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Synthesis of doped ZnO nanopowders in alcohol–water solvent for varistors applications
Materials Letters 121 (2014) 149–151
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis of doped ZnO nanopowders in alcohol–water solvent for varistors applications Mao-Hua Wang a,b, Xiao-Yu Ma a,b,n, Wen Jiang a,b, Fu Zhou a,b a b
School of Petrochemical Engineering, Changzhou University Changzhou, 213164, PR China Jiangsu Province Key Laboratory of Fine Petrochemical Engineering, Changzhou 213164, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 31 December 2013 Accepted 27 January 2014 Available online 3 February 2014
This work described the synthesis of the doped ZnO nanopowders employing alcohol–water mixture as solvent through a precipitation method. Several analytic techniques such as XRD, TEM, and SEM were used to make characterizations of the doped ZnO powders. The doped ZnO nanopowders were approximately spherical and monodisperse, the average diameter of the particles was about 30–50 nm. Additionally, the dense varistors were obtained according to the step-sintering process, with a nonlinear coefficient of about (27.57 0.4) and breakdown voltage (4087 5 V/mm). The experimental results showed the advantage of addition of the alcohol for controlling grain size, avoiding hard agglomeration and improving electrical performance of the varistors. Published by Elsevier B.V.
Keywords: ZnO Alcohol Adhension Ceramics Precipitation
1. Introduction ZnO varistors were polycrystalline electronic devices whose primary function was to sense and limit transient voltage surges [1–3]. Varistor manufacturing was usually based on the conventional solidstate route, where 493 mol% ZnO powder was homogeneously mixed with several metal oxide additives such as Bi, Sb, Co, Mn and Al and converted to free-flow granules by spray drying [4]. ZnO varistors were the most widely used in several applications from small current electronic circuits to large current transmission lines because of their high non-ohmic behavior in voltage–current characteristics [5]. The conventional method of making of ZnO varistors involves mixing of ZnO powder with various additives, in the past few year, various chemical methods have been widely applied to synthesize doped nanocrystalline ZnO powders [6–12]. Among these methods, precipitation was commonly used to synthesize doped ZnO nanopowders, but the powders obtained by this process were often heavily aggregated. In order to solve this problem, some additives were employed to control the agglomeration of the obtained powers. But the residual additives in the precursor needed higher decomposition temperature which would weaken the sinterability and dispersion of the obtained powders [13]. In this work, doped nanocrystalline ZnO powders for varistors applications were synthesized in alcohol–water solvent at room
n Corresponding author at: School of Petrochemical Engineering, Changzhou University Changzhou, 213164, PR China. Tel.: þ86 519 86330263. E-mail address: [email protected] (X.-Y. Ma).
0167-577X/$ - see front matter Published by Elsevier B.V. http://dx.doi.org/10.1016/j.matlet.2014.01.161
temperature, the alcohol was used as both the dispersant and solvent, replacing other additives [14]. The objective of the study was to improve the dispersity in the synthesis of doped nanocrystalline ZnO powders. Meanwhile, a novel approach of step-sintering was applied in this research to develop high-performance varistors. 2. Experimental Sample preparation: All reagents used in experiments were analytical grade and without further purification. Zinc nitrate hexahydrate (Zn(NO3)2 6H2O) and cobalt nitrate hexahydrate (Co (NO3)2 6H2O) were dissolved in distilled water. Since bismuth nitrate undergoes rapid hydrolysis, it was dissolved in concentrated HNO3 (2 mL). A certain amount of alcohol was added into water, which was used as both dispersant and solvent. Ammonium hydrogen carbonate was used as precipitant and dripped into 0.4 M metal ion solution at a rate of 1 mL min 1 under agitation at room temperature. During hydrolysis, the acidified bismuth solution was also added dropwise at a very slow rate. Then the precipitate was continually stirred for 18 h and washed with alcohol–water solvent for several times. The excess moisture present in the powders was removed by drying at 70 1C for 12 h. The synthesized powders were calcined at 600 1C for 1 h. For comparison, we also prepared the precursor in distilled water without addition of alcohol. The doped ZnO nanopowders were compacted into discs of diameter 12 mm and thickness 2.5 mm at a pressure of 45 MPa by uniaxial pressing. The green discs were sintered by a stepsintering method which was performed as follows. Firstly, the sintering temperature was raised to 1050 1C at a comparatively
