Preparation and electrical properties of copper–nickel manganite ceramic derived from mixed oxalate

Sensors and Actuators A 135 (2007) 472–475

Preparation and electrical properties of copper–nickel manganite ceramic derived from mixed oxalate Jun-Feng Gao, Dao-Lai Fang, Zhong-Bing Wang, Ping-Hua Yang ∗ , Chu-Sheng Chen Laboratory of Advanced Functional Materials and Devices, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, PR China Received 5 April 2006; received in revised form 12 September 2006; accepted 14 September 2006 Available online 17 October 2006

Abstract Cu0.3 Ni0.66 Mn2.04 O4 negative temperature coefficient (NTC) ceramic was prepared using mixed oxalate-derived oxide powder. The mixed oxalate was synthesized by milling a mixture of copper acetate, nickel acetate, manganese acetate and oxalic acid at room temperature. The spinel-structured oxide powder was obtained by calcining the mixed oxalate in air at 800 ◦ C for 2 h. The oxide powder compact was sintered at a relatively low temperature of 1100 ◦ C for 2.5 h, attaining a relative density of ∼98%. The sintered ceramic had a specific resistivity ρ25 ◦ C of 63.3  cm and the thermal constant B25/50 of 2740 K, and exhibited much reduced shift in electrical resistivity of ∼5% after annealing at 150 ◦ C for 500 h in air. The difference in the electrical property of the as-prepared ceramic is attributed to the fine-grained microstructure and the lower sintering temperature used in the present study. © 2006 Elsevier B.V. All rights reserved. Keywords: Solid-state coordination reaction; Mixed oxalate; Sintering activity; NTC thermistors; Electrical resistivity; Aging

1. Introduction Mixed transition-metal manganite semi-conducting ceramics with spinel structure are used for negative temperature coefficient (NTC) ceramic thermistors [1,2]. Among the NTC ceramics, copper–nickel manganite system possesses a relatively low electrical resistivity, thus it is particularly suitable for applications in suppression of inrush current in electronic devices [3,4]. Low-resistivity composition of copper–nickel manganites can also be used as infrared sensing bolometer or sputtering element form [5]. The electrical properties of the Cu-containing manganite ceramic are largely determined by its chemical composition, especially its Cu content [6]. Also, the electrical properties of the ceramic depend strongly on the sintering temperature [7]. Therefore, it is necessary to prepare the oxide powders with accurate stoichiometry and good sintering activity. The copper–nickel manganite oxides are generally prepared by thermal decomposition in air of mixed oxalates, which often

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Corresponding author. Tel.: +86 551 3602940; fax: +86 551 3601592. E-mail addresses: [email protected], [email protected] (P.-H. Yang). 0924-4247/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2006.09.005

are synthesized by the co-precipitation method [8]. However, the problem with this wet chemical method lies in its difficulty to control the chemical composition of precursors, due to the differences in solubility and precipitation rate between constituent cations under given pH, temperature and solvent [9]. It has been reported recently that the mixed oxalates with desired composition can be prepared by a solid-state coordination reaction [10]. In that study, binary oxalates of Ni and Mn are prepared, and the derived mixed oxide powders have accurate stoichiometry, narrow particle size distribution and high sintering activity. In the present study, this method is extended to the preparation of ternary system of Cu–Ni–Mn oxides. The sintering behavior of the derived oxide powder and the electrical properties of the resulting ceramic are reported. 2. Experimental procedure Analytical grade copper acetate Cu(CH3 COO)2 ·H2 O, nickel acetate Ni(CH3 COO)2 ·4H2 O, manganese acetate Mn(CH3 COO)2 ·4H2 O and oxalic acid H2 C2 O4 ·2H2 O were used as starting materials. The starting materials were ground into powders, respectively, and the powder mixture with a molar ratio of Cu2+ :Ni2+ :Mn2+ :oxalic acid of 0.3:0.66:2.04:3.3

