The Department of Defense (DoD) and the National Aeronautics and Space Administration (NASA) requested that the National Research Council's National Materials Advisory Board conduct a study to assess the current status of microwave processing technology, identify applications of microwave technology where resulting properties are unique or enhanced relative to conventional processing or where significant cost, energy or space savings can be realized, and to recommend future activities in microwave processing. A committee was established to perform the study and report on their findings. This paper is a summary developed from the committee's report, Microwave Processing of Materials (NMAB Report Number 473, Copyright 1994 by the National Academy of Sciences, National Academy Press, Washington, DC).

The use of microwave heating or microwave plasmas in the sintering of ceramics has often been observed to result in more rapid sintering than the same heating schedule in a conventional furnace; the increased sintering rate has sometimes been ascribed to the enhancement of diffusion by microwave heating/plasma. Some experiments on diffusion measurements under microwave heating have also been carried out in order to check this hypothesis. In this paper, we evaluate the experimental evidence for microwave enhanced diffusion, discuss the various models that have been proposed for the enhanced diffusion, and suggest some further experiments aimed at understanding these phenomena.

Electricity today plays a prominent role in many diverse processes in manufacturing and other industries. Understanding the fundamental principles behind a host of electrically driven processes is the remit of electroheat as an academic discipline. Centres of electricity utilisation at Universities, funded by the electricity utilities, should be encouraged and supported at the embryonic stage of development in order to facilitate the transfer of the electroheat technology between industry and academe.

Material parameters important for microwave processing are identified in case of powder synthesis from precursor compounds and in case of sintering Al2O3- as well as SiC-matrix ceramics. By varying the spatial distribution of the precursor in microwave transparent materials, different pyrolysis temperatures are obtained, which can be attributed to different heating rates due to selective microwave heating. The microwave sintering behavior of oxide ceramics is strongly influenced by the specific surface area of the powder and by aliovalent dopants. In contrast, for covalent ceramics, like SiC+TiC, no experimental evidence for similar effects was obtained.

Technology transfer is often described as the final stage in the process of technological change. This paper will attempt to demonstrate that this should not be the case, and technology transfer should be considered early in the development of any new technology. The paper will commence with a definition of technology, proceed to describe briefly the nature and effects of technological change and end with a description of how we hope to transfer microwave-assisted technology to the ceramic industry.

Successful applications of microwave heating technology result from much more than successful engineering and pilot demonstration. Of utmost importance is translating this information into meaningful customer satisfaction through business concepts intrinsic to the industry being served. Microwave heating companies and laboratories are poorly positioned to commercialize new applications. They do not have the depth of experience in each of the industries which they serve. Success comes only from in-depth knowledge of real customer needs.

Microwave vulcanization of rubber is one of the very successful applications. This paper uses the Rubber Industry as an example of the type of thinking necessary to develop a “customer” focus as opposed to a “microwave” focus.

Studies using laboratory test samples have shown that microwave heating produces sintered reaction-bonded silicon nitride materials with improved properties [1,2]. The final challenge for processing this material by microwave heating is the development of a technology for processing larger batch-size quantities of these materials. Initial microwave scale-up experiments were performed using powder compacts of a bucket tappet geometry. In experiments using microwave-transparent boron nitride sample crucibles, temperature gradients within some crucibles led to larger variations in the sample densities than were obtained with the conventionally processed samples. The use of a microwave-suscepter type crucible made of silicon carbide and boron nitride resulted in an improved temperature uniformity and in density variations comparable to those obtained for the control groups.

This paper reports on the use of high-power traveling wave tubes (TWTs) as a source of microwave energy for materials processing applications. Recent work by Oak Ridge National Laboratories and Microwave Laboratories personnel has demonstrated the usefulness of sweeping the microwave processing frequency over substantial (>20%) bandwidths in order to achieve uniformity of heating over volumes unattainable using conventional microwave sources ∼ e.g., magnetrons. Properly constructed high-power TWTs are a logical choice of microwave source in such systems. After briefly reviewing the basic operating principles of the TWT, the required characteristics of a TWT for materials processing applications and how those requirements affect the TWT's design are discussed. Comments on the present product lines and areas of development for all of the major TWT manufacturers are also presented. Finally, the issue of the ultimate potential cost of TWTs designed for microwave processing applications is addressed.

