Dielectric Aluminum Oxides: Nano-Structural Features and ...

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Figure 8. FE-SEM view of cut edge surface of a tab-strip after scintillation testing in ethylene glycol electrolyte The present work extends the study of these plumes to longer times and other current densities in search of microscopic evidence of the origin of the plumes. We have seen that brief scintillation voltage testing in ethylene glycol electrolytes can lead to plume growth on cut edges and possibly any surface where the original formed oxide is ruptured. In order to understand the implications of this mechanism for aluminum raw edges during aging of capacitors, the photograph in figure 9 depicts the corner between the formed surface and the raw, cut edge on a tabstrip which has been aged at 65C overnight at 390V in the same ethylene glycol electrolyte. The uncontrolled current density and the long application time of the current has produced a thick mat of plumes on the cut edge that is visible as a orangebrown deposit. Clearly, much charge is wasted in this parasitic reaction while the underlying oxide quality is suspect. Figure 9. Surface View of Formed Tabstrip Corner between Raw Edge [top] and Formed Surface [bottom] after Aging at 390V / 65C Overnight. 70
Figure 10. Surface View of Cut Edge of 1199 Tabstrip formed in EG electrolyte To 100V at 37C and 1.2mA/cm 2 . In order to elucidate a mechanism for the formation of these plumes, we examine whether their formation is associated with contaminants on the cut edges as well as the voltage dependence of their appearance. Figure 10 shows a view of the cut edge of a tabstrip formed in the same ethylene glycol electrolyte, but only to 100V. The anodic oxide exhibits a semi-regular porous structure, especially along cutting lines and apparent grain boundaries. A number of particles are embedded in this porous oxide, but no plumes could be found associated with them or anywhere on the cut edges. EDXA studies of these particles and the surrounding area detected only aluminum and oxygen - no appreciable level of contaminant elements such as iron, copper or silicon. An important clue as to the lack of the plume structures in Figure 10 can be found in the scintillation curve of Figure 11. At constant current, the voltage increases linearly to about 150V where another mechanism reduces the formation efficiency markedly until about 200V. Above 200V, the efficiency improves but does not return to the earlier level. Above 400V, scintillation sets in, preventing further oxide growth. 71
Page 1 and 2: Dielectric Aluminum Oxides: Nano-St
Page 3 and 4: Figure 1.Interior Surface SEM: Anod
Page 5 and 6: Figure 4. Cross-Section View of Etc
Page 7: The degree of success in converting
Page 11 and 12: Closer examination of a short plume
Page 13 and 14: Careful searching of the cut edge s

Dielectric Aluminum Oxides: Nano-Structural Features and ...

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