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Aerosol growth and photothermal ignition of multilayer graphene-encapsulated nickel nanoparticles
Powder Technology 364 (2020) 522–530
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Powder Technology journal homepage: www.elsevier.com/locate/powtec
Aerosol growth and photothermal ignition of multilayer graphene-encapsulated nickel nanoparticles Ji Hoon Kim a, Ho Sung Kim b, Soo Hyung Kim a,b,c,⁎ a b c
Research Center for Energy Convergence Technology, Pusan National University, 2, Busandaehak-ro 63beon-gil, Geumjeong-gu, Busan 46241, Republic of Korea Department of Nanofusion Technology, College of Nanoscience and Nanotechnology, Pusan National University, 2, Busandaehak-ro 63beon-gil, Geumjeong-gu, Busan 46241, Republic of Korea Department of Nanoenergy Engineering, College of Nanoscience and Nanotechnology, Pusan National University, 2, Busandaehak-ro 63beon-gil, Geumjeong-gu, Busan 46241, Republic of Korea
a r t i c l e
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Article history: Received 19 August 2019 Received in revised form 23 January 2020 Accepted 8 February 2020 Available online 11 February 2020 Keywords: Graphene Nickel nanoparticles Spray pyrolysis Thermal chemical vapor deposition Optical igniter
a b s t r a c t In this report, we propose an easy and viable fabrication method for growing multilayer graphene (MLG)-encapsulated nickel nanoparticles (Ni NPs) via a one-step continuous aerosol process comprising combined spray pyrolysis (SP) and thermal chemical vapor deposition (CVD). A bimodal size distribution was observed for the Ni NPs owing to droplet-to-particle and gas-to-particle conversions undergone in the SP at ~1000 °C. The number of graphene layers grown on the surface of Ni NPs in the thermal CVD was controlled by varying the reaction temperature of 300–700 °C, which perturbed the solubility and diffusion rates of carbon in Ni NPs. Finally, we demonstrated that the MLG-encapsulated Ni NP-added nanoenergetic materials were stably ignited by flash irradiation in a short ignition delay time (~267 μs) with fast burn rate (~200 m·s−1). This suggests that the MLG-encapsulated Ni NPs could play an important role as potential optical igniters for practical thermal engineering applications. © 2020 Published by Elsevier B.V.
1. Introduction Graphene has attracted a lot of attention in many fields including those involving lithium-ion batteries [1–4], proton-membrane fuel cells [5], supercapacitors [6,7], sensors [8–11], and lubricants [12] because of its two-dimensional (2D) carbon nanostructure with a unique honeycomb lattice. It offers outstanding physical properties such as a high specific surface area, excellent thermal and electrical conductivity, and high mechanical strength. However, it is inevitable that the aggregation of graphene layers owing to π-π stacking and van der Waals interactions (i.e., large sheet aspect ratio and stacking structure) results in decreased specific surface area so that the superior properties of individual graphene sheet cannot be fully exploited [13]. The aggregation is caused by the 2D structural characteristics of graphene; thus, it can be relieved to a large extent through introducing structural changes. Recently, many researchers have synthesized threedimensional (3D)-structured graphene, in the form of graphene balls [3–7,14], crumpled graphene balls [15–19], and graphene foam [20–22], employing various methods such as exfoliation, hydrothermal synthesis, chemical vapor deposition (CVD), spray pyrolysis (SP), and ⁎ Corresponding author at: Research Center for Energy Convergence Technology, Pusan National University, 2, Busandaehak-ro 63beon-gil, Geumjeong-gu, Busan 46241, Republic of Korea. E-mail address: [email protected] (S.H. Kim).
https://doi.org/10.1016/j.powtec.2020.02.023 0032-5910/© 2020 Published by Elsevier B.V.
spray drying. Among various metallic catalysts, Ni nanoparticles (NPs) are known as one of the promising catalysts for growing graphene layers, and they exhibit tunable mechanical, electrical, and optical properties depending on the size, shape and aggregation state [4,8,23]. The encapsulation of metal NPs with graphene layers can play the role of protecting the metal core from possible oxidation and simultaneously improving the electrical conductivity, chemical stability, and optical absorptivity of metal core. Graphene layers are formed on the surface of Ni NPs due to dissolution-precipitation mechanism based on high solubility of carbon in Ni at relatively high temperatures [4]. The preparation of 3D-structured graphene on the surface of metal NPs remains challenging in various synthetic processes. Heat treatment of metal-organic framework and hydrothermal synthetic processes can be suggested promising methods, but complex chemical treatment, secondary solvent extraction, and multiple heating processes are often required to obtain graphene layer-encapsulated metal NPs. In addition, the controlled growth of number of graphene layer is difficult because supply of carbon sources to the metal NPs are not easily controlled in those approaches [4,23]. In this report, we suggest a simple and scalable method for the synthesis of 3D multilayer graphene-encapsulated metal NPs in the gas phase. Specifically, in our study, nickel nanoparticles (Ni NPs) were synthesized by SP, and then multilayer graphene (MLG) was continuously grown on their surface using thermal CVD. The size of Ni NPs and thickness of MLG were controlled precisely by the operating conditions (i.e., initial concentration of Ni precursor
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solution and reaction temperature). We developed a new aerosol process to fabricate MLG-encapsulated Ni NPs with tunable particle sizes and graphene layer thicknesses. Finally, we demonstrate a potential application of the MLGencapsulated Ni NPs as optical igniters for triggering nanoscale energetic materials (nEMs) based on the enhanced photothermal effect [24,25], which can be occurred because of the metal NPs being encapsulated with multiple graphene layers. The nEMs are composites composed of nanoscale metallic fuels (e.g., Al, Mg) and inorganic oxidizers (e.g., CuO, Fe2O3, Bi2O3) which emits a huge amount of heat and pressure instantaneously through very fast redox reaction between fuel and oxidizer. It has attracted attention in various thermal engineering fields including explosives, propellants, and pyrotechnics due to the potential of developing multifunctional combustion systems with enhanced capabilities at low costs. Recently, various graphene-based materials (e.g., graphene [26,27], graphene oxide [28–30], functionalized graphene [31,32]) have been applied to the nEMs to improve the ignition and combustion characteristics utilizing unique physicochemical properties of graphene-based materials (i.e., high thermal and electrical conductivity, high specific surface area, functionalization, photothermal effect, etc.). In this study, Ni NPs with improved photothermal conversion characteristics by MLG encapsulation were applied to nEMs (i.e., Al/CuO NP) to induce successful optical ignition by photoflash irradiation.
2. Experimental 2.1. Aerosol synthesis of MLG on the surface of Ni NPs A simple one-step continuous aerosol process consisting of conventional SP and thermal CVD was employed to synthesize the MLG on the surface of Ni NPs as shown in Fig. 1. First, the precursor solution was prepared by dissolving nickel nitrate hexahydrate (Ni(NO3)2·6H2O, Sigma Aldrich) in deionized water at a concentration of 1–20 wt% and the precursor solution was aerosolized by a homemade ultrasonic nebulizer operated at 40 W and 60 Hz. The nickel nitrate aerosol droplets were then transferred to a silica-gel dryer under N2 flow of 3 standard liters per minutes (slpm). After passing through a silica-gel dryer, the dried nickel nitrate particles were transformed into nickel oxide NPs through solvent evaporation and thermal decomposition in the first
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tube furnace reactor (2.54 cm diameter, 30 cm heating length) heated at ~1000 °C. Subsequently, the nickel oxide NPs were reduced under a hydrogen flow of 100 standard cubic centimeters per minute (sccm) and finally pure Ni NPs were formed. The residence time of aerosol flow in the primary reactor was ~2.09 s. The resulting Ni NPs were then rapidly transported into the second tube furnace (5.48 cm diameter, 30 cm heating length) heated at 300–700 °C and reacted with acetylene (10 sccm) and hydrogen (100 sccm) gases over a residence time of ~13.64 s. Through a catalytic reaction of Ni NPs with acetylene, MLG was grown on the surface of Ni NPs. Finally, the resulting MLGencapsulated Ni NPs were collected on a membrane filter with pore size of 200 nm.
