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New negative temperature coefficient ceramics in Ca0.9Y0.1MoO4–CeNbO4 system
Materials Letters 264 (2020) 127319
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
New negative temperature coefficient ceramics in Ca0.9Y0.1MoO4– CeNbO4 system Yafei Liu a,b, Bo Zhang b,⇑, Zhilong Fu b,c, Aimin Chang b a
School of Physics Science and Technology, Xinjiang University, Urumqi 830046, China Key Laboratory of Functional Materials and Devices for Special Environments of CAS, Xinjiang Key Laboratory of Electronic Information Materials and Devices, Xinjiang Technical Institute of Physics & Chemistry of CAS, Urumqi 830011, China c University of Chinese Academy of Sciences, Beijing 100049, China b
a r t i c l e
i n f o
Article history: Received 27 November 2019 Received in revised form 4 January 2020 Accepted 6 January 2020 Available online 7 January 2020 Keywords: Electroceramics Structure Electrical properties NTC thermistors X-ray techniques
a b s t r a c t 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 Ca0.9Y0.1MoO4–xCeNbO4 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, b300/500 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 b300/500 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 (DR/R) is less than 2%, suggesting the materials show excellent stability. Ó 2020 Elsevier B.V. All rights reserved.
1. Introduction In recent years, complex oxides show interesting properties such as superconductivity [1,2], metal conductivity [3], photocatalytic [4–6], 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 ⇑ Corresponding author. E-mail address: [email protected] (B. Zhang). https://doi.org/10.1016/j.matlet.2020.127319 0167-577X/Ó 2020 Elsevier B.V. All rights reserved.
to investigate the structure and electrical properties of Ca0.9Y0.1MoO4–CeNbO4 by adjusting the CeNbO4 content. 2. Experimental procedure A polycrystalline powder of Ca0.9Y0.1MoO4–xCeNbO4 (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 300 MPa for 180 s was applied to improve the density of the sample. Finally, the sample was sintered at 1350 for 9 h. X-ray diffraction (XRD) was used to identify crystalline phases in the ceramics. The microstructure and composition distribution of the ceramics were recorded by a scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS), respectively. Xray photoelectron spectroscopy (XPS) was used to analyze the chemical states of the ceramics. To characterize the electrical properties, the sintered samples were polished, coated with a thin layer of Pt slurry, and heated at 900 °C for 30 min. The resistance was measured from 50 °C to 700 °C using a digital multimeter (HP 34401A). The aging coefficient was measured by the relative DR/
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Y. Liu et al. / Materials Letters 264 (2020) 127319
R of the sample, and the aging time can reach 200 h in different time periods in air at 500 °C. 3. Results and discussion Fig. 1(a) shows the XRD patterns of as-sintered Ca0.9Y0.1MoO4– xCeNbO4 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 sys-
tem 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) of CeNbO4 in Ca0.9Y0.1MoO4 lattice. Fig. 1(b) shows the zoom of the main peak of XRD patterns for the samples. The XRD pattern of single-phase Ca0.9Y0.1MoO4 (x = 0) has also been added in the Fig. 1 for comparison. It should also be noted that the XRD diffraction peak of the Ca0.9Y0.1MoO4 phase for Ca0.9Y0.1MoO4–xCeNbO4 ceramics (x 1.0) shifts to
Fig. 1. (a) XRD patterns of as-sintered ceramics. (b) The zoom of the main peak of XRD patterns for the ceramics.
Fig. 2. SEM micrographs of as-sintered ceramics. (a) x = 1.0; (b) x = 1.5; (c) x = 2.0; (d) x = 2.5; (e) x = 3.0. (f) EDS spectrum taken from the bright area (A) of the ceramics (x = 3.0). (g) EDS spectrum taken from the dark area (B) of the ceramics (x = 3.0).
