Stabilised copper particles

Abstract

Stabilised copper sub-micron scale particles are provided, which are suitable for use in the manufacture of an electrically conductive product. The particles have a mean diameter in the range of from 25nm to 1000nm, and each particle comprises (a) a metallic copper inner core and (b) a sulfated polymer outer layer. The particles can be compressed at room temperature to fabricate free-standing conductive copper films or can be deposited onto a substrate to form supported electrically conductive products.

Description

[0001] STABILISED COPPER PARTICLES

[0002] Field of the Invention

[0003] The present invention relates to stabilised copper particles and processes for producing such particles. The present invention also provides electrically conductive products formed from the stabilised copper particles, and methods for their manufacture.

[0004] Background to the Invention

[0005] Copper (Cu) has high conductivity (5.96 x 107S / m) and is available at relatively low cost when compared to other similarly conductive metals. However, full exploitation of the capabilities of Cu remains impeded because of its higher susceptibility to oxidation, which retards its long-term conductivity and therefore the stability and performance of resultant devices.

[0006] There are various methods used in the art to improve the stability of Cu particles. These include alloying, core-shell formation, and the use of stabilising ligands.

[0007] It is, in particular, known in the art to use stabilising agents comprising of long hydrocarbon chains, to mitigate oxidation and preserve its metallic properties. Typical organic ligands utilized in the stabilization of Cu micro / nanostructures include amine (J. Cure et al, J. Phys. Chem. C 2017, 121, 5253.), carboxylic acid (R. Tokura, Het al, Mater. Adv. 2022, 3, 4802) and thiol terminated alkyl groups (G. Di. Met al, Nat. Commun. 2017, 8).

[0008] Chitosan has also been used as a stabilising agent for copper (Zain et al, Carbohydrate Polymers. 2014; 112: 195-202 and Tokarek et al, European Journal of Inorganic Chemistry 2013 (28):4940-7) and various amino acids (Kumar et al, Appl. Surf. Sci. 2016, 385, 225-233), which are especially useful in the context of biological and pharmaceutical applications.

[0009] For conductive applications, copper particles are deposited using casting, coating or printing techniques to form a defined pattern of required thickness and then sintered at high temperature to promote interparticle coalescence, which results in the gradual fusion of particles to form an interconnected network. The greater the degree of fusion between the particles, the higher the electrical conductivity will be. During the sintering process most of the organic ligands, solvents and additives are decomposed, thereby facilitating contact between adjacent particles and enabling fusion.

[0010] Alternatively, photonic sintering can be employed where a light source (ultraviolet, infrared, laser or intense pulse sintering) is used to promote sintering (J. Jung, et al, ACS Appl. Mater. Interfaces 2019, 11, 15773; Y. R. Jang et al, Review on Intense Pulsed Light Sintering Technologies for Conductive Electrodes in Printed Electronics. Int. J. Precis. Eng. Manuf. - Green Technol. 2020, 1-37). Depending on the exact source used, the sintering time can be significantly reduced due to the high energy of the light source.

[0011] Oxidation of Cu can be promoted by oxygen and moisture in the ambient atmosphere, and therefore most sintering processes need to be performed in an inert atmosphere to mitigate expediated oxidation that would otherwise occur at elevated temperatures (W. Li, Qet al, The rise of conductive copper inks: challenges and perspectives. Appl. Mater. Today 2020, 18).

[0012] Ultimately, whichever sintering method is adopted, the existing protocols require high temperatures / energies or long sintering times in order to decompose the conventional stabilising agents, aiding complete sintering of individual particles so as to attain high conductivity.

[0013] This has become a significant obstacle in recent years, with the rapid growth of the wearable and flexible electronics industry. This is because most flexible substrates have low glass transition temperatures, and therefore can undergo irreversible physical deformation at typical temperatures used in the sintering of copper particles. Therefore conventional approaches are not fit for purpose when seeking to manufacture wearable and flexible electronics products.

[0014] Over the years, strategies have been developed to attempt to lower the required sintering temperature, which would then enable processing techniques that are more compatible with the aforementioned thermally-sensitive substrates. One such approach is to utilize smaller ligands that still promote stability, but which would also decompose at lower temperatures than typically used long hydrocarbon derivatives. Although some improvements have been reported, it has remained challenging to achieve very high conductivities when using sintering temperatures of lower than about 150-180°C. The use of the alpha amino acid 1-alanine as a stabilizing agent in the aqueous synthesis of submicron-sized metallic copper particles under ambient atmospheric conditions has been described. (H. Jessica Pereira et al, ACS Appl. Electron. Mater. 2022, 4, 5, 2502- 2515). Using this approach it was found possible to form conductive copper films at temperatures < 120°C; however the resistivities of these copper films were significantly higher than that of bulk metallic copper.

[0015] Thus, it remains the case that to be able to generate highly conductive films with reasonable stability, typical sintering technologies still require high temperatures, and inert or reducing atmospheres.

[0016] There is, therefore, an increased need for an alternative source of copper for the manufacture of conductive products, which is stable yet also does not require the application of high temperature / energy in order to obtain the desired electrical conductivity, comparable with bulk metallic copper.

[0017] Summary of the Invention

[0018] The invention provides, in a first aspect, stabilised copper sub-micron scale particles suitable for use in the manufacture of an electrically conductive product, wherein each particle comprises (a) a metallic copper inner core and (b) a sulfated polymer outer layer.

[0019] The particles are sub-micron scale and in this regard the particles have a mean particle diameter in the range of from 25nm to lOOOnm, such as from 25nm to 800nm, preferably from 25nm to 800nm and more preferably from 50nm to 500nm.

[0020] This size refers to the mean size of the stabilised copper sub-micron scale particles, which comprise the copper core and the polymer outer layer. Therefore the size reflects the dimensions of the particles as made up of the copper core and the sulfated polymer outer layer.

[0021] The diameter of the particles can be determined using a transmission electron microscope (TEM), such as a JEOL 21 OOF TEM operating at 200 kV equipped with a Gatan Orius CCD camera and JEOL scanning transmission electron microscopy (STEM) detectors. Software such as Image J software can be used to analyse the TEM images and compute particle size distributions. A sample of at least 100 particles may be taken and the average (mean) particle size determined.

[0022] Therefore it is preferred that the invention provides a product which comprises 100 or more particles as defined above.

[0023] In the particles of the first aspect, the sulfated polymer forms a stabilising layer at the surface of the particles. The particles are stable when exposed to water and oxygen, even at elevated temperatures. Of course, the skilled person will appreciate that the particles should not be heated to a temperature above the decomposition temperature of the sulfated polymer.

[0024] The use of a sulfated polymer has therefore been found to provide effective stabilisation for the metallic copper particles, i.e. Cu(0).

[0025] As discussed above, in the prior techniques which use a stabilising agent it is normally the case that sintering is necessary to remove the agent, at least in part, in order to convert the stabilised particles into an electrically conductive product. Surprisingly, however, for the present invention sintering is not necessary to transform the particles into an electrically conductive product. Without being bound by theory, it is believed that at least some pendant sulfate groups interact with the copper, and provide enough interaction for binding stabilization, but there is not a full and compact coating formed - instead the polymer layer is discontinuous. Thus there are uncoated areas present whereby the application of pressure at room temperature is sufficient to allow the copper in adjacent particles to get close enough to achieve good conductivity without needing to remove the stabilising agent. Therefore although sintering can be carried out if desired, it is not necessary.

[0026] The flexibility and freedom provided by not needing to apply high temperatures or light energies in a sintering process is clearly advantageous and opens up the ability to manufacture a large range of conductive products from copper. For example, wearable and flexible conductive products can be produced using the particles of the invention.

[0027] It is also beneficial to be able to use a wider range of conditions to form the desired conductive end products whilst still achieving the desired level of conductivity. In particular, the present invention facilitates the use of more environmentally friendly and lower cost production techniques, due to not needing to use such high temperatures or controlled / inert atmospheres.

[0028] In the present invention it is important that the copper is metallic copper Cu(0). The use of copper in other forms, e.g. copper oxide, would not provide the desired properties in terms of being highly electrically conductive.

[0029] Seaweed derived materials have been used as capping agents for silver and gold in relation to medical and environmental applications of such metals (Gupta, P. et al, International Journal of Biological Macromolecules 2021 172, 30-40; Elsupikhe, R.F. et al. Nanoscale Research Letters 2015 10:302; Wan, H. et al Colloids and Surfaces A: Physicochemical and Engineering Aspects 2021, 616, 126325). There has also been use as a capping agent for copper oxide, again with the purpose of being used in the fields of pharmaceuticals and food (Aboeita, N.M. et al, Pharmaceutics 2022, 14, 418; Halder, U. et al Chemical Engineering Journal Advances 2022 11, 100294; Kouhkan, M. et al Current Nanoscience, 2020, 16, 101-111; Rajeshkumar, S. et al, Journal of Molecular Structure 2021, 1242, 130724.)

