WO2011126706A2

WO2011126706A2 - Printable materials and methods of manufacture thereof

Info

Publication number: WO2011126706A2
Application number: PCT/US2011/028979
Authority: WO
Inventors: Derek Alexander Graham; Bin Wei; Riju Davis
Original assignee: Henkel Corporation; Henkel Ag & Co.Kgaa
Priority date: 2010-04-09
Filing date: 2011-03-18
Publication date: 2011-10-13
Also published as: TW201211054A; WO2011126706A3

Abstract

The present invention relates to printable materials and methods of manufacture thereof. Of particular interest are printable materials comprising metals such as copper, gold and silver. Silver is the preferred metal. The present invention relates to printable compositions of such metals and in particular silver. The materials when printed onto a substrate can form conductive pathways. Any suitable method of printing is of interest including ink-jet printing.

Description

TITLE

Printable Materials and Methods of Manufacture Thereof

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to United States Provisional Patent Application No. 61/322,457 filed April 9, 2010, the contents of which is incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The present invention relates to printable materials and methods of manufacture thereof. Of particular interest are printable materials comprising metals such as copper, gold and silver. Silver is the preferred metal. The present invention relates to printable compositions of such metals and in particular silver. The materials when printed onto a substrate can form conductive pathways. Any suitable method of printing is of interest including ink-jet printing.

BACKGROUND TO THE INVENTION

[0003] The potential for printing technology to pattern conductive circuits for example electronic circuits is of great interest. Of particular interest is printing onto low cost substrates such as plastic. Such processes can provide a significantly lower cost of fabrication than conventional processes such as those common in the electronics industry. Particular applications include those in printed displays, RFID tags and photovoltaics.

[0004] The applications of the conductive patterns require highly functional materials such as inks that will meet the performance requirements of the application. Ideally the materials of interest for forming conductive pathways are also easily applied, for example by printing, and it is further desirable that the materials can be processed at low temperatures.

[0005] One of the key challenges is the development of a suitable low cost printable conductive material. The so-called coinage metals (copper, silver and gold) possess the highest electrical conductivities and thus have suitable conductivities to form conductive pathways. However gold is considered too expensive whilst although significantly cheaper, copper does not have suitable environmental stability. For these reasons silver is usually regarded as the material of choice for such applications.

[0006] For example silver nanoparticles have been employed to make functional inks which can be ink-jet printed and then transformed thermally to dense highly conductive silver films. The films are formed based on low temperature sintering behaviour of the very small metal particles (typically <100nm) where essentially the smaller particle the lower the sintering temperature. Resistivities of the order 1.5 x 10^'5 ohm.cm from 30-50 nm particles at sintering temperatures of 150°C have been reported. Other methods have achieved resistivities of 9 x 10"⁶ ohmxm but at a lower temperature of 120°C.

[0007] While compositions based on silver nanoparticles are useful there are a number of issues to be dealt with when preparing and utilising such materials. The synthesis of the particles themselves can be complex for example with a considerable number of complex separations. The processes tend to be wasteful in the consumption of other chemicals such as stabiliser and solvent. Small metallic nanoparticles are very difficult to keep col!oidally stable which necessitates the inclusion of a suitable surfactant stabiliser. During the sintering operation the surfactant must either be volatilised or at the very least be removed from the silver particle surface. This leads to a difficult balance between colloidal stability and ease of sintering. This balance becomes more difficult to maintain as the particle size becomes smaller especially at sizes at or smaller than about 20 nm.

[0008] An alternative approach to the nanoparticle approach is the use of a precursor material which is transformed, typically by decomposition, and after printing, into the metal material. In effect the physical change achieved by sintering is replaced by a chemical change. The chemical change may be induced by heating, for example sufficient heat to cause decomposition of the metal precursor material to form the metal. In such a case the decomposition temperature can be controlled by precursor reactivity.

[0009] There are a number of advantages. There is no requirement to make and stabilise nanoparticles. Typically the materials are liquids containing no solids so there is no need to worry about aggregation, settling or blockage during application for example during printing. As Newtonian liquids they are considerably easier to formulate for application, e.g. by printing such as ink-jet printing.

