Antoni J. Canos

Accurate determination of the complex permittivity of materials with transmission reflection measurements in partially filled rectangular waveguides

An enhanced transmission reflection technique for the precise determination of the complex permittivity of dielectric materials partially filling the cross section of a rectangular waveguide is described. Dielectric properties are determined by an iterative procedure from two-port S-parameter measurements and a numerically generated propagation constant obtained from the analysis of a partially filled waveguide. Convergence of the solution is ensured from perturbational approximations. Unlike previous approaches, an uncertainty investigation is performed, taking into account all the parameters involved in the dielectric characterization. Permittivity accuracy values are presented and, hence, an optimum measurement setup can be established. Measurements of reference materials have been carried out to validate the method.

A novel technique for deembedding the unloaded resonance frequency from measurements of microwave cavities

A novel technique to extract the influence of coupling networks on the resonant frequency of cavities in one-port measurements is presented. The determination of the unloaded resonant frequency is performed directly from measurements without either the need to obtain the electromagnetic fields in the resonator or to deembed the delay of transmission lines from the measuring equipment to the resonator. The importance of the Fosters form on the modeling of the frequency detuning of the resonators is also discussed and a criterion for the choice of the appropriate Fosters form is suggested. The procedure is validated with simulations and experimental measurements of manufactured cavities

Dynamic Measurement of Dielectric Properties of Materials at High Temperature During Microwave Heating in a Dual Mode Cylindrical Cavity

A microwave cavity and heating system for microwave processing and in situ dynamic measurements of the complex permittivity of dielectric materials at high temperatures ( ~ 1000 °C) has been developed. The method is based on a dual-mode cylindrical cavity where heating and testing are performed by two different swept frequency microwave sources. A cross-coupling filter isolates the signals coming from both sources. By adjusting the frequency bandwidth of the heating source and the level of coupling to the cavity, an automatic procedure allows for the establishment of a desirable level of heating rate to the dielectric sample to reach high temperatures in short cycles. Dielectric properties of materials as a function of temperature are calculated by an improved cavity perturbation method during heating. Accuracy of complex permittivity results has been evaluated and an error lower than 5% with respect to a rigorous analysis of the cavity has been achieved. The functionality of the microwave dielectric measurement system has been demonstrated by heating and measuring glass and ceramic samples up to 1000 °C. 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.

Design rules for the optimization of the sensitivity of open-ended coaxial microwave sensors for monitoring changes in dielectric materials

Open-ended coaxial probes are widely used for non-destructive measurement of dielectric properties of materials, and also as microwave sensors for industrial processes and quality control applications. The main design parameters of these sensors are the coaxial radii and working frequency. In this paper, the influence of these variables on the final sensitivity of the coaxial sensor when monitoring dielectric materials is analysed, and a novel expression for this parameter selection is proposed. Moreover, a method to select the optimum parameters of experimental configurations is provided. Measurements demonstrate that high discrimination can be achieved with this method when monitoring changes in the dielectric properties of materials.

Full-Wave Analysis of Dielectric-Loaded Cylindrical Waveguides and Cavities Using a New Four-Port Ring Network

In this paper, a full-wave method for the electromagnetic analysis of dielectric-loaded cylindrical and coaxial waveguides and cavities is developed. For this purpose, a new four-port ring network is proposed, and the mode-matching method is applied to calculate the generalized admittance matrix of this new structure. A number of analyses on dielectric-loaded waveguide structures and cavities have been conducted in order to validate and to assess the accuracy of the new approach. The results have been compared with theoretical values, numerical modeling from the literature, and data from commercial electromagnetic simulators. The method has been also applied to the accurate determination of dielectric properties, and we provide an example of these measurements as another way to validate this new method.

Simulation of temperature distributions in pressure-aided microwave rubber vulcanization processes

In this paper microwave heat generation in materials surrounded by dielectric molds is studied. Particularly, the pressure-aided microwave rubber vulcanization process, where the use of dielectric molds that exert the proper pressure over the rubber samples is essential for this application. Temperature distributions of rubber and mold materials have been simulated using a discretization strategy based on the FDTD and FEM techniques for one and two-dimensional cases, respectively. Simulation results show that the dielectric and thermal properties of the pressing mold are of the utmost importance in order to obtain the desired temperature profiles throughout the volume of the rubber sample and therefore to achieve a uniform degree of vulcanization in the rubber Experimental measurements in a monomode cylindrical cavity with several configurations of mold materials, pressing low thermal conductivity rubber samples have been performed and used to validate the simulation results.

Noninvasive Monitoring of Polymer Curing Reactions by Dielectrometry

A microwave sensor system for the noninvasive monitoring of the curing process of a thermoset material placed inside a metallic mold is described. The microwave sensor is designed as an open-ended coaxial resonator with a curved surface adapted to the mold inner shape. The analysis of the microwave resonator comprises a recently developed method for deembedding the effect of coupling network in overcoupled resonators, so the range of permitted measurements encompass both low and high dielectric losses of polymeric materials. Results show that noninvasive, continuous monitoring of the microwave dielectric properties of the thermoset material can be performed in real time, allowing one to check initial conditions and to verify the evolution of the cure process.

A new empirical method for extracting unloaded resonant frequencies from microwave resonant cavities

Equivalent circuits traditionally used to model resonators and coupling networks in the vicinity of a resonance provide values of unloaded resonant frequencies which usually do not agree with experimental results. A new empirical method for the extraction of influence of coupling networks on resonant cavities is presented. The characterization of coupling structures is performed directly from measurements without the need of obtaining the electromagnetic fields inside the cavity, which is very interesting from a practical point of view. Results are validated with simulations and experimental measurements. The accuracy of some cavity applications, such as dielectric characterization techniques can be directly improved with this approach.

Microwave sensor system for continuous monitoring of adhesive curing processes

A microwave sensor system has been developed for monitoring adhesive curing processes. The system provides continuous, real-time information about the curing progress without interfering with the reaction. An open-coaxial resonator is used as the sensor head, and measurements of its resonance frequency and quality factor are performed during cure to follow the reaction progress. Additionally, the system provides other interesting parameters such as reaction rate or cure time. The adhesive dielectric properties can also be computed off-line, which gives additional information about the process. The results given by the system correlate very well with conventional measurement techniques such as differential scanning calorimetry, combining accuracy and rate with simplicity and an affordable cost.

Microwave Non-contact Sensor for On-Line Moisture Measurement of Laminate Paper

A microwave sensor, intended for on-line measurements of paper moisture, is presented in this paper. The sensor main body is a resonant structure, with a paper sheet passing through it. From the measurement of the resonant frequency and/or the quality factor of the resonator, the value of the humidity content of the paper can be obtained, with great accuracy. Very promising measurements have been carried out, leading to the use of the sensor at industrial level.