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Formation of nanosized particles encapsulated in boron nitride during low-temperature annealing of mechanochemically treated Fe–BN mixtures
Journal of Alloys and Compounds 372 (2004) 141–147
Formation of nanosized particles encapsulated in boron nitride during low-temperature annealing of mechanochemically treated Fe–BN mixtures B. Bokhonov∗ , M. Korchagin, Yu. Borisova Institute of Solid State Chemistry, Siberian Branch, Russian Academy of Sciences, Kutateladze 18, 630128 Novosibirsk, Russia Received 12 August 2003; received in revised form 29 September 2003; accepted 29 September 2003
Abstract Electron microscopic investigations and X-ray studies of mechanically activated mixtures of iron and boron nitride provide evidence of the transformation of hexagonal boron nitride into the amorphous state, which is shows up as a halo with maximum at d = 3.35 Å in the electron diffraction patterns. According to the obtained electron microscopic data, the particles of iron and boron nitride after mechanical activation form aggregates with a size of 300–1000 nm. The iron particles exhibit a rather broad size distribution (from 3–7 nm to 100 nm), while the minimal size of the boron nitride particles is several tens of nanometers. Annealing of iron samples with boron nitride after preliminary mechanical activation, at a temperature of 860 ◦ C, caused the formation of nanosized encapsulated particles of iron boride Fe2 B. The surface of the Fe2 B nanoparticles were coated with a shell composed of hexagonal boron nitride, 5–15 nm thick. The size of the encapsulated particles varies within a rather broad range from 30 to 50 nm to several hundred nanometers. © 2003 Elsevier B.V. All rights reserved. Keywords: Nanostructures; Mechanical alloying; Transmission electron microscopy; X-ray diffraction
1. Introduction The attention for obtaining nano- and low-dimensional materials, such as fullerenes, nanotubes, and encapsulated particles (metals, carbides, borides, nitrides) is increasing in recent years. Intensive investigations in this area have led to the fact that nanosized structures of this morphology are characteristic not only of carbon materials. The formation of nanostructures similar to fullerenes or nanotubes turned out to be possible for compounds based on boron nitride, as well as for molybdenum and tungsten chalcogenides [1]. Such a morphological similarity of nanosize formations is mostly due to the similarity of the crystal structures of BN, WS2 and MoS2 compounds with the graphite structure. The synthesis of nanotubes from boron nitride was performed for the first time by Chopra et al. in 1995 [2]. Since then, nanotubes of boron nitride with various morphologies were prepared (bamboo-like, tassel-like and tree-like nanostructures, etc.) [3–5]. Investigation of boron nitride nan∗
Corresponding author. E-mail address: [email protected] (B. Bokhonov).
0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2003.09.138
otubes showed differences in their physical and chemical properties from carbon-based nanodimensional structures. Some characteristics of a boron nitride nanoparticles even exceed those of carbon nanomaterials. Nevertheless, though the properties of carbon nanostructures have been investigated in a large number of works, much less papers deal with nanostructures based on boron nitride. In order to obtain nanomaterials based on boron nitride, similar to obtaining carbon nanostructures, arc discharge and laser ablation in the presence of catalytic additives are mainly used [1–5]. In our opinion, among the new developments in the methods of synthesis of nanoparticles based on boron nitride, the most interesting ones are those aimed at obtaining boron nitride nanotubes by low-temperature annealing of mechanically activated compositions. For example, it was demonstrated that boron nitride nanotubes are formed in a process which includes mechanical activation of hexagonal boron nitride or elemental boron in an atmosphere of ammonia, followed by low-temperature annealing of the activated composition in an atmosphere of nitrogen at a temperature of 1200–1300 ◦ C [6–11].
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According to the data obtained in [12,13], boron nitride (or boron) was transformed into disordered state by means of mechanical activation. Moreover, mechanical activation of the boron powder was accompanied both by the formation of the BN phase via the reaction: B + NH3 → BN (disordered) + H2 (gas) and by the formation of nanosized boron particles. Low-temperature annealing (T = 1000 ◦ C) of mechanically activated compositions led to the formation of nanotubes with a size of 20–100 nm via two processes:
2. Investigation of morphological, phase and structural characteristics of low-temperature (up to 1000 ◦ C) annealing of mechanically activated composition. The choice of the object of this investigation, iron–boron nitride, is explained by the possibility to obtain nanoparticles of different morphology in this system.
