Characterizing Aerogels

Editor’s Note: This article is under construction but we thought we’d post it as we go anyway so you can benefit from it as much as possible.

Characterization is the process of observing, measuring, and describing the properties a material has. There are lots of different ways to characterize a material. Classes of properties of interest for aerogels include:

• Visual properties: color, shape, general appearance, microstructure
• Optical properties: transparency of the material to different wavelengths of light, index of refraction, phosphorescence, fluorescence, photoluminescence
• Physical properties: bulk sample density, skeletal density, specific surface area, porosity, pore size statistics, particle size statistics, speed of sound
• Mechanical properties: monolithicity, ultimate compressive strength, compressive modulus, flexibility, modulus of flexure
• Chemical properties: chemical composition, surface chemistry, hydrophilicity/hydrophobicity/oleophilicity, flammability, reactivity
• Electronic properties: electrical conductivity, band gap, loss tangent
• Magnetic properties: magnetism, magnetic susceptibility

But what information does each of these properties tell us about a material (that is, why do we care)? And how do you measure these properties?

Visual Properties

Color, Shape, and General Appearance

These properties can be characterized with the eye and communicated either with prose or photography. For example,

“Silica aerogels produced by our process were translucent and exhibited a sky-blue cast. Cylindrical monoliths of up to 5 cm in diameter and 1 cm tall were prepared and only slightly cracked.”

or

“Isocyanate-crosslinked vanadia aerogels were opaque with a distinct avocado green color striped with lighter shades of green.”

A photograph is the best way to characterize visual properties. Use indirect lighting, preferably from a bright incandescent source. Placing a ruler in the photo or putting the sample on a sheet of graph paper is a great way to give a scale to your sample. For transparent aerogels, try photgraphing them on both matte black and white backgrounds–often times the results will be quite different!

Microstructure

Since the particles and pores that make up aerogels are generally sub-micron in size, an optical microscope is of little use for characterizing the microstructure of aerogels. Instead, an electron microscope such as a scanning electron microscope (SEM) or transmission electron microscope (TEM) should be used.

SEM generates an image by reflecting electrons off a sample and collecting them onto a charge-coupled device (CCD) sensor like a digital camera has. The image is analogous to a photograph but allows you to see features much smaller than with visible light. Most aerogels will just look like terrains of little spheres or other shapes unless a high-resolution SEM (HRSEM) is used, where you can really zoom in and see the particles up-close, for example the “string of pearls” morphology of base-catalyzed silica aerogels or the leaf-like structure of alumina aerogels made from alkoxide.

In general, though, to appreciate the microstructure of an aerogel you need to use TEM. TEM works by shining an electron beam through a powder or very thin layer of a sample on a special microgrid and taking a picture of the electron shadow. This technique allows for much higher resolution than SEM and allows you to see nanoparticles and nanopores clearly. TEM works best with conductive samples and so for some metal oxides, a thin layer of gold may need to be sputtered onto the aerogel before imaging. One drawback with TEM is that the samples are annoying to prepare, since the grids you have to put your sample on are so tiny and the sample must be very thin or a very fine powder. In fact, all of the area ever imaged by TEM could fit onto a single thumbnail.

Optical Properties

Transparency

Transparency is a measure of how much light passes through a material. Transparency depends on the wavelength of light. For example, silica aerogels are very transparent to red light but scatter blue light readily. Thus the percentage of red light transmitted is higher than blue light.

Transparency can be measured using an optical spectrometer. A sample of a regular shape (preferably a rectangular prism with a square cross section) is placed in the spectrometer. Inside the spectrometer, light from a light source such as an incandescent light bulb is focused through a series of lenses and directed into a prism where it is spread into a rainbow of colors in space. A narrow slit is placed in the path of the rainbow allowing a narrow band of wavelengths to pass through. This beam is then directed through the sample and onto a photocell that measures how bright the beam of light is. By comparing the brightness of the beam with and without the sample present, the the transparency of the sample as a function of wavelength can be measured. The results can then be plotted as a graph of % transmittance or % absorbance vs. wavelength.

Index of Refraction

Index refraction refers to how much a material bends the path of light passing through it. This is the same effect that causes the world to “shift” back and forth when taking a pair of glasses on and off, or why a straw looks bent when you stick it in a glass of water.

Index of refraction is also a function of wavelength but generally is constant over visible wavelengths. It can be measured by shining a highly-collimated beam of light (such as a laser) through a sample and measuring the position of the spot on a screen on the other side of the sample. Comparing the position of the spot with and without the sample present and knowing how thick the sample is and how long the beam path is, you can calculate the index of refraction.

Physical Properties

Bulk Density

Bulk density is useful to get an idea of how much of a sample is empty space. Additionally, many properties of aerogels such as compressive strength and electrical conductivity are related to bulk density.

Bulk density can be measured by weighing a sample, measuring the sample’s volume, and then dividing the mass by the volume. A microbalance (resolution of 0.01 to 0.001 g) should be used to weigh the sample since most aerogels are very low density and a sizable piece won’t weight very much. For regularly-shaped objects such as cubes and cylinders, dimensional analysis (that is, measuring the dimensions of the object) with calipers is typically used to figure out the volume, however for irregularly-shaped objects this can be harder. In these cases, an approximate volume can be reported or a displacement technique using solid particles or a viscous liquid can be used to estimate its volume. Keep in mind using a liquid displacement technique may destroy the sample, and so the liquid needs to be selected carefully based on the predicted hydrophilicity/hydrophobicity of the sample. Mercury displacement can also be used but requires special equipment.