By control of the nanosecond pulsation, energy input, and flow, it is possible to achieve commercial-level hydrogen peroxide (H2O2) concentrations using only water and plasma in a continuous process while minimizing thermal degradation. Time-resolved ultrafast Optical Emission Spectroscopy was employed to observe the formation of reactive species, shedding light on the underlying mechanisms. This study also found that thermal degradation has a critical role, which was effectively managed through quenching of the plasma zone. A parametric scan of pulse duration and pulse repetition frequency of the microwave power showed a significant influence on H2O2 formation, whereby the mean power also plays an important role. Additionally, the H2O2 concentration was found to be inversely proportional to the water flow rate. A maximum concentration of 0.17 wt % was achieved with 1.2 g/kWh based on the absorbed power at a flow rate of 0.2 mL/min. This plasma reactor technology shows promise for further development as a decentralized solution for the green chemical synthesis of H2O2.

Microwave heating is widely used in daily applications but is fundamentally limited by non-uniform temperature distribution. Despite extensive efforts to manipulate energy distribution around materials, achieving uniform heating remains elusive due to the intrinsic inhomogeneity of electromagnetic fields. Here, we report a self-regulating solution that adaptively modulates the absorbance distribution using a Negative Temperature Coefficient (NTC) Metamaterial Absorber (MA). Distinct from electromagnetic field-shaping strategies, our approach intrinsically suppresses overheating in high-temperature regions and redistributes energy to cooler areas. We demonstrate, for the first time, that uniform heating-quantified by over 90% reduction in the coefficient of variation-can be achieved across diverse configurations, including planar, polyhedral, curved, and multiple objects, as well as under power variations spanning two orders of magnitude. This work not only provides a theoretical resolution to the longstanding challenge of non-uniform microwave heating but also opens new avenues for development and application of temperature-adaptive metamaterials.

Fused filament fabrication (FFF) based 3D printing of continuous carbon fiber-reinforced thermoplastic composites (C-CFRTP) has emerged as a new manufacturing method and offered new avenues for the design and fabrication of complex composite structures. To ensure printing quality and improve mechanical performance of composites, the prevailing approach is producing pre-impregnated filaments before 3D printing. However, current manufacturing methods of filaments still rely on traditional pultrusion approaches, necessitating high facility investment and resulting in elevated prices. Here, we propose a low-cost C-CFRTP filament manufacturing method utilizing a two-step impregnation process. To ensure high filament quality while maintaining compactness, we introduce a liquid-solid two-step impregnation method aimed at flattening carbon fiber bundles during impregnation to achieve thorough and uniform infiltration. We successfully fabricated carbon fiber reinforced polyamide (PA) prepreg filaments and analysed its impregnation and mechanical properties. This study holds significant implications for enhancing impregnation efficiency and reducing the cost of C-CFRTP.

Filament winding is a widely employing technique for manufacturing carbon fiber reinforced polymer composite. However, the low winding speed of traditional filament winding methods hinder the improvement of production efficiency. In this study, a novel microwave-assisted high-speed winding method for the manufacturing process of continuous carbon fiber reinforced thermoplastics has been developed. The apparatus is established, and the maximum winding speed is measured based on experimental and finite elements calculations. The correlation between microwave power, winding speed and performance is investigated.

Estimating the effective permittivity of anisotropic fibrous media is critical for advancing electromagnetic applications, requiring detailed microstructural and orientation analyses. This study introduces innovative approaches for disclosing the orientation and microstructure of fibers, leading to mixing relations. It particularly focuses on two specific fiber configurations: 1. wave-curved fibers and 2. a collection of interconnected fibers. The first approach uses sinusoidal wave fibers, considering their curvature and direction. Conversely, the approach for the interconnected fibers operates on the principle of representing fibers as a collection of straight segments. Investigations on fibrous media for both approaches were performed using numerical calculations at the microwave frequency of 2.45 GHz. Each fibrous medium was treated as an effective medium by using fibers significantly smaller than the microwave wavelength. A thorough comparison was made between the proposed mixing relations, numerical data, and state-of-the-art mixing relations to assess their consistency and validity. The comparison of the proposed approaches with traditional models shows an improved accuracy of up to 70% and 8% for the real and imaginary components of the permittivity, respectively. Additionally, the root-mean-square errors were determined as 0.001 + j0.003 and 0.001 - j0.007 for the sinusoidal and interconnected straight fibers approaches, respectively. In addition, a woven alumina fabric was used to compare the experimental resonance frequency with that from simulations using the permittivity of the fabric estimated by the interconnected straight fibers approach. These findings advance the predictive accuracy of permittivity estimation in fibrous media, providing a robust foundation for engineering applications.

