MRS Online Proceedings Library (OPL)

The Microwave/Radio Frequency portion of the electromagnetic spectrum has been used principally for the transmission and reception of information in communications and radar. With the advent of low cost commercial and consumer items (satellite downlinks and microwave ovens) has also come a resurgence of interest in using the energy and properties of these frequencies for other important tasks.

The purpose of this paper is to give a short review of the basics of electromagnetic heating and an overview of new trends in this use of microwave/RF energy. These trends can be divided into two broad categories: power and metrology.

The 20th century can be called the Age of Technology and during the last 40 years the technology has doubled every 10 years, and microwave energy is one of the technologies. The utilization of microwave energy for the generation of heat was discovered accidentally during testing of magnetrons at the Microwave & Power Tube Division of Raytheon in 1950.

Basic interaction mechanisms are shown to depend strongly on the dielectric and magnetic properties of a process material. This causes a strong dependence of power absorption on frequency, material particle size, shape, temperature, and density. Sintering dynamics cause the microstructure of the treated material to change resulting in a change in microwave (1W) heating uniformity and rate. The concept of dielectric mixtures is introduced to predict the dielectric and heating properties of a host material with its inclusions in the form of shells, ellipsoids, spheres, disks and needles. Simplified models are described to give a process designer some insight into the behavior of MW sintering. Microwave power, by its very nature, gives better heating control and efficiency and provides internal heat to aid the material transport during sintering. No inherent temperature limit exists for MW sintering although refractory materials used to contain the process material create an artificial upper limit. It is shown that very high (1500–2000°C) temperatures in small samples can be readily achieved using commercial microwave ovens if appropriate MW transparent sample holders are used.

Theoretical considerations regarding microwave sintering of ceramics are discussed. It is shown how the rigorous application of multiple scattering theory may permit a better understanding of this interesting process. The possibility of thermal runaway being chaotic is discussed. Experimental results, and a possible connection to percolation theory, are also given.

Electromagnetic waves can discriminate between objects of different handedness due to their transverse nature, which implies that the origin of chirality need not necessarily be molecular as in the case of optically active media. Effectively chiral composites may, therefore, be constructed by embedding chiral microstructures in non-chiral host media.

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.

Appropriate sample preparation is essential to achieve both accuracy and precision in analysis of materials. This preliminary step is one of the most time-consuming parts of many analyses and has become the rate limiting step for such multi-element techniques as ICP, AA, GFAAS and ICPMS. Acid dissolution of biological and botanical samples can take from 4 to 48 hours using classical digestion techniques. Standard reflux methods used in EPA procedures may take from 24–96 hours. Many of these same samples require only 10 to 20 minutes with microwave digestions, dramatically reducing preparation time. Volatile elements such as selenium and phosphorus can be quantitatively retained in a sealed vessel using microwave decomposition prior to instrumental analysis (1). The technique has been tested on all the major sample types including biological, botanical, geological, alloy, and glassy samples and has demonstrated advantages for each of these sample groups. The development of microwave procedures for each of these sample types is currently an intense area of research.

Electromagnetic heating of an infinite half space is analyzed to determine temperature distributions for constant conductivity. Both steady state and transient profiles are presented.

Experimental methods for determining the high-temperature millimeter-wave dielectric properties of solids are described and the data obtained on a wide variety of polycrystalline ceramics are reviewed. In general, the observed increase in dielectric constants with temperature can be modeled with a macroscopic dielectric virial expansion and shown to be primarily caused by an increase in polarizability due to volume expansion. The room-temperature loss tangents in low-absorption ceramics are probably caused by impurity doping of the primary and secondary crystalline phases at grain junctions and along grain boundaries. The rapid increase in loss tangent at high temperatures commonly observed in polycrystalline ceramics is associated with softening of intergranular amorphous phases resulting in an increase in localized electrical conductivity.

A procedure is outlined for determining the complex permittivity components of solid materials of known densities from those properties of granular or pulverized samples measured at a few different bulk densities. Properties of the solid are obtained by extrapolation of functions of the dielectric constant and loss factor that are linearly related to the bulk density of the air-particle mixture. Examples are given for whole-kernel wheat, ground white rice, and pulverized goethite.

Microwave investigations on sub-pm (“mesoscopic”) metal crystals revealed a size-induced metal-insulator transition (SIMIT). The microwaves were applied to determine the sizedependent quasi-static conductivity of metal crystals dispersed in an insulating matrix. Choosing an indium colloid for a model system allowed an in-situ particle size variation from 10 nm up to about 1μm. The experiments were carried out in a microwave bridge at 10 GHz, a frequency where the oscillation time is much longer than the elastic scattering time of electrons in a metal (quasi-static limit). The conductivity of metal crystals was found to decrease approximately with their volumes in the above size range. The 3-dimensional confinement of the electron wave packets gives rise to quantum-mechanical interference effects leading to the SIMIT as the crystals become smaller than 1μm. Experimental details and results including the sizeaffected temperature dependence of the conductivity are presented. The universal significance of the SIMIT and its consequences for the engineering of novel materials and the ultimate size-reduction in microelectronic devices are discussed as well.,

Composites of metal filled particulates, Al, Cu, Si(doped), Ag and Ni, up to 45 volume percent in a polyester or epoxy matrix were made and their dielectric properties measured between 8.5 and 11.5 GHz. The particulate size was small relative to the wavelength of the microwave radiation, but in general large compared to the skin depth. For all systems the real part of the dielectric constant increases rapidly with volume fraction filler. Some correlation was also found between the dielectric constant and the average aspect ratio of the particulates, based on the number of particulates in contact. In addition particle type and size was important. The imaginary part of the dielectric constant was small except for samples that appeared to be on the verge of percolation.

