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Preparation of ultra-fine cobalt–nickel manganite powders and ceramics derived from mixed oxalate
Materials Research Bulletin 43 (2008) 1877–1882 www.elsevier.com/locate/matresbu
Preparation of ultra-fine cobalt–nickel manganite powders and ceramics derived from mixed oxalate Cui-Hong Zheng, Dao-Lai Fang * School of Materials Science and Engineering, Anhui Key Laboratory of Metal Materials and Processing, Anhui University of Technology, Ma’anshan, Anhui 243002, PR China Received 31 May 2007; accepted 28 June 2007 Available online 10 July 2007
Abstract Co0.30Ni0.66Mn2.04O4 negative temperature coefficient ceramics were derived from mixed oxalate Co0.30Ni0.66Mn2.04(C2O4)3nH2O. The mixed oxalate was synthesized by milling a mixture of cobalt acetate, nickel acetate, manganese acetate, and oxalic acid at room temperature. An ultra-fine Co0.30Ni0.66Mn2.04O4 powder was obtained by calcining the mixed oxalate in air at 800 8C for 3 h. The oxide powder compact was sintered at a relatively low temperature of 1100 8C for 5 h, achieving a relative density of 98%. The specific resistivity r25 8C and the thermal constant B25/85 8C were 765.2 V cm and 3604 K, respectively. The resistance drift after aging at 150 8C for 500 h was 1.5%. # 2007 Elsevier Ltd. All rights reserved. Keywords: A. Ceramics; B. Chemical synthesis; C. Thermogravimetric analysis; D. Electrical properties
1. Introduction Mixed transition-metal manganite ceramics with spinel structure are widely used as negative temperature coefficient (NTC) ceramic thermistors for temperature measurement and compensation due to their high sensitivity to temperature change and low cost [1–3]. The electrical properties of the ceramic thermistors are characterized by three basic exploitation parameters: room temperature resistivity r25 8C, thermal constant B25/85 8C, and resistance drift DR/R (%) (usually called aging coefficient). These exploitation parameters are mainly determined by their chemical and phase compositions, and preparation condition. The mixed oxide powder with both a high sintering activity and an accurate stoichiometry is crucial for fabricating the ceramic thermistors of high performance [4]. Conventional solid-state reaction and chemical co-precipitation are the two usual routes adopted to prepare the mixed oxide powder for NTC ceramic thermistors [5–7]. However, the oxide powder prepared by conventional solidstate reaction route is less active in sintering due to the high calcination temperatures used, and the compositional homogeneity of the powder is often insufficient due to the degree of mixing. Chemical co-precipitation route has a difficulty in achieving the desired stoichiometry of the powder due to the difference in solubility and precipitation rate between constituent cations under given pH, temperature, and solvent [8].
* Corresponding author. E-mail address: [email protected] (D.-L. Fang). 0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2007.06.061
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In our previous work [9], a solid-state coordination reaction route was successfully developed to synthesize binary system of mixed nickel–manganese oxalate, allowing the resulting nickel manganite powder with both accurate stoichiometry and high sintering activity. In this investigation, the method was extended to preparation of ternary system of mixed cobalt–nickel–manganese oxalate. Furthermore, we studied the sintering behavior of the derived oxide powders, and the electrical properties of the ceramic samples. 