CONDUCTIVE ELEMENT

Abstract

Methods for producing a conductive element precursor and a conductive element, such as a tape or wire, are provided. The methods comprise growing a plurality of carbon nanotubes on a metallic substrate and coating carbon nanotubes of the plurality of carbon nanotubes on the metallic substrate with a metallic material.

Claims

1-56. (canceled)

57. A method for producing a conductive element precursor, the method comprising the following steps: forming a plurality of carbon nanotubes on a metallic substrate; applying a shear force to the plurality of carbon nanotubes on the metallic substrate in a first direction; and coating carbon nanotubes of the plurality of carbon nanotubes with a metallic material.

58. The method of claim 57, wherein the step of applying the shear force occurs after the step of coating the carbon nanotubes

59. A method for producing a conductive tape, the method comprising the following steps: forming the conductive element precursor according to claim 57; compressing the conductive element precursor such as to increase its length and form the conductive tape.

60. A method for producing an insert, the method comprising the following steps: forming the conductive element precursor according to claim 57; and rolling up the substrate with the coated carbon nanotubes to form the insert.

61. A method for producing a conductive element, the method comprising the following steps: forming the insert of claim 60; and drawing the insert to increase its length and form a conductive element.

62. A method for producing a conductive element, the method comprising the following steps: forming the conductive element precursor according to claim 57, wherein the forming of the conductive element precursor further comprises the step of rolling up the substrate with the coated carbon nanotubes to form an insert; and wherein the shear force is applied to the plurality of carbon nanotubes on the metallic substrate in a first direction by drawing the insert to increase its length and form the conductive element.

63. A method for producing a conductive element, the method comprising the following steps: forming a plurality of carbon nanotubes on a metallic substrate; rolling up the substrate with the plurality of carbon nanotubes to form an insert; and drawing the insert to increase its length and form the conductive element; wherein the method further comprises the step of coating carbon nanotubes of the plurality of carbon nanotubes with a metallic material; and wherein the rolling up the substrate is a step of rolling up the substrate with the coated carbon nanotubes to form the insert.

64. The method of claim 62, wherein the rolling step comprises rolling the substrate around a metallic bobbin and wherein the metallic bobbin with the rolled substrate thereon is placed in a metallic sleeve to form the insert.

65. The method of claim 64, wherein the metallic bobbin and the metallic sleeve comprise copper.

66. The method of claim 64, wherein the substrate with the coated carbon nanotubes is rolled round the bobbin such that the first direction is substantially parallel to the rotational axis of the bobbin.

67. The method of claim 62, wherein the first direction is substantially parallel to a drawing direction of the drawing step.

68. The method of claim 57, wherein the step of forming a plurality of carbon nanotubes comprises chemical vapour deposition.

69. The method of claim 57, wherein the step of coating the carbon nanotubes comprises chemical vapour deposition.

70. The method of claim 57, wherein the step of coating the carbon nanotubes comprises decorating the carbon nanotubes with the metallic material via chemical vapour deposition and then subsequently electroplating the carbon nanotubes with the metallic material.

71. The method of claim 62, further comprising an annealing step following the drawing step.

72. The method of claim 62, wherein the conductive element is in the form of a wire.

73. The method of claim 57, wherein the metallic substrate is in the form of a foil.

74. The method of claim 57, wherein the metallic substrate and metallic material comprise copper.

75. A conductive element precursor formed by the method of claim 57.

76. A conductive tape formed by the method of claim 59.

77. An insert formed by the method of claim 60.

78. A conductive element formed by the method of claim 61.

Description

[0095] The present invention will now be described in relation to the following specific example along with the drawings.

[0096] FIG. 1 is an SEM image of the surface of the deposited silica layer on a copper foil.

[0097] FIG. 2 is an SEM image of the surface of the deposited silica layer on a copper foil where the silica layer has been cracked.

[0098] FIG. 3 is an SEM image of the carbon nanotubes grown on a copper foil.

[0099] FIG. 4 is an SEM image of carbon nanotubes with deposited copper particles.

[0100] FIG. 5 is a further SEM image of carbon nanotubes with deposited copper particles

[0101] FIG. 6 is another SEM image of carbon nanotubes with deposited copper particles.

[0102] FIG. 7 schematically depicts the step of applying a shear force to the grown carbon nanotubes.

[0103] FIG. 8 is a schematic depiction of the electroplating process.

[0104] FIG. 9 is a schematic depiction of the current profile applied for the electroplating step.

[0105] FIG. 10 is an SEM image of carbon nanotubes coated with copper following electroplating.

