U.S. patent application number 11/037329 was filed with the patent office on 2005-08-25 for silver powder made of silver particles, each to which fine silver particles adhere and process of producing the same.
This patent application is currently assigned to Mitsui Mining and Smelting Co., Ltd.. Invention is credited to Fujimoto, Taku, Kato, Masashi, Sakaue, Takahiko, Sasaki, Takuya, Yoshimaru, Katsuhiko.
Application Number | 20050183543 11/037329 |
Document ID | / |
Family ID | 34703082 |
Filed Date | 2005-08-25 |
United States Patent
Application |
20050183543 |
Kind Code |
A1 |
Sasaki, Takuya ; et
al. |
August 25, 2005 |
Silver powder made of silver particles, each to which fine silver
particles adhere and process of producing the same
Abstract
This invention is a silver powder having a low-temperature
sintering performance and dispersibility, which allows the powder
particles to be agglomerated to a small degree and be nearly in the
monodisperse state. Employed is silver powder of fine silver
particles each to which fine silver particles adhere, wherein fine
silver particles of nano-order particle size are adhered to the
surface of each silver powder particle. The powder particles of the
silver powder of fine silver particles each to which fine silver
particles adhere have excellent dispersibility. In the production
of the silver powder of fine silver particles each to which fine
silver particles adhere, a process of including the steps of:
adding a silver nitrate and a neutralizing agent into a slurry of
silver powder in a dispersing medium; dissolving the mixture while
stirring to allow fine silver oxide particles to be precipitated on
the surface of each silver powder particle; washing the resultant
silver powder; and exposing the fine silver oxide particles to UV
rays to reduce the same to fine silver particles.
Inventors: |
Sasaki, Takuya;
(Shimonoseki-shi, JP) ; Kato, Masashi;
(Shimonoseki-shi, JP) ; Fujimoto, Taku;
(Shimonoseki-shi, JP) ; Sakaue, Takahiko;
(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 and Smelting Co.,
Ltd.
Tokyo
JP
|
Family ID: |
34703082 |
Appl. No.: |
11/037329 |
Filed: |
January 19, 2005 |
Current U.S.
Class: |
75/371 ;
257/E23.075; 75/255 |
Current CPC
Class: |
Y02P 10/234 20151101;
B22F 1/0096 20130101; B22F 2999/00 20130101; H01L 23/49883
20130101; C22B 3/44 20130101; B22F 9/24 20130101; C22B 11/04
20130101; H05K 1/095 20130101; H05K 2201/0218 20130101; B22F 1/0014
20130101; Y02P 10/20 20151101; H01L 2924/0002 20130101; B22F
2999/00 20130101; B22F 1/0096 20130101; B22F 9/24 20130101; B22F
2999/00 20130101; B22F 1/0096 20130101; B22F 9/20 20130101; B22F
2202/11 20130101; H01L 2924/0002 20130101; H01L 2924/00
20130101 |
Class at
Publication: |
075/371 ;
075/255 |
International
Class: |
B22F 009/24; C22B
011/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 28, 2004 |
JP |
JP2004-189416 |
Oct 22, 2003 |
JP |
JP2003-361480 |
Claims
1. Silver powder made of silver particles, each whose center part
is regarded as a core material characterized in that: silver
particles each being finer than the silver particle of said center
part adheres to said core material.
2. The silver powder made of silver particles each to which fine
silver particles adhere according to claim 1, wherein the silver
powder is substantially spherical.
3. The silver powder made of silver particles each to which fine
silver particles adhere according to claim 2, wherein the silver
powder has the following powder characteristics a to c: a. an
average particle size of primary particles D.sub.IA obtained by
image analysis of a scanning electron microscope is 0.6 .mu.m or
less; b. a degree of agglomeration represented by
D.sub.50/D.sub.IA, where D.sub.IA is said average particle size of
primary particles and D.sub.50 is the average particle size
obtained by laser diffraction scattering particle size distribution
measurement method, is 1.5 or less; and c. a crystallite size is 10
nm or less.
4. The silver powder made of silver particles each to which fine
silver particles adhere according to claim 2, characterized in that
the silver powder has the following powder characteristics a to d:
a. an average particle size of primary particles D.sub.IA obtained
by image analysis of scanning electron microscope is 0.6 .mu.m or
less; b. a degree of agglomeration represented by
D.sub.50/D.sub.IA, where D.sub.IA is said average particle size of
primary particles and D.sub.50 is the average particle size
obtained by laser diffraction scattering particle size distribution
measurement method, is 1.5 or less; c. a crystallite size is 10 nm
or less; and d. a content of organic impurities is 0.25% by weight
or less in terms of amount of carbon.
5. The silver powder made of silver particles each to which fine
silver particles adhere according to claim 1, wherein each of
particles of the silver powder is substantially flat.
6. The silver powder made of silver particles each to which fine
silver particles adhere according to claim 1, wherein sinterable
temperature is 170.degree. C. or less.
7. A process for producing the silver powder made of silver
particles each to which fine silver particles adhere according to
claim 1, comprising the steps of: bringing a silver powder into
contact with a solution containing a silver complex, which is
obtained by mixing silver nitrate and a complexing agent and
dissolving the mixture while stirring; and adding a reducing agent
into the solution to allow fine silver particles to be precipitated
on the surface of each silver powder particle.
8. The process for producing the silver powder made of silver
particles each to which fine silver particles adhere according to
claim 7, wherein the complexing agent is a sulfite salt or an
ammonium salt.
9. A process for producing the silver powder made of silver
particles each to which fine silver particles adhere according to
claim 1, comprising the steps of: adding a silver nitrate and a
neutralizing agent into a slurry of silver powder in a dispersing
medium; dissolving the slurry mixture while stirring to allow fine
silver oxide particles to be precipitated on the surface of each
silver powder particle; washing the resultant silver powder; and
irradiating with UV rays to reduce the fine silver oxide particles
to fine silver particles.
10. The process for producing the silver powder made of silver
particles each to which fine silver particles adhere according to
claim 9, wherein the neutralizing agent is any one or two or more
selected from the group consisting of sodium hydroxide, potassium
hydroxide and aqueous ammonia.
11. A process for producing the silver powder made of silver
particles each to which fine silver particles adhere according to
claim 1, comprising the step of: using a silver powder of nearly
spherical powder particles which is obtained in the steps of:
preparing an aqueous solution of a silver complex by mixing and
reacting an aqueous solution of silver nitrate and a complexing
agent; contact-mixing an organic reducing agent with said aqueous
solution of a silver complex; allowing silver particles to be
precipitated by reduction in the solution after the mixing, while
keeping the silver concentration at 1 g/l to 6 g/l and the organic
reducing agent concentration at 1 g/l to 3 g/l; separating the
precipitated silver particles through a filter; and washing the
silver particles in water and then in an alcoholic solution.
12. A process for producing the silver powder made of silver
particles each to which fine silver particles adhere according to
claim 1, comprising the steps of: using silver powder of nearly
spherical powder particles which is obtained in the steps of:
preparing an aqueous solution of a silver complex by mixing and
reacting an aqueous solution of silver nitrate and a complexing
agent; contact-mixing an organic reducing agent with said aqueous
solution of a silver complex; allowing silver particles to be
precipitated by reduction in the solution after the mixing, while
keeping the silver concentration at 1 g/l to 6 g/l and the organic
reducing agent concentration at 1 g/l to 3 g/l; separating the
precipitated silver particles through a filter; and washing the
silver particles in water and then in an excess amount of alcoholic
solution.
Description
TECHNICAL FIELD
[0001] The present invention relates to a silver powder made of
silver particles, each to which fine silver particles adhere and a
process of producing the same.
BACKGROUND ART
[0002] Silver ink (silver paste) has been conventionally used not
only for forming circuit boards by co-firing it with a ceramic
substrate at a relatively high temperature but also for forming a
wiring circuit on a printed wiring board, via-hole-fillers through
such a board and adhesives for mounting electrical parts thereon by
mixing and curing the silver ink (silver paste) with various types
of resin ingredients, as described in the Patent Document 1. In the
latter application, in general, an electrical conductivity has been
obtained simply by mutually having the particles of silver powder
touched, as a conductive filler without mutually sintering the
particles.
[0003] However, there have been a demand in recent years for
lowering an electrical resistance for conductor portions formed
using the silver powder and improving connecting reliability in
order to realize lowering of the electrical resistance, and
consequently, more demand has occurred for silver ink or silver
paste to be formed by sintering a filler itself of the silver
powder while resin ingredients are being cured, resulting in that
the conductivity is exhibited. It goes without saying that for
meeting these demands, size-fining of each of the particles of the
silver powder as a conductive filler is necessary in order to lower
the sintering temperature.
