U.S. patent application number 11/066345 was filed with the patent office on 2005-09-01 for silver powder and method for producing same.
This patent application is currently assigned to DOWA MINING CO., LTD.. Invention is credited to Hasegawa, Yoshio, Ogi, Kozo.
Application Number | 20050188788 11/066345 |
Document ID | / |
Family ID | 34879607 |
Filed Date | 2005-09-01 |
United States Patent
Application |
20050188788 |
Kind Code |
A1 |
Ogi, Kozo ; et al. |
September 1, 2005 |
Silver powder and method for producing same
Abstract
There is provided a method for a silver powder capable of
decreasing the viscosity of a photosensitive paste using the silver
powder and improving the film state, sensitivity and linearity of
the paste even if the particle diameter of the silver powder is
small. The surface of a silver powder produced by a wet reducing
method is smoothed by a surface smoothing process for mechanically
causing particles of the silver powder to collide with each other,
and thereafter, silver agglomerates are removed by classification.
The surface smoothing process is carried out by putting a dried
silver powder into an apparatus, which is capable of mechanically
fluidizing particles, e.g., a mixer or mill such as a cylindrical
high-speed mixer, for mechanically causing the particles to collide
with each other.
Inventors: |
Ogi, Kozo; (Honjo-shi,
JP) ; Hasegawa, Yoshio; (Funabashi-shi, JP) |
Correspondence
Address: |
BACHMAN & LAPOINTE, P.C.
900 CHAPEL STREET
SUITE 1201
NEW HAVEN
CT
06510
US
|
Assignee: |
DOWA MINING CO., LTD.
|
Family ID: |
34879607 |
Appl. No.: |
11/066345 |
Filed: |
February 25, 2005 |
Current U.S.
Class: |
75/252 ;
257/E23.075; 420/501; 75/371 |
Current CPC
Class: |
B22F 2998/10 20130101;
B22F 9/24 20130101; C22C 5/06 20130101; H01L 2924/0002 20130101;
H01G 4/0085 20130101; H01L 2924/0002 20130101; B22F 2009/045
20130101; H01L 23/49883 20130101; B22F 9/04 20130101; B22F 2998/10
20130101; H01B 1/02 20130101; B22F 9/24 20130101; H01L 2924/00
20130101; C22B 11/04 20130101; B22F 9/04 20130101 |
Class at
Publication: |
075/252 ;
075/371; 420/501 |
International
Class: |
C22C 005/06; B22F
009/24 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 26, 2004 |
JP |
2004-051009 |
Claims
1. A method for producing a silver powder, said method comprising
the steps of: producing a silver powder by a wet reducing method;
smoothing a surface of the produced silver powder by a surface
smoothing process for mechanically causing particles to collide
with each other; and removing silver agglomerates by a
classification.
2. A method for producing a silver powder as set forth in claim 1,
wherein said wet reducing method comprises the steps of: adding an
alkali or complexing agent to an aqueous silver salt containing
solution to form a silver oxide containing slurry or an aqueous
silver complex salt containing solution; and thereafter, adding a
reducing agent to the slurry or solution to deposit a silver powder
by reduction.
3. A method for producing a silver powder as set forth in claim 1,
wherein said silver powder has a mean particle diameter of 0.1 to
10 .mu.m after said classification.
4. A method for producing a silver powder as set forth in claim 1,
wherein said silver powder has a mean particle diameter of not
greater than 5 .mu.m after said classification.
5. A method for producing a silver powder as set forth in claim 1,
wherein said classification removes silver agglomerates having a
size of greater than 15 .mu.m.
6. A method for producing a silver powder as set forth in claim 1,
wherein said classification removes silver agglomerates having a
size of greater than 11 .mu.m.
7. A method for producing a silver powder as set forth in claim 1,
wherein said surface smoothing process is carried out by means of a
high-speed mixer.
8. A silver powder having a mean particle diameter of 0.1 to 10
.mu.m and a maximum particle diameter of not greater than 15 .mu.m,
wherein said silver powder has a maximum particle diameter
D.sub.max measured by a grind gauge is not greater than 12.5 .mu.m
when said silver powder is used for preparing a paste, and wherein
a mixture, which is obtained by mixing and dispersing 80 wt % of
said silver powder in 20 wt % of an epoxy resin having a viscosity
of 0.2 to 0.6 Pa.multidot.sec at 25.degree. C., has a viscosity of
not greater than 135 Pa.multidot.sec when said viscosity is
measured by an E-type viscometer at 25.degree. C. and 1 rpm.
9. A silver powder as set forth in claim 8, wherein said maximum
particle diameter of said silver powder is not greater than 11
.mu.m, and said maximum particle diameter D.sub.max measured by
said grind gauge is not greater than 7.5 .mu.m.
10. A silver powder as set forth in claim 8, wherein said means
particle diameter of said silver powder is not greater than 5
.mu.m.
