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Preface |
Plasma is the fourth state of matter after solid, liquid and gas. It is an ionized gas with equal positive and negative charges. It is comprised mainly of ions, electrons and neutral atoms or molecules, and it is electrically neutral as a whole. It is estimated that more than 99％ of known matter in the universe is in the plasma state. Our sun, for instance, is essentially a huge, hot plasma body. The atmosphere of and interstellar spaces around many stars are also full of plasma. As far as the Earth is concerned, however, there are very few plasmas that exist naturally. The only natural plasma phenomena that can be observed on Earth are the lightning and auroras that appear in the atmosphere.
With the rapid development of science and technology, however, artificial plasmas have appeared in large numbers in laboratories and industries in recent years, and human beings have begun to rely more and more on plasma. Plasma research is not only essential to the development of fundamental physics but its limitless application possibilities promise excellent future prospects for the related technologies. Analysis by Markets and Markets estimates that the global plasma technology application market size was about 20.39 billion USD in 2021, and it is projected to reach 32.74 billion USD by 2023, meaning a compound annual growth rate (CAGR) as high as 60.3％. Semiconductor manufacturing is the largest field of application for plasma technology. The related output accounts for about 40％ of the value of the global plasma market and is expected to reach a scale of 13.67 billion USD by 2023. The life science and healthcare application markets are also considered to have considerable development potential and are expected to experience rapid growth in the next few years.
Atmospheric Plasma (AP) refers to plasma phenomena generated at or near atmospheric pressure. Unlike typical Low Pressure Plasma (LP) technology, atmospheric plasma systems do not need expensive vacuum equipment, so it not only has advantages in terms of cost but also simplifies the manufacturing process and meets environmental protection and energy conservation requirements. In addition, the equipment used for generating plasma in a normal pressure, low temperature environments can be easily designed to be portable. It also saves time because there is no need to pump a vacuum, and its application is safer, easier and more efficient. At present, atmospheric plasma technology is being widely used in surface treatment, cleaning and decontamination, air purification, sterilization and material synthesis, etc.. As for applications in fields related to people’s livelihoods, industries such as biomedicine, food processing, smart agriculture, textiles and shoe-making are expected to have great development prospects in the future. According to analysis conducted by Mordor Intelligence, the global atmospheric plasma application market was valued at approximately 790 million USD in 2019 and is predicted to reach 1.32 billion USD by 2025 with a compound annual growth rate of about 8.8％.
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In this issue, MA-tek has specially invited Professor Jeng-Gong Du, a top domestic scholar in the field of plasma research, to write an article for the “New Technology Channel | Collaboration Column” introducing the application technology and future development trends of atmospheric plasma. We hope to share with readers the progress being made in the academic research of this important field of science and technology. |
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MA-tek R&D Center Director Chris Chen 2023/4/5
How can we prevent nitrogen fertilizers from destroying our ecology? Atmospheric Plasma Technology
Department of Materials Science and Engineering, National Tsing Hua University
Professor Jeng-Gong Du
Postgraduate Researcher Yuantai Lai
(This article was provided by Professor Jeng-Gong Du and edited by MA-tek)
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Chemicals, ions, radiation and even electric fields from atmospheric plasma sources have significant influences and effects on the modification of material surfaces, overall reactions and doping. This technology has many applications in the fields of manufacturing processes, biochemistry and the micro-manufacturing of various materials. Atmospheric plasma does not require a fixed or closed cavity, so the size of subjects to be tested is not limited by the size of the cavity. It also has many other advantages, such as low equipment and operation costs, fast operating speeds, suitability for continuous process operation, the ability to be easily combined with other equipment to greatly improve production efficiency and more. It is already a very active topic of research in the industry. Using the dielectric discharge plasma process of atmospheric plasma technology, atmospheric plasma-activated water can be produced. This demonstrates this technology’s potential in agricultural seedling cultivation. Furthermore, the recycling of organic waste as water-soluble nitrogen fertilizer would open new horizons for circular agriculture.
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What is plasma? |
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Almost 99.9% of all matter in the universe exists in the form of plasma. Stars and interplanetary spaces, for instance, are comprised of plasma. Plasma can be produced via artificial methods such as nuclear fusion, nuclear fission, glow discharge and other discharge methods. |
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The internal structures of molecules and atoms are composed mainly of electrons and nuclei. The relationship between electrons and nuclei are relatively fixed. Electrons exist at different energy levels around the nucleus, where they have little potential or kinetic energy. When the matter is subjected to external energy (such as magnetism, electricity or heat), however, the potential energy of the outer electrons in the atom drops rapidly, and the electrons finally break away from the shackles of the nuclear field and escape to a distance. This is what we call ionization. At this point, the atom becomes two charged particles: a negatively charged electron and a positively charged ion. If all the molecules or atoms in a material are ionized (Figure 1), its original state changes to that of the fourth state of matter, plasma. |
Figure 1. Schematic Diagram of Plasma Production via Atom Ionization [1] |
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The plasma state is mainly a collection of electrons, positive ions and neutral molecules formed through the ionization of gas under the influence of high electric and magnetic fields. Plasma matter has extremely high activity and energy, which can trigger a series of chain reactions, including ionization, excitation, recombination, dissociation, and charge transfers, etc.. The characteristics of high energy, crack-able gas give the plasma process unlimited possibilities. Due to its high energy density and reaction characteristics, people have already begun applying plasma to various industries.
