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PTCR characteristic of gelcast BaTiO3 ceramic thermistor
Sensors and Actuators A 88 (2001) 67±70
PTCR characteristic of gelcast BaTiO3 ceramic thermistor Yunxiang Hu*, Dongxiang Zhou, Daoli Zhang, Wenzhong Lu Department of Solid State Electronics, Huazhong University of Science & Technology, Wuhan 430074, PR China Accepted 28 August 2000
Abstract This paper applied gelcasting, a novel forming method, to form BaTiO3-based ceramic thermistor. The resistivity±temperature characteristics and microstructures of gelcast samples from suspensions with different solids loading were investigated in comparison with those of die-pressed ones. It is found that gelcast ceramics exhibit stronger PTCR effect than die-pressed ones at the same sintering systems and that the solids loading in the range of 42±50 vol.% has slight in¯uence on the PTCR effect of gelcast ceramics. The two phenomena were explained. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Gelcasting; BaTiO3 ceramic thermistor; PTCR effect; Microstructure
1. Introduction Gelcasting is a novel method for fabricating near-netshape ceramic green bodies [1,2]. Fundamental researches of this process carried out by Omatete et al. [3±5] and Young et al. [6] showed its general feasibility and advantages in comparison with other forming processes. Microstructure investigation of gelcast alumina by Wasche et al. [7] revealed that gelcast samples are of homogeneous microstructures and high densities in comparison with die-pressed samples. Although gelcasting process is generic and has its advantages, no literature reports its application in functional ceramics except in structural ceramics such as alumina and silicon nitride. In this paper, we applied gelcasting process to form BaTiO3-based ceramic thermistor, a kind of functional ceramics, and investigated its positive temperature coef®cient of resistivity (PTCR) and microstructure. Also die-pressed samples with the same starting materials were prepared in order to observe the effect of the green body forming process on the PTCR characteristics of the ®nal products. 2. Experimental 2.1. Sample preparation The starting BaTiO3-based PTCR ceramic powder has a nominal composition of Ba0.94Ca0.06Ti1:01 O3 0:25 mol% * Corresponding author. Fax: 86-27-8754-2994. E-mail address: [email protected] (Y. Hu).
Y2 O3 2:50 mol%SiO2 0:08 mol%Mn(NO3)2, consisting of Mn(NO3)2 and the calcined mixture of BaCO3, CaCO3, TiO2, SiO2, and Y2O3, and has an average particle size of 1.0 mm. Green pellets with 15 mm in diameter and 2.5 mm in thickness were prepared via both gelcasting and traditional die-pressing route. As regards the die-pressing route, the ceramic powder was granulated with PVA as binder before die-pressed into pellets. The gelcasting process ¯owchart employed is shown in Fig. 1 [2,3,6]. Organic monomers, including acrylamide (AM) and N,N0 -methylenebisacrylamide (MBAM), and dispersant ammonium polymethacrylate (PMAA, a self-made 10% aqueous solution with an average molecular weight of 15,000) [8,9] were dissolved and mixed in distilled water to give premix solution. The content of AM and MBAM in the premix solution is 12.5% and the weight ratio of AM and MBAM is 95.6:4.4. The content of PMAA is 0.45% of the weight of ceramic powder. Then suspensions with different solids loading (42, 46, and 50 vol.%) were made by ball milling ceramic powder together with the premix solution. After deairing under vacuum, initiator (NH4)2S2O8 and catalyst N,N,N0 ,N0 -tetramethylethylenediamine were added and the suspensions were cast into the mould where the gelation took place in a few minutes. Then the gelated, wet green bodies were demoulded and dried carefully under room ambient. The green pellets by the two kinds of processes were sintered into PTCR ceramics under the same sintering systems and conditions. Two sintering systems were used: system A, sintered at 13008C for 10 min, cooled down to 11508C at a rate of 3008C/h, and soaked at 11508C for
0924-4247/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 0 ) 0 0 4 9 6 - 9
68
Y. Hu et al. / Sensors and Actuators A 88 (2001) 67±70 Table 1 Electrical properties of samples Sample Process
Fig. 1. Gelcasting process flowchart.