11. A silver powder having a mean particle diameter of 0.1 to 10
.mu.m and a maximum particle diameter of not greater than 15 .mu.m,
wherein said silver powder has a maximum particle diameter
D.sub.max measured by a grind gauge is not greater than 12.5 .mu.m
when said silver powder is used for preparing a paste, and wherein
a mixture, which is obtained by mixing and dispersing 80 wt % of
said silver powder in 20 wt % of an epoxy resin having a viscosity
of 0.2 to 0.6 Pa.multidot.sec at 25.degree. C., has a viscosity of
not greater than 90 Pa.multidot.sec when said viscosity is measured
by an E-type viscometer at 25.degree. C. and 3 rpm.
12. A silver powder as set forth in claim 11, wherein said maximum
particle diameter of said silver powder is not greater than 11
.mu.m, and said maximum particle diameter D.sub.max measured by
said grind gauge is not greater than 7.5 .mu.m.
13. A silver powder as set forth in claim 11, wherein said means
particle diameter of said silver powder is not greater than 5
.mu.m.
14. A silver powder produced by a method as set forth in claim 1,
wherein said silver powder has a maximum particle diameter
D.sub.max is not greater than 12.5 .mu.m when said maximum particle
diameter D.sub.max is measured by a grind gauge if said silver
powder is used for preparing a paste.
15. A silver powder as set forth in claim 14, wherein said maximum
particle diameter D.sub.max measured by said grind gauge is not
greater than 7.5 .mu.m.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention generally relates to a silver powder
and a method for producing the same. More specifically, the
invention relates to a silver powder for a conductive paste for use
in electronic parts, such as internal electrodes of multilayer
capacitors, conductive patterns of circuit boards, and electrodes
of substrates for plasma display panels, and a method for producing
the same.
[0003] 2. Description of the Prior Art
[0004] As a conventional conductive paste for use in electronic
parts, such as internal electrodes of multilayer capacitors,
conductive patterns of circuit boards, and electrodes of substrates
for plasma display panels (PDPs), there is used a silver paste
produced by mixing a silver powder and a glass frit in an organic
vehicle and kneading them so as to the silver powder in the
vehicle. In order to decrease the size of these electronic parts
and/or to form a conductive pattern having a high density and fine
lines, it is required that a silver powder for a conductive paste
has reasonably small particle diameters and a reasonably narrow
particle size distribution.
[0005] As a method for producing such a silver powder for a
conductive paste, there is known a wet reducing method for adding
an alkali or a complexing agent to an aqueous silver salt
containing solution to form a silver oxide containing slurry or an
aqueous silver complex salt containing solution, and thereafter,
adding a reducing agent to the silver oxide containing slurry or
the aqueous silver complex salt containing solution to deposit a
silver powder by reduction. As a method for producing a silver
powder having desired particle diameters for a conductive paste,
there is known a method for adding a complexing agent to an aqueous
silver salt containing solution to form an aqueous silver complex
salt containing solution (an aqueous silver ammine complex
solution), and thereafter, adding a reducing agent to the aqueous
silver salt containing solution in the presence of a very small
amount of organic metal compound to produce a silver powder having
desired particle diameters (see, e.g., Japanese Patent Laid-Open
No. 8-176620). According to this method, it is possible to obtain
spherical silver particles having desired particle diameters by
changing the amount of the organic metal compound to be added.
[0006] However, if a silver powder having small particle sizes is
produced by a conventional silver powder producing method, the
viscosity of a conductive paste using the silver powder increases
as the particle diameter of the silver powder decreases. That is,
there is a problem in that it is not possible to produce a silver
powder capable of decreasing the viscosity of a conductive paste
using the silver powder even if the particle diameter of the silver
powder is small.
[0007] In order to solve this problem, there is proposed a method
for producing a silver powder, which can smooth irregularities and
angular portions on the surface of particles of the silver powder
without substantially changing the particle diameter and particle
size distribution of the silver powder by carrying out a surface
smoothing process for mechanically causing the particles to collide
with each other, so that it is possible to decrease the particle
diameter of the silver powder and the viscosity of a conductive
paste using the silver powder (see, e.g., Japanese Patent Laid-Open
No. 2002-80901).
[0008] On the other hand, as a method for forming an electrode of a
substrate for a plasma display panel or the like, there is proposed
a method for forming a fine pattern by a photolithography method
using a photosensitive paste which is obtained by adding a
photosensitive resin serving as an organic component to a
conductive paste (see, e.g., Japanese Patent Laid-Open No.
11-339554). As a photosensitive paste (a photo paste) for use in
such a method for forming a fine pattern by the photolithography
method, it is possible to use a paste using a silver powder
produced by the method disclosed in Japanese Patent Laid-Open No.
2002-80901, and such a paste has a very excellent sensitivity. The
reason for this is not clear, but it is considered that, since the
surface of the silver powder produced by the method disclosed in
Japanese Patent Laid-Open No. 2002-80901 is smooth, it is possible
to decrease the irregular reflection of ultraviolet to precisely
cure a desired region of a film of the paste to its deep part.
[0009] However, in recent years, electronic parts, such as
electrodes of substrates for plasma display panels, are required to
have a pattern having a higher density and finer lines, so that
there are some cases where the film state and linearity of a
photosensitive paste are not good even if the photosensitive paste
uses a silver powder produced by the method disclosed in Japanese
Patent Laid-Open No. 2002-80901. Thus, there are some cases where
it is not possible to obtain a good burned film, so that it is not
possible to provide a pattern having a higher density and finer
lines.
