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Performance improvement of micron-sized fibrous metal filters by direct growth of carbon nanotubes
Carbon 44 (2006) 1930–1935 www.elsevier.com/locate/carbon
Performance improvement of micron-sized fibrous metal filters by direct growth of carbon nanotubes Seok Joo Park a
a,*
, Dong Geun Lee
b
Clean Energy System Research Center, Korea Institute of Energy Research, 71-2, Jang-dong, Yuseong-gu, Daejeon 305-343, Republic of Korea b Department of Mechanical Engineering, Chungnam National University, 220 Gung-dong, Yuseong-gu, Daejeon 305-764, Republic of Korea Received 15 November 2005; accepted 4 February 2006 Available online 20 March 2006
Abstract Carbon nanotubes were synthesized directly by thermal chemical vapor deposition onto the surface of micron-sized metallic fibers to improve the filtration performance of a conventional metal filter. Depending on the synthesis conditions, carbon nanotubes grew up to consist of microstructures like bushes surrounding the metal fibers or like webs crossing between the fibers. The carbon nanotubes grown around the fibers collected more particulate pollutants, so that the filtration efficiency increased without significant increase of pressure drop. Especially, the filtration performance was improved when the CNTs formed the microstructures having the morphology such as naps or weeds on the micron-fibers. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Carbon nanotubes; Catalytically grown carbon; Chemical vapor deposition; Scanning electron microscopy; Microstructure
1. Introduction Carbon nanotubes (CNTs) with large aspect ratio and specific surface area are suggested as a new material that may be applied for hydrogen storage, gas absorption, filtration and separation [1]. Up to now, so many studies have not been trying to fabricate new filter media by using CNTs. Walters et al. [2] produced the first macroscopic object comprised of highly aligned CNTs. This object is a thin membrane prepared by filtering the suspension of CNTs in the magnetic field to produce an aligned membrane of CNTs. Shimoda et al. [3] reported the formation of a membrane composed of the aligned single-wall carbon nanotube (SWNT) bundles on a soaked glass substrate by self-assembly in the SWNTs/water dispersion with natural vaporization of water. Vander Wal and Hall [4] and Johnson et al. [5] synthesized directly CNTs upon a metal mesh screen. Their approach does not need the several processes, required when ready-made CNTs are coated onto sub*
Corresponding author. Tel.: +82 42 860 3649; fax: +82 42 860 3134. E-mail address: [email protected] (S.J. Park).
0008-6223/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2006.02.005
strates, such as the dispersion, deposition and binding of CNTs. Srivastava et al. [6] developed a CNT-filter that could remove micron- to nano-scale contaminants from water and heavy hydrocarbons from petroleum. The CNT-filter was made by spraying benzene and ferrocene mixture into a tube-shaped quartz mould at 900 °C. Because the CNT-filter is entirely composed of CNTs, however, its pressure drop is considerably high and its scale-up is limited. In this study, we synthesized directly CNTs on a micron-fibrous metal filter and observed the morphologies of microstructures composed of CNTs grown on the micron-fibers. Finally, we have tried to suggest a new application of CNTs to fabricate novel filter media by the direct growth of CNTs onto a conventional micronsized fibrous metal filter. By using a metal filter as a substrate for CNT-growth, a secondary process for the generation of the catalytic metal nanoparticles that should be formed upon the surface of fibers is not necessary because CNTs can be grown directly from the catalytic sites activated on the metallic surface by the reduction using hydrogen gas.
