The role of chirality in synthetic microwave absorbers

Date
1998
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Publisher
Stellenbosch : Stellenbosch University
Abstract
ENGLISH ABSTRACT: Claims were made during the late 1980's that synthetic chiral materials can yield microwave absorbers with significantly improved electromagnetic and physical properties. Papers appeared in scientific literature and a number of patents were issued. These claims stimulated wide interest in microwave chiral absorbers. In 1992 Bohren et al. challenged the claims about chirality by asking the question, "Microwave absorbing chiral composites: is chirality essential or accidental?" They concluded that "although helices have some excellent properties in the design of composite absorbing materials, the resulting chirality is accidental and not essential." An experimental study by Cloete et al. seems to support the findings of Bohren et al. However, we are also of the opinion that fundamentally the role of chirality in the interaction between an electromagnetic wave and an absorbing medium is not yet fully understood, especially in the regime where the chiral inclusions are resonant. Hence this theoretical and experimental study of closely related uniaxial crystals of conducting wire structures of resonant length in the microwave regime, embedded in a dielectric host. Unit cells of synthetic material are designed to fit into a rectangular or square waveguide to allow accurate measurements of reflection and transmission coefficients. The three segment wire hook was chosen as basic element, instead of the helix, for the simple geometrical relation between the chiral enantiomorphs and the non-chiral structures. Four uniaxial crystals are described, and classified according to their point group symmetry: a chiral unit cell, consisting of four hooks of the same handedness; a non-chiral unit cell of four staples; a non-chiral unit cell of four cranks; and a racemic unit cell of enantiomorphous chiral hooks, two of each handedness. A finite difference time domain code was developed for the full-wave numerical analysis in a rectangular waveguide of the conducting thin wire structures, embedded in an absorbing host. The code uses a subcell thin wire formulation for conducting wires, including the effective material properties of the wires. In order to validate the FDTD code, wire crystals were manufactured and measured in S-­hand (2 4 GHz) rectangular waveguide. The FDTD subcell thin wire formulation for highly conducting wires was found to predict the scattering with respect to magnitude, phase and absorption, with good accuracy. Numerical experiments were performed in a square waveguide on the wire crystals to study their absorption properties. The following observations were made. The inclusion of wire structures in a microwave absorber can improve the absorption interaction of an electromagnetic wave with the host, provided that the inclusions are good conductors and of resonant length. This is observed whether the inclusions are chiral, or non-chiral. The unit cell of non-chiral cranks couples the incident field to the loss mechanisms of the host as effectively as the unit cell of chiral hooks and the racemic unit cell of enantiomorphous hooks. Chirality is a geometrical requirement for optical activity. However, no evidence in these experiments suggests that chirality is a geometrical requirement for absorption by synthetic microwave materials.
AFRIKAANSE OPSOMMING: Aansprake gedurende die laat tagtigerjare dat sintetiese chirale materiale as mikrogolfabsorbeerders merkwaardige eienskappe kan besit, het wye belangstelling gewek in chirale mikrogolfabsorbeerders. Artikels het in die wetenskaplike literatuur verskyn en 'n aantal patente is uitgereik. Bohren et al. het in 1992 hierdie aansprake rakende chiraliteit bevraagteken in hul artikel, "Microwave absorbing chiral composites: is chirality essential or accidental?" Hulle maak die opmerking dat, alhoewel 'n suspensie van metaalhelikse in 'n nie-chirale omgewing aantreklike eienskappe het vir die ontwerp van mikrogolfabsorbeerders, die chiraliteit toevallig is en nie noodsaaklik nie. 'n Eksperimentele studie deur Cloete et al. blyk die bevindings van Bohren et al. te ondersteun. Tog was daar 'n behoefte om fundamenteel die rol van chiraliteit in die interaksie tussen 'n elektromagnetiese golf en 'n absorberende medium te ondersoek, en spesifiek in die gebied waar die insluitings resonant is. Dit was die oogmerk van hierdie proefskrif. Hierdie proefskrif behels 'n teoretiese en eksperimentele studie, by mikrogolffrekwensies, van verwante uniaksiale kristalle van geleidende draadstrukture van resonante lengte, ingele in 'n dielektriese materiaal. Eenheidselle van die sintetiese materiaal is ontwerp om in 'n reghoekige of vierkantige golfleier te pas sodat akkurate metings van weerkaats- en transmissiekoëffisiente uitgevoer kan word. Die drie-segment draadhakie, in plaas van die heliks, is as basiese element gekies vanweë die eenvoudige geometriese verband tussen die chirale enantiomere en die nie-chirale strukture. Vier uniaksiale kristalle word beskryf en geklassifiseer volgens hul puntgroep-simmetrie: 'n chirale eenheidsel; twee nie-chirale eenheidselle; en 'n raseme eenheidsel. ‘n Eindige verskil tydgebied ("FDTD") rekenaarkode is ontwikkel om die chirale en nie­chirale geleidende dundraadstrukture, ingele in 'n absorberende materiaal, te bestudeer in reghoekige golfleier. Die kode gebruik 'n subsel-dundraadformulering vir geleidende drade, en sluit die effektiewe materiaaleienskappe van die drade in. Die rekenaarkode is getoets teen eksperimente met fisiese draadkristalle wat in S-band (2 4 GHz) reghoekige golfleier gemeet is. Die FDTD dundraadformulering vir hoogs-geleidende drade voorspel die interaksie van 'n draadkristal met 'n mikrogolf met goeie akkuraatheid ten opsigte van amplitude, fase en absorpsie. Numeriese eksperimente is vervolgens in vierkantige golfleier uitgevoer om die kristalle te bestudeer vir hulle absorpsie eienskappe. Die volgende waarnemings is gemaak. Die insluiting van draadstrukture in 'n mikrogolfabsorbeerder kan die absorpsie van 'n elektromagnetiese golf verbeter, mits die insluitings goeie geleiers is en van resonante lengte. Dit geld vir beide chirale en nie-chirale strukture. Die eenheidsel van nie-chirale hakies is net so effektief soos die eenheidsel van chirale hakies en die raseme eenheidsel van enantiomorfe hakies in terme van verbeterde absorpsie deur die mikrogolfabsorbeerder. Chiraliteit is 'n geometriese vereiste vir optiese aktiwiteit. Geen getuienis in hierdie eksperimente dui op chiraliteit as 'n geometriese vereiste vir absorpsie in sintetiese mikrogolfabsorbeerders nie.
Description
Dissertation (Ph.D.) -- University of Stellenbosch, 1998.
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