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a low angle compared to the single phase Ca0.9Y0.1MoO4 (x = 0), and then shifts to a high angle when x 2.5. The low angle shift is due to the ionic radius of Ce3+ (1.14 Å) and Nb5+ (0.48 Å) is larger than that of the Ca2+ (0.97 Å) and Mo6+ (0.41 Å), respectively. The high angle shift is caused by the extrusion of the CeNbO4 phase from Ca0.9Y0.1MoO4 lattice, which leads to the lattice contraction. Fig. 2 shows the SEM micrograph of the polished and thermally etched ceramic samples. It can be seen that all the ceramics show well-sintered and well grown grains. It is also observed that the grain size of the composite phase sample (d) is reduced compared to the single phase solid solution samples (c). The reduction in grain size can be attributed to the formation of CeNbO4 which consumes part of the energy of grain boundary migration and thus retarding the growth of the grains. The compositions taken from the bright (A) and dark (B) area were qualitatively identified by EDS, as shown in Fig. 2(f) and (g), respectively. It can be concluded that the bright regions at the grain boundary are CeO2-rich phase and the dark regions are mainly Ca0.9Y0.1MoO4 and CeNbO4 phases. The appearance of the CeO2 phase may be attributed to the thermal
Fig. 3. Relationship between lnq and 1000/T for the NTC thermistors.
etching that causes Ce4+ ion escaping from the grains and enriching at the grain boundary and thus forming little CeO2 phase. Fig. 3 shows the relationship between the natural logarithm of resistivity (lnq) and the reciprocal of the absolute temperature (1000/T) for the NTC thermistors. It can be seen that the Ca0.9Y0.1MoO4–xCeNbO4 thermistors show remarkable NTC characteristics. Table 1 lists the thermistor constant b, a and activation energy of the samples. The b can be derived as follows: h i N b ¼ TTT ln RRNT (1), where RT is the resistance of temperature T, T N RN is the resistance of temperature TN. The temperature coefficient of resistance a can be expressed according to the following equa-
tion: a ¼ ð1=RÞ½dðRÞ=dðT Þ ¼ b=T 2 (2). It can be seen that as the CeNbO4 increases, the b constant and activation energy all decrease, which should be attributed to the increase in Ce3+ content. The detail study is shown in the following XPS analysis. The b300/500 and Ea values of the NTC thermistors are in the range of 5768–6583 K, 0.497–0.568 eV, respectively. The b300/500 is significantly reduced by increasing the content of CeNbO4, and obtains its minimum (5768 K) at x = 3. The above results indicate that the electrical properties can be effectively adjusted to the desired values by changing the concentration of CeNbO4 in the compound. An aging test at 500 °C shows a change in the coefficient of aging (DR/R) of less than 2% during the 200 h aging period. These compounds show low b300/500 constant and high stability, could be used as the high-temperature thermistor with a wide temperature range. Fig. 4a shows the curve fitting example of the Ce 3d XPS peaks recorded for Ca0.9Y0.1MoO4–2.5CeNbO4 ceramics. The Ce 3d XPS spectra of other ceramic samples are very similar to the Ca0.9Y0.1MoO4–2.5CeNbO4 ceramics, so XPS spectra are not shown here. In the Fig. 4a, v, u peaks are mainly characteristic of Ce3+ ions, while u’, u0, v0 components are attributed to Ce4+. The hopping conductivity in these compounds may be attributed to the variable valence of cerium, basically Ce3+ and Ce4+ [12,14,15]. For CaCeNbMoO8, its structure will be stable only if cerium is Ce3+ in order to maintain the oxygen stoichiometry instead of its stable
Table 1 b300/500 constant, a, and activation energy for the Ca0.9Y0.1MoO4–xCeNbO4 NTC thermistors. x
1.0
1.5
2.0
2.5
3.0
b300/500 (K) a300 (K1) a500 (K1) Ea (eV)
6583 0.0200 0.0110 0.568
6114 0.0186 0.0102 0.527
6025 0.0184 0.0101 0.520
5986 0.0182 0.0100 0.516
5768 0.0176 0.0097 0.497
Fig. 4. (a) Ce 3d XPS spectra collected for Ca0.9Y0.1MoO4–2.5CeNbO4 ceramics. (b) Evolution of the mole number of Ce3+ as a function of for the ceramic samples.
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Ce4+ [12]. The excess oxygen arising from CeO2 will be released into the atmosphere, leaving behind electron in the lattice, and this electron will lead to the production of Ce3+ and hopping conductivity for these NTC ceramics. These processes can be described 0 1 by the following defect reactions: O O ! VO þ 2 O2 þ 2e (3), 0
Ce4þ þ e ! Ce3þ (4). This explanation is consistent with our XPS results. The concentration ratio of Ce3+/Ce is calculated as follows: 3þ u þAv (5), where Ai is the integrated area of Ce =Ce ¼ Au þAu AþA 0 þAv þAv 0
u
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.
