There have been recent indications that variable-frequency microwave sintering of ceramics provides several advantages over single-frequency sintering, including more uniform heating, particularly for larger samples. The Finite-Difference Time-Domain (FDTD) code at the University of Utah was modified and used to simulate microwave sintering using variable frequencies and was coupled with a heat-transfer code to provide a dynamic simulation of this new microwave sintering process. This paper summarizes results from the FDTD simulations of sintering in a variable-frequency cavity. FDTD simulations were run in 100-MHz steps to account for the frequency variation in the electromagnetic fields in the multimode cavity. It is shown that a variable-frequency system does improve the heating uniformity when the proper frequency range is chosen. Specifically, for a single ceramic sample (4 × 4 × 6 cm3), and for a variable-frequency range from f = 2.5 GHz to f = 3.2 GHz, the temperature distribution pattern was much more uniform than the heating pattern achieved when using a single-frequency sintering system at f = 2.45 GHz.

To help develop large-scale microwave-sintering processes and to explore the feasibility of the commercial utilization of this technology, we used the recently developed multi-grid 3D Finite-Difference Time-Domain (FDTD) code and the 3D Finite-Difference Heat-Transfer (FDHT) code to determine the electromagnetic (EM) fields, the microwave power deposition, and temperature-distribution patterns in layers of samples processed in large-scale multimode microwave cavities.

This paper presents results obtained from the simulation of realistic sintering experiments carried out in both 500 and 4000 liter furnaces operating at 2.45 GHz. The ceramic ware being sintered is placed inside a cubical crucible box made of rectangular plates of various ceramic materials with various electrical and thermal properties. The crucible box can accommodate up to 5 layers of ceramic samples with 16 to 20 cup-like samples per layer. Simulation results provided guidelines regarding selection of crucible-box materials, cruciblebox geometry, number of layers, shelf material between layers, and the fraction volume of the load vs. that of the furnace.

Results from the FDTD and FDHT simulations will be presented and various tradeoffs involved in designing an effective microwave-processing system will be compared graphically.

The Finite-Difference Time-Domain (FDTD) code available at the University of Utah has been used to simulate sintering of ceramics in single and multimode cavities, and many useful results have been reported in literature [1–4]. More detailed and accurate results, specifically around and including the ceramic sample, are often desired to help evaluate the adequacy of the heating procedure. In electrically large multimode cavities, however, computer memory requirements limit the number of the mathematical cells, and the desired resolution is impractical to achieve due to limited computer resources. Therefore, an FDTD algorithm which incorporates multiple-grid regions with variable-grid sizes is required to adequately perform the desired simulations. In this paper we describe the development of a three-dimensional multi-grid FDTD code to help focus a large number of cells around the desired region. Test geometries were solved using a uniform-grid and the developed multi-grid code to help validate the results from the developed code. Results from these comparisons, as well as the results of comparisons between the developed FDTD code and other available variable-grid codes are presented. In addition, results from the simulation of realistic microwave sintering experiments showed improved resolution in critical sites inside the three-dimensional sintering cavity. With the validation of the FDTD code, simulations were performed for electrically large, multimode, microwave sintering cavities to fully demonstrate the advantages of the developed multi-grid FDTD code.

Induction heating for the treatment of metals has been in commercial use since the mid 1960's. Traditional advantages of induction heating over the convection or radiation processes include speed of heating, possible energy savings, and the ability to customize the coil design to optimize the heating process. In this paper we used the Finite-Difference Time-Domain (FDTD) technique to simulate and analyze the induction heating process for highly conducting ceramics. In order to analyze frequency effects, simulations were performed at 300 kHz, 2 MHz, and 25 MHz. It is found that at higher frequencies coils with a pitch of 2″ or greater became capacitive and generate a large, axial, electric-field component. This new axial electric field, in addition to the normally encountered azimuthal field, causes an improvement in the uniformity of the power deposition in the ceramic sample. If the sample occupies a large portion of the coil, uniformity may also be improved by using a variable-pitch coil, or by extending the length of the coil a few turns beyond the length of the sample. In a production-line arrangement, where multiple sample are place inside the coil, it is shown that maximum uniformity is achieved when the samples are placed coaxially.

