Dissertations / Theses on the topic 'Microwave heating'

The formation and propagation of hotspots in a cylindrical medium that is undergoing microwave heating is studied in detail. A mathematical model developed by Garcia-Reimbert, C., Minzoni, A. A. and Smyth, N. in Hotspot formation and propagation in Microwave Heating, IMA, Journal of Applied Mathematics (1996), 37, p. 165-179 is used. The model consists of Maxwell's wave equation coupled to a temperature diffusion equation containing a bistable nonlinear term. When the thermal diffusivity is sufficiently small the leading order temperature solution of a singular perturbation analysis is used to reduce the system to a free boundary problem. This approximation accurately predicts the steady-state solutions for the temperature and electric fields in closed form. These solutions are valid for arbitrary values of the electric conductivity, and thus extend the previous (small conductivity) results of Garcia-Reimbert et.al. A time-dependent approximate profile for the electric field is used to obtain an ordinary differential equation for its relaxation to the steady-state. This equation appears to accurately describe the time scale of the electric field's evolution even in the absence of a temperature front (with zero coupling to the temperature), and can be of wider interest than the model for microwave heating studied here. With sufficiently small thermal diffusivity and strong coupling, the differential equation also accurately describes the time evolution of the temperature front's location. A closed form expression for the time scale of the formation of the hotspot is derived for the first time in the literature of hotspot modeling. Finally, a rigorous proof of the existence of steady-state solutions of the free boundary problem is given by a contraction mapping argument.

The numerical modelling of electromagnetic waves has been the focus of many research areas in the past. Some specific applications of electromagnetic wave scattering are in the fields of Microwave Heating and Radar Communication Systems. The equations that govern the fundamental behaviour of electromagnetic wave propagation in waveguides and cavities are the Maxwell's equations. In the literature, a number of methods have been employed to solve these equations. Of these methods, the classical Finite-Difference Time-Domain scheme, which uses a staggered time and space discretisation, is the most well known and widely used. However, it is complicated to implement this method on an irregular computational domain using an unstructured mesh. In this work, a coupled method is introduced for the solution of Maxwell's equations. It is proposed that the free-space component of the solution is computed in the time domain, whilst the load is resolved using the frequency dependent electric field Helmholtz equation. This methodology results in a timefrequency domain hybrid scheme. For the Helmholtz equation, boundary conditions are generated from the time dependent free-space solutions. The boundary information is mapped into the frequency domain using the Discrete Fourier Transform. The solution for the electric field components is obtained by solving a sparse-complex system of linear equations. The hybrid method has been tested for both waveguide and cavity configurations. Numerical tests performed on waveguides and cavities for inhomogeneous lossy materials highlight the accuracy and computational efficiency of the newly proposed hybrid computational electromagnetic strategy.

In this work we proposed the governing equations for describing the microwave heating process where the complex interactions between the thermo-mechanical and electromagnetic fields are taken into account. Starting point are the general balance laws of mechanics and electrodynamics. Transient and spatial temperature profiles of liquids (water and corn solution) inside a cylindrical container during microwave heating at 2450 MHz were measured. Transient temperature rise at a given location was almost linear. The slowest heating region was at the container bottom due to small energy penetration through the bottom. Numerical simulations were carried out for microwave heating of 2D cylinders of pure water with internal convection in the liquid regions. The results are found to be consistent with those of the experiments. A generalized theoretical model was formulated for the process of microwave heating of materials. Finally stability analysis was done on a 1-D model of microwave heating and the equations for the perturbations were obtained.

This thesis reports the synthesis of linear and star polycaprolactone via the ring-opening polymerisation of ε-caprolactone using R–OH / tin octanoate as the initiator / catalyst system. The aim of the work carried out in this thesis is to study the polymerisation when conducted using both conventional and microwave heating and develop new synthetic methods for the synthesis of structural polymers via ring-opening polymerisation.

Microwave processing has been studied as an alternate heating technique over conventional heating for industrial applications. Some advantages include quicker and more uniform heating. Also, microwave energy offers the advantage of localized heating and smaller-sized equipment. Many ceramics, however, are difficult to heat using microwave energy by reason of the strong temperature dependence of the dielectric loss. The ability of a ceramic to absorb microwave energy, a measure of its dielectric loss, increases with temperature which makes the material more susceptible to thermal runaway. The purpose of this research is to develop a model that accurately reproduces experimental data and can be used to explore new applicator designs for continuous processing of such ceramics. A two-dimensional numerical model, created for this purpose, assumes that the ceramic is a circular cylinder that moves either longitudinally through a microwave cavity of given dimensions. By adjusting the electromagnetic field so that the absorbed power matches the measured power, the model successfully imitates experimental results and avoids thermal runaway while achieving high temperatures.
Master of Science

The research conducted in this thesis aims to develop an efficient microwave delivery system employing miniature resonant microwave cavities, targeted at compact, flexible and ideally field-deployable microwave-assisted diagnostic healthcare applications. The system comprises a power amplifier as a solid-state microwave source and a load - as a single mode cavity resonator to hold the sample. The compactness of the practical microwave delivery system relies on the direct integration of the sample-holding cavity resonator to the power amplifier and inclusion of the built-in directional coupler for power measurements. The solid state power transistors used in this research (10W-LDMOS, 10W-GaN) were provided by the sponsoring company NXP Inc. In practical microwave delivery applications, the impedance environment of the cavity resonators change significantly, and this thesis shows how this can be systematically utilized to present the optimal loading conditions to the transistor by simply designing the series delay lines. This load transfer technique, which critically can be achieved without employing bulky, lossy and physically larger output matching networks, allows high performance of the power amplifier to be achieved through waveform engineering at the intrinsic plane of the transistor. Starting with the impedance observation of a rectangular cavity, using only series delay lines allowed the practical demonstration of the high power and high efficiency fully integrated inverse class-F (F-1) power amplifier. Temperature is an important factor in a microwave heating and delivery system as it changes the impedance environment of the cavity resonator. This natural change in both cavity and sample temperature can be accommodated through simplified series matching lines and the microwave heating system capable of working over substantial bandwidth was again practically demonstrated. The inclusion of the coupler maintained the compactness of the system. In the practical situations envisaged, the microwave delivery system needs to accommodate natural variation between sample volumes and consistencies for heating. The experimental work considered the heating of different sample volumes ii of water, and characterizing the change in the natural impedance environment of the cavity as a result. It was shown how the natural impedance variation can not only be accommodated, but also exploited, allowing ‘continuous’, high-efficiency performance to be achieved while processing a wide range of sample volumes. Specifically, using only transistor package parasitic, the impedance of the cavity itself together with a single series microstrip transmission line allows a continuous class-F-1 mode loading condition to be identified. Through different experiments, the microwave delivery systems with high-performance are demonstrated which are compact, flexible and efficient over operational bandwidth of the cavity resonators.

