HIGHLY COMPACT METAL-CNT COMPOSITES AND MANUFACTURE THEREOF

Abstract

A method for manufacturing metal-CNT composites is disclosed. The method comprises providing an agglomerate of CNTs, filling interstices of the CNT agglomerate in a plating solution, so as to form a metal phase, in which the CNTs are embedded. The CNT agglomerate is compressed with a clamping appliance when the metal phase is formed. A further aspect of the invention relates to metal-CNT composites with high CNT content.

Claims

1.-20. (canceled)

21. A method for producing a metal-CNT composite material, comprising: providing an agglomerate of carbon nanotubes filling interstices of the carbon nanotubes agglomerate in a plating solution, so as to form a metal phase, in which the carbon nanotubes are embedded; wherein the carbon nanotubes agglomerate is compressed with a clamping appliance when the metal phase is formed.

22. The method as claimed in claim 21, wherein the compression of the carbon nanotubes agglomerate inhibits or prevents swelling of the carbon nanotubes agglomerate during formation of the metal phase.

23. The method as claimed in claim 21, wherein the carbon nanotubes comprise a hydrophilic coating.

24. The method as claimed in claim 21, wherein the hydrophilic coating comprises polyphenol or poly(catecholamine) and at least one of metal ions crosslinking the polyphenol or the poly(catecholamine) and metal ions chelated by the polyphenol or the poly(catecholamine).

25. The method as claimed in claim 21, wherein the metal comprises copper.

26. The method as claimed in claim 21, wherein the clamping appliance comprises a first and a second plate holding the carbon nanotubes agglomerate there between.

27. The method as claimed in claim 26, wherein at least one of the first and a second plates comprises passageways for the plating solution.

28. The method as claimed in claim 27, wherein at least one of the first and second plates comprises a patterned pressure-transfer face turned towards the carbon nanotubes agglomerate, the patterned pressure-transfer face comprising raised areas and recessed areas, the recessed areas communicating with the passageways.

29. The method as claimed in claim 28, wherein compression of the carbon nanotubes agglomerate is effected at least predominantly via the raised areas, leading to higher compaction in regions of the carbon nanotubes agglomerate opposite the raised areas, to less compaction in regions of the carbon nanotubes agglomerate opposite the recessed areas and to formation of the metal phase at least predominantly within the less compacted regions of the carbon nanotubes agglomerate opposite the recessed areas.

30. The method as claimed in claim 29, comprising releasing the carbon nanotubes agglomerate from the clamping appliance and rinsing the carbon nanotubes from the previously more highly compacted regions less or not impregnated with the metal phase.

31. The method as claimed in claim 26, comprising arranging a membrane between at least one of the first and second plates and the carbon nanotubes agglomerate, the membrane being permeable for the plating solution.

32. The method as claimed in claim 21, wherein filling the interstices of the carbon nanotubes agglomerate in the plating solution is carried out by electroplating.

33. The method as claimed in claim 21, wherein filling the interstices of the carbon nanotubes agglomerate is carried out under sonication, e.g. under ultrasonication.

34. A metal-CNT composite material, comprising: an agglomerate of carbon nanotubes embedded in a metal phase occupying interstices of the carbon nanotubes agglomerate; characterized in that the metal-CNT composite material contains at least 45% by volume of carbon nanotubes.

35. The metal-CNT composite material as claimed in claim 34, wherein the metal-CNT composite material contains between 45% and 65% by volume of carbon nanotubes.

36. The metal-CNT composite material as claimed in claim 34, wherein the metal phase is continuous.

37. The metal-CNT composite material as claimed in claim 34, having a porosity of 5% or less.

38. The metal-CNT composite material as claimed in claim 34, wherein the carbon nanotubes agglomerate includes a mix of short and long carbon nanotubes, at least 30% by weight of the carbon nanotubes being short carbon nanotubes having a length in the range from 2.5 ?m to 50 ?m and at least 30% by weight of the carbon nanotubes being long carbon nanotubes having a length in the range from 75 ?m to 1500 ?m.

39. The metal-CNT composite material as claimed in claim 34, wherein the metal-CNT composite material is in the form of a mesh or in the form of a layer or foil.

