Nanotechnology for Improved Energy Transport, Conversion, and StorageDr. Timothy S. Fisher
Birck Nanotechnology Center, Purdue University
Nanotechnology offers promise to improve a wide range of processes that occur in (thermal) energy transport, direct (thermal-electric) energy conversion, and hydrogen storage for oil independence in transportation. First lets define some specific nanostructures of interest.
The carbon nanotube structure (which is a hexagonal graphene carbon structure that resembles a plate wrapped into a cylinder) and be multi walled, capped on the ends or uncapped. It was first observed in the 70s, but only recently has been the subject of intensive investigation. This is a fairly inert structure which has very good mechanical, electrical, and thermal properties.
A variant of the tube is a zigzag chiral structure (with a 3-d wall profile) which can exhibit varying electrical properties. The Chiral vector is about 1/3 metallic in nature and 2/3 semiconducting. In the tube structures all bonds are satisfied, making them chemically quite inert. There are also herringbone nanostructures of carbon that have lots of dangling bonds.
Mechanically nanotubes have a Young's modulus about 5x that of steel. Electrically they can be ballistic up to their length with a field emission of 1V/micron. Thermally the exhibit thermal conductivity about 8x that of copper in the axial direction (3000 W/mK) which is second only to pure diamond.
Growing nanotubes at the tip is almost always a catalytic process of plasma chemical vapor deposition. Base growth is a diffusion process which I believe (but cannot prove) has a precursor structure of acetylene.
Before discussing three potential applications of nanotubes, let me talk briefly about the Birck Nanotechnology Center on the Purdue University Campus (part of Discovery Park). It opened in 2005 at a cost of $58,000,000 which came almost entirely from private funds. It offers 25000 sq ft of clean room space (Classes 1, 10, and 100 - although design specs were an order of magnitude poorer). Ultrapure water humidifies the clean rooms, and there are 40 staff in the center. There is an electron microscope incubator, and facilities for academic and corporate visitors to use.
An application of nanotubes that has progressed the furthers is for heat transfer at thermal interfaces in the "IC chip" industry. Heat management is the main stumbling block to making denser faster processors. The heat from such chips must be transferred to proper heat sinks by conduction. Birck Center has been very instrumental in developing measurement techniques for such conduction using nanotubes. One such technique involves IR image sensors between two reference pars. Another is a photo acoustic technique with a laser pulse energy source, and various tuned frequencies of a acoustic pick up to obtain data. Currently it has been demonstrated that a one-sided nanotube interface can decrease thermal resistivity from between copper and silicon by nearly an order of magnitude (from 6 to .7). Another order of magnitude is possible if nanotubes are used for both sides of the interface (to .07). Currently this technology is under development by a start-up company Nanoconduction Inc., in Silicon Valley.
The second potential application of Nanotubes is Energy Conversion, but this is a long way from demonstration on a practical basis. Vacuum thermionic emission has its roots in the old vacuum tubes of early electronics. A cathode is excited by heat which frees up electrons which can then jump to an anode supplying potential for an electric current. This principle could be used for electric generation if efficiency becomes favorable. Herringbone nanofibers with potassium stuck between the planes (intercolation) has the potential to "shoot" electrons to a collector in a fairly precise 2 ev distribution which is fairly efficient. It remains to be seen if this can be developed into a direct heat to electricity converter with efficiency for power application (or even conversion directly from sunlight). There are many obstacles to overcome in this project.
The final potential application I will discuss is Hydrogen Storage. Right now the DOE has some very unrealistic targets set for 2010 for vehicular use of hydrogen. Material based storage (as opposed to pressure vessels) is attractive, but no current practical cost effective system exists. Sodium BoroHydrate systems are being looked at up to 1 KW, but there is a limitation of density. Metal hydride systems are not practical from the standpoint of "fast filling" (refueling) and because of exothermic reactions that require a sizable cooling system to be used when fueling. No one want to pump hydrogen in their car for 35 minutes at fuel stop. It is in this massive heat transfer problem that nanotubes (with their excellent thermal conductivity) may be able to provide solutions. The problem will be production of nanotubes in layers sized for this job. A few square cm (for chips is feasible) but not the massive surfaces of metal hydride storage systems. This seems to be a monumental problem and will probably not be realized for a decade or two. Another problem with hydrogen base transit is the short life of current fuel cells (only about 100 hours).