B-containing nanomaterial synthesis when a combustion wave moves within a packed bed of solid particles

Combustion and Flame 172 (2016) 271–279

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B-containing nanomaterial synthesis when a combustion wave moves within a packed bed of solid particles Hayk Nersisyan a,c, Tae Hyuk Lee b, Bung Uk Yoo b, Suk Cheol Kwon a,c, Hoyoung Suh d, Jin-Gyu Kim d, Jong-Hyeon Lee a,b,c,∗ a

Department of Materials Science & Engineering, Chungnam National University, 99 Daehakro, Yuseong-gu, Daejeon, Republic of Korea Graduate School of Energy Science & Technology, Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon, Republic of Korea Rasom, Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon, Republic of Korea d Division of Electron Microscopic Research, Korea Basic Science Institute (KBSI), 169-148 Gwahang-no, Yuseong-gu, Daejeon, Republic of Korea b c

a r t i c l e

i n f o

Article history: Received 24 March 2016 Revised 9 May 2016 Accepted 19 July 2016 Available online 6 August 2016 Keywords: Amorphous boron Boron carbide Boron nitride, Nanoparticles Combustion synthesis

a b s t r a c t This study deals with combustion behavior of B2 O3 /Mg/NH4 Cl/C complex systems for the synthesis of amorphous boron (B), boron carbide (B4 C), and boron nitride (BN) nanostructures. The raw mixtures used in the experiments were prepared on the base of a B2 O3 –Mg precursor mixture, which is sufficiently exothermic to maintain a self-propagating regime of the combustion reaction. Thermodynamic analysis of the combustion temperatures and experimental validation indicate that the 10 0 0–150 0 °C temperature range is very effective for synthesizing the nanostructures of B, B4 C, and BN. It was found that Bcontaining functional nanostructures are mainly spherical nanoparticles (B) or nanosheets (B4 C, BN). The phase composition and microstructural characteristics of the final products were evaluated based on the combustion temperature and solid/liquid phase changes. © 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

1. Introduction The preparation of controlled chemical composition nanoscale particles, plates, and fibers of amorphous boron (B) and its compounds, such as boron carbide (B4 C) and boron nitride (BN) is the subject of intense scientific research due to the potential applications of these materials in a large variety of fields ranging from engineering to the biomedical, military, and aerospace sciences. The wide absorption spectrum of boron makes it suitable for use as a neutron radiation shield [1,2]; boron neutron capture therapy can be used in cancer treatment [3,4]; and boron can be a high temperature ignition fuel in pyrotechnics [5–7]. BN nanostructures are electrically insulating, with an energy gap of ∼5.5 eV [8–10]; they are resistant to oxidation up to 800 °C [11,12]. Due to these key characteristics, BN nanostructures are very useful in structural applications, such as in reinforcing industrial ceramics (e.g., quartz, alumina, and silicon nitride) to improve their tolerance to thermal shocks [13,14]. Furthermore, BN nanostructures are

∗ Corresponding author at: Department of Materials Science & Engineering, Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon, Republic of Korea. E-mail address: [email protected] (J.-H. Lee).

non-abrasive, lubricating, and non-reactive towards molten metals [15,16]. B4 C nanoparticles are used in boron neutron capture therapy, in biomedical and bioscience areas, in coatings, in plastics, and in catalyst applications [17–19]. Boron nanoparticles are commercially produced by arc decomposition of B2 H6 [20], gas phase pyrolysis of B10 H14 [21], and gas phase reduction of BCl3 in an inductively coupled plasma [22]. The reported diameters of boron nanoparticles synthesized from the gas phase reaction are in the range of 10–150 nm. The production rate of commercial methods is from several- to tens- of tons/year; as-produced black boron nanopowder display about 99.9 wt% purity. The disadvantage of gas-phase methods is the very high cost of B nanopowder due to the expensive raw materials. Laboratory scale production of B nanopowders can be performed by solution reduction of BBr3 with sodium naphthalenide [23] and ball milling of coarse boron powder [24]. The solution reduction method has very low productivity and a complicated synthesis procedure; therefore, it cannot be considered as a prospective method for scale-up development. The disadvantages of ball milling technology are the non-uniformity of the produced nanoparticles and the high percentages of impurities. It is noteworthy to mention that boron can also be synthesized in the form of nanotubes [25], nanowires [26], and nanoribbons [27].

