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Table 2 shows the electrical conductivity measurement results of the as-received and heat-treated SiC-based ceramic fibers. The as-received ZE8 fiber had the highest electrical conductivity among the samples, which can be explained by its higher Si and C content when compared with the other two fibers. After heat treatment, the ZE8 fiber maintained this tendency, acquiring the highest conductivity value among the heat-treated fibers. The results of the electrical conductivity measurements of the as-received and heat-treated SiC-based ceramic fibers showed that electrical conductivity decreases significantly following heat treatment, accounted for by the formation of an oxide layer around the fibers. The lack of an explicit continuous oxide layer could have resulted in the lower electrical conductivity decrease in the ZE8-type fiber. Interaction of ceramic woven fabrics with electromagnetic radiation The parameters used to quantify the interaction of electromagnetic waves with the woven fabrics—reflection loss,RdB, and transmission loss, TdB—were measured in the 17–40 GHz frequency range using the free-space method. The fractions of reflected and transmitted waves were calculated using Eq. 2, where P0, PR, PTshow the incident, reflected, and transmitted wave powers, respectively. The changes in reflection loss in the as-received and heat-treated SiC-based ceramic woven fabrics are presented in Fig. 5a and b, respectively, measured in decibels (dB) as a function of frequency. A decrease in reflection loss (increasing negative dB) indicates that the reflected portion of the electromagnetic wave from the material surface is diminished. The ZE8 fabric recorded the highest reflection loss among all the as-received fabrics in the 17–40 GHz frequency range (Fig. 5a). Electrical conductivity has a marked effect on the reflection loss of ceramic woven fabrics. Increases in electrical conductivity result in more reflected fraction from the materials, and thus an increase in reflection loss, whereas lower electrical conductivity leads to less reflection from the material surface, and hence lower reflection loss. This relation can be explained by the increase in the electromagnetic impedance of the material as conductivity decreases. Consequently, the level of impedance mismatch to air becomes lower, reducing the reflection loss of the material. In line with this explanation, the ZE8 fabric retained the highest reflection loss following heat treatment due to its highest level of electrical conductivity (Fig. 5b). A comparison of reflection losses of the as-received and heat-treated fabrics revealed a distinct behavior of the PN fabric. At low frequencies, reflection loss of the as-received PN fabric was about -5 dB, corresponding to a ~31%reflection of the electromagnetic wave; while at similar frequencies reflection loss of the PN-H fabric decreased to -14 dB with a ~3% reflection of the electromagnetic wave. The PN-type fabric is composed of carbon-coated SiC-based fibers, and the high conductivity of the carbon yielded higher reflection losses in the as-received condition. However, after heat treatment in air, as a result of the loss of the carbon coating and the formation of an oxide layer the electrical conductivity of the fiber decreased, causing a considerable difference between the reflection losses of the as-received and PN-H fabrics. |
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表2所示为收货状态和经热处理的基于SiC的陶瓷纤维的电导率测量结果。收货状态的ZE8纤维在全部样件中电导率最高，可解释为Si和C含量高于其他2种纤维。热处理后，ZE8纤维维持这种趋势，获得热处理纤维中最高的电导率。收货状态和热处理后的基于SiC的陶瓷纤维的电导率测量结果表明，热处理后电导率显著下降，可解释为纤维周围氧化层的形成。缺乏明显的连续氧化层导致ZE8型纤维的电导率下降。 陶瓷织物与电磁辐射的相互作用 用于量化电磁波与织物间相互作用的参数-反射损失，RdB和传输损失，TdB在17-40GHz频率范围通过自由空间法测量。反射和传输波的部分通过式2计算，P0、PR、PT分别表示发射、反射和传输波功率。 收货状态和热处理后的基于SiC的陶瓷织物的反射损失变化分别如图5a、b所示，作为关于频率的函数测量其分贝值（dB）。反射损失的下降（负dB增加）表示材料表面的电磁波反射部分减弱。在17-40GHz频率范围上，ZE8织物在所有收货状态织物中显示出最高的反射损失（图5a）。电导率对陶瓷织物的反射损失有明显影响。电导率增加导致材料反射更多的电磁波，使反射损失增加，而低电导率使材料表面的反射减少，于是反射损失降低。材料的电磁阻抗增加将这种关系解释为电导率下降。于是，与空气之间的阻抗差距降低，减少了材料的反射损失。根据这种解释，ZE8织物因其最高的电导率水平在热处理后保持最高的反射损失（图5b）。 收货状态和热处理后织物的反射损失比较揭示了PN织物一个显著特性。低频率上，收货状态的PN织物的反射损失约-5dB，对应31%的电磁波反射；在相似的频率上，PN-H织物的反射损失降到-14dB，电磁波的反射为3%。PN型织物由涂碳的SiC基纤维构成，碳的高电导率导致收货状态的高反射损失。然而，经过空气中热处理，碳层的损失和氧化层的形成使纤维电导率下降，导致收货状态和热处理后PN织物的反射损失出现显著差距。 |
2楼2014-06-12 14:22:09