U.S. patent application number 10/566353 was filed with the patent office on 2007-04-12 for fine particulate silver powder and production method thereof.
This patent application is currently assigned to Mitsui Mining & Smelting Co., Ltd.. Invention is credited to Masashi Kato, Takuya Sasaki, Katsuhiko Yoshimaru.
Application Number | 20070079665 10/566353 |
Document ID | / |
Family ID | 34100960 |
Filed Date | 2007-04-12 |
United States Patent
Application |
20070079665 |
Kind Code |
A1 |
Sasaki; Takuya ; et
al. |
April 12, 2007 |
Fine particulate silver powder and production method thereof
Abstract
An object of the present invention is to provide a silver powder
which is an unprecedentedly fine particulate silver powder, has a
dispersibility more approximate to monodispersibility resulting
less aggregation, and has a low impurity content. To achieve the
above object, a fine particulate silver powder having unprecedented
powder properties in which: a. the average particle diameter
D.sub.IA of the primary particles obtained by image analysis of a
scanning electron micrograph is 0.6 .mu.m or less; b. the
crystallite diameter is 10 nm or less; c. the sintering starting
temperature is 240.degree. C. or less; and d. the carbon content is
0.25 wt % or less is obtained by allowing a silver ammine complex
aqueous solution S.sub.1 to flow in a certain flow path
(hereinbelow referred to as "first flow path") providing a second
flow path b which joins the first flow path a on its way and
allowing an organic reducing agent and additives S.sub.2, if
required, to flow into the first flow path a through the second
first flow path b, and contacting and mixing them at the joining
point of the first flow path a and the second flow path b to allow
reduction and deposition of particles, and washing the particles
with an excess amount of an alcohol.
Inventors: |
Sasaki; Takuya;
(Shimonoseki-shi, JP) ; Kato; Masashi;
(Shimonoseki-shi, JP) ; Yoshimaru; Katsuhiko;
(Shimonoseki-shi, 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.
11-1, Osaki 1-chome
Tokyo
JP
141-8584
|
Family ID: |
34100960 |
Appl. No.: |
10/566353 |
Filed: |
July 15, 2004 |
PCT Filed: |
July 15, 2004 |
PCT NO: |
PCT/JP04/10102 |
371 Date: |
July 21, 2006 |
Current U.S.
Class: |
75/255 ; 75/362;
75/371 |
Current CPC
Class: |
B22F 9/24 20130101; C22C
5/06 20130101 |
Class at
Publication: |
075/255 ;
075/362; 075/371 |
International
Class: |
B22F 9/24 20060101
B22F009/24 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 29, 2003 |
JP |
2003-281660 |
Claims
1. A fine particulate silver powder having low aggregation
properties, characterized in that the silver powder has the
following powder properties in which: a. the average particle
diameter D.sub.IA of the primary particles obtained by image
analysis of a scanning electron micrograph is 0.6 .mu.m or less; b.
the aggregation degree represented by D.sub.50/D.sub.IA using the
average particle diameter D.sub.IA of the primary particles and the
average particle diameter D.sub.50 by laser diffraction scattering
particle size distribution measurement is 1.5 or less; c. the
crystallite diameter is 10 nm or less; and d. the content of
organic impurities is 0.25 wt % or less on the basis of the amount
of carbon.
2. The fine particulate silver powder according to claim 1, wherein
the sintering starting temperature is 240.degree. C. or less.
3. A production method of a fine particulate silver powder
comprising obtaining an aqueous solution of silver ammine complex
by mixing and reacting an aqueous silver nitrate solution with
ammonia water, reducing and depositing silver particles by adding a
reducing agent thereto and performing filtering, washing and drying
the particles, characterized in that the silver particles are
reduced and deposited by contacting and mixing aqueous solution of
silver ammine complex with an organic reducing agent, and
maintaining the silver concentration to 1 g/l to 6 g/l, and the
concentration of the organic reducing agent to 1 g/l to 3 g/l in
the mixed solution, and the silver particles are then filtered,
washed with water, and washed with an excess amount of a methanol
solution.
4. The production method of a fine particulate silver powder
according to claim 3, characterized in that when the aqueous
solution of silver ammine complex is contacted and mixed with the
organic reducing agent, the silver ammine complex aqueous solution
flows in a certain flow path (hereinbelow referred to as "first
flow path"), a second flow path which joins the first flow path on
its way is provided, and the organic reducing agent is allowed to
flow into the first flow path through the second first flow path
and is contacted and mixed with the aqueous solution of silver
ammine complex at the joining point of the first flow path and the
second flow path.
5. The production method of a fine particulate silver powder
according to claim 3, characterized in that the method employs an
aqueous solution of silver ammine complex having a silver
concentration of 2 to 12 g/l obtained by mixing and reacting a
silver nitrate aqueous solution having a silver nitrate
concentration of 2.6 to 4.8 g/l with ammonia water.
6. The production method of a fine particulate silver powder
according to claim 3, wherein a dispersant is contained in the
organic reducing agent to be used.
