10 research outputs found
Oxygen- and Lithium-Doped Hybrid Boron-Nitride/Carbon Networks for Hydrogen Storage
Hydrogen storage
capacities have been studied on newly designed
three-dimensional pillared boron nitride (PBN) and pillared graphene
boron nitride (PGBN). We propose these novel materials based on the
covalent connection of BNNTs and graphene sheets, which enhance the
surface and free volume for storage within the nanomaterial and increase
the gravimetric and volumetric hydrogen uptake capacities. Density
functional theory and molecular dynamics simulations show that these
lithium- and oxygen-doped pillared structures have improved gravimetric
and volumetric hydrogen capacities at room temperature, with values
on the order of 9.1–11.6 wt % and 40–60 g/L. Our findings
demonstrate that the gravimetric uptake of oxygen- and lithium-doped
PBN and PGBN has significantly enhanced the hydrogen sorption and
desorption. Calculations for O-doped PGBN yield gravimetric hydrogen
uptake capacities greater than 11.6 wt % at room temperature. This
increased value is attributed to the pillared morphology, which improves
the mechanical properties and increases porosity, as well as the high
binding energy between oxygen and GBN. Our results suggest that hybrid
carbon/BNNT nanostructures are an excellent candidate for hydrogen
storage, owing to the combination of the electron mobility of graphene
and the polarized nature of BN at heterojunctions, which enhances
the uptake capacity, providing ample opportunities to further tune
this hybrid material for efficient hydrogen storage
Dimensional Crossover of Thermal Transport in Hybrid Boron Nitride Nanostructures
Although
boron nitride nanotubes (BNNT) and hexagonal-BN (hBN)
are superb one-dimensional (1D) and 2D thermal conductors respectively,
bringing this quality into 3D remains elusive. Here, we focus on pillared
boron nitride (PBN) as a class of 3D BN allotropes and demonstrate
how the junctions, pillar length and pillar distance control phonon
scattering in PBN and impart tailorable
thermal conductivity in 3D. Using reverse nonequilibrium molecular
dynamics simulations, our results indicate that although a clear phonon
scattering at the junctions accounts for the lower thermal conductivity
of PBN compared to its parent BNNT and hBN allotropes, it acts as
an effective design tool and provides 3D thermo-mutable features that
are absent in the parent structures. Propelled by the junction spacing,
while one geometrical parameter, e.g., pillar length, controls the
thermal transport along the out-of-plane direction of PBN, the other
parameter, e.g., pillar distance, dictates the gross cross-sectional
area, which is key for design of 3D thermal management systems. Furthermore,
the junctions have a more pronounced effect in creating a Kapitza
effect in the out-of-plane direction, due to the change in dimensionality
of the phonon transport. This work is the first report on thermo-mutable
properties of hybrid BN allotropes and can potentially impact thermal
management of other hybrid 3D BN architectures
Screw Dislocations in Complex, Low Symmetry Oxides: Core Structures, Energetics, and Impact on Crystal Growth
Determining the atomic structure
and the influence of defects on properties of low symmetry oxides
have long been an engineering pursuit. Here, we focus on five thermodynamically
reversible monoclinic and orthorhombic polymorphs of dicalcium silicates
(Ca<sub>2</sub>SiO<sub>3</sub>)î—¸a key cement constituentî—¸as
a model system and use atomistic simulations to unravel the interplay
between the screw dislocation core energies, nonplanar core structures,
and Peierls stresses along different crystallographic planes. Among
different polymorphs, we found that the α polymorphs (α-C<sub>2</sub>S) has the largest Peierls stress, corresponding to the most
brittle polymorph, which make it attractive for grinding processes.
