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論文題目「Synthesis of Metal Boride Nanoparticles by RF Induction Thermal Plasma Applying for Transparent Neutron Shielding Materials」
Liu Libei
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論文題目「Synthesis of Metal Boride Nanoparticles by RF Induction Thermal Plasma Applying for Transparent Neutron Shielding Materials」
Liu Libei
1. Introduction
Lead glasses are usually used for the observation window to prevent the operators from neutron exposure. Along with a wide range of nuclear industry and its application, Pb glasses have limitations due to the toxicity, brittleness and easily discolored. Tungsten boride nanoparticles (NPs) as a capable materials possess B which is widely used to adsorb thermal neutrons and inhibit the generation of secondary gamma rays and W which is effectively used to shield gamma rays because of its high density and multi-electron layer to capture photons. According to Rayleigh scattering theory, nanoparticles with a size below 20 nm are crucial to scatter light less vigorously and can be compromised to a transparent polymer matrix. These NPs well-dispersed into the polymer matrix promise high shielding performance and transparency that can replace Pb glasses.
RF induction thermal plasma has unique fluid dynamics, thermodynamics, and chemical features, such as high enthalpy which enhances reaction kinetics, high chemical reactivity, large plasma volume, high cooling rate, and long residence time. It has a wide range of operating parameters and the feature of its electrodeless] that can be considered as an innovative and powerful tool to obtain high purity NPs from refractory materials. This research aims to synthesize W boride NPs with a size under 20 nm that can achieve uniform distribution inside the polymer materials (i.e. epoxy resins) to obtain high shielding performance materials with excellent transparency.
2. Experiment
The experimental setup mainly consists of a plasma torch, a reaction chamber, a particle collection filter and a power supply of 4 MHz at a plate power of 20 kW. The precursors were transported by the carrier gas and instantaneously evaporated after injected into the torch, and then the vapors become supersaturated to get homogeneous nucleation. Then, the NPs are consequently synthesized from the gas phase.
The typical conditions for plasma processing are as follows: Ar was used as the sheath gas (60 L/min), inner gas (5 L/min) and carrier gas (3 L/min). The total system operated under atmospheric pressure. A fixed composition of W:B=4:1 of raw powders was conducted with additional N2 quenching gas with counter flow to investigate the effect on the phase composition and size distribution of the products.
The crystal structures of the synthesized NPs were analyzed by XRD and their morphology and size distribution were performed by TEM. The quantitative analysis was conducted by XPS.
3. Results and discussion
With the increasing quenching gas flow rate, the temperature gradient increased more than 100 times (up to 10^6 K/s) than the case without quenching gas, lead to shorter residence time, and then the formed borides decreased. B2O3 was suppressed. The BN was suspected where the peaks were covered by others.
The TEM images of NPs prepared with no quenching gas indicates that the products might have a core-shell structure. The core with dark colour might be W while the shell with light colour might be B or B2O3. After set for a few months, the dry product was observed a thicker shell owing to the gradual oxidation. The particle mean diameter was 14 nm with 84% under 20 nm while less than 10 nm with 97% under 20 nm in the case of quenching gas. Quenching gas has a significant effect on particle size reduction.
The quantitative analysis of 1-3 nm of the particle surface by XPS indicates that the existence of BN was confirmed. The B1s spectra are identified to be B(0) state of metal borides at 186.4 eV, B-N peak at 191 eV, and the W2B peak at 187.7 eV which is the highest amount of the products according to the XRD results. The peaks belong to other borides are at approximately 188.6 eV. The B-O/B-O-H peak at 193 eV which originates from the hydroxylation of some B atoms at the surface of the sample owing to the absorbed O2 and H2O vapor in the air. The oxidation of W also happened. The atomic ratio of B and N in products were 4:1 and 6.3:1 for the quenching gas of 14.3 and 30 L/min, respectively. The higher quenching gas flow rate caused a shorter reaction time so that the formed BN decreased. In addition, the high atomic ratio of B:W=5.5:1 on particle surface proved that the shell structure is B.
The ΔG of W boride formation showed that the WB and W2B formation become negative around 4500 K, while the WB4 and W2B5 formation become negative around 4100 K which means these reactions can occur spontaneously. Nitridation starts at low temperature region near the melting point of B which indicates that BN formation is also difficult.
Homogeneous nucleation temperatures of materials considered in the present study were estimated based on nucleation theory, in which indicates W nucleates first, and then B becomes saturated and co-condenses onto the W nuclei. Due to the large temperature gap of 749 K between the melting point of W and the boiling point of B, the reaction of W and B is quite difficult. During the condensation, the B atoms diffuse into the W lattice, where the diffusion rate is high enough at such high temperature even for diffusion in a solid. The boridation most possibly occurs during the diffusion process.
Due to the rapid quenching rate, the reaction time is quite short and a lot of unreacted B and W remain to form their single phases respectively. Therefore, the prepared nanoparticles have core-shell structure.
4. Conclusion
RF induction thermal plasmas provide an efficient and unique way to synthesize metal boride NPs, and their phase composition and size distribution are easily controllable by adjusting the plasma parameters.
Lead glasses are usually used for the observation window to prevent the operators from neutron exposure. Along with a wide range of nuclear industry and its application, Pb glasses have limitations due to the toxicity, brittleness and easily discolored. Tungsten boride nanoparticles (NPs) as a capable materials possess B which is widely used to adsorb thermal neutrons and inhibit the generation of secondary gamma rays and W which is effectively used to shield gamma rays because of its high density and multi-electron layer to capture photons. According to Rayleigh scattering theory, nanoparticles with a size below 20 nm are crucial to scatter light less vigorously and can be compromised to a transparent polymer matrix. These NPs well-dispersed into the polymer matrix promise high shielding performance and transparency that can replace Pb glasses.
