Lightweight, flaw-tolerant, and ultrastrong
nanoarchitected carbon
Xuan Zhang
a
, Andrey Vyatskikh
b
, Huajian Gao
c,1
, Julia R. Greer
b,1
, and Xiaoyan Li
a,d,1
a
Center for Advanced Mechanics and Materials, Applied Mechanics Laboratory, Department of Engineering Mechanics, Tsinghua University, 100084
Beijing, China;
b
Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA 91125;
c
School of Engineering, Brown
University, Providence, RI 02912; and
d
Center for X-Mechanics, Zhejiang University, 310027 Hangzhou, China
Contributed by Huajian Gao, February 9, 2019 (sent for review October 8, 2018; reviewed by Yonggang Huang and Christopher M. Spadaccini)
It has been a long-standing challenge in modern material design to
create low-density, lightweight materials that are simultaneously
robust against defects and can withstand extreme thermomechan-
ical environments, as these properties are often mutually exclusive:
The lower the density, the weaker and more fragile the material.
Here, we develop a process to create nanoarchitected carbon that
can attain specific strength (strength-to-density ratio) up to one to
three orders of magnitude above that of existing micro- and
nanoarchitected materials. We use two-photon lithography followed
by pyrolysis in a vacuum at 900 °C to fabricate pyrolytic carbon in
two topologies, octet- and iso-truss, with unit-cell dimensions of
∼
2
μ
m, beam diameters between 261 nm and 679 nm, and densities
of 0.24 to 1.0 g/cm
3
. Experiments and simulations demonstrate that
for densities higher than 0.95 g/cm
3
the nanolattices become insensi-
tive to fabrication-induced defec
ts, allowing them to attain nearly
theoretical strength of the constituent material. The combination of
high specific strength, low density
, and extensive deformability be-
fore failure lends such nanoarchitected carbon to being a particularly
promising candidate for applications under harsh thermomechanical
environments.
nanolattices
|
pyrolytic carbon
|
octet-truss
|
iso-truss
|
specific strength
L
ightweight porous materials, such as wood, bone,
Euplectella
sponges, diatoms, and bamboo, are ubiquitous in nature.
These natural structural materials have been extensively in-
vestigated (1
–
5) and have been shown to be resilient against
externally applied loads and powerful in absorbing and dissi-
pating impact energy. Such properties have been enabled by two
design principles: (
i
) a multiscale hierarchy of constituent ma-
terials and length scales, which generally consist of complex
multilevel architectures with characteristic dimensions from
nano- to macroscale (5) and (
ii
) their tolerance of flaws when the
characteristic material length scale falls below a critical value (4).
Both principles have been applied to engineering advanced
materials to various degrees of success (5, 6).
A general guideline for a material to be considered
“
light-
weight
”
is for its density to be less than that of water (i.e.,
ρ
≤
1.0 g/cm
3
) (1, 7). Recent breakthroughs in material processing
techniques, especially in 3D microfabrication and additive
manufacturing, provide a particularly promising pathway to
fabricate lightweight materials, which often possess a suite of
other beneficial properties such as high specific stiffness, high
specific strength, and good resilience/recoverability (7
–
27). A
penalty for the ultralight weight of such nano- and micro-
architected materials is a severe reduction in their stiffness and
strength through power law scaling:
σ
y
∼
(
ρ
/
ρ
s
)
m
,
E
∼
(
ρ
/
ρ
s
)
n
,
where
σ
y
is the yield strength,
E
the Young
’
s modulus,
ρ
the
density, and
ρ
s
the density of the fully dense constituent solid (1).
The exponents
m
and
n
are generally greater than 1, which
renders developing methodologies to create materials that are
simultaneously lightweight and strong/stiff while maintaining
their other properties (i.e., thermal stability, electrical conduc-
tivity, magnetism, recoverability, etc.) a grand unsolved challenge
because of restricted material choices and limited architectures.
