Technical field

The invention relates to a method for preparation of a bimodal mixture of copper nanoparticles and microparticles with a polymeric protective layer.

The invention also relates to a bimodal mixture of copper nanoparticles and microparticies with a polymeric protective layer prepared by this method and to a printing formulation containing this bimodal mixture.

Background art

Copper particles (including nanoparticles and microparticles), as well as silver and gold particles (including nanoparticles and microparticles) are the most significant metallic particles which have found wide application, especially in electronics and catalysis, due to their excellent electrical conductivity, catalytic abilities and high chemical stability. In the past 20 years a large number of different methods have been developed for the preparation of copper particles having dimensions in the range from tens of nanometers to units of micrometers, some of the methods being more suitable, others less suitable. However, apart from indisputable benefits, the methods which are currently used also exhibit a number of disadvantages.

The most widely used methods for preparation of copper particles are based on liquid-step reduction, in which metallic copper is produced by reduction from a suitable precursor dissolved in a suitable solvent by using a suitable reducing agent. The mean size of the particles thus prepared depends on the ratio of the copper precursor to the reducing agent, as well as on the specific reaction conditions (e.g. temperature, stirring conditions, order of mixing individual components together, addition of other agents, etc.). The most

SUBSTITUTE SHEETS (RULE 26) commonly used copper precursor is copper sulfate, and also copper chloride, copper nitrate, copper acetylacetonate, copper acetate, cuprous oxide, cupric oxide , etc. For their dissolution different solvent systems are used, most often on the basis of water, organic solvents (e.g. acetone, toluene, etc.), alkanes (e.g. n-hexane, n-heptane, n-octane, etc.), ethylene glycol, polyethylene glycols or various mixtures thereof. As a reducing agent especially sodium borohydrate (NaBH ₄ ) or potassium borohydrate (KBH ₄ ) is used (see, e.g., the article by I. Lisiecki et al.: "Control of the shape and the size of copper metallic particles", Journal of the Physical Chemistry, 100 (1996) 4160, or H.-X. Zhang et al.: "Facile Fabrication of Ultrafine Copper Nanoparticles in Organic Solvent", Nanoscale Research Letters 4 (2009) 705), as well as hydrazine (see, e.g., the article by S.H. Wu et al.: "Synthesis of high-concentration Cu nanoparticles in aqueous CTAB solutions", Journal of Colloidal Interface Science 273 (2004) 165), sodium hypophosphite (see, e.g., the article by Y. Lee et al.: "Large scale synthesis of copper nanoparticles by chemically controlled reduction of applications of inkjet-printed electronics", Nanotechnology 19 (2008) 415604), ascorbic acid (see, e.g., the article by W. Yu et al.: "Synthesis and characterization of monodispersed copper colloids in polar solvents", Nanoscale Research Letters 4 (2009) 465), amines higher than hydrazine, e.g., N,N- diethylamine, Ν,Ν-triethylamine, etc.

Furthermore, it is also possible to use a combination of different reducing agents. In this sense is known, e.g., the use of a combination of NaHB ₄ and ascorbic acid, but in these cases it is mostly a matter of selection of conditions ad hoc, rather than a systematic procedure.

The method for preparation of copper particles based on using liquid-step reduction was described, e.g., also in US 20120251381.

In the patent literature also other methods for preparing copper particles, based on very special procedures, have been disclosed. E.g., US 20080159902 describes the preparation of copper particles having a diameter below 100 nm by a sequence of dispersion of Cu ₂ 0 or CuO microparticles in a hot solution of an amine compound and subsequent chemical reduction by a mixture of oleic acid and formic acid; WO 2009040479 discloses the preparation of copper nanoparticles with a diameter ranging from 1 to 10 nm by means of copper chloride and NaBH ₄ in ethanol-chioroform solution.

The disadvantage of all these methods is the fact that they require long reaction times, a great excess of the reducing agents used, high temperatures and pressures and, above all, the use of exotic and often highly toxic substances, which makes these methods difficult to be upscated and used in an industrial scale. Moreover, the final price of the produced copper particles can be negatively influenced to a considerable extent by a potential use of other additives, such as surfactants used for better wetting of the produced particles, particle stabilizers, etc. Another drawback is the necessity to use a highly expensive process of bubbling inert gases through a reaction mixture in order to avoid undesirable oxidation of the produced particles.

