Method of forming a sintering powder comprising copper particles

Abstract

A method of forming a sintering powder comprising copper particles, the method comprising: providing a solution comprising copper ions, a base and a capping agent; contacting the solution with a reducing agent to provide the sintering powder; and recovering the sintering powder, wherein: prior to contacting the solution with the reducing agent, the solution has a pH of from 4 to 7; and the capping agent comprises an amino alcohol.

Description

[0001] Method of forming a sintering powder comprising copper particles

[0002] The invention relates to a method of forming a sintering powder comprising copper particles.

[0003] Sintered joints provide an alternative to soldered joints. A typical method of forming a sintered joint involves placing a metal powder, often in the form of a powder compact, between two work pieces to be joined and then sintering the metal powder. The resulting atomic diffusion of the metal atoms forms a bond between the two work pieces.

[0004] Metal nanopowders have been used to form sintered joints in the electronics industry and are useful alternatives to lead-free soldering. The differing behaviour between nanomaterials and the corresponding bulk material is thought to be due to nanomaterials having a higher surface-area-to-volume ratio. The formation of a conductive joint between two materials by use of metal nanoparticles has many benefits when compared with the traditional solder paste, in terms of its mechanical, electrical and thermal reliability.

[0005] Sintering powders containing silver nanoparticles are known. Sintered joints formed by atomic diffusion of silver nanoparticles can be processed at a temperature significantly lower than the melting temperature of the bulk and can also be used for high temperature applications. However, the sintering temperatures of such sintering powders are still too high for effective use in most electronics applications.

[0006] Sintering temperatures may be reduced by applying an external pressure during sintering. Pressure-assisted low-temperature sintering of silver paste has been shown to be a viable alternative to solder reflow as a die-attachment method. The application of high pressure has been shown to significantly lower the sintering temperature, and the desired properties for die attachment can be achieved at a relatively faster rate resulting in the formation of a sintered joint within a few minutes. However, a large external pressure makes automation of the process difficult. Furthermore, application of a large external pressure may result in damage to the work pieces.

[0007] Silver metal, due to its excellent electrical and thermal conductivity, has long been used in the electronics industries for die attach applications. However, the high cost of silver limits the wide-spread application of silver in many areas. Moreover, silver also faces the issue of ion migration at relatively high temperature and humidity, which is the other major cause of concern.

[0008] Copper is a much cheaper material in comparison to silver and also possesses a very high conductivity (only 6% less than that of Ag). In addition to its lower cost, copper also does not exhibit the problem of ion migration which gives copper an edge over silver. However, the inherent tendency of copper nanoparticles to oxidize is still a major problem.

[0009] There have been several reports presenting various approaches which demonstrate that copper nanoparticles can resist oxidation under ambient conditions if they are coated by a proper protective layer. The utility of these copper nanoparticles in a conductive die attach paste for die attach application is still very much unexplored. Hence, in the electronics market, there is a huge demand for a copper die attach paste which can give almost the same benefits of nanosilver, but at a much lower cost.

[0010] WO 2014 / 068299 relates to a sintering powder comprising a particulate having a mean longest diameter of less than 10 microns and is directed at silver nanoparticles with a bimodal particle size distribution. WO 2015 / 155542 relates to a low-pressure sintering powder and is also directed at silver nanoparticles with a bimodal particle size distribution. Since both documents make use of silver, the problem of ion migration may occur.

[0011] W02020 / 002890 relates to a sintering powder comprising copper particles. The particle size distribution of the copper particles is controlled in an attempt to reduce the level of oxidation. However, increasing the size of the particles adversely affects the sintering properties. The sintering powders are prepared in solutions with high pH (see, for example, the description of the manufacturing method of Example 1 , which quotes a pH > 7, likely in the region of 8).

[0012] There is a need to provide a copper sintering powder that is less prone to oxidation and / or agglomeration than conventional copper sintering powders but that exhibits similar sintering properties to that of silver sintering powders.

[0013] The present invention seeks to tackle at least some of the problems associated with the prior art or at least to provide a commercially acceptable alternative solution thereto.

[0014] In a first aspect, the present invention provides a method of forming a sintering powder comprising copper particles, the method comprising: providing an aqueous solution comprising copper ions, a base and a capping agent; contacting the solution with a reducing agent to provide the sintering powder; and recovering the sintering powder, wherein: prior to contacting the solution with the reducing agent, the solution has a pH of from 4 to 7; and the capping agent comprises an amino alcohol.

[0015] Each aspect or embodiment as defined herein may be combined with any other aspect(s) or embodiment(s) unless clearly indicated to the contrary. In particular, any features indicated as being preferred or advantageous may be combined with any other feature indicated as being preferred or advantageous. The inventors have surprisingly found that the resulting sintering powder may exhibit advantageous sintering properties while also being less prone to oxidation than conventional copper-containing sintering powders.

[0016] The copper particles of the resulting sintering powder may exhibit a thin outer layer I shell comprising copper oxide and copper hydroxide. Based on powder X- ray diffraction and XPS analysis, such a layer appears to be different in chemical composition and / or structure to a typical copper oxide layer that forms on a conventional copper particle when exposed to air. Without being bound by theory, it is considered that this thin outer layer may contribute to the sintering powder’s advantageous sintering properties and resistance to oxidation. In particular, this thin outer layer may be thick enough, and have a suitable chemical composition, to inhibit further oxidation of the copper particles in air and inhibit agglomeration. However, at typical sintering temperatures, the thin outer layer may be removed to enable sintering to take place. Again, without being bound by theory, it is considered that the presence of copper hydroxide in the thin outer layer may render the thin outer layer easier to reduce at typical sintering temperatures, for example when the sintering powder is in the form of a paste containing conventional activators.

[0017] Without being bound by theory, it is considered that the properties of the thin outer layer may be somewhat intermediate that of an oxide layer formed on, for example silver, and an oxide layer formed on aluminium. As will be appreciated, when metals are exposed to oxidising conditions such as air, an oxide layer forms on the surface of the metal. For certain metals such as silver, with continued exposure to air the metal will transform completely to its oxide. In contrast, for certain metals such as aluminium, the oxide layer acts as a barrier and prevents further oxidation of the bulk metal. In simple terms, this difference may be attributed to the volume of the formed oxide - when the volume of the metal increases on forming its oxide it acts as a barrier, whereas when the volume of the metal decreases on forming its oxide it exposes underlying metal to further oxidation. The inventors have speculated that the properties of the thin outer layer of the sintering powder of the present invention may result in the thin outer layer exhibiting more “barrier-like” properties in comparison to those of typical oxide layers formed on particles of conventional copper sintering powders when exposed to air.

[0018] The sintering powder formed using the method of the present invention may exhibit a particle size distribution that contributes to the favourable sintering properties. For example, the copper particles may exhibit: a D90 of from 270 to 470 nm (typically 270 to 350 nm or 420 to 470 nm), and / or a D50 of from 130 to 160 nm , and / or a D10 of from 90 to 110 nm.

[0019] Without being bound by theory, it is considered that the particle size distribution and / or the thin outer layer may result from the reaction kinetics. It is considered that during the reaction copper ions may react with the base to form some kind of complex or colloidal species, and that the nitrogen lone pair of the amino alcohol capping agent may be donated to the copper centre to form some kind of chelated species. Such an intermediate may be easier to reduce during the subsequent reduction step. The use of a pH of from 4 to 7 controls the proportion of copper ions that form the intermediate. In other words, the advantageous properties of the sintering powder may derive from the use of the amino alcohol capping agent in combination with a pH of from 4 to 7.

[0020] Advantageously, the use of a capping agent may help to reduce agglomeration of the particles. Such agglomeration is unfavourable, since it may increase the sintering temperature of the sintering powder. Accordingly, the use of a capping agent enables the formation of a sintered joint between work pieces at lower temperatures and pressures and, therefore, may help to reduce damage to a work piece caused by exposure to high sintering temperatures. In addition, the use of a capping agent may help to avoid degradation of the copper such as, for example, damage caused by exposure of the metal to air at temperatures below typical sintering temperatures. This may enable the sintering powder to retain its favourable sintering properties over time, i.e. after a period of storage.

[0021] The amino alcohol may evaporate and decompose under typical sintering conditions, for example 1 to 30 MPa and 200 to 300 °C. This may result in very low levels of organics in the sintered joint, typically substantially no organics in the sintered joint. This may improve the strength and mechanical properties of the sintered joint. In addition, the amino alcohol may decompose to produce reducing gases, for example CO, NO and NH3. Without being bound by theory, it is considered that the presence of such reducing gases during sintering may aid the sintering by reducing the thin outer protective shell on the copper particles and / or inhibiting oxidation of the copper particles.

[0022] The amine group of the amino alcohol may serve to interact with the copper and form the intermediate discussed above. The hydroxyl group may render the amino alcohol water-soluble.

[0023] Advantageously, the copper sintering powder provides equal or similar benefits of nanosilver in terms of sinterability, thermal conductivity, electrical conductivity and favourable mechanical properties of the final joint, but at a much lower cost. In addition, the problem of ion migration associated with nanosilver is overcome.

[0024] The sintering powder is typically air stable and non-agglomerated.

[0025] The particle size distribution of the sintering powder may be unimodal or bimodal.

[0026] Furthermore, the sintering powder enables the versatile joining of surfaces and in particular, the versatile joining of two or more surfaces of different finishes, such as: copper-to-copper; copper to silver, copper to brass, copper to tin, copper to gold, copper to copper-molybdenum, copper to copper-tungsten and copper-to- nickel. The sintering powder is particularly suitable for die, component, and package attach. The term “sintering powder” as used herein may encompass a powder capable of forming a sintered joint. Sintered joints are formed by atomic diffusion of metal particles placed between two work pieces to be joined. The term “sintering powder” may encompass a particulate.

[0027] The sintering powder comprises copper particles. The particles may comprise regular shaped particles (such as, for example, spheres) and / or irregular shaped particles (such as, for example, whiskers, faceted, plates, rods or flakes). Preferably at least some of the particles comprise faceted particles. Preferably the particles comprise both regular and irregular shaped particles.

[0028] The copper particles may be in the form of, for example, copper metal or copper alloy, preferably copper metal.

[0029] The sintering powder may comprise particles other than copper particles. However, the majority of the particles in the sintering powder are copper particles, typically substantially all of the particles are copper particles.

[0030] The method comprises providing an aqueous solution comprising copper ions, a base and a capping agent. The solution is an aqueous solution (e.g. the solvent is demineralised water) since water is non-toxic, low cost and widely available. The copper ions are typically provided in the form of a salt. Accordingly, in addition, to copper ions, base and capping agent, the solution typically further comprises the counterion(s) of the salt. Copper ions may be the only metal ions contained in the solution. Alternatively, other metal ions may be present in the solution. The other metal ions may be counter-ions of the salt. Preferably copper ions are the only metal ions in the solution. This may avoid unnecessary impurities in the sintering powder which may compromise the sintering properties. The copper ions may comprise copper (I) ions and / or copper (II) ions, preferably copper (II) ions. The term “capping agent” as used herein may encompass a species that, when present on the surface of metal particles, reduces agglomeration of the metal particles, enables particle size control during powder production and reduces particles’ surface oxidation or other contamination.

[0031] The method comprises contacting the solution with a reducing agent to provide the sintering powder. Without being bound by theory, it is considered that the reducing agent reduces the copper ions causing precipitation of copper metal particles. Such copper metal particles are “capped” by the capping agent, therefore hindering further agglomeration. Such particles are typically nanoparticles. Contacting the solution with a reducing agent may comprise, for example, adding the reducing agent to the solution. Alternatively, contacting the solution with a reducing agent may comprise, for example, adding the solution to a solution of the reducing agent.

[0032] The method comprises recovering the sintering powder. In other words, the method may comprise recovering the sintering powder from the solution. The sintering powder may be stored, for example, under vacuum or under an inert atmosphere. This may further hinder oxidation and / or agglomeration of the particles of the sintering powder.

[0033] Prior (i.e. immediately prior) to contacting the solution with the reducing agent, the solution has a pH of from 4 to 7. In other words, the step of providing an aqueous solution comprising copper ions, a base and a capping agent may comprise providing an aqueous solution comprising copper ions, a base and a capping agent having a pH of from 4 to 7. As will be appreciated, once the solution is contacted with the reducing agent, the pH of the solution may change and may be outside of this range.

[0034] The capping agent comprises an amino alcohol. The term “amino alcohol” as used herein may encompass an organic compound that contains both hydroxyl (-OH) and amino (-NH2, -NHR, and -NR2) functional groups on an alkane backbone. The amino alcohol may contain a single hydroxyl group or multiple hydroxyl groups. The hydroxyl group may be present anywhere between the C2 and the Cn position, the Cn position being the terminal position. The alkane backbone may be linear, branched or cyclic. The carbon chain is typically aliphatic but may be aromatic. The boiling point of the amino alcohol is typically above 50 °C. The amine group may be present at the primary or secondary position. Amino alcohols are sometimes referred to in the art as aminosubstituted alcohols or alkanolamines. The capping agent may consist of the amino alcohol. Alternatively, the solution may comprise capping agent(s) other than an amino alcohol. The capping agent may comprise a single type of amino alcohol or multiple types of amino alcohol.

