EM Modeling of Mode Stirrers in Microwave Applicators

Mode stirrers are mobile metallic elements that modify the electromagnetic (EM) boundary conditions within a microwave heating applicator, resulting in a temporal non-stationary electric field pattern over the processed materials. Due to this effect, mode stirrers are widely used at industrial level to improve the heating uniformity in multimode microwave ovens.

The EM modeling or computation of the EM fields inside multimode cavities with mode stirrers has not received much attention in the technical literature, with few exceptions, as reported in [1] and [2]. This has led to applicator designs based on the experience rather than on a deep comprehension of the electromagnetic problem.

Figure 1a (left) shows the schematic of a microwave chamber chosen for the EM modeling of the electric field distributions. Despite of the specific design of mode stirrers and microwave applicator depicted in the figure, this example can be extended to a general case of multimode applicators with any mode stirrer configuration.

Figure 1.

The microwave applicator is as a large metallic box with a dielectric sample on a tray and a waveguide feeding port from the top. Mode stirrers are designed as metallic blades tracing a synchronous angular continuous movement in order to establish non-uniform boundary conditions for the electric field distribution.

The calculation of the EM fields (electric and magnetic fields) at microwave frequencies requires the resolution of Maxwell’s equations with the specific boundary conditions depicted in the structure of the Figure. Several numerical methods can be employed to solve Maxwell’s equations in such multimode cavity. For complicated geometries, the most popular techniques are based on the finite-element method (FEM) and the finite-difference time-domain (FDTD) method. Out of the two previously mentioned methods, the FEM represents a more agile technique since calculations can be performed for complex and arbitrary structures without the staircase meshing used in FDTD. On the other hand, FDTD usually requires lower computational times.

Here, the multimode applicator has been discretized as 2-D rectangular enclosure excited by the TE10 mode with a standard WR-340 waveguide, centered at the top of the cavity, as can be seen in Figure 1b. The use of a 2-D approach versus the three-dimensional (3-D) reduces the required high computing times to achieve a good precision in the results without a big lack of rigor in the calculation of field. The PDE tool function included in MATLAB was employed in order to apply the FEM procedure to a mesh in the 2-D domain. Figure 2 (left) shows the mesh of the structure of Figure 1b, with a different size depending on the sharpness or dielectric properties of the specific zone. Some other commercial EM simulators could be also used to solve the EM fields in this structure.

Figure 2.

Figure 2b (right) shows the electric field distribution calculated by the FEM method at a given angle position of the blades. This static distribution might correspond to those that we would have if no mode stirrers were installed in the applicator. At the dielectric sample position, the electric field profile for the static situation would lead to unequal heating patterns (high intense E-fields –hot points- are represented by red color, whereas low E-fields –cold points- take blue color). It can be also appreciated by the color levels in Figure. 2b, that inside the sample the field strength is lower than in the cavity due to the energy reflected at the dielectric-air interface.

This E-field distribution depends on the blade angle position. The following video shows these electric patterns in the microwave cavity with the movement of the blades of the mode stirrer (32 different stirrer positions).

Figure 3.

The numerical calculation of the average E-field in the dielectric sample can be determined as the quadratic averaging of the EM fields or absorbed power in the material for different positions of the blades in the mode stirrer. For each spatial position; the wave equation is solved providing an instantaneous electric field distribution in both the cavity and sample.

Fig. 4 shows the average electric field distribution computed for the 32 different stirrer positions, along some dielectric materials placed in the microwave cavity, according to Figure 1. The same figure includes the electric field in the dielectric sample calculated with a generalized plane-wave approach (GPWA), for comparison purposes.

The GPWA assumes an isolated dielectric material and four different planes waves propagating toward the center of the material, from different orthogonal angles and considers the reflections at the dielectric–air interface.

From Figure 4, we can see that the average E-field inside the dielectric material is very close to the field provided by propagating plane waves, especially for medium and high dielectric loss materials. Different stirrer configurations lead to similar EM fields in cavity and dielectric sample. Therefore, the electric-field distribution in the multimode microwave applicator seems to be independent of the stirrer arrangement for medium and high dielectric loss materials, where the dielectric sample seems to enforce the electric field behavior, due to the similarities to the results provided by plane waves.

Thanks to the EM modeling, we learnt that the effect of mode stirrer on the average electric field distribution in the dielectric sample can be approximated by a simply electromagnetic problem with multi plane wave incidences where the interface dielectric-air plays the dominant role, especially for moderate and high loss materials.

This is an example that highlights once more the use of EM modeling for a better understanding of microwave heating applicators. More details of this example of modeling can be found in:

[1] J.M. Catalá-Civera , P. Plaza-González, J. Monzó-Cabrera, D. Sánchez-Hernández. “A New Approach for the Prediction of the Electric Field Distribution in Multimode Microwave Applicators with Mode Stirrers”. IEEE Transactions on Magnetics, ISSN 0018-9464. Vol. 40 (3), 2004, pp. 1672-1678.