0
peak i. The mole number of Ce3+ in the samples can be obtained from the Ce3+/Ce ratio, as shown in Fig. 4(b). From Fig. 4 (b), it can be seen that with the increase of CeNbO4, the total amount of Ce elements increases, and the mole number of Ce3+ also increases. That is why the resistivity decreases as the CeNbO4 increases (see Fig. 3). An increase in the Ce3+ will promote the rise in charge carriers and the electron hopping, thereby resulting in a decrease in the resistivity. 4. Conclusion In conclusion, we prepared a series of new Ca0.9Y0.1MoO4– xCeNbO4 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, b constant and activation energy all decrease, which should be attributed to the increase in Ce3+ content that has been proved by XPS analysis. The b300/500 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 (DR/R) of these materials is less than 2%, showing excellent stability. CRediT authorship contribution statement Yafei Liu: Investigation, Data curation, Writing - original draft. Bo Zhang: Conceptualization, Methodology, Writing - review &
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). References [1] M.A.L. Nobre, S. Lanfredi, Appl. Radiat. Isot. 82 (2003) 2284–2286. [2] P.A. Sobocinski, P.L. Grande, P. Pureur, Phys. C 517 (2015) 49–52. [3] E.M. Tejada-Rosales, J. Oró-Solé, P. Gómez-Romero, J. Solid State Chem. 163 (2002) 151–157. [4] W.C. Feng, J. Li, Z.M. Lei, Y. Liu, Y.Q. Shen, Z.X. Chen, J. Mater. Sci. – Mater. Electron. 17 (2019) 16049–16055. [5] D.T. Nguyen, S.S. Hong, J. Nanosci. Nanotechnol. 19 (2019) 7696–7701. [6] P. Hajra, S. Kundu, A. Maity, C. Bhattacharya, Chem. Eng. J. Soc. 374 (2019) 1221–1230. [7] N. Najafvandzadeh, R. Vali, Phys. B 572 (2019) 266–272. [8] S. Hajra, M. Sahu, V. Purohit, R. Panigrahi, R.N.P. Choudhary, Mater. Res. Express. 6 (2019) 1–20. [9] M. Zhou, C.X. Hou, Y. Xie, L. Wang, X.Y. Shu, D.D. Shao, X.R. Lu, J. Au Ceram. Soc. 55 (2019) 831–836. [10] P.S. Babu, P.P. Rao, T.S. Sreena, J. Mater. Sci. – Mater. Electron. 30 (2019) 16174–16183. [11] A. Feteira, J. Am. Ceram. Soc. 92 (2009) 967–983. [12] M. Deepa, P.P. Rao, S. Sumi, A.N.P. Radhakrishnan, P. Koshy, J. Am. Ceram. Soc. 93 (2010) 1576–1579. [13] B. Zhang, Q. Zhao, C.J. Zhao, A.M. Chang, J. Alloys Compd. 698 (2017) 1–6. [14] S.S. Pramana et al., J. Am. Ceram. Soc. 138 (2016) 1273–1279. [15] G.G. Zhang, W.B. Wan, P.Q. Long, Q. Li, C.Z. Deng, Z.G. Yi, J. Alloys Compd. 616 (2014) 328–332.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
New negative temperature coefficient ceramics in Ca0.9Y0.1MoO4– CeNbO4 system Yafei Liu a,b, Bo Zhang b,⇑, Zhilong Fu b,c, Aimin Chang b a
School of Physics Science and Technology, Xinjiang University, Urumqi 830046, China Key Laboratory of Functional Materials and Devices for Special Environments of CAS, Xinjiang Key Laboratory of Electronic Information Materials and Devices, Xinjiang Technical Institute of Physics & Chemistry of CAS, Urumqi 830011, China c University of Chinese Academy of Sciences, Beijing 100049, China b
a r t i c l e
i n f o
Article history: Received 27 November 2019 Received in revised form 4 January 2020 Accepted 6 January 2020 Available online 7 January 2020 Keywords: Electroceramics Structure Electrical properties NTC thermistors X-ray techniques
a b s t r a c t 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 Ca0.9Y0.1MoO4–xCeNbO4 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, b300/500 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 b300/500 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 (DR/R) is less than 2%, suggesting the materials show excellent stability. Ó 2020 Elsevier B.V. All rights reserved.
