- Email: [email protected]
Semi-continuous drying of RF gels with supercritical acetone
Letters to the Editor / Carbon 41 (2003) CO1–853 [8] Sandler J, Shaffer MSP, Prasse T, Bauhofer W, Schulte K, Windle AH. Development of dispersion process for carbon nanotubes in an epoxy matrix and the resulting electrical properties. Polymer 1999;40(21):5967–71. [9] Lourie O, Wagner HD. Evidence of stress transfer and formation of fracture clusters in carbon nanotube-based composites. Comp Sci Technol 1999;59(6):975–7. [10] Sun XK, Cong HT, Sun M, Yang MC. Preparation and mechanical properties of highly densified nanocrystalline Al. Metall Mater Trans A 2000;31(3A):1017–24.
851
[11] Liu C, Cong HT, Li F et al. Semi-continuous synthesis of single-walled carbon nanotubes by a hydrogen arc discharge method. Carbon 1999;37(11):1865–8. [12] Zhong R, Cong HT, Liu C. Fabrication of single-walled carbon nanotubes from multi-walled carbon nanotubes and carbon fibers. Carbon 2002;40(15):2970–3. [13] Hou PX, Liu C, Tong Y, Xu ST, Liu M, Cheng HM. Purification of single-walled carbon nanotubes synthesized by the hydrogen arc–discharge method. J Mater Res 2001;16(9):2526–9.
Semi-continuous drying of RF gels with supercritical acetone Guotong Qin,a , *, Wei Wei b , Shucai Guo b a
Department of Environmental Engineering, Beijing University of Aeronautics and Astronautics, 37 Xueyuan Road, Haidian District, Beijing 100083, China b Laboratory of Comprehensive Utilization for Carbonaceous Resources, Dalian University of Technology, 158 -90 Zhongshan Road, Dalian, China Received 30 September 2002; accepted 18 December 2002
Keywords: A. Carbon aerogel; C. Adsorption; Transmission electron microscopy (TEM); D. Microstructure
RF organic aerogels and their carbonized derivatives are new aerogels with characteristics different from inorganic oxide aerogels. RF aerogels and carbon aerogels are physiologically acceptable [1] and can be used as artificial organ components. Carbon aerogels can be used as electrodes owing to their conductivity [2,3]. Although these novel nano-porous materials have found wide application, their actual uses are limited because of their time-consuming production and high cost. Generally, RF aerogels are derived from a tedious procedure including aqueous sol– gel polymerization, solvent exchange, liquid carbon dioxide exchange and supercritical drying [4]. Carbon aerogels can be obtained by carbonization of RF aerogels. Aquogels with a nanoporous texture form during sol–gel polymerization. The aerogels can be obtained by removing the solvents in the pores of the aquogels in the absence of capillary forces, because even small capillary stress will cause a collapse of the gel skeleton. The removal is achieved by supercritical drying. Because water in the aquogels is not suitable for supercritical drying, solvent exchange is conducted. If acetone is used as the drying medium solvent exchange need 3–4 days. If carbon dioxide is used as the drying medium, the acetone in the pores of the gels must be exchanged with liquid carbon dioxide before supercritical drying. The carbon dioxide exchange needs a further 2–3 days. Therefore, most of the
*Corresponding author. Fax: 186-10-8231-6373. E-mail address: [email protected] (G. Qin,).
cost and a large fraction of the processing time of organic aerogels, their carbonized derivatives, and other carbon foams are associated with the drying of the polymeric gel precursors. In order to simplify the drying process, we previously reported intermittent supercritical drying with acetone [5] and an alcoholic sol–gel process [6]. In this paper, we successfully obtain the RF aerogels by a semicontinuous drying process. Solvent exchange and liquid carbon dioxide exchange are not required and the production procedure is effectively simplified. Resorcinol, formaldehyde and sodium carbonate are first dissolved with deionized distilled water. A typical gel formulation contains 0.29 M resorcinol, 0.58 M formaldehyde and 1.0–12.0 mM sodium carbonate as the catalyst. The solutions are decanted into 13 mm diameter3120 mm long glass ampoules. The ampules are sealed with a gas torch, and placed in a bath at 8561 8C for gelation and aging. After a 7-day cure, the RF gels are removed from the ampoules and immediately placed into an autoclave filled with acetone to minimize the evaporation of the water in the gel and to avoid the occurrence of gel cracking during pressure build-up. A specially designed stainless steel wire construction is used to keep the gel rod vertical in the center of the autoclave and to keep the complete rod surface in contact with the supercritical acetone. The acetone is continuously pumped into the autoclave and the pressure gradually increased to 6 MPa. The exit valve of the autoclave is then partially opened to ensure that the sequent heating process occurs under constant pressure. The autoclave is isobarically heated to
0008-6223 / 03 / $ – see front matter 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016 / S0008-6223(03)00016-2
