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

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

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

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 (Ca₂SiO₃)a key cement constituent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-C₂S) 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-C₂S > α_H-C₂S > α_L-C₂S > β-C₂S > γ-C₂S, and the following order in term of reactivity, α -C₂S > α_L-C₂S > γ-C₂S > α_H-C₂S > β-C₂S. 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

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

H₂, N₂, and CH₄ 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 H₂, N₂, and CH₄ 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 (H₂), ZIF-95 offers a superior adsorption capacity for large molecules (CH₄). Moderately sized molecules (N₂) 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 N₂ and CH₄ 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 poly­(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