FIELD OF THE INVENTION
The present invention relates to a process of forming a carbon nanotube yarn metal composite with enhanced mechanical strength and electrical conductivity. The invention further relates to a carbon nanotube yarn metal composite.
BACKGROUND OF THE INVENTION
Carbon is an essential element in chemistry and also the most important elements of life on earth. It exists in various forms including fullerenes, carbon nanotubes (CNT’s), graphene, etc. Among all the allotropes of carbon, CNT’s are technologically important. CNT’s are tubules of carbon formed by wrapping a graphene sheet. CNT’s are 5 µm to 1 cm in length and 5-100 nm in diameter. CNT’s exist either as single-walled (SWNT), double-walled (DWNT), thin-multi-walled (T-MWNT) or multi-walled (MWNT). Individual CNT’s have exceptional mechanical, electronic, photonic, and optical properties, which could be important to several applications. Further, conversion of nanostructured CNTs into macroscopic from i.e. yarn is an intriguing concept which can result in macro-scale applications of CNT-Y for electrical, defense and aerospace [1-3] industry.
The preparation of CNT-Y from CNT arrays or aerogel or solution does not result in the translation of the pristine CNT electrical and mechanical properties to CNT-Y. Hence, several inventors have disclosed methods to improve the electrical and mechanical properties of as-prepared CNT-Y. The disclosures on electrical property improvement of CNT-Y are novel and are capable of enhancing electrical conductivity. However, most of the processes involve corrosive gasses and acids including iodine, hydrochloric acid,

hydroiodic acid, nitric acid, etc. The corrosive nature of the gasses and acids pose an environmental hazard. The resultant effect being the acceptability for upscaling is narrow. Some of the most relevant disclosures related to the present invention are US 2013/0183439 A1 and WO 2013/109442 A1 which disclose the methods to improve the electrical conductivity of CNT-Y by chemical doping by bromine, iodine, chloroauric acid, hydrochloric acid, hydroiodic acid, nitric acid, etc. Invention WO 2012118836 A1 discloses methods to improve the electrical conductivity of CNT-Y by doping.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a process of forming carbon nanotube yarn metal composite with enhanced mechanical strength and electrical conductivity.
A further object of the invention is to propose a carbon nanotube yarn metal composite.
SUMMARY OF THE INVENTION
According to the present invention, there is provided a metal infiltration process, where the metal infiltration is conducted by nano-metal dispersed in CNT-Y by CNT wetting liquid with 60-80% volume percentage having a particle size of 2-10 nm. The nano-metal dispersed in the CNT wetting liquid is prepared by microwave technique. The nano-metal dispersed CNT-Y is laser processed and annealed under high temperature (800-1000 ◦C) and high vacuum (10-6 to 10-7 Torr)

The invention is also directed to a CNT-Y metal composite with enhanced mechanical strength and electrical conductivity.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 Electrical conductivity of (1) Cu, (2) CNT-Y-Metal composite, (3) CNT-Y.
Figure 2 Scanning electron microscope (SEM) image of CNT-Y-metal composite.
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to a process of preparing a CNT-Y-metal composite (4) with enhanced mechanical strength and electrical conductivity (3). The process includes a nano-metal infiltration into CNT-Y before densification by twisting. Preferably the nano-metal dispersed liquids are made of nanoparticles of size ranging between 2-10 nm. More preferably the nano-metal dispersions are prepared from and not limited to silver, copper, and gold in liquids that have good wettability with CNT. Preferably the liquids can be and not limited to isopropyl alcohol, ethanol, acetone, ethylene glycol. Infiltration by CNT wetting liquid is an efficient method to improve mechanical strength by densifying CNT-Y prepared by spinning from an aligned array of CNT. Preferably the wetting liquid is selected based on the metal salt that is used for the preparation of nano-metal particle dispersion by microwave heating. The reducing action of the liquid aids in the formation of nano-metal particles from the metal salts during microwave heating. Preferably the microwave heating is performed for 2-5 minutes with a power between 100-200 W so as to achieve a particle size of 2-10 nm.

