Beryllium oxide thermal conductivity

Although beryllium oxide [1304-56-9] is in many ways superior to most commonly used alumina-based ceramics, the principal drawback of beryUia-based ceramics is their toxicity thus they should be handled with care. The thermal conductivity of beryUia is roughly about 10 times that of commonly used alumina-based materials (5). BeryUia [1304-56-9] has a lower dielectric constant, a lower coefficient of thermal expansion, and slightly less strength than alumina. Aluminum nitride materials have begun to appear as alternatives to beryUia. Aluminum nitride [24304-00-5] has a thermal conductivity comparable to that of beryUia, but deteriorates less with temperature the thermal conductivity of aluminum nitride can, theoreticaUy, be raised to over 300 W/(m-K) (6). The dielectric constant of aluminum nitride is comparable to that of alumina, but the coefficient of thermal expansion is lower. 

Beryllium oxide, BeO, is used in place of Si02 or A1203 in performance-sensitive ceramic applications. It is distinguished by having the highest melting point (2507°C) combined with excellent thermal conductivity and poor electrical conductivity. 

Beryllium oxide shows excellent thermal conductivity, resistance to thermal shock, and high electrical resistance. Also, it is unreactive to most chemicals. Because of these properties the compound has several applications. It is used to make refractory crucible materials and precision resistor cores as a reflector in nuclear power reactors in microwave energy windows and as an additive to glass, ceramics and plastics. 

Beryllium is used in satellite structures in the form of both sheet and extruded tubing and is a very important material for all types of space optics. Beryllium oxide ceramic applications take advantage of high room temperature thermal conductivity, very low electrical conductivity, and high transparency to microwaves in microelectronic substrate applications. 

Some applications, however, must conduct heat but not electricity. In these applications the adhesive must permit high transfer of heat plus a degree of electrical insulation. Fillers used for achieving thermal conductivity alone include aluminum oxide, beryllium oxide, boron nitride, and silica. Table 9.9 lists thermal conductivity values for several metals as well as for beryllium oxide, aluminum oxide, and several filled and unfilled resins. 

Theoretically, boron nitride is an optimum filler for thermally conductive adhesives. However, it is difficult to fill systems greater than 40 percent by weight with boron nitride. Beryllium oxide is high in cost, and its thermal conductivity drops drastically when it is mixed with organic resins. Therefore, aluminum, aluminum oxide, and copper fillers are commonly used in thermally conductive adhesive systems. 

Titanium or beryllium oxide also provides a degree of improvement in thermal conductivity to epoxy systems. Magnesium oxide and aluminum oxide have also been commonly used for this purpose, although the degree of improvement is not as great as with the fillers discussed above. The effect of various fillers on the thermal conductivity of cured adhesive is shown in Fig. 9.6. The incorporation of metal fibers with metal powders has been shown to provide synergistic improvement to the thermal conductivity of adhesive systems,... 

Beryllium oxide ceramics exhibit the highest thermal conductivity of all the ceramic products and are the best electrical insulators at high temperatures. Despite these exceptional properties, beryllium oxide ceramics have only found limited application due to their high cost and poisonousness. They are manufactured by sintering dry or plastically pressed fine particulate beryllium oxide at 1400 to 1450°C in a hydrogen atmosphere. 

In modem electronic devices there is a need to manufacture materials which have high thermal conductivity and a high electrical resistance. The data in the Table 5.19 show that such a requirement can be easily fulfilled using boron nitride or beryllium oxide. Both fillers have excellent thermal conductivity and they are electrical insulators. 

Beryllium oxide is a high melting compound with an exceptionally high thermal conductivity, very low electrical conductivity, and a remarkable chemical stability. The breaking strength of BeO is comparable to that of a-Al203, however, the industrial production of BeO is more expensive. The high toxicity of beryllium compounds, for example abrasive dust of BeO, prevents a broad use of this material. 

A typical thermally conductive epoxy system used as an adhesive, as well as for other purposes, has a thermal conductivity of 0.0026 cal/cm/sec/°C and a volume resistivity of 1.5 x 10 ohm.cm (1.5 x 10 ohm.m). Fillers include alumina (aluminum oxide), beryllia (beryllium oxide), other unspecified inorganic oxides, boron nitride, and silica. Boron nitride is an excellent choice as a thermally conductive filler except that its content reaches a maximum at about 40% by weight in epoxy resins. The resultant products are always thixotropic pastes. BerylUa powder has excellent thermal conductivity by itself, but when mixed with a resin binder its conductivity drops drastically. It is also highly toxic and high in cost. Alumina is a commonly used filler to impart thermal conductivity in resins. ... 

Functional Fillers. A variety of fillers can be used to add specific properties. Metals, and beryllium and aluminum oxides, can be added to increase thermal conductivity (Table 3.33). Metals can be added to increase electrical conductivity (Table 3.34). Graphite increases lubricity and electrical conductivity. Mica increases elec-... 

S.2.5.2 Thermal Conductivity. Thennal conductivity can be increased to shorten molding cycles and to avoid overheating of electrical equipment. Silver, copper, and aluminum have conductivities 1000 times that of unfilled plastics loading them into plastics can increase conductivity considerably, in proportion to their volume fraction (Table 5.22). Beryllium oxide, boron nitride, aluminum oxide, aluminum nitride, and graphite are also quite effective. 

Beryllium oxide is used as a diluent for the UO2 to increase the diameter of the fuel rod and its thermal conductivity for improved thermal performance and also to improve fission product retention in the BeO-UO2 matrix over that possible with UO2 alone. 

Beryllium is used commercially in three major forms as a pure metal, as an alloy with other metals, and as a ceramic. The favorable mechanical properties of beryllium, e.g., its specific stiffness, have made it a major component for certain aerospace applications in satellites and spacecraft. As a modulator and reflector of neutrons, beryllium is of interest in fusion reactions and for nuclear devices that have defense applications. When a small amount of beryllium is added to copper, the desirable properties of copper (i.e., thermal and electrical conductivity) are kept but the material is considerably stronger. The superior thermal conductivity of beryllium oxide ceramics has made the product useful for circuit boards and laser tubes. A more complete discussion of the applications of beryllium was recently reviewed [2]. 

As can be seen in Table 4.2, the thermal conductivities of the Group rv carbides, nitrides, and borides are relatively close. They are also similar to those of the host metals and, from this standpoint, reflect the metallic character of these compounds. However, their conductivities are much lower than that of the best conductors such as Type II diamond (2000 W/m-K), silver (420 W/m-K), copper (385 W/m K), beryllium oxide (260 W/m-K), and aluminum nitride (220 W/m-K). 

Aluminum nitride has outstanding thermal conductivity and is an electrical insulator and heat sink in competition with beryllium oxide and more recently polycrystalline diamond (see Ch. 13). 

Beryllium oxide (BeO, beryllia) is the only material apart from diamond which combines high thermal-shock resistance, high electrical resistivity, and high thermal conductivity at a similar level. Hence its major application is in heat sinks for electronic components. BeO is highly soluble in water, but dissolves slowly... 

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