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Current-limiting property of $n-BaTiO_3$ ceramics

Kutty, TRN and Ravi, V (1994) Current-limiting property of $n-BaTiO_3$ ceramics. In: Materials Science & Engineering B, 25 (2-3). pp. 119-131.

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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 currentlimiting 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 $l_{lim}$ 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 hetero-junction 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.

Item Type: Journal Article
Publication: Materials Science & Engineering B
Publisher: Elsevier
Additional Information: The copyright of this article belongs to Elsevier.
Department/Centre: Division of Chemical Sciences > Materials Research Centre
Date Deposited: 15 Sep 2006
Last Modified: 19 Sep 2010 04:30
URI: http://eprints.iisc.ac.in/id/eprint/8148

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