Chapter 1

Introduction

Abstract

This book focuses on voids. All materials have voids; i.e., they are pervasive in all materials at some length scale. In this book we bring information from a number of different fields of study such as material science and engineering, physics, chemistry, and mechanics. We describe “different” types of voids such as intrinsic and intentional and highlight their commonalities; from processing and characterization to their effect in material properties. Along with theoretical exploration of voids we provide real-world applications of designed cellular solids.

Keywords

Cell structure; Closed cell; Defects; Foam; Free volume; Holes; Intentional void; Intrinsic void; Open cell; Reinforced void; Syntactic foam; Unreinforced void; Voids

1.1. Overview

So why write a book just on voids as the topic? The answer is simple and twofold—first, the juxtaposition of “empty space” adjacent to solid material seemed, to us, an interesting dichotomy. Second, depending on your perspective or desired outcome, voids can limit or enhance the performance of materials. Is your target a consolidated material or a foam? If your target is a foam, how do material properties change with the type and amount of voids; open cell versus closed cell, the mean size, size distribution, volume fraction of voids, etc.? If your target is a consolidated material, some important questions might be how do properties change with the volume percent, location, geometry, and size of voids? Another interesting question is how do those voids, on the subnanometer and nanometer scale, which are typically not characterized by foam researchers, play into the final properties. Voids are also very important to understand from a practical engineering standpoint. Even in the most highly engineered densified materials, defects, such as voids, will limit the design of real structures. So along with the theoretical exploration of voids in materials, this book will give examples of these real-world applications so the reader becomes aware of their prevalence in structures all around them. This book explores such dichotomies; solid versus empty and desired versus undesired aspects of voids and materials. Furthermore, this book sheds light on a “middle ground” of the smart use of voids to help in the optimization of part performance. By middle ground we mean a neutral look at the impact voids have on material/parts and use of voids as a design parameter for optimizing performance in multifunctional materials. There is much published work available on foams (Gibson and Ashby, 1997; Shutov, 2004). Even more numerous are those that provide a passing mention of voids when they are incidental/unwanted during the fabrication of nominally dense materials. This book, however, is not just about foams or residual porosity in materials, important though these contributions are; instead it focuses on the void itself. The fact is that all materials have voids; i.e., they are pervasive in all materials at some length scale. So in addition to voids in foams, this book brings in information from a number of different fields of study such as material science and engineering, physics and chemistry of materials, and mechanics of materials. This book treats all of these “different” types of voids equally and highlights their commonalities in all aspects; from processing, formation, and characterization to the resulting material properties. For the purpose of this book, a void has two essential properties, it must be (1) a volume measured in cube of some unit of length and (2) occupied by a vacuum or gas (i.e., solid/liquid materials are absent). In general, there is no size or shape requirement on a void; so they range form subnanometers to millimeters, sometimes even larger, in equivalent diameter. We should make it clear that voids in a liquid and gaseous medium will not be covered in this book. We devote the rest of this chapter to a general discussion on voids.

1.2. Descriptions

1.2.1. Intrinsic and Intentional Voids

Intrinsic voids appear in materials because of inherent structure, natural processes, processing limitations, and/or aging in service environments. At some length scale all real materials have intrinsic voids. At the atomic level, if we examine the Bohr atomic model, we see that most of the volume occupied by an atom is empty space. We will not be going into details of the Bohr model in this book but it is important to mention as an introduction. A general chemistry text is sufficient to review the general structure of atoms. When voids are thought of as defects, they are viewed as having a detrimental effect on material properties. However, some defects can be beneficial and are essential to specific material behavior, such as color centers and semiconducting properties. The intrinsic voids generally range from 10?15 to 10?3 m; examples include atomic vacancies, free volume, lattice holes, and process induced porosity. Intentional voids are incorporated by design into a solid material. Such materials are usually, but not always, referred to as foams. This is especially true when the voids are on a micrometer scale. In the early twenty first century, technology has evolved to an extent that we can control voids in the nanometer range. Materials with intentional voids can be classified in many ways. Some important examples are single-phase foams, composite and syntactic foams (Gladysz and Chawla, 2002). There are many ways to introduce an intentional void into a material, therefore the method of introducing these voids is highly dependent on the type of material they are introduced into as well as the desired properties needed in the finished material. Details of these processes will be covered in Chapters 4–6.
Figure 1.1 A hollow glass sphere illustrating nanometer scale intrinsic voids caused by processing and the intentional void, the hollow core of the sphere. When we discuss intentional voids in a material, it is important to remember that the intrinsic voids on the atomic and/or nanometer length scale may still be present. Figure 1.1 is an example of an intrinsic void in the wall of a hollow (intentional void) glass microspheres formed during the spray drying formation process. The hollow core of the sphere is in the micrometer range and the unintentional voids in the shell are in nanometers/submicrometers. This intrinsic void weakens the shell of the sphere and can lead to a premature failure during service. Independent of the material, voids can be categorized into reinforced or unreinforced and open or closed cell. We will discuss the concepts of open cell versus closed cell and reinforced versus unreinforced in Sections 1.2.2 and 1.2.3, respectively.

1.2.2. Closed and Open Cell

Whether discussing cells in foam or just general porosity, they are simply voids dispersed in a solid phase. Cells are made up of struts and faces, as shown schematically in Fig. 1.2, that surround the void space. In a closed cell, the face of the cell wall consists of a continuous solid phase. In an open cell material, a part of that wall is missing. In an open cell foam, see Fig. 1.3(a) and (b), gas can freely flow in and out of the cells when the structure is compressed or extended. Because the cell faces are discontinuous, these materials typically have a lower modulus and strength than closed cell foams. The reticulated (meaning weblike) foam in Fig. 1.3 is an extreme example of an open cell foam as it is composed entirely of struts without faces. This material is a candidate for an electrode in microbial fuel cell (Lepage et al., 2012). There are some general conditions needed for a material to be open cell. According to Shutov (Shutov, 2004), the following two criteria must be met for a predominantly open cell structure:
Figure 1.2 An idealized structure of a cell consisting of struts and faces.
Figure 1.3 Examples of the structure (a) low magnification and (b) higher magnification of open cell reticulated foam (Lepage et al., 2012) (c) a closed cell silicone foam. 1. Each polygonal cell must have at least two discontinuous or broken faces. 2. An overwhelming majority of the cell struts must be shared by at least three cells. From the above criteria it is clear that the physical structure of open cell foams and resulting properties can vary widely. We discuss the structure–property relationships in more detail in Chapter 6. In general, open cell foams exhibit good absorption capacity for water and good acoustic damping properties (Zhang et al., 2012) compared to closed cell foams. In closed cell foams (Fig. 1.3(c)) the faces are continuous, which leaves gases inside individual cells isolated from the surrounding cells. Because the faces are intact, closed cell foams typically have higher strength and modulus than open cell foams. In addition to superior mechanical properties, they are used extensively for their insulating properties (Jelle, 2011), because the air trapped in the cells is a good insulator.

1.2.3. Unreinforced and Reinforced Voids

Unreinforced voids...