[0004] Conventionally, a wet reduction process where an aqueous
solution of a silver/ammine complex using a silver nitrate solution
and an aqueous ammonia is produced; and an organic reducing agent
is added to the aqueous solution of the complex, as described in
the Patent Document 2, has been employed for production of a silver
powder, and the obtained silver powder has been made into a silver
paste. Additionally, in order to ensure more excellent
low-temperature sintering performance than conventional, the silver
ink containing silver nanoparticles, as disclosed in the Patent
Document 3, has been proposed.
[0005] [Patent Document 1]
[0006] Japanese Patent Laid-Open No. 2001-107101
[0007] [Patent Document 2]
[0008] Japanese Patent Laid-Open No. 2002-334618
[0009] [Patent Document 3]
[0010] Japanese Patent Laid-Open No. 2002-324966
DISCLOSURE OF THE INVENTION
[0011] However, in a metal powder such as a silver powder, it is
generally said that both the size-refinement of powder particles
and the dispersibility meant by that powder particles nearly in a
mono-disperse state are hard to become satisfied. For example, in
the case of the silver ink containing silver nanoparticles, as
disclosed in the above-described Patent Document 1, in order to
stabilize a state of the dispersion of the nanoparticles, it is
common to add a large amount of dispersant as a protective colloid
thereto. In such a case, generally the decomposition temperature of
the dispersant is higher than the sintering temperature of the
silver nanoparticles, resulting in that the low-temperature
sintering performance of the silver nanoparticles themselves cannot
be used to the full.
[0012] Further, in case of the silver ink containing the silver
nanoparticles, because the content of the filler is less than
conventional, a thick film is difficult to form while a thin film
is easy to form. This makes it difficult to use such silver ink for
application to form wiring circuits having a large cross-sectional
area sufficient to be usable for power-supply circuit such carries
a relatively large amount of current or for application to form a
low-resistance circuit. Further, in the use of the silver ink for
adhesives for mounting components thereon, not only its
conductivity, but also its adhesive strength is required severely.
Therefore it is indispensable to add a certain amount or more of
resin thereto which exhibits high-adhesive strength when the resin
is cured. Thus, there remain many problems that cannot be overcome
by conventional silver-nanoparticles-containing silver ink.
[0013] In the meantime, it goes without saying that silver powder
used for commonly-used silver pastes has had a limitation in terms
of its low-temperature sintering performance judging from its
particle size. Because in the particle-size of the silver powder
obtained by the conventional production processes, the actual
condition is that the average particle size between their primary
particles D.sub.IA is usually more than 0.6 .mu.m, the average
particle size D.sub.50 measured by a laser diffraction scattering
particle size distribution measurement method is more than 1.0
.mu.m, and the degree of agglomeration represented by
D.sub.50/D.sub.IA is more than 1.7 (Note that "D.sub.IA" is a
particle-size which can be calculated by a particle image from an
SEM image analysis.). Thus, the conventional silver powder has been
unsuitable when forming recent circuits where fine-pitched wiring
patterns are drawn.
[0014] Further, in the circuit formation in a case where a silver
paste using the conventional silver powder is employed, non-firing
type application or low-temperature sintering type one is often
used where heating temperature is 300.degree. C. or less. In such
circumstance, the use of silver powder having low crystallinity has
been considered to be preferable, in order to obtain high sintering
performance at a low temperature. However, in order to obtain
silver powder of low crystallinity, it is inevitable to employ a
reaction system where reduction is rapidly advanced in view of its
manufacturing condition. As a result, a silver powder has been
merely obtained such that its crystallinity is low, and
additionally its agglomeration significantly occurs easily. Thus,
in a market, there have been demands for silver powder which has a
low-temperature sintering performance, whose particles each is
finer than that of particles of any conventional silver powder, and
which has an excellent dispersibility such that little agglomerate
occurs regarding powder particles.
[0015] Also on the other hand, a silver powder has been required to
have few impurities. In the production of the silver powder, the
above-described wet reduction process has been employed, and the
reducing agent, etc. used in the process remains on the surface of
the powder particles. Thus, as long as the conventional production
process is employed, the problem regarding the impurities is
inevitable. And there has occurred another problem that in
accompanying with the increase of the amount of impurities on the
surface of the silver powder, an electrical resistance of the
conductor formed using the silver powder also becomes higher.
[0016] Thus, there have been demands in the market for a silver
powder which has an excellent low-temperature sintering performance
which has not existed until now, whose particles each is very fine
and have high dispersibility, and which has little impurities in
order to realize a low electrical resistance.
[0017] In the light of the above-described problems, the present
inventors have made eager research efforts as to any novel process
for fining of each of particles of the conventional silver powder
particles themselves. However, it is natural that the
currently-used technology has some inherent limitations at a level.
So then the inventors suppose that when sintering the silver powder
particles, an only outer portion of surface of each of silver
powder particles may be sufficiently sintered and connected.
Further, if so, they suppose that even the above-described
conventional silver powder could have a low-temperature sinterable
characteristic. Hereinafter, the present invention will be
described in terms of two separate points: "silver powder made of
silver particles, each to which fine silver particles are adhered"
and "process of producing the silver powder".
[0018] <Silver Powder Made of Silver Particles, Each to which
Fine Silver Particles Adhere>
[0019] The "silver powder made of silver particles whose center
part is regarded as a core material, in which silver particles each
being finer than the silver particle of the center part adheres to
the core material"(i.e., silver powder made of silver particles,
each to which fine silver particles adhere) relating to the present
invention is in other words expressed by "silver powder made of
silver particles each being made adhered to the surface of each of
the silver powder particles". Specifically, the surface of each
powder particle of the silver powder 2 as a core material is
further coated with much finer silver particles 3, just like the
image shown in FIG. 1. Thus, the fine silver particles 3 existing
on the surface of each particle of the silver powder 2, each allows
the fine silver particles 2 to exhibit a low-temperature sintering
performance, independent of the shape and size of the powder
particles of the core material, thereby making easier the sintering
of the adjacent powder particles of the silver powder made of
silver particles 1 made to adhere thereto.
[0020] The term "fine silver particles" herein used means silver
nanoparticles of nanometer-order particle size which exist only on
the surface of each particle of the silver powder 2. As
above-mentioned, when using the silver nanoparticles themselves for
a silver ink, generally a large amount of dispersant whose
decomposition temperature is higher than the sintering temperature
of silver nano-particles, is added to stabilize the dispersibility
of the nanoparticles. As a result, the characteristic of being
sintered at low temperatures which silver nanoparticles themselves
have cannot be used to the full. However, having much finer silver
particles 3 additionally adhered onto the surface of each powder
particle of the silver powder 2 enables the characteristic of being
sintered at low temperatures which silver nanoparticles have to be
sufficiently used, independent of the size and shape of the powder
particles of the silver powder as the core material. Accordingly,
even if the shape of the silver powder particles is substantially
spherical or the shape is flaky having particle size being several
tens .mu.m, the silver powder can be used as a core material.
[0021] The silver powder used as the core material may be
substantially spherical, flaky or flat, and if the production
conditions in conventionally-adopted production process are
modified, particle size distribution of sharpness and
dispersibility can be ensured, to some degree. Thus, once silver
powder is used in a form of the silver powder including silver
particles to which fine silver particles adhere, it has excellent
handleability and does not require any large amount of protective
colloid when formed into a paste, though silver nanoparticles have
poor dispersibility in themselves. And a silver paste prepared
using such silver powder can contain silver particles by an amount
being equivalent to an amount of conventional silver pastes,
resulting in making it possible to thickening coating when a wiring
pattern of a circuit is drawn.
[0022] In the silver powder made of silver particles, each to which
fine silver particles adhere as above-described, its sinterable
temperature is 170.degree. C. or less and it shows extraordinarily
excellent sintering characteristics. Thus, if a silver paste
(silver ink) is produced using the the silver powder made of silver
particles, each to which fine silver particles adhere, and the
wiring patterns of a circuit are drawn using such a silver paste
(silver ink), a coating thickness can be obtained which is
sufficient to provide a circuit that can flow a large amount of
current. And what is more, the ease of sintering the powder
particles substantially improves the characteristics of the silver
paste (silver ink) as a conductor, such as low electric resistance
and continuity reliability.