[0010] US Patent Publication 2006/001726 describes silver precursor materials which are reduced by a reducing agent to form the elemental silver metal. The precursor materials comprise silver nitrate and other silver materials as set out in Table 1 of that document. Copper formate, is mentioned as a possible additional component.

[0011] International (PCT) publication WO 2006/093398 describes the use of silver precursor systems based on carbamate complexes. The carbamate group provides a counter-ion in the from of the carbamate ion whilst the zwitterion form provides complexation allowing easy solubility in common solvents. The basic chemistry is shown below.

Λ ii /

n N— C - V II / *

NH^*- -R₄ N— C -O" Ag⁺ N - C -O- NH^*— R«

/ \ / \

R? /

R₅ n-1

+ X" NH⁴— R₄

n - 0.7 - 5.5

[0012) The so produced silver carbamate complexes are readily soluble in common solvents such as ethanol and can be ink jet printed to yield silver films with thicknesses in the range 100 ran to 1.5 mm. These films are said to have a resistivity of around 4 x 10^"6 ohm.cm when dried for a period of time from 10-30 minutes at 130-150°C. This represents a significant improvement over certain nanoparticle approaches.

(00131 The industry is ever more demanding on the processing temperature, ultimate resistivity and the time to achieve this, all of which are being driven ever lower. There is therefore a demand from industry to improve on one or more of temperature, time in formation of, and resistivity achieved in, the conductive pathway laid down.

SUMMARY OF THE INVENTION

[0014] The present invention provides compositions suitable for printing to form silver layers and processes for preparing those compositions.

[0015] The present invention provides a unique chemistry and process for the manufacture of silver based precursor inks. These can be readily applied by, e.g. spin coating or printing technologies, to a suitable substrate. They can then be thermally transformed to silver which attains excellent electrical properties at temperatures and for reaction times significantly lower than described in the prior art.

[0016) One particular end use application of the present invention is to provide for printing application of silver films. [0017) The technology of the present invention is suitable for printing onto many types of substrate including glass, plastics including polyesters such as PET (Polyethylene terephthalate) and tapes for use in the electronics industry including soldering tapes for example polyimide tapes such as those sold under the trade name apton tape by DuPont.

[0018) The technology of the present invention is suitable for many end-use applications including printing for example in applications as printable conductive inks, in ink-jet printing, in the manufacture of printed circuits, in the manufacture of flat panel displays and RFID devices.

[0019) The present invention provides a printable ink comprising:

(i) a liquid vehicle; and

(ii) a silver precursor which is a silver complex formed by reaction of at least one of silver formate or silver oxalate with a stabilising agent.

[0020] Compositions of the invention have been shown to demonstrate excellent properties when printed.

[0021) Desirably the silver precursor is present in an amount from 15 to 30 % by weight of the composition, for example 17 to 27% by weight of the composition desirably ideally 20 to 25% by weight of the composition.

[0022] Desirably the stabilising agent is selected from any suitable type including amines, oximes, hydrazones, guanidines, hydrazides, carbazones and combinations thereof.

[0023] In one embodiment the stabilising agent comprises amine. Suitable amines include for example primary, secondary amines and include cyclic amines and combinations thereof.

Suitable amines include aliphatic amines including C|-C|₆ amines including butyl amine and nonylamine, dibutyl amine, tributyl amine, diethylene amine, tetraethylene pentamine, octylamine and combinations thereof. Desirably the stabiliser is octylamine.

[0024] The amine may be an amino alcohol including C1-C16 amino alcohols. Examples include, ethanolamine, propanolamine including isopropanolamine and combinations thereof.

[0025] In one embodiment the stabilising agent comprises oxime. Suitable oximes include aldoximes and ketoximes and combinations thereof. Suitable oximes the oxime is an aldoxime or ketoxime optionally selected from including Ci-C^ aldoximes or ketoximes and combinations thereof. The oximes may include acetoneoxime, methylaldoxime, and methylethylketoxime and combinations thereof. [0026J Desirably the stabilising agent is present in an amount from 5 to 10% by weight of the ink composition.

|0027J Desirably the average nanoparticle size is the range from 5 nm to 10 nm.

