Behaviour of Microwave-Heated Al4SiC4 at 2.45 GHz

The ongoing development of high-temperature processes with the use of microwaves requires new microwave absorbers that are useful at these temperatures. In this study, we propose Al4SiC4 powders as important and efficient microwave absorbers. We investigated both the behavioural microwave heating and electrical permittivity characteristics of Al4SiC4 powders with various particle sizes at 2.45 GHz. The TE103 single-mode cavity indicated that Al4SiC4 powder samples yielded different heating behaviours and dielectric constants for each particle size compared with SiC. By microwave heating ∅50 mm × 5 mm disks of Al4SiC4 and SiC, we demonstrate that for specific sizes, Al4SiC4 can be heated at a higher temperature than SiC. Finally, the results of the two-dimensional two-colour thermometer show that an energy concentration appears at the interface of the microwave-heated Al4SiC4. These phenomena, which are inconsistent in individual physical property values, can be explained without contradicting microwave heating physics.


Introduction
High-temperature processes employing microwave heating have not been demonstrated by many researchers, despite the report by Roy et al. on microwave-based metal sintering [1]. Huang and Ishizaki et al. applied microwave heating to the steel reaction to obtain pig iron with low impurities at temperatures ≥1400 • C [2,3]. Microwave heating has been applied to the development of the hydrogen reduction method and the high-power irradiation method for iron ore [4,5]. Leonelli et al. used microwaves to provide heat for high-temperature processes in order to detoxify asbestos-containing materials [6,7]. Given that such materials have high-heat insulation properties, they can be heated more rapidly by employing microwave heating than conventional heating methods. By using microwaves to heat the inner parts of the refractory to attain high temperatures [8], the microwaves rapidly detoxify the asbestos fibres. In the future, microwave heating technology will continue to expand into the high-temperature-processing region.
To efficiently utilise the characteristics of microwaves in heating processes, materials that convert microwave energy into heat must be developed. When microwaves are used as a heat source for chemical synthesis, the raw material is heated, but the surrounding atmosphere remains cold. This feature is advantageous in chemical processes, such as amorphisation and vitrification, but for many other processes, it has detrimental effects, including decreased process efficiency and increased defective product rates. To prevent these adverse effects and take advantage of the positive effects, good microwave absorbers, such as silicon carbide (SiC) and ferrite, are placed near the material to be heated for many chemical processes [2,3,6,7]. Therefore, various researchers have developed microwave well absorbers [9][10][11][12][13].
Here, in order to evaluate the performance of Al 4 SiC 4 as a microwave absorber for high-temperature processes, the electrical permittivity and microwave heating behaviour of Al 4 SiC 4 were investigated. Materials used as microwave well absorbers for high-temperature operations must have high microwave absorption, high-temperature stability, and high resistance to oxidation. Al 4 SiC 4 possesses high oxidation resistance and a thermal coefficient closer to those of Al 2 O 3 than to those of SiC [14][15][16]. There is a trade-off between the absorption of microwaves into a substance and penetration depth. Given that Al 4 SiC 4 does not absorb microwaves as effectively as SiC, microwaves can penetrate more deeply even at high temperatures. Therefore, because Al 4 SiC 4 can be used to absorb microwaves that would be blocked by SiC, the substance to be heated can be directly heated by microwaves at high temperatures. If Al 4 SiC 4 indeed possesses appropriate microwave absorption, it would be a promising candidate as a microwave absorber for use at high temperatures with Al 2 O 3 as it could transmit at those temperatures. We first investigated both the microwave heating behaviour and electrical permittivity of Al 4 SiC 4 powders with various particle sizes at the frequency of 2.45 GHz. Sintered Al 4 SiC 4 was then placed in a multimode cavity similar to that used in furnaces and heated by microwaves at 2.45 GHz. This study thereby demonstrated that at a certain size Al 4 SiC 4 , can be heated to a higher temperature than SiC of the same size. Finally, the heating behaviour and deterioration of Al 4 SiC 4 were investigated using a two-dimensional two-colour thermometer. The results showed that Al 4 SiC 4 is an efficient microwave absorber.