Most work on micro/nanoarchitected materials to date has
been focused on hollow-beam-based architectures, which offer
exceptionally light weight with a concomitant high compliance
[e.g., nickel-based hollow-tube microlattices with a Young
’
s
modulus of 529 kPa and a compressive strength of
∼
10 kPa at a
density of
∼
0.010 g/cm
3
(7) and ceramic hollow-tube nanolattices
with Young
’
s moduli of 0.003 to 1.4 GPa and compressive
strengths of 0.07 to 30 MPa at densities of 0.006 to 0.25 g/cm
3
(10
–
14)]. These micro/nanoarchitected materials have a common
feature of length scale hierarchy, that is, relevant dimensions of
their structural elements span three to five orders of magnitude,
from tens of nanometers to hundreds of micrometers and even
greater. Structural features of nickel-alloy hollow-tube nanolattices
fabricated using large-area projection microstereolithography span
seven orders of magnitude in spatial dimensions, from tens of
nanometers to tens of centimeters. These nanolattices attain ten-
sile strains of
>
20% with a low modulus of 125 kPa and a low
tensile strength of
∼
80 kPa at a density of
∼
0.20 g/cm
3
,which
corresponds to the relative density of 0.15% (17). The deform-
ability of these nanolattices is attributed to a combination of
bending- and stretching-dominated hierarchical architectures
distributed over successive hierarchies and shell buckling, an
elastic instability characteristic of thin-walled hollow cylinders
(17). Among the thin-walled architectures, 3D periodic graphene
aerogel microlattices have been synthesized via direct ink writing;
Significance
A long-standing challenge in mod
ern materials manufacturing and
design has been to create porous mat
erials that are simultaneously
lightweight, strong, stiff, and fla
w-tolerant. Here, we fabricated
pyrolytic carbon nanolattices with designable topologies by a two-
step procedure: direct laser writi
ng and pyrolysis at high temper-
ature. The smallest characteristic size of the nanolattices
approached the resolution limits of the available 3D lithography
technologies. Due to the designable unit-cell geometries, re-
duced feature sizes, and high quality of pyrolytic carbon, the
created nanoarchitected carbon structures are lightweight, can
be made virtually insensitive to fabrication-induced defects, at-
tain nearly theoretical strength of the constituent material, and
achieve specific strength up to one to three orders of magnitude
above that of all existing micro/nanoarchitected materials.
Author contributions: H.G., J.R.G., and X.L. designed research; X.Z. and A.V. performed
research; X.Z., A.V., H.G., J.R.G., and X.L. analyzed data; and X.Z., H.G., J.R.G., and X.L.
wrote the paper.
Reviewers: Y.H., Northwestern University; and C.M.S., Lawrence Livermore
National Laboratory.
The authors declare no conflict of interest.
This open access article is distributed under
Creative Commons Attribution-NonCommercial-
NoDerivatives License 4.0 (CC BY-NC-ND)
.
1
To whom correspondence may be addressed. Email: Huajian_Gao@brown.edu, jrgreer@
caltech.edu, or xiaoyanlithu@tsinghua.edu.cn.
This article contains supporting information online at
www.pnas.org/lookup/suppl/doi:10.
1073/pnas.1817309116/-/DCSupplemental
.
Published online March 18, 2019.
www.pnas.org/cgi/doi/10.1073/pnas.1817309116
PNAS
|
April 2, 2019
|
vol. 116
|
no. 14
|
6665
–
6672
ENGINEERING
these materials are exceptionally lightweight (with a density of
0.031 to 0.123 g/cm
3
), compliant (with a modulus of 1 to 10 MPa),
and weak (with a low strength of 0.10 to 1.6 MPa) and exhibit
nearly complete recovery after compression to 90% strain (23).
