(a) Schematic illustration of
ultralong CNT bundles composed of continuous CNTs. (b) Scanning electron
microscopy (SEM) image of horizontally aligned ultralong CNTs. Inset:
high resolution transmission electron images of as-grown ultralong CNTs
with single, double, and triple walls. (c) Schematic of in situ
fabrication of CNTs by gas-flow focusing (GFF) method. (d) Simulation of
GFF. (e,f) SEM images of bundles of two and three CNTs. (g-i) TEM
images of CNT bundles. [Bai et al., Nature Nanotechnology (2018),
https://doi.org/10.1038/s41565-018-0141-z].
Nanoscale fibers boast impressive mechanical properties often
exceeding those of their bulk companions. However, larger-scale
materials created from those nanofibers do not always match up to
predictions. Now two pieces of research indicate promising strategies
for translating the exceptional attributes of nanoscale fibers like
carbon nanotubes and cellulose nanofibrils into macroscale materials.
Carbon nanotubes (CNTs) are touted as one of the strongest known
materials, but usable fibers made from nanotubes do not achieve the same
the impressive physical prowess. The reason is simple: the presence of
defects, impurities, random orientations, and different length nanotubes
add up to a fiber with compromised strength. Now, however, researchers
report that a simple stretching and relaxing process can release initial
non-uniform strains in CNT bundles and enable the fabrication of much
stronger fibers [Bai et al., Nature Nanotechnology (2018), https://doi.org/10.1038/ s41565-018-0141-z].
“CNTs [have] inherent tensile strength higher than 100 GPa but almost
all reported CNT fibers are fabricated using agglomerated CNTs or
vertically aligned CNT arrays with components shorter than hundreds of
microns and containing plenty of structural defects and impurities,
rendering their tensile strengths in the range of 0.5–8.8 GPa,” explains
Rufan Zhang of Tsinghua University.
Along with colleagues at Stanford University, the team used a simple
approach to produce centimeter-long bundles of ultralong, defect-free
CNTs with a tensile strength of over 80 GPa. The key to the strength of
the bundles is the way in which the CNTs are produced.
The researchers use gas-flow-directed chemical vapor deposition to
synthesize ultralong nanotubes, which have at least one perfectly
structured wall. A gas flow focusing strategy gradually assembles the
as-grown CNTs via van dear Waals forces into ultralong bundles. Next,
however, the researchers undertake a careful process of tightening and
relaxing the fiber bundles, which releases the internal strains as the
component nanotubes shrink and slip over each other. After repeated
cycles of stretching and relaxing, the nanotubes are more uniformly
aligned in the bundles and the internal strains are more similar.
The simple process appears to boost the tensile strength of nanotube
bundles from as little as 47 GPa to as much as 80 GPa. The researchers
believe that their approach could provide a way of synthesizing
superstrong fibers, although the issue of producing high quality,
ultralong CNTs remains.
“The researchers have made a nice step in terms of achieving bundles
of SWCNTs of very high quality that, through a method similar to
engineering methods used with bridge cables (bundles composed of many
individual wires that all bear load), could exhibit high intrinsic as
well as engineering strength,” comments Rodney Ruoff of Ulsan National
Institute of Science and Technology (UNIST) in Korea. “It is important
to note that these are bundles, not fibers, and that a significant
challenge remains in achieving very long fibers composed of CNTs that
would also exhibit exceptional strengths.”
Similar issues afflict cellulose nanofibrils, which are the most
abundant structural component in living systems like trees and plants.
Cellulose nanofibrils have high strength and stiffness but attempts to
produce artificial analogues have, to date, produced composite materials
up to 15 times weaker.
“One of the biggest challenges in fabricating engineering materials
that make use of the often-exceptional properties of nanoscale building
blocks is the retention of these properties [at the macroscale],” says
L. Daniel Söderberg of KTH Royal Institute of Technology in Sweden.
Together with colleagues at RISE Bioeconomy, DESY in Germany,
Stanford University and the University of Michigan in the USA, Söderberg
has fabricated an engineering material using nanocellulose that does
retain these exceptional mechanical properties [Mittal et al., ACS Nano (2018), https://doi.org/10.1021/ acsnano.8b01084].
The team created continuous fibers (or filaments) from very slender
fibrils of nanocellulose, derived from conventional paper pulp fibers.
The key to success is the alignment of the nanocellulose fibrils in
the fibers. The researchers first dispersed nanocellulose fibrils in
water and used a micro-fluidic concept called flow focusing to process
the dispersion into fibers. By excluding Brownian diffusion, which would
allow the fibrils to rotate, the process aligns the fibrils along the
length of the fibers. The aligned structure is then locked into a gel
network by lowering the pH. A continuous fiber can be extracted from the
gel, with no restriction on length.
“[Our] continuous, well-defined fibers (or filaments), made from 100%
bio-based components (with no fossil-based additives), have a
mechanical performance on the same level as glass and Kevlar fibers and
perform better than the attributed strength and stiffness of spider
dragline silk, widely thought of as the strongest bio-based material,”
points out Söderberg.
The process allows the excellent strength and stiffness of
nanocellulose fibrils to be translated into engineering-scale fibers.
Although the team is only making small amounts of fiber at the moment,
they are working with the Swedish research institute RISE Bioeconomy to
scale-up the process to produce fibers continuously at high speeds.
“Using these fibers, it will be possible to fabricate 100% bio-based
lightweight composites for structurally demanding applications such as
automotive products,” says Söderberg. “And because cellulose is
compatible with biological tissue, we envisage that materials with our
fibers as key components of scaffolds and load-bearing applications in
medicine.”
Söderberg believes that applications in medicine could come within
the next five years, with lightweight, load-bearing construction
applications taking slightly longer to realize.
Markus J. Buehler, McAfee Professor of Engineering at Massachusetts
Institute of Technology, agrees that many of the researchers’ ideas
could be translated to engineered materials.
“The study reports impressive results that showcase the translation
of a biological design paradigm into engineered materials, addressing
one of the most challenging problems today," he comments. "The unique
architecture is achieved by a clever engineering of the processing of
the material, similar to what we see in many other biomaterials such as
silk, where an interplay of fluid mechanics, chemistry, and the design
of the constituting building blocks leads to the final high-performance
material, and ultimate nano-level geometry control that is critical for
the outcome.”
Buehler believes that the work offers important insights into the
design of hierarchical materials that translate nanoscopic properties to
the macroscale.