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Microscale electrodes, on the order of 10-100 μm, are rapidly becoming critical tools for neuroscience and brain-machine interfaces (BMIs) for their high channel counts and spatial resolution, yet the mechanical details of how probes at this scale insert into brain tissue are largely unknown. Here, we performed quantitative measurements of the force and compression mechanics together with real-time microscopy for in vivo insertion of a systematic series of microelectrode probes as a function of diameter (7.5–100 μm and rectangular Neuropixels) and tip geometry (flat, angled, and electrochemically sharpened). Results elucidated the role of tip geometry, surface forces, and mechanical scaling with diameter. Surprisingly, the insertion force post-pia penetration was constant with distance and did not depend on tip shape. Real-time microscopy revealed that at small enough lengthscales (

Metal structures on insulators are essential components in advanced electronic and nanooptical systems. Their electronic and optical properties are closely tied to their crystal quality, due to the strong dependence of carrier transport and band structure on defects and grain boundaries. Here we report a method for creating patterned single-crystal metal microstructures on amorphous insulating substrates, using liquid phase epitaxy. In this process, the patterned metal microstructures are encapsulated in an insulating crucible, together with a small seed of a differing material. The system is heated to temperatures above the metal melting point, followed by cooling and metal crystallization. During the heating process, the metal and seed form a high-melting-point solid solution, which directs liquid epitaxial metal growth. High yield of single-crystal metal with different sizes is confirmed with electron backscatter diffraction images, after removing the insulating crucible. Unexpectedly, the metal microstructures crystallize with the [Formula: see text] direction normal to the plane of the film. This platform technology will enable the large-scale integration of high-performance plasmonic and electronic nanosystems.

Large biaxial strain is a promising route to tune the functionalities of oxide thin films. However, large strain is often not fully realized due to the formation of misfit dislocations at the film/substrate interface. In this work, we examine the growth of strained ceria (CeO2) thin films on (001)-oriented single crystal yttria-stabilized zirconia (YSZ) via pulsed-laser deposition. By varying the film thickness systematically between 1 and 430 nm, we demonstrate that ultrathin ceria films are coherently strained to the YSZ substrate for thicknesses up to 2.7 nm, despite the large lattice mismatch (∼5%). The coherency is confirmed by both X-ray diffraction and high-resolution transmission electron microscopy. This thickness is several times greater than the predicted equilibrium critical thickness. Partial strain relaxation is achieved by forming semirelaxed surface islands rather than by directly nucleating dislocations. In situ reflective high-energy electron diffraction during growth confirms the transi...

This study provides new insights into the relationship between radiation-dose-dependent structural damage due to natural U and Th impurities and the anisotropic mechanical properties (Poisson’s ratio, elastic modulus and hardness) of zircon. Natural zircon samples from Sri Lanka (see Muarakami et al. in Am Mineral 76:1510–1532, 1991) and synthetic samples, covering a dose range of zero up to 6.8 × 1018 α-decays/g, have been studied by nanoindentation. Measurements along the [100] crystallographic direction and calculations, based on elastic stiffness constants determined by Ozkan (J Appl Phys 47:4772–4779, 1976), revealed a general radiation-induced decrease in stiffness (~54 %) and hardness (~48 %) and an increase in the Poisson’s ratio (~54 %) with increasing dose. Additional indentations on selected samples along the [001] allowed one to follow the amorphization process to the point that the mechanical properties are isotropic. This work shows that the radiation-dose-dependent changes of the mechanical properties of zircon can be directly correlated with the amorphous fraction as determined by previous investigations with local and global probes (Rios et al. in J Phys Condens Matter 12:2401–2412, 2000a; Farnan and Salje in J Appl Phys 89:2084–2090, 2001; Zhang and Salje in J Phys Condens Matter 13:3057–3071, 2001). The excellent agreement, revealed by the different methods, indicates a large influence of structural and even local phenomena on the macroscopic mechanical properties. Therefore, this study indicates the importance of acquiring better knowledge about the mechanical long-term stability of radiation-damaged materials.

This study provides new insights into the relation between thermally induced structural reorganization and the macroscopic mechanical properties of radiation-damaged titanite. The natural sample contains ca. 30% amorphous fraction. Low-temperature annealing affects only slightly the sample stiffness and leads to a softening resulting from the defect annihilation in crystalline regions. In the high-temperature annealing regime, amorphous domains recrystallize and this leads to further recovery of defects, reduction of interfaces, grain growth, and, in general, an increase in the long-range order. The thermally induced recrystallization is accompanied by massive dehydration leading to considerable stiffening and hardening. This interpretation of the recrystallization process in titanite based on the correlation of new results from nanoindentation and Raman-spectroscopic measurements complementing previous investigations using thermogravimetric and gas analyses by Hawthorne et al. (1991) and infrared spectroscopy by Zhang et al. (2001). The new data combined with previous work leads to a detailed description of the annealing behavior of a radiation-damaged titanite, which is a complicated process that includes dehydration and atomic-scale structural reorganization. To minimize the influence of surface phenomena on the hardness measurements, the so-called “true” hardness was used instead of the standard hardness calculation (Oliver and Pharr 1992). A comparison shows that the Oliver and Pharr method clearly underestimates the hardness.