[0030] However, whilst some of these seaweed-derived capping agents are sulfated, such as carrageenan, there has not been any disclosure or teaching that they can be used to effectively stabilise metallic copper, whilst retaining the ability for the copper to have good electrical conductivity without needing sintering to remove the capping agent.

[0031] It is important to recognise that it is much more challenging to stabilise Cu as compared to silver, because of its inherent susceptibility to oxidation. In particular, it is challenging to balance good stability and high conductivity. In addition, gold, platinum and palladium are all also far more stable in atmospheric conditions than Cu, as well as being much less conductive compared to Cu.

[0032] It is surprising that it has been possible to effectively stabilise metallic copper, by use of the combination of stabilising agent and particle size as defined in the first aspect, and for these stabilised copper particles to be able to be readily and easily converted into electrically conductive products. The invention provides, in a second aspect, a method of manufacturing a free-standing electrically conductive product, the method comprising: providing stabilised copper sub-micron scale particles according to the first aspect; and applying energy to the particles, to form a free-standing electrically conductive product.

[0033] The skilled person will understand that a free-standing product has no supportive substrate. The free-standing electrically conductive product may, for example, be a film, disk, plate, sheet, or the like.

[0034] The energy that is applied to the particles may, in a preferred embodiment, be application of a load (pressure). The energy that is applied to the particles may, alternatively or additionally, be application of temperature. The energy that is applied to the particles may, alternatively or additionally, be application of light energy.

[0035] It is beneficial that the electrically conductive product can be obtained solely by the application of a load, but the skilled person will appreciate that this does not preclude the use of heat and / or light. Equally, in some embodiments only heat and / or only light might be used, i.e. the particles of the present invention can, if desired, be converted to an electrically conductive product using heat sintering and / or photonic sintering, using techniques known in the art.

[0036] A range of different size products can be made. The skilled person will appreciate that a range of different size die sets are available, and that pressure can be applied to particles in any such die-set.

[0037] In one embodiment, the free-standing electrically conductive product has a maximum thickness of 5 pm or more or 10 pm or more, such as 15 pm or more, or 20 pm or more. It may be that the maximum thickness is from 10pm to 250mm, such as from 15pm to 200mm, or from 20pm to 150mm. In one embodiment the product has a maximum thickness from 10pm to 25mm, such as from 15pm to 20mm, or from 20pm to 15mm. In one embodiment the product has a maximum thickness from 1 mm to 250 mm, such as from 2 mm to 200 mm, or from 3 mm to 150 mm. In one embodiment, the free-standing electrically conductive product has a width or a diameter of 3 mm or more, or 4 mm or more, such as 5 mm or more, or 7 mm or more. The maximum dimension of the product could be as small as 1 mm (e.g. in one embodiment the maximum dimension could be in the range of from 5 mm to 100 mm) or the maximum dimension of the product could be as large as 10 m or more (e.g. in one embodiment the maximum dimension could be in the range of from 1 m to 5 m); in general the skilled person will appreciate that the nature of the particles of the invention is such that they can be formed into a small product or a large product.

[0038] The method of the second aspect does not require the application of heat or light. It can be carried out at ambient temperatures. A free-standing electrically conductive product can be obtained solely from the application of pressure.

[0039] For example, the stabilised copper sub-micron scale particles according to the first aspect may be placed in a press, such as a die or bench press, and pressed to obtain a film or sheet.

[0040] The skilled person will appreciate that a pressure may be applied by applying a load to a surface area. In one embodiment, a load of 0.5 tons (500 kg) or more is applied, especially 1 ton or more, or 2 tons or more. In one embodiment, a load of from 1 to 10 tons (1000 to 10,000 kgs) is applied, such as from 4-5 tons or from 8-10 tons. The skilled person will appreciate that a suitable applied load can be selected that is sufficient to compact the sample to obtain a product having the desired thickness. In general, the thickness is controlled by the applied load as well as the amount of powder introduced to the die-set.

[0041] The conductivity and resistivity have been evaluated and, surprisingly, these values are comparable with standard values for bulk copper, even without the use of heat and / or light.

[0042] Nonetheless, it will be appreciated that the method of the second aspect can optionally be carried out with the application of heat (thermal sintering), as desired and / or the application of light (photonic sintering), as desired. In one embodiment, the method of the second aspect is carried out at a temperature of 120°C or less, such as 100°C or less, e.g. in the range of from 15°C to 100°C or from 15°C to 85°C.

[0043] In another embodiment, the method of the second aspect is carried out at a temperature of from 15°C to 300°C or from 15°C to 200°C; such as from 15°C to 180°C or from 15°C to 150°C.

[0044] The invention provides, in a third aspect, a method of manufacturing a supported electrically conductive product, the method comprising: providing stabilised copper sub-micron scale particles according to the first aspect; and applying the particles onto a substrate, to form an electrically conductive layer on the substrate.

[0045] The substrate may be flexible, e.g. paper or a flexible polymer, or may be rigid, e.g. ceramic or a rigid polymer. In one embodiment the substrate may be a circuit board and thus a printed circuit board can be obtained.

[0046] In one embodiment, the particles are provided in the form of a suspension, which may be an aqueous or non-aqueous suspension. This product may, therefore, be considered to be an ink. Thus, the particles may be printed or otherwise deposited onto the substrate, e.g. paper or cardboard or polymer. The particles may be applied to provide a pattern or design on the substrate.

[0047] It has been shown that the ink can be applied onto filter paper and printed circuit board substrates using an instrument such as a fountain pen or dip pen, or via screen printing, to provide highly conductive films patterned onto these substrates. A stencil or mesh screen may, for example, be used to apply the ink to the substrate in a desired pattern or design.

[0048] The method of the third aspect does not require the application of heat. It can be carried out at ambient temperatures. A supported electrically conductive product can be obtained solely from the application of pressure when the particles are deposited onto the substrate. This is significant because it allows the copper to be provided on a substrate that is temperature sensitive, e.g. because the material of the substrate itself is temperature sensitive, or because other temperature sensitive materials have been deposited on the substrate.

[0049] In one embodiment, the method of the third aspect is carried out at a temperature of 120°C or less, such as 100°C or less, e.g. in the range of from 15°C to 100°C or from 15°C to 85°C.

[0050] Nonetheless, it will be appreciated that the method of the third aspect can optionally be carried out with the application of heat, as desired. It may be that the substrate is not temperature sensitive, or that the substrate can be heated to a certain temperature without adverse effects. It can therefore be understood that in some embodiments the method of the third aspect can optionally be carried out with the application of heat (thermal sintering), as desired and / or the application of light (photonic sintering).

[0051] Therefore it may, for example, be that the method of the third aspect is carried out at a temperature of from 15°C to 300°C or from 15°C to 200°C; such as from 15°C to 180°C or from 15°C to 150°C.

[0052] The invention provides, in a fourth aspect, a method of manufacturing a 3D printed electrically conductive product, the method comprising: a) providing a particulate material comprising stabilised copper sub-micron scale particles according to the first aspect; b) depositing the particulate material; and c) selectively fusing the deposited particulate material; so as to obtain a three-dimensional object.

[0053] The depositing step may, for example, comprise depositing the material on a surface, such as a build platform.

[0054] Steps b) and c) may be repeated as required. Therefore, it may be that successive layers of particulate material are deposited and fused, with each subsequent layer being placed on the previously fused layer(s). This is of course standard and well known in 3D printing techniques such as Multi Jet Fusion (MJF) and Selective Laser Sintering (SLS). For example, when a build platform is used, after step c) the build platform lowers and then another layer is deposited (step b)) and fused (step c)) on top.

[0055] Thus the steps b) and c) can be repeated as many times as necessary to obtain the three- dimensional object in the desired shape and size.

[0056] The invention also provides, in a fifth aspect, a process for producing the particles according to the first aspect, the process comprising: providing a solution of the sulfated polymer; combining the polymer solution with a Cu(II) salt to form a mixture; adjusting the pH of the mixture to 9 or more; and adding a reducing agent, to reduce the Cu(II) to Cu(0), thereby obtaining particles according to the first aspect in the form of a suspension.

[0057] The particle synthesis can be carried out as a batch process (e.g. on a mg to g scale). The particle synthesis can also be carried out in continuous flow (e.g. on a g scale and higher).

[0058] The particles can be separated from the suspension, as desired, e.g. by centrifugation. The particles can suitably then be washed. The particles can then be dried to powder form, as desired, e.g. by drying under vacuum.

