Personal cooling units for firefighters in the offing

Firefighters entering burning buildings and athletes competing in the broiling sun may soon be able to carry their own, lightweight cooling units, thanks to a new nanowire array that cools, scientists say.
"Most electrocaloric ceramic materials contain lead. We try not to use lead," said Qing Wang from Pennsylvania State University in the US.
"Conventional cooling systems use coolants that can be environmentally problematic as well. Our nanowire array can cool without these problems," said Wang.
Electrocaloric materials are nanostructured materials that show a reversible temperature change under an applied electric field, researchers said.
Previously available electrocaloric materials were single crystals, bulk ceramics or ceramic thin films that could cool, but are limited because they are rigid, fragile and have poor processability, they said.

Ferroelectric polymers also can cool, but the electric field needed to induce cooling is above the safety limit for humans, researchers said.

They looked at creating a nanowire material that was flexible, easily manufactured and environmentally friendly and could cool with an electric field safe for human use.
Such a material might one day be incorporated into firefighting gear, athletic uniforms or other wearables, researchers said.
Their vertically aligned ferroelectric barium strontium titanate nanowire array can cool to about minus 15 degrees Celsius using 36 volts, an electric field level safe for humans, they said.
A 500 gram battery pack about the size of an IPad could power the material for about two hours, researchers said.

They grew the material in two stages. First, titanium dioxide nanowires were grown on fluorine doped tin oxide coated glass.
They used a template so all the nanowires grew perpendicular to the glass' surface and to the same height. Then they infused barium and strontium ions into the titanium dioxide nanowires.
Researchers applied a nanosheet of silver to the array to serve as an electrode.
They can move this nanowire forest from the glass substrate to any substrate they want - including clothing fabric - using a sticky tape.
"This low voltage is good enough for modest exercise and the material is flexible," said Wang.

"Now we need to design a system that can cool a person and remove the heat generated in cooling from the immediate area," he said.
This solid state personal cooling system may one day become the norm because it does not require regeneration of coolants with ozone depletion and global warming potentials and could be lightweight and flexible, researchers said.
The findings were published in the journal Advanced Materials.