Frequency dependent impedance response analysis of nanocrystalline ZnO chemiresistors.

ZnO is a widely studied gas sensor material and is used in many commercial sensor devices. However, selectivity towards any particular gas remains an issue due to lack of complete knowledge of the gas sensing mechanism of oxide surfaces. In this paper, we have studied the frequency dependent gas sensor response of ZnO nanoparticles of a diameter of nearly 30 nm. A small rise of synthesis temperature from 85 °C to 95 °C in the solvothermal process, shows coarsening by joining and thereby distinct loss of grain boundaries as seen from transmission electron micrographs. This leads to a substantial reduction in impedance, Z (GΩ to MΩ), and rises in resonance frequencyfres(from 1 to 10 Hz) at room temperature. From temperature dependent studies it is observed that the grain boundaries show a Correlated Barrier Hopping mechanism of transport and the hopping range in the grain boundary region is typically 1 nm with a hopping energy of 153 meV. On the other hand, within the grain, it shows a change of transport type from low temperature tunneling to beyond 300 °C as polaron hopping. The presence of disorder (defects) as the hopping sites. The temperature dependence offresagrees with different predicted oxygen chemisorbed species between 200 °C to 400 °C. As opposed to the traditional DC response, the AC response in the imaginary part of (Z″) shows gas specific resonance frequencies for each gas, such as NO2, ethanol, and H2. Among the two reducing gases, ethanol and hydrogen; the former shows good dependence on concentration in Z″ whereas the latter shows a good response infresas well as capacitance. Thus, the results of frequency dependent response allow us to investigate greater details of the gas sensing mechanism in ZnO, which may be exploited for selective gas sensing.

Perforator-Based Propeller Flap with Additional Venous Pedicle for Lower Limb Reconstruction.

Propeller flaps are effective reconstructive tools for small-to-medium sized defects of the lower limb and a reasonable alternative to free flaps. A major vessel of the lower limb remains undisturbed while raising the flap and the flap donor and recipient areas are addressed in the same operative field. Perforator-based propeller flaps are based on single perforator arising from a major vessel and during rotation of the larger paddle there is a possibility of kink in the venous component leading to congestion of flap. In our modification, one superficial vein of the lower limb namely the great saphenous or short saphenous vein was included in the flap territory to enhance the retrograde or antegrade venous drainage of the flap. We observed, no flap congestion or necrosis in the postoperative period. Also, the patency of the incorporated vein was confirmed using handheld and color Doppler.