Dielectric studies of layer structured sodium - calcium bismuth titanate mixed ceramics

Transcription

1 Indian Jurnal f Engineering & Materials Sciences Vl. 5, April 1998, pp Dielectric studies f layer structured sdium - calcium bismuth titanate mixed ceramics P S Rama Sastry, T Bhimasankaram, G S Kumar, G Prasad & SV Suryanarayana Department f Physics, Osmania University, Hyderabad , India Received 3 March 1997; accepted 4 June 1997 Dielectric prperties have been studied n ferrelectric mixed ceramics cmpnsmg bismuth cmpunds with layer structure (NaBi)05x Ca._xBi4Ti4 015 with x = 0,0.1,0.3,0.5,0.7 and 1. The prperties have been measured as a functin f frequency and temperature in the ranges 1 khz-io MHz between C. AC cnductivity is calculated 'using dielectric cnstant and lss tangents. Distrtin f the Octahedra f the pervskite layers alng plarisatin axis culd be cntributing t the spntaneus plarisatin. A cmbinatin f traditinal ceramic technlgy f mdem xide materials prduced rapid develpment f new electrnic materials, many f which are based n ferrelectric systems. Layered bismuth titanates, which are als ferrelectric are knwn t pssess a structure with the general frmula 1-3. (BizOz)2+ (Am_tBm03m+t)zr Am-tBiZBm03m+3 where A is a mn r di r trivalent in r a mixture. 0f them, B represents Fe, 3+ T 4+ I, NbS+, Ta 5+, W 6+ etc., and the mt~ger m takes any f the values frm 3 t 8 and dentes number f pervskite layers. These cmpunds are built up by regular intergrwth f (BizOz)z+ layers and pervskite layers (Am-tBm03m+ti-. Many cmpunds belnging t this family were synthesized by Smlenskii et al.4 and Subba RaS,6. T date there are mre than 50 cmpunds in the grup. There are 6 ne-layer structures, 12 tw-layer, 20 three-layer, and 9 fur-layer and 5 five-layer cmpunds. Bismuth xide layer is very strng and hence it is difficult t substitute fr bismuth in these layers. On the ther hand, the bismuth atms in the pervskite layer are readily replaced by large number f univalent, divalent and trivalent catins. The size requirements were n mre stringent than thse f pervskite structure itself. In all these cmpunds a and b values des nt shw much change. Hwever, there is an bserved change in c value13. Calcium mdified sdium bismuth titanate, i.e., (Nll.sBi.5kxCaxBi4Ti40,s cetamics were reprted t display excellent piezelectric and pyrelectric prperties 7,8. These ferrelectrics are characterised by lwer dielectric cnstant and higher Curie temperature (Tc), hence larger anistrpy in cupling factrs K33 and K3t as cmpared t cnventinal piezelectric cera-mics. These characteristics make them ideal candidates fr piezelectric applicatins at high temperatures and frequencies r pyrelectric materials with large figures f merit. These ceramics hwever have tw disadvantages in a pling prcess - ne is due t their crystal structure in which the rtatin f spntaneus plarisatin is tw dimensinally restricted and the ther having larger cercive field r higher Curie temperatures. S there is an intense activity t imprve strategy fr pling and als t reduce the Curie temperatures f basic cmpunds by substitutin f suitable alivalent ins. In this aspect a study f mixed systems having their rigins in the bismuth layered structured ferrelectrics (BLSF) will be rewarding. The present paper is a systematic study f dielectric prperties f sdium bismuth titanate-calcium bismuth titanate (NBT -CBT) mixed system. Experimental Prcedure Sample preparatin-plycrystalline samples f NBT -CBT mixed system with the general frmula (Nll.sBi.5)I-x CaxBi4Ti40, 5 where x=

