- Email: [email protected]
Nickel oxide coated carbon nanoparticles as temperature sensing materials
Materials Chemistry and Physics 148 (2014) 305e310
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Nickel oxide coated carbon nanoparticles as temperature sensing materials Chun-Chih Huang a, Pei-Chen Su b, Hao-Ming Hsiao c, Ying-Chih Liao a, * a
Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore c Department of Mechanical Engineering, National Taiwan University, Taipei, Taiwan b
h i g h l i g h t s Chemical deposition method is used to synthesize NiO coated carbon nanoparticles. The conductivity of the nanocomposites is highly sensitive to temperature variation. Surface morphology and crystal structures of the sintered thin films are examined. Electrical resistivity of the oxide thin films obeys the Arrhenius relation. An activation energy of 0.38 eV is found in the Arrhenius relation.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 April 2013 Received in revised form 18 September 2013 Accepted 27 July 2014 Available online 15 August 2014
In this research, carbon black nanoparticles were coated with nickel oxide chemically to prepare composite materials with a large temperature coefficient of resistance (TCR) and low resistivity for thermistor applications. Carbon black (CB) nanoparticles were first suspended in aqueous nickel chloride solution. After titrating with sodium hydroxide, nickel hydroxide was chemically depositing onto carbon black nanoparticles. The precipitates were filtered, collected, and calcined at 300 C to generate NiO/CB composites. The XRD results for the NiO/CB composites showed distinct NiO crystalline structures with an average grain size of 12 nm. Surface morphology of prepared elements was also examined with SEM to show that the CB particles were coated by NiO. The electrical resistivity of the composite NiO/CB material follows Arrhenius relation with activation energy of 0.38 eV in the range of 50e200 C and possesses a high temperature sensitivity with B-values more than 4000 K. © 2014 Elsevier B.V. All rights reserved.
Keywords: Oxides Chemical synthesis Coatings Thermoelectric effects Electrical conductivity
1. Introduction Negative temperature coefficient (NTC) thermistors are widely used as temperature sensors, temperature compensation devices, and many other electronic components and sensors [1e4]. The majority of commercial NTC thermistors are composed of nickel oxide, a transition-metal oxide, because of its large temperature coefficient of resistance (TCR). To fabricate reliable thermistors for temperature detection, nickel oxide has been mixed or doped with various materials to modify its resistivity, TCR, and/or environmental stability. For example, the electrical resistivity of NiO can be largely reduced while maintaining the large TCR after doping with
* Corresponding author. Tel.: þ886 2 3366 9688; fax: þ886 2 2362 3040. E-mail addresses: [email protected] (C.-C. Huang), [email protected] (P.-C. Su), [email protected] (H.-M. Hsiao), [email protected] (Y.-C. Liao). http://dx.doi.org/10.1016/j.matchemphys.2014.07.048 0254-0584/© 2014 Elsevier B.V. All rights reserved.
lithium. The doped lithium ions partially replace nickel atoms in the cation sites, and increase the amount of Ni3þ ions, which are responsible for the higher electrical conductivity [5e7]. The LixNi1xO composite materials possess a significantly higher conductivity (1e2 orders higher than pure NiO) and have nearly the same TCR values [6,8]. Another example is nickel manganite oxides, which have a cubic close packing of oxygen atoms with cations distributed in tetrahedral sites and octahedral sites. Due to an electron jump between the Mn3þ and Mn4þ cations located on the octahedral sites, nickel manganite oxides have better electron conduction than NiO [9,10]. Other oxides such as Co3O4, CuO, ZnO and Fe2O3 have also been used as dopants to improve the performance of thermistors, i.e., lower electric resistivity with higher TCR [11e14]. Although the aforementioned chemical doping methods produce oxide materials with lower electrical resistance, complicated dopant formulations and sintering processes are regularly required. To improve the performance of NiO in temperature sensing
306
C.-C. Huang et al. / Materials Chemistry and Physics 148 (2014) 305e310
applications, i.e., lower electrical resistivity and higher TCR, NiO are regularly doped with several dopants at the same time. These chemical synthetic routes usually need complicated formulation steps. Moreover, thermal treatments with precise temperature controls are required to sinter the oxides and to form specific crystalline structures. In this study, a simple synthetic method is proposed to prepare NiO composite material with low resistivity and high TCR. Nickel oxide particles mixed with carbon black (CB) nanoparticles [15e18], a conductive material, to reduce the resistivity. Because regular physical mixing or grinding of NiO with CB powders shows coagulation of CB and NiO domains (Fig. 1), the electron transfer mostly through the CB domains in the mixture material due to the low resistivity of carbon. Thus, the temperature sensing element fabricated from simple particle mixture shows low temperature sensitivity. To prevent the dominant carbon black domains on the electrical resistivity of nickel oxide, those carbon black nanoparticles need to be uniformly surrounded by nickel oxide with a coreeshell structure. Therefore, instead of mixing nickel oxide and carbon black physically, nickel oxide was coated on the surface of carbon black nanoparticles by depositing nickel hydroxide chemically onto carbon black nanoparticles and following with a thermal treatment to convert the hydroxide coating to oxide. Basic properties, such as surface morphology, crystal structures, and electrical conductivity, of the prepared nickel oxide coated carbon black (NiO/CB) nanoparticles with various molar ratios of Ni versus C are well characterized and presented in this article. The electrical performance of the prepared NiO/CB composite material exhibits the same temperature sensitivity as that of pure NiO but with a much lower resistance. This composite material can be further applied to temperature sensing element fabrication. 2. Experiments 2.1. Materials The NiO/CB composite powder was prepared by a chemical precipitation followed by thermal annealing (Fig. 1(a)) [19]. First,
