ULTRACONDUCTIVE COPPER WIRE: OVERVIEW OF WORLDWIDE RESEARCH AND DEVELOPMENT

H. Stillman and M. Burwell

International Copper Association 260 Madison Avenue

New York, NY 10016, USA hal.stillman@copperalliance.org

ABSTRACT

Over 60% of copper shipped each year is used in electrical applications.

Improving the electrical conductivity of copper could have a transformational effect on electrical and electronic devices and would be of immense benefit to society.

One potential approach for decreasing copper’s electrical resistivity is the

incorporation of carbon nanotubes into copper. This process has been shown to increase the ambient-temperature electrical conductivity of copper to more than 140% of that of pure copper. This nano-composite material is called UltraConductive copper.

Carbon nanotubes conduct electricity differently than metals: optimizing the

electrical conductivity of a copper/nanocarbon composite requires careful engineering on a nano-scale.

UltraConductive copper is being developed on a laboratory scale at several

organizations around the world. It is still many years away from being able to be mass-produced. Fabrication techniques being studied include electrolytic co-deposition, die casting, hot-extrusion, laser formation and fiber infiltration. This paper provides an overview of the world-wide research effort towards the development of UltraConductive copper wire, and reviews physical and mechanical properties being achieved. The state of development of the copper/nanocarbon composites is summarized and barriers to commercialization reviewed.

INTRODUCTION

For the past hundred years or so, two metals, copper and aluminum, have dominated applications that require good electrical conduction. As copper has the lower electrical resistivity of these two metals, it remains the preferred electrical conductor in many wiring applications, with more than 60% of the copper produced around the world being used in electrical markets.

Improving its electrical conductivity would continue the dominance of copper-

based materials in electrical applications, and could even have a transformational effect on electrical and electronic devices that would be of immense benefit to society. One potential method of achieving this is via the dispersion of carbon nanotubes (CNTs) in the copper (Figure 1). The unique electrical properties of CNTs enable the possibility of significantly improving the electrical conductivity [3,4,5,6,9,10,11,16,20] and ampacity [23] of copper. There has been much activity around the world recently to incorporate carbon nanotubes into copper, to produce a copper-based nano-composite material with improved ambient-temperature electrical conductivity that is termed UltraConductive Copper (UCC).

a) b)

Figure 1 – Morphology of UltraConductive wire based on a copper/CNT composite (a) longitudinally-sectioned wire showing regions of high (grey, axial) and low (copper-

colored near wire surface) electrical conductivity, and (b) electron-micrograph taken in the high conductivity area of Figure 1a displaying separated, dispersed and oriented

CNTs (yellow arrows) embedded in the copper matrix (Source: T. Nayfeh, A. Wiederholt at Cleveland State University)

There are few peer-reviewed scientific papers published in the area of UCC

science. There is, however, much significant scientific research taking place in the area. This situation comes about because UCC technology is far from stable, and because of the high potential reputational and commercial value expected if it can be stabilized. Until then, research discoveries remain veiled. Much, however, can be learned of UCC technology from the recent patent literature. It is from the literature and from the

authors’ ongoing informal links to the many UCC researchers worldwide that the contents of this paper are drawn.

BRIEF DESCRIPION OF CARBON NANOTUBES

Carbon nanotubes are allotropes of carbon having a long cylindrical structure.

The tubes have diameters typically in the range 1-100nm. The walls of nanotubes are composed of graphene, a one-atom thick layer of carbon atoms arranged in a hexagonal pattern. Carbon nanotubes can be fabricated either as single-wall (SWCNT) or multi-wall (MWCNT). A single-wall nanotube has the structure of a graphene sheet rolled into a seamless cylinder (as shown in Figure 2). Multi-wall nanotubes consist of tubes having two or more graphene-like layers in their walls. Nanotubes typically have lengths between several hundreds to many millions of times their diameters.