SUMMARY OF THE INVENTION
[0010] It is therefore an object of the present invention to
eliminate the aforementioned problems and to provide a silver
powder capable of decreasing the viscosity of a photosensitive
paste using the silver powder and improving the film state,
sensitivity and linearity of the paste even if the particle
diameter of the silver powder is small, and a method for producing
the same.
[0011] In order to accomplish the aforementioned and other objects,
according to one aspect of the present invention, there is provided
a method for producing a silver powder, the method comprising the
steps of: producing a silver powder by a wet reducing method;
smoothing a surface of the produced silver powder by a surface
smoothing process for mechanically causing particles to collide
with each other; and removing silver agglomerates by a
classification.
[0012] In this method, the wet reducing method may comprise the
steps of: adding an alkali or complexing agent to an aqueous silver
salt containing solution to form a silver oxide containing slurry
or an aqueous silver complex salt containing solution; and
thereafter, adding a reducing agent to the slurry or solution to
deposit a silver powder by reduction. The silver powder preferably
has a mean particle diameter of 0.1 to 10 .mu.m after the
classification, and more preferably has a mean particle diameter of
not greater than 5 m after the classification. The classification
preferably removes silver agglomerates having a size of greater
than 15 g m, and more preferably removes silver agglomerates having
a size of greater than 11 g m. The surface smoothing process is
preferably carried out by means of a high-speed mixer.
[0013] According to another aspect of the present invention, there
is provided a silver powder having a mean particle diameter of 0.1
to 10 .mu.m and a maximum particle diameter of not greater than 15
.mu.m, wherein the silver powder has a maximum particle diameter
D.sub.max measured by a grind gauge is not greater than 12.5 .mu.m
when the silver powder is used for preparing a paste, and wherein a
mixture, which is obtained by mixing and dispersing 80 wt % of the
silver powder in 20 wt % of an epoxy resin having a viscosity of
0.2 to 0.6 Pa.multidot.sec at 25.degree. C., has a viscosity of not
greater than 135 Pa.multidot.sec when the viscosity is measured by
an E-type viscometer at 25.degree. C. and 1 rpm.
[0014] Preferably, in this silver powder, the maximum particle
diameter of the silver powder is not greater than 11 .mu.m, the
maximum particle diameter D.sub.max measured by the grind gauge is
not greater than 7.5 .mu.m, and the means particle diameter of the
silver powder is not greater than 5 .mu.m.
[0015] According to a further aspect of the present invention,
where is provide a silver powder having a mean particle diameter of
0.1 to 10 .mu.m and a maximum particle diameter of not greater than
15 .mu.m, wherein the silver powder has a maximum particle diameter
D.sub.max measured by a grind gauge is not greater than 12.5 .mu.m
when the silver powder is used for preparing a paste, and wherein a
mixture, which is obtained by mixing and dispersing 80 wt % of the
silver powder in 20 wt % of an epoxy resin having a viscosity of
0.2 to 0.6 Pa.multidot.sec at 25.degree. C., has a viscosity of not
greater than 90 Pa.multidot.sec when the viscosity is measured by
an E-type viscometer at 25.degree. C. and 3 rpm.
[0016] Preferably, in this silver powder, the maximum particle
diameter of the silver powder is not greater than 11 .mu.m, the
maximum particle diameter D.sub.max measured by the grind gauge is
not greater than 7.5 .mu.m, and the means particle diameter of the
silver powder is not greater than 5 .mu.m.
[0017] According to a still further aspect of the present
invention, there is provided a silver powder produced by the above
described method, wherein the silver powder has a maximum particle
diameter D.sub.max is not greater than 12.5 .mu.m when the maximum
particle diameter D.sub.max is measured by a grind gauge if the
silver powder is used for preparing a paste.
[0018] Preferably, in this silver powder, the maximum particle
diameter D.sub.max measured by the grind gauge is not greater than
7.5 .mu.m.
[0019] According to the present invention, a surface smoothing
process for mechanically causing particles of a silver powder,
which is produced by a wet reducing method, to collide with each
other is carried out. Thus, it is possible to smooth irregularities
and angular portions on the surface of the silver powder without
substantially changing the particle diameter and particle size
distribution of the silver powder, so that it is possible to
produce a silver powder capable of reducing the viscosity of a
photosensitive paste and improving the sensitivity thereof when the
photosensitive paste uses the silver powder even if the particle
diameter of the silver powder is small. Moreover, if silver
agglomerates are removed by a classification, it is possible to
produce a silver powder capable of improving the film state and
linearity of a photosensitive paste using the silver powder. Thus,
it is possible to greatly improve the film state and linearity of a
photosensitive paste using a silver powder according to the present
invention, so that it is possible to provide an electronic part
having a pattern which has a high density and fine lines.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] The present invention will be understood more fully from the
detailed description given herebelow and from the accompanying
drawings of the preferred embodiments of the invention. However,
the drawings are not intended to imply limitation of the invention
to a specific embodiment, but are for explanation and understanding
only.