0.08 mol nickel chloride hexahydrate (NiCl2$6H2O, First Chemical Company, Taiwan) was dissolved in 80 mL ethanol (SigmaeAldrich, USA). Then, proper amount of CB nanoparticles (0.24 or 0.48 g) was added into the 1 M NiCl2 solution to prepare a solution of different Ni/C ratios. The resulting solution was kept in ultrasonic bath (DELTA DC300H) for 5 h to help CB particle suspension. Next, 1 M aqueous sodium hydroxide (Mallinckrodt Baker Inc.) solution was added dropwise to the aqueous NiCl2 solution containing CB under vigorous magnetic stirring until dark precipitation of Ni(OH)2/CB composites were formed. The precipitates were filtered under vacuum filtration and washed with deionized water three times to remove NaOH residues. The collected powder was dried in a vacuum oven at 50 C for 24 h. Finally, the Ni(OH)2/CB composites were calcined at 300 C for 2 h with temperature ramping of 10 C min1 to obtain the final product NiO/CB. NiO powder was also produced similarly by hydrothermal method (Fig. 1(b)). First, 0.01 M NaOH solution was added dropwise into 0.01 M NiCl2 solution until the green solution became colorless. The precipitate was filtered, washed, and calcined at 300 C for 2 h with temperature ramping of 10 C min1 to yield black NiO powder. 2.2. Specimen preparation To investigate the electrical performance of the synthetic materials, the prepared oxide powders were mixed with solvents to form pastes and were cast on glass slides to prepare specimens for electrical measurement. Either 0.5 g NiO/CB composite or 0.5 g NiO powder was mixed with 0.5 mL ethylene glycol (SigmaeAldrich, USA) and 0.2 mL ethanol (SigmaeAldrich, USA). The mixture was then manually ground in an agate mortar with a pestle to form a homogeneous paste. The paste was cast on a glass slide with a rectangular shape (10 mm wide and 5 mm long). The cast samples were first dried at 100 C for 5 h to create a smooth thin film under gradual solvent evaporation. Then, the samples were further sintered at 200 C for 10 h with temperature ramping of 10 C min1 to consolidate the cast films and totally remove the solvent residue.
Fig. 1. Schematic diagrams for (a) NiO/CB and (b) NiO þ CB synthesis.
C.-C. Huang et al. / Materials Chemistry and Physics 148 (2014) 305e310
The thickness of these specimens was measured by alpha stepper surface profilometry (Veeco Dektak 6M) for the calculation of electrical resistivity. 2.3. Characterization The thermal behavior of the prepared samples was characterized by a thermogravimetric analyzer (TGA) (Rigaku Thermo plus2 system TG8120, Rigaku, Japan). 10 mg of the prepared NiO/CB precipitates was tested with a purge air flow under a ramping rate of 10 C min1. The crystalline structures of sintered NiO/CB powders were characterized with an X-ray diffractometer (XRD) (Ultima IV, Rigaku, Japan) by using CuKa radiation in qe2q scan mode with 2q angle ranging from 20 to 80 . The microstructure of the prepared specimen was examined with scanning electron microscopy (Nova NanoSEM 230, FEI, USA) with an accelerating voltage of 5 kV. Prepared oxide specimens were connected to silver wires with silver paste, and the electrical resistance of the prepared specimens was measured at temperatures between 50 and 200 C with a multimeter (HP 3478A, HewlettePackard, USA). 3. Results and discussion 3.1. Thermal analysis To determine the calcination temperature for NiO composite, the transition temperature at which Ni(OH)2 precursors convert into NiO was characterized by TGA. Fig. 2 shows the TGA results for precipitates containing CB/Ni(OH)2 composite material (cf. Fig. 1a). A large weight loss occurs between 50 and 100 C due to the evaporation of solvents. Around 17 wt% of the precipitates was evaporated, and therefore a broad valley was observed in both differential thermal analysis (DTA) curves between 50 and 100 C. Another significant weight loss occurs between 100 and 300 C. At this stage, an exothermic peak at 290 C shows up in the DTA curve, indicating the endothermic transition of nickel hydroxide to nickel oxide. Because the precipitate has a molar ratio of Ni(OH)2/CB ¼ 2:1, the average molecular weight of the test material before and after the hydroxide/oxide transition are 65.8 to 53.8, respectively. Accordingly, the dry mass should decrease 18% after the conversion. In the TGA curve, the remaining mass of the test material drops from 83 wt% at 120 C to ~68 wt% at 300 C, which is in good agreement of the 18% loss for the hydroxide/oxide transition. When
307
the temperature keeps increasing, a third weight loss occurs between 450 and 600 C is observed with a great exothermic peak at 520 C, indicating the thermal decomposition of carbon black. Because the total mass of carbon black is about 7% in the material (NiO/C ¼ 2:1), the weight loss from 68 wt% at 300 C to 63 wt% at 600 C (63/68 ¼ 0.93) in the TGA curve indicates that the carbon contents can be totally removed at high temperatures. Based on the above thermal analysis, to create NiO coated CB nanoparticles, the annealing temperature needs to be higher than 290 C in order to completely convert Ni(OH)2 to NiO, but should be lower than 520 C to prevent carbon black decomposition. Thus, in the following sections, the powders were all prepared by annealing the precipitates at 300 C for 2 h. 3.2. Composition and crystalline structures To investigate the composition and crystalline structures of nickel oxide coated CB nanoparticles, the prepared NiO/CB powders are characterized by X-ray diffractometry (Fig. 3). In comparison, the structure of pure NiO and CB are also analyzed. As shown in Fig. 3, diffraction peaks at 24.6 and 43.2 attributed to the crystalline carbon structure are obtained for carbon black powders and is in good agreement with hexagonal structure carbon (JCPDS No. 75-1621). Similarly, well-defined diffraction peaks (37.3 , 43.4 , and 63.1 ) for cubic NiO (JCPDS, No. 71-1179) are also observed for pure NiO powders. However, the XRD pattern of NiO/CB composite only shows an abundant amount of cubic-phase nickel oxide. In order to investigate the disappearance of CB diffraction peaks in the XRD pattern of NiO/CB composite, NiO þ CB mixture with the same molar Ni/C ratio is also characterized for comparison. The XRD pattern of NiO þ CB mixture also shows no carbon black but only nickel oxide peaks regardless of the Ni/C ratios (Ni/C ¼ 2:1, 4:1, and 8:1). Moreover, the diffractogram of CB after heating at 300 C for 2 h is exactly the same as that of the untreated CB, indicating no structural changes in CB after thermal treatments. Therefore, the CB peak disappearance is most possibly a result of too little amount of carbon black in both NiO þ CB mixture and NiO/CB composite to be observed. The particle size can be roughly estimated form the Scherrer equation as,
D¼
Kl B2 cos q
(1)
where D is the grain size, K is a dimensionless shape factor with a typical value of about 0.9, l is the wavelength of the X-ray source
Fig. 2. TGA/DTA curves for precipitate Ni(OH)2/CB (Ni:C ¼ 2:1) at an annealing ramping rate of 10 C min1 with an operating temperature up to 800 C.
