The role of intergranular glassy phases in the microwave joining of alumina ceramic rods has been investigated. Alumina ceramics of different purities have been examined using a TE102 single mode rectangular resonant cavity operating at 2.45 GHz. Temperature and axial pressure were varied and the resultant join microstructures analyzed. The presence of a glassy grain boundary phase has been shown to improve the ability to microwave join alumina ceramics by increasing the dielectric loss of the ceramic and, when softened, by enabling the alumina grains to flow under the imposed load. Evidence has also been found for the sintering of individual grains where they have come into contact across the join line.

The plastic deformation of grains is shown to take place due to the vacancy drift perturbations caused by space charge formation in the near-surface layer and by surface permeability asymmetry with respect to the direction of the motion of atoms through it.

Microwave processing has been investigated as an alternative to conventional thermal method in processing polymer matrix composite materials. The main advantages of microwave processing over thermal processing are that: 1) microwave heating is volumetric, direct, selective, instantaneous, and controllable which offers advantages such as fast heating and minimization of temperature excursion; 2) microwave radiation can provide many desirable features in polymer and composite processing, such as enhanced polymerization rates and glass transition temperatures of thermosets, improved mechanical properties of composite materials, and increased adhesion between graphite fibers and matrix.

Microwave heating has been used in food processing, drying, material processing, waste treatment, and organic synthesis. This paper summarizes the current status of microwave technology for the processing of polymer matrix composite materials. The discussion will be focused on the use and development of batch and continuous techniques using tunable single mode resonant microwave cavities for processing polymer composites.

Microwave processing has been used to reduce the long process cycles normally associated with the curing cycles for highly ordered polyimides, such as BPDA-PDA. These materials have found applications in the electronics industry due to the excellent thermal stability and low coefficient of thermal expansion of this class of polymers. Thin polymer films - an application not normally considered for microwave curing - were processed and the dependence of the mechanical properties compared with conventional thermal processing.

Epoxy resin was cured by microwave heating using a variable frequency furnace. Curing behavior and uniformity were characterized by visual inspection, hardness, and differential scanning calorimetry. Frequency sweeping was used as a means of mode stirring to minimize power nonuniformities in the furnace cavity. Cure times were reduced by half and with the appropriate post-cure treatment (also done in the microwave furnace) the resin had a glass transition temperature of 165–175°C. Small amounts of additives such as carbon black and various oxide pigments were tested to determine if they would improve microwave coupling and further reduce cure time. There was little or no effect at a standard concentration of 1 wt% pigment.

Ethylene carbon monoxide copolymer (ECO) has physical and thermal properties essentially identical to those of low density polyethylene, including a melt temperature near 100C and a low water vapor transmission rate. Unlike polyethylene, ECO absorbs and can be heated and melted with microwave and radio frequency energy. A conventional method of sealing flexible polymer containers is to apply the external heat to the walls with heated dies. The time of sealing is directly related to the thickness of the wall structure, its composition, and the temperature of the heated dies. The die temperature is often limited by the softening or melt temperature of the external polymer layer and/or degradation of surface inks or applied graphics. High frequency energy applied to a properly designed polymer structure deposits the energy into the internal absorbing layers, which transform that energy into heat. No heated dies are required. External layers of high frequency non-absorbing polymer act as thermal insulators, keeping the generated heat in at the seal location. Sealing rates can be several times faster than by conventional means.

Cyanate ester networks are receiving considerable attention as potential candidates for high temperature adhesives and composite applications. The dicyanates ester monomers cyclo-trimerize1,2 via addition polymerization to form highly crosslinked polycyanurates (Scheme 1).

It is well known that when pores of hardened cement are filled with a suitable polymer, desirable properties are greatly enhanced. With microwave induced polymerization, significant increases in compressive strength are observed.

Mortar samples with a sand cement ratio of 2.5 and water cement ratios, 0.3, 0.4, and 0.5, were molded and hardened. After sufficient hydration, they were evacuated, filled with MMA and pressurized. The samples were then processed in a dynamic multimode applicator. Temperature was monitored and they were processed until the temperature exceeded 85°C.

The principal result is the increase in compressive strength as compared to an untreated sample. For a W/C=0.5, and a sample hydrated for one day, the strength increased by a factor of6.0. In addition, the highly porous 1 day polymerized samples exhibited significantly greater strength than 28 day unpolymerized material.

The microwave heatability of a low loss polymer (polypropylene) was enhanced by the addition of a conductive powder (iron). The effects of the amount (5–40% iron by volume) and size of the conductive particles on the microwave heatability were studied. The complex permittivity and complex permeability at microwave frequencies and the dc resistivity were measured. Samples were also heated in a multimode microwave cavity. Results show that microwave heating increased with increasing iron concentration. However, the penetration depth of the microwaves decreased with increasing iron content, and dramatically decreased when the polypropylene-iron composites became electrically conductive at the percolation threshold.

This paper reports on the use of variable frequency microwave processing to uniformly heat and post-cure large (>100 cm2) plates of thermoset polymer matrix composite (PMC) material consisting of isocyanate/epoxy mixtures. Prior work has indicated that microwave energy is capable of reducing the time or the temperature necessary to achieve full cure; however, single-frequency microwave heating results in non-uniformity of heating/curing in areas of over 12.5 cm2. By using a Variable Frequency Microwave Furnace and sweeping the microwave power over the 5 to 6 GHz frequency range at a power level of 150 Watts, uniform heating and curing of the PMC plates was achieved over an area of greater than 100 cm2-- and a volume of greater than 200 cm3. The overall temperature gradient across the surface of the plates was less than 10%. Finally, this paper also reports on observed, volume-related heating effects, which are postulated to be the result of heat loss from the sample surface.

Silicon carbide ceramic materials have been successfully formed from commercially available polycarbosilane using microwave radiation as the sole source of heat. Conventional thermal processing of the same polymeric precursor was also studied for comparison with microwave processing. Microwave heating enhances crystallinity at much lower curing temperature and within shorter times. Possible explanations for microwave enhanced processing are proposed.

Dielectric properties of poly(ethylene terephthalate) (PET) were measured over a frequency range of 10 KHz to 2.45 GHz and a temperature range of 20 to 110°C. Relaxation peaks were identified at 1) fixed frequency with variable temperatures, and 2) fixed temperature with variable frequencies. The crystallinity of poly (ethylene terephthalate) was measured using differential scanning calorimetry (DSC). Relationships between crystallinity, dielectric properties, and location of the dielectric relaxation peak on the frequency and temperature scales were studied for poly(ethylene terephthalate). Also, the dielectric loss factor decreases with increased crystallinity at 2.45 GHz and 4 GHz within the temperature range studied.

Studies of microwave-assisted curing of neat resins (epoxy) and unidirectional glass and carbon fiber laminates have shown that a substantial reduction in the curing time was obtained [1]. This may be explained by the penetration of microwave energy directly and throughout the laminate with enhancement of the kinetics of the chemical reaction.

Results of this work indicate that the microwave assisted curing of glass fiber laminates also shows a substantial reduction of the required curing time.

Microwave radiation of 2.45 GHz has been demonstrated to be an acceptable method to cure unidirectional carbon fiber laminates. Also, effective curing of crossply (0/90) laminates through this method was observed when proper rotation of the parts accompanied the curing process. This is in accordance with previous work [2,3].

Multidirectional carbon fiber/epoxy laminates demonstrate a lack of coupling during the curing process. A direct curing of these laminates was not possible by microwave radiation with the experimental approach used, in agreement with previous work [4[. Nevertheless, a moderate reduction in the curing time of these thin laminates was observed due to hybrid curing.