- Email: [email protected]
Al interfaces
Accepted Manuscript Full Length Article Synthesis and Exothermic Reactions of Ultra-fine Snowman-shaped Particles with Directly Bonded Ni/Al interfaces Gu Hyun Kwon, Kyung Tae Kim, Dong Won Kim, Jungho Choe, Jung Yeul Yun, Jong Man Kim PII: DOI: Reference:
S0169-4332(19)30076-5 https://doi.org/10.1016/j.apsusc.2019.01.068 APSUSC 41467
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
7 September 2018 30 December 2018 8 January 2019
Please cite this article as: G. Hyun Kwon, K. Tae Kim, D. Won Kim, J. Choe, J. Yeul Yun, J. Man Kim, Synthesis and Exothermic Reactions of Ultra-fine Snowman-shaped Particles with Directly Bonded Ni/Al interfaces, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.01.068
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis and Exothermic Reactions of Ultra-fine Snowman-shaped Particles with Directly Bonded Ni/Al interfaces
Gu Hyun Kwon1,2, Kyung Tae Kim1*, Dong Won Kim1 , Jungho Choe1, Jung Yeul Yun1, Jong Man Kim2* 1
Metal Powder Department, Korea Institute of Materials Science, 797 Changwon-daero, Seongsan-gu, Changwon, Gyeongnam 51508, Republic of Korea
2
Department of Nanoenergy Engineering, Pusan National University, San 30, Jangjeon-dong, Geumjung-gu, Busan 609-735, Republic of Korea
1
Abstract Nickel-nanoparticle-attached aluminum (Ni/Al) particles are synthesized by plating Ni onto Al particles with the surface oxide partly removed. The microstructure of the synthesized Ni/Al particles resembles a child’s snowman, with an approximately 200 nm spherical Ni nanoparticle bonded onto the surface of an Al particle about 800 nm in size. Particularly, the surface oxide is rarely observed at the Ni/Al interfaces unlike uncoated surface of the Al powder. Due to these microstructural characteristics, it was clearly noted that a self-propagating high-temperature synthesis (SHS) reaction of the snowman-shaped Ni/Al (SNA) particles occurs more actively compared to that which occurs when simply mixing particles of Ni and Al in the temperature range of 720-900 K. The formation of intermetallic compounds of NiAl or NiAl3 is confirmed in a comparison of XPS results after both samples were oxidized at 773 K. The exothermic enthalpy energy, 8,850 Jg-1 of SNA particles represents 118% of the value of 7,580 Jg-1 for the mixed particles. These results indicate that SNA particles are promising as advanced reactive materials when used in conjunction with an efficient SHS reaction.
Keywords: Reactive Metal Powder; Nickel Coating; Aluminum; SHS reaction *Corresponding
authors :
K.T.K.
E-mail:
[email protected]
2
[email protected],
and
J.K.
E-mail :
1. Introduction Energetic metals such as aluminum (Al), beryllium (Be), zirconium (Zr), boron (B) and titanium (Ti) have been widely utilized in civil and military applications as explosives, propellants, welding materials and pyrotechnic materials [1-3]. The origin of the energy release in energetic metals is the exothermic heat energy caused by their oxidation, and aluminum (Al) is especially well known to possess high volumetric thermal energy (31 kcal/cm3)
[4, 5]
. However,
unfortunately, the empirical amount of energy released by Al during combustion is generally lower than expected because re-passivation of the surface by natural oxidation prevents the continuous oxidation of the Al in powder form
[6]
. Furthermore, because the surface oxide film
has a high melting point of approximately 2300 K, it prevents direct contact between the external oxygen and internal Al at low temperatures [7]. Although the exothermic enthalpy energy can be increased due to the enlarged specific surface area with finer sizes of the Al particles, the limitation caused by re-passivation remains [8, 9]. Recently, several researchers have focused on the mixing or coating of additive materials to enhance the thermal oxidation behaviors of fine Al powders. Among the many additive candidates, nickel (Ni) materials are known to be promising agents for Al powders when used for coating or mixing due to the positive effect realized via self-propagating high-temperature synthesis (SHS) [10]. Despite the many approaches which have been tested thus far [11-13], research on Ni materials applied to sub-um-sized Al has rarely been conducted owing to the experimental difficulty involved when attempting the homogeneous coating or mixing of Ni nanoparticles 15]
[14,
. It is known that sub-um Al particles offer the simultaneous advantages of a low ignition
temperature and an appropriate energy release given the combined properties of micron- and
3
nano-scale particles. Thus, the development of novel approaches for efficiently using the SHS reaction between Ni and sub-um sized Al particles is closely related to the successful synthesizing of advanced reactive Al particles which show superior exothermic reaction outcomes. In this study, we designed a synthetic process of a novel material, referred to here as a snowman-shaped Ni/Al (SNA) particle, in which a Ni particle 200 nm in size is attached to the surface of a sub-um Al particle. The attachment of Ni nanoparticles is done in an effort to realize an enhanced SHS reaction during the oxidation or combustion process by inducing the direct bonding between Ni and Al. The enhanced exothermic energy in the SNA particles is compared to that of a powder sample created by simply mixing Ni nanoparticles and sub-um Al particles. In addition, the influence of the rapid SHS reaction on the thermal oxidation behaviors is investigated by analyzing the byproducts of intermetallic compounds from oxidized SNA particles.