[0035] Prior to contacting the solution with the reducing agent, the solution preferably has a pH of from 4.1 to less than 7, more preferably from 4.2 to 6.5, even more preferably from 4.5 to 6, still even more preferably from 4.8 to 5.5. This may assist in providing the favourable particle size distribution, particle shapes and / or thin outer layer.

[0036] The amino alcohol preferably comprises two or more hydroxyl groups more preferably two hydroxyl groups (i.e. only two hydroxyl groups). This may result in an increase in intermolecular hydrogen bonding between capping agent molecules, thereby increasing the oxidation protection.

[0037] The hydroxyl groups of the amino alcohol are preferably situated on a single aliphatic chain (typically alkyl chain), i.e. all of the hydroxyl groups of the amino alcohol are preferably situated on a single aliphatic chain. The amino alcohol preferably comprises two hydroxyl groups in a single aliphatic chain, i.e. only two hydroxyl groups and these are both present on the same aliphatic chain. The inventors have surprisingly found that the presence of hydroxyl groups on multiple alkyl chains (e.g. in triethanolamine as per W02020 / 002890) may create steric hindrance and / or orientation hindrance, and hence may result in a weaker chelating complex as well as less intramolecular hydrogen bonding. Furthermore, the presence of hydroxyl groups on multiple alkyl chains (e.g. in triethanolamine) may also result in poor reaction kinetic control, thereby producing larger particles. On the other hand, the presence of two hydroxyl groups on the same aliphatic chain may result in less steric hindrance, and hence result in a stronger chelating complex, thereby providing better reaction kinetic control to produce smaller size particles. Further, the presence of two hydroxyl groups in a single aliphatic chain may result in intramolecular hydrogen bonding, thereby further contributing to oxidation protection as well as agglomeration control of the copper particles.

[0038] The amino alcohol preferably comprises alkyl amino alcohol, more preferably dialkyl amino alcohol. The alkyl group is preferably methyl or ethyl, more preferably methyl. The amino alcohol more preferably comprises methyl amino alcohol, even more preferably dimethyl amino alcohol. Having the nitrogen atom of the amino group substituted with a smaller chain alkyl group, e.g. methyl or ethyl, preferably methyl, may create less steric hindrance than bigger chain alkyl groups (e.g. propyl, butyl etc). The presence of alkyl groups, preferably two alkyl groups, may also produce sufficient (+) inductive effect (+l effect) to donate the lone pair on nitrogen, and hence result in a sufficiently strong chelating complex compared to non-substituted amines (-NH2). In contrast, amino alcohols that are devoid of alkyl groups, such as ones comprising three alcohol groups, e.g. triethanolamine, may have less electron donation capability. In the case of triethanolamine, for example, the presence of three terminal -OH groups may reduce the +l effect.

[0039] The amino alcohol preferably does not comprise triethanolamine. The capping agent is preferably different to the base. The amino alcohol preferably comprises at least two hydroxyl groups on adjacent carbon atoms. Preferably, at least two of the substituents on the nitrogen of the amino alcohol are non-identical. The amino alcohol preferably has a pKa larger than 9, more preferably larger than 10, even more preferably larger than 11 , still even more preferably larger than 12, still even more preferably larger than 13. The amino alcohol preferably comprises from 3 to 12 carbon atoms. Such an amino alcohol may function as a particularly effective capping agent.

[0040] The amino alcohol may comprise di-hydrogen amino alcohol, i.e. it may comprise a — NH2 group.

[0041] The amino alcohol preferably comprises 3-(dimethyl amino)-1 ,2-propanediol (DMPA), 3-(amino)-1 ,2-propanediol (AP), and / or 3-(methyl amino)-1 ,2- propanediol (MAP), preferably 3-(dimethyl amino)-1 ,2-propanediol. Such amino alcohols may function as particularly effective capping agents.

[0042] 3-(dimethyl amino)-1 ,2-propanediol has the following chemical structure:

[0043] 3-(amino)-1 ,2-propanediol has the following chemical structure:

[0044] 3-(methyl amino)-1 ,2-propanediol has the following chemical structure:

[0045] Providing the solution comprising copper ions, a base and a capping agent preferably comprises providing a solution comprising copper ions and a base, and then contacting the solution comprising copper ions and a base with a capping agent. The copper ions in the solution are preferably provided in the form of a copper salt, more preferably selected from one or more of copper(ll) nitrate, copper(ll) sulphate, copper(ll) acetate, copper(ll) formate and copper(ll) hydroxide, even more preferably selected from one or both of copper(ll) nitrate and copper(ll) acetate, still even more preferably copper(ll) nitrate. Such species are particularly suitable sources of copper ions. The use of copper nitrate over copper acetate may enable the use of lower pH. This may be due to the fact that the ligand strength of nitrate may be lower than that of acetate, i.e. that copper nitrate is almost ionic whereas copper acetate is almost covalent.

[0046] The base preferably comprises an amine such as, for example, triethanolamine, diethanol amine or triethyl amine, preferably triethanolamine. The base may comprises a hydroxide in addition to the amine. The presence of these species may assist in forming the intermediate species discussed above and may be particularly suitable for providing the solution with the required pH. The hydroxide is preferably selected from one or more of sodium hydroxide, potassium hydroxide and ammonium hydroxide. Such hydroxides are particular suitable bases. More preferably the hydroxide is ammonium hydroxide. In comparison to metal hydroxides such as sodium hydroxide and potassium hydroxide, the use of ammonium hydroxide may result in lower levels of impurity metals in the sintering powder, which may hinder the sintering properties and / or compromise the properties of a sintered joint formed using the sintering powder.

[0047] The reducing agent preferably comprises one or more of hydrazine (e.g. hydrazine hydrate), sodium borohydride and lithium borohydride, more preferably hydrazine. Such species are particularly suitable reducing agents and may assist in providing in the favourable reaction kinetics discussed above. Hydrazine is a particularly suitable reducing agent and is particularly effective at producing the particles in the required size range.

[0048] Once the solution has been contacted with the reducing agent, the molar ratio of copper ions : base : capping agent : reducing agent in the solution is preferably from 0.2 to 1 : from 0.05 to 2 : from 0.2 to 1 .5 : from 1 to 10. Such ratios may assist in providing in the favourable reaction kinetics discussed above.

[0049] The solution comprising copper ions, a base and a capping agent is preferably contacted with the reducing agent for from 1 to 20 hours, preferably for from 2 to 10 hours, more preferably for from 3 to 5 hours, i.e. before the step of recovering the sintering powder. This may assist in proving the favourable particle size distribution, particle shape and / or outer layer discussed above. Shorter times may result in a reduced yield and / or the formation of unfavourably small particles. Longer times may result in the formation of unfavourably large particles or may cause unfavourable oxidation.

[0050] Recovering the sintering powder preferably comprises one or more of decanting the resulting solution from the sintering powder, washing the sintering powder and drying the sintering powder.

[0051] The drying is preferably carried out in a non-inert atmosphere or in an inert atmosphere, more preferably air or nitrogen.

[0052] The sintering powder formed by the method may be the sintering powder described below.

[0053] In a further aspect, the present invention provides a method of forming a sintering powder comprising copper particles, the method comprising: providing an aqueous solution comprising copper ions, a base and a capping agent, the aqueous solution having a pH of from 4 to 7; contacting the solution with a reducing agent to provide the sintering powder; and recovering the sintering powder, wherein the capping agent comprises an amino alcohol. The advantages and preferable features of the first aspect apply equally to this aspect.

[0054] In a further aspect, the present invention provides a sintering powder formed using the method described herein.

[0055] The advantages and preferable features of the first aspect apply equally to this aspect.

[0056] In particular, such a sintering powder may exhibit the particle size distribution discussed above, and the particles of the sintering powder may exhibit the particle shape and / or outer layer discussed above. As discussed above, such an outer layer is chemically different to an outer layer formed when conventional copper sintering powders are left in air.

[0057] In a further aspect, the present invention provides a sintering powder comprising copper particles, wherein at least some of the copper particles are capped with a capping agent comprising an amino-substituted alcohol.

[0058] The advantages and preferable features of the first aspect apply equally to this aspect.

[0059] At least some of the copper particles are capped with a capping agent comprising an amino-substituted alcohol. Typically, the majority (> 50 wt.%) of the copper particles are capped with a capping agent comprising an amino-substituted alcohol, more typically substantially all of the copper particles are capped with a capping agent comprising an amino-substituted alcohol.

[0060] Preferably, at least some of the copper particles are at least partially coated with a layer comprising copper oxide and copper hydroxide. In such a case, the capping agent coats this layer. Typically, the majority (> 50 wt.%) of the copper particles are at least partially coated with a layer comprising copper oxide and copper hydroxide, more typically substantially all of the copper particles are at least partially coated with a layer comprising copper oxide and copper hydroxide.

[0061] The layer (comprising copper oxide and copper hydroxide ) preferably has a thickness of from 20 to 60 nm, more preferably from 25 to 55 nm. The thickness may be a mean thickness and may be measured using the method described in the examples. Such a thickness may assist in providing the combination of favourable sintering properties and oxidation resistance discussed above.

[0062] The amino alcohol preferably comprises two or more hydroxyl groups, more preferably two hydroxyl groups. The amino alcohol preferably comprises from 3 to 12 carbon atoms. Preferably, the hydroxyl groups of the amino alcohol are situated on a single aliphatic chain. Preferably, the amino alcohol comprises two hydroxyl groups and both are situated on the same aliphatic chain. The amino alcohol preferably comprises alkyl amino alcohol, more preferably dialkyl amino alcohol. The amino alcohol preferably comprises methyl amino alcohol, more preferably dimethyl amino alcohol. The amino alcohol preferably comprises dihydrogen amino alcohol.

[0063] The amino alcohol preferably comprises 3-(dimethyl amino)-1 ,2-propanediol, 3- (amino)-1 ,2-propanediol, and / or 3-(methyl amino)-1 ,2-propanediol, preferably 3- (dimethyl amino)-1 ,2-propanediol.

[0064] The sintering powder preferably comprises from 0.1 to 12 wt.% capping agent based on the total weight of the sintering powder, more preferably up to 3 wt.%, even more preferably up to 1 wt.%. Lower levels of capping agent may be insufficient to provide the combination of favourable sintering properties and oxidation resistance discussed above. Higher levels may result in unfavourable levels of organics remaining in the sintered joint and / or may increase the cost of the method of manufacture.

[0065] The copper particles preferably exhibit: a D90 of from 200 to 500 nm, and / or a D50 of from 120 to 195 nm , and / or a D10 of from 55 to 115 nm.

[0066] The copper particles more preferably exhibit: a D90 of from 270 to 470 nm (preferably 270 to 350 nm or 420 to 470 nm), and / or a D50 of from 130 to 160 nm , and / or a D10 of from 90 to 110 nm.

[0067] Such particle size distributions may improve the sintering properties of the sintering powder. In addition, a sintered joint formed using such a sintering powder may exhibit favourable mechanical properties. Without being bound by theory, it is considered that such particle size distributions, including a significant number of smaller particles, my result in a particularly favourable packing density. By D90 it is mean the size at 90% in the cumulative particle size distribution, i.e. 90% of the particles have a size smaller than this value. By D50 it is mean the size at 50% in the cumulative particle size distribution (i.e. the mode), and by D10 it is mean the size at 10% in the cumulative particle size distribution. The particle size refers to a particle diameter. As is conventional in the art, the diameter corresponds to the equivalent spherical diameter, and the distribution corresponds to a volume distribution. The D90, D50 and D10 may be measured by suitable techniques known in the art such as, for example, laser diffraction, dynamic light scattering and SEM image analysis.

[0068] The copper particles preferably exhibit: a D90 of from 270 to 470 nm, a D50 of from 130 to 160 nm , and a D10 of from 90 to 110 nm.

[0069] In a particularly preferred embodiment, the copper particle exhibit: a D90 of from 420 to 470 nm, a D50 of from 130 to 160 nm , and a D10 of from 90 to 110 nm.

[0070] In another particularly preferred embodiment, the copper particle exhibit: a D90 of from 270 to 350 nm, a D50 of from 130 to 160 nm , and a D10 of from 90 to 110 nm.

[0071] The copper particles preferably comprise facet-shaped particles. Such particles may result in improved packing density in comparison to spherical shaped particles. Preferably at least 50 % by volume of the particles are facet-shaped, more preferably at least 75 % by volume, even more preferably at least 90 % by volume, still even more preferably from 30 to 90 % by volume.

