Breiss Hanadi, El Assal Aicha, Benzerga Ratiba, Méjean Chloé, Sharaiha Ala
Univ Rennes, CNRS, IETR-UMR 6164, F-35000 Rennes, France.
Micromachines (Basel). 2020 Dec 6;11(12):1081. doi: 10.3390/mi11121081.
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.
本文介绍了一种轻质环氧泡沫材料，其负载了重量百分比极低（≤0.5 wt.%）、不同长度（3毫米、6毫米和12毫米）的碳纤维（CF），用作电波暗室应用的宽带微波吸收材料。研究了CF长度对1至15吉赫兹频率范围内微波吸收的影响，特别是对吸收频带的影响。为了制备复合材料，使用了三种不同方法——刮刀法、剪切混合法和超声法——来分散CF。在分散步骤之后对这些CF的观察表明，与使用刮刀法或超声法时不同，使用剪切混合法时纤维断裂率很高。另一方面，对制备的复合材料的表征突出了混合方法、进而纤维断裂与复合材料测量的反射系数（反射损耗）之间的相关性。结果，当观察到纤维断裂时，反射系数的最小值向高频方向移动，这表明短CF在高频吸收，而长CF在低频吸收。从自由空间测量中提取的、由不同纤维长度（3毫米、6毫米和12毫米）制备的复合材料的介电性能证实，短CF（3毫米）在高频（约15吉赫兹）显示出最大损耗，而长CF（12毫米）在低频（低于4吉赫兹）显示出最大介电损耗。然而，对于这些负载极低CF率的多孔复合材料，相对介电常数实部未观察到随纤维长度的显著变化。制备并表征了一种包含不同CF长度混合的混合复合材料。基于这种混合复合材料的角锥吸收器的吸收性能模拟结果与基于负载单一长度碳纤维的复合材料的角锥吸收器的模拟结果进行了比较。基于混合复合材料的角锥吸收器预测具有最佳吸收性能，特别是在低频带。该吸收器的模拟反射系数在所有研究频率范围内均小于-12分贝，对于高于3吉赫兹的频率小于-40分贝。这一结果证实了使用不同长度碳纤维混合来实现宽带微波吸收器的意义。