2. Material and methods 2.1 Synthesis of SNA particles Spherical Al powder with an average size of 800 nm was used. The Ni2(SO4)3 precursor solution used (Adhemax Ni LFS 1,2, and 3, Atotech Ltd.) was solutionized into 91.0 ml of distilled water. The pH of the Ni precursor solution was adjusted to 9.0 with an ammonia hydroxide solution (NH4OH, 28.0-30.0 wt%). At the same time, an alkaline etching solution to remove the surface oxide was prepared by mixing 4.0 g of an ammonia solution and 30.0 ml of distilled water. The sub-um Al powder was added to the etching solution and was stirred for 3 min. The Ni precursor solution and etching solution were then merged to nucleate Ni
4
nanoparticles onto the Al particle heterogeneously after the removal of the surface oxide. Both chemical reactions, i.e., oxide etching and Ni plating, were simultaneously carried out for 30 min. at room temperature. Finally, the prepared powder was collected using a membrane filter, after which it was rinsed with water and then dried at 333 K for more than 24 hours in a vacuum oven. For comparison, Ni nanoparticles (AVENTION Co.) approximately 200 nm in size were mixed with the 800 nm Al particles by a ball milling process for 10 min. with 5mm-sized zirconia balls at a mixing ratio of 1 to 4 of Ni and Al, respectively.
2.2 Characterization The morphology of the powder was characterized by field-emission scanning electron microscope (FE-SEM, MIRA II LMH, Tescan). An elemental analysis of the sythesized particles was conducted using FE-SEM with energy dispersive spectroscopy (EDS). X-ray photon spectroscopy (XPS, Axis Supra) was used for a quantitative analysis to determine whether NiAl-based intermetallic compounds formed. A thermo-gravimetric analysis and a differential scanning calorimetry (TGA/DSC, TA Instruments, Q600) assessment were conducted at a heating rate of 10 Kmin-1 from 298 K to 1673 K in air and in an Ar atmosphere, respectively. Interfacial structures were characterized and an elemental analysis was conducted of the SNA particles using a field-emission transmission electron microscope (EDS-attached FE-TEM, JEM2100F, JEOL) operating at 200 kV. The TEM sample was prepared using a lift-off technique with a focused ion beam (FIB, NOVA200, FEI Inc.).
3. Results and discussion
5
3.1 Microstructure of SNA particles Fig. 1(a) shows a schematic illustration of the synthesis process used to create a Ninanoparticle-attached Al particle, referred to here as a snowman-shaped Ni/Al (SNA) particle. The surface oxide is partially removed by controlling both the amount of Al particles and the pH. The plating of the Ni materials preferentially occurs on the revealed Al surfaces due to the differences between the reduction potentials, with the result being spherical Ni particles. In the alkaline etching solution used to remove the Al surface oxide, the Al/Al2O3 interface of the particles reacts following the formula below (1):
2Al(s) + 6OH- + 6H+ → 2Al(OH)3(s) + 3H2(g)↑ (1)
The Ni2+ cations of the Ni precursor solution are initially reduced to Ni0 on the Al surface in the region where the oxide film is removed, after which seeds can form and grow. Ni ions can be reduced by the difference in the reduction potential when Al is exposed to electrons or by a reducing agent. Fig. 1(b) shows the microstructure of the synthesized SNA particles. The EDS results clearly show that a Ni nanoparticle comes directly into contact with an Al particle. Fig. 2(a) shows cross-sectional morphologies and the results of a corresponding elemental analysis of a SNA particle. The dark contrasting region corresponds to Ni, which is attached to the Al surface and from which oxide film is not observed at the Ni/Al interface. The phase of the attached Ni nanoparticle is confirmed as amorphous according to the results of a pattern analysis via selected area electron diffraction (SAED) of the Al and Ni/Al interface. Fig. 2(b) is an enlarged TEM image of an uncoated surface of an Al particle. The results provide evidence that a surface oxide layer approximately 7 nm thick is continuously present expect at the Ni/Al
6
interfaces. Fig. 2(c) is an enlarged TEM image of the Ni/Al interface. As shown in Fig. 2(d), the results of an EDS mapping analysis of the Ni/Al interface do not show oxide film.
