| dc.description.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. | |
