Views:0 Author:Site Editor Publish Time: 2019-09-10 Origin:Site
Lead zirconate titanate ceramics (PZT) have been widely used in polarized piezoelectric ceramic transducer due to their excellent properties. However, the piezoelectric ceramics are containing PZT have a relatively high sintering temperature of about 1200 °C. The oxidation temperature of lead oxide (PbO) is about 800 °C. Therefore, it is easy to cause the volatilization of PbO during the sintering process, which not only causes environmental pollution, but also causes the actual composition of the ceramic material to deviate from the designed formulation, resulting in deterioration of electrical properties. In addition, with the development of surface mount technology (SMT), multi-layer chip devices are favored by the market for their high efficiency, miniaturization, and functional integration. However, the high sintering temperature of the piezoelectric ceramic makes it difficult to achieve a co-firing of the ceramic material and the electrode material. If process improvement can be made from lowering the sintering temperature, it not only has important technical and economic value in suppressing PbO volatilization, which is ensuring material properties, reducing environmental pollution, prolonging equipment life, etc., but also replacing Pt and Pd with Ag, Ni, etc. The noble metal is used as the inner electrode to fire the multilayer chip piezoelectric component at a time, thereby greatly reducing the cost of the device and saving energy. In addition, in-depth research on low-temperature sintering can promote the development of sintering theory. At present, the process methods for reducing the sintering temperature of ceramics including the sintering process, the milling process, and the method of liquid phase sintering. Among them, the liquid phase sintering process is the simplest, the lowest cost, and is convenient for industrial application. It is one of the hot spots in the piezoelectric industry at home and abroad to study the low-temperature sintering of PZT piezoelectric ceramics by liquid phase sintering.
1 Mechanism of liquid phase sintering
By sintering a low-melting sintering aid such as Bi2O3, B2O3, SiO2, V2O5, etc. in a PZT piezoelectric base material, adding a substance is capable of forming a solid solution with PZT, the sintering of PZT piezo transducer piezoelectric ceramic belongs to a liquid phase. The liquid phase that occurs during the sintering process will wet the green body particles and fill the pores between the particles, which is creating a surface tension between the particles. The magnitude of the surface tension is related to the nature, quantity, and particle size of the liquid phase. Under the action of surface tension, the particles flow and change the original arrangement and rearrange them to obtain a closer packing between the green particles. At the same time, due to the presence of the liquid phase, the mass transfer process will proceed at the interface between the liquid phase and the solid phase, the small particles gradually dissolve to disappear, and the large particles grow continuously and eventually become uniform and dense porcelain. The presence of a liquid phase activates the atoms on the surface of the powder particles. The low-melting glass phase formed by various low-melting sintering aids is distributed along the contacting interface of each particle, and the atoms are transported by liquid phase diffusion, and the diffusion coefficient is large, so that the sintering process is greatly accelerated, and the sintering temperature can be made larger than when the liquid phase does not occur. To reduce that the following three requirements must be met as additives for liquid phase sintering: (1) liquid phase should appear at the sintering temperature; (2) good wettability to solids; (3) solid phase in liquid phase. There is significant solubility in it.
2 Ways to achieve liquid phase sintering
The biggest advantage of PZT piezoelectric ceramics is that they can be adjusted to a wide range of electrophysical properties by changing the composition or changing the external conditions to suit different needs. The use of other substances to achieve low-temperature sintering of PZT ceramics not only reduces the sintering temperature, but also optimizes its electrical properties. In addition, the liquid phase low-temperature sintering can be realized by the conventional solid phase method, and the process is simple and industrialized. The liquid phase sintering is mainly carried out in three ways: (1) the solid matter is formed into a solid solution with the sintered body; (2) the additive forms a liquid phase with the sintered body (3) the additive forms a transition liquid phase with the sintered body.
3.1 Addition to the sintered body to form a solid solution
When the admixture is close to the particle size, piezo crystal form and electricity price of the sintering phase, it can be mutually soluble to form a solid solution,which is causing the main crystal phase to be lattice-distorted, and the defects are increased, so that some structural elements are in a non-equilibrium state, which has a large energy and is convenient. Move to promote sintering.
2.2 The addition of the substance to the sintered body forms a liquid phase
Liquid phase sintering is due to the addition of additives to make the system have a low eutectic point. The temperature is significantly lower than the traditional sintering temperature of the piezo round disc transducer. The liquid phase is formed in the initial stage of sintering. The grain boundary can be enhanced due to grain rearrangement and strengthening contact in liquid phase sintering. The mobility is such that the pores are sufficiently discharged, thereby promoting the densification of the sintering and achieving the purpose of lowering the sintering temperature. However, the addition of a low-melting glass or oxide introduces a second phase, and the presence of too much second phase inevitably leads to a significant decrease in the dielectric constant of the ceramic, and an increase in the dielectric loss tan δ must be noted.
2. 3 The addition and the sintered body form a transitional liquid phase.
If the piezo ceramics can form a liquid phase at the initial stage of sintering, and the composition of the liquid phase can volatilize or re-enter the main crystal phase lattice at a high temperature, it can function as a doping modification. Such low melting point additives can not only greatly reduce the sintering temperature, but also avoid the formation of secondary phases by the addition of flux, thereby maintaining the good piezoelectric properties of the original base material, which is of great significance for energy saving and environmental pollution reduction.