- Email: [email protected]
Effects of heat treatment conditions on the thermal properties of mesophase pitch-derived graphitic foams
Letters to the Editor / Carbon 42 (2004) 1849–1852 [5] Sung SL, Tsai SH, Tseng CH, Chiang FK, Liu XW, Shih HC. Well-aligned carbon nitride nanotubes synthesized in anodic alumina by electron cyclotron resonance chemical vapor deposition. Appl Phys Lett 1999;74(2):197–9. [6] Haruyama J, Takesue I, Sata Y. Coulomb blockade in a single tunnel junction directly connected to a multiwalled carbon nanotube. Appl Phys Lett 2000;77(18):2891–3. [7] Han CC, Lee JT, Chang H. Thermal annealing effects on structure and morphology of micrometer-sized carbon tubes. Chem Mater 2001;13:4180–6. [8] Sui YC, Acosta DR, Gonzalez-leon JA, Bermudez A, Feuchtwanger J, Cui BZ, et al. Structure, thermal stability, and deformation of multibranched carbon nanotubes synthesized by CVD in the AAO template. J Phys Chem B 2001;105:1523–7.
1849
[9] Kyotani TB, Pradhan K, Tomita A. Synthesis of carbon nanotube composites in nanochannels of an anodic aluminum oxide film. Bull Chem Soc Jpn 1999;72(9):1957–70. [10] Qin DH, Lu M, Li HL. Magnetic force microscopy of magnetic domain structure in highly ordered Co nanowire arrays. Chem Phys Lett 2001;350(1–2):51–6. [11] Sha J, Niu J, Ma X, Xu J, Zhang X, Yang Q, et al. Silicon nanotubes. Adv Mater 2002;14(17):1219–21. [12] Scokart PO, Rouxhet PG. Comparison of the acid-basenproperties of various of oxides and chemically treated oxides. J Colloid Interface Sci 1982;86(1):96–104. [13] Jing L, Martin M, Tom HL. Nanoscale electroless metal deposition in aligned carbon nanotubes. Chem Mater 1998; 10:1963–7.
Effects of heat treatment conditions on the thermal properties of mesophase pitch-derived graphitic foams James W. Klett *, April D. McMillan, Nidia C. Gallego, Timothy D. Burchell, Claudia A. Walls Metals and Ceramics Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831, USA Received 2 April 2003; accepted 20 January 2004 Available online 17 March 2004
Keywords: A. Mesophase pitch, porous carbon; B. Heat treatment; D. Thermal conductivity
Carbon foam was first developed by researchers in the late 1960s as reticulated vitreous (glassy) carbon foams [1,2]. More recently, Klett and coworkers [3–5] at the Oak Ridge National Laboratory reported the first graphitic foams with bulk thermal conductivities greater than 58 W/m K; thermal conductivities up to 180 W/m K have recently been reported [6–8]. By combining an open cellular structure with a thermal conductivity to weight ratio (k=q) greater than 200 (as compared to 45 for copper), the development of high conductivity graphitic foam presents a unique opportunity to radically change the approach to many heat transfer problems. For many graphitic materials, the heating rate during graphitization is critical to optimizing the material properties. Therefore, in this project we examine the influence of heating rate during graphitization on key properties of the foam, such as density, thermal diffusivity, and crystallinity. Several billets of foam (~15 cm ~10 cm ~2.5 cm) were produced from Mitsubishi AR mesophase pitch powder with a softening point of 235 C. The mesophase pitch was foamed under 1000 psi pressure in aluminum pans utilizing two heating rates during (3.5 and 10 C/ min foaming rate) [9]. The foaming step consisted of *
Corresponding author. Tel.: +1-8655-745220; fax: +1-8655768424. E-mail address: [email protected] (J.W. Klett). 0008-6223/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.carbon.2004.01.057
heating under vacuum to 250 C and soaking for 1 h, applying the foaming pressure, and then heating at the specified heating rate to 600 C, soaking for 1 h, and then cooling to room temperature (while simultaneously reducing pressure) at approximately 1.25 C/min. All billets were marked for their position in the furnace (i.e. bottom or top of furnace) and then carbonized to 1000 C under an atmospheric nitrogen purge at a heating rate of 0.2 C/min. The carbonized billets were then cut into 1.59 cm cubes in a regular pattern throughout the foam billets and the Euclidian density was measured for each cube (ASTM C559). The foam cubes were separated into four different groups for each initial foaming rate (3.5 and 10 C/min) and subsequently heated to a graphitization temperature of 2800 C in four separate runs