(Nanowerk Spotlight) Ceramics boast impressive strength and durability, but their inherent rigidity and brittleness have long hindered applications demanding flexibility, such as filters, sensors, wearable devices and flexible electronics. Materials scientists have pursued methods to capture the desirable properties of ceramics like titania and zirconia in a flexible fiber form.
Electrospinning, which employs electric fields to draw ultrathin fibers from a liquid, has emerged as the leading approach to create ceramic nanofibers. However, the technique requires the starting solution to have sufficient viscosity – typically over 100 millipascal-seconds – to form a stable jet. This constraint has severely restricted the compositions and quality of ceramic fibers produced.
An electrospun ceramic fiber. (Image: Grobert Research Group, University of Oxford)
Conventional wisdom states that a polymer must be blended with ceramic precursors to provide the requisite viscosity for electrospinning. But the polymer leaves the final ceramic fibers riddled with pores and defects that severely compromise their mechanical integrity. Ceramic nanofibers made by this method are notoriously brittle and fragile, often crumbling under slight strain.
Researchers have explored countless strategies to improve the precursor solution, such as adjusting the ceramic-to-polymer ratio, incorporating additives, and precisely controlling the sol-gel reaction. Yet modifying the formula to bolster fiber strength inevitably disrupts its delicate rheology, causing the solution to clog the spinneret or the jet to break up into droplets.
Recognizing the dilemma, materials scientists began to suspect the polymer itself was preventing the formation of dense, defect-free ceramic nanofibers. However, directly electrospinning a polymer-free ceramic precursor sol had never succeeded due to its low viscosity of under 10 millipascal-seconds. A radically different approach was needed to bypass this seemingly insurmountable obstacle.
In a pioneering study, reported in ACS Nano (“Electrospinning Non-Spinnable Sols to Ceramic Fibers and Springs”), a University of Oxford team may have uncovered the solution. They developed an innovative coaxial electrospinning technique that simultaneously feeds two different liquids through a spinneret nozzle. A dilute ceramic precursor sol flows through the inner core, while a separate polymer solution flows around it, forming a protective shell. The shell’s viscoelasticity enables the formation of a stable jet, even with a core material that is far too fluid to electrospin on its own.
“The key is the coaxial configuration, which allows us to use a low-viscosity ceramic precursor that would be impossible to electrospin by itself,” Prof. Nicole Grobert, who led the study, explains to Nanowerk. “We can encapsulate this ‘non-spinnable’ sol inside a spinnable polymer shell. The polymer provides the rheological properties needed to create fibers, but it’s ultimately sacrificial—we remove it during calcination to obtain the pure ceramic fiber.”
TiO2 fibers and springs electrospun from a nonspinnable dilute sol using the sol/polymer coelectrospinning technique. (a) Schematic illustration of feeding an alkoxide sol and polymeric solution through a coaxial nozzle. Electrospinning two solutions simultaneously generates precursor fibers with a core–shell structure. Calcining the as-spun fiber yields high-quality ceramic fibers, where the chemical composition of the sol–gel reactants and solid fibers is highlighted. (b) Digital photo of the bicomponent solution droplet at the tip of the coaxial nozzle, showing a clear boundary between the core and shell solutions. Scanning electron microscope (SEM) images of the electrospun product of (c) viscous polymer solution and (d) dilute alkoxide sol. (e) Plot of solution viscosity at varied alkoxide sol/polymer solution mixed ratios, where the spinnability is indicated. (f) SEM image shows the surface patterns of the as-spun precursor fiber, and (g) transmission electron microscope (TEM) images revealing its core–shell structure. (h, i) SEM and Energy dispersive x-ray spectroscopy (EDS) elemental line scan across the fiber cross-section suggests a core and shell layers, where the intensity of the Ti signal is magnified three times for better visualization. (j, k) SEM images of the calcined TiO2 fibers in straight nanofiber and coiled “nanospring” morphologies. (l, m) SEM and EDS elementary profile of the cross-section of a TiO2 ceramic fiber. The scale bar in f, g, j, and k is 1 µm. (Image: reprinted from DOI:10.1021/acsnano.3c12659, CC-BY 4.0.) (click on image to enlarge)
As a proof-of-concept, the researchers applied their method to a sol of titanium isopropoxide, a precursor for titania (TiO2). After electrospinning the core-shell fluid and heat-treating the resulting fibers, transmission electron microscopy revealed a uniform, densely packed structure of titania nanocrystals. In stark contrast, nanofibers produced by the standard method contained an erratic porous interior due to phase separation of the polymer and ceramic during spinning.
Mechanical testing showcased the remarkable properties enabled by this controlled nanostructure. Titania nanofibers produced by the coaxial technique achieved a Young’s modulus up to 54.3 megapascals, nearly triple the stiffness of those made by conventional electrospinning. Whereas typical ceramic nanofibers are extremely fragile, the coaxial spun fibers withstood significant bending and folding without breaking.
“The benefits extended to other materials as well,” Dr Barbara Maciejewska, a co-first author of the paper, points out. “Zirconia nanofibers produced by the coaxial method attained an impressive Young’s modulus of 130.5 megapascals and toughness of 11.9 kilojoules per cubic meter – representing a five-fold leap over zirconia fibers made by standard electrospinning. Silica-containing formulations yielded higher-porosity fibers due to the lower reactivity of the silica precursor, demonstrating the tunability of our coaxial approach.”
Perhaps most surprisingly, the team discovered that nanofibers calcined without a supporting substrate coiled into tight spring-like spirals. Simulations revealed that asymmetric contraction between the ceramic core and polymer shell generates torsion that twists the fiber into a helix during heat treatment. These ‘nanosprings’ exhibited lower stiffness than straight fibers, but their toughness increased by a factor of 3-5 thanks to an elongated breakage strain.
“By removing the need for thickening polymers, our approach enables the fabrication of robust nanofibers from virtually any ceramic precursor sol,” says Dr. Shiling Dong, first author of the study. “We can now access compositions and microstructures that were previously unattainable, such as multi-component and high-entropy ceramics.”
The advent of strong, flexible ceramic nanofibers and springs opens the door to a host of applications, from filtration membranes and catalyst supports to optical sensors and structural composites. With further optimization, the coaxial technique could extend to additional classes of materials and nanostructures.
“Ceramic nanofibers have immense technological potential, but limitations in how they’re made have prevented their widespread use,” Grobert concludes. “Our coaxial electrospinning process provides a versatile platform to spin nanofibers from low-viscosity sols. This advance marks a step-change in our ability to design and tailor ceramic nanofibers for targeted applications.”
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