Ceramic-matrix composites of alumina and silicon carbide whiskers have recently found novel commercial application as electromagnetic absorbers. However, a detailed understanding of how materials issues influence the composite electrical response, which underpins this application, has been absent until now. In this project, such composites were electrically measured over a wide range of conditions and modeled in terms of various aspects of the microstructure in order to understand how they work. For this purpose, three types of composites were made by different methods from the same set of ceramic powder blends loaded with different volume fractions of whiskers. In doing so, the interfaces between whiskers, the preferred orientations of whiskers, and the structure of electrically-connected whisker clusters were varied; the whisker aspect-ratio distributions were the same for all methods. At the electrode interfaces, Schottky barriers at the junctions of the electrically-percolating wide-bandgap semiconductor whiskers on the surface were responsible for a significant portion of the total measured impedance. The associated electrical response was studied on the microscopic and macroscopic level, and the gap between these different scales was bridged. Also, a modeling approach was developed for the non-linear behavior of the composite which results from these barriers. In regards to the whiskers within the composite bulk, the effects of various factors on the wide-band frequency dependence of the dielectric response and dc conductivity were explained and contextualized for the electromagnetic absorber application. Such factors include whisker preferred orientation, electrical percolation and cluster structure, the interfaces between electrically-connected SiC whiskers, and porosity. A quantitative correlation between the anisotropy of the microstructure and that of the conductivity was found, and was understood in terms of the interfacial SiC-Al2O3-SiC conduction mechanism. This behavior was shown to differ from the behavior commonly observed for other disordered mixtures of relatively conductive particles dispersed inside insulating polymer hosts. A description of this new mechanism was developed based on an observed correlation between the temperature dependencies of the static and radio-frequency electrical responses. Also, the aforementioned non-linear response model was expanded upon to describe conduction through and across electrically-percolated clusters. The model demonstrates how loading and interface behavior influence the topology and the strength of the non-linear response of the clusters.

Os materiais absorvedores de radiação eletromagnética (MARE) têm recebido muita atenção nas últimas quatro décadas, devido à crescente utilização para atender as exigências das legislações governamentais no controle de níveis de radiação eletromagnética espúria emitida por equipamentos eletrônicos, das normas industriais de compatibilidade e interferência eletromagnética, envolvendo basicamente as indústrias de equipamentos eletrônicos e telecomunicações, bem como a demanda da sociedade na confiabilidade nos equipamentos eletrônicos. O presente trabalho tem como objetivo estudar a produção e a caracterização de MARE na faixa de freqüências compreendida entre 8,2 12,4 GHz, baseados em papel celulósico impregnado com negro de fumo e matriz tipo poliuretano, aplicado por meio de pistola de pintura. Seis diferentes condições de impregnação foram estudadas, variando distância de aplicação e número de demãos (uma em cada lado do papel celulósico): 25 cm 2 demãos, 25 cm 4 demãos, 30 cm 2 demãos, 30 cm 4 demãos, 40 cm -2 demãos e 40 cm 4 demãos. Os resultados médios obtidos da caracterização eletromagnética dos seis lotes, via medição dos parâmetros S (S11/S22 Energia refletida, Er e S12/S21 Energia transmitida, Et) e calculo da energia absorvida (Ea), mostram que para os lotes com duas demãos, quanto maior a distância de aplicação, menores são os valores de Er e Ea e maior é a Et, uma vez que a gramatura do MARE diminui. Para os lotes com quatro demãos, quanto maior a distância de aplicação, menor é a Er e maior a Et. Entretanto, o parâmetro Ea para o lote com quatro demãos não se comporta como o lote com duas demãos, pois entre as distâncias de aplicação 25 cm e 30 cm é observada a queda de Ea, mas com a distância de aplicação 40 cm o valor de Ea aumenta, pois a onda eletromagnética consegue penetrar mais efetivamente no material, ou