3.2 Thermal oxidation behaviors Fig. 3(a) and (b) present the results of a comparison of the TGA/DSC outcomes obtained from both a SNA particles and the mixed particles as a function of the temperature when it ranged from 298 K to 1673 K. As shown in Fig. 3(a) no major differences in the weight change caused by thermal oxidation between the SNA particles and mixed particles were noted. This likely stems from overall oxidation of similar amounts of Ni and Al particles. Fig. 3(b) presents DSC curves which show exothermic reaction peaks in both particles in the two different temperature ranges of 600-910 K (Zone Ⅰ) and 960-1,300 K (Zone Ⅱ). The SNA particles show evidence of slightly higher exothermic energy than the mixed particles in both peaks. Fig. 3 (c) shows the DSC measured in an Ar atmosphere to analyze the differences in the exothermic peaks. It was found that the mixed particles had exothermic energy of 340.2 Jg-1 and that the SNA particles had exothermic energy of 1,126.9 Jg-1. This indicates that the exothermic reaction which occurs in an inert gas atmosphere must be from the SHS reaction which makes an intermetallic compound between Al and Ni. Al and Ni can react as shown in the following formula (2):
Al (s) + Ni (s) → NiAl3 + 150.6 kJ mol-1
(2)
Fig. 3(d) compares the exothermic enthalpy energies of both particles obtained from the
7
each zone of the DSC peaks in the temperature range of 298-1473 K. In Zone Ⅰ, the energy levels for the mixed particles and for the SNA particles are approximately 2.4 kJg-1 and 2.7 kJg-1, respectively. In Zone Ⅱ, the exothermic energies for the SNA and mixed particles were determined to be 5.1 kJg-1 and 6.2 kJg-1, respectively. The total exothermic energy of the SNA particles at 8.9 kJg-1 is 118% of the value of 7.5 kJg-1 for the mixed particles below 1,400 K. This means that a rapid and more active SHS reaction readily activated the ignition of the oxidation of Al, leading to further oxidation. In other words, because the diffusion of Al in the mixed powder is limited by the oxide film present on the surface, a high temperature is required to form the intermetallic compounds compared to the SNA particles. Direct contact between Ni and Al feasibly forms intermetallic compounds such as NiAl3 or NiAl related materials, a process known to provide additional exothermic energy apart from the main oxidation of Al.
Fig. 4(a) shows the wide-scan results of the XPS analysis of the SNA and mixed particles after they were heat-treated at 773 K. As shown in Table 1, the ratio of Al is lower than that of the mixed particles, while the ratio of oxygen is relatively high, indicating that the SNA particles were oxidized further before melting. Fig. 4(b) and (c) show the results of an XPS analysis of the surfaces of both particles, revealing the ratio of the NiAl3 as obtained from the electronic binding energy of Ni2p. It was found that when the temperature is increased to 773 K, both the mixed and SNA particles form intermetallic compounds of NiAl3 and NiO, respectively. The formation ratios of NiAl3/NiO in the mixed particles and SNA particles were analyzed and found to be 15.5/84.5 and 26.7/73.3, respectively [16]. This occurs because the direct contact between Ni and Al facilitates the formation of the intermetallic compounds. Fig. 4(d) is a schematic illustration of the reaction mechanisms of the SNA particles at the Ni/Al interfaces during the combustion
8
process or the thermal oxidation reaction. As the temperature is increased to 700 K, some of the Ni is oxidized, resulting in Ni and NiO phase which are present in the Ni layer at the same time. At 720 K, Ni and NiO can diffuse into Al and the subsequent melting process forms the intermetallic compound NiAl3 with a simultaneous exothermic reaction. In addition, a small amount of oxygen existing in the Ni layer reacts with the internal Al and causes a small amount of oxidation. Moreover, after oxidation at 1100 K, Ni and NiO diffuse into the molten Al, causing the Ni layer to become thin or the Al to be exposed to external oxygen. Hence, external oxygen actively reacts with the Al core.
4.Conclusions In summary, snowman-shaped Ni/Al (SNA) particles were synthesized by removing the surface oxide while at the same time plating Ni particles. Ni nanoparticles approximately 200 nm in size were initially nucleated and then revealed spherical pores in an oxide-free region which formed on the Al surface during a subsequent etching process. The attachment of Ni nanoparticles onto Al particles prevented their separation during oxidation and combustion, but this phenomenon also provides an advantage with regard to the formation of intermetallic compounds due to the direct contact between the Al and Ni materials. During the SHS reaction, it was noted that NiAl3 phases are mainly formed in the SNA particles such that further exothermic energy is released. As a result of the thermal oxidation of SNA particles, the exothermic energy released amounted to 8.9 kJg-1, while that for the mixed powder was 7.5 kJg-1, with the former value representing 118% of the latter, thus improving the combustion efficiency. The improved exothermic reaction occurred due to the direct contact between the Ni nanoparticles and the Al particles, leading to the SHS reaction. This result can be used to
9
improve the performance of an object that requires a large amount of energy over a long period of time, with applications in the areas of rocket propulsion, given the improved oxidation characteristics of sub-um Al combustion.
Acknowledgments This study was supported by the Fundamental Research Program funded by the Agency for Defense Development of Korea (Contract No. UD160008GD) and K.T.K thanks for the financial support from the the Principal R&D project titled as ‘Development of fundamental technology for tailoring properties of materials based on metastable microstructure’ (Code No. PNK5590) of Korea Institute of Materials Science (KIMS) in Republic of Korea.
References 1. 2. 3. 4. 5.
6. 7.
8. 9.
