Microstructure and Electrical Properties of Mn-Ni-In Spinels

Microstructural investigations of negative temperature coefficient (NTC) ceramics in the Mn–Ni–In–O system have been performed using X-ray diffraction (XRD), scanning (SEM) and transmission electron microscopy (TEM). The electrical properties have been characterized by measurements of resistance, activation 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/cubic phase boundary. It is concluded that the Mn3+ concentration on octahedral sites increases due to aging. The domain configuration changes with increasing Ni content: samples with low Ni content reveal domain laths (∼100 nm width) with internal twinning (<10 nm), samples near to the boundary exhibit finer scaled (5–10 nm), weakly curved domains without twinning 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 microstructure as the corresponding non-aged ones, whereas aged Mn2.32Ni0.58In0.1O4 is more comparable with Mn2.40Ni0.50In0.1O4. Consequently, the choice of composition with respect to the phase boundary is decisive for the electrical and microstructural behaviour.