Fig. 3. XRD patterns of (a) carbon black, (b) synthesized NiO, (c) NiO/CB composite (Ni:C ¼ 2:1) and (d) NiO þ CB mixtures (Ni:C ¼ 2:1).
308
C.-C. Huang et al. / Materials Chemistry and Physics 148 (2014) 305e310
which is 0.154 nm for CuKa radiation, B is the full width at half maximum (FWHM) of the reflection in radian, and q is the Bragg angle. The particle sizes of NiO, NiO/CB composites and NiO þ CB mixtures calculated based on the (200) peak are about 12 nm in diameter (Table 1). Moreover, the ratios of (200) peak versus (111) peak of all samples are about 1.6. The same particles sizes and ratios of peak intensities reveal that the structure of NiO coating on CB is same as that of synthesized NiO.
resistivity but the resistivity barely changes with temperature at all. Manual ground NiO þ CB mixtures do have a much lower resistivity than that of pure NiO, but the temperature sensitivity is poor, possibly because the electron transfer path is mainly dominated by CB domains. In contrast, because the NiO/CB composites are composed of NiO coated CB particles, the electron have to pass both
3.3. Morphology To further investigate the microstructure of synthesized NiO and NiO/CB composite, the prepared specimens were examined by scanning electron microscopy. Fig. 4a shows the SEM images of synthesized NiO. The primary NiO powder consists of primary nanoparticles of 10e15 nm in diameter, and a few primary particles aggregate into large secondary particles. On the contrary, the carbon black particles are about 40 nm in diameter (Fig. 4b) and there are many cracks on the surface of carbon black which may provide active sites for NiO coating. The morphology of CB particles after heated at 300 C for an hour was also examined and was the same as those in Fig. 4b, indicating the chemical stability of the CB particles at high temperatures. According to Gao et al. [19], the addition of aqueous solution of NaOH into the mixed aqueous solution containing CB and NiCl2 induce the reaction to form Ni(OH)2, and part of Ni(OH)2 would coat on the surface of carbon. In order to prepare CB nanoparticles with good Ni(OH)2 coating coverage, NaOH solution was gradually added in a dropwise manner into the NiCl2 solution so that the heterogeneous nucleation leads to preferential attachment of Ni(OH)2 onto the CB nanoparticles. After the sintering process, a mass of particulate NiO aggregation were observed in Fig. 4c. The particle size of NiO in the composite NiO/CB material is about the same order as that of synthesized NiO, which is consistent with the results derived from XRD patterns. However, one cannot easily find the carbon nanoparticles possibly due to the good attachment of NiO particles on CB substrate. To identify the difference between the pure NiO and NO/CB materials, energydispersive X-ray (EDX) spectrum analysis was also performed (Fig. 5). The results showed that carbon can be found in the NiO/CB material but absent in the NiO. Thus, carbon nanoparticles did exist in Fig. 4c, but might be possibly buried inside the NiO agglomerations. 3.4. Electrical resistivity Fig. 6 shows the variation in electrical resistivity of the prepared specimens with temperature. To investigate the electrical performance of the nano-composite materials, all the specimens were heated and cooled between 50 C and 200 C with a 25 C interval, and the resistivity was calculated by the measured resistance and the cross-sectional area of the specimen. As shown in the figure, the heating and cooling cycles virtually overlap, indicating nearly no electrical resistance hysteresis regarding temperature variation. For pure NiO, the resistivity is the largest among all the specimens, but the resistivity varies with temperature sharply, i.e., a great temperature sensitivity. On the other hand, pure CB has the lowest
Table 1 Calculated particle sizes and ratios of peak intensities from XRD patterns. Material
Grain size
(200)/(111)
NiO NiO/CB composites NiO þ CB
12.20 12.18 11.77
1.61 1.65 1.56
Fig. 4. SEM images of (a) synthesized NiO, (b) carbon black and (c) NiO/CB composite (Ni:C ¼ 2:1).
C.-C. Huang et al. / Materials Chemistry and Physics 148 (2014) 305e310
309
r ¼ r∞ eEa =kT
(2)
where r∞ is the resistivity at infinite temperature, Ea is the activation energy, k is the Boltzmann constant, and T is absolute temperature. For pure NiO, the activation energy is estimated to be ~0.39 eV, which is in the range of 0.15e0.5 eV as a useful material for NTCR applications [20]. Alternatively, for a thermistor device of fixed dimensions, Eq. (2) often appears rewritten as:
R ¼ R∞ eB=T
(3)
where R∞ is the resistance of device at infinite temperature, B is the so-called material constant of a thermistor. The B-value can also be calculated from resistance measurements as follows:
B¼
ln R1=R 2
(4)
1=T 1=T 1 2
Fig. 5. Energy-dispersive X-ray (ERD) spectra of (a) synthesized NiO, and (b) NiO/CB composite (Ni:C ¼ 2:1).
where R1 and R2 are the resistances at the temperature T1 and T2 , respectively. The B-values between 50 C and 200 C of prepared specimens are listed in Table 2. The B-value of carbon black is 81 K, much lower than that of pure nickel oxide (B ¼ 4694 K), indicating that carbon black is insensitive to temperature. The B-value of manual ground NiO þ CB mixture with Ni/C molar ratio of 2:1 is 949 K, much higher than that of pure CB, indicating that the addition of NiO enhances the temperature sensitivity. With more nickel oxide (Ni/C molar ratio ¼ 4:1), the B-value of NiO þ CB mixture increases to 1864 K, but still not large enough for thermistor applications (B > 2000 K). In contrast, NiO/CB composites have the structure with carbon black nanoparticles uniformly surrounded by nickel oxide, preventing the dominant electron transfer through carbon black domains. Therefore, the B-values of NiO/CB composites (4177 K and 4350 K for Ni/C rations of 4:1 and 2:1, respectively) are nearly the same as that of NiO regardless of the Ni/ C ratios, and can be used in thermistor applications.