Interestingly, our analyses indicate that this polymorphs has the
lowest dislocation core energy, making it ideal for reactivity and
crystal growth. Generally, we identified the following order in terms
of grinding efficiency based on screw dislocation analysis, α-C<sub>2</sub>S > α<sub>H</sub>-C<sub>2</sub>S > α<sub>L</sub>-C<sub>2</sub>S > β-C<sub>2</sub>S > γ-C<sub>2</sub>S, and the following order in term of reactivity, α
-C<sub>2</sub>S > α<sub>L</sub>-C<sub>2</sub>S > γ-C<sub>2</sub>S > α<sub>H</sub>-C<sub>2</sub>S > β-C<sub>2</sub>S. This information, combined with other deformation-based
mechanisms, such as twinning and edge dislocation, can provide crucial
insights and guiding hypotheses for experimentalists to tune the cement
grinding mechanisms and reactivity processes for an overall optimum
solution with regard to both energy consumption and performance. Our
findings significantly broaden the spectrum of strategies for leveraging
both crystallographic directions and crystal symmetry to concurrently
modulate mechanics and crystal growth processes within an identical
chemical composition
Molecular Mechanistic Origin of Nanoscale Contact, Friction, and Scratch in Complex Particulate Systems
Nanoscale contact mechanisms, such
as friction, scratch, and wear,
have a profound impact on physics of technologically important particulate
systems. Determining the key underlying interparticle interactions
that govern the properties of the particulate systems has been long
an engineering challenge. Here, we focus on particulate calcium–silicate–hydrate
(C–S–H) as a model system and use atomistic simulations
to decode the interplay between crystallographic directions, structural
defects, and atomic species on normal and frictional forces. By exhibiting
high material inhomogeneity and low structural symmetry, C–S–H
provides an excellent system to explore various contact-induced nanoscale
deformation mechanisms in complex particulate systems. Our findings
provide a deep fundamental understanding of the role of inherent material
features, such as van der Waals versus Coulombic interactions and
the role of atomic species, in controlling the nanoscale normal contact,
friction, and scratch mechanisms, thereby providing de novo insight
and strategies for intelligent modulation of the physics of the particulate
systems. This work is the first report on atomic-scale investigation
of the contact-induced nanoscale mechanisms in structurally complex
C–S–H materials and can potentially open new opportunities
for knowledge-based engineering of several other particulate systems
such as ceramics, sands, and powders and self-assembly of colloidal
systems in general
Molecular Mechanistic Origin of Nanoscale Contact, Friction, and Scratch in Complex Particulate Systems
Nanoscale contact mechanisms, such
as friction, scratch, and wear,
have a profound impact on physics of technologically important particulate
systems. Determining the key underlying interparticle interactions
that govern the properties of the particulate systems has been long
an engineering challenge. Here, we focus on particulate calcium–silicate–hydrate
(C–S–H) as a model system and use atomistic simulations
to decode the interplay between crystallographic directions, structural
defects, and atomic species on normal and frictional forces. By exhibiting
high material inhomogeneity and low structural symmetry, C–S–H
provides an excellent system to explore various contact-induced nanoscale
deformation mechanisms in complex particulate systems. Our findings
provide a deep fundamental understanding of the role of inherent material
features, such as van der Waals versus Coulombic interactions and
the role of atomic species, in controlling the nanoscale normal contact,
friction, and scratch mechanisms, thereby providing de novo insight
and strategies for intelligent modulation of the physics of the particulate
systems. This work is the first report on atomic-scale investigation
of the contact-induced nanoscale mechanisms in structurally complex
C–S–H materials and can potentially open new opportunities
for knowledge-based engineering of several other particulate systems
such as ceramics, sands, and powders and self-assembly of colloidal
systems in general
Toughness Governs the Rupture of the Interfacial H‑Bond Assemblies at a Critical Length Scale in Hybrid Materials
The geometry and material property
mismatch across the interface
of hybrid materials with dissimilar building blocks make it extremely
difficult to fully understand the lateral chemical bonding processes
and design nanocomposites with optimal performance. Here, we report
a combined first-principles study, molecular dynamics modeling, and
theoretical derivations to unravel the detailed mechanisms of H-bonding,
deformation, load transfer, and failure at the interface of polyvinyl
alcohol (PVA) and silicates, as an example of hybrid materials with
geometry and property mismatch across the interface. We identify contributing
H-bonds that are key to adhesion and demonstrate a specific periodic
pattern of interfacial H-bond network dictated by the interface mismatch
and intramolecular H-bonding. We find that the maximum toughness,
incorporating both intra- and interlayer strain energy contributions,
govern the existence of optimum overlap length and thus the rupture
of interfacial (interlayer) H-bond assemblies in natural and synthetic
hybrid materials. This universally valid result is in contrast to
