Nanometric single-crystal silver pastes

Abstract

A conductive paste of nanometric silver particles, the paste comprising: (a) a carrier liquid; (b) a plurality of nanometric silver particles, in which at least a portion of the nanometric silver particles are single crystals, the plurality of nanometric silver particles having an average secondary particle size (at least one of d50 and D50) within a range of 60 to 200 nanometers, the nanometric silver particles disposed within the carrier liquid; and (c) at least one dispersant; wherein a concentration of the nanometric silver particles within the conductive paste is within a range of 75% to 92%, by weight.

Description

[0001] NANOMETRIC SINGLE-CRYSTAL SILVER PASTES

[0002] FIELD AND BACKGROUND OF THE INVENTION

[0003] The present invention relates to nanometric silver pastes, and to methods of producing such pastes.

[0004] Various commercially available silver pastes typically contain an appreciable proportion of non-nanometric silver particles, particularly at high silver loadings.

[0005] The present inventors have recognized a need for improved silver pastes, having high silver loadings, as well as a substantial content of single crystal nanoparticles.

[0006] SUMMARY OF THE INVENTION

[0007] According to aspects of the present invention there is provided a conductive paste comprising: (a) a carrier liquid; (b) a plurality of nanometric silver particles, in which at least a portion of the nanometric silver particles are single crystals, the plurality of nanometric silver particles having an average secondary particle size (at least one of dso and D50) within a range of 60 to 200 nanometers, the nanometric silver particles disposed within the carrier liquid; and (c) at least one dispersant; wherein a concentration of the nanometric silver particles within the conductive paste is within a range of 75% to 92%, by weight.

[0008] Other aspects of the invention are disclosed hereinbelow.

[0009] BRIEF DESCRIPTION OF THE FIGURES

[0010] The foregoing discussion will be understood more readily from the following detailed description of the invention, when taken in conjunction with the accompanying Figures, in which:

[0011] Figure 1 is a High-Resolution Scanning Electron Microscopy (HRSEM) image showing a typical field containing nanometric silver particles produced according to Comparative Example 19;

[0012] Figure 2 is an HRSEM image showing a typical field containing nanometric silver particles produced according to an embodiment of the present invention, described in Example 20;

[0013] Figure 3 is an HRSEM image showing a typical field containing nanometric silver particles produced according to an embodiment of the present invention, described in Example 21;

[0014] Figure 4 is an HRSEM image showing the field of Figure 1, along with markings illustrating the measurement of the longest diameter of the nano-particles;

[0015] Figure 5 is an HRSEM image showing a portion of the field of Figures 1 and 4, in which the diameters of the nano-particles have been measured; and

[0016] Figure 6 is a plot of a cumulative, number-based Particle Size Distribution based on the calculations of the particle diameters in the image of Figure 5.

[0017] DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] The principles of the inventive nanometric silver particle pastes, and the inventive methods of producing such pastes, may be better understood with reference to the drawings and the accompanying description.

[0019] Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting.

[0020] The manufacturing of electronic devices requires conductive lines or traces. The trend to make smaller devices with more functionalities leads to the need to make narrower and thinner patterns without detracting from the electrical properties. And at the same time, the industry is under ever-increasing pressure to lower production costs (e.g., by consuming less metal, attaining higher throughputs, etc.).

[0021] For example: solar cells require narrower patterns so as to increase cell efficiency, reduce metal consumption, while lowering contact resistance; PCBs require narrower and thinner patterns at lower costs without compromising the electrical properties; the automotive industry requires narrower and thinner conductive patterns for various applications, including windshield defrosters and autonomous driving sensors.

[0022] Various patterning technologies for the manufacturing of these conductive patterns are available. Sophisticated and very accurate technologies such as photolithography and mask Physical Vapor Deposition (PVD) are utilized in industry, but they are complicated, multi-step, costly technologies and involve extensive capital expenditure.

[0023] Printing of the patterns is a very common alternative technology, and may be achieved by screen printing (contact printing utilizing high viscosity formulations), inkjet printing (non-contact printing technology utilizing low viscosity inks) and dispensing (noncontact, utilizing inks over a wide range of viscosities). Inkjet printing has limited use for printing conductive patterns in the industry due to its lower throughput (inks have < 70% metal and usually only around 50% metal loading, which contributes to relatively slow production speeds). Higher metal concentrations may substantially increase the viscosity, rendering such formulations unsuitable for inkjet printing.

[0024] While screen printing may currently be the most widely implemented printing technology, screen printing may be reaching its limitations which relate to the available pastes as well as to the printing technology. There is now a demand for printing technologies to attain the performance achieved by photolithography and PVD, / .c. : 1) patterning on brittle substrates; 2) reduced metal consumption in producing the patterns; 3) high throughput; and 4) production of very narrow patterns.

[0025] New printing technologies are being developed that may overcome screen printing technology constraints. These include two high throughput, non-contact printing technologies that utilize high viscosity pastes: (a) dispensing via high throughput, multinozzle systems; and (b) laser-induced forward transfer (LIFT).

[0026] In the manufacture of conductive patterns, electrically conductive pastes based on metal nanoparticles are enablers for the dispensing and LIFT technologies, to fully exploit the capabilities of these technologies - contact-less printing of very narrow patterns below the minimum width achievable by screen printing. Metal nanoparticle based pastes also serve to improve the performance of screen printing. These pastes, when printed using any of these three printing technologies, may enable low temperature sintering. Moreover, the amount of metal deposited may be reduced by 25 to 50% with respect to state-of-the-art micronic and sub-micronic pastes, in part due to the decrease in the achievable width of printed patterns.

[0027] As understood in the art of metal pastes, a paste typically consists of finely dispersed solid particles that are evenly distributed in a liquid or gel-like carrier to form a thick, cohesive formulation.

[0028] In rheological terms, a paste is an example of a Bingham plastic fluid, z.e., a viscoplastic material that behaves as a rigid body at low stress, but flows as a viscous fluid at high stress. In contrast to Newtonian fluids, metal pastes typically show shear thinning as well as thixotropic behavior.

[0029] For various applications (e.g., production of ultra-thin and ultra narrow electrically conductive lines or traces), it would be advantageous to utilize silver pastes. However, silver pastes typically contain coarse particles that may deleteriously affect the sintering temperature and / or the electrical resistance of the conductive line. They may also increase the contact resistance between the pattern and substrate or connector (as large particles result in large voids between particles and between particles and the substrate).

[0030] Reducing the average particle size within the silver paste may reduce the sintering temperature as well as the electrical resistance of the conductive line. However, for a given silver particle content (weight %) within the paste, the reduction in average particle size may greatly increase — even exponentially — the viscosity of the paste formulation. Since various applications are limited by the maximum viscosity that can be printed, this effectively limits — disadvantageously — the silver particle content within the paste.

[0031] Without wishing to be bound by theory, the inventors believe that the higher specific surface area associated with smaller particles is a major contributor to this appreciable viscosity increase.

[0032] By way of example, in trying to produce a paste utilizing a prior art nanometric silver population having a characteristic average particle size of 60nm, the high specific surface area of the particles may exponentially increase the viscosity of the paste when increasing the silver particle concentration above 75%. The viscosity at 80% silver loading may be on the order of 109cP. Such a viscosity makes working with such pastes impossible for practical deposition technologies. In addition, as the silver loading is yet further increased, the paste may break down, yielding a wet powder-type consistency. Moreover, the inventors believe that in agglomerated materials, the particles within the agglomerate may be arranged in a disordered manner, resulting in irregular surfaces. Since agglomerated nanoparticles tend to have higher surface roughness with respect to nonagglomerated crystal nanoparticles of the same size, populations of such agglomerated nanoparticles will exhibit characteristically higher specific surface areas with respect to corresponding populations of non-agglomerated crystal nanoparticles. Similarly, amorphous silver nanoparticles may also have more severe and frequent surface defects.

[0033] This and more: the inventors believe that multi-crystal particles (e.g., crystalline particles composed of a plurality of crystallites) may exhibit somewhat higher specific surface areas than corresponding single crystals having the same size and geometry. This may be due, in part, to the grain boundaries and irregularities in the multi-crystal structure. The low specific surface areas may be further attributed to the thermodynamic stability of the single crystals, with respect to multi-crystals.

[0034] In addition, the inventors have found that in the production of high nano-silver loading pastes from dry nano-silver powders, it is very difficult to wet the particles and to homogeneously disperse them in the paste. W02012078590, entitled “Stable Dispersions of Monocrystalline Nanometric Silver Particles”, and which is incorporated by reference, as if fully set forth herein, discloses a method of producing such silver particles within a liquid medium, and further discloses a method of concentrating the silver particles so as to produce a concentrated nanometric dispersion of silver particles, with little agglomeration.

[0035] W02012078590 fails to disclose a method of producing pastes containing nanometric silver particles. This may be due to agglomeration that may ensue in the process of concentrating the fluid silver dispersions into silver pastes. But perhaps more fundamentally, the inventors have found that the method disclosed in W02012078590 may be unsuitable for producing nanometric silver pastes, and more particularly, nanometric silver pastes that are suitable for the production of ultra-thin and ultra narrow electrically conductive lines.

[0036] The present inventors have discovered that the particulate matter produced by the method disclosed in W02012078590 may contain an inordinate fraction of coarse particles, many of which are coarse agglomerated particles. Such particles tend to be at most an inconsequential problem in low-viscosity printing technologies, as their concentration tends to be low and the jetting systems have filters that protect the printing heads. By contrast, high viscosity pastes are very difficult to filter.

[0037] Moreover, in various systems used to apply the conductive pastes, such as screenprinting, the distinct non-homogeneity of the coarse particles may adversely influence the printing quality in various ways. For example, parts of the printed pattern may have different dimensions with respect to the design dimensions and the desired electrical properties. In addition, the coarse particles may clog the apertures of the screen in screen printing; in dispensing, the coarse particles may clog the nozzle; and in LIFT, paste may disadvantageously be left on the donor layer.

[0038] In addition, the inventors have found that these coarse agglomerated particles may disadvantageously contain substantial quantities of silver oxide (Ag2O). When the dispersions are concentrated into pastes, such coarse agglomerated particles serve to make the paste non-homogeneous, particularly with respect to properties relating to electrical conductivity.

[0039] The present inventors have discovered a method of producing inventive nanometric silver particle pastes having high silver loadings, as well as a substantial content of single crystal nanoparticles. In this method, the nanometric silver particles are always in a wetted state. In this inventive method, the production of such coarse agglomerated particles is mitigated, and the average and local concentrations of Ag2O are sufficiently low for the production of ultra-thin and ultra narrow electrically conductive lines as well as for various other applications having stringent specifications for the silver paste. The salient features of the inventive method will be evident from the description provided hereinbelow.