2. Experimental procedure Analytical-grade reagents of cobalt acetate Co(CH3COO)24H2O, nickel acetate Ni(CH3COO)24H2O, manganese acetate Mn(CH3COO)24H2O, and oxalic acid H2C2O42H2O, were used as starting materials. A powder mixture with a molar ratio of Co2+:Ni2+:Mn2+:oxalic acid of 0.30:0.66:2.04:3.3 was ball-milled at room temperature for 10 h. The milled mixture was then dried at 70 8C, and calcined for 3 h at 450 and 800 8C. For preparation of thermistor components, the 800 8C-calcined powder was uniaxially pressed at 60 MPa to form disk-shaped samples with a diameter of 6 mm and a thickness of 3 mm, and then isostatically pressed at 300 MPa. These powder compacts were sintered at 1100 8C for 5 h in air, subsequently cooled to room temperature in furnace. ˚ ) was used to analyze the phase A Philips X’pert Pro X-ray diffractometer with Cu Ka radiation (l = 1.5418 A compositions of the oxalate precursor, the oxide powder and the ceramic sample. Thermogravimetric analysis (TGA) of the oxalate powder was performed on a Shimadzu TGA-50H thermal analyzer. The experiment was performed in an air flow at a heating rate of 5 8C/min from room temperature to 1000 8C. The primary particle size of the 800 8Ccalcined oxide powder and the microstructure of the ceramic sample was analyzed by using a Hitachi X-600 scanning electron microscope (SEM). The chemical compositions of the powder and the ceramic sample were analyzed by inductively coupled plasma emission spectrometry (ICP). Its relative standard deviation is less than 1%. Cylindrical compacts of the 800 8C-calcined powder were made by isostatic pressing at 300 MPa. The length and diameter of the compacts were 10 mm and 6 mm, respectively. The sintering behavior of the compacts was studied using a NETZSCH DIL 402C dilatometer in an air flow at a heating rate of 5 8C/min. The bulk density rbulk of the powder compacts and the ceramic samples was determined by Archimedes method in mercury, and their relative density rrel was calculated from the formula rrel = rbulk/rth, where rth is the theoretical density obtained from XRD data of the oxide powders and ceramics. The disk-shaped ceramic samples were coated on the opposite surfaces with silver paste, and annealed at 850 8C for 20 min, followed by rapid cooling to room temperature. Ag wires were attached to these samples for electrical measurement. The resistances at 25 and 85 8C were measured with an Agilent 34401A digital multimeter, during which the ceramic thermistor samples were immersed in silicon oil. B25/85 8C values were calculated according to the formula B25/85 8C = 1778 ln(R25/R85), where R25 and R85 were the resistances at 25 and 85 8C, respectively. The values of specific resistivity r25 8C and B25/85 8C were the averages of five thermistor samples, and the standard deviation was less than 1%. The resistance drift, namely, aging coefficient, is denoted by the relative variation (R500 h – R0)/ R0 100%, in which R0 is the resistance value measured at 25 8C before annealing and R500 h is the resistance value measured at 25 8C after annealing at 150 8C for 500 h. 3. Results and discussion 3.1. Structure and thermal decomposition behavior of the as-prepared oxalate Fig. 1(a) shows the XRD pattern of the as-prepared cobalt–nickel–manganese oxalate. For comparison, the standard XRD patterns of cobalt oxalate (JCPDS: 25–0251), nickel oxalate (JCPDS: 25–0582), and manganese oxalate (JCPDS: 25–0544) are also given in Fig. 1. Obviously, the XRD pattern in Fig. 1(a) is not the simple superposition of the XRD patterns in Fig. 1(b–d). This gives the evidence that the cobalt–nickel–manganese oxalate synthesized by the solid-state coordination reaction is not a mechanical mixture of cobalt oxalate, nickel oxalate, and manganese oxalate. The XRD pattern of the cobalt–nickel–manganese oxalate is similar to that of nickel oxalate, distinctly different from that of manganese oxalate and cobalt oxalate, suggesting that the cobalt–nickel–manganese oxalate prepared may be a mixed oxalate, whose crystal structure is similar to that of b-form nickel oxalate (JCPDS: 25–0582). The characteristic peaks of the starting material acetates are not observed in the XRD pattern (Fig. 1(a)) of the dried oxalate powder, which is an indication of the completion of the solid-state coordination reaction.
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Fig. 1. XRD patterns of the dried oxalate powder: (a) Co0.30Ni0.66Mn2.04(C2O4)3nH2O, (b) CoC2O4nH2O, (c) NiC2O4nH2O, and (d) MnC2O4nH2O.
