WO2022174244A1

WO2022174244A1 - Conductive carbon nanotube-silver composite ink composition

Info

Publication number: WO2022174244A1
Application number: PCT/US2022/070612
Authority: WO
Inventors: Yongan Yan; Stefan Maat; Troy Robinson; Satyabrata Raychaudhuri
Original assignee: Yazaki Corporation
Priority date: 2021-02-11
Filing date: 2022-02-10
Publication date: 2022-08-18
Also published as: EP4291614A1

Abstract

The present invention is directed to an ink composition comprising: (a) silver (Ag)-metallized carbon nanotubes (CNTs) comprising CNTs and silver nanoparticles covalently bound to the CNTs, (b) silver particles, and (c) solvent. The present ink composition produces printed components with good thermoform elongation and high electrical conductivity, without the presence of polymeric organic binders. The present invention is also directed to a method for preparing Ag-metallized CNTs, and a method for preparing an ink comprising Ag-metallized CNTs.

Description

CONDUCTIVE CARBON NANOTUBE-SILVER COMPOSITE INK

COMPOSITION

FIELD OF THE INVENTION

The present invention relates to a carbon nanotube-silver composite ink composition. The present ink composition produces conductive prints with good thermoform elongation and high electrical conductivity, without adding polymeric organic binders.

BACKGROUND

In-mold structural electronics (IMSE) involve a process of integrating 2D printed electronic circuitry with thermoforming and injection molding to form 3D-shaped objects.

The capacity to print electronic circuitry on a 2D substrate prior to converting it into a functional 3D object requires that the printed wires/circuits have good thermoform elongation to survive the forming and molding steps, while retaining high electrical conductivity. Most of the thermoformable prints are prepared using silver-based conductive inks with various organic binders (Reference #1-5). However, the presence of an electrically insulating polymeric organic binder significantly reduces the electrical conductivity of the printed inks.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 depicts prints with the ink of the present invention. The sintered silver particles (nanoparticles and flakes) are cross-linked with metallized carbon nanotube (CNT) strands.

Figure 2 shows a schematic process to prepare a dispersion of CNT that are metallized with silver (Ag) nanoparticles (NP).

Figure 3 illustrates a general procedure for preparing and printing of the present ink composition.

Figure 4 shows a schematic drawing of plates for (left) stencil printing and (right) screen printing, to form a rectangular printed shape.

Figure 5 shows a SEM image of CNT that are metallized with silver (Ag) nanoparticles (NP) (Ag-metallized CNT, or Ag-CNT). Figure 6 (a and b) shows SEM images of prints prepared from the inks of the present invention after curing at 130°C.

Figure 7 shows micrographic images of prints without (left) or with (right) incorporation of 0.1 wt% Ag-CNT. The left shows large cracks and the right shows that the cracks were eliminated by incorporation of Ag-CNT.

Figure 8 shows SEM images of a silver print prepared from ink containing non- metallized CNT. Image at right is a close-up of boxed area in image at left.

Figure 9 shows schematic drawings of Ag nanoparticles attached to CNT surface via either van der Waals pi-pi interaction (left, comparative) or by covalent bonding to CNT carbon framework (right, the present invention).

Figure 10 is a block diagram showing the printing and thermoforming process.

Figure 11 shows thermoforming of a 2D printed panel into a 3D structure. The printed lines were elongated by between 0 and 20% depending on position.

Figure 12 is a graph showing line resistance vs. thermoform elongation for printed lines prepared using different types of silver-based inks.

DETAILED DESCRIPTION OF THE INVENTION

Ink Composition

The present invention provides a silver-based ink composition with CNTs as an additive; further, the CNTs are metallized with silver (Ag) nanoparticles (NP). The ink composition produces conductive prints with good thermoform elongation and high electrical conductivity.

The present ink composition comprises: (a) silver (Ag)-metallized CNTs comprising CNTs having covalently bound silver nanoparticles (Ag-CNT), (b) silver particles, and (c) a solvent. In one embodiment, the composition comprises 15 to 90 percent by weight (wt%) of silver, 0.001 to 5 wt% CNT, and 10 to 85 wt% of solvent. The wt% of silver in the composition includes silver nanoparticles bound to CNTs, and unbound silver particles. The wt% of CNT in the composition includes CNTs only, and does not include silver nanoparticles bound to the CNT. In a preferred embodiment, the composition comprises 0.01 to 0.5 wt% of CNT. In one embodiment, the composition comprises 45 to 85 wt% or 45 to 75 wt% of silver. In one embodiment, the composition comprises 15 to 55 wt% or 25 to 55 wt% of solvent. In a preferred embodiment, the composition comprises 65 - 85 wt% of silver. In a preferred embodiment, the composition comprises 15 - 35 wt% of solvent. In the Ag-metallized CNT compound, the weight ratio of CNT to silver is about 1:0.1 to 1:100, preferably 1:1 to 1:50, or 1:1 to 1:10.

