A metal-ceramic coaxial cable Fabry-Pérot microwave interferometer for monitoring fluid dielectric constant

Abstract

A metal-ceramic coaxial cable Fabry-Pérot interferometer (MCCC-FPI) has been developed as a new microwave sensor and demonstrated for fast and reliably measuring and continuously monitoring dielectric constants for pure and mixed liquids. The sensor only requires a simple reference scan of the interferogram for the FPI with its sensing chamber filled with air or under vacuum to determine the actual inter-reflector length. The sensor is validated by measuring room temperature dielectric constant ( r ) at high frequencies around 1.4 -4.7 GHz for three vegetable oils (corn, sesame, and olive), the synthetic Mobil ® -1 engine oil, and mixtures containing sesame oil and water. The sensor has been also successfully demonstrated for determining  r values for the Mobil-1 oil at various temperatures and for the welldispersed sesame oil-water mixtures of different compositions. The Bottcher's model of effective medium theory is shown to be better suited for predicting the  r of the sesame oil-water mixture than the Maxwell-Garnett and Bruggeman equations of spherical solutions.

Characterizing the Gas adsorption-dependent Dielectric Constant for Silicalite Nanoparticles at Microwave Frequencies by a Coaxial Cable Fabry-Pérot Interferometric Sensing Method

“…The microwave FPI can be established by introducing two dielectric perturbations into the electric insulation layer in a distance of "d" to act as weak reflectors in the coaxial transmission line. In the MCCC-FPI sensor of this work, the two dielectric perturbations are the interfaces at the two ends of a cavity created inline of the MCCC insulator where two materials of different e r values meet [10]. The basic structure of the zeolite particle-packed MCCC-FPI sensor is schematically depicted in Figure 4, which is a modified version of our recently reported coaxial cable FPI liquid sensor.…”

“…The position of the alumina tube remaining outside of the SS tube was physically locked by alumina-based ceramic adhesives to allow for restoring the exact position of the alumina tube end after the packed particles were removed. Based on the r e values of vacuum (1.0) or (dry air ~1.0003), the vacuumed or air-filled sample chamber can be used to accurately determine chamber length d either by scanning the microwave interferogram or more conveniently by recording the time domain spectrum of the FPI [10]. After determining the accurate chamber length d, the volume fractions of the zeolite particles (ϕ z ) and inter-particle voids (i.e.…”

“…the interfaces at the two ends of the sample chamber. The two reflections, U 1 and U 2 , interfere and the interference waveform is expressed by the following equation [10], (4) where Γ(f) is the frequency-dependent reflection coefficient of the reflectors; z signifies the propagation direction; α is the transmission attenuation coefficient; d 0 is the propagation distance prior to the first reflector; e r is the dielectric constant of the packed-bed between the reflectors; c is the speed of light in vacuum; and d is the sample chamber length. Equation (4) indicates that, for a given FPI with known component materials, structural parameters, and operation temperature, the resonant frequencies of the reflected microwave interferogram depends solely on the optical length (…”

“…The understanding of zeolites' dielectric behaviors and their relationship with molecular adsorption in the zeolitic pores of nanometer or sub-nanometer scales has been hindered by the lack of simple and reliable measurement technologies. Currently, the widely used capacitance measurement approach applies to low frequency ranges while the conventional techniques for measurements in microwave frequency ranges often involve challenging and costly device fabrication and encounter difficulties in controlling sample conditions during operation [9,10]. This paper reports the measurement of dielectric constant for pure silica MFI-type zeolite (i.e.…”

“…This paper reports the measurement of dielectric constant for pure silica MFI-type zeolite (i.e. silicalite) nanoparticles in the microwave frequency range of 1 -6 GHz by a new metalceramic coaxial cable Fabry-Pérot interferometer (MCCC-FPI) sensor approach, which was recently developed by the authors [9,10]. The MFI-type zeolite has been specifically selected for this study because it is a very versatile group of materials that has so far attracted the most extensive research efforts in exploration of new applications as catalysts, low-k electronic components, ion conductors, and photonic and microwave substrates, etc.…”