Microwave Transparent Material Research Articles

Artificial Intelligence Applied to Microwave Heating Systems: Prediction of Temperature Profile through Convolutional Neural Networks

Microwave heating, which is caused by the interaction of electromagnetic radiation and materials, has become an important component in industrial operations across numerous industries. Despite their importance, conventional numerical simulations of microwave heating are computationally intensive. Concurrently, advances in artificial intelligence (AI), particularly machine learning algorithms, have transformed data processing by increasing accuracy while decreasing computational time. This study tackles the difficulty of efficient and accurate modelling in microwave heating by combining convolutional neural networks (CNNs) with traditional simulation techniques. The major goal of this research is to use CNNs to forecast temperature profiles in a variety of industrial materials, including susceptors, semi-transparent, and microwave-transparent materials, under varying power settings and heating periods. This unique strategy greatly reduces prediction times, with up to 60-fold speed increases over standard methods. Our research is based on examining the electromagnetic and thermal responses of these materials under microwave heating. This study’s findings emphasise the need for extensive datasets and show the transformational potential of CNNs in optimising material processing. It uses artificial intelligence to pave the way for more effective and exact simulations, supporting breakthroughs in industrial microwave heating applications.

Microwave-Assisted Flow Chemistry for Green Synthesis and Other Applications

Abstract: Using combined microwave-assisted flow chemistry approaches is one of the most active areas of microwave chemistry and green synthesis. Microwave-assisted organic synthesis (MAOS) has contributed significantly to developing green synthetic methods, while flow chemistry applications are quite popular in industrial chemistry. The combination of the two has farreaching advantages. In early studies, the flow chemistry concept was applied in domestic microwave ovens already indicating strong potential for future applications. The relatively small diameter of the flow reactors can address the limited penetration depth of microwaves, which is a major impediment in large-scale batch reactors. With the commercial availability of dedicated microwave synthesizers with tunable frequencies and better temperature control, the possibilities to apply flow synthesis grew even broader. The developments focus on several issues; the two major ones are the design and application of reactors and catalysts. Common reactor types include microwave- absorbing, such as silicon carbide, and microwave-transparent materials, such as borosilicate glass, quartz, or Teflon, with the catalyst or solvent adjusted accordingly. Several heterogeneous catalysts are considered strong microwave absorbers that can heat the reaction from inside the reactor. Such materials include clays, zeolites, or supported metal catalysts. Here, the major advances in design and applications and the benefits gained will be illustrated by synthesizing fine chemicals, from organic compounds to nanoparticles and new materials.

Rapid hybrid microwave cladding of SiO2/TiO2 sol–gel derived composite coatings

UV protection and coatings for plastics are important in many applications. The cladding of low microwave (MW) absorption composite coatings on high MW transparent plastic substrates is a challenge due to their disparate dielectric properties. Moreover, an uneven heat energy conversion within the composite creates an additional hurdle in producing a coating with good surface integrity. In this study, a protocol was developed to overcome these difficulties based on a hybrid approach. The adverse effect of temperature mismatch between the coating and substrate was reduced through a two-way susceptor-aided heating mechanism. Low MW absorbing sol–gel derived composite coatings consisting of silicon dioxide (SiO2) and titanium dioxide (TiO2) were successfully cladded on the surface of MW and visual light transparent polycarbonate to produce a clear protective coating with UV-resistance. Nanoindentation tests were conducted to assess the effectiveness of the proposed protocol. Significant enhancement in the surface elastic modulus and hardness were achieved. Schematic of hybrid MW cladding of sol–gel derived SiO2/TiO2 coating on polycarbonate. The heat energy transferred to the sample is a combination of (1) the heat generated through MW coupling of the SiO2/TiO2 coating and (2) the heat transfer by conduction, convection, and radiation from the heated. This process promotes hydrolysis and condensation reactions, leading to fast densification of the coating. Moreover, the difficulty of processing low microwave absorbing and microwave transparent materials simultaneously has been overcome. As a result, a crack-free SiO2/TiO2 coating with enhanced surface mechanical properties can be produced within a minute.