[0028] Desirably the liquid medium comprises a polar solvent, including polar organic solvents and includes alcohols, such as ethyl alcohol, acetone and water and combinations thereof.

Desirably the liquid medium comprises ethyl alcohol.Typically the liquid medium forms 70 to 80 % by weight of the composition.

[0029J The composition may further comprise a conductivity-promoting component such as oxalic acid, formic acid and combinations thereof. It is to be noted however that such a component is not necessary as the conductivity achieved even without such a component may be close to that of bulk silver.

[0030] Where suitable, it will be appreciated that all optional and/or preferred features of one embodiment of the invention may be combined with optional and/or preferred features of another/other embodiment(s) of the invention.

[0031] The invention also relates to a process for manufacturing a printable ink comprising: a silver precursor which is a silver complex formed by reaction of at least one of silver formate or silver oxalate with a stabilising agent, in a suitable liquid medium.

[0032] The process of the invention may further comprise the step of reacting silver formate or silver oxalate with a stabilising agent to form the silver complex.

[0033] Desirably the liquid medium forms a liquid vehicle for printing the silver precursor onto a substrate. After reaction the amount of liquid of the liquid medium may be reduced to increase the silver precursor concentration. For example the liquid medium may be taken off, for example, drawn off by vacuum, desirably while being maintained at temperature of less than about 0 °C.

[0034] One specific process of the invention is a process for the manufacture of a silver complex formed by reaction of silver formate or oxalate with an amine comprising the step of: reacting the silver formate or oxalate with the amine in a suitable liquid medium at a temperature of less •than about 0 °C.

[0035] The invention also relates to a printer having a supply of printable ink, for example a reservoir of printable ink such as an ink cartridge, wherein the ink is as an ink of the present ^• invention. f0036J The invention further relates to an applied material including a printed material comprising the print product of a printable ink according to the present invention. The invention also relates to the print product of a printable ink of the present invention. The applied material may be treated, for example exposed to a fluid form of, a conductivity-promoting component. Desirably the conductivity-promoting component is an organic acid optionally selected from oxalic acid and formic acid and combinations thereof.

|0037J The invention further relates to a material which is formed by sintering of an ink composition of the present invention after application thereof to a substrate. Included within the present invention is a sintered material formed by ink applied to a substrate that has been treated with a conductivity-promoting component such as an acid. One suitable acid is formic acid. Desirably the material is sintered while being contacted with the conductivity-promoting component. For example the conductivity-promoting component may be in a liquid or gas phase including a vapour phase. For example sintering can take place where a substrate with ink applied thereto is sintering while in an enclosed atmosphere that has a gaseous form of the conductivity-promoting component. The conductivity-promoting component may be an organic acid optionally selected from oxalic acid and formic acid and combinations thereof.

[0038] The invention extends to a device having a conductive material applied thereto which conductive material is a product of an ink composition of the invention or a sintered material formed by the composition of the invention.

[0039J The invention further relates to a process for sintering an applied silver material which silver material is a silver complex formed by reaction of at least one of silver formate or silver oxalate with a stabilising agent which process includes the step of sintering the applied silver material while in contact with a conductivity-promoting component. The conductivity- promoting component is desirably an organic acid optionally selected from oxalic acid and formic acid and combinations thereof. The conductivity-promoting component may be in a liquid form for example in a liquid medium. Desirably the conductivity-promoting component is in a gaseous form for example in a gaseous medium.

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Additional features and advantages of the present invention are described in, and will be apparent from, the detailed description of the invention and from the drawings in which: [0041] Figure 1 is a schematic representation of a method for preparing a silver precursor (formate) solution according to the invention.

[0042] Figure 2 is an image of a modified rotary evaporator for solvent removal.

[0043] Figure 3 is a graph (Y axis is height in microns; X axis is scan range in microns) illustrating the measured profile and electrical properties of silver film prepared using silver formate and sintered at 100°C.

