FEATURES OF UTILITARIAN STONEWARE FIRED WITH MICROWAVE RADIATION

The energy dependence on fossil resources and the increasing competitiveness of the stoneware industry, which is a relevant natural gas consumer, leads to new and more environmentally friendly firing methods. Microwave radiation is herein presented as an alternative heating technology for stoneware firing. The samples were fired in a multimode furnace with 6 magnetrons in its core, each one operating at a nominal power of 900 W and frequency of 2.45 GHz. A pyrometer and a thermocouple were installed in the microwave furnace for temperature measuring, control and monitoring. Pyrometer was calibrated in an electric furnace for accurate temperature measurements. During calibration, the thermocouple used in the microwave furnace was installed in the electric furnace, giving a temperature difference from the control (electric furnace) of 2 to 5 ºC, from room temperature up to 1450 ºC. To help the stoneware firing, a silicon carbide (SiC) plate was used as microwave susceptor, also working as a support base for the stoneware samples (mugs). The microstructure of the microwave fired stoneware shows features similar to those of conventionally fired samples (gas and electric heating), with the microwave requiring lower firing temperature to reach an equal structure. X-Ray diffraction and scanning electron micrograph show the relevant transformations taking place for lower temperatures when using microwave heating.

ON THE POSSIBILITIES OF PERMITTIVITY CALCULATION IN A CERTAIN BANDWIDTH FROM SINGLE FREQUENCY RESULTS

The permittivity of a material can be obtained from resonant measurements in an accurate way [1] at a single frequency (where the resonance occurs). In figure (1) results for the Debye Model at 298K temperature can be seen in the 10MHz-50GHz frequency band for distilled water. In this work we explore the possibilities of obtaining the permittivity of materials from resonant measurements in a certain frequency bandwidth around the resonance frequency. With this purpose a Debye model jointly with a certain conductivity useful for polar liquids [1], are studied to evaluate this possibility jointly with inverse techniques.

Rapid Synthesis of highly luminescent Carbon Quantum Dots using Low-Pressurized Microwave Solvothermal Heating

Since the first serendipity of carbon quantum dots (CQDs)1, it is expected to be used for imaging materials for reusable living bodies (e.g. Hela cells). However, the reported CQDs synthetic methods have yet to be at the practical levels; the quantum yields is low, and synthetic condition is over 5 hrs under more than 30 atms. In this research, we ameliorated the problems of CQDs synthesis and luminescence (quantum yields) by the novel synthesis protocol using microwave chemistry. Specifically, we synthesized high quantum yields CQDs (61%) by utilizing a microwave chemical synthesis, synthesizing at low pressure condition (lower than 5 atom) and short reaction time (3 hrs). The achievement of this high quantum yields made it clear that the contribution of polyethylene glycol (PEG) shell to CQDs is large. It was confirmed from the DLS and TEM image that the particle size of the synthesized particles was 8 to 13 nm (Fig. 1). On the other hand, the relationship between the polymerization degree of added PEG and the quantum yields to the addition amount is summarized in Table 1. The quantum yields of CQDs without addition of PEG was 16.7 %, while it was improved at 61.1 % when 0.6 g of PEG6000 (Molecular weight: 6000) was added.We succeeded in remarkably improving the quantum yields by using PEG, which is usually used as a protective agent, as a shell. By using this method, we succeeded in improving the quantum yields of the existing report by approximately 3 times. From the surface modified structure of PEG, the mechanism of improvement of quantum yields will be considered.[1] X. Xu et al., J. Am. Chem. Soc., 2004, 126, 12736–12737.

