CARBON/METAL HYBRID FILLERS, ELECTRICALLY CONDUCTIVE ADHESIVE, METHODS AND USES

THEREOF

TECHNICAL FIELD

[0001] The present disclosure relates to nano-sized electrically conductive materials, in particular to carbon/metal hybrid, to be used as fillers for non-conductive matrices, in particular for polymeric resins, with the purposed objective of endowing electrical conductivity to the composite matrix-filler. This approach was pursued by noncovalent functionalization of the carbon nanotubes (CNTs) with molecules having metal chelating groups or by covalent functionalization of the CNTs with molecules having metal chelating groups. In particular, the metal chelating groups attached to the CNTs are then used to grow the metallic component of the hybrid filler.

[0002] Furthermore, this disclosure also relates to methods for the production of hybrid nano- carbon/nano-metal network fillers, where CNTs are functionalized with metal nanoparticles, through chemical coordination either to non-covalent or covalently bonded functional groups.

BACKGROUND

[0003] The search for new materials or formulations for electrical conductivity has been a challenge dating from early in the XXth century. The motivation behind such efforts is the interest in bridging certain physical, mechanical and other interesting properties of non-conductive materials, with electrical interconnection. Obvious applications for these materials are in the fields of electrical and electronic manufacturing or maintenance, but also range to high-end applications as sensors, power generation or miniaturized medical devices.

[0004] Successful innovations range from new alloys, such as in solders, to polymeric materials. Solders are actually the mainstay of the electronic assembly industry; however, in recent years, some commercial polymeric formulations were developed aiming the replacement of solder in PCB assembly.

[0005] Modern commercial adhesives for electrical interconnection are basically a formulation of a resin such as epoxy or silicone, with a conductive filler. Silver is the most common filler, dispersed in micro- and nanosized forms. The cost of silver and the large filler content required to allow conductivity through percolation, in particular 40 to 80 % in weight, make silver-containing adhesives costly.

[0006] Document US5891366 discloses an anisotropically conducting adhesive that includes a thermoplastic base material; and particles which include metal particles and metal ions, which are electrically conductive, and which are finely distributed within the thermoplastic base material below a percolation threshold, wherein the particles are enriched in certain regions under the influence of exposure to at least one of light and heat.

[0007] Commercial electrically conductive adhesives are gaining increasing interest and have been progressively integrated into electronic component fabrication. In the near future, with the development of Flexible PCB solutions and Printed Electronics, it is expected that these materials will witness a surge in demand for new and more demanding formulations.

[0008] These facts are disclosed in order to illustrate the technical problem addressed by the present disclosure.

GENERAL DESCRIPTION

[0009] The present disclosure relates to the development of carbon/metal hybrid nanomaterials, to be used as fillers for endowing electrical conductivity to an otherwise non-conductive matrix. In particular, this disclosure relates to functionalization of CNTs for metal fixation, wherein said functionalization of CNTs is a covalent functionalization or a noncovalent functionalization.

[0010] An aspect of the present disclosure relates to a filler for an electrically conductive adhesive, comprising: a carbon nanotube, a metal nanoparticle and a perylene linker or a pyrrolidine linker, wherein the linker is also bound to the metal nanoparticle via a reactive group selected from an amine, a thiol or a carboxyl.

[0011] In an embodiment, the linker may be covalently bound to the carbon nanotube and to the metal nanoparticle for the pyrrolidine linker.

[0012] In an embodiment, the linker may be noncovalently bound to the carbon nanotube and covalently bound to the metal nanoparticle for the perylene linker.

[0013] In an embodiment, the linker may comprise two reactive groups independently selected from an amine, a thiol or a carboxyl, preferably four reactive groups independently selected from an amine, a thiol or a carboxyl.

[0014] In an embodiment, the perylene linker is wherein each NR is independently selected from an amino acid H2NR, preferably wherein both R are equal.

[0015] In an embodiment, wherein each R may be selected independently from the following list:

r mixtures thereof.

