Preparation And Microwave Absorption Properties Of Doping NdMnO₃and Sr₂FeMoO₆

In this paper, Nd_1-xSr_xMnO₃, Nd_0.5Sr_0.5Co_xMn_1-xO₃and (Sr_2-xBa_x)FeMoO₆were synthesized bydry high energy ball milling and sequent heat treatment, Nd_0.7Sr_0.3MnO₃were synthesized by wet highenergy ball milling and sequent heat treatment, and their microwave absorption properties wereinvestigated systematically.The crystal structure, composition and surface morphologies of the samples were characterizedby XRD, EDS, SEM and TEM, respectively. The Rietveld refinement of the Nd_1-xSr_xMnO₃sinteredsamples’ x-ray powder diffraction profiles were investigated. The results showed that high energy ballmilling promotes the reaction between original powders, the crystalline grain size of powders canreduce to the nanoscale after long time ball milling, and the single perovskite or double-perovskitestructure can be obtained after sequent heat treatment. Powders by dry milling appeared to be nearlyspherical and the powders’ morphology of samples after wet grinding was irregular flaky shape. Thesize of the particles was about a few microns. The room temperature resistivity of sintered sampleswas measured by four-probe array method. The results showed that the resistivity of all samples waswithin semieonduetor range and changed with the doping contents.The complex permittivity and complex permeability of the samples were measured bymicrowave vector network analyzer, and the loss tangent and microwave reflectivity were calculatedin the frequency range from2to20GHz according to measurements. The effects of elementincorporation and thickness of the sample on the microwave-absorbing properties were investigated.The absorption mechanism of the materials were studied generally. The results showed that theabsorption peak is13.2dB at16.6GHz and the bandwidth less than10dB is1.1GHz when thethickness of Nd_1-xSr_xMnO₃dry milled samples is2.5mm and x=0.2. The absorption peak is24.6dB at12.6GHz and the bandwidth less than10dB is0.66GHz when the thickness of Nd_1-xSr_xMnO₃sintered samples is2.8mm and x=0.4. The absorption peak is23.9dB and the bandwidth less than10dB is1.7GHz when the thickness of Nd_0.7Sr_0.3MnO₃wet milled sample is2.4mm and x=0.3. Theabsorption peak is22.7dB at14.3GHz and the bandwidth is0.85GHz when the thickness ofNd_0.5Sr_0.5Co_xMn_1-xO₃sintered samples is2.5mm and x=0.6. When the thickness of (Sr_2-xBa_x)FeMoO₆sintered samples is5.4mm and x=0.4, the absorption peak is42.2dB at4.6GHz and15dB at14GHz,the bandwidth is3.15GHz; When x=0.5, the absorption peak is40.9dB at4.5GHz and14.3dB at13.6GHz, the bandwidth is3.33GHz. The analysis of absorption mechanism showed that the dielectric loss of Nd_1-xSr_xMnO₃andNd_0.5Sr_0.5Co_xMn_1-xO₃was mainly and the magnetic loss was weak.(Sr_2-xBa_x)FeMoO₆had both strongdielectric loss and magnetic loss. The dielectric and magnetic loss tangent of all samples changedinversely with the changes of microwave frequeney. It was probably caused by a mutualtransformation between ferromagnetic and antiferromagnetic clusters in the microwaveelectromagnetic field.