Thermogravimetric analysis (TGA) of the cobalt–nickel–manganese oxalate is given in Fig. 2. For the cobalt– nickel–manganese oxalate, two intense and sharp peaks of mass loss occur at 151 and 267 8C, respectively. In general, thermal decomposition behavior of single transition-metal oxalate presents two striking steps of mass loss: one in the lower temperature range corresponding to the dehydration of the oxalate and the other in the higher temperature range corresponding to decomposition of the dehydrated oxalate [10,11]. The onset decomposition temperature of the oxalates is closely related to the electronegativities of the centering metal ions in the coordination compound of the oxalates [12]. As a result, thermal decomposition behavior of a mixture of two single transition-metal oxalates generally exhibits four pronounced steps of mass loss [13,14]. The TGA traces (Fig. 2) of the cobalt–nickel– manganese oxalate present unambiguously only two steps of mass loss, behaving as the decomposition behavior of a single transition-metal oxalate. The TGA result confirms that the cobalt–nickel–manganese oxalate prepared is a single-phase mixed oxalate, rather than a mechanical mixture of cobalt oxalate, nickel oxalate, and manganese oxalate, which is consistent with the result of the XRD analysis (Fig. 1). XRD patterns of the 450 8C- and 800 8C-obtained oxide powders are given in Fig. 3(a and b), respectively. Fig. 3(a) shows that a single-phase oxide of spinel structure is formed at a low temperature of 450 8C. The peak broadening indicates that the crystallites in the powders are very fine and/or that the powder is not well crystallized. Formation of the spinel-structured oxide powder at a temperature as low as 450 8C comfirms that cobalt ions, nickel ions, and manganese ions in the prepared oxalate are homogeneously dispersed, implying the prepared oxalate is not a mechanical mixture of single-metal oxalate. The XRD result (Fig. 3(a)) is well in agreement with that of TGA (Fig. 2). With further elevation of calcination temperature to 800 8C, a well-crystallized spinel-structured oxide powder is achieved, as shown by XRD pattern in Fig. 3(b).
Fig. 2. Thermogravimetric analysis of the dried oxalate powder Co0.30Ni0.66Mn2.04(C2O4)3nH2O.
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Fig. 3. XRD patterns of the Co0.30Ni0.66Mn2.04O4 powder calcined at (a) 450 8C, (b) 800 8C, and (c) XRD pattern of the 1100 8C-sintered ceramic sample.
3.2. Characterization and sintering behavior of the 800 8C-obtained powders The results of ICP analysis of the 800 8C-obtained oxide powder indicate that the molar ratio of cobalt, nickel, and manganese ions in the oxide powder is 0.295:0.667:2.042, which is very close to the expected ratio of 0.30:0.66:2.04. In principle, the solid-state coordination route should cause no deviation in the resulting composition of the oxide powder. In a sense, the stoichiometry of the resulting oxide only depends on the purity of the starting materials. Therefore, the chemical formula of the spinel-structured oxide powder is denoted as Co0.295Ni0.667Mn2.042O4. Fig. 4 shows an SEM image of the 800 8C-calcined oxide powder. It can be observed that the primary particles of the powder are uniform and spherical-shaped with a particle size of 200 nm. Fig. 5 presents the dilatometric traces of the oxide powder compact. The compact starts to shrink at 735 8C and reaches the maximum shrinkage rate at 1054 8C, at which the linear shrinkage of the compact is about 10%. 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
Fig. 4. SEM image of the 800 8C-calcined Co0.30Ni0.66Mn2.04O4 powder.
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Fig. 5. Dilatometric analysis of Co0.30Ni0.66Mn2.04O4 powder compact.
compact [15]. Therefore, a sintering temperature of 1100 8C and duration of 5 h were adopted, achieving the resulting ceramics with a relative density of 98%. The XRD result (Fig. 3(c)) reveals the ceramic sample retains a pure spinel phase. The SEM analysis (Fig. 6) shows that the ceramic sample is well densified and microstructurally homogeneous. The solid-state coordination route allows a mixed oxide powder with a high sintering activity, which makes it possible to obtain well-densified and single-phase ceramics at a lower sintering temperature. In order to reach a relative density of 98%, the required sintering temperature for the oxide powder prepared by the conventional solidstate reaction or the co-precipitation method is at least 100 8C higher [16,17], usually resulting in a pronounced phase decomposition due to the higher sintering temperature adopted. Another advantage of the solid-state coordination route is its convenience in controlling the stoichiometry of the resulting ceramics. ICP analysis reveals that the chemical formula of the ceramics can be denoted as Co0.293Ni0.668Mn2.041O4, which is very close to the chemical formula of Co0.30Ni0.66Mn2.04O4 desired at the beginning.
Fig. 6. SEM image of the fractured surface of Co0.30Ni0.66Mn2.04O4 ceramics.
