Development of electromagnetic wave absorber with woodceramics

近年来，随着蜂窝电话、局域网等新型通信系统的不断涌现，对GHz频段高频电磁波的利用率不断提高。因此，来自这些新系统或机器的高频电磁波频繁地影响医疗和航空电子设备，并引发许多危险的错误动作和事故。因此，这类材料的快速发展不仅要屏蔽有害波，而且要完全吸收以上有害波。另一方面，木质陶瓷(WCS)是由木材或木质材料(如中密度纤维板)在真空炉中碳化后浸渍液体酚醛树脂制成的新型多孔炭材料。浸渍的酚醛树脂在碳化过程中转变为硬玻璃态碳，增强了中密度纤维板中木纤维生成的软炭。本研究旨在利用…技术研制出对这种有害波具有良好吸收效果的吸波材料。浸渍酚醛树脂的中密度纤维板在6 5 0~70 0℃碳化处理后，在约7 GHz处有明显的电磁吸收(约5 0d B)。用同轴电缆法和网络分析仪测量了由30%酚醛树脂粉和70%质量%木粉组成的新型粉末炭化法制备的木质复合材料的电磁波吸收特性。将质量分数为30%的酚醛树脂粉和质量分数为70%的木粉在650GHz碳化，制得的木质陶瓷在2℃左右的频率处有较大的吸收峰(约50d B)。在起始混合物中加入碳化硅粉末，可使烧结复合材料的吸收峰向更高的频率移动，对于含47%碳化硅的样品，其吸收峰向9 GHz移动。通过改变MDF法的碳化温度或粉末法制备的碳化硅粉末的用量，可以使吸波材料适用于1~10 GHz的电磁波。较少

Recently, the utilization of high frequency electromagnetic waves of GHz range has increased with increasing new communication systems such as cellular phone and local area network. As a result, high frequency electromagnetic waves from such new system or machines have frequently affected medical and avionics equipments and induced many dangerous incorrect actions and accidents. Therefore, the fast development of such materials to not only shield but also absorb completely above harmful waves has been needed.On the other hand, woodceramics (WCS) are new porous carbon materials which are made by carbonizing wood or woody materials such as MDFs (medium-density fiberboards) impregnated with liquid phenolic resin in a vacuum furnace. The impregnated phenolic resin changes into hard glassy carbon during carbonizing process and reinforces the soft charcoal which originated from wood fibers in the MDF. This study has been done in order to develop the superior absorber for such harmful wave by … More using WCS.The WCS boards obtained by carbonizing the phenolic resin impregnated MDF at 650-700℃ showed remarkable electromagnetic absorption (about 50 dB) at about 7 GHz. The boards carbonized at 750-800℃ showed the absorption of about 40 dB at the frequency of about IGHz.The electromagnetic wave absorption characteristics of the WCS made by the new powder method, namely by carbonizing the compacted mixtures of 30 mass% phenolic resin powder and 70 mass% wood powder, were measured by the coaxial cable method using a network-analyzer. The woodceramics made by carbonizing the compacted mixture of 30 mass% phenolic resin powder and 70 mass% wood powder at 650℃ showed the large absorption peak (about 50 dB) at the frequency of about 2 GHz. The addition of SiC powder to the starting mixture shifted the absorption peak of the sintered composite to the higher frequency, for example, 9 GHz for the specimen containing 47 % SiC.It is possible to make the absorbers suitable for the electromagnetic waves of 1-10 GHz by changing the carbonization temperature in the MDF method or the amount of SiC powder in the powder method. Less