Study On The Microwave Absorption Properties Of Transition Metal And TiO₂Nanocomposites

In this thesis, the nanocomposites composed of transition metals and TiO2wereprepared by the mechanochemical synthesis plus annealing. Crystalline structure,microstrain and microstructure of the samples were characterized by means of X-raydiffraction (XRD) and Scanning Electron Microscopy (SEM). The magnetic propertieswere measured using a Vibrating Sample Magnetometer (VSM). Measurement andanalysis of luminescence properties were performed by using Hitachi F-4500Fluorescence Spectrometer.The frequency dependence of electromagnetic parameters ofthe samples was measured by HP8722ES type Network Vector Analyzer in2-18GHzrange. We mainly focus on the effect of annealing on the changes of grain size,microstructure and microstrain, and the influence of these changes on magnetic,photoluminescence and microwave-absorption properties of the nanocomposites.The Ni/TiO2nanocomposites were prepared by mechanochemical synthesis plusannealing at500oC for half an hour. Annealing enlarges the grain size and releases thelarge microstrain in the Ni/TiO2nanocomposite prepared by mechanochemical synthesis.This greatly enhances the dielectric resonance and a clear Cole-Cole semicircle isobserved for the annealed sample, for which a maximum of reflection loss (RL) as largeas-45.2dB at12.6GHz with RL values exceeding10dB in the whole Ku-band isobtained for a layer of2.3mm thickness. In particular, a20dB bandwidth over thefrequency range from16.4to18GHz with a RL value of41dB is found at17.1GHzfor2.1mm thickness. The great improvement of microwave-absorption properties afterannealing can be ascribed to an enhanced dielectric resonance and an excellent match ofmagnetic and dielectric loss.Nanocomposites consisting of excessive Ni and TiO2were prepared bymechanochemical synthesis. The as-milld sample shows reflection peaks of hexagonal-structure rutile TiO2and face-centered cubic structure Ni. Annealing in vacuum releases the internal stress, decreases the lattice defects and enlarges the grain sizes of the metalNi, leading to an increase of the saturation magnetization and a decrease of the surfaceanisotropy. After annealing, the intrinsic, free exciton and bound exciton emission peaksof TiO2exhibit red shift. This comes from distortion of the band structure. The intensenonlinear dielectric resonance occurs at15GHz for the as-milled nanocomposite and aCole-Cole semicircle is presented in complex plane. A reflection loss (RL) exceeding-10dB was calculated in14-16GHz for an absorber thickness of8mm, the optimal RLwas-32dB. For annealed samples, the intensity of the natural resonance at2.8and5.2GHz was significantly strengthened so that the properties of microwave absorptionbetween2-6GHz ranges were significantly improved.NiCo/TiO2nanocomposites composed of tetragonal TiO2(rutile) and cubic NiCowere prepared by ball milling in Ar atmosphere and then annealed at500oC for half anhour. Phase compositions, morphology, magnetic properties and electromagnetismparameters of the NiCo/TiO2nanococomposites were examined by X-ray diffraction,scanning electron microscope, vibrating sample magnetometer and vector net workanalyzer. Compared with the as-milled samples, Bragg reflections of NiCo for theannealed sample were clearly narrowed, which suggested a larger grain size and asmaller microstrain, leading to an increase of saturation magnetization intensity, and adecrease of coercive force. The samples show the behavior of dielectric relaxation andmagnetic resonance, and annealing optimizes the microstructure, which leads tosignificant strengthen of interface polarization and polarization loss in the frequencyrange12-16GHz, while the optimal reflection loss decreases from-6.4dB to-14.8dBfor8mm thickness. After annealing the nature resonance peaks at2.6and5.3GHz, andthe exchange resonance peaks at10.5,13and15.6GHz are greatly enhanced.FeCo/TiO2nanocomposites, prepared by mechanochemical synthesis, exhibitfluctuation of ε′and an intense dielectric resonance peak of ε″at13.5GHz. Thedielectric resonance was greatly weakended after annealing at500°C, as a result ofwhich the clear Cole-Cole full circle disappeared. This may be mainly attributed to thechange of conductivity and the microstructure. In the two-phase interface of the nanocomposites, dielectric resonance phenomenon generated by the appropriateconductivity can make real and imaginary part of the complex permittivity (ε′-ε″)onthe complex plane Cole-Cole plot show a full circle. For the as-milled sample, amaximum of reflection loss (RL) as large as-38.3dB at12.4GHz for a layer of8.1mmthickness. Howerver, for the milled sample, a RL value exceeding-10dB was obtainedin11-16GHz, almost covering the entire Ku-band (12-18GHz). The great improvementof microwave-absorption properties for as-miiled sample can be ascribed to a dielectricresonance and an excellent match of magnetic and dielectric loss.