[0072] In a further aspect, the present invention provides a sintering powder comprising copper particles, wherein at least some of the copper particles are at least partially coated with a layer comprising copper oxide and copper hydroxide, the layer having a thickness of from 20 to 60 nm, preferably from 25 to 55 nm.

[0073] The advantages and preferable features of the first aspect apply equally to this aspect. The thickness may be measured using the method described in the examples.

[0074] In a further aspect, the present invention provides a sintering paste comprising the sintering powder described herein.

[0075] The advantages and preferable features of the first aspect apply equally to this aspect.

[0076] The paste may further comprise, for example, one or more of a solvent, an activator, a binder, a defoaming agent, a reducing agent, and a rheology modifier. In a further aspect, the present invention provides a sintering film comprising the sintering powder described herein.

[0077] The advantages and preferable features of the first aspect apply equally to this aspect.

[0078] The sintering film may further comprise, for example, a binder. The film may be a free-standing film. The film may comprise a release layer. The film may be on a polymeric substrate.

[0079] In a further aspect, the present invention provides a method of forming a joint between two or more work pieces, the method comprising: providing two or more work pieces to be joined, providing the sintering powder described herein and / or the sintering paste described herein and / or the sintering film described herein in the vicinity of the two or more work pieces, heating the sintering powder and / or sintering paste and / or sintering film to at least partially sinter the copper particles.

[0080] The advantages and preferable features of the first aspect apply equally to this aspect.

[0081] The two or more work pieces preferably comprise a die and a substrate. The heating is preferably carried out at a temperature of from 150 to 400 °C, more preferably from 200 to 300 °C, even more preferably from 230 to 280 °C. The heating may be carried out in the absence of an applied pressure. Alternatively, a pressure may be applied during the heating, preferably from 1 to 35 MPa, more preferably from 5 to 32 MPa, more preferably from 8 to 30 MPa. Higher temperatures and pressures may result in improved sintering and more favourable characteristics of the sintered joint. Lower temperatures and pressures are more energy efficient and reduce the likelihood of damage to the workpieces being joined. In a further aspect, the present invention provides use of the sintering powder described herein, and / or the sintering paste described herein, and / or the film described herein in a method selected from: die attachment, wafer-to-wafer bonding, hermetic and near hermetic sealing, sintering films comprising a sintering powder and a binder formed in a film on a backing layer, dispensing and the production of interconnect lines.

[0082] The advantages and preferable features of the first aspect apply equally to this aspect.

[0083] In a further aspect, the present invention provides use of the sintering powder described herein, and / or the sintering paste described herein, and / or the film described herein in the manufacture of additively printed structures, humanmachine interfacing (HMI) electronic devices, capacitive switches, wire harness, solderable electronic circuits and devices, IME structures, structural I embedded electronics, large area electronics, lighting I luminaires, large area lighting, flexible and formable displays, embedded sensors, printed battery assembly, printed heaters, flexible circuits and cables, Cu coated dielectric materials, dieattachment applications, or package attach and component attach applications.

[0084] The advantages and preferable features of the first aspect apply equally to this aspect.

[0085] In a further aspect, the present invention provides a sintered joint formed using the sintering powder described herein, and / or the sintering paste described herein, and / or the film described herein and / or the method described herein.

[0086] The advantages and preferable features of the first aspect apply equally to this aspect. In a further aspect, the present invention provides use of an amino alcohol comprising two hydroxyl groups as a capping agent for copper particles.

[0087] The advantages and preferable features of the first aspect apply equally to this aspect.

[0088] The invention will now be further described by reference to the following numbered clauses:

[0089] 1 . A sintering powder comprising copper particles, wherein at least some of the copper particles are capped with a capping agent comprising an aminosubstituted alcohol.

[0090] 2. The sintering powder of clause 1 , wherein the amino-substituted alcohol comprises two or more hydroxyl groups.

[0091] 3. The sintering powder of clause 1 or clause 2, wherein the aminosubstituted alcohol comprises from 3 to 12 carbon atoms.

[0092] 4. The sintering powder of any preceding clause, wherein the aminosubstituted alcohol comprises 3-(dimethyl amino)-1 ,2-propanediol.

[0093] 5. The sintering powder of any preceding clause, wherein sintering powder comprises from 0.1 to 12 wt.% capping agent based on the total weight of the sintering powder, preferably up to 3 wt.%, more preferably up to 1 wt.%.

[0094] 6. The sintering powder of any preceding clause, wherein the copper particles exhibit: a D90 of from 450 to 800 nm, and / or a D50 of from 100 to 150 nm , and / or a D10 of from 60 to 90 nm. Such a particle size distribution may provide favourable sintering properties.

[0095] 7. The sintering powder of any preceding clause, wherein the copper particles exhibit: a D90 of from 450 to 800 nm, and a D50 of from 100 to 150 nm , and a D10 of from 60 to 90 nm.

[0096] Such a particle size distribution may provide favourable sintering properties.

[0097] 8. The sintering powder of any preceding clause, wherein the copper particles comprise facet-shaped particles.

[0098] 9. A sintering powder comprising copper particles, wherein at least some of the copper particles are capped with a capping agent and wherein the copper particles exhibit: a D90 of from 450 to 800 nm, and a D50 of from 100 to 150 nm , and a D10 of from 60 to 90 nm.

[0099] Such a particle size distribution may provide favourable sintering properties.

[0100] 10. A sintering powder comprising copper particles, wherein at least some of the copper particles are capped with a capping agent comprising a polymer comprising a lactam, preferably a y-lactam.

[0101] Such a capping agent may be particularly capable of inhibiting oxidation and / or agglomeration of copper particles. However, in comparison to an amino alcohol capping agent, the wetting and / or activity may be lower.

[0102] 11 . The sintering powder of clause 10, wherein the lactam comprises 2- pyrrolidone. 12. The sintering powder of clause 10 or clause 11 , wherein the polymer comprising a lactam comprises polyvinylpyrrolidone.

[0103] 13. The sintering powder of any of clauses 10 to 12, wherein the polymer has a molecular weight of from 2 to 400 KDa, preferably from 5 to 50 KDa, more preferably from 7 to 35 KDa.

[0104] 14. The sintering powder of any of clauses 10 to 13, wherein the copper particles exhibit: a D90 of from 450 to 800 nm, and / or a D50 of from 100 to 150 nm , and / or a D10 of from 60 to 90 nm.

[0105] Such a particle size distribution may provide favourable sintering properties.

[0106] 15. The sintering powder of any of clause 10 to 13, wherein the copper particles exhibit: a D90 of from 450 to 800 nm, and a D50 of from 100 to 150 nm , and a D10 of from 60 to 90 nm.

[0107] Such a particle size distribution may provide favourable sintering properties.

[0108] 16. A sintering powder comprising copper particles, wherein the copper particles are substantially devoid of capping agent and wherein the copper particles exhibit: a D90 of from 1000 to 1800 nm, and a D50 of from 200 to 500 nm, and a D10 of from 120 to 160 nm.

[0109] Such a particle size distribution may provide favourable sintering properties. 17. The sintering powder of clause 16, wherein the sintering agent comprises less than 1 wt.% capping agent based on the total weight of the sintering powder, preferably less than 0.7 wt.%, more preferably less than 0.5 wt.%.

[0110] 18. The sintering powder of any preceding clause, wherein at least some of the copper particles have a longest dimension of from 100 to 1000 nm.

[0111] 19. The sintering powder of any preceding clause, wherein at least some of the copper particles are at least partially coated with a layer comprising copper oxide and / or copper hydroxide, the layer having a thickness of from 20 to 60 nm, preferably from 25 to 55 nm.

[0112] 20. A sintering powder comprising copper particles, wherein at least some of the copper particles are at least partially coated with a layer comprising copper oxide and / or copper hydroxide layer, the layer having a thickness of from 20 to 60 nm, preferably from 25 to 55 nm.

[0113] 21 . A method of forming a sintering powder, the method comprising: providing a solution comprising copper ions, a base and optionally a capping agent, the solution having a pH of from 4 to 7.5; contacting the solution with a reducing agent to provide the sintering powder; and recovering the sintering powder.

[0114] 22. The method of clause 21 , wherein providing the solution comprising copper ions, a base and optionally a capping agent comprises providing a solution comprising copper ions and a base and then contacting the solution comprising copper ions and a base with a capping agent.

[0115] 23. The method of clause 21 or clause 22, wherein the copper ions in the solution are provided in the form of a copper salt, preferably selected from one or more of copper(ll) nitrate, copper(ll) sulphate, copper(ll) acetate, copper(ll) formate and copper(ll) hydroxide, more preferably selected from one or both of copper(ll) nitrate and copper(ll) acetate.

[0116] 24. The method of any of clauses 21 to 23, wherein the base comprises triethanolamine and a hydroxide, preferably selected from one or more of sodium hydroxide, potassium hydroxide and ammonium hydroxide.

[0117] 25. The method of any of clauses 21 to 24, wherein the solution comprising copper ions, a base and optionally a capping agent has a pH of from 4 to 7, preferably from 4.2 to less than 7.

[0118] 26. The method of any of clauses 21 to 25, wherein the reducing agent comprises one or more of hydrazine, sodium borohydride and lithium borohydride, preferably hydrazine,

[0119] 27. The method of any of clause 21 to 26, wherein the molar ratio of copper ions : base : capping agent : reducing agent is from 0.2 to 1 : from 0.05 to 2: from 0.2 to 1.5: from 1 to 10.

[0120] 28. The method of any of clauses 21 to 27, wherein solution comprising copper ions, a base and optionally a capping agent is contacted with the reducing agent from 1 to 20 hours, preferably from 2 to 10 hours, more preferably from 3 to 5 hours.

[0121] 29. The method of any of clauses 21 to 28, wherein recovering comprises one or more of decanting the resulting solution from the sintering powder, washing the sintering powder and drying the sintering powder.

[0122] 30. The method of any of clauses 21 to 29, wherein the method is carried out in a non-inert atmosphere or in an inert atmosphere, preferably air or nitrogen. 31 . The method of any of clauses 21 to 30, wherein the sintering powder is according to any of clause 1 to 17.

[0123] 32. A sintering paste comprising the sintering powder of any of clause 1 to 20.

[0124] 33. A sintering film comprising the sintering powder of any of clause 1 to 20.

[0125] 34. A method of forming a joint between two or more work pieces, the method comprising: providing two or more work pieces to be joined, providing the sintering powder of any of clauses 1 to 20 and / or the sintering paste of clause 32 and / or the sintering film of clause 33 in the vicinity of the two or more work pieces, heating the sintering powder and / or sintering paste and / or sintering film to at least partially sinter the copper particles.

[0126] 35. The method of clause 34, wherein the two or more work pieces comprise a die and a substrate.

[0127] 36. Use of the sintering powder of any of clauses 1 to 20, and / or the sintering paste of clause 29, and / or the film of clause 30 in a method selected from: die attachment, wafer-to-wafer bonding, hermetic and near hermetic sealing, sintering films comprising a sintering powder and a binder formed in a film on a backing layer, dispensing and the production of interconnect lines.

[0128] 37. Use of the sintering powder of any of clauses 1 to 20, and / or the sintering paste of clause 32, and / or the film of clause 33 in the manufacture of additively printed structures, human-machine interfacing (HMI) electronic devices, capacitive switches, wire harness, solderable electronic circuits and devices, IME structures, structural I embedded electronics, large area electronics, lighting I luminaires, large area lighting, flexible and formable displays, embedded sensors, printed battery assembly, printed heaters, flexible circuits and cables, Cu coated dielectric materials, die-attachment applications, or package attach and component attach applications

[0129] 38. A sintered joint formed using the sintering powder of any of clauses 1 to 20, and / or the sintering paste of clause 32, and / or the sintering film of clause 33 and / or the method of clause 34 or 35.

[0130] 39. Use of an amino-substituted alcohol as a capping agent for copper particles.

[0131] The invention will be described in relation to the following non-limiting drawings in which:

[0132] Figure 1 shows FESEM images of: (a) Sample 1 , (b) Sample 2, (c) Sample 3, (d) Sample 4, (e) Sample 5, (f) Sample 8, (g) Sample 9, (h) Sample 10, and (i) Sample 12.

[0133] Figure 2 shows FESEM images of: (a) Sample 14, (b) Sample 15, (c) Sample 16, (d) Sample 17, (e) Sample 18, (f) Sample 19, (g) Sample 20, (h) Sample 21 , and (i) Sample 22.

[0134] Figure 3 shows photographs of: (a) Sample 4, (b) Sample 5, (c) Sample 14, (d) Sample 9, (e) Sample 10, and (f) Sample 16.

[0135] Figure 4 shows TEM micrographs of: (a) Sample 4, (b) Sample 5, (c) Sample 9, and (d) Sample 9.

