Spark plasma sintering of MgAl2O4–YCr0.5Mn0.5O3 composite NTC ceramics

Spark plasma sintering of MgAl2O4–YCr0.5Mn0.5O3 composite NTC ceramics

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ScienceDirect Journal of the European Ceramic Society xxx (2014) xxx–xxx

Spark plasma sintering of MgAl2O4–YCr0.5Mn0.5O3 composite NTC ceramics Bo Zhang a,b , Qing Zhao a , Aimin Chang a,∗ , Yiyu Li c , Yin Liu c , Yiquan Wu c,∗ 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, Urumqi 830011, China b University of Chinese Academy of Sciences, Beijing 100049, China c Kazuo Inamori School of Engineering, New York State College of Ceramics at Alfred University, Alfred, NY 14802, USA Received 11 September 2013; received in revised form 22 March 2014; accepted 26 March 2014

Abstract New high temperature negative temperature coefficient (NTC) thermistor ceramics based on a xMgAl2 O4 –(1 − x)YCr0.5 Mn0.5 O3 (x = 0.1, 0.4, 0.6) composite system have been successfully fabricated through spark plasma sintering (SPS) with a low sintering temperature and a short sintering period. The X-ray diffraction analysis indicates that the SPS-sintered composite ceramics consist of a cubic spinel MgAl2 O4 phase and an orthorhombic perovskite YCr0.5 Mn0.5 O3 phase isomorphic to YCrO3 . The SPS-sintered composite ceramics have high relative density ranging from 94.1 to 97.4% of the theoretical density. X-ray photoelectron spectroscopy analysis corroborates the presence of Cr3+ , Cr4+ , Mn3+ , and Mn4+ ions on lattice sites, which may result in the hopping conduction. The obtained ρ25 , B25–150 , and B700–1000 of the SPS-sintered composite NTC thermistors are in the range of 1.53 × 106 –9.92 × 109  cm, 3380–5172 K, and 7239–9543 K, respectively. These values can be tuned by adjusting the MgAl2 O4 concentration. © 2014 Elsevier Ltd. All rights reserved. Keywords: Spark plasma sintering; NTC thermistors; Spinel; Perovskite; Electrical properties

1. Introduction Negative temperature coefficient (NTC) thermistors have recently attracted attention due to their potential in high temperature applications (1000 ◦ C), such as exhaust gas and catalytic converter temperature sensing.1,2 NTC thermistors are thermally sensitive resistors whose resistance decreases with increasing temperature. The popular NTC materials based on spinel forming oxide systems (MMn2 O4 , where M = Ni, Co, Fe, Cu, Zn) exhibit the instability and changing electrical characteristics, and thus their application is commonly limited to temperatures below 300 ◦ C.3 Therefore, there is a need to develop new materials that have good electrical characteristics at high temperatures.

The literature suggests that a perovskite-type YCr0.5 Mn0.5 O3 has a high conductivity,2,4 and spinel-type MgAl2 O4 has a high resistivity.5 The conductivity of single phase YCr0.5 Mn0.5 O3 is too high for high temperature applications.4 However, composite ceramics consisting of both a high and a low resistivity phase (i.e. a NTC material), provide a high temperature thermistor with moderate resistivity and B (thermistor constant) value at working temperature.5 So, according to the electrical properties mixing rules,2 there is a possibility of obtaining a new material suitable for high temperature NTC thermistor applications by associating these two phases. Generally electrical properties mixing rules can be expressed by the following equation: ρ t = ρ s Vs + ρ i V i



Corresponding authors. Tel.: +1 607 871 2662; fax: +1 607 871 2354. E-mail addresses: [email protected] (A. Chang), [email protected] (Y. Wu).

(1)

where ρt , ρs , ρi are the resistivity of composites, semiconductive phase and insulating phase, respectively, and Vs , Vi are the volume fraction of semi-conductive phase and

http://dx.doi.org/10.1016/j.jeurceramsoc.2014.03.025 0955-2219/© 2014 Elsevier Ltd. All rights reserved.

