Study On Electromagnetic Shielding Properties Of Biomass Conductive Charcoal Composites

With the rapid development of modern electronic industry and the sharp increase of electronic products, electromagnetic radiation cannot be ignored. Meanwhile, biomass is renewable resource and abundant, especially crop stalk, with 700 million tons of annual output. Using crop stalk to prepare electromagnetic shielding materials can effectively control electromagnetic pollution, which is the new direction of biomass resource utilization.Conductive biomass charcoal was prepared from biomass carbonized at high temperature. By mixing conductive charcoal with high-density polyethylene (HDPE) matrix, a new kind of polymer composite was prepared. The effects of the precusor, content and particle size of charcoal on the conductivity and electromagnetic shielding efficiency (EMSE) of the composite were studied. Results showed that, when filled with cotton stalk charcoal or bamboo charcoal, the best EMSE was achieved. EMSE increased with the amount of biomass charcoal content. When the mass fraction of the charcoal was 60%, the EMSE reached up to 18 dB and 98.4% of electromagnetic wave was shielded at the frequency of 900 MHz. When the particle size was reduced, the EMSE of the composite increased before certain point and then decreased. The particle size of 125-150μm was the optimum for EMSE of the composite as well as for the conductivity.In order to improve the EMSE, effects of adding a small amount of carbon black and metal powder into the composite on the EMSE of biomass charcoal/HDPE composites were studied respectively. Effects of acid-washing and potassium hydroxide (KOH) activation of conductive charcoal respectively were also studied. Several results was obtained as following. (1) Adding 5% of carbon black into the composite could significantly improve the EMSE and the shielding effectiveness increased from 68.4% to 93.7% when the carbon mass fraction was 50%. (2) The EMSE of the composite was compared when different metal powder such as iron (Fe), chromium (Cr), nickel (Ni) and wolfram (W) were added respectively. The composite filled with Fe achieved the best electromagnetic shielding efficiency. Adding 5% of Fe into the composite could improve the shielding effectiveness from 60.2% to 92.1% when the conductive charcoal mass fraction was 40%. It was analyzed that when metal powder was chosen as addictive, the metal with low density and excellent magnetism was the best option for both electromagnetic shielding efficiency and absorption efficiency. (3) Hydrochloric acid (HCl) washing of conductive charcoal could improve the EMSE of the composite and the shielding effectiveness increased 12.2%; While nitric acid (HNO3) washing of conductive charcoal deteriorated the EMSE. (4) KOH activation could improve the conductivity and pore structure of conductive charcoal, thereby improving the EMSE of the composite. When the charcoal was activated by KOH, shielding effectiveness increased 32.7%.By mixing conductive charcoal with high-density polyethylene (HDPE) matrix at room temperature, conductive charcoal/CMC composites were prepared. The composite performed excellent electromagnetic shielding efficiency, as well as good pore structure which promotes the adsorption ability of organic pollutants. As a result, it was a kind of new multi-functional composite materials. When the mass fraction of the conductive charcoal was 93.0%, the electromagnetic shielding efficiency reached up to 35 dB and 99.97% of electromagnetic wave was shielded at the frequency of 900 MHz. For toluene adsorption, it could reach108mg/g.