As part of a program of investigation of microwave sintering, self-consistent CNDO/2 calculations are presented for diffusion barriers and potentials for the motion of interstitial atoms and vacancies in MgO. Clusters of 30 atoms are used in the calculations. Activation energies, diffusion barriers, shape of the potentials and electron densities are obtained.

Microwave processing was carried out on SiO2 -B2O3 solutions and gels prepared by sol-gel methods. Monolithic gels were prepared from alcoholic solutions of trimethylborate and tetraethylorthosilicate using a two-step hydrolysis process. A novel technique of Liquid State Processing (LSP) was employed for the first time, and it was found to be faster and more effective than the conventional processing techniques. The structural evolution of the dried products was followed using FTIR. The effect of processing was examined via surface area analysis (BET), electron microscopy, and FTIR. The microwave drying has been compared with conventional oven drying and vacuum drying techniques. Shorter processing times, improved microstructures, and unique properties have been obtained.

We describe an innovative microwave instrument, designed in collaboration with and owned by Raytheon Company. The instrument permits the manipulation of biological specimens in their fluid milieu during the actual period of rapid tissue fixation. The specimen chamber is designed for sample containers up to 1.7 cm in diameter and 4.5 cm in height. Reflected power is reproducibly low, limiting the need for pretuning the microwave output to the sample. Microwave exposure can be controlled in 1 msecond increments with a range of 10 mseconds to 10 seconds. Mammalian cells and tissues fixed by this microwave device were evaluated by light and electron microscopy. Preliminary findings show large regions of excellent preservation in tissues and in cell suspensions in -100 mseconds.

The use of microwave energies for polymerization of epoxy and other commercially available resins, used routinely in electron microscopy methodologies, shows promise for diagnostic pathology where rapid methods can be crucial. Many desirable embedment properties, such as specimen infiltration, color, hardness, rough trimming, and ability to stain, are necesssary. The properties necessary for cutting 1 micron semithin sections and 700 Angstrom ultrathin sections (that can withstand pentration by a 100 kV electron beam for image formation) must be maintained.

In this study, six epoxy resins and one unsaturated polyester resin were used, the latter in a variety of recipes. Each formulation was subjected to microwave power levels from 400 to 700 W for 1 to 15 minutes. Selected specimen embedments tested well and significantly reduced traditional convection oven polymerization time from two days to a few minutes.

Acute periodontal lesions generally respond to conventional therapy. In AIDS patients, however, further deterioration is the usual consequence. Candida albicans(CA) lesions of the oral mucosa have been observed in 88% of AIDS patients; 59% of male homosexual and IV drug abuser oral candidiasis patients subsequently developed AIDS. Indeed HIV may be hard to find in AIDS patients in whom CA lesions are very prominent.

Rapid reproducible silver stains have been developed in our laboratories to detect Gram-negative bacteria, fungi and protozoa. They demonstrate these microorganisms by the microwave-accelerated metallization of the aldehydes produced in their surface carbohydrates or lipopolysaccharides after periodic acid oxidation (Giammara and Hanker, US Patent No. 4,690,901, 1987; Sigma Diagnostics Kit No. HT-100). We thought that these stains might be useful to demonstrate by microscopy the large numbers of CA which culture studies had suggested were present in subgingival plaque at atypical periodontal disease sites of AIDS patients. Direct staining of plaque smears and their evaluation under the light microscope could be done in much less time than that required for culture. Any equivocal microorganisms could be positively identified as CA by immediate examination of the slide or coverslip by scanning electron microscopy. Although our PATS reaction was more valuable for identifying Gram-negative bacteria especially spirochetes and for studying substructure of CA and their epiphytic interactions with bacteria, the SIGMA DIAGNOSTICS Silver Stain HT-100 was much easier to use for rating CA concentration in disease sites. This test was applied to subgingival plaque smears from 28 periodontal disease sites of 22 HIV-seronegative patients and 45 sites of 12 HIV-seropositive patients. After staining, the smears were examined and scored (Davenport Index). None of the 12 HIVseropositive patients but 15 of the 22 HIV-seronegative patients had a score of zero at all sites. The score ranged from 0 (no CA) to 3 (large numbers of CA in all fields). In the seropositive patients the median score was 2 while in the seronegative group it was 0.

The much higher scores obtained in specimens from periodontal disease sites of the AIDS patients suggest that this simple rapid test might be valuable for screening individuals and indicating those requiring further testing or monitoring for AIDS.

Computer-assisted microscopic image analysis of silverstained subgingival plaque smears was shown to be far more dependable and reproducible than subjective scoring or rating by visual inspection to estimate the amounts of Candida 4ibicans(CA) present. High CA counts in subgingival plaques from atypical periodontal diseased sites could suggest that an individual be tested or monitored further for AIDS. This is especially the case if the individual is in a high risk group or if the CA count cannot be ascribed to immunosuppression resulting from diabetes, corticosteroid or broad-spectrum antibiotic therapy, cancer chemotherapy or blood transfusion for hemophilia. In many such situations this simple, rapid procedure may be useful in screening for AIDS when serologic procedures for the HIV virus may be equivocal.

