Following November 9, 2023,Hu LiangbingProfessor hairscienceAfter that, it will be reissued on November 29, 2023nature
Plasma, which can produce an ultra-high temperature reaction environment, can be used for the synthesis and processing of various materials. However, the limited volume, instability, and inhomogeneity of plasma make it challenging to produce large blocks of high-temperature materials on a large scale. Here, from the University of Maryland, USAZhao JichengJu YiguangHu Liangbingand other researchers proposedA plasma device, byConsists of a pair of carbon fiber tip reinforced electrodes, capable of being used at atmospheric pressureVertical orientationoflongShort carbon fiberCombinationProduces uniformityUltra-high temperatureswithStable plasma(up to 8,000 K). Related** titled "A stable atmospheric-pressure plasma for extreme-temperature synthesis" was published on November 29, 2023natureAbove.
plasma, which creates highly reactive and non-equilibrium environments that are used in the synthesis and processing of various materials. However, the manufacture of large-scale, especially high-melting, bulk materials requires a plasma discharge process that enables uniform high temperatures over large areas or volumes. Volumetric plasma, such as glow discharge, has been demonstrated by a variety of methods, although typically at low pressures (<150 torr), at which the neutral gas temperature (tg) of the plasma is much lower than the electron temperature (Te). As a result, glow discharge has a very limited ability to handle high-temperature materials. Arc discharge, on the other hand, is often used to achieve high-temperature plasma (up to 10,000 K or more) and enables fabrication at atmospheric pressure.
However, atmospheric arcing between conventional plate electrodes typically shrinks into narrow, random arc channels (about 1 mm), resulting in extremely uneven temperature distribution due to energy dissipation to the surrounding environment. In order to improve the stability and scale of high-temperature plasma discharge at atmospheric pressure, various electrode designs and processes have been explored. For example, random plasma discharges can be prevented for needle electrodes, in which the high curvature (radius of a few millimeters) increases the local electric field strength and promotes the thermionic emission of secondary electrons, resulting in a stable high-temperature plasma. However, the needle structure confines the arc plasma to a narrow channel with a limited plasma volume. Although the rotational slip arc can increase the discharge volume, the plasma channel is still a filament, and the temperature and active species are unevenly distributed. Therefore, as far as is known, the challenge of high-temperature plasma synthesis and processing at atmospheric pressure remains to be solved in order to achieve scalable fabrication of high-temperature bulk materials.
Here, the researchers realized the use at atmospheric pressureA pair of carbon-tipped reinforced electrodes, implemented oneHomogeneousUltra-high temperatures(up to 8000 k) andStable plasma(USP) (Figure 1A, B). The electrodes consist of high-density (about 105 cm2) vertically oriented short carbon fibers (about 10 m in diameter) and some long carbon fibers that extend into the gap between the two electrodes and form contact. When a voltage is applied, Joule heating intensifies in the defect area or contact point (with the highest resistance) of the long carbon fiber, reaching ultra-high temperatures until the fiber breaks, creating a very small gap (about 10 m). At the tips of these newly formed fibers, a locally enhanced electric field promotes secondary electron emission, resulting in a spark discharge through a narrow fiber gap (Figure 1b), which facilitates the initiation of the plasma at record-low breakdown voltages (about 40-45 V). At the same time, as shown in the scanning electron microscopy (SEM) images of Figure 1C, the densely spaced short carbon fibers generate tip-enhanced electric fields that merge on the electrode surface (Figure 1D), accelerating the transition from Townsend breakdown to arc, expanding the size and volume of the plasma, and increasing the homogeneity of the plasma. This expansion also creates a collective heating effect that helps stabilize the plasma.
Figure 1At atmospheric pressure, a uniform USP is achieved by a design of electrodes with long carbon fibers and a large number of small-diameter carbon fibers with densely packed tips
With this technique, researchers can easily achieve a continuous volumetric plasma (determined by the size of the electrode) with a highly controllable temperature between 3,000 K and 8,000 K and a uniform temperature distribution (Figure 1E). In contrast to other arc jets or needle-specific arc plasmas (Figure 1F), the USP process can achieve uniform large areas and high temperatures (e.g., 8,000 K) at atmospheric pressure with a moderate current input (about 45 A). It is worth noting that the carbon fiber tip remains stable even under these ultra-high temperature conditions due to the low heat capacity of the carbon electrode as well as the high thermal conductivity and emissivity. As a result, volumetric plasma can maintain stable operation for 10 minutes or more with a continuous power input.
