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Strong Nano Trusses
September 25, 2013
The
IBM Selectric typewriter was the standard
word processing tool when I first entered
science. I typed my
dissertation on a rented Selectric II, and the drafts of my first
published papers were typed on Selectrics. About 13 million Selectrics of all types were sold through 1986. I wrote an article about the Selectric on the occasion of its fiftieth anniversary (
IBM Selectric Typewriter, August 3, 2011).
The Selectric was especially useful for scientific
manuscripts, since it could type
mathematical symbols. This was possible, since the Selectric didn't use the massive array of type bars of traditional typewriters which limited the maximum number of possible
characters. The Selectric used an interchangeable type ball. The
surface of these balls was covered with the same impact characters found on the type bars.
A particular character was selected by
rotation and tilting of the ball to its position, followed by impact. The mathematical symbols were available by removing the type ball of
alphabetic characters and inserting the math ball. It was a tedious process, but it gave
employment to many
secretaries.
The math ball would need to be inserted quite often for some papers, so type balls were often dropped. Occasionally, a type ball wouldn't work properly after being dropped because one of its
index teeth, which were used to mechanically align the ball for perfect spacing, had broken off. I'm fairly sure that I wasn't the first to notice that the breakage was always at the tooth beneath an
underscore or
dash character (see figure).
The underscore character on a Selectric type ball.
Dropping a type ball would often break off the indexing tooth under the underscore or dash character, rendering the ball useless.
(Photo by author.)
The
molded underscore character was equivalent to putting a notch in the material. The notch is a
stress concentrator, and the tooth breaks for the same reason that a
glassblower scratches a
glass rod to facilitate a clean break. This demonstrates that not just the
chemical composition of a material, but also its
microstructure, is important to its
mechanical properties. Since our devices are shrinking to
nanosize, how does this affect their
strength?
As it turns out, smaller things tend to be less breakable because they have a smaller
possibility of containing a
flaw. This fact was first elucidated in
fracture studies by the
aeronautical engineer,
Alan Arnold Griffith. Griffith conducted
experiments on
freshly-drawn glass fibers that demonstrated that the
stress needed to fracture increases as the fiber
diameter decreases.
In his experiments, Griffith added a controlled flaw, a notch at the surface of the glass fibers. His data led to what's now called
Griffith's criterion, that the product of the stress at fracture and the square root of the notch length is nearly a constant, at least for
brittle materials; viz.,
σf√a ≈ C
where
σf is the
stress at
fracture,
a is the notch length, and
C is a constant that depends on the
energy required to create two new surfaces at each side of the fracture. Griffith's work was ranked in the top fifty of the greatest materials moments by the
The Minerals, Metals & Materials Society (TMS) in 2007.[2]
Scientists at the
California Institute of Technology (
Pasadena, California), the
Jet Propulsion Laboratory (Pasadena, California), and Caltech's
Kavli Nanoscience Institute have sharpened their nano pencils, and they've created nanoscale
truss-like structures that mimic the
siliceous skeletons of
microscopic organisms such as
diatoms and
radiolaria.[3-5] The lightweight, porous skeletons of these organisms have remarkably higher strength than man-made structures of the same composition.
The Caltech research team fabricated hollow
ceramic scaffolds of the same dimension and
hierarchical structure of these skeletons, relying on Griffith's prediction that the smaller pieces would have high strength when combined. The fabricated structures (see image) attained a
compressive strength of 1.75 GPa without failure, even after many
deformation cycles, and their experiments showed that this approach can be used to create lightweight, damage-tolerant, engineering materials.[3]
Three-dimensional, hollow nanotruss with tessellated octahedral geometry, fabricated from titanium nitride.
Each unit cell is about ten micrometers, and the diameter of each strut is less than one micrometer.
(Caltech image by Dongchan Jang and Lucas Meza.)
The structures, like their
bio-inspired counterparts, are more than 85% air.[4] The structure is comprised of repeating
octahedral unit cells, which mimics the periodic lattice structure of diatoms.[4] Says
Julia R. Greer,
professor of
materials science and mechanics at Caltech and
project leader,
"Inspired, in part, by hard biological materials and by earlier work... we designed architectures with building blocks that are less than five microns long, meaning that they are not resolvable by the human eye... Constructing these architectures out of materials with nanometer dimensions has enabled us to decouple the materials' strength from their density and to fabricate so-called structural metamaterials which are very stiff yet extremely lightweight."[4]
To fabricate these structures, the research team formed a three-dimensional
polymer lattice by
two-photon lithography. This polymer lattice was coated with thin layers of
titanium nitride (TiN), and the polymer core was removed, leaving the TiN ceramic nanolattice. The hollow struts had wall thickness of 75
nanometers or less.[4]
The research team has fabricated structures as large as a one-
millimeter cube.[4] They've also substituted
gold for the TiN. Says Greer, "Basically, once you've created the scaffold, you can use whatever technique will allow you to deposit a uniform layer of material on top of it."[4]
Such bio-inspired structures have great potential. Nanoscale metals can be fifty times stronger than their bulk, and the mechanical character of some
amorphous materials changes from brittle to
ductile. This process can be used to produce mechanically strong nanoscale
batteries and
implantable biomedical devices.[4] The research was funded by
DARPA's Materials with Controlled Microstructural Architecture program, and the
Army Research Office.[4]
References:
- A.A. Griffith, A. A. (1921), "The phenomena of rupture and flow in solids," Philosophical Transactions of the Royal Society of London, vol. A221 (1921), pp. 163–198. Available here, also.
- The Greatest Moments in Materials Science and Engineering, The Top 50 Moments in History, The Minerals, Metals & Materials Society (TMS).
- Dongchan Jang, Lucas R. Meza, Frank Greer and Julia R. Greer, "Fabrication and deformation of three-dimensional hollow ceramic nanostructures," Nature Materials, September 1, 2013, doi:10.1038/nmat3738.
- Kimm Fesenmaier, "Made-to-Order Materials - Caltech engineers focus on the nano to create strong, lightweight materials," California Institute of Technology Press Release, September 5, 2013
- Supplementary video for ref. 1. Uniaxial compression test on a unit cell of the octahedral titanium nitride nanotruss at five times the actual speed.
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