40. The method as claimed in claim 21, wherein the hydrophilic coating comprises polydopamine.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] By way of example, preferred, non-limiting embodiments of the invention will now be described in detail with reference to the accompanying drawings, in which:

[0031] FIG. 1: is a schematic cross-sectional view of a first clamping appliance holding a CNT agglomerate immersed in an electrodeposition bath;

[0032] FIG. 2: is a schematic cross-sectional view of a second clamping appliance holding a CNT agglomerate immersed in an electrodeposition bath;

[0033] FIG. 3: is a graph showing the swelling of uncompressed CNT agglomerates during electroplating as a function of the degree of pre-compression;

[0034] FIG. 4: is a perspective view of an example of the patterned face of a plate of a clamping appliance;

[0035] FIG. 5: is a photograph of a patterned Cu-CNT composite obtained by the method disclosed herein;

[0036] FIG. 6: shows SEM pictures of a) a metal-CNT composite obtained without using a clamping appliance (comparative example) and b) a metal-CNT composite obtained by using a clamping appliance;

[0037] FIG. 7: is a photograph of a patterned Cu-CNT composite, obtained in accordance with the present invention, exhibiting regions wherein the CNT agglomerate has remained unplated (dark dots on the left-hand side: before rinsing the CNTs away; light dots on the right-hand side: after removing the CNTs with a wipe soaked with ethanol);

[0038] FIG. 8: is a SEM cross section of a patterned Cu-CNT composite, showing an unplated region from which the CNTs can be rinsed away;

[0039] FIG. 9: is a SEM picture of a Cu-CNT composite region densely filled with copper.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0040] According to an embodiment of the invention, a CNT agglomerate in the form of a tissue (non-woven) of polydopamine-coated CNTs is provided. Alternatively, the CNT tissue is deposited onto one or two faces of an electrically conductive or insulating substrate. As illustrated in FIG. 1, the CNT agglomerate 10 is clamped between a first 12 and a second plate 14 of a clamping appliance 16, which is then immersed in a plating solution 18. The plates 12, 14 (both made of isolating material) of the clamping appliance 16 have passageways 20 therein, such that the plating solution 18 can come into contact with and that the metal ions may migrate to the CNT agglomerate 10. The CNT agglomerate 10 is electrically contacted with a working electrode terminal 22 and the interstices of the CNT agglomerate 10 are then filled with the metal phase 36, such that the CNTs become embedded therein. During the electrodeposition process, the clamping appliance 16 maintains the CNT agglomerate 10 compressed, such that swelling thereof is prevented or at least inhibited.

[0041] It is worthwhile noting that high pressures are not necessary, such that the plates 12, 14 of the clamping appliance and the tightening screws 24 could be made of plastic or like materials. A particularly convenient way to fabricate the plates would be by additive manufacturing (3D printing).

[0042] Each of the first and second plates 12, 14 comprises a patterned pressure-transfer face 26, turned towards the CNT agglomerate 10, with raised areas 28 and recessed areas 30. The recessed areas 30 fluidly communicate with the passageways 20. The raised areas 28 serve to transfer pressure on the CNT agglomerate 10. A first porous membrane 32 is arranged between the pressure-transfer face of the first plate 12 and the CNT agglomerate 10, a second porous membrane 34 is arranged between the pressure-transfer face of the second plate 14 and the CNT agglomerate 10. The membranes 32, 34 are permeable for the metal ions contained in the plating solution 18 and further serve to distribute the pressure more homogeneously on the CNT agglomerate 10 (and thus to confine the CNT agglomerate also in regions facing the recessed areas 30). The membranes could, e.g. be hydrophilic PTFE membranes with 10 ?m pore size, around 80% porosity and a thickness of 85 ?m. Highly porous membranes (porosity>60) may be preferred to facilitate ion mass transfer. Pore size is preferable selected small enough to avoid swelling of the CNTs into the pores but a compromise may be made to ascertain acceptable growth rate of the metal phase. Also, hydrophilic membranes may be preferred: in tests, hydrophilic membranes performed better than other candidate membranes.

