U.S. patent application number 13/489032 was filed with the patent office on 2012-09-27 for aqueous-based dispersions of metal nanoparticles.
This patent application is currently assigned to Yissum Research Development Company of the Hebrew University of Jerusalem. Invention is credited to Shai Aviezer, Michael Grouchko, Alexander Kamyshny, Shlomo Magdassi.
Application Number | 20120241693 13/489032 |
Document ID | / |
Family ID | 36046873 |
Filed Date | 2012-09-27 |
United States Patent
Application |
20120241693 |
Kind Code |
A1 |
Magdassi; Shlomo ; et
al. |
September 27, 2012 |
AQUEOUS-BASED DISPERSIONS OF METAL NANOPARTICLES
Abstract
The invention relates to a method for preparing an aqueous-based
dispersion of metal nanoparticles comprising: (a) providing an
aqueous suspension of a metal salt; (b) pre-reducing the metal salt
suspension by a water soluble polymer capable of metal reduction to
form a metal nuclei; and (c) adding a chemical reducer to form
metal nanoparticles in dispersion. The invention further relates to
aqueous-based dispersions of metal nanoparticles, and to
compositions such as ink comprising such dispersions.
Inventors: |
Magdassi; Shlomo;
(Jerusalem, IL) ; Kamyshny; Alexander; (Jerusalem,
IL) ; Aviezer; Shai; (Merkaz, IL) ; Grouchko;
Michael; (Jerusalem, IL) |
Assignee: |
Yissum Research Development Company
of the Hebrew University of Jerusalem
Jerusalem
IL
|
Family ID: |
36046873 |
Appl. No.: |
13/489032 |
Filed: |
June 5, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11813628 |
Apr 17, 2008 |
8227022 |
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PCT/IL06/00031 |
Jan 10, 2006 |
|
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13489032 |
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60642116 |
Jan 10, 2005 |
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Current U.S.
Class: |
252/512 ;
977/773 |
Current CPC
Class: |
C09D 11/322 20130101;
B82Y 30/00 20130101; Y10S 977/81 20130101; B22F 1/0018 20130101;
B22F 1/0022 20130101; B22F 2998/00 20130101; C09D 11/52 20130101;
Y10S 977/773 20130101; B22F 2998/00 20130101; C09D 5/24 20130101;
B22F 9/24 20130101; B22F 1/0022 20130101; C23C 18/30 20130101 |
Class at
Publication: |
252/512 ;
977/773 |
International
Class: |
H01B 1/02 20060101
H01B001/02 |
Claims
1. An aqueous-based dispersion comprising metal nanoparticles and
at least one water soluble polymer, said aqueous-based dispersion
is characterized by: (a) the concentration of said metal
nanoparticles in said dispersion is in the range 0.5-35 wt %; (b)
the size of said nanoparticles is below 20 nm in diameter; (c) the
weight ratio of said water soluble polymer to said metal
nanoparticles is below 0.1:1; and (d) up to 25%, based on the total
weight of the dispersing medium of organic solvents.
2. An ink composition, comprising the aqueous-based dispersion of
claim 1.
3. An aqueous-based dispersion comprising metal nanoparticles and
at least one water soluble dispersant, said aqueous-based
dispersion is characterized by: (a) the concentration of said metal
nanoparticles in said dispersion is in the range 5-80 wt %; (b) the
size of said nanoparticles is below 20 nm in diameter; (c) the
weight ratio of said water soluble dispersant to said metal
nanoparticles is below 0.1:1; and (d) up to 25%, based on the total
weight of the dispersing medium of organic solvents.
4. An ink composition, comprising the aqueous-based dispersion of
claim 3.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] The present application is a divisional of U.S. application
Ser. No. 11/813,628, filed Jul. 10, 2007, as a 371 national stage
application of PCT/IL06/00031, filed Jan. 10, 2006, which claims
the benefit of U.S. Provisional application No. 60/642,116, filed
Jan. 10, 2005. The entire contents of prior applications are herein
incorporated by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to the field of metal
nanoparticle dispersions. More particularly, the present invention
relates to aqueous-based dispersions of metal nanoparticles, a
method for their preparation and compositions such as ink
comprising such dispersions.
BACKGROUND OF THE INVENTION
[0003] Metallic nanoparticles draw intense scientific and practical
interest due to their unique properties, which differ from those of
bulk and atomic species. Such a difference is determined by
peculiarity of electronic structure of the metal nanoparticles and
extremely large surface area with a high percentage of surface
atoms. Metal nanoparticles exhibit a drastic decrease in melting
point compared to that of the bulk material, they are characterized
by enhanced reactivity of the surface atoms, high electric
conductivity and unique optical properties. Virtually, nanosized
materials are well-known materials with novel properties and
promising applications in electrochemistry, microelectronics,
optical, electronic and magnetic devices and sensors, in new types
of active and selective catalysts, as well as in biosensors.
Creation of stable concentrated nanocolloids of metals with low
resistivity offers new prospects in computer-defined direct-write
noncontact technologies, such as ink jet printing, for deposition
of metallic structures on various substrates. Microfabrications of
such structures by lithographic and electroless techniques are
time-consuming and expensive processes, and there is a real
industrial need for direct digital printing of conductive patterns.
Suggestions based on jetting small droplets of molten metals onto
the substrate have met several problems, such as difficulty of
adhering droplets onto a substrate, oxidation of the liquid metal,
and the difficulty of fabrication a droplet-ejection mechanism
compatible with high temperatures. Direct patterning by ink jet
printing, in addition to the conventional graphic applications, was
reported in the last decade for various applications, such as
fabrication of transistors and organic light emitting diodes,
polymer films, structural ceramics and biotechnology.
[0004] Conventional ink jet inks may contain two types of
colorants, dye or pigment, and are characterized by their main
liquid, which is the vehicle for the ink. The main liquid may be
water (water-based inks), or an organic solvent (solvent-based
inks)
[0005] The dye or pigment-based inks differ with respect to the
physical nature of the colorant. Pigment is a colored material that
is insoluble in the liquid, while the dye is soluble in the liquid.
Each system has drawbacks: pigments tend to aggregate, and
therefore clog the nozzles in the orifice plate, or the narrow
tubings in the printhead, thus preventing the jetting of the ink
while printing. Dyes tend to dry, and form a crust on the orifice
plate, thus causing failure in jetting and misdirection of
jets.
[0006] It is clear that the terms "dye" or "pigment" are the
general wordings for materials, which are soluble or insoluble,
respectively, in the solvents comprising the ink. Therefore, metal
nanoparticles may be considered, in this context, if introduced
into ink, as pigments of metal, having a size in the nanometer
range.
[0007] Conventional pigments in ink jet inks contain particles in
the size range of 100-400 nm. In theory, reducing the particle size
to 50 nm or less should show improved image quality and improved
printhead reliability when compared to inks containing
significantly larger particles.
[0008] The majority of inks in ink jet printers are water-based
inks. The use of metal nanoparticles as pigments requires the
elaboration of ink formulations containing stable concentrated
aqueous metal colloid. The synthesis of stable colloidal systems
with high metal concentration is a serious problem. A variety of
substances have been used to stabilize silver colloids: amphiphilic
nonionic polymers and polyelectrolytes, ionic and nonionic
surfactants, polyphosphates, nitrilotriacetate,
3-aminopropyltrimethoxysilane, and CS.sub.2. Stable water-soluble
silver nanoparticles were also obtained by reduction of silver ions
in the presence of amino- and carboxilate-terminated poly(amido
amine) dendrimers, and crown ethers. The preparations of stable
silver colloids, having low metal concentrations are described in
the literature, in procedures based on reduction of metal from
solution. The metal concentrations in these procedures amount only
to 10.sup.-2 M (about 0.1%) even in the presence of stabilizers (it
is almost impossible to obtain a stable aqueous silver colloid with
the metal concentrations higher then 10.sup.-3 M without an
additional stabilizer, due to fast particle aggregation). The
preparation of ink compositions having silver nanoparticle
concentration of up to about 1.5 wt % (during the reaction step) is
described in WO 03/038002.
[0009] The synthesis of concentrated silver nanoparticles is
described in: [0010] B. H. Ryu et al., Synthesis of highly
concentrated silver nanoparticles, assisted polymeric dispersant,
KEY ENGINEERING MATERIALS 264-268: 141-142 Part 1-3 2004; [0011]
Beyong-Hwan Ryu et al., Printability of the synthesized silver nano
sol in micro-patterning of electrode on ITO glass, Asia
display/IMID 04 Proceedings, pages 1-4; [0012] Ivan Sondi et al.,
Preparation of highly concentrated stable dispersions of uniform
silver nanoparticles, Journal of colloid and Interface Science, 260
(2003) 75-81; [0013] Dan V. Goaia et al., Preparation of
monodispersed metal particles, New J. Chem. 1998, pages
1203-1215.
[0014] Since ink jet ink compositions contain, in addition to dyes
or pigments, other additives, such as humectants, bactericides and
fungicides and binders (polymeric additives, which improve the dye
or pigment binding to substrate), the stabilizers should be
compatible with these substances and should not change noticeably
the physicochemical and rheological characteristics of inks (the
most important characteristics are viscosity and surface
tension).
