A simple set-up to measure the transient photoconductivity in the microwave frequency range is presented. The effective mobility is derived from the end of pulse transient photoconductivity. This can be used for the characterization of semiconductor films. Examples of measurements on a-Si:H films are given.

Measurements of the complex dielectric constant of microwave sintered, porous ZnO at 2.45 GHz are presented. The dielectric properties as a function of porosity do not obey the standard Maxwell-Garnet dielectric mixing law with the ceramic material as the major phase, but instead behave as if the ceramic grains always remain in relatively poor electrical contact even at very high densities. Electromagnetic simulations, carried out for a variety of microstructure geometries, are performed to explore this observation. A model which treats the ceramic as an array of grains and pores, with the grains separated from each other by nonor slightly-percolating, fractal-geometry surfaces, provides a good description of the experimental results.

To investigate the suitability of heavy clay bodies for microwave processing, a range of material properties needed to be investigated.

This paper presents results of the analyses, chemical, mineralogical and thermal, of a range of heavy clay bodies. The microwave dielectric properties of these bodies were measured, over a range of temperatures, using the cavity perturbation technique. The relationship between the dielectric properties and the results of DTA/TGA analysis is discussed and related to the potential use of microwave energy for the firing of these materials.

The permittivity and conductivity of suspensions of spherical sub micron size polystyrene particles were measured in the 10 kHz to 10 MHz frequency range using a HP 4192A Impedance Analyzer. Due to the high conductivity of the samples (0.1 to 0.6 S/m) every detail of the measurements had to be optimized. For each sample, the instrument was calibrated as an electric quadrupole, by means of short, open, and reference measurements, using for this last a standard electrolyte with a conductivity similar to that of the sample. The results obtained were combined with measurements in the 1 MNHz to 28 GHz range made in Goettingen, Germany, using precisely the same samples. The final spectra cover the broadest frequency range to date for these systems.

Resonant internal and radiated external electromagnetic fields of an open coaxial structure are analyzed using FDTD. These fields are characterized for two structures, with and without a quarter-wave choke. Techniques utilized to obtain a steady state solution are discussed. Good agreement with field measurements is obtained. This moveable open applicator is convenient for creating intense hot zones as required in ceramic joining. It is also suitable for the measurement of surface dielectric properties of planar materials at room or elevated temperatures.

A simple waveguide measurement technique is presented to determine the complex permittivity of printed circuit board material. The printed circuit board with metal coating removed from both sides and cut into size which is the same as the cross section of the waveguide is loaded in a short X-band rectangular waveguide. Using a network analyzer, the reflection coefficient of the shorted waveguide(loaded with the sample) is measured. Using the Finite Element Method(FEM) the exact reflection coefficient of the shorted wavguide(loaded with the sample) is determined as a function of dielectric constant. Matching the measured value of the reflection coefficient with the reflection value calculated using FEM and utilizing Newton-Raphson Method, an estimate of the dielectric constant of a printed circuit board material is obtained. A comparison of estimated values of permittivity constant obtained using the present approach with the available data.

This paper examines the design aspects and numerical modeling of single- and multi-mode, high power, microwave resonant cavity applicators. A novel numerical electromagnetic modeling approach, based on the finite difference solution of Maxwell's equations in the time domain, is utilized taking into account the finite conductivity and thickness of the cavity walls. Results obtained from the model enable an unambiguous choice of the optimum iris size for critical coupling; shape, dimension, orientation and number of radiating elements for a prescribed irradiation pattern of slotted waveguide feeds as well as dimensions and excitation mechanism of multimode cavities. The model also allows the evaluation of the volumetric distribution of the microwave energy at any point inside the load and may eventually be used to determine the dynamic transient temperature profiles induced during the curing process.

We describe a hybrid numerical method for modeling the electromagnetic interaction of a low-loss material in a single-mode waveguide applicator. The method we propose utilizes a combination of asymptotic and numerical techniques. The interaction between the applicator and the electromagnetic fields is described using scattering matrix theory and the interaction between the electromagnetic fields and the ceramic is determined numerically. Several simulations are presented to show the accuracy and simplicity of this method along with the relatively small amount of computer resources it requires.

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.

Normal modes and power absorption distributions in microwave cavities containing lossy dielectric samples were calculated for problems of interest in materials processing. The calculations were performed both using a commercially available finite-difference electromagnetic solver and by numerical evaluation of exact analytic expressions. Results obtained by the two methods applied to identical physical situations were compared. Our studies validate the accuracy of the finite-difference electromagnetic solver. Relative advantages of the analytic and finitedifference methods are discussed.

The Finite-Difference Time-Domain (FDTD) method has been used by our group to simulate a wide variety of Radio Frequency (RF) and induction-drying processes and realistic, microwave-sintering experiments. Many results were presented and some guidelines towards the effective use of the microwave and RF heating technologies of ceramic ware were developed.

