Research On The Preparation Process Of Niobium Suboxide Electrolytic Capacitor

The electrolytic capacitor has a huge market because of its high specific capacity,small volume and lightweight. Although aluminum electrolytic capacitor is relativelycheap, the capacitance changes a lot when it was used and its reliability is low. Thetantalum electrolytic capacitor has good performances and higher reliability, but itsprice is much higher since the shortage in resources of tantalum came up in1999.Niobium metal has similar physical and chemical properties with tantalum, as it appearsnext to tantalum in the periodic table. Niobium oreis much more abundant and is lessexpensive than tantalum. This has given the opportunity for tantalumcapacitormanufacturers to evaluate niobium as a potential alternative to tantalum metal.Butthe diffusion rate of oxygen from theNb2O5dielectric to niobium matrix is highercompared to tantalum, resulting in DCL instability.Niobium suboxide, especially NbO,is a hard ceramic material characterized by high conductivity, a property usuallyassociated with metals.The diffusion rate of oxygen from the dielectric to NbO matrix ismuch smaller, which means the performance of NbO capacitor was much better thanniobium metal capacitor. So the research on the manufacturing technology of NbOelectrolytic capacitor is of great academically and commercially valuable.The present research was based on themanufacturing technology of Tantalumelectrolytic capacitor. The production technology was optimized and related mechanismwas studied making use of the production line of Tantalum electrolytic capacitor.Firstly, the relationships between the press density, sintering temperature and theelectrical properties of NbO were studied. The differences of external features andelectrical performances between samples obtained after anodizing, MnO2decomposition and aging were studied. The temperature and time of heat treatment topellets after anodizing was optimized, and their influences on the electricalperformances of capacitor were studied. Frequency characteristics, resistance tosoldering heat and V-A characteristics of prepared NbO capacitors were tested.The influence of iterative heat treatment of impregnated aqueous Mn(NO3)2solution to the microstructure of MnO2produced has been investigated in the process tofabricate niobium suboxide capacitors.We separate the whole process to two stages: Atthe early stage of impregnationsin Mn(NO3)2solution (with specific density smaller than1.35g/cm3), produced MnO2grains mainly locate in the inner space and pores,which present equiaxed nanocrystalline morphology. As for impregnationsin Mn(NO3)2solution with specific density bigger than1.35g/cm3, MnO2grains in the inner spaceand pores continue to grow and present a hexagonal pyramid shape. In this stage, MnO2are start to be produced on the outer surface of pellets and exhibit a cluster morphology,which consists of MnO2grainshaving a size between30~80nm. The electrical propertiesof NbO capacitors are optimized by adjusting impregnation times and sequences. Byalternately impregnating in Mn(NO3)2solutions with specific density of1.23and1.35g/cm3, MnO2grains are better combined and internal space of the pellets are fully filled.Impregnation in Mn(NO3)2solutionwith small specific density(1.10and1.23g/cm3) indry atmosphere produces denser MnO2layer in the internal space, and improvedperformance of capacitors is obtained.Surface treatment was carried on pellets after MnO2decomposition, by usingcoupling agent solution (KH550). The binding force betweenMnO2and graphite layerwas reinforced and change rate of electrical properties was reduced. Benzyl alcoholcompounds and water were used as dispersing agent for colloidal graphite. Thedifferences in electrical performances of capacitors which respectively immersed inthese two kinds of graphite dispersion were studied.