U.S. patent application number 11/994921 was filed with the patent office on 2009-05-07 for highly crystalline silver powder and production method of highly crystalline silver powder.
This patent application is currently assigned to MITSUIMINING & SMELTING CO., LTD.. Invention is credited to Taku Fujimoto, Takuya Sasaki, Katsuhiko Yoshimaru.
Application Number | 20090116998 11/994921 |
Document ID | / |
Family ID | 37604510 |
Filed Date | 2009-05-07 |
United States Patent
Application |
20090116998 |
Kind Code |
A1 |
Fujimoto; Taku ; et
al. |
May 7, 2009 |
HIGHLY CRYSTALLINE SILVER POWDER AND PRODUCTION METHOD OF HIGHLY
CRYSTALLINE SILVER POWDER
Abstract
Object of the present invention is to provide a production
method of a highly crystalline silver powder containing powder
particles in fine particle region and having a good particle size
distribution as well as the highly crystalline silver powder
obtained by the production method. In order to achieve the object,
the present invention adopts a production method characterized in
that the method comprises preparation of a first aqueous solution
in which gelatin, silver nitrate and nitric acid are dissolved in
water, preparation of a second aqueous solution in which L-sorbic
acid and/or ascorbic acid and a water-soluble organic acid are
dissolved, adding of the second aqueous solution slowly to mix with
the first aqueous solution, stirring of the mixture to grow the
silver particles after finishing the mixing, keeping of the mixture
still to settle the silver particles, discarding of the
supernatant, filtration and rinsing to obtain the highly
crystalline silver powder.
Inventors: |
Fujimoto; Taku; (Yamaguchi,
JP) ; Sasaki; Takuya; (Yamaguchi, JP) ;
Yoshimaru; Katsuhiko; (Yamaguchi, JP) |
Correspondence
Address: |
ROTHWELL, FIGG, ERNST & MANBECK, P.C.
1425 K STREET, N.W., SUITE 800
WASHINGTON
DC
20005
US
|
Assignee: |
MITSUIMINING & SMELTING CO.,
LTD.
Tokyo
JP
|
Family ID: |
37604510 |
Appl. No.: |
11/994921 |
Filed: |
July 4, 2006 |
PCT Filed: |
July 4, 2006 |
PCT NO: |
PCT/JP2006/313320 |
371 Date: |
March 10, 2008 |
Current U.S.
Class: |
420/501 |
Current CPC
Class: |
B22F 9/24 20130101; B22F
1/0044 20130101 |
Class at
Publication: |
420/501 |
International
Class: |
C22C 5/06 20060101
C22C005/06 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 5, 2005 |
JP |
2005-196627 |
Claims
1. A production method of highly crystalline silver powder from a
silver ion containing solution by using L-sorbic acid and/or
ascorbic acid as a reducing agent, characterized in that the method
comprises preparation of a first aqueous solution in which gelatin,
silver nitrate and nitric acid are dissolved in water, preparation
of a second aqueous solution in which L-sorbic acid and/or ascorbic
acid and a water-soluble organic acid are dissolved, adding of the
second aqueous solution slowly to mix with the first aqueous
solution, stirring of the mixture to grow the silver particles
after finishing the mixing, keeping of the mixture still to settle
the silver particles, discarding of the supernatant, filtration and
rinsing to obtain highly crystalline silver powder.
2. The production method of the highly crystalline silver powder
according to claim 1, wherein the gelatin concentration in the
first aqueous solution is 2 g/l to 10 g/l.
3. The production method of the highly crystalline silver powder
according to claim 1, wherein the silver nitrate concentration in
the first aqueous solution is 50 g/l to 150 g/l as silver.
4. The production method of the highly crystalline silver powder
according to claim 1, wherein the free nitric acid concentration in
the first aqueous solution is adjusted to 40 g/l to 120 g/l by
adding the nitric acid aqueous solution.
5. The production method of the highly crystalline silver powder
according to claim 1, wherein the concentration of L sorbic acid
and/or ascorbic acid in the second aqueous solution is 45 g/l to
120 g/l.
6. The production method of the highly crystalline silver powder
according to claim 1, wherein the concentration of the
water-soluble organic acid in the second aqueous solution is 1 g/l
to 50 g/l.
7. The production method of the highly crystalline silver powder
according to claim 6, wherein the water-soluble organic acid is one
or combination of two or more kinds selected from DL-malic acid,
citric acid, formic acid, valeric acid, isovaleric acid, succinic
acid, propionic acid, lactic acid and adipic acid.
8. The highly crystalline silver powder characterized by obtained
by a production method according to claim 1.
9. The highly crystalline silver powder according to claim 8
characterized in that the powder has a primary particle diameter of
0.07 .mu.m to 4.5 .mu.m and a crystal diameter of 200 .ANG. or
more.
10. The highly crystalline silver powder according to claim 8
characterized in that the powder has a volume accumulated average
particle diameter D.sub.50 measured by laser diffraction
scattering-type particle size distribution measurement method of
0.1 .mu.m to 5.0 .mu.m and a particle size distribution index
(D.sub.90-D.sub.10)/D.sub.50 of not more than 1.5.
11. The highly crystalline silver powder according to claim 8
characterized in that the powder has a volume accumulated maximum
particle diameter D.sub.max measured by laser diffraction
scattering-type particle size distribution measurement method of
16.0 .mu.m or less.
12. The highly crystalline silver powder according to claim 8
characterized in that the powder has a specific surface area of 0.2
m.sup.2/g or more.
Description
TECHNICAL FIELD
[0001] The present invention relates to highly crystalline silver
powder among the silver powders and production method of the highly
crystalline silver powder.
BACKGROUND ART
[0002] Conventionally, highly crystalline silver powder (which has
a large crystal diameter) has been widely used for processing into
silver ink and silver paste for the reason that it is excellent in
thermal shrinkage resistance at the time of sintering. For example,
it has been used in wiring circuit of printed wiring board, via
hole filling, adhesive for mounting devices and the similar
applications in which a silver powder is mixed and cured with
various kinds of resin components as well as applications in which
a silver powder is sintered at relatively high temperatures such as
circuit formation by co-sintering with a ceramic substrate. Silver
powder excellent in thermal shrinkage resistance at the time of
sintering has been demanded from the point of view for improving
accuracy of shape as a conductor particularly in silver powder used
for silver ink and silver paste used for wiring and electrodes in
circuits.
[0003] The crystallinity of silver powder is greatly affected by
the production method. For example, as a method for producing
silver powder, atomize process as disclosed in Patent Document 1
(Japanese Patent Laid-Open No. 2003-286502) can be used. However,
even though silver powder having high crystallinity may be obtained
by the atomize process, it is difficult to obtain silver powder
consisting of fine particles with a sharp particle size
distribution. Although it can be surely supposed that silver powder
as a product having a sharp particle size distribution may be
obtained by performing classification repeatedly, it is not
preferable at all from the point of view of production cost.
Therefore, obtaining silver powder by a wet production process
described below has been examined.
