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Electrical properties of lead-free thick film NTC thermistors based on perovskite-type BaCoIIxCoIII2xBi1 − 3xO3
Materials Letters 65 (2011) 836–839
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Electrical properties of lead-free thick film NTC thermistors based on perovskite-type BaCoIIxCoIII2xBi1 − 3xO3 C.L. Yuan a,b,⁎, X.Y. Liu a,b, C.R. Zhou b, J.W. Xu b, B. Li b a b
College of Materials Science and Engineering, Central South University, Changsha 410083, PR China School of Material Science and Engineering, Guilin University of Electronic Technology, Guilin 541004, PR China
a r t i c l e
i n f o
Article history: Received 4 October 2010 Accepted 8 December 2010 Available online 15 December 2010 Keywords: Lead free Thermistors Thick films BaCoIIxCoIII2xBi1 − 3xO3 Electrical properties
a b s t r a c t Lead-free thick film negative temperature coefficient (NTC) thermistors based on perovskite-type BaCoIIxCoIII2xBi1 − 3xO3 (x ≤ 0.1) were prepared by mature screen-printing technology. The microstructures of the thick films sintered at 720 °C were examined by X-ray diffraction and scanning electron microscopy. The electrical properties were analyzed by measuring the resistance-temperature characteristics. For the BaBiO3 thick films, the room-temperature resistivity is 0.22 MΩ cm, while the room-temperature resistivity is sharply decreased to about 3 Ω cm by replacing of Bi with a small amount of Co. For compositions 0.02 ≤ x ≤ 0.1, the values of room-temperature resistivity (ρ23), thermistor constant (B25/85) and activation energy are in the range of 1.995–2.975 Ω cm, 1140–1234 K and 0.102–0.111 eV, respectively. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Negative temperature coefficient (NTC) thick-film resistors are widely used in various electronic applications pertaining to temperature monitoring, control and compensation. Previously, the researches on thick-film NTC thermistors were predominantly focused on spinel or spinel-like structure solid solutions [1–4]. However, it is very difficult to obtain low room-temperature resistivity (below 1000 Ω cm) for spinel-based thick film NTC thermistors. In the case of low-resistance thick film thermistors, it is more appropriate to use perovskite materials as target films because of their low value of ceramic resistivity [5,6]. The only shortcoming of perovskite materials is the high sintering temperatures which is essential for the fabrication of ceramic thick films. Thick film commonly consists of functional material, lead borosilicate glass frits and organic vehicle [7]. For thick-film technology, a glass phase is often mixed with the target film material to keep the adhesion with the alumina substrate or increase the conductivity of thick film. The glass frits are mainly based on high percentage of lead, cadmium besides glass forming agents such as boron, silicon and aluminum. The lead and cadmium are toxic and remain stable over time. Hence, they are hazardous for human being and also to the environment. Recently, the environmental protection and lead-free products are the issues more and more concerned thanks to the strict RoHS (Restriction of Hazardous Substances) ⁎ Corresponding author. College of Materials Science and Engineering, Central South University, Changsha 410083, PR China. Tel.: +86 773 229 1434; fax: +86 773 229 5903. E-mail address: [email protected] (C.L. Yuan). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.12.013
guidelines. Some researchers have initiated eliminating the hazardous elements from the thick film thermistors [8–10]. However, to our knowledge, the lead-free thick film thermistors are barely available as on date in the electronics. Taking account of the importance of the mentioned above, we initiated the research on lead-free thick film NTC thermistors with low room-temperature resistivity and sintering temperature. In the present work, low-resistance, low B-value and lead-free thick film NTC thermistors based on the BaCoIIxCoIII2xBi1 − 3xO3 materials were prepared by the mature screen-printing technology. The electrical properties and microstructures were also evaluated. The novel thermistor material can be used to decrease the room-temperature resistivity and sintering temperature, playing a similar role as the PbO (conventional glass frit) and the RuO2 (conductive phase) in preparation process of conventional NTC thick films. 