Novel Materials and Methods

The present invention relates to nanomaterials. In particular, the invention relates to nanoparticles having a metallic core and a protective layer bonded to the core.

The invention also concerns a method of sintering nanoparticles in order to form conductive patterns as well as uses of the present novel nanoparticles.

Printing methods for creating printed electronics on laminar substrate surfaces, in particular on paper, are developing fast and a variety of materials, products and printing applications have been introduced for serving this purpose. Rapid development of Information and Communication Technologies (ICT) provides for versatile communication, which rather than being dependent on time and location is based on consumer needs. If materials and methods for producing printed electronics on paper can be further developed, paper can be a good substrate for many electronic products and media of the next generation. In near future paper could serve as a substrate in applications for sophisticated identification, interactivity and information transfer.

Furthermore, the first steps towards achieving this goal include the creation of conductive patterns, which could enable combined use of embedded electronics and printed conductive patterns (e.g. near-field imaging applications) on paper surfaces.

Thus, there is a need to develop paper and similar fibrous substrates further to carrier substrate for cheap and recyclable electronics and electronic applications.

Nanoparticles are in general relatively easy to sinter because of their low melting points. Also deposition methods are available to nanoparticles and conductive inks have been synthesised (e.g. B. K. Park et al, Thin Solid Films 515 (2007) 7706-7711 and D. Mott, J. et al, Langmuir 2007, 23, 5740-5745). Metallic nanoparticle materials gain conductivity via sintering, when the particles adhere to each other to form a conductive stain. Gold, silver and copper particles have been studied recently (X. Sun et al, Mat. Chem. Phys. 96 (2006), 29-33; C. H. Walker et al, J. Am. Chem. Soc, 2001, 123, 3846-3847; H-H. Lee et al, Nanotechnology 16 (2005) 2436-2441; and D. Mott et al, Langmuir 2007, 23, 5740-5745). Metallic nanoparticles typically require stabilization, i.e. protection against oxidation, and several methods have been

suggested for this purpose. For example, in the article of X. Sun, polyelectrolytes including both branched and linear poly(ethylene imine) as stabilizing agents and reducing reagents in the preparation of gold nanoparticles at elevated temperature (80 ⁰ C). In such a process, poly(ethylene imine) will transfer to corresponding positively charged polyelectrolytes (protonated amines) as mixing up with the precursor tetrachloroauric acid (HAuCl ₄ ). During the reduction of auric acid to gold nanoparticles, amine groups will be oxidized into amides at elevated temperatures.

EP 1646095 describes silver nanoparticles and alloy nanoparticles like silver-copper, go Id- silver-copper nanoparticles. Organic stabilizers like thiol and amine or/and their mixtures are used. Also in this method, the preparation temperature of the nanoparticles is relatively high, typically 50-200 °C.

US 7081214 discloses a conductive paste which is composed of a varnish- like resin (preferably thermosetting resins), metal fillers (with an average particle diameter 0.5-20 micrometer, produced by grinding), and ultrafϊne metal particles with an average diameter 2- 100 nm composed of one or more metal out of silver, for example. The paste is manufactured from commercially available ultrafϊne silver particles, having a alkylamine (preferably C8- Cl 8 with terminal primary amine) or polyoxyalkyleneamine coat on the surfaces thereof. The resulting particles are contained in a paste and cannot be used in powder form.

US 2008/0003363 discloses a method for manufacturing metal nanoparticles by refluxing an aqueous solution of metal hydrazine carboxylate of target metal at about 80 °C.

One method and some materials for forming condutive patterns on paper substrates are described in the International Application PCT/FI2007/050419. The application discloses, e.g., a tin-bismuth microparticle powder sintered on paper and plastic films with a method resembling electrophotographic printing. Microparticles cannot generally be printed. Finnish Patent Application No. 20075676 discloses nanoparticles having a protective group or ligand derived from a thiol and being bonded to metal nanoparticles. These nanoparticles can be sintered and also printed.

The processability and the level of conductivity of structures produced using known nanomaterials is not satisfactory. In particular, the known methods for preparing metallic nanoparticles involve the use of elevated temperatures. A specific problem of prior art relates to the stabilizing copper nanoparticles, which oxidize very fast compared with most other metals. Thus, not all of the prior art methods are suitable for manufacturing protected copper nanoparticles.

Therefore, it is an aim of the invention to achieve an improved material suitable for manufacturing of conductive patterns on essentially planar substrates by printing and sintering. In particular, it is an aim of the invention to achieve a nanoparticle material, which exhibits good processability during manufacturing and applying and high conductivity when sintered. A particular aim of the invention is to achieve a method for producing nanoparticles, in particular copper nanoparticles, at room temperature.

It is also an aim of the invention to achieve an improved method for forming conductive patterns on substrates.

