11-NKN-BT-Materials chemistry and physics

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Materials Chemistry and Physics 126 (2011) 769–772

Contents lists available at ScienceDirect

Materials Chemistry and

Physics

j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /m a t c h e m p h y

Frequency and temperature dependent dielectric and conductivity behavior of 0.95(K 0.5Na 0.5)NbO 3–0.05BaTiO 3ceramic

Laijun Liu ?,Meixia Wu,Yanmin Huang,Zhao Yang,Liang Fang,Changzheng Hu

State Key Laboratory Breeding Base of Nonferrous Metals and Speci?c Materials Processing,Key laboratory of New Processing Technology for Nonferrous Metal and Materials,Ministry of Education,College of Material Science and Engineering,Guilin University of Technology,Guilin 541004,China

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

Received 2July 2010

Received in revised form 3October 2010Accepted 13December 2010Keywords:

Sodium potassium niobate Dielectric relaxation Conductivity

a b s t r a c t

Dielectric and conductivity measurements were carried out on the 0.95(K 0.5Na 0.5)NbO 3–0.05BaTiO 3(KNN–5BT)ceramic both as a function of temperature (～350–850K)and frequency (40Hz to 100MHz).A high-temperature dielectric relaxation above Curie temperature (～590K)was observed and ana-lyzed with the Cole–Cole function.Frequency dependent conductivity was analyzed with an augmented Jonscher relation and exhibited a universal conductivity behavior.The activation energy of dielectric relaxation was estimated to be 1.09eV.It could be attributed to the thermal motion of double ionized oxygen vacancy or to the formation of defect dipoles between the acceptor ion and charge compensating oxygen vacancies.The mechanism for both the high-temperature dielectric relaxation and the frequency dependent conductivity was proposed based on a possible mode of incorporation of Ba 2+and Ti 4+ions into the KNN lattice sites.

1.Introduction

There is an increasing demand for the environment friendly materials like lead-free ceramics for different piezoelectric device applications.Alkali niobate,K 0.5Na 0.5NbO 3(KNN)is considered one of the promising candidates for lead-free piezoelectric ceramics [1–3].However,the stable phase limit of KNN is 1140?C [4],and they would deliquesce once exposed to humidity due to the for-mation of extra phases [5,6].The main problem is the volatilization of potassium oxide (K 2O)at 800?C making the stoichiometry dif-?cult to control [6,7].Furthermore,oxygen de?ciency has been another problem in the preparation which results from high-temperature processing and gives rise to electronic conductivity [7].Therefore,alkaline-earth ions [8]and alkaline-earth titanate [9–21]have been introduced into KNN lattice in order to solve these problems.

The physical mechanism of hard doping is complex,since differ-ent hard doping ions affect different properties [22].A hard doping ion is considered an acceptor,since it causes oxygen vacancies in the perovskite lattice.The hard doping ion may be occupied either in A or B-site,depending upon the ionic radius of the doping ion,which usually has a lower or equal chemical valence or approxi-

?Corresponding author at:Guilin University of Technology,Key Lab.of Nonfer-rous Materials and New Processing Technology,Ministry of Education,12Jiangan Road,Guilin,Guangxi 541004,China.Tel.:+867735896290;fax:+867735896671.

E-mail address:ljliu2@https://www.360docs.net/doc/4f16236547.html, (L.Liu).mately similar ionic radii than that of the ion replaced in the lattice [23].Using element substitution,one can tailor the properties of the ceramics for any speci?c application desired.

In present article,introduction of donor bivalent (Ba 2+)cations at the A-site and,acceptor quadrivalent (Ti 4+)cations at the B-site in the piezoelectric sodium potassium niobate solid solutions were characterized.The dielectric and conductivity investigations both as a function of temperature and frequency were carried out on KNN–5BT ceramic using an impedance analyzer.The combined effects of these donor and acceptor dopants were analyzed and characterized.

2.Experimental procedure

Carbonates and oxides Na 2CO 3,K 2CO 3·1.5H 2O and Nb 2O 5were used as start-ing materials.KNN–5BT powder was obtained after milling 450rpm for 2h with mechanical alloying technique and then calcined at 850?C for 2.5h.The calcined powder was pressed into discs uniaxially of 10mm in diameter and 2mm in thick-ness under 300MPa and then pressed under 650MPa with a cool isostatic pressing method.The discs were sintered at 1070?C for 2h in an alumina crucible.

X-ray diffraction patterns were obtained using an automated diffractometer (XRD;SIEMENS D5000)with Cu K ?1radiation.Both sides of the samples were sput-tered gold electrodes.Electrical properties measurement was taken with an applied voltage of 500mV over the frequency range 40Hz to 100MHz from 350K to 850K with an impedance analyzer (Agilent 4294A).