“About” as used in this application, refers to ± 10% of the recited value.

In the wet ink composition, as well as in the printed and cured ink (e.g., an electrically conductive line, shape, pattern, or circuit printed onto a substrate) after the solvent has been removed from the wet ink composition, the amount of CNT in weight percent (wt%) in relation to the total amount of silver (including silver particles and silver in the Ag-metallized CNT) is about 0.01 to 5 wt%, and preferably 0.1 to 0.5 wt%. If the amount of CNT is too high in relation to the total amount of silver (excess CNT), the conductivity of the print is too low. If the amount of CNT is too low in relation to the total amount of silver (insufficient CNT), the stretchability of the print is too low. The conductivity and mechanical properties of the final prints are determined by the relative amounts of CNT and silver in the starting composition and in the final print. Therefore, the relative amounts of CNT and silver need to be selected for optimized performance of the ink in its targeted use.

In the present invention, the silver nanoparticles are covalently bound to the carbon nanotubes forming the Ag-metallized CNT. Prints with an ink composition having bare CNTs without covalently bound silver nanoparticles do not provide sufficient thermoform elongation, and do not reduce the tendency for printed dried ink to crack. In addition, bare CNTs and silver particles do not mix well in an ink composition, and the bare CNTs and silver particles remain separated in the ink composition. Whereas, the Ag metallization of CNTs makes them more compatible with silver particles, and improves the mixing of Ag- metallized CNTs and Ag particles in an ink composition.

In the present ink composition, the silver particles of (b) comprise silver nanoparticles (Ag NP), or silver flake particles (Ag FP), or a mixture thereof. Silver nanoparticles in general have a diameter of <100 nm; preferably <50 nm. For example, silver nanoparticles have a diameter of 1-100 nm, 1-50 nm, 1-20 nm, or 10-20 nm. Silver flake particles in general have a diameter in the range of 0.1-10 pm (widest dimension in-plane), with D50 (median particle size) about 1-7 pm, preferably 2 - 4 pm, and a thickness of about 0.05 to 0.2 pm.

In one embodiment, the weight % of Ag NP in the silver particles Ag (NP+FP) is in the range of 0-100%, preferably in the range of 20-80% or 25-80%, and more preferably in the range of 30-50 or 40-60%. The weight % of Ag FP in the silver particles is in the range of 0-100%, preferably in the range of 20-80% or 20-75%, and more preferably in the range of 40-60% or 50-70%.

In one embodiment, the silver particles Ag (NP+FP) comprise 5-60 wt% of silver nanoparticles (Ag NP) and 40-95 wt% of silver flake particles (Ag FP).

In one embodiment, the silver particles Ag (NP+FP) comprise 25-60 wt% of silver nanoparticles (Ag NP) and 40-75 wt% of silver flake particles (Ag FP).

In one embodiment, the silver particles Ag (NP+FP) comprise 30-60 wt% of silver nanoparticles (Ag NP) and 40-70 wt% of silver flake particles (Ag FP).

In one embodiment, the silver particles Ag (NP+FP) comprise 40-60 wt% of silver nanoparticles (Ag NP) and 40-60 wt% of silver flake particles (Ag FP).

In one embodiment, the silver particles Ag (NP+FP) comprise about 50 wt% of silver nanoparticles (Ag NP) and about 50 wt% of silver flake particles (Ag FP).

A greater proportion of Ag FP increases stretchability, but decreases conductivity, whereas a greater proportion of Ag NP increases conductivity, but decreases stretchability. This ink composition, having an approximately even mixture of Ag NP and Ag FP, provides a good combination of conductivity and stretchability in the resulting printed articles.

In one embodiment, the silver particles Ag (NP+FP) comprise primarily Ag FP (>95%) with only a small amount of Ag NP (<5%), or comprises only Ag FP (100%), with no Ag NP at all (0%). Such an ink composition is useful for applications in which high stretchability is desired and in which there is a tolerance for lower electrical conductivity.

In the present ink composition, the solvent is preferably an organic solvent, such as alcohols, esters, alcohol esters, glycols, glycol ethers, and ketones. Preferable organic solvents include isopropanol (IP A), 2-ethoxyethanol, methyl n-amyl ketone, diisobutyl ketone, 2-butoxy ethanol, 1 -(2-methoxypropoxy)-2-propanol (MPP), di(propylene glycol) methyl ether, 2-(2-ethoxyethoxy)ethanol, n-methyl-2-pyrrolidone, ethylacetate, diethylene glycol monobutyl ether, diethylene glycol, diethylene glycol n-butyl ether acetate, 2,2,4- trimethyl-l,3-pentanediol monoisobutyrate (TMPD-MIB, or Texanol), diethylene glycol dibutyl ether (DGDE), methyl 5-(dimethylamino)-2-methyl-5-oxopentanoate, glycerin, dibutyl tartrate, or any combination thereof.

