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LaNiO3 conducting particle dispersed NiMn2O4 nanocomposite NTC thermistor thick films by aerosol deposition
Journal of Alloys and Compounds 534 (2012) 70–73
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
LaNiO3 conducting particle dispersed NiMn2O4 nanocomposite NTC thermistor thick films by aerosol deposition Ju-Eun Kang, Jungho Ryu ⇑, Guifang Han, Jong-Jin Choi, Woon-Ha Yoon, Byung-Dong Hahn, Jong-Woo Kim, Cheol-Woo Ahn, Joon Hwan Choi, Dong-Soo Park Functional Ceramics Research Group, Korea Institute of Materials Science (KIMS), 66 Sangnam-Dong, Changwon, Gyeongnam 641-831, Republic of Korea
a r t i c l e
i n f o
Article history: Received 6 December 2011 Received in revised form 9 April 2012 Accepted 13 April 2012 Available online 26 April 2012 Keywords: Negative temperature coefficient (NTC) thermistor Thick film Aerosol deposition (AD) Conducting oxide LaNiO3
a b s t r a c t Nickel manganite spinel (NiMn2O4) based negative temperature coefficient (NTC) thermistors have good thermistor characteristics and stabilities. However, to achieve thermistors with a high B constant, the activation energy should be high, which results in high room temperature (RT) resistivity. To obtain a high B constant and low RT resistivity, NiMn2O4 based nanocomposite thick films with the conducting metal oxide, LaNiO3, were fabricated on a glass substrate by aerosol deposition at room temperature. The NiMn2O4–LaNiO3 nanocomposite thick films showed an RT resistivity as low as 35 kX cm, which is one order of magnitude lower than that of the NiMn2O4 films, while retaining the high B constant of NiMn2O4 of over 5000 K when 25 vol.% LaNiO3 was used without any post thermal treatment. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction NiMn2O4 (NMO) based spinels are the most popular negative temperature coefficient (NTC) thermistor materials and are widely used as temperature sensors, temperature compensation devices, and many other electronic components and sensors, because their electrical resistance shows a uniform exponential decrease with increasing temperature [1–5]. Various NMO bulk ceramics and thin/thick films with a wide range of dopants can be used in order to tune the device performance to the specific requirements [1,2,6– 8]. In general, the composition and processing of the NTC affect the distribution of cations in the spinel structure and, thus, significantly change the electrical properties. Therefore, numerous studies on the transition metal doping of NiMn2O4 based spinel NTC materials have been conducted in order to improve their NTC characteristics. However, to date, they have not been successful in obtaining a high B constant with low room temperature (RT) resistivity, because both are directly related to the activation energy for Mn3+/4+ polaron hopping [1,4,9–11]. The concepts of thick film NTC thermistors are attractive from a system design and manufacturing point of view [12,13]. In 2009, we reported the success of NMO thick films by using a room temperature powder spray in the vacuum process (so-called aerosol deposition (AD)) [14,15]. According to our previous report, AD ⇑ Corresponding author. E-mail address: [email protected] (J. Ryu). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.04.038
can be used to fabricate nano-grained NTC thermistors which have a high thermistor B constant, but their RT resistivity is also very high, because of the large area of grain boundaries compared to the bulk ceramics with the same composition [14,15]. The high RT resistivity of NTC thermistors is not desirable for practical applications, because it can render the interface circuitry complex. To overcome this trade-off, we report the fabrication of NMO nanocomposite thick films with LaNiO3 (LNO) conduction metal oxide on a glass substrate at RT and show the feasibility of obtaining a high B constant with a low RT resistivity from the same NTC thick film without the need for a post annealing process. 2. Experimental procedure The NMO powders used for the NTC thermistor matrix and LNO powders used for the conducting particles were individually synthesized by the conventional solid state reaction and modified sol–gel process, respectively. For the NMO powders, the composition selected was NMO + 0.05Co + 0.05Fe, since it shows not only a very high B constant (>5500 K), but also high RT resistivity (>100 kX cm) [16]. The detailed powder preparation procedures are described in our previous reports [9,14,15]. The average particle size of both NMO and LNO was in the vicinity of 1.5 lm. The prepared NMO and LNO powders were mixed by ball milling prior to AD. The LNO content was varied from 0 to 50 vol.% and the mixed slurries were rotary evaporated and sieved. The prepared NMO LNO powder mixtures were aerosol deposited at RT as in the case of other nanocomposite films fabricated by AD [16,17] and the thickness of the nanocomposite films was in the range of 5– 10 lm. The phases of the powder mixture and nanocomposite films were identified by X-ray diffraction (XRD: X’Pert-PRO, PANalytical, Philips, Netherland) and their microstructures were observed using scanning electron microscopy (SEM: JSM5800, JEOL CO., Tokyo, Japan) and scanning transmission electron microscopy
