This paper reviews the expanding application of microwaves for processing solid materials, both natural and synthetic. The Canadian interest ranges from food processing to forest products, ceramic drying and curing to the preprocessing of rare minerals and ores, with the zeolite family of particular industrial interest.

In all cases, across the world, the number of process variables is large and the known physical data are limited. Yet the engineer requires to know only a few specific properties which, in general, involve rate and limit functions. The problem posed here, which has to be considered as a part of the unification of the subject of electromagnetic heating, concerns the careful use of atomistic modelling to achieve a model-informed empiricism for process development. As an example, it is shown in this paper that the majority of solids have the same piecewise linear approximate temperature model for absorbing and reflecting power: more than twenty variables affecting the dielectric properties can then be reduced to five or six which need to be measured. Correctly interpreted, this approach (due, in its broad philosophic format, to M.F. Ashby, 1987) may help solve many electromagnetic engineering problems, including those associated with breakdown and thermal-runaway.

Materials or, more generally, industrial products may be processed and/or controlled by means of microwave techniques. This paper presents a brief overview of the recent french activity related to such industrial applications of microwaves. For some of these applications, microwave techniques are already currently used while, for some others, studies are still conducted at a research and development level.

Microwave ISM applications in Brazil are reviewed, with special emphasys on industrial. applications and measurement techniques of the dielectric properties of materials, where developments have been most sicgnificant.

Special techniques for measuring the properties of typical Brazilian soils for radiometric applications in satellite remote sensing are discussed.

High power microwave techniques are presented and analyzed.

Industrial applications in ceramics, chemical products, food processing, sterilization, production of alcohol, and many others are explained. The design of special applicators for the above techniques, including continuous processing apparatus, detectors, special loads and waveguide configurations are reviewed.

Various methods for obtaining the dielectric constant of solid and liquid substanlces are presented, and experimental techniques are analyzed.

Biological and medical applications are in early stages of development, but present a great potential in the coming years at various well equipped medical institutions.

In conclusion, the great potential of microwave non-communications applications in Brazil is explained, with promising achievements in the years ahead.

1. Powerful sources of millimeter wavelength (MMW) radiation are developed nowadays for plasma investigation, particularly for thermonuclear plasma heating. Powerful oscillators for plasma heating - gyrotrons - at wavelengths down to 2 mm with output of several hundreds kilowatt at CW operation and over 1 MW at pulsed operation (pulse duration up to `1 s) are created or being created.1 The existence of CW multi kilowatt gyrotrons allows for new possibilities for material processing. The MMW range is situated at the end of the microwave range close to infrared and naturally allows applications either in the multimode microwave-oven manner or in the form of wave beam energy fluxes similar to laser radiation. Gyrotrons are close to technological lasers from the view-point of high flux densities up to `106W/cm2. By their efficiency (`40%), gyrotrons are among the most effective electromagnetic devices and significantly exceed technological lasers in efficiency as well as in CW outputs. So gyrotrons can be used for various material transformation processes requiring great amounts of energy such as plasma chemistry2 and processing of solid materials.

Examples of the effects of microwave processing are presented that suggest that microwave ‘heating’ creates higher excitation levels and increased chemical and physical reactions within a material than would be expected if the only change that had been produced by the microwaves was a rise in temperature. The examples include a chemical coal-cleaning process. the destruction of coronene in alcohol, heating of high-temperature superconducting compounds. and a microwaveinduced photovoltaic effect.

This paper analyzes of the impact of a new generation of microwave sensors on the transverse control of conveyed products and materials. Such sensors, resulting from recent developments in microwave imaging technology, offer significant improvements in comparison to other more classical microwave sensors. The main features of the new sensors are presented and discussed.

We have developed a new technique to measure the thermal conductivity of materials using microwave energy. A thermal wave is induced in a material of unknown thermal conductivity using a pulse of microwave energy. This energy is incident on one side of the material. The corresponding temperature rise of the opposite side of the material is measured. The thermal diffusivity of the material can then be determined in the same way as ‘laser flash’[1]. Some of the advantages of the microwave system are the relatively low cost of the magnetron compared to the high energy laser, easily variable pulse length, and accurate measurement of the reflected energy.