In one embodiment, the solvent has a boiling temperature above 150°C, or between 150°C and 300°C. Such solvents may include MPP, a glycol ether, TMPD-MIB, or any combination thereof. In one embodiment, the solvent is a mixture of two or more solvents, and the mixture has a boiling temperature above 100°C, or between 100°C and 350°C, or preferably between 130°C and 280°C.

In the present ink composition, the CNT is single wall CNT (SWCNT), double wall CNT (DWCNT), multi-wall CNT (MWCNT), or any combination thereof. Preferably, the CNT is SWCNT or DWCNT. The length of the CNT strands dispersed in the ink is in general 1-500 pm, preferably 1-100 pm, more preferably 1-40 pm.

The present ink formulation preferably does not contain a polymeric organic binder or a resin binder such as a polyester, acrylic polymer, acrylic block copolymer, acrylic polymer having tertiary alkyl amide functionality, polysiloxane polymer, polystyrene copolymer, polyvinyl polymer, divinylbenzene copolymer, polyetheramide, polyvinyl acetal, polyvinyl butyral, polyvinyl acetal, polyvinyl alcohol, polyvinyl acetate, polyvinyl chloride, methylene polyvinyl ethers, cellulose acetates, styrene acrylonitrile, amorphous polyolefin, thermoplastic urethane, polyacrylonitrile, ethylene vinyl acetate copolymer, ethylene vinyl acetate terpolymer, functional ethylene vinyl acetate, ethylene acrylate copolymer, ethylene acrylate terpolymer, ethylene butadiene copolymer and/or block copolymer, styrene butadiene block copolymer, polyvinylpyrrolidone (PVP), acrylic resin, phenoxy resin, phenolic resin, epoxy resin, urethane resin, and polyhydroxyether resin. Polymeric organic binders or resin binders in general are highly electrically insulating and significantly reduce the electrical conductivity of the printed inks.

In one embodiment, the present ink composition does not contain a surfactant, such as sodium dodecylbenzene sulfonate, which is often used to disperse and stabilize CNTs in a liquid mixture. CNTs in the present ink composition are stable and remain dispersed in the mixture without settling for at least 6 months.

The present ink composition can be sintered or cured at 100-150°C for 30 minutes to one hour, and does not need a high sintering temperature (e.g., 250°C or above), which is not compatible with most types of plastic substrates such as polycarbonate (PC) or polyethylene terephthalate (PET).

Figure 1 illustrates prints with the present ink composition. The sintered silver particles (nanoparticles and flakes) are cross-linked with metallized CNT nanofibers. Process for Preparing the Ink Composition

The present invention is also directed to a process for preparing Ag-metallized CNT. The process comprises the steps of: (a) mixing a CNT dispersion comprising CNTs having covalently bonded carboxyl groups in a first solvent with ammonium hydroxide, ammonia, or an alkyl amine, and a silver salt to cause a chemical reaction resulting in a diamine silver (Ag(NH₃)₂ ⁺¹) complex, and (b) reacting the mixture of (a) with formic acid, resulting in CNTs having silver (Ag°) nanoparticles covalently bound on their surfaces, to form CNTs covalently bound with silver nanoparticles.

Figure 3 illustrates the general procedure for preparing the present ink composition, and for preparing printed components using the present ink composition.

The starting CNTs having carboxyl (COOH) groups can be prepared, for example, by reacting CNTs with a HN0₃ solution to form a dispersion comprising CNTs with carboxyl groups (Figure 3, step 1). The acidic water is then removed from the dispersion and the CNTs having carboxyl groups are ready for storage or use. This procedure generates COOH functional groups on the CNT surface, dissolves inorganic impurities such as iron catalyst residue, and allows the CNTs to be dispersed and remain suspended in a variety of solvents, including most organic solvents, and water if there are sufficient COOH groups.

To prepare an Ag NP metallized CNT dispersion, CNTs having carboxyl groups (Figure 3, step 1) are first dispersed in a proper solvent, for example, an alcohol such as isopropyl alcohol (IP A), ethyl alcohol, or MPP, a glycol ether such as 2-butoxyethanol or diethylene glycol, TMPD-MIB, or any combination thereof (Figure 3, step 2), to which an ammonium salt (such as ammonium hydroxide), ammonia, or an alkylamine (such as hexylamine) and a silver salt (Ag⁺¹, such as silver acetate or silver nitrate) are added (Figure 3, step 3).