http://dx.doi.org/10.1016/j.combustflame.2016.07.024 0010-2180/© 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

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There are a large number of reports that have focused on the laboratory scale preparation of various types of B4 C nanostructured materials such as nanoparticles, nanotubes, nanocapsules, nanohorns, nanoropes, and nanosheets. Shi et al. [28] discussed a low-temperature method to synthesize B4 C ultrafine particles through a new metallic Na co-reduction route; Yamada et al. [29] and Chang et al. [30] formed fine B4 C particles from an amorphous boron/graphite mixture using shock-wave and hightemperature furnace techniques, respectively. Chen et al. [31] reported on the synthesis of B4 C nanoparticles via a reaction of magnesium diboride (MgB2 ) with multiwalled carbon nanotubes at 1150 °C. Singh et al. [32] synthesized B4 C nanoparticles by heating an H3 BO3 –Mg–C mixture in an autoclave at 70 0–80 0 °C. Zhu et al. [33] reported the synthesis of B4 C nanowires by heating a B–C–Ni(NO3 )2 mixture at 1200 °C in argon flow. Su et al. [34] reported the synthesis of elongated B4 C nanostructures from the pyrolysis of a boric acid-sucrose mixture at 1500 °C. Jazirehpour et al. [35] also reported elongated nanostructures of B4 C (nanowires and nanobelts) via a catalyst-assisted thermal evaporation process using commercially available B4 C powder; however, in both of these cases, homogenous nanostructures were not achieved. Du et al. [36] reported the synthesis of B4 C nanoflakes from a bamboo-tree material via a thermal reduction method. Various approaches have also been reported for the synthesis of h-BN nanoparticles, including high-pressure calcination, chemical vapor deposition (CVD), solution combustion synthesis, and pyrolysis techniques. Meng et al. [37,38] synthesized h-BN nanoparticles under high-pressure conditions in a stainless steel autoclave. Lee et al. [39] reported a large-scale synthesis of BN nanosheets in a chemical vapor deposition (CVD) process by controlling the surface morphologies of the copper (Cu) catalysts. Tang et al. [40] used the CVD method in a two-step route: the resultant particles had average diameters of 90 nm and surface areas of 26.8 m2 g–1 . Xiong et al. [41] reported the synthesis of boron nitride nanoparticles with good aqueous dispersibility by means of a direct reaction of boric acid and ammonium chloride. Ferreira and co-workers [42] prepared BN nanotubes by calcination of a B+NH4 NO3 +Fe2 O3 mixture at 1300 °C in nitrogen gas flow. Xu et al. [43] proposed a metalassisted technique for the synthesis of triangular nanoplates in a closed autoclave; this technique uses NaNH2 , NH4 BF4 , and nickel sponge as the starting materials. As another technique, Zhao et al. [44] proposed a solution combustion process followed a heat treatment procedure in nitrogen at 110 0-140 0 °C. For this technique, H3 BO3 , CO(NH2 )2 , NaN3 , and NH4 Cl were used as raw materials.. Starting from NaBH4 , FeCl3 , and NaN3 , Wang et al. [45] synthesized BN nanoplates using a stainless steel autoclave of 20 ml capacity. BN nanosheets fabricated using this method have been demonstrated to be a good gold-nanoparticle support for selective oxidation. Zhong et al. [46] synthesized BN nanoplates (20 0–50 0 nm in size) from ammonium borane pyrolized at 1450 °C under 0.8 MPa argon pressure. The processing of B-containing nanostructured materials using the different synthesis techniques described above offers a large variety of choice for controlling the microstructure, shape, and size of the boron-containing nanostructures. Each synthesis approach is scientifically attractive and each has particular advantages and disadvantages. Even with the large variety of existing approaches, the development of a process that can produce a diversity of nanostructures in a simple manner and with low-cost will be welcome. In our recent reports [47–49], we demonstrated the synthesis of boron containing nanostructures using the solid wave combustion process. On the basis of our experimental results, the combustion parameters and the characteristics (morphology, phase composition, purity) of B, BN, and B4 C nanostructures were evaluated and reported. It was shown that in laboratory scale experiments that tens to hundreds of grams of nanopowders can

be prepared from low-cost raw materials. Compared to existing methods, the combustion reaction has large-scale production ability and shows interesting potential when compared with existing methods. The aim of this study is to summarize the previously reported data and determine the key factors that are responsible for Bbased functional nanostructure synthesis when a combustion wave moves within a packed bed of solid particles. We will discuss the relationship between the key factors and the morphological characteristics of the nanostructures. The combustion chemistry will be highlighted based on research data. Finally, we will formulate general concepts for the synthesis procedure and discuss this procedures practical usefulness for scale-up development.