4. Conclusions
Fig. 6. Resistivity of prepared specimens as a function of temperature.
the NiO and CB domains (Fig. 1). Thus, the NiO/CB composites possess the same temperature sensitivity as that of NiO, but also have a lower resistivity. The resistivity of all the prepared materials decreases with the increasing temperature, and obeys the Arrhenius relation:
Nickel oxide coated carbon black nanoparticles are synthesized to serve as an active material for thermistor applications. Ni(OH)2/ CB precursors were prepared by adding the aqueous solution of NaOH dropwise into aqueous NiCl2 solution with CB nanoparticles under vigorous magnetic stirring. Based on thermogravimetric analysis, the sintering temperature should be higher than 290 C in order to completely oxidize the precursors but lower than 520 C to prevent carbon black from decomposing. Thus, after filtration, washing and drying processes, the precursors were calcined at 300 C for 2 h to generate NiO/CB composites. The X-ray diffractograms showed that nickel hydroxide coated on carbon black was transferred into nickel oxide of cubic phase after the sintering process. The microstructures of composites were examined by SEM and it was observed that nickel oxide nanoparticles of 12 nm in diameter are closely packed around carbon black nanoparticles. With the addition of carbon black, the resistivity of NiO/CB
Table 2 The B-value of the prepared specimens. Material
NiO
NiO/CB composites (Ni:C ¼ 4:1)
NiO/CB composites (Ni:C ¼ 2:1)
NiO þ CB mixture (Ni:C ¼ 4:1)
NiO þ CB mixture (Ni:C ¼ 2:1)
CB
B (K)a(323e423 K) Activation energy (eV)
4694 0.40
4177 0.36
4350 0.38
1864 0.16
949 0.08
81 0.007
a
See Eq. (4) for details.
310
C.-C. Huang et al. / Materials Chemistry and Physics 148 (2014) 305e310
composites are reduced by an order of magnitude compared with that of NiO. The composites have great sensitivity to temperature which follows the Arrhenius relation with an activation energy of 0.36e0.39 eV. The B-values of composites between 50 C and 200 C are larger than 4000 K, almost the same as that of NiO. In summary, NiO coated CB nanoparticles exhibit high temperature sensitivity with a reduced resistivity, and is a promising material for temperature sensing applications. Acknowledgments This research was supported by Ministry of Science and Technology in Taiwan through Grants NSC 101-2221-E-002-177-MY2 and MOST 103-2221-E-002-185-MY3. References
[4] [5] [6] [7] [8] [9] [10] [11]
[12] [13] [14] [15] [16] [17] [18]
[1] M. Lee, M. Yoo, Sens. Actuators A 96 (2002) 97e104. [2] J. Huang, Y.D. Hao, H. Lin, D.L. Zhang, J.J. Song, D.X. Zhou, Mater. Sci. Eng. B 99 (2003) 523e526. [3] M. Hrovat, D. Belavic, J. Kita, J. Cilensek, L. Golonka, A. Dziedzic, J. Eur. Ceram. Soc. 25 (2005) 3443e3450.
[19] [20]
E. Strelcov, Y. Lilach, A. Kolmakov, Nano Lett. 9 (2009) 2322e2326. R.E. Newnham, Rep. Prog. Phys. 52 (1989) 123e156. E. Antolini, Mater. Chem. Phys. 82 (2003) 937e948. E. Antolini, Mater. Lett. 51 (2001) 385e388. C.C. Huang, P.C. Su, Y.C. Liao, Thin Solid Films 544 (2013) 348e351. E.G. Larson, R.J. Arnott, D.G. Wickham, J. Phys. Chem. Solids 23 (1962) 1771e1781. S. Fritsch, J. Sarrias, M. Brieu, J.J. Couderc, J.L. Baudour, E. Snoeck, A. Rousset, Solid State Ionics 109 (1998) 229e237. O.S. Aleksic, M.V. Nikolic, M.D. Lukovic, N. Nikolic, B.M. Radojcic, M. Radovanovic, Z. Djuric, M. Mitric, P.M. Nikolic, Mater. Sci. Eng. B Adv. 178 (2013) 202e210. K. Park, D.Y. Bang, J. Mater. Sci. Mater. Electron. 14 (2003) 81e87. S.G. Wang, A.M. Chang, H.M. Zhang, Q. Zhao, Mater. Chem. Phys. 110 (2008) 83e88. M.A. Gabal, Y.M. Al Angari, Mater. Chem. Phys. 115 (2009) 578e584. X.G. Liu, S.W. Or, C.G. Jin, Y.H. Lv, C. Feng, Y.P. Sun, Carbon 60 (2013) 215e220. X.Y. Wang, X.Y. Wang, L.H. Yi, L. Liu, Y.Z. Dai, H. Wu, J. Power Sources 224 (2013) 317e323. C. Mahendiran, T. Maiyalagan, K. Scott, A. Gedanken, Mater. Chem. Phys. 128 (2011) 341e347. F.A.A. Barros, A.J.R. Castro, J.M. Filho, B.C. Viana, A. Campos, A.C. Oliveira, J. Nanopart. Res. 14 (2012). B. Gao, C.Z. Yuan, L.H. Su, L. Chen, X.G. Zhang, J. Solid State Electrochem. 13 (2009) 1251e1257. A. Feteira, J. Am. Ceram. Soc. 92 (2009) 967e983.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Nickel oxide coated carbon nanoparticles as temperature sensing materials Chun-Chih Huang a, Pei-Chen Su b, Hao-Ming Hsiao c, Ying-Chih Liao a, * a
Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore c Department of Mechanical Engineering, National Taiwan University, Taipei, Taiwan b
h i g h l i g h t s Chemical deposition method is used to synthesize NiO coated carbon nanoparticles. The conductivity of the nanocomposites is highly sensitive to temperature variation. Surface morphology and crystal structures of the sintered thin films are examined. Electrical resistivity of the oxide thin films obeys the Arrhenius relation. An activation energy of 0.38 eV is found in the Arrhenius relation.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 April 2013 Received in revised form 18 September 2013 Accepted 27 July 2014 Available online 15 August 2014
In this research, carbon black nanoparticles were coated with nickel oxide chemically to prepare composite materials with a large temperature coefficient of resistance (TCR) and low resistivity for thermistor applications. Carbon black (CB) nanoparticles were first suspended in aqueous nickel chloride solution. After titrating with sodium hydroxide, nickel hydroxide was chemically depositing onto carbon black nanoparticles. The precipitates were filtered, collected, and calcined at 300 C to generate NiO/CB composites. The XRD results for the NiO/CB composites showed distinct NiO crystalline structures with an average grain size of 12 nm. Surface morphology of prepared elements was also examined with SEM to show that the CB particles were coated by NiO. The electrical resistivity of the composite NiO/CB material follows Arrhenius relation with activation energy of 0.38 eV in the range of 50e200 C and possesses a high temperature sensitivity with B-values more than 4000 K. © 2014 Elsevier B.V. All rights reserved.
