Feroelektrična domenska struktura piezoelektrične trdne raztopine BiFeO3-SrTiO3

Žiberna, Katarina (author), Rojac, Tadej (mentor), Meden, Anton (comentor)

Piezoelektrični materiali imajo široko uporabo v elektromehanskih aplikacijah kot senzorji, aktuatorji ali ultrazvočni pretvorniki. Ker je v večini komercialnih piezoelektričnih elementov prisoten tudi strupeni svinec, kot je Pb(Zr,Ti)O3, iščemo nove materiale, ki bi nadomestili piezoelektrike na osnovi svinca. Poleg tega je vse več raziskav usmerjenih v iskanje nesvinčenih piezoelektričnih materialov, ki delujejo pri visokih temperaturah. Med temi izstopa bizmutov ferit, BiFeO3 (BFO), z izjemno visoko Curie-jevo temperaturo. Zaradi slabih piezoelektričnih lastnosti in visoke prevodnosti, lastnosti BiFeO3 izboljšujejo s tvorbo trdnih raztopin z drugimi perovskitnimi materiali. xBiFeO3-(1-x)SrTiO3 z deležem bizmutovega ferita okrog t. i. morfotropne fazne meje je ena takih sestav, ki poleg značilnih feroelektričnih in piezoelektričnih lastnosti kaže še relaksorske značilnosti, kot na primer vrh v dielektričnem spektru, ki je frekvenčno odvisen. Ker so relaksorske lastnosti tesno povezane z domensko strukturo ter značilnimi polarnimi nano domenami, je bila v tem delu preiskovana domenska struktura keramičnega sistema xBiFeO3-(1-x)SrTiO3 v širokem območju sestav (0,575 ≤ x ≤ 0,70). Domenska struktura preiskovanih sestav kaže tako odvisnost od sestave, kot tudi odvisnost od jakosti zunanjega električnega polja. Z višanjem deleža stroncijevega titanata se domene manjšajo, kar pripisujemo večjemu kemijskemu neredu v kristalni rešetki in potemtakem tvorbi relaksorske faze. Sestava z najmanjšim deležem BFO ima drobno nanometrsko domensko strukturo. Po drugi strani ima sestava, ki je najbogatejša z BFO, značilne lamelaste domene z večkrat opaženo strukturo t. i. ribje kosti. V tej sestavi polje vpliva na reorganizacijo domen iz fine v bolj urejeno lamelasto strukturo. Iz analize nelinearnega dielektričnega odziva keramike je po polarizaciji vzorcev opazno povečanje nelinearnega odziva območja revnega z BFO, in nasprotno, zmanjšanje nelinearnega odziva sestav bogatih z BFO. Jasne povezave med tem nelinearnim odzivom in spremembo domenske strukture pod vplivom polja nismo opazili. Nelinearne odzive polarizirane keramike pripisujemo spremembam na ravni porazdelitve defektov znotraj materiala med samo polarizacijo, ki definirajo potencial za gibanje domenskih sten.

• Language: Slovenian
• Keywords: domene, domenske stene, keramika, BiFeO3-SrTiO3
• Work type: Master's thesis/paper
• Typology: 2.09 - Master's Thesis
• Organization: FKKT - Faculty of Chemistry and Chemical Technology
• Year: 2020
• PID: 20.500.12556/RUL-119398
• COBISS.SI-ID: 28750083
• Publication date in RUL: 08.09.2020

Secondary language

• Language: English
• Title: Ferroelectric domain structure of piezoelectric BiFeO3-SrTiO3 solid solution

Abstract

Piezoelectric materials are widely used in electromechanical applications such as sensors, actuators and ultrasonic transducers. Since most commercial piezoelectric elements consist of toxic lead, such as the widely used Pb(Zr,Ti)O3 ceramics, the research is strongly focused on the development of lead-free materials that would replace the lead-based counterparts. In addition, recent research has been focused on the development of lead-free materials that can withstand high temperatures, where bismuth ferrite, BiFeO3, is particularly promising because of its extremely high Curie temperature. However, in order to enhance the poor piezoelectric properties and suppress the high conductivity of pure bismuth ferrite, a commonly adopted strategy is to create solid solutions of bismuth ferrite with other perovskites. The xBiFeO3-(1-x)SrTiO3 with the content of bismuth ferrite close to the morphotropic phase boundary region is one of such compositions showing characteristic piezoelectric and ferroelectric properties along with relaxor behavior, evidenced by a frequency dependent peak in the dielectric spectrum. Since the relaxor properties are strongly correlated with the domain structure and the characteristic polar nanodomains, this work focuses on the domain structure analysis of the xBiFeO3-(1-x)SrTiO3 ceramics in a wide compositional range (0,575 ≤ x ≤ 0,70). The domain structure of the studied compositions revealed strong dependency on the composition, as well as on the applied external electric field. With the increase of the strontium titanate content, the domains become smaller due to the so-called chemical disorder in the crystal lattice and the formation of a relaxor phase. The composition with the lowest BFO content has a fine nano-domain structure. On the contrary, the composition with the highest BFO content has larger lamellar domains with a herring bone structure. In this composition we can observe a field induced reorganization from a fine domain structure to organized domain structure with lamellar domains. After poling, an increase of the nonlinear dielectric response for BFO-poor compositions was observed, and conversely, a decrease in the nonlinear response for BFO-rich compositions. There was no clear correlation between this response and electric-field induced changes within the domain structure. We explain the nonlinear response in poled samples to defect redistribution in the material, which define the energy potential for domain-wall motion.

• Keywords: domain, domain walls, ceramics, BiFeO3-SrTiO3