The greenhouse vegetable industry in Northern Europe is an extensive, high value and energy-intensive horticultural sector that is vital for food supply and security in Europe. Despite technical and legislative innovation over a period of thirty years, little improvement has been made in the reduction of absolute energy consumption for heating greenhouse growing space. A novel free-space microwave system for heating greenhouse crops volumetrically is proposed. and demonstrated. It is estimated that such a system can use one-third of the energy of a conventional heating system. This would allow for significant reductions in energy consumption, reduce environmental damage and provide financial savings of several billion Euros. Experiments on real plants demonstrate microwave heating utilising ISM band frequencies as a viable method for heating growing plants from seedling to fruition and reveal plant and fruit quality that is comparable to that attained by conventional heating. Non-uniformity of electric-field distribution within the plants, both between and within plant components, is identified as the main limiting factor during microwave heating and can result in localised burning. This can cause catastrophic failure when occurring on stems. The necessary reduction in power and the resultant decrease in average plant temperature delays development relative to conventionally-heated plants. Poor leaf heating and development is the main source of delay in fruit formation. A novel practical technique for improving inter- and intra-object heating uniformity is thus also presented and demonstrated. This method utilises circularly polarised incident waves to allow more regular heating of plant components and greater consistency of heating between different plant components. Furthermore, this technique can be applied generally to other dielectric heating scenarios where heating non-uniformity is a problem, specifically in industrial processes. The concept of plant sectors and sections is also defined to allow for valid comparison of the energy consumption of conventional and microwave heating systems, 1Il a commercial setting.

Microwave heating of food has increased rapidly as a food processing technique. This increases the concern that chemicals could migrate from food packaging to food. The specific effect of microwave heating in contrast to conventional heating on overall and specific migration from common plastic food storage boxes was studied in this work. The purpose was especially to determine the interaction effects of different plastics in contact with different types of foods during microwave heating. The study focused on polycarbonate (PC), poly(ethylene terephthalate) (PET), polypropylene homo-polymer (PP), co-polymer (PP-C) and random co-polymer (PP-R) packages. The migration determinations were evaluated at controlled times and temperatures, using a MAE device. The migrants were analyzed by GC-MS and HPLC. ESI-MS was evaluated as a new tool for migration determinations. Food/food simulant absorption and changes in degree of crystallinity during heating were also followed. Significant degradation of antioxidants Irgafos 168 and Irganox 1010 in PP packages occurred during microwave heating of the packages in food simulants containing ethanol, resulting in the formation of antioxidant degradation products. Degradation of PC by Fries chain rearrangement reaction leading to formation of 9,9-dimethylxanthene, and transesterification of PET leading to formation of diethyl terephthalate, were also observed after microwave heating the packages in ethanol and 90/10 isooctane/ethanol. These reactions were not observed during conventional heating of the packages at the same temperature, or after microwave heating of the packages in liquid food (coconut milk). The microwave heating also significantly increased the migration of cyclic oligomers from PET into ethanol and isooctane at 80 °C. Migration of compounds into coconut milk was slightly lower than calculated amounts using the EU mathematical model to predict migration of additives into foodstuffs. The results thus show that the use of ethanol as a fat food simulant during microwave heating can lead to a significant overestimation of migration as well as degradation of polymer or the incorporated additives. Some other detected migrants were dimethylbenzaldehyde, 4-ethoxy-ethyl benzoate, benzophenone, m-tert-butyl phenol and 1-methylnaphthalene. All identified migrants with associated specific migration limit (SML) values migrated in significantly lower amounts than the SML values during 1 h of microwave heating at 80 °C. The antioxidant diffusion coefficients in PP and PP co-polymers showed larger relative differences than the corresponding degrees of crystallinity in the same polymers and PP-R showed by far the fastest migration of antioxidants.
Mikrovågsuppvärmning av mat har ökat markant under de senaste åren. Detta ökar risken för att ämnen i plast migrerar från matförpackningar till mat. Den specifika effekten av mikrovågsvärmning i kontrast till konventionell värmning på total och specifik migrering från vanliga matförvaringslådor av plast studerades i denna avhandling. Syftet var i huvudsak att bestämma interaktionseffekter mellan olika typer av plaster och olika typer av mat under mikrovågsvärmning. Studien fokuserades på förpackningar av polykarbonat (PC), polyetentereftalat (PET), polypropylen homopolymer (PP), copolymer (PP-C) och random copolymer (PP-R). Migreringstesterna utfördes under kontrollerade tider och temperaturer genom att använda MAE. Migranterna analyserades med hjälp av GC-MS och HPLC. ESI-MS-analys utvärderades också som ny analysmetod för migreringstester. Absorption av mat- och matsimulanter samt förändringar i kristallinitetsgrad följdes också. Signifikant nedbrytning av antioxidanterna Irgafos 168 och Irganox 1010 i PP-förpackningar inträffade under mikrovågsvärmning av förpackningarna i etanol-innehållande matsimulanter, vilket resulterade i bildning av nedbrytningsprodukter från antioxidanterna. Nedbrytning av PC genom en Fries omfördelningsreaktion, vilket orsakade bildning av 9,9-dimetylxanten, samt transesterifikation av PET, vilket orsakade bildning av dietyltereftalat, observerades också efter mikrovågsvärmning av förpackningarna i etanol och 90/10 isooktan/etanol. Dessa reaktioner observerades ej efter konventionell värmning av förpackningarna under samma temperatur och ej heller efter mikrovågsvärmning av förpackningarna i riktig mat (kokosmjölk). Mikrovågsvärmningen ökade också betydelsefullt migrering av cykliska oligomerer från PET till etanol och isooktan under 80 °C. Specifika ämnens migrering till kokosmjölk var alla något lägre än migreringsvärden beräknade m. h. a. EU's officiella matematiska modell för förutsägelse av migrering från matförpackningar till mat. Dessa resultat visar att användandet av etanol som matsimulant för fet mat under mikrovågsvärmning kan leda till betydande överestimering av migrering, samt nedbrytning av polymer och additiv i polymeren. Andra detekterade migranter var till exempel dimetylbenzaldehyd, 4-etoxy-etylbenzoat, benzofenon, m-tertbutylfenol och 1-metylnaftalen. Alla identifierade migranter med tillhörande ‘specific migration limit’ (SML)-värden migrerade i betydelsefullt mindre mängder än ämnenas tillhörande SML-värden under 1 h mikrovågsvärmning under 80°C. Diffusionskoefficienterna för antioxidanterna i PP-förpackningarna visade större relativa skillnader än förpackningarnas motsvarande kristallinitetsgrader och migrering av antioxidanter var snabbast från PP-R.