The purpose of this paper is to broadly assess the industrial applications of microwave power and to perhaps formulate some of the ingredients for a successful application. Traditional applications are reviewed and the ingredients for their success are noted. Also discussed will be a few applications that were not successful. Future uses of microwave power are hypothesized based upon research and development efforts in progress throughout industry.

This paper reviews the results from two relatively new microwave heating studies under development at Ontario Hydro's Research Division. The first study involves the development of a hybrid kiln to process advanced ceramics at high temperatures. Using a combination of resistive heating elements and microwave heating, this new device has been operated to show significant process advantages in terms of time and energy savings. A description of the hybrid kiln is presented together with the test results for calcining and sintering a variety of ceramic materials.

The second investigation involves the use of microwave heating for the treatment of hazardous wastes. The selective heating ability of microwaves has been applied to reduce the environmental impact of wastes generated from industrial processes. An example is provided to illustrate the advantages and difficulties of applying this technology.

The characterization of single crystalline silicon wafers for application in (opto)electronic devices by transient photoconductivity measurements is investigated. To this aim is the transient photoconductivity in Si wafers after different treatments determined by the Time Resolved Microwave Conductivity ( TRMC ) method. This technique is non-evasive and contactless and so in-situ measurements are possible. Application of TRMC measurements for process control and quality control of relevant process steps in the production of (opto)electronic devices is discussed in view of the experimental results presented.

Microwave energy is commonly considered as a fast and efficient source of energy for heating materials. This perception can be easily verified by the proliferation of microwave ovens in household kitchens and the emerging usage of the technology in the food industry. From the standpoint of material science, coupling of microwave energy into materials during processing can be more than just shortening the heat-up time in conventional thermal treatment; this energy source can provide an additional degree of control over the properties developed during processing.

Polymers and polymer composites have been processed In a cylindrical resonant microwave applicator at a frequency of 2.45GHz. Stoichlometric mixtures of two epoxy/amine systems, DGEBA (Diglycidyl Ether of Bisphenol A)/DDS (4,4'-Diaminodiphenyl Sulfone) and DGEBA/mPDA(m-Phenylene Diamine), were microwave and thermally cured Isothermally using a thin film technique. FTIR was used to determine the extent of cure. Increased reaction rates were observed In microwave cure when compared to those of thermal cure. The rate Increase due to microwave effects was much greater for the DGEBA/DDS system than for DGEBA/mPDA. Also, crossply and unidirectional 24-ply and 72-ply graphite/epoxy laminates(AS4/3501-6 prepreg, Hercules Corp.). were processed using microwave radiation. The flexural properties of the microwave processed composites were strongly dependent on the resonant heating mode. Comparable flexural properties were obtained for the unpressurized microwave processed composites and the pressurized autoclave processed composites. Proper controlled-hybrid modes are required to process composites of high mechanical properties. The procedures for obtaining these controlled-hybrid modes are described.

Microwave processing has been utilized to process thermosetting polymeric materials. Specifically, fundamental studies relating epoxy network generation to processing conditions have been investigated in a tunable cylindrical cavity operating at a frequency of 2.45 GHz. These studies demonstrate that fully cured networks can be generated in ten minutes with the retention of good mechanical properties. Furthermore, toughened epoxy systems which utilize carefully designed amine terminated poly(arylene ether sulfone) thermoplastics as reactive oligomers have resulted in novel phase separated morphologies. In fact, it has been demonstrated that the morphology in these multiphase systems can actually be controlled by utilizing microwave processing. Bismaleimide toughened systems, devised by similar strategies, have demonstrated a 10–20 fold reduction in the time required to achieve full cure.

The microwave heatability of various thermoplastic polymers was investigated. The concept of microwave calorimetry was proposed to quantitatively illustrate how viscoelastic behavior controlled microwave heatability. Specifically, heating rate as a function of sample temperature revealed a distinct maximum which was identified as the Tg at 2.45 GHz. The critical temperature, Tc, necessary for rapid microwave heating was identified by drawing a tangent to the heating rate curve and extrapolating to a critical value at zero heating rate. In separate experiments, low frequency (100 kHz) dielectric measurements were made which show the frequency dependence of Tg by means of Arrhenius activation energy plots. In general, the larger the activation energy, the closer the critical heating temperature, TC, was to the Tg determined by DSC. The smaller the activation energy, the further dielectric loss shifted with increased frequency so that Tc was very far from Tg determined by DSC.

