Recently, electrolytic capacitor of tantalum have been demanded to be smaller in size and higher in performance. But there is few reports about sintering of tantalum. Then, in this work, the tantalum was sintered by hot isostatic pressing (HIP ping) without capsule, so-called a capsule-free HIP (CFHIP), under the high pressure of argon gas and was sintered also in vacuum furnace at 1523 and 1623 K for 1 h. The volume shrinkage with sintering was measured and the properties of pore of the sintered tantalum were measured by a mercury pressure porosimeter, too. The surface was observed by SEM and capacitance was measured. The sintering behavior and characteristics of pores of the porous tantalum compact sintered under CFHIP and in vacuum were investigated. The results obtained are as follows. The shrinkage of volume under both sintering conditions became larger with the higher sintering temperature. The volume shrinkage of tantalum sintered under CFHIP was samller than by sintering in vaccum. From the SEM image, it was obviously clear that tantalum powder was lumped larger by sintering in vaccum as compared with under CFHIP. The pore of these sintered compacts, in this experimental region, was open. The mean diameter of pores of the sintered compacts with CFHIP was smaller as compared with those sintered in vaccum and the specific surface area of the sintered compact with CFHIP was larger than that sintered in vacuum. The capacitance of the sintered compact increased with increase of its specific surface area and the capacitance of the specimen treated at 1623 K was larger.