Plasma treatments are used in many fields, and the conditions for generating plasma are very diverse. The application fields, equipment costs and gas pressure requirements are shown in Figure 2. It is not difficult to find out that the special thin films and other processes used for semiconductors, magnetic media and architectural glass need to be carried out in a high cost, high vacuum environment. This is the field of application for low pressure plasma, but it accounts for only a portion of plasma surface treatment technology applications. Limited by the cost of the manufacturing process, plasma treatment is discouraged in the water treatment and food processing industries. These plasma treatments also cannot be applied in the industrial cleaning and food processing industries due to limitations caused by the gas pressure of the vacuum process. It can be seen that, if the plasma treatment technology can be altered to work in a normal pressure environment, the range of applications could be greatly expanded [2]. |
Figure 2. Required Gas Pressure and Material Costs for Plasma Treatment Applications in Different Industrial Fields [2] |
Generally, low pressure plasma treatments use expensive and complex equipment. In contrast, atmospheric plasma treatments do not have to be matched with vacuum chambers and vacuum systems and can be carried out in normal pressure environments, giving it more potential applications, such as in the areas of water and waste water treatment. In addition, atmospheric plasma is low cost and the processing is fast, so it also has application potential in the food processing industry. In summary, compared to low pressure plasma, atmospheric plasma treatments have a wider range of application fields and more obvious application potential.
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The Principle Behind and Structure of Dielectric Atmospheric Plasma |
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When designing atmospheric plasma devices, types include Dielectric Barrier Discharge (DBD), Corona Discharges and more. However, because the treatment effect of corona discharges are weak and its electrodes are easily damaged, limiting the expansion of the technology, the following introduction will focus on DBD. |
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Dielectric barrier discharge is also known as Silent Discharge. It is when one to two dielectric materials (usually glass, quartz or ceramic) are placed between two electrodes. When a high voltage is applied, a plasma is generated in the gap between the electrode and the dielectric material or between the two dielectric materials. The whole device undergoes circuit matching via Capacitive Coupling. Due to the introduction of the dielectric layer, dielectric barrier discharge cannot use a DC power supply. Typically, pulse DC, RF or microwave power supply options are selected. The schematic diagram of the basic structure of dielectric barrier discharge plasma is shown in Figure 3. Dielectric barrier discharge can take a flat plate or cylindrical shape. The flat plate shape can modify the surfaces of large area materials, and the cylindrical shape can produce a higher density of excited particles. |
Figure 3. Schematic Diagram of Basic Dielectric Barrier Discharge Structure [3] |
Dielectric barrier discharge is usually driven by an Alternating Current (AC) high voltage power supply. As the supplied voltage increases, the state of the reactive gas in the system will undergo three stages of change, going gradually from the Insulation state to the Breakdown and ending with discharge. When the supplied voltage is low, some gases show some ionization and free diffusion. However, because the content is too low and the current is too small, it is not enough to cause the gas in the reaction zone to undergo a plasma reaction, and the current at this time is zero.
As the applied voltage is gradually increased, the electrons in the reaction zone also increase, though the Breakdown Voltage (Avalanche Voltage) of the reaction gas is not yet reached. At this point, the electric field between the two electrodes is too low to provide the electrons with enough energy to cause inelastic collisions among the gas molecules. This means the number of electrons does not increase significantly, so the reacting gas is still in an insulated state and cannot generate a discharge. At this time, the current increases slightly with the applied voltage, but it is still almost zero.
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If the applied voltage continues to increase, once the electric field between the two electrodes is great enough to cause inelastic collisions among the gas molecules, the ionized inelastic collisions will greatly increase the volume of the gas. When the electron density in the space rises above a critical value (at the Paschen breakdown voltage), there will be many Microdischarges between the two electrodes. At the same time, a Luminous phenomenon can be clearly observed in the system, and the current will increase rapidly as the applied voltage increases.
Paschen’s Law provides the relationship between the gas collapse voltage, the discharge gas pressure and the distance between the two electrodes: V = APd / ln(Pd)+B …………………………. (1) P is the discharge gas pressure, d is the distance between the electrodes, and A and B are constants that vary with the type of gas. According to this formula, as the P*d value increases, the gas breakdown voltage will remain proportional to the P*d value. When the P*d becomes smaller, however, there will be a Vmin value. Gases below this Vmin value will not collapse. Figure 4 shows the Paschen curve of common gases [4]. |
Figure 4. The Relationship Between Common Gas Breakdown Voltages, Discharge Gas Pressures and Electrode Distances (P*d) [4] |
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Dielectric Barrier Discharge Plasma—Production Form and Structure |
Dielectric barrier discharge can work at normal pressure and in a wide frequency range. Usually, the working pressure is one to ten atmospheres, and the power frequency can be anywhere from 50Hz to 1MHz. As mentioned above, the basic dielectric barrier discharge structure can have a variety of electrode designs. Different DBD electrode structures can be designed according to the needs of different applications to improve plasma treatment efficiency. There are three main variants: the flat plate array, the cylindrical plasma beam, and the cylindrical plasma beam array.