QC 20120530

Thesis (MScIng)--University of Stellenbosch, 2005.
ENGLISH ABSTRACT: Both coaxial probe and waveguide (WG) measurement systems for electric and magnetic material property extraction were investigated. These measurement techniques were used to determine electrical properties of an inhomogeneous rock sample in its solid and crushed states. A lumped element model of the probe was used and permittivity was determined by the inversion algorithm developed by Stuchly and Stuchly. To support this technique it was compared to a full wave inversion algorithm and referenced to properties of the same samples but determined by a resonant cavity technique. The Nicholson, Ross and Weir inversion algorithm was used to determine material properties from WG measurements. As a reference, the same techniques were applied to a well defined material. It was found that neither of the measurement techniques could measure low loss factors or conductive materials and literature values were used in these cases. Various simulation models of the multiphase ore in both its solid and crushed states are presented. These models were utilised in finite-difference time-domain (FDTD) simulations of different microwave (MW) cavities. Simulation and experimental S-parameter comparisons are presented. The level of accuracy achieved varies as a function of the geometrical representation and material properties. After an S-parameter comparison with simulation results it was concluded that the electrical properties of both the solid and crushed rocks have been well determined for MW cavity design. Predicted and measured field distributions in cavities were also compared and it is shown that accurate models of multiphase materials become especially important in the determination of field distributions in and around different rock phases. Recommendations for the suggested material property determination and verification processes are presented. A specific application of this work is in the field of microwave assisted comminution.
AFRIKAANSE OPSOMMING: ’n Koaksiale probe en golfgeleier (WG) stelsels vir die bepaling van materiaal eienskappe (elektries en magneties) word gebruik met die doel om ’n nie-homogene rotsmonster te karakteriseer. Die ekstraksie algoritme van Stuchly en Stuchly word gebruik om die materiaal eienskappe te bepaal vanaf die gemete S11-parameter. Hierdie ekstraksie metode word vergelyk met ’n vol golf ekstraksie van permitiwiteit vanaf dieselfde gemete data. Beide die ekstraksie metodes word dan vergelyk met resonante holte meetings van dieselfde materiale. Die Nicholson, Ross en Weir ekstraksie algoritme word toegepas op meetings wat gedoen is deur die golfgeleier stelsel. As ’n verwysing word dieselfde tegnieke toegepas op ’n bekende materiaal en daar is gevind dit stem goed ooreen behalwe dat nie een van die twee meet tegnieke lae verlies faktore kan meet nie. Verder kan nie een van die twee sisteme geleidende materiale meet nie. Vir sulke gevalle is waardes nageslaan. Verskeie simulasiemodelle van die rots word voorgestel vir beide soliede en vergruisde monsters. Hierdie modelle word gebruik in FDTD simulasies van verskeie mikrogolftoevoegers met die oog om ’n vergelyking te tref tussen gesimuleerde en gemete S-parameters. Verskillende vlakke van akkuraatheid is bereik en is ’n funksie van die geometrie en die materiaaleienskappe van die model. Nadat gemete en gesimuleerde S-parameters vergelyk is, is gevind dat die materiaal eienskappe van beide die soliede en vergruisde rots monsters goed bepaal is vir mikrogo lf toevoeger ontwerp. Voorspelde en gemete veldverspreidings word ook vergelyk en dit is veral hierso van belang om ’n realistiese model van die nie-homogene monster te gebruik. Sekere voorstelle word gemaak om die verskillende aspekte van die meet van ma teriaaleienskappe en simulasiemodelle te kan verfyn. ’n Spesifieke toepassing van hierdie werk is in mikrogolf ondersteunde skeiding van minerale en erts.

Includes bibliographical references.
General purpose pressure vessels for digestion in microwave ovens have been developed and their applications investigated. The vessels were manufactured from PTFE and polypropylene and included a safety valve. The easily manufactured vessels were found to be reliable for a wide range of samples. A small vessel of 10 ml capacity, also constructed from PTFE and polypropylene, was developed for very small samples. These were used for the digestion of blood. A design for a simple modification of microwave ovens for use in the laboratory has been investigated. A Sharp microwave oven was lined with polypropylene and an extraction system that worked by the Venturi effect was used to remove the fumes from the cavity of the oven. This modification was found to be adequate to prevent corrosion of the oven and to provide the necessary safety features required for a laboratory system. In another modification, a thermocouple and a controller were used for maintaining the temperatures of the samples. Ports were available at the top of the cavity for insertion of suitable vessels for a variety of investigations. A computer-controlled waveguide has been designed for general laboratory applications. The temperature of the samples could be monitored and controlled. The waveguide was used for investigating sample digestions and the heating characteristics of a wide range of materials. A cylindrical applicator has been developed for the microwave heating of large (ca. 300- 600 ml) samples. Temperature monitoring and control was achieved through the use of a thermocouple and a computer. The instrument was found useful for many laboratory investigations involving relatively large samples. A new applicator has been developed for the even heating of multiple laboratory samples. The vessels (tubes) were introduced into the multimode cavity through ports. A choke was developed to allow rotation of the ports and the samples inside the cavity. Vapours could be extracted from the vessels outside the cavity using a fume extraction system. This system was found to be safe in terms of microwave leakages and yielded very good evenness of heating.

Kyoto University (京都大学)
0048
新制・課程博士
博士(農学)
甲第15425号
農博第1810号
新制||農||979(附属図書館)
学位論文||H22||N4524(農学部図書室)
27903
京都大学大学院農学研究科地域環境科学専攻
(主査)教授 東 順一, 教授 二井 一禎, 教授 縄田 栄治
学位規則第4条第1項該当

2013 - 2014
Thermal processing is the major processing technology in the food industry and its purpose is to extend the shelf life of food products without compromising food safety. Apart from the positive effect of food treatments, such as the inactivation of pathogens, there are also some limitation by way of partial destruction of quality attributes of products, especially heat-labile nutrients, and sensory attributes. The technological revolution, nutritional awareness, and continuous demand of the new generation have necessitated search for new or improved food processing technologies. Presently, several new food processing technologies, including microwave heating, are investigated to improve, replace, or complement conventional processing technology. Microwave has been successfully used to heat, dry, and sterilize many food products. Compared with conventional methods, microwave processing offers the following advantages: 1) microwave penetrates inside the food materials and, therefore, cooking takes place throughout the whole volume of food internally and rapidly, which significantly reduces the processing time; 2) since heat transfer is fast, nutrients and vitamins contents, as well as flavor, sensory characteristics, and color of food are well preserved; 3) ultrafast pasteurization or sterilization of pumpable fluids minimizes nutrient, color, and flavor losses; 4) minimum fouling depositions, because of the elimination of the hot heat transfer surfaces, since the piping used is microwave transparent and remains relatively cooler than the product; 5) energy saving because of the absence of a medium between the sample and the MW; in addition, if the system is well projected, high efficiency can be reached (some authors showed the reduction of the energy costs during drying processes using microwaves, with a further improvement using air dryer and microwaves in sequence; moreover, consider the possibility to use alternative energy sources, eg. photovoltaic); 6) perfect geometry for clean-in-place system; 7) low cost in system maintenance; 8) space saving, if the system is compared with the traditional ones, based on boilers and surface heat exchangers. On the other hand, there are some problems which prevent the diffusion of this technique; among them: 1) uneven temperature patterns of the food processed, due to the uneven temperature field inside the microwave cavity; 2) temperature readout and control problems, because traditional probes fail: in particular, the thermocouples disturb the measurement and are damaged by the electric field, while fiberoptic probes allow to know the temperature only in few points; 3) difficulties in predicting the temperature field, because of coupling of three physical phenomena, that is, electromagnetic wave propagations, heat transfer and, in most of cases, fluid motion. Consider that sizing, during the design phase, and the control, during the operating phase, could be based on theoretical predictions, avoiding the so called “trial and error” approach. To address the critical points mentioned above, during the thesis work, theoretical models were developed and experimental tests were performed, with reference to “batch” and “continuous flow” processes. In particular, after a brief description of the principles of microwave heating, some batch processes have been analysed, that is, apple drying and in-package heating of water and oil. In both cases, the use of infrared technique allowed to obtain the spatial temperature distribution of the samples under test. ... [edited by Author]
XIII n.s.

