Porous Materials

Processing and Applications

Peisheng Liu, Guo-Feng Chen

• Publisher: Butterworth-Heinemann
• Year: 2014
• ISBN:
  9780124078376
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

Engineers and scientists alike will find this book to be an excellent introduction to the topic of porous materials, in particular the three main groups of porous materials: porous metals, porous ceramics, and polymer foams. Beginning with a general introduction to porous materials, the next six chapters focus on the processing and applications of each of the three main materials groups. The book includes such new processes as gel-casting and freeze-drying for porous ceramics and self-propagating high temperature synthesis (SHS) for porous metals. The applications discussed are relevant to a wide number of fields and industries, including aerospace, energy, transportation, construction, electronics, biomedical and others. The book concludes with a chapter on characterization methods for some basic parameters of porous materials. Porous Materials: Processing and Applications is an excellent resource for academic and industrial researchers in porous materials, as well as for upper-level undergraduate and graduate students in materials science and engineering, physics, chemistry, mechanics, metallurgy, and related specialties.

• A comprehensive overview of processing and applications of porous materials – provides younger researchers, engineers and students with the best introduction to this class of materials
• Includes three full chapters on modern applications - one for each of the three main groups of porous materials
• Introduces readers to several characterization methods for porous materials, including methods for characterizing pore size, thermal conductivity, electrical resistivity and specific surface area

Chapter One

General Introduction to Porous Materials

Abstract

Because porous materials are a new type of engineering materials, people are not generally aware of them. But they are worthy of note due to their valuable attributes of function and structure. This chapter gives a brief overview of the topic so that readers might have a rudimental understanding of it. Some primary concepts are introduced for porous materials, including porous metals, porous ceramics, and polymer foams, and their main groups and chief features.

Keywords

Porous materials

Primary concepts

Porous metals

Porous ceramics

Polymer foams

Porous materials widely exist around us and play a role in many aspects of our daily lives; among the fields they can be found in are energy management, vibration suppression, heat insulation, sound absorption, and fluid filtration. Highly porous solids have relatively high structural rigidity and low density of mass, so porous solids often serve as structural bodies in nature, including in wood and bones [1,2]; but human beings use porous materials more functionally than structurally, and develop many structural and functional integrative applications that use these materials fully [3,4]. This chapter will introduce the elementary concepts and features of this kind of material.

1.1 Elementary Concepts for Porous Materials

Just as their name implies, porous materials contain many pores. Porous solids are made of a continuously solid phase that forms the basic porous frame and a fluid phase that forms the pores in the solid. The latter can consist of gas, when there is a gaseous medium in the pore, or of liquid, when there is a liquid medium in the pore.

In that case, can all materials with pores be referred to as porous? Perhaps surprisingly, the answer is “no.” For instance, holes and crannies that are the result of defects will lower a material’s performance. This result is not what designers want, and so these materials cannot be termed porous. So-called porous materials must possess two essential characteristics: one is that the material contains a lot of pores, and the other is that the pores are designed specifically to achieve the expectant index of the material’s performance. Thus, the pore of porous materials may be thought as a functional phase what designers and users hope to come forth within the material, and it supplies an optimizing action for the performance of the material.

