Microwave-induced high-energy sites and targeted energy transition promising for efficient energy deployment

Abstract

Diverse interactions between microwaves and irradiated media provide a solid foundation for identifying novel organization pathways for energy flow. In this study, a high-energy-site phenomenon and targeted-energy transition mechanism were identified in a particular microwave heating (MH) process. Intense discharges were observed when microwaves were imposed on irregularly sized SiC particles, producing tremendous heat that was 8-fold the amount generated in the discharge-free case. Energy efficiency was thereby greatly improved in the electricity-microwaves-effective heat transition. Meanwhile, the dispersed microwave field energy concentrated in small sites, where local temperatures could reach 2000°C– 4000°C, with the energy density reaching up to 4.0 × 10⁵ W/kg. This can be called a high-energy site phenomenon which could induce further processes or reactions enhancement by coupling effects of heat, light, and plasma. The whole process, including microwave energy concentration and intense site-energy release, shapes a targeted-energy transition mechanism that can be optimized in a controlled manner through morphology design. In particular, the discharge intensity, frequency, and high-energy sites were strengthened through the fabrication of sharp nano/microstructures, conferring twice the energy efficiency of untreated metal wires. The microwave-induced high-energy sites and targeted energy transition provide an important pathway for high-efficiency energy deployment and may lead to promising applications.

Graphical abstract

Keywords

microwave discharge / high-energy sites / targeted-energy transition / morphology design / energy efficiency

1 Introduction

Since microwave heating (MH) has been found to be an effective alternative to conventional heating methods in 1940s, the microwave technology has been continuously finding applications in new fields. As is well-known, MH is based on energy dissipation due to alternating electromagnetic field-induced agitation of the molecules, charged ions, or electrons present in a material, offering practical advantages over conventional heating such as volumetric, selective, rapid, and noncontact heating [1,2]. Therefore, based on the efficient coupling of microwave with certain absorbers, such as SiC and activated carbon, the microwave technology has been widely adopted in various processes such as hydrogenation [3], pyrolysis and gasification [4–6], wastewater remediation [7], and material preparation [8], with the energy efficiency or reaction speed being continuously optimized.

On the other hand, due to the intrinsic high ionization degree, low electric fields required, high densities of reactive species and broad operation pressure range (from vacuum to high pressure above atmosphere pressure), microwave plasmas have been found to be practical for gas reforming [9–11], diamond deposition [12,13], preparation of nanomaterials (i.e., aligned carbon nanotubes (CNTs) [14] and metal-filled CNTs [15]), sterilization [16,17] and surface modification [18,19]. In addition, some special mechanisms underpinning microwave plasmas account for their excellent performance in reaction promotion. For instance, microwave plasmas make the generation of molecular plasmas possible with relatively low electron temperatures but high vibrational temperatures that can favor vibrational excitation in place of direct dissociation mechanisms by electron impact [20]. With the in-depth investigation of microwave plasma induction mechanisms, its applications are being continuously expanded.

As a special induction mode of microwave plasma, the intensive discharge phenomena can be triggered when certain targeted materials are subjected to microwave radiation [21], leading to the generation of drastic heating, plasma, and UV-visible-light effects. The discharge site is energy-intensive and reactive, as evidenced by the detection of melted material tips, reactive radicals, and UV-visible light [22–24]. This particular microwave-matter coupling effect offers a pathway for targeted energy transfer through a kind of high-energy site effect: the disperse microwave field energy first converges on some specific sites due to strong microwave-matter interactions; the concentrated energy then induces the discharge phenomena at the corresponding sites; subsequently, the energy is released in the form of combined effects of heat, plasma, and light. Notably, the high-energy site effect can play a crucial role in enhancing energy-usage efficiency and promoting chemical reactions in various applications. According to previous research, an energy-conversion efficiency greater than 80% for the conversion of inputted microwave energy to effective heat can be achieved through microwave-metal discharge [25]. Furthermore, the discharge greatly promotes the pyrolysis of waste electronic scraps and tires with significant reduction of activated energy compared to that of a conventional pyrolysis process, engendering it cost-competitive to recycle these types of wastes through microwave-assisted pyrolysis [26,27]. Therefore, the high-energy site and the targeted energy transfer mode are of high significance and worthwhile for systematical research and development.

