Negative-temperature coefficient of resistance (NTCR) thin films were prepared from (Ni 0.2 Mn 2.8-x Cu x )Cl 2 (0.010 ≤ x ≤ 0.040) solutions by liquid flow deposition (LFD) method. Influence of the Cu on the structural and electrical properties of the films annealed at °400°C was studied. It was found that the incorporation of copper (Cu) promoted an increase in both the crystallinity and grain size of the films. As Cu level increased, the absolute negative temperature coefficient of resistance (TCR) of the films was slightly decreased from 3.21% to 2.38%K −1 . On the other hand, the electrical resistivity (ρ) of the films significantly dropped with an increase of Cu, which was attributed to the improved carrier concentration rather than the enlarged grain size. The best electrical performance with ρ ~ 200 Ωcm was achieved in the film with x = 0.025 at room temperature. We provide the discussion on the conduction mechanism, particularly, for the high conduction behavior of the films via the changes of oxidation states of the manganese.
Toward the development of NTCR thermistors, nanocrystalline Mn–Ni–Cu–O powder was synthesized from a mixed chloride aqueous solution by a simple co-precipitation method.The introduction of an oxidizing agent (H2O2) into the solution led to the partial oxidation of Mn2+ ions into Mn3+ ions, which enabled the collected powder to be well crystallized at 650 °C. Such a low calcining temperature resulted in fine particles with a mean size of 60 nm, which significantly promoted densification of the resulting ceramics. As a result, a dense and homogenous microstructure with a relative density up to 97.2% was achieved for pellets sintered at 1100 °C. Furthermore, these sintered ceramics exhibited a room temperature resistivity (ρ25) of 67 Ω·cmand a thermistor constant (B25/85) of 2843 K, which make them suitable for use in industrial thermistors. In addition, electrical stability was greatly improved when the ceramics were prepared by a new two-step sintering method. The results suggest that the co-precipitation route with the introduction of H2O2 is suitable for the fabrication of cubic spinel thermistor nanopowders.
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