2 84 INDIAN J. ENG. MATER. SCI., APRIL 1998 ;; :; T... ;;. 0 " IS ~ 0 Fig I-X-ray diffractgram f sdium bismuth titanate (NBT) H- _H _H n ss 4S JS JS IS S 28,d~r" Fig 2-X-ray diffraetgram f calcium bismuth titanate (CBT). H E / Table I-Summary Ca Cntent Lattice Cmpund parameter a=5.411 BIT@ b=5.448 c=32.83 a=5.428 NBT 0 b=5.450 c=40.65 a=5.424 O.INCBT 0.1 b=5.448 c=40.63 a= NCBT 0.3 b=5.446 c=40.61 a=5.418 CBT b=5.428 c=40.75 *Literature values after reference Values after reference 12. f pertinent data fnbt - CBT mixed system Expt. density glee X-ray density g/cc Particle size * * r; cia * ,0.1,0.3,0.5,0.7 and 1 were prepared by slid state reactin f the AR grade (99% pure) cnstituent xides and carbnates. The cnstituents were mixed thrughly in an agate mrtar t make hmgeneus mixture and calcined at a temperature f 800'C fr 2 h. After calcinatin, the material was finely grund and mixed with 2% plyvinyl alchl and pressed int circular pellets f diameter 1 cm and 1-2 mm thickness at a pressure f 30 M Pa. These pellets were finally sintered at 1000'C fr 2 h and slw cled t 400'C at 60'CIh and then furnace cled. X-ray diffractin-the X-ray pwder diffractgram (XRD) f the samples was taken with CuKa radiatin (A,=1.548A). The XRD patterns f NBT and CBT are shwn in the Figs 1 and 2. The average densities f the samples was determined by using the Archemedis principle and are shwn in Table 1. The experimental density in NBT is arund 86% and that f CBT is 70% f the X-ray density. Hwever, the experimental densities f end cmpunds btained in the present study are higher than thse reprted earlier 6. The average particle size was calculated using the Scherrer's equatin P = 0.89AJPII2COSe where P = particle size, f... = wave length f the radiatin, P = half width f peak intensity, and e = Brag angle. Pling prcedure-all the cmpsitins were pled electrically prir t the dielectric measurements. The samples were pled at 150'C under DC field f 100 kv/cm using a stabilised pwer supply and cled t rm temperature in the presence f the field. Dielectric measurements-the large areas f the samples were painted with high purity, ultra fine silver paste and dried up with infrared lamp. The dielectric measurements were perfrmed using a tw terminal sample hlder with Hewlett Packard

3 RAMA SASTRY et al.: DlELECfRIC PROPERTIES OF FERROELECTRIC MIXED CERAMICS ' , 1800, NBT 0 1NaC8T 0 ) NaC8T D 0 5NaCBT 0 7 NCBT CBT N8T 0 1N(8T D 0.3N(BT C8T 1000 W L-~~~~~~~~~~~ ) 0 ) 5 ' 0 lg (f) KHz Fig 3-Variatin f dielectric cnstant with frequency fr NBT-CBT system. 01 N BT D C 8 T Temperat"re ;c Fig 5- Temperature variatin f dielectric cnstant at 100 khz fr different cmpsitins fnbt-cbt system. le-02r IE-OJ -"r'u, Ẹ! le-04 D N 8T C8T <.0 C 0 01 v r le-sj L-~_~_L_~_~_~_~~ Fig 4-Variatin CBTsystem. Lg(F) KHz )0 l5 4 0 f lss tangent with frequency fr NBT- HP 4192A impedance analyser interfaced t an IBM PC in the temperature range f C. Measurements f dielectric cnstant (E) and lss (Tan) were carried ut as a functin f frequency (1 khz -10 MHz) and temperature ( C). Results and Discussin Fig. 3 presents the vanatin f dielectric cnstant as a functin f frequency fr different cmpsitins fnbt - CBT mixed system. All the samples shw a decrease in dielectric cnstant with the increase f frequency attaining a cnstant value beynd 100 khz. Pure NBT samples shwed maximum dielectric cnstant at the lwest frequency. The value f the dielectric cnstant decreased with the increase f frequency in le-06i-=-_.,.." :-----::-'":' _---".L-'.,--_.