22P068-P
Formation Mechanism of Boron Rich Compounds Nanoparticles
in Induction Thermal Plasma Processing
誘導結合型熱プラズマにおける多ホウ化物ナノ粒子の生成機構
Jiro Matsuo, Sooseok Choi, and Takayuki Watanabe
松尾次郎, 崔 秀錫，渡辺隆行
Department of Environmental Chemistry and Engineering, Tokyo Institute of Technology
4259 G1-22, Nagatsuta, Yokohama, Kanagawa 226-8502, Japan
東京工業大学大学院総合理工学研究科 〒226-8502 横浜市緑区長津田町4259番地G1-22
Induction thermal plasma was used to prepare nano-sized boron rich compounds and the mechanism ofnanoparticle formation was investigated by estimating the homogeneous nucleation rate. In order tosynthesize AlB12 and YB66, the premixed feed powders of Al, B and YB4 were injected into the thermal
plasma in different compositions. High boron contents and thermal conductive helium were favorable toenhance AlB12 and YB66 yields in the final product due to a high boiling point of boron.

1. Introduction
Boron rich compounds are attractive materials
because of their high melting point, hardness,high electrical conductivity and high corrosion
resistance to molten material. Therefore, theirnanoparticles have received great attention for theapplications of polishing material, neutron shieldmaterial and catalyst of fuel cell. Some reports
about the preparation of boride nanoparticles
have
been
publishedpreviously;
YB 66
nanoparticles were prepared by plasma chemical
process using starting powders of YB 4 and boron[1].
Induction thermal plasmas have been widelyused for powder treatment of the nanoparticles
synthesis. Induction thermal plasma has someadvantages of high enthalpy to enhance reaction
kinetics, high chemical reactivity, large volume
with low velocity, oxidation, and reduction
atmospheres in accordance with required
chemical reactions and rapid quenching to
produce chemically non-equilibrium materials.
The purpose of this paper is to preparenano-sized boron rich compounds and to analyze
the formation mechanism of boron richcompounds.
2. Experimental procedure
Figure 1 shows a diagrammatic illustration ofexperimental apparatus. The set-up consists of aplasma torch, a reaction chamber and a power
supply (4 MHz). Plasma operating condition wereas follows; plate power of 30 kW, operatingpressure at atmosphere, sheath plasma gas mixtureof Ar-He, inner plasma gas of He, and carrier gas of
Ar. Raw powder material used in this study areboron (average particle size: 45 μm), YB4 (9.2 μm),and aluminum (15 μm). The composition ratio ofFig. 1. Schematic diagram of experimental set-up.

the feeding powders was changed in theexperiments to find an optimal condition.
Phase and size distribution of the preparednanoparticles were analyzed by x-ray powder
diffraction (XRD) and transmission electron
microscopy (TEM).
3. Experimental results and discussion
3.1 YB4 and boron system
The XRD patterns of the prepared nanoparticles
for YB4-B system are shown in Fig. 2. YB66 andYB4 are identified from the XRD spectrum. The
successful preparation of yttrium boride wasattributed to evaporation enhancement of raw
materials in thermally conductive helium as theinner gas.
3.2 Aluminum and boron system
The XRD patterns of the prepared nanoparticles
for Al-B system are shown in Fig. 3. AlB12, AlB10,boron, and aluminum are observed TEM
photograph of the prepared nanoparticles for Al-B

system is shown in Fig. 4. The particle distributionof Fig. 5 evaluated from the TEM presents theaverage particles size of 20.8 nm with geometricalstandard deviation of 1.03.

Fig. 4. TEM photograph of nanoparticles:
Al:B = 1:15.
0.2

10.8
0.15

0.6
0.1
0.4

0.05
0.201

10

Culmative Number Fraction [ - ]-1

Number Fraction per Unit Diameter [ nm ]3.3 Homogeneous nucleation temperature
The nucleation rate expression proposed by
Girshick et al. [2] was used for investigation of theformation mechanism. Particle formation can beobserved when the nucleation rate exceeds 1.0 cm-3
s-1. Therefore, the corresponding value of saturation
ratio is defined as the critical saturation ratio [3].
The nucleation temperature at the criticalsaturation ratio is presented in Fig. 6 according tothe raw materials. The nucleation temperature ofboron is higher than those of aluminum and yttrium,accordingly boron is nucleated in first. After theboron nucleation, AlB12 and YB66 are formed bycondensation of aluminum and yttrium to the boron
nuclei, because the boiling points of aluminum andyttrium is higher than boron nucleation temperature.

0100

Diameter [ nm ]Fig. 5. Particle size distribution of nanoparticles:
Al:B = 1:15.

Fig. 2. XRD spectrum chart of prepared particles for
YB4-B system; Composition ratio of Y and B:(a) 1:12, (b) 1:40, (c) 1:66(☆:YB4, △:YB66, □:B).
Fig. 6. Nucleation, boiling and melting temperature.

4. Conclusion
Nano-sized YB66 and AlB12 are successfully
synthesized by the induction thermal plasma.
Injection of helium as the inner gas promotes theyield of boron rich nanoparticles. High boron
contents in the raw material were favorable toenhance YB66 and AlB12 in the final product.

Fig. 3. XRD spectrum chart of prepared particles for
Al-B system; Composition ratio of Al and B:(a) 1:5, (b) 1:12, (c) 1:15(☆:AlB12, △:AlB10, □:B, ○:Al).

References[1] J.Y. Huang, et al.: J Mater Sci. 33 (1998) 4141.
[2] S.L. Girshick, et al.: Aerosol Sci and Technol. 13(1990) 465.
[3] S.K. Friedlander: Smoke, Dust and Haze,
Wiley-Intersience, (1977) 241.