| 1.The precursors of carbon cryogels were prepared by sol-gel process via freeze-drying technique and pyrolysis method through changing solvent content, the ratios of the reactants and catalytic amount. The porous carbons (PCs) with tunable morphology and pore size were obtained by the decomposing precursors under nitrogen atmosphere at 900℃. The results of field-emission scanning electron microscope (FESEM) showed the 3D interonnected carbon framework had been constructed in the samples and macro-and meso-pores were visible in the images. According to X-ray diffraction (XRD), Raman spectroscopy and nitrogen sorption analysis, the graphitization degree were barely influenced as the ratios of tert-butanol (T) to resorcinol (R) decreased, while the pore morphologies changed from the disordered slit-shaped pores to the uniform cage-like pores. Additionally, the degrees of graphitization were enhanced gradually with the increasing ratios of furfural (F) to resorcinol (R) and resorcinol (R) to Hexamine (H), the pore morphologies transformed from the disordered slit-shaped pores to the uniform cage-like and tubular pores, respectively. Dielectric properties of the as-prepared carbon samples were determined by a vector network analyzer in the frequency range of 8.2-12.4 GHz. Results showed the pore morphology and graphitization degree influenced on the dielectric loss performance of PCs. When the T/R ratio was 7.5, the sample with cage-like pores revealed the maximum values in the real part ε’ and the imaginary part ε" of complex permittivity, which were 13.2-6.5 and 15.6-10.1, respectively. When the F/R ratio was 2.5, the sample with higher graphitization degree and cage-like pores displayed the maximum ε’ and ε" values, which were 12.2-11 and 12.4-10, respectively. However, when the R/H ratio was 75, the sample with higher graphitization degree and tubular-like pores displayed the maximum ε" value, which was 4.5-2. The proposed mechanism for the effect of the pore morphologies and graphitization degree on microwave absorption performance was discussed.2.According to the FESEM analysis, the Co-doped PCs revealed the same 3D interconnected carbon framework, implying a barely influence on morphology by the Co level and temperature. The results of the XRD showed the pure Co and Co3C composite presented in the Co-doped PC samples in addition to the major graphite component. Moreover, the Co content and temperature impacted on the graphitization degree of the PCs. Electromagnetic properties of the as-prepared carbon samples with the different Co levels were determined by a vector network analyzer in the frequency range of 8.2-12.4 GHz. Results showed that values of ε’, ε" and dielectric loss factor tanδE of the PC with 5% Co-doped displayed the maximum, which were 10.75,4.5 and 0.45, respectively. Its dielectric loss performance was obviously improved as compared to the undoped PC sample. In addition, the dielectric loss performance of the PC sample with 10% Co-doped was improved as the temperature increasing. The values of ε’ and ε" reached to the maximum at 1100℃ and 900℃, which were 9.8 and 2.15, respectively. The magnetic loss performance of the PCs was barely influenced with the increasing Co-doped content. However, the real part μ’ and the imaginary part μ" of complex permeability of the 10% Co-doped PC sample increased with the reducing frequency at 800℃.3.According to the FESEM analysis, the Fe-doped PCs revealed the same 3D interconnected carbon framework, implying a barely influence on morphology by the Fe level and temperature. The results of the XRD showed the pure Fe and FeC composite presented in the Fe-doped PC samples in addition to the major graphite component. Moreover, the Fe content and temperature impacted on the graphitization degree of the PCs. Electromagnetic loss properties of the as-prepared carbon samples with the different Fe levels were determined by a vector network analyzer in the frequency range of 8.2-12.4 GHz. Results showed the PC with 10% Fe-doped displayed the maximum ε’ value, which was 12.75. However, the PC with 15% Fe-doped revealed the maximum ε" and tan8E, which was 4.5 and 0.45, respectively. Its dielectric loss performance was obviously improved as compared to the undoped PC sample. In addition, the dielectric loss performance of the PC sample with 10% Fe-doped was improved as the increasing temperature. The values of ε’ and ε" reached to the maximum at 1100℃, which were 13.6 and 5.0, respectively. The magnetic loss performance of the PCs was influenced by the Fe-doped content in the low frequency range. But it was barely affected by the temperature. |
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