Elsevier

Materials Letters

Volume 264, 1 April 2020, 127319
Materials Letters

New negative temperature coefficient ceramics in Ca0.9Y0.1MoO4–CeNbO4 system

https://doi.org/10.1016/j.matlet.2020.127319Get rights and content

Highlights

  • The new NTC ceramics in Ca0.9Y0.1MoO4–CeNbO4 system are investigated.

  • The resistivity decreases with increasing CeNbO4 content.

  • The decrease in the resistivity is attributed to the increase of Ce3+ content.

  • These materials show excellent high-temperature stability.

Abstract

The Ca0.9Y0.1CeNbMoO8 compounds are the solid solutions of Ca0.9Y0.1MoO4 and CeNbO4, and have a potential in the high temperature thermistor application. A concept has been proposed to adjust the negative temperature coefficient (NTC) properties of these thermistors by increasing the content of CeNbO4. The structure of as-sintered Ca0.9Y0.1MoO4xCeNbO4 ceramics is a single Ca0.9Y0.1MoO4 phase at x < 2.5, while the CeNbO4 phase begins to appear at x ≥ 2.5. The thermistor samples have remarkable NTC characteristics in the temperature of 50–700 °C. The resistivity, β300/500℃ constant and activation energy all decrease with increasing CeNbO4. The increase of Ce3+ content proved by XPS analysis is the reason for the decrease of the resistivity. The β300/500℃ and Ea values of the NTC thermistors are in the range of 5768–6583 K, 0.497–0.568 eV, respectively. After annealing at 500 °C in air for 200 h, the coefficient of aging (ΔR/R) is less than 2%, suggesting the materials show excellent stability.

Introduction

In recent years, complex oxides show interesting properties such as superconductivity [1], [2], metal conductivity [3], photocatalytic [4], [5], [6], dielectric properties [7], [8], nuclear radiation detectionand [9], photoluminescence [10], etc. Among them, powellite-type Ca-Ce-Nb-M-O (M = W or Mo) materials show the potential for negative temperature coefficient (NTC) thermistor application [11], [12]. It is believed that CaCeNbMoO8 compounds are the solid solutions of CaMoO4 and CeNbO4, and their conductivity is probably due to the conversion of Ce4+ to Ce3+ in the lattice [12]. However, this material shows a large thermistor constant, and thus limiting its wide temperature application. Bo Zhang et al. [13] have investigated the effects of rare earth Y doping on electrical properties of CaCeNbWO8, which show that the Y doping can adjust the NTC electrical properties. So considering the ionic radius of Y3+ and Ca2+ and the solid solutions characteristic of CaCeNbMoO8, the Y doping can adjust the NTC electrical properties of CaCeNbMoO8. Besides, in order to further reducing the thermistor constant of CaCeNbMoO8 NTC ceramics, a concept has been proposed to adjust the NTC properties of these thermistors by increasing the content of CeNbO4. The objectives of this study are to investigate the structure and electrical properties of Ca0.9Y0.1MoO4–CeNbO4 by adjusting the CeNbO4 content.

Section snippets

Experimental procedure

A polycrystalline powder of Ca0.9Y0.1MoO4xCeNbO4 (x = 1.0, 1.5, 2.0, 2.5, 3.0) was prepared by a conventional solid state reaction. Appropriate amounts of high purity Y2O3 (99.99%), CaCO3 (99%), CeO2 (99.99%), Nb2O5 (99.99%) and MoO3 (99.99%) were uniformly ground with agate and pre-fired in air at 1100℃ for 3 h. Then, the calcined powder was again ground uniformly, and pressed into a disk with a diameter of 10 mm and a thickness of 2.5 mm at die pressure of 10 MPa. Cold isostatic pressure of

Results and discussion

Fig. 1(a) shows the XRD patterns of as-sintered Ca0.9Y0.1MoO4xCeNbO4 ceramics. It can be seen that the sintered ceramic has a single Ca0.9Y0.1MoO4 (PDF no. 41–1431) solid solution phase described by the space group I41/a at x < 2.5. When x ≥ 2.5, the system reaches its solid solution limit and the CeNbO4 (PDF no. 33–0332) phase begins to appear. The Ca0.9Y0.1CeNbMoO8 compounds are the solid solutions of Ca0.9Y0.1MoO4 and CeNbO4. These results indicate that there is a solubility limit (x = 2.5)

Conclusion

In conclusion, we prepared a series of new Ca0.9Y0.1MoO4xCeNbO4 NTC ceramics through high-temperature solid state reaction. The sintered ceramic has a single Ca0.9Y0.1MoO4 phase at x < 2.5. When x ≥ 2.5, the CeNbO4 phase begins to appear, which retards the growth of the grains. As the CeNbO4 increases, the resistivity, β constant and activation energy all decrease, which should be attributed to the increase in Ce3+ content that has been proved by XPS analysis. The β300/500℃ and Ea values of

CRediT authorship contribution statement

Yafei Liu: Investigation, Data curation, Writing - original draft. Bo Zhang: Conceptualization, Methodology, Writing - review & editing, Supervision. Zhilong Fu: Software, Formal analysis. Aimin Chang: Supervision, Validation.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We would like to acknowledge financial support from the National Natural Science Foundation of China (Grant No. 61871377) and the Youth Innovation Promotion Association, CAS (Grant No. 2019424).

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