Heating Behaviour and Electrical Permittivity of Al 4 SiC 4 Powders
To investigate their heating behaviour, Al 4 SiC 4 powder samples (weight = 1.0 g, Tateho Chemical Industries Co., Ltd., Akou, Hyogo, Japan) with different particle radii (1 µm, 3 µm, 10.4 µm, 16.3 µm, and 1-2 mm) were heated by microwave irradiation from a magnetron oscillator at 2.45 GHz. The heating system consisted of waveguides (109.1 mm × 54.6 mm × 220.5 mm ± 5 mm) with a magnetron oscillator, an E-H tuner, a plunger, and an isolator. The microwaves were focused by an iris and formed a TE 103 wave in this cavity. The iris had a slit with a width of 52 mm and was parallel to the direction of the electric field. The plunger was placed at the end of the waveguide. This system enabled us to spatially separate the electric and magnetic fields of the microwaves [17]. The sample was placed at an electric field node (denoted by E max ) at which the magnetic field was zero. The temperature of the reactants was monitored using a radiation thermometer with a lower limit of 300 • C (FTZ9-P300-20K21, Japan Sensor Corporation, Tokyo, Japan). In these experiments, dry air flowed at a rate of 0.6 l min −1 , and the absorption microwave power was set at 200 W. These parameters were set so that the temperature of Al 4 SiC 4 of each particle size was constant at approximately 400-1200 • C. The irradiation time was set to 10 min to provide sufficient time for the temperature of the heated materials to stabilize to a constant value.
The real and imaginary parts of the relative permittivity (ε r and ε r ) of the Al 4 SiC 4 powders with various particle radii (1 µm, 3 µm, 10.4 µm, and 16.3 µm) were measured to evaluate the heating mechanism. In pre-experiments, particle radii were investigated by laser diffraction (SALD-2300, Shimazu Co. Ltd., Tokyo, Japan), as shown in Figure 1. Herein, the refractive index was 1.95, and the solvent was ultrapure water. We used the cavity perturbation method [18,19] to obtain ε r and ε r The measurement system consisted of a cylindrical cavity and a network analyser (8753D, Keysight Co. Ltd., Tokyo, Japan). When samples were moved in the cavity, they disturbed the Q factor and resonant frequency of the cavity because of their material properties (i.e., ε r and ε r ). The network analyser monitored the Q factor and resonant frequency of the system, and ε r and ε r were calculated based on these values. The cavity generated the TM 020 mode (in the TM mode, the Poynting vectors of the electromagnetic field are vertically controlled by the magnetic field). The perturbation coefficient for the estimation was assumed to be 4.318, which was assessed using a cavity without a sample. Here, the perturbation coefficient was calculated using the already measured dielectric constants of quartz and alumina rods.
The microwave heating behaviour was investigated with a multimode applicator (Nihon Koshuha, Kanagawa, Japan, maximum power = 6 kW, wave frequency = 2.45 GHz) with a maximum rated power of 6 kW. This multimode heating device was developed by Yoshikawa et al. [20], and the microwave heating device consisted of a hexagonal container. Therefore, this microwave heating furnace enables the formation of an appropriate electric field distribution inside it (Q factor = 10-500), and it can heat various materials, as shown in Figure 2. Sintered Al 4 SiC 4 and SiC disks with specific sizes and densities (∅30 mm × 5 mm, ∅50 mm × 5 mm, density: 99%) were placed in the centre of the furnace and irradiated with microwaves at 800 W and 3.2 kW, and their temperatures were measured (atmosphere: 10 L/min dry air). A radiation thermometer (FTZ6, Japan Sensor, Tokyo, Japan) was employed at high temperatures, whereas a fibre thermometer (FL-2000, FS100-2M-190-260 • C, Anritsu, Kanagawa, Japan) was used at low temperatures (0-260 • C). The microwave heating behaviour was investigated with a multimode applicator (Nihon Koshuha, Kanagawa, Japan, maximum power = 6 kW, wave frequency = 2.45 GHz) with a maximum rated power of 6 kW. This multimode heating device was developed by Yoshikawa et al. [20], and the microwave heating device consisted of a hexagonal container. Therefore, this microwave heating furnace enables the formation of an appropriate electric field distribution inside it (Q factor = 10-500), and it can heat various materials, as shown in Figure 2. Sintered Al4SiC4 and SiC disks with specific sizes and densities (∅30 mm × 5 mm, ∅50 mm × 5 mm, density: 99%) were placed in the centre of the furnace and irradiated with microwaves at 800 W and 3.2 kW, and their temperatures were measured (atmosphere: 10 L/min dry air). A radiation thermometer (FTZ6, Japan Sensor, Tokyo, Japan) was employed at high temperatures, whereas a fibre thermometer (FL-2000, FS100-2M-190-260 °C, Anritsu, Kanagawa, Japan) was used at low temperatures (0-260 °C).
