U.S. patent application number 10/588970 was filed with the patent office on 2009-01-22 for highly crystalline silver powder and method for producing the same.
This patent application is currently assigned to Mitsui Mining & Smelting Co., Ltd.. Invention is credited to Taku Fujimoto, Takuya Sasaki, Hiroyuki Shimamura, Katsuhiko Yoshimaru.
Application Number | 20090023007 10/588970 |
Document ID | / |
Family ID | 34836167 |
Filed Date | 2009-01-22 |
United States Patent
Application |
20090023007 |
Kind Code |
A1 |
Fujimoto; Taku ; et
al. |
January 22, 2009 |
HIGHLY CRYSTALLINE SILVER POWDER AND METHOD FOR PRODUCING THE
SAME
Abstract
An object of the present invention is to provide highly
crystalline silver powder which is characterized in fine particles,
showing high dispersibility, it's particle size distribution is not
excessively sharp but relatively broad and crystallites are large;
and a method for producing the same. In order to achieve the
object, a method for producing highly crystalline silver powder is
characterized in that mixing a first aqueous solution and a second
aqueous solution, wherein the first aqueous solution contains
silver nitrate, a dispersing agent and nitric acid, and the second
solution contains ascorbic acid. For dispersing agent,
polyvinylpyrrolidone or gelatin is preferred. Highly crystalline
silver powder produced by the above-described method is preferred
to be a crystallite diameter of 300 .ANG. or more, an average
particle diameter D.sub.50 in the range from 0.5 .mu.m to 10 .mu.m,
and a thermal shrinkage rate for the length direction after heating
at 700.degree. C. in the range from -3% to 3%. For ratio
D.sub.90/D.sub.10 of the silver powder is preferred to be in the
range from 2.1 to 5.0.
Inventors: |
Fujimoto; Taku; (Yamaguchi,
JP) ; Sasaki; Takuya; (Yamaguchi, JP) ;
Yoshimaru; Katsuhiko; (Yamaguchi, JP) ; Shimamura;
Hiroyuki; (Tokyo, JP) |
Correspondence
Address: |
ROTHWELL, FIGG, ERNST & MANBECK, P.C.
1425 K STREET, N.W., SUITE 800
WASHINGTON
DC
20005
US
|
Assignee: |
Mitsui Mining & Smelting Co.,
Ltd.
Tokyo
JP
|
Family ID: |
34836167 |
Appl. No.: |
10/588970 |
Filed: |
February 4, 2005 |
PCT Filed: |
February 4, 2005 |
PCT NO: |
PCT/JP05/01660 |
371 Date: |
August 10, 2006 |
Current U.S.
Class: |
428/546 ;
420/501; 75/370 |
Current CPC
Class: |
B22F 1/0044 20130101;
B22F 9/24 20130101; Y10T 428/12014 20150115 |
Class at
Publication: |
428/546 ; 75/370;
420/501 |
International
Class: |
C22C 5/06 20060101
C22C005/06; B22F 9/24 20060101 B22F009/24 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 10, 2004 |
JP |
2004-034121 |
Claims
1. A method for producing highly crystalline silver powder which is
characterized in that mixing a first aqueous solution and a second
aqueous solution, wherein the first aqueous solution contains
silver nitrate, a dispersing agent and nitric acid, and the second
solution contains ascorbic acid.
2. The method for producing highly crystalline silver powder
according to claim 1, wherein the dispersing agent is
polyvinylpyrrolidone.
3. The method for producing highly crystalline silver powder
according to claim 1, wherein the dispersing agent is a
gelatin.
4. The method for producing highly crystalline silver powder
according to claim 2, the first aqueous solution when it contains
100 parts by weight of silver nitrate, it further contains 5 parts
by weight to 60 parts by weight of polyvinylpyrrolidone and 35
parts by weight to 70 parts by weight of nitric acid.
5. The method for producing highly crystalline silver powder
according to claim 3, the first aqueous solution when it contains
100 parts by weight of silver nitrate, it further contains 0.5
parts by weight to 10 parts by weight of gelatin and 35 parts by
weight to 70 parts by weight of nitric acid.
6. The method for producing highly crystalline silver powder
according to claim 1, when the first aqueous solution contains 100
parts by weight of silver nitrate, ascorbic acid contained in the
second aqueous solution to be mixed with the first aqueous solution
is 30 parts by weight to 90 parts by weight.
7. The method for producing highly crystalline silver powder
according to claim 1, when the second aqueous solution contains 100
parts by weight of ascorbic acid, nitric acid contained in the
first aqueous solution to be mixed with the second aqueous solution
is 40 parts by weight to 150 parts by weight.
8. Highly crystalline silver powder which is characterized in that
the powder is produced by the method according to claim 1.
9. The highly crystalline silver powder according to claim 8,
wherein crystallite diameter of the powder is 300 .ANG. or
more.
10. The highly crystalline silver powder according to claim 8,
wherein an average particle diameter D.sub.50 of the powder is in
the range from 0.5 .mu.m to 10 .mu.m. (where D.sub.50 is a median
diameter (.mu.m) calculated as 50% of volume cumulative
distributions examined by a laser diffraction scattering particle
size distribution measuring method).
11. The highly crystalline silver powder according to claim 8,
wherein a thermal shrinkage rate of the powder after heating at
700.degree. C. is in the range from -3% to 3%.
12. The highly crystalline silver powder according to claim 8,
wherein a ratio D.sub.90/D.sub.10 of the powder is in the range
from 2.1 to 5.0 (where D.sub.10 is diameter (.mu.m) at 10% of
volume cumulative distributions and D.sub.90 is diameter (.mu.m) at
90% of volume cumulative distributions examined by a laser
diffraction scattering particle size distribution measuring method,
respectively).
13. Highly crystalline silver powder which is characterized in that
a crystallite diameter is 300 .ANG. or more, an average particle
diameter D.sub.50 is in the range from 0.5 .mu.m to 10 .mu.m, and a
thermal shrinkage ratio after heating at 700.degree. C. in the
length direction is in the range from -3% to 3%.