[0044] Figure 4 is a graph (Y axis is height in microns; X axis is scan range in microns) illustrating the measured profile and electrical properties of silver film prepared using silver formate and sintered at 130°C

[0045] Figure 5 is a graph (Y axis is height in microns; X axis is scan range in

microns)illustrating the measured profile and resistivity of a silver film prepared using silver formate and sintered at 100°C.

[0046] Figure 6 is a graph (Y axis is height in microns; X axis is scan range in microns) illustrating the measured profile and resistivity of a silver film prepared using silver formate and sintered at 130°C.

[0047] Figure 7 is a series of SEM (Scanning Electron Microscope) images of sectioned silver films on glass, top left 100°C for 1 minute, top right 100°C for 10 minutes, bottom left 130°C for 1 minute and bottom right 130°C for 10 minutes.

[0048] Figure 8 is a schematic representation of preparation of a silver oxalate ink.

DETAILED DESCRIPTION OF THE INVENTION

[0049] It should be readily apparent to one of ordinary skill in the art that the examples disclosed herein below represent generalised examples only, and that other arrangements and methods capable of reproducing the invention are possible and are embraced by the present invention.

[0050] We base our invention on well established chemistry of the Ag⁺ ion. It is well known that this readily complexes with N- donor ligands to yield compounds with coordination number in the range 2-4). In particular amines form linear 2 coordinate complexes of the type shown below:

[0051] Complexes are easily prepared by addition of the amine in the correct stoichiometry to suitable silver salt AgX in the required solvent.

Preparation of the Silver Precursor Solution

[0052] One aspect of the invention is to provide a desirable precursor chemistry and in these experiments a desirable amine precursor chemistry. In this the choice of a suitable silver salt is important. We have based our chemistry around silver formate Ag(HC0₂) which gives excellent results in providing the platform for the low temperature conversion to silver. Another example of a silver compound employed by the present invention is silver oxalate. These materials are not available commercially so we describe as part of the invention a synthetic method for their routine manufacture in high yield and purity.

[0053) Silver formate is prepared from silver nitrate and sodium formate in aqueous solution. The optimum stoichiometry is 1 :3 as follows,

3 NaHC0₂ + Ag Or*" AgHC0₂ + NaNO, + 2NaHC0₂

[0054] Silver formate is only slightly soluble in water and precipitates out of solution upon mixing. Silver formate is a metastable compound and decomposes to silver quite rapidly at any stoichiometry below 1 :2. Beyond 1 :3 no improvement in yield was observed. Yields are however substantially improved and decomposition limited (essentially to zero) by carrying the precipitation in the cold at or near 0°C. A typical preparation is as follows.

[0055] A solution of 1.71 g (10 mmol) of silver nitrate is dissolved in 10 ml of distilled water and placed in a domestic fridge or preferably an ice bath until it equilibrates. Similarly a solution of 3.4g of sodium formate (30 mmol) in 20 ml is also prepared. These are then mixed together whereupon a milky white precipitate forms. This is rapidly filtered and washed whilst cold (it helps to chill the filter funnel prior to use). Washing is carried out with 2 x 25 ml portions of ice cold water followed by 1 x 25 ml of ice cold ethanol. The yield of 80% is based on the initial quantity of silver nitrate. The remaining silver (i.e. not recovered as solid silver formate) remains in the filtrate as silver formate is slightly soluble even in cold water. This silver is easy to recover from the filtrate as simply leaving it for several hours (or overnight) results in its reduction to silver metal which can be readily separated washed and recycled.

[0056] Silver formate is light sensitive and will slowly turn grey then black on standing in air. This is exacerbated by higher temperature. For this reason the ethanol wet solid prepared above is not dried, it is preferably used immediately to make the precursor solution. It is however possible to store it (ethanol wet) in a domestic freezer for several weeks (in the dark) without any obvious decomposition.