(a) (b)  The microwave heating behaviour was investigated with a multimode applicator (Nihon Koshuha, Kanagawa, Japan, maximum power = 6 kW, wave frequency = 2.45 GHz) with a maximum rated power of 6 kW. This multimode heating device was developed by Yoshikawa et al. [20], and the microwave heating device consisted of a hexagonal container. Therefore, this microwave heating furnace enables the formation of an appropriate electric field distribution inside it (Q factor = 10-500), and it can heat various materials, as shown in Figure 2. Sintered Al4SiC4 and SiC disks with specific sizes and densities (∅30 mm × 5 mm, ∅50 mm × 5 mm, density: 99%) were placed in the centre of the furnace and irradiated with microwaves at 800 W and 3.2 kW, and their temperatures were measured (atmosphere: 10 L/min dry air). A radiation thermometer (FTZ6, Japan Sensor, Tokyo, Japan) was employed at high temperatures, whereas a fibre thermometer (FL-2000, FS100-2M-190-260 °C, Anritsu, Kanagawa, Japan) was used at low temperatures (0-260 °C).

Results and Discussion
The Al 4 SiC 4 powders show adequate microwave absorption characteristics at most particle sizes. In this measurement, to observe the electric field absorption of Al 4 SiC 4 for its heating behaviour, a TE 103 single-mode cavity which can heat materials depending on their electrical permittivity was used as the heating device. Figure 3 shows the time vs. temperature plots of 1.0 g of Al 4 SiC 4 with various particle radii (1 µm, 3 µm, 10.4 µm, 16.3 µm, and 1-2 mm) heated by a TE 103 single-mode cavity. Although the microwave power was set to 200 W in all experiments, the maximum temperature increased at all particle radii except for 3 µm. When the Al 4 SiC 4 powders were irradiated, their temperature increased rapidly to 400-500 • C. The time vs. temperature curve shows an inflection point within this range and has a gentle slope at the temperature of the inflection point or higher. The Al 4 SiC 4 powders with radii of 1-2 mm enhanced plasma ignition with high reproducibility (N = 3).

