A D 3
212
Fig.3: Micrographs of Cug2Nig.1Co1.4Mn; 304 ceramic samples sintered at: a - 1370 K (x500); b -
1470 K (x400); ¢ - 1570 K (x400 magnification).
Ceramic samples obtained at (0.75-0.85)7, (regime 2) have large grains uniformly distributed on
the surface (fig.3b). Micrographs clearly reveal the rounded pores localized on the boundaries and,
rarely, inside of grains. The amount of these pores is not so large as in the case of regime 1 but their
effective diameter is bigger. It is evident that above mentioned structural changes at the increasing
of sintering temperature are caused by the coalescence and repair of smaller pores. This process
strongly depends on the dimensions of initial pores. If the initial pores are larger of the surrounding
crystallite grains than repair takes place owing to viscous run off under the influence of capillary
pressure and volume diffusion of the substance into the pore. If pores are smaller of crystallites,
only the repair mechanism with volume diffusion takes place. Mainly, ceramic composites sintered
at regime 2 are homogeneous solid solutions with spinel structure. Such materials have stable
recurrent parameters and they are proper for the development of NTC thermistors. Met
High-temperature sintering at 7,>0.857 (regime 3) leads to the repeated recrystallization of
semiconducting ceramic composites which is caused by thermally activated movement of intergrain
boundaries. This conclusion is confirmed by large diameter of grains, existence of pores as inside of
grains as on the boundaries, clear orientation of separate grains, broadening and spheroidization of
pores, increasing of amount of external inclusions situated on the boundaries and in the grains
(fig.3c). Repeated recrystallization leads to the decrease of density and extracting of additional
phases that is confirmed by X-ray phase analysis. Thus, as a rule, ceramics sintered at regime 3 is
multiphase and can not be used for the manufacturing of thermistor elements. Moreover, for
ceramic compositions with large concentration of Co and Mn the clasterization of spinel structure
on the grain level takes place.
Conclusions
Thus it is shown that optimum for practical application in NTC thermistors are single phase solid
solutions of Cu,Nij.xyCo2yMnz2.,O4 semiconducting ceramics possessing statistically uniform
distribution of crystalline grains. The optimal temperature of high-temperature sintering is
determined by the cationic composition and lies in the region of (0.75-0.85) 7m.
References
(1) M. Hosseini: Ceramics International. 26 (2000) 245.
(2) T. Battault, R. Legrous, A. Rousset: J. Mater. Synt. & Proces. 4 (1996), 361.
(3) 1.V. Hadzaman, A P. Kovalsky, O.Ya. Mrooz, O.1. Shpotyuk: Materials Letters. 29 (1996), 195.
