The characteristics of discharge plasma generated by piezoelectric transformers (PTs) and the practical applications of PT-based plasma reactors are reviewed in this paper. PTs of Pb(Zr•Ti)O 3 generate high voltage by the piezoelectric effect, which can cause excitation and ionization of atoms and molecules, resulting in the generation of discharge plasma. When a narrow gap exists between the PT surface and a dielectric layer having a metal back electrode, dielectric barrier discharge (DBD) occurs at atmospheric pressure and above. The mechanical vibration of the PT and the resultant surface potential, which relate to electro-mechanical energy conversion by the piezoelectric effect, are investigated using a laser Doppler vibrometer and probe method, respectively. The characteristics of DBD are examined based on light patterns detected by the intensified charge coupled device camera, and the fundamental parameters of the filamentary discharge are determined. PT-generated DBD is shown to be applicable for ozone generation and as excimer lamps, and high-power operation is demonstrated using a parallel-driven hexad plasma reactor.

Light emission is observed in the silent discharge that produced by a piezoelectric transformer (PT) in nitrogen. The spatial distribution of light emission is observed in the λ mode vibration of the PT and compared with the potential distribution on the PT surface. In the case of λ mode vibration, light patterns appear at four areas around the PT. The spatial emission and potential distributions match clearly, indicating that the intensity of light emission is related to the electric field intensity around the PT. It is observed that luminous patterns alternate around the PT by sinusoidal driving voltages, as induced by the piezoelectric effect of elastic vibration on the PT surface. The temporal variation of emission intensity follows the induced electric field on the PT.

The dependence of ozone generation on the types of dielectric electrode material has been investigated using an ozone generator constructed with the piezoelectric transformer developed in our laboratory. The ozone generator is based on the excitation of the dielectric barrier discharge (DBD), which has the advantage of a compact configuration for generating ozone. Four kinds of dielectric materials are prepared for dielectric barrier electrodes. Electrical properties of the DBD and the ozone generation characteristics are investigated for the different dielectric materials. Differences in the discharge mode among the barrier electrode materials are recognized and discussed on the basis of the results of the Lissajous figures and voltage-current waveforms. During the continuous running of the generator, a temporal decrease in ozone concentration is observed owing to the temperature increase inside the reactor. Although the ozone generation characteristics are influenced by many properties of dielectrics, two important factors for achieving high-efficiency ozone generation are identified in this study. One is the use of a high-thermal conductivity material for the dielectric electrode, which functions well as a heat sink for transferring the generated heat to the outside through the material. The other factor is the control of the discharge mode. Our results show that the discharge mode that is considered as Townsend-like DBD is suitable for high-efficiency ozone generation.

A novel generation method of a dielectric barrier discharge (DBD) using a piezoelectric transformer (PT) is proposed. Spatiotemporal variations of microdischarges were investigated and discussed on the basis of the observations using a chargecoupled device (CCD) camera. As an example of a practical application of the DBD to a plasma reactor, an ozonizer using the PT was developed and ozone productions were performed in air and oxygen. Maximum ozone concentrations of 3.38 and 20.3 g/Nm 3 were obtained in air and oxygen, respectively. The ozonizer can be designed in a compact configuration and driven with low applied voltages because the PT serves as both a high-voltage generator and a discharge electrode.

A new method for absolute measurement of surface potential distributions on a Rosen-type piezoelectric transformer (PT) was investigated using a sensing electrode proposed in this study. The sensing electrode consists of a dielectric layer having striped back electrodes arrayed in the longitudinal direction of the PT, which is basically similar to the electrode for generating a dielectric barrier discharge (DBD). An equivalent circuit was defined for each striped electrode and a constant in the circuit necessary for the estimations of the absolute surface potential distributions was determined experimentally. The absolute measurements of the surface potential distributions of the PT were performed at (1/2)λ mode vibration both with and without a potential divider (PD) connected to the secondary terminal. Surface potential distribution without the PD connected, i.e., the unloaded condition of the PT, is in good agreement with the theoretical curve of the surface potential distribution. Moreover, the surface potential distribution while the DBD appears and the discharge power distribution of the DBD generated by the PT was also determined at absolute values using the same electrode configurations.