J.P. Agrawal, Recent trends in high-energy materials, Prog. Energy Combust. Sci. 24 (1998) 1-30. P.F. Pagoria, G.S. Lee, A.R. Mitchell,R.D. Schmidt, A review of energetic materials synthesis, Thermochimica Acta 384 (2002) 187-204. D. Badgujar, M. Talawar, S. Asthana,P. Mahulikar, Advances in science and technology of modern energetic materials: an overview, J. Hazard. Mater. 151 (2008) 289-305. D.W. Kim, K.T. Kim, T.S. Min, K.J. Kim,S.H. Kim, Improved Energetic-Behaviors of Spontaneously Surface-Mediated Al Particles, Sci. Rep. 7 (2017) 4659. J. Zhi, L. ShuFen, Z. Feng Qi, L. Zi Ru, Y. Cui Mei, L. Yang,L. Shang‐ Wen, Research on the combustion properties of propellants with low content of nano metal powders, Propellants, Explos., Pyrotech. 31 (2006) 139-147. S. Hasani, M. Panjepour,M. Shamanian, The oxidation mechanism of pure aluminum powder particles, Oxid. Met. 78 (2012) 179-195. M.A. Trunov, M. Schoenitz, X. Zhu,E.L. Dreizin, Effect of polymorphic phase transformations in Al2O3 film on oxidation kinetics of aluminum powders, Combust. Flame 140 (2005) 310-318. Y. Ohkura, P.M. Rao,X. Zheng, Flash ignition of Al nanoparticles: Mechanism and applications, Combust. Flame 158 (2011) 2544-2548. A.P. Il'in, A.A. Gromov, V.I. Vereshchagin, E. Popenko, V. Surgin,H. Lehn, Combustion of ultrafine aluminum in air, Combust., Explos. Shock Waves 37 (2001) 664-668.
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10. 11.
12. 13. 14. 15.
16.
D.W. Kim, G.H. Kwon, K.T. Kim, Synthesis of Nickel Nanoparticle-adsorbed Aluminum Powders for Energetic Applications, J. Korean Powder Metall. Inst. 24 (2017) 242-247 X. Guo, X. Li, H. Li, D. Zhang, C. Lai,W. Li, A Comprehensive investigation on the electrophoretic deposition (EPD) of Nano-Al/Ni energetic composite coatings for the combustion application, Sur. Coat. Technol. 265 (2015) 83-91. S.L. Vummidi, Y. Aly, M. Schoenitz,E.L. Dreizin, Characterization of Fine NickelCoated Powder as Potential Fuel Additive, J. Propul. Power 26 (2010) 454-460. K.T. Kim, J.-Y. Woo, Y.J. Choi,C.K. Kim, Influence of Nickel-Coating on the Thermal Oxidation Behaviors of Aluminum Powders, Korean J. Met. Mater. 53 (2015) 873-882. V. Babuk,V. Vasilyev, Model of aluminum agglomerate evolution in combustion products of solid rocket propellant, J. Propul. Power 18 (2002) 814-823. V.A. Arkhipov,A.G. Korotkikh, The influence of aluminum powder dispersity on composite solid propellants ignitability by laser radiation, Combust. Flame 159 (2012) 409-415. A.K.-V. Alexander V. Naumkin, Stephen W. Gaarenstroom, and Cedric J. Powell, NIST Standard Reference Database 20, Version 4.1, in, 2010.
11
Table 1. Quantitative analysis on contents of each components obtained from the XPS analysis of SNA and mixed particles heat-treated at 773 K
Contents of Components (at.%) Sample Ni 2p
C 1s
Al 2p
O 1s
SNA particles
1.3%
18.4%
27.5%
52.8%
Mixed particles
5.8%
18.8%
32.9%
42.5%
12
Fig. 1. (a) Schematic illustration showing the synthetic process of the snowman-shaped Ni/Al (SNA) particles. (b) Surface morphology of the synthesized SNA particles and results of the SNA particles enlarged in the inset photo, and (c) a cross-sectional TEM imgae and depiction of an SNA particles with energy-dispersive spectroscopy(EDS) results at the Ni/Al interface.
13
Fig. 2. (a) A cross-sectional TEM image and SAED patterns of the SNA particle. (b) Enlarged TEM image of an Al particle showing Al surface oxide in the yellow box in (a). (c) A TEM image of the Ni/Al interface as indicated by the green-lined box in (a), and (d) EDS mapping results showing the distribution of the Al, Ni, O and P atoms in SNA particles.
14
Fig. 3. Comparison of (a) TGA curves and (b) DSC curve of both SNA and mixed particles thermally oxidized in an air atmosphere. (c) Variation of the DSC curves with an increase in the temperature under an Ar atmosphere. (d) Comparison of exothermic enthalpy values between SNA and mixed particles calculated to the Zones.
15
Fig. 4. (a) XPS analysis of SNA and mixed particles heat-treated at 773 K. Enlarged XPS curves of (b) mixed particles and (c) of SNA particles, respectiely, to determine the ratio of NiAl3 formed by the SHS reaction in Ni2p2/3. (d) Schematic illustrations and FE-SEM images of the thermally oxidized SNA particles according to the temperature to show SHS reaction and oxidation reactivity at the Ni/Al interface.
