This technique permits to measure the pore volume and size by forcing mercury to penetrate inside the open porosity. Mercury is used because it behaves as a non-wetting liquid with a large number of materials. This technique is not advisable when the sample contains metals reacting with mercury (i.e. gold, aluminum, etc.) and forming amalgams. Mercury is forced to enter into the pores by applying a controlled increasing pressure. As the sample holder is filled with mercury under vacuum conditions (mercury surrounds the sample without entering the pores due to the very low residual pressure), during the experiment the pressure is increased and the volume of mercury penetrated is detected by means of a capacitive system. The decreasing volume of mercury in the sample holder represents the pore volume. The penetration pressure is directly related to the pore access size by a well known mathematical model: the Washburn equation.
R = -2 γ cos (θ) / Pc
Where:
γ: surface tension of pure mercury (480 dyne/cm)
θ: contact angle between mercury ad the solid (average value 140°)
Pc: mercury penetration equilibrated pressure
R: pore radius
The distribution of pore size as well as the total porosity, bulk and apparent density and the specific pore volume can be obtained by the relationship between the pressure necessary for penetration (the pore dimension) and the volume of penetrated mercury (pore volume). There are some main assumptions necessary when applying the Washburn equation: pores are assumed to be of a cylindrical shape and the sample is pressure stable. The latter because, in mercury porosimetry it is possible to reach very high pressures, up to 400 MPa, and not all the materials can tolerate such a stress without collapsing of compressing. From the Washburn equation it is clear that the pore size range that can be investigated by mercury porosimetry is directly related to the pressure range.
The Pascal porosimeters pore size determination is ranging between 300,000 nm (under vacuum conditions) and 1.8 nm (at 400 MPa) pore radius.
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