Last Updated on June 13, 2023 by You Ling
Piezoelectric ceramics must go through electrode application and polarization before they can be used properly. Electrode application is a crucial process that determines how piezoelectric ceramics will be assembled with other components. Furthermore, many of our customers’ requirements are closely related to the electrode application of piezoelectric ceramics. Therefore, through the following article, we would like to introduce the knowledge about electrode application of piezoelectric ceramics.
1、Main process flow of piezoelectric ceramics:
Ingredient preparation – Grinding – Filtration, drying – Pre-firing – Secondary grinding – Filtration, drying – Sieving – Shaping – Debinding – Sintering – Refinement – Electrode application – Silver firing – Polarization – Testing.
(1) Raw material processing:
First, prepare the ingredients according to the chemical reaction formula. The materials used are mostly metal oxides, and a few can also be carbonates (which decompose into oxides during pre-firing). To ensure a smooth chemical reaction for the production of piezoelectric ceramics, the fineness of the raw materials should not exceed 2μm (average diameter). Improving the purity of the raw materials is beneficial for enhancing product quality, but this is not an absolute requirement. By using raw materials with lower purity and selecting appropriate process conditions, high-performance products can still be produced.
Usually, a rotating ball mill or a vibrating ball mill is used for raw material mixing and grinding. In addition, air jet milling is often used in production, which utilizes the strong crushing effect of high-pressure airflow to form the powder into a mist. Since no grinding media is used, impurities can be avoided, and the efficiency is improved.
Reaction process during pre-firing: The pre-firing process generally goes through four stages: linear expansion (room temperature to 400°C), solid-phase reaction (400-750°C), shrinkage (750-850°C), and grain growth (800-900°C and above).
(2) Shaping and debinding:
After pre-firing, the raw materials form a solid solution compound. After another round of grinding, they can be shaped. Different shaping methods such as tape casting, pressing, or isostatic pressing can be used according to specific requirements. A binder needs to be added before shaping. For tape casting, the binder is usually around 15%-20% of the mass of the powder, while for pressing, only about 5% is needed. Excessive binder can reduce the density of the final product. After shaping, the binder and moisture in the green body must be heated and removed, a process called debinding.
The debound green bodies are placed back into the kiln for sintering. There are many factors that affect the sintering process. Firstly, the chemical composition of the formulation plays a significant role. When there are sufficient mobile ions in the formulation, sintering can proceed smoothly. For example, zirconium ions in PZT have low mobility, so increasing the zirconium content in PZT will raise the sintering temperature and make the process more challenging.
Additives play a crucial role in improving the performance and sintering of piezoelectric ceramics. “Soft” additives have the common characteristic of shifting the ceramic properties towards being “soft,” which means increasing the elastic compliance, reducing the mechanical quality factor Qm, increasing the dielectric constant, and enhancing dielectric loss. These “soft” additives create A-site vacancies when incorporated into the solid solution, hence they are also known as A-site vacancy-generating additives. The formation of cation vacancies during the sintering process greatly accelerates ion diffusion and promotes the sintering of piezoelectric ceramics.
“Hard” additives refer to metal ions such as K+, Na+ entering the A site, and Fe2+, Co2+, Mn2+, Ni2+, Mg2+, etc. entering the B site. The function of “hard” additives is opposite to that of “soft” additives. They can promote the development of piezoelectric ceramics towards being “hard,” which means reducing dielectric loss, increasing the mechanical quality factor Qm, and enhancing the dielectric constant. The primary effect of “hard” additives in the crystal lattice is to generate oxygen vacancies, causing cell contraction and reducing diffusion speed, making it difficult to sinter piezoelectric ceramics.
The addition of additives that generate a liquid phase (such as MgO, MnO) can lower the sintering temperature but narrow the sintering temperature range. Additives that limit grain growth (such as Fe3+, Al3+, Cr3+) can form limited solid solutions. As the sintering process progresses, lattice defects are corrected, solubility decreases, and the previously solid-solved additives precipitate at grain boundaries, forming a second phase. This can inhibit grain growth and improve the ceramic’s flexural strength.
The sintering of piezoelectric ceramics should be conducted in an oxidizing atmosphere. Especially when additives such as La3+ and Nb5+ are present in the composition, they often require post-sintering oxidation treatment to generate cation vacancies, reducing the number of free electrons and improving piezoelectric performance.
