Fundamentals of Ceramics

Michel Barsoum

• Verlag: CRC Press
• Jahr: 2019
• ISBN:
  9781000397956
• Auflage: 2
• Thema: Ciencias físicas
• Thema: Física

Fundamentals of Ceramics presents readers with an exceptionally clear and comprehensive introduction to ceramic science. This Second Edition updates problems and adds more worked examples, as well as adding new chapter sections on Computational Materials Science and Case Studies.

The Computational Materials Science sections describe how today density functional theory and molecular dynamics calculations can shed valuable light on properties, especially ones that are not easy to measure or visualize otherwise such as surface energies, elastic constants, point defect energies, phonon modes, etc. The Case Studies sections focus more on applications, such as solid oxide fuel cells, optical fibers, alumina forming materials, ultra-strong and thin glasses, glass-ceramics, strong and tough ceramics, fiber-reinforced ceramic matrix composites, thermal barrier coatings, the space shuttle tiles, electrochemical impedance spectroscopy, two-dimensional solids, field-assisted and microwave sintering, colossal magnetoresistance, among others.

1

Introduction

All that is, at all

Lasts ever, past recall,

Earth changes,

But thy soul and God stand sure,

Time’s wheel runs back or stops:

Potter and clay endure.

Robert Browning

1.1 Introduction

The universe is made up of elements, which in turn consist of neutrons, protons and electrons. There are roughly 100 elements, each possessing a unique electronic configuration determined by their atomic number, Z, and the spatial distribution and energies of their electrons. What determines the latter requires some understanding of quantum mechanics and is discussed in greater detail in the next chapter.

One of the major triumphs of quantum theory was a rational explanation of the periodic table (see inside front cover) of the elements that had been determined from experimental observation long before the advent of quantum mechanics. The periodic table places the elements in horizontal rows of increasing atomic number and vertical columns or groups, so that all elements in a group display similar chemical properties. For instance, all elements of group 17, known as halides, exist as diatomic gases characterized by very high reactivity. Conversely, the elements of group 18, the noble gases, are monoatomic and are chemically extremely inert.

A large fraction of the elements are solids at room temperature, and because they are shiny, ductile, and good electrical and thermal conductors, they are considered metals. A fraction of the elements—most notably, N, O, H, the halides and the noble gases—are gases at room temperature. The remaining elements are predominantly covalently bonded solids that, at room temperature, are either insulators (B, P, S, C¹) or semiconductors (Si, Ge). These elements, are typically referred to as metalloids.

Few elements are used in their pure form; most often they are alloyed with other elements to create engineering materials. The latter can be broadly classified as metals, polymers, semiconductors or ceramics, with each class having distinctive properties that reflect the differences in the nature of their bonding.

In metals, the bonding is predominantly metallic, where delocalized electrons provide the “glue” that holds the positive ion cores together. This delocalization of the bonding electrons has far-reaching ramifications since it is responsible for properties most associated with metals: ductility, thermal and electrical conductivity, reflectivity and other distinctive properties.

Polymers consist of very long, for the most part, C-based chains, to which other organic atoms (for example, C, H, N, Cl, F) and molecules are attached. The bonding within the chains is strong, directional and covalent, while the bonding between chains is relatively weak. Thus, the properties of polymers, as a class, are dictated by the weaker bonds, and consequently they possess lower melting points, have higher thermal expansion coefficients and are less stiff than most metals or ceramics.

Semiconductors are covalently bonded solids that, in addition to Si and Ge, already mentioned, include GaAs, CdTe and InP, among many others. The usually strong covalent bonds holding semiconductors together render their mechanical properties similar to those of ceramics (i.e., brittle and hard).

Now that these distinctions have been made, it is possible to answer the nontrivial question: What is a ceramic?

1.2 Definition of Ceramics

Ceramics can be defined as solid compounds that are formed by the application of heat, and sometimes heat and pressure, comprising at least two elements provided one of them is a non-metal or a metalloid. The other element(s) may be a metal(s) or another metalloid(s). A somewhat simpler definition was given by Kingery, who defined ceramics as “the art and science of making and using solid articles, which have, as their essential component, and are composed in large part of inorganic, nonmetallic materials”. In other words, what is neither a metal, a semiconductor or a polymer is a ceramic.

