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Sintering temperature and XPS analysis of Co2.77Mn1.71Fe1.10Zn0.42O8 NTC ceramics
Materials Chemistry and Physics 239 (2020) 122098
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Sintering temperature and XPS analysis of Co2.77Mn1.71Fe1.10Zn0.42O8 NTC ceramics Bing Wang a, b, Junhua Wang a, b, Dunsheng Shang a, Aimin Chang a, b, Jincheng Yao a, b, * a
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, 40-1 South Beijing Road, Urumqi, China b University of Chinese Academy of Sciences, Beijing, 100049, China
H I G H L I G H T S
� We first revealed the effects of sintering temperature of the CMFZ ceramics. � The optimum sintering temperature of 1200 � C was determined. � CMFZ ceramics was confirmed to be an n-type semiconductor. A R T I C L E I N F O
A B S T R A C T
Keywords: NTC thermistors XPS analysis Sintering Microstructure Electrical properties
Microstructure and electrical properties of Co2.77Mn1.71Fe1.10Zn0.42O8 (CMFZ) ceramics as a function of tem perature were studied in the sintering process. The optimum sintering temperature of 1200 � C is determined to obtain the optimum microstructure and electrical properties. Both microstructure and electrical properties are sensitive to the different sintering temperatures. In particular, the appropriately increased sintering temperature enhances the densification and the homogeneity of grain size. However, the accelerated grain growth with a large number of closed pores appears when above the optimum sintering temperature. The electrical resistivity first increases and then decreases with the increased sintering temperatures. The ρ25 and B25/50 constant are in the range of 3072–4238 Ω cm and 4021–4056 K, respectively. X-ray photoelectron spectroscopy (XPS) was analyzed the evolution of resistivity clearly and proved the n-type semiconductor of CMFZ ceramics.
1. Introduction Thermosensitive ceramics with negative temperature coefficients (NTC) are semiconductive ceramics that usually composed of Manganese-based transition metal oxides [1–4]. Due to the high sensi tivity, fast response, low cost, and convenience in use, Mn-based NTC ceramics have attracted extensive attention and achieved a wide range of applications in temperature control and measurement [5,6]. In the past decades, various sintering techniques are used, such as traditional sintering [7], liquid-phase sintering [8], spark plasma sintering [9], microwave sintering [10], two-step sintering and cold sintering process [11–13], which have attracted much attention in recent years. However, it is worth noting that different sintering techniques have different sintering temperature regimes, and the control of sintering temperature on all sintering methods is crucial. Recently, some researches have centered on the effects of sintering temperature on the properties of
ceramics [14–16]. For instance, Ma et al. [17] have investigated the influences of the sintering temperature on the electrical properties of Co0.98Mn2.02O4 ceramics and found that the resistivity decreases monotonically with the increased sintering temperature. Unfortunately, the analysis for the variation of resistivity is only conjecture, but the mechanism of cationic valence states and concentration variation is not depth. However, there are few reports focusing on the effect of sintering process on the electrical properties and stability of Co2.77Mn1.71 Fe1.10Zn0.42O8 ceramics. It is an effective way to investigate the cationic oxidation states and concentration distributions in ceramics with XPS. Therefore, the effects of sintering temperature on the properties of Co2.77Mn1.71Fe1.10Zn0.42O8 NTC ceramics were systemically in the present work.
* Corresponding author. University of Chinese Academy of Sciences, Beijing, 100049, China. E-mail addresses: [email protected], [email protected] (J. Yao). https://doi.org/10.1016/j.matchemphys.2019.122098 Received 16 June 2019; Received in revised form 26 August 2019; Accepted 28 August 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.
B. Wang et al.
Materials Chemistry and Physics 239 (2020) 122098
Co2.77Mn1.71Fe1.10Zn0.42O8. And then, the weighed powders were ballmilled, dried and calcinated at 850 � C for 2 h. Later the obtained pow ders were ground and sieved using a 100-mesh sieve. Subsequently, the powders were shaped into disks of 2 mm thickness and 10 mm diameter and pressed with a pressure of 350 MPa using the cold isostatic press. Then the disks were sintered at 1150 � C–1250 � C for 2 h. Both sides of the sintered disks were polished and coated with a silver paste to investigate the electrical properties. X-ray diffraction (XRD) results on CMFZ ceramics were recorded utilizing a Bruker D8 Advance diffractometer. The valence states and ionic concentration of CMFZ ceramics were analyzed using XPS (KAlphaþ). The surface microstructure and elemental distribution were characterized using the scanning electron microscope (SEM, Zeiss Supra55VP) and energy dispersive spectroscopy (EDS, Bruker Xflash5010). 3. Results and discussion Fig. 1 presents XRD images of CMFZ samples sintered at 1150 � C, 1175 � C, 1200 � C, 1225 � C, and 1250 � C. It is clear that every sample exhibits the cubic spinel phase, and no secondary phase appeared. Ori entations including (111), (220), (311), (222), (400), (422), (511) and (400) can be detected, with the strongest orientation being at (311). Besides, the intensity of the XRD peak increases first and then decreases with the increased sintering temperatures. The increased peak intensity, mainly attributed to the increased temperature from 1150 � C to 1200 � C, enhances the diffusion velocity of atoms, which improves the binding activity of tetrahedron and octahedron in the sintering process.
Fig. 1. X-ray diffraction patterns of CMFZ ceramics sintered at different temperatures.
