C. Gillot et al., MICROSCOPIC ORIGIN OF THE PTC EFFECT IN NIOBIUM-DOPED BARIUM-TITANATE, Journal of the American Ceramic Society, 80(4), 1997, pp. 1043-1046
The influence of bulk conductivity on the PTC effect of Nb-doped bariu
m titanate is investigated. Both macroscopic and microscopic resistanc
e measurements are performed as a function of temperature. The macrosc
opic measurement refers to the whole sample while the microscopic one,
obtained by photolithography techniques, concerns only several grains
. The results show that the increase of resistivity occurs in two step
s: first, an abrupt variation at the Curie temperature T-c which is as
cribed to the phase transition in the bulk; second, a continuous incre
ase above T-c which is usually explained from grain boundary potential
barriers only. However, our results suggest that a nonnegligible cont
ribution of the bulk is also involved in this increase above T-c.