14. The highly crystalline silver powder according to claim 13,
wherein a ratio D.sub.90/D.sub.10 of the powder is in the range
from 2.1 to 5.0 (where D.sub.10 is diameter (.mu.m) at 10% of
volume cumulative distributions and D.sub.90 is diameter (.mu.m) at
90% by volume of cumulative distributions examined by a laser
diffraction scattering particle size distribution measuring method,
respectively).
Description
TECHNICAL FIELD
[0001] The present invention relates to highly crystalline silver
powder and a method for producing the same, and more specifically,
to highly crystalline silver powder preferable for production of a
conductive paste that can significantly reduce the size of the
electrode or circuit of, for example, a chip devices, plasma
display panel and the like with high density, high accuracy and
high reliability. In particular it can enable to form a minute
wiring or a thin and flat coating film with high density, high
accuracy and high reliability. It is because the silver powder is
composed of fine particles, has high dispersibility, particle size
distribution is not excessively sharp but is relatively broad, and
crystallites are large. So when it is used as the material for the
conductive paste, the dispersibility of the silver powder and the
filling properties of the paste are excellent, and then the size of
the electrode or circuit formed from the thick silver film can be
reduced, and the thick silver film obtained from the conductive
paste excels in thermal shrinkage resistance, and has low specific
resistance (resistivity).
BACKGROUND ART
[0002] Heretofore, as a method for forming an electrode or circuit
of an electronic part or the like, there has been known a method to
form a circuit by printing a conductive paste wherein silver
powder, which is a conductive material, is dispersed in a paste, on
a substrate, and baking or curing the paste to form a thick silver
film. However in recent years, due to the improvement in functions
of electronic equipment, so called down sizing and wiring density
increase of electronic devices have been demanded; consequently, it
has been desired that silver powder, which is the material for the
conductive paste, excels in filling properties and dispersibility
despite of fine particles when used in the conductive paste. In the
present invention, dispersibility means the difficulty of
aggregating the primary particles of the silver powder with each
other, unless otherwise specified such as the dispersibility of
silver powder in a paste. For example, the state of high
dispersibility means the state wherein there is little or no
proportion of primary particles are aggregated with each other; and
the state of low dispersibility means the state wherein there is
much proportion or all of aggregated primary particles are
aggregated with each other.
[0003] A substrate on which the above-described conductive paste is
printed is normally used in a part of a ceramic substrate where
heat generation is large, such as the IC package. However, when the
conductive paste is printed on the ceramic substrate, since the
thermal shrinkage of the ceramic substrate is generally different
from the thermal shrinkage of the thick silver film formed from the
conductive paste, there is possibility wherein the thick silver
film is separated from the ceramic substrate, or the substrate
itself is deformed. Therefore, it is preferable that the rate of
the thermal shrinkage of the ceramic substrate is as close to the
rate of the thermal shrinkage of the thick silver film formed from
the conductive paste.
[0004] As a cause of the thermal shrinkage of the above-described
thick silver film in such baking process, it is considered that the
silver powder in the conductive paste causes sintering during
baking. Specifically, it is considered that the silver powder is a
polycrystalline construction composed of fine crystallites, and
when the conductive paste containing silver powder is baked for the
formation of the thick silver film, the fine crystallites in the
silver powder are sintered, and dimension change between before and
after formation of the sintered thick silver film cause thermal
shrinkage. Therefore, in order to obtain a conductive paste
containing silver powder with less thermal shrinkage, it is
desirable that the crystallites in the silver powder are as large
as possible so as to minimize the sintering of the
crystallites.
[0005] In recent years, the improvement of the high-frequency
signal characteristics of circuits and the improvement of
dimensional stability of substrates before and after sintering are
demanded, and for this, as a substrate on which the thick silver
film is formed, an LTCC (low temperature co-baked ceramic)
substrate has been used substituting the above-described ordinary
ceramic substrates. Furthermore, since the LTCC substrate is
obtained by sintering a green sheet of the LTCC substrate with a
conductive paste containing a low-resistance conductor such as
silver powder together, compared with the technique to form the
circuit of a thick silver film by printing a conductive paste using
the above-described ordinary ceramic substrate, the number of
sintering steps is less, the film thickness of the ceramic
dielectric can be easily controlled, the conductor resistance of
the circuit formed from the conductive paste is lowered, and the
coplanarity of the substrate can be easily improved. However, since
the LTCC is much excellent in dimensional stability, silver powder,
which is the material of the conductive paste used in it is
strongly required to have less thermal shrinkage, and therefore, it
is strongly desired that the crystallites in the silver powder be
large.
[0006] If the crystallites in the silver powder are large as
described above, the content of impurities in the silver powder is
generally lowered, and thereby the specific resistance of the
circuit formed from the thick silver film is easily lowered;
therefore, this is also preferable in the aspect that the
conductive paste containing silver powder can be used not only in
the circuit formed by baking as described above, but also in the
circuit formed without baking.
[0007] As described above, it is desired for silver powder used in
the conductive paste, that the silver powder which is composed of
fine particles, having high dispersibility, whose particle size
distribution is not excessively sharp but relatively broad, and the
crystallites are large.
[0008] Whereas in Patent Document 1 (Japanese Patent Laid-Open No.
2000-1706), a method for producing highly crystalline silver powder
wherein an aqueous solution of silver nitrate and a solution
prepared by dissolving acrylic acid monomer in an aqueous solution
of L-ascorbic acid are mixed and allowed to react simultaneously.
According to the above-described method, highly crystalline silver
powder whose crystallite size is 400 .ANG. or more, and the range
of the particle diameters is as narrow as 2 to 4 .mu.m can be
obtained.