150
M.-H. Wang et al. / Materials Letters 121 (2014) 149–151
high heating rate (10 1C min 1) and the samples were soaked for 30 min at 1050 1C. Subsequently, the temperature was minimized to 950 1C with the cooling rate of 4 1C min 1, the samples were kept for 2 h, and then furnace-cooled. Silver paste was printed on both sides of the sintered samples, and then calcined at 600 1C in air for 10 min in preparation for the experiment to determine its electrical characteristics. Sample characterization: The doped ZnO powders were determined by X-ray diffraction (XRD; Rigaku D/MAX-YA) using CuKa radiation, λ ¼ 0.154 nm, scans were performed from (2θ) 201 to 601 by rate 5 1/min. Particle morphology was investigated by transmission electron microscope (TEM; HITACHI H-600, Japan), and the microstructures of the as-sintered ceramic samples were characterized using scanning electron microscope (SEM; HITACHI S-570, Japan). Densities of sintered discs were measured by Archimedes method. The electrical (I–V) properties of the varistors were measured by DC high voltage tester (CJ1001, China).
101
▲ ▼
Intensity(a.u)
100 002
600-1h
110
102
▲ ▲
ß - Bi2 O3
- a - Bi2 O3
▲ ▲
▲▲
500-1h ▼
▼
▼
▼
400-1h
20
30
40
50
60
2 θ Degree Fig. 1. XRD patterns of doped ZnO nanopowders synthesized in alcohol–water solvent calcined at 400, 500 and 600 1C for 1 h.
3. Results and discussion Fig. 1 showed the multi-plot XRD patterns of powders synthesized in alcohol–water solvent calcined at different temperature for 1 h. The peaks showed the phase composition of wurtzite ZnO, which could be assigned to (100), (002), (101), (102) and (110) planes and were in good agreement with the standard JCPDS Card of ZnO (no. 36-1451). In case of the sample 500-1 h, there were extra peaks in XRD patterns, which were similar to the standard patterns reported for the α-Bi2O3 (JCPDS no. 71-2274). And in case of the sample 600-1 h, extra peaks corresponding to the β-Bi2O3 (JCPDS no. 78-1793) was observed. It was known that β-Bi2O3 was often reported as being present at room temperature in varistors, forming at the multiple grain junctions on cooling after sintering. The metastable β-Bi2O3 usually existed at high temperatures, transforming to the stable monoclinic α form on cooling to ambient temperatures. In this research, the presence of β-Bi2O3 might be due to non-equilibrium cooling rates, allowing it to be quenched to room temperature [15]. Fig. 2 presented the TEM images of the as-synthesized ZnO powders synthesized in different solvent calcained at 600 1C for 1 h. It was obvious that the products showed hard agglomeration in the absence of alcohol, with diameter ranging from 90 to 120 nm. However, it could be seen from Fig. 2b that the product consisted of well-dispersed spherical nanopaticles with a diameter of 30–50 nm. It was identified that the present of alcohol in the precipitation process could avoid hard agglomeration and decrease the size of the powders to form well-dispersed doped ZnO nanoparticles. To our knowledge, the dielectric constant drastically influences the kinetic of nucleation and growth of ZnO. Alcohol with weak dielectric constant favours the nucleation step (small crystallites), whereas water with high dielectric constant favours the growth step (large crystallites). And alcohol could lower the surface tension of the colloidal particle and avoid the particle aggregation occurring when the precipitated precursors were dried milled and calcined [16,17]. In a word, the alcohol on the surface
Fig. 2. TEM image of the doped ZnO nanopowders synthesized in (a) water solvent and (b) alcohol–water solvent calcined at 600 1C for 1 h.
Fig. 3. Microstructure of ZnO varistors using the powders synthesized in different solvent (a) water and (b) alcohol–water.
