Enhanced gradient crystal-plasticity study of size effects in a β-titanium alloy

A calibrated model of enhanced strain-gradient crystal plasticity is proposed, which is shown to characterize adequately deformation behaviour of b.c.c. single crystals of a β-Ti alloy (Ti-15-3-3-3). In this model, in addition to strain gradients evolving in the course of deformation, incipient strain gradients, related to a component's surface-to-volume ratio, is accounted for. Predictive capabilities of the model in characterizing a size effect in an initial yield and a work-hardening rate in small-scale components is demonstrated. The characteristic length-scale, i.e. the component's dimensions below which the size effect is observed, was found to depend on densities of polar and statistical dislocations and interaction between them

Reaction and Growth Mechanisms in Al₂O₃ deposited via Atomic Layer Deposition: Elucidating the Hydrogen Source

In this work, we have quantitatively elucidated the source of the hydrogen content in the atomic layer deposition of Al₂O₃ at different temperatures (80–220 °C), by replacing the H₂O precursor with heavy water (D₂O) to use as a tracer and discern between the H coming from the unreacted metal precursor ligands and that from the unreacted −OD (hydroxyl) groups coming from the (heavy) water. The main source of impurities arises from the unreacted hydroxyl groups (−OD), reaching ∼18 atom % of deuterium at a deposition temperature of 80 °C. Reconsidering carefully our own and literature experimental data, we concluded that the generally accepted mechanism of steric hindering by monodentate Al­(CH₃)₂ adsorbates (dimethylaluminum) cannot be solely responsible for the retention of hydroxyls during atomic layer deposition (ALD). On this regard, we propose two additional mechanisms that can lead to sterically hinder hydroxyl groups which will then remain unreacted in the film: surface rehydroxylation resulting in the reconfiguration of bidentate or tridentate adsorbates into monodentate adsorbates and hindered subsurface hydroxyl groups during the (heavy) water pulse and the hydroxylation of sterically hindered dissociated methyl chemisorbed species. Based on these three steric hindrance mechanisms, we constructed a growth model that consists of the initial chemisorption configurations of trimethyl-aluminum molecules with the alumina surface and the subsequent reconfiguration of the resulting adsorbates into a monodentate configuration that consequently leads to sterically hindered hydroxyl groups. The fraction of AlOx adsorbates arranged in monodentate and bidentate configurations entails a specific number of O/Al atoms and unreacted hydroxyl groups inside the film. This model was able to explain the deuterium content, the O/Al ratio, and the density obtained from Rutherford back-scattering and heavy ion elastic recoil detection analysis measurements. Furthermore, this model was able to predict more accurately the growth per cycle to what has been reported to be the ALD window of alumina. Our findings will spur further detailed investigations of the reaction and growth modes in ALD films

Compression of Nanowires Using a Flat Indenter: Diametrical Elasticity Measurement

A new experimental approach for the characterization of the diametrical elastic modulus of individual nanowires is proposed by implementing a micro/nanoscale diametrical compression test geometry, using a flat punch indenter. A 250 nm diameter single crystal silicon nanowire is compressed inside of a scanning electron microscope. Since silicon is highly anisotropic, the wire crystal orientation in the compression axis is determined by electron backscatter diffraction. In order to analyze the load-displacement compression data, a two-dimensional analytical closed-form solution based on a classical contact model is proposed. The results of the analytical model are compared with those of finite element simulations and to the experimental diametrical compression results and show good agreement

Compression of Nanowires Using a Flat Indenter: Diametrical Elasticity Measurement

A new experimental approach for the characterization of the diametrical elastic modulus of individual nanowires is proposed by implementing a micro/nanoscale diametrical compression test geometry, using a flat punch indenter. A 250 nm diameter single crystal silicon nanowire is compressed inside of a scanning electron microscope. Since silicon is highly anisotropic, the wire crystal orientation in the compression axis is determined by electron backscatter diffraction. In order to analyze the load-displacement compression data, a two-dimensional analytical closed-form solution based on a classical contact model is proposed. The results of the analytical model are compared with those of finite element simulations and to the experimental diametrical compression results and show good agreement

Combinatorial Reactive Sputtering with Auger Parameter Analysis Enables Synthesis of Wurtzite Zn₂TaN₃

