During the past decade, many exploratory studies and experiments have been performed on the microwave heating and processing of ceramics and composite materials. Much of this effort was stimulated by the unique and potential benefits that microwave energy can provide over conventional processing methods. While microwave processing of ceramics is still in an early developmental stage, there are many areas yet to be explored, challenges to be met, and economic and commercial payoffs to be substantiated.

Since the first MRS International Symposium on Microwave Processing in 1988, interest in this field has grown and many new developments have occurred. The purpose of this paper is to highlight some of the recent advances, to discuss the current state-of-the-art, and to suggest some future directions.

This paper surveys some of the recent research into the microwave processing of polymers. The mechanism of coupling of the radiation with polymers is described and a historical perspective is presented on the development of this technology from the 1960's when the processing of rubber was first initiated, to current research in high performance materials and composites. The controversial area of microwave effects on processing is also discussed.

A brief review of the current status of microwave processing in the UK is provided, focussing on ceramic and polymer processing, materials synthesis, catalyst preparation and dielectric property measurement. The central role of the Materials and Microwave Processing Group is also discussed.

A two dimensional model for computer simulation of microwave heating of ceramics has been developed. The effects of microwave power, properties of ceramics, and thermal insulation on dynamic temperature distributions of ceramics under various processing conditions have been simulated. The practical solutions for eliminating thermal runaway and improving temperature homogeneity under microwave rapid heating can be approached using this computation technique.

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.

The use of single-mode cavities has been recognized as a viable technique for sintering and joining ceramics and for creating microwave plasma for materials processing. It is often reported, however, that tuning these cavities at lower and higher temperatures critically depends on the types of materials being processed, the variation of ε* as a function of temperature, the size and shape of samples, and the overall heating system configuration with and without insulation.

In this paper we use the finite-difference time-domain technique to address the performance of the single-mode cavity heating system under a wide variety of heating conditions and system configurations. Specifically, practical issues such as limiting values of sample sizes, the dependence of optimal sample sizes on the complex permittivity of materials under test, the role of sample shape on enhancing uniformity of heating patterns, and various tradeoffs involved in utilizing various cavity modes and/or SiC rods as stimulus for heating large samples are evaluated.

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.

The unavoidable way of a better conception of cavities for industrial applications is electromagnetic field numerical computation with the aim of placing the fields E and H into the material, and furthermore to obtain some characteristics like the input impedance of the cavity or the dissipated power in the sample. From Maxwell's equations and using a Finite Element Method with 3D edge elements we get a linear system from which, after resolution, we can obtain several kinds of results: research of eigen modes of an empty or partially dielectric loaded cavity, that is, for a bounded domain, all the frequencies that could be excited, using only dimensionnal (L,l,h) and physical (εʹ,εʺ) characteristics, or response of a loaded cavity to an external excitation, that is, setting a TE10 mode in a waveguide feeding a cavity, obtain the distribution of field inside the oven and the dielectric. An original approach allows us, by projection of the excitation on eigen modes, to understand the behavior of the cavity and the interaction wave-product. Our computation is in very good agreement with analytical results we can obtain in some specific configurations (eigenvalues of an empty cavity, or transmission of only the TE10 mode in a correctly shaped waveguide).

We describe a new type of microwave processing furnace in which the frequency can be varied continuously from 4 to 8 GHz and the power level varied from zero up to 2.5 kW. The extraordinary bandwidth of this furnace is achieved by using a traveling wave tube (TWT) amplifier originally developed for electronic warfare applications. The TWT is a linear beam device characterized by a traveling electromagnetic wave that continuously extracts energy longitudinally along the path of an electron beam. The TWT, unlike other microwave tubes such as the magnetron, klystron, gyrotron, and others, does not depend upon resonant RF fields and is therefore capable of wide bandwidth operation.

Traditional microwave heating and measuring are mostly conducted using two different channels, i.e., a single frequency high power channel for heating and a sweeping frequency low power channel for measuring. Here, a different single frequency system is discussed, by which heating and measuring are conducted simultaneously using one high power source. The theoretical basis emphasizing Q determination at a single frequency is presented. A re-entrant resonator serves as the applicator. Without the normally required swept frequency channel, the experimental system is greatly simplified.

Coupling of the input power to a dielectric in a rectangular microwave cavity by using a compound iris having both inductive and capacitive slits, has been investigated. The compound iris is used in combination with a variable tuning plunger to maintain a resonant frequency of 2.45 GHz and minimize the reflected power. Under certain conditions, the compound iris has been observed to achieve an improved coupling of the input power, resulting in higher temperature of cylindrical 0.95 cm rods of silicon carbide (SiC) and alumina (A12O3). By placing the SiC perpendicular to the applied E field (with SiC enclosed in a porous insulation partially filling the applicator), coupling with the compound iris by adjusting both inductive and capacitive slits was achieved for a wide range of values compared to the use of a single pair of inductive slits. The coupling was more sensitive to a precise positioning of the single inductive slit. Such a difference was not observed for the case where the sample was placed parallel to the E field. In temperatures over 1600°C have been reached with a 600 W of power injected into the cavity. In tests with low loss alumina samples (99.8% pure) placed parallel to the E field, the compound iris permitted a greater coupling and higher temperature. The presence of alumina insulation had a significant effect in achieving higher temperatures, and occassionally preventing the initiation of arcing.

A new theory has been developed to calculate the microwave absorption and resultant temperature profile within a sphere positioned in a single mode rectangular cavity. This theory is an extension of a total absorption model based on Mie scattering results. Temperature profiles have been computed for alumina spheres at the center of a rectangular cavity excited in the TM354 mode. Parametric studies reveal significant structure in those profiles under special conditions that are associated with electromagnetic resonances inside the spheres. Anomalous behavior similar to thermal runaway occurs at moderate temperatures when there is enhanced absorption associated with resonant conditions in the sphere.

