With the rapid development of modern science and technology, especially the electronics industry technology, electromagnetic radiation of different frequencies fills people’s living spaces, destroys the good ecological environment of humans, and causes serious electromagnetic pollution. Therefore, research and development can solve electromagnetic radiation pollution. The microwave absorbing materials have become the focus of attention. In addition, in order to meet the needs of modern high-tech, three-dimensional warfare, the research of stealth materials has been highly valued by military powers in the world. As stealth technology increases the performance requirements of absorbing materials, the research and development of new absorbing materials has become the focus of research on absorbing materials.
The development of a new generation of stealth weapons and equipment has put forward higher and higher requirements on the thickness, density, absorbing frequency band, and absorbing performance of absorbing materials. Traditional absorbents can no longer meet this demand, and new absorbents must be found. As far as the development of absorbing materials is concerned, composite absorbing materials that make full use of the excellent properties of each component are the requirements for the development of absorbing materials; both magnetic loss and electrical loss absorbing materials will represent the development direction of absorbing materials .
Ferrite is a kind of magnetic composite metal oxide composed of iron oxide and iron group or rare earth group. It is a ferrimagnetic material. Broadly speaking, ferrite is a magnetic oxide or magnetic ceramic. The absorbing performance of ferrite comes from its both ferrimagnetic and dielectric properties. Its relative permeability and relative conductivity are both in plural form, which can produce both dielectric loss and magnetically induced loss. Therefore, iron Oxygen absorbing materials have good microwave performance.
Ferrite absorbing material is a more mature and well-researched absorbing material. Because of its high permeability and resistivity at high frequencies, electromagnetic waves are easy to enter and decay quickly, and are widely used in In the field of radar absorbing materials. Due to the high resistivity of ferrite (1～1012Ω·cm), it can avoid the skin effect of metal conductors at high frequencies, so it can still maintain high magnetic permeability at high frequencies. In addition, its dielectric constant is small, and it can be mixed with other absorbents to adjust the electromagnetic parameters of the coating. It is an important electromagnetic wave absorbent.
The main mechanism for ferrite to absorb electromagnetic waves is natural resonance. The so-called natural resonance refers to the fact that the eigenfrequency determined by the angular frequency of the incident alternating magnetic field and the magnetic anisotropy equivalent field of the crystal is equal to produce precession (a self-rotating object is subjected to a constant magnetic field). The external force causes its rotation axis to rotate around a certain center. This phenomenon is called precession) resonance, thereby absorbing a large amount of electromagnetic wave energy. Compared with magnetic metal powder, ferrite material has better frequency characteristics, its relative permeability is larger, and the relative permittivity is smaller. It is suitable for making matching layer and has good application prospects in low frequency and broad band. The main disadvantages are high density and poor temperature stability. To this end, researchers expect to improve the loss characteristics and reduce the density by adjusting the chemical composition, particle size and distribution, particle morphology and dispersion technology of the material itself. At the same time, the nano-ferrite particles are combined with other nanoparticles and micro-particles, and the magnetic loss absorbing materials, resistive loss absorbing materials, and dielectric loss absorbing materials are effectively combined, and the composition and electromagnetic parameters can be designed. Gradient function absorbing materials, which are beneficial to wave impedance matching and absorption, are also effective ways to achieve strong absorption. In order to reduce its density and improve its dispersion, Mu et al. wrapped barium ferrite on ceramic hollow spheres with a sol-gel method. The particle size is 80nm, the maximum absorption is 31dB, and the absorption bandwidth is 4GHz greater than 10dB. The density is only 1.8g/cm3. X.Shen et al. prepared one-dimensional nano-iron composite materials on carbon nanotubes. The outer diameter of pure nanotubes is 13nm, and the diameter of nano-iron composite tubes can reach 35nm. The composite materials have μ', μ'at 2GHz. , Ε'and ε'' are 2.64, 1.63, 12 and 2.04, respectively.
Application of ferrite wave absorbing materials
The U.S. applies the absorbing coating made of ferrite to the F-117A stealth fighter; NEC of Japan has improved the ferrite absorbing material, and when the frequency bandwidth is 7GHz (6～13GHz), the attenuation is 10dB; When the attenuation reaches 20dB when the bandwidth is 3.7GHz (8.5～12.2GHz), the total thickness is 4.7mm and the mass per unit area is 8kg/m2. The problems of ferrite absorbing coatings are high density, insufficient absorbing frequency, thicker coatings, and poor high-temperature characteristics. Its development direction is to compound with ultrafine nano powder and conductive magnetic fiber absorbing agent to improve the absorbing performance.