2. Experimental procedures CMFZ ceramics were prepared using a traditional ceramic fabrica tion route. Analytical reagent Co3O4, Mn3O4, Fe2O3, and ZnO were used as raw materials and weighed according to the nonstoichiometric ratio
Fig. 2. Scanning electron microscope micrographs of the CMFZ ceramics sintered at (a) 1150 � C, (b) 1175 � C, (c) 1200 � C, (d) 1225 � C, and (e) 1250 � C. 2
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Materials Chemistry and Physics 239 (2020) 122098
Fig. 3. Elemental mapping of the CMFZ ceramic sample sintered at 1200 � C.
However, the higher sintering temperature may induce phase decom position from this phase into other cubic spinel phase, and hence the decreased peak intensity appeared. The phase decomposition at high temperatures can easily explain in theory. As the increase of sintering temperature, the oxygen partial pressure around the sample decreases, so the sample is easy to lose oxygen. Therefore, phase decomposition is easily to happen at higher sintering temperatures.
Fig. 2 shows surface microstructure images of the CMFZ samples sintered at various temperatures. The microstructure evolution exhibits notable temperature dependence [18–20]. Compared with the applied sintering temperature of 1150 � C, some pores gradually disappear, and the as-sintered ceramics presents a homogeneous and close-packed grain while the sintering temperature increased from 1150 � C to 1200 � C. It is associated with the increased temperature that promotes the shrinkage 3
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Materials Chemistry and Physics 239 (2020) 122098
B25=50 ¼ � 1
ln ρρ50
T50
(1)
25
1
=T25
where ρ25 and ρ50 are the resistivity obtained at 25 � C and 50 � C, respectively, T is the absolute temperature, and k is the Boltzmann constant. As shown in Fig. 5, the values of ρ25 and B25/50 constant are in the range of 3072–4238 Ω cm and 4021–4056 K, respectively. It can be seen that the difference of B25/50 constant was insignificant for CMFZ samples sintered at various temperatures. However, ρ25 increases first and later decreases as the increase of the sintering temperatures. This evolution might induce by the variation of hopping conduction mechanism be tween Mn3þ and Mn4þ ions originated from the difference of cation distribution. The XPS investigation was conducted to clarify the mech anism of resistivity variation clearly. The chemical valence of every cation (i.e., Co, Mn, Fe, and Zn) in CMFZ ceramics were analyzed. The standard C 1s peak (248.6 eV) was applied to calibrate every binding energy for eliminating the charge effect. Fig. 6 shows the high-resolution XPS spectra of CMFZ samples sintered at various temperatures. It can be seen that the characteristic peaks of every cation present similar binding energies when sintered the samples at various temperatures. It indicates that the cation distribu tions in A and B sites of the CMFZ compounds are similar even at different temperatures. Fig. 6 (a) shows the Co 2p spectrum obtained from CMFZ samples at various sintering temperatures. The Co 2p3/2 major peak and its satellite peak located at the 780.0 eV and 786.4 eV, respectively, and the Co 2p1/2 major peak and its satellite located at 795.5 eV and 802.8 eV, respectively. The splitting between 2p3/2 and 2p1/2 due to spin-orbit coupling is nearly 15.5 eV. The obtained results well agree with other reports [21,23,24]. More importantly, the satellite peaks of Co 2p3/2 are a useful way to confirm the valence states of Co ions. In general, the satellite of Co2þ ions typically near 786 eV, while Co3þ ions are known to 790 eV. Through the analysis above, the satellite of Co 2p3/2 located at 786.4 eV. Therefore, the satellite peak of Co 2p3/2 level strongly suggests that only Co2þ exist in the CMFZ ceramics. Fig. 6 (b) shows the Mn 2p core-level spectrum. Two major characterized peaks, Mn 2p3/2, and Mn 2p1/2 are located at 641.7 eV and 653.5 eV, respectively. The calculated Full Width at Half Maximum (FWHM) of the Mn 2p level was 3.91, 4.17, 4.10, 4.30, and 4.22 for different sintering temperatures, respectively. The obtained FWHM values are higher than that of mono-valance state Mn ions, i.e., the FWHM of MnO is 3.2 eV, Mn2O3 is 3.0 eV, and 2.5 eV corresponds to MnO2 [25]. Hence, the larger FWHM values indicate that Mn ions exist in a multivalent state (the specific valence state and its distribution will be analyzed in detail in Fig. 7). The Fe 2p spectrum is shown in Fig. 6 (c), which consists of a spin-orbit doublet Fe 2p1/2, and Fe 2p3/2 around at 724.8 eV and 711.1 eV with the satellite corresponds to 733.6 eV and 718.6 eV, respectively. Only Fe3þ ions exist in CMFZ compounds owing to the binding energy is higher than 710 eV [23]. Fig. 6 (d) shows the Zn 2p core-level spectrum. The binding energy of 1044.2 eV and 1021.1 eV assign to the Zn 2p1/2 and Zn 2p3/2 levels, respectively. Clearly, Zn ions in CMFZ ceramics have only a single oxidation state of Zn2þ. To sum up, the conductive behavior of CMFZ ceramics might be mainly induced by the hopping conduction among multivalent manganese ions. Due to Mn 2p3/2 signals has a relatively high sensitivity to distinguish the valence states and concentration variations of Mn ions, so Mn 2p3/2 spectra are selected for fitting and analysis [26,27]. Fig. 7 shows the fitted results of Mn2p3/2 XPS signals of CMFZ samples sintered at different temperatures. After the experimental data were normalized, the peak intensities are estimated using the peak synthesis method that includes Mn4þ (642.84 eV), Mn3þ (641.57 eV), and Mn2þ (641.0 eV). And then, the Mn 2p3/2 signals were fitted employing a least-squares method. The fitted results presented in Fig. 7 and the calculated values of the relative concentration of Mn ions shown in Table 1. It can
Fig. 4. Plots of the lnρ versus 1000/T for the CMFZ ceramics sintered at different temperatures.