High power gyrotrons have been developed for application to plasma heating in the program of magnetically confined nuclear fusion research. Gyrotron power levels of up to 1 MW in long pulse operation (>ls) and up to 200 kW in true continuous operation (CW) have been demonstrated at frequencies in the 8 to 140 GHz range. The status of high power gyrotron development is reviewed. One current goal of the worldwide gyrotron effort is the development of 1 MW,CW gyrotrons at a frequency of about 170 GHz for heating the proposed international tokamak ITER to ignition. Gyrotrons are also now being used in a variety of other applications including materials processing. Improved gyrotrons with features such as wide range tuning could be developed for industrial applications.

The Klystrode® (IOT) is a new high power electron tube which combines the features of Klystron and the Tetrode. During the past six years it has virtually taken over the UHF-TV market and has demonstrated its capabilities in high power research applications. This paper describes the principles of Klystrode IOT operation and the power and frequency ranges where it is most useful. Field experience with over fifty (50) transmitters in UHF TV service is reported as well as test results from high power scientific devices operated at 267 MHz, 250 kW (CW) and 425 MHz, 500 kW (pulse). Finally, future improvements in the Klystrode IOT to reduce cost, improve efficiency and optimize the tube for industrial applications.

This talk will summarize the present state of the art of the quasioptical gyrotron (QOG), an alternative gyrotron configuration which has been under development primarily in the U.S. and Switzerland for heating fusion plasmas in tokamaks and other high-power millimeter-wave applications. The QOG features an open-mirror Fabry-Perot resonator instead of the conventional waveguide cavity used in conventional gyrotrons. This gives the QOG the potential for wide tunability, advantages for high-power operation, and facilitates the use of a depressed collector for spent electron beam energy recovery. An experimental QOG has been tuned from 85 to 130 GHz by varying the applied magnetic field. QOGs have produced peak powers up to 600 kW for 13 μs pulses and 100 kW for 10 ms pulses. Electronic efficiencies up to 22% have been achieved at 85 GHz, and operation with a depressed collector yielded an overall efficiency of 30%. The design of a multi-kW CW QOG tunable from 80 to 120 GHz is discussed.

The Injection Locked Magnetron (ILM) has been used as a source of coherent power in a number of radar systems. It has a number of characteristics that make it particularly suitable to heating applications, above competing tube types used in radar applications.

Power available is dependent upon the frequency of operation, ranging from several hundred watts at Ku-band to tens of kilowatts at L-band. Operation could be either cw or pulsed, but at higher power levels pulsed operation would be required. Designs for tubes at 1 GHz and 15 GHz will be presented.

ILMs could be made at any frequency from 800 MHz to 20 GHz, beyond which the magnetron becomes either too large to be practical or too small to fabricate. Locking bandwidths are typically 0.5%, sufficient to be able to guarantee the tube remains locked at the required operating frequency. A tuner would increase the tube life and compensate for frequency drift effects.

The tube life is dependent on the desired frequency of operation, ranging from several hundred hours at high frequency to an order of magnitude higher at low frequency. Adding a tuner will increase this several fold. ILMs are inherently rugged and may be run in any orientation, having been developed for airborne radar applications. Tubes are operated directly into a circulator and consequently can operate into high VSWR.

The magnetron is a low cost tube, hence its use in domestic microwave ovens. ELMs have a similar part count and complexity. Efficiency varies from approximately 45% in Ku-band to 80% at L-band. Operating voltages are lower for ILMs than other vacuum tubes, at 1 GHz a 100 kW system would require 20 kV and for a 15 GHz 200 watt system, a 2 kV power supply would be required. These would run in the region of 10% duty cycle, 1 kHz PRF. Tubes would have integral rare earth magnets and require cooling water. The required quantity of tubes would be the most significant cost driver.

The use of microwave energy for industrial processing applications has been ongoing since the early 1950's. A number of high power tubes were developed in the mid-1960's at 805, 915 and 2,450 MHz for industrial uses. One super power crossed-field amplifier (CFA), the QKS1262, was available in either 50 or 100 kW continuous wave (CW) format. This CFA was designed with a platinum emitter which was pulse started with 100 W average power. The CFA operates at ∼20 kV at 5 amps (50 kW - CW output) or 10 amps (100 kW - CW output/65% efficiency).

This paper will describe the operating characteristics of the QKS1262 and the options in design to attain high power with high efficiency and long life (25,000 hours).

The measurement of the thermal noise transmitted by lossy materials in the microwave frequency range is, at the present time, the only one thermometric method able to bring information about the temperature distribution inside a material without insertion of any sensor. In this paper, we first briefly recall the principles of the method, and the present state of the experimental set-ups. This technique interest quite different domains of applications such as industrial thermometry, biomedicai engineering, domotics and microelectronics. In other words, the presentation is related to different situations:

∼ material at uniform or non uniform temperatures

∼ temperatures over or beneath the room, or reference temperature

∼ different materials: high or low losses, homogeneous, multilayered or heterogeneous

∼ measurement of a thermal noise temperature or of a correlation temperature.

