Influence of yttrium oxide addition on the characteristics of SnO 2 based ceramics

In the presented paper the microstructure and the electrical characteristics of (99.9- x ) SnO 2 - x Y 2 O 3 - 0.05 Nb 2 O 5 - 0.05 Cr 2 O 3 (mol. %, x = 0, 0.3, 0.5 and 0.7) varistor ceramics in a wide range of current densities 10 −9 - 10 2  A∙cm −2 have been considered. In all samples with Y 2 O 3 addition the SnO 2 cassiterite phase with rutile-type structure and Y 2 Sn 2 O 7 phase with pyrochlore-type structure were observed. The values of the some parameters of ceramics have been calculated and their average values have been found. The lowest shrinkage and average grain size and the highest grain resistivity have the samples with 0.5 mol. % Y 2 O 3 addition. Such ceramics has the largest values of nonlinearity coefficient 48.6, electric breakdown field 14.6 kV∙cm −1, grain boundary breakdown voltage 2.3 V∙barrier −1, activation energy of electrical conduction 0.92 eV and the lowest ones of leakage current 0.09 μA and low-field electrical conductivity 2.9∙10 −12 Ohm −1 ∙cm −1. The simultaneously coordinated changes of structural and electrical parameters of samples prove the barrier mechanism of electrical conductivity in SnO 2 - Y 2 O 3 – Nb 2 O 5 - Cr 2 O 3 varistors. The addition of yttrium oxide to tin-oxide ceramics allows increasing the nonlinearity of voltage-current characteristics and considerable decreasing the leakage current of samples. These effects were due to an increase of the amount of yttrium stannate secondary precipitated pyrochlore phase Y 2 Sn 2 O 7 in ceramics.