[0106] FIG. 11 schematically depicts a bobbin that can be used with the present invention.

[0107] FIGS. 12 and 13 schematically depict the metallic substrate being wound onto a recess in the bobbin.

[0108] FIG. 14 schematically depicts a sleeve that can be used with the present invention.

[0109] FIG. 15 schematically depicts a cross-section of the sleeve containing the bobbin.

[0110] FIG. 16 schematically depicts the wire-drawing step.

[0111] FIG. 17 schematically depicts a cross-section through the final wire.

[0112] FIG. 18 is a secondary SEM image and a backscattered SEM image of a cross-section of drawn wire.

[0113] FIG. 19 is a detail of the carbon nanotube layers in FIG. 18.

[0114] FIG. 20 is a SEM image of an etched cross-section of the drawn wire.

[0115] FIG. 21 is an SEM image of an etched longitudinal section of the drawn wire.

[0116] The substrate used is a thin copper foil ribbon that is clamped into a copper, or brass, sample holder. The term “ribbon” is used due to the copper foil's long length relative to its width. The sample holder is introduced in a first reactor chamber through a side door, where it sits on the rails that will ensure its translation to the next chamber. The deposition chamber is closed and evacuated and backfilled with argon several times to remove most of the oxygen and moisture. The pressure is then set to a value of about 5 mbar with a steady argon flow of 1 SLM.

Silica Deposition

[0117] An electric current is run in between the two rails, through the sample holder and the copper foil ribbon. When the ribbon temperature reaches 650° C., the precursor injection can take place. The injection frequency is 50 Hz, with an opening time of 0.7 ms. A solution of 0.1 M TEOS in anhydrous toluene is injected in the evaporating vessel, which is heated at 190° C. A 2 SLM Ar carrier gas flow is run through the evaporator. After 15 minutes of injection, the flow of precursor is interrupted, and the chamber evacuated several times to remove the remaining traces of precursor solution.

[0118] The obtained silica layers are 400 nm thick on average, and very smooth, as the SEM micrograph of FIG. 1 shows. FIG. 2 demonstrates a section where the silica layer has been deliberately cracked to expose the underlying copper foil for illustration purposes. In practice, to avoid cracking, the elevated temperature of the metallic substrate can be maintained between the deposition of the silica layer and the next carbon nanotube forest growth step.

Carbon Nanotube Forest Growth

[0119] Once the cleaning is finished, the pressure in the chamber is raised by filling with argon gas and once atmospheric pressure is reached, the sample holder is transferred to the next chamber through a gate valve. Once the sample is in the second chamber and the gate valve locked, the carbon nanotube injection process can begin. The precursor of a 3% wt solution of ferrocene in toluene, injected along the same process as the silica precursor. The injection parameters are 0.7 ms opening time, 25 Hz frequency and 3 SLM Ar carrier gas flow. The pre-heating furnace taking place in between the evaporator and the deposition chamber is heated at 725° C. The process lasts for 10 minutes. Once the process is finished, the copper ribbon is cooled down, and the chamber is evacuated and filled back with argon to remove the traces of precursor remaining.

[0120] The carbon nanotube forests grown by this process have about 400 μm thickness, with a carbon nanotube density of about 10.sup.8 carbon nanotubes per cm.sup.2 and very good alignment as shown in FIG. 3.

Loose Carbon Nanotube Cleaning

[0121] Once the carbon nanotube forest growth process is finished, the sample holder is transferred to an intermediate cleaning chamber through another gate valve. There, it is submitted to a high argon flow to blow away any loose CNT. Once this step completed, it is transferred to the third deposition chamber.

Copper Seeding

[0122] When the sample is in position in the third chamber, the pressure is lowered again to 160 mbar, and a stream of precursor is injected in the chamber along the same process as for the silica deposition. The precursor solution injected is a 0.25 M solution of Cu(acac).sub.2 in toluene, with a pulse length of 0.7 ms and a frequency of 25 Hz. The carbon nanotube forest is then decorated by copper nanoparticles, as shown in FIG. 4, FIG. 5 and FIG. 6. This step allows a better deposition of the copper into the thickness of the carpet during the next step.

[0123] The third deposition chamber is then evacuated, flushed with argon and increased to atmospheric pressure. The sample holder is then extracted through a side door.

Optional Halogen Doping

[0124] Iodine is used as a doping halogen and is injected using a solution of iodoethane (C.sub.2H.sub.5I) in toluene: two parts of iodoethane for one part of toluene. The injection takes place during the copper seeding step. Firstly the copper precursor is injected for 20 minutes, then the iodine-containing solution is injected using the same parameters until 15 ml of solution has been injected, then copper injection is resumed.

