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

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It has been known for many years that growth of oxide in a glycol forming environment can be current density sensitive. Therefore, an exploration of the effects of current density on the nano-structures formed on these cut edges is warranted. Figure 15 reveals that the plume formation is already well underway at 100V when the current density is increased to 12mA/cm 2 whereas none are found at the standard scintillation test current density of 1.2mA/cm 2 . Figure 15. Surface View of Cut Edge of 1199 Tabstrip formed in EG electrolyte To 100V at 37C and 12.0mA/cm 2 . When the current density is lowered by a factor of 10 to 0.12mA/cm2, large plumes are very rare up to the scintillation voltage. Instead, there is a tendency for the porous structure to be more well-developed with small structures sometimes appearing in the pore itself as seen in Figure 16. The edges of the pore frequently have a small disc lifted off of the surface similar to a ruptured blister. Figure 16. Surface View of Cut Edge of 1199 Tabstrip Formed in EG electrolyte To 400V at 37C and 0.12mA/cm 2 . 74
Careful searching of the cut edge surface at higher magnification reveals a few pores which still retain delicate “flower-like” filament structures like those seen in Figure 17. The existence of these structures imply that the great majority of the pores form them during the build up to scintillation voltage and that the missing oxide film surrounding the pore is carried away by the growing filament, part of which is retained at the filament tip. The implication of these photographs is that once pores begin growing, a parasitic reaction which depends upon voltage and current density can lead to filament growth from the pore. At higher current density, these filaments probably develop quickly and become entangled with nearby filaments to merge into the braided appearance of the larger plumes seen earlier. Hence, these delicate flower-like structures are likely to be the incipient stage of plume formation. They can still be seen in figure 17 due to the low current density favoring barrier growth over filament formation. Figure 17. High Magnification Surface View of Cut Edge of 1199 Tabstrip Formed in EG electrolyte To 400V at 37C and 0.12mA/cm 2 . CONCLUSIONS Despite the commercial exploitation of anodic aluminum oxide films as dielectrics for nearly a century, we continue to find novel features in these films. We have shown here that nanocomposite barrier oxides can be produced by a novel combination of porous anodizing and crystalline barrier layer formation techniques. This oxide is a nano-composite of amorphous oxide and crystalline oxide that shows tremendous resistance toward the standard boiling water stability test without resort to the heavy phosphate doping characteristic of standard amorphous oxides. Furthermore, the nano-composite film can be produced with capacitance values approaching standard crystalline alumina thereby avoiding most of the capacitance penalty associated with amorphous alumina barrier layers. We have also demonstrated that the inflection seen in the scintillation and formation curves of anodic alumina produced in ethylene glycol based electrolytes is associated with the development of porosity and nano-structural filaments on the growing oxide layer leading to microscopic plumes of anodic material under some conditions. These plume-like agglomerates have been seen macroscopically for many years as orange-brown deposits on 75
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 and 8: The degree of success in converting
Page 9 and 10: Figure 10. Surface View of Cut Edge
Page 11: Closer examination of a short plume

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

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