J. Cent. South Univ. (2014) 21: 900−903 DOI: 10.1007/s11771­014­2016­2

Preparation of amorphous nano­boron powder with high activity by combustion synthesis

DOU Zhi­he(豆志河), ZHANG Ting­an(张廷安), HE Ji­cheng(赫冀成), HUANGYang(黄杨)

Key Laboratory for Ecological Metallurgy of Multi­metallic Mineral of Ministry of Education (Northeastern University), Shenyang 110819, China

© Central South University Press and Springer­Verlag Berlin Heidelberg 2014

Abstract: The preparation process of amorphous nanometer boron powders through combustion synthesis was investigated, and the effects of the reactant ratio, the heating agent and the milling rate on the activity and particle size of amorphous boron powders were studied. The results show that the boron powders exist in the form of an amorphous phase which has the crystallinity lower than 30.4%, and the particle size of boron powder decreases with an increase of the high­energy ball milling rate. The purity of amorphous boron powder is 94.8% and particle sizes are much smaller than 100 nm when the mass ratio of B2O3/Mg/KClO3 is 100:105:17 and the ball milling time is 20 min with the milling rate of 300 r/min. At the same time, the amorphous boron nano­fibers appear in the boron powders.

Key words: amorphous nano­boron powder; high activity; combustion synthesis; high­energy ball milling

1 Introduction

In the elemental semiconductor field, boron is one of the least understood elements about its structure and performance [1−2]. Elemental boron has a low density, a high melting point (about 2300 °C, about 1000 °C higher than that of Si), a high boiling point, and a low volatility. Its hardness is only smaller than that of diamond [1]. Besides, boron is one of the few elements that can be used in nuclear reactions, in spacecraft as a reinforcing material, in high­temperature semiconductors as a protective material, and in high­temperature thermo­ electric applications. In addition, the ultrafine amorphous boron powder is an excellent high­energy component in the oxygen­rich solid propellant, and is a high­energy solid fuel [3−4]. Amorphous boron powder is used as a rich boron fuel and the airbags initiator because of its large specific surface area, high combustion heat, and so on [4]. Recently, ultrafine amorphous boron powder is used in the fields of energy storage and utilization of solar energy. WANG et al [5] studied the utilization of the amorphous boron powder to store hydrogen. ABU­HAME et al [6] used B2O3­Mg system for solar energy storage and conversion.

At present, the preparation methods of the amorphous boron powder include salt electrolysis, pyrolysis of diborane, hydrogen reduction of halogenated boron and metal thermal reduction [7]. Amorphous boron

powder can be prepared by self­propagating high­ temperature reduction reaction (magnesium thermal reduction) between magnesium powders and boron oxide [8−9]. SONG et al [10] obtained boron powder with the purity higher than 90% from the molten salt system of potassium chloride­potassium fluoride­boric acid­boron oxide, which was not seen a further development due to its low efficiency of the current utilization, poor working conditions and non­continuous production. For the use of B2H6 and BX3 as the raw materials (X represents Cl or Br), high­purity boron powder can be obtained, while it is limited due to the harsh operating environment and pollution. At present, boron powder with the content of boron less than 90% and the particle size above 2 µm can be only obtained by the metal thermal reduction, due to technical limitations [8, 11−13]. A new method was put forward to prepare amorphous nano­boron powder with a high activity by the use of combustion synthesis. The effects of the high­energy ball milling rate and the heating agent content on the activity and the particle size of amorphous boron powder were investigated. Amorphous nano­boron powders were characterized by XRD, TEM and Raman spectroscopy, etc.

2 Experimental

2.1 Preparation of samples The reaction materials are as follows: 1) magnesium

powder with the purity of 99% and the particle size

Foundation item: Project(51002025) supported by the National Natural Science Foundation of China Received date: 2012−09−13; Accepted date: 2013−04−11 Corresponding author: ZHANG Ting­an, Professor, PhD; Tel: +86−24−83681563; E­mail: zta2000@163.net

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smaller than 149 µm; 2) B2O3 powder with the purity of 98% and the particle size smaller than 149 µm. The reaction equation is expressed as follows:

B2O3+3Mg = 2B+3MgO (1)

The detailed conditions for combustion synthesis were listed in Table 1.

Table 1 Combustion synthesis conditions (mass ratio, %) Mass ratio/%

No. B2O3 Mg KClO3

Milling rate/ (r∙min −1 )

Time/ min

1 100 100 − −

2 100 105 17 250 20

3 100 110 17 250 20

4 100 105 17 300 20

For Trial 1, the raw materials were mixed for 3 h by the ordinary blender, the total reactant was 1.6 kg, and magnesium ingredient amount was 97% of the theoretical amount; For Trial 2, the raw materials were mixed for 20 min by the high­energy ball milling, the milling speed was 300 r/min, the total reactant was 0.265 kg, and the magnesium ingredient amount was 90% of the theoretical amount; For Trial 3, the raw materials were mixed for 20 min by the high­energy ball milling, the milling speed was 250 r/min, the total reactant was 0.404 kg, and the magnesium ingredient amount was 95% of the theoretical amount; For Trial 4, the raw materials were mixed for 20 min by the high­energy ball milling, the milling speed was 300 r/min, the reactant total was 0.396 kg, and the magnesium ingredient amount was 90% of theoretical amount.

The mixed powders were then placed in a vacuum equipment (with volume of 40 L) to perform the combustion synthesis. The reactions were induced by tungsten filament through electric heating. The combustion products were acid­leached. The boron powders were obtained after filtration and drying.