The influence of the native oxide films, which is essential for the understanding of the microwave heating of metal powder, is analyzed numerically based on the experimental data on temperature, dilatation and resistance of compacted copper powder samples. The indicative thickness of the oxide films, determined from the resistivity data using an effective medium approximation, is shown to decrease from ≥ 1 μm to below 1 nm. The effective complex dielectric permittivity and magnetic permeability of the copper powder with oxide films on the particles are calculated within the recently developed models. The dielectric permittivity exhibits a percolation behavior in the temperature range of the oxide decomposition, viz. 210 – 250 °C when microwave heating is carried out in nitrogen and 330 – 375 °C – in argon. The efficiency of absorption of the incident 30 GHz microwave radiation in a slab of powder is assessed separately for the electric and magnetic-type losses, the contribution of the latter being predominant until the percolation transition. The energy flux density of the incident microwave radiation required to sustain the prescribed heating rate is shown to increase during the microwave heating process from ∼ 20 W/cm2 to > 1 kW/cm2 due to increasing reflection. The additional microwave energy input required for the completion of the endothermic oxide decomposition reaction is shown to be significant at the initial stage of the process (below 200 °C).

Microwave-assisted 3D printing based on the fused filament fabrication (FFF) method is an emerging technology to print lightweight continuous carbon fiber reinforced thermoplastics (CCFRP) at high speed. Different from traditional FFF, microwave offers selective and volumetric heating properties to melt thermoplastic materials instantaneously, and the microwave printing head and nozzle remain at room temperature. These advantages can increase the printing speed of CCFRP significantly, while the belt slippage of a printing bed is noticed during microwave-assisted 3D printing. The slippage of the moving belt happens because the cold nozzle moves at high speed and encounters resistance from the rough surface of the printed filament. To solve this problem, this paper presents a hierarchical digital twin (DT) framework, consisting of core and basic DTs, for the prevention of belt slippage induced printing malfunction. The core DTs use MATLAB Simscape multibody models to simulate the printing process virtually and the printing G-code is corrected before printing. In addition, an accelerometer installed on the printing bed is connected to the core DTs. Abnormal vibration signals due to belt slippage can be measured and communicated with core DTs for interrupting and correcting the process. By monitoring the temperature of the heated filament and the output microwave power, the service life of the nozzle and microwave cable are evaluated in the basic DTs.

The aim of this work was to study the densification kinetics and to evaluate the electrical resistivity of hematite nanopowders (30 nm) during sintering by high-frequency microwave dilatometry (30 GHz) in multimodal cavity, as the purpose of providing new reference data on the kinetic behavior of the densification of high dielectric loss ceramic materials when subjected to microwave sintering, which are few in the literature to date. To analyze the densification kinetics, non-isothermal methods were used, such as the classic models of Woolfrey-Bannister and Dorn for the initial stage of sintering, and the model of Wang-Raj for the intermediate stage, both at heating rates ranging from 5 to 20°C/min. The results show an extreme very low activation energies, both for the initial stage (39–66 kJ/mol) and for the intermediate stage (68 kJ/mol), proved to be sufficient for material densification with less grain growth, compared to lower frequencies (2.45 GHz). In addition, the in situ resistance measurements revealed a decrease in electrical resistivity as a function of material densification, as well as a semiconductor behavior of the sintered hematite.

During cold start of vehicles with gasoline combustion engines, conversion of pollutants in the exhaust gas to inert products is very low due to low catalyst temperature. Only above the light-off temperature, significant conversion can be achieved. Previous strategies to reduce cold-start emissions have been focused on developing catalysts with a low light-off temperature. Electric catalyst heating systems have also been discussed repeatedly. A disadvantage of such systems is the required volume flow through the catalyst, which is necessary for heat transfer to the catalyst. In contrast, microwave-assisted heating allows direct introduction of thermal power into the catalyst due to dielectric losses of the catalyst materials. This work analyses simulation-based the influence of the material on the heatability by microwaves. The focus is on the substrate materials rather than the catalytically active coatings, since the substrate represents the part in the TWC where most of the dielectric losses occur. For this purpose, the temperature-dependent dielectric material properties of cordierite and silicon carbide (SiC) are investigated. The determined material properties are then transferred to a simulation model that calculates heat distribution and heat insertion based on the electromagnetic field distribution. The heat propagates better throughout the monolith due to the higher thermal conductivity of SiC compared to cordierite. In summary, SiC leads to a homogeneous heating of the entire catalyst material. The fact that dielectric losses of SiC decrease with temperature may help to self-limit the catalyst temperature.

