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The transmission losses of the as-received and oxidized woven fabrics are plotted in Fig. 6a and b, respectively, as a function of frequency. The decreasing fraction of the transmitted wave through the material corresponds to the weakening of the transmission loss (higher negative dB). The lowest transmission losses were seen in the ZE8 fabric in both as-received and heat-treated conditions. Similar to its reflection loss, the PN fabric recorded the largest difference in transmission loss between its as-received and heat-treated states. Moreover, as a general tendency, it was observed that woven fabrics that have a higher reflectivity of electromagnetic waves also recorded lower transmission(higher reflection loss corresponds to lower transmission loss). In line with the principle of energy conservation, incident electromagnetic radiation energy is either reflected,transmitted, or absorbed (dissipated and converted into heat) by a material. Accordingly, the absorption percentages of the woven fabrics were calculated using Eq. 3,where %R and %T represent the reflected and transmitted portion of the electromagnetic wave, respectively. Table 3 shows the calculated absorption percentages (%) of the ceramic woven fabrics at various frequencies. The absorption percentages of single-layer SiC-based ceramic woven fabrics were \40%, which made them highly unsuitable as electromagnetic absorbing materials in commercial applications. In order to achieve a better electromagnetic wave absorption potential, several double-layer combinations of the as-received and heat-treated ceramic woven fabrics were constructed. In the first case, combinations of as-received woven fabrics were measured, among which combinations of S8 woven fabrics recorded good absorption characteristics, with both reduced reflection and transmission loss. Figure 7 shows the absorption potentials of double-layer combinations containing S8-type woven fabrics as one of the layers. A low-conductivity layer(S8 woven fabric) reduced the reflection loss at the air/low- conductivity layer interface, while most of the electromagnetic wave transmitted through the woven fabric was reflected from the surface of the second layer. The low-conductivity first layer behaved as an absorbing layer, whereas the high-conductivity second layer behaved as a reflective layer, binding electromagnetic energy within the material system. In the low¡ú high electrical conductivity layer combinations higher absorption potentials were achieved. In this respect, the highest conductivity mismatch between the S8 and ZE8 fabrics resulted in the maximum absorption potential. Additionally, the ordering of the layers also affected the absorption potential of a given combination; for example, the absorption potential of the S8¨CZE8 combination was different to that of the ZE8¨CS8 combination, although both combinations contained identical woven fabric layers. In the case of the S8¨CZE8 combination, the electromagnetic wave was first confronted with the low-conductivity S8 woven layer,before passing through the ZE8 woven layer; whereas inthe ZE8¨CS8 combination, the ZE8 layer was the first layer to act upon the electromagnetic wave, followed by the S8 layer. Due to the high reflection loss of the ZE8 fabric, the absorption potential was weaker in the ZE8¨CS8 combination. |
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wanghaiju: ½ð±Ò+37, ·ÒëEPI+1 2014-06-12 18:11:01
wanghaiju: ½ð±Ò+37, ·ÒëEPI+1 2014-06-12 18:11:01
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