TECHNOLOGICAL FIELD

This invention generally relates to copper flakes for use in conductive inks and processes for their preparation.

BACKGROUND

Additive manufacturing of electronic devices, known also as printed electronics is a growing field in both academia and the electronics industry. These technologies are considered as methods alternative to the conventional lithography and etching of laminated sheets to form electrical circuitry (e.g. printed circuit boards - PCB). The conventional processes have three main limitations: many process steps, formation of large amounts of toxic waste, and a low material efficiency as only a small portion of a metal that enters a PCB factory on laminated FR4 sheets is present in the final product.

Conductive inks should enable printing conductive patterns on various substrates, and therefore they should have a high metal loading. There are two main types of metal- based conductive inks: inks containing metal particles that are printed and sintered, and inks containing metal precursors that are printed and decomposed to form a conducting metal. Currently, most available conductive inks are based on silver. However, the high cost of this metal limits commercial use and therefore conductive inks based on low cost metals, such as copper, are required. An important drawback of copper is its oxidation at ambient conditions that prevents obtaining conductive printed patterns. Therefore, there is an unmet need for a stable copper ink which can provide good conductivity.

Many methods have been developed for preparing copper particles in top-down and bottom-up approaches. In top-down methods, such as pulsed wire discharge and pulsed laser ablation, particles are formed by breaking bulk material into smaller particles. In bottom-up methods, particles are synthesized by reducing copper ions to copper atoms in solutions, and formed copper atoms aggregate to particles ("wet chemistry" approach). Synthesis by reduction of copper ions is now the most common route for preparation of copper particles. In order to form small particles and to prevent their aggregation, dispersing agents should be added. This means that the formed particles are coated with an insulating material that prevents their close contact in a printed layer and therefore harms the obtained conductivity. An alternative and barely used method to form copper particles is thermal decomposition of copper salts in a dry environment. Rosenband et al [1] demonstrated the formation of copper and nickel particles by decomposition of their metal formate, but did not report their use in any application. Copper formate, a coper salt that has been previously used by the authors as a copper precursor in conductive inks decomposes rapidly at temperatures above 180 °C with self-reduction of the copper ion. In an inert environment, the only solid product of the decomposition is pure copper. Decomposition follows the general Scheme (1).

Cu(HCOO ) _{2 (s)} ® Cu _(s) + H _{2 (g)} + 2C0 _{2 (g)} (1)

Conventionally, conductive inks are based on spherical metal nanoparticles. However, in past years there is a growing interest in the use of flakes [2, 3] which are considered as excellent conducting material due to the increased contact area between the stacked particles. Currently, silver flakes are wildly used in conductive inks (e.g. for printing electrodes in solar cells) and conductive adhesives.

Copper flakes are usually formed by processing metal particles in a ball mill. During milling, when two grinding balls collide, a particle trapped in between may undergo fracturing (size reduction) or plastic deformation, depending on the ductility/brittleness of the material. As copper is relatively ductal, the particles are plastically deformed. In addition, when two metal particles are pressed together they can undergo cold welding, causing agglomeration. Direct metal contact can be prevented during milling of copper particles by adding an organic surfactant. This prevents welding of small particles to form larger ones, so the individual particles undergo only the shape deformation with reducing thickness and an increasing diameter.

Carboxylic acids are often added to the milling process as surfactants/dispersing agents as they prevent agglomeration. When mixed with a solvent and copper, these fatty acids can adsorb on the copper surface through a chemical bond between one or more of the oxygen atoms of the carboxyl group and a copper atom. This bond anchors the acid to the particle, positioning the alkyl chains facing away from the particle surface, thus coating the particles. The alkyl chains weaken the friction between particles, which is affected by the chain length of the fatty acid: the longer the chain, the lower the friction

[4]. Sze et al. [2] prepared copper flakes by dry ball milling (without a solvent) of copper particles, and found that both prolonged milling duration and addition of decanoic acid reduced the obtained flakes diameter. Wu et al. [3] prepared Cu flakes with a diameter of 9 pm by synthesizing copper particles followed by dry milling. Cipolloni et al. [5] used high energy mechanical milling to follow microstructural evolution of copper powders in both continuous and interrupted processes, forming flakes with a diameter of 50-300 pm. It was reported that when continuously milled, the particles were first flattened and then welded. In order to avoid welding, the milling was interrupted periodically to enable cooling down of the system.

REFERENCES

[1] Rosenband, V.; Gany, A., Preparation of nickel and copper submicrometer particles by pyrolysis of their formates. J. Mater. Process. Technol. 2004, 153, 1058.

[2] Tam, S. K.; Fung, K. Y.; Ng, K. M., Copper pastes using hi moda l particles for flexible printed electronics. Journal of Materials Science 2016, 51 (4), 1914.

[3] Wu, S.; Gao, R.; Xu, L., Preparation of micron-sized flake copper powder for base-metal-electrode multi-layer ceramic capacitor. J. Mater. Process. Technol. 2009, 209

[3], 1129.

[4] Hardy, W.; Doubleday, I. In Boundary lubrication.— The paraffin series, Proc. R. Soc. Lond. A, The Royal Society: 1922; pp 550.

[5] Cipolloni, G.; Menapace, C.; Pellizzari, M.; Ischia, G., Differences of the microstructural evolution of Cu powder during continuous and interrupted mechanical milling. Powder Metall. 2017, 60 (3), 232.

[6] Zhao, Y.; Horita, Z.; Langdon, T.; Zhu, Y., Evolution of defect structures during cold rolling of ultrafine-grained Cu and Cu-Zn alloys: Influence of stacking fault energy. Materials Science and Engineering: A 2008, 474 (1-2), 342.

[7] Daniel, S. G., The adsorption on metal surfaces of long chain polar compounds from hydrocarbon solutions. Transactions of the Faraday Society 1951, 47, 1345.

[8] Loehle, S.; Matta, C. ; Minfray, C.; Le Mogne, T.; Iovine, R.; Obara, Y.; Miyamoto, A.; Martin, J., Mixed lubrication of steel by Cl 8 fatty acids revisited. Part I: Toward the formation of carboxylate. Tribology International 2015, 82, 218. [9] Loehle, S.; Matta, C. ; Minfray, C; Le Mogne, T.; Iovine, R.; Obara, Y.; Miyamoto, A.; Martin, J., Mixed lubrication of steel by C18 fatty acids revisited. Part II: Influence of some key parameters. Tribology International 2016, 94, 207.

[10] Wood, M. H.; Casford, M. T.; Steitz, R.; Zarbakhsh, A.; Welbourn, R.; Clarke, S. M., Comparative adsorption of saturated and unsaturated fatty acids at the iron oxide/oil interface. Langmuir 2016, 32 (2), 534.