The discovery of new functional materials is one of the key challenges in materials science. Combinatorial high-throughput approaches using reactive sputtering are commonly employed to screen unexplored phase spaces. During reactive combinatorial deposition, the process conditions are rarely optimized, which can lead to poor crystallinity of thin films. In addition, sputtering at shallow deposition angles can lead to off-axis preferential orientation of the grains. This can make the results from a conventional structural phase screening ambiguous. Here, we perform a combinatorial screening of the Zn–Ta–N phase space with the aim to synthesize the novel semiconductor Zn2TaN3. While the results of the X-ray diffraction (XRD) phase screening are inconclusive, including Auger parameter analysis in our workflow allows us to see a very clear discontinuity in the evolution of the Ta binding environment. This is indicative of the formation of a new ternary phase. In additional experiments, we isolate the material and perform a detailed characterization confirming the formation of single-phase wurtzite Zn2TaN3. Besides the formation of the new ternary nitride, we map the functional properties of ZnxTa1–xN and report previously unreported clean chemical state analysis for Zn3N2, TaN, and Zn2TaN3. Overall, the results of this study showcase common challenges in high-throughput materials screening and highlight the merit of employing characterization techniques sensitive toward changes in the materials’ short-range order and chemical state

Understanding and Controlling Nucleation and Growth of TiO₂ Deposited on Multiwalled Carbon Nanotubes by Atomic Layer Deposition

Controlled deposition of thin conformal oxide films on carbon nanotubes (CNTs) by atomic layer deposition (ALD) for applications in solar energy and photocatalysis is still challenging, as the early stages of nucleation and subsequent growth are not yet well understood. In this work, we employed ALD to grow TiO₂ on multiwalled carbon nanotubes (MW-CNTs). The effects of deposition temperature (120–240 °C), number of ALD cycles (20–750), and surface pretreatment of the MW-CNTs with oxygen plasma on the morphology and crystallinity of the TiO₂ were systematically studied using transmission electron microscopy (TEM). By tuning the deposition conditions, controllable nucleation and growth of TiO₂ on CNTs was achieved. In particular, high-quality crystalline anatase conforming to the CNTs was obtained with an ALD growth temperature as low as 200 °C. Direct observation using aberration-corrected atomic-resolution TEM imaging at 120 keV revealed an island structure of crystalline TiO_<i>x</i> at the very early stage of nucleation, followed by coalesced growth of crystalline anatase at this temperature. The study also paves the way to understand the interface between the two materials on an atomic level

Atomic Layer Deposition of Titanium Oxide on Single-Layer Graphene: An Atomic-Scale Study toward Understanding Nucleation and Growth

Controlled synthesis of a hybrid nanomaterial based on titanium oxide and single-layer graphene (SLG) using atomic layer deposition (ALD) is reported here. The morphology and crystallinity of the oxide layer on SLG can be tuned mainly with the deposition temperature, achieving either a uniform amorphous layer at 60 °C or ∼2 nm individual nanocrystals on the SLG at 200 °C after only 20 ALD cycles. A continuous and uniform amorphous layer formed on the SLG after 180 cycles at 60 °C can be converted to a polycrystalline layer containing domains of anatase TiO₂ after a postdeposition annealing at 400 °C under vacuum. Using aberration-corrected transmission electron microscopy (AC-TEM), characterization of the structure and chemistry was performed on an atomic scale and provided insight into understanding the nucleation and growth. AC-TEM imaging and electron energy loss spectroscopy revealed that rocksalt TiO nanocrystals were occasionally formed at the early stage of nucleation after only 20 ALD cycles. Understanding and controlling nucleation and growth of the hybrid nanomaterial are crucial to achieving novel properties and enhanced performance for a wide range of applications that exploit the synergetic functionalities of the ensemble

Nonaqueous Sol–Gel Synthesis of Anatase Nanoparticles and Their Electrophoretic Deposition in Porous Alumina

Titanium dioxide (TiO₂) nanoparticles were synthesized by nonaqueous sol–gel route using titanium tetrachloride and benzyl alcohol as the solvent. The obtained 4 nm-sized anatase nanocrystals were readily dispersible in various polar solvents allowing for simple preparation of colloidal dispersions in water, isopropyl alcohol, dimethyl sulfoxide, and ethanol. Results showed that dispersed nanoparticles have acidic properties and exhibit positive zeta-potential which is suitable for their deposition by cathodic electrophoresis. Aluminum substrates were anodized in phosphoric acid in order to produce porous anodic oxide layers with pores ranging from 160 to 320 nm. The resulting nanopores were then filled with TiO₂ nanoparticles by electrophoretic deposition. The influence of the solvent, the electric field, and the morphological characteristics of the alumina layer (i.e., barrier layer and porosity) were studied

Microscale Fracture Behavior of Single Crystal Silicon Beams at Elevated Temperatures

The micromechanical fracture behavior of Si [100] was investigated as a function of temperature in the scanning electron microscope with a nanoindenter. A gradual increase in <i>K</i>_C was observed with temperature, in contrast to sharp transitions reported earlier for macro-Si. A transition in cracking mechanism via crack branching occurs at ∼300 °C accompanied by multiple load drops. This reveals that onset of small-scale plasticity plays an important role in the brittle-to-ductile transition of miniaturized Si