Computer modeling and numerical simulation provide a valuable tool for providing guidelines towards a successful routine experimentation with microwave sintering of ceramics. It is also expected that continued efforts in numerical simulation will lead to establishing procedures for the scale up and commercial utilization of this new technology.

In this paper, we utilize the FDTD technique to model sintering ceramics in multimode microwave cavities. The role of using process stimulus such as SiC rods on improving the uniformity of the microwave sintering process are also simulated. To help experimentally validate the obtained results, the FDTD electromagnetic power deposition results were combined with a 3D heat transfer program to calculate temperature distribution in samples and surrounding insulation. Results from the FDTD codes and comparisons with experimental measurements of sintering experiments are presented.

To improve the efficiency of the simulation and achieve more accurate results, we developed a variable mesh FDTD code to help focus the numerical results and hence improve the resolution in critical sites inside the sintering oven. Detailed solution procedures are described. We solved some test geometries with the uniform grid and the developed variable mesh codes and compared the obtained results to validate and check the accuracy of the solution procedure. Results from these comparisons are presented.

The Department of Defense (DoD) and the National Aeronautics and Space Administration (NASA) requested that the National Research Council's National Materials Advisory Board conduct a study to assess the current status of microwave processing technology, identify applications of microwave technology where resulting properties are unique or enhanced relative to conventional processing or where significant cost, energy or space savings can be realized, and to recommend future activities in microwave processing. A committee was established to perform the study and report on their findings. This paper is a summary developed from the committee's report, Microwave Processing of Materials (NMAB Report Number 473, Copyright 1994 by the National Academy of Sciences, National Academy Press, Washington, DC).

Open-ended coaxial probes have been used in broadband dielectric properties measurements for several years. To aid in the ongoing numerical simulation and microwave sintering research at the University of Utah, we have found it necessary to make dielectric properties measurements up to temperatures as high as 1400 °C. The available cavity perturbation techniques were unsuitable in this application due to their relatively narrow band, and the available metal probes are also unsuitable due to the differential thermal expansions of the inner and outer conductors, which makes it difficult to carry out accurate and on-line calibration procedures for these probes.

To help us achieve both broadband and high-temperature dielectric properties measurements, we have developed a new metallized ceramic coaxial probe. The detailed design of this probe is described and the metallization procedure is discussed.

Also to optimize the design of the probe and in particular to increase the penetration of fields in samples under test and hence improve the probe sensitivity to variation in properties of a larger class of materials, and to determine the required minimum thickness of various samples to obtain accurate results, we modeled and simulated the probe performance using the Finite-Difference Time-Domain (FDTD) method. Results from the FDTD simulation are presented and some guidelines that may be used to optimize the design of the probe are discussed.

Analysis of heating patterns in microwave sintering experiments provide information on the contributions of the various heat transfer components to the overall temperature pattern. Measured temperature patterns provide limited information on overall effects. Numerical simulations provide a cost effective way from which the effect of geometry, material properties and the presence of stimulus such as SiC rods or sheets on the heating pattern can be studied separately. Parametric studies allow us to identify the most significant properties and provide guidelines for the routine successful utilization of microwave sintering experiments. These guidelines may also facilitate the scale up and commercialization of microwave sintering.

In this paper we describe a thermal model that calculates the temperature distribution in ceramic samples and insulation under realistic microwave sintering conditions. The calculation process involves a two-step procedure. The first step is to calculate the microwave power deposition in the sample and surrounding insulation. 3D FDTD calculations, described in a companion paper[1,2], are used for this purpose. The other step involves calculation of the temperature distribution using a 3D finite-difference heat-transfer program developed in our Departments[3]. Results illustrating the effect of thickness of insulation and the placement of SiC rod susceptors in picket-fence arrangement are presented.