[0021] In the drawings:
[0022] FIG. 1 is an illustration for explaining silver agglomerates
observed in a film state;
[0023] FIG. 2 is an illustration for explaining a comb-shaped
pattern used for evaluating sensitivity; and
[0024] FIGS. 3A through 3C are illustrations for explaining
linearity, which are enlarged views showing a line portion of the
comp-shaped pattern of FIG. 2.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0025] In a preferred embodiment of a method for producing a silver
powder according to the present invention, after a surface
smoothing process for mechanically causing particles of a silver
powder, which is produced by a wet reducing method, to collide with
each other is carried out, silver agglomerates are removed by a
classification.
[0026] The wet reducing method may be a method comprising the steps
of adding an alkali or complexing agent to an aqueous silver salt
containing solution to form a silver oxide containing slurry or an
aqueous silver complex salt containing solution, and thereafter,
adding a reducing agent to the slurry or solution to deposit a
silver powder by reduction. In order to prevent the secondary
cohesion of the silver powder to obtain monodisperse silver
particles to improve characteristics of an electronic part which
uses a conductive paste using the silver powder, the wet reducing
method may include a process for adding a dispersing agent to a
silver slurry obtained by deposition due to reduction, or a process
for adding a dispersing agent to a water reaction system containing
at least one of a silver salt and silver oxide before the silver
powder is deposited by reduction. The dispersing agent may be one
or more selected from the group consisting of fatty acids, fatty
acid salts, surface active agents, organic metals, chelating agents
and protective colloids.
[0027] The surface smoothing process is carried out by putting a
dried silver powder into an apparatus, which is capable of
mechanically fluidizing particles, to mechanically cause particles
of the silver powder to collide with each other. In practice, a
mixer or mill, such as a cylindrical high-speed mixer, e.g., a
Henchel mixer, may be used for carrying out the surface smoothing
process. The input amount of the silver powder, the revolving speed
and kind of blades of the mixer or mill, and the processing time
may be controlled to optimize the fluidization of particles and the
smoothing of the shape of the surface due to collision. By this
surface smoothing process, it is possible to smooth irregularities
and angular portions on the surface of particles of the silver
powder to decrease the viscosity of a conductive paste using the
silver powder without substantially changing the particle diameter
and particle size distribution of the silver powder. By this
surface smoothing process, it is also possible to greatly improve
the sensitivity of a photosensitive paste using the silver
powder.
[0028] The classification may be a method capable of removing large
silver agglomerates. As such methods, there are a method for
causing particles of a silver powder to pass through meshes having
a predetermined size (e.g., a method utilizing a sieve shaker or
inplane sieve), and a method for separating particles of a silver
powder by air flow. In view of the precision of removal of
agglomerates, an air classification for separating a group of
particles by air flow is preferably carried out. The air
classification may be carried out by using any one of commercially
available apparatuses based on gravity, inertia, centrifugal force
or the like. Any one of such commercially available apparatuses may
be suitably chosen in accordance with the desired size of particles
to be removed, the particle size distribution of particles after
removing agglomerates, the classification speed, the yields of the
silver powder and so forth. For example, such air classification
apparatuses include a variable impactor, an elbow jet, a cyclotron
and an acu-cut. In addition, any one of various pulverizers or
mills may be used if it has an air classification function although
pulverization or milling is not intended to be carried out. For
example, such pulverizers or mills include a CF mill and a jet
mill. Moreover, the above described air classification apparatuses
may be combined.
[0029] Examples of a silver powder and a method for producing the
same according to the present invention will be described below in
detail.
EXAMPLE 1
[0030] To 3600 ml of an aqueous solution containing 12 g/l silver
nitrate as silver ions, 375 ml of industrial aqueous ammonia was
added to form an aqueous silver ammine complex solution. To the
aqueous silver ammine complex solution thus formed, 75 g of sodium
hydroxide was added to control the pH of the solution. Then, 96 ml
of industrial formalin serving as a reducing agent was added to the
solution. Immediately thereafter, 1.5 g of oleic acid was added to
the solution to obtain a silver slurry. Then, the silver slurry
thus obtained was filtered, washed with water, dried to obtain a
silver powder. Then, the surface of the silver powder thus obtained
was smoothed by a surface smoothing process using a high-speed
mixer, and the silver powder thus smoothed was classified to remove
silver agglomerates having a greater diameter than 8 .mu.l m.
[0031] The particle diameters of the silver powder thus obtained
were measured by Microtrack. As a result, D.sub.10 was 0.8 .mu.m,
and the mean particle diameter D.sub.50 was 1.4 .mu.m. In addition,
D.sub.90 was 2.5 .mu.m, and the maximum particle diameter D.sub.max
was 6.5 .mu.m. Moreover, the specific surface area of the silver
powder was 0.75 m.sup.2/g, and the tap density of the silver powder
was 5.0 g/ml.
