- Nano Express
- Open Access
ZnO Nanowire-Based Corona Discharge Devices Operated Under Hundreds of Volts
© Yang et al. 2016
- Received: 25 October 2015
- Accepted: 27 December 2015
- Published: 16 February 2016
Minimizing the voltage of corona discharges, especially when using nanomaterials, has been of great interest in the past decade or so. In this paper, we report a new corona discharge device by using ZnO nanowires operated in atmospheric air to realize continuous corona discharge excited by hundreds of volts. ZnO nanowires were synthesized on microelectrodes using electric-field-assisted wet chemical method, and a thin tungsten film was deposited on the microchip to enhance discharging performance. The testing results showed that the corona inception voltages were minimized greatly by using nanowires compared to conventional dischargers as a result of the local field enhancement of nanowires. The corona could be continuously generated and self-sustaining. It was proved that the law of corona inception voltage obeyed the conventional Peek’s breakdown criterion. An optimal thickness of tungsten film coated over ZnO nanowires was figured out to obtain the lowest corona inception voltage. The ion concentration of the nanowire-based discharger attained 1017/m3 orders of magnitude, which is practicable for most discharging applications.
- Corona discharge
- Corona inception voltage
- Electric field enhancement
A corona discharge arises from an ionization and partial breakdown of surrounding atmospheric gases at a highly curved electrode within a strong electric field. It can bring about the stationary generation of ions with relatively low electric current and has been used as an ionizer in various areas such as aerosol measurements , degradation of organic compounds , preparation of some types of nanomaterials , mass spectrometers , and micromotors . Most of these discharge schemes are achieved by using needle-to-plane electrode geometries, where needles with smaller diameters can generate a greater local electric field enhancement when other parameters are fixed. Accordingly, it is of great interest to develop a miniaturized corona ionizer whose corona inception voltage could be reduced while the ion concentration could be maintained at an available level with the aid of minimizing needles. For instance, ionization gas sensors realized their on-chip operation due to the contribution of various nanostructures including nanotubes , silicon nanowires , gold nanowires , ZnO nanowires , and other nanostructures [10, 11]. These discharges in gas sensors do not need to be maintained continuously, but only the breakdown voltages are detected to discriminate ambient gases.
Recent studies on the ionization of atmospheric air have paid their attentions to continuous corona discharges with microscale or nanoscale needles which permit the reduction in physical size and cost of the corona ionizers. Eifert et al.  used the nanostructured edges of gold and aluminum foils as electrodes with 5-mm inter-electrode spacing for corona discharges. The lowest corona inception voltage reached 1.2 kV in ambient air. Hsu et al.  fabricated a cantilever-like corona electrode with a tip radius of about 500 nm and electrode gaps of 1–6 mm. The smallest corona onset voltages were in the range of 2–2.5 kV. Chua et al.  developed a micromachined corona ionizer with a point-to-grid geometry and millimeter electrode gaps. The corona inception voltages were in the range from 1.4 to 2 kV. Park et al.  designed a chip-type unipolar discharger with the inter-electrode gap of 165 μm. The corresponding corona inception voltage was 1.5 kV, and the estimated ion concentration was in the range from 6.5 × 1012/m3 to 4.3 × 1013/m3. Bo et al. developed nanomaterial-based corona discharges using suspended carbon nanotubes  and graphene sheets grown on metallic wires  to realize a continuous corona discharge at a corona inception voltage of about 0.1 and 3.2 kV, respectively. The maximum surface current density of the corona discharge they obtained using the suspended carbon nanotubes was 0.095 A/m2 at the voltage of 0.9 kV.
Nanowires are excellent candidates to enhance local electric fields for corona discharges, and among them, ZnO nanowires have the advantages of efficient excitonic emission [18, 19] and high temperature resistance, which could efficiently prevent the electrodes from damage due to ion bombardment and unsteady ion currents. The employment of ZnO nanowires to carry out a continuous corona discharge was demonstrated by Yang et al. . Later, Park et al.  reported a ZnO nanowire charger with an interstitial gap of 0.9 mm between electrodes, whose corona starting voltage was 1.6 kV and the generated ion concentrations were 6.7 × 108/cm3–4.3 × 109/cm3 at the applied voltages of 2–3.1 kV.
In this paper, we presented a comprehensive design and analysis on the continuous corona dischargers using ZnO nanowires, which were synthesized on microelectrodes using an electric-field-assisted wet chemical method and coated by a thin tungsten film to enhance their conductivity and discharging performance. Electric field enhancements of ZnO nanowires were studied, and the corona inception voltages were experimentally tested. The performances of the corona discharge devices using ZnO nanowires demonstrate their promising prospects for versatile applications.
Growth of ZnO Nanowire on Microelectrodes
ZnO nanowires as the anodes of ionizers were grown on microelectrodes using an electric-field-assisted wet chemical method, which is with low temperature, simplicity, low cost, capable of easy scale-up compared with other methods, and easy to incorporate with micromachined electrode structures [22, 23]. The electric field was used to control the growth position and growth direction of nanowires on the electrodes by applying a tailor-designed electric field. The shape of the microelectrodes was designed as a coplanar comb-finger pair which was favorable for the growth of nanowires .
Assembly of Devices
Electric Field Enhancement of ZnO Nanowires
Values of field enhancement factor β calculated by different methods
“hemisphere on post” model
Further increasing voltage applied on the discharger with the tungsten film could yield a continuous corona discharge, while that with the gold film could only achieve a transient spark. As a result, tungsten was selected as the coating film.
Corona Discharge Phenomenon
Corona Inception Voltage
Critical electric field strength calculated by Eq. (6) and the numerical method (the unit in the table is V/m)
Type A, tungsten film 80 nm
Type B tungsten film 80 nm
Type A tungsten film 160 nm
Type B tungsten film 160 nm
Calculated by Eq. (6)
Ion Concentration Induced by Corona Discharge
Continuously self-sustained corona discharges using ZnO nanowires as anode tips of dischargers are realized. The corona inception voltage is greatly reduced to hundreds of volts in virtue of the local field enhancement by nanowires as electrodes. It is proved that the law of corona inception voltage obeys Peek’s breakdown criterion. An optimal thickness of tungsten film coated on ZnO nanowires is figured out to obtain the lowest corona inception voltage. The ion concentration of the developed corona discharger using nanowires attains 1017/m3 orders of magnitude, which is sufficient for most applications.
This work was supported by the National Key Project of Scientific Instrument and Equipment Development (2012YQ030261), the National Natural Science Foundation of China (NSFC) under Grant 91123017, and the Fundamental Research Funds for the Central Universities (FRF-TP-15-028A1).
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