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Solution combustion synthesis of carbon nanotube loaded nickel foams
Materials Letters 73 (2012) 126–128
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Solution combustion synthesis of carbon nanotube loaded nickel foams Lori J. Groven ⁎, Jan A. Puszynski South Dakota School of Mines & Technology, Chemical and Biological Engineering Department, 501 East Saint Patrick Street, Rapid City, SD 57701, USA
a r t i c l e
i n f o
Article history: Received 9 November 2011 Accepted 8 January 2012 Available online 14 January 2012 Keywords: Carbon nanotubes Solution combustion synthesis Nickel Metal foams Porous materials
a b s t r a c t Nickel–carbon nanotube (Ni–CNT) foams were synthesized using a solution combustion method. In this method CNTs were added directly to the nickel nitrate–glycine solution followed by combustion on a hot plate at 300 °C under atmospheric conditions. No heat treatment of the resulting foams was necessary to yield relatively high purity (99.7%) foam as determined by X-ray diffraction, energy dispersive spectroscopy and standard combustion techniques. Raman spectroscopy and transmission electron microscopy analyses confirmed that there was no visible degradation of the carbon nanotubes in the reacted product. By combining solution combustion with carbon nanotube addition Ni–CNT foams were easily formed. This approach shows promise for being a flexible, general, route for the synthesis of a wide range of transition metal, intermetallic, cermet, and oxide–CNT materials. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The fabrication of carbon nanotube (CNT) reinforced metals and intermetallics has been an area of significant research during the past few years as CNTs have been considered the ultimate composite reinforcement [1–7]. However, the processing methodologies for incorporating CNTs into such materials have been non ideal at best and have typically relied on ball milling followed by hot pressing or in the case of single metals melt processing or thermal spraying, often resulting in significant damage to the CNTs. Most recently we have demonstrated that CNTs may be incorporated into reactive nanopowders followed by solid combustion synthesis to form dense to porous CNT reinforced nickel aluminides with improved microhardness [8,9]. Significant progress has been made in the formation of metallic or intermetallic foam like materials in the field of combustion synthesis. Both self-propagating high temperature synthesis [10–12] and the most recent investigations using solution combustion synthesis [13–15] have been shown to be suitable approaches. The solution combustion method allows synthesis of pure metals, alloys, and cermets by adjusting the initial fuel/oxidizer ratio in the solution [13–15]. This unique technique, where reactants are mixed at the molecular level, lends itself well to the addition of inert inclusions, such as carbon nanotubes. Since mixing occurs within the entire reactant volume the product species will inherently form around such reinforcement. Therefore, in this study our objective was to determine if CNTs can be included in a solution combustion regime, without destruction, and by that open a new processing methodology
⁎ Corresponding author at: School of Mechanical Engineering, Purdue University, 500 Allison Road, West Lafayette, IN, USA. E-mail address: [email protected] (L.J. Groven). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.01.033