with different heating rates (1, 5, 10, and 15 C/min) under an argon purge. Key material properties such as crystallographic structure, thermal diffusivity (measured by ASTM C714) and density were determined. Table 1 reports the density uniformity for the billets processed under different foaming rates and in different positions within the furnace. As can be seen, there is little effect on uniformity (smaller deviations between the maximum and minimum values) due to the heating rate during foaming. However, a significant effect on density uniformity with the position in the furnace was observed. While there appeared to be some effect on density uniformity due to the foaming rate, this effect
1850
Letters to the Editor / Carbon 42 (2004) 1849–1852
Table 1 Density uniformity of billets Foaming rate [C/min]
Bottom pana
Top pana 3
3.5 10
0.474 0.434
10.6 13.1
[%]
Average density [g/cm3 ]
Max deviationb [%]
0.448 0.430
3.7 3.8
Bottom pan was located at the bottom of the furnace and top pan was located at the top. Defined as 100 ðmaximum density minimum densityÞ=mean. 0.3366
Crystal Interlayer Spacing [d002-nm]
was small compared to the effect of the position of the pan in the furnace. It is known that a temperature gradient within the furnace exists as the exit path for offgasses is at the top of the furnace. Hence, buoyancy effects cause the gasses from the bottom pan to rise and pass the upper pans on the way out, thus increasing their temperature. Perhaps, like steaming food, the gasses rising through the furnace result in much more uniform temperatures within the billets at the top of the furnace. Since the billet at the bottom does not get this effect, it may still contain large temperature non-uniformities during the foaming process, thus results in non-uniform foam. Verification of this gradient is extremely difficult as site specific safety requirements will not allow retrofit of this furnace for multiple thermocouples. The interlayer spacing (d0 0 2 ), crystal stacking height (Lc;0 0 2 ), and the crystal coherence length (or crystallite size) (La;1 0 0 ) are indicators of crystalline perfection, and thus might be expected to improve with lower heating rates since the elimination of defects is temperature and time dependent. Fig. 1 shows the effect of heating rate on these parameters. It is seen that the d-spacing for all the samples (all heating rates) was between 0.3358 and 0.3368 nm and similar to that witnessed in previous studies with the foam [5]. However, there does not appear to be a correlation between either d0 0 2 or Lc with foaming rate, position in furnace, or graphitization rate. However, La;1 0 0 does show a strong correlation with foaming rate, position in furnace, and graphitization rate. The thermal properties of the samples were then measured in all three directions (z; y; x). The z-direction is the vertical orientation of the billet and corresponds to the foaming direction. It is important to note here the mechanisms of pore formation. During foaming, the mesophase pitch is molten and begins to decompose. The decomposition gasses rise to the surface (bubble percolation) where they coalesce into larger bubbles and eventually rupture, like boiling water. This boiling action causes some orientation in the mesophase crystals, resulting in a preferred orientation of the a-direction of the crystals in this bubbling direction (z-direction). However, the decomposition of the pitch causes polymerization and increases its viscosity, thus impeding the flow of the bubbles to the surface. As the balance between decomposition and viscosity increase reaches a temperature unique to each pitch, the bubbles no longer
0.3364
0.3362
0.3360
0.3358
3.5 ˚C/min Base 10 ˚C/min Base 10˚C/min Top 3.5 ˚C/min Top
0.3356 0
2
4
6
8
10
12
14
16
Heat Treatment Rate during Graphitization [ ˚C/min] 240
Crystal Stack Height [Lc,002-nm]
b
Max deviation
3.5 ˚C/min Base 10 ˚C/min Base 10˚C/min Top 3.5 ˚C/min Top
220 200 180 160 140 120 100 0
2
4
6
8
10
12
14
16
Heat Treatment Rate during Graphitization[˚C/min] 90 3.5 ˚C/min Base 10 ˚C/min Base 10 ˚C/min Top 3.5 ˚C/min Top
80
Crystallite Size [La,100-nm]
a
Average density [g/cm ]
b
70
60
50
40
30 0
2
4
6
8
10
12
14
16
Heat Treatment Rate during Graphitization[˚C/min]
Fig. 1. X-ray diffraction results: interlayer spacing, d0 0 2 ; stacking height, Lc ; coherence length, La .