GENERAL DESCRIPTION

As a person versed in the art would realize, currently, all suggested methods for preparing copper flakes require the use of expensive copper particles, or the use of preparation methods, e.g., wet synthesis, that are difficult to scale-up to industrial throughput. Therefore, a new simple and scalable method is required for preparing copper flakes.

The inventors of the technology disclosed herein have thus embarked on the development of a simple and easily scalable process for the production of copper flakes; a process which could not only be modified to produce particles of various shapes and compositions, but also a process that produces particles that find utility in a broad spectrum of industries.

In a first of its aspects, the invention provides a process for preparing copper metal flakes, the process comprising mechanically shaping copper metal particles to obtain the copper flakes, wherein the copper metal particles are prepared by reduction of at least one copper metal precursor.

In some embodiments, the process further comprises obtaining at least one copper metal precursor and reducing same to obtain copper metal particles.

Thus, the invention further provides a process for preparing copper metal flakes, wherein the process comprises:

- reducing at least one copper metal precursor to obtain copper metal particles; and

- mechanically shaping the copper metal particles;

to obtain the copper metal flakes.

As disclosed herein, mechanical shaping is carried out on copper metal particles that have been obtained by reduction of a copper metal precursor, as defined herein. As the mechanical shaping need not be carried out immediately following production of the metal particles, these may be prepared well in advance of the actual mechanical shaping or may be prepared at a time immediately preceding the step of mechanical shaping.

To obtain the metal particles, at least one copper metal precursor is used. This metal precursor is a copper-based material which can be transformed by reduction into the corresponding metallic form, namely into the copper metal particles. Such a copper- based material may be any copper salt or any copper complex, as known in the art. In some embodiments, the at least one copper metal precursor is a copper salt; and in some other embodiments, the at least one copper metal precursor is a copper complex.

It is known in the art that copper salts, as copper complexes, can appear in different combinations and variations. The salts may be selected amongst organic copper salts (wherein the anions are organic anions), inorganic salts (wherein the anions are inorganic anions) or mixed form salts (wherein the one anion is an organic anion and another is an inorganic anion). Hence, non-limiting examples of such copper salts include copper bromide, copper bromide dimethyl sulfide, copper chloride anhydrous, copper chloride dehydrate, copper cyclohexanebutyrate, copper fluoride, copper D-gluconate, copper hydroxide, copper iodide, copper iodide anhydrous, copper nitrate, copper acetate, copper butyrate, copper carbonate, copper acetate, copper chlorate, copper chromate, copper chromite, copper citrate, copper formate, copper glycinate, copper ferrocyanide, copper hexafluorosilicate, copper oleate, copper oxalate, copper oxide, copper phosphate, copper selenite, copper stearate, copper lactate, copper glycolate, copper sulfate, copper sulfate pentahydrate, copper sulfide, copper tartrate, copper perchlorate hexahydrate, copper pyrophosphate hydrate, copper tartrate hydrate, copper tetrafluoroborate, copper tetrafluoroborate hydrate, copper thyocianate, copper tungstate and copper mercuric iodide.

In some embodiments, the copper salt is selected from copper formate, copper acetate, copper oxalate, copper oleate, copper glycolate and copper lactate.

In some embodiments, the copper salt is an organic copper metal.

In some embodiments, the copper salt is copper formate.

To achieve reduction of a copper metal precursor one may utilize one of many available transformations. Some involve a radiation treatment or a thermal treatment and others may involve a chemical treatment. In some embodiments, reduction of the at least one copper metal precursor can be achieved by exposing the at least one copper metal precursor, e.g., copper formate, to actinic radiation, thermal radiation, plasma treatment or to a chemical treatment.

In some embodiments, the reduction is by exposing the at least one copper metal precursor to thermal radiation or to a chemical treatment.

In some embodiments, the reduction is by exposing the at least one copper metal precursor to thermal radiation.

In some embodiments, the reduction is by exposing the at least one copper metal precursor to a reducing environment.

When a copper metal precursor, such as a copper formate, is exposed to a thermal radiation it undergoes "thermal decomposition" . Such a decomposition is achievable at a decomposition temperature which may vary based on the copper metal precursor that is used and on additives such as organic ligands. As the thermal decomposition temperatures of metal precursors are known in the art, a practitioner will know to determine the appropriate temperature based on the selected metal precursor or precursor combination. Decomposition may be achieved in the neat or under inert conditions, e.g., to avoid competing oxidation processes.

In some embodiments, decomposition is achieved by adding an organic ligand to the copper metal precursor, e.g., copper formate, at a temperature compatible to the combination of copper metal precursor and the organic ligand. In some embodiments, decomposition is achieved by exposing the copper metal precursor, e.g., copper formate to nitrogen at a high temperature.

In some embodiments, the decomposition temperature used is between 50 °C and 350 °C. In some embodiments, the thermal treatment is continued for a period ranging from 1 minute to 180 minutes.

In some embodiments, copper formate is the metal salt. Copper formate may be decomposed under nitrogen at a temperature ranging between 190 °C and 210 °C. In some embodiments, the temperature is between 100 °C and 150 °C.

In some embodiments, thermal treatment is continued for a period of between 10 and 60 minutes.

Once the metal particles are formed as disclosed herein, they may be mechanically treated to transform their shape into flakes. Thus, the process comprises“ mechanically shaping” (or mechanically treating) the copper metal particles to transform the shape of the particles into flakes. The copper metal particles which shape is to be transformed into metal flakes may be of any size and shape and thus may require a mechanical means that is effective to structurally manipulate (or transform) any particle size and shape. The copper metal particles may be in the form of microparticles and/or nanoparticles and may be shaped as spherical particles, rod-like particles, polypod particles, elliptical particles, triangular particles, cubic particles, hexagonal particles, helical particles and prism-like particles.

In some embodiments, the copper metal particles are spherical particles.

In some embodiments, the copper metal particles are zero dimension particles (0- D), one dimension particles (1-D), two dimension particles (2-D) or three dimension particles (3-D).

In some embodiments, the copper metal particles may be classified as zero- dimension (0-D classification). In some other embodiments, the zero-dimension particles are zero-dimension (0-D) spheres.

In some embodiments, the copper metal particles are three-dimensionally amorphous.

Thus, to achieve a structural transformation of the particles one may involve or use any such mechanical means as mechanical milling, mechanical crushing, mechanical bending and mechanical flattening. In some embodiments, mechanical milling comprises or is selected from bed milling, box milling, column milling, c-frame milling, floor milling, gantry milling, horizontal boring milling, Jig borer milling, knee milling, knee- and-column milling, planer milling, bead milling, ram milling and turret milling.

