Unifying the size effect observed in micropillar compression experiments

Micropillar compression experiments show size effect, σp/μ = A(d/b)n, where σp, is the flow stress, μ is the resolved shear modulus, d is the pillar diameter and b is Burgers’ vector. With fcc metals n ≈ –0.67 and A ≈ 0.7; however, with bcc metals there is greater variation, with n closer to zero. Here we propose a different but similar empirical relation of σp/μ = σb/μ + A’(d/b)n’, where σb is a size independent resistance stress. In which case there must be a strong correlation between the original constants, A and n. This hypothesis is found to be true for the published data from a large number of bcc metals, ionic solids that possess the rock salt crystal structure, and some covalent bonded semiconductors. This correlation is shown to predict a universal power law with the exponent in the range, −1.0 n’ A’ close to 1. These values are very similar to the empirical relation that can be used to describe the behaviour of fcc metals tested in micropillar compression with σb = 0. This universality of the empirical relation provides strong evidence for a common mechanism for the micropillar size effect across a range of materials.</p

Graphene-Based Transparent Flexible Strain Gauges with Tunable Sensitivity and Strain Range

Monolayers of graphene oxide, assembled into densely packed sheets at an immiscible hexane/water interface, form transparent conducting films on polydimethylsiloxane membranes after reduction in hydroiodic acid (HI) vapor to reduced graphene oxide (rGO). Prestraining and relaxing the membranes introduces cracks in the rGO film. Subsequent straining opens these cracks and induces piezoresistivity, enabling their application as transparent strain gauges. The sensitivity and strain range of these gauges is controlled by the cracked film structure that is determined by the reducing conditions used in manufacture. Reduction for 30 s in HI vapor leads to an array of parallel cracks that do not individually span the membrane. These cracks do not extend on subsequent straining, leading to a gauge with a usable strain range >0.2 and gauge factor (GF) at low strains ranging from 20 to 100, depending on the prestrain applied. The GF reduces with increasing applied strain and asymptotes to about 3, for all prestrains. Reduction for 60 s leads to cracks spanning the entire membrane and an increased film resistance but a highly sensitive strain gauge, with GF ranging from 800 to 16,000. However, the usable strain range reduces to <0.01. A simple equivalent resistor model is proposed to describe the behavior of both gauge types. The gauges show a repeatable and stable response with loading frequencies >1 kHz and have been used to detect human body strains in a simple e-skin demonstration

Ductile deformation in alumina/silicon carbide nanocomposites

A transmission electron microscope study on cross sections obtained from ground and polished surfaces has revealed that ductile deformation is dominated by dislocations in alumina/silicon carbide nanocomposites containing 1, 5 and 10 vol% silicon carbide particles, and by twinning in unreinforced alumina. The dispersed silicon carbide particles in alumina/silicon carbide nanocomposites restrict the motion of dislocations. A dislocation pinning model is used to compare the possible mechanisms of deformation in alumina and the nanocomposites. Cr-fluorescence piezospectroscopy has been used to characterise the residual stress levels in the materials studied. The measured broadening of the Al2O3/Cr3+ fluorescence peak indicates a dislocation density of 7.3 - 9.7 x 1016 m-2 under the indentations in the nanocomposites, whilst the beneath indentations in alumina is 1-2 orders of magnitude smaller

Influence of Gas Phase Equilibria on the Chemical Vapor Deposition of Graphene

We have investigated the influence of gas phase chemistry on the chemical vapor deposition of graphene in a hot wall reactor. A new extended parameter space for graphene growth was defined through literature review and experimentation at low pressures (≥0.001 mbar). The deposited films were characterized by scanning electron microscopy, Raman spectroscopy, and dark field optical microscopy, with the latter showing promise as a rapid and nondestructive characterization technique for graphene films. The equilibrium gas compositions have been calculated across this parameter space. Correlations between the graphene films grown and prevalent species in the equilibrium gas phase revealed that deposition conditions associated with a high acetylene equilibrium concentration lead to good quality graphene deposition, and conditions that stabilize large hydrocarbon molecules in the gas phase result in films with multiple defects. The transition between lobed and hexagonal graphene islands was found to be linked to the concentration of the monatomic hydrogen radical, with low concentrations associated with hexagonal islands

