Md. Aminul Islam 1*), M. A. Gafur 2, M. Saidul Islam 1

Transcription

1 Science of Sintering, 47 (215) doi: /SOS152175I UDK ; Sintering Characteristics of La/Nd doped Bi 4 Ti 3 O 12 Bismuth Titanate Ceramics Md. Aminul Islam 1*), M. A. Gafur 2, M. Saidul Islam 1 1 Dept. of Materials Science and Engineering, Rajshahi University, Bangladesh 2 BCSIR, Dhaka, Bangladesh. Abstract: A good understanding about the properties of La/Nd doped Bismuth Titanate (BIT) ceramics at high temperature is very important as the new materials being developed based on the BIT. Pure BIT, La doped (BLT), Nd doped (BNT) and La and Nd co-doped BIT (BLNT) powders were synthesized by solid state reaction method. Prepared powders were calcined at different temperatures and structural properties measured by XRD. For pure BIT better crystal quality was obtained at 75 C and for both BLT and BNT better result obtained at 8 C. Calcined powders were formed into pellets and sintered at different temperatures and its dielectric properties were characterized. Optimum sintering temperature for both BLT and BNT showed was C and La and Nd co-doped bismuth titanate (BLNT) revealed optimum sintering temperature of C. Therefore, optimum sintering temperature of bismuth titanate was increased due to La and Nd doping. Keywords: Bismuth Titanate, La/Nd doping, Sintering, XRD, Dielectric properties. 1. Introduction Bismuth Titanate (Bi 4 Ti 3 O 12 ) (BIT) is an attractive material for their lead free character with high curie temperature, high dielectric constant, high remnant polarization, excellent piezoelectric and electro-optical properties, relatively low possessing temperature and high breakdown strength and have become a potential candidates for the piezoelectric applications (transducers and actuators, ultrasonic devices, medical imaging detectors), Non volatile memory storage, and optical displays[1,2]. However, BIT suffers from not enough remnant polarization(2p r ), high coercive field (2E c ) and high leakage current and domain pinning due to defects such as Bi vacancies accompanied by oxygen vacancies which is an obstacle for industrial applications. A large number of researches have been performed dynamically in both thin film and bulk ceramics and studies are still in steps forwarded to overcome the hitch of BIT. It is well known that, ion doping by suitable cations (La, Nd, Pr etc.) is a best approach for the improvement of ferroelectric performance of BIT [3-6]. Among these cations, lanthanide elements, Nd [r(nd 3+ )=1.27Å] and La [r(la 3+ )=1.36Å] have been recognized as the materials to substitute Bi [r(bi 3+ )=1.4Å] in the A-site of perovskite structure. The radii size of Bi 3+ ion is compatible with that of Nd 3+ and La 3+ ions, suggested that both materials can eagerly for this substitution in perovskite bismuth titanate. Recently La-substituted BIT having composition of (Bi 3.15 La.75 Ti 3 O 12 ) (BLT) is regarded as one of *) Corresponding author: aminulmse@gmail.com