Given the aforementioned background, exploring a novel method to strengthen microwave-induced discharges and developing a means of optimizing the high-energy site phenomenon in a controlled manner are critical to achieving an efficient energy utilization and fully deve-loping targeted energy-use pathways. SiC has been widely used as a microwave absorber, but has been rarely used to induce microwave discharges. In fact, as a semiconductor, SiC can trigger discharges in a microwave field. The coupling effect of microwave absorption and discharges may provide a new pathway to fully optimize the microwave energy conversion and targeted energy transfer, albeit has been scarcely researched before. Furthermore, the microwave-induced discharge and its intensity are influenced by various factors, such as the microwave power, properties of the targeted initiators (including the size, quantity, geometry, and surface morphology), and dielectric properties of the surrounding medium [21,28], among which macroscopic parameters rather than microscopic ones have been paid more attention to. The microwave-induced discharge intensity can be enhanced by increasing the microwave power and initiator amount [21,28]. Nevertheless, optimization of the discharge process at a certain microwave power and initiator amount is critical for efficient microwave-energy utilization. Microwave-induced discharge originates from electron emission, which is dominated by lattice defects and grain boundaries [29,30]. Note that nanostructures with sharp tips can reduce the turn-on electric field of electron emission by several orders of magnitude and therefore exhibit excellent electron emission characteristics [31–34]. Thus, there is a substantial potential to boost electron emission by suitably designing the initiator surface micro-morphology. However, to date, few studies have been systematically conducted in this regard.

In this paper, to characterize the high-energy site effect of microwave-induced discharge, the discharge phenomena occurring during the interaction between SiC and microwaves were fully investigated. The heating effects of the SiC particles with and without discharge were quantitatively measured by utilizing a direct calorimetric method, with surprising results revealing a magnitude difference of heating effects in the two cases. The temperature profiles of the high-energy sites were determined through numerical calculation. To utilize the high-energy sites in a controlled manner, the potential for intensifying the effects of microwave discharges and high-energy sites via the microscopic surface morphology design was specifically investigated at a given microwave power, using copper wires as discharge initiators. The copper wires were treated by using various wet chemical processes to form nano- or microscale structures. The discharge process was evaluated based on its heating effect and light intensity; the former was characterized by the direct calorimetric method [22], whereas the latter was recorded online using optical fibers connected to an oscilloscope. Finally, the mechanisms underpinning the microwave-induced discharge and high-energy sites were assessed to optimize the microwave energy-usage efficiency and promote industrial applications of MH.

2 Experimental methods

2.1 Material preparation

Silicon carbide with different particle sizes (5, 18, 125, 700, 2000, and 4000 μm) was used as the microwave-energy-converging target material. The SiC particles were subjected to microwave irradiation after dispersion in air, water, ethanol, and paraffin oil. Both the ethanol and the paraffin were purchased from Aladdin Co., Ltd. The properties of the SiC particle and dispersion media used are given in Table 1.

In addition to SiC particles, copper wires (diameter: 1 mm) purchased from Aladdin Co., Ltd. and cut into 1 cm pieces were used as the electrodes to trigger microwave discharge. To strengthen microwave-metal discharge through morphology design, the aforementioned metal wires were treated by two methods. In the first method, the copper wires were first micro-roughened by immersion in an ultrasonic aqueous bath consisting of 100 g/L of iron (III) chloride (FeCl₃, anhydrous, 98 wt.%) and 80 g/L hydrogen chloride (HCl, 37 wt.%) at room temperature for 10 min. After being rinsed with deionized water, the copper wires were incubated in a 0.03 mol/L ammonia solution (NH₃·H₂O, 28 wt.%) at approximately 5°C for 48–192 h [35]. Further, the wires were thoroughly washed with deionized water and dried at 180°C for 2 h to covert the Cu(OH)₂ into stable CuO by completing the dehydration reaction. In the second method, the copper wires were immersed in a 6 mol/L HCl solution for ultrasonic treatment for 10 min to remove the surface oxide film. Then, they were rinsed with absolute ethanol and immersed in an ethanolic solution of n-tetradecanoic acid (0.01 mol/L) at approximately 20°C for 1–5 days. The samples were then rinsed with deionized water and ethanol and dried in air. The surface morphologies of the metal wires were characterized by scanning electron microscopy (SEM).

