AND METHODS FOR USING SAME

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application makes reference to and claims the priority benefit of the following co-pending U.S. Provisional Patent Application No. 62/468,910, filed March 8, 2010. The entire disclosure and contents of the foregoing Provisional Application is hereby incorporated by reference.

Field of the Invention

[0002] The present invention broadly relates to electrically conductive adhesive compositions comprising one or more epoxy resins, one or more nanoparticulate conductive metals having a functionalized coating, and one or more electrically conductive nanoscopic carbon materials having a coating of one or more liquid polymeric phosphite plasticizers. The present invention also broadly relates to kits for forming such electrically conductive adhesive compositions. The present invention further broadly relates to methods for forming activated electrically conductive adhesive compositions.

BACKGROUND

[0003] Polymer-based electrically conductive adhesives (ECAs) have drawn much attention as an environmentally friendly solution for lead-free interconnects due to the advantages of environmental friendliness, mild processing conditions, fewer processing steps, and especially, the fine pitch interconnect capability due to the availability of small-sized conductive fillers. Electrically conductive adhesives (ECAs) are composites of polymeric matrices, electrically conductive fillers and processing aids. The polymeric resin provides physical and mechanical properties such as adhesion, mechanical strength, impact strength, and the filler (such as, silver, gold, nickel, carbon particles or copper) conducts electricity. The conductive fillers provide the composite with electrical conductivity through physical contact between the conductive particles.

[0004] ECAs have been widely used in bonding the flex circuit of a glass liquid crystal display (LCD) to the matching conducting electrode traces on a printed circuit board (PCB), or attaching a component lead to a matching pad on a thermally-sensitive printed circuit board, etc. ECAs materials are needed to replace the estimated 50,000 metric tons of tin-lead solder currently used each year because of toxicity concerns, but there are no "drop-in" replacements for eutectic tin-lead solder. Fine-pitch interconnection is one of the most important technologies for device miniaturization. Metal solders, including eutectic tin/lead solder and lead-free alloys, are the most common interconnect materials in the electronic/photonic packaging areas. Conventional eutectic tin/lead solder or lead-free solders cannot meet the requirements for fine-pitch assembly due to their stencil printing resolution limit and bridging issues of solders in fine-pitch bonding, which is an intrinsic characteristic of metal solders.

SUMMARY

[0005] According to one broad aspect of the present invention, there is provided a composition comprising an electrically conductive adhesive, the electrically conductive adhesive comprising: one or more epoxy resins; from about 40 to about 65% by weight of the epoxy resins of one or more electrically conductive metal nanoparticulates, the metal nanoparticulates having a functionalized coating of one or more C4-C20 dicarboxylic acids, or C4-C20 diols; and

from about 0.01 to about 5% by weight of the epoxy resins of one or more electrically conductive nanoscopic carbon materials, the nanoscopic carbon materials having a coating of one or more liquid polymeric phosphite plasticizers.

[0006] According to second broad aspect of the present invention, there is provided a kit for forming an activated electrically conductive adhesive composition, the kit comprising: a Part A comprising one or more epoxy resins;

a Part B comprising one or more epoxy resin curing agents; and

a Part C comprising an electrically conductive component having: one or more electrically conductive metal nanoparticulates, the metal nanoparticulates having a functionalized coating of one or more C4-C20 dicarboxylic acids, or C4-C20 diols; and one or more electrically conductive carbon materials, the nanoscopic carbon materials having a coating of one or more liquid polymeric phosphite plasticizers.

[0007] According to third broad aspect of the present invention, there is provided method for forming an activate electronically conductive adhesive composition suitable for subsequent use in joining two or more components of a device and for forming an electrically conductive network, the method comprising the steps of:

(a) providing one or more epoxy resins;

(b) providing one or more epoxy resin curing agents, and

(c) providing an electrically conductive mixture having: one or more electrically conductive metal nanoparticulates, the metal nanoparticulates having a functionalized coating of one or more C4-C ₂ 0 dicarboxylic acids, or C4-C20 diols; and one or more electrically conductive nanoscopic carbon materials, the nanoscopic carbon materials having a coating of one or more liquid polymeric phosphite plasticizers; and

(d) combining the epoxy resins of step (a), the epoxy curing agent of step (b) and the electrically conductive mixture of step (c) together to form the activated electrically conductive adhesive composition.