S.J. Park, D.G. Lee / Carbon 44 (2006) 1930–1935
2. Experimental The metal filter was mounted into a quartz tube of thermal chemical vapor deposition (CVD) apparatus operated at atmospheric pressure. The metal filter (BEKAERT, BEKIPORÒ ST 7CL4) made of stainless steel fibers was used as a substrate on which CNTs are grown directly. An electric furnace heated the quartz tube including the substrate with supplying argon gas. When the temperature inside the quartz tube reached to a predetermined condition, hydrogen gas was supplied along with the argon gas during a sufficient time to make catalytic sites activated on the surfaces of metal fibers. After finishing the reduction of metallic surface, acetylene gas as a source of CNTs was supplied additionally into the quartz tube with the argon and hydrogen gases for a predetermined time to synthesize CNTs. After finishing the synthesis, the quartz tube was cooled down to room temperature in argon ambient. The morphologies of CNTs grown onto the micron-filter were observed by scanning electron microscope (SEM; HITACHI S-4700). To investigate in detail the CNT structure, CNTs were separated from the synthesized filter by sonication in alcohol for 1 day, and then the colloid solution of CNTs was dropped on TEM grids for transmission electron microscope (TEM) analysis. Field emission transmission electron microscope (FE-TEM; FEI Technai G2 F30 S-TWIN) was used to investigate the structure and crystallinity of the CNTs. The pressure drop and filtration efficiency of the filters on which the CNTs were grown directly were measured by using a filter test unit consisted of an atomization aerosol generator (AAG; TSI 3079), a diffusion drier, a filter holder, a pressure difference gauge, a differential mobility analyzer (DMA; TSI 3081) and an ultra condensational particle counter (UCPC; TSI 3025A). Sodium chloride (NaCl) solid particles as test particles were generated by atomizing 0.2 M NaCl aqueous-solution from the AGG
1931
and drying the atomized droplets through the diffusion drier. The test particles were sampled at the inlet and outlet of the filter holder, and the concentrations and size distributions of test particles were measured by the DMA and the UCPC, and filtration efficiencies were calculated from the number concentrations along particle diameter measured at the upstream and downstream through the filter. At the end, we took SEM images to know how the CNTs capture the test particles. 3. Results and discussion The fiber diameter and pore size of the micron-fibrous metal filter were measured from SEM images. The metal filter has the porous structure of the nonwoven matrix composed of the stainless steel fibers of about 12 lm in diameter and the pores distributed from 10 to 50 lm. The metal fibers are sintered each other layer by layer without any binders. Fig. 1 shows the morphologies of the CNTs grown onto micron-fibers. As written in Table 1, the syntheses of CNTs were performed at different H2 gas flow rate of 200, 400 and 600 sccm when the flow rates of C2H2 and Ar gases were 10 sccm and 1 slm, the furnace temperature was 600 °C, and the synthesis time was 6 min. The CNTs do not fill up the original micro-pores of the raw filter, which are grown surrounding around micron-fibers or networking between them. Fig. 1(a) shows that the CNTs are synthesized like bushes covered around the fibers at H2 gas flow rate of 200 sccm in the case of ‘CNT-filter A’. Since the growth layer of CNTs is not so dense as shown in Fig. 1(a), it is expected that pollutant nanoparticles may reach into the nano-pores (or nano-gaps) formed among the CNTs and be captured easily by the CNTs. When the flow rate of H2 gas increased to 400 sccm such as the case of ‘CNT-filter B’, some CNTs were grown like webs crossing the micron-fibers as shown in Fig. 1(b). However, the
Fig. 1. SEM images of CNTs grown directly onto micron-fibers along the different flow rate of hydrogen gas such as the synthesis conditions of (a) CNTfilter A, (b) CNT-filter B, and (c) CNT-filter C.
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Table 1 Synthesis conditions for CNT-growth and the morphologies of the microstructure composed of CNTs on the metal filters Case CNT-filter CNT-filter CNT-filter CNT-filter CNT-filter CNT-filter CNT-filter a b
A B C D E F G
QC2 H2 a (sccm)
QH2 (sccm)
QArb (slm)
Ts (°C)
ts (min)
SEM image
Microstructure morphology
10 10 10 5 10 10 10
200 400 600 400 400 400 400
1 1 1 1 5 1 1
600 600 600 600 600 700 600
6 6 6 6 6 6 3
Fig. 1(a) Figs. 1(b) and 3(a) Fig. 1(c) Fig. 3(b) Fig. 3(c) Fig. 3(d) Fig. 3(e)
Bush-like Sufficient web-like Deficient web-like Bush-like with naps Fine dense bush-like Coarse sparse bush-like Bush-like with weeds
Sccm means standard cubic centimeter per minute. Slm means standard cubic liter per minute.
Fig. 2. TEM images of CNTs separated from the filters synthesized at the condition ‘CNT-filter B’.
