James A. R. Ball

Productivity improvement through the use of industrial microwave technologies

Microwave processing of materials is a relatively new technology advancement alternative that provides new approaches for enhancing material properties as well as economic advantages through energy savings and accelerated product development. This paper presents a state-of-the-art review of microwave technologies, processing methods and industrial applications. The characteristics of microwave interactions with materials are outlined together with the challenges that are difficult to process the materials present. To fully realise the potential benefits of microwave and hybrid processes, it is essential to scale-up process and system designs to large batch or continuous processes. This necessitates computational modelling and simulation, system design and integration and a critical assessment of the costs and benefit analysis. Impediments to industrial applications are identified and development opportunities that take advantage of unique performance characteristics of microwaves are discussed. Clearly, advantages in utilising microwave technologies for processing materials include penetrating radiation, controlled electric field distribution and selective and volumetric heating.The aim of the work presented in this paper is to help guide those interested in using microwaves to improve current materials processing. Microwave fundamentals are described to provide a brief awareness of the advantages and limitations of microwaves in the processing of materials. Furthermore, the limitations in current understanding are included as a guide for potential users and for future research and development activities. Examples of successful applications are given to illustrate the characteristics of materials, equipment and processing methods applicable to industrial microwaves. Economic considerations are described and costs are provided as guidelines in determining the viability of using microwaves for processing materials.

Microwave Facilities for Welding Thermoplastic Composites and Preliminary Results

The wide range of applications of microwave technology in manufacturing industries has been well documented (NRC, 1994; Thuery, 1992). In this paper, a new way of joining fibre reinforced thermoplastic composites with or without primers is presented. The microwave facility used is also discussed. The effect of power input and cycle time on the heat affected zone (HAZ) is detailed together with the underlying principles of test piece material interactions with the electromagnetic field. The process of autogenous joining of 33% by weight of random glass fibre reinforced Nylon 66, polystyrene (PS) and low density polyethylene (LDPE) as well as 23.3% by weight of carbon fibre reinforced PS thermoplastic composites is discussed together with developments using filler materials, or primers in the heterogenous joining mode. The weldability dependence on the dielectric loss tangent of these materials at elevated temperatures is also described.

Resolving ambiguity in broadband waveguide permittivity measurements on moist materials

The dielectric properties of moist materials depend strongly on free water content. Moisture content may be conveniently sensed using microwaves, provided that calibration data is available. One common method utilizes network analyzer transmission measurements on waveguide cells filled with samples of known water content. Since this technique yields multiple solutions for the complex permittivity, a simple procedure has been developed to identify the correct solution. This has proved to be highly reliable and lends itself to measurement automation.

Permittivity Measurement of Thermoplastic Composites at Elevated Temperature

The material properties of greatest importance in microwave processing of a dielectric are the complex relative permittivity ε = ε’-jε”, and the loss tangent, tan 8= This paper describes two convenient laboratory based methods to obtain e’, e” and hence tan 3 of fibre-reinforced thermoplastic (FRTP) composites. One method employs a microwave network analyzer in conjunction with a waveguide transmission technique, chosen because it provides the widest possible frequency range with high accuracy. The values of the dielectric constant and dielectric loss of glass fibre reinforced (33%) low density polyethylene, LDPE/GF (33%), polystyrene, PS/GE (33%), and Nylon 66/GE (33%), were obtained. Results are compared with those obtained by another method using a high-temperature dielectric probe.

Variable frequency microwave processing of thermoplastic composites

Abstract The range of applications for variable frequency microwave (VFM) facilities (2–18 GHz) has been extended to thermoplastic composites. Five thermoplastic polymer matrix composites are processed and discussed, including 33 wt-% random carbon fibre reinforced polystyrene [PS–CF (33%)], and low density polyethylene [LDPE–CF (33%)]; 33 wt-% random glass fibre reinforced polystyrene [PS–GF (33%)], low density polyethylene [LDPE–GF (33%)]and Nylon 66 [Nylon 66–GF (33%)]. Bond strengths of lap joints were tested in shear and results were compared with those obtained using fixed frequency (2·45 GHz) microwave processing. The primer or coupling agent used was a 5 min, two part adhesive containing 100%liquid epoxy and 8% amine, which was more readily microwave reactive than the composites themselves. The VFM was operated under software control, which provided automatic data logging facilities. Results indicate that VFM can produce strong bonds for PS and LDPE.

