Effect of Dispersant and Binder on Fabrication of BZT-0.5BCT Piezoelectric Wafers by Tape Casting Technique

Abstract

Recently, piezoelectric wafers/substrates have opened new opportunities for nondestructive evaluation (NDE) of structures in defence, aerospace and industrial sectors. Such wafers can be easily made by tape casting of slurries where dispersants and binders play a critical role. Generally, lead zirconate titanate (PZT) based compositions are used for piezo wafers/substrates. However, recent concern about environmental pollution from lead-based materials has generated renewed interest in developing lead-free ferroelectrics like Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 [BZT-0.5BCT] having high dielectric constant (εr>3500) and piezoelectric properties (d33> 500pC/N). To date, there are no reports on the fabrication of BZT-0.5BCT wafers by tape casting and their electrical characterization.
The objective of the thesis is to develop a dense warpage free BZT-0.5BCT wafer/substrate with suitable dielectric and piezoelectric property. The BZT-0.5BCT powder was synthesized by solid-state reaction route. BZT-0.5BCT tapes were fabricated by a non-aqueous tape casting method. Tape casting slurries of BZT-0.5BCT powder were prepared using methyl ethyl ketone-ethanol (MEK-EtOH) (solvent), phosphate ester / polyvinyl butyral (PVB) (dispersant), PVB (binder), polyethylene glycol (PEG) and butyl benzyl phathalate (BBP) (plasticizer). The optimum amount of dispersant was determined from the minimum in slurry viscosity and sedimentation height. The role of the two dispersants (phosphate ester and PVB) in making stable slurry of BZT-0.5BCT powder vis-a-vis the final properties of the fired wafers was studied. It was confirmed that 1 wt.% phosphate ester is sufficient for effective dispersion of BZT-0.5BCT to get stable slurry and the amount was 0.5 wt.% when PVB was used as dispersant in place of phosphate ester. The role of the binder (PVB) on the fabrication of wafer was also studied. Variation of PVB content shows that at least 3wt% PVB required to get the workable green tape. Up to 3.5 wt% PVB (binder) addition, solid loading can be maintained at 65%. Improved dielectric (εr =1750 at 1 kHz), ferroelectric (Pr = 6.3 µC/cm2), and piezoelectric property (d33 =174 pC/N) were observed for wafers made without phosphate ester.