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was ball-milled in a polyethylene bottle at room temperature for 5 h using zirconia ball as grinding medium. The obtained cream-like mixture was dried at 70 ◦ C and calcined in air at 800 ◦ C for 2 h. The prepared oxide powder was ball-milled, dried, blended with binder. Powder compacts were prepared by isostatic pressing at 300 MPa, and sintered in air at 1100 ◦ C for 2.5 h, and furnace-cooled. The phase composition of oxalate powders, calcined oxide powders and the resulting ceramics were analyzed with Philip X’pert Pro X-ray diffractometer using Cu K␣ radia˚ The microstructure of the oxide power and tion (λ = 1.5418 A). ceramic samples were analyzed using Field emission scanning electron microscope (JSM-6700F). The thermogravimetric analysis (TGA) of the oxalate powders was performed in air current with SHIMADZU TGA-50H thermal analyzer. The dilatometric analysis of the powder compact was carried on NETZSCH DIL 402C dilatometer using a heating rate of 10 ◦ C/min in airflow. The densities of the sintered ceramic were determined by Archimedes method in mercury. To measure the electrical resistance, the electrodes were mounted on the two opposite surfaces of disk-shaped ceramic by the classic method of ‘serigraphy’ metallization, including coating Pt paste on the surfaces of ceramic disk, and annealing at 850 ◦ C for 15 min, followed by rapid cooling to room temperature. The ceramic thermistors were obtained by soldering Ag wires to the Pt electrode. The resistances of the thermistor samples at 25 and 50 ◦ C were measured with an Agilent 34401A digital multimeter. According to the Arrhenius formula ρ = ρ0 exp[B/T] [6], the B25/50 constant can be calculated by the following equation: B25/50 = (ln(R25 /R50 ))/(1/T25 − 1/T50 ) = 3853.89 ln(R25 /R50 ), in which R25 and R50 are the resistances measured at 25 and 50 ◦ C, respectively. The aging coefficient is characterized by the relative variation of resistance R/R (%) = [(R − R0 )/R0 ] × 100, in which R0 is the resistance value measured at 25 ◦ C before annealing; R is the resistance value measured at 25 ◦ C after annealing at 150 ◦ C for 500 h. The thermal constant B25/50 and specific resistivity ρ25 ◦ C are the average of six thermistor samples.

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Fig. 1. XRD patterns of the four different oxalates: (a) Cu0.3 Ni0.66 Mn2.04 (C2 O4 )3 ·nH2 O; (b) CuC2 O4 ·nH2 O; (c) NiC2 O4 ·nH2 O; (d) MnC2 O4 ·nH2 O.

been reported that mixed Cu–Ni–Mn oxalates cannot be formed unless the nickel content (expressed in the derived spinel oxide) is greater than ∼0.1, and that the crystal structure of the mixed oxalate transforms from ␣ to ␤ phase with increasing nickel content [8,11]. The characteristic peaks of the starting material acetates are not observed in the XRD pattern of the dried powder, which is an indication of the completion of the formation reaction of the mixed oxalate. Fig. 2 shows the thermogravimetric traces of the as-prepared Cu–Ni–Mn oxalate powder. It can be seen that the thermal decomposition in air of the oxalate powder presents two intense and sharp peaks of mass loss, one at ∼154 ◦ C corresponding to the dehydration of the oxalate, and the other at ∼288 ◦ C, actually due to the decomposition of anhydrous oxalate. It has been reported that the behavior of the thermal decomposition of the mixed oxalate is much different from that of the mechanical mixture of oxalates [8,10,11]. The thermal decomposition of the mixed oxalate normally presents two intense and sharp peaks of mass loss, while that of the mechanical mixture of oxalates exhibits more than three peaks of mass loss. Combination of

