GB/T 19587-2017Determination of the specific surface area of solids by gas adsorption using the BET method (English PDF)

6.1 Sample preparation

Sampling shall be carried out in accordance with ISO 8213 and ISO 14488. Prior to the determination of an adsorption isotherm, remove physically adsorbed material from the sample surface by degassing, while avoiding irreversible changes to the surface.

Ascertain the maximum temperature at which the sample is not affected by thermogravimetric analysis (see Figure 3), by spectroscopic methods, or by trial experiments using different degassing conditions of time and temperature. When vacuum conditions are used, degassing to a residual pressure of approximately 1 Pa or better is usually sufficient. Degassing of the sample can also be performed at elevated temperature by flushing with an inert gas (e.g., helium). Degassing is complete when a steady value of the residual gas pressure p, of its composition or of the sample mass is reached.

Using the vacuum technique, isolate the heated sample container from the pump and trap (at time t i in Figure 4). If the pressure is nearly constant over a period of 15 min ~

30 min, degassing is complete. Almost invariant pressure also confirms the absence of leaks. The specific surface area should be related to the mass of the degassed sample.

After degassing, the sample container is cooled to the measuring temperature. It should be noted that, at low gas pressures, the temperature of the sample needs some time to equilibrate due to the reduced thermal conductivity within the sample cell.

For sensitive samples, a pressure-controlled heating (see Figure 5) is recommended.

This procedure consists in varying the heating rate in relationship to the gas pressure evolved from a porous material during the degassing under vacuum conditions. When a fixed pressure limit, pL (usually around 7 Pa ~ 10 Pa), is surpassed due to the desorbed material from the sample surface, the temperature increase is stopped and the temperature is kept constant until the pressure falls below the limit. At that point, the system continues the temperature ramp. This procedure is particularly suitable for avoiding structural changes in microporous materials, when fast heating rates can damage fragile structures due to a vigorous vapour release. In addition, the method is very safe in preventing sample elutriation when water or other vapours are released from the pores in very fine powder materials.