The mechanical behavior of submicron body-centered cubic (BCC) micro-pillars is investigated by three-dimensional dislocation dynamics (DD) simulations to better understand the governing mechanisms for size dependent plasticity under uniaxial tension and pure torsion. A formula is developed to compute the incremental plastic twist due to dislocation motion in DD simulations. The DD simulations show that different dislocation microstructures are created depending on the loading conditions, which leads to different size dependent mechanical behavior. While in tension plasticity is mainly governed by the kinetics of dislocation motion controlled partly by the surface dislocation sources, plastic flow in torsion is controlled by dislocation pile-ups associated with strain gradients. The simulation results also reveal a Bauschinger effect and plastic recovery under cyclic twist, which have been observed in recent experiments.

Microscale electrodes are rapidly becoming critical tools for neuroscience and brain-machine interfaces (BMIs) for their high spatial and temporal resolution. However, the mechanics of how devices on this scale insert into brain tissue is unknown, making it difficult to balance between larger probes with higher stiffness, or smaller probes with lower damage. Measurements have been experimentally challenging due to the large deformations, rapid events, and small forces involved. Here we modified a nanoindentation force measurement system to provide the first ultra-high resolution force, distance, and temporal recordings of brain penetration as a function of microwire diameter (7.5 µm to 100 µm) and tip geometry (flat, angled, and electrosharpened). Surprisingly, both penetration force and tissue compression scaled linearly with wire diameter, rather than cross-sectional area. Linear brain compression with wire diameter strongly suggest smaller probes will cause less tissue damage upon insertion, though unexpectedly no statistical difference was observed between angled and flat tipped probes. These first of their kind measurements provide a mechanical framework for designing effective microprobe geometries while limiting mechanical damage.

A qualitative understanding of the behaviors of point defects can be established by considering atoms as hard spheres packed together to form the crystal. Crude as the hard sphere model may seem, it can be used to explain many of the observations made about point defects. In Section 4.1, we define the hard sphere radius of an atom and show its influence on the site preference of solute atoms. In Section 4.2, we use the hard sphere model to show the type of the distortions (spherically symmetric or not) in the host crystal around a solute atom. This allows us to explain why certain solutes have a much stronger solid solution hardening effect than others. We then need to go beyond the hard sphere model in order to be more quantitative. In Section 4.3, we define the Seitz radius, which is more useful than the hard sphere radius for keeping track of the volume occupied by atoms of different kinds in solid solutions. We will see that atoms often appear to take on a different radius as a solute atom in another crystal compared to the radius it takes in its own crystal. In Section 4.4, we apply elasticity theory to predict the elastic fields around a solute atom. For simplicity, the size of the point defect is shrunk to zero and is modeled as force dipoles acting on a point in an elastic medium. In Section 4.5, a more realistic model is developed, in which the solute atom is modeled as an elastic sphere to be inserted into a hole inside an elastic medium. Elastic fields arise because the initial size of the sphere is larger than the initial size of the hole. Even though many atomistic and electronic details concerning point defects are ignored, the models developed in this chapter are increasingly more quantitative and can be used to explain a large number of behaviors of point defects. Hard sphere model Hard sphere radius It is common to treat atoms in a crystal as undeformable spheres and to calculate the atomic sizes from the lattice parameters (measured using X-ray diffraction).We call this the hard sphere approach.

Here, we report the full coding sequence of rhinovirus C47 (RV-C47), obtained from a patient respiratory sample collected during an acute respiratory illness investigation in Butte County, California, in January 2017. This is the first whole-genome sequence of RV-C47 to be reported.

Three-dimensional dislocation dynamics (DD) simulations are performed to investigate the size-dependent plasticity in submicron face-centered cubic (FCC) micropillars under torsion. By using a previously implemented surface nucleation algorithm within DD, we show that the plastic behavior of FCC micropillars under torsion is strongly affected by the crystallographic orientation: In 〈110〉 oriented submicron pillars, coaxial dislocations nucleate and pile up near the axis, leading to homogeneous deformation along the pillars. In contrast, in 〈100〉 and 〈111〉 oriented pillars, heterogeneous plasticity has been observed due to the formation of localized dislocation arrays. As a result of the existence of a coaxial slip plane in 〈110〉 oriented pillars, stronger size-dependent plasticity is observed in this case compared with those in other orientations.

Three dimensional dislocation dynamics (DD) simulations are performed to investigate the governing mechanism of size dependent plastic deformation in submicron face-centered cubic (fcc) micropillars under uniaxial loading. Based on previous atomistic simulations, we introduce an algorithm for dislocation nucleation at the free surface as a function of stress and temperature in the DD simulation. The simulation results show stochastic behaviors in agreement with experimental observations, and reveal that dislocation nucleation at the free surface is the dominant mechanism of plastic flow in small pillars with diameters less than 200 nm, while the operation of truncated dislocation sources is the governing mechanism in large pillars with diameters exceeding 1 μm. In between, both mechanisms come into play in a stochastic way.