[0004] For example, Patent Document 2 (Japanese Patent Publication
No. 57-21001) discloses a method to deposit fine silver powder by
adding a fatty acid to an aqueous mixture solution of a silver
nitrate and formalin in an amount of 0.1 wt % to 5.0 wt % against
to amount of silver to be deposited while stirring followed by
adding an alkaline solution into the mixture solution. Patent
Document 2 also discloses the production method which gives fine
silver powder having an average particle diameter of 0.8 .mu.m to
0.9 .mu.m.
[0005] In addition, Patent Document 3 (Japanese Patent Laid-Open
No. 04-323310) discloses a method for forming fine spherical
particles by dissolving a metal, an alloy or a metal salt in an
aqueous solvent, followed by adjusting pH by adding a base chemical
thereto and adding a reducing agent to this to deposit fine metal
powder. In which, the temperature of the solution is adjusted to a
range of 10.degree. C. to 30.degree. C. to obtain spherical fine
powder. When the temperature is adjusted to a range of 50.degree.
C. or more, fine polyhedron-like metal powder can be obtained. In
addition, the particle size distribution of the obtained metal
powder is approximately from 0.3 .mu.m to 2.0 .mu.m by the
production method.
[0006] Since the deposition of crystals in the silver powders
obtained by the production methods disclosed in above-mentioned
Patent Document 2 and Patent Document 3 are not controlled, thermal
shrinkage when sintering process was performed was significant.
Therefore, in order to solve such problem, Patent Document 4
(Japanese Patent Laid-Open No. 2000-1706) discloses a method for
producing the highly crystalline silver powder wherein UV is
irradiated at the time of the reaction when 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
react simultaneously. The highly crystalline silver powder obtained
by the method consists of high crystals single crystals and
quasi-crystals whose crystal diameter is 2 .mu.m to 4 .mu.m. In
addition, the Patent Document 4 discloses that if the crystal
diameter is not more than 2 .mu.m, the shrinkage factor at the time
of sintering is large and if the crystal diameter exceeds 4 .mu.m,
irregularity of the conductor surface is large to result problems
such as increase in losses as an electric circuit. The highly
crystalline silver particle as referred to here indicates those
having a crystal diameter of 400 .ANG. or more calculated from
half-value width of (1.1.1) peak by X-ray diffraction method.
[0007] Further, Patent Document 5 (Japanese Patent Laid-Open No.
2003-49202) discloses silver particles having a crystal diameter of
400 .ANG. to 600 .ANG., tap density of 5 g/cm.sup.3 or more and
specific surface area of 0.15 m.sup.2/g or less. And it is clearly
described that the silver particle is obtained by a production
method characterized by reacting an alkaline aqueous solution
(ammonium hydroxide aqueous solution+one or more kind selected from
a group consisting of sodium hydroxide and potassium hydroxide)
containing a silver ion and a hydrogen peroxide aqueous solution
(added with one or more kind selected from a group consisting of
fatty acids, fatty acid salts and derivatives thereof as required).
And it is clearly described that the range of the crystal diameter
of silver particles obtained is from 400 .ANG. to 600 .ANG. and, if
it is less than 400 .ANG., crystallinity is low for silver
particles for high temperature sintering conductive paste, and if
it exceeds 600 .ANG., shape of the silver particles may show
deviation. Also, Patent Document 5 discloses that tap density of
the silver particles should be 5 g/cm.sup.3 or more and specific
surface area of the silver particles should be 0.15 m.sup.2/g or
less. But these are not sufficient as properties to identify the
powder because there is no description about particle diameter even
though the both conditions should be inherently based on relations
with particle diameter.
[0008] [Patent Document 1] Japanese Patent Laid-Open No.
2003-286502
[0009] [Patent Document 2] Japanese Patent Publication No.
57-21001
[0010] [Patent Document 3] Japanese Patent Laid-Open No.
04-323310
[0011] [Patent Document 4] Japanese Patent Laid-Open No.
2000-1706
[0012] [Patent Document 5] Japanese Patent Laid-Open No.
2003-49202
DISCLOSURE OF THE INVENTION
[0013] L ascorbic acid, which is used as a reducing agent in the
production method disclosed in above Patent Document 4 relating to
high crystalline silver powder, is expensive and may result
increasing of the product price. As for the highly crystalline
silver particle obtained by substantially using ascorbic acid as a
reducing agent and irradiating ultraviolet in the reductive
reaction, particle diameter and crystal diameter has an
approximately proportional relationship. And products having
constantly stable quality can be obtained when large powder
particles of 2 .mu.m to 4 .mu.m are aimed. However, when fine
silver powder particles having a particle diameter less than 2
.mu.m with a crystal diameter of more than 400 .ANG. are aimed,
lack of process stability of the production method may result wide
variation in the crystal diameter. Further, it has been confirmed
in many case that the fine particles cannot satisfy thermal
shrinkage resistance because shrinkage factor after sintering of
the fine silver powder particles less than 2 .mu.m is too
large.
[0014] In addition, the solution used in the production method
disclosed in Patent Document 5 consists of ammonia aqueous solution
and ammonium nitrate. They are strongly odorous substances, and
therefore, it may pollute the working atmosphere as well as causing
a problem of accelerated damage on the apparatuses made of copper
applied in the facilities. Besides, the method uses a hydrogen
peroxide aqueous solution whose ability tends to change largely and
lacks in stability of the quality of the solution. Therefore, the
average particle diameter of the obtained silver powder deviates
significantly and controlling of the particle size and particle
size distribution is difficult.
[0015] As described above, the highly crystalline silver powder
consisting of fine particles has been demanded but there has not
been the highly crystalline silver powder which sufficiently
satisfies market needs.
[0016] Accordingly, the present inventors have conducted intensive
studies to solve the problems, and consequently have found that the
silver powder obtained with the production method described below
has high crystallinity with the fine particle level of which
conventional silver powder has never achieved.
[0017] Production method of the highly crystalline silver powder:
The production method of the highly crystalline silver powder of
the present invention is characterized by comprising followings.
Preparation of a first aqueous solution in which gelatin, silver
nitrate and nitric acid are dissolved in water. Preparation of a
second aqueous solution in which L-sorbic acid and a water-soluble
organic acid are dissolved. Adding of the second aqueous solution
slowly to mix with the first aqueous solution with stirring of the
mixture to make the particles to grow to form silver particles
after the addition is completed. Keeping of the mixture still to
settle the silver particles. Discarding of the supernatant.
Filtration and rinsing. Then the highly crystalline silver powder
is obtained.
[0018] In the production method of the highly crystalline silver
powder of the present invention, it is preferable that gelatin
concentration in the first aqueous solution is 2g/l to 10 g/l.
[0019] In the production method of the highly crystalline silver
powder of the present invention, it is preferable that the silver
nitrate concentration in the first aqueous solution is 50 g/l to
150 g/l as silver.
