Phase transformations induced by spherical indentation in ion-implanted amorphous silicon

The deformation behavior of ion-implanted (unrelaxed) and annealed ion-implanted (relaxed) amorphous silicon(a-Si) under spherical indentation at room temperature has been investigated. It has been found that the mode of deformation depends critically on both the preparation of the amorphous film and the scale of the mechanical deformation.Ex situmeasurements, such as Raman microspectroscopy and cross-sectional transmission electron microscopy, as well as in situ electrical measurements reveal the occurrence of phase transformations in all relaxed a-Si films. The preferred deformation mode of unrelaxed a-Si is plastic flow, only under certain high load conditions can this state of a-Si be forced to transform. In situ electrical measurements have revealed more detail of the transformation process during both loading and unloading. We have used ELASTICA simulations to obtain estimates of the depth of the metallic phase as a function of load, and good agreement is found with the experiment. On unloading, a clear change in electrical conductivity is observed to correlate with a “pop-out” event on load versus penetration curves

Nanoindentation-induced deformation of Ge

The deformation mechanisms of crystalline (100) Ge were studied using nanoindentation, cross sectional transmission electron microscopy (XTEM) and Raman microspectroscopy. For a wide range of indentation conditions using both spherical and pointed indenters, multiple discontinuities were found in the force–displacement curves on loading, but no discontinuities were found on unloading. Raman microspectroscopy, measured from samples which had plastically deformed on loading, showed a spectrum shift from that in pristine Ge, suggesting only residual strain. No evidence (such as extra Raman bands) was found to suggest that any pressure-induced phase transformations had occurred, despite the fact that the material had undergone severe plastic deformation.Selected area diffraction pattern studies of the mechanically damaged regions also confirmed the absence of additional phases. Moreover, XTEM showed that, at low loads, plastic deformation occurs by twinning and dislocation motion. This indicates that the hardness of Gemeasured by indentation is not primarily dominated by phase transformation, rather by the nucleation and propagation of twin bands and/or dislocations

Letter from James P. Munroe, Lexington, Massachusetts, to Anne Whitney Boston, Massachusetts, 1907 May 8

https://repository.wellesley.edu/whitney_correspondence/2804/thumbnail.jp

Plasma-sprayed nickel splats on chromium substrates: The role of substrate preheating and thermal conductivity

The effect of chromium substrate preheating on the spreading behavior of plasma-sprayed nickel splats was characterized. The interfacial features along the splat-substrate interface were analyzed by focused ion beam (FIB) microscopy and transmission electron microscopy (TEM). In addition, the transient droplet spreading process was modelled in a simulation study. The results showed that the presence of a large fraction of surface moisture on the substrates induced splat fragmentation. However, splat fragmentation was completely constrained on chromium preheated to a low temperature (373 K) where a certain amount of surface moisture was still present. The high thermal conductivity of chromium substrates increased droplet solidification rates, decreasing the interaction time between the spreading droplet and the vaporized gas layer. Accordingly, splat fragmentation was suppressed and disk-shaped splats were formed. Fast solidification not only refined the final splat microstructure, but also promoted the formation of finger-splashed disk splats. In addition, the extent of elemental diffusion across the splat-substrate interface was decreased due to the low interfacial temperature

Coral-like V<inf>2</inf>O<inf>5</inf> nanowhiskers as high-capacity cathode materials for lithium-ion batteries

Coral-like V2O5 nanowhiskers were prepared by a direct electrolytic synthesis method. The as-prepared V2O5 nanowhiskers are approximately 1 μm in length and 50-60 nm in width, which was confirmed by scanning electron microscopy and transmission electron microscopy analysis. When applied as cathode materials in lithium-ion batteries and combined with an ionic liquid electrolyte, the V2O5 nanowhiskers exhibited an initial capacity of 461 mAh g-1, which is a significant enhancement compared to commercial V2O5 powders. The high rate performance of the V2O5 nanowhiskers was further improved at an elevated working temperature of 50 °C. The V2O5 nanowhiskers demonstrated a high specific capacity and an excellent high-rate performance at elevated temperatures. © 2013 The Royal Society of Chemistry

Formation and growth of nanoindentation-induced high pressure phases in crystalline and amorphous silicon

Nanoindentation-induced formation of high pressure crystalline phases (Si-III and Si-XII) during unloading has been studied by Raman micro-spectroscopy, cross-sectional transmission electron microscopy (XTEM), and postindentation electrical measurements. For indentation in crystalline silicon(c-Si), rapid unloading (∼1000 mN∕s) results in the formation of amorphous silicon(a-Si) only; a result we have exploited to quench the formation of high pressure phases at various stages during unloading to study their formation and evolution. This reveals that seed volumes of Si-III and Si-XII form during the early stages of unloading with substantial volumes only forming after the pop-out event that occurs at about 50% of the maximum load. In contrast, high pressure phases form much more readily in an a-Si matrix, with substantial volumes forming without an observable pop-out event with rapid unloading. Postindentation electrical measurements have been used to further investigate the end phases and to identify differences between indentations which otherwise appear to be identical from the XTEM and Raman analyses.This research was funded by the Australian Research Council and by WRiota Pty. Ltd

