Scale-up studies were performed in which microwave heating was used to fabricate reactionbonded silicon nitride and sintered reaction-bonded silicon nitride (SRBSN). Tests were performed in both a 2.45 GHz, 500 liter and a 2.45 GHz, 4000 liter multimode cavities. A variety of sizes, shapes, and compositions of silicon preforms were processed in the studies, including bucket tappets and clevis pins for diesel engines. Up to 230 samples were processed in a single microwave furnace run. Data were collected which included weight gains for nitridation experiments, and final densities for nitridation and sintering experiments. For comparison, nitridation and sintering studies were performed using a conventional resistance-heated furnace.

The conversion efficiency of electrical to microwave power by magnetrons operating at 896 / 915 MHz is significantly greater than that of magnetrons operating at 2.45 GHz. The increased efficiency at this frequency improves the prospects of microwave firing being adopted as a commercially viable process but the change in frequency alters the microwave-material interactions that take place.

This paper discusses the influence and implications of using 896 / 915 MHz for firing ceramics and compares dielectric property data at both frequencies. The results suggest that using the lower frequency will result in more efficient heating.

A practical microwave ceramic sintering technology has been developed. To achieve the highest merit from microwave sintering, susceptors were not used. In this paper, the ceramic sintering process using the 28GHz mm-wave energy generated by a Gyrotron oscillator is described. Large sized (Φ 200 × 200 ) and complex shaped material of Si3N4, A12O3, and ZrO2 were successfully and reproducibly sintered. The sintered bodies exhibited high density and fine crystalline structure. The processed materials were found to have exceptional strength and toughness.

The critical processing technology developed dealt with the means by which uniform and defect free sintered bodies are obtained. The processing stressed the importance of achieving a uniform electric field within the applicator and to decrease the sintering dependence on the materials’ temperature based dielectric loss properties. With the use of innovative support and thermal insulation structures, the processing system allowed for very precise temperature controls.

A review of some market-driven research, process applications and systems development is provided. The variable frequency microwave processing concepts are briefly described. Industrial processing using variable frequency microwave energy in the areas of polymerization, composite processing, bonding and plasma is discussed. Analytical applications inherent in the use of variable frequency and its control are demonstrated in the areas of materials signature analysis for volumetric cure monitoring.

To help develop large-scale microwave-sintering processes and to explore the feasibility of the commercial utilization of this technology, we used the recently developed multi-grid 3D Finite-Difference Time-Domain (FDTD) code and the 3D Finite-Difference Heat-Transfer (FDHT) code to determine the electromagnetic (EM) fields, the microwave power deposition, and temperature-distribution patterns in layers of samples processed in large-scale multimode microwave cavities.

This paper presents results obtained from the simulation of realistic sintering experiments carried out in both 500 and 4000 liter furnaces operating at 2.45 GHz. The ceramic ware being sintered is placed inside a cubical crucible box made of rectangular plates of various ceramic materials with various electrical and thermal properties. The crucible box can accommodate up to 5 layers of ceramic samples with 16 to 20 cup-like samples per layer. Simulation results provided guidelines regarding selection of crucible-box materials, cruciblebox geometry, number of layers, shelf material between layers, and the fraction volume of the load vs. that of the furnace.

Results from the FDTD and FDHT simulations will be presented and various tradeoffs involved in designing an effective microwave-processing system will be compared graphically.

In the field of plastic and composite materials the radio frequency dielectric heating is more and more used. Compared to traditional techniques such as conduction and convection heating, the radio frequency technology is interesting, because it allows fast heating of thick materials and heat insulation materials.

As bonding techniques are more and more integrated in production lines, the polymerization of glues must be realized in a very short time. The 50 use of the Ω radio frequency technology makes this heating process possible.

The authors describe the industrial application of this technology to the CITROEN ZX and CITROEN XANTIA cars. Steps involved in implementing this industrial process (laboratory - pilot -industrial equipment) are presented and analysis the technical and economic results of this application.

Throughout the world car industry has been using window encapsulation for a few years now. This technology is mainly used in production lines and is called RIM for polyurethane reaction injection moulding.

This technology, however brings about some problems such as:

– glass breaking during mould closure,

– high production cost,

– systematic rough edges.

The PSA Group (Peugeot-Citroën), a pioneer in this field, in collaboration with SAIREM has launched a new innovating process for window encapsulation by using the 50 Ω RF technology for gelling PVC Plastisol.

