Sorry if this question is in the wrong community but anyway, why don't people use foam aluminium for capacitors? Its got a large surface area to my understanding (however probably not as much surface area as activated carbon) which should give it a good capacitance, right? Thanks for reading.
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\$\begingroup\$ I don't know the answer but I can make some guesses. 1. What is the characteristic size of the voids in aluminum foam? Many capacitors have dimensions measured in fractions of a mm. 2. Does aluminum foam maintain some conductivity through the bulk? Because if it does that would defeat the purpose of using a capacitor in many situations. \$\endgroup\$– The PhotonCommented Jan 5, 2016 at 1:49
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4\$\begingroup\$ It's not just about large surface area. You need a large surface area between two conductors that are very close, but slightly separated by a dielectric. Getting another conductor close to the aluminium foam will probably be challenging, compared to a flat aluminium foil that can be anodized and rolled up. \$\endgroup\$– Oleksandr R.Commented Jan 5, 2016 at 2:01
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\$\begingroup\$ Thanks for the feedback, honesty, I never thought of having to roll it up :). And only some aluminium foam is closed cell, you can get open cell. \$\endgroup\$– Bob SmithCommented Jan 5, 2016 at 2:22
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\$\begingroup\$ Interesting question. Tantalum capacitors make use of a similar idea where the high surface area of a volume of sintered tantalum provides high capacitance. The sintered tantalum is the anode, and the cathode is either an electrolyte that saturates the bulk or a conductive layer chemically built up throughout the porous bulk. Perhaps it's hard to make foam aluminum with a sufficiently fine structure; perhaps aluminum foam has been tried but was unsuccessful for other reasons -- but maybe you have an idea that's worth investigating further. \$\endgroup\$– Fred SchleiferCommented Jan 11, 2016 at 6:29
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Because only a small fraction of that surface area woyld be exposed to the other plate of the capacitor. A way would have to be found to coat the foam in an insulating substance then in another conductive substance throughout its whole volume. Also, it would not be as efficient as an electrolytic capacitor which has an insulating layer molecules thick, which is chemically formed after assembly.
