@inproceedings{AckerDuckeRietigetal., author = {Acker, J{\"o}rg and Ducke, Jana and Rietig, Anja and M{\"u}ller, Tim and Eisert, Stefan and Reichenbach, Birk and L{\"o}ser, Wolfgang}, title = {Segregation, grain boundary milling, and chemical leaching for the refinement of metallurgical-grade silicon for photovoltaic application}, series = {Silicon for the Chemical and Solar Industry XII, Trondheim, 2014}, booktitle = {Silicon for the Chemical and Solar Industry XII, Trondheim, 2014}, editor = {Oye, Harald A. and Brekken, Harald and Rong, Harry and Tangstad, Merete and Tveit, Halvard}, publisher = {Department of Materials Science and Engineering, Norwegian University of Science and Technology}, address = {Trondheim}, isbn = {978-82-997357-8-0}, pages = {177 -- 188}, abstract = {The present work describes a completely new approach to the solidification refinement of metallurgical-grade silicon. The new process comprises the following steps: (i) The first step involves adding auxiliary metals to the molten silicon in order to segregate the metallic and non-metallic impurities in the secondary phase after cooling. (ii) The melt is rapidly cooled in the cellular solidification regime. This generates a Si microstructure with a defined cell size in which all cell boundaries are surrounded by the secondary phase. Furthermore, the secondary phase should form an interconnected three-dimensional network. (iii) The solids are crushed by shockwaves using electrohydraulic fragmentation techniques. The shockwaves lead to preferential crushing at the interface between the silicon and the secondary phase. (iv) The secondary phases are fast and effectively removed by microwave-assisted high-pressure leaching that was newly developed for this process. The potential of the new refinement procedure is demonstrated with auxiliary metals Ca, Al, and Ti. This new procedure yields a significant decrease in phosphorous and metal impurities.}, language = {en} } @inproceedings{RietigAcker, author = {Rietig, Anja and Acker, J{\"o}rg}, title = {A new and fast method for determination of boron, phosphorus and other trace elements in metallurgical grade silicon}, series = {Silicon for the Chemical and Solar Industry XIII, Kristiansand, 2016}, booktitle = {Silicon for the Chemical and Solar Industry XIII, Kristiansand, 2016}, editor = {Nygaard, Lars and Pachaly, Bernd and Page, Ingrid Gamst and Rong, Harry and Tangstad, Merete and Tveit, Halvard}, publisher = {Department of Materials Science and Engineering, Norwegian University of Science and Technology}, address = {Trondheim}, pages = {95 -- 106}, abstract = {A new method for accurate and precise determination of non-metallic and metallic impurities in silicon was developed and statistically validated. The first step is the fast dissolution of silicon in a microwave-assisted high pressure system to minimize a loss of phosphorus. The essential innovation is the use of the concentrated digestion solution for ICP-OES measurements. This approach avoids the common removal of the silicon and acid matrix by volatilization, which can cause considerable losses of boron. Finally, for the ICP-OES measurements in such high-silicon matrices the optimum measuring conditions were determined and a careful selection of emission lines with respect to selectivity, spectral and non-spectral inferences and matrix effects was performed. The method of matrix matched calibration (MMC) is used for quantification of the impurities' contents. For Al, Mg, Ca, Ti, Cr, Mn, Fe, Ni, Cu, Zr and P the validation was performed against certified reference materials (IPT134, IPT135, NIST57b). To validate the determination of boron 9 silicon samples of different boron contents from three interlaboratory comparisons were used. The new procedure allows the determination of impurities of 4N-silicon (12 elements) with high precision and accuracy.