2. Experimental procures BaCoIIxCoIII2xBi1 − 3xO3 powders with x = 0.0–0.1 were prepared by solid-state reaction of stoichiometric mixtures of reagent grade BaCO3, Co3O4 and Bi2O3 (purity N 99.9%) with submicronic particle size. The mixtures were homogenized in a ball mill using ethanol, and then dried in oven. The dried mixtures were calcined in high-alumina crucibles at 700 °C for 4 h. The powders obtained were then manually ground again, used as target material for thick-film paste formation. Thick-film paste was prepared by mixing perovskite-type BaCoIIx CoIII2xBi1 − 3xO3 (70 wt.%) and organic vehicle (30 wt.%) in agate mortar for 60 min (the sample numbers were shown in Table 1). Organic vehicle was a solution of ethyl cellulose, lecithin and 2-(2butoxy ethoxy-ethyl) acetate. The resultant thick-film thermistor
C.L. Yuan et al. / Materials Letters 65 (2011) 836–839
837
Table 1 Resistivity at 23 °C, thermistor constant and activation energy of the thick-film thermistors having different compositions. Samples
Compositions (x)
ρ23 (Ω cm)
B25/85 (K)
Ea (eV)
K0 K1 K2 K3 K4
x = 0.00 x = 0.02 x = 0.04 x = 0.08 x = 0.10
0.22 × 106 2.005 1.995 2.395 2.975
3914 1140 1190 1234 1184
0.337 0.102 0.107 0.111 0.106
pastes were screen-printed onto a 98% alumina substrate with 11 × 5 × 0.08 mm size using a stainless screen of 250 mesh size, then settled down and dried under infrared lamp for 10–15 min. This printing process was repeated ten times and the thickness of each printing was 5–6 μm. The printed patterns were fired at 720 °C with a dwell time of 60 min in a tube furnace. For electrical contacts, Ag thick-film conductor paste was applied at the two ends by screen printing and fired at 550 °C for 40 min before carrying out resistancetemperature measurements. As the key parameters of thermistor films, thermistor constant (B) and activation energy (Ea) were calculated by the following equations [11]: B25/85 = [In(ρ25/ρ85)]/ [(1/T25) − (1/T85)] (1); Ea = B·kB (2), where ρ25 and ρ85 are the resistivity measured at temperatures of 25 °C (T25) and 85 °C (T85), respectively. kB is Boltzmann constant. The fired thick films were analyzed by X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The electrical resistances of thermistor films were measured using a digital multimeter (Fluke 45) from 23 to 250 °C in a step of 5 °C.
3. Results and discussions XRD patterns of polycrystalline thick-film samples of BaCoIIxCoIII2x Bi1 − 3xO3 with x = 0.0 – 0.1 were shown in Fig. 1a and b. The figures illustrate the structural evolution of BaCoIIxCoIII2xBi1 − 3xO3 with the Co content. The spectra were indexed on a perovskite-type unit cell with space group I2/m, in a similar manner to that of monoclinic BaBiO3 [12,13]. The residual peaks show that some amounts of Al2O3 from the alumina substrates and unreacted Bi2O3 are present. In addition, the crystalline structure is affected by the substituted Co2+ and Co3+. In the BaCo II x Co III 2x Bi 1 − 3x O 3 samples, the highest diffraction peak corresponding to the BaBiO3 phase slightly shifts towards higher angles with increasing Co content (as shown in Fig. 1b), which indicates that the lattice parameters change after the addition of Co element. It is well known that BaBiO3 shows a distorted perovskite lattice with monoclinic unit cell (I2/m) characterized by two different B sites, occupied respectively by Bi3+ and Bi5+ [14]. From the XRD patterns, it is believed that the substituted Co is present on Bi sites in the monoclinic perovskite lattice to form substitution solid solutions. The Bi–O band length in BaBiO3, to a certain extent, will show shorter band distance after partial substitution of Co for Bi as a result of the difference of ionic radii between Co and Bi (The ionic radii of Co2+ and Co3+ are 0.072 and 0.063 nm, while those of Bi3+ and Bi5+ are 0.074 and 0.096 nm, respectively). Consequently, the reduced band length leads to the shift of higher angles for (200). The surface morphologies of thick-film thermistors with various amount of Co were shown in Fig. 2a–e. The thick films printed onto the alumina substrate adhere very well to the substrate. For compositions x = 0.02–0.1, the distribution of thick-film grains is clearly visible in the SEM images and the agglomerated particles of the thick films is deduced within the range of 1.0–2.0 μm. Some white particles are also seen in Fig. 2b and d. The whites are Al2O3 from the alumina substrate during the incising process of thick films. For the BaBiO3 thick film, some pores or holes are observed on the surface of the films, which will deteriorate the electrical properties.