Fig. 5. Evolution of ρ25 and B constant as a function of sintering temperature of CMFZ ceramics.
of the pores and eventually leads to densification. As the temperature increases further, however, the grains grow gradually. More impor tantly, when the sintering temperature reaches 1250 � C, some closed pores appeared and accompanied by a loss in densification. It is due to the higher sintering temperature reaching the secondary recrystalliza tion temperature of the samples, which makes the motion velocity of grain boundaries faster than that of pores [21,22]. Consequently, it makes the grain boundary separate from the pores, making the grain grow up further and some pores trapped in the grains. Fig. 3 shows the elemental mapping of the CMFZ ceramic sample sintered at 1200 � C. It can observe that every element is scattered ho mogeneously over the SEM images of CMFZ compounds. Fig. 4 shows plots of the lnρ versus 1000/T for the CMFZ ceramics sintered at different temperatures. A good linear relationship between lnρ and 1000/T exists over the measured temperature region. It is indicative of the NTC characteristics with the hopping conduction mechanism. To illustrate the variation of electrical properties precisely, the evolution of resistivity at room temperature (ρ25) and the material constant (B25/50) as a function of sintering temperature are presented in Fig. 5. The B25/50 constant is calculated using Eq. (1). 4
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Materials Chemistry and Physics 239 (2020) 122098
Fig. 6. XPS spectra of CMFZ ceramics sintered at different temperature (1150 � C � T � 1250 � C) for (a) Co 2p, (b) Mn 2p, (c) Fe 2p, and (d) Zn 2p levels.
see that the numbers of Mn2þ ions first decrease and then basically unchanged. Further, the relative concentration of Mn3þ/Mn4þ couples is 0.99, 0.75, 0.68, 0.68, and 0.67, which tends to increase initial and then maintain invariable upon the sintering temperature goes from 1150 � C to 1200 � C–1250 � C. So now the mechanism of resistivity variation can be clearly explained in Fig. 5. From 1150 � C to 1200 � C, the disordered movement of ions in the thermal field becomes more intense, which makes Mn2þ ions migrate from A to B sites. To maintain electrical neutrality near the B sites, Mn3þ is converted to Mn4þ, hence reducing Mn3þ/Mn4þ couples and increasing resistivity. When the sintering temperature continues to rise, however, Mn3þ/Mn4þ couples doesn’t change. But, we noticed that the grain size increased hugely when the sintering temperature increased from 1225 � C to 1250 � C in Fig. 2 (d)–(e). Therefore, the decrease of resistivity after 1200 � C is due to the effect of grain size. In other words, the accelerated grain growth exceedingly reduces the number of grain boundaries, which effectively reduces the potential barrier of grain boundaries and hence reduces the hopping resistance between Mn3þ and Mn4þ. Finally, the resistivity goes down. Besides, the value of thermopower Q associated with Mn3þ/Mn4þ couples is a very effective method to determine the conductive type of thermosensitive ceramics [28]. The calculated Q values are 20.1 μV, 44.1 μV, 52.5 μV, 52.5 μV, and 53.8 μV at different sintering temperatures. All negative values indicate that the CMFZ ceramics is an n-type semiconductor. The thermopower can be calculated using the following equation [28].
Q¼
� � � � 1 ½Mn3þ � kB=e ln β ½Mn4þ �
(2)
where β ¼ 5/4 is a spin degeneracy factor, e is the electronic charge, kB is the Boltzmann constant, [Mnxþ] is the concentration of Mn ions (where x ¼ 3 or 4). Fig. 8 shows the relationship between the resistance drift ΔR/R and the aging time for CMFZ ceramics sintered at different temperatures. It can be observed that the as-sintered CMFZ ceramics presented excellent thermal stability, i.e., the resistance drift of all samples was less than 0.25%. Moreover, the resistance drift of all samples firstly displayed an increasing trend with fluctuation. However, when the aging time ex ceeds 500 h, the resistance no longer changes with aging time. It will guide pre-aging treatment in practical applications of the CMFZ ce ramics. Besides, the as-sintered CMFZ ceramics exhibit the lowest resistance drift when sintered the samples at 1200 � C. It mainly attrib uted to the optimized microstructure stabilized the configuration of the cation distribution. Similar results have reported by Li et al. [29]. 4. Conclusion The microstructural morphologies and electrical properties of Co2.77Mn1.71Fe1.10Zn0.42O8 ceramics sintered at 1150–1250 � C were investigated systemically in this paper. The sintering temperature was found to have a profound effect on the microstructure of CMFZ ceramics.
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Materials Chemistry and Physics 239 (2020) 122098
Fig. 7. XPS fitting of Mn 2p3/2 signals of CMFZ ceramics sintered at different temperatures.