[0016] In an embodiment, the carbon nanotube may be selected from single-walled carbon nanotubes (SWCNT), double walled carbon nanotubes (DWCNT), multi-walled carbon nanotubes (MWCNT), or mixtures thereof.

[0017] In an embodiment, the metal nanoparticle may be selected from silver, gold, copper, or mixtures thereof.

[0018] In an embodiment, the metal nanoparticle concentration may be less than 80 % (wtmetai/wtfiiier), preferably between 5-60 %( w ai/wtimer), more preferably 5-40 % (w ai/wtimer).

[0019] Another aspect of the present disclosure relates to an electrically conductive adhesive comprising the filler of the present disclosure and a polymeric matrix.

[0020] In an embodiment, the filler nanoparticles may be less than 20 % (wt _f ni _er /wt _adhesive ) preferably between 2-15 %( wt _f nier/wtad _hesive ), more preferably 5-10 % (wt _f mer/wtad _hesive ).

[0021] In an embodiment, the electrically conductive adhesive may further comprise graphite, preferably wherein the graphite concentration may be less than 5 % (wt _g raphite/wt _a dhesive). The graphite addition decreases the viscosity of the electrically conductive adhesive while maintained the conductivity.

[0022] In an embodiment, the polymeric matrix is a thermosetting resin, in particular the thermosetting resin is selected from a list consisting of: epoxy, silicone, acrylate, or mixtures thereof.

[0023] In an embodiment, the covalent functionalization of the CNTs was performed by the 1,3 dipolar cycloaddition. In particular, the 1,3 dipolar cycloaddition may be carried out using an amino acid, such as acid N-benzyloxycarbonylglycine, to introduce pyrrolidine groups (-CH2-NH-CH2-) at the surface of the CNTs. In this embodiment, the amine nitrogen can be a suitable linkage to the metal. The metal may be further used to initiate the growth of metal nanowires from the surface of the CNTs.

[0024] In an embodiment, the noncovalent functionalization of the CNTs was performed using perylene derivatives modified with amino acids, in particular wherein the amino acids are selected from cysteine, phenylalanine or lysine. The advantage of the noncovalent functionalization is that it is less damaging to the CNTs electronic structure.

[0025] In an embodiment, metals attached to the CNTs, covalently and noncovalently functionalized, are selected from copper and/or silver and/or gold.

[0026] In an embodiment, the metal is then used to initiate the growth of metal nanowires from the surface of the CNTs, independently of the CNTs functionalization, covalent or noncovalent.

[0027] In an embodiment, CNTs may be prepared/combined with graphite, as is or exfoliated in water with a pyrene modified molecule. The advantage of this preparation/combination is that the electrical properties of the composite are maintained, while the rheological properties are improved, meaning that the viscosity of the composite of CNTs combined with graphite decreases versus the viscosity of the composite of CNTs not combined with graphite, making it easier to work/manipulate the CNTs composite.

[0028] In an embodiment, CNTs may be replaced by carbon nanofibers, chopped carbon fibers, or graphite nanoplatelets, while all the remaining features are maintained.

[0029] In an embodiment, carbon nanofibers, chopped carbon fibers, or graphite nanoplatelets may be prepared/combined with graphite, as is or exfoliated in water with a pyrene modified molecule.

[0030] In an embodiment of the disclosure, it is provided a covalent functionalization of CNTs, for grafting of metal nanoparticles on the surface of the CNTs, said embodiment comprising:

a CNTs network of either a rolled single-walled graphene sheet, in particular a SWCNT structure, or multiple rolled concentric graphene sheets, in particular DWCNT or MWCNT structures;

an intermediary chemical compound, in particular an intermediary binder, with capacity to covalently bond to the CNTs, in particular to its outer surface, while additionally having another chemical group in the structure that allows retention of a metal particle; and

metal nanoparticles of either gold, copper or silver, in a form that is retainable by the intermediary binder and are later grown into a more complex nanostructure such as, but not only, a metal nanowire. [0031] In an embodiment of the disclosure, it is provided the noncovalent functionalization of CNTs, for grafting of metal nanoparticles on the surface of the CNTs, said embodiment comprising the following arrangement:

a non-modified CNTs network of either single-walled graphene sheet, in particular a SWCNT structure, or multiple rolled concentric graphene sheets, in particular a DWCNT or MWCNT structure;

an intermediary chemical compound with the ability to bind noncovalently to the CNT by p-p interaction, in particular to the CNT outer network, while additionally having another chemical group in the structure that allows anchoring of a metal particle; and

metal nanoparticles of either gold, copper or silver, in a form that is retainable by the coordinating compound and are later grown into a more complex nanostructure such as, but not only, a metal nanowire.