Fig. 1. XRD patterns of BaCoIIxCoIII2xBi1 − 3xO3 (x = 0.0 − 0.1) thick films in the 2-theta range (a) from 20 to 80°, and (b) from 27 to 31°.
The electrical resistance versus temperature characteristics was performed on thick films within the temperature range from 20 to 250 °C. The relation between the film resistivity and temperatures was plotted in Fig. 3a. For all the samples, a monotonic decrease of the resistivity is observed with the increase of temperature. Fig. 3b shows the plots (Arrhenius plot) of the logarithms of the electrical resistivity (log ρ) against the reciprocal of the absolute temperature (1000/T) for the BaCoIIxCoIII2xBi1 − 3xO3 (x = 0.00, 0.02, 0.04, 0.08, and 0.10) thickfilm NTC thermistors sintered at 720 °C. From Fig. 3b, it can be seen that all the thick film NTC thermistors operate steadily with the straight-line relationships between these parameters over a wide range of temperatures, indicating an excellent NTC thermistor characteristics. The values of Ea obtained from the linear portion of Fig. 3b and Eq. (2), were listed in Table 1, together with the B25/85 constant and the
838
C.L. Yuan et al. / Materials Letters 65 (2011) 836–839
Fig. 2. SEM micrographs of thick-film samples: (a) K1, (b) K2, (c) K3, (d) K4 and (e) K0.
resistivity at 23 °C (ρ23). The table shows that the values of ρ23, B25/85 constant and Ea of the thick-film NTC thermistors with different compositions (x = 0.02–0.1), which are the most important characteristics of technical interest for NTC thermistors are in the range of 1.995– 2.975 Ω cm, 1140–1234 K, and 0.102–0.111 eV, respectively. The roomresistivity of BaBiO3 NTC thick-film thermistors is about 0.22 MΩ cm and it decreases to about 3 Ω cm as the bismuth ions replaced by a small amount of Co2+ and Co3+ ions (i.e., BaCoII0.02CoIII0.04Bi0.94O3 thick film). Thus, it is believed that the cooperative substitution of Co2+ and Co3+ leads to the decrease of film resistivity. For the composition x = 0.04, the resistivity at 23 °C decreases to the lowest value. With the further
increase in amount of Co3O4 (i.e. x N 0.04), the film resistivity increases slightly, as shown in Fig. 3 and Table 1. In BaBiO3 compound, the semiconductor-semimetal transition occurs with partial substitutions of Co2+ and Co3+ for Bi3+ or Bi5+ site, which is similar to the behavior of Pb substitution for Bi site [15,16]. Furthermore, as shown in the XRD analysis, the cooperative substitutions of Co2+ and Co3+ for Bi3+ or Bi5+ in BaBiO3 reduce the band length of Bi–O. This promotes the electronic transport of Bi–O chains and thus a drastic decrease in the resistivity [17]. However, the existence of excess Co2+ and Co3+ ions (i.e. x N 0.04) gives a chance to the decrease in the ratio of pair Bi5+/Bi3+ ions on octahedral sites,
Fig. 3. (a) Relation between the electrical resistivity and temperature for the BaCoIIxCoIII2xBi1 − 3xO3 (x = 0.0 − 0.1) thick-film NTC thermistors. (b) Arrhenius plots of resistivity for the BaCoIIxCoIII2xBi1 − 3xO3 (x = 0.0 − 0.1) thick-film NTC thermistors.
C.L. Yuan et al. / Materials Letters 65 (2011) 836–839
which are responsible for the semiconducting behavior of BaBiO3 [14], resulting in a slight increase in the thick-film resistivity. 4. Conclusions The thick-film NTC thermistors with compositions of BaCoIIxCoIII2x Bi1 − 3xO3 were fabricated by screen-printing process. The films based on the BaCoIIxCoIII2xBi1 − 3xO3 paste was deposited onto alumina substrate and sintered at 720 °C. The substituted Co2+ and Co3+ in BaBiO3 form the BaCoIIxCoIII2xBi1 − 3xO3 solid solution with monoclinic perovskite structure. The values of ρ23, B25/85 constant and activation energy of the thickfilm NTC thermistors are 1.995–2.975 Ω cm, 1140–1234 K, and 0.102– 0.111 eV, respectively. The Co2+ and Co3+ substitutions lead to a significant decrease in ρ23 and B25/85 as compared with BaBiO3 thick film. It is concluded that the BaCoIIxCoIII2xBi1 − 3xO3 thick films can be useful for low-resistance applications as lead-free thick film NTC thermistors over a wide temperature range.