The paper analyses also the reasons why these method have not still much penetrated the industrial market and explains the arguments for a future extension of these kinds of applications.

The results of an in depth study of the dielectric properties of various rubber compounds at different microwave frequencies are discussed. Subsequently, the influence of carbon black volume percentage on the dielectric properties of rubber is studied in a wide frequency range (5 -24 GHz). The results show that the real and imaginary parts of rubber dielectric constant increase as a function of increasing carbon black percentage. Frequencies around 5 GHz show more sensitivity to small changes in the carbon volume content. The presence of curatives in uncured rubber samples is detected, which is an indication of the sensitivity of microwaves to the chemical reaction triggered by these curatives. Similar approach is used to measure the air content in specially prepared model material (plastic microbaloon-filled epoxy) samples with varying degrees of uniformly distributed air inclusion. The results of these measurements for a wide range of frequencies is also reported. Measurement analysis resulted in detecting variation in the air volume fraction of around 1.5%. The results confirm the utility of microwave NDE techniques for local porosity detection and evaluation in dielectric composites.

Microwave multiport sensors have been shown to provide some unique capabilities to achieve real-time testing of products conveyed at high speed. In many applications, quantitative measurements of physical quantities such as moisture content, density, etc… are required, either to guarantee reliable production or to optimally control a fabrication/transformation process. In this paper, different ways of extracting such physical quantities from microwave measurements performed by multiport sensors are presented. Model approaches are used, based on polynomial expansions of the physical quantities to be measured as a function of the microwave amplitude and phase data. Calibration procedures have been investigated for both paper and wood material samples. Comparisons between in-situ, microwave and conventional, measurements are analysed.

Using the modulated scatterer technique allows us to measure the electromagnetic field in an applicator. The design of a new sensor modulated at 25 Hz is described. The operating conditions and the performance are presented.

The sensor can be used for measuring high microwave electric fields up to 10 kV/m in an industrial applicator supplied by any industrial magnetron.

In this study, we evaluated the electrical properties of the bonded silicon-on-insulator (SOI) wafers with lifetime measurements using a non-contact laser-microwave method. We prepared one group that consisted of bonded SOI wafers with different active layer thicknesses (I0,30,100μm) and another group consisting of bonded SOI wafers with different buried oxide layer thicknesses(0,0.01,0.1,0.75μm). Primary mode lifetime (τ1) was measured by the photoconductivity decay (PCP) method using the laser diode (λ= 774nm) as a carrier-injected light source. Steady-state change in the conductivity was measured by the photoconductivity modulation (PCM) method using a He-Ne laser (λ = 633nm) as a carrier-injected light source. τ1 decreases as the active layer thickness decreases. The PCM intensity also decreases with decreasing active layer thickness. Surface and interface recombination rates of the SOI are increased with decreasing layer thickness. The PCM intensity also decreases as the buried layer thickness decreases.

A microwave based “wall probe” has been developed which is capable of nondestructive evaluation of architectural structures. By using microwaves in the 8 to 12 GHz range this probing instrument can detect subsurface characteristics through concrete, brick, wood or other building materials to depths in excess of 12 inches.

The instrument interrogates a structure from a single side by transmitting a microwave signal into the surface at some angle of incidence and receiving the reflected signal some distance away on the same side of the structure. The transmitted signal is partially reflected at each internal boundary of different dielectric constant, giving a composite reflection which contains information from each internal layer. The reflected composite signal is compared in phase and amplitude to the transmitted signal and that reading is considered the “signature” of the structure under test. Computer algorithms analyze the signature for recognizable features and nonstandard construction.

The object of this project was the evaluation of microwaves as a non-invasive tool for determining the thicknesses of the fat layers directly beneath the hides of beef cattle. The motive for the project is the elimination of the cost of feeding cattle beyond the point at which the muscle is optimally marbled. The nature of the animals and the conditions for operation require a simple, rugged, non-invasive system. Modified open-ended microwave S-band coaxial cavities were applied as contact radiators to the surface of a three-layer sample composed of lean meat and fat tissue, which simulated the configuration of hide, fat and muscle on the outside of an animal. The lean and fat layers loaded the cavity, affecting the resonance frequency, bandwidth, and center-frequency reflection coefficient. Measurements were made with a network analyzer. An exact analysis of the microwave circuit has not been possible, but the in vitro laboratory tests show that a system based on this device can be used to measure subcutaneous fat layer thicknesses up to 16 mm beneath hides up to 16 mm thick.