Electromagnetic-heating enhancement of source rock permeability for high recovery

Source rock shales have permeability in the nano-Darcy range and require drilling long horizontal wells and extensive hydraulic fracturing to commercially produce hydrocarbons. Recently, an electromagnetic (EM) thermal method was proposed as a primary, or secondary, stimulation method to produce light hydrocarbons from extremely tight reservoirs. This method was based on the fact that EM heating rapidly elevates the pore-water pressure in tight rocks to fracture the reservoir. Since only the in-situ pore-water is needed, this is considered to be a water-free stimulation method, which is especially desirable for regions with limited water resources. Herein this further study provides an in-depth and quantitative investigation of the effects of laboratory EM stimulation on the permeability of the source rock shales under hydrostatic confinement and its dependence on the rock water content. In order to maintain hydrostatic pressure, a special pressure cell was built using nearly microwave transparent materials. Permeability of the samples before and after EM stimulation was acquired using a novel method capable of simultaneously measuring both matrix and fracture permeability. Microwave heating was found to generate extensive fractures in the source rock samples with a moderate amount of pore-water, which resulted in an increase of more than three orders of magnitude in permeability. This study provides further comprehensive evidence suggesting that EM thermal stimulation can be an efficient production method for source rock reservoirs and can result in both a high recovery rate and large estimated ultimate recovery of hydrocarbons.

Microstructure and Dielectric Property of 3D BNf/Si3N4 Fabricated by CVI Process

As potential wave-transparent materials applied at high temperatures, 3D BNf/Si3N4 ceramic matrix composites were prepared by low pressure chemical vapor infiltration or deposition (LPCVI/ CVD) process from SiCl4-NH3-H2-Ar gas precursor at 800 °C. The densification process, microstructure and dielectric properties of 3D BNf/Si3N4 composites were investigated. The results indicated that 3D BNf/ Si3N4 was successfully fabricated by LPCVI/CVD, with final open porosity of 2.37% and density of 1.89 g/ cm3. Densification kinetics of 3D BNf/Si3N4 is a typical exponential pattern. The Si3N4 matrix was uniformly infiltrated into porous BNf preform. The deposited Si3N4 matrix was amorphous by XRD analysis. Introduction of BN fiber into Si3N4 ceramic lowered the permittivity of Si3N4. The fabricated BNf/Si3N4 composites possess low permittivity of 3.68 and low dielectric loss of lower than 0.01, which are independent of temperature below 400 °C. Transmission coefficient of BNf/Si3N4 composite is 0.57 and keeps stable below 400 °C. BNf/Si3N4 can be fabricated at low temperature and may be candidates for the microwave transparent materials.

Boron doped diamond films: A microwave attenuation material with high thermal conductivity

Microwave attenuation materials with high thermal conductivity are required for developing high power microwave technology. In this paper, boron-doped diamond films with different doping concentrations were prepared by microwave plasma chemical vapor deposition. Complex permittivity of the samples was measured in the K-band by using the transmission/reflection method. It was found that the complex permittivity of the diamond films increased with an increase in the boron doping concentration and the diamond films were transformed from a microwave transparent material into a microwave absorbing material. In addition, although thermal conductivity decreased with increasing boron concentration, it remained at a fairly high level. Therefore, boron-doped diamond films could be developed into a microwave attenuation material with extremely high thermal conductivity. Mechanism analysis revealed that the increase in the real part of permittivity mainly resulted from the hopping polarization of bound charges, while the increase in the imaginary part was due to both hopping polarization and valence band conduction. It was found that with the increase in the boron doping concentration, the proportion of the dielectric loss of the first mechanism increased.

