Synthesis and characterisation of textured lead-free piezoelectric ceramics

Abstract

Lead-based piezoelectric ceramics are applied extensively in electromechanical devices. While lead-free alternatives are available, their performance and/or reliability have not yet been developed to the extent that they are suitable for commercialisation and the widespread replacement of lead-based piezoceramics. The aim of this thesis is to investigate preferential grain orientation (texture) as a route for improving lead-free piezoelectric ceramics based on K0.5Na0.5NbO3 (KNN) and Ca, Zr-doped BaTiO3. Importantly, this thesis demonstrates that electromechanical property improvement through texturing is contingent on utilising the direction of highest texture, which must also be the crystallographic direction in which the piezoelectric response is maximised.

KNN-based materials are known to suffer from processing challenges. In this work, spray pyrolysis was used to produce large amounts of fine (~100 nm) KNN powder. Despite starting with fine powders, challenges similar to those reported for conventionally prepared KNN powders (low densification and non-stoichiometry) were encountered. Intentionally non-stoichiometric KNN was also synthesised to study the effect of non-stoichiometry on powder and densification properties; neither excess alkali nor Nb improved the density. Large cuboidal grains and rapid densification at low temperatures (~650 °C) observed in alkali-excess KNN suggest the presence of a liquid phase during heating to the sintering temperature. This liquid phase is proposed to consist of alkali hydroxides and/or carbonates from reactions of the excess alkali with atmospheric water and CO2. Critically, the liquid formed through the melting of these species during heating results in coarsening, which consequently limits the further densification of alkali-excess KNN. Furthermore, it is proposed that both the formation of cuboidal grains and sintering challenges in stoichiometric KNN are also caused by the same mechanisms as for alkali-excess KNN. In addition, this mechanism also provides an explanation for the increased density and more uniform microstructures previously observed when sintering KNN in reducing atmospheres, as one would expect the alkali hydroxides and/or carbonates to evaporate in these conditions. As a consequence of alkali oxide evaporation at high temperatures, a Nb-rich secondary phase was observed after sintering of nominally stoichiometric KNN. The distribution of this secondary phase was dependent on the dwell time and cooling rate, and showed clear indications of the presence of a high-temperature liquid phase during sintering.

Texturing has previously been demonstrated to substantially improve the piezoelectric response of KNN-based materials. By tape casting with needle-like KNN templates followed by homotemplated grain growth, 94 % dense KNN ceramics with a strong <100>pc texture (Lotgering factor F = 86 %) parallel to the tape cast direction and a moderate (F = 28 %) <001>pc texture normal to the tape cast plane were obtained. Enhanced piezoelectric response (d33 = 125 ± 3 pC/N, Smax/Emax 300 pm/V) and the highest degree of domain reorientation was observed parallel to the tape cast direction compared to non-textured KNN (d33 of 107 ± 4 pC/N, Smax/Emax 200 pm/V). Furthermore, texture reduced the dielectric nonlinearity at subcoercive fields. This suggests that <100>pc texture enhances the piezoelectric response in KNN (in accordance with rotator ferroelectricity), introduces an advantageous domain configuration, and facilitates non-180° domain reorientation at high fields. Interestingly, when the electric field was applied normal to the tape cast plane (the direction of moderate <001>pc), lower response (d33 = 89 ± 3 pC/N, Smax/Emax 130 pm/V) compared to both non-textured ceramics and parallel to the tape cast direction was observed. This suggests that, within a textured ceramic, strong texture in one direction might restrict the domain reorientation parallel to other directions.