J.-E. Kang et al. / Journal of Alloys and Compounds 534 (2012) 70–73
71
(STEM: JEM-2100F, JEOL Co., Tokyo, Japan) at 200 kV. Two parallel Pt electrodes separated by a distance of 0.25 mm were dc sputtered onto the film surfaces in order to enable the in-plane resistivity vs. temperature measurements to be conducted. The area of the film for B constant measurement was 5 10 mm2. Wires were attached with silver paste to the sputtered Pt. The resistivity (q) vs. temperature (T) characteristics were measured at different temperatures in the range of 243–403 K (30–130 °C) using a precise digital multimeter (Model 2700, Keithley, Cleveland, OH) during the cooling process in a constant humidity chamber. The NTC figure of the B constant was calculated from the resistivity vs. temperature characteristics using Eq. (1):
BT 1 =T 2 ¼
lnðR1 R2 Þ ð1=T 1 Þ ð1=T 2 Þ
ð1Þ
where R1 and R2 are the resistances measured at T1 and T2, respectively [9].
3. Results and discussion The phases of the NMO and LNO powder mixtures and the deposited films were identified by X-ray diffraction and the results are presented in Fig. 1. The patterns obtained from the NMO and LNO powder mixture (Fig. 1a) show a clear tendency for the LNO increment with composition. The NMO phase has a pure cubic spinel structure, while LNO has a perovskite structure. After AD, the as-deposited NMO LNO nanocomposite films showed relatively weak and broad peak patterns compared to those of the powder mixtures. Most of the AD-ceramic films showed similar behavior and such broad XRD peaks are known to generally indicate a lower crystallinity due to the nano-sized grains. Because of that, the asdeposited films showed only a few of the strongest spinel peaks from NMO and perovskite peaks from LNO. With increasing content of LNO, the peak intensity of the LNO perovskite increased, which means that the AD film composition followed the initial powder mixture composition. Almost all highly conducting metal oxides, including LNO, are prone to decomposition at high temperature (>800 °C) [18,19], thus no high temperature annealing process was conducted in this study, in order to prevent any undesirable phase decomposition and inter-diffusion of the phases. Fig. 2 shows the fractured cross-sectional and surface SEM micrographs of the NMO (a), (b) and NMO + 25 vol.% LNO (c) (d) thick films. The films were highly dense and very little pores were observed in the cross-sectional micrographs of either film. This is also confirmed by the surface SEM micrographs of each sample. The surface micrographs showed the typical dense microstructure of the AD film surfaces. From the cross-sectional micrographs of the films, the film thickness, which was controlled by the amount of repetitive deposition cycles, was measured to be in the range of 5–10 lm. The distribution and size of the LNO conducting particles in the NMO matrix were not possible to identify from these SEM micrographs. The STEM micrographs and EDX mapping were used to examine the distribution of LNO in the NMO matrix and the results are presented in Fig. 3. The bright field images of the asdeposited NMO + 25 vol.% LNO film showed nano-scaled crystallites with a size of several nanometers formed by the collision of the accelerated particles with high kinetic energy during deposition. The LNO particles distributed in the NMO matrix comprised crystallites with a size of several tens of nanometers. The twodimensional EDX mapping of La, Mn, and Ni elements, as shown in Fig. 3b–d, confirmed that the LNO conducting particles were homogeneously distributed in the NMO matrix without forming any conducting path chains or severe inter-diffusion between the two phases. If conducting path chains were formed in the NMO matrix, it would harm the NTC thermistor characteristics, especially the B constant, which is related to the activation energy of polaron hopping. Therefore, achieving a homogeneous distribution of conducting particles, without the formation of the conduction paths which is formed at the lower limit of the percolation threshold, is the most important factor in this kind of nanocomposite material.