[0020] In the production method of the highly crystalline silver
powder of the present invention, it is preferable that nitric acid
is added to arrange the free nitric acid concentration of 40 g/l to
120 g/l in the first aqueous solution by adjusting the nitric acid
aqueous solution.
[0021] In the production method of the highly crystalline silver
powder of the present invention, it is preferable that the L-sorbic
acid concentration in the second aqueous solution is 45 g/l to 120
g/l.
[0022] In the production method of the highly crystalline silver
powder of the present invention, it is preferable that the
water-soluble organic acid concentration in the second aqueous
solution is 1 g/l to 50 g/l.
[0023] In the production method of the highly crystalline silver
powder of the present invention, it is preferable for the
water-soluble organic acid which is one or combination of two or
more kinds selected from DL-malic acid, citric acid, formic acid,
valeric acid, isovaleric acid, succinic acid, propionic acid,
lactic acid and adipic acid.
[0024] Highly crystalline silver powder: The highly crystalline
silver powder of the present invention is the silver powder
obtained through reductive deposition by adding L-sorbic acid
and/or ascorbic acid as a reducing agent and a water-soluble
organic acid to a silver ion containing solution.
[0025] The highly crystalline silver powder is characterized in
that the powder has a primary particle diameter of 0.07 .mu.m to
4.5 .mu.m and a crystal diameter of 200 .ANG. or more.
[0026] It is also preferable that the highly crystalline silver
powder shows powder characteristics as followings. Volume
accumulated average particle diameter D.sub.50 of 0.1 .mu.m to 5.0
.mu.m when measured by laser diffraction scattering-type particle
size distribution measurement method and particle size distribution
index (D.sub.90-D.sub.10)/D.sub.50 of not more than 1.5.
[0027] Furthermore, the highly crystalline silver powder of the
present invention has the volume accumulated maximum particle
diameter D.sub.max measured by laser diffraction scattering-type
particle size distribution measurement method of 16.0 .mu.m or
less, even usually contains coarse particles in a certain
ratio.
[0028] In addition, the specific surface area of the highly
crystalline silver powder of the present invention is preferably
0.2 m.sup.2/g or more.
[0029] The production method of the highly crystalline silver
powder of the present invention is suitable for the production of
the highly crystalline silver powder whose particles have fine
diameter to large diameter by using a water-soluble organic acid
together with a reducing agent. In particular, the process is hard
to be affected by process changes such as temperature variation,
concentration variation at the time of the reductive reaction and
the process enables production of the highly crystalline silver
powder at a high yield.
[0030] The highly crystalline silver powder shows powder
characteristics as followings. Volume accumulated average particle
diameter D.sub.50 of 0.1 .mu.m to 5.0 .mu.m when measured by laser
diffraction scattering-type particle size distribution measurement
method and particle size distribution index
(D.sub.90-D.sub.10)/D.sub.50 of not more than 1.5. As is apparent
from the powder characteristics, the powder has a sharp particle
size distribution at a level which has not been conventionally
achieved in the particle diameter range (2 .mu.m to 5 .mu.m) of the
conventional particles. And the powder has a stably large crystal
diameter and a sharp particle size distribution even if it is a
product having a particle diameter less than 2 .mu.m. Therefore,
conductor layers formed by a silver paste produced by using the
highly crystalline silver powder of the present invention are
excellent in thermal shrinkage resistance and have a smoother
surface roughness.
BEST MODE FOR CARRYING OUT THE INVENTION
[0031] Best modes for carrying out the inventions of the highly
crystalline silver powder of the present invention and the
production method thereof are described below.
[0032] Production method of the highly crystalline silver powder:
The production method of the highly crystalline silver powder of
the present invention is characterized by comprising followings.
Preparation of a first aqueous solution in which gelatin, silver
nitrate and nitric acid are dissolved in water, a second aqueous
solution in which L-sorbic acid and a water-soluble organic acid
are dissolved. Adding of the second aqueous solution slowly to mix
with the first aqueous solution with stirring of the mixture to
make the particles to grow to form silver particles after the
addition is completed. Then keep the mixture still to settle the
silver particles, discarding the supernatant, followed by
filtration with rinsing. Then the highly crystalline silver powder
is obtained.
[0033] The first aqueous solution is described first. The first
aqueous solution is a silver salt containing solution in which
gelatin, silver nitrate and nitric acid are dissolved in water. The
term gelatin herein is used as a concept which includes glue
inferior to gelatin in purification degree. The gelatin is used to
control the reduction rate (reaction rate) in the reductive
deposition of the highly crystalline silver powder. Gelatin also
performs as a steric hindrance agent to restrain aggregation of the
reductively deposited particles and restrains cohesion between the
particles which have been reductively deposited. As a result, the
particle size distribution of the obtained the highly crystalline
silver powder is made sharp, which is preferable. And it is
preferable that the gelatin concentration in the above first
aqueous solution is 2 g/l to 10 g/l. When the gelatin concentration
is less than 2 g/l, the rate of the reduction reaction when the
first aqueous solution reacts with the second aqueous solution is
too fast to make the crystal diameter of the deposited silver
particles grow large and the cohesion between particles becomes
remarkable. Accordingly, silver powder having a sharp particle size
distribution cannot be obtained. On the other hand, when the
gelatin concentration exceeds 10 g/l, the rate of the reduction
reaction becomes too slow not to satisfy industrial productivity
and the variation in the crystal diameter becomes large.
[0034] And it is preferable that the silver nitrate concentration
in the above first aqueous solution is 50 g/l to 150 g/l as silver.
When the silver nitrate concentration (as silver) is less than 50
g/l, the amount of the reductively deposited silver is small and
the particle diameter of the deposited silver is too small
resulting small crystal diameter. On the other hand, when the
silver nitrate concentration (as silver) exceeds 150 g/l, judging
from balance with a reducing agent, silver ions not reduced remain
leading to waste of resources. Also, reductive reaction sites
increase too much and the aggregation of the deposited particles is
remarkable and the yield of the highly crystalline silver powder
having a volume accumulated average particle diameter D.sub.50 of
0.1 .mu.m or more, which is excellent in particle dispersibility,
decreases.
[0035] Furthermore, it is preferable to adjust the free nitric acid
concentration in the first aqueous solution to be within 40 g/l to
120 g/l by adding a nitric acid aqueous solution. The free nitric
acid concentration is a condition to be managed to prevent the
silver ions from formation of a sludge in the first aqueous
solution, which is a silver salt containing solution, and to
efficiently perform the reductive deposition of silver particles
with a reducing agent.
[0036] And it is preferable that the solution temperature of the
first aqueous solution is 45 to 55.degree. C. when the solution
temperature is less than 45.degree. C., it is difficult to mix
gelatin, silver nitrate and nitric acid in a short time, and the
rate of the reduction reaction by adding the second aqueous
solution (described later in the present description) becomes too
slow and thus making it difficult to produce the highly crystalline
silver powder having an appropriate particle size distribution. On
the other hand, when the solution temperature exceeds 55.degree.
C., decomposition of gelatin is promoted to make solution life
shorter and silver powder having an appropriate crystal diameter
cannot be obtained.
