材料化学(论文)

Journal of the Korean Physical Society,Vol.61,No.6,September2012,pp.891∼894

Phase Structure and Piezoelectric Properties of Li-modified NKLN Lead-free

Piezoelectric Ceramics

Sin-Woong Kim,Sung-Chan Lee,Min-Soo Kim,Soon-Jong Jeong,In-Sung Kim and Jae-Sung Song∗

Korea Electrotechnology Research Institute,Changwon641-120,Korea

(Received20March2012,infinal form2July2012)

Through the low-temperature sintering method,a sintered body with excellent characteristics

was produced in an eco-friendly niobate-based piezoelectric ceramic,whose application was low in

expectation due to poor sinterability.Li2CO3was added in excess to(Na0.49K0.45Li0.06)NbO3,and

ceramics were manufactured using a commercial sintering method.Then,the sinterability and the

piezoelectric properties of the specimens containing varying amounts of Li2CO3were investigated.

The microstructure demonstrated the typical abnormal grain growth tendencies with the addition

of Li2CO3,and this was explained through changes in the critical driving force in the interface

reaction-controlled nucleation and growth theory.When the specimen had been sintered at1000◦C

for4hours in air after the addition of1.5mol%Li2CO3,the sintered body showed outstanding

characteristics with a piezoelectric coefficient of180pC/N,an electromechanical coupling coefficient

of0.32,and a dielectric constant of975.These results showed that eco-friendly niobate-based

ceramics,whose use in applications was expected to be difficult in spite of their excellent properties,

could be used to produce piezoelectric materials with outstanding properties through a commercial

low-temperature sintering method using additives.

PACS numbers:77.65.-j,77.80.Bh,77.84.Dy,77.80.-e

Keywords:Lead-free,Piezoelectric,Microstructure,NKN-based ceramics

DOI:10.3938/jkps.61.891

I.INTRODUCTION

Pb(Zr1−x Ti x)O3(PZT)is currently the most out-standing piezoelectric material and at present,many studies are focusing on the utilization of piezoelectric ac-tuators,piezoelectric transducers,sensors and resonators [1].However,although PZT ceramics are outstanding in terms of piezoelectric properties,they contain lead, which leads to problems in terms of environmental pol-lution and economic feasibility.Thus,much research is being performed on eco-friendly piezoelectric ceramics, which do not contain the hazardous element lead[2–10]. In particular,among the lead-free piezoelectric ceram-ics,(Na0.5K0.5)NbO3(NKN)has a high phase transition temperature(420◦C),low coercive electricfield(5kV/ cm)and a high remanent polarization(30µC/cm2)and is,therefore,considered as one of the most outstanding materials that could replace piezoelectric ceramics con-taining lead[2–5].Unfortunately,however,because of the high hygroscopicity and volatility of its components, Na2CO3and K2CO3,during sintering,manufacturing a NKN-based sintering body with high density through the general commercial sintering method is known to be difficult.Research on the properties of the previous ∗E-mail:jssong@keri.re.kr;Fax:+82-55-280-1590

NKN-based ceramics was performed for costly manufac-turing processes such as hot press,spark plasma sinter-ing,and template grain growth[2–6],and recently,there have been reports of various new compositions with their piezoelectric properties improved through the addition of various compounds such as SrTiO3,BaTiO3,LiSbO3, LiTaO3,and LiNbO3by using a commercial sintering method[7–9].One notable report was that when6mol% of Li is employed in(1−x)(Na0.5K0.5)NbO3-x LiNbO3,a morphotropic phase boundary is formed and high piezo-electric properties are achieved[10].However,the sin-terability of NKN-based ceramics is still insufficient and needs to be investigated further in order to be applied in industry.

As for PZT piezoelectric ceramics,they are highly volatile,which poses a problem,and addition of excess lead was proposed as a means to solve this problem[11].

Excess lead ultimately affects grain growth as it forms a liquid phase during heat treatment and is positioned be-tween the grain boundaries[11–14].The sintering behav-ior of ceramics that have undergone liquid phase sinter-ing is highly influenced by the liquid phase composition.

Addition of PbO to Pb(Mg1/3Nb2/3)O3-PbTiO3leads to abnormal grain growth while addition of SiO2to BaTiO3 results in an increase in the sizes of abnormally grown particles[11,12].Moreover,adding Nb2O5to SrTiO3or -891-