This Thesis is focused on scientific research on composite materials electromagnetic characterization and electric conductive polymers applications. Mainly two different composite materials types are taken into account, those based on epoxy-resin and those achieved through pyrolisis of a phenolic-resin more often known as Carbon-Carbon. The use of such structures is relevant in aerospace/aeronautics, for electromagnetic (EM) protection from natural phenomena (lightning), and intentional interference with radar absorbing materials (RAM), in nuclear physics, for nuclear EM pulses (NEMP) protection, in electromagnetic compatibility (EMC), for equipment-level shielding, high-intensity radiated fields (HIRF) protection, anechoic chambers (for the realizations of wedges and pyramidal arrays), and human exposure mitigation. In order to modulate the electromagnetic characteristics, like electrical conductivity and microwave absorbing capability, the epoxy-resin composite materials taken into account, are reinforced using carbon nanomaterials in different weight percentage. The microwave absorbing capability of these fancy materials is analyzed, and numerical design of wide frequency band microwave absorbing structures and microwave shielding structures are presented and discussed in details in terms of both microwave reflection loss and transmission attenuation i.e., shielding effectiveness. In this Thesis, different branches of research field are applied: nanotechnology, electromagnetic wave propagation theory, composite materials manufacturing, evolutionary computation, and all of them are used to design the “quasi perfect absorber” from electromagnetic point of view. Traditional composites are loaded by graphene/graphite micrometric mixtures. In this work, we propose an inhomogeneous multilayer absorber made of micrometric graphite (at different wt%), and nanometric carbon particles (SWCNTs, MWCNTs, CNFs, at different wt%). Thus, an improvement of the traditional absorbers has been achieved upon optimization through an in-house genetic algorithm (GA), Particle swarm Optimization (PSO), and winning particle optimization (WPO), this last appositely developed. Main goal of the work is to achieve lower values (< -10 dB) of both reflection and transmission coefficients for angular apertures within 40°. The evolutionary computation codes are flexible in the selection of the algorithm parameters such as frequency band, incidence angular range, overall maximum multilayer thickness, possibility to decide if the design optimization procedure must privilege thickness minimization and/or losses maximization. With respect to the present literature, the developed method considers the absorbing capability taking into account both the reflection and the transmission properties of the entire multilayer structure. Moreover, the absorbing properties of the multilayer structures have been analyzed considering oblique incidence at fixed angles within a finite range. This work is organized into six main chapters. Chapter 1 describes electromagnetic theory of plane multilayer structures made of lossy materials. Electromagnetic theory about propagation in no-lossy and lossy materials is also discussed using examples to clarify concepts. Reflection and Transmission Coefficients are discussed, oblique incidence and Snell’s law, Transverse Impedance, Brewster angle and Critical Angle, Complex Waves, Zenneck Waves, are introduced. At the end, Surface Plasmons are analyzed and simulated using genetic algorithm. Chapter 2 describes composite materials manufacturing, chemical/physical analysis, and problems in manufacturing large tiles of composite materials. Composite materials considered here are based on epoxy matrix reinforced with several species of filler in particular carbon nanomaterials are considered. These latter have been chosen taking into account the lowest market prices: the economic aspects, normally neglected in small laboratory applications, are on the contrary important in real applications where the amount of carbon nanopowders could be relatively high. In such scenario a good compromise in terms of cost/performances has been obtained using industrial grade multiwall carbon nanotubes (MWCNTs, about 300 $/kg), graphite micropowder (about 40 $/kg), and carbon nanofibers (CNFs, about 30 $/g). As far as composite materials manufacturing is concerned, the main problem discussed is nanopowders dispersion in relatively high weight percentages within the epoxy-matrix. In fact, microwave absorption properties of the composites are definitively compromised if dispersion is not good enough. Chapter 3 is related to the electromagnetic characterization of composite materials used to build microwave absorbing and shielding structures. The electromagnetic characterization of composite materials consists in determining the dielectric properties like electrical permittivity, which in turns can be used in order to compute microwave electrical conductivity, skin depth penetration, etc. Several measuring methods are possible: wave guide, coaxial line, free space antennas, resonant cavities, and so on. In this work, the wave guide method has been adopted: the reason for such choice is due to the problems intrinsically existing with other methods where mechanical machining of composite materials is required, thus affecting the final dielectric permittivity values determination. Meanings of microwave scattering parameters, electrical conductivity, and permittivity are discussed. Main algorithms used to convert values of scattering parameters measured by Vector Network Analyzer into permittivity are shown. Chapter 4 deals with the algorithms adopted for the numerical design of microwave absorbing and shielding structures. In order to modeling absorbing structures where the microwave absorbing performances are the best obtainable in a wider frequency band and for all possible microwave incidence angles, transmissions line equations have been applied to multilayer structures. Here in particular each layer can assume the dielectric properties of one particular composite material in the data base composed by all composite materials electromagnetically characterized. In such model, the number and the thickness of each layer determine the entire multilayer structure electromagnetic wave absorbing properties. Frequency band considered is in the range 5-18 GHz. Two main design scenarios have been considered, the first classically called radar absorbing material (RAM) where the multilayer structure is supposed baked with a perfect electric conductor (PEC), the second baptized microwave shielding structure (MSS) where at the end of multilayer structure there are again free space conditions for microwave propagation. Such last scenario is useful in application where the composite material posses also mechanical structural properties and is used in place of metal structure (aircraft structure applications). Since the absorber’s overall thickness is sometimes an important constraint in the design process, then the design and optimization algorithms are capable to take into account simultaneously for both, i.e., electromagnetic performances and overall thickness of the multilayer structure. For such kind of problems, evolutionary computation represents a promising method, assuring at the same time good global performances and reasonable computation time. In this work, a new algorithm called winning particle optimization (WPO) is presented and applied. In order to check the soundness of WPO results, an in-house built genetic algorithm (GA) and Particle Swarm Optimization (PSO) are presented and applied too, and final results compared. Chapter 5 presents the experimental validation of the developed electromagnetic absorbing and shielding mathematical theoretical model. Validation is obtained comparing measurements and simulations of reflection loss (RL), and shielding effectiveness of some realized microwave absorbing and shielding structures based on carbon nanostructured composite materials. Measurements of (RL) in free space using NRL Arch technique are performed on large RAM multilayer structure tiles obtained by numerical design and optimization process. Measurements of shielding effectiveness in free space using directional shielding effectiveness measurement (DSEM), developed by us and Università Politecnica delle Marche (Dipartimento di Elettromagnetismo e Bioingegneria), are performed on materials and multilayer structures obtained by numerical design and optimization techniques presented. All the cited equipments i.e., NRL arch system, DSEM system, sample holders system, have been appositely in-house manufactured. Chapter 6, is focused on carbon-carbon (CC) composite materials. Electromagnetic characterization is shown and electrical conductivity, absorbing and shielding properties discussed. NRL arch and DSEM measurements are presented and analyzed. Due to high electrical conductivity of CC, measurements using wave-guide methods do not permit us to determine the absorption and electrical conductivity properties in a precise way. Then a microwave wave-guide has been built using CC, and the attenuation of microwave signal measured using vector network analyzer. Using the measured attenuation values, the electrical conductivity of CC has been computed.