839
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
Kanade SA, Puri V. Mater Res Bull 2008;43:819–24. Kanade SA, Puri V. Mater Lett 2006;60:1428–31. Park K. J Mater Sci Mater Electron 2003;14:81–7. Schmidt R, Brinkman AW. J Eur Ceram Soc 2005;25:3027–31. Gutierrez D, Peña O, Duran P, Moure C. J Eur Ceram Soc 2002;22:567–72. Gutierrez D, Peña O, Duran P, Moure C. J Eur Ceram Soc 2002;22:1257–62. Prudenziati M, Middelhoek S. Handbook of sensors and actuators. Thick Film Sensors. Amsterdam: Elsevier; 1994. Rane S, Prudenziati M, Morten B. Mater Lett 2007;61:595–9. Kshirsagar A, Rane S, Mulik U, Amalnerkar D. Mater Chem Phys 2007;101:492–8. Kanade SA, Puri V. J Alloy Compd 2009;475:352–5. Macklen ED. Thermistors. Ayr, Scotland: Electrochemical Publications Ltd; 1979. Thornton G, Jacobson AJ. Acta Crystallogr B 1978;34:351–4. Cox DE, Sleight AW. Acta Crystallogr B 1979;35:1–10. Cox DE, Sleight AW. Solid State Commun 1976;19:969–73. Hashimoto T, Kawazoe H, Shimamura H. Phys C 1994;223:131–9. Pei S, Jorgensen JD, Dabrowski B, Hinks DG, Richards DR, Mitchell AW, et al. Phys Rev B 1990;41:4126–41. Kambea S, Shime I, Ohshima S, Okuyama K, Sakamoto K. Solid State Ionics 1998;108:307–13.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Electrical properties of lead-free thick film NTC thermistors based on perovskite-type BaCoIIxCoIII2xBi1 − 3xO3 C.L. Yuan a,b,⁎, X.Y. Liu a,b, C.R. Zhou b, J.W. Xu b, B. Li b a b
College of Materials Science and Engineering, Central South University, Changsha 410083, PR China School of Material Science and Engineering, Guilin University of Electronic Technology, Guilin 541004, PR China
a r t i c l e
i n f o
Article history: Received 4 October 2010 Accepted 8 December 2010 Available online 15 December 2010 Keywords: Lead free Thermistors Thick films BaCoIIxCoIII2xBi1 − 3xO3 Electrical properties
a b s t r a c t Lead-free thick film negative temperature coefficient (NTC) thermistors based on perovskite-type BaCoIIxCoIII2xBi1 − 3xO3 (x ≤ 0.1) were prepared by mature screen-printing technology. The microstructures of the thick films sintered at 720 °C were examined by X-ray diffraction and scanning electron microscopy. The electrical properties were analyzed by measuring the resistance-temperature characteristics. For the BaBiO3 thick films, the room-temperature resistivity is 0.22 MΩ cm, while the room-temperature resistivity is sharply decreased to about 3 Ω cm by replacing of Bi with a small amount of Co. For compositions 0.02 ≤ x ≤ 0.1, the values of room-temperature resistivity (ρ23), thermistor constant (B25/85) and activation energy are in the range of 1.995–2.975 Ω cm, 1140–1234 K and 0.102–0.111 eV, respectively. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Negative temperature coefficient (NTC) thick-film resistors are widely used in various electronic applications pertaining to temperature monitoring, control and compensation. Previously, the researches on thick-film NTC thermistors were predominantly focused on spinel or spinel-like structure solid solutions [1–4]. However, it is very difficult to obtain low room-temperature resistivity (below 1000 Ω cm) for spinel-based thick film NTC thermistors. In the case of low-resistance thick film thermistors, it is more appropriate to use perovskite materials as target films because of their low value of ceramic resistivity [5,6]. The only shortcoming of perovskite materials is the high sintering temperatures which is essential for the fabrication of ceramic thick films. Thick film commonly consists of functional material, lead borosilicate glass frits and organic vehicle [7]. For thick-film technology, a glass phase is often mixed with the target film material to keep the adhesion with the alumina substrate or increase the conductivity of thick film. The glass frits are mainly based on high percentage of lead, cadmium besides glass forming agents such as boron, silicon and aluminum. The lead and cadmium are toxic and remain stable over time. Hence, they are hazardous for human being and also to the environment. Recently, the environmental protection and lead-free products are the issues more and more concerned thanks to the strict RoHS (Restriction of Hazardous Substances) ⁎ Corresponding author. College of Materials Science and Engineering, Central South University, Changsha 410083, PR China. Tel.: +86 773 229 1434; fax: +86 773 229 5903. E-mail address: [email protected] (C.L. Yuan). