Negative temperature coefficient resistors NTC thermistors

The TCR of a semiconductor is expected to be negative (see Section 2.6) whether the conducting electrons move in a conduction band, as for example in SiC, or hop between localized sites as is believed to occur in lithium-doped NiO (see Section 2.6.2). In each case the resistivity p depends on temperature according to 

Mn304 is a normal spinel with Mn2+ on tetrahedral sites and Mn3+ on octahedral sites. Therefore it is not very conductive since it does not contain ions of the same element with differing charges on similar sites, as required for electron hopping. The substitution of Ni for Mn increases the conductivity. Ni2+ 

Mixtures of the rare earth oxides, e.g. 70 cat.% Sm and 30 cat.% Tb, can be used at temperatures up to 1000 °C as they have no tendency to lose or gain oxygen. Since their room temperature resistivity is of the order of 107 Qm, they are not convenient for room temperature use. At 600 °C the resistivity is about 25 Om with B = 6500 K, giving aR = —0.9% K-1. 

Electrodes are commonly silver fired on with an admixture of glaze, but electroless nickel and vacuum-deposited metals can also be used. A final coating of a glaze over the whole body improves the long-term stability. 

Miniature bead thermistors, which are particularly valuable as temperature sensors, can be made by arranging two sets of fine platinum wires at right angles with a separation of a fraction of a millimetre between the sets. The intersections of the wires are then enclosed by small beads of paste containing the thermistor material in powder form. The beads are dried out and sintered in the same way as bulk units or fused individually with a laser beam or an oxidizing flame and annealed. The beads are then separated with two platinum lead wires which can be attached to a probe. Their small mass enables them to reach a rapid thermal equilibrium with their surroundings. 

---