M.-H. Wang et al. / Materials Letters 121 (2014) 149–151
of ZnO nanoparticles played a significant role in preventing the grains from aggregating. The ZnO varistors were obtained according to the stepsintering process in static air [18–21]. The representative microstructure of the as-prepared ZnO varistor using the powders synthesized in different solvent and calcined at 600 1C obtained by step-sintering was presented in Fig. 3. As shown in Fig. 3a, some pores in the grain boundary could be observed and the average grain size of 8–12 μm was determined in this case. The sintered density of the sample was only 86.5% of its theoretical density. According to Fig. 3b, the average grain size of samples reduced to 3–5 μm in diameter, the samples attained 5.409 g/cm3 sintered density, corresponding to 96.5% of theoretical density of pure ZnO. To confirm the electric properties of the ZnO varistors made from the powders synthesized in alcohol–water solvent, correlative parameters were investigated. Experiment results demonstrated that it possessed good electrical characteristics, with nonlinear coefficient (27.570.4), breakdown voltage (40875 V/mm), and leakage current (1.0570.06 μA), the value of breakdown voltage was higher than that of varistors made from the powders synthesized in water solvent [22]. This enhancement in breakdown voltage might be due to better densification of the present samples, which resulted from better ZnO matrix grain to grain contact, better homogeneous distribution of dopants and low grain boundary thickness [23,24]. A detailed understanding of the as-prepared ZnO varistor's electronic properties requires further investigation, which is being undertaken. 4. Conclusions In summary, we had synthesized doped ZnO nanopowders employing alcohol–water mixture as solvent, through a normal precipitation method. The doped ZnO nanopowders were spherical and monodisperse, with diameters ranging from 30 nm to 50 nm. The as-prepared varistors possessed good electrical characteristics, with
151
nonlinear coefficient (27.570.4), breakdown voltage (40875 V/mm) and leakage current (1.0570.06 μA). The research showed that the alcohol played an important role in preventing the ZnO nanoparticles from aggregating. The synthetic procedure is suitable for large-scale fabrication and the alcohol–water solvent could also be recycled, showing potential application in industry.
Acknowledgements This work was supported by the Changzhou Science and Technology Innovation Project (CC20120031, CC20110048) and 2013 Research and Innovation Project for College Graduates. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
Wang Jinmin, Gao Lian. J Mater Chem 2003;51:129–35. Gupta TKJ. Am Ceram Soc 1990;73:1817. Clarke DRJ. Am Ceram Soc 1999;82:485. Hembram K, Sivaprahasam D, Rao. TN. J Eur Ceram Soc 2011;31:1905. Farrauto RJ, Heck RM. Catal Today 2000;55:179–83. Hembram K, Vijay RJ. Nanosci Nanotechnol 2009;9:76–82. Shi J, Yunge QC, Huang WY. Mater Sci Eng B 2003;99:344. Hwang CC, Wu TY. Mater Sci Eng B 2004;111:49. Chen JF, Shen. ZG. Scr Mater 2003;49:509–14. Cui B, Wang. X. Adv Funct Mater 2007;28:1–5. Woudenberg FCM, Sager. WFC. Thin Solid Films 2005;471:134–9. Xu Jinbao, Zhai Jiwei, Yao Xi. J Alloy Compd 2009;467:567–71. Ikegami T, Li JG, Moriyoshi. Y. J Am Ceram Soc 2002;85:1725. Poothayala Mukundan, Shuklaa Sanjeev K, Ananthakumara S. Acta Mater 2007;55:5792. Cui Bin, Yu Pengfei, Wang Xue. Mater Chem Phys 2008;459:589–93. Rege SU, Yang. RT. Chem Eng Sci 2001;56:3781. Baudrin Emmanuel, Delobel Bruno. Solid State Ion 2010;181:623–30. Shi Jianzhang, Cao Quanxi, Wei Yunge. Mater Sci Eng B 2003;111:344. Banerjee Arundhati, Ramamohan TR, Patni MJ. Mater Res Bull 2001;33:1259. Cao Yongge, Huang Zhi, Tang Fei. Mater Chem Phys 2011;128:44–9. Pillai SC, Kelly JMJ. Mater Chem 2003;13:2586. Wu J, Xie C, Bai. Z. Mater Sci Eng 2002;B95:157. Straumal Boris B, Mazilkin Andrei A. Philos Mag 2013;93:1371–83. Straumal BB, Protasova SG. JEPT Lett 2013;97:367–677.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis of doped ZnO nanopowders in alcohol–water solvent for varistors applications Mao-Hua Wang a,b, Xiao-Yu Ma a,b,n, Wen Jiang a,b, Fu Zhou a,b a b
School of Petrochemical Engineering, Changzhou University Changzhou, 213164, PR China Jiangsu Province Key Laboratory of Fine Petrochemical Engineering, Changzhou 213164, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 31 December 2013 Accepted 27 January 2014 Available online 3 February 2014
This work described the synthesis of the doped ZnO nanopowders employing alcohol–water mixture as solvent through a precipitation method. Several analytic techniques such as XRD, TEM, and SEM were used to make characterizations of the doped ZnO powders. The doped ZnO nanopowders were approximately spherical and monodisperse, the average diameter of the particles was about 30–50 nm. Additionally, the dense varistors were obtained according to the step-sintering process, with a nonlinear coefficient of about (27.57 0.4) and breakdown voltage (4087 5 V/mm). The experimental results showed the advantage of addition of the alcohol for controlling grain size, avoiding hard agglomeration and improving electrical performance of the varistors. Published by Elsevier B.V.