The article by A. Sinha et al.: "Preparation of copper powder by glycerol process", Materials Research Bulletin 37 (2002) 407 further describes a special type of a reduction reaction in the liquid step - the so-called polyol method, in which in the presence of poiyhydric alcohol, e.g. glycerol or polyethylene glycol, metallic copper is reduced, in particular most often from cuprous oxide or organic precursor of copper, such as copper acetylacetonate. This reaction takes place at temperatures higher than the boiling point of water, typically in the range of 100 to 240 °C, whereby the used poiyhydric alcohol plays a synergistic role of a solvent and at the same time a reducing agent. In some cases, it is possible to add another reducing agent to the solution in order to accelerate reaction kinetics - see, e.g., the article by S. Jeong et al.: "Controlling the Thickness of the Surface Oxide Layer on Cu Nanoparticles for the Fabrication of Conductive Structures by Ink-Jet Printing", Ad. Funct. Mater. 15 (2008) 679. The disadvantages of these polyol methods include a very long reaction time (in the order of tens of hours), the necessity of highly laborious and expensive washing of the produced copper particles for removal of the reaction residues, as well as the high price of the input chemicals.

Beside these methods, other known methods for preparing copper nanoparticles include solid step reduction by milling cupric chloride (CUCI2) and sodium chloride (NaCI) with sodium in a ball mill. As stated e.g. in the article by J. Ding et al., "Uitrafine Cu particles prepared by mechanochemical process", Journal of Alloys and Compounds 234 (1996) L1 , after 16 hours of milling it is possible to obtain copper nanoparticles with a cubic structure and a diameter of 25 to 100 nm. On the other hand, the disadvantage of this method is that it is highly time-consuming, whereby due to usual capacity of ball mills only very limited quantities of nanoparticles can be produced - in the order of units of grams. A further disadvantage of these methods is also the fact that the reaction mixture often contains incorporated cations which are very difficult to remove, such as Na ⁺ , and which negatively affect the purity of the final product and its electrical conductivity. At the same time, these cations unfavourably contribute to a change in the surface potential of the produced copper nanoparticles and their excessive clustering, which complicates their further application.

For most of the applications of copper particles, including those in printing formulations for printing electrically conductive layers, these particles must be completely metallic and must not be even partially oxidized. However, as is mentioned, e.g., in the article by S. Magdassi et al.: "Copper Nanoparticles for Printed Electronics: Routes Towards Achieving Oxidation Stability", Materials 3 (2010) 4626-4638, particles and especially copper nanoparticles and microparticles are very unstable to air and tend to oxidize on their surface, forming cuprous oxide. Although deterioration of electrical conductivity of one nanoparticle is not particularly strong, in the functional layers where there are enormous quantities of nanoparticles (in the order of millions to a milliard pieces/cm ² ), it is noticeable and strongly worsens electron transport.

One of the possibilities of protecting the surface of copper particles from undesirable oxidation is forming a protective layer of a chemically stable substance immediately after preparing these particles. The suitable chemically stable substance is, e.g., an organic substance, such as polymer, alken, etc., the advantage of which is the fact that after applying copper particles to the required substrate it can be removed by suitable treatment - most often by exposure to a temperature in the range from 200 to 400 °C, when the protective layer is decomposed by pyrolysis and the copper particles are electrically conductive interconnected. However, the shortcoming of this method is that these temperatures are too high for some substrates to which copper nanoparticles are applied and which have recently found increasingly wide application in the sphere of printed electronics (e.g. thin polymeric films, fabrics, etc.). Moreover, removal of the protective layer is not always quantitative, and very often its residues or reaction products arising during its removal may lead to undesirable contamination of the layer of copper particles being formed.

In addition to the above-mentioned organic compounds, it is also possible to use e.g. graphene, silicon dioxide, amorphous carbon, etc., for forming the protective layer. Nevertheless, the disadvantage of these substances is that they can only be removed from the surface of the copper particles with great difficulty and for some applications their use is principally inadmissible.

