Ultraconductus: Innovative Electrical Conductors

Creating Revolutionary Electrical Conductors

If technology were music, then electricity would be one of its greatest symphonies. From 1950 to 2008, annual worldwide electrical power production and consumption increased more than 14-fold, from slightly less than 1,000 billion kilowatt-hours to 14,028 billion kilowatt-hours.
The world relies on conductors made primarily of copper and aluminum for transmitting and carrying electrical power, fabricating motors and generators, and myriad other applications. Such conductors, because they have nonzero (small but not exactly zero) electrical resistance, dissipate or lose a small portion of the power they transport. As global energy consumption increases, power transmission and subsequently losses increase, costing consumers more and more money each year.

What type of material would be ideal as an electrical conductor? The obvious answer is something with zero electrical resistance—a superconductor. But super-conductors are not without limitations, in particular, their need for cryogenic operating temperatures and quench effects related to magnetic field and current capacity. What if there were a material that had electrical conductivity better than that of common metals and approaching that of superconductors, but without their operating constraints? On a normalized cost basis, it would be more cost effective to use this new type of material than to use either metals or superconductors. It also would have a tensile strength greater than steel or graphite fibers yet be easily formed and cost much less than superconducting materials. It would not be subject to current density, magnetic field, or temperature quench. Sounds like science fiction? It was—until recently.

James Maxwell and his team at Los Alamos National Laboratory have developed a process known as Ultraconductus, which produces these revolutionary electrical conductors. These new types of conductors can be easily formed into required shapes, such as wires or cables, allowing for greatly enhanced conductivity over existing metallic conductors. But perhaps most important, these "ultraconductors" are more cost effective than copper-alloy conductors and simultaneously minimize the use of expensive or rare materials.

Damascus Steel pic

Used to produce blades such as this one from approximately 300 BC to 1700 AD, Damascus steel is characterized by distinctive patterns of branding and mottling that look like flowing water. Although the original method of producing Damascus steel is not known, National Geographic and others have reported that nanowires and carbon nanotubes played a role in the making of such steel.

Understanding Carbon Nanotubes

Carbon is one of the most interesting and versatile elements in the periodic table. Not only is carbon the key ingredient of all organic chemistry and essential for all living things, it also forms allotropes such as diamond, graphite, and carbon nanotubes (CNTs). All these types of allotropes exhibit high strength. Found throughout history in meteorites, blades made from Damascus steel, and flue soot, CNTs perhaps were the first form of carbon on a growing Earth three to five billion years ago.

CNTs consist of cylindrical sheets of graphene (another carbon allotrope) or extended hexagonal arrays of sp2-hybridized carbon with a conjugated π-system. A CNT's side walls are arranged in a helical fashion around the tube axis and are considered single-dimensional objects because of their small outer diameters (in the nanometer range) and high length-to-width aspect ratio, which is typically greater than 100.

Because carbon-carbon covalent bonds are among the strongest in nature, it follows that structures based on such bonds form very strong materials. In theoretical and experimental studies, scientists have discovered that CNTs with a tensile strength that ranges from 100 to 600 GPa are approximately two orders of magnitude stronger than high-strength carbon fibers. Moreover, a CNT's density of 1.3 g/cm3 is lower than that of commercial carbon fibers (1.8–1.9 g/cm3). The significant reduction of density and volume brought about by replacing carbon fibers with CNTs has important implications in aerospace and other high-performance composite applications. CNTs also have a high stiffness-to-weight ratio, with a Young's modulus of 1,000+ GPa, which is higher than that of carbon fibers.

CNTs also possess beneficial electrical properties. For example, a nanotube's chirality (i.e., twist) determines whether a CNT functions as a metal or a semiconductor. Metallic CNTs have ballistic transport (i.e., zero resistance along the tube), which means that they can produce conductivities 1,200 times greater than that of copper.

Graph of tenoerature-dependent electrical conductivity

This graph plots the temperature-dependent electrical conductivity of Ultraconductus (blue circles) vs. copper (red crosses) over a useful operating range; 300 Kelvin is room temperature.

A metallic CNT wire's conductance does not depend on its length. Unlike traditional metal wires, in which electrical conductance is inversely proportional to the wire length (i.e., G = A/pL), the quantum conductivity of carbon nanotubes is G = 2e2/h, where e is the fundamental charge of an electron and h is Planck's constant. We call these "quantum" conductors. Note that there is no length specified in the CNT conductance equation. In such cases, there are only two states: either the metallic CNT conducts this value of current, or charge, or it conducts nothing. Hence, a CNT can act as an ideal conduit for electrical current.

Left: Growth of aligned CNTs along the length of a wire using Ultraconductus. Observe the catalyst nanoparticles at the tips of the individual CNTs. The aligned nanotubes are subsequently coated with proprietary matrix metals. Right: Bundles of vertically aligned nanotubes grown selectively from a substrate that uses catalytic nanoparticles. Each "pixel" (i.e., column) contains roughly 370 million nanotubes in a cross-sectional area of only 25 x 25 microns—less than half the diameter of a human hair.

Ultraconductus Process

The Ultraconductus process can grow very long metallic CNTs (100s of mm to 10s of cm) while simultaneously cladding them within a metal matrix. Embedding CNTs in a metal matrix facilitates current flow between tubes along with ballistic transport from end to end, thereby increasing the net electrical conductivity of the metal matrix. This "nanocomposite" material accretes the benefits of both the CNTs and the added metal, providing both increased conductivity and structural strength over that of the metal conductor by itself. A conductor produced using the Ultraconductus process possesses an increase in conductivity by a factor of 10 to 100. The improvement is even greater with increasing temperature.