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1.Flat Plate Array |
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Generally, the electrode is in direct contact with the plasma, so the high energy particles in the plasma will etch into the electrode’s surface, leading to electrode consumption. To deal with the above problem, a dielectric barrier discharge plasma generation device with a dielectric layer introduced between the two electrodes was developed. Due to the introduction of the dielectric layer, the magnitude of the current can be limited, and the generation of arc can be suppressed. In addition, the whole device is circuit-matched via Capacitive Coupling. Therefore, DC power supplies cannot be used for dielectric barrier discharge. Typically, pulse DC, radio frequency or microwave power supplies are selected, as shown in Figure 5. |
Figure 5. Schematic Diagram of Flat Plate Dielectric Barrier Discharge Device [5-7] |
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2.Cylindrical Plasma Beam |
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The plasma beam is a widely used electrode design. It is shown in Figure 6. This design can process various forms of materials, including solids and liquids. The advantage of the plasma beam is that it can generate high-density, excited particles to perform strong and effective plasma treatments on surfaces. At the same time, the temperature of the plasma generated is low, and the energy dissipated in the form of heat is also less. |
Figure 6. Plasma Beam Electrode [8] |
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3.Cylindrical Plasma Beam Array |
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Figure 7 shows plasma beams arranged into an array to improve processing efficiency [9-11]. In this method, air is introduced below the surface of the water to generate bubbles, and plasma is generated inside those bubbles when they pass through the electrodes. Finally, the bubbles rise to the water’s surface. The reaction with the water is completed while floating to prevent active substances from escaping into the air and to achieve maximum treatment efficiency. |
Figure 7. Schematic Diagram of a Plasma Beam Array [17] |
In practical applications in the industry, cylindrical and tubular electrode structures are widely used for various chemical reactors, whereas the flat plate electrode structure is used in the modification of plates, powders and metal films, polymer grafting, surface tension improvement and cleaning, and hydrophilic modification.
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Atmospheric Plasma Advantages and Bottlenecks |
If plasma could be generated in the pressures and temperatures of the normal environment in which human beings live, the technology would be both economical and efficient. It would eliminate the need for many of the systems required for maintaining high vacuums, such as chambers and pumps, while also reducing maintenance costs and time. Because there are no chamber limitations, size limitations are reduced accordingly, and it is easy for the process to carry out continuous operations, which can greatly improve processing efficiency. In addition, this friendly environment would offer many fascinating characteristics to work with. Not only could the plasma be excited by using the surrounding air, but it could even decompose substances that pollute the environment into non-polluting gases, making it a promising technology for preventing future environmental crises.
Plasma generation requires sufficient power to stimulate the reaction. After the electrons absorb the energy of the electric field, if the energy is sufficient, they will disassociate the colliding gas molecules. At the same time, the number of electrons increases, and the newborn electrons will undergo a similar reaction, leading to a chain reaction. However, at atmospheric pressure, the gas molecules are so numerous that collisions are very frequent. At this point, the mean free path of the gas (the effective collision distance between the gas molecules) is quite small. It is difficult for the energy to accumulate, so it is difficult to excite the plasma. There are two main solutions:
(1) Increase the potential of the external power supply
(2) Increase the current
The two concepts above are both directed at increasing the energy input, but how to increase the energy supply while establishing low cost and high efficiency atmospheric plasma technology is a topic that scholars are still committed to researching.
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Plasma in Agriculture |
The introduction of new technologies in the agricultural sector is often slow. However, due to the climate change in recent years, people have become more aware of the dangers facing food production, especially considering that traditional agriculture itself causes a certain degree of harm to the environment. However, as far as agricultural production is concerned, nitrogen is one of the most important production factors. Areas with poor soils in particular rely on the use of nitrogen fertilizers to control and increase yields. However, the pursuit of large yields and a lack of rational consideration in the use of nitrogen fertilizers have led to excessive investments in the production of nitrogen fertilizers, resulting in excessive amounts of residual nitrogen compounds in the environment, which, in turn, are destroying the Earth’s ecology and nitrogen cycle. To ensure a stable food supply in the future, both sustainable management and the acceleration and improvement of agricultural fertilization technology are important issues. The author’s laboratory is working on injecting air into the atmospheric plasma system. With the concept of nitrogen fixation, converting nitrogen in the ambient air into fertilizer to optimize and provide an ideal, alternative approach to the environmental nitrogen cycle. The following will further introduce the current fertilizer problems and the application of atmospheric plasma into fertilizer manufacturing.
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Environmental Impact of Traditional Farming (Soil Tilling) Using Chemical Fertilizers |
So far, the main product of the world’s chemical industry is still chemical fertilizer. The statistics show that the global organic fertilizer market will grow by 1.36 billion USD from 2019 to 2023 with a Compound Annual Growth Rate (CAGR) of up to 14%. Compared to 2019, this would be a 14.01% increase. The global organic fertilizer market sells 2.5 to 3 million tons a year. It is estimated that the market will exceed 30 billion USD in the future. The largest demand is in the Asian market, which accounts for about 41% of global usage, as shown in Figure 8. Due to the soilless cultivation of fresh fruits and vegetables in Europe, they have the largest market demand for water-soluble fertilizers, accounting for about 33% of the global usage. The market size will grow by 3.97 billion USD at a compound annual growth rate (CAGR) of 6%, as shown in Figure 9. This enormous fertilizer market indirectly causes environmental pollution.