2. Experimental 1. interaction of boron nanoparticles with nitrogen (nitridation of residual B powder) by the reaction (B + N2 → hexagonal-BN (powders + filaments)); 2. recrystallization of the disordered BN phase formed during mechanical activation, BN (disordered) → hexagonal-BN (filaments). The mechanochemical method in combination with low-temperature annealing could find wide application in preparing nanostructured materials based on boron nitride because of the simplicity of activation and annealing. However, in agreement with the experimental data reported in several papers [6–11], the time of preliminary mechanical activation of boron nitride or boron is 16–150 h, which prejudices the reasonability of this method for preparing nanosized structures. From this point of view, the possibility to use high-energy ball milling can allow decreasing the time necessary for the formation of the required products by several orders of magnitude, as it was demonstrated, for example in our works [14]. On the other hand, the temperature of annealing mechanically activated mixtures of boron nitride (or boron) was above 1000 ◦ C, which can hardly be considered as low-temperature annealing. It should also be noted that no data on structural and phase changes in catalyst (mainly iron) particles stimulating the growth of nanotubes are reported in papers dealing with the use of mechanical activation. For instance, it is accepted that the growth of bamboo-like and frame nanotubes is due only to surface diffusion processes and capillary effects. Along with the possibility to form boron nitride nanotubes under annealing of mechanically activated mixtures, not less interest exists in the possibility of the formation of nanosized particles encapsulated with boron nitride in these processes. It is known that the nanoparticles of this morphology can be formed in arc discharge, arc-melting or chemical methods. Using these methods nanosized boron-encapsulated particles were obtained from Fe, Co, Ni, CoOx , LaB6 , YB6 , FeB, Fe2 O3 , NiB, AlB10 , TiN, VN, Ag and Au. The whole set of data on the application of mechanochemical methods in the formation of nano- and low-dimensional structures based on boron nitride allowed us to formulate the goals of the present study: 1. Investigation of structural and phase transformations under high-energy ball milling of iron–boron nitride composition.
Iron and boron nitride powders of 99.99% purity grade with particle size 5–10 m were used in mechanochemical experiments. Mechanical activation was carried out in a planetary centrifugal mill at an acceleration of 600 m/s2 in stainless steel vials. The ratio of ball mass to the mass of powder to be activated was 6:1. The iron to boron nitride ratio in mechanochemical experiments was 50 at.% of iron to 50 at.% of boron nitride. To prevent oxidation, mechanical activation was carried out in argon atmosphere. Following milling, subsequent annealing of selected powders was carried out under Ar gas flow or vacuum at a temperature of 300 ◦ C. The structure of the milled and annealed samples was investigated by means X-ray diffraction using Co K␣ radiation at room temperature. Transmission electron microscopy was performed using JEOL JEM-2000FXII operated at 200 kV.
3. Results and discussion Our investigations showed that mechanical activation of the iron–boron nitride composition is accompanied by substantial structural and phase changes. For instance, the reflections of crystalline hexagonal boron nitride are completely absent from the X-ray diffraction patterns of the mixture of iron and boron nitride powders activated for 6 min (Fig. 1). In this case, a rather broad halo of the amorphous state is observed in the diffraction patterns in the angle region 2θ = 25–35◦ . At the same time, the iron reflections (1 1 0) and (2 0 0) are broadened substantially. In our opinion, the reasons leading to these changes in diffraction patterns are amorphization of hexagonal boron nitride, decrease in the size of the crystal blocks of iron, and its deformation. The possibility of the formation of a solid solution of boron nitride in iron cannot be excluded, too. The annealing of this composition at a temperature of 860 ◦ C for 30 min leads to narrowing of the ␣-Fe reflections. In addition, the reflections of the crystalline hexagonal boron nitride and Fe2 B boride phase are also present in the X-ray diffraction patterns (Fig. 1). Electron microscopic investigations of mechanically activated samples containing iron and boron nitride provide another evidence of the transformation of hexagonal boron nitride into the amorphous state, which is confirmed by the presence of a halo with a maximum at d = 3.35 Å in the
B. Bokhonov et al. / Journal of Alloys and Compounds 372 (2004) 141–147
143
Fe Fe2B
140
h-BN 120
INTENSITY (a.u.)
100
Fe + h-BN 6 min. MA amorphous BN
80
60
Fe + h-BN 6 min. MA 0 860 C 0.5 h
40
20
0 20
30
40
50
60
70
80
90
2θ DEGREES Fig. 1. X-ray powder diffraction patterns (Co K␣ irradiation) of iron-boron nitride samples after mechanical activation for 6 min and annealing at 860 ◦ C (0.5 h).
electron diffraction patterns (Fig. 2). According to the electron microscopic data obtained, the particles of iron and amorphous boron nitride form aggregates after mechanical activation for 6 min; the size of these aggregates
Fig. 2. SAD patterns of iron–boron nitride samples after mechanical activation for 6 min amorphous halo corresponding to the transformation of crystalline BN to the amorphous state.