DETAILED DESCRIPTION

[0008] It is advantageous to define several terms before describing the invention. It should be appreciated that the following definitions are used throughout this application.

Definitions

[0009] Where the definition of terms departs from the commonly used meaning of the term, applicant intends to utilize the definitions provided below, unless specifically indicated.

[0010] For the purposes of the present invention, the term "electrically conductive adhesive composition" refers to a composition which comprises one or more epoxy resins, and optionally one or more electrically conductive metal nanoparticulates having a functionalized coating and/or one or more electrically conductive nanoscopic carbon materials having a coating one or more liquid polymeric phosphite plasticizers.

[0011] For the purposes of the present invention, the term "activated electrically conductive adhesive composition" refers to electrically conductive adhesive composition in which the epoxy resin which been activated by inclusion, combination with, etc., one or more epoxy resin curing agents.

[0012] For the purposes of the present invention, the term "epoxy resin" refers to a class of reactive prepolymers and polymers which contain epoxide groups. Suitable epoxy resins may include one or more of: glycidyl ethers of C ₂ -C ₂ 8 diols; Ci-C ₂₈ alkyl- or poly-phenol glycidyl ethers; polyglycidyl ethers of pyrocatechol, resorcinol, or hydroquinone; 4,4'- dihydroxydiphenyl methane (or bisphenol F, such as RE-303-S or RE-404-S available commercially from Nippon Kayuku, Japan); 4,4'-dihydroxy-3,3'-dimethyldiphenyl methane; 4,4'-dihydroxydiphenyl dimethyl methane (or bisphenol A); 4,4'-dihydroxydiphenyl methyl methane; 4,4'-dihydroxydiphenyl cyclohexane; 4,4'-dihydroxy-3,3'-dimethyldiphenyl propane; 4,4'-dihydroxydiphenyl sulfone; tris(4-hydroxyphenyl)methane; polyglycidyl ethers of transition metal complexes; chlorination and bromination products of the foregoing diphenols; polyglycidyl ethers of novolacs; polyglycidyl ethers of diphenols obtained by esterifying ethers of diphenols obtained by esterifying salts of an aromatic hydrocarboxylic acid with a dihaloalkane or dihalogen dialkyl ether; polyglycidyl ethers of polyphenols obtained by condensing phenols and long-chain halogen paraffins containing at least two halogen atoms; phenol novolac epoxy; cresol novolac epoxy; etc. See, for example, U.S. Pat. No. 9,589,693 (Dreezen et al), issued March 7, 2017, the entire disclosure and contents of which is herein incorporated by reference, for suitable epoxy resins for use in embodiments of the present invention.

[0013] For the purposes of the present invention, the term "epoxy resin curing agent" (interchangeable with the terms "epoxy resin hardening agents" and "epoxy resin accelerator agents") refers to those compositions, compounds, etc., which activate the epoxy resin. Suitable epoxy resin curing agent for use in embodiments of the present invention may include one or more amines, including: primary amines; secondary amines; primary diamines; secondary diamines; etc. These amine curing agents may be aliphatic, cycloaliphatic, aromatic, etc., may be difunctional, polyfunctional, etc., may be substituted or unsubstituted, etc. Suitable amine curing agents for use in embodiments of the present invention may include, for example: polyoxypropylenediamine; meta-xylenediamine; polyetheramines; amidoamines; etc. The ratio between the curing agent and the epoxy resin may be calculated and optimized by those skilled in this art for desired curing properties.

[0014] For the purposes of the present invention, the term "electrically conductive" refers to materials which have the property, capability, etc., to conduct an electric current. Electrically conductive materials may include conductive metals such as copper, silver, nickel, aluminum, gold, etc., as well as combinations thereof; carbon materials such as carbon nano fibers, graphene nanoplatelets, etc., as well as combinations thereof.

[0015] For the purposes of the present invention, the term "conductive metal nanoparticulates" refers to nanoscopic particulates {e.g., nanoparticles, nanorods, nanowhiskers, etc) which are formed from electrically conductive metals such as silver, copper, nickel, aluminum, etc., or combinations such metals, and which may be dispersed in a medium such as a paste, paint, ink, etc. The conductive metal nanoparticulates {e.g., silver nanoparticulates) may have an average diameter of from about 1 nanometer to about 10000 nanometers and can have spherical or flake-like shape. The particle size is defined herein as the average diameter of the silver-containing particles, as determined by TEM (transmission electron microscopy). The conductive metal nanoparticulates may be formed from one metal, or alloys, may be in the form of shells, flakes, wires, strands, etc.

