A new experiment, conducted by scientists from France, Switzerland, and the UK, has recorded the largest ever change in a bulk material’s electrical resistance brought about by stretching the material at room temperature. Piezoresistance
is one of several phenomena in which a resistance change, prompted by a change in another physical parameter, can be used in making sensitive sensors. In magnetoresistance, for example, the force from a tiny magnetic domain can alter the resistance of a circuit in a scanner directly overhead. A pronounced form of this effect, giant magnetoresistance, is at the heart of the billion-dollar hard-drive industry, earning two pioneer scientists the Nobel prize in physics in 2007.
In piezoresistance, by contrast, it’s not a tiny magnetic field but the tiny mechanical stretching of a material that alters the resistance, which in turn registers as an electrical signal. Piezoresistive devices have been in use for some time. In simple metal-foil versions, the kind used in monitoring the integrity of concrete walls or in monitoring prosthetic limbs, the change in resistance per unit of strain (a ratio referred to as the gage factor) typically has a value of about 2. For the more expensive silicon-based piezoresistors, the kind used in cell phones and airbag accelerometers, the gage factor is usually about 100.
In the new experiment a metal/silicon hybrid piezoresistance sample yielded a gage factor of 900, the largest ever seen at room temperature in a bulk material. (Larger gage factors have been observed at impractically low temperatures where quantum effects accentuate the piezoresistance.) Giant piezoresistive structures should be good news for the designers of MEMS (microelectromechanical systems) devices where it is important to measure ultra-small accelerations, or atomic-scale deflections. Alternatively, higher sensitivity to movement can be translated into lower power requirements when (as in cell phones) battery energy is at a
One of the researchers, Alistair Rowe of the Ecole Polytechnique in Palaiseau, France (email@example.com, 33-169-3347-87) says that the giant-piezoresistance materials probably wouldn’t be used directly as a storage medium but more likely as a method for reading mechanically-stored information in devices like IBM’s “Millipede.” (Rowe et al., Physical Review Letters, 11 April 2008)