Historically, coatings were processed from natural oils, fats, and resins; the first well-known and widely used being lacquer [Meir-Westhues, 2007]. In the 20th century, synthetic resins were developed to achieve coatings with improved properties. Of these coating compositions, polyurethanes (PURs) were one of the most prevalent. Polyurethanes became possible in 1937 when Otto Bayer developed the diisocyanate polyaddition process [Randall et al, 2002]. Since that time, literally thousands of PUR compositions have been used commercially. The primary application of interest in this study is that of coatings for wood substrates. It is well-known among materials researchers that there can be a number of differences between microwave and conventional materials treatment techniques [Clark et al, 1996], including enhanced reaction rates, lowered processing temperatures for some products, and selective interactions in composite systems. The primary goals of this research were to determine (1) whether microwave energy affected the cure rate in a water-based, aliphatic PUR, and (2) if there was an effect of microwave frequency on the cure rate. The primary tool for determining extent of cure in the PUR samples was Fourier transform infrared spectroscopy (FTIR). Using this characterization method, the changes in intensities of four bonds specific to the PUR composition were followed. It was determined that, in the particular PUR composition studied, microwave energy had an effect on the cure rate when compared with conventional heating, and that there was a frequency effect on the cure rate. Additionally, a deeper understanding of the use of FTIR spectroscopy techniques for studying cure kinetics was developed. / Master of Science

There is growing interest in the use of microwaves to process materials. The main application of microwave processing of materials is in heating. The most important characteristic of microwave heating is {\it volumetric} heating, which is quite different from conventional heating where the heat must diffuse in from the surface of the material. Volumetric heating means that materials can absorb microwave energy directly and internally and convert it to heat. It is this characteristic that leads to advantages such as rapid, controlled, selective, and uniform heating. However, some problems hinder the widespread use of microwave energy. One of these problems is called thermal runaway, which is a type of thermal instability due to the interaction between the electromagnetic waves and materials. As thermal runaway occurs, the temperature of the heated material rises uncontrollably. The normal consequence of thermal runaway is the damage of the processed materials. The origins of thermal runaway are different under different processing conditions. When processing ceramic materials, thermal runaway is mainly due to the positive temperature dependence of dielectric loss of the material. These materials absorb more microwave energy as they are being heated. The most plausible explanation of this phenomenon is the so-called "S-curve" theory. However, prior to this work, no direct experimental evidence has been published to verify this theory. In this dissertation, we report the direct experimental evidence of the so-called "S-curve" by heating thermal runaway materials in a microwave resonant cavity applicator. A complete discussion of how the experimental results were achieved is presented. From the experimental results, we find that by the use of the cavity effects thermal runaway can be controlled. To explain the experimental findings, a theoretical model based on equivalent circuit theory is developed. Also, a coupled heat transfer and electromagnetic field model is developed to simulate the heating process. Both models give reasonably good comparison with our experimental results. Finally, a method to control thermal runaway is described. / Ph. D.

Microwave processing of ceramics has seen a growth in research and development efforts throughout the past decade. One area of interest is the exploration of improved heating control through experiments and numerical modeling. Controlled heating may be used to counteract non-uniform heating and avoid destructive phenomena such as cracking and thermal runaway. Thermal runaway is a potential problem in materials with temperature dependent dielectric properties. As the material absorbs electromagnetic energy, the temperature increases as does its ability to absorb more energy. Controlled processing of the material may be achieved by manipulating the applied field. The purpose of this research is to model the interaction of the EM-field with a thin ceramic fiber to investigate possible mechanisms that may affect the heating process. The fiber undergoes microwave heating in a single-mode resonant applicator. Maxwell's equations for the fields within the cavity are solved using mode-matching techniques taking into account the field interaction of the fiber and an arbitrarily shaped coupling aperture. Effects of varying the aperture shape on the field distribution are explored. The coupled nature of the electromagnetic solution with the material's temperature-dependent properties, including an analysis of non-uniform heating, is also discussed. / Master of Science

