Beryllia properties

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. 

Curium trifluoride cun be reduced tn Ihc metal hy healing ill 275 C in a beryllia crucible wilh barium vapor. The metal is silvery in color and has the properties of an electropositive element in common wilh the other Actinide elements. 

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). 

Modern ceramic fabrication techniques and equipment can be used on high purity, sinterable-grade BeO powder to produce beryllia shapes with improved properties at reduced cost. 

Table IV summarizes the effect of various oxides on the resulting fiber properties [30-31]. The addition of increasing amounts of alumina, beryllia, yttria or nitride has already been discussed in Chapter 4.1.2. These modifiers tend to increase the modulus or stiffness of a glass fiber. In addition, alumina and baria tend to increase the density, alumina and strontia tend to increase the refractive index, and zinc oxide and zirconia tend to increase the alkali resistance of a fiber.

Beryllia materials are fabricated in much the same way as alumina compounds, although the toxic properties of the powders mandate that they be processed in laboratories equipped to handle them safely. 

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). 

This chapter primarily considers the properties of ceramics used in LTCC circuits, HTCC circuits, and the more standard ceramics, including aluminum oxide (alumina, AI2O3), beryllium oxide (beryllia, BeO), and aluminum nitride (AIN). 

Despite its favorable properties, use of beryllia is limited because it is toxic for approximately 1 percent of the populace when submicrometer-size particles are ingested into the lung. " There is no test for an individual s susceptibility to the resultant condition, beryl-hosis, and no known cure. Thus, users must take stringent precautions to filter out dust... 

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

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. 

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... 

Certain refractories are grouped imder special refractories. Examples are zir-conia, thoria, and beryllia. They possess special properties that make them useful in special applications. For example, thoria is a nuclear fuel that can sustain radiation, damage, and high temperatures. 

An examination of the red Siberian lead ore led Klaproth to the rediscovery of chromium independently of Vauquelin (see p. 553). He investigated gado-linite (1800), and determined the properties of yttria and beryllia he adopted the name Beryllerde which he says had already been recommended by Link instead of Vauquelin s name (Glykine, Siisserde) (see p. 553). Klaproth discovered ceria ( ochroite , from its colour), which he regarded as an earth, independently of Berzelius (see Vol. IV), who regarded it as the oxide of a metal, cerium. Berzelius sent his paper to Gehlen for publication and was told that Klaproth s was to be published in the next number of Journaly Berzelius... 

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