| Microwave heating technology has aroused widespread attention in the application field of activated carbon preparation,due to its high efficiency,uniformity,selectivity,etc..However,the thermal runaway phenomenon also has occurred in the microwave heating process, as the material’s strong microwave absorption ability brings about a rapid rise of temperature,so that the reactant is likely to be burned, which reduces the microwave safety and efficacy.The dielectric property of material determines the degree of microwave absorption in the preparation process of activated carbon. There is a close relation between dielectric property with its structure and composition,while the material structure and composition varies as the temperature of carbonization and activation process,raw material,as well as the activator.Correspondingly,the dielectric property varies as those factors.Therefore,it is very important to study the dielectric property of the process of preparing activated carbon in order to apply the microwave heating technology correctly and effectively and avoid thermal runaway phenomenon.In this thesis,raw experimental materials,such as corncob,coconut shell and bituminous coal,were prepared into the corresponding char samples through carbonization temperature of 300~900℃, then prepared activated carbon samples at different temperatures(corncob,coconut shell,800℃,900℃;coal,700℃,900℃)with different activators(KOH and ZnCl2).The dielectric properties of the samples were determined by using vector network analyzer in the frequency range of 2~18GHz, and the characterization methods such as X-ray diffraction, Fourier transform infrared spectroscopy and BET specific surface area test method were used,in order to analyze bio-chars in the process of structure evolution,and explore the effect mechanism of different conditions on the dielectric properties of bio-chars and the mechanism of microwave attenuation from the micro perspective.The carbonization of raw materials showed that, with carbonization temperature increases,the dielectric parameters( ?’,?" and tan?)show a increasing trend.(1)XRD analysis result shows that,with temperature increases,bio-chars crystallite carbon crystal plane spacing gradually decreases,while the crystal stacking height increases,the carbon crystallite structure of chars tend to orderly develop overall, which makes the dielectric properties increases with carbonization temperature increasing;(2)FT-IR analysis result shows that bio-chars aromatic degree and the ordered structure of the degree increased, which lead to the dielectric performance enhancement.(3)For three materials carbonized at given temperature of 300℃,500℃,dielectric parameters are small. As the carbonization temperature increases further,different materials have the different dielectric properties,namely,for corncob and coconut shell biomass materials,the dielectric parameters turn to increase at 700℃,but for bituminous coal,the turning temperature is 900℃.The activation showed that,The dielectric parameters of activated carbons in all kinds of raw materials show an increasing trend with the temperatureincrease in the activation process.(1)The effect of pore structure on dielectric properties is obtained:the dielectric properties of corncob,coconut shell are activated by 800℃,900℃ and bituminous coal are activated by 900℃are less than the same material chars at the corresponding temperature,While specific surface area is contrary to law.It concludes that the more developed the pore structure of activated carbon,the smaller the dielectric parameters.(2)Study on the effect of KOH,ZnCl2 two activators on the dielectric properties of activated carbon,the results show that the former of the dielectric properties of activated carbon were less than that of the latter, but specific surface area and the adsorption capacity showed the opposite pattern,suggesting that alkali metals K while enrich pore structure has an inhibitory effect on the dielectric properties of activated carbon samples. |
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