[0009] Patent Document 1: Japanese Patent Laid-Open No. 2000-1706
(page 1)
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0010] However, although the silver powder described in Patent
Document 1 is composed of fine particles with large crystallites,
it is difficult that the thermal shrinkage at a high temperature of
for example about 700.degree. C. is sufficiently reduced. The
silver powder has a large thermal shrinkage at a high temperature
even if the crystallites are sufficiently large, and the reason is
estimated to be caused by that the range of the particle diameters
of the silver powder is between 2 and 4 .mu.m, and since the
particle size distribution is excessively sharp, gaps are formed
between the particles of the silver powder, and the filling of the
silver powder is lowered. Therefore, when the silver powder was
used to prepare a conductive paste to form a thick silver film or
to form a circuit using an LTCC substrate, the dimension change
between before and after forming the circuit was enlarged causing a
problem of warpage in an ordinary ceramic substrate or LTCC
substrate, particularly the LTCC substrate.
[0011] Therefore, it is an object of the present invention to
provide highly crystalline silver powder composed of fine
particles, having high dispersibility, whose particle size
distribution is not excessively sharp but relatively broad, and the
crystallites are large; and a method for producing the same.
Means for Solving the Problems
[0012] Under such a situation, the present inventors carried out
keen studies, and found to complete the present invention. It is
that if silver powder is produced using a method to mix a first
aqueous solution containing silver nitrate, a dispersing agent and
nitric acid, and a second aqueous solution containing ascorbic
acid, highly crystalline silver powder composed of fine particles,
having high dispersibility, whose particle size distribution is not
excessively sharp but relatively broad, and the crystallites are
large. And it can make the thick silver film obtained from the
conductive paste excel in thermal shrinkage resistance, can be
obtained.
[0013] Specifically, the present invention provides a method for
producing highly crystalline silver powder which is characterized
in that mixing a first aqueous solution and a second aqueous
solution, wherein the first aqueous solution contains silver
nitrate, a dispersing agent and nitric acid, and the second
solution contains ascorbic acid.
[0014] The present invention also provides the method for producing
highly crystalline silver powder characterized in that the
dispersing agent is polyvinylpyrrolidone.
[0015] The present invention function provides the method for
producing highly crystalline silver powder characterized in that
the dispersing agent is a gelatin.
[0016] The present invention still further provides the method for
producing highly crystalline silver powder characterized in that
the first aqueous solution when it contains 100 parts by weight of
silver nitrate, it further contains 5 parts by weight to 60 parts
by weight of polyvinylpyrrolidone and 35 parts by weight to 70
parts by weight of nitric acid.
[0017] The present invention also provides the method for producing
highly crystalline silver powder characterized in that the first
aqueous solution when it contains 100 parts by weight of silver
nitrate, it further contains 0.5 parts by weight to 10 parts by
weight of gelatin and 35 parts by weight to 70 parts by weight of
nitric acid.
[0018] The present invention further provides the method for
producing highly crystalline silver powder characterized in that
the first aqueous solution contains 100 parts by weight of silver
nitrate, ascorbic acid contained in the second aqueous solution to
be mixed with the first aqueous solution is 30 parts by weight to
90 parts by weight.
[0019] The present invention still further provides the method for
producing highly crystalline silver powder characterized in that
the second aqueous solution contains 100 parts by weight of
ascorbic acid, nitric acid contained in the first aqueous solution
to be mixed with the second aqueous solution is 40 parts by weight
to 150 parts by weight.
[0020] The present invention also provides highly crystalline
silver powder characterized in that the powder is produced by the
method for producing highly crystalline silver powder.
[0021] The highly crystalline silver powder produced by the method
for producing highly crystalline silver powder is characterized in
that the powder has a crystallite diameter of the powder is 300
.ANG. or more.
[0022] The highly crystalline silver powder is characterized in
that an average particle diameter D.sub.50 of the powder is in the
range from 0.5 .mu.m to 10 .mu.m. (where D.sub.50 is a median
diameter (.mu.m) calculated as 50% of volume cumulative
distributions examined by a laser diffraction scattering particle
size distribution measuring method).
[0023] The highly crystalline silver powder is characterized in
that a thermal shrinkage rate of the powder after heating at
700.degree. C. is in the range from -3% to 3%.
[0024] The highly crystalline silver powder is characterized in
that a ratio D.sub.90/D.sub.10 of the powder is in the range from
2.1 to 5.0 (where D.sub.10 is diameter (.mu.m) at 10% of volume
cumulative distributions and D.sub.90 is diameter (.mu.m) at 90% of
volume cumulative distributions examined by a laser diffraction
scattering particle size distribution measuring method,
respectively).
[0025] The highly crystalline silver powder is characterized in
that a crystallite diameter is 300 .ANG. or more, an average
particle diameter D.sub.50 is in the range from 0.5 .mu.m to 10
.mu.m, and a thermal shrinkage ratio after heating at 700.degree.
C. in the length direction is in the range from -3% to 3%.
[0026] The highly crystalline silver powder is characterized in
that a ratio D.sub.90/D.sub.10 of the powder is in the range from
2.1 to 5.0 (where D.sub.10 is diameter (.mu.m) at 10% of volume
cumulative distributions and D.sub.90 is diameter (.mu.m) at 90% by
volume of cumulative distributions examined by a laser diffraction
scattering particle size distribution measuring method,
respectively).
ADVANTAGE OF THE INVENTION
[0027] Since the highly crystalline silver powder according to the
present invention is of fine particles, has high dispersibility,
and whose particle size distribution is not excessively sharp and
relatively broad, and crystallites are large. And when it is used
as a material for a conductive paste, the dispersibility of the
silver powder to the conductive paste and the filling property of
the conductive paste with the silver powder in can be excellent; an
electrode, circuit and the like can be finer; the thick silver film
obtained from the conductive paste can be excellent in thermal
shrinkage resistance; and the resistivity thereof can be lowered.
The method for producing the highly crystalline silver powder
according to the present invention can efficiently produce the
above-described highly crystalline silver powder according to the
present invention.
