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 DC 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 at the anode 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 thickness of oxide growth, one third grows out and two thirds grows in. Due to the limits of oxide growth, there is a limit on the maximum voltage rating of tantalum oxide for each of the presently available tantalum powders (see Figure 3). The dielectric layer thickness generated by the forming voltage is directly proportional to the voltage proof of electrolytic capacitors. Electrolytic capacitors are manufactured with a safety margin in oxide layer thickness, which is the ratio between voltage used for electrolytical creation of dielectric and rated voltage of the capacitor, to ensure reliable functionality. The safety margin for solid tantalum capacitors with manganese dioxide electrolyte is typically between 2 and 4. That means that for a 25 V tantalum capacitor with a safety margin of 4 the dielectric voltage proof can withstand 100 V to provide a more robust dielectric. This very high safety factor is substantiated by the failure mechanism of solid tantalum capacitors, "field crystallization". For tantalum capacitors with solid polymer electrolyte the safety margin is much lower, typically around 2. Cathode The next stage for solid tantalum capacitors is the application of the cathode plate (wet tantalum capacitors use a liquid electrolyte as a cathode in conjunction with their casing). This is achieved by pyrolysis of manganese nitrate into manganese dioxide. The "pellet" is dipped into an aqueous solution of nitrate and then baked in an oven at approximately 250 °C to produce the dioxide coat. The chemical equation is: Mn(NO3)2 → MnO2 + 2 NO2 This process