[0040] The electrically conductive paste of the present invention comprises (a) a carrier liquid; (b) a plurality of nanometric silver particles, in which at least 20% (and more typically, at least 40%, at least 60%, or at least 80%) of the nanometric silver particles are single crystals, wherein the plurality of nanometric silver particles have an average secondary particle size (at least one of dso and D50, and typically both) within a range of 60 to 200 nanometers, wherein the nanometric silver particles are disposed within the carrier liquid; and (c) at least one dispersant; wherein a concentration of the nanometric silver particles within the conductive paste is within a range of 75 to 92% (and more typically, 78 to 92%, 80 to 92%, or 82 to 92%), by weight; wherein the plurality of nanometric silver particles have a number fraction of coarse particles (Fcoarse), defined by a characteristic particle size (Sc oarse) : S Coarse 2 • D50, and wherein the paste optionally has at least one (and more typically, at least two or all three) of the following structural properties:

[0041] (I) the fraction of agglomerated particles within the coarse particles is Aggcoarse; and wherein the product of Fc oarse and Aggc oarse (Fc oarse • Aggc oarse ) is at most 0.05;

[0042] (II) Fc oarse is at most 0.05; and

[0043] (III) the viscosity of the paste at 25°C fulfills at least one of the following structural properties:

[0044] (i) at a shear rate of 0.02 sec1, the viscosity is at most 950,000,000cP;

[0045] (ii) at a shear rate of 0.2 se1, the viscosity is at most 450,000,000cP;

[0046] (iii) at a shear rate of 1.0 sec1, the viscosity is at most 50,000,000cP; and

[0047] (iv) at a shear rate of 10 sec1, the viscosity is at most l,000,000cP.

[0048] In embodiments, Fc oarse • Aggc oarse is at most 0.04, at most 0.03, at most 0.02, or at most 0.01.

[0049] In embodiments, Fcoarse is at most 0.04, at most 0.03, or at most 0.02.

[0050] In the concentrated pastes of the present invention, the nanometric silver particles may be mostly or predominantly single crystals, on a weight basis. The presence of single crystals was qualitatively demonstrated by means of Electron Back Scattered Diffraction (EBSD). Quantification of the results was achieved by performing a plurality of scans at randomly chosen points, as described in greater detail hereinbelow. We have found that in our inventive pastes, at least 20% of the nanometric silver particles are single crystals. More typically, at least 40%, at least 50%, at least 65%, at least 80%, or at least 90% are single crystals.

[0051] The inventive pastes may be characterized by low specific resistivity values, typically 3-50 x 10'6ohm»cm, as measured according to ASTM procedure F 390 - 98.

[0052] Production Method

[0053] The method of producing an electrically conductive, nanometric silver paste, according to aspects of the present invention, may include the following steps:

[0054] Step a: reacting at least one soluble silver compound with an alkali metal hydroxide in an aqueous medium, in a presence of a first dispersant, to produce a dispersion of silver oxide (Ag2O) particles;

[0055] Step b: reacting the Ag2O particles with hydrogen peroxide (H2O2) in an aqueous medium, in a presence of a second dispersant, while controlling the reaction conditions (e.g., utilizing a relatively low molar ratio of H2O2 to AgNCf), to produce a dispersion of nanometric silver particles, the silver particles typically having an average secondary particle size below 200 nanometers;

[0056] Step c: washing the silver particles of Step b in an aqueous washing medium;

[0057] Step d: introducing at least one volatile organic solvent, to the washed silver particles, and replacing most of the aqueous medium therewith;

[0058] Step e: replacing most of the volatile organic solvent by at least one, typically non-volatile or low-volatile organic solvent;

[0059] Step f: concentrating the silver nanoparticle dispersion to produce the electrically conductive, nanometric silver paste;

[0060] Step g (optional): introducing additives to the organic solvent dispersion (e.g., in any of Steps a to f), or to the paste in Step h;

[0061] Step h (optional): homogenizing the paste e.g., in a planetary centrifugal mixer); and Step i (optional): passing the paste through a (typically ~11pm) filter, using pressure.

[0062] An anti-foaming agent may be introduced to control foaming in any of process steps a to g, as necessary.

[0063] Various embodiments of the inventive method of producing a nanometric silver product will now be described in further detail.

[0064] Step a At least one soluble silver compound (typically AgNCf) is dissolved in an aqueous solvent to form a first solution. The alkaline hydroxide (e.g., sodium or potassium hydroxide) may then be added, under vigorous stirring to this first solution. However, it may be advantageous to prepare a second solution of the alkaline hydroxide. The second solution may then be introduced to the first solution, under vigorous stirring, and in the presence of a dispersant, to form a fine silver oxide precipitate having an average secondary particle size below 800 nanometers, and more typically, below 400nm.

[0065] The resulting dispersion is preferably agitated, and an anti-foaming agent may be added to prevent or reduce foaming.

[0066] The vigorous mixing may be effected in an ultrasonic bath, typically maintained within a temperature range of 10°C to 35°C.

[0067] Various dispersants may be used to contribute to the quality of the inventive nanometric silver product, including polyvinylpyrrolidone (PVP), gum arabic, polyvinyl alcohol (PVA), polyacrylic acid (PAA), polyallylamine (PAAm), polysodium styrene sulfonate (PSS), 3 -(aminopropyl)trimethoxy silane (APS), fatty acids, such as stearic, palmitic, lauric, oleic, etc.; lauryl amine, cetyltrimethylammonium bromide (CTAB), tetraoctylammonium bromide (TOAB), sorbitol, silicon oil, and various commercially available dispersants such as BYK® 104, BYK® 104s, BYK®411, and Lubrizol® 85000.

[0068] In Step a, the weight ratio of the dispersant (e.g., PVP) to the soluble silver salt (e.g., AgNCh), is typically within the range of 0.05 to 1.

[0069] Step b

[0070] A reducing agent, typically a peroxide such as hydrogen peroxide (H2O2) may be added, under vigorous mixing, to a dispersion containing fine silver oxide particles, whereby the oxide particles are reduced, producing a second dispersion containing nanometric silver particles. The presence of a dispersant in Step b may reduce or substantially inhibit agglomeration. The dispersant may be the previously added dispersant from Step a, or may be a mixture of the previously added dispersant from Step a and a newly added dispersant, which may be identical to the dispersant used in Step a, or may be a different chemical species.

[0071] The vigorous mixing may be effected in an ultrasonic bath.

[0072] Preferably, the reduction reaction is conducted within a temperature range of 20°C to 40°C.

[0073] The present inventors have discovered that a portion of the Ag2O, albeit minute, may disadvantageously remain unreacted in Step b. Moreover, adding a larger excess of H2O2 with respect to the Ag20, in addition to being deleterious from economic and environmental standpoints, may fail to remedy this problem, such that Ag2O or Ag2O- containing particles ultimately remain in the silver paste.

[0074] Moreover, the present inventors have discovered that the reaction of Ag2O with reducing agents such as H2O2 may produce a relatively small but significant fraction of coarse, agglomerated, silver-containing particles within the conductive silver paste.

[0075] The present inventors have discovered, however, that by slow, gradual addition of the H2O2 to the stirred reaction vessel (preferably over the course of at least 6 hours, at least 3 hours or at least 1.5 hours), substantially all of the Ag2O reacts with the peroxide.

[0076] This and more: the present inventors have counterintuitively discovered that by adding a relatively low reactant molar ratio Rreactants of peroxide to soluble silver source (e.g., TfcCh / AgNCh), the conversion of Ag2O may actually be improved.

[0077] Thus, in embodiments, Rreactants is within a range of 1.1-3.0, 1.1-2.6, or 1.1-2.3. In any of these embodiments, Rreactants may be at least 1.3, at least 1.5, or at least 1.8.

[0078] Moreover, the present inventors have discovered the importance of monitoring the pH within the stirred reaction vessel, which begins at a mildly basic pH and has been found to drop as the reaction progresses. By monitoring the pH and controlling the feed rate of the H2O2 to the stirred reaction vessel such that the addition of the H2O2 is curtailed only after the pH has been reduced to about 6 or below, substantially all of the Ag2O may react with the peroxide and the disadvantageous formation of the coarse, agglomerated, silver- containing fraction of particles may be largely or completely mitigated. While this fraction may represent only a small weight fraction of the silver particle formulation, the inventors have found that its presence is highly detrimental to the properties of the conductive silver paste.

[0079] The present inventors have found in order to obtain silver nanoparticles having the requisite properties, the weight ratio RD of the concentration of the at least one dispersant to the silver nitrate concentration in Step b is preferably at least 0.30.

[0080] In embodiments, RD is at least 0.32, at least 0.35, or at least 0.40.

[0081] In embodiments, RD is at most 0.75, at most 0.65, or at most 0.60.

[0082] In Step b, the concentration of silver particles produced within the reaction mixture is typically between 0.5% to 5%, by weight, and more typically, between 1% to 3%, by weight.

[0083] In addition to water, an additional solvent can be introduced in Step a and / or Step b. Typically, the additional solvent includes a polar solvent such as a polar organic solvent. It is generally advantageous for the additional solvent to be relatively volatile, soluble in water, and to dissolve substantial amounts of the dispersants used.

[0084] Preferably, alcohols like methanol, ethanol, and isopropyl alcohol (IP A) can be used as polar solvents. Various glycols and the like may also be used.

[0085] Step c

[0086] Typically, water or an aqueous solvent may be used to purify the resulting dispersion of Step b in a suitable purification system. The aqueous medium may also be partially removed from the particles, to concentrate the dispersion. In this concentration process, the introduction of water or aqueous solvent to the purification system may be controlled to replace the spent aqueous liquor, while maintaining the concentration of the silver particles, at any time, below a desired value (e.g., below 70% by weight). As a result, substantially all the salts and most of the dispersant in the aqueous liquor may be removed without deleteriously changing the shapes or agglomerating the silver particles.

[0087] The aqueous solvent may contain, in addition to water, an organic solvent such as a polar organic solvent. The streams produced in Step c typically include a concentrate containing most of the nanometric silver particles, and a relatively dilute stream containing a lower concentration of the silver nano-particles, and preferably, containing substantially none of the silver nano-particles. Usually, particular values of the final concentration of the salts (based on the silver weight), of the dispersant (based on the silver weight), and of the silver particles (based on the dispersion weight) may be preset, and the operation of Step c is considered finished when these preset values are met.

[0088] Step c may be conducted in a microfiltration system such as a membrane purification system having at least one membrane capable of separating the silver particles from the aqueous liquor, without losing a large fraction of the silver particles in the aqueous phase that would make the process economically unviable. Alternatively or additionally, Step c may be conducted in a centrifuge purification system having at least one centrifuge, such as a decanter centrifuge.

[0089] A microfiltration system and method of general relevance to the present invention is disclosed by Pagana et al., “Applied Pilot-Scale Studies on Ceramic Membrane Processes for the Treatment of Wastewater Streams” (Global NEST Journal, Vol. 8, No. 1, pp 23-30, 2006), and is incorporated in its entirety by reference into the specification, as if fully set forth herein.

[0090] At least one membrane of the membrane purification system should be capable of filtering off the nanometric silver particles in the dispersion. To this end, the characteristic pore size of this membrane may be selected to be within a range that is suitable to retain the nanometric silver particles. The membranes may be made of a metallic material, ceramic material, polymeric material, or of other materials that may be known to those of ordinary skill in the art.