Questa tesi raccoglie il lavoro di tre anni di ricerche e studi nel settore dei nanomateriali, nanostrutture ed in generale dei compositi avanzati effettuati presso la Scuola di Ingegneria Aerospaziale della “Sapienza” Università di Roma. In particolare lo scopo è stato quello di approfondire l’interazione tra campi elettromagnetici ed alcune tipologie di compositi avanzati basati essenzialmente su strutture in carbonio e nanomateriali. Questo tentativo ha richiesto un approccio multidisciplinare tra diversi settori scientifici che comprendono quello dei materiali, delle strutture, dei processi di fabbricazione, delle nanotecnologie e dell’elettromagnetismo, i cui concetti di base sono, in questo contesto, dati per acquisiti e per il cui approfondimento si rimanda a testi specifici. L’obiettivo principale è stato quello di utilizzare queste conoscenze trasversali per progettare e costruire nuovi materiali/strutture in grado di assorbire efficacemente i campi elettromagnetici in un ampio intervallo di frequenze ed angoli d’incidenza con molteplici applicazioni anche se l’ambito su cui si è lavorato è quello aerospaziale. Per ottimizzare questi materiali/strutture si è fatto ricorso all’utilizzazione di algoritmi evoluzionistici che sono entrati a pieno titolo nello studio multidisciplinare con uno stretto collegamento tra la teoria sviluppata e le prove di laboratorio atte a validare sperimentalmente i modelli matematici proposti.