[0015] Several methods of the metallic image generation with the
use of ink jet technology have been described.
[0016] One known method is based on an ink containing a reducing
agent and receiving material containing the reducible silver
compound (AgNO.sub.3 or silver di(2-ethylhexyl)-sulphosuccinate),
and, on the contrary, an ink and a receiving support containing a
silver compound and reducer, respectively. Heating the receiving
support during or after the ink deposition resulted in an image
formed by silver metal (U.S. Pat. No. 5,501,150 to Leenders, et al;
U.S. Pat. No. 5,621,449 to Leenders, et al).
[0017] Another approach for the deposition of metallic structures
is based on ink jet printing of organometallic precursor dissolved
in organic solvent with subsequent conversion of the precursor to
metal at elevated temperatures (-300.degree. C.). To increase the
metal (silver) loading of ink and to obtain higher decomposition
rates, silver or other metal nanoparticles may be added to the ink
along with the organometallic precursor. Near-bulk conductivity of
printed silver films has been achieved with such compositions
(Vest, R. W.; Tweedell, E. P.; Buchanan, R. C. Int. J. Hybrid
Microelectron. 1983, 6, 261; Teng, K. F.; Vest, R. W. IEEE Trans.
Indust. Electron. 1988, 35, 407; Teng, K. F.; Vest, R. W. IEEE
Electron. Device Lett. 1988, 9, 591; Curtis, C.; Rivkin, T.;
Miedaner, A.; Alleman, J.; Perkins, J.; Smith, L.; Ginley, D. Proc.
of the NCPV Program Review Meeting. Lakewood, Colo., USA, 14-17
October 2001, p. 249).
[0018] Fuller et al. demonstrated ink jet printing with the use of
colloidal inks containing 5-7 nm particles of gold and silver in an
organic solvent, a-terpineol, in order to build electrically and
mechanically functional metallic structures. When sintered, the
resistivity of printed silver structures was found to be 3
.mu..OMEGA.cm, about twice of that for bulk silver (Fuller, S. B.;
Wilhelm, E. J.; Jacobson, J. M. J. Microelectromech. Syst. 2002,
11, 54).
[0019] The inventors have previously described the preparation of
stabilized nanodispersions with silver concentration up to 1.5 wt
%, at the reaction step which were shown to be suitable pigments
for water-based ink jet inks (WO 03/038002; Magdassi, S.; Bassa,
A.; Vinetsky, Y.; Kamyshny, A. Chem. Mater. 2003, 15, 2208). The
stabilizers used were ionic polymeric materials such as
carboxymethyl cellulose (CMC) and polypyrrole (PPy), the silver
nanoparticles size did not exceed 100 nm.
[0020] There is a widely recognized need and it will be highly
advantageous to have a new method for obtaining aqueous-based
dispersion of metal nanoparticles, preferably silver nanoparticles,
which is simplified in production, which enables production of
metal nanodispersion characterized by small diameter of the
nanoparticles and high nanoparticle concentration and yet which is
physically stable (i.e. does not undergo caking or agglomeration
and can be easily redispersed if present as a sediment or a
powder). Additionally it would be highly advantageous to have an
aqueous based dispersion of metal nanoparticles with improved
properties such as high electric conductivity when applied onto a
substrate.
SUMMARY OF THE INVENTION
[0021] The present invention relates to a method for preparing an
aqueous-based dispersion of metal nanoparticles comprising: [0022]
(a) providing an aqueous suspension of a metal salt; [0023] (b)
pre-reducing said metal salt suspension by a water soluble polymer
capable of metal reduction to form metal nuclei; and [0024] (c)
adding a chemical reducer to form metal nanoparticles in
dispersion.
[0025] The present invention additionally relates to an
aqueous-based dispersion comprising metal nanoparticles and at
least one water soluble polymer, said aqueous-based dispersion is
characterized by: [0026] (a) the concentration of said metal
nanoparticles in said dispersion is in the range 0.5-35 wt %;
[0027] (b) the size of said nanoparticles is below 20 nm in
diameter; and [0028] (c) the weight ratio of said water soluble
polymer to said metal nanoparticles is below 0.1:1.
[0029] The present invention further relates to an aqueous-based
dispersion comprising metal nanoparticles and at least one water
soluble dispersant, said aqueous-based dispersion is characterized
by: [0030] (a) the concentration of said metal nanoparticles in
said dispersion is in the range 5-80 wt %; [0031] (b) the size of
said nanoparticles is below 20 nm in diameter; and [0032] (c) the
weight ratio of said water soluble dispersant to said metal
nanoparticles is below 0.1:1.
BRIEF DESCRIPTION OF FIGURES
[0033] FIG. 1 shows TEM images of nanoparticles in concentrated Ag
dispersions (FIG. 1A-8 wt %; FIG. 1B-20 wt %).
[0034] FIG. 2 shows silver patterns printed by ink jet printer onto
polyimide film (ink formulation contains 8 wt % silver and 0.5% BYK
348 as a wetting agent). On the left side (FIG. 2A), the part of
the line (12 mm length, 1.5 mm width, 3.5 um thickness), on which
the conductivity was measured, is shown (magnified in FIG. 2B).
[0035] FIG. 3 shows High Resolution SEM micrographs of the image
obtained with silver nanodispersion deposited on glass slide, dried
and sintered at various temperatures (60.degree. C., 150.degree.
C., 260.degree. C., 320.degree. C.).
[0036] FIG. 4 displays the conductivity (.sigma.) of deposited and
sintered samples relative to the conductivity of bulk silver
(6.310.sup.7 ohm.sup.-1m.sup.-1).
[0037] FIG. 5 shows optical and HR-SEM images of a ring formed on a
glass substrate.
[0038] (Down left) Optical microscope image of a 2 mm diameter ring
formed by drying a drop of silver dispersion and (up left) HR-SEM
top-view image of the same ring, showing also the adjacent inner
area enclosed by the ring and the gradual decrease in the particle
density towards the center of the ring. (Right) HR-SEM image of the
particles in the ring.
[0039] FIG. 6 presents the view of multiply twined nanoparticles
prepared as described in Example 3.
DETAILED DESCRIPTION OF THE INVENTION
[0040] According to the present invention it is possible to obtain
aqueous-based dispersion of metal nanoparticles from an aqueous
suspension of a metal salt by using water soluble polymers, which
have a dual function: One function is as initiators of the metal
reduction process by providing a "pre-reduction" step while forming
the metal nuclei, which serve as nucleation centers for subsequent
reduction by reducing agent. Another function is as stabilizers of
the formed metal nuclei and the resulting nanoparticles (preventing
their agglomeration). The aqueous based dispersions of the present
invention are characterized by high metal content, low particles
size at the same or even lower stabilizer (water soluble
polymer)-to-metal ratio compared to the prior art, high
conductivity (after deposition onto substrates and drying).
[0041] The aqueous-based dispersions obtained by using an aqueous
suspension of metal salt and these dual effect stabilizers (water
soluble polymers) show the following advantageous properties:
[0042] 1) The concentration of metal nanoparticles during the
reaction step (before separation step) can be up to about 35 wt %.
Since a powder of redispersible metal nanoparticles (preferably
silver nanoparticles) can be prepared according to this patent
application, concentrated dispersions (up to 80% wt %) may be
prepared. [0043] 2) The ratio of the water-soluble polymer to the
metal can be decreased below that in the prior art in spite of much
higher concentration of silver nanoparticles. This is highly
advantageous since it enables production of a more pure product
having low content of organic material which may interfere with the
high conductivity properties. [0044] 3) The metal nanoparticle
dispersions can form, upon drying of droplets, densely packed
rings, which are conductive even at room temperature, without
further heat induced sintering. [0045] 4) The metal nanoparticle
dispersions are prepared by a pre-reduction of a metal salt
(present in the form of aqueous suspension) with a proper water
soluble polymer, which functions as a reducer and a stabilizer,
followed by full reduction (exhaustive reduction) obtained by a
chemical reducer, such as tri-sodium citrate, ascorbic acid,
di-sodium tartrate, hydrazine, sodium borohydride, or mixtures
thereof. This enables production of nanoparticles having high
concentrations in dispersion and small size. [0046] 5) The
preferred silver salt used in the preparation of the nanoparticles
is silver acetate. Possibly due to: a) the action of acetate ion in
aggregation of the nanoparticles. This allows easy separation of
nanoparticles from the aqueous medium at the end of the reaction
process (after full reduction with the chemical reducer). b) due to
the low solubility of the silver acetate salt, the silver ion
concentration is kept below silver salt saturation value (for
example about 2.5 wt % at 95.degree. C.), thus the undissolved
silver acetate serves as reservoir of silver ion. The low
concentration of silver ion enables production of smaller particles
at high metal salt concentrations. [0047] 6) The nanoparticle size
of the obtained dispersions after separation is preferably below 20
nm and can be as low as 5-8 nm. Due to the smaller particle size
the sedimentation rate is very slow, and is hindered by the
Brownian motion. This is advantageous when long term stability is
required. Additionally the sintering temperature can be lowered
compared to particles having a larger size.
