Synthesis & characterization of modified barium titanate ferroelectrics by modified solid state reaction and microwave sintering routes

Abstract

These following five series have been synthesized by modified solid state reaction (MSSR) route:
(i) Ba1-xCaxTiO3/BCT
(ii) Ba1-xMgxTiO3/BMT
(iii) Ba1-xSrxTiO3 /BST
(iv) Ba1-xSmxTi(1-x/4)O3 /BSmT
(v) Ba1-xLaxTi(1-x/4)O3 /BLT where x=0.02, 0.04, 0.06 & 0.08.
(In the present work for x=0.02, 0.04. 0.06 & 0.08, we have used BCT1, BCT2, BCT3, BCT4, BMT1, BMT2, BMT3, BMT4, BST1, BST2, BST3, BST4, BSmT1, BSmT2, BSmT3, BSmT4 & BLT1, BLT2, BLT3, BLT4 notations). In the modified solid state reaction route (MSSR) barium acetate trihydate, calcium acetate/strontium acetate/magnesium acetate/lanthanum acetate/samarium acetate (all from Aldrich, USA with 99.8% purity) and fumed TiO2 (Degussa) were used as starting precursors. Calcination temperature for single perovskite phase formation of iso-valent modified BT ceramic samples was optimized at 900°C for 4 hrs whereas for higher% of off-valent substituted BT samples, calcination temperature is little higher (950°C for 4 hrs). For better density and morphology, iso-valent & off-valent substituted BT systems pellets were sintered at 1300 o C for 4 hrs. Calcination and sintering temperature of the iso-valent and off-valent substituted BT systems synthesized by MSSR route is significantly lower than the same systems processed through SSR route (calcination ~1200 0 C & sintering temperature ~1450 o C). Lowering of sintering temperature is attributed to the fine particle size of the precursors. In BMT and BST systems a tetragonal structure is found. The tetragonality (c/a ratio) decreases with the increase in Mg & Sr substitution% in BMT & BST systems. In BCT ceramic samples tetragonality increases with the increase of Ca substitution%. In BLT ceramics, the tetragonality (c/a ratio) decreases with the increase of La substitution% and in BLT2, BLT3 & BLT4 systems the structure is found to be cubic. In BSmT ceramic samples the tetragonality (c/a ratio) decreases with the increase of Sm substitution fraction and in BSmT4 system the structure is found to be cubic. Surface micrographs (SEM) of all the iso-valent and off-valent substituted BT systems showed well developed grains. Presences of nearly pore free uniform grains in these ceramic samples suggest the advantage of using MSSR route over conventional SSR route. Grain size increases with the increase of Ca and decreases with the increase of Mg, Sr, La & Sm substitution% in modified BT systems. Densities of iso-valent and offvalent modified BT samples synthesized by MSSR route at significantly lower processing temperature is better than the same samples synthesized by SSR route. Variation of r and tan at different frequencies (1kHz- 1MHz) as a function of temperature of all the iso-valent and off-valent substituted BT system synthesized by MSSR route have been studied. Room temperature (RT) variation of r and tan as a function of frequency of the modified BT system has also been studied. Variation of dielectric properties with frequency shows the usual behaviour of dielectric materials i.e decrease of the value of r with the increase of frequency. With the increase of substitutions% of Ca in BCT system, position of Tc increases slightly. With the increase in Sr & Mg substitution% in BST & BMT systems, the position of Tc decreases but remains above RT. Temperature coefficient of capacitance from RT to 70 o C in BCT1, BCT2 & BCT3 samples is almost negligible compare to unmodified BT samples, which signifies the importance of Ca modification in BT system for its use in multilayer capacitors. Whereas, with the increase of La and Sm substitution% in BLT & BSmT systems, the position of Tc decreases drastically giving rise to high value of r at RT. High value of r at RT in BLT1 system suggested it as a useful material for MLC applications. Diffusivity factor () is increasing slightly with the increase of Ca substitution concentration in BCT ceramic samples but remains near to 1, which represents normal phase transition. This establishes the good solubility of Ca in BT system and as a result less compositional