Tunable nanomechanical performance regimes in ceramic nanowires

At the macroscopic size regime, ceramic materials exhibit brittle fracture and catastrophic failure when they are subjected to mechanical loads that exceed their characteristic strength. In this report, we present recoverable plasticity in alpha-phase, potassium stabilized manganese dioxide nanowire (α-K_0.13MnO₂ NW) crystals when they are subjected to atomic force microscopy (AFM) based three-point bending tests at very low loading rates. The force-deflection curves and AFM scans obtained from these measurements reveal yielding and extended plasticity in the NWs during the loading process, while the large plastic deformation is recovered spontaneously during the unloading process. However, the same material system exhibits failure via fracture at substantially higher strengths when it is subjected to bending tests at nearly an order of magnitude higher loading rates. These results highlight an important new pathway to controllably tune the nanomechanical performance of these technologically important nanoceramics for application-specific needs: either achieve self-reversible and ultra-large plasticity, or achieve substantially higher fracture strengths that approach the intrinsic limits of the material system.

Plastic recovery and self-healing in longitudinally twinned SiGe nanowires

This paper reports on plastic recovery and self-healing behavior in longitudinally-twinned and [112] orientated SiGe nanowire (NW) beams when they are subjected to large bending strains. The NW alloys are comprised of lamellar nanotwin platelet(s) sandwiched between two semi-cylindrical twins. The loading curves, which are acquired from atomic force microscope (AFM) based three-point bending tests, reveal the onset of plastic deformation at a characteristic stress threshold, followed by further straining of the NWs. This ductility is attributed to dislocation activity within the semi-cylindrical crystal portions of the NW, which are hypothesized to undergo a combination of elastic and plastic straining. On the other hand, the lamellar nanoplatelets undergo purely elastic stretching. During the unloading process, the release of internal elastic stresses enables dislocation backflow and escape at the surface. As a result, the dislocations are predominantly annihilated and the NW samples evidenced self-healing via plastic recovery even at ultra-large strains, which are estimated using finite-element models at 16.3% in one of the tested devices. Finite element analysis also establishes the independence of the observed nanomechanical behavior on the relative orientation of the load with respect to the nanoplatelet. This first observation of reversible plasticity in the SiGe material system, which is aided by a concurrent evolution of segmented elastic and plastic deformation within its grains during the loading process, presents an important new pathway for mechanical stabilization of technologically important group-IV semiconductor nanomaterials.

Brittle fracture to recoverable plasticity: polytypism-dependent nanomechanics in todorokite-like nanobelts

Atomic force microscopy (AFM) based nanomechanics experiments involving polytypic todorokite-like manganese dioxide nanobelts reveal varied nanomechanical performance regimes such as brittle fracture, near-brittle fracture, and plastic recovery within the same material system. These nanobelts are synthesized through a layer-to-tunnel material transformation pathway and contain one-dimensional tunnels, which run along their longitudinal axis and are enveloped by m × 3 MnO₆ octahedral units along their walls. Depending on the extent of material transformation towards a tunneled microstructure, the nanobelts exhibit stacking disorders or polytypism where the value for m ranges from 3 to up to ∼20 within different cross-sectional regions of the same nanobelt. The observation of multiple nanomechanical performance regimes within a single material system is attributed to a combination of two factors: (a) the extent of stacking disorder or polytypism within the nanobelts, and (b) the loading (or strain) rate of the AFM nanomechanics experiment. Controllable engineering of recoverable plasticity is a particularly beneficial attribute for advancing the mechanical stability of these ceramic materials, which hold promise for insertion in multiple next-generation technological applications that range from electrical energy storage solutions to catalysis.

Super-giant magnetoresistance at room-temperature in copper nanowires due to magnetic field modulation of potential barrier heights at nanowire-contact interfaces

We have observed a super-giant (∼10 000 000%) negative magnetoresistance at 39 mT field in Cu nanowires contacted with Au contact pads. In these nanowires, potential barriers form at the two Cu/Au interfaces because of Cu oxidation that results in an ultrathin copper oxide layer forming between Cu and Au. Current flows when electrons tunnel through, and/or thermionically emit over, these barriers. A magnetic field applied transverse to the direction of current flow along the wire deflects electrons toward one edge of the wire because of the Lorentz force, causing electron accumulation at that edge and depletion at the other. This lowers the potential barrier at the accumulated edge and raises it at the depleted edge, causing a super-giant magnetoresistance at room temperature.

