Mechanical properties of nickel manganites-based ceramics used as negative temperature coefficient thermistors (NTC)

A series of nickel manganite ceramics of composition NixMn3−xO4 (0.28 ≤ x ≤ 1) has been prepared by sintering powders obtained using two different ways: the classic solid/solid reaction and the co-precipitation method (soft chemistry). The evolution of the three-point bend strength has been correlated to the density and the structure of the ceramics. As expected, the strength decreases with increasing porosity. A correlation has been observed between the flexure strength and the structure of the spinel phase. The bend strength increases as the structural distortion (c/a ratio) increases, probably due to a lowering of symmetry rather then a modification of the bond strength. Moreover, we show that a modification of the oxidation state does not affect the value of the flexure strength.