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Figure 8. Organic Fertilizer Market [12] |
Figure 9. Water-Soluble Fertilizer Market [13] |
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Modern agriculture requires crops to produce higher quality products and higher yields per unit area. To achieve this goal, the physical conditions and nutrient supply of the farmlands must be able to fully meet the needs of the crops. The physical conditions depend on tillage, and the nutrient supply depends on soil fertility, fertilization and irrigation. However, the nutrients in almost all arable soils are nitrogen deficient, and the supplementation of nutrients is usually only possible through the application of chemical fertilizers. Modern agriculture cannot be realized without chemical fertilizers. In recent centuries, chemical fertilizers have contributed to the production of a large amount of high quality food and the general extension of human life expectancy [14]. However, urea is by far the most commonly used nitrogen fertilizer in traditional soil tillage and open farming. Urea is easily soluble in water and quickly hydrolyzes into ammonia in the soil, which, in turn, quickly oxidizes into nitric acid. To slow down these reactions, urea is often coarsened or mixed with nitrification inhibitors to increase the efficiency of fertilizer absorption and utilization by crops. However, there are inevitably many nutrients left over, and the more they are applied, the greater their impact on the environment. Statistics show that only 20% to 50% of fertilizers are actually absorbed by crops, as shown in Figure 10. The excess nitrogen and phosphorous nutrients is carried into rivers and lakes by rainwater, polluting the water and the environment [15]. |
Figure 10. Pollution Caused by Agriculture [16] |
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Environmentally Friendly Farming Methods—Closed Loop Soilless Cultivation |
Traditional soil farming is considered a type of open system agriculture. It is not easy to achieve precise control of either fertilization or fertilizer loss. Soilless cultivation, on the other hand, is a mostly closed system in which the nutrients the plants need can be supplemented accurately. Generally, the methods for supplementing chemical liquid fertilizers are as follows:
- Perform testing to determine the concentration and level of the nutrient solution. First test the reduction of NO3- in the nutrient solution then calculate the reduction of other elements in proportion. Then supplement them to keep the nutrient solution at the proper concentration and nutrient level.
- Make calculations according to the decrease in water volume. First investigate the relationship between the water consumption and nutrient absorption of different crops in soilless cultivation. Then calculate the supplementary amount of nutrients according to the amount of water reduction and make the necessary adjustments. Make adjustments according to the change measured in the conductivity of the nutrient solution. This is a common method in production. There is a positive correlation between the conductivity and the concentration of the nutrient solution.
- Therefore, once the conductivity value of the working solution is determined, it can be used to calculate the concentration of the nutrient solution. Based on that, the amount of nutrient solution that needs to be supplemented with chemical substances can also be calculated.
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Can atmospheric plasma water be beneficial to plant life? |
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Using the air and water resources of the natural environment, atmospheric plasma technology can produce fertilizers that provide plants with what they need to grow, including active substances such as nitrogen and ammonia, which promote plant growth and stimulate crop development. Considering the important nutrients needed for plant growth, nitrogen fertilizer is the most important nutrient source, and it is comprised mainly of NH4+ and NO3-. NO3- is the best for plant absorption. During plant growth, these molecules act as important reaction and metabolic signaling factors. When the intake of these ions is insufficient, plants develop slowly and poorly. The author’s laboratory team designed a plasma jet which they used to inject air and generate Plasma-Activated Water (PAW). They confirmed via ionizer analysis that this method can effectively produce the key nitrogen fertilizer mentioned above. The reaction mechanism is shown in Figure 11. |
Figure 11. Plasma-Activated Water Reaction Mechanism |
By further adjusting the process parameters and processing time, the concentration can be kept precisely controlled within a certain range. Figure 12 shows the conversion of water with different pH levels into plasma water via atmospheric plasma treatment. In Figure 12 (a), the treatment time is 0, meaning it is the original water without plasma treatment. It can be seen that, as the plasma treatment time increases, the concentration of nitrogen fertilizer (NO3-), which is beneficial to plant growth and development, also increases. Figure 13 (b) shows that the NO3- produced is also fairly stable in water. In fact, a certain concentration of it can be maintained in the water for several days.
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Figure 12. NO3- concentration in water with different pH levels (a) produced after different atmospheric plasma treatment times, and (b) changes in the concentration after undergoing 15 minutes of atmospheric plasma treatment then being left for several days [17] |
Furthermore, hydrogen peroxide (H2O2) is also generated during the plasma treatment process, as shown in Figure 13. In regards to agronomy and its development, hydrogen peroxide can promote germination and eliminate most bacteria, viruses and fungi, thereby improving crop yields.
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Figure 13. The concentration of H2O2 in water with different pH levels (a) produced by different atmospheric plasma treatment times, and (b) the changes in concentration after undergoing 15 minutes of atmospheric plasma treatment then being left for several days [17] |
Plasma water that has undergone 15 minutes of atmospheric plasma treatment was used for lettuce seedling irrigation. The results are shown in Figure 14. It can be clearly observed that, compared with tap water, irrigation with plasma-treated water can shorten seedling development from 9 days to 5~6 days, greatly improving the efficiency of agricultural seedling cultivation.