H2-reduction Behavior of FeS-CaO Mixture during Microwave Heating

Microwave irradiation is an energy-efficient and a rapid-heating method to decrease the activation energy of chemical reactions via both thermal and non-thermal effects of microwave photons 1). Recently, hydrogen-reduction during microwave heating has been proposed for magnetite reduction to combine the advantages of microwave irradiation and using H2 as a reducing agent during iron production 2). In the present study, as a novel idea, the traditional microwave heating system was equipped with thermobalance to investigate the kinetics of H2-reduction of FeS-CaO mixture (FeS(s) + CaO(s) + H2(g) = Fe(s) + CaS(s) + H2O(g)) under microwave heating at 2.45 GHz to further mitigate CO2 emission and prevent SO2 release during iron production from a sulfide mineral. Microscope observations revealed that the un-reacted core model can be employed for such a kinetic study. Linearity (R2) of different rate-controlling mechanisms after a 10-minute reduction reaction demonstrated that the gas diffusion in micropores of reduced metallic Fe is a dominant rate-controlling mechanism while the interfacial chemical reaction is progressed rapidly. This is attributed to extraordinary effects of microwave irradiation on speeding up the chemical reactions 3), while the formation of Fe shell on the surface of FeS/FeO particles decreases the accessibility of gas to un-reacted parts, resulting in a lower rate of gas diffusion in micropores. Moreover, the diffusion coefficients (De) at 460, 570, and 750 °C were calculated from the plot of the gas diffusion, as illustrated in Fig. 1, wherein the X is reduction degree: where Wi (g) is the initial weight of the sample, Wt (g) is the weight of the sample after treatment for t seconds, Wht (g) is the weight change of the sample owing to the dehydration reaction, and WO (-) is the stoichiometric weight ratio of oxygen in the sample, which is 0.111. Consequently, the activation energy of 22.3 kJ.mol-1 was attained from the Arrhenius equation for the hydrogen-reduction reaction of FeS-CaO mixture under microwave heating.

MATHEMATICAL MODELING OF MICROWAVE HEATING OF BALL-SHAPED PRODUCTS

Microwave heating is widely used in the energy, construction, forestry, chemical and food industries, etc. There are many publications that discuss the main mechanisms that occur during microwave heating. For a better understanding of these processes and the development of high-performance microwave installations, mathematical modeling is necessary. As a rule, nonlinear models that most adequately describe these phenomena use a numerical algorithm for calculations. The authors of this report are engaged in approximate analytical approaches for microwave heating and microwave drying of bodies, which, with a controlled decrease in accuracy, nevertheless allow you to display the main processes and evaluate such heating and drying parameters as: temperature and moisture distribution, heating time, drying speed, reaching maximum values, etc. In this work, we consider a model of microwave heating in the form of a ball with uniform irradiation of microwave energy in the conditions of radiation-convective interaction of the product with the environment. The absorption of the microwave inside the material is given by the law of the Bouguer. In this case, a number of simplifications were made: the electrophysical and thermophysical properties of the material are constant, the material is homogeneous in composition and properties.

MILLIMETER WAVE ABSORPTION IN HYDROXYAPATITE AND 3YSZ CERAMICS IN WIDE TEMPERATURE RANGE

In the field of ceramic-based materials processing the last three decades has been marked by significant academic and industry interest in Additive Manufacturing (AM) technology due to its capability to produce ceramic parts with complex geometry and customizable materials properties. Conceptually, AM technology is a layer-by-layer fabrication of three dimensional physical parts directly from computer-aided design [1]. Solidification of the parts prepared from substances containing ceramic powder may be performed either by conventional heat treatment of a part as whole or by directed energy deposition. Both these strategies can be implemented using gyrotron-based millimeter-wave facilities allowing alternatively both the uniform heating of large-size parts in multi-mode cavities and local heating by focused wave-beams [2]. Hydroxyapatite- and yttria-stabilized zirconia-based ceramics are widely used in biomedical applications due to their high biocompatibility. The knowledge of their microwave absorption variation with temperature and porosity as the materials are densified, is necessary to optimize the scheme of microwave heating. 8 mm diameter disks for the measurements were prepared by uniaxial compacting from commercially available hydroxyapatite (HA) powder and yttria-stabilized zirconia (3YSZ) powder (Tosoh corp.). The measurements were performed at 24 GHz 3 kW gyrotron system. Samples for measurements were placed into the gyrotron system applicator and surrounded with porous alumina based thermal insulation. The design of the applicator and insulation allowed performing optical measurements of both the sample size and temperature distribution over the surface of the sample using a digital monochrome CCD camera. Measurements were made by the calorimetric method, when the microwave power absorbed in the sample is determined basing on the difference of the heating/cooling rates at the moments of intentional abrupt change of the microwave power at different sample temperatures. Absorption coefficient was determined as a division of the absorbed power to the incident microwave power. Special calibration experiments were made for determining microwave power density in the applicator and inside the thermal insulation. The method allows to measure absorption coefficients in situ during the sintering process. Absorption coefficients of HA were obtained in the range of 200 C - 1200 C, and for 3YSZ - in the range of 400 C - 1400 C both in situ during sintering and for as sintered samples. Dependencies of the absorption coefficients on the temperature and porosity are discussed. References Vaezi, M., et al., Int. J. Adv. Manuf. Technol., 2013, 67, 1721–1759. Bykov, Yu., Eremeev, A., et al., IEEE Trans. Plasma Science, 2004, 32, 67–72.

