Welding of carbon single-walled nanotubes by microwave treatment

The present invention relates to methods for the preparation of carbon single-walled nanotubes, in particular, the synthesis of interconnected nanotubes.

Carbon nanotubes are hexagonal networks of carbon atoms forming seamless tubes with each end capped with half of a fullerene molecule. They were first reported in 1991 by Sumio Iijima who produced multi-layer concentric tubes or multi-walled carbon nanotubes by evaporating carbon in an arc discharge. In 1993, lijima\'s group and an IBM team headed by Donald Bethune independently discovered that a single-wall nanotube could be made by vaporizing carbon together with a transition metal such as iron or cobalt in an arc generator (see Iijima et al Nature 363:603 (1993); Bethune et al., Nature 363: 605 (1993) and U.S. Pat. No. 5,424,054). The original syntheses produced low yields of non-uniform nanotubes mixed with large amounts of soot and metal particles.

Presently, there are three main approaches for the synthesis of single- and multi-walled carbon nanotubes. These include the electric arc discharge of graphite rod (Journet et al. Nature 388: 756 (1997)), the laser ablation of carbon (Thess et al. Science 273: 483 (1996)), and the chemical vapor deposition of hydrocarbons (Ivanov et al. Chem. Phys. Lett 223: 329 (1994); Li et al. Science 274:1701 (1996)). Multi-walled carbon nanotubes can be produced on a commercial scale by catalytic hydrocarbon cracking while single-walled carbon nanotubes are still produced on a gram scale.

Generally, single-walled carbon nanotubes are preferred over multi-walled carbon nanotubes because they have unique mechanical and electronic properties. Defects are less likely to occur in single-walled carbon nanotubes because multi-walled carbon nanotubes can survive occasional defects by forming bridges between unsaturated carbon valances, while single-walled carbon nanotubes have no neighboring walls to compensate for defects. Defect-free single-walled nanotubes are expected to have remarkable mechanical, electronic and magnetic properties that could be tunable by varying the diameter, number of concentric shells, and chirality of the tube.

The synthesis of SWNTs by any of the methods described above produces individual tubular structures where the carbon atoms comprising the hexagonal rings are sp³ hybridized. For some applications, such as electrical applications, electrochemical applications, and for applications requiring mechanical strength, interconnected SWNTs may be preferred. The interconnected SWNTs have large surface area which is advantages for applications, such as for super-capacitors, high energy density batteries, high density catalyst support applications, and the like.

The tubular SWNTs can be joined with “X,” “Y,” and/or “T” type molecular junctions that require the carbon atoms at the junction be sp² hybridized. U.S. Pat. No. 6,495,258 to Chen et al. describes a method of creating a three-dimensional density distribution of carbon nanotubes, where a substrate having a network of randomly oriented fibers is created, and carbon nanotubes are dispersed in the network. The fibers are nickel fibers sintered together at their crossing points. Thus, in the method of Chen, the carbon nanotubes are not directly connected with each other; instead, the substrate is used to provide the 3-dimensional distribution of nanotubes. In another method described by Imholt et al. (2003) Chem. Mater. 15: 3969-3970, nanotubes are heated to a temperature of at least 1500° C. using a microwave field that fuses the nanotubes together. A method described by Terrones et al. (2002) Phys. Rev. Lett. 89:75505-1 dissolves SWNTs in ethanol which are then subjected to electron irradiation at high temperatures in a transmission electron microscope that resulting in cross-linking between the tubes. The methods of Imholt et al. and Terrones et al. use high temperature to link carbon nanotubes.

These methods either do not directly connect the nanotubes or employ harsh conditions and are expensive for application to large scale synthesis of interconnected SWNTs. Thus, there is a need for simple and reliable methods for producing interconnected SWNTs. Accordingly, the present invention provides methods and processes for joining carbon nanotubes.

The present invention provides methods and processes for welding single-walled carbon nanotubes (SWNTs). SWNTs can be synthesized by any one of the art methods, and can be fibers, bundles, or soot. The SWNTs are exposed to the electrical field (E) component of microwave energy which heats the SWNTs to about 1200° C., thereby welding together the SWNTs.

In one aspect, the present invention provides methods for producing interconnected single-walled carbon nanotubes (SWNTs) wherein a sample of SWNTs is placed in a microwave cavity wherein the electric field is near maximum; and the sample is then exposed to microwave field at a temperature less than about 1400° C. The microwave field has a frequency between 1 GHz and about 5 GHz, preferably a frequency between about 2 GHz and about 3 GHz, and power between about 100 watts and about 450 watts. The sample can be heated to a temperature between about 1000° C. and about 1400° C., preferably between about 1000° C. and about 1200° C.

In another aspect, the methods for producing interconnected single-walled carbon nanotubes (SWNTs) involve placing a sample of SWNTs in a microwave cavity wherein the electric field is near maximum; and exposing the sample to microwave field at a temperature less than about 1400° C., wherein the microwave field has a frequency between 1 GHz and about 3 GHz, and power between about 150 watts and about 400 watts. The SWNTs are interconnected by a “X,” “Y,” or “T” type junction.

These and other aspects of the present invention will become evident upon reference to the following detailed description. In addition, various references are set forth herein which describe in more detail certain procedures or compositions, and are therefore incorporated by reference in their entirety.