Development of Lead-Free PTC Thermistor Materials

The electric properties of bismuth-and sodium-modified barium titanate, Ba_1-x(Bi_1/2Na_1/2)_xTiO₃ (BBNT) were studied as a lead-free PTC (positive temperature coefficient of resistivity) thermistor material usable over 130degC. Two semiconductorization processes and their electric properties are described. Lanthanum(La)-doped BBNT ceramics sintered in air still show dielectric behaviors, but Nb-doped ones have a low resistivity _PRT on the order of 10³ Omegacm at room temperature and show a PTC behaviors. Sintering under low O₂ atmosphere produces BBNT ceramics with less than 10² Omegacm compared to those prepared in air. A trial PTC heater constructed using the BBNT semiconducting ceramics demonstrates a comparable performance of those made of the Pb-contained PTC ones. Our current research shows the BBNT ceramics with a switching temperature T_s around 210degC by increasing the (Bi_1/2Na_1/2) content x of the ceramics.