[2] José M. Catalá Civera, Pedro J.Plaza, Juan Monzó, David Sánchez. “Effect of Mode-Stirrer Configurations on Dielectric Heating Performance in Multimode Microwave Applicators”. IEEE Transactions on Microwave Theory and Techniques, Vol. 53 No. 5, 2005, pp. 1699-1706.


Paper about microwave synthesis in Thermochimica Acta

A new paper entitled “Reactive synthesis of Ti–Al intermetallics during microwave heating in an E-field maximum” has recently been published in the scientific journal Thermochimica Acta. This work describes some experiments carried out by the Electromagnetic Wave Processing Group of the Swiss Federal Laboratories for Materials Testing and Research – EMPA- in collaboration with the Microwave Area of the Institute ITACA.

The experiments deal about the time-resolved X-ray diffraction synchrotron radiation technique used in combination with E-field microwave heating to study in situ the kinetics of intermetallic phase formation in the Ti–Al system.

The reaction of Ti with Al is triggered by the melting and spreading of Al onto the surface of Ti particles. The tetragonal TiAl3phase is the primary reaction product, formed by instantaneous nucleation at the interface between the unreacted Ti cores and the Al melt. The growth of TiAl3 layers is diffusion-controlled.

These preliminary results demonstrate that microwave heating can readily be used to rapidly synthesise intermetallic phases from high-purity elemental powders constituents.

The paper is published on line and can be found in the website of Thermochimica Acta.

For more information about the experiments described in the paper, please contact directly with the authors.

Dielectric Characterization of Water-in-Oil Emulsions

Water in oil (W/O) emulsions appear in the sludges of many industries. These pollutants must be treated before discharge, and microwave heating is considered as an environmental-friendly technique to separate these emulsions. The amount of heat generated in the emulsion greatly depends on the emulsion dielectric properties. Precise characterization of emulsions is essential to predict their interaction with microwave radiation.

Previous studies on emulsions dielectric properties have been conducted at RF frequencies, which do not include the standard heating frequency (2.45 GHz). Now, with the Vials Dielectric Kit developed by DIMAS, accurate characterization of different W/O emulsions (mineral and vegetable) at microwave frequencies has been performed. Also, an open-ended coaxial sensor has been designed for dielectric characterization of this type of emulsions while flowing through a pipe. Additional information given by Cryo-SEM micrographs (see Fig.) has allowed correlating the measured dielectric properties with other chemical properties of the studied emulsions (polar/non-polar molecules, amphipatic character, droplets size, etc.).

Two types of W/O emulsions were considered, using motor oil SAE 40 in the mineral oil emulsions, and oleic acid (C18H34O2) in vegetable oil emulsions. The concentrations of water in samples were 15%, 30%, 40% and 50% by volume. Results show that dielectric properties of emulsions depend not only on the water volume fraction, but also on the chemical properties of the oils. This is due to the existence of a carboxile group which is present in vegetable oil. This group is polar, and tends to form hydrogen bonds with other polar molecules, such as water. Therefore, emulsions prepared with vegetable oil have higher polarity than emulsions made with mineral oil (formed by non-polar hydrocarbonated chains). Higher polarity involves higher losses due to dipolar rotation of molecules, when an external field is applied.

From the dielectric characterization, interesting results have been also derived about microwave power distribution in emulsions samples (see figures). Penetration depth is lower for vegetable oil emulsions. This means that, in vegetable oil emulsions, a bigger amount of energy is absorbed and dissipated in heat. However, the penetration depth of these emulsions is lower, and heating may be limited to the emulsion surface. Conversely, mineral oil emulsions have a higher penetration depth, but the heating rate may be low. This is caused by the trade-off between power loss density and penetration depth.

These and other related results were presented at the 12th International Conference on Microwave and High Frequency Heating (AMPERE 2009).

Dielectric Characterization of Liquid Mixtures

Liquid mixtures are widely used for very different purposes, with applications in medicine, biology, chemical engineering, environmental science, etc. Characterization of their dielectric properties at microwave frequencies yields a wealth of information about molecular motions, kinetic processes and liquid structure which is often not available by other characterization methods. An increasing need for dielectric data, characterizing the interaction of materials with microwaves also arises from emerging technical applications such as heating, moisture sensors and process control.

The dielectric measurement (dielectric constant and loss factor) can be easily performed with the Vials Dielectric Kit developed by DIMAS at a microwave frequency around 2.5 GHz. Only 8 mL of liquid mixture inside a standard tube is enough to obtain accurate results in a fast and convenient way. There is no need of calibration with other liquids of known properties, which makes it very appropriate for non-expert personnel.

