Here we provide some insight into the fascinating and varied structures of solids as background for the discussions of the many applications in the chapters of Part Five. We start with the atoms of the elements as the basic building blocks of all substances. Atoms of some elements will chemically combine (as we've discussed) in definite ratios to form the molecules of compounds, like aluminum and oxygen combining to form aluminum oxide, Al2O3, where two aluminum atoms chemically bond together with three oxygen atoms.
Most solids are crystalline. They have atoms or molecules that stack themselves in neat three-dimensional lattices called crystals, like a box of carefully, tightly packed oranges all of the same size. Quartz, which may have the same chemical composition as glass, is crystalline. Diamonds are crystals of carbon atoms. Sapphires and rubies are crystals of Al2O3 molecules, crystals that take on different colors depending on the presence of a relatively small number of atoms of other elements as impurities. Gems are cut or cleaved from large single crystals, which have one stacking order and one stacking orientation throughout.
Often substances are polycrystalline, consisting of millions of small crystalline grains that have grown together as a molten material solidified, with each grain having the same internal stacking of atoms, but with each grain having a different orientation.
In some cases the atoms are bonded in some way to their nearest neighbors, often forming hard crystals that tend to be electrical insulators. Metals (composed of just one metallic element) are held together in a metallic bond as solids or liquids, that is, by a “sea” of electrons spread out evenly but not attached to any particular atom, though attracted to the nuclei in their vicinity. Because in many solid metals the stacked layers of atoms can easily shear over each other (like the cards in deck of playing cards), these metals are malleable and ductile and can be rolled, drawn, or hammered into various shapes. When this is done, structural defects are created within the grains which prevent the shearing, and thereby work-harden the metal. (If you've ever bent a metal clothes hanger back and forth until it hardened, became brittle, and broke, you've had a demonstration of work-hardening.) The metal can then be annealed by heating it to a temperature where (without any melting) new grains form and grow to larger sizes, gathering atoms from the old grains at the boundaries between them and shifting the boundaries in the process. In this way, the older grains with the defects lose atoms to the growing new grains and eventually disappear, producing a relatively soft material once again.
The molecules in crystalline compound solids not only are neatly stacked but also tend to be bonded together molecule to molecule, producing hard, brittle substances.
Other solids are amorphous: the atoms or molecules comprising the solid have no particular stacking order. They are like very thick viscous liquids, so viscous that they tend to retain their shape. Glass is a good example. But, if we were to wait long enough, panes of glass would eventually, very slowly, flow like liquids down into a puddle.
Solids, liquids, and gases are different phases of a substance. The atoms or molecules of some solid substances can stack themselves in two or more different geometrical arrays. We say that these different stackings are different solid phases of that substance. Solids having different stacking orders are distinctly different phases of a substance, just as are melted liquid phases or evaporated gas phases. (I hope that you are unfazed by all of this!)
The atoms of more than one metallic element can often be combined into what is called an alloy. This is often accomplished by melting the elements together to form a solution (like sugar dissolved in water), which in the case of an alloy sometimes solidifies as a single-phase solid solution but often solidifies in such a way that grains of two or more different solid-solution phases are produced, each phase having its own separate proportions of the different elements and its own form of stacking. Sometimes, if the alloy is cooled fast enough from the melt or anneal, these second phases don't have time to form but can be caused to appear during later heat treating (heating but not melting) of the alloy. Bronze, for example, is an alloy of copper, tin, and zinc. Sometimes an alloy contains nonmetallic elements, and some of the phases can then also include a stacking of molecules (much like the stacking of molecules of the compound Al2O3 described earlier). Steels, for example, are alloys of iron and carbon and/or other elements (e.g., chromium and nickel for stainless steels).
For just a moment, I will discuss steel to illustrate some of the sophistication of modern man-made materials. Steels typically contain several different finely intermingled solid phases, each of a different chemical composition depending on the initial overall chemical composition of the steel alloy, the history of its melting, the rate of cooling to a solid, and then the heat treating to cause different phases to form. So it is that metallurgists have, for example, been able to create steels that have malleable phases that can be easily rolled into thin sheets and then cut into the form of razor blades, only then to provide a heat treatment that precipitates out finely divided solid phases within the original phases. These precipitates (like nails driven through a deck of cards to prevent the cards from sliding [shearing] over each other) make the overall alloy hard and stiff. For example, razor blades are made from steel that is soft when it is rolled into thin sheets and cut to make the blades, but then that same steel is hardened with a heat treatment that makes it easy to sharpen the blades by grinding and makes them slow to wear out and become dull.
Some phase changes can even take place suddenly at low temperatures without heating (by the shearing of one type of crystal lattice to form another). And so it was that some of our Liberty ships carrying cargo during WWII were found to break apart, because of unanticipated phase changes, when their hulls (of the wrong composition of steel) became exposed to the frigid cold of the North Atlantic and transformed into brittle materials.
In the next chapter we'll consider the electrical properties of solids. It is often both the electrical and mechanical properties that need to be considered in the invention of new materials and devices, as described in Part Five of this book, soon to follow.