Carbon-fiber-reinforced plastics (CFRPs) are of increasing popularity in a wide range of applications, and microwave curing promises significant reduction in processing times. However, for the design of an efficient microwave curing system, the composites' effective material parameters must be known. This work presents a measurement system using a wall perturbation approach with a coaxial cavity to determine the effective conductivity of a CFRP along the fiber direction.

Near-field focus (NFF) antennas have been recently used in several applications for different purposes. In this work, the time-reversal (TR) concept is used to shape the phase distribution of the phased array elements for the NFF of the electromagnetic field strength. It is shown that the TR concept is equivalent to the known ray optic method for the NFF. A slotted waveguide phased array antenna operating at 5.8 GHz is designed to provide the maximum electric field strength at the near-field region of the phased array. It is shown that the application of the full-wave simulation allows for an antenna design that provide high strength of the electromagnetic field and sufficient steerability even at near-field conditions close to the phased array antenna.

When using the statistical inversion framework in microwave tomography (MWT), generally, the real and imaginary parts of the unknown dielectric constant are treated as uncorrelated and independent random variables. Thereby, in the maximum a posteriori estimates, the two recovered variables may show different structural changes inside the imaging domain. In this work, a correlated sample-based prior model is presented to incorporate the correlation of the real part with the imaginary part of the dielectric constant in the statistical inversion framework. The method is used to estimate the inhomogeneous moisture distribution (as dielectric constant) in a large cross section of polymer foam. The targeted application of MWT is in industrial drying to derive intelligent control methods based on tomographic inputs for selective heating purposes. One of the features of the proposed method shows how to integrate lab-based dielectric characterization, often available in MWT application cases, in the prior modeling. The method is validated with numerical and experimental MWT data for the considered moisture distributions.

Ultrafast pulsation of microwave power for CO2 conversion using plasmas is a mean to improve the efficiency of the process. Nevertheless, the fundamental phenomena involved need deeper understanding in order to design optimal plasma based devices. Therefore, detailed parametric scans of the plasma torch performance are per-formed with plasma diagnostics to unravel the underlying mechanisms limiting the CO yield. Very short pulsed plasmas have low CO2 conversion because of the energy cost needed to generate the plasma itself. For power pulses longer than 2-3 mu s, excess energy is spent in gas heating up to 7000 K. Few mu s (both ON and OFF times) have the best efficiency and gas temperatures of about 3000 K are measured at the beginning of the pulse. Power modulation and appropriate gas flow residence times allow dissociating CO2 also in the power-OFF phase and therefore to optimize the efficiency of the process. 2D cylindrical symmetric simulations of the plasma torch give insight in the gas flow dynamics and estimation for a gas residence time in the plasma volume. The gas in the regimes with OFF times close to or longer than the residence time leads to under-processing of the CO2 flow. The plasma is destabilized by the gas flow itself depending on pulsed regime. The combination of capacitive coupling for ignition (confirmed by frequency harmonics generation) and inductive power absorption lead to complex plasma dynamics.

This paper proposes a new approach to relate the effective thermal conductivity of open-cell solid foams to their porosity. It is based on a recently published approach for estimating the dielectric permittivity of isotropic porous media. A comprehensive assessment was performed comparing the proposed mixing relation with published experimental data for thermal conductivity and with numerical data from state-of-the-art relations. The mixing relation for the estimation of thermal conductivities based on dodecahedrons as building blocks shows good agreement with experimental data over a wide range of porosity.

Microwave-based dielectric heating is a suitable method for energy- and time-efficient processes. Considering the energy required in the production of carbon fibers, it is evident that microwave-based dielectric heating during the different phases of the production needs to be considered too. Nevertheless, the dielectric properties of the processed material needs to be known for the design of an appropriate microwave applicator. When looking at the first stage in the production, the stabilization stage of the PAN fiber, the important data about the dielectric properties is very limited in literature. For this reason, first in-situ temperature-dependent measurements of the dielectric properties during the stabilization stage are presented. The impact of raising temperatures and chemical reactions on the dielectric properties of the heated PAN fiber is discussed. Secondly, the steps taken to set up the reaction kinetics from the dielectric loss point of view are given. This enables determination of the reaction degree as a function of the measured dielectric loss for the first time. The established correlation opens the potential for the application to processes such as an in-situ quality determination. The strong temperature impact on the process is shown, and reaction kinetics are analyzed accordingly. In a final third step, a heat transfer model is presented. It utilizes the evaluated reaction kinetics data and microwave heating, creating a first modelling approach for monitoring and controlling the desired fiber temperature, leading towards an online process.