Carbon Nanotube Orientation

[0125] The coated foil ribbons are removed from the sample holder, and the carbon nanotube forest of both sides are laid down by passing the coated ribbon 2 in between two rotating smooth quartz cylinders 4, 6 along the width dimension of the ribbons, as illustrated in FIG. 7. This allows the carbon nanotubes to be oriented coaxially to the subsequent drawing process.

Copper Infiltration

[0126] The ribbon 2 with the oriented carbon nanotube forest is then installed in a rack and dipped into an electroplating bath 7, as depicted in FIG. 8. This bath is composed of a solution of 0.56 mol/l CuSO.sub.4 aqueous solution, with 0.67 mol/l sulphuric acid, 0.0027 mol/l sodium chloride. The volume of this solution in the bath is 250 ml with an addition of 5 ml of N-methyl pyrrolidone (NMP), 5 ml of methanol and 0.1 g sodium dodecyl sulphate (SDS). A current is established in between the coated copper ribbon and a pure copper anode in order to electroplate the carbon nanotube forest with copper. The current is imposed and the potential adjusted, as usually done for copper electroplating, and described in Schneider, Weiser, Dörfer et al. (2012), Surface Engineering, vol. 28, issue 6, pages 435 to 441. In order to improve the copper deposition inside the forest, the current follows a pulse-reversed pattern (as described in “Mechanical Properties of Carbon Nanotubes/Metal Composites” doctoral dissertation by Ying Sun, University of Central Florida, 2010), with a slight offset to maintain an electromigration force applied to the copper ions during the non-depositing time. FIG. 9 shows the typical pattern used, where I.sub.c is the plating current, I.sub.s is the stripping current, with the offset allowing electromigration of the copper ions without plating. The fully plated carbon nanotubes are illustrated in FIG. 10.

Wire Drawing

[0127] The insert is optionally first compressed (e.g. by using a rotational swaging machine or hot or cold rolling machine) until all voids in the insert are substantially eliminated.

[0128] The wire drawing process is carried out using a copper substrate that has a length of 300 mm and a width of 100 mm. This substrate has undergone the above steps to form laid down copper nanotubes on the two major opposing surfaces of the substrate. A bobbin 8 with a recessed region 10 for receiving the wound substrate is provided as shown in FIG. 11. The substrate 6 is then wound onto a bobbin 8 to form a bobbin of 18 mm in diameter. This is depicted in FIGS. 12 and 13. This bobbin 8 is slid into a sleeve 12 by sliding it into a cavity in one of the sleeve's ends. The sleeve has an outer diameter of 22 mm and a length of 500 mm and is depicted in FIG. 14. The sleeve 12 with the bobbin 8 inside is depicted in FIG. 15. The carbon nanotubes have been laid down to be substantially aligned along the length of the insert.

[0129] The insert is then drawn on a drawbench to achieve a 10% reduction in the insert's diameter, as depicted in FIG. 16. The drawn billet is then annealed in an argon atmosphere at a temperature of 550° C.

[0130] The steps of drawing and annealing are repeated until the insert's diameter has been reduced to 8 mm and the length has been increased to 3.75 m.

[0131] X-ray analysis is then conducted on the insert's ends so as to identify and cut off the sections that are pure copper (due to the greater length of the billet compared to the substrate prior to drawing). This insert is then run through a rod breakdown machine to reduce to the diameter to 2 mm. This is then drawn down to 1 mm using a wire drawing machine and the wire is then spooled. The final length of the wire in this example is approximately 50 m.

[0132] FIG. 17 shows a depiction of the final wire, where there is a copper matrix 14, within which there are layers of carbon nanotubes separated by layers which do not contain carbon nanotubes. The pattern of the carbon nanotube layers was introduced in the rolling up process.

[0133] FIG. 18 illustrates an SEM image of a wire drawn down to 1.5 mm. The image is a cross-section of the wire and shows at least 13 layers of carbon nanotubes along the radius of the wire. Detail of these layers is depicted in FIG. 19. An etched sample of this cross-section is shown in FIG. 20, where the alignment of the ends of the carbon nanotubes can be appreciated by the collection of carbon nanotubes at the centre of the image.

[0134] FIG. 21 illustrates a longitudinal section of the wire with substantial alignment of the carbon nanotubes in the centre of the images along the longitudinal direction of the carbon nanotubes.

[0135] In addition to the combination of features recited in the claims, the various features described herein can be combined in any compatible manner.