Keywords: Oxides Chemical synthesis Coatings Thermoelectric effects Electrical conductivity
1. Introduction Negative temperature coefficient (NTC) thermistors are widely used as temperature sensors, temperature compensation devices, and many other electronic components and sensors [1e4]. The majority of commercial NTC thermistors are composed of nickel oxide, a transition-metal oxide, because of its large temperature coefficient of resistance (TCR). To fabricate reliable thermistors for temperature detection, nickel oxide has been mixed or doped with various materials to modify its resistivity, TCR, and/or environmental stability. For example, the electrical resistivity of NiO can be largely reduced while maintaining the large TCR after doping with
* Corresponding author. Tel.: þ886 2 3366 9688; fax: þ886 2 2362 3040. E-mail addresses: [email protected] (C.-C. Huang), [email protected] (P.-C. Su), [email protected] (H.-M. Hsiao), [email protected] (Y.-C. Liao). http://dx.doi.org/10.1016/j.matchemphys.2014.07.048 0254-0584/© 2014 Elsevier B.V. All rights reserved.
lithium. The doped lithium ions partially replace nickel atoms in the cation sites, and increase the amount of Ni3þ ions, which are responsible for the higher electrical conductivity [5e7]. The LixNi1xO composite materials possess a significantly higher conductivity (1e2 orders higher than pure NiO) and have nearly the same TCR values [6,8]. Another example is nickel manganite oxides, which have a cubic close packing of oxygen atoms with cations distributed in tetrahedral sites and octahedral sites. Due to an electron jump between the Mn3þ and Mn4þ cations located on the octahedral sites, nickel manganite oxides have better electron conduction than NiO [9,10]. Other oxides such as Co3O4, CuO, ZnO and Fe2O3 have also been used as dopants to improve the performance of thermistors, i.e., lower electric resistivity with higher TCR [11e14]. Although the aforementioned chemical doping methods produce oxide materials with lower electrical resistance, complicated dopant formulations and sintering processes are regularly required. To improve the performance of NiO in temperature sensing
306
C.-C. Huang et al. / Materials Chemistry and Physics 148 (2014) 305e310
applications, i.e., lower electrical resistivity and higher TCR, NiO are regularly doped with several dopants at the same time. These chemical synthetic routes usually need complicated formulation steps. Moreover, thermal treatments with precise temperature controls are required to sinter the oxides and to form specific crystalline structures. In this study, a simple synthetic method is proposed to prepare NiO composite material with low resistivity and high TCR. Nickel oxide particles mixed with carbon black (CB) nanoparticles [15e18], a conductive material, to reduce the resistivity. Because regular physical mixing or grinding of NiO with CB powders shows coagulation of CB and NiO domains (Fig. 1), the electron transfer mostly through the CB domains in the mixture material due to the low resistivity of carbon. Thus, the temperature sensing element fabricated from simple particle mixture shows low temperature sensitivity. To prevent the dominant carbon black domains on the electrical resistivity of nickel oxide, those carbon black nanoparticles need to be uniformly surrounded by nickel oxide with a coreeshell structure. Therefore, instead of mixing nickel oxide and carbon black physically, nickel oxide was coated on the surface of carbon black nanoparticles by depositing nickel hydroxide chemically onto carbon black nanoparticles and following with a thermal treatment to convert the hydroxide coating to oxide. Basic properties, such as surface morphology, crystal structures, and electrical conductivity, of the prepared nickel oxide coated carbon black (NiO/CB) nanoparticles with various molar ratios of Ni versus C are well characterized and presented in this article. The electrical performance of the prepared NiO/CB composite material exhibits the same temperature sensitivity as that of pure NiO but with a much lower resistance. This composite material can be further applied to temperature sensing element fabrication. 2. Experiments 2.1. Materials The NiO/CB composite powder was prepared by a chemical precipitation followed by thermal annealing (Fig. 1(a)) [19]. First,
0.08 mol nickel chloride hexahydrate (NiCl2$6H2O, First Chemical Company, Taiwan) was dissolved in 80 mL ethanol (SigmaeAldrich, USA). Then, proper amount of CB nanoparticles (0.24 or 0.48 g) was added into the 1 M NiCl2 solution to prepare a solution of different Ni/C ratios. The resulting solution was kept in ultrasonic bath (DELTA DC300H) for 5 h to help CB particle suspension. Next, 1 M aqueous sodium hydroxide (Mallinckrodt Baker Inc.) solution was added dropwise to the aqueous NiCl2 solution containing CB under vigorous magnetic stirring until dark precipitation of Ni(OH)2/CB composites were formed. The precipitates were filtered under vacuum filtration and washed with deionized water three times to remove NaOH residues. The collected powder was dried in a vacuum oven at 50 C for 24 h. Finally, the Ni(OH)2/CB composites were calcined at 300 C for 2 h with temperature ramping of 10 C min1 to obtain the final product NiO/CB. NiO powder was also produced similarly by hydrothermal method (Fig. 1(b)). First, 0.01 M NaOH solution was added dropwise into 0.01 M NiCl2 solution until the green solution became colorless. The precipitate was filtered, washed, and calcined at 300 C for 2 h with temperature ramping of 10 C min1 to yield black NiO powder. 2.2. Specimen preparation To investigate the electrical performance of the synthetic materials, the prepared oxide powders were mixed with solvents to form pastes and were cast on glass slides to prepare specimens for electrical measurement. Either 0.5 g NiO/CB composite or 0.5 g NiO powder was mixed with 0.5 mL ethylene glycol (SigmaeAldrich, USA) and 0.2 mL ethanol (SigmaeAldrich, USA). The mixture was then manually ground in an agate mortar with a pestle to form a homogeneous paste. The paste was cast on a glass slide with a rectangular shape (10 mm wide and 5 mm long). The cast samples were first dried at 100 C for 5 h to create a smooth thin film under gradual solvent evaporation. Then, the samples were further sintered at 200 C for 10 h with temperature ramping of 10 C min1 to consolidate the cast films and totally remove the solvent residue.
