TY - THES

T1 - Thermal treatment of continuous alumina fibers

AU - Gruber, Fabian Ingo

N1 - embargoed until 02-05-2029

PY - 2024

Y1 - 2024

N2 - Today´s challenges faced in different sectors of industry require materials working at high temperatures exposed to chemically aggressive environments. All-Oxide Ceramic Matrix Composites (OCMC) have proven to be promising materials due to their stability under high temperatures and oxidizing conditions. The properties of the OCMCs are largely depending on the properties of the used alumina fibers and therefore the processing of continuous alumina fibers is of special interest. This thesis focuses on the thermal treatments of green continuous alumina fibers and the characterization of green, pyrolyzed and sintered fibers, aiming to understand the influence of temperature, heating rate, holding time, thermal radiation and applied tension on the chemical, physical and mechanical properties of the obtained fibers. Green continuous alumina fibers based on aluminum chlorohydrate were analyzed using DTA, TGA and SEM. The results of the DTA and TGA showed that between 100°C - 600°C volatile components leave the fiber and a mass loss of 58% was observed. The transformation to ¿-alumina happened at 875°C and the transformation to ¿-alumina at 1130°C. No peaks for ¿-alumina or ¿-alumina were observed. Microscopical investigations revealed different abnormalities of the green fibers. The mean filament diameter was 21 µm. Fibers which were pyrolyzed at 825°C and 875°C, at slow and fast heating rates with 5 min and 15 min holding time had mean filament diameters between 15 µm - 16 µm. Only at 15 min holding time ¿-alumina was the only crystalline phase present. The microstructure of the fibers consisted of grains smaller than 250 nm. The densities were with 2,123 g/cm³ - 2,377 g/cm³ very low and the tensile strengths were between 380 MPa - 692 MPa. Longer holding times and applied tension lead to higher densities with same grain sizes but not to higher tensile strengths. Radiation did not affect the properties of pyrolyzed fibers. The fibers which were sintered between 1100°C - 1700°C for 1 min ¿ 9 min had mean filament diameters of 13 µm -18 µm. Fibers which were sintered at 1200°C, 9 min; 1300°C, 6 min; 1400°C, 3 min; 1400°C, 5 min; 1500°C, 3 min; 1700°C, 2 min; consisted of an ¿-alumina microstructure with grain sizes smaller than 250 nm. Densities of 2,234 g/cm³ -3,007 g/cm³ were measured and tensile strengths of 179 MPa -326 MPa. The mechanical properties of the sintered fibers fell below the properties of pyrolyzed fibers. It was found out that even at long holding times, no proper densification was observed. No tension could be applied during the sintering. Radiation did lead to a different arrangement of the grains which was observed on the fracture surfaces of the filaments. The findings of this thesis provide detailed information of the effect of different parameters during the thermal treatment on the chemical, physical and mechanical properties of alumina fibers and may help fabricating continuous alumina fibers with desired properties for the final application.

AB - Today´s challenges faced in different sectors of industry require materials working at high temperatures exposed to chemically aggressive environments. All-Oxide Ceramic Matrix Composites (OCMC) have proven to be promising materials due to their stability under high temperatures and oxidizing conditions. The properties of the OCMCs are largely depending on the properties of the used alumina fibers and therefore the processing of continuous alumina fibers is of special interest. This thesis focuses on the thermal treatments of green continuous alumina fibers and the characterization of green, pyrolyzed and sintered fibers, aiming to understand the influence of temperature, heating rate, holding time, thermal radiation and applied tension on the chemical, physical and mechanical properties of the obtained fibers. Green continuous alumina fibers based on aluminum chlorohydrate were analyzed using DTA, TGA and SEM. The results of the DTA and TGA showed that between 100°C - 600°C volatile components leave the fiber and a mass loss of 58% was observed. The transformation to ¿-alumina happened at 875°C and the transformation to ¿-alumina at 1130°C. No peaks for ¿-alumina or ¿-alumina were observed. Microscopical investigations revealed different abnormalities of the green fibers. The mean filament diameter was 21 µm. Fibers which were pyrolyzed at 825°C and 875°C, at slow and fast heating rates with 5 min and 15 min holding time had mean filament diameters between 15 µm - 16 µm. Only at 15 min holding time ¿-alumina was the only crystalline phase present. The microstructure of the fibers consisted of grains smaller than 250 nm. The densities were with 2,123 g/cm³ - 2,377 g/cm³ very low and the tensile strengths were between 380 MPa - 692 MPa. Longer holding times and applied tension lead to higher densities with same grain sizes but not to higher tensile strengths. Radiation did not affect the properties of pyrolyzed fibers. The fibers which were sintered between 1100°C - 1700°C for 1 min ¿ 9 min had mean filament diameters of 13 µm -18 µm. Fibers which were sintered at 1200°C, 9 min; 1300°C, 6 min; 1400°C, 3 min; 1400°C, 5 min; 1500°C, 3 min; 1700°C, 2 min; consisted of an ¿-alumina microstructure with grain sizes smaller than 250 nm. Densities of 2,234 g/cm³ -3,007 g/cm³ were measured and tensile strengths of 179 MPa -326 MPa. The mechanical properties of the sintered fibers fell below the properties of pyrolyzed fibers. It was found out that even at long holding times, no proper densification was observed. No tension could be applied during the sintering. Radiation did lead to a different arrangement of the grains which was observed on the fracture surfaces of the filaments. The findings of this thesis provide detailed information of the effect of different parameters during the thermal treatment on the chemical, physical and mechanical properties of alumina fibers and may help fabricating continuous alumina fibers with desired properties for the final application.

KW - Oxidische Endlosfasern

KW - Endlosfasern

KW - Fasern

KW - Aluminiumoxid

KW - CMC

KW - Verbundwerkstoff

KW - Wärmebehandlung

KW - thermische Behandlung

KW - alumina fibers

KW - continuous alumina fibers

KW - fibers

KW - CMC

KW - composite

KW - thermal treatment

KW - alpha alumina

KW - aluminum oxide

M3 - Master's Thesis

ER -