[0048] Thus, the present invention is based on the findings that it
is possible to obtain aqueous-based dispersions of metal
nanoparticles by a new method which comprises metal ions reduction
in an aqueous suspension of metal salt, using water soluble
polymers which are capable of metal reduction followed by full
reduction using a chemical reducer. The method includes two-step
reduction, first with a water soluble polymer to obtain metal
nuclei, and then exhaustive reduction with a chemical reducer to
form nanoparticles in dispersion.
[0049] The new method of preparation enables formation of
physically stable dispersions (i.e. which do not undergo caking and
agglomeration). Separation step may be very simple due to
spontaneous formation of a sediment as a result of nanoparticle
aggregation. Therefore centrifugation step may be omitted. The new
method enables formation of aggregated nanoparticles which can be
easily separated from the aqueous medium and redispersed. The
formed sediment can be easily redispersed in a liquid (after
separating from the aqueous medium) by using a suitable dispersing
agent to form a stable and more concentrated dispersion.
[0050] Thus, the formed nanoparticles in the dispersion may be in
an aggregated form (i.e. the nanoparticles maybe partially or
mostly in an aggregated form).
[0051] The method utilizes metal salts (preferably silver salts)
having low solubility in water (preferably up to 5% w/w, at a
temperature of 100.degree. C.) which results in low concentration
of metal ions in the solution phase of the reaction mixture. At
these conditions, small metallic nanoparticles are formed even at
low concentrations of water soluble polymer (stabilizer).
[0052] This method allows obtaining much higher concentration of
metallic silver, at low stabilizer:silver ratio, after completion
of reduction with a chemical reducer compared to all known
procedures. This is highly advantageous since a more pure
dispersion is obtained at the end of the process. This is important
for example in applications where formation of conductive patterns
is required.
[0053] The weight ratio of the water soluble polymer (stabilizing
polymer, protective agent) to metal that can be used according to
this new method is much lower than in all known procedures, and may
be only 0.01:1.
[0054] This is highly advantageous especially when low viscosity
aqueous dispersion are required and if direct contact between the
particles is required after application, for example, electrical
conductivity and metallic appearance.
[0055] The new method enables to obtain nanodispersions or
nanopowders (after the separation step) with organic: metal
(preferably silver) weight ratio below 0.07:1 and this ratio can be
as low as 0.03:1-0.05:1. Therefore, the obtained product is more
pure and can be successfully used, for example, for formation of
conductive patterns (due to low content of insulating organic
material).
[0056] The size (diameter) of silver nanoparticles may be as low as
5-8 nm.
[0057] Rings produced by depositing drops of the obtained silver
dispersion onto a substrate display high electric conductivity (up
to 15% of that for bulk silver) at room temperature, without
sintering at elevated temperatures. Various types of conductive
patterns can be obtained by deposition of arrays of said rings by
various means such as ink jet printing.
[0058] Thus, the present invention relates to a method for
preparing an aqueous-based dispersion of metal nanoparticles
comprising: [0059] (a) providing an aqueous suspension of a metal
salt; [0060] (b) pre-reducing of said metal salt suspension by a
water soluble polymer capable of metal reduction to form metal
nuclei; and [0061] (c) adding a chemical reducer to form metal
nanoparticles in dispersion.
[0062] The term "aqueous-based" as used herein, means that the
dispersing medium of the dispersion comprises either water or an
aqueous liquid or solution. Most preferably, the aqueous medium
(dispersing medium) is all water, however the dispersing medium may
also contain small amounts (preferably up to about 25 wt %, based
on the total weight of the dispersing medium) of organic solvents
which are miscible with water.
[0063] By the term "pre-reducing" in step (b) is meant that the
water soluble polymer initiates metal reduction and reduces part
the metal ions in the aqueous suspension.
[0064] Full reduction of the remaining metal ions is obtained in
step (c).
[0065] By the term "metal nuclei" is meant an intermediate
nanoparticle, wherein the average size of said nuclei is below the
average size of the nanoparticles obtained in step (c).
[0066] The dispersion preparation may be also conducted by double
jet method. (consisting of mixing two jets: of the dispersion
obtained from step (b) and the chemical reducer from step
(c).).
[0067] The method may further comprise at least one step (i.e. a
step or repeated steps) of separating the nanoparticles obtained in
step (c) from the aqueous medium of said dispersion and
redispersing in a liquid to form a dispersion of nanoparticles.
[0068] Thus, according to a preferred embodiment of the present
invention the method comprises: [0069] (a) providing an aqueous
suspension of a metal salt; [0070] (b) pre-reducing of said metal
salt suspension by a water soluble polymer capable of metal
reduction to form metal nuclei; [0071] (c) adding a chemical
reducer to form metal nanoparticles in dispersion; and [0072] (d)
at least one step of separating the nanoparticles obtained in step
(c) from the aqueous medium of said dispersion and redispersing in
a liquid to form a dispersion of nanoparticles.
[0073] According to a preferred embodiment of the present
invention, the separation is selected from centrifugation,
decantation, filtration, ultrafiltration, and a combination
thereof.
[0074] Further according to a preferred embodiment of the present
invention, the redispersing is performed using a suitable
dispersing agent and optionally a wetting agent. The wetting agent
may be added before or after the separation, preferably before the
separation.
[0075] Preferably the dispersing agent is a water soluble
dispersant.
[0076] Still further according to a preferred embodiment of the
present invention, the dispersing agent is selected from
surfactants, water soluble polymers, and mixtures of any of the
above.
[0077] Additionally according to a preferred embodiment of the
present invention, the water soluble polymer is a
polyelectrolyte.
[0078] Further according to a preferred embodiment of the present
invention, the polyelectrolyte (dispersing agent) is selected from
Disperbyk 190, Solsperse 40000, and mixtures of any of the
above.
[0079] Disperbyk 190 is a High-molecular-weight block copolymer
with acidic affinic groups (acid value 10 mg KOH g.sup.-1), which
can be obtained from BYK Chemie Germany.
[0080] Solsperse 40000 is a Water-soluble anionic phosphated
alkoxylated polymer, which can be obtained from Avecia,
England.
[0081] The wetting agent may be a surfactant. The surfactant may be
for example BYK-154, BYK-348, Disperbyk 181, Disperbyk 184, LABS
(such as LABS-W-100), and LABS salts, and mixtures of any of the
above.
[0082] BYK-154 is an Ammonium salt of an acrylate copolymer, which
can be obtained from BYK Chemie, Germany.
[0083] BYK-348 is a Polyether modified poly-dimethyl siloxane,
which can be obtained from BYK Chemie, Germany.
[0084] Disperbyk 181 is an Alkanolammomium salt of a polyfunctional
polymer (acid value 30 mg KOH g.sup.-1), which can be obtained from
BYK Chemie, Germany. Disperbyk 184 is a High-molecular-weight block
copolymer with pigment affinic groups (acid value 10 mg KOH
g.sup.-1), which can be obtained from BYK Chemie, Germany.
[0085] LABS is a Linear alkyl benzene sulphonic acid which may have
different chain length.
[0086] LABS-W-100 is a Linear alkyl benzene sulphonic acid, which
can be obtained from Zohar-Dalia, Israel.
[0087] The wetting agent may be for example a surfactant.
[0088] Preferably the liquid is an aqueous liquid (the liquid used
for redispersing of the nanoparticles after separation from the
aqueous medium).
[0089] The method may further comprise at least one step (i.e. a
step or repeated steps) of separating the nanoparticles obtained in
step (c) from the aqueous medium of said dispersion followed by
removal of the water in order to obtain a powder of metallic
particles. The removal of the water can be obtained by various
methods such as loyphilization, spray drying, oven drying, vacuum
drying etc. Prior to removal of the aqueous phase (medium)
redispersing agents may be added such as wetting agents, dispersant
etc.
[0090] The powder may be further redispersed in a liquid such as an
aqueous liquid or non-aqueous liquid (such as organic solvents,
oils etc.).
[0091] Preferably the obtained powder of metal nanoparticles is
characterized by a weight ratio of the organic material to the
metal nanoparticles of below 0.1:1, more preferably below 0.07:1,
still more preferably in the range 0.03:1-0.05:1. Such a powder is
capable of redispersing in a liquid (aqueous liquid or non aqueous
liquid such as organic solvents, or mixtures thereof), preferably
without addition of a dispersant. The particle size after
redispersion is preferably less that 20 nm in diameter.
[0092] Preferably step (b) includes incubation for a period of at
least 5 minutes. i.e. the aqueous metal suspension and the water
soluble polymer are incubated for at least 5 minutes to form a
metal nuclei. (Preferably for 5-15 minutes).
[0093] Preferably the aqueous metal suspension and the water
soluble polymer are incubated for a period of at least 5 minutes
(preferably for 5 to 15 min) while stirring.
[0094] According to another preferred embodiment of the present
invention, step (b) is conducted at a temperature range of
20-100.degree. C. More preferably step (b) is conducted at a
temperature range of 50-95.degree. C.
[0095] Additionally according to a preferred embodiment of the
present invention, step (c) is conducted at a temperature range of
20-100.degree. C. More preferably step (c) is conducted at a
temperature range of 50-95.degree. C.
[0096] Preferably step (c) further includes a cooling step.