7. The production method of a fine particulate silver powder
according to claim 3, wherein hydroquinone is used as the organic
reducing agent.
8. The production method of a fine particulate silver powder
according to claim 3, wherein alcohol is used in an amount of 5 L
or more per kg of the obtained silver particles.
Description
TECHNICAL FIELD
[0001] The invention of this application relates to a fine
particulate silver powder and a production method of the fine
particulate silver powder. Particularly, the present invention
relates to a fine particulate silver powder having a low impurity
content.
BACKGROUND ART
[0002] Conventionally, for the production of a silver powder, wet
reduction process has been adopted in which an aqueous solution of
silver ammine complex is produced with a silver nitrate solution
and ammonia water, and an organic reducing agent is added to this
as described in Patent Document 1. In late years, these silver
powders are used for forming electrodes and circuits of chip parts,
plasma display panel, etc. as main field of application. [0003]
[Patent document 1]: Japanese Patent Laid-Open Publication No.
2001-107101
[0004] Therefore, as for such electrodes and circuits, drastic fine
production of the electrodes and circuits are demanded, and high
reliability has come to be demanded along with high density and
high accuracy of electric wiring.
PROBLEMS TO BE SOLVED BY THE INVENTION
Disclosure of the Invention
[0005] However, the powder particles of silver powder obtained by
this conventional production method have an average particle
diameter of primary particles D.sub.IA usually exceeding 0.6 .mu.m
and an average particle diameter D.sub.50 by laser diffraction
scattering particle size distribution measurement method exceeding
1.0 .mu.m, and the aggregation degree expressed by
D.sub.50/D.sub.IA exceeds 1.7 in actual condition. Therefore they
were unsuitable for the recent fine-pitched circuit formation, and
made a significant factor of decreasing product yield.
[0006] In the meantime, the following problems have arisen from a
viewpoint of usage of silver powders. It has been conventionally
considered that a low crystalline silver powder is desirable for
achieving a high degree of sinterbility at a low temperature
because non-sintering or low-temperature-sintering type in which
the heat temperature is 300.degree. C. or less has been largely
used in the circuit formation using a silver paste. However, fast
reduction reaction systems have had to be adopted in the production
condition to obtain a low crystalline silver powder, and, as a
result, only silver powders which have low crystallinity but are
remarkably aggregating have been able to be obtained.
[0007] Under these circumstances, silver powders which are
unprecedentedly fine particulate silver powders and have a
dispersibility more approximate to monodispersibility resulting
less aggregation and also have excellent low-temperature sintering
properties have been desired to be provided in the market.
[0008] In the meantime, it has been demanded that the silver powder
has a low impurity content. That is to say, the above-mentioned wet
reduction process has been adopted for the production of a silver
powder and the reducing agent and the like used in the process
remain on the surface of the powder particles of the silver powder.
Accordingly, this is an inevitable problem as long as the
conventional production process is adopted. And as the impurity
content increases, the electric resistance of the conductor formed
of the silver powder increases.
[0009] As a result, the market has demanded a silver powder which
is fine, highly dispersible, and low in the impurity content.
[0010] Accordingly, the present inventors have conducted intensive
studies and made the best use of inventiveness in the production
method based on a conventional production method in which an
aqueous solution of silver ammine complex is obtained by mixing and
reacting an aqueous silver nitrate solution and ammonia water, and
an organic reducing agent is added to this to allow reduction and
deposition of silver particles which are then filtered, washed and
dried. As a result, fine particulate silver powders of a level
unattainable can be obtained by a conventional production method,
and fine particulate silver powders never present conventionally
can be obtained by significantly reducing the impurity content of
the fine particulate silver powders. Further, the production method
according to the present invention attained a production method
which enables the fine particulate silver powders to be obtained
stably at a high yield. Hereinbelow, the present invention will be
described as divided into the "fine particulate silver powder" and
the "production method".
<Fine Particulate Silver Powder>
[0011] First, the fine particulate silver powder according to the
present invention is described. The fine particulate silver powder
according to the present invention is mainly characterized in that
it has the following powder properties a. to c. As for these powder
properties, they are enumerated as the most conspicuous properties
and common in the fine particulate silver powder according to the
present invention under current powder measurement techniques.
Hereinbelow, each of the properties is described.
