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Structural characterization of CuO-doped alkali niobate piezoelectric ceramics by electron paramagnetic resonance spectroscopy

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  1. Tez No: 696437
  2. Yazar: EBRU ERÜNAL
  3. Danışmanlar: PROF. DR. RÜDİGER-A. EİCHEL
  4. Tez Türü: Doktora
  5. Konular: Fizik ve Fizik Mühendisliği, Kimya, Kimya Mühendisliği, Physics and Physics Engineering, Chemistry, Chemical Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 2011
  8. Dil: İngilizce
  9. Üniversite: Albert-Ludwigs-Universität Freiburg im Breisgau
  10. Enstitü: Yurtdışı Enstitü
  11. Ana Bilim Dalı: Fizikokimya Ana Bilim Dalı
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 157

Özet

Alkali niobate ceramics are promising 'lead-free' alternatives for currently used Pb(Zr,Ti)O3 piezoelectric materials. In this thesis, the defect structure of CuO-doped alkali niobate ceramics were investigated depending on the alkali and niobium non-stoichiometry, process conditions (calcination and sintering) and CuO-doping amount by means of multi-frequency Electron Paramagnetic Resonance (EPR) Spectroscopy. Moreover, the atomic scale information provided through EPR analysis was supported by theoretical DFT calculations and compared to a microstructural (with XRD and SEM characterization) analysis. The samples under study were prepared through a mixed-oxide-carbonate route. The CuO-doping content was kept comparatively low at 0.25 mol% to minimize the secondary-phase formation effects. In consideration of a complete picture, the technologically promising morphotropic-phase boundary materials (K0:5Na0:5)NbO3 (KNN 50/50) were studied together with the end members of the pseudo-binary solid-solution system, KNbO3 (KN) and NaNbO3 (NN). Besides, in order to transfer the determined set of spin Hamiltonian parameters into structural information, a semi-empirical relationship between the spin-Hamiltonian parameters and the defect structure of Cu2+ systems with low spin (S = 1/2) was developed. By this way, the existence of 'isolated' Cu2+ centers in a complete oxygen octahedron or the formation of a dimeric defect complex (V O 􀀀 Cu2+) or a trimeric defect complex (V O 􀀀Cu2+􀀀V O ) with charge-compensating oxygen vacanies (V O ) can be directly assigned. The alkali or niobium non-stoichiometry has a strong influence on the defect structure for CuO-doped NN and KNN 50/50. As the niobium amount increased in the system, more Cu2+ incorporation is observed. Furthermore, the site of incorporation of Cu2+ has been detected as the Nb5+ site. This triggers the formation of mutually compensating dimeric and trimeric defect complexes. However, their relative concentrations depend strongly on the alkaline (Na+, K+) and niobium (Nb5+) non-stoichiometry. Differently from the KN and KNN 50/50, for NN samples a secondary phase invoking a Cu2+- Cu2+-dimeric functional center was obtained. Apart from this, for KNN 50/50, as the doping amount of CuO is increased above 0.25 mol%, the formation of a secondary phase (K4CuNb8O23) was distinguished both by XRD and EPR. Lastly, the nature of chemical bonding of Cu2+ with the Na+ ion in CuO-doped KNN 50/50 was analyzed by 'Hyperfine Sublevel Correlation Experiments' (HYSCORE). The transferred spin density to the Na+ ions in KNN 50/50, showed a considerably reduced covalent bonding to the A-site ion than in CuO doped PZT system. KEYWORDS:Lead-Free Piezoelectrics,CuO doping, KNN,KN,NN,Solid State EPR, Defect Chemistry, Oxygen Vacancies

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