BEST MODE FOR CARRYING OUT THE INVENTION
(Highly Crystalline Silver Powder According to the Present
Invention)
[0028] The highly crystalline silver powder according to the
present invention is substantially granular powder. The average
particle diameter D.sub.50 of the highly crystalline silver powder
according to the present invention is 0.5 .mu.m to 10 .mu.m,
preferably 1 .mu.m to 5 .mu.m. It is preferable that the average
particle diameter D.sub.50 is within the above-described range
because when the silver powder is used in a conductive paste, the
filling of the silver powder in the conductive paste is excellent,
and the circuit or the like formed from the thick silver film can
be made finer. On the other hand, the average particle diameter
D.sub.50 less than 0.5 .mu.m is not preferable because the
collection of the silver powder becomes difficult; and the average
particle diameter D.sub.50 exceeding 10 .mu.m is not preferable
because the silver powder is often aggregated. Here, the average
particle diameter D.sub.50 means the volume average particle
diameter measured by a laser diffraction scattering method, that
is, the particle diameter at 50% cumulative distribution.
[0029] The highly crystalline silver powder according to the
present invention has a crystallite diameter of 300 .ANG. or more,
preferably 350 .ANG. to 600 .ANG.. It is preferable that the
crystallite diameter is within the above-described range because
when the conductive paste containing the silver powder is applied
onto a ceramic substrate, and baked to form a circuit or the like
composed of a thick silver film, the thermal shrinkage of the thick
silver film between before and after baking becomes close to the
thermal shrinkage of the ceramic substrate, and the effect to
suppress the separation of the thick silver film from the ceramic
substrate, or the deformation of the ceramic substrate due to the
dimension change of the thick silver film is large.
[0030] On the other hand, it is not preferable that the crystallite
diameter is less than 300 .ANG., because when the conductive paste
is applied onto a ceramic substrate, and baked to form a circuit or
the like composed of a thick silver film, the shrinkage of the
thick silver film between before and after baking becomes larger
than the shrinkage of the ceramic substrate, and the thick silver
film is easily separated from the ceramic substrate or the ceramic
substrate is easily deformed due to the dimension change of the
thick silver film. Here, the crystallite diameter means the average
crystallite diameter obtained from the half-value width of the
diffraction angle of each crystal face obtained by conducting X-ray
diffraction for silver powder sample.
[0031] The D.sub.90/D.sub.10 of the highly crystalline silver
powder according to the present invention is normally 2.1 to 5.0,
preferably 2.5 to 4.7. In the present invention, D.sub.10 is
diameter (.mu.m) at 10% by volume cumulative distributions, and
D.sub.90 is diameter (.mu.m) at 90% by volume cumulative
distributions examined by a laser diffraction scattering particle
size distribution measuring method, respectively. The
D.sub.90/D.sub.10 is an indicator to indicate fluctuation, and a
large D.sub.90/D.sub.10 indicates that the fluctuation of particle
size distribution is large. It is preferable that D.sub.90/D.sub.10
is within the above-described range, because the particle size
distribution of the silver powder is not excessively sharp, but is
relatively broad, and when a circuit is formed with a conductive
paste using the silver powder, the filling properties of the silver
powder and the thermal shrinkage resistance of the circuit becomes
excellent, specifically the dimension change of the circuit between
before and after baking is easily reduced.
[0032] On the other hand, it is not preferable that
D.sub.90/D.sub.10 is less than 2.1, because the particle size
distribution becomes excessively sharp, and when a circuit is
formed with a conductive paste using the silver powder, the filling
properties of the silver powder easily becomes inferior,
specifically, the dimension change of the circuit between before
and after baking easily becomes large. It is also not preferable
that D.sub.90/D.sub.10 exceeds 5.0, because the particle size
distribution of the silver powder becomes excessively broad to make
silver powder filling properties inferior when forming a circuit
using a conductive paste using the silver powder, and thus the
thermal shrinkage resistance of the circuit is easily deteriorated,
specifically, the dimension change of the circuit between before
and after baking easily becomes large.
[0033] The thermal shrinkage of the highly crystalline silver
powder according to the present invention at 700.degree. C. in the
length direction is normally in the range from -3% to 3%,
preferably in the range from -2% to 2%. In the present invention,
the thermal shrinkage at 700.degree. C. in the length direction
means the thermal shrinkage of a pellet in the length direction
measured using thermomechanical analysis (TMA) in a sample in which
silver powder is formed into a pellet.
[0034] In the highly crystalline silver powder according to the
present invention, the resistivity of the silver coating film baked
at a relatively low temperature, for example 300.degree. C., is
low. Specifically, even if the highly crystalline silver powder is
sintered at a low temperature, the resistivity of the sintered
article can be easily reduced. The reason why the resistivity of
the silver coating film thus baked at 300.degree. C. is estimated
to be because the movement of electrons in the silver powder is
smoothened due to large crystallite diameter.
[0035] The specific surface area of the highly crystalline silver
powder according to the present invention is normally 0.10
m.sup.2/g to 1.0 m.sup.2/g, preferably 0.20 m.sup.2/g to 0.90
m.sup.2/g. The specific surface area of less than 0.10 m.sup.2/g is
not preferable because the formation of the finer electrode or
circuit composed of a thick silver film tends to be difficult. The
specific surface area exceeding 1.0 m.sup.2/g is also not
preferable because the formation of paste of silver powder tends to
be difficult. The specific surface area in the present invention is
the BET specific surface area.
[0036] The tap density of the highly crystalline silver powder
according to the present invention is normally 3.8 g/cm.sup.3 or
more, preferably 4.0 cm.sup.3 to 6.0 cm.sup.3. The tap density
within the above-described range is preferable because the filling
of the silver powder in the paste of the highly crystalline silver
powder is favorable in the preparation of the conductive paste to
facilitate the preparation of the conductive paste, and on the
coating film formation of the conductive paste, adequate gaps are
formed between the particles of the highly crystalline silver
powder to facilitate binder removal from the coating film on baking
the coating film and to improve the density of the baked film, and
as a result, the resistance of the thick silver film can be easily
lowered. The highly crystalline silver powder according to the
present invention can be produced by the following method.
(Method for Producing the Highly Crystalline Silver Powder
According to the Present Invention)
[0037] The method for producing the highly crystalline silver
powder according to the present invention is to mix a first aqueous
solution containing silver nitrate, a dispersing agent and nitric
acid, and a second aqueous solution containing ascorbic acid.