number of CNTs grown like webs was reduced as the flow rate of H2 gas increased to 600 sccm in the case of ‘CNTfilter C’ as shown in Fig. 1(c). TEM images show the CNTs crystallized with multiwalled structures and hollows inside as shown in Fig. 2. The catalytic metal nanoparticles are distributed in the hollows of the CNTs, which means that these catalytic nanoparticles are detached from the catalytic sites activated on the surface of the metal fibers. Some catalytic particles are located at the tips of CNTs and others at the middle of the hollows. Fig. 2(b) shows the high-resolution TEM (HR-TEM) image, indicating the multiwalled graphene layer with good crystallinity of the CNT. Fig. 2 also shows that the diameter of CNTs is ranged from 20 to 50 nm. As written in Table 1, the syntheses of CNTs were performed as changing the other experimental conditions except for the flow rate of hydrogen gas from the case ‘CNT-filter B’ where the CNTs are grown satisfactorily to be the web-like microstructure. Fig. 3 shows SEM images of the filters produced at different synthesis conditions, compared with Fig. 3(a), which is the same with Fig. 1(b), obtained from the case ‘CNT-filter B’. Fig. 3(b) shows the SEM image obtained from the case ‘CNT-filter D’ where the flow rate of C2H2 gas only is changed to 5 sccm and other conditions are identical with the case ‘CNT-filter B’. Fig. 3(c) is obtained from ‘CNT-filter E’ where the flow rate of Ar gas only is changed to 5 slm, Fig. 3(d) from ‘CNT-filter F’ where the synthesis tempera-
ture only is changed to 700 °C, and Fig. 3(e) from ‘CNT-filter G’ where the synthesis time only is reduced to 3 min. As shown in Fig. 3(b)–(d), any web-like CNTs spanning the micron-fibers were not observed when the flow rate of C2H2 or Ar gas or the synthesis temperature was changed from the condition of the case ‘CNT-filter B’. Even so the states of CNT-growth on the fibers do not show the same morphologies. CNTs like naps are appeared on the layer of bush-like CNTs in Fig. 3(b). Fine CNTs are covered densely around the micron-fiber in Fig. 3(c) and relatively coarse CNTs are sparsely in Fig. 3(d). The length of web-like CNTs is remarkably decreased as the synthesis time decreased as shown in Fig. 3(e); however, some CNTs ready to grow webs are shown such as weeds standing out beyond the surface of micron-fibers. Consequently, the CNTs may be grown on micron-filters with various microstructures along synthesis conditions, as summarized in Table 1, and with the microstructure like webs at certain specific conditions as well. Fig. 4 shows the pressure drops of the raw metal filter and the metal filters on which the CNTs were grown at different synthesis conditions. The solid circle symbol shows the pressure drop of the raw filter before growing CNTs. The other open symbols describe the pressure drops of the filters that the CNTs were grown on and various microstructures were built up on. The pressure drops of the filters that web-like CNTs are formed on are much higher than that of the raw filter since the CNTs are grown across
S.J. Park, D.G. Lee / Carbon 44 (2006) 1930–1935
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Fig. 3. SEM images of CNTs grown directly onto micron-fibers at several conditions such as (a) CNT-filter B, (b) CNT-filter D, (c) CNT-filter E, (d) CNT-filter F, and (e) CNT-filter G.
30
1.0
Pressure drop (mmH2 O)
25
20
15
Filtration efficiency
CNT-filter B CNT-filter C CNT-filter D CNT-filter E CNT-filter F CNT-filter G Raw filter
10
0.9
0.8
CNT-filter B CNT-filter C CNT-filter D
5
CNT-filter E CNT-filter F CNT-filter G Raw filter
0.7 0
0 0
1
2
3
4
Face velocity (cm/s)
Fig. 4. Pressure drops versus filtration face velocities of the filters on which the CNTs are grown at different synthesis conditions.
the whole micro-pores distributed depth-wise through the metal filter. However, it is expected that the metal filter fabricated by web-like CNTs may be produced with lower pressure drop if a metal filter of thinner thickness or with larger micro-pores is used as a support. On the other hand, the filters with bush-like CNTs have lower pressure drops compared to the filters with web-like CNTs. Especially, the ‘CNT-filter D’ with the bush-like CNTs including the nap-likes, synthesized at the condition that the flow rate of carbon source gas is lower, does not have a significant difference in pressure drop, compared to the raw filter. This is because the growth of CNT-layer is weakened by the decrease of carbon source. Fig. 5 shows the filtration efficiencies of the raw filter and the filters synthesized at several conditions. The minimum filtration efficiency of the raw filter is low to 75% at
100
200
300
400
500
Particle diameter (nm) Fig. 5. Filtration efficiencies of the filters on which the CNTs are grown at different synthesis conditions.
130 nm in test particle diameter and 3 cm/s in filtration face velocity. However, the minimum efficiency of the filter increased to above 90% by growing directly CNTs on it and to 98% by the growth of sufficient web-like CNTs, especially. The filtration efficiencies of nanoparticles below 200 nm in diameter were particularly improved for the filter with web-like CNTs. The reason why the filters with grown CNTs have higher efficiencies is because the CNTs grown onto the micronfilter act as the porous structure of the filter media holding a lot of particulate pollutants. Fig. 6 shows SEM images that NaCl test particles of cubic shape are captured by the CNTs grown with bush-like (see Fig. 6(a)) or web-like (see Fig. 6(b)) microstructures upon the micron-fibers. Nanoparticles are generally collected onto the fibers of a filter by the dominant filtration mechanisms such as the
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S.J. Park, D.G. Lee / Carbon 44 (2006) 1930–1935
Fig. 6. SEM images of NaCl particles collected onto the filter that the CNTs are grown with (a) bush-like and (b) web-like microstructures onto the micron-fibers.