Germanium is one of the promising alloying anode (Si, Ge, Sn) materials for high capacity lithium ion batteries. Since it is isostructural with crystalline silicon, crystalline Ge is expected to show intriguing lithiation-induced phenomena similar to Si, such as anomalous volume expansion and fracture. Here, we present the study of lithiation of Ge micropillars, and we compare the findings to silicon pillar lithiation. The critical pillar diameter ∼1.2μm associated with lithiation-induced fracture of 〈111〉 Ge pillars, determined through our statistical investigation, is much greater than the critical dimension for fracture of 〈111〉 silicon pillars (∼300 nm). This larger critical size for lithiation-induced fracture of Ge likely arises from lower tensile stress concentrations at the surface due to the more inherently isotropic expansion that Ge undergoes during lithiation. Upon lithiation, Ge displays only slight anisotropy in its volume expansion, with the 〈110〉 directions exhibiting radial expansion that is only 1.1 times larger than that along 〈111〉 directions. Despite its relatively weak anisotropy in volume expansion, however, Ge pillars above the critical dimension still show anisotropic fracture, with favored fracture sites residing between the 〈110〉 directions on the pillar sidewall, similar to Si. We believe that this study provides better understanding of lithiation of Ge for designing high-density anode for Li-ion batteries.

Nanoindentation tests, spanning various length scales ranging from 200 nm to 5 μm deep, were performed on a sample of organic-rich Woodford shale in both the bedding plane normal and bedding plane parallel directions. Focused ion beam milling, scanning electron microscopy, and energy dispersive X-ray spectroscopy were utilized to characterize the shale at the scale of the nanoindentation testing as being comprised predominantly of clay and other silicate minerals suspended in a mixed organic/clay matrix. The nanoindentation tests reveal the mechanical properties of the relatively homogeneous constituent materials as well as those of the highly heterogeneous composite material. Loads on the order of a few millinewtons produced shallower indents and demonstrated the elastic–plastic deformation response of the constituent materials, whereas higher loads of as much as a few hundred millinewtons produced deeper indents revealing the response of the composite matrix. In both cases, significant creep was observed. We use nonlinear finite element modeling utilizing an isotropic critical state theory with creep to capture the indentation response by calibrating plastic material parameters to the laboratory measurements. The simulations provide a means of extracting plastic material parameters from the nanoindentation measurements and reveal the capabilities as well as limitations of an isotropic model in capturing the response of an inherently anisotropic material.

The time-independent and time-dependent mechanical behavior of electrochemically lithiated silicon was studied with nanoindentation. As indentation was performed with continuous stiffness measurements during loading and load-hold, new insight into the deformation behavior of lithiated silicon is furnished. Supporting other research, Young's modulus and the hardness of lithiated silicon are found to decline with increasing lithium content. However, the results of this study indicate that Young's modulus of the fully lithiated phase, at 41 GPa, is in fact somewhat larger than reported in some other studies. Nanoindentation creep experiments demonstrate that lithiated silicon creeps readily, with the observed viscoplastic flow governed by power law creep with large stress exponents (>20). Flow is thought to occur via local, shear-driven rearrangement at the scale of the Li15Si4 molecular unit volume. This research emphasizes the importance of incorporating viscoplasticity into lithiation/delithiation models. Additionally, more broadly, the work offers insight into nanoindentation creep methodology.

Background The rotavirus disease burden has declined substantially since rotavirus vaccine was introduced in the United States in 2006. The aim of this study was to determine the viral etiology of acute gastroenteritis (AGE) in US children aged Methods The New Vaccine Surveillance Network (NVSN) of geographically diverse US sites conducts active pediatric population-based surveillance in hospitals and emergency departments. Stool samples were collected from children aged Results Detection rates of pathogens in children with AGE versus those of HCs were, respectively, 23.0% versus 6.6% for norovirus (P < .01), 23.0% versus 16.0% for adenovirus (P = .08), 11.0% versus 16.0% for parechovirus A (P = .09), 11.0% versus 9.0% for enterovirus (P = .34), 7.0% versus 3.0% for sapovirus (P = .07), 3.0% versus 0.3% for astrovirus (P = .01), and 3.0% versus 0.4% for rotavirus (P = .01). A high prevalence of adenovirus was detected at 1 surveillance site (49.0% for children with AGE and 43.0% for HCs). Norovirus GII.4 New Orleans was the most frequently detected (33.0%) norovirus genotype. Codetection of >1 virus was more common in children with AGE (16.0%) than in HCs (10.0%) (P = .03). Conclusions Norovirus, astrovirus, sapovirus, and rotavirus were detected significantly more in children with AGE than in HCs, and norovirus was the leading AGE-causing pathogen in US children aged