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Figure 14. Plasma Water Treated with Atmospheric Plasma for 15 Minutes is Used to Irrigate Lettuce Seedlings [17] |
In another case study [18], the flat plate plasma array discussed above was used to treat ice cauliflower seeds dryly. The plasma array structure is shown in Figure 15. Ice cauliflower is a high value crop. In studies where the seedlings underwent plasma treatment via nitrogen gas for 30 to 180 seconds (N30 to N180), it was found that the 60 second treatment had the highest germination rate, increasing it from 60% to 75%. Through analysis via Fourier Transform Infrared Spectroscopy (FTIR), as shown in Figure 16, it was found that N-H vibrational bonding (~3340 cm -1) was generated on the surface of seeds treated for 60 seconds. This meant that the seeds got extra nutrients and therefore had the highest germination rate. Treating the seeds for too long, however, resulted in damage to the surfaces of some seeds, which led to a drop in germination rate to 67%.
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Figure 15. Flat Plate Plasma Array Used for the Study: (a) Top View and (b) Side View [18] |
Figure 16. Surface Characteristics of Ice Cauliflower Seeds After Different Lengths of Plasma Treatment Time |
From the two research studies above, it can be seen that atmospheric plasma treatment of seeds, whether wet or dry, is of considerable benefit to the improvement of germination.
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Using Atmospheric Plasma Water and Organic Waste to Rapidly Generate Organic Fertilizer |
If it is possible to do so, replacing/partially replacing chemical fertilizers with organic fertilizers would be extremely beneficial to the nitrogen cycle of the environment. Broadly speaking, organic fertilizers include all natural organisms that contribute to the physical, chemical and biological properties of the soil and their derivatives. After an organism dies and enters the soil, it is decomposed by microorganisms. This releases the plant nutrients contained in the material, which are then absorbed and utilized by other plants. In the early days, before chemical fertilizers became commonplace, organic materials from the environment were the only fertilizers available. After the development of the Haber method, chemical fertilizers largely replaced organic materials because they were cheaper, worked quickly, took up less space and were easy to apply. However, organic fertilizers have many environmentally friendly advantages. They can not only recycle the limited renewable and nonrenewable resources of the Earth but also save energy and reduce carbon emissions, thereby improving the quality of the environment and increasing human food production. These characteristics are all closely connected to the important issue of Earth’s sustainability.
Figure 17 shows the traditional composting process, which is the biochemical process of using microorganisms to convert compost materials into compost. Factors that influence this process include: the microbial nutritional properties of the compost materials, the water activity in the materials, the alkalinity during composting, and the maintenance of aerobic properties. Inoculation with a large amount of composting bacteria can create favorable conditions for composting, and thermophilic microorganisms can be used to eliminate pests and diseases. It is also beneficial to eliminate low molecular-weight metabolites and increase high molecular-weight polymers to improve maturity and avoid harming the crop cultivation.
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Figure 17. Traditional Composting Process [19] |
Composting cannot be completed using only one microorganism. Rather, it is the result of continuous decomposition by multiple microorganisms. The most active at the beginning of the process is mold, which reproduces quickly and consumes the sugars and amino acids in matter. When the mold increases rapidly, the heat released by the excessive respiration makes the surrounding temperatures rise. When it reaches forty degrees Celsius, the mold dies, and high-temperature actinomycetes become the main actors. Actinomycetes begin to break down the fibrous tissues that the mold cannot digest. As this bacteria grows more active, the temperature of the environment can rise to nearly sixty degrees Celsius. Once the hard tissues are decomposed, the activity of actinomycetes decreases, and the temperature drops with it. When the temperature becomes suitable, various other bacteria will continue to decompose the soft, fibrous tissues. The entire composting process is time-consuming and labor-intensive. It also takes up space. The process includes the crushing and mixing of materials, the adjusting of moisture, controlling the compost volume and environmental and turning the compost. The time required depends on the organic fertilizer accumulation method used. At present, closed and ventilated composting takes about 2 months, while simple composting takes about 3 months [19].
The author’s laboratory team built on the basic principles of Plasma-Activated Water (PAW), combining it with organic waste materials such as soy bean dregs [20-21] and coffee grounds [22] to provide crops with more sufficient NO3- and other trace elements. By replacing traditional microorganisms with the active components in atmospheric plasma, Reactive Nitrogen Species (RNS) and Reactive Oxygen Species (ROS), for the decomposition of organic waste materials, the time and space needed for production can be greatly decreased. Doing so also consumes the excess nitrogen produced in the world and promotes a better nitrogen cycle for the Earth.
Take, for example, the research findings that the author’s research team recently presented to the journal IEEE Transactions on Plasma Science [22]. The study showed that soaking coffee grounds in plasma-activated water treated with different gases and letting it sit for 1 hour produces different concentrations of nitrogen fertilizer ions, as shown in Figure 18. Among them, plasma water treated with argon (P-ar) produced a lower NO3- concentration because only a small amount of the N2 and O2 in the environment participate in the reaction.