The invention is based on the idea of protecting metallic nanoparticles with a wrapping derived from aliphatic amines. In particular, the wrapping may comprise small organic ligands or polymers, which are derived from polymeric polyamines. The protective agent may form a continuous wrapping around the core, thus effectively preventing agglomeration of the metal particles during manufacturing and/or use.

Thus, there are provided both a novel intermediate nanoparticle product for the manufacture of sintrable nanoparticles and final sintrable nanoparticles. The intermediate nanoparticles product comprises nanoparticles having a metallic core and a protective, at least partly volatile wrapping attached to the core, said protective wrapping being derived from aliphatic amines. Generally, the amount of such volatile components is more that 30 %, in particular more than 50 % of the total weight of the intermediate particles. The intermediate product typically comprises also certain amounts of reducing agent used in particle synthesis.

The final product comprises metallic nanoparticles derived from such intermediate product by purification. This means that at least part of the volatile components, an optionally some synthesis agents, are removed. Generally, the amount of volatile components is reduced to a level of less than 20 %. However, depending on the type of the wrapping, the percentage or

the residual amine derivative may be as low as 0.5 % or even lower. Removal of the excess volatiles typically takes place while washing away the reducing agent from the synthesis mixture.

The final product may, at least in the case of copper particles and depending on the protective agent and synthesis parameters, contain minor amounts of oxide of the metal used. Although there exists a thin protective layer on the metallic core, the final product is preferably stored in a container including inert gas, preferably nitrogen for preventing any further oxidation.

According to one embodiment only oligomers and polymers of amines are used to produce the wrapping. No other protection-forming components, such as thiols, are required.

According to one embodiment, the metallic core is a copper core. According to a further embodiment, the protective agent is tetraethylene pentamine (TEPA) or a derivative thereof. According to alternative embodiment, the protective agent is polyethylene imine (PEI) or a derivative thereof. In these compounds the the imine groups are dominant rather than the terminal primary amines.

In the method according to the invention, the nanoparticles of the above kind are applied on a substrate, which may be a paper, cardboard or some polymeric material and sintered at an elevated temperature, preferably 150 - 300 ⁰ C, and pressure. Under these circumstances, the protective wrappings of the nanoparticles volatilize and the cores of the nanoparticles fuse, i.e., sinter. The resulting conductive pattern generally exhibits resistivity of less than 5*10 ⁵

ωm.

More specifically, the aspects of the present invention are characterized by what is stated in the independent claims.

The invention offers considerable advantages. The nanoparticle material according to the invention is not only non-agglomerating by nature, but is also easy to sinter with constant quality in process conditions well withstood by various substrates, such as papers, cardboards and other fibrous materials, as well as many polymer substrates. In particular copper particles have found to aggregate severely without proper protection.

Further, copper nanoparticles as such oxidize very easily in air. The problems of aggregation and oxidation can be circumvented efficiently using the present invention without compromising conductance and processability. Gold and silver withstand oxidation better than copper, and can also be used within the present invention, but copper is still very alluring candidate for future conducting materials because of its abundance and cheapness. The suggested method can be conducted at room temperature, which is a significant advantage with respect to most known stabilization methods.

The resulting chemical bonding of the wrapping keeps the protection at room temperatures but is sufficiently weak to be removed by heating, which is essential in many practical applications.

The present nanoparticles are generally recovered and stored in powder form. Although the protective wrapping typically reduces oxidation as it reduces the surface area of the metal particle exposed to air, it is preferred to store the particles in inert gas after purification. This is particularly true when the amount of wrapping material is very low, such as is the case with TEPA generally.

Copper has previously been found to form various copper sulphides when sintered in the presence of sulphur. The interesting feature of these sulphides is the fact that they are semiconductors and quite resistant to oxidation in air. However, the conductivity properties of sulphides are not as good as those of pure metal obtainable by the present sulphur- free process.

In connection with the present invention it has been found that the electrically conductive pattern produced according to the present invention is of a structurally unique type, which reflects in its improved electrical properties. For example, using polymer-coated copper nanoparticles resistivities close to those of pure metals and comparable to those of silver nanoinks, have been achieved. This is a clear indication of improved sintering properties of the present materials as compared with known nanomaterials. The properties of the present novel materials and structures are described in more detail in the experimental section below.

Surprisingly, the results as concerns the quality of the sintered patterns have been found to be better when printed on paper than when printed on plastic. This makes the present nanomaterials very suitable for being used in printed electronics applications.

Naturally, applications of the above mentioned kinds will strengthen the position of paper and offer easier market entries for more intelligent media products.

Other possible substrates include textiles, non- woven materials, circuit boards of electronics industry, moulded articles, glass, construction materials, such as wallpapers and floor coatings, unfϊred and fired ceramics, (bio)polymer bases and composites. Each one of the listed substrates has its own application areas and advantages.

In particular, the present method involving nip sintering is suitable for substrate has a shattering or deformation point below 300 ⁰ C, in particular below 250 ⁰ C, even below 200 ⁰ C, that is, at least various paper and plastic grades not tolerating high temperatures.