Fig. 1. XRD patterns of (a) NMO LNO powder mixtures and (b) aerosol-deposited nanocomposite films on glass substrate at RT.
Fig. 2. (a) Fractured cross-sectional and (b) surface SEM micrographs of NMO, (c) and (d) show those of the NMO + 25 vo% LNO nanocomposite thick film.
The resistivities (q) of the NMO LNO films were evaluated in terms of the temperature dependence and are plotted in Fig. 4. To determine its effect on the q–T characteristics, the LNO content was varied from 0 to 50 vol.%. The slopes of the plots relate to the thermistor B constant. Fig. 5 shows the NTC thermistor properties (B40/100 constant and q at 40 °C) and schematics of microstructural evolution for the NMO-LNO nanocomposite films as a
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Fig. 3. (a) STEM bright field image and (b)–(d) EDS mapping image of NMO + 25 vol.% LNO nanocomposite thick film. (b) La, (c) Mn, and (d) Ni.
Fig. 4. Resistivity variation of NTC-LNO nanocomposite thermistor thick films as a function of temperature.
Fig. 6. Resistance change with time at 130 °C for the Co and Fe co-doped and 25 vol.% LNO added NMO nanocomposite NTC thermistor films.
and did not form the complete network of conducting path as shown in Fig. 3, thus the NTC constant was not affected while the q was decreased by rule of mixture in the case of regions 1 and 2. However, when the LNO content exceeded 30 vol.% (region 3), both B and q were drastically degraded, due to the percolation of the LNO particles inside the NMO matrix. Once the conduction network completed by percolation, the NTC characteristics can be dominated by the LNO conduction network, thus the composite film showed very low resistivity and thermistor constant. In the 25 vol.% LNO added nanocomposite film, the B constant remained above 5000 K, while the RT resistivity was 35 kX cm, which is one order of magnitude lower than that of the NMO films. Therefore, it can be concluded that 25% LNO conducting particles within the NMO matrix were very effective in lowering the resistivity, while keeping the thermistor parameter B constant up to the percolation threshold limit. To characterize the long term stable operation which is an essential requirement for real applications, the resistance change was measured for the time variation at 130 ± 1 °C and is depicted in Fig. 6. Both the Co and Fe co-doped and 25 vol.% LNO added nanocomposite NTC thermistor films maintain almost the same resistance over 1100 min with less than 1% deviation. This high resistivity-time stability may be associated to the highly dense microstructure of the NTC films. 4. Conclusion
Fig. 5. NTC thermistor properties (B40/100 constant and q at 40 °C) and schematics of microstructural evolution for the NMO-LNO nanocomposite thermistor thick films as a function of the LNO content.
function of the LNO content. The q value varied gradually with increasing LNO content, but the B constant did not change noticeably up to 25 vol.% LNO (regions 1 and 2). It is believed that the LNO particles were homogeneously dispersed in the NTC matrix
In summary, nanocomposite NTC thermistor thick films composed of NMO + LNO were deposited on a glass substrate by AD at RT. Dense, nano-crystalline, composite NMO films with homogeneously dispersed nano-scale LNO conducting particles were effectively formed by AD and exhibited a high B constant and low resistivity. The percolation threshold limit occurs between 25 and 30 vol.%. The B constant of the deposited NMO + 25 vol.% LNO nanocomposite film remained above 5000 K, but the resistivity was one order of magnitude lower than that of the NMO films. This indicates that the LNO conducting particles in the MNO NTC matrix were very effective in lowering the resistivity, while keeping the thermistor B constant up to percolation threshold limit. Acknowledgements This research was supported by a Grant from the Fundamental R&D Program for Core Technology of Materials funded by the Min-
J.-E. Kang et al. / Journal of Alloys and Compounds 534 (2012) 70–73
istry of Knowledge Economy, Republic of Korea. The authors deeply appreciate reviewer’s invaluable, thoughtful suggestions and comments on the manuscript. References [1] A. Feteira, J. Am. Ceram. Soc. 92 (2009) 967. [2] J.L.M. De Vidales, P. Garcia-Chain, R.M. Rojas, E. Vila, O. Garcia-Martinez, J. Mater. Sci. 33 (1998) 1491. [3] G.D.C. Csete de Gyoergyfalva, A.N. Nolte, I.M. Reaney, J. Eur. Ceram. Soc. 19 (1999) 857. [4] R. Schmidt, A. Basu, A.W. Brinkman, Z. Klusek, P.K. Datta, Appl. Phys. Lett. 86 (2005) 073501. [5] A. Veres, J.G. Noudem, O. Perez, S. Fourrez, G. Bailleul, Solid State Ionics 178 (2007) 423. [6] O. Shpotyuk, A. Kovalskiy, O. Mrooz, L. Shpotyuk, V. Pechnyo, S. Volkov, J. Eur. Ceram. Soc. 21 (2001) 2067. [7] A. Veres, J.G. Noudem, O. Perez, S. Fourrez, G. Bailleul, J. Eur. Ceram. Soc. 27 (2007) 3873. [8] C. Zhao, B. Wang, P. Yang, L. Winnubst, C. Chen, J. Eur. Ceram. Soc. 28 (2008) 35.
