Browsing by Author "Zienert, Tilo"

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FOR 3010: Thermal Expansion and Phase Transformation up to 1200 °C of Metastable Aluminas Produced by Flame Spraying
(Technische Universität Bergakademie Freiberg, 2026-10-27) Zienert, Tilo; Aneziris, Christos G.
The transition aluminas δ1 and the cubic, non-spinel-type η-Al2O3 were detected in addition to α-Al2O3 in ﬂame-sprayed material. Their transitions from room temperature up to 1200 °C were investigated by high-temperature XRD measurements. Structural changes with time and temperature were observed for all transition aluminas (η-, δ1- and θ-Al2O3). The phases followed the expected transition sequence of η→δ1→θ→α and showed mainly linear, temperature-independent transition rates. Based on the determined thermal expansion of the phases, it is proposed that the metastable transitions are at least partly mechanically induced. In addition, a second-order phase transition from η→θ might be indicated around 1050–1075 °C by the determined trends of density.
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FOR 3010: Thermal Expansion of Electrofused MgO-Based Spinel Systems Containing Fe2O3, Al2O3-Fe2O3, Al2O3-Cr2O3-Fe2O3 and Al2O3-NiO-Fe2O3
(Technische Universität Bergakademie Freiberg, 2026-10-27) Zienert, Tilo; Borges, Otavio H.; Pandolfelli, Victor C.; Aneziris, Christos C.
Magnesia-spinel multicomponent materials have been used as refractories for a long time. In addition to a few binary systems, the influence of spinel phases on the thermal expansion (α) of MgO or the resulting compound has not been studied so far. As α is critical for refractories in application, this work investigates the thermal expansion of complex MgO-based spinel systems using X-ray diffraction (XRD) in combination with Rietveld refinement in the temperature range between 30 °C and 1200 °C. All studied periclase solid solutions, in contact with spinels of the systems Mg1.01(Al0.23Cr1.64Fe0.13)O4, Fe3O4–MgFe2O4, NiFe2O4–NiAl2O4, MgAl2O4–MgFe2O4, Fe3O4–FeAl2O4 and Fe3O4 · NiFe2O4 ·2 MgAl2O4 showed α trends below plain MgO, or even decreasing values above 1000 °C. Many spinels showed large negative thermal expansion coefficients. It was found that the structural change in spinels is constrained, leading to a common analytical expression to calculate the lattice parameter of spinels with temperature, which was used to study the nature of the investigated spinels in more detail. The work highlights that Cr-free MgO-spinel systems show similar or even better high-temperature behaviour than commonly used magnesia–chrome aggregates.
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FOR3010: High-Temperature Compressive Behavior of Refractory Alumina–Niobium Composite Material
(Technische Universität Bergakademie Freiberg, 2026-09-23) Günay, Gökhan; Zienert, Tilo; Endler, Dirk; Aneziris, Christos G.; Biermann, Horst; Weidner, Anja
The development of coarse-grained refractory composites combining refractory ceramics with refractory metals provides new approaches for high-temperature applications. In particular, coarse-grained Nb-Al2O3 composites are widely interested due to their promising functional properties such as of high thermal shock resistance, low shrinkage and good electrical conductivity. In order to identify and release the potential of these materials in advanced applications, their mechanical behavior should be evaluated. This study presents room- and high-temperature compression behavior up to 1500 °C of Nb-Al2O3 refractory composites manufactured via castable and extrusion technology. The influences of production method, particle size, open porosity as well as temperature and strain rate are discussed. For comparison, pure niobium and alumina specimens are used as reference materials. The results indicate that Nb-Al2O3 refractory composites show high ductility at high temperatures. This behavior is also verified with microstructural investigations by scanning electron microscope (SEM).
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FOR3010: Influence of particle size and fabrication method on mechanical properties of Nb-Al2O3 refractory composites under compressive loads at high temperatures
(Technische Universität Bergakademie Freiberg, 2026-09-23) Günay, Gökhan; Zienert, Tilo; Endler, Dirk; Aneziris, Christos G.; Biermann, Horst; Weidner, Anja
The present study explores the properties of both fine- and coarse-grained refractory composites made from niobium and alumina, designed for high performance applications at extreme temperatures, in abrasive environments, and under mechanical stresses. The effects of various production methods, including casting and extrusion, and different alumina particle sizes (fine or coarse) on the mechanical properties under compressive loads are investigated. Compression tests, as well as stress-relaxation and creep tests under compressive loads, are conducted at room temperature and high temperatures up to 1500 °C. The results are analysed concerning initial open porosities and particle sizes. The findings reveal that both particle size and manufacturing method significantly affect the mechanical properties of Nb-Al2O3 refractory composites. Additionally, the choice of production technique substantially influences the microstructure of the composites, leading to noticeable differences in mechanical performance. This research highlights the essential role of optimizing particle size and production methodologies to enhance the operational properties of Nb-Al2O3 refractory composites.
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GRK 2802: Investigations on the corrosion of 316L steel composite materials with MgO/TiO2 ceramic immersed in molten cryolite
(Technische Universität Bergakademie Freiberg, 2026-06-25) Yaroshevskyi, Serhii; Weigelt, Christian; Kerber, Florian; Brachhold, Nora; Zienert, Tilo; Adamczyk, Alexander; Vogt, Daniel; Charitos, Alexandros; Aneziris, Christos G.
Composites based on a steel with 40 vol% magnesia or titania were produced with the ceramics-derived extrusion and pressureless sintering. The materials were tested in a laboratory-scale fused-salt electrolysis cell with a synthetic cryolite in order to identify their potential use as electrode material in the Hall-Heroult process. The highly corrosive atmosphere and salt melt initiated certain corrosion effects in both material variants. The corrosion depth was determined with 1160 μm for magnesia and 463 μm for titania, respectively, after 8 h corrosion test. The initial corrosion includes the complete penetration of the specimens with cryolite and the dissolution of the ceramic component. A pre-oxidation of the specimens containing magnesia depressed the corrosion depth by 75% due to the formation of an aluminium oxide layer in the composite material during corrosion tests. The reduction in corrosion depth by pre-oxidation was less ronounced for the TiO2 composite materials (- 15%).
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GRK 2802: Steel ceramic composite anodes based on recycled MgO–C lining bricks for applications in cryolite/aluminum melts
(Technische Universität Bergakademie Freiberg, 2026-02-16) Yaroshevskyi, Serhii; Weigelt, Christian; Malczyk, Piotr; Roungos, Vasileios; Hubalkova, Jana; Zienert, Tilo; Kraft, Bastian; Wagner, Stefan; Aneziris, Christos
Novel manufacturing route for composite inert anodes containing 60:40 of 316 L stainless steel and MgO powder obtained from recycled MgO-C brick material has been developed and evaluated. After burnout of residual carbon from the recycled MgO-C powder, MgO and steel were granulated and pre-sintered in order to generate agglomerates of composite material acting as coarse grains within the composite material, and thus lowering the sintering-related shrinkage. The pre-sintered granules were mixed with raw steel and MgO powder in order to achieve a high particle packing and subsequently cold isostatically pressed in the form of electrodes. All manufactured anode samples were subjected to sintering at 1350 ◦C and pre-oxidation at different temperatures – 800 °C, 900 °C, and 1000 °C. Afterwards, mechanical and electrical properties of the manufactured electrodes were characterized. The results show that upcycling of the MgO-C material enables manufacturing of sophisticated electrode products, which can be applied in the aluminum industry.