Figure 2USP plasma generation using a carbon-tip enhanced electrode design
Figure 2A shows a schematic of the electrode setup used for the USP generation process at atmospheric pressure. The two disc-shaped electrodes are made of carbon felt (diameter 25.).4 mm). The researchers then aligned the two carbon felt electrodes vertically in an argon environment at atmospheric pressure (Figure 2a), about 3 mm apart from each other. In this setup, some long carbon fibers protrude from the surface of the carbon felt and form contact between the two electrodes (Figure 2B). In addition, the electrode surface features a large number of short, vertically arranged carbon fibers with passivated tips separated by horizontally arranged fibers with a bundle spacing of approximately 200 microns.
The diameter of the carbon fiber tip is about 10 microns (Figure 2c), which is much smaller than the micron to centimeter scale of the metal needle electrodes traditionally used to generate arc plasma. The resulting USP plasma emits extremely bright light and requires a neutral density filter to visualize the plasma (Figure 2D). The plasma remained stable for at least 10 min until the investigator turned off the power (Figure 2D). The researchers found that the carbon fiber tip was sharpened during plasma generation (Figure 2e), possibly because of the concentration of the electric field. The sharp tip should also further increase the surrounding local electric field, facilitating the discharge process. Finally, the investigators conducted a control experiment using two graphite plate electrodes of the same size, but with only a bundle of long fibers (i.e., no short fiber tip array) pasted between the electrodesFigure 2g), only spark discharges were observed, with no continuous or extended plasma formed.
Figure 3USP Identification
Figure 3 and its subplots illustrate the current-voltage (I-V) characteristics of the USP discharge process using a tip-enhanced carbon felt electrode, as well as the corresponding images for the different stages (Figure 3b). At the same time, the plasma electric field strength between the electrodes was measured using the in-situ electric field-induced second harmonic (e-fish) method (Figure 3c). As the upper carbon felt bias increases from 0 volts to about 33 volts (Fig. 3a, i), a bright filamentous discharge between the electrodes is observed (Fig. 3b, i), which is due to Joule heating of the long carbon fiber contact. Then from about 33-42 volts, the I-V curve has no current signal (Fig. 3a, ii) and the fibers do not emit light (Fig. 3b, ii). This is because excessive Joule heating causes the physical breakage of the long fibers in contact, creating micro-gaps and cutting off the current (Fig. 3b, ii).
As the voltage is further increased to about 42-45 volts (Fig. 3a, iii), the electric field is further increased (Fig. 3c) and microelectric spark discharges are observed (Fig. 3b, iii). These discharges occur in narrow gaps between broken fibers, utilizing an electric field enhanced by the tip. This process helps to reduce the breakdown voltage of USP by facilitating electron collision ionization and the Townsend avalanche effect (Fig. 3B, iv). With the transition from microelectric spark to arc discharge, the voltage rapidly drops to about 20 volts due to the increase in electron number density and conductivity in the plasma (Figure 3a,c) and the current surges to 18 amperes (i.e., arc discharge breakdown current;Figure 3a). After breakdown, the measured electric field between the two electrodes is very low (about 5 volt mm), indicating that it is a hot plasma. The arc discharge volume then expands rapidly between the electrodes (Fig. 3b, v). When the current reaches about 45 amperes (after Figure 3a, b, vi), the USP produces a temperature of about 7700 K (Figure 2F).
Subsequently, the researchers gradually reduced the current from 45 amperes (Fig. 3a, b, vii), even though the current was about 7 amperes, much lower than the gas discharge breakdown current (about 18 amperes;Figure 3a), the arc discharge remains stable. This is a typical hysteresis phenomenon of arc discharge. Overall, the researchers found that the breakdown voltage of the USP process was significantly lower than the previously reported plasma breakdown value and was highly reproducible (about 42.2.).6 volts, based on 15 experiments). The researchers also found that volumetric plasma can be quickly turned on and off by simply adjusting the voltage and current. For example, the researcher used a programmable power supply by repeatedly setting the applied voltage to 45 volts 05 seconds, then back to 0 volt 05 sec (Figure 3D) to achieve pulsed plasma. This process increases the plasma current to 35 amps in 1 second. As a result, the researchers were able to cycle the plasma temperature between 1000K and 6000K in less than 1 second, with a heating and cooling rate of about 103K s. This remarkable adjustability is due to the fact that the tip-enhanced electrode reduces the voltage barrier for arc plasma conversion. This ability to pulse the plasma to high temperature and then quickly cool back to low temperature for a short period of time suggests that USP can be used to control the reaction pathways of various non-equilibrium synthetics, where sharp temperature changes (e.g., rapid cooling) are required.