[0012] The property a. is that the average particle diameter
D.sub.IA of the primary particles obtained by image analysis of a
scanning electron microscope image is 0.6 .mu.m or less. Here, the
"average particle diameter D.sub.IA of the primary particles
obtained by image analysis of a scanning electron microscope image
" is an average particle diameter obtained by image analysis of an
image of the silver powder observed by a scanning electron
microscope (SEM) (wherein it is preferable to observe at a
magnification of 10,000 times in the case of a fine particulate
silver powder according to the present invention and at a
magnification of 3000-5000 times in the case of a conventional
silver powder.). The image analysis of the silver powder observed
by a scanning electron microscope (SEM) in the present
specification is performed by obtaining average particle diameter
D.sub.IA by using IP-1000PC manufactured by Asahi Engineering Co.,
Ltd. and conducting around particle analysis assuming the circular
threshold as 10, and the overlapping degree as 20. Because the
average particle diameter D.sub.IA obtained by the image analysis
of the observed image is directly obtained by the SEM observation
image, the average particle diameter of the primary particles can
be surely obtained. The D.sub.IA as used in the present invention
is mostly falls within the range of 0.01 .mu.m to 0.6 .mu.m, as far
as the present inventors observe it, but more minute particle size
may be observed in reality, and therefore, no specific lower limit
is described on purpose.
[0013] As for the property b., because the fine particulate silver
powder according to the present invention shows a high
dispersibility never shown by the conventional silver powders,
"aggregation degree" as an index of showing this dispersibility is
used.
[0014] The "aggregation degree" as used in this specification is
the value represented by D.sub.50/D.sub.IA using the
above-mentioned average particle diameter D.sub.IA of primary
particles and the average particle diameter D.sub.50 by laser
diffraction scattering particle size distribution measurement
method. Here, D.sub.50 is particle size at 50% weight accumulation
obtained by laser diffraction scattering particle size distribution
measurement method, and the value of this average particle diameter
D.sub.50 is not by directly and truly observing the diameter of the
powder particles one by one but it can be said that an average
particle diameter by assuming an aggregated powder particle as one
particle (aggregated particle) is calculated. This is because
powder particles of a real silver powder are usually considered not
to be so-called monodisperse powder in which each individual
particle is completely separated but in a condition in which
several powder particles are aggregated. However, it is usual that
the value of average particle diameter D.sub.50 becomes smaller as
less aggregation of powder particles is present, and they are more
approximate to monodisperse. D.sub.50 of the fine particulate
silver powder to be used in the present invention is a range of
about 0.25 .mu.m to 0.80 .mu.m, and a fine particulate silver
powder is provided having an average particle diameter D.sub.50 of
the range which has not been obtained at all by a conventional
production method. The laser diffraction scattering particle size
distribution measurement method as used in this specification is
performed by mixing 0.1 g of fine particulate silver powder with
ion-exchange water, and dispersing it with an ultrasonic
homogenizer (a product of Nippon Seiki Seisaku-sho Co., Ltd.,
US-300T) for five minutes and measuring with a laser diffraction
scattering particle size distribution measuring apparatus MicroTrac
HRA 9320-X 100 type (a product of Leeds & Northrup
company).
[0015] In contrast, the "average particle diameter D.sub.IA of the
primary particles obtained by image analysis of a scanning electron
micrograph" is an average particle diameter obtained by image
analysis of an image of the silver powder observed by a scanning
electron microscope (SEM) and the average particle diameter of the
primary particles is surely observed without considering state of
aggregation.
[0016] As a result, the present inventors have decided to take the
value calculated by D.sub.50/D.sub.IA calculated from the average
particle diameter D.sub.50 by laser diffraction scattering particle
size distribution measurement method and the average particle
diameter D.sub.IA obtained by image analysis as the aggregation
degree. That is to say, assuming that the D.sub.50 and D.sub.IA
values can be measured with same precision in a fine particulate
silver powder of the same lot, the value of D.sub.50 allowing the
state of aggregation to be reflected in the measured value is
supposed to be larger than that of D.sub.IA based on the theory
stated above. The value of D.sub.50 approaches the value of
D.sub.IA limitlessly, and the value of D.sub.50/D.sub.IA which is
the aggregation degree will approach 1 as the state of aggregation
of the powder particles of the fine particulate silver powder is
reduced. At the stage where the aggregation degree becomes 1, the
powder can be said to be monodisperse powder in which the state of
aggregation of the powder particles is not present at all.
[0017] Accordingly, the present inventors have tried to examine the
correlation among the aggregation degree, viscosity of fine
particulate silver powder pastes produced with a fine particulate
silver powder of different aggregation degree and surface
smoothness of the conductor obtained by sintering. As a result, it
has been found that an extremely good correlation can be obtained.
As is understood from this, it can be judged that free control of
viscosity of a fine particulate silver powder paste is possible by
controlling the aggregation degree of the fine particulate silver
powder used therein. Besides, it has been found that if the
aggregation degree is maintained to be 1.5or less, fluctuation of
the viscosity of fine particulate silver powder paste, and the
surface smoothness after sintering processing can be retained in an
extremely small region. In addition, as the aggregation state is
eliminated, the film apparent density of the conductor obtained by
sintering the fine particulate silver oxide powder improves and, as
a result, it comes to be possible to reduce the electric resistance
of the formed sintered conductor.
[0018] In addition, when the aggregation degree is actually
calculated, there are some cases where the value less than 1 is
shown. It is considered that this is due to the assumption that
D.sub.IA used for calculation of aggregation degree is a truth
sphere, and although the value of aggregation degree cannot be a
value under 1 theoretically, it is supposed that such a value of
aggregation degree less than 1 can be obtained in reality because
it is not a truth sphere.
