Screen printing of co-precipitated NiMn2O4+δ for production of NTCR thermistors

Screen printing and sintering procedures have been established to produce ∼25 μm thick, uniform ceramic NiMn2O4+δ films on alumina (thick film grade) substrates. NiMn2O4+δ source powder was prepared by calcination of NiMn2(C2O4)3·6 H2O co-precipitated from nickel and manganese oxalate precursor solutions of precisely determined concentration (by titration methods). Decomposition of NiMn2(C2O4)3·6 H2O into its constituent oxides in air was found to take place at ∼300 °C using thermal gravimetric analysis. Formation of phase pure NiMn2O4+δ occurred at temperatures above 800 °C and was accompanied by a significant increase in grain size. At constant temperature grain growth displayed an approximately linear dependence on time. Heating the precursor to 850 °C for 30 min was found to be near the optimum, ensuring that the reaction had gone to completion while simultaneously minimising grain growth. Two types of film were fabricated; one in which the source NiMn2O4+δ powder was only mixed with organic binders and a second set to which a glass phase and a dispersing agent had been added as well. Printed films were sintered at 850 °C for 30 min. Films with the added glass phase were found by SEM and laser profileometry to have superior morphology and to be denser than films without glass. Resistance vs. temperature (R–T) characteristics were measured over the temperature range 122 K –345 K and indicated that conduction was well described by a variable - range - hopping model, in which electron transfer takes place between Mn3+ and Mn4+ cations. Films containing the glass phase were found to be slightly more resistive and marginally less temperature sensitive.