[0136] Figure 5 shows powder X-ray diffraction patterns of Sample 4, Sample 5, Sample 9 and Sample 14 along with PDF data of Cu, CU2O and CuO.

[0137] Figure 6 shows XPS survey scans of: (a) Sample 14, (b) Sample 4, (c) Sample 5, and (d) Sample 9. Figure 7 shows high-resolution and deconvoluted Cu2p XPS spectra of: (a) Sample 14, (b) Sample 4, (c) Sample 5, and (d) Sample 9.

[0138] Figure 8 shows Depth profiling (etch depth vs atomic % of carbon, oxygen and copper) of: (a) Sample 14, (b) Sample 4, (c) Sample 5, and (d) Sample 9.

[0139] Figure 9 shows TGA characterization results of “copper sintering powders”: the weight-loss due to the decomposition of organic cappants as well as absorbed volatile species of, (a) Example 4, Example 5, Example 9 and Example 10, and

[0140] (b) Example 16 and Example 17, as a function of temperature.

[0141] Figure 10 shows optical images of free-standing, sintered copper foils prepared using a laboratory-press at different pressures (a) 10 MPa, (b) 20 MPa and (c) 30 MPa at 260 °C for 2 min.

[0142] Figure 11 shows the electrical resistivities (surface resistances and volume resistivities) of various free-standing, sintered copper foils as prepared at different pressures: 1 , 5, 10, 15, 20, 25 and 30 MPa (260 °C for 2 min) as well as the electrical resistivity of bulk copper.

[0143] Figure 12 shows FE-SEM images of the microstructures of various crosssectioned free-standing, sintered copper foils sintered at (a) 30 MPa, (b) 20 MPa,

[0144] (c) 10 MPa, (d) 5 MPa and (e) 1 MPa.

[0145] The invention will now be described in relation to the following non-limiting examples.

[0146] Preparation of sintering powders

[0147] Sample 1: Copper(ll) nitrate trihydrate (100 g) and triethanolamine (30 g) were added in a 5 L beaker and were dissolved in deionised water (1200 ml). The reaction mixture was then stirred for 15 mins to make homogeneous solution and pH of this solution mixture was 3.9-4.1 . Next, hydrazine hydrate (250 g) was added dropwise via dropping funnel to this reaction mixture with vigorous stirring (~1500- 1600 rpm). The resulting reaction mixture was then stirred (~1300 rpm) for 4 hours at room temperature (22 °C) followed by addition of 1200 ml de-ionised water and stirred for additional 15 minutes. Then the reaction mixture was allowed to stand for 15 min for setting of the precipitate. Standard laboratory filtration set-up and process (e.g. Buchner funnel) are used to separate as synthesized copper sintering powder from the reaction mixture followed by repeated washing with water, water-acetone mixture, and acetone, sequentially. The wet copper sintering powder as collected after filtration and washing are dried in a standard box oven at a temperature range of 40 °C in air. The dried copper sintering powder are packed in a container with vacuum sealing for further storage, characterization, handling, and use.

[0148] Sample 2:

[0149] Copper(ll) nitrate trihydrate (100 g) and triethanolamine (30 g) were added in a 5 L beaker and were dissolved in deionised water (1200 ml). The reaction mixture was then stirred for 15 mins to make homogeneous solution followed by dropwise addition of ammonium hydroxide (50 g) with vigorous stirring. The reaction mixture was continued to stir for another 15 minutes, and the pH of this solution mixture was 7.2-7.5. Next, hydrazine hydrate (200 g) was added dropwise via dropping funnel to this reaction mixture with vigorous stirring (~1500-1600 rpm). The resulting reaction mixture was then stirred (~1300 rpm) for 4 hours at room temperature (22 °C) followed by addition of 1200 ml de-ionised water and stirred for additional 15 minutes. Then the reaction mixture was allowed to stand for 15 min for setting of the precipitate. Standard laboratory filtration set-up and process (e.g. Buchner funnel) are used to separate as synthesized copper sintering powder from the reaction mixture followed by repeated washing with water, water-acetone mixture, and acetone, sequentially. The wet copper sintering powder as collected after filtration and washing are dried in a standard box oven at a temperature range of 40 °C in air. The dried copper sintering powder are packed in a container with vacuum sealing for further storage, characterization, handling, and use.

[0150] Sample 3:

[0151] Copper(ll) nitrate trihydrate (100 g) and triethanolamine (30 g) were added in a 5 L beaker and were dissolved in deionised water (1200 ml). The reaction mixture was then stirred for 15 mins to make homogeneous solution followed by dropwise addition of ammonium hydroxide (35 g) with vigorous stirring. The reaction mixture was continued to stir for another 15 minutes, and the pH of this solution mixture was 5-5.2. Next, hydrazine hydrate (250 g) was added dropwise via dropping funnel to this reaction mixture with vigorous stirring (~1500-1600 rpm). The resulting reaction mixture was then stirred (~1300 rpm) for 4 hours at room temperature (22 °C) followed by addition of 1200 ml de-ionised water and stirred for additional 15 minutes. Then the reaction mixture was allowed to stand for 15 min for setting of the precipitate. Standard laboratory filtration set-up and process (e.g. Buchner funnel) are used to separate as synthesized copper sintering powder from the reaction mixture followed by repeated washing with water, water-acetone mixture, and acetone, sequentially. The wet copper sintering powder as collected after filtration and washing are dried in a standard box oven at a temperature range of 40 °C in air. The dried copper sintering powder are packed in a container with vacuum sealing for further storage, characterization, handling, and use.

[0152] Sample 4:

[0153] Copper(ll) nitrate trihydrate (100 g) and triethanolamine (30 g) were added in a 5 L beaker and were dissolved in deionised water (1200 ml). The reaction mixture was then stirred for 15 mins to make homogeneous solution followed by dropwise addition of ammonium hydroxide (10 g) with vigorous stirring. The reaction mixture was continued to stir for another 15 minutes, and the pH of this solution mixture was 4.3-4.5. Next, hydrazine hydrate (200 g) was added dropwise via dropping funnel to this reaction mixture with vigorous stirring (~1500-1600 rpm). The resulting reaction mixture was then stirred (~1300 rpm) for 4 hours at room temperature (22 °C) followed by addition of 1200 ml de-ionised water and stirred for additional 15 minutes. Then the reaction mixture was allowed to stand for 15 min for setting of the precipitate. Standard laboratory filtration set-up and process (e.g. Buchner funnel) are used to separate as synthesized copper sintering powder from the reaction mixture followed by repeated washing with water, water-acetone mixture, and acetone, sequentially. The wet copper sintering powder as collected after filtration and washing are dried in a standard box oven at a temperature range of 40 °C in air. The dried copper sintering powder are packed in a container with vacuum sealing for further storage, characterization, handling, and use.

[0154] Sample 5:

[0155] Copper(ll) nitrate trihydrate (100 g) and triethanolamine (30 g) were added in a 5 L beaker and were dissolved in deionised water (1200 ml). The reaction mixture was then stirred for 15 mins to make homogeneous solution followed by dropwise addition of ammonium hydroxide (24 g) with vigorous stirring. The reaction mixture was continued to stir for another 15 minutes, and the pH of this solution mixture was 4.6-4.9. Next, hydrazine hydrate (110 g) was added dropwise via dropping funnel to this reaction mixture with vigorous stirring (~1500-1600 rpm). The resulting reaction mixture was then stirred (~1300 rpm) for 4 hours at room temperature (22 °C) followed by addition of 1200 ml de-ionised water and stirred for additional 15 minutes. Then the reaction mixture was allowed to stand for 15 min for setting of the precipitate. Standard laboratory filtration set-up and process (e.g. Buchner funnel) are used to separate as synthesized copper sintering powder from the reaction mixture followed by repeated washing with water, water-acetone mixture, and acetone, sequentially. The wet copper sintering powder as collected after filtration and washing are dried in a standard box oven at a temperature range of 40 °C in air. The dried copper sintering powder are packed in a container with vacuum sealing for further storage, characterization, handling, and use. Sample 6:

[0156] Copper(ll) acetate monohydrate (100 g) and triethanolamine (30 g) were added in a 5 L beaker and were dissolved in deionised water (1200 ml). The reaction mixture was then stirred for 15 mins to make homogeneous solution followed by drop-wise addition of ammonium hydroxide (90 g) with vigorous stirring. The reaction mixture was continued to stir for another 15 minutes, and the pH of this solution mixture was 8-8.1 . Next, hydrazine hydrate (250 g) was added dropwise via dropping funnel to this reaction mixture with vigorous stirring (~1500-1600 rpm). The resulting reaction mixture was then stirred (~1300 rpm) for 4 hours at room temperature (22 °C) followed by addition of 1200 ml de-ionised water and stirred for additional 15 minutes. Then the reaction mixture was allowed to stand for 15 min for setting of the precipitate. Standard laboratory filtration set-up and process (e.g. Buchner funnel) are used to separate as synthesized copper sintering powder from the reaction mixture followed by repeated washing with water, water-acetone mixture, and acetone, sequentially. The wet copper sintering powder as collected after filtration and washing are dried in a standard box oven at a temperature range of 40 °C in air. The dried copper sintering powder are packed in a container with vacuum sealing for further storage, characterization, handling, and use.

[0157] Sample 7:

[0158] Copper(ll) acetate monohydrate (100 g) and triethanolamine (30 g) were added in a 5 L beaker and were dissolved in deionised water (1200 ml). The reaction mixture was then stirred for 15 mins to make homogeneous solution followed by drop-wise addition of ammonium hydroxide (15 g) with vigorous stirring. The reaction mixture was continued to stir for another 15 minutes, and the pH of this solution mixture was 4.3-4.5. Next, hydrazine hydrate (200 g) was added dropwise via dropping funnel to this reaction mixture with vigorous stirring (~1500-1600 rpm). The resulting reaction mixture was then stirred (~1300 rpm) for 4 hours at room temperature (22 °C) followed by addition of 1200 ml deionised water and stirred for additional 15 minutes. Then the reaction mixture was allowed to stand for 15 min for setting of the precipitate. Standard laboratory filtration set-up and process (e.g. Buchner funnel) are used to separate as synthesized copper sintering powder from the reaction mixture followed by repeated washing with water, water-acetone mixture, and acetone, sequentially. The wet copper sintering powder as collected after filtration and washing are dried in a standard box oven at a temperature range of 40 °C in air. The dried copper sintering powder are packed in a container with vacuum sealing for further storage, characterization, handling, and use.

[0159] Sample 8:

[0160] Copper(ll) nitrate trihydrate (100 g) and triethanolamine (30 g) were added in a 5 L beaker and were dissolved in deionised water (1200 ml). The reaction mixture was then stirred for 15 mins to make homogeneous solution followed by dropwise addition of 3-(Dimethyl amino)-1 ,2-propanediol (DMPA) (24 g) with vigorous stirring. The reaction mixture was continued to stir for another 15 minutes, and the pH of this solution mixture was 4.1 -4.3. Next, hydrazine hydrate (200 g) was added dropwise via dropping funnel to this reaction mixture with vigorous stirring (~1500-1600 rpm). The resulting reaction mixture was then stirred (~1300 rpm) for 4 hours at room temperature (22 °C) followed by addition of 1200 ml deionised water and stirred for additional 15 minutes. Then the reaction mixture was allowed to stand for 15 min for setting of the precipitate. Standard laboratory filtration set-up and process (e.g. Buchner funnel) are used to separate as synthesized copper sintering powder from the reaction mixture followed by repeated washing with water, water-acetone mixture, and acetone, sequentially. The wet copper sintering powder as collected after filtration and washing are dried in a standard box oven at a temperature range of 40 °C in air. The dried copper sintering powder are packed in a container with vacuum sealing for further storage, characterization, handling, and use.

[0161] Sample 9:

[0162] Copper(ll) nitrate trihydrate (100 g) and triethanolamine (30 g) were added in a 5

[0163] L beaker and were dissolved in deionised water (1200 ml). The reaction mixture was then stirred for 15 mins to make homogeneous solution followed by dropwise addition of 3-(Dimethyl amino)-1 ,2-propanediol (DMPA) (48 g) with vigorous stirring. The reaction mixture was continued to stir for another 15 minutes, and the pH of this solution mixture was 4.6-4.9. Next, hydrazine hydrate (200 g) was added dropwise via dropping funnel to this reaction mixture with vigorous stirring (~1500-1600 rpm). The resulting reaction mixture was then stirred (~1300 rpm) for 4 hours at room temperature (22 °C) followed by addition of 1200 ml deionised water and stirred for additional 15 minutes. Then the reaction mixture was allowed to stand for 15 min for setting of the precipitate. Standard laboratory filtration set-up and process (e.g. Buchner funnel) are used to separate as synthesized copper sintering powder from the reaction mixture followed by repeated washing with water, water-acetone mixture, and acetone, sequentially. The wet copper sintering powder as collected after filtration and washing are dried in a standard box oven at a temperature range of 40 °C in air. The dried copper sintering powder are packed in a container with vacuum sealing for further storage, characterization, handling, and use.