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3.3. Electrical properties of the NTC ceramics The electrical resistances of the ceramic sample measured at 25 and 85 8C yielded the specific electrical resistivity r25 8C of 765.2 V cm and the thermal constant B25/85 8C of 3604 K, which are close to those reported by Metz [17]. The resistance drift of the ceramic sample after aging at 150 8C for 500 h was measure to be 1.5%, which is remarkably lower than the value 5% given in Metz’s work [17]. This shows that the ceramic sample derived from the mixed oxalate has a higher electrical stability. It may be attributed to the higher density and lower porosity of the ceramic sample we prepared, as shown by SEM image (Fig. 6). Its dense microstructure restrains the adsorption of oxygen into the ceramic sample in the duration of subsequent cooling after sintering or/and annealing for metallization, consequently preventing formation of cationic vacancies in spinel lattice, and allowing the prepared ceramic sample with a higher electrical stability [6,18]. 4. Conclusion A mixed cobalt–nickel–manganese oxalate was synthesized by milling a mixture of cobalt acetate, nickel acetate, manganese acetate, and oxalic acid at room temperature. The mixed oxalate-derived oxide powder has not only an accurate stoichiometry but also a high sintering activity, allowing the resulting ceramic sample with a desired composition of Co0.30Ni0.66Mn2.04O4 and a high density of 98% at a lower sintering temperature of 1100 8C. The obtained ceramic sample exhibits a higher electrical stability, probably due to that its dense microstructure restrains the adsorption of oxygen, consequently, formation of cationic vacancies in spinel lattice. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
H. Okuda, J. Electron. Eng. 32 (344) (1995) 23–26. G. Lavenuta, Sensors 14 (5) (1997) 46–55. R.K. Kamat, G.M. Naik, Sensor Rev. 22 (4) (2002) 334–340. J.G. Fagan, V.R.W. Amarkoon, Am. Ceram. Soc. Bull. 72 (1) (1993) 70–76. C. Chanel, S. Fritsch, C. Drouet, A. Rousset, Mater. Res. Bull. 35 (3) (2000) 431–439. W.A. Groen, C. Metzmacher, P. Huppertz, S. Schuurman, J. Electroceram. 7 (2) (2001) 77–87. D. Houivet, J. Bernard, J.M. Haussonne, J. Eur. Ceram. Soc. 24 (6) (2004) 1237–1241. X. Kuang, G. Carotenuto, L. Nicolais, Adv. Perform. Mater. 4 (1997) 257–274. D.-L. Fang, Z.-B. Wang, P.-H. Yang, W. Liu, C.-S. Chen, J. Am. Ceram. Soc. 89 (1) (2006) 230–235. A.M. Donia, D. Dollimore, Thermochim. Acta 290 (1996) 139–147. C. Drouet, P. Alphonse, A. Rousset, Solid State Ionics 123 (1999) 25–37. M.A. Gabal, A.A. El-Bellihi, S.S. Ata-Allah, Mater. Chem. Phys. 81 (2003) 84–92. C. Laberty, P. Alphonse, J.J. Demai, C. Sarda, A. Rousset, Mater. Res. Bull. 32 (2) (1997) 249–261. C. Laberty, J. Pielaszek, P. Alphonse, A. Rousset, Solid State Ionics 110 (3–4) (1998) 293–302. P. Duran, M. Villegas, F. Capel, P. Recio, C. Moure, J. Eur. Ceram. Soc. 16 (9) (1996) 945–952. K. Park, S.J. Kima, J.G. Kimb, S. Nahmc, J. Eur. Ceram. Soc. 27 (2007) 2009–2016. R. Metz, J. Mater. Sci. 35 (2000) 4705–4711. A. Basu, A.W. Brinkman, R. Schmidt, J. Eur. Ceram. Soc. 24 (2004) 1247–1250.
