A finite element numerical model is used to predict the thermal and mechanical response of mineral-bearing ores irradiated by microwave energy. The model considers a small, spherical, pyrite particle surrounded by a matrix of calcite. Power density data are determined from the dielectric properties of the mineral and host rock materials at typical microwave frequencies and power capabilities. The effects of varying power density and mineral particle diameter are studied. Using power densities within the expected achievable range for pyrite, significant temperature differences are predicted between the mineral particle and host rock. These temperature gradients lead to circumferential tensile stresses in the host rock well in excess of the reported uniaxial tensile strength of common rock materials. It is shown that, for a fixed microwave energy source, both the temperature difference between the mineral and host rock, and the peak tensile stress in the host rock are reduced as the mineral particle size is reduced. Recent experimental efforts to corroborate this numerical study are briefly described.

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.

Experiments on microwave sintering of ceramic fibers in a single-mode cavity have revealed the presence of thermal spikes and “hot spots” which sometimes travel along the fiber and eventually disappear. They are triggered by relatively small increases in microwave power, and thus have obvious implications for the development of practical microwave-based fiber processing systems. These hot spots are conjectured to originate at slight irregularities in the tow morphology, and propagate as the result of solid phase transitions which take place at elevated temperatures and reduce the dielectric loss coefficient є”.

An elementary mathematical model of the heat transfer process was developed which reproduces the essential features of the observed phenomena, thus lending support to our conjecture. This model is based on the assumption of one-dimensional heat conduction along the axis of the fiber tow, and radiation losses at the surface.

Quantities of untreated hazardous wastes are growing at alarming rates. Intensive searches are underway to develop more effective and energy efficient solutions that can reduce or eliminate the impact of these wastes. One potential answer involves the use of microwave heating. Incorporated into process equipment, microwave heating can provide a number of important attributes that contribute to productivity improvement. These characteristics include: selective heating associated with the potential for recovery of value-added product; volumetric heating to improve processing time; the potential for remote operation to limit personnel exposure to hazardous waste; and, in situ application to reduce the risk of airborne contamination caused during material transfer. Successful applications appear in niche areas using microwave heating to provide an economic advantage over conventional surface heating techniques particularly in areas such as infectious waste, soil decontamination, solvent recovery and microwave ‘catalysis’. The perception of high cost and ‘fear-of-the-unknown’ remain obstacles preventing more widespread adoption of these technologies as in the case of other relatively new microwave applications. An understanding of the reasons for the success of existing commercial systems helps in targeting opportunities for future applications in hazardous waste treatment. This review illustrates these factors by highlighting examples of existing applications, new research and development initiatives and future opportunities.

An experimental investigation was carried out to determine the regeneration characteristics of adsorbent beds using microwave heating. Tests were performed with water on F-200 activated alumina and on types 4A and 13X zeolite molecular sieves and with methanol on 4A and 13X zeolites. For these systems, microwave heating produced very high rates of regeneration with either gas purge or vacuum purge. Dielectric properties of both the adsorbent and the adsorbate affect the heating response. Temperature profiles in the adsorbent bed show that microwave heating eliminates the mass transfer front characteristic of conventional regeneration. This may have potential implications for improved bed life. The ultimate goal of the program is the recovery of volatile organic compounds (VOCs) by regenerating adsorbents with little or no dilution in a stripping medium.

More than 18 million tons of nitrogen oxide (NO and NO2) are emitted into the air over the United States each year. These oxides contribute to the formation of acid rain and street level ozone, and are most concentrated in areas of industrial and commercial activity. The response of the Environmental Protection Agency to the problem has been to legislate anti-pollution laws and regulations which will be enforced in increasingly strict stages beginning in 1997. Existing NOx abatement technologies do not meet the long range NOx abatement needs of the processing industries and utilities.

Char, a heat treated and volatile form of coal, can effectively adsorb NOx from waste gas. Microwave energy, when properly applied, can convert the NOx and carbon via reduction to nitrogen and carbon dioxide. The basic principles of this technology are taught as well as some of the pertinent development aspects currently under investigation.