[0164] Sample 10:

[0165] Copper(ll) nitrate trihydrate (100 g) and triethanolamine (30 g) were added in a 5 L beaker and were dissolved in deionised water (1200 ml). The reaction mixture was then stirred for 15 mins to make homogeneous solution followed by dropwise addition of 3-(Dimethyl amino)-1 ,2-propanediol (DMPA) (96 g) with vigorous stirring. The reaction mixture was continued to stir for another 15 minutes, and the pH of this solution mixture was 4.8-5. Next, hydrazine hydrate (200 g) was added dropwise via dropping funnel to this reaction mixture with vigorous stirring (~1500-1600 rpm). The resulting reaction mixture was then stirred (~1300 rpm) for 4 hours at room temperature (22 °C) followed by addition of 1200 ml deionised water and stirred for additional 15 minutes. Then the reaction mixture was allowed to stand for 15 min for setting of the precipitate. Standard laboratory filtration set-up and process (e.g. Buchner funnel) are used to separate as synthesized copper sintering powder from the reaction mixture followed by repeated washing with water, water-acetone mixture, and acetone, sequentially. The wet copper sintering powder as collected after filtration and washing are dried in a standard box oven at a temperature range of 40 °C in air. The dried copper sintering powder are packed in a container with vacuum sealing for further storage, characterization, handling, and use.

[0166] Sample 11:

[0167] Copper(ll) nitrate trihydrate (100 g) and triethanolamine (30 g) were added in a 5 L beaker and were dissolved in deionised water (1200 ml). The reaction mixture was then stirred for 15 mins to make homogeneous solution followed by dropwise addition of 3-(Dimethyl amino)-1 ,2-propanediol (DMPA) (48 g) with vigorous stirring. The reaction mixture was continued to stir for another 15 minutes followed by drop-wise addition of ammonium hydroxide (10.5 g) with vigorous stirring. The reaction mixture was continued to stir for another 15 minutes, and the pH of this solution mixture was 5.1 -5.3. Next, hydrazine hydrate (200 g) was added dropwise via dropping funnel to this reaction mixture with vigorous stirring (~1500-1600 rpm). The resulting reaction mixture was then stirred (~1300 rpm) for 4 hours at room temperature (22 °C) followed by addition of 1200 ml deionised water and stirred for additional 15 minutes. Then the reaction mixture was allowed to stand for 15 min for setting of the precipitate. Standard laboratory filtration set-up and process (e.g. Buchner funnel) are used to separate as synthesized copper sintering powder from the reaction mixture followed by repeated washing with water, water-acetone mixture, and acetone, sequentially. The wet copper sintering powder as collected after filtration and washing are dried in a standard box oven at a temperature range of 40 °C in air. The dried copper sintering powder are packed in a container with vacuum sealing for further storage, characterization, handling, and use.

[0168] Sample 12:

[0169] Copper (II) acetate monohydrate (100 g) and triethanolamine (30 g) were added in a 5 L beaker and were dissolved in deionised water (1200 ml). The reaction mixture was then stirred for 15 mins to make homogeneous solution followed by drop-wise addition of 3-(Dimethyl amino)-1 ,2-propanediol (DMPA) (48 g) with vigorous stirring. The reaction mixture was continued to stir for another 15 minutes, and the pH of this solution mixture was 5-5.2. Next, hydrazine hydrate (200 g) was added dropwise via dropping funnel to this reaction mixture with vigorous stirring (~1500-1600 rpm). The resulting reaction mixture was then stirred (~1300 rpm) for 4 hours at room temperature (22 °C) followed by addition of 1200 ml de-ionised water and stirred for additional 15 minutes. Then the reaction mixture was allowed to stand for 15 min for setting of the precipitate. Standard laboratory filtration set-up and process (e.g. Buchner funnel) are used to separate as synthesized copper sintering powder from the reaction mixture followed by repeated washing with water, water-acetone mixture, and acetone, sequentially. The wet copper sintering powder as collected after filtration and washing are dried in a standard box oven at a temperature range of 40 °C in air. The dried copper sintering powder are packed in a container with vacuum sealing for further storage, characterization, handling, and use.

[0170] Sample 13:

[0171] Copper(ll) nitrate trihydrate (100 g) and triethanolamine (30 g) were added in a 5 L beaker and were dissolved in deionised water (1200 ml). The reaction mixture was then stirred for 15 mins to make homogeneous solution followed by dropwise addition of ammonium hydroxide (90 g) with vigorous stirring. The reaction mixture was continued to stir for another 15 minutes, and the pH of this solution mixture was 8-8.1 . Next, hydrazine hydrate (250 g) was added dropwise via dropping funnel to this reaction mixture with vigorous stirring (~1500-1600 rpm). The resulting reaction mixture was then stirred (~1300 rpm) for 4 hours at room temperature (22 °C) followed by addition of 1200 ml de-ionised water and stirred for additional 15 minutes. Then the reaction mixture was allowed to stand for 15 min for setting of the precipitate. Standard laboratory filtration set-up and process (e.g. Buchner funnel) are used to separate as synthesized copper sintering powder from the reaction mixture followed by repeated washing with water, water-acetone mixture, and acetone, sequentially. The wet copper sintering powder as collected after filtration and washing are dried in a standard box oven at a temperature range of 40 °C in air. The dried copper sintering powder are packed in a container with vacuum sealing for further storage, characterization, handling, and use.

[0172] Sample 14:

[0173] Copper(ll) acetate monohydrate (100 g) and triethanolamine (20 g) were added in a 5 L beaker and were dissolved in deionised water (1200 ml). The reaction mixture was then stirred for 15 mins to make homogeneous solution followed by drop-wise addition of sodium hydroxide (20 g) dissolved in de-ionised water (200 ml) with vigorous stirring. The reaction mixture was continued to stir for another 15 minutes, and the pH of this solution mixture was 7.2-7.5. Next, hydrazine hydrate (200 g) was added dropwise via dropping funnel to this reaction mixture with vigorous stirring (~1500-1600 rpm). The resulting reaction mixture was then stirred (~1300 rpm) for 4 hours at room temperature (22 °C) followed by addition of 1200 ml de-ionised water and stirred for additional 15 minutes. Then the reaction mixture was allowed to stand for 15 min for setting of the precipitate. Standard laboratory filtration set-up and process (e.g. Buchner funnel) are used to separate as synthesized copper sintering powder from the reaction mixture followed by repeated washing with water, water-acetone mixture, and acetone, sequentially. The wet copper sintering powder as collected after filtration and washing are dried in a standard box oven at a temperature range of 40 °C in air. The dried copper sintering powder are packed in a container with vacuum sealing for further storage, characterization, handling, and use.

[0174] Sample 15:

[0175] Copper(ll) acetate monohydrate (100 g) and triethanolamine (20 g) were added in a 5 L beaker and were dissolved in deionised water (1200 ml). The reaction mixture was then stirred for 15 mins to make homogeneous solution followed by drop-wise addition of sodium hydroxide (20 g) dissolved in de-ionised water (200 ml) with vigorous stirring. The reaction mixture was continued to stir for another 15 minutes, and the pH of this solution mixture was 7.2-7.5. The reaction mixture was then stirred for 15 mins to make homogeneous solution followed by slow addition of polyvinyl pyrrolidone (PVP 10 KDa; 28 g) dissolved in water with stirring. The reaction mixture was continued to stir for another 15 minutes. Next, hydrazine hydrate (250 g) was added dropwise via dropping funnel to this reaction mixture with vigorous stirring (~1500-1600 rpm). The resulting reaction mixture was then stirred (~1300 rpm) for 4 hours at room temperature (22 °C) followed by addition of 1200 ml de-ionised water and stirred for additional 15 minutes. Then the reaction mixture was allowed to stand for 15 min for setting of the precipitate. Standard laboratory filtration set-up and process (e.g. Buchner funnel) are used to separate as synthesized copper sintering powder from the reaction mixture followed by repeated washing with water, water-acetone mixture, and acetone, sequentially. The wet copper sintering powder as collected after filtration and washing are dried in a standard box oven at a temperature range of 40 °C in air. The dried copper sintering powder are packed in a container with vacuum sealing for further storage, characterization, handling, and use.

[0176] Sample 16:

[0177] Copper(ll) acetate monohydrate (100 g) and triethanolamine (20 g) were added in a 5 L beaker and were dissolved in deionised water (1200 ml). The reaction mixture was then stirred for 15 mins to make homogeneous solution followed by drop-wise addition of sodium hydroxide (20 g) dissolved in de-ionised water (200 ml) with vigorous stirring. The reaction mixture was continued to stir for another 15 minutes, and the pH of this solution mixture was 7.2-7.5. The reaction mixture was then stirred for 15 mins to make homogeneous solution followed by slow addition of polyvinyl pyrrolidone (PVP 10 KDa; 56 g) dissolved in water with stirring. The reaction mixture was continued to stir for another 15 minutes. Next, hydrazine hydrate (250 g) was added dropwise via dropping funnel to this reaction mixture with vigorous stirring (~1500-1600 rpm). The resulting reaction mixture was then stirred (~1300 rpm) for 4 hours at room temperature (22 °C) followed by addition of 1200 ml de-ionised water and stirred for additional 15 minutes. Then the reaction mixture was allowed to stand for 15 min for setting of the precipitate. Standard laboratory filtration set-up and process (e.g. Buchner funnel) are used to separate as synthesized copper sintering powder from the reaction mixture followed by repeated washing with water, water-acetone mixture, and acetone, sequentially. The wet copper sintering powder as collected after filtration and washing are dried in a standard box oven at a temperature range of 40 °C in air. The dried copper sintering powder are packed in a container with vacuum sealing for further storage, characterization, handling, and use.

[0178] Sample 17:

[0179] Copper(ll) acetate monohydrate (100 g) and triethanolamine (20 g) were added in a 5 L beaker and were dissolved in deionised water (1200 ml). The reaction mixture was then stirred for 15 mins to make homogeneous solution followed by drop-wise addition of sodium hydroxide (20 g) dissolved in de-ionised water (200 ml) with vigorous stirring. The reaction mixture was continued to stir for another 15 minutes, and the pH of this solution mixture was 7.2-7.5. The reaction mixture was then stirred for 15 mins to make homogeneous solution followed by slow addition of polyvinyl pyrrolidone (PVP 10 KDa; 84 g) dissolved in water with stirring. The reaction mixture was continued to stir for another 15 minutes. Next, hydrazine hydrate (250 g) was added dropwise via dropping funnel to this reaction mixture with vigorous stirring (~1500-1600 rpm). The resulting reaction mixture was then stirred (~1300 rpm) for 4 hours at room temperature (22 °C) followed by addition of 1200 ml de-ionised water and stirred for additional 15 minutes. Then the reaction mixture was allowed to stand for 15 min for setting of the precipitate. Standard laboratory filtration set-up and process (e.g. Buchner funnel) are used to separate as synthesized copper sintering powder from the reaction mixture followed by repeated washing with water, water-acetone mixture, and acetone, sequentially. The wet copper sintering powder as collected after filtration and washing are dried in a standard box oven at a temperature range of 40 °C in air. The dried copper sintering powder are packed in a container with vacuum sealing for further storage, characterization, handling, and use.

[0180] Sample 18:

[0181] Copper(ll) acetate monohydrate (100 g) and triethanolamine (20 g) were added in a 5 L beaker and were dissolved in deionised water (1200 ml). The reaction mixture was then stirred for 15 mins to make homogeneous solution followed by drop-wise addition of sodium hydroxide (20 g) dissolved in de-ionised water (200 ml) with vigorous stirring. The reaction mixture was continued to stir for another 15 minutes, and the pH of this solution mixture was 7.2-7.5. The reaction mixture was then stirred for 15 mins to make homogeneous solution followed by slow addition of polyvinyl pyrrolidone (PVP 30 KDa; 56 g) dissolved in water with stirring. The reaction mixture was continued to stir for another 15 minutes. Next, hydrazine hydrate (250 g) was added dropwise via dropping funnel to this reaction mixture with vigorous stirring (~1500-1600 rpm). The resulting reaction mixture was then stirred (~1300 rpm) for 4 hours at room temperature (22 °C) followed by addition of 1200 ml de-ionised water and stirred for additional 15 minutes. Then the reaction mixture was allowed to stand for 15 min for setting of the precipitate. Standard laboratory filtration set-up and process (e.g. Buchner funnel) are used to separate as synthesized copper sintering powder from the reaction mixture followed by repeated washing with water, water-acetone mixture, and acetone, sequentially. The wet copper sintering powder as collected after filtration and washing are dried in a standard box oven at a temperature range of 40 °C in air. The dried copper sintering powder are packed in a container with vacuum sealing for further storage, characterization, handling, and use.