[0037] Next, the second aqueous solution is described. The second
aqueous solution is an aqueous solution containing a reducing
agent. The term [L-sorbic acid and/or ascorbic acid] which are
reducing agents means that either one of L-sorbic acid or ascorbic
acid may be used independently as a reducing agent, or L-sorbic
acid and ascorbic acid may be used in combination. Therefore, the
concentration of L-sorbic acid and/or ascorbic acid described below
is a concept including the case where L-sorbic acid and ascorbic
acid are used together. When they are used together, weight ratio
[L-sorbic acid]:[ascorbic acid] can be 0.1:9.9 to 9.9:0.1,
preferably [L-sorbic acid]:[ascorbic acid]=0.5:9.9 to 9.9:0.5
considering more stable process stability and further preferably
[L-sorbic acid]:[ascorbic acid]=1:9 to 9:1 considering further
stable process stability.
[0038] It is preferable that the concentration of L-sorbic acid
and/or ascorbic acid in the above second aqueous solution is 45 g/l
to 120 g/l. The reducing agent concentration is decided in relation
with the silver content of the first aqueous solution. Both the
concentration of silver in the first aqueous solution and the
concentration of the reducing agent in the second aqueous solution
in the appropriate range makes reductive deposition of silver
powder consisting of fine particles which has a particle diameter
of 2 .mu.m or less possible. That is, when the concentration of
L-sorbic acid and/or ascorbic acid in the second aqueous solution
is less than 45 g/l, the reduction of silver ions in the first
aqueous solution may be insufficient to result waste of resources,
and the particle size distribution of the obtained silver powder
may be broad and the highly crystalline silver powder having a good
crystal diameter cannot be obtained. On the other hand, when the
concentration of L-sorbic acid and/or ascorbic acid in the second
aqueous solution exceeds 120 g/l, the amount of the reducing agent
exceeds the amount necessary for the reduction of silver ions in
the first aqueous solution to result waste of the reducing agent,
and the reduction reaction proceeds so fast to make the crystal
diameter small.
[0039] In addition, the second aqueous solution has a major
characteristic in the point to use a water-soluble organic acid as
well as the reducing agent. The water-soluble organic acid performs
to increase the crystal diameter of the reductively deposited
silver crystals and enables to increase the crystal diameter even
if the fine particle silver powder has a particle diameter of less
than 2 .mu.m. Further, the water-soluble organic acid improves the
particle size distribution of the reductively deposited silver
powder to result sharp particle size distribution excellent in
particle dispersibility. The water-soluble organic acid as referred
to herein is one or combination of two or more kinds selected from
DL-malic acid, citric acid, formic acid, valeric acid, isovaleric
acid, succinic acid, propionic acid, lactic acid and adipic
acid.
[0040] It is preferable that the concentration of the water-soluble
organic acid is 1 g/l to 50 g/l. when the concentration of the
water-soluble organic acid in the second aqueous solution is less
than 1 g/l, the effect to increase the crystal diameter of the
reductively deposited silver crystals cannot be obtained and
purpose to use a water-soluble organic acid cannot be achieved. On
the other hand, when the concentration of the water-soluble organic
acid exceeds 50 g/l, the effect to increase the crystal diameter is
saturated and rather a tendency to deteriorate the particle size
distribution of the obtained silver powder may arise.
[0041] The second aqueous solution is slowly added to the first
aqueous solution over 10 to 60 minutes. When the first aqueous
solution and the second aqueous solution are mixed all at once, the
particle size distribution of the obtained silver powder might be
broad. Also, a product having a sharp particle size distribution
cannot be obtained, and formation of coarse particles may be
remarkable. Therefore, when the mixing time is less than 10
minutes, the situation is similar to that by the addition all at
once, the particle size distribution of the obtained silver powder
is broad, and formation of coarse particles increases. On the other
hand, when the mixing time exceeds 60 minutes, it may only down
productivity, and no improvement in the particle size distribution
can be expected even if the addition is
[0042] As for the amount of the second aqueous solution to be added
to the first aqueous solution, the basic amount of a reducing agent
to be added corresponds to the reaction equivalent to the reduction
against to the amount of silver contained in the first aqueous
solution and it is sufficient at the minimum. At this time, adding
of an excess amount of a reducing agent corresponds to the reaction
equivalent to the reduction against to the amount of silver
contained in the first aqueous solution causes no problem at all.
The temperature at the time of the reductive reaction is not
limited in particular, but it is preferable to adopt a range from
room temperature to 50.degree. C. because it hardly causes
significant change in the temperature of the first aqueous
solution. When a temperature more than 50.degree. C. is adopted,
evaporation of water becomes remarkable and tends to cause change
in the composition while mixing of the first aqueous solution and
the second aqueous solution.
[0043] After finishing the addition of the second aqueous solution
to the first aqueous solution, the mixture is kept being stirred
for 3 to 5 minutes to grow particles to form silver particles. When
the stirring time is less than 3 minutes, the reductive reaction
may not be completely finished, so it is not preferable. On the
other hand, when the stirring time is more than 5 minutes, the
reductive reaction has already been completely finished and such a
stirring is meaningless from a practical point of view.
[0044] When it is assumed that the addition of the second aqueous
solution to the first aqueous solution is finished and reductive
deposition of silver has not occurred, the composition balance of
the first aqueous solution and the second aqueous solution in the
reduction reaction at this stage is gelatin concentration of 2 g/l
to 10 g/l, silver nitrate concentration (as silver) of 50 g/l to
150 g/l, free nitric acid concentration of 40 g/l to 120 g/l,
concentration of reducing agent(s), L-sorbic acid and/or ascorbic
acid, of 45 g/l to 120 g/l, water-soluble organic acid
concentration of 1 g/l to 50 g/l.
[0045] After finishing the above reducing procedure, the mixture is
kept still to settle the silver particles. Then the supernatant
liquid is discarded, and filtration and rinsing are performed and
thus the highly crystalline silver powder can be obtained.
[0046] Highly crystalline silver powder: The highly crystalline
silver powder of the present invention is the silver powder
obtained by adding L-sorbic acid and/or ascorbic acid as a reducing
agent and a water-soluble organic acid to the silver ion containing
solution to cause reductive deposition.