Design of Microwave Absorbing Structure and Microwave Shielding Structure by using Composite Materials, Nanomaterials and Evolutionary Computation / Micheli, Davide. - STAMPA. - (2011).

Design of Microwave Absorbing Structure and Microwave Shielding Structure by using Composite Materials, Nanomaterials and Evolutionary Computation

MICHELI, DAVIDE
01/01/2011

Abstract

This Thesis is focused on scientific research on composite materials electromagnetic characterization and electric conductive polymers applications. Mainly two different composite materials types are taken into account, those based on epoxy-resin and those achieved through pyrolisis of a phenolic-resin more often known as Carbon-Carbon. The use of such structures is relevant in aerospace/aeronautics, for electromagnetic (EM) protection from natural phenomena (lightning), and intentional interference with radar absorbing materials (RAM), in nuclear physics, for nuclear EM pulses (NEMP) protection, in electromagnetic compatibility (EMC), for equipment-level shielding, high-intensity radiated fields (HIRF) protection, anechoic chambers (for the realizations of wedges and pyramidal arrays), and human exposure mitigation. In order to modulate the electromagnetic characteristics, like electrical conductivity and microwave absorbing capability, the epoxy-resin composite materials taken into account, are reinforced using carbon nanomaterials in different weight percentage. The microwave absorbing capability of these fancy materials is analyzed, and numerical design of wide frequency band microwave absorbing structures and microwave shielding structures are presented and discussed in details in terms of both microwave reflection loss and transmission attenuation i.e., shielding effectiveness. In this Thesis, different branches of research field are applied: nanotechnology, electromagnetic wave propagation theory, composite materials manufacturing, evolutionary computation, and all of them are used to design the “quasi perfect absorber” from electromagnetic point of view. Traditional composites are loaded by graphene/graphite micrometric mixtures. In this work, we propose an inhomogeneous multilayer absorber made of micrometric graphite (at different wt%), and nanometric carbon particles (SWCNTs, MWCNTs, CNFs, at different wt%). Thus, an improvement of the traditional absorbers has been achieved upon optimization through an in-house genetic algorithm (GA), Particle swarm Optimization (PSO), and winning particle optimization (WPO), this last appositely developed. Main goal of the work is to achieve lower values (< -10 dB) of both reflection and transmission coefficients for angular apertures within 40°. The evolutionary computation codes are flexible in the selection of the algorithm parameters such as frequency band, incidence angular range, overall maximum multilayer thickness, possibility to decide if the design optimization procedure must privilege thickness minimization and/or losses maximization. With respect to the present literature, the developed method considers the absorbing capability taking into account both the reflection and the transmission properties of the entire multilayer structure. Moreover, the absorbing properties of the multilayer structures have been analyzed considering oblique incidence at fixed angles within a finite range. This work is organized into six main chapters. Chapter 1 describes electromagnetic theory of plane multilayer structures made of lossy materials. Electromagnetic theory about propagation in no-lossy and lossy materials is also discussed using examples to clarify concepts. Reflection and Transmission Coefficients are discussed, oblique incidence and Snell’s law, Transverse Impedance, Brewster angle and Critical Angle, Complex Waves, Zenneck Waves, are introduced. At the end, Surface Plasmons are analyzed and simulated using genetic algorithm. Chapter 2 describes composite materials manufacturing, chemical/physical analysis, and problems in manufacturing large tiles of composite materials. Composite materials considered here are based on epoxy matrix reinforced with several species of filler in particular carbon nanomaterials are considered. These latter have been chosen taking into account the lowest market prices: the economic aspects, normally neglected in small laboratory applications, are on the contrary important in real applications where the amount of carbon nanopowders could be relatively high. In such scenario a good compromise in terms of cost/performances has been obtained using industrial grade multiwall carbon nanotubes (MWCNTs, about 300 $/kg), graphite micropowder (about 40 $/kg), and carbon nanofibers (CNFs, about 30 $/g). As far as composite materials manufacturing is concerned, the main problem discussed is nanopowders dispersion in relatively high weight percentages within the epoxy-matrix. In fact, microwave absorption properties of the composites are definitively compromised if dispersion is not good enough. Chapter 3 is related to the electromagnetic characterization of composite