The use of novel technologies is essential to achieve climate targets. The EuroStars project “Material Advancements for Solar Fuels Technology (MAfoS)” is developing critical equipment and materials for the first industrial solar-to-fuel pilot plant. Concentrated solar energy, water, CO2, and CH4 will be used to produce sustainable liquid fuel through the pilot plant. A subfield of this project includes the development of tubes made of high temperature oxide ceramic fibre composite (high temperature CMC – ceramic matrix composite), which are used in a temperature range of 1200 °C-1500 °C. In particular, the Fraunhofer Center HTL has the task of developing oxide fibres that exhibit very good thermomechanical properties above 1300 °C.
 
Mullite fibres (3Al2O3 * 2SiO2) are known for very good thermo-mechanical properties such as creep resistance, thermal shock resistance, chemical stability and high temperature resistance. The problem with oxide ceramics with a polycrystalline structure is that diffusion of cations and oxygen along the grain boundaries leads to grain growth. The consequences of grain growth are strength losses. Slowing down grain growth is possible with the help of ZrO2 doping in the fibres. Besides the property of slowing down grain growth, ZrO2 doping can also improve densification.
The mullite fibres with addition of ZrO2 in the MAfoS project are produced by a water-based sol-gel and a dry-spinning process. On the one hand, the type, quantity and size of ZrO2 doping particles needed for optimum inhibition of grain growth are investigated, and on the other hand, the temperatures and atmosphere for the thermal treatments, such as calcination and sintering, are determined. The heating rate, maximum temperature, dwell time and atmosphere are varied to achieve the phase transition to mullite and a dense microstructure. After thermal treatment, the mullite fibres are examined for crystal size & phase and porosity using XRD and SEM. In addition, the tensile strength and creep rate at ambient temperature of the ZrO2 doped mullite fibres are analysed.