is 300–1000 nm. The iron particles exhibit a rather broad size distribution: from 3 to 7 nm to several tens nanometers, whereas the minimal size of amorphous boron nitride particles is several tens of nanometers. The annealing at 860 ◦ C of the samples containing iron with boron nitride subjected to the preliminary mechanical activation caused the formation of encapsulated particles with their surface coated with a shell 5–15 nm thick. The size of the encapsulated particles varies within a rather broad range from 30 to 50 nm to several hundreds of nanometers (Fig. 3a). The dark-field electron microscopic investigations (Fig. 3b and c) and the HREM image (Fig. 4). showed that the shells of the formed nanoparticles are composed of crystalline hexagonal boron nitride, while the particle itself is composed of the Fe2 B phase. Reflections of the hexagonal boron nitride and iron boride (Fe2 B) are present in the electron diffraction patterns (Fig. 5). In order to explain the observed sequence of the formation of BN-encapsulated nanosized Fe2 B particles during the annealing of mechanically activated iron–boron nitride compositions, it is necessary to consider the data available in the literature on the stage sequence and assumed mechanisms of formation of encapsulated nanoparticles of similar composition in arc discharge and arc-melting processes. It was demonstrated in [15] that arc-melting of lanthanum hexaboride LaB6 in the atmosphere of nitrogen and argon results in the formation of LaB4 nanoparticles encapsulated in boron nitride. In case if a mixture of lanthanum
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Fig. 3. (a) TEM images of boron nitride encapsulated iron boride particles formed during the annealing of a mechanically activated iron–boron nitride mixture. (b) Dark-field electron microscopic images of encapsulated iron boride particles in crystalline boron nitride reflection. (c) Dark-field electron microscopic images of encapsulated iron boride particles in crystalline iron boride–Fe2 B reflection.
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Fig. 4. HREM images of iron boride particles encapsulated in boron nitride.
hexaboride LaB6 with boron was used as initial components, arc-melting of the composition in the atmosphere of nitrogen caused the formation of LaB6 nanoparticles encapsulated in boron nitride. The authors proposed the following
Fig. 5. SAD patterns of iron–boron nitride samples after annealing at 860 ◦ C (0.5 h).
sequence of stages for the formation of lanthanum boride nanoparticles encapsulated in boron nitride: • the lanthanum hexaboride particles pass into the liquid state during melting, • the crystallization of LaB6 is accompanied by its decomposition with the formation of the LaB4 phase coated with a amorphous boron shell, • amorphous boron interacts with nitrogen to form crystalline layers of hexagonal boron nitride on the surface of the LaB4 nanoparticles. The stage sequence of the formation of LaB6 particles encapsulated in boron nitride in the mixture of hexaboride and boron is similar to the process considered above. However, unlike for the formation of encapsulated LaB4 particles, the intermediate compound formed in the arc-melting and crystallization of a mixture of lanthanum hexaboride and boron is the LaB6+x phase coated with a shell of amorphous boron. By means of arc-melting in a N2 /Ar atmosphere, AlB10 , TiN and VN nanoparticles encapsulated in boron nitride have been obtained [16]. The initial reagents were mixtures of boron with Al, TiB2 and VB2 powders. According to the data presented in that paper, the formation of AlB10 , TiN and VN particles encapsulated in boron nitride nanocapsules proceeds via the formation of intermediate products during arc-melting. These intermediate products have the following compositions: amorphous boron coating the surface of the AlB12−x particles—for a B + Al mixture; amorphous boron
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coating the surface of TiB2−x particles—for a B + TiB2 mixture, and amorphous boron coating the surface of the VB2−x particles—for a B + VB2 mixture. The result of the interaction of these intermediate compositions with gaseous nitrogen is the formation of AlB10 , TiN and VN nanoparticles encapsulated in boron nitride nanocapsules. The formation of particles encapsulated in BN occurs not only with the participation of boride phases. For instance, particles of iron and cobalt oxides encapsulated in BN are formed in arc discharge and arc-melting of mixtures of these oxides with boron in an atmosphere of nitrogen and argon [17–19]. The authors assume that liquid nanodrops composed of a mixture of an oxide and boron are formed during arc discharge. In the opinion of the authors, boron in the liquid drop does not interact with cobalt oxide, whereas it reacts with gaseous nitrogen forming boron nitride, which is deposited as a shell on the cobalt oxide surface. Encapsulation of cobalt nanoparticles in boron nitride nanocapsules have been obtained from Co and boron powders in the H2 /NH3 atmosphere under heating to 800 ◦ C [20]. It was found that cobalt particles play the role of a catalyst in the following chemical reaction: 2B + 2NH3g → 2BN + 3H2g + 93 kcal/mol. But this equation cannot help one to establish the role of cobalt in the formation of the encapsulated particles. The formation of intermediate products is proposed in the works in which the formation of nanostructures occurs with the application of chemical methods of synthesis. For example, the growth of BN nanotubes was observed under annealing of nanosized ␣-FeB particles at 1100 ◦ C in an atmosphere composed of ammonia and nitrogen. In the opinion of Huo et al. [21], the centers