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Figure 18. Concentration of NO3- in Plasma Water Produced by Injecting Different Gases into Plasma and the Concentration of NO3- After Adding Coffee Grounds to the Plasma Water (P-DI: De-ionized Water Without Plasma Treatment) [22] |
Figure 19. Concentration of NO3- in Plasma Water Produced by Injecting Different Gases into Plasma and the Concentration of NO3- After Adding Amino Acids to the Plasma Water (P-DI: De-ionized Water Without Plasma Treatment) [22] |
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When air is added to the plasma jet (P-ar/air), more reactive ions are produced because of the 22% O2 and 78% N2 in the air, which then greatly increases the NO3- concentration. Interestingly, soaking coffee grounds in plasma-activated water can further increase the NO3- concentration by a significant amount (C-ar/air). The main reason for this is that the water-soluble amino acids in the coffee grounds degrade into the smaller NO3- ions in the plasma-activated water. This was proven through experiments with amino acids. Soaking the amino acids contained in coffee grounds alone in plasma-activated water resulted in the same concentration trend, as shown in Figure 19.
A deeper understanding of the reaction trends of coffee grounds or their water-soluble amino acids in plasma water can be gained from a look at Oxidation-Reduction Potential (ORP). ORP can be used to judge the role or strength of redox reactions between substances [23]. According to the literature, a high ORP comes from the active substances, such as H2O2 and NO2, in the plasma solution [24]. Since there are more active nitrogen and oxygen substances participating in the reaction, plasma water has the highest ORP value after air treatment (P-ar/air). That value also represents greater oxidizing power, as shown in Figure 20. |
Figure 20. ORP Before and After Coffee Grounds are Added to the Plasma Water [22] |
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The higher the organic contents of plasma water, the lower the ORP [25]. When the oxidants present in plasma water are consumed by organic matter, the organic matter is converted into NO3- ions, which are at a higher oxidation state. This is accompanied by a decrease in the ORP to a value close to equilibrium. Comparing the ORP before and after the addition of coffee grounds reveals that the greater the difference, the higher the driving force. This also explains why the Ar/Air treated plasma water produced the most nitrogen fertilizer (NO3-) after having coffee steeped in it. Finally, in addition to nitrogen, the nutrients needed for plant growth also include a variety of ions. Inductively Coupled Plasma Mass Spectrometry (ICP-MS) can be used to analyze the plasma solution resulting from soaking coffee grounds in de-ionized water and various types of plasma water, as shown in Figure 21.
From the results of the analysis, it can be seen that the overall composition is similar to that of chemical fertilizers, which means that it has considerable potential. It could replace chemical fertilizers—possibly even traditional organic fertilizers. In the future, its efficiency and potential will be verified through practical application in the planting of crops. |
Figure 21. Fertilizer Contents of Solutions Resulting from Soaking Coffee Grounds in De-ionized Water and Various Types of Plasma Water [22] |
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Conclusion |
Atmospheric plasma is the result of using the surrounding air to excite plasma. It is one of the technologies with great potential for enabling humans to solve the environmental crisis in the future. As such, it has gradually attracted the attention of various scientific and technological fields. Its ability to work under general atmospheric conditions, low temperatures and low pressures promises to open up many great possibilities for the biotechnology industry, whether it’s for medical cleaning, the manufacturing of semiconductor panels and other processes, or even agricultural applications. Its high activity and high reactivity can produce key, beneficial and nondestructive effects on the surfaces of treated objects. Many advanced countries already have considerable literature on the use of plasma water in agriculture.
The author’s laboratory team first revealed the potential of atmospheric plasma in the rapid production of domestic liquid organic fertilizers. They used plasma-activated water and organic waste materials for the reaction. It only took 1 hour to convert amino acids from organic matter into nitrate ions (NO3-) that can be easily absorbed by plants. This process significantly improves the time-consuming and space-consuming problems faced by traditional organic fertilizer manufacturing. In this era of environmental awareness and resource management, it provides a new direction for development for a circular agriculture economy.
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Postscript from the MA-tek Editorial Team |
Plasma has a unique form similar to that of flame, but it has greater energy and activity. Particles of matter in the plasma state are often formed under extremely high ambient temperature and pressure conditions, such as those caused by lightning, solar winds and nuclear fusion reactions. In a plasma system, the atoms and molecules of matter are decomposed into charged ions and free electrons, etc.. These undergo frenzied collisions and interactions and exhibit a variety of incredible physical phenomena, such as fluorescence, electrical discharge, self-organization, plasma wind and magnetic pressure stress, etc..
The plasma phenomenon was first observed by renowned British scientist Sir William Crookes in the vacuum cathode ray Crookes Tube that he developed. He discovered the phenomenon in 1879 and called it Luminous Material. The official English name of “Plasma” was formally proposed by American scientist Dr. Irving Langmuir in 1928. After more than a hundred years of research and development, plasma technology has become a key to the transformation of human life. According to analysis conducted by Grand View Research, it is estimated that the revenue of the global plasma application market will reach approximately 38.57 billion USD in 2028, and it is expected to continue growing in the future.
Plasma has a wide variety of applications. With the rapid development of science and technology, plasma technology has become widely used in fields such as industry, daily living, military, academic research and medical treatment. For example, in the semiconductor industry, plasma technology has been widely used for etching, deposition, cleaning, ion implantation, surface treatment and other processes to improve the quality and stability of wafer manufacturing. According to analysis conducted by Mordor Intelligence, the global market size of plasma applications in the field of semiconductor manufacturing reached about 10.6 billion USD in 2019, and it is expected to grow to 14.3 billion USD by 2025 with a CAGR of 4.9％.
