Flexible and efficient direct-insertion microwave heating with a dielectric wedge

Nowadays, microwave heating has evolved into a common tool for chemists based on its numerous advantages over conventional conductive heating. Surprisingly, the efficiency of microwave-assisted heating is still rather unexplored. In this contribution, we report our investigations concerning the heating efficiency of a variety of solvents including polar and apolar substances. Moreover, the effects of adding salt or passive heating elements on the microwave heating efficiency will be addressed. Finally, the heating efficiency of demineralized water is discussed at different volumes and with different microwave power levels in both monomode and multimode microwave synthesizers, demonstrating maximum average heating efficiencies of 10% for small-scale vessels (5 mL), 20% for medium-scale (50 mL), and 30% for large-scale microwave heating (400 mL).

Variations in load shape, volume, and permittivity significantly impact microwave energy conversion efficiency within multimode heating cavities. This study introduces a wedge dielectric plate that enables asymmetric microwave propagation when being in contact with a load, thereby enhancing heating efficiency and robustness under load variations. First, based on the transformation optics theory, an asymmetric waveguide is established by the wedge dielectric plate. Experimental results show varying microwave transmission efficiencies in both forward and reverse directions. Second, a microwave multimode cavity with a dielectric substrate is designed. When the dielectric wedge, dielectric substrate, and load are in physical contact, an efficient microwave transmission path will be formed to enable directed propagation toward the load via surface waves. Simulation results show over 90% heating efficiency with loads of varying volumes, dielectric properties, and shapes. Finally, microwave heating experiments are conducted and compared with a domestic oven using different loads. The proposed method achieves approximately 40% higher temperature increase at 750 W. This method is advantageous due to its low manufacturing cost, high power, robustness, and versatility, making it highly suitable for industrial applications.

Microwave heating mechanism and Self-healing performance of scrap tire pyrolysis carbon black modified bitumen

This paper proposed a method based on transformation optics, implicit function, and level set methods to solve the challenge of multi-physics simulation of a microwave heating cavity with two different motion modes. A 3D computation model with a rotating turntable, a lifting support rod, and a sample is proposed as a detailed demonstration. Based on the theory of transformation optics, the rotating turntable is surrounded by two circles with a time-varying, inhomogeneous and anisotropy medium, and the electric field in the moving region is rotated by controlling the two mediums. The implicit function and level set methods compute the lifting motion by setting the properties of the lifting region as a function of space and time. The correctness of the proposed method is verified by comparing the proposed method’s results with the discrete position’s results, and then its accuracy is further verified by experiment. Subsequently, compared with the implicit function and level set methods only, the proposed method is more accurate. Finally, the effects of lifting motion, rotating motion and lifting motion (i.e., spiral motion) on microwave heating uniformity and heating efficiency were analyzed, respectively.

The contamination of soils by mineral oils presents a significant environmental challenge, particularly due to the widespread use of petroleum products in various industrial sectors. This study investigates the efficiency of microwave heating compared to conventional resistance heating for the thermal desorption of mineral oil from contaminated soils. The experimental results demonstrated that microwave heating offers superior performance in terms of pollutant removal efficiency, energy consumption, and the preservation of soil’s physical and chemical properties. This study further conducted a kinetic analysis of the desorption process, revealing that microwave heating follows a first-order kinetic model and requires lower activation energy than conventional methods. The findings suggest that microwave-assisted thermal desorption is a highly effective and energy-efficient technology for soil remediation, providing a potential alternative to conventional techniques. This research offers valuable insights into optimizing thermal desorption processes for environmental remediation, with implications for broader applications in soil treatment.

For the desorption of methane, thermal stimulation is an alternative to develop coalbed methane (CBM) when it is difficult to reduce the formation pressure. Microwave heating is a promising thermal method to increase the gas recovery of CBM especially for the CBM reservoir with high heterogeneity and low water content. The current study aims to establish a fully coupled numerical model to understand the enhanced gas recovery (EGR) mechanism of CBM under microwave heating. In the study, the CBM reservoir model was developed first. Then a mathematical model considering electromagnetic excitation, heat transfer, mass transfer, Langmuir adsorption, and fluid flow was built. Moreover, some important parameters were set as temperature-dependent to achieve the coupling effects among the multiphysics. Based on the above two models, a fully coupled electromagnetic-thermal-hydraulic-mechanical model was solved by the finite element, so that the distributions of electric field, reservoir temperature, methane concentration were able to be investigated. Finally, a sensitivity analysis including water content, microwave power and microwave heating mode was done based on the heating efficiency and EGR. Under microwave heating, the electric field distributes near the microwave heater with the maximum electric intensity as 1.07×103 V/m. The high electric intensity and low thermal conductivity easily enables microwaves to generate the required temperature region within CBM reservoir, so that 200 W power was applied to continuous heat the formation. Under 1 day, the maximum temperature of CBM reservoir increased to 81 °C, enabling the desorption of methane. Moreover, heating efficiency is controlled by the dielectric properties as well as electric field intensity of the CBM reservoir, although the existence of water content increases the dielectric constant within the CBM reservoir. In addition, by setting the temperature-dependent properties, microwave heating shows the ability to induce the pore volume changes by generating thermal stress, so that the porosity and permeability of CBM reservoir near the heater increase from 0.15 to 0.24 and from 0.36 mD to 1.47 mD, respectively. Based on the above positive effects of microwave heating, the CBM recovery could be significantly enhanced. Finally, in order to transfer the heat deeper into the reservoir, the feasibility of stepwise microwave heating mode has been successfully proven based on the temperature distribution within the CBM reservoir. In the study, microwave has showed great potential in enhancing the CBM recovery resulting from its high heating efficiency and pore induction effect. The results presented in this paper can provide comprehensive guidance for the optimization of microwave heating parameters.

