The crystal structure of Be is closed-packed hexago­nal with a c/a ratio of 1.5671 and lattice parameters a = 0.22866nm and c = 0.35833 nm.3 Table 1 shows the basic properties of Be.4,5 It weighs only about
two-thirds as much as aluminum (Al), and both its melting point and its specific heat capacity are quite high for a light metal. It is widely known for its high Young’s modulus and other elastic coefficients. Its nucleus is small in neutron absorption cross-section and relatively large in scattering cross-section, both of which are advantageous for use as a moderator or reflector. Its superior high-temperature dynamical

Table 1 Basic properties of Be

properties are also advantageous for use in nuclear reactors. It emits neutrons under g-ray irradiation and can thus be used as a neutron source. Its soft X-ray absorption is less than one-tenth that of Al, making it highly effective as a material for X-ray tube windows.

Figure 1 shows the temperature dependence of the specific heat capacities of various Be samples.3 The following equations describing the specific heat capacity of Be are reported.3

CP = 11.8 + 9.12 x 10-3T

(JK-1g-1 atom, from 600 to 1560 K)

CP = 25.4 + 2.15 x 10-3T

(JK-У1 atom, from 1560 to 2200K)

Temperature dependences of the thermal expansion coefficient and the electrical resistivity of Be3 are given in Figures 2 and 3, respectively. Figure 4 shows the temperature dependence of the thermal conductivities of various Be samples.3,6 Be exhibits relatively high thermal conductivity values around 200 W m-1 K-1 at room temperature, and the values decrease with temperature. The effect of high-dose neutron irradiation on the thermal conductibility of Be has been investigated.7,8 It is reported by Chakin et al.7 that neutron irradiation at 303 K to a neutron fluence of 2 x 1022 cm-2 (E > 0.1 MeV) leads to sharp decrease of thermal conductivity, in particular at 303 K, the thermal conductivity decreases by a factor

of five, but short-term high-temperature annealing (773 K for 3 h) leads to partial recovery of the thermal conductivity.

In addition to the data listed in Table 1, the thermodynamic properties of Be have been reported recently,9 in which the temperatures of transforma­tion Ttr and melting Tm, and the enthalpies of trans­formation AtrH and melting DmH are measured
by difference thermal analysis and by anisothermal calorimetry. It is reported by Kleykamp9 that the results for hcp-bcc transformation of Be are Ttr = 1542 ± 1K and DtrH = 6.1 ± 0.5 kJ mol-1 and those for the melting process are Tm = 1556 ± 2K and DmH = 7.2 ± 0.5 kJ mol-1.

A fine, transparent BeO film of about 10-6cm thickness forms on Be in air, and it therefore retains

its metallic gloss when left standing. This results in its passivation in dry oxygen at up to 923 K, but the oxidized film breaks down at temperatures above about 1023 K and it thus becomes subject to progres­sive oxidation.10 It reacts with nitrogen at 1173 K or higher, forming Be2N3, and with NH3 at lower tem — peratures.10 Be undergoes passivation in dry CO2 at up to 973 K, but only up to 873 K in moist CO2.11,12 Its resistance to corrosion by water varies with tem­perature, dissolved ion content, pH, and other factors; it is reportedly poor in water containing CP (1-10ppm), SO2~ (5—15ppm), Cu2+ (0.1-5ppm), Fe2+ (1-10 ppm), or other such ions.10

Among the various compounds formed by Be, BeO and Be2C may be taken as typical. The basic properties of BeO are shown in Table 2.4 Its melting point and thermal conductivity are both high,13 its heat shock resistance is excellent, its thermal neutron absorption cross-section is small, and its corrosion resistance to CO2 at high temperatures is also excellent. Be2C is formed by reaction of Be or BeO with C. Its basic properties are density, 2.44 gcm ; specific heat capac­ity, 41.47J K-1 moP1 (303-373 K); thermal expansion coefficient, 10.5 x 10-6K-1 (298-873 K); and electric resistivity, 0.063 O m (303 K). It is reportedly unstable in moist air.10

Intrinsically, BeO is an excellent moderator and reflector material in nuclear reactors. Various utiliza­tions of BeO in reactors14 and behavior of BeO under neutron irradiation have been reported.15 Especially,

the effect of neutron irradiation on the thermal conduc­tivity of BeO has been widely studied.16,17 Figure 5 shows the temperature dependence of the thermal con­ductivity of unirradiated and irradiated BeO.17 It is observed that irradiation of BeO with neutrons consid­erably reduces the thermal conductivity. It has also been reported that the irradiation-induced change in thermal conductivity can be removed by thermal annealing, but complete recovery is not achieved until an annealing temperature of 1473 K is reached.

One further important property of Be that must be noted is its high toxicity. The effect of Be dust, vapor, and soluble solutes varies among individuals,

0

260 280 300 320 340 360

Temperature, T (K)

Figure 5 Temperature dependence of the thermal conductivity of unirradiated and irradiated BeO. Reproduced from Pryor, A. W.; et al. J. Nucl. Mater. 1964, 14, 208.

but exposure may cause dermatitis and contact or absorption by mucous membrane or respiratory tract may result in chronic beryllium disease, or ‘berylliosis.’ Maximum permissible concentrations in air were established in 1948 and include an 8-h average concen­tration of 2 pgm~3, a peak concentration of 25 pgm~3 in plants, and a peak concentration of 0.01 pgm~3 in plant vicinities.18 In relation to workplace health and safety, particular care is necessary in the control of fine powder generated during molding and mechan­ical processing. Dust collectors must be installed at the points of generation, and dust-proof masks, dust — proofgoggles, and other protective gear must be worn during work. InJapan, Be is subject to the Ordinance on Prevention of Hazards due to Specified Chemical Substances.