The resistivity first increases and then decreases with the increased sintering temperatures. XPS analysis revealed that the dominant factor of resistivity evolution is transfer from Mn3þ/Mn4þ couples to the grain size effect. CMFZ ceramics was also confirmed to be n-type semi conductor according to XPS peak fitting and the calculated thermo power results. All samples exhibited excellent thermal stability. This research will be essential to guide the advancement and popularization of novel ceramic materials and sintering techniques in the future.
Table 1 The relative concentration of manganese ions and the thermopower Q of CMFZ ceramics sintered at different temperatures. Sintering Temperature (� C)
Mn2þ (%)
Mn3þ (%)
Mn4þ (%)
Mn3þ/ Mn4þ
1150 1175 1200 1225 1250
14.1 25.5 46.4 47.1 43.3
42.8 31.9 21.7 21.4 22.7
43.1 42.6 31.9 31.5 34.0
0.99 0.75 0.68 0.68 0.67
Q (μV) 20.1 44.1 52.5 52.5 53.8
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Materials Chemistry and Physics 239 (2020) 122098 [10] C. Meunier, F. Zuo, N. Peillon, S. Saunier, S. Marinel, D. Goeuriot, In situ study on microwave sintering of ZTA ceramic: effect of ZrO2 content on densification, hardness, and toughness, J. Am. Ceram. Soc. 100 (2017) 929–936. [11] M. Mazaheri, A.M. Zahedi, S.K. Sadrnezhaad, Two-step sintering of nanocrystalline ZnO compacts: effect of temperature on densification and grain growth, J. Am. Ceram. Soc. 91 (2008) 56–63. [12] J. Guo, A.L. Baker, H. Guo, M. Lanagan, C.A. Randall, Cold sintering process: a new era for ceramic packaging and microwave device development, J. Am. Ceram. Soc. 100 (2017) 669–677. [13] Y. Zhao, S.S. Berbano, L. Gao, K. Wang, J. Guo, K. Tsuji, J. Wang, C.A. Randall, Cold-sintered V2O5-PEDOT: PSS nanocomposites for negative temperature coefficient materials, J. Eur. Ceram. Soc. 39 (2019) 1257–1262. [14] J. Yang, L. Wang, X. Tan, Q. Zhi, R. Yang, G. Zhang, Z. Liu, X. Ge, E. Liang, Effect of sintering temperature on the thermal expansion behavior of ZrMgMo3O12p/2024Al composite, Ceram. Int. 44 (2018) 10744–10752. [15] J. Chen, J. Chen, X. Yao, Z. Chen, X. Liu, Z. Huang, Effect of sintering temperature on electrical properties of SiC/ZrB2 ceramics, J. Eur. Ceram. Soc. 38 (2018) 3083–3088. [16] J.B. Xia, Q. Zhao, B. Gao, A.M. Chang, B. Zhang, P.J. Zhao, R.J. Ma, Sintering temperature and impedance analysis of Mn0.9Co1.2Ni0.27Mg0.15Al0.03Fe0.45O4 NTC ceramic prepared by W/O microemulsion method, J. Alloy. Comp. 617 (2014) 228–234. [17] C.J. Ma, H. Gao, TEM and electrical properties characterizations of Co0.98Mn2.02O4 NTC ceramic, J. Alloy. Comp. 749 (2018) 853–858. [18] M. Karunanithy, G. Prabhavathi, A.H. Beevi, B.H.A. Ibraheem, K. Kaviyarasu, S. Nivetha, N. Punithavelan, A. Ayeshamariam, M. Jayachandran, Nanostructured metal tellurides and their heterostructures for thermoelectric applications—a review, J. Nanosci. Nanotechnol. 18 (2018) 6680–6707. [19] M.V. Arularasu, M. Anbarasu, S. Poovaragan, R. Sundaram, K. Kanimozhi, C. M. Magdalane, K. Kaviyarasu, F.T. Thema, D. Letsholathebe, G.T. Mola, M. Maaza, Structural, optical, morphological and microbial studies on SnO2 nanoparticles prepared by Co-precipitation method, J. Nanosci. Nanotechnol. 18 (2018) 3511–3517. [20] K. Kaviyarasu, P.A. Devarajan, S.S.J. Xavier, S.A. Thomas, S. Selvakumar, One pot synthesis and characterization of cesium doped SnO2 nanocrystals via a hydrothermal process, J. Mater. Sci. Technol. 28 (2012) 15–20. [21] N.V. Bini Pathrose, P. Radhakrishnan, A. Mujeeb, Stability, size and optical properties of silver nanoparticles prepared by femtosecond laser ablation, J. Nanomater. Mol. Nanotechnol. 5 (2016) 10–80. [22] K. Kaviyarasu, E. Manikandan, Z.Y. Nuru, M. Maaza, Quantum confinement of lead titanate nanocrystals by wet chemical method, J. Alloy. Comp. 649 (2015) 50–53. [23] H. Han, S. Mhin, K.R. Park, K.M. Kim, J.I. Lee, J.H. Ryu, Fe doped Ni-Mn-Co-O ceramics with varying Fe content as negative temperature coefficient sensors, Ceram. Int. 43 (2017) 10528–10532. [24] K.J. Kim, J.W. Heo, Electronic structure and optical properties of inverse-spinel MnCo2O4 thin films, J. Korean Phys. Soc. 60 (2012) 1376–1380. [25] J. Wu, Z. Huang, W. Zhou, C. Ouyang, Y. Hou, Y. Gao, R. Chen, J. Chu, Investigation of cation distribution, electrical, magnetic properties and their correlation in Mn2-xCo2xNi1-xO4 films, J. Appl. Phys. 115 (2014), 113703. [26] H.W. Nesbitt, D. Banerjee, Interpretation of XPS Mn(2p) spectra of Mn oxyhydroxides and constraints on the mechanism of MnO2 precipitation, Am. Mineral. 83 (1998) 305–315. [27] W.W. Kong, L. Chen, B. Gao, B. Zhang, P.J. Zhao, G. Ji, A.M. Chang, C.P. Jiang, Fabrication and properties of Mn1.56Co0.96Ni0.48O4 free-standing ultrathin chips, Ceram. Int. 40 (2014) 8405–8409. [28] R. Dannenberg, S. Baliga, R.J. Gambino, A.H. King, A.P. Doctor, Resistivity, thermopower and the correlation to infrared active vibrations of Mn1.56Co0.96Ni0.48O4 spinel films sputtered in an oxygen partial pressure series, J. Appl. Phys. 86 (1999) 514–523. [29] D.F. Li, S.X. Zhao, K. Xiong, H.Q. Bao, C.W. Nan, Aging improvement in Cucontaining NTC ceramics prepared by co-precipitation method, J. Alloy. Comp. 582 (2014) 283–288.