[0032] Another aspect of the present disclosure relates to a printed circuit board comprising the electrically conductive adhesive composition of the present disclosure.

[0033] The printed circuit board may be incorporated in medical device, car device or electronic equipment, among others.

[0034] Another aspect of the present disclosure relates to a method for manufacturing a printed circuit board comprising the step of applying a filler of the present disclosure for binding a component or components to the printed circuit board.

[0035] Another aspect of the present disclosure relates to the use of the filler of the present disclosure for binder of a component or components to the printed circuit board.

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The following figures provide preferred embodiments for illustrating the description and should not be seen as limiting the scope of the present disclosure.

[0037] Figure 1 Scheme of the reaction of perylenetetracarboxylic dianhydride with an amino acid (H2N-R) to produce the perylene bisimide derivatives, wherein H2NR stands for cysteine, lysine or phenylalanine.

[0038] Figure 2: Schematic representation of the present disclosure concerning the metal-coated CNTs as fillers for epoxy-based adhesive, wherein 1 corresponds to the synthesis of linkers; 2 corresponds to metal-linker bonding; 3 corresponds to linker-CNT deposition; 4 corresponds to nanowire growth; 5 corresponds to hybrid nano-carbon/nano-metal network fillers.

[0039] Figure 3: l-V curves of the composites prepared with the hybrid fillers DR102, covalent functionalized MWCNT with silver a), and DR104, covalent functionalized MWCNT with copper b). For each material were prepared a composite with the hybrid filler alone, and with added graphite, and with added graphite and SWCNTs.

[0040] Figure 4: SEM imaging a) to h) with different magnification and detection modes, and EDS plots i) and j) of the composites of hybrid filler DR 102, covalent functionalized MWCNT with silver attached 2 wt. %.

[0041] Figure 5: SEM imaging with different magnification and detection modes of the composites of hybrid filler DR 102, covalent functionalized MWCNT with silver attached, 2 wt. %.and graphite 5 wt. %.

[0042] Figure 6: SEM imaging with different magnification and detection modes of the composites of hybrid filler DR 102, covalent functionalized MWCNT with silver attached, 2 wt. %, graphite 5 wt. % and SWCNT 0.2 wt. %.

[0043] Figure 7: SEM imaging with different magnification and detection modes of the composites of hybrid filler DR 104, covalent functionalized MWCNT with copper attached 5 wt. %

[0044] Figure 8: SEM imaging with different magnification and detection modes of the composites of hybrid filler DR 104, covalent functionalized MWCNT with copper attached 5 wt. % and. graphite 5 wt. %.

[0045] Figure 9: SEM imaging with different magnification and detection modes of the composites of hybrid filler DR 104, covalent functionalized MWCNT with copper attached 5 wt. %, graphite 5 wt. % and SWCNT 0.2 wt. %.

DETAILED DESCRIPTION

[0046] The present disclosure relates to a filler for an electrically conductive adhesive, comprising a carbon nanotube, a metal nanoparticle, and a perylene linker or a pyrrolidine linker bound to the carbon nanotube; wherein the linker is also bound to the metal nanoparticle via a reactive group selected from an amine, a thiol or a carboxyl. [0047] The present disclosure also relates to an electrically conductive adhesive, a printed circuit board comprising the electrically conductive adhesive and a method for manufacturing a printed circuit board.