2.2 Experimental methods

The experiments were performed in a modified household microwave oven (Midea X3-233A) whose input power and frequency were 1300 W and 2450 MHz, respectively, and the output power was adjustable from 0 to 900 W at intervals of 100 W.

2.2.1 Microwave-SiC discharge experiments

To reveal the discharge phenomenon and heating effect of the SiC particles, 2 g of SiC, with a particle size ranging from 5 to 4000 μm, were placed separately in either air (in a 20 mL crucible) or 120 g of paraffin oil, water, or ethanol (in a 200 mL quartz beaker). The target materials were held in the center of the microwave oven and irradiated at a microwave power of 700 W. A wireless webcam (Decrypters W20) was fitted to the oven to permit the in situ and real-time capture of the discharge phenomenon during the heating process. For discharge process analysis, the recorded video was further examined in slow motion (24 frames per second) using Ulead Video Studio.

In microwave-SiC interactions, regardless of whether discharges occur, the input power is eventually transformed into heat. To measure the overall heating effect, an optimized direct calorimetric method was adopted, in which liquid paraffin oil was used to store the heat generated from the interactions between the microwaves and the target materials. The increases in the temperature of the paraffin oil were measured using a digital K-type thermocouple with an accuracy of ±0.1°C, which was inserted into the paraffin oil after the microwave oven was switched off to avoid the interaction between the microwaves and the thermocouple. The heat generated Q was calculated according to the temperature rise of paraffin, along with estimates of the errors and considerations of all the possible heat losses from the vessels and the oil, using the method described [22]. By utilizing this calorimetric methodology, the heating effects were measured and studied for different SiC materials with wide-range particle sizes and aggregation degrees. The initial temperature of both the target material and the oil was controlled at 25°C.

2.2.2 Microwave-metal discharge experiments

To reveal the effect of the metal-surface morphology on the discharge intensity, for each experiment, a 2 g metal wire was immersed in 8 g of paraffin oil in a small quartz glass crucible (inner diameter: 30 mm, height: 50 mm) and irradiated with microwaves for a certain time period (Fig. 1(a)). The temperature changes of the paraffin oil were recorded. Each experiment was repeated at least three times, and the values were averaged for comparison. To reduce the experimental error, both the room temperature and the initial paraffin oil temperature were controlled at 25°C.

To probe the effect of the metal-surface morphology on the discharge intensity and characteristics, the discharge light intensity was recorded. First, 4 g of metal wires were evenly distributed at the bottom of Beaker 1 (50 mL), which contained 30 mL of paraffin. Then, Beaker 1 was placed in Beaker 2 (250 mL), which was filled with quartz sand, as illustrated in Fig. 1(b). Further, Polymethyl methacrylate (PMMA) optical fibers (Ø2 mm) were buried in the quartz sand, beneath Beaker 1, and used for capturing the light intensity through a photoresistance located outside the microwave oven. To shield the optical fibers from the substantial heat released by the discharge, a piece of transparent glass was placed between Beaker 1 and the sand. Therefore, the optical fibers were actually buried between the transparent glass and the sand. Finally, the photoresistance was connected in series with a DC power supply (12 V) and a resistance (1.2 MΩ) through holes in the oven and Beaker 2 to detect the light intensity of the microwave discharge. The light intensity was recorded at a rate of 10 counts per second as a V–t graph, using an oscilloscope (Tektronix TBS1104).