Letters to the Editor / Carbon 41 (2003) CO1–853
852 Table 1 Characteristics of RF aerogels* No.
1 2 3 4
Concentration of catalyst (mM) 1.45 2.9 5.9 11.6
Density (g cm 23 )
Shrinkage (%, DV /V )
BET surface area (m 2 g 21 )
A
B
A
B
A
B
0.068 0.070 0.083 0.199
0.058 0.064 0.092 0.162
49 66 66 89
24 45 57 66
280 393 525 677
545 – – 718
*The data of RF aerogels from the present process are in column A. The data of RF aerogels from the supercritical CO 2 drying process are in column B.
250 8C at a heating rate of 1 8C min 21 . The gel is continuously flushed with the supercritical acetone at 250 8C and 6 MPa and after 1 h, the feed pump is turned off. The autoclave is isothermally decompressed to atmospheric pressure at a rate of 0.1 MPa min 21 . Then the autoclave is cooled to room temperature under an argon flow and the RF aerogels obtained. The general drying procedure only takes about 6–7 h. The resulting aerogels are monolithic, crack free and black in color. Table 1 shows the density, drying shrinkage and BET surface area of aerogels from the present process. These properties are compared with characteristics of aerogels from the intermittent supercritical carbon dioxide drying process. With an increase of catalyst concentration, the density, drying shrinkage and BET surface area increased. The density of the aerogel can be as low as 0.068 g cm 23 . Compared to the supercritical carbon dioxide drying process the drying shrinkage of gels is larger. As a result the density of aerogels by the present process is larger. The surface area is less than that. The results of transmission electron microscopic (TEM) analysis (Fig. 1) show that the RF aerogels by this new method have structural entities (i.e., cells, pores, and particles) smaller than 100 nm. The solid phase is composed of particles with diameter about 10 nm. The pore size is typically smaller than 50 nm. This microstructure is similar
to that of RF aerogels from the intermittent supercritical drying process with acetone or carbon dioxide as drying medium [4,5]. The nitrogen adsorption isotherms of aerogels are presented in Fig. 2. All the isotherms are type IV. This shows that the pores of these aerogels are typically mesopores. The isotherm for sample 1 possesses a small hysteresis loop compared with the others. The pore size distributions (Fig. 3) show that the pore diameter is typically smaller than 40 nm which agrees with the TEM results. The shrinkage of gels during drying is primarily caused by desorption of the solvent adsorbed in the pores of the gel during depressurizing [7]. The adsorbed supercritical acetone causes the gels to remain in a swollen state. Desorption of supercritical acetone during depressurizing reduce the adsorption force and results in compression. The adsorption stresses are strongly dependent on pore size. Since the aerogels prepared with a higher catalyst concentration have smaller pores, the adsorption stress is bigger. Therefore, the shrinkage increased as the catalyst concentration increase. All these aerogels are black in color. The acetone in the outlet of autoclave gives off a tar smell and is dark brown in color. These indicate that the gel was partially pyrolyzed during drying. The decomposition of RF gels results in the escape of small molecules from the network. This is similar to desorption of solvent.
Fig. 1. TEM of RF aerogel (sample No. 1; the line in the micrograph is 100 nm).
Fig. 2. Nitrogen adsorption isotherms on RF aerogels.
Letters to the Editor / Carbon 41 (2003) CO1–853
853
method, the process is effectively simplified. The resulting aerogels have a typical nano-structure.
Acknowledgements This work is conducted in Dalian University of Technology financially supported by the National Natural Science Foundation of PRC.
References
Fig. 3. Pore size distributions of RF aerogels.