[0182] Sample 19: Copper(ll) acetate monohydrate (100 g) and triethanolamine (20 g) were added in a 5 L beaker and were dissolved in deionised water (1200 ml). The reaction mixture was then stirred for 15 mins to make homogeneous solution followed by drop-wise addition of sodium hydroxide (20 g) dissolved in de-ionised water (200 ml) with vigorous stirring. The reaction mixture was continued to stir for another 15 minutes, and the pH of this solution mixture was 7.2-7.5. The reaction mixture was then stirred for 15 mins to make homogeneous solution followed by slow addition of polyvinyl pyrrolidone (PVP 30 KDa; 28 g) dissolved in water with stirring. The reaction mixture was continued to stir for another 15 minutes. Next, hydrazine hydrate (250 g) was added dropwise via dropping funnel to this reaction mixture with vigorous stirring (~1500-1600 rpm). The resulting reaction mixture was then stirred (~1300 rpm) for 4 hours at room temperature (22 °C) followed by addition of 1200 ml de-ionised water and stirred for additional 15 minutes. Then the reaction mixture was allowed to stand for 15 min for setting of the precipitate. Standard laboratory filtration set-up and process (e.g. Buchner funnel) are used to separate as synthesized copper sintering powder from the reaction mixture followed by repeated washing with water, water-acetone mixture, and acetone, sequentially. The wet copper sintering powder as collected after filtration and washing are dried in a standard box oven at a temperature range of 40 °C in air. The dried copper sintering powder are packed in a container with vacuum sealing for further storage, characterization, handling, and use.

[0183] Sample 20:

[0184] Copper (II) nitrate trihydrate (100 g) and triethanolamine (20 g) were added in a 5 L beaker and were dissolved in deionised water (1200 ml). The reaction mixture was then stirred for 15 mins to make homogeneous solution followed by dropwise addition of sodium hydroxide (20 g) dissolved in de-ionised water (200 ml) with vigorous stirring. The reaction mixture was continued to stir for another 15 minutes, and the pH of this solution mixture was 7.2-7.5. The reaction mixture was then stirred for 15 mins to make homogeneous solution followed by slow addition of polyvinyl pyrrolidone (PVP 30 KDa; 56 g) dissolved in water with stirring. The reaction mixture was continued to stir for another 15 minutes. Next, hydrazine hydrate (250 g) was added dropwise via dropping funnel to this reaction mixture with vigorous stirring (~1500-1600 rpm). The resulting reaction mixture was then stirred (~1300 rpm) for 4 hours at room temperature (22 °C) followed by addition of 1200 ml de-ionised water and stirred for additional 15 minutes. Then the reaction mixture was allowed to stand for 15 min for setting of the precipitate. Standard laboratory filtration set-up and process (e.g. Buchner funnel) are used to separate as synthesized copper sintering powder from the reaction mixture followed by repeated washing with water, water-acetone mixture, and acetone, sequentially. The wet copper sintering powder as collected after filtration and washing are dried in a standard box oven at a temperature range of 40 °C in air. The dried copper sintering powder are packed in a container with vacuum sealing for further storage, characterization, handling, and use. Sample 21:

[0185] Copper (II) nitrate trihydrate (100 g) and triethanolamine (20 g) were added in a 5 L beaker and were dissolved in deionised water (1200 ml). The reaction mixture was then stirred for 15 mins to make homogeneous solution followed by dropwise addition of sodium hydroxide (20 g) dissolved in de-ionised water (200 ml) with vigorous stirring. The reaction mixture was continued to stir for another 15 minutes, and the pH of this solution mixture was 7.2-7.5. The reaction mixture was then stirred for 15 mins to make homogeneous solution followed by slow addition of polyvinyl pyrrolidone (PVP 10 KDa; 56 g) dissolved in water with stirring. The reaction mixture was continued to stir for another 15 minutes. Next, hydrazine hydrate (250 g) was added dropwise via dropping funnel to this reaction mixture with vigorous stirring (~1500-1600 rpm). The resulting reaction mixture was then stirred (~1300 rpm) for 4 hours at room temperature (22 °C) followed by addition of 1200 ml de-ionised water and stirred for additional 15 minutes. Then the reaction mixture was allowed to stand for 15 min for setting of the precipitate. Standard laboratory filtration set-up and process (e.g. Buchner funnel) are used to separate as synthesized copper sintering powder from the reaction mixture followed by repeated washing with water, water-acetone mixture, and acetone, sequentially. The wet copper sintering powder as collected after filtration and washing are dried in a standard box oven at a temperature range of 40 °C in air. The dried copper sintering powder are packed in a container with vacuum sealing for further storage, characterization, handling, and use.

[0186] Sample 22:

[0187] Copper (II) acetate monohydrate (100 g) and triethanolamine (20 g) were added in a 5 L beaker and were dissolved in deionised water (1200 ml). The reaction mixture was then stirred for 15 mins to make homogeneous solution followed by drop-wise addition of sodium hydroxide (20 g) dissolved in de-ionised water (200 ml) with vigorous stirring. The reaction mixture was continued to stir for another 15 minutes, and the pH of this solution mixture was 7.2-7.5. The reaction mixture was then stirred for 15 mins to make homogeneous solution followed by slow addition of polyvinyl pyrrolidone (PVP 360 KDa; 56 g) dissolved in water with stirring. The reaction mixture was continued to stir for another 15 minutes. Next, hydrazine hydrate (250 g) was added dropwise via dropping funnel to this reaction mixture with vigorous stirring (~1500-1600 rpm). The resulting reaction mixture was then stirred (~1300 rpm) for 4 hours at room temperature (22 °C) followed by addition of 1200 ml de-ionised water and stirred for additional 15 minutes. Then the reaction mixture was allowed to stand for 15 min for setting of the precipitate. Standard laboratory filtration set-up and process (e.g. Buchner funnel) are used to separate as synthesized copper sintering powder from the reaction mixture followed by repeated washing with water, water-acetone mixture, and acetone, sequentially. The wet copper sintering powder as collected after filtration and washing are dried in a standard box oven at a temperature range of 40 °C in air. The dried copper sintering powder are packed in a container with vacuum sealing for further storage, characterization, handling, and use.

[0188] Characterisation of samples

[0189] Details of various characterization techniques as well as detailed characterization results of samples 1-22 are summarized below:

[0190] (i) Instrument details: a. JEOL JSM-7800F prime Field Emission Scanning Electron microscope (FE-SEM) was used for the morphological investigations and particle size measurements. b. The surface microstructure was investigated by Transmission Electron Microscopy (TEM, T2O ST) operated at 200 KV. c. The powder X-Ray Diffraction Patterns (p-XRD) of samples were collected by using Rigaku SmartLab X-Ray diffractometer operating at 40 kV and 30 mA with Cu Ka radiation (A= 0.15406 nm). d. Particle Size Distribution (PSD) was measured by Dynamic Light Scattering technique (Microtrac, Model Name: Nanotrac ULTRA™ ). e. X-ray Photo Electron Spectroscopic (XPS) measurements were carried out using Thermo Fisher, K-Alpha instrument, where monochromatic Al Ka X- ray used. f. Thermo-gravimetric Analyser (TGA) PerkinElmer TGA 8000 instrument was used to quantify the organic cappants as well as adsorbed volatile species in the sample. Since, copper is very prone to oxidation, TGA measurements were carried out in a inert atmosphere (N2 atmosphere).

[0191] (ii) Sample preparation methods: a. FE-SEM samples were prepared by sprinkling copper sintering powders on a carbon tape or by drop casting of copper sintering powders dispersed in ethanol on a Si-wafer followed by drying at 45 °C. b. TEM samples were prepared by drop casting of copper sintering powders dispersed in ethanol (5 x10’5mg / l) on a carbon coated copper TEM grid followed by drying at 45 °C in an oven and overnight drying in vacuum at room temperature. c. DLS analyses were carried out using very diluted well dispersed copper sintering powders in ethanol as prepared using probe sonication. d. XPS analyses were carried out using samples were prepared by drop casting of copper sintering powders dispersed in ethanol (5 x10-5 mg / l) on an ethanol on a Si-wafer followed by drying at 45°C.

[0192] (iii) Morphological characterization and particle size distribution measurements using FE-SEM, TEM and DLS:

[0193] Field Emission Scanning Electron Microscope (FE-SEM) and Transmission Electron Microscope (TEM) were used for the morphology characterization of various copper sintering powders. For example, Figure 1 summarize the FE-SEM micrographs of uncapped (Figure 1 (a-e), Samples 1-5, respectively) and amino alcohol (DMPA) capped (Figure 1 (f-i), Samples 8-10 and 12, respectively) copper sintering powders. Further, Figure 2(a) shows the FE-SEM micrographs of uncapped (Sample 14), and Figure 2(b-i) shows the FE-SEM micrographs of PVP capped copper sintering powders (Samples 15-22, respectively). Figure 3 shows the physical appearance of: (a) Sample 4, (b) Sample 5, (c) Sample 14, (d) Sample 9, (e) Sample 10, and (f) Sample 16. Table 1 below summarizes the physical appearance as well as average particle size (FE-SEM analysis) of Sample 1-22.

[0194]

[0195] Table 1

[0196] Figure 4(a and b) shows TEM micrographs of uncapped copper sintering powders (Samples 4 and 5, respectively), while Figure 4(c and d) shows the DMPA capped copper sintering powder (Sample 9). Further, Table 2 summarize average particle size (TEM analysis) of Samples 4, 5 and 9.

[0197] Table 2 Further, DLS technique was used to measure the particle size distribution (PSD), as summarized in Table 3, of Example 1 , 3-5, 8-10, 12 and 14-22.

[0198] Table 3

[0199] Based on FE-SEM, TEM and DLS characterization revealed the following characteristics of copper sintering powders, when organic cappants, such as PVP and DMPA are used during synthesis: a. Mean Particle Size (d50): 100 - 150 nm as measured & characterized using DLS technique, b. Shape: Mixture of facet shaped (“faceted particles”) along with both regular and irregular shaped nanoparticles are observed and characterized using FE-SEM, c. Size Distribution: o Significant quantities of particles with size <50 nm are present as measured & characterized using TEM, o Larger / longer sized particles up-to 450-800 nm are present as measured & characterized using DLS, TEM & FE-SEM, o DLS characterization shows the particle size distribution to be:

[0200] ■ d10: 90 - 110 nm

[0201] ■ d50: 130 - 160 nm

[0202] ■ d90: 270 - 470 nm

[0203] Further, based on FE-SEM, TEM and DLS characterization revealed the following characteristics of copper sintering powders, when no organic cappants, are used during synthesis: a. Mean Particle Size (d50): 200 -500 nm as measured & characterized using DLS technique, b. Shape: Mixture of facet shaped (“faceted particles”) along with both regular and irregular shaped nanoparticles are observed and characterized using FE-SEM, c. Size Distribution: o Significant quantities of particles with size 150 - 250 nm are present as measured & characterized using TEM, o Larger / longer sized particles up-to 1000-1800 nm are present as measured & characterized using DLS, TEM & FE-SEM, o DLS characterization shows the particle size distribution to be:

[0204] ■ d10: 120-160 nm

[0205] ■ d50: 200 - 700 nm d90: 1000 - 1800 nm

[0206] (iv) X-Ray diffraction (p-XRD) characterization:

[0207] To evaluate the crystallinity of copper sintering powders, p-XRD analysis was performed. Figure 5 shows the p-XRD pattern of various copper sintering powders (Sample 4, Sample 5, Sample 9 and Sample 14). The p-XRD diffraction patterns of copper sintering powders are also compared with the standard (PDF data) p-XRD data of Cu(0) (PDF 01-071-4609), Cu2O (PDF 01-073-6237) and CuO (PDF 01-078-0428) and are shown in Figure 5. p-XRD patterns of copper sintering powders as shown in Figure 5 can be indexed as face centered cubic (fee) crystal structure of copper, where intense peaks due to Cu (111 ), Cu (200) and Cu (220) planes are clearly visible at (29 value) 43.3°, 50.4°, 74.1 °, respectively. A minor peak for CuxO at 36.5° is also observed in all the samples, while no other significant oxides peaks are visible. This indicates that a very small quantities of copper oxides (Cu2O or CuO) can present as a passivating layer for copper sintering powders.

[0208] (v) XPS characterization:

[0209] To evaluate the chemical composition, XPS characterization of copper sintering powders was performed. For example, Figure 6 shows the XPS survey scan of Sample 14 [Figure 6(a)], Sample 4 [Figure 6(b)], Sample 5 [Figure 6(c)] and Sample 9 [Figure 6(d)], The presence of carbon, copper, oxygen, and nitrogen species were confirmed in all samples. However, N1s peak intensity isn very weak for Sample 14 [Figure 6(a)] than the other samples, which indicates the presence of nitrogen is very less in Sample 14 as compared to other samples. Further, Figure 7 shows the high-resolution Cu2p spectra of Sample 14 [Figure 7(a)], Sample 4 [Figure 7(b)], Sample 5 [Figure 7(c)] and Sample 9 [Figure 7(d)], Since, different oxidation states of copper species appeared in different binding energies(eV), the deconvolution of high resolution Cu2p spectra helps to understand the presence of different oxidation states of copper [Cu(0), Cu(l) and Cu(ll)].