2.2. Material characterization The MLG-encapsulated Ni NPs were characterized by scanning electron microscopy (SEM; ZEISS, Model No. SUPRA40VP) operated at 15 kV, Cs-corrected scanning transmission electron microscopy (STEM; JEOL, Model No. JEM-2100) operated at 200 kV, X-ray diffraction (XRD; PANalytical, Model No. Empyrean series2) using Cu Κα radiation, Raman spectroscopy (Horiba Jobin Yvon, Model No. LabRam Aramis) using a 514.5 nm (2.41 eV) Ar-ion laser at 100 μW power, and X-ray photoelectron spectroscopy (XPS; Kratos analytical, Model No. AXIS Supra) using Al Kα radiation (1486.6 eV). The electrical conductivity of the MLG-encapsulated Ni NPs and pure Ni NPs was measured using a two-probe electrical resistance measurement system and powder compaction devices [33,34]. Briefly, 100 mg of sample powders were filled in a hollow polytetrafluoroethylene (PTFE) cylinder with an inner diameter of 5 mm, and compressed between two Cu pistons. The Cu pistons were connected to a digital multimeter (Keysight, Model No. 34465A) to measure the electrical resistance. The pressure on the device was applied using an automatic press machine (Japan Instrumentation System, Model No. JSV H1000) equipped with a 100 N load cell. The maximum pressure reached during the test was ~5 MPa using a constant displacement speed of 1 mm min−1. Thermogravimetric and differential scanning calorimetry (TG-DSC) analyses were also conducted at temperatures ranging from 30 °C to 1000 °C at a heating rate of 10 °C min−1 in air for 10 mg of samples. The MLG-encapsulated Ni NP-added Al and CuO NP composites were characterized using scanning electron microscopy (SEM; Hitachi,
Fig. 1. Schematic of experimental setup for aerosol synthesis of MLG-encapsulated Ni NPs.
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Model No. S4700) operated at 15 kV and Cs-corrected scanning transmission electron microscopy (STEM; JEOL, Model No. JEM-2100) operated at 200 kV. 2.3. Fabrication of MLG-encapsulated Ni NP-added Al/CuO composites Commercially available Al NPs (Nanotechnology Inc.) and CuO NPs (Sigma Aldrich) with average primary particle sizes of approximately ~80 nm and ~100 nm were used as a fuel and an oxidizer, respectively. The MLG-encapsulated Ni NPs were used as optical igniters. First, the MLG-encapsulated Ni NP-added Al and CuO NP composite powder was prepared. Briefly, Al NPs (fuel) were mixed with CuO NPs (oxidizer) in an ethanol (EtOH) solution. The mixing ratio of the fuel and oxidizer
was fixed at Al:CuO = 30:70 by weight to ensure strong explosion reactivity [35]. The MLG-encapsulated Ni NPs fabricated were then added to the Al/CuO-dispersed solution at a mixing ratio of 1 wt%. To ensure that the MLG-encapsulated Ni NP/Al NP/CuO NP was homogeneously mixed in the EtOH solution, the solution was ultrasonicated at 200 W and 40 kHz for approximately 30 min. Subsequently, the EtOH was dried in a convection oven at 80 °C for 30 min. Finally, the MLGencapsulated Ni NP-added Al/CuO composite powder was obtained. 2.4. Optical ignition test To examine the potential role of the MLG-encapsulated Ni NPs as optical igniters, flash irradiation was undertaken on the various fabricated
Fig. 2. (a) Low-resolution (LR)-SEM and (b) high-resolution (HR)-SEM images, (c) primary particle size distributions, and (d) LR-TEM and (e) HR-TEM images of MLG-encapsulated Ni NPs formed at 700 °C in the thermal CVD process. (Dp is the average particle size.)
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materials, namely, the Ni NPs, MLG-encapsulated Ni NPs, Al/CuO NPs, and MLG-encapsulated Ni NP (1 wt%)-added Al/CuO NPs. Briefly, sample powders were placed on a glass stage and exposed to a light generated using a camera flash under the glass stage. A light emitting hole with a diameter of 1 mm was made on the surface of camera flash covered with the glass stage and Al tape, and the sample powder was aligned (mass: 10 mg, particle length: 20 mm, width: 2 mm) thereon to measure the burn rate and ignition delay time for the MLG-encapsulated Ni NP (1 wt%)-added Al/CuO NP composite [36]. The reaction undergone by each sample was recorded using a high-speed camera (Photron, Model No. FASTCAM SA3 120 K) at a frame rate of 30 kHz. 3. Results and discussion SEM images presented in Fig. 2a and b show that spherical MLGencapsulated Ni NPs were synthesized by the combination of the SP and thermal CVD processes. The MLG-encapsulated Ni NPs showed a bimodal particle size distribution, in which the major peaks appeared at ~579 ± 11 nm for the large primary particles and ~23.5 ± 0.8 nm for the small primary particles (Fig. 2c). The particles formed in the SP process underwent two different growth mechanisms of droplet-to-particle and gas-to-particle conversions [37–39]. The aerosolized droplets containing metal nitrates turned into a solid particle during the SP and reduction processes. Most of the large primary particles were generated by the droplet-to-particle conversion, whereas the small primary particles seemed to arise from the gas-to-particle conversion. During the droplet-to-particle conversion of nickel nitrate-containing droplets in the first tube furnace reactor, the solid particles evaporated rapidly so that the vapors of solid particles formed new small primary particles by nucleation and condensation. The vapor pressures of the Ni and
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NiO at 1000 °C in the SP was calculated to ~2.19 × 10−10 and ~2.02 × 10−11 atm, respectively [40–42]. The small Ni NPs were formed by the nucleation of Ni vapors evaporated from the large Ni NPs and the reduction of NiO NPs formed by NiO vapors. The solid particle formation is strongly dependent on various parameters, including the physicochemical properties and concentrations of precursors and the various operating conditions such as the pressure, temperature, flow rate, and cooling rate involved in the evaporation, nucleation, and condensation of particles [39–43]. The particle size distribution was mostly controlled by the initial concentration of the precursor solution. As the initial concentration of the precursor solution was increased from 1 to 20 wt%, the size of the large primary particle increased gradually, wherein a linear relationship was observed between the logarithms of the precursor concentration and the particle diameter [43] (i.e.,log(dp,v) = 1/3 ⋅ log C + log (dd,v[M/ρp]1/3), where dp,v = volumetric particle diameter [m], dd,v = volumetric droplet diameter [m], C = precursor concentration [mol m−3], M = molecular weight of particle [kg mol−1], and ρp = density of precursor [kg m−3]). This was confirmed by SEM analyses for the fabricated solid particles, as shown in Fig. S1 in Supporting Information. TEM images shown in Fig. 2d and e reveal that Ni core particles were clearly encapsulated by the MLG shells, formed by concentric carbon precipitation on the entire surface of the Ni NPs. The mechanism for the formation of MLG shells on the surface of Ni NPs can be explained as follows. Carbon atoms first dissolved into Ni NPs by catalytic reactions undergone under the supply of hydrocarbon sources (i.e., C2H2). When the concentration of the carbon dissolved in Ni NPs exceeded the carbon solubility of the Ni NPs, carbon atoms began to precipitate on the surface of Ni NPs. The Ni NPs were fully covered by very thin graphene layers, and the formation of MLG-encapsulated Ni NPs was then finally complete [44,45].
