Electrical properties of SnO₂-based varistor ceramics with solid-phase and liquid-phase sintering

Abstract

The varistor effect takes place in SnO2-Co3O4-Nb2O5-Cr2O3 ceramics with CaO, SrO or BaO additions prepared by solid-phase sintering and in the same ceramics with CuO, V2O5 or Bi2O3 additions prepared by liquid-phase sintering. This effect is accompanied by a humidity-sensitive effect at the low-field electrical conductivity. The latter is observed when air relative humidity increases from 10 to 93%. The lowest humidity-sensitive effect was found in the liquid-phase sintering samples. For the investigated samples the values of the nonlinearity coefficient 24-50 at high electric fields (E1 = 2.8 − 11 kV cm −1 ) and the humidity sensitivity coefficient 1.8•10 3 -3.2•10 5 at low electric fields were calculated. Such properties of tin oxide based ceramics can be explained within the model of electrical conduction controlled by the grain-boundary potential barriers and the dissociative adsorption of water molecules on the samples surface. This leads to a low decrease of the intergranular potential barrier height in the humid air and an increase in the conductivity in low electric fields. At the same time in the high electric fields the barrier height decreases more intensively with an increase in the electric field and this leads to the varistor effect.

Electrical properties of tin oxide based varistors with PbO addition in humid air

“…The samples with such additions have liquid phases in the process of sintering, that is why the sensitivity of these ceramics to the environment decreases. But the humidity sensitivity coefficient of these samples remains quite large [3].…”

“…Tin oxide SnO 2 is widely used for the production of optically transparent conducting materials, gas sensitive sensors and varistors [1][2][3]. One of the important properties of SnO 2 based semiconductor materials is their great sensitivity to the relative air humidity [3][4][5][6][7][8][9][10]. The electric conductivity of tin oxide ceramics in the low electric field grows with the increase of air humidity [3][4][5][6][7][8][9].…”

“…One of the important properties of SnO 2 based semiconductor materials is their great sensitivity to the relative air humidity [3][4][5][6][7][8][9][10]. The electric conductivity of tin oxide ceramics in the low electric field grows with the increase of air humidity [3][4][5][6][7][8][9]. The growth of electrical conductivity is undesirable when tin oxide based varistors are produced, especially when they are designed for electronic device overvoltage protection.…”

“…So, such effect should be avoided or minimized. For this purpose, the surface of ceramic samples is protected with different moisture-resistant films [4,6] or different additions, mainly those which decrease the moisture influence on SnO 2 crystallite boundaries [3,5,[7][8][9].…”

“…Nowadays, the actual task is to decrease the sensitivity of this system to the air humidity. Earlier, this effect was achieved with adding Bi 2 O 3 , V 2 O 2 and CuO to the SnO 2 -Co 3 O 4 -Nb 2 O 5 -Cr 2 O 3 system [3,5,[7][8][9]. Bi 2 O 3 , V 2 O 2 and CuO oxides have low melting temperatures, which are less than burning temperatures of ceramics.…”