Ferroelectrics

The Effect of Excessive Sm3+ doping on the Ferroelectric properties of Lead Zirconate Titanate Ceramics T. Anil Babu*, K. V. Ramesh+, V. Raghavendra [email protected] and D. L. Sastry* * Physics Department, Andhra University, Visakhapatnam – 530 003 (AP) + Department of Engineering Physics, GITAM University, Visakhapatnam – 530 045(AP) @ UGC-DAE Consortium for Scientific Research, University Campus, Indore-(MP) Email: [email protected] com Abstract. Polycrystalline samples of Sm modified Pb Zrx Ti(1-x) O3 [PZT] ceramics ( where x = 0. 6,0. 7,0. 8,0. ) have been prepared by a high energy ball milling technique, which was followed by calcination at 9500 C and sintering at 11500 C. As Sm3+ concentration is increased more than 0. 1 mole% considerable pyrochlore phase has been formed.This phase has been identified as Sm2Ti2O7 from its X-ray diffraction peaks. X-ray diffraction (XRD) and Scanning electron mcroscopy (SEM) studies have been carried out to determine the structural phase and morphological modifications that occur with the change in Zr/Ti ratio. The ferroelectric phase transition temperature (Tc) decreases and increases where as the dielectric constant (? ) increases and decreases. The saturation polarization Ps (? c/cm2), remnant polarization Pr (? c/cm2) and coercive field Ec (kv/cm) values of the perovskite in the presence of the pyrochlore are presented. Key Words: Ferroelectric Properties, Dielectric Constant, Sm3+ doping, Lead Zirconium Titanate, Pyrochlore PACS: 77. 80. -e, 77. 80. B-, 77. 22. -d; 77. 80dj; 77. 84Cg. Introduction LEAD ZIRCONATE TITANATE IS A PEROVSKITE MATERIAL OF ABO3 STRUCTURE WITH SEVERAL APPLICATIONS SUCH AS ACTUATORS, TRANSDUCERS, PYROELECTRIC DETECTORS, ELECTROOPTIC MODULATORS, RANDOM ACCESS MEMORIES ETC.IT IS A SOLID SOLUTION OF FERROELECTRIC PBTIO3 AND ANTIFERROELECTRIC PBZRO3 WITH A MORPHOTROPIC PHASE BOUNDARY EXISTING AT ZR/TI RATIO OF 48/52. THE MATERIALS HAVE VERY INTERESTING PHYSICAL PROPERTIES AT THE COMPOSITION AROUND MORPHOTROPIC PHASE BOUNDARY (MPB). HOWEVER ABOVE AND BELOW MPB ALSO THE MATERIALS HAVE INTERESTING PHYSICAL PROPERTIES USEFUL FOR DEVICE APPLICATIONS. THE PHYSICAL PROPERTIES OF PZT CERAMIC COMPOUNDS ARE GREATLY INFLUENCED BY THE SUBSTITUTION OF DIVALENT OR TRIVALENT IONS IN THE PLACE OF PB2+.THE METHOD OF PREPARATION SUCH AS SOLID STATE REACTION OR WET CHEMICAL METHODS OR MECHANOCHEMICAL SYNTHESIS ARE ALSO SUPPOSED TO INFLUENCE THE PHYSICAL PROPERTIES AND HENCE THE PARAMETERS THAT ARE USEFUL FOR DEVICE FABRICATIONS. IN THE PRESENT WORK WE HAVE CHOSEN PZT WITH ZR/TI IN THE RATIO OF 50/50 WHICH IS ON THE TETRAGONAL SIDE OF THE MPB WHICH IS A FERROELECTRIC PHASE AT THE ROOM TEMPERATURE. THE METHOD OF PREPARATION CHOSEN IS HIGH ENERGY BALL MILLING COMBINED WITH CALCINATION AT 9500C. IN THIS MATERIAL SM3+ RARE EARTH ION IS SUBSTITUTED IN THE PLACE OF PB TO GIVE PB1-X SMX (ZR0. TI0. 5) O3 (PSZT) WHERE X IS VARIED FROM 0. 1 TO 0. 4. IN THE PRESENT COMMUNICATION RESULTS OBTAINED FOR PSZT WHERE SM3+ CONCENTRATION FOR X= 0. 1 AND 0. 4 ARE PRESENTED. EXPERIMENTAL The samarium substituted (PSZT) materials were prepared by taking high purity PbO, ZrO2, TiO2 and Sm2O3 in the required stoichiometric ratio and milled at room temperature using a INSMART systems planetary ball mill for nearly 24hours adding methanol to obtain a homogeneous mixture of the starting materials. Tungsten carbide balls were employed in the milling process and milling speeds of 300rpm were used.Milling was stopped every one hour for 10minutes to allow the contents to cool. The tungsten carbide balls were of 8mm&10mm diameter and the ball to powder ratio employed was 10:1. The as prepared samples were calcined at 950oC for 3hrs so as to completely react any remaining unreacted ingredients. The calcined powders were thoroughly ground adding a small amount of PVA and made into pellets of 10mm diameter under a pressure of 3ton/mt2 using hydraulic press. The pellets were sintered in alumina crucibles at a temperature of 11500C for 4hrs.The X-ray diffractograms of the calcined powders were recorded using Xpert-Pro X-ray diffractometer with CuK? (? =1. 