In some embodiments, mechanical milling is bead milling. In some embodiments, bead milling is performed in the presence of a solvent, in a so-called wet milling processes, as disclosed hereinbelow. In some embodiments, the bead milling is or comprises wet milling or dry milling.

Analysis of the copper particles indicated that the particles’ surfaces were primarily oxidized (Cu ²⁺ ). Thus, while the bulk of the particles consisted of pure copper, the outer shell consisted copper oxide (CuO). In order to prevent agglomeration which could be caused by oxidation of the copper particles and in order to avoid cold welding of the copper particles in the milling process, an acid is added to the copper particles before milling is commenced. The addition of an acid to the copper particles also prevents formation of large and thick copper flakes; thereby resulting in smaller and thinner copper flakes. Thus, in another of its aspects, the invention provides a process for preparing copper flakes, the process comprising wet milling of copper metal particles prepared by thermal decomposition of copper formate; to thereby obtain the copper flakes.

As used herein, "wet milling" is a process which can reduce a material or a product into finer particles in the micron and submicron (nanometer) particle size range. In this process, particles are dispersed in a liquid medium by attrition, impact, crushing or shearing, thereby producing smaller particles in different shapes (according to the milling type). Different materials can be used as a grinding medium. The beads can be made of glass, plastic, ceramics, tungsten carbide and steel. Also, different media milling processes can be used, including comminution (reduction to minute particles) and de agglomeration (separating particles that are clustered together). Factors that may influence the particle size are the size of the grinding media, the time the products spend in the grinding chamber, the number of passes through the mill, the copper precursor type, ratio of amount of milling beads to the copper precursor and the speed of agitation. Using the right grinding media and equipment, one can cost-effectively create fine particles with almost no contamination.

The shape of the resulting flakes depends on the size of the initial copper particles, the milling conditions and additives. Thus, in some embodiments the morphology of the flakes described herein may be that of rounded-edged flakes as shown in Figs. 3A-I.

Wet milling may be conducted in the presence of a solvent and a milling agent (which may be a surfactant, a polymer, an organic acid or any other dispersing agent). Likewise, wet milling can be performed without the addition of an acid or a milling agent, thus merely in the presence of a solvent.

In some embodiments the solvent may be a deep eutectic solvent or an ionic liquid. A "deep eutectic solvents " are a class of solvents obtained by mixing solid compounds (that are not necessarily salts), while obtaining a eutectic mixture having a melting point that is much lower than that of the individual components.

The "ionic liquid” is an ionic material, in a liquid form, generally regarded a mixture of organic salts with low melting points (<100 °C), composed of organic cations and organic or inorganic anions. As known in the art, ionic liquids comprise an asymmetrically substituted cation (e.g., imidazolium, pyridinium, ammonium and pyrrolidinium) and a plurality of anions, especially halogen based anions (e.g., Cl, Br, I, BF4, A1C14, PF6). Non-limiting examples of ionic liquids and deep eutectic solvents (which exhibit the formula: Hydrogen Bond Acceptor : Hydrogen Bond Donor, where the molar ratio is depicted between the terms) include choline acetate 1 :1.5 glycerol, choline fluoride 1:2 urea, choline nitrate 1 :2 urea, choline chloride 1 :2 l-methylurea, choline chloride 3:7 imidazole, choline chloride 1 :2 acetamide, choline chloride 1 :2.5 2,2,2- trifluoroacetamide, choline chloride 1 :3 l,4-butanediol, choline chloride 1 :2 glycine, choline chloride 1 : 1 glucose, choline chloride 1 : 1 xylitol, choline chloride 1 : 1 sorbitol, choline chloride 1 :2 urea, choline chloride 1 :2 ethylene glycol, NH ₄ Al(S0 ₄ ) ₂ 7:2 urea, A1(N(¾) ₃ 2:2.3 urea, AlCb anhydrous 2.4:2 urea, lactic acid 9:1 alanine, lactic acid 2: 1 betaine and menthol 5:5 camphor.

In some embodiments, the solvent used in the wet milling process is an organic solvent. Non-limiting examples of such organic solvents include acetic acid, acetone, acetonitrile, benzene, 1 -butanol, 2-butanol, 2-butanone, t-butyl alcohol, carbon tetrachloride, chlorobenzene, chloroform, cyclohexane, 1 ,2-dichloroethane, diethylene glycol, diethyl ether, diglyme (diethylene glycol dimethyl ether), 1 ,2-dimethoxyethane (glyme, DME), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), l,4-dioxane, ethanol, ethyl acetate, ethylene glycol, glycerin, heptane, hexamethylphosphoramide (HMPA), hexamethylphosphorous triamide (HMPT), hexane, methanol, methyl t-butyl ether (MTBE), methylene chloride, N-methyl-2-pyrrolidinone (NMP), nitromethane, pentane, petroleum ether (ligroine), 1 -propanol, 2-propanol, pyridine, tetrahydrofuran (THF), toluene, triethyl amine, water, o-xylene, m-xylene, p-xylene, acetic anhydride, acetonitrile, benzonitrile, butyl acetate, tert-butyl methyl ether, carbon disulfide, 1- chlorobutane, butyl chloride, cyclohexane, cyclopentane, 1, 2-dichlorobenzene, 1 ,2- dichloroethane, dichloromethane, di(ethylene glycol) diethyl ether, 2-ethoxyethyl ether, N,N-dimethylacetamide, N,N-dimethylformamide, l,4-dioxane, ether, ethyl alcohol, ethylene glycol dimethyl ether, monoglyme, hexanes, 2-methoxyethanol, 2-methoxyethyl acetate, 2-methylbutane, 3-methyl- 1 -butanol, isoamyl alcohol, 4-methyl-2-pentanone, methyl isobutyl ketone, 2-methyl- 1 -propanol, isobutyl alcohol, 2-methyl-2-propanol, 1- methyl-2-pyrrolidinone, methyl sulfoxide, terpineol, Nitromethane, l-Octanol, 3- pentanone, propylene carbonate, tetrachloroethylene, tetrahydrofuran, 1,1,2- trichlorotrifluoroethane, 2,2,4-trimethylpentane and di(propylene glycol) methyl ether (DPM).

In some embodiments, the organic solvent is DPM. The "milling agent " may be any additive known in bead milling and/or other in other types of mills, to increase throughput, improve energy efficiency, to modify the quality of the material to be milled and to improve the performance of the mill. The milling agent may alternatively or additionally be used to prevent re-agglomeration of the particles and, consequently, help reduce caking and incrustations in the mill, increase throughput speed, improve milling fineness and the reactivity of the material to be milled. The flowability of the copper particles to be milled can be improved as well. Without wishing to be bound by theory, the milling agent interacts with the surface of the particles to be milled and neutralizes electrical surface charges and/or generates charges on their surfaces. Consequently, the attraction between the individual particles is reduced or eliminated.