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[9] J. Ryu, K.-Y. Kim, J.-J. Choi, B.-D. Hahn, W.-H. Woon, B.-K. Lee, D.-S. Park, C. Park, J. Am. Ceram. Soc. 92 (2009) 3084. [10] S. Fritsch, J. Sarrias, M. Brieu, J.J. Couderc, J.L. Baudour, E. Snoek, A. Rousset, Solid State Ionics 109 (1998) 229. [11] R. Schmidt, A. Basu, A.W. Brinkman, Phys. Rev. B 72 (2005) 115101. [12] A. Ikegami, H. Arima, H. Tosaki, Y. Matsuoka, M. Ai, H. Minorikawa, Y. Asahino, IEEE Trans. Components, Hybrid Manuf. Technol. CHMT-3 (1980) 541. [13] R. Schmidt, A. Stiegelschmitt, A. Roosen, A.W. Brinkman, Key Eng. Mater. 206– 213 (2002) 1417. [14] J. Ryu, D.-S. Park, R. Schmidt, J. Appl. Phys. 109 (2011) 113722. [15] J. Ryu, C.-W. Baek, G. Han, J.-Y. Cho, Y.-S. Lim, D.-S. Park, D.-Y. Jeong, J. Nanosci. Nanotechnol, in press. [16] C.-S. Park, J. Ryu, J.-J. Choi, D.-S. Park, C.-W. Ahn, S. Priya, Jpn. J. Appl. Phys. 48 (2009) 08204. [17] J. Ryu, K.-Y. Kim, B.-D. Hahn, J.-J. Choi, W.-H. Yoon, B.-K. Lee, D.-S. Park, C. Park, Catal. Commun. 10 (2009) 596. [18] J.-J. Choi, J. Ryu, B.-D. Hahn, W.-H. Yoon, B.-K. Lee, J.-H. Choi, D.-S. Park, J. Am. Ceram. Soc. 93 (2010) 1614. [19] G. Han, J. Ryu, W.-H. Yoon, J.-J. Choi, B.-D. Hahn, D.-S. Park, Ceram. Int. 38S (2012) S241.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
LaNiO3 conducting particle dispersed NiMn2O4 nanocomposite NTC thermistor thick films by aerosol deposition Ju-Eun Kang, Jungho Ryu ⇑, Guifang Han, Jong-Jin Choi, Woon-Ha Yoon, Byung-Dong Hahn, Jong-Woo Kim, Cheol-Woo Ahn, Joon Hwan Choi, Dong-Soo Park Functional Ceramics Research Group, Korea Institute of Materials Science (KIMS), 66 Sangnam-Dong, Changwon, Gyeongnam 641-831, Republic of Korea
a r t i c l e
i n f o
Article history: Received 6 December 2011 Received in revised form 9 April 2012 Accepted 13 April 2012 Available online 26 April 2012 Keywords: Negative temperature coefficient (NTC) thermistor Thick film Aerosol deposition (AD) Conducting oxide LaNiO3
a b s t r a c t Nickel manganite spinel (NiMn2O4) based negative temperature coefficient (NTC) thermistors have good thermistor characteristics and stabilities. However, to achieve thermistors with a high B constant, the activation energy should be high, which results in high room temperature (RT) resistivity. To obtain a high B constant and low RT resistivity, NiMn2O4 based nanocomposite thick films with the conducting metal oxide, LaNiO3, were fabricated on a glass substrate by aerosol deposition at room temperature. The NiMn2O4–LaNiO3 nanocomposite thick films showed an RT resistivity as low as 35 kX cm, which is one order of magnitude lower than that of the NiMn2O4 films, while retaining the high B constant of NiMn2O4 of over 5000 K when 25 vol.% LaNiO3 was used without any post thermal treatment. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction NiMn2O4 (NMO) based spinels are the most popular negative temperature coefficient (NTC) thermistor materials and are widely used as temperature sensors, temperature compensation devices, and many other electronic components and sensors, because their electrical resistance shows a uniform exponential decrease with increasing temperature [1–5]. Various NMO bulk ceramics and thin/thick films with a wide range of dopants can be used in order to tune the device performance to the specific requirements [1,2,6– 8]. In general, the composition and processing of the NTC affect the distribution of cations in the spinel structure and, thus, significantly change the electrical properties. Therefore, numerous studies on the transition metal doping of NiMn2O4 based spinel NTC materials have been