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Domain structure of [(Na 0.7K 0.2Li 0.1)0.5Bi 0.5]TiO 3ceramics studied by piezoresponse force microscopy

Jiangtao Zeng ?,Kunyu Zhao,Huarong Zeng,Liaoying Zheng,Gurong Li,Qingrui Yin

The State Key Laboratory of High Performance Ceramics and Super ?ne Microstructure,Shanghai Institute of Ceramics,Chinese Academy of Sciences,Shanghai 200050,PR China

a b s t r a c t

a r t i c l e i n f o Article history:

Received 8February 2009Accepted 24March 2009Available online 5April 2009Keywords:Ceramics

Ferroelectrics

Atomic force microscopy Perovskites

Piezoelectric materials

The domain structure of lead-free ceramics [(Na 0.7K 0.2Li 0.1)0.5Bi 0.5]TiO 3was studied by piezoresponse force microscopy (PFM)method.The complicated curved domain structure was observed in the ceramics,and there are some nano domains in the sub-microsized domains,which indicate the relaxor nature of the material.The mechanism for the strong relaxation of the material was discussed in the letter.The reversal behavior of the domain was also studied by PFM method.Only part of the domains reversed after the poling process,and domains of the ceramics reversed back from the center of the domains at ?rst.

?2009Elsevier B.V.All rights reserved.

1.Introduction

Owing to their excellent electromechanical properties,Lead zirconate titanate (PZT)ceramics have been widely used in sensors and actuators;however,the large amount of toxic PbO in PZT ceramics can cause severe environmental problem [1].Many efforts have been devoted to develop the lead-free ceramics in the last decades.Na 0.5Bi 0.5TiO 3(NBT)based ceramics and K 0.5Na 0.5NbO 3(KNN)based ceramics are the most intensively studied ceramics among all the candidates [2,3].

NBT shows the rhombohedra structure at room temperature.Pure NBT ceramics have very high remnant polarization and also high coercive ?eld.The piezoelectric properties of pure NBT ceramics are quite low and they can be enhanced by doping and grain orientation technique.NBT can form morphotropic phase boundary (MPB)with BaTiO 3,(K 0.5Bi 0.5)TiO 3and (Li 0.5Bi 0.5)TiO 3[4–6].Among them,the [(Na 0.7K 0.2Li 0.1)0.5Bi 0.5]TiO 3ceramics have the best piezoelectric properties [6].Although the electrical properties of NBT-based ceramics have been intensively studied,the domain structures of NBT-based ceramics have rarely been revealed.Few researchers observed the domain structure of NBT-based single crystal or ceramics by optic method or TEM method and strip-like domain structure was revealed in these materials [7,8].Besides optic method or TEM method,piezoresponse force microscopy (PFM)is a powerful method to image the ferroelectric domain with nanometer scale resolution [9].In this letter,the domain structure of [(Na 0.7K 0.2Li 0.1)0.5Bi 0.5]TiO 3ceramics was studied by PFM and the complicated curved domain structure was observed which is very different from previous reports.

2.Experimental

[(Na 0.7K 0.2Li 0.1)0.5Bi 0.5]TiO 3ceramics (abbreviated as BNKLT),were prepared by the solid-state reaction method.The raw materials used,Na 2CO 3,K 2CO 3,Li 2CO 3,Bi 2O 3and TiO 2,are all of high purity grade.The raw materials in the stoichiometric ratio of the compositions were mixed thoroughly in ethanol by ball-milling for 8h,and then dried and calcined at 850°C for 2h in an alumina crucible.After the calcination,the mixture was ground and ball-milled again,and then pressed into disk samples with a diameter of 12mm and a thickness of 2mm.The disk samples were ?nally sintered at 1020–1100°C for 3h in ambient surrounding.The sample was polished for the PFM test after sintering.The polished sample was thermal etched for SEM examination.

The microstructure was examined by the ?eld emission scanning electron microscopy (FE –SEM,Model JSM-6700F).An atomic force microscope (AFM)(SPA 400,SPI3800N,Seiko Inc.Japan)was used in this experiment.A 2-μm-thick,90-μm-long Ti/Pt-coated Si cantilever (Micro Masch,NSC12-B)with the spring constant of 14N/m and resonance frequency of 315kHz was used.The static force applied on the tip was set to 7nN.Domain visualization was performed under an applied ac voltage with amplitude V ac =9V and frequency f =8kHz.3.Results and discussion

Fig.1a shows the SEM micrograph of the BNKLT ceramics.It can be seen from the SEM micrograph that the ceramics was well densi ?ed and the grains are irregular shape with their sizes ranging from 0.5μm to 2μm.Fig.1b shows PFM image of BNKLT ceramics at 20μm×20μm scale.Different contrast can be seen in the PFM image indicative its domain structure.There are many curved domain pattern in the view.