-J /TI(-1 Fig 6-AC cnductivity as a functin f,temperature fr NBT-CBT inverse general. All the samples except 0.1 NCBT attained a frequency independent value f abut 140 at higher frequencies. Variatin f lss tangent as a functin f frequency (n lg-lg scale) is presented in Fig. 4. The 0.1 NCBT, 0.3 NCBT samples cmparatively shwed lwer lsses. All the samples shwed frequency dependent variatin f lss tangent. The samples shwed small variatin f dielectric cnstant with temperature up t 300 C. Beynd this temperature there is an increasing trend in the value f dielectric cnstant shwing a peak between 586 C and 600 C (Fig. 5). The peak in the dielectric cnstant versus temperature shifts twards higher temperature with the increase f Ca cntent. Beynd x = 0.3 mle fractin f Ca, the peak in dielectric cnstant culd nt be recrded within the temperature range f the present measurements. But CBT is reprted t have a transitin temperature f 785 C 8. In all f

4 86 INDIAN J. ENG. MATER. SCI., APRIL 1998 Table 2-Activatin energy fr AC cnductin fnbt-cbt mixed system Cmpsitin f calcium Activatin energy, ev these samples, we have bserved pyrelectric current under shrt circuit cnditin when heated linearly with time. Dielectric peaks alng with pyrelectric signal cnfirms the ferrelectric nature f the present system f samples. Using dielectric cnstant and lss tangent data, AC cnductivity f these samples is calculated and pltted as a functin f temperature fr NBT and CBT samples in Fig. 6. These graphs shw tw slpes. Frm lge AC vs 10 3 IT plts activatin energy fr cnductin befre the nset f ferrelectric phase transitin (at 400 C) IS calculated and shwn in Table 2. The high densed ceramics f NBT -CBT family f bismuth layer structured cmpunds with density f abut 90% f X-ray density were btained frm the duble sintering rute f sample preparatin. X-ray pwder diffractin patterns f this system indicate a single phase with pseud rthrhmbic symmetry. The particle size calculated frm XRD data is indicated in Table 1. The particle size btained increased with the increase f calcium cntent except fr 0.1 NCBT samples. The structure f the present cmpunds cnsists f (Bi20 2 i+ layers, interleaved with pervskite like [(Na.sBi.s)"Ca\."Bi4 Ti40 13 t layers. The bismuth xide layer is almst invilate. On the ther hand, the bismuth atms in the pervskite layer can be readily replaced by a large number f univalent, divalent and trivalent catins. The size requirements are n mre stringent than the pervskite structure itself. (Bi 2 02)2+ layers are made up f square pyramidal, Bi04 grups shearing their basal edges. All the cmpunds are pseud rthrhmbic. Present cmpunds cnsists f fur pervskite like layers with Ti0 6 ctahedra and spntaneus plarisatin takes place in these layers. Inic plarisatin makes an imprtant cntributin t the permanent diple mment in the ferrelectric state, while in the pervskite layers, the inic plarisatin wuld invlve the displacement f the highly charged B site catins away frm the centers f their crdinatin ctahedra. The structural cnsequence f this plarisatin is t distrt the envirnment f the B catins frm the nearly regular ctahedra t distrted ctahedra cnfiguratin thereby lwering 69 the crystal symmetry,. The mlecular frmula fr the system (NaBi).s"Ca\."Bi4Ti40ls,can als be written in the frm f Bi [ {(NaBi).s"Ca\.x)114 Bi2/4 ~\/4} Ti0 3 ] The later shwn in the square bracket bviusly belngs t AB0 3 type. The in ill "A" psitin is (NaBi).s"Cal."Bi2/4~\/4 where ~ indicates catin vacancy. These vacancies intrduces strain int the lattice. Larger number f Bi ins in "A" psitin and their larger plarisability may cntribute t the ferrelectricity\\. Oxygen ctahedran within the