One of possible applications of copper particles is their conversion into different forms of printing formulations, which are afterwards used for creating electrically conductive layers by printing - see, e.g., the articles by B. Lee et al.: "A low-cure-temperature copper nano ink for highly conductive printed electrodes", Current Applied Physics 9 (2009) e157-e160, Y. Lee et al.: "Large- scale synthesis of copper nanoparticles by chemically controlled reduction for applications of inkjet-printed electronics", Nanotechnology 19 (2008) 415604, as well as WO 20101 14769 and US 20140009545. The layer of copper particles created by printing is subsequently sintered by using sintering methods, thus producing a homogeneous compact layer with similar or the same uniform electrical conductivity throughout the layer's volume, which has been achieved so far only by standard methods of metal plating. Furthermore, the advantage of printing is the fact that it enables continuous and high-speed application of specific patterns to different flexible as well as rigid substrates. A number of methods which are based on power supply to the printed copper particles are used for the purpose of sintering, in order to cause their bulk melting or surface melting and their subsequent sintering. These methods include, e.g., hot air drying, exposure to infrared radiation, exposure to UV radiation, exposure to microwave radiation, photonic sintering - i.e. exposure to a laser beam with an appropriate wavelength, electric current sintering - by contact with the aid of direct current or contactlessly with the aid of high-frequency AC voltage, exposure to plasma, etc. In all these cases, beside the source of energy, it is especially the size and surface composition of the copper particles that play an essential role.

The disadvantage of existing printing formulations containing only an addition of copper nanoparticles or microparticles is that in the first case they are not capable of forming a sufficiently strong and mechanically resistant layer, and in the second case the formed layer is highly porous and has uneven electrical conductivity.

Therefore, the aim of the invention is to eliminate the disadvantages of the background art and propose a method for preparation of copper particles with a suitable distribution in size, and also to ensure protection of the copper particles from undesired surface oxidation by a layer of material which can be easily and quickly removed.

In addition, the aim of the invention is also these particles and a printing formulation which contains them.

Principle of the invention

The goal of the invention is achieved by a method for preparation of a bimoda! mixture of copper nanoparticles and microparticles with a polymeric protective layer, whose principle consists in that in the first step is prepared a reaction mixture containing at least one precursor of copper, an aqueous solution of at least one monohydric and/or polyhydric alcohol and at least one organic polymer in a weight ratio precursor (precursors) of copper : alcohol (alcohols) : organic polymer (polymers) 1 : 5-500 : 0.05-0.5, and in the second step at least one organic reducing agent is quantitatively added to this reaction mixture under intensive stirring, whereby the weight ratio precursor (precursors) of copper : organic reducing agent (agents) is 1 : 1-20. As a result of synergy between quantitative addition of organic reducing agent and intensive stirring of the reaction mixture, copper nanoparticles and microparticles are precipitated from the reaction mixture upon the reduction of the copper precursor, being provided with a protective layer of organic polymer or a mixture of organic poiymers. The resulting bimodal mixture then contains a fraction of copper nanoparticles having a diameter of approximately 1 to 200 nm, as well as a fraction of copper microparticies having a diameter of approximately 0.5 to 3 μιη. This combination has a highly favourable influence on the electric conductivity of the layer formed, e.g., by printing or spraying the printing formulation which contains this mixture, since the nanoparticles fill the void spaces between the microparticies, thereby increasing the conductivity of the layer being created, or, in other words, they even out differences in its conductivity in the entire volume, reducing its resulting porosity and increasing its compactness.

The best results are achieved if the weight ratio precursor (precursors) of copper : alcohol (alcohols) : organic polymer (polymers) in the reaction mixture is 1 : 5-50 : 0,1-0,3 and/or the weight ratio precursor (precursors) of copper : organic reducing agent (agents) is 1 : 2-10.

Suitable precursors of copper include mainly copper sulfate, copper chloride, copper nitrate, copper acetylacetonate, copper acetate, cuprous oxide, cupric oxide, whereby it is possible to use also any mixture of at least two of them.

A suitable alcohol is especially methanol, ethanol, propanol, butanol, isopropanol, ethylene glycol, propylene glycol, diethylene glycol, butandiol, glycerol, whereby it is possible to use also any mixture of at least two of them.