Electromagnetic heating (EMH) processes are being increasingly used in the industrial and domestic sectors, yet they receive relatively little attention in the thermal engineering domain. Time-temperature characteristics in EMH are qualitatively different from those in conventional heating techniques due to the additional parameters (viz dielectric properties of the material, size and shape of the product and process frequency). From a unified theory perspective, a multi-purpose model has been developed in order to obtain the heating characteristics for an arbitrary processing situation. Theoretical analyses of various EMH processes in materials of various regular geometries and a range of physical properties have been undertaken. Despite the wide spread usage of microwave energy in the food engineering sector. few understand microwaves and their interactions with foods. Much of the published research is largely focussed from the view point of an electrical engineer and aimed at the oven designer. However, trial-and-error methods are usually employed when developing microwavable food products and when using microwave ovens. The presented thesis is focussed from the view-point of the thermal engineer and aimed primarily at food developers and end users. The multi-purpose model was then modified specifically for simulating the heating of food materials in a microwave oven. The validity of the commonly made assumptions was investigated; in particular the variation of dielectriC properties during the heating processes and their likely influence on the model's predictions. Experimental data available in the literature were compiled and analysed to form a set of equations for predicting the dielectric properties of various food materials. Also available correlations for thermal properties were evaluated for a selected set of experimental data of different food materials. Analyses were undertaken to demonstrate and evaluate the effects of various parameters on the heating characteristics of different food materials commonly heated/cooked in microwave ovens. A qualitative comparison of model predictions and experimental measurements is provided to validate the physical basis of the model. Findings from the model lead to a better understanding of the interactions between foods and microwaves. [...cont.]

Maple sap was successfully transformed into maple syrup and maple syrup products by evaporation of water by microwave heating. Pulsed power supply with duty cycles of 100%, 75% and 60% were used for the microwave application. The dielectric properties of maple syrup at different moisture contents during the process were determined at 25$ sp circ$C. The products obtained were of excellent quality and were comparable to the highest grade prescribed by the industry. Pulsed power supply was found to have better efficiency of heating, but it increased the total time required for the process. The total time was also found to be dependent on the initial mass of the load. The behavior of the dielectric properties of the maple syrup was found to be fairly linear with moisture content and were found to be in close agreement with an empirical model found in literature. Microwave heating seems to have an enormous potential for production of high quality maple syrup.

L'objectif principal de cette thèse est l'évaluation de la technologie de chauffage par micro-ondes et son applicabilité dans l'étape de densification, dans le cadre d’un procédé de recyclage des déchets nucléaires à très longue vie et ensuite le suivi du procédé de frittage de l'oxyde de cérium, simulant de l’oxyde de plutonium. Dans ce travail, nous avons développé un système permettant de déterminer les propriétés diélectriques de l'oxyde de cérium et avons fait une étude comparative entre le frittage par chauffage micro-ondes dans une cavité monomode et le frittage conventionnel dans un dilatomètre classique pour deux poudres de cérine: l’une de taille micrométrique, la seconde de taille nanométrique. En outre, nous avons effectué des simulations numériques sur la base d’un modèle couplant électromagnétisme et transfert de chaleur. Ces travaux ont montré l’effet de la taille d’une particule sphérique modèle sur le champ électrique (E) dans la particule et autour d’elle. Dans le cadre d’empilements modèles, nous avons montré que la présence d’un cou entre ces particules, leur orientation par rapport au champ E et le nombre de particules constitutives de cet empilement jouent un rôle déterminant sur l’intensité du champ E, ce qui a des conséquences sur le chauffage des particules
The main objective of this thesis is the evaluation of the heating by microwave technology and its applicability in the densification step, as a part of nuclear long life wastes recycling process and then the following up of the sintering of cerium oxide, a non-radioactive simulant of plutonium oxide. In this work, we developed a system for determining the dielectric properties of cerium oxide and made a comparative study between the sintering by microwave heating in a single-mode cavity and the conventional sintering in a dilatometer for two different powders of ceria: the first one has a micrometric particle size and the second has a nanometric one. In addition, we performed several numerical simulations on the basis of a model coupling electromagnetics and heat transfer. In these works, we have shown the effect of the size of a spherical model particle on the electric field (E) inside and around the particle. In the framework of a packing model of particles, the presence of a neck between these particles, their orientation relative to E field, and the number of these particles showed a decisive role in the intensity of the E field which has a direct impact on the heating of the particles

This study investigated the potential application of formulating pharmaceutical products using microwave heating methods alongside associated analytical investigations. Firstly, the interaction between three functionally related drugs, ibuprofen, ketoprofen and flurbiprofen, with two distinct forms of cyclodextrin at three temperatures, 298, 303 and 310K was investigated using isothermal titration calorimetry (ITC). In all cases, the associated changes in Gibbs free energy, enthalpy, and entropy are presented along with the stoichiometry and binding constant. It was found that binding always occurred at a 1:1 ratio with an associated negative enthalpy and Gibbs free energy with the formation of the complex enthalpically, rather than entropically driven. The data further demonstrated a clear relationship between the thermodynamic behaviour and logP of the drug molecules and provides an insight into the chemistry of drug-excipient binding for the compounds under investigation in this work. Secondly, four drugs, ibuprofen, ketoprofen, flurbiprofen and paracetamol were formulated using microwave and conventional heating, with and without the presence of water, with four excipients, namely, stearic acid (SA), β-cyclodextrin (BCD), 2-(hydroxypropyl)-β-cyclodextrin (2HPBCD) and polyvinylpyrrolidone (PVP). Three different analytical techniques were employed to determine whether the formulation method made a significant difference to the appearance and behaviour of the product. For example, the thermal behaviour of the drug and excipient, was investigated by differential scanning calorimetry (DSC). Scanning electron microscopy was utilised to determine if the formulation method illustrated any physical differences between the formulations and lastly, a thermal activity monitor was used to investigate the stability of the different formulations. Overall, it was found that the formulation method can make a significant difference to the character of the resultant formulations with a change in thermal behaviour or physical appearance observed in certain formulations but with a consistent stability seen across all products. Lastly, each of the resultant formulations were subjected to dissolution analysis to determine if the presence of water or choice of heating method, i.e. conventional heating vs. microwave heating affected the dissolution profile obtained. It was found that in the majority of cases water increased drug dissolution, which may have occurred because of a reduction in particle size. In summary, the application of microwave heating for pharmaceutical formulations has been thoroughly investigated and found to be a potential alternative to conventional heating with several distinct benefits for industry and the patient.

