Project objective

Research into a new generation of carbon-bonded products based primarily on gelatine with possible tannin or lactose additives with Al₂O₃-C and MgO-C recyclates is developing a novel approach to 'Green Refractories'. The structural modification in terms of high thermal shock resistance is achieved a) by nanoscale and semiconducting additives or b) by recycled carbon fibres or recycled carbon fibre structures. The addition of nanoscale and semiconducting additives is intended to increase the residual carbon content, while the carbon fibre structures are intended to act as crack deflection centres.

The thermal shock resistance and the chemical interactions with the steel are being researched in a special steel casting simulator. AFA (Automatic Feature Analysis) in the P-SEM is used to determine the significant inclusion populations in the steel. By simultaneously pouring the molten steel through a reference pouring nozzle and a nozzle based on the environmentally friendly refractory products, wetting and, in particular, anti-clogging phenomena can be investigated in terms of material and flow technology.

Project objective

The stress relaxation and creep behaviour as well as the thermocyclic and mechanical fatigue behaviour of the recyclate materials at high temperatures up to 1500 °C are being investigated as part of this doctoral project. Of particular interest here is the influence of impurities compared to the use of new raw materials as well as the evaluation of other functional additives, such as nanoscale additives or recycled fibre structures. The use of environmentally friendly binders in the refractory materials and their effect on the thermomechanical properties also play an important role. The high-temperature properties are correlated with damage mechanisms that occur, such as fatigue damage or cracking.

Project objective

Refractories recyclates exhibit strong changes in thermophysical material data, in particular heat conductivity, compared to the original materials (impurities, changes in structure and phase composition, etc.) due to their initial use at high temperatures and melt contact. The aim is to predict the temperature-dependent thermal conductivity and the associated variables of thermal diffusivity, heat capacity and thermal shock behaviour of the recyclates and composite materials as a function of their composition and history. Methods are the utilisation and further development of different thermal conductivity measurement methods for the fastest possible evaluation in a wide temperature range, coupled with the development of mathematical models for the quantitative evaluation of the most important influencing factors.

Project objective

The focus of this doctoral project is to investigate the interaction of electrical steels (transformer steels), with high-silicate purity ladle slags free of running slag, with MgO-C products based on recyclates and environmentally friendly binders. The formation of ladle glaze can be studied by immersing a MgO-C product in a tracer-labelled slag during an immersion test. Investigations of the thermophysical properties of the liquid, killed/unkilled electrical steels and liquid slag as a function of temperature and SiO₂ content, combined with thermodynamic calculations using FactSage software, make it possible to determine their influence on the interaction with MgO-C and ladle glaze formation. The influence of ladle glaze on the oxidic inclusion population, on the pick-up effect (undesirable increase of alloying elements) and reoxidation of the alloying elements is investigated by ladle glaze dissolution in the electric steel melt accompanied by chemical analysis and optical microscopy in combination with AFA (Automatic Feature Analysis) in the P-SEM. Research into the effect of the oxidic, non-metallic inclusions that form from ladle glaze on clogging in the spout nozzle based on recycled Al₂O₃-C is another focus.

Project objective

The focus of this doctoral project is to investigate the interaction of a low-sulphur manganese-boron steel MBW1500 and a highly basic desulphurisation slag with MgO-C products based on recyclates and environmentally friendly binders. In order to determine the influence of thermophysical properties on the interaction with new refractory materials, the viscosity, surface tension and density of the liquid aluminium-killed MBW1500 steel in undesulphurised and desulphurised condition and the slags with high sulphur capacity are investigated as a function of [S], (SiO₂), (MgO), (S) and temperature. By finger testing a MgO-C product in the molten steel and slag, the refractory samples are obtained for further analysis of the interactions using optical microscopy and SEM. The (S)/[S] distribution between the MBW1500 steel and the slag is investigated in a crucible of MgO-C products based on recyclates in the MFG-40. The inclusion population of the steel samples after examination in MFG-40 is interpreted by chemical analysis or analysed by optical examination methods such as light microscopy combined with AFA (Automatic Feature Analysis) in P-SEM. The spinel formation between recycled FF material and liquid steel is specifically studied in a confocal laser scanning microscope.

Project objective

The aim of the project is to establish a simulation tool, which supports the research of novel MgO-C, Al₂O₃-C refractories based on recycled refractories and environmentally friendly binders on the modelling side. It is primarily intended to enable the solid-state mechanical evaluation of the thermal shock resistance of the materials considered holistically in the Research Training Group. In addition to the evaluation of the refractory materials already produced, it should also be possible to make predictions of the thermomechanical behaviour in the sense of a virtual laboratory. With the help of numerical experiments (parameter studies, sensitivity analyses) based on experimentally calibrated and validated models, key influencing variables for the thermal shock behaviour, such as the proportion of recycled material, microstructure, properties of recycled grain, binders and their boundary layer, microcrack distributions, etc., will then be identified,

Methodologically, methods of fracture and damage mechanics as well as cohesive zone and phase field modelling are pursued, whereby there is a particular need for research into their combination and extension to transient, thermomechanical phenomena in microheterogeneous materials. An elementary component of the project is therefore the broad-based doctoral training in state-of-the-art methods of continuum mechanical modelling (theoretical modelling) and simulation (numerical implementation) of the behaviour of modern high-tech materials, with particular reference to refractory materials.

