Jens Bauer/PNAS
Plot of compressive strength versus density.
Materials shape human progress – think stone age or bronze age. The 21st
century has been referred to as the molecular age, a time when
scientists are beginning to manipulate materials at the atomic level to
create new substances with astounding properties.
Taking a step in that direction, Jens Bauer at the Karlsruhe Institute
of Technology (KIT) and his colleagues have developed a bone-like
material that is less dense than water, but as strong as some forms of
steel. “This is the first experimental proof that such materials can
exist,” Bauer said.
Material world
Since the Industrial Revolution our demand for new materials has
outstripped supply. We want these materials to do many different things,
from
improving the speed of computers to withstanding the heat when
entering Mars' atmosphere.
However, a key feature of most new materials still remains in their
strength and stiffness – that is, how much load can they carry without
bending or buckling.
All known materials can be represented quite neatly in one chart (where
each line means the strength or density of the material goes up ten
times):
The line in the middle at 1000kg/m3 is the density of water –
all materials to its left are lighter than water and those on the right
are heavier. No solid material is lighter than water unless it is
porous. Porous materials like wood and bone exhibit exquisite structures
when observed under a microscope, and they served as inspiration for
Bauer’s work.
For many years, material scientists have thought that some empty areas
on the compressive strength-density chart should be filled by materials
that theory predicts. Computer simulations could be used to indicate an
optimum microstructure that would give a material the right properties.
However, nobody had tools to build materials with defined patterns at
the scale of a human hair.
With recent developments in lasers and 3D printing, however, a German
company called Nanoscribe started offering lasers that could do just
what Bauer wanted. Nanoscribe’s system involves the use of a polymer
that reacts when exposed to light and a laser that can be neatly focused
on a tiny spot with the help of lenses.
A drop of a honey-like polymer is placed on a glass slide and the laser
is turned on. A computer-aided design is fed into the system and the
slide carefully moves such that the laser’s stationary focus touches
only those points where the material is to be made solid. Once complete,
the extra liquid is washed away, leaving behind materials with
intricate internal structures.
However, these materials on their own are not as strong as Bauer wanted.
So he coats them with a thin layer of alumina (aluminium oxide) before
subjecting them to stress tests. Based on the tests, he was able to
improve the theoretical models he used to design the internal structure
of the materials. Their results were just published in the
Proceedings of the National Academy of Sciences.
Even though alumina layers increase the density of these materials, all
of them remain lighter than water. Bauer’s strongest material has a
specific honeycomb internal structure and is coated with a 50
nanometre-thick (billionth of a metre) layer of alumina. It beats all
natural and man-made materials that are lighter than 1000kg/m3,
being able to withstand a load of 280MPa (mega pascals is a unit of
measuring pressure), which makes it as strong as some forms of steel.
There are limitations. Nanoscribe’s system can only make objects that
are tens of micrometres in size. “One of their newer machines can make
materials in the milimetre-range, but that’s about it for now”, Bauer
added. But that is not enough for any real-life application.
However, there have been rapid improvements in all the areas this work
relies on: 3D printing, new polymers and laser technology. That means we
may soon have a suite of new, super lightweight materials for
everything from skis to aircraft parts. If nothing else, Bauer’s work
shows that we are definitely in the molecular age.