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.12.013
guidelines. Some researchers have initiated eliminating the hazardous elements from the thick film thermistors [8–10]. However, to our knowledge, the lead-free thick film thermistors are barely available as on date in the electronics. Taking account of the importance of the mentioned above, we initiated the research on lead-free thick film NTC thermistors with low room-temperature resistivity and sintering temperature. In the present work, low-resistance, low B-value and lead-free thick film NTC thermistors based on the BaCoIIxCoIII2xBi1 − 3xO3 materials were prepared by the mature screen-printing technology. The electrical properties and microstructures were also evaluated. The novel thermistor material can be used to decrease the room-temperature resistivity and sintering temperature, playing a similar role as the PbO (conventional glass frit) and the RuO2 (conductive phase) in preparation process of conventional NTC thick films. 2. Experimental procures BaCoIIxCoIII2xBi1 − 3xO3 powders with x = 0.0–0.1 were prepared by solid-state reaction of stoichiometric mixtures of reagent grade BaCO3, Co3O4 and Bi2O3 (purity N 99.9%) with submicronic particle size. The mixtures were homogenized in a ball mill using ethanol, and then dried in oven. The dried mixtures were calcined in high-alumina crucibles at 700 °C for 4 h. The powders obtained were then manually ground again, used as target material for thick-film paste formation. Thick-film paste was prepared by mixing perovskite-type BaCoIIx CoIII2xBi1 − 3xO3 (70 wt.%) and organic vehicle (30 wt.%) in agate mortar for 60 min (the sample numbers were shown in Table 1). Organic vehicle was a solution of ethyl cellulose, lecithin and 2-(2butoxy ethoxy-ethyl) acetate. The resultant thick-film thermistor
C.L. Yuan et al. / Materials Letters 65 (2011) 836–839
837
Table 1 Resistivity at 23 °C, thermistor constant and activation energy of the thick-film thermistors having different compositions. Samples
Compositions (x)
ρ23 (Ω cm)
B25/85 (K)
Ea (eV)
K0 K1 K2 K3 K4
x = 0.00 x = 0.02 x = 0.04 x = 0.08 x = 0.10
0.22 × 106 2.005 1.995 2.395 2.975
3914 1140 1190 1234 1184
0.337 0.102 0.107 0.111 0.106
pastes were screen-printed onto a 98% alumina substrate with 11 × 5 × 0.08 mm size using a stainless screen of 250 mesh size, then settled down and dried under infrared lamp for 10–15 min. This printing process was repeated ten times and the thickness of each printing was 5–6 μm. The printed patterns were fired at 720 °C with a dwell time of 60 min in a tube furnace. For electrical contacts, Ag thick-film conductor paste was applied at the two ends by screen printing and fired at 550 °C for 40 min before carrying out resistancetemperature measurements. As the key parameters of thermistor films, thermistor constant (B) and activation energy (Ea) were calculated by the following equations [11]: B25/85 = [In(ρ25/ρ85)]/ [(1/T25) − (1/T85)] (1); Ea = B·kB (2), where ρ25 and ρ85 are the resistivity measured at temperatures of 25 °C (T25) and 85 °C (T85), respectively. kB is Boltzmann constant. The fired thick films were analyzed by X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The electrical resistances of thermistor films were measured using a digital multimeter (Fluke 45) from 23 to 250 °C in a step of 5 °C.