2 176 Md. Aminul Islam et al. /Science of Sintering, 47 (215) promising candidate materials having enhanced 2Pr of 12 µc/cm 2 with excellent fatigue endurance [3]. One the other hand Kim and et.al [4] have reported Nd doped BIT (BNT) ceramics having composition of (Bi 3.15 Nd.75 Ti 3 O 12 ) belonging remnant polarization (2P r ) of 11.2µC/cm 2 and coercive force (2E c ) of 42KV/cm. Moreover in the case of thin film, it has been reported that, the 2P r and 2E c values of La and Nd-substituted BIT (Bi 3.25 La.75 Ti 3 O 12, BLT and Bi 3.15 Nd.85 Ti 3 O 12, BNT) films are 3 and μc cm -2, and and 269 kv/cm, respectively[5,6]. In both thin film and ceramic forms the 2P r is either large along with serious large 2E c or 2E c is small along with the not enough large 2P r. Thus, balancing of 2E c and 2P r is still challenging which may overcome by co-doping with La and Nd within BIT. However, most of the structural and dielectric properties of ferroelectric ceramic, like density, dielectric constant, dielectric loss, leakage current, polarization and piezoelectricity etc are strongly depend on the (processing) temperature[7]. To achieve the further progress in developing of ferroelectric material based on BIT, it is gainful to have a patent understanding about the properties of these BLT/BNT ceramics at high temperature as the new materials being developed based on the BIT. Consequently it is crucial to explore the effect of processing temperature on the properties of BIT, BLT and BNT ceramics. This paper will assist in understanding the change of behavior of BIT ceramics at high temperature with doping. 2. Materials and Methods: 2.1 Starting Materials and Reagents Pure BIT, La doped BIT (BLT) [Bi 3.25 La.75 Ti 3 O 12 ], Nd doped BIT (BNT) [Bi 3.15 Nd.85 Ti 3 O 12 ] and, La and Nd co-doped BIT [Bi 3 (La x Nd 1-x )Ti 3 O 12 ] (BLNT x =.5) samples were prepared by solid-state reaction with the starting materials, bismuth oxide (Bi 2 O 3 ), Titanium oxide (TiO 2 ), Lanthanum oxide (La 2 O 3 ) and Neodymium oxide (Nd 2 O 3 ). 2.2 Preparation of Powder and Ceramic Specimen The high purity raw materials, Bi 2 O 3, TiO 2, La 2 O 3 and Nd 2 O 3 powdered were mixed according to a stoichiometric ratio with the nominal compositions and then followed by ball milling in ethanol for 24 hours for proper mixing. 1 mol% excess bismuth oxide (Bi 2 O 3 ) was added within the mixture to compensate the bismuth loss during high temperature processing [8]. Then the solution was settled down for 24 h and ethanol was separated out. The well mixed powders were dried and then grounded in a mortar pestle to form into fine powder. The powders were calcinated at different temperatures (6- C) for 2 h. Calcined powder was weighted and mixed with 2.5% PVA solution and dried powers uniaxially pressed at 6 KN to form pellet having radius and thickness about.6cm and.15cm respectively. The compact pellets were sintered in air at different temperatures (8- C) for 1 h. 2.3 Experimental Methods The phase purity and crystal structures of calcined powders were analyzed by XRD (X Pert-PRO, Philips, Japan) using Cu Kalpha radiation (λ=1.54a ). The scanning drive axis is taken as two-theta and the scan was recorded in between 2 to 6 of 2 values for the samples of pure BIT, BLT, BNT and BLNT. The dielectric properties were measured by precision impedance analyzer, (Model 4294A, Agilent Technologies, Japan) within the frequency range of Hz -1MHz at AC signal of 3 mv. Before measurement of dielectric properties, silver past was coated on the both surface of pellets.

3 Md. Aminul Islam et al./science of Sintering, 47 (215) Results and discussion 3.1. Characterization of the powder before sintering The X-ray diffraction patterns of Pure Bismuth titanate (BIT) powder calcined at different temperatures i.e. 6, 65, 7, 75 and 8 C respectively for 2 hours in open air are shown in Fig.1. From the spectra it is shown that the line width and intensity of diffraction line decreases and increases, respectively with the increase of calcinations temperature. It is found that with the increase of calcination temperature the intensity of (117) peak has been enhanced rapidly comparable with other peaks and crystal become (117) oriented. Finally at 75 C the powder is almost fully crystallized and no impurity phase other than the perovskite phase, with a predominant (117) peak. Further increase of calcining temperature, the peak intensities have decreases that may be due to high temperature bond breaking [9]. So optimum calcinations temperature for Pure BIT is 75 C. Intensity (arb. unit) BIT 7 BIT 65 BIT θ (degree) (a) Intensity (arb. unit) (8) (111) (1) (117) (2)&(2) (14) (28)&(28) (1115) BIT 8 (214) (317) BIT θ (degree) (b) Fig. 1. The X-ray diffraction patterns of pure BIT powder calcined at (a) 6, 65 and7 C (b) 75and 8 C temperature for 2 hours.

