Role of metallic and ceramic supports on enhanced microwave heating processes

Analysis has been carried out to study efficient heating due to microwaves (MWs) for the samples placed on metallic and ceramic supports (Al₂O₃, SiC). The analysis is carried out for water which exhibits greater dielectric loss and oil which possesses small dielectric loss. A preliminary analysis on enhanced MW heating of samples has been carried out via average power within a sample vs. sample thickness diagram for various cases. The maxima in power, also termed as ‘resonances’ are observed for specific sample thicknesses and the two consecutive maxima in average power are termed as R₁ and R₂ modes. During both R₁ and R₂ modes, the greater intensity of resonances for power absorption in samples occurs in presence of metallic support. For a specific resonance mode in water samples, the maxima with metallic supports correspond to smaller sample thickness only. In contrast, for oil samples, the metallic support corresponds to greater average power for all sample thicknesses (>1 cm). Ceramic supports correspond to lower average power and Al₂O₃ is a suitable support for water whereas SiC support may cause local runaway heating for oil sample. The spatial distributions of power illustrate that the regime connected with the metallic support is heated at a lower rate specially for oil samples. The efficient and enhanced heating strategy is further exercised with metallic-ceramic composite supports. It is observed that Al₂O₃-metallic for water and SiC-metallic for oil are optimal support assemblies for efficient heating. In addition, the optimal SiC-metallic support would avoid the local runaway heating of oil samples. The choice of support may not be trivial due to complex dielectric response of sample-support assembly. Our analysis is carried out for two limiting cases due to water and oil, and we have recommended efficient heating strategy for both water and oil. The heating strategy can be suitably extended for heating of any materials on a support in custom MW ovens.     

Computational characterisation of microwave heating of fibre preforms for CVI of SiCf/SiC composites

Silicon carbide fibre reinforced silicon carbide matrix composites (SiC_f/SiC) are known as materials with high-performance mechanical properties for the aerospace industry. Microwave-enhanced (ME) chemical vapour infiltration (CVI) heating of ceramic matrix composites is potentially an energy efficient production technique capable of yielding near fully dense SiC_f/SiC composites in a much shorter time span. This paper reports on the output of computational analysis of electromagnetic (EM) and thermal characteristics of the ME CVI process occurring with thin circular SiC fibre preform in a Labotron microwave system from SAIREM. Computer simulation is performed with the use of the finite-difference time-domain technique implemented in QuickWave computational environment. Multiple puzzling phenomena observed in the earlier experimental work are illuminated in the present study and the causes for the formation of microwave-induced temperature fields are clarified. With the use of the developed EM model, resonant and non-resonant frequencies of the Labotron system for different temperatures of the processed samples are analysed to explain the differences and variability in heating rates. This showed that when microwave processing of small SiC samples, energy coupling is extremely sensitive to frequency: a change of the reflection coefficient from 0.05 (absorbing) to 0.75 (reflective) could be made by a drift as small as 0.003–0.005 GHz, respectively, indicating the importance of scaling the microwave cavity to the sample size and the ability to precisely control the frequency of the microwave source. Moreover, energy coupling is temperature-dependent: low reflections produces very high heating rates (greater than 550 ℃ min^-1); the opposite is true for high reflections where heating rates are significantly slower. Temperature fields in the SiC fibre preforms are computed with the coupled EM-thermal model at different frequencies. It is shown that while being highly non-uniform in the beginning of the process, temperature patterns evolve to being fairly homogeneous by its end. Overall, the results suggest a means for better control of the equipment to pave the way to more efficient, controlled, and repeatable implementations of the ME CVI technology to produce high quality SiC_f/SiC composites.     

Design of double-layer ceramic absorbers for microwave heating

We propose a guide for designing double-layer ceramic absorbers in microwave heating by optimizing the thickness based on the analysis of reflection loss (RL) of a double-layer absorber consisting of a high-loss SiC layer and a low-loss Al₂O₃ layer. The calculated reflection losses for individual layers of SiC and Al₂O₃ show that the former with a thickness of 0.0054 m has the maximum microwave absorption while the latter in the thickness range up to 0.1 m is identified as a poor microwave absorbing material with RL larger than −0.4 dB. By using a 0.0054-m-thick SiC layer as the susceptor, the absorption in the Al₂O₃ layer and of the entire double-layer absorber increases significantly. The results demonstrate that high microwave absorption throughout the heating process can only be achieved in a sample with a small thickness in which a slight absorption peak shift during heating (less than one eighth-wavelength in the medium) occurs.     