To illustrate, consider the following examples: Magnesia,² or MgO, is a ceramic since it is a solid compound of a metal, Mg, bonded to the nonmetal, oxygen, O. Silica is also a ceramic since it combines a metalloid, Si, with a nonmetal. Similarly, TiC and ZrB₂ are ceramics since they combine metals (Ti, Zr) and a metalloid (C, B). SiC is a ceramic because it combines two metalloids. Ceramics are not limited to binary compounds: BaTiO₃, YBa₂Cu₃O₃, and Ti₃SiC₂ are all perfectly respectable class members.

It follows that the oxides, nitrides, borides, carbides, and silicides (not to be confused with silicates) of all metals and metalloids are ceramics; which, needless to say, leads to a large number of compounds. This number becomes even more daunting when it is appreciated that the silicates are also, by definition, ceramics. Because of the abundance of oxygen and silicon in nature, silicates are ubiquitous; rocks, dust, clay, mud, mountains, sand, in short, the vast majority of the earth’s crust is composed of silicate-based minerals. When it is also appreciated that cement, bricks, and concrete are essentially silicates, a case could be made that we live in a ceramic world.

In addition to their ubiquitousness, silicates were singled out above for another reason, namely, as the distinguishing chemistry between traditional and modern ceramics. Before that distinction is made, however, it is important to briefly explore how atoms are arranged in three dimensions.

1.2.1 Crystalline versus Amorphous Solids

The arrangement of atoms in solids, in general, and ceramics, in particular, will exhibit long-range order, only short-range order, or a combination of both.³ Solids that exhibit long-range order⁴ are referred to as crystalline, while those in which that periodicity is lacking are known as amorphous, glassy or noncrystalline solids.

The difference between the two is illustrated schematically in Fig. 1.1. From the figure, it is obvious that a solid possesses long-range order when the atoms repeat with a periodicity that is much greater than the bond lengths or the distance between the atoms. Most metals and ceramics, with the exception of glasses and glass-ceramics (see Chap. 9), are crystalline.

FIGURE 1.1 (a) Long-range order; (b) short-range order in silica.

Since, as discussed throughout this book, the details of the lattice patterns can strongly influence the macroscopic properties of ceramics, it is imperative to understand the rudiments of crystallography.

1.3 Elementary Crystallography

As noted above, long-range order requires that atoms be arrayed in a three-dimensional (3D) pattern that repeats. The simplest way to describe a pattern is to describe a unit cell within that pattern. A unit cell is defined as the smallest region in space that, when repeated, completely describes the 3D pattern of atoms in a crystal. Geometrically, it can be shown that there are only seven unit cell shapes, or crystal systems, that can be stacked together to fill three-dimensional space. The seven systems, shown in Fig. 1.2, are cubic, tetragonal, orthorhombic, rhombohedral, hexagonal, monoclinic and triclinic. These systems are distinguished from one another by the lengths of the unit cell edges and the angles between the edges, collectively known as the lattice parameters or lattice constants (a, b, c, α, β and γ in Fig. 1.2).

FIGURE 1.2 Geometric characteristics of 7 crystal systems and 14 Bravais lattices.

It is useful to think of a given crystal system as a “brick” of a certain shape. For example, the bricks can be cubes, hexagons, parallelepipeds, etc. And while the shape of the bricks is an important descriptor of a crystal structure, it is insufficient. In addition to the brick shape, it is important to know the symmetry of the lattice pattern within each brick, as well as the actual location of the atoms on these lattice sites. Only then would the description be complete.

It turns out that if one considers only the symmetry within each unit cell, the number of possible permutations is limited to 14. The 14 arrangements, shown in Fig. 1.2, are also known as the Bravais lattices. A lattice can be defined as an indefini...

Inhaltsverzeichnis

• Cover
• Half-Title
• Series
• Title
• Copyright
• Dedication
• Contents
• Series Preface
• Preface to the Second Edition
• Preface to First Edition
• Author
• 1 Introduction
• 2 Bonding in Ceramics
• 3 Structure of Ceramics
• 4 Effect of Chemical Forces on Physical Properties
• 5 Thermodynamic and Kinetic Considerations
• 6 Defects in Ceramics
• 7 Diffusion and Electrical Conductivity
• 8 Phase Equilibria
• 9 Formation, Structure and Properties of Glasses
• 10 Sintering and Grain Growth
• 11 Mechanical Properties: Fast Fracture
• 12 Creep, Subcritical Crack Growth and Fatigue
• 13 Thermal Properties
• 14 Linear Dielectric Properties
• 15 Magnetic and Nonlinear Dielectric Properties
• 16 Optical Properties
• Index