Materials Letters 63(2009)1468–1470

?Corresponding author.Tel.:+862152412034;fax:+862152413122.E-mail address:zjt@https://www.wendangku.net/doc/a71678008.html, (J.

Zeng).0167-577X/$–see front matter ?2009Elsevier B.V.All rights reserved.doi:

10.1016/j.matlet.2009.03.053

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Some of them disappear or merge,forming a complicated structure.The contrast is determined by the out-of-plane component of polarization,where dark and bright areas correspond to the opposite directions of polarization.Because it is a polycrystalline ceramics,and there are different grains with various directions.There are some brightest areas and some darkest areas and some grey areas.The brightest areas indicate that the polarization is upwards and perpendicular to the surface.The darkest areas indicate that the polarization is downwards and perpendicular to the surface.The grey areas indicate that the polarization has an angle to the surface.It is clear that the domains are antiparallel domains and the typical width of the domains determined from the piezoresponse image is in the range 400–800nm.The domain structure in this study is similar to that in relaxor ferroelectrics such as PMN –PT.Fig.1c is an enlargement of square outlined in Fig.1b.It can be seen that the sub-microsized domains are not uniform.There are some grey “points ”in the bright area,and there are also some bright “points ”in the dark area.The size of such domain ranges from 10nm –50nm determined from the line pro ?le (shown as Fig.1d).It was known that nanodomains exist in PMN –PT and PZN –PT single crystal as a result of transition from

the

Fig.1.(a)SEM micrograph of BNKLT ceramics;(b)PFM image of of BNKLT ceramics at 20μm ×20μm scale;(c)magni ?ed PFM image at 5μm ×5μm scale and (d)line pro ?le in

(c).

Fig.2.(a)PFM image of a partially electroded sample after poling;(b)magni ?ed PFM image of (a).

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J.Zeng et al./Materials Letters 63(2009)1468–1470

relaxor to classic ferrorelectrics[10,11].BNKLT is also typical relaxor ferroelectrics.It is reasonable that there are some nanodomains in the ceramics.

The results in this study were very different from other NBT systems such as NBT–BT crystal and NBT ceramics.Jiang studied NBT–BT single crystal and the strip-like domain patterns were revealed by optic method[7].Dorcet studied the NBT ceramics by TEM method and also the strip-like domain patterns were revealed[8].The curved domain pattern in this study indicates that there are random internal ?elds and nanopolar clusters interaction in the ceramics[12].For NBT system,different atoms occupy the A-site of ABO3perovskite lattice and A-site cation disorder may exist.Bi3+,Na+,and Ba2+disorder in NBT–BT or NBT ceramics is the reason for their relaxor phase transition,and the different valence between Bi3+and Na+(Ba2+) will cause the random?eld,but the random?eld may be small,so the domain patterns still keep straight.In BNKLT ceramics,not only the cation disorder(Na+,K+,Li+,Bi3+),but also there are other mechanisms cause a more relaxor state.The Li+has much small ionic radius than K+and the oxygen cage is too big for it.When Li+ occupy the A-site,it will shift to an off-center position.It can cause strong relaxor feature.This phenomenon can also be found in other materials such as Li+doped KTaO3crystals and Ca2+doped BaTiO3 crystals[13,14].

The sample was partially electroded with silver paste and poled at the electric?eld of4kV/mm.The silver paste was washed away by acetone before PFM test.Fig.2a shows the PFM image of the partially poled BNKLT sample.It can be seen that the poled area shows bright contrast compared with unpoled area,but there are still some small dark areas exist in the poled area which indicate that only part of the domain reversed.The number of domains in the poled area decreased considerably,which indicate that some domains reversed and coalesced.Fig.2b shows the magni?ed image outlined in Fig.2a. The large domain has the out of plane polarization,but there are some dark points in the domain.Since the180°domain is easy to reverse, the dark points most probably caused by the domain reversed back after the removal of the electrical?eld.It can also be noticed that most of the dark points appeared in the center of the domain,which indicate that the center of the domain is more likely to reverse.

4.Conclusions

The domain structure of lead-free ceramics BNKLT was studied by piezoresponse force microscopy method.The complicated curved domain structure was observed in the ceramics,and there are some nano domains in these sub-microsized domains,which indicate its relaxor nature.The displacement of Li+is the possible reason for the strong relaxation.The domain structure of a poled sample was also studied by PFM method.Only part of the domains reversed after the poling process,and the domains?rstly reversed back from the center of the domains after the removal of the electric?eld. Acknowledgements

This work was supported by the Ministry of Sciences and Technology of China through973-project(no.2009CB623305),the National Advanced Materials Committee of China(863-project no. 2007AA03Z330),and the Natural Science Foundation of China (50702068and50675218).

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