Results and Discussion
The Al4SiC4 powders show adequate microwave absorption characteristics at most particle sizes. In this measurement, to observe the electric field absorption of Al4SiC4 for its heating behaviour, a TE103 single-mode cavity which can heat materials depending on their electrical permittivity was used as the heating device. Figure 3 shows the time vs. temperature plots of 1.0 g of Al4SiC4 with various particle radii (1 μm, 3 μm, 10.4 μm, 16.3 μm, and 1-2 mm) heated by a TE103 single-mode cavity. Although the microwave power was set to 200 W in all experiments, the maximum temperature increased at all particle radii except for 3 μm. When the Al4SiC4 powders were irradiated, their temperature increased rapidly to 400-500 °C. The time vs. temperature curve shows an inflection point within this range and has a gentle slope at the temperature of the inflection point or higher. The Al4SiC4 powders with radii of 1-2 mm enhanced plasma ignition with high reproducibility (N = 3).  Figure 4 indicates the variations in temperature vs. the real and imaginary parts of the relative electrical permittivity of Al4SiC4 samples at various particle radii (1, 3, 10.4, 30.3, and 100 μm). In this case, the cavity was TM020, the atmosphere was 6N-grade N2, and their densities ranged from 1.0 to 1.8 g/cm 3 . The permittivity was measured in an inert gas to rule out the effects of the oxidation of Al4SiC4, although powders with radii of 1-2 mm could not be loaded into the instrument and could not be measured. As shown, the real part of the dielectric constant increased as the temperature increased. At room temperature (25 °C), the real part of the dielectric constant was high and followed the order of 30.3 μm > 3 μm > 10.4 μm > 1 μm, but as the temperature of the Al4SiC4 increased, the order changed to 3 μm > 1 μm > 30.3 μm > 10.4 μm > 100 μm. Similarly, the imaginary part of the dielectric constant increased with the increasing temperature. At room temperature, the real part of the dielectric constant was also high, but it followed the order of 100 μm > 10.4 μm = 30.3 μm > 3 μm. As the temperature of Al4SiC4 increased, the order changed to 3 μm > 1 μm = 30.3 μm > 10.4 μm.  Figure 4 indicates the variations in temperature vs. the real and imaginary parts of the relative electrical permittivity of Al 4 SiC 4 samples at various particle radii (1, 3, 10.4, 30.3, and 100 µm). In this case, the cavity was TM 020 , the atmosphere was 6N-grade N 2 , and their densities ranged from 1.0 to 1.8 g/cm 3 . The permittivity was measured in an inert gas to rule out the effects of the oxidation of Al 4 SiC 4 , although powders with radii of 1-2 mm could not be loaded into the instrument and could not be measured. As shown, the real part of the dielectric constant increased as the temperature increased. At room temperature (25 • C), the real part of the dielectric constant was high and followed the order of 30.3 µm > 3 µm > 10.4 µm > 1 µm, but as the temperature of the Al 4 SiC 4 increased, the order changed to 3 µm > 1 µm > 30.3 µm > 10.4 µm > 100 µm. Similarly, the imaginary part of the dielectric constant increased with the increasing temperature. At room temperature, the real part of the dielectric constant was also high, but it followed the order of 100 µm > 10.4 µm = 30.3 µm > 3 µm. As the temperature of Al 4 SiC 4 increased, the order changed to 3 µm > 1 µm = 30.3 µm > 10.4 µm.
The Al 4 SiC 4 powder samples with diameters of 3 µm could not be easily heated by microwaves with high accuracy. The temperature of a material heated by microwaves is determined by the difference between the microwave absorption energy in the material and the thermal energy that is lost from the materials. Given that the microwave absorption energy W is proportional to the imaginary part of the relative permittivity ε" r ( w ∼ 1 2 ωε 0 ε r |E| 2 dV at E max , where ε 0 : electrical permittivity, E: strength of electrical field, and ω: angular velocity), the imaginary part increases with temperature, as shown in Figure 4.  The Al4SiC4 powder samples with diameters of 3 μm could not be easily heated by microwaves with high accuracy. The temperature of a material heated by microwaves is determined by the difference between the microwave absorption energy in the material and the thermal energy that is lost from the materials. Given that the microwave absorption energy W is proportional to the imaginary part of the relative permittivity ε′′r : electrical permittivity, E: strength of electrical field, and : angular velocity), the imaginary part increases with temperature, as shown in Figure 4. Notably, the imaginary part of the electrical permittivity of Al4SiC4 is an order of magnitude lower than that of SiC reported in previous studies [12,13]. This value is an index which quantifies how microwave energy is absorbed per unit material volume. The higher this value is, the easier the material can absorb microwaves [8,13]. However, as this value increases with increasing temperature, it becomes more difficult for microwaves to penetrate deeply into the material. Thus, the ratio between the dielectric constants of the material and air becomes larger, and microwaves are more easily reflected by the material. Therefore, SiC and carbon, which are known as microwave well absorbers, act as microwave reflectors in processes at temperatures ≥600 °C [12,13]. However, the electrical permittivity (imaginary part) of Al4SiC4 remains constant at high temperatures; thus, Al4SiC4 can be expected to behave as an absorber even at high temperatures.