[0057] Preparation of the precursor solution from the silver formate described above is not a trivial matter if significant decomposition is to be avoided. Complexation of silver formate with e.g. amine ligands results in a considerable exotherm. This can lead to premature decomposition of the precursor and clearly must be avoided. A procedure was developed to do just this, which avoids any decomposition. It is desirable to have a precursor solution with as high a silver loading as possible as this will deposit more silver down per print cycle and should lead ultimately to denser films upon solvent evaporation and thermal treatment. However in order to avoid decomposition due to the exothermic complex formation reaction, it is desirable to keep the system as dilute as possible to help dissipate the heat. Furthermore it is again desirable to run the process cold as again this mitigates the heat from the exotherm. The following procedure was developed which resolves these issues a schematic of which is shown below.

[0058] An open beaker containing lOg of ethanol is placed in a domestic freezer for several hours to equilibrate. The silver formate prepared using the procedure described above

(approximately 1.2g) is then added and mixed thoroughly to disperse it. This is then placed back in the freezer and left for 1 hour to equilibrate. At the same time a quantity (2-3g) of butylamine is placed in a separate beaker and allowed to equilibrate. To the alcohol slurry is then added 1.15g of the chilled butylamine. This is well mixed and replaced back into the freezer. After about 1-2 hours the milky white solution becomes clear. At this point a dilute solution of precursor has now been prepared.

[0059] In order to concentrate the precursor (or replace the solvent) it is necessary to remove all or part of the ethanol. This can be carried out using a modified rotary evaporator as in Figure 2. The apparatus has an ice bath and a C0₂ trap to maintain stability of the precursor.

[0060] In this arrangement the water bath of the rotary evaporator is filled with ice/water. The in-built condenser is not used. Instead the ethanol is isolated using an in line dry ice trap. The procedure is relatively slow and allows the option for all or some of the ethanol to be removed. This allows simple concentration to the desired level of silver or complete removal for formulation into another solvent combination. Our standard procedure is to remove sufficient ethanol to leave an ethanol amine complex with approximately 20% w/w silver. 10061] The present invention has also been carried out using silver oxalate which can produce films which have a similar conductivity to those formed using silver formate and when annealed at 140 °C. With an additional acid treatment (see below), sintering at only 100 °C and for 10 min achieves the same level of conductivity.

[0062] Silver oxalate can be prepared from silver nitrate and oxalic acid dihydrate in aqueous solution (as set out also in Figure 8). An exemplary stoichiometry is 2:3.

(COOH)₂ ^■ 2H₂0 + AgN0₃→ Ag₂(COO)₂ + HN0₃ + H₂0

10063] Oxalic acid dihydrate 30 g is dissolved in 350 ml water. Separately 30 g of silver nitrate is dissolved in 120 ml water. Then the silver nitrate solution is added to the oxalic acid solution in a dropwise manner, while the oxalic acid solution is stirred. Silver oxalate instantly formed and white precipitation immediately appeared as the oxalic acid solution is added. When the reaction was complete, the solution was filtered to remove the water, using 1 μπι filter paper. The filtrate was washed with water twice to remove the soluble components and any residual acid. Then the filtrate was further washed with ethanol twice. The filtrate cake was finally crushed into powdery state and vacuum dried overnight at room temperature. White powder of silver oxalate was thus obtained. The yield of silver oxalate is > 95%w.

[0064] To prepare the precursor ink, the silver oxalate powder was dissolved in a mixture of solvents. 25 g of silver oxalate powder was weighed out and put into 50 g of ethanol. The white powder did not dissolve at this stage and precipitated to the bottom. The suspension was continuously stirred and cooled with an ice bath. Then 50 g of isopropanolamine was added to the suspension in a drop manner in about 10 min. Furthermore, 12.5 g of octylamine was added to the mixture. The silver oxalate started dissolving. Then the sample was taken out of the ice bath and continued to be stirred at room temperature for about 2 hours until a pale yellow solution was obtained. The solution was aged at room temperature for up to 2 weeks. Some dark precipitation was observed during the aging. After aging, the solution was centrifuged at 9000 rpm for 1 hour to remove the solid content. A transparent ink in which was light yellow was thus obtained. The ink is stable at room temperature for more than a month. Nonetheless as silver oxalate is sensitive to light it is recommended to store the ink in darkness.