2. Experimental Among the large variety of B-containing systems, we addressed amorphous boron (B), boron carbide (B4 C), and boron nitride (BN) nanostructures due to their practical importance. All three compounds were synthesized through a combustion reduction method using boron oxide powder (95% pure, particle size 50–300 μm, Junsei Chemical Co., Ltd., Japan), magnesium and NH4 Cl powders (99% pure, particle size: 50–200 μm, Samchun Chemicals and Metals Co., Ltd., Korea), and polyvinylchloride powder (C2 H3 Cl)n (99% pure, particle size: 50–300 μm, Junsei Chemical Co., Japan). All precursors were used as received. In a typical experiment, a reaction mixture of about 100 g, prepared by hand mixing of raw materials in a ceramic mortar, was hand-compacted into a quartz pipe (diameter: 4.0 cm, height: 8– 9 cm). During compaction, two -shaped tungsten-rhenium thermocouples (WR-26/WR-5), 100 μm in diameter, were placed inside the sample near the center. Individual thermocouples were coated with a thin layer of Al2 O3 (∼5–10 μm) to increase their resistance to oxidation and to avoid a possible interaction between the thermocouples and the powder bed at elevated temperatures. Approximately 2-3 g of Ti + 0.9C (black soot) + 0.1[(C2 F4 )n ] was placed on top of the reaction sample as an ignition agent. The cup containing the reaction mixture and the thermocouples was subsequently placed under a nickel/chromium coil in the combustion chamber. The chamber was then filled with argon (for B and B4 C) or nitrogen (for BN) and the combustion reaction was conducted under 1.0–2.5 MPa gas pressure. Local ignition of the reaction sample was achieved within 1–2 s using a nickel-chromium filament that was electrically heated to 90 0–10 0 0 °C. A computer-assisted data logger (GL100A, Graphtec Co., Japan) continuously recorded the temperature-time history of the process at a rate of 10 Hz. After completion of the combustion, the burnt sample was cooled to room temperature. Its surface layer (1–2 mm) was mechanically removed and the main sample was transferred to a 500 mL beaker for further purification. The reaction by-products were removed by mixing the sample with diluted HCl in a 500 mL glass beaker on a hot magnetic stir plate. The acid-leached powder was rinsed with distilled water and dried at 80–100 °C. The crystal structures and morphology of the final powders were characterized using an X-ray diffractometer with Cu Kα radiation (Siemens D50 0 0, Germany), and a transmission electron microscope (TEM, JEOL JEM-1400, Japan). Raman scattering spectra were obtained at room temperature using a Horiba Jobin Yvon LABRAM-HR800 laser micro-Raman spectrometer with a 633-nm laser. The adsorption/desorption isotherms and the specific surface area of the samples were determined using an SA-9600 BrunauerEmmett-Teller (BET) surface area analyzer (Horiba, USA). The oxygen concentrations were determined using a Thermo Scientific FLASH 20 0 0 Analyzer (formerly EA112, USA).

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Fig. 1. (a) A set of typical recorded images indicating the propagation of combustion wave on B2 O3 + 1.5Mg+0.125(C2 H3 Cl)n system in argon; (b) images of as-combusted samples and corresponding fine powders derived from the same samples.