[0091] Step d

[0092] A volatile organic solvent may replace most of the aqueous liquor of the purified dispersion obtained in Step c, in a method similar to the method utilized in Step c. The same purification system may be used. In displacing the aqueous liquor, a further purification of the silver particles is achieved, which may be essential for various products and applications.

[0093] The volatile organic solvent may advantageously be soluble in water, and may readily dissolve the dispersant or dispersants remaining from Step c. Various solvents may be appropriate as solvents for Step d of the process, either alone or mixed with at least one additional solvent. These solvents include, but are not limited to alcohols such as methanol, ethanol, propanol, isopropanol, and a butanol such as 1 -butanol; acetonitrile; dimethyl sulfoxide (DMSO); alkylamines such as butylamine; ethylene diamine; dimethyl acetamide; 1,4-butanediol; formic acid; and acetic acid.

[0094] Steps e and f:

[0095] Solvent is evaporated in a rotavapor under low pressure conditions. The temperature is adapted to the pressure achieved in order to evaporate the solvent. Special care needs to be taken to make the process continuous and smooth, therefore it is preferred to set a temperature that is as low as possible to evaporate the solvent, and the pressure is gradually lowered as the solvent evaporates. The amount of solvent is monitored by measuring the volume of the evaporated solvent in the receiving flask. The target amount of solvent to evaporate may be pre-calculated, such that when the amount of the solvent evaporated reaches the target amount, the pressure is gently increased and the evaporation process is stopped, and the flask containing the paste may then be detached from the evaporator apparatus.

[0096] A second organic solvent, whose identity and properties may be dictated by market requirements, may be used to replace most of, and typically at least 80% of, or at least 90% or 95% of, the volatile organic solvent of the dispersion obtained in Step d. The method of solvent replacement or displacement may be similar to the method utilized in Step c and / or Step d, and the purification system may be similar or identical. However, the second organic solvent may replace the volatile organic solvent in an evaporation system in which the volatile organic solvent is evaporated, with a concomitant addition of the desired organic solvent, in order to maintain a concentration of the silver particles below a particular, desired value. Typically the concentration of the silver particles is at most 90%, at most 85%, or at most 80%.

[0097] Various solvents may be appropriate as solvents for Step e of the process, either alone or mixed with at least one additional solvent. Exemplary solvent families include, but are not limited to: glycol ethers, aliphatic solvents, alcohols, acetates, glycol-polyols, as well as miscellaneous organic solvents. Examples of specific solvents are provided hereinbelow:

[0098] Glycol ethers

[0099] Ethylene glycol, Propylene glycol, Hexylene glycol, Diethylene glycol, Dipropylene glycol, Tripropylene glycol, Tripropylene glycol monomethyl ether (TPM), Dipropylene glycol monomethyl ether, Diethylene glycol monomethyl ether, Diethylene glycol monoethyl ether, Triethylene glycol monomethyl ether, Triethylene glycol monoethyl ether, Diethylene glycol monobutyl ether, Propylene glycol monomethyl ether, Propylene glycol monopropyl ether, Propylene glycol monobutyl ether, Ethylene glycol monomethyl ether, Ethylene glycol monoethyl ether, Ethylene glycol monoisopropyl ether, Ethylene glycol monopropyl ether, Ethylene glycol monobutyl ether, Ethylene glycol monohexyl ether, N-methyl-2-pyrrolidinone, 1,2-Butylene carbonate, Propylene carbonate, N,N- dimethylformamide, (Mono) ethanol amine.

[0100] Aliphatic Solvents

[0101] Hexane, heptane, xylene.

[0102] Alcohols a-Terpineol (90%), Terpineol mixture (a~65%; ~10%; y~25%), Benzyl alcohol, Glycerol, Water, Iso-Butanol, Isopropanol, Ethanol.

[0103] Acetates

[0104] DB Acetate, DPM acetate, EB Acetate, Ethyl Acetate, Ethyl Aceto Acetate, PM Acetate, Iso-Butyl Acetate, Iso Propyl Acetate, N-Butyl Acetate, N-Propyl acetate, 2-Ethyl Hexyl Acetate, Propylene glycol methyl ether acetate, Dipropylene glycol methyl ether acetate, Diethylene glycol ethyl ether acetate, butyl carbitol acetate.

[0105] Glycol s / Polyols Diethylene Glycol, Ethylene Glycol, Hexylene Glycol, Polyethylene Glycol, Triethylene Glycol, Tripropylene Glycol.

[0106] Miscellaneous Solvents

[0107] N-methyl-2-pyrrolidone (NMP); ; various epoxy resins; TPM / NMP mixtures.

[0108] The first purifying step (Step c), as well as Steps e and f, may advantageously be effected or augmented by nano-separation or nano-filtration using nano-separation membranes. Such processes may be excessively and impractically slow when the size of the filtrate species approaches that of the membrane pore or opening. Moreover, nano- filtration membrane processes may even be substantially impossible when the size of the filtrate species equals, or exceeds, the size of the membrane opening.

[0109] Certain dispersants, such as PVP, may have an elongated or needle-like structure. While the characteristic long dimension or diameter of such molecules is much too large to pass through the openings of nano-separators, and may be considerably larger than the silver nano-particles themselves, the characteristic narrow dimension or diameter of such molecules may be orders of magnitude smaller. Thus, the structure of the dispersant can be tailored to satisfy the process requirements for advantageously effecting a nanoseparation of the dispersant from the silver nano-particles.

[0110] Ceramic nano-filtration membranes have been advantageously employed, but polymeric and / or metallic nano-filtration membranes may also be fundamentally suitable. The membrane systems may be static or dynamic (e.g., having a vibrational mechanism for facilitating the separation).

[0111] Typical ceramic nano-separation or nano-filtration membranes for use in conjunction with the method of the present invention have one or more pores that are typically cylindrical, with a high length to width aspect ratio, through which the water / solvent and fine matter can pass through. In many cases, the membrane is typically shaped like a long cylinder.

[0112] Nano-membranes having a pore diameter of less than 200 nanometers may be suitable for use in the process of the present invention. In some applications, the preferred pore diameter is less than 150 nanometers, less than 120 nanometers or less than 100 nanometers. Generally, the pore diameter or nominal pore diameter of the membrane may be at least 20 nanometers, and often, at least 30-50 nanometers, so as to enable various species to pass through the membrane openings, and so that the separation kinetics are not prohibitively slow. The dispersant size and shape and the size of the membrane openings may be selected such that the silver nano-particles pass through the openings, while passage of the dispersant through the openings is hindered or substantially prevented.

[0113] Downstream of the separation step, much smaller membrane openings may be selected, such that the passage of the silver nano-particles through the openings is hindered or substantially prevented, while smaller molecules such as water, ethanol, etc., pass through the openings with relative facility.

[0114] Step g (optional): adding additives to the organic solvent dispersion

[0115] Various functional additives can be added to the solvent-based dispersions before evaporating the volatile solvent. For example, additives may be added to enhance adhesion properties, hydrophobic additives may be added to enhance stability of printed patterns to humidity, and rheological additives such as surfactants may be added to effect rheological modifications, especially viscosity reduction.

[0116] We have found that in most cases, introducing small amounts of additives (less than 7% w / w, including dispersants) are enough to obtain the desired properties, without appreciably detracting from the electrical properties. Specific chemical families of additives, as well as exemplary species of these families, are provided below:

[0117] UV curable

[0118] (e.g., for UV-curing of the printed pattern to pin the trace width and / or to improve adhesion to the substrate)

[0119] Solsperse 71000, Solsperse 41000, Solsperse 74000, Solsperse 85000, Solsperse 36000, Solsperse 86000, Solsperse 32000, Solsperse 75000, Solsperse 88000, Solsperse 87000, Solsperse 39000, BYK UV 3500, BYK UV 3505, BYK UV 3511, BYK UV 3576, BYK UV 3590, BYK UV 3510, BYK UV 3519, Tilo-39000.

[0120] Aromatic / ester mixtures (surfactants and dispersants)

[0121] Solsperse 36600, Solsperse 84500, Solsperse 38500, Solsperse M386, Solsperse 32600, Solsperse 2800, Bykumen, Disperbyk 107, Disperbyk 161, Tilo-17250.

[0122] Aromatic (surfactants and dispersants)

[0123] Solsperse 26000, Solsperse 24000, Solsperse 13240, BYK-P 105, Disperbyk 168, Tilo- 32500.

[0124] Aliphatic (surfactants and dispersants) Solsperse 3000, Solsperse 8000, Solsperse 9000, Solsperse 13300, Solsperse 16000, Solsperse 13940, Disperbyk 102, Disoerbyk 163, Disperbyk 162 TF, Tilo-17940, Tilo- 13940.

[0125] Alcohol (surfactants and dispersants)

[0126] Solsperse 45000, Solsperse 20000, Solsperse 54000, BYK P 104 S SG, BYK P 104 SG, Tilo-185, Tilo-516, Tilo-5163.

[0127] Universal (surfactants and dispersants)

[0128] Solsperse 64000, Solsperse 66000, Solsperse 53095, Solsperse 67000, Solsperse 65000, BYK P 104, Tilo-40101.

[0129] Step h (optional): homogenizing the paste

[0130] The paste may be rigorously homogenized, e.g., in a planetary centrifugal mixer such as a Thinky ARM-310.

[0131] Step i (optional): filtering the paste

[0132] Coarse filtering, mainly to remove foreign materials, may be performed, typically by means of pressure filtration, (e.g., using an 11pm filter).

[0133] EXAMPLES

[0134] Reference is now made to the following Examples, which together with the above description, illustrate the invention in a non-limiting fashion.

[0135] The chemicals used in effecting these Examples are identified hereinbelow:

[0136] AgNCh - Metalor

[0137] KOH - Aldrich hydrogen peroxide (aqueous solution of 34-36%) - Adama-Makhteshim (Israel) anti -foaming agent Contraspum 1012 - Zschimmer & Schwarz (Germany) ethanol abs. - Aldrich isopropyl alcohol (IP A) - Aldrich tripropyleneglycol methyl ether (TPM ) - Aldrich butyl carbitol acetate (BCA) - Aldrich caprylic acid - Aldrich

[0138] PVP - Sokalan K30P BASF

[0139] Ethlylene glycol butyl ether (EB) Aldrich

[0140] Orthophosphoric acid (88%) Aldrich Epoxy XY8000 -Japan Epoxy Resins Co., Ltd. (Japan). Epoxy XY8000 may be identified by CAS No. 30583-72-3, and has the chemical name cyclohexanol, 4,4-(l- methylethylidene)bis-, polymer with (chloromethyl)oxirane.

