Improved High-Yield PMMA/Graphene Pressure Sensor and Sealed Gas Effect Analysis

Graphene with atomic thickness possesses excellent mechanical and electrical properties, which hold great potential for high performance pressure sensing. The exposed electron of graphene is always cross-sensitive to any pollution absorbed or desorbed on the surface, from which the long-term stability of the graphene pressure sensor suffers a lot. This is one of the main obstacles towards graphene commercial applications. In this paper, we utilized polymethylmethacrylate (PMMA)/graphene heterostructure to isolate graphene from the ambient environment and enhance its strength simultaneously. PMMA/graphene pressure sensors, with the finite-depth cavities and the through-hole cavities separately, were made for comparative study. The through-hole device obtained a comparable sensitivity per unit area to the state of the art of the bare graphene pressure sensor, since there were no leaking cracks or defects. Both the sensitivity and stability of the through-hole sensor are better than those of the sensor with 285-nm-deep cavities, which is due to the sealed gas effect in the pressure cavity. A modified piezoresistive model was derived by considering the pressure change of the sealed gas in the pressure cavity. The calculated result of the new model is consistent with the experimental results. Our findings point out a promising route for performance optimization of graphene pressure sensors.

Experimental Investigation of Cavity Patterns and Noise Characteristics

When a marine propeller with the wing shape rotates at high speed underwater, local pressure on the blade decreases and various types of cavitation inevitably occur in where the local pressure falls below the vapor pressure. Cavity reduces the efficiency, erodes the propeller surface, and generate vibration and serious noise. Especially, underwater noise caused by cavitation is directly connected to the comfort of commercial ships and also the survivability of naval vessels. In order to reduce the occurrence of the cavitation and to design low noise propeller, it is demanded to figure out the correlation of noise characteristics with growth patterns of the cavity. In this paper, we observed global behavior of partial cavities generated on two-dimensional hydrofoils and made a map of cavity patterns. We also measured pressure fluctuations and investigated noise characteristics directly connected with the process of occurrence of the cavity.

Design and Fabrication of an Improved MEMS-Based Piezoresistive Pressure Sensor

An improved piezoresistive pressure sensor is designed for harsh environment application. The highlight of this design is that the Wheatstone bridge circuit is put in lower surface of pressure diaphragm and sealed in the vacuum pressure cavity. The bridge circuit is led out by embedded Al electrodes on bonding surface. ANSYS software has been used to analyze the stress distribution of the diaphragm. By using the MEMS technology, the pressure sensor with the dimension of 1.5mm×1.5mm×500µm is fabricated. The performance of piezoresistive pressure sensor, including output, sensitivity, and nonlinearity, are investigated. The test results show that sensitivity is 20mV/V-MPa and maximum nonlinearity is 2.73%, which meet the requirements for the modern industry.

An Experimental Investigation of Wall Effects on Supercavitating Hydrofoils of Finite Span

A geometrically similar family of three half-span, elliptical-planform, supercavitating hydrofoils was tested in the M.I.T. variable-pressure water tunnel. Lift, drag, moment, tunnel speed, ambient pressure, cavity pressure and cavity length were measured for attack angles from 8 to 21 deg and a variety of ambient pressure settings. For the small and medium foils, it was sufficient to correct only for the effect on downwash of the images of the trailing vortices. The large foil data, however, required further correction; the corrected force and moment data for the large foil plotted slightly lower than did the data for the two smaller foils, while the cavity length data for the large foil indicated cavity lengths significantly larger than theoretical predictions or the cavity length data for the two smaller foils. Existing two-dimensional corrections may be used to bring the force data for the large foil into close agreement with the force data for the two smaller foils, but no correction factors exist for the cavity length data.