La_1-yA_yxK_xMnO₃(0<y<0.4, A=Ca, Sr ) Study On The Preparation Of Polycrystalline Ceramics And Their Electrical Transport Properties

Perovskite manganese oxides with formula of RE_1-xA_xMn O₃(RE=rare-earth,e.g.La and Pr;A=alkaline-earth or alkali-metals like K and Sr)have attracted extensive attention due to their ferromagnetic metal to paramagnetic insulator(FMM-PMI)transition,lattice effect,as well as colossal magnetoresistance(CMR)effect,and so on.Owing to high temperature coefficient of resistivity(TCR)and magnetoresistance(MR)near the FMM-PMI transition,they can be used to fabricate uncooled infrared bolometers and magnetic sensors.It would be helpful to improve the sensitivity and stability of uncooled infrared bolometers and magnetic sensor devices through optimized the TCR and MR values of REAMO ceramics by A-sites mixed-valence co-doping.Because the Jahn-Teller(JT)effect,the double-exchange(DE)mechanism,and the phenomenological percolation(PP)model together largely determine the TCR and MR,it is necessary to combine existing theories and models to explain the TCR and MR properties of perovskite manganese oxides.Herein,La_1-ySr_y-xK_xMn O₃(A=Ca,Sr)ceramics material are synthesized via the sol-gel process,and the structure and properties of samples were measured by using X-ray diffraction(XRD),field emission scanning electron microscope(FESEM),etc.The main work is as follows:The electric neutrality can be achieved by changing valence of manganese under the replacement of trivalent La³⁺ion by monovalent K⁺ion.The ionic radius of K⁺is much larger than that of La³⁺and the potassium element is low-boiling dopant.The A-sites vacancy and disorder degree(?²)of A-sites cations are generated by these factors,and it will cause change of Mn³⁺/Mn⁴⁺content and distortion of Mn O₆ octahedron.In addition,the La_0.9-xK_xCa_0.1Mn O₃ ceramics can be achieved dynamic broad A-sites vacancy and?² of A-sites cations.This change remarkably improved the DE mechanism,JT effect and PP model.Hence,the utilization of A-sites vacancy and?² of A-sites cations to optimize room-temperature TCR and MR of La_0.9-xK_xCa_0.1Mn O₃ ceramics.At the optimal doping content x=0.15,the maximum TCR value is 9.7%K^-1 at 291.4 K,and the maximum MR value is 35.2%at 295.03 K,respectively.Large-radius K⁺ions occupying A-sites of La_1-ySr_y-xK_xMn O₃ ceramics,which will affect the crystal structure,concentration of Mn³⁺-O^2--Mn⁴⁺pairs,and distortion of Mn O₆ octahedron.Therefore,the DE mechanism,JT effect,and PP model can be improved by modulating the stoichiometric ratio of K⁺,Sr²⁺,and La³⁺at A-sites.The co-occupation of La³⁺,Sr²⁺and K⁺ions at A-sites can enhance the room-temperature TCR of polycrystalline La_1-ySr_y-xK_xMn O₃ ceramics.Herein,La_1-ySr_y-xK_xMn O₃ceramics are synthesized via a facile sol-gel method.The optimized x content for the polycrystalline La_0.8Sr_0.2-xK_xMn O₃ ceramics is determined to be x=0.08.At this x value,the peak TCR of the resistivity measured 9.11%K^-1 at 292.44 K.Peak TCR of the La_0.81Sr_0.09K_0.1Mn O₃ ceramics is found to be 11.56%K^-1 at 297.15 K.The TCR_max of La_0.7K_0.3-xSr_xMn O₃ ceramics increases from 6.4%K^-1 at270.2 K(x=0.00)to 11.9%K^-1 at 291.2 K(x=0.05),and the low-field(1 T)MR_max reaches 30.5%at294.5 K.With previous works,this paper combines the electrical and magnetic properties of La_1-xCa_xMn O₃,La_1-xSr_xMn O₃,and La_1-xK_xMn O₃ using Sol-gel method to prepare the sample.The replacement of trivalent La³⁺,Ca²⁺,and Sr²⁺ions by monovalent K⁺ions,significantly influence the crystal structure,formation of Mn³⁺/Mn⁴⁺,and distortion of Mn O₆ octahedron.Herein,polycrystalline La_0.8K_0.04Ca_0.16-xSr_xMn O₃ ceramics are successfully synthesized via the sol-gel process.The electrical transport property of ceramics is analyzed via a combination of DE mechanism,JT effects and PP model.At optimal doping content of 0.12,peak TCR of the resistivity was found to be 12.04%K^-1 at 290.38 K,which is the best reported TCR for A-site mixed-valence co-doped perovskite manganese oxides.These results confirm that high room-temperature TCR values can be achieved by optimizing A-site mixed-valence co-doping and demonstrate the potential of polycrystalline La_0.8K_0.04Ca_0.04Sr_0.12Mn O₃ceramic in uncooling infrared bolometers.