Influence of Nb2O5 on the varistor behavior of TiO2–Cr2O3 system

• Published in: Journal of materials science. Materials in electronics Vol. 24; no. 3; pp. 938 - 944
• Main Authors: Follador Neto, Ricardo Guido, de Souza, Eder Carlos Ferreira, de Andrade, André Vitor Chaves, Antunes, Sandra Regina Masetto, Antunes, Augusto Celso
• Format: Journal Article
• Language: English
• Published: Boston Springer US 01-03-2013; Springer
• Subjects: Applied sciences; Characterization and Evaluation of Materials; Chemistry and Materials Science; Electronic equipment and fabrication. Passive components, printed wiring boards, connectics; Electronics; Exact sciences and technology; Fractures; Instruments, apparatus, components and techniques common to several branches of physics and astronomy; Materials; Materials Science; Mechanical properties and methods of testing. Rheology. Fracture mechanics. Tribology; Metals. Metallurgy; Optical and Electronic Materials; Physics; Scanning probe microscopes, components and techniques; Niobium Oxide; Potential Barrier Height; Nb2O5; Breakdown Electric Field; TiO2; Potential barrier; Ternary system; Material processing; Doping; Porous material; Grain boundary; Low voltage; Cooling by air; Protection system; Niobium oxide; Damaging; Densification; Ball mill; Cooling rate; Protective device; Homogenization; Varistance; Low energy; Electrical measurement; Electric field; Chromium oxide; Sintering; Electric field effect; Non linear effect; Low-power electronics; Titanium oxide
• ISSN: 0957-4522; 1573-482X
• DOI: 10.1007/s10854-012-0854-y

This work investigated the influence of Nb
2
O
5
dopant on the varistor behavior of the ternary system (99.95 − x)%TiO
2
, 0.05 %Cr
2
O
3
, x%Nb
2
O
5
, where x = 0.10; 0.15; 0.20; 0.25 % in mol. The processing was carried out using the conventional oxide mixture method. The initial oxides were homogenized in alcoholic media in a ball mill, for 3 h, dried in oven and isostatically pressed at 210 MPa and sintered at 1,400 °C for 2 h in air atmosphere and cooled at 5 °C/min cooling rate, resulting in pellets with high densification. Electrical measurements in continuous current at different temperatures revealed that for the lowest Nb
2
O
5
concentration the breakdown electric field, E
B
 = 4.41 V/cm and non-linear coefficient, α = 4.6 were obtained, and for the highest Nb
2
O
5
concentration the breakdown electric field, E
B
 = 9.71 V/cm and non-linear coefficient, α = 15.3 were obtained. These low values in the breakdown electric field enable these varistor systems to be used in protection systems for low-voltage energy grids. Changes in the potential barrier present in the grain–grain boundaries could also be observed, in which the height increased and the width decreased with the increase in the dopant concentration. However, for x = 0.25 % in mol Nb
2
O
5
, a significant reduction in the potential barrier height and the voltage per barrier was obtained. Furthermore, the tendency to increase the volume of the unitary cell with the increase of dopant concentration, which was interrupted in the system with 0.25 % Nb
2
O
5
, was another evidence that suggested the occurrence of Nb
2
O
5
segregation in the grain boundaries, or even that the reduction in the average grain size could possibly dilute Nb
2
O
5
concentration in the grain boundaries. The average grain size was calculated through SEM micrographs and ranged from 7 to 16 μm, with larger sizes occurring for lower dopant concentrations and presenting higher porosity and lower uniformity in the grains shape.