page_title: Aluminum electrolytic capacitor
section_title: History
hierarchical_section_title: Aluminum electrolytic capacitor / History
caption_attribution_description: Photographed at Radio Relay Station Former USAF microwave relay station ACE-High system at Lefkada, Greece
context_page_description: Aluminium electrolytic capacitors are polarized electrolytic capacitors whose anode electrode is made of a pure aluminum foil with an etched surface. The aluminum forms a very thin insulating layer of aluminium oxide by anodization that acts as the dielectric of the capacitor. A non-solid electrolyte covers the rough surface of the oxide layer, serving in principle as the second electrode of the capacitor. A second aluminum foil called “cathode foil” contacts the electrolyte and serves as the electrical connection to the negative terminal of the capacitor.
  Aluminum electrolytic capacitors are divided into three subfamilies by the type of electrolyte:
  non-solid aluminum electrolytic capacitors,
  solid manganese dioxide aluminum electrolytic capacitors, and
  solid polymer aluminium electrolytic capacitors.
  Aluminum electrolytic capacitors with non-solid electrolyte are the most inexpensive type and also those with widest range of sizes, capacitance and voltage values. They are made with capacitance values from 0.1 µF up to 2,700,000 µF, and rated voltages values from 4 V up to 630 V. The liquid electrolyte provides oxygen for re-forming or self-healing of the dielectric oxide layer.
context_section_description: In 1875, French researcher Eugène Ducretet discovered that certain "valve metals" (aluminum and others) can form an oxide layer that blocks an electric current from flowing in one direction but allows it to flow in the reverse direction.
  Karol Pollak, a producer of accumulators, found out that the oxide layer on an aluminum anode remained stable in a neutral or alkaline electrolyte, even when the power was switched off. In 1896 he obtained a patent for an Electric liquid capacitor with aluminium electrodes (de: Elektrischer Flüssigkeitskondensator mit Aluminiumelektroden) based on the idea of using the oxide layer in a polarized capacitor in combination with a neutral or slightly alkaline electrolyte.
  The first electrolytic capacitors realized industrially consisted of a metallic box used as cathode, filled with a borax electrolyte dissolved in water, in which a folded aluminum anode plate was inserted. Applying a DC voltage from outside, an oxide layer was formed on the surface of the anode. The advantage of these capacitors was that they were significantly smaller and cheaper than all other capacitors at this time with respect to realized capacitance value. This construction with different styles of anode construction but with a case as cathode and a container as the electrolyte was used up to the 1930s and was called a "wet" electrolytic capacitor, referring to its high water content.
  The first common application of wet aluminum electrolytic capacitors was in large telephone exchanges, to reduce relay hash (noise) on the 48 volt DC power supply. The development of AC-operated domestic radio receivers in the late 1920s created a demand for large-capacitance (for the time) and high-voltage capacitors for the valve amplifier technique, typically at least 4 microfarads and rated at around 500 volts DC. Waxed paper and oiled silk film capacitors were available, but devices with that order of capacitance and voltage rating were bulky and prohibitively expensive.
  The ancestor of the modern electrolytic capacitor was patented by Samuel Ruben in 1925, who teamed with Philip Mallory, the founder of the battery company that is now known as Duracell International. Ruben's idea adopted the stacked construction of a silver mica capacitor. He introduced a separate second foil to contact the electrolyte adjacent the anode foil instead of using the electrolyte-filled container as the cathode of the capacitor. The stacked second foil got its own terminal additional to the anode terminal and the container had no longer an electrical function. This type of electrolytic capacitor with one anode foil separated from a cathode foil by a liquid or gel-like electrolyte of a non-aqueous nature, which is therefore dry in the sense of having a very low water content, became known as the "dry" type of electrolytic capacitor. This invention, together with the invention of wound foils separated with a paper spacer 1927 by A. Eckel, Hydra-Werke (Germany), reduced the size and the price significantly, which helped make the new radios affordable for a broader group of customers.
  William Dubilier, whose first patent for electrolytic capacitors was filed in 1928, industrialized the new ideas for electrolytic capacitors and started large-scale commercial production in 1931 in the Cornell-Dubilier (CD) factory in Plainfield, New Jersey. At the same time in Berlin, Germany, the "Hydra-Werke", an AEG company, started the production of electrolytic capacitors in large quantities.
  Already in his patent application of 1886 Pollak wrote that the capacitance of the capacitor increased if the surface of the anode foil was roughened. A number of methods have since been developed for roughening the anode surface, mechanical methods like sand blasting or scratching, and chemical etching with acids and acid salts forced by high currents. Some of these methods were developed in the CD factory between 1931 and 1938. Today (2014), electro-chemically etching of low voltage foils can achieve up to a 200 fold increase in surface area compared to a smooth surface. Progress r
caption: A "dry" electrolytic capacitor with 100 µF and 150 V
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