Microstructural differences between microwave and conventionally annealed solid solution TiO2-SnO2 and TiO2-SnO2-(Al2O3)0,02-samples demixed by spinodal decomposition are reported. The decomposition is retarded upon microwave annealing as compared to conventional heat treatment. This result is in accordance with theoretical predictions, if a superposition of the microwave field with an internal periodic electrostatic potential resulting from kinetic effects is assumed.

The influence of the microwave heating on chemical reactions were investigated. The kinetic of the Diels Alder reaction were studied under Microwave irradiation at a frequency of 2.45 GHz in a single mode cavity and were compared to the kinetic obtained by a conventional heating. Experiments were carried out in a liquid solvent in order to have a better control of the medium temperature measurement. In a second part, the presence of a catalytic solid phase was introduced. Some thermal fluctuations which are due to an heterogeneity of the electric field were detected in the medium. They reduce the precision of the results and cause problems of experimental reproducibility. A thermoluminescent material allow a good visualisation of these phenomena.

In addition, the profiles of the electric field intensity were modelled by a 2D finite elements method in our reactor in the presence of a solvent. Despite the small size of the sample and the use of a monomode cavity which both limited the heterogeneities of the medium temperature, we showed a great heterogeneity of the electric field intensity and as a result the heterogeneity of the temperature in our sample.

In order to avoid these phenomena which induce a lack of reproducibility, a stirring device was developed.

The values of the kinetics obtained under the 2 heating modes with the introduction of the stirring device. So, it induces a good control of the medium temperature.

All those investigations prompted us to the conclusion that there is no difference between microwave heating and a classical heating in the studied reaction.

We describe computer simulations of a microwave-induced driving force for ionic transport. The simulations are based on a model which predicts rectification of ionic fluxes at interfaces and a resulting depletion or accumulation of defects near the interface. Some details of the model are discussed, results of the simulations are presented, and the impact of these effects on sintering and diffusion is discussed.

This work deals with kinetic description of carbothermic reduction of iron oxides using microwaves as power source. Previous researches shown that it is possible to conduct this kind of processes successfully, but real kinetic comparisons between conventional and microwaves procedure have been presented partially. The aim of this work is to describe reduction kinetics, taking into account how the iron oxide is reduced by microwaves compared with conventional energy supply. In this study we used iron ore in pellet shape and dust. We found that both, pellet and dust reduction stops when it reaches approximately 40%, even at whole power

The potential for microwave processing of a single phase material is often limited due to the dependence of dielectric losses upon the chemical bonding and temperature of the material. We will present results showing the effect of microwave absorption on bulk crystallization of glasses belonging to M2O-CaO-SiO2-ZrO2 system (where M+ Li, Na, K). The glass samples were devitrified using both microwave and conventional heating. The effect of Li+, Na+, K+ on crystallization is quite remarkable and is a function of ion size. This is true especially in the microwave heating where the important dielectric losses observed in silicate glasses are related to the motion of alkali cations throughout the glass matrix. X-ray diffraction analysis was performed on the powdered samples to determine crystalline phases. Results of microstructure and microanalysis on these glass-ceramic samples will also be presented.

Results of the theoretical study of surface effects in ionic crystalline solids under the action of high-frequency electric fields of moderate intensity are presented. The averaged ponderomotive action of the electric field on the charged vacancies within the crystal causes directional mass transport that leads to development of a surface instability. The analysis shows that the proposed effect can result in the formation of a periodic profile on the surface.

A comparative study of grain growth behavior in silicon nitride under conventional and microwave annealing is presented. Microwave annealed specimens showed a faster growth rate as indicated by the quantitative microstructural analysis. The phenomenon was used in combination with seeding techniques to develop a silicon nitride exhibiting a bi-modal microstructure. Microwave annealing was carried out using a microwave radiation frequency of 28 GHz.

The formation kinetics of BaTiO3 from the solid-state BaCO3 and TiO2 powder in a microwave field was investigated. The quantitative XRD analysis and the model considered the volume change between reactant and product were used in this experiment. Results show that the formation rate of BaTiO3 in a microwave field is much faster than upon conventional heating. The activation energy of the solid state reaction for BaTiO3 was measured as 58 kJ/mol. This indicates the enhancement of diffusion by the microwave heating process.

Processing conditions such as temperature, soak time, and heating rate affect the final density of conventionally-sintered and microwave-sintered ceramics. Of additional importance is the question of whether microwave-sintered ceramics display intrinsically superior macroscopic engineering properties compared with conventionally-sintered control specimens. An analysis using the Yates algorithm indicates that the processing condition which has the largest impact on the density of the specimen is the heating method (microwave vs. conventional). The microwave-sintered specimens resulted in higher densities and higher fracture strengths. However, it was determined that the higher fracture strengths were due to the higher sintered densities rather than a significantly different microstructure.

The recent theory of averaged ponderomotive action of microwave E-field in solids is expanded to describe quasistationary ionic currents driven by that action. New experimental results on the dynamics of the microwave-induced currents in AgCl and NaCi are presented. Agreement between experiment and theory provides further and stronger evidence for general validity of the theoretical model.

This investigation compares microstructures and mechanical properties of both Gelcast B4C and “conventionally” die-pressed B4C, using both microwave and conventional sintering methods. The microstructures and final mechanical properties of B4C specimens are discussed.