20 min; system B, sintered at 13308C for 10 min, cooled down to 11508C at a rate of 3008C/h, and soaked at 11508C for 40 min. Some of the sintered bodies were rubbed with zinc±silver electrode paste on both surfaces and heat-treated to form ohmic contact. 2.2. Property measurements The resistivity±temperature characteristic measurements were carried out on samples with electrode using a computer aided system in the temperature range of room temperature to 2508C. The densities of ceramic bodies were measured by Archimedes method. The densities of green bodies were obtained by measuring the masses and the dimensions of the samples.
rmin (O cm)
rmax/rmin
a (1/8C)
A1 A2 A3 A4
42 vol.%, Gelcasting, 13008C 46 vol.%, Gelcasting, 13008C 50 vol.%, Gelcasting, 13008C Die-pressing, 13008C
77.6 77.7 74.9 62.9
3.5 2.5 2.2 1.3
105 105 105 105
18.71% 18.66% 18.62% 17.64%
B1 B2 B3 B4
42 vol.%, Gelcasting, 13308C 46 vol.%, Gelcasting, 13308C 50 vol.%, Gelcasting, 13308C Die-pressing, 13308C
185.0 168.1 159.6 154.3
5.2 3.9 2.8 2.3
104 104 104 104
17.84% 17.26% 16.73% 16.62%
curves of gelcast and die-pressed samples sintered at 13008C are shown in Fig. 2. It is evident that, at the same sintering system (A or B), samples from both gelcasting and diepressing nearly have the same minimum resistivity (rmin) and all gelcast samples have a high ratio of the maximum resistivity to the minimum (rmax/rmin) and a high temperature coef®cient of resistivity, a, in comparison with the die-pressed one. Apparently, the gelcast ceramics exhibit stronger PTCR effect than the die-pressed ones at the same sintering systems. In order to explain this phenomenon, we investigated the microstructures and densities of the ceramic bodies via the two forming routes. The SEM micrographs of samples sintered at 13008C are shown in Fig. 3. It is clearly seen that gelcast samples have more homogeneous microstructures than die-pressed one. Within gelcast ceramic bodies the grains in size are homogeneous and the size of the examined maximum micro-pore is the same magnitude of order as that of grains, while the die-pressed sample contains grains with different sizes and very large, elongated pores. The microstructure homogeneity of gelcast ceramic bodies may be attributed to the good dispersion and the in-situ gelation of the suspension. Ceramic PTCR effect is the synthetic behavior of all grain boundaries [10,11], so the homogeneous microstructure of gelcast samples may result
2.3. Observation of microstructures The microstructures of fractured surfaces of ceramic bodies were examined by means of a scanning electron microscope (SEM, SX-40, Japan). 3. Results and discussion 3.1. PTCR characteristics of gelcast ceramics The measurement results of resistivity±temperature characteristics of PTCR ceramics prepared from different processes are shown in Table 1. The resistivity±temperature
Fig. 2. Resistivity±temperature curves of gelcast and die-pressed ceramics sintered at 13008C.
Y. Hu et al. / Sensors and Actuators A 88 (2001) 67±70
69
Fig. 3. SEM micrographs of fractured surfaces of samples sintered at 13008C: (a) 50 vol.%, gelcasting; (b) 46 vol.%, gelcasting; (c) 42 vol.%, gelcasting; (d) die-pressing.
in strong PTCR effect. On the other hand, it is worth mentioning that the relative densities of gelcast ceramic samples are slightly higher than that of the die-pressed one (Fig. 4). We know according to Kuwabara [12] that, for PTCR ceramics, when density is high (above about 80% theoretical density), the magnitude of PTCR effect decreases as density increases because of the incomplete grain boundary of ceramics with higher density. Therefore, the slightly higher densities of gelcast samples may reduce the PTCR effect. Of both the homogeneous microstructure and slightly
Fig. 4. Ceramic body density, green body density wet green body drying shrinkage, and green body sintering shrinkage for both die-pressing and gelcasting routines.