Table 2 Summary of penetrations, pressure drops, and filter qualities of each filter for 130 nm in test particle diameter and 3 cm/s in face velocity Filter Raw filter CNT-filter CNT-filter CNT-filter CNT-filter CNT-filter CNT-filter
B C D E F G
SEM image
Penetration (%)
Pressure drop (mmH2O)
Filter quality (mmH2O)1
– Figs. 1(b) and 3(a) Fig. 1(c) Fig. 3(b) Fig. 3(c) Fig. 3(d) Fig. 3(e)
24.6 2.3 7.1 6.2 10.0 10.5 7.1
9.0 22.9 16.0 11.7 14.8 13.5 13.9
0.156 0.165 0.165 0.238 0.156 0.167 0.190
interception and Brownian diffusion [7,8]. Theses filtration mechanisms of the nanoparticles flowing around the micron-fibers on which the microstructures such as the bundles of CNTs are constructed are considered to be different from the mechanisms around the micron-fibers without any microstructures of CNTs. A filter has higher performance when it has lower pressure drop as well as higher filtration efficiency because its pressure drop is related directly with the energy expended for filtration. The performance of a filter can be evaluated by a measure called the quality factor [9] or filter quality qF [10] defined as Eq. (1) qF ¼
lnðP Þ Dp
quality is highest for ‘CNT-filter D’, secondly highest for ‘CNT-filter G’, and lowest for the raw filter. On the other hand, the filter quality is not so improved for ‘CNT-filter B’ on which CNTs were grown with web-like microstructures. From these results, the filter quality may be more improved when the CNTs would rather grow standing out beyond the surface of micron-fibers like naps or weeds than covering uniformly on the surface. On the other hand, the exceeding growth of CNTs like webs across the whole pores through a filter may increase the pressure drop of the filter and reduce the filter quality, consequently. 4. Conclusions
ð1Þ
where Dp is the pressure drop and P is the penetration meaning the fraction of entering particles that exit or penetrate the filter. The greater the value of qF the better the filter. The filter qualities of each filter were calculated by Eq. (1) and summarized in Table 2 for 130 nm in particle diameter (the diameter showing a minimum efficiency for the raw filter) and 3 cm/s in face velocity. The filter
Microstructures composed of CNTs were built upon a conventional micron-sized fibrous metal filter by the direct growth of CNTs. The CNTs were synthesized on the metal filter by thermal chemical vapor deposition from acetylene gas. The CNTs were grown up to form the microstructures of various morphologies such as bushes around the micron-fibers or webs across the fibers. The filtration efficiencies of the filters on which the CNTs were grown
S.J. Park, D.G. Lee / Carbon 44 (2006) 1930–1935
directly were measured and compared with the raw metal filter without CNTs, so that the metal filters fabricated by CNTs were produced with higher filtration efficiency without significant increase in pressure drop compared to the raw metal filter. The filter quality was improved as the CNTs were grown like naps or weeds standing out beyond the surfaces of micron-fibers. In conclusion, growing appropriately the microstructures composed of CNTs onto micron-fibrous filters can develop the novel filters with higher performance. References [1] Park C, Engel ES, Crowe A, Gilbert TR, Rodriguez NM. Use of carbon nanofibers in the removal of organic solvents from water. Langmuir 2000;16(21):8050–6. [2] Walters DA, Casavant MJ, Qin XC, Huffman CB, Boul PJ, Ericson LM, et al. In-plane-aligned membranes of carbon nanotubes. Chem Phys Lett 2001;338(1):14–20.
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[3] Shimoda H, Fleming L, Horton K, Zhou O. Formation of macroscopically ordered carbon nanotube membranes by self-assembly. Physica B 2002;323(1–4):135–6. [4] Vander Wal RL, Hall LJ. Carbon nanotube synthesis upon stainless steel meshes. Carbon 2003;41(4):659–72. [5] Johnson DF, Craft BJ, Jaffe SM. Adhered supported carbon nanotubes. J Nanopart Res 2001;3:63–71. [6] Srivastava A, Srivastava ON, Talapatra S, Vajtai R, Ajayan PM. Carbon nanotube filters. Nature Mater 2004;3(9):610–4. [7] Friedlander SK. Smoke, dust, and haze; fundamentals of aerosol dynamics. New York: Oxford University Press; 2000, p. 58–93. [8] Brown RC. Air filtration; an integrated approach to the theory and applications of fibrous filters. New York: Pergamon Press; 1993, p. 73–119. [9] Brown RC. Air filtration; an integrated approach to the theory and applications of fibrous filters. New York: Pergamon Press; 1993, p. 5–9. [10] Hinds WC. Aerosol technology; properties, behavior, and measurement of airborne particles. New York: John Wiley & Sons; 1982, p. 164–72.