4 178 Md. Aminul Islam et al. /Science of Sintering, 47 (215) Fig.2 shows the XRD patterns of BLT ceramic powders calcined at 7, 75, 8 and C temperature. The peaks position for BLT has slightly changed for example (117) peak at degree for BIT which is at degree for BLT. This is caused due to La doping. XRD patterns can be also indexed according to the standard powder diffraction data with (reference code ), which indicates that the obtained BLT ceramic is base centered orthorhombic polycrystalline and has almost a single phase of bismuth-layered perovskite structure with preferred (117) and (l) orientations. For Nd doped BIT a similar feature has been observed that is shown at Fig.3. For both BLT and BNT, it is shown that peak intensities have been increased due to increase of calcination temperature and hence crystallinity has been increased. Highest intensity for both BLT and BNT is observed at 8 C. The optimum calcination temperature of BIT has been greater than before due to La and Nd doping that may be caused for enlarge of reaction temperature as a result of La and Nd doping. (8) (111) (1) (117) (2)&(2) (14) (28)&(28) (1115) BLT (214) (317) Intensity (arb. unit) BLT 8 BLT 75 BLT θ (degree) Fig. 2. The X-ray diffraction patterns of BLT powder calcined at 7, 75 and 8 C temperature for 2 hours. Intensity (arb. unit) (117) (8) (111) (115) (1) (2)&(2) (26) (14) (28) (22) (1115) BNT 8 (214) (137) BNT 75 BNT θ (degree) Fig. 3. The X-ray diffraction patterns of BNT powder calcined at 7, 75and 8 C temperature for 2 hours.

5 Md. Aminul Islam et al./science of Sintering, 47 (215) According to the previous effect the La and Nd co-doped Bismuth Titanate [Bi 3 (La x Nd 1x )Ti 3 O 12 ], where x =.5 powder was calcined at 8 C temperature for 2 hours in open air. The XRD spectrogram is shown at Fig. 4 there is no impurity phase other than the perovskite phase. Intensity (Arb. Unit) (8) (111) (115) (1) (2)&(2) (14) (28)&(28) (1115) (214) (317) (117) Bi 3 (La x Nd 1-x )Ti 3 O 12 x= θ (degree) Fig. 4. The X-ray diffraction patterns of Lanthanum (La) and Neodymium (Nd) co-doped Bismuth titanate BLNT Bi 3 (La x Nd 1-x )Ti 3 O Characterization of Ceramic sample after the sintering process: Measurement of Dielectric Constant Ceramic powders of BIT having calcinations temperature of 75 C and for ceramic BLT, BNT and BLNT having calcinations temperature of 8 C are formed into pellet and sintered at different temperatures. Fig. 5 shows the variation of the dielectric constant with frequency at room temperature for pure BIT sample sintered at 8, and C temperature for 1 hour Dielectric constant Fig. 5. Frequency dependent dielectric constant of BIT samples sintered at different temperatures (8, and C). It is observed that the dielectric constant of the samples is high at lower frequency region,

6 18 Md. Aminul Islam et al. /Science of Sintering, 47 (215) decreases with increase of frequency by approaching approximately a more or less constant value above 1 KHz. It is also observed that, dielectric constant increases with the increase of sintering temperature, due to the grain growth and enhanced crystallinity but at too much high temperature above C, dielectric constant was decreased due to the evaporation of volatile Bi 3+ ions [1]. The better result was obtained at C. From the preceding result both BLT and BNT ceramics were sintered at, and C temperature for 1 hour. The dielectric behavior of both BLT and BNT ceramic is similar to that of BIT except the increased magnitude that is shown at Fig. 6 and Fig. 7. From the Fig.s it is observed that, the higher the sintering temperature, dielectric constant increases due to the larger grains growth [11] and highest dielectric constant is obtained at C for both BLT and BNT, further increase of sintering temperature dielectric constant has been decreased owing to evaporation of bismuth oxide at higher temperature. Dielectric constant and optimum sintering temperature for both La and Nd doped BIT were increased due to increase of lattice distortion [12] and raise of reaction temperature respectively upon substitution as shown in Fig.6 and Fig Dielectric constant Fig. 6. Frequency dependent dielectric constant of BLT samples sintered at different temperatures (, and C). 4 Dielectric constant Fig. 7. Frequency dependent dielectric constant of BNT samples sintered at different temperatures(, and C).