[0016] For the purposes of the present invention, the term "functionalized coating" for the conductive metal nanoparticulates refers to a coating formed on the surface of the nanoparticulates with one or more C ₄ -C ₂ 4 dicarboxylic acids (such as C ₄ -C ₇ dicarboxylic acids), or C4-C24 diols (such as C4-C7 diols). Suitable functionalized coatings of these dicarboxylic acids/diols for these conductive metal nanoparticulates may be formed with, for example, glutaric acid, adipic acid, pimelic acid, etc. The molar ratio of conductive metal nanoparticulates {e.g., silver nanoparticles) to C3-C6 dicarboxylic acids/diols {e.g., glutaric acid) may be in the range of from about 0.1 : 1 to about 1 : 1. The resulting nanoparticulates may also have a metal {e.g., silver) content of 80% by weight or more.

[0017] For the purposes of the present invention, the term "carbon material" refers to materials made of carbon, and which may function as one or more of: electrically conductive materials; structural reinforcement materials; nanoscopic particulate fillers; etc. Carbon materials may include one or more of: carbon nanofibers (including carbon-based nanotubes); graphite; graphite flakes; carbon black; graphene; graphene-like materials; {e.g., reduced graphene oxide, functionalized graphene, graphene oxide, partially reduced graphene oxide, etc); etc.

[0018] For the purposes of the present invention, the term "liquid polymeric phosphite plasticizers" refers to polymeric phosphite plasticizers which are liquid at room temperature and which can coat the electrically conductive nanoscopic carbon materials, such as the carbon nano fibers, carbon nanotubes, vapor growth carbon fibers (VGCFs), etc., and thus keep such nanoscopic carbon materials separated and from clumping together. Suitable liquid polymeric phosphite plasticizers for use in embodiments of the present invention may include alkylphenol free-liquid polymeric phosphite stabilizers illustrated by Structure IV and the accompanying description at column 4, line 50 through column 5, line 10 of U.S. Pat No. 8,563,637 (Jakupca et al), issued October 22, 2013, the entire disclosure and contents of which is herein incorporated by reference, such as the alkylphenol free-liquid polymeric phosphite plasticizer known by the tradename Doverphos LGP-11 sold by Dover Chemical Corp.

[0019] For the purposes of the present invention, the term "carbon nanofibers (CNFs)" refers to cylindrical nanostructures having graphene layers arranged as stacked cones, cups, plates, etc. Carbon nanofibers with graphene layers wrapped and arranged as cylinders are commonly referred to as carbon nanotubes. Carbon nanofibers (CNFs) may be produced either in a vapor-grown form or by electrospinning. Vapor-grown carbon nanofibers may be in the form of a free-flowing powder {e.g., wherein 99% of the carbon mass is in a fibrous form) known as multi-walled carbon nanotubes (MWCN) or stacked-cup carbon nanotubes (SCCNT) where the graphene plane surface is canted from the fiber axis, thus exposing the plane edges present on the interior and exterior surfaces of the carbon nanotubes, and may be produced by the floating catalyst method in the vapor phase by decomposing carbon- containing gases, such as methane, ethane, acetylene, carbon monoxide, benzene, coal gas, etc., in presence of floating metal catalyst particles inside a high-temperature reactor. Ultrafine particles of the catalyst may be either carried by the floating gas into the reactor or produced directly in the reactor by decomposing of the catalyst precursor. One such catalyst is iron, which may be produced by the decomposition of ferrocene. However other metals alone or in combination may be utilized as well as catalysts. Carbon nanofibers suitable for use herein may have an average diameter in the range of from about 20 to about 150 nm {e.g., from about 60 to about 150 nm) depending upon the grade and may have lengths of, for example, in the range of from about 3 to about 100 microns {e.g., about 30 to about 100 microns). Carbon nanofibers may undergo post treatment after production, including removing impurities on their surface, such as tar and other aromatic hydrocarbons, by a process called pyro lytic stripping, that involves heating, for example, to about 1000°C in a reducing atmosphere. Sometimes heating, for example, to 3000°C may be used to impart higher tensile strength and tensile modulus by graphitizing the surface of the carbon fibers. However, the heat treatment which may achieve an improved combination of mechanical and electrical properties may be found at a temperature of, for example, about 1500°C. In embodiments of the composites of the present invention, commercially available sources of suitable carbon nanofibers may be obtained, for example, from Applied Sciences Inc. as grade PR-24XT-LHT, PR-24XT-HHT as well as Aldrich product 719803, Grupo Antolin carbon nanofibers (GANF1 and GANF3), etc.