Fig. 3. (a) XRD patterns, (b) Raman spectra of Ni NPs with and without MLG encapsulation, (c) C1s XPS spectrum of Ni NPs with MLG encapsulation, and (d) electrical conductivity of Ni NPs with and without MLG encapsulation. (The MLG was synthesized at 700 °C in the thermal CVD process.)
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Fig. 4. (a) TG and (b) DSC analyses of Ni NPs with and without MLG encapsulation. (The MLG was synthesized at 700 °C in the thermal CVD process.)
To examine the structural characteristics of Ni NPs with and without MLG encapsulation, X-ray diffraction (XRD) and Raman spectra analyses were performed. For the as-prepared Ni NPs without MLG encapsulation, strong X-ray diffraction peaks resulting from the presence of Ni were clearly observed, as shown in Fig. 3a. In addition, relatively weak peaks owing to NiO were observed. This was attributed to the surface oxidation of Ni NPs by the ambient airafter the SP and reduction process. However, for the MLG-encapsulated Ni NPs, only strong peaks for Ni appeared without any peaks for NiO, suggesting that the presence of MLG prevented the oxidation of Ni NPs by coating their entire surfaces. The X-ray diffraction peaks for MLG were not clearly observed in Fig. 3a due to the small fraction of carbon content compared to that of Ni. The Raman spectrum of Fig. 3b clearly exhibits that there were no carbon structures on the as-prepared Ni NPs that were formed by the SP process. However, the MLG formed on the surface of Ni NPs using the combination of SP and CVD showed two strong peaks at ~1350 cm−1 (D mode, disorder-induced band originating from defects or carbon impurities) and ~1600 cm−1 (G mode, stretching mode in the graphite plane). The ratio of signal intensities (i.e., ID/IG) at the Raman shifts of 1350 and 1600 cm−1 was found to be approximately 1.05, indicating that the degree of graphitization was relatively low for the carbon layer formed using this approach. This phenomenon was suggested to be due to the structural defects and amorphous nature of the carbon of the graphene layers grown on the spherical surface of Ni NPs, which resulted in the formation of wavy and discontinuous graphene layers with very small graphene domains [4,7,45]. In addition, a 2D peak was also detected at ~2700 cm−1, which was a second-order D peak with regard to the number of graphene layers [46–49], and another peak was observed for the presence of D + D’, which is the combination mode of D and D’ [45–50]. Here, the D’ peak arose from the double-phonon resonance process in the graphene layer containing defects [50]. The pure D’ peak was not clearly observed due to the superposition of the D’ peak with the G peak. XPS analysis is used to confirm the reduction status of graphene in the MLG-encapsulated Ni NPs. Fig. 3c shows the C1s XPS spectra of MLG-encapsulated Ni NPs. The C1s signals of the MLG-encapsulated Ni NPs deconvoluted into several signals for C\\Ni (~283.9 eV), C-C/C=C (~284.8 eV), C-OH/C-O-C (~286.5 eV), and O-C=O (~288.9 eV) bonds. The content of C-C/C=C
Fig. 5. TEM images of various MLG-encapsulated Ni NPs formed at (a) 300 °C, (b) 350 °C, (c) 400 °C, (d) 450 °C, (e) 500 °C, (f) 600 °C, and (g) 700 °C in the thermal CVD process. (h) Number of graphene layers grown on the surface of Ni NPs as a function of temperature in the thermal CVD process.
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bond corresponding to sp2 carbon in the MLG was confirmed to 78.71% except for C\\Ni content. Fig. 3d presents the electrical conductivity of the Ni NP and MLG-encapsulated Ni NP powders under compression (0.1 to 5 MPa). The electrical conductivity (σ) was calculated using σ = l/(AR), where l is the powder column height, R is the electrical resistance and A is the cross sectional area of the Cu piston. The electrical resistance of the compaction device (i.e. Cu pistons) was also measured without powder insertion. This was found to be ~5 × 10−4 Ω m, hence not compromising the measurement of electrical conductivity. As the compaction pressure increased, the electrical conductivity of MLGencapsulated Ni NPs and Ni NPs increased and finally reached ~47.3 and ~2.1 × 10−4 S m−1 at 5 MPa, respectively. The electrical resistance of MLG-encapsulated Ni NPs was much higher than that of Ni NPs because the MLG has a higher electrical conductivity. The electrical conductivity of graphene powder was reported to have 107–108 S m−1 in isolated single particle condition and ~262 S m−1 under compression of 5 MPa [33]. Relatively low electrical conductivity for the Ni NPs without MLG encapsulation seemed to be the presence of the NiO shell on the surface of Ni NPs. To examine the MLG content and thermal properties of the MLGencapsulated Ni NPs, thermogravimetric and differential scanning calorimetry (TG-DSC) analyses were performed, as shown in Fig. 4. For the
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Ni NPs without MLG encapsulation, the exothermic reaction began at b600 °C with associated increase in weight due to the oxidation of the NPs. Although the oxidation temperature of bulk Ni is known to be ~600 °C, it decreased drastically owing to the enhanced reactivity resulting from the nano-sized Ni particles [51,52]. The first and second exothermic reactions began at ~200 °C and ~400 °C, respectively, which was attributed to the oxidation of small and large primary Ni NPs with average diameters of ~24 nm and ~580 nm, respectively. The smaller Ni NPs having average particle diameter (Dp ) of ~24 nm were firstly oxidized at ~200 °C and the larger Ni NPs (Dp=580 nm) were oxidized at ~400 °C due to the nanosize effect. The oxidation of Ni NPs terminated at ~600 °C and the Ni NPs were fully converted to NiO NPs so that the final total mass of Ni NPs increased by up ~124% of the original. The NiO shell content for the initial Ni NPs was theoretically determined to be ~12 wt% (see Supporting Information for detailed calculation). The presence of the NiO shell in the as-prepared Ni NPs was also confirmed by STEM analysis, as shown in Fig. S2 in Supporting Information. For the MLG-encapsulated Ni NPs, the first exothermic reaction began at a higher temperature of ~300 °C because the MLG shell formed on the surface of Ni NPs prevented the oxidation of Ni NP cores to some extent. However, the oxidation of Ni NP cores was finally initiated by the breakage of MLG shells at N~300 °C due to oxidation of the graphene layers.
Fig. 6. (a) LR-SEM image, (b) HR-SEM image, and (c) STEM image and elemental mapping results of MLG-encapsulated Ni NP-added Al and CuO NP composite powders. (The thermal CVD temperature for the MLG-encapsulated Ni NP was 700 °C.)
