The modern era of aluminum electrolytic capacitors

The first article in this series [1] covered the early history of electrolytic capacitors, from their invention around 1880 to the invention of the modern Al electrolytic capacitor structure in 1925. To summarize the early history, "valve metals" were recognized in the 1880s for their ability to conduct in one direction but impede current flow in the opposite direction as a result of oxide growth when submersed in an appropriate electrolyte. Early attempts to use the oxide as a dielectric so formed were frustrated by acidic electrolytes, which damaged the oxide layer and resulted in short capacitor life. Charles Pollak, a manufacturer of rechargeable batteries, realized the importance of the electrolyte and found that a neutral or basic electrolyte provides much greater stability of the oxide layer. In his 1896 German patent application for an electrolytic capacitor, Pollak described his invention as a "liquid condenser with aluminum electrodes, which are covered with a uniformly insulating layer generated by forming with a weak current, characterized by using an alkaline or neutral electrolyte. Since the insulating layer is very thin, the condenser has a very high capacitance and could be used as a polarized capacitor in a DC circuit." Early electrolytic capacitors consisted of an Al electrode in a "bath" of electrolyte. The resistance of the electrolyte resulted in a relatively high equivalent series resistance (ESR), and the capacitors were both bulky and heavy, although not relative to the alternatives at the time. Mains-operated radio receivers, introduced around 1927, created a large consumer market for capacitors, which were required to produce ripple-free DC voltages. Wax paper capacitors, limited to 1 or 2 μF, had to be combined with bulky chokes (inductors) to produce efficient power frequency filters. Electrolytic capacitors provided much greater capacitance so that the chokes were not necessary, but early electrolytic capacitors used in radio receivers still consisted of an oxidized anode in a bath of electrolyte.