Formation and retention of low Σ interfaces in PTC thermistors

The effect of SiO2 addition on the development of low-Σ grain boundaries in PTC thermistors

“…However in most other cases any variation is difficult to confirm due to statistical noise associated with the small numbers involved, although when grain boundaries in the range 5-29 are considered together, there is an overall decrease in the total population from 8.4% to 6.8%. Despite this progressive reduction in the proportion of low-grain boundaries with SiO 2 addition, the 3 twin population remained relatively constant throughout, lying in the range 3.0 ± 0.3%, which is consistent with a previous study that noted the twin density is controlled by mean grain size, and reflects a relaxation of accumulated stress 18 . In all of the samples studied here there are significantly more 3 grain boundaries than would be expected to occur by chance within a random distribution of grain orientations (around 2.7%): an observation that is in agreement with the findings of earlier workers 17,18 .…”

“…Despite this progressive reduction in the proportion of low-grain boundaries with SiO 2 addition, the 3 twin population remained relatively constant throughout, lying in the range 3.0 ± 0.3%, which is consistent with a previous study that noted the twin density is controlled by mean grain size, and reflects a relaxation of accumulated stress 18 . In all of the samples studied here there are significantly more 3 grain boundaries than would be expected to occur by chance within a random distribution of grain orientations (around 2.7%): an observation that is in agreement with the findings of earlier workers 17,18 . We also note that with SiO 2 addition there are progressive decreases, both in the proportion of 3 grain boundaries, and in the total number of grain boundaries indexable in the range 5-29.…”

Direct probing of semiconductor barium titanate via electrostatic force microscopy

“…Space loads are developed in the neighborhood of the grains' applied voltage limit (10 Volts) due a flow of electrons starting from inside the semiconducting grains to regions of high resistivity in their vicinity. This transition has been explained in terms of constant width layer of the space load, independent of grain size [6,11,13,17].…”

“…It has been accepted that the PTC in donor-doped BaTiO 3 originates from the potential barrier that is formed at the grain boundaries increasing the electrical resistance [4-8, 10, 11]. These materials display an electrical behavior characterized by an abrupt increase in resistivity as a function of temperature close to the Curie temperature (T C ) [7,8,11]. This effect occurs in barium titanate during the shift from cubic to tetragonal phase when cooling to temperatures below 130 °C and it is a grain boundary effect that depends strongly on the microstructure [11][12][13][14][15].…”

“…These materials display an electrical behavior characterized by an abrupt increase in resistivity as a function of temperature close to the Curie temperature (T C ) [7,8,11]. This effect occurs in barium titanate during the shift from cubic to tetragonal phase when cooling to temperatures below 130 °C and it is a grain boundary effect that depends strongly on the microstructure [11][12][13][14][15]. The most widely accepted models attribute the PTC phenomena to the formation of potential barriers at grain boundaries, increasing the resistivity when temperatures * Apresentado no V Encontro da Sociedade Brasileira de Pesquisa em Materiais, Florianópolis, SC (2006) …”