Session KW71: Plasma Enabled Synthesis of Novel Materials

Hydrogen Plasma Reduction of Silicates for Lunar Oxygen Liberation

As crewed missions to the moon become closer, the focus on in-situ resource utilization efforts has increased. Since lunar soil has abundant oxygen on the surface of the moon, work has been done to process this resource in an efficient manner. Hydrogen reduction of lunar regolith has emerged as a method of liberating oxygen that can be used for fuel and water production. Traditional reduction techniques require high temperatures and increased complexity when dealing with high amounts of silicates. Plasma can reduce silicates efficiently and thus has become of high interest for use in the lunar highlands where silicates exist in high quantities. NASA’s Kennedy Space Center is exploring the use of low temperature plasmas for the reduction of silicates. This work shows that exposing silicates to a hydrogen plasma produces water. Residual gas analysis and optical emission spectroscopy were used to measure the relative quantities of water vapor production and OH (A-X) emission for hydrogen plasma interactions with lunar highland simulant and silica. X-ray photo-electron spectroscopy and scanning electron microscopy/energy-dispersive X-ray spectroscopy were also used to observe surface modifications to the silicates that reflect the key chemical reactions that occurred. From this data, plasma has emerged as a promising solution to produce oxygen on the lunar surface.   

New frontiers in high-entropy transition metal borides and carbides made using microwave plasma-assisted boro/carbothermal reduction

A novel approach is demonstrated for synthesis of high entropy transition metal borides and carbides using a single heating step enabled by microwave-induced plasma. For borides, an argon-rich plasma allows rapid boro-carbothermal reduction of a consolidated powder mixture containing five metal oxides blended with graphite and boron carbide (B₄C) as reducing agents. Plasma exposure of the powder bed to temperatures as low as 1800 °C for 45min resulted in the hexagonal AlB₂-type structure characteristic of a high-entropy boride: (Zr,Mo,Hf,Ti,Ta)B₂ with an average particle size of 165 nm and with uniform distribution of the five metal cations in the microstructure. In contrast to primarily convection-based (e.g. vacuum furnace) methods that involve slow thermal reduction steps followed by conversion to the single high-entropy phase at elevated temperature, the microwave plasma approach enables rapid heating rates and reduced processing time as a single heating step. The potential roles that microwave heating combined with plasma surface interactions have on thermal reduction to the high entropy phase will be discussed. More recently, we attempted carbothermal reduction via microwave plasma in order to form high entropy transition metal carbide: (Zr,Mo,Hf,Ti,Ta)C₂. This resulted in a solid solution with FCC structure characteristic of the high entropy carbide phase. The benefits of microwave plasma heating to allow highly efficient reduction reactions for synthesis of high entropy borides and carbides in a single heating step are expected to accelerate progress in the field of high entropy ceramic materials used in extreme conditions such as is needed in hypersonic or atmospheric re‐entry vehicles, rocket propulsion, etc.   

Experimental study of plasma-chemical synthesis of optically active substances from C2H5OH, NH3, H2O, and CO2

This work is focused on the study of optical activity, revealed by the rotation of the polarization plane of light by matter, exhibited by the organic substances synthesized using a plasma-liquid system with a rotating gliding discharge submerged in a liquid. The synthesis was conducted using ethanol, ammonia, water, and carbon dioxide as reactants. The experiments have shown the possibility of plasma-chemical synthesis of the optically active substances when all of the starting reagents are not active. The samples collected during the experiments without the plasma activation of the gas reactant featured significant optical activity. However, this optical activity was observed only for several hours after the treatment and disappeared within a day of the experiment. The same process with the plasma activation of the gas reactant produced the substances with a significant optical activity, which could still be observed for several thousands of hours after the treatment. Additionally, the study has shown that during the plasma activation of a gas reactant the rotation angle of the light polarization plane is significantly influenced by the direction of the rotation of the carbon dioxide flow.   

In-situ observation of inkjet droplets in atmospheric-pressure radio frequency argon plasma and demonstration for synthesis of monodisperse submicron gold particles

  In recent decades, plasmas in contact with liquids have been extensively studied in a wide range of fields such as material synthesis, surface treatment, agriculture, and medicine [1]. Some short-lived reactive species such as O₃ and OH radicals are abundant near the plasma-liquid interface [2], and by using micro-sized droplets with a large specific surface area, magnified interfacial reactions different from that of bulk liquid phase can be promoted. Here, we generated microdroplets by means of an inkjet device with high reproducibility and exposed to atmospheric-pressure radio frequency argon plasma. By controlling the ejection timing of the inkjet droplets and illuminating the droplets by a flash lamp, in-situ observation of each droplet in plasma at various positions became possible by charge-coupled device camera. The droplet size in the plasma decreased rapidly compared to in the atmosphere, and the temporal behavior of the solvent evaporation rate was measured. Furthermore, synthesis of monodisperse gold particles via atmospheric-pressure plasma process with inkjet droplets of chloroauric acid (HAuCl₄) aqueous solution was demonstrated [3]. The variation of inkjet droplets size was within 1–2%, thus synthesized particles had a very narrow size distribution (3–9 % standard deviation). Their diameters can be controlled by adjusting the concentration of the solution. Further details will be presented at the conference.

[1] P. J. Bruggeman et al., Plasma Sources Sci. Technol. 25 (2016) 053002.

[2] C. Chen et al., Plasma Chem. Plasma Process 34 (2014) 403.

[3] K. Nitta et al., J. Phys. D: Appl. Phys. 54 (2021) 33LT01.   

Cost-effective synthesis of silicon carbide in atmospheric pressure microwave plasma

Silicon carbide (SiC), due to its wide bandgap, is widely used in microelectronics applications such as high-power, -temperature, -voltage devices, and optoelectronics. Over the past decades, many production options based on epitaxial growth, physical and chemical vapor deposition have been implemented. Despite the variety of synthesis methods, such processes remain expensive and energy-consuming. This work proposes a simplified version of SiC synthesis based on the deposition of reaction products from the plasma phase of a microwave discharge. Tetramethylsilane feedstock diluted in a controlled flow of argon was used as a source. The reaction took place inside a quartz tube under the influence of an external microwave field. A sample of the material was deposited on a substrate in the air at atmospheric pressure and room temperature. The yield was an agglomerate of SiC particles encapsulated in a carbon shell with a 10-15 nm characteristic size. The used production method did not allow achieving stoichiometry of SiC due to the Si/C ratio of 1:4 in the precursor, which led to the formation of residual amorphous carbon and contamination of the specimen. This problem can be eliminated by further post-processing of the obtained material or by changing the precursor to an argon-silane gas mixture, which will be demonstrated.