Background Feeding interventions for preterm infants aim to reduce the physiologic stress of feeding to promote growth. Heart rate variability (HRV) is a potential noninvasive measure of physiologic stress that may be useful for evaluating efficacy of feeding interventions. Purpose To evaluate whether HRV is a sensitive measure of physiologic stress compared with standard physiologic outcomes in the context of a feeding intervention study. Methods This was a secondary analysis of a within-subjects, cross-over design study comparing usual care feeding with a gentle, coregulated (CoReg) feeding approach in 14 infants born less than 35 weeks' postmenstrual age. HRV indices were calculated from electrocardiogram data and compared with standard physiologic outcomes, including oxygen saturation (Spo2), respiratory rate (RR), apnea, heart rate (HR), and bradycardia. Data were analyzed using linear mixed modeling. Results Infants fed using the CoReg approach had fewer apneic events and higher RR, suggesting they were able to breathe more during feeding. No statistically significant differences were found in SpO2, HR, bradycardia, or high frequency power (the most commonly reported measure of HRV). Infants fed using the usual care approach had significantly higher SD12, a measure of HRV indicating randomness in the HR, which is a potential indicator of elevated stress. Implications for practice SD12 was more sensitive to stress than SpO2, HR, and bradycardia. The utility of HRV as a measure of feeding outcomes in clinical practice needs further exploration. Implications for research Further exploration of HRV as an intervention outcome measure is needed, particularly evaluating nonlinear indices, such as SD12.

Recent research on the electrochemical lithiation of amorphous silicon nanoparticles shows that amorphous silicon is more fracture resistant than crystalline silicon during lithiation. Nanoparticles of amorphous silicon can be lithiated and delithiated without any fracture at all. To fully exploit the potential of using amorphous silicon as electrodes for lithium ion batteries it is important to determine if larger, micron-sized, amorphous silicon structures can be lithiated and delithiated without fracture. Here we study the morphologies of initially amorphous silicon micropillars (∼2.3 μm tall) both before and after electrochemical lithiation and delithiation. No internal or external cohesive cracking is detected in lithiated pillars for any of the pillar sizes studied. Delithiated pillars exhibit some delamination at the interface between the pillar and the underlying nickel substrate. For larger diameter pillars, the initiated interfacial crack is driven upward into the delithiated pillar as the crack propagates radially inward. However, no cohesive fracture unrelated to interfacial cracking is seen in even the largest delithiated pillars. Finite element modeling provides support for the observation that the cohesive fracture resistance of amorphous silicon micropillars is representative of the fracture resistance of amorphous silicon microparticles of comparable dimensions.

The chemical equilibrium distribution of point defects modeled as non-overlapping, spherical inclusions with purely positive dilatational eigenstrain in an isotropically elastic solid is derived. The compressive self-stress inside existing inclusions must be excluded from the stress dependence of the equilibrium concentration of the point defects, because it does no work when a new inclusion is introduced. On the other hand, a tensile image stress field must be included to satisfy the boundary conditions in a finite solid. Through the image stress, existing inclusions promote the introduction of additional inclusions. This is contrary to the prevailing approach in the literature in which the equilibrium point defect concentration depends on a homogenized stress field that includes the compressive self-stress. The shear stress field generated by the equilibrium distribution of such inclusions is proved to be proportional to the pre-existing stress field in the solid, provided that the magnitude of the latter is small, so that a solid containing an equilibrium concentration of point defects can be described by a set of effective elastic constants in the small-stress limit.

Nanoindentation high-resolution mapping has been used to probe the mechanical properties [elastoplastic factor (S2/P, where S is the contact stiffness and P is the load), indentation hardness (H), and elastic modulus (E)] of a natural, highly zoned zircon (ZrSiO4). The zoning, on a scale of ∼ 5 to 400 μm, is due to variations in the U and Th concentrations, resulting in a range of α-decay event doses of ∼ 3.7× to 7.5 × 1018 α-decays/g. Thus, this single, zoned zircon crystal can be used to investigate the effects of α-decay radiation-damage on mechanical properties. The results also illustrate how multilayered ceramics accommodate volume expansion and change in mechanical properties as a function of radiation dose. Further, the detailed investigation of fractures in the lesser damaged, higher crystalline domains provides a better understanding of crack propagation in the initially crystalline material due to the strain induced by heterogeneous damage distribution. This is an important consideration in designing materials for the immobilization of plutonium from dismantled nuclear weapons, as plutonium decays by α-decay events. The directly measurable stiffness2/load (S2/P) provides a useful estimate of the degree of radiation-damage. The evolution of E provides experimental evidence for the predicted second percolation transition that denotes the end of percolation of the crystalline fraction.

The study of grain boundaries is the foundation to understanding many of the intrinsic physical properties of bulk metals. Here, the preparation of microscale thin-film gold bicrystals, using rapid melt growth, is presented as a model system for studies of single grain boundaries. This material platform utilizes standard fabrication tools and supports the high-yield growth of thousands of bicrystals per wafer, each containing a grain boundary with a unique111tilt character. The crystal growth dynamics of the gold grains in each bicrystal are mediated by platinum gradients, which originate from the gold-platinum seeds responsible for gold crystal nucleation. This crystallization mechanism leads to a decoupling between crystal nucleation and crystal growth, and it ensures that the grain boundaries form at the middle of the gold microstructures and possess a uniform distribution of misorientation angles. It is envisioned that these bicrystals will enable the systematic study of the electrical, optical, chemical, thermal, and mechanical properties of individual grain boundary types.