Results of research on the use of microwave energy for several waste remediation applications are discussed. Studies include; processing of simulated nuclear waste glass frits, the destruction/vitrification of electronic circuitry from commercial and defense applications, the decomposition of organic compounds and the possible use of microwave energy in off-gassing operations during the ashing of electronic circuitry. Results of leach tests on simulated nuclear waste glass are presented as well as results from preliminary tests on organic wastes.

Previous research work in the area of microwave heating for low temperature thermal desorption of contaminates has led to a novel method of delivering heat energy to contaminated materials (soils) that have poor electrical characteristics for the absorption of microwaves. This method involves the use of a heating facilitator. The facilitator, which in this case is a ferromagnetic material (magnetite), is admixed with the soil or sludge to be treated. When exposed to microwave energy, the facilitator preferentially absorbs the energy and transfers heat to the contaminated matrix. Using this method, it has been possible to achieve temperatures in excess of 900 °C (1650 °F) in test soils. At these temperatures, even the highest molecular weight hydrocarbon contaminants are compelled to desorb from the soil.

The advantage of using magnetite is that it can be separated by conventional separation methods for recycle and reuse. The technology has implications of being useful in several chemical processing operations.

This survey paper introduces the basics of RF and microwave device (or component) testing. It establishes a foundation of concepts and terminology.

In high temperature microwave systems, property variations of the process material can cover orders of magnitude causing significant changes in microwave coupling efficiency, heating rate, impedance matching, field strength and penetration depth. We created some first-order, temperature dependent models of these system parameters for a number of materials and calculated typical values over the temperature range of 0–1000 C. The results show that the input impedance of a resonator, the relative energy dissipation the power density and the penetration depth in the process material are strongly influenced by the temperature dependent dielectric constant. Process control algorithms require models of the system parameters and hence our models are useful for automated microwave processing systems. Moreover, it is shown that the relative dielectric loss factor of the process material can be monitored via the reflection coefficient of the microwave resonator containing the material.

Two different microwave ECR sources have been examined for use in dry etching of GaAs, InP and related compounds. The first, a multipolar, tuned-cavity disk source (Wavemat) has been in operation for ∼4 years and provides stable, uniform discharges of BCl3/Ar, CH4/H2, SF6/Ar, HI/H2 and CCl2F2 for smooth, anisotropie pattern transfer into the III-V materials. Electron densities of -5 × 1011 cm-3 are achieved at 1 mtorr for microwave powers of >700W, as determined by microwave interferometry. Additional rf (13.56 MHz) biasing of the sample position is necessary for achieving practical etch rates. The second source is a Astex ECR high-profile geometry, with a drift-tube and secondary magnets near the sample position to improve the plasma confinement and etching uniformity. Similar electron densities are found as with the resonant cavity source, and optimization of the top and bottom magnet currents produces similar etch uniformities. Examples of the etching characteristics of III-V's as a function of microwave power, rf-induced dc bias and sample temperature are given.

This paper reports on the results of experimental studies of the effect of bandwidth on the uniformity of energy distribution in a multi-mode cavity as well as on the development of a theoretical model to, at least qualitatively, describe the effect of varying the processing frequency. Using a mapping method reported in earlier work, Microwave Laboratories and Oak Ridge National Laboratory personnel have graphically measured the effect of processing bandwidth on heating uniformity. These results can be compared to the effects of other attempts to improve uniformity e.g., “mode stirrers.” Additional insight into the effect of bandwidth on time-averaged energy distribution within the cavity can also be gained from the simple theoretical model, which is presented for comparison with the experimental results. Finally, this paper uses the presented results to comment on the scalability of the variable frequency microwave processing methodology for large-scale applications.

This paper gives details of microwave firing experiments carried out with a large 17.6kW 2450 MHz microwave kiln on a range of ceramic materials.

The design of the large kiln is discussed, together with results of large batches of ceramic products. The design of successful heating experiments in terms of thermal insulation and microwave susceptors is discussed in relationship to the dielectric properties of the material to be fired.

Results are given of the fired properties from batches of products fired in the kiln, and the uniformity of the fired properties of large batches is discussed.

