Pultrusion is the only continuous process available for the manufacture of fibre reinforced thermosetting composites. It its simplest form it requires that dry reinforcement inthe form of continuous rovings or fabrics is pulled through a shallow bath of resin and then through a heated metal die wherein the resin crosslinks to a rigid network. The rate of manufacture (pulling speed) depends on two factors; the rate of heat transfer to and through the composite by conduction from the (heated) die wall and the rate of heat generated internally by the (exothermic) crosslinking reaction.

Recently a small, minimally-perturbing, isotropic, and easy-to-use fiberoptic microwave E-field probe has been developed. This probe senses microwave energy by measuring the Joule heating of a susceptor material. It consists of two Luxtron Fluoroptic sensors. One measures the temperature of the susceptor formed as a thin-walled spherical shell, the other measures the temperature of the air around the susceptor so that any ambient temperature change can be subtracted. The differential temperature (δT) obtained between the two sensors is directly related to the microwave E-field strength or the power density corresponding to that field strength at the probe.

Computer modeling and numerical techniques provide an attractive, accurate, fast, and cost-effective means for calculating power deposition patterns and temperature distributions in materials during and after their processing using microwaves. Besides giving information on heating rates and patterns, numerical modeling and computational techniques may provide valuable information that may result in better control and optimization of theheating process. The optimum material/insulation combination for improved uniformity and efficiency of microwave sintering of ceramics and the development of procedures that may prevent problems such as temperature runaways are typical examples of attainable benefits from numerical modeling and computational techniques.

The present note introduces new trends in electromagnetic spectroscopy in complex media.When an electromagnetic wave propagates in heterogeneous media, some questions arise about both physical meaning and validity range of the traditional analysis. The aim of our advanced research is related to the generalisation of Maxwell's equations able todescribe both homogeneous and heterogeneous media from an unique point of view.

We present the results of our recent studies of the possible origin of the reported increase in sintering efficiency brought about by the application of microwaves. We study the phenomenon of thermal runaway in ceramic materials undergoing microwave heating and present a theory on the basis of a simple temperature-time equation. The nonlinear evolution inherent in the equation arises naturally from physical considerations. The theory isapplied to experimental observations reported on several different materials including silica, alumina, strontium titanate, zinc oxide and iron oxide, and shown to be in good agreement with the data.

The detailed transient temperature distribution in an inhomogeneous model of a cross section through the prostate region of the human body undergoing hyperthermia treatment forcancer has been calculated. The finite element method has been used to solve the bioheattransfer equation. A commercially available finite element software package called ANSYS® has been adapted to the present problem.

The model consists of 523 triangular elements and incorporates a tumor in the prostate.The hyperthermia device under test is an Annular Phased Array consisting of dipole antennas. The model is surrounded by a bolus of deionized water. The calculated electromagnetic energy distribution is input into the bioheat transfer equation and the resulting temperature distributions calculated.

The increase in blood perfusion rates due to heating is incorporated into the model. Detailed transient temperature profiles in the finite element model are presented for various values of blood perfusion rates in the tumor and surrounding tissues. It is observed that the Annular Phased Array is effective in raising the temperature of the tumor to therapeutic values.

Many technological applications based on the use of intensive microwave radiation are connected with high temperature material processing. As a rule the microwave power being absorbed in the material must provide heating of the processed zone up to rather high temperatures (> 100º c) within fairly small time intervals. hese requirementsare dictated either by the nature of the process ( surface transformation hardening, joining ceramics and metals, cutting materials ) or by necessity to provide sufficiently large process output ( coating of materials, joining of dielectrics, melting, etc.). The upper value uf the exploited radiation intensity is limited by effects of gas breakdown near the treated surface. The considerable part of microwave rad.ation can be absorbed and dispersed by the discharge plasma, hence microwave energy supplied to the material significantly decreases.

The dielectric properties of different ranks of Argonne Premium coal samples have been measured using an automated network analyzer system in the frequency range 0.3MHz to 3GHz. These measurements indicate that the inherent moisture present in the various ranks ofcoal has dielectric properties which are different from that of free water. The dielectric properties were also observed to increase with the moisture content as well as with the amount of oxygen present in the coal sample. A mixture model was successfully used topredict these dielectric properties.

The measured dielectric properties have been incorporated into numerical calculations ofthe distribution of absorbed energy in coal irradiated with electromagnetic waves. The numerical technique used is the Method of Moments. Subsequently, the temperature distribution obtained in the coal is evaluated using a Finite Difference technique to solve the heat conduction equation. These calculations should prove very useful in the microwave drying of coal where it is difficult to monitor the temperature using probes.

Ceramics processing with microwave energy is dominated by empirical testing of small samples. Concepts and models for reconciling information between different types of heatingtrials, and for predicting heating performance for large parts are discussed.

The Generalized Pulse—Spectrum Technique (GPST), a versatile, efficient and stable Newton-like iterative numerical inversion algorithm with Tikhonov regularization. GPSThas been been successfully applied to many practical inverse problems, including the inverse problems of equations of the diffusion type. Methodology in applying GPST for solving the inverse problems in microwave heating of ceramic samples is described.

