Biomimetic Self-Organization and Self-Healing - Engineered Biomimicry - page 346

tough, strong composite material [54, 59] . The
hybrid ceramic material has a yield strength of
200 MPa and a fracture toughness of 30 MPa m 1/2
and is thus comparable to aluminum. The frac-
ture toughness measured as fracture energy is
more than 300 times what is known for any of
the constituents. The material is produced by
first freezing a slurry of alumina in water [59, 60]
under controlled conditions. When water freezes,
ice crystals grow vertically from the cooling sur-
face, as shown in Figure 13.20 . The ceramic par-
ticles are repelled by the ice crystals and become
trapped in the space between the ice dendrites.
In this way, vertical layers of ceramic particles
are made. The ice is removed by freeze-drying
and the ceramic particles are sintered. Compact-
ing the structure can reduce the size of the void
regions, and then infiltrating the ceramic struc-
ture with the polymer makes the composite.
Remarkable properties have been achieved,
but so far it has not been possible to fabricate

structures as small as in nacre. Bricks 5-10 μ m
wide and 20-100 μ m long are separated
by polymer layers of 1-2 μ m thickness,
although submicrometer structures are found
in large areas. The layer formation is self-orga-
nized, apart from the necessary compacting
required to achieve the narrow spaces for the
polymer.
A remarkable feature of nacre that adds to the
desirable combination of strength and toughness
is the almost defect-free structure where all space
is occupied by either aragonite or protein. In con-
trast, a high density of defects in common cement
is the reason for its relatively poor structure
properties. Even the compressive strength of
about 40 MPa is much less than what can be
achieved with polymers, ceramics, and metal
[61] . The reason is that the many trapped-air
cavities in cement function as crack initiators.
The cavities originate from trapped air and
dehydrated areas.

FIGURE 13.20 The principle of freeze casting, based on the description in Ref. 59 .