Effects of the Susceptor Dielectric Properties on the Microwave Sintering of Alumina

Microwave sintering of alumina has been carried out using SiC and Y‐ZrO₂ based susceptors. The microstructure of the sintered samples has been rigorously compared and correlated to the heating behaviour of the susceptor used. It was found that the nature of the susceptor highly influences the sintering behaviour of alumina. The results show that at high temperatures, the electrical conductivity of the SiC susceptor tends to screen the incident electric field, thus resulting in a surface heating of the alumina sample material. When using ZrO₂ susceptor, the microstructure analysis of the sintered alumina samples reveals a volumetric heating, which is a signature of the microwave dielectric loss mechanism. This could be explained by the lower ZrO₂ electrical conductivity compared to the SiC one. The simulation results confirm this behaviour, particularly showing that in the presence of ZrO₂, the intensity of the electric field within the sample is higher than the one when SiC susceptor is used. Basically, the results of the simulation data are in good agreement with our experimental results. Although the SiC based susceptor is usually used in the microwave heating of materials, the ZrO₂ based susceptor presents numerous advantages over the SiC one, especially in term of direct microwave heating contributions.     

Ultrasonic-assisted preparation of complex-shaped ceramic cutting tools by microwave sintering

Complex-shaped ceramic tools were prepared by ultrasonic-assisted molding technology and microwave sintering. A uniaxial ultrasonic vibration compaction system was designed. The effects of ultrasonic vibration and molding pressures on mechanical properties and microstructures of A₂O₃ and Al₂O₃/SiC complex-shaped milling cutters were investigated by simulations and experiments. The results indicated that the density was distributed unevenly within the green ceramic tool samples, especially poor in the tool edges. These drawbacks were well improved by ultrasonic-assisted molding technology and microwave sintering. Applying ultrasonic vibration during the compaction process of green ceramic tool samples could improve the densification, hardness and reduce the randomness of mechanical properties of the sintered ceramic tool materials, especially for the difficult-to-sinter composite ceramics (with poor density). For A₂O₃ ceramic tool, fully dense microstructure was achieved, and for Al₂O₃/SiC ceramic tool, the cutting edges showed better density than that of the interior body, which was beneficial to improve the abrasion resistance of cutting tool.     

High-performance Al2O3 ceramics prepared by DCC-HVCI via microwave heating

A novel and rapid fabrication method for Al₂O₃ ceramics by the DCC-HVCI method via microwave heating was proposed. Effects of microwave heating temperature on coagulation time, micromorphology, as well as performance of the green body and ceramic sample were studied. As the microwave heating temperature rises, the coagulation time gradually reduced and compressive strength of green sample decreased while relative density and flexural strength of ceramics rose at the beginning and then dropped. The 50 vol.% Al₂O₃ suspension was coagulated and demolded after treating at 60°C for 800 s by microwave heating. The compressive strength of green samples reached 1.12 ± 0.13 MPa. The relative density of Al₂O₃ ceramic samples reached 99.39%. And the flexural strength of Al₂O₃ ceramics reached 334.55 ± 26.41 MPa. The Weibull modulus of Al₂O₃ ceramics reached 19. In contrast with the ceramic samples heated through water bath, the ceramic samples treated through microwave possessed uniform microstructures. Microwave heating could reduce the coagulation time by 77%. Meanwhile, it could significantly raise the compressive strength of green bodies by 65%. Additionally, it could increase the flexural strength of ceramics by 30%.     

Synthesis,Characterization, and Microstructure of ZrC/SiC Composite Ceramics via Liquid Precursor Conversion Method

The ceramic precursor for ZrC/SiC was prepared via solution‐based processing using polyzirconoxane, polycarbosilane, and divinylbenzene. The precursor could be transformed into ZrC/SiC ceramic powders at relative low temperature (1500°C). The cross‐linking process of precursor was studied by FT–IR. The conversion from precursor into ceramic was investigated by TGA, XRD. The ceramic compositions and microstructures were identified by element analysis, Raman spectra, SEM, and corresponding EDS. The results indicated that the ceramic samples remained amorphous below 1000°C and t–ZrO₂ initially generated at 1200°C. Further heating to 1400°C led to the formation of ZrC and SiC with the phase transformation of ZrO₂ and almost pure ZrC/SiC could be obtained upon heat‐treatment at 1500°C. During heat treatments, the ceramic sample changed from compact to porous due to carbothermal reduction. The ceramic powders with particle size of 100 nm~400 nm consisted of high crystalline degree ZrC and SiC phases, and Zr, Si, C were well distributed at the different sites in ceramic powders. The free carbon content was lowered to 1.60 wt% in final ZrC/SiC composite ceramics.     