Thesis (PhD)--University of Stellenbosch, 2004.
ENGLISH ABSTRACT: Microwave heating technology is extensively used in households, is well-established in certain industries and is being explored in others. lts attraction is that it is quick, energy efficient, clean and can be used to heat materials of both high and low thermal conductivities. Successful application of the technology requires knowledge of both material properties and microwave equipment. Commercial fixtures for measuring dielectric properties are not widely available and are usually created in-house. The domestic oven magnetron is easily available and can be used in prototype microwave systems but applicators for specific applications need development. This thesis covers three main areas, namely; dielectric measurement fixtures, microwave applicators and chokes, and drying of wood. The first part of the thesis presents four dielectric measurement fixtures, one commercial and the others in-house. The design and calibration of the in-house fixtures is presented. An intercomparison of the performance of the four fixtures is carried out by measuring the permittivities of well characterised dielectrics: teflon and perspex. The most convenient fixture is used to measure the dielectric properties of wood at different moisture contents. The second part covers the design of a slotted waveguide fed microwave applicator and a choke. The design of the slotted feed is carried out analytically with and without mutual coupling between the slots. A semi-empirical design using a finite element package is done independent of the analytical approach and the results are compared. The choke is rigorously characterised using a procedure which avoids de-embedding. The third and last section reports on the wood drying experiments carried out in the applicator, simulated and mapped heating patterns of drying wood, and a mock-up industrial wood drying facility.
AFRIKAANSE OPSOMMING: Mikrogolfverhittingstegnologie word wyd gebruik in huishoudings, is goed gevestig in sekere nywerhede en moontlike gebruike daarvan op ander gebiede word gedurig ondersoek. Die vernaamste voordele is dat dit vinnig, energiedoeltreffend en skoon is en dat dit gebruik kan word om materiale van beide hoë en lae termiese geleidingsvermoë te verhit. Suksesvolle toevoeging van die tegnologie vereis kennis van beide materiale-eienskappe en mikrogolftoerusting. Kommersiële apparaat vir die meet van diëlektriese eienskappe is nie wyd beskikbaar nie en word gewoonlik "binnenshuis" ontwikkel. Die huishoudelike mikrogolfoond magnetron is geredelik beskikbaar en kan in prototipe mikrogolfstelsels gebruik word, maar toevoegers geskik vir spesifieke gebruike moet ontwikkel word. Hierdie proefskrif dek drie hoofgebiede, naamlik: diëlektriese metingsapparaat, mikrogolftoevoegers en smoorders, en die droging van hout. Die eerste deel van die proefskrif handeloor vier diëlektriese metingsapparate, een kommersiëel en die ander "binnenshuis". Die ontwerp en kalibrasie van die "binnenshuis" apparate word aangebied. 'n Tussenvergelyking van die verrigtinge van die vier apparate word gedoen deur die meting van baie bekende diëlektrikums: teflon en perspex. Die mees gerieflike apparaat word gebruik vir die meet van die diëlektriese eienskappe van hout teen verskillend voginhoude. Die tweede deel dek die ontwerp van 'n smoorder en 'n mikrogolf toevoeger wat deur 'n gegleufde golfgeleier gevoer word. Die ontwerp van die gegleufde golfgeleier word beide met en sonder wedersydse koppeling tussen die gleuwe, analities gedoen. 'n Semi-empiriese ontwerp wat van 'n eindige-element pakket gebruik maak, is onafhanklik van die analitiese benadering gedoen. Die resultate word dan vergelyk. Die smoorder word deur 'n prosedure wat "de-embedding" vermy, streng gekarakteriseer. Die derde en laaste afdeling behandel die eksperimentele droging van hout binne die toevoeger. Simulasies en verhittingspatrone van die hout wat gedroog word, sowel as die "mock-up(model van die voorgestelde)" nywerheids houtdroogkamer, word gegee.

In 2011-2012, wheat production in Canada was 20.1 Mt. As a major staple food globally, wheat is one of the grain widely used in baking industry as well. Both the physical and chemical characteristics of flours affect their quality and the subsequent products from them. The chemical compounds, namely starch and protein contribute to the functional and strength property of dough. Freshly harvested wheat may need a few years of storage to enhance the quality of these compounds. However, heat treatment with microwave is an efficient method to accelerate the process of aging. The physical properties such as ash content and gelatinization temperature of starch play an important role in the dough stability during baking and other characteristics of final products as well. In this study, two factors which include temperature and moisture content was applied. There were three levels of each: temperatures were 60 oC, 75 oC and 90 oC while moisture contents were set at 12%, 15% and 18% on wet basis. Combinations with different factors studied were obtained from the JMP 10 software (SAS Institute Inc., Cary, NC, USA) using a Central Composite Design (CCD). Total protein content was determined by bicinchoninic acid method, while starch content was estimated by enzymatic method. A muffler furnace was used to burn the wheat at 585 oC in order to measure the mineral content of wheat flour. The gelatinization temperature of starch was determined by using the differential scanning calorimetry (DSC). Data were analyzed by analysis of variance (ANOVA) with JMP 10. It was found that, following the increase inin temperature, the gelatinization temperature increased compared to the untreated samples. Starch and protein were increased initially followed by the decreasing trend when the heating temperature reached 75 oC. Similarly, ash content increased first and decreased when the temperature reached 90 oC. However, moisture content was found to be insignificant in the range used in this study. In conclusion, the effects of microwave heating treatment on starch, protein and gelatinization temperature were determined which are important for baking. The model explained the effects of the factors on the parameters, which helps its use in baking industry.
En 2011-2012, le Canada a produit 20.1 Tg de blé. Une denrée alimentaire de base à l'échelle mondiale, le blé est une des céréales les plus utilisées dans l'industrie de la boulangerie. Les caractéristiques physiques et chimiques des farines de blé en affectent la qualité ainsi que les produits qui peuvent en découler. Les composés chimiques, notamment l'amidon et les protéines contribuent aux propriétés fonctionnelles et à la résistance de la pâte. Le blé nouvellement récolté peut gagner à être entreposé pour quelques années afin d'affiner la qualité de ces composés. Cependant, un traitement thermique par microondes présente une méthode efficace pour accélérer le processus de vieillissement. Les propriétés physiques tel la teneur en cendres et la température de gélatinisation de l'amidon jouent un rôle important dans la stabilité de la pâte durant la cuisson, ainsi que d'autres propriétés des produits finis. Deux facteurs expérimentaux, dont la température et la teneur en humidité, furent appliqués à trois niveaux chacun: températures de 60 °C, 75 °C et 90 °C, teneurs en humidité de 12%, 15% et 18% par rapport au poids frais. Les combinaisons des différents facteurs à l'étude dans un design expérimental composite central furent analysées avec le logiciel JMP 10 du SAS Institute. La teneur globale en protéine fut dosée par la méthode de l'acide bicinchoninique, tandis que la teneur en amidon fut dosée par une méthode enzymatique. Un four à moufle opérant à 585 °C servit à brûler la farine de blé afin d'en mesurer la teneur en minéraux. La température de gélatinisation de l'amidon fut déterminée par analyse calorimétrique différentielle (DSC). Une analyse de variance (ANOVA) des données utilisant JMP 10 indiqua qu'une augmentation de la température par rapport au traitement témoin donna lieu à une augmentation de la température de gélatinisation. Une hausse de la température jusqu'à 75°C donna lieu à une augmentation, par rapport au témoin, des teneurs en protéines, amidon et cendres, mais une tendance décroissante au delà de cette température. Cependant, la teneur en humidité n'eut aucun effet significatif. Les effets d'un traitement de la farine de blé par microondes eut donc un effet sur la teneur en protéines, en amidon et sur la température de gélatinisation de l'amidon, tous des facteurs importants dans la cuisson du pain. Le modèle statistique expliqua l'effet des différents facteurs du système de traitement aux microondes sur les paramètres étudiés, permettant à l'industrie de la boulangerie d'en entrevoir les avantages.

Thesis (M.S.)--West Virginia University, 2006.
Title from document title page. Document formatted into pages; contains xi, 84 p. : ill. (some col.). Includes abstract. Includes bibliographical references.