incomplete grain boundary for gelcast samples in comparison with die-pressed ones, maybe the homogeneous microstructure plays a dominant role in the PTCR effect, and therefore, gelcast samples show stronger PTCR effect than die-pressed ones. 3.2. The effect of solids loading on PTCR characteristic From Table 1and Fig. 2, it is seen that the solids loading of the suspension for gelcasting slightly affects PTCR characteristic of gelcast ceramics sintered at the same systems. As the solids loading increases greatly from 42 to 50 vol.%, rmax/rmin and a decrease slightly, and rmin nearly remains the same. This result is in good agreement with that from SEM examination and density measurement. The SEM micrographs (Fig. 3a±c) show that the samples made from suspensions with different solids loading nearly have similar microstructures, and Fig. 4 shows the ceramic body density very slightly increases as the solids loading increases. As mentioned above, the very slight increase of ceramic body density may result in the very slight incomplete grain boundary, and therefore, results in the very slight decrease of the magnitude of ceramic PTCR effect. This insensitivity of PTCR effect to solids loading of suspension is helpful to the manufacture of ceramic thermistor via gelcasting because it permits us to use not too dense suspension.
70
Y. Hu et al. / Sensors and Actuators A 88 (2001) 67±70
In order to understand the fact that the suspensions with greatly increased solids loading, via gelcasting, lead to ceramic bodies with nearly similar microstructures and very slightly increased densities, we investigated the wet green body drying shrinkage in diameter, Dd/d, green body density, green body sintering shrinkage in diameter, and ceramic body density versus solids loading, respectively (Fig. 4). It is found that, as solids loading increases from 42 to 50 vol.%, the wet green body drying shrinkage decreases greatly and the resulting green body density increases slightly, and the following green body sintering shrinkage decreases further and the resulting ceramic body density increases very slightly. That is to say, a high drying shrinkage and a high sintering shrinkage make the sample from a low solids loading suspension obtain a density approaching up to the density of the sample from a high solids loading suspension. 4. Conclusions BaTiO3 ceramic thermistors were prepared via gelcasting route. Due to homogeneous microstructure, gelcast ceramics exhibits stronger PTCR effect than die-pressed one. For gelcast ceramics, the solids loading of suspension in the range of 42±45 vol.% has only slight in¯uence on the PTCR effect because the resulting ceramic bodies nearly have similar microstructures.
References [1] M.A. Janney, et al., Method for molding ceramic powders, US Patent 4,894,194 (1990). [2] M.A. Janney, et al., Method for molding ceramic powders using a water-based gel casting, US Patent 5,028,362 (1991). [3] O.O. Omatete, et al., Gelcasting Ð a new ceramic forming process, Am. Ceram. Soc. Bull. 70 (10) (1991) 1641±1649. [4] O.O. Omatete, et al., Gelcast reaction bonded silicon nitride composites, Ceram. Eng. Sci. Proc. 12 (7/8) (1991) 1257±1264. [5] O.O. Omatete, et al., Gelcast zirconia±alumina composites, Ceram. Eng. Sci. Proc. 12 (9/10) (1991) 2084±2094. [6] A.C. Young, et al., Gelcasting of alumina, J. Am. Ceram. Soc. 74 (3) (1991) 612±618. [7] R. Wasche, et al., The microstructure of gelcast alumina, cfi/Ber. DKG 72 (1/2) (1995) 24±27. [8] J.-H. Jean, et al., Dispersion of aqueous barium titanate suspensions with ammonium salt of poly(methacrylic acid), J. Am. Ceram. Soc. 81 (6) (1998) 1589±1599. [9] J.-H. Jean, et al., Effect of solids loading, pH, and polyelectrolyte addition on the stabilization of concentrated aqueous BaTiO3 suspensions, J. Am. Ceram. Soc. 83 (2) (2000) 277±280. [10] W. Heywang, Barium titanate as a PTC thermistor, Solid State Electr. 3 (1961) 51±58. [11] G.H. Jonker, Some aspect of semiconducting barium titanate, Solid State Electr. 7 (1964) 895±903. [12] M. Kuwabara, Effect of microstructure on the PTCR effect in semiconducting barium titanate ceramics, J. Am. Ceram. Soc. 64 (11) (1981) 639±644.