Superior piezoelectric response has been reported in Ba0.85Ca0.15Zr0.1Ti0.9O3 (BCZT), yet the structure of this composition is still debated. Rietveld refinements of x-ray diffractograms of BCZT in the temperature range -100 °C to 150 °C were performed to investigate the structure. Upon cooling, the phase sequence cubic, tetragonal, tetragonal + rhombohedral, and rhombohedral was observed. This phase sequence is consistent with previous reports of two-phase coexistence at room temperature, and extends the temperature range of the phase-coexistence to well below room temperature. In addition, the diffractograms also showed evidence of thermal history-dependent strain from domain walls

Texture was studied in Ba0.92Ca0.08TiO3 (BCT) due to the wider temperature range of the tetragonal phase in this composition compared to BCZT. <100>-textured (F of up to 99 %) and <111>-textured (F of up to 84 %) BCT were prepared by tape casting and templated grain growth using BaTiO3 platelet templates. The largest piezoelectric response was obtained in the <100>-textured BCT (Smax/Emax of up to 621 pm/V and d33 of up to 238 pC/N), and the lowest in the <111>-textured (Smax/Emax of up to 234 pm/V and d33 of up to 113 pC/N), compared to non-textured BCT (Smax/Emax of up to 336 pm/V and d33 of up to 155 pC/N). The large response in the <100>-textured BCT persisted upon heating to 100 °C, which contributes to its potential as a viable lead-free piezoelectric. The order of response with the type of grain configuration correlates with a hypothesis of extender ferroelectricity in BCT, based on the behaviour of BaTiO3 at temperatures significantly above the tetragonal-orthorhombic transition. This demonstrates that considerations of the piezoelectric anisotropy and rotator/extender ferroelectricity are necessary in the optimisation of textured lead-free piezoelectrics. Remarkably, poling at high temperatures improved the d33 of the <111>-textured while decreased the d33 of the <100>-textured and non-textured BCT ceramics.

Framstilling og karakterisering av kornorienterte blyfrie piezoelektriske materialer

Elektroniske instrumenter og utstyr, inkludert det meste av forbrukerelektronikk, inneholder en rekke helse- og miljøskadelige stoffer som for eksempel blyoksider i piezoelektriske materialer. Piezoelektriske materialer deformeres når de påføres elektrisk spenning, eller motsatt, genererer elektrisk spenning når de deformeres. Denne egenskapen gjør at disse materialene anvendes for eksempel til å nøyaktig dosere ut blekk i blekkskrivere eller til å sende og motta ultralydbølger til medisinsk avbildning. Det finnes blyfrie piezoelektriske materialer, men ennå ikke med tilstrekkelig funksjonalitet til å kunne erstatte de dominerende blyholdige.

Denne doktoravhandlingen omhandler i hovedsak orientering av kornene – eller byggesteinene - i blyfrie keramiske piezoelektriske materialer. Hvis alle kornene i materialet er orientert i samme retning, kan man påføre elektrisk spenning i den mest optimale retningen og dermed oppnå bedre respons enn når alle kornene er tilfeldig orientert.

Blyfrie piezoelektriske keramiske materialer med en spesiell kornorientering ble i dette arbeidet framstilt ved å orientere store, plate- eller nåleformede partikler i samme retning i en blanding av små partikler. Ved påfølgende varmebehandling gror de små partiklene i samme retning som de store, og materialet får en ordnet struktur. To ulike typer keramiske materialer ble undersøkt, ett bestående av grunnstoffene kalium, natrium, niob og oksygen, det andre av barium, titan, kalsium og oksygen. I begge systemene førte den spesielle orienteringen til høyere piezoelektrisk respons (mer deformering ved påført elektrisk spenning) i noen retninger sammenlignet med materialer med en tilfeldig kornorientering.

I tillegg til arbeidet med å lage en spesiell kornorientering ble strukturen på atomnivå studert ved bruk av røntgenstråler med høy energi, og forbedrede fremstillingsmetoder for tette, blyfrie piezoelektriske keramer ble utviklet. Alt i alt bidrar denne doktoravhandlingen til å øke forståelsen av blyfrie piezoelektriske materialer og med det bidra til utviklingen av mer miljøvennlig elektronikk.