flow to the surface, but rather simply form and enlarge, resulting in further alignment of the crystals along the cell walls due to biaxial extension. The foaming action is restrained by the container (and gravity), thus yielding a
Letters to the Editor / Carbon 42 (2004) 1849–1852
slight elongation of the bubbles in the z-direction. This elongation, along with the effect of the preferred orientation of the crystallites, results in a higher thermal conductivity in the z-direction than in the x–y direction. Fig. 2 shows the relationship of heating rate during graphitization on the x, y, and z thermal conductivities. From the plots, it is clear that the z-direction thermal conductivity is directly related to the graphitization
160
Thermal Conductivity [W/m·K]
140 120 100 80 60
Foaming Rateand 3.5 ˚C/min Bottom
40
10 ˚C/min Bottom 10 ˚C/min Top
20
3.5 ˚C/min Top
0 0
(a)
5
10
15
20
Heat Treatment Rate during Graphitization [˚C/min]
Thermal Conductivity [W/m·K]
60
50
40
30 Foaming Rate and
20
Position in Furnace 3.5 ˚C/min Bottom 10 ˚C/min Bottom
10
10 ˚C/min Top 3.5 ˚C/min Top
0 0
(b)
5
10
15
20
Heat Treatment Rate during Graphitization [˚C/min]
heating rate. As anticipated in the previous section, the higher graphitization rate affects the crystal structure, thus decreasing the thermal conductivity in the direction with the greatest preferred orientation (z-direction). However, it is unclear why the x-direction thermal conductivity increased with increasing graphitization rate. This phenomenon was not seen in the y-direction, and this was also only seen in the samples made at the bottom of the furnace, not the top. Fig. 3 shows the z-direction thermal conductivity versus coherence length, La , and, as expected, there is a general trend that as the coherence length increases, the thermal conductivity increases. Thus, processing parameters which affect the coherence length will directly affect the final thermal conductivity. However, it is unclear at this time why there does not appear to be a correlation between La and the x- and y thermal conductivities. Future studies will be performed to characterize this phenomena. The effect on the final thermal properties of the location in the foaming furnace is also noteworthy. It is clear that in the z-direction (the direction with the highest conductivity), both billets that were located in the top of the furnace exhibited the highest overall thermal conductivity, irrespective of foaming rate. Moreover, it is also evident that the foaming rate does affect the final thermal properties, with the lower foaming rates yielding graphitized foams exhibiting the higher thermal conductivities. While these trends are also true for the x- and y-directions, the effects are less noticeable than those in the z-direction. Several conclusions may be drawn from this study. The foams produced at the lower position in foaming furnace have a less uniform density than those foamed in the upper position. Moreover, the foaming rate of the precursor pitch is critical with samples foamed at lower heating rates. It was also observed that the graphitization rate was critical to the thermal properties and that higher heating rates tended to disrupt the crystal 160 Thermal Conductivity [W/m·K]
70
Thermal Conductivity [W/m·K]
60 50 40 30 Foaming Rate and Position in Furnace
20
3.5 ˚C/min Bottom 10 ˚C/min Bottom
10
10 ˚C/min Top
0
5
10
15
140 120 100 80 60 40 20 0
3.5 ˚C/min Top
0
0
(c)
1851
20
Heat Treatment Rate during Graphitization [˚C/min]
Fig. 2. Thermal conductivities in the (a) z-direction, (b) y-direction, (c) x-direction.