Figure 2 – Schematic of a single-wall carbon nanotube

Carbon nanotubes are among the strongest and stiffest materials yet discovered. However, of more interest to copper producers and users are their electrical properties. Carbon nanotubes can be fabricated to have either metallic or semi-conductive properties, depending on the chirality of the carbon atoms’ arrangement in the tubes. Metallic nanotubes have the capability to be ballistic electrical conductors, allowing the unimpeded flow of electrons along their lengths. With conventional metallic conductors, free electrons collide and are scattered by crystalline lattice defects and impurities, defects or atoms within the metal. This causes the traveling electrons to lose momentum, and this loss of momentum is what at a macro-level constitutes electrical resistance. With carbon nanotubes, however, the electrons can travel along the nanotubes without experiencing scattering, resulting in very low resistance or dissipation of energy. This is referred to as ballistic conduction, and it is this ballistic conduction behavior of carbon nanotubes that provides the potential to significantly increase the electrical conductivity of copper. The actual mechanism of ballistic conduction is not fully understood, but for SWCNTs, some researchers postulate that electrons travel through the cloud of non-hybridized π-orbitals on the inner and outer tube surfaces. This is believed to be described by Luttinger-state transport models [1,2]. For MWCNTs, electrons may travel in the spaces between tube layers. Because of their deleterious effects on the charge density distribution near the CNT, defects in the lattice structure of nanotubes significantly affect electrical conductivity.

ULTRACONDUCTIVE COPPER

Pure copper has an electrical conductivity of 0.58 MS/m (which corresponds to

an electrical resistivity of 1.72μΩcm at 20oC). The electrical conductivity of other materials is commonly referred to as a “% IACS” ratio, which compares it to the conductivity of copper. For example, copper used for most electrical applications has an electrical conductivity of 100% IACS and ultra-pure aluminum has an electrical conductivity of 65% IACS. Ultra-pure copper can have up to about 103% IACS. The production of a composite of aligned carbon nanotubes mixed into pure copper provides the potential of significantly increasing electrical conductivity above 100% IACS. This concept was proposed in 2004 by Hjortstam et al [3], who predicted that composites of single-walled carbon nanotubes dispersed in copper could provide a composite material having at least twice the electrical conductivity of pure copper. Their predictions are reproduced in Figure 3, which suggests that adding aligned carbon nanotubes to pure copper can decrease resistivity below 1.72μΩcm. The dashed horizontal line in Figure 3 represents an electrical resistivity of 0.86μΩcm (i.e., half that of pure copper, or a conductivity of 200% IACS), and the calculations from Hjortstam et al [3] predict that this resistivity can be achieved after mixing 30-40 vol% CNTs (4-6 wt%) into pure copper. Unpublished experimental results from NanoRidge, Cleveland State University and the University of Central Florida suggest that this theoretical wt% may be an order of magnitude higher than what is actually needed to produce the effect.

Figure 3 – Calculated resistivity for a composite of copper and single-walled nanotubes. The resistivity is shown as a function of volume fraction of single-walled carbon

nanotubes (the filling factor) for three different geometries [3]. The dashed horizontal line is a resistivity level that is 50% below the resistivity of copper at room temperature

In their 2004 paper, Hjortstam et al [3] suggested that three factors are crucial to the successful development of UltraConductive copper; (1) the use of high quality, non-deformed CNTs, (2) the development of methods to disperse and align the CNTs in the pure copper and (3) the identification of techniques that provide excellent electrical contact between the CNTs and the matrix copper metal. These three factors, are at the roots of current development programs.

MANUFACTURING APPROACHES FOR ULTRACONDUCTIVE COPPER

A number of different approaches are currently being explored around the world for the synthesis of UltraConductive copper in ways that could enable scale-up to mass quantities. These production processes are far from stable today and produce inconsistent results. All researchers have obtained tantalizing small-scale results with conductivities >130% IACS, at least one of which has been independently verified. Some of the more promising approaches are described here, and their properties summarized. Electrolytic Co-Deposition

Several research teams are examining different approaches to produce

UltraConductive copper using electrolytic co-deposition. In this method, a solution containing both copper ions and CNTs is placed in an electrolytic cell, and the application of an electric current causes a composite of pure copper containing CNTs to be deposited at the cathode.

Chen at the University of Central Florida (USA) provides some details about this

approach [4]. The CNT’s are positively charged via treatment with a surfactant (such as cetyl trimethyl ammonium bromide) that keeps them dispersed within the solution. As the copper ions within the solution are also positively charged, co-deposition of the copper and the CNTs occurs at the cathode of the electrolytic cell (Figure 4). The highest deposition rate reported by Chen is 1μm per minute.

Figure 4: Schematic drawing showing the electrolytic cell reported by Chen [4]

Chen has measured the physical and mechanical properties of UltraConductive copper produced via electrolytic co-deposition. Table 1 shows the electrical resistivity of 10μm thick samples, indicating that the Cu-SWCNT composite has an electrical conductivity 41% higher than that of pure copper.