3. Results and discussion 3.1. Initial system characterization, and key factors of combustion process All raw mixtures used in our experiments were constructed on a base of B2 O3 -Mg precursor mixture, which is sufficiently exothermic to maintain a self-propagating regime of the combustion reaction. Moreover, this mixture is inexpensive and can produce Bcontaining functional nanomaterials at low-cost. The composition of the raw mixtures used in the combustion experiments were as follows: - For boron synthesis: B2 O3 + α Mg; here, α is the mole number of Mg, which was controlled in the range of 1.0≤ α ≤2.0. - For boron carbide synthesis: B2 O3 + (α +α /24)Mg + (α /12)(C2 H3 Cl)n , for α =1.3–2.0. - For BN synthesis: B2 O3 + (3 + 0.5k)Mg + kNH4 Cl for k=2-7 (k is the mole number of NH4 Cl). The following key factors were taken into account when designing the compositions of the mixtures: (1) combustion temperature, (2) melting point of precursor materials, and (3) concentration of chemical additives used in the experiments. 3.2. Combustion process and burned down samples The propagation character of the combustion wave in the designed system can be seen in Fig. 1a, in which a set of typical images of the combustion wave in a B2 O3 + 1.5Mg+0.125(C2 H3 Cl)n mixture in argon are shown. After local ignition of the sample by an incandescent filament (1), a combustion wave is formed and propagates through the cold sample (2–8). According to the presented images, the combustion wave shows a discrete character and consists of a number of bright spots travelling in both longitudinal and transverse directions. The reaction mixture, compacted into a quartz pipe (height-6.0 cm), had a total combustion time of about 40–45 s, thus indicating that the combustion speed is about 0.1–0.15 cm/s. The small flame at the bottom of the pellet (image 8) after 40 s. indicates that the combustion reaction has completed. After the passing of the combustion wave through the entire sample (8), the light intensity from the sample surface remains stable during a period of tens of seconds. This observation provides visual evidence of the afterburning stage, in which prolonged bulk postcombustion occurs. Images of samples after the combustion reaction and cooling are shown in Fig. 1b. It is clear that the samples maintain their primary size and shape. The fine powders obtained

after grinding and leaching of the reaction samples display brown, grey, and white colors, thus indicating the successful formation of B, B4 C, and BN phases, respectively (Fig. 1b). 3.3. Combustion temperatures: thermodynamic calculation and experiment The combustion temperature is a key variable that affects the morphological characteristics of the reaction products. Therefore, the change of the adiabatic combustion temperature (Tad ) was analyzed using the software ‘Thermo’ [50]; results of this measurement were compared with experimentally measured data (Tc ). Figure 2a shows the changes of Tad and Tc for the B2 O3 +α Mg system as a function of Mg concentration. As can be seen in Fig. 2a, a linear increase in Tc occurred from 1300 to 1750 °C with the increase of the concentration of Mg from 1.0 to 2.0 mol. The change of Tad in the specified interval of α is from 1167 (α =1) to 1980 °C (α = 2). The difference between Tad and Tc for α =1.5 is about 35– 40 °C. However, at higher or lower α (α = 1 and 2), this difference become ∼150–200 °C. The most common reasons given for this discrepancy are inaccuracy of the measurements, and certain kinetic factors that cannot be measured during the thermodynamic calculation. The average increase rate of Tc is 450 °C per 1 mol of Mg. This result indicates that by the concentration of magnesium we may achive a drastic change of the combustion temperature in the B2 O3 +α Mg system. For the B2 O3 + (α +α /24)Mg + (α /12)(C2 H3 Cl)n system (Fig. 2b), the effect of Mg concentration on Tc was also strong. Temperature change from 1200 to 800 °C was observed for the α = 1–2 mol range of k. Consequently, the average increase rate of the combustion temperature was ∼ 550–600 °C per 1 mol of Mg. For the B2 O3 + (3 + 0.5k)Mg + kNH4 Cl system, control over the combustion temperature was achieved by changing the concentration of NH4 Cl. At the starting point (k = 2), the combustion temperature showed its highest value (Tc =1750 °C) (Fig. 2c). As expected, the increase in the initial concentration of NH4 Cl (k) was accompanied by a rapid decrease in the temperature. As shown in Fig. 2c, an almost linear decrease in Tc from 1750 to 950 °C occurred with increasing of the concentration of NH4 Cl from 2 to 7 mol. The average decrease rate of the temperature can be estimated from the temperature slope, and is found to be about 160 °C per mol of NH4 Cl. The low decreasing rate of the temperature can be explained by supplementary exothermic reactions that may occur between Mg and NH4 Cl and between B and NH4 Cl. The values of the combustion velocity (Uc ) for the specified systems are shown in Table 1. The combustion velocity (Uc ) in the

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Fig. 2. The change of Tc and Tad in B-containing systems: (a) B2 O3 +α Mg; (b) B2 O3 + (α +α /24)Mg + α /12 (C2 H3 Cl)n ; (c) B2 O3 + (3 + 0.5k)Mg + kNH4 Cl.