After adding ammonium hydroxide (NH4OH), ammonia (NH₃), or an alkylamine to the carboxylated-CNT dispersion, some ammonia attaches to the carboxyl groups in the CNT, and some dissolves in the solvent. Due to the presence of ammonia in the CNT dispersion, a silver salt added to the dispersion can completely dissolve by forming silver ions (Ag⁺¹) coordinated to ammonia, i.e. Ag(NH₃)₂ ⁺¹ on the CNT surface and in the dispersion, for example:

AgCfbCC (solid) + 2NH OH Ag(NH₃)₂ ⁺¹ + CTBCOO ¹ + 2H₂0 When formic acid HCOOH is added to the above dispersion in step (b), formic acid reduces the Ag⁺¹ in Ag(NH₃)2⁺¹ ions to metallic Ag (Ag°) nanoparticles which are covalently bonded to the CNT surface through carboxyl linkages:

HCOOH + 2Ag(NH₃)₂ ⁺¹ + 20H ¹ -> 2Ag (metallic) + 4NH₃ + C0₂ + 2H₂0

The molar ratio for Ag⁺¹ion: NH4OH: formic acid is about 1: 0.2-20: 0.1-5, and preferably about 1:4:1.

In the present process, some silver particles are covalently bonded to the CNT surface while some may remain in the dispersion in the form of nanoparticles.

The present invention is also directed to a process for preparing the present ink composition. The process comprises the steps of: (a) mixing a first CNT dispersion comprising CNTs having carboxyl groups in a first solvent with ammonium hydroxide, ammonia, or an alkyl amine, and a silver salt, and reacting for a period of time; (b) reacting the mixture of (a) with formic acid to form CNTs having silver nanoparticles covalently bound on the surface to obtain a second CNT dispersion comprising CNTs covalently bound with silver nanoparticles; and (c) mixing a solution comprising silver particles with the second CNT dispersion to form the ink composition.

In one embodiment, between step (b) and step (c), the dispersion comprising CNTs covalently bound with silver particles (Ag-metallized CNT) is processed via three-roll milling or through a high-shear, high-pressure process, or both, in order to reduce the size of CNT aggregates in the dispersion. In one embodiment, the dispersion is processed via three- roll milling using a 5 - 20 pm gap. In one embodiment, the dispersion is high-shear, high- pressure processed by passing it through an orifice or channel 80 - 250 pm in diameter for one or more passes.

To prepare an ink composition, a solution comprising silver particles is mixed with the Ag NP metallized CNT dispersion (Figure 3, step 4). The silver particles can be silver nanoparticles, silver flake particles, or a mixture thereof. The silver particles are first dispersed in an organic solvent such as an alcohol such as isopropyl alcohol (IP A), ethyl alcohol, or MPP, a glycol ether such as 2-butoxyethanol or diethylene glycol, TMPD-MIB, or any combination thereof, before mixing with the Ag NP metallized CNT dispersion. In one embodiment, the mixing of Ag NP metallized CNT with silver particles is accomplished by using planetary mixing, 3-roll milling, or by a high pressure, high shear process.

Prior to printing with the prepared ink composition, an adjustment to the solvent amount in the composition is optionally applied, to achieve the most appropriate viscosity of the ink for the printing process to be used (Figure 3, step 5). This adjustment may entail either addition of solvent (to reduce viscosity) or removal of solvent through evaporation (to increase viscosity).

Application of the Ink Composition

The present ink composition can be used to print various electrically conducting components such as lines, shapes, patterns, and circuits on flat substrates by using stencil printing, screen printing, or inkjet printing (Figure 3, step 6). The printed components on the substrates can be formed into a 3D shape by thermoforming or mechanical-forming processes, while maintaining their electrical conductance without cracking. Printed components made with the present ink composition have high thermoform elongation and electrical conductivity, and are superior to printed components produced from inks containing polymeric organic binders.

The printed components typically have a thickness >3 pm, preferably >10 pm, and more preferably >15 pm. The printed components have electrical conductivity relative to that of bulk silver of > 5%, preferably > 10% or > 15%. The printed components exhibit a thermoform elongation of > 5%, preferably > 10% or > 15%.

Printed components may be produced with the present ink compositions via, for example, stencil printing, screen printing, gravure printing, flexographic printing, offset printing, and inkjet printing, among others. Stencil printing and screen printing are common and similar techniques that transfer inks onto a substrate through a stencil plate having apertures matching the design of the desired component. The apertures in stencil printing are completely open, while in screen printing the apertures are filled with a wire mesh (Figure 4). For a plate of a given thickness, stencil printing produces relatively thicker prints while screen printing produces thinner prints but with relatively high resolution.

The inventors discovered that in order to prevent the CNT in the ink composition from clogging the screen mesh, the maximum length of CNT strands should be less than about 50% of the circumference of the wire of the screen mesh, preferably less than about 1/3 of the circumference of the screen mesh wire. Also, the maximum length of CNT strands should be less than about 50% of the width of the mesh opening between wires. For example, for a size 200 mesh screen having wire diameter of 38 pm and mesh opening of 89 pm, the maximum length of a CNT strand would be about 40 pm. For example, the length of CNT strands should be between about 1 pm and 40 pm, or between about 1 pm and 30 pm.

The following examples further illustrate the present invention. These examples are intended merely to be illustrative of the present invention and are not to be construed as being limiting.

EXAMPLES

Example 1. Nitric acid treatment of as-received SWCNT

This Example is summarized in Figure 2, Step 1.

