Screen printing of co-precipitated NiMn2O4+δfor production of NTCR thermistors

Screen printing and sintering procedures were established to produce about 25 micron thick, uniform ceramic NiMn2O4+delta films on alumina (thick film grade) substrates. NiMn2O4+delta source powder was prepared by calcination of NiMn2(C2O4)3.6H2O co-precipitated from nickel and manganese oxalate precursor solutions of precisely determined concentration (by titration methods). Decomposition of NiMn2(C2O4)3.6H2O into constituent oxides in air occurred at about 300 C. Formation of phase pure NiMn2O4+delta occurred above 800 C, accompanied by a significant increase in grain size. At constant temperature, grain growth showed an approximately linear dependence on time. Heating the precursor to 850 C for 30 min was optimal, ensuring that the reaction was completed while simultaneously minimising grain growth. Two types of film were fabricated: one in which the source NiMn2O4-delta powder was only mixed with organic binders, and a second set to which a glass phase and a dispersing agent was also added. Printed films were sintered at 850 C for 30 min. SEM and laser profilometry showed that films with the added glass phase had superior morphology and were denser than films without glass. Resistance vs. temperature (R-T) characteristics were measured over the range 122 K-345 K and showed 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 slightly more resistive and marginally less temperature sensitive. 15 refs.