[0210] The Cu2p spectra for Sample 14, can be deconvoluted in two spectra as shown in Figure 7(a), which can be assigned the presence of Cu(0) and copper(l) oxide oxidation states. The peak intensity of Cu(0) is significantly stronger than the peak intensity copper(l) oxide, which signifies that the presence of Cu(0) is the dominant oxidation state in Example 14.

[0211] Furthermore, several literature reports confirms that the presence of satellite peaks in Cu2p spectrum [Figure 7(a)] can be assigned to the oxidation of copper (i.e. copper oxide species). Further, the intense satellite peak can be assigned due to Cu(ll) oxidation state, whereas the weak satellite peak can be assigned to Cu(l) oxidation state. Thus, the weak satellite peak shown in Figure 7(a) can be assigned to the presence of CU2O species in Sample 14. Thus, it can be concluded that a very thin shell of copper(l) oxide over copper(O) is present in Sample 14.

[0212] In a sharp contrast, the Cu2p spectra for Sample 4, Sample 5 and Sample 9 can be deconvoluted in three spectra as shown in Figure 7(b-d) and can be assigned as Cu(0), both copper(l) oxide and / or copper (II) oxides (CuxO) as well as copper(ll) hydroxides [Cu(OH)2] species.

[0213] For example, the deconvoluted Cu2p spectra of Sample 4 [Figure 7(b)] shows the presence of an intense satellite signals, which clearly signifies the presence of Cu(ll) oxides as the major species (as discussed earlier). However, the peak intensity of Cu(0) is weaker as compared to the peak intensity of copper(ll) oxide as well as copper(ll) hydroxides in Sample 4. Thus, it can be concluded that a very thick or strong shell of copper(ll) oxide as well as copper(ll) hydroxides over copper(O) is present in Sample 4. Further, the Cu2p spectra of Sample 5 and Sample 9 are shown in Figure 7(c- d). Interestingly, the peak intensity of Cu(0) are significantly stronger than the peak intensity of copper(l & II) oxide as well as of copper(ll) hydroxides in both Sample 5 and Sample 9. Thus, it can be concluded that a thin shell of copper(l&ll) oxide as well as copper(ll) hydroxides over copper(O) is present in Sample 5 and Sample 9.

[0214] (vi) Thickness of the oxide layer in copper sintering powders (depth profiling technique followed by simultaneous acquisition of XPS):

[0215] The thickness of the oxide layer in copper sintering powders was analysed using depth profiling technique followed by simultaneous acquisition of XPS data. Depth profiling technique uses an ion beam (monatomic) EX-06 source operated at 1000 V (source energy) to etch layers of the surface revealing sub-surface information. In this study, a total 157 nm etching was done in six steps and each etching step was carried out for 55 sec.

[0216] Firstly, a sample was mounted on the sample holder and a spectrum was recorded of the surface of the sample and the atomic composition was analysed. Next, the sample was etched by the ion beam over a square area (2 x 2 mm) of the sample and a spectrum was recorded of the surface of the sample and the atomic composition was analysed. Subsequently, this process, that is the etching of a square area (2 x 2 mm) of the sample using the ion beam followed by recording of the spectrum was repeated for another five times and the atomic composition was analysed after each steps.

[0217] Figure 8 shows the depth profiling (etch depth vs atomic % of carbon, oxygen, and copper) of different copper sintering powders: Sample 14 [Figure 8(a)], Sample 4 [Figure 8(b)], Sample 5 [Figure 8(c)] and Sample 9 [Figure 8(d)],

[0218] It can be clearly seen that with increasing the etch depth, the atomic % of oxygen decreases, while the atomic % copper increases in these samples. For example, the atomic % of oxygen are found to decrease from 40 atomic% to ~4, 16, 10 and 7 atomic % of oxygen as a function of etch depth from 0 to 158 nm for Sample 14 [Figure 8(a)], Sample 4 [Figure 8(b)], Sample 5 [Figure 8(c)] and Sample 9 [Figure 8(d)], respectively.

[0219] The thickness of the oxidized copper layer (e.g. CU2O, CuO or Cu(OH)2) surrounding copper(O) nanoparticles can be estimated by identifying the starting point (nm) of atomic % of oxygen starts saturating (as marked in Figure 8 to guide the eye). Further, the variation of atomic % of oxygen from these points do not show any significant changes as etch depth (nm) increases. For example, the thickness of the oxidized copper layer (e.g. CU2O, CuO or Cu(OH)2) surrounding copper(O) nanoparticles can be calculated as 26 nm, 52 nm, 52 nm, and 40 nm, respectively for Sample 14 [Figure 8(a)], Sample 4 [Figure 8(b)], Sample 5 [Figure 8(c)] and Sample 9 [Figure 8(d)], respectively. Thus, we can conclude that the oxidized copper thickness varies over the range~ 20 - 60 nm for the copper sintering powder samples measured.

[0220] (vii) Thermogravimetric analysis (TGA):

[0221] Figure 9 shows the weight-loss due to the decomposition of organic cappants as well as absorbed volatile species of various copper sintering powders as a function of temperature. For example, Figure 9(a) shows the weight-loss due to the decomposition of organic cappants as well as absorbed volatile species of Sample 4, Sample 5, Sample 9 and Sample 10.

[0222] Since, copper is very prone to oxidation, even a trace presence of oxygen (ppm level) can convert the metallic copper to copper oxide species above ~>150 °C. Thus, the weight gain in Figure 9(a) above 150 - 175 °C can be attributed due to the formation of copper oxide species.

[0223] 3-(dimethyl amino)-1 ,2-propanediol (DMPA) as well as any other volatile adsorbed species, such as solvents or triethanolamine, would be expected to be removed below ~< 170 °C, any weight loss in TGA analysis can be attributed to quantify the organic cappants as well as adsorbed volatile species in the sample. Thus, when no additional capping agents were added during the synthesis of Sample 4 and Sample 5 (uncapped copper sintering powders), the observed weight losses in TGA [Figure 9(a)] are found to be in the order of <0.2 % till ~150 °C. This weight can be attributed to the removal of volatile adsorbed species, while the copper oxide species formation can be seen as a sharp weight gain > 150 °C.

[0224] In comparison, when 3-(dimethyl amino)-1 ,2-propanediol (DMPA) was added as a capping agent during the synthesis of Sample 9 and Sample 10 (DMPA capped copper sintering powders), the observed weight losses in TGA [Figure 9(a)], are found to be in the order of ~<0.6 % till ~180 °C. This weight can be attributed to the removal of DMPA as well as volatile adsorbed species, while the copper oxide species formation can be seen as a sharp weight gain > 180 °C.

[0225] Since, PVP is stable till 400-450 °C, to analyze the PVP capped copper sintering powders, a small quantity of Sample 16 and 17 samples were kept at 150 °C in a oven for 10 h. This low temperature heat treatment was used to ensure to convert all the metallic copper species to copper oxide species. This ensures that no additional oxidation will occur during TGA analysis, any weight loss can only be attributed to the removal of capped PVPs from the copper sintering powders. Interestingly, Figure 9(b) shows the weight-loss characteristics in the order of ~10-12% till ~500 °C due to the decomposition of PVP 10 KDa from various PVP capped copper sintering powders (Sample 16 and 17) as a function of temperature.

[0226] (viii) Discussion:

[0227] The combination of these reactant mixtures as well as their mole ratios are found to be highly effective in controlling the kinetics of the reaction that can produce copper sintering powder with desired advantages. The effect of the synthesis conditions (nature of cappant and / or pH) on the morphology and colour of the copper sintering powders are summarized below:

[0228] (a) Uncapped copper sintering powders:

[0229] • Following are the examples of uncapped copper sintering powders: Sample 1 , Sample 2, Sample s, Sample 4, Sample s, Sample 6, Sample 7, Sample 13 and Sample 14.

[0230] • When reaction medium pH is > 5.2: These copper sintering powders are formed as deep brown to black coloured, irregular shaped, hard in nature, porous, bigger in size (0.7-2) pm, except Sample 14.

[0231] • When reaction medium pH is < 5.2: These copper sintering powders are formed as brown to deep brown-coloured, hard in nature, irregular shaped (spherical, plate, rod), not faceted, bigger in size (0.2-1 .3 pm). These particles contain higher oxides and higher particle size distribution (PSD) than desired limit.

[0232] (b) 3-(dimethyl amino)-1 ,2-propanediol (DMPA) capped copper sintering powders:

[0233] • Following are the examples of 3-(dimethyl amino)-1 ,2-propanediol (DMPA) capped copper sintering powders: Sample 8, Sample 9, Sample 10, Sample 11 and Sample 12.

[0234] • These copper sintering powders are formed as nice copper like colour (pinkish to light brown), nicely faceted, soft in nature, smaller in size ((0.1 to 0.8 pm), some are smaller than 0.05 pm).

[0235] • These particles contain lower surface oxides and desired PSD.

[0236] (c) Water-soluble polymer capped (PVP) copper sintering powders:

[0237] • Following are the examples of water-soluble polymer capped (PVP) capped copper sintering powders: Sample 15, Sample 16, Sample 17, Sample 18, Sample 19, Sample 20, Sample 21 and Sample 22. • These copper sintering powders are formed as deep brown to black coloured particles, spherical but not faceted, soft in nature, very smaller in size 0.05 to 0.3 pm.

[0238] • These particles contain higher surface oxides and very low and narrow PSD.

[0239] Sintering applications of copper sintering powders:

[0240] Free-standing, sintered copper foils were prepared by using a stencil printable, copper nanoparticle paste, where the copper sintering powders were used as the key ingredient to demonstrate the sintering activity. Electrical resistivities, densities, and thermal conductivities of such free-standing, sintered copper foils were measured as a function of applied sintering pressure. Microstructures of such free-standing, sintered copper foils were analyzed using FE-SEM to understand the densification as a function of applied sintering pressure.

[0241] (a) Preparation of copper nanoparticle paste:

[0242] A stencil printable, copper nanoparticle paste was prepared by mixing copper sintering powder (e.g. Sample 9) with terpineol, an epoxy acrylate resin, an organic acid as an activator, and a dispersing additive and these ingredients were homogenized using a laboratory high-speed roll mill. The homogeneous copper nanoparticle paste was then stored in a refrigerator at 4-8 °C.

[0243] (b) Preparation of free-standing, sintered copper foils:

[0244] Firstly, the copper nanoparticle pastes were printed by a stencil on FR4 board. Secondly, the printed copper nanoparticle pastes were dried at 80 - 120 °C using a laboratory box oven. Thirdly, the printed and dried copper nanoparticle pastes were further sintered to prepare free-standing, sintered copper foils using a laboratory-press at different pressures (1 , 5, 10, 15, 20, 25 and 30 MPa) at 260 °C for 2 min. For example, Figure 10 shows optical images of free-standing, sintered copper foils prepared using a laboratory-press at different pressures (a) 10 MPa, (b) 20 MPa and (c) 30 MPa at 260 °C for 2 min.

[0245] (c) Electrical resistivity measurement of free-standing, sintered copper foils:

[0246] The electrical resistivities (surface resistances and volume resistivities) of various free-standing, sintered copper foils were measured using a four-probe method. Measurements were carried out using a Jandel (RM 3000+) four probe measurement system with 50 g constant probe pressure for better connection. For electrical measurements free-standing, sintered copper foils samples were mounted on a flat FR4-PCB board or on a glass plate for electrical resistivity measurements.

[0247] The thicknesses of free-standing, sintered copper foils were measured using a HEIDENHAIN film thickness measurement system.

[0248] Using the resistance value and dimensions of the film, the bulk resistivity (p), surface resistance (Rs) and electrical conductivity (C) were obtained using the formulas given below:

[0249] Bulk resistivity, p = Rx A / l, Surface resistance, Rs= R x W / l R=Resistance of the foil

[0250] A= width (W) x thickness (t) (of foil) 1= length of foil W=width of foil

[0251] Electrical conductivity (<J)= 1 / p

[0252] The electrical resistivities (surface resistances and volume resistivities) of various free-standing, sintered copper foils as prepared at different pressures: 1 , 5, 10, 15, 20, 25 and 30 MPa (260 °C for 2 min) were measured and compared with the electrical resistivity of bulk copper (Figure 11 and Table 4).