Performance improvement of micron-sized fibrous metal filters by direct growth of carbon nanotubes Seok Joo Park a
a,*
, Dong Geun Lee
b
Clean Energy System Research Center, Korea Institute of Energy Research, 71-2, Jang-dong, Yuseong-gu, Daejeon 305-343, Republic of Korea b Department of Mechanical Engineering, Chungnam National University, 220 Gung-dong, Yuseong-gu, Daejeon 305-764, Republic of Korea Received 15 November 2005; accepted 4 February 2006 Available online 20 March 2006
Abstract Carbon nanotubes were synthesized directly by thermal chemical vapor deposition onto the surface of micron-sized metallic fibers to improve the filtration performance of a conventional metal filter. Depending on the synthesis conditions, carbon nanotubes grew up to consist of microstructures like bushes surrounding the metal fibers or like webs crossing between the fibers. The carbon nanotubes grown around the fibers collected more particulate pollutants, so that the filtration efficiency increased without significant increase of pressure drop. Especially, the filtration performance was improved when the CNTs formed the microstructures having the morphology such as naps or weeds on the micron-fibers. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Carbon nanotubes; Catalytically grown carbon; Chemical vapor deposition; Scanning electron microscopy; Microstructure
1. Introduction Carbon nanotubes (CNTs) with large aspect ratio and specific surface area are suggested as a new material that may be applied for hydrogen storage, gas absorption, filtration and separation [1]. Up to now, so many studies have not been trying to fabricate new filter media by using CNTs. Walters et al. [2] produced the first macroscopic object comprised of highly aligned CNTs. This object is a thin membrane prepared by filtering the suspension of CNTs in the magnetic field to produce an aligned membrane of CNTs. Shimoda et al. [3] reported the formation of a membrane composed of the aligned single-wall carbon nanotube (SWNT) bundles on a soaked glass substrate by self-assembly in the SWNTs/water dispersion with natural vaporization of water. Vander Wal and Hall [4] and Johnson et al. [5] synthesized directly CNTs upon a metal mesh screen. Their approach does not need the several processes, required when ready-made CNTs are coated onto sub*
Corresponding author. Tel.: +82 42 860 3649; fax: +82 42 860 3134. E-mail address: [email protected] (S.J. Park).
0008-6223/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2006.02.005
strates, such as the dispersion, deposition and binding of CNTs. Srivastava et al. [6] developed a CNT-filter that could remove micron- to nano-scale contaminants from water and heavy hydrocarbons from petroleum. The CNT-filter was made by spraying benzene and ferrocene mixture into a tube-shaped quartz mould at 900 °C. Because the CNT-filter is entirely composed of CNTs, however, its pressure drop is considerably high and its scale-up is limited. In this study, we synthesized directly CNTs on a micron-fibrous metal filter and observed the morphologies of microstructures composed of CNTs grown on the micron-fibers. Finally, we have tried to suggest a new application of CNTs to fabricate novel filter media by the direct growth of CNTs onto a conventional micronsized fibrous metal filter. By using a metal filter as a substrate for CNT-growth, a secondary process for the generation of the catalytic metal nanoparticles that should be formed upon the surface of fibers is not necessary because CNTs can be grown directly from the catalytic sites activated on the metallic surface by the reduction using hydrogen gas.