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Furthermore, the heat flow generated by the first exothermic reaction of the MLG-encapsulated Ni NPs was observed to be much higher than that noted for Ni NPs without MLG encapsulation, because the heat energy generated by MLG oxidation was added to the heat energy released by the first exothermic reaction of the Ni NP cores. However, the heat flow generated by the second exothermic reaction decreased due to the decreased mass fraction of Ni NPs in the MLG-encapsulated Ni NPs compared to that of the Ni NPs without MLG encapsulation. Finally, after complete oxidation of the MLG-encapsulated Ni NPs at N~600 °C, the MLG was thermally decomposed, and the exposed Ni cores were then fully transformed into NiO, resulting in an increase in weight to ~121% of the original. The MLG content in the MLG-encapsulated Ni NPs was finally determined to be ~5 wt% (see Supporting Information for detailed calculation). The exothermic reactions of Ni NPs with and without MLG encapsulation were similarly observed to be between 200 and 600 °C, and the total heat energy determined by integrating the heat flow curves was ~3.3 kJ·g−1 for the Ni NPs without MLG and ~4.3 kJ·g−1 for the Ni NPs with MLG encapsulation. This suggests that the MLG increased the exothermic reaction of Ni NPs and played the role of both a protecting layer preventing oxidation at low temperatures of ≤300 °C and a source of heat energy that promoted the combustion of the Ni NPs at higher temperatures of N300 °C. For the MLG-encapsulated Ni NPs, the MLG was oxidized at ~250 °C; at that temperature, they began to release the heat energy. After the removal of the MLG, oxidation of the Ni NPs was initiated. Therefore, the MLG-encapsulated Ni NPs released higher heat energy than the Ni NPs without MLG encapsulation. The enthalpy of carbon oxidation (C(s) + O2(g) → CO2(g), ΔfHo298 = 393.5 kJ·mol−1) is much higher than that of Ni oxidation (Ni(s) + 1/2 O2(g) → NiO(s), ΔfHo298 = 244.3 kJ·mol−1). In addition, the preserved Ni surface covered by MLG contributed to the higher heat energy release.
We observed the formation of graphene layers by varying the reaction temperature from 300 °C to 700 °C in the thermal CVD process, as shown in Fig. 5a-g. The number of graphene layers formed on the surface of Ni NPs changed considerably with the reaction temperature in the thermal CVD process. The graphene layer was not formed at all until the temperature reached 400 °C, and then the number of graphene layers formed increased considerably at ≥450 °C, as shown in Fig. 5h. To understand this observation, a simple graphene growth model was established, based on the bulk diffusion of carbon in Ni, and the number of graphene layers was theoretically predicted by the carbon solubility and diffusivity of Ni [53–55] (see Fig. S3 in Supporting Information for detailed calculation). The theoretical calculations showed that the number of graphene layers increased exponentially with increase in the CVD reaction temperature. In particular, it was observed that the theoretical prediction on abrupt increase in the number of graphene layers at 400–450 °C agreed very well with the experimental results. However, the experimentally determined number of graphene layers deviated significantly from the theoretical prediction at N450 °C. This was presumably due to the surface of Ni NPs being rapidly covered by the graphene layer so that carbon dissolution in the Ni NPs under catalysis by the hydrocarbon sources (i.e., C2H2) was not effective. Consequently, the number of graphene layers formed on the surface of Ni NPs could be controlled by simply controlling the reaction temperature in the thermal CVD process. As proof-of-concept of potential application as optical igniters, we employed the fabricated MLG-encapsulated Ni NPs in energetic composite materials. We fabricated MLG-encapsulated Ni NP-added Al (fuel) and CuO (oxidizer) NP composite powders. The degree of intermixing and the structure of the as-prepared MLG-encapsulated Ni NP-added Al and CuO NP composite powders were examined using SEM and STEM analyses, as shown in Fig. 6. Al NPs were observed to be located close to CuO NPs at the nanoscale level, and the MLG-encapsulated Ni NPs (potential
Fig. 7. Schematic of flash ignition tests for (a) MLG-encapsulated Ni NPs and (b) MLG-encapsulated Ni NP-added Al and CuO composite powders, and (c) high-speed camera snapshots and suggested mechanisms for ignition and combustion of MLG-encapsulated Ni NPs and MLG-encapsulated Ni NP (1 wt%)-added Al and CuO NP composites ignited by flash irradiation. (The thermal CVD temperature for the MLG-encapsulated Ni NP was 700 °C.)
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optical igniters) were homogeneously distributed throughout the Al and CuO NP-based energetic composite powders. To investigate the effect of MLG on the flash ignition of Ni NPs, flash ignition tests were performed for Ni NPs with and without MLG encapsulation. The MLG-encapsulated Ni NPs were successfully ignited by flash irradiation as shown in Fig. 7a and c. Many small flames were initiated and subsequently propagated throughout the entirety of MLG-encapsulated Ni NPs, suggesting that they could act as optical igniters [56–58]. The local ignition of MLG-encapsulated Ni NPs occurred because the MLG effectively absorbed the flash energy and rapidly heated up the surrounding air molecules, a phenomenon termed the photothermal effect [23–25]. Then, the MLG rapidly transferred and concentrated the heat energy into the Ni NP core. This sufficiently increased the temperature, resulting in the oxidation of both the core (Ni catalyst) and shell (MLG) structures. However, the pure Ni NPs without MLG encapsulation showed a very weak response to the flash irradiation as shown in Fig. S4 in Supporting Information, because they could not generate sufficient heat energy for ignition. To examine the role of MLG-encapsulated Ni NPs as potential optical igniters for nEMs (i.e., Al NP/CuO NP), flash ignition tests were also conducted, as shown in Fig. 7b and c. The MLG-encapsulated Ni NP (1 wt%)-added Al and CuO NP composite powders fabricated in this study were stably ignited by absorbing the flash irradiation, and then the combustion flame propagated rapidly through the aligned composite powders. However, the nEMs without added MLG-encapsulated Ni NPs were not ignited at all by repeated flash irradiation (see Fig. S4 in Supporting Information). Based on the high-speed camera snapshot analysis, the resulting burn rate and ignition delay time of the MLG-encapsulated Ni NP (1 wt%)-added Al and CuO NP composite powders were determined to be ~200 m·s−1 and ~267 μs, respectively. Here, the burn rate is defined by the total length of the aligned powder sample divided by the total flame propagation time, and the ignition delay time is defined as the time taken by the powder sample for ignition after flash irradiation. The addition of a very small amount of MLG-encapsulated Ni NPs into Al/CuO NPs could stably induce optical ignition without significant degradation of the combustion characteristics; the burn rate of pure Al/CuO NP ignited by hotwire igniter was measured to be ~220 m·s−1 (see, Fig. S5 in Supporting Information). It confirms the potential applicability of MLG-encapsulated Ni NPs as optical igniters for nEMs. 4. Conclusions In this study, MLG-encapsulated Ni NPs were synthesized by a simple one-step continuous aerosol process comprising SP and thermal CVD. Pure Ni NPs were synthesized using SP, and then MLG was continuously formed on the surface of Ni NPs by the thermal CVD process. The resulting MLG-encapsulated Ni NPs showed a bimodal particle size distribution with an average size of ~579 ± 11 nm for the large primary particles and ~23.5 ± 0.8 nm for the small primary particles. The number of graphene layers could be controlled by varying the CVD reaction temperature of 300–700 °C, which strongly affected the solubility and diffusion rate of carbon in the Ni NPs. The MLG-encapsulated Ni NPs were finally applied as optical igniters for nanoenergetic materials (nEMs). It was demonstrated that the MLG-encapsulated Ni NPs (1 wt%)-added Al/CuO NP composite powders could be successfully ignited by flash irradiation. The fast local heating of MLG-encapsulated Ni NPs by photoflash absorption successfully induced the ignition of surrounding Al/CuO NPs in a very short ignition delay of ~267 μs. The burn rate of the MLG-encapsulated Ni NPs (1 wt%)added Al/CuO NP composite was determined to be ~200 m·s−1. This suggests that the MLG-encapsulated Ni NPs can be employed as optical igniters for practical thermal engineering applications. Author contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
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Acknowledgments This study was financially supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education (2018R1A6A3A01012566). Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Powder Technology journal homepage: www.elsevier.com/locate/powtec
Aerosol growth and photothermal ignition of multilayer graphene-encapsulated nickel nanoparticles Ji Hoon Kim a, Ho Sung Kim b, Soo Hyung Kim a,b,c,⁎ a b c
Research Center for Energy Convergence Technology, Pusan National University, 2, Busandaehak-ro 63beon-gil, Geumjeong-gu, Busan 46241, Republic of Korea Department of Nanofusion Technology, College of Nanoscience and Nanotechnology, Pusan National University, 2, Busandaehak-ro 63beon-gil, Geumjeong-gu, Busan 46241, Republic of Korea Department of Nanoenergy Engineering, College of Nanoscience and Nanotechnology, Pusan National University, 2, Busandaehak-ro 63beon-gil, Geumjeong-gu, Busan 46241, Republic of Korea
a r t i c l e
i n f o
Article history: Received 19 August 2019 Received in revised form 23 January 2020 Accepted 8 February 2020 Available online 11 February 2020 Keywords: Graphene Nickel nanoparticles Spray pyrolysis Thermal chemical vapor deposition Optical igniter
a b s t r a c t In this report, we propose an easy and viable fabrication method for growing multilayer graphene (MLG)-encapsulated nickel nanoparticles (Ni NPs) via a one-step continuous aerosol process comprising combined spray pyrolysis (SP) and thermal chemical vapor deposition (CVD). A bimodal size distribution was observed for the Ni NPs owing to droplet-to-particle and gas-to-particle conversions undergone in the SP at ~1000 °C. The number of graphene layers grown on the surface of Ni NPs in the thermal CVD was controlled by varying the reaction temperature of 300–700 °C, which perturbed the solubility and diffusion rates of carbon in Ni NPs. Finally, we demonstrated that the MLG-encapsulated Ni NP-added nanoenergetic materials were stably ignited by flash irradiation in a short ignition delay time (~267 μs) with fast burn rate (~200 m·s−1). This suggests that the MLG-encapsulated Ni NPs could play an important role as potential optical igniters for practical thermal engineering applications. © 2020 Published by Elsevier B.V.