3. Results and discussion 3.1. Structure and thermal decomposition behavior of oxalates Fig. 1a shows that the XRD pattern of the as-prepared Cu–Ni–Mn oxalate. For comparison, the XRD patterns of single copper oxalate, nickel oxalate and manganese oxalate prepared in the same way are also given in Fig. 1. Obviously, the XRD pattern in Fig. 1a is not the simple superposition of the XRD patterns in Fig. 1b–d. This gives the evidence that the Cu–Ni–Mn oxalate synthesized by the solid-state coordination reaction is not a mechanical mixture of nickel oxalate, manganese oxalate and copper oxalate. The XRD pattern of the Cu–Ni–Mn oxalate is similar to that of nickel oxalate, distinctly different from that of manganese oxalate and copper oxalate; this suggests that the Cu–Ni–Mn oxalate is a mixed oxalate, whose crystal structure is similar to that of ␤-form nickel oxalate (JCPDS: 25-0582). It has

Fig. 2. Thermogravimetric analysis (TGA) and its first derivative of as-prepared Cu0.3 Ni0.66 Mn2.04 (C2 O4 )3 ·nH2 O in air current at a heating rate of 10 ◦ C/min.

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Fig. 3. XRD patterns of (a) Cu0.3 Ni0.66 Mn2.04 O4 ceramic.

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oxide

powder

and

(b)

as-sintered

XRD and thermogravimetric analysis provides strong evidence that the Cu–Ni–Mn oxalate is a single-phase mixed oxalate. 3.2. Characterization and sintering behavior of oxalate-derived oxide powders The oxide powder was derived by calcining the Cu–Ni–Mn oxalate at 800 ◦ C for 2 h. Examination of the XRD pattern (Fig. 3a) reveals that the oxide powder is a single-phase spinel oxide without the presence of ␣-Mn2 O3 and ilmenite NiMnO3 . The formation of the spinel phase at a relatively low temperature of 800 ◦ C is apparently due to that the homogeneous mixture at the molecular level of the constituent cations in the oxalate. The SEM observation shows that the as-derived oxide powder consists of fine particles of size ∼300 nm (Fig. 4). Fig. 5 presents the dilatometric traces of the oxide powder compact. The compact starts to shrink at ∼733 ◦ C and reaches the maximum shrinkage rate at ∼1005 ◦ C at which the linear

Fig. 4. SEM picture of the Cu0.3 Ni0.66 Mn2.04 O4 powders obtained by calcining the mixed oxalate precursors for 2 h at 800 ◦ C.

Fig. 5. Dilatometric analysis of the 800 ◦ C-calcined oxide powder compact in air at a heating rate of 10 ◦ C/min.

shrinkage of the compact is about 8% (the presence of the only one maximum in the shrinkage rate curve confirms the single mode of sintering behavior and a narrow pore size distribution in the power compact [12]). Therefore, a sintering temperature of 1100 ◦ C and duration of 2.5 h was used, and the as-sintered attained a relative density of ∼98%. Backscattered electron image (BEI) of the sintered ceramic is given in Fig. 6. The BEI analysis shows that the ceramic is well densified and microstructurally homogeneous with a grain size of ∼3 ␮m. Since BEI is sensitive to the nature of elements, it reveals that copper, nickel and manganese elements are distributed homogeneously in the ceramic. The XRD pattern of the ceramic (Fig. 3b) reveals the ceramic sample as a single-phase spinel phase. To reach a similar relative density, the sintering temperature required for a powder prepared by conventional solid-state reaction is 100 ◦ C higher [4], and 80–160 ◦ C higher for a powder derived by the co-precipitation method [11,13]. In comparison with other preparation methods, the oxide powder derived by solid-state coordination reaction has a higher sintering activity. In addition, it is worthwhile to note that the resulting ceramic

Fig. 6. BEI picture of a fractured surface of the ceramics sintered for 2.5 h at 1100 ◦ C.