[0048] In an embodiment, the synthesis of the perylene derivatives was carried out as follows. The reaction represented in Figure 1 was used for the preparation of the perylene derivatives. The method used was the combination of perylenetetracarboxylic dianhydride, in particular with 2 to 5 equiv. of the amino acid (H2NR) and 1 to 25 equiv. of imidazole, wherein the amino acid is selected from cysteine, lysine or phenylalanine. The reaction mixture was kept at a constant temperature, in particular at a temperature of 95 to 125 °C with magnetic stirring for 1 to 12 h. Distilled water was added to the mixture and the unreacted perylene dianhydride was removed by filtration. The perylene bisimide (PBI) was precipitated from the aqueous solution by addition of acid. The solid was then filtered and washed with distilled water. The product was dried, in particular the product was dried for 24 h at 100 °C under vacuum.

[0049] In an embodiment, anchoring of metal ions, in particular copper, silver, gold or other, onto the perylene derivatives was carried out as follows: a solution with the synthesized PBI, in particular 0.1 to 3 mmol, in DMF, in particular 4 ml of DMF, and another solution with the corresponding metal salt, in particular 0.1 to 3 mmol, in ethanol, in particular 2-8 ml, the two solutions were mixed and the resulting solution was stirred, in particular at room temperature for 2 to 7 days. At the end of the reaction diethyl ether and n-hexane, were added to the mixture and the solid formed was filtered, washed with diethyl ether and n-hexane and dried, in particular for 24 h at 95 °C under vacuum.

[0050] In an embodiment, the functionalization of the CNTs (SWCNT, DWCNT or MWCNT) with the perylene-metal complexes (for example copper-PBI, silver-PBI or gold-PBI) was performed by mixing a solution of the synthesized metal-PBI, in particular 5 to 25 mg, in ethanol, in particular 5 ml of ethanol, and a suspension of the CNTs (SWCNT, DWCNT or MWCNT), in particular 10 to 75 mg in DMF, in particular 2-10 ml. The mixture was stirred, in particular at room temperature for 3 to 7 days. At the end, diethyl ether and n-hexane were added, and the solid was filtered and dried, in particular for 24 h at 95 °C under vacuum.

[0051] In an embodiment and for even better results, the perylene modified with phenylalanine presents a simpler synthesis and the nature of the side chain of the inserted amino acid may promote a better interaction with the CNT surface. [0052] In an embodiment, in order to improve the dispersion of the perylene-metal complexes on the CNT surface and increase the amount of metal retained on the CNTs surface, the mixture was sonicated in an ultra-sounds bath, in particular for 10 minutes, more in particular twice a day.

[0053] In an embodiment, the amount of metal retained on the CNTs surface increases when sonication is used in the functionalization procedure.

[0054] In an embodiment, for the perylene functionalized with cysteine and complexed with silver, the amount of metal on the CNT surface increases 5 times when compared with the experimental approach in the absence of sonication.

[0055] In an embodiment, for the perylene functionalized with phenylalanine and complexed with copper, the amount of metal on the CNT surface is doubled when compared with the experimental approach in the absence of sonication.

[0056] In an embodiment, the presence of the metal, in particular gold, copper or silver, was confirmed by EDS, as presented in Figure 4 i) and j).

[0057] In an embodiment, the synthesis of covalently functionalized CNTs was carried out by a 1,3 dipolar cycloaddition reaction of an a-amino acid, in particular N-benzyloxycarbonylglycine (Z-Gly- OH), and paraformaldehyde following a previously described procedure [1], wherein the pyrrolidine group is incorporated on the CNT surface. The CNTs, the paraformaldehyde, and the Z-Gly-OH were combined, in particular in 1:5:1 mass ratio. The neat mixture was heated, in particular at 250 °C during 12- 14 h. The resulting product was washed with acetone, hexane, ethanol and methanol, filtered and dried in the vacuum oven.