The effect of microwave heating on the microstructure and the mechanical properties of reaction-bonded boron carbide

Reaction-bonded boron carbide composites were fabricated by both microwave (under Ar/10% H₂) and conventional heating (under vacuum or Ar/10% H₂). Silicon carbide (SiC) formation occurred in all cases and was slightly favored in the case of microwave heating under Ar/H₂. The resulting microstructures were influenced by the heating process and atmosphere; the SiC existed in the form of needles with conventional heating under vacuum. SiC small polygonal grains were present after microwave heating under Ar/H₂. Both the atmosphere and the electromagnetic field influence the SiC morphology. Despite this difference, the hardness and toughness of composites obtained by both heating techniques were similar.     

Heat transfer characterization of gas-liquid flows in a trickle-bed

Instantaneous local fluid-solid heat transfer coefficient (h_t) in a laboratory scale trickle-bed was measured using a constant-voltage anemometry technique. It was observed that convective heat transfer rate in the liquid-rich pulses was approximately 4 times that in gas-continuous bases for the air-water system. Time-averaged heat transfer rate was found to be positively influenced by both gas and liquid flow rates, with a stronger dependence on the latter. Heat removal efficiency, taking pressure drop penalty into account, suggested an optimum at intermediate liquid flow rate. Based on the measurements, a four-parameter heat transfer model featuring heat transfer coefficients in liquid-rich pulses (h_tp) and gas-continuous bases (h_tb), pulsing frequency and pulse fraction was developed to characterize transient h_t under various flow regimes. This model can be used in any trickle-bed reactor simulation that accounts for the dynamic interactions of catalytic reactions and heat transfer. It was found that while h_tp and h_tb correspond to liquid-solid and gas-solid heat transfer, respectively, and are determined mainly by the fluid properties, pulsing frequency and pulse fraction are the factors characterizing different flow regimes. Pulsing frequency, which can significantly impact reaction, may be tuned by selecting appropriate packing size, since smaller sizes generate higher frequency pulses. For example, a two-fold higher frequency was detected in packing as compared to that with packing. Flow regime evolution along the column axial location was identified visually, while the dispersed bubbling flow retreating to pulsing flow owing to gas bubble coalescence was evidenced by the heat transfer measurements.     

Influences of pre-forming on preparation of SiC by microwave heating

Silicon carbide (SiC) crystals were synthesized by microwave sintering using coal and tetraethoxysilane (TEOS) as raw materials. A sol-gel method was carried out to coat coal mineral particles with silicon dioxide (SiO₂). The mixed raw powders were pre-formed by uniaxial pressing into cylindrical pellets in dimension of ~ 30?×?3?mm². The pre-forming pressure was selected at 0?MPa, 1?MPa, 2?MPa, 3?MPa, 4?MPa and 5?MPa respectively, which led to different apparent density of the green pellets. The influence of apparent density of green pellets on microwave heating behavior was investigated. Different microwave thermal effects were analyzed. Techniques of XRD、SEM were carried out to characterize samples. It was found that pre-forming pressure showed crucial influences on microwave thermal effects and electric field (E-field) intensification. No SiC crystal could be formed without pre-forming pressure. Pre-forming pressure might be the prerequisite for synthesis of SiC by microwave heating. Five consecutive and indispensable heating stages including accumulation of residual air, microwave plasma generation, complex chemical reactions, nucleation and grain growth of SiC crystallites could be distinguished for samples under pre-forming pressure. Different pre-forming pressure leads to changes in heating behavior as well as morphologies of SiC crystals. ~ 4?MPa might be the optimized pre-forming pressure for both microwave plasma effects and E-field intensification.     

Fabrication of uniform 4H-SiC mesopores by pulsed electrochemical etching

In this letter, the uniform 4H silicon carbide (SiC) mesopores was fabricated by pulsed electrochemical etching method. The length of the mesopores is about 19 μm with a diameter of about 19 nm. The introduction of pause time (T_off) is crucial to form the uniform 4H-SiC mesopores. The pore diameter will not change if etching goes with T_off. The hole concentration decreasing at the pore tips during the T_off is the main reason for uniformity.     