1. Introduction Graphene has attracted a lot of attention in many fields including those involving lithium-ion batteries [1–4], proton-membrane fuel cells [5], supercapacitors [6,7], sensors [8–11], and lubricants [12] because of its two-dimensional (2D) carbon nanostructure with a unique honeycomb lattice. It offers outstanding physical properties such as a high specific surface area, excellent thermal and electrical conductivity, and high mechanical strength. However, it is inevitable that the aggregation of graphene layers owing to π-π stacking and van der Waals interactions (i.e., large sheet aspect ratio and stacking structure) results in decreased specific surface area so that the superior properties of individual graphene sheet cannot be fully exploited [13]. The aggregation is caused by the 2D structural characteristics of graphene; thus, it can be relieved to a large extent through introducing structural changes. Recently, many researchers have synthesized threedimensional (3D)-structured graphene, in the form of graphene balls [3–7,14], crumpled graphene balls [15–19], and graphene foam [20–22], employing various methods such as exfoliation, hydrothermal synthesis, chemical vapor deposition (CVD), spray pyrolysis (SP), and ⁎ Corresponding author at: Research Center for Energy Convergence Technology, Pusan National University, 2, Busandaehak-ro 63beon-gil, Geumjeong-gu, Busan 46241, Republic of Korea. E-mail address: [email protected] (S.H. Kim).
https://doi.org/10.1016/j.powtec.2020.02.023 0032-5910/© 2020 Published by Elsevier B.V.
spray drying. Among various metallic catalysts, Ni nanoparticles (NPs) are known as one of the promising catalysts for growing graphene layers, and they exhibit tunable mechanical, electrical, and optical properties depending on the size, shape and aggregation state [4,8,23]. The encapsulation of metal NPs with graphene layers can play the role of protecting the metal core from possible oxidation and simultaneously improving the electrical conductivity, chemical stability, and optical absorptivity of metal core. Graphene layers are formed on the surface of Ni NPs due to dissolution-precipitation mechanism based on high solubility of carbon in Ni at relatively high temperatures [4]. The preparation of 3D-structured graphene on the surface of metal NPs remains challenging in various synthetic processes. Heat treatment of metal-organic framework and hydrothermal synthetic processes can be suggested promising methods, but complex chemical treatment, secondary solvent extraction, and multiple heating processes are often required to obtain graphene layer-encapsulated metal NPs. In addition, the controlled growth of number of graphene layer is difficult because supply of carbon sources to the metal NPs are not easily controlled in those approaches [4,23]. In this report, we suggest a simple and scalable method for the synthesis of 3D multilayer graphene-encapsulated metal NPs in the gas phase. Specifically, in our study, nickel nanoparticles (Ni NPs) were synthesized by SP, and then multilayer graphene (MLG) was continuously grown on their surface using thermal CVD. The size of Ni NPs and thickness of MLG were controlled precisely by the operating conditions (i.e., initial concentration of Ni precursor
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solution and reaction temperature). We developed a new aerosol process to fabricate MLG-encapsulated Ni NPs with tunable particle sizes and graphene layer thicknesses. Finally, we demonstrate a potential application of the MLGencapsulated Ni NPs as optical igniters for triggering nanoscale energetic materials (nEMs) based on the enhanced photothermal effect [24,25], which can be occurred because of the metal NPs being encapsulated with multiple graphene layers. The nEMs are composites composed of nanoscale metallic fuels (e.g., Al, Mg) and inorganic oxidizers (e.g., CuO, Fe2O3, Bi2O3) which emits a huge amount of heat and pressure instantaneously through very fast redox reaction between fuel and oxidizer. It has attracted attention in various thermal engineering fields including explosives, propellants, and pyrotechnics due to the potential of developing multifunctional combustion systems with enhanced capabilities at low costs. Recently, various graphene-based materials (e.g., graphene [26,27], graphene oxide [28–30], functionalized graphene [31,32]) have been applied to the nEMs to improve the ignition and combustion characteristics utilizing unique physicochemical properties of graphene-based materials (i.e., high thermal and electrical conductivity, high specific surface area, functionalization, photothermal effect, etc.). In this study, Ni NPs with improved photothermal conversion characteristics by MLG encapsulation were applied to nEMs (i.e., Al/CuO NP) to induce successful optical ignition by photoflash irradiation.
2. Experimental 2.1. Aerosol synthesis of MLG on the surface of Ni NPs A simple one-step continuous aerosol process consisting of conventional SP and thermal CVD was employed to synthesize the MLG on the surface of Ni NPs as shown in Fig. 1. First, the precursor solution was prepared by dissolving nickel nitrate hexahydrate (Ni(NO3)2·6H2O, Sigma Aldrich) in deionized water at a concentration of 1–20 wt% and the precursor solution was aerosolized by a homemade ultrasonic nebulizer operated at 40 W and 60 Hz. The nickel nitrate aerosol droplets were then transferred to a silica-gel dryer under N2 flow of 3 standard liters per minutes (slpm). After passing through a silica-gel dryer, the dried nickel nitrate particles were transformed into nickel oxide NPs through solvent evaporation and thermal decomposition in the first
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tube furnace reactor (2.54 cm diameter, 30 cm heating length) heated at ~1000 °C. Subsequently, the nickel oxide NPs were reduced under a hydrogen flow of 100 standard cubic centimeters per minute (sccm) and finally pure Ni NPs were formed. The residence time of aerosol flow in the primary reactor was ~2.09 s. The resulting Ni NPs were then rapidly transported into the second tube furnace (5.48 cm diameter, 30 cm heating length) heated at 300–700 °C and reacted with acetylene (10 sccm) and hydrogen (100 sccm) gases over a residence time of ~13.64 s. Through a catalytic reaction of Ni NPs with acetylene, MLG was grown on the surface of Ni NPs. Finally, the resulting MLGencapsulated Ni NPs were collected on a membrane filter with pore size of 200 nm.