The study was followed by an industrial prototype. Industrial equipment was then installed at WEBASTO HEULIEZ for window encapsulation of the sunshine roof for the Citroën Xantia.

The authors describe the principle of window encapsulation and the different existing processes. He describes the 50 ΩRF technology, an industrial installation and the constraints of this technology in order to get maximum efficiency.

In the conclusion they present a technical and economical analysis of the different solutions for window encapsulation. They also present the advantages of the 50 Ω RF technology and the new opportunities it offers.

In the conclusion they present a technical and economical analysis of the different solutions for window encapsulation. They also present the advantages of the 50 Ω RF technology and the new opportunities it offers.

The variety of possible microstructures obtained upon microwave sintering of hardmetals is described with regard to microwave specific heating mechanisms. While the reduction of processing time and temperature is evident, the influence of the microwave sintering process on the microstructure is not fully understood. From microwave sintering experiments at different power levels processes that depends on the local field strength are identified. The mechanical properties of microwave sintered tool bits are compared with commercial WC-Co hardmetals. An incising wear resistance of MW-sintered hardmetals for speed turning of cast iron is found.

This paper reviews the major concerns related to the development of microwave sensing techniques. For their practical importance, testing and measurement of conveyed products via microwave multipoint sensors are more particularly considered. After reporting some promising sensing configurations and applications, technical and non technical aspects involved in the development of microwave sensors are analysed. Some of these aspects are encountered in any technology transfer process, while some others are more particularly specific to microwave sensors. In the conclusion, some general guidelines are provided for a successfull development of microwave sensing technology.

Wideband S-parameter measurements have been used to investigate the properties of electrically non-conductive materials using a vector network analyzer. Using S21 scattering parameters, amplitude and phase shift measurements were taken for a variety of both homogeneous and heterogeneous materials. With these initial S-parameter measurements as a reference, surface defects were modeled on the test samples and the S-parameter measurements were reevaluated. The effect of varying levels of moisture absorption in certain materials was also investigated. The results of this study indicate that there is indeed potential for the use of a vector network analyzer as a nondestructive testing tool to evaluate the structural integrity and/or moisture content of electrically non-conductive materials through the transmission of microwave energy.

Two nondestructive microwave methods for sensing and mapping the local loss factor ε” of conducting solid films are described. Example results of both methods are compared to the low frequency sheet resistance as measured by two- and four-point probe methods. Specimens ranged from 26 to 10,000 ohms per square. Depending on the test frequency, spatial resolution of about 1 cm is achievable, as demonstrated by maps of ε”.

Product quality control is a crucial part of manufacturing and usually involves materials characterization and diagnosis. Though various microwave assisted nondestructive evaluation (MA-NDE) systems have been fabricated for materials inspection, none of the systems can be applied to materials within a mold or reactor. A broadband variable frequency microwave based, resonant mode MA-NDE was studied as an alternative for characterization of materials within a cavity. The main advantage of the resonant mode MA-NDE are non-intrusive and volumetric diagnosis of the material inside a mold. The principles and possible applications of the resonant mode MA-NDE are discussed. Resonant mode MA-NDE was fully demonstrated by using Vani-Wave to trace material status during microwave curing of Diglycidyl Ether of Bisphenol A (DGEBA)/Diaminodiphenylsulphone (DDS) epoxy samples.

The separation measurements of a bulk lifetime υo, front and back surface recombination velocities S0, Sw have been investigated for both 620 μm and 1.08 mm thick Si wafers with various surface treatments. The separation was made by applying bisurface photocoductivity decay method to the photoconductivity decay curves measured by 500 MHz microwave reflection under the bias light illumination. The bulk lifetime and the surface recombination velocities have been determined, respectively, to be 624–659 μs and 18.4–66.1 cm/s for the oxidized sliced-surface and 6285 cm/s for the sliced surface. And they have been also determined, respectively, to be 412–422 μs and <1 cm/s for the oxidized mirror polished-surface or the oxidized etched surface, 565–626 μs and 1042–1112 cm/s for the oxidized sandblasted-surface. The noncontact microwave BSPCD method with bias lights makes it possible to determine separately the bulk lifetime and surface recombination velocities in the front surface and backsurface in Si wafers with the wafer thickness for the practical use.