Preparation of ultra-fine cobalt–nickel manganite powders and ceramics derived from mixed oxalate Cui-Hong Zheng, Dao-Lai Fang * School of Materials Science and Engineering, Anhui Key Laboratory of Metal Materials and Processing, Anhui University of Technology, Ma’anshan, Anhui 243002, PR China Received 31 May 2007; accepted 28 June 2007 Available online 10 July 2007
Abstract Co0.30Ni0.66Mn2.04O4 negative temperature coefficient ceramics were derived from mixed oxalate Co0.30Ni0.66Mn2.04(C2O4)3nH2O. The mixed oxalate was synthesized by milling a mixture of cobalt acetate, nickel acetate, manganese acetate, and oxalic acid at room temperature. An ultra-fine Co0.30Ni0.66Mn2.04O4 powder was obtained by calcining the mixed oxalate in air at 800 8C for 3 h. The oxide powder compact was sintered at a relatively low temperature of 1100 8C for 5 h, achieving a relative density of 98%. The specific resistivity r25 8C and the thermal constant B25/85 8C were 765.2 V cm and 3604 K, respectively. The resistance drift after aging at 150 8C for 500 h was 1.5%. # 2007 Elsevier Ltd. All rights reserved. Keywords: A. Ceramics; B. Chemical synthesis; C. Thermogravimetric analysis; D. Electrical properties
1. Introduction Mixed transition-metal manganite ceramics with spinel structure are widely used as negative temperature coefficient (NTC) ceramic thermistors for temperature measurement and compensation due to their high sensitivity to temperature change and low cost [1–3]. The electrical properties of the ceramic thermistors are characterized by three basic exploitation parameters: room temperature resistivity r25 8C, thermal constant B25/85 8C, and resistance drift DR/R (%) (usually called aging coefficient). These exploitation parameters are mainly determined by their chemical and phase compositions, and preparation condition. The mixed oxide powder with both a high sintering activity and an accurate stoichiometry is crucial for fabricating the ceramic thermistors of high performance [4]. Conventional solid-state reaction and chemical co-precipitation are the two usual routes adopted to prepare the mixed oxide powder for NTC ceramic thermistors [5–7]. However, the oxide powder prepared by conventional solidstate reaction route is less active in sintering due to the high calcination temperatures used, and the compositional homogeneity of the powder is often insufficient due to the degree of mixing. Chemical co-precipitation route has a difficulty in achieving the desired stoichiometry of the powder due to the difference in solubility and precipitation rate between constituent cations under given pH, temperature, and solvent [8].
* Corresponding author. E-mail address: [email protected] (D.-L. Fang). 0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2007.06.061
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In our previous work [9], a solid-state coordination reaction route was successfully developed to synthesize binary system of mixed nickel–manganese oxalate, allowing the resulting nickel manganite powder with both accurate stoichiometry and high sintering activity. In this investigation, the method was extended to preparation of ternary system of mixed cobalt–nickel–manganese oxalate. Furthermore, we studied the sintering behavior of the derived oxide powders, and the electrical properties of the ceramic samples. 2. Experimental procedure Analytical-grade reagents of cobalt acetate Co(CH3COO)24H2O, nickel acetate Ni(CH3COO)24H2O, manganese acetate Mn(CH3COO)24H2O, and oxalic acid H2C2O42H2O, were used as starting materials. A powder mixture with a molar ratio of Co2+:Ni2+:Mn2+:oxalic acid of 0.30:0.66:2.04:3.3 was ball-milled at room temperature for 10 h. The milled mixture was then dried at 70 8C, and calcined for 3 h at 450 and 800 8C. For preparation of thermistor components, the 800 8C-calcined powder was uniaxially pressed at 60 MPa to form disk-shaped samples with a diameter of 6 mm and a thickness of 3 mm, and then isostatically pressed at 300 MPa. These powder compacts were sintered at 1100 8C for 5 h in air, subsequently cooled to room temperature in furnace. ˚ ) was used to analyze the phase A Philips X’pert Pro X-ray diffractometer with Cu Ka radiation (l = 1.5418 A compositions of the oxalate precursor, the oxide powder and the ceramic sample. Thermogravimetric analysis (TGA) of the oxalate powder was performed on a Shimadzu TGA-50H thermal analyzer. The experiment was performed in an air flow at a heating rate of 5 8C/min from room temperature to 1000 8C. The primary particle size of the 800 8Ccalcined oxide powder and the microstructure of the ceramic sample was analyzed by using a Hitachi X-600 scanning electron microscope (SEM). The chemical compositions of the powder and the ceramic sample were analyzed by inductively coupled plasma emission spectrometry (ICP). Its relative standard deviation is less than 1%. Cylindrical compacts of the 800 8C-calcined powder were made by isostatic pressing at 300 MPa. The length and diameter of the