of nucleation and growth of BN nanotubes are defects on the ␣-FeB surface. It is assumed that the mechanism of the formation of nanotubes in the presence of ␣-FeB nanoparticles is similar to a well-known vapor–liquid–crystal (VLC) mechanism. A substantial difference with the mechanism proposed in studies is that the components of the growing boron nitride nanotubes originate not only from the gas phase, like nitrogen from a NH3 /N2 gas mixture, but are released, like boron from the catalytic ␣-FeB particle. So, ␣-FeB is not a catalyst in the direct meaning of this word, because changes its composition during the chemical reaction. According to the data shown above, at 1100 ◦ C, the FeB “catalyst” occurs in the form of liquid drops. When a NH3 /N2 gas mixture is decomposed on the surface of such a drop, the formation of nitrogen atoms adsorbed on the surface is observed; this nitrogen diffuses into the “catalyst” forming a BN/FeB1−x solid solution. Substantial supersaturation is achieved during the process, which stimulates the growth of boron nitride nanotubes. Such a mechanism of the growth of boron nitride nanotubes involving YB6 particles in the arc-melting of powder in N2 /Ar atmosphere was proposed in [22]. It was discovered that the growth of boron nitride nanotubes is accompanied by changes in the composition of the hexaboride, with
the formation of an intermediate off-stoichiometric YB6−x compound; the final product of the reaction, along with nanotubes, is yttrium boride YB2 . The formation of boron nitride nanotubes was observed under heating from 1000 to 1300 ◦ C in a mixture of boron and nickel boride NiB (used as a catalyst) deposited on aluminum oxide [23]. According to the data obtained, the “catalytic” growth of BN nanotubes was connected with NiB melting during heating. The interaction of a liquid drop of NiB with nitrogen resulted in the formation of a eutectic NiB/BN mixture. In the opinion of Tang et al. [23], the decomposition of such a eutectic composition leads to the growth of boron nitride nanotubes. Summing up the above-mentioned literature data, we may distinguish a common feature of all the processes (arc discharge, arc-melting, chemical synthesis) which result in the formation of nanosized particles encapsulated in boron nitride nanocapsules: 1. It is assumed that the initial stages of the formation of encapsulated nanoparticles and nanotubes involve the formation of an intermediate product, for example an off-stoichiometric boride, a eutectic-type product, or a supersaturated solution. 2. The direct formation of encapsulated particles or nanotubes occurs either as a consequence of decomposition of the intermediate product or due to its transformation in the interaction with gaseous nitrogen. As a rule, this stage is accompanied by changes in the stoichiometry of the intermediate product, for example, a decrease in boron concentration is observed for metal boride. Undoubtedly, arc discharge and arc-melting are accompanied by such changes in the state of the substance which can hardly be achieved by mechanical activation of solids. Nevertheless, the possibility of the formation of intermediate metastable products during mechanical activation of solids is not unusual. It is well known that in many cases rather high supersaturation (much higher than the equilibrium concentrations) can be achieved by means of mechanical activation [24]. It is natural that the annealing of such a non-equilibrium state should bring the system into the equilibrium. We have implemented such an approach based on the creation of an intermediate state by means of mechanical activation in metal/carbon systems when the annealing of mechanically activated compositions was accompanied by the formation of graphite-encapsulated iron and nickel nanoparticles [14]. We proposed the following mechanism of the formation of encapsulated nanoparticles: 1. Me + amorphous C → mechanical activation → Mex Cy (metastable compound). 2. Mex Cy → annealing → Me crystalline C.
B. Bokhonov et al. / Journal of Alloys and Compounds 372 (2004) 141–147
According to the proposed scheme, the stage of mechanical activation (1) of a mixture of amorphous carbon and metal involves dispersing and activation of the components, as well as the formation of a metastable compound of a metal with carbon, for example, carbide or supersaturated solid solution of carbon in the metal. The subsequent low-temperature annealing (2) leads to the decomposition of the metastable state and the subsequent formation of metal nanoparticle encapsulated in graphite. Drawing an analogy between the above-considered sequence of stages leading to the formation of nanoparticles encapsulated with graphite and the experimental and theoretical data available from literature we may assume that the formation of nanosized Fe2 B particles encapsulated in boron nitride follows a similar mechanism: Fe + BN → Fex (BN)y
or
Fe + BN → (Fex B/BN) + N2 Fex (BN)y → Fe2 B/BN + N2
(1) (2)
A metastable Fex (BN)y product is formed during mechanical activation of the Fe–BN composition (1). Low-temperature annealing (2) of this metastable state is accompanied by its decomposition with the formation of Fe2 B nanoparticles, their surfaces coated with hexagonal BN. So, our investigations of the low-temperature annealing of a mechanically activated mixture of iron and hexagonal boron nitride demonstrate the principal possibility to use this method for the synthesis of BN-encapsulated nanosized particles. In our opinion, due to the simplicity of the mechanical activation and the low-temperature annealing processes, this method can be widely used for obtaining BN-encapsulated nanosized particles of both borides and nitrides of different metals.
