Type of crystal

Some properties of crystals depend on the binding force that binds molecules into solids. These forces usually involve the interaction of the outermost electrons (or valence electrons) of atoms or molecules. If the binding force is strong, the crystal has a higher melting point. If they are weaker, the crystal will have a lower melting point, and may be more prone to bending and deformation. If they are very weak, crystals can only be formed at very low temperatures, at which time the available energy of molecules is not much. There are four main crystal bonds. Ionic crystals are composed of positive ions and negative ions, which are bound together by the gravitational force (ionic bond) between different charges. Sodium chloride is an example of ionic crystals. The atoms or molecules of an atomic crystal (covalent crystal) share their valence electrons (covalent bonds). Diamond, germanium and silicon are important covalent crystals. Metal crystals are metal atoms that turn into ions and are surrounded by free valence electrons. They can easily move from one atom to another and can be vividly described as being immersed in the ocean of free electrons (gold bonds). When these electrons all move in the same direction, their motion is called current. The molecules of molecular crystals do not share their electrons at all. Their binding is due to a slight change in the electric field from one end of the molecule to the other. Because the binding force is very weak (van der Waals force and hydrogen bond), these crystals melt at very low temperatures and have very low hardness. Typical molecular crystals are solid oxygen and ice. In an ionic crystal, electrons are transferred from one atom to another. The atoms of a covalent crystal share their valence electrons. A metal atom has a small negative charge at one end and a small positive charge at the other end. A weak electric force holds the molecules in place. One of the technologies used to make crystals for industrial use is to grow from molten liquid. Seed crystals can be used to promote the formation of single crystals. In this process, the seed crystal falls into the container containing the molten material. When the molten liquid around the seed crystal cools, its molecules attach to the seed crystal. These new crystal molecules follow the orientation of seed crystal to form a large single crystal. The basic component of sapphire and ruby is aluminum oxide, which has a high melting point. It is difficult to make a container to contain its molten liquid. Synthetic sapphires and rubies are made by the Werner method (flame melting method), that is, aluminum oxide powder and a small amount of titanium, iron or chromium powder for coloring are dropped onto the seed crystal through the flame. The flame melts the powder and then recrystallizes on the seed crystal. The growth of artificial diamonds requires a temperature higher than 1600 ℃ and 60000 times atmospheric pressure. Synthetic diamond sands are small and black, so they are suitable for industrial applications. The zone melting process is used to purify silicon crystals in the semiconductor industry. A single crystal is suspended vertically on the top of the silicon rod. When heated at the contact between the two, the top of the bar melts and recrystallizes on a single crystal, and then slowly move the heated point down the bar.