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insulating phase, respectively. However, these two materials show poor sinterability and are difficult to densify under ambient atmospheric conditions or through pressureless sintering techniques.6–8 Spark plasma sintering (SPS) is a relatively novel consolidation technique that relies on the simultaneous application of axial pressure and elevated temperature, generated by a high current flow.9 It has been convincingly shown that the SPS process provides significant advantages both by lowering the required sintering temperature, shortening sintering periods and generally forming high density ceramics.10 Although SPS has frequently been used to produce dense transparent and structural ceramics, there are few reports focusing on the application of this technique to prepare dense ceramics for NTC thermistor applications.11 In the present study, a new series of MgAl2 O4 –YCr0.5 Mn0.5 O3 composite ceramics were prepared by SPS to investigate their NTC electrical properties. Fig. 1. XRD patterns of SPS-sintered ceramics.

2. Experimental procedure Stoichiometric amounts of Y2 O3 (Sigma–Aldrich, USA, 99.99%), Cr2 O3 (Sigma–Aldrich, USA, 99%), and MnO2 (Sigma–Aldrich, USA, 99%) were well mixed, ground, and calcined at 1200 ◦ C for 2 h to yield YCr0.5 Mn0.5 O3 . Then, the mixture of xMgAl2 O4 –(1 − x)YCr0.5 Mn0.5 O3 (MgAl2 O4 , Alfa Aesar, USA, 99.9%) with x = 0.1, 0.4, 0.6, was milled again for 6 h. The sintering was carried out under vacuum (6 Pa) with an SPS apparatus (FCT Systeme GmbH, FCT, Rauenstein, Germany). The mixed powders were placed in a cylindrical graphite die with an inner diameter of 18.75 mm, separated by graphite foil. The temperature was increased to 1000 ◦ C at a heating rate of 100 ◦ C/min and then further increased to 1200 ◦ C within 4 min. A uniaxial pressure of 30 MPa was applied at 1000 ◦ C and kept constant until the final sintering temperature of 1200 ◦ C was reached, and then released at the end of the holding time (20 min). The cooling rate was 50 ◦ C/min. During SPS processing, the temperature was measured on the surface of the graphite die using an optical pyrometer. The mixed powders were pressed into a disk and then sintered in an electrical resistance furnace (LHT08/18, Nabertherm, Germany) at 1600 ◦ C for 5 h for comparison. The density and porosity of the sintered samples were determined by the Archimedes method. X-ray diffraction (XRD; BRUKER D2-ADVANCE) using CuK␣ radiation was used to identify crystalline phases in the sintered ceramics. The microstructure and phase composition of the sintered ceramics were observed by the Scanning Electron Microscope (SEM; FEI Quanta 200) in combination with energy dispersive spectroscopy (EDS). X-ray photoelectron spectroscopy (XPS; Thermo Fisher, ESCALAB 250) was used to analyze the chemical states of the sintered ceramics. For the characterization of electrical properties, sintered pellets were polished, coated with a thin layer of non-fluxed Pt paste, and then heated to 900 ◦ C for 30 min. The resistances were measured from 25 up to 1000 ◦ C, using a digital multimeter (Keithley 2000).

3. Results and discussion The XRD patterns of the as-sintered xMgAl2 O4 –(1 − x)YCr0.5 Mn0.5 O3 are shown in Fig. 1. The main phases in the composite ceramics were indexed to be a cubic spinel MgAl2 O4 (JCPDF Card No. 21-1152) phase and an orthorhombic perovskite YCr0.5 Mn0.5 O3 phase isomorphic to YCrO3 (JCPDF Card No. 34-0365). It is noted that the peaks corresponding to the perovskite phase in the composite ceramic samples slightly shifted toward higher angles with respect to the YCr0.5 Mn0.5 O3 with an increase in MgAl2 O4 content. These results may be due to the substitution of the smaller Al3+ for Mn3+ and Cr3+ at perovskite lattice positions. This can be confirmed by the following EDS result. Fig. 2(a)–(c) shows the SEM images of the SPS-sintered composite ceramics. It can be seen that all of the ceramics were highly dense. The grain sizes of the sintered samples ranged from 0.5 to 2 ␮m. The theoretical densities, 3.584 g/cm3 for MgAl2 O4 8 and 5.70 g/cm3 for YCr0.5 Mn0.5 O3 ,2 were used to calculate the theoretical densities of the SPS-sintered composite ceramics by using the mixing rule.12 The relative densities were 95.5%, 97.4% and 94.1% of the theoretical density for x = 0.1, 0.4, 0.6, respectively. The porosity was 1.98%, 1.43% and 3.21% for x = 0.1, 0.4, 0.6, respectively. The compositions taken from the brighter and darker grains of polished and thermally etched surfaces for the 0.6MgAl2 O4 –0.4YCr0.5 Mn0.5 O3 ceramics (Fig. 2(d)) were qualitatively identified by EDS, as shown in Fig. 2(e) and (f), respectively. It can be seen that two tone contrasts were observed in Fig. 2(d), which indicate the two different phases of the composite ceramics. From Fig. 2(e) and (f), it can be concluded that the brighter regions are mainly the YCr0.5 Mn0.5 O3 phase and the darker is the MgAl2 O4 . It was also observed that there were Al and Mg elements in YCr0.5 Mn0.5 O3 phase, and Cr and Mn elements in MgAl2 O4 phase. Considering the ionic radii and the lattice sites, the interdiffusion of Al3+ , Mg2+ , Mn3+ and Cr3+ ions may occur between these two