Fig. 1. Schematic diagrams for (a) NiO/CB and (b) NiO þ CB synthesis.
C.-C. Huang et al. / Materials Chemistry and Physics 148 (2014) 305e310
The thickness of these specimens was measured by alpha stepper surface profilometry (Veeco Dektak 6M) for the calculation of electrical resistivity. 2.3. Characterization The thermal behavior of the prepared samples was characterized by a thermogravimetric analyzer (TGA) (Rigaku Thermo plus2 system TG8120, Rigaku, Japan). 10 mg of the prepared NiO/CB precipitates was tested with a purge air flow under a ramping rate of 10 C min1. The crystalline structures of sintered NiO/CB powders were characterized with an X-ray diffractometer (XRD) (Ultima IV, Rigaku, Japan) by using CuKa radiation in qe2q scan mode with 2q angle ranging from 20 to 80 . The microstructure of the prepared specimen was examined with scanning electron microscopy (Nova NanoSEM 230, FEI, USA) with an accelerating voltage of 5 kV. Prepared oxide specimens were connected to silver wires with silver paste, and the electrical resistance of the prepared specimens was measured at temperatures between 50 and 200 C with a multimeter (HP 3478A, HewlettePackard, USA). 3. Results and discussion 3.1. Thermal analysis To determine the calcination temperature for NiO composite, the transition temperature at which Ni(OH)2 precursors convert into NiO was characterized by TGA. Fig. 2 shows the TGA results for precipitates containing CB/Ni(OH)2 composite material (cf. Fig. 1a). A large weight loss occurs between 50 and 100 C due to the evaporation of solvents. Around 17 wt% of the precipitates was evaporated, and therefore a broad valley was observed in both differential thermal analysis (DTA) curves between 50 and 100 C. Another significant weight loss occurs between 100 and 300 C. At this stage, an exothermic peak at 290 C shows up in the DTA curve, indicating the endothermic transition of nickel hydroxide to nickel oxide. Because the precipitate has a molar ratio of Ni(OH)2/CB ¼ 2:1, the average molecular weight of the test material before and after the hydroxide/oxide transition are 65.8 to 53.8, respectively. Accordingly, the dry mass should decrease 18% after the conversion. In the TGA curve, the remaining mass of the test material drops from 83 wt% at 120 C to ~68 wt% at 300 C, which is in good agreement of the 18% loss for the hydroxide/oxide transition. When
307
the temperature keeps increasing, a third weight loss occurs between 450 and 600 C is observed with a great exothermic peak at 520 C, indicating the thermal decomposition of carbon black. Because the total mass of carbon black is about 7% in the material (NiO/C ¼ 2:1), the weight loss from 68 wt% at 300 C to 63 wt% at 600 C (63/68 ¼ 0.93) in the TGA curve indicates that the carbon contents can be totally removed at high temperatures. Based on the above thermal analysis, to create NiO coated CB nanoparticles, the annealing temperature needs to be higher than 290 C in order to completely convert Ni(OH)2 to NiO, but should be lower than 520 C to prevent carbon black decomposition. Thus, in the following sections, the powders were all prepared by annealing the precipitates at 300 C for 2 h. 3.2. Composition and crystalline structures To investigate the composition and crystalline structures of nickel oxide coated CB nanoparticles, the prepared NiO/CB powders are characterized by X-ray diffractometry (Fig. 3). In comparison, the structure of pure NiO and CB are also analyzed. As shown in Fig. 3, diffraction peaks at 24.6 and 43.2 attributed to the crystalline carbon structure are obtained for carbon black powders and is in good agreement with hexagonal structure carbon (JCPDS No. 75-1621). Similarly, well-defined diffraction peaks (37.3 , 43.4 , and 63.1 ) for cubic NiO (JCPDS, No. 71-1179) are also observed for pure NiO powders. However, the XRD pattern of NiO/CB composite only shows an abundant amount of cubic-phase nickel oxide. In order to investigate the disappearance of CB diffraction peaks in the XRD pattern of NiO/CB composite, NiO þ CB mixture with the same molar Ni/C ratio is also characterized for comparison. The XRD pattern of NiO þ CB mixture also shows no carbon black but only nickel oxide peaks regardless of the Ni/C ratios (Ni/C ¼ 2:1, 4:1, and 8:1). Moreover, the diffractogram of CB after heating at 300 C for 2 h is exactly the same as that of the untreated CB, indicating no structural changes in CB after thermal treatments. Therefore, the CB peak disappearance is most possibly a result of too little amount of carbon black in both NiO þ CB mixture and NiO/CB composite to be observed. The particle size can be roughly estimated form the Scherrer equation as,
D¼
Kl B2 cos q
(1)
where D is the grain size, K is a dimensionless shape factor with a typical value of about 0.9, l is the wavelength of the X-ray source
Fig. 2. TGA/DTA curves for precipitate Ni(OH)2/CB (Ni:C ¼ 2:1) at an annealing ramping rate of 10 C min1 with an operating temperature up to 800 C.