[0141] BYK® additives (BYK® division, Altana group)

[0142] Lubrizol® additives (Lubrizol® Corporation)

[0143] Nano-silver powder:

[0144] (1) Inframat Advanced Materials, 47MN-0001; (Ag, 99.95%, 150 nm)

[0145] (2) SkySpring Nanomaterials, Inc. 0121XH Silver (Ag) nanoparticles / nanopowder (Ag, 99.9%, ~50-60nm)

[0146] Screen printed pastes: Solaronix Elcosil S-L / SP (57.9% solids) and Solaronix Elcosil SG / SP (89.4% solids).

[0147] Aqueous solutions were prepared by using deionized water using an lonex water purification system (PuriTech, Dessel, Belgium). All reagents and solvents were used without further purification.

[0148] The instruments used in conjunction with the Examples are identified hereinbelow:

[0149] Particle size measurements were performed using a LUMiSizer® Dispersion Analyzer. The particle size distribution (including the volume-weighted dso) is calculated from the sedimentation rate of the particles while applying high rpm (centrifugal forces).

[0150] High Resolution Scanning Electron Microscopy (HRSEM) images were generally obtained using an HRSEM Ultra Plus Zeiss Gemini (Inlens Detector), having an instrument magnification of xl00,000.

[0151] Electron Back Scattered Diffraction (EBSD) Patterns were obtained using an E- SEM Quanta™ 200 (FEI, Hillsboro, Oregon). The instrument was equipped with an attachment for orientation image microscopy (OIM) Channel 5 (Oxford Instruments, England).

[0152] Evaporation was performed using a R-215 Rotavapor® equipped with a heating bath (BUCHI Labortechnik AG, Flawil, Switzerland).

[0153] Filtration of dispersions was conducted by using a membrane system that included ceramic membranes (JM Separations BV, The Netherlands).

[0154] EXAMPLE 1

[0155] 1575g AgNOa were dissolved in 2000ml Dl-water (solution A). 500g PVP (MW=55,000, k=29.2) were dissolved in 16,000ml water (solution B). 550g KOH were dissolved in 2000ml water (solution C). Solution A was poured into solution B under vigorous stirring. Then solution C was added to the combined and mixed solutions A and B, under vigorous stirring, forming a colloidal precipitate of Ag2O at ambient temperature (25°C).

[0156] EXAMPLE 2

[0157] After stirring the product of Example 1 for 10 minutes, about 2000ml H2O2 (33%) were slowly pumped into the reaction vessel under stirring at a temperature range between about 25°C to about 50°C, as the reaction was exothermic. The pH, which initially was around 8.5, was monitored during the slow and gradual addition of the H2O2, slowly decreased to 6 after about 4.5 hours, at which point the addition of H2O2 was curtailed. A dispersion of silver nanoparticles was formed. The dispersion was stirred for an additional 15 minutes, and was then transferred to a storage tank.

[0158] A particle size analysis using the LUMiSizer® Dispersion Analyzer yielded an average particle size (dso) of about 80nm.

[0159] COMP ARA TIVE EXAMPLE 3

[0160] 52g AgNCh and 3.3g PVP (same as in Example 1) were dissolved in a mixture of 780ml ethanol and 80ml water (solution A). 17g KOH were dissolved in 140ml water (solution B). Solution B was poured into solution A under vigorous stirring in an ultrasonic bath, forming a colloidal precipitate of Ag2O at ambient temperature. After stirring the dispersion for 10 minutes, 180ml H2O2 (33%) were slowly pumped into the dispersion under stirring at the temperature range between about 25°C to about 60°C, forming silver nanoparticles. The dispersion was stirred for additional 15 minutes, and was transferred to a storage tank.

[0161] A particle size analysis using the LUMiSizer® Dispersion Analyzer yielded an average particle size (dso) of about 80nm.

[0162] COMP ARA TIVE EXAMPLE 4

[0163] After stirring the product of Example 1 for 10 minutes, about 2000ml H2O2 (33%) were slowly pumped into the reaction vessel under stirring, over the course of 10 minutes. The temperature, initially at about 25°C, rapidly increased to almost 60°C. A dispersion of silver nanoparticles was formed. The dispersion was stirred for an additional 15 minutes, and was then transferred to a storage tank.

[0164] EXAMPLE 5: Concentrating a Dispersion

[0165] 1000 ml of the product dispersion from Example 2 were pumped from the storage tank. The dispersion was washed in a membrane separation system by feeding gradually and continuously about 20 liters of water into the membrane system, and simultaneously, by gradually and continuously withdrawing a similar volume of spent wash liquor from the membrane system in such a manner that the concentration of the silver particles, never exceeded 70%, on a weight basis.

[0166] The membrane system included ceramic membranes (JM Separations BV) having separation pores or capillaries having a nominal pore diameter of 100 nanometers. Water was added, and ionic matter and dispersant, selectively, passed through the membranes, leaving the nanometric silver particles in the dispersion.

[0167] The washing process was continued until the salts in the dispersion were, practically eliminated, and the dispersant was reduced to a preset concentration of 3.5% of the weight of the silver particles.

[0168] As the mass balance of the water feed to the membrane system and the spent wash liquor flowing out of the membrane system was, deliberately, changed along this washing step in order to effect an efficient washing operation, and in order to obtain a concentrated dispersion, the resulting washed silver dispersion contained about 25% solids, on a weight basis. A particle size analysis of the washed silver particles yielded an average particle size (dso) of about 80 nanometers; no significant change in particle size between the unwashed and washed nano-silver product was observed.

[0169] EXAMPLE 6: Replacing Water with a Volatile Organic Solvent (Water-Ethanol solvent exchange)

[0170] A 1000 ml portion of a dispersion of silver particles in water, containing about 150g of the silver particles, which was prepared in a similar manner to that in Example 5, was concentrated to 500 ml using the same membrane separation system as in Example 5. 400 ml ethanol were then added, and the dispersion was concentrated once again to 500 ml by withdrawing the requisite volume of liquid. This cycle, in which 400 ml ethanol is added and about 400 ml ethanol-water mixture is withdrawn, was repeated until the concentration of the ethanol reached 94%-95% by weight (which is close to the composition of an azeotropic mixture of ethanol-water). The resulting 500 ml silver dispersion contained about 150g of nanometric silver and about 300g of the ethanol-water mixture.

[0171] A particle size analysis of the silver dispersion after the water-ethanol exchange yielded an average particle size (dso) of about 80 nanometers.

[0172] A majority of the particles obtained were single crystals (by EBSD determination, as elaborated in the Example below). From HRSEM images, it is evident that the single- crystal silver particles include crystals having triangular faces, square faces, and hexagonal faces.

[0173] EXAMPLE 7

[0174] The presence of single crystals was qualitatively demonstrated by means of Electron Back Scattered Diffraction (EBSD).

[0175] EBSD produces a diffraction pattern from the surface of the sample of silver nanoparticles. The procedure, which will be readily understood to those of ordinary skill in the art of EBSD, is as follows:

[0176] 1. The sample is scanned using a scanning electron microscope (SEM) Quanta™ 200, typically at a working distance of 18mm, and at 20 KeV, to obtain a diffraction pattern or image. The spot size is 4.5; the probe current is about 0.5 nA; the collection time for EBSD pattern: 300 msec; integration: 50.

[0177] 2. Interpretation of the diffraction image is performed using the instrument software (comparing basic silver crystallographic data);

[0178] 3. The "solution" of the diffraction is matched, depicting the orientation of the crystal, correlating every kikuchi line to its fitting crystallographic plane in the lattice. If there is a perfect match between the kikuchi lines and the crystallographic planes (according to the theoretical data), the diffraction determines the orientation of a single crystal.

[0179] In the case of nanometric silver particles, a perfect solution may not always be obtained; sometimes there is no solution at all. This may indicate that the beam is located on a grain boundary. Alternatively, a perfect solution may not be obtained when the beam is located between two grains.

[0180] EXAMPLE 8

[0181] Following the basic procedure of Example 7, we quantified the presence of nanometric silver single crystals within each sample of nanometric silver. Quantification was achieved by performing a plurality (at least 5, and preferably at least 10) of scans at randomly chosen points. In testing the various dispersions of the present invention, at least 30% or at least 50% of the scans produce a substantially perfect match for a silver single crystal. More typically, at least 80%, at least 90%, or substantially 100% of the scans produce a substantially perfect match for a silver single crystal.

[0182] It has been disclosed that if at least 30% of the scans produce a substantially perfect match for a silver single crystal, then a majority of the nanometric silver particles are single crystals (based on the number of particles). If at least 50% of the scans produce a substantially perfect match for a silver single crystal, then at least 60%, and typically at least 70% of the nanometric silver particles are single crystals. If at least 60% of the scans produce a substantially perfect match for a silver single crystal, then at least 70%, and typically at least 80% of the nanometric silver particles are single crystals. If at least 80% of the scans produce a substantially perfect match for a silver single crystal, then at least 90%, and typically at least 95% of the nanometric silver particles are single crystals.

[0183] In theory, these quantitative EBSD scanning methods may provide a quantitative evaluation of a top layer or cross-section of the sample. In practice, however, this quantitative evaluation closely reflects the fraction of silver particles that are single crystals, particularly for samples that do not have an extremely broad particle size distribution.

[0184] EXAMPLE 9: Replacing Water with a Volatile Organic Solvent (IP A)

[0185] Example 6 was repeated, but with isopropyl alcohol (IP A) instead of ethanol.

[0186] A particle size analysis of the silver dispersion after the water-isopropyl alcohol exchange yielded an average particle size (dso) of about 80 nanometers.

[0187] A majority of the particles obtained were single crystals (as determined by EBSD).

[0188] EXAMPLE 10: Replacing the Volatile Organic Solvent with an Organic Non-Volatile Solvent (Ethanol-TPM solvent exchange)

[0189] The 500 ml of silver dispersion from Example 6, containing about 150g of silver particles and about 300g of solvent (ethanol -water mixture), were transferred into a 1 -liter flask. 150g tripropyleneglycolmethylether (TPM) were added to the flask (in order to ultimately obtain a final dispersion containing about 50% solids, on a weight basis). The flask was connected to a Rotavapor® apparatus, and the ethanol was evaporated under vacuum (at 20mm Hg; 60°C; 80 rpm). The resulting silver dispersion in TPM contained 49.5% (by weight) silver with an average particle size (dso) of about 80 nanometers (as well as traces of ethanol and water). From the HRSEM images, it is evident that the general appearance of the silver particles has not been noticeably changed with respect to that of the particles obtained in Example 6.

[0190] EXAMPLE 11

[0191] A solvent mixture was prepared as follows: 2.60g DGME, 5.20g DGBE and 5.20g EB were vigorously mixed at room temperature, until a completely homogenous mixture is obtained (Solution A). Solution A was added dropwise with vigorous stirring at room temperature to 174.0g of single crystal nano silver (50% w / w) dispersion in ethanol, produced as described in Example 6 (Dispersion B). Dispersion B was then transferred to a flask which is connected to a Rotavapor® apparatus, and the ethanol was evaporated under vacuum (at 20 mbar; bath 60°C; 120 rpm). The resulting silver paste, 100g, contained 87% (w / w) nanosilver particles, a large fraction of which were single crystals. The resulting paste was homogenized in a planetary centrifugal mixer (Thinky ARM-310), for two cycles of 2 minutes at 1600 rpm.