Some efforts have also been dedicated to the synthesis and
development of mechanical properties of micro- and nano-
architected materials that are composed of nonhollow beams of
various materials, achieving greater stiffness and higher densities
compared with their hollow-beam counterparts. Most of these
studies have been on architectures composed of core-shell types of
beams, usually with an acrylic polymer core and a thin (from tens
of nanometers to several hundred nanometers), rigid outer coat-
ing. For example, triangular-truss microlattices with polymer-core-
alumina-shell beams have been synthesized by combining two-
photon lithography (TPL) direct laser writing (DLW) and atomic
layer deposition and sustained a modulus of
∼
30 MPa at a low
fracture strain of
∼
4 to 6% and a density of 0.42 g/cm
3
(16). Octet-
truss nanolattices made up of 262- to 774-nm-diameter polymer
beams with sputtered 14- to 126-nm-thick high-entropy alloy
(HEA) coatings were reported to have a Young
’
smodulusof16to
95 MPa and a compressive strength of 1 to 10 MPa at densities
between 0.087 and 0.865 g/cm
3
(20). Samples with HEA thick-
nesses less than 50 nm completely recovered after being com-
pressed for
>
50% (20). Beyond core-shell-beamed nano- and
microarchitected materials, several reports exist on the fabrication
and deformation of 3D structural metamaterials with monolithic
beams. For example, nanocrystalline nickel octet-truss nano-
lattices with 300- to 400-nm-diameter monolithic beams and 2-
μ
m
unit cells, created via TPL on custom-synthesized resins followed
by pyrolysis, exhibited a modulus of
∼
90 MPa, a compressive
strength of 18 MPa, and a high fracture strain of
>
20% at a
density of 2.5 g/cm
3
(20). Reports on vitreous carbon octet-truss
microlattices with beam diameters of
∼
100
μ
m, fabricated by py-
rolyzing a UV-mask patterned polymer template, reported a
modulus of 1.1 GPa, a compressive strength of 10.2 MPa, and a
fracture strain of only
∼
3% at a density of 0.19 g/cm
3
(24). Glassy
carbon microlattices with rhombic dodecahedron unit-cell and
beam diameters of 50 to 150
μ
m, fabricated using stereo-
lithography and pyrolysis, had densities of 0.03 to 0.05 g/cm
3
,
moduli of 5 to 25 MPa, compressive strengths of 0.08 to 0.35 MPa,
and fractured at a strain of
∼
5% (25). Glassy carbon nanolattices
with tetrahedral unit cells created via TPL and pyrolysis had
smaller dimensions (0.97- to 2.02-
μ
m unit cells and beam diame-
ters of
∼
200 nm), a modulus of 3.2 GPa, and a compressive
strength of
∼
280 MPa at a density of
∼
0.35 g/cm
3
(18). These
advances highlight a strong coupling between the density and
compliance of architected materials: The lower the density, the
softer and the weaker the material.
We developed an approach to fabricate nanoarchitected py-
rolytic carbon and to demonstrate two prototype unit-cell ge-
ometries, octet- and iso-truss, shown in Fig. 1, using TPL and
pyrolysis. The octet-truss architecture has cubic anisotropy and
superior overall properties compared with other conventional
lattices, such as triangular, tetrahedral, or cubic trusses and
foams (28), whereas the iso-truss structure is isotropic and has
been theorized to possess optimal stiffness compared with tra-
ditional lattice topologies (29). Uniaxial compression experi-
ments revealed their Young
’
s moduli to be 0.34 to 18.6 GPa,
their strengths to be 0.05 to 1.9 GPa, and prefailure deform-
ability of 14 to 17% at density varying from 0.24 to 1.0 g/cm
3
. The
highest specific strength is up to 1.90 GPa
·
g
−
1
·
cm
3
, which out-
performs all other reported mechanically robust lightweight
meso/micro/nanolattices (7
–
27). We attribute this distinction to
the optimized unit-cell geometries, reduced feature sizes, and
high-quality pyrolytic carbon.
4 nm
500 nm
2
m
2
m
500 nm
d
=435 nm
d
2
=523 nm
d
1
=460 nm
d
d
1
d
2
IP-Dip
photoresist
Vacuum
900°C
A
D
B
E
C
F
Polymer
Pyrolytic carbon
High-speed galvo mode
Pyrolysis for 5 hours
Volume shrinkage
>98%
Fig. 1.
Fabrication and microstructural characterization of pyrolytic carbon nanolattices. (
A
) Schematic illustration of the fabrication process of pyrolytic
carbon nanolattices. (
B
and
C
) CAD rendition of an octet- and iso-truss unit cell. (
D
and
E
) SEM images of an octet nanolattice with a strut diameter of
d
=
435 nm and an iso-truss nanolattice fabricated with a vertical strut diameter of
d
1
=
460 nm and a slanted strut diameter of
d
2
=
523 nm. (
F
) An HRTEM image
of pyrolytic carbon extracted from the nanolattice, which indicates an amorphous nature of the pyrolytic carbon. Initial detectable structural imp
erfections
caused by fabrication process are circled in
D
and
E
.
6666
|
www.pnas.org/cgi/doi/10.1073/pnas.1817309116
Zhang et al.