}, language = {en} } @misc{SieberDuckeRietigetal., author = {Sieber, Tim and Ducke, Jana and Rietig, Anja and Langner, Thomas and Acker, J{\"o}rg}, title = {Recovery of Li(Ni0.33Mn0.33Co0.33)O2 from Lithium-Ion Battery Cathodes: Aspects of Degradation}, series = {Nanomaterials}, volume = {9}, journal = {Nanomaterials}, number = {2}, issn = {2079-4991}, doi = {10.3390/nano9020246}, pages = {246 -- 259}, abstract = {Nickel-manganese-cobalt oxides, with LiNi0.33Mn0.33Co0.33O2 (NMC) as the most prominent compound, are state-of-the-art cathode materials for lithium-ion batteries in electric vehicles. The growing market for electro mobility has led to a growing global demand for Li, Co, Ni, and Mn, making spent lithium-ion batteries a valuable secondary resource. Going forward, energy- and resource-inefficient pyrometallurgical and hydrometallurgical recycling strategies must be avoided. We presented an approach to recover NMC particles from spent lithium-ion battery cathodes while preserving their chemical and morphological properties, with a minimal use of chemicals. The key task was the separation of the cathode coating layer consisting of NMC, an organic binder, and carbon black, from the Al substrate foil. This can be performed in water under strong agitation to support the slow detachment process. However, the contact of the NMC cathode with water leads to a release of Li+ ions and a fast increase in the pH. Unwanted side reactions may occur as the Al substrate foil starts to dissolve and Al(OH)3 precipitates on the NMC. These side reactions are avoided using pH-adjusted solutions with sufficiently high buffer capacities to separate the coating layer from the Al substrate, without precipitations and without degradation of the NMC particles.}, language = {en} } @misc{RietigLangnerAcker, author = {Rietig, Anja and Langner, Thomas and Acker, J{\"o}rg}, title = {A revised model of silicon oxidation during the dissolution of silicon in HF/HNO₃ mixtures}, series = {Physical chemistry, chemical physics}, volume = {21}, journal = {Physical chemistry, chemical physics}, issn = {1463-9076}, doi = {10.1039/c9cp04429a}, pages = {22002 -- 22013}, abstract = {The stoichiometry of wet chemical etching of silicon in concentrated HF/HNO₃ mixtures was investigated. The formation of nitrogen species enriched in the etching mixture and their reactivity during the etching process was studied. The main focus of the investigations was the comprehensive quantification of the gaseous reaction products using mass spectrometry. Whereas previously it could only be speculated that nitrogen was a product, its formation was detected for the first time. The formation of hydrogen, N₂, N₂O and NH₄⁺ showed a dependence on the etching bath volume used, which indicates the formation of nitrogen compounds by side reactions. Simultaneously, the ratio of the nitrogen oxides, NO and NO₂, formed decreases with increasing etching bath volume, while nitric acid consumption increases, so that the formation of NO₂ could also be identified as a side reaction. Based on the stoichiometries obtained, a new reaction scheme for the reduction of nitric acid during etching in HF/HNO₃ mixtures and an electron balance for the oxidation of silicon is presented.}, language = {en} } @misc{SieberRietigDuckeetal., author = {Sieber, Tim and Rietig, Anja and Ducke, Jana and Acker, J{\"o}rg}, title = {Direkte Feststoffanalyse von Hauptkomponenten in Kathodenmaterialien von Lithiumbatterien mittels HRCS-GF-AAS}, series = {Colloquium Analytische Atomspektroskopie - CANAS 2019, Book of Abstracts}, volume = {2019}, journal = {Colloquium Analytische Atomspektroskopie - CANAS 2019, Book of Abstracts}, editor = {Vogt, Carla}, edition = {1. Auflage}, publisher = {TU Bergakademie Freiberg}, address = {Freiberg}, pages = {S1/4}, abstract = {Zur Bestimmung der metallischen Hauptkomponenten in Lithium-Batterie-Kathodenmaterialien ist der nasschemische Aufschluss mit anschließender