[0192] The paste had a viscosity of 180,284,000cP at a shear rate of 0.02sec-1, and 3,136,750cP at a shear rate of l.Osec’1. The flow point of the paste was 4,812 Pa, G’ was 10,103 Pa, and G” was 745.4 Pa.

[0193] EXAMPLE 12

[0194] 2.0 g Lubrizol Solsperse™ 85000 (surfactant) were dissolved in 3.0 g ethanol, under vigorous stirring, at room temperature (Solution A). A solvent mixture was prepared as follows: 6.6 g DGBE and 2.4 g terpineol were vigorously mixed at room temperature until the solvent mixture was completely homogenous (Solution B). Solution A was added dropwise with vigorous stirring at room temperature to 178 g of the nanosilver (50% w / w) dispersion in ethanol made as described in Example 6 (Dispersion C). Solution B was then added dropwise with vigorous stirring at room temperature to Dispersion C to produce Dispersion D. Dispersion D was transferred to a flask which was connected to a Rotavapor® apparatus, and the ethanol was evaporated under vacuum (at 20 mbar; bath 60°C; 120 rpm). The resulting silver paste, 100g, contained 87% (w / w) nanosilver particles, a large fraction of which were single crystals. The resulting paste was homogenized in a planetary centrifugal mixer (Thinky ARM-310), for two cycles of 2 minutes at 1600 rpm.

[0195] The paste had a viscosity of 5,463,250cP at a shear rate of 0.02sec-1, and 949,530cP at a shear rate of l.Osec1. The flow point of the paste was 625.8 Pa, G’ was 593.5 Pa, and G” was 118.1 Pa.

[0196] It must be emphasized that different embodiments of this procedure are possible in which additional additives (e.g. rheology controlling additives, binders, electrically conductive additives, etc.) may be added to Solution A and / or to solution B. Also, Solution B can be made of one solvent or multiple solvents.

[0197] EXAMPLE 13

[0198] A solvent mixture was prepared as follows: 3.36g DGME, 6.73g DGBE and 6.73g EB were vigorously mixed at room temperature, until a completely homogenous mixture is obtained (Solution A). Solution A was added dropwise with vigorous stirring at room temperature to 166.38g of single crystal nano silver (50% w / w) dispersion in ethanol, produced as described in Example 6 (Dispersion B). Dispersion B was then transferred to a flask which is connected to a Rotavapor® apparatus, and the ethanol was evaporated under vacuum (at 20 mbar; bath 60°C; 120 rpm). The resulting silver paste, 100g, contained 80% (w / w) nanosilver particles, a large fraction of which were single crystals. The resulting paste was homogenized in a planetary centrifugal mixer (Thinky ARM-310), for two cycles of 2 min. at 1600 rpm.

[0199] The paste had a viscosity of 217,322cP at a shear rate of 0.02sec-1, and 29,194cP at a shear rate of 1.Osec’1. The flow point of the paste was 11.5 Pa, G’ was 84.0 Pa, and G” was 43.1 Pa.

[0200] COMP ARA TIVE EXAMPLE 14

[0201] A solvent mixture was prepared as follows: 16.8 g DB and 6.2 g terpineol were vigorously mixed at room temperature, until a completely homogenous mixture was obtained (Solution A). 2.0 g Lubrizol 85000 were dissolved in solution A at room temperature, under vigorous stirring to produce Solution B. Solution B was then added dropwise to 75g of silver nanoparticle powder (SkySpring Nanomaterials, Inc.) at room temperature, under vigorous stirring, to produce Dispersion C. The resulting mixture was homogenized in the planetary centrifugal mixer, as above. The resulting silver paste, 100g, contained about 75% (w / w) nanosilver particles.

[0202] The paste had a viscosity of 431,882,000cP at a shear rate of 0.02sec-1, and 5,208,350cP at a shear rate of l.Osec’1. The flow point of the paste was 9,000 Pa, G’ was 2,007,415 Pa, and G” was 144,266 Pa.

[0203] COMPARATIVE EXAMPLES 15 - 18

[0204] Silver nanoparticle concentrates were formulated according to Comparative Example 14, but with increased nanosilver loadings: 78.8%, 82.6%, 86.8%, and 90.8% (w / w). Unlike the concentrate of Example 14, which formed a highly viscous paste, the concentrates of Examples 15-18 failed to form pastes, and instead had the consistency of agglomerated powders.

[0205] COMP ARA TIVE EXAMPLE 19

[0206] 1.260kg AgNCh were dissolved in 1.6 liters of DI water (solution A). 0.640kg PVP (MW=55,000, k=29.2) were dissolved in 18.0 liters of Dl-water (solution B). 0.420kg KOH were dissolved in 3.40 liters of DI- water (solution C). Solution A was then poured into solution B under vigorous stirring, and Solution C was added to the mixed solution (A&B) at ambient temperature, under vigorous stirring, to form a colloidal precipitate of Ag2O.

[0207] After stirring the dispersion for 10 minutes, around 4.4 liters of H2O2 (33%) were pumped into the dispersion at a rate of just above 160 ml / min, under vigorous stirring, for about 25 minutes.

[0208] This dispersion was pumped out from the top of the storage tank in which it was kept, without any mixing, to ensure that any large particles would settle to the bottom of the storage tank. As above, the dispersion was washed in a membrane separation system by feeding gradually and continuously water into the membrane system, and simultaneously, by gradually and continuously withdrawing a similar volume of spent wash liquor from the membrane system in such a manner that the concentration of the silver particles, never exceeded 70% (and preferably less than 60%), on a weight basis. Water was added, and ionic matter and dispersant, selectively, passed through the membranes, leaving the nanometric silver particles in the dispersion.

[0209] The washing process was continued until the salts in the dispersion were practically eliminated, and the dispersant was reduced to a preset concentration of 3% of the weight of the silver particles. In this example, 144 liters of water were added, over the course of 12.67 hours. 0.687 kg nano silver particles were obtained, corresponding to about 85.9% of the theoretical yield. The final dispersant concentration (PVP) was about 3.4%. The dispersion contained unreacted Ag2O (filtration was impossible).

[0210] An HRSEM image showing a typical field of the nanometric particles produced is provided in Figure 1.

[0211] EXAMPLE 20

[0212] 1.575kg AgNCh were dissolved in 2.0 liters of Dl-water (solution A). 0.550kg PVP (MW=55,000, k=29.2) were dissolved in 16.0 liters of Dl-water (solution B). 0.550kg KOH were dissolved in 2.0 liters of Dl-water (solution C). Solution A was then poured into solution B under vigorous stirring, and Solution C was added to the mixed solution (A&B) at ambient temperature, under vigorous stirring, to form a colloidal precipitate of Ag2O.

[0213] After stirring the dispersion for 10 minutes, around 8.0 liters of H2O2 (33%) were pumped into the dispersion at a rate of around 22.2 ml / min under vigorous stirring for 6 hours. The pH was monitored throughout the slow addition of the H2O2, until the pH dropped below 6, down to about 5.5.

[0214] The above was allowed to rest overnight. As above, the dispersion was then pumped out from the top of the storage tank and washed in a membrane separation system as described hereinabove. Water was added, and ionic matter and dispersant, selectively, passed through the membranes, leaving the nanometric silver particles in the dispersion.

[0215] The washing process was continued until the salts in the dispersion were, practically eliminated, and the dispersant was reduced to a preset concentration of 3% of the weight of the silver particles. In this example, 104 liters of water were added over the course of 6.7 hours. 0.717 kg nano silver particles were obtained, corresponding to about 71.7% of the theoretical yield. The final dispersant concentration (PVP) was about 2.3%. Substantially no unreacted Ag2O was left over, and the filtration proceeded with ease.

[0216] An HRSEM image showing a typical field of nanometric particles produced is provided in Figure 2.

[0217] EXAMPLE 21

[0218] 1 ,800kg AgNCh were dissolved in 2.0 liters of Dl-water (solution A). 0.600kg PVP (MW=55,000, k=29.2) were dissolved in 16.0 liters of Dl-water (solution B). 0.660kg KOH were dissolved in 2.0 liters of Dl-water (solution C). Solution A was then poured into solution B under vigorous stirring, and Solution C was added to the mixed solution (A&B) at ambient temperature, under vigorous stirring, to form a colloidal precipitate of Ag2O.

[0219] After stirring the dispersion for 10 minutes, around 9.6 liters of H2O2 (33%) were pumped into the dispersion at a rate of around 23.4 ml / min under vigorous stirring for 6:50 hours, while controlling the temperature to be well below 60°C.

[0220] The pH was monitored throughout the slow addition of the H2O2, until the pH dropped below 6 (down to about 5.5); the results are provided in the table below: The dispersion was allowed to rest overnight. As above, the dispersion was then pumped out from the top of the storage tank and washed in a membrane separation system as described hereinabove. Water was added, and ionic matter and dispersant, selectively, passed through the membranes, leaving the nanometric silver particles in the dispersion.

[0221] The washing process was continued until the salts in the dispersion were, practically eliminated, and the dispersant was reduced to a preset concentration of 3% of the weight of the silver particles. In this example, 120 liters of water were added over the course of 9 hours. 1.012 kg nano silver particles were obtained, corresponding to about 88.5% of the theoretical yield. The final dispersant concentration (PVP) was about 1.7%. Substantially no unreacted Ag2O was left over, and the filtration proceeded with ease.

[0222] An HRSEM image showing a typical field of nanometric particles produced is provided in Figure 3.

[0223] EXAMPLE 22

[0224] 1.575kg AgNCf were dissolved in 2.0 liters Dl-water (solution A). 0.500kg PVP (MW=55,000, k=29.2) were dissolved in 16.0 liters Dl-water (solution B). 0.550kg KOH were dissolved in 2.0 liters Dl-water (solution C). Solution A was poured into solution B under vigorous stirring. Then solution C was added to the mixed solutions A&B, under vigorous stirring forming a colloidal precipitate of Ag2O at ambient temperature.

[0225] After stirring the dispersion for 10 minutes, 5.5 liters of H2O2 (33%) were pumped into the dispersion at a rate of 19.6 ml / min under vigorous stirring for 280 minutes, while controlling the temperature to be close to ambient.

[0226] The pH was monitored while adding the H2O2, until the pH dropped below 6, down to about 5.9; the results are provided in the table below:

[0227] EXAMPLE 23

[0228] 1.575kg AgNCf were dissolved in 2.0 liters of Dl-water (solution A). 0.500kg PVP (MW=55,000, k=29.2) were dissolved in 16.0 liters of Dl-water (solution B). 0.550kg KOH were dissolved in 2.0 liters of Dl-water (solution C). Solution A was then poured into solution B under vigorous stirring, and Solution C was added to the mixed solution (A&B) at ambient temperature, under vigorous stirring, to form a colloidal precipitate of Ag2O.