7 Md. Aminul Islam et al./science of Sintering, 47 (215) Dielectric constant Fig. 8. Frequency dependent dielectric constant of Bi 3 (La x Nd 1-x )Ti 3 O 12 where x =.5, sample sintered at different temperatures(, and C). Fig. 8 shows the frequency dependent dielectric constant of Bi 3 (La.5 Nd.5 )Ti 3 O 12 sintered a temperature,, and C for 1 hour. Maximum dielectric constant at higher frequency range was observed at C that means optimal sintering temperature has been increased owing to increase of reaction temperature upon co-doping with La and Nd simultaneously Measurement of Dielectric Loss From the Fig.9 Fig.12 shows the variation of the dielectric loss with frequency at room temperature for BIT, BLT BNT and BLNT samples sintered at different temperatures for 1 hour. The dielectric loss of all samples is high at lower frequency region (due to contribution of all kinds of loss process at low frequency) decreases with increase of frequency by approaching more or less a constant value above 1 KHz. It is possible that this decrease in the dielectric loss in this frequency range is due to cheese of ion migration and ion vibration effect on the dielectric loss at higher frequencies [1] Dielectric Loss Fig. 9. Frequency dependent dielectric loss of BIT sintered at different temperature(8, and C).

8 182 Md. Aminul Islam et al. /Science of Sintering, 47 (215) It is observed that, due to increase of sintering temperature dielectric loss decreases gradually due to the larger grains growth [11] and improve crystal quality at high temperature, but at too much high temperature above a certain degree Celsius( C) dielectric loss further increases owing to evaporation of bismuth oxide at higher temperature. For pure BIT, dielectric loss at C was lowest as shown in the Fig.9 and further increase of sintering temperature lose tangent was increased due to evaporation of bismuth ion and oxygen ion at high temperature Dielectric Loss Fig. 1. Frequency dependent dielectric loss of BLT sintered at different temperature (, and C). The loss tangent behavior of BLT sintered at, and C in response of frequency was shown at Fig.1 where improved loss was observed at C. Fig.11 exposed the frequency dependent dielectric loss of BNT for different sintering temperature. The loss tangent property was improved with the increase of sintering temperature up to C and then decreased due increase of vacancies at high temperature Dielectric Loss Fig. 11. Frequency dependent dielectric loss of BNT sintered at different temperature (,, C).

9 Md. Aminul Islam et al./science of Sintering, 47 (215) Dielectric Loss Fig.12. Frequency dependent dielectric loss of Bi 3 (La x Nd 1-x )Ti 3 O 12 where x =.5, sample sintered at different temperatures(, and C). Fig.12 show the dielectric loss of La and Nd co doped BIT sintered at, and C temperature, where better result was at C Measurement of frequency dependent AC conductance The effect of sintering temperature on ac conductivity of Pure BIT, BNT, BLT and BLNT ceramic can be observed in the Fig.13 to Fig.16 where conductivity has been increased with frequency. These increase of conductivity can be explain by the conductivity equation (1) 1, σ = ωε where σ is conductivity, ω= 2πf (f =frequency) and ε is dielectric loss factor. From this equation it is obvious that the conductivity is directly related with frequency [1] which is reasonable with obtained results. AC conductivity decreases with the increase of sintering temperature. These could be due to the enhancement of grain and improvement of crystal quality[11]. But at too much high temperature above a certain C, the AC conductivity further increases. This increase of conductivity can be explained by considering the evaporation of volatile Bi 3+ ions at high temperature. During heat treatment volatile Bi 3+ ions are evaporated and some intrinsic oxygen vacancies appear which produce electrons according to the equation (2) O o = V ** + 2e +1/2O 2, 2 Where O o is the oxygen ion at parent crystal site, V ** is the intrinsic oxygen vacancy and e is the free electron. From this equation it is obvious that, free electrons increases with the increase of sintering temperature and eventually conductivity increases. AC conductance of pure BIT, BLT and BNT were studied that was shown at Fig.13, Fig. 14 and Fig. 15 respectively where it was observed that ac conducting property of pure BIT, BLT and BNT was improved with sintering temperature up to C for BIT, and C for La and Nd doped BIT then move backed.

10 184 Md. Aminul Islam et al. /Science of Sintering, 47 (215) a-c Conductance (S)x Fig. 13. Frequency dependent ac conductance of BIT [Bi 4 Ti 3 O 12 ] ceramic sintered at different temperature. a-c Conductance (S)x Fig. 14. Frequency dependent ac conductance of BLT [Bi 3.25 La.75 Ti 3 O 12 ] ceramic sintered at different temperature. 5 a-c Conductance (S)x Fig. 15. Frequency dependent ac conductance of BNT [Bi 3.15 Nd.85 Ti 3 O 12 ] ceramic sintered at different temperature.

