CRYOGENIC SENSORS FROM SEMICONDUCTING BARIUM-TITANATE CERAMICS WITH STRONG NEGATIVE TEMPERATURE-COEFFICIENT OF RESISTANCE

Stable sensors for low-temperature applications are realized from dono r-doped BaTiO3 polycrystalline ceramics having a strong negative tempe rature coefficient of resistivity (NTCR) below the orthorhombic-rhombo hedral phase-transition point, from 10(2)-10(3) Omega cm at 190 K to 1 0(10)-10(13) Omega cm at approximate to 50 K, and a thermal coefficien t of resistance, alpha = 20-23% K-1. The temperature region of the NTC R can be modified by substituting isovalent ions in the lattice. Highl y non-linear current-voltage (I-V) curves are observed at low temperat ures, with a voltage maximum followed by the negative differential res istance. The I-V curves are sensitive to dissipation, so that cryogeni c sensors can be fabricated for liquid-level control, flow-rate monito ring, radiation detection or in-rush voltage limiting. The observed NT CR of n-BaTiO3 ceramics at low temperatures is made use of in evaluati ng the composition of aliphatic hydrocarbon (petrol)-alcohol mixtures through the determination of freezing points, since these liquid mixtu res are used as an alternative fuel in internal combustion engines.