1   Scope

This International Standard provides guidelines for the determination of the total specific external and internal surface area of disperse or porous (pore diameter > 2 nm) fine ceramic materials by measuring the amount of physically adsorbed gas according to the method of Brunauer, Emmet and Teller (BET method). General guidelines of the method are described in ISO 9277. This International Standard only focuses on specific details relevant to fine ceramic materials. It should further be noted that the BET method cannot be applied to type I isotherms (microporous materials or chemisorption behaviour) or when the solid absorbs the measuring gas.

NOTE For further details on those subjects please see IUPAC references, or the textbook by Gregg and Sing.

2   Normative references

The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

ISO 9277:1995, Determination of the specific surface area of solids by gas adsorption using the BET method

3   Terms and definitions

For the purposes of this document, the following terms and definitions apply.

3.1

adsorption

enrichment of the adsorbate at the external and accessible internal surfaces of a solid

3.2

physisorption

weak bonding of the adsorbate, reversible by small changes in pressure or temperature

3.3

adsorbate

measuring gas to be adsorbed

3.4

adsorbent

solid, which adsorbs the measuring gas

3.5

surface area

area of external surface of a fine ceramic powder plus the internal surface of its accessible macro- and mesopores

3.6

adsorption isotherm

relation between the quantity of adsorbate and the equilibrium pressure of the adsorbate at constant temperature

Principle

The method specified involves the determination of the amount of adsorbate required to cover the external and accessible internal pore surface of a solid with a complete monolayer of adsorbate (see Figure 1).

Any gas may be used provided it is physically adsorbed by weak bonds at the surface of the solid (van der Waals forces), and can be desorbed by a decrease in pressure at the same temperature. Nitrogen at its boiling point (about ) is the most frequently used adsorbate. Krypton or argon at liquid nitrogen temperature are two other gases that are frequently used. The latter two gases offer an improved sensitivity for the analysis of low surface-area samples (less than about to total surface area of the sample). Results of measurements with different adsorbates may deviate from each other because of different molecular cross sectional areas, different accessibility to pores and different measuring temperatures.

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