[0043] Due to the patterned pressure-transfer faces 26, compression of the CNT agglomerate occurs predominantly opposite the raised areas 28, which leads to higher compaction in those regions of the CNT agglomerate 10. Although the membranes transfer part of the pressure also into the regions of the CNT agglomerate 10 opposite the recessed areas 30, these regions are less compacted (lower CNT density and wider interstices). Consequently, formation of the metal phase 36 takes place predominantly within the less compacted regions of the CNT agglomerate 10, i.e. opposite the recessed areas 30. Thanks to the patterned pressure-transfer faces 26, the ratio between metal and CNTs can thus be spatially modulated. By adjusting the process parameters (exerted pressure, type of membrane, deposition time, thickness of the CNT agglomerate, etc.) one may obtain a metal-CNT composite wherein the highly compacted regions are so little impregnated with the metal phase that after removal of the CNT agglomerate from the clamping appliance and from the plating bath, the CNTs can be rinsed away, leaving a patterned metal-CNT composite (e.g. a mesh or shaped pieces). FIG. 7 is a picture of a patterned metal-CNT composite. The left-hand side of FIG. 7 shows a portion of the composite as retrieved from the electroplating bath, while the right-hand side shows a portion wherein the CNTs have been removed from the unplated regions using a tissue wetted with ethanol. FIGS. 8 and 9 are SEM pictures of an unplated region and a plated region of a patterned metal-CNT composite, respectively.

[0044] It may be worthwhile noting that one or both membranes 32, 34 may not be needed in all applications.

[0045] FIG. 2 illustrates an alternative setup, which differs from the setup of FIG. 1 in several aspects. With the setup of FIG. 2, one aims at a substantially uniform distribution of the metal phase throughout the CNT agglomerate 10. The first and second plates 12, 14 are in this setup held together by C-clamps 16. The second plate 14 has a conductive pressure-transfer face 26, which is contacted by the working electrode terminal 22 of the voltage source. The CNT agglomerate 10 rests directly on the second plate 14 (i.e. there is no porous membrane between the CNT agglomerate and the pressure-transfer face 26 of the second plate).

[0046] Only the first plate 12 of the clamping appliance has passageways 20 therein, such that the plating solution 18 can come into contact with and that the metal ions may migrate to the CNT agglomerate 10. The pressure-transfer face 26 of the first plate 12 is essentially flat, i.e. the areas of the passageway openings are relatively small. A porous membrane 32 is arranged between the pressure-transfer face 26 of the first plate 12 and the CNT agglomerate 10 in order to distribute both the pressure and the metal ions homogeneously on the CNT agglomerate 10.

Examples

A. Production of CNTs Coated with Polydopamine Crosslinked with Copper Ions

[0047] 100 mg of oxidized CNTs were added to 625 ml of a solution comprised of dopamine hydrochloride solution (DA, 0.1 mg/ml) and CuSO.sub.4.5H.sub.2O (0.6 mg/ml). The solution was ultra-sonicated during 10 s, then left under vigorous stirring for 30 minutes. 375 ml of Tris-HCl (10 mM) were added to the solution. The solution was ultrasonicated during 10 s and then left under vigorous stirring for 24 hours. 12.5 ml of NaOH (1 M) were added and the solution was filtrated. The filtrated CNTs were then dispersed in EtOH (ethyl alcohol). The resulting CNTs were coated with polydopamine (Pda) crosslinked with copper ions.

[0048] The CNTs used in this example were multi-walled CNTs (MWCNTs) with lengths from 2.5 ?m to 8 ?m (6-13 nm diameter), and/or with length around 200 ?m (5-30 nm diameter) and/or with lengths around 800 ?m (70-80 nm diameter).

[0049] The amounts of DA and CuSO.sub.4.5H.sub.2O in the coating solution can be decreased to 0.0125 mg/ml and 0.075 mg/ml, respectively. The resulting CNT agglomerates are less sticky and can be more easily (pre-) compressed.

[0050] Other coating protocols may be followed, e.g. those disclosed in WO 2020/043590 A1.

B. Preparation of a CNT Agglomerate (in the Form of a Layer)

[0051] 20 ml of the ethanol solution, produced as described above, containing 0.25 g/ml of the CNTs coated with Pda crosslinked with copper ions, was filtrated on a PTFE (polytetrafluoroethylene) membrane (5 cm diameter). The CNT agglomerate obtained by filtration was recovered by peeling. The CNT agglomerate was compressed at 400 kg/cm.sup.2 during 5 min.