3 Results and discussion

3.1 Microwave-SiC discharge

Since the discharge induced by the interaction between the microwave and SiC has rarely been studied before, this study began by investigating the influencing factors of microwave-SiC discharge, including the dispersion medium, the particle size and aggregation degree of the target material, and the physical characteristics of the target material. The discharge phenomenon was photographically recorded using a wireless webcam.

Dispersion medium: Photographic studies of the SiC dispersed in air, water, organic solvents, and liquid paraffin show that discharge sparks occur when the dispersion medium is air or paraffin, which do not absorb microwaves. However, this phenomenon does not occur when the medium is water or ethanol, which can absorb microwaves.

Particle size and aggregation degree of the target material: Both the particle size of the target material and the distance between the particles determine whether the microwave discharge phenomenon can be induced. In studies using 2 g of the SiC target material, it was found that when the particle sizes were smaller than 125 μm, no discharge phenomena were observed, regardless of whether the particles were in close contact or evenly dispersed. When the particle sizes were greater than 700 μm, discharge sparks readily occurred under aggregation conditions. Particularly, the discharge phenomena were intense when the SiC particles were 1–3 cm in size and presented irregular tips, as shown in Fig. 2(a).

Physical characteristics of the target material particles: The geometry and surface properties of the SiC particles were found to have an influence on the discharge phenomena. Specifically, it was more likely to trigger the microwave discharge on irregular SiC particles, and the discharge became more intense in accordance with the surface roughness and/or shape irregularities of the particles or particle aggregates, which were typically characterized by their edges and corners.

Characteristics of the discharge process: In addition to confirming the importance of the properties of both the target materials and the dispersion medium in facilitating the microwave-induced discharge phenomenon, it is confirmed that the microwave-induced discharge process is pulsed in general, as evidenced in the discharge photography of Fig. 2. The images in Fig. 2 indicate that, at the start of a microwave discharge, only minute sparks are observed. This is followed by the widening of the active regions and the increasing of the light intensity. Finally, intensive lighting with a distinct yellow outer halo is observed. The time period of a discharge cycle ranges from a fraction of a second to several seconds, and the time intervals between two contiguous cycles also varies.

3.2 Heating effects of SiC with and without discharges

Currently, there is no effective method to detect the temperature of a hot spot or a discharge high-energy site in the microwave field. The calorimetric method using microwave-transparent paraffin as the dispersion and heat-storage medium offers a direct pathway to quantify the total heating effect of different target materials subjected to microwave radiation in a certain period, regardless of whether the discharge phenomenon occurs or not. Figure 3 depicts the heating effects of 2 g SiC materials of varied particle sizes (5–4000 μm) subjected to fixed-power microwave radiation for 1 min. The only differences among the experiments in the series were the particle sizes and extent of aggregation of the SiC particles. It is observed that heating effects of different magnitudes occur when the SiC particles of the same mass but different sizes are subjected to microwave irradiation. When the SiC particles are relatively small (<125 μm), no sparks or arcs are observed as a result of the irradiation, regardless of whether the particles are fully dispersed or aggregated. Moreover, the heat generated is<600 J and is attributable only to the effects of microwave absorption. With larger particle sizes (700–4000 μm), corresponding to fully dispersed particles, similar results are obtained, and the heat output is<600 J. However, if the particles are in contact in the aggregates, flickering discharges are observed, which increase in accordance with the extent of aggregation. In particular, the heat output is significantly greater, with a maximum of approximately 4800 J when the aggregation of the target material particles is maximized. This high heat output can be largely attributed to the microwave-induced discharge effects.

The influence of the aggregation degree of the target material on the heat output was further studied on two sizes of SiC particles: 20 and 4000 μm. The heat outputs when the same batch of target material is fully dispersed and aggregated with varying degrees in paraffin are demonstrated in Fig. 4. For all aggregation states of the small particles (20 μm), no discharge is observed from microwave irradiation, and the heat output is<600 J. For the larger particle size (4000 μm), increased discharge and heat output are observable in accordance with the increased extent of aggregation. The greatest heat output (approximately 4800 J) occurs when the SiC particle contact is maximized by the containment in the bottom of a small quartz crucible as illustrated in Fig. 4(e).