In regards to matter analysis, plasma can decompose gas molecules into positively charged ions and free electrons. Then analytical instruments such as mass spectrometers can be used to conduct material composition identification or contamination detection, etc.. Plasma also has many important applications In the biomedical field, such as sterilization, disinfection, promotion of wound healing and even cancer treatment. In terms of environmental protection, plasma can be used for air purification, water pollution control, and waste recycling treatments, etc.. For example, exhaust gas emitted by factory chimneys and automobile tailpipes can be decomposed into environmentally friendly gases via plasma ion bombardment. At present, considerable progress has been made in the research of plasma technologies such as plastic decomposition and modification as well as CO2 resource utilization. It can even be applied to military technology in shielding technology that absorbs radar waves to make aircrafts invisible, advanced, high-speed communications, electromagnetic pulse weapons, and optical interference equipment, etc.. Truly, plasma technology has irreplaceable potential. In addition, plasma may play an important role in future energy technologies (including new technologies such as nuclear fusion, magnetic fluid power generation, plasma propulsion, renewable energy, combustion control and solar cells) for the generation of cleaner, safer and more efficient sustainable energy and meet the huge demand for energy of future applications.
Plasma has many unique physical properties, and these properties are essential to understanding plasma physics and its applications and related technologies. An in-depth understanding of all of the characteristics of plasma will help us design more efficient plasma devices and achieve better application results. First, plasma has the ability to shield against external electric fields and keep itself electrically neutral. If two plates connected to the two ends of a battery are placed in plasma, electrons and ions will be attracted to the plates connected to the positive and negative electrodes respectively. As a result, the electric field will only exist in a thin layer around the electric plate with a thickness of the Debye shielding length. In other regions of the plasma, the electric field generated by the flat plate will tend towards 0. This unique barrier effect is called Debye Shielding. There will be a layer with a thickness of several Debye Shielding lengths near the plate boundary which is generally called the Plasma Sheath. In this sheath, the density of charged particles increases, creating a positive ion layer and a negative electron layer, and these layers prevent more electrons and ions from entering the conductor.
The plasma sheath can substantially affect the interactions between the plasma and its surrounding environment, so it has many important applications in the fields of semiconductor manufacturing, surface treatment, biomedicine and environmental cleansing. In semiconductor manufacturing, for instance, plasma sheaths can be used to deposit thin films and clean processes. In biomedicine, plasma sheaths can be used to change the properties of the surfaces of materials such as artificial hearts and joints, increasing the affinity of cells and bio-molecules. In the textile industry, plasma sheaths can be used to change the properties of textile surfaces, making them more waterproof and more resilient to contamination and abrasions, thereby enhancing the functionality of products such as sportswear, military uniforms and firefighting clothing.
Collective Behavior is another important plasma characteristic. Strictly speaking, the particles that make up plasma (ions and electrons) have their own electric fields, and, when the particles move, they generate magnetic fields. They’ll also be influenced by electromagnetic fields. When the number of charged particles in the plasma is large enough, they will interact and change each other’s trajectories. This collective behavior manifests as the macroscopic properties of the plasma, such as its physical resistance and capacitance properties. Plasma will also demonstrate unique physical effects, such as plasma oscillation, plasma fluctuation and plasma instability, when excited by external electromagnetic fields.
For example, when excited by a high-frequency electric field, the charged particles in the plasma will start to resonate and oscillate like a spring, resulting in the plasma oscillation phenomenon. This kind of collective behavior is very important to the study of the physical properties and applications of plasma. In nuclear fusion experiments, for example, control and maintenance of the fusion reactions can be achieved by exciting plasma oscillations. Then there is plasma instability, which refers mainly to when the energy distribution, plasma density, magnetic field distribution and other parameters in the plasma change and cause the state of the plasma system to become unstable. This causes the movement of charged particles in the plasma to become unbalanced, resulting in certain non-linear phenomena such as plasma explosions and plasma turbulence, etc..
Furthermore, every plasma system has its own set of frequencies referred to as the Plasma Frequency. This is another essential characteristic of plasma. The plasma frequency refers to the frequency generated by the resonant vibration of electrons, ions and neutral particles under the influence of a given electric or magnetic field. For an ionized gas with a very low degree of ionization and high particle density, the collision frequency between charged and neutral molecules is very high, so the average collision frequency between particles is higher than the plasma frequency. At this time, physical properties would be determined by the collisions of the individual parts and not by the collective behavior. Such a system cannot be called a plasma. The conditions necessary for the establishment of a plasma system is that the particle collision frequency must be lower than the plasma frequency. This helps maintain a stable plasma presence in the system.
Plasma frequency is an important parameter that determines the electromagnetic wave and energy transition speed in plasma. When a beam of electromagnetic waves hits the surface of a plasma, if the frequency of the electromagnetic wave is lower than the plasma frequency, the electromagnetic waves will be prevented from entering the plasma. When an electromagnetic wave encounters a higher frequency plasma, its energy is absorbed by electrons in the plasma and then converted into plasma waves and finally dispersed. This phenomenon is called the Plasma Shielding Effect. Therefore, plasma can be used as a shield against high frequency magnetic waves to, for instance, protect aircrafts from lightning, reduce the noise for radio telescopes and protect electronic equipment from the impact of electromagnetic waves, etc..