[0047] The powder is characterized in that the primary particle
diameter is 0.07 .mu.m to 4.5 .mu.m and the crystal diameter is 200
.ANG. or more. The highly crystalline silver powder of the present
invention can be obtained from a silver ion containing solution by
adding L-sorbic acid and/or ascorbic acid as a reducing agent with
a water-soluble organic acid. Use of a water-soluble organic acid
in combination in such a way enables to obtain the highly
crystalline silver powder having a large crystal diameter. Also, a
good crystal diameter and a sharp particle size distribution even
when the primary particle diameter is in a fine particle region as
low as less than 2.0 .mu.m. Thus, high crystallinity with a crystal
diameter of 200 .ANG. or more in a wide particle diameter range of
0.07 .mu.m to 4.5 .mu.m can be achieved. Here, the primary particle
diameter is the value determined as an average of the particle
diameters observed with a scanning electron microscope on the high
crystalline silver powder and directly measuring the particle
diameters for 100 particles exist in the scope. In the range of
primary particle diameter, particles which can be referred to as
fine particles are in the range of 0.07 .mu.m to less than 2.0
.mu.m, preferably 0.07 .mu.m to 1.5 .mu.m, more preferably 0.07
.mu.m to 1.0 .mu.m. Generally, the particle diameter and the
crystal diameter have a proportional relationship but crystal
diameter more than 400 .ANG. can be obtained when the primary
particle diameter exceeds 0.3 .mu.m. Further, in the case of the
highly crystalline silver powder of the present invention, crystal
diameter of 200 .ANG. to 300 .ANG. can be stably obtained even when
the particle diameter is in the range of 0.07 .mu.m to 0.3 .mu.m.
As described above, a fine silver powder in such a fine particle
size range having a crystal diameter of 200 .ANG. or more has not
been present conventionally. By the way, RINT2000 X-ray diffraction
device manufactured by Rigaku Corporation was used for the
measurement of the crystal diameter mentioned in the present
invention and the measurement was performed by Wilson method
(measuring method of crystal diameter by X-ray diffraction).
[0048] Furthermore, when high crystalline silver powder having the
above powder characteristics is measured by a laser diffraction
scattering-type particle size distribution measurement method, it
shows powder characteristics as followings. Volume accumulated
average particle diameter D.sub.50 of 0.1 .mu.m to 5.0 .mu.m, and
particle size distribution index (D.sub.90-D.sub.10)/D.sub.50 of
not more than 1.5.
[0049] That is, the accumulated average particle diameter D.sub.50
measured by a laser diffraction scattering-type particle size
distribution measurement method corresponding to the primary
particle diameter range of from 0.07 .mu.m to 4.5 .mu.m is measured
as a value in the range approximately from 0.1 .mu.m to 5.0 .mu.m.
As for products having a primary particle diameter not less than
0.07 .mu.m and less than 2.0 .mu.m, which are classified as fine
particles by the range of the primary particle diameter, the volume
accumulated average particle diameter D.sub.50 mostly falls within
the range of 0.1 .mu.m to 1.6 .mu.m. As for products having a
primary particle diameter of 0.07 .mu.m to 1.5 .mu.m, the volume
accumulated average particle diameter D.sub.50 mostly falls within
the range of 0.1 .mu.m to 1.2 .mu.m. As for products having a
primary particle diameter of 0.07 .mu.m to 1.0 .mu.m, the volume
accumulated average particle diameter D.sub.50 mostly falls within
the range of 0.1 .mu.m to 0.7 .mu.m.
[0050] Furthermore, the highly crystalline silver powder of the
present invention has dispersibility (D.sub.90-D.sub.10)/D.sub.50
value of not more than 1.5 which is an index representing the
particle dispersibility. Here, (D.sub.90-D.sub.10)/D.sub.50 is a
value obtained by dividing the difference between D.sub.90, volume
accumulated particle diameter at 90% and D.sub.10, volume
accumulated particle diameter at 10%, by accumulated average
particle diameter D.sub.50. That is, it is a value showing how many
times is the particle size distribution range against to the volume
accumulated average particle diameter D.sub.50 as a basic property.
So, the value approaches to 1 when the frequency curve of the
particle size distribution becomes sharp. Therefore, a state with
the value (D.sub.90-D.sub.10)/D.sub.50 of less than 1.5 means that
the particle size distribution is considerably sharp, and it may be
said that the particle diameter of most particles is equal to or
less than 1.5 times of the volume accumulated average particle
diameter D.sub.50. On the other hand, the silver powder having a
crystal diameter more than 400 .ANG. can be also obtained in
conventional cases where a reducing agent such as ascorbic acid
alone is used when attention is paid only to crystal diameter.
However, when the primary particle diameter is less than 1.6 .mu.m
(about 2.0 .mu.m as volume accumulated average particle diameter
D.sub.50), the particle size distribution of the obtained silver
powder may so broad to have a wide particle size distribution with
a value of (D.sub.90-D.sub.10)/D.sub.50 exceeding 2.0. When a
conductive film is formed with a paste of silver powder having such
a broad particle size distribution, the surface of the conductive
film may become irregular, so it is not preferable.
[0051] Furthermore, the highly crystalline silver powder obtained
from silver ion containing solution by using L-sorbic acid and/or
ascorbic acid as a reducing agent with a water-soluble organic acid
usually contains coarse particles in a certain ratio, wherein the
volume accumulated maximum particle diameter D.sub.max is 16.0
.mu.m or less. The maximum particle diameter can be recognized as a
coarse particle in a certain meaning, and may be eliminated from
the product by classification as required.
[0052] The specific surface area of the highly crystalline silver
powder of the present invention obtained by the above-mentioned
production method is in the range of 0.2 m.sup.2/g or more. Judging
from the results of the studies by the present inventors, it is in
the range of 0.2 m.sup.2/g to 3.5 m.sup.2/g. The value of the
specific surface area decreases as the surface of the particles
becomes smooth and the viscosity of a paste or ink made of the
particles can be reduced.
EXAMPLE 1
[0053] Preparation of the first aqueous solution: The solution was
prepared by mixing 1.0 g of gelatin, 50 g of silver nitrate and
26.4 g of nitric acid to 250 g of pure water and elevating the
temperature of the mixture to 50.degree. C. with stirring to finish
dissolving.
[0054] Preparation of the second aqueous solution: The solution was
prepared as a solution in which 26.4 g of L-sorbic acid as a
reducing agent and 4.2 g of DL-malic acid as a water-soluble
organic acid were dissolved in 250 g of pure water.
[0055] Reductive deposition of the highly crystalline silver
powder: The second aqueous solution was slowly added to the first
aqueous solution at a solution temperature of 50.degree. C. for 30
minutes. After finishing addition of the second aqueous solution to
mix with the first aqueous solution, stirring was kept for five
minutes to grow deposited silver particles.
[0056] Collection by filtration of the highly crystalline silver
powder: After finishing the five-minute stirring, the mixture was
kept still to settle generated silver powder, supernatant liquid
was discarded and filtration and rinsing were performed by popular
methods, and the highly crystalline silver powder was obtained. The
powder characteristics of the highly crystalline silver powder are
shown in Table 1 along with those of the other Examples and
Comparative Examples.
EXAMPLE 2
[0057] Preparation of the first aqueous solution: Same as in
Example 1 and description is omitted to avoid redundancy.
[0058] Preparation of the second aqueous solution: The solution was
prepared as a solution in which 26.4 g of L-sorbic acid as a
reducing agent and 3.6 g of citric acid as a water-soluble organic
acid were dissolved in 250 g of pure water.