S.J. Park, D.G. Lee / Carbon 44 (2006) 1930–1935
2. Experimental The metal filter was mounted into a quartz tube of thermal chemical vapor deposition (CVD) apparatus operated at atmospheric pressure. The metal filter (BEKAERT, BEKIPORÒ ST 7CL4) made of stainless steel fibers was used as a substrate on which CNTs are grown directly. An electric furnace heated the quartz tube including the substrate with supplying argon gas. When the temperature inside the quartz tube reached to a predetermined condition, hydrogen gas was supplied along with the argon gas during a sufficient time to make catalytic sites activated on the surfaces of metal fibers. After finishing the reduction of metallic surface, acetylene gas as a source of CNTs was supplied additionally into the quartz tube with the argon and hydrogen gases for a predetermined time to synthesize CNTs. After finishing the synthesis, the quartz tube was cooled down to room temperature in argon ambient. The morphologies of CNTs grown onto the micron-filter were observed by scanning electron microscope (SEM; HITACHI S-4700). To investigate in detail the CNT structure, CNTs were separated from the synthesized filter by sonication in alcohol for 1 day, and then the colloid solution of CNTs was dropped on TEM grids for transmission electron microscope (TEM) analysis. Field emission transmission electron microscope (FE-TEM; FEI Technai G2 F30 S-TWIN) was used to investigate the structure and crystallinity of the CNTs. The pressure drop and filtration efficiency of the filters on which the CNTs were grown directly were measured by using a filter test unit consisted of an atomization aerosol generator (AAG; TSI 3079), a diffusion drier, a filter holder, a pressure difference gauge, a differential mobility analyzer (DMA; TSI 3081) and an ultra condensational particle counter (UCPC; TSI 3025A). Sodium chloride (NaCl) solid particles as test particles were generated by atomizing 0.2 M NaCl aqueous-solution from the AGG
1931
and drying the atomized droplets through the diffusion drier. The test particles were sampled at the inlet and outlet of the filter holder, and the concentrations and size distributions of test particles were measured by the DMA and the UCPC, and filtration efficiencies were calculated from the number concentrations along particle diameter measured at the upstream and downstream through the filter. At the end, we took SEM images to know how the CNTs capture the test particles. 3. Results and discussion The fiber diameter and pore size of the micron-fibrous metal filter were measured from SEM images. The metal filter has the porous structure of the nonwoven matrix composed of the stainless steel fibers of about 12 lm in diameter and the pores distributed from 10 to 50 lm. The metal fibers are sintered each other layer by layer without any binders. Fig. 1 shows the morphologies of the CNTs grown onto micron-fibers. As written in Table 1, the syntheses of CNTs were performed at different H2 gas flow rate of 200, 400 and 600 sccm when the flow rates of C2H2 and Ar gases were 10 sccm and 1 slm, the furnace temperature was 600 °C, and the synthesis time was 6 min. The CNTs do not fill up the original micro-pores of the raw filter, which are grown surrounding around micron-fibers or networking between them. Fig. 1(a) shows that the CNTs are synthesized like bushes covered around the fibers at H2 gas flow rate of 200 sccm in the case of ‘CNT-filter A’. Since the growth layer of CNTs is not so dense as shown in Fig. 1(a), it is expected that pollutant nanoparticles may reach into the nano-pores (or nano-gaps) formed among the CNTs and be captured easily by the CNTs. When the flow rate of H2 gas increased to 400 sccm such as the case of ‘CNT-filter B’, some CNTs were grown like webs crossing the micron-fibers as shown in Fig. 1(b). However, the
Fig. 1. SEM images of CNTs grown directly onto micron-fibers along the different flow rate of hydrogen gas such as the synthesis conditions of (a) CNTfilter A, (b) CNT-filter B, and (c) CNT-filter C.
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S.J. Park, D.G. Lee / Carbon 44 (2006) 1930–1935
Table 1 Synthesis conditions for CNT-growth and the morphologies of the microstructure composed of CNTs on the metal filters Case CNT-filter CNT-filter CNT-filter CNT-filter CNT-filter CNT-filter CNT-filter a b
A B C D E F G
QC2 H2 a (sccm)
QH2 (sccm)
QArb (slm)
Ts (°C)
ts (min)
SEM image
Microstructure morphology
10 10 10 5 10 10 10
200 400 600 400 400 400 400
1 1 1 1 5 1 1
600 600 600 600 600 700 600
6 6 6 6 6 6 3
Fig. 1(a) Figs. 1(b) and 3(a) Fig. 1(c) Fig. 3(b) Fig. 3(c) Fig. 3(d) Fig. 3(e)
Bush-like Sufficient web-like Deficient web-like Bush-like with naps Fine dense bush-like Coarse sparse bush-like Bush-like with weeds
Sccm means standard cubic centimeter per minute. Slm means standard cubic liter per minute.
Fig. 2. TEM images of CNTs separated from the filters synthesized at the condition ‘CNT-filter B’.