fluctuations. Whereas with the increase of Mg and Sr substitution concentration in modified BT systems the value of  factor increases, indicating the increase in diffusive nature of phase transition. Similarly, in BSmT & BLT series the value of  factor is more than 1 and indicates the diffuse phase transition nature of these modified BT systems. Existence of good dielectric properties in the modified BT ceramic samples synthesized by MSSR route at comparably lower processing temperature than SSR route & using cheap chemicals comparable to chemical routes suggested the advantage of using MSSR route in the present study. P-E hysteresis loops of all the iso-valent and off-valent substituted BT systems synthesized by MSSR route have been studied. Value of coercive field (Ec) and remnant polarization (Pr) is decreasing with the increase of Ca and Sm substitution% in BCT & BSmT systems. Whereas, with the increase of La substitution % in BLT system, the P-E hysteresis loops shows the disappearance of ferroelectric nature of the samples. With the increase of Mg substitution% in BMT ceramic samples the ferroelectric nature decreases. With the increase of Sr substitution% in BST ceramic samples the value of Pr increases. The value of remnant polarization (Pr) of modified BT ceramic samples synthesized by MSSR route is comparable to the same systems synthesized by other routes. This again hints towards the advantage of using MSSR route in the present study. Longitudinal strain versus bipolar electric field (S-E) behavior of iso-valent and off-valent modified BT ceramic samples have also been studied. The appearance of a butterfly shape loop confirms the piezoelectric nature. Strain as high as 0.13% has been observed in unmodified BT ceramic samples synthesized by MSSR route. BLT & BCT SAMPLES SYNTHESIZED BY MW PROCESSING TECHNIQUE The pellets of polycrystalline calcined ceramic samples of BCT and BLT systems obtained from carbonate and oxide powders were sintered by microwave processing technique. The microwave sintering of the BCT & BLT ceramic samples was carried out at 1100 0 C for 1 h with a heating rate of 25 0 C/min by placing the pellets in the centre of a 4.4 kW, 2.45-GHz multi mode microwave cavity. The microwave furnace temperature was recorded by using a Raytek non-contact sensor (XRTG5). In the MW processed BCT samples, with the increase in Ca concentration the splitting at (200) plane decreases indicating the decrease in tetragonality. Whereas in the MW processed BLT samples with the increase in La concentration the splitting near 2=45 o merges into singlet indicating the structure changes from tetragonal to cubic. The MW processed BLT1 samples were indexed with tetragonal structure, whereas BLT2, BLT3 & BLT4 samples were indexed with cubic structure. Densities of the microwave sintered BCT and BLT ceramics were measured using Archimedes’ principle. The density measurements showed a better densified and less porous product of the microwave processed BCT and BLT samples at significantly lower sintering temperature and time. The obtained densities of MW sintered modified BT samples sintered at significantly lower sintering temperature and time are higher than the earlier reports on modified and unmodified BT ceramic samples processed through other processes including MW technique. It is evident that the microwave processing technique takes only a fraction of the time required in the conventional processing to achieve single-phase dense material. Grain size of microwave sintered Ca & La modified BT ceramic samples is lower than the same systems processed by SSR route. Better densification with the formation of finer and uniform grains in the case of microwave sintered samples hints towards the advantage of using this technique. Temperature variation of εr and tan at different frequencies of microwave sintered BCT and BLT ceramic samples were studied. For various substitutions of Ca in BCT system, position of Tc decreases slightly, whereas with the increase of La substitution% in BLT system, the position of Tc decreases drastically giving rise to high value of r at RT. In BLT1 and BLT2 systems, the temperature coefficient