A COMPARATIVE STUDY OF MICROWAVE AND BARRIER DISCHARGE PLASMA FOR THE REGENERATION OF SPENT ZEOLITE CATALYSTS

Due to their acid characteristics and pore structure, which can induce high product selectivity; zeolite catalysts are used extensively in industry to catalyse reactions involving hydrocarbons. However, these catalysts can suffer from deactivation due to cracking reactions that result in the deposition of carbon leading to poisoning of the acid sites and blocking of the pores [1]. Depending upon the reaction and the particular catalyst involved this deactivation may take place over several months or even years but in some cases occurs in minutes. Therefore, zeolite catalysts are frequently reactivated / regenerated. This generally involves a thermal treatment involving air which results in oxidation of the carbon [2]. However, the oxidation of carbon is highly exothermic, and if not carefully controlled, results in the generation of exceedingly high localized temperatures which can destroy the zeolite structure and result in subsequent loss of catalyst activity. More conservative thermal treatments can result in incomplete regeneration and again a catalyst displaying inferior activity. This paper explores the use of non-thermal plasma which had been either generated using microwaves or via a barrier discharge to regenerate spent zeolite catalysts. The catalyst, H-mordenite, was tested for the disproportionation of toluene (Figure 1) using conventional heating. The spent catalyst was then regenerated using a plasma or conventional thermal treatment before having its activity re-evaluated for the toluene disproportionation reaction as previous. Fig. 1. Reaction Scheme for Toluene Disproportionation. Interestingly, not only is plasma regeneration highly effective but also catalysts can be regenerated in greatly reduced times. There is an additional advantage in that plasma regeneration can impart physical properties that result in a zeolite that is resistant to further deactivation. However, the results are highly dependent upon the experimental conditions involved for plasma regeneration. References Wu J, Leu L., Appl. Catal., 1983; 7:283-294. M. Guisnet and P. Magnoux, Deactivation of Zeolites by Coking. Prevention of Deactivation and Regeneration. In: Zeolite Microporous Solids: Synthesis, Structure, and Reactivity. E.G. Derouane, F Lemos, C. Naccache, F. Ramôa Ribeiro, Eds. Pages 437-456. Springer 1992.

ANALYSIS OF HIGH-FREQUENCY C-CORE MAGNETIC FLUX LEAKAGES FOR BONE TUMOR WITH INDUCTION HEATING BY USING MULTI-COIL

High-frequency magnetic field has been developed pervasively. The induction of heat from the magnetic field can help to treat tumor tissue to a certain extent. Normally, treatment by the low-frequency magnetic field needed to be combined with magnetic substances. To assist in the induction of magnetic fields and reduce flux leakage. However, there are studies that have found that high frequencies can cause heat to tumor tissue. In this paper present, a new magnetic application will focus on the analysis of the high-frequency magnetic nickel core with multi-coil. In order to focus the heat energy using a high-frequency magnetic field into the tumor tissue. The magnetic coil was excited by 915 MHz signal and the combination of tissues used are muscle, bone, and tumor. The magnetic power on the heating predicted by the analytical model, the power loss density (2.98e-6 w/m3) was analyzed using the CST microwave studio.

SINTERING OF MLCC’S BARIUM TITANATE WITH MICROWAVES

A comparison of microwave and conventional, in an electric resistance furnace, sintered layers of dielectric base barium titanate (BaTiO3) of the kind employed for multilayer ceramic capacitors (MLCC) was performed. Two kinds of samples were used for each processing method; the layers alone without electrodes, and the green MLCC with the layers and electrodes interdigitated. Samples were exposed to microwaves for 20 minutes and heated up to 1050°C and 1150°C for sintering in a crucible with graphite that acted as reduction agent and microwave susceptor. Conventional sintering was performed in the same arrangement but lasted 120 minutes since it was found that 20 minutes was not enough time to achieve sintering. Heating rate in both cases was 10 °C/min. It was observed that the layers without the electrodes achieve about the same densification for both processes, while in the case of the green MLCC’s the results were variable, ranging from sample that became dust, to cracked samples and some well sintered ones. At least in the microwave case, it is possible that the variability of the results is due to the importance of the location of the sample in the cavity that in turn affects the electric field pattern, especially because the presence of the electrodes that can cause overheating around them.