[0020] For the purposes of the present invention, the term "graphene-like material" refers to a material, substance, etc., which may have a layered structure the same or similar to graphene. Graphene-like materials may include one or more of: graphene; functionalized graphene; graphene oxide; partially reduced graphene oxide; graphite flakes; graphene nanoplatelets; etc.

[0021] For the purposes of the present invention, the term "graphene" refers to pure or relatively pure carbon in the form of a relatively thin, nearly transparent sheet, which is one atom in thickness {i.e., a monolayer sheet of carbon), or comprising multiple layers (multilayer carbon sheets), having a plurality of interconnected hexagonal cells of carbon atoms most of which are present in sp ² hybridized state and which form a honeycomb like crystalline lattice structure. In addition to hexagonal cells, pentagonal and heptagonal cells (defects), versus hexagonal cells, may also be present in this crystal lattice.

[0022] For the purposes of the present invention, the term "functionalized graphene" refers to graphene which has incorporated into the graphene lattice a variety chemical functional groups such as -OH, -COOH, -NH _¾ etc., in order to modify the properties of the graphene.

[0023] For the purposes of the present invention, the term "graphene oxide" (also known as "graphitic acid" and " graphite oxide") refers interchangeably to a compound of carbon, oxygen, and hydrogen which may exist in variable ratios of these three atoms, and which may be obtained by treating graphite with strong oxidizers. [0024] For the purposes of the present invention, the term "partially reduced graphene oxide" refers to graphene oxide that, upon reduction, contains from about 5 about 30% oxygen by weight of the graphene oxide.

[0025] For the purposes of the present invention, the term "graphene nanoplatelets (NGPs)" and "nanosheets" refer interchangeably to platelets of graphene, and may also refer to platelets and sheets comprised of other graphene-like materials such as graphene oxide, partially reduced graphene oxide, functionalized graphene, etc., having a thickness in the range of from about 0.34 to about 100 nm and may include one material or in any combination.

[0026] For the purposes of the present invention, the term "flakes" refers to particles in which two of the dimensions {i.e., width and length) are significantly greater compared to the third dimension {i.e., thickness).

[0027] For the purposes of the present invention, the term "nanoscopic" refers to materials, substances, structures, etc., having a size in at least one dimension {e.g., diameter, thickness, etc) of from about 1 to about 1000 nanometers, such as from about 1 to about 100 nanometers. Nanoscopic materials, substances, structures, etc., may include, for example, nanoplatelets, nanotubes, nanowhiskers, flakes, etc.

[0028] For the purposes of the present invention, the term "closely- spaced stack-like arrangement" refers to an atomic arrangement in a crystalline phase wherein covalently or ionically bonded atoms form layered structures, which arrange themselves in close proximity and parallel to each other. These layers are weakly bound by Van der Waals forces.

[0029] For the purposes of the present invention, the term "conductive metal nanoparticulates" refers to nanoscopic particulates {e.g., nanoparticles, nanorods, nanowhiskers, etc) which are formed from the electrically conductive metals.

[0030] For the purposes of the present invention, the term "liquid" refers to a non-gaseous fluid composition, compound, substance, material, etc., which may be readily flowable at the temperature of use {e.g., room temperature) with little or no tendency to disperse and with a relatively high compressibility.

[0031] For the purposes of the present invention, the term "solid" refers to non-volatile, non-liquid components, compounds, materials, etc., which may be in the form of, for example, particulates, particles, powders, etc. [0032] For the purposes of the present invention, the term "room temperature" refers to refers to the commonly accepted meaning of room temperature, i.e., an ambient temperature of from about 20° to about 25°C.

[0033] For the purposes of the present invention, the term "substantially uniform" refers to a composition, dispersion, material, substance, etc., which is substantially uniform in terms of composition, texture, characteristics, properties, etc.

