INSIGHTS | PERSPECTIVESsciencemag.org SCIENCEPHOTOS: XUET AL. (^1
)By Manish Chhowalla^1 and Deep Jariwala^2M
aterials that operate in extreme en-
vironments, such as aerospace ap-
plications that require operation at
high temperatures and in reactive
atmospheres, must be ultralight,
very mechanically strong, and
thermally insulating. Achieving such dispa-
rate functionalities requires rational design
not only of the material itself but also of
hierarchical structures at multiple lengthscales that can respond in the desired way
to extreme environmental factors in real
time. On page 723 of this issue, Xu et al. ( 1 )
report the synthesis of a multifunctional
structure with hyperbolic surfaces (saddle
shapes with negative curvature) in the form
of an aerogel where the solid medium is anetwork of atomically thin sheets of hex-
agonal boron nitride (h BN). By careful me-
chanical design of the microstructure, the
authors report that their aerogels exhibit
extraordinary mechanical and thermal re-
sistance properties far superior to those
of current aerogels. Their discovery opens
new pathways for the integration of ratio-
nally designed ultralightweight materials
with the correct combination of mechani-
cal and thermal properties for a variety of
extreme environments.Aerogels are mixtures or composites of
air or free space and a ceramic, metal, par-
ticulate, powder, or carbon solid medium,
where the proportion of air or free space is
>99%. Thus, aerogels can be exceptionally
lightweight with densities approaching 0.1
mg cm–3 ( 2 ). Ceramic aerogels possess many
of the requisite properties for operation at
high temperatures in corrosive environ-
ments, such as low density, excellent thermal
insulation, and chemical stability. As a result,
ceramic aerogels have been widely investi-gated for such applications, particularly in
aerospace components that demand extreme
material property requirements ( 3 ).
However, aerogels of typical ceramic
materials such as silica, alumina, and
silicon carbide are highly brittle and are
fragile under stress, especially at high tem-
peratures or under abrupt thermal shock
( 3 ). Conventional strategies for mitigating
the brittleness of ceramic aerogels often
lead to degradation of other properties,
such as an increase in thermal conductiv-
ity. Unconventional strategies attempt to
achieve a material that has both a negative
Poisson’s ratio (such that it would expand
along the normal direction when stretched)
and a negative thermal expansion coef-
ficient (such that it would contract upon
heating). These properties can be realized
through internal hierarchical structuring
(metamaterial architectures) that can in
principle enhance the fracture toughness
and mitigate the brittleness of ceramics.
However, the synthesis of metamaterial
aerogels with rationally designed hierar-
chical structures is challenging with bulk
three-dimensional (3D) ceramics because
of processing limitations.
Recent work on aerogels based on 2D
graphene (a single atomic layer of graph-
ite) provides the basic design principles for
realizing ultralow-density, highly deform-
able, and thermally insulating aerogels ( 4 ,
5 ). Aerogels obtained from 2D materials
feature a face-to-face stacking of the 2D
nanosheets so that the cell walls of the
aerogel consist of minimally thick mate-
rial with exceptionally high mechanical
strength. The 3D hierarchical structure ob-
tained from 2D nanosheets also divides the
aerogels into tiny cells so that convection
of air between them is practically reduced,
thereby enabling the realization of thermal
conductivities below that of air.
Although graphene is not suitable for
high-temperature applications in air, the
fundamental knowledge obtained from its
processability and design principles for
achieving exceptional aerogel properties
can be applied to other 2D materials to re-
alize unexpected functionalities. Xu et al.
have in fact rationally designed hyperbolic
aerogels with hBN, a 2D ceramic. Their
hBN aerogel comprises a defective crystal-
line lattice of atomically thin planes inter-
connected to form a cellular network. ToCERAMICSHyperbolic 3D architectures with 2D ceramics
A hexagonal boron nitride aerogel has high resistance to thermal and mechanical shock
(^1) Department of Materials Science and Metallurgy, University
of Cambridge, Cambridge CB3 0FS, UK.^2 Department
of Electrical and Systems Engineering, University of
Pennsylvania, Philadelphia, PA 19104, USA. Email: mc209@
cam.ac.uk; [email protected]
Hyperbolic ceramics that keep their cool. An aerogel made from thin hexagonal boron nitride sheets by Xu et al.
has exceptional mechanical strength and thermal insulation properties. This material (white block) can insulate a
flower from the heat of an open flame for several minutes.
700°C
350°C
–20°C
694 15 FEBRUARY 2019 • VOL 363 ISSUE 6428
Published by AAAS
on February 14, 2019^
http://science.sciencemag.org/
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