Niobium capacitor
The development of solid electrolyte tantalum capacitors began in the early 1950s as a miniaturized, more reliable low-voltage support capacitor to complement the newly invented transistor. The solution found by R. L. Taylor and H. E. Haring of the Bell Labs was based on experience with ceramics. They ground down tantalum to a powder, pressed this powder into a cylindrical form and then sintered the powder particles into a pellet (“slug”) at high temperatures, between 1500 and 2000 °C, under vacuum conditions. These first sintered tantalum capacitors used a non-solid electrolyte not consistent with the concept of solid state electronics. 1952 a targeted search in the Bell Labs for a solid electrolyte by D. A. McLean and F. S. Power led to the invention of manganese dioxide as a solid electrolyte for a sintered tantalum capacitor.
Niobium capacitor
The phenomenon that can electrochemically form an oxide layer on aluminum and metals like tantalum or niobium, blocking an electric current in one direction but allowing it to flow in the other direction, was discovered in 1875 by the French researcher Eugène Ducretet. He coined the term "valve metal“ for such metals. Charles Pollak (born Karol Pollak) used this phenomenon for an idea of an polarized “Electric liquid capacitor with aluminum electrodes”. In 1896 Pollak obtained a patent for the first electrolytic capacitor. 
The first tantalum electrolytic capacitors with wound tantalum foils and non-solid electrolyte were developed in 1930 by Tansitor Electronics Inc., USA, and used for military purposes.