In the cases of maximum aggregation without (Fig. 4(d)) and with (Fig. 4(e)) using the small crucible, the experimental discharge phenomenon and heat output are different. When the quartz reactor is not used, the discharge phenomenon is only observed in the early stages of irradiation. However, instead of intensifying, it weakens and eventually disappears in approximately 30 s. The aggregation extent decreases after microwave radiation. It is inferred that the loss of close contact between the particles upon microwave irradiation is responsible for the unsustained microwave discharge, and partial discharge heating effect is converted into momentum. In the case of aggregation at the bottom of the small quartz reactor, the discharge is sustained during the whole irradiation process and results in the maximum heating effect.

3.3 High-energy-site effects of microwave-induced discharge

It is difficult to characterize the extent of high-energy sites by experimental methods. In this study, a numerical calculation was used to probe the depth and extent of high-energy site effects arising from MH in which the microwave-induced discharge phenomenon occurs. To illustrate the effects of the high-energy sites and simplify the numerical calculation, the experimental heating effect results mentioned above were adopted.

As exhibited in Figs. 3 and 4, the heat generated by 2 g of SiC particles after 60 s of microwave irradiation is approximately 560 J when there is no discharge, and approximately 4800 J when there is discharge. Therefore, the average heat generation powers P will be approximately 4.7 × 10³ W/kg (without discharge) and approximately 4.0 × 10⁴ W/kg (with discharge). This calculation is based on the assumption that the heat is generated evenly within the whole particle. However, discharges will actually only be generated at the tips, edges, or corners of the irregular large particle. If it is assumed that only 10% of the mass fraction of the particle is involved in the discharge, the actual heat generation power P when the discharge occurs will be an order of magnitude greater, at approximately 4.0 × 10⁵ W/kg. This value is close to that for 1 kg of coal releasing all of its heat in 1 s, which is a significantly high energy density.

The heat generated by microwave discharge effects can only be transferred inside the SiC particle or to the surrounding paraffin oil. However, no way exists to estimate the amount of heat that is delivered to the surroundings and how much of it is absorbed by the particle to cause the intense heating effects. If the cases considered are those in which 90%, 50%, and 10% of the heat generated is retained within the discharge fraction of the SiC particle, the temperature increase of the discharge caused by the retained heat can be calculated by using Eq. (1).

(1)P_{\mathrm{h}\mathrm{g}}\times m\times R_{\mathrm{p}}\times t=C\times m\times \left(T_{\mathrm{f}}-T_{\mathrm{i}}\right),

where P_hg is the heat generation power of the discharge fraction of SiC, R_p denotes the percentage of the energy retained by the discharge fraction of the SiC particle, t represents the time span of the heat generation, C is the specific heat capacity of SiC, m is the mass of the discharge fraction of SiC, T_f is the final temperature of the discharge fraction, and T_i is the initial temperature of the SiC particle.

The results regarding the temperature increase of the discharge fraction caused by the retained heat, in accordance with time, are manifested in Fig. 5, which clearly shows the very sharp temperature increases that occur at high-energy sites on SiC particles over short periods. When R_p = 90%, the local temperature reaches approximately 2000°C in 2 s and over 8000°C in 10 s. However, even for R_p = 50%, which should be very close to the actual case, the local temperatures at high-energy sites could reach 2000°C–4000°C in 5–10 s.

In short, the discharges of SiC and the accompanied intense heat release should be given greater focus in applications. Specifically, under the same input power and other conditions, the energy utilization efficiency will be greatly improved by accumulating particles instead of enabling them to exist in a decentralized state. This is critical for microwave thermal utilization in a highly-efficient, energy-saving approach, which would confer direct economic benefits in industrial production. Furthermore, certain high-energy sites (points of contact or tips, edges, and corners of irregular, large particles) are formed because the heat cannot be rapidly transferred away from the source either inside the SiC particle or to the surrounding paraffin oil. The occurrence of more high-energy sites in discharge cases than in cases without the discharge means a greater potential in chemical catalysis. Therefore, the identification of the discharge phenomena will broaden the options for leveraging the interaction between microwave irradiation and semiconductors in scientific research.