3.Results and discussion

The X-ray diffraction (XRD)pattern of KNN–5BT ceramic is shown in Fig.1.All the peaks were index to monoclinic perovskite structure.It is suggested that BT diffuses into the KNN lattices

770

L.Liu et al./Materials Chemistry and Physics

126 (2011) 769–772

Fig.1.X-ray powder diffraction pattern obtained at room temperature for KNN–5BT and indexed with monoclinic indices.

to form a homogeneous solid solution,in which Ba 2+enters the (Na 0.5K 0.5)+sites while Ti 4+occupy the Nb 5+sites.

The temperature dependence of the real part of the dielec-tric permittivity at a few representative frequencies is shown in Fig.2(a).The data shown in Fig.2(a)and (b)are taken during the cooling process of a thermal cycle.The ferroelectric-orthorhombic to ferroelectric-tetragonal phase transition may shifts to below 400K,while the ferroelectric-tetragonal to paraelectric-cubic phase transition shifts to 590K.The upward arrow in Fig.2(a)at the temperature denotes the dielectric anomaly corresponding to paraelectric to ferroelectric phase transition.The Curie tempera-ture shifted to low temperature by 103K to that of undoped KNN (T C =693K)[8,9].However,the dielectric constant of the sample at room temperature was enhanced signi?cantly compared with undoped KNN.

It is seen from Fig.2(a)that at any given temperature other than the anomaly region,the dielectric permittivity decreases with an increase in frequency and the dependence of dielectric permittivity with frequency increasingly varies with temperature.The sub-sequent monotonous increase of dielectric permittivity at higher temperatures is due to the conductivity increase of the sample.In addition,the value of dielectric loss factor increases very strongly with temperature (shown in Fig.2(b)).This dielectric dispersion is related to a conductivity phenomenon which obeys an Arrhenius-type thermal activation law.The relaxation frequency increases with the increase in

temperature.

Fig.2.Temperature dependence of the dielectric permittivity (a)and dielectric loss

factor (b)of KNN–5BT at different

frequencies.

https://www.360docs.net/doc/4f16236547.html,plex-plane impedance plots for KNN–5BT at different temperatures.

To understand this dielectric relaxation,the semiempirical com-plex Cole–Cole equation was used to analyze the impedance data.Fig.3shows the impedance data for KNN–5BT ceramic in the form of Cole–Cole plots (a plot draw between imaginary and real part of the impedance)at different temperatures.It can be seen from the ?gure that as temperature increases,the graphs turn from a pitch arc to semicircles.This trend indicates that the sample has insulat-ing behavior at the low temperature (<570K).Not all the plots are depressed semicircles and the centers of the circles do not fall on the real axis but fall on the below of the real axis.With increasing temperature,the Cole–Cole plots for all the samples show smaller semicircles and,the angle of depression decrease with the increase in temperature.

Fig.4(a)and (b)shows real (Z )and imaginary (Z )parts of impedance with frequency at various temperatures.It is evident that Z decreases with increase in both frequency as well as tem-perature.The Z values for all temperature merge above 100kHz;the dispersion at low frequency may be due to a thermally assisted electric or ionic relaxation.From Fig.4(b)it reveals that Z also increase with increase in both frequency as well as temperature and

reach a maximum (Z max

)for the temperature above 740K,indicat-ing a single relaxation process in the system.It is obviously that the Z max shifts to higher frequency side with increasing temperature.

And the Z max

values for all temperature also merge together above 100kHz.

The normalized imaginary parts Z /Z max of impedance as a func-tion of frequency in KNN–5BT ceramic at different temperatures

is shown in Fig.5.It seems from the ?gure that at high tempera-ture triggers a relaxation process.The Z /Z max parameter exhibits

Fig.4.Frequency dependences of the real part (a)Z and imaginary part Z (b)of impedance of KNN–5BT at various temperatures.

L.Liu et al./Materials Chemistry and Physics 126 (2011) 769–772

771

Fig.5.Normalized imaginary parts,Z /Z

max of impedance as a function of frequency.

symmetric peak at each temperature especially at higher tempera-tures.The relaxation frequency obeys the Arrhenius relation given by:

ωmax =ω0exp ?

E rel

k B T

(1)

where ω0is pre-exponential factor and k B is Boltzmann constant.The activation energy E rel calculated from the ln ω0?1/T data in Fig.6.The calculated activation energy and pre-exponential factor of KNN–5BT are 1.09eV and 6.78×1011Hz,respectively.

The frequency dependence of AC conductivity ( ac )at four tem-peratures is shown in Fig.7.The conductivity shows a dispersion which shifts to the higher-frequency side with the increase of tem-perature.It is seen from Fig.7that ac decreases with decreasing frequency and becomes independent of frequency after a certain value.Extrapolation of this part towards lower frequency will obtain direct current conductivity ( dc ).Obviously,it could be described by the so-called “universal dielectric response”law: (f )= dc + 0f s

(2)

where dc is the dc bulk conductivity and f is the frequency.Eq.(2)is typical of thermally assisted tunneling between localized states.This law describes one phenomenon that is associated with many-body interactions between charges and

dipoles.