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Fig. 2. Secondary-electron (SE) and backscattered (BS) SEM images of the xMgAl2 O4 –(1 − x) YCr0.5 Mn0.5 O3 ceramics sintered by SPS: (a) SE-SEM, x = 0.1; (b) SE-SEM, x = 0.4; (c) SE-SEM, x = 0.6; (d) BS-SEM, x = 0.6. (e) EDS spectrum taken from the brighter grain (A). (f) EDS spectrum taken from the darker grain (B).

phases. This is in agreement with our XRD analysis and previous literature.13 Fig. 3 displays the XPS spectra of Cr 2p and Mn 2p regions of the 0.4MgAl2 O4 –0.6YCr0.5 Mn0.5 O3 ceramics sintered by SPS. As can be seen in Fig. 3, the peaks in the Cr 2p and Mn 2p spectra can be split into two peaks through Gaussian–Lorentzian curve fitting. The Cr 2p3/2 spectrum shown in Fig. 3(a) clearly shows two peaks at 576.6 eV and 577.5 eV, which can be resolved into the contribution from Cr3+ and Cr4+ . The XPS results presented herein are in agreement with the discussion conducted previously by Isao Ikemoto.14 Analyzing the Mn 2p spectrum, it is possible to note that the Mn 2p3/2 peak shows two distinctive peaks: a main peak at 641.4 eV and a second peak at 642.3 eV, which are attributed to different Mn valences and environments.

According to the binding energy values,15 the main peak can be resolved into the contribution of Mn3+ and the second, to Mn4+ . These results indicate the presence of Cr3+ , Cr4+ , Mn3+ and Mn4+ on lattice sites. The relationship between the natural logarithm of the resistivity (ln ρ) and the reciprocal of the absolute temperature (1000/T) for the SPS-sintered xMgAl2 O4 –(1 − x)YCr0.5 Mn0.5 O3 NTC thermistors was obtained, as shown in Fig. 4(a). The results clearly reveal that the electrical behavior is NTC. It is apparent that the plotting curves are not linear over the measured temperature range, change around 600 ◦ C. This result may be due to there is either a new or an additional mechanism which controls the conduction mechanism at high temperature.2,16 The linear dependence in

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Fig. 3. XPS spectra of (a) Cr 2p regions and (b) Mn 2p regions of 0.4MgAl2 O4 –0.6YCr0.5 Mn0.5 O3 ceramics sintered by SPS.

this plot is characteristic of the small-polaron hopping transport which is often observed in chromium based perovskites.16 This conducting phenomenon is generally given by   A −Ea σ = exp (2) T kT where σ is electrical conductivity, A is the charge carrier concentration, Ea is the activation energy of hopping, and k is the Boltzmann constant.16,17 In addition, the thermistor constant B, which indicates a sensitivity to temperature excursions and is given by18 B=

Ea k

(3)

As shown in previous XPS results, the presence of Cr4+ and Mn4+ must distort the crystal structure accompanied by the formation of Cr3+ O Cr4+ and Mn3+ O Mn4+ bonds due to the differences of ionic radii among Cr3+ , Cr4+ , Mn3+ and Mn4+ .19 Thus, we speculate that the electrical conduction in these ceramics may be due to the electron jumps between Cr3+ and Cr4+ ions on one hand, and between Mn3+ and Mn4+ ions on the other hand. This explanation is consistent with previous studies.6,16,20 Here one can observe that the resistivity increased as the MgAl2 O4 content increased. This result can be explained based on the

