Sanjit Bhowmick

A Sinusoidally Architected Helicoidal Biocomposite.

A fibrous herringbone-modified helicoidal architecture is identified within the exocuticle of an impact-resistant crustacean appendage. This previously unreported composite microstructure, which features highly textured apatite mineral templated by an alpha-chitin matrix, provides enhanced stress redistribution and energy absorption over the traditional helicoidal design under compressive loading. Nanoscale toughening mechanisms are also identified using high-load nanoindentation and in situ transmission electron microscopy picoindentation.

3D Porous Graphene by Low-Temperature Plasma Welding for Bone Implants

3D scaffolds of graphene, possessing ultra-low density, macroporous microstructure, and high yield strength and stiffness can be developed by a novel plasma welding process. The bonding between adjacent graphene sheets is investigated by molecular dynamics simulations. The high degree of biocompatibility along with high porosity and good mechanical properties makes graphene an ideal material for use as body implants.

Role of Atomic Layer Functionalization in Building Scalable Bottom-Up Assembly of Ultra-Low Density Multifunctional Three-Dimensional Nanostructures

Building three-dimensional (3D) structures from their constituent zero-, one-, and two-dimensional nanoscale building blocks in a bottom-up assembly is considered the holey grail of nanotechnology. However, fabricating such 3D nanostructures at ambient conditions still remains a challenge. Here, we demonstrate an easily scalable facile method to fabricate 3D nanostructures made up of entirely zero-dimensional silicon dioxide (SiO2) nanoparticles. By combining functional groups and vacuum filtration, we fabricate lightweight and highly structural stable 3D SiO2 materials. Further synergistic effect of material is shown by addition of a 2D material, graphene oxide (GO) as reinforcement which results in 15-fold increase in stiffness. Molecular dynamics (MD) simulations are used to understand the interaction between silane functional groups (3-aminopropyl triethoxysilane) and SiO2 nanoparticles thus confirming the reinforcement capability of GO. In addition, the material is stable under high temperature and offers a cost-effective alternative to both fire-retardant and oil absorption materials.

Optimization of clamped beam geometry for fracture toughness testing of micron-scale samples

Fracture toughness measurements at the small scale have gained prominence over the years due to the continuing miniaturization of structural systems. Measurements carried out on bulk materials cannot be extrapolated to smaller length scales either due to the complexity of the microstructure or due to the size and geometric effect. Many new geometries have been proposed for fracture property measurements at small-length scales depending on the material behaviour and the type of device used in service. In situ testing provides the necessary environment to observe fracture at these length scales so as to determine the actual failure mechanism in these systems. In this paper, several improvements are incorporated to a previously proposed geometry of bending a doubly clamped beam for fracture toughness measurements. Both monotonic and cyclic loading conditions have been imposed on the beam to study R-curve and fatigue effects. In addition to the advantages that in situ SEM-based testing offers in such tests, FEM has been used as a simulation tool to replace cumbersome and expensive experiments to optimize the geometry. A description of all the improvements made to this specific geometry of clamped beam bending to make a variety of fracture property measurements is given in this paper.

Atomically thin gallium layers from solid-melt exfoliation

A unique way to synthesize innovative 2D gallenene. Among the large number of promising two-dimensional (2D) atomic layer crystals, true metallic layers are rare. Using combined theoretical and experimental approaches, we report on the stability and successful exfoliation of atomically thin “gallenene” sheets on a silicon substrate, which has two distinct atomic arrangements along crystallographic twin directions of the parent α-gallium. With a weak interface between solid and molten phases of gallium, a solid-melt interface exfoliation technique is developed to extract these layers. Phonon dispersion calculations show that gallenene can be stabilized with bulk gallium lattice parameters. The electronic band structure of gallenene shows a combination of partially filled Dirac cone and the nonlinear dispersive band near the Fermi level, suggesting that gallenene should behave as a metallic layer. Furthermore, it is observed that the strong interaction of gallenene with other 2D semiconductors induces semiconducting to metallic phase transitions in the latter, paving the way for using gallenene as promising metallic contacts in 2D devices.

