Structural Properties of Porous Materials and Powders Used in Different Fields of Science and Technology

Paper is a fibrous sheet material made from fibers based on a variety of materials: wood cellulose, synthetic polymers, mineral fibers (glass, basalt, and asbestos), and other material (wool, mica, metallic “whiskers,” and graphite) [1]. The most common type of paper is writing paper. Its main feature is the capillary-porous structure that allows the absorption of inks, dyes, and graphite pencil powder. Paper (the Italian bambagia—cotton) is a fibrous sheet material. Paper with weight exceeding 250 g per m² is called cardboard. Distinguishing between general-purpose paper (mass and non-mass) and special, a decision was made to divide paper into a number of classes for printing (newsprint, offset, etc.), writing, typing, drawing and crayon, for paper money, for vehicles (punch-card, ticker-tape, etc.), electrical (cable, capacitor, etc.), wrapping and packaging, etc.

Classification of porous materials (PM), their properties and characteristics is studied, a comparative analysis is done. An overview of a number of modeling-analytical descriptions of the physical properties of porous bodies is provided. The general issues for producing porous materials by powder metallurgy techniques including various methods of molding and sintering processes are under consideration. The potential applications of porous materials are demonstrated.

In this section, some classifications of porous thermal insulation materials are given and different techniques for porosity investigation are described. Special attention is focused on the standard contact porosimetry, which provides no destruction of the samples and gives a possibility to determine pores in a wide diapason of sizes. Owing to these advantages, the technique allows us to research evolution of porous structure at different stages of the product preparation and identify the synthesis phase, when functional properties of the material are transformed to diametrically opposite ones. Effect of porosity on such properties as thermal conductivity and compression strength is estimated, the appropriated correlations are represented. The information dealt to research of thermal conductivity is given, the heat transfer through porous media is considered. It is noted, that the main way to reduce thermal conductivity is to increase porosity of the material, the contribution of solid phase can be diminished by this manner. This principle is used for manufacture of most of thermal insulators such as widespread polymer and inorganic foams. They are characterized by extremely low thermal conductivity, the order of magnitude of which is 10⁻² and 10⁻¹ W m⁻¹ K⁻¹, respectively. Modern approaches to development of new thermal insulating materials and structures are considered. These approaches are based on a decrease in heat conductivity of gaseous phase. In order to minimize the fluid contribution, inert gases, which are characterized by lower conductivity in a comparison with air, can be encapsulated in closed pores. Other ways are degassing of thermal insulating materials and decrease of their pore sizes, simultaneously high porosity has to be provided.

Medical substances (MS) used in the pharmaceutical technology are microheterogeneous solid-phase powder materials that underwent dispersion procedures in the gas phase. Many properties of these powders, particularly their stability and ability for subsequent processing into medications in forms acceptable to the consumer such as tablets, capsules, or suspensioms depend not only on their chemical composition, but also on their structure. MS have a high specific surface which results in increased inter-particle interactions and therefore to a great extent influences their chemical and technological properties. In order to optimize their further processing it is important to evaluate their structural properties by well-defined standard measuring procedures. In the pharmaceutical industry, besides MS different secondary, auxiliary substances are used which per se have no medical influence but help to produce the final medication. From the point of view of evaluating their structural properties these auxiliary substances are equivalent to the MS.

Free volume is an important property of polymers. When the size and connectivity of “microcavities” in polymers increase they often indicate the microporosity of polymers. In this review, an attempt is made to consider in a systematic manner the nature of free volume in polymers and different methods of their estimation. Several original approaches for evaluation of microporosity of polymeric gas separation membrane materials and sorbents are also discussed. It is important that the first industrially produced gas separation membrane was based on poly(vinyltrimethyl silane), the first glassy polymer with relatively high free volume. At present, the attention of researchers is attracted to several classes of membrane materials distinguished by high free volume. In turn, hypercrosslinked polystyrene sorbents are manufactured by several companies now, and they are widely used in chemical, food, and water treatment industries. In conclusion, the authors tried to emphasize common features and differences between free volume and microporosity in polymers.

This section is devoted to characterization of a porous structure of hydrocarbon-bearing fields using methods of standard contact porosimetry and atomic force microscopy. Multidimensioned peculiarities of the fields and of the oil production process are also discussed. The model for calculation of concentration profiles of the suspended particles along the depth of the filtration bed in running time is developed. The model simulates the oil reservoir with retarded proppant particles. A good agreement between experimental and theoretical values of numerical concentration of the proppant particles as functions of coordinate and time is established. The model can be applied for evaluation of the dynamic porosity of rocks as well as for changing their specific permeability. The experimental data describing change in saturation of displacing fluid in the pore space of a rectangular porous block filled with more viscous fluid are obtained. The mathematical model of instability of a two-phase flow in the form of propagation of “fingers” of displacing fluid is investigated. Modeling results are compared with the experimental data. It is shown that the influence of interfacial forces must be taken into account for adequate description of two-phase filtration.