fact that MgAl2 O4 is an electrical insulator. As the amount of MgAl2 O4 in the thermistors increases, that of YCr0.5 Mn0.5 O3 decreases, leading to a decrease in the amount of charge carriers, which are responsible for hopping and conductivity, thus resulting in an increase in the resistivity. As mentioned above, the slope of the (ln ρ) versus (1000/T), representing the thermistor constant B, changes around 600 ◦ C. We have calculated two thermistor constants (B) the one for temperature ranging from 25 to 150 ◦ C (B25–150 ) and the second from 700 to 1000 ◦ C (B700–1000 ), together with the resistivity (ρ) and activation energy, as shown in Table 1. B can be calculated by the following equation3 :     ρT TTN ln B= (4) TN − T ρN where ρT is the resistivity at temperature T, and ρN is the resistivity at temperature TN . Evolution of resistivity and B values as a function of “x” for the SPS-sintered samples is plotted in Fig. 4(b). It can be seen that B25–150 increased with increasing MgAl2 O4 content, whereas B700–1000 decreased. These results indicate that there is a gradual change from the nonlinear to linear in resistance-temperature curves, as shown in Fig. 4(a). These electric behaviors should be ascribed to the various compositions of the grain/grain boundary and related defect characteristics.21 To understand the nature of the conduction behavior in this

Fig. 4. (a) Relationship between ln ρ and 1000/T for the SPS-sintered samples. (b) Evolution of ρ25 and B constants as a function of MgAl2 O4 content (x) for the SPS-sintered samples.

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Table 1 Resistivity, B constant, and activation energy for the SPS-sintered samples. x

ρ25 ( cm)

ρ700 ( cm)

B25/150 (K)

B700/1000 (K)

Ea25−150◦ C (eV)

Ea700−1000◦ C (eV)

0 0.1 0.4 0.6 1

8.14 × 105

117 184 914 6.45 × 103 3.12 × 107

3198 3380 3699 5172 2576

9840 9543 7690 7239 19873

0.276 0.291 0.319 0.446 0.222

0.848 0.823 0.663 0.624 1.713

1.53 × 106 1.62 × 107 9.92 × 109 1.55 × 1012

system, further investigation is necessary and desirable. As shown in Table 1 and Fig. 4(b), the values of ρ25 , B25–150 , B700–1000 , Ea25−150◦ C and Ea700−1000◦ C of the SPS-sintered xMgAl2 O4 –(1 − x)YCr0.5 Mn0.5 O3 composite NTC thermistors were in the range of 1.53 × 106 –9.92 × 109  cm, 3380–5172 K, 7239–9543 K, 0.291–0.446 eV, and 0.624–0.823 eV, respectively. It can be seen that the Ea700−1000◦ C is significantly higher than Ea25−150◦ C , which may be due to the random distribution of the differently charged cations at high temperature, making hopping more difficult and thus increasing the activation energy.22 According to previous research,2,3 in order to make small NTC resistors with a resistance value ranging from several hundreds k at room temperature to several  at 1000 ◦ C, resistivities ranging from 1 to 100 M cm are required. Therefore, the resistivities from our results may satisfy the high temperature applications through adjusting the MgAl2 O4 content. Moreover, the values of B700–1000 are slight higher than the values (about 6000 K) reported by D. Houivet et al.2 The NTC thermistors with higher B constants are more sensitive for measuring temperature variation, especially for high temperature thermistor applications.23 Fig. 5 shows the plots of natural logarithm of the resistivity (ln ρ) versus the reciprocal of the absolute temperature (1000/T) for the conventionally sintered samples. The resistivity, B constant, and activation energy for the conventionally sintered samples are given in Table 2. From Fig. 5 and Table 2, it can be seen that these resistivities were somewhat lower than those sintered by SPS. There are two possible reasons for the decrease in the resistivity: (1) during conventional sintering process, a higher sintering temperature and a longer sintering period lead to the volatility of more chromium at high temperatures and leaving an excess of O2− ions, which will cause the increase of Cr4+ and Mn4+ ions as a result of charge compensation, thus promoting the electron hopping and then decreasing the resistivity24 ; (2) an increase in grain size (see Fig. 6) leads to a decrease in grain boundary area, which results in an increase in the time between electron scattering events of charge carriers and thus decreasing the resistivity.18 In order to gain insight into the conduction behavior of the system as well as interdiffusion between these two phases, the

