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Polycrystalline mullite fibers and novel zirconia-toughened mullite (ZTM) fibers with average diameters between 9.7 and 10.3 μm containing 3, 7 and 15 wt.-% tetragonal ZrO2 (ZTM3, ZTM7, ZTM15) in the final ceramic were prepared via dry spinning followed by continuous calcination and sintering in air. A shift in the formation of transient alumina phases and tetragonal ZrO2 to higher temperatures with increasing amounts of ZrO2 was observed. Concomitantly, the mullite formation temperature was lowered to 1229 °C for ZTM15 fibers. X-ray diffraction revealed formation of the desired tetragonal crystal structure of ZrO2 directly from the amorphous precursor. Room temperature Weibull strengths of 1320, 1390 and 1740 MPa and Weibull moduli of 9.5, 7.1 and 9.0 were determined for mullite, ZTM3 and ZTM15 fibers, respectively. Average Young’s moduli ranged from 190 to 220 GPa. SEM images revealed crack-free fiber surfaces and compact microstructures independent of the amount of ZrO2.

Polycrystalline zirconia toughened alumina (ZTA) fibers were prepared from aqueous solutions of aluminum hydroxide chloride 2.5-hydrate and zirconium oxychloride octahydrate. Fiber processing was accomplished via dry spinning. Poly(vinylpyrrolidone) (PVP) was used as spinning aid. Polycrystalline ZTA fibers were obtained by calcination of the green fibers followed by sintering at defined temperatures in air. Ceramic fibers were 10 μm in diameter and had an average tensile strength of 1010 (±363) MPa with peak values reaching 1535 MPa. Differential scanning calorimetry/thermogravimetric analysis coupled with mass spectrometry (DSC/TGA-MS) showed an exothermic peak at 1156 °C assigned to the crystallization of α-Al 2 O 3 and tetragonal ZrO 2 and an overall ceramic yield of 41.7% at 1400 °C. X-ray diffraction (XRD) analysis showed that tetragonal ZrO 2 can be obtained at 1350 °C directly from the amorphous precursor whereas pure-phase α-Al 2 O 3 is formed stepwise via transition alumina phases.

Polycrystalline calcium phosphate ((Cl/OH)Ap = Ca5(PO4)3(OH/Cl); TCP = Ca3(PO4)2) fibres were prepared from aqueous solutions of calcium chloride and phosphoric acid using poly(ethylene oxide) (PEO) as spinning aid. Generation of nonwoven materials was accomplished via rotary jet spinning. Polycrystalline (Cl/OH)Ap fibres 10–25 μm in diameter were obtained with 37% ceramic yield by pyrolysis of the green fibres followed by sintering at 1150 °C in air. X-ray diffraction (XRD) analysis provided evidence for apatite formation starting at 650 °C while (Cl/OH)Ap ceramic fibres were obtained at 1100 °C via transformation through intermediate dicalcium dichloride hydrogen phosphate (Ca2Cl2(HPO4)) and calcium pyrophosphate (Ca2P2O7) phases. A glass-forming Al-based additive was applied to enhance the mechanical properties of the Cl/OH)Ap ceramic fibres and indeed resulted in the formation of (Cl/OH)Ap/Al2O3 fibres with improved mechanical stability. Finally, TCP, (Cl/OH)Ap and (Cl/OH)Ap/Al2O3 fibres were subjected to seeding with mesenchymal stem cells. Negligible cytotoxicity is observed.

Polycrystalline yttrium aluminum garnet (YAG, Y 3 Al 5 O 12 ) fibers were prepared from aqueous solutions of aluminum chlorohydrate and yttrium chloride. Fiber processing was accomplished via dry spinning. Poly(vinylpyrrolidone) (PVP) was used as spinning aid. Polycrystalline YAG fibers were obtained by pyrolysis of the green fibers followed by sintering at defined temperatures in air. Ceramic fibers were 9–16 μm in diameter. Differential scanning calorimetry/thermogravimetric analysis coupled with mass spectrometry (DSC/TGA-MS) showed an exothermic peak at 920 °C assigned to the crystallization of YAG and an overall ceramic yield of 38% at 1400 °C. X-ray diffraction (XRD) analysis showed that phase-pure YAG can be obtained at 1600 °C after intermediate formation of Y 2 O 3 and monoclinic yttrium aluminum oxide (YAM, Y 4 Al 2 O 9 ) phases.