The difficulties with using CNTs to make an ultraconductor are fourfold: (1) there must be a route through which electrons can enter and leave the nanotube's conductive path, (2) there must be a means for electrical conduction from nanotube to nanotube within the bundle, (3) it must be possible to make long and continuous nanotubes, and (4) sufficiently high percentages of metallic nanotubes must be created.

The Ultraconductus process resolves all four issues. Like everything else in the universe, nanotubes possess defects. These defects/impurities can serve as routes through which electrons can enter and leave a nanotube's conductive path. A common defect in carbon nanotubes is a "diode" in which structures in a pair exist side by side, one with a five-member ring (the "n" material) and the other with a seven-member ring (the "p" material).

During synthesis, these diodes can be created by mismatches and damage to the lattice. Moreover, impurities and missing atoms from the lattice also provide routes of entry to and egress from the quantum conductor. CNTs are also somewhat unique in that they can be readily doped with boron and nitrogen, thus providing stable n- and p-type diode materials for entry and egress. Maxwell and his team create conductive paths in and out of the CNTs, as well as paths between the nanotubes, by appropriately doping the CNTs, coating them lightly with proprietary metals, and then embedding them in a metallic matrix.

To fabricate long conductive nanotubes in a matrix, Ultraconductus employs laser-induced chemical reactions and selective chemistry to first form nanotubes and then physically and chemically infiltrate a metal matrix between the tubes. The Ultraconductus process begins when a primary set of laser beams is focused on a pressurized chamber containing a retractable mandrel coated with catalytic nanoparticles. Hydrogen and an appropriate hydrocarbon then flow through a nozzle onto the laser foci, where vertically aligned carbon nanotubes are grown into the laser beams. If the beams remain stationary, CNTs will grow into their respective beams along the laser axis. When the focused laser spots are drawn backward, the CNTs follow, thus yielding long strands of newly grown material. In each laser focus, there are many millions of CNT strands.

Once the strands reach critical length, a second set of laser beams is focused near the lower end of the bundles while simultaneously flowing trace quantities of metallic precursor gases across these laser foci. A chemical reaction occurs at this second set of foci, resulting in the formation of a metal matrix between the nanotube strands. This two-step process then continues onward, as the newly formed nanocomposite wires are drawn backward and spooled outside the chamber through a vapor trap.

This figure shows the fabrication method for Ultraconductus cabling

This figure shows the fabrication method for Ultraconductus cabling. The aligned carbon nanotubes are approximately 10 nm in diameter. The tubes are embedded in a metal cladding, thus ensuring conduction horizontally between tubes. There is very little resistance along their length (i.e., they are ballistic or quantum conductors).

Applications for Ultraconductus Products

Ultraconductus represents a leap forward in technology comparable to Thomas Edison's first economically viable system of central generation and distribution of electric light, heat, and power. The revolutionary Ultraconductus manufacturing technology easily produces wires and cables that have greater conductivity than any other metal alloy, possess 10 times the tensile strength and up to 100 times the conductivity of copper, operate at both room temperature and high-temperature environments, do not require cooling, and are not subject to current density and magnetic field or temperature quench. Additionally, the normalized cost of Ultraconductus cables, expressed in terms of dollars per meter for 100-ampere capacity, is at least four times less expensive than copper and at least 25–30 times less expensive than high-temperature superconductors.

This technology has a wide range of applications, for example, high-voltage cables used to transmit power to homes and businesses around the world, motors and generators that power everything from simple electronics to complex manufacturing systems, electrical wires used in everything from simple electronic devices such as cell and specialized phones and televisions, and specialized applications in which the tensile strength of copper or aluminum is insufficient.

One of the limiting factors of high-voltage, high-power transmission lines, for example, is the sag that occurs under heavy load and high ambient temperatures. Under these conditions, overheated lines can sag to restrictively low levels requiring taller towers, additional conductors, or reduced power transmission. Tower heights and load currents need to be sized to prevent the lines from sagging too low and endangering people or equipment. An improved solution for this case is one from the company 3M, which sells a product called ACCR aluminum matrix cable, which is designed to carry more power—that is, more current (I2R loss) with minimal sagging. These cables are marketed as a means to upgrade the power capability of existing transmission lines without replacing towers or impacting existing rights-of-way. Their conductivity remains the same as that of aluminum, so they can be run hotter while sagging within acceptable limits. Note that Ultraconductus-produced cables completely eliminate all these problems.

By replacing just one-half of present-day power transmission systems with Ultraconductus-produced cables and devices, the United States alone could achieve annual energy savings of approximately 150 billion kilowatt-hours of energy and an associated $15 billion in cost savings.

As technology in specialized areas continues to mature, products created using Ultraconductus will play a pivotal role in their implementation. For example, Ultraconductus can be used to fabricate nano and/or micro tubes designed as conductors or insulators. Such devices could be used for biological sensors in which critical elements include scale (the smaller the better) and the ability to select various molecules. Other possible applications include microscale tubes and heaters that can be placed on tumors to heat and destroy them and nano- and micro-fibers designed to purify water and gas.

As the world continues to face ever-increasing power demands, Ultraconductus will play a pivotal role in solving constraints associated with energy generation and consumption.

–James L. Maxwell, Chris R. Rose, and Octavio Ramos Jr.

Cable image

A cable produced using the Ultraconductus process yields ultraconductive branching without the need for connectors or terminals.

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