Browsing by Author "Shahsavari, Rouzbeh"
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Item Balancing strength and toughness of calcium-silicate-hydrate via random nanovoids and particle inclusions: Atomistic modeling and statistical analysis(Elsevier, 2016) Zhang, Ning; Shahsavari, RouzbehAs the most widely used manufactured material on Earth, concrete poses serious societal and environmental concerns which call for innovative strategies to develop greener concrete with improved strength and toughness, properties that are exclusive in man-made materials. Herein, we focus on calcium silicate hydrate (C-S-H), the major binding phase of all Portland cement concretes, and study how engineering its nanovoids and portlandite particle inclusions can impart a balance of strength, toughness and stiffness. By performing an extensive +600 molecular dynamics simulations coupled with statistical analysis tools, our results provide new evidence of ductile fracture mechanisms in C-S-H – reminiscent of crystalline alloys and ductile metals – decoding the interplay between the crack growth, nanovoid/particle inclusions, and stoichiometry, which dictates the crystalline versus amorphous nature of the underlying matrix. We found that introduction of voids and portlandite particles can significantly increase toughness and ductility, specially in C-S-H with more amorphous matrices, mainly owing to competing mechanisms of crack deflection, voids coalescence, internal necking, accommodation, and geometry alteration of individual voids/particles, which together regulate toughness versus strength. Furthermore, utilizing a comprehensive global sensitivity analysis on random configuration-property relations, we show that the mean diameter of voids/particles is the most critical statistical parameter influencing the mechanical properties of C-S-H, irrespective of stoichiometry or crystalline or amorphous nature of the matrix. This study provides new fundamental insights, design guidelines, and de novo strategies to turn the brittle C-S-H into a ductile material, impacting modern engineering of strong and tough concrete infrastructures and potentially other complex brittle materials.Item Boron nitride nanochannels encapsulating a water/heavy water layer for energy applications(Royal Society of Chemistry, 2019) Shayeganfar, Farzaneh; Beheshtian, Javad; Shahsavari, RouzbehWater interaction and transport through nanochannels of two-dimensional (2D) nanomaterials hold great promises in several applications including separation, energy harvesting and drug delivery. However, the fundamental underpinning of the electronic phenomena at the interface of such systems is poorly understood. Inspired by recent experiments, herein, we focus on water/heavy water in boron nitride (BN) nanochannels – as a model system – and report a series of ab initio based density functional theory (DFT) calculations on correlating the stability of adsorption and interfacial properties, decoding various synergies in the complex interfacial interactions of water encapsulated in BN nanocapillaries. We provide a comparison of phonon vibrational modes of water and heavy water (D2O) captured in bilayer BN (BLBN) to compare their mobility and group speed as a key factor for separation mechanisms. This finding, combined with the fundamental insights into the nature of the interfacial properties, provides key hypotheses for the design of nanochannels.Item Bottom-Up Synthesis of Mechanically Enhanced Industrial Composites(2018-08-20) Hwang, Sung Hoon; Shahsavari, RouzbehFine-tuning physicochemical properties of nano- or submicron materials and utilizing them as fundamental building blocks for a larger, multifunctional material is now the common strategy in the field of materials engineering. The remarkable depth and breadth of advanced synthesis and characterization techniques have now enabled this “bottom-up” fabrication of materials for diverse industries, which consistently demand strong and tough materials with novel functional properties. Most importantly, this bottom-up approach has proposed new solutions for overcoming inherent limitations of ceramic materials, which are strong but also, highly brittle. Despite being a common synthetic approach, there is still a myriad of industrial materials or composites, for which the aforesaid bottomup approach is yet to be applied. Therefore, my Ph.D research is focused on engineering and evaluating properties of nano- and submicron-sized particles, for example, size, porosity and mechanical properties of ultrafine calcium silicate particles and chemical properties of two dimensional materials such as graphene and boron nitride and assembling them via various synthetic techniques to produce a composite with enhanced mechanical properties. Unprecedented synthetic pathways coupled with effective fine-tuning of properties at submicron scale and hybridization of building units, such as calcium silicate porous particles integrated with bisphenol A diglycidyl ether will propose new strategies to apply the advanced bioinspired concept to low-cost and abundant calcium