In this paper we describe an experimental effort which was used to validate the FDTD simulation results. Specifically an experimental RF dryer, Thermax Model No. T3GB operating at 25 MHz, was used to dry ceramic ware of various materials, sizes, and shapes and the temperature distribution pattern was monitored using six fiber-optic temperature probes. The measured heating patterns were then compared with the FDTD simulation results. Many of the guidelines developed using the numerical simulations were confirmed experimentally.

Results from various comparisons between simulation and experimental data will be presented. Additional results from the simulation efforts illustrating possible procedures for improving the efficiency and the uniformity of RF drying will also be described

Numerical simulations were used to investigate the effectiveness for materials processing of recently announced microwave spot-heating applicators. These devices can produce heating in depth over a region a centimeter or two in diameter. Computed 3-D time-dependent temperature distributions for both stationary and moving samples suggest that such applicators could be very useful for joining of ceramics, such as alumina. Moving the beam relative to the workpiece at an appropriate rate avoids thermal runaway.

A new software package is described for 2-Dimensional modelling of microwave heating. It is particularly suited to the high temperature heating of ceramics, where electrical and thermal properties are strongly dependent on temperature. The described program, x_tlm, runs under Xwindows, which allows a microwave cavity and its contents to be entered graphically and stored as a schematic diagram. The diagram is automatically converted into a mesh format for simulation using a topology independent Transmission Line Modelling formulation. Temperature dependencies of material properties are specified as polynomial functions of temperature. Surface heat loss by the radiation mechanism, which dominates at higher temperatures, is also introduced into the model.

A single mode microwave resonant cavity used for materials heating is simulated using the FDTD method. The simulation includes both the resonant cavity structure and the input microwave power coupling structure. An overlapping grid method with interpolation between the conformal grid systems developed for each section of the microwave system is used to simulate geometrically complex microwave structures. Comparisons of the FDTD solution for a simplified, lossy-loaded cavity is made with a known analytical solution.

Precise knowledge of millimeter-wave oven properties and design studies have to be obtained by 3D numerical field calculations. A simulation code solving the electromagnetic field problem based on a covariant raytracing scheme (MiRa-Code) has been developed. Time dependent electromagnetic field-material interactions during sintering as well as the heat transfer processes within the samples has been investigated. A numerical code solving the nonlinear heat transfer problem due to millimeter-wave heating has been developed (THESIS3D-Code). For a self consistent sintering simulation, a zip interface between both codes exchanging the time advancing fields and material parameters is implemented. Recent results and progress on calculations of field distributions in large overmoded resonators as well as results on modeling heating of materials with millimeter waves are presented in this paper. The calculations are compared to experiments.

The microwave heating and ignition of a combustible material is modeled and analyzed in the small Biot number and large activation energy regimes. Both the temporal and spatial evolution of the temperature within the material are described. The ignition characteristics are determined by a localized equation for the perturbation to the inert temperature, which is shown to exhibit thermal runaway behavior. Analysis of this local equation provides explicit ignition conditions in terms of the physical parameters in the problem.

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.

Correlation between the rate of densification in powder ceramic materials and specific absorbed microwave power is determined by the experimental method. The approach is based on a comparison of the densification curves obtained at different rates of heating. The changes in the ramping rate are provided by varying the microwave power fed into the microwave furnace. Using the energy balance for the microwave heated samples, the correlation between the apparent energy of activation at the initial stage of densification and the value of the specific microwave power absorbed in heated materials are found. The experiments with silicon nitride-based ceramics allowed to determine the reduction in the value of the activation energy resulted from an increase in the specific absorbed microwave power.

915 MHz microwave heating has been used to drive the CO oxidation reaction over Pd/Al2O3 without significantly affecting the reaction kinetics. As compared to an identical conventionally heated system, the activation energy, pre-exponential factor, and reaction order with respect to CO were unchanged. Temperature was measured using a thermocouple extrapolation technique. Microwave-induced thermal gradients were found to play a significant role in kinetic observations.

A theoretical model of microwave absorption in linear dielectric (non-ferroelectric) ionic crystals that takes into account the presence of point defects was synthesized and verified using NaCl single crystals. In the next stage of this research, we will introduce a controlled density of dislocations into the single crystal NaCl samples and study the effect on the microwave absorption mechanisms (ionic conduction, dielectric relaxation and multi-phonon processes) both theoretically and experimentally. Qualitative outlines of this modified theory are presented. The loss factor ε’ has been measured in the dislocation-free case by a cavity resonator insertion technique and the experimental results are in good agreement with the theoretical model. We describe the sample preparation technique that will be used to produce a controlled dislocation density in single crystal samples that will also be studied in our cavity resonator insertion system.