Fabrication Of Lithium Ferrites For X-band Phase Shifter And Study On The Electromagnetic Properties

Lithium ferrites have been used as the preferred material of the ferrite phaseshifter for their high Curie temperature and high remanence ratio, and have beenwidely applied in the microwave field. However, their high saturationmagnetization, high coercivity, high dielectric loss and low density can notsatisfy the developments of the microwave devices, such as miniaturization andlower dielectric loss. In this paper, we tried to improve the overall performanceof lithium ferrite by three ways: substituting Ni-Ti ions, changing formulationsand adding Bi₂O₃sintering aid. The properties of sintered ferrite ceramic sampleswere characterized by X-ray diffraction （XRD）, scanning electron microscope（SEM）, laser granularity analyzer, integrated magnetic properties tester andvector network analyzer respectively. Here are the results.With Ni-Ti ions substitution, Li_{0.5-0.5x+0.5y}Ni_xTi_yFe_{2.5-0.5x-1.5y}O₄（x=0.2,y=0.05,0.10,0.15,0.20,0.25） ferrites were fabricated by oxide ceramicprocessing. All the specimens exhibited single spinal-phase without anyimpurities, and were crystallized well, after sintering at1050℃for7h.Successfully, saturation magnetizations are adjusted to the range from1752Gs to2412Gs, while coercivety decreased remarkably from5.5Oe to2.9Oe. At thelevel of y value at0.10, the sample showed better magnetic properties whensaturation magnetizations4πMs at2162Gs, remanence ratio at0.86, coercivetywhich was a little larger at4.6, ε at14.68, and with a higher dielectric loss thattanδεat6.1×10^-3.The two aspects researches of the formula changings were lithium surplusand iron deficiency respectively, both were based on the results ofLi0.45Ni0.2Ti0.1Fe2.25O4ferries. All the studies were carried out in order to reducethe dielectric loss of Li0.45Ni0.2Ti0.1Fe2.25O4ferrites. The results showed that irondeficiency formulas were more effective to reduce the dielectric loss than lithiumsurplus formulas. Trace of lithium surplus could compensate lithium loss andincrease densities， but most of the lithium-surpluse samples showedunsatisfactory overall properties with large dielectric losses. On iron deficiencyformula side, appropriate iron deficiency content could effectively improve microstructure and dielectric properties. The samples with iron deficiencycontent at0.06exhibited single spinal phase without impurities, and werecrystallized well. Apparent density of the samples was4.67g·cm^-3，ε at4.56andtanδεat2.1×10^-3. Compared to those samples with no iron deficiency(tanδε=6.1×10^-3), its dielectric loss had significantly decreased.Ferrites of Li0.45Ni0.2Ti0.1Fe2.25-δO4, doped with different contents of Bi₂O₃onthe basis of iron deficiency content δ at0.06were fabricated by oxide ceramicprocessing, and exhibited pure spinal phase after sintering at1050℃for5h.Appropriate amount of Bi₂O₃not only promoted liquid formation to acceleratemass transfer, helped ferrite samples sintered well, but also was good to improvemagnetic properties and lower dielectric loss. The ferrite sample adding1.5wt%Bi₂O₃aids showed well comprehensive performance, such as good microstructure,low porosity and high density. Its optimum properties that ρ at4.72g·cm-3,4πMsat2275Gs, R at0.853, Hc at2.9Oe, ε at14.16and tanδεat5.4×10-4, could meetthe requirements of the X-band ferrite phase shifter.