Terminal

Hermetically Sealed Electrical Terminals

CeramTec high-alumina ceramic terminals are used to pass electrical conductors through the case or housing of electrical apparatus for connection to outside cables or leads. Applications include transformers, capacitors, filters, power supplies, and cable terminations.

The terminals insulate the conductors from the case and provide a hermetic seal between the inside of the apparatus, which is usually oil or gas-filled, and the outside atmosphere.

The insulation within the terminal is a high-alumina ceramic body, usually of tubular or disc shape. The body provides the terminal with its superior insulating properties.

Bonded to the top of the tubular ceramic body (or the inside diameter of disc-shaped bodies) is a metal cap-and-stud or cap-and-lug assembly which forms the connection point to the outside cable or lead. A metal flange is similarily bonded to the ceramic near the bottom (or the outside diameter of disc-shaped bodies) for mounting to the apparatus case.

On some terminal designs, the cap-stud or cap-lug assembly is integral with a conductor passing through the hollow inside of the body. This conductor is used for the internal connection in the apparatus. On other designs, this internal conductor is inserted by the customer and joined with the cap upon installation of the terminal.

Terminal mounting flanges are joined to the apparatus housing by various welding, brazing, and soft soldering techniques.

CeramTec's Special Bonding Process

The highly successful performance of CeramTec high-alumina ceramic terminals and assemblies stems largely from 1) the excellent electrical and mechanical properties of the alumina ceramic, and 2) the high-vacuum tightness, strength, and high-temperature resistance of the ceramic-to-metal bond.

CeramTec creates this bond with its own proprietary sealing process - a variation of the active alloy technique. The bond between metal and ceramic is chemical, rather than mechanical. Repeated rupture tests have shown that fracture of CeramTec terminals occurs in the ceramic rather than the bond.

Ceramaseal terminals and assemblies remain hermetic in extreme environments. Some designs can survive high-temperature bakeouts; also continuous operation at 350°C in air.

Resistance to thermal shock is excellent, some Ceramaseal® terminals have remained leak tight after repeated direct transfer from liquid nitrogen (-196°C) to molten solder at 240°C.

High-Alumina Ceramic

The insulating bodies on most terminals and assemblies in this catalog are made of high- strength, high-alumina (Al₂O₃) ceramic. Alumina content ranges from 85 to 99.5 percent, depending on the unit and its application.

Several properties of high-alumina ceramic make it excellent for use in insulating terminals:

1. It possesses high dielectric strength, even at high temperatures.
2. High thermal conductivity and high mechanical strength enable the ceramic to survive severe thermal shock.
3. Alumina is an extremely dense material, is gas tight in thicknesses of 0.005 inch and over, and absorbs no water. Because of these properties, it provides reliable insulation even in the presence of contamination and extreme humidity.

Alumina ceramic blanks for terminal bodies are made by a number of methods, including extrusion, iso-static pressing, and die pressing. After forming, the ceramic blanks are fired in high-temperature kilns, then coated with a high-temperature glaze which matches the ceramic in expansion coefficient. In service, the glazed surface stays cleaner than unglazed ceramic, thus permitting maximum voltage before corona or flashover occurs. And when cleaning does become necessary, the operation is easier with glazed ceramic.

Caps and Flanges

Normally, the caps and flanges on CeramTec high-alumina terminals and assemblies are made of Kovar® or a nickel-iron alloy containing coefficients close to that of the ceramic. Table 2 shows the compositions of Kovar® and of the nickel-iron alloy normally used in CeramTec's Ceramaseal® products.

CeramTec has also produced assemblies with copper hardware. High-alumina has been successfully bonded to such metals as nickel, platinum, and cold-rolled and stainless steel.

Caps and flanges are supplied with or without hot dip solder finish, as specified by the customer. Solder dipping is performed in accordance with Specification MIL-F-14072, finish number M-258 Type II.

Vacuum-Tube-Grade Bonding Alloys

Choice of brazing alloy is important in obtaining good ceramic-to-metal bonds. Only vacuum-tube-grade alloys are used in Ceramaseal® products. The typical CeramTec terminal is bonded with a silver-copper eutectic with a melting point of 780°C. Other metals and alloys have also been used successfully by CeramTec, including pure silver, melting at 961°C, and a gold-nickel alloy melting at 950°C.

Photos	Sub-section
	Fluted & Low Profile Insulators
	Straight Wall Insulator