(Co)-Doping of Lead-Free Piezoceramics

• Main Author: Blömker, Martin Manfred
• Format: Others
• Language: English; en
• Published: 2016
• Online Access: https://tuprints.ulb.tu-darmstadt.de/5297/8/06_Complete_Body_008_Martin_Bl%C3%B6mker.pdf; Blömker, Martin Manfred <http://tuprints.ulb.tu-darmstadt.de/view/person/Bl=F6mker=3AMartin_Manfred=3A=3A.html> (2016): (Co)-Doping of Lead-Free Piezoceramics.Darmstadt, Germany, Technische Universität Darmstadt, [Ph.D. Thesis]

This work encompassed an in depth investigation of the defect chemistry and piezoelectric properties of Bi0.5(Na(1-w)Kw)0.5Ti(1-x-y)CuxVyO3 with w = 0.1, 0.2 and 0.3 and x and y < 0.01, synthesized by a solid state process. A pseudocubic perovskite phase with no detectable impurities was verified by X-ray diffraction, while doping tended to reduce the rhombohedral character of Bi0.5(Na0.9K0.1)0.5TiO3 (BNKT10) ceramics. Doping BNKT10 was shown to result in more homogenous grain size distributions, elucidated by analyzing scanning electron microscopy images. Small and large signal piezoelectric measurements also revealed that the tetragonal phase of Bi0.5(Na0.7K0.3)0.5TiO3 is disproportionally affected by doping. The transition temperature from ferroelectric to relaxor phase (Tf-r), the coercive field, and the remanent polarization drop for compositions with high K-content, while the maximum strain is mostly increased. The general trend of decreasing Tf-r in these high K tetragonal materials can be rationalized by A-site defects and the resulting increased distribution of random fields. Impedance spectroscopy at resonance indicates a high electromechanical coupling factor of planar samples in thickness mode (kt up to 0.56), which is useful for sensing applications in combination with the observed low mechanical quality factor (QM) of 9 in thickness direction. Impedance spectroscopy at elevated temperatures revealed that doping resulted in a decrease in the activation energy (EA) of 110±10 meV from 1.37 eV of undoped Bi0.5(Na0.9K0.1)0.5TiO3 for both single element doping with Cu or V, as well as simultaneous doping with both Cu and V. Furthermore, the resistivities of doped BNKT10 ceramics (3.3×103 Ωm to 1.3×10E7 Ωm) were consistently lower than those of the undoped BNKT10 ceramics (1.6×10E5 Ωm to 2.0×10E8 Ωm). The variation of the oxygen partial pressure from 0.21 bar (synthesis condition) to 1 bar, 2.1×10E-6 bar and < 10-18 bar likewise lead to consistently decreased EA and resistivities with values as low as 0.23 eV and 1.9×10E0 Ωm to 2.7×10E3 Ωm, respectively. Electron paramagnetic resonance spectroscopy (EPR) and X-ray photoelectron spectroscopy (XPS) indicated the presence of Cu2+, V4+ and V5+ in the doped ceramics, while lower oxidation states of Cu and V could be excluded. EPR spectroscopy indicated Cu2+ and V4+ in a rhombic environment with major distortion in one direction and minor distortion in the other directions. While V4+ is introduced into the B-site of the bulk, Cu2+ was shown to dominantly segregate from the bulk, most likely at the grain boundary due to the creation of a liquid phase during sintering. This correlates with the increasing density with increasing Cu content and the observed trend of a lowered QM with increasing Cu content, presumably due to leakage. The solubility limit of Cu in the bulk was found to be < 0.05 at.%. All doped BNKT10 ceramics, containing Cu, were shown to possess Cu in two different rhombic electronic environments, namely CuO6 and CuO4. For V doping, a discernable, albeit, small (<400 meV) increase in Fermi level was determined by XPS, suggesting a donor doping effect corroborated by the lower resistivities and EA of these samples likely due to charge compensation induced electrons. The overall defect chemistry of the Cu and V doped ceramics was revealed to be influenced by (I) bismuth, sodium and potassium acceptor vacancies, resulting from the host material, (II) additional n-type doping induced electrons, (III) few bulk Cu2+ acceptor centers, resulting from Cu doping, (IV) V5+ donor states, introduced through V doping, and (V) in all cases the concentration and mobility of oxygen vacancies.