[0038] The first aqueous solution in the present invention is an
aqueous solution containing silver nitrate, a dispersing agent and
nitric acid. As the water used for the preparation of the first
aqueous solution, pure water, ion-exchanged water, ultra-pure water
or the like is preferable for preventing impurities to be mixed.
The silver nitrate used in the present invention is not
specifically limited, but either solid or an aqueous solution can
be used.
[0039] The examples of dispersing agents used in the present
invention include polyvinylpyrrolidone (PVP), gelatin, polyethylene
glycol, polyvinyl alcohol and the like. In the present invention,
the term "gelatin" is used in the concept including glue. Among the
dispersing agents used in the present invention,
polyvinylpyrrolidone and gelatin are preferable because the thermal
shrinkage resistance of silver powder can be particularly elevated.
In the present invention, by compounding the dispersing agent in
the first aqueous solution, the dispersion of the silver powder can
be improved, and there is the effect of making the silver powder
have fine particles, and making the particle size distribution not
excessively sharp and relatively broad.
[0040] The nitric acid used in the present invention is not
specifically limited, but either concentrated nitric acid or
diluted nitric acid can be used. In the present invention, since
the rate of the reaction to form silver from silver ions is
controlled to be relatively slow by compounding sulfuric acid in
the first aqueous solution, there is the effect of making the
particle size distribution not excessively sharp and relatively
broad, and enlarging the crystallites. If silver powder is produced
without compounding nitric acid, the rate of the reaction to form
silver from silver ions is excessively rapid and the reaction
starts immediately, the obtained silver powder has a smaller
particle diameters, and the crystallite diameter tends to be
reduced compared with the case to produce the silver powder by
compounding nitric acid as in the present invention.
[0041] When the dispersing agent is polyvinylpyrrolidone, the first
aqueous solution contains normally 5 parts by weight to 60 parts by
weight, preferably 15 parts by weight to 50 parts by weight, and
more preferably 20 parts by weight to 40 parts by weight of
polyvinylpyrrolidone to 100 parts by weight of silver nitrate. It
is preferable that the compounding quantity of polyvinylpyrrolidone
is within the above-described range, because the dispersion of the
silver powder is improved, and the particle size distribution of
the silver powder tends to be not excessively sharp but to be
relatively broad. On the other hand, it is not preferable that the
compounding quantity of polyvinylpyrrolidone is less than 5 parts
by weight, because the obtained silver powder is easily aggregated;
and it is not preferable that the compounding quantity of
polyvinylpyrrolidone exceeds 60 parts by weight, because the
impurity concentration in the obtained silver powder is easily
elevated, environment is easily contaminated, and the production
costs tend to be high.
[0042] When the dispersing agent is gelatin, the first aqueous
solution contains normally 0.5 part by weight to 10 parts by
weight, preferably 1 part by weight to 8 parts by weight, and more
preferably 2 parts by weight to 6 parts by weight of gelatin to 100
parts by weight of silver nitrate. It is preferable that the
compounding quantity of gelatin is within the above-described
range, because the dispersion of the silver powder is improved, and
the particle size distribution of the silver powder tends to be not
excessively sharp but to be relatively broad. On the other hand, it
is not preferable that the compounding quantity of gelatin is less
than 0.5 parts by weight, because the obtained silver powder is
easily aggregated; and it is not preferable that the compounding
quantity of gelatin exceeds 10 parts by weight, because the
impurity concentration in the obtained silver powder is easily
elevated, environment is easily contaminated, and the production
costs tend to be high.
[0043] When the dispersing agent is polyvinylpyrrolidone, the first
aqueous solution contains normally 1 part by weight to 10 parts by
weight, preferably 2 parts by weight to 4 parts by weight of
gelatin to 100 parts by weight of water. It is preferable that the
compounding quantity of polyvinylpyrrolidone is within the
above-described range, because the dispersion of the silver powder
is improved, and the particle size distribution of the silver
powder tends to be not excessively sharp but to be relatively
broad. On the other hand, it is not preferable that the compounding
quantity of polyvinylpyrrolidone is less than 1 parts by weight,
because the obtained silver powder is easily aggregated; and it is
not preferable that the compounding quantity of
polyvinylpyrrolidone exceeds 10 parts by weight, because the
impurity concentration in the obtained silver powder is easily
elevated, environment is easily contaminated, and the production
costs tend to be high.
[0044] When the dispersing agent is gelatin, the first aqueous
solution contains normally 0.1 parts by weight to 5 parts by
weight, preferably 0.4 parts by weight to 2 parts by weight of
gelatin to 100 parts by weight of water. It is preferable that the
compounding quantity of gelatin is within the above-described
range, because the dispersion of the silver powder is improved, and
the particle size distribution of the silver powder tends to be not
excessively sharp but to be relatively broad. On the other hand, it
is not preferable that the compounding quantity of gelatin is less
than 0.1 part by weight, because the obtained silver powder is
easily aggregated; and it is not preferable that the compounding
quantity of gelatin exceeds 5 parts by weight, because the impurity
concentration in the obtained silver powder is easily elevated,
environment is easily contaminated, and the production costs tend
to be high.
[0045] The first aqueous solution contains normally 35 parts by
weight to 70 parts by weight, preferably 40 parts by weight to 60
parts by weight, and more preferably 48 parts by weight to 54 parts
by weight of nitric acid to 100 parts by weight of silver nitrate.
It is preferable that the compounding quantity of nitric acid is
within the above-described range, because the dispersion of the
silver powder is improved, and the particle size distribution of
the silver powder tends to be not excessively sharp but to be
relatively broad. On the other hand, it is not preferable that the
compounding quantity of nitric acid is less than 35 parts by
weight, because the crystallization ability of the silver powder is
easily lowered; and it is not preferable that the compounding
quantity of nitric acid exceeds 70 parts by weight, because the
obtained silver powder is easily aggregated. In the present
invention, the compounding quantity of nitric acid means the
compounding quantity converted to concentrate nitric acid of a
concentration of 61%.
[0046] The second aqueous solution in the present invention is an
aqueous solution containing ascorbic acid. As the water used for
the preparation of the first aqueous solution, pure water,
ion-exchanged water, ultra-pure water or the like is preferable for
preventing impurities to be mixed. As the ascorbic acid used in the
present invention, either L-isomer or D-isomer can be used.