(0065] To evaluate the conductivity, we dip coated the silver oxalate ink on glass slides. We simply dipped a side of the glass slide (25x25 mm) into a pool of ink held in a Petri dish.

Immediately the coated slide was annealed at 140 °C for 30 minutes on a hot plate. The resistance was measured with a standard 4-wire probe (discussed in detail below). The film thickness was measured with a Veeco white light interferometer (discussed in more detail below). The results of representative examples are listed in the table below.

(0066] A process was then developed to decrease the sintering temperature of the silver oxalate ink. Low temperature sintering expands the application of the ink to temperature sensitive substrates such as plastic and paper. We found that promoting sintering using acid can effectively decrease the sintering temperature by at least 10 °C. Furthermore, the acid treatment can also accelerate the sintering. Films have been found highly conductive after annealing at 100 °C or lower for only 10 min.

(0067] We developed two processes to decrease the sintering of silver oxalate and increase the conductivity of the sintered films: (1) Promoting the sintering with formic acid vapor; (2) Promoting the sintering with formic acid liquid.

[0068] The following is an example of Process (1): A hot plate was placed in a vapor chamber saturated with formic acid vapour. In this example, the formic acid vapour was generated with boiling formic acid inside the chamber. (The formic acid vapour can also be generated outside of the chamber and be fed into the chamber for example with a carrier gas such as air.) The temperature of the hot plate was controlled to be at 130 °C. After dip coating with the silver oxalate ink, the film was dried at 130 °C for about 2 minutes on another hot plate in air free of acidic vapour. Then the film was transferred to the hot plate inside the chamber and was annealed for 30 minutes. The data in the table below clearly illustrate that the annealing in an environment with formic acid improves the conductivity of the films. Formic Acid Resistance (Ω)

Control no vapor > 0.13 Ω*

Example with vapor 0.032

* The resistance was not consistent for the films annealed at this temperature, ranging from 0.13 Ω to values reaching mega Ohms.

[0069J The following is an example of Process (2): A film of silver oxalate ink was dip coated on a glass slide. Then the film was annealed on a hot plate. The hot plate temperature was 130 °C, 120 °C or 100 °C. The annealing time was 10, 20, or 30 minutes. The film was taken away from the hot plate and cooled down to room temperature. The cooled film was dipped into formic acid liquid. Finally the film was dried again on the hot plate for 2 minutes. The data in the table below again illustrate that the formic acid promoted the sintering of the film. The resistance decreased from values in the region of mega ohms to the level of 10^*2 ohms. Meanwhile, the sintering temperature was decreased from 1 0 °C to 130 °C, or even 100 °C and below. The annealing time was decreased from 30 minutes to 10 minutes or shorter.

Properties of the Silver Films

[0070] The concentrated precursor described above, although not formulated for ink jet printing, readily wets common substrates such as glass. This allows us to use spin coating as a quick and convenient technique for assessing the film properties. Samples of the precursor prepared above were spin coated onto glass slides and these were then subsequently heated at temperatures at both 100°C and 130°C for 10 minutes. The electrical resistance of the coated slides was measured by a standard 4-wire method to eliminate contact resistance. The 4- wire measurement system consists of a Lucas Labs 302 Resistivity Stand (Lucas Signatone Corp, Gilroy, CA) and a Keithley 2010 multimeter (Keithley Instruments Inc., Cleveland, OH). A line was then etched in the film so that the substrate was exposed and served as the baseline for the thickness measurement. The step height of the intact part of the film relative to the exposed substrate is measured with Wyko NT9100 Optical Profiler (Veeco Instruments Inc., Plainview, NY). After measuring the dimensions of the samples (typically long rectangle), the resistivity can be calculated asp = Rt- -C , where R is the resistance measured, t is the film thickness, and Ci is

In 2

a geometric correction factor. This procedure allows a fairly accurate measure of film resistivity. The results for a typical silver butylamine formate precursor are shown in Figure 3.

[0071 J The most significant outcome from this work is the attainment of highly conductive films at low temperatures as shown in Figure 4. As far as the present inventors are aware there is no other printing technology which can achieve sub 10^"5 ohm.cm at temperatures any where near 100°C and/or times any where near 10 minutes. The films despite the excellent electrical properties are somewhat rough.

