Thomas Kiefer1, Hawraa Kariem1, and Josef Füssl1
1)  Institute for Mechanics of Materials and Structures, Technical University of Vienna, Karlsplatz 13,
A-1040 Vienna
e-mail: {thomas.kiefer, josef.fuessl, hawraa.kariem}

Keywords: Thermal conductivity, Multiscale material model, Fired clay brick.

Abstract. The need to reduce heat losses through building facades motivates the optimization of fired clay bricks regarding their thermal conductivity. Such an optimization can be done at a macroscopic level, e.g. by optimizing geometries of vertically perforated bricks, or at a microscopic level, adding different pore-forming agents. Basically, through a reduction of the density can a lower thermal conductivity can be reached. However, significant differences in the thermal conductivity of fired clay bricks with the same density can be observed, suggesting that beside the density, a complex pore system and micro-structural properties, such as morphology, size and thermal conductivity of the constituent phases have a significant effect on the macroscopic thermal conductivity [2, 10, 11, 15]. To relate those micro-structural characteristics with the macroscopic thermal conductivity tensor, a multiscale material model is established. The identification of all required microstructural properties requires a large set of experiments, ranging from Scanning Electron Microscopy coupled with an energy dispersive X-ray spectroscopy to identify morphology and composition of the material, to Scanning Thermal Microscopy used for the determination of the thermal conductivity of its constituent phases. Consequently, results for the homogenized thermal conductivity from the multiscale material model are compared to measurements carried out on brick ceramic bodies, showing a very good agreement. Thereby, a validated, physically sound material model in terms of thermal conductivity can be presented, allowing for a more targeted optimization process of fired clay bricks.