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wanghaiju

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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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wanghaiju: 金幣+36, 翻譯EPI+1, ★★★★★最佳答案 2014-06-12 18:10:33

表2所示為收貨狀態(tài)和經(jīng)熱處理的基于SiC的陶瓷纖維的電導(dǎo)率測(cè)量結(jié)果。收貨狀態(tài)的ZE8纖維在全部樣件中電導(dǎo)率最高，可解釋為Si和C含量高于其他2種纖維。熱處理后，ZE8纖維維持這種趨勢(shì)，獲得熱處理纖維中最高的電導(dǎo)率。收貨狀態(tài)和熱處理后的基于SiC的陶瓷纖維的電導(dǎo)率測(cè)量結(jié)果表明，熱處理后電導(dǎo)率顯著下降，可解釋為纖維周圍氧化層的形成。缺乏明顯的連續(xù)氧化層導(dǎo)致ZE8型纖維的電導(dǎo)率下降。

陶瓷織物與電磁輻射的相互作用

用于量化電磁波與織物間相互作用的參數(shù)-反射損失，RdB和傳輸損失，TdB在17-40GHz頻率范圍通過自由空間法測(cè)量。反射和傳輸波的部分通過式2計(jì)算，P0、PR、PT分別表示發(fā)射、反射和傳輸波功率。
收貨狀態(tài)和熱處理后的基于SiC的陶瓷織物的反射損失變化分別如圖5a、b所示，作為關(guān)于頻率的函數(shù)測(cè)量其分貝值（dB）。反射損失的下降（負(fù)dB增加）表示材料表面的電磁波反射部分減弱。在17-40GHz頻率范圍上，ZE8織物在所有收貨狀態(tài)織物中顯示出最高的反射損失（圖5a）。電導(dǎo)率對(duì)陶瓷織物的反射損失有明顯影響。電導(dǎo)率增加導(dǎo)致材料反射更多的電磁波，使反射損失增加，而低電導(dǎo)率使材料表面的反射減少，于是反射損失降低。材料的電磁阻抗增加將這種關(guān)系解釋為電導(dǎo)率下降。于是，與空氣之間的阻抗差距降低，減少了材料的反射損失。根據(jù)這種解釋，ZE8織物因其最高的電導(dǎo)率水平在熱處理后保持最高的反射損失（圖5b）。
收貨狀態(tài)和熱處理后織物的反射損失比較揭示了PN織物一個(gè)顯著特性。低頻率上，收貨狀態(tài)的PN織物的反射損失約-5dB，對(duì)應(yīng)31%的電磁波反射；在相似的頻率上，PN-H織物的反射損失降到-14dB，電磁波的反射為3%。PN型織物由涂碳的SiC基纖維構(gòu)成，碳的高電導(dǎo)率導(dǎo)致收貨狀態(tài)的高反射損失。然而，經(jīng)過空氣中熱處理，碳層的損失和氧化層的形成使纖維電導(dǎo)率下降，導(dǎo)致收貨狀態(tài)和熱處理后PN織物的反射損失出現(xiàn)顯著差距。

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2樓2014-06-12 14:22:09