Piezoelectric nonthermal plasma — or cold plasma — may sound as if it comes out of a science fiction show. However, it has been used for decades in many different industries for different forms of surface treatment, as well as other applications. Cold plasma is relatively inexpensive and efficient to produce, while on the other hand, thermal plasma requires substantial amounts of energy to produce.

Cold plasma — more accurately cold atmospheric pressure plasma — is created when electrons are thermalized. Cold plasma is created when gas is ionized and has conductive properties. The most effective way to create cold plasma is through piezoelectric direct discharge.

This type of plasma is often used for the surface treatment that is needed for the secure bonding of plastics, metals, and other materials.

This is increasingly important with the adoption of 3D printers by design engineers working on prototypes and products. As a result, there has been increasing interest further lowering the costs of creating cold plasma, with compact, light-weight, and easy-to-use cold plasma generators that can be operated with standard batteries. The first portable cold plasma product on the market was the piezobrush PZ2, developed by Relyon Plasma, a subsidiary of TDK Electronics.

Treating Difficult Surfaces

3D printers have increasingly become a conventional part of the design and manufacturing process by design engineers. As an additive manufacturing process, 3D printers allow engineers to immediately visualize their design, make it tangible, and test the product before engaging in expensive manufacturing processes — all for a low cost. However, the demands on quality, diversity of materials, and robustness of the products are growing — particularly in bonding plastic parts.

Four individual parts produced in 3D printing; surface activation with the piezobrush PZ2; finished glued trim. (Credit: TDK Electronics)

Most often, individual parts are printed and then glued together. In order to create a secure bond between two plastic parts, surface treatment becomes a vital part of the bonding process. Cold plasma can be used to activate the surfaces of predefined joint geometries with maximum bonding areas before being glued together.

When there are long or narrow parts, the strength and effectiveness of the bond is crucial, as the bonding surface is minimal. Plasma treatment used to activate a surface results in fine cleaning from organic contaminants. It also increases the surface energy for improved wettability by adhesives. In effect, bond strength is optimized before an adhesive is applied through surface activation by cold plasma.

The piezobrush PZ2 is based on TDK’s CeraPlas piezo plasma generator and is a wear-free cold atmospheric pressure plasma source that needs no external processing gas. This element creates the plasma that can be used to activate temperature-sensitive materials. By using this device to activate the bonding surfaces, bonds can be up to three times stronger than without surface activation.

Fig. 1 - The input side of a Rosen-type design of ceramic piezo transformer and plasma generator features a multilayer structure with copper inner electrodes, while the output side of the element has a monolithic structure.

Utilizing Piezoelectric Ceramics

Multiple technologies are needed to create such devices. First, in order to reduce the size, a single, ceramic solution that is built using multilayer ceramic technology is required. This allows cold plasma to be generated efficiently. This also removes the need for a Rosen-type piezoelectric transformer — which generates the voltages otherwise required to create cold plasma — as the components can both generate plasma and perform this function due to the multilayer properties of the ceramic (see Figure 1). In this case, the plasma generation takes place on the monolithic output side.

The input side features a ceramic made of lead zirconate titanate (PZT) that can be co-fired with internal copper electrodes. The electromechanical coupling results in low losses that can with-stand a wide range of strain due to a highly stable mechanical quality.

A standing acoustic wave transforms the low voltage from the input side to a high voltage at the mechanically coupled output side at resonant frequency. Without disturbing its mechanical movement and due to the device operating a second harmonic vibration mode, the element can be contacted and mounted at the vibration nodal points.

An electrical discharge between two electrodes that are separated by a dielectric barrier during the dielectric barrier discharge (DBD) process is caused during normal plasma generation. This discharge occurs on the surface of the element on the output side because of the dielectric electrode properties of the ceramic component and generated voltage.

Charges collect on the surface of the dielectric material and are discharged in microseconds where high-electric fields are situated on the material. As a result, there is no need for a traditional discharge electrode and reform elsewhere on the surface. Since the element functions as a dielectric electrode, the energy source provides sufficient ionization and plasma is created and sustained.

Making Components Compact

Ceramic components achieve a more effective surface activation, consume less power, and are typically more compact compared to conventional plasma generation components. Because of this, there is less need for bulky, costly external high-voltage safety components. This results in devices that perform optimally in regard to plasma temperature and surface activation within a smaller device. Such devices can be used in a wider range of applications.

The first portable cold plasma product on the market was the piezobrush PZ2, developed by Relyon Plasma, a subsidiary of TDK Electronics. (Credit: TDK Electronics)

To further improve the efficiency of these devices, a driver is needed to optimize the piezoelectric plasma generator and the piezoelectric transformer’s resonance frequency. This reduces potential stress to the component and tunes the element, which in turn, keeps the operation stable and allows the device to immediately react to load and environmental changes.

By employing these techniques, a plasma generator may be created with a low-voltage power supply, a driver, a ceramic component, and a supply of gas. The cleaning and bonding of individual parts needed to create and test a design is more easily done, and, by activating the bonding surfaces, bonds are stronger than ever before.

This article was written by Sonja Taylor Brown, Sr. Product Manager for Piezo Products, TDK Electronics Inc. For more information, visit here .