Thus, at high temperatures and certain particle sizes, Al4SiC4 is superior to SiC as a microwave absorber, possibly because microwaves can penetrate further into a material with moderate microwave absorption properties. Figure 5a shows time vs. temperature plots of sintered Al4SiC4 samples (∅50 mm × 5 mm, ∅30 mm × 5 [mm]) irradiated by microwaves at 800 W, where the percentage means the purity of the sample. The temperatures of the sintered Al4SiC4 and SiC were measured by a fibre thermometer. During the microwave heating of the Al4SiC4 and SiC disk (∅50 mm × 5 mm) shown in Figure 5a, it was necessary to turn off the microwave output to avoid damaging the thermometer as the sample approached 240 °C. Therefore, the irradiation time was decided accordingly. A preliminary test confirmed that an alumina disk of the same size was not heated by microwaves. A sintered Al4SiC4 disk with a diameter of 30 mm exhibited a temperature rise in the range of 50-100 °C under microwave irradiation, and the heating rate of sintered Al4SiC4 depended on the density. By contrast, the sintered Al4SiC4 disk with a diameter of 50 mm heated to a higher temperature than a disk with an equivalent density but a diameter of 30 mm. Considering that the sintered Al4SiC4 (∅50 mm × 5 mm) density is relatively close to that of the sintered Al4SiC4 (∅30 mm × 5 mm) disk, this finding implies Notably, the imaginary part of the electrical permittivity of Al 4 SiC 4 is an order of magnitude lower than that of SiC reported in previous studies [12,13]. This value is an index which quantifies how microwave energy is absorbed per unit material volume. The higher this value is, the easier the material can absorb microwaves [8,13]. However, as this value increases with increasing temperature, it becomes more difficult for microwaves to penetrate deeply into the material. Thus, the ratio between the dielectric constants of the material and air becomes larger, and microwaves are more easily reflected by the material. Therefore, SiC and carbon, which are known as microwave well absorbers, act as microwave reflectors in processes at temperatures ≥600 • C [12,13]. However, the electrical permittivity (imaginary part) of Al 4 SiC 4 remains constant at high temperatures; thus, Al 4 SiC 4 can be expected to behave as an absorber even at high temperatures.
Thus, at high temperatures and certain particle sizes, Al 4 SiC 4 is superior to SiC as a microwave absorber, possibly because microwaves can penetrate further into a material with moderate microwave absorption properties. Figure 5a shows time vs. temperature plots of sintered Al 4 SiC 4 samples (∅50 mm × 5 mm, ∅30 mm × 5 [mm]) irradiated by microwaves at 800 W, where the percentage means the purity of the sample. The temperatures of the sintered Al 4 SiC 4 and SiC were measured by a fibre thermometer. During the microwave heating of the Al 4 SiC 4 and SiC disk (∅50 mm × 5 mm) shown in Figure 5a, it was necessary to turn off the microwave output to avoid damaging the thermometer as the sample approached 240 • C. Therefore, the irradiation time was decided accordingly. A preliminary test confirmed that an alumina disk of the same size was not heated by microwaves. A sintered Al 4 SiC 4 disk with a diameter of 30 mm exhibited a temperature rise in the range of 50-100 • C under microwave irradiation, and the heating rate of sintered Al 4 SiC 4 depended on the density. By contrast, the sintered Al 4 SiC 4 disk with a diameter of 50 mm heated to a higher temperature than a disk with an equivalent density but a diameter of 30 mm. Considering that the sintered Al 4 SiC 4 (∅50 mm × 5 mm) density is relatively close to that of the sintered Al 4 SiC 4 (∅30 mm × 5 mm) disk, this finding implies that the disk size affects the microwave heating behaviour. Notably, the heating rate of the sintered SiC disk was higher than that of sintered Al 4 SiC 4 disk, which indicates that SiC exhibits higher microwave absorption than Al 4 SiC 4 at low temperatures. However, as shown in Figure 5b, the 3.2 kW microwaves heated the sintered Al 4 SiC 4 to approximately 200 • C, which was higher than that of the sintered SiC. Further, at higher temperatures, Al 4 SiC 4 is heated more effectively than SiC, which indicates that Al 4 SiC 4 may be superior to SiC as a microwave absorber in the high-temperature