To use the salt-assisted SHS technique to prepare B powders was proposed. Calculation results found that the adiabatic combustion temperature of the B2O3-Mg reaction system was 2604 K, higher than the 1800 K criterion of self-propagating temperature, which meant that the SHS application was feasible. When 0, 10%, 20%, 30%, 40%, 50%, and 60% NaCl content were added, the adiabatic combustion temperature of the reaction system decreased linearly. When 60% NaCl content was added, the adiabatic combustion temperature was 1799 K (lower than 1800 K), unsuitable for self-propagating reaction, which was consistent with the experimental results. Through scanning electron microscope (SEM), energy disperse spectroscopy (EDS), and particle size analysis, the influence of different addition of NaCl on the morphology, average particle size, and purity of prepared B powder was investigated. EDS and chemical analysis indicated that the purity of prepared B powder was over 96% and the average particle size was within the range of 0.4~0.8
Amorphous boron is attractive material with applications in fuel additives [
Traditional methods to prepare amorphous boron powder include molten salt electrolysis, diborane pyrolysis, boron hydrogen chloride reduction, and magnesium thermal reduction [
SHS (self-propagating high-temperature synthesis) is a self-propagating process to realize reactions between the powders, which shortens and simplifies the procedures in comparison to traditional preparation processes. Once being started via ignition, no more outside energy is needed. Compared to other processes, this self-propagating technique has many advantages, such as simple process, high purity, small particle size, and high activity of product.
A new technique was proposed in this paper to produce boron powder and optimize the preparation process by combining the SHS technique and the traditional magnesium thermal reduction method and choosing NaCl as a diluter. The adiabatic temperature of SHS technique is rather high. The reaction occurs in a high-temperature state for a long time, which is liable to bring about the side reactions, reduce purity, and lead to particle aggregation. NaCl can effectively reduce the adiabatic combustion temperature, suppress side reactions, increase product purity, and reduce particle size. Moreover, the process is easier to be applied and realize industrialization. In this paper, effect of NaCl addition on morphology, size, and phase of submicron amorphous boron powders was investigated.
Commercial grade B2O3 (>99.0 wt%, Liaoning Pengda Science and Technology Ltd., Yingkou), Mg (99.5 wt%, Kunshan Fuerbang New Material Technology Co., Ltd., Kunshan), and NaCl (99.0 wt%, Nanjing Dongde Chemical Technology Co., Ltd., Nanjing) were mixed in certain proportion. Materials were thoroughly mixed by a planetary ball mill and pressed into a cylinder. The tabletting pressure was 10 Mpa. A self-made reactor was used to successfully produce the ultrafine boron powders via the salt-assisted SHS technique. The combustion synthesis of the 60% NaCl system was also tried in the experiment, which however resulted in incomplete reaction.
The main equation of combustion synthesis is as follows:
If no NaCl was added in the reaction materials, the product obtained after combustion was relatively hard and needs grinding. This was because, without NaCl, severe agglomeration occurred. If NaCl was added, the product obtained was a fluffy, block-like material. This was because the product was coated by NaCl and thus easy to crush. The obtained products contained boron powder, diluter NaCl, byproduct MgO, and so forth. The acid leaching-alkali leaching-acid leaching-water leaching process was adopted to produce high purity boron powder. The leaching agents included HCl, NaOH, and distilled water. After filtration and drying, the amorphous B powder was finally obtained.
The leaching reaction equation is as follows:
The morphology and elementary composition of B powder were examined by scanning electron microscope (JSM-6700, JEOL) equipped with an energy dispersive spectrometer (EDS) and chemical analysis. The phase analysis of the powder samples was investigated by X-ray diffraction (XRD, D/max-2400, Rigaku) and selected area electron diffraction (SAED, JEM-1200EX, FEI USA). The average particle size of sample was measured by laser particle size analyzer (Nano Series Nano-ZS, ZETASIZER).
