PZT/Ti composite piezoceramic membranes for liquid filtration: Fabrication and self-cleaning properties

A novel composite membrane with a built-in self-cleaning capability was fabricated, characterized and tested. The membrane consists of a piezoelectric lead-zirconate-titanate (PZT) separation layer and a macroporous Ti support. The Ti support provides mechanical strength to the membrane and is also used as one of the electrodes needed to operate the membrane in self-cleaning mode. The PZT layer serves as a size selective membrane and as the component that generates ultrasound (US) by application of an alternating voltage (AV). The membrane structure was made by dry pressing of PZT particles on the porous Ti, and heat-treated at 650 °C to achieve sufficient mechanical integrity. A low treatment temperature was chosen to avoid adverse effects of Ti support oxidation, differential shrinkage and thermal expansion mismatch between the support and the PZT layer. Characterizations were performed with X-ray photoelectron spectroscopy, thermogravimetry, scanning electron microscopy, and mechanical strength measurements. These led us to conclude that the PZT reacted with a thin film of TiO₂, formed on the Ti surface, resulting in excellent interfacial strength. The membranes' performance was assessed from cross-flow water purification of oil in water (O/W) emulsions. For membrane structures with a poled PZT layer, operated with an AV of 60 V, the permeance became stationary at 92% of the initial value. For other membranes without US generation that number was ∼64%. The higher stationary permeance is ascribed to the fact that oil droplet accumulation on the membrane is avoided due to the action of in-situ generated US. The combination of Ti support and PZT membrane layer appeared particularly suitable because of minimal thermal expansion mismatches, excellent operational stability, and simplicity of the design.