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ISO 12800:2017 Nuclear fuel technology — Guidelines on the measurement of the specific surface area of uranium oxide powders by the BET method
来源: | From: Gold APP Instruments | Published Date: 2023-02-02 | 337 Time(s) of View | 分享到:
This document gives guidelines on the determination of the specific surface area of as-fabricated uranium dioxide powder by volumetric or gravimetric determination of the amount of nitrogen adsorbed on the powder, and can be applied to other similar materials, e.g. U3O8, UO2-PuO2 powders, and other bodies with similar surface areas, e.g. powder granules or green pellets, provided that the conditions described are fulfilled. Modifications using other adsorbing gases are included.
This document gives guidelines on the determination of the specific surface area of as-fabricated uranium dioxide powder by volumetric or gravimetric determination of the amount of nitrogen adsorbed on the powder, and can be applied to other similar materials, e.g. U3O8, UO2-PuO2 powders, and other bodies with similar surface areas, e.g. powder granules or green pellets, provided that the conditions described are fulfilled. Modifications using other adsorbing gases are included.

The method is relevant as long as the expected value is in the range between 1 m2/g and 10 m2/g.


Summary of the BET method

The BET method is based on the determination of the amount of gas necessary to cover the surface by a monomolecular layer. This amount is determined from the isothermal adsorption curve of nitrogen (N2) at the temperature of liquid nitrogen (77,4 K) according to Reference. The amount of N2 adsorbed at a given pressure is determined by volumetric or gravimetric measurement. In order to remove surface contamination of the adsorbent, the sample has to be evacuated and heated under appropriate conditions before the measurement is performed.

isotherm types

Sample preparation

Impurities on the sample surface, especially water vapour, shall be removed before the adsorption measurement. Conditions for removing impurities (vacuum, temperature, time) have to be found which are compatible with the powder type. Chemical reactions (decomposition), sintering, change of crystal structure and other processes on the surface shall be avoided. Long evacuation periods are needed for highly porous powders. In order to shorten the heating time, the optimum temperature should be determined. In most cases, the measured specific surface area first increases with an increase in the heating temperature and then decreases, e.g. by sintering of the powder.


Volumetric measurement

The pre-treated sample of known mass is placed in a bulb of calibrated volume, which is filled with nitrogen at a defined temperature and pressure. At ambient temperature and pressure, measurable adsorption does not occur. The closed bulb is cooled to the temperature of liquid nitrogen. The adsorbed amount of nitrogen can be calculated from the amount of nitrogen enclosed in the bulb, the volume, the temperature, and the pressure drop. Accurate volumetric measurements can be obtained by measuring the difference in pressure between the sample-containing bulb and an empty reference bulb.

ISO 12800:2017 Nuclear fuel technology — Guidelines on the measurement of the specific surface area of uranium oxide powders by the BET method

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