The microwave system consists of a 2.45 GHz magnetron that is pulse modulated to energies as high as 10 J. A typical pulse lasts for 1 ms with rise and fall times of 10µs. This is achieved by a high voltage source (5 – 8 kV at 1 – 2 A), switched by a high power, rf transmitting tube connected in the filament circuit of the magnetron.

This laboratory is characterizing the mechanisms underlying the potential applications of microwave treatment to the remediation of hazardous waste streams under an Environmental Protection Agency Basic Research grant. Applications that have been investigated encompass the following processes(1-14): volatilization of organics from substrates; ‘chemical fixation’ of non-volatile organics in substrates; ‘chemical fixation’ of heavy metals in substrates; kinetics of decomposition of organics in the gas phase using fixed and ‘lossy’ beds. To-date the work has been carried out on a bench-scale level. Pilot plant studies are now beginning. This paper will focus on studies carried out on non-volatile organics and heavy metals, specifically, chromium.

Microwave-assisted oxidation of trichloroethane (TCE) performed at 500-580°C has been found to be significantly more efficient than conventional oxidation methods. Experiments were conducted using a 6 kW, 2.45 GHz power supply and a 6 inch bed of silicon carbide granules in a 1 inch diameter quartz reactor tube which in turn was placed in a microwave cavity. After heating the reactor to a given temperature a TCE-air stream was passed through the silicon carbide bed. TCE was almost completely detoxified (98-99%) in a single pass through the silicon carbide bed at 500-580°C. The oxidation products are HC1, CO2 and CO. By comparison the corresponding single-pass detoxification using conventional thermal methods results in only partial conversion. The principal products being dichloroethylene (C2H2C12) and HCl.

Microwave resonators have been found to be sensitive devices for sorting dielectric objects of irregular shapes according to their volumes, for detecting voids inside objects of similar shape or for determining moisture content in objects of varying volumes. Fundamental principles of operation as well as experimental results are discussed in the paper.

After a general survey of characterization techniques the use of transient photoconductivity measurements in the microwave frequency range for the characterization of semiconductors and semiconductor devices for (opto)electronic applications is treated. Experimental details and applications of these measurements are given.

Research conducted at the Bureau of Mines, Tuscaloosa Research Center has resulted in a process of forming tungsten carbide (WC) in a microwave induced plasma (MIP). This process allows for the formation of 99.96 pet WC with a 0.04 pet free carbon content and grain sizes on the order of 0.05 µm in approximately 10 minutes. Commercial methods of producing WC require that a mixture of metallic tungsten and elemental carbon be heated to temperatures of 1,500°C for up to 7 hours. Therefore, the new Bureau developed method is significantly less energy intensive, requiring only 13 pct of the energy used in conventional WC processing methods, and is less expensive although initial capital outlay may be high.

The Bureau process involves using a 915 MHz, 30 kW variable power microwave generator to sustain a carbon monoxide plasma. The reaction chamber is constructed of a WR 975 rectangular waveguide that is short circuited at the end. The reaction takes place inside the waveguide at an electric field maximum. After the reaction chamber is readied, the microwave generator is turned on which ignites the plasma within seconds. In approximately 10 minutes, the reaction is complete resulting in a commercially acceptable product.

Patterning has been successfully performed on diamond films deposited on polished silicon wafers from CH4 and H2 gases using microwave plasma chemical vapor deposition. The selectivity was achieved using a lift off process on silicon wafers which had been polished with diamond paste. The lift off process involves deposition and patterning of a thin ZnO film followed by deposition of amorphous silicon using a glow discharge technique. This is followed by microwave plasma CVD of the diamond film. The film is deposited selectively on the crystalline silicon areas of the substrate. Quality and patterning resolution of the resulting diamond films have been determined.