[0253] Table 4

[0254] For example, volume resistivities of free-standing, sintered copper foils, prepared using 1 and 5 MPa pressure, 260 °C temperature and 2 minutes, are 17 x 10’08Q.m and 4.1 x 1O’08Q.m, respectively. Further, volume resistivities of freestanding, sintered copper foils prepared using 10 to 30 MPa pressure, 260 °C temperature and 2 minutes are found to be within 3.6 x 10’08Q.m to 3.2 x 10’08Q.m.

[0255] Interestingly, volume resistivities of free-standing, sintered copper foils prepared using >10 MPa pressure and at 260 °C are found ~<2 times higher than the volume resistivity of bulk copper. This is possibly as pressure increases the coalescence of the copper sintering powder increases, resulting in increase in the degree of densification during sintering process.

[0256] (d) Thermal conductivity measurements of free-standing, sintered copper foils:

[0257] The in-plane thermal diffusivities (a) of the free-standing, sintered copper foils were measured using a Netzsch Laser Flash Apparatus (LFA-447). A special in-plane sample holder was used that directs the thermal energy along the sample giving the corresponding in-plane thermal diffusivity values (a). The samples were cut into an appropriate size and placed into a special stage and sample holder. For in-plane thermal diffusivity measurements, surfaces need to be rough; they must have low emissivity, hence free-standing, sintered copper foils were sprayed with graphite to deposit a very thin layer of graphite.

[0258] The thermal conductivity (K) is obtained from the equation given below:

[0259] K= paCp where p is the apparent mass density of the free-standing, sintered copper foils and CPis the specific heat capacity of the film. Specific heat capacity of copper is known to be 0.385 J.g’1.°C’1.

[0260] The apparent mass density (p) of each of the free-standing, sintered copper foils were calculated using the formula, p =m / v equation, where “p” is the apparent mass density, “m” is the mass and “v” is the volume. This apparent mass density is required to calculate thermal conductivity (K) as per the above equation.

[0261] To calculate apparent mass density (p), firstly each of the free-standing, sintered copper foils were cut into (I) 1 .2 cm X (w) 1 .2 cm. Secondly, the average thickness of each of the free-standing, sintered copper foils were calculated by measuring the thicknesses (t) at nine different positions. Thirdly, the volume (v) of each of the free-standing, sintered copper foils were calculated by multiplying length (I), width (w), and thickness (t). Finally, the weight (m) of each of the freestanding, sintered copper foils were measured using a standard laboratory balance.

[0262] Table 5 summarizes typical apparent mass densities (p), thermal diffusivities (a) and thermal conductivities (K) values of 99.5 % pure commercial copper foil, POCO graphite as a standard as well as various free-standing, sintered copper foils foils prepared at different processing conditions (30, 20 and 10 MPa pressure respectively, at 260 °C for 2 min).

[0263] Table 5

[0264] The measured thermal diffusivities (a) and thermal conductivities (K) of values pure commercial copper foil and POCO graphite are found to be well in agreemenet with the reported literature. Interestingly, the densities, thermal diffusivities and thermal conductivities of free-standing, sintered copper foils are found to increase as a function of increasing sintering pressure, indicates the degree of densification increases as a function of sintering pressure. The thermal conductivities of the free-standing, sintered copper foils as disclose in this invention show > 60 W.rrr1.K’1.

[0265] (e) Microstructure analysis of free-standing, sintered copper foils:

[0266] Figure 12(a-e) shows the FE-SEM images of the microstructures of various cross-sectioned free-standing, sintered copper foils sintered at 30 MPa, 20 MPa, 10 MPa, 5 MPa and 1 MPa, respectively. The cross-sectional analysis of freestanding, sintered copper foils show very good densification, especially when pressure is > 10 MPa. The densification of copper sintering powder increases considerably with the increase in sintering pressure. For example, the diffusion and densification of samples prepared using 30 MPa is superior to the samples prepared using 1 MPa or 10 MPa. Furthermore, the highest densification is observed for the sintering pressure of 30 MPa, while the lowest densification is evident for the sintering pressure of 1 MPa. The higher pressure facilitates the neck growth, grain growth, shrinkage and eliminate the pores to create continuous layer of metal. Hence it is evident that the observed superior electrical resistivities as well as thermal conductivities of free-standing, sintered copper foils are resulted due to the densification of copper sintering powder during sintering.

[0267] The combination of the above characteristics is one of the key aspects of copper sintering powder. These features allow to achieve a very close-pack and high- density sintered structure using moderate to low sintering conditions, such as temperature: < 300°C & pressure: <35 MPa.

[0268] These features will be useful to develop copper sintering inks for several practical applications, such for producing high-current carrying and solderable electronic circuits, joining of two metal pieces, attaching die or electronic components to various metalized substrates, such as DBC and wafers bonding.

[0269] The foregoing detailed description has been provided by way of explanation and illustration and is not intended to limit the scope of the appended claims. Many variations in the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art and remain within the scope of the appended claims and their equivalents.

Claims

Claims:1 . A method of forming a sintering powder comprising copper particles, the method comprising: providing an aqueous solution comprising copper ions, a base and a capping agent; contacting the solution with a reducing agent to provide the sintering powder; and recovering the sintering powder, wherein: prior to contacting the solution with the reducing agent, the solution has a pH of from 4 to 7; and the capping agent comprises an amino alcohol.

2. The method of claim 1 , wherein prior to contacting the solution with the reducing agent, the solution has a pH of from 4.1 to less than 7, preferably from 4.2 to 6.5, more preferably from 4.5 to 6, even more preferably from 4.8 to 5.5.

3. The method of claim 1 or claim 2, wherein the amino alcohol comprises two hydroxyl groups.

4. The method of any preceding claim, wherein the hydroxyl groups of the amino alcohol are situated on a single aliphatic chain.

5. The method of claim 1 , wherein the amino alcohol comprises two hydroxyl groups and both are situated on the same aliphatic chain.

6. The method of any preceding claim, wherein the amino alcohol comprises alkyl amino alcohol, preferably dialkyl amino alcohol.

7. The method of any preceding claim, wherein the amino alcohol comprises methyl amino alcohol, preferably dimethyl amino alcohol.

8. The method of any of claim 1 to 3, wherein the amino alcohol comprises di-hydrogen amino alcohol.

9. The method of any preceding claim, wherein the amino alcohol comprises one or more of 3-(dimethyl amino)-1 ,2-propanediol, 3-(amino)-1 ,2-propanediol, and 3-(methyl amino)-1 ,2-propanediol, preferably 3-(dimethyl amino)-1 ,2- propanediol.

10. The method of any preceding claim, wherein providing the aqueous solution comprising copper ions, a base and a capping agent comprises providing a solution comprising copper ions and a base, and then contacting the solution comprising copper ions and a base with a capping agent.11 . The method of any preceding claim, wherein the copper ions in the solution are provided in the form of a copper salt, preferably selected from one or more of copper(ll) nitrate, copper(ll) sulphate, copper(ll) acetate, copper(ll) formate and copper(ll) hydroxide, more preferably selected from one or both of copper(ll) nitrate and copper(ll) acetate, even more preferably copper(ll) nitrate.

12. The method of any preceding claim, wherein the base comprises triethanolamine solely or together with a hydroxide, preferably selected from one or more of sodium hydroxide, potassium hydroxide and ammonium hydroxide, more preferably ammonium hydroxide.

13. The method of any preceding claim, wherein the reducing agent comprises one or more of hydrazine, sodium borohydride and lithium borohydride, preferably hydrazine,14. The method of any preceding claim, wherein once the solution has been contacted with the reducing agent, the molar ratio of copper ions : base : cappingagent : reducing agent in the solution is from 0.2 to 1 : from 0.05 to 2 : from 0.2 to1.5 : from 1 to 10.

15. The method of any preceding claim, wherein the solution comprising copper ions, a base and a capping agent is contacted with the reducing agent for from 1 to 20 hours, preferably for from 2 to 10 hours, more preferably for from 3 to 5 hours.

16. The method of any preceding claim, wherein recovering the sintering powder comprises one or more of decanting the resulting solution from the sintering powder, washing the sintering powder and drying the sintering powder.

17. The method of any preceding claim, wherein the drying is carried out in a non-inert atmosphere or in an inert atmosphere, preferably air or nitrogen.

18. A sintering powder formed using the method of any preceding claim.

19. A sintering powder comprising copper particles, wherein at least some of the copper particles are capped with a capping agent comprising an amino alcohol.

20. The sintering powder of claim 19, wherein at least some of the copper particles are at least partially coated with a layer comprising copper oxide and copper hydroxide.21 . The sintering powder of claim 19 or claim 20, wherein the layer has a thickness of from 20 to 60 nm, preferably from 25 to 55 nm.

22. The sintering powder of any of claims 19 to 21 , wherein the amino alcohol comprises two or more hydroxyl groups.

23. The sintering powder of any of claims 19 to 22, wherein the amino alcohol comprises from 3 to 12 carbon atoms.

24. The sintering powder of any of claims 19 to 23, wherein the hydroxyl groups of the amino alcohol are situated on a single aliphatic chain.

25. The sintering powder of any of claims 19 to 24, wherein the amino alcohol comprises two hydroxyl groups and both are situated on the same aliphatic chain.

26. The sintering powder of any of claims 19 to 25, wherein the amino alcohol comprises alkyl amino alcohol, preferably dialkyl amino alcohol.

27. The sintering powder any of claims 19 to 26, wherein the amino alcohol comprises methyl amino alcohol, preferably dimethyl amino alcohol.

28. The sintering powder of any of claim 19 to 25, wherein the amino alcohol comprises di-hydrogen amino alcohol.

29. The sintering powder of any of claims 15 to 19, wherein the amino alcohol comprises one or more of 3-(dimethyl amino)-1 ,2-propanediol, 3-(amino)-1 ,2- propanediol, and 3-(methyl amino)-1 ,2-propanediol, preferably 3-(dimethyl amino)-1 ,2-propanediol.

30. The sintering powder of any of claims 19 to 29, wherein sintering powder comprises from 0.1 to 12 wt.% capping agent based on the total weight of the sintering powder, preferably up to 3 wt.%, more preferably up to 1 wt.%.31 . The sintering powder of any of claims 19 to 30, wherein the copper particles exhibit: a D90 of from 270 to 470 nm, and / or a D50 of from 130 to 160 nm , and / or a D10 of from 90 to 110 nm.

32. The sintering powder of any of claims 19 to 31 , wherein the copper particles exhibit: a D90 of from 270 to 470 nm, a D50 of from 130 to 160 nm , and a D10 of from 90 to 110 nm.

33. The sintering powder of any of claims 19 to 32, wherein the copper particles exhibit: a D90 of from 420 to 470 nm, a D50 of from 130 to 160 nm , and a D10 of from 90 to 110 nm.

34. The sintering powder of any of claims 19 to 32, wherein the copper particles exhibit: a D90 of from 270 to 350 nm, a D50 of from 130 to 160 nm , and a D10 of from 90 to 110 nm.

35. The sintering powder of any of claims 19 to 34, wherein the copper particles comprise facet-shaped particles.

36. A sintering powder comprising copper particles, wherein at least some of the copper particles are at least partially coated with a layer comprising copper oxide and copper hydroxide, the layer having a thickness of from 20 to 60 nm, preferably from 25 to 55 nm.

37. A sintering paste comprising the sintering powder of any of claim 18 to 36.

38. A sintering film comprising the sintering powder of any of claim 18 to 36.

39. A method of forming a joint between two or more work pieces, the method comprising: providing two or more work pieces to be joined, providing the sintering powder of any of claims 18 to 36 and / or the sintering paste of claim 37 and / or the sintering film of claim 38 in the vicinity of the two or more work pieces, heating the sintering powder and / or sintering paste and / or sintering film to at least partially sinter the copper particles.

40. The method of claim 39, wherein the two or more work pieces comprise a die and a substrate.41 . Use of the sintering powder of any of claims 18 to 36, and / or the sintering paste of claim 37, and / or the film of claim 38 in a method selected from: die attachment, component attachment, package attachment, electronic device attachment, wafer-to-wafer bonding, hermetic and near hermetic sealing, sintering films comprising a sintering powder and a binder formed in a film on a backing layer, dispensing and the production of interconnect lines.

42. Use of the sintering powder of any of claims 18 to 36, and / or the sintering paste of claim 37, and / or the film of claim 38 in the manufacture of additively printed structures, human-machine interfacing (HMI) electronic devices, capacitive switches, wire harness, solderable electronic circuits and devices, IME structures, structural I embedded electronics, large area electronics, lighting I luminaires, large area lighting, flexible and formable displays, embedded sensors, printed battery assembly, printed heaters, flexible circuits and cables, Cu coated dielectric materials, die-attachment applications, or package attach and component attach applications.

43. A sintered joint formed using the sintering powder of any of claims 18 to 36, and / or the sintering paste of claim 37, and / or the sintering film of claim 38 and / or the method of claim 39 or 40.

44. Use of an amino alcohol comprising two hydroxyl groups as a capping agent for copper particles.

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