Nitric acid solution (~3N) was prepared by adding 180 g of 70% nitric acid to 408 g of deionized water and stirring for at least 5 min. The nitric acid solution was mixed with 20 g of as-received SWCNT. The mixture was simultaneously sonicated and mechanically stirred at 50°C for 4 h and then the dispersion was stirred overnight. This process served both to generate COOH groups on the SWCNT and to dissolve inorganic impurities. The mixture was then vacuum-filtered to remove most of the acidic water.

Then, the CNT wet cake was dispersed in 800 g deionized water, sonicated and stirred for at least 30 min and then vacuum-filtered to remove water. These procedures were repeated 5 times to remove acid and dissolved inorganic impurities.

Then, a dispersion of the COOH-functionalized and purified CNT in isopropanol (IP A) was prepared at a concentration of 0.5 wt% CNT, using a high pressure, high shear process.

The resulting dispersion of CNT in IPA was then used as described in Example 2.

Example 2. Metallize CNT surface with Ag NP

This Example is shown in Figure 2, Step 2, which is referred to as bonding of Ag nanoparticles to the CNT surfaces.

The Ag-metallized CNT dispersion was prepared by adding 465.6 g of 0.5 wt% CNT- IPA dispersion into a glass container, to which 60.4 g ammonium hydroxide (NFEOH) (30 wt%) were added while mechanically stirring at 800 rpm. After mixing for at least 20 min, 21.6 g silver acetate were added and mixing was continued for an additional 2 h. The weight ratio of CNT:Ag was 1:6.

Finally, 6.27 g formic acid were added to the mixture and stirred overnight (at least 16 h.). The dispersion contained 0.42 wt% SWCNT and 2.52 wt% Ag. The molar ratio of Ag: NH4OH: formic acid was 1:4:1.

A SEM image of the Ag-metallized CNT (Ag-CNT) is shown in Figure 5.

Example 3. Ink containing Ag (NP+FP) and Ag-metallized CNT (Ag-CNT)

An ink composition was obtained composed of about 60 wt% silver nanoparticles (Ag NP) 10-20 nm in size, and about 40 wt% solvent, the solvent comprising 2-butoxy ethanol, diethylene glycol, ethanol, or a combination thereof. The ink composition contained no polymeric binder material.

Commercially available silver flakes (Ag FP) were obtained in powder form, with flake particle size in the range of 0.1 - 10 pm, D50 of about 4 pm and thickness of 0.1 pm.

An ink composition containing Ag (FP+NP) and Ag-CNT was then prepared by adding 50 g of Ag-CNT dispersion in IPA (Example 2) and 52.5 g Ag FP to 87.5 g of the Ag NP ink, and then mechanically mixing at 3200 rpm for 60 seconds, 3 times, with about 1 minute gap between each mixing. Approximately 45 g of solvent were then removed from the mixture by evaporation. The solvent content was then adjusted by adding 5 g butoxy ethanol and 2.5 g diethylene glycol, and then mixing again at 3200 rpm for 30 seconds, 3 times.

The prepared ink (150 g) contained silver particles composed of 50 wt% Ag FP and 50 wt% Ag NP, (i.e., a 1 : 1 ratio of NP:FP) and Ag-CNT. The weight ratio of CNT to Ag (NP + FP) was 0.2 wt%, and solvent content in the ink was about 30 wt%.

Example 4. SEM images of ink prints

An ink composition was prepared according to Example 3 except the silver particles consisted entirely of silver flakes (Ag FP), and the amount of CNT (as Ag-CNT) was 0.067 wt% relative to the amount of Ag FP. The amount of Ag as nanoparticles bonded to CNT was 0.4 wt% relative to the amount of Ag FP. A SEM image of a stencil print prepared from this ink and cured at 130°C is shown in Figure 6a. The image illustrates that the silver nanoparticles bonded to the CNT surfaces also effectively bonded the CNT to the flake surfaces, and the CNT cross-linked the silver flake particles together without using organic binder.

Another ink composition was prepared according to Example 3 except the Ag-CNT to Ag (NP + FP) weight ratio was increased to 0.3 wt%, and the CNT was a commercial variety having a shorter strand length of < 40 pm, with average strand length of about 5 pm. Figure 6b is an SEM image of a screen print prepared using this ink and cured at 130°C. The Ag- metallized CNT are visibly attached to and bridging across the individual Ag NP and Ag FP, creating a strong cross-linking effect between the Ag particles.

Example 5: Effect of Ag-CNT in the prints to eliminate cracks

A silver ink containing 60 wt% Ag NP and 40 wt% solvent was stencil printed as 16 pm thick lines on a polycarbonate substrate and cured at 130°C for 30 min. The prints showed long and wide (100 x 20 pm) cracks (Figure 7, left panel). When 0.1 wt% of Ag- metallized CNT (Ag-CNT) was added to the ink, the cracked lines in the prints were completely eliminated (Figure 7, right panel). The print containing the Ag-CNT had a thickness of ~25 pm.

