A scanning electron microscope study of oxide replicas of aluminum electrolytic capacitor foils has provided information on tunneling produced by a given etch. Tunnel morphology depends on microstructure of the aluminum and on etching conditions. With increasing current density in an electrochemical etch, the number of etching tunnels per unit area increases and tunnel diameter decreases. A simplified model predicts the number of tunnels per unit area and the tunnel diameter needed for maximum capacitance for a given anodic Al2O3 film thickness.

The light microscope has long been used to reveal the surface morphology of aluminum foil etched for electrolytic capacitors (1), but its effectiveness has been p a r t l y limited by insufficient depth of focus at high magnifications. The electron microscope with its greater depth of focus and higher resolution has been used to reveal considerable detail in configurations like those produced by tunnel corrosion in a 2S aluminum alloy (2) and tunnel electroetching in aluminum foil of 99.8% purity (3), but the anodic oxide replica samples for this technique have to be relatively transparent to electrons.The scanning electron microscope (SEM), on the other hand, has neither of these limitations. It has a depth of focus generally 50-100 times that of the optical microscope and samples m a y be thick and opaque to electrons. Nonconducting samples, however, require the addition of a thin conducting layer such as vapordeposited carbon or gold. In fact, gold-coated anodic oxide replicas are found to be excellent SEM samples for studying the morphology of etched aluminum. Illustrations have been reported for lightly etched foil (4, 5). The present communication gives a brief description of the preparation of SEM samples for studying both lightly and heavily etched foil.Lightly etched foil and final oxide replicas were obtained in the following way. A specimen of annealed 99.99% A1 foil was given an oxide-removing treatment in a NaOH bath, a 5-sec electroetch at 10 A/in. 2 in a hot NaC1 electrolyte, and a thorough wash in distilled water. An anodic film of about 600A thickness was then formed in an aqueous solution of ammonium pentaborate. Small sheared pieces were placed in a b r o m i n e -m e t h a n o l solution to dissolve the aluminum matrix and thus free the oxide replicas. Each replica was mounted with the o x i d e -m e t a l interface up. Vapor deposition of about 500A of gold in the normal direction completed the preparation of the SEM samples.

Superconducting Tl-Ca-Ba-Cu-oxide films have been prepared on yttria stabilized zirconia substrates via the reaction of T1 2 O vapor with precursor Ca-Ba-Cu-oxide films prepared by the spray pyrolysis of a solution of the metal nitrates. The vapor reaction process, evaluated in both air and oxygen ambients, was carried out in a two-zone reactor which permitted the independent control of the temperatures of the sample and of a boat containing thallium oxide. Sample temperatures of 865-905 °C and boat temperatures of 775-870 °C were investigated. X-ray diffraction analysis of the best samples, which were prepared in an oxygen ambient at sample temperatures of 895-900 °C and boat temperatures of 805-810 °C, revealed the presence of a highly oriented Tl 2 Ca2Ba 2 Cu3O 10+) , phase with a trace of the Tl 2 CaBa 2 Cu 2 0 8+r phase. FWHM's of ~ 2° 9 were obtained in x-ray rocking curve analyses of the oriented 2223 phase. T c (0) values of 95-105 K were measured for the better samples, and a zero field critical current density of 28 800 amps/cm 2 was measured at 77 K for the best sample. SEM micrographs taken of the highest J c samples reveal a highly textured structure consisting of platelike crystals.

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