Structural Evolution in Sol–Gel‐Derived Yttrium Aluminum Garnet–Alumina Precursor Fibers

Abstract

A sol leading to a eutectic Al2O3–Y3Al5O12 composition was spun into fibers. These fibers were dried and pyrolyzed between 200° and 850°C in a nitrogen or water‐vapor atmosphere. Pyrolysis in nitrogen resulted in dense, amorphous fibers with considerable residual carbon content. In water vapor, fibers also remained amorphous, but organics were almost completely removed. The loss of organics created micropores that grew as the pyrolysis temperature increased. The amorphous fiber structures were examined by nitrogen adsorption, helium pycnometry, and transmission electron microscopy. Young's moduli of the pyrolyzed fibers were measured, and porosities of the fibers were calculated from both nitrogen‐adsorption data and elastic behavior. Sintering tests showed the best sinterability for fibers pyrolyzed in water vapor to 385°–500°C.

Microstructure of sol–gel derived TiO2 thin films characterized by atmospheric ellipsometric porosimetry

“…The film material obviously undergoes the same structural changes as already observed for xerogels [26], sol-gel fibers [27] and "sol powders" isolated from the TiO 2 coating solutions [18]: Samples annealed below 300°C presented no open porosity, due to large amount of residual organics. First at 300°C a certain amount of micropores (4%) is generated by the thermal decomposition of organic residues as indicated by a type-I isotherm.…”

From Sol-Gel Processing to Bio-Inspired Materials Synthesis

“…In water vapor atmosphere the removal of organic residues is promoted by continued hydrolysis reactions, but the fibers subsequently show approximately 50% porosity. Nevertheless these materials undergo complete densification at 1700 o C, whereas porosity is created in the N 2 -pyrolyzed samples during sintering presumably through Cburnout during sintering [48].…”

“…(4). Illustration of the sol-gel processing of YAG-alumina fibers [48]. It should be noted that for practical reasons after spinning fiber bundles are handled batchwise for drying, pyrolysis and sintering.…”

“…Mixed YAG-corundum ("duplex") compositions are investigated to combine the good mechanical properties ofalumina with the creep resistance of YAG [48] (see 2.2.1).…”

“…Solid sol-gel precursors leading to an eutectic Al 2 O 3 -Y 3 Al 5 O 12 composition were synthesized from yttrium(III)acetate and aluminum sec-butoxide with hexanoic and propionic acid as complexing agent, respectively [48]. The solid mass liquefies and becomes spinnable between 160 o C and 180 o C. The resulting fibers are collected into bundles for the further batchwise treatments, a schematic representation of the processing steps is given in Fig.…”