for achieving metal–CNT and oxide–CNT composite powders that can be further developed and is quite versatile. 2. Materials and methods 2.1. Materials and synthesis The following materials were utilized as received from the manufacturer: Multi-walled carbon nanotubes (MWNTs) (5–15 nm diameter, Cheap Tubes Inc.), Single-walled carbon nanotubes (SWNTs) (1.3 nm diameter, Helix Materials Inc.), Ni(NO3)2·6H2O (Fisher Scientific), Glycine (Acros Organics). Carbon nanotube addition to the metal nitrate-fuel solutions was accomplished by dispersing the desired quantity in 20 mL deionized water with 2.0-wt.% nonionic dispersant (1C-Coulter) for 10 min by ultrasonication (Sonics VCX 130, 50% amplitude) and then pouring the nanotube slurry into the boiling solution prior to foaming and combustion. Combustion was conducted on a hot plate set at 300 °C. Air atmosphere was used. The stoichiometry was adjusted to be fuel rich (1.44 instead of 1.33) to yield foams with the highest purity of the desired metal rather than a mixture of Ni–NiO [13,14]. 2.2. Material characterization Transmission electron microscopy was conducted with a Tecnai F30 field-emission gun transmission electron microscope (at 300 kV) at the Center for Integrated Nanotechnologies, Sandia National Laboratory, Albuquerque, NM. Samples were prepared by crushing the material, dispersing in methanol and dropping the resulting solution onto a 3 mm carbon coated copper grid. Raman spectroscopy was conducted using a Renishaw 1000 with a 779.5 nm laser source or with an Aramis confocal microscope with a 532 nm source. Scanning electron microscopy was conducted using
L.J. Groven, J.A. Puszynski / Materials Letters 73 (2012) 126–128
a Zeiss Supra VP-45. X-ray diffraction (XRD) was conducted using a Rigaku Ultima-Plus Powder Diffractometer with a Cu-Kα source. The purity of the materials was determined by energy dispersive spectroscopy (EDS) and LECO Nitrogen/Oxygen/Hydrogen (THS600) and LECO Carbon/Sulfur (CS600) analysis. 3. Results and discussion For the sake of comparison, Ni foams were first synthesized without CNT addition using the same approach as Erri et al. [13] and Kumar et al. [14]. The resulting material appeared to be a three-dimensional structure contained within the volume of the beaker. However, upon removal the metal foam broke into small pieces (powder like) and did not have any structural strength. Energy dispersive spectrometry (EDS) of the Ni-foam indicated that the material did not have any detectable residual carbon present and a secondary analysis using a LECO Carbon/Sulfur analyzer also did not detect any residual carbon indicating that all excess fuel was consumed during the reaction. An average oxygen content of 0.3-wt.% was found for the Ni-foam. Therefore, the same stoichiometry was used for the synthesis of Ni– CNT foams. Ni–CNT foams were synthesized with 1.0, 2.0 and 3.0-wt.% CNT addition relative to final powder mass. XRD patterns of the 3.0wt.% MWNT Ni-foam and Ni-foam were identical; no CNT peak was observed and no nickel carbides were detected. SEM confirmed that without MWNT addition the grain sizes appear to be ~200–300 nm as shown in Fig. 1, but with increased MWNT addition there was an increase in grain growth up to 1 μm (Fig. 1d). In addition, with increased MWNT addition there appeared to be more connectivity of the Ni grains and reduction of microporosity (poresb 1 μm in size). Only in the case of the 1.0-wt.% MWNT-Ni foam could protruding MWNTs be readily detected at higher magnifications (Fig. 1c). With the increase in grain size it appears that the MWNTs are encapsulated by the Ni formed during the reaction. The resulting material showed a significant difference in its structural integrity and foams with MWNT addition could easily be removed from the