[0059] The process of the fifth aspect is advantageous because it can, if desired, be carried out under ambient atmospheric conditions. Controlled / inert conditions are not required for any of the essential steps. It should be noted that the drying step is not essential.

[0060] In the process of the fifth aspect, only the optional drying step needs to be performed under atmospheric conditions that are controlled. The drying step may, for example, be carried out under vacuum conditions (pressure below normal atmospheric pressure).

[0061] The remaining essential steps (and non-essential steps) can all be conducted on the benchtop. This is clearly advantageous in terms of being a “greener” process, as well as more cost-effective, flexible and straightforward to implement and scale up. In one embodiment, all steps of the process, apart from the optional drying step, are carried out at a temperature of 120°C or less, such as 100°C or less, e.g. in the range of from 15°C to 100°C or from 15°C to 85°C.

[0062] The invention further provides, in a sixth aspect, an electrically conductive product formed from particles according to the first aspect. The product may, for example, have been made by the method of the second or third or fourth aspect. The product will comprise metallic copper and the sulfated polymer.

[0063] In one embodiment, the product is a free-standing product, such as a film, disk, plate or sheet, or a 3D printed article, the free-standing product comprising metallic copper and the sulfated polymer.

[0064] In another embodiment, the product is a supported product, such as a printed circuit board or a printed paper item, e.g. a paper-based electronic device, wherein the supported product comprises metallic copper and the sulfated polymer on a flexible or rigid substrate. The copper may be provided as a layer or a pattern on a surface of the substrate.

[0065] The substrate may, for example, be paper or ceramic or polymer or fabric (woven or nonwoven), or combinations thereof. It may be that the substrate is a composite material, such as a composite comprising a matrix (such as resin, e.g. epoxy resin), and a reinforcement material (such as fabric or glass fibres and / or paper), and optionally a filler.

[0066] The product may be a supported product that comprises metallic copper and the sulfated polymer on a flexible or rigid substrate and that further comprises additional layers or components, e.g. dielectric layers and / or additional electrode layers and / or outer coating layers.

[0067] The product may be an electronic device. The product may comprise an electrode that is formed from the metallic copper and the sulfated polymer on a flexible or rigid substrate.

[0068] In one embodiment, the electrically conductive product has a maximum thickness of 5 pm or more or 10 pm or more, such as 15 pm or more, or 20 pm or more. It may be that the maximum thickness is from 10pm to 250mm, such as from 15pm to 200mm, or from 20pm to 150mm. In one embodiment the product has a maximum thickness from 10pm to 25mm, such as from 15 m to 20mm, or from 20pm to 15mm. In one embodiment the product has a maximum thickness from 1 mm to 250 mm, such as from 2 mm to 200 mm, or from 3 mm to 150 mm.

[0069] In one embodiment, the electrically conductive product has a width or a diameter of 3 mm or more, or 4 mm or more, such as 5 mm or more, or 7 mm or more. The maximum dimension of the product could be as small as 1 mm (e.g. in one embodiment the maximum dimension could be in the range of from 5 mm to 100 mm) or the maximum dimension of the product could be as large as 10 m or more (e.g. in one embodiment the maximum dimension could be in the range of from 1 m to 5 m); in general the skilled person will appreciate that the nature of the particles of the invention is such that they can be formed into a small product or a large product.

[0070] The invention provides, in a seventh aspect, the use of a sulfated polymer to provide stabilised copper sub-micron scale particles that are suitable for use in the manufacture of an electrically conductive product without requiring sintering.

[0071] Detailed description of the Invention

[0072] Sulfated polymer:

[0073] The present invention uses a sulfated polymer as a stabilising agent for the copper particles. The sulfated polymer may, for example, be a sulfated biopolymer. In a preferred embodiment it is a sulfated polysaccharide. The sulfated polymer provides a plurality of pendent sulfate groups -OSO3’.

[0074] The polymer may, for example, have a degree of polymerization (DP) over 40, preferably from about 50 to about 100,000, more preferably from about 500 to about 50,000.

[0075] It is desirable, but not essential, that the sulfated polymer has a weight average molecular weight in the range of from about 10 kDa to about 10,000 kDa, preferably from about 50 kDa to about 5,000 kDa, more preferably from about 100 kDa to about 1,000 kDa, such as from about 250 kDa to about 750 kDa. The sulfated polymer may, for example, have an average of 0.25 or more sulfate groups per monomer (on average, at least one sulfate for every four monomers), or 0.33 or more sulfate groups per monomer (on average, at least one sulfate for every three monomers), preferably an average of 0.5 or more sulfate groups per monomer (on average, at least one sulfate for every two monomers). It may be that there is an average of 1 or more sulfate groups per monomer, or 1.5 or more sulfate groups per monomer.

[0076] In one preferred embodiment, the sulfated polymer has an average of 0.25 to 2 sulfate groups per monomer, preferably from 0.5 to 2 sulfate groups per monomer, such as from 0.5 to 1.5 sulfate groups per monomer. It is believed that the presence of this quantity of sulfate groups will provide sufficient interaction with the copper so as to form a stabilising layer of polymer on the copper. However, there is not a full and complete coating formed; instead the polymer layer is discontinuous. Thus there are uncoated (exposed) areas present whereby the application of pressure is sufficient to allow the copper in adjacent particles to get close enough to achieve good conductivity without needing to remove the stabilising agent. Therefore although sintering can be carried out if desired, it is not necessary.

[0077] In one embodiment the sulfated polymer is a sulfated polysaccharide comprising one or more repeat units (e.g. two or more repeat units) selected from the group consisting of saccharides having 5 carbon atoms and saccharides having 6 carbon atoms. Each saccharide may be a pentose, a hexose, a furanose or a pyranose. Examples of saccharides include, but are not limited to, mono- and di-saccharides such as isomaltose, isomaltotriose, isomaltotetraose, glucose, fructose, galactose, xylose, mannose, arabinose, rhamnose, maltose, sucrose, lactose, maltulose, ribose, lyxose, allose, altrose, gulose, idose, talose, trehalose, nigerose, kojibiose, lactulose.

[0078] The polysaccharides may also include sugar alcohols as the repeat units. Non-limiting examples of sugar alcohols include sorbitol, erythritol, arabitol, xylitol, threitol, pentaerythritol, mannitol and galactol and the like.

[0079] The polysaccharides can be linear, or branched in a variety of ways, such as 1-2, 1-3, 1- 4, 1-6, 2-3 and mixtures thereof. The sulfated polymer may optionally include further anionic groups in addition to the sulfate groups, for example there may also be hydroxyl groups present and / or there may be carboxyl groups present.

[0080] Carrageenan is known to be a thermo-reversible polymer and to have a structural change with respect to temperature. Without being bound by theory, it is believed that the present invention may utilise these characteristics. The use of sulfated polysaccharides that undergo temperature-based conformational change may support the ability of these materials to provide a protective coating, giving stability, at elevated temperatures, yet provide gaps / spacing at ambient temperature, which then presents sufficient coppercopper close contact in order to facilitate good electrical conductivity upon application of pressure.

[0081] The sulfated polymer may be naturally occurring or may be synthetic.

[0082] The sulfated polymer is preferably water soluble. In this regard, it is desired that the polymer fully dissolves in water at room temperature and pressure with stirring within 5 minutes. However, it may also be that the sulfated polymer requires some heating in order to be water soluble, e.g. a polymer may be used that fully dissolves in water at a temperature of 30 to 80°C, at room pressure, with stirring within 5 minutes.

[0083] A synthetic polymer may be a polymer that has sulfate groups on some or all of the monomeric material when the monomer is polymerised, or it may be that sulfate groups are added following the polymer being formed. It may, for example, be that the sulfated polymer is a sulfated biopolymer obtained by reacting a sulphation agent with a saccharide-based biopolymer in a reaction medium comprising an ionic liquid, e.g. as taught in W02007 / 112304. Non-limiting examples of saccharides that could be modified so as to be sulfated include starch, chitin, chitosan, dextran, maltodextran, dextrin, maltodextrin, gums, agar, alginates, and the like. Suitable agents for sulphation include, but are not limited to, chlorosulfonic acid, [SO3. pyridine] complex, sulfuric acid, sulfamic acid, and SO3.