ICP-OES-Analyse oft das Mittel der Wahl. Da dieses Verfahren jedoch recht zeitaufwendig ist und den Einsatz starker S{\"a}uren erfordert, wurde eine Methode zur direkten Feststoffanalyse mittels HRCS-GF-AAS (high resolution continuum source graphit furnace atom absorption spectrometry) nach dem STPF-Konzept (stabilized temperature platform furnace) entwickelt. Die hohen Analytkonzentrationen erfordern dabei die Messung auf den vergleichsweise wenig intensiven Linien Li = 323,2657 nm, Ni = 294,3912 nm, Mn = 321,6945 nm und Co= 243,5823 nm. Zus{\"a}tzlich wird das Probenmaterial einer Feststoffverd{\"u}nnung mit matrixverwandten Komponenten unterzogen. Die Verd{\"u}nnung senkt zum einen die Konzentration und die Gefahr der Verschleppung der Analyten und beg{\"u}nstigt zum anderen die Freigabe des Analyten aus der Probenmatrix. Durch Aufnahme von Extinktions-Zeit-Verl{\"a}ufen im Temperaturbereich von 200 - 2600 °C konnten die Freisetzungstemperaturen f{\"u}r jeden Analyten bestimmt werden. Nach anschließenden Optimierungen der Pyrolyse- und Atomisierungstemperaturen wurde mithilfe der Einzeloxide f{\"u}r jeden Analyten die Linearit{\"a}t des Messsignals gepr{\"u}ft und der Arbeitsbereich festgelegt. Durch Vermessung von variierenden Oxidmischungen und Mischoxiden, sowie Zusatz m{\"o}glicher weiterer Interferenten, wie dem Bindermaterial PVDF wurden Spezifit{\"a}t, Selektivit{\"a}t und Robustheit der Methode {\"u}berpr{\"u}ft. Abschließend erfolgte anhand realer Proben (Recyclinggut aus Lithium-Batterie-Kathoden) ein Vergleich zwischen den Ergebnissen der direkten Feststoffanalyse mittels HRCS-GF-AAS und dem bereits etablierten Verfahren der ICP-OES Analyse nach nasschemischem Aufschluss. Nach umfangreicher Methodenentwicklung kann ein Verfahren der direkten Feststoffanalyse von Recylinggut aus Kathodenmaterialien von Lithium-Ionen-Batterien mittels HRCS-GF-AAS bereitgestellt werden, das eine schnelle und pr{\"a}zise Analyse der Hauptkomponenten Li, Ni, Mn und Co erlaubt.}, language = {de} } @misc{LangnerRietigAcker, author = {Langner, Thomas and Rietig, Anja and Acker, J{\"o}rg}, title = {Raman spectroscopic determination of the degree of dissociation of nitric acid in binary and ternary mixtures with HF and H2SiF6}, series = {Journal of Raman Spectroscopy}, volume = {51}, journal = {Journal of Raman Spectroscopy}, number = {2}, doi = {10.1002/jrs.5769}, pages = {366 -- 372}, abstract = {The oxidizing effect of nitric acid in aqueous solutions depends on the concentration of undissociated nitric acid. This makes the concentration of undissociated nitric acid an essential parameter to monitor and control the quality of silicon etching in the industrial manufacturing of solar cells. In the present study, a method known already is extended in such a way that the degree of dissociation of nitric acid can be determined by Raman spectroscopy in HF/HNO3/H2SiF6 acid mixtures over a broad concentration range for the first time and without using an internal or external standard to compensate the typical time-dependent drift of a Raman spectrometer. The method developed requires the calculation of a peak area ratio from the areas of the unimpeded Raman signals assigned to nitrate (νN - O) at 1,048 cm-1 and to undissociated HNO3 (νN - OH) at 957 cm-1. The correlation between the peak ratio and the degree of dissociation of nitric acid revealed can be described by a simple empirical equation. Using this equation, the degree of dissociation of nitric acid can be determined over a broad concentration range in binary and ternary mixtures of HNO3 with HF and H2SiF6. The impact of the acids HF and H2SiF6 and the total water content in the degree of dissociation of nitric acid is discussed.