materials used to build microwave absorbing and shielding structures. The electromagnetic characterization of composite materials consists in determining the dielectric properties like electrical permittivity, which in turns can be used in order to compute microwave electrical conductivity, skin depth penetration, etc. Several measuring methods are possible: wave guide, coaxial line, free space antennas, resonant cavities, and so on. In this work, the wave guide method has been adopted: the reason for such choice is due to the problems intrinsically existing with other methods where mechanical machining of composite materials is required, thus affecting the final dielectric permittivity values determination. Meanings of microwave scattering parameters, electrical conductivity, and permittivity are discussed. Main algorithms used to convert values of scattering parameters measured by Vector Network Analyzer into permittivity are shown. Chapter 4 deals with the algorithms adopted for the numerical design of microwave absorbing and shielding structures. In order to modeling absorbing structures where the microwave absorbing performances are the best obtainable in a wider frequency band and for all possible microwave incidence angles, transmissions line equations have been applied to multilayer structures. Here in particular each layer can assume the dielectric properties of one particular composite material in the data base composed by all composite materials electromagnetically characterized. In such model, the number and the thickness of each layer determine the entire multilayer structure electromagnetic wave absorbing properties. Frequency band considered is in the range 5-18 GHz. Two main design scenarios have been considered, the first classically called radar absorbing material (RAM) where the multilayer structure is supposed baked with a perfect electric conductor (PEC), the second baptized microwave shielding structure (MSS) where at the end of multilayer structure there are again free space conditions for microwave propagation. Such last scenario is useful in application where the composite material posses also mechanical structural properties and is used in place of metal structure (aircraft structure applications). Since the absorber’s overall thickness is sometimes an important constraint in the design process, then the design and optimization algorithms are capable to take into account simultaneously for both, i.e., electromagnetic performances and overall thickness of the multilayer structure. For such kind of problems, evolutionary computation represents a promising method, assuring at the same time good global performances and reasonable computation time. In this work, a new algorithm called winning particle optimization (WPO) is presented and applied. In order to check the soundness of WPO results, an in-house built genetic algorithm (GA) and Particle Swarm Optimization (PSO) are presented and applied too, and final results compared. Chapter 5 presents the experimental validation of the developed electromagnetic absorbing and shielding mathematical theoretical model. Validation is obtained comparing measurements and simulations of reflection loss (RL), and shielding effectiveness of some realized microwave absorbing and shielding structures based on carbon nanostructured composite materials. Measurements of (RL) in free space using NRL Arch technique are performed on large RAM multilayer structure tiles obtained by numerical design and optimization process. Measurements of shielding effectiveness in free space using directional shielding effectiveness measurement (DSEM), developed by us and Università Politecnica delle Marche (Dipartimento di Elettromagnetismo e Bioingegneria), are performed on materials and multilayer structures obtained by numerical design and optimization techniques presented. All the cited equipments i.e., NRL arch system, DSEM system, sample holders system, have been appositely in-house manufactured. Chapter 6, is focused on carbon-carbon (CC) composite materials. Electromagnetic characterization is shown and electrical conductivity, absorbing and shielding properties discussed. NRL arch and DSEM measurements are presented and analyzed. Due to high electrical conductivity of CC, measurements using wave-guide methods do not permit us to determine the absorption and electrical conductivity properties in a precise way. Then a microwave wave-guide has been built using CC, and the attenuation of microwave signal measured using vector network analyzer. Using the measured attenuation values, the electrical conductivity of CC has been computed.
2011
File allegati a questo prodotto
Non ci sono file associati a questo prodotto.

I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.

Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11573/495297
 Attenzione

Attenzione! I dati visualizzati non sono stati sottoposti a validazione da parte dell'ateneo

Citazioni
  • ???jsp.display-item.citation.pmc??? ND
  • Scopus ND
  • ???jsp.display-item.citation.isi??? ND
social impact