Among electrolytic capacitors, traditional aluminum electrolytic capacitors use electrolyte as cathode material, which cannot get rid of the dangerous phenomenon of thermal expansion and leakage due to physical properties, making aluminum electrolytic capacitors face unprecedented pressure and challenges. Some markets pessimistically believe that aluminum electrolytic capacitors have reached the end of their road and will withdraw from the passive component stage in the future. In addition, traditional tantalum electrolytic capacitors use manganese dioxide as cathode material. In addition to the danger of burning due to voltage problems, the future market is greatly limited due to environmental issues. In addition, since the organic semiconductor TCNQ is a cyanide, it is easy to volatilize highly toxic cyanide gas at high temperatures, which will be restricted in production and use. Similarly, traditional tantalum capacitors also have many inherent disadvantages due to the choice of manganese dioxide as the cathode. Disadvantages of MnO2 as the cathode of tantalum capacitors: a. The conductivity is small, about 0.1~1S/cm, which makes the equivalent series resistance (ESR) too large, limiting the high-frequency characteristics of tantalum capacitors. b. The stress generated by the difference in thermal expansion coefficients between MnO2 and the dielectric layer will damage the dielectric layer during the high-temperature coating process. c. MnO2 material has a high oxygen content and is prone to spontaneous combustion during operation. Solid aluminum electrolytic capacitors that use polymer conductive materials to replace traditional electrolytes have the advantages of high frequency and low impedance (10 milliohms), high temperature stability (-50 degrees ~ 125 degrees), fast discharge, reduced volume, no leakage, and a lifespan of up to 40,000 hours in an 85°C working environment. The cathode materials of polymer solid capacitors can be polypyrrole, polyaniline and polythiophene. a. Polyaniline (PAn): The conductivity can reach 10S/cm-125S/cm,but polyaniline will produce benzidine in the process of forming organic conductive polymerization, which is a toxic substance. Because this problem has not been well solved, polyaniline as a cathode material for tantalum electrolytic capacitors has been subject to certain restrictions. b. Polypyrrole (PPY): Polypyrrole has good stability, and its conductivity can usually reach about 100 S/cm, but in high temperature and high humidity environment, the stability of PEDOT is better than that of polypyrrole. c. Poly (3,4-polyethylenedioxythiophene) (PEDOT): PEDOT has the characteristics of thermal stability, high conductivity (300S/cm), simple processing technology, etc. These advantages exceed the same type of materials, so PEDOT has been studied the most and become the mainstream. Polyaniline, a kind of polymer compound, has special electrical and optical properties. After doping, it can have conductivity and electrochemical properties. After certain treatment, various equipment and materials with special functions can be produced, such as urease sensors that can be used as biological or chemical sensors, electron field emission sources, electrode materials with better reversibility during charging and discharging than traditional lithium electrode materials, selective membrane materials, antistatic and electromagnetic shielding materials, conductive fibers, anti-corrosion materials, etc. Polyaniline has been widely studied and applied due to its easy availability of raw materials, simple synthesis process, and good chemical and environmental stability.
Polypyrrole is a common conductive polymer. Pure pyrrole monomer is a colorless oily liquid at room temperature. It is a C, N five-membered heterocyclic molecule with a boiling point of 129.8℃ and a density of 0.97g/cm3. It is slightly soluble in water and non-toxic.
Polypyrrole has the characteristics of high conductivity, good environmental stability and easy synthesis. It has always received widespread attention and is an ideal substitute for MnO2. The structure of the tantalum anode is relatively complex, and there is a layer of Ta2O5 dielectric oxide film on the surface. Therefore, how to minimize the damage to the dielectric oxide film and form a uniform and complete polymer film layer with high conductivity and good stability on its surface is one of the key technologies for manufacturing polymer tantalum electrolytic capacitors. The state of the polymer film layer on the surface of tantalum oxide can be controlled by controlling the electrolyte concentration, the amount of impregnation, and the addition of additives. Compared with MnO2, polypyrrole can be synthesized by a simple method, does not require thermal decomposition, and has less damage to the dielectric oxide film layer. In addition, polypyrrole tantalum electrolytic capacitors have extremely low equivalent series resistance, very small loss value, high upper limit of application frequency, good capacity-frequency characteristics and impedance-frequency characteristics, wide operating temperature range and large ripple current resistance, and are an excellent cathode material.
Polythiophene PEDT has the following advantages:
1. It has high transmittance and high conductivity in the visible spectrum
2. Minimum surface resistance up to 150Ω/cm2 (depending on manufacturing conditions)
3. Better hydrolysis resistance, photostability and thermal stability
4. At high pH, conductivity does not decrease. In addition, the conductivity of the solid-state capacitor using the conductive polymer product PEDT as the cathode material can reach 100S/, which is 100 times that of TCNQ salt and 10,000 times that of electrolyte, and there is no pollution. Solid polymer conductor capacitors also have good temperature characteristics and can withstand high temperatures of more than 300°C, so they can be installed using the SMT chip process and are also suitable for large-scale production. The safety of the solid polymer conductor capacitor is better, and when it encounters high temperatures, the electrolyte only melts without explosion, so it does not have explosion-proof grooves like ordinary aluminum electrolyte capacitors.