When justifying the remediation projects of aquifers from pollution, as a rule, field studies are in preference. In comparison with laboratory experiments, they cover a larger volume of rocks that contain groundwater, and therefore better reflect the average properties of the reservoir. The most important parameter determined in field trials is the porosity. It determines migration velocity of the contamination front and characterizes the amount of pollutants in the reservoir, etc. Depending on the objectives of the study, it can be determined under field conditions in different ways: injection of chemical or thermal indicators, or vice versa pumping or injection–pumping. There are examples of three studies presented below. In the first example, one well-experienced results of injection–pumping of clean water into the contaminated aquifer are presented. As a result, the active porosity of gravel and pebble reservoir with sand filling is equal to 0.15. The data obtained by the experience have been required to support the interception of contaminated flow. The second example shows the results of pumping heated water into the reservoir folded by fractured carbonate sediments (limestones and dolomites). The resulting value of the active porosity is 0.005. Field trial was needed to validate the use of groundwater for cooling of process equipment. The third example is related to the processing of man-made pumping oil. As a result, we obtained porosity of 0.39, residual water saturation of 0.05, and residual oil saturation of 0.25. The need to determine the parameters has been associated with the need to optimize the remediation works.

The current chapter is dedicated to the research on water state in food objects of various origins. It is shown that the bound water in such objects is not homogeneous. This phenomenon is of great importance for the food industry. Studies illustrating this problem in relation to several food products, such as various grains and its processing goods and sugar, including foodstuff based on those products, are given in this chapter. Modern analytical methods are used. Among them are NMR and DSC methods. Both methods have given a high correlation of results in the separating of bound water for “simply bound” and “strongly bound.” Based on the data provided by the research of water state in various products of plant origin, moisture sorption isotherms are obtained and a mathematical model, describing the moisture state in such products, is developed.

The interaction of soil and root system of plants is considered in this chapter. Particularly, the hierarchical structure of soils and soil agents, which stabilize it at different organization levels, pore classifications, depending on their size, origin, and functionality are presented. Regarding the plants, soil pores were shown to act as a medium, in which the root system is formed. The pores provide a transport of water and nutrients on the one hand and gas exchange on the other hand. The roots are involved to aggregation of soil particles, and the waste products of plant stabilize their shape. During growth and development of the root system, the waste products are the medium for the expansion of symbiotic and nonsymbiotic nitrogen-fixing bacteria. The structure of root and its constituent tissues has been considered. Special attention was paid to the porous structure of cell membranes, which carry out, in particular, accumulating and transporting functions. Mass transport in the soil–root system has been also analyzed. It is shown, that higher hydraulic conductivity of the roots in a comparison with the soil is the necessary condition to provide water maturation by the plants.

The section summarizes information about the structure of collagen—the basis of natural leather, which is characterized by a hierarchical organization of the main structural elements and chemical multifunctional nature. It is shown that the native collagen comprises the elements and pores of various sizes in the range from 1 nm to 200 μ forming micro-, meso-, and macro levels. In leather manufacture, collagen undergoes structural transformations because of an influence of a variety of chemical materials. This phenomenon contributes to the leather structure formation with the necessary complex of functional properties and due to porosity.

The current variety of filtration theories suggests that the porous medium and the fluid filling it form some continuous medium. That is, the elements of the porous medium-fluid system, though considered physically infinitesimal, however, are large enough compared to the size of pores and particles (grains, fibers) forming a porous medium. Averaged characteristics of the porous medium, which are introduced for mathematical description, may be sufficiently substantiated only for the volume with a large number of enclosed pores and particles. In terms of the elementary theory of filtration, the meaning of the solid skeleton of the porous medium is, above all, geometric—the skeleton limits the region of space in which the fluid moves. In more complex cases, we have seen strong interaction between the skeleton and the adjacent layers of the fluid. Therefore, the properties of the porous medium in the theory of filtration are usually described by a set of geometric averages. In this chapter, several cell models are considered for calculation of hydrodynamic permeability of porous media. It is assumed that porous media in general may consist of partially porous spherical or cylindrical particles. Different limiting cases are investigated and theoretical results are compared with experimental data.