, anti-agglomerating agent, and reduced amounts of AgCl is now
separated, as by filtration, from the liquid medium and dried. As
previously stated, the powder is of particles less than about 5
microns and usually less than about 1 micron. In addition, the
homogeneity of dispersion of the individual ingredients is far
beyond that achievable by simple comminution and/or alloying.
Furthermore, it has been found that in most instances, as indicated
by X-ray defraction techniques, the silver is alloyed with the
metal.
In a typical procedure for conducting the second alternative
technique described hereinabove the solutions of metal and silver,
preferably in nitrate form, are formed and admixed in a manner
similarly as described in the first alternative technique. Gold
powder having a particle size of less than about 5 microns is then
added to the common solution. Preferably, as stated, the gold
powder is that as produced by my afore-cited copending application.
The anti-agglomerating agent is added either prior to,
simultaneously with, or after the gold powder is added and the
particles of this agent and gold are slurried in the solution with
vigorous agitation.
A reducing agent in the before-described amounts is then added to
precipitate the metal and silver and the homogeneous powder is
obtained and processed similarly as described with respect to the
first alternative technique. The product of the second alternative
technique differs markedly from the first alternative technique
because no chloride (or cyanide) is used to dissolve the gold and
thus no chloride (or cyanide) is present to form a contaminating
insoluble salt in the powder. As previously stated, this is of
significant value in the microelectronic resistor art.
The following examples are presented as illustrative of this
invention.
EXAMPLE 1
FIRST ALTERNATIVE TECHNIQUE
A gold reactant solution (HAuCl.sub.4) was prepared from pure gold
sponge. This was accomplished by reacting 20 gms. of Au in an
aqua-regia solution and diluting the solution to 160 ml. with
H.sub.2 O. A palladium nitrate solution was prepared by dissolving
53.8 grams of the metal in 150 ml (70%) HNO.sub.3. The resultant Pd
(NO.sub.3).sub.2 solution had a volume of 160 ml. A silver nitrate
solution was formed by dissolving 115.3 grams of silver nitrate
crystals in 150 ml. of distilled water. The total volume of the
resultant solution was 175 ml.
To start the process 80 ml. of the Pd (NO.sub.3).sub.2 solution, 87
ml. of the AgNO.sub.3 solution, and 100 ml. of H.sub.2 O were
charged into a reactor. The reactor employed was a 1 liter baffled
glass kettle provided with a pitched marine-type propeller agitator
capable of operating at up to 500 rpm. A cooling bath surrounded
the kettle and reaction temperature was measured by a thermometer
immersed in the reaction solution.
After mixing the initial charge thoroughly, an admixture containing
3.75 gms. of CAB-O-SIL (ultrafine silica) in 50 ml. of H.sub.2 O
was added and slurried. With continued agitation the reduction
reaction was carried out by adding 170 ml. of a solution containing
75 grams of NaHSO.sub.3 to the reactor. The NaHSO.sub.3 addition
took place over a 14 minute time interval and the reaction media
was maintained at 38.degree. C.
Agitation was continued to slurry the precipitated silver and
palladium metals with the anti-agglomerant and the slurry was
prepared for HAuCl.sub.4 addition and reduction by the addition
thereto of 15 grams of Na.sub.2 SO.sub.3 in 100 ml. of H.sub.2 O.
This was followed by rapid addition of 80 ml. of the HAuCl.sub.4
solution. An additional 100 ml. of the Na.sub.2 SO.sub.3 solution
was added to insure complete reduction of the gold. After 5 minutes
of further mixing, the agitator was shut off.
After permitting the reactants to settle out for 30 minutes, the
solids were separated from the liquid by filtration. The solids
were washed by slurrying in H.sub.2 O and filtering several times.
The final wash was followed by drying at 110.degree. C overnight. A
finely divided, homogeneous powder was obtained. The powder
analyzed as containing on a metal basis by weight, 47.4% Ag, 14.55%
Au, and 38.05% Pd. The powder also contained 13.02% by weight
chloride salt, assuming all chlorides to be silver chloride.
EXAMPLE 2
SECOND ALTERNATIVE TECHNIQUE
250 gms. of a powder consisting of by weight 52% Ag, 35% Pd, and
13% Au were prepared by the following technique.
A Pd(NO.sub.3).sub.2 solution was prepared by adding 87.5 grams of
palladium metal to 245 ml. of a 70% HNO.sub.3 solution. The
addition of Pd to the acid was conducted slowly and the nitrous
oxide gas formed was vented. An AgNO.sub.3 solution was prepared by
adding 204.3 grams of AgNO.sub.3 crystals to 500 ml. of distilled
water with stirring. The gold powder employed had a particle size
of less than 5 microns and was produced in accordance with Example
1 of my aforementioned copending application. 32.5 grams of the
powder were washed free of emulsifier in acetone and thoroughly
dried for use hereinafter.
The AgNO.sub.3 solution was charged into a reaction vessel which
consisted of a 5 liter glass baffled flask with a propeller type
agitator. The reaction vessel was provided with a heating and
cooling bath to regulate the temperature. One liter of distilled
water was added and the solution mixed rapidly by the propeller
agitator. At this time, 12.5 gms. of CAB-O-SIL (ultrafine
SiO.sub.2) anti-agglomerating agent was suspended in the reaction
medium. Five minutes of vigorous stirring were then allowed to
proceed.
Next, the Pd(NO.sub.3).sub.2 solution was added and the 32.5 gms.
of gold powder suspended in the combined solution with continuing
agitation. An additional 375 ml. of distilled water were also added
to aid in the washing in of the gold powder.
The reduction reaction was initiated, with continued vigorous
agitation, by adding to the reaction mass, dropwise, 390 gms. of a
50% hypophosphorous acid solution diluted in 400 ml. of distilled
water. The total time to add all of the H.sub.3 PO.sub.2 was about
1/2 hour. During that time the resulting exothermic reaction was
maintained at about 50.degree. C by cooling water. After the
reducing agent had been added the reaction media was held at the
reaction temperature with mixing for 1 hour to insure completeness
and optimum homogeneity of the metal slurry.
Thereafter, the metal slurry was filtered and washed with about two
liters of distilled water and placed in a 110.degree. C drying oven
overnight.
The resulting dry powder was a microscopically homogeneous black
mass having substantially no particles greater than about 5
microns. Analysis indicated the powder to consist on a metal basis
of, by weight, 52% Ag, 35% Pd, and 13% Au and further indicated
that no contaminating salts were present therein. This powder, as
evidenced by the examples in the aforementioned copending
Greenstein application, may be used as a valuable starting material
for microelectronic resistor production.
Once given the above disclosure many other features, modifications,
and improvements will become apparent to the skilled artisan. Such
other features, modifications, and improvements are therefore
considered to be a part of this invention, the scope of which is to
be determined by the following claims.
* * * * *