Keywords: ZnO Alcohol Adhension Ceramics Precipitation
1. Introduction ZnO varistors were polycrystalline electronic devices whose primary function was to sense and limit transient voltage surges [1–3]. Varistor manufacturing was usually based on the conventional solidstate route, where 493 mol% ZnO powder was homogeneously mixed with several metal oxide additives such as Bi, Sb, Co, Mn and Al and converted to free-flow granules by spray drying [4]. ZnO varistors were the most widely used in several applications from small current electronic circuits to large current transmission lines because of their high non-ohmic behavior in voltage–current characteristics [5]. The conventional method of making of ZnO varistors involves mixing of ZnO powder with various additives, in the past few year, various chemical methods have been widely applied to synthesize doped nanocrystalline ZnO powders [6–12]. Among these methods, precipitation was commonly used to synthesize doped ZnO nanopowders, but the powders obtained by this process were often heavily aggregated. In order to solve this problem, some additives were employed to control the agglomeration of the obtained powers. But the residual additives in the precursor needed higher decomposition temperature which would weaken the sinterability and dispersion of the obtained powders [13]. In this work, doped nanocrystalline ZnO powders for varistors applications were synthesized in alcohol–water solvent at room
n Corresponding author at: School of Petrochemical Engineering, Changzhou University Changzhou, 213164, PR China. Tel.: þ86 519 86330263. E-mail address: [email protected] (X.-Y. Ma).
0167-577X/$ - see front matter Published by Elsevier B.V. http://dx.doi.org/10.1016/j.matlet.2014.01.161
temperature, the alcohol was used as both the dispersant and solvent, replacing other additives [14]. The objective of the study was to improve the dispersity in the synthesis of doped nanocrystalline ZnO powders. Meanwhile, a novel approach of step-sintering was applied in this research to develop high-performance varistors. 2. Experimental Sample preparation: All reagents used in experiments were analytical grade and without further purification. Zinc nitrate hexahydrate (Zn(NO3)2 6H2O) and cobalt nitrate hexahydrate (Co (NO3)2 6H2O) were dissolved in distilled water. Since bismuth nitrate undergoes rapid hydrolysis, it was dissolved in concentrated HNO3 (2 mL). A certain amount of alcohol was added into water, which was used as both dispersant and solvent. Ammonium hydrogen carbonate was used as precipitant and dripped into 0.4 M metal ion solution at a rate of 1 mL min 1 under agitation at room temperature. During hydrolysis, the acidified bismuth solution was also added dropwise at a very slow rate. Then the precipitate was continually stirred for 18 h and washed with alcohol–water solvent for several times. The excess moisture present in the powders was removed by drying at 70 1C for 12 h. The synthesized powders were calcined at 600 1C for 1 h. For comparison, we also prepared the precursor in distilled water without addition of alcohol. The doped ZnO nanopowders were compacted into discs of diameter 12 mm and thickness 2.5 mm at a pressure of 45 MPa by uniaxial pressing. The green discs were sintered by a stepsintering method which was performed as follows. Firstly, the sintering temperature was raised to 1050 1C at a comparatively
150
M.-H. Wang et al. / Materials Letters 121 (2014) 149–151
high heating rate (10 1C min 1) and the samples were soaked for 30 min at 1050 1C. Subsequently, the temperature was minimized to 950 1C with the cooling rate of 4 1C min 1, the samples were kept for 2 h, and then furnace-cooled. Silver paste was printed on both sides of the sintered samples, and then calcined at 600 1C in air for 10 min in preparation for the experiment to determine its electrical characteristics. Sample characterization: The doped ZnO powders were determined by X-ray diffraction (XRD; Rigaku D/MAX-YA) using CuKa radiation, λ ¼ 0.154 nm, scans were performed from (2θ) 201 to 601 by rate 5 1/min. Particle morphology was investigated by transmission electron microscope (TEM; HITACHI H-600, Japan), and the microstructures of the as-sintered ceramic samples were characterized using scanning electron microscope (SEM; HITACHI S-570, Japan). Densities of sintered discs were measured by Archimedes method. The electrical (I–V) properties of the varistors were measured by DC high voltage tester (CJ1001, China).