[0019] The property c. is that crystallite diameter is 10 nm or
less and there is a very close relationship between this
crystallite diameter and sintering starting temperature. That is to
say, in the comparison between the silver powders having equal
average particle diameter, the sintering temperature can be lower
as the crystallite diameter is smaller. Therefore, the fine
particulate silver powder of the present invention having a large
surface energy due to its small particle size, and having a small
crystallite diameter of 10 mm or less can reduce the sintering
starting temperature. Here, no lower limit is defined for the
crystallite diameter, and the reason therefor is that a certain
measurement error occurs depending on measuring apparatus,
measurement condition and so on. It is difficult. to demand high
reliability in the measured values in the range where the
crystallite diameter is less than 10 nm and if the lower limit is
dared to be determined, it is supposed to be around 2 nm obtained
as a result of study of the present inventors.
[0020] The property d. is that the content of organic impurities is
0.25 wt % or less in terms of the amount of carbon. Here, the
carbon content is used as an index of the content of organic
impurities and is to estimate the amount of impurities adhered to
the powder particles of the silver powder. The carbon content at
this time is measured by mixing 0.5 g of a fine particulate silver
powder, 1.5 g of tungsten powder and 0.3 g of tin powder and
placing the mixture in a porcelain crucible and performing the
measurement by burn up infrared absorption method using EMIA-320V
manufactured by Horiba, Ltd. The carbon content of a silver powder
obtained by a conventional production method will be that
containing more than 0.25 wt % carbon no matter how strongly
washing is enhanced.
[0021] The fine particulate silver powders according to the present
invention have the powder properties a. to d. as stated above, and
they can be taken as fine particulate silver powders never present
conventionally. In addition, from a viewpoint of sintering starting
temperature, the fine particulate silver powder according to the
present invention can be said to be fine particulate silver powder
for which sintering can be started at a low temperature as low as
240.degree. C. or less. No lower limit is particularly defined for
this sintering starting temperature either, but it is almost
impossible to attain a sintering starting temperature less than
170.degree. C. and this is supposed to be said temperature
corresponding to the lower limit in consideration of the study
performed by the present inventors and general technique common
sense.
[0022] Furthermore, tap bulk density of the fine particulate silver
powder according to the present invention is as high as 4.0
g/cm.sup.3 as an effect resulted by the powder properties mentioned
above. The tap bulk density as used herein is measured by a method
comprising accurately weighing 200 g of fine particulate silver
powder, placing the powder in a measuring cylinder of 150 cm.sup.3,
tapping by dropping at a stroke of 40 mm, 1,000 times repeatedly,
and then measuring the volume of the fine particulate silver
powder. This tap bulk density will be obtained as a higher value as
the powder has theoretically more minute particle size, and it is
in a state of higher dispersibility without aggregation among the
particles. Considering that the tap bulk density of the
conventional silver powders was less than 4.0 g/cm.sup.3, this
supports that the fine particulate silver powder according to the
present invention is very fine and excellent in dispersibility.
<Production Method of Fine Particulate Silver Powder>
[0023] The production method according to the present is a method
in which an aqueous solution of silver ammine complex is obtained
by contacting and reacting an aqueous silver nitrate solution and
ammonia water, and an organic reducing agent is added to this to
allow reduction and deposition of silver particles which are then
filtered, washed and dried, significantly characterized in that the
reducing agent, silver nitrate and ammonia water are used in
amounts so that they may be diluted after they are added.
Conventionally, it was common to mix a reducing agent solution and
a silver ammine complex aqueous solution in a tank at once and
therefore, large amounts of silver nitrate, reducing agent and
ammonia water should be added in order to make the silver
concentration to be 10 g/l or more, and otherwise productivity for
the scale of facilities was not able to be secured.
[0024] The first characteristic of the production method according
to the present invention is that the concentration of the organic
reducing agent after contacting and reacting a silver ammine
complex aqueous solution and an organic reducing agent is low, and
it is possible to decrease the organic reducing material which may
be adsorbed and left on the surface of the powder particles of the
generated silver powder, or taken in the powder particles in the
growing process of the powder particles. Therefore, it is the most
preferable to maintain the concentration of the organic reducing
agent to 1 g/l to 3 g/l and adjust the silver concentration to 1
g/l to 6 g/l in the mixed solution.
[0025] Here, there is a proportional relationship between the
silver concentration and naturally quantitatively a larger amount
of silver powders can be obtained as the silver concentration is
higher. However, when the silver concentration exceeds 6 g/l, there
arises a tendency that the deposited silver particle becomes coarse
particles, and the particle diameter will not be different at all
from that of the conventional silver powders and silver powders
having high dispersion properties by the present invention cannot
be obtained. In contrast, if the silver concentration here is less
than 1 g/l, extremely fine particulate silver powder can be
obtained but because the powder is too fine, oil absorption
increases and the viscosity of the paste rises, which brings about
necessity to increase the amount of the organic vehicle, and leads
to decrease in the film density of the finally formed sintered
conductor, and tendency of increase in the electric resistance. In
addition, it will not satisfy the necessary industrial
productivity.