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Formation of nanosized particles encapsulated in boron nitride during low-temperature annealing of mechanochemically treated Fe–BN mixtures B. Bokhonov∗ , M. Korchagin, Yu. Borisova Institute of Solid State Chemistry, Siberian Branch, Russian Academy of Sciences, Kutateladze 18, 630128 Novosibirsk, Russia Received 12 August 2003; received in revised form 29 September 2003; accepted 29 September 2003
Abstract Electron microscopic investigations and X-ray studies of mechanically activated mixtures of iron and boron nitride provide evidence of the transformation of hexagonal boron nitride into the amorphous state, which is shows up as a halo with maximum at d = 3.35 Å in the electron diffraction patterns. According to the obtained electron microscopic data, the particles of iron and boron nitride after mechanical activation form aggregates with a size of 300–1000 nm. The iron particles exhibit a rather broad size distribution (from 3–7 nm to 100 nm), while the minimal size of the boron nitride particles is several tens of nanometers. Annealing of iron samples with boron nitride after preliminary mechanical activation, at a temperature of 860 ◦ C, caused the formation of nanosized encapsulated particles of iron boride Fe2 B. The surface of the Fe2 B nanoparticles were coated with a shell composed of hexagonal boron nitride, 5–15 nm thick. The size of the encapsulated particles varies within a rather broad range from 30 to 50 nm to several hundred nanometers. © 2003 Elsevier B.V. All rights reserved. Keywords: Nanostructures; Mechanical alloying; Transmission electron microscopy; X-ray diffraction
1. Introduction The attention for obtaining nano- and low-dimensional materials, such as fullerenes, nanotubes, and encapsulated particles (metals, carbides, borides, nitrides) is increasing in recent years. Intensive investigations in this area have led to the fact that nanosized structures of this morphology are characteristic not only of carbon materials. The formation of nanostructures similar to fullerenes or nanotubes turned out to be possible for compounds based on boron nitride, as well as for molybdenum and tungsten chalcogenides [1]. Such a morphological similarity of nanosize formations is mostly due to the similarity of the crystal structures of BN, WS2 and MoS2 compounds with the graphite structure. The synthesis of nanotubes from boron nitride was performed for the first time by Chopra et al. in 1995 [2]. Since then, nanotubes of boron nitride with various morphologies were prepared (bamboo-like, tassel-like and tree-like nanostructures, etc.) [3–5]. Investigation of boron nitride nan∗
Corresponding author. E-mail address: [email protected] (B. Bokhonov).
0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2003.09.138
otubes showed differences in their physical and chemical properties from carbon-based nanodimensional structures. Some characteristics of a boron nitride nanoparticles even exceed those of carbon nanomaterials. Nevertheless, though the properties of carbon nanostructures have been investigated in a large number of works, much less papers deal with nanostructures based on boron nitride. In order to obtain nanomaterials based on boron nitride, similar to obtaining carbon nanostructures, arc discharge and laser ablation in the presence of catalytic additives are mainly used [1–5]. In our opinion, among the new developments in the methods of synthesis of nanoparticles based on boron nitride, the most interesting ones are those aimed at obtaining boron nitride nanotubes by low-temperature annealing of mechanically activated compositions. For example, it was demonstrated that boron nitride nanotubes are formed in a process which includes mechanical activation of hexagonal boron nitride or elemental boron in an atmosphere of ammonia, followed by low-temperature annealing of the activated composition in an atmosphere of nitrogen at a temperature of 1200–1300 ◦ C [6–11].
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According to the data obtained in [12,13], boron nitride (or boron) was transformed into disordered state by means of mechanical activation. Moreover, mechanical activation of the boron powder was accompanied both by the formation of the BN phase via the reaction: B + NH3 → BN (disordered) + H2 (gas) and by the formation of nanosized boron particles. Low-temperature annealing (T = 1000 ◦ C) of mechanically activated compositions led to the formation of nanotubes with a size of 20–100 nm via two processes:
2. Investigation of morphological, phase and structural characteristics of low-temperature (up to 1000 ◦ C) annealing of mechanically activated composition. The choice of the object of this investigation, iron–boron nitride, is explained by the possibility to obtain nanoparticles of different morphology in this system.