Thesis (PhD)--Stellenbosch University, 2002.
ENGLISH ABSTRACT: One of the most difficult problems in microwave dielectric heating is the generation and control of field and heating patterns. A technique allowing the synthesis of different, pre-determinable heating patterns by interference modulation is proposed. The proposed concept may be described by the term 'interference modulation'. Interference modulation is a technique which enables particular patterns, called features, to be obtained by signals from several sources interfering with each other. The relative phases of the signals are modulated, by which process known features may be selected. Weights are assigned to these features, which may be combined over time to form a new heating pattern. Phase changes may then be used to switch to specific, known features, with weights which will determine the contribution of each feature to the desired overall pattern. In the practical implementation described, magnetron tubes are the sources. Each of these narrow-band high-power sources was injection locked to a low power control signal. The control signals are derived from a reference source and their phase is set to select a corresponding feature. Calculation and measurement showed that reliable locking occurs with a control signal power of at least 3% of the magnetron's emitted power. Measurements of patterns were carried out with materials formed into sheets and blocks. Some were chemically prepared to reveal the overall heating pattern. The observed patters, simulations and field measurements concur, thus validating the concept and operation of the proposed topology.
AFRIKAANSE OPSOMMING: Een van die moeilikste probleme in mikrogolf diëlektriese verhitting is die opwekking en beheer van veld- en verhittingspatrone. 'n Tegniek wat voorsiening maak vir die sintese van verskillende, voorafbepaalde verhittingspatrone deur interferensie word hier voorgestel. Die voorgestelde beginsel kan beskryf word deur die term "interferensie modulasie". Interferensie modulasie is 'n tegniek wat spesifieke patrone, genoem kenmerke, moontlik maak deur seine van verskillende bronne met mekaar te laat interfereer. Bekende kenmerke kan geselekteer word deur die relatiewe fases van die seine te moduleer. Gewigte word aan hierdie kenmerke toegeken wat oor tyd gekombineer kan word om nuwe verhittingspatrone te vorm. Faseveranderings kan dan gebruik word om na 'n spesifieke, bekende kenmerk te skakel met gewigte wat die bydrae van elke kenmerk van die verlangde algehele patroon bepaal. Magnetrons word gebruik as bronne in die praktiese implimentering wat beskryf word. Elkeen van hierdie nouband, hoë drywing bronne is injeksie-gesluit met 'n lae drywing beheersein. Die beheerseine is afgekoppel van 'n verwysingsbron en hul fases is gestelom 'n ooreenstemmende kenmerk te verkry. Berekening en meting toon dat betroubare sluiting voorkom wanneer die beheersein ten minste 3% van die magnetron se uittree-drywing is. Metings van patrone is gemaak met materiale wat in lae en blokke gevorm is. Sommiges is met chemikalië voorberei sodat die algehele verhittingspatroon gesien kan word. Die waargeneemde patrone, simulasies en veldmetings stem goed ooreen en bevestig die beginsel en werking van die voorgestelde topologie.

There are many heavy oil reservoirs with thin pay zones (less than 10 m) in the world and in Turkey. Conventional steam injection techniques are not costeffective for such reservoirs, due to excessive heat loss through the overburden. Heat losses can be minimized through controlled heating of the pay zone. One such way is to introduce heat to the reservoir in a controlled manner is microwave heating. Laboratory studies on microwave heating of a scaled model of a heavy oil reservoir with a thin pay zone are presented with an economical feasibility of the method. In this thesis, three different conceptual oil reservoirs from south east Turkey are evaluated: Bati Raman (9.5 API) and Ç
amurlu (12 API) heavy crude oils and paraffinic Garzan (26 API)crude oil. Using a graphite core holder packed with crushed limestone with crude oil and water microwave effects of operational parameters like heating time and waiting period as well as rock and fluid properties like permeability, porosity, wettability, salinity, and initial water saturation are studied. The main recovery mechanisms for the experiments are viscosity reduction and gravity drainage. An analytical model is developed by coupling heat equation with the electromagnetic dissipated power per unit of volume based in Maxwell'
s equation successfully models the experiments for temperatures less than the pyrolysis temperature is presented. Also the experiments are scaled to the model by geometric similarity concept. In economic evaluation, the cost of oil is calculated based on domestic electricity prices.

The objective of this research project was to investigate the effect of microwave treatment on the conversion of pyrite in coal in order to enhance its magnetic properties and to aid subsequent desulphurisation by magnetic separation. Key areas of this investigation include:- a review of the fundamental theoretical aspects of heating in an applied microwave field, a detailed literature survey of the potential applications of microwave heating to aid coal desulphurisation, a study of the transformation mechanisms of pyrite by microwave and conventional heating methods, a comprehensive review of the influence of heterogeneity on the dielectric properties of coals and the effect of high power microwave heating on the decomposition of pyrite in a high sulphur coal. A detailed survey of the literature has shown that although a significant amount of work has been reported on the microwave heating of coal, little information is available which examines the mechanism through which different coals heat in an applied microwave field. A study is therefore presented in this thesis which quantifies, for the first time, the structural ordering of carbon in a range of coals of various ranks and the relationship with their dielectric properties. The results show a clear relationship between the structural ordering of carbon in coal and dielectric loss factor. Essentially, as the structural ordering increases, the level of free charge (per unit volume) increases together with electron mobility which results in an increase in dielectric loss factor. The transformation of pyrite to form the more magnetic pyrrhotite and other related decomposition products by conventional radiant heating has been the focus of much research. However, little attention has been given to the effect of microwave heating on the mechanisms of pyrite transformation in inert and oxygen-containing atmospheres. The study presented in this thesis investigates the effect of temperature, particle size and heat treatment time to establish how these affect the transformation process and the subsequent nature of the final product. This study showed that exposure of pyrite to both conventional and microwave heating methods enhanced its magnetic properties as a result phase alterations at the particle surface. Conventionally heated pyrite samples showed the highest magnetic susceptibility increases, speculated to be due difficulties surrounding the temperature control and measurement of the microwave heated samples. From a review of the literature in this field of research and by SEM observations made for each sample test in this work a general pyrite transformation model as a result of microwave exposure is proposed. The effect of microwave radiation on the conversion of in-situ coal derived pyrite was also investigated. The study presented here extends previous work by others by assessing quantitatively the influence of higher applied microwave power levels on the conversion of pyrite in coal. Samples of run-of-mine high pyritic sulphur content (~12%wt) Kentucky coal were microwave pre-treated at varying exposure times using 8kW and 15kW of microwave power operating at a frequency of 2.45GHz. This work included initial material characterisation which consisted of TGA, optical microscopy and dielectric property measurement, followed by SEM/EDAX characterisation of both microwave treated and untreated sample specimens. This is the first time that microwave treated coal, embedded in a carnauba wax mounting medium, has been observed by this method with focus on the thermal alterations of the pyrite phase. It was demonstrated that with sufficient power and residence time, the conversion of pyrite to the more magnetic pyrrhotite is achievable using microwave irradiation. The amount of pyrrhotite formed as a result of microwave exposure varies with microwave residence time and forward power level. Comparisons of change in the iron sulphide species in the coal was made between microwave treated and untreated material. Using a multimode microwave cavity it was found that significant amounts of pyrrhotite were formed in 20s when using 15kW of microwave power. Treatment at lower power levels in this type of cavity was found to be less effective. The overall effect of treatment on the coal proximate character was minimal.