region. Sintered Al 4 SiC 4 compounds behave as a better microwave absorber than SiC at temperatures >600 • C for two reasons. One is the size of sintered Al 4 SiC 4 . A material with good microwave absorption rapidly absorbs microwave energy. Thus, as the size increases, the microwaves do not heat the interior but heats only the surface. Conversely, in a material with moderate microwave absorption properties, microwaves can penetrate further into the interior. Therefore, most of the microwave energy can be absorbed even if the imaginary part of the relative electrical permittivity is small. Thus, as the size increases, SiC loses this advantage in microwave heating. The other reason is that SiC exhibits a high microwave reflection response at high temperatures. High-temperature SiC has many free electrons on the Fermi surface, so it behaves metallically toward microwaves [12,13]. Therefore, graphite and SiC reflect microwaves at high temperatures. For these two reasons, it is speculated that sintered Al 4 SiC 4 outperforms SiC as a high-temperature microwave absorber. In addition, Al 4 SiC 4 has a coefficient of thermal expansion similar to that of Al 2 O 3 [14], which is often used as a refractory material in high-temperature processes. This is another important reason why Al 4 SiC 4 would be a useful candidate for microwave absorbers in such processes.
Microwave heating of Al 4 SiC 4 in air causes another interesting phenomenon at the particle interfaces. Figure 6 shows the time vs. temperature plots of Al 4 SiC 4 particles (2 mm) during microwave heating in a 100 W electric field, along the two-dimensional two-colour thermometer measurement results. As shown in this figure, when Al 4 SiC 4 is heated in air, the particle surface temperature becomes approximately 200 • C higher than that in the interior. This temperature difference can possibly be attributed to the formation of oxide films on the Al 4 SiC 4 surfaces when it is heated in air. Thus, these oxide films exhibit different heating responses. Interestingly, this interfacial gradient cannot be explained without assuming a thermal conductivity that is several hundred times lower than that of air according to the law of heat conduction.
Scanning electron microprobe (SEM) and energy-dispersive X-ray (EDX) analyses of the Al 4 SiC 4 before and after microwave heating (dry air, 1000 • C, 10 min) provide deeper insights into this interfacial gradient, as shown in Figure 7. Before microwave heating, the surface of Al 4 SiC 4 exhibited strong Kα rays indicating C and Si, with the latter being homogeneously distributed. However, after microwave heating, white particles adhered to the surface of the Al 4 SiC 4 . In addition, the amount of carbon decreased, but the oxygen increased after heating. This result suggests that a special electric field concentration is formed by the surface coating, according to Maxwell's formula [21]. Scanning electron microprobe (SEM) and energy-dispersive X-ray (EDX) analyses of the Al4SiC4 before and after microwave heating (dry air, 1000 °C, 10 min) provide deeper insights into this interfacial gradient, as shown in Figure 7. Before microwave heating, the surface of Al4SiC4 exhibited strong Kα rays indicating C and Si, with the latter being homogeneously distributed. However, after microwave heating, white particles adhered to the surface of the Al4SiC4. In addition, the amount of carbon decreased, but the oxygen increased after heating. This result suggests that a special electric field concentration is formed by the surface coating, according to Maxwell's formula [21]. Remarkably, more Si seemed to evaporate during heating than Al. Because Al has a lower ionisation energy than Si, Al has a higher probability of plasma ionisation in the presence of an electric field. However, the SEM analysis indicates that Si may have evaporated, which implies that the evaporation of the interfacial elements follows a thermodynamic mechanism.  Remarkably, more Si seemed to evaporate during heating than Al. Because Al has a lower ionisation energy than Si, Al has a higher probability of plasma ionisation in the presence of an electric field. However, the SEM analysis indicates that Si may have evaporated, which implies that the evaporation of the interfacial elements follows a thermodynamic mechanism.