2. Experimental 1. interaction of boron nanoparticles with nitrogen (nitridation of residual B powder) by the reaction (B + N2 → hexagonal-BN (powders + filaments)); 2. recrystallization of the disordered BN phase formed during mechanical activation, BN (disordered) → hexagonal-BN (filaments). The mechanochemical method in combination with low-temperature annealing could find wide application in preparing nanostructured materials based on boron nitride because of the simplicity of activation and annealing. However, in agreement with the experimental data reported in several papers [6–11], the time of preliminary mechanical activation of boron nitride or boron is 16–150 h, which prejudices the reasonability of this method for preparing nanosized structures. From this point of view, the possibility to use high-energy ball milling can allow decreasing the time necessary for the formation of the required products by several orders of magnitude, as it was demonstrated, for example in our works [14]. On the other hand, the temperature of annealing mechanically activated mixtures of boron nitride (or boron) was above 1000 ◦ C, which can hardly be considered as low-temperature annealing. It should also be noted that no data on structural and phase changes in catalyst (mainly iron) particles stimulating the growth of nanotubes are reported in papers dealing with the use of mechanical activation. For instance, it is accepted that the growth of bamboo-like and frame nanotubes is due only to surface diffusion processes and capillary effects. Along with the possibility to form boron nitride nanotubes under annealing of mechanically activated mixtures, not less interest exists in the possibility of the formation of nanosized particles encapsulated with boron nitride in these processes. It is known that the nanoparticles of this morphology can be formed in arc discharge, arc-melting or chemical methods. Using these methods nanosized boron-encapsulated particles were obtained from Fe, Co, Ni, CoOx , LaB6 , YB6 , FeB, Fe2 O3 , NiB, AlB10 , TiN, VN, Ag and Au. The whole set of data on the application of mechanochemical methods in the formation of nano- and low-dimensional structures based on boron nitride allowed us to formulate the goals of the present study: 1. Investigation of structural and phase transformations under high-energy ball milling of iron–boron nitride composition.
Iron and boron nitride powders of 99.99% purity grade with particle size 5–10 m were used in mechanochemical experiments. Mechanical activation was carried out in a planetary centrifugal mill at an acceleration of 600 m/s2 in stainless steel vials. The ratio of ball mass to the mass of powder to be activated was 6:1. The iron to boron nitride ratio in mechanochemical experiments was 50 at.% of iron to 50 at.% of boron nitride. To prevent oxidation, mechanical activation was carried out in argon atmosphere. Following milling, subsequent annealing of selected powders was carried out under Ar gas flow or vacuum at a temperature of 300 ◦ C. The structure of the milled and annealed samples was investigated by means X-ray diffraction using Co K␣ radiation at room temperature. Transmission electron microscopy was performed using JEOL JEM-2000FXII operated at 200 kV.
3. Results and discussion Our investigations showed that mechanical activation of the iron–boron nitride composition is accompanied by substantial structural and phase changes. For instance, the reflections of crystalline hexagonal boron nitride are completely absent from the X-ray diffraction patterns of the mixture of iron and boron nitride powders activated for 6 min (Fig. 1). In this case, a rather broad halo of the amorphous state is observed in the diffraction patterns in the angle region 2θ = 25–35◦ . At the same time, the iron reflections (1 1 0) and (2 0 0) are broadened substantially. In our opinion, the reasons leading to these changes in diffraction patterns are amorphization of hexagonal boron nitride, decrease in the size of the crystal blocks of iron, and its deformation. The possibility of the formation of a solid solution of boron nitride in iron cannot be excluded, too. The annealing of this composition at a temperature of 860 ◦ C for 30 min leads to narrowing of the ␣-Fe reflections. In addition, the reflections of the crystalline hexagonal boron nitride and Fe2 B boride phase are also present in the X-ray diffraction patterns (Fig. 1). Electron microscopic investigations of mechanically activated samples containing iron and boron nitride provide another evidence of the transformation of hexagonal boron nitride into the amorphous state, which is confirmed by the presence of a halo with a maximum at d = 3.35 Å in the
B. Bokhonov et al. / Journal of Alloys and Compounds 372 (2004) 141–147
143
Fe Fe2B
140
h-BN 120
INTENSITY (a.u.)
100
Fe + h-BN 6 min. MA amorphous BN
80
60
Fe + h-BN 6 min. MA 0 860 C 0.5 h
40
20
0 20
30
40
50
60
70
80
90
2θ DEGREES Fig. 1. X-ray powder diffraction patterns (Co K␣ irradiation) of iron-boron nitride samples after mechanical activation for 6 min and annealing at 860 ◦ C (0.5 h).
electron diffraction patterns (Fig. 2). According to the electron microscopic data obtained, the particles of iron and amorphous boron nitride form aggregates after mechanical activation for 6 min; the size of these aggregates
Fig. 2. SAD patterns of iron–boron nitride samples after mechanical activation for 6 min amorphous halo corresponding to the transformation of crystalline BN to the amorphous state.