Table 1 Combustion parameters and chemical composition of B, B4 C and BN nanoparticles. No

1 2 3 4 5 6 7 8 9 10

System

B(α =1.0) B(α =1.5) B(α =2) B4 C(C2 H3 Cl)n (α =1.3) B4 C(C2 H3 Cl)n (α =1.5) B4 C(C2 H3 Cl)n (α =2.0) BN(k=2) BN(k=4) BN(k=5) BN(k=7)

Tc , ° C

1300 1580 1750 1450 1570 1730 1880 1450 1250 1030

Uc , cm/s

0.08 0.18 0.25 0.11 0.13 0.18 0.31 0.3 0.25 0.25

Impurities concentrations, wt%

Particle shape

B

O

Mg

Ctotal

Cfree

N2

95 88.5 84.7 – – – – – – –

1.75 0.5 0.2 1.1 0.9 0.7 2.2 3.5 3.9 4.5

3.25 11 15 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5

– – – 20.0 20.3 20.2 – – – –

– – – 1.9 1.6 1.4 – – – –

– – – – – – 41.9 40.8 40.5 40.0

B producing system is a function of the temperature, i.e. an increase in the temperature from 1300 to 1750 °C generates a corresponding increase in the wave velocity (from 0.08 to 0.25 cm/s) (Table 1, 1–3). For the B4 C system, the change in temperature from 1450 to 1730 °C resulted in an increase in the value of Uc from 0.11 to 0.18 cm/s. For the BN systems, the maximum combustion temperature has a minor impact on the wave propagation velocity at k=2-4: Uc shows a stable propagation rate (∼0.3 cm/s); however, at k = 5–7 (Tc =1250–1030 °C), Uc decreased to 0.25 cm/s. The values of the combustion velocity obtained in the experiments are generally in the preferred range in terms of the synthesis procedure. 3.4. Combustion environment and reaction phases In our study, we attempted to produce B-containing structures at low combustion temperatures, namely in the 10 0 0–150 0 °C temperature range. As noted above, these temperatures were reached by changing the concentration of Mg (in the B and B4 C systems) and by combining B2 O3 +3Mg and NH4 Cl+0.5Mg mixtures (for BN synthesis). Generally, low temperatures promote the formation of

Nanoparticles Nanoparticles Crystalline Nanosheets Nanosheets Nanosheets Nanosheets Nanosheets Nanosheets Nanosheets

nanoparticles, but can also negatively affect the completeness of the combustion reaction, because the low termperatures slow the rate of the diffusion processes. However, our approach makes it possible to overcome these negative circumstances because the combustion temperatures are always higher than the melting point of the precursor materials. When this is the case, the main reactions leading to B, B4 C, and BN formation were found to occur in the molten reaction medium, in which the diffusion mobility of the molecules is high compared to their mobility in solid phase. The XRD patterns of B, B4 C, and BN synthesized at temperatures of 1420 °C (α =1.2), 1570 °C (α =1.5), and 1450 °C (k = 4), respectively, show the effectiveness of the developed system at producing phase-pure nanostructures (Fig. 3A). Only in the B4 C system was a small amount of free carbon detected by XRD analysis. The chemical analysis revealed ∼1.4–1.9 wt % free carbon in the B4 C samples (Table 1). The concentrations of oxygen, magnesium, and other impurities in the B-containing samples are also summarized in Table 1. The ability of the designed system to produce a singlephase product is not limited by the temperatures shown above. For instance, as can be seen in Fig. 3B, the lowest combustion

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Fig. 3. XRD patterns of B-containing systems: A. (a) – B (α =1.2); (b) - B4 C (α =1.5); (c) - BN (k=4). B. (a) - BN (k = 5); (b) BN (k = 7).

Fig. 4. TEM, HR-TEM and corresponding SAED patterns of B nanoparticles prepared from B2 O3 +1.2Mg mixture.