A Microwave Induced Plasma has been used to investigate the processing of metallurgically significant refractory oxides in a non-equilibrium or “cold” plasma. The main emphasis of this work has centered on the reduction of TiO2 in a hydrogen plasma. The plasma was maintained using microwave energy (2.45 GHz) in a single mode resonant cavity. The reaction was monitored for volatile species with a quadrupole mass spectrometer, while the extent of reaction was determined by an external standard X-Ray diffraction technique. The effect of process variables (absorbed power by the plasma, chamber pressure, and time) on the extent of reaction was investigated. Hydrogen reduction in a plasma has produced up to 60% conversion of TiO2 to Ti2O3 in 11 minutes with the sample on a grounded support rod, while only an 8% conversion is observed for a sample on an insulated support rod. Sample temperatures were below 735 K for most plasma conditions, based upon thermodynamic considerations.

Plasma activated CVD in a remote operation mode is applied to the deposition of dielectric layers on extended substrates. Layer thickness uniformity of ±1% over areas of 80mm diameter is demonstrated. The microwave applicator sustaining the plasma source of condensable species is compared in its deposition characteristic to conventional evaporation sources. Deposition efficiencies up to 70% are reached. The layer materials used are SiO2 and TiO2. Multilayer structures are prepared and optically characterized.

An ECR microwave plasma etching system and an alternating bias voltage source with variable frequencies ( 10kHz to 10 MHz ) have been built for the study of the effects of the substrate bias on the submicron feature etching. A quadrupole mass spectrometer was used for the end-point monitoring and the plasma chemistry analysis. A Langmuir probe was used to characterize the etching plasma. The system can generate plasmas with densities ranging from 1×1015m−3 to 5×1016m−3 corresponding to gas pressures from 1×10−4 torr to 6×10−3torr at a fairly low microwave power. This condition results in a stable electron temperature. The magnetic field profile can be adjusted to obtain a constant plasma density of 2.8×1016m−3 and an electron temperature in kTe of 4 eV to 5 eV. The plasma beam is of 75mm in diameter with the ion current intensity fluctuation of about ±5% at the etching position for the gas pressures ranging from 4×10−4 torr to 3×10−3 torr at an absorbed power of 70Watts. The plasma density, the electron temperature, the breakdown power, and the sustaining power have been studied as functions of gas pressure in order to optimize the etching performance.

The dielectric properties of carbon imply that heating with microwave energy should not be significant at elevated temperatures. It has been demonstrated, however, that temperatures in excess of 1900°C can be achieved with low powers. The effect of particle size on the heating rate will be presented and a heat absorption mechanism is postulated to explain the high temperatures observed.

Under controlled conditions, conventional and microwave heating has been used to observe growth of the carbon crystallite size up to a temperature of 1400°C. Under these conditions, no enhanced crystal growth was observed for microwave heating.

Reports of enhanced sintering rates associated with microwave heating may be due to nonthermal lattice fluctuation statistics. Recent theoretical analyses reviewed in this paper confirm the feasibility of this phenomenon for a wide variety of situations involving very different microwave absorption mechanisms. For materials with weak microwave absorption coefficients, the effect is expected to be uniformly distributed throughout the volume. For strongly absorbing materials, however, the effect is expected to be concentrated near the material surface, with a characteristic exponential penetration depth of Lnt ∼ 10 - 100 μm. An “observable” nonthermal effect depends on the relative magnitude of the microwave electric field strength |E| and the lattice ion energy relaxation rate γ with the most pronounced effects occurring for larger values of |E| and smaller values of γ.

A sublinear dependence of the microwave heating rate on the incident microwave power implying a saturation effect appears to have been observed recently. We present simple model calculations to address this observation on the basis of an idea of spatial confinement of the absorbing charges in grain boundary regions. Two natural lengths exist in this model: the spatial extent of the confining region, and the maximum distance an absorbing charge travels under the combined action of damping and of the oscillating microwave field. We suggest that a mismatch of these lengths results in the observed saturation, more generally, in the observed decrease in absorption efficiency.

Electrically polarizable materials in an inhomogeneous electric field experience a dielectrophoretic force which is proportional to the gradient of the square of the electric field. For high frequency radiation, the time averaged electromagnetic field provides a dc force. The force exerted on a sapphire and a metal sphere by the microwave field in a single mode resonant cavity has been measured. The force measurements have been made at equally spaced points along three orthogonal axes centered in a cylindrical cavity operating in the TE111 mode using 20 watts of microwave power at about 4.84 GHz. These data are compared to the simple theory of dielectrophoresis on a small sphere and verify that, for this mode and in the absence of other forces, there is a unique and stable equilibrium position at the center of the cavity. Measurements of the quality factor of the cavity provide a means of making an absolute check to the theory. The TE111 mode is one of an infinite set of modes predicted to have stable equilibrium positions that exist away from the cavity walls. It is proposed that dielectrophoresis in a microwave resonant cavity can be used to position samples for material processing applications in a microgravity environment. Knowledge of these forces may also be important for interpreting thermogravimetric measurements made in a microwave oven.

Microwave energy generated by the electron beam in the superconducting cavities of the Continuous Electron Beam Accelerator Facility (CEBAF) is absorbed by special loads fabricated with a novel lossy ceramic material (AIN-glassy carbon) developed especially for this application. Strict environmental constraints (ultra-high vacuum compatibility, operation at 2 K, brazeability, etc.) are imposed on the materials which can be used. Several other ceramics were sintered with AIN and various minority conductive powders to obtain the desired electrical properties according to the ‘artificial dielectric’ model. Dielectric permittivity data and results of low temperature measurements are reported.