Reliable and accurate temperature measurement during microwave processing of ceramic bodies is controversial. Although thermocouples are routinely used in conventional thermal furnaces, their presence in microwave furnaces can locally distort the electromagnetic field, conduct heat away from the sample, induce thermal instabilities and microwave breakdown, and lead to serious measurement errors. These thermocouple effects have been studied and found to be more pronounced in low-and medium-loss ceramic materials. To decrease the thermocouple effects during the processing of advanced ceramic materials, an optical, noncontact temperature sensing system has been developed, calibrated, and incorporated into a computer-controlled microwave furnace.

No abstract

The thermal conductivity of ZnO with different particle sizes (micrometer, submicrometer, and nanometer) was measured using the laser flash technique. As the "green" samples were heated from room temperature to 600°C (and 1000°C) and then cooled down to room temperature, the thermal conductivity was measured in situ. A model for interparticle neck growth was developed based on mass transfer to the neck region of a powder as a result of known temperature. By combining this model with a three-dimensional numerical code, the thermal conductivity of ZnO was calculated. Excellent agreement between the theoretical calculation and experimental data was found.

During microwave sintering of compacted ceramic powders, the electric field distribution within the ceramic body on a macroscale is determined by a combination of the operating frequency, the sample shape, and its permittivity. In contrast, our studies show that on a microscopic scale, the local electric fields are disproportionately intense close to grain boundaries and rough surfaces due to strong focusing. Also, the electric field in the interparticle contact zone exhibits preferred polarization directions despite illumination by a randomly polarized wave. This can lead to a highly nonuniform energy deposition and accelerated mass transfer rates via ponderomotive diffusion and plasma generation.

No abstract

Temperature gradients that develop in ceramic materials during microwave heating are known to be strongly dependent on the applied microwave frequency. To gain a better understanding of this dependence, identical samples of ZnO powder compacts were microwave heated at three distinct widely separated frequencies of 2.45, 30, and 83 GHz and the core and surface temperatures were simultaneously monitored. At 2.45 GHz, the approximately uniform “volumetric” heating tends to raise the temperature of the sample as a whole, but the interior becomes hotter than the exterior because of heat loss from the surface. At 30 and 83 GHz, this interior to exterior temperature difference was found to be reversed, especially for high heating rates. This reversal resulted from increased energy deposition close to the sample's surface associated with reduced skin depth. A model for solving Maxwell's equations was incorporated into a newly developed two‐dimensional (2‐D) heat transport simulation code. The numerical simulations are in agreement with the experimental results. Simultaneous application of two or more widely separated frequencies is expected to allow electronic tailoring of the temperature profile during sintering.

In this paper, a unique processing approach for producing a tailored, externally controlled microstructure in zinc oxide using very high heating rates (to 4900°C/min) in a microwave environment is discussed. Detailed data on the densification, grain growth, and grain size uniformity as a function of heating rate are presented. With increasing heating rate, the grain size decreased while grain size uniformity increased. At extremely high heating rates, high density can be achieved with almost complete suppression of grain growth. Ultrarapid microwave heating of ZnO also enhanced densification rates by up to 4 orders of magnitude compared to slow microwave heating. The results indicate that the densification mechanisms are different for slow and rapid heating rates. Since the mechanical, thermal, dielectric, and optical properties of ceramics depend on microstructure, ultrarapid heating may lead to advanced ceramics with tailored microstructure and enhanced properties.

Sheet electron beams focused by periodically cusped magnetic (PCM ) fields are stable against low-frequency velocity-shear instabilities (such as the diocotron mode). This is in contrast to the more familiar unstable behavior in uniform solenoidal magnetic fields. A period-averaged analytic model shows that a PCM-focused beam is stabilized by ponderomotive forces for short PCM periods. Numerical particle simulations for a semi-infinite sheet beam verify this prediction and also indicate diocotron stability for long PCM periods is less constraining than providing for space-charge confinement and trajectory stability in the PCM focusing system. In this article the issue of beam matching and side focusing for sheet beams of finite width is also discussed. A review of past and present theoretical and experimental investigations of sheet-beam transport is presented. I. lNTROllUCTlONA strong motivation for the use of thin ribbon or sheet electron beams in coherent radiation sources or accelerators derives from the ability to transport large currents at reduced current density through thin clearance spaces or in close proximity to walls or structures. This feature is a result of the opportunity to add current to the beam at constant current density by increasing one wide transverse beam dimension, while keeping the other beam transverse dimension very small. A historically strong disincentive to using sheet electron beams in the above-mentioned applications is their known susceptibility to the disruptive diocotron instability occurring in the presence of a uniform solenoidal magnetic (focusing) field.Recent research appears to have identified a solution to this decades-old problem, paving the way for implementation of sheet beams in both relativistic and nonrelativistic applications. The essence of the solution is to use ponderomotive focusing achieved with one of several configurations of spatially periodic magnetic fields.In this paper we present an organized review of the physics and recent results of research of periodically focused sheet electron beams, and we describe new results of simulation studies of beam stability and emittance growth. II. HISTORICAL REVIEWThe advantage of using sheet electron beams for high current applications was first noted over three decades ago.' However, around the same time, experiments with both thin annular2"1 and planar4 sheet beams identified a filamentation instability when the beams were propagated parallel to a uniform solenoidal magnetic focusing field. The simplest theoretical model was derived for a very thin, monoenergetic, nonrelativistic, planar sheet beam, and *Paper 212, Bull. Am. Phys. Sot. 38, 1901Sot. 38, (1993. 'Invited speaker. considered only low-frequency, quasistatic perturbations transverse to the magnetic field axis.5 Since then, both the experimental and theoretical details have become considerably more sophisticated, including finite beam thickness, thermal velocity spread, relatistic beam energies, nearby conducting boundaries, ion-space-charge neutra...