Low-cost, high-density Mn–Co spinel coatings for stainless steel interconnect via efficient microwave heating

Decoupling microwave heating efficiency from food property variations: A reduced-gradient index metamaterial strategy for superior efficiency and uniformity

High entropy alloy nanoparticles (HEA-NPs) are reported to have superior performance in catalysis, energy storage, and conversion due to the broad range of elements that can be incorporated in these materials, enabling tunable activity, excellent thermal and chemical stability, and a synergistic catalytic effect. However, scaling the manufacturing of HEA-NPs with uniform particle size and homogeneous elemental distribution efficiently is still a challenge due to the required critical synthetic conditions where high temperature is typically involved. In this work, we demonstrate an efficient and scalable microwave heating method using carbon-based materials as substrates to fabricate HEA-NPs with uniform particle size. Due to the abundant functional group defects that can absorb microwave efficiently, reduced graphene oxide is employed as a model substrate to produce an average temperature reaching as high as ∼1850 K within seconds. As a proof-of-concept, we utilize this rapid, high-temperature heating process to synthesize PtPdFeCoNi HEA-NPs, which exhibit an average particle size of ∼12 nm and uniform elemental mixing resulting from decomposition nearly at the same time and liquid metal solidification without diffusion. Various carbon-based materials can also be employed as substrates, including one-dimensional carbon nanofibers and three-dimensional carbonized wood, which can achieve temperatures of >1400 K. This facile and efficient microwave heating method is also compatible with the roll-to-roll process, providing a feasible route for scalable HEA-NPs manufacturing.

The interaction of incident microwaves with heterogeneous catalysts presents unique opportunities for selective and energy-efficient heating in catalytic and materials processes. Recently, microscopic selective heating phenomena have emerged as a promising strategy to revolutionize microwave-assisted systems by enabling targeted energy absorption and improved control over local reaction environments. However, since the interaction between catalysts and microwaves is highly dependent on the temperature, catalysts that induce such localized heating at elevated temperatures do not necessarily absorb microwave energy efficiently at room temperature. Typically, catalysts that do not absorb microwaves well at room temperature require the addition of microwave susceptors such as SiC to initiate heating. However, under high-temperature reaction conditions, these susceptors can interfere with the efficient delivery of microwave energy directly to the catalyst. In this work, to propose a new material heating method that leverages structural transformations during microwave irradiation, the structural transformation of indium oxide (In2O3) deposited MFI-type zeolites under H2 condition was investigated. At lower temperature (<200 °C), In2O3 played a role of moderate microwave absorber. Once the sample reached 200 °C, In2O3 was reduced to form In+ cations, which migrate into zeolite pores via reductive solid-state ion exchange. This results in a sharp increase in heating efficiency, reaching 500 °C with only 40 W of microwave power. In contrast, thermally pretreated samples showed minimal heating under the same conditions. These findings reveal that In+ cations act as atomic-scale microwave hotspots at elevated temperatures, potentially applicable to thermally-effective catalysis. This discovery provides a new strategy for the design of practical catalysts tailored for microwave heating, enabling more efficient and selective catalytic processes without relying on external susceptors.

An efficient synthesis of various N-substituted 3-aminoindazoles using Buchwald-Hartwig C-N coupling reaction is described. Several parameters were varied, including the nature of the halogen atom and the protecting group of the starting materials, as well as the effects of the catalyst system, base, solvent, and reaction time. The efficiency of microwave versus conventional heating was also compared to test the outcome of the reaction. Thus, by applying this recent knowledge about metal-catalyzed aminations, an alternative for the direct synthesis of primary 3-aminoindazoles has been provided.

In order to achieve a significant size reduction to get ultrasmall upconverting nanoparticles (UCNPs) following a thermal coprecipitation pathway, we identified two critical points: the UCNP precursor mixing and high-temperature heating steps. Significant differences could be observed according to the way the inorganic sodium and fluoride sources were mixed to the rare-earth oleate before the high-temperature heating step. More interestingly, accurate monitoring of the high-temperature heating step using microwave (MW) dielectric heating yielded major improvement toward ultrasmall UCNPs. Thus, hexagonal, Tm-doped sub-5-nm UCNPs with an unusual Na(Yb-Gd)F4 matrix with 53% Yb were produced, displaying satisfactory luminescence. Noticeably, MW heating was achieved in a weakly MW-absorbing oleic acid (OA)/octadecene mixture, and the influence of the OA content composition on the MW heating efficiency is discussed in this report.

Microwave heating based on two rotary waveguides to improve efficiency and uniformity by gradient descent method

Multi-physical field coupling modeling of microwave heating and reduction behavior of zinc oxide

This study designs a metal surface movable-microwave heating cavity with a turntable to choose an efficient and uniform microwave heating method according to the shape of the ceramic. The movement of the metal surface affects the standing wave in the cavity, causing the electric field in the cavity to change constantly during heating due to the rotating turntable and metal surface motion. This results in the absorption mode of ceramic is not constant. However, it is challenging to mesh the dynamic position of ceramic; therefore, this study proposes a transform optics method to simulate the rotating motion of the turntable and the moving motion of the metal surface. Experiments verify the correctness of the simulation model. The heating effects of fixed heating, turntable rotating and metal surface moving are compared. Finally, the effects of microwave heating on ceramics at different speeds and moving rates are discussed.