[0059] Reductive deposition of the highly crystalline silver
powder: The second aqueous solution was slowly added to the first
aqueous solution at 50.degree. C. for 30 minutes. After finishing
addition of the second aqueous solution to mix with the first
aqueous solution, stirring was kept for five minutes to grow
deposited silver particles.
[0060] Collection by filtration of the highly crystalline silver
powder: After finishing the five-minute stirring, the mixture was
kept still to settle generated silver powder, supernatant liquid
was discarded and filtration and rinsing were performed by popular
methods, and the highly crystalline silver powder was obtained. The
powder characteristics of the highly crystalline silver powder are
shown in Table 1 along with those of the other Examples and
Comparative Examples.
EXAMPLE 3
[0061] Preparation of the first aqueous solution: The solution was
prepared by mixing 3.3 g of gelatin, 55 g of silver nitrate and 27
g of nitric acid to 550 g of pure water and elevating the
temperature of the mixture to 50.degree. C. with stirring to finish
dissolving.
[0062] Preparation of the second aqueous solution: The solution was
prepared as a solution in which 28.1 g of L-sorbic acid as a
reducing agent and 4.47 g of DL-malic acid as a water-soluble
organic acid were dissolved in 250 g of pure water.
[0063] Reductive deposition of the highly crystalline silver
powder: The second aqueous solution was slowly added to the above
first aqueous solution at 50.degree. C. for 30 minutes. After
finishing addition of the second aqueous solution to mix with the
first aqueous solution, stirring was kept for five minutes to grow
deposited silver particles.
[0064] Collection by filtration of the highly crystalline silver
powder: After finishing the five-minute stirring, the mixture was
kept still to settle generated silver powder, supernatant liquid
was discarded and filtration and rinsing were performed by popular
methods, and the highly crystalline silver powder was obtained. The
powder characteristics of the highly crystalline silver powder are
shown in Table 2 along with those of the other Examples and
Comparative Examples.
EXAMPLE 4
[0065] Preparation of the first aqueous solution: Same as in
Example 3 and description is omitted to avoid redundancy.
[0066] Preparation of the second aqueous solution: The solution was
prepared as a solution in which 28.1 g of L-sorbic acid as a
reducing agent and 3.83 g of citric acid as a water-soluble organic
acid were dissolved in 550 g of pure water.
[0067] Reductive deposition of the highly crystalline silver
powder: The second aqueous solution was slowly added to the above
first aqueous solution at 50.degree. C. for 30 minutes. After
finishing addition of the second aqueous solution to mix with the
first aqueous solution, stirring was kept for five minutes to grow
deposited silver particles.
[0068] Collection by filtration of the highly crystalline silver
powder: After finishing the five-minute stirring, the mixture was
kept still to settle generated silver powder, supernatant liquid
was discarded and filtration and rinsing were performed by popular
methods, and the highly crystalline silver powder was obtained. The
powder characteristics of the highly crystalline silver powder are
shown in Table 2 along with those of the other Examples and
Comparative Examples.
EXAMPLE 5
[0069] Preparation of the first aqueous solution: The solution was
prepared by adding 4.0 g of gelatin, 66 g of silver nitrate and
32.4 g of nitric acid to 700 g of pure water and elevating the
temperature of the mixture to 50.degree. C. with stirring to finish
dissolving.
[0070] Preparation of the second aqueous solution: The solution was
prepared as a solution in which 33.8 g of ascorbic acid as a
reducing agent and 4.6 g of citric acid as a water-soluble organic
acid were dissolved in 700 g of pure water.
[0071] Reductive deposition of the highly crystalline silver
powder: The second aqueous solution was slowly added to the above
first aqueous solution at 50.degree. C. for 30 minutes. After
finishing addition of the second aqueous solution to mix with the
first aqueous solution, stirring was kept for five minutes to grow
deposited silver particles.
[0072] Collection by filtration of the highly crystalline silver
powder: After finishing the five-minute stirring, the mixture was
kept still to settle generated silver powder, supernatant liquid
was discarded and filtration and rinsing were performed by popular
methods, and the highly crystalline silver powder was obtained. The
powder characteristics of the highly crystalline silver powder are
shown in Table 3 along with those of the other Examples and
Comparative Examples.
EXAMPLE 6
[0073] Preparation of the first aqueous solution: Same as in
Example 5 and description is omitted to avoid redundancy.
[0074] Preparation of the second aqueous solution: The solution was
prepared as a solution in which 33.8 g of ascorbic acid as a
reducing agent and 6.0 g of DL-malic acid as a water-soluble
organic acid were dissolved in 700 g of pure water.
[0075] Reductive deposition of the highly crystalline silver
powder: The second aqueous solution was slowly added to the above
first aqueous solution at 50.degree. C. for 30 minutes. After
finishing addition of the second aqueous solution to mix with the
first aqueous solution, stirring was kept for five minutes to grow
deposited silver particles.
[0076] Collection by filtration of the highly crystalline silver
powder: After finishing the five-minute stirring, the mixture was
kept still to settle generated silver powder, supernatant liquid
was discarded and filtration and rinsing were performed by popular
methods, and the highly crystalline silver powder was obtained. The
powder characteristics of the highly crystalline silver powder are
shown in Table 3 along with those of the other Examples and
Comparative Examples.
EXAMPLE 7
[0077] Preparation of the first aqueous solution: Same as in
Example 5 and description is omitted to avoid redundancy.
[0078] Preparation of the second aqueous solution: The solution was
prepared as a solution in which 16.9 g of ascorbic acid and 16.9 g
of L-sorbic acid as reducing agents and 6.0 g of DL-malic acid as a
water-soluble organic acid were dissolved in 720 g of pure
water.
[0079] Reductive deposition of the highly crystalline silver
powder: The second aqueous solution was slowly added to the above
first aqueous solution at 50.degree. C. for 30 minutes. After
finishing addition of the second aqueous solution to mix with the
first aqueous solution, stirring was kept for five minutes to grow
deposited silver particles.
[0080] Collection by filtration of the highly crystalline silver
powder: After finishing the five-minute stirring, the mixture was
kept still to settle generated silver powder, supernatant liquid
was discarded and filtration and rinsing were performed by popular
methods, and the highly crystalline silver powder was obtained. The
powder characteristics of the highly crystalline silver powder are
shown in Table 4 along with those of the other Examples and
Comparative Examples.
EXAMPLE 8
[0081] Preparation of the first aqueous solution: Same as in
Example 5 and description is omitted to avoid redundancy.
[0082] Preparation of the second aqueous solution: The solution was
prepared as a solution in which 16.9 g of ascorbic acid and 16.9 g
of L-sorbic acid as reducing agents and 4.6 g of citric acid as a
water-soluble organic acid were dissolved in 720 g of pure
water.
[0083] Reductive deposition of the highly crystalline silver
powder: The second aqueous solution was slowly added to the above
first aqueous solution at 50.degree. C. for 30 minutes. After
finishing addition of the second aqueous solution to mix with the
first aqueous solution, stirring was kept for five minutes to grow
deposited silver particles.