[0229] After stirring the dispersion for 10 minutes, 4.5 liters of H2O2 (33%) were pumped into the dispersion at a rate of around 12 ml / min under vigorous stirring for 375 minutes, while controlling the temperature to be at most 41°C.

[0230] The pH was monitored while adding the H2O2, until the pH dropped below 6, down to about 5.7; the results are provided in the table below:

[0231] EXAMPLE 24 — Procedure for D50 Determination

[0232] (1) Measure and count the particles in a SEM image (magnification 100k), in a field area of 617,000-650,000 nm2, (by defining a square area in the image, using the scale in the image to calculate the size of the area in mm, and marking the area), where the particles cover at least 65% of the area, and the area contains at least 150 particles.

[0233] (2) Measure the diameter with a suitable mm ruler, if the particles are not spherical, measure the longest diameter. Count the particles for each size. Particles that are on the borders of the area are to be counted if at least 50% of the particle area is within the border of the (image) area. Use the scale in the SEM to calculate the sizes in nm. Calculate the D50 of the sample and solve

[0234] EXAMPLE 25 — Procedure for Determination of Aggcoarse

[0235] As used herein in the specification and in the claims section that follows, the term “Aggcoarse” refers to the fraction of agglomerated particles within the coarse particles of the paste. The same field area used to determine D50 is utilized to determine Aggcoarse. All of the particles within the Scoarse fraction are evaluated by one skilled in the art of particle identification. Crystalline coarse particles are generally identifiable by their regular faces, which are also typically smooth relative to their agglomerated counterparts.

[0236] Similarly, the concentration, by number, of triangular-faced nanometric silver particles within the plurality of nanometric silver particles, CT, is determined by manual counting of particles in SEM fields.

[0237] Figure 4 is an HRSEM image showing the field of Figure 1, along with markings illustrating the measurement of the longest diameter of select nano-particles, according to this procedure. The selected particles have a long dimension of 174nm and 291nm, respectively.

[0238] Figure 5 is an HRSEM image showing a portion of the field of Figures 1 and 4, in which the diameters of the nano-particles have been manually measured, according to the procedure stipulated in Example 24. The D50, which is a particle number averaged parameter, and not a particle volume averaged parameter, was calculated to be 34nm, yielding a value of 68nm for Sc oarse •

[0239] Figure 6 is a plot of a cumulative, number-based Particle Size Distribution based on the calculations of the particle diameters in the image of Figure 5, from which D50 may be extracted.

[0240] The term "average secondary particle size", as used herein, in the specification and in the claims section that follows, is used with regard to the silver oxide and the silver particles, and refers to the mean diameter of the silver oxide and silver particles, and is specifically meant to include the diameters of agglomerated particles.

[0241] As used herein in the specification and in the claims section that follows, the term "mean diameter", used with regard to silver oxide and silver particles, refers to a volume- weighted equivalent spherical particle size (dso), calculated by a LUMiSizer® Dispersion Analyzer from the sedimentation rate of the particles while applying centrifugal force, or if unavailable, by a functionally-equivalent particle size analyzer, or if unavailable, by a Brookhaven 90Plus particle size analyzer (Brookhaven Instruments Corporation, Holtsville, New York), or if unavailable, by a functionally-equivalent particle size analyzer suited for measuring equivalent spherical particle size throughout the range of 5 to 1000 nanometers.

[0242] In determining this mean diameter, the particle size analysis is performed in a professional and reproducible manner using the particle size analyzer, by personnel trained and qualified to operate the particle size analyzer. A representative sample of the solid particles (silver oxide, silver) is to be taken.

[0243] As used herein in the specification and in the claims section that follows, the term "D50", used with regard to a population of silver particles, refers to a number-based median diameter based on the nanoparticles as they appear in a SEM image, and determined according to Example 24. As used herein in the specification and in the claims section that follows, the term "polyvinylpyrrolidone", or PVP, refers to a water-soluble polymer having or including the following molecular structure:

[0244] The market of PVP dispersants includes polymers produced by attaching (e.g., grafting) PVP onto other moieties. As used herein in the specification and in the claims section that follows, the term "polyvinylpyrrolidone" includes such dispersants.

[0245] As used herein in the specification and in the claims section that follows, the term "silver compound", and the like, is meant to include an inorganic silver salt, an organic silver salt, or an organo-silver complex.

[0246] As used herein in the specification and in the claims section that follows, the term "soluble silver compound", and the like, refers to a silver compound having a solubility of at least 10 grams / liter in water or in ethanol at 25°C. Preferably, the soluble silver compound has a solubility of at least 25 grams / liter in water or in ethanol at 25°C, and more preferably, a solubility of at least 50 grams / liter in water or in ethanol at 25°C.

[0247] As used herein in the specification and in the claims section that follows, the term “volatile solvent”, such as an organic volatile solvent, refers to a solvent that, in pure form, has a boiling point below 105°C, and typically 100°C or less, at atmospheric or ambient pressure.

[0248] As used herein in the specification and in the claims section that follows, the term “non-volatile solvent”, such as an organic non-volatile solvent, refers to a solvent that, in pure form, has a boiling point above 105°C, and typically above 110°C, at atmospheric or ambient pressure.

[0249] As used herein in the specification and in the claims section that follows, the term “single-crystal” and the like, with respect to silver particles, refers to a single-crystal silver particle as determined by the standard Electron Back Scattered Diffraction (EBSD) method described in Example 7 hereinabove. Any quantitative assessment of a fraction or percentage of single-crystal particles within a sample, as used herein in the specification and in the claims section that follows, may be performed according to the quantitative EBSD determination method described in Example 8. While in theory, this quantitative EBSD scanning method provides a quantitative evaluation of a top layer or cross-section of the sample, in practice, this quantitative evaluation closely reflects the single crystal fraction of silver particles, particularly for samples that do not have an extremely broad particle size distribution.

[0250] The silver content of the paste is characterized as follows: about 0.1g of the paste (weighed in an analytical balance) is deposited in a crucible, the crucible is placed in a furnace at 600°C for 5 minutes, taken out left to cool to room temperature and the residue weighed in the analytical balance; the silver content is calculated from the weight difference. In this analytical procedure, the weight of any silver oxide in the sample (which tends to be insignificant) is represented solely by its silver content. Any dispersant present in the silver particles is combusted, and does not contribute to the silver content of the paste, nor to the concentration of silver particles.

[0251] As used herein in the specification and in the claims section that follows, the term “concentration of silver particles” and the like (e.g., “concentration of nanometric silver particles within the conductive paste”) is determined according to the above-provided silver content analysis. By way of example, in a nanometric silver paste sample containing 78% Ag° and 1.2% Ag2O, by weight, the “concentration of nanometric silver particles” within the paste would be [78% + 1.2%»(2» 107.87) / 231.74], or 79.1%. There is an insignificant difference — 0.1% — between the weight of the silver particles with or without the oxide of the Ag2O. Thus, as used herein in the specification and in the claims section that follows, the term “majority”, with respect to silver particles, refers to at least one of the following: at least 30% of the randomly-selected EBSD scans produce a substantially perfect match for a silver single crystal, according to the procedure described in Example 8, or more than 50% of the silver particles, based on the number of silver particles.

[0252] As used herein in the specification and in the claims section that follows, the term “standard storage modulus” refers to the storage modulus (G1) measured in an amplitude sweep at a fixed frequency of 10s-1and at 25°C.

[0253] Similarly, as used herein in the specification and in the claims section that follows, the term “standard loss modulus” refers to the loss modulus (G”) measured under the same conditions.

[0254] Similarly, as used herein in the specification and in the claims section that follows, the term “flow point” refers to the point at which G' and G" intersect, at 25°C. While pastes have flow points, fluid dispersions do not. Rheology measurements are performed using a TA Instruments HR10 Discovery Hybrid Rheometer apparatus using the standard programs supplied with therewith, or e.g., if unavailable, by a functionally similar rheometer. A ~l. lg sample of the paste is used, and the sample is left to stabilize for a few minutes within the apparatus before the rheology measurements are performed. For viscosity measurements of the paste, a flow test is performed using a plate-plate geometry and temperature control over the bottom plate (set at 25°C). The shear rate range is 0.01 sec1to 100 se1, for a duration of 900 seconds, points per decade = 3. All viscosity characterizations as used herein refer to this 25°C viscosity.

[0255] Modulus (G’, G”) and flow point measurements are performed using the same equipment as above. Oscillation test parameters: Constant Frequency of 1 Hz, Torque range of 0.03 uN-m to 100000 uN-m, points per decade = 5.

[0256] The silver content of the paste is characterized as follows: about 0.1 g of the paste (weighed in an analytical balance) is deposited in a crucible, the crucible is placed in a furnace at 600°C for 5 minutes, taken out, left to cool to room temperature; the residue is weighed in the analytical balance; the silver content is calculated from the weight difference.

[0257] INVENTIVE CONCEPTS

[0258] The present disclosure includes the following Inventive Concepts: Concept 1. A conductive paste comprising:

[0259] (a) a carrier liquid;

[0260] (b) a plurality of nanometric silver particles, in which at least 20% of said nanometric silver particles are single crystals (e.g., EBSD-determined single crystals), said plurality of nanometric silver particles having an average secondary particle size (at least one of dso and D50) within a range of 60 to 200 nanometers, said nanometric silver particles disposed within said carrier liquid; and

[0261] (c) at least one dispersant; wherein a concentration of said nanometric silver particles within the conductive paste is within a range of 80 to 92%, by weight; wherein said plurality of nanometric silver particles have a number fraction of coarse particles (F coarse), defined by a characteristic particle size (Scoarse) :

[0262] Scoarse > 2 • D50 and wherein Fc oarse is at most 0.05. Concept 2. A conductive paste comprising:

[0263] (a) a carrier liquid;

[0264] (b) a plurality of nanometric silver particles, in which at least 20% of said nanometric silver particles are single crystals (e.g., EBSD-determined single crystals), said plurality of nanometric silver particles having an average secondary particle size (at least one of dso and D50) within a range of 60 to 200 nanometers, said nanometric silver particles disposed within said carrier liquid; and

[0265] (c) at least one dispersant; wherein a concentration of said nanometric silver particles within the conductive paste is within a range of 80 to 92%, by weight; wherein said plurality of nanometric silver particles have a number fraction of coarse particles (F coarse), defined by a characteristic particle size (Scoarse) :

[0266] Scoarse > 2 • D50 wherein the fraction of agglomerated particles within said coarse particles is Aggcoarse; and wherein the product of Fc oarse and Aggc oarse (Fc oarse • Aggc oarse ) is at most 0.05.

[0267] Concept 3. A conductive paste comprising:

[0268] (a) a carrier liquid;

[0269] (b) a plurality of nanometric silver particles, in which at least 60% of said nanometric silver particles are single crystals (e.g., EBSD-determined single crystals), said plurality of nanometric silver particles having an average secondary particle size (at least one of dso and D50) within a range of 60 to 200 nanometers, said nanometric silver particles disposed within said carrier liquid; and

[0270] (c) at least one dispersant; wherein a concentration of said nanometric silver particles within the conductive paste is within a range of 78% to 92%, by weight.