cmpunds, with Bi layer structure cnsisting f even number f pervskite layers, will exhibit a zig-zag like pattern alng c-axis with mirrr symmetry \ 2. As the catin Ca 2 +, with small inic radius is used t replace (NaBi)~~ in "A" psitin, the angle f zig-zag wuld be small. It is seen frm ur data that the replacement f (Na.sBis)2+ with calcium in increases Curie cnstant and decreases density. The inic radius f Na' is 0.97A, the inic radius f cl+ is 1.04 A and that f BiH is 1.2 A. Calcium has the lwest plarisability whereas bismuth has the highest plarisability. Hence it is expected that CBT shuld shw lwer dielectric cnstant, high T, as bserved in the present measurements. The sample 0.1 NCBT shwed maximum experimental density and minimum particle size and hence least dielectric cnstant and lss tangents. The fitting Curie - Weiss law gave a value f Curie cnstant f the rder f 1OS1 C and Curie temperature arund 600 C. The present samples have been fired in air at abut 1000 C. At these temperatures xygen is expected t escape frm the sample creating xygen vacancies. During subsequent cling the xygen enters the lattice again but cannt ttally cmpensate fr the lss. This is the case with all titanates like BaTi0 3, Sr Ti0 3 etc 12 The bserved cnductivity in the present samples is due t the mbility f xygen vacancies. The xygen mvement takes place within the ctahedra and als frm ne ctahedra t anther within pervskite layer. The cnductivity curves shwed tw slpes and the

5 RAMA SASTRY et al.: DIELECfRIC PROPERTIES OF FERROELECfRIC MIXED CERAMICS 87 activatin energies in the high temperature regin is given in the Table 2. The activatin energy fr cnductin is calculated at 400 C befre the nset f transitin. Lw temperature cnductivity is basically because f the defects and reflects space charge. This is als indicated by dielectric cnstant - frequency and tand - frequency graphs. In the lw temperature regin, since there is large cncentratin f defects and vacancies and their agglmeratins with fewer free charges, it is expected that the cnductivity and activatin energy fr cnductin wuld be less because f multiple and cmplex variety f relaxatins present in the sample. The vacancies present in the sample (bth at A and B psitins f AB0 3 structure) and lattice defects culd be respnsible fr the bserved cnductivity. The sample O.lNCBT shwed minimum activatin energy. The larger activatin energy, higher space charge plarisatin, higher lss tangent indicate higher cnductivity fr NBT samples as bserved in the present measurements. sintering rute. These samples shwed dielectric anmalies and Curie cnstant is fund t be f the rder f C. Increase f Ca cntent increases Curie temperature. Cnductivity in these samples is dminated by vacancies and defects. References I AurivilliusB,ArkKemi, I (1949) Aurivillius B,Ark Kemi, I (1949) Jan'les A R, Kumar G S, Bhimasankaram T & Suryanarayana S Y, Bull Mater Sci, 17 (1994) Smlenskii G A, Isupv Y A & Agranskaya A I, Fiz, Tverdg Tela, 3 (1961) 895. [Sviet Physics-slid state, 3 (1961) 651]. 5 Subba Ra E C, JAm Ceram Sc, 45 (1962) Subba Ra E C, J Phys Chem Slids, 23 (1962) Takenaka T & Sakata K, Sensrs Mater, 1 (1988)35. 8 TakenakaT&SakataK,Ferrelectrics, 118(1991) Yadav K L & Chudhary R N P, Ferrelectrics, 141 (1993) Seiji Ivegami & Ichir Ued,Jpn, J Appl Physics, 13 (1974) Chen Da Ren, lee Trans Ferr FreqCntrl, 86 (1986) Newnham R E, Wlfe R W & Drrin J F, Mater Res Bull, Cnclusins 6 (1971) The fur layered NBT-CBT mixed system f 13 Chen Da Ren & Gu Yanyi., Electrnic elements Mater, 1 cmpunds have been prepared by duble (1982) 25.