[0084] Collection by filtration of the highly crystalline silver
powder: After finishing the five-minute stirring, the mixture was
kept still to settle generated silver powder, supernatant liquid
was discarded and filtration and rinsing were performed by popular
methods, and the highly crystalline silver powder was obtained. The
powder characteristics of the highly crystalline silver powder are
shown in Table 4 along with those of the other Examples and
Comparative Examples.
Comparative Examples
Comparative Example 1
[0085] In this Comparative Example, the silver powder was produced
in the condition that the water-soluble organic acid (DL-malic
acid) in the second aqueous solution of Example 1 was omitted and
the other conditions were same as in Example 1. The powder
characteristics of the silver powder are shown in Table 1 along
with those of the other Examples and Comparative Examples.
Comparative Example 2
[0086] In this Comparative Example, the silver powder was produced
in the condition that the water-soluble organic acid (DL-malic
acid) in Example 3 of the second aqueous solution was omitted and
the other conditions were same as in Example 1. The powder
characteristics of the silver powder are shown in Table 2 along
with those of the other Examples and Comparative Examples.
Comparative Example 3
[0087] In this Comparative Example, the silver powder was produced
in the condition that the water-soluble organic acid (citric acid)
in Example 5 of the second aqueous solution was omitted and the
other conditions were same as in Example 1. The powder
characteristics of the silver powder are shown in Table 3 along
with those of the other Examples and Comparative Examples.
Comparative Example 4
[0088] In this Comparative Example, the silver powder was produced
in the condition that the water-soluble organic acid (DL-malic
acid) in Example 7 of the second aqueous solution was omitted and
the other conditions were same as in Example 1. The powder
characteristics of the silver powder are shown in Table 4 along
with those of the other Examples and Comparative Examples.
Comparison between Examples and Comparative Examples
Comparison between Examples 1 and 2 and Comparative Example 1
[0089] Example 1 and Example 2 are different in that the
water-soluble organic acid used with a reducing agent is DL-malic
acid or citric acid, and Comparative Example 1 does not use the
water-soluble organic acid used in Example 1. Therefore, all of
these are shown in Table 1 for comparison.
TABLE-US-00001 TABLE 1 Primary particle Crystallite diameter
D.sub.10 D.sub.50 D.sub.90 D.sub.max SSA diameter Sample .mu.m
m.sup.2/g .ANG. (D.sub.90 - D.sub.10)/D.sub.50 Example 1 3.96 2.83
4.05 6.98 13.1 0.29 525 1.02 Example 2 3.86 2.54 4.12 7.02 15.6
0.28 522 1.08 Comparative 3.33 2.38 4.02 8.46 18.5 0.26 546 1.51
Example 1
[0090] In the Table 1, primary particle diameter, D.sub.10,
D.sub.50, D.sub.90, D.sub.max by the laser diffraction
scattering-type particle size distribution measurement method,
specific surface area (SSA), crystal diameter and a value of
(D.sub.90-D.sub.10)/D.sub.50 are shown. Firstly, when attention is
paid to the primary particle diameter, it is understood that silver
powder having a slightly small primary particle diameter was
obtained in Comparative Example 1, in which no water-soluble
organic acid was used, as compared with Example 1 and Example 2. In
the comparison between Examples 1 and 2 and Comparative Example 1
on each value of D.sub.10, D.sub.50, D.sub.90, D.sub.max, specific
surface area (SSA) and crystal diameter, it cannot be said that
there are significant differences between the values. In contrast,
the value of (D.sub.90-D.sub.10)/D.sub.50 are definitely smaller in
Examples 1 and 2 than in Comparative Example 1. That is, the
particle size distributions in Examples 1 and 2 have sharper
particle size distributions as compared with the particle size
distribution in Comparative Example 1. It means that the particle
diameters in the Examples 1 and 2 exist in a narrower range than
the particle diameters in the Comparative Example 1.
[0091] From examination on the results, the silver powder obtained
in Comparative Example 1 tends to be smaller in primary particle
diameter as compared with those of Examples 1 and 2, but
aggregation of the reductively deposited silver particles is so
remarkable that practical use thereof is difficult. In contrast,
since the highly crystalline silver powders obtained by the
production method in Examples 1 and 2 are hard to aggregate, they
may form only a small amount of coarse particles to be a product
with good balance and excellent in particle dispersibility.
Comparison between Examples 3 and 4 and Comparative Example 2
[0092] Example 3 and Example 4 are different in that the
water-soluble organic acid used with a reducing agent is DL-malic
acid or citric acid, and Comparative Example 2 does not use the
water-soluble organic acid used in Example 3, and therefore all of
these are shown in Table 2 for comparison.
TABLE-US-00002 TABLE 2 Primary Particle Crystallite Diameter
D.sub.10 D.sub.50 D.sub.90 D.sub.max SSA diameter Sample .mu.m
m.sup.2/g .ANG. (D.sub.90 - D.sub.10)/D.sub.50 Example 3 0.89 0.62
1.02 1.88 3.6 0.85 438 1.24 Example 4 0.94 0.69 1.08 1.98 3.9 0.83
432 1.19 Comparative 0.74 0.45 1.16 2.84 5.5 0.86 438 2.06 Example
2
[0093] In the Table 2, primary particle diameter, D.sub.10,
D.sub.50, D.sub.90, D.sub.max by the laser diffraction
scattering-type particle size distribution measurement method,
specific surface area (SSA), crystal diameter and a value of
(D.sub.90-D.sub.10)/D.sub.50 are shown. Firstly, when attention is
paid to the primary particle diameter, it is understood that silver
powder having a small primary particle diameter was obtained in
Comparative Example 2, in which no water-soluble organic acid was
used, as compared with Example 3 and Example 4. Here, respective
values of D.sub.10, D.sub.50, D.sub.90, D.sub.max, specific surface
area (SSA) and crystal diameter of Examples 3 and 4 and Comparative
Example 2 are compared. Firstly, when attention is paid to the
primary particle diameter and to D.sub.10, the values in Example 3
and Example 4 are larger than in Comparative Example 2. When
attention is paid to D.sub.50, there is no significant difference
between Examples 3 and 4 and Comparative Example 2. When attention
is paid to D.sub.90 and D.sub.max, the value in Example 3 and
Example 4 are smaller than in Comparative Example 2. At this stage,
it can be expected that the particle size distributions in Example
3 and Example 4 are better and sharper than that in Comparative
Example 2. Next, it cannot be said that there is no significant
difference in specific surface area (SSA) and crystal diameter.
Meanwhile, the value of (D.sub.90-D.sub.10)/D.sub.50 are definitely
smaller in Examples 3 and 4 than in Comparative Example 2. That is,
the particle size distributions in Examples 3 and 4 have sharper
particle size distributions as compared with the particle size
distribution in Comparative Example 2. It means that the particle
diameters in the Examples 3 and 4 exist in a narrower range than
the particle diameters in the Comparative Example 2.