silicate-based materials. The project will potentially impact diverse industries ranging from refractory and insulation materials to construction industry and bone-tissue engineering, where the common or newly rising materials suffer from shortcomings in mechanical properties, which prevent their widespread commercialization.Item Deep Learning Method to Accelerate Discovery of Hybrid Polymer-Graphene Composites(Springer Nature, 2021) Shayeganfar, Farzaneh; Shahsavari, RouzbehInterfacial encoded properties of polymer adlayers adsorbed on the graphene (GE) and silicon dioxide (SiO2) have been constituted a scaffold for the creation of new materials. The holistic understanding of nanoscale intermolecular interaction of 1D/2D polymer assemblies on substrate is the key to bottom-up design of molecular devices. We develop an integrated multidisciplinary approach based on electronic structure computation [density functional theory (DFT)] and big data mining [machine learning (ML)] in parallel with neural network (NN) and statistical analysis (SA) to design hybrid polymers from assembly on substrate. Here we demonstrate that interfacial pressure and structural deformation of polymer network adsorbed on GE and SiO2 offer unique directions for the fabrication of 1D/2D polymers using only a small number of simple molecular building blocks. Our findings serve as the platform for designing a wide range of typical inorganic heterostructures, involving noncovalent intermolecular interaction observed in many nanoscale electronic devices.Item Deformation Mechanisms of Cement-Based Materials: Atomistic Simulation of Screw Dislocations, Global/Local Deformations and Heat/Radiation-Induced Damage(2017-10-05) Tao, Lei; Shahsavari, RouzbehCement is the most widely used material in the world. Billions of tons of cement are consumed every year. Since cement manufacturing is one of the most carbon dioxide intensive industries, the high cement consumption becomes a serious problem. The demand for lower cement consumption and more reliable infrastructure requires development of high performance cementitious materials. With the advent of nanotechnology and emerging advanced computational tools, it is now possible to fundamentally understand and change the mechanics of cement-based materials from the nano scale up, providing key design guidelines for experiments. The main focus of this thesis is on the behavior of cement hydrate product, calcium silicate hydrate (C-S-H), and its hybrid derivative, Hexagonal Boron Nitride/C-S-H composite. C-S-H is the main source of strength and durability in all Portland cement concretes. Having a deeper understanding of the mechanical properties and deformation mechanisms of C-S-H is the basis for the development of new cementitious materials. Using molecular dynamics (MD) simulation, C-S-H is modeled by the layered tobermorite structure - a mineral analog of C-S-H. First, screw dislocations are simulated to evaluate the dislocations’ effect on the plastic deformation of C-S-H. The screw dislocations with different Peierls stress are identified, with which the plastic deformation of cement can be modulated. Next, by comparing various global deformations (e.g. shear, compression and tension) and a local deformation (e.g. nano-indentation), it is found that the global deformations lead to size-independent mechanical properties while the local deformation results in size-dependent mechanical properties at the nanometer scales. Three key mechanisms govern the deformation and thus mechanics of the layered C-S-H: diffusive-controlled deformation mechanism, displacive-controlled deformation mechanism, and local phase transformations with strain gradient. Together, these elaborately classified mechanisms provide deep fundamental understanding and new insights on the relationship between the macro-scale mechanical properties and underlying molecular deformations, providing new opportunities to control and tune the mechanics of layered crystals and other complex materials such as glassy C-S-H, natural composite structures, and manmade laminated structures. Finally, a hexagonal boron nitride (h-BN) reinforced cement is investigated for its high thermal and radiation-resistance. The rapid development of nuclear power plants (NPP) all over the world requires more advanced cementitious materials for radiation shielding and safety protection. Because of h-BN’s exceptional hardness, high thermal conductivity, and high neutron absorbing efficiency, the h-BN/C-S-H composite possesses higher strength, thermal tolerance and radiation-resistance. The radiation damage of h-BN, C-S-H, and h-BN/C-S-H composite are examined through a series of radiation cascade simulations. By assessing their strength degradation under different radiation dosages and temperatures, h-BN is found to help preserve more residual strength under extreme heat and radiation conditions. The proposed “thermal-radiation shock maps”, akin to thermal shock maps, for the first time uncovers the coupled effect of radiation and temperature on the strength of the structures, guiding science-based engineering