The piezoelectric direct discharge (PDD) is a comparatively new type of atmospheric pressure gaseous discharge for production of cold plasma. The generation of such discharge is possible using the piezoelectric cold plasma generator (PCPG) which comprises the resonant piezoelectric transformer (RPT) with voltage transformation ratio of more than 1000, allowing for reaching the output voltage >10 kV at low input voltage, typically below 25 V. As ionization gas for the PDD, either air or various gas mixtures are used. Despite some similarities with corona discharge and dielectric barrier discharge, the ignition of micro-discharges directly at the ceramic surface makes PDD unique in its physics and application potential. The PDD is used directly, in open discharge structures, mainly for treatment of electrically nonconducting surfaces. It is also applied as a plasma bridge to bias different excitation electrodes, applicable for a broad range of substrate materials. In this review, the most important architectures of the PDD based discharges are presented. The operation principle, the main operational characteristics and the example applications, exploiting the specific properties of the discharge configurations, are discussed. Due to the moderate power achievable by PCPG, of typically less than 10 W, the focus of this review is on applications involving thermally sensitive materials, including food, organic tissues, and liquids.

A new large volume microwave plasma source has been used for the production of a hydrogen plasma. The source consists of an annular waveguide cavity with axial slots on the inner side which acts as a field applicator to sustain a plasma at 2.45 GHz. The plasma is contained in a fused silica bell jar of 16 cm in diameter and 20 cm in height. The distance between the slots corresponds to a waveguide wavelength. The source is able to generate a highly dissociated (up to 90%) hydrogen plasma for cleaning purposes. Stable operation of the plasma source is shown for a pressure range of 0.1–1.3 mbar and a power range of 600–2000 W. The plasma can be ignited over the entire examined pressure range, and the power needed for discharge ignition is below 1.7 kW. The minimum ignition power is 1050 W for a pressure of 0.7 mbar. A double Langmuir probe and optical emission spectroscopy were used to characterize the hydrogen plasma as a function of microwave power, pressure, and position. The results indicated a typical ion density of 1.5×1011 cm−3 which is an order of magnitude less than that obtained for argon under similar conditions. The typical electron temperature is 2.5 eV for microwave power of 2 kW and pressure of 0.7 mbar.

The removal of different lubricants (oils and greases) from metal surfaces by use of a capacitively coupled 13.56 MHz low-pressure oxygen discharge has been examined. Stainless steel, tool steel, brass, aluminum, and copper were used as a substrate materials. The ion bombardment and chemical reactions occurring at the metal surfaces in such a discharge allow for high cleaning speed. A hollow-cathode discharge has been used to increase the plasma density in contrast to a plane cathode discharge configuration. The cleaning process was characterized by use of emission spectroscopy and mass spectroscopy. Influence of rf power (up to 1 kW), pressure (from 0.01 to 1 mbar), and magnetic field (up to 20 mT) on the process duration and surface cleanliness has been investigated. The criteria for detection of the process end point are formulated and checked. The spectral information is useful for end point detection only in restricted regions of the process conditions. Results indicate a strong influence of the lubricant temperature on the process duration. The removal rate of N62 oil at 140 °C is twice as high as that at 50 °C. For temperatures higher than 140 °C the creation of a hardly removable solid residue occurs. The influence of the initial lubricant area and lubricant thickness on the process duration has been investigated. The increase of the lubricant area results in an increase of the process time (loading effect for cleaning process). The increase of the lubricant film thickness results in a proportional increase of the process time.