Studies on the synthesis, stability and properties of a new ferroelectric ceramic

• Main Author: Zhu, Na
• Published: University of Sheffield 2009
• Subjects: 620.14042
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.505340

Recently there has been great interest in a new ferroelectric phase, BaThOs (BT2), due to its high permittivity. The crystal structure and thermal stability of BT2 have been previously characterised but difficulties arose due to the challenging synthesis conditions, particularly that phase-pure BT2 can not be synthesised using a simple, conventional solid-state reaction method. Wet chemical synthesis of this phase has been successful in some cases although accurate and effective experimental methods remain unclear. BT2 can be obtained by quenching the eutectic composition from above 1320 °C and has also been prepared as single crystals grown from melts. This indicates that BT2 may, in fact, be a thermodynamically stable phase over a narrow range of temperatures close to melting, We have therefore started to reinvestigate the BaO-Ti02 phase diagram to determine whether BT2 is thermodynamically stable at high temperatures only and modify the phase diagram. Subsequently, we have researched the electrical properties and the iD-f1uence of dopants on the synthesis process and the electrical properties. The stability range ofBT2 has been identified thus: BT2 is kinetically stable but thermodynamically metastable below 1150 °C; however, BT2 is thermodynamically stable over a narrow range of temperatures between 1220 and 1230 °C; BT2 does not appear at temperatures close to the melting point (- 1325 °C), which agrees with recent phase diagrams; the formation and decomposition kinetics are very temperature dependent, especially at temperatures close to the stability range, 1220 to 1230 °C. Based on the stability of BT2, the BaO-TiOz phase diagram was modified and BT2 was synthesised both below 1150 °C, and between 1220 °C and 1230 °C using solid state reaction for the first time. BT2 ceramic sintered at 1220 °C for 207 hours shows excellent properties: e'max is 190; Tc is 426°C. The e'max value is two orders of magnitude lower than for single crystals along the b-axis, but is higher than for BT2 ceramic synthesised by -sol-gel method with lower sintering temperature. T c is composition-dependent. Tano remains low from room temperature to 400 DC ® 0.09), and are lower than the values for single crystal BT2. A higher sintering temperature (but one which avoids BT2 decomposition) produces a BT2 ceramic with higher a', lower Ea. Iso- and alio-valent dopants on A- and B-sites have an effect on the stability, structure and the electrical properties ofBT2 to different degrees. Ge and Al reduce, but Zr increases, the stable temperature range ofBT2, Ca does not influence it. There is an approximately linear relationship between ionic radius difference and cell volume difference for Ge4+, Sn4+ and Zr4+. a'max values for doped BT2 ceramics range from 30 to 57. All samples are highly insulating. Ge increases Tc ofBT2; Al, Sn, Zr and Nb reduce Te by different amounts; Ca does not change Te. Tano data are low ® 0.007) from room temperature to 400 DC. La- and Nb-doped BT2 obtain a mixture phases with more BT than BT2; they have almost temperature-independent permittivity (-30). BT2 can not be synthesized from BaC03 with Ba6Th7040 (B6T17) and Ba4Ti13030 ( (B4T13) in a solid state reaction. The permittivity for B6T17 and B4T13 are - 46 and - 37, respectively. Tano values for B6T17 and B4T13 are below 0.01 between room temperature and 500 DC.