In some embodiments, the milling agent is a carboxylic acid. Non-exhaustive list of representatives acids may include methanoic acid, ethanoic acid, ethanedioic acid, oxoethanoic acid, 2-hydroxyethanoic acid, propanoic acid, prop-2-enoic acid, 2- propynoic acid, propanedioic acid, 2-hydroxypropanedioic acid, oxopropanedioic acid, 2,2-dihydroxypropanedioic acid, 2-oxopropanoic acid, 2-hydroxypropanoic acid, 3- hydroxypropanoic acid, 2,3-dihydroxypropanoic acid, 2-oxiranecarboxylic acid, butanoic acid, 2-methylpropanoic acid, 2-oxobutanoic acid, 3-oxobutanoic acid, 4-oxobutanoic acid, (E)-butenedioic acid, (Z)-butenedioic acid, But-2-ynedioic acid, oxobutanedioic acid, hydroxybutanedioic acid, 2,3-dihydroxybutanedioic acid, (E)-but-2-enoic acid, pentanoic acid, 3-Methylbutanoic acid, pentanedioic acid, 2-oxopentanedioic acid, hexanoic acid, hexanedioic acid, 2-hydroxypropane-l,2,3-tricarboxylic acid, prop-l-ene- l,2,3-tricarboxylic acid, l-hydroxypropane-l,2,3-tricarboxylic acid, (2E,4E)-hexa-2,4- dienoic acid, heptanoic acid, heptanedioic acid, cyclohexanecarboxylic acid, benzenecarboxylic acid, 2-hydroxybenzoic acid, octanoic acid, benzene- l,2-dicarboxylic acid, nonanoic acid, benzene- 1, 3, 5 -tricarboxylic acid, (E)-3-phenylprop-2-enoic acid, decanoic acid, decanedioic acid, undecanoic acid, dodecanoic acid, benzene-l,2,3,4,5,6- hexacarboxylic acid, tridecanoic acid, tetradecanoic acid, pentadecanoic acid, hexadecanoic acid, heptadecanoic acid, octadecanoic acid, (9Z)-octadec-9-enoic acid, (9Z,l2Z)-octadeca-9,l2-dienoic acid, (9Z,l2Z,l5Z)-octadeca-9,l2,l5-trienoic acid, (6Z,9Z, 12Z)-octadeca-6,9, 12-trienoic acid, (6Z,9Z, 12Z, 15Z)-octadeca-6,9, 12,15- tetraenoic acid, nonadecanoic acid, eicosanoic acid, (5Z,8Z,l lZ)-eicosa-5,8,l l-trienoic acid, (5Z,8Z,l lZ,l4Z)-eicosa-5,8,l l,l4-tetraenoic acid, heneicosanoic acid, docosanoic acid, (4Z,7Z,l0Z,l3Z,l6Z,l9Z)-docosa-4,7,l0,l3,l6,l9-hexaenoic acid, tricosanoic acid, tetracosanoic acid, pentacosanoic acid and hexacosanoic acid.

In some embodiments, the carboxylic acid is selected from hexanoic acid, octanoic acid, nonanoic acid, decanoic acid, pentadecanoic acid, palmitic acid, stearic acid, 3-pentanoic acid, palmitoleic acid, oleic acid, linoleic acid and erucic acid.

In some embodiments, the carboxylic acid is an oleic acid.

The ratio between the copper particles and acid during milling is chosen based on measurements of the conductivity of various oleic acid/copper flakes ratios. Therefore, in some embodiments, the ratio between the amount of oleic acid and the amount of copper in a conductive ink prepared by a process of the invention may be between about 12.5 mg of oleic acid / 1 gr of Cu and about 250 mg of oleic acid / 1 gr of Cu. In some embodiments, the ratio between the amount of oleic acid and the amount of copper in a conductive ink prepared by a method of the invention is between about 25 mg of oleic acid / 1 gr of Cu and about 250 mg of oleic acid / 1 gr of Cu. In some embodiments, the ratio is 50 mg of oleic acid / 1 gr of Cu.

It is also known in the art that, in general, milling duration affects the morphology of the particles, transforming the spherical or amorphous particles to flakes. Thus, the milling time may be chosen or adapted to achieve flakes of a desired shape and size. In some embodiments, the milling time may range from about 5 minutes to about 240 minutes. In other embodiments, the milling time may range from about 40 minutes to about 240 minutes. In other embodiments, the milling time may range from about 80 minutes to about 240 minutes or from 120 minutes to 240 minutes.

The copper metal flakes thus obtained may be of different thicknesses. In some embodiments, the average flake thickness may range from about 20 nm to about 500 nm. In some embodiments, the average flake thickness may range from about 20 nm to about 400 nm, or from about 20 nm to about 300 nm, or from about 20 nm to about 200 nm, or from about 20 nm to about 100 nm.

In some embodiments, the average flake thickness may range from about 25 nm to about 75 nm, or from about 20 nm to about 50 nm, or from about 50 nm to about 100 nm, or from about 75 nm and 150 nm, or from 100 nm and 200 nm, or from 150 nm and 200 nm, or from 200 nm and 250 nm, or from 250 to 300 nm, or from 300 to 350 nm, or from 350 to 400 nm. In another aspect of the invention, the process is carried out on copper formate as an exemplary salt, i.e., an example of a copper metal precursor. In such a case, the process comprises obtaining copper formate and thermally decomposing the copper formate to obtain copper metal particles.

In some embodiments, the process comprises:

-thermally decomposing the copper formate to obtain copper metal particles; and

-wet milling the copper metal particles;

to obtain the copper flakes.

In yet another of its aspects, the invention provides a process for the preparation of copper flakes, wherein the process comprises:

-thermally decomposing copper formate to obtain copper metal particles; and

-wet milling of the copper metal particles;

to obtain the copper flakes.

Further provided are copper particles obtained by a process of the invention.

The invention further provides copper flakes obtained by a process comprising mechanically treating copper metal particles prepared by reduction of at least one copper metal precursor; to thereby obtain the copper flakes.

Also provided are copper metal flakes characterized by rounded-shape edges and having an averaged thickness ranging from about 20 nm to about 500 nm.

In some embodiments, the flakes have an average flake thickness as disclosed hereinabove. In some embodiments, the thickness is between 25 nm and 75 nm, or between 20 nm and 50 nm, or between 50 nm and 100 nm, or between 75 nm and 150 nm, or between 100 nm and 200 nm, or between 150 nm and 200 nm, or between 200 nm and 250 nm, or between 250 to 300 nm, or between 300 to 350 nm, or between 350 to 400 nm.