Alloying anodes such as silicon are promising electrode materials for next-generation high energy density lithium-ion batteries because of their ability to reversibly incorporate a high concentration of Li atoms. However, alloying anodes usually exhibit a short cycle life due to the extreme volumetric and structural changes that occur during lithium insertion/extraction; these transformations cause mechanical fracture and exacerbate side reactions. To solve these problems, there has recently been significant attention devoted to creating silicon nanostructures that can accommodate the lithiation-induced strain and thus exhibit high Coulombic efficiency and long cycle life. In parallel, many experiments and simulations have been conducted in an effort to understand the details of volumetric expansion, fracture, mechanical stress evolution, and structural changes in silicon nanostructures. The fundamental materials knowledge gained from these studies has provided guidance for designing optimized Si electrode structures and has also shed light on the factors that control large-volume change solid-state reactions. In this paper, we review various fundamental studies that have been conducted to understand structural and volumetric changes, stress evolution, mechanical properties, and fracture behavior of nanostructured Si anodes for lithium-ion batteries and compare the reaction process of Si to other novel anode materials.

Recent micropillar experiments have shown strong size effects at small pillar diameters. This “smaller is stronger” phenomenon is widely believed to involve dislocation motion, which can be studied using dislocation dynamics (DD) simulations. In the present paper, we use a three-dimensional DD model to study the collective dislocation behavior in body-centered cubic micropillars under compression. Following the molecular dynamics (MD) simulations of Weinberger and Cai, we consider a surface-controlled cross-slip process, involving image forces and non-planar core structures, that leads to multiplication without the presence of artificial dislocation sources or pinning points. The simulations exhibit size effects and effects of initial dislocation density and strain rate on strength, which appear to be in good agreement with recent experimental results and with a simple dislocation kinetics model described here. In addition, at the high strain rates considered, plasticity is governed mainly by the kinetics of dislocation motion, not their elastic interactions.

To utilize high-capacity Si anodes in next-generation Li-ion batteries, the physical and chemical transformations during the Li-Si reaction must be better understood. Here, in situ transmission electron microscopy is used to observe the lithiation/delithiation of amorphous Si nanospheres; amorphous Si is an important anode material that has been less studied than crystalline Si. Unexpectedly, the experiments reveal that the first lithiation occurs via a two-phase mechanism, which is contrary to previous understanding and has important consequences for mechanical stress evolution during lithiation. On the basis of kinetics measurements, this behavior is suggested to be due to the rate-limiting effect of Si-Si bond breaking. In addition, the results show that amorphous Si has more favorable kinetics and fracture behavior when reacting with Li than does crystalline Si, making it advantageous to use in battery electrodes. Amorphous spheres up to 870 nm in diameter do not fracture upon lithiation; this is much larger than the 150 nm critical fracture diameter previously identified for crystalline Si spheres.

Large biaxial strain is a promising route to tune the functionalities of oxide thin films. However, large strain is often not fully realized due to the formation of misfit dislocations at the film/substrate interface. In this work, we examine the growth of strained ceria (CeO

The high theoretical specific capacity of Si as an anode material is attractive in lithium-ion batteries, al- though the issues caused by large volume changes during cycling have been a major challenge. Efforts have been de- voted to understanding how diffusion-induced stresses cause fracture, but recent observations of anisotropic volume expansion in single-crystalline Si nanostructures re- quire new theoretical considerations of expansion behavior during lithiation. Further experimental investigation is also necessary to better understand the anisotropy of the lithia- tion process. Here, we present a method to reveal the crys- talline core of partially lithiated Si nanopillars with three dif- ferent crystallographic orientations by using methanol to dissolve the Li atoms from the amorphous Li-Si alloy. The exposed crystalline cores have flat {110} surfaces at the pillar sidewalls; these surfaces represent the position of the reaction front between the crystalline core and the amor- phous Li-Si alloy. It was also found that an amorphous Si structure remained on the flat surfaces of the crystalline core after dissolution of the Li, which was presumed to be caused by the accumulation of Si atoms left over from the removal of Li from the Li-Si alloy.

In situ transmission electron microscopy (TEM) is used to study the electrochemical lithiation of high-capacity crystalline Si nanoparticles for use in Li-ion battery anodes. The lithiation reaction slows down as it progresses into the particle interior, and analysis suggests that this behavior is due not to diffusion limitation but instead to the influence of mechanical stress on the driving force for reaction.