Although electrostatic transferring has been found to provide significant advantages, other forms of printing may be employed as well. Of these methods, particularly screen printing, gravure printing, flexographic printing, offset printing, and relief printing are mentioned. Common to all these methods is,that no additional screening during particle deposition or particle removal after deposition is needed, whereby no loss or recirculation of particles takes place. That is, the particles can be initially deposited in a formation that is ready to be sintered in a subsequent two-roll nip or the like.

Next, embodiments of the invention are described more closely with reference to the attached drawings.

In the drawings, Fig. Ia illustrates an X-ray diffraction (XRD) spectrum of Cu-PEI.

Fig. Ib illustrates an X-ray diffraction (XRD) spectrum of sintered Cu-PEI. Fig. 2a shows Cu-PEI reaction mixture as a TEM image. Fig. 2b shows Cu-PEI reaction mixture as a SEM image. Fig. 2c shows final Cu-PEI product as a TEM image.

Fig. 2d shows final Cu-PEI product as a SEM image.

Fig. 3 is an energy-dispersive X-ray spectrum (EDS) of Cu-PEI final product. Fig. 4a shows an image of a sample sintered from Cu-TEPA nanoparticles. Fig. 4b shows an image of a samples sintered from Cu-PEI nanoparticles. Fig. 5 depicts the results of Cu-PEI Wide angle X-ray scattering (WAXS) measurements. : a) full spectra at 30° C. b) Copper oxide region during heating, c) Metallic copper region during heating.

Fig. 6a depicts Cu-TEPA reaction mixture as a SEM image. Fig. 6b depicts final Cu-TEPA product as a SEM image. Fig. 7a shows an EDS image of Cu-TEPA reaction mixture. Fig. 7b shows an EDS image of final Cu-TEPA product. Fig. 8a shows a TEM image of final Cu-TEPA product. Fig. 8b shows another TEM image of final Cu-TEPA product.

Fig. 9a depicts the results of WAXS measurement for a Cu-TEPA sample across a wide range.

Fig. 9b depicts the results of WAXS measurement for a Cu-TEPA sample across the copper range during heating. Fig. 10 shows a comparative graph of resistivities of conductive patterns achieved using various materials, including Cu-PEI after sintering.

As stated above, the present invention concerns substantially spherical nanoparticles, which can be manufactured, for example, using the processes described later in more detail.

The average size of the metallic core of the particles varies usually from 1 to 20 nm, in particular from 1 to 10 nm. The core material is most advantageously copper, but can also be aluminium, zinc, nickel, cobalt and indium or any mixture thereof.

The present nanoparticles in reaction mixture (unpurified) typically have an average particle size which, measured at the outer surface of the protective wrapping, is 200 - 1500 nm, in particular 500 - 1000 nm. The wrapping typically weighs 0.5 - 20 times the weight of the core, in average. The wrapping is bonded to the metal core by coordination forces or by covalent bonds.

The amount of volatile compounds, ie. the protecting agent, in the final product depends on the starting material. For Cu-PEI, a typical percentage of volatile compounds of the structure is 10 - 15 %, whereas for Cu-TEPA, percentages of as low as 0.1 - 5 %, in particular 0.5 - 2 % were obtained.

The amount of protective agent on the surface of the copper core seems to correlate with on the number of amine groups in the protective agent.

The average diameter of the particles, as far as they can be discerned in a TEM image of the final Cu-PEI product, is was 5 - 60 nm. The Cu-TEPA product comprised aggregates of some hundreds of nanometers in size, essentially consisting of smaller particles ofundiscernible size.

When the temperature of the particles is raised above a certain point, typically between 150 and 300 ⁰ C, the wrapping starts to volatilize, allowing the cores to touch each other, melt at least partly and fuse together, i.e., sinter or coalesce. If the temperature rise is carried out under significant pressure, typically 5 - 50 kN, the cores will form a uniformly conducting layer of fused metal particles. The experiments have shown that the time this process usually takes depends on the layer thickness and varies from lms to 10 s in order for significant fusing to take place and that no further conductivity increase takes place after a certain time, usually about 10 - 12O s.

Typically, the application of the heat and pressure is stopped when a resistivity of the rreessuullttiinngg ccoonndduuccttiivvee ppaalttern of 10 ^"5 to 10 ^"7 ωm is achieved, that is, after lms - 1 s of the start of the sintering process.

According to WAXS calculations, the crystal size of Cu-PEI particles in end product was about 8.5 nm in room temperature and 19.4 nm for Cu-TEPA.