Project author: M.Sc. Serhii Yaroshevskyi

Project objective

A new generation of inert electrodes based on coarse and fine-grained MgO recyclates and Cr-Ni steel is being researched with the aid of image-forming extrusion at room temperature, sintering and targeted subsequent oxidation. To achieve sufficient electrical conductivity, the contribution of Ni/NiO/TiO₂ additives as well as the interface design with SEM/FIB/EBSD is being investigated. Another promising variant for adjusting the electrical conductivity is the use of MgO-C recyclates and the addition of pre-synthesised or in-situ generation of MAX phases, e.g. based on Ti₃AlC₂, Ti₂AlC or Ti₃SiC₂ during sintering firing. In addition to celluloses, the contributions of protein- and sugar-based excipients are also being investigated as binders, particularly in the MAX phase-based formulations. Coatings in the Al₂O₃/MgO/TiO₂ system are applied using flame spraying technology to create a targeted oxidation passivation layer. Cyclic mercury pressure porosimetry, computed tomography images and HT-XRD will help to clarify the microstructure evolution as a function of the extrusion and sintering parameters.

Project objective

Field-assisted sintering / spark plasma sintering (FAST/SPS) is a technology that is very suitable for the rapid production of compact composite materials.

In order for it to be used in the future for the production of composites with desired microstructure and properties, relationships between microstructural features, electrical conductivity at elevated temperatures and microstructural change through diffusion processes, phase reactions, phase formation, grain growth and recovery of microstructural defects must be understood. This results in the following objectives of the project:

• Description of temperature-induced microstructural changes in relevant composite materials (in refractory materials to be produced from recyclates),
• Description of the electrical resistance of a multiphase material taking into account the formation of electrically conductive paths, the scattering of electrons at phase boundaries, grain boundaries and at microstructural defects as well as taking into account the temperature dependence of the electrical resistance and the temperature-induced microstructural changes.

Project objective

Raman spectroscopy (RS) is used to investigate phases, functionalised surfaces and possible impurities of refractory recyclates for the production of novel refractory materials using environmentally friendly binders. Using in-situ RS as a function of temperature, phase transitions can be detected and intermediate or by-products can be identified. The characterisation of carbon-bonded materials using Raman spectroscopy and, in particular, an understanding of the reactions during the coking of the environmentally friendly gelatine-based binders as a function of temperature is a key area of work.

Inert, metalloceramic anode materials based on refractory recyclates are characterised by means of temperature-dependent electrical conductivity measurements. As far as possible, information from RS (e.g. spinel formation of nickel ferrites) and the microstructure design of composite materials is used by FAST/SPS to model the measurement data obtained. The aim is to understand temperature-induced structural and compositional changes in the metalloceramic anode materials.

Project objective

Inert anodes based on recycled material have a significant ecological impact, as the avoidance of CO₂ and CF₄ emissions can be combined with recycling approaches. The focus is on characterising the behaviour of inert anodes based on the sintered composite material MgO with Cr-Ni steel/316 L and other additives, including Ni/NiO/TiO₂, in a high-temperature laboratory electrolysis cell. The corrosion interactions that take place at the anode-corrosive gas-phase and anode-electrolyte (here cryolite) interfaces are investigated.

The relationship between current density, temperature and electrochemical corrosion rate is recorded in long-term experiments. The electrical conductivity of the interface (anode-electrolyte) is being researched. Furthermore, the wettability of the cryolite melt with the anode is being investigated on an existing sessile drop system (good wettability is crucial for electrical conductivity). Another focus is research into other electrolyte systems, such as KF-AlF₃, with the aim of increasing the corrosion resistance of inert electrodes through lower operating temperatures.

Project objective

Carbon has a positive effect on the electrical conductivity of anodes and is contained in recycled lining materials. To reduce the CO₂ footprint, the formation of CO, CO₂, CF₄ on carbon-containing anodes must be minimised/avoided. The focus is on the evaluation of low-carbon anodes based on metalloceramic composite materials with MAX phases and C residues from MgO-C recyclates. The measurement of the oxidation rate of the carbon is determined by means of continuous CO/CO₂ measurement. The oxidation mechanisms and corrosion processes are investigated by XRF, XRD and SEM/EDX. The electrical conductivity at the anode-electrolyte interface is recorded. The low-C anodes are compared with conventional graphite anodes in the high-temperature laboratory electrolysis cell. Another focus is researching the low-carbon composite materials with flame-sprayed oxidation passivation layer in the electrolytic cell with regard to sufficient conductivity and thermomechanical and chemical stability.

Project objective

The focus is on researching the thermomechanical properties of metalloceramic composites with MAX phases and C residues with and without a flame-sprayed oxidation passivation layer. In addition to the stress relaxation and creep behaviour, the damage behaviour of these materials under compressive and bending stress, including strain rate cycling tests, is also being investigated. Of particular interest here is the influence of reaction products of the two grain sizes (e.g. MAX phases) on the thermomechanical properties. Any damage mechanisms that occur are investigated post mortem using scanning electron microscopy.