| The microwave absorbing materials play an important role in the military and civil fields by which the incidence electromagnetic wave can be absorbed and electromagnetic radiation is attenuated. The aim of this paper is to prepare the high quality microwave absorbers. The preparation, structures and properties of metallic Fe chips, M-type barium ferrite, modified cenospheres and ZnO doped by Al and Co were investigated systematically. Electromagnetic loss and microwave absorbing mechanism of these materials in microwave range were discussed.Flaky Fe powders with thickness about 0.5μm were obtained by pyrolyzing the existing oil on the surface of the industrial waste Fe chips. The results of microwave electromagnetic tests show that Fe powders possess magnetic loss performance in the frequency range of 2-18 GHz. The real partμ′and the imaginary partμ″of permeability and magnetic loss factor tanδM of the samples are 0.770-1.283,0.029-0.471 and 0.031-0.517, respectively. The results of reflection show that attenuation of electromagnetic wave in the sample increases with the increasing content of Fe powders in the rubber, and the frequency of maximum attenuation shifts to lower frequency. When the Fe powder content is 33.3%, there is the maximum attenuation of -14.371dB. The frequency width of the attenuation less than -10dB is 2.56GHz. The attenuation of electromagnetic wave of the sample increases as the thickness of sample increases, and the frequency of maximum attenuation also shifts to lower frequency. When the thickness of sample is 5.4mm, there is the maximum attenuation of -15.719dB. The frequency width of the attenuation less than -10dB is 1GHz.The precursors of hexagonal barium ferrite and ferrite yellow coated by colloid BaCO3 were prepared via co-precipitation molten-salt and precipitation-topotactic reaction methods, respectively. The flaky and needle-like hexagonal barium ferrite powders were obtained by calcining the precursors at 850℃and 1000℃, respectively. The magnetic loss properties show the values ofμ',μ" and tanδM for the flaky and needle-like samples are similar in the frequency range of 2-18 GHz. The magnetic loss of the flaky sample is greater than that of needle-like one on the whole. The values ofμ',μ" and tanδM for the flaky sample are 0.847-1.110,0.031-0.224 and 0.028-0.231, respectively. The results of reflection test show that both the reflection of the flaky and needle-like samples are less than -5dB in the frequency range of 2-18 GHz. The minimum reflections are -18.197dB and -18.282dB, respectively. The frequency width less than -10dB is up to 10.24GHz.The surfaces of cenosphere particles were activated by colloid solution of Sn-Pd, and the metallic Co, Ni, Co-Fe and Ni-Fe thin films were plated on these surfaces by electroless method, respectively. The continuous and uniform coatings deposited on the cenospheres are composed of nano-particles. The properties of magnetic loss show that the values ofμ',μ" and tanδM for the modified cenospheres have no obvious increases in the frequency range of 2.6-12GHz, but they increase obviously in the frequency range of 12-18GHz. The results of reflection show the attenuation of electromagnetic wave in the cenospheres deposited by Co and Co-Fe thin films are obviously greater than that of the cenospheres deposited by Ni and Ni-Fe thin films, where the maximum attenuation of the cenosphere with Co film reaches to -24dB at 6.48GHz and the frequency width less than -10dB is maximal, with the value of 8.88GHz.The precursor of ZnO was prepared by the solid state reaction. The effect of the calcined temperature, holding time and synthesis atmosphere on the microwave dielectric properties of ZnO powders were investigated. Results show that the wurtzite structure ZnO powders were produced by calcining the precursors at 400-800℃with holding time of 1.5h, at 600℃with different holding times and at 800℃in air, hydrogen and nitrogen atmospheres. The results of dielectric properties show that both the real partε′and the imaginary partε″of permittivity of the samples increase first and then decrease as the calcined temperature increases in the frequency range of 8.2-12.4 GHz, reaching a maximum values at 700℃, which are 2.80 and 0.52 in average, respectively. The maximum value of dielectric loss factor tanδE is about 0.19 at 800℃. The valuesε′andε″of the ZnO powders prepared with different holding times increase slightly with the increasing time, which are 2.52-2.62 and 0.4-0.49, respectively. The value of tanδE has no obvious change with increasing time. The ZnO powder prepared in hydrogen atmosphere has the greatest values inε′,ε″and tanδE than the samples obtained in air and nitrogen atmospheres, which are about 7.5, 2.5 and 0.3, respectively.The band structure, state density and complex permittivity of the pure ZnO and the Al-doped ZnO were studied by using the first-principle ultrasoft pseudopotential approach of the plane wave based on the density function theory. Results show that the volume of super cell has no obvious change and Fermi energy level introduces into conduction band through introducing Al ions. The values ofε′andε″for the ZnO with doping Al ions increase at low energy.The precursors of ZnO with doping different Al content were prepared by the solid state reaction. The wurtzite structure ZnO powders were prepared by calcining the 10at% Al-doped precursors under 400-800℃for 1.5h and the precursors with different Al content at 800℃for 1.5h, respectively. The results of XPS (X-ray photoelectric spectrum) show that the partial Zn2+ ions are substituted by Al3+ ions in the Al-doped ZnO. The results of dielectric properties for Al-doped ZnO powders show that all theε',ε" and tanδE of the 10at% Al-doped ZnO powder prepared at different calcined temperatures increase firstly and then decrease with the increasing temperatures in the frequency range of 8.2-12.4 GHz, reaching maximum values at 600℃which are about 4.32, 2.34 and 0.56, respectively. The value ofε',ε" and tanδE increases obviously as Al-doped content increases. Bothε' andε" reveal the greatest values when the Al-doped content is 20at% which are about 4.73 and 2.51, respectively. The value of tanδE reaches maximum value of 0.56 at 10at% content. The experimental results show that bothε′andε" of the samples are improved by Al doping, in agreement with the result calculated by first-principle.The precursors of ZnO with doping different Co content were also prepared by the solid state reaction. The wurtzite structure ZnO powders were prepared by calcining the 5at% Co-doped precursors at 400-800℃for 1.5h , the precursors with different Co content at 600℃for 1.5h and the 5at% Co-doped precursor at 800℃in different atmospheres, respectively. The results of XPS show that the partial Zn2+ ions are substituted by Co2+ or Co2+ and Co3+ ions in the Co-doped ZnO prepared at 600oC, and that the partial Zn2+ ions are substituted by Co2+ ions in the Co-doped ZnO prepared at 800oC in air and nitrogen atmospheres. But the partial Zn2+ ions are substituted by Co2+ and Co3+ ions in the Co-doped ZnO prepared at 800oC in hydrogen atmosphere.The results of electromagnetic loss for Co-doped ZnO powders show that all theμ',μ" and tanδM of the 5at% Co-doped ZnO powder prepared at different calcined temperature increase firstly and then decrease with the increasing temperature in the frequency range of 8.2-12.4 GHz, reaching maximum values at 600℃, which are about 0.98, 0.46 and 0.52, respectively. The values ofμ' andμ" of the samples increase gradually with the increasing Co content, reaching maximum values of 1.03 and 0.54 at 15at% content, respectively. But the value of tanδM increases firstly and then decrease, reaching maximum values of 0.54 at 10at% content. The sample prepared in hydrogen atmosphere has the greater values inμ′,μ″and tanδM than the samples prepared in air and nitrogen atmospheres, which are 9.68, 6.37 and 0.94, respectively. The sample prepared in hydrogen atmosphere has also the greater values inε′,ε″and tanδE than the samples prepared in air and nitrogen atmospheres, which are 5.27, 1.21 and 0.239, respectively.
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