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beaker without disintegration as shown by the cubes cut from such, an image of such may be found in the Supplemental information. Resulting densities were on the order of 0.05 g cm − 3. These CNT–Ni foams could be further processed by cold-pressing (78 MPa) to yield high porosity (66+%), low density (2.95 g cm − 3) Ni–CNT monoliths. Though CNTs have very high specific surface areas (200–300 m 2 g − 1) this was not a characteristic imparted to the metal foam. Instead there was an observed decrease in specific surface area with increased CNT addition. The SSA's were 2.25, 1.13, 0.94, and 0.75 m 2 g − 1 for 0, 1, 2, and 3 wt.% MWNT addition respectively. This trend follows the observation of greater structural integrity with increased CNT addition. To verify the chemical and structural integrity of the CNTs after the combustion is completed TEM and Raman spectroscopy analyses were conducted. TEM imaging of the 3.0-wt.% MWNT–Ni foam as presented in Fig. 2 shows that the MWNTs were clearly retained and were not destructed during the combustion synthesis. However, the Ni particles did not appear to be bonded to the tube surface and no strong interfacial interaction was observed. This may be in part to how TEM samples were prepared and crushing/sonication of the material breaks the Ni away from the encapsulated CNTs. As shown in Fig. 2a, the nanotubes were quite well distributed and were not found in large bundles. Raman spectroscopy was conducted on foams synthesized with 3.0-wt.% SWNTs (Renishaw 1000, 779.5 nm source) and 3.0-wt.% MWNTs (Aramis, 532 nm source). Prior to Raman analysis the material was subjected to dissolution in concentrated nitric acid and the residue filtered and washed with deionized water and then dried. The Raman spectrum was compared to that of the original SWNTs and MWNTs. As shown in Fig. 3a,b the spectra clearly indicate that the chemical and structural integrity of the SWNTs and MWNTs were preserved as indicated by the appearance of the D (~1308 cm − 1) and G bands (~ 1580 cm − 1). If there was degradation of the CNTs due to their exposure to the high temperatures during the combustion synthesis there would be
Fig. 1. SEM images of solution combustion synthesized Ni foams with a) 0.0-wt.% MWNTs, b) 1.0-wt.% MWNTs, c) 2.0-wt.% MWNTs and d) 3.0-wt.% MWNTs. White arrows indicate protruding nanotubes.
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L.J. Groven, J.A. Puszynski / Materials Letters 73 (2012) 126–128
Fig. 2. TEM images of 3.0-wt.% MWNT–Ni foam synthesized by solution combustion synthesis.
a visible increase in the ID/G ratio. One of the most important aspects of coupling solution combustion synthesis with the inclusion of CNTs is that the typical dispersion and mixing concerns associated with CNT reinforced metals are reduced. Due to the nature of the process the CNTs are dispersed in the solution prior to ignition which results in the metal forming around the reinforcement and hence, a porous structure that is essentially a network of metal with dispersed CNTs is formed.
4. Conclusion For the first time, a flexible solution combustion approach for the synthesis of foam-like Ni–CNT composites was demonstrated. This method can be easily extended to the formation of a wide range of transition metals, intermetallics, cermets, and oxide–CNT materials. At this time Co–CNT, Co3O4–CNT, Ni–CNT, NiO–CNT, and Al2O3–CNT materials have been synthesized. The possible applications are abundant ranging from electromagnetic absorption materials, catalysis, fuel cells, hydrogen storage, and unique insulation, to name a few. Supplementary materials related to this article can be found online at doi:10.1016/j.matlet.2012.01.033.
Acknowledgments The authors wish to acknowledge the National Science Foundation Graduate Fellowship program (Groven) and the Center for Integrated Nanotechnologies (User Proposal: TEM study of interfaces in in-situ densified CNT-reinforced combustion synthesized materials, Lead CINT Scientist: Jianyu Huang) for HRTEM.
Fig. 3. Raman spectra for CNTs as received and recovered after the combustion synthesis of Ni–CNT foams. a) SWNTs: 1 — as received, 2 — recovered, and b) MWNTs: 1 — as received, 2 — recovered.