Fig. 3. XRD patterns of (a) carbon black, (b) synthesized NiO, (c) NiO/CB composite (Ni:C ¼ 2:1) and (d) NiO þ CB mixtures (Ni:C ¼ 2:1).
308
C.-C. Huang et al. / Materials Chemistry and Physics 148 (2014) 305e310
which is 0.154 nm for CuKa radiation, B is the full width at half maximum (FWHM) of the reflection in radian, and q is the Bragg angle. The particle sizes of NiO, NiO/CB composites and NiO þ CB mixtures calculated based on the (200) peak are about 12 nm in diameter (Table 1). Moreover, the ratios of (200) peak versus (111) peak of all samples are about 1.6. The same particles sizes and ratios of peak intensities reveal that the structure of NiO coating on CB is same as that of synthesized NiO.
resistivity but the resistivity barely changes with temperature at all. Manual ground NiO þ CB mixtures do have a much lower resistivity than that of pure NiO, but the temperature sensitivity is poor, possibly because the electron transfer path is mainly dominated by CB domains. In contrast, because the NiO/CB composites are composed of NiO coated CB particles, the electron have to pass both
3.3. Morphology To further investigate the microstructure of synthesized NiO and NiO/CB composite, the prepared specimens were examined by scanning electron microscopy. Fig. 4a shows the SEM images of synthesized NiO. The primary NiO powder consists of primary nanoparticles of 10e15 nm in diameter, and a few primary particles aggregate into large secondary particles. On the contrary, the carbon black particles are about 40 nm in diameter (Fig. 4b) and there are many cracks on the surface of carbon black which may provide active sites for NiO coating. The morphology of CB particles after heated at 300 C for an hour was also examined and was the same as those in Fig. 4b, indicating the chemical stability of the CB particles at high temperatures. According to Gao et al. [19], the addition of aqueous solution of NaOH into the mixed aqueous solution containing CB and NiCl2 induce the reaction to form Ni(OH)2, and part of Ni(OH)2 would coat on the surface of carbon. In order to prepare CB nanoparticles with good Ni(OH)2 coating coverage, NaOH solution was gradually added in a dropwise manner into the NiCl2 solution so that the heterogeneous nucleation leads to preferential attachment of Ni(OH)2 onto the CB nanoparticles. After the sintering process, a mass of particulate NiO aggregation were observed in Fig. 4c. The particle size of NiO in the composite NiO/CB material is about the same order as that of synthesized NiO, which is consistent with the results derived from XRD patterns. However, one cannot easily find the carbon nanoparticles possibly due to the good attachment of NiO particles on CB substrate. To identify the difference between the pure NiO and NO/CB materials, energydispersive X-ray (EDX) spectrum analysis was also performed (Fig. 5). The results showed that carbon can be found in the NiO/CB material but absent in the NiO. Thus, carbon nanoparticles did exist in Fig. 4c, but might be possibly buried inside the NiO agglomerations. 3.4. Electrical resistivity Fig. 6 shows the variation in electrical resistivity of the prepared specimens with temperature. To investigate the electrical performance of the nano-composite materials, all the specimens were heated and cooled between 50 C and 200 C with a 25 C interval, and the resistivity was calculated by the measured resistance and the cross-sectional area of the specimen. As shown in the figure, the heating and cooling cycles virtually overlap, indicating nearly no electrical resistance hysteresis regarding temperature variation. For pure NiO, the resistivity is the largest among all the specimens, but the resistivity varies with temperature sharply, i.e., a great temperature sensitivity. On the other hand, pure CB has the lowest
Table 1 Calculated particle sizes and ratios of peak intensities from XRD patterns. Material
Grain size
(200)/(111)
NiO NiO/CB composites NiO þ CB
12.20 12.18 11.77
1.61 1.65 1.56
Fig. 4. SEM images of (a) synthesized NiO, (b) carbon black and (c) NiO/CB composite (Ni:C ¼ 2:1).
C.-C. Huang et al. / Materials Chemistry and Physics 148 (2014) 305e310
309
r ¼ r∞ eEa =kT
(2)
where r∞ is the resistivity at infinite temperature, Ea is the activation energy, k is the Boltzmann constant, and T is absolute temperature. For pure NiO, the activation energy is estimated to be ~0.39 eV, which is in the range of 0.15e0.5 eV as a useful material for NTCR applications [20]. Alternatively, for a thermistor device of fixed dimensions, Eq. (2) often appears rewritten as:
R ¼ R∞ eB=T
(3)
where R∞ is the resistance of device at infinite temperature, B is the so-called material constant of a thermistor. The B-value can also be calculated from resistance measurements as follows:
B¼
ln R1=R 2
(4)
1=T 1=T 1 2
Fig. 5. Energy-dispersive X-ray (ERD) spectra of (a) synthesized NiO, and (b) NiO/CB composite (Ni:C ¼ 2:1).
where R1 and R2 are the resistances at the temperature T1 and T2 , respectively. The B-values between 50 C and 200 C of prepared specimens are listed in Table 2. The B-value of carbon black is 81 K, much lower than that of pure nickel oxide (B ¼ 4694 K), indicating that carbon black is insensitive to temperature. The B-value of manual ground NiO þ CB mixture with Ni/C molar ratio of 2:1 is 949 K, much higher than that of pure CB, indicating that the addition of NiO enhances the temperature sensitivity. With more nickel oxide (Ni/C molar ratio ¼ 4:1), the B-value of NiO þ CB mixture increases to 1864 K, but still not large enough for thermistor applications (B > 2000 K). In contrast, NiO/CB composites have the structure with carbon black nanoparticles uniformly surrounded by nickel oxide, preventing the dominant electron transfer through carbon black domains. Therefore, the B-values of NiO/CB composites (4177 K and 4350 K for Ni/C rations of 4:1 and 2:1, respectively) are nearly the same as that of NiO regardless of the Ni/ C ratios, and can be used in thermistor applications.