The benefits of microwave technology in materials processing is well documented and researched. It offers many potential advantages over conventional processing such as rapid heating, faster processing times and more consistent product quality. However the actual implementation of this technology has been lacking and the benefits have gone largely unrealised. This is due largely in part to the non-uniform heating obtained in multimode cavities in conventional microwave processing. Recently, a new processing method dubbed the Variable Frequency Microwave (VFM) Technique has been developed to overcome the inherent problems associated with conventional microwave processing. By sweeping through a bandwidth of frequencies, the limitations observed in conventional processing, and specifically the problem of heat uniformity, are avoided. With the increase in research activities in alternative processing methods for new and current materials that will provide better product quality as well as time and cost savings, the VFM technique has the potential to rejuvenate interest in microwave processing. This thesis documents the research work undertaken on the VFM technique with emphasis on its characterization, optimisation and implementation to suitable applications in particular in the upcoming area of Microfabrication. A commercial Variable Frequency Microwave with an operating bandwidth of 2.5-8.0 GHz was investigated through modelling and experimental work to determine the energy distribution within a multimode cavity and to provide an insight of the mechanisms of the method. Modelling was found to be an efficient and cost-effective tool to simulate VFM and to examine the reported advantages of this new technique. Results obtained confirm the superiority of the VFM method over the conventional fixed-frequency processing showing a marked improvement in the heating uniformity achieved. Quantitative analysis of the three major VFM parameters that influence heat uniformity - Sweep Rate, Bandwidth and Central Frequency - indicate that although slight variation in heat uniformity was observed when changing these parameters, these variations are only small which implies that the VFM technique is quite insensitive to changes in the parameters making it quite a robust system. An analytical model of the Variable Frequency Microwave technique was developed and it was found that the heating uniformity could be further optimised using a sweep rate that varies as the inverse of the frequency squared (weighted-sweep). In this study, VFM Technique was successfully extended to the Micro-Electro- Mechanical Systems (MEMS) industry as an alternative method for the processing of a polymer system - negative-tone SU8 photoresist - which is gaining widespread use in Microfabrication. The VFM method was compared to conventional hotplate curing as well as a new hybrid curing method introduced in this work and the product quality assessed optically and by thermal analysis. Results from this work indicate that the Variable Frequency Microwave technique is a viable alternative to the conventional cure currently used in practice. With proper optimisation of the VFM parameters, VFM was found to provide samples that are comparable or better than conventionally cured samples in terms of properties and microstructure quality. Using the VFM method, enhancement in cure rates and drying rates, which are described by others as microwave effects, were observed and investigated. A significant increase on the degree of cure of up to 20% greater than conventional cure was observed when VFM was utilized and an apparent enhancement in solvent evaporation in the thin SU8 films observed. Experiments undertaken show that microwaves irradiation can enhance diffusion rates of cyclopentanone in the SU8 system by approximately 75-100%. The findings signify that SU8 curing at lower temperatures or rapid curing are possible and long drying times could be reduced significantly thus alleviating many of the problems associated with conventional thermal curing. Outcomes of this study demonstrate the ability of the new VFM technique to provide uniform heating which is essential for materials processing. Its application to the emerging field of Microfabrication exhibits its unique advantages over conventional curing methods and establishes itself to be a versatile and robust processing tool. The experimental observations made under microwave irradiation are further proof of the existence of specific microwave effects which is one of the most debatable topics in the Microwave processing field. A mechanism based on the Cage Model by Zwanzig [1983] was put forward to explain the increase in transport rates.

Transparent conducting oxides (TCOs) have become an integral part of modern life through their essential role in touchscreen technology. The growing demand for cheap and superior transparent conducting layers, primarily driven by the smart phone market, has led to renewed efforts to develop novel TCOs. Currently, the most widely used material for transparent conducting applications is Sn-doped indium oxide (ITO), which has outstanding optical and electrical properties. This material is expensive though, due to the extensive use of In, and efforts to develop new low-cost transparent conducting oxides (TCO) have become increasingly important. Similarly attempts to reduce the cost of the fabrication and post-sintering steps used in making doped metal oxide thin films through innovative technologies have gained a lot of attention. With these points in mind, this research project has focused on the development of a novel low-cost aerosol assisted physical deposition method for TCO thin film fabrication and the development of new highly conducting materials to replace the expensive ITO for TCO applications. In this study, a new and simple aerosol assisted vapour deposition technique (i.e AACT) is developed to fabricate TCO films using TCO nanoparticle suspensions. Firstly, to test the validity of the method, ITO thin films are fabricated on float glass substrates from a nanoparticle suspension. The influence of the deposition parameters on the structural and opto-electronic properties of the thin films are investigated to understand the intricacies of the process. In order to investigate the fabrication of replacement materials for ITO, a range of doped zinc oxide powders are synthesised and processed using microwave radiation. Nominally, Al doped ZnO (AZO), Ga doped ZnO (GZO), Si doped ZnO (SZO), Cu doped ZnO (CZO) and Mn doped ZnO (MZO) singly doped ZnO powders are all investigated to determine the best metal dopants for transparent conducting ZnO. AZO and GZO pellets are found to present the best electrical conductivity for the singly doped microwave fabricated powders with values of 4.4 x 10-3 and 4.3 x 10-3 Ω.cm achieved reproducibly. In an effort to further improve the properties of ZnO, co-doping experiments, utilising the two best dopants from the previous work (i.e. Al and Ga) is investigated. ZnO structures that are co-doped with Al and Ga (AGZO) are found to exhibit significantly enhanced electrical properties than the singly doped powders. Typically, electrical conductivity value of 5.6 x 10-4 Ω.cm is obtained for AGZO pellets, which is an order of magnitude better than the previously fabricated materials. Finally, the best AZO, GZO and AGZO materials are utilised to fabricate thin films using the previously verified AACT technique. Further investigations into the opto-electrical properties of the resulting thin films is presented prior to the utilisation of the best films in a practical application. Transparent heaters are fabricated using the best AGZO thin films, which are capable of reaching a mean temperature of 132.3 °C after applying a voltage of 18 V for 10 min. This work highlights the potential for using highly conducting AGZO, particularly fabricated by the microwave synthesis route, as a potential alternative for ITO in a wide variety of applications. The research also highlights the advantages of using microwaves in the thermal processing of TCO materials which significantly reduces the energy impact of the production process.