[0058] In an embodiment, anchoring metals on covalently functionalized CNTs was carried out as follows. CNTs (single wall, double wall or multi wall carbon nanotubes) are functionalized by the 1,3- dipolar cycloaddition reaction, procedure described elsewhere (1), or any suitable method for covalent bonding of amine or thiol groups. Bonding of the metal ions, in particular copper, silver, gold or others, to the functionalized CNTs, was carried out by combining a suspension of the functionalized CNTs, in particular 5 to 75 mg in DMF, in particular 4ml DMF, and a solution of the metal salt, in particular 0.1 to 3 mmol of the solution of the metal salt, in ethanol, in particular 2 to 10 ml of ethanol. The mixture was stirred, in particular at room temperature for 1 to 7 days. The reaction medium may be sonicated in an ultrasound bath for a short time, in particular for 5-30 min, more in particular 2 to 5 times a day. At the end diethyl ether and n-hexane were added and the solid was filtered, washed with diethyl ether and n-hexane, and dried, in particular for 24 h at 95 °C under vacuum. [0059] In an embodiment, growth of metal nanowires on the CNT surface was carried out as follows. Copper, silver, gold or other metal nanowires on CNTs were prepared using experimental procedures previously described in the literature for growing the metal nanowires (2-3) modified to include the CNTs with the same metal anchored, as seeding points for the metal nanowires growth.

[0060] In an embodiment, the functionalized CNTs were dispersed in a resin, in particular in an epoxy resin, silicone, acrylate, or mixtures thereof.

[0061] In an embodiment, the functionalized CNTs, after being dispersed in a resin, were cured, in particular at 80 to 180 °C for 20 to 60 min.

[0062] In an embodiment, thermogravimetric analysis (TGA) was used to verify and quantify the formation of the metal complex by the perylene derivatives, to determine the level of functionalization of the CNTs with the perylene derivatives, with and without metal, and to quantify the composition of the nanocomposites prepared.

[0063] In an embodiment, the noncovalent functionalized CNTs have an amount of metal of at least 10 % (w/w).

[0064] In an embodiment, Nuclear Magnetic Resonance, NMR, technique was used for the characterization of the amino acid modified perylenes.

[0065] In an embodiment, Scanning Electron Microscopy, SEM, imaging with integrated microanalysis X-ray system (EDS - energy dispersive spectrometer) and Electron Backscatter Diffraction (EBSD) were used for the morphological characterization of the nanomaterials. EDS allowed the semi-quantitative analysis of the constituent chemical elements. The combination of the two techniques made it possible to observe the nanomaterials morphology and to map the chemical composition of the nanomaterials surface. The conjugation of SEM and EDS allowed confirmation of the anchoring of metals onto the modified perylene molecules and their adsorption on the CNTs surface.

[0066] In an embodiment, the electrical characterization of the cured epoxy composite electrical properties was done by determination of the volume resistivity and/or surface resistivity of the material, from measurements of the current intensity at an applied potential, and also by the current intensity vs. potential (l-V) curves.

[0067] In an embodiment, the electrical properties of CNT and CNT/silver epoxy composites with 5 wt. % of graphite nanoplatelets, cured at 80 °C during 1 h were determined. The results are displayed in Table 1. [0068] Table 1: Electrical properties of CNT and CNT/silver epoxy composites with 5 wt. % of graphite nanoplatelets, cured at 80 °C during 1 h.

a) Samples cured at 80 °C and post-cured at 180 °C; b) Samples cured at 180 °C

[0069] In an embodiment, the characterization of the cured epoxy composite mechanical properties was performed by tensile testing on a universal testing machine, and by dynamic mechanical analyses (DMA) using three-point bending mode.

[0070] In an embodiment, the rheological characterization of the composite was performed using an ARES-G2 TA Instruments (USA).

[0071] In an embodiment, the dispersion of commercial carbon nanoparticles in the epoxy resin produced composites with resistivity in the order of 10 Q.cm at an overall filler content around 5 wt. %, the results are summarized in Table 2.

[0072] In an embodiment, the dispersion of hybrid particle compositions was tested using hybrid fillers of carbon and silver and of carbon and copper and of carbon and gold, and mixtures thereof. The hybrid fillers comprise the CNTs functionalized with copper, silver, gold or combinations thereof. The composites were prepared with only the hybrid particles, and also with the hybrid particles and graphite (5 wt. %), and with graphite and SWCNT (5 and 0.2 wt. % respectively). The values obtained are presented in Table 2 and the corresponding l-V curves in Figure 1.