References [1] Salvetat J-P, Briggs GAD, Bonard J-M, Bacsa RR, Kulik AJ, Stöckli T, et al. Elastic and shear moduli of single-walled carbon nanotube ropes. Phys Rev Lett 1999;82:944–7. [2] Lupo F, Kamalakaran R, Scheu C, Grobert N, Rühle M. Microstructural investigations on zirconium oxide–carbon nanotube composites synthesized by hydrothermal crystallization. Carbon 2004;42:1995–9. [3] Kuzumaki T, Ujiie O, Ichinose H, Ito K. Mechanical characteristics and preparation of carbon nanotube fiber-reinforced Ti composite. Adv Eng Mater 2000;2:416–8. [4] Zhong R, Cong H, Hou P. Fabrication of nano-Al based composites reinforced by single-walled carbon nanotubes. Carbon 2003;41:848–51. [5] Kim KT, Lee KH, Cha SI, Mo C-B, Hong SH. Characterization of carbon nanotubes/Cu nanocomposites processed by using nano-sized Cu powders. Mater Res Soc Symp Proc 2004;821:P3.25. [6] Pang L-X, Sun K-N, Ren S, Sun C, Fan R-H, Lu Z-H. Fabrication and microstructure of Fe3Al matrix composite reinforced by carbon nanotube. Mater Sci Eng A 2007;447:146–9. [7] Thostenson ET, Karandikar PG, Chou T-W. Fabrication and characterization of reaction bonded silicon carbide/carbon nanotube composites. J Phys D: Appl Phys 2005;38:3962. [8] Groven LJ, Puszynski JA. Effect of carbon nanotube addition on morphology of SHS synthesized materials. Int J Self Propag High Temp Synth 2007;16:189–98. [9] Groven LJ. Combustion synthesis of carbon nanotube reinforced composites. PhD Dissertation: South Dakota School of Mines & Technology, Rapid City, SD; 2009. p. 1–302. [10] Hunt EM, Pantoya ML, Jouet RJ. Combustion synthesis of metallic foams from nanocomposite reactants. Intermetallics 2006;14:620–9. [11] Tappan BC, Huynh MH, Hiskey MA, Chavez DE, Luther EP, Mang JT, et al. Ultralowdensity nanostructured metal foams: combustion synthesis, morphology, and composition. J Am Chem Soc 2006;128:6589–94. [12] Tappan BC, Steiner SA, Luther EP. Nanoporous metal foams. Angew Chem Int Ed 2010;49:4544–65. [13] Erri P, Nader J, Varma A. Controlling combustion wave propagation for transition metal/alloy/cermet foam synthesis. Adv Mater 2008;20:1243–5. [14] Kumar A, Wolf EE, Mukasyan AS. Solution combustion synthesis of metal nanopowders: nickel—reaction pathways. AIChE J 2011;57:2207–14. [15] Kumar A, Wolf EE, Mukasyan AS. Solution combustion synthesis of metal nanopowders: copper and copper/nickel alloys. AIChE J 2011;57:3473–9.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Solution combustion synthesis of carbon nanotube loaded nickel foams Lori J. Groven ⁎, Jan A. Puszynski South Dakota School of Mines & Technology, Chemical and Biological Engineering Department, 501 East Saint Patrick Street, Rapid City, SD 57701, USA
a r t i c l e
i n f o
Article history: Received 9 November 2011 Accepted 8 January 2012 Available online 14 January 2012 Keywords: Carbon nanotubes Solution combustion synthesis Nickel Metal foams Porous materials
a b s t r a c t Nickel–carbon nanotube (Ni–CNT) foams were synthesized using a solution combustion method. In this method CNTs were added directly to the nickel nitrate–glycine solution followed by combustion on a hot plate at 300 °C under atmospheric conditions. No heat treatment of the resulting foams was necessary to yield relatively high purity (99.7%) foam as determined by X-ray diffraction, energy dispersive spectroscopy and standard combustion techniques. Raman spectroscopy and transmission electron microscopy analyses confirmed that there was no visible degradation of the carbon nanotubes in the reacted product. By combining solution combustion with carbon nanotube addition Ni–CNT foams were easily formed. This approach shows promise for being a flexible, general, route for the synthesis of a wide range of transition metal, intermetallic, cermet, and oxide–CNT materials. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The fabrication of carbon nanotube (CNT) reinforced metals and intermetallics has been an area of significant research during the past few years as CNTs have been considered the ultimate composite reinforcement [1–7]. However, the processing methodologies for incorporating CNTs into such materials have been non ideal at best and have typically relied on ball milling followed by hot pressing or in the case of single metals melt processing or thermal spraying, often resulting in significant damage to the CNTs. Most recently we have demonstrated that CNTs may be incorporated into reactive nanopowders followed by solid combustion synthesis to form dense to porous CNT reinforced nickel aluminides with improved microhardness [8,9]. Significant progress has been made in the formation of metallic or intermetallic foam like materials in the field of combustion synthesis. Both self-propagating high temperature synthesis [10–12] and the most recent investigations using solution combustion synthesis [13–15] have been shown to be suitable approaches. The solution combustion method allows synthesis of pure metals, alloys, and cermets by adjusting the initial fuel/oxidizer ratio in the solution [13–15]. This unique technique, where reactants are mixed at the molecular level, lends itself well to the addition of inert inclusions, such as carbon nanotubes. Since mixing occurs within the entire reactant volume the product species will inherently form around such reinforcement. Therefore, in this study our objective was to determine if CNTs can be included in a solution combustion regime, without destruction, and by that open a new processing methodology
⁎ Corresponding author at: School of Mechanical Engineering, Purdue University, 500 Allison Road, West Lafayette, IN, USA. E-mail address: [email protected] (L.J. Groven). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.01.033
for achieving metal–CNT and oxide–CNT composite powders that can be further developed and is quite versatile. 2. Materials and methods 2.1. Materials and synthesis The following materials were utilized as received from the manufacturer: Multi-walled carbon nanotubes (MWNTs) (5–15 nm diameter, Cheap Tubes Inc.), Single-walled carbon nanotubes (SWNTs) (1.3 nm diameter, Helix Materials Inc.), Ni(NO3)2·6H2O (Fisher Scientific), Glycine (Acros Organics). Carbon nanotube addition to the metal nitrate-fuel solutions was accomplished by dispersing the desired quantity in 20 mL deionized water with 2.0-wt.% nonionic dispersant (1C-Coulter) for 10 min by ultrasonication (Sonics VCX 130, 50% amplitude) and then pouring the nanotube slurry into the boiling solution prior to foaming and combustion. Combustion was conducted on a hot plate set at 300 °C. Air atmosphere was used. The stoichiometry was adjusted to be fuel rich (1.44 instead of 1.33) to yield foams with the highest purity of the desired metal rather than a mixture of Ni–NiO [13,14]. 2.2. Material characterization Transmission electron microscopy was conducted with a Tecnai F30 field-emission gun transmission electron microscope (at 300 kV) at the Center for Integrated Nanotechnologies, Sandia National Laboratory, Albuquerque, NM. Samples were prepared by crushing the material, dispersing in methanol and dropping the resulting solution onto a 3 mm carbon coated copper grid. Raman spectroscopy was conducted using a Renishaw 1000 with a 779.5 nm laser source or with an Aramis confocal microscope with a 532 nm source. Scanning electron microscopy was conducted using
L.J. Groven, J.A. Puszynski / Materials Letters 73 (2012) 126–128
a Zeiss Supra VP-45. X-ray diffraction (XRD) was conducted using a Rigaku Ultima-Plus Powder Diffractometer with a Cu-Kα source. The purity of the materials was determined by energy dispersive spectroscopy (EDS) and LECO Nitrogen/Oxygen/Hydrogen (THS600) and LECO Carbon/Sulfur (CS600) analysis. 3. Results and discussion For the sake of comparison, Ni foams were first synthesized without CNT addition using the same approach as Erri et al. [13] and Kumar et al. [14]. The resulting material appeared to be a three-dimensional structure contained within the volume of the beaker. However, upon removal the metal foam broke into small pieces (powder like) and did not have any structural strength. Energy dispersive spectrometry (EDS) of the Ni-foam indicated that the material did not have any detectable residual carbon present and a secondary analysis using a LECO Carbon/Sulfur analyzer also did not detect any residual carbon indicating that all excess fuel was consumed during the reaction. An average oxygen content of 0.3-wt.