[0047] In the production method according to the present invention,
the above-described first aqueous solution and second aqueous
solution are mixed to deposit highly crystalline silver powder in
the blended solution. The examples of mixing modes include a method
wherein the first aqueous solution is agitated and the second
aqueous solution is mixed thereto. As the method to add the second
aqueous solution, the entire quantity of the second aqueous
solution can be added to the first aqueous solution at once, or the
second aqueous solution can be gradually added to the first aqueous
solution a little at a time. When the dispersing agent in the first
aqueous solution is polyvinylpyrrolidone, the method to add the
entire quantity of the second aqueous solution to the first aqueous
solution is preferable because the silver powder composed of fine
particles, whose particle size distribution is not excessively
sharp but is relatively broad can be easily obtained; when the
dispersing agent in the first aqueous solution is gelatin, the
method to add the second aqueous solution gradually to the first
aqueous solution a little at a time is preferable because the
particle diameter of silver powder can be easily controlled.
[0048] In mixing the first aqueous solution and the second aqueous
solution, the solutions are mixed so that the quantity of ascorbic
acid contained in the second aqueous solution is normally 30 parts
by weight to 90 parts by weight, preferably 40 parts by weight to
80 parts by weight, and more preferably 50 parts by weight to 75
parts by weight to 100 parts by weight of silver nitrate contained
in the first aqueous solution. It is preferable that the
compounding quantity of ascorbic acid to silver nitrate is within
the above-described range because the yield of the silver powder is
easily elevated. On the other hand, it is not preferable that the
compounding quantity of ascorbic acid to 100 parts by weight silver
nitrate is less than 30 parts by weight, because reduction is
insufficient and the yield of the silver powder is easily lowered;
and it is not preferable that the compounding quantity of ascorbic
acid to 100 parts by weight silver nitrate exceeds 90 parts by
weight, because environment is easily contaminated, and the
production costs tend to be high.
[0049] In mixing the first aqueous solution and the second aqueous
solution, the solutions are mixed so that the silver ion
concentration in the obtained mixed solution is normally 10 g/l to
80 g/l, preferably 30 g/l to 65 g/l. It is preferable that the
silver ion concentration in the blended solution is within the
above-described range because the yield of the silver powder is
high and the obtained silver powder is difficult to aggregate. On
the other hand, it is not preferable that the silver ion
concentration is less than 10 g/l, the productivity of the silver
powder tends to be worsened; and it is not preferable that the
silver ion concentration exceeds 80 g/l, the silver powder is
easily aggregated.
[0050] In mixing the first aqueous solution and the second aqueous
solution, the solutions are mixed so that the quantity of nitric
acid contained in the first aqueous solution is normally 40 parts
by weight to 150 parts by weight, preferably 50 parts by weight to
120 parts by weight, and more preferably 65 parts by weight to 100
parts by weight to 100 parts by weight of ascorbic acid contained
in the second aqueous solution. It is preferable that the
compounding quantity of nitric acid to ascorbic acid is within the
above-described range because the yield of the silver powder is
easily elevated. On the other hand, it is not preferable that the
compounding quantity of nitric acid to 100 parts by weight ascorbic
acid is less than 40 parts by weight, because it is difficult to
sufficiently increase the crystallite diameter of the obtained
silver powder; and it is not preferable that the compounding
quantity of nitric acid to 100 parts by weight ascorbic acid
exceeds 150 parts by weight, the obtained silver powder is easily
aggregated.
[0051] It is preferable that silver powder deposited in the blended
solution by mixing the first aqueous solution and the second
aqueous solution is grown in the blended solution by continuing
agitation normally for further 3 minutes or more, preferably 5
minutes to 10 minutes, because the particle diameter, particle size
distribution and dispersion of the silver powder are easily within
the specific ranges of the silver powder according to the present
invention. After filtering the silver powder obtained in the
blended solution using filtering means, for example, a Nutsche, the
filtered product is washed with pure water and dried to obtain the
highly crystalline silver powder according to the present
invention.
[0052] The above-described highly crystalline silver powder
according to the present invention can be used as the material for
a conductive paste that can form electrodes or circuits of, for
example, chip devices, plasma display panels, glass ceramic
packages, ceramic filters and the like; in particular, it can be
suitably used as the material for a conductive paste not only for
ordinary ceramic substrates, but also for LTCC substrates as
substrates forming the circuit, utilizing the very small thermal
shrinkage of the silver powder. The method for producing highly
crystalline silver powder according to the present invention can
also be used for producing highly crystalline silver powder
according to the present invention.
[0053] Although the examples will be described below, these
examples should not be construed to limit the present
invention.
EXAMPLE 1
[0054] 10 g of PVP (K-value: 30), 50 g of silver nitrate and 24.6 g
of concentrated nitric acid (concentration: 61 wt %) were added
into 500 g of pure water at room temperature, and dissolved by
stirring to prepare a first aqueous solution (first aqueous
solution A). In addition, 35.8 g of ascorbic acid was added into
500 g of pure water at room temperature, and dissolved by stirring
to prepare a second aqueous solution (second aqueous solution A).
The compositions of the first aqueous solution and the second
aqueous solution are shown in Table 1 and Table 2.
[0055] Next, the second aqueous solution A was added to the
stirring first aqueous solution A at once, after that, stirring was
continued for 5 minutes to grow particles in the blended solution.
Thereafter, stirring was stopped to settle the particles in the
blended solution. After settling the particles, the supernatant of
the blended solution was disposed, and the rest of blended solution
was filtered using a Nutsche. Then the filtered product was rinsed
with pure water and dried, and then highly crystalline silver
powder was obtained.
[0056] For the obtained silver powder, D.sub.10, D.sub.50,
D.sub.90, D.sub.100, SD, crystallite diameter, specific surface
area, tap density, thermal shrinkage and resistivity were measured
using the following methods and D.sub.90/D.sub.10 was calculated.
The results are shown in Table 3 to Table 6.
