We report the carbothermal synthesis of sturdy, highly porous (>85%) SiC and Si 3 N 4 monolithic aerogels from compressed polyurea-cross-linked silica xerogel powders. The high porosity in those articles was created via reaction of core silica nanoparticles with their carbonized polymer coating toward the new ceramic framework and CO that escaped. Sol−gel silica powder was obtained by disrupting gelation of a silica sol with vigorous agitation. The grains of the powder were about 50 μm in size and irregular in shape and consisted of 3D assemblies of silica nanoparticles as in any typical silica gel. The individual elementary silica nanoparticles within the grains of the powder were coated conformally with a nanothin layer of carbonizable polyurea derived from the reaction of an aromatic triisocyanate (TIPM: triisocyanatophenyl methane) with the innate −OH, deliberately added −NH 2 groups, and adsorbed water on the surface of silica nanoparticles. The wet-gel powder was dried at ambient temperature under vacuum. The resulting free-flowing silica/polyurea xerogel powder was vibrationsettled in suitable dies and was compressed to convenient shapes (discs, cylinders, donut-like objects), which in turn were converted to same-shape SiC or Si 3 N 4 artifacts by pyrolysis at 1500 °C under Ar or N 2 , respectively. The overall synthesis was time-, energy-, and materials-efficient: (a) solvent exchanges within grains of powder took seconds, (b) drying did not require high-pressure vessels and supercritical fluids, and (c) due to the xerogel compactness, the utilization of the carbonizable polymer was at almost the stoichiometric ratio. Chemical and materials characterization of all intermediates and final products included solid-state 13 C and 29 Si NMR, XRD, SEM, N 2 -sorption, and Hg intrusion porosimetry. Analysis for residual carbon was carried out with TGA. The final ceramic objects were chemically pure, sturdy, with compressive moduli at 37 ± 7 and 59 ± 7 MPa for SiC and Si 3 N 4 , respectively, and thermal conductivities (using the laser flash method) at 0.16 3 ± 0.010 and 0.070 ± 0.001 W m −1 K −1 , respectively. The synthetic methodology of this report can be extended to other sol−gel derived oxide networks and is not limited to ceramic aerogels. A work in progress includes metallic Fe(0) aerogels.

Electrochemical methods were used to study the role of Cr(III) on the anti-corrosion behavior of a trivalent chromium process conversion coating on AA2024-T3. The same conversion coating was investigated with and without Cr(III) added to the bath. Polarization curves (naturally-aerated 0.5 M Na 2 SO 4 + 0.1% NaCl) revealed similar anodic current suppression by both coatings. Cathodic currents were suppressed more by the coating with Cr(III). Rotating disk voltammetric data revealed that cathodic currents for oxygen reduction were invariant with the rotation rate (rpm) 1/2 for the alloy coated with the Cr(III) conversion coating. The trivalent chromium process coating appears to inhibit oxygen reduction by providing a diffusional barrier and by blocking sites (Cr(OH) 3 ) for O 2 chemisorption on cathodically-active intermetallics.

Organic aerogels are a class of material most suited for their transformation into electrically conducting porous carbon networks.

The detection of H 2 O 2 formed in naturally-aerated electrolyte solution contacting uncoated and TCP-coated AA20243-T3 alloy surfaces is reported on. This molecule is produced from the reduction of dissolved oxygen and it has been implicated in the mechanism of transient formation of Cr(VI) in trivalent chromium process (TCP) conversion coatings. Linear sweep voltammetry and a spectrophotometric assay involving 2',7'-dichloro-dihydrofluorescein diacetate (DCFH-DA) were used to detect H 2 O 2 produced in 0.5 M Na 2 SO 4 . The activation of DCFH-DA to form the redox probe, DCFH, and its oxidation by H 2 O 2 to the fluorescent dichlorofluorescein was used for detection. Immersion tests were performed in naturally-aerated 0.5 M Na 2 SO 4 + 0.1 mM DCFH-DA for periods of time up to 96 h under open circuit conditions. The H 2 O 2 concentration was greater in solution exposed to uncoated than to TCP-coated AA2024-T3 for equivalent times due to the inhibition of the oxygen reduction reaction kinetics by the conversion coating. With this direct evidence of H 2 O 2 production, the following mechanism for transient formation of Cr(VI) in TCP coatings is confirmed: (i) dissolved oxygen is reduced to H 2 O 2 on the alloy surface presumably at cathodic intermetallic sites, (ii) the H 2 O 2 then diffuses to nearby coating sites to oxidize insoluble Cr(OH) 3 to soluble Cr(VI) species (e.g., CrO 4 2− ) and (iii) the transiently formed Cr(VI) species diffuses to nearby corroding sites on the alloy where the inhibitor gets reduced back to passivating Cr(OH) 3 .