[0032] To 8 g of the silver powder thus obtained, 2 g of an epoxy
resin (Epicoat produced by Japan Epoxy Resin Co., Ltd., (Grade 819
having a viscosity of 0.2 to 0.6 Pa.multidot.sec at 25.degree. C.))
was added to prepare a paste. The viscosities of the paste thus
prepared were measured by an E-type viscometer at 25.degree. C. and
at 0.5 rpm, 1 rpm and 3 rpm, respectively. As a result, the
viscosities were 153 Pa.multidot.sec, 118 Pa.multidot.sec and 79
Pa.multidot.sec, respectively.
[0033] The particle size of silver particles contained in the paste
thus obtained was evaluated by a grind gauge. As a result, the
maximum particle diameter D.sub.max was 4 .mu.m, the fourth scratch
(the fourth particle diameter from the maximum particle diameter
when the particle size of silver particles in the paste was
measured by the grind gauge) was 3 .mu.m, and the mean particle
diameter D.sub.50 was 2 .mu.m.
COMPARATIVE EXAMPLE 1
[0034] A silver powder was produced by the same method as that in
Example 1, except that the classification was not carried out. The
particle diameters of the silver powder thus obtained were measured
by Microtrack. As a result, D.sub.10 was 0.9 .mu.m, and D.sub.50
was 1.4 .mu.m. In addition, D.sub.90 was 2.6 .mu.m, and D.sub.max
was 6.5 .mu.m. Moreover, the specific surface area of the silver
powder was 0.77 m.sup.2/g, and the tap density of the silver powder
was 5.0 g/ml.
[0035] To 8 g of the silver powder thus obtained, 2 g of the same
epoxy resin as that in Example 1 was added to prepare a paste. The
viscosities of the paste thus prepared were measured by the E-type
viscometer at 25.degree. C. and at 0.5 rpm, 1 rpm and 3 rpm,
respectively. As a result, the viscosities were 159
Pa.multidot.sec, 122 Pa.multidot.sec and 81 Pa.multidot.sec,
respectively.
[0036] The particle size of silver particles contained in the paste
thus obtained was evaluated by a grind gauge. As a result, the
maximum particle diameter D.sub.max was 15 .mu.m, the fourth
scratch was 8 .mu.m, and the mean particle diameter D.sub.50 was 2
.mu.m.
EXAMPLE 2
[0037] To 3600 ml of an aqueous solution containing 12 g/l silver
nitrate as silver ions, 180 ml of industrial aqueous ammonia was
added to form an aqueous silver ammine complex solution. To the
aqueous silver ammine complex solution thus formed, 1 g of sodium
hydroxide was added to control the pH of the solution. Then, 192 ml
of industrial formalin serving as a reducing agent was added to the
solution. Immediately thereafter, 0.1 g of stearic acid was added
to the solution to obtain a silver slurry. Then, the silver slurry
thus obtained was filtered, washed with water, dried to obtain a
silver powder. Then, the surface of the silver powder thus obtained
was smoothed by a surface smoothing process using a high-speed
mixer, and the silver powder thus smoothed was classified to remove
silver agglomerates having a greater diameter than 11 .mu.m.
[0038] The particle diameters of the silver powder thus obtained
were measured by Microtrack. As a result, D.sub.10 was 1.7 .mu.m,
and D.sub.50 was 3.1 .mu.m. In addition, D.sub.90 was 5.0 .mu.m,
and D.sub.max was 11.0 .mu.m. Moreover, the specific surface area
of the silver powder was 0.28 m.sup.2/g, and the tap density of the
silver powder was 5.4 g/ml.
[0039] To 8 g of the silver powder thus obtained, 2 g of the same
body resin as that in Example 1 was added to prepare a paste. The
viscosities of the paste thus prepared were measured by the E-type
viscometer at 25.degree. C. and at 0.5 rpm, 1 rpm and 3 rpm,
respectively. As a result, the viscosities were 119
Pa.multidot.sec, 108 Pa.multidot.sec and 83 Pa.multidot.sec,
respectively.
[0040] The particle size of silver particles contained in the paste
thus obtained was evaluated by a grind gauge. As a result, the
maximum particle diameter D.sub.max was 6 .mu.m, the fourth scratch
was 5 .mu.m, and the mean particle diameter D.sub.50 was 3
.mu.m.
COMPARATIVE EXAMPLE 2
[0041] A silver powder was produced by the same method as that in
Example 2, except that pulverization was carried out by means of a
food mixer in place of the surface smoothing process and that the
classification was not carried out. The particle diameters of the
silver powder thus obtained were measured by Microtrack. As a
result, D.sub.10 was 2.0 .mu.m, and D.sub.50 was 4.0 .mu.m. In
addition, D.sub.90 was 7.1 .mu.m, and D.sub.max was 15.6 .mu.m.
Moreover, the specific surface area of the silver powder was 0.26
m.sup.2/g, and the tap density of the silver powder was 5.4
g/ml.
[0042] To 8 g of the silver powder thus obtained, 2 g of the same
epoxy resin as that in Example 1 was added to prepare a paste. The
viscosities of the paste thus prepared were measured by the E-type
viscometer at 25.degree. C. and at 0.5 rpm, 1 rpm and 3 rpm,
respectively. As a result, the viscosities were 170
Pa.multidot.sec, 142 Pa.multidot.sec and 101 Pa.multidot.sec,
respectively.
