SURFACE-CHEMISTRY AND SINTERING CHARACTERISTICS OF NICKEL ULTRAFINE POWDERS

The surface chemistry of nickel ultrafine powders (UFPs) was examined by temperature programmed desorption (TPD) measurements. Nickel UFPs e xposed to air contained NiO and Ni(OH)2. The oxide phase increased dur ing heating in vacuum mainly due to the adsorbed gases and to the deco mposition of the Ni(OH)2. Sintering characteristics of three types of nickel UFP in vacuum and a stream of hydrogen were examined by measuri ng their dimensional changes, specific surface areas, crystallite size s, and structural changes. In the case of vacuum sintering of clean ni ckel UFPs, neck growth by surface diffusion occurs even at room temper ature and densification by grain boundary and/or lattice diffusion sta rts below 400 K. On the other hand, the sintering diagram of the nicke l UFPs exposed to air can be divided into three regions: the first reg ion where only gas desorption is observed, the second where only the g rowth of surface oxide is observed, and the third where both oxide and nickel sinter. The sintering shrinkage rate in hydrogen showed two pe aks. One is due to the rearrangement by a reduction reaction of surfac e oxide and the other is due to densification by grain boundary and/or lattice diffusion. For the latter peak, the smaller the particle size the lower is the peak temperature.