% was found for the Ni-foam. Therefore, the same stoichiometry was used for the synthesis of Ni– CNT foams. Ni–CNT foams were synthesized with 1.0, 2.0 and 3.0-wt.% CNT addition relative to final powder mass. XRD patterns of the 3.0wt.% MWNT Ni-foam and Ni-foam were identical; no CNT peak was observed and no nickel carbides were detected. SEM confirmed that without MWNT addition the grain sizes appear to be ~200–300 nm as shown in Fig. 1, but with increased MWNT addition there was an increase in grain growth up to 1 μm (Fig. 1d). In addition, with increased MWNT addition there appeared to be more connectivity of the Ni grains and reduction of microporosity (poresb 1 μm in size). Only in the case of the 1.0-wt.% MWNT-Ni foam could protruding MWNTs be readily detected at higher magnifications (Fig. 1c). With the increase in grain size it appears that the MWNTs are encapsulated by the Ni formed during the reaction. The resulting material showed a significant difference in its structural integrity and foams with MWNT addition could easily be removed from the
127
beaker without disintegration as shown by the cubes cut from such, an image of such may be found in the Supplemental information. Resulting densities were on the order of 0.05 g cm − 3. These CNT–Ni foams could be further processed by cold-pressing (78 MPa) to yield high porosity (66+%), low density (2.95 g cm − 3) Ni–CNT monoliths. Though CNTs have very high specific surface areas (200–300 m 2 g − 1) this was not a characteristic imparted to the metal foam. Instead there was an observed decrease in specific surface area with increased CNT addition. The SSA's were 2.25, 1.13, 0.94, and 0.75 m 2 g − 1 for 0, 1, 2, and 3 wt.% MWNT addition respectively. This trend follows the observation of greater structural integrity with increased CNT addition. To verify the chemical and structural integrity of the CNTs after the combustion is completed TEM and Raman spectroscopy analyses were conducted. TEM imaging of the 3.0-wt.% MWNT–Ni foam as presented in Fig. 2 shows that the MWNTs were clearly retained and were not destructed during the combustion synthesis. However, the Ni particles did not appear to be bonded to the tube surface and no strong interfacial interaction was observed. This may be in part to how TEM samples were prepared and crushing/sonication of the material breaks the Ni away from the encapsulated CNTs. As shown in Fig. 2a, the nanotubes were quite well distributed and were not found in large bundles. Raman spectroscopy was conducted on foams synthesized with 3.0-wt.% SWNTs (Renishaw 1000, 779.5 nm source) and 3.0-wt.% MWNTs (Aramis, 532 nm source). Prior to Raman analysis the material was subjected to dissolution in concentrated nitric acid and the residue filtered and washed with deionized water and then dried. The Raman spectrum was compared to that of the original SWNTs and MWNTs. As shown in Fig. 3a,b the spectra clearly indicate that the chemical and structural integrity of the SWNTs and MWNTs were preserved as indicated by the appearance of the D (~1308 cm − 1) and G bands (~ 1580 cm − 1). If there was degradation of the CNTs due to their exposure to the high temperatures during the combustion synthesis there would be
Fig. 1. SEM images of solution combustion synthesized Ni foams with a) 0.0-wt.% MWNTs, b) 1.0-wt.% MWNTs, c) 2.0-wt.% MWNTs and d) 3.0-wt.% MWNTs. White arrows indicate protruding nanotubes.
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Fig. 2. TEM images of 3.0-wt.% MWNT–Ni foam synthesized by solution combustion synthesis.
a visible increase in the ID/G ratio. One of the most important aspects of coupling solution combustion synthesis with the inclusion of CNTs is that the typical dispersion and mixing concerns associated with CNT reinforced metals are reduced. Due to the nature of the process the CNTs are dispersed in the solution prior to ignition which results in the metal forming around the reinforcement and hence, a porous structure that is essentially a network of metal with dispersed CNTs is formed.