The evaporation of PbO during the sintering process has a significant impact on the quality of the product. The volatilization of PbO disrupts the chemical composition of the formulation, leading to the decomposition of Pb(Zr, Ti)O3 and the appearance of non-ferroelectric phase ZrO2, increasing the porosity inside the ceramic and making sintering difficult.
(4) Electrode application:
After refinement, grinding, and cleaning, the sintered ceramic can be coated with electrodes. Typically, electrode coating is done by applying silver paste and then drying it. The coated ceramics are placed in a furnace, heated to 750°C, and held for 10-20 minutes to reduce the silver oxide in the paste to metallic silver. The silver then diffuses and bonds firmly to the ceramic surface. Other methods such as vacuum deposition or chemical deposition can also be used for electrode application. Once the electrodes are applied, the products can undergo manual polarization treatment.
Piezoelectric ceramics must undergo polarization to exhibit piezoelectric properties. Polarization is the process of orienting the domains along the direction of an applied DC electric field. To ensure sufficient polarization, the polarization field should be increased. The magnitude of the polarization field depends mainly on the coercive field strength and saturation field strength. The polarization field strength must be greater than the coercive field strength to induce domain reversal. However, increasing the field strength can lead to breakdown, limiting the increase of the polarization field.
2、Several methods for electrode application in piezoelectric ceramics:
The main methods for electrode application in piezoelectric ceramics are chemical plating and vacuum plating:
(1) Chemical plating:
Chemical plating is a self-catalytic process that does not require electricity. Based on the principle of oxidation-reduction reactions, strong reducing agents are used in a solution containing metal ions to reduce the metal ions to metal and deposit a dense coating on various material surfaces.
(2) Vacuum Plating
Vacuum plating mainly includes several types such as vacuum evaporation, sputtering, and ion plating. They all involve depositing various metal and non-metal films on the surface of the object in a vacuum environment through distillation or sputtering. This method can achieve very thin surface coatings and has the advantages of fast speed and good adhesion. However, it is relatively expensive, has a limited range of applicable metals, and is generally used for functional coatings on high-end products.
3、 Differences between Chemical Plating and Vacuum Plating for Piezoelectric Ceramics
Electroless plating is widely used due to its simple process and less stringent equipment and environmental requirements compared to vacuum ion plating.
So far, many heavy metals can be deposited from aqueous solutions through the process of electroless copper plating. From an economic perspective, electroless copper plating has the lowest cost and is therefore widely adopted. The appearance of the electroless copper layer is copper-red and cannot be used as a decorative or protective layer. It is usually used as a conductive layer for non-metallic materials, printed circuit board hole metallization, and other electroplating thickening layers. Electroless copper plating can only provide a thin conductive layer for piezoelectric ceramic components. If other layers need to be further thickened, the electroless copper layer must be electroplated with copper to increase its thickness. Acidic copper plating or alkaline copper plating can be used for electroplating copper. After electroplating copper on the piezoelectric ceramic component, other metal layers can be electroplated as needed.
In contrast to chemical plating, vacuum plating is more environmentally friendly, safe, energy-efficient, and does not generate wastewater. Vacuum plating allows for better control of coating thickness and enables the preparation of various functional coatings. The bond between vacuum-plated coatings and the substrate is stronger and less prone to detachment. Vacuum plating provides dense, pure coatings with better adhesion.
Currently, the most commonly used method in the production process of piezoelectric ceramics is chemical plating. Besides the low cost, another reason is for better soldering of the piezoelectric ceramic components. The metal coating deposited by chemical plating is thicker, making it easier for soldering. However, chemical plating on the surface of piezoelectric ceramics can sometimes leave watermarks or scratches, which may affect the aesthetics.
Vacuum plating is a more advanced technology but requires specialized equipment and involves more complex operations. Generally, we use vacuum magnetron sputtering to deposit nickel metal onto piezoelectric ceramics, followed by a choice of further gold or copper plating. The vacuum-plated coating on piezoelectric ceramics is thinner, which is not conducive to soldering. However, this can be resolved by multiple plating cycles, although it increases the cost. Additionally, the surface of vacuum-plated piezoelectric ceramics is very smooth and aesthetically pleasing.
Chemical plating and vacuum plating are the main methods for electrode application on piezoelectric ceramics. Currently, chemical plating is the preferred method due to its low cost and simplicity. However, vacuum plating is a more advanced technology, albeit with higher equipment costs and the need for multiple coating cycles. With technological advancements in the future, vacuum plating is expected to become the primary method for electrode application on piezoelectric ceramics.