The process of removing the entrained water within an organic polymer has been one focus of RF and microwave energy use in the plastics industry. This processing is critically dependent upon the absorption of the energy which in turn is dependent upon the frequency and the material's dielectric loss factor. Knowledge, therefore, of this value is of great importance and is the subject of this study. Measurements were made on several plastic samples as a function of both temperature and frequency.

Metal alloys, when exposed to a salt/organic environment at elevated temperatures, corrode resulting in a decrease in the surface conductivity. This decrease can be monitored and assessed via the measurement of the incident and reflected microwave signals impinging upon the corroded surface. Several metallic alloys, stainless steels and inconels, were treated with a salt/organic mixture (proprietary) and heat treated at 1100 F. Periodically, the metals were removed from the furnace, allowed to cool to room temperature, and measured electrically. The samples were re-coated with the salt/organic mixture and re-heat treated. The electrical measurements showed a generally increased power absorption as corrosion thickness increased.

Novel morphologies were produced in phase segmented, toughened epoxies via microwave processing. Novel and exciting chemistries have been demonstrated through the specificity of delivery of electromagnetic radiation in tuned cavities.

Dielectric heating techniques have been applied to plastics drying applications, polymer blending, wood flour drying, and to an existing biomass conversion process. While both radio frequency and microwave equipment is expensive per unit of capacity, certain advantages may be gained in terms of the speed of drying, quality control, and the overall capitalized cost which may make dielectric heating feasible.

Microwave heating has for years been successfully used for some polymer applications, especially in preheating or continous vulcanization of rubber and also for crosslinking of thermosets. It may therefore seem surprising to find how little woork has been reported on the use of dielectric heating of thermoplastic resins. Certainly a major reason is that very few thermoplastics have dielectric properties making them respond to high frequency electromagnetic fields, or their response is inadequate for practical applications.

Many processes could benefit from the many advantages of dielectric heating if the most suitable polymer could be made to absorb such energy. Therefore a study was undertaken to find additives which in small amounts could render virtually all polymers, especially the thermoplastic ones, responsive to microwave or radio frequency electromagnetic energy without significantly changing processing and physical properties.

A series of such sensitizers was developed. Some of these are now commercially available as white, fine particle size, freeflowing powders and others are colorless liquids. Their use will depend on polymer type and application, but their common objective is to substantially increase the material's loss factor.

Selected tests with difficult to process ultrahigh molecular weight polyethylene (UHWPE) and polyphenylene sulfide (PPS) proved that the sensitizers in low concentration will allow dielectric heating of unresponsive polymers with potential for important advantages in cost and quality.

We have measured the microwave-induced damage to the near-surface region of a graphite/epoxy composite material for 1.1-μs pulses at a frequency of 2.856 GHz and a pulse power of up to 8 MW. Rectangular samples were irradiated by single-pass TE10 traveling wave pulses inside a WR-284 waveguide, and in situ and post irradiation studies were performed to characterize the material modifications induced by the microwave pulses. The results of the time-resolved optical measurements in vacuo show that surface decomposition of the epoxy resin occurs for incident pulse powers exceeding 1.1 MW, and that the surface damage is accompanied by a large increase in the reflected microwave power. Simultaneous with the onset of surface decomposition, significant light emission from the sample and a large enhancement of the gas pressure in the test cell were observed. The large increments in both the reflected power and light emission are attributed to the formation of a plasma due to electrical breakdown of the gas at (or near) the sample surface.

We have used 1.1-μs microwave pulses at a frequency of 2.856 GHz to rapidly heat the near-surface region of arsenic-implanted silicon. The samples were irradiated inside a WR-284 waveguide by single-pass TE10 traveling wave pulses. Post-irradiation studies show that surface melting occurs for incident pulse powers exceeding about 3 MW. Time-resolved measurements of the microwave reflectivity (R) show that there is an abrupt and large increase in R for microwave pulse powers greater than the melt threshold. Significant light emission was also observed from the test cell, which is most likely due to the relaxation of a microwave-induced plasma formed by electrical breakdown of gas. Using secondary ion mass spectrometry, we measured the depth profile of the implanted arsenic and found that the penetration of the melt front in the near-surface region is not spatially homogeneous over the silicon surface.

During microwave sintering of ceramics, the input microwave power and specimen temperature are limited by the breakdown of the gas in the cavity. The ionization potential of the gas, the temperature, and the specimen shape are all factors that influence the onset of breakdown. Thus, in the sintering of SiC rods in a TE111 cylindrical cavity at 1 atm argon or nitrogen, breakdown occurred at temperatures less than 1300 C and 1700 C, respectively, which are well below the required sintering temperature.

Breakdown can be suppressed by applying either high vacuum or high pressure. Sintering in high vacuum would exacerbate the problems of decomposition and vaporization of ceramics such as Si3N4 and SiC at high temperatures. Pressurization not only suppresses breakdown but inhibits decomposition and vaporization.

The TE111 cylindrical cavity has been adapted for pressurization to 1.38 MPa. The allowable input microwave power and specimen temperature have been markedly increased under high pressure conditions.