Fig. 5. Plots of natural logarithm of the resistivity (lnρ) versus the reciprocal of the absolute temperature (1000/T) for the conventionally sintered samples.

resistivities of the individual components were obtained, and the experimental resistivity measured on composites were compared with estimates obtained applying Eq. (1), as shown in Fig. 7. It can be seen that the experimental and calculated resistivities of composite ceramics were between those of YCr0.5 Mn0.5 O3 and MgAl2 O4 . This indicates that electrical properties mixing rules control the resistivity.2 Here one can observe that experimental resistivity was smaller than calculated resistivity for the SPS-sintered composite ceramics. This result may be due to the interdiffusion between these two phases during SPS processing, which can be supported by previous XRD and EDS analysis. From Fig. 2(d), the big spinel grains of the SPS-sintered composite ceramics were separated by two-phase regions composed of fine grained spinel and YCr0.5 Mn0.5 O3 . These structures show the interdependent grain growth of constituted phases in composites, which may be due to the mass transfer during grain growth and the interdiffusion between these two phases during SPS processing,25 thus leading to a lower experimental resistivity. When Cr3+ and Mn3+ ions are substituted for Al3+ ions in MgAl2 O4 , the electron hopping takes place between Cr3+ and Cr4+ ions on one hand, and between Mn3+ and Mn4+

Table 2 Resistivity, B constant, and activation energy for the conventionally sintered samples. x

ρ25 ( cm)

ρ700 ( cm)

B25/150 (K)

B700/1000 (K)

Ea25−150◦ C (eV)

Ea700−1000◦ C (eV)

0.1 0.4 0.6

6.85 × 104

3.52 98.7 8.39 × 102

3196 3012 2742

6930 7176 9101

0.276 0.260 0.236

0.597 0.619 0.785

1.53 × 106 2.51 × 107

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ions on the other hand, which will contribute to a decrease in resistivity.26 When Mg2+ ion is substituted for Cr3+ and Mn3+ ions in YCr0.5 Mn0.5 O3 , which will cause the formation of Cr4+ and Mn4+ ions as a result of charge compensation, thus decreasing the resistivity.6 Therefore, the combined effect of these two results leads to a decrease in experimental resistivity for composite ceramics. Further works will be concentrated on high temperature aging property to qualify as a thermistor device. 4. Conclusion

Fig. 6. Backscattered SEM image of the xMgAl2 O4 –(1 − x)YCr0.5 Mn0.5 O3 composite ceramics sintered by conventional method: (a) x = 0.4; (b) x = 0.6.

The structure and NTC electrical properties of xMgAl2 O4 –(1-x)YCr0.5 Mn0.5 O3 composite ceramics were investigated. The major phases presented in the SPS-sintered composite ceramics are the cubic spinel MgAl2 O4 phase and the orthorhombic perovskite YCr0.5 Mn0.5 O3 phase. The relative densities for the SPS-sintered composite ceramics are 95.5%, 97.4% and 94.1% of the theoretical density for x = 0.1, 0.4, 0.6, respectively. The brighter regions are mainly the YCr0.5 Mn0.5 O3 and the darker is the MgAl2 O4 . There is ion diffusion between these two phases. The resistivity of the conventionally sintered samples is somewhat lower than those sintered by SPS. The interdiffusion between these two phases leads to a decrease in experimental resistivity in comparison to the calculated resistivity. The electrical conduction in these ceramics may be due to the electron jumps between Cr3+ and Cr4+ ions on one hand, and between Mn3+ and Mn4+ ions on the other hand. The obtained ρ25 , B25–150 , and B700–1000 values of the SPS-sintered composite NTC thermistors are in the range of 1.53 × 106 –9.92 × 109  cm, 3380–5172 K, and 7239–9543 K, respectively. These composite ceramics could be used as potential candidates for NTC thermistors, especially for high temperature thermistor applications. Acknowledgments This study was supported by the National Natural Science Foundation of China (Grant No. 51102276), the High Technology Research and Development Program of Xinjiang of China (Grant No. 201116147), and the National High Technology Research and Development Program of China (Grant No. 2012AA091102). The author, Bo Zhang, would also like to acknowledge her scholarship from the China Scholarship Council. References

Fig. 7. Plots of natural logarithm of the resistivity of individual components, experimental and calculated resistivity versus the reciprocal of the absolute temperature for the xMgAl2 O4 –(1 − x)YCr0.5 Mn0.5 O3 samples sintered by SPS.

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