Example 6: Ink prints containing non-metallized CNT (comparative example)

The 0.1 wt% Ag-metallized CNT in Example 5 was replaced with 0.15 wt% dispersed CNT without Ag metallization. The print prepared from this ink had a thickness of 16 pm and cracks were continually present in the prints (Figure 8). This result indicates that the non- metallized CNT do not provide the necessary strong surface interaction between CNT and silver particles that was demonstrated by the Ag-metallized CNT in Example 5.

Example 7. Inks containing CNTs with non-covalently bonded silver nanoparticles (comparative example)

This ink was prepared by a similar method as described in Example 1 of US 9,418,769. About 100 ml of ethanol and 1.18 ml of benzyl mercaptan were mixed; then, 8 ml of the resultant solution and 1000 ml of ethanol containing 3.4 g of AgNCh were added at a molar ratio of benzyl mercaptan to Ag of 1:25 to prepare Ag nanoparticles functionalized with benzyl mercaptan. About 50 mg ultrasonically dispersed SWCNT in ethanol were mixed with 150 ml of the above solution of Ag nanoparticles functionalized with benzyl mercaptan. The weight ratio of AgNCUCNT was 10.2:1 or Ag:CNT was 6.4:1. The silver nanoparticles were weakly bound to the surfaces of the CNT by the aromatic hydrocarbon group via p-p interactions between the benzyl and CNT surfaces. The silver nanoparticles were not attached to the CNT by covalent bonds.

The 0.1 wt% Ag NP metallized CNT in Example 5 was replaced with 0.1 wt% CNT non-covalently bonded with silver particles prepared in this Example. The print prepared from this ink had a thickness of 16 pm and cracks were continually present in the prints similar to those shown in Figure 8. The result indicates that a strong covalent chemical bond is needed between Ag and the CNT surfaces. The bonding of Ag to CNT surfaces via weak p- p interactions of the aromatic hydrocarbon groups did not eliminate print cracks, and it did not improve the mechanical properties, such as stretchability or thermoform elongation on plastic substrates after curing.

Figure 9 shows schematic drawings of Ag nanoparticles attached to CNT surface via either p-p interaction (this comparative example) or by covalent bonding to CNT carbon framework (the present invention).

Example 8: Evaluation of the thermoform elongation of printed lines

Electrically conductive lines were printed on flat polycarbonate (PC) substrates using the ink composition described in Example 3. The printed PC was then heated to about 185°C for about 80 seconds and a vacuum was applied over an arc-shaped porous mold to form a 3D curved shape. The process is illustrated by the block diagram in Figure 10. The printed lines were stretched in the range of 0-20% depending on the line position on the 3D arc, as shown in Figure 11. The printed lines prepared from this ink composition exhibited, in the unstretched state, electrical conductivity of 11% relative to that of bulk silver. The printed lines exhibited thermoform elongation of 15%, meaning that the electrical conductivity was maintained in the printed lines until the elongation exceeded 15%.

Conductive lines were also printed using the ink composition of Example 3, after further processing the ink by three-roll milling. After three-roll milling of the ink, the elongation of the printed lines increased to 18.3-20%.

Example 9: Silver inks containing polymeric organic binder (comparative example)

Five batches of silver-based inks A - E, each containing 70 wt% silver particles and about 30 wt% solvents were prepared as shown in Table 1. All inks contained 67 g AgNP (10- 20 nm) and 33 g Ag FP (as described in Example 3) (i.e., a 2: 1 weight ratio of Ag NP: Ag FP). One ink batch (A) contained neither Ag-metallized CNT nor a polymeric binder. Three of the ink batches (B - D) contained one of three types of polymeric organic binders, including polyurethane, polyether polyol, or ethyl cellulose, which was added respectively to the ink composition at 1 wt% relative to the total weight of Ag particles (NP + FP) . The fifth batch of silver ink (E) was formulated as in Example 3, but containing 0.07 wt% Ag-metallized CNT without organic binder.

Table 1: Composition and properties of five batches of silver-based inks containing polymeric organic binder and metallized CNT.

^¨Conductivity of pure bulk silver is 6.3 x 10⁵ S/cm. Each ink was stencil printed onto a flat PC substrate and the conductivity and thermoform elongation of the resulting prints were determined. The silver ink without Ag- metallized CNT or binder showed electrical conductivity of 18.7% of that of bulk silver and thermoform elongation of about 1%. Adding 1% polymeric binder reduced the conductivity considerably to 12.2-13.4% of that of bulk silver and slightly increased the thermoform elongation to about 2%. By adding 0.07% of Ag-metallized CNT without organic binder, the inks showed a high conductivity of 16% of that of bulk silver and high thermoform elongation of 9%. The data show that the silver inks containing Ag-metallized CNT and no binder had both higher conductivity and greater elongation than inks containing polymeric binders. Example 10: Comparison of commercial silver inks (with or without polymeric organic binder) with the ink of the present invention