[0026] And it is the most suitable condition for obtaining fine
particulate silver powder according to the present invention in a
high yield to maintain the concentration of the organic reducing
agent to 1 g/l to 3 g/l and the silver concentration to 1 g/l to
6g/l. Here, the reason for adjusting the concentration of the
organic reducing agent to 1 g/l to 3 g/l is to select it as the
range most suitable for obtaining a silver powder of fine particles
in a relationship with the silver concentration in the silver
ammine complex aqueous solution. When the concentration of the
organic reducing agent exceeds 3 g/l, the amount of the reducing
agent liquid to be added to the silver ammine complex aqueous
solution decreases, but the progress of aggregation of the powder
particles of the reduced and deposited silver powder comes to be
significant, and the amount of impurity contained in the powder
particles (In this specification, the amount of impurity is taken
as carbon content.) begins to increase rapidly. On the other hand,
when the concentration of the organic reducing agent is adjusted to
less than 1 g/l, the total liquid volume of the reducing agent to
be used increases and the amount of waste water treatment grows
significantly, and it cannot satisfy the industrial economic
efficiency.
[0027] The "organic reducing agent" as used herein is hydroquinone,
ascorbic acid, glucose, etc. Among these, it is preferable to use
hydroquinone for an organic reducing agent selectively.
Hydroquinone is comparatively excellent in reactivity in the
present invention in comparison with other organic reducing agents,
and it may be said that it is the agent having a reaction rate most
suitable for obtaining low crystalline silver powder having a small
crystallite diameter.
[0028] And the other additives can be used in combination with the
above-mentioned organic reducing agents. The additives as used
herein are glue such as gelatine, amine-based polymeric agent,
cellulose and so on and desirably they stabilize the reduction
deposition process of silver powder and have functions to be a
certain dispersing agent at the same time and can be used suitably
selectively in accordance with the organic reducing agent, type of
production process and so on.
[0029] And according to the present invention, it is desirable to
adopt a method of contacting and reacting the resulting silver
ammine complex aqueous solution and an organic reducing agent to
reduce and deposit a fine particulate silver powder, wherein the
silver ammine complex aqueous solution S.sub.1 flows in a certain
flow path (referred to as "the first flow path" in the above and
the following) and the second flow path b is provided which joins
the first flow path a on its way as shown in FIG. 1 and the organic
reducing agent and an additive S.sub.2, if required, are allowed to
flow into the first flow path a though the second first flow path b
and are contacted and mixed at the joining point m of the first
flow path a and the second first flow path b to reduce and deposit
a silver powder (hereinafter referred to as "interflow mixing
method").
[0030] By adopting such interflow mixing method, mixing of the two
liquids is achieved in the shortest time and the reaction proceeds
in a uniform state within the system a, and therefore, powder
particles of the uniform shape can be formed. In addition, because
the amount of the organic reducing agent after mixing is small when
observed as the whole solution, the amount of the organic reducing
agent adsorbed and left on the surface of the powder particles of
the reduced and deposited silver powder decreases. As a result, the
amount of impurities adsorbed on each of the fine particulate
silver powder obtained by filtering and drying can be reduced. This
reduction of the amount of impurities adsorbed on the fine
particulate silver powder also enables to reduce the electric
resistance of the sintered conductor which is formed through a
silver paste.
[0031] Furthermore, it is desirable to obtain a silver ammine
complex aqueous solution at a silver concentration of 2 g/l to 12
g/l using a silver nitrate aqueous solution of 6 g/l to 48 g/l when
contacting and reacting a silver nitrate aqueous solution and
ammonia water to obtain a silver ammine complex aqueous solution.
The prescription of the concentration of a silver nitrate aqueous
solution here is the same as prescribing the liquid volume of the
silver nitrate aqueous solution and in consideration that the
silver concentration of the silver ammine complex aqueous solution
is adjusted to 2 g/l to 12g/1, the concentration and the liquid
volume of ammonia water to be added thereto will be necessarily
decided. Although clear technical reasons have not been clear in
the present stage, a fine particulate silver powder showing the
best production stability and stable in quality can be obtained by
using a silver nitrate aqueous solution of 2.6 g/l to 48 g/l.
[0032] The second characteristic of the production method according
to the present invention is washing which is finally performed and
is very important. As for the washing at this time, a combination
of water washing and alcohol washing may be performed or alcohol
washing alone may be used but the washing by alcohol washing is
enhanced. That is to say, usually washing is performed with about
100 ml of pure water and then with about 50 ml of alcohol for 40 g
of a fine particulate silver powder which has been reduced and
deposited. In contrast, according to the present invention, alcohol
washing is performed with 200 ml or more of alcohol, that is,
washing is performed with an excess amount of alcohol of 5L or more
per kg of a fine particulate silver powder.