Fig. 6.Temperature dependence of the most probable relaxation frequency obtained from the normalized imaginary part of impedance plots for KNN–5BT.The squares are the experimental points and the solid line is the least-squares straight-line

?t.

Fig.7.Frequency dependence of the real part of ac conductivity of KNN–5BT.The

solid lines are the ?t according to Eq.(2).

The temperature variation of dc thus obtained follows the Arrhenius law given by: dc =T

0exp ?

E con

k B T

(3)

with activation energy E con =0.82eV,as shown in Fig.8.

It is observed that the activation energy for relaxation frequency of charge carriers is more than that for conduction.It is known that the activation energy for conduction (E con )is the sum of both the creation of charge carriers and migration or hopping free energy of charge carriers over a long distance while the activation energy for relaxation or hopping frequency of charge carriers (E rel )is equal to the migration free energy of charge carriers and hopping of these charge carriers between the adjacent lattice sites [24].The activa-tion energy for dielectric relaxation and the activation energy of conduction are consistent with those activation energies of oxy-gen vacancies in perovskite.Based on the results,the difference between the activation energy for relaxation or hopping frequency of charge carriers (E rel )and the activation energy for conduction (E con )is attributable to the creation of free energy,which shows that the carrier concentration in temperature dependent and these car-riers are dissociated from traps or defects (cation vacancy)present in the sample.

In oxide ferroelectrics,doubly charged oxygen vacancies (V O

)are the most mobile charges and play an important role in the con-duction process [24].The motion of oxygen vacancies is well known to give rise to activation energy of about 1eV [25]in perovskite oxides at high temperature.Thus,the calculated activation

energy

Fig.8.Arrhenius plot of the dc conductivity of KNN–5BT.The squares are the exper-imental points and the solid line is the least-squares straight-line ?t.

772L.Liu et al./Materials Chemistry and Physics126 (2011) 769–772 of1.09eV is attributed to the thermal motion of double ionized

oxygen vacancy or to the formation of defect dipoles between the

acceptor ion and charge compensating oxygen vacancies[26–28].

The sodium oxide evaporation(the same reactions to potassium

oxide)and a possible Ti4+ion reduction result in a defect structure

can be written as:

Na Na→Na+V Na+h?(4)

Ba2+→Ba Na+V Na(5)

Ba2+→Ba?Na+e (6)

Ti4+

Ti →Ti Nb+1

V??

(7)

where V Na and V

Na are neutral and ionized sodium vacancies,

respectively;h?is the electron hole,V??

O is doubly ionized oxygen

vacancies,e is the free electron,Ba?

Na is single ionized barium which

occupy on the Na site,Ti Nb is quadruply ionized titanium which

occupy on the Nb site,and Ti4+

Ti is quadruply ionized titanium ions.

The volatilization of these A-site elements will result in the de?-ciency in A-site cations,and thereafter the generation of oxygen vacancies due to the valence balance,as happened in undoped KNN ceramics[29]and PbZr x Ti1?x O3-based materials[30],which may alter the lattice distortion and thus in?uence the dielectric behav-iors of these ceramics.The donor Ba2+is expected to reduce the oxygen vacancies mobility in the KNN lattice and therefore reduce the concentration of domain-stabilizing defect pairs.The term ele-ment substitution means that cations in a perovskite lattice,i.e.Na+, K+and Nb5+are replaced partially by other cations with the approx-imately same chemical valence and similar ionic radii as those of the replaced ions.The new substitution cation usually occupies the position of the replaced cation in the perovskite lattice and a sub-stitutional solid solution is thus formed.Nb5+was replaced by Ti4+, the negatively charged defect Ti Nb will be formed and the corre-sponding half number of positively charged oxygen vacancies will be required to satisfy the charge neutrality condition.The change of activation energy from～1.47eV for undoped KNN[29]to～1.09 for KNN–5BT could be attributed to the formation of(Ti Nb?V??O)?defect complexes with charge compensating oxygen vacancies.The defect dipole will be studied in detail by means of electron param-agnetic resonance spectroscopy in the future work.

4.Conclusions

The frequency-dependent dielectric dispersion of KNN–5BT ceramic synthesized by the mechanical alloying method was inves-tigated in the temperature range from350to850K.An analysis of the real part and imaginary part of impedance with frequency was performed assuming single relaxation time.The frequency depen-dent maxima in the imaginary part of impedance are found to obey an Arrhenius law with activation energy1.09eV.Existence of oxygen vacancies and defect dipoles were postulated.Electrical conductivity showed thermally activated behavior.Ionic conduc-tivity dominating in the high temperature range was ascribed to oxygen vacancy or defect dipole dynamics.

Acknowledgments

This work was supported by Natural Science Foundation of China(no.51002036),Natural Science Foundation of Guangxi(no. C013002),and Guangxi Key Lab for the Advanced Materials and New Processing Technology(0842003).

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