Beryllia, thermal properties

Beryllium oxide, BeO, (Beryllia). Beryllia has excellent thermal properties (almost 10 times better than alumina at 250°C), and also has a lower dielectric constant (6.5). 

Beryllia, BeO, is commonly used as a substrate material in electronic packaging as a result of its low dielectric loss (0.02 percent) and combined high electrical and low thermal resistivities. Its electrical and thermal properties are somewhat better than those of alumina. This material is relatively easily sintered into dense substrates. Beryllia is higher in cost than alumina because of the higher cost of its raw materials and its higher-temperature fabrication = 2570°C). The limiting factor keeping it from widespread use is the toxicity of Be-base powders. Beryllia has been used in klystron devices and power diodes. 

Heat dissipation can be effectively dealt with by using substrate materials such as aluminum nitride, beryllia and, more recently, diamond which combine electrical insulation with high thermal conductivity. The relevant properties of these three materials are shown in Table 14.1. 

Beryllia ceramics offer the advantages of a unique combination of high thermal conductivity and heat capacity with high electrical resistivity (9). Thermal conductivity equals that of most metals at room temperature, beryllia has a thermal conductivity above that of pure aluminum and 75% that of copper. Properties illustrating the utility of beryllia ceramics are shown in Table 2. 

NISTCERAM National Institute of Standards and Techology Gas Research Institute, Ceramics Division mechanical, physical, electrical, thermal, corrosive, and oxidation properties for alumina nitride, beryllia, boron nitride, silicon carbide, silicon nitride, and zirconia... 

Beryllia has broadly similar properties to alumina (Table 5.3) but its thermal conductivity is 5-10 times greater. It is therefore used when thermal dissipation combined with electrical isolation is of major importance, e.g. in high-power... 

Sintered beryllia. This material exhibits an extraordinarily high thermal conductivity, only surpassed by graphite and metals — hence the high resistance to thermal shock which together with high chemical inertness is its main practically utilized property. In the application of sintered BeO in nuclear reactors, use is made of its low absorption cross section and of high scattering cross section of neutrons (moderators, reflectors). 

TABLE 2.4 Mechanical, Thermal, and Electtical Properties of Coors Alumina and Beryllia Ceramics... 

Beryllia ceramics have these characteristics extremely high thermal conductivity, particularly in the lower temperature range excellent dielectric properties outstanding resistance to wetting and corrosion by many metals and nonmetals mechanical properties only slightly less than those of 96% alumina 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 alumina and some other ceramics, beryllia is readily metallized by a variety of thick and thin film techniques. 

Although beryllia usually is selected for a desirable combination of properties, key to most applications is the material s comparatively high thermal conductivity. Even at the highest temperatures, its thermal conductivity is four times that of dense alumina and from room temperature to 5(X)°C, seven to eight times greater. BeO s thermal conductivity is quite dependent on purity. For example, increasing purity from 99% to 99.8% results in a 10-15% rise in conductivity. Physical Properties... 

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