}, language = {en} } @misc{RietigAcker, author = {Rietig, Anja and Acker, J{\"o}rg}, title = {Kinetic studies on acidic wet chemical etching of silicon in binary and ternary mixtures of HF, HNO3 and H2SiF6}, series = {Physical Chemistry Chemical Physics}, volume = {25}, journal = {Physical Chemistry Chemical Physics}, number = {38}, issn = {1463-9084}, doi = {10.1039/d3cp03188h}, pages = {26245 -- 26257}, language = {en} } @inproceedings{RietigLangnerAcker, author = {Rietig, Anja and Langner, Thomas and Acker, J{\"o}rg}, title = {Dissolution of silicon in HF/HNO3 mixtures: A revised model}, series = {9th European Silicon Days, 9-12 September 2018, Saarbr{\"u}cken, Germany, Book of abstracts}, booktitle = {9th European Silicon Days, 9-12 September 2018, Saarbr{\"u}cken, Germany, Book of abstracts}, editor = {Scheschkewitz, David and Kickelbick, Guido}, publisher = {Universit{\"a}t des Saarlandes}, address = {Saarbr{\"u}cken}, pages = {S. 199}, abstract = {The dissolution of Si in HF/HNO3 consists of a set of complex reactions and thus a large number of reaction products. The most comprehensive picture of this reaction, the role of the involved reaction products and the reactivity of the HF/HNO3 mixtures depending on their composition is drawn by Steinert et al..[1]-[3] Based on the first systematic investigations on hydrogen formation by Hoffmann et al.[4], Acker et al. succeeded a first mass and electron balance for the reaction of silicon in HF/HNO3.[5] However, there is still a lack in interpretation of the mass end electron balances arising from several nitrous oxides. So far, the identified nitrogen oxides NO, NO2 and N2O were considered in sum[5] and neither separated nor individually studied or quantified. The aim of this work is to complete the mass and electron balance by the contribution of the individual nitrous oxides and to identify their individual formation pathways. Kinetic measurements of the NO and NO2 formation during the dissolution of Si, NO2 turns out as a result from the oxidation of the primary product NO by the HNO3 in the etching mixtures. Subsequently, NO and NO2 react to N2O3 dissolved in the acid mixture. The kinetics of both reactions were individually studied by bubbling NO in HF/HNO3 mixtures of different composition. The already identified intermediary species N4O62+ turns out to be formed by disproportionation of dissolved NO2 via N2O4 without dissolution of silicon. A detailed kinetic studied showed, that only dissolved N2O3 and not the intermediate N4O62+ contribute to the dissolution rate of silicon in HF/HNO3 acid mixtures. Finally, kinetic measurements revealed that the formed H2 reduces gaseous NO yielding to the final gaseous reaction products N2, N2O as well as to ammonium ions which all are formed with identical reaction rate. This reaction is assumed to proceed via NH2OH as intermediate. As result of the identification and quantification of intermediary and final reaction products a new reaction scheme needs to be established leading to a new approach to the mass and electron balance for the oxidation of silicon during the dissolution in HF/HNO3 mixtures.}, language = {en} } @incollection{RietigAcker, author = {Rietig, Anja and Acker, J{\"o}rg}, title = {Ressourcensicherung durch Recycling von Sekund{\"a}rrohstoffen}, series = {Systemwissen f{\"u}r die vernetzte Energie- und Mobilit{\"a}tswende}, booktitle = {Systemwissen f{\"u}r die vernetzte Energie- und Mobilit{\"a}tswende}, edition = {2. Auflage}, publisher = {Vereinigung f{\"u}r Betriebliche Bildungsforschung e.V.}, address = {Berlin}, isbn = {978-3-9816861-8-0}, pages = {141 -- 152}, language = {de} } @misc{RietigGrafeAcker, author = {Rietig, Anja and Grafe, Hans-Joachim and Acker, J{\"o}rg}, title = {Understanding the Reasons for Erroneous Determinations of Boron in Silicon following Wet Chemical Digestion in HF/HNO3}, series = {Proceedings of the Silicon for the Chemical \& Solar Industry XVI}, journal = {Proceedings of the Silicon for the Chemical \& Solar Industry XVI}, isbn = {978-82-692919-0-2}, doi = {10.2139/ssrn.4123058}, pages = {14}, language = {de} }