Influence of SiO2 addition on the PTCR characteristics of Ba0.92(Bi0.5K0.5)0.08TiO3 ceramics

Abstract

As a candidate of lead-free positive temperature coefficient of resistivity (PTCR) materials, Ba 0.92 (Bi 0.5 K 0.5 ) 0.08 TiO 3 -0.0005Nb 2 O 5 -xSiO 2 (x = 0-0.03) ceramics were prepared by the conventional solid state reaction method. The effects of SiO 2 addition on the crystal structure, microstructure and PTCR characteristics have been investigated. For x C 0.01, the secondary phases of Ba 2 TiSi 2 O 8 and BaSiO 3 were found. SEM-EDS spectra revealed that the secondary phases existed at intergranular regions as liquid phase resulted in the enhancement of densification and the optimization of PTC effect. Scanning electron microscopy observations revealed that the grain size did not change greatly for all samples. With the increasing SiO 2 content, the room temperature resistivity increased. For x = 0.001, the Curie temperature of the sample increased to 149.5°C. A further study was conducted by impedance spectroscopy so as to study the effects of SiO 2 addition on the resistivity of grains and grain boundaries.

Abnormal resistivity-temperature characteristic in fluorite type Bi/K-substituted ceria ceramics

“…However, due to the environmental pollution of Pb-contained compounds, the (Ba,Pb)TiO 3 -based products are going to be prohibited in the electric devices. Some new types of lead-free PTC materials such as BaTiO 3 -(Bi 1/ 2 Na 1/2 )TiO 3 and BaTiO 3 -(Bi 1/2 K 1/2 )TiO 3 systems have been developed in past decade [9][10][11]. The related PTC conduction mechanisms were found to follow the Heywang-Jonker model, in which the grain boundary effect plays a key role in the resistivity jump around the Curie temperatures, as shown in the BaTiO 3 -based thermistors [6][7][8].…”