is 300–1000 nm. The iron particles exhibit a rather broad size distribution: from 3 to 7 nm to several tens nanometers, whereas the minimal size of amorphous boron nitride particles is several tens of nanometers. The annealing at 860 ◦ C of the samples containing iron with boron nitride subjected to the preliminary mechanical activation caused the formation of encapsulated particles with their surface coated with a shell 5–15 nm thick. The size of the encapsulated particles varies within a rather broad range from 30 to 50 nm to several hundreds of nanometers (Fig. 3a). The dark-field electron microscopic investigations (Fig. 3b and c) and the HREM image (Fig. 4). showed that the shells of the formed nanoparticles are composed of crystalline hexagonal boron nitride, while the particle itself is composed of the Fe2 B phase. Reflections of the hexagonal boron nitride and iron boride (Fe2 B) are present in the electron diffraction patterns (Fig. 5). In order to explain the observed sequence of the formation of BN-encapsulated nanosized Fe2 B particles during the annealing of mechanically activated iron–boron nitride compositions, it is necessary to consider the data available in the literature on the stage sequence and assumed mechanisms of formation of encapsulated nanoparticles of similar composition in arc discharge and arc-melting processes. It was demonstrated in [15] that arc-melting of lanthanum hexaboride LaB6 in the atmosphere of nitrogen and argon results in the formation of LaB4 nanoparticles encapsulated in boron nitride. In case if a mixture of lanthanum
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Fig. 3. (a) TEM images of boron nitride encapsulated iron boride particles formed during the annealing of a mechanically activated iron–boron nitride mixture. (b) Dark-field electron microscopic images of encapsulated iron boride particles in crystalline boron nitride reflection. (c) Dark-field electron microscopic images of encapsulated iron boride particles in crystalline iron boride–Fe2 B reflection.
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Fig. 4. HREM images of iron boride particles encapsulated in boron nitride.
hexaboride LaB6 with boron was used as initial components, arc-melting of the composition in the atmosphere of nitrogen caused the formation of LaB6 nanoparticles encapsulated in boron nitride. The authors proposed the following
Fig. 5. SAD patterns of iron–boron nitride samples after annealing at 860 ◦ C (0.5 h).
sequence of stages for the formation of lanthanum boride nanoparticles encapsulated in boron nitride: • the lanthanum hexaboride particles pass into the liquid state during melting, • the crystallization of LaB6 is accompanied by its decomposition with the formation of the LaB4 phase coated with a amorphous boron shell, • amorphous boron interacts with nitrogen to form crystalline layers of hexagonal boron nitride on the surface of the LaB4 nanoparticles. The stage sequence of the formation of LaB6 particles encapsulated in boron nitride in the mixture of hexaboride and boron is similar to the process considered above. However, unlike for the formation of encapsulated LaB4 particles, the intermediate compound formed in the arc-melting and crystallization of a mixture of lanthanum hexaboride and boron is the LaB6+x phase coated with a shell of amorphous boron. By means of arc-melting in a N2 /Ar atmosphere, AlB10 , TiN and VN nanoparticles encapsulated in boron nitride have been obtained [16]. The initial reagents were mixtures of boron with Al, TiB2 and VB2 powders. According to the data presented in that paper, the formation of AlB10 , TiN and VN particles encapsulated in boron nitride nanocapsules proceeds via the formation of intermediate products during arc-melting. These intermediate products have the following compositions: amorphous boron coating the surface of the AlB12−x particles—for a B + Al mixture; amorphous boron
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coating the surface of TiB2−x particles—for a B + TiB2 mixture, and amorphous boron coating the surface of the VB2−x particles—for a B + VB2 mixture. The result of the interaction of these intermediate compositions with gaseous nitrogen is the formation of AlB10 , TiN and VN nanoparticles encapsulated in boron nitride nanocapsules. The formation of particles encapsulated in BN occurs not only with the participation of boride phases. For instance, particles of iron and cobalt oxides encapsulated in BN are formed in arc discharge and arc-melting of mixtures of these oxides with boron in an atmosphere of nitrogen and argon [17–19]. The authors assume that liquid nanodrops composed of a mixture of an oxide and boron are formed during arc discharge. In the opinion of the authors, boron in the liquid drop does not interact with cobalt oxide, whereas it reacts with gaseous nitrogen forming boron nitride, which is deposited as a shell on the cobalt oxide surface. Encapsulation of cobalt nanoparticles in boron nitride nanocapsules have been obtained from Co and boron powders in the H2 /NH3 atmosphere under heating to 800 ◦ C [20]. It was found that cobalt particles play the role of a catalyst in the following chemical reaction: 2B + 2NH3g → 2BN + 3H2g + 93 kcal/mol. But this equation cannot help one to establish the role of cobalt in the formation of the encapsulated particles. The formation of intermediate products is proposed in the works in which the formation of nanostructures occurs with the application of chemical methods of synthesis. For example, the growth of BN nanotubes was observed under annealing of nanosized ␣-FeB particles at 1100 ◦ C in an atmosphere composed of ammonia and nitrogen. In the opinion of Huo et al. [21], the centers of nucleation and