Scientists have developed a method of allowing materials, commonly used in aircraft and satellites, to self-heal cracks at temperatures well below freezing.
The paper, published in Royal Society Open Science, is the first to show that self-healing materials can be manipulated to operate at very low temperatures (-60°C).
The team, led by the University of Birmingham (UK) and Harbin Institute of Technology (China), state that it could be applied to fibre-reinforced materials used in situations where repair or replacement is challenging such as offshore wind turbines, or even ‘impossible’, such as aircraft and satellites during flight.
Self-healing composites are able to restore their properties automatically, when needing repair. In favourable conditions, composites have yielded impressive healing efficiencies. Indeed, previous research efforts have resulted in healing efficiencies above 100%, indicating that the function or performance of the healed material can be better than that prior to damage.
However, until this paper, healing was deemed insufficient in adverse conditions, such as very low temperature.
Similarly to how some animals in the natural world maintain a constant body temperature to keep enzymes active, the new structural composite maintains its core temperature.
Three-dimensional hollow vessels, with the purpose of delivering and releasing the healing agents, and a porous conductive element, to provide internal heating and to defrost where needed, are embedded in the composite.
Yongjing Wang, PhD student at the University of Birmingham, explained, “Both of the elements are essential. Without the heating element, the liquid would be frozen at -60°C and the chemical reaction cannot be triggered. Without the vessels, the healing liquid cannot be automatically delivered to the cracks.”
A healing efficiency of over 100% at temperatures of -60°C was obtained in a glass fibre-reinforced laminate, but the technique could be applied across a majority of self-healing composites.
Tests were run using a copper foam sheet or a carbon nanotube sheet as the conductive layer. The latter of the two was able to self-heal more effectively with an average recovery of 107.7% in fracture energy and 96.22% in peak load.
The healed fibre-reinforced composite, or host material, would therefore have higher interlaminar properties – that is the bonding quality between layers. The higher those properties, the less likely it is that cracks will occur in the future.
Mr Wang added, “Fibre-reinforced composites are popular due to them being both strong and lightweight, ideal for aircraft or satellites, but the risk of internal micro-cracks can cause catastrophic failure. These cracks are not only hard to detect, but also to repair, hence the need for the ability to self-heal.”
The group will now look to eliminate the negative effects that heating elements have on peak load by using a more advanced heating layer. Their ultimate goal, however, is to develop new healing mechanisms for more composites that can recover effectively regardless the size of faults in any condition.
Electronic materials have been a major stumbling block for the advance of flexible electronics because existing materials do not function well after breaking and healing. A new electronic material created by an international team, however, can heal all its functions automatically even after breaking multiple times. This material could improve the durability of wearable electronics.
“Wearable and bendable electronics are subject to mechanical deformation over time, which could destroy or break them,” said Qing Wang, professor of materials science and engineering, Penn State. “We wanted to find an electronic material that would repair itself to restore all of its functionality, and do so after multiple breaks.”
Self-healable materials are those that, after withstanding physical deformation such as being cut in half, naturally repair themselves with little to no external influence.
In the past, researchers have been able to create self-healable materials that can restore one function after breaking, but restoring a suite of functions is critical for creating effective wearable electronics. For example, if a dielectric material retains its electrical resistivity after self-healing but not its thermal conductivity, that could put electronics at risk of overheating.
The material that Wang and his team created restores all properties needed for use as a dielectric in wearable electronics — mechanical strength, breakdown strength to protect against surges, electrical resistivity, thermal conductivity and dielectric, or insulating, properties. They published their findings online in Advanced Functional Materials.
The Harbin Institute of Technology (abbreviation: HIT, known colloquially in Chinese as Hā GōngDà) is a research university and a member of the C9 League in China consisting of three campuses, which nearly span the country from north to south: the Harbin campus in Heilongjiang Province, the Weihai campus in Shandong Province and the Shenzhen graduate school in Guangdong Province.
HIT undertakes research and numerous projects covered by official secrets (e.g. in space science and defense-related technologies) which may have a bearing on its international ranking, although it is widely recognized as one of the top universities in the country, especially when it comes to local science and engineering league tables. HIT is one of only ten universities in the world that have designed, built, and launched their own satellites (in 2004, 2008 and 2013). It made the largest contribution to the success of the Shenzhou series spacecraft and Kuaizhou series spacecraft. One minor planet (#55838) is named after the Harbin Institute of Technology and nicknamed “Hagongda Star” by the International Astronomical Union for HIT’s achievements in science and engineering.