2.2. Material characterization The MLG-encapsulated Ni NPs were characterized by scanning electron microscopy (SEM; ZEISS, Model No. SUPRA40VP) operated at 15 kV, Cs-corrected scanning transmission electron microscopy (STEM; JEOL, Model No. JEM-2100) operated at 200 kV, X-ray diffraction (XRD; PANalytical, Model No. Empyrean series2) using Cu Κα radiation, Raman spectroscopy (Horiba Jobin Yvon, Model No. LabRam Aramis) using a 514.5 nm (2.41 eV) Ar-ion laser at 100 μW power, and X-ray photoelectron spectroscopy (XPS; Kratos analytical, Model No. AXIS Supra) using Al Kα radiation (1486.6 eV). The electrical conductivity of the MLG-encapsulated Ni NPs and pure Ni NPs was measured using a two-probe electrical resistance measurement system and powder compaction devices [33,34]. Briefly, 100 mg of sample powders were filled in a hollow polytetrafluoroethylene (PTFE) cylinder with an inner diameter of 5 mm, and compressed between two Cu pistons. The Cu pistons were connected to a digital multimeter (Keysight, Model No. 34465A) to measure the electrical resistance. The pressure on the device was applied using an automatic press machine (Japan Instrumentation System, Model No. JSV H1000) equipped with a 100 N load cell. The maximum pressure reached during the test was ~5 MPa using a constant displacement speed of 1 mm min−1. Thermogravimetric and differential scanning calorimetry (TG-DSC) analyses were also conducted at temperatures ranging from 30 °C to 1000 °C at a heating rate of 10 °C min−1 in air for 10 mg of samples. The MLG-encapsulated Ni NP-added Al and CuO NP composites were characterized using scanning electron microscopy (SEM; Hitachi,
Fig. 1. Schematic of experimental setup for aerosol synthesis of MLG-encapsulated Ni NPs.
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Model No. S4700) operated at 15 kV and Cs-corrected scanning transmission electron microscopy (STEM; JEOL, Model No. JEM-2100) operated at 200 kV. 2.3. Fabrication of MLG-encapsulated Ni NP-added Al/CuO composites Commercially available Al NPs (Nanotechnology Inc.) and CuO NPs (Sigma Aldrich) with average primary particle sizes of approximately ~80 nm and ~100 nm were used as a fuel and an oxidizer, respectively. The MLG-encapsulated Ni NPs were used as optical igniters. First, the MLG-encapsulated Ni NP-added Al and CuO NP composite powder was prepared. Briefly, Al NPs (fuel) were mixed with CuO NPs (oxidizer) in an ethanol (EtOH) solution. The mixing ratio of the fuel and oxidizer
was fixed at Al:CuO = 30:70 by weight to ensure strong explosion reactivity [35]. The MLG-encapsulated Ni NPs fabricated were then added to the Al/CuO-dispersed solution at a mixing ratio of 1 wt%. To ensure that the MLG-encapsulated Ni NP/Al NP/CuO NP was homogeneously mixed in the EtOH solution, the solution was ultrasonicated at 200 W and 40 kHz for approximately 30 min. Subsequently, the EtOH was dried in a convection oven at 80 °C for 30 min. Finally, the MLGencapsulated Ni NP-added Al/CuO composite powder was obtained. 2.4. Optical ignition test To examine the potential role of the MLG-encapsulated Ni NPs as optical igniters, flash irradiation was undertaken on the various fabricated
Fig. 2. (a) Low-resolution (LR)-SEM and (b) high-resolution (HR)-SEM images, (c) primary particle size distributions, and (d) LR-TEM and (e) HR-TEM images of MLG-encapsulated Ni NPs formed at 700 °C in the thermal CVD process. (Dp is the average particle size.)
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materials, namely, the Ni NPs, MLG-encapsulated Ni NPs, Al/CuO NPs, and MLG-encapsulated Ni NP (1 wt%)-added Al/CuO NPs. Briefly, sample powders were placed on a glass stage and exposed to a light generated using a camera flash under the glass stage. A light emitting hole with a diameter of 1 mm was made on the surface of camera flash covered with the glass stage and Al tape, and the sample powder was aligned (mass: 10 mg, particle length: 20 mm, width: 2 mm) thereon to measure the burn rate and ignition delay time for the MLG-encapsulated Ni NP (1 wt%)-added Al/CuO NP composite [36]. The reaction undergone by each sample was recorded using a high-speed camera (Photron, Model No. FASTCAM SA3 120 K) at a frame rate of 30 kHz. 3. Results and discussion SEM images presented in Fig. 2a and b show that spherical MLGencapsulated Ni NPs were synthesized by the combination of the SP and thermal CVD processes. The MLG-encapsulated Ni NPs showed a bimodal particle size distribution, in which the major peaks appeared at ~579 ± 11 nm for the large primary particles and ~23.5 ± 0.8 nm for the small primary particles (Fig. 2c). The particles formed in the SP process underwent two different growth mechanisms of droplet-to-particle and gas-to-particle conversions [37–39]. The aerosolized droplets containing metal nitrates turned into a solid particle during the SP and reduction processes. Most of the large primary particles were generated by the droplet-to-particle conversion, whereas the small primary particles seemed to arise from the gas-to-particle conversion. During the droplet-to-particle conversion of nickel nitrate-containing droplets in the first tube furnace reactor, the solid particles evaporated rapidly so that the vapors of solid particles formed new small primary particles by nucleation and condensation. The vapor pressures of the Ni and
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NiO at 1000 °C in the SP was calculated to ~2.19 × 10−10 and ~2.02 × 10−11 atm, respectively [40–42]. The small Ni NPs were formed by the nucleation of Ni vapors evaporated from the large Ni NPs and the reduction of NiO NPs formed by NiO vapors. The solid particle formation is strongly dependent on various parameters, including the physicochemical properties and concentrations of precursors and the various operating conditions such as the pressure, temperature, flow rate, and cooling rate involved in the evaporation, nucleation, and condensation of particles [39–43]. The particle size distribution was mostly controlled by the initial concentration of the precursor solution. As the initial concentration of the precursor solution was increased from 1 to 20 wt%, the size of the large primary particle increased gradually, wherein a linear relationship was observed between the logarithms of the precursor concentration and the particle diameter [43] (i.e.,log(dp,v) = 1/3 ⋅ log C + log (dd,v[M/ρp]1/3), where dp,v = volumetric particle diameter [m], dd,v = volumetric droplet diameter [m], C = precursor concentration [mol m−3], M = molecular weight of particle [kg mol−1], and ρp = density of precursor [kg m−3]). This was confirmed by SEM analyses for the fabricated solid particles, as shown in Fig. S1 in Supporting Information. TEM images shown in Fig. 2d and e reveal that Ni core particles were clearly encapsulated by the MLG shells, formed by concentric carbon precipitation on the entire surface of the Ni NPs. The mechanism for the formation of MLG shells on the surface of Ni NPs can be explained as follows. Carbon atoms first dissolved into Ni NPs by catalytic reactions undergone under the supply of hydrocarbon sources (i.e., C2H2). When the concentration of the carbon dissolved in Ni NPs exceeded the carbon solubility of the Ni NPs, carbon atoms began to precipitate on the surface of Ni NPs. The Ni NPs were fully covered by very thin graphene layers, and the formation of MLG-encapsulated Ni NPs was then finally complete [44,45].