Two types of commercial silver-based inks were stencil printed as 105 mm long and 1 mm wide lines on PC substrates. The composition and properties of the ink and prints are listed in Table 2. The commercial Ink #1, containing silver particles without binder, showed high electrical conductivity but low thermoform elongation. Commercial Ink #2 containing both silver particles and polymeric organic binders showed the lowest electrical conductivity but high thermoform elongation. Ink #3 was prepared by combining Inks #1 and #2 in a 1: 1 ratio. This mixed ink composition showed only slightly better thermoform elongation than Ink #1, and only slightly better electrical conductivity compared with Ink #2. Combining the two inks did not provide any significant synergistic effect. The ink of the present invention described in Example 3 showed both high electrical conductivity (11.0% of that of bulk Ag) and high thermoform elongation (18.3%). Table 2: Comparison of commercial silver inks (with or without polymeric organic binder) with ink of the present invention comprising Ag-metallized CNT.

Figure 12 shows electrical line resistance vs. thermoform elongation for typical printed lines of different Ag-based inks, after undergoing a thermoforming process.

Commercial ink #1 without organic binder or CNT shows low line resistance. However, flexibility was poor and the printed lines showed cracks and damage when elongation exceeded 3.8%. Commercial ink #2, with organic binder (but no CNT) showed high flexibility, but also high line resistance (i.e., low electrical conductivity). The Ag-CNT ink of the present invention, as described in Example 3, show both low resistance and high elongation. The printed lines prepared from this ink composition exhibited electrical conductivity in the unstretched state of 11 % relative to that of bulk silver, and a thermoform elongation of up to 18.3%.

Example 11: Silver ink with long CNT strands for stencil printing

A silver ink is formulated containing 55 wt% silver particles (consisting of silver nanoparticles and silver flake particles in a 1:1 ratio), 0.4 wt% (relative to the weight of silver) Ag-metallized SWCNT, and about 44.6 wt% solvent. The length of the SWCNT strands after dispersion is in the range of 1-500 pm. High quality 1 mm wide printed lines with high electrical conductivity (-10% of that of bulk silver) and high thermoform elongation (-20%) are produced from the ink by using a stencil printing process. The results indicate that inks appropriate for stencil printing are formulated with CNT having strands up to 500 pm in length, possibly longer.

However, when the ink was used for a screen-printing process by using a size 200 mesh screen with a wire diameter of 38 pm and mesh opening of 89 pm, clogging of the screen mesh was observed after printing five consecutive samples. Microscopic observation revealed CNT strands attached to and wound around the mesh wires, suggesting that the strands were too long to be used for repetitive screen printing of the composite ink.

Example 12. Silver ink with short CNT strands for screen printing

A silver ink is formulated with the same composition as that of Example 11, except the CNT used is a commercial variety having a shorter strand length of < 40 pm, with average strand length of about 5 pm. The length of the SWCNT strands after dispersion is in the range of 1 - 30 pm. High quality 1 mm wide printed lines with high electrical conductivity (11% of that of bulk silver) and good thermoform elongation (18%) are continuously produced from this ink by using a screen-printing process. The screen used for printing is a size 200 mesh with a wire diameter of 38 pm and mesh opening of 89 pm. References

1. Dorfman, 1/2016, US009245666B2. “Thermoformable polymer thick film silver conductor and its use in capacitive switch circuits”

2. Crumpton, 10/2013, US 8,562,808 B2, “Polymer thick film silver electrode composition for use as a plating link”

3. Gao, 12/2015, US 2015/0322276 Al, “Flexible conductive ink”

4. Chopra, 12/2104, US 2014/0374671 Al “Conductive metal inks with polyvinylbutyral and polyvinylpyrrolidone binder”

5. Xiao, 11/2001, US6322620B1, “Conductive ink composition” 6. Chung, 8/2016, US 9.418,769 B2, “Conductive carbon nanotube-metal composite ink:”

7. Ma, J. Mater. Chem., 2011, 21, 7070-7073

8. Zhao, Smart Materials and Structures, Volume 21, Issue 11, November 2012, Pages 71-75

9. Wang, X.; Guo, W.; Zhu, Y.; Liang, X.; Wang, F.; Peng, P. Electrical and Mechanical Properties of Ink Printed Composite Electrodes on Plastic Substrates. Appl. Sci. 2018, 8, 2101.

10. Hu Dan, Zhu Wei, Peng Yuncheng, Shen Shengfei, Deng Yuan. Flexible carbon nanotube-enriched silver electrode films with high electrical conductivity and reliability prepared by facile screen printing. J. Mater. Sci. Techno!., 2017, 33(10): 1113-1119.

From the foregoing, it will be appreciated that specific embodiments of the invention have been described herein for purposes of illustration, but that various modifications may be made without deviating from the scope of the invention.

