Optimization Of Absorbing Property Of Carbonyl Iron Micro-powder Composites

In the rapid development of the modern society, a mass of electromagnetic wave, which is produced in our living environment because of the widely use of various electronic devices, has led to the electromagnetic pollution of our space. The electromagnetic wave has not only interference with each other but also perniciousness to our human body. So, microwave absorbing materials have also been used for people's livelihood from military application.The information society requests for fast and high quality data transmission which requires much higher transmission frequency of the electromagnetic wave. Then the working frequency of microwave absorbing materials should be improved correspondingly. Carbonyl iron makes a good microwave absorbing agent due to its high Curie temperature (775℃), high saturation magnetization (210 emu/g) and low coercivity (15 Oe). But these traditional materials have a low cut-off frequency, high permittivity and bad impedance matching. So, it is possible to optimize the microwave absorbing properties of these materials. In this thesis, in order to meet the requirements of the microwave absorbing materials such as:thin thickness, wide band, light weight and strong absorbing peak, different processing and modification method (oxidation, ball-milling, coating and magnet aligned by rotating magnetic field) were used to improve the microwave absorbing properties of the Carbonyl iron composite materials.The purpose of the oxidation process is to cover the particles with a thin layer of oxide, which will lower the dielectric constant. In order to control the oxidation process more easily, hydrogen peroxide was used as oxidant during the oxidation process. Anisotropic absorbing materials with increased cut-off frequency and permeability can be obtained with magnetic field orientation method using the flake-like particles, which are obtained by a mechanical ball-milling. And coating titanium dioxide can further reduce the dielectric constant and increase impedance matching, which will increase the microwave absorbing performance.(1)Influence of surface oxidation of the spherical carbonyl iron particles:first, the spherical carbonyl iron is mixed with paraffin to obtain the composite materials with certain absorbing performance. After the oxidation process, dielectric constant is effectively reduced, which improves the impedance matchment and hence a brodened absorption bandwidth (The bandwidth lower than -5 dB has been increased from 8.5 GHz to 10.1 GHz for the 30% in volume sample), an increase of absorption intensity has also obtained (from-10 dB to-15 dB). This means that by means of oxidation, we can control the electromagnetic parameters of the absorbing materials, thereby improve the absorbing properties of the composite materials.(2) Morphology of carbonyl iron particles:Through the balling, get flaky carbonyl iron particles, increased permeability of the material, which is conducive to absorbing composite materials, and make the microwave absorbing coating thinner (in the 2-4 GHz reduced from the 4-7 mm band thickness of 2 mm or so). However, after milling, the sample dielectric constant increased a lot. To solve this problem, we carry on the surface oxidation process. According to the experimental result, it can reduce the permittivity but can not match the magnetic permeability which value we required. Then, we should make a further improvement on magnetic permeability of the sample. It can be improved 1.5 times more than original. The orientation effect is very good according to the experiment result the 33% of original magnetic permeability was improved from 7 to 10.8. The magnetic permeability increasing improved the impedance matching however it still has a big gap between magnetic permeability and dielectric constant which needs further treatment.(3) Carbonyl iron particles coated TiO2:In order to effectively reduce the dielectric constant of composite material made with flake carbonyl iron, we coated titanium dioxide on the flake carbonyl iron particles surface. After this coating process, the reflection loss of the sample is maintaining lower-10 dB absorption intensity at 0.1-4 GHz. And reduce the thickness of the absorbing coating. This to reduce the weight of absorbing coatings coated in flying has an important role. The experiment result reached to reduce the dielectric constant. There appears the best point on impedance matching the sample with 5 mm thickness which is around 0.8 GHz can be reached-45 dB. This indicates that we can improve the materials' microwave absorbing properties with coating.(4) Influence of the binder in the composite materials:Spherical carbonyl iron used as the same absorbing agent, and paraffin, epoxy resin and organ-silicon epoxy resin as different binder to produce samples. In experiments, the volume fraction of absorbing agent is 40%. Results show that the absorbing property of the sample made with organ-silicon epoxy resin is the best. The minimum reflection loss reaches -41.1 dB (the 99.9% energy of electromagnetic wave is absorbed) at 3.2 GHz. The band width with the reflection loss lower than-5 dB (70% energy) is 4.5 GHz (1.8-6.3 GHz), and the one lower than-10 dB (90% energy) is 1.9 GHz (2.4-4.3GHz). At last we make and study the composite materials of carbonyl iron/organ-silicon epoxy resin with different volume fraction. It can keep the absorption peak at using frequency by controlling the volume fraction of carbonyl iron powder and the thickness of composite materials.