PTCR characteristic of gelcast BaTiO3 ceramic thermistor Yunxiang Hu*, Dongxiang Zhou, Daoli Zhang, Wenzhong Lu Department of Solid State Electronics, Huazhong University of Science & Technology, Wuhan 430074, PR China Accepted 28 August 2000
Abstract This paper applied gelcasting, a novel forming method, to form BaTiO3-based ceramic thermistor. The resistivity±temperature characteristics and microstructures of gelcast samples from suspensions with different solids loading were investigated in comparison with those of die-pressed ones. It is found that gelcast ceramics exhibit stronger PTCR effect than die-pressed ones at the same sintering systems and that the solids loading in the range of 42±50 vol.% has slight in¯uence on the PTCR effect of gelcast ceramics. The two phenomena were explained. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Gelcasting; BaTiO3 ceramic thermistor; PTCR effect; Microstructure
1. Introduction Gelcasting is a novel method for fabricating near-netshape ceramic green bodies [1,2]. Fundamental researches of this process carried out by Omatete et al. [3±5] and Young et al. [6] showed its general feasibility and advantages in comparison with other forming processes. Microstructure investigation of gelcast alumina by Wasche et al. [7] revealed that gelcast samples are of homogeneous microstructures and high densities in comparison with die-pressed samples. Although gelcasting process is generic and has its advantages, no literature reports its application in functional ceramics except in structural ceramics such as alumina and silicon nitride. In this paper, we applied gelcasting process to form BaTiO3-based ceramic thermistor, a kind of functional ceramics, and investigated its positive temperature coef®cient of resistivity (PTCR) and microstructure. Also die-pressed samples with the same starting materials were prepared in order to observe the effect of the green body forming process on the PTCR characteristics of the ®nal products. 2. Experimental 2.1. Sample preparation The starting BaTiO3-based PTCR ceramic powder has a nominal composition of Ba0.94Ca0.06Ti1:01 O3 0:25 mol% * Corresponding author. Fax: 86-27-8754-2994. E-mail address: [email protected] (Y. Hu).
Y2 O3 2:50 mol%SiO2 0:08 mol%Mn(NO3)2, consisting of Mn(NO3)2 and the calcined mixture of BaCO3, CaCO3, TiO2, SiO2, and Y2O3, and has an average particle size of 1.0 mm. Green pellets with 15 mm in diameter and 2.5 mm in thickness were prepared via both gelcasting and traditional die-pressing route. As regards the die-pressing route, the ceramic powder was granulated with PVA as binder before die-pressed into pellets. The gelcasting process ¯owchart employed is shown in Fig. 1 [2,3,6]. Organic monomers, including acrylamide (AM) and N,N0 -methylenebisacrylamide (MBAM), and dispersant ammonium polymethacrylate (PMAA, a self-made 10% aqueous solution with an average molecular weight of 15,000) [8,9] were dissolved and mixed in distilled water to give premix solution. The content of AM and MBAM in the premix solution is 12.5% and the weight ratio of AM and MBAM is 95.6:4.4. The content of PMAA is 0.45% of the weight of ceramic powder. Then suspensions with different solids loading (42, 46, and 50 vol.%) were made by ball milling ceramic powder together with the premix solution. After deairing under vacuum, initiator (NH4)2S2O8 and catalyst N,N,N0 ,N0 -tetramethylethylenediamine were added and the suspensions were cast into the mould where the gelation took place in a few minutes. Then the gelated, wet green bodies were demoulded and dried carefully under room ambient. The green pellets by the two kinds of processes were sintered into PTCR ceramics under the same sintering systems and conditions. Two sintering systems were used: system A, sintered at 13008C for 10 min, cooled down to 11508C at a rate of 3008C/h, and soaked at 11508C for
0924-4247/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 0 ) 0 0 4 9 6 - 9
68
Y. Hu et al. / Sensors and Actuators A 88 (2001) 67±70 Table 1 Electrical properties of samples Sample Process
Fig. 1. Gelcasting process flowchart.