Recent micropillar compression tests of fcc and bcc single crystals have shown that ‘Smaller is Stronger’ even in the absence of significant strain gradients, an effect that is empirically characterised by a power-law relation. When a micropillar contains a dislocation network, this power-law relation has been explained in terms of the size-dependent operation stress of the weakest single arm dislocation sources. This single arm dislocation source model has successfully captured the power-law relation for the strength of a few fcc micropillars, but a physical interpretation has not been made by comparing different materials. We applied the model, not only to fcc but also to bcc micropillars, to understand quantitatively why different materials have different power-law exponents. Here, the different power-law exponents are interpreted by comparing material parameters that are size-independent properties. Also, by rearranging these parameters such that the formulation becomes independent of material paramet...

We use a unique transmission electron microscope (TEM) technique to show that Si nanowires (NWs) with diameters in the range of a few hundred nanometers can be fully lithiated and delithiated without fracture, in spite of the large volume changes that occur in this process. By analyzing the stresses associated with lithiation and delithiation we conclude that the process does not occur by the growth of discrete crystalline phases; rather it occurs by amorphization of the Si NWs followed by diffusion of Li into the structure. By accounting for the large deformation associated with this process and by including the effects of pressure gradients on the diffusion of Li, we show that Si NWs with diameters less than about 300 nm could not fracture even if pre-existing cracks were present in the NW. These predictions appear to be in good agreement with the experiment.

With its high specific capacity, silicon is a promising anode material for high-energy lithium-ion batteries, but volume expansion and fracture during lithium reaction have prevented implementation. Si nanostructures have shown resistance to fracture during cycling, but the critical effects of nanostructure size and native surface oxide on volume expansion and cycling performance are not understood. Here, we use an ex situ transmission electron microscopy technique to observe the same Si nanowires before and after lithiation and have discovered the impacts of size and surface oxide on volume expansion. For nanowires with native SiO(2), the surface oxide can suppress the volume expansion during lithiation for nanowires with diameters∼50 nm. Finite element modeling shows that the oxide layer can induce compressive hydrostatic stress that could act to limit the extent of lithiation. The understanding developed herein of how volume expansion and extent of lithiation can depend on nanomaterial structure is important for the improvement of Si-based anodes.

Single-crystal Au microparticles on a sapphire substrate were deformed under compression. Most of microparticles yield with a large strain burst, and there is a strong dependence of the yield strength on microparticle size. With the help of molecular dynamics simulations and finite-element analysis we conclude that the deformation is dislocation nucleation-controlled and that the stress levels reached at the onset of plasticity approach the theoretical shear strengths of Au. A significant size effect is identified in both the experimentally measured and computed strength of the microparticles; the smaller microparticles yield at higher compressive stresses. We propose a stress-gradient nucleation model relating this size effect to stress gradients along the slip plane.

We have studied the effects of focused-ion-beam (FIB) irradiation and prestraining on the mechanical properties of nearly defect-free Au microparticles on a sapphire substrate. The Au microparticles, which were produced by a solid-state diffusion dewetting technique, were FIB-irradiated and/or prestrained, the latter using a nanoindenter with a flat ended punch operating under a nanohammering mode. Also, the prestrained Au microparticles were exposed to FIB to examine the effects of ion-beam damage on the properties of crystals containing mobile dislocations. We found that both FIB irradiation and prestraining reduced the yield strength of pristine Au microparticles significantly and made the stress–strain curves jerky. However, FIB irradiation does not affect the mechanical properties of prestrained Au microparticles very significantly. Once a microparticle contains mobile dislocations, its mechanical properties are not influenced much by the defects generated by FIB irradiation, even at the submicrometer scale.

Silicon is a promising candidate for the anode material in lithium-ion batteries due to its high theoretical specific capacity. However, volume changes during cycling cause pulverization and capacity fade, and improving cycle life is a major research challenge. Here, we report a novel interconnected Si hollow nanosphere electrode that is capable of accommodating large volume changes without pulverization during cycling. We achieved the high initial discharge capacity of 2725 mAh g(-1) with less than 8% capacity degradation every hundred cycles for 700 total cycles. Si hollow sphere electrodes also show a Coulombic efficiency of 99.5% in later cycles. Superior rate capability is demonstrated and attributed to fast lithium diffusion in the interconnected Si hollow structure.

A previously reported method for extracting the thin film hardness from nanoindentation into a film on an elastically mismatched substrate was applied to four different cases of extreme mismatch in elastic properties: Parmax, Ultem, Polysulfone and Perfluorocyclobutyl polymer thin films on Si substrates. All of these cases represent extremely compliant films on a stiff substrate, where the ratio of film shear modulus to substrate shear modulus ranged from 0.008 to 0.036. Analyzing the nanoindentation data into these film/substrate systems poses a significant limitation when using the Oliver and Pharr method as the hardness increases rapidly with indentation depth. Therefore, a method involving the measured contact stiffnesses to more accurately determine the correct contact areas was used to extract the true hardness of the polymer thin films. The results indicate that our method is able to remove the substrate effects as well as the complications arising from pile-up and surface roughness to yield a wide plateau in hardness despite the extreme elastic mismatch conditions.