temperature at which the synthesis of single-phase BN was still achieved was 1030 °C (k = 7). For amorphous boron the lowest synthesis temperature was 1300 °C (α =1.0) and for B4 C it was 1450 °C (k = 1.3) (Table 1). 3.5. Nanostructures and their specific features Having detailed the reaction mixture induced changes in the combustion temperatures and in the phase behavior, we now describe the morphology of the corresponding nanostructures. Typical nanostructures of the boron samples, obtained by TEM analysis technique, are shown in Fig. 4. Most of the particles have spherical shapes, as can be seen in Fig. 4a. The diameter of these particles is approximately 25–100 nm, as estimated from the micrograph. Figure 4b shows an HR-TEM image of a single boron nanoparticle. No crystalline fringes can be identified in the HR-TEM image at the lattice-resolved scale. This indicates that the boron nanoparticles are amorphous. The SAED pattern (Fig. 4b, inset) of the nanoparticles, which shows only some diffuse rings, confirms that the boron nanowires are completely amorphous. Next to the spherical particles, a small portion of nanocrystalline particles were also detected by low-magnification TEM observation (Fig. 4c). The size of the

crystallites can vary from 100 to 500 nm. In order to show the microstructure of the crystallite more clearly, a HR-TREM image was taken from the middle part of the crystallite sample. Because boron is prone to incorporate impurity elements and to form boron-rich compounds with other elements, it is not safe to say that these crystalline nanoparticles are purely boron. In order to prove that the crystalline nanoparticles are boron nanoparticles, SAED studies were carried out. HR-TEM observation and selected-area electron diffraction (SAED) pattern analysis (inset of Fig. 4d) indicate that these nanocrystallites have a β -rhombohedral boron structure with a growth direction along the axis. Among the structures of B4 C preliminarily obtained with different carbon sources [47], the most interesting case was the formation of B4 C nanosheets when PVC was applied as a carbon source. Therefore, TEM analysis was applied to find out the details of the microstructure of the B4 C sample prepared at k = 1.5. The results of the analysis are shown in Fig. 5. Different morphological fragments of the B4 C nanosheets can be detected in the TEM images: round and hexagonal shaped nanosheets with edge lengths of less than 100 nm (Fig. 5a, b), and imperfectly-shaped nanosheets with edge lengths of more than 100 nm (Fig. 5c). Most of the nanosheets have thicknesses from 3 to 40 nm and edge lengths of less than 500 nm. In some fragments, we detected larger size nanosheets of B4 C that were folded from one side (Fig. 5d). The edge lengths were more than 1 μm, as estimated from the micrograph. The HR-TEM image (Fig. 5b, inset) shows a 4.53 A˚ value for d-spacing; that value corresponds to the (101) plane of boron carbide. The selected area electron diffraction (SAED) patterns of B4 C, shown in the inset of Fig. 5b, c, and d, indicate that all the rings belong to the rhombohedral B4 C phase. The existence of a noticeable amount of free carbon in the B4 C sample was also confirmed by the TEM analysis data shown in Fig. 5e, f, and g. One can see that carbon particles are present in the form of nanotubes. The diameters of the nanotubes are from 30 to 100 nm and the length of the nanotubes is several hundreds of nm. An HR-TEM image of a carbon tube with an SAED diffraction pattern is shown in Fig. 5e. We calculated that there were about 30 atomic layers in the wall of the carbon tube. This observation may open a promising method for the synthesis of single- and multi-walled carbon nanotubes via the combustion approach developed and presented here. For the BN samples, the most interesting observation was that k increased from 4 to 7. The maximum combustion temperatures (Tc ), estimated from the temperature slopes (Fig. 2c), were 1450, 1250, and 1030 °C for k = 4, 5, and 7, respectively. Figure 6a shows a TEM image of the BN particles synthesized at k = 4. The morphology of most of the synthesized BN nanoparticles is spherical (or elliptical) plate. The thickness of the plates is about 20–30 nm and the diameter is in the range of 10 0–50 0 nm. For k = 5.0 and 7.0, the TEM images revealed nanoostructures of BN

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Fig. 5. (a, b, c, d) TEM images of B4 C nanostructures prepared with (C2 H3 Cl)n : (e, f) Carbon nanotubes formed along with B4 C nanoparticles.

