Jérôme Acker
Karlsruhe Institute of Technology
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Featured researches published by Jérôme Acker.
Physical Chemistry Chemical Physics | 2009
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
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
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
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
Jérôme Acker; Hans Kungl; Michael J. Hoffmann
Physical Review B | 2011
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
Jérôme Acker; Hans Kungl; Roland Schierholz; Susanne Wagner; Rüdiger-A. Eichel; Michael J. Hoffmann
Journal of The European Ceramic Society | 2013
Jérôme Acker; Hans Kungl; Michael J. Hoffmann
MRS Proceedings | 2009
Ebru Erünal; Rüdiger-A. Eichel; Jérôme Acker; Hans Kungl; Michael J. Hoffmann
The 5th International Congress on Ceramics ICC5 | 2014
Hans Kungl; Micheal J. Hoffmann; Roland Schierholz; Peter Jakes; Jérôme Acker; Rüdiger-A. Eichel