1. Introduction In recent years, complex oxides show interesting properties such as superconductivity [1,2], metal conductivity [3], photocatalytic [4–6], 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 ⇑ Corresponding author. E-mail address: [email protected] (B. Zhang). https://doi.org/10.1016/j.matlet.2020.127319 0167-577X/Ó 2020 Elsevier B.V. All rights reserved.
to investigate the structure and electrical properties of Ca0.9Y0.1MoO4–CeNbO4 by adjusting the CeNbO4 content. 2. Experimental procedure A polycrystalline powder of Ca0.9Y0.1MoO4–xCeNbO4 (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 300 MPa for 180 s was applied to improve the density of the sample. Finally, the sample was sintered at 1350 for 9 h. X-ray diffraction (XRD) was used to identify crystalline phases in the ceramics. The microstructure and composition distribution of the ceramics were recorded by a scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS), respectively. Xray photoelectron spectroscopy (XPS) was used to analyze the chemical states of the ceramics. To characterize the electrical properties, the sintered samples were polished, coated with a thin layer of Pt slurry, and heated at 900 °C for 30 min. The resistance was measured from 50 °C to 700 °C using a digital multimeter (HP 34401A). The aging coefficient was measured by the relative DR/
2
Y. Liu et al. / Materials Letters 264 (2020) 127319
R of the sample, and the aging time can reach 200 h in different time periods in air at 500 °C. 3. Results and discussion Fig. 1(a) shows the XRD patterns of as-sintered Ca0.9Y0.1MoO4– xCeNbO4 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 sys-
tem 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) of CeNbO4 in Ca0.9Y0.1MoO4 lattice. Fig. 1(b) shows the zoom of the main peak of XRD patterns for the samples. The XRD pattern of single-phase Ca0.9Y0.1MoO4 (x = 0) has also been added in the Fig. 1 for comparison. It should also be noted that the XRD diffraction peak of the Ca0.9Y0.1MoO4 phase for Ca0.9Y0.1MoO4–xCeNbO4 ceramics (x 1.0) shifts to
Fig. 1. (a) XRD patterns of as-sintered ceramics. (b) The zoom of the main peak of XRD patterns for the ceramics.
Fig. 2. SEM micrographs of as-sintered ceramics. (a) x = 1.0; (b) x = 1.5; (c) x = 2.0; (d) x = 2.5; (e) x = 3.0. (f) EDS spectrum taken from the bright area (A) of the ceramics (x = 3.0). (g) EDS spectrum taken from the dark area (B) of the ceramics (x = 3.0).
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a low angle compared to the single phase Ca0.9Y0.1MoO4 (x = 0), and then shifts to a high angle when x 2.5. The low angle shift is due to the ionic radius of Ce3+ (1.14 Å) and Nb5+ (0.48 Å) is larger than that of the Ca2+ (0.97 Å) and Mo6+ (0.41 Å), respectively. The high angle shift is caused by the extrusion of the CeNbO4 phase from Ca0.9Y0.1MoO4 lattice, which leads to the lattice contraction. Fig. 2 shows the SEM micrograph of the polished and thermally etched ceramic samples. It can be seen that all the ceramics show well-sintered and well grown grains. It is also observed that the grain size of the composite phase sample (d) is reduced compared to the single phase solid solution samples (c). The reduction in grain size can be attributed to the formation of CeNbO4 which consumes part of the energy of grain boundary migration and thus retarding the growth of the grains. The compositions taken from the bright (A) and dark (B) area were qualitatively identified by EDS, as shown in Fig. 2(f) and (g), respectively. It can be concluded that the bright regions at the grain boundary are CeO2-rich phase and the dark regions are mainly Ca0.9Y0.1MoO4 and CeNbO4 phases. The appearance of the CeO2 phase may be attributed to the thermal
Fig. 3. Relationship between lnq and 1000/T for the NTC thermistors.
