Sonogelling in boron alkoxides and visualization of "wet" boria microstructures by Cryo-SEM

Gelling in tri-isopropoxy boron and a direct imaging characterization of it s "wet" microstructures during sonogelling is reported for the first time i n this article. The batch composition corresponds to an intimate blend of t ri-isopropoxy boron, ethyl acetoacetate chelating agent, and isopropyl alco hol molecularly mixed using intense, contact-mode probe-type ultrasonicatio n at 135 watts ultrasonic power. Chelation serves to retard the runaway alk oxide hydrolysis and sonogel the boria sol, gelling in which has so far not been reported due to its small molecular size and its amphoteric character istics. The hydrolysis water was added at 0.4ml/h/g of alkoxide under contr olled conditions of temperature, atmosphere and humidity. About 100 mu l of samples in varying stages of sonogelling were injected between two 300-mes h transmission electron microscopy (TEM) grids mounted on 0.1mm copper plat es and plunged into melting freon at -160 degrees C to cause a rapid solidi fication without vitrification. The sample was cryo-transferred into a scan ning electron microscope (SEM), equipped with a liquid N-2 cold Stage maint ained at -170 degrees C. Cryo-SEM imaging at the sonogelling point shows a polymeric networking similar to that in alumina gels synthesized from alumi num sec-butoxide. (C) Elsevier Science Inc., 1999. All rights reserved.