A New Liquid Polyester Electrolyte for the Production of Extremely High Voltage Anodic Oxide Films on Aluminum

摘要

The valve-oxide forming properties of aluminum were discovered in the 1850's. Over the course of the next fifty years, researchers found that most neutral or near neutral pH aqueous salt solutions, which are free from halogens and sulfate, may be used to grow anodic films on aluminum with varying degrees of success. By the 1920's, aqueous boric acid / borax solutions had become the industry standard for the anodizing of aluminum capacitor foil and remained the most widely used anodizing electrolyte solutions for aluminum until the late 1970's and early 1980's. A detailed description of the early development of aqueous anodizing electrolytes with particular emphasis on the boric acid / borax compositions is given in Georgiev's book on electrolytic capacitors (1). The maximum anodizing voltage which could be reliably produced on aluminum capacitor anode foil for many years was approximately 800 volts through the use of boric acid solutions without borax (2). The solutions generally contain approximately 150 grams of boric acid per liter of solution and melt at temperatures in excess of 80°C. Relatively recently it has been found that dilute aqueous solutions of organic acid salts may be used to anodize aluminum to the 900-1000 volt range by minimization of the halogen content of the electrolyte solution (3) and by careful control of the hydration of state of the anodic oxide, both general oxide layer hydration and hydration faults through the barrier film (4) (5) (6). Even considering these recent improvements in aqueous aluminum anodizing, the maximum attainable anodizing voltage is approximately 1,000 volts.