101
▲ ▼
Intensity(a.u)
100 002
600-1h
110
102
▲ ▲
ß - Bi2 O3
- a - Bi2 O3
▲ ▲
▲▲
500-1h ▼
▼
▼
▼
400-1h
20
30
40
50
60
2 θ Degree Fig. 1. XRD patterns of doped ZnO nanopowders synthesized in alcohol–water solvent calcined at 400, 500 and 600 1C for 1 h.
3. Results and discussion Fig. 1 showed the multi-plot XRD patterns of powders synthesized in alcohol–water solvent calcined at different temperature for 1 h. The peaks showed the phase composition of wurtzite ZnO, which could be assigned to (100), (002), (101), (102) and (110) planes and were in good agreement with the standard JCPDS Card of ZnO (no. 36-1451). In case of the sample 500-1 h, there were extra peaks in XRD patterns, which were similar to the standard patterns reported for the α-Bi2O3 (JCPDS no. 71-2274). And in case of the sample 600-1 h, extra peaks corresponding to the β-Bi2O3 (JCPDS no. 78-1793) was observed. It was known that β-Bi2O3 was often reported as being present at room temperature in varistors, forming at the multiple grain junctions on cooling after sintering. The metastable β-Bi2O3 usually existed at high temperatures, transforming to the stable monoclinic α form on cooling to ambient temperatures. In this research, the presence of β-Bi2O3 might be due to non-equilibrium cooling rates, allowing it to be quenched to room temperature [15]. Fig. 2 presented the TEM images of the as-synthesized ZnO powders synthesized in different solvent calcained at 600 1C for 1 h. It was obvious that the products showed hard agglomeration in the absence of alcohol, with diameter ranging from 90 to 120 nm. However, it could be seen from Fig. 2b that the product consisted of well-dispersed spherical nanopaticles with a diameter of 30–50 nm. It was identified that the present of alcohol in the precipitation process could avoid hard agglomeration and decrease the size of the powders to form well-dispersed doped ZnO nanoparticles. To our knowledge, the dielectric constant drastically influences the kinetic of nucleation and growth of ZnO. Alcohol with weak dielectric constant favours the nucleation step (small crystallites), whereas water with high dielectric constant favours the growth step (large crystallites). And alcohol could lower the surface tension of the colloidal particle and avoid the particle aggregation occurring when the precipitated precursors were dried milled and calcined [16,17]. In a word, the alcohol on the surface
Fig. 2. TEM image of the doped ZnO nanopowders synthesized in (a) water solvent and (b) alcohol–water solvent calcined at 600 1C for 1 h.
Fig. 3. Microstructure of ZnO varistors using the powders synthesized in different solvent (a) water and (b) alcohol–water.