For illustration purposes, the following figures show some examples of the dielectric characterization of liquid mixtures. Two mixtures with very different behavior have been selected: mixtures of water in glycerin and mixtures of alcohol in oil. As the figure shows, the volumetric proportion of water in glycerin and the proportion of alcohol in oil present a great influence on the dielectric behavior of the studied samples with very different responses. These examples show the potential of the Dielectric Kit for analysis of mixtures in the sectors of medicine, biology, chemical engineering, environmental science, etc.

For more information, please contact with the research team.

SLS Measurements

Researchers of the Swiss Federal Laboratories for Materials Testing and Research – EMPA – lead by Dr. Sebastien Vaucher in collaboration with researchers of the Microwave Division of the research institute ITACA are performing since 2006 time-resolved experiments pioneering in situ microwave heating experiments at the Suisse Light Source, Materials Science Beamline (SLS-MS).

The Swiss Light Source (SLS) at the Paul Scherrer Institut (PSI) is a third-generation synchrotron light source. In the design of SLS a high priority was given to the items quality (high brightness), flexibility (wide wavelength spectrum) and stability (very stable temperature conditions) for the primary electron beam and the secondary photon beams. With an energy of 2.4 GeV, it provides photon beams of high brightness for research in materials science, biology and chemistry. The SLS is the Swiss National Source, open for international research groups as well. It offers unique research opportunities to academic research teams as well as industrial research groups.

The fast frame rate of the MYTHEN detector enables experiments to be carried out in which the structural and microstructural evolution of solids under microwave application can be accurately followed in near-to-real time, while monitoring the microwave heating processes and eventually fine tuning the microwave application for processing for a broad variety of materials. The high-temperature time-resolved powder diffraction experiments provide a detailed set of informations on the phase transformation sequence(s) and kinetics during the exposure of the materials to microwaves.

The picture below shows the facilities of SLS and the microwave equipment of the measurements

In addition, 3D microtomography is a modern powerful instrument to observe the morphology of conventional or microwave sintered parts. Synchrotron-based X-ray tomographic experiments are performed at the TOMCAT beamline of the Swiss Light Source to investigate the inner structure of metal-diamond composites manufactured by liquid metal infiltration. The access to the cross-sections of bulk sintered materials offers a deeper insight into the interface reactions taking place during microwave heating.

This opens new possibilities for the evaluation of microwave effects at the interface between the dissimilar constituents of composite materials and supports the improvement of the microwave heating powder processing route towards sintered parts with high uniformity and improved thermal contact at the metal-diamond interfaces.

An excellent example of these measurements is the efficient microwave-assisted carbothermal reduction of magnetite Fe3O4 to iron, a process of high interest for the steel industry. A transient iron oxide phase was found which intermediates the transition from magnetite, Fe3O4, to wüstite, FeO. The kinetics of this phase transformation provides a deeper understanding of volumetric heating by microwaves.

Some of these measurements have been published in the following scientific journals.

  • Nicula R, Ishizaki K, Stir M, Catala,Civera J,M, Vaucher S , “Microwave energy absorption driven by dynamic structural and magnetization states in Fe85B15 metallic glass ribbons”, APPLIED PHYSICS LETTERS 95, 174104 (2009)
  • Nicula R, Stir M, Ishizaki K, Catala,Civera J,M, Vaucher S, “Nanocrystallization of amorphous alloys using microwaves: in situ timeresolved synchrotron radiation studies”, JOURNAL OF PHYSICS: CONFERENCE SERIES 144, 012109 (2009)
  • Nicula R, Stir M, Ishizaki K, Catala,civera JM, Vaucher S , “Rapid nanocrystallization of softmagnetic amorphous alloys using microwave induction heating”, SCRIPTA MATERIALIA 60, 120 (2009)

Experl Project

The microwave division (DIMAS) of the research institute ITACA participates in the ExPerl project co-financed by the EU in the framework of 7PM. (Theme NMP-2008-4.0-5: Innovative concepts and processes for strategic mineral supply and for new high added value mineral-based products).

Among the various industrial minerals needed by the EU industry, perlite is highly important both from technological and economic point of view with many applications in Construction, Chemical industry and in horticultural applications. Conventionally expanded perlite has a number of favourable properties (inert, fire resistant and incombustible, good sound and thermal insulation) but is also characterised by low strength, lack of durability, high porosity and easy deterioration that limit the range of its applications and affect the quality of conventional perlite based products. These unfavourable properties originate from the sponge-like open structure of expanded perlite granules, which results from the technology of the expansion process.

However, with the application of innovative expansion techniques based on electrical heating and microwave heating, it is possible to produce micro-sized perlite particles with spherical shape and closed structure (CSP), which have all the favourable properties of the conventionally expanded ones and also enhanced mechanical (strength, durability) and physical properties ( porosity, thermal conductivity).