In some embodiments, such flakes are prepared by a process comprising mechanically treating copper metal particles prepared by reduction of at least one copper metal precursor. In some embodiments, the process is a process of the invention.

In another one of its aspects, the invention provides an ink formulation comprising a plurality of copper flakes of the invention or copper flakes obtained by a process of the invention. In some embodiments, the ink formulation may comprise an amount of the copper flakes, a liquid carrier, optionally a binder (which may also be used as a rheology modifier) and further optionally a solvent.

In some embodiments, the concentration of the copper flakes in an ink formulation of the invention may be between about 10% wt and about 50% wt. In some embodiments, the copper flakes concentration is between about l0%wt and about 35%wt. In other embodiments, the copper flakes concentration is about 30% wt.

The "binder" , being a part of the ink formulation, assists in holding the flakes together and enables an efficient printing process and good adhesion of the ink to the substrate. The binder is selected according to the ink composition, and should be compatible with the solvent composition. In some embodiments, the binder is selected from acacia tragacanth, gelatin, starch paste, pregelatinized starch, alginic acid, cellulose, methyl cellulose, ethyl cellulose, hydroxy propyl methyl cellulose (HPMC), hydroxy Propyl cellulose, sodium carboxy methyl cellulose, polyvinyl pyrrolidone (PVP), polyethylene glycol (PEG), polyvinyl alcohols, polymethacrylates, polyurethane, epoxy- based polymer, cyano acrylates, UV-polymerizable compositions, polyesters, phenolic resins, vinyl chloride co- and terpolymers.

The binder can be further selected from resins utilized in inks and paint formulations, for example and without being limited thereto, those described in

materials/resins solvent-based-liquid-inks/ or m https://www.wacker.eom/cms/media/publications/downioads/7529 EN.pdf.

In some embodiments, the binder is ethyl cellulose (EC).

In some embodiments, the solvent which is used to dissolve ethyl cellulose is selected from the list of organic solvents as described hereinabove.

In some embodiments, the solvent is terpineol.

In some embodiments, the concentration of EC in a terpinol-EC mixture is between about l%wt and about 5%wt. In some embodiments, the concentration of EC in a terpinol-EC mixture is between about 2.5%wt and about 5%wt.

In some embodiments, the concentration of terpinol in a terpinol-EC mixture is between about 99%wt and about 95%wt. In some embodiments, the concentration of terpinol in a terpinol-EC mixture is between about 95%wt and about 97.5%wt. In some embodiments, the ink formulation may further comprise at least one dispersing agent that is selected according to the liquid compositions and the particles surface. The dispersing agent may be physically bound to the surface of the particles to aid the milling process and the removal of surface oxides while enabling high conductivity upon the application of the flakes in conductive inks. In some embodiments, the dispersing agent forms (spontaneously) a chemical bond with the surface of the particles. Non-limiting examples of such dispersing agents include Efka® 7701, Efka® 7731, Efka® 7732 (as reported in http://www.dispersions- pigments.basf.com/portal/load/fid77H23/), Disperbyk 180 and 110 from Byk-Chemie as

dispersing .himl , super dispersants such as Solsperse from Lubrizol as presented in htfps://www.lubrizoi.com/Coatmgs/Brands/Soi¾perse-Hyperdisp ersants alginates, methylcellulose, hydroxyethylcellulose, carboxymethylcellulose, sodium carboxymethylcellulose, microcrystalline cellulose, acacia, tragacanth, xanthan gum, bentonite, carbomer, carageenan, powdered cellulose, gelatin and carboxylic acids.

In some embodiments, the selection of dispersants depends on the milling liquid and on the interaction of the dispersant with the precursor particles.

In some embodiments, the dispersing agent is an organic acid.

In some embodiments, the organic acid is a carboxylic acid, as selected hereinabove.

In some preferred embodiments, the carboxylic acid is oleic acid.

In some embodiments, the ink formulation of the invention is for use as a printing ink, e.g., in printing of a variety of patterns. In some embodiments, the printing is of a conductive pattern.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to better understand the subject matter that is disclosed herein and to exemplify how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

Fig. 1 depicts a two-step process according to the invention.

Figs. 2A-C: Fig. 2A presents a photograph of the copper formate powder before and after decomposition. Fig. 2B presents a SEM micrograph of copper formate. Fig. 2C is a SEM image of formed copper particles after decomposition, in which the particle size range is 200-500 nm.

Figs. 3A-I presents SEM images of copper particles during transformation from spherical to flake morphology. Fig. 3A shows the copper particles before milling. Figs. 3B-H shows the copper particles after milling. Fig. 3B presents an image of the copper particles after one minute of milling. Fig. 3C presents an image of the copper particles after 5 minutes of milling. Fig. 3D presents an image of the copper particles after 20 minutes of milling. Fig. 3E presents an image of the copper particles after 40 minutes of milling. Fig.3F presents an image of the copper particles after 80 minutes of milling. Fig. 3G presents an image of the copper particles after 120 minutes of milling. Fig. 3H presents an image of the copper particles after 240 minutes of milling. Fig. 31 is an enlarged image of copper particles after wet milling for 5 minutes, showing the mechanism of flake formation, from cold welding of small particles to the formation of flakes.

Figs. 4A-H: Figs. 4A-F are micrographs of cross section of lines printed with copper particles milled for various time durations. Fig. 4A presents the image of cross section before milling. Fig. 4B presents the image of cross section after 5 minutes of milling. Fig. 4C presents the image of cross section after 20 minutes of milling. Fig. 4D presents the image of cross section after 80 minutes of milling. Fig. 4E presents the image of cross section after 120 minutes of milling. Fig. 4F presents the image of cross section after 240 minutes of milling. Fig. 4G is the image of copper flakes after 120 minutes of milling. Fig. 4H describes the average flake thickness as a function of milling duration based on measurements in four micrographs for each sample.

Fig. 5 presents the conductivity (% of bulk copper conductivity) obtained with inks formulated with copper particles milled for various durations with oleic acid and DPM. The first point was prepared with copper particles that were not milled.

Figs. 6A-F presents x-ray diffraction patterns of copper particles at different process steps. Fig. 6A refers to copper particles formed by thermal decomposition of copper formate. Fig. 6B refers to copper flake powder after milling, drying on a glass slide, and scraping the pattern off the glass slide, forming free powder. Fig. 6C refers to copper flakes after milling and drying on a glass slide. Fig. 6D refers to printed copper flake ink on glass slide. Fig. 6E refers to same as sample D after sintering for 30 minutes at 250 °C. Fig. 6F is the reference line of pure fee copper: The vertical red lines indicate the position and relative intensity of pure copper, the vertical blue lines indicate the position and relative intensity of copper oxide, which is not present in measurements describes in Figs. 6A-E.