(D.sub.10, D.sub.50, D.sub.90, D.sub.100, SD): 10%, 50%, 90% and
100% were indicated as D.sub.10 (.mu.m), D.sub.50 (.mu.m), D.sub.90
(.mu.m), D.sub.100 (.mu.m) are particle diameters when the
cumulative distributions measured by a laser diffraction scattering
method using [Micro Track HRA] manufactured by Nikkiso Co., Ltd.
were, respectively, and SD is the standard deviation in the
particle size distribution. (Crystallite diameter): X-ray
diffraction on the powder was performed using an X-ray diffraction
apparatus [RINT 2000/PC] manufactured by Rigaku Corporation, and
the crystallite diameter was calculated from the half-value width
of the peak of diffraction angle obtained on each crystal faces.
(Specific surface area): It is a B.E.T. specific surface area
measured by [Monosorb] manufactured by Yuasa-Ionics Co., Ltd. (Tap
density): Tap density was measured by tapping the sample using [Tap
Denser] manufactured by Kuramochi Kagaku Kikai Seisakusyo Co., Ltd.
(Thermal shrinkage): A columnar pellet was prepared by compressing
the silver powder, and the TMA analysis of the pellet was conducted
using TMA/SS 6300 manufactured by Seiko Instruments Inc., in air at
a temperature elevation rate of 10.degree. C./min within the range
between room temperature and 850.degree. C. to measure thermal
shrinkage of the pellet in the length direction. The measuring
temperatures were 300.degree. C., 500.degree. C. and 700.degree. C.
(Resistivity): A mixed solvent was prepared by mixing 95 parts by
weight of terpineol and 5 parts by weight of ethyl cellulose, a
paste was prepared by mixing 15 parts by weight of the mixed
solvent and 85 parts by weight of the sample powder, and the paste
was baked at 300.degree. C. to prepare a silver coating film having
a thickness of about several .mu.m. Other silver coating films were
prepared in the same manner as described above except that the
baking temperatures were 400.degree. C. and 500.degree. C. instead
of 300.degree. C.
[0057] Then, after measuring the resistance (.OMEGA.) of the silver
coating films by four-terminal network method using (MILLIOHM METER
manufactured by Hewlett-Packard), the resistivity .rho. (.OMEGA.m)
was obtained from the cross-sectional area of the silver coating
films and the length between the terminals.
EXAMPLE 2
[0058] 20 g of PVP (K-value: 30), 50 g of silver nitrate and 24.6 g
of concentrated nitric acid (concentration: 61%) were added into
500 g of pure water at room temperature, and dissolved by stirring
to prepare a first aqueous solution (first aqueous solution B). In
addition, 35.8 g of ascorbic acid was added into 500 g of pure
water at room temperature, and dissolved by stirring to prepare a
second aqueous solution (second aqueous solution A). The
compositions of the first aqueous solution and the second aqueous
solution are shown in Table 1 and Table 2.
[0059] Next, the second aqueous solution A was added to the first
aqueous solution B in at once, after that, stirring was continued
for 5 minutes to grow particles in the blended solution.
Thereafter, stirring was stopped to settle the particles in the
blended solution. After settling the particles, the supernatant of
the blended solution was filtered using a Nutsche. Then, the
filtered product was rinsed with pure water and dried, and then
highly crystalline silver powder was obtained.
[0060] For the obtained silver powder, D.sub.10, D.sub.50,
D.sub.90, D.sub.100, SD, crystallite diameter, specific surface
area, tap density, thermal shrinkage and resistivity were measured
in the same manner as in Example 1 using the following methods and
D.sub.90/D.sub.10 was calculated. The results are shown in Table 3
to Table 6.
COMPARATIVE EXAMPLE 1
[0061] 10 g of PVP (K-value: 30) and 50 g of silver nitrate were
added into 500 g of pure water at room temperature, and dissolved
by stirring to prepare a first aqueous solution (first aqueous
solution C). In addition, 26 g of ascorbic acid was added into 500
g of pure water at room temperature, and dissolved by stirring to
prepare a second aqueous solution (second aqueous solution B). The
compositions of the first aqueous solution and the second aqueous
solution are shown in Table 1 and Table 2.
[0062] Next, the second aqueous solution B was added to the
stirring first aqueous solution C at once, after that, stirring was
continued for 5 minutes to grow particles in the blended solution.
Thereafter, stirring was stopped to settle the particles in the
blended solution. After settling the particles, the supernatant of
the blended solution was disposed, and the rest of blended solution
was filtered using a Nutsche. Then the filtered product was rinsed
with pure water and dried, and then highly crystalline silver
powder was obtained.
[0063] For the obtained silver powder, D.sub.10, D.sub.50,
D.sub.90, D.sub.100, SD, crystallite diameter, specific surface
area, tap density, thermal shrinkage and resistivity were measured
in the same manner as in Example 1 using the following methods and
D.sub.90/D.sub.10 was calculated. The results are shown in Table 3
to Table 6.
EXAMPLE 3
[0064] 1.0 g of gelatin (manufactured by Nitta Gelatin Inc.), 50 g
of silver nitrate and 24.6 g of concentrated nitric acid
(concentration: 61%) were added into 250 g of pure water at room
temperature, and then, heated up to 50.degree. C. and dissolved by
stirring to prepare a first aqueous solution (first aqueous
solution D). In addition, 26.4 g of ascorbic acid was added into
250 g of pure water at room temperature, and dissolved by stirring
to prepare a second aqueous solution (second aqueous solution C).
The compositions of the first aqueous solution D and the second
aqueous solution C are shown in Table 1 and Table 2.
[0065] Next, the second aqueous solution C at room temperature was
gradually added to the stirring first aqueous solution D at
50.degree. C. in 30 minutes, after that, stirring was continued for
5 minutes to grow particles in the blended solution. Thereafter,
stirring was stopped to settle the particles in the blended
solution. After settling the particles, the supernatant of the
blended solution was disposed, and the rest of blended solution was
filtered using a Nutsche. Then the filtered product was rinsed with
pure water and dried, and then highly crystalline silver powder was
obtained.