The objective of this proposal is the production of micro-sized closed structure perlite with the application of breakthrough perlite expansion technologies and the development of a new generation high added value end- products based on CSP, including preformed insulating products (panels, boards and bricks), mortars and functional fillers tailored for the Construction, Manufacturing and Chemical industry The new CSP-based end-products will present a good number of favourable properties as they will be lightweight, inert, non toxic, recyclable, of high strength, with improved insulating properties, incombustible, unchangeable over time under and of low cost highly attractive for the European consumer. More information about the project is given in the project website

[A complete list of the projects developed by our team is included in the ITACA website]

Project about electromagnetic heating of rubber compounds

Rubber is one of the basic materials used in the manufacture of soles and sole components for footwear (soles, top-pieces, heels, socks, etc.). The many possibilities which the material offers with regard to the adaptation to different wear purposes, from high performance footwear (military, professional, sports), to medium or low performance footwear (street, fashion, indoors, etc.), makes it an indispensable material in this industrial sector, with a consumption level exceeding 25% of the total footwear produced. However, the rubber has to be vulcanised in order for it to be used subsequently and this operation is traditionally carried out in the footwear industry statically way by means of hot-plate presses, which unfavourably increases manufacturing times and energy consumption. This involves slow, unflexible and costly manufacturing processes which greatly limit production.

To overcome these drawbacks, a research project was initiated with the main objective to develop an alternative system to the traditional processes of rubber vulcanisation in the footwear industry, by means of microwave heating technology. The project entitled as “Vulcanisation of footwear components by electromagnetic techniques” was co-financed by the EU in the framework of 6PM with the participation of the microwave division (DIMAS) of the research institute ITACA in a cooperation research completed in 2007. Reference of the project: BRST985314

To accomplish this objective, a pilot scale microwave oven prototype was developed as well as microwave transparent moulds and rubber compounds sensitive to their heating action. All these elements allowed a successful vulcanisation of rubber components for footwear in a continuous manufacturing system.

Microwave technology offered many advantages as compared to traditional processes: direct heating of the rubber mass and not of the moulds, faster, more even and selective heating, higher performance, less energy consumption, better quality and reproducibility of the finished product, less energy leakage to the environment, cleaner, less space for the installation, easier maintenance, electronic operation and control, automation capacity and a more comfortable working environment.

The following picture shows the pilot plant developed for the continuous vulcanization process. The top of the oven shows the pneumatic actuators to close the dielectric transparent moulds inside the oven, for molding the rubber samples, simultaneous to the microwave heating.

The versatility of the above mentioned prototype made the application of this technology possible not only for the vulcanisation processes related to the footwear sector, but for other industrial sectors involved in rubber vulcanising processes, latex or other polymer materials.

[A complete list of the projects developed by our team is included in the ITACA website]

Paper about cure monitoring by dielectrometry

A new microwave sensor system for the noninvasive monitoring of the curing process of thermoset materials has been developed by DIMAS, in cooperation with the research institute INESCOP, and main results have been recently published in IEEE Sensor Journal. The picture shows the sensor system.

The thermoset material is placed inside a mould and the microwave sensor is designed with a curved surface adapted to the mold inner shape. In particular, an open-ended coaxial resonator has been designed as the microwave sensor head.

The effect of the coupling network for the resonator with overcoupled feeding conditions is removed by a novel method recently developed by DIMAS, so the unload resonance can be de-embedded from the measurements. This allows that the range of permitted measurements encompass polymeric  materials with both low and high dielectric losses.

Noninvasive, continuous monitoring of the microwave dielectric properties (Dielectric Constant and loss factor) of several thermoset materials has been performed in real time, allowing verification of the cure process.

The following Figure shows a cure reaction of Foam Polyurethane which takes place in about 40 seconds.  The effect of the Polyurethane formation can be seen in the change of the measured dielectric properties. During the cure process, the increase of viscosity of the liquid mix cause a drop of the molecular mobility, and thus, the dielectric properties of the material are drastically decreased. Since the change of permittivity is directly produced by the change of material’s viscosity, therefore, the rate of reaction can be followed by simple inspection of the traces vs time. Moreover, the sensor provides accurate information about other parameters such as the reaction kinetics, cure time, etc.

Although other types of dielectric cure monitoring devices are available, the main tarjet with this sensor was the need to operate in real time and in the rather unclean industrial environment, which is often hostile to sophisticated equipment. Robustness in both hardware and software is an additional requirement which has been fulfilled. For practical applications, the system has been demonstrated to be advisable not only for accuracy and rate but also for simplicity and cost. This allows the system to be used as a production monitoring and control tool as well as for laboratory studies.