We investigated the effect of microwave radiation on the physico-chemical properties of some semiconductor materials (CdxHgl-xTe, GaAs) and metal (Al, Pt, Cr, Mo, W) - GaAs Schottky barrier diode structures. From comparison with the results of the other treatments (heat annealing, g-irradiation) some suggestions are made concerning possible mechanisms of the interaction between microwave radiation and the objects studied.

The magnetic field sensor with sensitivity 10−3 Oe and work temperature 77°K is created on the base of YBaCuO ceramics having a sharp jump of the diamagnetic moment in the region of the first Josephson critical magnetic field. The magnetic field measurement methods and the sensor principle are described. Ways to improve the obtained results are indicated.

Microwave power for heating applications is normally generated in the designated ISM frequency bands which occupy a band of 4%. Actual microwave generators, such as are used in domestic ovens utilise only a small fraction of this bandwidth. It is being demonstrated that spreading the power uniformly over the full ISM band by controlled frequency modulation dramatically reduces all levels of potential electromagnetic interference. With such controlled modulation telecommunication channels can operate within the ISM bands without serious problems because the leakage levels are reduced by 20 to 30 dB with no additional shielding costs. One simple (though not optimum) modulating waveform is a large ripple voltage on the magnetron power supply. Frequency modulation that spreads the energy over the full ISM band also improves the overall energy efficiency in multimode heating applications.

There have been recent indications that variable-frequency microwave sintering of ceramics provides several advantages over single-frequency sintering, including more uniform heating, particularly for larger samples. The Finite-Difference Time-Domain (FDTD) code at the University of Utah was modified and used to simulate microwave sintering using variable frequencies and was coupled with a heat-transfer code to provide a dynamic simulation of this new microwave sintering process. This paper summarizes results from the FDTD simulations of sintering in a variable-frequency cavity. FDTD simulations were run in 100-MHz steps to account for the frequency variation in the electromagnetic fields in the multimode cavity. It is shown that a variable-frequency system does improve the heating uniformity when the proper frequency range is chosen. Specifically, for a single ceramic sample (4 × 4 × 6 cm3), and for a variable-frequency range from f = 2.5 GHz to f = 3.2 GHz, the temperature distribution pattern was much more uniform than the heating pattern achieved when using a single-frequency sintering system at f = 2.45 GHz.

Adhesive bonding through the application of variable frequency microwave (VFM) radiation has been evaluated as an alternative curing method for joining composite materials. The studies showed that the required cure time of a thermosetting epoxy adhesive is substantially reduced by the use of VFM when compared to conventional (thermal) curing methods. Variable frequency microwave processing appeared to yield a slight reduction in the required adhesive cure time when compared to processing by the application of single frequency microwave radiation. In contrast to the single frequency processing, the variable frequency methodology does not readily produce localized overheating (burnt or brown spots) in the adhesive or the composite. This makes handling and location of the sample in the microwave oven less critical for producing high quality bonds and allows for a more homogeneous distribution of the cure energy. Variable frequency microwave processing is a valuable alternative method for rapidly curing thermoset adhesives at low input power levels.

Propagation of a sharp temperature wave was observed during microwave heating of porous zinc oxide in nitrogen and argon atmospheres. This wave initiated from the center of the sample and traveled at an average velocity of 0.2 cm/min towards its surface. This temperature wave was attributed to an anomalous peak in the imaginary part of the complex permittivity possibly caused by desorption of chemisorbed oxygen from the surfaces of ZnO crystallites.

A rapid feedback control system was designed to operate with a traveling wave tube amplifier for regulating the sintering temperature of tows and tubes in a single mode microwave cavity. The control system regulated the microwave frequency and power absorbed by the sample in order to maintain the sample temperature. Testing with NICALON tows and mullite tubes demonstrated that the control scheme worked well for stationary and slowly moving (<10mi/min) samples, but failed for fast moving samples. Difficulty with measuring sample temperatures was resolved by using a light sensor to measure the emitted light intensity and to gauge the relative degree of heating. This work was supported by the Department of Energy, Office of Industrial Technologies, Advanced Industrial Materials Program.

A new process is in development that uses high power millimeter wave energy to separate specific components from a composite material stream. The component to be separated from the material stream must have a combination of high loss tangent and low permittivity with respect to the other materials involved. In this case, when the material is subjected to a high power, high frequency RF source, the desired component can be vaporized and extracted without transferring significant heat to the remaining material. Since absorption increases significantly with frequency, it is important to use a millimeter wave source. A vacuum pump with sufficient pumping speed can transport the vaporized component for collection. The bulk material can either be collected or recycled through the system for further extraction.

The microwave sintering of nanocrystalline alumina compacts is currently under investigation at NRL. This paper will discuss an overmoded microwave furnace based on a 35 GHz pulsed gyrotron which is currently being set up to extend ongoing microwave sintering experiments at 2.45 GHz to 35 GHz. The gyrotron operates at 70 kV and currents up to 10 A. It is driven by a hard tube, variable pulse length (1–15 μs) modulator at repetition rates up to 1 kHz. The gyrotron can produce peak powers up to 100 kW at an efficiency of 20%, and average powers up to 200 W. The gyrotron output is transported via pressurized Ka-Band waveguide to an overmoded resonator containing the workpiece. In initial experiments, the resonator will consist of a piece of WR-284 waveguide. The operation of the system in preliminary sintering experiments is described.