16
Fabrication and Exothermic Reactions of Snowman-shaped Ultra-fine Particles with Directly Bonded Ni/Al interface
Gu Hyun Kwon1,2, Kyung Tae Kim1*, Dong Won Kim1, Jungho Choe1, Jung Yeul Yun1, Jong Man Kim2 1
Metal Powder Department, Korea Institute of Materials Science, 797 Changwon-daero, Seongsan-gu, Changwon, Gyeongnam 51508, Republic of Korea
2
Department of Nano Fusion Technology, Pusan National University, San 30, Jangjeon-dong, Geumjung-gu, Busan 609-735, Republic of Korea
Highlight - Snowman-shaped Ni/Al particles are synthesized by removing Al oxide and plating Ni. - The Ni nanoparticle is only coated on the surface oxide removed region of Al surface. - The SNA particle shows high exothermic enthalpy energy during thermal oxidation. - The directly bonded Ni/Al interface produces a rapid and much intermetallic compounds
17
18
S0169-4332(19)30076-5 https://doi.org/10.1016/j.apsusc.2019.01.068 APSUSC 41467
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
7 September 2018 30 December 2018 8 January 2019
Please cite this article as: G. Hyun Kwon, K. Tae Kim, D. Won Kim, J. Choe, J. Yeul Yun, J. Man Kim, Synthesis and Exothermic Reactions of Ultra-fine Snowman-shaped Particles with Directly Bonded Ni/Al interfaces, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.01.068
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis and Exothermic Reactions of Ultra-fine Snowman-shaped Particles with Directly Bonded Ni/Al interfaces
Gu Hyun Kwon1,2, Kyung Tae Kim1*, Dong Won Kim1 , Jungho Choe1, Jung Yeul Yun1, Jong Man Kim2* 1
Metal Powder Department, Korea Institute of Materials Science, 797 Changwon-daero, Seongsan-gu, Changwon, Gyeongnam 51508, Republic of Korea
2
Department of Nanoenergy Engineering, Pusan National University, San 30, Jangjeon-dong, Geumjung-gu, Busan 609-735, Republic of Korea
1
Abstract Nickel-nanoparticle-attached aluminum (Ni/Al) particles are synthesized by plating Ni onto Al particles with the surface oxide partly removed. The microstructure of the synthesized Ni/Al particles resembles a child’s snowman, with an approximately 200 nm spherical Ni nanoparticle bonded onto the surface of an Al particle about 800 nm in size. Particularly, the surface oxide is rarely observed at the Ni/Al interfaces unlike uncoated surface of the Al powder. Due to these microstructural characteristics, it was clearly noted that a self-propagating high-temperature synthesis (SHS) reaction of the snowman-shaped Ni/Al (SNA) particles occurs more actively compared to that which occurs when simply mixing particles of Ni and Al in the temperature range of 720-900 K. The formation of intermetallic compounds of NiAl or NiAl3 is confirmed in a comparison of XPS results after both samples were oxidized at 773 K. The exothermic enthalpy energy, 8,850 Jg-1 of SNA particles represents 118% of the value of 7,580 Jg-1 for the mixed particles. These results indicate that SNA particles are promising as advanced reactive materials when used in conjunction with an efficient SHS reaction.
Keywords: Reactive Metal Powder; Nickel Coating; Aluminum; SHS reaction *Corresponding
authors :
K.T.K.
E-mail:
[email protected]
2
[email protected],
and
J.K.
E-mail :
1. Introduction Energetic metals such as aluminum (Al), beryllium (Be), zirconium (Zr), boron (B) and titanium (Ti) have been widely utilized in civil and military applications as explosives, propellants, welding materials and pyrotechnic materials [1-3]. The origin of the energy release in energetic metals is the exothermic heat energy caused by their oxidation, and aluminum (Al) is especially well known to possess high volumetric thermal energy (31 kcal/cm3)
[4, 5]
. However,
unfortunately, the empirical amount of energy released by Al during combustion is generally lower than expected because re-passivation of the surface by natural oxidation prevents the continuous oxidation of the Al in powder form
[6]
. Furthermore, because the surface oxide film
has a high melting point of approximately 2300 K, it prevents direct contact between the external oxygen and internal Al at low temperatures [7]. Although the exothermic enthalpy energy can be increased due to the enlarged specific surface area with finer sizes of the Al particles, the limitation caused by re-passivation remains [8, 9]. Recently, several researchers have focused on the mixing or coating of additive materials to enhance the thermal oxidation behaviors of fine Al powders. Among the many additive candidates, nickel (Ni) materials are known to be promising agents for Al powders when used for coating or mixing due to the positive effect realized via self-propagating high-temperature synthesis (SHS) [10]. Despite the many approaches which have been tested thus far [11-13], research on Ni materials applied to sub-um-sized Al has rarely been conducted owing to the experimental difficulty involved when attempting the homogeneous coating or mixing of Ni nanoparticles 15]
[14,
. It is known that sub-um Al particles offer the simultaneous advantages of a low ignition
temperature and an appropriate energy release given the combined properties of micron- and
3
nano-scale particles. Thus, the development of novel approaches for efficiently using the SHS reaction between Ni and sub-um sized Al particles is closely related to the successful synthesizing of advanced reactive Al particles which show superior exothermic reaction outcomes. In this study, we designed a synthetic process of a novel material, referred to here as a snowman-shaped Ni/Al (SNA) particle, in which a Ni particle 200 nm in size is attached to the surface of a sub-um Al particle. The attachment of Ni nanoparticles is done in an effort to realize an enhanced SHS reaction during the oxidation or combustion process by inducing the direct bonding between Ni and Al. The enhanced exothermic energy in the SNA particles is compared to that of a powder sample created by simply mixing Ni nanoparticles and sub-um Al particles. In addition, the influence of the rapid SHS reaction on the thermal oxidation behaviors is investigated by analyzing the byproducts of intermetallic compounds from oxidized SNA particles.
