Near-Field Microwave Microscopy of Materials Properties

Near-field microwave microscopy has created the opportunity for a new class of electrodynamics experiments of materials. Freed from the constraints of traditional microwave optics, experiments can be carried out at high spatial resolution over a broad frequency range. In addition, the measurements can be done quantitatively so that images of microwave materials properties can be created. We review the five major types of near-field microwave microscopes and discuss our own form of microscopy in detail. Quantitative images of microwave sheet resistance, dielectric constant, and dielectric tunability are presented and discussed. Future prospects for near-field measurements of microwave electrodynamic properties are also presented.Comment: 31 pages, 9 figures, lecture given at the 1999 NATO ASI on Microwave Superconductivity Changes suggested by editor, including full reference

A new method for the precise multiband microwave dielectric measurement using stepped impedance stub

This article presents a new method of wideband dielectric measurement at microwave frequencies. This method can be used to determine the complex dielectric properties of solid and semisolid materials from 0.9GHz to 4.5GHz, including the ISM bands of 915 MHz and 2450MHz. The new method is based on the scattering parameter measurement of a stepped impedance open circuited micro-strip stub, partly loaded with dielectric test material. Current microwave wideband spectroscopy techniques generally measure dielectric materials over a wide range of frequencies but their accuracy is limited. In contrast, narrowband techniques generally measure dielectric properties to a high accuracy but only at a single frequency. This new technique is capable of measuring dielectric properties over a wide range of frequencies to a high accuracy. The technique has been verified by the empirical characterisation of the dielectric properties of Teflon and Duroid 5880 materials. Empirical results were in good agreement with values in the manufacturer’s data sheets. The complex permittivity data will be useful for further microwave processing of the materials

Structural analyses of sintered MT and BZT ceramics

Development of dielectric materials is increasing with a rapid progress in mobile and satellite communications systems, where magnesium titanates find their place owing to good dielectric properties. Recently it has been established that, these materials, which are based on binary magnesium titanates (MgTi03 and Mg2Ti04) can be applied in MW engineering. These materials differ extremely low dielectric loss in the microwave range and high dielectric constant. On the other hand, barium-titanate compounds have attracted great attention for their specific microwave properties, as well. They were commonly used as parts of resonators, filters and multilayer ceramic capacitors, in the microwave region. The crystal phase with the structure BaZn2Ti40 11 is present in various commercial microwave dielectric materials based on barium-titanate compounds. Taking all this into account, in this article, the influence of mechanical activation of the MgO-Ti02 and BaCOr ZnO-Ti02 systems on phase composition, crystal structure and microstructure before and after sintering process, has been reported

Relevance of Dielectric Properties in Microwave Assisted Processes

Microwaves are electromagnetic radiation with wavelength ranging from 1 mm to 1 m in free space with a frequency from 300 GHz to 300 MHz, respectively. International agreements regulate the use of the different parts of the spectrum; the frequencies 915 MHz and 2.45 GHz are the most common among those dedicated to power applications for industrial, scientific and medical purposes (Metaxas & Meredith, 1983). Although microwaves have been firstly adopted for communications scope, an increasing attention to microwave heating applications has been gained since after World War II (Meredith, 1998; Chan & Reader, 2002). Reasons for this growing interest can be found in the peculiar mechanism for energy transfer: during microwave heating, energy is delivered directly to materials through molecular interactions with electromagnetic field via conversion of electrical field energy into thermal energy. This can allow unique benefits, such as high efficiency of energy conversion and shorter processing times, thus reductions in manufacturing costs thanks to energy saving. Moreover, other effects have been pointed out, such as the possibility to induce new structural properties to irradiated materials (development of new materials) and to apply novel strategies in chemical syntheses (green techniques). Crucial parameters in microwave heating are the dielectric properties of matter; they express the energy coupling of a material with electromagnetic microwave field and, thus, the heating feasibility (Metaxas & Meredith, 1983; Schubert & Regier 1995; Tang et al., 2002). On the bases of dielectric properties, microwave devices (applicators) can be adopted in heating operations and optimized working protocols can be used. This chapter is divided into four sections dealing with: i. fundamentals of microwave heating and relevance of dielectric properties of materials; ii. different techniques used in dielectric properties measurements of materials (test fixtures characteristics, technique applicability, advantages and disadvantages); iii. application of the open-ended coaxial-probe method in dielectric properties measurements of food, pharmaceutical ingredients, living materials, to understand specific heating phenomenology and, thus, to optimize thermal treatments / to define safety limits of exposition; iv. basics of heat and mass transfer modeling in microwave assisted processes

Determination of Dielectric Property Profile in Cement-Based Materials using Microwave Reflection and Transmission Properties

Microwave characterization methods are effective means for evaluating dielectric properties of materials and correlating them to their important physical, chemical and mechanical properties. For characterization purposes most materials are considered homogeneous and the measurement of their dielectric properties is fairly straightforward. However, certain materials may be considered inhomogeneous in such a way that their dielectric properties vary in a preferred direction within the material. To evaluate the dielectric property profile of these materials an electromagnetic model is necessary that can be used along with their measured reflection and transmission properties. This paper presents the development of such a model which is subsequently used to determine the dielectric property profile in mortar samples exposed to cyclical ingress of salt solution

In Situ Monitoring of Microwave Processing of Materials at High Temperatures through Dielectric Properties Measurement

[EN] Microwave-assisted processes have recognized advantages over more conventional heating techniques. However, the effects on the materials¿ microstructure are still a matter of study, due to the complexity of the interaction between microwaves and matter, especially at high temperatures. Recently developed advanced microwave instrumentation allows the study of high temperature microwave heating processes in a way that was not possible before. In this paper, different materials and thermal processes induced by microwaves have been studied through the in situ characterization of their dielectric properties with temperature. This knowledge is crucial in several aspects: to analyze the effects of the microwave field on the reaction pathways; to design and optimize microwave-assisted processes, and to predict the behavior of materials leading to repeatable and reliable heating processes, etc.García Baños, B.; Catalá Civera, JM.; Peñaranda Foix, FL.; Plaza González, PJ.; Llorens Vallés, G. (2016). In Situ Monitoring of Microwave Processing of Materials at High Temperatures through Dielectric Properties Measurement. Materials. 9(5):1-10. doi:10.3390/ma9050349S1109