forming method of piezoelectric ceramic embryo (2)

 Granulation is a process in which a porcelain PZT material parameter is mixed with a binder to prepare coarser particles (about 20 mesh/inch) having good fluidity. This kind of granule is called a pellet to show the difference.

1) The role of granulation
Since the ground porcelain powder of sensitive piezoelectric ceramics is fine and light,which has a large specific surface area, and has a large volume, the fluidity is poor, the packing density and the compaction density are not high. Therefore, the effect of granulation lies in the binder in the uniform porcelain powder, increasing its particle size, specific gravity and fluidity, and it is increasing the density of the shaped body.

2) Common granulation methods and characteristics .Ordinary manual granulation method is add ing appropriate amount (such as 4-5wt%) of 5% concentration PVA aqueous solution binder to the porcelain, and mix it evenly in the mortar, then sieve. The method is simple in operation, but labor intensity is large, mixing is difficult to be uniform, pellet quality is not high, and efficiency is low, and it is only suitable for laboratory and small-scale production.

3 Pressurized granulation method
The porcelain of piezoelectric ring crystal was added to a 4-5 wt% 5% strength PVA aqueous solution binder, and uniformly stirred in a mixer to pass through a 20 mesh/twist sieve. Then, it was pressed on a hydraulic press with a stamper at a pressure of 180-250 kg/cm 2 for 1 minute to form a round cake. The crumbs were crushed with a crusher until they were all passed through a 20 mesh/twist screen. The granularity of the corporation is large, and the mechanical strength of the green body is high, which can meet the molding requirements of various large and special-shaped products. However, the output is small, the efficiency is not high, and the process operation requirements are strict, which is suitable for laboratory experiments and medium-volume production.

4 spray drying granulation
The porcelain mixed with the binder is first made into a slurry, and then spray-dried by a spray dryer to be granulated and collected for use. The agglomerate thickness of Pzt ceramic ring transducer can be controlled by atomization related conditions. The particles of the method are spherical,which have good fluidity, good quality, large yield, continuous production, it is high efficiency, low labor intensity and improved conditions. Suitable for mass production, but the equipment costs are high.

(5) Mold design
The design of the electronic transducer piezo ceramics mold is based on the shape, size, pressure, powder compression, sintering shrinkage, roughness, etc. of the product. The design of the mold: 1 matching accuracy (such as three-level precision matching); 2 finish (such as working surface) 3 hardness (such as HRC 57-62); 4 Demoulding taper (generally not less than 1%); 5 shape, size; 6 materials (tool steel, such as Cr12 steel).

(6) The factors are affecting the quality of the piezo ceramic disk forming are the adhesives by using PVA adhesives as examples. The main influencing factors are:

1 degree of polymerization n. n is too large, the chain length is long, the adhesion and elasticity are large, which is not conducive to granulation and molding; n is too small, the chain link is light, the elasticity and the bonding force are small, and the binder is not used. n Generally, 1750±50 is suitable.
2 dosage. Too little is not conducive to molding. Too much, after the displacement, there are many pores, which affect the density and electromechanical properties of the body and porcelain.