The important parameters of dielectric material for capacitors are 1. its dielectric constant or permittivity, which indicates the electric charge storage capacity, that can vary from 6 for steatite to 300 - 20,000 for today’s complex ferroelectrics; 2. the loss factor; 3. the insulation resistance; 4. the dielectric breakdown voltage; and 5. the sensitivity of these on temperature, signal frequency and electric field strength. High dielectric constant, electrically insulating ceramics are ideal dielectrics for energy storage capacitors. However, a trade-off has to be made because these high dielectric constant ceramics are more temperature, frequency and electric field sensitive. Conventional barium titanate is now mainly used in discrete and multilayer capacitors, as it has been displaced in other applications because of its lowish Curie temperature of 120 ºC compared with say PZT. However, additives can be used to tailor the barium titanate to extend its useful temperature range, and barium titanate derivatives can achieve huge dielectric constants (up to 100,000) by optimising the boundary layers between grains of the ceramic. But as these boundary layers are only some 1 to 2 microns thick they cannot withstand voltages above around 50 volts. Other highly specialised ceramics successfully used for capacitors include lead lanthanum titanium zirconate.

Ceramic capacitors have to compete with alternatives, and the methods of manufacture reflect this. There are two methods, in the first the powdered raw material, sometimes spray-dried, is compacted and shaped by pressing in steel or carbide dies prior to sintering. Alternatively the raw materials have organic binders, solvents and plasticisers added, allowing the ceramic to be cast continuously on a substrate of silicone-treated paper. The solvent is rapidly removed so the ceramic sheet can be rolled up. The ceramic is then removed from the substrate and when leather hard cut or punched into wafers or discs. After sintering they are silvered, leads applied and encapsulated. “Discrete” capacitors may have a dielectric thickness of 50 to 1000 microns and are typically shaped as discs (2 to 30 mm diameter), rectangles or tubes (5 to 60 mm long).

“Multilayer” capacitors are usually made from very thin (3 to 8 microns), homogenous continuous ceramic cast tape. The tape is cut into sheets, typically 150 mm square, onto which patterns of electrodes are screen-printed. The individual layers are stacked and diced to form the shape required and the necessary overlapping of electrodes to obtain electrical contact. The stacks are then sintered ensuring there is no movement between the various layers. Very high value capacitors can be made up of layers having a surface area of only 0.5 mm by 1 mm, but having several hundred of them that are as thin as 4 microns. Terminations are made by conductive coating that is painted on to the ends of the device and then fired at about 800 ºC. Very high dielectric coefficients can also be achieved using lead-based niobate systems that require relatively lower firing.

Surface mount capacitors on printed
circuit board

The higher the dielectric constant is, the smaller the component can be made for the same capacitance, very important for high-density printed circuit boards (pcbs). Multilayer capacitors are very volume efficient, typically with an area as small as 0.6 mm x 0.3 mm, and the most numerous type produced are surface mount devices in printed and hybrid circuits. Reliability figures for ceramic capacitors are very high, as is necessary for use in large circuit boards with many components. One hundred billion multi-layer ceramic capacitors were made in 1987, mainly made of titanium dioxide or barium titanate. By 2003 capacitor demand had reached 300 billion components, and by 2005, 750 billion. A huge number, but explicable when you consider that as early as 1989 a top of the range automobile could contain over 1000 ceramic capacitors.