[0097] Moreover according to a preferred embodiment of the present
invention step (c) is conducted at a temperature range of
20-100.degree. C., more preferably 50-95.degree. C., followed by
cooling to a temperature range 10-30.degree. C., more preferably
15-25.degree. C.
[0098] Further according to a more preferred embodiment of the
present invention, the metal nanoparticles are selected from silver
nanoparticles, gold nanoparticles, platinum nanoparticles,
palladium nanoparticles and a mixture of any of the above.
[0099] Most preferably the metal nanoparticles are silver
nanoparticles.
[0100] The metal salt is preferably a silver salt or a gold salt,
most preferably a silver salt.
[0101] The metal salt may also be a platinum salt or palladium
salt.
[0102] Moreover according to a preferred embodiment of the present
invention, the metal salt have low water solubility.
[0103] Moreover according to a preferred embodiment of the present
invention, the metal salt have a solubility (solubility in water)
of up to 5% w/w at a temperature of 100.degree. C.
[0104] According to an additional preferred embodiment of the
present invention, the metal salt (preferably silver salt) is
selected from silver acetate, silver sulfate, silver carbonate, and
mixtures of any of the above. Most preferably the metal salt is
silver acetate.
[0105] Preferably the metal salt is a metal acetate salt, and most
preferably the metal acetate salt is silver acetate.
[0106] Further according to a preferred embodiment of the present
invention, the content of the metal salt in the suspension is in
the range of 1.0 to 50 wt % (based on the total weight of the
suspension).
[0107] The concentration of said metal salt in said suspension may
be in the range of 15 to 35 wt %, and preferably in the range of 15
to 25 wt % (based on the total weight of the suspension).
[0108] Additionally, according to a preferred embodiment of the
present invention, the concentration of said metal nanoparticles in
said dispersion is in the range 0.5-35 wt %, based on the total
weight of the dispersion.
[0109] The concentration of said metal nanoparticles in said
dispersion (prior to the separation step) may be in the range
1.5-35 wt %, preferably 2-30 wt %, more preferably 3-25 wt %, and
most preferably 5-25 wt %, based on the total weight of the
dispersion. The concentration may also be in the range 3-35 wt %,
preferably 5-35 wt %, more preferably 5-30 wt %, still more
preferably 5-25 wt %, based on the total weight of the
dispersion.
[0110] As used in the present invention by the term particle size
below a certain value in diameter, for example by the term
"particle size of said nanoparticles is below 50 nm in diameter" is
meant that the mean particle diameter according to 90% by number of
the particles (d.sub.90) is under 50 nm, as measured by Dynamic
Light Scattering.
[0111] Similarly, as used herein by the term "particle size of said
nanoparticles is below 20 nm in diameter" is meant that 90% of mean
particle diameter calculated by number is under 20 nm, as measured
by Dynamic Light Scattering.
[0112] The particle size of the nanoparticles may be below 50 nm in
diameter, preferably below 40 nm in diameter. More preferably the
particle size is below 20 nm in diameter, even more preferably
below 18 nm in diameter, still more preferably in the range 5-15 nm
in diameter, and most preferably in the range 5-8 nm in
diameter.
[0113] The nanoparticles of the present invention may be spherical,
rod-like shaped or a combination thereof. Most preferably the
nanoparticles are spherical shaped.
[0114] In the case where the nanoparticles are rod shaped,
preferably the width of said particles is below 20 nm, the length
to width ratio is up to 1:5 (preferably the length to width ratio
in the range of 1:1.2 to 1:3)
[0115] The nanoparticles of the present invention may be multiply
tweened particles (mtp).
[0116] Preferably the multiply twined nanoparticles are capable of
sintering at the temperature range of 90-320.degree. C., more
preferably at the temperature range of 100-160.degree. C.
[0117] The formed nanoparticle dispersion obtained after step (c)
in the method described above, may be in the form of aggregates of
nanoparticles which are physically stable (i.e. do not undergo
caking in case a sediment is formed) and can be easily redispersed
in a liquid following separation from the aqueous dispersion, thus
allowing formation of a more concentrated stable dispersions.
[0118] Further according to a preferred embodiment of the present
invention, the concentration of said metal nanoparticles in the
dispersion (the obtained dispersion after separation step) is in
the range 5-80 wt %, based on the total weight of the dispersion,
and more preferably in the range 10-80 wt %, based on the total
weight of the dispersion. The concentration of said metal
nanoparticles in the dispersion may be in the range 10-60 wt %, and
more preferably in the range 20-60 wt %. The concentration of the
metal nanoparticles in the dispersion may also be in the range
35-80 wt % and more preferably 40-60 wt %. (This preferred
embodiment refers to the dispersion obtained after separating the
nanoparticles obtained in step (c) from the aqueous medium of the
dispersion and redispersing in a liquid to form a dispersion of
nanoparticles.).
[0119] As may be understood by any person skilled in the art, there
is a plurality of water soluble polymers (stabilizers), which are
appropriate for use in the composition (aqueous-based dispersions)
of the present invention and a man versed in the art can select for
appropriate water soluble polymers using the following
criteria:
[0120] Suitable water soluble polymers are those meeting the
following criteria:
[0121] 1) Lack of Gel Formation in the Presence of Metal Ions.
[0122] Water soluble polymers, which do not form a gel at
concentrations required to initiate metal reduction and form a
metal nuclei, are selected. The concentration of the polymers in
the obtained dispersion will depend on the type of the polymer and
can be lower than 0.5 wt % for a polymer such as polypyrole (and
higher up to 10 wt % for a polymer such as Sokolan HP80).
[0123] 2) Stabilization of Metal Nanoparticles.
[0124] Water soluble polymers, which are also capable of
stabilizing the formed metal (such as silver) nuclei were chosen.
Such protective agents are water soluble polymers possessing
electrostatic and steric effects of stabilization.
[0125] After formation of nanoparticles in dispersion, the water
soluble polymer stabilizes that dispersion, such that the
nanoparticles can be easily redispersed (i.e. prevents caking of
the dispersion).
[0126] 3) Pre-Reduction of Metal Ions with Formation of Metal
Nuclei.
[0127] Polymers should pre-reduce metal (such as silver) ions with
formation of metal nuclei, which serve as seeds for following
formation of metal nanoparticles in dispersion after addition of
the main chemical reducer.
[0128] Water soluble polymers, which fulfill all the above
criteria, are chosen to be used in the aqueous dispersions of the
present invention.
[0129] Preferably the water soluble polymer carries functional
groups such as pyrrole, alkoxy, etheric, glycol, hydroxyl, amine
groups, and combinations thereof. Such functional groups are
capable of reducing metal ion. According to a preferred embodiment
of the present invention, the water soluble polymer is selected
from polypyrrole, Sokalan HP80, Solsperse 40000, poly(ethylene
glycol), and mixtures of any of the above.
[0130] Sokalan HP 80 is a Polycarboxylate ether, which can be
obtained from BASF, Germany.
[0131] Solsperse 40000 is a Water-soluble anionic phosphated
alkoxylated polymer, which can be obtained from Avecia,
England.
[0132] Most preferably the water soluble polymer is
polypyrrole.
[0133] Moreover according to a preferred embodiment of the present
invention the metal salt is silver acetate and the water soluble
polymer is polypyrrole.
[0134] Additionally according to a preferred embodiment of the
present invention, the concentration of said water soluble polymer
is in the range of 0.1-10.0 wt %.
[0135] The weight ratio of the water soluble polymer to the metal
may be in the range of 0.01:1 to 1:1. Preferably the weight ratio
of the water soluble polymer to the metal is below 0.1:1
(preferably in the range 0.01:1-0.1:1), more preferably in the
range 0.01:1-0.06:1, even more preferably in the range
0.01:1-0.04:1, and most preferably in the range 0.01:1-0.025:1.
[0136] In case the water soluble polymer is polypyrole, the
preferred concentration range is 0.1-1.0 wt %.
[0137] In case the water soluble polymer is Sokalan HP80, the
preferred concentration range is 5.0-10.0 wt %.
[0138] Further according to a preferred embodiment of the present
invention, the chemical reducer is selected from tri-sodium
citrate, ascorbic acid, di-sodium tartrate, hydrazine, sodium
borohydride, and mixtures of any of the above. Most preferably the
chemical reducers are ascorbic acid and hydrazine.
[0139] Preferably the method further comprises adding a colorant to
the dispersion.
[0140] The method may further comprise adding to the dispersion an
additive selected from humectants, binders, surfactants,
fungicides, rheology modifiers, pH adjusting agents, co-solvents,
and mixtures thereof.
[0141] Preferably the aqueous-based dispersion is useful in
preparing ink compositions, paints, or coatings.
[0142] Preferably the ink composition is for use in ink jet
printing.
[0143] The aqueous-based dispersion may be used in coating
compositions to provide for example an optical effect on a
substrate.
[0144] Moreover according to a more preferred embodiment of the
present invention, the dispersion is for use in obtaining
conductive patterns by deposition of the dispersion on a substrate
and optionally followed by sintering.
[0145] In case conductive rings are obtained as will be detailed
below the step of sintering can be omitted.