[0073] Table 2 - Electrical properties of epoxy composites with hybrid particles, covalently functionalized MWCNT with metal nanowires, cured at 80 °C during 1 h.

a) The nanowires were grown with the functionalized MWCNT without isolation of the meta decorated MWCNT

[0074] In an embodiment, the characterization of the composites by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) were performed to evaluate the actual presence, distribution and dispersion of the different fillers and these results are presented in Figures 4 to 9.

[0075] In an embodiment, the SEM images of the composites containing hybrid filers from the covalent functionalization of MWCNT with both silver and copper show a homogenous distribution of the fillers added. For the hybrid silver composite in Figure 4 the images show the presence of some large particles of silver, Figure 4 f) g) and h), with sizes near 500 nm, and a small agglomerate of CNTs is also observed; in i) and j) the EDS for the two zones identified in g) is presented, confirming the presence of silver in both regions.

[0076] In an embodiment, the SEM of the composites of the same hybrid with graphite (5 wt. %) are presented in Figure 5 and present an overall homogeneity of the nanoparticles dispersion in the composite. The images obtained with the backscattered electrons detector, f) and h), of the same areas as e) and g), show again large silver particles along with graphene structures.

[0077] In an embodiment, for the DR102 hybrid silver composite with graphite and SWCNT (5 and 0.2 wt. %, respectively) presented in Figure 6, the images depict a homogenous dispersion of the nanoparticles, and large particles of silver are observed in backscatter mode f). The presence of SWCNT is also evident in the higher magnifications, d) e) and f).

[0078] In an embodiment, the SEM images of the hybrid copper composite in Figure. 7 present a homogenous distribution of the filler, and the same was observed for the composites with graphite and with graphite and SWCNT in Figure 8 and Figure 9. Another common feature of the three composites is that in none of then it was possible to identify the presence of copper by EDS analysis, due to its small signal intensity. The filler itself was observed by SEM / EDS and the presence of copper was positively identified. For example, sample DR93, noncovalently functionalized MWCNT with copper attached, contained approximately 3 wt.% of copper.

[0079] In an embodiment, the presence of graphite nanomaterials as filler in conjunction with carbon nanotubes does not change significantly the electrical properties. However, the presence of graphite may have the advantage of reducing the viscosity of the resin. Therefore, the rheological properties may be improved while the electrical properties are maintained.

[0080] In conclusion, the synthesis of perylene derivatives was successful using tree different amino acids. All derivatives were able to bond to metals, copper and silver, and suitable to modify the carbon nanotubes non-covalently and anchoring metal at the CNT surface. Covalently modified CNT ^' s also anchored metals at the surface. Composites of all carbon nanomaterials with filler amounts below 10% were prepared, using a three-roll mill. Electrical resistivity of a few Q.cm was obtained under these conditions.

[0081] The term "comprising" whenever used in this document is intended to indicate the presence of stated features, integers, steps, components, but not to preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

[0082] It will be appreciated by those of ordinary skill in the art that unless otherwise indicated herein, the particular sequence of steps described is illustrative only and can be varied without departing from the disclosure. Thus, unless otherwise stated the steps described are so unordered meaning that, when possible, the steps can be performed in any convenient or desirable order.

[0083] The disclosure should not be seen in any way restricted to the embodiments described and a person with ordinary skill in the art will foresee many possibilities to modifications thereof. [0084] The above described embodiments are combinable. The following claims further set out particular embodiments of the disclosure.

[0085] References

1. ACS Nano, 2010, 4 (12), pp 7379-7386. DOI: 10.1021/nnl022523

2. Chem. Commun., 2014, 50, 2562—2564; DOI: 10.1039/c3cc48561g

3. Materials Science and Engineering B 223 (2017) 1-23; DOI 10.1016/j.mseb.2017.05.002