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.
Capacitor
Since the beginning of the study of electricity non conductive materials like glass, porcelain, paper and mica have been used as insulators. These materials some decades later were also well-suited for further use as the dielectric for the first capacitors. 
Paper capacitors made by sandwiching a strip of impregnated paper between strips of metal, and rolling the result into a cylinder were commonly used in the late 19th century; their manufacture started in 1876, and they were used from the early 20th century as decoupling capacitors in telecommunications (telephony).