Anomalous heat-releasing phenomenon from bubbles in aluminum induced by electron beam irradiation

Abstract

In the early 1990s, Japanese scholars unexpectedly observed that single crystal changes into polycrystal in deuterium-implanted aluminum under electron irradiation, but never found the same phenomenon in the hydrogen-implanted aluminum. However, previous study of our group has proved that the polycrystalline phenomenon can also be observed in hydrogen-implanted aluminum during electron irradiation. In this paper, the behavior of inert gas bubbles in aluminum under electron irradiation is investigated, aiming to further explore the effects of ion species, electron voltage and the pressure of bubbles on the anomalous heat-releasing reaction of bubbles induced by electron irradiation. In the experiment, the transmission electron microscope (TEM) samples of pure aluminum were implanted with He+, Ne+, Ar+ respectively by ion accelerator at room temperature. The TEM is used to in-situ observe and investigate the evolution of microstructure and the change of selected electron diffraction patterns of gas bubbles during electron irradiation. The results show that gas bubbles form in aluminum sample after ion implantation. During 200 keV electron irradiation TEM results show that the three kinds of inert gas bubbles all coalesce, grow up and bust separately. Finally, lots of nanoscale black dots appear inside them. At the same time, the electron diffraction patterns change from single crystal diffraction spots to polycrystalline diffraction rings. The dark field images indicate that the diffraction rings are induced by these black dots. Moreover, from the characterization of the diffraction rings, it is known that these black dots are pure aluminum rather than aluminum oxide. Therefore, the possibility that the diffraction rings result from aluminum oxide is eliminated. It is assumed that a certain kind of heat-releasing reaction should happen when the gas bubbles are irradiated by electrons, which leads to the poly-crystallization of aluminum after electron irradiation. However, while helium bubbles are irradiated by electrons with an energy of 80 keV, no diffraction ring is observed after electron irradiation. The same phenomenon as that in the case of helium bubbles irradiated by 80 keV electrons is observed. When helium and argon mixed bubbles with polygonal shape are irradiated by 200 keV electrons, no diffraction ring is observed after electron irradiation either. The reason might be related to the energy of the electron beam and the pressure of gas bubbles separately. There should be a threshold value of electron voltage for the heat-releasing reaction. In addition, the pressure of the gas bubbles is also a key factor for the heat-releasing reaction. The heat-releasing phenomenon of gas bubbles reminds us of the sonoluminescence phenomenon. By model calculation, it is predicted that there is a plasma core in the bubble during sonoluminescence. According to the hint from researches of sonoluminescence, an assumption is made to explain the mechanism of heat-releasing reaction of gas bubbles during electron irradiation. It is that the implanted gas in high pressure bubbles in aluminum is excited into plasma during electron irradiation. When the energy of plasma in the bubbles is accumulated to a certain degree, the plasma is extinguished suddenly. In this process, a lot of heat is released to melt the aluminum, thus leading the aluminum to recrystallize.