A suitable organic reducing agent is especially ascorbic acid, glucose, fructose, sucrose, acetaldehyde, dimethyl ketone, whereby it is possible to use also any mixture of at least two of them.

A suitable organic polymer is especially polyvinylpyrrolidone, polyvinyl alcohol, polyethylene glycol, arabic gum, xanthan gum, hydroxypropyl cellulose, acetyl cellulose, hydroxyethyl cellulose, carboxymethyl cellulose, whereby it is possible to use also any mixture of at least two of them.

The goal of the invention is also achieved by bimodal mixture of copper nanoparticles and microparticies with a protective polymeric layer which is prepared by this method. This mixture then contains a fraction of nanoparticles ranging in size from 1 to 200 nm and a fraction of microparticles ranging in size from 0.5 to 3 prn.

The goal of the invention is further achieved by a printing formulation for printing electrically conductive layers, whose principle consists in that it contains 55 to 85 % by weight, preferably 70 to 80 % by weight of the bimodal mixture of nanoparticles and microparticles with a protective polymeric layer.

Description of the drawings

In the accompanying drawings Fig.1 is an image of a fraction of copper nanoparticles of a bimodal mixture of copper nanoparticles and microparticles prepared by the method according to the invention, taken by an electron microscope with 100,000x magnification, Fig. 2 is an image of a fraction of copper microparticles of a bimodal mixture of copper nanoparticles and microparticles prepared by the method according to the invention taken by an electron microscope with 5,000x magnification, and Fig. 3. is an image of a layer of bimodal copper particles printed on a glass substrate, taken by an electron microscope with 650x magnification.

Specific description

The method for preparation of copper particles with a polymeric protective layer and suitable distribution of their sizes according to the invention is based on the reduction of copper cations from at least one suitable copper precursor in an aqueous solution of at least one suitable monohydric and/or polyhydric alcohol by at least one suitable organic reducing agent, in the presence of at least one suitable organic polymer.

Substantially, any common salt of copper can be used as a precursor of copper, such as copper sulfate, copper chloride, copper nitrate, copper acetylacetonate, copper acetate, etc., or copper oxide, e.g., cuprous oxide, cupric oxide, etc., or possibly, a mixture of at least two of these precursors. The alcohol used can be substantially any common monohydric alcohol, such as methanol, ethanol, propanol, butanol, isopropanol, etc., or any common polyhydric alcohol, such as ethylene glycol, propylene glycol, diethylene glycol, butanediol, glycerol, etc., or any mixture containing at least two monohydric alcohols, or at least two polyhydric alcohols, or at least one monohydric and at least one polyhydric alcohol.

As an organic reducing agent is used, e.g., ascorbic acid, glucose, fructose, sucrose, acetaldehyde, dimethyl ketone, etc., or any mixture of at least two of them. The organic reducing agent is fed quantitatively to the reaction mixture, i.e. all the batch in the form of solid particles or an aqueous solution is added to it at a time, under very intensive stirring, which can be achieved by using a magnetic stirrer or reactor equipped with a mixing propeller (e.g. torax), whereby as a result of this procedure elementary copper is precipitated in the form of nanoparticles and microparticles, achieving a yield of up to 98 %.

As an organic polymer it is possible to use e.g. polyvinylpyrrolidone, polyvinyl alcohol, polyethylene glycol, arabic gum, xanthan gum, hydroxypropyl cellulose, acetyl cellulose, hydroxyethyl cellulose, carboxymethyl cellulose, or any mixture of at least two of them. This organic polymer/these organic polymers ensures/ensure better wettability of the resulting copper particles by the reaction mixture, which is advantageous for the rapid completion of the reaction, and at the same time - already during the reduction of elementary copper - forms a thin film on the surface of the resulting particles, whereby the film protects them from undesirable surface oxidation and prevents their agglomeration.

In the first step of the method for preparation of copper particles with a suitable distribution of size according to the invention, a reaction mixture is prepared, the reaction mixture containing at least one precursor of copper, an aqueous solution of at least one monohydric and/or at least one polyhydric alcohol as well as at least one organic polymer, whereby for diluting all these components the mixture is stirred at higher temperatures (at the speed of the stirrer below 200 mm). The ratio (by weight) of the individual components of the reaction mixture precursor (precursors) of copper : alcohol (alcohols) : polymer (polymers) is 1 : 5-500 : 0.05-0.5; preferably 1 : 5-50 : 0.1-0.3.

