Structure, bonding, and phase relations in Bi2Sn2O7 and Bi2Ti2O7 pyrochlores: new insights from high pressure and high temperature studies

One of the key points of interest in pyrochlore materials containing bismuth derives from the dielectric properties of some such materials that are linked to the displacements of the bismuth atoms from the ideal site. This study uses high pressure to probe the variations in, and causes of, these displacements. Under compression Bi(2)Ti(2)O(7) does not undergo any phase changes, but Bi(2)Sn(2)O(7) undergoes a similar series of changes to those observed during heating. The trigonal β-Bi(2)Sn(2)O(7) structure is solved from high temperature powder neutron diffraction data and hence the sequence of phases observed in Bi(2)Sn(2)O(7) is discussed for the first time. The variation in Bi displacements can be considered in terms of the frustration of the tetrahedral lattice that accommodates them. It can also be inferred that the main driver for Bi displacement is a deficiency in the bond valence sum of bismuth.     

Electrochemical behavior and differential pulse voltammetric detection of thiobencarb on 2-(4-((4-ethoxyphenyl)diazenyl)phenylamino)ethanol-modified carbon paste electrode

Cyclic voltammetry, chronoamperometry, and rotating disk electrode voltammetry were used to investigate the electrochemical behavior of thiobencarb (TB) at carbon paste electrode modified with an azo dye, 2-(4-((4-ethoxyphenyl)diazenyl)phenylamino)ethanol (EDPE), EDPE/modified carbon paste electrode (MCPE). The modified electrode showed high electrocatalytic activity toward thiobencarb. The current was enhanced significantly relative to the situation prevailing when a bare glassy carbon electrode was used. The kinetics parameters of this process were calculated, the apparent electron transfer rate constant k _s and α (charge transfer coefficient between electrode and EDPE) were 14.6 s⁻¹ and 0.48, respectively. The experimental parameters were optimized, and the mechanism of the catalytic process was discussed. The best defined cathodic peak was obtained with 0.1 M acetate buffer (pH 3.0). The response of the sensor was very quick, and response time was approximately 5 s. The differential pulse voltammetry response of the MCPE was linear against the concentration of TB in the range of 0.96 to 106 μg L⁻¹. The limit of detection was found to be 0.025 μg L⁻¹. The precision was examined by carrying out eight replicate measurements at a concentration of 25 μg L⁻¹ TB; the relative standard deviation was 2.9%.     

Flexible Probes for Characterizing Surface Topology: From Biology to Technology

In nature, several species use flexible probes to actively explore their environment and acquire important sensory information, such as surface topology, texture, and water or air flow velocity. For example, rats and other rodents have an array of facial vibrissae (or whiskers) with which they gather tactile information about the external world. The complex mechanisms by which mechanical deformations of the probe lead to neuronal activity in the animal’s nervous system are still far from being completely understood. This is due to the intricacy of the deformation mechanics of the flexible sensors, the processes responsible for transforming the deformation to electrical activity, and the subsequent representation of the sensory information by the nervous system. Understanding how these mechanosensory signals are transduced and extracted by the nervous system promises great insight into biological function, and has novel technological applications. To understand the mechanical aspect of sensory transduction, here we monitored the deformation of a rat’s vibrissa as it strikes rigid objects with different topologies (surface features) during locomotion, using high-speed videography. Motivated by our observations, we developed detailed numerical models to study the mechanics of such flexible probes. Our findings elucidate how active sensation with vibrissae might provide sensory information and in addition have direct implications for several technological areas. To put this in perspective, we propose strategies in which flexible probes can be used to characterize surface topology at high speeds, which is a desirable feature in several technological applications such as memory retrieval.     

Dynamic crushing and energy absorption of regular,irregular and functionally graded cellular structures

The in-plane dynamic crushing of two dimensional honeycombs with both regular hexagonal and irregular arrangements was investigated using detailed finite element models. The energy absorption of honeycombs made of a linear elastic-perfectly plastic material with constant and functionally graded density were estimated up to large crushing strains. Our numerical simulations showed three distinct crushing modes for honeycombs with a constant relative density: quasi-static, transition and dynamic. Moreover, irregular cellular structures showed to have energy absorption similar to their counterpart regular honeycombs of same relative density and mass. To study the dynamic crushing of functionally graded cellular structures, a density gradient in the direction of crushing was introduced in the computational models by a gradual change of the cell wall thickness. Decreasing the relative density in the direction of crushing was shown to enhance the energy absorption of honeycombs at early stages of crushing. The study provides new insight into the behavior of engineered and biological cellular materials, and could be used to develop novel energy absorbent structures.