[0023] In the silver powder made of silver particles, each to which
fine silver particles adhere according to the present invention,
the silver powder is used as a core material. Accordingly, the
particle size and dispersibility of the powder particles, each to
which fine silver particles adhere have a much more tendency to
depend on the silver powder as the core material. In other words,
it is preferable to select, as the core material, a silver powder
which ensures a particle size distribution and dispersibility at a
high level. Also it is more preferable to select silver powder
containing few impurities, which would be a hindrance when lowering
electric resistance. So, the present inventors have made eager
research efforts toward the size-refinement of each of particles of
silver powder itself and finally obtained the silver powder having
the powder characteristics as below. And the inventors have arrived
at an idea that excellent products can be obtained by producing the
silver powder made of silver particles, each to which fine silver
particles adhere using the above-described silver powder as the
core material.
[0024] The above-described silver powder basically has the
following powder characteristics: a. the average particle size of
primary particles D.sub.IA obtained by the analysis of the images
of scanning electron microscope is 0.6 .mu.m or less; b. the degree
of agglomeration represented by D.sub.50/D.sub.IA, where D.sub.IA
is the above-described average particle size of primary particles
and D.sub.50 is the average particle size obtained by a laser
diffraction/scattering particle size distribution measurement
method, is 1.5 or less; and c. the crystallite size is 10 nm or
less. And another type of silver powder has not only the
above-described powder characteristics a to c, but the following
powder characteristic: d. the content of organic purities is 0.25%
by weight in terms of amount of carbon. These two types of silver
powder are different in content of purities due to their different
production conditions. The silver powder having such powder
characteristics has dispersibility at a level which cannot be
obtained by conventional production processes. In the following,
the powder characteristics of the silver powder used as the core
material will be described.
[0025] The characteristic of a is that the average particle size of
primary particle D.sub.IA obtained by the analysis of the image of
a scanning electron microscope (SEM) is 0.6 .mu.m or less. The term
"the average particle size of primary particles D.sub.IA obtained
by the analysis of the images of scanning electron microscope" is
the average particle size of silver powder obtained by analyzing
the images observed with a scanning electron microscope (SEM)
(preferably the fine silver powder used in the present invention is
observed at .times.10,000 magnification while conventional silver
powder at .times.3,000 to .times.5,000 magnification). In the image
analysis of fine silver powder observed with a SEM, round particle
size analysis is conducted by using IP-1000 PC of Asahi Engineering
Co., Ltd. and setting the roundness threshold value and the overlap
degree at 10 and 20, respectively to obtain the average particle
size D.sub.IA. The average particle size D.sub.IA obtained by
analyzing the images of observed fine silver powder will represent
a reliable value of the average particle size of primary particles,
because it is obtained directly from the observed SEM images.
Almost all the D.sub.IA values of the fine silver powder used in
the present invention fall in the range of 0.01 .mu.m to 0.6 .mu.m
as far as the inventors observe; however in reality, silver powder
having much smaller particle sizes may also be confirmed. That is
why the minimum value is not shown clearly.
[0026] The characteristic of b is described by the "degree of
agglomeration", which is an index of the dispersibility of
particles, because the fine silver powder used as a core material
in the present invention shows excellent dispersibility which
conventional silver powder has never had. The "degree of
agglomeration" herein used means the value represented by
D.sub.50/D.sub.IA, where D.sub.IA is the above-described average
particle size of primary particles and D.sub.50 is the average
particle size obtained by a laser diffraction scattering particle
size distribution measurement method. The D.sub.50 means that the
particle size at volume accumulation of 50% obtained by laser
diffraction scattering particle size distribution measurement
method and its value is calculated not by directly observing each
particle size, but by considering an agglomerated powder particle
as a single particle (an agglomerate). This is because, in reality,
the particles of silver powder do not lie in the monodisperse
state, where particles are completely separated from each other,
but typically in state where a plurality of particles agglomerate.
However, the less, powder particles agglomerate and the more, they
lie closely in the monodisperse state, the smaller the average
particle size D.sub.50 becomes. The D.sub.50 of the fine silver
powder used in the present invention is in the range of 0.25 .mu.m
to 0.80 .mu.m, which fine silver powder produced by a conventional
production process has never had. In the laser diffraction
scattering particle size distribution measurement method herein
described, the average particle size of fine silver powder is
measured with a laser diffraction scattering particle size
distribution meter, Micro Trac HRA Model 9320-X100 (by
Leeds+Northrup) after mixing 0.1 g of fine silver powder made of
ion-exchanged water and dispersing the silver powder made of an
ultrasonic homogenizer (Nippon Seiki Co., Ltd.) for five
minutes.
[0027] On the other hand, "the average particle size of primary
particles D.sub.IA obtained by the analysis of the images of
scanning electron microscope" is the average particle size of
silver powder obtained by analyzing the images observed with a
scanning electron microscope (SEM), it represents a reliable value
of the average particle size of primary particles obtained without
considering over the particles in the agglomerated state.
[0028] Thus, the inventors take the value calculated by
D.sub.50/D.sub.IA, where D.sub.IA is the average particle size
obtained by image analysis and D.sub.50 is the average particle
size obtained by laser diffraction scattering particle size
distribution measurement method, as the degree of agglomeration.
Specifically, according to the above-described theory, the value
D.sub.50, which reflects the existence of agglomerates, should be
larger than the value D.sub.IA, assuming that the values D.sub.50
and D.sub.IA can be measured with the same accuracy in the fine
silver powder of the same lot. And the value D.sub.50 becomes
infinitely close to the value D.sub.IA with decreasing agglomerates
of the fine silver powder particles; as a result, the value
D.sub.50/D.sub.IA, the degree of agglomeration, becomes infinitely
close to 1. At a stage where the degree of agglomeration becomes 1,
it is said that the silver powder is monodisperse powder where no
agglomerate of powder particles exists.
[0029] Then, the inventors examined the correlation between the
degree of agglomeration of the fine silver powder has and the
viscosity of the paste produced using the fine silver powder as
well as the correlation between the degree of agglomeration and the
smoothness of the conductor obtained by sintering the paste, with
fine silver powder having different degrees of agglomeration. The
examination revealed that there is very excellent correlation
between the above factors. This indicates that viscosity of a paste
produced using fine silver powder can be freely controlled by
controlling the degree of agglomeration the fine silver powder has.
The examination also revealed that if the degree of agglomeration
is kept 1.5 or less, then the fluctuations in the viscosity of the
paste of the fine silver powder and in the smoothness of the
conductor obtained by sintering the silver powder can also be kept
in a very narrow range. It can also be said that the less, the
agglomerates of fine silver powder, the more, the film density of
the conductor obtained by sintering the fine silver powder is
improved, thereby making it possible to lower an electrical
resistance of conductor formed by sintering the silver powder.
[0030] However, in reality, the calculated value of the
agglomeration degree can sometimes be less than 1. This can be
possibly because the calculation is made under the assumption that
the value D.sub.IA used for the calculation of the agglomeration
degree is the value of a true sphere. In theory, the degree of
agglomeration cannot be 1, but in reality fine silver particles are
not true spheres, thereby yielding a calculated value which is less
than 1.
[0031] The characteristic of c is that the crystallite size is 10
nm or less. The crystallite size and the sinterable temperature
mutually have very closely relationship. Specifically, comparing
different types of silver powder having the substantially same
average particle size, those having smaller crystallite size are
more capable of being sintered at low temperatures. Thus, fine
silver powder having large surface energy due to its fineness and
having a crystallite size being 10 nm or less, just like the fine
silver powder according to the present invention, makes it possible
to lower the sinterable temperature of the fine silver powder
itself, as the core material. The reason why the lower limit of the
crystallite size is not provided here, is that certain measuring
errors can be produced depending on the measuring device, measuring
conditions or the like. In addition, it is difficult to require
extreme reliability of measured values in the range where the
crystallite size is less than 10 nm. The lower limit dares to be
set, the inventors would say, based on their study, that the lower
limit would be about 2 nm.
[0032] The characteristic of d is that the content of organic
impurities in the silver powder is 0.25% or less by weight in terms
of amount of carbon. Here the carbon content is used as an index of
the content of organic impurities and as a measure of the amount of
impurities adhering to the fine silver powder particles. The carbon
content was measured with EMIA-320V manufactured by Horiba, Ltd. by
a combustion-infrared absorption method in such a manner as to mix
0.5 g of fine silver powder, 1.5 g of tungsten powder 1.5 g and 0.3
g of tin powder and thereafter put the mixture into a porcelain
crucible. The carbon content in silver powder obtained by a
conventional production process is more than 0.25% by weight even
if washing the silver powder is reinforced, too much.