Plasma has many important properties in addition to those discussed above, and its applications continue to expand as science and technology advance. For example, when the plasma density, electric field or magnetic field exceed a certain critical value, the behavior of plasma becomes non-linear. These non-linear changes can be used to create many advanced applications, such as high power lasers and high energy particle accelerators, etc.. Furthermore, under certain conditions, plasma can even exhibit superconductivity, meaning it can conduct electricity without loss of energy. This superconducting phenomenon can be used to manufacture magnetic levitation vehicles, nuclear magnetic resonance instruments, superconducting electromagnets, superconducting quantum computer equipment and more. In addition, when plasma is placed inside a magnetic field, the magnetic field will generate an inward force known as magnetic pressure. This force compresses the plasma, increasing its density and temperature. This characteristic has been applied to nuclear fusion reactions and is widely used in the field of nuclear energy research. Other important plasma properties include non-thermal equilibrium, non-locality, modulation, expansion and the volume effect, etc..
Plasma is distinguished by the environment in which it is formed into two types: Atmospheric Plasma (AP) and Low Pressure Plasma (LP). Atmospheric plasma is a new type of plasma technology first discovered by Japanese scholar Kenichiro Hashimoto in 1998. Hashimoto’s research team used an experimental device called a dielectric resistive discharger to apply a high-frequency electric field to the surface of a medium, causing a discharge phenomenon that resulted in a high density plasma. This significant discovery led the global industry and academia to jointly pursue the exploration of atmospheric plasma applications. Atmospheric plasma does not require expensive vacuum equipment and has the advantages of low costs, high safety and simple systems. The fact that it can be formed directly in a normal atmospheric environment, in particular, gives it a definite advantage when it comes to applications in certain fields, including air pollution control, surface cleaning, sterilization and disinfection, and large area coating, etc.. Furthermore, because atmospheric plasma has essential characteristics such as a high particle collision frequency, it can provide low ion bombardment energy for substrates. As such, it is more suitable for processing softer and high dielectric materials.
Its reliance on ordinary, low temperature and normal pressure conditions give atmospheric plasma broad development prospects in biomedicine and daily living applications. It can be used in food processing for processes such as sterilization, disinfection, prolonging shelf life and improving taste. It can also react with the surfaces of fruits and vegetables to form oxides, thereby killing hidden bacteria and fungi. For textiles, on the other hand, it can be applied to improving antibacterial and antistatic properties as well as optimizing texture and more. It can also improve the hydrophilicity and lipophilicity of textile surfaces, making them easier to clean. In terms of sewage treatment, harmful substances such as oxidized pollutants, organic pollutants, bacteria, viruses and heavy metals can be removed from water through oxidation reduction reactions or the generation of free radicals and ozone, etc.. As for healthcare, it can be applied to sterilization, wound healing, tooth whitening and even cancer treatment. Furthermore, the adhesion of materials in 3D printing can be enhanced through plasma action so as to improve the quality of printed objects.
At present, plasma technology applications have become more popular and are attracting a lot of attention in many fields. Its development promises much to look forward to. With its unique physical characteristics and application advantages, it is sure to provide humanity with many more applications in the future. This article provided a comprehensive introduction to the field of plasma and the results of the recent research into the applications of atmospheric plasma in smart agriculture. This type of research will not only help to solve the problem of soil nitrogen fertilizer pollution but also increase the speed of crop production. It has a great chance to become a major development direction for this field in the future.
The first author of this article, Professor Jeng-Gong Du, has been teaching in the Department of Materials Science and Engineering at Tsing Hua University since earning his Ph.D. from Purdue University in 1983. During this time, he has successively served as the provost of Tsing Hua University, the convener of the Materials Science Department of the National Science Council, and the chairman of the Taiwan Association for Coating and Thin Film Technologies. He has also won the Tsing Hua University Outstanding Teaching Award several times in addition to the National Science Council Outstanding Research award and other honors. He has made great contributions to the development of domestic materials technologies. Professor Du has led his team to publish many important research findings in internationally renowned journals over the years, with over 460 papers and more than 25 patents. His academic achievements are truly outstanding. Professor Du has retired (as of 2022). However, he still serves as an honorary professor in his department at Tsing Hua University and continues to guide master and doctoral students. He remains committed to advanced research in the fields of electronic packaging, thin film materials, plasma technology and various energy materials.
The expertise of Dr. Yuantai Lai, the second author of this article, lies in atmospheric plasma fertilizer manufacturing, plant optical design and agricultural power integration planning, etc.. At present, Dr. Lai is working as a researcher in the Department of Materials Engineering at Tsing Hua University, the founder of A Farm Technology Co, Ltd., and a member of the review committee for the Ministry of Economic Affairs’ University Research Cooperation, Innovation and Entrepreneurship program. Dr. Lai has been committed to the development of various green circular economy technologies based on the agricultural power symbiosis framework for many years. He has made substantial contributions to the domestic promotion of green energy and environmental protection in smart agriculture.
MA-tek is honored to be able to carry out a second industry-university collaboration project with Professor Du this year, providing his team with all the analytical services needed for advanced technology and material research. MA-tek has the complete testing equipment and professional technical experience to meet the various analysis and testing needs of electronic materials, manufacturing processes and packaging research.
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