2.2 Analytical methods The phase compositions of the combustion products

and the acid­leached products were analyzed by X­ray diffractometer (XRD, Model D8, Bruke, Germany; working conditions: Cu Kα1, 40 kV, 40 mA). The powder morphologies were observed by scanning electron microscopy (SEM, Model Nano SEM, Holand). The Raman patterns of boron powder were analyzed by Raman spectrometer (model HR800, Japan, test conditions: laser wave length 488 nm, power 17 mW, slitwidth 300 nm, lens 50 times; integration 30 s, number of integration 2 times; scan wave length range 200− 3000 cm −1 ). The chemical compositions of samples were determined by inductively coupled plasma atomic

emission spectrometry (model ICP­Prodigy, Optima 4300 DV, Lehman, USA).

3 Results and discussion

3.1 XRD analysis Figure 1 shows the XRD patterns of Samples 1−4. It

can be seen that crystalline diffraction peaks emerge obviously in the Sample 1, and Samples 3 and 4 exist weak crystalline diffraction peaks at the 2θ of 36.96°, 44.32°, 51.64°, 59.48° and 65.3°, respectively, which are mainly the diffraction peaks of crystalline boron. The crystallinity degrees are calculated with Diffrac Suite Eva software to be 72.46%, 31.5%, 32.0% and 30.4%, respectively, for Samples 1−4. Based on the results of XRD and crystallinity calculation results, it can be concluded that the Samples 2−4 are mainly amorphous. The adiabatic temperature of reaction (1) is 2604 K [8], which belongs to strong exothermic and fast redox reaction.

Fig. 1 XRD patterns of samples

Therefore, once the reaction is triggered, it instantly releases a great amount of reaction heat, and the reaction temperature sharply rises to around 2000 °C. In fact, the larger the amount of the reactant is, the higher the temperature of the actual reaction is. A long period of high temperature provides a sufficient condition for the development and the growth of crystals of the reaction products. Since the SHS reactor volume is only 40 L, due to the limit of its cooling capacity and cooling speed, the greater amount the reactants are, the longer the high temperature maintains in the reaction process. The amount of reaction material of Sample 1 which is 1.6 kg is 4−6 times the amount of reaction material of Samples 2−4. Therefore, the conditions for development and growth of crystals of Sample 1 are far better than those of Samples 2−4, which is unfavorable for obtaining amorphous boron powder of high activity.

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3.2 SEM analysis Figure 2 shows the SEM images of Samples 1−4.

Among them, the reaction materials of Sample 1 were not pretreated by high­energy ball milling, the reactants of Samples 2 and 3 were milled by high­energy ball milling at the speed of 250 r/min for 20 min, and the reactants of Sample 4 were milled by high­energy ball milling at the speed of 300 r/min for 20 min. Figure 2 shows that the average particle size of boron powder of Sample 1 is at the sub­micron level, and a large number of rod­like crystals (length of about 1 µm, diameter of 100−200 nm) appear. The average particle sizes of boron powder of Samples 2−4 are at the nano­scale level. Among them, the average particle size of Sample 2 is about 100 nm, and that of Sample 3 is less than 100 nm. In addition, there are also structures similar to the nano­whiskers or nanowires (shown in circle area). The average particle size of Sample 4 is about 50 nm. Combined with XRD analysis, it can be concluded that long high temperature state not only makes the boron powder particles fully grow up, but also leads to a substantial increase of the crystal boron in the products. To compare Sample 1 to Sample 4, it is concluded that particle size of boron powder becomes significantly smaller with the increase of the milling rate. Therefore, the high­energy ball milling pretreatment is very effective in obtaining nanometer and smaller particle size.

3.3 Raman spectroscopy Figure 3 shows the Raman spectroscopy results of

Sample 3. It can be seen that, Raman peaks appear near 350, 450, 770, 1370 and 1560 cm −1 . Furthermore, Raman peaks above 1000 cm −1 become wider. According to Refs. [14−15], Raman peaks above 1000 cm −1 are icosahedral atomic vibrational modes, Raman peaks between 550 cm −1 and 1000 cm −1 are icosahedron body vibration mode, and low corresponding value of Raman peak is icosahedral rotation mode. Raman peaks near 1370 cm −1 and 1560 cm −1 are gradually broadened, and the intensities are significantly strengthened. This change is caused by gradual tapering of particle size of boron powder.

3.4 TEM Figure 4 shows the TEM images and electron

diffraction pattern of Sample 3. As can be seen from Fig. 4(a), the average particle size of boron powder is very small, less than 50 nm. Figure 4(b) shows its morphology observed by transmission electron microscopy (TEM), left by morphology of the sample melting. From the melting traces, it can be seen more clearly that the particle size of the monomer particle is less than 100 nm. The melting phenomenon of sample under TEM irradiation fully shows that the amorphous boron powder has a very high activity and its purity is 94.8%.

Fig. 2 SEM micrographs of boron powders: (a) Sample 1; (b) Samlpe 2; (c) Sample 3; (d) Sample 4

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Fig. 3 Raman shift of product of Sample 3

Fig. 4 TEM images and EDS pattern of product of Sample 3

4 Conclusions

1) The high­energy ball milling pretreatment is very effective in obtaining nanometer and smaller particle size. The particle sizes of boron powder decrease with the increase of the high­energy ball milling rate, and particle sizes are much smaller than 100 nm. At the same time, the amorphous boron nano­fibers appear in the boron powders.

2) The boron powders exist in the form of amorphous phase with the crystallinity less than 30.4%, and its purity is 94.8%.

3) In future, the relationship between the reaction

activity of boron powder and its particle size will be studied, and then the control model on the reaction activity of boron powder will be established.

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(Edited by YANG Bing)