1.2 Main Groups of Porous Materials

The number of pores (i.e., porosity) will vary for different porous materials. Porous materials can be classified as low porosity, middle porosity, or high porosity based on the number of pores. Generally, porous materials with low and middle porosity have closed pores (Figure 1.1) which behave like a phase of impurity. For porous materials with high porosity (Figures 1.2–1.4), there are two different cases according to various morphologies of the pore and the continuous solid phase. In the first case, the continuous solid constructs a two-dimensional array of polygons; the pore is isolated in space, taking on polygonal columniations accordingly; and the cross-sectional shape of the pore is commonly triangle, quadrangle, or hexagon (Figure 1.2). This structure looks similar to the hexagonal cell of a honeycomb, and such two-dimensional porous materials are called honeycomb materials. Porous materials with directional pores [5], which are called lotus-type porous materials, have a similar structure as honeycomb materials, but the cross-sectional shape of the pores for these materials is circular or elliptic, and the pore often cannot run through it, resulting in less uniformity of distribution and a lower density of the array. In the second case, the continuous solid presents a three-dimensional reticulated structure (Figure 1.3), and such porous materials can be termed three-dimensional reticulated foamed materials. These materials have connective pores that are of a typical open-cell structure. In the third case, the continuous solid shows the cell wall structure of pores of sphericity, elliptical sphericity, or polyhedron shape (Figure 1.4), and such three-dimensional porous materials can be called bubblelike foamed materials. Within these materials, the cell wall may separate many isolated closed pores or cells, forming a closed-cell, bubblelike foamed substance (Figure 1.4a). The cell wall may make up open-cell, bubblelike foamed material as well (Figure 1.4b). In the literature, three-dimensional, reticulated foamed materials are referred to as “open-cell foamed materials,” closed-cell, bubblelike foamed materials are called “closed-cell foamed materials,” and open-cell, bubblelike foamed materials are “half open-cell foamed materials.”

Figure 1.1 Porous composite oxide ceramics, which is a low-porosity material, shown as a cross-sectional image.

Figure 1.2 Two-dimensional honeycomb materials: (a) conductive honeycomb TiC ceramics with quasi-square pores [6]; (b) thermal storage of honeycomb ceramics with square pores (with dimensions of 100 mm × 100 mm × 100 mm, cell-wall thickness of 1 mm, and square-pore side length of 2.5 mm) [7].

Figure 1.3 Three-dimensional reticulated foamed materials: (a) nickel foam; (b) iron foam.

Figure 1.4 Bubblelike foamed materials: (a) a closed-cell bubblelike foamed material of aluminum foam [8]; (b) an open-cell bubblelike foamed material of iron foam.

Porous solids include two types of porous bodies (i.e., natural and artificial). Natural porous solids can be found universally [1], such as bones that support the bodies and limbs of animals and human beings (see Figure 1.5), plant leaves, wood, sponge, coral (Figure 1.6), pumice (Figure 1.7), and lava (Figure 1.8). Lava is a sort of natural porous material that can be used in construction or for creating artwork (Figure 1.9). It is not accurate to refer to the natural, porous solids of living animal bones and tree trunks as “natural porous materials.” However, when a tree is cut down to make materials used by human beings to make things like furniture, it becomes natural porous materials. The fluid phase contained in the pores of plant leaves and living tree trunks always consists of liquid (namely sap), while that within artificial porous materials is mostly gas. Artificial porous materials can be subclassified further into porous metals, porous ceramics, and polymer foams.

Figure 1.5 Cross-sectional view of a reticulated porous bone of a whale.

Figure 1.6 An optical photograph showing the porous morphology of coral.

Figure 1.7 An image showing the porous morphology of pumice.

Figure 1.8 Cross-sectional view of the porous morphology of lava.

Figure 1.9 A vase made of lava.

1.3 Porous Metals

Porous metals are a relatively new class of engineering materials that can serve functional and structural purposes [9–11]. They have undergone rapid development over the last thirty years. These lightweight materials not only have the typical characteristics of metals (weldability, electrical conductivity, and ductibility), but also possess other useful characteristics, such as low bulk density, great specific surface area, low thermal conductivity, good penetrability, energy management, mechanical damping, vibration suppression, sound absorption, noise attenuation, and electromagnetic shielding. Consequently, these materials have increasing applications, and have emerged as a focus of great attention in the international material field [12]. The next sections describe the main characteristics of these types of metals [11,13–15].

1.3.1 Powder-Sintering Type

The powder-sinte...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• About the Authors
• Preface
• Chapter One: General Introduction to Porous Materials
• Chapter Two: Making Porous Metals
• Chapter Three: Application of Porous Metals
• Chapter Four: Special Porous Metals
• Chapter Five: Fabricating Porous Ceramics
• Chapter Six: Applications of Porous Ceramics
• Chapter Seven: Producing Polymer Foams
• Chapter Eight: Applications of Polymer Foams
• Chapter Nine: Characterization Methods: Basic Factors
• Chapter Ten: Characterization Methods: Physical Properties
• Index