Rugged conductive and transparent silver fibre mesh for flexible substrates
In a paper titled “Room-temperature processing of silver submicron fiber mesh for flexible electronics” published in npj Flexible Electronics, the researchers note that silver precursor AgNO3 in solution in Polyvinylpyrrolidone (PVP) can be used as a carrier for continuous draw spinning of ultrathin fibres with a diameter of about 200nm and several kilometres long (at a speed of up to 8m/s). Instead of relying on a heat-curing process like they did in previous research, the authors irradiated the interwoven precursor fibres with a strong UV light which leads to the formation and integration of silver nanoparticles. With irradiation times up to 5 hours, they found that more silver particles were reduced and gradually bonded together while the PVP carrier gradually decomposed. The result was a network of silver particles compactly packed together within each fibre, yielding a fully connected mesh, while a residual carbon-based polymer material filled the gaps within the fibres, enhancing their flexibility and toughness.
This UV-curing approach is interesting as it allows the silver fibres to be directly intertwined on most flexible substrates such as polyethylene terephthalate (PET) and polyimide (PI), which are generally thermosensitive materials and can’t be submitted to typical heat-sintering processes.
The authors transferred their silver fibre mesh onto a polyethylene terephthalate (PET) substrate and connected a LED circuit through it for bending and stretching tests.
When gradually bending the mesh down to a 1mm radius, the sheet resistance of the mesh increased by 41%. For comparison, they note that copper films subjected to a bending radius of 5mm experienced a 1500% sheet resistance increase. When repeatedly bending the samples up to 1000 times at a 7mm bending radius, the researchers observed a sheet resistance increase of 120%, while the sheet resistance of the copper films increased to 1500% (after only 500 bending cycles).
In another experiment, the silver fibre mesh was attached to a polydimethylsiloxane (PDMS) film for a stretching test. When stretched to 130% of its original length, the mesh’s sheet resistance only increased up to 120%, while the sheet resistance of copper films increased to 15,000% after it was stretched to 115% of its original length.
To strengthen the silver fibre mesh, the authors sealed one of its sides on a PET substrate with a heat-sealing apparatus, with ethylene-vinyl acetate as the hot melt adhesive to firmly secure the mesh. They found that once sealed, the mesh remained stable even in acidic and alkaline conditions.
The transparent conductive film was also proven in a flexible electrochromic window application and an infrared stealth film.
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