Mechanical property characterization of film-fabric laminate for stratospheric airship envelope

A stratospheric airship can offer a cost effective alternative to earth orbit satellites for telecommunication and science observation. To float in the stratosphere with very thin air density, the airship needs to be designed with envelope materials that are extremely light, flexible and of high strength. Film-fabric laminates, which consist of single plain woven fabric layer impregnated in polymer matrix and are laminated with thin films for environmental and leakage protection, have unique material characteristics compared to conventional composite structures that can be exploited in the design of the high altitude airship envelope materials. In this study, the material characterization of a film-fabric laminate developed for stratospheric airship envelop was conducted. First, uni-axial tests were performed for fabricated specimens to obtain tensile properties. The tests were done in the thermal chamber at low, room, and high temperatures to investigate the temperature dependency. Next, the effective tensile properties were predicted by geometrically nonlinear finite element analyses. The analyses were done for unit cells where the detailed fiber tow architecture was modeled discretely. The analysis results were compared to the test results and investigated focusing on the geometrical nonlinearity and the micro-structural behavior of the laminates.     

Numerical analysis on electroosmotic flows in a microchannel with rectangle-waved surface roughness using the Poisson–Nernst–Planck model

The present study has numerically investigated two-dimensional electroosmotic flows in a microchannel with dielectric walls of rectangle-waved surface roughness to understand the roughness effect. For the study, numerical simulations are performed by employing the Nernst–Planck equation for the ionic species and the Poisson equation for the electric potential, together with the traditional Navier–Stokes equation. Results show that the steady electroosmotic flow and ionic-species transport in a microscale channel are well predicted by the Poisson–Nernst–Planck model and depend significantly on the shape of surface roughness such as the amplitude and periodic length of wall wave. It is found that the fluid flows along the surface of waved wall without involving any flow separation because of the very strong normal component of EDL (electric double layer) electric field. The flow rate decreases exponentially with the amplitude of wall wave, whereas it increases linearly with the periodic length. It is mainly due to the fact that the external electric-potential distribution plays a crucial role in driving the electroosmotic flow through a microscale channel with surface roughness. Finally, the present results using the Poisson–Nernst–Planck model are compared with those using the traditional Poisson–Boltzmann model which may be valid in these scales.     

A study on an equivalent model of the threaded fasteners in complex structures through tightening and loosening analysis

Journal of Mechanical Science and Technology - The threaded fasteners are typical machine components in tightening of the machine parts and structures. In addition, as threaded fasteners are easy...     

PR-MAC: A practical approach for exploiting relay transmissions in multi-rate WLANs

In this paper, we propose a new MAC protocol, called proxy relay-enabled MAC (PR-MAC), to improve the capacity of multi-rate WLANs. PR-MAC introduces a new entity called Proxy Relay Point (PRP), which serves as a relay between the AP and stations. The cooperation of the AP with PRP replaces direct transmissions for low-rate stations with fast two-hop transmissions while the stations think that they communicate directly with the AP. Our experiment and simulation results showed that PR-MAC can significantly improve the throughput of legacy stations without any modification on the MAC operation of the stations.     

Metalorganic Chemical Vapor Deposition of CdTe(133) Epilayers on Si(211) Substrates

Single-crystalline CdTe(133) films have been grown by metalorganic chemical vapor deposition on Si(211) substrates. We studied the effect of various growth parameters on the surface morphology and structural quality of CdTe films. Proper oxide removal from the Si substrate is considered to be the principal factor that influences both the morphology and epitaxial quality of the CdTe films. In order to obtain single-crystalline CdTe(133) films, a two-stage growth method was used, i.e., a low-temperature buffer layer step and a high- temperature growth step. Even when the low-temperature buffer layer shows polycrystalline structure, the overgrown layer shows single-crystalline structure. During the subsequent high-temperature growth, two-dimensional crystallites grow faster than other, randomly distributed crystallites in the buffer layer. This is because the capturing of adatoms by steps occurs more easily due to increased adatom mobility. From the viewpoint of crystallographic orientation, it is assumed that the surface structure of Si(211) consists of (111) terrace and (100) step planes with an interplanar angle of 54.8°. This surface structure may provide many preferable nucleation sites for adatoms compared with nominally flat Si(100) or (111) surfaces. The surface morphology of the resulting films shows macroscopic rectangular-shaped terrace—step structures that are considered to be a (111) terrace with two {112} step planes directed toward 〈110〉.     

Power Performance of AlGaN/GaN HEMTs Grown on SiC by Ammonia-MBE at 4 and 10 GHz

In this letter, we report on the microwave power and efficiency performance of AlGaN/GaN high-electron mobility transistors (HEMTs) grown by ammonia molecular beam epitaxy (ammonia-MBE) on SiC substrates. At 4 GHz, an output power density of 11.1 W/mm with an associated power-added efficiency (PAE) of 63% was measured at V _ds = 48 V on passivated devices. At 10 GHz, an output power density of 11.2 W/mm with a PAE of 58% was achieved for V _ds = 48 V. These results are the highest reported power performance for AlGaN/GaN HEMTs grown by ammonia-MBE and the first reported for ammonia-MBE on SiC substrates.     

Hybrid film of chemically modified graphene and vapor-phase-polymerized PEDOT for electronic nose applications

A hybrid of graphene and conducting polymer holds great potential as the active materials for high performance chemical sensor application. In this work, a thin hybrid film of reduced graphene oxide (RG-O) and poly(3,4-ethylenedioxythiophene) (PEDOT) was fabricated by means of vapor phase polymerization and explored as active material for chemical sensors. The chemical sensors based on hybrid film of RG-O and PEDOT are capable of detecting electrical signals caused by the absorption of trace levels of different analyte vapors with high sensitivity, selectivity and fast response.     

Active object search in an unknown large-scale environment using commonsense knowledge and spatial relations

Intelligent Service Robotics - In this study, the goal is to efficiently and actively search for a target object in a previously unknown large-scale environment. To this end, we develop a...