Figs. 7A-C: Figs. 7A-B present the two extreme positions of samples during a bending cycle, and the measured relative resistance as a function of the number of cycles. Fig. 7C presents the relative resistance of the copper lines as a function of bending cycles.

Figs. 8A-B are images of NFC antenna demonstrator printed with an ink containing copper flakes.

Figs 9A-B: Fig. 9A presents the X-ray diffraction pattern of copper particles formed by thermal decomposition of copper formate indicating pure copper. Fig. 9B presents X-ray photoelectron spectra (area of Cu 2p2/3 peak) of the same, indicating oxidation (CuO) on the particle surface.

Figs 10A-B: Fig. 10A depicts conductivity obtained with inks with a set Cu wt% (30%), and varying amount of EC in the Terpineol- EC mixture. Fig. 10B presents the adhesion of the copper samples (prepared with flakes milled with oleic acid) to the glass substrate was tested by a standard tape test performed according to ISO 2409, showing that addition of ethyl cellulose improves adhesion.

Fig. 11 presents the conductivity obtained with inks where the % Cu is varied between 10 and 50%, mixed with a 2.5 wt% EC Terpineol- EC mixture.

Fig. 12 presents the electrical resistance and Conductivity obtained with inks where the % Cu is varied between 10 and 35%, and the EC/Cu ratio was kept constant.

Figs. 13A-C shows x-ray photoelectron spectra (area of Cu 2p2/3 peak) of copper particles at different process steps. All indicating pure fee copper without CuO on the surface. Fig. 13A presents peaks of copper flakes after milling and drying from solvent. Fig. 13B presents peaks of copper flakes after formulation in the ink, and printing. Fig. 13C is the same as B after sintering for 30 minutes at 250 °C.

Fig. 14A-D presents x-ray photoelectron spectra (area of O ls peak) of copper particles at different process steps. Fig. 14A shows peaks of copper particles formed by thermal decomposition of copper formate. Fig. 14B shows peaks of copper flakes after milling and drying from solvent. Fig. 14C shows peaks of copper flakes after formulation in the ink, and printing. Fig. 14D is the same as Fig. 14C after sintering for 30 minutes at 250 °C. Figs 15A-D: Fig. 15A presents x-ray photoelectron spectra (area of Cu 2p2/3 peak) of copper flakes formed by milling without addition of a carboxylic acid. Fig. 15B presents peaks of copper flakes formed with oleic acid. Figs. 15C-D are SEM images of the flakes measured in A,B, indicating that the flakes formed without an acid are larger than those formed by milling with an acid.

Figs. 16A-E show cross section micrographs of lines printed with copper milled with various oleic acid concentrations. With 25 - 250 mg acid / gr Cu, a similar brick and mortar morphology was obtained on the final printed sample. However with the lower acid concentration of 12.5 mg acid / gr Cu, aggregated flakes formed a disarrayed layer with large voids, indicating that this concentration did not enable good dispersing and low friction between the flakes. Fig. 16A presents a concentration ratio of 12.5 mg of oleic acid / 1 gr of Cu. Fig. 16B presents a concentration ratio of 25 mg of oleic acid / 1 gr of Cu. Fig. 16C presents a concentration ratio of 50 mg of oleic acid / 1 gr of Cu. Fig. 16D presents a concentration ratio of 100 mg of oleic acid / 1 gr of Cu. Fig. 16E presents a concentration ratio of 250 mg of oleic acid / 1 gr of Cu. Fig. 16F presents the conductivity (% of bulk copper conductivity) obtained with inks formulated with copper particles milled with various acid concentrations.

DETAILED DESCRIPTION OF EMBODIMENTS

Copper flakes were prepared by a two-step process illustrated in Fig. 1. Anhydrous copper formate was decomposed under nitrogen at 200 °C for 40 minutes, resulting in copper particles. These particles were then processed in a wet bead mill (ultrafine wet milling), resulting in a shape transformation from spheres to flakes.

An image of the copper formate powder before and after decomposition is presented in Fig. 2A. SEM images of the copper formate (Fig. 2B) and formed copper particles (Fig 2C), reveal a substantial decrease in particle size from copper formate particles with a size range of 3-50 pm, to aggregates containing smaller copper particles, with a diameter of 200-500 nm. XRD pattern of the formed particles (Fig. 9A) corresponds to crystalline FCC copper, without any peak corresponding to copper oxide. However, XPS analysis indicates (Figs 9B) that copper at the particle surface is primarily in the oxidized state (90% Cu ²⁺ ). Thus, while the bulk of the particles consists of pure copper, the outer shell consists of copper oxide (CuO). In order to form flake shaped particles, the obtained copper particles were milled with a solvent (DPM) and oleic acid as a milling agent. DPM, which is widely used in ink formulations, was chosen as a solvent because of its relatively low viscosity, while oleic acid is known to strongly bind to copper surfaces. The process of copper morphology transformation during milling is presented in Fig. 3. In general, the milling duration affects the morphology of the particles, changing from spherical to flakes. After 40 minutes mostly flakes are observed. After 80, 120 and 240 minutes there are only flakes, with a diameter in the range of 2-6 pm.

Three types of particles are clearly seen in the enlarge image (Fig. 31) after 5 minutes of milling: small spherical particles, flakes, and aggregates of single particles welded together. It can be concluded that during the flake formation, original spherical particles aggregate and undergo cold welding followed by plastic deformation to thin flakes.

In order to optimize the conductive ink formulation, a series of screen printable inks containing flakes at different stages of milling were prepared by drying and dispersing 30 wt% flakes in terpineol with ethyl cellulose as a rheology modifier and binder (results of experiments that lead to the optimal formulation used in this paper are presented in Figs. 10, 11 and 12 along with a detailed explanation). These inks were screen printed and the obtained conductive patterns were evaluated. The screen printed patterns were inspected before sintering to evaluate printing quality. Uneven printed lines were observed with an ink containing the original particles (not milled). This ink was lumpy due to the aggregation between the copper particles. Inks with flakes milled for 20 minutes or more had an even consistency (without lumps) and were printed evenly, thus demonstrating the advantage of using the milled flakes compared to the intermediate copper particles. Fig. 4 presents FIB micrographs of cross-sections of the printed lines, showing that prolonged milling resulted in thinner flakes. The thinner flakes are stacked in a more dense structure resembling a brick and mortar structure. An enlarged image of the sample milled for 120 minutes is presented in Fig. 4G showing that the thickness of the thinnest flake observed is about 22 nm. Fig. 4H presents the average flake thickness in each sample, showing that very thin particles were obtained, with an average thickness of 48 ± 23 nm and 35 ± 15 nm after 120 and 240 minutes respectively.