Therefore, the shrinkage must be enhanced by the decomposition of RF gels. In addition, the further condensation of RF gels during partial pyrolysis may be another factor causing their shrinkage. Sample 1 shows a different isotherm and pore size distribution from the others. This may be associated with the low catalyst concentration used during sol–gel polymerization. With an extremely low catalyst concentration the gels shows low cross linkage and skeleton rigidity to resist the drying shrinkage. In conclusion, RF aerogels can be obtained by semicontinuous drying with supercritical acetone. By this
[1] Berg A, Droege MW, Fellmann JD, Klaveness J et al. WO 95 / 01165, 1995. [2] Wang J, Angnes L, Tobias H. Carbon aerogel composite electrodes. Anal Chem 1993;65:2300–3. [3] May ST, Pekala RW, Kaschmitter JL. The aerocapacitor: an electrochemical double-layer energy-storage device. J Electrochem Soc 1993;140:446–51. [4] Pekala RW. Organic aerogels from the polycondensation of resorcinol with formaldehyde. J Mater Sci 1989;24:3221–7. [5] Qin G, Guo S. Drying of RF gels with supercritical acetone. Carbon 1999;37:1168–9. [6] Qin G, Guo S. Preparation of RF organic aerogels and carbon aerogels by alcoholic sol–gel process. Carbon 2001;39:1935–7. [7] Rangarajan B, Lira CT. Interpretation of aerogel shrinkage during drying. Mater Res Soc Symp Proc 1992;271:559–66.
New structure of carbon nanofibers after high-temperature heat-treatment Guo-Bin Zheng*, Hideaki Sano, Yasuo Uchiyama Department of Materials Science and Engineering, Faculty of Engineering, Nagasaki University, Bunkyo machi 1 -14, Nagasaki 852 -8521, Japan Received 12 December 2002; accepted 18 December 2002 Keywords: A. Carbon fibers, Carbon nanotubes; B. Graphitization; D. Microstructure
In the last decade, carbon nanotubes have attracted great interest because of their unique properties [1–4]. Less attention has been paid to carbon nanofibers, which were developed before the discovery of carbon nanotubes. Baker [5] synthesized carbon nanofibers which were found to have three types of structures, platelet, herringbone and tubular. These nanofibers have been found to possess good *Corresponding author. Tel. / fax: 181-95-847-9773. E-mail address: [email protected] (G.-B. Zheng).
properties in hydrogen storage [6] and electrochemistry [7], but little work has been involved with their heattreatment. This letter reports structural changes in nanofibers after high-temperature heat-treatment. The synthesis of the carbon nanofibers was carried out in a horizontal alumina reactor with an inner diameter of 50 mm, and a length of 1100 mm. An alumina plate used as the substrate was first immersed in 0.2 M nickel nitrate / ethanol solution, and was removed and dried. Subsequently, it was placed in the reactor and reduced in hydrogen at
0008-6223 / 02 / $ – see front matter 2002 Elsevier Science Ltd. All rights reserved. doi:10.1016 / S0008-6223(02)00443-8
851
[11] Liu C, Cong HT, Li F et al. Semi-continuous synthesis of single-walled carbon nanotubes by a hydrogen arc discharge method. Carbon 1999;37(11):1865–8. [12] Zhong R, Cong HT, Liu C. Fabrication of single-walled carbon nanotubes from multi-walled carbon nanotubes and carbon fibers. Carbon 2002;40(15):2970–3. [13] Hou PX, Liu C, Tong Y, Xu ST, Liu M, Cheng HM. Purification of single-walled carbon nanotubes synthesized by the hydrogen arc–discharge method. J Mater Res 2001;16(9):2526–9.
Semi-continuous drying of RF gels with supercritical acetone Guotong Qin,a , *, Wei Wei b , Shucai Guo b a
Department of Environmental Engineering, Beijing University of Aeronautics and Astronautics, 37 Xueyuan Road, Haidian District, Beijing 100083, China b Laboratory of Comprehensive Utilization for Carbonaceous Resources, Dalian University of Technology, 158 -90 Zhongshan Road, Dalian, China Received 30 September 2002; accepted 18 December 2002
Keywords: A. Carbon aerogel; C. Adsorption; Transmission electron microscopy (TEM); D. Microstructure
RF organic aerogels and their carbonized derivatives are new aerogels with characteristics different from inorganic oxide aerogels. RF aerogels and carbon aerogels are physiologically acceptable [1] and can be used as artificial organ components. Carbon aerogels can be used as electrodes owing to their conductivity [2,3]. Although these novel nano-porous materials have found wide application, their actual uses are limited because of their time-consuming production and high cost. Generally, RF aerogels are derived from a tedious procedure including aqueous sol– gel polymerization, solvent exchange, liquid carbon dioxide exchange and supercritical drying [4]. Carbon aerogels can be obtained by carbonization of RF aerogels. Aquogels with a nanoporous texture form during sol–gel polymerization. The aerogels can be obtained by removing the solvents in the pores of the aquogels in the absence of capillary forces, because even small capillary stress will cause a collapse of the gel skeleton. The removal is achieved by supercritical drying. Because water in the aquogels is not suitable for supercritical drying, solvent exchange is conducted. If acetone is used as the drying medium solvent exchange need 3–4 days. If carbon dioxide is used as the drying medium, the acetone in the pores of the gels must be exchanged with liquid carbon dioxide before supercritical drying. The carbon dioxide exchange needs a further 2–3 days. Therefore, most of the
*Corresponding author. Fax: 186-10-8231-6373. E-mail address: [email protected] (G. Qin,).