Computational evaluation of food carrier designs to improve heating uniformity in microwave assisted thermal pasteurization

Abstract Microwave assisted thermal pasteurization system (MAPS) is a novel food safety technology that employs carriers made from stainless steel to move pre-packaged foods inside 915 MHz single mode microwave cavities to eliminate bacterial and viral pathogens. This paper studied the performance of metal carriers for 16 oz and 10 oz food packages in the MAPS. A simulation model built with Quick-wave software was developed to analyze the electromagnetic field distribution inside a MAP system as affected by the presence of the metal food carriers. Computer simulations were validated using a mashed potato model food processed in a pilot scale MAP system; heating patterns of the samples were detected by a chemical-marker based computer vision method. Results showed that different designs of the food carriers could be used to modify electric field distribution to obtain relatively uniform heating patterns within the microwave cavities. Simulation results also illustrated that magnetron frequency variations between 900 MHz and 920 MHz do not affect the heating patterns of food packages processed using carriers containing metal parts. The results demonstrated that the MAPS with moving metal carriers has stable and predictable heating patterns. Industrial relevance Microwave transparent materials such as plastics have been used to make transport carriers for food packages in microwave assisted thermal sterilization (MATS) (Tang, 2015). However, polymers may have a short life in the high-temperature processing conditions and thus may not be desirable in an industrial setting. The simulations and experiments conducted in this work showed that the novel concept of metal tray carrier is an effective mechanism for transporting pre-packaged foods in microwave heating systems. The validated computer simulation model presented in this work will be a time efficient, economical and convenient tool to evaluate tray carrier designs for efficient and uniform microwave heating in MAPS.

Nearly stoichiometric BN fiber with low dielectric constant derived from poly[(alkylamino)borazine

Nearly stoichiometric boron nitride (BN) fiber with low dielectric constant was prepared by curing, pyrolysis of a novel poly[(alkylamino)borazine] (PAAB) under NH 3 up to 1200 °C in a ceramic yield of 60 wt.%. Based on IR, XRD, XPS and elemental analysis (EA), the fiber with a composition of BN 1.09 shows a characteristic of turbostratic BN ( t-BN). The tensile strength is approximately 0.6 GPa with 13 μm in diameter. Moreover, the fiber possesses very low real part ( ε′) of 2.66 and loss tangent (tan δ) of 0.0012 at 10 GHz, respectively, making it possible to be used in microwave-transparent material.

Porous Silicon Nitride Microwave Transparent Material with Compounded Component Pore Former

It is an effective way to decrease dielectric constant of microwave transparent materials by improving porosity rate. Network -Si3N4 porous transparent materials were prepared by gas pressure sintering (GPS) with Y2O3 additions. Porosity rate of silicon nitride using pore former with different content starch, polytetrafluoroethylene (PTEE) powder respectively and their compounded component were discussed in this paper. It is concluded that porosity rate can be improved by all the three, starch is most favorable to improve porosity but great volume rebound after cold iso-static pressing were happened and easily lead to crack of green body. Method of pore-making by step on principle of pore formers discharging in different temperature range used to get high strength and porosity green body. Series of porous silicon nitride microwave transparent materials were prepared with porosity rate from 48% to 67%, bending strength from 162MPa to 59MPa and dielectric constant from 3.41 to 2.37.

Characteristics of Aluminum and Magnesium Based Nanocomposites Processed Using Hybrid Microwave Sintering

Powder metallurgy is one of the highly established methods to synthesize metals, alloys and composites. Sintering is one of the important steps in powder metallurgy methodology and is usually realized through conventional resistance furnaces. The sintering usually takes a few hours to realize density in excess of 90%. The present study highlights the use of energy efficient and environment friendly microwave sintering route to synthesize pure aluminum, magnesium and magnesium based nanocomposites. Three reinforcements were targeted: a) silicon carbide, a microwave susceptor, b) alumina, a microwave transparent material and c) copper, a conducting material. Composites were prepared using blend-compact-microwave sintering-extrusion methodology. Process evaluation revealed that microwave assisted sintering can lead to a reduction of 86% in sintering time and energy savings of 96% when compared to conventional sintering. Moreover, microwave assisted sintering of metal compacts in this study was carried out in air, in the absence of any protective atmosphere, without compromising the mechanical properties of the materials. Results revealed that properties of magnesium can be convincingly enhanced using the said processing methodology and the materials formulations selected. Most importantly, the study established the viability of microwave sintering approach used in place of conventional sintering for magnesium based formulations.