Fig. 3. (a) XRD patterns, (b) Raman spectra of Ni NPs with and without MLG encapsulation, (c) C1s XPS spectrum of Ni NPs with MLG encapsulation, and (d) electrical conductivity of Ni NPs with and without MLG encapsulation. (The MLG was synthesized at 700 °C in the thermal CVD process.)
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Fig. 4. (a) TG and (b) DSC analyses of Ni NPs with and without MLG encapsulation. (The MLG was synthesized at 700 °C in the thermal CVD process.)
To examine the structural characteristics of Ni NPs with and without MLG encapsulation, X-ray diffraction (XRD) and Raman spectra analyses were performed. For the as-prepared Ni NPs without MLG encapsulation, strong X-ray diffraction peaks resulting from the presence of Ni were clearly observed, as shown in Fig. 3a. In addition, relatively weak peaks owing to NiO were observed. This was attributed to the surface oxidation of Ni NPs by the ambient airafter the SP and reduction process. However, for the MLG-encapsulated Ni NPs, only strong peaks for Ni appeared without any peaks for NiO, suggesting that the presence of MLG prevented the oxidation of Ni NPs by coating their entire surfaces. The X-ray diffraction peaks for MLG were not clearly observed in Fig. 3a due to the small fraction of carbon content compared to that of Ni. The Raman spectrum of Fig. 3b clearly exhibits that there were no carbon structures on the as-prepared Ni NPs that were formed by the SP process. However, the MLG formed on the surface of Ni NPs using the combination of SP and CVD showed two strong peaks at ~1350 cm−1 (D mode, disorder-induced band originating from defects or carbon impurities) and ~1600 cm−1 (G mode, stretching mode in the graphite plane). The ratio of signal intensities (i.e., ID/IG) at the Raman shifts of 1350 and 1600 cm−1 was found to be approximately 1.05, indicating that the degree of graphitization was relatively low for the carbon layer formed using this approach. This phenomenon was suggested to be due to the structural defects and amorphous nature of the carbon of the graphene layers grown on the spherical surface of Ni NPs, which resulted in the formation of wavy and discontinuous graphene layers with very small graphene domains [4,7,45]. In addition, a 2D peak was also detected at ~2700 cm−1, which was a second-order D peak with regard to the number of graphene layers [46–49], and another peak was observed for the presence of D + D’, which is the combination mode of D and D’ [45–50]. Here, the D’ peak arose from the double-phonon resonance process in the graphene layer containing defects [50]. The pure D’ peak was not clearly observed due to the superposition of the D’ peak with the G peak. XPS analysis is used to confirm the reduction status of graphene in the MLG-encapsulated Ni NPs. Fig. 3c shows the C1s XPS spectra of MLG-encapsulated Ni NPs. The C1s signals of the MLG-encapsulated Ni NPs deconvoluted into several signals for C\\Ni (~283.9 eV), C-C/C=C (~284.8 eV), C-OH/C-O-C (~286.5 eV), and O-C=O (~288.9 eV) bonds. The content of C-C/C=C
Fig. 5. TEM images of various MLG-encapsulated Ni NPs formed at (a) 300 °C, (b) 350 °C, (c) 400 °C, (d) 450 °C, (e) 500 °C, (f) 600 °C, and (g) 700 °C in the thermal CVD process. (h) Number of graphene layers grown on the surface of Ni NPs as a function of temperature in the thermal CVD process.
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bond corresponding to sp2 carbon in the MLG was confirmed to 78.71% except for C\\Ni content. Fig. 3d presents the electrical conductivity of the Ni NP and MLG-encapsulated Ni NP powders under compression (0.1 to 5 MPa). The electrical conductivity (σ) was calculated using σ = l/(AR), where l is the powder column height, R is the electrical resistance and A is the cross sectional area of the Cu piston. The electrical resistance of the compaction device (i.e. Cu pistons) was also measured without powder insertion. This was found to be ~5 × 10−4 Ω m, hence not compromising the measurement of electrical conductivity. As the compaction pressure increased, the electrical conductivity of MLGencapsulated Ni NPs and Ni NPs increased and finally reached ~47.3 and ~2.1 × 10−4 S m−1 at 5 MPa, respectively. The electrical resistance of MLG-encapsulated Ni NPs was much higher than that of Ni NPs because the MLG has a higher electrical conductivity. The electrical conductivity of graphene powder was reported to have 107–108 S m−1 in isolated single particle condition and ~262 S m−1 under compression of 5 MPa [33]. Relatively low electrical conductivity for the Ni NPs without MLG encapsulation seemed to be the presence of the NiO shell on the surface of Ni NPs. To examine the MLG content and thermal properties of the MLGencapsulated Ni NPs, thermogravimetric and differential scanning calorimetry (TG-DSC) analyses were performed, as shown in Fig. 4. For the
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Ni NPs without MLG encapsulation, the exothermic reaction began at b600 °C with associated increase in weight due to the oxidation of the NPs. Although the oxidation temperature of bulk Ni is known to be ~600 °C, it decreased drastically owing to the enhanced reactivity resulting from the nano-sized Ni particles [51,52]. The first and second exothermic reactions began at ~200 °C and ~400 °C, respectively, which was attributed to the oxidation of small and large primary Ni NPs with average diameters of ~24 nm and ~580 nm, respectively. The smaller Ni NPs having average particle diameter (Dp ) of ~24 nm were firstly oxidized at ~200 °C and the larger Ni NPs (Dp=580 nm) were oxidized at ~400 °C due to the nanosize effect. The oxidation of Ni NPs terminated at ~600 °C and the Ni NPs were fully converted to NiO NPs so that the final total mass of Ni NPs increased by up ~124% of the original. The NiO shell content for the initial Ni NPs was theoretically determined to be ~12 wt% (see Supporting Information for detailed calculation). The presence of the NiO shell in the as-prepared Ni NPs was also confirmed by STEM analysis, as shown in Fig. S2 in Supporting Information. For the MLG-encapsulated Ni NPs, the first exothermic reaction began at a higher temperature of ~300 °C because the MLG shell formed on the surface of Ni NPs prevented the oxidation of Ni NP cores to some extent. However, the oxidation of Ni NP cores was finally initiated by the breakage of MLG shells at N~300 °C due to oxidation of the graphene layers.
Fig. 6. (a) LR-SEM image, (b) HR-SEM image, and (c) STEM image and elemental mapping results of MLG-encapsulated Ni NP-added Al and CuO NP composite powders. (The thermal CVD temperature for the MLG-encapsulated Ni NP was 700 °C.)