Adiabatic temperature (
Figure
Change of
The NaCl melting point is 800.8°C, lower than the calculated adiabatic temperature. NaCl is in liquid state in reaction. It absorbs heat in the melting process, so the addition of NaCl can help in reducing
Melting points of reactants and products.
Reactants and products | Melting points/°C |
---|---|
NaCl | 800.8 |
Mg | 650.0 |
B | 2079.0 |
B2O3 | 450.0 |
MgO | 2800 ± 13 |
According to the measurement of the adiabatic temperature and the initial reaction temperature, the initial reaction temperature of B2O3-Mg-NaCl system was higher than the melting points of B2O3, Mg, and NaCl. We can therefore infer that the salt-assisted SHS technique is a liquid-liquid reaction mechanism.
Figure
Morphologies of leached products obtained by salt-assisted SHS synthesis with different content of diluter. (a) 0% NaCl; (b) 10 wt% NaCl; (c) 20 wt% NaCl; (d) 30 wt% NaCl; (e) 40 wt% NaCl; (f) 50 wt% NaCl.
It indicates that the morphology and particle size could be controlled effectively through properly changing the addition of the diluter NaCl.
Figure
Particle size distribution of products obtained by combustion synthesis with different contents of diluter. (a) 0% NaCl; (b) 10 wt% NaCl; (c) 20 wt% NaCl; (d) 30 wt% NaCl; (e) 40 wt% NaCl; (f) 50 wt% NaCl.
When NaCl increased from 0% to 50%, the average particle sizes measured by laser particle size analyzer were 3.88
From Figures
Comparisons of particle sizes between amorphous B powder produced with 50% NaCl in this experiment with B powder produced by SB Boron are shown in Table
Comparison of particle sizes between amorphous B powder produced with 50% NaCl and B powder produced by SB boron.
Samples | Samples in |
SB boron 90 | SB boron 95 |
---|---|---|---|
Average particle size/ |
0.4~0.8 | 1.0 | 0.8~1.0 |
B content/% | >96 | 90~92 | 95~97 |
From the comparison of B powder sample produced with 50% NaCl and those produced by SB boron in Table
An EDS analysis of six samples was made. The statistical results are shown in Table
Element contents in leached products prepared with different contents of diluter.
Samples |
B (wt%) |
O (wt%) |
Mg (wt%) |
B (wt%) |
---|---|---|---|---|
0 | 94.08 | 2.32 | 3.59 | 94.72 |
10 | 96.09 | 0.15 | 3.77 | 96.02 |
20 | 96.01 | 0.43 | 3.56 | 96.00 |
30 | 98.22 | 0.25 | 1.53 | 97.96 |
40 | 97.43 | 0.44 | 2.13 | 97.12 |
50 | 96.26 | 1.16 | 2.58 | 96.16 |
Figure
XRD patterns for leached boron samples.
Figure
SEAD chart of leached B powder.
To prepare submicron B powder by the salt-assisted SHS technique was proposed in this paper. Calculation results show that the addition of diluter NaCl into the B2O3-Mg system can effectively reduce the adiabatic combustion temperature. With the increase of NaCl content, the adiabatic combustion temperature of the reaction system linearly declined. When 60% NaCl was added, the adiabatic combustion temperature was 1799 K, lower than 1800 K, unsuitable for the self-propagating reaction, which was consistent with results of experiment.
With the increase of NaCl content, the average particle size of leached products decreased. When NaCl content increased from 10% to 50%, the average particle size decreased from 2.07
The XRD and SEAD of products indicated that the prepared B powder was amorphous. The obtained B powder had small particle size and high activity and thus should be stored in the inert atmosphere.
This method could be further industrialization and become a common approach to prepare various inorganic materials. We hope that the present work is useful for the popularization and application of the amorphous boron powders.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was funded by National Natural Science Foundation of China (51164022) and the Natural Science Foundation B of Gansu Province, China (148RJZA003).