Sintering system

In sintering process the metallic (or polymer) powder particles are sintered together to form a continuous, conductive structure. The sintering procedure utilizes simply pressure and temperature (either in a roll or plate configuration). This is used to exceed the melting and sintering temperature of the used conductor material. The sintering is preferably

carried out in a sintering nip comprising two opposing rolls, plates or belts. The surface materials of the heated material should tolerate the temperature used (e.g., 50 C - 250 C) without deformation. Possible surface materials for the roll are e.g. tungsten carbide, hard chrome, PTFE covers and its derivatives and ceramic material with anti-sticking properties (low surface energy). Either one of both of the rolls, plates or belts in the sinter nip may be heated. The sintering may occur in direct contact with the heated roll or the heat may be transferred through the substrate material. Also both contacting rolls may be heated to increase heat transfer in the nip. Our studies indicate, that for maximizing the affixation of the particles to the substrate, it is preferred that at least the roll or plate coming into contact with the surface of the substrate not comprising the particle-formed pattern (second roll) is heated. The roll in contact with the powder (first roll) may be in considerably lower temperature, even unheated and cooled. Durable patterns are, however, achieved by heating that roll (first roll) only.

Important roll surface properties are a very high smoothness (preferably, Ra < 1 μm) and low surface energy (preferably, < 100 mN/m). This is to decrease the sticking tendency of melted metals to the roll.

The heating can be made also with laser, or by induction, IR heating and microwave heating prior to compression in a nip.

An exemplary sintering equipment is disclosed in WO 2008/006941, which is incorporated herein by reference.

Generally speaking, the disclosed technique can be varied such that some other sintering technique instead of nip sintering is used. Thus, in a general form, the method comprises forming a conductive pattern on a planar insulating substrate in particular so that

- particle-type conductive matter is transferred onto a surface of the substrate into a predefined pattern, and - the particle-type conductive matter is at least partially sintered at elevated temperature and pressure in order to convert the particle-formed pattern into a continuously conducting pattern affixed to the substrate.

As described above, preferably an electrostatic transfer method and transfer means capable of screenless transferring of the particle-type conductive matter onto the surface of the

substrate into a predefined pattern are used, but other printing methods can be employed too. Thus, all embodiments described in this document and those defined in the appended claims may be freely combined with the basic concept disclosed above.

Initially, the non-sintered nanoparticles are typically isolating or semiconducting, implying that the wrapping acts to some degree as an insulator between the cores. However, the wrapping can also be treated so as to be at least slightly conductive. Generally speaking, notwithstanding the initial conductivity, one of the aims of the sintering process is to increase the conductivity of the nanoparticle layer. Another main object is to bind the metal contained in the nanoparticles to the substrate for forming permanent and durable wirings and the like patterns. It has been observed, that 95 - 99.9 %, usually at least 98 % of the protective wrapping is volatilized during the sintering process, thus resulting in a relatively pure metallic layer.

The nanoparticles as such are isolating or semiconductive and can be brought on a substrate as a thin layer for example by printing technologies or lithography. Typically such substrates can be selected from group of webs and sheets of paper and cardboard and similar fibrous substrates, and various polymer materials present as films or sheets.

The present copper nanoparticles can be manufactured by

- providing a CuCl ₂ precursor,

- reducing the precursor in the presence of polyethylene imine (PEI) or tetraethylene pentamine (TEPA) or a derivative thereof for producing nanoparticulate wrapped copper matter, and

- washing the reaction mixture under inert gas such as nitrogen, typically with degassed water, until the supernatant is neutral.

Preferably, reducing reagent, such as NaBH ₄ , is added to the reaction mixture at some point of preparation before the washing stage.

The particle synthesis is preferably carried out at a temperature less than 35 °C, preferably essentially at room temperature.

Application of nanoparticles

In the following, suitable transfer methods for applying the nanoparticles onto a substrate are described:

Electrophotographic transfer

Conductive separate particles are used for screenless electrostatic printing on insulating substrate material. Such a system utilizes charging of particles with a high voltage electrode in a powder carrier. Charged particles are transferred from the powder carrier by a first electric field on the surface of a transfer roll. The transfer roll is formed from electrodes , which are in different potentials. The charged particles attach to the transfer roll according to a potential difference. For example particles with positive charge attach to the areas where the potential is negative or grounded. The charge of the particles is essentially maintained on the surface of the transfer roll. From the transfer roll the charged particles move to a transfer nip. In this nip the particles are moved to the substrate by an attractive force generated by an electrode roll. Thus, transfer in the nip occurs due to a second electric field between the transfer roll and the electrode roll. As understood by a person skilled in the art, the transfer may also be carried out by transfer and electrode plates or belts, or other suitable members, instead of rolls by using the described principle.

The transfer can be achieved by an apparatus comprising electrostatic transfer means for transferring the particle-type conducting matter onto the surface of the substrate directly into a predefined pattern. As the pattern is in the predefined form, it can be sintered in a sintering nip.

If electrostatic transfer is used, the transfer means typically includes a tool comprising a transfer member (e.g. a roll, plate or belt) having at least one transfer electrode embedded therein and an even dielectric surface layer for preventing discharging of the charged conducting particles transferred on the surface of the member by a voltage applied to the transfer electrode. An apparatus of the above kind allows accurate and convenient screenless transferring of the particles to the substrate.

