Preparation And High Frequency Research On Micro And Nano Magnetic Materials

In recent years, electromagnetic pollution became an increasingly concerned topic and the hot issues for the people actually living due to the rapid development of wireless communications and microwave circuits in the GHz frequency range. In the military field, the rapid development of modern electronic information technology and radar systems greatly increases the ability to target search, tracking, attack in the wars. Conventional weapon systems and defense platforms face a severe challenge. Stealth technology of Anti-radar, infrared, visible light, acoustic, smoke, track detection has come into being. Radar stealth technology has become one of the commanding heights of the military strategy after World War II. The development of the stealth coating materials is the current focus of the study besides the structural design to reduce the radar cross section technology. In order to systematically study the mechanism of microwave absorption and microwave absorption properties of magnetic micro-and nanometer coating materials, several works have been done in the thesis:First, we focused on the study of the morphology-controlled preparation and microwave absorption properties of Cobalt particles. Cobalt microspheres, nanobelts, micron chains, micron flowers had been synthesized by using the sodium dodecyl benzene sulfonate and sodium tartrate surfactants since which could be easily formatted to targeted micelles. The magnetic research result shows that the shape anisotropy is the key factor to determine the different coercivity for various morphology samples. The study of the mechanism of microwave absorption in co micro flowers proves that the triple dielectric loss should come from the Lorentz resonance rather than the Debye dipolar relaxation process. The resonance peaks in permeability are due to the natural resonance and exchange resonance.Second, Microwave absorption properties of octahedron-shaped iron-oxide and Mn1-xZnxFe2O4were investigated. The study on the microwave absorption of iron oxide particles shows that, hydrothermally synthesised iron oxide particles have well crystallization and an octahedral morphology. In the low frequency and thicker matching thickness, the microwave absorption can be less than-10dB for iron oxide micron particles, but the absorption band is narrow. The study on the microwave absorption of Mn1-xZnxFe2O4shows that, the higher magnetic ratio in the sample, the better microwave absorption properties. Microwave absorption properties of the sample have no direct relationships with Zn content in the sample or the static magnetic properties of the samples. This result also shows that the static magnetic property and the anisotropy cannot directly determine the microwave absorption performance of the sample. Other factors are key factors, which determine the microwave absorption properties of samples. Third, the microwave absorption mechanism and performance of four types of composites have been studied. Category â… :Iron-cobalt alloy and cobalt ferrite composite, the research result shows that FeCo/CoFe2O4(FeCo wt.77%) samples are strong-broadband microwave absorbers. The RL minimum value is-51dB, the microwave absorption band where RL is less than-10dB was about16GHz. Category â…¡:Barium titanate/carbonyl iron powder and Barium titanate/Ni-Co-P ferroelectric/ferromagnetic nanocomposite, the research result shows that, strong microwave absorption could be obtained in a wide frequency range and the reflection loss (RL) was enhanced since the ferroelectric nano BaTiO3had been introduced into the carbonyl iron/BTO composite. In addition, the absorbing frequency range of RL<-10dB had been extended to2-8GHz and the effective absorbing bandwidth reached16GHz. The excellent microwave-absorption properties mainly resulted from proper EM matching, the strong natural/exchange resonance, as well as the dipole-polarization of the different phase interface. Category â…¢:We studied the carbonyl iron and zinc oxide composite. The minimum microwave reflection absorption reached-60dB. RL<-20dB could be obtained within8.4-17.9GHz with a bandwidth of9GHz. At this point, matched microwave absorption thickness of the absorber is very small (1.3-2.3mm), which reflects that the carbonyl iron and zinc oxide composite is a good microwave absorbing material. It was expected to be widely used in practical applications. The study of the relationship between the microwave absorption and the quarter-wavelength Law shows that, impedance matching is the fundamental determinants in single-layer microwave absorber. Category â…£:SiO2-Ni dielectric-magnetic composite with core-shell structures. The study found that effective permittivity and effective permeability of core-shell structure composite materials is determined by the radius of the core/shell thickness and dielectric and magnetic mass percentage. By adjusting composite structures, the phase ratio, the effective permittivity and permeability of the samples will be adjusted where the input impedance of the sample will achieve a good impedance matching. This adjusting will also provides guidance to design broadband and strong microwave absorbers.Last, from the impedance matching equation in the real domain, we first defined the concept of impedance matching degree. The concept of impedance matching degree is the difference of the input impedance between the single-layer absorber and the air input impedance. This difference is larger, the degree of impedance matching is worse. Smaller the difference, better the sample impedance matching degree, and the microwave absorption performance are better. This research on the relationship between the impedance matching degree and microwave reflection from microwave frequency and complex permittivity/permeability shows that. This relationship has no impact on the complex permeability, the complex permittivity and the microwave absorption frequencies.When reflection absorption approaches to smallest, the impedance matching degree also has the smallest values. The relationship between impedance matching degree and the microwave absorption meets the exponential growth law.