[0034] For the purposes of the present invention, the term "powder" refers to a solid material which is comprised of a large number of fine particles.

[0035] For the purposes of the present invention, the term "exfoliation" refers to the chemical and/or physical process of separation of layers of a material {e.g., graphite flakes).

[0036] For the purposes of the present invention, the term "sonication" refers to applying sound energy {e.g., sound waves) to agitate, stir, mix, etc., for example, one or more liquids, solid particles, etc. Sonication may also be used to facilitate the process of exfoliation.

[0037] For the purposes of the present invention, the term "volumetric resistivity" (also known interchangeably as "electrical resistivity," "resistivity," "specific electrical resistance," "volume resistivity," etc) refers to the degree to which a material resists the flow of electrical current, and is measured herein in units of Ohmxcm.

[0038] For the purposes of the present invention, the term "comprising" means various compositions, compounds, components, elements, steps, etc., may be conjointly employed in embodiments of the present invention. Accordingly, the term "comprising" encompasses the more restrictive terms "consisting essentially of and "consisting of."

[0039] For the purposes of the present invention, the terms "a" and "an" and similar phrases are to be interpreted as "at least one" and "one or more." References to "an" embodiment in this disclosure are not necessarily to the same embodiment.

[0040] For the purposes of the present invention, the term "and/or" means that one or more of the various compositions, compounds, components, elements, steps, etc., may be employed in embodiments of the present invention.

[0041] Unless otherwise specified, all percentages given herein are by weight. Description [0042] Recently, polymer-based nanomaterials have been intensively studied to replace metal-based interconnect materials, as well to enhance the properties of various electrically conductive adhesives (ECAs). Among different metal particles, silver in a form microscopic flakes is commonly used as an electrically conductive material for current commercial ECAs because of the high conductivity and the maximum contact between such flakes. In addition, silver is unique among all the cost-effective metals by nature of its conductive oxide. Silver also has the highest room temperature electrical and thermal conductivity among all the conductive metals. Silver-containing ECAs usually have high filler loadings (e.g., about 70% wt. or more) that weaken the overall mechanical strength. But the disadvantage associated with associated with ECAs containing microscopic silver flakes may include high cost, decreased mechanical strength, brittleness, lower adhesion strength, etc.

[0043] Nano-sized silver particulates (silver nanoparticulates) may be used in ECAs to replace the micro-sized silver flakes. Due to its small size for a fixed amount of addition, the silver nanoparticulates contain a larger number of particles when compared with micro-sized particles. This large number of particles for the silver nanoparticulates should be beneficial to the connection between such particles. For silver nanoparticulates, sintering behavior may occur at much lower temperatures, such that the use of silver nanoparticulates in ECAs might be promising for higher electrical performance of ECA joints by eliminating the interface between metal particulates. The application of silver nanoparticulates may also increase the number of conductive materials on each bond pad and result in more contact area between silver nanoparticulates and bond pads. Disadvantages associated with silver nanoparticulates may include higher cost (compared to micro-silver flakes), limited shelf life, segmentation, etc.

[0044] In embodiments of the ECAs of the present invention, it has been found that such silver nanoparticles may be more uniformly dispersed in a polymer matrix when the surface of these silver nanoparticles has a functionalized coating. This functionalized coating may be provided by certain interfacial modifiers that include, for example, one or more C ₃ -C ₆ dicarboxylic acids, or C ₃ -C ₆ diols, such as glutaric acid.

[0045] In embodiments of the ECAs of the present invention, it has been found to be extremely attractive to partially replace expensive conductive metal nanoparticulates such as silver nanoparticulates with certain conductive nanoscopic carbon materials. In addition, in order to go below about 0.1 Ω-cm in volume resistivity, it has been found that a combination of conductive nanoscopic carbon materials with conductive metal nanoparticulates such as silver nanoparticulates is required to achieve such low volume resistivity for ECAs used in most of applications related to electronic devices.

[0046] Attractive candidates for such ECAs comprising combinations of conductive nanoscopic carbon materials with conductive metal nanoparticulates are vapor-grown carbon fibers (VGCF), as well as carbon nano fibers with a diameter of 100-200 nm and a length of 10-100 microns. VGCFs are hollow-core filaments that may be produced by catalytic chemical vapor deposition (CVD). In order to remove a top layer with poor electrical properties, these carbon fibers may be either carbonized at 1500°C (Pyrograf I PR-24-XT- LHT) or graphitized at 2500-3000°C (Pyrograf III PR-24-XT-HHT).