3.4 Morphology design for optimizing discharge high-energy sites

The intense heating effect of discharges and the concomitant high-energy sites provide a great potential for enhancing energy efficiency and process catalysis. It is, therefore, crucial to intensify the discharge in a controlled manner. In this study, the microwave-induced discharge was strengthened through surface morphology design by selecting copper wires rather than SiC particles as the discharge initiators, based on the consideration that the wave absorption in metal wires is rare compared with SiC, which makes the investigation and quantification of the discharge heating effect more direct. In addition, it is much easier to fabricate morphologies in a controlled manner on metal surfaces. In this study, the copper wires were treated using different wet chemical processes to form nanoscale or microscale structures.

Figure 6 shows the surface morphology of the copper wires which is first treated with an HCl+ FeCl₃ solution and then immersed in a 0.03 mol/L ammonia solution at different time intervals. Compared with the untreated copper wire, the surface morphology significantly changes in accordance with the increased immersion time, presenting incrementally increased surface roughness. Particularly, when the copper wires are immersed in the ammonia solution for 192 h, a few holes are observed on the copper surface and several sisal-like nanoribbon clusters develop perpendicularly from the curved surfaces of these microstructures (Fig. 6(e)). In accordance with the increase in surface roughness, the discharge intensity is greatly strengthened, as confirmed by the heating effect (Fig. 7). The discharge heating effect due to the presence of sisal-like nanoribbon clusters was almost twice that of the untreated copper wires, indicating a significant enhancement in the discharge intensity and energy conversion. The main reason for this is that the rougher the particle surface morphology, the lower the electric field threshold that could cause a field emission, and the higher the likelihood of a breakdown.

Similar results were obtained when copper wires were treated with n-tetradecanoic acid. Specifically, clusters of microwires formed on the substrate and exhibited an interesting flower-like architecture, as illustrated in Fig. 8. For short immersion times (a few hours or less), a few small clusters of copper carboxylate microwires were distributed sparsely on the copper surface [36]. For increased immersion times, the copper carboxylate microwires and self-assembled clusters enlarged and densified. After immersion for 72 h, some of the microwires began extending upward, as in the petals of a chrysanthemum. Consequently, the discharges were notably strengthened, presenting a quasilinear increase in the discharge heating effect in accordance with the immersion time, as shown in Fig. 8(e).

In addition to the overall heating effect, the effect of surface morphology on discharge intensity is also reflected in discharge light. Although the discharge process is non-uniform and difficult to reproduce, the differences in the discharge intensity and frequency were obvious when the random discharge processes of the copper wires treated with n-tetradecanoic acid for different immersion times were recorded and compared, as illustrated in Fig. 9. Overall, the discharge process was pulsed and not continuous. The discharge process of the untreated copper wire was relatively weak in intensity and considerably intermittent. As the formation of microwire clusters provides more sites for discharge initiation, the discharge intensity and frequency were strengthened. In particular, the discharge process appeared to be periodical in intensity and duration when the copper wires were treated with n-tetradecanoic acid for four days. This result establishes the potential for optimizing the discharge intensity, high-energy site effect, and pulse property through morphology design. A periodical pulsed and strengthened discharge process is significant in certain chemical processes, e.g., the synthesis of metal-cored nanoparticles with a graphitized carbon shell, because the instantaneous release of immense heat in pulse discharge can promote the graphitization of carbon, while nanoparticle growth in a long nanotube can be effectively limited by the intermittent discharge heat supply [37].