Fig. 8. Relationship between ΔR/R and the aging time for CMFZ ceramics sintered at different temperatures.
Acknowledgement This work was supported by the National Key Research and Devel opment Program of China (Grant No. 2017YFB0406405). References [1] M. Schubert, C. Munch, S. Schuurman, V. Poulain, J. Kita, R. Moos, Characterization of nickel manganite NTC thermistor films prepared by aerosol deposition at room temperature, J. Eur. Ceram. Soc. 38 (2018) 613–619. [2] Q. Wang, W.W. Kong, J.C. Yao, A.M. Chang, Fabrication and electrical properties of the fast response Mn1.2Co1.5Ni0.3O4 miniature NTC chip thermistors, Ceram. Int. 45 (2019) 378–383. [3] F. Guan, X.J. Lin, H. Dai, J.R. Wang, X. Cheng, S.F. Huang, LaMn1-xTixO3-NiMn2O4 (0�x�0.7): a composite NTC ceramic with controllable electrical property and high stability, J. Eur. Ceram. Soc. 39 (2019) 2692–2696. [4] L.H. Omari, L. Hajji, M. Haddad, T. Lamhasni, C. Jama, Synthesis, structural, optical and electrical properties of La-modified Lead Iron Titanate ceramics for NTCR thermo-resistance based sensors, Mater. Chem. Phys. 223 (2019) 60–67. [5] A. Feteira, Negative temperature coefficient resistance (NTCR) ceramic thermistors: an industrial perspective, J. Am. Ceram. Soc. 92 (2009) 967–983. [6] C.J. Ma, Y.F. Liu, Y.N. Lu, H. Gao, H. Qian, J.X. Ding, Effect of Zn substitution on the phase, microstructure and electrical properties of Ni0.6Cu0.5ZnxMn1.9-xO4 (0�x�1) NTC ceramics, Mater. Sci. Eng., B 188 (2014) 66–71. [7] X.B. Zhang, W. Ren, W.W. Kong, Q. Zhou, L. Wang, L. Bian, J.B. Xu, A.M. Chang, C. Q. Jiang, Effect of sputtering power on structural, cationic distribution and optical properties of Mn2Zn0.25Ni0.75O4 thin films, Appl. Surf. Sci. 435 (2018) 815–821. [8] M.Y. Chen, J. Juuti, H. Jantunen, Sintering behavior, microstructure and dielectric performance of BaTiO3 with 60-65 wt% addition of B2O3-Bi2O3-SiO2-ZnO glass, J. Alloy. Comp. 737 (2018) 392–397. [9] B. Zhang, Q. Zhao, A.M. Chang, Y.Q. Wu, H.Y. Li, Spark plasma sintering of MgAl2O4-LaCr0.5Mn0.5O3 composite thermistor ceramics and a comparison investigation with conventional sintering, J. Alloy. Comp. 675 (2016) 381–386.
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Materials Chemistry and Physics 239 (2020) 122098
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Sintering temperature and XPS analysis of Co2.77Mn1.71Fe1.10Zn0.42O8 NTC ceramics Bing Wang a, b, Junhua Wang a, b, Dunsheng Shang a, Aimin Chang a, b, Jincheng Yao a, b, * a
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, 40-1 South Beijing Road, Urumqi, China b University of Chinese Academy of Sciences, Beijing, 100049, China
H I G H L I G H T S
� We first revealed the effects of sintering temperature of the CMFZ ceramics. � The optimum sintering temperature of 1200 � C was determined. � CMFZ ceramics was confirmed to be an n-type semiconductor. A R T I C L E I N F O
A B S T R A C T
Keywords: NTC thermistors XPS analysis Sintering Microstructure Electrical properties
Microstructure and electrical properties of Co2.77Mn1.71Fe1.10Zn0.42O8 (CMFZ) ceramics as a function of tem perature were studied in the sintering process. The optimum sintering temperature of 1200 � C is determined to obtain the optimum microstructure and electrical properties. Both microstructure and electrical properties are sensitive to the different sintering temperatures. In particular, the appropriately increased sintering temperature enhances the densification and the homogeneity of grain size. However, the accelerated grain growth with a large number of closed pores appears when above the optimum sintering temperature. The electrical resistivity first increases and then decreases with the increased sintering temperatures. The ρ25 and B25/50 constant are in the range of 3072–4238 Ω cm and 4021–4056 K, respectively. X-ray photoelectron spectroscopy (XPS) was analyzed the evolution of resistivity clearly and proved the n-type semiconductor of CMFZ ceramics.
