BET

Model:

Sorptionsgerät Autosorb 1-MP (Quantachrom)

Application

Determination of ad- and desorption isotherms with water vapour and non corrosive gases in porous materials and calculation of specific surfaces (BET-surfaces), pore volumes and pore size distributions. 

Theory

By cooling a sample on a constant temperature and adding a determined amount of gas into the sample holder, a part of the gas molecules will be attached at the sample surface. Depending on the amount of added gas and the surface of the sample, this leads to a certain equilibrium pressure of the gas in the sample holder. These equilibrium pressures as a function of added gas amount deliver the adsorption isotherm which by the BET-Equation enables to calculate the amount of gas which is adsorbed as a monolayer. From size and number of gas molecules at monolayer adsorption the specific surface of the sample material can be determined. With further gas adsorption accessing the monolayer adsorption, successively, pore with increasing diameters are filled. Since the temperature in the sample holder is equal the boiling point of the adsorbed gas, capillary condensation occur and lead to an step in the adsorption isotherm. In dependence from pressure and amount of adsorbed gas at the pore size and the pore volume of the respective size can be determined. The standard method for surface and porosity determination is the adsorption of liquid nitrogen (at 77 K) but depending on sample characteristics also other non corrosive gases as O2, Ar, Kr or CO2 may be used.

Description

The AUTOSORB-1 is a fully automated analyzer for surface area and pore size measurements. It is equipped with one measurement and two sample degassing stations which work simultaneously. In the sample degassing stations the sample surface contaminants are removed by heating up to 450°C under vacuum. For measurements the adsorbate is fully automated exactly dosed into the measurement cell until a specific chosen equilibration pressure is reached. Equilibration time and pressure tolerance may be set by the user. Measurement cells are available in various geometric forms and outer diameter of 6, 9, and 12 mm in order to adapt an optimal ratio of sample amount and void volume. Furthermore, this device allows to measure water vapour sorption and the determination of thermodynamical parameters describing interactions between solids and water (hygroscopic, hysteresis, hydration, water activity). The shape of vapour isotherms may for instance give information about changes in the material during water sorption.