the previous reports that correlate shear strength with rupture of
H-bonds assemblies at a finite overlap length. Overall, this work
establishes a unified understanding to explain the interplay between
geometric constraints, interfacial H-bonding, materials characteristics,
and optimal mechanical properties in hybrid organic–inorganic
materials
Toughness Governs the Rupture of the Interfacial H‑Bond Assemblies at a Critical Length Scale in Hybrid Materials
The geometry and material property
mismatch across the interface
of hybrid materials with dissimilar building blocks make it extremely
difficult to fully understand the lateral chemical bonding processes
and design nanocomposites with optimal performance. Here, we report
a combined first-principles study, molecular dynamics modeling, and
theoretical derivations to unravel the detailed mechanisms of H-bonding,
deformation, load transfer, and failure at the interface of polyvinyl
alcohol (PVA) and silicates, as an example of hybrid materials with
geometry and property mismatch across the interface. We identify contributing
H-bonds that are key to adhesion and demonstrate a specific periodic
pattern of interfacial H-bond network dictated by the interface mismatch
and intramolecular H-bonding. We find that the maximum toughness,
incorporating both intra- and interlayer strain energy contributions,
govern the existence of optimum overlap length and thus the rupture
of interfacial (interlayer) H-bond assemblies in natural and synthetic
hybrid materials. This universally valid result is in contrast to
the previous reports that correlate shear strength with rupture of
H-bonds assemblies at a finite overlap length. Overall, this work
establishes a unified understanding to explain the interplay between
geometric constraints, interfacial H-bonding, materials characteristics,
and optimal mechanical properties in hybrid organic–inorganic
materials
H<sub>2</sub>, N<sub>2</sub>, and CH<sub>4</sub> Gas Adsorption in Zeolitic Imidazolate Framework-95 and -100: Ab Initio Based Grand Canonical Monte Carlo Simulations
A multiscale approach
based on ab initio and grand canonical Monte
Carlo (GCMC) simulations is used to report the H<sub>2</sub>, N<sub>2</sub>, and CH<sub>4</sub> uptake behaviors of two zeolitic imidazolate
frameworks (ZIFs), ZIF-95 and -100, with exceptionally large and complex
colossal cages. The force fields describing the weak interactions
between the gas molecules and ZIFs in GCMC simulations are based on
ab initio MP2 level of theory aimed at accurately describing the London
dispersions. We report the total and excess gas uptakes up to 100
bar at 77 and 300 K. Our results unravel the interplay between the
uptake amount, pore volume, guest molecule size, temperature, chlorine
functional group, and isosteric heat of adsorption in ZIFs. We found
that while the uptake capacity of ZIF-100 outperforms ZIF-95 for small
molecules (H<sub>2</sub>), ZIF-95 offers a superior adsorption capacity
for large molecules (CH<sub>4</sub>). Moderately sized molecules (N<sub>2</sub>) exhibit a more complex uptake behavior depending on the
temperature. Furthermore, we show that the induced dipole interactions,
such as those caused by −Cl functional groups, play a vital
role on gas adsorption behaviors. This work provides the first report
on the N<sub>2</sub> and CH<sub>4</sub> uptake of ZIF-95 and -100
using ab initio based GCMC simulations
Biomimetic, Strong, Tough, and Self-Healing Composites Using Universal Sealant-Loaded, Porous Building Blocks
Many
natural materials, such as nacre and dentin, exhibit multifunctional
mechanical properties via structural interplay between compliant and
stiff constituents arranged in a particular architecture. Herein,
we present, for the first time, the bottom-up synthesis and design
of strong, tough, and self-healing composite using simple but universal
spherical building blocks. Our composite system is composed of calcium
silicate porous nanoparticles with unprecedented monodispersity over
particle size, particle shape, and pore size, which facilitate effective
loading and unloading with organic sealants, resulting in 258% and
307% increases in the indentation hardness and elastic modulus of
the compacted composite. Furthermore, heating the damaged composite
triggers the controlled release of the nanoconfined sealant into the
surrounding area, enabling moderate recovery in strength and toughness.
This work paves the path towards fabricating a novel class of biomimetic
composites using low-cost spherical building blocks, potentially impacting
bone-tissue engineering, insulation, refractory and constructions
materials, and ceramic matrix composites
Microwave Heating of Functionalized Graphene Nanoribbons in Thermoset Polymers for Wellbore Reinforcement
Here, we introduce a systematic strategy
to prepare composite materials for wellbore reinforcement using graphene
nanoribbons (GNRs) in a thermoset polymer irradiated by microwaves.
We show that microwave absorption by GNRs functionalized with polyÂ(propylene
oxide) (PPO-GNRs) cured the composite by reaching 200 °C under
30 W of microwave power. Nanoscale PPO-GNRs diffuse deep inside porous
sandstone and dramatically enhance the mechanics of the entire structure
via effective reinforcement. The bulk and the local mechanical properties
measured by compression and nanoindentation mechanical tests, respectively,
reveal that microwave heating of PPO-GNRs and direct polymeric curing
are major reasons for this significant reinforcement effect