with bifurcating curved fringes (Fig. 6b, c). The lateral dimensions of the as-synthesized nanosheets ranged from 50 to 100 nm, as estimated from the TEM image. In Fig. 6f, parallel line features can be observed along the edge of the sheet under high magnification. The number of layers in one sheet is from 5 to 10, as estimated from Fig. 6f. This allows not only the counting of the number of layers of the nanosheets but also the measurement of the interlayer distance, which was found to be around 0.33 nm, consistent with the reported value for the h-BN structure. The SAED patterns of the BN nanosheets shown in the inset of Fig. 6f suggest that all rings on the diffraction patterns belong to the hexagonal BN phase, that the as-synthesized nanosheets have a good crystalline nature, ˚ and that the lattice parameters are a = 2.5044 A˚ and c = 6.6562 A. 3.6. Adsorption-desorption isotherms and BET surface Adsorption-desorption isotherms and surface area (S) of the asprepared powders were characterized by BET analysis. The corresponding data are shown in Fig. 7. The nitrogen adsorption– desorption isotherm plots were recorded and classified as type II (according to the format recommended by IUPAC), with the very narrow hysteresis loops that are associated with capillary conden-

Table 2 BET characteristics of B, B4 C and BN nanopowders. System

S, m2 g−1

Dp , nm

Vp , cm3 g−1

B B4 C BN

9.09 12.77 23.5

10.8 49.3 19.5

0.025 0.157 0.013

sation and that are related to the presence of mesopores in the material. The surface area values of the B, B4 C, and BN nanostructures, calculated using the BET method, were 9.09, 12.77, and 23.5 m2 /g, respectively. These narrow hysteresis loops are generally attributed to the aggregation of nanoparticles during the synthesis. More characteristics of nanopowders, such as average pore diameter (dp ) and total pore volume (Vp ), are shown in Table 2. 3.7. Process chemistry and main outcomes The basic exothermic reaction that supports the self-sustaining combustion process in the B, B4 C, and BN systems is the reduction of B2 O3 by Mg. This reduction reaction in the stoichiometric point

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Fig. 6. TEM/HR-TEM microstructures and SAED pattern of BN nanostructure: a – k = 4; b- k = 5; c, d – k = 7.

(B2 O3 +3Mg) is highly exothermic; the adiabatic combustion temperature, calculated by the “Thermo”, software is about 2127 °C. This temperature is too high to support the formation of nanostructures in the specified systems. This means that an approach to decrease the combustion temperature is required. In this study, temperature decrease was achieved in two ways: by lowering the concentration of Mg (in the B and B4 C systems) and by combining the B2 O3 +3Mg mixture with NH4 Cl+0.5Mg (in the BN systems). Experimentally, these approaches were useful in optimizing the combustion temperature in a range between 10 0 0 and 1500 °C. Therefore, the basic reactions in the above designed systems can be given by the following equations:

B2 O3 + α Mg → (2α /3)B + α MgO +(α /3)B2 O3 ,

α =1.0-2.0

(1)

(3), MgCl2 also melts in the combustion wave (Tmelt. =714 °C). Therefore, the B2 O3 (liquid)+Mg(liquid) mechanism of the reaction can be considered the main driving force of the synthesis process. The reaction rate between the liquid phases at low temperatures is much faster than that between the solid phases. This has a positive effect on the phase composition of the combustion products; particularly, in the specified interval of the parameters, single-phase products were easily achieved. The morphology of the as-synthesized B-containing products exhibits a strong correlation with the combustion temperatures, melting processes, and type of additives. Thus, the following attractive features of B-nanostructures can be summarized, emphasizing the process conditions:

(3)

- The predominant shape of boron nanoparticles is spherical with a diameter of ∼10–100 nm. - The predominant shape of the B4 C nanoparticles is sheet-type, with thicknesses of 10 to 40 nm and edge lengths of less than 500 nm. - The predominant shape of the BN nanoparticles is spherical (or elliptical) sheet-type. The number of atomic layers is from 5 to somewhere in the tens. The thickness of the nanosheets is about ∼2–30 nm and the diameter is in the 10 0–50 0 nm range.

The combustion temperatures of reactions (1)–(3) are higher than the melting points of the raw materials: B2 O3 (Tmelt =450 °C), Mg (Tmelt =650 °C), (C2 H3 Cl)n (Tmelt. =250 °C), and NH4 Cl (Tmelt. =338 °C). Therefore, all these components melted in the early stage of the reaction, namely in the preheating zone of the combustion wave. Moreover, in the BN system formed by reaction

Generally, only gas-phase reactions [20–22] are known to produce well dispersed and spherically shaped boron nanoparticles. Therefore, the fabrication of spherical shape boron particles is one of the important outcomes of the combustion reaction. The second important outcome is the formation of high quality two-dimensional nanosheets of B4 C. It should be noted that the combustion synthesis of B4 C with other carbon materials (black