Basically electrostatic transfer can be carried out as described above (dry/traditional electrophotography). Other forms include liquid electrophotography, in which a solvent is used. The particles to be sintered are deposited in the solvent. The solvent is evaporated or absorbed by the substrate (in particular paper or board), whereafter the sintering is carried out for (almost) dry particles. Possible transfer mechanisms, in addition to those described above, include cascade development (two-component development), fur brush development, magnetic brush development, impression development, powder-cloud development, liquid spray development, liquid electrophoretic development, heat development, liquid film development, selective toner release and others.

Screen printing

Liquid-form "colour" is transferred to the substrate through a web-like screen means (cloth or metal) or through a stencil. The nanocopper particles are dispersed into a solvent or carrier medium, which is used for applying the particles as a liquid paste onto the substrate. The solvent is evaporated or absorbed into the substrate before sintering.

Gravure printing, Flexographic printing. Offset printing. Ink-jet printing and Relief printing

Nanocopper particles are dispersed in a solvent or carrier medium, which is used for applying the particles as a liquid paste onto the substrate according to the technique in question. The solvent is evaporated or absorbed into the substrate before sintering.

As will be discussed in more detail in the example below, two samples of copper nanoparticles with different compositions were prepared using different starting materials for the wrappings. Both of the nanoparticle materials prepared showed excellent conductivity properties, easy sintrability and formed durable patterns.

According to one embodiment, the protective wrapping is derived from organic amines, in particular organic polyamines, used for protecting the metal atom or metal core, in practice often copper. The amines can be monomeric, but it is preferred to have oligomeric or polymeric amines. Thus, the amines contain at least 3, preferably at least 4 repeating units, and at least 2, preferably at least 3 amine functions.

The organic amines typically exhibit a plurality of amine functions selected from the group of primary, secondary and tertiary amine functions and combinations thereof.

According to one embodiment, the amines exhibit both primary and secondary amine functions.

According to another embodiment, the amines exhibit primary, secondary and tertiary amine functions.

Examples of suitable amine compounds include poly(alkylene imine)s, wherein the alkylene unit comprises 1 to 4 carbon atoms, such as polyethylene imine (PEI) and derivatives thereof. Other suitable amines include tetra-alkylene polyamines, such as tetraethylene pentamine (TEPA) and derivatives thereof.

According to one embodiment, the molar ratio of primary to secondary amine groups is in the range of 10-50 : 10-100 mol, in particular about 10 - 30 : 30 - 80, preferably about 15 - 25 :

25 - 35.

According to another embodiment, the molar ratio of primary, secondary and tertiary amine groups of a poly(alkylene imine) is in the range of 20-30 : 40-60 : 20-30.

The molecular weight of the amine derivative is typically less than 1800 g/mol, but higher than about 120 g/mol. For polymeric amine derivatives, such as poly(alkylene imine)s, the molecular weight is preferably 800 - 1600 g/mol, in particular when PEI is concerned.

The following detailed examples and comparative examples illustrate the invention.

Examples

Particle synthesis

The following procedures were used in the particle synthesis of two amine derivatives according to the invention, namely PEI and TEPA, and copper. Particles were then purified to

remove residues of a reducing agent used and excess polymer. The product particles were sintered with different parameters to get a conductive layer pressed on paper.

Cu-PEI1200(l/l)

The monomer of polyethylene imine consists of a three-membered ring. Two corners of the molecule include -CH ₂ - linkages. The third corner is a secondary amine group, =NH. In the presence of a catalyst this monomer is converted into a highly branched polymer with about 25% primary amine groups, 50% secondary amine groups, and 25% tertiary amine groups. This product is sometimes called "pure polyethylene imine" in order to differentiate it from certain copolymers of ethylene imine and acryl amide. The latter mixture, which can also be used within the meaning of this invention can be copolymerized to produce so-called "modified PEI".

Polyethylene imine (PEI) used, was branched, containing primary, secondary, and tertiary amine groups in approximately 25/50/25 ratio. Mw=1200. In a typical upscaled PEI synthesis 2400mg (2 mmol) of PEI1200 was dissolved in 150 ml of UHQ H2O. 269 mg (2 mmols) of CuCl ₂ and 757 mg (20 mmols) OfNaBH ₄ were weighted in septa sealed vials. The solution, vials and some solvent were carefully degassed with N ₂ for about 30 minutes. 5 ml of degassed solvent was used to transfer copper chloride in the reaction flask, producing an intense blue color due to complexation of the copper and the polymer. Sodium borohydride was added dropwise with 5 ml of degassed water. The reaction mixture turned typically in 4 hours to a pitch black liquid, which was then washed under nitrogen with degassed water until the supernatant was neutral. Separations were performed using a centrifuge. The yield of the process was 57.6%.

