Fiber medicine

Simple and inexpensive method to protect carbon fiber

Yongfeng Lu. Credit: Craig Chandler | University communication

For the past 50 years, manufacturers have considered carbon fiber to be a dream material: although the individual fibers are finer than a strand of human hair, they can be twisted together and fused with a matrix material to form a lightweight composite that is stronger than steel, twice as stiff and a good conductor of heat. And, unlike metals, the material does not crack over time. It has been used in a wide range of applications including aircraft and spacecraft, cars, buildings, medical devices and sports equipment.

But carbon fiber has a major drawback, said Husker engineer Yongfeng Lu, an expert in carbon materials. Under extreme temperatures, regularly encountered in the aerospace industry, for example, carbon fiber oxidizes, which means it reacts with oxygen in the air and burns, just as wood does when it is combined with sufficient heat and oxygen. Oxidation quickly diminishes the dreamlike qualities of carbon fiber, especially its strength.

“One of the weaknesses of carbon fibers is that they burn easily if temperatures and oxygen are high enough,” said Lu, Lott University professor emeritus of electrical engineering and computer science. “If we could make them non-flammable, so they don’t burn when exposed to fire, that would be exciting.”

In a recent article published in PNAS, Lu’s team describes a major step towards this goal. He and his colleagues at the University of Nebraska-Lincoln and the Institute of Condensed Matter Chemistry in Bordeaux in France have developed an inexpensive and scalable method to protect carbon fiber from oxidation. The approach represents a significant improvement over other anti-oxidation processes which are laborious, slow and expensive.

“We are trying to add surface layers that can separate carbon fibers from oxygen so that even at high temperatures they are not burned,” Lu said. “Carbon fibers can be used in many ways. ways – woven into textiles and into parts of buildings, aircraft, electronic equipment – but if they are flammable, this poses a new risk to the system and greatly limits these applications.”

To eliminate the flammability, Lu’s team came up with a simple, one-step process that starts with melting a salt chemically very similar to table salt. Once the salt crystals have become liquid, the researchers add titanium and chromium powders, which are known to withstand high temperatures. Carbon fibers are then added to the mixture.

After a spontaneous reaction, the process results in a three-layer coating – made up of chromium carbide and titanium carbide – which acts as a barrier against oxidation. The coating is multi-layered because titanium and chromium each have different reaction behaviors and rates in the molten salt, leading to three distinct layers of end product. This triple coating gives extra protection compared to a single layer.

When the researchers evaluated the coated carbon fibers against extreme temperatures – about 2,200 degrees Fahrenheit – and extreme environmental conditions that they simulated with an oxyacetylene flame, they found that the carbon material maintained its structure. Lu said the next step is to identify how flame retardant coated fibers are compared to their unprotected counterparts, and how long they can retain their most valuable properties under extreme conditions.

Lu’s team isn’t the first to explore methods of protecting carbon fibers from oxidation, but if successful in further testing, the approach would be the first to be viable on a large scale. Previous approaches, such as chemical vapor deposition, involve expensive equipment, multiple steps, and difficult-to-control chemical reactions. The molten salt approach circumvents these pitfalls by using inexpensive base materials that undergo a spontaneous process at a relatively low temperature of around 1,800 degrees Fahrenheit.

The process is also quick and clean, making it ready for widespread industrial use.

“We found a recipe that can form three layers in one state,” Lu said. “With one dip, we can get three layers of coating.”

New optimization approach enables design of lighter carbon fiber composite materials

More information:
Loic Constantin et al, Spontaneous formation of multi-layered refractory carbide coatings in a molten salt medium, Proceedings of the National Academy of Sciences (2021). DOI: 10.1073/pnas.2100663118

Provided by the University of Nebraska-Lincoln

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