[0146] Further according to a preferred embodiment of the present
invention, the method further comprises placing or jetting drops of
the dispersion as described in the present invention onto a
substrate to obtain conductive rings.
[0147] According to a preferred embodiment of the present invention
the conductive rings have high electrical conductivity at room
temperature.
[0148] Moreover according to a preferred embodiment of the present
invention, the method further comprises dispensing a plurality of
drops of the dispersion as described in the present invention onto
a substrate to form arrays of conductive rings.
[0149] The arrays of conductive rings form a conductive
pattern.
[0150] The substrate may be for example plastics, paper,
photo-paper, films (such as polyimide films), glass or PCB (printed
circuits boards).
[0151] The invention further relates to an aqueous-based dispersion
comprising metal nanoparticles and at least one water soluble
polymer capable of initiating metal reduction, wherein the
concentration of said metal nanoparticles in said dispersion is in
the range 0.5-35 wt % and wherein the size of said nanoparticles is
below 20 nm in diameter.
[0152] Such a dispersion is highly advantageous because of the
combination of high nanoparticle concentration and low particle
size of the nanoparticles provides superior properties to the
dispersion such as high conductivity.
[0153] Moreover, formulating a dispersion characterized by high
nanoparticles concentration and small particle size, and yet which
is physically stable (does not undergo caking and aggregation) is a
formulatory endeavor, and is non-obvious to obtain.
[0154] According to a preferred embodiment of the present invention
the aqueous-based dispersion is a substantially pure aqueous-based
dispersion. By a "substantially pure aqueous-based dispersion" is
meant that the weight ratio of the water soluble polymer to the
metal nanoparticles is preferably below 0.1:1 wt %, and most
preferaby in the range 0.01:1 to 0.025:1 wt %.
[0155] Preferably the aqueous-based dispersion consists essentially
of metal nanoparticles and at least one water soluble polymer
capable of initiating metal reduction, wherein the concentration of
said metal nanoparticles in said dispersion is in the range 0.5-35
wt % and wherein the size of said nanoparticles is below 20 nm in
diameter.
[0156] The present invention additionally relates to an
aqueous-based dispersion comprising metal nanoparticles and at
least one water soluble polymer, said aqueous-based dispersion is
characterized by: [0157] (a) the concentration of said metal
nanoparticles in said dispersion is in the range 0.5-35 wt %;
[0158] (b) the size of said nanoparticles is below 20 nm in
diameter; and [0159] (c) the weight ratio of said water soluble
polymer to said metal nanoparticles is below 0.1:1.
[0160] Preferably the concentration of said metal nanoparticles in
said dispersion is in the range 1.5-35 wt %, more preferably in the
range 2-30 wt %, more preferably in the range 3-25 wt %, and most
preferably in the range 5-20 wt %.
[0161] The concentration may also be in the range 3-35 wt %,
preferably 5-35 wt %, more preferably 5-30 wt %, still more
preferably 5-25 wt %, based on the total weight of the
dispersion.
[0162] According to a preferred embodiment of the present
invention, the metal nanoparticles are selected from silver
nanoparticles, gold nanoparticles, platinum nanoparticles,
palladium nanoparticles and a mixture of any of the above.
[0163] Most preferably the metal nanoparticles are silver
nanoparticles.
[0164] The water soluble polymer is capable of initiating metal
reduction.
[0165] Preferably the water soluble polymer carries functional
groups such as pyrrole, alkoxy, etheric, glycol, hydroxyl, amine
groups, and combinations thereof. Such functional groups are
capable of reducing metal ion.
[0166] Further according to a preferred embodiment of the present
invention, the water soluble polymer is selected from polypyrrole,
Sokalan HP80 (Polycarboxylate ether), Solsperse 40,000
(Water-soluble anionic phosphated alkoxylated polymer),
poly(ethylene glycol), and mixtures of any of the above.
[0167] Most preferably the water soluble polymer is
polypyrrole.
[0168] The water soluble polymer is capable of initiating metal
reduction to form metal nuclei during preparation of the
dispersion. The water soluble polymer also functions as a
stabilizer during preparation of the dispersion and is capable of
preventing metal nuclei aggregation and agglomeration after the
pre-reduction step.
[0169] The water soluble polymer is further characterized in that
it does not form a gel in the presence of metal ions, at
concentrations used to prepare the dispersion.
[0170] Additionally according to a preferred embodiment of the
present invention, the weight ratio of said water soluble polymer
to said nanoparticles is below 0.1:1.
[0171] Preferably the weight ratio of the water soluble polymer to
the nanoparticles is in the range 0.01:1-0.1:1.
[0172] More preferably the weight ratio of said water soluble
polymer to said nanoparticles is in the range 0.01:1-0.06:1, even
more preferably the weight ratio of said water soluble polymer to
said nanoparticles is in the range 0.01:1-0.04:1. Most preferably
the weight ratio of said water soluble polymer to said
nanoparticles is in the range 0.01:1-0.025:1.
[0173] Further according to a preferred embodiment of the present
invention, the size of said nanoparticles is below 18 nm in
diameter.
[0174] Preferably the size of said nanoparticles is in the range
5-15 nm in diameter, more preferably the size of the nanoparticles
is in the range 5-8 nm in diameter.
[0175] The aqueous dispersion may further comprise an organic
solvent. The organic solvent may be for example dipropyleneglycol
methyl ether (DPM), 2-methoxyethyl ether (diglyme),
triethyleneglycol dimethyl ether (triglyme), propylene glycol,
sulfolane, polyethylene glycol, glycerol. The concentration of the
organic solvent may be up to 20 wt %, based on the total weight of
the dispersion.
[0176] Moreover according to a preferred embodiment of the present
invention, the aqueous dispersion is characterized in that the
conductivity of the dispersion deposited onto substrate can be as
high as 50% of that of the bulk metal.
[0177] The invention additionally relates to an aqueous-based
dispersion comprising metal nanoparticles and at least one water
soluble dispersant, wherein the concentration of said metal
nanoparticles in said dispersion is in the range 5-80 wt % and
wherein the size of said nanoparticles is below 20 nm in
diameter.
[0178] Preferably the aqueous-based dispersion is a substantially
pure aqueous-based dispersion. By the term "substantially pure
aqueous-based dispersion" is meant that weight ratio of said water
soluble dispersant to said nanoparticles is below 0.1:1.
[0179] Preferably the aqueous-based dispersion consists essentially
of metal nanoparticles and at least one water soluble dispersant,
wherein the concentration of said metal nanoparticles in said
dispersion is in the range 5-80 wt % and wherein the size of said
nanoparticles is below 20 nm in diameter.
[0180] The present invention additionally relates to an
aqueous-based dispersion comprising metal nanoparticles and at
least one water soluble dispersant, said aqueous-based dispersion
is characterized by: [0181] (a) the concentration of said metal
nanoparticles in said dispersion is in the range 5-80 wt %; [0182]
(b) the size of said nanoparticles is below 20 nm in diameter; and
[0183] (c) the weight ratio of said water dispersant to said metal
nanoparticles is below 0.1:1.
[0184] According to a preferred embodiment of the present
invention, the metal nanoparticles are selected from silver
nanoparticles, gold nanoparticles, platinum nanoparticles,
palladium nanoparticles and mixtures of any of the above.
[0185] Most preferably the metal nanoparticles are silver
nanoparticles.
[0186] Further according to a preferred embodiment of the present
invention, the water soluble dispersant is selected from
surfactants, water soluble polymers, and mixtures of any of the
above.
[0187] Still further according to a preferred embodiment of the
present invention, the water soluble polymer is a
polyelectrolyte.
[0188] Preferably the weight ratio of said water soluble dispersant
to said nanoparticles is below 0.1:1, more preferably below
0.075:1, and most preferably in the range 0.04:1-0.06:1.
[0189] The weight ratio of the water soluble dispersant to the
nanoparticles may also be in the range 0.04:1-0.1:1, and more
preferably in the range 0.04:1-0.075:1.
[0190] Preferably the polyelectrolyte (dispersant) is selected from
Disperbyk 190, Solsperse 40000, and mixtures of any of the
above.
[0191] Moreover according to a more preferred embodiment of the
present invention, the aqueous-based dispersion is characterized in
that the conductivity of the dispersion deposited onto substrate
can be as high as 50% of that of the bulk metal.
[0192] Further according to a preferred embodiment of the present
invention, the size of said nanoparticles is below 18 nm in
diameter.
[0193] Preferably the size of said nanoparticles is in the range
5-15 nm in diameter, more preferably the size of the nanoparticles
is in the range 5-8 nm in diameter.
[0194] The nanoparticles may be spherical, rod-like shaped (as
described above) or a combination thereof. Most preferably the
nanoparticles are spherical shaped.
[0195] The nanoparticles may be multiply tweened particles
(mtp).
[0196] Preferably the multiply twined nanoparticles are capable of
sintering at the temperature range of 90-320.degree. C., more
preferably at the temperature range of 100-160.degree. C.
[0197] The aqueous-based dispersions of the present invention may
further comprise a water soluble metal salt (such as silver
salt).
[0198] The water soluble silver salt may be for example silver
acetate, silver nitrate, silver sulfate, silver carbonate, silver
lactate, silver perchlorate, or mixtures thereof.