Figure 5a shows stress-strain curves, indicating that the strength of the Cu-

SWCNT composite is more than twice that of pure copper, but that the ductility is significantly lower. Figure 5b reproduces data for the coefficient of thermal expansion, showing that the Cu-SWCNT composite has a value ranging between 4 to 5.5x10-6/oC (versus 17x10-6/oC for pure copper). In all cases, Chen did not report the percentage of CNTs in these samples but did indicate that material produced using single-walled CNTs had higher electrical conductivity and higher strength than material produced using multi-walled CNTs.

Table 1 – Measured electrical resistivity of samples produced by Electrolytic Co-

Deposition [4]

a) b)

Figure 5 - Other mechanical and physical properties reported by Chen [4] a) Strength and ductility

b) Coefficient of thermal expansion

NanoRidge, a US company focusing on nanomaterials, has also described an electrolytic co-deposition process for the production of UltraConductive copper [5], in their case focusing on circuit board applications. Nanoridge has produced a copper-nanomaterial composite conductor with a current carrying capacity of 5.6x104 amps/cm2, compared with a similar circuit fabricated from 99.9% pure copper that has a capacity of

Material Deposited Electrical Resistivity

(μΩcm) Electrical Conductivity

(% IACS)

Cu/SWCNT composite 1.22 141

Pure copper 1.72 100

only 3.9x104 amps (i.e., the copper-nanomaterial composite has a 44% higher ampacity) [5] .

Figure 6 shows an electronmicrograph from electrolytically-synthesized UCC

wire. This surface was produced by fracturing a 2mm diameter UCC wire after it had been cryogenically cooled. It is proposed that the deformation features on the surface of the fracture represent a map of the embedded SWCNTs. Ongoing work at higher resolution is underway to better characterize the distribution of CNTs in this form of UCC.

Figure 6 – Electronmicrograph of the surface of cryogenically-fractured UCC containing SWCNTs (Source: NanoRidge Materials Inc. – TeraCopper® material)

The electrodeposition of UCC powder is also under development at several organizations around the world [6]. In these methods, the equipment is very similar to the electrolytic deposition of UCC plate described above: CNTs are dispersed in a copper ion solution and a DC current is applied between electrodes inserted in the solution. However, by applying high electrode current densities, UCC composite powder forms on the plates and is sloughed off to the bottom of the electrolytic cell from where it is harvested (Figure 7). The electrolytic production of copper powders has been done since 1886 [7]. Mass production of these powders is possible in continuous-flow deposition cells [8]. Production of UCC in this way is straightforward, but leaves the challenge of forming the powders into strip or wire (see “Conversion to Wire” section later in this paper). Challenges with oxidation and porosity are currently the focus of this work.

Figure 7 - Schematic of “point electro co-deposition” for production of CNT reinforced

copper nanocomposite powder [6] Die Casting

A die casting approach to UCC synthesis has been examined by researchers at Cleveland State University in the USA [9]. Their system involves pre-loading CNTs into a cartridge, which is then placed into the shot sleeve of a cold chamber die casting machine (Figure 8). Liquid copper is poured into the shot sleeve, which mixes with the CNTs, and the composite material is then injected into a die and the copper solidified. To provide mixing and alignment, three zones are defined in their casting system. In the first zone the material is agitated to mix the nanotubes into the liquid copper. In the second zone, the flow of the liquid copper become laminar, where the goal is to align the carbon nanotubes parallel to the axis of the casting. In the final zone, heat is extracted causing the liquid copper to solidify. Samples of copper-CNT composite material produced using this die casting approach have shown an electrical conductivity of 113% IACS [9].

