Preparation And Electromagnetic Microwave Absorbing Properties Of Nanosized Nickel Ferrite And Its Composites

Microwave absorbing materials have been widely used in military and civilian applications. They are the hotspot in the field of stealth technology and electromagnetic compatibility technology. In most of the absorbers, ferrite is one of the main components because of its good performance and low cost. Its main disadvantage is high density which limits its application in light absorbers. In response to the requirement for high performance absorbers, the spinel type nanocrystalline nickel ferrite was prepared by the polyacrylamide gel method with acrylamide as the monomer and N,N’-methylenediacrylamide as the lattice agent. Then the composites of nanocrystalline nickel ferrite with hollow microspheres, porous ceramics, polyaniline and polystyrene were fabricated. These new microwave absorbing materials are light weight and have good electromagnetic, wide band and high absorbing properties.Nanocrystalline NiFe2O4 ferrite was prepared using the polyacrylamide gel method. The effects of ratio of acrylamide/N,N’-methylenediacrylamide, reaction temperature and reaction time on characteristics of the polycrylamide-gel were studied. XRD results show that the dry gel is amorphous, and after calcining at 500℃or a higher temperature, it transforms into spinel nickel ferrite. TEM observation shows that particle sizes of the spinel nickel ferrite obtained by calcination at 500℃,600℃,700℃,800℃and 900℃, are 8 nm,20 nm,45 nm,80 nm and 120 nm, respectively. FT-IR analysis illustrates that the characteristic absorption peak of the M-O shifts from 590 cm-1 to 613 cm-1. The nanocrystallne spinel nickel ferrite has both dielectric loss and magnetic loss in the range of 8.2-12.4 GHz, and its dielectric loss, magnetic loss and reflection loss increase with increasing heat treatment temperature.Hollow glass microspheres were coated with nickel ferrite using the polyacrylamide gel method. The obtained composite powder is composed of nickel ferrite, quartz and mullite. A continuous NiFe2O4 coating was obtained at a microsphere content of 25 wt%. The composite with 25 wt% microspheres exhibits better dielectric loss and magnetic loss properties in the X band. Its dielectric loss tangent and magnetic loss tangent are 0.30 and 0.10, respectively. The composite shows a minimum reflection loss of-13.7 dB with a bandwidth of-10 dB over the extended frequency range of 2.1 GHz. It is also low in density, implying its potential application as a good and light electromagnetic wave absorbing material.SiC porous ceramics were coated with nickel ferrite using the polyacrylamide gel method. Spinel structured NiFe2O4 forms on the porous ceramics at 600℃. With the increase of coating layers, the coating becomes more uniform and homogenous, and the loading of nickel ferrite coatings increases slowly. The nickel ferrite coating prepared at 800℃adheres well to the surface of SiC. The porous ceramics coated with nickel ferrite exhibit lower dielectric loss and higher magnetic loss compared with the porous ceramics matrix. Its maximum tanδm is 0.55, indicating a better magnetic loss property. The tanδm of the porous ceramics coated with nickel ferrite decreases with increasing frequency in the X band.Polyaniline with good electric conductivity was prepared by an ultrasoonic and emulsion polymerization process using aniline as monomer, dodecylbenzene sulfonic acid as dopant and ammonium peroxydisulfate as oxidant. The nickel ferrite/ polyaniline composites were prepared by the in situ polymerization method under ultrasonic field. The composite with a content of 15 wt% nickel ferrite has a maximum conductivity of 0.8455 S/cm. Dielectric loss tangent and magnetic loss tangent of the nickel ferrite/ polyaniline composites are higher than those of the polyaniline. The composites with a ferrite content of 5 wt% and 15 wt% have the maximum tanδεvalue and tanδm value, respectively. The composite with 15 wt% ferrite content shows better electromagnetic wave absorbing property, with a minimum reflection loss of-23.4 dB and a bandwidth of-8 dB over the extended frequency range of 5.73 GHz.A nickel ferrite/polystyrene composite and a carbonyl-iron/polystyrene composite were made by hot pressing. The complex permittivity values and complex permeability values of the nickel ferrite/polystyrene composite show an increasing trend with the increase of nickel ferrite content. The complex permittivity values and complex permeability values of the carbonyl-iron/polystyrene composite show an increasing trend with the increase of the carbonyl-iron. The material pressed at 145 MPa has the minimum reflection loss and the best microwave absorption. The double layer structure composed of the nickel ferrite/polystyrene and the carbonyl- iron/polystyrene has an effect impedance coupling, widening the band and improving minimum reflection loss, so that the electromagnetic wave absorbing properties can be improved compared to those of the single materials.