2. Material and methods 2.1 Synthesis of SNA particles Spherical Al powder with an average size of 800 nm was used. The Ni2(SO4)3 precursor solution used (Adhemax Ni LFS 1,2, and 3, Atotech Ltd.) was solutionized into 91.0 ml of distilled water. The pH of the Ni precursor solution was adjusted to 9.0 with an ammonia hydroxide solution (NH4OH, 28.0-30.0 wt%). At the same time, an alkaline etching solution to remove the surface oxide was prepared by mixing 4.0 g of an ammonia solution and 30.0 ml of distilled water. The sub-um Al powder was added to the etching solution and was stirred for 3 min. The Ni precursor solution and etching solution were then merged to nucleate Ni
4
nanoparticles onto the Al particle heterogeneously after the removal of the surface oxide. Both chemical reactions, i.e., oxide etching and Ni plating, were simultaneously carried out for 30 min. at room temperature. Finally, the prepared powder was collected using a membrane filter, after which it was rinsed with water and then dried at 333 K for more than 24 hours in a vacuum oven. For comparison, Ni nanoparticles (AVENTION Co.) approximately 200 nm in size were mixed with the 800 nm Al particles by a ball milling process for 10 min. with 5mm-sized zirconia balls at a mixing ratio of 1 to 4 of Ni and Al, respectively.
2.2 Characterization The morphology of the powder was characterized by field-emission scanning electron microscope (FE-SEM, MIRA II LMH, Tescan). An elemental analysis of the sythesized particles was conducted using FE-SEM with energy dispersive spectroscopy (EDS). X-ray photon spectroscopy (XPS, Axis Supra) was used for a quantitative analysis to determine whether NiAl-based intermetallic compounds formed. A thermo-gravimetric analysis and a differential scanning calorimetry (TGA/DSC, TA Instruments, Q600) assessment were conducted at a heating rate of 10 Kmin-1 from 298 K to 1673 K in air and in an Ar atmosphere, respectively. Interfacial structures were characterized and an elemental analysis was conducted of the SNA particles using a field-emission transmission electron microscope (EDS-attached FE-TEM, JEM2100F, JEOL) operating at 200 kV. The TEM sample was prepared using a lift-off technique with a focused ion beam (FIB, NOVA200, FEI Inc.).
3. Results and discussion
5
3.1 Microstructure of SNA particles Fig. 1(a) shows a schematic illustration of the synthesis process used to create a Ninanoparticle-attached Al particle, referred to here as a snowman-shaped Ni/Al (SNA) particle. The surface oxide is partially removed by controlling both the amount of Al particles and the pH. The plating of the Ni materials preferentially occurs on the revealed Al surfaces due to the differences between the reduction potentials, with the result being spherical Ni particles. In the alkaline etching solution used to remove the Al surface oxide, the Al/Al2O3 interface of the particles reacts following the formula below (1):
2Al(s) + 6OH- + 6H+ → 2Al(OH)3(s) + 3H2(g)↑ (1)
The Ni2+ cations of the Ni precursor solution are initially reduced to Ni0 on the Al surface in the region where the oxide film is removed, after which seeds can form and grow. Ni ions can be reduced by the difference in the reduction potential when Al is exposed to electrons or by a reducing agent. Fig. 1(b) shows the microstructure of the synthesized SNA particles. The EDS results clearly show that a Ni nanoparticle comes directly into contact with an Al particle. Fig. 2(a) shows cross-sectional morphologies and the results of a corresponding elemental analysis of a SNA particle. The dark contrasting region corresponds to Ni, which is attached to the Al surface and from which oxide film is not observed at the Ni/Al interface. The phase of the attached Ni nanoparticle is confirmed as amorphous according to the results of a pattern analysis via selected area electron diffraction (SAED) of the Al and Ni/Al interface. Fig. 2(b) is an enlarged TEM image of an uncoated surface of an Al particle. The results provide evidence that a surface oxide layer approximately 7 nm thick is continuously present expect at the Ni/Al
6
interfaces. Fig. 2(c) is an enlarged TEM image of the Ni/Al interface. As shown in Fig. 2(d), the results of an EDS mapping analysis of the Ni/Al interface do not show oxide film.
3.2 Thermal oxidation behaviors Fig. 3(a) and (b) present the results of a comparison of the TGA/DSC outcomes obtained from both a SNA particles and the mixed particles as a function of the temperature when it ranged from 298 K to 1673 K. As shown in Fig. 3(a) no major differences in the weight change caused by thermal oxidation between the SNA particles and mixed particles were noted. This likely stems from overall oxidation of similar amounts of Ni and Al particles. Fig. 3(b) presents DSC curves which show exothermic reaction peaks in both particles in the two different temperature ranges of 600-910 K (Zone Ⅰ) and 960-1,300 K (Zone Ⅱ). The SNA particles show evidence of slightly higher exothermic energy than the mixed particles in both peaks. Fig. 3 (c) shows the DSC measured in an Ar atmosphere to analyze the differences in the exothermic peaks. It was found that the mixed particles had exothermic energy of 340.2 Jg-1 and that the SNA particles had exothermic energy of 1,126.9 Jg-1. This indicates that the exothermic reaction which occurs in an inert gas atmosphere must be from the SHS reaction which makes an intermetallic compound between Al and Ni. Al and Ni can react as shown in the following formula (2):
Al (s) + Ni (s) → NiAl3 + 150.6 kJ mol-1
(2)
Fig. 3(d) compares the exothermic enthalpy energies of both particles obtained from the
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each zone of the DSC peaks in the temperature range of 298-1473 K. In Zone Ⅰ, the energy levels for the mixed particles and for the SNA particles are approximately 2.4 kJg-1 and 2.7 kJg-1, respectively. In Zone Ⅱ, the exothermic energies for the SNA and mixed particles were determined to be 5.1 kJg-1 and 6.2 kJg-1, respectively. The total exothermic energy of the SNA particles at 8.9 kJg-1 is 118% of the value of 7.5 kJg-1 for the mixed particles below 1,400 K. This means that a rapid and more active SHS reaction readily activated the ignition of the oxidation of Al, leading to further oxidation. In other words, because the diffusion of Al in the mixed powder is limited by the oxide film present on the surface, a high temperature is required to form the intermetallic compounds compared to the SNA particles. Direct contact between Ni and Al feasibly forms intermetallic compounds such as NiAl3 or NiAl related materials, a process known to provide additional exothermic energy apart from the main oxidation of Al.
