Guangbin Zhou

Fabrication of a carbon nanotube-based gas sensor using dielectrophoresis and its application for ammonia detection by impedance spectroscopy

This paper describes a new method for fabricating a gas sensor composed of multi-wall carbon nanotubes (MWCNTs) using dielectrophoresis (DEP). MWCNTs dispersed in ethanol were trapped and enriched in an interdigitated microelectrode gap under the action of a positive DEP force that drove the MWCNTs to a higher electric field region. During the trapping of MWCNTs, the electrode impedance varied as the number of MWCNTs bridging the electrode gap increased. After the DEP process, the ethanol was evaporated and the microelectrode retaining the MWCNTs was exposed to ammonia (NH3) gas while the electrode impedance was monitored. It was found that the electrode impedance was altered by ppm-levels of ammonia at room temperature. The ammonia exposure decreased the sensor conductance, while the capacitance increased. The sensor showed a reversible response with a time constant of a few minutes. The conductance change was proportional to ammonia concentration below 10 ppm and then gradually saturated at higher concentrations. Effects of the number of trapped MWCNTs on sensor response were also discussed.

Dielectrophoretic filter for separation and recovery of biological cells in water

Dielectrophoresis (DEP) is the electrokinetic motion of dielectrically polarized particles in nonuniform electric fields. DEP has found many useful technological applications including separation, levitation, and characterization of dielectric particles such as biological cells. In this study, the authors demonstrated continuous separation and recovery of biological cells suspended in water using a DEP filter. The DEP filter consists of an electrode system that is filled up with many glass beads. These glass beads modify the electric field distribution in the electrode system so that strong DEP force is generated on their surfaces. If the DEP force is stronger than drag force exerted by liquid flow in the filter, the suspended particles can be trapped and eliminated from the flowing liquid. The DEP filter can control trapping and releasing process just by changing electrode energizing AC signal and the resultant DEP force. It was experimentally confirmed that the DEP filter could continuously eliminate yeast cells suspended in water. The cell density decreased from 10/sup 6/ to 10/sup 1/ cells/mL in about 1 h. The electrical conductivity of the medium was a crucial parameter that influenced the liquid temperature by Joule heating and DEP force. Furthermore, the selective separation of viable and nonviable yeast cells was demonstrated by utilizing the viability dependency of the DEP force.

Calibration methods of carbon nanotube gas sensor for partial discharge detection in SF/sub 6/

The authors proposed a new type of gas sensor for high sensitive detection of decomposition products generated by partial discharge (PD) in SF/sub 6/ gas. The sensor employed carbon nanotubes (CNTs) as gas sensing transducer and was fabricated by electrokinetic manipulation of CNTs using dielectrophoresis. Due to complicated gas decomposition process of SF/sub 6/ gas, calibration of the CNT gas sensor is an essential and challenging subject in order to realize reliable and stable detection of PD under practical conditions. In this paper, two methods for the CNT gas sensor calibration were proposed and tested. The first method was based on initial conductance dependence of the CNT gas sensor response. The CNT gas sensor response to PD increased almost linearly with the amount of CNT trapped onto the sensor electrode, which could be quantified by the initial conductance of the sensor. However, the calibration accuracy of this method was about 50 % and was not high enough for practical use. The second method employed NO/sub 2/ (nitrogen dioxide) as calibration gas because of the similarity in the CNT sensor response to PD decomposition products and NO2. It was found that the accuracy of the NO/sub 2/ calibration was about 10 % and far better than the first method. Finally, effectiveness of the NO/sub 2/ calibration was demonstrated by a PD monitoring test in which two CNT sensors were simultaneously exposed to SF/sub 6/ decomposition products with identical concentration.

Production of carbon nanoparticles using pulsed arc discharge triggered by dielectric breakdown in water

This paper describes a simple production method for carbon nanoparticles by pulsed arc discharge in water, which is triggered by the electrical breakdown of water between graphite electrodes. High-resolution transmission electron microscopy (HRTEM) observation revealed that the main products obtained by the proposed method were onion-like nanoparticles; while carbon nanotubes were not formed. The carbon nanoonions uniformly dispersed in water forming micron-sized aggregates. The effects of the arc discharge mode (dc or pulse) on the nanoparticle formation mechanism are discussed.