of NPP concretes. This dissertation establishes a comprehensive understanding of cementitious materials at the atomic scale, providing fundamental understanding and guiding hypotheses for modern engineering of high performance cement-based materials.Item Diffusive, Displacive Deformations and Local Phase Transformation Govern the Mechanics of Layered Crystals: The Case Study of Tobermorite(Springer Nature, 2017) Tao, Lei; Shahsavari, RouzbehUnderstanding the deformation mechanisms underlying the mechanical behavior of materials is the key to fundamental and engineering advances in materials' performance. Herein, we focus on crystalline calcium-silicate-hydrates (C-S-H) as a model system with applications in cementitious materials, bone-tissue engineering, drug delivery and refractory materials, and use molecular dynamics simulation to investigate its loading geometry dependent mechanical properties. By comparing various conventional (e.g. shear, compression and tension) and nano-indentation loading geometries, our findings demonstrate that the former loading leads to size-independent mechanical properties while the latter results in size-dependent mechanical properties at the nanometer scales. We found three key mechanisms govern the deformation and thus mechanics of the layered C-S-H: diffusive-controlled and displacive-controlled deformation mechanisms, and strain gradient with local phase transformations. Together, these elaborately classified mechanisms provide deep fundamental understanding and new insights on the relationship between the macro-scale mechanical properties and underlying molecular deformations, providing new opportunities to control and tune the mechanics of layered crystals and other complex materials such as glassy C-S-H, natural composite structures, and manmade laminated structures.Item Insights on synergy of materials and structures in biomimetic platelet-matrix composites(AIP Publishing, 2018) Sakhavand, Navid; Shahsavari, Rouzbeh; Smalley Institute for Nanoscale Science and TechnologyHybrid materials such as biomimetic platelet-matrix composites are in high demand to confer low weight and multifunctional mechanical properties. This letter reports interfacial-bond regulated assembly of polymers on cement-an archetype model with significant infrastructure applications. We demonstrate a series of 20+ molecular dynamics studies on decoding and optimizing the complex interfacial interactions including the role and types of various heterogeneous, competing interfacial bonds that are key to adhesion and interfacial strength. Our results show an existence of an optimum overlap length scale (∼15 nm) between polymers and cement crystals, exhibiting the best balance of strength, toughness, stiffness, and ductility for the composite. This finding, combined with the fundamental insights into the nature of interfacial bonds, provides key hypotheses for selection and processing of constituents to deliberate the best synergy in the structure and materials of platelet-matrix composites.Item Interlaced, Nanostructured Interface with Graphene Buffer Layer Reduces Thermal Boundary Resistance in Nano/Microelectronic Systems(American Chemical Society, 2017) Tao, Lei; Sreenivasan, Sreeprasad; Shahsavari, Rouzbeh; Smalley Institute for Nanoscale Science and TechnologyImproving heat transfer in hybrid nano/microelectronic systems is a challenge, mainly due to the high thermal boundary resistance (TBR) across the interface. Herein, we focus on gallium nitride (GaN)/diamond interface—as a model system with various high power, high temperature, and optoelectronic applications—and perform extensive reverse nonequilibrium molecular dynamics simulations, decoding the interplay between the pillar length, size, shape, hierarchy, density, arrangement, system size, and the interfacial heat transfer mechanisms to substantially reduce TBR in GaN-on-diamond devices. We found that changing the conventional planar interface to nanoengineered, interlaced architecture with optimal geometry results in >80% reduction in TBR. Moreover, introduction of conformal graphene buffer layer further reduces the TBR by ∼33%. Our findings demonstrate that the enhanced generation of intermediate frequency phonons activates the dominant group velocities, resulting in reduced TBR. This work has important implications on experimental studies, opening up a new space for engineering hybrid nano/microelectronics.Item Mechanics of Platelet-Matrix Composites across Scales: Theory, Multiscale Modeling, and 3D Fabrication(2015-04-24) Sakhavand, Navid; Shahsavari, Rouzbeh; Nagarajaiah, Satish; Ajayan, PulickelMany natural and biomimetic composites - such as nacre, silk and clay-polymer - exhibit a remarkable balance of strength, toughness, and/or stiffness, which call for a universal measure to quantify this outstanding feature given the platelet-matrix structure and material characteristics of the constituents. Analogously, there is an urgent need to quantify the mechanics of emerging electronic and photonic systems such as stacked heterostructures, which are composed of strong