[0033] The reason why the impurities can be reduced by such
enhanced washing is that a technique is adopted in the contact
reaction of an aqueous solution of silver ammine complex and a
reducing agent for obtaining a fine particulate silver powder in
which technique a reaction system at a very low concentration is
adopted and the amount of the organic reducing agent is retained to
low for the whole solution after mixing.
[0034] The fine particulate silver powder according to the present
invention is fine as never present conventionally, has a high
dispersibility and has a small impurity content, and it can be
understood that such a silver powder is an unprecedented fine
particulate silver powder. In addition, it is enabled to
efficiently obtain the fine particulate silver powder according to
the present invention by adopting the production method described
above.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] FIG. 1 is a drawing expressing the concept of mixing a
silver ammine complex aqueous solution and a reducing agent;
[0036] FIG. 2 is a scanning electron micrograph of a fine
particulate silver powder according to the present invention;
[0037] FIG. 3 is a scanning electron micrograph of a fine
particulate silver powder according to the present invention;
[0038] FIG. 4 is a scanning electron micrograph of a fine
particulate silver powder according to a conventional production
method; and
[0039] FIG. 5 is a scanning electron micrograph of a fine
particulate silver powder according to a conventional production
method.
BEST MODE FOR CARRYING OUT THE INVENTION
[0040] Hereinbelow, the best mode of the present invention will be
described in detail by comparing with comparative examples.
EXAMPLE 1
[0041] In this example, a fine particulate silver powder was
produced using the production method stated above and the
properties of the obtained fine particulate silver powder were
measured. And further, a silver paste was produced with the fine
particulate silver powder and a test circuit was formed and the
conductor resistance and sintering starting temperature were
measured.
[0042] First, 63.3 g of silver nitrate was dissolved in 9.7 liters
of pure water to prepare a silver nitrate aqueous solution, and 235
ml of 25 wt % concentration ammonia water was added thereto at once
and agitated and a silver ammine complex aqueous solution was
obtained.
[0043] This silver ammine complex aqueous solution was introduced
into the first flow path a of 13 mm inside diameter shown in FIG. 1
at a flow rate of 1,500 ml/sec and a reducing agent was flowed from
the second flow path b at a flow rate of 1,500 ml/sec, and they
were contacted at the joining point m at a temperature of
20.degree. C. and a fine particulate silver powder was reduced and
deposited. A hydroquinone aqueous solution in which 21 g of
hydroquinone was dissolved in 10 liters of pure water was used as
the reducing agent on this occasion. Therefore, hydroquinone
concentration at the time point when mixing was finished was about
1.04 g/l, and it was a very low concentration.
[0044] Filtration was performed with a Nutsche to separate 40 g of
the resulting fine particulate silver powder, which was washed with
100 ml of water and 600 ml of methanol and further dried at
70.degree. C..times.5 hours and a fine particulate silver powder
was obtained. The photomicrograph of this obtained fine particulate
silver powder by scanning electron microscope was shown in FIG.
2.
[0045] The powder properties of the fine particulate silver powder
obtained as above are shown in Table 1 along with the powder
properties of the silver powders obtained in Example 2 and
Comparative Examples. Therefore, measuring method and the like
unidentified in the explanation described above are clarified here.
The sintering starting temperature in Table 1 was measured by
accurately weighing 0.5 g of a fine particulate silver powder with
a balance and pressing this into a shape of a pellet at a pressure
of 2 t/cm.sup.2 for one minute and performing measurement with
TMA/SS6000 which was a thermomechanical analysis equipment (TMA
equipment) manufactured by SEIKO Instruments Corporation and in a
condition of air flow rate of 200cc/min, temperature elevating rate
of 2.degree. C./min and retention time of 0 minute in the range of
ordinary temperature to 900.degree. C. The conductor resistance
described in Table 1 was measured by using a 1 mm width circuit
obtained by producing a silver paste with each of the silver
powders and drawing circuit on a ceramic substrate and sintering it
at a temperature of 180 to250.degree. C. The composition of this
silver paste was 85 wt % of a fine particulate silver powder, 0.75
wt % of ethyl cellulose, 14.25 wt % of terpineol. FIB analysis
measured the dimension of deposited crystal particles and was used
for measurement of crystallite diameter.
EXAMPLE 2
[0046] In this example, a fine particulate silver powder was
produced using the production conditions different from those of
Example 1 and the properties of the obtained fine particulate
silver powder were measured. And further, a silver paste was
produced with the fine particulate silver powder and a test circuit
was formed and the conductor resistance and sintering starting
temperature were measured.
[0047] First, 63.3 g of silver nitrate was dissolved in 3.1 liters
of pure water to prepare a silver nitrate aqueous solution, and 235
ml of 25 wt % concentration ammonia water was added thereto at once
and agitated and a silver ammine complex aqueous solution was
obtained.