[0033] The fine silver powder according to the present invention is
directed to a fine silver powder that conventional production
process has never produced, in terms of having its powder
characteristics: a to c; or a to d. In view of characteristics of
sintering temperature, the fine silver powder used as the core
material in the present invention is capable of starting a
sintering process at as low temperatures as 240.degree. C. or less.
The term "sinterable temperature" herein used means the lowest
temperature at which a circuit can be drawn on an alumina substrate
with a silver paste prepared using silver powder can undergo
sintering by a resistor-measurable extend. Although the lower limit
of the sinterable temperature has not been particularly specified,
either, considering the studies by the present inventors and the
common knowledge in this art, it is almost impossible for the fine
silver powder, as the core material, itself to be sintered at
temperatures lower than 170.degree. C. Thus, 170.degree. C. can be
taken as the lower limit of the sinterable temperature.
[0034] The effect of the above-described powder characteristics the
fine silver powder according to the present invention has is to
increase the tap density of the fine silver powder to as high as
4.0 g/cm.sup.3. The "tap density" herein used means the density
determined in such a manner as to weigh 200 g of fine silver powder
accurately, put the weighed powder into a 150-cm.sup.3 measuring
cylinder, tap the measuring cylinder by repeating the 40 mm-stroke
dropping of the cylinder by 1,000 times, and measure the volume of
the fine silver powder. Powder has a high tap density when it has a
theoretically fine particle size and is in the highly dispersed
state where its particles are not agglomerated. Considering the tap
density of conventional silver powder is less than 4.0 g/cm.sup.3,
the above-described tap density value has proved that the fine
silver powder according to the present invention is very fine and
excels in dispersibility.
[0035] The aforementioned fine silver powder, which is used as the
core material, is a very fine powder and excels in dispersibility,
thereby having low-temperature sintering performance allowing the
sinterable temperature of the fine silver powder itself to be
240.degree. C. or lower. And silver powder made of silver particles
made to adhere thereto, which is very fine and excels in
dispersibility, compared with conventional silver powder, and has a
low-temperature sintering performance that allows the sinterable
temperature to be 170.degree. C. or lower, is produced by making
silver nanoparticles adhere to the surface of the particles of the
above fine silver powder. A ratio of a size of each of the silver
nanoparticles to a size of the core material is substantially 1/5
to 1/100. The ratio thereof is not limited especially. Namely, the
ratio is changed corresponding to various conditions such as pH,
temperature, rate of reaction, and the like regarding as used
solutions.
[0036] <Process for Producing Silver Powder Made of Silver
Particles Each to which Fine Silver Particles Adhere>
[0037] In the following the process for producing silver powder
made of silver particles each to which fine silver particles adhere
according to the invention will be described in terms of two major
types: production process 1 and production process 2. The fine
silver powder suitably used in the production processes 1 and 2
will be described separately in the section "Process for producing
fine silver powder".
[0038] Production Process 1:
[0039] This production process is "a process for producing silver
powder made of silver particles each to which fine silver particles
adhere, characterized by bringing silver powder into contact with a
solution containing a silver complex which is obtained by mixing
silver nitrate and a complexing agent and dissolving the mixture
during stirring; and adding a reducing agent to the resultant
solution to allow fine silver particles to be precipitated on the
surface of each silver powder particle".
[0040] When making the silver powder into the above slurry, the
amount of silver powder contained is not particularly limited.
However, unless the amount of silver powder in the slurry is
defined, the amount of chemicals used in the production cannot be
clearly specified. Thus, the production process 1 will be described
as a process for producing silver powder made of silver particles
each to which fine silver particles adhere, wherein silver
nanoparticles are made to adhere to the surface of each silver
powder particle in a slurry of 50 g of silver powder dispersed in 1
liter of deionized water. The production process is based on the
assumption that the average particle size of primary particles
D.sub.IA of the silver powder used, which is obtained by the image
analysis of the scanning electron microscope, is 1 .mu.m or
less.
[0041] First, the "solution containing a silver complex which is
obtained by mixing silver nitrate and a complexing agent and
dissolving the mixture while stirring" will be described. To treat
the above-described amount of silver powder, 8 g to 26 g of silver
nitrate is used. If the amount of silver nitrate used is less than
8 g, a practically sufficient rate of coating with fine silver
particles cannot be obtained, whereas even if the amount of silver
nitrate used is more than 26 g, the coating rate cannot be
improved. The complexing agent used is a sulfite or an ammonium
salt. When using potassium sulfite, the amount used is in the range
of 50 g to 150 g. If the amount of potassium sulfite added is less
than 50 mg, the complexation of silver does not fully progress, and
therefore, a complete silver complex cannot be formed. On the other
hand, more than 150 mg of potassium sulfite well exceeds a
sufficient amount for forming a silver complex, and adding the
excess amount of potassium sulfite does not accelerate the
complexiation, resulting in being not economical. The solution
containing a silver complex is obtained by dissolving the
above-described amount of silver nitrate in 1 liter of deionized
water, adding a complexing agent to the solution and fully stirring
the mixed solution.
[0042] Then, 50 g of silver powder is added to the solution
containing a silver complex obtained as above and the slurry of
silver powder is fully stirred. A reducing agent is then added to
the slurry to cause a reduction reaction, so that fine silver
powder of nano-order particle size is allowed to be precipitates
uniformly on the surface of each silver powder particle. The
reducing agent used here is hydrazine, DMAB, SBH, formalin or
hypophosphoric acid. When using hydrazine, 5 g to 50 g of hydrazine
is dissolved in 200 ml or less (including 0 ml) of deionized water
and the prepared solution is added within 60 minutes (including the
case where the solution is added for a very short time). In the
present description, it is noted that "an operation that solution
is added for a very short time" is meant by an operation of
establishing a chemical reaction between different solutions as
rapid as possible. If the amount of hydrazine added is less than 5
g, the reduction does not sufficiently progress, and the surface of
each of silver powder particles of the silver powder cannot be
coated uniformly with fine silver powder. The addition of more than
50 g of hydrazine does not especially accelerate the reduction
reaction and is merely uneconomical.
[0043] The solution temperature during the reduction reaction lies
in the range of a room temperature to 45.degree. C. If the solution
temperature is higher than 45.degree. C., the reduction reaction
progresses so rapidly that the precipitation of fine silver powder
on the surface of each silver powder particle is likely to be
non-uniform, resulting in inferior particle size distribution of
the resultant silver powder made of silver particles each to which
fine silver particles adhere. Preferably, the time the addition of
a reducing agent takes is about 5 minutes to 40 minutes in the
above-described reducing agent concentration range. If the reaction
time is shorter than 5 minutes, the produced powder particles tend
to be agglomerated more strongly, whereas if for the addition of a
reducing agent, it takes not shorter than 40 minutes, a
satisfactorily uniform coating can be obtained.
[0044] After allowing fine silver powder to precipitate on the
surface of each silver powder particle through reduction reaction
in the above-described manner, the silver powder is separated
through a filter, washed, dehydrated, and dried to yield silver
powder made of silver particles each to which fine silver particles
adhere according to the present invention. The separation through a
filter, washing, dehydration, and drying may be carried out by
various procedures, and the procedure and conditions employed are
not limited to any specific ones.
[0045] Production Process 2:
[0046] This production process 2 is "a process for producing silver
powder made of silver particles each to which fine silver particles
adhere, characterized by including: adding silver nitrate and a
neutralizing agent into a slurry of silver powder in a dispersing
medium; dissolving the slurry mixture while stirring to allow fine
silver oxide particles to be precipitated on the surface of each
silver powder particle; washing the resultant silver powder; and
exposing the silver powder made of silver oxide particles on its
surface to UV rays to reduce the fine silver oxide particles to
fine silver particles".
[0047] In the above slurry of silver powder, the amount of silver
powder contained is not particularly limited. However, unless the
amount of silver powder in the slurry is not clearly defined, the
amount of chemicals used in the production cannot be specified.
Thus, the production process 2 will be described as a process for
producing silver powder made of silver particles each to which fine
silver particles adhere, wherein silver nanoparticles are made to
be adhered to the surface of each of silver powder particles in a
slurry of 50 g of silver powder dispersed in 1,500 g of dispersion
medium. The production process is based on the assumption that the
average particle size of primary particles D.sub.IA of the silver
powder used, which is obtained by the image analysis of the
scanning electron microscope, is 1 .mu.m or less.
[0048] First, the "slurry of silver powder added to a dispersing
medium" will be described. The dispersion medium used here is
ethylene glycol (including monoethylene glycol, diethylene glycol
and triethylene glycol), butanediol (including 1,4-butanediol,
1,2-butanediol and 2,3-butanediol) or glycerin. Here, the case
where ethylene glycol is used as the dispersion medium will be
described. Accordingly, the slurry of the silver powder is prepared
by adding 50 g of silver powder to 1,500 g of ethylene glycol and
stirring the mixture.