conducted in order to improve their NTC characteristics. However, to date, they have not been successful in obtaining a high B constant with low room temperature (RT) resistivity, because both are directly related to the activation energy for Mn3+/4+ polaron hopping [1,4,9–11]. The concepts of thick film NTC thermistors are attractive from a system design and manufacturing point of view [12,13]. In 2009, we reported the success of NMO thick films by using a room temperature powder spray in the vacuum process (so-called aerosol deposition (AD)) [14,15]. According to our previous report, AD ⇑ Corresponding author. E-mail address: [email protected] (J. Ryu). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.04.038
can be used to fabricate nano-grained NTC thermistors which have a high thermistor B constant, but their RT resistivity is also very high, because of the large area of grain boundaries compared to the bulk ceramics with the same composition [14,15]. The high RT resistivity of NTC thermistors is not desirable for practical applications, because it can render the interface circuitry complex. To overcome this trade-off, we report the fabrication of NMO nanocomposite thick films with LaNiO3 (LNO) conduction metal oxide on a glass substrate at RT and show the feasibility of obtaining a high B constant with a low RT resistivity from the same NTC thick film without the need for a post annealing process. 2. Experimental procedure The NMO powders used for the NTC thermistor matrix and LNO powders used for the conducting particles were individually synthesized by the conventional solid state reaction and modified sol–gel process, respectively. For the NMO powders, the composition selected was NMO + 0.05Co + 0.05Fe, since it shows not only a very high B constant (>5500 K), but also high RT resistivity (>100 kX cm) [16]. The detailed powder preparation procedures are described in our previous reports [9,14,15]. The average particle size of both NMO and LNO was in the vicinity of 1.5 lm. The prepared NMO and LNO powders were mixed by ball milling prior to AD. The LNO content was varied from 0 to 50 vol.% and the mixed slurries were rotary evaporated and sieved. The prepared NMO LNO powder mixtures were aerosol deposited at RT as in the case of other nanocomposite films fabricated by AD [16,17] and the thickness of the nanocomposite films was in the range of 5– 10 lm. The phases of the powder mixture and nanocomposite films were identified by X-ray diffraction (XRD: X’Pert-PRO, PANalytical, Philips, Netherland) and their microstructures were observed using scanning electron microscopy (SEM: JSM5800, JEOL CO., Tokyo, Japan) and scanning transmission electron microscopy
J.-E. Kang et al. / Journal of Alloys and Compounds 534 (2012) 70–73
71
(STEM: JEM-2100F, JEOL Co., Tokyo, Japan) at 200 kV. Two parallel Pt electrodes separated by a distance of 0.25 mm were dc sputtered onto the film surfaces in order to enable the in-plane resistivity vs. temperature measurements to be conducted. The area of the film for B constant measurement was 5 10 mm2. Wires were attached with silver paste to the sputtered Pt. The resistivity (q) vs. temperature (T) characteristics were measured at different temperatures in the range of 243–403 K (30–130 °C) using a precise digital multimeter (Model 2700, Keithley, Cleveland, OH) during the cooling process in a constant humidity chamber. The NTC figure of the B constant was calculated from the resistivity vs. temperature characteristics using Eq. (1):
BT 1 =T 2 ¼
lnðR1 R2 Þ ð1=T 1 Þ ð1=T 2 Þ
ð1Þ
where R1 and R2 are the resistances measured at T1 and T2, respectively [9].
