CURRENT-LIMITING PROPERTY OF N-BATIO3 CERAMICS

The current-voltage (I-V) relations of donor-doped BaTiO3 ceramics have four distinct regions in increasing order of applied potentials: (i) a linear portion at lower voltages which includes a current maximum, (ii) a negative differential conductivity (NDC) region, (iii) a nearly constant current segment and (iv) a slow upturn behaviour. For the bulk compositions having the isovalent lattice substituents, the NDC region is broadened out and the body temperatures T(b) attained vary with the shift in Curie point T(C). In the case of ceramics with mixed-phase character (tetragonal and cubic), the linear portion directly gives way to current-limiting behaviour without any maximum. The T(b) values attained during the current-limiting process are much lower than T(C). When T(a) > T(C), the I-V characteristics change from current limiting to voltage limiting (varistor). Under these thermal conditions, T(b) is always greater than T(C). The varistor behaviour could be achieved at room temperature by using isovalent substituents. The present results show that the current-limiting behaviour cannot be treated as a mere consequence of the positive temperature coefficient of resistance (PTCR). This is because the limiting currents I(lim) calculated from the resistivity-temperature relations in conjunction with body temperatures are three to five orders of magnitude lower than the measured values. Furthermore, current-limiting behaviour has been noted for certain titanate ceramics having no PTCR. The present observations indicate that current-limiting behaviour arises from the combined influence of Joule heating and field effect. Joule heating increases the cubic phase content much below T(C) owing to diffuse phase transformation behaviour in semiconducting BaTiO3, a fact which has been experimentally demonstrated in the present investigation. The charge-trapping behaviours of the midband gap states, particularly of the acceptor type, are different for the tetragonal and the cubic phases. This imparts a heterojunction character to the tetragonal-cubic interface regions. Tunnelling across such an asymmetric barrier, generated under Joule heating, will account for the current-limiting characteristics, whereas tunnelling across a more symmetric barrier, at T(a) > T(C), leads to voltage-limiting behaviour.