cost and a large fraction of the processing time of organic aerogels, their carbonized derivatives, and other carbon foams are associated with the drying of the polymeric gel precursors. In order to simplify the drying process, we previously reported intermittent supercritical drying with acetone [5] and an alcoholic sol–gel process [6]. In this paper, we successfully obtain the RF aerogels by a semicontinuous drying process. Solvent exchange and liquid carbon dioxide exchange are not required and the production procedure is effectively simplified. Resorcinol, formaldehyde and sodium carbonate are first dissolved with deionized distilled water. A typical gel formulation contains 0.29 M resorcinol, 0.58 M formaldehyde and 1.0–12.0 mM sodium carbonate as the catalyst. The solutions are decanted into 13 mm diameter3120 mm long glass ampoules. The ampules are sealed with a gas torch, and placed in a bath at 8561 8C for gelation and aging. After a 7-day cure, the RF gels are removed from the ampoules and immediately placed into an autoclave filled with acetone to minimize the evaporation of the water in the gel and to avoid the occurrence of gel cracking during pressure build-up. A specially designed stainless steel wire construction is used to keep the gel rod vertical in the center of the autoclave and to keep the complete rod surface in contact with the supercritical acetone. The acetone is continuously pumped into the autoclave and the pressure gradually increased to 6 MPa. The exit valve of the autoclave is then partially opened to ensure that the sequent heating process occurs under constant pressure. The autoclave is isobarically heated to
0008-6223 / 03 / $ – see front matter 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016 / S0008-6223(03)00016-2
Letters to the Editor / Carbon 41 (2003) CO1–853
852 Table 1 Characteristics of RF aerogels* No.
1 2 3 4
Concentration of catalyst (mM) 1.45 2.9 5.9 11.6
Density (g cm 23 )
Shrinkage (%, DV /V )
BET surface area (m 2 g 21 )
A
B
A
B
A
B
0.068 0.070 0.083 0.199
0.058 0.064 0.092 0.162
49 66 66 89
24 45 57 66
280 393 525 677
545 – – 718
*The data of RF aerogels from the present process are in column A. The data of RF aerogels from the supercritical CO 2 drying process are in column B.
250 8C at a heating rate of 1 8C min 21 . The gel is continuously flushed with the supercritical acetone at 250 8C and 6 MPa and after 1 h, the feed pump is turned off. The autoclave is isothermally decompressed to atmospheric pressure at a rate of 0.1 MPa min 21 . Then the autoclave is cooled to room temperature under an argon flow and the RF aerogels obtained. The general drying procedure only takes about 6–7 h. The resulting aerogels are monolithic, crack free and black in color. Table 1 shows the density, drying shrinkage and BET surface area of aerogels from the present process. These properties are compared with characteristics of aerogels from the intermittent supercritical carbon dioxide drying process. With an increase of catalyst concentration, the density, drying shrinkage and BET surface area increased. The density of the aerogel can be as low as 0.068 g cm 23 . Compared to the supercritical carbon dioxide drying process the drying shrinkage of gels is larger. As a result the density of aerogels by the present process is larger. The surface area is less than that. The results of transmission electron microscopic (TEM) analysis (Fig. 1) show that the RF aerogels by this new method have structural entities (i.e., cells, pores, and particles) smaller than 100 nm. The solid phase is composed of particles with diameter about 10 nm. The pore size is typically smaller than 50 nm. This microstructure is similar
to that of RF aerogels from the intermittent supercritical drying process with acetone or carbon dioxide as drying medium [4,5]. The nitrogen adsorption isotherms of aerogels are presented in Fig. 2. All the isotherms are type IV. This shows that the pores of these aerogels are typically mesopores. The isotherm for sample 1 possesses a small hysteresis loop compared with the others. The pore size distributions (Fig. 3) show that the pore diameter is typically smaller than 40 nm which agrees with the TEM results. The shrinkage of gels during drying is primarily caused by desorption of the solvent adsorbed in the pores of the gel during depressurizing [7]. The adsorbed supercritical acetone causes the gels to remain in a swollen state. Desorption of supercritical acetone during depressurizing reduce the adsorption force and results in compression. The adsorption stresses are strongly dependent on pore size. Since the aerogels prepared with a higher catalyst concentration have smaller pores, the adsorption stress is bigger. Therefore, the shrinkage increased as the catalyst concentration increase. All these aerogels are black in color. The acetone in the outlet of autoclave gives off a tar smell and is dark brown in color. These indicate that the gel was partially pyrolyzed during drying. The decomposition of RF gels results in the escape of small molecules from the network. This is similar to desorption of solvent.