[0084] In one preferred embodiment, however, the sulfated polymer is a naturally occurring polymer, such as a sulfated polysaccharide derived from marine organisms, e.g. seaweeds, marine invertebrates and sea grasses. Carrageenans are natural linear polysaccharides extracted from edible red seaweeds (Rhodophyta) and these are water soluble. Carrageenan comprises a repeating disaccharide sequence, with alternating A and B monomers, each based on galactopyranose residues, at least some of which are sulfated. In general, there is a repeating structure of alternating p-d-galactose and 3,6-anhydro-a-d-galactose units connected via a-(l,3) and P-(l,4) glycosidic linkages. The three main types of carrageenan that can be easily obtained are kappa (K), iota (r), and lambda (X) that correspond to one, two, and three sulfate groups per disaccharide, respectively.

[0085] The skilled person will be aware that other forms of carrageenan are also available, and these could be used. For example, mu- and nu-carrageenans are available and these are precursors of K- and r-carrageenans. X-carrageenan can be alkali treated to obtain 0- carrageenan. Furcellaran is also considered to be a carrageenan.

[0086] Agarans are also natural polysaccharides which are sulfated galactans that are water soluble and can be extracted from the red algae. Agarans are in the L-configuration whereas carrageenans are in the D-configuration. Porphyran is an example of an agaran, and this exhibits a linear backbone of alternating 3-linked P-D-galactose and 4-linked a- L-galactose-6-sulfate or 3,6-anhydro-a-L-galactose units. The regular agaran backbone may be interrupted by different O-linked substitutions, including methyl and xylosyl groups, in addition to the sulfate groups. Agarans that have P-D-xylosyl groups attached at the 0-6 position of D-galactose units are known, and may be referred to as xylogalactans.

[0087] Further red seaweed-derived sulfated polysaccharides are DL-hybrids, which have 4- linked D- and L-galactose sugars distributed within the same polysaccharide molecules.

[0088] Sulfated polysaccharides with mannose in their backbones are also known, such as xylomannnan from Scinaia hatei consisting primarily of a backbone of a-( 1 -^3)-linkcd D-mannose residues substituted at C-6, C-4, and C-2 with P-D-xylosyl residues.

[0089] Further natural polysaccharides which are sulfated galactans that are water soluble can be extracted from the red algae B. occidentalis . These sulfated galactans have a linear backbone made of alternating 3-linked P-d-Galp and 4-linked a-d-Galp residues. Two- thirds of the total a-units are 2,3-di-sulfated and one-third are 2-mono-sulfated.

[0090] Ulvans are natural sulfated polysaccharides that are mainly obtained from green algae and these are also water soluble. These comprise a repeating disaccharide sequence normally based on monomers selected from rhamnose, uronic acid, glucoronic acid, and xylose, at least some of which are sulfated. They include ulvanobiuronic acids and ulvanbioses.

[0091] Uv nobiu nic Adds

[0092] A water-soluble heteroglycuronan can be obtained from the green algae Enteromorpha compressa; this comprises ( 1 -^2.4)-linkcd rhamnose, ( l ^4)-linkcd xylose, and (1— >4)- linked glucuronic acid units and can be sulfated at the C-3 of rhamnose and the C-2 of xylose.

[0093] Sulfated galactans can also be obtained from green algae, such as Codium species. For example, C. fragile and C. cylindricum are known as sources of sulfated arabinogalactan and sulfated glucogalactan,

[0094] Fucans are natural sulfated polysaccharides based on fucose and these are water soluble. These are obtained from brown algae. Fucoidans are well-known fucans. Fucans are classified into two groups: (1) fucans with central chains composed of 1,3-linked a-1- fucopyranose residues and (2) fucans with central chains composed of 1,3- and 1,4-linked a-l-fucopyranose residues. In one embodiment, the sulfated polymer is a sulfated polysaccharide that is derived from algae e.g. red algae, green algae or brown algae, and especially from red, green or brown seaweed.

[0095] In one embodiment, the sulfated polymer is a sulfated polysaccharide that is selected from: carrageenans, agarans, ulvans and fucans.

[0096] In one embodiment, the sulfated polymer is a sulfated polysaccharide that is selected from: carrageenans, ulvans and fucans (e.g. fucoidans).

[0097] In one embodiment, the sulfated polymer is a carrageenan. In one preferred embodiment, the sulfated polymer is kappa carrageenan.

[0098] Sub-micron scale particles

[0099] The particles are sub-micron scale and in this regard have an average (mean) diameter in the range of from 25nm to lOOOnm, such as from 25nm to 800nm, preferably from 25nm to 800nm and more preferably from 50nm to 500nm. This size reflects the overall diameter of the particles, which comprise a copper core and a sulfated polymer layer (which partially coats the core).

[0100] In one preferred embodiment, the particles have an average diameter in the range of from 50nm to 400nm, more preferably from 50nm to 300nm, such as from 75nm to 200nm.

[0101] The diameter of the particles can be determined using a transmission electron microscope (TEM), such as a JEOL 21 OOF TEM operating at 200 kV equipped with a Gatan Orius CCD camera and JEOL scanning transmission electron microscopy (STEM) detectors. Software such as Image J software can be used to analyse the TEM images and compute particle size distributions. A sample of at least 100 particles may be taken and the average (mean) particle size determined.

[0102] The particles may, in one embodiment, be spherical, but other shapes are not precluded.

[0103] The particles may, in one embodiment, comprise from 95 to 99.9wt% metallic copper, such as from 96 to 99.8wt%, and preferably from 97 to 99.7wt%. It may be that the particles comprise from 98 to 99.6wt% metallic copper, preferably from 98.5 to 99.5wt% metallic copper.

[0104] The particles may, in one embodiment, comprise from 0.1 to 5wt% sulfated polymer, such as from 0.2 to 4wt%, and preferably from 0.3 to 3wt%. It may be that the particles comprise from 0.4 to 2wt% sulfated polymer, preferably from 0.5 to 1.5wt%.

[0105] The skilled person will appreciate that the proportion of copper and polymer can be determined by elemental analysis.

[0106] Another technique is that a sample of the particles can be weighed, then high temperature sintering can be carried out to remove all of the polymer, and then the sample weighed again, to determine the weight of (burnt off) polymer and the (remaining) weight of copper.

[0107] Form of the particles:

[0108] The particles may usefully be provided in the form of a powder. However, it is also contemplated that they could be provided in combination with a liquid carrier, which in one embodiment is an aqueous carrier but could alternatively be a non-aqueous carrier.

[0109] In one embodiment, the particles may be provided as a suspension. In one embodiment, the particles are provided in aqueous suspension. In one embodiment, the particles are provided in a non-aqueous suspension.

[0110] In particular, if the particles are to be printed onto a substrate, such as paper or cardboard or a circuit board, the provision of the particles in combination with a liquid carrier may be advantageous.

[0111] It may be that the liquid carrier is water. However, the invention is not limited by the nature of the liquid carrier and both aqueous and non-aqueous carriers are foreseen for use.

[0112] It may be that the liquid carrier is provided together with an additive, such as an antioxidant (e.g. ascorbic acid); this may improve the stability of the suspension. Other additives, such as surfactants, dispersants and / or thickening agents, can also be contemplated.

[0113] The suspension of the particles may usefully have a relatively high solid content, such as 30 mg / ml or more, or 40 mg / ml or more, or 50 mg / ml or more.

[0114] Process for producing the particles according to the invention:

[0115] The solvent used in the process of the fifth aspect may be an aqueous solvent, although any solvent that can dissolve the polymer can be contemplated. In one preferred embodiment, water is used. In another embodiment, a non-aqueous solvent is foreseen.

[0116] The polymer and solvent may be heated to assist with dissolving the polymer in the solvent, if required. In one embodiment heating to a temperature in the range of 30 to 90°C is used.

[0117] The polymer and solvent may be stirred to assist with dissolving the polymer in the solvent, if required.

[0118] The copper salt used in the process of the fifth aspect is not particularly limited. Examples of copper salts that may be used include: copper acetate, copper chloride, copper carbonate, copper nitrate, copper sulfate, or combinations thereof.

[0119] The pH of the mixture is adjusted to 9 or more using a base, such as NaOH or KOH. In one embodiment, the pH is adjusted to 10 or more, such as 11 or more, e.g. about 12. The skilled person will appreciate that pH can be measured using a pH meter.

[0120] The reducing agent used in the process of the fifth aspect is not particularly limited. Reducing agents that are capable of reducing Cu(II) to Cu(0) are well known in the art. Examples of reducing agents include, but are not limited to: ascorbic acid, formic acid, glucose, hydrazine hydrate, oleyl amine, sodium borohydride, lithium aluminium hydride, vanadium sulfate, 1,2-hexadecanediol, sodium hypophosphite, H2, CO, and forming gas (H2 and N2). In one embodiment, the reducing agent is ascorbic acid or formic acid. The optional separation of the particles can be carried out by centrifugation, e.g. at 5,000- 10,000 rpm for from 5-30 minutes.