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possesses an accurate stoichiometry, because of no loss of metal ions in the course of preparation. 3.3. Electrical properties of the NTC ceramics The electrical resistances of the ceramic sample were measured at 25 and 50 ◦ C yielding the averaged specific electrical resistivity ρ25 ◦ C of 63.3  cm and the thermal constant B25/50 of 2740 K; these values are higher than those reported by other researchers [11,14]. The shift in electrical resistivity upon aging at 150 ◦ C for 500 h was only 5.4%, much lower than the value ∼15% reported by Metz [15]. The difference in electrical properties is likely due to the fine-grained microstructure [16] and lower sintering temperature used. It is known that Cu+ and Cu2+ both can exist in solid oxides at elevated temperatures, and reducing the temperature favors the oxidation of Cu+ to Cu2+ accompanying the incorporation of oxygen into the lattice [17]. In the present work, due to the use of a relatively low sintering temperature the concentration of Cu2+ ion in the as-obtained ceramic is increased at the expense of Cu+ . The higher Cu2+ concentration in the as-prepared ceramic results in a lower Mn4+ concentration and thus a smaller concentration product of Mn4+ and Mn3+ ; consequently, the electrical conductivity of the sample is decreased, for according to the small polaron conduction model the electrical conductivity is proportional to the concentration product [4,15]. Moreover, the aging of the ceramic, which is triggered by the oxidation of Cu+ to Cu2+ , is suppressed due to the lower Cu+ and higher Cu2+ concentration in the as-prepared ceramic. 4. Conclusion Cu0.3 Ni0.66 Mn2.04 O4 ceramic with relative density of ∼98% can be prepared using mixed oxalate-derived oxide powder. The mixed oxalate can be synthesized by milling a mixture of copper acetate, nickel acetate, manganese acetate and oxalic acid at room temperature. The as-derived oxide powder possesses accurate stoichiometry and allows the ceramic to sinter at a lower temperature of ∼1100 ◦ C. And the as-prepared ceramic has a higher electrical resistivity and improved aging behavior, which probably can be attributed to the lower concentration of Cu+ in the ceramic due to the reduced sintering temperature. References [1] H. Okuda, NTC thermistors offer accuracy, reliability, and satisfy tougher requirements, J. Electron. Eng. 32 (344) (1995) 23–26. [2] R.K. Kamat, G.M. Naik, Thermistors—in search of new applications, manufacturers cultivate advanced NTC techniques, Sensor Rev. 22 (4) (2002) 334–340. [3] J. Plewa, M. Brunner, H. Altenburg, O. Shpotyuk, M. Vakiv, Chemicaltechnological approach to the selection of ceramic materials with predetermined thermistor properties, Key Eng. Mater. 206–213 (2001) 1497–1500. [4] E. Elbadraoui, J.L. Baudour, F. Bouree, B. Gillot, S. Fritsch, A. Rousset, Cation distribution and mechanism of electrical conduction in nickel–copper manganite spinels, Solid State Ionics 93 (1997) 219–225. [5] B. Gillot, M. Kharroubi, R. Metz, A. Rousset, Thermal stability, crystallographic and electrical properties in undoped and Ba-doped Cu–Ni manganite spinels, Solid State Ionics 48 (1991) 93–99.

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Biographies Jun-Feng Gao received his BE degree in metallurgy engineering from Northeastern University, China, in 2003. Currently, he is a graduate student in the Department of Materials Science and Engineering, University of Science and Technology of China (USTC). His research interest focuses on the NTC ceramic thermistors. Dao-Lai Fang graduated in physics from Anhui Normal University in 1995, and received his PhD degree in materials science from USTC in 2005. Currently, he is a Lecturer in the Department of Materials Science and Engineering at Anhui Technical University, China. Zhong-Bing Wang graduated in applied chemistry from Anhui University in 1998, and received his PhD degree in materials science from USTC in 2005. He is a Lecturer in the Department of Chemical Engineering at Hefei University of Technology, China. Ping-Hua Yang graduated in inorganic chemistry in 1976 from USTC, and since then has been working at the same university. She is a Senior Research Scientist in the Department of Materials Science and Engineering at USTC. Chu-Sheng Chen received his BSc and MSc degree in inorganic chemistry from USTC in 1983 and 1986, respectively, and PhD degree in materials science from University of Twente in The Netherlands in 1994. Since 1995, he has been working in the Department of Material Science and Engineering at USTC. Currently, he is Professor and Executive Dean of School of Chemistry and Materials Science. His field of work includes transport of ions and electrons in solid oxides, ceramic membranes and processes in energy systems, electrical ceramics and sensors.