Isotropically conductive adhesives (ICAs) and inks are potential candidates for low cost interconnect materials and widely used in electrical/electronic packaging applications. Silver (Ag)filled ICAs and inks are the most popular due to their high conductivity and good reliability. However, the price of Ag is a significant issue for the wider exploitation of these materials in low cost, high volume applications such as printed electronics. In addition, there is a need to develop systems compatible with temperature sensitive substrates through the use of alternative materials and heating methods. Copper (Cu) is considered as a more cost-effective filler for ICAs and in this work, Cu powders were treated to remove the oxide layer and then protected with a self-assembled monolayer (SAM). The coating was found to be able to limit the re-oxidation of the Cumicron particles. The treated Cu powderswerecombined with one of two different adhesive resins to form ICAs that were stencil printed onto glass substrates before curing. The use of conventional and microwave assisted heating methods under an inert atmosphere for the curing of the Cu loaded ICAs was investigated in detail. The samples were characterised for electrical performance, microstructure and shrinkage as a function of curing temperature (80-150°C) and time. Tracks with electrical conductivity comparable to Ag filled adhesives were obtained for both curing methods and with both resins. It was found that curing could be accelerated and/or carried out at lower temperature with the addition of microwave radiation for one adhesive resin, but the other showed almost no absorption indicating a difference in curing mechanism for the two formulations.

No description available.

The lithium disilicate (LS2) glass system provides the basis for a large number of useful glass-ceramic products. Microwave processing of materials such as glass-ceramics offers unique benefits over conventional processing techniques. Variable frequency microwave (VFM) processing is an advanced processing technique developed to overcome the hot spot and the arcing problems in microwave processing. In general, two main questions are addressed in this dissertation: 1. How does microwave energy couple with a ceramic material to create heat? and, 2. Is there a "microwave effect" and if so what are the possible explanations for the existence of that effect? The results of the present study show that VFM processing was successfully used to crystallize LS2 glass at a frequency other than 2.45 GHz and without the aid of other forms of energy (hybrid heating). Crystallization of LS2 glass using VFM heating occurred in a significantly shorter time and at a lower temperature as compared to conventional heating. Furthermore, the crystallization mechanism of LS2 glass in VFM heating was not exactly the same as in conventional heating. Although LS2 crystal phase (Orthorhombic Ccc2) was developed in the VFM crystallized samples as well as in the conventionally crystallized samples as x-ray diffraction (XRD) confirmed, the structural units of SiO4 tetrahedra (Q species) in the VFM crystallized samples were slightly different than the ones in conventionally crystallized samples as the Raman spectroscopy revealed. Moreover, the observed reduction in the crystallization time and apparent temperature in addition to the different crystallization mechanism observed in the VFM process both provided experimental evidence to support the presence of the microwave effect in the LS2 crystallization process. Also, the molecular orbital model was successfully used to predict the microwave absorption in LS2 glass and glass-ceramic. This model was consistent with experiments and indicated that microwave-material interactions were highly dependent on the structure of the material. Finally, a correlation between the Fourier transform infrared reflectance spectroscopy (FTIRRS) peak intensities and the volume fraction of crystals in partially crystallized LS2 glass samples was established. / Ph. D.

No description available.