In contrast to the method disclosed in the abovementioned article of X. Sun et al. when polyethylene imine stabilizer mixes up with the precursor copper chloride (CuCl ₂ ) in the synthesis of copper nanoparticles, it can neither form polyelectrolyte nor reduce copper ions to metallic copper particles at room temperature. In fact, an intensive blue complexation forms between copper ions and polymers, which effect is taken into account by the application of additional reducing reagent NaBH ₄ .

Cu-TEPA(l/10)

Tetraethylene pentamine (TEPA) was used to protect copper. The general formula of the TEPA monomer used was m _{/χ λ} NH

H, N V MH V HK ₅

Ratio used was 1 to 10 due to behavior encountered with other small protecting agent, DACH. The synthesis procedure was the following: 946.5 mg (5 mmols) of TEPA was dissolved in 36 ml of UHQ water and degassed. 189 mg (5 mmols) OfNaBH ₄ and 67 mg (0,5 mmols) of CuCl ₂ were degassed separately. After thorough degassing, CuCl ₂ was dissolved in 2 ml degassed water and injected into the reaction flask. Intensive colored complex formed immediately. NaBH ₄ was also transferred with 2 ml of degassed solvent to the reaction vessel. Quick transformation to darker color was observed. Within 4 hours the solvent turned clear, as copper colored precipitate formed in the flask. Purification was done under N ₂ : particles were washed with water until the supernatant was neutral. The yield of the process was 71.9%.

Particle characterisation

Thermo Gravimetric Analyzes (TGA) for the particles manufactured were done using

Mettler-Toledo TGA850 equipment with STARe software. Temperature range was 25-600° C, using nitrogen gas. SEM measurements were performed with Hitachi S4800 field emission scanning electron microscope.

Cu-PEI1200(l/l)

The synthesis procedure yielded dark particles with a metallic copper colored glare. Sintered particles exhibited good conductive capabilites with resistance values less than 1 ohm at best, see Table 1. Also, it was noticed that 4h reaction yielded better conductivity results than a overnight synthesis, see Table 2, probably because of the increased interparticle aggregation with increased synthesis time. Smaller copper particles will fuse more easily together than bigger particles, affecting the quality of the sintered sample.

Table 1. Resistance results for sintered 4 hrs synthesis Cu-PEI 1200.

Table 2: Resistance results for sintered 24 hrs synthesis Cu-PEI1200.

According to TGA measurements, 24 hour synthesis posessed 12% and 4 hour 15% volatile components, indicating that longer synthesis time will not affect the amount of protection. The samples were purified until the supernatant removed was neutral. Purifying these particles is still a problem: during the process great amount of fine black particles are lost. Also, during the purification some amount of copper colored particles are precipitated, or coated, on the centrifugation tube. This effect is especially strong, when the pH of the supernatant is nearing neutral value. This is probably due to the loss of the protecting agent, causing unprotected particles to aggregate and precipitate out.

XRD measurements

Heated XRD measurements were performed for the material. During the heating in air from 25° C to 350 ⁰ C peaks from the metallic copper grew narrower, corresponding to the particles coupling and forming larger structures, as shown in Fig. Ia. Interesting transformation is observed at 250° C, where Cu(I)oxide- signals start to diminish simultaneously as Cu(II)oxide-signals get stronger. This temperature apparently accelerates the conversion of copper(I)oxide to copper(II)oxide. The sample loses its metallic copper colored glare during this heating experiment in air and looks like black powder, supporting the idea of

copper(II)oxide formation. Sintered sample was also studied with XRD, the results being shown in Fig. Ib. Although the paper, on which the material was sintered on, gave many peaks, definite metallic copper peaks were observed. Although the sintered sample had been exposed to air for long periods of time, only some oxidation can be seen.

Electron microscopy

SEM and TEM measurements were done to three samples taken from one Cu-PEI synthesis. The samples were taken from the reaction mixture right after synthesis (Sl), after one water wash (S2) and from the purified final product (S3). The SEM image taken from Sl revealed spherical structures probably formed by the polymer wrapping itself around copper particles, as shown in Fig. 2b. The diameter of the spheres seemed to be close to lμm. With reference to Fig. 2a, also TEM imaging for Sl also showed the spheres. Unfortunately the contents of the spheres could not be verified, as EDS measurements were not possible due to extensive charging of the sample. Electron diffraction patterns of the spheres didn't reveal any details, as can be seen also from Fig. 2b. Entirely purified S3 sample seemed to consist of large copper regions. TEM imaging, however, revealed that the large copper chunks are aggregates of finer particles, Fig. 2c. This was expected, because the material is easily sintered in relatively low temperature. Electron diffraction showed a definate diffraction pattern, as can be seen from Fig. 2d. The S2 sample looked very similar to S3. EDS measurement for S3 showed definite metallic copper peaks with some oxygen present, as shown in Fig. 3.

