RESUMO
Nanopore brings extraordinary properties for a variety of potential applications in various industrial sectors. Since manufacturing of solid-state nanopore is first reported in 2001, solid-state nanopore has become a hot topic in the recent years. An increasing number of manufacturing methods have been reported, with continuously decreased sizes from hundreds of nanometers at the beginning to ≈1 nm until recently. To enable more robust, sensitive, and reliable devices required by the industry, researchers have started to explore the possible methods to manufacture nanopore array which presents unprecedented challenges on the fabrication efficiency, accuracy and repeatability, applicable materials, and cost. As a result, the exploration of fabrication of nanopore array is still in the fledging period with various bottlenecks. In this article, a wide range of methods of manufacturing nanopores are summarized along with their achievable morphologies, sizes, inner structures for characterizing the main features, based on which the manufacturing of nanopore array is further addressed. To give a more specific idea on the potential applications of nanopore array, some representative practices are introduced such as DNA/RNA sequencing, energy conversion and storage, water desalination, nanosensors, nanoreactors, and dialysis.
RESUMO
Conjugate heat transfer is numerically investigated using a three-dimensional computational fluid dynamics approach in various microchannel geometries to identify a high-performance cooling method for piezoelectric ceramic stacks and spindle units in high-precision machines. Straight microchannels with rectangular cross sections are first considered, showing the performance limitations of decreasing the size of the microchannels, so other solutions are needed for high applied heat fluxes. Next, many microchannel designs, focusing on streamwise geometric variation, are compared to straight channels to assess their performances. Sinusoidally varying channels produce the highest heat transfer rates of those studied. Thus, their optimization is considered at a channel width and height of 35 and 100 µm, respectively. Heat transfer increases as the amplitude and spatial frequencies of the channels increase due to increased interfacial surface area and enhanced Dean flow. The highest performance efficiencies are observed at intermediate levels of amplitude and frequency, with efficiency decreasing as these geometric parameters are increased further at the onset of flow separation. The sinusoidal channel geometries are then optimized with respect to minimizing the system's pressure drop for all applied heat fluxes between 5690 and 6510 kW/m2. Doing so created an optimal geometry curve and showed that all geometries in this region had amplitudes close to 40 µm. Therefore, imposing a fixed heat flux requirement for a case study of cooling piezoelectric ceramics, the optimized sinusoidal geometry decreases the system pressure drop by 79% relative to a straight channel while maintaining a larger minimum feature size.
RESUMO
Surface texturing is an effective approach to improve the tribological performance of artificial joints. In this paper, the frictional performance of Ultra-High-Molecular-Weight-Polyethylene and Cobalt-Chromium-Molybdenum material combination with micro grooves fabricated on the metal bearings is studied. The results show that grooves with width of 500 µm, depth of 4.5 µm and pitch distance of 3 mm could provide the optimized tribological performance, the coefficient of friction of which can be down to 0.05 showing a reduction of 51.9% compared to that of polished samples without micro grooves. A two-dimensional simulation of hydrodynamic pressure, based on Reynolds equation, is conducted. It is concluded that hydrodynamic pressure has little effect on the improved tribological performance of textured bioimplants. Otherwise, second lubrication effect induced by the polymer plastic deformation is proved to play a major role in the reduction of coefficient of friction.
Assuntos
Cromo , Cobalto , Fricção , Lubrificação , Teste de MateriaisRESUMO
The atmospheric pressure plasma polishing (APPP) is a novel precision machining technology. It performs the atom scale material removal based on low temperature plasma chemical reactions. As the machining process is chemical in nature, it avoids the surface/subsurface defects usually formed in conventional mechanical machining processes. APPP firstly introduces a capacitance coupled radio frequency (RF) plasma torch to generate reactive plasma and excite chemical reactions further. The removal process is a complicated integrating action which tends to be affected by many factors, such as the gas ratio, the RF power and so on. Therefore, to improve the machining quality, all the aspects should be considered and studied, to establish the foundation for further model building and theoretical analysis. The atomic emission spectroscopy analysis was used to study the process characteristics. A commercial micro spectrometer was used to collect the spectrograms under different parameters, by comparing which the influence of the RF power and gas ratio was initially studied. The analysis results indicate that an increase in RF power results in a higher removal rate within a certain range. The gas ratio doesn't show obvious influence on the removal rate and surface roughness in initial experiments, but the element compositions detected by X-ray photoelectron spectroscopy technology on the machined surfaces under different ratios really indicate distinct difference. Then the theoretical analysis revealed the corresponding electron transition orbits of the excited reactive fluorine atoms, which is necessary for further mechanism research and apparatus improvement. Then the initial process optimization was made based on the analysis results, by which the Ra 0.6 nm surface roughness and 32 mm3 x min(-1) removal rate were achieved on silicon wafers.