[0047] While graphitized VGCFs possess excellent intrinsic electric properties, VGCFs may not adhere well in thermoset matrixes that comprise ECAs. In order to improve the dispersion and carbon fiber-epoxy matrix adhesion, a dispersant aid in the form of one or more liquid polymeric phosphite plasticizers is used to apply a thin coating to the VGCFs prior to immersion in the epoxy resin present in the ECAs.

[0048] In one of embodiment of the present invention there is provided an electrically conductive adhesive composition. The electrically conductive adhesive composition comprises: one or more epoxy resins; from about 1 to about 99% (such as from about 40 to about 65%) by weight of the epoxy resins of one or more electrically conductive metal nanoparticulates, the metal nanoparticulates having a functionalized coating of one or more C4-C20 dicarboxylic acids, or C4-C20 diols; and from about 0.01 to about 5% (such as from about 0.5 to about 5%) by weight of the epoxy resins of one or more electrically conductive nanoscopic carbon materials, the nanoscopic carbon materials having a coating of one or more liquid polymeric phosphite plasticizers.

[0049] In another embodiment of the present invention, there is provided a kit for forming an activated electrically conductive adhesive composition. The kit comprises: a Part A comprising one or more epoxy resins; a Part B comprising one or more epoxy resin curing agents; and a Part C comprising an electrically conductive component having: one or more electrically conductive metal nanoparticulates, the metal nanoparticulates having a functionalized coating of one or more C4-C20 dicarboxylic acids, or C4-C20 diols; and one or more electrically conductive carbon materials, the nanoscopic carbon materials having a coating of one or more liquid polymeric phosphite plasticizers. In some embodiments of the kit, Part A comprises at least a portion of Part C. In other embodiments of the kit, Part B comprises at least a portion of Part C. In yet other embodiments of the kit, Part A comprises a first portion of Part B, and Part B comprises a second portion of Part C. In still other embodiments of the kit, Part C is separate from both Part A and Part B.

Example of Method for Making Epoxy Composite

[0050] Coating of VGCFs with liquid polymeric phosphite plasticizer: Vapor grown carbon fiber (VGCF) is coated with a liquid polymeric phosphite plasticizer (Doverphos LGP-11 from Dover Chemical Corp.). The mixture of the VGCFs (25% wt), liquid polymeric phosphite plasticizer (10% wt.) and chloroform as the solvent is stirred using a magnetic stirrer for 1 hour. The chloroform solvent is then completely removed under reduced pressure. The resulting coated VGCFs are dried in an oven at 70°C for overnight.

[0051] Silver nanoparticles coated by glutaric acid: The silver nanoparticles are sonicated in chloroform as the solvent for one hour in order to remove the existing polymer coating. Then the solution is centrifuged and the chloroform solvent layer is removed by decantation. Glutaric acid (9 mmol/L) is dissolved in ethyl alcohol as the solvent, then silver nanoparticles (9 mmol/L) are added to this solution and the resulting solution is sonicated for 2 hours. This solution is then centrifuged and the top ethanol solvent layer is decanted. After rinsing the coated nanoparticles with ethanol several times, these coated nanoparticles are dried in a vacuum chamber for 24 hours at room temperature.

[0052] Formulation of the epoxy composite: An epoxy resin system is formed from two parts (Part A and Part B). Part A comprises the epoxy resin as a low molecular weight pre- polymer or higher molecular weight polymer which contains at least two epoxide groups. Part B comprises the epoxy resin curing agent.

[0053] Part A is prepared by the mixing of a Bisphenol A epoxy resin (Epon 825 (29.2%), defoamer Byk-A530 (0.2%) and polypropylene glycol diglycidyl ether (9.6%)), and Doverphos LGP-11 (1%) on a high speed planetary mixer (HSM). After that, the conductive materials are added to the epoxy resin in the following order: first, Silver Micro Flakes (48%), then VGCFs coated with Doverphos LGP-11 (1.97%). After addition of the conductive materials, the mixture is stirred with a high speed mixer (HSM) for 5 min at 2500 rpm. In the next step, silver nanoparticles coated with glutaric acid (10%) and graphene nanopowder (0.03%) are then added to this mixture. The resulting mixture is then stirred one more time for 10 min using the HSM at 2500 rpm, providing the epoxy system in the form of a silver grey and very smooth thixotropic paste. [0054] Curing epoxy system with conductive materials: Polyoxypropylenediamine is used as the epoxy resin curing agent. The curing agent is added to the epoxy system in an epoxy/amine equivalent ratio of 1 to 3 and the resulting mixture is mixed on the HSM for 5 minutes at 2500 rpm. The resulting mixture is cured at 150°C for 1 hour to form an epoxy composite.