[0121] The particles can then be washed, e.g. with water, and can be dried to powder form, e.g. under vacuum. The resulting powder can be stored in sealed vessels.

[0122] Alternative process for producing the particles:

[0123] The process of the fifth aspect is beneficial in that it is straightforward, flexible and can be easily carried out on the lab bench and can be scaled readily.

[0124] However, the particles of the first aspect can alternatively be manufactured using other processes, such as those known in the art for manufacturing carrageenan capped gold and silver particles, by adjusting the starting materials to instead be copper-based starting materials, e.g. copper nitrate.

[0125] Methods using the particles:

[0126] A key benefit of the particles of the present invention is that the application of pressure is sufficient to allow the copper in adjacent particles to get close enough to achieve good conductivity without needing to remove the stabilising agent. Therefore although sintering (heat or light based) can be carried out if desired, it is not necessary.

[0127] The particles may be used to form a free-standing electrically conductive product, such as a film, disk, plate, sheet, or the like. For example, pressure may be applied to the particles using a press, such as a die press or bench press, to form a free-standing electrically conductive product from the particles of the invention.

[0128] Sintering may optionally be carried out, e.g. through the application of heat, but this is not essential.

[0129] The thickness of the product can be controlled by adjusting the quantity of particles (powder) used.

[0130] For example, when a 30 mg sample of the Cu particles has a load applied in the range of 8000-10000 kg, with the particles being provided in a die-set with a 7 mm circular crosssection, a circular shaped Cu disk with a 25-30 micrometre thickness and with a diameter of 7 mm was obtained. The load applied may also be expressed as applied pressure, where 8000 - 10000 kg is equivalent to 9.75E+02 to 1.22E+03 bar.

[0131] The size and shape of the product can be varied by changing the size and shape of dieset used. For example, a product that is a sheet with rectangular or square or circular cross section can be obtained by use of a correspondingly shaped die-set.

[0132] In one embodiment, the product is a rectangular or square shaped sheet or plate.

[0133] In one embodiment, the product is a Cu foil.

[0134] The particles may also be used to form a free-standing electrically conductive product by the use of 3D printing (additive manufacturing). Additive manufacturing techniques include, but are not limited to, selective laser sintering (SLS), powder / binder jetting techniques such as multi jet fusion (MJF), extrusion-based techniques such as paste extrusion, fused deposition modelling, inkjet-based 3D printing (IJ3DP), stereolithographic techniques, and electrophotographic techniques. Additive manufacturing can be used to produce a 3D shaped article from the particles of the invention. Photonic sintering or heat sintering may be used if needed; this may be appropriate if the shape of the article is such that it is not straightforward to apply pressure to bring the particles together and achieve sufficient conductivity.

[0135] The particles may be used to form a supported electrically conductive product, such as by applying the particles to a flexible or rigid substrate.

[0136] The substrate may, for example, be paper or ceramic or polymer or fabric (woven or nonwoven), or combinations thereof. It may be that the substrate is a composite material, such as a composite comprising a matrix (such as resin, e.g. epoxy resin), and a reinforcement material (such as fabric or glass fibres and / or paper), and optionally a filler.

[0137] The product may be a supported product that comprises metallic copper and the sulfated polymer on a flexible or rigid substrate and that further comprises additional layers or components. The product may be an electronic device. The product may comprise an electrode that is formed from the metallic copper and the sulfated polymer on a flexible or rigid substrate.

[0138] The particles may be provided in the form of a suspension or “ink” that is applied to the substrate, e.g. to form a pattern or layer on the substrate. The ink may be deposited onto the substrate in a pattern, e.g. using screen printing. Examples of instruments that may be used to form a pattern include a stencil screen or a mesh screen.

[0139] Pressure may be applied directly to the ink, e.g. using a pen or other instrument, as it is applied to the substrate and / or pressure may be applied to the entire substrate or portions thereof once the ink has been deposited on the substrate, e.g. by using a press.

[0140] In one embodiment the applied load may be in the range of from 2000 - 10000 kg, which is equivalent to 2.44E+02 to 1.22E+03 bar as an applied pressure.

[0141] The skilled person will appreciate that a suitable load or pressure to be applied can be selected based on the substrate, for example a paper substrate may have an applied load in the range of from 2000-5000 kg (equivalent, as applied pressure, to 2.44E+02 to 6.10E+02 bar), whilst a rigid polymer substrate may have an applied load in the range of from 4000 - 5000 kg (equivalent, as applied pressure, to 4.88E+02 to 6.10E+02 bar).

[0142] Sintering may optionally be carried out, e.g. through the application of heat, but this is not essential.

[0143] Examples of products that could be produced are printed circuit boards and wearable electronics products and flexible electronics products.

[0144] The invention will now be further described, in a non-limiting manner, by reference to the following examples and figures.

[0145] The Cu particles have been synthesised in a batch process at several different scales (mg- scale to g-scale), hence the following example provides reagent ratios instead of absolute values. Furthermore, the synthesis is also effective in a continuous flow reactor.

[0146] Example 1: synthesis of particles The polymer K-carrageenan was dissolved in water at 85°C and with stirring at 800rpm followed by the addition of Cu(II) chloride dihydrate (provided as a 0.02M solution).

[0147] Three different addition amounts of carrageenan were used. These are identified as Cu- KCGN-0.5, CU-KCGN- 1.0 and CU-KCGN- 1.5, where the number signifies the amount of K-carrageenan introduced to the reaction mixture as a percentage of weight by volume.

[0148] As the skilled person will appreciate, therefore, as the amount given in the form of 0.5, 1.0 and 1.5 % (w / v) refers to weight per volume percentage, this means that the 0.5 %

[0149] (w / v) product (CU-KCGN-0.5) had 0.5 g of carrageenan in 100 ml of water and the 1.0 %

[0150] (w / v) product (CU-KCGN- 1.0) had 1.0 g of carrageenan in 100 ml of water and the 1.5 %

[0151] (w / v) product (CU-KCGN- 1.5) had 1.5 g of carrageenan in 100 ml of water.

[0152] This light blue reaction mix was then basified to pH 12 using NaOH, upon which the solution turned milky blue.

[0153] L-ascorbic acid was then added until a pH of 4 was reached, and this reduced the Cu(II) precursor, first to Cu(I) and then to Cu(0), which was manifested by the formation of a red colour solution. The Cu particles appeared as a viscous reaction mixture, with particles well-isolated from one another.

[0154] The mixture was transferred to centrifuge tubes and the particles were separated using centrifugation at 8000rpm for 15 minutes. The particles were then washed and dried under vacuum.

[0155] The particles were then analysed.

[0156] Figure 1 shows representative transmission electron microscopy (TEM) images, with the corresponding particle size distributions. The particles were spherical in shape for all proportions of K-carrageenan, with the particle size decreasing as the amount of K- carrageenan increased.

[0157] This was also observed in UV-Vis absorption spectra, which showed a blue shift in the plasmonic peak as the amount of K-carrageenan increased. Figure 2 shows: (a) Powder XRD patterns; (b) TGA analysis (in air); and XPS analysis and corresponding peak assignments for Cu 2p and O ls core levels for the synthesised (c) CU-KCGN-0.5, (d) CU-KCGN- 1.0 and (e) CU-KCGN-01.5 powders. Inset of (b) is a photograph of the CU-KCGN- 1.0 powder. The powder XRD patterns in (a) have been offset along the y axis for clarity.

[0158] No evidence of surface oxides was observed in the powder XRD pattern. All peaks were of very good intensity and align perfectly with the standard, suggesting good stability in an ambient environment.

[0159] Thermogravimetric analysis (TGA) was used to study the oxidative stability of the synthesised particles. The temperature was increased under an oxygenated environment which resulted in an increase in weight over time, caused by surface oxidation resulting by the formation of CU2O and finally CuO. The temperature at which oxidation commences (as indicated by the point where the weight begins to rise) increased as the amount of K-carrageenan is increased, and was highest for CU-KCGN- 1.5 where no increase in weight is observed until about 260°C. It was also evident that although temperature induced oxidation is delayed for CU-KCGN- 1.5, these particles undergo subsequent oxidation at a higher rate compared to CU-KCGN- 1.0. The latter commences oxidation at about 183°C but oxidises at a lower rate, as apparent from the lower gradient. This could be attributed to the smaller size of the particles in CU-KCGN- 1.5 which undergo relatively faster oxidation compared to CU-KCGN- 1.0, because of the higher surface to volume ratio. Therefore a balance between particle size and amount of stabilising agent can be sought for optimised stability.