Characteristic impedance of unbalanced TDR probes

Time domain reflectometry (TDR) may be used to make simultaneous measurements of both dielectric constant and conductivity by means of a probe inserted into the medium. The air-spaced characteristic impedance of the probe is required in order to estimate the conductivity from the final value of the TDR waveform. Unbalanced probes derived from a coaxial line by replacing the outer shield with regularly spaced wires are favored for many applications because they eliminate the need for a balun. Until now, it has been necessary either to measure the probe characteristic impedance or to calibrate it in solutions of known conductivity. This paper shows how to deduce an expression for the probe characteristic impedance by means of a conformal transformation. For thin wires, a simpler formula has been derived which is suitable for hand calculation. The accuracy of these analytic expressions has been assessed by a method of moments numerical solution which exhibits very rapid convergence as extra basis functions are added. The characteristic impedance formula obtained by conformal transformation and its thin wire approximation are found to be accurate to 0.1% and 1%.

Microwave processing and permittivity measurement of thermoplastic composites at elevated temperature

[Abstract]: The material properties of greatest importance in microwave processing of a dielectric are the complex relative permittivity e = e′- je″, and the loss tangent, tan δ = e″/ e′. The real part of the permittivity, e′, sometimes called the dielectric constant, mostly determines how much of the incident energy is reflected at the air-sample interface, and how much enters the sample. This paper shows the reflection coefficient of a material and the depth of penetration of a dielectric. Therefore the larger the value of the real part of the complex permittivity, the more the incident energy will be reflected by a dielectric but the energy that enters the material will penetrate further than in a dielectric with the same e″ but lower e′. However, the most important property in microwave processing is the dielectric loss, e″ which predicts the ability of the material to convert the penetrating energy into heat. Measurements of e′ and e″ are therefore critical in the microwave processing of materials with or without primer. The dielectric constant, e′, dielectric loss, e″, and hence complex relative permittivity, e and loss tangent, tan δ, of some commonly used thermoplastics have been measured at various temperatures and frequencies. These results may be used to determine whether various fibre-reinforced thermoplastic (FRTP) composites are suitable for microwave processing. This paper describes a convenient laboratory based method to obtain e′, e″ and hence tan δ. The method employs a network analyser together with a waveguide transmission technique chosen because it provides the widest possible frequency range with high accuracy; the hardware and software of the method is also readily available in the electronic laboratory of the University of Southern Queensland. The required data were collected at a range of elevated temperatures and over a band of frequencies.

Characterisation of thermoplastic matrix composites using variable frequency microwave

Abstract In most industrial microwave processing operations, the frequency of the microwave energy launched into the waveguide or cavity containing the sample is fixed. This brings with it inherent heating uniformity problems. This paper describes a new technique for microwave processing, known as variable frequency microwave (VFM) processing, which alleviates the problems brought about by fixed frequency microwave processing. In VFM processing, microwave energy over a range of frequencies is transmitted into the cavity in a short time, e.g. 20 μs. It is therefore necessary to determine the best frequency range for processing a material. The best range frequency for microwave processing of five different thermoplastic matrix composites using the VFM facilities has been determined. The optimum frequency band for microwave processing of these five materials was in the range 8–12 GHz. This data enables bonding of the materials using microwave energy under the most favourable conditions.

Mode-matching analysis of a shielded rectangular dielectric-rod waveguide

Rectangular cross-sectional dielectric waveguides are widely used at millimeter wavelengths. In addition, shielded dielectric resonators having a square cross section are often used as filter elements; however, there is almost no information available on the effect of the shield. Rectangular or square dielectric waveguide is notoriously difficult to analyze because of the singular behavior of the fields at the corners. Most published analyses are for materials with a low dielectric constant, and do not include the effects of a shield. This paper describes a numerically efficient mode-matching method for the analysis of shielded dielectric-rod waveguide, which is applicable to both low and high dielectric-constant materials. The effect of the shield on the propagation behavior is studied. The shield dimensions may be selected such that the shield has a negligible effect so that results can be compared with free-space data. The results are verified by comparison with several sets of published data, and have been confirmed by measurement for a nominal /spl epsiv//sub r/ of 37.4.

Surface-wave propagation on a grounded dielectric slab covered by a high-permittivity material

A grounded dielectric slab covered by a higher permittivity material would not normally be expected to support surface waves. This conclusion must be modified when the covering material is sufficiently lossy. Assuming a thin slab, approximate analysis shows that the fundamental TM/sub 0/ surface wave is able to propagate if the cover loss is high enough. A numerical analysis has verified these conclusions. Propagation of higher order TM and TE modes is found to be possible above cutoff frequencies, which reduce as cover loss is increased. These results are of significance when printed circuit transmission lines such as microstrip or slotline are used as contact sensors, e,g., for moist materials.