La(CaSr)MnO 3 Polycrystalline Material Prepared By Sol-gel Method And Its Properties

The R_1-xA_xMnO₃ as a perovskite manganite,which was studied extensively because of the colossal magneto resistance?CMR?effect and other properties,was used in magneto resistive transducers,spin electronics devices,magnetic recording devices and other sensitive devices with great values.La1-xCaxMnO₃ as an important number of R_1-xA_xMnO₃,which has notable magneto resistance effect.It is necessary that the narrow range of Tp or applied in additional magnetic fields for the maximum values of CMR,which limited the practical application of the materials.Hence,it is very important to improve the Tp?metal-insulator transition temperature?and TCR?the temperature coefficient of resistance?for researching.With previous works,combined the electrical and magnetic properties of La1-xCaxMnO₃?LCMO?with La1-xSrxMnO₃?LSMO?using Sol-gel method to prepare La_1-y(Ca_y-xSr_x)MnO₃?y = 3/8,LCSMO?polycrystalline ceramics and[La0.625?Ca0.315Sr0.06?MnO₃]1-z:Agz polycry-stalline composite ceramics.The structure,morphology and properties were confirmed by X-ray diffraction,Scanning electron microscopy,X-ray Photoelectron Spectroscopy,Inductively coupled plasma mass spectrometry and so on.Experimental results showed that all the samples of La_1-y(Ca_y-xSr_x)MnO₃?y?3/8,x=0?y/25?2y/25?3y/25?4y/25?y/3?y/2?2y/3?are in a single phase with any impurities.The crystal structure of samples transform from orthorhombic phase?Pnma space group?to rhombohedral phase?R3c space group?.With the increasing of Sr,the values of TCR and MR reduced.The Tp reached 300.4 K and the MR was 12%,300 K with x=4y/25.The Ca2+ was replaced by Sr2+,which leaded to the average radius of A-site cationic increasing and the enhancement of the double exchange interaction between the Mn3+-O2--Mn4+ ions.Hence,the values of Tp and MR were tuned to the room temperature.Then,we changed the sintering temperature of La_1-y(Ca_y-xSr_x)MnO₃?y = 3/8,x = 4y/25?in the range of 1000-1400 ?.The grain size of polycrystalline ceramics increased and the stress in the crystal lattice was released,as the sintering temperature increasing,which made the characteristic peak to the low angle deviation.In addition,the resistivity decreased and TCR enhanced as the improvement of sample crystal quality and microstructure.The results of XPS showed that changed the sintering temperature from 1200 ? to 1400 ? had not effect on the scale of Mn4+.However,the variation of grain size and density had a negative influence on the double exchange interaction and electronic spin-polarized tunneling.Therefore,Tp and Tk decreased and MR increased with the increasing of the sintering temperature.In order to study that the Ag doping of La_1-y(Ca_y-xSr_x)MnO₃ effect on the property of samples and the relationship between Ag content and sintering temperature.Using Sol-gel method prepared the[La0.625?Ca0.315Sr0.06?MnO₃]1-z:Agz?z?0,0.01,0.03,0.05,0.07,0.10,0.15,0.20,0.30?on the basis of room temperature material at 1000 ?.Then preliminary optimization of Ag doping amount according to the test results to research the relationship between performance and sintering temperature.Combined the XRD with XPS results showed that the characteristic peak deviated and lattice changed with Ag+ substituted the A-site ion.The substitution promoted the double exchange and made the Mn4+ ion concentration increased,which led the Tp and Tk enhanced.In addition,the doping of Ag formed an electric conduction channel and affected the magnetic disordered state between the particles and material's MR.The evaporation of Ag content increased with the increasing of sintering temperature and the distribution was more homogeneous.Moreover,the scattering of electrons is reduced,so the change of the sample resistivity showed the differentiation.The Ag doping amount and La0.625?Ca0.315Sr0.06?MnO₃ molar ratio of 1:4 with 1400 ? which could achieve the polycrystalline composite materials with low resistivity and room temperature Tp,Tk and TCR is 9.38%.