Permittivity of materials is of utmost importance for microwave applicators’ design and to predict high-frequency dielectric heating of materials. In the case of aromatic plant biomass, however, there are few data that help researchers design microwave applicators for microwave-assisted extraction. In this work, the permittivity of cypress and rockrose biomass samples were measured versus temperature, density, and moisture content. A resonant technique based on a coaxial bi-reentrant microwave cavity was employed to obtain the complex permittivity of biomass samples as a function of those magnitudes around the 2.45 GHz ISM frequency. The obtained measurements show that large variations for permittivity values can be found with moisture content and density changes for both cypress and rockrose biomass. Temperature also has effects in a lesser degree, although it has an important influence on the cypress biomass loss factor. Polynomial expressions fitting the experimental data were provided in order to facilitate the estimation of intermediate values, which were not explicitly arranged in this work. As a general trend, the permittivity of cypress and rockrose biomass increases with increasing values of moisture content and density, whereas the biomass loss factor increases when temperature rises.
For a large variety of food products, the moisture content can be indirectly determined by measuring their dielectric properties. In the case of coffee, the permittivity knowledge can be applied to determine the moisture content indirectly or for online moisture meters in automatic control of coffee dryers. However, there is little data on the dielectric properties of roasted ground coffee near the 2.45 GHz ISM band. In this contribution, the permittivity was measured versus temperature, bulk density, and moisture content. A resonant technique based on a coaxial microwave cavity was employed to obtain the complex permittivity as a function of those magnitudes near the 2.45 GHz ISM band. In addition, the permittivity of the coffee particle kernel has been estimated from a complex refractive index mixture equation, thus calculating the permittivity of the coffee particle kernel from the permittivity values of the mixture air-coffee (ground coffee) at different bulk densities. The results showed that both dielectric constant and loss factor increase for increasing temperature, bulk density, and moisture content values. Furthermore, expressions fitting the experimental data were provided, thereby facilitating the estimation of the values throughout the studied temperature, bulk density, and moisture content ranges.
Metallic stirrers are commonly used in mode-stirred reverberation chambers but are less common in microwave ovens, because the use of a metallic axis creates a coaxial port in the cavity wall where the axis is introduced, which may result in significant microwave leakage levels. However, due to electromagnetic, mechanical, temperature, or chemical requirements, metallic axes must be used in some cases. In this paper, we present a high power coaxial filter that enables the use of metallic stirrers in microwave applications. The filter employs two-port coaxial cavities that are analyzed to determine their design parameters. These cavities are coupled via coaxial lines to achieve proper attenuation levels. The cavities were designed using commercial electromagnetic (EM) software, and a filter prototype was validated using vector network analyzer (VNA) measurements. The simulated and measured results show that a three-stage configuration can achieve attenuations greater than 70 dB at the 2.45 GHz ISM band, allowing the use and external handling of the metallic axis inserted into microwave ovens with negligible microwave radiation leakage.
Many efforts are being made to find innovative ways of recycling rubber from end-of-life tires (ELTs), also called ground tire Rubber (GTR). Recycling through devulcanization allows the reintroduction of rubber back into the manufacturing industry. Such a process requires providing enough energy to break the sulfur links, while preventing damage to the polymeric chain. Microwave heating is controllable, efficient, and it does not rely on conventional heating mechanisms (conduction, convection) which may involve high heating losses, but rather on direct dielectric heating. However, to adequately control the microwave-assisted devulcanization performance, a thorough knowledge of the GTR permittivity versus temperature is required. In this work, GTR permittivity was monitored during its devulcanization. A resonant technique based on a dual-mode cylindrical cavity was used to simultaneously heat rubber and measure its permittivity at around 2 GHz. The results show sharp changes in the GTR permittivity at 160 and 190 °C. After the GTR cooled down, a shifted permittivity evidences a change in the GTR structure caused by the devulcanization process. Microwave-assisted devulcanization effectiveness is proven through time-domain nuclear magnetic resonance (NMR) measurements, by verifying the decrease in the cross-link density of processed GTR samples compared to the original sample.
The main problems of the traditional foundry dewaxing processes in fine arts workshops are the emission of gases, the loss of 80% of the wax, the high electrical consumption, and the high risks for the operators. The introduction of the microwave technology for dewaxing of ceramic shell molds allows to minimize some of these problems, although the use of electromagnetic susceptors that capture the radiated energy and transform it into heat is required. This article describes different microwave dewaxing tests using TiO2 and graphite as susceptors. The results obtained show that this technique is viable, allowing the casting process to be carried out with a low percentage of breakage problems in the mold and significantly reducing the emitted gases and electricity consumption. The technique allows to recover in the same operation around 90% of the wax used in small and medium format objects. The tests show that the selection of the material used as a susceptor, the area of application and the power regimes, are fundamental to enable a controlled, soft and non-aggressive dewaxing, both for the art molds and for the environment, as opposed to the traditional Flash Dewaxing technique. In this way, it is possible to change the foundry of ceramic shells for artworks to achieve high levels of performance and safety, and to save energy, time and materials.
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