[0271] Concept 4. A conductive paste comprising:

[0272] (a) a carrier liquid;

[0273] (b) a plurality of nanometric silver particles, in which at least 30% of said nanometric silver particles are single crystals (e.g., EBSD-determined single crystals), said plurality of nanometric silver particles having an average secondary particle size (at least one of dso and D50) within a range of 60 to 200 nanometers, said nanometric silver particles disposed within said carrier liquid; and (c) at least one dispersant; wherein a concentration of said nanometric silver particles within the conductive paste is within a range of 78% to 92%, by weight; and wherein the viscosity of the conductive paste at 25°C fulfills at least one of the following structural properties:

[0274] (i) at a shear rate of 0.02 sec1, the viscosity is at most 950,000,000cP;

[0275] (ii) at a shear rate of 0.2 sec1, the viscosity is at most 450,000,000cP;

[0276] (iii) at a shear rate of 1.0 sec1, the viscosity is at most 50,000,000cP; and

[0277] (iv) at a shear rate of 10 sec1, the viscosity is at most l,000,000cP.

[0278] Concept 4A. The paste of any of Concepts 1 to 4, wherein the single crystals are EBSD- determined single crystals.

[0279] Concept 5. The paste of any of Concepts 2 to 4A, wherein Fcoarse is at most 0.30.

[0280] Concept 6. The paste of Concept 5, wherein Fc oarse is at most 0.25.

[0281] Concept 7. The paste of Concept 5, wherein Fc oarse is at most 0.20.

[0282] Concept 8. The paste of Concept 5, wherein Fc oarse is at most 0.12.

[0283] Concept 9. The paste of any one of the previous Concepts, wherein Fc oarse is at most

[0284] 0.08.

[0285] Concept 9A. The paste of Concept 9, wherein Fc oarse is at most 0.065.

[0286] Concept 9B. The paste of Concept 9, wherein Fc oarse is at most 0.05.

[0287] Concept 9C. The paste of Concept 9, wherein Fc oarse is at most 0.04.

[0288] Concept 9D. The paste of Concept 9, wherein Fc oarse is at most 0.03.

[0289] Concept 9E. The paste of Concept 9, wherein Fc oarse is at most 0.02.

[0290] Concept 10. The paste of any one of the previous Concepts, wherein said plurality of nanometric silver particles have a number fraction of coarse particles (Fcoarse), defined by a characteristic particle size (Sc oarse)

[0291] Scoarse > 2 • D50 wherein the fraction of agglomerated particles within said coarse particles is Aggcoarse; and wherein Fc oarse Aggc oarse is at most 0.04.

[0292] Concept 11. The paste of Concept 10, wherein Fc oarse • Aggc oarse is at most 0.03.

[0293] Concept 12. The paste of Concept 10, wherein Fc oarse • Aggc oarse is at most 0.02.

[0294] Concept 13. The paste of any of the previous Concepts, wherein the flow point of the paste is at most 12,000Pa.

[0295] Concept 13 A. The paste of any of the previous Concepts, wherein the flow point of the paste is at most 9,000Pa. Concept 13B. The paste of any of the previous Concepts, wherein the flow point of the paste is at most 7,500Pa.

[0296] Concept 14. The paste of any of the previous Concepts, wherein the flow point of the paste is at least 8Pa.

[0297] Concept 15. The paste of Concept 13 or 14, wherein the flow point of the paste is at least lOOPa.

[0298] Concept 16. The paste of Concept 15, wherein the flow point of the paste is at least l,500Pa.

[0299] Concept 16A. The paste of Concept 15, wherein the flow point of the paste is at least

[0300] 3,500Pa.

[0301] Concept 17. The paste of any of the previous Concepts, wherein the 0.02-sec-1-viscosity at 25°C is at most 700,000,000cP.

[0302] Concept 18. The paste of Concept 17, wherein the 0.02-sec_1-viscosity is at most 400,000,000cP.

[0303] Concept 19. The paste of Concept 17, wherein the 0.02-sec_1-viscosity is at most 200,000,000cP.

[0304] Concept 20. The paste of Concept 17, wherein the 0.02-sec-1-viscosity is at most 100,000,000cP.

[0305] Concept 20A. The paste of Concept 17, wherein the 0.02-sec-1-viscosity is at most 20,000,000cP.

[0306] Concept 20B. The paste of Concept 17, wherein the 0.02-sec-1-viscosity is at most 8,000,000cP.

[0307] Concept 21. The paste of any of the previous Concepts, wherein the 0.02-sec_1-viscosity at 25°C is at least 50,000cP.

[0308] Concept 22. The paste of Concept 21, wherein the 0.02-sec-1-viscosity is at least 125,000cP.

[0309] Concept 23. The paste of Concept 21, wherein the 0.02-sec-1-viscosity is at least 250,000cP.

[0310] Concept 24. The paste of any of the previous Concepts, wherein the 0.2-sec-1-viscosity at 25°C is at most 350,000,000cP.

[0311] Concept 25. The paste of Concept 24, wherein the 0.2-sec-1-viscosity is at most 250,000,000cP.

[0312] Concept 26. The paste of Concept 24, wherein the 0.2-sec-1-viscosity is at most 150,000,000cP. Concept 27. The paste of Concept 24, wherein the 0.2-sec-1-viscosity is at most 75,000,000cP.

[0313] Concept 28. The paste of any of the previous Concepts, wherein the 0.2-sec-1-viscosity at 25°C is at least 50,000cP.

[0314] Concept 29. The paste of Concept 28, wherein the 0.2-sec-1-viscosity is at least 750,000cP.

[0315] Concept 30. The paste of any of the previous Concepts, wherein the 1.0-sec_1-viscosity at 25°C is at most 40,000,000cP.

[0316] Concept 31. The paste of Concept 30, wherein the 1.0-sec_1-viscosity is at most

[0317] 25,000,000cP.

[0318] Concept 32. The paste of Concept 30, wherein the 1.0-sec_1-viscosity is at most

[0319] 15,000,000cP.

[0320] Concept 33. The paste of Concept 30, wherein the 1.0-sec_1-viscosity is at most

[0321] 10,000,000cP.

[0322] Concept 34. The paste of any of the previous Concepts, wherein the 1.0-sec_1-viscosity at 25°C is at least 20,000cP.

[0323] Concept 35. The paste of Concept 34, wherein the 1.0-sec_1-viscosity is at least

[0324] 350,000cP.

[0325] Concept 36. The paste of any of the previous Concepts, wherein the percentage of said nanometric silver particles that are single crystals, PCs, is at least 30%.

[0326] Concept 37. The paste of Concept 36, wherein PCs is at least 40%.

[0327] Concept 38. The paste of Concept 36, wherein PCs is at least 50%.

[0328] Concept 39. The paste of Concept 36, wherein PCs is at least 65%.

[0329] Concept 40. The paste of Concept 36, wherein PCs is at least 75%.

[0330] Concept 41. The paste of Concept 36, wherein PCs is at least 85%.

[0331] Concept 41 A. The paste of any of the previous Concepts, wherein the percentage of said nanometric silver particles that are single crystals, PCs, is at most 90%.

[0332] Concept 41B. The paste of Concept 41 A, wherein PCs is at most 87%.

[0333] Concept 42. The paste of any of the previous Concepts, wherein the concentration of silver oxide (CAg2O) within the paste is at most 5%, on a silver (metal) to total silver basis. Concept 43. The paste of Concept 42, wherein CAg2O is at most 3%.

[0334] Concept 44. The paste of Concept 42, wherein CAg2O is at most 1.5%.

[0335] Concept 45. The paste of Concept 42, wherein CAg2O is at most 0.7%.

[0336] Concept 46. The paste of Concept 42, wherein CAg2O is at most 0.3%. Concept 47. The paste of any of the previous Concepts, wherein the standard storage modulus (G’) is at most 1,500,000 Pa.

[0337] Concept 48. The paste of Concept 47, wherein G’ is at most 1,100,000 Pa.

[0338] Concept 49. The paste of Concept 47, wherein G’ is at most 100,000 Pa.

[0339] Concept 50. The paste of Concept 47, wherein G’ is at most 30,000 Pa.

[0340] Concept 51. The paste of any of Concepts 47-50, wherein G’ is at least 50 Pa.

[0341] Concept 52. The paste of Concept 51, wherein G’ is at least 2,000 Pa.

[0342] Concept 53. The paste of Concept 51, wherein G’ is at least 25,000 Pa.

[0343] Concept 51. The paste of any of the previous Concepts, wherein the standard loss modulus (G”) is at most 250,000 Pa.

[0344] Concept 52. The paste of Concept 51, wherein G” is at most 180,000 Pa.

[0345] Concept 53. The paste of Concept 51, wherein G” is at most 50,000 Pa.

[0346] Concept 54. The paste of any of Concepts 51-53, wherein G” is at least 30 Pa.

[0347] Concept 55. The paste of Concept 54, wherein G” is at least 20,000 Pa.

[0348] Concept 56. The paste of any of the previous Concepts, wherein the 10.0-sec_1-viscosity is at most 2,000,000cP.

[0349] Concept 57. The paste of Concept 56, wherein the 10.0-sec-1-viscosity is at most l,000,000cP.

[0350] Concept 58. The paste of Concept 56, wherein the 10.0-sec-1-viscosity is at most 300,000cP.

[0351] Concept 59. The paste of any of Concepts 56-58, wherein the 10.0-sec_1-viscosity is at least 5,000cP.

[0352] Concept 60. The paste of Concept 59, wherein the 10.0-sec_1-viscosity is at least 100,000cP.

[0353] Concept 61. The paste of Concept 59, wherein the 10.0-sec_1-viscosity is at least 200,000cP.

[0354] Concept 62. The paste of any of the previous Concepts, wherein the concentration, by number, of triangular-faced nanometric silver particles within the plurality of nanometric silver particles, CT, is at least 2%.

[0355] Concept 70. The paste of Concept 62, wherein CT is at least 4%.

[0356] Concept 71. The paste of Concept 62, wherein CT is at least 7%.

[0357] Concept 72. The paste of Concept 62, wherein CT is at least 10%.

[0358] Concept 73. The paste of Concept 62, wherein CT is at least 13%.

[0359] Concept 74. The paste of Concept 62, wherein CT is at least 16%. Concept 75. The paste of Concept 62, wherein CT is at least 18%.

[0360] Concept 76. The paste of Concept 62, wherein CT is at least 20%.

[0361] Concept 77. The paste of Concept 62, wherein CT is at least 25%.

[0362] Concept 78. The paste of any of Concepts 62 to 77, wherein CT is at most 80%.

[0363] Concept 79. The paste of Concept 78, wherein CT is at most 70%.

[0364] Concept 80. The paste of Concept 78, wherein CT is at most 60%.

[0365] Concept 81. The paste of Concept 78, wherein CT is at most 45%.