compacts were 10 mm and 6 mm, respectively. The sintering behavior of the compacts was studied using a NETZSCH DIL 402C dilatometer in an air flow at a heating rate of 5 8C/min. The bulk density rbulk of the powder compacts and the ceramic samples was determined by Archimedes method in mercury, and their relative density rrel was calculated from the formula rrel = rbulk/rth, where rth is the theoretical density obtained from XRD data of the oxide powders and ceramics. The disk-shaped ceramic samples were coated on the opposite surfaces with silver paste, and annealed at 850 8C for 20 min, followed by rapid cooling to room temperature. Ag wires were attached to these samples for electrical measurement. The resistances at 25 and 85 8C were measured with an Agilent 34401A digital multimeter, during which the ceramic thermistor samples were immersed in silicon oil. B25/85 8C values were calculated according to the formula B25/85 8C = 1778 ln(R25/R85), where R25 and R85 were the resistances at 25 and 85 8C, respectively. The values of specific resistivity r25 8C and B25/85 8C were the averages of five thermistor samples, and the standard deviation was less than 1%. The resistance drift, namely, aging coefficient, is denoted by the relative variation (R500 h – R0)/ R0 100%, in which R0 is the resistance value measured at 25 8C before annealing and R500 h is the resistance value measured at 25 8C after annealing at 150 8C for 500 h. 3. Results and discussion 3.1. Structure and thermal decomposition behavior of the as-prepared oxalate Fig. 1(a) shows the XRD pattern of the as-prepared cobalt–nickel–manganese oxalate. For comparison, the standard XRD patterns of cobalt oxalate (JCPDS: 25–0251), nickel oxalate (JCPDS: 25–0582), and manganese oxalate (JCPDS: 25–0544) are also given in Fig. 1. Obviously, the XRD pattern in Fig. 1(a) is not the simple superposition of the XRD patterns in Fig. 1(b–d). This gives the evidence that the cobalt–nickel–manganese oxalate synthesized by the solid-state coordination reaction is not a mechanical mixture of cobalt oxalate, nickel oxalate, and manganese oxalate. The XRD pattern of the cobalt–nickel–manganese oxalate is similar to that of nickel oxalate, distinctly different from that of manganese oxalate and cobalt oxalate, suggesting that the cobalt–nickel–manganese oxalate prepared may be a mixed oxalate, whose crystal structure is similar to that of b-form nickel oxalate (JCPDS: 25–0582). The characteristic peaks of the starting material acetates are not observed in the XRD pattern (Fig. 1(a)) of the dried oxalate powder, which is an indication of the completion of the solid-state coordination reaction.
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Fig. 1. XRD patterns of the dried oxalate powder: (a) Co0.30Ni0.66Mn2.04(C2O4)3nH2O, (b) CoC2O4nH2O, (c) NiC2O4nH2O, and (d) MnC2O4nH2O.
Thermogravimetric analysis (TGA) of the cobalt–nickel–manganese oxalate is given in Fig. 2. For the cobalt– nickel–manganese oxalate, two intense and sharp peaks of mass loss occur at 151 and 267 8C, respectively. In general, thermal decomposition behavior of single transition-metal oxalate presents two striking steps of mass loss: one in the lower temperature range corresponding to the dehydration of the oxalate and the other in the higher temperature range corresponding to decomposition of the dehydrated oxalate [10,11]. The onset decomposition temperature of the oxalates is closely related to the electronegativities of the centering metal ions in the coordination compound of the oxalates [12]. As a result, thermal decomposition behavior of a mixture of two single transition-metal oxalates generally exhibits four pronounced steps of mass loss [13,14]. The TGA traces (Fig. 2) of the cobalt–nickel– manganese oxalate present unambiguously only two steps of mass loss, behaving as the decomposition behavior of a single transition-metal oxalate. The TGA result confirms that the cobalt–nickel–manganese oxalate prepared is a single-phase mixed oxalate, rather than a mechanical mixture of cobalt oxalate, nickel oxalate, and manganese oxalate, which is consistent with the result of the XRD analysis (Fig. 1). XRD patterns of the 450 8C- and 800 8C-obtained oxide powders are given in Fig. 3(a and b), respectively. Fig. 3(a) shows that a single-phase oxide of spinel structure is formed at a low temperature of 450 8C. The peak broadening indicates that the crystallites in the powders are very fine and/or that the powder is not well crystallized. Formation of the spinel-structured oxide powder at a temperature as low as 450 8C comfirms that cobalt ions, nickel ions, and manganese ions in the prepared oxalate are homogeneously dispersed, implying the prepared oxalate is not a mechanical mixture of single-metal oxalate. The XRD result (Fig. 3(a)) is well in agreement with that of TGA (Fig. 2). With further elevation of calcination temperature to 800 8C, a well-crystallized spinel-structured oxide powder is achieved, as shown by XRD pattern in Fig. 3(b).