Microwave ovens have long been recognised as a means of reducing heating times versus conventional convection ovens. The principle design feature is based on the procurement of uniform heating within any material placed in the interior of the microwave cavity oven. Materials within the oven are subjected to a degree of heating dependent on their electromagnetic properties. For many applications, it is desirable to maintain control over the distribution of heat deposition. This can be achieved through focussing of the electromagnetic field within the cavity. Two new mechanisms are identified where an increased level of control over the heating pattern and its location could be advantageous. The research described within this thesis aims to improve heating selectivity in microwave cavity ovens by the identification and enhanced control of modal patterns in degenerate and non-degenerate resonators. This is achieved through the analysis of two novel oven arrangements. The first of these addresses the requirement for highly selective heating in hyperthermia treatment. It is demonstrated that proper selection of a forced degenerate mode set can lead to an enhancement in field focussing within the centre of the cavity through constructive and destructive interference of the fields in each mode pattern. It is found that a highly selective peak of field can be produced within the centre of a large cylindrical waveguide cavity for the purpose of hyperthermia treatment. The peak is produced using a quasi degenerate mode set excited at approximately 1:3GHz. The second example presents an open oven design for the curing of epoxy and encapsulant materials within the micro-electronics packaging industry. It is intended that the oven be placed on the arm of a precision alignment machine such that the curing and placement stages of production be combined, suggesting an increase in production efficiency. Two excitation schemes are presented based on the coupling of quasi degenerate mode sets through a wide frequency range and the excitation of a single high order mode enabling uniform field distribution for heating of encapsulant material and increased selective heating through spatial alignment of modal field peaks, respectively. Experimental results demonstrate the viability of the open-ended microwave oven for curing. Both proposed excitation methods within the open oven design are investigated with results presented. Optimisation of the heating fields is achieved through inclusion of lowloss materials within the oven. Curing of an encapsulant material covering a commercial chip package is achieved and the overall design validated.

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

[EN] Zirconia has become a widely utilized structural ceramic material with important applications in dentistry due to its superb mechanical properties, biocompatibility, aesthetic characteristics and durability. Zirconia needs to be stabilized in the t-phase to obtain improved mechanical properties such as hardness and fracture toughness. Fully dense yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) materials are normally consolidated through the energy-intensive processing of powders at very high temperatures (>1000 °C). Innovative non-conventional approaches are being developed to reduce time and energy consumption and, consequently, environmental impact in ceramic powder processing. Microwave sintering is one such approach aimed at fully-densifying ceramics by using a different heating mechanism based on the material's inherent dielectric properties. The main purpose of this work is to obtain highly dense Y-TZP dental materials from commercial and lab-prepared sources via microwave sintering with mechanical and microstructural qualities that are similar or even improved with respect to their conventionally sintered counterparts. Therefore, its effect on important aspects related to dental applications has been studied. First, Y-TZP ceramics have been characterized upon sintering to determine whether the resulting properties meet the minimum mechanical requirements for structural dental applications. Second, the influence of microwave sintering on hydrothermal degradation, a spontaneous ageing phenomenon that affects zirconia materials in wet conditions, has been investigated. And third, the behavior under fretting wear conditions of microwave and conventionally sintered materials has been assessed to evaluate their durability and performance. The main conclusions indicate that microwave sintering allows proper consolidation of dental Y-TZP materials resulting in a finer microstructure due to shorter processing time and mechanical properties comparable, and in some cases enhanced, to those obtained in conventional sintered materials at lower dwell temperatures. Additionally, a higher resistance to hydrothermal degradation has been determined for microwave sintered materials due to a finer grain size and lower sintering temperatures that reduce the presence of cubic phase, which is responsible for destabilizing neighboring tetragonal phase grains. Finally, a similar wear rate has been obtained between microwave and conventional sintering of zirconia materials under fretting wear conditions. In addition, humidity can reduce the wear volume loss due to the lubricative effect of water and wear of degraded materials might increase the resistance due to the formation of a protective debris layer. In general, microwave sintering can be an interesting alternative for obtaining fully-densified Y-TZP dental materials providing certain advantages over conventional methods. Nonetheless, more studies are still necessary to have a better understanding of the advantages and disadvantages of microwave sintering of zirconia ceramics.
[ES] La circona es un material ampliamente utilizado como cerámica estructural con aplicaciones en el ámbito dental debido a sus propiedades mecánicas, biocompatibilidad, características estéticas y durabilidad. Para poder aprovechar las altas propiedades mecánicas de la circona, es necesario estabilizarla en su fase tetragonal. Los materiales de circona policristalina estabilizada con itria (Y-TZP) se consolidan normalmente a través de polvos mediante procesos energéticamente intensivos a altas temperaturas (>1000 °C). Actualmente, se están desarrollando técnicas basadas en métodos no convencionales para reducir el tiempo y el consumo energético en el procesado de polvos cerámicos. La sinterización por microondas tiene por objetivo la densificación completa mediante la utilización de mecanismos de calentamiento basados en las propiedades dieléctricas del material. El objetivo principal es la obtención de materiales dentales de Y-TZP altamente densos mediante la sinterización por microondas con propiedades mecánicas y microestructurales similares, o incluso por encima de las obtenidas por el método convencional. Para ello, se estudian aspectos relevantes al ámbito dental. En primer lugar, los materiales son caracterizados con el fin de determinar si las propiedades finales cumplen con los requisitos mecánicos para aplicaciones dentales. Además, se ha investigado la influencia de la sinterización por microondas en la degradación hidrotérmica, un fenómeno espontáneo de envejecimiento que afecta a los materiales de circona en condiciones de humedad. Finalmente, se ha evaluado el comportamiento en condiciones de desgaste fretting de los materiales sinterizados para determinar su durabilidad. Las conclusiones principales indican que la sinterización por microondas permite la consolidación adecuada de estos materiales, resultando en una microestructura más fina debido a los tiempos más cortos de procesado y en propiedades mecánicas comparables a las de materiales obtenidos mediante el método convencional, incluso a temperaturas más bajas. Una mayor resistencia a la degradación hidrotérmica se ha determinado en materiales sinterizados por microondas. Al emplear temperaturas más bajas se reduce la presencia de fase cúbica, la cual es responsable por la desestabilización de granos adyacentes de fase tetragonal. Tasas de desgaste similares han sido observadas entre materiales sinterizados por microondas y convencionalmente bajo condiciones de desgaste fretting. Adicionalmente, la humedad puede reducir sustancialmente la pérdida de volumen de desgaste debido al efecto lubricante del agua y los materiales degradados pueden aumentar la resistencia a este tipo de desgaste como consecuencia de la formación de una capa protectora de material que se desprende más fácil. En general, la sinterización por microondas es una alternativa interesante para obtener materiales dentales de Y-TZP altamente densos con ciertas ventajas sobre los métodos convencionales pero deben considerarse también las desventajas de esta técnica.
[CAT] La circona és un material àmpliament utilitzat com a ceràmica estructural amb aplicacions en l'àmbit dental a causa de les seues propietats mecàniques, biocompatibilidad, característiques estètiques i durabilitat. Per a poder aprofitar les altes propietats mecàniques de la circona, és necessari estabilitzar-la en la seua fase tetragonal. Els materials de circona policristalina estabilitzada amb itria (Y-TZP) es consoliden normalment mitjançant processos energèticament intensius a altes temperatures (>1000 °C). Actualment, s'estan desenvolupant tècniques basades en mètodes no convencionals per a reduir el temps i el consum energètic en el processament de la pols ceràmicas. La sinterització per microones té per objectiu la densificació completa mitjançant la utilització de mecanismes d'escalfament basats en les propietats dielèctriques del material. L'objectiu principal d'aquesta tesi és l'obtenció de materials dentals de Y-TZP altament densos mitjançant la sinterització per microones amb propietats mecàniques i microestructurals superiors a les obtingudes per mètodes convencionals. En primer lloc, els materials seràn caracteritzats per a determinar si les propietats finals compleixen amb els requisits mecànics per a aplicacions dentals. En segon lloc, s'investigarà la influència de la sinterització per microones en la degradació hidrotèrmica, un fenomen espontani d'envelliment que afecta als materials de circona en condicions d'humitat. I en tercer lloc, s'avaluarà el comportament en condicions de desgast fretting dels materials sinteritzats per a determinar la seua durabilitat. Les conclusions principals indiquen que la sinterització per microones permet la consolidació adequada i millorada de materials de Y-TZP, amb una microestructura més fina a causa dels temps més curts de processament i propietats mecàniques comparables a les de materials obtinguts mitjançant el mètode convencional, fins i tot a temperatures més baixes. Un factor positiu ha sigut la major resistència a la degradació hidrotèrmica en materials sinteritzats per microones. A més, al emprar temperatures més baixes es redueix la presència de fase cúbica, la qual és la responsable de la desestabilització de grans adjacents de fase tetragonal. Finalment, sota condicions de desgast fretting, s'han observat taxes de desgast similars entre materials sinteritzats per microones i via convencional. Addicionalment, en condicions de 100% d'humitat es pot reduir substancialment la pèrdua de volum de desgast a causa de l'efecte lubrificant de l'aigua i materials degradats, els quals poden augmentar la resistència a aquest tipus de desgast com a conseqüència de la formació d'una capa protectora de material que es desprèn amb més facilitat. En general, la sinterització per microones és una alternativa molt interessant per a obtindre materials dentals Y-TZP òptims i amb certes avantatges sobre els mètodes convencionals, però han de considerar-se també algunes desavantatges d'aquesta tècnica.
Presenda Barrera, Á. (2016). ADVANCED CERAMIC MATERIALS FOR DENTAL APPLICATIONS SINTERED BY MICROWAVE HEATING [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/68510
TESIS