M.-H. Wang et al. / Materials Letters 121 (2014) 149–151
of ZnO nanoparticles played a significant role in preventing the grains from aggregating. The ZnO varistors were obtained according to the stepsintering process in static air [18–21]. The representative microstructure of the as-prepared ZnO varistor using the powders synthesized in different solvent and calcined at 600 1C obtained by step-sintering was presented in Fig. 3. As shown in Fig. 3a, some pores in the grain boundary could be observed and the average grain size of 8–12 μm was determined in this case. The sintered density of the sample was only 86.5% of its theoretical density. According to Fig. 3b, the average grain size of samples reduced to 3–5 μm in diameter, the samples attained 5.409 g/cm3 sintered density, corresponding to 96.5% of theoretical density of pure ZnO. To confirm the electric properties of the ZnO varistors made from the powders synthesized in alcohol–water solvent, correlative parameters were investigated. Experiment results demonstrated that it possessed good electrical characteristics, with nonlinear coefficient (27.570.4), breakdown voltage (40875 V/mm), and leakage current (1.0570.06 μA), the value of breakdown voltage was higher than that of varistors made from the powders synthesized in water solvent [22]. This enhancement in breakdown voltage might be due to better densification of the present samples, which resulted from better ZnO matrix grain to grain contact, better homogeneous distribution of dopants and low grain boundary thickness [23,24]. A detailed understanding of the as-prepared ZnO varistor's electronic properties requires further investigation, which is being undertaken. 4. Conclusions In summary, we had synthesized doped ZnO nanopowders employing alcohol–water mixture as solvent, through a normal precipitation method. The doped ZnO nanopowders were spherical and monodisperse, with diameters ranging from 30 nm to 50 nm. The as-prepared varistors possessed good electrical characteristics, with
151
nonlinear coefficient (27.570.4), breakdown voltage (40875 V/mm) and leakage current (1.0570.06 μA). The research showed that the alcohol played an important role in preventing the ZnO nanoparticles from aggregating. The synthetic procedure is suitable for large-scale fabrication and the alcohol–water solvent could also be recycled, showing potential application in industry.
Acknowledgements This work was supported by the Changzhou Science and Technology Innovation Project (CC20120031, CC20110048) and 2013 Research and Innovation Project for College Graduates. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
Wang Jinmin, Gao Lian. J Mater Chem 2003;51:129–35. Gupta TKJ. Am Ceram Soc 1990;73:1817. Clarke DRJ. Am Ceram Soc 1999;82:485. Hembram K, Sivaprahasam D, Rao. TN. J Eur Ceram Soc 2011;31:1905. Farrauto RJ, Heck RM. Catal Today 2000;55:179–83. Hembram K, Vijay RJ. Nanosci Nanotechnol 2009;9:76–82. Shi J, Yunge QC, Huang WY. Mater Sci Eng B 2003;99:344. Hwang CC, Wu TY. Mater Sci Eng B 2004;111:49. Chen JF, Shen. ZG. Scr Mater 2003;49:509–14. Cui B, Wang. X. Adv Funct Mater 2007;28:1–5. Woudenberg FCM, Sager. WFC. Thin Solid Films 2005;471:134–9. Xu Jinbao, Zhai Jiwei, Yao Xi. J Alloy Compd 2009;467:567–71. Ikegami T, Li JG, Moriyoshi. Y. J Am Ceram Soc 2002;85:1725. Poothayala Mukundan, Shuklaa Sanjeev K, Ananthakumara S. Acta Mater 2007;55:5792. Cui Bin, Yu Pengfei, Wang Xue. Mater Chem Phys 2008;459:589–93. Rege SU, Yang. RT. Chem Eng Sci 2001;56:3781. Baudrin Emmanuel, Delobel Bruno. Solid State Ion 2010;181:623–30. Shi Jianzhang, Cao Quanxi, Wei Yunge. Mater Sci Eng B 2003;111:344. Banerjee Arundhati, Ramamohan TR, Patni MJ. Mater Res Bull 2001;33:1259. Cao Yongge, Huang Zhi, Tang Fei. Mater Chem Phys 2011;128:44–9. Pillai SC, Kelly JMJ. Mater Chem 2003;13:2586. Wu J, Xie C, Bai. Z. Mater Sci Eng 2002;B95:157. Straumal Boris B, Mazilkin Andrei A. Philos Mag 2013;93:1371–83. Straumal BB, Protasova SG. JEPT Lett 2013;97:367–677.