Figure 8 - Schematic of the die casting approach [9]

As part of this process, the researchers at Cleveland State University have also

proposed the use of magnesium chloride as an addition to the raw CNTs in the die cast chamber. Magnesium metal is known to wet carbon nanotubes (copper does not) and is soluble in copper, so this addition may provide a tie layer between the CNTs and the copper to improve the composite’s conductivity. Acoustic Assisted Coating

Researchers at Los Alamos National Laboratory (LANL) have acoustically

engineered a resonant system that deposits high-quality coatings of CNTs on the outside surface of copper wire substrates [10]. Their process involves immersing short lengths of copper wire in a tube containing CNTs suspended in a fluid. Acoustic excitation is then applied in the vicinity of a high order resonance that corresponds to concentric ring and nodal line formation along the central axis of the tube. By sweeping the excitation frequency near such a resonance the contents of the tube are consecutively agitated and concentrated near the central wire, leading to continual embedding of the CNTs in a growing coating on the copper. The researchers have been able to produce very high quality coatings of high conductivity (see Figure 9). However, they have yet to demonstrate a composite wire with >100% IACS. Ongoing research is concentrating on implementing a reel-to-reel experimental set-up. The goal is to produce wires with >100% IACS, but even if successful there is still much research to be performed to address issues associated with manufacturing, joining, thermal shock, insulation, and coating adhesion under bending and corrosive conditions.

Figure 9 - Electrical performance of composite wires produced at LANL

CNT Fiber Infiltration

Researchers at Cambridge University (UK) are also examining a process for the production of long lengths of UltraConductive copper wire. The Cambridge University researchers have already developed a process to produce a continuous yarn of CNTs and they plan to use either vapor deposition or electro-deposition to penetrate copper into the yarn, thereby producing a continuous length of Cu-CNT composite material. This work, however, is still at the early stages of development, and the researchers have yet to publish any properties for copper-CNT composites produced in this manner. A European FP7 development project (called “Ultrawire”) [20] led by Cambridge University begins in 2013 and will accelerate discoveries in this area..

Other UCC Synthesis Processes

Two other processes have been investigated for forming UCC. Neither has been

promising, and investigation into both has stopped:

Laser Formation. Technology developed at Los Alamos National Laboratory (LANL) used a high pressure laser chemical deposition process. SWCNTs are built-up by laser-induced breakdown of gas precursors with simultaneous laser-induced cladding with metal. This process involves the growth of CNTs that are hundreds of millimeters long and the material (called Ultraconductus) is created in the form of a continuous wire [11]. Although the Los Alamos researchers claim increases in electrical conductivity of >10,000% IACS, this work has yet to be verified and has stopped due to the high costs of both further investigation and final scale-up.

Powder Metallurgical Synthesis. Various researchers [12-14] have investigated

ball-milling of CNTs into the surface of copper particles followed by spark plasma sintering of those particles into a fused solid. None of these researchers has achieved samples with conductivities greater than 100% IACS.

CONVERSION TO WIRE

Many of the processes for synthesizing UCC result in forms of material that are

not directly useful commercially. For example, electrolytic UCC comes in sheet form and die cast UCC comes in short rods. The most widely-used form of copper for electrical applications is wire. Accordingly, considerations of turning synthesized UCC into UCC wire are important.

Most commonly in industry today, the wire-making process begins with the continuous casting of molten copper into rod. Ideally, this first-step would be used with UCC to allow the reuse of the capital investment and know-how involved in the existing process. However, this route is not straightforward for UCC wire, since, because of carbon/copper density differences, the nanocarbon separates from the copper if the copper is melted (Figure 10). Work is ongoing to prevent this separation by using high shear mixing of the molten copper with the high viscosities found near the melting point.

Figure 10 – Separation of nanocarbon from UCC on melting and re-solidification

(Source: AGH University of Science and Technology, Krakow, Poland, material supplied by NanoRidge)

Alternatives to melt-processing are being investigated in which the composite

material is mechanically deformed, but never molten. The following section describes the main processes being investigated. To date, none of these processes have produced wire above 100% IACS for more than very short lengths. Some unverified samples have shown above 200% IACS over 1-2mm lengths and of over 115% IACS over centimeter lengths.

Consolidation and hot extrusion is an alternative to converting to wire through a

melting route. The UCC is cold or hot pressed into a billet which is then hot extruded into wire (Figure 11). This process should align the CNTs in the extruded product. Work is continuing to perfect this process and to assess its affects and the alignment of CNTs on the wire’s level of conductivity.

a) b) c)

Figure 11 – Consolidation of UltraConductive foil into a 16 mm diameter billet and hot extrusion into 2 mm diameter wire (a) before pressing, (b) after pressing, (c) after hot

extrusion. (Source: Ohio University, material supplied by NanoRidge)

UCC produced as nano-composite powders can be formed into useful products such as wire, strip and bar through a number of sintering and consolidation processes. Hot-pressing, isotactic pressing, spark plasma sintering and direct powder extrusion have been used [12]. In one technique borrowed from the nano-ceramic extrusion industry [15] the process used starts by encapsulating the powder in a copper can, hot degassing in dynamical vacuum and then sealing the can (eg. by electron beam welding). The resulting sealed can of powder is then placed in a hot extrusion press and wire is extruded into an inert gas atmosphere. In another technique, powder is placed in a tube and the tube compressed in a die to consolidate the powder (Figure 12) [16].

