Sintering of lead-free piezoelectric materials

Abstract

Piezoelectric ceramics are used in many types of devices, and today, lead zirconate titanate (PZT) dominates the market due to its superior properties. Environmental and health concerns have made the search for lead-free piezoelectric ceramics an important issue. Potassium sodium niobate (KNN) is a lead-free ceramic material, where good piezoelectric properties are possible through compositional engineering or texturing, but sintering is challenging due to coarsening and alkali evaporation.

The formation of a liquid phase consisting of alkali hydroxides and carbonates at low temperatures is proposed to be the cause of the abnormal grain growth observed in KNN materials. A reducing atmosphere should destabilize these alkali hydroxides and carbonates, which will limit the coarsening and therefore improve the densification and enhance the piezoelectric performance.

The effect of the sintering atmosphere in nominally stoichiometric KNN and KNN with 3 mol% nominal alkali excess was investigated. Conventional sintering and dilatometry was conducted in synthetic air, nitrogen and a reducing atmosphere. Characterization of the sintered pellets was done by XRD, SEM, infrared spectroscopy and piezoelectric measurements.

This work confirms that the liquid phase forming in KNN materials at low temperatures during sintering (or calcination) consists of alkali carbonates and hydroxides. The reducing atmosphere limits the degree of coarsening and gives higher densities. In stoichiometric KNN the coarsening was especially limited, resulting in a microstructure consisting of small cubic grains.

For piezoelectric applications, the samples had to be reoxidized, but this did not deteriorate the microstructure significantly. The piezoelectric measurements show that high density and proper sample preparation is important. The obtained values of the normalized strain and piezoelectric coefficient were generally low compared to the reported values in the literature. The polarization loops were round and had large leakage currents. However, the results suggest that a reducing atmosphere during sintering might be beneficial for enhanced piezoelectric performance.