20
40
60
Crystallite Size [L
a,100
80
100
- nm]
Fig. 3. z-Direction thermal conductivity as a function of coherence length showing a strong dependence between the two parameters.
1852
Letters to the Editor / Carbon 42 (2004) 1849–1852
structure (specifically the coherence length) and hence decrease the thermal properties compared to slower heat rates. Finally, it was found that the position in the furnace, and the foaming rate, exhibited a larger effect on the thermal properties than the graphitization rate. Thus, it appears that temperature uniformities within the billet during foaming are the most critical aspect of the process. Because the foaming stage (when the pitch is fluid) is critical to the formation and orientation of the crystallites (similar to mesophase pitch fibers [10]), further studies should focus on the foaming stage to allow tailoring the foam properties to the final application.
References [1] Ford W. Method of making cellular refractory thermal insulating material. US Patent 3,121,050, 1964. [2] Googin J, Napier J, Scrivner M. Method for manufacturing foam carbon products. US Patent 3,345,440, 1967.
[3] Klett J. High thermal conductivity, mesophase pitch-derived carbon foam. Extended Abstracts, Proceedings of the 1998 43rd International SAMPE Symposium and Exhibition, Part 1 (of 2), Anaheim, California, USA, 1998. p. 745–55. [4] Klett J. Pitch-based carbon foam and composites. US Patent 6,261,485, 2001. [5] Klett J et al. High-thermal-conductivity, mesophase-pitch-derived carbon foams: effect of precursor on structure and properties. Carbon 2000;38(7):953–73. [6] Klett J, Klett L, Burchell T, Walls C. Graphitic foam thermal management materials for electronic packaging. Society of Automotive Engineers Technical Paper Series 2000 (00FCC-117). [7] Gallego N, Klett J, McMillan A. Effects of processing conditions on properties of graphite foams. Extended Abstracts, Proceedings of The International Conference on Carbon in 2002, Beijing, China, 2002. [8] Klett J, McMillan AD, Stinton D. Modeling geometric effects on heat transfer with graphite foam. Extended Abstracts, The 26th Annual Conference on Ceramic, Metal, and Carbon Composites, Materials, and Structures, Cocoa Beach, Florida, 2002. [9] Klett J. Process for making carbon foam. US Patent 6,033,506, 2000. [10] Edie DD, Stoner EG. The effect of microstructure and shape on carbon fiber properties. In: Buckley JD, editor. Carbon–carbon materials and composites. Noyes Publications.
Amorphous carbon nanotubes produced by a temperature controlled DC arc discharge Yongning Liu a
a,*
, Song Xiaolong a, Zhao Tingkai a, Zhu Jiewu a, Michael Hirscher b, Fritz Philipp b
Department of Materials Science and Engineering, State Key Laboratory for Mechanical Behaviour of Materials, Xi’an Jiaotong University, Xi’an 710049, China b Max Plank Institute for Metal Research, Heisenbergstrasse, D-70569 Stuttgart, Germany Received 12 July 2003; accepted 27 January 2004 Available online 16 March 2004
Keywords: A. Carbon nanotubes; B. Arc discharge; C. X-ray diffraction
Since their discovery by Iijima in 1991 [1], carbon nanotubes (CNTs) have been studied all over the world. They have been synthesized mainly by three methods: DC (direct current) arc discharge, chemical vapour deposition and laser ablation. Both single-walled carbon nanotubes (SWNTs) and multi-walled nanotubes (MWNTs) can be produced by arc discharge [2–4]. Generally, the temperature in the vacuum chamber is not controlled and the walls of the chamber are cooled by water. After arcing, the soot deposited on the chamber walls consists of useless metal catalyst and carbonaceous particles other than few CNTs. Most CNTs are produced in the center of the core deposited at *
Corresponding author. Tel.: +86-29-8266-9071; fax: +86-29-82663453. E-mail address: [email protected] (YN. Liu). 0008-6223/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2004.01.069
the head of the cathode and also around the anode rod [5]. Hence, this procedure has a very low yield, generally on the scale of mg. It is believed that the structure of both single- and multi-walled nanotubes is like that of a hollow graphite cylinder [6] and that this structure is crystalline. This theory has been proven by many TEM analyses. However, recent research has shown that carbon nanotubes can also have an amorphous structure when prepared by CVD [7–10]. In this paper we report that the amorphous carbon nanotubes can also be produced by DC arc charge. A schematic diagram of the DC arc discharge furnace with a temperature controlling system is shown in Fig. 1. In the vacuum chamber, there is an inner stainless steel chamber, 200 mm in diameter and 300 mm in length, around which is mounted an electric heating apparatus.