3. Results and discussions XRD patterns of polycrystalline thick-film samples of BaCoIIxCoIII2x Bi1 − 3xO3 with x = 0.0 – 0.1 were shown in Fig. 1a and b. The figures illustrate the structural evolution of BaCoIIxCoIII2xBi1 − 3xO3 with the Co content. The spectra were indexed on a perovskite-type unit cell with space group I2/m, in a similar manner to that of monoclinic BaBiO3 [12,13]. The residual peaks show that some amounts of Al2O3 from the alumina substrates and unreacted Bi2O3 are present. In addition, the crystalline structure is affected by the substituted Co2+ and Co3+. In the BaCo II x Co III 2x Bi 1 − 3x O 3 samples, the highest diffraction peak corresponding to the BaBiO3 phase slightly shifts towards higher angles with increasing Co content (as shown in Fig. 1b), which indicates that the lattice parameters change after the addition of Co element. It is well known that BaBiO3 shows a distorted perovskite lattice with monoclinic unit cell (I2/m) characterized by two different B sites, occupied respectively by Bi3+ and Bi5+ [14]. From the XRD patterns, it is believed that the substituted Co is present on Bi sites in the monoclinic perovskite lattice to form substitution solid solutions. The Bi–O band length in BaBiO3, to a certain extent, will show shorter band distance after partial substitution of Co for Bi as a result of the difference of ionic radii between Co and Bi (The ionic radii of Co2+ and Co3+ are 0.072 and 0.063 nm, while those of Bi3+ and Bi5+ are 0.074 and 0.096 nm, respectively). Consequently, the reduced band length leads to the shift of higher angles for (200). The surface morphologies of thick-film thermistors with various amount of Co were shown in Fig. 2a–e. The thick films printed onto the alumina substrate adhere very well to the substrate. For compositions x = 0.02–0.1, the distribution of thick-film grains is clearly visible in the SEM images and the agglomerated particles of the thick films is deduced within the range of 1.0–2.0 μm. Some white particles are also seen in Fig. 2b and d. The whites are Al2O3 from the alumina substrate during the incising process of thick films. For the BaBiO3 thick film, some pores or holes are observed on the surface of the films, which will deteriorate the electrical properties.
Fig. 1. XRD patterns of BaCoIIxCoIII2xBi1 − 3xO3 (x = 0.0 − 0.1) thick films in the 2-theta range (a) from 20 to 80°, and (b) from 27 to 31°.
The electrical resistance versus temperature characteristics was performed on thick films within the temperature range from 20 to 250 °C. The relation between the film resistivity and temperatures was plotted in Fig. 3a. For all the samples, a monotonic decrease of the resistivity is observed with the increase of temperature. Fig. 3b shows the plots (Arrhenius plot) of the logarithms of the electrical resistivity (log ρ) against the reciprocal of the absolute temperature (1000/T) for the BaCoIIxCoIII2xBi1 − 3xO3 (x = 0.00, 0.02, 0.04, 0.08, and 0.10) thickfilm NTC thermistors sintered at 720 °C. From Fig. 3b, it can be seen that all the thick film NTC thermistors operate steadily with the straight-line relationships between these parameters over a wide range of temperatures, indicating an excellent NTC thermistor characteristics. The values of Ea obtained from the linear portion of Fig. 3b and Eq. (2), were listed in Table 1, together with the B25/85 constant and the
838
C.L. Yuan et al. / Materials Letters 65 (2011) 836–839
Fig. 2. SEM micrographs of thick-film samples: (a) K1, (b) K2, (c) K3, (d) K4 and (e) K0.