[0366] Concept 82. The paste of Concept 78, wherein CT is at most 35%.

[0367] Concept 83. The paste of any of Concepts 69-82, wherein CT is determined by manual counting of particles.

[0368] Concept 84. The paste of Concept 83, wherein CT is determined by manual counting of particles in SEM fields.

[0369] Concept 85. The paste of any of the previous Concepts, wherein the total concentration of additives is at most 7%, by weight.

[0370] Concept 86. The paste of any of the previous Concepts, wherein the total concentration of additives is at most 7%, said additives consisting of at least one of a binding agent, rheology modifier, surfactant, dispersant, and conductivity enhancer.

[0371] Concept 87. The paste of Concept 85 or 86, wherein the total concentration of the additives is at most 5.5%, by weight.

[0372] Concept 88. The paste of Concept 87, wherein the total concentration of the additives is at most 4.5%, by weight.

[0373] Concept 89. The paste of any of the previous Concepts, wherein the concentration of water (Cw) within the carrier liquid is at most 20%.

[0374] Concept 90. The paste of Concept 89, wherein Cw is at most 12%.

[0375] Concept 91. The paste of Concept 89, wherein Cw is at most 6%.

[0376] Concept 92. The paste of Concept 89, wherein Cw is at most 2%.

[0377] Concept 93. The paste of any of the previous Concepts, wherein the concentration of water (Cw) within the carrier liquid is at least 0.05%.

[0378] Concept 94. The paste of Concept 93, wherein Cw is at least 0.5%.

[0379] Concept 95. The paste of Concept 93, wherein Cw is at least 1.5%.

[0380] Concept 96. The paste of any of Concepts 89-91, wherein Cw is at least 4%.

[0381] Concept 97. The paste of Concept 89 or 90, wherein Cw is at least 8%.

[0382] Concept 98. The paste of any of the previous Concepts, wherein at least one of dso and

[0383] D50 is at most 180nm. Concept 99. The paste of Concept 98, wherein at least one of dso and D50 is at most 160nm.

[0384] Concept 100. The paste of Concept 98, wherein at least one of dso and D50 is at most 140nm.

[0385] Concept 101. The paste of Concept 98, wherein at least one of dso and D50 is at most 120nm.

[0386] Concept 102. The paste of Concept 98, wherein at least one of dso and D50 is at most

[0387] HOnm.

[0388] Concept 103. The paste of any of the previous Concepts, wherein at least one of dso and D50 is at least 65nm.

[0389] Concept 104. The paste of Concept 103, wherein at least one of dso and D50 is at least 70nm.

[0390] Concept 105. The paste of Concept 103, wherein at least one of dso and D50 is at least 75nm.

[0391] Concept 106. The paste of any of the previous Concepts, wherein the concentration of said nanometric silver particles within the paste CNS is at least 82%.

[0392] Concept 107. The paste of Concept 106, wherein CNS is at least 83%.

[0393] Concept 108. The paste of Concept 106, wherein CNS is at least 84%.

[0394] Concept 109. The paste of Concept 106, wherein CNS is at least 85%.

[0395] Concept 110. The paste of any of the previous Concepts, wherein the concentration of said nanometric silver particles within the paste CNS is at most 90%.

[0396] Concept 111. The paste of Concept 110, wherein CNS is at most 89%.

[0397] Concept 112. The paste of Concept 110, wherein CNS is at most 88%.

[0398] Concept 113. The paste of Concept 110, wherein CNS is at most 87%.

[0399] Concept 114. The paste of any of the previous Concepts, wherein the concentration of said nanometric silver particles within the paste CNS is within a range of 80 to 86%.

[0400] Concept 115. The paste of Concept 114, wherein the viscosity at a shear rate of 0.02sec-1is within a range of 120,000 to 5,000,000cP.

[0401] Concept 116. The paste of Concept 115, wherein the viscosity is at most 3,000,000cP.

[0402] Concept 117. The paste of Concept 115, wherein the viscosity is at most l,500,000cP.

[0403] Concept 118. The paste of Concept 115, wherein the viscosity is at most 800,000cP.

[0404] Concept 119. The paste of Concept 114, wherein the viscosity at a shear rate of Isec1is within a range of 15,000 to 3,500,000cP. Concept 120. The paste of Concept 119, wherein the viscosity at a shear rate of Isec1is at most l,000,000cP.

[0405] Concept 121. The paste of any of the previous Concepts, wherein a surfactant concentration is within a range of 0.2% to 2.7%.

[0406] Concept 122. The paste of any of the previous Concepts, wherein the total concentration of the at least one dispersant Ctd is at most 5%.

[0407] Concept 123. The paste of Concept 122, wherein Ctd is at most 3.5%.

[0408] Concept 124. The paste of Concept 122, wherein Ctd is at most 2.5%.

[0409] Concept 125. The paste of Concept 122, wherein Ctd is at most 2.0%.

[0410] Concept 126. The paste of Concept 122, wherein Ctd is at most 1.8%.

[0411] Concept 127. The paste of any of Concepts 122-126, wherein Ctd is at least 1.6%.

[0412] Concept 128. The paste of any of Concepts 122-125, wherein Ctd is at least 1.8%.

[0413] Concept 129. The paste of any of Concepts 122-124, wherein Ctd is at least 2.0%.

[0414] Concept 130. The paste of any of Concepts 122-129, wherein Ctd is at least 1.6%.

[0415] Concept 131. The paste of any of Concepts 122-130, wherein a ratio of Ctd to the concentration of the nanometric silver particles within the paste Rd-Ag is within a range of

[0416] 0.016 to 0.05.

[0417] Concept 132. The paste of Concept 131, wherein Rd-Ag is at most 0.04.

[0418] Concept 133. The paste of Concept 131, wherein Rd-Ag is at most 0.035.

[0419] Concept 134. The paste of Concept 131, wherein Rd-Ag is at most 0.03.

[0420] Concept 135. The paste of Concept 131, wherein Rd-Ag is at most 0.025.

[0421] Concept 136. The paste of any of Concepts 131-135, wherein Rd-Ag is at least 0.018.

[0422] Concept 137. The paste of Concept 136, wherein Rd-Ag is at least 0.02.

[0423] Concept 138. The paste of Concept 136, wherein Rd-Ag is at least 0.023.

[0424] Concept 139. The paste of any of the previous Concepts, wherein the at least one dispersant includes PVP.

[0425] Concept 140. The paste of Concept 139, wherein a PVP weight fraction WPVP of the PVP in the at least one dispersant is at least 0.50.

[0426] Concept 141. The paste of Concept 139, wherein WPVP is at least 0.85.

[0427] Concept 142. The paste of Concept 139, wherein WPVP is at least 0.95.

[0428] Concept 143. The paste of any one of Concepts 139-142, wherein an average molecular weight (AMWPVP) of the PVP is at most 100,000Da.

[0429] Concept 144. The paste of Concept 143, wherein AMWPVP is at most 80,000Da.

[0430] Concept 145. The paste of Concept 143, wherein AMWPVP is at most 65,000Da. Concept 146. The paste of Concept 143, wherein AMWPVP is at most 55,000Da.

[0431] Concept 147. The paste of Concept 143, wherein AMWPVP is at most 45,000Da.

[0432] Concept 148. The paste of Concept 143, wherein AMWPVP is at most 35,000Da.

[0433] Concept 149. The paste of any one of Concepts 143-148, wherein AMWPVP is at least

[0434] 5,000Da.

[0435] Concept 150. The paste of Concept 149, wherein AMWPVP is at least 8,000Da.

[0436] Concept 151. The paste of Concept 149, wherein AMWPVP is at least 10,000Da.

[0437] Concept 152. The paste of Concept 149, wherein AMWPVP is at least 15,000Da.

[0438] Concept 153. A method of producing a conductive silver paste, substantially as described herein.

[0439] Concept 154. The method of Concept 122, the method utilizing any feature or features of in Concepts 1 to 152.

[0440] It will be appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination. Similarly, the content of a claim depending from one or more particular claims may generally depend from the other, unspecified claims, or be combined with the content thereof, absent any specific, manifest incompatibility therebetween.

[0441] Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims. All publications, patents and patent applications mentioned in this specification, including U.S. Patent No. 6,277,169 and WO Patent Publication Nos. 2003 / 080,231 and 2012 / 078,590, 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. In addition, citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art to the present invention.

Claims

1. WHAT IS CLAIMED IS:

1. A conductive paste comprising:(a) a carrier liquid;(b) a plurality of nanometric silver particles, in which at least 20% of said nanometric silver particles are single crystals, said plurality of nanometric silver particles having an average secondary particle size (at least one of dso and D50) within a range of 60 to 200 nanometers, said nanometric silver particles disposed within said carrier liquid; and(c) at least one dispersant; wherein a concentration of said nanometric silver particles within the conductive paste is within a range of 80 to 92%, by weight; wherein said plurality of nanometric silver particles have a number fraction of coarse particles (F coarse), defined by a characteristic particle size (Scoarse):Scoarse > 2 • D50 and wherein Fc oarse is at most 0.05.

2. The paste of claim 1, wherein the fraction of agglomerated particles within said coarse particles is Aggc oarse i and wherein the product of Fc oarse and Aggc oarse (Fc oarse • Aggc oarse ) is at most 0.04.

3. The paste of claim 1 or claim 2, wherein the viscosity of the conductive paste at 25°C, at a shear rate of 0.02 sec1, is at most 950,000,000cP.

4. The paste of any one of claims 1 to 3, wherein the viscosity of the conductive paste at 25°C, at a shear rate of 1.0 se1, is at most 50,000,000cP.

5. The paste of any one of claims 1 to 4, wherein the flow point of the conductive paste at 25°C is at most 7,500Pa.

6. The paste of any one of claims 1 to 5, wherein the standard storage modulus (G’) is at most 1,500,000 Pa.

7. The paste of any one of claims 1 to 6, wherein the concentration of said nanometric silver particles within the conductive paste CNS is within a range of 80 to 86%.

8. The paste of claim 7, wherein the viscosity at a shear rate of 0.02SCC1is within a range of 120,000 to 5,000,000cP.

9. The paste of claim 7, wherein the viscosity at a shear rate of l.Osec1is within a range of 15,000 to 3,500,000cP.

10. The paste of claim 9, wherein the viscosity at said shear rate of l.Osec’1is at most l,000,000cP.

11. The paste of any one of claims 1 to 10, wherein the surfactant concentration is within a range of 0.5% to 2.7%.

12. The paste of any one of claims 1 to 11, wherein the percentage of said nanometric silver particles that are single crystals, PCs, is at least 40%.

13. The paste of claim 12, wherein PCs is at least 50%.

14. The paste of any one of claims 1 to 13, wherein said concentration of said nanometric silver particles within the conductive paste is at least 82%, by weight.

15. The paste of any one of claims 1 to 14, wherein said concentration of said nanometric silver particles within the conductive paste is at most 89%, by weight.

16. The paste of any one of claims 1 to 15, wherein said single crystals are EBSD- determined single crystals.

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