[Image above] Controlling “runaway” thermal reactions, such as combustion and pyrolysis, not only improves safety but allows for finer processing of materials. Credit: Cullan Smith, Unsplash
Fire is a prime example of a double-edged sword. When used in a controlled manner, fire can help cook food, power industrial processes, and maintain healthy ecosystems. But when fire spreads too quickly and is no longer contained, it can have devastating consequences, such as the prescribed burn in New Mexico that turned into the state’s largest recorded wildfire.
Despite the evident safety reasons for keeping fire under control, current methods for guiding or directing thermal reactions remain rather rudimentary.
For example, when synthesizing materials through “runaway” thermal reactions such as combustion (burning) or pyrolysis (thermal decomposition), the reaction rate can be controlled by modulating the amount of fuel, oxygen, and heat available in the system.
However, this approach does not offer the ability to control the direction in which the flame spreads (ignition propagation). It also does not allow for different reaction rates to be achieved based on location within the sample. Such an ability would allow for an assortment of chemical transformations to take place, resulting in various materials and/or structures.
Nonflammable coatings, such as SiO2, have demonstrated potential to provide location-based control over reaction rates during materials processing. Studies such as here and here used nonflammable coatings to control the heat release during pyrolysis, allowing for the successful transformation of organic fibers into carbon tubes.
In a recent study, researchers led by North Carolina State University developed a new nanocoating that provides even greater control over reaction rate and ignition propagation during combustion.
The NC State-led group includes collaborators from Iowa State University and the University of British Columbia. Their alkysilane-based coating, unlike the coatings mentioned above, not only influences location-based reaction rates but also the direction of ignition propagation within organic cellulose fibers.
Their coating achieved this “surface-then-core” ignition order because of a chemical transformation that takes place within the coating upon heating.
Initially, the alkysilane-based coating features flammable alkyl tails on the outer surface. When these tails combust upon heating, the inner surface of the coating transforms into a nonflammable silicate glass. This glass limits the amount of oxygen that can access the cellulose fibers, leading the cellulose to burn slowly from the inside out rather than bursting into flames.
In other words, this transformation of the coating from a flammable to nonflammable substance causes the combustion (high oxidant) process to turn into a pyrolysis (low oxidant) process.
The researchers call this coating transformation and subsequent combustion-to-pyrolysis conversion the Inverted Thermal Degradation (ITD) process as a nod to the slow inside-out burn that the fibers undergo.
The ITD process allowed the researchers to synthesize carbon nano- and microtubes with tunable wall dimensions and lengths. They gained further control over the final tube structure by varying the size of the starting fiber, adjusting the amount of oxygen in the system, and introducing various salts into the fibers.
In an email, senior author and NC State professor Martin Thuo explains that the added salts not only take up oxygen, thus reducing what is available to the fibers, but they also control (increase or decrease) the amount of heat available for the reaction, which in turn tunes how much the fibers burn.
“For example, potassium salts (KCl) are a heat sink, hence slows down the burning, while magnesium salts (MgCl2) increase the heat when they oxidize,” he explains.
In the conclusion, the researchers state that their “facile and highly tunable process” provides a convenient way to produce graphitic/graphene-oxide-dominated materials of various sizes and shapes.
Additionally, “We believe this method can be further extended into applications in electronics, semiconductors, and catalysis fields,” they conclude.
In an NC State press release, Thuo says the group is open to working with the private sector to explore various practical uses, such as developing engineered carbon tubes for oil–water separation.
The paper, published in Angewandte Chemie, is “Spatially directed pyrolysis via thermally morphing surface adducts” (DOI: 10.1002/anie.202308822).