1849
[9] Kyotani TB, Pradhan K, Tomita A. Synthesis of carbon nanotube composites in nanochannels of an anodic aluminum oxide film. Bull Chem Soc Jpn 1999;72(9):1957–70. [10] Qin DH, Lu M, Li HL. Magnetic force microscopy of magnetic domain structure in highly ordered Co nanowire arrays. Chem Phys Lett 2001;350(1–2):51–6. [11] Sha J, Niu J, Ma X, Xu J, Zhang X, Yang Q, et al. Silicon nanotubes. Adv Mater 2002;14(17):1219–21. [12] Scokart PO, Rouxhet PG. Comparison of the acid-basenproperties of various of oxides and chemically treated oxides. J Colloid Interface Sci 1982;86(1):96–104. [13] Jing L, Martin M, Tom HL. Nanoscale electroless metal deposition in aligned carbon nanotubes. Chem Mater 1998; 10:1963–7.
Effects of heat treatment conditions on the thermal properties of mesophase pitch-derived graphitic foams James W. Klett *, April D. McMillan, Nidia C. Gallego, Timothy D. Burchell, Claudia A. Walls Metals and Ceramics Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831, USA Received 2 April 2003; accepted 20 January 2004 Available online 17 March 2004
Keywords: A. Mesophase pitch, porous carbon; B. Heat treatment; D. Thermal conductivity
Carbon foam was first developed by researchers in the late 1960s as reticulated vitreous (glassy) carbon foams [1,2]. More recently, Klett and coworkers [3–5] at the Oak Ridge National Laboratory reported the first graphitic foams with bulk thermal conductivities greater than 58 W/m K; thermal conductivities up to 180 W/m K have recently been reported [6–8]. By combining an open cellular structure with a thermal conductivity to weight ratio (k=q) greater than 200 (as compared to 45 for copper), the development of high conductivity graphitic foam presents a unique opportunity to radically change the approach to many heat transfer problems. For many graphitic materials, the heating rate during graphitization is critical to optimizing the material properties. Therefore, in this project we examine the influence of heating rate during graphitization on key properties of the foam, such as density, thermal diffusivity, and crystallinity. Several billets of foam (~15 cm ~10 cm ~2.5 cm) were produced from Mitsubishi AR mesophase pitch powder with a softening point of 235 C. The mesophase pitch was foamed under 1000 psi pressure in aluminum pans utilizing two heating rates during (3.5 and 10 C/ min foaming rate) [9]. The foaming step consisted of *
Corresponding author. Tel.: +1-8655-745220; fax: +1-8655768424. E-mail address: [email protected] (J.W. Klett). 0008-6223/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.carbon.2004.01.057
heating under vacuum to 250 C and soaking for 1 h, applying the foaming pressure, and then heating at the specified heating rate to 600 C, soaking for 1 h, and then cooling to room temperature (while simultaneously reducing pressure) at approximately 1.25 C/min. All billets were marked for their position in the furnace (i.e. bottom or top of furnace) and then carbonized to 1000 C under an atmospheric nitrogen purge at a heating rate of 0.2 C/min. The carbonized billets were then cut into 1.59 cm cubes in a regular pattern throughout the foam billets and the Euclidian density was measured for each cube (ASTM C559). The foam cubes were separated into four different groups for each initial foaming rate (3.5 and 10 C/min) and subsequently heated to a graphitization temperature of 2800 C in four separate runs with different heating rates (1, 5, 10, and 15 C/min) under an argon purge. Key material properties such as crystallographic structure, thermal diffusivity (measured by ASTM C714) and density were determined. Table 1 reports the density uniformity for the billets processed under different foaming rates and in different positions within the furnace. As can be seen, there is little effect on uniformity (smaller deviations between the maximum and minimum values) due to the heating rate during foaming. However, a significant effect on density uniformity with the position in the furnace was observed. While there appeared to be some effect on density uniformity due to the foaming rate, this effect