number of CNTs grown like webs was reduced as the flow rate of H2 gas increased to 600 sccm in the case of ‘CNTfilter C’ as shown in Fig. 1(c). TEM images show the CNTs crystallized with multiwalled structures and hollows inside as shown in Fig. 2. The catalytic metal nanoparticles are distributed in the hollows of the CNTs, which means that these catalytic nanoparticles are detached from the catalytic sites activated on the surface of the metal fibers. Some catalytic particles are located at the tips of CNTs and others at the middle of the hollows. Fig. 2(b) shows the high-resolution TEM (HR-TEM) image, indicating the multiwalled graphene layer with good crystallinity of the CNT. Fig. 2 also shows that the diameter of CNTs is ranged from 20 to 50 nm. As written in Table 1, the syntheses of CNTs were performed as changing the other experimental conditions except for the flow rate of hydrogen gas from the case ‘CNT-filter B’ where the CNTs are grown satisfactorily to be the web-like microstructure. Fig. 3 shows SEM images of the filters produced at different synthesis conditions, compared with Fig. 3(a), which is the same with Fig. 1(b), obtained from the case ‘CNT-filter B’. Fig. 3(b) shows the SEM image obtained from the case ‘CNT-filter D’ where the flow rate of C2H2 gas only is changed to 5 sccm and other conditions are identical with the case ‘CNT-filter B’. Fig. 3(c) is obtained from ‘CNT-filter E’ where the flow rate of Ar gas only is changed to 5 slm, Fig. 3(d) from ‘CNT-filter F’ where the synthesis tempera-
ture only is changed to 700 °C, and Fig. 3(e) from ‘CNT-filter G’ where the synthesis time only is reduced to 3 min. As shown in Fig. 3(b)–(d), any web-like CNTs spanning the micron-fibers were not observed when the flow rate of C2H2 or Ar gas or the synthesis temperature was changed from the condition of the case ‘CNT-filter B’. Even so the states of CNT-growth on the fibers do not show the same morphologies. CNTs like naps are appeared on the layer of bush-like CNTs in Fig. 3(b). Fine CNTs are covered densely around the micron-fiber in Fig. 3(c) and relatively coarse CNTs are sparsely in Fig. 3(d). The length of web-like CNTs is remarkably decreased as the synthesis time decreased as shown in Fig. 3(e); however, some CNTs ready to grow webs are shown such as weeds standing out beyond the surface of micron-fibers. Consequently, the CNTs may be grown on micron-filters with various microstructures along synthesis conditions, as summarized in Table 1, and with the microstructure like webs at certain specific conditions as well. Fig. 4 shows the pressure drops of the raw metal filter and the metal filters on which the CNTs were grown at different synthesis conditions. The solid circle symbol shows the pressure drop of the raw filter before growing CNTs. The other open symbols describe the pressure drops of the filters that the CNTs were grown on and various microstructures were built up on. The pressure drops of the filters that web-like CNTs are formed on are much higher than that of the raw filter since the CNTs are grown across
S.J. Park, D.G. Lee / Carbon 44 (2006) 1930–1935
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Fig. 3. SEM images of CNTs grown directly onto micron-fibers at several conditions such as (a) CNT-filter B, (b) CNT-filter D, (c) CNT-filter E, (d) CNT-filter F, and (e) CNT-filter G.
30
1.0
Pressure drop (mmH2 O)
25
20
15
Filtration efficiency
CNT-filter B CNT-filter C CNT-filter D CNT-filter E CNT-filter F CNT-filter G Raw filter
10
0.9
0.8
CNT-filter B CNT-filter C CNT-filter D
5
CNT-filter E CNT-filter F CNT-filter G Raw filter
0.7 0
0 0
1
2
3
4
Face velocity (cm/s)
Fig. 4. Pressure drops versus filtration face velocities of the filters on which the CNTs are grown at different synthesis conditions.
the whole micro-pores distributed depth-wise through the metal filter. However, it is expected that the metal filter fabricated by web-like CNTs may be produced with lower pressure drop if a metal filter of thinner thickness or with larger micro-pores is used as a support. On the other hand, the filters with bush-like CNTs have lower pressure drops compared to the filters with web-like CNTs. Especially, the ‘CNT-filter D’ with the bush-like CNTs including the nap-likes, synthesized at the condition that the flow rate of carbon source gas is lower, does not have a significant difference in pressure drop, compared to the raw filter. This is because the growth of CNT-layer is weakened by the decrease of carbon source. Fig. 5 shows the filtration efficiencies of the raw filter and the filters synthesized at several conditions. The minimum filtration efficiency of the raw filter is low to 75% at
100
200
300
400
500
Particle diameter (nm) Fig. 5. Filtration efficiencies of the filters on which the CNTs are grown at different synthesis conditions.
130 nm in test particle diameter and 3 cm/s in filtration face velocity. However, the minimum efficiency of the filter increased to above 90% by growing directly CNTs on it and to 98% by the growth of sufficient web-like CNTs, especially. The filtration efficiencies of nanoparticles below 200 nm in diameter were particularly improved for the filter with web-like CNTs. The reason why the filters with grown CNTs have higher efficiencies is because the CNTs grown onto the micronfilter act as the porous structure of the filter media holding a lot of particulate pollutants. Fig. 6 shows SEM images that NaCl test particles of cubic shape are captured by the CNTs grown with bush-like (see Fig. 6(a)) or web-like (see Fig. 6(b)) microstructures upon the micron-fibers. Nanoparticles are generally collected onto the fibers of a filter by the dominant filtration mechanisms such as the
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Fig. 6. SEM images of NaCl particles collected onto the filter that the CNTs are grown with (a) bush-like and (b) web-like microstructures onto the micron-fibers.