In the second step, after complete dissolution of all components, to the reaction mixture thus prepared is added at least one organic reducing agent under intensive stirring (at a speed of the stirrer of above 200 rpm, preferably, however, above 400 rpm) and the process of reduction and precipitation of copper particles is started, during which time the reaction mixture is still very intensively stirred. The weight ratio of the precursor(s) of copper to the organic agent(s) is 1 : 1-20; preferably 1 : 2-10. As a result of synergy between the quantitative addition of an organic agent (agents) and the intensive stirring of the reaction mixture is prepared a mixture of spheric - or substantially spheric - copper nanoparticles and micropartic!es with a polymeric protective layer (formed by the used organic polymer or the mixture of polymers), which are subsequently separated from the reaction mixture by filtration or centrifugation (instead of expensive and volume-limited ultracentrifugation, necessary for the preparation by the methods known from the background art), and at the same time soluble impurities of the used precursor(s) of copper and organic pollutants are removed effectively from the mixture. Preferably, the separated mixture of copper nanoparticies and microparticles is washed with an organic solvent, e.g. isopropanol. After drying, this mixture is stored separately or in a solution, which prevents oxidation of copper nanoparticles and microparticles, e.g. in a solution of alcohol.

The method for preparation of a mixture of copper nanoparticles and microparticles according to the invention is based on using basic, commonly available and therefore inexpensive chemicals, without unnecessary admixtures of cations and anions of parasitic elements, which results in very high yields (up to 98 %) at low manufacturing costs. Surprisingly, in all alternatives and combinations of input materials, the result is always a bimodal mixture of copper nanoparticles and microparticles with a polymeric protective layer, i.e. a mixture whose particle size distribution curve has a bimodal character - this mixture contains both a fraction of copper nanoparticles having a diameter of approximately 1 to 200 nm (see Fig. 1 , which shows a fraction of nanoparticles of a bimodal mixture prepared by the method according to Example 6 given below, whose diameter ranges from 48.9 to 97.3 nm) and a fraction of copper microparticles having a diameter of approximately 0.5 to 3 μηη (see Fig. 2, which shows a fraction of microparticles of a bimodal mixture prepared by the method prepared according to Example 6 given below, whose diameter is in the range from 0.87 to 2.09 μιη). This combination of copper nanoparticles and microparticles has subsequently a very positive influence on the electrical conductivity of the layer formed e.g. by the printing or by spray coating of a printing formulation containing this mixture. The main building blocks of the layer thus formed are copper microparticles, whereby the voids between them are filled up with copper nanoparticles, which even out the differences in the electrical conductivity of the formed layer throughout its entire volume, and at the same time reduce its final porosity and increase its compactness. For this reason, for real applications, the best option is the use of the bimodal mixture of copper nanoparticles and microparticles in the form prepared by the method according to the invention; however, in case of need, it is possible to separate the individual fractions from each other and use them separately.

