Papers by Author: Xiao Wei Liu

Abstract: Antiwetting BNNT films have been achieved by milling-ink method. Superhydrophobic (CA <5°) are demonstrated on films with stainless steel as substrate. The high density and purity are confirmed by EDX and NEXAFS. There are only a few oxygen point defects in the form of nitrogen vacancies due to ink and annealing process in air.

Abstract: In this paper a high-performance closed-loop fourth-order sigma-delta (ΣΔ) micro-accelerometer is presented. After a introduction of sigma-delta accelerometer, system-level analysis and design of a fourth-order sigma-delta micro-accelerometer is given. The simulation result shows that an accelerometer with 107dB signal to noise ratio (SNR) and 17.5 bits effective number of bits (ENOB) is achieved. Through the root locus analysis, it is got that accelerometer is stable when quantization gain is bigger than 0.262. The accelerometer gets a good linearity and it becomes overload when input signal level is greater than -5dBFS.

Abstract: A new manufacture mainly for polymethyl methacrylate (PMMA) microfluidic chips is presented in this paper. In this technique, polymer microfluidic microchannels were fabricated by microcutter which temperature is controlled and stabilized by PID methord. There are so many techniques, such as hot embossing, laser direct-write, for mass-production of polymer microfluidic chip. However, we may feel different kinds of shortages when we use these techniques. In this paper, the experiment result shows that microcutter’s movement velocity and temperature have effert on microfluidic microchannel’s roughness.

Abstract: The piezoresistive and ohmic contact properties of polycrystalline silicon nano thin films were investigated in this paper. The polycrystalline silicon films with different thicknesses and doping concentrations were deposited by LPCVD and doped with boron highly, and then the cantilever beam samples were fabricated by photolithography and wet etching. By measuring the gauge factor and specific contact resistivity, the specific contact resistivity of Al contacts can reach 2.4×10^-3Ω·cm² after the alloying at 450 °C for 20 min; the enhanced piezoresistive effect of highly doped polycrystalline silicon nano thin films was discovered. The conclusions indicated that the enhanced piezoresistive sensitivity of PNTFs is due to the modification of depletion region barrier by ultra high doping and film thickness thinning and the enhancement of tunneling piezoresistive effect. The distinct piezoresistive phenomenon of PNTFs could be utilized for the development and fabrication of miniature piezoresistive sensors.

Abstract: In this paper, silicon-based micro direct methanol fuel cells (µDMFCs) with different anode structures were designed and investigated experimentally, the current collectors were fabricated using micro-electromechanical system (MEMS) technology. a novel encapsulation method was also designed using PDMS. The study of the effect of different anode flow fields suggested that the serpentine flow field has the optimal performance. The experimental results also revealed that an open ratio of around 47% led to the best cell performance, a maximum of power density of 13.2 mW/cm² was achieved with 1.0 M methanol solution.

Abstract: The previous research showed that highly doped polysilicon nanofilms (PSNFs) have a high gauge factor and stable temperature characteristics, promoting their applications in piezoresistive sensors. For correcting the resistance deviation and improving the resistance matching of sensing elements, the electrical trimming (ET) characteristics of PSNFs deposited by LPCVD at different temperatures were studied. By the interstitial-vacancy (IV) model, the resistance falloff in trimming phenomenon is considered as a result of the decrease of scattering centers caused by the recombination of IV pairs. By modeling grain boundaries (GBs) as the accumulation of IV pairs (basic defects), it is believed that IV pairs can recombine layer-by-layer under the energy excitation of joule heat generated by high current conduction. The experiment results show that the threshold current density for ET of PSNFs is an order of magnitude lower than that of polysilicon common films, while the trimming accuracy and stability of directly deposited PSNFs are superior to the recrystallized ones. Finally, it is gained that reducing amorphous phases at GBs by optimizing deposition temperature can improve ET characteristics of PSNFs.

Abstract: Membrane electrode assembly (MEA) is the key component of direct methanol fuel cell (DMFC), the structure and its preparation methods may bring great effects on the cell performances. Due to the requirement of the high performance of the MEA for the micro direct methanol fuel cell (DMFC), we provide a novel double-catalyst layer MEA using CCM-GDE (Catalyst Coated Membrane，CCM；Gas Diffusion Electrode，GDE) fabrication method. The double-catalyst layer is formed with an inner catalyst layer (in anode side: PtRu black as catalyst, in cathode side: Pt black as catalyst) and an outer catalyst layer (in anode side: PtRu/C as catalyst, in cathode side: Pt/C as catalyst). The fabrication procedures are important to the new structured MEA, thus three kinds of fabrication methods are studied, including CCM-GDE, GDE-Membrane and CCM-GDL methods. It was found that the CCM-GDE technology may enhance the contact properties between the catalyst and PEM, and increase the electrode reaction areas, resulted in increasing the performance of the DMFC.

Abstract: PMMA, Capillary Electrophoresis, Thermal bonding, Factor, Deformation. Abstract. In this paper, using the powder hot embossing machine, the effects of the bonding temperature, pressure and time on the deformation of microchannel dimensions is studied systematically. The bonding temperature is mainly influence factor on the deformation of microchannel determined by the method of orthogonal experiment. Some mathematical models are described to optimize the thermal bonding process parameters. According theory and experimental curve, the optimal experimental model (83°C, 0.4Mpa and 7 min) is suggested. Under the process parameters the deformation of crossing section dimension before and after thermal bonding is 37.8%. The channel is successfully sealed underneath with a thin foil 30μm thickness PMMA for contactless conductivity detection. The entire fabrication methodology may also be useful for preparation of other polymer microfluidic systems. Finally, the performance of the PMMA chip is demonstrated to separate continuously K+ ions using the contactless conductivity detection.

Abstract: Based on the visualization technology, we investigated experimentally the effect of different flow patterns on anode fluid behaviors of the μDMFC (Micro Direct Methanol Fuel Cell) with a transparent material under the same condition. Stainless steel mesh was utilized as the current collector which was distinct from the carbon cloth or carbon paper. Four dissimilar flow patterns were developed and tested. The observation of the effect of different flow patterns revealed that movements of dilute methanol solutions and CO2 gas bubbles in the dot and parallel flow fields represented more difficult, which could result in a decline of the μDMFC performance. The study also showed that a channel blocking in the single-serpentine flow field would be extremely terrible which could lead to a fuels leaking of the μDMFC, meanwhile the liquid-gas flow was more fluent and stable in a double-serpentine flow field. Therefore, due to its advantages, a double-serpentine flow pattern is more suitable for the μDMFC application compared with the other flow patterns.

Abstract: Quadrature error and Coriolis offset are main errors of micro-gyroscopes which caused by manufacturing imperfections. They deteriorate the overall performance of micro-gyroscopes. For inside sense outside drive frame micro-gyroscope, quadrature error and Coriolis offset have their own characters. Based on the mechanical analysis of inside sense outside drive frame micro-gyroscope in imperfect condition, the dynamic equations of condition with quadrature error and Coriolis offset are investigated, and an equivalent electrical model with quadrature error and Coriolis offset is presented. It is convenient to observe the effect of quadrature error and Coriolis offset by equivalent electrical model simulating. Three conditions are simulated and compared. The results indicate that the Coriolis offset reduces the amplitude of instantaneous displacement to a very small extent. Its effect can be neglected. Quadrature error causes an error displacement with the amplitude much bigger than the prefect displacement’s amplitude. It is the overwhelming error in ISOD frame micro-gyroscope.