1. Introduction Thermosensitive ceramics with negative temperature coefficients (NTC) are semiconductive ceramics that usually composed of Manganese-based transition metal oxides [1–4]. Due to the high sensi tivity, fast response, low cost, and convenience in use, Mn-based NTC ceramics have attracted extensive attention and achieved a wide range of applications in temperature control and measurement [5,6]. In the past decades, various sintering techniques are used, such as traditional sintering [7], liquid-phase sintering [8], spark plasma sintering [9], microwave sintering [10], two-step sintering and cold sintering process [11–13], which have attracted much attention in recent years. However, it is worth noting that different sintering techniques have different sintering temperature regimes, and the control of sintering temperature on all sintering methods is crucial. Recently, some researches have centered on the effects of sintering temperature on the properties of
ceramics [14–16]. For instance, Ma et al. [17] have investigated the influences of the sintering temperature on the electrical properties of Co0.98Mn2.02O4 ceramics and found that the resistivity decreases monotonically with the increased sintering temperature. Unfortunately, the analysis for the variation of resistivity is only conjecture, but the mechanism of cationic valence states and concentration variation is not depth. However, there are few reports focusing on the effect of sintering process on the electrical properties and stability of Co2.77Mn1.71 Fe1.10Zn0.42O8 ceramics. It is an effective way to investigate the cationic oxidation states and concentration distributions in ceramics with XPS. Therefore, the effects of sintering temperature on the properties of Co2.77Mn1.71Fe1.10Zn0.42O8 NTC ceramics were systemically in the present work.
* Corresponding author. University of Chinese Academy of Sciences, Beijing, 100049, China. E-mail addresses: [email protected], [email protected] (J. Yao). https://doi.org/10.1016/j.matchemphys.2019.122098 Received 16 June 2019; Received in revised form 26 August 2019; Accepted 28 August 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.
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Materials Chemistry and Physics 239 (2020) 122098
Co2.77Mn1.71Fe1.10Zn0.42O8. And then, the weighed powders were ballmilled, dried and calcinated at 850 � C for 2 h. Later the obtained pow ders were ground and sieved using a 100-mesh sieve. Subsequently, the powders were shaped into disks of 2 mm thickness and 10 mm diameter and pressed with a pressure of 350 MPa using the cold isostatic press. Then the disks were sintered at 1150 � C–1250 � C for 2 h. Both sides of the sintered disks were polished and coated with a silver paste to investigate the electrical properties. X-ray diffraction (XRD) results on CMFZ ceramics were recorded utilizing a Bruker D8 Advance diffractometer. The valence states and ionic concentration of CMFZ ceramics were analyzed using XPS (KAlphaþ). The surface microstructure and elemental distribution were characterized using the scanning electron microscope (SEM, Zeiss Supra55VP) and energy dispersive spectroscopy (EDS, Bruker Xflash5010). 3. Results and discussion Fig. 1 presents XRD images of CMFZ samples sintered at 1150 � C, 1175 � C, 1200 � C, 1225 � C, and 1250 � C. It is clear that every sample exhibits the cubic spinel phase, and no secondary phase appeared. Ori entations including (111), (220), (311), (222), (400), (422), (511) and (400) can be detected, with the strongest orientation being at (311). Besides, the intensity of the XRD peak increases first and then decreases with the increased sintering temperatures. The increased peak intensity, mainly attributed to the increased temperature from 1150 � C to 1200 � C, enhances the diffusion velocity of atoms, which improves the binding activity of tetrahedron and octahedron in the sintering process.
Fig. 1. X-ray diffraction patterns of CMFZ ceramics sintered at different temperatures.
2. Experimental procedures CMFZ ceramics were prepared using a traditional ceramic fabrica tion route. Analytical reagent Co3O4, Mn3O4, Fe2O3, and ZnO were used as raw materials and weighed according to the nonstoichiometric ratio
Fig. 2. Scanning electron microscope micrographs of the CMFZ ceramics sintered at (a) 1150 � C, (b) 1175 � C, (c) 1200 � C, (d) 1225 � C, and (e) 1250 � C. 2
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Materials Chemistry and Physics 239 (2020) 122098
Fig. 3. Elemental mapping of the CMFZ ceramic sample sintered at 1200 � C.
However, the higher sintering temperature may induce phase decom position from this phase into other cubic spinel phase, and hence the decreased peak intensity appeared. The phase decomposition at high temperatures can easily explain in theory. As the increase of sintering temperature, the oxygen partial pressure around the sample decreases, so the sample is easy to lose oxygen. Therefore, phase decomposition is easily to happen at higher sintering temperatures.