Quasi-stable temperature of the steady state of microwave heated hematite

Microwave heated materials often reach a quasi-stable temperature resulting in thermal runaway. To control steady state in microwave processing, it is important to predict the quasi-stable temperature of the steady state. We demonstrated that the microwave heating behavior of hematite varies significantly with its initial temperature. In microwave heating, hematite samples could not be heated from room temperature, whereas hematite samples preheated to 410 °C or higher was heated to a temperature of 1020 °C. The microwave heating behavior can be accurately predicted by considering the steady-state energy balance.     

Replication of jute stem for porous cellular ceramics/composites in the SiO2–SiC–C System

The present work demonstrates that carbonaceous template preforms derived from jute stem by thermal and microwave processing can be infiltrated at room temperature under vacuum with silica sol, obtained by acid catalyzed hydrolysis of tetra ethyl orthosilicate (TEOS). The microwave processed carbon-preforms are better suited for exercising control of SiO₂ infiltration. The oven-dried hybrid C/SiO₂ material accumulates silica as SiO₂·nH₂O (n lying approximately between 1 and 3) and may be suitably converted, depending on the modes of processing, into macroporous cellular SiO₂ or SiC ceramics or SiC/SiO₂/C or SiC/C or SiC/SiO₂ composites with tailored composition and microstructures. The processes for synthesis of the specific products have been developed and their phase compositions and microstructures have been examined through characterization by bulk density measurements, X-ray diffraction and scanning electron microscopy.     

Separating Test Artifacts from Material Behavior in the Oxidation Studies of HfB2–SiC at 2000°C and Above

Oxidation characteristics of HfB₂‐15 vol% SiC prepared by field‐assisted sintering was examined at 2000°C by heating it in a zirconia‐resistance furnace and by direct electrical resistance heating of the sample. Limitations of the material and the direct electrical resistance heating apparatus were explored by heating samples multiple times and to temperatures in excess of 2300°C. Oxide scales that developed at 2000°C from both methods were similar in that they consisted of a SiO₂/HfO₂ outer layer, a porous HfO₂ layer, and a HfB₂ layer depleted of SiC. But they differed in scale thicknesses, impurities present, scale morphology/complexity. Possible test artifacts are discussed.     

Reaction-bonded B4C/SiC composites synthesized by microwave heating

The reaction-bonding technique was used to synthesize boron carbide (B₄C) - silicon carbide (SiC) composites by microwave heating. Preforms of porous B₄C were obtained by compaction followed or not by partial densification. Then, the material was infiltrated by molten silicon under a microwave heating. The influence of the thermal cycles (T: 1400-1500°C, t: 5-120 minutes) is low. The hardness of boron carbide is comparable to that of alumina (15-19 GPa) for a much lower density (≈2.5 g/cm³ for B₄C-based material instead of 3.95 g/cm³ for alumina). These properties make this composite, obtained by microwave heating, a good candidate for ballistic applications.     

Microwave sintering of dense and lattice 3Y-TZP samples shaped by digital light processing

Nowadays it is possible to produce ceramic parts with solid and complex shapes with rapid and efficient shaping and sintering techniques. In this paper, 3mol% yttria stabilized zirconia (3Y-TZP) dense and lattice parts were shaped by Digital light processing method (DLP) and densified by conventional (CV) and microwave (MW) sintering. 3Y-TZP samples were MW sintered up to 1550 °C with different heating rates (10, 30, and 50 °C/min) for the dense samples and 30 °C/min for the lattice samples. Controlled thermal cycles with a homogenous heating and no thermal runaway was reached. CV sintering was carried out at 10 °C/min up to 1550 °C. No inter-layer delamination was detected after sintering by the two methods. Both dense and lattice MW-sintered samples reached high final densities (equivalent to obtained values with CV-sintered samples, i.e., ≥98% T.D.), but exhibited a lower average grain size than CV-sintered materials. The different architectures between dense and lattice samples resulted in a different specific absorbed power: the power absorbed by the dense sample is lower than that absorbed by the lattice one meaning that this sample architecture heats up easily.     

