Development of electromagnetic wave absorber with woodceramics

近年来，随着蜂窝电话和局域网等新型通信系统的兴起，对GHz频段高频电磁波的利用日益增加。因此，这些新系统或机器产生的高频电磁波经常影响医疗和航空电子设备，并引起许多危险的错误动作和事故。因此，需要快速发展既能屏蔽又能完全吸收以上有害波的材料。另一方面，木陶瓷（WCS）是将木材或木质材料如MDFs（中密度纤维板）在真空炉中浸渍液体酚醛树脂而制成的新型多孔碳材料。浸渍的酚醛树脂在炭化过程中变成坚硬的玻璃碳，强化了中纤板中源自木质纤维的软炭。本文的研究目的是为了开发一种更好的吸收这种有害波的材料。酚醛树脂浸渍MDF在650 ~ 700℃炭化得到的WCS板在约7 GHz处有显著的电磁吸收（约50 dB）。750 ~ 800℃炭化板在约IGHz频率处的吸收约为40 dB。采用新型粉末法，即用质量为30%的酚醛树脂粉和质量为70%的木粉进行炭化，用同轴电缆法测定了其电磁波吸收特性。将质量分数为30%的酚醛树脂粉与质量分数为70%的木粉混合在650℃下炭化而成的木陶瓷在2 GHz频率处有较大的吸收峰（约50 dB）。在起始混合物中加入SiC粉末使烧结复合材料的吸收峰向更高的频率移动，含47% SiC的试样的吸收峰为9 GHz。通过改变MDF法的碳化温度或粉末法中SiC粉的用量，可以使吸收器适用于1-10 GHz的电磁波。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