Controlled thermistor effect in the system CuxNi1–x–yCo2yMn2–yO4
Introduction
Spinel-structured semiconducting ceramics (SSSC) with high value of negative temperature coefficient (NTC) of resistance are the most known materials for NTC thermistors manufacturing.1, 2, 3 Traditionally a solid solutions based on transition metal oxides are used. In particular, the sets of NTC thermistors with a wide range of exploitation parameters, a low dependence of electrical properties on cation content and, as a result, a high level of technological reproducibility can be obtained in the limits of ternary Mn–Cu–Co, Mn–Co–Ni and Mn–Cu–Ni oxide systems.4 But sometimes it is impossible to obtain by this way the suitable materials for industrial manufacturing of NTC thermistors, possessed a predetermined range of rated resistances, a high temperature sensitivity (determined by a material-specific constant B) and a high level of reliability and reproducibility. Thus, it is important to work out a correct approach to the problem on the selection of SSSC for practical applications.
With the above aim, we consider in this paper the selection possibilities in quaternary Cu–Ni–Co–Mn oxide system, restricted by cubic spinels CuMn2O4, MnCo2O4 and NiMn2O4. Being within the limits of CuMn2O4–MnCo2O4–NiMn2O4 concentration triangle (which can be presented as CuxNi1–x–yCo2yMn2–yO4 chemical system at 0.1⩽x⩽0.8 and 0.1⩽y⩽0.9–x) and having precisely determined technological conditions, we can select the continuous solid solutions, which are the most suitable for thermistors manufacturing. The attempts to resolve partially this problem have been carried out previously for some chosen SSSC compositions from this chemical system.5
The final essence of this work is to establish for the CuxNi1–x–yCo2yMn2–yO4 system the general relationships between SSSC chemical composition, from the first side, technological features of sintering for single-phase ceramics preparation, from the second side, and electrical properties of the prepared thermistors, from the third side, using the method of simplex matrices of the 4-fold D-optimum plan.6
Section snippets
Experimental
Fig. 1 shows the whole variety of the investigated SSSC compositions, being uniformly distributed in the limits of CuMn2O4–MnCo2O4–NiMn2O4 concentration triangle.
The traditional ceramic technology was used in order to obtain the investigated samples.4, 5 The high purity and tested raw materials (CuCO3·mCu(OH)2, NiCO3·mNi(OH)2·nH2O, CoCO3mCo(OH)2·nH2O and MnCO3·mMn(OH)2·nH2O) were chosen as initial components. By changing the relative content of these components in a wide range, determined by x
Results and discussion
The experimentally obtained temperature dependences of electrical conductivity show that all NTC samples of CuxNi1–x–yCo2yMn2–yO4 (0.1⩽x⩽0.8; 0.1⩽y⩽0.9–x) chemical system are characterized by strong thermistor effect. The main electrical parameters, such as electrical conductivity σ25 at 25°C and activation energy ΔE20–70 for ceramic NTC thermistors are determined, in the first hand, by sintering temperature Ts. This tight correlation is connected with the structural peculiarities of
Conclusions
Thus, we conclude at the basis of the obtained experimental results, that promising electroceramics with beforehand-determined parameters of controlled thermistor effect can be obtained, using the quaternary Cu–Ni–Co–Mn oxide system with smooth content deviations of the main cubic spinels (CuMn2O4, MnCo2O4 and NiMn2O4). It means that the continuous solid solutions of cation-variable compositions in CuxNi1–x–yCo2yMn2–yO4 (0.1⩽x⩽0.8; 0.1⩽y⩽0.9–x) system, obtained at the optimally determined
Acknowledgments
This work was carried out in the framework of STCU Project No. 2080.
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