Preferably, the mixture of nanoparticles and microparticles prepared by the method according to the invention is used for preparing a printing formulation (paste) for printing electrically conductive layers, whereby the content of the mixture of copper nanoparticles and microparticles therein is in the range from 55 to 85 %, preferably from 70 to 80% of the total weight of the printing formulation. To this mixture are gradually added standard components of currently used printing formulations (see, e.g., examples 2, 3, 5 and 6 below) and all the components are thoroughly mixed together. In order for the nanoparticles and microparticles to maintain their high antioxidant stability even after printing, it is advantageous to irradiate the printed layer by high-frequency high-energy laser pulse or by high-energy pulse xenon source. In both cases, due to local action of high energy, copper nanoparticles and microparticles are partially melted only locally, the organic polymer is removed and the copper nanoparticles and microparticles are sintered, which enhances contact between the individual particles and improves the electrical properties of the entire layer, without negatively affecting its mechanical resistance or without damaging the substrate to which the layer was applied. Nevertheless, in general, any of the known methods can be used for removing the organic polymer and for the sintering of the copper nanoparticles and microparticles, e.g. exposure to infrared radiation, exposure to UV radiation, exposure to microwave radiation, photonic sintering - i.e. exposure to a laser beam with an appropriate wavelength, electric current sintering - by contact with the aid of direct current or contactlessly with the aid of high-frequency AC voltage, exposure to plasma or drying in a furnace with an inert atmosphere, etc. In any case, the result is a highly homogeneous layer of copper nanoparticles and microparticles having a thickness in the order of units to tens of pm, which exhibits a very low sheet resistance - as low as 0.02 Ω/m ² - and which, unlike the layers formed using copper nanoparticles or microparticles prepared by known methods, does not require for achieving the desired mechanical rigidity an additional high temperature biscuit firing (above 300°C), which usually results in lowered conductivity of the prepared layers caused by the oxidation of copper, which is, of course, undesirable for their applications. Using a mixture of copper nanoparticles and microparticles prepared by the method according to the invention finally not only leads to reducing costs of the printing formulation preparation, but also to reducing costs of the production of the required conductive layers, as well as to lower demands for technology. Due to this, the bimodal mixture of copper nanoparticles and microparticles prepared by the method according to the invention, or the printing formulations which contain these mixtures, find application in electronics and electrical technology - everywhere, where there is a need for forming thin conductive layers, particularly in various printed electronic circuits and connections, radio frequency antennas, displays, sensors, etc.

Below, for illustration, there are six specific examples of preparation of a bimodal mixture of copper nanoparticles and microparticles with a polymeric protective layer by the method according to the invention, with the yield achieved up 90 to 98 %, including the description of the preparation of a printing formulation containing this mixture. Nevertheless, these are merely illustrative examples, whereby, as is apparent to a person skilled in the art from aforementioned description, beside the raw materials which are explicitly mentioned in the description in the particular examples, it is possible to use also other raw materials and combinations thereof listed in the previous general description, whereby the result will always be identical or substantially identical.

Example 1

30 ml of demineralized water, 170 ml of ethylene glycol and 1.24 g of copper acetate monohydrate were mixed in a 400 mi beaker. The mixture thus formed was heated to a temperature of 50 °C and after dissolving all the components, 0.21 g of polyvinylpyrrolidone was added to it. This mixture was stirred at a constant temperature of 50 °C until complete dissolution. After that, 6 g of ascorbic acid were added quantitatively into it, which resulted in the reduction and precipitation of copper particles, whereby during the reduction the reaction mixture was intensively stirred with a stirrer at a rotation speed of 500 rpm. These particles were filtered through a glass frit and the resulting filter cake was washed with 200 ml of isopropanol. Thus 0.29 g of copper particles with a protective layer of polyvinylpyrrolidone was obtained. These particles were then spilt over with 200 ml of isopropanol for preserving them for characterization and for further processing.

Subsequently, during measurement by SE , two fractions of copper particles were identified - nanoparticles having a mean diameter of 50 nm and microparticles having a mean diameter of 1500 nm.

Example 2

30 ml of demineralized water, 170 ml of ethylene glycol and 15.58 g of copper sulfate pentahydrate were mixed in a 400 ml beaker. The mixture thus obtained was heated to a temperature of 50 °C and after dissolving all the components, 2.1 g of arabic gum was added. This mixture was stirred at a constant temperature of 50 °C until complete dissolution. After that, 6 g of ascorbic acid were added quantitatively into it, which resulted in the reduction and precipitation of copper particles, whereby during the reduction the reaction mixture was intensively stirred with a stirrer at a rotation speed of 600 rpm. These particles were filtered through a glass frit and the resulting filter cake was washed with 200 ml of isopropanol. Thus 2.4 g of copper particles with a protective layer of arabic gum were obtained. These particles were then spilt over with 200 ml of isopropanol for preserving them for characterization and for further processing.

Subsequently, during measurement by SEM, two fractions of copper particles were identified - nanoparticles having a mean diameter of 80 nm and microparticles having a mean diameter of 1800 nm.

1 g of the thus prepared mixture of copper nanoparticles and microparticles was mixed with 0.1 g of polyvinylpyrrolidone, 0.35 g of alpha- terpineol and 0.3 g of ethanoi to produce a printing formulation (paste). This formulation was then applied through a mesh with a density of 77 fibres/cm to a glass substrate, thereby creating a strip with a width of 2 mm on the substrate. The glass substrate with the applied printing formulation was sintered for 20 minutes at a temperature of 450 °C in an inert-atmosphere furnace, whereby a compact layer of copper nanoparticles and microparticles was formed from the printing formulation, the sheet resistance of the resulting layer measured by a four-point method being 0.04 Ω/m ² .

