Analysis of the admittance-frequency and capacitance–voltage of dense SnO2⋅CoO-based varistor ceramics

Abstract

This article describes the admittance-frequency feature of a class of SnO 2 •CoO-based polycrystalline ceramics with high nonlinear current-voltage characteristics ͑nonlinear coefficients above 50͒. Broad relaxation peaks caused by the presence of deep trap states were characterized based on the admittance response of different systems doped with La 2 O 3 , Pr 2 O 3 , and CeO 2. The calculation of the energy of this deep trap level revealed not only that all the compositions share the same value but also that this value could be attributed to an oxygen vacancy or to Co Sn like defects. The values of barrier height and density of states obtained from a capacitance-voltage analysis are in good agreement with the nonlinear coefficients. The highest nonlinear coefficients are found in compositions with greater barrier height values and higher density of deep trap states at the grain boundary interface.

Electrostatic force microscopy as a tool to estimate the number of active potential barriers in dense non-Ohmic polycrystalline SnO2 devices

“…͑1͒ combined with a complex capacitance analysis. [6][7][8] Table I presents the comparative values and other typical non-Ohmic parameters of the MOV system studied here. Therefore, ␣ and electric breakdown field ͑E b ͒ were found to be about 75 and 6900 V cm −1 , respectively.…”

“…useful in calculating the barrier height value and in establishing a good relationship between potential barrier height and non-Ohmic properties. 6,7 However, in most situations, the barrier height values may be poorly estimated if the materials contain few active barriers, mainly because the calculated p value would be higher than the real value. In other words, the p value is a source of error in the calculation of the potential barrier height and Eq.…”

“…The cause of GB oxidation is related to oxidized transition metals, e.g., Co, Mn, Cr, and Pr, segregated in grain boundary regions. 6,7 The increase in non-Ohmic properties due to the higher degree of oxidation has already been proven to cause an increase in the mean barrier height values and in the amount of active ͑or effective͒ barriers. [6][7][8] Therefore, the number of active barriers is an important parameter for the non-Ohmic properties, mainly because they are very sensitive to structural and compositional variations.…”

“…3,6 The Schottky-type barrier is composed of negative charges trapped in the GB region, balanced by positive charges in the depletion layer in the zone adjacent to the grains. 6 It is widely accepted that the oxygen species adsorbed at the interface between grains 6 increase the negative charge trap and, therefore, the height of the potential barrier, 6,7 promoting an increase in non-Ohmic behavior. The cause of GB oxidation is related to oxidized transition metals, e.g., Co, Mn, Cr, and Pr, segregated in grain boundary regions.…”

“…9,10 A larger number of active barriers due to GB oxidation, for example, is the cause of decreases in leakage current. 6,7 It is therefore important to describe how the back-to-back Schottky-type barrier height can be estimated based on the voltage dependence of the GB capacitance. Calculations of the GB capacitance are made from the high frequency intercept associated with high frequency relaxation in the complex capacitance plane.…”