MnO2 counter-electrode structure in Ta capacitors: A TEM study

The structure of the MnO2 counter-electrode at the core of solid electrolytic tantalum capacitors has been investigated by transmission electron microscopy. The MnO2 layer presented well-crystallized grains with sizes between 100 and 200 nm. Extensive De Wolff disorder was observed within these crystallites. The structure has been identified by electron microdiffraction as randomly faulted pyrolusite, in which half ramsdellite slabs are generally isolated from each other, i.e., isolated double-chain layers are present in a single-chain matrix and no real intergrowth exists. De Wolff defects corresponding to two orientation variants can coexist in the same pyrolusite crystallite. No evidence for microtwinning on {0 1 1} or {0 3 1} planes has been found.