[0199] Most preferably the silver salt is silver acetate.
[0200] The silver salt is preferably added to the final dispersion
(at a concentration range of preferably 0.05-5 wt %) to achieve
further increase in conductivity of printed pattern, which
decomposes during sintering that results in formation of metallic
(silver) additive acting as "glue" for sintering silver
nanoparticles.
[0201] The aqueous-based dispersions of the invention comprises an
aqueous medium which can be either water, an aqueous liquid or an
aqueous solution.
[0202] According to additional preferred embodiment of the present
invention, the aqueous-based dispersions of the present invention
further comprising at least one member selected from humectants
(such as dipropyleneglycol methyl ether (DPM), 2-methoxyethyl ether
(diglyme), triethyleneglycol dimethyl ether (triglyme), propylene
glycol, sulfolane, polyethylene glycol, glycerol), binders (such as
polyvinylpyrrolidone (PVP), acrylic resins, acrylic latexes),
surfactants (such as silwet L-77, BYK 348, BYK 346, BYK 333),
fungicides, rheology modifiers (such as colloidal silica, clays,
water soluble polymers), deformers (such as silicon derivatives),
pH adjusting agents (such as acids and bases), and mixtures of any
of the above.
[0203] BYK-333 is a Polyether modified poly-dimethyl polysiloxane,
which can be obtained from BYK Chemie, Germany.
[0204] BYK-346 is a Polyether modified poly-dimethyl-siloxane,
which can be obtained from BYK Chemie, Germany.
[0205] Silwet L-77 is a Polyalkylencoxide modified
Heptamethyltrisiloxane and Allyloxypolyethyleneglycol methyl ether
solution, which can be obtained from Helena Chemical Company,
USA.
[0206] Preferably the aqueous-based dispersions are characterized
by organic material: metal weight ratio of below 0.1:1, more
preferably below 0.07:1. This ratio can be as low as 0.03:1-0.05:1.
Therefore, the obtained product is more pure and can be
successfully used, for example, for formation of conductive
patterns (due to low content of insulating organic material).
[0207] According to another preferred embodiment of the present
invention, the aqueous-based dispersions of the present invention
further comprising a colorant.
[0208] The colorant may be for example organic dye or pigments.
[0209] The present invention additionally provides an ink
composition comprising an aqueous based dispersion as described in
the present invention.
[0210] The ink of the present invention may be characterized by the
following: The metal nanoparticles concentrations in the ink are as
high as 20 wt % if low viscosity ink is required (up to 5 cps) and
can be up to 70-80 wt %, if high viscosity is required (up to 20
cps at jetting temperature).
[0211] Thus, the present invention provides compositions and
methods for preparation of water-based inks (preferably ink jet
inks), in which the pigments are nanoparticles of metal, and
composition and methods for preparing stable, concentrated
dispersions of metallic nanoparticles. The ink composition of the
present invention overcomes a common problem in pigment containing
ink-jet inks, namely sedimentation, since the particle size is very
small, preferably below 20 nm in diameter, thus the sedimentation
rate is very slow, and is hindered by the Brownian motion.
[0212] It should be mentioned that the nanoparticles, due to their
very small size, would behave differently, when compared to large
particles. For example, nanoparticles have a lower melting point
than bulk metal, and a lower sintering temperature than that of
bulk metal. This property is of particular importance when
sintering is needed in order to obtain electrical conductivity.
[0213] It is clear that the metallic patterns obtained by the
aqueous dispersions of the present invention can be used for
decoration purposes, even if the resulting pattern is not
electrically conductive. Another aspect of the invention is that
the resulting pattern of the silver nanoparticles has an
antimicrobial effect, due to the presence of silver nanoparticles,
thus eliminating the need for antimicrobial agents which are often
introduced into water based ink jet inks
[0214] In addition, we recently discovered a new approach to obtain
conductive patterns based on the so called "coffee stain effect"
(Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S.
R.; Witten, T. A. Nature, 1997, 389, 827), which becomes apparent
when a spilled drop of coffee dries on a solid surface. This effect
caused by capillary forces, results in formation of a dense ring
along the perimeter of the drying droplet. We discovered that while
drying droplets of silver dispersion, a very dense ring is formed
at the perimeter of the droplet. This ring is composed of tightly
packed silver nanoparticles, and it was surprisingly found that
high electric conductivity of this ring is obtained even at room
temperature.
[0215] Further, the present invention provides conductive rings
produced by placing or jetting drops of a dispersion as described
in the present invention onto a substrate.
[0216] According to a preferred embodiment of the present invention
the conductive rings have high electrical conductivity at room
temperature.
[0217] Moreover, the present invention provides conductive patterns
obtained by dispensing a plurality of drops of a dispersion as
described in the present invention onto a substrate to form arrays
of conductive rings.
[0218] Ink jet printing of conductive patterns by placing or
jetting of dispersion droplets on a proper substrate may be applied
in microelectronic industry.
[0219] Patterns can be used in microelectronics, for smart card
obtaining, decorative coatings.
[0220] Preferably the high electrical conductivity is in the range
of 5-50% of bulk silver for printed patterns (after sintering at
150-320.degree. C.), in the range of 10-15% of bulk silver for
deposited rings at room temperature and in the range of 15-50% of
bulk silver for deposited rings (after sintering at 150-320.degree.
C.).
[0221] The present invention further provides a powder of metal
nanoparticles characterized by a weight ratio of the organic
material to the metal nanoparticles of below 0.1:1, more preferably
below 0.07:1, still more preferably in the range 0.03:1-0.05:1.
Such a powder is capable of redispersing in a liquid (aqueous
liquid or non aqueous liquid such as organic solvents, or mixtures
thereof), preferably without addition of a dispersant. The particle
size after redispersion is preferably less that 20 nm in
diameter.
[0222] Thus a solvent-based dispersion can be obtained by
dispersing the powder in a solvent or a solvent mixture. The
dispersion may optionally include binders, surfactants and rheology
modifiers etc. and may be for use in ink jet inks
Preparation of Nanoparticles and Dispersions
[0223] Fine metal particles from micrometer to nanometer size can
be synthesized by both physical methods (formation in gas phase,
laser ablation) and chemical methods (sonochemical or photochemical
reduction, electrochemical synthesis, chemical reduction), as are
known in the art. The former methods provide fine metal particles
by decreasing the size by applying energy to the bulk metal, while
in the latter methods, fine particles are produced by increasing
the size from metal atoms obtained by reduction of metal ions in
solution.
[0224] In the present invention, the chemical method for the
preparation of silver nanoparticles is preferably employed, namely,
fine particles were produced by reduction of silver salt in a
solution or a suspension with the use of a proper reducing agent
according to the following scheme:
Me.sup.n++nRed.fwdarw.Me.sup.o+nOx.sup.+
[0225] Two step reduction was employed, first with a water soluble
polymer and second with a chemical reducer.
[0226] Silver nanoparticles can be prepared with the use of various
reducing agents (chemical reducers), such as sodium borohydride,
trisodium citrate, hydrazine, ascorbic acid, sugars and gaseous
hydrogen.
[0227] Two principal stages are included in the procedure of
preparation of concentrated and stable silver nanodispersions: a)
synthesis step: pre-reduction of a silver salt by a water soluble
polymer (synthetic or natural polymer), which is also a stabilizing
agent, resulting in formation of silver nuclei; such nuclei serve
as seeds for formation of silver nanoparticles (which can be in an
aggregated form in dispersion).
[0228] after addition of a proper chemical reducer; b) separation
and concentration step: centrifugation is followed by decantation
and redispersion of formed the silver nanoparticles in a proper
dispersing medium. Such a method allows preparation of water-based
nanodispersions with silver concentration as high as 10-80 wt %.
The separation step can be also performed by an ultrafiltration
process. The water from the dispersion can be further removed (by
lyophiliztion, spray drying, vacuum drying oven drying etc.) and
the obtained powder can be redispersed again in a small volume of
water or organic solvent that results in formation of highly
concentrated silver nanodispersion. The advantage of such
dispersion is the low content of organic materials. Using the
present invention, conductive patterns with conductivity of about
50% of the conductivity of bulk silver, can be obtained.
EXAMPLES
1. Preparation of Silver Nanodispersions Via Silver Salt
Suspension
Examples 1-3
Materials and Reagents
[0229] Polypyrrole, 5% aqueous solution (PPy) Ascorbic acid Silver
acetate (AgAc) Dispersing agent Solsperse 40,000 (Avecia, England)
Triple distilled water (TDW)
TABLE-US-00001 Silver acetate 99% Sigma- CAS 563-63-3 Aldrich PPy
(Polypyrrole) Doped, 5 wt % Aldrich CAS 30604-81-0 solution in
water Acsorbic acid 99% Sigma CAS 50-81-7 Solsperse 40,000 84.1%
Avecia
Instruments
[0230] Hot plate with a stirrer Centrifuge (Sorvall superspeed
RC2-B) Ultrasound bath (42 kHz) DSL (Dynamic Light scattering)
(Malvern HPPS/NanoSizer) Oven for heating at 600.degree. C.