Fig. 4(a) shows the wide-scan results of the XPS analysis of the SNA and mixed particles after they were heat-treated at 773 K. As shown in Table 1, the ratio of Al is lower than that of the mixed particles, while the ratio of oxygen is relatively high, indicating that the SNA particles were oxidized further before melting. Fig. 4(b) and (c) show the results of an XPS analysis of the surfaces of both particles, revealing the ratio of the NiAl3 as obtained from the electronic binding energy of Ni2p. It was found that when the temperature is increased to 773 K, both the mixed and SNA particles form intermetallic compounds of NiAl3 and NiO, respectively. The formation ratios of NiAl3/NiO in the mixed particles and SNA particles were analyzed and found to be 15.5/84.5 and 26.7/73.3, respectively [16]. This occurs because the direct contact between Ni and Al facilitates the formation of the intermetallic compounds. Fig. 4(d) is a schematic illustration of the reaction mechanisms of the SNA particles at the Ni/Al interfaces during the combustion
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process or the thermal oxidation reaction. As the temperature is increased to 700 K, some of the Ni is oxidized, resulting in Ni and NiO phase which are present in the Ni layer at the same time. At 720 K, Ni and NiO can diffuse into Al and the subsequent melting process forms the intermetallic compound NiAl3 with a simultaneous exothermic reaction. In addition, a small amount of oxygen existing in the Ni layer reacts with the internal Al and causes a small amount of oxidation. Moreover, after oxidation at 1100 K, Ni and NiO diffuse into the molten Al, causing the Ni layer to become thin or the Al to be exposed to external oxygen. Hence, external oxygen actively reacts with the Al core.
4.Conclusions In summary, snowman-shaped Ni/Al (SNA) particles were synthesized by removing the surface oxide while at the same time plating Ni particles. Ni nanoparticles approximately 200 nm in size were initially nucleated and then revealed spherical pores in an oxide-free region which formed on the Al surface during a subsequent etching process. The attachment of Ni nanoparticles onto Al particles prevented their separation during oxidation and combustion, but this phenomenon also provides an advantage with regard to the formation of intermetallic compounds due to the direct contact between the Al and Ni materials. During the SHS reaction, it was noted that NiAl3 phases are mainly formed in the SNA particles such that further exothermic energy is released. As a result of the thermal oxidation of SNA particles, the exothermic energy released amounted to 8.9 kJg-1, while that for the mixed powder was 7.5 kJg-1, with the former value representing 118% of the latter, thus improving the combustion efficiency. The improved exothermic reaction occurred due to the direct contact between the Ni nanoparticles and the Al particles, leading to the SHS reaction. This result can be used to
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improve the performance of an object that requires a large amount of energy over a long period of time, with applications in the areas of rocket propulsion, given the improved oxidation characteristics of sub-um Al combustion.
Acknowledgments This study was supported by the Fundamental Research Program funded by the Agency for Defense Development of Korea (Contract No. UD160008GD) and K.T.K thanks for the financial support from the the Principal R&D project titled as ‘Development of fundamental technology for tailoring properties of materials based on metastable microstructure’ (Code No. PNK5590) of Korea Institute of Materials Science (KIMS) in Republic of Korea.
References 1. 2. 3. 4. 5.
6. 7.
8. 9.
J.P. Agrawal, Recent trends in high-energy materials, Prog. Energy Combust. Sci. 24 (1998) 1-30. P.F. Pagoria, G.S. Lee, A.R. Mitchell,R.D. Schmidt, A review of energetic materials synthesis, Thermochimica Acta 384 (2002) 187-204. D. Badgujar, M. Talawar, S. Asthana,P. Mahulikar, Advances in science and technology of modern energetic materials: an overview, J. Hazard. Mater. 151 (2008) 289-305. D.W. Kim, K.T. Kim, T.S. Min, K.J. Kim,S.H. Kim, Improved Energetic-Behaviors of Spontaneously Surface-Mediated Al Particles, Sci. Rep. 7 (2017) 4659. J. Zhi, L. ShuFen, Z. Feng Qi, L. Zi Ru, Y. Cui Mei, L. Yang,L. Shang‐ Wen, Research on the combustion properties of propellants with low content of nano metal powders, Propellants, Explos., Pyrotech. 31 (2006) 139-147. S. Hasani, M. Panjepour,M. Shamanian, The oxidation mechanism of pure aluminum powder particles, Oxid. Met. 78 (2012) 179-195. M.A. Trunov, M. Schoenitz, X. Zhu,E.L. Dreizin, Effect of polymorphic phase transformations in Al2O3 film on oxidation kinetics of aluminum powders, Combust. Flame 140 (2005) 310-318. Y. Ohkura, P.M. Rao,X. Zheng, Flash ignition of Al nanoparticles: Mechanism and applications, Combust. Flame 158 (2011) 2544-2548. A.P. Il'in, A.A. Gromov, V.I. Vereshchagin, E. Popenko, V. Surgin,H. Lehn, Combustion of ultrafine aluminum in air, Combust., Explos. Shock Waves 37 (2001) 664-668.