Processing oxide-based ceramics using microwave heating leads to a number of unexpected results, which can only be interpreted in terms of enhanced diffusion. Enhanced sintering has been observed in alumina and zirconia. Accelerated grain growth in dense, hot-pressed alumina has been demonstrated. Increased diffusion coefficients have been observed for diffusion of oxygen in sapphire. As yet, a satisfactory theory to account for these phenomena has not been developed. This paper reviews the experimental work conducted at the Oak Ridge National Laboratory during the past four years on the processing of oxides in both 2.45 and 28 GHz microwave furnaces.

Microwave sintering of oxides has been studied, owing to its great flexibility for thermal treatments, which may influence in particular the microstructure and the electrical properties of the so sintered material. In order to point out the influence of the structure and electrical properties we studied the microwave sintering : i) of insulating complexe oxides with the perovskite structure, ii) of close packed structure such as spinelsand iii) of high Tc superconductors. The differents behaviours of those materials in a microwave field are compared and discussed.

A microwave sintering technology has been developed for the production of metal oxide varistors. The electrical properties (leakage current and non-linearity coefficient) of the microwave sintered devices were found to be comparable to those obtained for conventionally sintered varistors of identical composition. Additionally, the reference voltages were greater (by a factor of two) and the biaxial moduli of rupture were higher for the microwave sintered specimens. These differences have been attributed to the smaller grain size associated with the microwave sintered specimens.

Microwave sintering possesses unique attributes and has the potential to be developed asa new technique for controlling microstructure to improve the properties of advanced ceramics. 1–6 Because microwave radiation penetrates most ceramics, uniform volumetric heating is possible. Thermal gradients, which are produced during conventional sintering because of conductive and radiative heat transfer to and within the part, can be minimized. By eliminating temperature gradients, it is possible to reduce internal stresses, which contribute to cracking of parts during sintering, and to create a more uniform microstructure, which may lead to improved mechanical properties and reliability. With uniform, volumetric temperatures, the generation of nonuniform particle/grain growth due to temperature gradients and associated sintering gradients can be regulated.

A method has been developed to reproducibly sinter high purity alumina (>99.8%) to densities in excess of 98% of theoretical. Sintering times of between 6 and 120 minutes have been investigated and a uniform grain structure is only obtained with sintering times greater than 30 minutes.

Microwave sintering produces a product with a smaller, more uniform grain structure witha more narrow size distribution. The alumina is also tougher than the conventionally sintered material.

When scaled—up to sinter batches of 400 grams, an energy consumption of 3.8 kWh/kg is required, which is estimated to be a 90% savings over conventional sinteringin an electric resistance furnace.

Microwave sintering of Si3N4—based materials showed improved densification as compared to samples heated conventionally under similar conditions. Accelerated nitridation of Si in the microwave furnace to produce Si3N4 was also observed. Dense Si3N4, annealed by microwave heating, exhibited enhanced grain growth; however preferential coupling of the microwave power to the grain—boundary phases in the present experiments resulted in their degradation.

Nanocrystalline TiO2 compacts having initial approximate mean grain sizes of 14 nm and approximate green densities of 70% of theoretical were sintered by short-time exposure in a 2.45 GHz microwave cavity to maximum temperatures of 800, 1000 or 1200 ºC. Sample densities were measured before and after exposure to microwaves using Archimede's method. Transmission electron microscopy and x-ray diffraction were utilized to monitor grain growth and phase changes. Rutherford backscattering was used to monitor any changes in oxygen stoichiometry. The results of this study indicate that enhanced densification behavior is obtained for microwavesintered samples relative to samples sintered using conventional pressureless-sintering techniques.

The microwave heating behaviour of composite systems is of considerable interest for theprocessing of ceramics or the creation of materials with interesting dielectric properties. In this investigation, the heating response of different mixtures of metallic and insulating powders is described. The heating rate as a function of metal type, mass fraction and powder types will be presented for pressed pellet samples.

From the results, three distinct heating regimes have been identified. At low temperatures. induced electric currents cause the metal particles to heat. As the temperature increases, exothermic oxidation of the metal powder particles increases the heating rate. Eventually, the insulating powder (alumina) begins to absorb microwave energy more efficiently and this dominates the high temperature behaviour. The relative importance of thesethree effects for different temperatures depends on the particular powders which are used.

The concept of 'hybrid heating with microwave (MW) energy at 2.45 GHZ.'for ultra rapid sintering of alumina is being introduced. This technique is a combination of MW - materual interaction as well as conventional radiant/conduction mechanisms, and facilitates the attainment of perhaps, the highest possible heating rates in a multimode MW cavity at 2.45 GJZz. (1500ºC in 120 sees.). Rapid sintering of pure.undoped alumina with this novel techniQue culminates in uniform, homogeneous microstructures and mechanical property enhancements vis-a-vis conventional fast firing.

The role of green microstructure (particle size) on MW(hybrid) heating and processing variables (temperature, time) on the MW (hybrid) heating phenomena vs. conventional fast firing were investigated. Hybrid heated samples showed accelerated densification with comparable grain sizes when compared with the conventionally fast fired samples. The effectof particle size on the microwave (hybrid) heating phenomena was found to be analogous to conventional sintering.

A variety of ceramic materials have been heated in various microwave excited plasmas at reduced pressures. Temperatures were measured by optical fiber thermometry. Specimen heating was accomplished by direct contact with the plasma, as well as direct coupling withthe microwave energy, although the latter was screened heavily by the plasma. Higher specimen temperatures were achieved in polyatomic gases which have greater reactional enthalpy. In general, insulating materials achieved higher temperatures than semiconducting or covalently bonded materials.