[0049] Silver nitrate and a neutralizing agent are added to the
slurry of silver powder obtained as above and dissolved while
stirring to allow fine silver oxide particles to be precipitated on
the surface of each silver powder particle. Preferably, the silver
nitrate and the neutralizing agent added are in the form of a
solution of silver nitrate or neutralizing agent. The reason is
that to do so prevents the maldistribution of chemicals in the
slurry of silver powder and allows a neutralization reaction to
uniformly occur in the slurry of silver powder. Preferably used is
an aqueous solution of silver nitrate prepared by dissolving 2.50 g
to 33.34 g of silver nitrate in 500 g of deionized water
(equivalent to silver nitrate concentration of 3% by weight to 40%
by weight). If the amount of silver nitrate added is less than 2.50
g, a sufficient amount of silver oxide to coat the surface of each
particle of the above-described silver powder uniformly will not be
precipitated. Even if the amount of silver nitrate is more than
33.34 g, the amount of silver oxide adhering to the surface of each
silver powder particle does not change very much, and rather
resulting in bringing about inferior particle size distribution and
dispersibility of the powder particles.
[0050] The neutralizing agent may be an alkali metal salt such as
sodium hydroxide and potassium hydroxide. It goes without saying
that the amount of the neutralizing agent added depends on the
amount of silver nitrate to be neutralized. Assuming that sodium
hydroxide is used, the amount is selected from those in the range
of 0.588 g to 7.840 g to corresponding amount of silver nitrate.
Sodium hydroxide is also preferably used in the form of an aqueous
solution. Thus, a solution of sodium hydroxide in 500 g of
deionized water is used.
[0051] After allowing fine silver oxide particles to be adhered to
the surface of each silver powder particle in the above-described
manner, the fine silver oxide particles are washed. This washing
has to be carried out to remove the solution having been used for
the neutralizing reaction and also fully remove water. Accordingly,
it is most preferable to employ washing in water and washing in
alcohol in combination. In order to wash the silver powder made of
fine silver oxide particles adhering thereto obtained under the
above-described conditions, at least 500 g or more of water or the
largest possible amount of water is used. It is preferably to
dehydrate the washed silver powder. This is done to remove
impurities as much as possible. In order to ensure that water is
removed, washing in alcohol is carried out. For the washing in
alcohol, ethyl alcohol, methyl alcohol or isopropyl alcohol may be
used. The amount of alcohol used is not particularly limited, as
long as the amount used is sufficient to remove water.
[0052] After completion of the washing, the silver powder made of
fine silver oxide particles adhering thereto is immediately exposed
to UV rays without being dried. Silver powder made of silver
particles adhering thereto can be obtained by reducing the fine
silver oxide particles on the surface of each silver powder
particle to fine silver particles. Exposure to UV rays accelerates
the conversion of the fine silver oxide particles to fine silver
particles and prevents non-uniform reduction from occurring. For UV
rays used, their wavelengths are not strictly limited and, for
example, UV light used for sterilization can be used. After
completion of the exposure to UV rays, the silver powder is fully
dried to produce silver powder made of silver particles each to
which fine silver particles adhere according to the present
invention.
[0053] Process for Producing Silver Powder Preferably Used as Core
Material:
[0054] A process will be described for producing silver powder (of
nearly spherical shape) suitably used as the core material for the
silver powder made of silver particles each to which finer silver
particles adhere according to the present invention. The production
process described here is for producing silver powder having the
above-described powder characteristics: "a. the average particle
size of primary particles D.sub.IA obtained by the image analysis
of the scanning electron microscope is 0.6 .mu.m or less"; "b. the
degree of agglomeration represented by D.sub.50/D.sub.IA, where
D.sub.IA is the above-described average particle size of primary
particles and D.sub.50 is the average particle size obtained by
laser diffraction scattering particle size distribution measurement
method, is 1.5 or less"; and "c. the crystallite size is 10 nm or
less." Accordingly, the silver powder described here can be said to
a fully-fine powder in comparison with conventional silver powder
produced using an aqueous solution of silver nitrate.
[0055] The process for producing the silver powder is a process
including: preparing an aqueous solution of a silver complex by
mixing and reacting an aqueous solution of silver nitrate and a
complexing agent; making an organic reducing agent contact with the
above aqueous solution for reaction to allow silver particles to be
precipitated by reduction; filtering the precipitate-containing
solution; washing the resultant silver powder; and drying the same,
characterized in that the reducing agent, silver nitrate and the
complexing agent are added in such amounts that allow each of the
above chemicals to be more dilute after the addition. It is
conventionally common that a solution of a reducing agent and an
aqueous solution of a silver complex are mixed for a very short
time in a bath and the concentration of silver is generally kept 10
g/l or more. Thus, silver nitrate, a reducing agent and a
complexing agent have needed to be added in large amounts to ensure
productivity which balances the scale of facilities. In the
following, the production process will be described more
specifically, taking an example of the process in which aqueous
ammonia is used as a complexing agent.
[0056] The most important characteristic of the production process
according to the present invention is in that after the contact
reaction of an aqueous solution of an ammine complex and an organic
reducing agent, the concentration of the organic reducing agent is
low, and therefore, the amount of the organic reducing agent
remaining adsorbed on the surface of the produced silver powder
particle or the amount of the organic reducing agent taken in the
inside of each of the powder particles during their growing process
can be reduced. Accordingly, in the solution produced by mixing a
solution of a reducing agent and an aqueous solution of a silver
complex, it is most preferable to keep the concentration of the
organic reducing agent be 1 g/l to 3 g/l, while keeping the
concentration of silver be 1 g/l to 6 g/l.
[0057] The concentration of silver has a proportional relationship
with relative to the amount of the reducing agent, and it goes
without saying that the higher the concentration of silver becomes,
the larger amount of silver powder can be obtained. However, if the
concentration of silver exceeds 6 g/l, precipitating silver
particles tend to be coarser, and the resultant silver powder has a
particle size not different from that of conventional silver
powder. Thus, fine silver powder having an excellent
dispersibility, at which the present invention aims, cannot be
obtained. On the other hand, if the concentration of silver is less
than 1 g/l, very fine silver powder is certainly obtained; however,
too fine silver powder absorb a larger amount of oil and causes the
viscosity of its paste to be increased. This in turn requires a
larger amount of vehicle to be added, and in the sintered conductor
formed as a final product, its film density is low and its
electrical resistance tends to be increased. And besides,
industrial productivity required cannot be satisfied.
[0058] To obtain fine silver powder of the present invention in a
good yield, an optimum requirement condition is to keep the
concentration of the organic reducing agent be 1 g/l to 3 g/l,
while keeping the concentration of silver be 1 g/l to 6 g/l. The
concentration of the organic reducing agent in the range of 1 g/l
to 3 g/l is selected as an optimum range for obtaining fine silver
powder in the relationship with the silver concentration of an
aqueous solution of a silver/ammine complex. If the concentration
of the organic reducing agent exceeds 3 g/l, the amount of the
reducing solution added to the aqueous solution of a silver/ammine
complex is certainly decreased; however, the agglomeration of the
powder particles of the silver powder precipitated through
reduction starts to progress significantly and the amount of
impurities (herein the amount of impurities is taken as the carbon
content) contained in the powder particles starts to be increased
rapidly. If the concentration of the organic reducing agent is less
than 1 g/l, the total amount of the reducing solution used
increases, thereby increasing the amount of a waste water to be
treated. This does not meet the industrial economy.
[0059] The term "organic reducing agent" herein used means
hydroquinone, ascorbic acid, glucose or the like. Of these organic
reducing agents, hydroquinone is preferably selectively used in the
present invention. Hydroquinone excels in reactivity compared with
other organic reducing agents in the present invention and can
bring about an optimum reaction rate for obtaining silver powder of
small crystallite size and low crystallizability.
[0060] Other additives can also be used in combination with the
above-described organic reducing agent. The term "additives" herein
used means glues such as gelatin, amine-group polymers, celluloses
or the like. Additives are desirably selected which stabilize the
process of the precipitation of silver powder through reduction,
and at the time, perform a certain function as a dispersant. Any
proper additive can be selectively used depending on the type of
organic reducing agent, process or the like.
