Synthesis of AlB12 and YB66 Nanoparticles by RF Thermal Plasmas

Boron-rich compounds of AlB12 and YB66 nanoparticles were synthesized in radio frequency (RF) thermal plasmas. Yttrium tetraboride or aluminium powders with 10 μm in diameter and boron powder with 45 μm in diameter were evaporated in the high temperature region of the thermal plasma, and then metal boride nanoparticles were formed in the tail region of the plasma with rapid quenching. Boron-rich compounds were identified by X-ray diffractometry in the product. Polyhedral shaped nanoparticles about 20 nm in size were measured in Al-B system by transmission electron microscopy, while YB66 particles about 50 nm in size were cubic in morphology. The composition of raw powder and the input power of RF thermal plasma were controlled to enhance the content of boron-rich metal borides in as-prepared nanoparticles. Since boron has very high melting point and evaporation temperature, high boron content in the raw powder and high input plasma power were preferable to synthesize AlB12 and YB66 nanoparticles.


Introduction
Boron-rich metal borides composed of polyhedral boron subunits and metal atoms have excellent material properties of high melting point, hardness, corrosion resistance, thermionic electron emission, and neutron absorption ability with low density [1]. Since they have various applications including mechanical tools, chemical catalysts, electronic devices and nuclear materials, typical production methods such as chemical vapour deposition and a long-time solid reaction in high temperature have been introduced [2,3]. Although boride nanoparticles are strongly required in many applications due to advantages of nanoparticles in optical, chemical, mechanical, electromagnetic characteristics, they are difficult to be obtained from solution which is typically used for the preparation of nanoparticles. In this work, boron-rich compounds of AlB 12 and YB 66 were prepared by radio frequency (RF) induction thermal plasmas. The RF thermal plasma is receiving much attention to synthesize nanoparticles, because it has sufficiently high temperature for the evaporation of any kinds of precursor and rapid quenching rate for nucleation and condensation [4]. In order to increase the content of boron-rich compounds in final products, effects of boron composition ratio in raw feeding powders and the input power of RF thermal plasma on the synthesis of AlB 12 and YB 66 were examined in the present work.

Experimental setup
Experimental setup and operating conditions are presented in figure 1 and table 1, respectively. Raw materials composed of boron (size: 45 m, purity: 99%, Kojundo Chemical Laboratory Co. Ltd.) powder and aluminum (size: 10 m, purity: 99%, Kojundo Chemical Laboratory Co. Ltd.) or yttrium tetraboride (size: 10 m, purity: 99%, Japan New Metals Co. Ltd.) powder are injected into the RF thermal plasma by a powder feeder. The composition ratio of metal to boron was 1:2, 1:12, and 1:15 for Al-B system, while it was controlled at 1:12, 1:40, 1:66, and 1:100 for Y-B system. The feed rate of raw material was fixed at 100 mg/min. The RF thermal plasma was operated at atmospheric pressure and the input power was controlled from 24 kW to 33 kW. Argon was used for the carrier gas of the raw powder, and argon and helium were used for plasma supporting gases. Evaporated boron and metal were nucleated and condensed in the RF thermal plasma producing nanoparticles that were found on a water cooling inner chamber and a filter.