[0072J The results are rather counter intuitive in that lower temperatures lead to better film properties, not only lower resistivity but higher film density (films prepared at 130°C had a resistivity of 9 x 10^"6 ohm.cm). This has not been observed in the sintering of nanoparticles nor with any of the materials/compositions of prior art compositions.

[0073] The reason why this might be the case is a matter for speculation at this stage however it is thought to arise from the relatively low boiling point of the solvent (in this case ethanol).

[0074] Although the precursor chemistry and the procedure for its manufacture lead to desired low resistivity at low temperatures and in a short time it was felt that that this approach could be exploited further. This arose from some fundamental work carried out on these systems with a view to understanding the mechanism of transformation from a liquid precursor to a solid silver film.

[0075] From the outset it has always been believed that precursor decomposition proceeds initially with the formation of silver nanoparticles. As the decomposition continues these individual particles get bigger and then merge into one another and ultimately form a network of connected sintered particles.

[0076] It was noted that during decomposition of the precursor there is no stabiliser/dispersant present which prevents growth and aggregation of freshly formed silver particles.

[0077] There is no requirement for a stabiliser/dispersant of the type conventionally used with silver nanoparticles and therefore the skilled person would omit same. In any event the skilled person would expect that any stabiliser/dispersant would, because of its nature, deleteriously affect the growth of silver particles.

[0078J Despite this, the present inventors decided to add a particular stabiliser and were surprised to find that a more desirable silver product was formed.

[0079] The inventors added octylamine which is known to act as a stabiliser. Octylamine is known to be a good capping agent and stabiliser and has been used successfully in the preparation of stable silver nanoparticles.

(0080] To the solution precursor described above, was added 10% w/w (on Ag) octylamine. This material was spin coated onto glass as before and the same thermal treatment regime, 130°C and 100°C for 10 minutes applied. The resistance and profiles were measured as before. The results are shown in Figure 5 and Figure 6.

Results

[0081] At 100°C we have approximately 3x bulk silver resistivity whilst at 130°C we have 2x bulk silver. Given that these figures are achieved within 10 minutes the results are remarkable and as far as the present inventors are aware there is nothing in the prior art that is any where near this good.

(0082) Incorporation of octylamine (whether it is a nanoparticle stabiliser or not) has significantly changed the films properties. The resulting films are much thinner and much smoother which are further attractive features of this technology. It is clear that the films have got much denser with respect to those without the additive and is strong although not conclusive evidence that some form of ordering possibly through in-situ generation of nanostructures has occurred.

(0083] Figure 7 shows SEM images of sectioned silver films (prepared as described above utlising solver formate and octylamine) on glass and exposed to heating regime as follows: top left 100°C for 1 minute, top right 100°C for 10 minutes, bottom left 130°C for 1 minute and bottom right I30°C for 10 minutes. The SEMs showed that within 1 minute, the ink formed nanoparticles. In such a short time the particles had already started sintering together to form a continuous conductive network. Longer annealing time (10 min) resulted in that the particles being completely fused together and forming large continuous domains.

(0084] The words "comprises/comprising" and the words "having/including" when used herein with reference to the present invention are used to specify the presence of stated features, integers, steps or components but do not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

[00851 It >^s appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable subcombination.

Claims

1. A printable ink comprising:

(i) a liquid vehicle;

2. A printable ink according to Claim 1 wherein the silver precursor is present in an amount from 15 to 30% by weight of the composition.

3. A printable ink according to Claim 1 or Claim 2 wherein the silver precursor is present in an amount from 17 to 27% by weight of the composition.

4. A printable ink according to Claim 1 or Claim 2 wherein the silver precursor is present in an amount from 20 to 25% by weight of the composition

5. A printable ink according to any preceding claim wherein the stabilising agent is selected from amines, oximes, hydrazones, guanidines, hydrazides, carbazones and combinations thereof.