Table 2 Summary of penetrations, pressure drops, and filter qualities of each filter for 130 nm in test particle diameter and 3 cm/s in face velocity Filter Raw filter CNT-filter CNT-filter CNT-filter CNT-filter CNT-filter CNT-filter
B C D E F G
SEM image
Penetration (%)
Pressure drop (mmH2O)
Filter quality (mmH2O)1
– Figs. 1(b) and 3(a) Fig. 1(c) Fig. 3(b) Fig. 3(c) Fig. 3(d) Fig. 3(e)
24.6 2.3 7.1 6.2 10.0 10.5 7.1
9.0 22.9 16.0 11.7 14.8 13.5 13.9
0.156 0.165 0.165 0.238 0.156 0.167 0.190
interception and Brownian diffusion [7,8]. Theses filtration mechanisms of the nanoparticles flowing around the micron-fibers on which the microstructures such as the bundles of CNTs are constructed are considered to be different from the mechanisms around the micron-fibers without any microstructures of CNTs. A filter has higher performance when it has lower pressure drop as well as higher filtration efficiency because its pressure drop is related directly with the energy expended for filtration. The performance of a filter can be evaluated by a measure called the quality factor [9] or filter quality qF [10] defined as Eq. (1) qF ¼
lnðP Þ Dp
quality is highest for ‘CNT-filter D’, secondly highest for ‘CNT-filter G’, and lowest for the raw filter. On the other hand, the filter quality is not so improved for ‘CNT-filter B’ on which CNTs were grown with web-like microstructures. From these results, the filter quality may be more improved when the CNTs would rather grow standing out beyond the surface of micron-fibers like naps or weeds than covering uniformly on the surface. On the other hand, the exceeding growth of CNTs like webs across the whole pores through a filter may increase the pressure drop of the filter and reduce the filter quality, consequently. 4. Conclusions
ð1Þ
where Dp is the pressure drop and P is the penetration meaning the fraction of entering particles that exit or penetrate the filter. The greater the value of qF the better the filter. The filter qualities of each filter were calculated by Eq. (1) and summarized in Table 2 for 130 nm in particle diameter (the diameter showing a minimum efficiency for the raw filter) and 3 cm/s in face velocity. The filter
Microstructures composed of CNTs were built upon a conventional micron-sized fibrous metal filter by the direct growth of CNTs. The CNTs were synthesized on the metal filter by thermal chemical vapor deposition from acetylene gas. The CNTs were grown up to form the microstructures of various morphologies such as bushes around the micron-fibers or webs across the fibers. The filtration efficiencies of the filters on which the CNTs were grown
S.J. Park, D.G. Lee / Carbon 44 (2006) 1930–1935
directly were measured and compared with the raw metal filter without CNTs, so that the metal filters fabricated by CNTs were produced with higher filtration efficiency without significant increase in pressure drop compared to the raw metal filter. The filter quality was improved as the CNTs were grown like naps or weeds standing out beyond the surfaces of micron-fibers. In conclusion, growing appropriately the microstructures composed of CNTs onto micron-fibrous filters can develop the novel filters with higher performance. References [1] Park C, Engel ES, Crowe A, Gilbert TR, Rodriguez NM. Use of carbon nanofibers in the removal of organic solvents from water. Langmuir 2000;16(21):8050–6. [2] Walters DA, Casavant MJ, Qin XC, Huffman CB, Boul PJ, Ericson LM, et al. In-plane-aligned membranes of carbon nanotubes. Chem Phys Lett 2001;338(1):14–20.
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[3] Shimoda H, Fleming L, Horton K, Zhou O. Formation of macroscopically ordered carbon nanotube membranes by self-assembly. Physica B 2002;323(1–4):135–6. [4] Vander Wal RL, Hall LJ. Carbon nanotube synthesis upon stainless steel meshes. Carbon 2003;41(4):659–72. [5] Johnson DF, Craft BJ, Jaffe SM. Adhered supported carbon nanotubes. J Nanopart Res 2001;3:63–71. [6] Srivastava A, Srivastava ON, Talapatra S, Vajtai R, Ajayan PM. Carbon nanotube filters. Nature Mater 2004;3(9):610–4. [7] Friedlander SK. Smoke, dust, and haze; fundamentals of aerosol dynamics. New York: Oxford University Press; 2000, p. 58–93. [8] Brown RC. Air filtration; an integrated approach to the theory and applications of fibrous filters. New York: Pergamon Press; 1993, p. 73–119. [9] Brown RC. Air filtration; an integrated approach to the theory and applications of fibrous filters. New York: Pergamon Press; 1993, p. 5–9. [10] Hinds WC. Aerosol technology; properties, behavior, and measurement of airborne particles. New York: John Wiley & Sons; 1982, p. 164–72.