Results and discussion
TEM images of synthesized nanoparticles in Al-B and Y-B systems are presented in figures 2 and 3, respectively. These images were obtained at the highest boron composition ratio in the raw powder among used conditions in the present work at the fixed input plasma power of 30 kW. In a RF thermal plasma process for the synthesis of nanoparticles, spherical shaped products are typically produced. In the present work, however, polyhedral nanoparticles which were about 20 nm in average diameter were synthesized in the case of Al-B system as seen in figure 2. Such polyhedral morphology seems to be caused by the structure of icosahedral B 12 and B 20 units which compose AlB 12 [5]. In figure 3, rectangular shaped nanoparticles about 50 nm in the length of each side were found in the product when YB 4 and B powders were used as raw materials. The cubic shape is unique morphology for YB 66 nanoparticles [6]. Therefore, successful synthesis of boron-rich metal borides nanoparticles of AlB 12 and YB 66 were confirmed from the observation of TEM images.  In order to increase the quantity of boron-rich compound in the product, the composition ratio of boron in the raw powder and the input plasma power were changed. Figures 4 and 5 present XRD data for the final product from the RF thermal plasma treatment of Al and B raw powders. First, the intensity of AlB 12 is increased with increasing boron composition ratio in the raw powder as seen in figure 4. The nucleation temperature of boron is higher than that of aluminum according to the synthesis mechanism of metal borides based on the homogeneous nucleation model [7,8]. As a result, boride nanoparticles are produced from the condensation of boron and aluminum monomers on boron nuclei. Therefore, enough quantity of boron nuclei and monomers should be secured by increasing the composition ratio of boron in the raw powder for the synthesis of boron-rich metal boride nanoparticles. In addition, the peak for AlB 10 is also produced together with AlB 12 as shown in figure  4, because both AlB 12 and AlB 10 nanoparticles are composed of icosahedral B 12 units. Therefore, boron clustering from boron nuclei and monomers takes place at first, followed by chemical reactions between boron clusters and Al monomers in the high temperature environment of RF thermal plasma forming AlB 12 and AlB 10 nanoparticles.  In the synthesis of boride nanoparticles in the RF thermal plasma, boron vapour should be reach at the saturation vapour pressure for nucleation. Since boron has very high melting point and evaporation temperature of 2 349 K and 4 200 K, respectively, high temperature and enthalpy of the RF thermal plasma are required to produce large amount of boron nuclei and monomers. Therefore, the content of AlB 12 in the product is increased with increasing the input plasma power as shown in figure 5.
Experimental results on the synthesis of YB 66 from evaporated YB 4 and B powders are presented in figures 6 and 7. The composition ratio of boron in the raw powder was controlled at the fixed input plasma power of 30 kW. As shown in figure 6, peak intensity for YB 66 is increased with increasing the composition ratio of boron in the raw material. Although YB 66 -related peaks at 22° and 25° for 2-theta values are appeared in the lowest boron contents in the raw material as shown in figure 6 (a), these two peaks also correspond to YB 4 . In addition, XRD pattern only for YB 66 was very weak to be detected in figure 6 (a). Therefore, high boron content in the raw material is essentially required for the synthesis of YB 66 nanoparticles in the RF thermal plasma. However, the peak intensity of YB 66 is still weak compared with that of YB 4 even in the highest boron composition ratio in the raw material as depicted in figure 6 (d). In order to synthesize YB 66 nanoparticles in the RF thermal plasma, large amount of boron nuclei and monomers are required because YB 66 is composed of 13-icosahedron unit of (B 12 ) 12 B 12 which is called supericosahedron [9].   Since YB 66 has unique cubic morphology, TEM images for nanoparticles produced in Y-B system were analyzed to evaluate the ratio of YB 66 nanoparticles in the final product. From the analysis of 200 particles for each condition, the ratio of rectangular nanoparticle in TEM images is presented in figure  7. The number of YB 66 nanoparticle is increased with increasing the composition ratio of boron in the raw material, and this result is in a good agreement with XRD results in figure 6.

Conclusion
Boron-rich metal boride nanoparticles of AlB 12 and YB 66 were successfully synthesized from the vapour phase of raw powders in the RF thermal plasma. Boron nuclei and monomers form boron clusters, and they react with metal monomers in the high temperature environment of the RF thermal plasma producing boron-rich metal boride nanoparticles. Since AlB 12 is composed of icosahedral B 12 and B 20 units, polyhedral nanoparticles is observed in the TEM image of final products synthesized from Al and B raw powders. Whereas cubic YB 66 nanoparticles is found when YB 4 and B powders is used as the raw material. Because boron clusters are required to form boron-rich compound, the content of boride nanoparticles in final products is increased with increasing the composition ratio of boron in the raw material. In addition, boron-rich compound is more easily synthesized in the condition of high input plasma power due to high melting point and evaporation temperature of boron.