in-plane bonding networks but weak interplanar bonding matrices. In this regard, development of a universal composition-structure-property map for natural platelet-matrix composites, and stacked heterostructures opens up new doors for designing materials with superior mechanical performance. In this dissertation, a multiscale bottom-up approach is adopted to analyze and predict the mechanical properties of platelet-matrix composites. Design guidelines are provided by developing universally valid (across different length scales) diagrams for science-based engineering of numerous natural and synthetic platelet-matrix composites and stacked heterostructures while significantly broadening the spectrum of strategies for fabricating new composites with specific and optimized mechanical properties. First, molecular dynamics simulations are utilized to unravel the fundamental underlying physics and chemistry of the binding nature at the atomic-level interface of organic-inorganic composites. Polymer-cementitious composites are considered as case studies to understand bonding mechanism at the nanoscale and open up new venues for potential mechanical enhancement at the macro-scale. Next, sophisticated mathematical derivations based on elasticity and plasticity theories are presented to describe pre-crack (intrinsic) mechanical performance of platelet-matrix composites at the microscale. These derivations lead to developing a unified framework to construct series of universal composition-structure-property maps that decode the interplay between various geometries and inherent material features, encapsulated in a few dimensionless parameters. Finally, after crack mechanical properties (extrinsic) of platelet-matrix composites until ultimate failure of the material at the macroscale is investigated via combinatorial finite element simulations. The effect of different composition-structure-property parameters on mechanical properties synergies are depicted via 2D and 3D maps. 3D-printed specimens are fabricated and tested against the theoretical prediction. The combination of the presented diagrams and guidelines paves the path toward platelet-matrix composites and stacked-heterostructures with superior and optimized mechanical properties.Item Merger of Energetic Affinity and Optimal Geometry to Boost Hydrogen Storage in Porous Materials: Ab initio based Multiscale Simulations(2016-06-20) Zhao, Shuo; Shahsavari, RouzbehHydrogen is an ideal alternative fuel for various applications such as automobiles and portable devices because it is lightweight, abundant, and its oxidation product (water) is environmentally benign. However, its utilization is impeded by the lack of a safe and efficient storage device. Herein, we investigate and propose a new building block approach for an exhaustive search of optimal hydrogen uptakes in a series of low density boron nitride (BN) nanoarchitectures via an extensive 3868 multiscale simulations based on ab initio results. By probing various geometries, temperatures, pressures, and doping ratios, our results demonstrate a maximum uptake of 8.65 wt% at 300K, the highest hydrogen uptake on sorbents at room temperature without doping. Next, we investigate the Li+ doping of the nanoarchitectures exhibiting a set of optimal combinations of gravimetric and volumetric uptakes, surpassing the US DOE targets. Our findings suggest that the non-intuitive merger of energetic affinity and optimal geometry in BN building blocks overcomes the intrinsic limitations of sorbent materials, putting hybrid BN nanoarchitectures on equal footing with hydrides while demonstrating a superior capacity-kinetics-thermodynamics balance. Finally, we propose a novel methodology to improve the stability and accuracy of the fitting of empirical force- field parameters against ab initio data. Overall, the proposed building block approach, combined with the novel concepts and strategies for exhaustive search of optimum structure-property relationships in adsorption, opens up an entirely new phase space for making efficient high performance gas storage materials.Item Multifunctional Nanofluids with 2D Nanosheets for Thermal Management and Tribological Applications(2013-11-22) Taha Tijerina, Jaime; Ajayan, Pulickel M.; Barrera, Enrique V.; Shahsavari, RouzbehConventional heat-transfer fluids such as water, ethylene glycol, standard oils and other lubricants are typically low-efficiency heat-transfer fluids. Thermal management plays a critical factor in many applications where these fluids can be used, such as in motors/engines, solar cells, biopharmaceuticals, fuel cells, high voltage power transmission systems, micro/nanoelectronics mechanical systems (MEMS/NEMS), and nuclear cooling among others. These insulating fluids require superb filler dispersion, high thermal conduction, and for certain applications as in electrical/electronic devices also electrical insulation. The miniaturization and high efficiency of electrical/electronic devices in these fields demand successful heat management and energy-efficient fluid-based heat-transfer systems. Recent advances in layered