Si<sub>3</sub>N<sub>4</sub>-BN-SiO<sub>2</sub> Based Microwave-Transparent Materials

The Si-B-O-N microwave-transparent materials were prepared by gas pressure sintering (GPS).The effect of BN and nano-SiO2 contents on the mechanical and dielectric properties of the composites was studied. The microstructural characteristic and reinforced mechanism of the composites were also investigated. The results showed that a series of Si-B-O-N wave-transparent materials could be obtained by controlling the contents of raw materials and technological parameter. The bending strength of composites is from 74.7MPa to 174.83MPa, the dielectric constant is from 3.5 to 4.2 and the tangent of loss angle is from 0.5×10-3 to 4.5×10-3.

Ablation Performance and Surface Texture of the Nitride Composites Reinforced by the Braided Silica Fibers

Braided silica fibers reinforced nitride composite (SFRN), which was prepared by the polymeric precursor infiltration and pyrolysis (PIP) process with the precursor polyborosilazane (PSBZ), was a new typed microwave transparent material with high mechanical and ablation resistance performance for high-temperature application. The thermal ablation performance of the SFRN was evaluated by the ablation equipment with the kerosene and liquid oxygen as the heating source. The ablation surface texture of the SFRN including macrostructure and roughness were measured by Three-dimensional Macrostructure and Contour Scale System (TMCSS). Results showed that there are no concurrent observation of thermal delaminations or cracks and the specimen remains intact. The SFRN has an excellent thermal shock resistance and good ablation resistance with the linear recession rate of 0.038mm/s. The ablation surface texture of the SFRN can be well illuminated by the TMCSS. And the ablation performance will be improved by enhancing material density and homogeneous intertextures.

Ablation and radar-wave transmission performances of the nitride ceramic matrix composites

The 2.5 dimensional silica fiber reinforced nitride matrix composites (2.5D SiO2f/Si3N4-BN) were prepared through the preceramic polymer impregnation pyrolysis (PIP) method. The ablation and radar-wave transparent performances of the composite at high temperature were evaluated under arc jet. The composition and ablation surface microstructures were studied by X-ray diffraction (XRD) and scanning electron microscope (SEM). The results show that the 2.5D SiO2f/Si3N4-BN composites have a linear ablation rate of 0.33 mm/s and high radar-wave transparent ratio of 98.6%. The fused layer and the matrix are protected by each other, and no fused layer accumulates on the ablation surface. The nitride composite is a high-temperature ablation resistivity and microwave transparent material.

Novel Routes to Microwave Processing of Ceramic Materials

ABSTRACTMaterial parameters important for microwave processing are identified in case of powder synthesis from precursor compounds and in case of sintering Al2O3- as well as SiC-matrix ceramics. By varying the spatial distribution of the precursor in microwave transparent materials, different pyrolysis temperatures are obtained, which can be attributed to different heating rates due to selective microwave heating. The microwave sintering behavior of oxide ceramics is strongly influenced by the specific surface area of the powder and by aliovalent dopants. In contrast, for covalent ceramics, like SiC+TiC, no experimental evidence for similar effects was obtained.

Druckaufschluß im Mikrowellenofen

Decompositions of inorganic and organic materials can be carried out within a few minutes by means of the microwave technique. For dissolving substances with volatile components in the microwave oven, low pressure containers were developed from microwave transparent materials and an excess pressure security system. Results and limitations of the method are explained by examples.