Silicon Nanotubes – one-dimensional objects

Nanotubes

The world of physics became very excited with the advent of silicon nanotubes (also known as nanowires) – describing them as wonderful, but what do we do with them?

Well, two uses are of real interest to us.  The first is the ability to turn waste heat in to energy and the second is a new generation of very efficient batteries. However, what is a one-dimensional object?  Well the prefix of the word Nanotubes tells you that these are microscopic objects. They are one-dimensional structures: that means that the nanotubes have length but no width or height from a physical standpoint. One-dimensional objects (like carbon nanotubes) are ideal for conducting heat or power because it is difficult to scatter or deflect whatever is being transported. It’s like a maglev train for molecules.

The Lawrence Berkeley National Labs is California estimate that some 55%-60% of all energy consumed in the USA is dissipated, much of it as waste heat.  The concept of converting heat to electricity is not new Approximately 90 per cent of the world’s power is generated by heat engines that use fossil fuel combustion as a heat source and typically operate at only 30–40 per cent efficiency, releasing roughly 15 terawatts of heat to the environment. If this “wasted heat” could be recycled at even the 5% level, the impact globally would be enormous.  The first attempts to produce nanotubes resulted in tubes with a ‘rough’ surface – initially disappointing researchers.  However, thermoelectric testing showed that rough nanowires allowed current to flow from a heat source toward a cold source but, astonishingly, the heat did not flow. By optimizing the roughness of the wires they were able to reduce the room temperature thermal conductivity by a factor of 100. While the physics behind the effect is not completely understood at this time, it maybe that thermal transport is impeded as heat waves simply bounce off the rough contours on the surface of the nanowires, but current flow is not impeded, as electrons are not similarly slowed down.  One can imagine recharging a mobile phone with electricity produced by the user’s body heat or reclaiming the heat that is released through a car’s exhaust system to power electronic devices.

The other area of excitement is in a new generation of batteries. The new technology produces 10 times the amount of electricity of existing lithium-ion, known as Li-ion, batteries. A laptop that now runs on a  battery for two hours could operate for 20 hours, a boon to all of us who love our mobile devices.  This has been described not as a small improvement, but a revolutionary development.  Material scientists are already very familiar with silicon, and therefore it should not be long (it is predicted within five years) that we see electric vehicles whose new silican nanotube-based batteries give them the kind of performance we have come to expect from their fossil-fuelled cousins.