resistivity at 23 °C (ρ23). The table shows that the values of ρ23, B25/85 constant and Ea of the thick-film NTC thermistors with different compositions (x = 0.02–0.1), which are the most important characteristics of technical interest for NTC thermistors are in the range of 1.995– 2.975 Ω cm, 1140–1234 K, and 0.102–0.111 eV, respectively. The roomresistivity of BaBiO3 NTC thick-film thermistors is about 0.22 MΩ cm and it decreases to about 3 Ω cm as the bismuth ions replaced by a small amount of Co2+ and Co3+ ions (i.e., BaCoII0.02CoIII0.04Bi0.94O3 thick film). Thus, it is believed that the cooperative substitution of Co2+ and Co3+ leads to the decrease of film resistivity. For the composition x = 0.04, the resistivity at 23 °C decreases to the lowest value. With the further
increase in amount of Co3O4 (i.e. x N 0.04), the film resistivity increases slightly, as shown in Fig. 3 and Table 1. In BaBiO3 compound, the semiconductor-semimetal transition occurs with partial substitutions of Co2+ and Co3+ for Bi3+ or Bi5+ site, which is similar to the behavior of Pb substitution for Bi site [15,16]. Furthermore, as shown in the XRD analysis, the cooperative substitutions of Co2+ and Co3+ for Bi3+ or Bi5+ in BaBiO3 reduce the band length of Bi–O. This promotes the electronic transport of Bi–O chains and thus a drastic decrease in the resistivity [17]. However, the existence of excess Co2+ and Co3+ ions (i.e. x N 0.04) gives a chance to the decrease in the ratio of pair Bi5+/Bi3+ ions on octahedral sites,
Fig. 3. (a) Relation between the electrical resistivity and temperature for the BaCoIIxCoIII2xBi1 − 3xO3 (x = 0.0 − 0.1) thick-film NTC thermistors. (b) Arrhenius plots of resistivity for the BaCoIIxCoIII2xBi1 − 3xO3 (x = 0.0 − 0.1) thick-film NTC thermistors.
C.L. Yuan et al. / Materials Letters 65 (2011) 836–839
which are responsible for the semiconducting behavior of BaBiO3 [14], resulting in a slight increase in the thick-film resistivity. 4. Conclusions The thick-film NTC thermistors with compositions of BaCoIIxCoIII2x Bi1 − 3xO3 were fabricated by screen-printing process. The films based on the BaCoIIxCoIII2xBi1 − 3xO3 paste was deposited onto alumina substrate and sintered at 720 °C. The substituted Co2+ and Co3+ in BaBiO3 form the BaCoIIxCoIII2xBi1 − 3xO3 solid solution with monoclinic perovskite structure. The values of ρ23, B25/85 constant and activation energy of the thickfilm NTC thermistors are 1.995–2.975 Ω cm, 1140–1234 K, and 0.102– 0.111 eV, respectively. The Co2+ and Co3+ substitutions lead to a significant decrease in ρ23 and B25/85 as compared with BaBiO3 thick film. It is concluded that the BaCoIIxCoIII2xBi1 − 3xO3 thick films can be useful for low-resistance applications as lead-free thick film NTC thermistors over a wide temperature range.
839
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
Kanade SA, Puri V. Mater Res Bull 2008;43:819–24. Kanade SA, Puri V. Mater Lett 2006;60:1428–31. Park K. J Mater Sci Mater Electron 2003;14:81–7. Schmidt R, Brinkman AW. J Eur Ceram Soc 2005;25:3027–31. Gutierrez D, Peña O, Duran P, Moure C. J Eur Ceram Soc 2002;22:567–72. Gutierrez D, Peña O, Duran P, Moure C. J Eur Ceram Soc 2002;22:1257–62. Prudenziati M, Middelhoek S. Handbook of sensors and actuators. Thick Film Sensors. Amsterdam: Elsevier; 1994. Rane S, Prudenziati M, Morten B. Mater Lett 2007;61:595–9. Kshirsagar A, Rane S, Mulik U, Amalnerkar D. Mater Chem Phys 2007;101:492–8. Kanade SA, Puri V. J Alloy Compd 2009;475:352–5. Macklen ED. Thermistors. Ayr, Scotland: Electrochemical Publications Ltd; 1979. Thornton G, Jacobson AJ. Acta Crystallogr B 1978;34:351–4. Cox DE, Sleight AW. Acta Crystallogr B 1979;35:1–10. Cox DE, Sleight AW. Solid State Commun 1976;19:969–73. Hashimoto T, Kawazoe H, Shimamura H. Phys C 1994;223:131–9. Pei S, Jorgensen JD, Dabrowski B, Hinks DG, Richards DR, Mitchell AW, et al. Phys Rev B 1990;41:4126–41. Kambea S, Shime I, Ohshima S, Okuyama K, Sakamoto K. Solid State Ionics 1998;108:307–13.