seja, há uma determinada concentração de centros absorvedores no substrato que permite maior interação do material com a onda eletromagnética. A partir dos valores de Ea, Er e Et foram selecionados 3 condições de impregnação para montagem de um MARE baseado em 8 tubos de celulose montados lado a lado formando um quadrado, sendo cada tubo com dimensões de 50 cm de comprimento e diâmetro de 7,6 cm, dobrados ao meio para promover múltiplas reflexões do sinal e assim aumentar a atenuação do sinal. As condições de impregnação foram escolhidas a partir da análise dos resultados dos seis lotes com base no menor valor de energia refletida e maior energia absorvida, na seguinte ordem crescente: 25 cm 4 demãos, 25 cm 2 demãos e 40 cm 2 demãos. Os resultados de refletividade no arco NRL mostram que a maior atenuação é obtida no lote com menor valor de energia refletida (40 cm 2 demãos, -8,20 dB/84,8% em média) e a menor atenuação no lote com maior valor de energia refletida (-2,84 dB/48,0%). Estes resultados demonstram que para um MARE baseado em múltiplas reflexões possuir alta eficiência de absorção de energia, o mesmo deve possuir baixos valores de energia refletida, pois a onda eletromagnética deve ter capacidade de penetrar na estrutura do material, fazendo com que a reflexão seja diminuída. / The electromagnetic absorbers has been receiving a lot of attention in the last four decades, due to their growing use in order to attend the demands of governmental control laws of electromagnetic radiation spurious levels emitted by electronic devices, industrial standards of electromagnetic compatibility (EMC) and electromagnetic interference (EMI), basically involving electronic and telecommunication industries, as well the demand of the society on the reliability of electronic equipments. The aim of the present work is to study the production and characterization of electromagnetic absorbers in the frequency range of 8,2 12,4 GHz, based on cellulose paper impregnated with carbon black and poliuretanic matrix, applied with spray gun. Six different conditions of impregnation were studied by varying the application distance and number of coats (one coat on each paper side): 25 cm 2 coats, 25 cm 2 coats, 25 cm 4 coats, 30 cm 2 coats, 30 cm 4 coats, 40 cm - coats and 40 cm 4 coats. The results of scattering parameters (S11/S22 reflected energy, Er and S12/S21 transmitted energy, Et) and calculated absorbed energy (Ea) for the six conditions showed that the conditions with two coats, the larger coat application distance, the smaller are the values of Er and Ea and the larger is Et, due to the decrease of paper gramature. The conditions with four coats, the larger coat application distance, the smaller is the value of Er and larger is the Et. However, the values of Ea for the condition with four coats has different behavior when compared with the condition with two coats, because between application distances of 25 cm and 30 cm is observed the decrease of Ea values, but with the application distance of 40 cm the Ea value increases, because the electromagnetic wave can penetrate more effectively in the material bulk, or be, there is a concentration of absorbers centers inside the material that allows a stronger interaction with the electromagnetic wave. Based on the values of Ea, Er and Et for the six conditions, were selected three conditions to assemble an electromagnetic absorber based on eight cellulose tubes, places side by side in order to form a square, with each tube with dimensions of 50 cm length and 7,6 cm diameter, bended in the middle portion to promote multiple reflections of electromagnetic wave in order to increase the signal attenuation. The conditions of tubes impregnation were chosen by the analysis of the six conditions results, based on the smaller value of Er and the larger value of Ea: 25 cm 4 coats, 25 cm 2 coats e 40 cm 2 coats. The reflectivity results of NRL arch shows that the larger attenuation is observed with the condition that present the smaller reflected energy (40 cm 2 coats, -8,20 dB/84,8%) and the smaller attenuation in the lot with the larger value of reflected energy (-2,84 dB/48,0%). These results shows that for an electromagnetic absorber based on multiple reflections possess high efficiency of energy absorption, the material must possess lower values of reflected energy, because the electromagnetic wave must have the capacity to penetrate in the bulk of the material, decreasing the reflection.