A model of micro-pillar plasticity controlled by the nucleation of dislocations at free surfaces was developed using methods of dislocation dynamics innovated by Johnston and Gilman 50 years ago. It is shown that the stress dependence of the rate of nucleation can be related to the dependence of the flow strength on the diameter of the pillar. The steady state flow stress in this model depends only on the kinetics of nucleation, whereas the transient deformation behavior depends on the initial dislocation density and the dislocation mobility, as well as the rate of nucleation. The model can describe not only the dependence of the strength on pillar diameter, but also recent experiments on fcc gold crystals, wherein pre-straining leads to softening and annealing leads to hardening. The model does not account for the stochastic, jerky nature of plastic flow in micro-pillars and is not meant to apply to pillars large enough to support substructure formation.

A size effect study was conducted on [0 0 1] oriented vanadium nanopillars that were synthesized from both a thin film and a bulk crystal. The results indicate that a size-dependent deformation behavior exists for vanadium; the smaller nanopillars displayed discrete strain bursts and higher stresses during deformation. The size effect exponent is found to be 0.79, and the results are compared with previous reports on other body-centered cubic (bcc) metals: Nb, Ta, Mo and W.

Background In May 2018, an outbreak of enterovirus A71 (EV-A71) neurologic disease was detected at Children’s Hospital Colorado (CHCO) prompting a public health investigation. We characterized clinical, laboratory, and radiologic findings during this outbreak. Methods A case was defined as meningitis, encephalitis, or acute flaccid myelitis with EV-A71 identified from a biologic specimen in a child examined at CHCO after March 1, 2018. Biologic specimens from children with neurologic disease and EV identified by clinical reverse-transcription polymerase chain reaction (RT-PCR) were typed by VP1 sequencing at CDC. Results As of July 20, 2018, 28 cases of EV-A71 neurologic disease were identified. This report describes the clinical, laboratory, and radiologic findings for the first 13 children identified with EV-A71 neurologic disease, for whom complete information is available. The median age was 13 months (range = 10 days–35 months) and 11 (85%) were male. Neurologic presentations included 12 (92%) with meningitis, 9 (69%) with encephalitis, and 3 (23%) with acute flaccid myelitis (AFM). All 13 children had fever and irritability; 3 (23%) had hand, foot, and mouth disease. Neurologic signs included encephalopathy (n = 7, 54%), ataxia (n = 7, 54%), myoclonus (n = 6, 46%), limb weakness (n = 4, 31%), cranial nerve deficits (n = 2, 15%), and seizures (n = 1, 8%). Nine (90%) of 10 children with cerebrospinal fluid (CSF) specimen analyzed had a pleocytosis (>5 white blood cells/uL); 6 of 8 (75%) children who had brain imaging showed abnormalities, with 5 (63%) in the brainstem, 3 (38%) in the cerebellum, and 3 (38%) in the spinal cord. All 13 children had EV-A71 identified in nasopharyngeal, pharyngeal, or fecal specimens; only 2 of 11 (18%) tested had EV identified in CSF. All 13 children were hospitalized and 4 (31%) required intensive care. The 3 (23%) children with AFM had residual limb weakness at time of discharge. All children survived. Conclusion EV-A71 should be considered when children present with myoclonus, ataxia, or limb weakness in the setting of a febrile illness. Testing of nonsterile sites (respiratory, pharyngeal, or fecal) should be considered when CNS disease associated with EV is suspected and initial CSF testing is negative. Disclosures All authors: No reported disclosures.

Two in literature predicted percolation transitions in radiation-damaged zircon (ZrSiO4) were observed experimentally by measurement of the indentation hardness as a function of density and their correlation with the elastic moduli. Percolations occur near 30% and 70% amorphous fractions, where hardness deviates from its linear correlation with the elastic modulus (E), the shear modulus (G) and the bulk modulus (K). The first percolation point pc1 generates a cusp in the hardness versus density evolution, while the second percolation point is seen as a change of slope.

A new function that describes the shape of three-sided pyramidal indenters is introduced. This function differs from the polynomial tip shape function conventionally used in nanoindentation tip shape calibration in that the new function includes fewer fitting parameters with clearer physical meaning. Specifically, two of the fitting parameters integrated into the new function are the indenter’s tip radius and the slope of the indenter’s equivalent cone. Tip shape calibration data were collected with four different diamond indenter tips, and both the new function and the standard polynomial function were fit to the data. It is found that the new function can fit calibration data nearly as well as the standard polynomial function and better than existing physically based functions. Although the tip radius parameter obtained by fitting the new model to data deviates from the tip radius determined from Hertzian elastic contact, the two values are correlated.

The Young's modulus (E) of a material is a key parameter for mechanical engineering design. Silicon, the most common single material used in microelectromechanical systems (MEMS), is an anisotropic crystalline material whose material properties depend on orientation relative to the crystal lattice. This fact means that the correct value of E for analyzing two different designs in silicon may differ by up to 45%. However, perhaps, because of the perceived complexity of the subject, many researchers oversimplify silicon elastic behavior and use inaccurate values for design and analysis. This paper presents the best known elasticity data for silicon, both in depth and in a summary form, so that it may be readily accessible to MEMS designers.