RF induction thermal plasma has unique fluid dynamics, thermodynamics, and chemical features, such as high enthalpy which enhances reaction kinetics, high chemical reactivity, large plasma volume, high cooling rate, and long residence time. It has a wide range of operating parameters and the feature of its electrodeless] that can be considered as an innovative and powerful tool to obtain high purity NPs from refractory materials. This research aims to synthesize W boride NPs with a size under 20 nm that can achieve uniform distribution inside the polymer materials (i.e. epoxy resins) to obtain high shielding performance materials with excellent transparency.
2. Experiment
The experimental setup mainly consists of a plasma torch, a reaction chamber, a particle collection filter and a power supply of 4 MHz at a plate power of 20 kW. The precursors were transported by the carrier gas and instantaneously evaporated after injected into the torch, and then the vapors become supersaturated to get homogeneous nucleation. Then, the NPs are consequently synthesized from the gas phase.
The typical conditions for plasma processing are as follows: Ar was used as the sheath gas (60 L/min), inner gas (5 L/min) and carrier gas (3 L/min). The total system operated under atmospheric pressure. A fixed composition of W:B=4:1 of raw powders was conducted with additional N2 quenching gas with counter flow to investigate the effect on the phase composition and size distribution of the products.
The crystal structures of the synthesized NPs were analyzed by XRD and their morphology and size distribution were performed by TEM. The quantitative analysis was conducted by XPS.
3. Results and discussion
With the increasing quenching gas flow rate, the temperature gradient increased more than 100 times (up to 10^6 K/s) than the case without quenching gas, lead to shorter residence time, and then the formed borides decreased. B2O3 was suppressed. The BN was suspected where the peaks were covered by others.
The TEM images of NPs prepared with no quenching gas indicates that the products might have a core-shell structure. The core with dark colour might be W while the shell with light colour might be B or B2O3. After set for a few months, the dry product was observed a thicker shell owing to the gradual oxidation. The particle mean diameter was 14 nm with 84% under 20 nm while less than 10 nm with 97% under 20 nm in the case of quenching gas. Quenching gas has a significant effect on particle size reduction.
The quantitative analysis of 1-3 nm of the particle surface by XPS indicates that the existence of BN was confirmed. The B1s spectra are identified to be B(0) state of metal borides at 186.4 eV, B-N peak at 191 eV, and the W2B peak at 187.7 eV which is the highest amount of the products according to the XRD results. The peaks belong to other borides are at approximately 188.6 eV. The B-O/B-O-H peak at 193 eV which originates from the hydroxylation of some B atoms at the surface of the sample owing to the absorbed O2 and H2O vapor in the air. The oxidation of W also happened. The atomic ratio of B and N in products were 4:1 and 6.3:1 for the quenching gas of 14.3 and 30 L/min, respectively. The higher quenching gas flow rate caused a shorter reaction time so that the formed BN decreased. In addition, the high atomic ratio of B:W=5.5:1 on particle surface proved that the shell structure is B.
The ΔG of W boride formation showed that the WB and W2B formation become negative around 4500 K, while the WB4 and W2B5 formation become negative around 4100 K which means these reactions can occur spontaneously. Nitridation starts at low temperature region near the melting point of B which indicates that BN formation is also difficult.
Homogeneous nucleation temperatures of materials considered in the present study were estimated based on nucleation theory, in which indicates W nucleates first, and then B becomes saturated and co-condenses onto the W nuclei. Due to the large temperature gap of 749 K between the melting point of W and the boiling point of B, the reaction of W and B is quite difficult. During the condensation, the B atoms diffuse into the W lattice, where the diffusion rate is high enough at such high temperature even for diffusion in a solid. The boridation most possibly occurs during the diffusion process.
Due to the rapid quenching rate, the reaction time is quite short and a lot of unreacted B and W remain to form their single phases respectively. Therefore, the prepared nanoparticles have core-shell structure.
4. Conclusion
RF induction thermal plasmas provide an efficient and unique way to synthesize metal boride NPs, and their phase composition and size distribution are easily controllable by adjusting the plasma parameters.
国際学会
- Libei Liu, Yuta Tanoue, Tadashi Nonaka, Manabu Tanaka, and Takayuki Watanabe: Synthesis of Transition Metal Boride Nanoparticles by Induction Thermal Plasma, 24th International Symposium on Plasma Chemistry, P3-43 (2019.6.13 Naples, Italy).
- Yuta Taoue, Libei Liu, Manabu Tanaka, and Takayuki Watanabe: Synthesis of Heavy Metal Borides Nanoparticles by Argon-Nitrogen Induction Plasma under Atmospheric Pressure, The 11th Asia-Pacific International Symposium in the Basics and Applications of Plasma Technology, P1-9 (2019.12.12 Kanazawa).
- Libei Liu, Yuuta Tanoue, Manabu Tanaka, and Takayuki Watanabe: Formation Mechanism of Tungsten Boride Nanoparticles by Induction Thermal Plasma, Material Research Meeting 2019, H2-O12-003 (2019.12.12 Yokohama).
国内学会
- 田上優太, 劉麗蓓, 野中侃, 田中学, 渡辺隆行: 高周波熱プラズマによる重金属ホウ化物ナノ粒子の合成,プラズマ・核融合学会九州・沖縄・山口支部第22回支部大会研究発表論文集, p.25-26, P-5 (2018.12.15 九州大学筑紫キャンパス).
