The production process of piezoelectric ceramics is similar to that of ordinary ceramics, but the production process of piezoelectric ceramics has its own characteristics. PZT namely Pb (Zr, Ti) O3 pottery urn is mainly used here to introduce the necessary process and production method of piezoelectric ceramics. Main technological process of piezoelectric ceramics:
Ingredients -- ball mill -- filtering, drying -- pre-burning -- secondary ball mill -- filtering, drying -- sieving -- forming--plasticizing -- sintering -- finishing -- upper electrode -- silver burning -- polarization -- test.
1. Raw material processing
First of all, according to the chemical reaction formula ingredients. Most of the raw materials used are metal oxides, and a few can also be carbonates (decomposed into oxides during pre-sintering). In order to make the chemical reaction of generating piezoelectric ceramics proceed smoothly, the fineness of the raw materials must not exceed 2 μm (average diameter). Increasing the purity of raw materials is conducive to improving product quality, but this problem is not absolute. Using low-purity raw materials and selecting appropriate process conditions can also produce products with excellent performance.
Usually, a rotating ball mill or a vibrating ball mill is used to mix and crush the raw materials. In addition, the jet pulverization method is often used in production, and the strong crushing action of high-pressure airflow is used to form the powder into a mist. Since no ball stone is used, the mixing of impurities can be avoided and the efficiency is improved.
The reaction process in pre-sintering: the pre-sintering process generally has to go through four stages: linear expansion (room temperature～400℃), solid phase reaction (400～750℃), shrinkage (750～850℃) and grain growth (800～ Above 900°C).
2. Forming and molding
After the raw material is pre-burned, the solid solution compounds are synthesized. After another crushing, it can be formed. Forming can be according to different requirements by rolling film, pressure or isostatic pressure. Adhesive is added before forming. For rolling film, the adhesive is generally 15% to 20% of the quality of powder, on the pressure profile of only 5% or so. Too much binder will reduce the density of the product. After forming the green adhesive, water must be heated to discharge, known as plastic or plastic discharge.
The green body after plastic discharging is reloaded into the furnace for sintering. There are many factors that affect sintering. The first is the chemical composition of the formula. When there are enough active ions in the formula composition, sintering is easy to proceed. For example, the activity of zirconium ions in PZT is poor, so the content of zirconium in PZT increases, the sintering temperature rises, and sintering is difficult.
The additive plays an important role in improving the performance and sintering of piezoelectric ceramics. The common feature of "soft" additives is that the ceramic properties can be changed to "soft", that is, the elastic compliance coefficient can be improved, the mechanical quality factor Qm value can be reduced, the dielectric constant can be increased, and the dielectric loss can be increased. Since the "soft" additive forms A vacancy when it is added to the solid solution, it is also called to produce A vacancy additive. Cationic vacancy is formed in the sintering process, which greatly accelerates the ion diffusion process and promotes the sintering of piezoelectric ceramics.
"Hard" additives refer to metal ions such as K and Na entering the A position and Fe2 , Co2 , Mn2 , Ni2 , Mg2 and the like entering the B position. The effect of "hard" additives is just opposite to the effect of "soft" additives, which can make the performance of piezoelectric ceramics develop in the direction of "hard", that is to say, reduce the dielectric loss and increase the Qm value of the mechanical quality factor. Increase the dielectric constant. The role of "hard" additives is the main role. In the crystal lattice, it causes oxygen vacancies, shrinks the unit cell, reduces the diffusion rate, and makes it difficult for piezoelectric ceramics to sinter.
Adding additives that produce a liquid phase (such as MgO, MnO, etc.) can reduce the sintering temperature, but the sintering temperature range is narrowed. Adding additives that can restrict grain growth (such as Fe3 , Al3 , Cr3 , etc.) can form a limited solid solution. With the sintering process, the lattice defects are corrected and the solubility is reduced, making the solid solution additives in the crystal The boundary precipitates and forms the second phase, which can inhibit the growth of crystal grains, thereby improving the flexural strength of the ceramic.
The firing of piezoelectric ceramics should be carried out in an oxidizing atmosphere. Especially when there are additives such as La3 and Nb5 in the composition, they are often oxidized after sintering to ensure the generation of cation vacancies, reduce the number of free electrons, and improve the piezoelectric performance.
The volatilization of PbO during the sintering of piezoelectric ceramics has a great influence on the quality of the product. Due to the volatilization of PbO, the chemical composition of the formula is destroyed, leading to the decomposition of Pb (Zr, Ti) O3 and the appearance of non-ferroelectric phase ZrO2, which increases the porosity of the ceramic and makes it difficult to sinter.
Piezoelectric ceramic products
4. On the electrode
The fired ceramics can be covered with electrodes after being refined, ground and cleaned. The coated electrode is generally dried with coated silver paste, then installed in a furnace, heated to 750°C, and kept for 10-20 minutes, so that the silver oxide in the silver paste is reduced to silver and burned to the ceramic surface to form a strong bonding layer. It can also be electrodeposited by vacuum evaporation or chemical deposition. The products on the electrode can be artificially polarized.
Piezoelectric ceramics must be polarized to have piezoelectricity. Polarization is to orient the domains in the direction of the electric field under the action of a DC electric field. In order to make the polarization fully proceed, the polarization field should be increased. The polarization electric field mainly depends on the coercive field strength and the saturation field strong. The polarization field strength must be greater than the coercive field strength in order to reverse the electrical domains. But increasing the field strength can easily cause breakdown, which limits the increase in the polarization electric field.