Partially Sintered Aerogels

The permeability of the composite aerogel can be estimated from the Carman-Kozeny equation applied to a gel [25], an approximation that relates the permeability to the pore size and which can be written as  

from the relative density and the mean pore radius, one can estimate the permeability of the partially sintered aerogel and the composite aerogel. The permeability of the composite aerogel is roughly 60 times higher than for the sintered aerogel (Table 29.1). 

This new kind of composite aerogels is the more appropriate as host matrix because of its improved mechanical properties and permeability. 

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Figure 29.2. Set of partially sintered aerogels from aerogel (relative density is 0.15, for the sample on the right) to fully sintered aerogel (transparent silica glass on the left, relative density is 1).

Table 29.1. Relative density (pr), mean pore size, and calculated permeability of partially sintered aerogel and composite aerogel...

TABLE 5-1. SPECIFIC SURFACE AREA EVOLUTION AS A FUNCTION OF DENSITY FOR PARTIALLY SINTERED AEROGELS... 

NMR imaging of gas adsorption/desorption in nanoporous solids, such as Y-AI2O3 and ZnO powders and partially sintered ceramics of these materials, as well as Vycor porous glass was analysed using Brunauer-Emmett-Teller (BET) theory. Visualization of gaseous xenon and methane in the void spaces of aerogels offered unique information and insights into the pore structure and molecular diffusivities of occluded sorbates. ... 

Another critical point are pores with narrow entrances that result in only slow pressure eqmlibration examples are carbon aerogels that have been treated at temperatures above 1,000°C or samples that have been partially sintered. In these cases, artifacts can arise due to insufficient equilibration of the gas presstue. 

The potentialities of this material (Cerenkov detector, acoustic or thermal insulator, host matrix for catalysts) are increased if considered not only as an end product, but as a precursor. By a set of sintering treatments, the silica aerogels can be easily transformed into pure silica glass (Woignier, 1990) and appropriate heat treatments lead to Partially Dense Aerogel (PDA) which can be used as a host matrix for the synthesis of doped glasses or composites. 

The densification of the aerogels is obtained by heat treatment at a temperature of 1050°C and, as a function of the sintering time, the bulk density increases. The samples are labeled PDAxx (Partially Dense Aerogel) where xx is the bulk density expressed in g/cm. The PCAxx samples (Partially Compressed Aerogel) correspond to gels whose density has been increased by room temperature compression. 

Figure 5-9. Measured oscillations of SAKS intensity for initial (0.186 f/cm density) and partially sintered (0.36 and 0.57) base catalysed silica aerogels.

Figure 5-11. SAXS measurements obtained for a series of fractal partially densified aerogels. The sintering treatment is performed at 1000° C as a function of time. decreases while a increases (see arrows). The slope (fractal dimension) is drawn as guides for eyes.

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