Figure 4Application of USP in the synthesis of various high-temperature materials
Figure 4 illustrates the application of ultra-high temperature plasma (USP) in the synthesis of high-temperature materials. Using USP, the researchers successfully synthesized and sintered HF(C,N) ultra-high temperature ceramics with extremely high melting points (Fig. 4A,B), and SEM images of USP-treated HFC HFN particles confirmed the successful sintering of the powder precursor (Fig. 4C,D). The X-ray diffraction results showed that the HF(C,N) single phase formed the halite structure (Fig. 4E). In addition, USP is used in the synthesis of tungsten-based refractory alloys from metallic element powders to form dense alloys with uniform elemental distribution, as well as in the preparation of carbon nanotubes by direct heating of carbon black without the need for catalysts. The rapid quenching capability of USP can be used to synthesize high-melting amorphous materials, such as the conversion of crystalline magnesium oxide (MGO) to an amorphous phase by rapid cooling (Fig. 4F-H).
The scalability and adaptability of USP technology is manifested in the ability to make larger carbon felt electrodes to handle larger samples and to achieve specific heating channels by changing the electrode design. For example, a long, volumetric plasma channel can be created by placing a carbon felt rod into a carbon felt tube to form a coaxial structure (Figure 4i,j). In addition, the USP settings can be adjusted to focus the plasma in a very small area, making it suitable for precision applications such as additive manufacturing (3D printing) (Figure 4K,L). Taken together, the researchers report a cutting-edge enhanced carbon electrode design capable of forming a homogeneous, large-area volumetric plasma at record-low breakdown voltages at atmospheric pressure, while achieving ultra-high temperatures of up to 8,000 K, overcoming the typical spatial inhomogeneities and/or instabilities and the limited temperature range of conventional atmospheric plasma.
Carbon is particularly well-suited as an electrode material for plasma-producing because it is highly conductive, can withstand much higher temperatures than various metals, and can be fabricated cost-effectively with different fiber structures to achieve a tip-enhanced electric field effect. In addition, the USP setup requires very low currents and voltages (50 A and 50 V in the investigator's experiments) without the need for expensive high-power power supplies and controls required by traditional arc melting systems. Due to its low cost and ease of setup, USP enables laboratories around the world to synthesize extreme materials that can now only be manufactured using highly specialized equipment, or are not needed at all.
Investigators can also quickly turn the USP process on and off, cycling between low (e.g., 1,000 K) and high (e.g., 6,000 K) levels in less than 1 second. This tunability enables non-equilibrium synthesis to control the reaction product or phase by rapidly lowering the temperature to avoid unnecessary reaction processes. The researchers demonstrated the ubiquity of USP in the synthesis of ceramics, alloys, and carbon nanotubes. This carbon tip-reinforced plasma offers various advantages over other plasma technologies used in material synthesis and fabrication, such as arc melting, including a large and uniform heating area, enabling the synthesis of materials on a larger scale. The flexibility of the USP electrode also allows it to be adapted to different synthesis and manufacturing applications. The researchers expect that this technology can help solve a range of challenges in high-temperature synthesis and facilitate the discovery of materials in reactive plasma environments. Finally, the researchers noted that this high-temperature volumetric plasma technology can be powered by renewable electricity, suggesting that it has the potential to green manufacture a wide range of materials at scale, including those that can withstand extreme environments such as ultra-high temperatures, pressure, and corrosion. As a result, USP can provide a platform for the synthesis of sustainable materials that are widely used in the future.
References
xie, h., liu, n., zhang, q. et al. a stable atmospheric-pressure plasma for extreme-temperature synthesis. nature 623, 964–971 (2023).
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