3. Results and discussion The phases of the NMO and LNO powder mixtures and the deposited films were identified by X-ray diffraction and the results are presented in Fig. 1. The patterns obtained from the NMO and LNO powder mixture (Fig. 1a) show a clear tendency for the LNO increment with composition. The NMO phase has a pure cubic spinel structure, while LNO has a perovskite structure. After AD, the as-deposited NMO LNO nanocomposite films showed relatively weak and broad peak patterns compared to those of the powder mixtures. Most of the AD-ceramic films showed similar behavior and such broad XRD peaks are known to generally indicate a lower crystallinity due to the nano-sized grains. Because of that, the asdeposited films showed only a few of the strongest spinel peaks from NMO and perovskite peaks from LNO. With increasing content of LNO, the peak intensity of the LNO perovskite increased, which means that the AD film composition followed the initial powder mixture composition. Almost all highly conducting metal oxides, including LNO, are prone to decomposition at high temperature (>800 °C) [18,19], thus no high temperature annealing process was conducted in this study, in order to prevent any undesirable phase decomposition and inter-diffusion of the phases. Fig. 2 shows the fractured cross-sectional and surface SEM micrographs of the NMO (a), (b) and NMO + 25 vol.% LNO (c) (d) thick films. The films were highly dense and very little pores were observed in the cross-sectional micrographs of either film. This is also confirmed by the surface SEM micrographs of each sample. The surface micrographs showed the typical dense microstructure of the AD film surfaces. From the cross-sectional micrographs of the films, the film thickness, which was controlled by the amount of repetitive deposition cycles, was measured to be in the range of 5–10 lm. The distribution and size of the LNO conducting particles in the NMO matrix were not possible to identify from these SEM micrographs. The STEM micrographs and EDX mapping were used to examine the distribution of LNO in the NMO matrix and the results are presented in Fig. 3. The bright field images of the asdeposited NMO + 25 vol.% LNO film showed nano-scaled crystallites with a size of several nanometers formed by the collision of the accelerated particles with high kinetic energy during deposition. The LNO particles distributed in the NMO matrix comprised crystallites with a size of several tens of nanometers. The twodimensional EDX mapping of La, Mn, and Ni elements, as shown in Fig. 3b–d, confirmed that the LNO conducting particles were homogeneously distributed in the NMO matrix without forming any conducting path chains or severe inter-diffusion between the two phases. If conducting path chains were formed in the NMO matrix, it would harm the NTC thermistor characteristics, especially the B constant, which is related to the activation energy of polaron hopping. Therefore, achieving a homogeneous distribution of conducting particles, without the formation of the conduction paths which is formed at the lower limit of the percolation threshold, is the most important factor in this kind of nanocomposite material.
Fig. 1. XRD patterns of (a) NMO LNO powder mixtures and (b) aerosol-deposited nanocomposite films on glass substrate at RT.
Fig. 2. (a) Fractured cross-sectional and (b) surface SEM micrographs of NMO, (c) and (d) show those of the NMO + 25 vo% LNO nanocomposite thick film.
The resistivities (q) of the NMO LNO films were evaluated in terms of the temperature dependence and are plotted in Fig. 4. To determine its effect on the q–T characteristics, the LNO content was varied from 0 to 50 vol.%. The slopes of the plots relate to the thermistor B constant. Fig. 5 shows the NTC thermistor properties (B40/100 constant and q at 40 °C) and schematics of microstructural evolution for the NMO-LNO nanocomposite films as a
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J.-E. Kang et al. / Journal of Alloys and Compounds 534 (2012) 70–73
Fig. 3. (a) STEM bright field image and (b)–(d) EDS mapping image of NMO + 25 vol.% LNO nanocomposite thick film. (b) La, (c) Mn, and (d) Ni.