[0043] The particle size of silver particles contained in the paste
thus obtained was evaluated by a grind gauge. As a result, the
maximum particle diameter D.sub.max was 14 .mu.m, the fourth
scratch was 7 .mu.m, and the mean particle diameter D.sub.50 was 3
.mu.m.
COMPARATIVE EXAMPLE 3
[0044] A silver powder was produced by the same method as that in
Example 2, except that the classification was not carried out. The
particle diameters of the silver powder thus obtained were measured
by Microtrack. As a result, D.sub.10 was 1.7 .mu.m, and D.sub.50
was 3.2 .mu.m. In addition, D.sub.90 was 5.2 .mu.m, and D.sub.max
was 11.0 .mu.m. Moreover, the specific surface area of the silver
powder was 0.26 m.sup.2/g, and the tap density of the silver powder
was 5.8 g/ml.
[0045] To 8 g of the silver powder thus obtained, 2 g of the same
epoxy resin as that in Example 1 was added to prepare a paste. The
viscosities of the paste thus prepared were measured by the E-type
viscometer at 25.degree. C. and at 0.5 rpm, 1 rpm and 3 rpm,
respectively. As a result, the viscosities were 113
Pa.multidot.sec, 103 Pa.multidot.sec and 86 Pa.multidot.sec,
respectively.
[0046] The particle size of silver particles contained in the paste
thus obtained was evaluated by a grind gauge. As a result, the
maximum particle diameter D.sub.max was 14 .mu.m, the fourth
scratch was 12 .mu.m, and the mean particle diameter D.sub.50 was 4
.mu.m.
EXAMPLE 3
[0047] To 3600 ml of an aqueous solution containing 12 g/l silver
nitrate as silver ions, 180 ml of industrial aqueous ammonia was
added to form an aqueous silver ammine complex solution. To the
aqueous silver ammine complex solution thus formed, 7.5 g of sodium
hydroxide was added to control the pH of the solution. Then, 192 ml
of industrial formalin serving as a reducing agent was added to the
solution. Immediately thereafter, 1.5 g of oleic acid was added to
the solution to obtain a silver slurry. Then, the silver slurry
thus obtained was filtered, washed with water, dried to obtain a
silver powder. Then, the surface of the silver powder thus obtained
was smoothed by a surface smoothing process using a high-speed
mixer, and the silver powder thus smoothed was classified to remove
silver agglomerates having a greater diameter than 8 .mu.m.
[0048] The particle diameters of the silver powder thus obtained
were measured by Microtrack. As a result, D.sub.10 was 1.0 .mu.m,
and D.sub.50 was 1.8 .mu.m. In addition, D.sub.90 was 3.0 .mu.m,
and D.sub.max was 6.5 .mu.m. Moreover, the specific surface area of
the silver powder was 0.46 m.sup.2/g, and the tap density of the
silver powder was 5.4 g/ml.
[0049] To 8 g of the silver powder thus obtained, 2 g of the same
epoxy resin as that in Example 1 was added to prepare a paste. The
viscosities of the paste thus prepared were measured by the E-type
viscometer at 25.degree. C. and at 0.5 rpm, 1 rpm and 3 rpm,
respectively. As a result, the viscosities were 138
Pa.multidot.sec, 115 Pa.multidot.sec and 82 Pa.multidot.sec.
respectively.
[0050] The particle size of silver particles contained in the paste
thus obtained was evaluated by a grind gauge. As a result, the
maximum particle diameter D.sub.max was 5 .mu.m, the fourth scratch
was 4 .mu.m, and the mean particle diameter D.sub.50 was 2
.mu.m.
COMPARATIVE EXAMPLE 4
[0051] A silver powder was produced by the same method as that in
Example 3, except that pulverization was carried out by means of a
food mixer in place of the surface smoothing process and that the
classification was not carried out. The particle diameters of the
silver powder thus obtained were measured by Microtrack. As a
result, D.sub.90 was 1.1 .mu.m, and D.sub.50 was 2.3 .mu.m. In
addition, D.sub.90 was 4.0 .mu.m, and D.sub.max was 11.0 .mu.m.
Moreover, the specific surface area of the silver powder was 0.45
m.sup.2/g, and the tap density of the silver powder was 4.7
g/ml.
[0052] To 8 g of the silver powder thus obtained, 2 g of the same
epoxy resin as that in Example 1 was added to prepare a paste. The
viscosities of the paste thus prepared were measured by the E-type
viscometer at 25.degree. C. and at 0.5 rpm, 1 rpm and 3 rpm,
respectively. As a result, the viscosities were 173
Pa.multidot.sec, 144 Pa.multidot.sec and 111 Pa.multidot.sec,
respectively.
[0053] The particle size of silver particles contained in the paste
thus obtained was evaluated by a grind gauge. As a result, the
maximum particle diameter D.sub.max was 15 .mu.m, the fourth
scratch was 12 .mu.m, and the mean particle diameter D.sub.50 was 3
.mu.m.