Generating a rigorous, description of the effective permittivity of anisotropic media requires an appropriate consideration of its microstructural geometry and orientation. In this study, we use anisotropic geometrical models made of filaments along with electromagnetic wave propagation calculations to investigate the effect of filament orientation on the effective permittivity. As a result, we have identified a correlation tensor parameter that, in conjunction with the filament orientation tensor provides efficient and reliable estimates of the effective permittivity of filamentary structures with solid volume fractions below 10 %.

3D printing technologies offer attractive solutions for manufacturing complex microwave components for high-power applications, such as satellite antennas. Metal additive manufacturing usually requires expensive power and printing equipment, and the components are very heavy. How to produce lightweight microwave components in a fast and low-cost way is a challenge that needs to be solved. In this paper, we present a novel method to manufacture microwave filters by electroplating 3D printed thermoplastics and thermoset materials. The design concept of generating slots for better electroplating of the inner walls of microwave components is introduced. Since the slots do not cut the current flow, microwave field distribution is not influenced. Filter structures based on 3D printed plastics, coated with copper skin are produced. The coaxial filters with or without slots are studied, and a comparison with CST results is conducted to understand the benefits and limitations of this technology.

This paper presents the integration of electrical capacitance tomography (ECT) with a moisture controller for the microwave drying of polymer foam. The proportional–integral (PI) control and the linear quadratic Gaussian (LQG) control are employed in designing the controller. The control objective in this process is that the moisture of polymer foam after the drying process reaches the desired set point. The permittivity distribution of polymer foam after the drying process is estimated in real-time using a designed ECT sensor and transferred as feedback to the controller. Since the permittivity and the moisture are strongly correlated, the material moisture can be controlled by controlling the permittivity. A state-space model is derived for the microwave drying process based on a system identification approach using the experimental data from the process. The derived model is employed in designing the LQG controller and adjusting the parameters of the PI controller. The designed controllers are implemented on a testbed microwave oven, and the experimental results show that the designed controllers are able to follow the desired set point moisture. The performance of the system with both controllers is compared, and their advantages and disadvantages are discussed. Moreover, the benefits of having a moisture controller for the microwave drying process are shown in simulation studies compared to an uncontrolled system.

In many energy and process engineering systems where fluids are processed, droplet-laden gas flows may occur. As droplets are often detrimental to the system's operation, they need to be removed. Compact engineering solutions for the removal of entrained droplets are difficult to achieve with conventional flow control and heat transfer approaches and thus droplet removal devices are hence often costly and bulky. In this study, we analyzed the potential of a compact technology based on droplet capture and in situ evaporation by microwave heating. For that, we designed a microwave applicator containing a porous droplet separator for capturing and evaporating droplets. The application of open-cell ceramic foams as filter medium reduced 99.9% of the volumetric flow of droplets, while additional microwave exposure increases reduction to 99.99%. In addition, microwave-heated foams prevent droplet re-entrainment and structure-borne liquid accumulation within foams, thus avoiding water clogging and flooding.

Microwave tomography (MWT)-based control is a novel idea in industrial heating systems that demands fast data acquisition (DAQ) and real-time imaging algorithms. Uniform diffraction tomography (UDT) is one such technique that can provide real-time imaging. However, its single-input single-output data-based inverse scattering formulation can lead to time-consuming DAQ. In this article, a multistatic uniform diffraction tomography (MUDT) imaging algorithm is proposed for a fixed array MWT system. The MWT system is integrated into the industrial heating unit HEPHAISTOS to estimate the moisture distribution in a polymer foam. In addition, a technique is presented to retrieve the electrical properties of the targets using reconstructed information from MUDT and by investigating the singular values of multistatic scattering data. Through numerical and experimental data for the considered moisture scenarios, the MUDT approach is tested, and its comparison with the UDT approach is shown. Reconstructed results show that in comparison to UDT, the MUDT approach: 1) eliminates the need for mechanical scanning; 2) provides aliasing-free images by following Nyquist sampling criteria; and 3) can resolve multiple targets in the imaging media with significant improvement in the spatial resolution that further augments in the correct retrieval of the dielectric constants of the target.