Current-limiting property of n-BaTiO 3 ceramics

Abstract

The current-voltage ( I–V) relations of donor-doped BaTiO 3 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 BaTiO 3, 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.