Thermodynamics and kinetics of nucleation, crystallization and growth processes of corrosion products
Ferritic-martensitic materials used as heat exchanger or boiler tube material in combustion based power plants are subject to an aggressive corrosion caused by the reaction of the alloy with reactive gases containing CO2, SO2/SO3, H2O, and O2. The corrosion mechanism in general is a multi-parametric process and only a little is known about the early stage of corrosion, proceeding within the first 250 hours of reaction time, but influencing the further degradation mechanism of the material, sensitively.
The series of projects comprises with several experimental studies dealing with the fundamental investigations of initial corrosion mechanisms of high temperature alloys in multiple reactive atmospheres. In a present study combined oxidation with sulfidation is studied on ferritic-martensitic high temperature alloys. Nucleation, crystallization and growth mechanisms of corrosion products are studied by ex-situ and in-situ experiments. Conventional ageing experiments are performed on model alloys (Fe-based) and technical steels in single and multiple model gas atmospheres containing SO2/O2/H2O/CO2 using tubular furnaces for ageing times higher 24h. Corrosion products are characterized on various lengths scales by different microscopic techniques (optical and electron microscopy), X-ray diffraction (XRD) and µ-focused X-ray absorption near edge structure analysis (XANES) at the metal (Fe, Cr, Mn…)-K- absorption edge. To study phase kinetics quantified phase analysis is performed by Rietveld analysis of XRD data and by µ-XANES. The latter uses reference substances and reference spectra to quantify phase contents by a linear combination fitting procedure to the collected XANES spectra.
For in-situ experiments energy dispersive X-ray diffraction (EDXRD) is combined with ageing experiments in a special designed reactor. These experiments are performed in the very early state of corrosion with ageing times lower 24 h. The formation of the oxide layers and other scale phases can be studied by measuring the Bragg peaks of these phases in-situ. The phases present in the reaction layer are analyzed by their dhkl values using the energy dispersive Bragg equation. The diffraction signal gives a broad spectrum of information about the structural characteristics of the present phases. The energetic position of the Bragg-reflections yield information regarding phase content or lattice deformations. The extracted intensities of specific Bragg reflections are used to follow the reaction progression and to study the rate laws of the specific phases.
Juniorprofessor for Technical Mineralogy of Energy Materials (W1/S) at Freie Universität Berlin, Department of Earth Science and Federal institute for materials research and testing (BAM), Department of Materials engineering
Postdoc/Instrument Scientist, HZB, Berlin
Ph.D. student, HZB, Berlin
Freie Universität Berlin, Department of Earth Science, Ph.D. thesis: “Structural trends in off-stoichiometric Cu(In,Ga)(Se,S)2 compound semiconductors” , Dr. rer. Nat.
Mineralogy at Leipzig University, Faculty of chemistry and mineralogy, Diploma thesis:
“Influence of selenium ion beam energy on structure and microstructure of polycrystalline Cu(In,Ga)Se2 thin films”
07/2011 – 08/2011 Akademie der Wissenschaften, Kishinev, Moldova
research stay in the frame of an EU project (PIRSES-2010-269167)
09/2006 – 03/2007 Universidade Nova de Lisboa, Lissabon, Portugal
C. Stephan, D. Greiner, S. Schorr, C.A. Kaufmann, The influence of sodium on the point defect characteristics in off stoichiometric CuInSe2, J Phys Chem Solids, 98 (2016) 309-315.
T. Scherb, S.A.J. Kimber, C. Stephan, P.F. Henry, G. Schumacher, S. Escolastico, J.M. Serra, J. Seeger, J. Just, A.H. Hill, J. Banhart, Nanoscale order in the frustrated mixed conductor La5.6WO12-[delta], Journal of Applied Crystallography, 49 (2016) 997-1008.
C. S. Schnohr, H. Kammer, T. Steinbach, M. Gnauck, T. Rissom, C. A. Kaufmann, C. Stephan and S. Schorr, Composition-dependent nanostructure of Cu(In,Ga)Se2 powders and thin films, Thin Solid Films 582, 356-360 (2015).
S. Schorr, C. Stephan und C. A. Kaufmann, Chalcopyrite thin film Solar-Cell Devices, in: J. G. Kearley V. and K. Peterson (Eds.) Neutron Applications in Materials for Energy, Springer, 2014
S. Eckner, H. Kämmer, T. Steinbach, M. Gnauck, A. Johannes, C. Stephan, S. Schorr, and C. S. Schnohr, Atomic-scale structure, cation distribution and bandgap bowing in Cu(In,Ga)S2 and Cu(In,Ga)Se2, Appl Phys Lett 103 (8), 081905 (2013)
C. Stephan, T. Scherb, C.A. Kaufmann, S. Schorr, and H. W. Schock, A structural perception of cationic point defects in CuGaSe2, Appl Phys Lett 101 (10), 101907(1) (2012).
L. Gütay, D. Regesch, J. K. Larsen, Y. Aida, V. Depredurand, A. Redinger, S. Caneva, S. Schorr, C. Stephan, J. Vidal, S. Botti, and S. Siebentritt, Feedback mechanism for the stability of the band gap of CuInSe2, Phys Rev B 86 (4), 045216 (2012).
C. S. Schnohr, H. Kammer, C. Stephan, S. Schorr, T. Steinbach, and J. Rensberg, Atomic scale structure and band gap bowing in Cu(In, Ga)Se2, Phys Rev B 85 (24) (2012).
S. Schorr, C. Stephan, T. Thörndahl, R. Mainz, X-Ray and neutron diffraction on materials for thin film solar cells, in: D. Abou-Ras, T. Kirchhartz, U. Rau (Eds.) Advanced Characterization Techniques for Thin Film Solar cells, Wiley VCH, 2011.
S. Schorr, C. Stephan, R. Mainz, H. Rodriguez-Alvarez, M. Tovar, Neutrons and photons in materials research for thin film solar cells, Adv Eng Mater, 13, 737-741 (2011).
C. Stephan, S. Schorr, M. Tovar, H.W. Schock, Comprehensive insights into point defect and defect cluster formation in CuInSe2, Appl Phys Lett 98 (9), 091906 (1) (2011).
C. Stephan, S. Schorr, H.W. Schock, New structural investigations in the Cu2Se(S)-In2Se(S)3/ Cu2Se(S)-Ga2Se(S)3 phase diagram, Thin-Film Compound Semiconductor Voltaics-2009, 1165 (2010) 411-417, 430.
R. Hesse, J. R. K. Kamdoum, R. Caballero, C. A. Kaufmann, C. Stephan, S. Schorr, D. Abou-Ras, T. Unold, and H. W. Schock, Structural investigations of copper incorporation into In-Ga-Se precursor layers for Cu(In,Ga)Se2 thin films, Physica Status Solidi C - Current Topics in Solid State Physics, Vol 6, No 5 6 (5), 1249 (2009).