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Featured researches published by Jérôme Acker.


Physical Chemistry Chemical Physics | 2009

Local variations in defect polarization and covalent bonding in ferroelectric Cu2+-doped PZT and KNN functional ceramics at the morphotropic phase boundary

Rüdiger-A. Eichel; Ebru Erünal; Michael D. Drahus; Donald M. Smyth; Johan van Tol; Jérôme Acker; Hans Kungl; Michael J. Hoffmann

Cu(2+)-doped Pb[Zr(0.54)Ti(0.46)]O(3) (PZT) and Cu(2+)-doped [K(0.5)Na(0.5)]NbO(3) (KNN) ferroelectrics with a dopant concentration of 0.25 mol% were investigated by means of multi-frequency and multi-pulse electron paramagnetic resonance (EPR) spectroscopy. Through the use of high magnetic fields and pulsed microwave fields an enhanced resolution was achieved yielding valuable information about the structural distortion at the dopant site. The results obtained suggest that Cu(2+) substitutes for both systems as an acceptor centre for the perovskite B-site. For reasons of local charge compensation, different kinds of defect associates invoking one and two oxygen vacancies are formed. These two kinds of extended defects differ in their electric and elastic properties. The results obtained are analyzed in order to characterize differences of the local structure in the Cu(2+)-defect center for morphotropic phase boundary compositions between the two systems. In particular, it is found that Cu(2+)-doping in KNN creates 50% more oxygen vacancies than the same amount of copper in PZT. Furthermore, local differences in covalent and ionic bonding are monitored.


Applied Physics Letters | 2013

Interactions of defect complexes and domain walls in CuO-doped ferroelectric (K,Na)NbO3

Rüdiger-A. Eichel; Ebru Erünal; Peter Jakes; Sabine Körbel; Christian Elsasser; Hans Kungl; Jérôme Acker; Michael J. Hoffmann

“Lead-free” piezoelectric sodium potassium niobate has been studied with respect to its defect structure when doping with CuO. The results indicate that two kinds of mutually compensating charged defect complexes are formed, ( Cu ′ ′ ′ Nb − V O • • ) ′ and ( V O • • − Cu ′ ′ ′ Nb − V O • • ) • . Concerning the interplay of these defect complexes with the piezoelectric materials properties, the trimeric ( V O • • − Cu ′ ′ ′ Nb − V O • • ) • defect complex primarily has an elastic dipole moment and thus is proposed to impact the electromechanical properties, whereas the dimeric ( Cu ′ ′ ′ Nb − V O • • ) ′ defect possesses an electric dipole moment in addition to an elastic distortion. Both types of defect complexes can impede domain-wall motion and may contribute to ferroelectric “hardening.”


Functional Materials Letters | 2010

DEFECT STRUCTURE OF COPPER DOPED POTASSIUM NIOBATE CERAMICS

Ebru Erünal; Rüdiger-A. Eichel; Sabine Körbel; Christian Elsasser; Jérôme Acker; Hans Kungl; Michael J. Hoffmann

The defect structure resulting from copper doping of potassium niobate (KNbO3) ceramics was investigated by a combined analysis of electron paramagnetic resonance (EPR) spectroscopy and first-principles calculations based on density functional theory (DFT). The results indicate that under atmospheric oxygen partial pressure, Cu preferentially substitutes on the Nb sites where it can trap one or two oxygen vacancies. Correspondingly, for 0.25 mol% Cu doped KNbO3 ceramics, two types of defect associates are formed — and . The association of Cu impurities and oxygen vacancies lowers the defect formation energy by 1.0 eV to 2.7 eV, depending on the Fermi level. Owing to the opposite charges of these two types of defects, overall charge neutrality is theoretically possible by mutual compensation of the defect associates, without formation of additional non-associated oxygen vacancies.


Archive | 2012

Influence of the A/B Stoichiometry on Defect Structure, Sintering, and Microstructure in Undoped and Cu-Doped KNN

Michael J. Hoffmann; Hans Kungl; Jérôme Acker; Christian Elsasser; Sabine Körbel; Pavel Marton; Rüdiger-A. Eichel; Ebru Erünal; Peter Jakes

Development of ceramics based on the alkaline niobate (KNN) system is one of the major lines of current research pointing to substitution of the lead containing ferroelectrics by lead‐free materials. Sodium potassium niobate (K0.5Na0.5)NbO3 is a prototype material of lead‐free alkaline‐transition metal ferroelectrics with \({\rm A}^{1+}{\rm B}^{5+}{\rm O}_3^{2-}\) perovskite structure. Processing procedures for KNN‐based ceramics are however challenging due to the hygroscopic behavior of sodium‐ and potassium carbonates and the evaporation of alkalines at the elevated processing temperatures, which make it difficult to control the stoichiometry of the ceramics. Alkaline (A‐site) or niobium (B‐site) excess results in pronounced qualitative differences of the microstructure in KNN ceramics.


Journal of the American Ceramic Society | 2010

Influence of Alkaline and Niobium Excess on Sintering and Microstructure of Sodium‐Potassium Niobate (K0.5 Na0.5)NbO3

Jérôme Acker; Hans Kungl; Michael J. Hoffmann


Physical Review B | 2011

CuO-doped NaNbO3antiferroelectrics: Impact of aliovalent doping and nonstoichiometry on the defect structure and formation of secondary phases

Ebru Erünal; Peter Jakes; Sabine Körbel; Jérôme Acker; Hans Kungl; Christian Elsasser; Michael J. Hoffmann; Rüdiger-A. Eichel


Journal of The European Ceramic Society | 2014

Microstructure of sodium-potassium niobate ceramics sintered under high alkaline vapor pressure atmosphere

Jérôme Acker; Hans Kungl; Roland Schierholz; Susanne Wagner; Rüdiger-A. Eichel; Michael J. Hoffmann


Journal of The European Ceramic Society | 2013

Sintering and microstructure of potassium niobate ceramics with stoichiometric composition and with potassium- or niobium excess

Jérôme Acker; Hans Kungl; Michael J. Hoffmann


MRS Proceedings | 2009

Structural Characterization of Cu 2+ Functional Centers In ‘Lead-Free’ KNN Piezoelectrics

Ebru Erünal; Rüdiger-A. Eichel; Jérôme Acker; Hans Kungl; Michael J. Hoffmann


The 5th International Congress on Ceramics ICC5 | 2014

The influence of Cu-Doping on defect- and microstructure of sodium potassium niobate depending on alkaline/niobium non-stiochiometry

Hans Kungl; Micheal J. Hoffmann; Roland Schierholz; Peter Jakes; Jérôme Acker; Rüdiger-A. Eichel

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Hans Kungl

Karlsruhe Institute of Technology

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Michael J. Hoffmann

Karlsruhe Institute of Technology

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Peter Jakes

Forschungszentrum Jülich

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Susanne Wagner

Karlsruhe Institute of Technology

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