Microwave-assisted sintering of dental porcelains

An investigation was made of hybrid microwave-assisted sintering of dental porcelains, using five commercial ceramic frits employed in the production of dental porcelains. The powders were characterized, transformed into prismatic test specimens, and subjected to conventional and microwave sintering. Microwave sintering was performed at a frequency of 2.45 GHz, using a susceptor material and in the absence of vacuum. The apparent density and apparent porosity of the sintered samples were characterized based on the Archimedes principle. They were also analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), and their flexural strength and microhardness were determined by the Vickers method. The powders, which showed a broad particle size distribution with a high fraction of particles of dimensions larger than 30 μm, were composed of amorphous phase and leucite particles. Microwave sintering yielded ceramic bodies whose apparent porosity (t-test, p<0.05) was the same or very similar to that of the conventionally sintered samples, while the apparent density (t-test, p<0.05) of most of the microwave sintered samples was the same or slightly lower. Although the microwave sintered samples showed larger average pore sizes (t-test, p<0.05), four of the five samples used in this study showed the same flexural strength (t-test, p<0.05) and all the ceramics under study showed the same surface microhardness (t-test, p<0.05).     

Highly-efficient preparation of anisotropic ZrB2–SiC powders and dense ceramics with outstanding mechanical properties

Highly-dense ZrB₂–SiC ceramics with excellent mechanical properties including Vickers hardness of 24.5 GPa and fracture toughness of 4.8 MPa/m^1/2 were successfully prepared, by spark plasma sintering of the raw powders synthesized by a novel molten-salt and microwave co-assisted boro/carbothermal reduction (MSM-BCTR) method. Compared with the processing conditions required for synthesizing ZrB₂–SiC by conventional reduction method, the present MSM-BCTR method possessed a variety of significant merits including the smaller material cost, lower processing temperature (1200°C), and remarkably higher efficiency (soaking time as short as 20 minutes). More importantly, the ZrB₂–SiC powders, resultant from MSM-BCTR treatment, were verified to have single-crystalline nature and uniform well-grown anisotropic morphologies (rod-like ZrB₂ and sheet-like SiC) as well as great potential in promoting the mechanical properties of their bulk counterparts. This great achievement was mainly ascribed to the specific MSM-BCTR conditions characterized by microwave heating and molten-salt medium.     

Thermal stability under laser heating of hot-pressed (Hf1-X ZrX)B2/SiC powder mixtures obtained by mechano-synthesis

The paper studied ultra-refractory (Hf_1-X Zr_X)B₂/SiC ceramics fabricated by hot-pressing (HP) of mechano-chemically assisted precursors synthesis, and tested their thermal stability under laser heating. Fully dense materials were obtained after HP. Microstructures of the sintered materials were analyzed by XRD and SEM-EDS, while 4-pt flexure strength in air at room temperature up to 1773 K was measured. The thermal stability was tested using a diode laser source. A SiC-free fully dense ZrB₂ ceramic was used as benchmark to identify the potential of the laser heating technique and separate the effects related to the addition of SiC into a diboride ceramic matrix. Infrared thermo-camera and 2-color pyrometer provided the real-time variation of the surface temperature vs time. For surface temperatures below 1900 K reached by the SiC-containing samples, SiC acted as a key player and provided excellent protection to the diboride matrices against oxidation: extensive post-test microstructural analyses documented this response.     

Conventional and microwave-heated oxygen pulsing techniques on metal-doped activated carbons

Conventional and microwave-heated oxygen pulsing techniques on metal-doped activated carbons to achieve a controlled meso/micropore structure were investigated. The gas pulsing experiments consisted of repeated cycles. Each cycle consists of an oxidising stage, under O₂/Ar atmosphere and constant temperature, and a burn-off stage, under N₂ atmosphere at variable temperature (heating and cooling). The porosity of the carbons was analysed by nitrogen adsorption at ?196 °C. Two different activated carbons, Cu-doped BPL (BPL-Cu) and ASC, were used as raw materials. The commercial activated carbon ASC showed higher reactivity towards O₂, due to the catalytic effect of the metals (mainly, Cu and Cr) that are on the carbon surface and their better dispersion. After several pulses, ASC underwent a moderate increase in the micropore volume and a significant increase in mesopore volume. BPL-Cu showed a higher increase in microporosity than mesoporosity, giving rise to a meso/micropore volume ratio lower than that of the original BPL-Cu. Oxygen pulsing technique was carried out in a conventional furnace and in a microwave oven. Conventional and microwave-heated oxygen pulsing on ASC yielded similar textural development. However, the time required under microwave heating was remarkably reduced respect to conventional heating (around 2.5 times less), which suggests that microwave-heated oxygen pulsing technique would be an interesting alternative to conventional activation.