Fig. 2 shows surface microstructure images of the CMFZ samples sintered at various temperatures. The microstructure evolution exhibits notable temperature dependence [18–20]. Compared with the applied sintering temperature of 1150 � C, some pores gradually disappear, and the as-sintered ceramics presents a homogeneous and close-packed grain while the sintering temperature increased from 1150 � C to 1200 � C. It is associated with the increased temperature that promotes the shrinkage 3
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Materials Chemistry and Physics 239 (2020) 122098
B25=50 ¼ � 1
ln ρρ50
T50
(1)
25
1
=T25
where ρ25 and ρ50 are the resistivity obtained at 25 � C and 50 � C, respectively, T is the absolute temperature, and k is the Boltzmann constant. As shown in Fig. 5, the values of ρ25 and B25/50 constant are in the range of 3072–4238 Ω cm and 4021–4056 K, respectively. It can be seen that the difference of B25/50 constant was insignificant for CMFZ samples sintered at various temperatures. However, ρ25 increases first and later decreases as the increase of the sintering temperatures. This evolution might induce by the variation of hopping conduction mechanism be tween Mn3þ and Mn4þ ions originated from the difference of cation distribution. The XPS investigation was conducted to clarify the mech anism of resistivity variation clearly. The chemical valence of every cation (i.e., Co, Mn, Fe, and Zn) in CMFZ ceramics were analyzed. The standard C 1s peak (248.6 eV) was applied to calibrate every binding energy for eliminating the charge effect. Fig. 6 shows the high-resolution XPS spectra of CMFZ samples sintered at various temperatures. It can be seen that the characteristic peaks of every cation present similar binding energies when sintered the samples at various temperatures. It indicates that the cation distribu tions in A and B sites of the CMFZ compounds are similar even at different temperatures. Fig. 6 (a) shows the Co 2p spectrum obtained from CMFZ samples at various sintering temperatures. The Co 2p3/2 major peak and its satellite peak located at the 780.0 eV and 786.4 eV, respectively, and the Co 2p1/2 major peak and its satellite located at 795.5 eV and 802.8 eV, respectively. The splitting between 2p3/2 and 2p1/2 due to spin-orbit coupling is nearly 15.5 eV. The obtained results well agree with other reports [21,23,24]. More importantly, the satellite peaks of Co 2p3/2 are a useful way to confirm the valence states of Co ions. In general, the satellite of Co2þ ions typically near 786 eV, while Co3þ ions are known to 790 eV. Through the analysis above, the satellite of Co 2p3/2 located at 786.4 eV. Therefore, the satellite peak of Co 2p3/2 level strongly suggests that only Co2þ exist in the CMFZ ceramics. Fig. 6 (b) shows the Mn 2p core-level spectrum. Two major characterized peaks, Mn 2p3/2, and Mn 2p1/2 are located at 641.7 eV and 653.5 eV, respectively. The calculated Full Width at Half Maximum (FWHM) of the Mn 2p level was 3.91, 4.17, 4.10, 4.30, and 4.22 for different sintering temperatures, respectively. The obtained FWHM values are higher than that of mono-valance state Mn ions, i.e., the FWHM of MnO is 3.2 eV, Mn2O3 is 3.0 eV, and 2.5 eV corresponds to MnO2 [25]. Hence, the larger FWHM values indicate that Mn ions exist in a multivalent state (the specific valence state and its distribution will be analyzed in detail in Fig. 7). The Fe 2p spectrum is shown in Fig. 6 (c), which consists of a spin-orbit doublet Fe 2p1/2, and Fe 2p3/2 around at 724.8 eV and 711.1 eV with the satellite corresponds to 733.6 eV and 718.6 eV, respectively. Only Fe3þ ions exist in CMFZ compounds owing to the binding energy is higher than 710 eV [23]. Fig. 6 (d) shows the Zn 2p core-level spectrum. The binding energy of 1044.2 eV and 1021.1 eV assign to the Zn 2p1/2 and Zn 2p3/2 levels, respectively. Clearly, Zn ions in CMFZ ceramics have only a single oxidation state of Zn2þ. To sum up, the conductive behavior of CMFZ ceramics might be mainly induced by the hopping conduction among multivalent manganese ions. Due to Mn 2p3/2 signals has a relatively high sensitivity to distinguish the valence states and concentration variations of Mn ions, so Mn 2p3/2 spectra are selected for fitting and analysis [26,27]. Fig. 7 shows the fitted results of Mn2p3/2 XPS signals of CMFZ samples sintered at different temperatures. After the experimental data were normalized, the peak intensities are estimated using the peak synthesis method that includes Mn4þ (642.84 eV), Mn3þ (641.57 eV), and Mn2þ (641.0 eV). And then, the Mn 2p3/2 signals were fitted employing a least-squares method. The fitted results presented in Fig. 7 and the calculated values of the relative concentration of Mn ions shown in Table 1. It can
Fig. 4. Plots of the lnρ versus 1000/T for the CMFZ ceramics sintered at different temperatures.
