| Electromagnetic-wave absorbing materials. are functional materials that may attenuate electromagnetic-wave energy, have been widely used in military and micro-electronic industries. M-type hexaferrite and carbonyl iron are two kinds of electromagnetic-wave absorbers with excellent property, which have been extensively studied and widely applied. However, they both have some insuperable disadvantages and cannot intensively absorb multi-frequency electromagnetic waves in a broad frequency band. There are some basic issues for M-type hexaferrite to need to study systematically such as effects of ball-milling on the structure and properties of micro-sized M-type fexaferrite, precipitation mechanism and its affecting factors during synthesizing submicro-sized M-type hexaferrite by chemical coprecipitation method, structure and properties of non-chiometric M-type hexaferrite with barium surplus, synthesis condition of single-phase nano-sized M-type hexaferrite with stable structure, and effects of crystal grain size on electromagnetic-wave absorbing property of M-type hexaferrite. In addition, it is also an unclear and unreported issue that electric and magnetic interaction mechanisms take place in the composite of M-type hexaferrite and carbonyl iron and how they affect electromagnetic-wave absorbing property of the composite. At the same time, how to improve the oxidation resistance of carbonyl iron is still not resolved.The thesis has investigated above-mentioned basic issues by two special studies: (1) Synthesis, characterization and properties of micro-, submicro- and nano-sized single-phase BaCoTiFe10O19 (namely multi-scale M-type hexaferrite), (2) Synthesis, characterization and properties of iron-SiO2/M-type hexaferrite core-nanoshell composite particles. The first one has mainly discussed some basic issues relative to M-type hexaferrite microwave absorber, and searched for process conditions of fabricating M-type hexaferrite nanoshell on the surface of carbonyl iron and iron-SiO2 core-nanoshell composite particles. The second one has for the first time studied the electric-magnetic interaction mechanism in the composite of carbonyl iron and M-type hexaferrite. The main results from these studies are summarized as following:1. Synthesis, characterization and properties of multi-scale M-type hexaferrite (1) The synergetic coprecipation effect of cations and coordination effect ofanions or complex ions, occurring during the coprecipitation reaction of preparing BaCoTiFe10O19 precursor, was for the first time discovered by systemically investigating the coprecipitation mechanism of Fe3+, Ti4+, Co2+ and Ba2+ cations. The conclusion has important guiding significance in studying coprecipitation condition of other multi-cations system.(2) A series of single-phase non-chiometric M-type hexaferrite with barium surplus Ba1+xCoTiFe10O19+x(x=0.00, 0.05, 0.10, 0.15, 0.20) have been for the first time successfully synthesized at 900℃ heating for 2 h by chemical coprecipitation method. It was discovered that specific saturation magnetization value (σs) of Ba1+xCoTiFe10O19+x are almost no change and remnant magnetization (σr) and coercivity (Hc) reach maxima when x=0.05, and that the electric domain polarization around excess barium may result in resonance dielectric loss at about 12.48 and 15.28 GHz. These results have important significance in manufacturing new electromagnetic-wave absorbing materials with multi-frequency bands in the range GHz.(3) Micro-, submicro-, and nano-sized single-phase BaCoTiFe10O19 were synthesized by conventional ceramic method, chemical coprecipiation method, and sol-gel process, respectively. The dielectric properties in the range 2-18 GHz show that the intrinsic electric energy of BaCoTiFe10O19 is inversely proportional to grain diameter in the range 2-8 GHz, and that a polarization resonation peak of BaCoTiFe10O19 nanoparitcles, whose formation mechanism is the same as that of micro-sized BaCoTiFe10O19 at about 16.24 GHz after having been ball-milled for 48 hours, occurs at ab...
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