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Furthermore, the heat flow generated by the first exothermic reaction of the MLG-encapsulated Ni NPs was observed to be much higher than that noted for Ni NPs without MLG encapsulation, because the heat energy generated by MLG oxidation was added to the heat energy released by the first exothermic reaction of the Ni NP cores. However, the heat flow generated by the second exothermic reaction decreased due to the decreased mass fraction of Ni NPs in the MLG-encapsulated Ni NPs compared to that of the Ni NPs without MLG encapsulation. Finally, after complete oxidation of the MLG-encapsulated Ni NPs at N~600 °C, the MLG was thermally decomposed, and the exposed Ni cores were then fully transformed into NiO, resulting in an increase in weight to ~121% of the original. The MLG content in the MLG-encapsulated Ni NPs was finally determined to be ~5 wt% (see Supporting Information for detailed calculation). The exothermic reactions of Ni NPs with and without MLG encapsulation were similarly observed to be between 200 and 600 °C, and the total heat energy determined by integrating the heat flow curves was ~3.3 kJ·g−1 for the Ni NPs without MLG and ~4.3 kJ·g−1 for the Ni NPs with MLG encapsulation. This suggests that the MLG increased the exothermic reaction of Ni NPs and played the role of both a protecting layer preventing oxidation at low temperatures of ≤300 °C and a source of heat energy that promoted the combustion of the Ni NPs at higher temperatures of N300 °C. For the MLG-encapsulated Ni NPs, the MLG was oxidized at ~250 °C; at that temperature, they began to release the heat energy. After the removal of the MLG, oxidation of the Ni NPs was initiated. Therefore, the MLG-encapsulated Ni NPs released higher heat energy than the Ni NPs without MLG encapsulation. The enthalpy of carbon oxidation (C(s) + O2(g) → CO2(g), ΔfHo298 = 393.5 kJ·mol−1) is much higher than that of Ni oxidation (Ni(s) + 1/2 O2(g) → NiO(s), ΔfHo298 = 244.3 kJ·mol−1). In addition, the preserved Ni surface covered by MLG contributed to the higher heat energy release.
We observed the formation of graphene layers by varying the reaction temperature from 300 °C to 700 °C in the thermal CVD process, as shown in Fig. 5a-g. The number of graphene layers formed on the surface of Ni NPs changed considerably with the reaction temperature in the thermal CVD process. The graphene layer was not formed at all until the temperature reached 400 °C, and then the number of graphene layers formed increased considerably at ≥450 °C, as shown in Fig. 5h. To understand this observation, a simple graphene growth model was established, based on the bulk diffusion of carbon in Ni, and the number of graphene layers was theoretically predicted by the carbon solubility and diffusivity of Ni [53–55] (see Fig. S3 in Supporting Information for detailed calculation). The theoretical calculations showed that the number of graphene layers increased exponentially with increase in the CVD reaction temperature. In particular, it was observed that the theoretical prediction on abrupt increase in the number of graphene layers at 400–450 °C agreed very well with the experimental results. However, the experimentally determined number of graphene layers deviated significantly from the theoretical prediction at N450 °C. This was presumably due to the surface of Ni NPs being rapidly covered by the graphene layer so that carbon dissolution in the Ni NPs under catalysis by the hydrocarbon sources (i.e., C2H2) was not effective. Consequently, the number of graphene layers formed on the surface of Ni NPs could be controlled by simply controlling the reaction temperature in the thermal CVD process. As proof-of-concept of potential application as optical igniters, we employed the fabricated MLG-encapsulated Ni NPs in energetic composite materials. We fabricated MLG-encapsulated Ni NP-added Al (fuel) and CuO (oxidizer) NP composite powders. The degree of intermixing and the structure of the as-prepared MLG-encapsulated Ni NP-added Al and CuO NP composite powders were examined using SEM and STEM analyses, as shown in Fig. 6. Al NPs were observed to be located close to CuO NPs at the nanoscale level, and the MLG-encapsulated Ni NPs (potential
Fig. 7. Schematic of flash ignition tests for (a) MLG-encapsulated Ni NPs and (b) MLG-encapsulated Ni NP-added Al and CuO composite powders, and (c) high-speed camera snapshots and suggested mechanisms for ignition and combustion of MLG-encapsulated Ni NPs and MLG-encapsulated Ni NP (1 wt%)-added Al and CuO NP composites ignited by flash irradiation. (The thermal CVD temperature for the MLG-encapsulated Ni NP was 700 °C.)
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optical igniters) were homogeneously distributed throughout the Al and CuO NP-based energetic composite powders. To investigate the effect of MLG on the flash ignition of Ni NPs, flash ignition tests were performed for Ni NPs with and without MLG encapsulation. The MLG-encapsulated Ni NPs were successfully ignited by flash irradiation as shown in Fig. 7a and c. Many small flames were initiated and subsequently propagated throughout the entirety of MLG-encapsulated Ni NPs, suggesting that they could act as optical igniters [56–58]. The local ignition of MLG-encapsulated Ni NPs occurred because the MLG effectively absorbed the flash energy and rapidly heated up the surrounding air molecules, a phenomenon termed the photothermal effect [23–25]. Then, the MLG rapidly transferred and concentrated the heat energy into the Ni NP core. This sufficiently increased the temperature, resulting in the oxidation of both the core (Ni catalyst) and shell (MLG) structures. However, the pure Ni NPs without MLG encapsulation showed a very weak response to the flash irradiation as shown in Fig. S4 in Supporting Information, because they could not generate sufficient heat energy for ignition. To examine the role of MLG-encapsulated Ni NPs as potential optical igniters for nEMs (i.e., Al NP/CuO NP), flash ignition tests were also conducted, as shown in Fig. 7b and c. The MLG-encapsulated Ni NP (1 wt%)-added Al and CuO NP composite powders fabricated in this study were stably ignited by absorbing the flash irradiation, and then the combustion flame propagated rapidly through the aligned composite powders. However, the nEMs without added MLG-encapsulated Ni NPs were not ignited at all by repeated flash irradiation (see Fig. S4 in Supporting Information). Based on the high-speed camera snapshot analysis, the resulting burn rate and ignition delay time of the MLG-encapsulated Ni NP (1 wt%)-added Al and CuO NP composite powders were determined to be ~200 m·s−1 and ~267 μs, respectively. Here, the burn rate is defined by the total length of the aligned powder sample divided by the total flame propagation time, and the ignition delay time is defined as the time taken by the powder sample for ignition after flash irradiation. The addition of a very small amount of MLG-encapsulated Ni NPs into Al/CuO NPs could stably induce optical ignition without significant degradation of the combustion characteristics; the burn rate of pure Al/CuO NP ignited by hotwire igniter was measured to be ~220 m·s−1 (see, Fig. S5 in Supporting Information). It confirms the potential applicability of MLG-encapsulated Ni NPs as optical igniters for nEMs. 4. Conclusions In this study, MLG-encapsulated Ni NPs were synthesized by a simple one-step continuous aerosol process comprising SP and thermal CVD. Pure Ni NPs were synthesized using SP, and then MLG was continuously formed on the surface of Ni NPs by the thermal CVD process. The resulting MLG-encapsulated Ni NPs showed a bimodal particle size distribution with an average size of ~579 ± 11 nm for the large primary particles and ~23.5 ± 0.8 nm for the small primary particles. The number of graphene layers could be controlled by varying the CVD reaction temperature of 300–700 °C, which strongly affected the solubility and diffusion rate of carbon in the Ni NPs. The MLG-encapsulated Ni NPs were finally applied as optical igniters for nanoenergetic materials (nEMs). It was demonstrated that the MLG-encapsulated Ni NPs (1 wt%)-added Al/CuO NP composite powders could be successfully ignited by flash irradiation. The fast local heating of MLG-encapsulated Ni NPs by photoflash absorption successfully induced the ignition of surrounding Al/CuO NPs in a very short ignition delay of ~267 μs. The burn rate of the MLG-encapsulated Ni NPs (1 wt%)added Al/CuO NP composite was determined to be ~200 m·s−1. This suggests that the MLG-encapsulated Ni NPs can be employed as optical igniters for practical thermal engineering applications. Author contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
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Acknowledgments This study was financially supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education (2018R1A6A3A01012566). Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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