La_1-yA_yxAg_xMnO₃ (A=Ca, Sr, 0<y<0.5 ) Preparation Of Polycrystalline Ceramics And Their Electromagnetic Properties

Due to its unique electronic and magnetic properties,perovskite manganites Ln_1-xA_xMnO₃(Ln represents La,Ce,Nd and other trivalent rare earth ions,A represents Ca,Sr,Ba and other divalent alkali metal ions)has been attracted the attention of many scientific researchers in the field of condensed matter physics and materials science.According to the chemical composition and microstructure of the materials,the ferromagnetic-paramagnetic temperature transition region in manganites could cause unusual physical effects,such as phase transition,charge ordering,spin ordering,orbital ordering and giant magnetoresistance effect,and so on.The temperature coefficient of resistivity and magnetoresistance are important factors to evaluate the performance of perovskite manganite materials.The peak value of the temperature coefficient of resistance(TCR)and magnetoresistance(MR)in manganite ceramics usually occur near the metal-insulator transition temperature(T_MI).Manganites have large TCR and MR values at room temperature and low magnetic fields,and have broad application prospects in scientific and technological fields such as information storage,magnetic sensors,magnetic refrigeration and uncooled infrared bolometers.A large number of studies have shown that Ag doping can significantly reduce the proportion of glass phase and improve the crystalline quality of the material.At the same time,the addition of Ag can significantly reduce the resistivity of the material.Obtaining TCR at room temperature and MR under low field are the key issue for studying manganites with perovskite structure.Ag doping can significantly increase the TCR value at room temperature.However,Ag doping in the second phase has no obvious adjustment effect on T_MI,and the actual doping amount cannot be controlled.Therefore,according to the stoichiometric ratio,La_1-yA_y-xAg_xMnO₃(A=Ca?Sr,0(27)y(27)0.5)polycrystalline ceramics have been quantitatively doped with Ag by using the sol-gel method,which can control the content of Ag in manganites.La_0.67Ca_0.09Ag_0.24Mn O₃ polycrystalline ceramics were prepared by sol-gel method.Because the melting point of Ag is 962°C,a higher sintering temperature could lead to the volatilization of Ag,but for ceramics to have excellent properties,they must be prepared at a higher temperature(above 1000°C),so how to balance their impact is the key issue.By changing the sintering temperature,under the final sintering temperature of 1000°C?1100°C?1300°C?1400°C and 1450°C,the influence of temperature on the performance of La_0.67Ca_0.09Ag_0.24Mn O₃ ceramics were studied,which found the best sintering temperature of this series of polycrystalline ceramic samples.According to the best temperature obtained by the experiment,La_0.67Ca_0.33-xAg_xMnO₃ polycrystalline ceramics were prepared.The data from XPS,XRD and EDS revealed that silver existed as Ag⁺ions in the lattice matrix position of LCAMO ceramics.Broad T_MI values ranging from 267.0 K(x=0)to 302.6 K(x=0.24)were obtained for LCAMO specimens prepared with variable Ag⁺added contents.TCR enhanced from 4.1%K^-1 at 263.2 K(for x=0)to 10.9%K^-1 at 278.5 K(for x=0.18),and reached 7.5%K^-1 at room-temperature(295.9 K)for x=0.24.Meanwhile,MR of materials reached 17.7%at room-temperature(299.2 K)for x=0.24.Overall,these findings demonstrated that Ag doping was beneficial for improving electrical and magnetic properties of LCAMO materials.Secondly,La0.67Sr0.33_-xAg_xMnO₃(x=0.00,0.15,0.18,0.19,0.20,0.21 and 0.24)polycrystalline ceramics were prepared.XRD spectrums reveal that all ceramics specimens crystallized in rhombohedron perovskite structure belonging to space group of R3(?)c.FESEM display that the grain size first increased and then decreased with the increase of Ag content.Large grain size could improve the electrical properties of LSAMO ceramics.The element mapping and EDS results show that specimens contain La,Sr,Ag,Mn and O elements without any impurity.The presence of Ag element has confirmed by XPS spectra,corresponding to XRD and EDS results.As x=0.19,the TCR peak value reached 16.19%K^-1,and TCR peak temperature reached room temperature(297.43 K).The results indicate that Ag doping has a significant changing on the double exchange(DE)effect and plays an important role in improving the electrical transport properties of LSAMO ceramics.The stoichiometric ratio of Ag is 0.2,and the crystal structure,surface morphology,electrical transport and magnetic properties of La_0.8-ySr_yAg_0.2MnO₃ polycrystalline ceramics were analyzed.Under the condition of bivalent Sr doping,the structural distortion of specimens was confirmed by the shift of the main diffraction peak of X-ray diffraction to low angle and the red-shift of Fourier transform infrared spectrum.The fitting result of the Mn 2p_3/2 spectrum confirmed the increase of Mn⁴⁺ion concentration,indicating that the double exchange(DE)effect was significantly enhanced.The value of TCR is increased from 2.5%K^-1(211.5 K)when y=0.00 to 16.9%K^-1(299.8 K)when y=0.175,which provides a theoretical and experimental basis for the application of the material in uncooled infrared bolometers and magnetic sensors.