1850
Letters to the Editor / Carbon 42 (2004) 1849–1852
Table 1 Density uniformity of billets Foaming rate [C/min]
Bottom pana
Top pana 3
3.5 10
0.474 0.434
10.6 13.1
[%]
Average density [g/cm3 ]
Max deviationb [%]
0.448 0.430
3.7 3.8
Bottom pan was located at the bottom of the furnace and top pan was located at the top. Defined as 100 ðmaximum density minimum densityÞ=mean. 0.3366
Crystal Interlayer Spacing [d002-nm]
was small compared to the effect of the position of the pan in the furnace. It is known that a temperature gradient within the furnace exists as the exit path for offgasses is at the top of the furnace. Hence, buoyancy effects cause the gasses from the bottom pan to rise and pass the upper pans on the way out, thus increasing their temperature. Perhaps, like steaming food, the gasses rising through the furnace result in much more uniform temperatures within the billets at the top of the furnace. Since the billet at the bottom does not get this effect, it may still contain large temperature non-uniformities during the foaming process, thus results in non-uniform foam. Verification of this gradient is extremely difficult as site specific safety requirements will not allow retrofit of this furnace for multiple thermocouples. The interlayer spacing (d0 0 2 ), crystal stacking height (Lc;0 0 2 ), and the crystal coherence length (or crystallite size) (La;1 0 0 ) are indicators of crystalline perfection, and thus might be expected to improve with lower heating rates since the elimination of defects is temperature and time dependent. Fig. 1 shows the effect of heating rate on these parameters. It is seen that the d-spacing for all the samples (all heating rates) was between 0.3358 and 0.3368 nm and similar to that witnessed in previous studies with the foam [5]. However, there does not appear to be a correlation between either d0 0 2 or Lc with foaming rate, position in furnace, or graphitization rate. However, La;1 0 0 does show a strong correlation with foaming rate, position in furnace, and graphitization rate. The thermal properties of the samples were then measured in all three directions (z; y; x). The z-direction is the vertical orientation of the billet and corresponds to the foaming direction. It is important to note here the mechanisms of pore formation. During foaming, the mesophase pitch is molten and begins to decompose. The decomposition gasses rise to the surface (bubble percolation) where they coalesce into larger bubbles and eventually rupture, like boiling water. This boiling action causes some orientation in the mesophase crystals, resulting in a preferred orientation of the a-direction of the crystals in this bubbling direction (z-direction). However, the decomposition of the pitch causes polymerization and increases its viscosity, thus impeding the flow of the bubbles to the surface. As the balance between decomposition and viscosity increase reaches a temperature unique to each pitch, the bubbles no longer
0.3364
0.3362
0.3360
0.3358
3.5 ˚C/min Base 10 ˚C/min Base 10˚C/min Top 3.5 ˚C/min Top
0.3356 0
2
4
6
8
10
12
14
16
Heat Treatment Rate during Graphitization [ ˚C/min] 240
Crystal Stack Height [Lc,002-nm]
b
Max deviation
3.5 ˚C/min Base 10 ˚C/min Base 10˚C/min Top 3.5 ˚C/min Top
220 200 180 160 140 120 100 0
2
4
6
8
10
12
14
16
Heat Treatment Rate during Graphitization[˚C/min] 90 3.5 ˚C/min Base 10 ˚C/min Base 10 ˚C/min Top 3.5 ˚C/min Top
80
Crystallite Size [La,100-nm]
a
Average density [g/cm ]
b
70
60
50
40
30 0
2
4
6
8
10
12
14
16
Heat Treatment Rate during Graphitization[˚C/min]
Fig. 1. X-ray diffraction results: interlayer spacing, d0 0 2 ; stacking height, Lc ; coherence length, La .