materials enable large scale synthesis of various two-dimensional (2D) structures. Some of these 2D materials are good choices as nanofillers in heat transfer fluids; mainly due to their inherent high thermal conductivity (TC) and high surface area available for thermal energy transport. Among various 2D-nanostructures, hexagonal boron nitride (h-BN) and graphene (G) exhibit versatile properties such as outstanding TC, excellent mechanical stability, and remarkable chemical inertness. The following research, even though investigate various conventional fluids, will focus on dielectric insulating nanofluids (mineral oil - MO) with significant thermal performance. It is presented the plan for synthesis and characterization of stable high-thermal conductivity nanofluids using 2D-nanostructures of h-BN, which will be further incorporated at diverse filler concentrations to conventional fluids for cooling applications, without compromising its electrical insulating property. For comparison, properties of h-BN based fluids are compared with conductive fillers such as graphene; where graphene has similar crystal structure of h-BN and also has similar bulk thermal conductivity. Moreover, bot h-BN and graphene are exfoliated through the same method. In essence, this project, for the first time, unravels the behavior of the exfoliated h-BN effect on reinforced conventional fluids under the influence of atomistic scale structures (particularly, electrically insulating and lubricant/cutting fluids), thereby linking the physical, electrical and mechanical properties of these nanoscale materials. The innovative experimental approach is expected to result in de novo strategies for introducing these systems for new concepts and variables to engineer nanofluid properties suitable for very promising industrial applications.Item Screw and Edge Dislocations in Cement Phases: Atomic Modeling(2013-10-09) Chen, Lu; Shahsavari, Rouzbeh; Nagarajaiah, Satish; Duenas-Osorio, LeonardoCement is the key strengthening and the most energy-intensive ingredient in concrete. With increasing pressure for reducing energy consumption in cement manufacturing, there is an urgent need to understand the basic deformation mechanisms of cement. In this thesis, we develop a computational framework based on molecular dynamics to study two common types of defects, namely screw and edge dislocations, in complex, anisotropic crystalline polymorphs of cement clinkers and cement hydration products. We found the likelihood of these defects in regions with higher Young moduli. We also found the preferred cement polymorphs that require less energy for grinding via analysis of Peierls stresses. Together, the results provide a detailed understanding of the role and type of defects in cement phases, which impact the rate of hydration, crystal growth and grinding energy. To our knowledge, this is the first study with atomic-resolution on deformation-based mechanisms in cement crystalline phases.Item Screw-Dislocation-Induced Strengthening–Toughening Mechanisms in Complex Layered Materials: The Case Study of Tobermorite(American Chemical Society, 2017) Zhang, Ning; Carrez, Philippe; Shahsavari, Rouzbeh; The Richard E. Smalley Institute for Nanoscale Science and TechnologyNanoscale defects such as dislocations have a profound impact on the physics of crystalline materials. Understanding and characterizing the motion of screw dislocation and its corresponding effects on the mechanical properties of complex low-symmetry materials has long been a challenge. Herein, we focus on triclinic tobermorite, as a model system and a crystalline analogue of layered hydrated cement, and report for the first time how the motion of screw dislocation can influence the strengthening–toughening relationship, imparting brittle-to-ductile transitions. By applying shear loading in tobermorite systems with single and dipole screw dislocations, we observe dislocation jogs around the dislocation core, which increases the yield shear stress and the work-of-fracture when the dislocation lines are along the [100] and [010] directions. Our results demonstrate that the dislocation core acts as a bottleneck for the initial straight gliding to induce intralaminar gliding, which consequently leads to a significant improvement in the mechanical properties. Together, the fundamental knowledge gained in this work on the role of the motion of the dislocation core on the mechanical properties provides an improved understanding of deformation mechanisms in cementitious materials and other complex layered systems, providing new hypotheses and design guidelines for the development of strong, ductile, and tough materials.Item Shape-controlled cement hydrate synthesis and self-assembly(2019-10-15) Shahsavari, Rouzbeh; Ebrahimpourmoghaddam, Sakineh; Whitmire, Kenton Herbert; Rice University; United States Patent and Trademark OfficeIn some embodiments, the present disclosure pertains to methods of forming calcium-silicate-hydrate particles by mixing a calcium source with a silicate source. In some embodiments, the mixing comprises sonication. In