POWER PIPS

The ability to convert energy between different forms—and the capacity to use it—is the hallmark of modern civilization. Humanity will face a crisis in the coming decades due to the rate at which fossil fuels are being used and the impact this is having on the environment. Nanotechnology almost certainly has a role in resolving this crisis, and mechanical engineers are perfectly situated to capitalize on the opportunity.

It is widely recognized that renewable sources of energy such as solar electricity and biomass will gain importance in the future. However, cost is a hurdle to the effectiveness of such technologies. For example, the cost of photovoltaics must be an order of magnitude lower than its current value to make the technology competitive with fossil fuels. This could be possible if, for example, silicon-based devices, which are currently manufactured in high-temperature processes, were replaced by nanostructured plastic-based photovoltaics.

If thermoelectric devices made of materials such as silicon and germanium perform about 10 to 20 percent of the Carnot limit, they could be as competitive as cost-effective solid-state energy conversion devices. This can only occur if the semiconductors are nanostructured to control heat and charge transport. Mechanical engineers who understand these challenges better than other technologists, will almost certainly devise the solution.

The particles making up a silicon carbide film at top average only 20 nm across. A polymer material (second from top) is made of nanoscale layers. Each "carrot" made by Georgia Tech researchers (third from top) contains thousands of silica wires grown from a gallium droplet. Another group grows nanowires by depositing metals on a porous alumina membrane (bottom).

One of the biggest environmental challenges that humanity faces today is clean water. Nanostructured filters used for ion exchange hold promise for removing contaminants. Their manufacture however, must be inexpensive, and the science of nanofluidics must be understood to make these filters effective for cleaning water. Mechanical engineers can collaborate with biologists and public health researchers to resolve both these issues.

Another area of expertise for mechanical engineers, instrumentation, is also key to tapping the potential of nanotechnology. Instruments that can probe the environment with increased resolution and sensitivity lead to breakthroughs in science and engineering. The scanning probe microscopes invented in the 1980s are electromechanical devices, which require precision actuation with Angstrom resolution, microfabrication of cantilever probes, force sensing with resolution measured in piconewtons, and a fundamental understanding of dynamics and control to increase imaging speed and spatial resolution.

These stringent requirements are not limited to the microscopes, but apply to any nanoscale measurement. For example, there is a tremendous need for instrumentation in high-throughput imaging and measurement in nanomanu-facturing processes to enable automation and process control. These issues offer opportunities for mechanical engineers to provide a system-level understanding of such instruments.

Other potential applications require a mix of skills. Nanoparticles and nanowires exist on a scale similar to biomolecules such as DNA and proteins. This suggests that the biological sciences can provide crucial insights to the behavior of such material and that nanoscale devices may be used for medical applications. For instance, nanoparticles may be used as markers to study very small samples of DNA or proteins. Achieved in a high-throughput manner, this could form the basis for biomolecular analysis in extremely small volumes (on the order of single cells), which has major implications for diagnosis of disease.

In addition, the combination of nanostructures and mechanical sensors such as cantilever beams could be used in chemical or biological defense. Nanostructures such as particles and polymeric dendrimers could be designed as drug delivery systems.

Biomolecules could be used to perform non-biological tasks. Possibilities include manufacturing, energy conversion, signal amplification, and information processing. Many of these functions are already achieved in an extremely efficient manner within a cell, thanks to the genius of natural selection, but to exploit them in non-biological conditions is a nontrivial problem.

Nevertheless, applications such as manufacturing and energy conversion have always been strengths of mechanical engineers. Can we harness the power of the biomolecular machinery for mechanical applications? Research in this direction has already started.

Another field where nanotechnology may need mechanical engineers is information processing and storage. When transistors reach the scales of 20 to 30 nano-meters (a scale that will be necessary to keep up with Moore's law) quantum effects such as electron tunneling will lead to electron leakage, and this will cost power. Higher speeds will also require electromagnetic isolation, which will necessitate the use of materials that have extremely low thermal conductivities. In addition, novel cooling technologies that directly interface with electronic and optoelectronic chips must be developed.

To create chip designs that solve these thermal problems, technologists will need a basic understanding of how heat flows in nanostructures and across interfaces. Mechanical engineers have just this sort of expertise.