Double perovskite oxides Gd 0.8 Ca 0.2 BaCo 2 O 5+δ have been successfully prepared by EDTA-citrate complexation process. The prepared samples were characterized by means of thermogravimetry–differential analysis, X-ray diffraction, Fourier transform infrared spectroscopy techniques. These analyses suggested that pure Gd 0.8 Ca 0.2 BaCo 2 O 5+δ phase could be formed when the pH value was maintained above three and calcination temperature was higher than 850 °C. The conductivity properties analyzed by means of four probes method, scanning electron microscopy and X-ray photoelectron spectroscopy indicated an improvement in conductivity with a high value of nearly 1200 S/cm, which was due to an increase in the number of small polaron hopping conduction units, Co 3+ –O–Co 4+ . Interestingly, the Gd 0.8 Ca 0.2 BaCo 2 O 5+δ samples exhibited a semiconducting behavior through a wide range of temperature below 100 °C and a metallic behavior at 100–500 °C, thereby demonstrating that Gd 0.8 Ca 0.2 BaCo 2 O 5+δ could meet the requirements of a smart material.
For high temperature thermistor applications, the MgAl 2 O 4 exhibits poor conductivity and requires high sintering temperature for ceramic densification. In this study, we have reported on the preparation and characterization of spinel Mg 0.5 Mn 0.5 AlFeO 4 ceramics by solid solution modification. The resultant ceramics exhibit a combination of high temperature stability and semi-conductivity, which are the characteristics of MgAl 2 O 4 and MnFe 2 O 4 . The material has a moderate thermistor constant, which is in favor of application at high temperatures and a wide temperature range. The electrical conductivity is due to the small-polaron hopping in different valence states of manganese and iron ions . Our findings demonstrate a promising approach to extend the applicability of spinel ceramics for high temperature thermistors.
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