[0048] This silver ammine complex aqueous solution was introduced
into the first flow path a of 13 mm inside diameter shown in FIG. 1
at a flow rate of 1,500 ml/sec and a reducing agent was flowed from
the second flow path b at a flow rate of 1,500 ml/sec, and they
were contacted at the joining point m at a temperature of
20.degree. C. and a fine particulate silver powder was reduced and
deposited. A hydroquinone aqueous solution in which 21 g of
hydroquinone was dissolved in 3.4 liters of pure water was used as
the reducing agent on this occasion. Therefore, hydroquinone
concentration at the time point when mixing was finished was about
3.0 g/l, and it was a very low concentration.
[0049] 40 g of the resulting fine particulate silver powder was
filtered with a Nutsche as in Example 1, washed with 100 ml of
water and 600 ml of methanol and further dried at 70.degree.
C..times.5 hours and a fine particulate silver powder was obtained.
The photomicrograph of this obtained fine particulate silver powder
by scanning electron microscope was shown in FIG. 3. The powder
properties of the fine particulate silver powder obtained as above
are shown in Table 1 along with the powder properties of the silver
powders obtained in Example 1 and Comparative Examples.
COMPARATIVE EXAMPLE 1
[0050] Only the condition for washing is changed from Example 1 in
this Comparative Example, and only the condition for washing is
described to avoid repetition of description. 40 g of the fine
particulate silver powder obtained in Example 1 was filtered with a
Nutsche, washed with 100 ml of water and 50 ml of methanol and
further dried at 70.degree. C..times.5 hours and a fine particulate
silver powder was obtained. The photomicrograph of this obtained
fine particulate silver powder by scanning electron microscope was
as shown in FIG. 2. The powder properties of the fine particulate
silver powder obtained as above are shown in Table 1 along with the
powder properties of the silver powders obtained in the other
Examples and Comparative Examples.
COMPARATIVE EXAMPLE 2
[0051] Only the condition for washing is changed from Example 2 in
this Comparative Example, and only the condition for washing is
described to avoid repetition of description. 40 g of the fine
particulate silver powder obtained in Example 2 was filtered with a
Nutsche, washed with 100 ml of water and 50 ml of methanol and
further dried at 70.degree. C..times.5 hours and a fine particulate
silver powder was obtained. The photomicrograph of this obtained
fine particulate silver powder by scanning electron microscope was
as shown in FIG. 3. The powder properties of the fine particulate
silver powder obtained as above are shown in Table 1 along with the
powder properties of the silver powders obtained in the other
Examples and Comparative Examples.
COMPARATIVE EXAMPLE 3
[0052] In this example, a fine particulate silver powder was
produced using the production method shown below and the properties
of the obtained fine particulate silver powder were measured. And
further, a silver paste was produced with the fine particulate
silver powder and a test circuit was formed and the conductor
resistance and sintering starting temperature were measured.
[0053] First, 63.3 g of silver nitrate was dissolved in 1.0 liter
of pure water to prepare a silver nitrate aqueous solution, and 235
ml of 25 wt % concentration ammonia water was added thereto at once
and agitated and a silver ammine complex aqueous solution was
obtained.
[0054] And this silver ammine complex solution was placed into a
reaction tank and a hydroquinone aqueous solution in which 21 g of
hydroquinone was dissolved in 1.3 liters of pure water was added
thereto at once as the reducing agent and agitated while the liquid
temperature was maintained at 20.degree. C. for allowing reaction
and a fine particulate silver powder was reduced and deposited. The
hydroquinone concentration at the time point when mixing was
finished was about 8.23 g/l, and it was a high concentration.
[0055] The resulting fine particulate silver powder was filtered
with a Nutsche as in Example 1, washed with 100 ml of water and 50
ml of methanol and further dried at 70.degree. C..times.5 hours and
a fine particulate silver powder was obtained. The photomicrograph
of this obtained fine particulate silver powder by scanning
electron microscope was shown in FIG. 4. The powder properties of
the fine particulate silver powder obtained as above are shown in
Table 1 along with the powder properties of the silver powders
obtained in the above-mentioned Examples and the second Comparative
Example.
COMPARATIVE EXAMPLE 4
[0056] In this example, a fine particulate silver powder was
produced using the production method shown below and the properties
of the obtained fine particulate silver powder were measured. And
further, a silver paste was produced with the fine particulate
silver powder and a test circuit was formed and the conductor
resistance and sintering starting temperature were measured.
[0057] First, 63.3 g of silver nitrate was dissolved in 300 ml of
pure water to prepare a silver nitrate aqueous solution, and 235 ml
of 25 wt % concentration ammonia water was added thereto at once
and agitated and a silver ammine complex aqueous solution was
obtained.
