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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→ 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–ZE8 combination was different to that of the ZE8–S8 combination, although both combinations contained identical woven fabric layers. In the case of the S8–ZE8 combination, the electromagnetic wave was first confronted with the low-conductivity S8 woven layer,before passing through the ZE8 woven layer; whereas inthe ZE8–S8 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–S8 combination. |
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reko34
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wanghaiju: 金币+37, 翻译EPI+1 2014-06-12 18:11:01
wanghaiju: 金币+37, 翻译EPI+1 2014-06-12 18:11:01
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收货状态和氧化后织物关于频率的传输损失如图6a、6b所示。发射波穿过材料的减弱部分对应传输损失的弱化（较高的负dB）。ZE8织物的收货状态和热处理状态都展现出最低的传输损失。与反射损失相似，PN织物的收货状态和热处理状态之间的传输损失区别最大。此外观察到一个普遍趋势，较高电磁波反射率的织物同时也具有较低的传输率（高反射损失对应低传输损失）。 按照能量转化原理，发射电磁辐射能量被材料反射、传输或吸收（耗散或转化为热）。相应地，织物的吸收百分比用公式3计算，%R和%T分别代表电磁波的反射和传输部分。表3所示为陶瓷织物在不同频率上计算得到的吸收百分比（%）。单层基于SiC的陶瓷织物的吸收百分比是\40%，导致它们不适合用作商业应用中的电磁波吸收材料。 为了获得更好的电磁波吸收能力，用收货状态和热处理后的陶瓷织物制作了几种双层组合。第1例中测量了收货状态的织物组合，S8织物组合的反射和传输损失均减小，显示出良好的吸收特性。图7所示为含有一层S8型织物的双层组合的吸收特性。低电导率层（S8织物）减小空气/低电导率层界面的反射损失，大部分穿过织物的电磁波被第2层表面反射。低电导率的第1层作为吸收层，高电导率的第2层作为反射层，将电磁能量约束在材料系统内部。低→高电导率层的组合实现了较高的吸收能力。据此，S8和ZE8织物间的最高电导率差导致最大化的吸收能力。此外，层的顺序也影响组合的吸收能力；例如S8-ZE8组合的吸收能力不同于ZE8-S8，尽管2种组合含有相同的织物层。对于S8-ZE8的组合，电磁波在穿过ZE8织物层之前先接触低电导率的S8织物层；而对于ZE8-S8组合，ZE8层先和电磁波发生作用，然后是S8层。由于ZE8织物的高反射损失，其吸收能力弱于ZE8-S8组合。 |
2楼2014-06-12 13:42:38