some embodiments, the mixing occurs in the presence of a surfactant and a solvent. In some embodiments, the methods of the present disclosure further comprise a step of controlling the morphology of the calcium-silicate-hydrate particles. In some embodiments, the step of controlling the morphology of calcium-silicate-hydrate particles comprises selecting a stoichiometric ratio of the calcium source over the silicate source. In some embodiments, the formed calcium-silicate-hydrate particles have cubic shapes. In some embodiments, the formed calcium-silicate-hydrate particles have rectangular shapes. In some embodiments, the formed calcium-silicate-hydrate particles are in the form of self-assembled particles of controlled shapes. Additional embodiments of the present disclosure pertain to compositions that contain the calcium silicate-hydrate particles of the present disclosure.Item Spacecraft Attitude Estimation Integrating the Q-Method into an Extended Kalman Filter(2013-09-16) Ainscough, Thomas; Spanos, Pol D.; Dick, Andrew J.; Shahsavari, Rouzbeh; Zanetti, RenatoA new algorithm is proposed that smoothly integrates the nonlinear estimation of the attitude quaternion using Davenport's q-method and the estimation of non-attitude states within the framework of an extended Kalman filter. A modification to the q-method and associated covariance analysis is derived with the inclusion of an a priori attitude estimate. The non-attitude states are updated from the nonlinear attitude estimate based on linear optimal Kalman filter techniques. The proposed filter is compared to existing methods and is shown to be equivalent to second-order in the attitude update and exactly equivalent in the non-attitude state update with the Sequential Optimal Attitude Recursion filter. Monte Carlo analysis is used in numerical simulations to demonstrate the validity of the proposed approach. This filter successfully estimates the nonlinear attitude and non-attitude states in a single Kalman filter without the need for iterations.Item Synergistic Behavior of Tubes, Junctions, and Sheets Imparts Mechano-Mutable Functionality in 3D Porous Boron Nitride Nanostructures(American Chemical Society, 2014) Sakhavand, Navid; Shahsavari, Rouzbeh; Richard E. Smalley Institute for Nanoscale Science and TechnologyOne-dimensional (1D) boron nitride nanotube (BNNT) and 2D hexagonal BN (h-BN) are attractive for demonstrating fundamental physics and promising applications in nano-/microscale devices. However, there is a high anisotropy associated with these BN allotropes as their excellent properties are either along the tube axis or in-plane directions, posing an obstacle in their widespread use in technological and industrial applications. Herein, we report a series of 3D BN prototypes, namely, pillared boron nitride (PBN), by fusing single-wall BNNT and monolayer h-BN aimed at filling this gap. We use density functional theory and molecular dynamics simulations to probe the diverse mechanomutable properties of PBN prototypes. Our results demonstrate that the synergistic effect of the tubes, junctions, and sheets imparts cooperative deformation mechanisms, which overcome the intrinsic limitations of the PBN constituents and provide a number of superior characteristics including 3D balance of strength and toughness, emergence of negative Poisson's ratio, and elimination of strain softening along the armchair orientation. These features, combined with the ultrahigh surface area and lightweight structure, render PBN as a 3D multifunctional template for applications in graphene-based nanoelectronics, optoelectronics, gas storage, and functional composites with fascinating in-plane and out-of-plane tailorable properties.Item Universal composition-structure-property maps for natural and biomimetic platelet-matrix composites and stacked heterostructures(Nature Publishing Group, 2015) Sakhavand, Navid; Shahsavari, Rouzbeh; Smalley Institute for Nanoscale Science and TechnologyMany natural and biomimetic platelet–matrix composites—such as nacre, silk, and clay-polymer—exhibit a remarkable balance of strength, toughness and/or stiffness, which call for a universal measure to quantify this outstanding feature given the structure and material characteristics of the constituents. Analogously, there is an urgent need to quantify the mechanics of emerging electronic and photonic systems such as stacked heterostructures. Here we report the development of a unified framework to construct universal composition–structure–property diagrams that decode the interplay between various geometries and inherent material features in both platelet–matrix composites and stacked heterostructures. We study the effects of elastic and elastic-perfectly plastic matrices, overlap offset ratio and the competing mechanisms of platelet versus matrix failures. Validated by several 3D-printed specimens and a wide range of natural and synthetic materials across scales, the proposed universally valid diagrams have important implications for science-based engineering of numerous platelet–matrix composites and stacked heterostructures.