Nano-Porous Graphene as Free-Standing Membranes
59
by reducing its fracture strength and its elastic modulus [124–126]. Like polyamide
active layers, nano-porous graphene must also lie on top of a porous substrate. It has
been shown that the maximum pressure in supported sheets of graphene decreases
as a function of nanopore size and substrate pore size [98]. Moreover, the continuous presence of water may further impact the fracture toughness of graphene as
a RO membrane [127]. RO process relies on the molecular-level separation of salt
ions from water molecules, thus, mechanical failures at the small scales may undermine the entire system [123]. Considering d, as the thickness of the membrane, the
stress in a thin membrane under pressure scales with d
−2/3 [128], and realizing that
graphene is ~1,000 times thinner than the polyamide layers used in conventional TFC
membranes, the important question arises: Is nano-porous graphene strong enough
to withstand similar pressures without fracturing?
The combined role of applied pressure, membrane morphology, elastic properties,
fracture stress and the effect of water have been systematically studied in [128].
Therein, using MD simulations and continuum fracture mechanics, it is demonstrated
that the maximum pressure that nano-porous graphene can withstand depends on
(1) the size and spacing of the nanopores and (2) the radius of the pores in the
substrate material. Therefore, a nano-porous graphene membrane can maintain its
mechanical integrity in RO but the choice of substrate is crucial to this execution.
An appropriate substrate with pores smaller than 1 µm would allow nano-porous
graphene to withstand pressures exceeding 57 MPa (570 bar). Furthermore, it is
revealed that nano-porous graphene membranes with greater porosity may help to
withstand even higher pressures [128].
While experimental tuning of the shape of nanopores at the atomic level remains
a challenge, Qin et al. [129], performed MD simulations on rectangular nanopores
with different areas, aspect ratios (length/width ratios) and orientations to explore
their effect on the tensile strength of defective graphene. It was shown that defective graphene with a larger area of rectangular nanopore shows a bigger drop in
tensile strength. Furthermore, changes in aspect ratio and orientation of rectangular
nanopore can either decrease or increase the tensile strength of defective graphene,
dependent on the tensile direction [129]. Simulations of nano-porous graphene were
performed by distributing vacancy defects, randomly and uniformly, across the lattice
by Anastasi et al. [130]. It was concluded that the fracture stress decreases substantially with increasing defect density and nano-porous graphene sheets with uniformly
distributed defects can withstand higher loads when compared to their counterparts
with random defects. They found that the fracture stress decreased substantially with
increasing defect density. The elastic modulus was found to be constant up to around
5% of vacancy defects and decreased for higher defect densities [130].
Structural response and fracture characteristics of nano-porous graphene
membranes subjected to shear loading were studied by Fang et al. [131]. Although
shear loading or strength is not directly a problem in RO membranes, considering
nano-porous graphene over a support substrate, examining these effects may be
helpful in design parameters. A shear load is a force that tends to generate a sliding
failure on a material along a plane that is parallel to the direction of the force. When a
paper is cut with scissors, the paper fails in shear. The effects of porosity, temperature,
59
by reducing its fracture strength and its elastic modulus [124–126]. Like polyamide
active layers, nano-porous graphene must also lie on top of a porous substrate. It has
been shown that the maximum pressure in supported sheets of graphene decreases
as a function of nanopore size and substrate pore size [98]. Moreover, the continuous presence of water may further impact the fracture toughness of graphene as
a RO membrane [127]. RO process relies on the molecular-level separation of salt
ions from water molecules, thus, mechanical failures at the small scales may undermine the entire system [123]. Considering d, as the thickness of the membrane, the
stress in a thin membrane under pressure scales with d
−2/3 [128], and realizing that
graphene is ~1,000 times thinner than the polyamide layers used in conventional TFC
membranes, the important question arises: Is nano-porous graphene strong enough
to withstand similar pressures without fracturing?
The combined role of applied pressure, membrane morphology, elastic properties,
fracture stress and the effect of water have been systematically studied in [128].
Therein, using MD simulations and continuum fracture mechanics, it is demonstrated
that the maximum pressure that nano-porous graphene can withstand depends on
(1) the size and spacing of the nanopores and (2) the radius of the pores in the
substrate material. Therefore, a nano-porous graphene membrane can maintain its
mechanical integrity in RO but the choice of substrate is crucial to this execution.
An appropriate substrate with pores smaller than 1 µm would allow nano-porous
graphene to withstand pressures exceeding 57 MPa (570 bar). Furthermore, it is
revealed that nano-porous graphene membranes with greater porosity may help to
withstand even higher pressures [128].
While experimental tuning of the shape of nanopores at the atomic level remains
a challenge, Qin et al. [129], performed MD simulations on rectangular nanopores
with different areas, aspect ratios (length/width ratios) and orientations to explore
their effect on the tensile strength of defective graphene. It was shown that defective graphene with a larger area of rectangular nanopore shows a bigger drop in
tensile strength. Furthermore, changes in aspect ratio and orientation of rectangular
nanopore can either decrease or increase the tensile strength of defective graphene,
dependent on the tensile direction [129]. Simulations of nano-porous graphene were
performed by distributing vacancy defects, randomly and uniformly, across the lattice
by Anastasi et al. [130]. It was concluded that the fracture stress decreases substantially with increasing defect density and nano-porous graphene sheets with uniformly
distributed defects can withstand higher loads when compared to their counterparts
with random defects. They found that the fracture stress decreased substantially with
increasing defect density. The elastic modulus was found to be constant up to around
5% of vacancy defects and decreased for higher defect densities [130].
Structural response and fracture characteristics of nano-porous graphene
membranes subjected to shear loading were studied by Fang et al. [131]. Although
shear loading or strength is not directly a problem in RO membranes, considering
nano-porous graphene over a support substrate, examining these effects may be
helpful in design parameters. A shear load is a force that tends to generate a sliding
failure on a material along a plane that is parallel to the direction of the force. When a
paper is cut with scissors, the paper fails in shear. The effects of porosity, temperature,