The Oak Ridge National Laboratory (ORNL) Waste Handling and Packaging Plant is developing a microwave process to reduce and solidify remote-handled transuranic (RH-TRU) liquids and sludges presently stored in large tanks at ORNL. Testing has recently begun on an in-drum microwave process using nonradioactive RH-TRU surrogates. The microwave process development effort has focused on an in-drum process to dry the RH-TRU liquids and sludges in the final storage container and then melt the salt residues to form a solid monolith. A 1/3-scale proprietary microwave applicator was designed, fabricated, and tested to demonstrate the essential features of the microwave design and to provide input into the design of the fullscale applicator. Conductivity cell measurements suggest that the microwave energy heats near the surface of the surrogate over a wide range of temperatures. The final wasteform meets the waste acceptance criteria for the Waste Isolation Pilot Plant, a federal repository for defense transuranic wastes near Carlsbad, New Mexico.

The availability of high power high frequency microwave sources of up to tens of kW has opened up important technological and industrial applications of microwave radiation. The particular use of gyrotrons for generating the high power microwaves is considered.

The application of high power microwaves for fusing wear resistant coatings onto refractory materials is discussed, together with the brazing of ceramics, welding together of thick metal parts and the rapid heating and curing of polymers for high reliability long life environmental coatings.

It is shown that the use of high power gyrotron radiation enables considerable improvements to be made on conventional technologies for high temperature processing of materials and enables the development of new and efficient processes.

A model for microwave heating of a ceramic preform is presented. Microwave power and external cooling are manipulated to obtain desired temperature profiles for effective chemical vapor infiltration (CVI) of the preform. A mass transport and chemical reaction model is developed to demonstrate preform densification. Conditions affecting matrix deposition are evaluated.

Using a new microwave small-sample quasi-TEM mode applicator, heating rates up to 700°C/s were obtained for various oxides and ceramic materials at 915 MHz. Design details are presented. A 60 W solid state power source is used to supply the microwave energy and control material temperature. Temperature is measured using either infrared or thermocouple techniques. The applicator is a TEM resonator with a TM mode gap field modified by a hollow variable radius center conductor acting as a waveguide below or near cutoff. This design creates a microwave materials analyzer equally suited for high or low temperature material studies, is scaleable to different frequencies and can be used to measure dielectric properties up to a temperature of at least 1500°C. Solid, granular, liquid, gas or plasma samples can be accommodated. By using microwave transparent refractory materials around the sample, temperatures greater than 15000 C can be maintained in some materials. Perturbation of the resonator by the inserted material causes a frequency shift which is nearly linear over a permittivity range of at least 1–70. This is an order of magnitude improvement over the conventional perturbation approach. For microwave joining or sintering of ceramic rods, the energy can be concentrated into a symmetrical hot zone as narrow as 1 mm.

Whenever possible, the modular approach to microwave applicator design presents several advantages, both economical and operational, particularly in countries where high-power microwave generators are not manufactured on a commercial scale. In this paper, examples are shown of industrial applicators of simple, modular design, for the processing of liquid and solid materials, coupled to generators built with domestic microwave oven components.

A vector network analyzer combined with an open ended coaxial probe provides a way to make non-destructive measurements of complex permittivity. These measurements can be made on both solids and liquids.

Many authors have done work on using an open ended coaxial line for making these measurements[1][2][3]. They have presented different techniques for modelling the coaxial probe. This paper does not focus on the area of modelling, but deals instead with the use of a vector network analyzer and coaxial probe in making measurements. To enhance measurement accuracy, the technique described adapts a calibration technique currently in use in vector network analysis.

An HP 8720B vector network analyzer was used for the development and measurements presented in this paper.

It is well known that a leaky wave in free space may show a decay rate even less than zero, or, in other words, its amplitude may grow instead of decay [1,2,3,4]. The above behaviour is very attractive from the point of view of deep processing of materials [5,6]; thus we extended the leaky-wave antenna theory to the lossy (half) space case and analyzed in detail the channel guide applicator [7,8,9].