# Effect of secondary-phase segregation on the positive temperature coefficient in resistance characteristics of $n-BaTio_3$ ceramics

Hari, NS and Kutty, TRN (1998) Effect of secondary-phase segregation on the positive temperature coefficient in resistance characteristics of $n-BaTio_3$ ceramics. In: Journal of Materials Science, 33 (13). 3275 -3284.

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## Abstract

Modifications in the positive temperature coefficient in resistance (PTCR) of $n-BaTi0_3$ ceramics are brought about by specific additives such as $Al_2O_3,\hspace{2mm}B_2O_3\hspace{2mm}or\hspace{2mm}SiO_2$, leading to the segregation of secondary phases such as $BaAl_6TiO_12,\hspace{2mm}BaB_6TiO_12\hspace{2mm}or\hspace{2mm}BaTiSu_3O_9$ at the grain boundaries. Segregation of barium aluminotitanates resulted in broad PTCR curves, whereas $B_2O_3$ addition gave rise to steeper jumps and $SiO_2$ addition did not result in much broadening compared with donor-only doped samples. Microstructural studies clearly show the formation of a structurally coherent expitaxial second phase layer of barium aluminotitanate surrounding the $BaTiO_3$ grains. Electron paramagnetic resonance investigations indicated barium vacancies, $V_{Ba}$, as the major electron trap centres which are activated across the tetragonal-to-cubic phase transition according to the process $V^X_\hspace{3mm}{Ba^+e^\prime\rightleftharpoons} V^\prime_\hspace{2mm}{Ba}$. The grain size dependence of the intensity of the $V^\prime_{Ba}$ signal indicated the concentration of these trap centers in the grain-boundary layer (GBL) regions. Further, the charge occupancy of these centres is modified by the secondary phases formed through grain-boundary segregation layers. $BaAl_6TiO_12$ gave rise to $Al-O^-$ hole centres whereas no paramagnetic centres corresponding to boron could be detected on $B_2O_3$ addition. Such secondary phases, forming epitaxial layers over the $BaTiO_3$ grains, modify the GBL region, rich in electron traps, surrounding the grain core. The complex impedance analyses support this three-layer structure, showing the corresponding contributions to the total resistance which can be assigned as $R_g, R_{gb} and R_{secondary phase}$. The epitaxial second phase layers bring about inhomogeneity in the spatial distribution of acceptor states between the grain boundary and the grain bulk resulting in extended diffuse phase transition characteristics for the GBL regions in $n-BaTiO_3$ ceramics. This can cause the GBL regions to have different transition temperatures from the grain bulk and a spread in energy levels of the associated GBL trap states, thus modifying the PTCR curves. An attempt has been made to explain the results based on the vibronic interactions applied to the mid-band-gap states in $n-BaTiO_3$.

Item Type: Journal Article Publisher Copyright of this article belongs to Springer Netherlands. Division of Chemical Sciences > Materials Research Centre 30 Aug 2006 02 May 2011 08:38 http://eprints.iisc.ernet.in/id/eprint/8093