The tensile strength of yarns prepared by spinning from an aligned nanotube array is limited to the Van der Waals forces. However, the yarn specific strength is higher due to the tube alignment and entanglement generated in the spinning process. Whereas the limitation i.e. tensile strength beyond certain value resides in weak inter tube shear interactions. Hence increasing strength is necessary for developing CNT-Y for commercial applications. And the inclusion of nano-metal particles into CNT-Y can benefit in connecting the adjacent CNT and benefit in load transfer and in enhancing mechanical strength and electrical conductivity. Hence the disclosed process can result in improving the mechanical strength and electrical conductivity.
After metal infiltration, the CNT-Y is spun with appropriate speed. The spinning leads to the formation of entangled and densified CNT-Y. The metal infiltrated and densified CNT-Y thus formed, is processed to achieve compaction and reduction of voids formed during spinning. Preferably the heating is performed with a laser in a vacuum atmosphere or in the presence of inert gas. Processing in high vacuum (>10-6 Torr) prevents oxidation of CNT and also aids in the removal of any de-absorbed species during laser heating. The laser is scanned across the length of CNT-Y. The high thermal conductivity of the nanotubes results in uniform heating, melting and formation of thin metallic layer connecting the adjacent nanotubes and benefit in load transfer and in enhancing mechanical strength. The reduction of voids and formation of conducting layer path across the volume of the CNT-Y combined with the conduction channels of CNT can result in an enhancement in electrical conductivity (2). Further during evaporation of the liquids (while laser processing) a large capillary force may be generated to draw adjacent nanotubes together. Preferably resulting in enhancement in mechanical and electrical properties.

The laser processed CNT-Y is further heat treated in high vacuum (>10-6 Torr) and high temperature (800-1000◦C) environment. The heating is performed to anneal the material and minimize the stress. Depending on the requirement, the annealing may also be performed in the presence of H2, Ar etc. The following examples are presented to further illustration the invention, but it is not being considered as limited thereto.
Example 1
Metal infiltration into CNT-Y while spinning from an array of CNT is described. Aligned CNT array is grown from the iron catalyst (5 nm deposited by RF sputtering with argon as the process gas) on p-type silicon wafers having a native oxide 30 nm. The aligned nanotube array is grown in a low-pressure chemical vapor deposition system with C2H2, Ar, and H2. The array is transferred to a spinner system, and spinning is performed. In this example the metal used for infusing CNT-Y is silver. The nano-silver is prepared by microwave heating of AgNO3 with ethanol and DI water. The volume percentage of Ag in ethanol is varied by increasing the AgNO3 content. The microwave heating of the mixture is performed at 400 W for less than 1 minute in a closed vessel with a Milestone Srl system. The particle size analysis of nano-silver dispersed in ethanol is carried out with a Malvern particle size analyzer. The particle size distribution is observed to be between 2-10 nm. Through scanning electron microscopy analysis, the morphology of the Ag nanoparticles was observed to be spherical. The nano-Ag dispersed in ethanol in the form a jet with controlled velocity is used to infuse the CNT web just before the web is twisted and transformed into CNT-Y. Optimization of Ag infusion into CNT-Y is performed by with Nano-Ag of varying volume percentage in ethanol. Optimal value is determined through the measured electrical conductivity of the CNT-Y-Ag composite.

