sintering conditions of piezoelectric ceramics

Determination of sintering conditions:

To determine the sintering conditions, we should pay attention to the relationship between the two aspects: one is the relationship between the sintering conditions and the microstructure and properties of the porcelain; the other is the relationship between the sintering conditions and the shape and size of the blank. The following is a description of the PZT material piezoelectric ceramics of sintering temperature is  the PZT-based ceramic component, which mainly depends on the chemical composition of the formulation,they are the grain size of the ceramic, the shape and size of the green body, the molding method, the heating rate, and the holding time. The best firing is generally determined by measuring the shrinkage, density, microstructure and electrical properties of the components at various sintering temperatures during the trial firing. Generally speaking,piezo ceramics with a wide sintering temperature range (±30 °C) should be sintered at the upper limit of sintering temperature, but the holding time should not be too long; the sintering temperature range is narrow (±10 °C) for porcelain. The lower limit temperature is sintered; and the holding time can be appropriately extended. (2) Heating rate and holding time: When the heating rate is fast (about 300 °C/h), the sintering temperature should be upper limit, the holding time should not be extended; when the heating rate is slow (about 200 °C/h), sintering The temperature should be lower than the lower limit, and the holding time can be extended appropriately. In view of the fact that PbO is volatile at high temperatures, small-sized blanks of simple shape are more rapid and short-term (≤1h). However, for large and complex shapes, the temperature should be slowed down and the holding time should be extended, and the sealing and sintering should be strived to make the product evenly sintered and avoid lead loss.(3) Cooling speed and cooling method: The cooling speed and cooling method mainly depend on the shape and size of the blank and the structure of the sintering furnace. Generally, small and medium-sized blanks can be cooled by natural cooling in the furnace. Small blanks with simple shape, in order to refine the grain, avoid reducing blackening,which can open the furnace door slightly and cool by air;sensitive piezoelectric ceramics also adopt the method of air cooling and rapid cooling, and then carry out natural cooling with the furnace below 850 °C. Large-sized blanks with complex shapes are generally controlled to cool at a cooling rate of 100°C / h to avoid deformation cracking.

(1)Preparation before sintering:

1) Inspection: 1 sintering furnace heating body good or bad, thermocouple position, current and voltage balance, insulation performance and other furnace conditions; 2 sintered refractory parts, such as the integrity of the enamel and cover, the degree of lead absorption, good or bad Wait.

2) Mounting and feeding, the furnace should be gently and gently pushed to avoid the stacking of the blank. 

Set the sintering program.According to the sintering conditions specified by the process, a sintering program is set on the temperature controller, and the sintering is performed by electric heating.

(3) Regularly observe, adjust, and record the operating conditions of the sintering furnace, and repeatedly check, measure, and correct the temperature to prevent temperature deviation near the optimum sintering temperature and holding time.

(4) After the end of the heat preservation, power off, cool naturally or cool down by other cooling methods. It can be baked below 200 C.

Judging the quality of sintering:

In addition to determining the density, electrical properties and microstructure, the production of piezo ceramic bimorph piezoelectric ceramics can be judged by simpler and more direct visual judgment. According to this judgment method, the porcelain of the preferred sintered quality should have the following characteristics:

color at room temperature: uniform and shiny;

bonding phenomenon: the porcelain pieces are slightly bonded, but easy to separate; 3 shrinkage rate about 12%;

ink diffusion: no ink penetration, no diffusion; 5 sound: there is a crisp metal sound when struck; 6 mechanical strength: high strength, not easy to be broken;

section condition: uniform, dense, no pores, the shell is shell-like, shiny in the same furnace, porcelain in the same crucible, the above characteristics are basically the same, there is no significant difference. Main Factors are affecting sintering .The influence of raw materials is that different raw materials of different specifications and grades contain different types and quantities of impurities. The activities of different origins and batches of piezoelectric ultrasonic tranducers are also different, and the sintering conditions will also change accordingly. (2) The influence of zirconium-titanium ratio (Zr/Ti) with the increase of Zr/Ti has the sintering temperature increases, and it is even difficult to sinter. (3) The influence of additives. The additives that affect sintering mainly have the following categories:

1 A "soft" additive (such as La3+, Nb5+, etc.) that produces A-site defects. Conducive to ion diffusion, the sintering temperature range is expanded, and the sintering speed is increased.

2 "hard" additives that produce O vacancies (such as Na+, Fe3+, Sc3+, etc.). The lattice shrinkage is not conducive to ion diffusion and is difficult to sinter.

3 small radius equivalent substitution of the substitution of the A site ion (such as Sr2+, etc.). The A bit is loosened, the lattice is distorted to facilitate ion diffusion, and it is easy to sinter.

4 A liquid phase additive (such as SiO2, Bi2O3, etc.) is formed. In the long liquid phase in the green body, ions are easily diffused, which can lower the sintering temperature.which can limit the grain boundary segregation additives (such as Fe3+, Al3+, Cr3+, Ni3+, etc.). A part of the additive is solid-dissolved in the piezo discs piezoceramic transducer crystal to modify; a part is condensed on the grain boundary, limiting grain growth, resulting in a high-density microcrystalline structure, improving the electromechanical properties of the porcelain piece, and particularly improving the mechanical strength.