Abstract
Materials with good irradiation damage tolerance are necessary for modern unclear equipment as core components while those with good irradiation sensitivity can be applied as sensors and detectors for irradiation environment. The study of irradiation effect under various conditions is a prerequisite to screen suitable candidates for both applications. As for the electron irradiation, besides the typical linac irradiation devices, TEM is an alternatively simple and effective method in studying electron irradiation effect, especially for in-situ study[
Alumina is one of the most extensively used structural ceramics and its electron irradiation effects have been well studied. Under conventional TEM observation (accelerate voltage no more than 300 kV, room temperature, and ordinary vacuum of ~10-5 Pa) a relative stable nature of alumina was found and only lattice defects like surface faceting[
Recently, we reported that transparent alumina ceramic could be easily sintered by SPS with AlF3 doping[
1 Experimental
The 0.1wt% AlF3 doped alumina ceramic was sintered by SPS with parameters same to our previous report[
2 Results and discussion
Fig. 1(a) is a clean grain surface stripped from surrounding grains by ultrasonic break. The surface was stable under low magnification observation and no attached second phase was observed. However, once the magnification exceeded 200 k magnification, the surface became unstable. Moveable and growing precipitates were emitted from the surface under electron irradiation after several to dozens of seconds. The precipitates were sphere-like separated out at the beginning and generally transformed to faceted shape when lowering magnification or moving observation area. The diameter of these precipitates ranged from 5 to 50 nm. The typical feature of these precipitations displayed in Fig. 1(b). Arrows in Fig. 1(c-d) exhibited the growing of a precipitate accompanied with the decrease of another. The whole process finished within less than 5 s. Moreover, as a general case, when two growing precipitates contacted with each other, instantaneously they coalesced into a whole. The drop-like moving and coalescing behavior indicates a doubtable liquid-like nature of these precipitates. After the spherical-to-faceted transition, lattice fringes were easy to be seen. The 0.235 nm fringe space of the precipitation corresponds to the 0.234 nm-spacing (111) plane of metallic Al (Fig. 1(e)). The precipitates from original alumina matrix were further confirmed to be metallic Al by EELS study of the Al-L23 edges of the precipitation layers. In Fig. 2, Each feature peaks are identical with the reference spectrum of Al metal.
Figure 1.TEM observations of a grain surface of 0.1wt% AlF3 doped sample (a) Original surface before high magnification observation; (b) Surface after high magnification observation, where sphere precipitates separated out; (c-d) Black arrow shows the growing of a new precipitate under irradiation; (e) Lattice fringes of a precipitated crystal
Though it was reported that α-alumina is relatively stable under conventional TEM electron irradiation, the decomposition to Al under electron irradiation is a feature of AlF3 as described in previous studies[
Figure 2.EELS of the precipitation and its reference
It will be easy to understand this behavior if the AlF3 was preferentially segregated at the surface. To further understand the phase-separation behavior and make clear the character inside a grain, another HRTEM and selected area diffraction were conducted with JEM-2100F. However, the results shown in Fig. 3(a-d) don’t seem to favor this assumption. Fig. 3(a) shows an original grain before irradiation induced Al precipitation. The white arrows point to the dark liner contrasts which suggesting the existence of planer defects. Diffraction pattern of the original grain is shown in Fig. 3(c), where the bright streaks identical with the liner contrasts in Fig. 3(a) confirm the existence of planer defects through the whole grain. It is interesting to note that, once after the Al precipitation by electron irradiation (Fig. 3(b)), the streaks in diffraction pattern of the same grain disappeared (Fig. 3(c)), indicating the precipitation of Al involved the reconstruction of lattice and finally resulted in a defect free state. Fig. 3(e) is the magnification of area i in Fig. 3(a), where the white dotted box traces a burgers circuit of an edge dislocation and the white arrow points the Burgers vector. An extra half atom plane can be clearly seen in the center of the box. Fig. 3(f) is the inversed FFT of area (ii) in Fig. 3(a) from (21¯1¯0)/(2¯110) diffraction spots, where many extra half-planes with dislocation mark can be seen, indicating the highly defected nature.
Figure 3.HRTEM images and selected area diffraction of AlF3 doped alumina grain (a) HRTEM images of AlF3 doped alumina grain before Al precipitation. White arrows points to the planer defects; (b) The same grain after Al precipitation; (c, d) Diffraction patterns correspond to the grain before and after Al precipitation, respectively. The zone axis is along [0001] direction; (e) Magnification of area i in (a). The dotted box traces a dislocation core and the parallel lines show the distortion contrast composed of three or four layers; (f) Inversed FFT of the area ii in (a) from (21¯1¯0)/(2¯110) diffraction spots
Two kinds of defects reactions often involve in the doping process, namely substitution-vacancy and interstitial processes. The reaction equation can be written as below in the present case:
Since the Al precipitated without holing the obvious matrix, it is hard to image the vacancy type reaction (1) dominates since the precipitated Al would be otherwise easier to settle down at Al vacancies. On the other hand, oxide ceramics sintered by SPS often accompanies oxygen vacancies due to the graphite surroundings and chamber vacuum[
Considering that the bright streaks in diffraction pattern (Fig. 3(c)) and the corresponding liner contrasts in HRTEM (Fig. 3(a)) are quite similar to the feature of GP zones[
Fig. 4(a) shows an ideal cation lattice diagram of alumina as suggested by Tomokiyo, et al[
Figure 4.(a) Schematic diagram of cation sub-lattice in
It is also interesting to note that, LiF has a similar but more sensitive behavior under electron exposure like AlF3[
3 Conclusions
Until now, we found a novel fast electron-irradiation- induced phase-separation behavior in AlF3 doped alumina ceramic, which is never reported before. The original grain of the spark plasma sintered sample carried many planer defects with HRTEM observation, Al nano-crystal separated out with these planer defects disappearing. A defects assisted interstitial segregation mechanism was proposed to explain this behavior.
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