Fig. 2. Thermogravimetric analysis of the dried oxalate powder Co0.30Ni0.66Mn2.04(C2O4)3nH2O.
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Fig. 3. XRD patterns of the Co0.30Ni0.66Mn2.04O4 powder calcined at (a) 450 8C, (b) 800 8C, and (c) XRD pattern of the 1100 8C-sintered ceramic sample.
3.2. Characterization and sintering behavior of the 800 8C-obtained powders The results of ICP analysis of the 800 8C-obtained oxide powder indicate that the molar ratio of cobalt, nickel, and manganese ions in the oxide powder is 0.295:0.667:2.042, which is very close to the expected ratio of 0.30:0.66:2.04. In principle, the solid-state coordination route should cause no deviation in the resulting composition of the oxide powder. In a sense, the stoichiometry of the resulting oxide only depends on the purity of the starting materials. Therefore, the chemical formula of the spinel-structured oxide powder is denoted as Co0.295Ni0.667Mn2.042O4. Fig. 4 shows an SEM image of the 800 8C-calcined oxide powder. It can be observed that the primary particles of the powder are uniform and spherical-shaped with a particle size of 200 nm. Fig. 5 presents the dilatometric traces of the oxide powder compact. The compact starts to shrink at 735 8C and reaches the maximum shrinkage rate at 1054 8C, at which the linear shrinkage of the compact is about 10%. 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
Fig. 4. SEM image of the 800 8C-calcined Co0.30Ni0.66Mn2.04O4 powder.
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Fig. 5. Dilatometric analysis of Co0.30Ni0.66Mn2.04O4 powder compact.
compact [15]. Therefore, a sintering temperature of 1100 8C and duration of 5 h were adopted, achieving the resulting ceramics with a relative density of 98%. The XRD result (Fig. 3(c)) reveals the ceramic sample retains a pure spinel phase. The SEM analysis (Fig. 6) shows that the ceramic sample is well densified and microstructurally homogeneous. The solid-state coordination route allows a mixed oxide powder with a high sintering activity, which makes it possible to obtain well-densified and single-phase ceramics at a lower sintering temperature. In order to reach a relative density of 98%, the required sintering temperature for the oxide powder prepared by the conventional solidstate reaction or the co-precipitation method is at least 100 8C higher [16,17], usually resulting in a pronounced phase decomposition due to the higher sintering temperature adopted. Another advantage of the solid-state coordination route is its convenience in controlling the stoichiometry of the resulting ceramics. ICP analysis reveals that the chemical formula of the ceramics can be denoted as Co0.293Ni0.668Mn2.041O4, which is very close to the chemical formula of Co0.30Ni0.66Mn2.04O4 desired at the beginning.
Fig. 6. SEM image of the fractured surface of Co0.30Ni0.66Mn2.04O4 ceramics.
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3.3. Electrical properties of the NTC ceramics The electrical resistances of the ceramic sample measured at 25 and 85 8C yielded the specific electrical resistivity r25 8C of 765.2 V cm and the thermal constant B25/85 8C of 3604 K, which are close to those reported by Metz [17]. The resistance drift of the ceramic sample after aging at 150 8C for 500 h was measure to be 1.5%, which is remarkably lower than the value 5% given in Metz’s work [17]. This shows that the ceramic sample derived from the mixed oxalate has a higher electrical stability. It may be attributed to the higher density and lower porosity of the ceramic sample we prepared, as shown by SEM image (Fig. 6). Its dense microstructure restrains the adsorption of oxygen into the ceramic sample in the duration of subsequent cooling after sintering or/and annealing for metallization, consequently preventing formation of cationic vacancies in spinel lattice, and allowing the prepared ceramic sample with a higher electrical stability [6,18]. 4. Conclusion A mixed cobalt–nickel–manganese oxalate was synthesized by milling a mixture of cobalt acetate, nickel acetate, manganese acetate, and oxalic acid at room temperature. The mixed oxalate-derived oxide powder has not only an accurate stoichiometry but also a high sintering activity, allowing the resulting ceramic sample with a desired composition of Co0.30Ni0.66Mn2.04O4 and a high density of 98% at a lower sintering temperature of 1100 8C. The obtained ceramic sample exhibits a higher electrical stability, probably due to that its dense microstructure restrains the adsorption of oxygen, consequently, formation of cationic vacancies in spinel lattice. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
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