Mesoporous hexagonal Co<inf>3</inf>O<inf>4</inf> for high performance lithium ion batteries

© 2014 Macmillan Publishers Limited. Mesoporous Co3O4 nanoplates were successfully prepared by the conversion of hexagonal β-Co(OH) 2 nanoplates. TEM, HRTEM and N2 sorption analysis confirmed the facet crystal structure and inner mesoporous architecture. When applied as anode materials for lithium storage in lithium ion batteries, mesoporous Co3O4 nanocrystals delivered a high specific capacity. At 10 C current rate, as-prepared mesoporous Co3O4 nanoplates delivered a specific capacity of 1203 mAh/g at first cycle and after 200 cycles it can still maintain a satisfied value (330 mAh/g). From ex-situ TEM, SAED and FESEM observation, it was found that mesoporous Co3O4 nanoplates were reduced to Li2O and Co during the discharge process and re-oxidised without losing the mesoporous structure during charge process. Even after 100 cycles, mesoporous Co3O4 crystals still preserved their pristine hexagonal shape and mesoporous nanostructure

The effects of Al and Ti additions on the structural stability, mechanical and electronic properties of D8m-sStructured Ta₅Si₃

In the present study, the influence of substitutional elements (Ti and Al) on the structural stability, mechanical properties, electronic properties and Debye temperature of Ta₅Si₃ with a D8m structure were investigated by first principle calculations. The Ta₅Si₃ alloyed with Ti and Al shows negative values of formation enthalpies, indicating that these compounds are energetically stable. Based on the values of formation enthalpies, Ti exhibits a preferential occupying the Ta⁴b site and Al has a strong site preference for the Si⁸h site. From the values of the bulk modulus (B), shear modulus (G) and Young’s modulus (E), we determined that both Ti and Al additions decrease both the shear deformation resistance and the elastic stiffness of D8m structured Ta₅Si₃. Using the shear modulus/bulk modulus ratio (G/B), Poisson’s ratio (υ) and Cauchy’s pressure, the effect of Ti and Al additions on the ductility of D8m-structured Ta5Si3 are explored. The results show that Ti and Al additions reduce the hardness, resulting in solid solution softening, but improve the ductility of D8m-structured Ta₅Si₃. The electronic calculations reveal that Ti and Al additions change hybridization between Ta-Si and Si-Si atoms for the binary D8m-structured Ta₅Si₃. The new Ta-Al bond is weaker than the Ta-Si covalent bonds, reducing the covalent property of bonding in D8m-structured Ta5Si3, while the new strong Ti⁴b-Ti⁴b anti-bonding enhances the metallic behavior of the binary D8m-structured Ta₅Si₃. The change in the nature of bonding can well explain the improved ductility of D8m-structured Ta₅Si₃ doped by Ti and Al. Moreover, the Debye temperatures, ΘD, of D8m-structured Ta₅Si₃ alloying with Ti and Al are decreased as compared to the binary Ta₅Si₃Linlin Liu, Jian Cheng, Jiang Xu, Paul Munroe and Zong-Han Xi

Slowing Heart Rate Protects Against Pathological Cardiac Hypertrophy.

We aimed to determine the pathophysiological impact of heart rate (HR) slowing on cardiac function. We have recently developed a murine model in which it is possible to conditionally delete the stimulatory heterotrimeric G-protein (Gαs) in the sinoatrial (SA) node after the addition of tamoxifen using cre-loxP technology. The addition of tamoxifen leads to bradycardia. We used this approach to examine the physiological and pathophysiological effects of HR slowing. We first looked at the impact on exercise performance by running the mice on a treadmill. After the addition of tamoxifen, mice with conditional deletion of Gαs in the SA node ran a shorter distance at a slower speed. Littermate controls preserved their exercise capacity after tamoxifen. Results consistent with impaired cardiac capacity in the mutants were also obtained with a dobutamine echocardiographic stress test. We then examined if HR reduction influenced pathological cardiac hypertrophy using two models: ligation of the left anterior descending coronary artery for myocardial infarction and abdominal aortic banding for hypertensive heart disease. In littermate controls, both procedures resulted in cardiac hypertrophy. However, induction of HR reduction prior to surgical intervention significantly ameliorated the hypertrophy. In order to assess potential protein kinase pathways that may be activated in the left ventricle by relative bradycardia, we used a phospho-antibody array and this revealed selective activation of phosphoinositide-3 kinase. In conclusion, HR reduction protects against pathological cardiac hypertrophy but limits physiological exercise capacity