Materials Science for Electrical and Electronic Engineers PDF

Introduction

This book is about the materials which are used by electrical and electronic engineers. I will be dealing with their electrical properties and with their mechanical properties. Their electrical properties are why an electrical or electronic engineer might wish to use them, but their mechanical properties control how they are used and how they are made. We shall see that both electrical and mechanical properties depend very directly on the bonding between the atoms in the material. Nearly all the materials described in this book are solids. It is the bonds between the atoms in a solid which hold it together. Bonds are of different types (three strong, two weak in fact). The type of bond which forms between atoms in a solid depends on which atoms are present.

Take copper, for example. Copper is a metal, and the most commonly used electrical conductor. It is easily bent or squashed and so when things are made from copper they are usually made by working (rolling, or extrusion, or wiredrawing, etc.). Copper is also an element—that is, all the atoms in a piece of copper are of the same type: they are copper atoms. They are held together by a type of bond called (not surprisingly) the metallic bond. Now consider a common insulator: alumina. Alumina is aluminium oxide, a compound of aluminium and oxygen and so alumina contains two types of atoms: aluminium atoms and oxygen atoms. These are held together by ionic bonds. Anyone who has handled alumina knows that it is a hard brittle substance whose shape cannot be changed (except by breaking pieces off it). Alumina insulators have to be made in their final shape (precisely how will come much later in this book—in Chapter 7).

A third and final example. Silicon, like copper, is an element. A piece of silicon contains just silicon atoms. Silicon is very different from copper, however. The atoms are bound together via the covalent bond. Silicon is a poor conductor of electricity (it is called a semiconductor: halfway between a conductor and an insulator). It is also very brittle. As with alumina, articles made from silicon have to be made in their final shape.

We will discover later on that it is possible, however, to make flexible insulators, by exploiting especially weak bonds in plastics.

Why? Why do copper atoms join together via metallic bonding to form a metal, whereas silicon atoms join together via covalent bonds to form a semiconductor? Why do aluminium atoms and oxygen atoms join together to form an insulator? Why can we draw wires from copper, but ionically bonded alumina and covalently bonded silicon will not, cannot, suffer a shape change?

The answer to all of these questions lies in the bonding between the atoms. To understand why this is so, and how and why bonds form, we need to understand a little bit about the structure of the atom. I will start this chapter, therefore, by describing how atoms are constructed.