COMPARATIVE EXAMPLE 5
[0054] A silver powder was produced by the same method as that in
Example 3, except that the classification was not carried out. The
particle diameters of the silver powder thus obtained were measured
by Microtrack. As a result, D.sub.10 was 0.9 .mu.m, and D.sub.50
was 1.8 .mu.m. In addition, D.sub.90 was 3.3 .mu.m, and D.sub.max
was 9.3 .mu.m. Moreover, the specific surface area of the silver
powder was 0.43 m.sup.2/g, and the tap density of the silver powder
was 5.0 g/ml.
[0055] To 8 g of the silver powder thus obtained, 2 g of the same
epoxy resin as that in Example 1 was added to prepare a paste. The
viscosities of the paste thus prepared were measured by the E-type
viscometer at 25.degree. C. and at 0.5 rpm, 1 rpm and 3 rpm,
respectively. As a result, the viscosities were 132
Pa.multidot.sec, 110 Pa.multidot.sec and 85 Pa.multidot.sec,
respectively.
[0056] The particle size of silver particles contained in the paste
thus obtained was evaluated by a grind gauge. As a result, the
maximum particle diameter D.sub.max was 18 .mu.m, the fourth
scratch was 12 .mu.m, and the mean particle diameter D.sub.50 was 3
.mu.m.
COMPARATIVE EXAMPLE 6
[0057] A silver powder was produced by the same method as that in
Example 3, except that pulverization was carried out by means of a
food mixer in place of the surface smoothing process. The particle
diameters of the silver powder thus obtained were measured by
Microtrack. As a result, D.sub.10 was 1.0 .mu.m, and D.sub.50 was
2.2 .mu.m. In addition, D.sub.90 was 3.5 .mu.m, and D.sub.max was
7.8 .mu.m. Moreover, the specific surface area of the silver powder
was 0.57 m.sup.2/g, and the tap density of the silver powder was
5.4 g/ml.
[0058] To 8 g of the silver powder thus obtained, 2 g of the same
epoxy resin as that in Example 1 was added to prepare a paste. The
viscosities of the paste thus prepared were measured by the E-type
viscometer at 25.degree. C. and at 0.5 rpm, 1 rpm and 3 rpm,
respectively. As a result, the viscosities were 166
Pa.multidot.sec, 138 Pa.multidot.sec and 106 Pa.multidot.sec,
respectively.
[0059] The particle size of silver particles contained in the paste
thus obtained was evaluated by a grind gauge. As a result, the
maximum particle diameter D.sub.max was 4 .mu.m, the fourth scratch
was 3 .mu.m, and the mean particle diameter D.sub.50 was 2
.mu.m.
EXAMPLE 4
[0060] To 3600 ml of an aqueous solution containing 12 g/l silver
nitrate as silver ions, 100 ml of industrial aqueous ammonia was
added to form an aqueous silver ammine complex solution. To the
aqueous silver ammine complex solution thus formed, 60 ml of
industrial aqueous hydrogen peroxide serving as a reducing agent
was added to the solution. Immediately thereafter, 1.5 g of
succinic acid was added to the solution to obtain a silver slurry.
Then, the silver slurry thus obtained was filtered, washed with
water, dried to obtain a silver powder. Then, the surface of the
silver powder thus obtained was smoothed by a surface smoothing
process using a high-speed mixer, and the silver powder thus
smoothed was classified to remove silver agglomerates having a
greater diameter than 11 .mu.m.
[0061] The particle diameters of the silver powder thus obtained
were measured by Microtrack. As a result, D.sub.10 was 1.4 .mu.m,
and D.sub.50 was 2.4 .mu.m. In addition, D.sub.90 was 4.4 .mu.m,
and D.sub.max was 9.3 .mu.m. Moreover, the specific surface area of
the silver powder was 0.46 m.sup.2/g, and the tap density of the
silver powder was 4.4 g/ml.
[0062] To 8 g of the silver powder thus obtained, 2 g of the same
body resin as that in Example 1 was added to prepare a paste. The
viscosities of the paste thus prepared were measured by the E-type
viscometer at 25.degree. C. and at 0.5 rpm, 1 rpm and 3 rpm,
respectively. As a result, the viscosities were 132
Pa.multidot.sec, 120 Pa.multidot.sec and 86 Pa.multidot.sec,
respectively.
[0063] The particle size of silver particles contained in the paste
thus obtained was evaluated by a grind gauge. As a result, the
maximum particle diameter D.sub.max was 7 .mu.m, the fourth scratch
was 6 .mu.m, and the mean particle diameter D.sub.50 was 3
.mu.m.
[0064] The results in Examples 1 through 4 and Comparative Examples
1 through 5 are shown in Tables 1 and 2.
