Ca2+ doping tuned structure, oxygen content and electrical transport in Sr0.725Y0.275CoO3-δ for NTCR thermosensitive materials

A series of (Sr 1- x Ca x ) 0.725 Y 0.275 CoO 3-δ ceramic materials were synthesized via the sol-gel method. X-ray diffraction (XRD) measurements revealed that all the samples crystallized into a tetragonal phase with the I4 / mmm space group, and an excessive doping concentration of Ca 2+ ( x  > 0.42) would induce the formation of the Ca 3 Co 2 O 6 secondary phase. X-ray photoelectron spectroscopy (XPS) results demonstrated an increase in the Co 4+ concentration, and iodometric titration further confirmed that appropriate doping with Ca 2+ can reduce oxygen vacancies (δ) (with oxygen content increasing from 2.60 to 2.64). This led to a significant decrease in electrical resistivity owing to the elevated carrier concentration and reduced hole scattering. All the samples exhibited a negative temperature coefficient of resistance (NTCR), a typical semiconducting characteristic. For the sample with x  = 0.14, the absolute value of the NTCR reached the maximum, and its value remained less than −4%·K −1 over the operating temperature range of 230 K to 330 K, coupled with a low electrical resistivity. This sample thus holds broad application prospects as a high-stability and high-responsivity thermosensitive material for uncooled infrared detection.