[0058] And this silver ammine complex solution was placed into a
reaction tank and a hydroquinone aqueous solution in which 3 g of
gelatine was added to 200 ml of pure water and further 21 g of
hydroquinone was dissolved in 700 ml of pure water was added
thereto at once as the reducing agent and agitated while the liquid
temperature was maintained at 20.degree. C. for allowing reaction
and a fine particulate silver powder was reduced and deposited. The
hydroquinone concentration at the time point when mixing was
finished was about 14.5 g/l, and it was a high concentration.
[0059] The resulting fine particulate silver powder was filtered
with a Nutsche as in Example 1, washed with 100 ml of water and 50
ml of methanol and further dried at 70.degree. C..times.5 hours and
a fine particulate silver powder was obtained. The photomicrograph
of this obtained fine particulate silver powder by scanning
electron microscope was shown in FIG. 5. The powder properties of
the fine particulate silver powder obtained as above are shown in
Table 1 along with the powder properties of the silver powders
obtained in the above-mentioned Examples and the second Comparative
Example.
COMPARATIVE EXAMPLE 5
[0060] In this example, a fine particulate silver powder was
produced using the production method shown below and the properties
of the obtained fine particulate silver powder were measured. And
further, a silver paste was produced with the fine particulate
silver powder and a test circuit was formed and the conductor
resistance and sintering starting temperature were measured.
[0061] First, 20 g of polyvinylpyrrolidone was dissolved in 260 ml
of pure water and further 50 g of silver nitrate was dissolved to
prepare a silver nitrate aqueous solution, and 25 g of nitric acid
was added thereto at once and agitated and anitric acid solution
containing silver was obtained. The ascorbic acid concentration at
the time point when mixing was finished was about 36.0 g/l.
[0062] In the meantime, 35.8 g of ascorbic acid was added and
dissolved to 500 ml of pure water as a reducing agent to prepare a
reducing agent solution.
[0063] And this nitric acid solution containing silver was placed
into a reaction tank and the above-mentioned reducing agent
solution was added thereto at once and agitated while the liquid
temperature was maintained at 25.degree. C. for allowing reaction
and a fine particulate silver powder was reduced and deposited.
[0064] The resulting fine particulate silver powder was filtered
with a Nutsche as in Example 1, washed with 100 ml of water and 50
ml of methanol and further dried at 70.degree. C..times.5 hours and
a fine particulate silver powder was obtained. The powder
properties of the fine particulate silver powder obtained as above
are shown in Table 1 along with the powder properties of the silver
powders obtained in the above-mentioned Examples and Comparative
Examples.
<Comparison and Reviewing of Examples and Comparative
Examples>
[0065] The above-mentioned Examples and Comparative Examples are
compared referring to Table 1. In addition, it is supposed that the
particle diameter of the primary particles of a powder particle can
be understood distinctly if the scanning electron microscope
photographs shown in FIG. 2 to FIG. 5 are referred to.
TABLE-US-00001 TABLE 1 Properties of Powder Properties Sintered
Conductor Tap Bulk Crystallite Conductor Sintering SSA Density
D.sub.50 D.sub.LA Diameter Carbon Resistance Starting Sample
m.sup.2/g g/cm.sup.3 .mu.m D.sub.50/D.sub.LA nm Content % .mu.-cm
Temperature.degree. C. Example 1 2.54 4.2 0.31 0.30 1.03 7 0.15 4.6
160 Example 2 1.68 4.7 0.55 0.49 1.12 7 0.21 5.9 190 Comparative
2.89 4.3 0.29 0.28 1.04 7 0.28 8.5 160 Example 1 Comparative 0.55
4.0 3.90 2.20 1.77 7 0.32 7.9 190 Example 2 Comparative 1.18 4.3
1.78 1.02 1.75 9 0.88 Not 250 Example 3 Measurable Comparative 0.55
4.0 3.90 2.20 1.77 8 0.89 Not 250 Example 4 Measurable Comparative
0.62 4.0 3.03 1.20 2.53 38 0.30 Not 350 Example 5 Measurable
[0066] As is apparent from this Table 1, it will be appreciated
that the fine particulate silver powders obtained in the above
Examples are extremely fine and highly dispersible and contain a
small amount of impurities as compared with the silver powders
produced with a conventional production method by comparing each of
the powder property values and they are fine particulate powders
never existed in the conventional silver powders. In addition, as
for the sintered conductor properties, when the circuit is formed
by using a fine particulate silver powder according to the present
invention, the film density is high, the impurity content is low,
and the electric resistance is reduced. In the case of each of the
Comparative Examples, it can be seen that the conductor resistance
is too high to be measured.
INDUSTRIAL APPLICABILITY
[0067] The fine particulate silver powder according to the present
invention is composed of fine powder particles as could never be
supposed in the conventional silver powder, and the aggregation
degree of the powder particle is low, and shows very excellent
dispersibility as compared with the conventional silver powders. In
addition, by adopting a production method of a fine particulate
silver powder according to the present invention, the residual
organic matter in the obtained fine particulate silver powder has
been decreased, which effects along with the high film density due
to fine particulate silver powder and, as a result, enables to
reduce the electric resistance of the obtained conductor.
* * * * *