Conductivities (presented as a percentage of bulk copper conductivity, 5.96xl0 ⁷ S/m) obtained with various inks are presented in Fig. 5. The samples prepared with original particles (the first data point in the graph - 0 minutes) had the lowest conductivity (0.29 %), and up to 80 minutes the increase in milling time led to an increase in conductivity. At time duration more than 80 min, the conductivity remained constant. Therefore, it can be concluded that the flake morphology is preferable for obtaining better electrical conductivity compared to spherical morphology of the original particles. This can be explained by a larger contact area between neighboring flakes and denser packing of the flakes in the printed layer. In view of these findings, the subsequent experiments were performed with flakes obtained after 120 min of milling.

When a thin film of flakes (after milling, without ink formulating) was placed on a glass slide and dried at 70 °C, the formed film had electrical conductivity with a sheet resistance as high as 800 kD/sq (flakes milled for 120 minutes), indicating that some electrically conducting contact points between the flakes were formed even without sintering. Higher conductivity was obtained by formulating the flakes into an ink, and heating the printed samples to 250 °C. This result can be explained by findings of Law et al. ²⁴ that heating a copper surface coated with oleic acid (b. p. 360 °C) to 200 °C or higher induces evaporation of unreacted (not directly adhered) acid; thus heating of the copper flake samples in this study probably results in the same effect, enabling better contact between the flakes, and thus improving the sample conductivity.

As seen from Fig. 5, the best conductivity was found to be less than 20% of the bulk copper conductivity that can be explained by oxidation of the copper flake surface, coating of the particles surface by an insulating layer of oleic acid, or the lack of tight packing of flakes in the printed layer. To evaluate the effect of the various factors on conductivity, the effects of the oxidation state of the flakes and the type of carboxylic acid were studied.

The oxidation state of flakes milled with oleic acid for 120 minutes was analyzed at various steps of copper pattern formation: after drying on a glass slide, after formulating an ink and printing, and after thermal sintering. XRD patterns (Fig. 6) of the flakes at the three steps correspond to crystalline copper, without peaks of copper oxides. The ratio between the intensities of XRD peaks for printed copper flake ink on a glass substrate (Fig. 6D, E) differ from the standard pattern for the bulk copper (Fig. 6F- vertical blue lines, PDF number 04-004-8486), with a stronger peek at the (220) plain, and without peeks at the (111) and (200) plains. This deviation from tabulated data is called an 'orientation' or 'texture' and indicates that the crystallites have a preferred orientation, and most particles are located so that (220) plains are parallel to the measurement plain (here: the plain of the glass substrate). As cross-section micrographs (Fig. 4G) indicated that the flakes in the printed patterns are horizontally oriented to the plain of the glass substrate, they have a preferred crystal structure with (220) plain perpendicular to the flakes' thickness. The texture is observed to a lower extent with flakes deposited on a glass slide from the dispersion (i.e. flakes not formulated in an ink, Fig. 6C), however when this layer is removed from the glass slide, forming a powder in which the particles are in random orientations, the texture is lost (Fig. 6B). In addition, the texture is not observed for the copper particles formed after the precursor decomposition (Fig. 6A), thus the orientation is induced by the milling and the arrangement of the flakes on the substrate. This type of texture was previously reported by Zhao [6] for copper treated by cold rolling, which includes rotation of crystallites to the preferred orientation during deformation. This indicates that the same plastic deformation mechanism occurs both during cold rolling and bead milling.

The surface of the copper flakes was examined for oxidation by X-ray photoelectron spectroscopy (XPS, Fig. 13). For the three process stages, peaks corresponding to Cu 2p 3/2 (Cu° or Cu ¹⁺ , 932.7 eV) were identified. The 932.7 eV peek corresponds to both, Cu° and Cu ¹⁺ (it is not possible to differentiate these two states with XPS). Furthermore, in the O ls region (Fig. 14), there is a small peak at 530.7 eV, indicating that some of the oxygen is bonded to copper. These findings correlate with two options: the chemisorption of the oleic acid to the copper surface via the carboxylic group, forming a surface ester, or due to the presence of CU2O. Characteristic peaks for CuO (Cu ²⁺ , 934 eV) were not found, indicating the absence of this surface oxide as in the case of the copper particles before milling (Fig. 9B) that may be explained by removal of the oxide by oleic acid, as it can rapidly react with copper oxide, forming a complex of binuclear copper oleate. The formed complex was probably washed away from the particle surface into the surrounding solvent during the milling. After removal of the oxide, the bound oleic acid may act as a capping agent, preventing further oxidation that explains the absence of peak corresponding to CuO.

Milling the copper particles without an additive resulted in oxidized flakes, which were larger in size compared to flakes milled with an acid. Figs. 15 A, B present the XPS results comparing flakes milled with and without acid. The obtained data indicates that the flakes formed without acid are partially oxidized (30% of the surface copper was at a Cu ⁺² state). XRD measurements (not shown) of the flakes prepared without acid indicated the presence of CuO, thus demonstrating the need for addition of an acid to the milling process to prevent oxidation. Figs. 15C and D present SEM images of these flakes showing that flakes formed without acid were substantially larger than flakes obtained with oleic acid. This result was expected as the oleic acid was added to the milling process to prevent agglomeration and cold welding.

Thus, the added acid acted as a capping agent preventing cold welding, and oxidation of the copper flakes. As the molecular structure of an adsorbed carboxylic acids can affect its packing and orientation on a copper surface (as published for nickel [7] and iron oxide [8-10] surfaces), as well as milling effectivity and contacts between neighboring particles after printing, a series of milling experiments was performed with saturated and unsaturated carboxylic acids with different chain lengths. The milling was performed for 120 minutes, with a weight ratio between copper and acid of 50 mg acid/g Cu (reasoning for this ratio is detailed in the SI). The flakes prepared with the various acids were formulated into inks (composition as above), printed and sintered. The electrical conductivity as a percentage of bulk copper conductivity obtained with each acid presented in Table 1. When the particles were milled with saturated acids, the best conductivity was obtained with acids with a relatively short chain length, C6-C10. With an ink containing flakes milled with octanoic acid, a conductivity of -22% bulk copper conductivity was obtained (corresponding to a resistivity of 7.5 mW-cm). It is unclear if a chain length of 6-10 is also optimal with unsaturated acids, as unsaturated carboxylic acids with this length are not commercially available. When comparing saturated and unsaturated acids with the same chain length (C16 vs C16: l, C18 vs C18: l and 08:2) the obtained conductivities were similar. This is an interesting finding, as double bonds in carboxylic acids are known to affect the density of the layer adsorbed on metal surfaces. Thus, the dominant factor influencing the electrical performance is the length of the chain, not its saturation.

Table 1: Describes conductivity (presented as a percentage of bulk copper conductivity) of copper patterns with inks milled with various carboxylic acids.