In the second variant, 15 g of the mixture of copper nanoparticles and microparticles obtained by the above-described method were mixed with 1.3 g of polyvinyl alcohol, 4.5 g of demineralized water, 1 g of glycerol, 1 g of ethanoi and 0.01 g of a surface active substance (SAS) to produce a printing formulation (paste). This formulation was then applied through a mesh with a density of 77 fibres/cm to a glass substrate, thereby creating a strip with a width of 2 mm on the substrate. The glass substrate with the applied printing formulation was sintered for 20 minutes at a temperature of 450 °C in an inert- atmosphere furnace, whereby from the printing formulation was formed a compact layer of copper nanoparttcles and microparticles, the sheet resistance of which measured by a four-point method was 0.034 Ω/m ² . Example 3

60 ml of ethylene glycol and 5 g of copper sulfate pentahydrate were mixed in a 150 mi beaker. The mixture thus formed was heated to a temperature of 65 °C and after dissolving all its components, 0.7 g of polyvinyl alcohol was added to it. This mixture was stirred at a constant temperature of 65 °C until complete dissolution. After that a solution of 6 g of ascorbic acid in water (20 g of ascorbic acid and 25 ml of demineralized water) was added quantitatively into it at a temperature of 65 °C, which resulted in the reduction and precipitation of copper particles, whereby during the reduction, the reaction mixture was intensively stirred with a stirrer at a rotation speed of 800 rpm. These particles were filtered through a glass frit and the resulting filter cake was washed with 100 ml of isopropanol. Thus, 1.2 g of copper particles with a protective layer of polyvinyl alcohol was obtained. These particles were then spilt over with 100 ml of isopropanol for preserving them for characterization and for further processing.

Subsequently, during measurement by SEM, two fractions of copper particles were identified - nanoparticles having a mean diameter of 100 nm and microparticles having a mean diameter of 1200 nm.

10 g of the thus prepared mixture of copper nanoparticles and microparticles were mixed with 0.8 g of polymethylmetacrylate, 5 g of toluene and 1 g of 2-butanon to produce a printing formulation (paste). This formulation was then applied through a mesh with a density of 77 fibres/cm to a glass substrate, thereby creating a strip with a width of 2 mm on the substrate. The glass substrate with the applied printing formulation was sintered for 20 minutes at a temperature of 450 °C in an inert-atmosphere furnace, whereby from the printing formulation was formed a compact layer of copper nanoparticles and microparticles, the sheet resistance of which measured by a four-point method was 0.037 Ω/m ² Example 4

50 ml of propylene glycol, 20 ml of propanol and 5 g of copper sulfate pentahydrate were mixed in a 150 ml beaker. The mixture thus formed was heated to a temperature of 95 °C and after dissolving all components, 0.7 g of carboxymethyl cellulose was added into it. This mixture was stirred at a constant temperature of 95 °C until complete dissolution. Subsequently, a solution of ascorbic acid in water (20 g of ascorbic acid and 25 ml of demineralized water) was added quantitatively into it at a temperature of 95 °C, which resulted in the reduction and precipitation of copper particles, whereby during the reduction the reaction mixture was intensively stirred with a stirrer at a rotation speed of 600 rpm. These particles were filtered through a glass frit and the resulting filter cake was washed with 100 ml of isopropanol. In this manner, 1.1 g of copper particles with a protective layer of carboxymethyl cellulose was obtained. These particles were then spilt over with 100 ml of isopropanol for preserving them for characterization and for further processing.

Subsequently, during measurement by SEM, two fractions of copper particles were identified - nanoparticies having a mean diameter of 150 nm and microparticles having a mean diameter of 2300 nm.