Structural, Mechanical, Imaging and in Vitro Evaluation of the Combined Effect of Gd³⁺ and Dy³⁺ in the ZrO₂–SiO₂ Binary System

Mechanical strength and biocompatibility are considered the main prerequisites for materials in total hip replacement or joint prosthesis. Noninvasive surgical procedures are necessary to monitor the performance of a medical device in vivo after implantation. To this aim, simultaneous Gd³⁺ and Dy³⁺ additions to the ZrO₂–SiO₂ binary system were investigated. The results demonstrate the effective role of Gd³⁺ and Dy³⁺ to maintain the structural and mechanical stability of cubic zirconia (<i>c</i>-ZrO₂) up to 1400 °C, through their occupancy of ZrO₂ lattice sites. A gradual tetragonal to cubic zirconia (<i>t</i>-ZrO₂ → <i>c</i>-ZrO₂) phase transition is also observed that is dependent on the Gd³⁺ and Dy³⁺ content in the ZrO₂–SiO₂. The crystallization of either ZrSiO₄ or SiO₂ at elevated temperatures is delayed by the enhanced thermal energy consumed by the excess inclusion of Gd³⁺ and Dy³⁺ at <i>c</i>-ZrO₂ lattice. The addition of Gd³⁺ and Dy³⁺ leads to an increase in the density, elastic modulus, hardness, and toughness above that of unmodified ZrO₂–SiO₂. The multimodal imaging contrast enhancement of the Gd³⁺ and Dy³⁺ combinations were revealed through magnetic resonance imaging and computed tomography contrast imaging tests. Biocompatibility of the Gd³⁺ and Dy³⁺ dual-doped ZrO₂–SiO₂ systems was verified through in vitro biological studies

Supercapacitor Electrodes from the in Situ Reaction between Two-Dimensional Sheets of Black Phosphorus and Graphene Oxide

Two-dimensional materials show considerable promise as high surface area electrodes for energy-storage applications such as supercapacitors. A single sheet of graphene possesses a large specific surface area because of its atomically thin thickness. However, to package this area efficiently in a device, it must be confined within a finite three-dimensional volume without restacking of the sheet faces. Herein, we present a method of maintaining the high surface area through the use of a hybrid thin film in which few-layer-exfoliated black phosphorus (BP) reduces graphene oxide (GO) flakes. When the film is exposed to moisture, a redox reaction between the BP and the GO forms an interpenetrating network of reduced GO (RGO) and a liquid electrolyte of intermediate phosphorus acids H_<i>x</i>PO_<i>y</i>. The presence of the liquid H_<i>x</i>PO_<i>y</i> electrolyte in the RGO/H_<i>x</i>PO_<i>y</i> film stabilizes and preserves an open-channel structure enabling rapid ion diffusion, leading to an excellent charging rate capability (up to 500 mV s^–1 and retaining 62.3% of initial capacitance at a large current density of 50 A g^–1) when used as electrodes in supercapacitors

Two-Step Electrochemical Intercalation and Oxidation of Graphite for the Mass Production of Graphene Oxide

Conventional chemical oxidation routes for the production of graphene oxide (GO), such as the Hummers’ method, suffer from environmental and safety issues due to their use of hazardous and explosive chemicals. These issues are addressed by electrochemical oxidation methods, but such approaches typically have a low yield due to inhomogeneous oxidation. Herein we report a two-step electrochemical intercalation and oxidation approach to produce GO on the large laboratory scale (tens of grams) comprising (1) forming a stage 1 graphite intercalation compound (GIC) in concentrated sulfuric acid and (2) oxidizing and exfoliating the stage 1 GIC in an aqueous solution of 0.1 M ammonium sulfate. This two-step approach leads to GO with a high yield (>70 wt %), good quality (>90%, monolayer), and reasonable oxygen content (17.7 at. %). Moreover, the as-produced GO can be subsequently deeply reduced (3.2 at. % oxygen; C/O ratio 30.2) to yield highly conductive (54 600 S m^–1) reduced GO. Electrochemical capacitors based on the reduced GO showed an ultrahigh rate capability of up to 10 V s^–1 due to this high conductivity