INFLUENCE OF THE MICROSTRUCTURE ON ELECTRICAL STABILITY OF NTC THERMISTORS

Thermistors with negative temperature coefficient (N.T.C.) made of spi nel structure transition metal manganites usually display ageing pheno mena under thermal constraint. The resistance drift depends on the com position, structure (cubic or tetragonal) and thermal treatments. Rela tionships between thermal treatments and ageing of the ceramics have b een studied by electron microscopy. Usually the grains of slowly coole d ceramics are free from lattice defects such as dislocation or planar defects. Otherwise quenched ceramics are always multiphase with a hig h density of dislocations and planar defects. These two dimensional de fects result in lamellar (microtwins or exsolutions) or very thin << t weed >> structures. Since ageing is believed to be correlated to atomi c diffusion in the spinel lattice, these intergranular defects could a ct as barriers against ion mobility thus explaining the better thermal stability of quenched ceramics.