Fig. 1. TEM of RF aerogel (sample No. 1; the line in the micrograph is 100 nm).
Fig. 2. Nitrogen adsorption isotherms on RF aerogels.
Letters to the Editor / Carbon 41 (2003) CO1–853
853
method, the process is effectively simplified. The resulting aerogels have a typical nano-structure.
Acknowledgements This work is conducted in Dalian University of Technology financially supported by the National Natural Science Foundation of PRC.
References
Fig. 3. Pore size distributions of RF aerogels.
Therefore, the shrinkage must be enhanced by the decomposition of RF gels. In addition, the further condensation of RF gels during partial pyrolysis may be another factor causing their shrinkage. Sample 1 shows a different isotherm and pore size distribution from the others. This may be associated with the low catalyst concentration used during sol–gel polymerization. With an extremely low catalyst concentration the gels shows low cross linkage and skeleton rigidity to resist the drying shrinkage. In conclusion, RF aerogels can be obtained by semicontinuous drying with supercritical acetone. By this
[1] Berg A, Droege MW, Fellmann JD, Klaveness J et al. WO 95 / 01165, 1995. [2] Wang J, Angnes L, Tobias H. Carbon aerogel composite electrodes. Anal Chem 1993;65:2300–3. [3] May ST, Pekala RW, Kaschmitter JL. The aerocapacitor: an electrochemical double-layer energy-storage device. J Electrochem Soc 1993;140:446–51. [4] Pekala RW. Organic aerogels from the polycondensation of resorcinol with formaldehyde. J Mater Sci 1989;24:3221–7. [5] Qin G, Guo S. Drying of RF gels with supercritical acetone. Carbon 1999;37:1168–9. [6] Qin G, Guo S. Preparation of RF organic aerogels and carbon aerogels by alcoholic sol–gel process. Carbon 2001;39:1935–7. [7] Rangarajan B, Lira CT. Interpretation of aerogel shrinkage during drying. Mater Res Soc Symp Proc 1992;271:559–66.
New structure of carbon nanofibers after high-temperature heat-treatment Guo-Bin Zheng*, Hideaki Sano, Yasuo Uchiyama Department of Materials Science and Engineering, Faculty of Engineering, Nagasaki University, Bunkyo machi 1 -14, Nagasaki 852 -8521, Japan Received 12 December 2002; accepted 18 December 2002 Keywords: A. Carbon fibers, Carbon nanotubes; B. Graphitization; D. Microstructure
In the last decade, carbon nanotubes have attracted great interest because of their unique properties [1–4]. Less attention has been paid to carbon nanofibers, which were developed before the discovery of carbon nanotubes. Baker [5] synthesized carbon nanofibers which were found to have three types of structures, platelet, herringbone and tubular. These nanofibers have been found to possess good *Corresponding author. Tel. / fax: 181-95-847-9773. E-mail address: [email protected] (G.-B. Zheng).
properties in hydrogen storage [6] and electrochemistry [7], but little work has been involved with their heattreatment. This letter reports structural changes in nanofibers after high-temperature heat-treatment. The synthesis of the carbon nanofibers was carried out in a horizontal alumina reactor with an inner diameter of 50 mm, and a length of 1100 mm. An alumina plate used as the substrate was first immersed in 0.2 M nickel nitrate / ethanol solution, and was removed and dried. Subsequently, it was placed in the reactor and reduced in hydrogen at
0008-6223 / 02 / $ – see front matter 2002 Elsevier Science Ltd. All rights reserved. doi:10.1016 / S0008-6223(02)00443-8