[0160] To evaluate how much of K-carrageenan was actually present in the Cu particles, elemental analysis was conducted for the three powders. The amount of C as a percentage was found to be 0.71, 1.14 and 1.48% and H was 0.19, 0.21 and 0.22% for CU-KCGN- 0.5, CU-KCGN- 1.0 and CU-KCGN- 1.5 respectively.

[0161] The increase in C content between CU-KCGN-0.5 and CU-KCGN- 1.0 is 0.43% and that between CU-KCGN- 1.0 and CU-KCGN- 1.5 is 0.34%. Therefore although the amount of C increases proportionately depending on the amount of K-carrageenan introduced to the reaction mixture, the successive increment in C content is gradually lowering, suggesting that there is a limited amount of K-carrageenan that can effectively coordinate with the surface of the Cu atoms and the excess is washed away during the purification process.

[0162] The TGA data and elemental analysis both show that the amount of K-CGN in each sample is very low (C < 1.5 %) and therefore the observed thermal and ambient oxidative stability is impressive.

[0163] X-ray Photoelectron Spectroscopy (XPS) analysis was performed to further assess the surface stability of the CU-KCGN powders. The presence of characteristic Cu 2p3 / 2 (932.56 ± 0.05 eV) and Cu 2pi / 2 (952.52 ± 0.05 eV) peaks confirmed the presence of metallic Cu, which was also corroborated by Cu LMM Auger electron spectroscopy. Notably, apart from a small amount of Cu(OH)2 which is likely to be a result of strong base reacting with minute amounts of the metal precursor, all powders showed very good stability under ambient conditions.

[0164] A synthesis in the absence of K-CGN was also carried out as a control experiment. However, as evidenced by SEM, the particles showed very poor uniformity in both size and shape.

[0165] Example 2: Formation of free-standing electrically conductive products

[0166] The particles from Example 1 were gently ground using a mortar and pestle. The colour of the powder gradually changed from a dull red to a lustrous red colour and formed a coarse coating on the mortar. A multimeter reading showed that this reflective layer of Cu was very conductive - less than 10 ohms.

[0167] Films were made using each of the three powders from Example 1. The ground powder was transferred into a stainless-steel quick press die-set and pressed using a bench press. A load of 8-10 tons (8000 - 10000 kg) was applied for a minute. The load may also be expressed as applied pressure, where 8000 - 10000 kg is equivalent to 9.75E+02 to 1.22E+03 bar. The film thickness was between 25-30 pm.

[0168] In each case, the result was a highly reflective Cu film, which was very conductive as evident by the extremely low resistance when measured with a multimeter. The four-terminal resistance of the films as fabricated were measured using the van der Pauw method. The foils were benchmarked against commercially available Cu foil (99.8% metal content) of similar thickness (25 gm). Foil was chosen as a suitable candidate for comparison because it is widely used in many diverse applications, particularly electrical applications, because it exhibits very good properties similar to that of bulk Cu.

[0169] The corresponding resistivities and conductivities obtained at ambient temperature conditions (24-28 °C) are given in Table 1 below.

[0170] Table 1

[0171] It can be seen that the films derived from all three types of Cu particles show very low resistivities (and correspondingly high conductivities), which are very close to that of Cu foil and bulk Cu (resistivity for bulk Cu: 1.689 x 10’8Q m; conductivity for bulk Cu: 5.96 x 107S / m).

[0172] Therefore in this example, the synthesised Cu particles were rendered conductive using only the application of pressure (no heat or photonic sintering). This was unexpected.

[0173] Two important factors could influence the measured resistance: the organic content and the particle size. The amount of organic matter can effectively increase the resistance of the film because K-carrageenan itself is an insulator and therefore addition of more of this material will contribute towards lowering the conductivity. The particle size is an important factor as it controls and strongly influences the packing density of the film. Well-packed particles will have excellent contact with adjacent particles and negligible porosity, contributing to improved conductivity.

[0174] Figure 3 shows representative Scanning Electron Microscope (SEM) images showing the surface morphology of the films prepared using (a) CU-KCGN-0.5 ; (b) CU-KCGN- 1.0 and (c) CU-KCGN- 1.5 powders. The inset of (a) shows the photograph of a typical Cu film prepared in this work.

[0175] These SEM images show that the pressed samples of all three types of Cu have a flat surface and are in good contact with neighbouring particles, which renders very high conductivity (same order of magnitude as bulk Cu for both CU-KCGN-0.5 and CU-KCGN- 1.0).

[0176] Considering the three samples, the film made using CU-KCGN- 1.0 particles showed the highest conductivity, and, as shown by the SEM images, this sample is very compact with negligible / no porosity and excellent packing density. The porosity for this product was computed using ImageJ software from SEM images as being 1.76 %.

[0177] The film composed of CU-KCGN-0.5 also has similar properties in terms of packing but the electrical conductivity is slightly lower compared to CU-KCGN- 1.0, even though it has the lowest organic content. This observation could therefore be caused by the higher particle size and broader distribution which affects packing as this is the only other change between these two types. The CU-KCGN- 1.5 which has the lowest particle size has relatively higher resistance which is an order of magnitude greater than that of bulk Cu. This could be a synergistic effect of higher organic content and lower particle size. In the context of the latter, as the grain size becomes smaller the measured conductivity deviates progressively from that of the bulk, mainly because of the increased number of grain boundaries. Additionally, as is evident from the SEM images corresponding to this film, there are more interparticle spaces in this sample which increases the barrier for electron transport.

[0178] Remarkably, the Cu films prepared according to the invention show good agreement with the resistivity of Cu. For example, films derived from CU-KCGN- 1.0 and CU-KCGN-0.5 are only 1.21 and 1.38 times the resistivity of bulk Cu, which is extraordinary and unexpected. It is surprising that the resistivity of these films (p20) prepared via a sintering-free room-temperature compression approach closely matches that of the bulk metal. The high conductivities obtained using compressed particles according to the invention is enhanced by the oxidative stability of the particles, which retards formation of an outer oxide layer that could otherwise deplete film conductivity, and the formation of a smooth planar surface which minimises unfavourable electron scattering caused by surface roughness.

[0179] When considering resistivity values at 20°C (computed from the linear plots in Figure 5), the commercial Cu foil yielded an identical resistivity value to that of bulk Cu (1.689 x 10"8Q m) and the films according to the invention provide 2.050 x 10"8Q m, 2.336 x 10"8Q m and 1.949 x 10"7Q m for CU-KCGN-0.5, CU-KCGN- 1.0 and CU-KCGN- 1.5 respectively.

[0180] The pressure applied to the films was varied in order to determine the effect of the applied load. The loads applied ranged from 2,000 to 10,000 kg. The film was circular in shape, with a 7 mm diameter.

[0181] Figure 4 shows the electrical properties of the CU-KCGN- 1.0 film as the applied load is varied. Figure 4 shows graphs for (a) applied load vs resistivity; and corresponding (b) applied load vs conductivity. Note: At all loads the resistivities (and conductivities) are the same order of magnitude as bulk Cu (1.689 x 10’8Q m).

[0182] The applied load was shown to have a significant effect on the conductivity of the film. An increased load therefore improved how well the particles were connected. Although the conductivity increased with increase in the applied load, even the lowest load provided a very low resistivity and therefore provided a useful conductive film.

[0183] Metals are known for their positive temperature coefficient, which means that the measured resistance increases with increasing temperature. For a pure metal this is a highly linear relationship from which the temperature coefficient of resistance (a) can be derived. The a value is a measure of how much the resistance will change for a unit rise in temperature. This measured a value can vary depending on particle size, mode of sintering, degree of oxidation; however for pure Cu, values in the range of 0.00369 - 0.00409 °C-1 (at 20 °C) are given in the literature.

[0184] In order to investigate the metallic property of the manufactured films, the four-terminal resistance over a temperature range of -130 to 130°C (145 to 400 K) was measured. Plotting resistivity vs temperature enables the deduction of resistivity of Cu at 20 °C; (p20) for these samples. Figure 5 shows: Four-terminal sheet resistance vs temperature measurements for films prepared using (a) CU-KCGN-0.5 ; (b) CU-KCGN- 1.0; (C) CU-KCGN- 1.5 and (d) Com-Cu- Foil. Inset in (c) shows the four-terminal sheet resistance response for the entire temperature range of measurement and the graph shows the linear range. Resistivity vs temperature measurements for films prepared using (e) CU-KCGN-0.5 ; (f) CU-KCGN- 1.0; (g) CU-KCGN- 1.5 and (h) Com-Cu-Foil. Inset in (g) shows the resistivity response for the entire temperature range of measurement and graph shows the linear range.