Fig. 4. Resistivity variation of NTC-LNO nanocomposite thermistor thick films as a function of temperature.
Fig. 6. Resistance change with time at 130 °C for the Co and Fe co-doped and 25 vol.% LNO added NMO nanocomposite NTC thermistor films.
and did not form the complete network of conducting path as shown in Fig. 3, thus the NTC constant was not affected while the q was decreased by rule of mixture in the case of regions 1 and 2. However, when the LNO content exceeded 30 vol.% (region 3), both B and q were drastically degraded, due to the percolation of the LNO particles inside the NMO matrix. Once the conduction network completed by percolation, the NTC characteristics can be dominated by the LNO conduction network, thus the composite film showed very low resistivity and thermistor constant. In the 25 vol.% LNO added nanocomposite film, the B constant remained above 5000 K, while the RT resistivity was 35 kX cm, which is one order of magnitude lower than that of the NMO films. Therefore, it can be concluded that 25% LNO conducting particles within the NMO matrix were very effective in lowering the resistivity, while keeping the thermistor parameter B constant up to the percolation threshold limit. To characterize the long term stable operation which is an essential requirement for real applications, the resistance change was measured for the time variation at 130 ± 1 °C and is depicted in Fig. 6. Both the Co and Fe co-doped and 25 vol.% LNO added nanocomposite NTC thermistor films maintain almost the same resistance over 1100 min with less than 1% deviation. This high resistivity-time stability may be associated to the highly dense microstructure of the NTC films. 4. Conclusion
Fig. 5. NTC thermistor properties (B40/100 constant and q at 40 °C) and schematics of microstructural evolution for the NMO-LNO nanocomposite thermistor thick films as a function of the LNO content.
function of the LNO content. The q value varied gradually with increasing LNO content, but the B constant did not change noticeably up to 25 vol.% LNO (regions 1 and 2). It is believed that the LNO particles were homogeneously dispersed in the NTC matrix
In summary, nanocomposite NTC thermistor thick films composed of NMO + LNO were deposited on a glass substrate by AD at RT. Dense, nano-crystalline, composite NMO films with homogeneously dispersed nano-scale LNO conducting particles were effectively formed by AD and exhibited a high B constant and low resistivity. The percolation threshold limit occurs between 25 and 30 vol.%. The B constant of the deposited NMO + 25 vol.% LNO nanocomposite film remained above 5000 K, but the resistivity was one order of magnitude lower than that of the NMO films. This indicates that the LNO conducting particles in the MNO NTC matrix were very effective in lowering the resistivity, while keeping the thermistor B constant up to percolation threshold limit. Acknowledgements This research was supported by a Grant from the Fundamental R&D Program for Core Technology of Materials funded by the Min-
J.-E. Kang et al. / Journal of Alloys and Compounds 534 (2012) 70–73
istry of Knowledge Economy, Republic of Korea. The authors deeply appreciate reviewer’s invaluable, thoughtful suggestions and comments on the manuscript. References [1] A. Feteira, J. Am. Ceram. Soc. 92 (2009) 967. [2] J.L.M. De Vidales, P. Garcia-Chain, R.M. Rojas, E. Vila, O. Garcia-Martinez, J. Mater. Sci. 33 (1998) 1491. [3] G.D.C. Csete de Gyoergyfalva, A.N. Nolte, I.M. Reaney, J. Eur. Ceram. Soc. 19 (1999) 857. [4] R. Schmidt, A. Basu, A.W. Brinkman, Z. Klusek, P.K. Datta, Appl. Phys. Lett. 86 (2005) 073501. [5] A. Veres, J.G. Noudem, O. Perez, S. Fourrez, G. Bailleul, Solid State Ionics 178 (2007) 423. [6] O. Shpotyuk, A. Kovalskiy, O. Mrooz, L. Shpotyuk, V. Pechnyo, S. Volkov, J. Eur. Ceram. Soc. 21 (2001) 2067. [7] A. Veres, J.G. Noudem, O. Perez, S. Fourrez, G. Bailleul, J. Eur. Ceram. Soc. 27 (2007) 3873. [8] C. Zhao, B. Wang, P. Yang, L. Winnubst, C. Chen, J. Eur. Ceram. Soc. 28 (2008) 35.
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