Dependence of Young's modulus on the sodium content within the structural tunnels of a one-dimensional Na-ion battery cathode

We report on the Young's modulus (YM) of single-crystalline Na4Mn9O18 (or Na0.44MnO2) nanowires (NWs), which have shown promise as reversible sodium-ion (Na(+)) intercalation cathodes with high capacity and excellent cyclability. In addition, acid treatment of this material yielded proton stabilized Na(0.44-y)MnO2 (y ∼ 0.23) NWs with a 74% increase in the YM. The tight correlation between YM and ionic content within the crystalline tunnels is particularly significant, since it points to the strong dependence of elastic properties on state-of-charge (SOC) within battery materials.

MD Concept: a model for integrating medical knowledge

Many integrated clinical information systems depend on large knowledge bases containing dictionary of terms as well as specific information about each term and the relationships between terms. We propose a knowledge base model called MD Concept which is based on a semantic network and uses an object-oriented paradigm and relational tables. A prototype has been developed which integrates the Unified Medical Language System (UMLS) with other databases including the Systematized Nomenclature of Medicine (SNOMED II), the Diagnostic and Statistical Manual of Mental Disorders (DSM-IIIR) and a pharmaceutical database. We demonstrate how a user can easily navigate in this knowledge world using a browser.

PureMD: a Computerized Patient Record software for direct data entry by physicians using a keyboard-free pen-based portable computer

This paper describes the data acquisition features of the PureMD Computerized Patient Record (CPR) software designed specifically for physicians. The physician uses a stylus to point, draw and handwrite on a Dynamic Dialog Interface that provides the same flexibility as the paper record and numerous other advantages. The clinical data thus entered is highly organized, easily legible and retrievable in many ways. The underlying Medical Knowledge Base (MKB) was optimized for rapid, intuitive and consistent data entry and automatic coding with minimum handwriting.

Endemic focus of Fasciolopsis buski infection in Bangladesh

Stool surveys were conducted on children 2--10 years of age in 27 villages within Dacca District and around this index area (1,668 children were sampled), revealing an endemic focus of Fasciolopsis buski infection to the south and the east of Dacca District. In order to determine the seasonal variation in the total snail populations and the natural rate of F. buski infection in the snails, two species of planorbid snails, Segmentina (Trochorbis) trochoideus and Hippeutis (Helicorbis) umbilicalis, were periodically sampled for 12 months from a village endemic for F. buski infection. Gymnocephalous cercariae were found in S. (T.) trochoideus snails during August, September and October. The size of the snail population (n = 1,275) was significantly correlated with inches of rainfall (r = +0.62; P less than 0.05) and water temperature (r = +0.59; P less than 0.05). The natural infection rate of F. buski in the snails ranged from 0.5--2%. Snails from non-endemic areas were exposed to 3--10 miracidia. A total of 13 of 49 (27%) of H. (H.) umbilicalis and 6 of 14 (43%) of S. (T.) trochoideus had gymnocephalous cercariae present 4 to 6 weeks after exposure to miracidia. Thus, snail strain variation is unlikely to be a barrier to F. buski transmission.

Single dose piperazine therapy for Ascaris lumbricoides: an unsuccessful method of promoting growth

One-hundred eighty-five Bangladeshi children age 1 1/2 to 8 yr with no Ascaris lumbricoides infection or with light, moderate, or heavy infection were randomly assigned to treatment of placebo groups, with treatment given in a double-blind fashion. The groups were comparable for nutritional and socioeconomic parameters. Treatment consisted of a single dose of piperazine citrate administered twice within a 2-wk period. The cure rates for the low, moderate, and heavy A. lumbricoides infected subgroups were 53, 31, and 36%, respectively. With more severe infections, worm eradication was more difficult and the rate of reinfection after treatment was more rapid. The rate of reinfection was significantly different for the low A. lumbricoides infected treatment and placebo subgroups for 5 months after treatment, for the moderate treatment and placebo subgroups for 3 months after treatment, and for the heavy A. lumbricoides infected treatment and placebo subgroups there was a difference, although not significant, for 1 month after treatment. Anthropometric measurements were obtained for a period of 11 months. Analysis of covariance revealed no significant difference for change of weight, change of height, weight-for age, weight-for-height, height-for-age, triceps skinfold, midarm circumference, and the abdominal girth to chest circumference ratio between the treatment and placebo groups after drug administration. The results of this study do not support single dose worm therapy as a means to enhance growth.