Pycnometric Determination of Porosity Characteristics

Pycnometric methods of characterization of the pore structure of solids, based on density measurements, provide data on the pore size distribution. Here we report results of open porosity determination via weighing of a dry sample, followed by evacuation and saturation with distilled water at atmospheric pressure; via pressure infiltration with water using a manostat system; and by mercury intrusion porosimetry. We have studied porous nickel samples prepared by a powder metallurgy process involving sintering of green compacts consisting of mixtures of nickel nanopowder and ammonium bicarbonate (NH₄HCO₃) powder as a pore former, with volume fractions of 80 and 20%, respectively. The particle size ranges of the pore former were 63–125, 140–200, and 250–315 μm. For the three methods used to determine open porosity, we theoretically evaluated the size of pores accessible to infiltrating liquid. The results demonstrate that, in the case of saturation with water after evacuation, the liquid can penetrate only pores more than 3 μm in size. Moreover, in the case of porous structures with a large fraction of submicron pores, the open porosity determined via saturation with distilled water after evacuation is considerably underestimated. The larger the fraction of small pores in the material, the larger the underestimation. The difference between the open porosity values obtained via pressure infiltration with water and by mercury porosimetry is insignificant. We demonstrate that, of the three porosity determination methods, only saturation with distilled water after evacuation cannot be used to analyze structures with submicron pores. The results we obtained can be useful in designing porous functional materials and articles with a tailored pore structure.