The thickness of the flakes prepared with seven different acids was compared with cross-section imaging (FIB), and an analysis of variance showed the absence of statistically significant difference. Therefore, the variation in conductivity is not a result of the flake thickness. Perhaps, the reason of such difference is the rate of acids evaporation during sintering at 250 °C, as longer aliphatic chains have a higher BP. In addition, the longer chain length may prevent direct contact between the neighboring particles and so increase the insulation between them.

Durability to bending of printed patterns was tested by measuring the resistance change after multiple bending cycles of a Kapton substrate with printed copper flakes. Fig. 7 presents the two extreme positions of the sample during a bending cycle, and the measured relative resistance as a function of the number of cycles. After 1000 bending cycles, the relative resistance (r/ro) increased by only 32% and 27% for samples with flakes milled with oleic and nonanoic acid respectively. This high durability is most likely due to the extended contact points between the flakes.

As a demonstrator, an NFC antenna on Kapton was printed. After sintering of the antenna, an NFC chip was added with conductive glue, forming a functioning NFC tag (Fig. 8) that communicated well with a smartphone, as demonstrated in the SI movie. This shows that the ink with the copper flakes can be used for printing functional and flexible electrical circuits.

To conclude, thin copper flakes suitable for printed electronics were prepared with a simple two step method. First, copper formate was decomposed, resulting in the formation of copper particles. This method for preparing copper particles has two main advantages compared to the wet synthesis: it results in pure copper particles that are not contaminated by capping agents, and it can easily be scaled up, without forming any chemical waste. The formed copper particles were then milled by ultrafine wet bead milling to form flakes. The use of wet milling resulted in substantially thinner flakes than those reported in the literature after dry milling. The milling duration was shown to be the most important parameter affecting the morphology of the obtained flakes and the final electrical conductivity of metallic patterns printed with flake-based inks. Copper flakes milled for 120 minutes with oleic acid had an average thickness of 48+23 nm, without any CuO on the surface. Various carboxylic acids had a limited effect on the obtained particles resulting in similar flake thickness; however, there was some effect on the obtained conductivity. Copper patterns with very good electrical conductivity were obtained after formulating inks, printing and sintering (up to 22% bulk copper conductivity). The ink could be used to print conductive patterns on glass and Kapton, with good adhesion to both substrates, and had a high durability to bending enabling its use in flexible printed electronics. A working NFC antenna was printed on Kapton demonstrating the excellent conductivity.

Methods

Copper particle formation: Copper formate hydrate (Hunan Heaven materials, China) was thermal dehydrated by heating for three days in a drying oven at 70 °C. The decomposition of the copper precursor can be performed at various time durations and temperature ranges. Full dehydration was confirmed by TGA (no weight loss when heated to 100 °C). XRD pattern indicated the formation of anhydrous copper formate. The dried copper formate was than decomposed in a tube oven at 200 °C for 40 minutes under a nitrogen environment to induce decomposition of the copper formate, resulting in formation of copper particles.

Copper flake formations: The copper powder was milled in a bead mill (DYNO®-MILL RESEARCH LAB, WAB, bead size: 0.3mm) by adding 20 g of copper powder to 60 g Dipropylene glycol monomethyl ether -DPM and 1 g carboxylic acid (unless otherwise stated). Milling was performed for duration of 1 to 240 minutes at 4500 RPM, resulting in the formation of copper flakes (in most experiments the samples were milled for 120 minutes). The dispersion was centrifuged at 1000 RPM for 30 minutes to separate the particles from most of the DPM and free acid used during milling. Ink formulation: A Terpineol-EC mixture was formed by dissolving 2.5 wt% Ethyl cellulose (48% ethoxyl, viscosity 46 cP, 5 % in toluene/ ethanol 80:20, Sigma, USA) in 97.5 wt% Terpineol (Sigma, USA) for several days before use (with periodic mixing). Mixtures with other wt% of EC in Terpineol were prepared and used for inks and samples presented in the SI. Conductive screen-printable inks were prepared by drying the copper flakes from DPM on a hot plate at 70 °C followed by dispersing the flakes in the Terpineol-EC by in a Thinky mixer (Thinky, Japan) for 3 min.

Printing and sintering: The inks were screen printed with a V 250-030 mesh (NBC, Japan; Positioned and patterned by Punger 2000 ltd, Israel) in a pattern of five 1*24 mm lines. The printed patterns were dried for 10 minutes on a hot plate at 70 °C in the fume hood, and sintered in a tube oven under nitrogen at a temperature of 250 °C for 30 minutes.

Electrical performance: Electrical resistance (R) of the formed copper lines was measured by two probes placed on a single line with 22 mm distance (L) between probes, and the cross section area (A) of the lines was measured with a mechanical profilometer (Dektak XT). Resistivity was calculated with the following formula: p=R*A/L, Conductivity as a percent of bulk copper conductivity was calculated by: 100% * p (Bulk copper = 1.68 mW·ah)/ p (sample)

Morphology analysis: Top view SEM micrographs were taken with a MagellanT XHR SEM. HR-TEM images were obtained with a Tecnai F20 G2 transmission electron microscope operating at 200 kV for samples prepared on a carbon-coated nickel grid. Cross-section images were taken using a focused ion beam system coupled with a scanning electron microscope (FIB-SEM, Helios 460F1, FEI). Average flake thickness was measured from four micrographs (enlarged to Xl00,000).

Chemical analysis: XRD measurements were performed with a X-ray diffractometer D8 Advance of Bruker AXS. X-ray photoelectron spectroscopy (XPS) analyses were performed using an Kratos AXIS Ultra spectrometer (Kratos Analytical Ltd., Manchester, UK). Spectra were acquired using monochromated Al KD (1486.6 eV) X-ray radiation source with 90° take-off angle (normal to analyzer). The XPS spectra were collected with pass energy 20 eV and step size 0.1 eV. Data analyses were done using Casa XPS (Casa Software Ltd.) and Vision data processing program (Kratos Analitycal Ltd.) Adhesion characterization: Adhesion was characterized with a standard tape test according to ISO 2409:2007. Samples were prepared by screen printing a 1.5*3 cm square, followed by sintering at 250 °C for 30 minutes. After sintering the samples were cut with a six-blade instrument (BYK, Germany). ISO 2409 adhesive tape (Elcometer, France) was firmly adhered to the copper samples, and then peeled off with a single swift motion at a 60-degree angle to the substrate.

Bending test: Durability to bending was tested by measuring the resistivity of a sample as a function of the number of bending cycles. The bending was performed in an in-house prepared devise that smoothly shifts the distance between two connection points, inducing bending of the flexible substrate between them with a curvature radius of 15 mm.