Paper at the European Microwave Week

Dimas group participated in the European Microwave Week that was held in Nuremberg in October 2017, with the paper entitled “Analysis of an Overmoded Re-Entrant Cavity“. It was authored by David Marques-Villarroya, Felipe Penaranda-Foix, Beatriz Garcia-Banos, Jose Manuel Catala-Civera, Jose Daniel Gutierrez-Cano.

The paper deals with the well knows reentrant cavity, but taking into account the complete set of modes and presenting a chart to fully charaterize the cavity. It is based on the Circuit Analisys Method, being developed in the Microwave Division of ITACA Research Institute since some years ago.

The paper was selected for a short video presentation that can be found here and also in this YouTube link:

New PhD at DIMAS Laboratory

DIMAS proudly announces that a staff member, Mr. Pedro J. Plaza-González presented in January 14Caratula TESIS PEDRO-v01th, 2016 his PhD, and he got the maximum qualification. All the staff at DIMAS group congratulates Dr. Pedro J. Plaza for his great research activity and contribution.

The thesis is entitled “Temperature Control in Microwave Heating Systems“, and covers most of the main issues when dealing with Microwave Heating applicators at high power and high temperature. It has been devolped completely at DIMAS laboratory facilities and next is the summary of the thesis:

Material heat processing systems using microwave energy have been used for more than 60 years. Design and implementation techniques have greatly evolved during this time, but a precise control in material temperature is still difficult to achieve due to theoretical and practical reasons.

This difficulty arises, in many cases, because a deep knowledge in several technical fields is needed in order to design the process properly, being microwave engineering only one of them. Usually it’s necessary to combine knowledge in microwaves with material technology, chemistry, and other fields, in order to have a clear idea about how the process should be.

The main aim of this work is the development of experimental equipment that allows the heat treatment of material samples using microwave energy, while providing a great control over the sample temperature and the energy absorbed. Using such an equipment, very valuable data can be obtained for the process dynamics when using microwave technology.

With this objective in mind, in a first step different suitable types of microwave applicators have been studied, as well as several optimization techniques for the temperature distribution within the sample.

Advantages and disadvantages of multimodal applicators have been analyzed, and a detailed study about the effect of mode stirrers in the field uniformity has been carried out, which is the more common technique for this aim.

A next step was the study of thermal effects in materials under high power electromagnetic fields. Heat transfer, convection and phase change phenomena have been studied in order to analyze their effect in the sample temperature.

The thermal runaway effect has a special importance in the processing of some materials, mainly when dielectric losses increase with temperature. This phenomenon has been analyzed, as well as some suitable techniques that can be used to avoid it, or at least to reduce its effects. Also, different types of temperature sensors have been reviewed to study its
usability in microwave systems, and as a result infrared temperature sensors has been chosen as the more suitable technology.

In microwave heating systems the temperature increment in the sample is determined by the microwave power absorbed by the applicator, and for this reason an accurate control over this parameter is required for a good temperature control. It should be remarked that microwave heating processes are dynamic, evolving with the changes in material temperature and properties. For this reason, different procedures for absorbed power control has been analyzed, with highlight in the work carried out regarding the development of a dynamic impedance matching system.

Moreover, other strategies for absorbed power regulation have been studied, using different control parameters as the generated power, the cavity tuning or the frequency sweep span. Two different systems have been developed. One is based on a tunable monomode cavity with a mechanic tuning system; the second is based on a non-tunable cavity and the use of a variable frequency generator. Both systems integrate temperature sensors and the equipment required to measure the power delivered to the sample. An automated process control algorithm based on PID has been implemented, allowing autonomous working during the experiments.

Both developed systems have been used in a high number of experiments with different nature of samples. Some of these results are presented in this work, showing the excellent performance of the systems and the valuable information that can be obtained for the studied materials.

As a final point, several future research lines are proposed in order to continue the work developed up to now“.

New paper at IEEE Trans. on MTT

DIMAS Group published a new paper at the IEEE Tras. on Microwave Theory and Techniques [1]

This paper presents a new system developed at DIMAS laboratory and consists of a new microwave cavity and heating system for microwave processing and in situ dynamic measurements of the complex permittivity of dielectric materials at high temperatures (around 1000 ºC).The method is based on a dual-mode cylindrical cavity where heating and testing are performed by two different swept frequency microwave sources and a cross-coupling filter that isolates the signals coming from both sources.

This system provides the dielectric propertieOLYMPUS DIGITAL CAMERAs of materials as a function of temperature by an improved cavity perturbation method during heating, with an accuracy of the complex permittivity better than 5% with respect to a rigorous analysis (full wave method) of the cavity.

The functionality of the microwave dielectric measurement system has been demonstrated by heating and measuring glass and ceramic samples up to 1000 ºC.

Additionally, the correlation of the complex permittivity with the heating rate, temperature, absorbed power, and other processing parameters can help to better understand the interactions that take place during microwave heating of materials at high temperatures compared to conventional heating.

 

[1] José M. Catalá-Civera, Antoni J. Canós, Pedro Plaza-González, José D. Gutiérrez, Beatriz García-Baños, and Felipe L. Peñaranda-Foix, “Dynamic Measurement of Dielectric Properties of Materials at High Temperature During Microwave Heating in a Dual Mode Cylindrical Cavity”. IEEE Trans. on MTT, Vol. 63, No. 9, Sep. 2015, pp. 2905-2914. DOI: 10.1109/TMTT.2015.2453263

More than 9000 downloads

Book "Passive Microwave Components and Antennas"

Book “Passive Microwave Components and Antennas”

In April 2010 Intech published the book “Passive Microwave Components and Antennas“.

The chapter 7 was written by the researchers of DIMAS Felipe L. Penaranda-Foix and Jose M. Catala-Civera and entitled “Circuital Analysis of Cylindrical Structures Applied to the Electromagnetic Resolution of Resonant Cavities“.
This chapter has reached more than 9000 downloads in February 2014 and the authors want to thank the editors and also the researchers this number and really hope that it will be useful for future research activities.

Paper about a new element in Circuit Theory in IEEE MTT Journal

A new paper entitled “Full-Wave Analysis of Dielectric-Loaded Cylindrical Waveguides and Cavities Using a New Four-Port Ring Network” has just been published in the scientific journal IEEE Transactions on Microwave Theory and Techniques. This paper describes the theory of a new 4-port element to be used in the library developed by the group to analyse large structures with the Circuit Theory. This Circuit Theory, as well as additional network elements like the new one recently proposed, are available in the free book Passive Microwave Components and Antennas, chapter 7.

This paper has been written in the frame of the close collaboration between ITACA-UPV and NIST.

The work is available in the ieeexplore website, and for more information about the experiments described in the paper, please contact directly with the authors.

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.