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 Epsilon = Epsilon' - jEpsilon'', and the loss tangent, tan Delta = Epsilon'/Epsilon''. This paper describes two convenient laboratory based methods to obtain Epsilon', Epsilon'' and hence tan Delta of fibre-reinforced thermoplastic (FRTP) composites. One method employs a microwave network analyser 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/GF (33%), and Nylon 66/GF (33%), were obtained. Results are compared with those obtained by another method using a high-temperature dielectric probe

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 TM0 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

The application of microwave sensing to the measurement of cheese curd moisture

There is a need in the dairy industry for instrumentation capable of providing on-line information about the moisture content of cheese during manufacture. Present measurement techniques are usually performed off-line and can be susceptible to human error. It is demonstrated that microwave-based moisture sensing techniques offer a number of potential advantages over conventional methods due to the strong interaction of microwaves with water. The permittivity of cream cheese curd and low-fat cheddar cheese curd has been measured over a range of frequencies and moisture contents in order to establish the relationship between these variables. A vector reflection coefficient measurement engine based on a six-port reflectometer has been built and tested. A suitable sensing head has been fabricated from a short length of microstrip transmission line. Two sensor characterisation models have been developed and compared with measured data. A novel algorithm has been developed to resolve the ambiguity inherent in many permittivity measurement techniques. It has been discovered that surface waves can propagate on a grounded dielectric slab covered by a material with a higher dielectric constant, provided the loss factor of the covering medium is greater than zero. It has also been found that the dominant mode of microstrip can radiate when the line is covered by a high-permittivity material, although this can be suppressed if the covering material is sufficiently lossy. There are three principal conclusions to draw from the investigation in this thesis. Firstly, changes in the moisture content of cheese curd during manufacture produce measurable variations in permittivity. Secondly, these changes can be measured accurately and cheaply using off-the-shelf microwave hardware. Finally, considerable attention must be paid to the characterisation of the sensing head if the instrument is to achieve its full potential. Promising results have been obtained in this area, however certain issues pertaining to the propagation of multiple dominant modes and higher order modes have not been fully resolved and would repay further theoretical analysis

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 ε = ε′- jε″, and the loss tangent, tan δ = ε″/ ε′. The real part of the permittivity, ε′, 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 ε″ but lower ε′. However, the most important property in microwave processing is the dielectric loss, ε″ which predicts the ability of the material to convert the penetrating energy into heat. Measurements of ε′ and ε″ are therefore critical in the microwave processing of materials with or without primer. The dielectric constant, ε′, dielectric loss, ε″, and hence complex relative permittivity, ε 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 ε′, ε″ 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

An important step in microwave processing of materials: permittivity measurements of thermoplastic composites at elevated temperatures

[Abstract]: The dielectric constant, ε′, dielectric loss, ε″, and loss tangent, tan δ, of some commonly used thermoplastics have been measured at various temperatures and frequencies. They do give some clues to the suitability of microwave processing of certain fibre-reinforced thermoplastic (FRTP) composites but cannot provide definite answers to the problems. Few measurements, if any, of the ε or the tan δ of FRTP composites have been reported in the literature. This paper describes a convenient laboratory based method designed to obtain ε′, ε″ and hence tan δ. The method employs an automatic logic network analyser and is called the waveguide transmission technique or transmission line method and is chosen because it provides the widest possible frequency range with high accuracy and utilised available the hardware and software. The required data were collected at a range of elevated temperatures and over a band of frequencies. The relationship between ε′ and ε″, or the tan δ of the composites and their weldability by microwave energy is also studied and discussed