BET

Measurement of the specific surface area

The measurement of the specific surface area of solid or porous materials gives important information on their physical structure. The area of a material’s surface determine how that solid will interact with its environment. Many properties, such as dissolution rates, retention of moisture, and reactivity, are often correlated to a material’s surface area. 

We at RISE use the BET Theory (Brunauer-Emmett-Teller theory) to measure the specific surface area. BET is one of the most widely used techniques in material characterisation. 

Method

Before performing the measurement, contaminants already adsorbed by the material (water and carbon dioxide, for example) must be removed from the solid surface. During this pretreatment, the solid is typically heated under vacuum to remove any initially adsorbed contaminants (the temperature is adapted to the material to avoid any alterations of the material). If the material is too sensitive to vacuum a flow of inert gas can be used instead.

To determine the surface area, the solid is first cooled under vacuum to cryogenic temperature using liquid nitrogen. This is because most gases and solids interact weakly at usual temperatures. The temperature of the solid sample is kept constant while an adsorbing gas is dosed to the solid in controlled increments. After each dose, the pressure is allowed to equilibrate, and the quantity of gas adsorbed is determined. The quantity of gas adsorbed is plotted as a function of pressure.  From this plot, the quantity of gas required to form a monolayer over the external surface of the solid is determined. The surface area can be calculated from the quantity of gas required to form a monolayer, using the BET theory.

To ensure the validity of the results, we analyse in parallel a reference material with a known specific surface area.

We use a Nova 800 from Anton Paar.

Delivery

The client will receive a detailed report containing:

• the sample’s handling,
• the raw data
• the data treatment
• the results
• the explanation of what the results mean
• conclusions

The sample(s) can be sent back.