A SEM images of s sintered PEI sample is shown in Fig. 4a. The iamge shows small copper particles fused together. Also aggregates with size of several micrometers are present in the sintering layer.

WAXS measurements

The sintered Cu-PEI material was also subjected to wide angle x-ray studies, using a SAXS - device specially modified for WAXS use. Measurements were performed in an inert atmosphere with a heater. Measurements are presented in Figs. 5a - 5c. Apparently, the sample was easily oxidized during the sample processing: Cu ₂ O signal can be see in Fig. 5b (theoretically at 2.59, 2.83 and 2.96 I/A.). Metallic copper 111 diffraction is seen as a strong peak around 3. I/A. During the heating experiment the Cu ₂ O -signal disappears. Metallic

copper signal get narrower, suggesting crystal size growth. Table 3 shows the crystal sizes calculated using Scherrer's equation for both TEPA and PEI samples. At 200 ⁰ C the crystal size starts to increase in an expedited manner. From 250° C the crystal size is so large, that the accuracy of the evaluation suffers. Evidently, this explains the successful sintering experiments, as the material fuses together at this temperature.

Table 3: Cu-PEI and Cu-TEPA crystal sizes as measured by WAXS.

Cu-TEPA(l/10)

The synthesis procedure for Cu-TEPA particles is in general simpler than the Cu-PEI, as the product particles are easily precipitated. The magnetic stirring bar gets coated with metallic copper coloured material during the synthesis, which lowers the yield. However, the TEPA samples also showed quite encouraging resistance values, as shown in Table 4. TGA measurements indicated very slight amount of protecting agent present in the final product, only 1% of volatile components.

Table 4. TEPA resistance results.

Electron microscopy

SEM-measurements were done to the reaction mixture and the final product after the purification to the sample. Large differences were not observed, except the amount of microstructure seemed to increase in the final product, Figs. 6a. EDS-measurements of the samples indicated several elements present in the reaction mixture, corresponding to all the impurities present: reductant and protecting agent residues, see Fig. 7a. EDS of final product, shown in Fig. 7b, indicated that the material is mainly copper, however small amount of oxygen was also present. TEM imaging indicated finestructured aggregates of some hundred nanometers, as shown in Figs. 8a and 8b. The fine structure forming the aggregates seems larger than that of the PEI sample and SAED image gives brighter diffraction image, compared to that of PEI, indicating larger particles.

A SEM images of s sintered TEPA sample is shown in Fig. 4b. The image shows small copper particles fused together.

WAXS measurements

The results of the measurement is shown in Figs. 9a and 9b. Only a minor bump is noticed after 2.5 I/A, indicating very slight oxidation during sample processing. Heating experiment does not show significant narrowing of the metallic copper signals, suggesting that no dramatic change in crystal size occurs. Table 3 shows the crystal sizes calculated with the Scherrer's equation: apparently the TEPA sample does not fuse together, as PEI does. Probably the crystals are too large for the temperature range used. The slight change in crystal size can be attributed as thermal expansion of the material.

According to the experiments, the PEI and TEPA samples possess the best conductivity properties. Both agents enable nanomaterials with varying degrees of conductive properties, depending on the sintering conditions, as can be seen from Table 5.

Table 5. Cu-PEI and Cu-TEPA compared resistivity results.

These results indicate that Cu-TEPA has improved properties compared to Cu-PEI. Nanoparticle formation is enabled by the protecting agent during the synthesis: the protecting agent probably covers the growing nanoparticles and prevents them from aggregating too efficiently. Upon purification the reducing agent is removed. The protecting agent is also removed, in the case of TEPA almost completely. PEI samples lose most of the protecting agent. WAXS measurement indicates that PEI and TEPA have crystal sizes of 8,5 nm and 19,4 nm, respectively. This difference in crystal size results in the different sintering result of the materials. Smaller Cu-PEI particles fuse together around 200° C, abruptly improving the conductivity. The conductivity, however is not as high as in the case of TEPA-samples, probably due to polymer residues and oxidation of copper in the Cu-PEI-sample. Cu-TEPA particles, according to WAXS studies, will not fuse together at this temperature range, possibly due to larger crystal size. Smaller protecting agent is completely removed from the material and larger crystals undergo oxidation more slowly. Oxidation of the PEI sample indicates, that the protecting agent residue present in the final product won't protect the material from oxidation. Strangely TEPA sample didn't oxidize as much as PEI during sample preparation. There are several possibilities for this: With TEPA samples larger crystal size might enable slower oxidation, or the hydrophilic polymer residue present in the PEI - sample itself enhances the oxidation by attracting moisture from the air.

Comparative examples

Cu-S-Ol(I/!)