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10. 11.
12. 13. 14. 15.
16.
D.W. Kim, G.H. Kwon, K.T. Kim, Synthesis of Nickel Nanoparticle-adsorbed Aluminum Powders for Energetic Applications, J. Korean Powder Metall. Inst. 24 (2017) 242-247 X. Guo, X. Li, H. Li, D. Zhang, C. Lai,W. Li, A Comprehensive investigation on the electrophoretic deposition (EPD) of Nano-Al/Ni energetic composite coatings for the combustion application, Sur. Coat. Technol. 265 (2015) 83-91. S.L. Vummidi, Y. Aly, M. Schoenitz,E.L. Dreizin, Characterization of Fine NickelCoated Powder as Potential Fuel Additive, J. Propul. Power 26 (2010) 454-460. K.T. Kim, J.-Y. Woo, Y.J. Choi,C.K. Kim, Influence of Nickel-Coating on the Thermal Oxidation Behaviors of Aluminum Powders, Korean J. Met. Mater. 53 (2015) 873-882. V. Babuk,V. Vasilyev, Model of aluminum agglomerate evolution in combustion products of solid rocket propellant, J. Propul. Power 18 (2002) 814-823. V.A. Arkhipov,A.G. Korotkikh, The influence of aluminum powder dispersity on composite solid propellants ignitability by laser radiation, Combust. Flame 159 (2012) 409-415. A.K.-V. Alexander V. Naumkin, Stephen W. Gaarenstroom, and Cedric J. Powell, NIST Standard Reference Database 20, Version 4.1, in, 2010.
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Table 1. Quantitative analysis on contents of each components obtained from the XPS analysis of SNA and mixed particles heat-treated at 773 K
Contents of Components (at.%) Sample Ni 2p
C 1s
Al 2p
O 1s
SNA particles
1.3%
18.4%
27.5%
52.8%
Mixed particles
5.8%
18.8%
32.9%
42.5%
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Fig. 1. (a) Schematic illustration showing the synthetic process of the snowman-shaped Ni/Al (SNA) particles. (b) Surface morphology of the synthesized SNA particles and results of the SNA particles enlarged in the inset photo, and (c) a cross-sectional TEM imgae and depiction of an SNA particles with energy-dispersive spectroscopy(EDS) results at the Ni/Al interface.
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Fig. 2. (a) A cross-sectional TEM image and SAED patterns of the SNA particle. (b) Enlarged TEM image of an Al particle showing Al surface oxide in the yellow box in (a). (c) A TEM image of the Ni/Al interface as indicated by the green-lined box in (a), and (d) EDS mapping results showing the distribution of the Al, Ni, O and P atoms in SNA particles.
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Fig. 3. Comparison of (a) TGA curves and (b) DSC curve of both SNA and mixed particles thermally oxidized in an air atmosphere. (c) Variation of the DSC curves with an increase in the temperature under an Ar atmosphere. (d) Comparison of exothermic enthalpy values between SNA and mixed particles calculated to the Zones.
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Fig. 4. (a) XPS analysis of SNA and mixed particles heat-treated at 773 K. Enlarged XPS curves of (b) mixed particles and (c) of SNA particles, respectiely, to determine the ratio of NiAl3 formed by the SHS reaction in Ni2p2/3. (d) Schematic illustrations and FE-SEM images of the thermally oxidized SNA particles according to the temperature to show SHS reaction and oxidation reactivity at the Ni/Al interface.
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Fabrication and Exothermic Reactions of Snowman-shaped Ultra-fine Particles with Directly Bonded Ni/Al interface
Gu Hyun Kwon1,2, Kyung Tae Kim1*, Dong Won Kim1, Jungho Choe1, Jung Yeul Yun1, Jong Man Kim2 1
Metal Powder Department, Korea Institute of Materials Science, 797 Changwon-daero, Seongsan-gu, Changwon, Gyeongnam 51508, Republic of Korea
2
Department of Nano Fusion Technology, Pusan National University, San 30, Jangjeon-dong, Geumjung-gu, Busan 609-735, Republic of Korea
Highlight - Snowman-shaped Ni/Al particles are synthesized by removing Al oxide and plating Ni. - The Ni nanoparticle is only coated on the surface oxide removed region of Al surface. - The SNA particle shows high exothermic enthalpy energy during thermal oxidation. - The directly bonded Ni/Al interface produces a rapid and much intermetallic compounds
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