Physics of Solids

A crystalline solid has its atoms, ions or molecules arranged in a regular, repeating pattern; an amorphous solid does not. This is the distinction the papers lean on most often, because it decides the rest of the chapter. The repeating unit of a crystal is the unit cell, the smallest block that tiles to build the whole lattice. Because the arrangement repeats in fixed directions, many crystalline properties depend on direction: the coefficient of thermal expansion, for instance, is one of the properties that changes with direction in a crystal, while qualities such as viscosity do not behave that way. There are seven crystal systems, sorted by the geometry of the arrangement. A worked example of a crystal is ruby, which is aluminium oxide with a trace of chromium, $\text{Al}_2\text{O}_3:\text{Cr}$. An amorphous solid such as Teflon is used to coat engine parts and cooking utensils because of its low friction and its stability at higher temperatures; when any solid is heated, its molecules vibrate more rapidly rather than becoming denser.

Running alongside the structure is a small vocabulary that the papers test directly. Elasticity is the property that lets a body return to its original shape once the stress is removed. Ductility is the ability to deform plastically before breaking. Brittleness is breaking with little or no plastic deformation. Strength is the maximum stress a solid can withstand before it breaks. Stiffness is a different idea again: it is resistance to deformation, and it is measured by Young's modulus, not by how much load a specimen survives.