Preparation And Electromagnetic Microwave-Absorbing Properties Studies On Spinel Nanocrystalline Ferrites

Microwave absorber materials are of great value in the military field and have significant effect on the future war and the development of weapons. The synthesis of nano complex mental oxide is studied and the mechanism of the absorption of electromagnetic wave is discussed in this paper to establish the theoretical basis for the practical application of stealth materials.The spinel nanocrystalline ferrites were prepared by polyacrylamide gel method with acrylamide as monomer and N, N'-methylenediacrylamide as lattice agent. The electromagnetic and microwave absorber properties for the prepared materials were studied. The achievements are as follows:(1) Nanocrystalline NiFe2O4 ferrite was prepared using polyacrylamide gel method. The effects of ratio of Acrylamide/N,N'-Methylenediacrylamide, reacting temperature and reacting time on characteristics of polycrylamide-gel were discussed. The analysis methods of DSC-TG, FT-IR, XRD, TEM and wave-guide were used to characterize the gel and the products after calcining. The XRD study showed that the dried gel was amorphous, the spinel nickel ferrite formed at and above 500℃. When the calcining temperatures were 500℃,600℃,700℃,800℃and 900℃, the grain sizes were 8,20,45,80 and 120nm identified by TEM, respectively. The FT-IR spectra illustrated that the M-O characteristic absorption peaks shifted from 590 cm-1 to 613 cm-1. The nanocrystalline had both dielectric loss and magnetic loss in the range of 8.2～12.4GHz. Dielectric loss, magnetic loss and reflection loss of spinel-type nanocrystalline increased with the increasing of heat treatment temperature. The reflectivity of spinel nanocrystalline increased with the increasing of thickness. The peak value of reflectivity shifted from high frequency to low frequency with the thickness increasing.(2) Nanocrystalline ZnFe2O4, Ni0.5Zn0.5Fe2O4 and Nio.4Coo.4Zno.2Fe2O4 ferrite was prepared using polyacrylamide gel method. The gel and the products after calcining were characterized by FT-IR, XRD and wave-guide method. Effects of calcining temperature on FT-IR characteristic absorption peaks, electromagnetic loss and reflectivity of the materials were investigated. The results indicate that FT-IR characteristic absorption peaks for ZnFe2O4 and for Ni0.5Zn0.5Fe2O4 shift 25cm-1 and 23cm-1 from low frequency to high frequency with the increasing of calcining temperature, respectively. Dielectric loss, magnetic loss and reflection loss of spinel-type nanocrystalline materials increase with the increasing of heat treatment temperature. The mechanism of spinel nanocrystalline was also discussed in this article. Besides the resonance energy loss, the process of laxity delay became the main channel of energy loss. Except the energy loss channel mentioned above, the special effects such as small size effect and interface effect for nano-sized materials were also the reason for the energy loss of microwave.(3) Nanocrystalline NixZn1-xFe2O4(x=0.2,0.5 and 0.8) and CoxZn1-xFe2O4(x=0.2,0.5 and 0.8) ferrite was prepared using polyacrylamide gel method. Effects of Ni2+ and Co2+ content on FT-IR characteristic absorption peak, the lattice constant, electromagnetic loss and reflectivity of the materials were investigated. When x of the NixZn1-xFe2O4 and CoxZn1-xFe2O4 shifted from 0.2 to 0.8, the M-0 characteristic absorption peaks of NixZn1-xFe204 shifted from 567 cm-1 to 609 cm-1 and that of CoxZn1-xFe2O4 shifted from 561 cm-1 to 584 cm-1, detected by FT-IR spectra, respectively, which is the effect of the increase of the bond constant and the change of stretching vibration of the Fe-0 peak when the lattice type transfered from normal spinel to inverse spinel. The lattice constant of NixZn1-xFe2O4 decreased from 0.8442nm to 0.8364 and that of CoxZn1-xFe2O4 decreased from 0.8451nm to 0.8409nm, which suggests the increase of electromagnetic loss and reflectivity.(4) Nanocrystalline nickel and nickel zinc ferrites with substitution of Fe3+ by rare-earth ions, according to the formula NiLaxFe2-xO4 (with x=0,0.05,0.1 and 0.15) and Ni0.5Zn0.5RE0.05Fe1.95O4(with RE=Ce, La and Y), were prepared by polyacrylamide gel method. The variational La3+ content and different rare-earth ions for ferrites on the structure and property of materials were investigated. To NiLaxFe2-xO4, according to XRD, the pure spinel-type crystal structure of the NiLaxFe2-xO4 (x=0 and 0.05) was obtained at 500℃, which shows that La3+ substituted for part of the Fe3+ in the spinel-type crystal structure. When x=0.1 and 0.15, however, the spinel-type crystal structure was partially destroyed, the characteristic peak of spinel weakened and LaFeO3 was formed. TEM results show that the particles of NiLaxFe2-xO4 are in regular spherical shape when x=0. With the increase of x value, the shape of particles becomes irregular. The results revealed that the nano-sized NiLaxFe2-xO4 had both dielectric loss and magnetic loss in X-bond (8.2-12.4 GHz) frequency range. When the x of NiLaxFe2-xO4 increased from 0 to 0.1, the dielectric loss and magnetic loss of the ferrites vary slightly. The absence of LaFeO3 probably cause some resonance peaks on NiLaxFe2-xO4 (with x=0.1 and 0.15) ferrite powder. The introduce of RE in Ni0.5Zn0.5RE0.05Fe1.95O4 (RE=Ce, La and Y) led to the increase of the electromagnetic loss and reflectivity, which caused by the introduce of La3+ is larger than that by Y3+ and less than that by Ce3+. This is because that the no-twin elctrons in 4f layer of Ce3+ increased the saturated magnetic moment, which increased the microwave absorbing property.(5) Nanocrystalline hematite was prepared by Polyacrylamide gel method. Through TEM, the average size of Nanocrystalline hematite was 40nm and the particles with size distribution in the range from 25nm to 40nm was 70%, which corresponded to the results of full width at half maximum method of XRD.