The properties of an aerogel depend primarily on the size of its pores: Cellulose aerogels can be transparent or opaque depending on the pore size. The thermal insulation properties of aerogels are also strongly influenced by pore size. Aerogel researchers are therefore constantly trying to obtain pores that are most suitable for the application. Mostly, these are the so-called “mesopores”, i.e. pores of medium size.
Why is a medium pore size particularly useful? On the one hand, it allows good mass transport of other substances through the structure of the aerogel (better than through small “micropores”, which are found in activated carbon, for example), and on the other hand, it provides a large internal surface area for a wide variety of applications (which is larger than in materials with large “macropores”, such as sponges).
Even though the pores are already medium-sized, they are so tiny that we cannot see them with the naked eye! That’s why special scanning electron microscopes are needed to see the pores at all. Their diameter ranges from 5 – 50 nanometers, where a nanometer is only one millionth of a millimeter.
| Designation | Symbol | Conversion |
|---|---|---|
| Millimeter | mm | |
| Micrometer | μm | 1 μm = 1/1,000 mm |
| Nanometer | nm | 1 nm = 1/1,000 μm = 1/1,000,000 mm |
Task:
Below you will see microscopy images of different aerogel pores. In each image, you will find the following information: 1. the material 2. what the aerogel looks like 3. a scale in nanometers (nm).
Imagine you are an aerogel researcher and have to evaluate the different porous structures based on whether they are aerogel structures with favorable properties such as a high specific surface area. To do this, arrange the images in the appropriate order: You need to assess which aerogel consists only of small mesopores (5-50 nm) and as few larger pores and gaps as possible: The more fine mesopores there are, the better.