4. Conclusions
Fig. 6. Resistivity of prepared specimens as a function of temperature.
the NiO and CB domains (Fig. 1). Thus, the NiO/CB composites possess the same temperature sensitivity as that of NiO, but also have a lower resistivity. The resistivity of all the prepared materials decreases with the increasing temperature, and obeys the Arrhenius relation:
Nickel oxide coated carbon black nanoparticles are synthesized to serve as an active material for thermistor applications. Ni(OH)2/ CB precursors were prepared by adding the aqueous solution of NaOH dropwise into aqueous NiCl2 solution with CB nanoparticles under vigorous magnetic stirring. Based on thermogravimetric analysis, the sintering temperature should be higher than 290 C in order to completely oxidize the precursors but lower than 520 C to prevent carbon black from decomposing. Thus, after filtration, washing and drying processes, the precursors were calcined at 300 C for 2 h to generate NiO/CB composites. The X-ray diffractograms showed that nickel hydroxide coated on carbon black was transferred into nickel oxide of cubic phase after the sintering process. The microstructures of composites were examined by SEM and it was observed that nickel oxide nanoparticles of 12 nm in diameter are closely packed around carbon black nanoparticles. With the addition of carbon black, the resistivity of NiO/CB
Table 2 The B-value of the prepared specimens. Material
NiO
NiO/CB composites (Ni:C ¼ 4:1)
NiO/CB composites (Ni:C ¼ 2:1)
NiO þ CB mixture (Ni:C ¼ 4:1)
NiO þ CB mixture (Ni:C ¼ 2:1)
CB
B (K)a(323e423 K) Activation energy (eV)
4694 0.40
4177 0.36
4350 0.38
1864 0.16
949 0.08
81 0.007
a
See Eq. (4) for details.
310
C.-C. Huang et al. / Materials Chemistry and Physics 148 (2014) 305e310
composites are reduced by an order of magnitude compared with that of NiO. The composites have great sensitivity to temperature which follows the Arrhenius relation with an activation energy of 0.36e0.39 eV. The B-values of composites between 50 C and 200 C are larger than 4000 K, almost the same as that of NiO. In summary, NiO coated CB nanoparticles exhibit high temperature sensitivity with a reduced resistivity, and is a promising material for temperature sensing applications. Acknowledgments This research was supported by Ministry of Science and Technology in Taiwan through Grants NSC 101-2221-E-002-177-MY2 and MOST 103-2221-E-002-185-MY3. References
[4] [5] [6] [7] [8] [9] [10] [11]
[12] [13] [14] [15] [16] [17] [18]
[1] M. Lee, M. Yoo, Sens. Actuators A 96 (2002) 97e104. [2] J. Huang, Y.D. Hao, H. Lin, D.L. Zhang, J.J. Song, D.X. Zhou, Mater. Sci. Eng. B 99 (2003) 523e526. [3] M. Hrovat, D. Belavic, J. Kita, J. Cilensek, L. Golonka, A. Dziedzic, J. Eur. Ceram. Soc. 25 (2005) 3443e3450.
[19] [20]
E. Strelcov, Y. Lilach, A. Kolmakov, Nano Lett. 9 (2009) 2322e2326. R.E. Newnham, Rep. Prog. Phys. 52 (1989) 123e156. E. Antolini, Mater. Chem. Phys. 82 (2003) 937e948. E. Antolini, Mater. Lett. 51 (2001) 385e388. C.C. Huang, P.C. Su, Y.C. Liao, Thin Solid Films 544 (2013) 348e351. E.G. Larson, R.J. Arnott, D.G. Wickham, J. Phys. Chem. Solids 23 (1962) 1771e1781. S. Fritsch, J. Sarrias, M. Brieu, J.J. Couderc, J.L. Baudour, E. Snoeck, A. Rousset, Solid State Ionics 109 (1998) 229e237. O.S. Aleksic, M.V. Nikolic, M.D. Lukovic, N. Nikolic, B.M. Radojcic, M. Radovanovic, Z. Djuric, M. Mitric, P.M. Nikolic, Mater. Sci. Eng. B Adv. 178 (2013) 202e210. K. Park, D.Y. Bang, J. Mater. Sci. Mater. Electron. 14 (2003) 81e87. S.G. Wang, A.M. Chang, H.M. Zhang, Q. Zhao, Mater. Chem. Phys. 110 (2008) 83e88. M.A. Gabal, Y.M. Al Angari, Mater. Chem. Phys. 115 (2009) 578e584. X.G. Liu, S.W. Or, C.G. Jin, Y.H. Lv, C. Feng, Y.P. Sun, Carbon 60 (2013) 215e220. X.Y. Wang, X.Y. Wang, L.H. Yi, L. Liu, Y.Z. Dai, H. Wu, J. Power Sources 224 (2013) 317e323. C. Mahendiran, T. Maiyalagan, K. Scott, A. Gedanken, Mater. Chem. Phys. 128 (2011) 341e347. F.A.A. Barros, A.J.R. Castro, J.M. Filho, B.C. Viana, A. Campos, A.C. Oliveira, J. Nanopart. Res. 14 (2012). B. Gao, C.Z. Yuan, L.H. Su, L. Chen, X.G. Zhang, J. Solid State Electrochem. 13 (2009) 1251e1257. A. Feteira, J. Am. Ceram. Soc. 92 (2009) 967e983.