[0061] In the process for contact reacting the aqueous solution of
a silver/ammine complex and the reducing agent obtained as above to
precipitate fine silver powder through reduction, it is desirable
to use a process in the present invention in which, as shown in
FIG. 2, a certain pass through which an aqueous solution S.sub.1 of
a silver/ammine complex is flowed through (hereinbefore and
hereinafter referred to as "first pass") and a second pass b which
joins the first pass a midway along the pass are provided, an
organic reducing agent and optionally additives S.sub.2 are flowed
into the first pass a through the second pass b so that S.sub.1,
the organic reducing agent and optionally S.sub.2 are contact mixed
at the juncture m of the first pass a and the second pass b to
precipitate silver particles through reduction (hereinafter
referred to as "joining mixing process").
[0062] Employing such joining mixing process makes it possible to
complete the mixing of the two solutions for a shortest-mixing time
and allow the reaction to progress while keeping the reaction
system uniform, which leads to the formation of uniformly shaped
powder particles. Further, if the solution after the mixing
contains a decreased amount of organic reducing agent, the amount
of the organic reducing agent remaining adsorbed on the surface of
each particle of the fine silver particle precipitated through
reduction is also decreased. This makes it possible to decrease the
amount of impurities attached on the fine silver powder obtained
through flitration and drying. The decrease in the amount of
impurities attached on the fine silver powder in turn makes it
possible to lower an electric resistance of the sintered conductor
formed of a silver paste which uses the silver powder.
[0063] Further, when obtaining an aqueous solution of a
silver/ammine complex by contact-reacting an aqueous solution of
silver nitrate and aqueous ammonia, it is desirable to use an
aqueous solution of silver nitrate whose silver nitrate
concentration is 2.6 g/l to 48 g/l and yield an aqueous solution of
a silver/ammine complex whose silver concentration is 2 g/l to 12
g/l. Specifying the concentration of an aqueous solution of silver
nitrate is, in other words, specifying the amount of the aqueous
solution of silver nitrate. Considering over the silver
concentration of an aqueous solution of a silver/ammine complex
being kept 2 g/l to 12 g/l, the concentration and amount of aqueous
ammonia to be added thereto are inevitably determined. Although the
technological reasons have not been clarified yet at present, use
of an aqueous solution of silver nitrate whose silver nitrate
concentration is 2.6 g/l to 48 g/l makes it possible to obtain a
fine silver powder having the best production stability and stable
quality.
[0064] The resultant fine silver powder is then washed and dried to
yield silver powder as a core material. The washing may be carried
out using washing in water and washing in alcohol in combination or
using washing in alcohol alone. Washing process is not particularly
limited. The drying may also be carried out by any suitable
process.
[0065] To provide the fine silver powder not only with the
above-described characteristics a to c, which are obtained by the
above-described production process, but also with the
characteristic d. the content of organic impurities is 0.25% by
weight or less in terms of amount of carbon", the washing process
must be changed. In the following, the washing process will be
described.
[0066] To decrease the amount of impurities contained in the fine
silver powder, washing carried out at the final stage is very
important. The washing may be carried out either by using washing
in water and washing in alcohol in combination or by using washing
in alcohol alone; however, the washing is reinforced in washing in
alcohol. Thus, washing is carried out for 40 g of fine silver
powder by using about 100 ml of deionized water and then using
about 50 ml of alcohol. However, in the present invention, when
carrying out washing in alcohol, 200 ml or more of alcohol is used
for 40 g of fine silver powder, in other words, 1 kg of fine silver
powder is washed using an excessive amount of alcohol, that is, 5 L
or more of alcohol.
[0067] An amount of impurities by reinforcement of washing can be
decreased just because, in the contact reaction between an aqueous
solution of a silver/ammine complex and a reducing agent, the
present invention employs a technique for keeping the amount of
organic reducing agent remaining in the solution after mixing small
by employing a low-concentration-reaction system of low
concentration of organic reducing agent.
[0068] The silver powder made of silver particles each to which
fine silver particles adhere according to the present invention has
a low-temperature sintering performance at a level which
conventional silver powder has never had, because it is constructed
by making to adhere fine silver particles (silver nanoparticles) to
the surface of each of the silver powder particles of the silver
powder. Further, it can have a particularly excellent
low-temperature sintering performance, because the silver powder is
very fine, excellent in dispersibility and has few impurities,
which conventional silver powder has never had, as its core
material.
[0069] In the meantime, the process for producing silver powder
made of silver particles each to which fine silver particles adhere
according to the present invention is excellent in running
stability through the production process, and therefore, it can
produce silver powder made of silver particles to each which fine
silver particles adhere very effectively. The process can also
produce silver powder used as the core material effectively,
because it employs a process for producing silver powder using the
above-described dilute solution.
BRIEF DESCRIPTION OF THE DRAWINGS
[0070] FIG. 1 is a schematic sectional view of a powder particle of
silver powder made of silver particles each to which fine silver
particles adhere; and
[0071] FIG. 2 is a view showing the concept of mixing of an aqueous
solution of a silver/ammine complex and a reducing agent.
BEST MODE FOR CARRYING OUT THE INVENTION
[0072] In the following, the preferred embodiment of the present
invention will be described in detail, comparing with Comparative
Examples. In the following examples, silver powder used as a core
material was produced first. Then, silver powder made of silver
particles each to which fine silver particles adhere, a silver
paste using the above silver powder made of silver particles each
to which fine silver particles adhere, and test circuits using the
silver paste were produced. The specific resistance and sinterable
temperature were measured for the produced test circuits.
EXAMPLE 1
[0073] Process for Producing Silver Powder Used as a Core
Material:
[0074] In this example, first silver powder (whose particle is
spherical) used as a core material was produced. The production
process was as follows.
[0075] First, 63.3 g of silver nitrate was dissolved in 9.7 liter
of deionized water to prepare an aqueous solution of silver
nitrate. Then, 235 ml of 25% by weight aqueous ammonia was added
for a very short time to the aqueous solution of silver nitrate and
stirred to give an aqueous solution of a silver ammine complex.
[0076] The aqueous solution of a silver ammine complex was
introduced into a first pass a having an inner diameter 13 mm,
shown in FIG. 2, at a flow rate of 1500 ml/sec and a reducing agent
was allowed to flow through a second pass b at a flow rate of 1500
ml/sec so that the solution and the agent came into contact with
each other at a juncture m while being kept at 20.degree. C. to
precipitate fine silver powder through reduction. The reduction
agent used here was a solution of 21 g of hydroquinone in 10 liter
of deionized water. Accordingly, the concentration of hydroquinone
at the time of completion of the mixture was about 1.04 g/l, which
is very low concentration.
[0077] To superlatively pick up 40 g of the resultant fine silver
powder, the solution was filtered using a nutsche, and the
separated fine silver powder was washed with 100 ml of water and
600 ml of methanol and dried at 70.degree. C. for 5 hours to yield
fine silver powder.
[0078] The powder characteristics of the silver powder obtained as
above correspond to those of Comparative Example 1, which are shown
in Table 1 together with those of the other examples and
Comparative Examples. Now the term "sinterable temperature" herein
used will be described, because its contents, such as measuring
method, has not been clearly defined by the description so far. The
term "sinterable temperature" shown in Table 1 means that the
lowest temperature at which a silver paste is produced using each
of the silver powder and each having been used for drawing a wiring
pattern of a circuit on an alumina substrate can be sintered into
products to such an extend that the electrical resistance of the
sintered products is measurable. The sintering temperature was
selected from the temperatures in the rage of 150 to 250.degree. C.
Then, specific resistance was measured for the circuits 1 mm width
pattern-circuit which was obtained by sintering the silver pastes.
To judge whether the sintering was done well or not, the sintered
state was also observed with a scanning electron microscope. The
composition of silver pastes was: 85 wt % of fine silver powder and
15 wt % of terpineol. FIB analysis was used for measuring the size
of the precipitated crystal grains to determine the crystallite
size.
[0079] Production of Silver Powder Made of Silver Particles Each to
which Fine Silver Particles Adhere:
[0080] Silver powder made of silver particles each to which fine
particles adhere was produced in accordance with the
above-described production process 1 using the silver powder, as a
core material obtained as above.
[0081] First, "a solution containing a silver complex which is
obtained by mixing silver nitrate and a complexing agent and
dissolving the mixture while stirring" was prepared so as to make
fine silver particles adhere onto the surface of each particle of
50 g of the above-described silver powder. The solution containing
a silver complex was prepared by first dissolving 17 g of silver
nitrate in 1 liter of deionized water and then adding 86 g of
potassium sulfite, as a complexing agent, to the solution.