Stock Solutions
[0231] Ascorbic acid 30 wt %
Solsperse 40000 5 wt %
Example 1
Procedure
Nucleation Step:
[0232] 1 g of AgAc was added to 10.605 ml of TDW in a 28 ml vial.
The vial was heated in a hot bath to 95.degree. C. while stirring.
After 5 min of stirring, 0.32 g of Ppy (5 wt %) was added.
Reaction:
[0233] 15 min after addition of PPy, 0.865 g of ascorbic acid (30
wt %) was added, and reaction mixture was heated at 95.degree. C.
for 5 min while stirring and then was cooled in ice bath. A
spontaneous formation of sediment is obtained as a result of
nanoparticles aggregation.
Separation Process:
[0234] Cold Ag dispersion was centrifuged for 10 min at 5000 rpm,
and all the supernatant liquid was decanted. 0.114 g (0.12 ml) of
30% Solsperse 40,000 was added to the rest. The resulting
dispersion was treated in ultrasonic bath for 10 min and
vortexed.
Mass Balance: (in the Reaction)
[0235] Silver concentration: 5 wt % PPy concentration relative to
silver: 2.5 wt % Solsperse 40000 concentration relative to silver:
5.7 wt %
Characteristics of Obtained Dispersion:
Silver Concentration:
[0236] A precise amount of silver dispersion was placed in glass
vial and heated at 600.degree. C. for 30 min. Silver content in
obtained dispersion was found to be 6.4 wt %.
Yield:
[0237] The silver yield is 95.2%.
Particle Size:
[0238] See Table 1 (measured by DLS).
Example 2
Procedure
Nucleation:
[0239] 2 g of AgAc was added to 8.45 ml of TDW in a 28 ml vial. The
vial was heated in a hot bath to 95.degree. C. while stirring.
After 5 min of stirring, 0.64 g of Ppy (5 wt %) was added.
Reaction:
[0240] 15 min after addition of PPy, 1.73 g of ascorbic acid (30 wt
%) was added, and reaction mixture was heated at 95.degree. C. for
5 min while stirring and then was cooled in ice bath. A spontaneous
formation of sediment is obtained as a result of nanoparticles
aggregation.
Separation Process:
[0241] Cold Ag dispersion was centrifuged for 10 min at 5000 rpm,
and all the supernatant liquid was decanted. 0.228 g (0.12 ml) of
30% Solsperse 40,000 was added to the rest. The resulting
dispersion was treated in ultrasonic bath for 10 min and
vortexed.
Mass Balance: (in the Reaction)
[0242] Silver concentration: 10 wt % PPy concentration relative to
silver: 2.5 wt % Solsperse 40000 concentration relative to silver:
5.7 wt %
Characteristics of Obtained Dispersion:
Silver Concentration:
[0243] A precise amount of silver dispersion was placed in glass
vial and heated at 600.degree. C. for 30 min. Silver content in
obtained dispersion was found to be 14.05 wt %.
Yield:
[0244] The silver yield is 97.3%.
Particle Size:
[0245] See Table 1 (measured by DLS).
Example 3
Procedure
Nucleation:
[0246] 2 g of AgAc was added to 3.415 ml of TDW in a 28 ml vial.
The vial was heated in a hot bath to 95.degree. C. while stirring.
After 5 min of stirring, 0.256 g of Ppy (5 wt %) was added.
Reaction:
[0247] 15 min after addition of PPy, 1.73 g of ascorbic acid (30 wt
%) was added, and reaction mixture was heated at 95.degree. C. for
5 min while stirring and then was cooled in ice bath. A spontaneous
formation of sediment is obtained as a result of nanoparticles
aggregation.
Separation Process:
[0248] Cold Ag dispersion was centrifuged for 10 min at 5000 rpm,
and all the supernatant liquid was decanted. 0.228 g (0.12 ml) of
30% Solsperse 40,000 was added to the rest. The resulting
dispersion was treated in ultrasonic bath for 10 min and
vortexed.
Mass Balance: (in the Reaction)
[0249] Silver concentration: 17.3 wt % PPy concentration relative
to silver: 1 wt % Solsperse 40000 concentration relative to silver:
5.7 wt %
Characteristics of Obtained Dispersion:
Silver Concentration:
[0250] A precise amount of silver dispersion was placed in glass
vial and heated at 600.degree. C. for 30 min. Silver content in
obtained dispersion was found to be 18 wt %.
Yield:
[0251] The silver yield is more than 97.3%.
Particle Size:
[0252] See Table 1 (measured by DLS).
Examples 4-5
Materials and Reagents
Sokalan HP80
[0253] Ascorbic acid Silver acetate (AgAc) Dispersing agent
Solsperse 40,000 (Avecia, England) Triple distilled water (TDW)
TABLE-US-00002 Silver acetate 99% Sigma- CAS 563-63-3 Aldrich
Sokalan HP80 40 wt % BASF solution in water Acsorbic acid 99% Sigma
CAS 50-81-7 Solsperse 40,000 84.1% Avecia
Instruments:
[0254] Hot plate with a stirrer Centrifuge (Sorvall superspeed
RC2-B) Ultrasound bath (42 kHz) DSL (Dynamic Light scattering)
(Malvern HPPS/NanoSizer) Oven for heating at 600.degree. C.
Stock Solutions:
[0255] Ascorbic acid 15 wt % Ascorbic acid 30 wt %
Solsperse 40000 5 wt %
Sokalan HP80 50 wt %
Example 4
Procedure
Nucleation Step:
[0256] 1 g of AgAc was added to 5.3 ml of TDW in a 28 ml vial. The
vial was heated in a hot bath to 95.degree. C. while stirring.
After 5 min of stirring, 3.26 g of Sokalan HP80, 50 wt %, was
added.
Reaction:
[0257] 5 min after addition of Sokalan HP80, 3.46 g of ascorbic
acid (15 wt %) was added, and reaction mixture was heated at
95.degree. C. for 5 min while stirring and then was cooled in the
ice bath.
Separation Process:
[0258] Cold Ag dispersion was centrifuged for 10 min at 5000 rpm,
and all the supernatant liquid was decanted. 0.127 g (0.12 ml) of
30% Solsperse 40,000 was added to the rest. The resulting
dispersion was treated in ultrasonic bath for 10 min and
vortexed.
Mass Balance: (in the Reaction)
[0259] Silver concentration: 5 wt % Sokalan HP80 concentration
relative to silver: 100 wt % Solsperse 40000 concentration relative
to silver: 7.4 wt %
Characteristics of Obtained Dispersion:
Silver Concentration:
[0260] A precise amount of silver dispersion was placed in glass
vial and heated at 600.degree. C. for 30 min. Silver content in
obtained dispersion was found to be 2.87 wt %.
Yield:
[0261] The silver yield is 67.6%.
Particle Size:
[0262] See Table 1 (measured by DLS).
Example 5
Procedure
Nucleation:
[0263] 2 g of AgAc was added to 4.3 ml of TDW in a 28 ml vial. The
vial was heated in a hot bath to 95.degree. C. while stirring.
After 5 min of stirring, 3.26 g of Sokalan HP80, 50 wt %, was
added.
Reaction:
[0264] 5 min after addition of Sokalan HP80, 3.46 g of ascorbic
acid (15 wt %) was added, and reaction mixture was heated at
95.degree. C. for 5 min while stirring and then was cooled in the
ice bath.
Separation Process:
[0265] Cold Ag dispersion was centrifuged for 10 min at 5000 rpm,
and all the supernatant liquid was decanted. 0.22 g (0.21 ml) of
30% Solsperse 40,000 was added to the rest. The resulting
dispersion was treated in ultrasonic bath for 10 min and
vortexed.
Mass Balance: (in the Reaction)
[0266] Silver concentration: 10 wt % PPy concentration relative to
silver: 100 wt % Solsperse 40000 concentration relative to silver:
.about.7 wt %
[0267] Characteristics of obtained dispersion:
Particle Size:
[0268] See Table 1 (measured by DLS).
TABLE-US-00003 TABLE 1 Particle size as measured by Dynamic Light
Scattering (DLS) Silver concentration Stabilizer/silver in the
reaction ratio in the d.sub.90 d.sub.95 Example (wt %) reaction
(wt/wt) (nm) (nm) 1 5 1/40 4.2 4.85 2 10 1/40 15.7 18.2 3 17.3
1/100 15.7 18.2 4 5 1/1 6.5 6.5 5 10 1/1 4.85 5.6 Values in nm,
represent mean diameter particle size. d.sub.90 means that 90% of
mean particle diameter calculated by number is below the indicated
value. d.sub.95 means that means that 95% of mean particle diameter
calculated by number is below the indicated value.
2. Preparation of Silver Nanopowder
[0269] The obtained concentrated silver nanodispersion can be
further lyophilized to yield a powder, optionally in the presence
of a wetting agent (which optionally is added before
lyphilization). This powder can be easily redispersed in water, to
yield a much more concentrated silver nanodispersion, up to 20-80
wt % of silver without change in the average particle size of
silver nanoparticles compared to original dispersion (FIG. 1, right
(FIG. 1B)).
