Chapter 1
Introduction to Porous Structures and Additive Manufacturing

1.1 INTRODUCTION

Porous structures, from a physical standpoint, can be viewed as composites comprising base materials that confer porosity and the surrounding media that fill the interstitial spaces. In the context of this discourse, we will refer to the individual pores as "cells" and the spaces they create as "voids." The concept of porous structures finds its inspiration in nature, where numerous natural constructs, such as bones and cork, inherently exhibit porosity. This characteristic arises from the biological process of constructing these structures through the growth of single cells. These natural constructs exhibit a wide spectrum of properties, enabling them to adapt to applications.

For example, nature employs porosity to optimize the structure and function of various organisms. In avian biology, bird bones exhibit a unique porous structure, combining strength with weight reduction, essential for aerial locomotion. This intricate network of interconnected pores within the bone matrix not only minimizes weight but also enhances the bone's overall strength. The porous architecture distributes stress evenly, preventing fractures and maximizing the bone's load-bearing capacity [1, 2].

For plant structure, porosity plays a crucial role in the efficient transport of water and minerals throughout the plant body. Trees, in particular, rely on a complex porous structure of xylem vessels, which are characterized by their porous structure. These vessels, composed of dead, hollow cells, facilitate the upward movement of water from the roots to the leaves. The porous nature of the xylem vessels allows for capillary action, enabling water molecules to adhere to the vessel walls and be drawn upward against gravity. Additionally, the porosity of the xylem vessels provides a large surface area for the absorption of water and minerals, ensuring adequate nutrient supply to the entire plant.

Although the existence of porous structures dates back to ancient times, they have recently garnered significant attention, leading to the development of a novel class of materials in contemporary engineering. These materials offer a unique opportunity for achieving high performance relative to their weight, a characteristic highly sought after in advanced structural and other engineering applications.

By selectively designing the pore structure, it is possible to derive new desired properties from a base material that lacks such properties in its bulk form. This selective design essentially creates a composite of voids and solid materials. For instance, by manipulating the porosity, one can achieve materials with properties such as negative thermal expansion and apparent negative thermal conductivity. These properties can be tailored to specific applications, such as high-efficiency energy absorption [3, 4] and thermal insulation [5, 6].

Historically, the mastery of metals has significantly shaped human civilization, marking distinct epochs such as the Bronze Age and the Iron Age. The development and utilization of porous metal structures continue this legacy, presenting a modern frontier in material science and engineering.

As such, porous metals, in particular, represent a fascinating class of materials characterized by high surface area, high permeability, and tunable pore size and distribution. These attributes make them suitable for a diverse array of applications, including catalysis, filtration, energy storage, and biomedical engineering. Compared to polymers, porous metals exhibit superior mechanical stability, are lightweight, and have additional physical properties such as good electrical and thermal conductivities.

On the other hand, the high surface area of porous metals facilitates a greater number of chemical reactions at active sites, rendering them ideal for catalytic applications. In filtration, such as gas separation and water purification, the ability to tune pore size and distribution allows for the efficient separation of particles of different sizes. In energy storage, porous metals are used in batteries and supercapacitors due to their ability to provide high surface area and permeability, enhancing ion transport and storage capacity.

In biomedical applications, porous metals are employed for bone implants because their structure can promote bone tissue regeneration and integration.

How can porous materials exhibit extraordinary properties and characteristics that are absent in their bulk base materials? To address this question, it is essential to investigate the unique microstructural features of porous materials, including their geometry, connectivity, and distribution, which govern their ability to achieve such remarkable functionalities. Porous structures exhibit a hierarchical organization that spans multiple length scales, from the atomic to the macroscopic. At the atomic level, the arrangement of atoms and the types of chemical bonds play a crucial role in determining the intrinsic properties of the material. Moving to the microscale, the size, shape, and distribution of the pores significantly influence the mechanical, thermal, and transport properties. Macroscale considerations include the overall geometry and connectivity of the porous network, which can produce a new collective property.

The phenomenal properties of porous materials may include materials with a negative Poisson's ratio, also known as auxetic materials, which expand laterally when stretched, a behavior opposite to that of conventional materials. This property can be engineered by designing the pore structure according to a specific function, leading to applications in protective gear, medical devices, and flexible electronics.

Recently, the development of materials with the capacity to exhibit anomalous thermal behavior, such as apparent negative thermal conductivity, represents a significant advancement in the field of thermal management. These materials are engineered to manipulate heat flow in unconventional manners, allowing them to redirect thermal energy in ways that are not typically achievable with conventional materials. The redirection and dissipation of heat enable these materials to enhance the efficiency of cooling systems in electronic devices, ensuring that they operate within safe temperature ranges. Furthermore, these materials hold great potential for providing superior thermal insulation in extreme environments, such as space exploration or deep-sea applications, where traditional insulation methods may fall short.

The true industrial and engineering potential of porous metals began to be recognized in the early 20th century. The first commercially available porous metals were produced using sintered powder technology. This process involves compacting and heating metal powders to a temperature just below their melting point, allowing the particles to coalesce and form a solid mass with a network of interconnected pores. The unique properties of these sintered metals, including their high surface area and permeability, made them ideal for applications such as filters, batteries, and self-lubricating bearings. These early applications demonstrated the potential of porous metals to enhance performance in high-volume industrial processes, a promise that continues to be realized today in various fields.

Historically, the concept of porous metals is not a modern invention. The earliest known reference to man-made porous metals can be traced back to the work of Pliny the Elder in 77 AD, who documented a process known as granulation. This technique was notably used by Etruscan goldsmiths, who applied it to create intricate patterns and textures on jewelry, a process that involved the creation of fine, porous structures on the surface of metals [7, 8]. Although these early applications were primarily esthetic, serving decorative purposes in jewelry and religious artifacts, they laid the foundation for the understanding of porous metals and their potential applications 9..

The concept of metallic foams, which involves creating a metal structure with high porosity through the introduction of gas bubbles or other foaming agents, was first formally introduced in a French patent in 1925. Metallic foams offer a distinct combination of lightweight, high strength, and excellent energy absorption characteristics, making them suitable for applications ranging from lightweight structural components to impact protection and thermal insulation. However, despite the early conceptualization, the commercialization of metallic foams did not begin in earnest until the late 1950s in the United States. This delay was primarily due to the need for extensive research and development to understand and optimize the foaming processes and to tailor the properties of the foams to specific applications [10, 11].

During the initial wave of research and development that started extensively from the 1950s, significant efforts were made to explore the potential applications of metallic foams across various industries [12, 13]. This period saw the development of foaming techniques for different metals and alloys, as well as the investigation of their mechanical and thermal properties. The versatility of metallic foams became apparent, leading to their adoption in a range of applications, including lightweight panels for aerospace structures, energy-absorbing layers in automotive crash protection systems, and high-efficiency heat exchangers.

Moreover, porous...