Ceramics have characteristics that enable them to be used in a wide variety of applications including:

high heat capacity and low heat conductance

corrosion resistance

electrically insulating, semiconducting, or superconducting

nonmagnetic and magnetic

hard and strong, but brittle

The diversity in their properties stems from their bonding and crystal structures.

Ceramics are probably best known as electrical insulators. Some ceramic insulators (such as BaTiO3) can be polarized and used as capacitors. Other ceramics conduct electrons when a threshold energy is reached, and are thus called semiconductors. In 1986, a new class of ceramics was discovered, the high Tc superconductors. These materials conduct electricity with essentially zero resistance. Finally, ceramics known as piezoelectrics can generate an electrical response to a mechanical force or vice versa.Table 3: Electrical Resistivity of different materials.Type

Material

Resistivity ( -cm)

Metallic conductors:

Copper

1.7 x 10-6

CuO2

3 x 10-5

Semiconductors:

SiC

10

Germanium

40

Insulators:

Fire-clay brick

108

Si3N4

> 1014

Polystyrene

1018

Superconductors:

YBa2Cu3O7-x

< 10-22 (below Tc)

Anyone who has used a portable cassette player, personal computer, or other electronic device is taking advantage of ceramic dielectric materials. A dielectric material is an insulator that can be polarized at the molecular level. Such materials are widely used in capacitors, devices which are used to store electrical charge. The structure of a capacitor is shown in the diagram.

Figure 8: Diagram of capacitor.

The charge of the capacitor is stored between its two plates. The amount of charge (q) that it can hold depends on its voltage (V) and its capacitance (C).

q = CV

The dielectric is inserted between the plates of a capacitor, raising the capacitance of the system by a factor equal to its dielectric constant, k.

q = (kC)V

Using materials that have large dielectric constants allows large amounts of charge to be stored on extremely small capacitors. This is a significant contribution to the continuing miniaturization of electronics (e.g., lap top computers, portable CD players, cellular phones, even hearing aids!).

The dielectric strength of a material is its ability to continuously hold electrons at a high voltage. When a capacitor is fully charged, there is virtually no current passing through it.

But sometimes very strong electric fields (high voltages) excite large numbers of electrons from the valence band into the conduction band. When this happens current flows through the dielectric and some of the stored charge is lost. This may be accompanied by partial breakdown of the material by melting, burning, and/or vaporization. The magnetic field strength necessary to produce breakdown of a material is its dielectric strength. Some ceramic materials have extremely high dielectric strengths. For example, electrical porcelain can handle up to 300 volts for every .001 inches (mil) of the material!

Dear Prof  Karunesh Tiwari..Thank you for your help...please can you provide a reference that explain somehow a relation between high values of dielectric constant  of superconductors and their corrosion resistance?