Ceramic materials often are used as electrical insulators. They are largely clay, and are far less conductive than metals. Ceramic insulators are used in many types of circuits to block currents or protect delicate sections from overheating. Ceramic coatings also are used in larger applications to protect metal against heat transfer and unwanted current. Many insulation properties of ceramics depend on how the ceramic material is made and what compounds are used to make it.
Dielectric is a term used to describe objects like ceramic materials that can allow an electromagnetic field while not conducting the current. This property makes ceramic materials useful in many electrical applications. Dielectric strength is a term used to describe how well materials like ceramics can withstand high voltages.
Ionic covalent bonding
Many materials tend to have either ionic or covalent bonding, but the atoms of ceramic materials are held by both types of bonding (hence "dielectric"). Not only does this make ceramic material harder than metal, but it also means ceramic materials have few if any free electrons. Materials without free electrons cannot conduct electricity. The bonding properties make ceramics good insulators.
Not all ceramic materials are designed specifically to be insulators. Some manufacturers doctor ceramics with additional elements to make them into semiconductors. Zinc oxide, for instance, is used to make ceramics into varistors, or variable resistors. These are resistors that still resist electrical current, but not in a linear manner, so the ceramics can conduct up to a certain point but then protect a system against more powerful electric shock.
Rarer types of ceramics are made into superconductors, or conductors that have almost no electrical resistance. These super conductors are made out of rare combinations of materials, such as Lanthanum (yttrium)-barium-copper ceramics, which act as a superconductor up to 138 Kelvin, higher than many other superconductors.
Thermal insulation properties
The same dual-bond structure that makes normal (or "fine") ceramics so resistant to electrical current also make the materials resistant to thermal conductivity, or the passage of heat. This allows ceramics to serve as useful thermal insulators, which is why ceramic coatings are used in various types of engines. Ceramics have a high melting point and natural heat resistance.