1 TABLE 1 particle diameter specific (Microtrack) surface D.sub.10
D.sub.50 D.sub.90 D.sub.max area tap density smoothing
classification (.mu.m) (.mu.m) (.mu.m) (.mu.m) (m.sup.2/g) (g/ml)
Ex. 1 x x 0.8 1.4 2.5 6.5 0.75 5.0 Ex. 2 x x 1.7 3.1 5.0 11.0 0.28
5.4 Ex. 3 x x 1.0 1.8 3.0 6.5 0.46 5.4 Ex. 4 x x 1.4 2.4 4.4 9.3
0.46 4.4 Comp. 1 x 0.9 1.4 2.6 6.5 0.77 5.0 Comp. 2 2.0 4.0 7.1
15.6 0.26 5.4 Comp. 3 x 1.7 3.2 5.2 11.0 0.26 5.8 Comp. 4 1.1 2.3
4.0 11.0 0.45 4.7 Comp. 5 x 0.9 1.8 3.3 9.3 0.43 5.0 Comp. 6 x 1.0
2.2 3.5 7.8 0.57 5.4
[0065]
2 TABLE 2 viscosity particle size (Pa .multidot. sec) (grind gauge)
0.5 rpm 1 rpm 3 rpm D.sub.max (.mu.m) 4th (.mu.m) D.sub.50 (.mu.m)
Ex. 1 153 118 79 4 3 2 Ex. 2 119 108 83 6 5 3 Ex. 3 138 115 82 5 4
2 Ex. 4 132 120 86 7 6 3 Comp. 1 159 122 81 15 8 2 Comp. 2 170 142
101 14 7 3 Comp. 3 113 103 86 14 12 4 Comp. 4 173 144 111 15 12 3
Comp. 5 132 110 85 18 12 3 Comp. 6 166 138 106 4 3 2
[0066] Then, the silver powder obtained in each of Examples 1
through 4 and Comparative Examples 1 through 5 was used for
preparing a photosensitive paste of composition shown in Table 3.
The preparation of the photosensitive paste was carried out by
pre-kneading materials of composition shown in Table 3, and
thereafter, kneading them by means of a three-roll mill so as to
disperse the silver powder therein.
3 TABLE 3 parts by weight metal powder 70 photosensitive resin 20
monomer 5 photopolymerization 1 initiator 1 photopolymerization 3
initiator 2 diluent solvent 10 glass frit 1 stabilizer 1 defoaming
agent 0.2
[0067] As the photosensitive resin, an acrylic copolymer resin
having acrylic and carboxyl groups (solid content: 48 wt %, acid
value: 115, double bond equivalent: 450) was used. At the monomer,
etoxylated trimethylolpropane triacrylate was used. As the
photopolymerization initiator (1),
2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropane-1-on was
used. As the photopolymerization initiator (2), 2,4,6-trimethyl
benzoyl diphenyl phosphine oxide was used. As the diluent solvent,
butylcarbitol acetate was used. As the glass frit,
SiO.sub.2.multidot.B.sub.2O.sub.3.multidot.ZnO glass frit
(softening point: 580.degree. C.) was used. As the stabilizer,
malonic acid was used. As the defoaming agent, a silicon defoaming
agent was used.
[0068] The photosensitive paste thus obtained was printed on a
glass substrate by using a 400 mesh stainless screen (emulsion
thickness: 5 .mu.m), and dried at 80.degree. C. for thirty minutes
by means of a hot gas dryer. Then, the film state, sensitivity and
linearity of the dried film thus prepared were evaluated.
[0069] The evaluation of the film state was carried out by
observing a sample, which was obtained by exposing the dried film
to ultraviolet of 300 mJ/cm.sup.2, by means of an optical
microscope, and determining whether agglomerates 12 having a size
of about tens micrometers exist on a uniform film 10 as shown in
FIG. 1.
[0070] The evaluations of the sensitivity and linearity were
carried out with respect to a sample obtained by putting a
comb-shaped pattern chromium mask 100 of FIG. 2 on the dried film,
exposing the dried film and spray-developing the exposed portion of
the dried film with an aqueous solution containing 0.5 wt % of
Na.sub.2CO.sub.3 at 30.degree. C. The sensitivity was evaluated by
observing the residual portion 106 of the pattern by an optical
microscope when the dried film was developed by using the
comb-shaped pattern 100 which has lines 102 having a width L
(.mu.m) and spaces 104 having a width S (.mu.m) and which satisfies
L/S=50/50. The linearity was evaluated by observing the presence of
bulging portion (shown in FIG. 3C) and/or broken portion (shown in
FIG. 3B) in the developed linear residual portion 106 by means of
an optical microscope, and was evaluated to be good if no bulging
and broken portions were observed as shown in FIG. 3A.
[0071] As a result, in the case of the photosensitive paste
obtained from the silver powder in Examples 1 through 4, no
agglomerates were observed, so that the film state was good. In
addition, the sensitivity was good, and the linearity was also good
since the bulging and broken portions of the line were not
observed.
[0072] In the case of the photosensitive paste obtained from the
silver powder in Comparative Examples 1, 3 and 5, agglomerates were
observed, so that the film state was bad. Although the sensitivity
was good, the linearity was bad since the bulging and broken
portions of the line were observed.
[0073] In the case of the photosensitive paste obtained from the
silver powder in Comparative Examples 2 and 4, some agglomerates
were observed, so that the film state was bad. In addition, the
sensitivity was insufficient, and the linearity was also bad since
the bulging and broken portions of the line were observed.
[0074] In the case of the photosensitive paste obtained from the
silver powder in Comparative Example 6, no agglomerates were
observed, so that the film state was good. In addition, the bulging
and broken portions of the line were not observed, so that the
linearity was good. However, the sensitivity was insufficient.
* * * * *