| Two dimensional(2D)materials are currently at the forefront of fundamental cutting edge materials research.The pioneering work on graphene by Novoselov et al.brought significant interest in the discovery and understanding of several other 2D materials with unique properties originating from their peculiar 2D atomic layers structure.The resulting progress of research for over a decade long period has reported the synthesis of several of these materials,including phosphorene,borophene,transition metal dichalcogenides(TMDs),and hexagonal boron nitride(hBN)to name a few,which formed a hugely diverse group of materials in terms of their electrical,functional and mechanical properties.Of these,hBN boasts immense promise for various cutting edge specialized applications owing to its impressive suite of thermo-mechanical and functional properties including excellent thermal and chemical stability.In fact,it is one of the strongest electrically insulating materials,with significantly better interlayer integrity than its more popular counterpart of graphene.However,the work towards realizing its immense potential for next generation applications is still at its infancy,while several key challenges regarding the fundamental understanding of its structural and mechanical characteristics still remain under active research.2D materials are frequently considered and studied as flat single atomic layers,and in that regard their properties have been investigated extensively either along its atomic layers or perpendicular to the atomic layers.However,hBN has a multilayered form,with intra-layer covalent bonds and inter-layer van der Waals(vd W)interactions,similar to many other 2D layered materials like graphene.In practice,they are most commonly produced and employed as multilayered specimens,owing to the difficulties in obtaining isolated single atomic layers through either meticulous exfoliation technique from bulk samples or through chemical vapor deposition with careful design of substrate and control of synthesis.This becomes crucial when it comes to scaling up of the technologies in order to produce for broader commercial engineering applications,which require economic and efficient synthesis of the materials.Thus,in order to incorporate the common multilayered structure of hBN in various engineering applications,there arises a crucial need for precise characterization and understanding of the variation of its properties in different crystallographic directions corresponding to its multilayer unit cell between the conventional in-plane and out-of-plane(perfectly perpendicular to the atomic plane)direction.In this regard it becomes crucial to expand our understanding of its properties by fully incorporating the variation due to its structural anisotropy in threedimensions(3D),similar to our approach to study bulk materials,and model their properties.The occurrence of a layered structure further introduces the possibilities of having wide differences in its typical characteristics and behavior,due to various inter-layer interactions and deformation mechanisms that are unique to layered materials,like ripples,which need to be properly understood in order to achieve its practical and optimum applications.Ripples are parallel repeating wrinkle-like features that are a form of a common deformation mode in layered materials and structures in all length scales,also referred to as ripplocations in the literature.An interesting property of such out-of-plane corrugations when it comes to 2D materials like hBN is that they break the 2D symmetry of the atomic layers at the global as well as the local bonding level.This local asymmetry in structure due to ripples can have interesting consequences for hBN,for both single and multilayered structures,and can potentially open up novel routes to optimize their properties.In this context,it is of utmost importance to characterize the bonding and mechanical properties of hBN considering the effect of anisotropy and the induced-asymmetry in its structure,which is one of the crucial challenges from a practical standpoint to realize its different engineering applications.Thus,for this dissertation,the bonding and mechanical properties of multilayer hBN in context of the effects of structural anisotropy and asymmetry were studied,using a combination of state-of-the-art transmission electron microscopy(TEM)assisted characterization techniques,and large scale molecular dynamics(MD)simulations,in order to investigate three main scientific issues as discussed below.Firstly,key insights regarding characterization of the anisotropic bonding structure in hBN in different crystallographic orientations were obtained using advanced analytical TEM.Here I have used Electron energy loss spectroscopy(EELS)to probe the electron excitation energies from the core-shell to the higher energy outer shell of the material.EELS is a powerful analytical technique in a TEM that can give valuable information regarding the bonding environment,structure as well as the phase of the material.However,the use of EELS for quantitative analysis in hBN is severely restricted in absence of a thorough and robust strategy for characterizing its significant anisotropic nature,owing to the highly oriented layered bonding structure of hBN.In this study,calculation of the relative weights of π and σ peaks using Gaussian peak fitting was done showing strong correlation with the theoretically predicted trend,especially for calculations on the B-K edge.This serves to experimentally validate the use of core loss EELS as a quantitative measure of the arbitrary tilt of layered materials like hBN.These results provide crucial advance in understanding the experimental tilt-dependent core-loss EELS response of hBN.Secondly,I modelled the complete anisotropic nature of mechanical and failure properties of multilayer hBN using molecular dynamics(MD)simulations.Anisotropic strength theories,which use a polynomial function of stresses to give a scalar value in order to predict the failure point of the material,could prove to be immensely helpful to model the complete failure response of hBN without the need of a comprehensive description of the various possible failure modes of the material.A general multiaxial strength criteria was optimized in full stress-space and employed,in order to describe the anisotropic strength and failure properties of multilayer hBN,by using atomistic fracture data corresponding to both principal axes and asymmetric orientations of hBN.Several parameters were theoretically derived to interpret the different values of a failure function,which could be used to estimate the accuracy of any strength criteria quantitatively.Thus,the predictions of the optimized anisotropic strength criteria for hBN were verified and found to agree well with the atomistic failure data.Furthermore,this study illustrates a practical application strategy of the optimized failure criteria to anisotropic orientations and use it to assess the strength under complex loading conditions,and interpret the results to gain insights into the anisotropic deformation and failure behavior of multilayer hBN.Additionally,these results illustrate the suitable tests to optimize the anisotropic strength criteria effectively for other generally orthotropic materials as well,including the highly-oriented class of 2D materials.Moreover,comparative study between several failure results using the optimized strength criterion revealed significant strengthening of hBN caused by ripples in its atomic layers formed during deformation.Thirdly,I investigated further in greater depth the potential positive effect in the fracture properties of hBN due to ripples,after carrying out detailed experimental characterization of ripples in sintered hBN samples using high resolution TEM(HRTEM).The directions of the observed ripples in multilayer hBN were both along the zigzag and armchair chiralities of hBN,which were stable under ambient conditions.A novel intrinsic strengthening and toughening mechanism in hBN was revealed using detailed atomistic simulations of rippled and flat hBN in conjunction with local principal stress analysis.This novel mechanism was found to be governed by the degree of structural asymmetry in rippled hBN in the form of distortion of bond angles and bond lengths.Increasing strength(maximum stress until permanent deformation)and toughness(fracture resistance)is crucial in engineering applications for all materials,but improving strength often lowers the material’s capacity to deform without fracturing,and vice versa.This conflict passes on uniquely to layered van der Waals materials,which are generally brittle.In this study,a novel mechanism was found that can lead to simultaneous intrinsic toughening and strengthening in both mono-and multilayer hBN governed by amplified lattice distortions due to ripples,providing resistance against crack nucleation.These results demonstrate an unprecedented strategy that can overcome the conflict between strengthening and toughening in a singlephase brittle material,while improving the bond strength. |
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