[0185] The CU-KCGN-0.5 and CU-KCGN- 1.0 films show a remarkably linear relationship with temperature that is similar to that of commercial Cu foil. The CU-KCGN- 1.5 sample shows linearity in a slightly lower temperature range of -75 to 80 °C. This observation is not unexpected as the measured conductivity showed the highest deviation from bulk Cu for CU-KCGN- 1.5 based films. The corresponding a values were calculated using the linear plot and equation R =Rref[l +a(T-Tref)], where R is the resistance at a given temperature T, and Rref, the resistance at a given reference temperature 7' / (. / for which the a value is calculated. For the calculation, the reference temperature was selected at 20°C because a values are usually provided for this temperature.

[0186] The calculated a values (acai) are 0.0023, 0.0018 and 0.0012 °C1for CU-KCGN-0.5, CU- KCGN- 1.0 and CU-KCGN- 1.5 respectively. For commercial Cu foil a value of 0.0026 °C1was obtained.

[0187] All values deviate from the expected value for pure Cu; this is expected because these formulations are not purely Cu but instead contain variable amounts of organic material and also have been processed under different conditions. However, what is important is the trend that is observed, which shows that an increase in organic content / decrease in the particle size causes progressive reduction of the calculated a value. The observed differences between the three films according to the invention are too significant to be solely caused by the amount of K-carrageenan, particularly because the loadings are varied by only 0.05% w / v increments. Thus, a far more plausible explanation is the effect of grain boundaries; the higher the number of grain boundaries, the higher the deviation will be. Grain boundaries are defects in a polycrystalline material and are usually located at the interface between two crystallites. Although the crystal structure and the composition of the adjacent grains are the same, the orientation is different resulting in a planar defect which increases scattering of electrons. Therefore grain boundaries are effectively barriers, limiting conductivity and the degrading the resistance dependence on temperature. When the particle size is higher, the number of particles required to pack a given area is lower, generating a lower number of grain boundaries per unit area compared to smaller particles which will involve more particles to achieve a similar coverage. Hence the effect of grain boundaries become more pronounced for smaller particles.

[0188] The entire processing of the powder into films was performed under normal atmospheric conditions. Therefore the oxidative stability of the prepared Cu films was tested following their formation.

[0189] Figure 6 shows: XPS analysis and corresponding peak assignments for Cu 2p and O Is core levels for the films prepared using (a) CU-KCGN-0.5 ; (b) CU-KCGN- 1.0; and (c) Cu- KCGN- 1.5 powders.

[0190] Similar to that of the corresponding dried powder, characteristic peaks of Cu 2p 3 / 2 and Cu 2p 1 / 2 are clearly visible with some Cu(OH)2 and also small amount of CU2O indicating onset of surface oxidation. However, the peak area is very small and no satellite peak is observed for all three types. Considering the O ls spectra the notable exception relative to the powders is that the peak corresponding to the O-Cu interaction is far more pronounced in the film than it was in the powder.

[0191] In order to investigate the long-term stability, a film prepared using the CU-KCGN- 1.0 powder, which shows the best electrical conductivity, was left in an ambient atmosphere for 7.5 months and compared with a similarly aged Cu foil sample followed by a subsequent XPS analysis. The four-terminal sheet resistance was also monitored for the CU-KCGN- 1.0 sample over time to further understand the extent of stability.

[0192] Figure 7 shows: XPS analysis and peak assignments for Cu 2p core levels for (a) Cu films prepared using the CU-KCGN- 1.0 powder; (b) Com-Cu-Foil left under ambient atmospheric conditions for 7.5 months; (c) overlay of graphs given in (a) and (b); (d) long-term 4-terminal sheet resistance measurements for Cu films prepared using Cu- KCGN- 1.0 powder and Com-Cu-Foil; and (e) total reflectance measurements for Cu films prepared using CU-KCGN-0.5, CU-KCGN- 1.0 and CU-KCGN- 1.5 compared with that of Com-Cu-Foil.

[0193] The XPS data showed that the stability of the Cu film according to the invention was considerably better than the Cu foil, with significantly low amounts of oxidative products including Cu(OH)2, CU2O and CuO.

[0194] Importantly, the distinct Cu 2p peaks are still very prominent in the sample according to the invention, whereas in the conventional foil it has merged with that of the oxidized version of Cu (Cu2+), together with the appearance of a very significant satellite peak indicating significant oxidation.

[0195] Further, as depicted in the graph in Figure 7(d), the sheet resistance is very stable with only a 1.5% change in resistance, over the measured period of 100 days corroborating very high stability under ambient conditions. The stability of the film is significantly better compared to a similarly exposed Com-Cu-Foil sample.

[0196] It is important to note that the particles and resulting film according to the invention only contained K-carrageenan as a stabilizing agent, and no other additives to enhance stability. Thus, the oxidative stability as demonstrated by XPS and four-terminal sheet resistance measurements clearly validates that K-carrageenan is able to provide excellent stability, especially considering that the processing was completely carried out under ambient atmospheric conditions (rather than controlled / inert conditions).

[0197] Without being bound by theory, it may be that this impressive stability exhibited by the Cu film according to the invention is a result of the supramolecular arrangement of carrageenan polysaccharide helices which accumulate at the interparticle gaps, i.e. at the grain boundaries, when compressed under high pressure, passivating these sites which are most susceptible to oxidation. Additionally, when compressed under high pressure, particles are forced into place, rather than the gradual coalescence observed in conventional sintering, enabling controlled densification, minimizing interparticle gaps and pores, there by restricting ingress and lateral diffusion of oxygen.

[0198] Example 3: Formation of supported electrically conductive products a) Paper substrate

[0199] The particles according to the invention (CU-KCGN- 1.0 particles) were deposited on filter paper as the substrate, to provide Cu patterns of various designs.

[0200] In order to facilitate deposition, the particles were suspended in a liquid carrier, which was a dilute aqueous solution of L-ascorbic acid (15 mM), to provide an “ink”. The ink formulation was quite viscous, with a high solid content (60 mg / ml), to ensure good coverage of particles.

[0201] The ink was deposited on the filter paper in a pattern using a fountain pen or a calligraphy dip pen. The liquid carrier of the ink was gradually absorbed by the paper, leaving behind a dense Cu layer on the paper. This Cu layer was subsequently dried under vacuum to completely remove the residual liquid carrier.

[0202] The Cu patterned paper was then pressed. A load of 4-5 tons (4000 - 5000 kg) was applied for 1 minute. This is equivalent to an applied pressure of: 4.88E+02 to 6.10E+02 bar.

[0203] This gave rise to a highly reflective and very conductive paper electrode. The paper electrode was highly flexible and could be subjected to physical deformation without delamination of the deposited Cu.

[0204] Optical microscope and SEM images showed that the Cu patterned paper consisted of a random distribution of cellulose fibres which were packed together giving rise to porous areas where the Cu particles packed densely.

[0205] Depending on the thickness of the Cu layer and the arrangement of the cellulose fibres, some fibres were fully covered, whereas others were less covered.

[0206] As shown by four-terminal electrical measurements, the Cu features are very conductive, and the observed resistance is a measure of the length and thickness of the Cu pattern.

[0207] Figure 8: Photographs showing (a) the Cu ink used to prepare patterned electrodes on paper and PCB substrates; (b) various Cu patterns on filter paper; (c) physical deformation of an electrode on filter paper - no delamination of the Cu film occurs upon bending; (d) a typical sample used for four-terminal electrical measurements of films on paper and the corresponding optical microscope image; SEM images of (e) filter paper; and (f) Cu films on filter paper (of increasing magnification); (g) cross sectional EDXS analysis of a Cu film on paper showing the overlayed image and elemental composition: Al (from the SEM stub), C, O and Cu.

[0208] Further paper substrates were successfully provided with the Cu ink according to the invention, using a nib or dip pen. For finer writing and greater control over line thickness, this use of a nib or dip pen was found to be highly effective as an instrument for depositing the ink onto the substrate, especially because the nib size and shape can be changed according to the requirement.

[0209] It is noted that these experiments all used unmodified filter paper. However, appropriate surface modifications could be further considered for the substrate before the Cu is deposited. Surface modifications of paper substrates are already known in the art in various paper-based electronics. For example, a plasma treatment or a chemical treatment could be foreseen. These treatments could be used to enhance the adherence of the ink. b) PCB substrate

[0210] Conventionally, printed circuit boards are made by depositing a layer of Cu, followed by lithography / masking and etching to remove the unwanted coverage, resulting in a circuit of a desired pattern.

[0211] The ink comprising particles according to the invention (CU-KCGN- 1.0 particles) was deposited to provide Cu patterns of various designs on printed circuit boards.