In tight gas sand reservoir, formation damages cause severe reduction of gas production rate. In this study, the microwave heating method is used to heat tight sandstones and investigates how microwave heating influences the petrographic and petrophysical properties of these samples by using various experimental techniques and numerical simulation. It is demonstrated that removing formation damage and enhancing reservoir quality with microwave heating is feasible in the near wellbore region.

The main purpose of this thesis is to develop a model that can predict the temperature profile inside a frozen food sample during microwave tempering and infrared assisted microwave tempering processes. Another goal of the study is to compare the tempering time of frozen foods by using microwave, infrared assisted microwave, and ohmic heating methods. Frozen potato puree was used as the food sample for all studies. Three different microwave power levels (30%, 40%, and 50%) were used for microwave tempering studies. Three different microwave power levels (30%, 40%, and 50%) and three different infrared power levels (10%, 20%, and 30%) were combined for infrared assisted microwave tempering. As a control, tempering was done by keeping the sample at 4°
C. The increase in microwave power level and infrared power level reduced tempering time in infrared assisted microwave tempering. For the ohmic tempering studies, three different frequencies (10 kHz, 20 kHz, and 30 kHz) and three different salt contents (0.50%, 0.75%, and 1.00%) were used. The increase in frequency of ohmic heating and salt content also decreased tempering times. Microwave tempering and infrared assisted microwave tempering of frozen foods were simulated by using finite difference method. For this purpose, the change in heat capacity and the dielectric properties of frozen potato puree with respect to time were measured. The temperature distribution inside the sample was modeled, and the predicted results were compared with experimental results. The predicted temperatures showed good agreement with the experimental data (r2 >
0.985). It was possible to decrease tempering times by about 75%, 90%, and 95% using ohmic, microwave, and infrared assisted microwave tempering methods, respectively as compared to control.

Thesis (M.S.) -- Worcester Polytechnic Institute.
Keywords: uniformity of heating; optimization; optimal process; modeling; microwave pulsing; microwave heating; FDTD method; coupled problem. Includes bibliographical references (leaves 50-59).

Conventional thermal kinetics of enzyme inactivation and microbial destruction in fruit juices were studied in the pasteurization temperature range (50 to 90°C). Pectin methylesterase (PME), as the most heat resistant enzyme, in orange juice and Saccharomyces cerevisiae and Lactobacillus plantarum, as the most common spoilage yeast and bacteria, respectively, in apple juice used as indicators were subjected to heat treatment in a well-agitated water bath. Based on gathered time-temperature profiles, effective portions of the come-up (CUT) and come-down (CDT) times (lags) were determined for inclusion in kinetic data handling. The inactivation/destruction kinetics followed typical first-order rate of reactions.
A continuous-flow microwave heating system was set up and evaluated for obtaining kinetic parameters under microwave heating conditions. The outlet temperature was characterized as a function of fluid flow rate, heating volume and initial temperature.
Kinetics of enzyme inactivation and microbial destruction at various temperatures under continuous-flow microwave heating conditions were then evaluated using the technique established above. The rates of inactivation/destruction varied depending on temperature. Taking into consideration the effectiveness of the CUT and contributory thermal inactivation during the CDT, the D-values were found to vary from 38.5 s at 55°C to 1.32 s at 70°C (pH 3.7) for PME, 4.75 s at 52.5°C to 0.378 s at 60°C (pH 3.4) for S. cerevisiae (ATCC 16664) and 14.1 s at 57.5°C to 0.327 s at 65°C (pH 3.4) for L. plantarum (ATCC 14917).
Some non-thermal microwave effects were hypothesized to exist and responsible for such differences between the two heating modes. Enzyme inactivation and microbial destruction were then studied further to evaluate the non-thermal effects. A continuous-flow microwave heating system was developed operating at full power while maintaining sample temperatures below 40°C by circulating a microwave-transparent liquid (kerosene) for immediate removal of heat produced in the juice during microwave exposure.
In order to explain and better characterize the faster rate of inactivation/destruction associated with microwave heating conditions observed in kinetic studies, additional tests were carried out using the second set-up described above, but without the cooling heat exchanger. The temperature of samples of different sizes were allowed to progressively increase under carefully controlled conditions. Inactivation of PME in orange juice (pH 3.7) and destruction of S. cerevisiae in apple juice (pH 3.4) were again used as parameters. The results once again clearly demonstrated superior inactivation/destruction effects under microwave heating which increased with temperature and decreased with sample size. (Abstract shortened by UMI.)

A model for microwave heating is considered. The behaviour of the solution is analysed and a procedure to calculate the solution is presented. This procedure is based on an eigenfunction expansion. Combining both analytical argument and numerical results, it is then shown that the qualitative behaviour of the solution can be deduced from the fundamental-mode. An extension to a model of porous medium combustion is presented. It is again shown that the qualitative behaviour of the solution is captured by the first eigenmode. The corresponding model for traveling combustion waves is examined and a numerical solution is sought. The algorithm for computation is based on a shooting method used in an existence proof. The idea is to cut the infinite domain into two semi-infinite intervals and apply the shooting technique on both sides. This two-sided shooting method is then applied to compute traveling combustion waves of a solid material.