Microstructural investigations of negative temperature coefficient (NTC) ce
ramics in the Mn-Ni-In-O system have been performed using X-ray diffraction
(XRD), scanning (SEM) and transmission electron microscopy (TEM). The elec
trical properties have been characterized by measurements of resistance, ac
tivation energy and aging. The replacement of Mn by In in Mn2.9-xNixIn0.1O4
(x = 0.50-0.66) leads to higher resistivities and thermal constants, both
decreasing with increasing Ni content, and minimum aging of 0.1% for x = 0.
58. Microstructural changes deduced from the a/c ratio and caused by aging
are observable for Mn2.32Ni0.58In0.1O4 which is nearest to the tetragonal/c
ubic phase boundary. It is concluded that the Mn3+ concentration on octahed
ral sites increases due to aging. The domain configuration changes with inc
reasing Ni content: samples with low Ni content reveal domain laths (simila
r to 100 nm width) with internal twinning (<10 nm), samples near to the bou
ndary exhibit finer scaled (5-10 nm), weakly curved domains without twinnin
g and for high Ni contents a slightly increased domain width of 5-15 nm is
observed. With the exception of x = 0.58, all aged samples show the same mi
crostructure as the corresponding non-aged ones, whereas aged Mn2.32Ni0.58I
n0.1O4 is more comparable with Mn2.40Ni0.50In0.1O4. Consequently, the choic
e of composition with respect to the phase boundary is decisive for the ele
ctrical and microstructural behaviour.