of capacitance is very small. In BLT1 system, the capacitance variation with temperature at different frequencies is almost negligible from RT to 70 o C. Temperature coefficient of εr from RT to 80 o C is almost zero for BCT2 and BCT3 systems. P-E hysteresis loops of microwave sintered BCT and BLT systems were studied. Value of coercive field (Ec) and remnant polarization (Pr) is decreasing with the increase of Ca substitution% whereas, with the increase of La substitution % in BT systems, the PE hysteresis loop study shows the disappearance of ferroelectric nature in the BLT2, BLT3 & BLT4 ceramic samples. Longitudinal strain versus bipolar electric field (S-E) behavior of MW processed modified BT ceramic samples has also been studied. The appearance of a butterfly shape loop confirms hints towards the piezoelectric nature of the samples. Strain as high as 0.15% has been observed in BCT3 & BLT1 ceramic samples. Finally, in the present study, MSSR and microwave processing have decreased the processing temperatures and time. High value of r and low value of tan have been obtained at RT in Ca & La modified BT systems synthesized by MSSR route. MSSR route promises desired dielectric properties at lower processing temperatures. High density with the presence of pore free uniform grains at lower sintering temperature & time suggested the advantage of using MW processing technique. Temperature coefficient of εr from RT to 80 o C was almost zero for MW processed BCT2 and BCT3 systems indicating increase in temperature stability of these compounds and their usefulness in MLC applications. This work is reported in eight chapters. First chapter describes a short review of the historical developments of ferroelectricity. This chapter also describes different structures of ferroelectric materials and establishes the motivation and aim of the present work.
Chapter II describes a detailed review about the investigated parameters.
Chapter III describes about the methodology of sample preparation and characterization techniques used for studying structural, micro-structural, dielectric, ferroelectric, piezoelectric properties and strain induced by electric field behaviors.
Chapter IV discusses about the perovskite phase evolution, structural, microstructural and density properties.
Chapter V discusses about the dielectric properties.
Chapter VI discusses about the ferroelectric & piezoelectric properties and strain induced by electric field behaviors.
Chapter VII discusses about the synthesis & characterization of modified BT ceramics synthesized by MW process.
Chapter VIII presents the conclusions of the present work and recommendations for future work in this field. This work has resulted in the following publications:
1. Low Temperature Perovskite Phase Formation in PCT 90/10 System, Sonia et.al, Applied Surface Science, 255 (2009) 5686–5689.
2. Synthesis and characterization of calcium modified barium titanate ceramics by partial precipitation route, Sonia et.al, Materials Science: An Indian Journal, 5 [3] (2009) 1. 3. Synthesis and Characterization of Isovalent Substituted BaTiO3 Ceramics by Modified Chemical Route, Sonia et.al, Integrated Ferroelectrics, 118:1 (2010) 106.
4. Structural, Dielectric & Ferroelectric Study of Microwave Sintered Lanthanum Substituted BaTiO3 Ceramics, Sonia et.al, Materials Chemistry & Physics, 130 (2011) 191– 195.
5. Effect of Sm and La off-valent Ion Substitution in BT Ceramics Synthesized by Modified Solid State Route, Sonia et.al, International J Mat. Sci. 6 [2] (2011) 133–140.
6. Effect of Microwave Processing on Structural, Dielectric and Ferroelectric Properties of Calcium Doped BaTiO3 Ceramics, Sonia et.al, J. Ceramic Progress & Research (In Press).
7. Characterizations of BT ceramics synthesized by modified solid state route, Sonia et.al, AIP Conf. Proc. 1372, (2011) 116.
8. Phase, dielectric and ferroelectric properties of microwave sintered la & ca modified batio3 ceramics, Sonia et.al, AIP Conf. Proc. 1372, (2011) 57.
9. Low temperature synthesis and dielectric, ferroelectric and piezoelectric study of microwave sintered of BaTiO3 ceramics, Sonia et.al, Ceramic International, 38 (2012) 1585–1589.