[0094] From examination on the results, the silver powder obtained
in Comparative Example 2 tends to be smaller in primary particle
diameter as compared with those of Examples 3 and 4, but
aggregation of the reductively deposited silver particles is so
remarkable that practical use thereof is difficult. In contrast,
since the highly crystalline silver powders obtained by the
production method in Examples 3 and 4 are hard to aggregate, they
may form only a small amount of coarse particles to be a product
with good balance and excellent in particle dispersibility.
Comparison between Examples 5 and 6 and Comparative Example 3
[0095] Example 5 and Example 6 are different in that the
water-soluble organic acid used with a reducing agent is DL-malic
acid or citric acid, and Comparative Example 3 does not use the
water-soluble organic acid used in Example 5, and therefore all of
these are shown in Table 3 for comparison.
TABLE-US-00003 TABLE 3 Primary particle Crystallite diameter
D.sub.10 D.sub.50 D.sub.90 D.sub.max SSA diameter Sample .mu.m
m.sup.2/g .ANG. (D.sub.90 - D.sub.10)/D.sub.50 Example 5 0.85 0.65
0.98 1.82 3.5 0.90 408 1.19 Example 6 0.84 0.62 0.96 1.74 3.5 0.89
405 1.16 Comparative 0.68 0.50 1.10 2.94 3.5 0.95 407 2.22 Example
3
[0096] In the Table 3, primary particle diameter, D.sub.10,
D.sub.50, D.sub.90, D.sub.max by the laser diffraction
scattering-type particle size distribution measurement method,
specific surface area (SSA), crystal diameter and a value of
(D.sub.90-D.sub.10)/D.sub.50 are shown. Firstly, when attention is
paid to the primary particle diameter, it is understood that silver
powder having a small primary particle diameter was obtained in
Comparative Example 3, in which no water-soluble organic acid is
used, as compared with Example 5 and Example 6. Here, respective
values of D.sub.10, D.sub.50, D.sub.90, D.sub.max, specific surface
area (SSA) and crystal diameter of Examples 5 and 6 and Comparative
Example 3 are compared. When attention is paid to D.sub.10, the
values in Example 5 and Example 6 are larger than in Comparative
Example 3. When attention is paid to D.sub.50, there is no
significant difference between Examples 5 and 6 and Comparative
Example 3. When attention is paid to D.sub.90 and D.sub.max, the
values in Example 5 and Example 6 are smaller than in Comparative
Example 3. At this stage, it can be expected that the particle size
distributions in Example 5 and Example 6 are better and sharper
than that in Comparative Example 3. Next, it cannot be said that
there is no significant difference in specific surface area (SSA)
and crystal diameter. Meanwhile, the value of
(D.sub.90-D.sub.10)/D.sub.50 are definitely smaller in Examples 5
and 6 than in Comparative Example 3. That is, the particle size
distributions in Examples 5 and 6 have sharper particle size
distributions as compared with the particle size distribution in
Comparative Example 3. It means that the particle diameters in the
Examples 5 and 6 exist in a narrower range than the particle
diameters in the Comparative Example 3.
[0097] From examination on the results, the silver powder obtained
in Comparative Example 3 tends to be smaller in primary particle
diameter as compared with those of Examples 5 and 6, but
aggregation of the reductively deposited silver particles is so
remarkable that practical use thereof is difficult. In contrast,
since the highly crystalline silver powders obtained by the
production method in Examples 5 and 6 are hard to aggregate, they
may form only a small amount of coarse particles to be a product
with good balance and excellent in particle dispersibility.
Comparison between Examples 7 and 8 and Comparative Example 4:
[0098] Example 7 and Example 8 are different in that the
water-soluble organic acid used with a reducing agent is DL-malic
acid or citric acid, and Comparative Example 4 does not use the
water-soluble organic acid used in Example 7, and therefore all of
these are shown in Table 4 for comparison.
TABLE-US-00004 TABLE 4 Primary particle Crystallite diameter
D.sub.10 D.sub.50 D.sub.90 D.sub.max SSA diameter Sample .mu.m
m.sup.2/g .ANG. (D.sub.90 - D.sub.10)/D.sub.50 Example 7 0.82 0.58
0.94 1.68 2.3 0.99 401 1.17 Example 8 0.86 0.59 0.92 1.58 2.1 1.01
400 1.08 Comparative 0.65 0.53 1.09 3.33 5.5 0.99 403 2.57 Example
4
[0099] In the Table 4, primary particle diameter, D.sub.10,
D.sub.50, D.sub.90, D.sub.max by the laser diffraction
scattering-type particle size distribution measurement method,
specific surface area (SSA), crystal diameter and a value of
(D.sub.90-D.sub.10)/D.sub.50 are shown. Firstly, when attention is
paid to the primary particle diameter, it is understood that silver
powder having a small primary particle diameter was obtained in
Comparative Example 4, in which no water-soluble organic acid is
used, as compared with Example 7 and Example 8. Here, respective
values of D.sub.10, D.sub.50, D.sub.90, D.sub.max, specific surface
area (SSA) and crystal diameter of Examples 7 and 8 and Comparative
Example 4 are compared. As for D.sub.90 and D.sub.max, the values
are definitely larger in Comparative Example 4 than in Example 7
and Example 8 and it is considered that coarse particles are formed
in a higher ratio. It cannot be said that there is no significant
difference in the other values. Meanwhile, the value of
(D.sub.90-D.sub.10)/D.sub.50 are definitely smaller in Examples 7
and 8 than in Comparative Example 4. That is, the particle size
distributions in Examples 7 and 8 have sharper particle size
distributions as compared with the particle size distribution in
Comparative Example 4. It means that the particle diameters in the
Examples 7 and 8 exist in a narrower range than the particle
diameters in the Comparative Examples 4.
[0100] From examination on the results, the silver powder obtained
in Comparative Example 4 tends to be smaller in primary particle
diameter as compared with those of Examples 7 and 8, but
aggregation of the reductively deposited silver particles is so
remarkable that practical use thereof is difficult. In contrast,
since the highly crystalline silver powders obtained by the
production method in Examples 7 and 8 are hard to aggregate, they
may form only a small amount of coarse particles to be a product
with good balance and excellent in particle dispersibility.
INDUSTRIAL APPLICABILITY
[0101] The highly crystalline silver powder described above is a
product which is the highly crystalline and has a sharp particle
size distribution in the whole range from fine particles to
particles with large diameter as compared with conventional highly
crystalline silver powder. Therefore, conductor films formed by a
silver paste produced by using the highly crystalline silver powder
of the present invention are excellent in thermal shrinkage
resistance and have a smoother surface roughness. Therefore,
quality of a conductor formed with the conductive paste can be
improved.
[0102] In addition, the production method of the highly crystalline
silver powder of the present invention comprises adding a solution
which contains a reducing agent and a water-soluble organic acid to
a solution which contains a silver salt to cause reductive
deposition. The presence of the water-soluble organic acid enables
production of the highly crystalline silver powder having high
crystallinity with a sharp particle size distribution and provides
a process suitable for industrial production process.
* * * * *