Characterization of new negative temperature coefficient thermistors based on Zn–Ni–O system

Journal of Advanced Ceramics 2226-4108 Characterization of new negative temperature coefficient thermistors based on Zn-Ni-O system Xiang SUN Hong ZHANG 0 1 Ya LIU 1 Jia GUO Zhicheng LI 0 1 0 State Key Laboratory of Powder Metallurgy, Central South University , Changsha 410083 , China 1 School of Materials Science and Engineering, Central South University , Changsha 410083 , China Y2O3-doped Zn1xNixO (x = 0, 0.3, 0.4, 0.5, 0.6, 0.7, and 0.9) powders were prepared by a wet chemical synthesis method, and the related ceramics were obtained by the traditional ceramic sintering technology. The phases and related electrical properties of the ceramics were investigated. The analysis of X-ray diffraction (XRD) indicates that the prepared ceramics with Ni substitution have a cubic crystalline structure. The resistance-temperature feature indicates that all the ceramics show a typical effect of negative temperature coefficient (NTC) of resistivity with the thermal constants between 3998 and 5464 K, and have high cyclical stability in a temperature range from 25 to 300 ℃. The impedance analysis reveals that both grain effect and grain boundary effect contribute collectively to the NTC effect. The electron hopping and band conduction models are proposed for the grain (bulk) conduction, and the thermally activated charge carrier transport overcoming the energy barrier is suggested for the grain boundary conduction. Zn1xNixO; thermistors; microstructure; electrical properties; conduction mechanism - Negative temperature coefficient (NTC) thermistors are widely used in industrial and domestic devices. At present, the NTC thermistors for the commercial applications are always the solid solutions based on the transition metal oxides and have the spinel structure, such as Mn–Ni–O, Ni–Cu–Mn–O, and Mn–Co–N–O systems [1,2]. In the AB2O4-type spinel structure, there are two sites available for the cations: a tetrahedral site, A-site, and an octahedral site, B-site. Generally, Mn3+ occupies predominantly the B-site while Mn2+ locates at the A-site, and almost all Ni2+ goes to the B-site. In the spinel manganite ceramics, the conduction is generally  * Corresponding author. E-mail: believed to propagate by an electron hopping mechanism taking place between the Mn3+ and Mn4+ cations locating at the B-sites in the octahedron induced by lattice vibrations [3,4]. The exact oxidation states of the Mn cations in the spinel structure closely depend on sintering conditions including oxygen partial pressure, sintering temperature, and procedure. So those ceramics typically exhibit a considerable difference in resistivity for a minor change of oxygen partial pressure during sintering or annealing [5,6]. On the other hand, the structural relaxation of those kinds of NTC compounds limits their applications at temperatures below 200 ℃. In NixMn3xO4 ceramics, the lowest electrical resistivity can be obtained at x = 0.66, and the value is about 1900 Ω·cm [7,8]. In order to further decrease the resistivity, Cu is doped into nickel manganites, e.g., the resistivity of Cu0.2Ni0.5Mn2.3O4 decreases to 70 Ω·cm. However, the doping of Cu will cause a harmful consequence, i.e., the resistivity drift rate is 14.9% after ageing at 150 ℃ for 500 h in air [9]. For improving the ageing characteristic, much work has been done by cation doping and microstructure modification [9–11]. For instance, Ma et al. [12] reported that the resistivity drift for Ni0.6Cu0.5ZnxMn1.9xO4 after annealing at 150 ℃ for 500 h in air decreases sharply from 10.2% to 4.8% with x increases from 0 to 0.25. For meeting the real applications, some new types of NTC thermistors have been developed, e.g., doped Bi2O3-based ceramics, inverse spinel type Zn7Sb2O12, pyrochlore type Bi3Zn2Sb3O14 compounds, and even modified simple oxides such as SnO2 and CuO [13–17]. One of the distinct advantages of the semiconductive oxides used for the NTC applications is that the room-temperature conductivity can be effectively adjusted by element doping and the related thermal-sensitive constants can also be improved by substituting suitable impurities [17–20]. Zinc oxide (ZnO), as a semiconductor with the band gap of 3.37 eV, has attracted much attentions for several decades due to its potential applications in light emitting, solar cells, spintronic, transparent conducting, semiconductors, biosensors, and photocatalytic materials [21–24]. NiO is also one typical semiconductive material with the band gap of 3.6–4.0 eV and has aroused numerous interests for various applications [25–28]. Recently, many reports on Zn–Ni–O materials have mainly focused on the crystalline structure, optical properties, and magnetic properties, etc. [29–31]. To the best of our knowledge, little work is focused on the temperature dependence of resistivity. During investigating the influence of Ni and Y doping on the electrical property and crystal structure of the ZnO ceramics, the authors found that the Z (...truncated)