CNT-Y infused with Ag is collected on an alumina cylinder, which is later transferred into a vacuum chamber for laser processing. The CNT-Y-Ag composite is prepared by heating the CNT-Y (Diameter: 20-50 µm) infused with Ag with a diode laser (beam size ~20-50 µm) in a vacuum. The power of the laser is varied from 100-400 W. Laser processing is done in a high vacuum of 10-6 Torr and also in the presence of Argon. Argon gas of ~500 sccm is used during the laser processing. Further, the outlet of the vacuum chamber is connected to a vacuum pump which evacuates the Argon gas continuously. As seen in figure 1 the electrical conductivity of as prepared CNT-Y without metal infusion is 2.6E01 S/cm, whereas the CNT-Y-Ag composite is 6.12 E05 S/cm and it is on par with a copper conductivity of 5.9E05 S/cm.
Example 2
In example 1 the heating of the CNT-Y-Ag composite is done with the only laser. In this instance, in addition to laser processing the heating of CNT-Y-Ag composite is performed in high temperature (800-1000◦C) and high vacuum (>10-6 Torr) or in the presence of Ar, H2 or N2 atmosphere. For the heating step, the CNT-Y-Ag composite collected on an alumina or quartz cylinder is placed inside a three zone tube furnace and heating is performed in steps either in a vacuum or in Ar, H2 or N2 atmosphere for ~8-10 Hrs. The additional heating step helps in annealing of the composite thus enhancing the mechanical strength and electrical conductivity.
Although various embodiments of this invention have been shown and described, it should be understood that various modifications and substitutions, as well arrangements and combinations of the preceding

embodiments can be made by those skilled in the art, without departing from novel spirit and scope of the invention.
REFERENCES CITED

b) NON - PATENT LITERATURE
1. Chaffee. J., Lashmore. D., Lewis. D., Mann. J., Schaeur, and White. B, Direct synthesis of CNT yarns and Sheets, NSTI-nanotech, 3, 118-121 (2008).
2. Agnieszka. L. R., Lukasz. K., Peng. X., Krzysztof. K., Towards the development of carbon nanotube based wires, Carbon, 68, 597-609 (2014).
3. Feng. C., Liu. K., Wu. S. J., Liu. L., Cheng. J. S., Zhang. Y., Sun. Y., Li. Q., Fan. S., and Jiang. K., Flexible, Stretchable, Transparent Conducting Films Made from Superaligned Carbon Nanotubes, Adv. Funct. Mater., 20, 885–891 (2010).

WE CLAIM :
1. A process of forming a carbon nanotube yarn metal composite with enhanced mechanical strength and electrical conductivity, comprising metal infiltration into the carbon nanotube yarn with CNT wetting liquid having nano-metal dispersion prepared by microwave heating technique; laser processing of the yarn; and high temperature and high vacuum annealing of the CNT-Y infused with metal.
2. The process as claimed in claim 1, wherein the nano-metal dispresed liquids are prepared by microwave technique with salts of respective metals.
3. The process as claimed in claim 1, wherein the nano-metals are prepared by heating salts of Ag, Au, Cu in a reducing liquid capable of wetting CNT.
4. The process as claimed in claims 1 to 3, wherein the metal salts are heated by 2.5 GHz microwave for 2-5 minutes between 200-800 W power.
5. The process as claimed in claim 1, wherein the metal particles is not limited to Ag,Au,Cu.
6. The process as claimed in claim 5, wherein the size of metal particles is 2-10 nm.
7. The process as claimed in claim 1, wherein the volume precentage of nano metal is 60-80% in liquid.

8. The process as claimed in claim 1, wherein the nano-metal particles are dispresed in CNT wetting liquid including and not limited to isopropyl alcohol, acetone, ethanol, ethylene glycol.
9. The process as claimed in claim 1, wherein the nano-metal dispresed liquid is infiltrated before the yarn is twisted.
10. Thr process as claimed in claim 1, wherein the metal infiltrated CNT-Y is processed with laser of varying power dependent on diameter of CNT-Y, and wherein the laser is not limited to CO2 or diode laser.
11. The process as claimed in claim 10, wherein the laser processing is either performed in high vacuum (>10-6 Torr) or in presence of gasses including and not limited to Ar,H2,O2,N2, and wherein the laser scan speed can be and not limited to 1-10 mm/sec.
12. The process as claimed in claim 1, wherein the metal infiltrated CNT-Y is processed at high temperature (800-10000C) and high vacuum (>10-6 Torr).
13. A nanotube yarn metal composite with high mechanical strength and electrical conductivity produced in a process as claimed in any of claims 1 to 12.