M(1-x)CaxMnO3( M=La, Y) thin films for high temperature thin film thermocouple applications

In this study, rare earth manganates have been investigated for high temperature thin film thermocouple applications. Thin films of La(1-x)CaxMnO3 and Y(1-x)CaxMnO3 were deposited on to sapphire substrates using pulsed laser ablation technique and annealed at temperatures ranging from 1173 K to 1573 K in air. Their thermal stability was studied in terms of phase, microstructure and composition using x-ray diffraction, atomic force microscopy and electron spectroscopy for chemical analysis respectively. It was observed that manganate thin films were stable up to 1373 K temperatures above which, breakdown in chemical composition occurs. XRD studies also indicate that these materials exist as a single phase perovskite structure up to 1373 K beyond which mixed phases of individual oxides were detected; Electrical conductivity and seebeck coefficients were measured in-situ as a function of temperature up to 1023 K. Electrical conductivity results indicated that these manganate thin films conduct using a thermally activated small polaron hopping conduction mechanism. Stable and fairly high seebeck coefficients along with high electrical conductivity indicate that these materials offer significant improvements over conventional Pt/PtRh thermocouples and hence, hold excellent promise for high temperature thin film thermocouple applications.