Self-healing circuits

Materials World magazine
1 Nov 2009
carbon nanotubes

Capsules placed on circuit boards that contain suspensions of conductive carbon nanotubes could make burnt out electronics a thing of the past.

The shells are being designed to erode under conditions of high electrical potential, temperature spikes, mechanical damage or other appropriate stimuli to release their contents where needed, thus restoring current in damaged electrical conductors.

The US Department of Energy and scientists from the University of Illinois at Urbana-Champaign, also in the USA, have created microcapsules composed of poly(urea-formaldehyde) shell walls. They encapsulate single walled carbon nanotubes (CNTs) suspended in chlorobenzene and ethyl phenylacetate.  

The researchers have tested the capsules at laboratory scale and measured the contents’ ability to conduct electricity between two electric probes separated by around 100µm. The CNTs migrated towards the probe tips, aligned with the electric field and completed the circuit. The best capsules were between 280 and 350µm, with the smaller ones too difficult to break and the larger varieties breaking too easily.

The researchers hope to test their concept on other conductive materials, such as charge-transfer salts and polymers. By exploring a variety of triggers to promote rupture, the encapsulated core material may also provide another function, such as preventing thermal runaway in batteries.

It is hoped that the new technology could prevent fires caused by mobile phones and laptops which contain lithium ion batteries that ignite when circuits inside fail. It could also be applied in situations where circuits cannot be easily manually repaired.

Mary Caruso, one of the researchers on the project, says, ‘The next step is for these capsules to be incorporated into a layered device to be tested. The capsules will release the nanotube suspensions into the crackplane of the device when damaged. The initial and final resistivity of the conductive layer in the device will be compared to show restoration efficiencies.’

Caruso adds, ‘For computers, the capsules would have to be scaled appropriately in size to be incorporated into small areas and remain undamaged until the moment of damage/failure’.  

James Rohan, Electrochemical Deposition and Micropower Activity Leader at Tyndall, National Institute, Cork, Ireland, agrees. He says, ‘The approach could be very useful in battery systems if the material could be incorporated without influencing the system until required and then be utilised at the necessary site. It is probably easier to see how capsule destruction could be achieved to shut down an unwanted growth which could lead to a short if untreated’.

The the researchers at Illinois believe  that the cost of introducing the nanotubes to circuits for self repair should be low as the solvents used are inexpensive and the labour to make the microcapsules can be scaled up quite easily, it is thought the only large cost would come from the CNTs.