Suntan Tantalum Capacitor Dielectric Construction
The dielectric is then formed over all the tantalum particle surfaces by the electrochemical process of anodization. To achieve this, the “pellet” is submerged into a very weak solution of acid and voltage is applied. The total dielectric thickness is determined by the final voltage applied during the forming process. Initially the power supply is kept in a constant current mode until the correct voltage (i.e. dielectric thickness) has been reached, it then holds this voltage and the current decays to close to zero to provide a uniform thickness throughout the device and production lot. The chemical equations describing the dielectric formation process are as follows:
2 Ta → 2 Ta5+ + 10 e−
2 Ta5+ + 10 OH− → Ta2O5 + 5 H2O
The oxide forms on the surface of the tantalum but it also grows into the material. For each unit of oxide two thirds grows out and one third grows in. It is for this reason that there is a limit on the maximum voltage rating of tantalum oxide for each of the present technology powders.