3.5 Discussion

Microwave-induced discharge initiated with an initiator (metal electrode, solid particle, etc.) presents a high application potential since it can be produced at low levels of incident microwave power and over a wide range of operating pressures [38]. In this study, the discharges induced by SiC particles and the accompanied intense heat release were elucidated to broaden the microwave energy-usage modes, enhance energy efficiency, and employ high-energy sites for process enhancement. In addition to microwave absorption, discharge (sparks or arcs) could be induced when the SiC particles reached a certain size and were aggregated, which was accompanied by drastic energy transformation, bright light, and plasmas. The experimental results showed that the amount of heat generated with discharges was 8-fold than that of the same sample without discharges, indicating that discharge is more efficient in energy conversion than wave absorption. Adequate evidence was obtained which showed that a high-energy site was formed by the discharge, and the numerical results indicated that the site temperature could amount to several thousand degrees Celsius.

The microwave-induced discharge process and associated high-energy-site effect can be optimized in a controlled manner through surface design, since the discharge is fundamentally related to the field emission of electrons from the initiator, which depends on the thermal and electric fields. When an initiator is exposed to a microwave field, the first step is the generation of a strong electric field by increasing the applied electric field, thereby inducing an increase in the electron density to initiate the electron breakdown avalanche. The field enhancement produced by an initiator depends on the geometry and dimensions of the initiator [39]. The field enhancement factor is generally computed as a function of h/r, where h is the height and r is the radius of curvature of the initiator tip. For several highly elongated geometrical shapes on a planar conducting surface, the field enhancement factor can be approximated by 2+ h/r, provided that h/r>5 [39]. Therefore, a thinner/sharper initiator tip can induce a greater field enhancement compared to a rounder one. This means that an initiator with several sharp tips perpendicular to the surface can induce a greater number of field-enhanced sites, consequently increasing the electron tunneling probability.

Owing to the field enhancement, the ohmic heating of the conductive electrons and the dielectric heating of the initiator are also strengthened, allowing for higher temperatures [39]. As a result, additional electrons in the initiator tip can occupy states above the Fermi level and possess a higher tunneling probability. In this case, both the field and thermal emission of electrons from the initiator tip can be induced. When electrons are emitted from the initiator tip, the initial seed electrons multiply under the influence of the supercritical electric field, leading to a stronger ionization with the formation of energetic electrons, ions, and reactive radicals. From that point, the formation of a plasma filament or so-called discharge occurs, with the release of heat as a result of Joule heating of the partial or full breakdown current and photons as a result of activated electrons transitioning back to their ground state. Subsequently, the effective electron collision frequency decreases to a low level several microseconds after the discharge [40], meaning that the discharge cycle has finished. The entire process occurred in a discharge cycle is schematically illustrated in Fig. 10.

4 Conclusions

In summary, the nature of high-energy-site phenomenon and targeted energy transition mechanism in a microwave-induced discharge process was evaluated, with the disclosure of some interesting findings. The heat generated when intense discharges were induced on irregularly sized SiC particles was tremendous, which was 8-fold the amount generated in a discharge-free case, indicating that the discharge heating effect is more remarkable than the wave absorption effect. As a result, high-energy sites, with the energy density of up to 4.0 × 10⁵ W/kg can be shaped to promote reactions, with the energy efficiency greatly enhanced. Therefore, it is important to optimize the high-energy-site phenomenon and targeted-energy transition pathway in a controlled manner.

This work proved that the fabrication of sharp nano/microstructures perpendicular to the initiator surface can boost the discharge intensity and high-energy sites, and thereby lead to a higher energy efficiency and controlled target-oriented energy conversion. The high-energy sites and greater energy efficiencies induced by the optimized microwave discharges will reveal novel microwave energy utilization modes and enable practical applications of microwave technology in a greater number of fields, such as the synthesis of metal-cored carbon nanomaterial through periodically pulsed discharge, tar reforming through the intensified heating and plasma effects, and the photocatalytic degradation of pollutants by strengthened discharge lighting. This study is expected to be an important reference for the smart use of microwave-matter interactions to realize targeted catalysis, process enhancement, and efficient energy use.

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