Investigation On High Gyromagnetic LiZn Ferrite And Its Application

LiZn ferrite, which performs a wide range of variable saturation magnetization, highCurie temperature, low stress sensitivity and low fabrication cost, is apt to be fabricatedinto microwave/millimeter wave devices, e.g. ferrite phase shifter. Ka-band ferrite phaseshifter is a crucial component of phase array radar, and the LiZn ferrite applied in itshould have excellent gyromagnetic property （high saturation magneticzation）, goodsoft magnetic property （low coercivity） and rectangular characteristic （high remanenceratio）. Furthermore, low microwave losses （low ferromagnetic resonance line width anddielectric loss） are required to reduce insertion loss. For the features given above, thepreparation process, the influence mechanism of the static magnetic characteristics andmicrowave properties, and its application of high gyromagnetic LiZn ferrite areresearched and analyzed.Firstly, the auto-combustion method used for preparation of LiZn ferrite nanopowder was researched and the sintering characteristic of LiZn ferrite nano powder wasanalyzed. The results are as follows.1) High activity LiZn ferrite nano powder can beobtained by auto-combustion process at low temperature （about500℃）, and anappropriate ratio of citric acid and metal nitrates （r=1:1） is the prerequisite to obtain thesingle crystalline phase LiZn ferrite;2) LiZn ferrite nano powder is so active thatlithium volatilization ocurrs when sintered at800℃, which causes the Fe3+precipitatedand Fe₂O₃phase formed;3) When the reaction temperature off Sol-Gel is heighten, thepowder size and the specific saturation magnetization decrease slightly;4) The sinteredbulk sample, derived from self-propagating nano powder which is fabricated by theSol-Gel method, has high porosity （>10%） and low density （about4.4g·cm-3）.And then， the comparation between Sol-Gel process and oxide process isperformed, And the oxide process was choosed to fabricate LiZn-ferrite materials usedfor Ka band phase shifter. The advantages of oxide process to fabricate LiZn-ferritewere analyzed. The influence of calcining temperature, second ball milling time,sintering conditions and atmosphere on the material performance was discussed. Theresults are as follows.1) When the calcining temperature is800℃, the LiZn ferrite powder posesses a high activity, and the sintered LiZn ferrite has a perfectmicrostructure, uniform grain size, less pore, high saturation magnetization and highremanence ratio, relatively low coercivity, and low ferromagnetic resonance linewidth;2) When sintered at1100℃, if the second ball milling time is more than4hours, it haslittle impact on saturation magnetization. But sintered at1160℃, saturationmagnetization decreases and coercivity increases when second ball milling time exceeds4hours;3) For the1wt%Bi₂O₃doped LiZn-ferrite, in order to get a rectangularproperty so that high remanence can be obtained, it should be sintered below1100℃;4)When sintered in oxygen atmosphere, the resistivity of LiZn-ferrite is enhancedobvously. Sintered at a negative air pressure is benefit to increase saturationmagnetization and decrease coercivity.The effect of Zn substitution on the ions distribution and superexchange interactionof LiZn ferrite was researched. The effect of ions distribution and superexchangeinteraction on the magnetic parameter and temperature characteristic was analyzed,together with the effect of iron deficiency and Li overdose on the microwave loss ofLiZn ferrite. The results are as follows.1) The Zn2+can enhance the saturationmagnetization and decrease the coercivity and ferromagnetic resonance linewidth, but ata price of decreasing the remanence ratio and Curie temperature;2) In the main recipe, amoderate iron deficiency can suppress the appeareance of Fe2+effectively. And then thedielectric loss can be decreased;3) A proper Li overdose in the main recipe can reducethe dielectric loss, but the existence of too much Li will cause a drop in the saturationmagnetization and remanence ratio, and then make the ferromagnetic resonancelinewidth and coercivity to increase.In addition, the paper carried out a series of additives research systematically,including V2O5, CuO, Bi₂O₃, Mn₃O₄, and NiO on grain growth, sintering characteristics,magnetic property. Furthermore, the effect of sintering temperature and Bi₂O₃concentration on Li volatilization is investigated. The results are as follows.1)V2O5caneffectively promote sintering densification and grain growth and decrease coercivity, butnonmagnetic V5+ions substitution can result in a decline in saturation magnetizationsignificantly. So V2O5concentration should be controlled below0.25wt%, in order toobtain high saturation magnetization LiZn ferrite materials;2)Sintering at1000～1100℃, CuO doping can effectively promote sintering densification and grain growth, reduce the sintering temperature, enhance density, saturation magnetization, andremanence, reduce coercive force, but coercive force is still higher. Sintering at1130℃,the CuO aid is not obvious;3)Sintered at1000℃, the appropriate dosage of Bi₂O₃is2～3wt%. Li volatilization in1100℃sintered magnet decreases from8wt%to1wt%by doping3wt%Bi₂O₃;4) content of Mn₃O₄and adding method have great impact onthe performance for LiZn ferrite. In920℃sintered sample, adding appropriate amountof Mn₃O₄into the raw materials can enhance saturation magnetization and remanence,and reduce coercive force. Whereas adding Mn₃O₄into the presintering powder affectslittle on saturation magnetization and remanence. Sintered at950℃, coercive force ofthe samples fabricated by the both Mn₃O₄adding methods decreases significantly,furthermore, the coercivity is higher when adding Mn₃O₄into the raw materials. For theboth adding methods, Moderate Mn₃O₄content can improve resistivity via inhibitingthe electron hopping between Fe2+and Fe3+. Both resistivities ofLi0.35Zn0.30Fe2.29MnxO4±δ（0.02≤x≤0.08） ferrite reach the peak value when x=0.06,furthermore, resistivity is higher when adding Mn₃O₄into the raw materials. Mn₃O₄stillhelps to improve resistivity sintering at1000℃, but the resistivity declines comparedwith that of950℃sintered samples;5)A small amount of NiO helps to improveremanence ratio, but it increases coercivity.Finally, the suitable main recipe and process are given and the application isinvestigated for high gyromagnetic LiZn ferrite. Simulation design for ferrite phaseshifter with back ridged waveguide is developed. The results are as follows.1)Modifying the section size of grooved waveguide and ferrite rectangular helps todecrease insertion loss;2) To insure differential phase shift being360°in the wholerange of-60～+100℃, the length of ferrite rectangular loop should be at least25.5mm;3) Ferrite phase shifter with29.6mm ferrite rectangular was fabricated, and420°differential phase shift was achieved at25℃. VSWR and insertion loss at33～35GHzis less than1.4and1.3dB, respectively.