High Plastic Strain of Silica Microparticles under Electron Beam Irradiation

The studies of irradiation damage in silica are of significant interest because of its application in nuclear reactors, nuclear waste containers, optical fibers, and semiconductor devices [1,2]. Although there are a number of publication showing the effect of electrons, ions, protons, alpha-particles irradiation on microstructural changes of silica, understanding deformation behavior under applied stress of irradiated sample is still lacking [2,3]. In this work, we investigate plastic flow and failure behavior of amorphous silica particles under compressive stress inside a scanning electron microscopy (SEM).

A Combined Effect of Electron Beam and Stress on Plastic Flow of Amorphous Silica Microparticles

Radiation induced plastic flow in amorphous silica glass is an important subject in glass science and technology and have been studied for decades by many researchers using high energy ions and particles. However, the deformation behavior of such material irradiated by low energy electrons is not well understood. In comparison to heavier particles and ions, electrons have much higher penetration depths and therefore can generate uniform damage and structural changes throughout the sample. In this study, we investigate plastic flow of silica particles under a combined effect of compressive stress and electron beam inside a scanning electron microscopy. To prepare the particle samples for compression experiments, silica microparticles of diameter ~1 μm were mixed in water, ultrasonicated for 10 minutes, and dispersed on silicon substrates. In situ compression experiments were conducted using a PI 85 SEM PicoIndenter (Hysitron, Inc., Minneapolis, MN) with 5 μm flat punch diamond probe. TriboScan software was used to record and analyze load-displacement data. The load-displacement plots and real-time video of deformation were synchronized and captured during the experiment, which aided post-experimental analysis. Quasistatic compression experiments were conducted at Pmax = 0.05 mN, 1 mN and 4 mN under different beam intensities.

Mechanical Properties of Ultralow Density Graphene Oxide/Polydimethylsiloxane Foams

Low-density, highly porous graphene/graphene oxide (GO) based-foams have shown high performance in energy absorption applications, even under high compressive deformations. In general, foams are very effective as energy dissipative materials and have been widely used in many areas such as automotive, aerospace and biomedical industries. In the case of graphene-based foams, the good mechanical properties are mainly attributed to the intrinsic graphene and/or GO electronic and mechanical properties. Despite the attractive physical properties of graphene/GO based-foams, their structural and thermal stabilities are still a problem for some applications. For instance, they are easily degraded when placed in flowing solutions, either by the collapsing of their layers or just by structural disintegration into small pieces. Recently, a new and scalable synthetic approach to produce low-density 3D macroscopic GO structure interconnected with polydimethylsiloxane (PDMS) polymeric chains (pGO) was proposed. A controlled amount of PDMS is infused into the freeze-dried foam resulting into a very rigid structure with improved mechanical properties, such as tensile plasticity and toughness. The PDMS wets the graphene oxide sheets and acts like a glue bonding PDMS and GO sheets. In order to obtain further insights on mechanisms behind the enhanced mechanical pGO response we carried out fully atomistic molecular dynamics (MD) simulations. Based on MD results, we build up a structural model that can explain the experimentally observed mechanical behavior.

In Situ Study of High-Temperature Mechanical Properties of Carbon Nanotube Scaffolds

Arrays of vertically aligned carbon nanotubes (CNTs), also known as forests, turfs or scaffolds, have received significant attention recently for potential use in applications such as thermal, electrical, interface materials, interconnects, energy absorbing foams, biologically inspired adhesives, filtration structures, and composite reinforcement agents [1-4]. In this work, blocks of aligned CNTs were grown by chemical vapor deposition using ferrocene and xylene on silicon wafer substrates. Prior to the growth process, the surface of the substrate was sputter by 10 nm aluminum and 1.5 nm iron films. The density of the 3D structure was measured to be 0.13–0.32 mg/mm. The nucleation source agent, time, and temperature generally determine the dimension, morphology, density, and tortuosity of the scaffold. To conduct in situ pillar compression tests, micropillars of dimensions 15 μm x 15 μm in cross-section and 25-30 μm in height were prepared from the middle of the sample by focused ion beam (FIB).

Temperature Dependence of Fracture Initiation in Silicon from In-situ SEM

Silicon has a rich history of technological importance, as well as serving as an ideal model material for studying mechanical behavior in semi-metallic materials. Of particular interest is the rapid transition in deformation mechanisms as a function of scale and temperature, where such concepts as dislocation character1, nucleation/propagation control2 and possible core structure3 changes may contribute. The result is that under certain loading conditions, sizes, temperatures and doping, silicon can display a wide variety of response from highly brittle cleavage to over 50% plastic strain4.