1.05.2.1 Introduction to Radiation Damage in Alumina and Spinel

a-Al2O3 and MgAl2O4 are two of the most important engineering ceramics. They are both highly refrac­tory oxides and are used as dielectrics in electrical applications (capacitors, etc.). Both a-Al2O3 and MgAl2O4 have been proposed as potential insulat­ing and optical ceramics for application in fusion reactors.1-3 In a magnetically confined fusion device, these applications include (1) insulators for lightly shielded magnetic coils; (2) windows for radio­frequency heating systems; (3) ceramics for structural applications; (4) insulators for neutral beam injectors; (5) current breaks; and (6) direct converter insula­tors.3 Such devices in a fusion reactor environment will experience extreme environmental conditions, including intense radiation fields, high heat fluxes and heat gradients, and high mechanical and electri­cal stresses. A special concern is that under these extreme environments, ceramics such as a-Al2O3 and MgAl2O4 must be mechanically stable and resis­tant to swelling and concomitant microcracking.

Over the last 30 years, many radiation damage experiments have been performed on a-Al2O3 and MgAl2O4 under high-temperature conditions by a number of different research teams. Figure 1 shows the results of one such study, where the high tem­perature, neutron irradiation damage responses of a-Al2O3 and MgAl2O4 are compared. The plot in Figure 1 was adapted from Figure 1 in an article by Kinoshita and Zinkle,4 based on experimental data obtained by Clinard et a/. and Garner et a/.5-

The neutron (n) fluence on the lower abscissa in Figure 1 refers to fission or fast neutrons, that is, neutrons with energies greater than ~0.1 MeV. Figure 1 also shows the equivalent displacement damage dose on the upper abscissa, in units of

displacements per atom (dpa). These dpa estimates are based on the approximate equivalence (for cera­mics) of 1 dpa per 1025nm~2 (En > 0.1 MeV).9

Figure 1 shows a stark contrast between the radiation damage behavior, particularly the volume swelling behavior, between a-Al2O3 and MgAl2O4. Specifically, MgAl2O4 spinel exhibits no swelling in the temperature range 658-1100 K, for neutron fluences ranging from ^3 x 1026 to 2.5 x 1027nm~2 (3-200 dpa). On the contrary, a-Al2O3 irradiated at temperatures between 925-1100 K exhibits signifi­cant volume swelling, ranging from ~1 to 5% over a fluence range of 1 x 1025 to 3 x 1026nm~2 (1-30 dpa). The purpose of the following discussion is to reveal the reasons for the tremendous dispar­ity in radiation-induced volume swelling between alumina and spinel.

Figure 2 shows a bright-field (BF) transmission electron microscopy (TEM) image that reveals the microstructure of a-Al2O3 following fast neutron irra­diation at T= 1050 K to a fluence of 3 x 1025nm~2

(^3dpa). The micrograph reveals a high density of small voids (2-10 nm diameter), arranged in rows along the c-axis of the hexagonal unit cell for the a-Al2O3. When voids are arrayed in special crystallo­graphic arrangements, as in Figure 2, the overall structure is referred to as a void lattice. Figure 2 shows the underlying explanation for the pronounced volume swelling of a-Al2O3 shown in Figure 1, namely the formation of a void lattice with increasing neutron radiation dose. This phenomenon is well known in many irradiated materials, both metals and ceramics, and is referred to as void swelling. Susceptibility to void swelling is a very undesirable material trait and basically disqualifies such a mate­rial from use in extreme environments (in this case, high temperature and high neutron radiation fields). It should be noted that TEM micrographs (not shown here) obtained from MgAl2O4 spinel irradiated under similar conditions to those in Figure 2 show no evidence of voids of any size.