6. Tape cast porosity-graded piezoelectric ceramics
We have recently reported the optimization needed to produce a flat, 400 µm thick,
functionally-graded porous Nb-doped PZT material (Pb0.988(Zr0.52Ti0.48)0.976Nb0.024O3, PZTN)
by tape casting (Mercadelli et al., 2010). Casting, lamination, debinding and sintering are
necessary steps to obtain multilayer structures with this technique. Any variation of each of
these steps strongly affects the final product. Therefore each of them (slurry formulation,
lamination and thermal treatments) has to be thoroughly investigated.
Combining the sacrificial template method with tape casting, an engineered porosity could
be produced. The choice of the right pore former deeply influences the size and morphology
of the pores and, as a consequence, the final electrical properties of the piezoelectric product.
A preliminary study was conducted to identify the most suitable pore former to obtain
micrometric and isotropic porosity. A literature analysis indicated rice starch (RS) and
carbon black (CB) as the more appropriate pore formers to obtain fine porosity. Sintered
tapes with 30%vol of CB and RS lead to about the same amount of total porosity (35%).
However the tapes fracture sections in figure 5 clearly show that CB produces uniform
micrometric pores while RS leads to 5-6 µm size pores. The CB was therefore chosen as pore
former for the fabrication of porous piezoelectric layers and multilayers.