3. Preparation of Ink-Jet Inks Containing Silver Nanoparticles
[0270] The suitability of prepared silver nanodispersions as
pigments for ink jet inks was evaluated with the use of Lexmark
Z602 ink jet printer. Several ink jet formulations are described in
the following examples. Each formulation was capable of printing.
Printing was performed on various substrates, such as paper,
photo-paper, polyimide films, transparency, glass and PCB (printed
circuits boards). In general, the new ink jet ink contains the
silver nanoparticles, and aqueous solution which may contain
surfactants, additional polymers, humectants, cosolvents, buffering
agent, antimicrobial agents and defoamers in order to ensure proper
jetting and adhesion of the ink to specific substrates. FIG. 2
presents an example of silver electrodes pattern printed onto
polyimide film (ink formulation contains 8 wt % silver, 0.6 wt %
Disperbyk 190 as a dispersing agent and 0.5% BYK 348 as a wetting
agent). On the left side, the part of the line (12 mm length, 1.5
mm width, 3.5 .mu.m thickness), on which the conductivity was
measured, is shown. It should be emphasized that that printer
requires inks with very low viscosities, a few cps. However,
industrial printhead such as those produced by Spectra, are
functional at viscosities as high as 15-20 cps. Therefore, for such
printheads more concentrated dispersions of silver nanoparticles
can be utilized. A silver dispersion having a silver content higher
than 20% (up to about 80% w/w) can be prepared by redispersion the
silver nanoparticles powder in a proper amount of aqueous
phase.
Examples for Ink Compositions
Example 1
[0271] Silver nanodispersion (8 wt %) prepared as described above,
containing 0.2 wt % BYK 346 and 5 wt % DPM.
Example 2
[0272] Silver nanodispersion (8 wt %) and containing 0.5 wt % BYK
346 and 10 wt % DPM.
Example 3
[0273] Silver nanodispersion (8 wt %) with 0.2 wt % BYK 346 and 20
wt % DPM.
Example 4
[0274] Silver nanodispersion (8 wt %) with 0.5 wt % BYK 346 and 15
wt % DPM.
Example 5
[0275] Silver nanodispersion (8 wt %) with 0.5 wt % BYK 346 and 5
wt % DPM.
Example 6
[0276] Silver nanodispersion (8 wt %) with 1 wt % BYK 346 and 10 wt
% DPM.
Example 7
[0277] Silver nanodispersion (8 wt %) with 0.2 wt % BYK 346.
Example 8
[0278] Silver nanodispersion (8 wt %) with 0.2 wt % BYK 348.
Example 9
[0279] Silver nanodispersion (8 wt %) with 5 wt % DPM.
Example 10
[0280] Silver nanodispersion (8 wt %) with 0.5 wt % BYK 348.
Example 11
[0281] Silver nanodispersion (8 wt %) with 0.5 wt % BYK 348 and 5
wt % Diglyme.
Example 12
[0282] Silver nanodispersion (8 wt %) with 0.5 wt % BYK 348 and 5
wt % Triglyme.
Example 13
[0283] Silver nanodispersion (8 wt %) with 0.5 wt % BYK 348 and 5
wt % Propylene glycol.
Example 14
[0284] Silver nanodispersion (8 wt %) with 0.5 wt % BYK 348 and 5
wt % Polyethylene glycole 200.
Example 15
[0285] Silver nanodispersion (8 wt %) with 0.5 wt % BYK 348 and 5
wt % Glycerol.
Example 16
[0286] Silver nanodispersion (8 wt %) with 0.5 wt % BYK 348 and 0.2
wt % PVP (polyvinylpyrollidone) 10,000.
Example 17
[0287] Silver nanodispersion (8 wt %) with 0.5 wt % BYK 348 and 0.2
wt % PVP 40,000.
Example 18
[0288] Silver nanodispersion (8 wt %) with 0.5 wt % BYK 348 and 0.2
wt % PVP 55,000.
Example 19
[0289] Silver nanodispersion (8 wt %) with 0.5 wt % BYK 348 and 0.1
wt % PVP 10,000.
Example 20
[0290] Silver nanodispersion (8 wt %) with 0.5 wt % Sulfolane.
Example 21
[0291] Silver nanodispersion (25 wt %) with 0.05 wt % BYK 348.
Example 22
[0292] Silver nanodispersion (25 wt %) with 0.1 wt % BYK 348.
Example 23
[0293] Silver nanodispersion (37 wt %) with 0.05 wt % BYK 348.
Example 24
[0294] Silver nanodispersion (37 wt %) with 0.1 wt % BYK 348.
Example 25
[0295] Silver nanodispersion (25 wt %) with 0.1 wt % BYK 348 and
0.2 wt % PVP 40,000.
Example 26
[0296] Silver nanodispersion (35 wt %) with 0.4 wt % silver
acetate.
Example 27
[0297] Silver nanodispersion (35 wt %) with 1.0 wt % silver
acetate.
4. Obtaining the Conductive Patterns
[0298] The conductive patterns can be obtained either by the direct
printing (that can be repeated for several times) followed by
sintering at a proper temperature (not higher than 320.degree. C.)
or/and by using the first metallic pattern to induce formation of
additional metal layers, such as encountered in "electroless
process". To improve the interconnection between nanoparticles and
to increase the conductivity, a decomposable silver salt, such as
silver acetate or silver nitrate, silver sulfate, silver carbonate
and silver lactate, silver perchlorate can be added to the ink
formulation. Printing may be also followed by additional dipping in
electroless bath, or by printing the electroless solution onto the
printed pattern. Actually, the printed nanoparticles can be used as
templates for further crystallization and precipitation of other
materials.
[0299] It has been found that the use of formulations described in
Examples 1, 6, 10 and 16, as ink jet inks, allows obtaining printed
silver patterns, which were characterized by electric conductivity
(the resistance of lines of 12 mm length, 1.5 mm width and 3.5-5
.mu.m thickness printed 1 to 10 times, was measured). The
conductivity was shown to increase with the increase in the number
of printed layers as well as with the increase in sintering
temperature (Table 2).
[0300] Further increase in conductivity of printed pattern can be
achieved by addition of silver acetate to the final dispersion,
which decomposes during sintering that results in formation of
metallic (silver) additive acting as "glue" for sintering the
silver nanoparticles.
[0301] 40 .mu.l of formulation of Example 10 was spread and dried
on glass slide. Then the silver strip (70 mm length and 7 mm width)
was sintered at 150.degree. C. and 320.degree. C. It has been found
that addition of silver acetate to the ink formulation results in
decrease in resistance of silver strip from 9.3 to 7.0.OMEGA. at
150.degree. C. and from 1.4 to 1.1.OMEGA. at 320.degree. C.
[0302] To observe the changes in the silver layer after sintering,
we viewed silver dispersion deposited onto glass slides, dried and
heated at various temperatures (60.degree. C., 150.degree. C.,
260.degree. C., 320.degree. C.), by High Resolution SEM (FIG. 3).
At 320.degree. C. the electric conductivity can reach about 50% of
that for the bulk metal (FIG. 4). The lower conductivity of printed
lines compared to that of the deposited lines may result from
defects and voids in the printed pattern.
TABLE-US-00004 TABLE 2 Resistance of silver lines (15 mm length,
1.5 mm width) printed onto polyimide films. Sintering Number of
temperature Sintering Resistance of Example No. printings (.degree.
C.) time (min) printed line (.OMEGA.) 1 5 320.degree. 10 10 1 10
320.degree. 10 1.9 6 10 320.degree. 10 7.6 10 10 150.degree. 240
4.8 10 10 200.degree. 60 4.0 10 10 250.degree. 60 2.4 10 1
320.degree. 10 252 10 10 320.degree. 10 2.6 16 10 150.degree. 240
7.7 16 10 200.degree. 60 4.3 16 10 250.degree. 60 3.2 16 1
320.degree. 10 73.3 16 10 320.degree. 10 2.4
[0303] Formation of conductive rings while drying the drops of
silver dispersion is another approach to obtaining the conductive
patterns. It was found that during drying of individual drop of the
silver dispersion of nanoparticles, a dense ring is formed at its
perimeter.
[0304] The ring preparation was performed as follows. A dispersion
of silver nanoparticles containing 8 wt % of metal and 0.1 wt % of
PPy was diluted 200 times, and the resulting concentrations of Ag
and PPy were 0.04 and 0.0005 wt %, respectively. Then a drop of
this dispersion (3 .mu.l) was placed on glass slide and dried. The
ring formed after drying the drop was shown to be composed of
closely packed silver nanoparticles (FIG. 5). Such rings were shown
to possess high electric conductivity (up to 15% of that for bulk
silver) already at room temperature without any additional
treatment (e.g. sintering).
[0305] While this invention has been shown and described with
reference to preferred embodiments thereof, it will be understood
by those skilled in the art that many alternatives, modifications
and variations may be made thereto without departing from the
spirit and scope of the invention. Accordingly, it is intended to
embrace all such alternatives, modifications and variations that
fall within the spirit and broad scope of the appended claims.
[0306] All publications, patents and patent applications mentioned
in this specification are herein incorporated in their entirety by
reference into the specification, to the same extent as if each
individual publication, patent or patent application was
specifically and individually indicated to be incorporated herein
by reference.
* * * * *