[0082] Then, 50 g of the above-described silver powder was added to
the solution containing a silver complex obtained as above and
stirred for 1 minute. A reducing agent was added to the above
silver powder-containing solution so that a reduction reaction was
caused to precipitate fine silver powder of nano-order particle
size uniformly. The reduction reaction was carried out by adding a
solution of 10 g of hydrazine in 90 ml of deionized water, as a
reducing agent, for a very short time at a solution temperature of
40.degree. C. for 10 minutes. After fine silver particles were
precipitated by reduction on the surface of each powder particle
through reduction in the above-described manner, the precipitate
was separated through a filter, washed, dehydrated and dried to
yield silver powder made of silver particles each to which fine
silver particles adhere according to the present invention.
[0083] The powder characteristics of the silver powder made of
silver particles each to which fine silver particles adhere
obtained as above were determined in the same manner as in the case
of the silver powder used as a core material. Then, a silver paste
was prepared using the above silver powder and a test circuit was
formed using the silver paste. Then, the specific resistance and
sinterable temperature were measured for the formed test circuit.
The resultant characteristics are shown in Table 1 as those of
Example 1.
EXAMPLE 2
[0084] Process for Producing Silver Powder Used as a Core
Material:
[0085] In this example, first silver powder (which is substantially
spherical) used as a core material was produced. The production
conditions were as described below.
[0086] Silver powder was produced under production conditions
different from those of Example 1 and the powder characteristics of
the resultant silver powder were determined. Then, a silver paste
was prepared using the above silver powder and a test circuit was
formed using the silver paste. Then, the specific resistance of the
conductor and sinterable temperature were measured for the formed
test circuit.
[0087] First, 63.3 g of silver nitrate was dissolved in 3.1 liter
of deionized water to prepare an aqueous solution of silver
nitrate. Then, 235 ml of 25% by weight aqueous ammonia was added
for a very short time to the aqueous solution of silver nitrate and
stirred to give an aqueous solution of a silver ammine complex.
[0088] The aqueous solution of a silver ammine complex was
introduced into a first pass a having an inner diameter of 13 mm,
shown in FIG. 2, at a flow rate of 1500 ml/sec and a reducing agent
was allowed to flow through a second pass b at a flow rate of 1500
ml/sec so that the solution and the agent came into contact with
each other at a juncture m while being kept at 20.degree. C. to
precipitate fine silver powder through reduction. The reduction
agent used here was a solution of 21 g of hydroquinone in 3.4 liter
of deionized water. Accordingly, the concentration of hydroquinone
at the time of completion of the mixture was as low as about 3.0
g/l, which is very low concentration.
[0089] To superlatively pick up 40 g of the resultant fine silver
powder in the same manner as in Example 1, the solution was
filtered using a nutsche, and the separated silver powder was
washed with 100 ml of water and a large volume of, that is, 600 ml
of methanol and dried at 70.degree. C. for 5 hours to yield fine
silver powder as a core material. The powder characteristics of the
silver powder obtained as above correspond to those of Comparative
Example 2, which are shown in Table 1 together with those of the
other examples and Comparative Examples.
[0090] Production of Silver Powder Made of Silver Particles Each to
which Fine Silver Particles Adhere:
[0091] Silver powder made of silver particles each to which fine
silver particles adhere was produced in accordance with the
above-described production process 2 using the silver powder
obtained as above as a core material. First, a slurry of silver
powder was prepared by adding 50 g of the above silver powder to
1500 g of ethylene glycol, as a dispersion medium, and fully
stirring the mixture to disperse the silver powder in the
medium.
[0092] Then, silver nitrate and a neutralizing agent were added to
the resultant slurry of silver powder and dissolved by stirring to
precipitate fine silver oxide particles on the surface of each
silver powder particle. Silver nitrate was added first using an
aqueous solution of 16.67 g of silver nitrate in 500 g of deionized
water (equivalent to 30 wt % silver nitrate concentration). Then a
neutralizing agent was added and fully stirred. The neutralizing
agent used was a solution of 3.92 g of sodium hydroxide in 500 g of
deionized water. Fine silver oxide particles were made to adhere to
the surface of each silver powder particle in this manner.
[0093] The silver powder made of fine silver oxide particles
adhering thereto was then separated by filtration and washed. The
washing was carried out using washing in water and washing in
alcohol in combination. First washing in water was performed. The
silver powder made of fine silver oxide particles adhering thereto
obtained under the above-described conditions was washed in 500 g
of water to remove impurities on the powder as much as possible and
dehydrated. Then, to ensure that water is removed, the powder was
washed in 500 g of isopropyl alcohol.
[0094] Immediately after completion of the washing, the silver
powder made of fine silver oxide particles adhering thereto was
exposed to UV rays without being dried to reduce the fine silver
oxide particles on the surface of each silver powder particle to
fine silver particles. The exposure to UV rays was conducted using
CL15-A by Toshiba Corporation, which is usually used as a
bactericidal lamp, for 3 hours so as to accelerate the rapid
conversion of the fine silver oxide particles to fine silver
particles and prevent the occurrence of non-uniform reduction.
After that, drying was carried out by conventional procedure to
yield silver powder made of silver particles each to which fine
silver particles adhere according to the present invention on which
little impurities were attached.
[0095] The powder characteristics of the silver powder made of
silver particles each to which fine silver particles adhere
obtained as above were determined in the same manner as in the case
of the silver powder used as a core material. Then, a silver paste
was prepared using the above silver powder and a test circuit was
formed using the silver pastes. Then the specific resistance and
sinterable temperature were measured for the formed test circuit.
The resultant characteristics are shown in Table 2 as those of
Example 2.
EXAMPLE 3
[0096] Process for Producing Silver Powder Used as a Core
Material:
[0097] In this example, first silver powder (of nearly spherical
shape) having a large crystallite size was produced using the
process shown below, and the powder characteristics of the
resultant silver powder were determined. Then, a silver paste was
prepared using the above silver powder and a test circuit was
formed using the silver paste. Then the specific resistance and
sinterable temperature were measured for the formed test
circuit.
[0098] First, 20 g of polyvinyl pyrrolidone was dissolved in 260 ml
of deionized water and 50 g of silver nitrate was dissolved to
prepare an aqueous solution of silver nitrate. Then, 25 g of nitric
acid was added for a very short time to the above solution and
stirred to yield a silver-containing nitric acid solution. At the
time of completion of the mixing, the concentration of ascorbic
acid was about 36.0 g/l.
[0099] A reducing solution was prepared by adding and dissolving
35.8 g of ascorbic acid, as a reducing agent, in 500 ml of
deionized water.
[0100] The silver-containing nitric acid solution was put into a
reaction bath and then the above reducing solution was also added
for a very short time to the reaction bath. Silver powder was
precipitated through reduction by stirring the mixed solution,
while keeping the solution temperature 25.degree. C., to cause a
reaction.
[0101] The resultant fine silver powder was separated by filtration
using a nutsche, and the separated silver powder was washed with
100 ml of water and 500 ml of methanol and dried at 70.degree. C.
for 5 hours to yield silver powder as a core material. The powder
characteristics of the silver powder obtained are shown in Table 1
as those of Comparative Example 3.
[0102] Production of Silver Powder Made of Silver Particles Each to
which Fine Silver Particles Adhere:
[0103] Silver powder made of silver particles each to which fine
silver particles adhere was produced in the same manner as in the
above-described Example 2 using the silver powder obtained above as
a core material. To avoid the repetition, the detailed description
of the production process will be omitted here.
[0104] The powder characteristics of the resultant silver powder
made of silver particles each to which fine silver particles adhere
were determined in the same manner as in the case of the silver
powder used as a core material. A silver paste was produced using
the silver powder made of silver particles each to which fine
silver particles adhere and a test circuit was formed using the
silver paste. The specific resistance and sinterable temperature
were measured for the test circuit. The results are shown in Table
1 as those of Example 3.
EXAMPLE 4
[0105] Process for Producing Flake Silver Powder Used as a Core
Material:
[0106] In this example, flake-shaped silver powder was produced by
machining silver powder which is substantially spherical, and the
resultant silver powder was used as a core material. The powder
characteristics of the flake silver powder are shown in Table 2 as
those of Comparative Example 4.
[0107] Production of Flake Silver Powder Made of Silver Particles
Each to which Fine Silver Particles Adhere:
[0108] Flake silver powder made of silver particles each to which
fine silver particles adhere was produced in the same manner as in
the above-described production process 1 using the flake silver
powder obtained above as a core material. The production conditions
employed were the same as those of Example 1. To avoid the
repetition, the detailed description of the production conditions
will be omitted here.
