Beryllium oxide

A colorless to white crystalline powder of the composition beryllium oxide, also called beryl-lia. It has a specific gravity of 3.025, a high melting point, about 2585°C, and a Knoop hardness of 2000. It is used for polishing hard metals and for making hot-pressed ceramic parts. Its high heat resistance and thermal conductivity make it useful for crucibles, and its high dielectric strength makes it suitable for high-frequency insulators. Single-crystal beryllia fibers, or whiskers, have a tensile strength above 6800 MPa.

Ceramic parts with beryllia as the major constituent are noted for their high thermal conductivity, which is about three times that of steel, and second only to that of the high-conductivity metals (silver, gold, and copper). They also have high strength and good dielectric properties. Properties of typical grades of beryllia ceramics are tensile strength, 96 MPa; compressive strength, 2068 MPa; hardness (micro), 1300 Knoop; maximum service temperature, 2400°C; dielectric strength, 5.8. Beryllia ceramics are costly and difficult to work with. Above 1650°C they react with water to form a volatile hydroxide. Also, because beryllia dust and particles are toxic, special handling precautions are required. Beryllia parts are used in electronic, aircraft, and missile equipment. A more recent application has been the use of beryllia as thermocouple insulators in vacuum furnace equipment operating below 1650°C.

Beryllium oxide powder is available in three particle size ranges: (1) submicron to 1 to 2 mm, used for fabricating both ceramic components and BeO-UO2 nuclear fuel elements, (2) 2 to 8 mm, used primarily for fabricating beryllia bodies of 96 to 99.5% purity, and (3) ultrahigh-density grains of specific size distribution for admixing with resins and other organics to provide very high thermal conductivity coatings and potting compounds.

Beryllia ceramics have these characteristics: outstanding resistance to wetting and corrosion by many metals and nonmetals; mechanical properties only slightly less than those of 96% Al2O3 ceramics; valuable nuclear properties, including an exceptionally low, thermal neutron absorption cross section; and ready availability in a wide variety of shapes and sizes. Like Al2O3 and some other ceramics, beryllia is readily metallized by a variety of thick- and thin-film techniques.

Major markets for beryllium oxide ceramics are: microwave tube parts such as cathode supports, envelopes, spacers, helix supports, collector isolators, heat sinks, and windows; substrates, mounting pads, heat sinks, and packages for solid-state electronic devices; and bores or plasma envelopes for gas lasers.

Other uses include klystron and ceramic electron tube parts, radiation and antenna windows, and radar antennae. The exceptional resistance of beryllia to wetting (and thus corrosion) by many molten metals and slags makes it suitable for crucibles for melting uranium (U), thorium (Th), and beryllium.

The high general corrosion resistance of beryllia has helped it capture new applications in the chemical and mechanical fields. And other uses in aircraft, rockets, and missiles are predicted.

Beryllium oxide is tapped for nuclear reactor service because of its refractoriness, high thermal conductivity, and ability to moderate (slow down) fast neutrons. The "thermal" neutrons that result are more efficient in causing fusion of uranium-235. Nuclear industry uses for beryllia include reflectors and the matrix material for fuel elements. When mixed with suitable nuclear poisons, beryllium oxide may be a new candidate for shielding and control rod assembly applications.

The market for electrically insulating, heat-conductive encapsulants based on beryllia grain-polymer mixtures is both small and restricted. Although these composites have thermal conductivities 10 to 20 times higher than those of other filled plastics, the handling restrictions necessitated by the presence of beryllia limit their use.

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