Example 5

60 ml of glycerol and 5.9 g of copper nitrate hexahydrate were mixed in a 150 ml beaker. The mixture thus formed was heated to a temperature of 65 °C and after dissolving all components, 0.7 g of xanthan gum was added into it. This mixture was stirred at a constant temperature of 65 °C until complete dissolution. After that, a solution of glucose in water (20 g of glucose and 25 ml of demineralized water) was added quantitatively into it at a temperature of 65 °C, which resulted in the reduction and precipitation of copper particles, whereby during the reduction the reaction mixture was intensively stirred with a stirrer at a rotation speed of 600 rpm. These particles were filtered through a glass frit and the resulting filter cake was washed with 100 ml of isopropanol. Thus 1.15 g of copper particles with a protective layer of xanthan gum was obtained. These particles were then spiit over with 100 ml of isopropanol for preserving them for characterization and for further processing.

Subsequently, during measurement by SEM, two fractions of copper particles were identified - nanoparticies having a mean diameter of 90 nm and microparticles having a mean diameter of 1600 nm.

1 g of the thus prepared mixture of copper nanoparticies and microparticles was mixed with 0.1 g of polyvinylpyrrolidone, 0.25 g of ethylene glycol, 0.25 g of propanediol and 0.1 g of butanol, to produce a printing formulation (paste). This formulation was then applied through a meshpolyhyd with a density of 77 fibres/cm to a glass substrate, thereby creating a strip with a width of 2 mm on the substrate. The glass substrate with the applied printing formulation was sintered for 20 minutes at a temperature of 450 °C in an inert- atmosphere furnace, whereby from the printing formulation was formed a compact layer of copper nanoparticies and microparticles, the sheet resistance of which measured by a four-point method was 0.038 Ω/m ² .

In the second variant, 12 g of the mixture of copper nanoparticies and microparticles obtained by the above-described method were mixed with 1.8 g of hydroxyethyl cellulose, 6 g of demineralized water, 0.5 g of 1-propanol, 1.5 g of ethylene glycol and 0.03 g of SAS, to produce a printing formulation (paste). This formulation was then applied through a mesh with a density of 77 fibres/cm to a glass substrate, thereby creating a strip with a width of 2 mm on the substrate. The glass substrate with the applied printing formulation was sintered at a temperature of 450 °C in an inert-atmosphere furnace for 20 minutes, whereby from the printing formufation was formed a compact layer of copper nanoparticies and microparticles, the sheet resistance of which measured by a four-point method was 0.034 D/m ² . Example 6

250 g of copper sulfate pentahydrate and 3 litres of propylene glycol were mixed in a 6 litre Keller flask provided with an anchor stirrer, a temperature probe and a cooler. The mixture thus formed was heated to a temperature of 65 °C and after dissolving all the components, 33.5 g of polyvinylpyrrolidone were slowly added into it. This mixture was stirred at a constant temperature of 50 °C until complete dissolution. Subsequently, a solution of glucose in water (1 kg of glucose and 1.25 litre of demineralized water) was added quantitatively into it at a temperature of 65 °C, which resulted in the reduction and precipitation of copper particles, whereby during the reduction, the reaction mixture was intensively stirred with a stirrer at a rotation speed of 600 rpm. These particles were filtered through a glass frit and the resulting filter cake was washed with 3 x 300 ml of isopropanol. Thus, 56 g of copper particles with a protective layer of polyvinylpyrrolidone were obtained. These particles were then spilt over with 500 mi of isopropanol for preserving them for characterization and for further processing.

Subsequently, during measurement by SEM, two fractions of copper particles were identified - nanoparticles having a mean diameter of 75 nm and micropartic!es having a mean diameter of 1400 nm - see Fig. 1 and Fig. 2.

10 g of the thus prepared mixture of copper nanoparticles and microparticles were mixed with 0.5 g of polyvinyl chloride, 3.7 g of cyciohexanone and 0.9 g of 2-butanone, to produce a printing formulation (paste). This formulation was then applied through a mesh with a density of 77 fibres/cm to a glass substrate, thereby creating a strip with a width of 2 mm on the substrate. The glass substrate with the applied printing formulation was sintered for 20 minutes at a temperature of 450 °C in an inert-atmosphere furnace, whereby from the printing formulation was formed a compact layer of copper nanoparticles and microparticles with a thickness of 65 μιη - see Fig. 3, the sheet resistance of which measured by a four-point method was 0.032 Ω/m ² .