[0052] Alternatively, CNT agglomerates could be obtained by spraying a dispersion of the Pda-coated CNT on a substrate. In case the metal phase is to be produced by electrodeposition, the substrate could be the working electrode of the electrodeposition step or a porous membrane. The substrate could also be a temporary substrate, from which the layer is removed after formation. Examples of CNT layer preparation by spray coating are described in detail in WO 2020/043590 A1.

C. Pre-Compression of the CNT Agglomerate

[0053] The decrease of porosity in the CNT agglomerate during the pre-compression step is dependent of the pressure applied (max pressure tested: 800 kg/cm.sup.2).

[0054] Pre-compression alone of the CNT agglomerate is not sufficient to increase the CNT volume content of the metal-CNT composite. The diagram of FIG. 3 illustrates that the thickness of a CNT layer after electroplating is essentially independent of the degree of pre-compression (when compression is not maintained during the electroplating step): the higher the pre-compression, the greater was the swelling during electroplating.

[0055] The diagram was obtained using MWCNT (2.5-8 ?m long, 6-13 nm in diameter) and shows the swelling, expressed as the ratio of the CNT layer thickness after electroplating, F, to the CNT layer thickness after any pre-compression but before immersion in the electroplating bath, C, as a function of the compression ratio, i.e. the ratio of the original, uncompressed CNT layer thickness (before any pre-compression), U, to the CNT layer thickness after any pre-compression but before immersion in the electroplating bath, C. It can be seen that F?1.4 U, independently of the degree of pre-compression.

D. Clamping Appliance Used for Testing

[0056] The clamping appliance comprised a patterned plate and a flat supporting plate. FIG. 4 shows the pressure-transfer face of the patterned plate. The recessed areas 30 (forming a hexagonal mesh) are shown in black whereas the raised areas 28 are shown in white. The recessed areas are fluidly connected to passageways allowing passage of the plating solution across the patterned plate. The flat supporting plate was a silicon/Al.sub.2O.sub.3 substrate.

E. Fabrication of a Patterned Metal-CNT Composite

[0057] The dried CNT agglomerate prepared as described above was clamped between the plates of the clamping appliance. A porous PCA membrane was inserted between the plates and the CNT agglomerate.

[0058] Electrical contact with the working electrode terminal was made directly on the CNT agglomerate. The clamping appliance with the fixed CNT agglomerate was immersed in an aqueous copper plating solution (CuSO.sub.4 0.63 M, H.sub.2SO.sub.4 0.1M,HCl 50 ppm, bis(sodiumsulfopropyl) disulphide 15 ppm, PEG (polyethylene glycol) 100 ppm). Plating was carried out using galvanic pulses (35 mA/cm.sup.2/0 mAON/OFF 0.02 s/0.1 s) during the time needed to fill the interstices between the CNTs with copper. As used herein, ppm means parts per million in terms of mass fraction.

[0059] The duration of the electroplating step depends (amongst other parameters) on the level of compression of the CNT agglomerate (more pore volume means more metal to deposit and hence higher deposition time) and CNT layer thickness (in the inventors' experiments, about 3 hours deposition time was needed to obtain a 20 ?m thick metal-CNT composite layer).

[0060] At the end of the electrodeposition step, the deposited copper formed a pattern in the CNT agglomerate. The CNTs present in the areas that were not plated could be rinsed away with water. FIG. 5 is the photograph of a prototype metal-CNT composite shown next to a pen for scale.

[0061] A porous membrane can be added between the patterned plate and the CNT agglomerate to enhance the volume percentage CNTs in the patterned composite.

F. Fabrication of a Metal-CNT Composite Foil

[0062] A pre-compressed CNT agglomerate produced as described above was placed on a flat titanium substrate covered with a seed layer of copper (100 nm). A porous membrane was placed on top of the CNT layer. A capillary layer (tissue) was added on top of the membrane. A perforated plate was then placed on top of the capillary layer and fixed to the substrate with screws. Electrical contact with the working electrode terminal was taken on the conductive substrate. The clamping appliance with the fixed CNT agglomerate was immersed in an aqueous copper plating solution (CuSO.sub.4 0.63 M, H.sub.2SO.sub.4 0.1 M, HCl 50 ppm, bis(sodiumsulfopropyl) disulphide 15 ppm, PEG 100 ppm). Plating was carried out using galvanic pulses (35 mA/cm.sup.2/0 mAON/OFF 0.02 s/0.1 s) during the time needed to fill the interstices between the CNTs with copper.