Figure 12 - Cross-sections of Fe/Cu-CNT composite wire fabricated using fine

copper powder with diameter less than 1 μm. CNT is SWNT and mechanical alloying has been performed for 30 minutes before packing into an iron tube [16]

A final method of producing wire combines UCC synthesis and wire formation

into a single process. Attempts have been made for over 100 years to directly deposit copper electrolytically into wire [17,18]. These attempts have been technically

successful, but the process costs and corrective post-processing needed have prevented it being commercially exploited. However, it may offer a route for making UCC wire directly. One particular option is to take the SWCNT continuous fiber produced by Cambridge University’s CVD process or Rice University’s wet-spinning [19] and directly deposit copper into it, either electrolytically or by molten copper applications.

STATUS AND FUTURE

The UltraConductive copper described in this paper falls into a class of materials

called metals-matrix composites (MMCs). A MMC consists of two separate materials, typically a ceramic and a metal, and MMCs are produced by mixing together the two materials. The purpose of producing a MMC is to dramatically extend the properties of metallic materials, to obtain properties not achievable with monolithic metals and alloys, while retaining many of the advantages of metallic alloys (strength, ductility, good electrical and thermal conductivity, etc.). Common MMCs include aluminum-silicon carbide, aluminum-aluminum oxide, and titanium-silicon carbide. MMCs are used in a number of markets including ground transportation, electronics, aerospace and defense.

However, although MMCs have been under development for quite some time

(decades, in some cases), they still have not achieved widespread commercial acceptance, tending to be used only in niche applications. For example, although the world market for MMCs for all applications grew 44% from 1999 to 2005 (from 2.8 to 4.0 thousand tons) [21,22], this is still only a small fraction (less than 0.03%) of the total amount of copper produced in 2005 (14.9 million tons).

Unlike UCC, these MMCs do not address very large markets. The main issue

limiting the widespread application of MMCs is the high cost of both the materials used, as well as the high cost of processes used to produce the composites. The challenge is to reduce these costs through learning-curve effects from mass-manufacturing, something that does not exist for these non-UCC applications. As many MMCs are more expensive than monolithic alloys, MMCs tend only to be used where the higher cost can be justified by improved properties and performance. It is likely that copper-CNTs composites will face the same issues.

Although CNTs are the focus of research around the world, the prices of CNTs

remain high. Recent data indicate that the price of MWCNTs are typically around US$200-$1,000/kg, and as high as $50,000-$100,000/kg for SWCNTs. Commoditization and scaleup of CNT production is currently held back by the lack of high-volume applications, so today’s prices are an upper limit. As CNTs become used in larger and larger volumes, it is likely that their costs will come down closer to the raw material cost of other forms of carbon (eg. high quality gaphitis rods cost around $60/kg). If CNTs could be produced at the cost of graphite, the combined raw material costs of UCC might approach 110% that of pure copper.

Clearly there are a number of challenges that must be met before copper-CNT nano-composites will achieve widespread commercial acceptance. First, the price and

availability of CNTs must be reduced to a level where they can be incorporated into wire products produced at high volume. Second, the fabrication techniques developed here are still laboratory-scale and need significant development before they might be capable of producing commercial quantities of material. Finally, UCC wire-manufacturing costs will also need to be significantly lowered. All three of these developments will take time, and so the commercialization of these copper-CNT composites is probably some 5-10 years away.

Similar to other MMCs, it is doubtful that material and fabrication costs for

copper-CNT composites will ever be as low as those for pure copper. In the long run, as mass-manufacturing processes are developed, it is possible that UCC manufacturing costs may become much closer to the costs of pure copper, but it is unlikely that UltraConductive copper will replace monolithic copper in all applications. However, it is likely that copper-CNT composites will first achieve penetration into high-end markets, such as defense, aerospace and high performance electronics, and this will provide an opportunity for producers and users to continue to extend the applications for copper and its alloys into larger-volume markets.

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