The economy of utilizing 915 MHz vs. 2450 MHz microwave equipment has long been acknowledged for many industrial processes. A severe roadblock to advancing 915 MHz processing has been the lack of proper equipment to run test trials. At 2450 MHz low power equipment is readily available and inexpensive, even resorting to the use of the consumer microwave ovens. At 915 MHz, however, prototyping systems have only been available using very expensive 5 kW magnetrons. Experimenters have been forced to use 2450 MHz low power equipment to explore scale-up potential for high power 915 MHz systems. Because of wavelength and dielectric property differences, this procedure may give highly erroneous results, sometimes leading to the purchase of production equipment which does not work.

This paper describes an inexpensive, low power, microwave system for prototype testing at 915 MHz. A 600 watt generator is coupled to a specially designed evanescent mode applicator/cavity for the trial testing prior to making a scale-up decision. A description of the generator and evanescent cavity design are presented.

A formula to calculate the photoconductivity decay curve influenced by the microwave distribution within a silicon wafer has been derived analytically. The difference of photoconductivity decay curves measured at 500 MHz and 10 GHz has been explained by the skin effect and by the comparing the experimental curves to those estimated by the analytical formula. The wafer resistivity and wafer thickness dependent photoconductivity decay curves calculated based on the skin effect have been demonstrated. The results have led to the conclusion that 10 GHz microwave probe scarcely influences on the photoconductivity decay for the silicon wafer with the resistivity >10 Ωcm and a wafer thickness <0.7 mm. For the wafer with a resistivity<10 Ωcm and a wafer thickness>0.7 mm, the photoconductivity decay curve is influenced by the skin effect at 10 GHz. However, the apparent lifetime (or effective lifetime) obtained from the gradient of the tail decay is out of the influence by the microwave skin effect.

Ceramic-metal composites have been produced through the reduction of copper oxide (CuO) and the oxidation of aluminum in a reducing atmosphere. Fine powders of CuO and Al (<38μm) were mixed with yttria stabilized zirconia, ball milled and uniaxially pressed into disc samples. Microwave hybrid heating was used to process samples using β-SiC and activated carbon powder as susceptors. Pyrolysis of the carbon provided the reducing atmosphere. The resulting Ctt/Al2O3/Y-ZrO2 composite was analyzed for density, compressive strength and resistivity.

The necessary energy for processing ceramics usually comes from gas firing or electricity. With gas firing, ceramics processing is conducted at relative low temperatures. In the case of electric arc furnace, temperatures are high enough to smelt the material and the reaction takes place in a liquid state. In this work we use microwave radiation for conducting reactions between oxides to produce ceramic materials, mainly spinels. The microwave energy was supplied using an 800W magnetron operating at 2.45 GHz. The microstructure and the mineral composition were studied by means of SEM and X-Ray diffractometer respectively.

In an attempt to accelerate the process of joining of metals to ceramics, a new rapid brazing technology has been developed. In this process, referred to as Microwave Induced Plasma (MIP) brazing, a microwave plasma is used to rapidly heat the ceramic and metal to the melting temperature of the reactive braze material. The heating rate obtained by MIP could be many times faster than those achieved by conventional resistive heating in a tube furnace. The fast heating rate has no detrimental effect on the joint quality and in fact results in the formation of a thick interfacial film suggesting significant interdiffusion between the braze and ceramic, possibly stimulated by the microwave radiation. In this paper the experimental arrangement of the MIP system is described. The unique capability of the MIP heating is demonstrated by successful joining of hot pressed silicon nitride to stainless steel using reactive metal brazing. The results of microstructural characterisation of the joints carried out by SEM and EDS will also be presented.

Initial results for the continuous microwave processing of wide webs using a new microwave applicator are presented. The results show that acceptable uniformity can be obtained using this system and that with the appropriate controls, a very tight process window can be maintained to produce preimpregnated glass cloth (prepreg) for use in circuit board manufacture.

Glasses of the system AgI-Ag2O-(0.95B2O3:0.05SiO2) have been formed by microwave processing using a domestic multi-mode oven operating at 900 watts and 2.45 GHz. Microwave heating resulted in rapid melting times with homogeneity in the quenched glasses equivalent to or better than conventional melting at 730°C. The glass forming region in this pseudo-ternary system is compared with the conventionally melted glass forming region in the system AgI-Ag2O-B2O3. A reversible color difference has been observed between glasses conventionally melted and those melted by microwave for all glass compositions in our system.