11 Md. Aminul Islam et al./science of Sintering, 47 (215) a-c Conductance x1-6 (S) Frequency x 1 5 (Hz) Fig. 16. Frequency dependent ac conductance of BLNT [Bi 3 (La x Nd 1-x )Ti 3 O 12 ], X =.5 sintered at different temperature. Fig. 16 showed the AC conducting behavior of La and Nd co-doped BIT sintered at,, and 15 C, where best result was obtained at C. From the above feature it could be concluded that the optimum result of AC conductance for BIT, BLT, BNT and BLNT ceramics were obtained at,, and C respectively that means the stability of bismuth layered structure at high temperature was sustained with the addition of La and Nd by suppressing the bismuth evaporation. 4. Conclusion Pure Bismuth titnate (BIT) Lanthanum doped (BLT), Neodymium doped, (BNT) and Lanthanum and Neodymium co-doped (BLNT) ceramics were fabricated by using solid state reaction method. The physical, structural, dielectric and electrical properties of fabricated ceramics were studied using XRD and impedance analyzer. Based on the experimental results of those properties the following conclusions can be made: (a) All ceramics were polycrystalline belonging a single phase of bismuth layered perovskite structure with preferred (117) and (l) orientations and have no other impurity phase. (b) Pure BIT showed better crystallinity at calcining temperature of 75 C. (c) BLT, BNT and BLNT ceramics show better crystal quality at calcining temperature of 8 C, which implies that the reaction temperature of bismuth titanate has been increased due to La and Nd doping. (d) Pure BIT optimum sintering temperature was C. (e) Optimum sintering temperature for both BLT and BNT showed was C and (g) La and Nd co-doped bismuth titanate (BLNT) revealed optimum sintering temperature of C. Therefore, sintering temperature of bismuth titanate also was increased due to La and Nd doping. 5. References 1. J. F. Dorrian, R. E. Newnham, K. K. Smith, Ferroelectrics, 3, 17 (1971). 2. Z. Lazarević, B. D. Stojanović, J. A. Varela, Science of Sintering, 37, 199 (25) 3. B. H. Park, B. S. Kang, S. D. Bu, T.W. Noh, J. Lee, and W. Jo, Nature, 41, 682 (1999).

12 186 Md. Aminul Islam et al. /Science of Sintering, 47 (215) X.L. Zhong, J B. Sun, C. B. Tan, X. J. Zheng and Y. C. Zhou, Appl. Phy. Lett., 9, 1296 (27). 5. U. Chon, G. C. Yiand H. M. Jang, Appl. Phys. Lett., 78, 658 (21). 6. U. Chon,H. M. Jang, M. G. Kim and C. H. Chang, Phy. Rev. Lett., 89, 8761(22). 7. V. V. Mitic, V. Paunovic, V. Pavlovic and Lj. Zivkovic, Science of Sintering, 43, 277, (211). 8. M. A. Islam, M. A. Gafur and M. S. Islam, Asian Journal of Applied Science and Engineering, 1(2), 65 (212). 9. W. Zhang, B. Yang, and J. Chen, International Journal of Photo energy, 212, 8, (212). 1. W.D. Kingery, Introduction to Ceramic, John Wiley and Sons, Inc. New York, C. Fu, X. Long, W. Cai, G. Chen and X. Deng, Ferroelectrics, 445, 114 (213) 12. A. Roy, R. Prasad, S. Auluck, and A. Garg, Appl. Phys. Lett. 12, (213) Садржај: Добро разумевање својстава керамике бизмут титаната (BIT) допираног La/Nd на високим температурама је јако важно због нових материјала на бази BIT. Чист BIT, (BLT) допиран La, (BNT) допиран Nd и BIT допиран La и Nd (BLNT) синтетизовани су реакцијама у чврстој фази. Припремљени прахови су калцинисани на различитим температурама и структурна својства су им одређена рендгенском дифракцијом. За чист BIT боља кристалиничност је постигнута на 75 C а за BLT и BNT бољи резултати постигнути су на 8 C. Калцинисани прахови су испресовани и синтеровани на различитим температурама и одређена су њихова диелектрична својства. Оптимална температура синтеровања за BLT и BNT је C а La и Nd кодопиран бизмут титанат (BLNT) има оптималну температуру синтеровања од C. Стога, оптимална температура синтеровања бизмут титаната расте са додатком допаната La и Nd. Кључне речи: бизмут титанат, допанти La/Nd, синтеровање, рендгено структурна анализа, диелектрична својства