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5 Chess rook fabricated in stereolithography from epoxy/carbon nanotubes mixture [89] 2.4.3 Ceramics/polymers manufacturing at the University of Oxford The extensive use of high real permittivity materials in technology sectors related to electronics and communication has emerged the development of ceramic/polymers composite materials. Such composites combine the high real part of permittivity of ceramics together with the mechanical properties of polymers. The Department of Materials at the University of Oxford has established studies on the microwave dielectric properties of Barium titanate ABS/ BaTiO3 composites in Fused Deposition Modeling 3D printing. The work in [90] explicitly provide the detailed relative permittivity measurements of ABS/BaTiO3 composites with many different weight (wt) percent of BaTiO3. BaTiO3 microparticles were dispersed in ABS to form filaments with loadings of 0 to 70 wt% of BaTiO3 with increment of 10 wt%. The permittivity measurement on solid discshaped samples of all the filaments in a Split-Post Dielectric Resonator (SPDR) at 15 GHz showed a considerable increase of M as the fraction of BaTiO3 in the composite increases.
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The interaction between an impinging electromagnetic wave and a target results in ascattered wave, which depends upon the frequency, the size, shape and electromagnetic propertiesof the target. The measurement of a scattered signal can be used to detect or characterizea target, and the applications range from radar systems at large scale to optical...
Citations
... In [121], measurements and numerical methods such as the superposition T-matrix method, DDA and MoM were compared in amplitude and phase of the scattered field for an aggregate, giving comparable results for different frequencies or wavelengths. Furthermore, in [115], measurements and numerical simulations of the scattered field of spheroids in T-matrix and FEM gave also equivalent results in terms of intensity and phase. These results proved the validation of our simulation methods compared to other models and our measurements. ...
... Thereby, the size parameter and the refractive index of the real particle should be conserved in order to have the same scattering properties for the analog particle (see Figure 1. 30 for an example of a real particle of size a and its analog particle that has been multiplied by a factor f in size, f a, and incident wavelength, f λ, while conserving the same refractive index). It should be noted that the scaling can be applied to produce larger analogs as it is the case for the previous example or smaller analogs, for example centimetric analogs of trees [115]. ...
The first stages of planetary formation begin with the agglomeration of protoplanetary dust to form bigger bodies. However, this dust growth is not fully understood and the standard accretion scenario model still contains different barriers inhibiting the growth of this dust. Several scenarios have been proposed to overcome these barriers, for example irregular dust, e.g., fractal aggregates and grains, contrary to compact spherical dust that was accepted to simplify models. Scattered light observations of protoplanetary disks can be done with nowadays telescopes, obtaining indirect information on this dust. But how can we interpret these scattering information to know if this dust has different morphologies and therefore help to improve the understanding of these barriers? how can we do this when we do not have direct information about dust? One solution is to study the scattering of dust analogs with laboratory experiments where the control of the experimental conditions is possible and therefore the interpretation of scattering information is also possible. This thesis is dedicated to provide more realistic tools to interpret protoplanetary disk observations with microwave scattering experiments, where our dust analogs are geometrically controlled thanks to additive manufacturing, using a refractive index similar to astronomical silicate. The size of these analogs is chosen to be proportional to real dust compared to the used wavelengths to do the observations, in order to respect the electromagnetic scale invariance rule and thus reproduce similar scattering behaviors as real dust. During my PhD I studied the scattering parameters (in the Mie scattering regime) such as the phase function, degree of linear polarization and other Mueller matrix elements of three dust morphologies, e.g. fractal aggregates, and two families of grains with different types of roughness. The goal was to understand their scattering properties and thus give insights or tools to understand the indirect information that is gathered with scattered light observations. Measurements of dust analog were performed in the anechoic chamber of CCRM and cross-validated with numerical simulations. Thanks to the multi-orientation and multi-wavelength that our setup provides, two types of analyses were performed with the three types of morphologies: first, scattering parameters averaged over several orientations of analogs at different wavelengths, and second, scattering parameters including a power-law size distribution. Based on these two analyses, I was able to identify characteristic scattering properties of each morphology, showed with their scattering parameters. I identified the differences of their scattering parameters within a given morphology and between the different morphologies, and I compared them with scattering parameters of similar morphologies found in literature, verifying the coherence of our results. Based on our results, I proved that the control of geometry, refractive index and orientation of our analogs are key to interpret their scattering properties, providing unique scattering measurements thanks to our microwave experiment in CCRM and to the additive manufacturing. Furthermore, these results suggest that porosities of our aggregates and roughness of our compact grains clearly affect in specific ways their scattering properties. Moreover, I showed the interest to continue the instrumental development of telescopes to obtain more than the total scattered intensity (phase function) and degree of linear polarization. Indeed, the other scattering parameters (which are related to the non-sphericity, the degree of circular polarization and the polarization at 45°) can give more clues about the morphology of dust in protoplanetary disks. Finally, I suggested to increase the size of our analogs and test other refractive indices to obtain closer scattering parameters to observations of forming disks, as well as to perform measurements at backscattering angles.
... They also demonstrate our ability to measure fields with low amplitude. This has been made possible thanks to the careful optimization of the measurement setup that we have performed throughout the years [26]. Fig. 10. ...
Controlling the electromagnetic properties of dielectric objects is demanded in microwave applications such as radar cross section studies, metamaterial design and antennas prototyping. Additive manufacturing has made the fabrication of desired shapes easier, but there is still room for improvement in controlling the permittivity. This paper proposes a novel approach to control the permittivity of 3D printed objects, in particular the ones with low permittivity contrasts. The effective permittivity is set by locally varying the material density. The object is first meshed using tetrahedral meshing software. Then its air percentage is controlled by adjusting the diameter of cylinders, which are positioned at each edge of the mesh. By tuning the volume fraction, one can achieve the required effective permittivity. Design, manufacturing and characterization steps are discussed for the specific case of spheres. With such a canonical shape, the effective permittivity can be directly retrieved by comparing far field electromagnetic scattering measurements with Mie computations. Bistatic Radar Cross Section measurements and simulations are provided and discussed. They enable us to assess the validity of the proposed methodology, in particular the good adequacy between the adequately chosen unstructured air/material distribution, the desired relative permittivity and the good homogenization of the object.
... or month for Ref. [7]. AQ:6 = Please confirm the author name and title for Ref. [9]. Also, provide the department name for the same. ...
For foliage penetrating (FoPen) radar development, we previously developed a hybrid volume-surface model, named Domain dEcomposition Model (DEMOS), to evaluate the electromagnetic scattering from large scenes composed by targets (metallic objects) placed in a natural environment (dielectric object). In this letter, we compare the scattered field obtained by DEMOS with the quasi-monostatic measurements done in an anechoic chamber on scaled models composed of dielectric and metallic structures. For all measurements, we consider both polarizations, HH and VV. Our final objective is to determine the optimal configurations for the detection of a target placed in a forest environment in the very high-frequency (VHF)-UHF frequency bands.