growth of BN nanotubes are defects on the ␣-FeB surface. It is assumed that the mechanism of the formation of nanotubes in the presence of ␣-FeB nanoparticles is similar to a well-known vapor–liquid–crystal (VLC) mechanism. A substantial difference with the mechanism proposed in studies is that the components of the growing boron nitride nanotubes originate not only from the gas phase, like nitrogen from a NH3 /N2 gas mixture, but are released, like boron from the catalytic ␣-FeB particle. So, ␣-FeB is not a catalyst in the direct meaning of this word, because changes its composition during the chemical reaction. According to the data shown above, at 1100 ◦ C, the FeB “catalyst” occurs in the form of liquid drops. When a NH3 /N2 gas mixture is decomposed on the surface of such a drop, the formation of nitrogen atoms adsorbed on the surface is observed; this nitrogen diffuses into the “catalyst” forming a BN/FeB1−x solid solution. Substantial supersaturation is achieved during the process, which stimulates the growth of boron nitride nanotubes. Such a mechanism of the growth of boron nitride nanotubes involving YB6 particles in the arc-melting of powder in N2 /Ar atmosphere was proposed in [22]. It was discovered that the growth of boron nitride nanotubes is accompanied by changes in the composition of the hexaboride, with
the formation of an intermediate off-stoichiometric YB6−x compound; the final product of the reaction, along with nanotubes, is yttrium boride YB2 . The formation of boron nitride nanotubes was observed under heating from 1000 to 1300 ◦ C in a mixture of boron and nickel boride NiB (used as a catalyst) deposited on aluminum oxide [23]. According to the data obtained, the “catalytic” growth of BN nanotubes was connected with NiB melting during heating. The interaction of a liquid drop of NiB with nitrogen resulted in the formation of a eutectic NiB/BN mixture. In the opinion of Tang et al. [23], the decomposition of such a eutectic composition leads to the growth of boron nitride nanotubes. Summing up the above-mentioned literature data, we may distinguish a common feature of all the processes (arc discharge, arc-melting, chemical synthesis) which result in the formation of nanosized particles encapsulated in boron nitride nanocapsules: 1. It is assumed that the initial stages of the formation of encapsulated nanoparticles and nanotubes involve the formation of an intermediate product, for example an off-stoichiometric boride, a eutectic-type product, or a supersaturated solution. 2. The direct formation of encapsulated particles or nanotubes occurs either as a consequence of decomposition of the intermediate product or due to its transformation in the interaction with gaseous nitrogen. As a rule, this stage is accompanied by changes in the stoichiometry of the intermediate product, for example, a decrease in boron concentration is observed for metal boride. Undoubtedly, arc discharge and arc-melting are accompanied by such changes in the state of the substance which can hardly be achieved by mechanical activation of solids. Nevertheless, the possibility of the formation of intermediate metastable products during mechanical activation of solids is not unusual. It is well known that in many cases rather high supersaturation (much higher than the equilibrium concentrations) can be achieved by means of mechanical activation [24]. It is natural that the annealing of such a non-equilibrium state should bring the system into the equilibrium. We have implemented such an approach based on the creation of an intermediate state by means of mechanical activation in metal/carbon systems when the annealing of mechanically activated compositions was accompanied by the formation of graphite-encapsulated iron and nickel nanoparticles [14]. We proposed the following mechanism of the formation of encapsulated nanoparticles: 1. Me + amorphous C → mechanical activation → Mex Cy (metastable compound). 2. Mex Cy → annealing → Me crystalline C.
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According to the proposed scheme, the stage of mechanical activation (1) of a mixture of amorphous carbon and metal involves dispersing and activation of the components, as well as the formation of a metastable compound of a metal with carbon, for example, carbide or supersaturated solid solution of carbon in the metal. The subsequent low-temperature annealing (2) leads to the decomposition of the metastable state and the subsequent formation of metal nanoparticle encapsulated in graphite. Drawing an analogy between the above-considered sequence of stages leading to the formation of nanoparticles encapsulated with graphite and the experimental and theoretical data available from literature we may assume that the formation of nanosized Fe2 B particles encapsulated in boron nitride follows a similar mechanism: Fe + BN → Fex (BN)y
or
Fe + BN → (Fex B/BN) + N2 Fex (BN)y → Fe2 B/BN + N2
(1) (2)
A metastable Fex (BN)y product is formed during mechanical activation of the Fe–BN composition (1). Low-temperature annealing (2) of this metastable state is accompanied by its decomposition with the formation of Fe2 B nanoparticles, their surfaces coated with hexagonal BN. So, our investigations of the low-temperature annealing of a mechanically activated mixture of iron and hexagonal boron nitride demonstrate the principal possibility to use this method for the synthesis of BN-encapsulated nanosized particles. In our opinion, due to the simplicity of the mechanical activation and the low-temperature annealing processes, this method can be widely used for obtaining BN-encapsulated nanosized particles of both borides and nitrides of different metals.
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