[0066] For the obtained silver powder, D.sub.10, D.sub.50,
D.sub.90, D.sub.100, SD, crystallite diameter, specific surface
area, tap density, thermal shrinkage and resistivity were measured
in the same manner as in Example 1 using the following methods and
D.sub.90/D.sub.10 was calculated. The results are shown in Table 3
to Table 6.
EXAMPLE 4
[0067] 3.0 g of gelatin (manufactured by Nitta Gelatin Inc.), 50 g
of silver nitrate and 24.6 g of concentrated nitric acid
(concentration: 61%) were added into 500 g of pure water at room
temperature, and then, heated up to 50.degree. C. and dissolved by
stirring to prepare a first aqueous solution (first aqueous
solution E). In addition, 25.9 g of ascorbic acid was added into
500 g of pure water at room temperature, and dissolved by stirring
to prepare a second aqueous solution (second aqueous solution D).
The compositions of the first aqueous solution and the second
aqueous solution are shown in Table 1 and Table 2.
[0068] Next, the second aqueous solution D at room temperature was
gradually added to the stirring first aqueous solution E in 30
minutes, after that, stirring was continued for 5 minutes to grow
particles in the blended solution. Thereafter, stirring was stopped
to settle the particles in the blended solution. After settling the
particles, the supernatant of the blended solution was disposed,
and the rest of blended solution was filtered using a Nutsche. Then
the filtered product was rinsed with pure water and dried, and then
highly crystalline silver powder was obtained.
[0069] For the obtained silver powder, D.sub.10, D.sub.50,
D.sub.90, D.sub.100, SD, crystallite diameter, specific surface
area, tap density, thermal shrinkage and resistivity were measured
in the same manner as in Example 1 using the following methods and
D.sub.90/D.sub.10 was calculated. The results are shown in Table 3
to Table 6.
TABLE-US-00001 TABLE 1 Kind of Kind Dispersing Silver Concentrated
first of dispersing agent nitrate nitric acid aqueous Water (g)
agent (g) (g) (g) solutions Example 1 500 PVP 10 50 24.6 A Example
2 500 PVP 20 50 24.6 B Comparative 500 PVP 10 50 0 C Example 1
Example 3 250 Gelatin 1.0 50 26.4 D Example 4 500 Gelatin 3.0 50
24.6 E
TABLE-US-00002 TABLE 2 Kind of Ascorbic second Water acid aqueous
(g) (g) solutions Example 1 500 35.8 A Example 2 500 35.8 A
Comparative 500 26.0 B Example 1 Example 3 250 26.4 C Example 4 500
25.9 D
TABLE-US-00003 TABLE 3 D.sub.100 D.sub.10 (.mu.m) D.sub.50 (.mu.m)
D.sub.90 (.mu.m) (.mu.m) D.sub.90/D.sub.10 SD Example 1 2.97 6.35
10.75 22.0 3.6 3.01 Example 2 1.30 3.03 5.67 15.6 4.4 1.59
Comparative 2.14 2.83 4.08 9.3 1.9 0.71 Example 1 Example 3 2.72
4.36 7.33 18.5 2.7 1.71 Example 4 0.76 1.27 2.28 4.6 3.0 0.57
TABLE-US-00004 TABLE 4 Crystallite Specific diameter surface area
Tap density (.ANG.) (m.sup.2/g) (g/cm.sup.3) Example 1 441 0.30 4.1
Example 2 377 0.62 4.0 Comparative 258 0.62 3.8 Example 1 Example 3
545 0.20 4.4 Example 4 441 0.72 4.8
TABLE-US-00005 TABLE 5 Thermal Thermal Thermal shrinkage shrinkage
shrinkage percentage at percentage at percentage at 300.degree. C.
500.degree. C. 700.degree. C. (%) (%) (%) Example 1 0.13 -2.13 -2.2
Example 2 0.09 -2.68 -2.9 Comparative 0.84 -4.02 -7.82 Example 1
Example 3 0.27 1.08 1.13 Example 4 -0.58 -1.51 -1.35
TABLE-US-00006 TABLE 6 Resistivity of Resistivity of Resistivity of
silver coating silver coating silver coating film baked at film
baked at film baked at 300.degree. C. .rho.(.OMEGA. m) 400.degree.
C. .rho.(.OMEGA. m) 500.degree. C. .rho.(.OMEGA. m) Example 1 4.1
.times. 10.sup.-5 2.0 .times. 10.sup.-5 9.9 .times. 10.sup.-6
Example 2 5.2 .times. 10.sup.-5 1.5 .times. 10.sup.-5 1.2 .times.
10.sup.-5 Comparative 7.2 .times. 10.sup.-4 8.9 .times. 10.sup.-6
4.8 .times. 10.sup.-5 Example 1 Example 3 9.4 .times. 10.sup.-6 8.3
.times. 10.sup.-6 9.9 .times. 10.sup.-6 Example 4 1.0 .times.
10.sup.-5 8.8 .times. 10.sup.-6 4.8 .times. 10.sup.-5
[0070] From Table 1 to Table 5, it is clearly shown that silver
powder prepared by using both dispersing agent and nitric acid is
highly crystalline with a large crystallite diameter, and thermal
shrinkage after heating at 700.degree. C. is small. Especially when
gelatin is used as the dispersing agent, thermal shrinkage after
heating at 700.degree. C. is especially small. From Table 6, it is
found that silver powder prepared by using both dispersing agent
and nitric acid show lower resistivity .rho. on the silver coating
film after baked at 300.degree. C. when compared with the silver
powder prepared without using nitric acid. The reason why is
suspected that the movement of electrons in the silver powder of
the invention is more smooth because of it is large crystallite
diameter.
INDUSTRIAL APPLICABILITY
[0071] The highly crystalline silver powder according to the
present invention is useful for the material to be contained in a
conductive paste for forming electrodes and/or circuits, for
example, chip devices, plasma display panels, glass ceramic
packages, ceramic filters and the like. Also the method for
producing highly crystalline silver powder according to the present
invention is useful. Especially, it is shows good performance on
LTCC substrate.
* * * * *