ZnO films have been prepared by rf sputtering a Zn target in a planar magnetron system with controlled Ar/O2 gas mixtures. The films were deposited on unheated glass substrates which were either stationary in front of the target or in constant motion. Both the system pressure and plasma impedance changed when an oxide layer formed on the target surface. This occurred at an oxygen flow rate which increased almost linearly with rf power; at 500 W, the required flow rate was 9 ml/min and the pressure increased from 0.1 to 1.2 Pa due to the reduced oxygen gettering. High resistance ZnO films were deposited at oxygen flow rates above this threshold value. The target self-bias voltage increased by 30 V at this value; it is affected by both the system pressure and the power. The deposition rate increased linearly with power at approximately 0.03 (μm/min)/(W/cm2) which appears to be typical of sputtering from a ZnO layer or target. For continuous substrate motion, the average rate was approximately 7% of this value. All the films were polycrystalline ZnO with a preferred orientation, the c axis of the hexagonal structure being within a small angle of the substrate normal; this orientation was improved by motion of the substrate past the target. Films deposited at pressures of approximately 0.4 Pa had a large internal stress, as revealed both by substrate bending and x-ray measurements. Increasing the pressure to 4.7 Pa decreased the stress by an order of magnitude. SEM analysis showed that this was associated with the development of a columnar structure. The refractive indices obtained from guided wave measurements were 1.940±0.006 and 1.962±0.003, which correspond to 97% of the single crystal values. The resistivity measured normal to the film plane was greater than 107Ω cm. The changes in film stress and structure are similar to effects in metal films. The electromechanical coupling coefficients obtained from SAW measurements are approximately half the best reported value.
The ability to control cell-surface interactions in order to achieve binding of specific cell types is a major challenge for microfluidic immunoaffinity cell capture systems. In the majority of existing systems, the functionalized capture surface is constructed of solid materials, where flow stagnation at the solid-liquid interface is detrimental to the convection of cells to the surface. We study the use of ultra-high porosity (99%) nanoporous micro-posts in microfluidic channels for enhancing interception efficiency of particles in flow. We show using both modelling and experiment that nanoporous posts improve particle interception compared to solid posts through two distinct mechanisms: the increase of direct interception, and the reduction of near-surface hydrodynamic resistance. We provide initial validation that the improvement of interception efficiency also results in an increase in capture efficiency when comparing nanoporous vertically aligned carbon nanotube (VACNT) post arrays with solid PDMS post arrays of the same geometry. Using both bacteria (~1 μm) and cancer cell lines (~15 μm) as model systems, we found capture efficiency increases by 6-fold and 4-fold respectively. The combined model and experimental platform presents a new generation of nanoporous microfluidic devices for cell isolation.
Microwaves at the ISM frequency of 2450 and 5800 MHz have been exploited to prepare FeCoNiCrAl-\ud family high entropy alloys by direct heating of pressed mixtures of metal powders. The aim of this work is\ud to explore a new microwave assisted near-net-shape technology, using powder metallurgy approach for\ud the preparation of high entropy alloys, able to overcome the limits of current melting technologies\ud (defects formation) or solid state ones (time demanding). Results show that direct microwave heating of\ud the powder precursors occurs, and further heating generation is favored by the ignition of exothermal\ud reactions in the compound. Microwave processing, exploited both for the ignition and sustaining of such\ud reactions, has been compared to reactive sintering in laboratory furnace and mechanical alloying in a\ud planetary ball milling. Results demonstrate that microwave required the shortest time and lowest energy\ud consumption, thus it is promising time- and cost-saving synthetic route
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