Doped Mesoporous Silica Fibers: A New Laser Material

632 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 1999 0935-9648/99/0806-0632 $ 17.50+.50/0 Adv. Mater. 1999, 11, No. 8 After this the polymer was rigid and could be peeled off carefully from the silicon master. The suspensions were prepared by adding powder to distilled water under constant stirring. The pH was always kept between 4 and 5 by dropwise adding a 2 M solution of hydrochloric acid (Titrisol, Merck, Darmstadt, Germany). Thereafter the suspensions were ball milled with alumina milling balls for 18 h. Two drops of 1-octanol (puriss, Fluka, Buchs, Switzerland) were added to reduce the surface tension of the suspension before degassing for 15 min at 90 mbar under constant rotation. For the formation of the ceramic samples a few drops of the particle suspension were poured onto the PDMS template and dried in room atmosphere. No separating agent was applied onto the mold. The suspension layer on the template structure was kept relatively thin (about 1 mm) in respect to its diameter (about 5 mm) to get quasi uniaxial shrinkage in the thickness direction to prevent high lateral shrinking rates during drying. After drying, the ceramic parts were carefully lifted from the molds and sintered. The PDMS templates were cleaned in an ultrasonic bath after processing and could be reused. The alumina samples were heated with 300 K/h to 1550 C and held there for 3 h. The boehmite samples were heated with 300 K/h to 300 C, then with 60 K/h to 600 C to allow the phase transition of boehmite from g-AlOOH to g-Al2O3 and then with 300 K/h to 1250 C and held there for 4 h. Suspensions of three powders with different particle sizes were prepared. Their properties are listed in Table 2. For a good pattern replication the suspensions needed to have a relatively low viscosity of about 0.5 Pa s which limits the amount of powder that can be dispersed. The achieved solids loadings decreased with decreasing particle size of the powders. Solids loadings in volume percent are shown in Table 2.