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

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Figure 5. Cross-Section of ICD Foil Tunnel: Nano-Composite Oxide Produced by Anodizing, Hydrating, and Forming. Further study of nano-composite oxide using Focused Ion Beam [FIB] thinning techniques prior to TEM examination in Figure 6 shows greater detail in the layered structures obtained. This sample was deliberately produced with excess porous oxide prior to hydration to illustrate the nearly total conversion of the porous oxide incorporated into the barrier layer during the pore filling steps of hydration and formation in the prior example. Porous oxide above the barrier forming voltage is not converted. The greater sensitivity of the TEM to subtle differences in density and crystallinity reveal that the crystalline layer is not monolithic but is itself a composite of at least two layers. Electron diffraction patterns of each region confirm the relative crystallinity in a qualitative sense. 6 5 4 3 1 2 Figure 6. TEM Photo of Nano-Composite Oxide Cross-section: 1: Residual Hydrate; 2: Residual Porous Oxide; 3: Mostly Crystalline Oxide; 4: Highly Crystalline Oxide; 5: Amorphous Oxide; 6: Base Aluminum Metal 68
The degree of success in converting the porous alumina to largely crystalline alumina begs the question of how much residual tensile stress is found in the nano-composite oxide due to this conversion. The results of a standard 2-hour boil test are shown in the “ramp” curve in Figure 7. In this case, no appreciable coulombs are devoted to repair of the oxide at the lower voltages as in the standard crystalline oxide of Figure 3. The absence of a large peak low in the curve may be interpreted as a lack of delamination of the amorphous layer from the base metal or of delamination of the crystalline layer from the amorphous oxide. The lack of an appreciable “baseline” of charge during the build-up to the effective formation voltage [EFV], implies a lack of vertical cracks in the oxide typical of stress relaxation and a rarity of exposed voids normally found in the upper layer of high voltage crystalline alumina. In short, this boil test ramp is typical of those expected from totally amorphous anodic oxides used in heavy duty photoflash and garage door opener capacitors. In this case, however, the capacitance sacrifice versus crystalline oxide is much less. The authors feel that this high degree of anodic oxide stability will eventually find acceptance in some severe duty commercial applications. 400 Charge/Volt [uC/V] 300 200 100 0 0 100 200 300 400 500 600 Volts Figure 7. 2 Hour Boil Test “Ramp” Curve: Nano-Composite Oxide on ICD-Style Etched Foil. Rise Time = 1.8 sec. Nano-Structures in E.G. Formation/Aging: Another fascinating area of study of nano-structures of anodic alumina was introduced by Stevens 7 , et.al. in a study of the effects of non-aqueous fill [or working] electrolytes on formation and deformation. Long, convoluted plumes of anodic residue were observed to be produced on raw cut surfaces of formed aluminum tabstrip when subjected to a scintillation voltage test in ethylene glycol electrolyte. Other researchers 8-11 have noticed odd deposits on aluminum formed in various organic solvents with low water content, but this report has some of the first microscopic evidence connecting nano-scale anomalies and larger plume or network structures formed in an ethylene glycol electrolyte. Figure 8 shows these plume structures as formed on the cut edge of formed high purity aluminum tabstrip at 1.25 mA/cm2 at 37C in an ethylene glycol electrolyte containing boric acid and azelaic acid salts and having less than 3% water. Local electrochemical activity of a parasitic nature appears to be taking place on these cut edges. Speculation has centered on the presence of impurities or contaminants on the aluminum substrate. 69
Page 1 and 2: Dielectric Aluminum Oxides: Nano-St
Page 3 and 4: Figure 1.Interior Surface SEM: Anod
Page 5: Figure 4. Cross-Section View of Etc
Page 9 and 10: Figure 10. Surface View of Cut Edge
Page 11 and 12: Closer examination of a short plume
Page 13 and 14: Careful searching of the cut edge s

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