Study On The Microwave Absorption And Optical Properties Of Transition Metal Doped Al₂O₃

In this paper, Ni/Al2O3and Fe/Al2O3nanocomposites were successfully preparedby ball milling method. Al2O3and Cr3+doped Al2O3thin films were grown onsingle-crystalline silicon substrates by means of pulsed laser deposition (PLD). Themicrostructure, surface appearance, magnetic and optical properties were characterizedby X-ray diffraction (XRD), scanning electron microscope (SEM), spectrophotometerand vibrating sample magnetometer (VSM). The microwave-absorption was measuredby vector network analyzer. We studied the effect of annealing on the microwave-absorption properties of Ni/Al2O3and Fe/Al2O3nanocomposites. Meanwhile theinfluence of different deposition conditions on luminescence properties of the Al2O3thin film was analyzed.1. Ni/Al2O3nanocomposites were prepared by the mechanochemical synthesismethod, and then annealed at100C and700C at Ar atmosphere. Annealing enlargesthe grain size of both the metal Ni and the insulating Al2O3in the as-millednanocomposite and leads to an increase of the saturation magnetization and a decreaseof the surface anisotropy. An optimal reflection loss (RL) of-23dB is obtained in theas-milled nanocomposite at17.8GHz for an absorber thickness of6.6mm. Theannealed sample at700C exhibits a RL exceeding-20dB in the whole Ku-band for anabsorber thickness of6.6-9.7mm with an optimal RL of-54.7dB at13.2GHz for alayer thickness of9.3mm. The excellent microwave-absorption properties are aconsequence of a larger dielectric loss and magnetic loss.2. Fe/Al2O3nanocomposites were prepared by the ball milling method, and thenannealed for0.5h at Ar atmosphere at700C. Effect of annealing on the microstructureand microwave-absorption properties of Fe/Al2O3nanocomposites was studied.Annealing enlarges the grain size, decreases the lattice defects and releases themicrostrain; meanwhile, the strength of photoluminescence peaks of Al2O3decreasessignificantly, and the peaks at398nm and484nm exhibit a blue shift. An optimal reflection loss of-11.4dB is obtained in the as-milled sample at15.5GHz with anabsorber thickness of7.7mm. After annealing, both dielectric loss and magnetic lossincrease and the microwave absorption properties have been remarkably improved,leading to an optical reflection loss of-35.5dB at17GHz for a layer thickness of6.4mm for the annealed sample.3. Al2O3and0.3%,1%Cr3+-doped Al2O3thin films were prepared by means ofpulsed laser deposition (PLD). We have studied the film structure and the luminescenceproperties before and after annealing, and analyzed effect of different preparationconditions on the optical properties of thin films. The result shows that, defects andoxygen vacancies have been reduced after annealing, when the distance between targetand substrate is40mm, oxygen vacancy obtains minimum. Photoluminescence peaksare observed at678nm,692.6nm and697nm for0.3%Cr3+-doped Al2O3thin film.Compared with0.3%Cr3+-doped Al2O3thin film, photoluminescence peaks for1%Cr3+-doped Al2O3thin film are weaken and exhibits a red shift. Comparing the0.3%Cr3+-doped Al2O3thin films prepared by XeCl excimer laser with the films prepared byall-solid-state Nd:YAG laser, we find the quality of films prepared by Nd:YAG laser isrelatively better, and the intensity of photoluminescence peak is stronger.