Real-time Imaging of High-Temperature Sintering and Associated Oxygen Isotope Exchange Reactions by Confocal Hyperspectral Raman Spectroscopy

• Applicant: Professor Dr. Thorsten Geisler-Wierwille
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: from 2017 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 390544388

Since humankind's earliest history pottery has played an important role in daily and cultural life in the form of craft and art objects. Pottery is produced by firing clay with various mineral additives at temperatures of up to 1400°C, whereby the minerals in a ceramic green body transform via different inter- and intra-grain transformation reactions to a new phase assemblage. Understanding the reaction sequence and the interplay between the microstructure, the phase assemblage, and the bulk material properties is essential to optimize production technologies and design new ceramic materials with specific physical properties. Such knowledge is also needed in archaeology to, e.g., reconstruct ancient pottery production technologies. Modern state-of-the-art confocal Raman spectrometer systems principally permit two-dimensional imaging of mineral reactions and textures in situ, i.e. while they are proceeding, and thus the study of the mechanisms underlying the various thermal sintering reactions. In addition, thermodynamic and kinetic information about the growth and breakdown of distinct phases, including metastable phases, can be gained with a high temperature and time resolution. To the best of the applicants' knowledge, the imaging capability of confocal hyperspectral Raman spectroscopy (CHRS) has not yet been used to study high temperature solid-solid or solid-melt reactions. A prove of concept for such investigations is given in this application by the results of a first experiment that was designed to image sintering reactions in a kaolinite-feldspar-quartz-calcite-based green body at temperatures between ~750 and 1150°C. CHRS imaging allowed visualizing the evolution of the textural relationship between gehlenite, wollastonite, anorthite, and pseudowollastonite with increasing temperature. The great potential of CHRS for in situ investigation of sintering reactions is further expanded by the possibility to image the distribution of 18-O within and among different phases at the micrometer scale when using 18-O-labeled precursor reactants. The principle behind analyzing the 18-O content in condensed matter by vibrational spectroscopy is that the energies or frequencies associated with vibrational motions are dependent on the masses of the vibrating atoms. In the present project, it is intended to systematically study, for the first time, in situ high-temperature sintering reactions in two-phase systems as well as in synthetically blended kaolin-based green bodies by CHRS imaging using either 18-O-labeled calcite or quartz as isotope tracer reactant.

• DFG Programme: Research Grants