Decomposition of NiMn 2 O 4 spinels
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Thermal analysis and x-ray diffraction have confirmed that single-phase, cubic-spinel-structured NiMn2O4 (2:1 Mn2O3:NiO) begins to decompose into a rocksalt and a second spinel-structured phase above 907 °C. The decomposition product in samples air-quenched from 900 and 1200 °C was therefore investigated using transmission electron microscopy. Samples quenched from 900 °C (below decomposition temperature) did not show the expected single-phase microstructure, but instead grains contained nanoregions of a lenticular fringe contrast. Samples quenched from 1050 to 1200 °C were generally composed of Mn-rich spinel grains in addition to grains containing Mn-rich spinel precipitates (30–50 nm) surrounded by a Ni-rich rocksalt matrix. As temperature increased, the spinel grains and precipitates became clearly tetragonal, exhibiting a ferroelastic domain structure arising from a cooperative Jahn–Teller distortion. A decomposition mechanism based on the degree of inversion is proposed to explain these microstructures. Slow cooling samples from 1250 °C resulted in partial recomposition, leading to a microstructure principally composed of cubic spinel and regions of much smaller spinel structured precipitates (50–120 nm) in a rocksalt structured matrix. The slow-cooled samples showed a larger increase in resistance over time than single-phase samples did when held at 400 °C. X-ray diffraction measurements carried out before and after electrical characterization showed a reduction in the amount of rocksalt structured material present in slow-cooled samples.
I. INTRODUCTION
Negative temperature coefficient (NTC) thermistors are widely used in industrial and domestic applications where cost-efficient but reliable temperature sensing, fluid flow rate, or pressure-sensing devices are required. NTC devices are also used to protect circuitry against surges in current, exploiting the self-heating characteristics of a resistor. Early thermistors were based on simple transition-metal oxides such as Fe3O4 and CoO but were susceptible to oxidation and therefore unreliable. Today over 80% of commercial NTC devices are fabricated from manganese-oxide-based ceramics combined with other transition-metal ions such as iron, cobalt, copper, and nickel. Modifying composition allows tuning of the resistivity of the materials (measured in Ohm meters) and also of their temperature coefficient (measured in Kelvin and termed the B-value). The spinel structure can be viewed as a 2 × 2 × 2 array of face-centered-cubic oxygen sub-unit cells in which 16 out of the 32 octahedral and 8 out of 64 tetrahedral sites are occupied. NiMn2O4 is a predominantly inverse cubic spinel (Fd3¯m) but as x increases in the solid solution, Ni(1−x)Mn(2+x)O4, the structure transforms to a tetragonal (I41/amd), normal spinel1 (Fig. 1). The phase boundary J. Mater. Res., Vol. 18, No. 6, Jun 2003
at room temperature between cubic and tetragonal is nominally considered to be at a Mn:Ni ratio of approximately 41⁄4:1.2 The tetragonal distortion arises from the cooperative intera
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