A geometrical analysis of 3D dislocation dynamics simulations of FCC micro-pillar plasticity shows that the insertion of jogged dislocations before relaxation or enabling cross-slip during plastic flow are prerequisites for the formation of potentially strong natural pinning points and single arm dislocation sources. Absent these conditions, we argue that mobile dislocation starvation will occur naturally in the course of plastic flow and lead to a state in which plastic flow is controlled by the nucleation of dislocations, most likely at the free surfaces.

Multilayer thin films with bilayer thicknesses in the nanometer range have been reported to have very high strengths. A previous study has shown that Al–Al3Sc multilayers, with bilayer thicknesses as small as 6 nm, have hardnesses as high as � 3 GPa as measured by sharp tip nanoindentation. In the present study, we have avoided some of the complications associated with sharp tip nanoindentation by directly measuring the yield strengths and strain hardening/softening properties of Al–Al3Sc multilayers using microcompression testing methods with a nanoindenter. The results show the expected trend of increasing yield strength with decreasing bilayer thickness, and compare favorably with estimates of the yield strengths based on sharp tip nanoindentation. During deformation, the Al–Al3Sc multilayer pillars with smaller bilayer spacings experience considerable strain softening, resulting in a ‘‘flat-top mushroom” shape after deformation. We have developed a numerical model to account for this inhomogeneous deformation behavior and to calculate stress–strain relationships during strain softening. A new transmission electron microscopy study of a deformed pillar shows that the softening is a result of destruction of the layered structure due to shearing and rotation. 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The size-dependent strength of face-centered cubic (fcc) metals, as revealed by uniaxial compression of nanopillars, suggests that plasticity is dislocation source-controlled, with fewer sources in smaller pillars producing a “smaller is stronger” effect. To further investigate this phenomenon we have studied the effects of prestraining and annealing on the deformation properties of [0 0 1] Au nanopillars. By making pillars from an epitaxial film of [0 0 1] Au on [0 0 1] MgO, using focused ion beam machining, we are able to create both puck-shaped pillars that can be stably prestrained and pillars with a high aspect ratio, which can be tested in uniaxial compression. We find that prestraining dramatically reduces the flow strength of nanopillars while annealing restores the strength to the pristine levels. These unusual effects are not seen in bulk fcc metals, which behave in the opposite way. We discuss their possible causes in terms of dislocation densities using transmission electron microscopy.

In our previous paper [Feng et al., Acta Mater.55, 2929 (2007)], an analytical model is proposed to estimate the stress field around an elastoplastic indentation/contact, matching nicely with the finite element analysis. The model is related to an embedded center of dilatation (ECD) in a half-space. In this paper, we focus on determining the ECD strength B* and the ECD depth ξ. By matching an expanding cavity model and the ECD model, we find that B* ≈ Yc3/6 and ξ ≈ 0.4c, where Y is the yield strength and c is the plastic zone radius. We provide a method to predict Y, c, and thereby B* as well as ξ through nanoindentation data, and we also demonstrate that pileup is the physical reason for the existence of the upper limit for the ratio of hardness to Y. Thus, our ECD model is completed by combining our previous paper (the analytical expression) and this paper (the essential parameters).

A novel physical model and a simulation algorithm are used to predict electromigration (EM)-induced stress evolution in dual inlaid copper interconnects. The aim of the current simulation was to investigate the dual effect of the microstructure, which consists of the effect of grain boundaries (GBs) and the effect of texture-related variations of the modulus of elasticity on the stress evolution in copper lines caused by EM. The major difference between our approach and the previously described ones is the accounting of additional stress generated by the plated atoms. The results of the numerical simulation have been proven experimentally by EM degradation studies on fully embedded dual inlaid copper interconnect test structures and by subsequent microstructure analysis, mainly based on electron backscatter diffraction (EBSD) data. The virtual EM-induced void formation, movement, and growth in a copper interconnect were continuously monitored in an in situ scanning electron microscopy experiment. The copper microstructure, particularly the orientation of grains and GBs, was determined with EBSD. For interconnects with interfaces that resist atomic transport and where GBs are the important pathways for atom migration, degradation and failure processes are completely different for microstructures with randomly oriented GBs compared with ldquobamboolikerdquo microstructures. The correspondence between simulation results and experimental data indicates the applicability of the developed model for optimization of the physical and electrical design rules.

The storage and loss stiffnesses for the composite response of the sample, indenter, and load frame during dynamic nanoindentation are derived. In the first part of the analysis, no physical model is assigned to the composite system. It is shown that this case is equivalent to the conventional nanoindentation analysis. In the second part of the analysis, the sample is modeled as a standard linear solid in series with the indenter and load frame. The results for the storage and loss stiffnesses as computed by the two methods differ by at most ∼3% for the elastomeric system under consideration. Results for the storage and loss moduli are also similar. The relative merits and weaknesses of each analysis are discussed.