[0212] The PCB substrate was initially UV / O3 treated to make the surface more hydrophilic and then the ink was deposited, using a dipping pen, before being dried and pressed. A load of 4-5 tons (4000 - 5000 kg) was applied for 1 minute. This is equivalent to an applied pressure of: 4.88E+02 to 6.10E+02 bar.

[0213] The Cu particles adhered well to the underlying PCB substrate.

[0214] The PCB as obtained had a very smooth, highly reflective top surface and had excellent conductivity. Fig 9: Photographs showing (a) smooth and highly reflective Cu patterns on PCB substrates; (b) a typical sample used for four-terminal electrical measurements of films on PCB substrates; SEM images of (c) PCB substrate; and (d) Cu films on PCB substrates (of increasing magnification).

[0215] Further testing:

[0216] Similar testing has been carried out using the following seaweed derived polysaccharides, instead of K-carrageenan, and similar results were obtained: r-carrageenan

[0217] X-carrageenan the fucoidan shown below:

[0218] This fucoidan is obtained from brown algae.

[0219] The synthesis was successful, yielding submicron copper particles. These particles can be used to form free-standing electrically conductive products and supported electrically conductive products as discussed above.

[0220] The invention can be put into effect using different sulfated polysaccharides, including sulfated polysaccharides extracted from red algae, from brown algae and from green algae (carrageenans, fucans, ulvans). Thus the invention can be implemented using a wide range of naturally derived sulfated polymers. Conclusions:

[0221] Sulfated polysaccharides, such as carrageenan and fucoidan, have been found to be highly effective stabilising agents in the synthesis of metallic Cu particles, with excellent stability. The presence of well-isolated particles suggests that polysaccharide helices partially passivate the Cu particles, preventing agglomeration.

[0222] Beneficially, they can be obtained via a very simple synthesis that is carried out in an aqueous medium under ambient atmospheric conditions using vitamin C as the reducing agent. The synthesis is therefore cost effective and environmentally friendly.

[0223] The isolated Cu powders can be pressed under a high load, at room temperature, to fabricate free-standing (substrate-free) conductive copper films. The films exhibit very high conductivity, close to that of bulk Cu. This is remarkable considering the sample is not subjected to any heat treatment to achieve this conductivity.

[0224] The morphology as seen by SEM and AFM show the particles are densely packed, forming a well-connected network of compressed particles with a very smooth surface which minimizes scattering and enhances electron transport.

[0225] The pressed films also exhibit excellent stability, as evidenced by XPS analysis of aged films, and show long-term conductivity with only a 1.5% change in four-terminal sheet resistance over 100 days of exposure to ambient atmospheric conditions.

[0226] The particles have also been successfully provided in substrate-mounted form, being deposited on both paper and PCB substrates. The resulting patterned substrates show excellent conductivity, flexibility and mechanical stability, and the copper particles adhered well to the substrate. Again, this is remarkable considering the sample is not subjected to any heat treatment to achieve this conductivity.

[0227] The present invention combines conductivity and oxidative stability, low energy synthesis (which, in embodiments, may be from renewable resources) and ease of film formation on a range of substrates. Applications which make use of copper inks for printable electronics are, in particular, foreseen. Therefore the invention beneficially provides a new route to a range of useful electrically conductive products, having excellent electrical conductivity and high metallic character, without the need for high temperature or controlled / inert conditions. The approach is relatively simple and greener, supporting sustainable chemistry and processing.

Claims

CLAIMS1. Stabilised copper sub-micron scale particles suitable for use in the manufacture of an electrically conductive product, wherein the particles have a mean diameter in the range of from 25nm to lOOOnm, and wherein each particle comprises (a) a metallic copper inner core and (b) a sulfated polymer outer layer.

2. The particles of claim 1, wherein the particles are in the form of a powder.

3. A formulation comprising the particles as defined in claim 1 in combination with a liquid carrier, such as an aqueous carrier.

4. A method of manufacturing a free-standing electrically conductive product, the method comprising: providing stabilised copper sub-micron scale particles as defined in claim 1; and applying energy to the particles, to form a free-standing electrically conductive product.

5. The method of claim 4, wherein the energy is applied by application of a load, and / or by application of heat, and / or by application of light.

6. The method of claim 4 or claim 5, wherein a load is applied to the particles to press the particles, to form the free-standing electrically conductive product.

7. The method of claim 6, wherein the particles are placed in a press, such as a die or bench press, and pressed to obtain a film, disk, plate or sheet.

8. The method of any one of claims 4 to 7, wherein the method is carried out at a temperature of 120°C or less, such as 100°C or less.

9. The method of any one of claims 4 to 7, wherein the method is carried out at a temperature of from 120°C to 500°C, such as from 150°C to 300°C.

10. A method of manufacturing a supported electrically conductive product, the method comprising:providing stabilised copper sub-micron scale particles as defined in claim 1 or claim 2; and applying the particles onto a substrate, to form an electrically conductive layer on the substrate.1 1. The method of claim 10, wherein the substrate is:(a) flexible, e.g. paper, or(b) rigid, e.g. ceramic.

12. The method of claim 10 or claim 1 1, wherein the particles are provided in the form of a suspension, and this suspension is deposited onto the substrate.

13. The method of any one of claims 10 to 12, wherein the method is carried out at a temperature of 120°C or less, such as 100°C or less.

14. The method of any one of claims 10 to 12, wherein the method is carried out at a temperature of from 120°C to 500°C, such as from 150°C to 300°C.

15. A method of manufacturing a 3D printed electrically conductive product, the method comprising: a) providing a particulate material comprising stabilised copper sub-micron scale particles as defined in claim 1 or claim 2; b) depositing the particulate material; and c) selectively fusing the deposited particulate material; so as to obtain a three-dimensional object.

16. The method of claim 15, wherein heat and / or light is applied to selectively fuse the particles.

17. A process for producing the particles as defined in claim 1, the process comprising: providing a solution of the sulfated polymer; combining the polymer solution with a Cu(II) salt to form a mixture; adjusting the pH of the mixture to 9 or more; and adding a reducing agent, to reduce the Cu(II) to Cu(0),thereby obtaining particles as defined in claim 1 in the form of a suspension.

18. The process of claim 17, wherein the particles are separated from the suspension, e.g. by centrifugation.

19. The process of claim 18, wherein the particles are (i) washed and then (ii) dried to powder form, e.g. by drying under vacuum.

20. The process of any one of claims 17 to 19, wherein all steps of the process, apart from the optional drying step (ii), are carried out at a temperature of 120°C or less, such as 100°C or less.

21. An electrically conductive product formed from particles as defined in claim 1 or claim 2.

22. The product of claim 21, which is a free-standing product, such as a film, disk, plate or sheet, or a 3D printed article, the free-standing product comprising metallic copper and the sulfated polymer.

23. The product of claim 22, which is a supported product, such as a printed circuit board or a printed paper item, e.g. a paper-based electronic device, wherein the supported product comprises metallic copper and the sulfated polymer on a flexible or rigid substrate.

24. The use of a sulfated polymer to provide stabilised copper sub-micron scale particles that are suitable for use in the manufacture of an electrically conductive product without requiring sintering.

25. The invention of any one of the preceding claims, wherein the sulfated polymer is a sulfated polysaccharide.

26. The invention of any one of the preceding claims, wherein the sulfated polymer has an average of 0.25 to 2 sulfate groups per monomer.

27. The invention of any one of the preceding claims, wherein the sulfated polysaccharide is derived from marine organisms, e.g. seaweeds, marine invertebrates and sea grasses, such as red, green or brown algae.

28. The invention of any one of the preceding claims, wherein the sulfated polymer is a sulfated polysaccharide that is:(a) selected from carrageenans, agarans, ulvans and fucans; or(b) selected from carrageenans (e.g. kappa carrageenan), ulvans and fucans (e.g. fucoidans); or(c) selected from carrageenans (e.g. kappa carrageenan) and fucans (e.g. fucoidans); or(d) kappa carrageenan.

29. The invention of any one of the preceding claims, wherein the particles have a mean diameter in the range of:(a) from 25nm to 800nm; or(b) from 50nm to 500nm; or(c) from 50nm to 400nm; or(d) from 50nm to 300nm; or(e) from 75nm to 200nm.

30. The invention of any one of the preceding claims, wherein the particles comprise:(a) from 95 to 99.9wt% metallic copper; or(b) from 97 to 99.7wt% metallic copper; or(c) from 98.5 to 99.5wt% metallic copper.

31. The invention of any one of the preceding claims, wherein the particles comprise:(a) from 0.1 to 5wt% sulfated polymer; or(b) from 0.3 to 3wt% sulfated polymer; or(c) from 0.5 to 1.5wt% sulfated polymer.

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