B2 O3 + (α +α /24)Mg + (α /12)(C2 H3 Cl)n → (α /6)B4 C + α MgO + (α /3)B2 O3 + α /24MgCl2 + α /8H2 ,

α =1.3-2.0 (2)

B2 O3 + (3 + 0.5k)Mg + kNH4 Cl → 2BN + 0.5kMgCl2 + 3 MgO + 2kH2 + (k-2)/2N2 , k=2-7

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Fig. 7. Adsorption-desorption isotherms of B-containing functional nanomaterials: a – B; b - B4 C; and c – BN.

soot, polyethylene, etc.) was not successful in producing particles of a sheet type morphology. More likely, the mechanism of sheettype nanostructure formation is based on the decomposition behavior of the (C2 H3 Cl)n polymer during the synthesis. Particularly, sheet-type carbon fragments can be formed after the fast melting of (C2 H3 Cl); these fragments may serve as a base to grow B4 C nanosheets. Another favorable mechanism for B4 C nanosheet formation is the dissolution-precipitation mechanism. Particularly, the melted B2 O3 formed during the process may serve as a reaction medium, from which the nucleation and growth of B4 C nanosheets may take place. During the dissolution–precipitation processes, the liquid phase prevents physical contact between individual nanosheets of B4 C and the grain growth processes. Therefore, B4 C nanosheets produced in a temperature range of 1400– 1570 °C have a small thickness. The third outcome is the formation of few-atomic-layer BN nanosheets from the designed system. The key factors that promote the formation of the few-atomic-layer BN nanosheets are the low combustion temperatures and the large portion of molten MgCl2 . The low combustion temperatures (10 0 0–120 0 °C), and the large portion of molten MgCl2 eventually inhibited the grain growth process; therefore, after the purification, a few atomic layer BN nanosheets were ptroduced Finally, based on the experimental findings the next conceptual points can be highlighted:

➢ Preferable values of the combustion temperature for B, B4 C, and BN nanostructure synthesis are located in the 10 0 0–150 0 °C range. ➢ Control over the temperature can be achieved by varying the concentration of Mg (for B and B4 C) and of the NH4 Cl+0.5Mg mixture (for BN).

➢ Control of the behavior of the reaction medium is essential for morphological shaping of the nanostructures and to preserve their nanodimensions. Liquid phase medium was found to be very effective for our purposes. Therefore, low melting point raw materials (Mg, B2 O3 , NH4 Cl) were used in the combustion experiments. Also, MgCl2, which forms during the combustion reaction, must be added to the list of these salts. ➢ The type and concentration of the additives ((C2 H3 Cl)n and NH4 Cl+0.5Mg) have a strong effects on the mechanism of the chemical reaction and, consequently, on the morphology of the nanostructures. The application of (C2 H3 Cl)n as a unique carbon source may extend the ability of the reaction to produce new morphological structures. The NH4 Cl+0.5Mg mixture was found to be effective at increasing the nitrogen concentration in the combustion wave, and at producing MgCl2 , which acts as a molten reaction medium for the B nitridation reaction. ➢ The combustion approach based on the B2 O3 +α Mg mixture can be easily scaled up to produce kg quantities of B, BN, and B4 C nanomaterials. 4. Conclusions In conclusion, we have demonstrated a combustion approach for synthesizing nanostructures of amorphous boron and its compounds from the B2 O3 /Mg/NH4 Cl/C complex systems. Control of the reaction temperature in the 10 0 0–150 0 °C range was found to make it possible to obtain single-phase nanostructures of B, B4 C, and BN. The morphology of the as-synthesized nanostructures was found to exhibit a strong correlation with the combustion temperatures, melting processes, and type of additives. The following attractive features of the obtained nanostructures can be summarized, emphasizing the process conditions: the predominant shape of boron nanoparticles is a spherical, with a diameter

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of ∼10–100 nm; B4 C nanoparticles are of sheet- type, with thicknesses of 10 to 40 nm and edge lengths of less than 500 nm; and BN nanoparticles were in the form of few-atomic-layer nanosheets ∼2–30 nm in thickness and 10 0–50 0 nm in diameter. The process chemistry and the main outcomes are discussed and highlighted. Acknowledgments This work was supported by a National Research Foundation of Korea (NRF) (NRF-2014R1A2A1A11049997) grant funded by the Korean government (MSIP).

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