Packing-dependent viscous sintering of glass powder from wet deposition

• Applicants: Professor Dr.-Ing. Joachim Deubener; Dr. Susanne Selle
• Subject Area: Glass, Ceramics and Derived Composites ; Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials
• Term: from 2018 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 405699578

• Final Report Year: 2023

Final Report Abstract

The spraying and firing of a dried coating of glass powders onto metallic substrates is an established technology and results in glassy thick-film products (enamels), which are indispensable for sheet metal, pipes, reactors and large containers in the chemical industry and in supply and energy technology. A bottleneck in this respect is the lack of a fundamental understanding of the sintering kinetic parameters and the resulting empirically guided material and process development, which is prone to defects (waviness, bubble and crack formation). The research hypothesis is therefore based on the use of modern X-ray tomography (XCT) and high-resolution X-ray microscopy (XRM) to study the processes of thermal densification of practical powders from wet deposition, which differ from "ideal" glass sintering powders in terms of possible leaching in the aqueous slurry, low green density (no mechanical pre-compaction), thermal history of the rapidly quenched frit particles and multimodal particle distribution. The results show that the 3D imaging of XCT and XRM is suitable to shed light on the local processes during sintering, as the analysis of the local relative density revealed that the expected decrease in spatial heterogeneity from the initial (neck formation) to the final (closed porosity) sintering stage is interrupted by an intermediate stage with highest spatial heterogeneity and a characteristic bimodal density distribution. The bimodality of the intermediate stage is due to spatially separated denser arrangements within the representative volume element with closed pores and less dense arrangements with a network of interconnected or open pores that is biased normal to the layer due to friction caused by adhesion with the substrate. During the leaching of glass powders, the expected interdiffusion of alkali and alkaline earth cations of the glass with water species of the solution occurs. However, the modelling of diffusion during sintering showed that the alkali depletion (Na+) on the particle surface is compensated from the particle interior before sintering and dissolved water evaporates, but the depletion of alkaline earth cations (Ca2+) on the surface is maintained during sintering, so that the observed sintering delay can be described as the effect of a composition gradient, which in turn leads to a viscosity gradient, whereby the outer part of the glass particles is less prone to viscous flow due to the higher silica content and the lower amount of network modifiers. In contrast, no influence of the thermal history of the glass particles could be determined, which suggests that they are already structurally relaxed at the onset of sintering.

Publications

• Local relative density fluctuation in the early stage of a sintered glass sphere compact determined by X-ray computed tomography. Journal of Non-Crystalline Solids, 546, 120316.
  Fernandes, Roger Gomes; Selle, Susanne; Wagner, Thomas; Menzel, Matthias; Bornhöft, Hansjörg; Höche, Thomas & Deubener, Joachim
  
• Packing dependent viscous flow sintering of glass powder from wet deposition – part 1, XRM Workshop Fraunhofer IMWS, Halle 2020
  Gomes Fernandes R.
  
• Packing dependent viscous flow sintering of glass powder from wet deposition – Part 2, XRM Workshop Fraunhofer IMWS, Halle 2020
  Selle S.
  
• Microstructure investigation of sintered glass compacts by 3D Imaging – part 1, Annual Meeting of the German Enameller Association (DEV), Bad Sassendorf 2021
  Selle S.; Gomes Fernandes R.; Menzel M.; Wagner T.; Patzig C. & Deubener J.
  
• Effect of acid leaching on the viscous sintering of glass powder compacts, 26th International Congress on Glass (ICG), Berlin 2022
  Gomes Fernandes R.
  
• Microstructure investigation of glass sintered compacts by 3D imaging, 26th International Congress on Glass (ICG), Berlin 2022
  Gomes Fernandes R.
  
• Microstructure Investigation of Sintered Glass Compacts by 3D Imaging – part 2, Annual Meeting of the German Enameller Association (DEV), Bad Pyrmont 2022
  Selle S.; Gomes Fernandes R.; Menzel M.; Wagner T.; Patzig C. & Deubener J.
  
• Mikrostrukturelle Untersuchungen an gesinterten Glaspresslingen mittels 3D Röntgentomographie, Email 71, 2 (2023) 27–33, ISSN 09 38-9865
  Selle S.; Gomes Fernandes R.; Menzel M.; Wagner T.; Patzig C. & Deubener J.
  
• Sintern von Email-Biscuits, Annual Meeting of the German Enameller Association (DEV), Münster 2023
  Selle S.
  
• Viscous Sintering of Acid Leached Glass Powders. Glass Europe, 1, 37-53.
  Gomes, Fernandes Roger; Al-Mukadam, Raschid; Bornhöft, Hansjörg; Reinsch, Stefan; Müller, Ralf; Selle, Susanne & Deubener, Joachim
  
• Local fluctuation of the relative density of glass powders over the entire viscous sintering process. Journal of Non-Crystalline Solids, 625, 122746.
  Fernandes, Roger Gomes; Selle, Susanne; Wagner, Thomas; Menzel, Matthias & Deubener, Joachim