Over the last two decades the sol gel technique has become very popular because of its ability to yield a multicomponent inorganic glass at low temperatures[1,2]. More recently, techniques to incorporate organic species into a sol gel network of metal alkoxides has been developed and studied[3-7]. One technique developed in our laboratory is to incorporate oligomers endcapped with metal alkoxide functionalities into a reaction mixture of TEOS, water and acid catalyst. The resulting materials, often cured at ambient temperatures, can be tough and transparent monoliths. This technique is important in that it helps to overcome some of the defects of pure sol gel silicate structures such as cracks, voids and brittleness. The high temperature structural properties of these materials, however, are limited because of the organic component. This approach provides the ability to make a wide range of materials with vastly different properties depending upon the polymer and metal alkoxide chosen. Novel properties displayed by these materials make them attractive for certain technological applications such as coatings, films and fibers. One of the main drawbacks of the ambient temperature curing process is the relatively long reaction time which hinders immediate utilisation of the product. As would be expected, the extent of reaction has been found to be dependent on the T, of the oligomer under consideration. In the case of high Tg oligomers (e. g. polysulfones) the reaction has to be completed at elevated temperatures (above the Tg.) in order to acheive a high extent of reaction. In the case of ceramers containing functionalized poly(tetramethyleneoxide) (PTMO) oligomer, curing time can vary between 48 to 100 hours before the final gel can be successfully handled and tested. In order to enhance the rates of reaction and determine the effect of such an acceleration on the physical properties, samples were reacted at elevated temperatures using microwave heating and conventional oven curing. When microwaves were used the curing time has been found in some cases to be reduced dramatically from 60 hours to twenty minutes. In this paper structure-property relationships of these "microwaved" materials will be compared and contrasted with similar materials cured in a conventional oven at equivalent elevated temperatures as well as room temperature cured materials. Time-temperature studies of the reactants were also conducted in a travelling wave guide. These studies were done primarily to gain some insight as to the effect of microwaves on specific reactants. analyzer.

The effect of microwave processing on the chemical interactions occurring between the carbon fiber surface and the epoxy matrix constituents was investigated using X-ray Photoelectron Spectroscopy (XPS). Monofunctional model compounds of the matrix constituents were exposed to the carbon fibers at temperatures similar to those encountered during composite processing. The microwave treatment resulted in a substantial increase in the amount of chemical interaction between the fiber surface and the epoxy resin but little difference for the amine component of the matrix when compared to thermal processing. An epoxy resin/amine hardener adduct compound used to determine the hydroxyl group interaction with the carbon fiber surface indicated a low level of chemical interaction of the hydroxyl with the carbon fiber surface under the conditions used in this study.

Composite laminates have been cured under pressure in a tunable microwave cavity. A 7-in. diam cavity with a sliding short was tuned to produce single mode or controlled hybrid mode standing waves at 2.45 GHz. Graphite/epoxy laminates were processed with power inputs as low as 2 W/g.

A low loss press was designed and built to operate inside the cavity. The press can apply 100 psi pressure on a 9-in. square laminate. An empirical approach has been used to develop processing conditions for high quality laminates. Graphite/epoxy composites were cured in 90 min or less. A simple on-off temperature control loop modulated power input to the cavity with a coaxial switch.

Mechanical testing and differential scanning calorimetry have been used to evaluate the composites. The material properties have been related to processing conditions.

Traditionally, polymeric composite parts are heated and consolidated in an autoclave. For large parts, such as transport aircraft fuselages or submarine hulls, size becomes a limiting factor. To overcome this limitation and to reduce labor costs we are developing an automated tape placement process. In this process we build composite parts one layer at a time with tape containing carbon fibers impregnated with a thermoplastic. As the tape comes into contact with the part, we apply heat to melt the thermoplastic and apply pressure to consolidate the tape to the part. To support this effort we have developed a proprietary microwave applicator that is suitable for rapidly heating carbon-fiber composites in an automated tape placement process. Small carbon-fiber/poly(aryl-ether-ether-ketone) parts made using the microwave applicator have interlaminar shear strengths of 100 MPa (14.5 ksi), which is almost equal to the 103 MPa (15.0 ksi) obtained using an autoclave.

The hydrolysis rate of adenosine-5'-triphosphate (ATP) has been reported to be 25 times greater during microwave heating than during conventional heating at comparable temperatures (100 to 105 °C). This remarkable rate increase was both attributed to and cited as evidence for a nonthermal microwave effect. Attempts to replicate those findings under carefully controlled microwave exposure conditions and with special attention being paid to accurate temperature measurements lead to the conclusion that the hydrolysis rate is related solely to temperature.

This paper reviews measurement techniques and sources of data for the dielectric constant and loss factor of solid materials. Accurate values of such properties are basic to intelligent design of materials processing schemes utilizing electromagnetic energy. Emphasis is on techniques and data for these properties in the “microwave” range of frequencies of roughly 108 to 1011 Hz and at elevated temperatures.

Texas Instruments Defense Systems and Electronics Group develops radars, missile systems, infra-red systems and electronic systems for the Defense Department. Materials having special microwave properties are needed for radar systems, missile systems that home on microwave emissions, microwave emitter locator systems, and jamming systems. The microwave materials appear in the form of antennas, lenses, radomes, windows, dielectric loading materials, potting materials, dielectric transmission lines, microwave absorbing materials, adhesives, sealants, coatings, and foams. Naturally, the complex permittivity and permeability of these materials need to be known to enhance their application and control their properties in manufacturing. Texas Instruments has a microwave materials test laboratory that includes permittivity and permeability measurements as well as physical and thermal properties. The samples come in many different forms, measurement at different frequencies are needed and the electrical properties cover a wide span (conductors to dielectrics and foams to ferrites). For this reason, many different methods of dielectric and magnetic measurement are used. These various methods will be discussed below.