4. Conclusion For the first time, a flexible solution combustion approach for the synthesis of foam-like Ni–CNT composites was demonstrated. This method can be easily extended to the formation of a wide range of transition metals, intermetallics, cermets, and oxide–CNT materials. At this time Co–CNT, Co3O4–CNT, Ni–CNT, NiO–CNT, and Al2O3–CNT materials have been synthesized. The possible applications are abundant ranging from electromagnetic absorption materials, catalysis, fuel cells, hydrogen storage, and unique insulation, to name a few. Supplementary materials related to this article can be found online at doi:10.1016/j.matlet.2012.01.033.
Acknowledgments The authors wish to acknowledge the National Science Foundation Graduate Fellowship program (Groven) and the Center for Integrated Nanotechnologies (User Proposal: TEM study of interfaces in in-situ densified CNT-reinforced combustion synthesized materials, Lead CINT Scientist: Jianyu Huang) for HRTEM.
Fig. 3. Raman spectra for CNTs as received and recovered after the combustion synthesis of Ni–CNT foams. a) SWNTs: 1 — as received, 2 — recovered, and b) MWNTs: 1 — as received, 2 — recovered.
References [1] Salvetat J-P, Briggs GAD, Bonard J-M, Bacsa RR, Kulik AJ, Stöckli T, et al. Elastic and shear moduli of single-walled carbon nanotube ropes. Phys Rev Lett 1999;82:944–7. [2] Lupo F, Kamalakaran R, Scheu C, Grobert N, Rühle M. Microstructural investigations on zirconium oxide–carbon nanotube composites synthesized by hydrothermal crystallization. Carbon 2004;42:1995–9. [3] Kuzumaki T, Ujiie O, Ichinose H, Ito K. Mechanical characteristics and preparation of carbon nanotube fiber-reinforced Ti composite. Adv Eng Mater 2000;2:416–8. [4] Zhong R, Cong H, Hou P. Fabrication of nano-Al based composites reinforced by single-walled carbon nanotubes. Carbon 2003;41:848–51. [5] Kim KT, Lee KH, Cha SI, Mo C-B, Hong SH. Characterization of carbon nanotubes/Cu nanocomposites processed by using nano-sized Cu powders. Mater Res Soc Symp Proc 2004;821:P3.25. [6] Pang L-X, Sun K-N, Ren S, Sun C, Fan R-H, Lu Z-H. Fabrication and microstructure of Fe3Al matrix composite reinforced by carbon nanotube. Mater Sci Eng A 2007;447:146–9. [7] Thostenson ET, Karandikar PG, Chou T-W. Fabrication and characterization of reaction bonded silicon carbide/carbon nanotube composites. J Phys D: Appl Phys 2005;38:3962. [8] Groven LJ, Puszynski JA. Effect of carbon nanotube addition on morphology of SHS synthesized materials. Int J Self Propag High Temp Synth 2007;16:189–98. [9] Groven LJ. Combustion synthesis of carbon nanotube reinforced composites. PhD Dissertation: South Dakota School of Mines & Technology, Rapid City, SD; 2009. p. 1–302. [10] Hunt EM, Pantoya ML, Jouet RJ. Combustion synthesis of metallic foams from nanocomposite reactants. Intermetallics 2006;14:620–9. [11] Tappan BC, Huynh MH, Hiskey MA, Chavez DE, Luther EP, Mang JT, et al. Ultralowdensity nanostructured metal foams: combustion synthesis, morphology, and composition. J Am Chem Soc 2006;128:6589–94. [12] Tappan BC, Steiner SA, Luther EP. Nanoporous metal foams. Angew Chem Int Ed 2010;49:4544–65. [13] Erri P, Nader J, Varma A. Controlling combustion wave propagation for transition metal/alloy/cermet foam synthesis. Adv Mater 2008;20:1243–5. [14] Kumar A, Wolf EE, Mukasyan AS. Solution combustion synthesis of metal nanopowders: nickel—reaction pathways. AIChE J 2011;57:2207–14. [15] Kumar A, Wolf EE, Mukasyan AS. Solution combustion synthesis of metal nanopowders: copper and copper/nickel alloys. AIChE J 2011;57:3473–9.