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Porosimetry measures the pore volume and pore volume distribution of the entire composite along the thickness direction of the material. Moreover, in many of the composite filters, one of the layers is very thin, and the low pore volume associated with this layer is not detectable by porosimetry.
Measuring Individual LayersA new technology based on the principles of flow porometry has been developed to measure pore structure of individual layers of a composite. To test a sample using this new method, the sample’s pores are filled spontaneously with a wetting liquid for which the liquid/sample surface free energy is less than the gas/sample surface free energy. Pressure of a non-reacting gas is slowly increased on one side of the sample so as to displace the liquid from the pores and increase gas flow through the sample. When the wetting liquid is displaced from the pore, the gas/sample interfacial area increases at the expense of the liquid/sample area, and the free energy of the system increases. Gas can displace the liquid only when the work done by the gas is equal to the increase in surface free energy. Equating the two energy terms,1 the differential pressure, p, required for displacement of a low surface tension wetting liquid at a location in a pore is given by Equation 1 shown above.2
It follows from Equation 2 that the pressure required to empty a pore is the smallest for the largest pore. Consequently, gas flow rate, which is zero at the beginning, starts at the pressure required to empty the largest pore and increases with increasing pressure because the smaller pores are emptied.
The TechnologyThe sketch of the two layered composite used in this study is outlined in Figure 2. Layer 1 is thin and has small pores, while Layer 2 is thick and has large pores. The gas pressure required to empty pores in such a material filled with a wetting liquid is determined by pore size.
This basic principle is used to determine the pore structures of the two layers of the ceramic composite (Figure 2). The pores are filled with a wetting liquid, and gas pressure under Layer 2 is increased. If gas is allowed to flow in the z-direction, flow porometry measures the size of the constricted parts of pores along the z-direction. Because the pores in the two layers are in series, the small pores present in Layer 1 act as constricted parts of z-direction pores. Therefore, the Layer 1 pores are measured. On the other hand, if gas is allowed to flow in the x-y plane, gas will tend to flow simultaneously in pores of both layers, which are in parallel. Flow porometry will first detect the large pores in Layer 2. At higher pressures, the small pores of Layer 1 will be detected. Thus, pore structures of both layers can be separately determined.
Results and DiscussionPore Structure of Layer 1
Layer 1 had smaller pores. To find its pore structure, a sample saturated with silwick (g = 20.1 dynes/cm) was placed between two O-rings in the sample chamber (Figure 4), and the air pressure under Layer 2 was slowly increased. The air flowed only along the z-direction and escaped to the atmosphere because the O-rings prevented air from flowing in the x and y directions. The flow rates are shown in Figure 7. The largest pore diameter was calculated from the bubble point pressure. The mean flow pore diameter was calculated from the mean flow pressure.
The area under the curve in any size range gives the percentage flow in that range (see Equation 4). The amount of flow is determined by pore diameter and pore size. The pore distribution suggests that most of the pores are in the range of about 0.7 to 0.1 microns.
Pore Structure of Layer 2
Layer 2 had large pores. To analyze the pore structure, the sample was soaked in silwick, sandwiched between two non-porous plates and loaded in the sample chamber (Figure 5). A small central hole in the bottom plate (next to Layer 2) allowed entry of air to the sample. Air could not escape in the z-direction because of the non-porous top plate, but could escape to the atmosphere by flowing in the x and y directions in the sample.
The flow rates through wet and dry samples are shown in Figure 1. The largest pore diameter and the mean flow diameter are listed in Table 1. As expected, the pore diameters in Layer 2 are much larger than those in Layer 1.
Mercury PorosimetryThe composite was also examined by mercury intrusion porosimetry. The results simply showed the pore volume of Layer 2. No indication of the pore volume of Layer 1 was obtained.
Accurately Determining Pore StructureUsing a new technique based on flow porometry, the largest pore diameter, the mean flow pore diameter and the pore size distribution of each layer of a two-layered ceramic composite were determined. In comparison, porosimetry could not measure any property of Layer 1.
As multi-layered ceramics and functionally graded ceramic materials are increasingly used for high-tech applications, this new technique can be used to accurately determine the pore structures of the individual layers of these composites, thus ensuring the quality of the final product.