flow to the surface, but rather simply form and enlarge, resulting in further alignment of the crystals along the cell walls due to biaxial extension. The foaming action is restrained by the container (and gravity), thus yielding a
Letters to the Editor / Carbon 42 (2004) 1849–1852
slight elongation of the bubbles in the z-direction. This elongation, along with the effect of the preferred orientation of the crystallites, results in a higher thermal conductivity in the z-direction than in the x–y direction. Fig. 2 shows the relationship of heating rate during graphitization on the x, y, and z thermal conductivities. From the plots, it is clear that the z-direction thermal conductivity is directly related to the graphitization
160
Thermal Conductivity [W/m·K]
140 120 100 80 60
Foaming Rateand 3.5 ˚C/min Bottom
40
10 ˚C/min Bottom 10 ˚C/min Top
20
3.5 ˚C/min Top
0 0
(a)
5
10
15
20
Heat Treatment Rate during Graphitization [˚C/min]
Thermal Conductivity [W/m·K]
60
50
40
30 Foaming Rate and
20
Position in Furnace 3.5 ˚C/min Bottom 10 ˚C/min Bottom
10
10 ˚C/min Top 3.5 ˚C/min Top
0 0
(b)
5
10
15
20
Heat Treatment Rate during Graphitization [˚C/min]
heating rate. As anticipated in the previous section, the higher graphitization rate affects the crystal structure, thus decreasing the thermal conductivity in the direction with the greatest preferred orientation (z-direction). However, it is unclear why the x-direction thermal conductivity increased with increasing graphitization rate. This phenomenon was not seen in the y-direction, and this was also only seen in the samples made at the bottom of the furnace, not the top. Fig. 3 shows the z-direction thermal conductivity versus coherence length, La , and, as expected, there is a general trend that as the coherence length increases, the thermal conductivity increases. Thus, processing parameters which affect the coherence length will directly affect the final thermal conductivity. However, it is unclear at this time why there does not appear to be a correlation between La and the x- and y thermal conductivities. Future studies will be performed to characterize this phenomena. The effect on the final thermal properties of the location in the foaming furnace is also noteworthy. It is clear that in the z-direction (the direction with the highest conductivity), both billets that were located in the top of the furnace exhibited the highest overall thermal conductivity, irrespective of foaming rate. Moreover, it is also evident that the foaming rate does affect the final thermal properties, with the lower foaming rates yielding graphitized foams exhibiting the higher thermal conductivities. While these trends are also true for the x- and y-directions, the effects are less noticeable than those in the z-direction. Several conclusions may be drawn from this study. The foams produced at the lower position in foaming furnace have a less uniform density than those foamed in the upper position. Moreover, the foaming rate of the precursor pitch is critical with samples foamed at lower heating rates. It was also observed that the graphitization rate was critical to the thermal properties and that higher heating rates tended to disrupt the crystal 160 Thermal Conductivity [W/m·K]
70
Thermal Conductivity [W/m·K]
60 50 40 30 Foaming Rate and Position in Furnace
20
3.5 ˚C/min Bottom 10 ˚C/min Bottom
10
10 ˚C/min Top
0
5
10
15
140 120 100 80 60 40 20 0
3.5 ˚C/min Top
0
0
(c)
1851
20
Heat Treatment Rate during Graphitization [˚C/min]
Fig. 2. Thermal conductivities in the (a) z-direction, (b) y-direction, (c) x-direction.
20
40
60
Crystallite Size [L
a,100
80
100
- nm]
Fig. 3. z-Direction thermal conductivity as a function of coherence length showing a strong dependence between the two parameters.
1852
Letters to the Editor / Carbon 42 (2004) 1849–1852
structure (specifically the coherence length) and hence decrease the thermal properties compared to slower heat rates. Finally, it was found that the position in the furnace, and the foaming rate, exhibited a larger effect on the thermal properties than the graphitization rate. Thus, it appears that temperature uniformities within the billet during foaming are the most critical aspect of the process. Because the foaming stage (when the pitch is fluid) is critical to the formation and orientation of the crystallites (similar to mesophase pitch fibers [10]), further studies should focus on the foaming stage to allow tailoring the foam properties to the final application.
