ABSTRACT
Experimental measurements and tests of properties of porous materials are the main criteria for the estimation of their technical merit, their choice for various needs, estimation of the correctness and the usefulness of the relevant theoretical models, and economic forecasting regarding the perspective of the industrial and/or domestic applications of the tested materials. For studying porous materials, in principle, one may use all existing methods of study of condensed phase and multiphase systems. The main idea of such experiments is the comparison of measured characteristics of a porous solid sample with those of the similar nonporous sample having the same chemical composition and structure as the continuous phase of the porous sample. The difference between those results of measurements is attributed to the contribution of porosity. In the case of a homogeneous porous sample, this phenomenological comparison can be presented as the following equation:
X(P, ξ)=X0(P, ξ=0)+Xξ(ξ) (2.1)
where P is the measured parameter, X is the result of the measurement, ξ is the porosity, X0 is the result of the measurement for the nonporous sample, and Xξ is the contribution of the homogeneous porosity. The form of the analogous equation for the measurements of heterogeneous porosity is very similar to Eq. (2.1) but comprises a sum over all factors of heterogeneity:
X(P, Σ ξ)=X0(P, ξ=0)+ΣXξ(ξ) (2.2)
In principle, for the homogeneous porosity Eq. (2.1) should provide
(2.3)
where the sum is taken over all accounted powers of derivatives of X. Equation (2.2) means that two or more experiments of the same kind accomplished on the same sample should provide the same results. However, this is not always right (for micropores one can claim that it is never right!), and in real conditions X(P, ξ) depends also on time and
the number of measurements. This well-known experimental fact is caused by two phenomena:
1. “Aging” of pores 2. Hysteresis of properties of porous structure
B. Aging
Aging of a pore can be defined as a spontaneous change of its structure. The driving force for aging is the excess internal energy of the pore. As it was discussed in Chapter 1, micropores are characterized by the upper level of the internal energy. Therefore, the effect of aging is especially important for micropores. The increase of the characteristic size of a pore reduces its energy and, respectively, the influence of aging. For macropores, errors related to aging are so minor they can be neglected in comparison with the level of the error of the measuring equipment.
