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Long Carbon Fibers for Microwave Absorption: Effect of Fiber Length on Absorption Frequency Band

机译：用于微波吸收的长碳纤维：纤维长度对吸收频带的影响

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This work presents lightweight epoxy foams loaded with very low weight percentages (≤0.5 wt.%) of carbon fibers (CFs) with different lengths (3 mm, 6 mm, and 12 mm) as broadband microwave absorbing materials for anechoic chamber application. The effect of CF length on microwave absorption, especially on the absorption frequency band, is investigated for frequencies between 1 and 15 GHz. For the elaboration of composites, three different methods—spatula, shear mixing, and ultrasounds—are used for the dispersion of CFs. The observation of these CFs, after the dispersion step, shows a high fiber breakage rate when shear mixing is used, unlike when spatula or ultrasounds methods are used. On the other hand, the characterization of the elaborated composites highlights a correlation between the mixing methods, hence the fiber brakeage, and the measured reflection coefficient (reflection loss) of the composites. As a result, the minimum value of the reflection coefficient is shifted toward the high frequencies when the fiber breakage is observed, suggesting that short CFs absorb at high frequencies while long CFs absorb at low frequencies. Dielectric properties, extracted from the measurement in free space, of composites elaborated with different fiber lengths (3 mm, 6 mm, and 12 mm) confirm that short CFs (3 mm) show maximum losses at high frequencies (around 15 GHz) while long CFs (12 mm) show maximum dielectric losses at low frequencies (below 4 GHz). However, no significant variation is observed on the real part of the relative permittivity, as a function of fiber length, for these porous composites loaded with very low CF rates. A hybrid composite, with a mix of different CF lengths, is prepared and characterized. The simulation of the absorption performance of a pyramidal absorber, based on this hybrid composite, is compared to the one of pyramidal absorber based on composites loaded with a single length of carbon fibers. The pyramidal absorber-based hybrid composite predicts the best absorption performance, especially at the low frequency band. The simulated reflection coefficient of this absorber is less than −12 dB in all the studied frequency range, and less than −40 dB for frequencies higher than 3 GHz. This result confirms the interest of using a mix of carbon fiber lengths to achieve a broadband microwave absorber.
机译：该工作呈现了具有非常低的重量百分比（≤0.5重量％）的轻质环氧泡沫（≤0.5重量％）碳纤维（CFS），其具有不同长度（3mm，6mm，12mm）的宽带微波吸收材料，用于解开腔室应用。 CF长度对微波吸收的影响，特别是在吸收频带上，针对1至15GHz之间的频率进行研究。为了阐述复合材料，三种不同的方法 - 刮刀，剪切混合和超声用于CFS的分散。在分散步骤之后，观察这些CFS，显示使用剪切混合时的高纤维破裂率，与使用刮刀或超声波方法时不同。另一方面，精制复合材料的表征突出了混合方法之间的相关性，因此是复合材料的纤维增压和测量的反射系数（反射损耗）之间的相关性。结果，当观察到纤维破裂时，反射系数的最小值朝向高频移位，表明短CFS在高频时吸收，而长CF在低频上吸收。从自由空间中的测量中提取的介电性质，复合材料用不同的纤维长度（3mm，6mm，12mm）确认，短CFS（3mm）显示高频（长约15GHz）的最大损耗CFS（12 mm）显示低频下的最大介电损耗（低于4 GHz）。然而，对于具有非常低CF速率的多孔复合材料，在相对介电常数的实际部分上没有观察到具有显着的变化。制备和表征具有不同CF长度的混合物的杂种复合材料。基于该杂合复合材料的金字塔型吸收器的吸收性能的模拟与基于装有单个碳纤维的复合材料的金字塔镜吸收器之一进行比较。基于金字塔吸收剂的混合组合材料预测了最佳吸收性能，尤其是在低频带。在所有研究的频率范围内，该吸收器的模拟反射系数小于-12dB，对于高于3GHz的频率小于-40dB。该结果证实使用碳纤维长度混合来实现宽带微波吸收器的兴趣。

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