etching that causes Ce4+ ion escaping from the grains and enriching at the grain boundary and thus forming little CeO2 phase. Fig. 3 shows the relationship between the natural logarithm of resistivity (lnq) and the reciprocal of the absolute temperature (1000/T) for the NTC thermistors. It can be seen that the Ca0.9Y0.1MoO4–xCeNbO4 thermistors show remarkable NTC characteristics. Table 1 lists the thermistor constant b, a and activation energy of the samples. The b can be derived as follows: h i N b ¼ TTT ln RRNT (1), where RT is the resistance of temperature T, T N RN is the resistance of temperature TN. The temperature coefficient of resistance a can be expressed according to the following equa-
tion: a ¼ ð1=RÞ½dðRÞ=dðT Þ ¼ b=T 2 (2). It can be seen that as the CeNbO4 increases, the b constant and activation energy all decrease, which should be attributed to the increase in Ce3+ content. The detail study is shown in the following XPS analysis. The b300/500 and Ea values of the NTC thermistors are in the range of 5768–6583 K, 0.497–0.568 eV, respectively. The b300/500 is significantly reduced by increasing the content of CeNbO4, and obtains its minimum (5768 K) at x = 3. The above results indicate that the electrical properties can be effectively adjusted to the desired values by changing the concentration of CeNbO4 in the compound. An aging test at 500 °C shows a change in the coefficient of aging (DR/R) of less than 2% during the 200 h aging period. These compounds show low b300/500 constant and high stability, could be used as the high-temperature thermistor with a wide temperature range. Fig. 4a shows the curve fitting example of the Ce 3d XPS peaks recorded for Ca0.9Y0.1MoO4–2.5CeNbO4 ceramics. The Ce 3d XPS spectra of other ceramic samples are very similar to the Ca0.9Y0.1MoO4–2.5CeNbO4 ceramics, so XPS spectra are not shown here. In the Fig. 4a, v, u peaks are mainly characteristic of Ce3+ ions, while u’, u0, v0 components are attributed to Ce4+. The hopping conductivity in these compounds may be attributed to the variable valence of cerium, basically Ce3+ and Ce4+ [12,14,15]. For CaCeNbMoO8, its structure will be stable only if cerium is Ce3+ in order to maintain the oxygen stoichiometry instead of its stable
Table 1 b300/500 constant, a, and activation energy for the Ca0.9Y0.1MoO4–xCeNbO4 NTC thermistors. x
1.0
1.5
2.0
2.5
3.0
b300/500 (K) a300 (K1) a500 (K1) Ea (eV)
6583 0.0200 0.0110 0.568
6114 0.0186 0.0102 0.527
6025 0.0184 0.0101 0.520
5986 0.0182 0.0100 0.516
5768 0.0176 0.0097 0.497
Fig. 4. (a) Ce 3d XPS spectra collected for Ca0.9Y0.1MoO4–2.5CeNbO4 ceramics. (b) Evolution of the mole number of Ce3+ as a function of for the ceramic samples.
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Y. Liu et al. / Materials Letters 264 (2020) 127319
Ce4+ [12]. The excess oxygen arising from CeO2 will be released into the atmosphere, leaving behind electron in the lattice, and this electron will lead to the production of Ce3+ and hopping conductivity for these NTC ceramics. These processes can be described 0 1 by the following defect reactions: O O ! VO þ 2 O2 þ 2e (3), 0
Ce4þ þ e ! Ce3þ (4). This explanation is consistent with our XPS results. The concentration ratio of Ce3+/Ce is calculated as follows: 3þ u þAv (5), where Ai is the integrated area of Ce =Ce ¼ Au þAu AþA 0 þAv þAv 0
u
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.
0
peak i. The mole number of Ce3+ in the samples can be obtained from the Ce3+/Ce ratio, as shown in Fig. 4(b). From Fig. 4 (b), it can be seen that with the increase of CeNbO4, the total amount of Ce elements increases, and the mole number of Ce3+ also increases. That is why the resistivity decreases as the CeNbO4 increases (see Fig. 3). An increase in the Ce3+ will promote the rise in charge carriers and the electron hopping, thereby resulting in a decrease in the resistivity. 4. Conclusion In conclusion, we prepared a series of new Ca0.9Y0.1MoO4– xCeNbO4 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, b constant and activation energy all decrease, which should be attributed to the increase in Ce3+ content that has been proved by XPS analysis. The b300/500 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 (DR/R) of these materials is less than 2%, showing excellent stability. CRediT authorship contribution statement Yafei Liu: Investigation, Data curation, Writing - original draft. Bo Zhang: Conceptualization, Methodology, Writing - review &
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