A thorough understanding of fundamental microwave absorption mechanisms in ionic crystalline solids is important for microwave sintering of ceramics, as well as the design of high speed electronic packaging, advanced radomes, etc. Of particular importance to these applications are how the density and type of crystalline defects affect the dominant microwave absorption mechanisms. We have designed experiments to measure microwave absorption in NaCl samples with controlled variations in defect conditions(pure, point defects, dislocations and grain boundaries) at different temperatures (20–400 °C) and frequencies (2–20 GHz). Initial results are reported and discussed.

Recently the heating of a thin ceramic cylinder in a single mode applicator was modeled and analyzed assuming a small Biot number and a known uniform electric field through out the sample. The resulting simplified mathematical equations explained the mechanism for the generation and growth of localized regions of high temperature. The results predicted that a hot-spot, once formed, will grow until it consumes the entire sample. Although this phenomenon is seen in some experiments, others show that the hot-spot stabilizes and moves no further.

A new model is proposed which incorporates the dependence of the thermal conductivity and the effective heat transfer coefficient upon temperature, and the nonuniformity of the electric field along the fiber axis. The resulting simplified mathematical equations indicate that these effects may stabilize the growth of hot-spots.

We describe experiments to test for microwave enhancement of bulk vacancy diffusion in NaCl, bulk interstitial diffusion in AgCl, and grain boundary diffusion in NaCl. Experimental results for bulk vacancy diffusion in NaCl indicate that intrinsic vacancy mobility is not enhanced by microwave field intensities typical of microwave sintering experiments. Nevertheless, a microwave effect is observed with features consistent with a recent theory for microwave-enhanced driving forces.

Microwave energy for processing materials is emerging as a vital manufacturing technology for the nineties and beyond. Research to date has shown significant advantages in several areas, including drying and sintering, joining, surface modification and waste remediation. Increased processing rates, improved physical and mechanical properties and, in some cases, reduced hazardous emissions have sparked the interest of many manufacturers in the ability to integrate microwave processing techniques into existing and future manufacturing operations. This presentation will provide an overview of the microwave processing research and development work in progress at the University of Florida.

A comparison of microwave and conventional processing of silicon nitride-based ceramics was performed to identify any differences between the two, such as improved fabrication parameters or increased mechanical properties. Two areas of thermal processing were examined: (1) sintered silicon nitride (SSN) and (2) sintered reaction-bonded silicon nitride (SRBSN). The SSN powder compacts showed improved densification and enhanced grain growth. SRBSN materials were fabricated in the microwave with a one-step process using cost-effective raw materials. The SRBSN materials had properties appropriate for structural applications. Observed increases in fracture toughness for the microwave processed SRBSN materials were attributable to enhanced elongated grain growth.

Effects of microwave heating were investigated for calcining metal-oxide precursors that were supported on an activated carbon. To obtain the heterogeneous metal oxide/activated carbon catalysts, calcination of the precursors impregnated onto the activated carbon was executed for 10 minutes at 300°C within a cavity of 2.45 GHz microwave. Such an electromagnetic treatment was compared with the calcination in a conventional muffle furnace. Activities for the oxidation of the benzene or oil adsorbed on the catalysts prepared by the two methods were examined with a micro-pulse reactor as well as with thermogravimetric analysis. Experiments revealed that the time required for the calcination in the electromagnetic field is much shorter than that in the muffle furnace. The catalytic activities in the low-temperature oxidation were also enhanced by the application of microwave in the calcination. In addition, the oxidative reaction of the adsorbates could be obtained without the activated carbon being burned. The microwave effects were outstanding for Co3O4/activated carbon catalysts.

Since the volatilization of lead oxides is inevitable at high temperatures, the problems of maintaining stoichiometry and air pollution are of great concern in conventional synthesis of lead-barium titanate. We have demonstrated that the reaction between PbO, BaO and TiO2 could be greatly accelerated by using microwave energy. The synthesis of (PbxBa1-x)TiO3 can be completed in 10 minutes at about 750°C by microwave heating, while it takes at least 2 hours at 850°C in a conventional heating. The weight loss of PbO during microwave synthesis was much less than that in conventional synthesis. Thus the microwave synthesis technique may find wide applications for the preparation of various lead-containing materials in the production of electronic ceramics.