[0109] The powder characteristics of the resultant flake silver
powder made of silver particles each to which fine silver particles
adhere were determined in the same manner as in the case of the
flake silver powder used as a core material. A silver paste was
produced using the silver powder made of silver particles each to
which fine silver particles adhere and a test circuit was formed
using the silver paste. The specific resistance and sinterable
temperature were measured for the test circuit. The results are
shown in Table 2 as those of Example 4.
COMPARATIVE EXAMPLES
Comparative Example 1
[0110] The silver powder described in Example 1 and used as a core
material was also used as Comparative Example. The powder
characteristics etc. of the silver powder are shown in Table 1 as
those of Comparative Example 1.
Comparative Example 2
[0111] The silver powder described in Example 2 and used as a core
material was also used as Comparative Example. The powder
characteristics etc. of the silver powder are shown in Table 1 as
those of Comparative Example 2.
Comparative Example 3
[0112] The silver powder described in Example 3 and used as a core
material was also used as Comparative Example. The powder
characteristics etc. of the silver powder are shown in Table 1 as
those of Comparative Example 3.
Comparative Example 4
[0113] The silver powder described in Example 4 and used as a core
material was also used as Comparative Example. The powder
characteristics etc. of the silver powder are shown in Table 1 as
those of Comparative Example 4.
[0114] <Comparative Examination of Examples and Comparative
Examples>
[0115] The above-described Examples 1 to 3 and Comparative Examples
1 to 3 are compared while referring to Table 1.
[0116] [Table 1]
1 TABLE I Powder Charactaristics Silver Powder made of Silver
Particles, each to Sinterd Conductor Core Material which Fine
Silver Charactaristics Tap Crystallite Particles adhere Spesific
Sinterable SSA Density D.sub.50 D.sub.max D.sub.IA Size Carbon
D.sub.50 D.sub.max SSA Resistance Temparature Sample m.sup.2/g
g/cm.sup.3 .mu.m D.sub.50/D.sub.IA nm Content % .mu.m m.sup.2/g
.mu..OMEGA. .multidot. cm .degree. C. Example 1 2.54 4.2 0.31 0.97
0.30 1.03 7 0.32 0.29 0.97 3.73 3.4 150 Example 2 1.68 4.7 0.55
1.86 0.49 1.12 7 0.21 0.57 1.85 2.85 5.3 150 Example 3 0.62 4.0
3.03 11.0 1.20 2.53 38 0.22 3.26 11.0 0.99 7.9 150 Comparative 2.54
4.2 0.31 0.97 0.30 1.03 7 0.28 -- 7.9 180 Example 1 Comparative
1.68 4.7 0.55 1.86 0.49 1.12 7 0.21 -- 5.9 190 Example 2
Comparative 0.62 4.0 3.03 11.0 1.20 2.53 38 0.30 -- not 250 Example
3 available
[0117] Comparing the powder characteristics of the silver powder as
a core material and those of silver powder made of silver particles
each to which fine silver particles adhere for the cases of Example
1 and Comparative Example 1, Example 2 and Comparative Example 2,
and Example 3 and Comparative Example 3 while referring to Table 1,
it is apparent that even if fine silver particles are made to
adhere to the core material, the powder characteristics of the core
material are hardly changed. Particularly from the fact that there
was almost no change in the value D.sub.max before and after the
adhesion of fine silver particles, it is apparent the
dispersibility which the silver powder used as the core material
has is maintained in the silver powder made of silver particles
each to which fine silver particles adhere. This indicates that a
silver powder as fine as possible having excellent dispersibility
is advantageously used as the core material. The reason of this is
that the crystallite size of the silver powder as the core material
is kept unchanged, though the crystallite size of the silver powder
made of silver particles each to which fine silver particles adhere
is not described in the Tables.
[0118] Comparing the sintered conductor characteristics for the
cases of Example 1 and Comparative Example 1, Example 2 and
Comparative Example 2, and Example 3 and Comparative Example 3, it
is obvious that sinterable temperature is so decreased by making
fine silver particles adhere to silver powder as the core material
that conventional knowledge cannot explain. Particularly in the
cases of Examples 1 to 3, though the powder characteristics of both
core material and silver powder made of silver particles each to
which fine silver particles adhere are different from example to
example, the sinterable temperature is 150.degree. C. for all the
cases. On the other hand, in the cases of Comparative Examples 1 to
3, there exists no fine silver particle layer on the silver powder,
the sintered conductor characteristics are largely affected by the
powder characteristics, and in the case of Comparative Example 3,
it is impossible to measure the specific resistance. The comparison
so far confirms that the silver powder made of silver particles
each to which fine silver particles adhere according to the present
invention is not affected by the powder characteristics of the core
material and it can be sintered at low temperatures, because of the
fine silver particles adhering to the surface of each of particles
of the silver powder as a core material.
[0119] Then, the above-described example 4 and Comparative Example
4 will be compared while referring to Table 2. For the flake silver
powder as core material and the flake silver powder made of silver
particles each to which fine silver particles adhere, the powder
characteristics are drawn which are different from those of the
nearly sphere-shaped powder particles shown in Table 1. For the
flake powder, the measurement of crystallite size and carbon
content was omitted because its crystallite size undergoes changes
by physical machining and its surface is contaminated by lubricants
used in the machining. The value D.sub.IA obtained by the
observation with a scanning electron microscope is also excluded
from measurement items, because of its large fluctuations in a
field of view. Instead, the measurements by laser diffraction
scattering particle size distribution measurement method are often
used.
[0120] [Table 2]
2 TABLE II Powder Charactaristics Flake Silver Powder made of
Silver Flake Silver Powder Particles, each to which Sinterd
Conductor as Core Material Fine Silver Particles Adhere
Charactaristics Tap Tap Spesific Sinterable SSA Density D.sub.10
D.sub.50 D.sub.90 D.sub.max SSA Density D.sub.10 D.sub.50 D.sub.90
D.sub.max Resistance Temparature Sample m.sup.2/g g/cm.sup.3 .mu.m
m.sup.2/g g/cm.sup.3 .mu.m .mu..OMEGA. .multidot. cm .degree. C.
Example 4 0.29 2.6 8.91 16.7 28.9 67.9 0.72 2.7 9.17 18.8 29.8 74.0
21 200 Comparative -- 109 200 Example 4
[0121] Comparing the powder characteristics of flake silver powder
as the core material of Comparative Example 4 and those of flake
silver powder made of silver particles each to which fine silver
particles adhere of Example 4 with reference to Table 2, it is
apparent that even if fine silver particles are made to adhere to
the core material, the powder characteristics of the core material
do not change very much, just like the case of the powder of the
substantially spherical particles shown in Table 1. The value
D.sub.max seems to be increased after the adhesion of fine silver
particles; however, it cannot be necessarily asserted that there
exist very large fluctuations, in view of including any measuring
errors.
[0122] Comparing over the sintered conductor characteristics of
Example 4 with Comparative Example 4, it is apparent that
sinterable temperature is decreased by making fine silver powder
adhere to the core material. This is the same as in the cases of
Examples 1 to 3. The comparison so far confirms that the flake
silver powder made of silver particles each to which fine silver
particles adhere according to the present invention is not affected
by the powder characteristics of the core material and it can be
sintered at low temperatures, because of the fine silver particles
adhering to the particle surface of the flake silver powder as a
core material.
INDUSTRIAL APPLICABILITY
[0123] The silver powder made of silver particles each to which
fine silver particles adhere according to the present invention is
constructed by making fine silver powder (silver nanoparticles)
adhere to the surface of each silver powder particle. Such
construction enables the silver powder of the present invention to
exhibit a low-temperature sintering performance at a level which
conventional silver powder has never had. Because of its stable
low-temperature sintering performance, which conventional silver
powder has never had, a drastic expansion of applications in which
silver powder is used is expected and a drastic reduction in energy
cost during the sintering process will be made possible. Further,
use of the silver powder, which is very fine, excels in
dispersibility and contains less impurities compared with any
conventional silver powder, as the core material for the silver
powder made of silver particles each to which fine silver particles
adhere makes it possible to realize an especially excellent
low-temperature sintering performance and the formation of a
low-resistant sintered conductor.
[0124] In the meantime, the process for producing silver powder
made of silver particles each to which fine silver particles adhere
according to the present invention is excellent in running
stability through the production process, and therefore, it can
produce silver powder made of silver particles each to which fine
silver particles adhere very effectively. Thus, it can provide
inexpensive and high-quality silver powder in the market, thereby
contributing to the expansion of applications in which the silver
powder made of silver particles each to which fine silver particles
adhere according to the present invention is used.
* * * * *