Fig. 5. Evolution of ρ25 and B constant as a function of sintering temperature of CMFZ ceramics.
of the pores and eventually leads to densification. As the temperature increases further, however, the grains grow gradually. More impor tantly, when the sintering temperature reaches 1250 � C, some closed pores appeared and accompanied by a loss in densification. It is due to the higher sintering temperature reaching the secondary recrystalliza tion temperature of the samples, which makes the motion velocity of grain boundaries faster than that of pores [21,22]. Consequently, it makes the grain boundary separate from the pores, making the grain grow up further and some pores trapped in the grains. Fig. 3 shows the elemental mapping of the CMFZ ceramic sample sintered at 1200 � C. It can observe that every element is scattered ho mogeneously over the SEM images of CMFZ compounds. Fig. 4 shows plots of the lnρ versus 1000/T for the CMFZ ceramics sintered at different temperatures. A good linear relationship between lnρ and 1000/T exists over the measured temperature region. It is indicative of the NTC characteristics with the hopping conduction mechanism. To illustrate the variation of electrical properties precisely, the evolution of resistivity at room temperature (ρ25) and the material constant (B25/50) as a function of sintering temperature are presented in Fig. 5. The B25/50 constant is calculated using Eq. (1). 4
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Materials Chemistry and Physics 239 (2020) 122098
Fig. 6. XPS spectra of CMFZ ceramics sintered at different temperature (1150 � C � T � 1250 � C) for (a) Co 2p, (b) Mn 2p, (c) Fe 2p, and (d) Zn 2p levels.
see that the numbers of Mn2þ ions first decrease and then basically unchanged. Further, the relative concentration of Mn3þ/Mn4þ couples is 0.99, 0.75, 0.68, 0.68, and 0.67, which tends to increase initial and then maintain invariable upon the sintering temperature goes from 1150 � C to 1200 � C–1250 � C. So now the mechanism of resistivity variation can be clearly explained in Fig. 5. From 1150 � C to 1200 � C, the disordered movement of ions in the thermal field becomes more intense, which makes Mn2þ ions migrate from A to B sites. To maintain electrical neutrality near the B sites, Mn3þ is converted to Mn4þ, hence reducing Mn3þ/Mn4þ couples and increasing resistivity. When the sintering temperature continues to rise, however, Mn3þ/Mn4þ couples doesn’t change. But, we noticed that the grain size increased hugely when the sintering temperature increased from 1225 � C to 1250 � C in Fig. 2 (d)–(e). Therefore, the decrease of resistivity after 1200 � C is due to the effect of grain size. In other words, the accelerated grain growth exceedingly reduces the number of grain boundaries, which effectively reduces the potential barrier of grain boundaries and hence reduces the hopping resistance between Mn3þ and Mn4þ. Finally, the resistivity goes down. Besides, the value of thermopower Q associated with Mn3þ/Mn4þ couples is a very effective method to determine the conductive type of thermosensitive ceramics [28]. The calculated Q values are 20.1 μV, 44.1 μV, 52.5 μV, 52.5 μV, and 53.8 μV at different sintering temperatures. All negative values indicate that the CMFZ ceramics is an n-type semiconductor. The thermopower can be calculated using the following equation [28].
Q¼
� � � � 1 ½Mn3þ � kB=e ln β ½Mn4þ �
(2)
where β ¼ 5/4 is a spin degeneracy factor, e is the electronic charge, kB is the Boltzmann constant, [Mnxþ] is the concentration of Mn ions (where x ¼ 3 or 4). Fig. 8 shows the relationship between the resistance drift ΔR/R and the aging time for CMFZ ceramics sintered at different temperatures. It can be observed that the as-sintered CMFZ ceramics presented excellent thermal stability, i.e., the resistance drift of all samples was less than 0.25%. Moreover, the resistance drift of all samples firstly displayed an increasing trend with fluctuation. However, when the aging time ex ceeds 500 h, the resistance no longer changes with aging time. It will guide pre-aging treatment in practical applications of the CMFZ ce ramics. Besides, the as-sintered CMFZ ceramics exhibit the lowest resistance drift when sintered the samples at 1200 � C. It mainly attrib uted to the optimized microstructure stabilized the configuration of the cation distribution. Similar results have reported by Li et al. [29]. 4. Conclusion The microstructural morphologies and electrical properties of Co2.77Mn1.71Fe1.10Zn0.42O8 ceramics sintered at 1150–1250 � C were investigated systemically in this paper. The sintering temperature was found to have a profound effect on the microstructure of CMFZ ceramics.
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Materials Chemistry and Physics 239 (2020) 122098
Fig. 7. XPS fitting of Mn 2p3/2 signals of CMFZ ceramics sintered at different temperatures.
The resistivity first increases and then decreases with the increased sintering temperatures. XPS analysis revealed that the dominant factor of resistivity evolution is transfer from Mn3þ/Mn4þ couples to the grain size effect. CMFZ ceramics was also confirmed to be n-type semi conductor according to XPS peak fitting and the calculated thermo power results. All samples exhibited excellent thermal stability. This research will be essential to guide the advancement and popularization of novel ceramic materials and sintering techniques in the future.
Table 1 The relative concentration of manganese ions and the thermopower Q of CMFZ ceramics sintered at different temperatures. Sintering Temperature (� C)
Mn2þ (%)
Mn3þ (%)
Mn4þ (%)
Mn3þ/ Mn4þ
1150 1175 1200 1225 1250
14.1 25.5 46.4 47.1 43.3
42.8 31.9 21.7 21.4 22.7
43.1 42.6 31.9 31.5 34.0
0.99 0.75 0.68 0.68 0.67
Q (μV) 20.1 44.1 52.5 52.5 53.8
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Fig. 8. Relationship between ΔR/R and the aging time for CMFZ ceramics sintered at different temperatures.
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