Nanosheets have a surface-texture advantage (image credit: Anna Demming)

The ability to monitor fluid flow is crucial for a number of applications but current methods use ‘hot wires’ that are brittle and prone to break under operating conditions. Now researchers at the University of Waterloo have found a simple synthesis technique for producing stable silver nanosheets that perform well in place of these hot wires. The nanosheet devices are cheap and robust to high temperatures and electrical potentials, as well as mechanical force.

Synthesis of stable nanosheets for robust flow meters could be as simple as “mix and scoop” according to Ehsan Marzbanrad, a researcher at the Centre for Advanced Materials Joining at the University of Waterloo in Canada. He and his colleagues produce silver nanosheets for their devices by mixing silver nitrate with a reducing solution and then scooping out the resulting nanosheets onto aluminium oxide substrates.

“It’s very fast,” says Marzbanrad. “It takes just one minute at room temperature, which is amazing – and we can change the morphology to produce nanobelts, porous nanosheets, nanoflakes and spherical nanoparticles.”

What is more, the nanoparticles form with silver’s most stable crystallographic orientation – the (111) plane – at the surface. As a result they are resilient to heat degradation, electromigration and mechanical vibrations, making them excellent materials for microscale air flow sensors that cost little more than a dollar per device.

The nanosheet flow sensors operate by the same principle as the hot wires used at present. A heater attached to the device heats the sensor material so that air or fluid flowing past cools the material down. The change in temperature is readily detected in the electronic response of the material, just as in a hot wire. However the mechanical properties of the nanosheet and the aluminium substrate supporting them prevent the nanomaterial from breaking under vibrations caused by the air flow.

An edgy kind of synthesis

The nanosheets are produced as hexagonal and triangular nanoparticles formed in the solution join together. “This was by design – triangular and hexagonal silver nanoparticles are most interesting for me because they have the same (111) surface,” says Marzbanrad.

The solution contains a polymer surfactant so that the nanoparticles cannot join at the surfaces but readily join at the edges, even when sintered at a relatively low temperature of 150 °C. “When the nanoparticles make contact they will join together – that’s the interesting property of these materials and we were surprised by that,” says Marzbanrad.

Previous work comparing molecular dynamics calculations with experiments provided a good understanding of the joining process so that synthesis could be tailored to produce nanosheets with optimum properties for flow sensors. The process can be readily scaled up to meet industry requirements.

In addition simply adjusting the mixing parameters provides control over the morphology of the nanoparticles formed. Silver nanobelts formed by the same process are currently being investigated as an conductive adhesive alternative to tin solder, and Marzbanrad expects the silver nanosheets will also find use in sensors for strain, hydrogen peroxide and other applications as well.

Full details of the research are available in Nanotechnology 26 445501, which features in Nanotechnology Select.

Source and copyright: Nanotechweb