References [1] Ford W. Method of making cellular refractory thermal insulating material. US Patent 3,121,050, 1964. [2] Googin J, Napier J, Scrivner M. Method for manufacturing foam carbon products. US Patent 3,345,440, 1967.
[3] Klett J. High thermal conductivity, mesophase pitch-derived carbon foam. Extended Abstracts, Proceedings of the 1998 43rd International SAMPE Symposium and Exhibition, Part 1 (of 2), Anaheim, California, USA, 1998. p. 745–55. [4] Klett J. Pitch-based carbon foam and composites. US Patent 6,261,485, 2001. [5] Klett J et al. High-thermal-conductivity, mesophase-pitch-derived carbon foams: effect of precursor on structure and properties. Carbon 2000;38(7):953–73. [6] Klett J, Klett L, Burchell T, Walls C. Graphitic foam thermal management materials for electronic packaging. Society of Automotive Engineers Technical Paper Series 2000 (00FCC-117). [7] Gallego N, Klett J, McMillan A. Effects of processing conditions on properties of graphite foams. Extended Abstracts, Proceedings of The International Conference on Carbon in 2002, Beijing, China, 2002. [8] Klett J, McMillan AD, Stinton D. Modeling geometric effects on heat transfer with graphite foam. Extended Abstracts, The 26th Annual Conference on Ceramic, Metal, and Carbon Composites, Materials, and Structures, Cocoa Beach, Florida, 2002. [9] Klett J. Process for making carbon foam. US Patent 6,033,506, 2000. [10] Edie DD, Stoner EG. The effect of microstructure and shape on carbon fiber properties. In: Buckley JD, editor. Carbon–carbon materials and composites. Noyes Publications.
Amorphous carbon nanotubes produced by a temperature controlled DC arc discharge Yongning Liu a
a,*
, Song Xiaolong a, Zhao Tingkai a, Zhu Jiewu a, Michael Hirscher b, Fritz Philipp b
Department of Materials Science and Engineering, State Key Laboratory for Mechanical Behaviour of Materials, Xi’an Jiaotong University, Xi’an 710049, China b Max Plank Institute for Metal Research, Heisenbergstrasse, D-70569 Stuttgart, Germany Received 12 July 2003; accepted 27 January 2004 Available online 16 March 2004
Keywords: A. Carbon nanotubes; B. Arc discharge; C. X-ray diffraction
Since their discovery by Iijima in 1991 [1], carbon nanotubes (CNTs) have been studied all over the world. They have been synthesized mainly by three methods: DC (direct current) arc discharge, chemical vapour deposition and laser ablation. Both single-walled carbon nanotubes (SWNTs) and multi-walled nanotubes (MWNTs) can be produced by arc discharge [2–4]. Generally, the temperature in the vacuum chamber is not controlled and the walls of the chamber are cooled by water. After arcing, the soot deposited on the chamber walls consists of useless metal catalyst and carbonaceous particles other than few CNTs. Most CNTs are produced in the center of the core deposited at *
Corresponding author. Tel.: +86-29-8266-9071; fax: +86-29-82663453. E-mail address: [email protected] (YN. Liu). 0008-6223/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2004.01.069
the head of the cathode and also around the anode rod [5]. Hence, this procedure has a very low yield, generally on the scale of mg. It is believed that the structure of both single- and multi-walled nanotubes is like that of a hollow graphite cylinder [6] and that this structure is crystalline. This theory has been proven by many TEM analyses. However, recent research has shown that carbon nanotubes can also have an amorphous structure when prepared by CVD [7–10]. In this paper we report that the amorphous carbon nanotubes can also be produced by DC arc charge. A schematic diagram of the DC arc discharge furnace with a temperature controlling system is shown in Fig. 1. In the vacuum chamber, there is an inner stainless steel chamber, 200 mm in diameter and 300 mm in length, around which is mounted an electric heating apparatus.