Electromagnetic drying processes have been used in industrial applications for many years. New materials with different properties and structures have been recently developed which required the development of new drying systems capable of effectively drying the new materials. In this paper, we use Finite-Difference Time-Domain (FDTD) to model and simulate realistic electromagnetic drying processes including induction heating and RF drying. The power deposition patterns obtained from the FDTD code is also used in a finite-difference heat transfer code to evaluate the temperature distribution pattern and optimize rates of drying. Sample parameters used to analyze the RF drying and the induction heating processes include the sample conductivity, dielectric constant along with size, orientation and the geometric arrangement. Specifically, we analyzed the operating frequency, coil pitch, sample to coil diameter ratio, and the possible placement of multiple samples in an induction heating process. Effects investigated in a parallel plate RF drying system include: electrode spacing, electrode length and width, and the spacing between multiple samples. Results of electromagnetic power distributions and temperature patterns are presented.

Computer modeling and simulation of microwave processing systems provide significant advantages that are expected to play an important role in the development, scale-up, commercialization, and effective utilization of microwave heating technology. These advantages include the following:

• Understanding of the physical aspects of microwave interaction with materials. Microwave heating is a complex process, and computer modeling may help identify the role of materials properties, dimensions, input power, heating rates, etc. on the effective use of microwave heating procedures. Hybrid microwave heating often refers to the use of SiC rods, or lining the insulation surrounding the sample with thin SiC coatings, to help stimulate the microwave heating process. This is particularly important when microwaves are used to sinter low-loss materials that do not effectively absorb electromagnetic radiation. This complex hybrid heating process may lead to unpredictable results in routine microwave heating experiments; and many aspects of its procedure may be simulated, evaluated, and possibly optimized using computer modeling. For example, the number, dimensions, and locations of SiC rods in a typical picketfence arrangement and the thickness of SiC sheets that may be used to allow effective stimulation of the heating process, while still not shielding the sample from the microwave energy, may be better understood, and guidelines for experimentations may be developed using numerical simulation.

This paper discusses theoretical and practical aspects of the development and implementation of various measurement techniques for high-temperature broadband microwave characterization of materials at the University of Utah. Objectives include materials measurements in the frequency range from 45 MHz to 12 GHz and for temperatures as high as 1000°C.

Microwave sintering of ceramics in multimode cavities, particularly the use of picket-fence arrangements, has recently received considerable attention. Various types of ceramics have been successfully sintered and, in some cases, a desirable and unique “microwave effect” has been observed.

At present, various aspects of the sintering process such as sample sizes and shapes, types of insulations, and the desirability of including a process stimulus such as SiC rods are considered forms of art and highly dependent on human expertise. The simulation of realistic sintering experiments in a multimode cavity may provide a better understanding of critical parameters involved and allow for the development of guidelines towards the optimization of the sintering process.

In this paper, we utilize the FDTD technique to model various geometrical arrangements and material compatibility aspects in multimode microwave cavities and to simulate realistic sintering experiments. The FDTD procedure starts with the simulation of a field distribution in multimode microwave cavities that resembles a set of measured data using liquid crystal sheets. Also included in the simulation is the waveguide feed as well as a dielectric loading plate placed at the base of the cavity. The FDTD simulation thus provides realistic representation of a typical sintering experiment. Aspects that have been successfully simulated include types of insulation, role of SiC rods on the uniformity of the resulting fields, problems that may result from presence of thermocouples, and the possible shielding effects that may result from excessive use of SiC. These results as well as others showing the electromagnetic fields and power-deposition patterns in multiple ceramic samples are presented.

