“Certain classes of explosives have unique thermal characteristics that help to identify explosive vapours in presence of other vapours,” said researcher Thomas Thundat, of the US Oak Ridge National Laboratory (ORNL).
Micromachined sensors, based on resonating cantilevers coated with selective absorbers, have been made before. They are highly sensitive, but are incapable of distinguishing certain explosives from certain non-explosives because the absorbers are not selective enough.
Instead, Thundat’s technique – developed with colleagues at the University of Tennessee, ORNL, and the Technical University of Denmark – uses an electrically-heated micro bridge that measures its own heating profile, modified by whatever has condensed onto it.
Fabricated on a silicon-on-insulator wafer, the bridge is 500µm long, 100µm wide, and 550nm thick.
Buried in it are three TiAu wires: two 35µm wide heating tracks with a 10µm wide thermistor-like measuring track between them.
The low thermal mass of the structure means its thermal time constant is well below a millisecond, and allows it to be heated to hundreds of degrees C in a few microseconds.
The thermistors of the bridge and a second bridge which is not exposed to vapours are connected to form a potentiometer.
The output voltage of this potentiometer represents the thermal lag of the vapour-exposed bridge compared with the isolated bridge.
Applying a 50ms long linear voltage ramp to the heaters results in a 104 deg C/s thermal ramp.
The thermistor potentiometer output is a smooth voltage curve for neutral materials.
However explosives are energetic and their curves have bumps in where various bonds break exothermically at certain temperatures.
While neutral material curves are all very similar, the curves from explosives – TNT, PETN, and RDX were all tested – have clear signatures in their deviations from the smooth.
Altering the concentration of the explosive alters only the amplitude of the signature and not its basic form.
The technique has worked down to 600picogramme of material.
“They are now improving the sensitivity and making a prototype device, which they expect to be ready for field testing later this year,” said ORNL.