20 min; system B, sintered at 13308C for 10 min, cooled down to 11508C at a rate of 3008C/h, and soaked at 11508C for 40 min. Some of the sintered bodies were rubbed with zinc±silver electrode paste on both surfaces and heat-treated to form ohmic contact. 2.2. Property measurements The resistivity±temperature characteristic measurements were carried out on samples with electrode using a computer aided system in the temperature range of room temperature to 2508C. The densities of ceramic bodies were measured by Archimedes method. The densities of green bodies were obtained by measuring the masses and the dimensions of the samples.
rmin (O cm)
rmax/rmin
a (1/8C)
A1 A2 A3 A4
42 vol.%, Gelcasting, 13008C 46 vol.%, Gelcasting, 13008C 50 vol.%, Gelcasting, 13008C Die-pressing, 13008C
77.6 77.7 74.9 62.9
3.5 2.5 2.2 1.3
105 105 105 105
18.71% 18.66% 18.62% 17.64%
B1 B2 B3 B4
42 vol.%, Gelcasting, 13308C 46 vol.%, Gelcasting, 13308C 50 vol.%, Gelcasting, 13308C Die-pressing, 13308C
185.0 168.1 159.6 154.3
5.2 3.9 2.8 2.3
104 104 104 104
17.84% 17.26% 16.73% 16.62%
curves of gelcast and die-pressed samples sintered at 13008C are shown in Fig. 2. It is evident that, at the same sintering system (A or B), samples from both gelcasting and diepressing nearly have the same minimum resistivity (rmin) and all gelcast samples have a high ratio of the maximum resistivity to the minimum (rmax/rmin) and a high temperature coef®cient of resistivity, a, in comparison with the die-pressed one. Apparently, the gelcast ceramics exhibit stronger PTCR effect than the die-pressed ones at the same sintering systems. In order to explain this phenomenon, we investigated the microstructures and densities of the ceramic bodies via the two forming routes. The SEM micrographs of samples sintered at 13008C are shown in Fig. 3. It is clearly seen that gelcast samples have more homogeneous microstructures than die-pressed one. Within gelcast ceramic bodies the grains in size are homogeneous and the size of the examined maximum micro-pore is the same magnitude of order as that of grains, while the die-pressed sample contains grains with different sizes and very large, elongated pores. The microstructure homogeneity of gelcast ceramic bodies may be attributed to the good dispersion and the in-situ gelation of the suspension. Ceramic PTCR effect is the synthetic behavior of all grain boundaries [10,11], so the homogeneous microstructure of gelcast samples may result
2.3. Observation of microstructures The microstructures of fractured surfaces of ceramic bodies were examined by means of a scanning electron microscope (SEM, SX-40, Japan). 3. Results and discussion 3.1. PTCR characteristics of gelcast ceramics The measurement results of resistivity±temperature characteristics of PTCR ceramics prepared from different processes are shown in Table 1. The resistivity±temperature
Fig. 2. Resistivity±temperature curves of gelcast and die-pressed ceramics sintered at 13008C.
Y. Hu et al. / Sensors and Actuators A 88 (2001) 67±70
69
Fig. 3. SEM micrographs of fractured surfaces of samples sintered at 13008C: (a) 50 vol.%, gelcasting; (b) 46 vol.%, gelcasting; (c) 42 vol.%, gelcasting; (d) die-pressing.
in strong PTCR effect. On the other hand, it is worth mentioning that the relative densities of gelcast ceramic samples are slightly higher than that of the die-pressed one (Fig. 4). We know according to Kuwabara [12] that, for PTCR ceramics, when density is high (above about 80% theoretical density), the magnitude of PTCR effect decreases as density increases because of the incomplete grain boundary of ceramics with higher density. Therefore, the slightly higher densities of gelcast samples may reduce the PTCR effect. Of both the homogeneous microstructure and slightly
Fig. 4. Ceramic body density, green body density wet green body drying shrinkage, and green body sintering shrinkage for both die-pressing and gelcasting routines.