6. A printable ink according to Claim 5 wherein the stabilising agent comprises amine.

7. A printable ink according to Claim 6 wherein the amine is selected from primary, secondary and cyclic amines and combinations thereof.

8. A printable ink according to Claim 6 or Claim 7 wherein the amine is selected from aliphatic amines including C|-C|₆ amines including butylamine and nonyla ine, dibutyl amine, tributyl amine, diethylene amine, tetraethylene pentamine, octylamine and combinations thereof.

9. A printable ink according to any one of Claims 6 to 8 wherein the amine is octylamine.

10. A printable ink according to Claim 6 or Claim 7 wherein the amine is an amino alcohol including C1-C16 amino alcohols.

1 1. A printable ink according to Claim 10 wherein the amino alcohol is selected from

ethanolamine, propanolamine including isopropanolamine and combinations thereof.

12. A printable ink according to any preceding claim wherein the stabilising agent comprises oxime.

13. A printable ink according to Claim 12 wherein the oxime is an aldoxime or ketoxime optionally selected from C|-C₎₆ aldoximes or C|-C|₆ ketoximes and combinations thereof.

14. A printable ink according to Claim 12 or Claim 13 wherein the oxime is selected from acetoneoxime, methylaldoxime, and methylethyl ketoxime and combinations thereof.

15. A printable ink according to any preceding claim wherein the stabilising agent is present in an amount from 5 to 10% by weight of the ink composition.

16. A printable ink according to any preceding claim wherein the liquid vehicle comprises a polar solvent based vehicle.

17. A printable ink according to Claim 16 wherein the liquid vehicle comprises alcohol

18. A printable ink according to Claim 17 wherein the alcohol is ethyl alcohol.

19. A process for manufacturing a printable ink comprising:

(i) providing a silver precursor which is a silver complex formed by reaction of at least one of silver formate or silver oxalate with a stabilising agent, in a suitable liquid medium.

20. A process according to Claim 19 further comprising the step of reacting silver formate or silver oxalate with a stabilising agent to form the silver complex.

21. A process according to Claim 19 or Claim 20 wherein the liquid medium forms a liquid . vehicle for printing the silver precursor onto a substrate.

22. A process according to any one of Claims 19 to 21 wherein the amount of liquid of the liquid medium is reduced to increase the silver precursor concentration.

23. A process according to Claim 22 wherein the liquid medium is drawn off under vacuum while being maintained at temperature of less than about 0 °C.

24. A process for the manufacture of a silver complex formed by reaction of silver formate or oxalate with an amine comprising the step of:

(i) reacting the silver formate or oxalate with the amine in a suitable liquid medium at a temperature of less than about 0 °C.

25. A printer having a supply of printable ink wherein the ink is as defined in any one of Claims 1 to 18.

26. The product of a printable ink according to any one of Claims 1 to 18 when applied to a substrate.

27. A product according to Claim 26 wherein the printed material has been treated with a conductivity-promoting component.

28. A product according to Claim 27 wherein the conductivity-promoting component is an organic acid optionally selected from oxalic acid and formic acid and combinations thereof.

29. A sintered material which is formed by sintering of the ink according to any one of Claims 1 to 18 after application thereof to a substrate.

30. A sintered material according to Claim 29 wherein the ink is sintered while in contact with a conductivity-promoting component.

31. A sintered material according to Claim 30 wherein the conductivity-promoting component is an organic acid optionally selected from oxalic acid and formic acid and combinations thereof.

32. A device having a conductive material applied thereto which conductive material is the product of any one of Claims 25 to 28 or the sintered material of any one of Claims 29 to 31.

33. A process for sintering an applied silver material which silver material is a silver complex formed by reaction of at least one of silver formate or silver oxalate with a stabilising agent which process includes the step of sintering the applied silver material while in contact with a conductivity-promoting component.

34. A process according to Claim 33 wherein the conductivity-promoting component is an organic acid optionally selected from oxalic acid and formic acid and combinations thereof.

35. A process according to Claim 33 or Claim 34 wherein the conductivity-promoting

component is in a liquid form.

36. A process according to Claim 33 or Claim 34 wherein the conductivity-promoting

component is in a gaseous form.