5406A0) target. The pellets were finely ground and polished. A silver paste was applied on both sides for measurements of frequency dependent dielectric and conductivity. The samples were poled at 1200C for an hour by applying 3kv/cm for hysteresis loop measurements. [pic] Fig1. X-ray diffractogram of PSZT for Sm3+ = 0. 1mole% RESULTS AND DISCUSSION Fig. 1 shows the X-ray diffractograms of the calcined powders for x=0. 1 of PSZT. All peaks corresponding to the tetragonal phase of PZT were observed.Table 1 gives the lattice parameters of the tetragonal phase of the PZT for different Sm3+ dopant concentrations. As the Sm3+ concentration is increased, peaks other than those corresponding to PZT are found to increase in intensity. By comparison with JCPDS-data these peaks are identified as due to the pyrochlore Sm2Ti2O7which has a cubic structure. The lattice parameters of the pyrochlore phase are also given in Table1. To estimate the ratio of the pyrochlore phase to that of the perovskite phase the intensity of (222) peak corresponding to the pyrochlore (not shown in Fig. 1. and intensity of (101) peak corresponding to the perovskite are used. This intensity ratio increases from 0. 05:1 for Sm3+ concentration 0. 1mole% to 0. 56:1 for Sm3+ concentration of 0. 4 mole%. The intensity ratio of x-ray diffraction lines do not give the actual intensity ratio as x-ray intensities not only depend on the quantity of the phase present but also on other factors like structure factor, Loentz factor etc. which may vary from system to system. But the growth of x-ray diffraction lines due to the pyrochlore indicates the growth of this phase as Sm3+ concentration increases.The pyrochlore is generally an unwanted phase as its presence deteriorates the parameters of the perovskite which are desirable for good device performance. Nevertheless it is interesting to investigate the ferroelectric properties of the PSZT in the presence of the pyrochlore. It can be seen that as Sm3+ concentration (or as pyrochlore phase concentration) is increasing Tc decreases from 3700C to 3500C and increases to 3800C. Conversely the dielectric constant corresponding to 10 KHz at Tc increases from 106 to 527 and decreases again to 106.The results of hysteresis studies are shown on Table. 2. The saturation polarization, remnant polarization and co-ercive field values decrease as as Sm3+ dopant concentration is increasing. The effect of increase in pyrochlore formation can be two fold. (1) All of Sm3+ used may not replace Pb2+. (2) Pyrochlore formed may remove some Ti4+ (or Zr4+) from the perovskite lattice thus creating several lattice defects. This can have a substantial influence on the ferroelectric properties of the perovskite phase as it affects the DC and AC conductivity of the system.These results are under study. Table1. Sm3+ concentration, lattice parameters of PSZT and pyrochlore phases, Tc, dielectric constant ? ’, saturation Ps and remnant polarizations Pr (µC/cm2). |Sm3+ |a(A0) |b(A0) |Pyrochlore |Tc,(0C) |? ’ at Tc| |mole% | | |a(A0) | | | |0. 1 |4. 033 |3. 971 |10. 277 |370 |106 | |0. 2 |4. 039 |3. 990 |10. 267 |350 |527 | |0. 3 |4. 050 |4. 019 |10. 230 |360 |207 | |0. 4 |4. 20 |3. 997 |10. 218 |380 |106 | Table2. Sm3+ concentration, saturation Ps and remnant polarizations Pr (µC/cm2) and co-ercive field Ec (kV) values of PSZT. |Sm3+ mole% |Ps (µC/cm2) |Pr (µC/cm2) |Ec (kV) | |0. 1 |1. 076 |0. 448 |4. 98 | |0. 2 |1. 776 |0. 982 |4. 21 | |0. 3 |0. 669 |0. 264 |4. 03 | |0. 4 |0. 096 |0. 004 |0. 22 | REFERENCES [1] B. Jaffe, W.R. Crook, and H. Jaffe, Piezoelectric Ceramics,Academic Press, New York, NY, USA, 1971. [2] B. V. Hiremath, A. I. Kingon, and J. V. Biggers, Journal of the AmericanCeramic Society, vol. 66, no. 11, pp. 790–793, 1983. [3] C. Pramila, T. C. Goel, and P. K. C. Pillai, Materials Science andEngineering B, vol. 26, no. 1, pp. 25–28, 1994. [4] R. Khazanchi, S. Sharma, and T. C. Goel, “Journal of Electroceramics, vol. 14, no. 2, pp. 113–118, 2005. [5] S. K. S. Parashar, R. N. P. Choudhary, and B. S. Murty, Journal of Nanoscience and Nanotechnology, vol. 9, pp. 1-6, 2008.

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