| Because of the M-type barium ferrite material have high magnetic anisotropy field and larger saturation magnetization, so it can be widely used in self-biased millimeter wave ferrite circulator. With the development of microwave integrated circuits, thin / thick film circulator is urgent needed for integration, so the research of cirulator fabricated with ferrite in thin film / thick film form integrated with substrate cause much attention.In this article, barium ferrite thick film is prepared by screen printing method,also the effects of process conditions on properties of barium ferrite thick film has been studied. The results show that milling time has little effect on the surface morphology of BaM thick film, with the increase of milling time, the remanence ratio and coercivity of samples are not obviously changed. But with the rolling time growth, the pore size and number of the samples download. Remanence ratio of the samples gradually raise to 0.75 from 0.6.The composition and the proportion of organic carrier in the barium ferrite paste have a greater impact on the thick film samples.Barium ferrite thick film with single solvent , which surface pores reduced and remanence ratio increased gradually from 0.6 to 0.7 with the lower content of organic carrier, but the coercive force slow reduced from 3.27kOe to 3.19kOe. the solvent mixture consisting of terpineol and tributyl citrate can adjust the volatilization characteristic of the paste so that to reduce the porosity and improve saturated magnetization of the barium ferrite thick films. While the remanence ratio can be reached to 0.66 and Ms is 1600Gs. The dying time has little effects on the morphology of barium ferrite thick films because the solvent mixture has smooth volatilization process. And the magnetic properties of the sample is almost unchanged.The sintering temperature can significantly affect the performance of thick film samples. With the sintering temperature growing up, the sample's porosity gradually reduced, but the grain will gradually grow up, from 1150℃to 1200℃the main phase of the samples changes from the BaM to Fe2O3. BaM decomposed completely above 1250℃. Saturation magnetization of samples down from about 1700Gs to about 400Gs. At 1150℃and 1200℃coercive force is 3.4kOe and 3.34kOe but at 1250℃and 1300℃it reduced to 0.3kOe and 0.25kOe.Through process optimization, the prepared barium ferrite thick film with remanence ratio of 0.75, a coercivity of 2800 Oe.With 100 micron thick barium ferrite thick film prepared on 200 micron thick substrate,we through theoretical calculation and HFSS software simulation, model's structure parameters is: the radius of the center conductor is 1mm, the length of first matching section microstrip is 0.95mm, width is 0.1mm, the length of the section II matching microstrip is 0.1mm, width is 1.6mm, the length of section III matching microstrip is 1mm, width is 0.19mm.The simulation results show that the designed circulator at 25GHz has a good performance, the insertion loss is about 2dB, isolation is about 20dB.Finally the ferrite thick film self-biased circulator is fabricated by using micromachining. In the absence of an external magnetic field, a vector network analyzer is used to test the circulator's microwave transmission characteristics. The final test results show that at the 25.6GHz it has a good performance, the device insertion loss is about 14.9dB, isolation is about 22.4dB, the difference is about 7.5dB. |
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