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Sintering of lead-free piezoelectric materials

Bakken, Kristine
Master thesis
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URI
http://hdl.handle.net/11250/2377302
Date
2015
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  • Institutt for materialteknologi [2407]
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.
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NTNU

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