This paper dicusses experimental arrangements, describes measurement techniques and presents experimental results for hightemperature broadband dielectric properties measurements of ceramics. The cavity perturbation technique and the open-ended coaxial line method are used in these experimental measurements. The design and construction details of cavities and probes are described and representative results of measurements on zirconia and alumina samples (green and sintered) are presented. Results of measurements made on insulating materials are shown. In general measurements are made in the frequency band 1 to 10 GHz and temperatures up to 1000°C.

Analysis of dynamic development of heating patterns during microwave sintering provides vital information on the evolution of the heating process and the contributions from the various components in a complex sintering arrangement (picket fence) to the heat-transfer mechanism. Measured heating patterns often provide overall effects, and it is difficult to isolate and control the various contributions. To this end, results from numerical simulation may be significant.

In this paper we describe a thermal model that calculates the temperature distribution in ceramic samples and insulation under realistic sintering conditions. The calculation process involves a two-step procedure. The first step is to calculate the microwave power deposition in the sample and surrounding insulation. 3D FDTD calculations described in a companion paper are used for this purpose [1] The other step involves calculation of the temperature distribution using a 3D finite-difference heat-transfer program developed in our department.Results illustrating the effect of thickness of insulation and the placement of SiC rods in picket-fence arrangement are presented. Also, the need to measure additional parameters such as thermal conductivity and density of green samples as a function of temperature during sintering is discussed.

This proved to be a very insightful discussion, thanks to an enthusiastic audience and the team of panelists listed below:

Peter Angelini, Oak Ridge National Laboratory

Raymond F. Decker, Wavemat/Paton Tek Inc.

E. Eugene Eves, III, Raytheon

Frank D. Gac, Los Alamos National Laboratory

John F. Gerling, Gerling Laboratories

Magdy F. Iskander, University of Utah

Bernard Krieger, Cober Electronics

Richard Smith, Electric Power Research Institute

William B. Snyder, Microwave Materials Technology, Inc.

Wayne R. Tinga, University of Alberta

Although the panelists represented a mix of individuals from government, academia, and industry, only one aspect of industry was represented, namely microwave system manufacturers. For future panel discussions, it is recommended that the materials manufacturing (i.e., microwave user) sector also be represented.

Computer modeling and numerical techniques provide an attractive, accurate, fast, and cost-effective means for calculating power deposition patterns and temperature distributions in materials during and after their processing using microwaves. Besides giving information on heating rates and patterns, numerical modeling and computational techniques may provide valuable information that may result in better control and optimization of theheating process. The optimum material/insulation combination for improved uniformity and efficiency of microwave sintering of ceramics and the development of procedures that may prevent problems such as temperature runaways are typical examples of attainable benefits from numerical modeling and computational techniques.

The feasibility of using electromagnetic (EM) energy in industrial, medical, or in biological applications relies heavily on accurate knowledge of the dielectric properties of materials and on detailed understanding of the underlying interaction mechanisms. This chapter deals with the medical and biological applications of EM radiation. It therefore starts with a brief review of the dielectric properties of various tissues and a description of some of the basic interaction mechanisms. The techniques used to measure these properties are of particular interest to the microwave processing of materials research. Therefore, some of the time- and frequency-domain measurements methods of the dielectric properties of materials are discussed.

Available techniques for calculating and measuring the absorption of EM radiation by humans and other biological models are also described. The impact of the obtained EM dosimetry results on modifying the ANSI Safety Standard is outlined. It will be noted that many of the dosimetric measurements procedures are highly relevant to experiences in the microwave processing of materials research.

In addition, some of the medical applications in both the diagnostics and therapeutic areas are described. These applications basically exploit some of the unique dielectric characteristics of biological tissues. Specifically, noninvasive and interstitial EM hyperthermia techniques for cancer treatment are discussed, and other potential microwave methods for medical diagnostics are briefly reviewed. Future research needs to further the understanding of the various interaction mechanisms of EM radiation with materials are outlined. Every effort is made to relate experiences in the medical and biological research areas to research in the microwave processing of materials.