Claims

What is claimed is:

1. An ink composition comprising: (a) silver (Ag)-metallized carbon nanotubes (CNTs) comprising CNTs having covalently bound first silver nanoparticles, (b) silver particles, and (c) one or more solvents, wherein the composition comprises 15 to 90 weight percent (wt%) of total silver, 0.001 to 5 wt% CNT, and 10 to 85 wt% of the one or more solvents, and the wt% of total silver includes the first silver nanoparticles bound to CNTs and unbound silver particles, and the wt% of CNT includes CNTs only, and does not include the first silver nanoparticles bound to the CNTs.

2. The ink composition of claim 1, which comprises 45-85 wt% of total silver, 0.01 to 0.5 wt% of CNT, and 15 to 55 wt% of solvent.

3. The ink composition of claim 1, wherein the amount of CNT in weight percent, relative to the total amount of silver, is about 0.01 to 5 wt%.

4. The ink composition of claim 1, wherein the weight ratio of CNT to silver in the Ag- metallized CNT is about 1:0.1 to 1:100.

5. The ink composition of claim 4, wherein the weight ratio of CNT to silver in the Ag- metallized CNT is about 1:1 to 1:10.

6. The ink composition of any one of claims 1-5, wherein the silver particles of (b) comprise second silver nanoparticles and silver flake particles.

7. The ink composition of claim 6, wherein the second silver nanoparticles have a diameter of 1-100 nm, and the silver flake particles have a diameter of 0.1-10 pm.

8. The ink composition of claim 6, wherein silver particles of (b) comprise 30-60 wt% of the second silver nanoparticles and 40-70 wt% of the silver flake particles.

9. The ink composition of any one of claims 1-5, wherein the silver particles of (b) comprise silver flake particles and do not comprise silver nanoparticles.

10. The ink composition of claim 1, wherein each of the first silver nanoparticles is covalently bound to CNTs through a carboxyl linkage.

11. The ink composition of claim 1, wherein the one or more solvents are organic solvents and are selected from the group consisting of: isopropanol (IP A), 2-ethoxy ethanol, methyl n-amyl ketone, diisobutyl ketone, 2-butoxy ethanol, l-(2-methoxypropoxy)-2- propanol (MPP), di(propylene glycol) methyl ether, 2-(2-ethoxyethoxy)ethanol, n-methyl-2- pyrrolidone, ethylacetate, diethylene glycol monobutyl ether, diethylene glycol, diethylene glycol n-butyl ether acetate, 2,2,4-trimethyl-l,3-pentanediol monoisobutyrate (TMPD-MIB, or Texanol), diethylene glycol dibutyl ether (DGDE), methyl 5-(dimethylammo)~2 -methyl-5- oxopentanoate, glycerin, dibutyl tartrate, and any combination thereof.

12. The ink composition of claim 1, wherein the length of CNT strands, bundles, or individual CNT dispersed in the ink is about 1-500 pm.

13. The ink composition of claim 12, wherein the length of CNT strands, bundles, or individual CNT dispersed in the ink is between about 1 pm and 40 pm.

14. The ink composition of claim 1, wherein the CNT is SWCNT, DWCNT, MWCNT, or any mixture thereof.

15. An electrically conductive line, shape, pattern, or circuit printed onto a substrate, wherein the electrically conductive line, shape, pattern, or circuit comprises the printed and cured ink composition of claim 1.

16. The electrically conductive line, shape, pattern, or circuit of claim 15, having a thickness >3 pm.

17. The electrically conductive line, shape, pattern or circuit of claim 15, having a conductivity of > 5% relative to that of pure bulk silver, and a thermoform elongation of > 5%.

18. A method for screen printing an electrically conductive line, shape, pattern or circuit on a substrate, comprising transferring an ink composition onto a substrate through a screen plate comprising apertures filled with mesh wire, wherein the length of each CNT strand in the ink composition is less than 50% of the circumference of the mesh wire.

19. A method for preparing Ag-metallized CNTs, comprising the steps of:

(a) mixing a CNT dispersion comprising CNTs having carboxyl groups in a first solvent with ammonium hydroxide, ammonia, or an alkyl amine, and a silver salt, and reacting for a period of time; (b) reacting the mixture of (a) with formic acid to form CNTs having silver nanoparticles covalently bound on the CNT surfaces to form CNTs covalently bound with silver nanoparticles.

20. A method for preparing an ink composition, comprising the steps of: (a) mixing a first CNT dispersion comprising CNTs having carboxyl groups in a first solvent with ammonium hydroxide, ammonia, or an alkyl amine, and a silver salt, and reacting for a period of time;

(b) reacting the mixture of (a) with formic acid to form CNTs having silver nanoparticles covalently bound on the CNT surfaces to form a second CNT dispersion comprising CNTs covalently bound with silver nanoparticles; and

(c) mixing a solution comprising silver particles with the second CNT dispersion to form the ink composition.