Tantalum is often used to make capacitors because the anodized surface forms an amorphous oxide film with a high dielectric constant. Tantalum wire is used as the anode lead for capacitors. One of the key indicators of capacitors is the leakage current performance. The hardness characteristics of tantalum wire have a great influence on the leakage current. High hardness tantalum wire cannot be used to manufacture capacitors.
Tantalum wire can be divided into three types according to different tensile strength :(1) soft state (M), tensile strength σb = 300 ~ 600MPa; (2) Semi-hard (Y2), tensile strength σb = 600 ~ 1000MPa; (3) Hard (Y), tensile strength σb> 1000MPa. The traditional tantalum wire manufacturing process is as follows:
Powder forming (bar making)→ sintering → rolling → surface oxidation film forming → large drawing → intermediate annealing → small drawing (straightening)→ cleaning → rewinding → packing and warehousing
Annealing of tantalum wire can eliminate the residual stress and improve the microstructure by recrystallization. At present, tantalum wire annealing is generally carried out in vacuum heat treatment furnace with the annealing temperature basically at 1400-1500℃. After annealing, the finished product is straightened by wire drawing machine. This method can be used to produce hard tantalum wire, but when producing semi-hard or soft tantalum wire, it is difficult to straighten the annealed tantalum wire and its flatness is low. The flatness of tantalum wire refers to the ratio of the distance between the center line of the two endpoints of 200mm tantalum wire and the center line of tantalum wire after the natural loosening and the length of tantalum wire. The flatness is a dimensionless value, and the smaller the value, the straighter the tantalum wire. There are strict requirements for the flatness of tantalum wire for preparing capacitors, so it is necessary to find a suitable annealing device and method.
