Preparation and characterization of piezoelectric ceramic–polymer composite thick films by aerosol deposition for sensor application

Abstract

Lead zirconate titanate (PZT)-based piezoelectric ceramic and polyvinylidene fluoride (PVDF) organic composite thick films (∼20 μm in thickness) were deposited on a 4-in. diameter Pt/Ti/SiO₂/Si substrate and a 2-in. diameter Ag–Pd/sapphire substrate by aerosol deposition using a single 4-in. size nozzle for use as a tactile-sensor device. The PVDF content was varied from 3 to 10 vol% and the films deposited on silicon and sapphire were annealed at 700 and 900 °C, respectively. The annealed films showed a relatively high density, even at a 10 vol% of PVDF-piezoelectric ceramic composition, as well as good adhesion to the substrate after annealing. The relative dielectric constant of the composite film decreased sharply with increasing PVDF content, while the piezoelectric d₃₃ coefficient was conserved at a relatively high value. Therefore, the g₃₃ coefficient increased and showed the highest value at 3 vol% PVDF. The g₃₃ values of the as-deposited, 700 °C annealed and 900 °C annealed piezoelectric ceramic-PVDF composite films were 7.77, 16.3 and 41.5 (10⁻³ m²/N), respectively.

Introduction

Piezoelectric ceramic films have a wide range of sensor and actuator applications that are important in a variety of fields for precise sensing and controlling small forces and displacements [1]. For applications such as motion sensors, actuators, ultrasound and acoustic devices, the film thickness is one of the most important requirements because the piezoelectric displacement is proportional to the film thickness [2], [3].

Pb(Zr,Ti)O₃ (PZT) is the most common piezoelectric material used for sensor and actuator applications on account of its unique ferroelectric and electromechanical properties. PZT films with thicknesses ranging from 5 to 150 μm fill the gap between PZT thin films (t = 100 nm to 2 μm) and bulk materials (t > 200 μm). Films of such thickness are difficult to fabricate using conventional thin film production techniques such as sol–gel, sputtering, chemical vapor deposition or pulsed laser deposition, due to the relatively slow deposition rates and high levels of stress generated during processing [4]. On the other hand, the sintering of PZT thick films using conventional powder processes is still very difficult due to the following: the lack of lateral material transport, PbO evaporation during sintering, thermal expansion mismatch between the PZT thick film and substrate material, and reactivity with the substrate [5]. There were studies for fabrication of piezoelectric ceramic–polymer 0–3 composite bulk or thick films by screen printing to increase the film thickness and decrease the processing temperature [6], [7], [8]. However the matrix of the composite was mostly polyvinylidene fluoride (PVDF)-based polymers, therefore the piezoelectric d₃₃ constant is very low (<10 pC/N) and the wear resistance characteristic is bad.

The aerosol deposition (AD) process is a recently developed room-temperature gas deposition method that employs fine particles mixed with a carrier gas to form a colloid aerosol flow. The aerosol flow is accelerated by the high pressure difference between two chambers and is ejected through a nozzle. The accelerated particles in the aerosol flow collide at high speed with the substrate and form a dense ceramic film [9]. The AD process has a high deposition rate (>5 μm/min) and no difficulties such as PbO evaporation or reactions with the substrate because almost full densification (>95%) can be achieved at room-temperature. However, the PZT film consolidated by the AD process has an extremely small crystalline size, generally from 5 to 20 nm, and exhibits poor piezoelectric properties [10]. For actual applications to enhance piezoelectric properties, a post-annealing of the film at temperature >600 °C is required. At the post-annealing temperature, which is far lower than the temperature required for conventional powder sintering, PbO evaporation or reactions with the substrate is not such a severe problem. However, thermal expansion mismatch of the PZT thick film and the substrate material sometimes cause the film to crack or delaminate from the substrate, which becomes more severe when the film thickness is increased.

In previous studies, 20∼100 μm thick PZT-based piezoelectric films were fabricated using powders containing excess PbO [11] or organic residue derived from the sol–gel route [12] to relieve the thermal stress formed during the post-annealing step as a result of PbO evaporation or organic decomposition. However, the inhomogeneous PbO evaporation or the high powder preparation cost for the sol–gel route is still an issue that needs to be overcome, particularly for large area deposition. In this study, ∼20 μm thick piezoelectric composite films were prepared using relatively cheap commercial PZT-based piezoelectric ceramics and PVDF organic powder, and the structural changes and electrical properties were characterized with different PVDF contents and post-annealing temperatures. The piezoelectric ceramic–PVDF composite film showed excellent structural stability against thermal stress, as well as an excellent piezoelectric voltage coefficient (g₃₃) comparable to that of the bulk material.