[0063] At the end of the plating time, the deposited copper formed a continuous metal phase having the CNTs embedded therein. The capillary layer was removed. The membrane was peeled off the copper-CNT composite foil and the latter was peeled off the titanium substrate.

[0064] The porous membrane is susceptible to becoming partially trapped by the metal, which can begin to grow into its pores. Removal of the membrane can be achieved by chemical dissolution (e.g. by immersion in dichloromethane in case of a polycarbonate membrane) and/or by mechanical polishing (e.g. in case of a PTFE membrane).

G. Calculation of the Volume Percentage of CNTs in the Composite

[0065] The volume percentage of CNTs in a metal-CNT composite can be calculated by:

[00001]$\begin{array}{cc}\mathrm{CNT}\mathrm{vol}.\%=\frac{V_{\mathrm{CNT}}}{V_{\mathrm{composite}}}.Math.100=\frac{V_{\mathrm{composite}}-V_{\mathrm{metal}}}{V_{\mathrm{composite}}}.Math.100& \left(\mathrm{Eq}.1\right)\end{array}$$\begin{array}{cc}{V}_{\mathrm{metal}}=\frac{m_{\mathrm{metal}}}{{?}_{\mathrm{metal}}}=\frac{m_{\mathrm{composite}}-m_{\mathrm{CNT}}}{{?}_{\mathrm{metal}}}& \left(\mathrm{Eq}.2\right)\end{array}$

where: [0066] V.sub.composite is the measured volume of the composite (e.g. in cm.sup.3), [0067] V.sub.metal, obtained by Eq. 2, is the volume of the metal contained in the composite, [0068] m.sub.composite is the measured mass of the composite (e.g. in g), [0069] m.sub.CNT is the measured mass of the (dry) CNT agglomerate before plating (e.g. in g) [0070] ?.sub.metal is the specific mass of the metal (e.g. in g/cm.sup.3),

[0071] In case of a metal-CNT composite in the form of a layer, the volume percentage of CNTs in the composite can, alternatively, be calculated using the following equations:

[00002]$\begin{array}{cc}\frac{{?}_{\mathrm{composite}}-{?}_{\mathrm{CNT}}}{{?}_{\mathrm{metal}}}=t_{\mathrm{metal}}& \left(\mathrm{Eq}.1\right)\end{array}$$\begin{array}{cc}{t}_{\mathrm{composite}}-t_{\mathrm{metal}}=t_{\mathrm{CNT}}& \left(\mathrm{Eq}.2\right)\end{array}$$\begin{array}{cc}\mathrm{CNT}\mathrm{vol}\%=\frac{t_{\mathrm{CNT}}}{t_{\mathrm{composite}}}.Math.100& (\mathrm{Eq}.3\end{array}$

where: [0072] ?.sub.composite is the measured surface density of the composite (e.g. in g/cm.sup.2), [0073] ?.sub.CNT is the measured surface density of the (dry) CNT layer before plating (e.g. in g/cm.sup.2), [0074] ?.sub.metal is the specific mass of the metal (e.g. in g/cm.sup.3), [0075] t.sub.composite is the measured thickness of the composite, [0076] t.sub.metal, as obtained from Eq. 1 is the equivalent thickness of the metal inside the composite, and [0077] t.sub.CNT, as obtained from Eq. 2, is the equivalent thickness of CNTs inside the composite.

H. Results

[0078] Continuous metal-CNT composites with high volume percentages of CNTs could be obtained by the method described above. The same plating processes carried out without compressing the CNT agglomerate led to composites with less than 10% by volume of CNTs therein (cf. FIG. 6). Furthermore, it was observed that, without using the compressive system, the thickness of the resulting composites was independent of the level of pre-compression of the CNT agglomerates, meaning that if the CNT agglomerate is more compressed, then the swelling is more important. In contrast, when the CNT agglomerates were compressed during electroplating, the swelling was suppressed, i.e. the thickness of the CNT agglomerates remained essentially constant during the electroplating step.

[0079] While specific embodiments have been described herein in detail, those skilled in the art will appreciate that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention, which is to be given the full breadth of the appended claims and any and all equivalents thereof.