[0055] The resistivity of the resulting epoxy composite is determined according to "Standard Test Method for Volume Resistivity of Conductive Adhesives" and MIL-STD 883/5011. Two strips of a Scotch tape are applied onto a pre-cleaned glass slide. The paste is coated between the two strips (5mm wide). Then the strips were removed and the conductive paste is cured at 150°C for 1 hour. The thickness of the cured film is determined by Mitutyo (thickness measuring equipment, Japan). The obtained resistivity is in the range 0.01-0.0001 Ω-cm.

[0056] Instead of the VGCFs used in the above example, other types of electrically conductive nanoscopic carbon materials may be used, for example, nanoscopic carbon fibers, carbon nanotubes including single (SWCN) and multiwall (MWCN) carbon nanotubes, graphene nanoplatelets (G P), reduced graphene oxide (RGO), carbon black, micronized graphite, amorphous carbon, etc. These electrically conductive nanoscopic carbon materials may be subjected to various treatments such as chemical modification, including oxidation, reduction, attachment of functional groups, grafting, thermal treatments such as oxidation, reduction, carbonization, graphitization, etc.

[0057] The dispersant aid may be selected from any aliphatic or aromatic dicarboxylic acids, their aliphatic or aromatic esters, amides or anhydrides; any high molecular weight liquid polymeric phosphite plasticizer; any low molecular weight condensation product of dicarboxylic acids and ethylene/diethylene glycols; any alkyl pelargonates; any polypropylene glycol monoalkylethers; any N-alkylpyrrolidones or N-arylpyrrolidones; any liquid polymeric phosphite plasticizer used in combination with any phenolic antioxidants or any thioesters and any synthetic elastomers and their combinations.

[0058] Besides chloroform and ethyl alcohol, suitable organic alcohols for use herein can be one or more of: methanol; isopropanol; isobutyl alcohol; acetone; butyl acetate; methyl ethyl ketone; methyl amyl ketone; hexane; toluene; benzene; cyclohexane; pentane; bromobenzene; chlorobenzene; etc. [0059] Besides high speed mixers (HSMs) and sonication, other mixing devices such as magnetic stirrers, mechanic stirrers, etc. The silver flakes and VGCFs may be added to the suspension in any order. The load amount of silver flakes may in the range of from about 25 to about 80% wt. of the final cured epoxy. The silver nanoparticulates and/or VGCFs may be at least partially present in epoxy resin curing agent. The amount of silver nanoparticles in the formulation may be from about 1 to about 20% wt. of the final cured epoxy. The amount of VGCFs in the formulation may be from about 0.001 to about 10% wt. of the final cured epoxy.

[0060] All documents, patents, journal articles and other materials cited in the present application are hereby incorporated by reference.

[0061] Although the present invention has been fully described in conjunction with several embodiments thereof with reference to the accompanying drawings, it is to be understood that various changes and modifications may be apparent to those skilled in the art. Such changes and modifications are to be understood as included within the scope of the present invention as defined by the appended claims, unless they depart therefrom.

[0062] In addition, the purpose of the Abstract of the Disclosure in this application is to enable the U.S. Patent and Trademark Office, as well as the public generally, including any scientists, engineers and practitioners in the art who may not be familiar with patent or other legal terms or phraseology to determine the what the technical disclosure of the application describes. Accordingly, while the Abstract of the Disclosure may be used to provide enablement for the following claims, it is not intended to be limiting as to the scope of those claims in any way.

[0063] Finally, it is the applicant's intent that only claims which include the express language "means for" or "step for" be interpreted under 35 U.S.C. § 112, paragraph 6. Accordingly, claims that do not expressly include the phrase "means for" or "step for" are not to be interpreted as being within the purview of 35 U.S.C. § 112, paragraph 6, or to be construed as being subject to any case law interpreting the meaning of these phrases.