Synthesis of CuS nano flakes was done as described by Zhang et al in Mat. Chem. Phys. 98 (2006) 298. 130 mg of copper acetylacetonate, 16 mg of elementar sulpfur and 7,2 ml of oleylamine were degassed separately. With a strong stirring oleylamine was added on the solids. The mixture was heated to 110° C. The solution turned dark green and the color turned dark brown in a couple of minutes. The reaction time was 1,5 h. After the reaction the solution was black with a faint of greenish tint. Ethanol was added and the solution, which was then centrifuged and the final product was purified until the supernatant was clear, requiring two iterations. The black precipitate was dispersed in chloroform producing a black liquid with a small greenish tint. Solution couldn't pass 0,45 micrometer filter, and was collected and evaporated with N ₂ .

Samples resistance values are shown in Table 6.

Table 6. Cu-S-Ol(I/!) resistance results.

Cu-DHDP(I/!)

Typical 1 :1 copper to protecting agent ratio. 62,6 mg (0,25 mmol) of 3,3'-dihydroxydiphenyl disulfide was dissolved in solvent mixture 10ml EtOH, 10 ml UHQ water. System was carefully degassed and 33,6 mg (0,25 mmol) of CuC12 was added in the system. Upon addition of 95 mg (2,5 mmol) NaBH4 the reaction mixture turned quickly from pale transparent blue to dark brown turbid color. Purification was done with a centrifuge, using degassed tubes. The reaction mixture was made slightly acidic with addition of HCl, resulting in precipitation in the reaction mixture. Particles were washed with water until the supernatant was neutral. Particles were observed to disperse in EtOH and THF, both with some undispersing chunks. Samples were collected in a sample vial using THF, which was

evaporated under N2 flow. Samples with different protection ratios were prepared to compare the conductivity with different amount of protecting agent.

Different batches were attempted, and most showed no conductivity at any sintering temperature. At best, the resistance was larger than 6 Mohm.

Cu-DACH(l/10)

Diamino cyclohexane (DACH) was also used to protect copper. Copper to DACH ratio of 1/10 was used, because experiment with 1 to 1 ratio caused almost immediate precipitation, probably due to too low amount of protecting agent. In a typical synthesis 571 mg (5 mmols) of DACH was dissolved in 36 ml of UHQ water and degassed. 189 mg (5 mmols) OfNaBH ₄ and 67 mg (0,5 mmols) of CuC12 were degassed separately. After thorough degassing, CuCl ₂ was dissolved in 2 ml degassed water and injected into the reaction flask. Intensive colored complex formed immediately. NaBH ₄ was also transferred with 2 ml of degassed solvent to the reaction vessel. Quick transformation to darker color was observed. Within 2 hours the solvent turned clear, as dark precipitate formed in the flask. Purification was done under N ₂ : particles were washed with water until the supernatant was neutral.

The first synthesis with a ratio of 1/1 Cu:DACH resulted in a very quick precipitation within minutes of the addition of NaBH4. The ratio was adjusted to 1/10 to increase the amount of copper complexing amine groups. The conductivity results are shown in Table 7.

Table 7. DACH resistance results.

Cu-Sug

Saccharose was also used to test the production of nanoparticles. 6000 mg (17,5 mmol) of saccharose was dissolved 30 ml UHQ water. After thorough degassing 67 mg (0.5 mmol) of

CuCl ₂ was dissolved in 5 ml of water and transferred into the reaction flask. No color change indicating complexation was observed. Sodium borohydride (190 mg, 5 mmol) was also dissolved in 5 ml of water and added to the reaction. Quick color change from brown to black and finally, within minutes, precipitation occured. Resulting particles were purified with water until the supernatant was neutral. Yield 81 ,4%.

TGA indicated that the material contained 3% of volatile components. Only resistance measurements were done, yielding similar results to TEPA -samples. Material was also conductive even prior to sintering, and the sintering process did not improve the resistance much.

Applications Various kinds of printed electrical circuits, in particular wirings between electrical components, and electrically readable markings, such as RFID tags can be manufactured using the present invention. Conductive patterns are basis for printed electronics, sensors, simple electronics, simple user interfaces, and opens possibilities also for interactivity and detection.

In the area of printed electronics, the present materials can be used especially for flexible synthetic polymer based substrates the focus is on transistors, field effect transistors, radio frequency indentification (RFID) devices, paper-like displays and creating batteries to offer power for above mentioned systems. Multicolored displays or displays that could operate while bended are of particular interest. The display may be flexible but the information to the displays still comes in inflexible, conventional way, e.g. via wire.

The potentiality with paper and printed simple electronics lies in applications for sophisticated identification, interactivity and information transfer. Utilizing embedded electronics and simple printed electronics can create a variety of new products which carry the information of traditional print, but also contain above mentioned intelligent features. This will strongly add value and create new markets to the paper based products. Remote readable identification circuits and sensors are desired to packages for ensuring safety, authentication and enable item level tracking. However, these systems require adding of chips to the printed antenna. In future remote readable identification may be possible to implement by printing. Before that,

interactive features could be created in packaging and advertisement products by utilizing printing of conductive, semi-conductive and non-conductive structures and embedding the more sophisticated electronic components.