incomplete grain boundary for gelcast samples in comparison with die-pressed ones, maybe the homogeneous microstructure plays a dominant role in the PTCR effect, and therefore, gelcast samples show stronger PTCR effect than die-pressed ones. 3.2. The effect of solids loading on PTCR characteristic From Table 1and Fig. 2, it is seen that the solids loading of the suspension for gelcasting slightly affects PTCR characteristic of gelcast ceramics sintered at the same systems. As the solids loading increases greatly from 42 to 50 vol.%, rmax/rmin and a decrease slightly, and rmin nearly remains the same. This result is in good agreement with that from SEM examination and density measurement. The SEM micrographs (Fig. 3a±c) show that the samples made from suspensions with different solids loading nearly have similar microstructures, and Fig. 4 shows the ceramic body density very slightly increases as the solids loading increases. As mentioned above, the very slight increase of ceramic body density may result in the very slight incomplete grain boundary, and therefore, results in the very slight decrease of the magnitude of ceramic PTCR effect. This insensitivity of PTCR effect to solids loading of suspension is helpful to the manufacture of ceramic thermistor via gelcasting because it permits us to use not too dense suspension.
70
Y. Hu et al. / Sensors and Actuators A 88 (2001) 67±70
In order to understand the fact that the suspensions with greatly increased solids loading, via gelcasting, lead to ceramic bodies with nearly similar microstructures and very slightly increased densities, we investigated the wet green body drying shrinkage in diameter, Dd/d, green body density, green body sintering shrinkage in diameter, and ceramic body density versus solids loading, respectively (Fig. 4). It is found that, as solids loading increases from 42 to 50 vol.%, the wet green body drying shrinkage decreases greatly and the resulting green body density increases slightly, and the following green body sintering shrinkage decreases further and the resulting ceramic body density increases very slightly. That is to say, a high drying shrinkage and a high sintering shrinkage make the sample from a low solids loading suspension obtain a density approaching up to the density of the sample from a high solids loading suspension. 4. Conclusions BaTiO3 ceramic thermistors were prepared via gelcasting route. Due to homogeneous microstructure, gelcast ceramics exhibits stronger PTCR effect than die-pressed one. For gelcast ceramics, the solids loading of suspension in the range of 42±45 vol.% has only slight in¯uence on the PTCR effect because the resulting ceramic bodies nearly have similar microstructures.
References [1] M.A. Janney, et al., Method for molding ceramic powders, US Patent 4,894,194 (1990). [2] M.A. Janney, et al., Method for molding ceramic powders using a water-based gel casting, US Patent 5,028,362 (1991). [3] O.O. Omatete, et al., Gelcasting Ð a new ceramic forming process, Am. Ceram. Soc. Bull. 70 (10) (1991) 1641±1649. [4] O.O. Omatete, et al., Gelcast reaction bonded silicon nitride composites, Ceram. Eng. Sci. Proc. 12 (7/8) (1991) 1257±1264. [5] O.O. Omatete, et al., Gelcast zirconia±alumina composites, Ceram. Eng. Sci. Proc. 12 (9/10) (1991) 2084±2094. [6] A.C. Young, et al., Gelcasting of alumina, J. Am. Ceram. Soc. 74 (3) (1991) 612±618. [7] R. Wasche, et al., The microstructure of gelcast alumina, cfi/Ber. DKG 72 (1/2) (1995) 24±27. [8] J.-H. Jean, et al., Dispersion of aqueous barium titanate suspensions with ammonium salt of poly(methacrylic acid), J. Am. Ceram. Soc. 81 (6) (1998) 1589±1599. [9] J.-H. Jean, et al., Effect of solids loading, pH, and polyelectrolyte addition on the stabilization of concentrated aqueous BaTiO3 suspensions, J. Am. Ceram. Soc. 83 (2) (2000) 277±280. [10] W. Heywang, Barium titanate as a PTC thermistor, Solid State Electr. 3 (1961) 51±58. [11] G.H. Jonker, Some aspect of semiconducting barium titanate, Solid State Electr. 7 (1964) 895±903. [12] M. Kuwabara, Effect of microstructure on the PTCR effect in semiconducting barium titanate ceramics, J. Am. Ceram. Soc. 64 (11) (1981) 639±644.