Research On Preparation And Electromagnetic Properties Of La_0.67Ca_0.33-xSr_xMn_1-yB_yO₃:Ag_z(B=Fe,Co,Ni) Polycrystalline Ceramics

Perovskite manganate has a high temperature coefficient of resistance(TCR)and a large magnetoresistance(MR)effect,making it a novel multifunctional material.La1-x Cax Mn O3,a typical perovskite manganate,can be used in thermal radiation/infrared detecting,magnetic refrigeration,magnetic storage,etc.,therefore has received extensive attention in recent decades.TCR,MR indicates the sensitivity of the material's electric resistance responding to the changes in temperature and magnetic field respectively.To achieve high TCR value and simultaneously giant MR value is a practically key point in this field,especially to obtain high TCR and MR near room temperature is a big challenge in uncooled thermal radiation/infrared detecting and magnetic sensors.This thesis dedicates to improving the electric,magnetic transport properties of perovskite manganates,to enhancing the peak value of TCR(TCRpeak)and MR(MRpeak),and obtain high TCR and MR materials at/near room temperature.Taking the initial composition as La1-x Cax Mn O3 with x=0.33,which is the representative of perovskite manganites and studied most extensively.Polycrystalline ceramics are prepared by the sol-gel routine,the electrical and magnetic properties after element doping are systematically studied.The main results are as follows:(1)For the basic composition of La0.67Ca0.33Mn O3,the TCRpeak is 40.1%·K^-1,the MRpeak is 60.3%,and the metal-insulator transition temperature(TMI)is 266.6 K.Three series of polycrystalline ceramics were prepared by replacing Mn site with Fe,Co,and Ni respectively,i.e,La0.67Ca0.33Mn1-y By O3(B=Fe,Co,Ni).As for MR and TCR,Fe,Co,and Ni show their own characteristics.At low doping,Fe,Co,and Ni are all beneficial to simultaneously increasing the value of TCR and MR.Fe doping can increase the grain size while reducing lattice distortion,and it also can drastically enhance MR.When Fe doping is 0.06,MRpeak reaches 81.1%.(2)Ag is added in La0.67Ca0.33Mn1-y By O3,i.e.,La0.67Ca0.33Mn1-y By O3:Agz.In La0.67Ca0.33Mn0.98Fe0.02O3:Agz series,when Ag doping is 0.15,TCRpeak increases from 45.2%·K^-1,without Ag doping,to 75.4%·K^-1,while MRpeak increases to87.0%;In La0.67Ca0.33Mn0.97Co0.03O3:Agz series,when Ag doping 0.1,TCRpeak increases from 44.2%·K^-1 to 65.2%·K^-1,and MRpeak reaches 82.6%when Ag doping is 0.05;In the La0.67Ca0.33Mn0.97Ni0.03O3:Agz series,when the Ag content is0.05,TCRpeak reaches 56.7%·K^-1,while MRpeak is 78.1%.For Mn-site doping,Ag doping can further effectively increases both TCRpeak and MRpeak,especially increases TCR value drastically.The good crystallinity and uniformity induced by Ag doping are the main reasons for the enhancement of TCR and MR.(3)When Ag amount is fixed at 0.15,Fe,Co,and Ni replaces 0.02 Mn respectively,that is,La0.67Ca0.33Mn0.98B0.02O3:Ag0.15(B=Fe,Co,Ni).The substitution of Fe,Co,and Ni for Mn all promotes the growth of the grain size,which is more obvious in the case of Fe doping.Mn-site doping elements have different effects on the properties of La0.67Ca0.33Mn O3:Ag0.15,all in all MR has been increased,in which Fe doping presents the most promising potential in increasing MR.(4)On the basis of Mn-site doping and Ag doping to optimize the physical properties of La0.67Ca0.33Mn O3,TMI,which determines the applicating temperatureis of La0.67Ca0.33Mn O3 compound,is further tailored by Sr doping,namely La0.67Ca0.33-x Srx Mn O3:Ag0.15 and La0.67Ca0.33-x Srx Mn0.98Fe0.02O3:Ag0.15.After Sr doping,the grain size increses.With the increase of Sr doping,TMI moves to high temperature/room temperature as expected,while the TCRpeak and MRpeak gradually decreases.For La0.67Ca0.33-x Srx Mn O3:Ag0.15 series,when Sr doping is 0.04,the TMI of the system increases to 302.2 K,and TCRpeak and MRpeak are 24.0%·K^-1 and 49.2%,respectively.By Sr doping,polycrystalline perovskite manganate ceramics with high TCR and high MR values at near room temperature can be obtained.In conclusion,through Mn-site element doping and Ag doping,the TCR and MR of La0.67Ca0.33Mn O3 polycrystalline can be effectively improved.Among them,the optimization effect of Fe and Ag co-doping is the most significant;further doping of Sr can move TMI to near room temperature,while maintaining high TCR and MR,La0.67Ca0.33Mn O3 is used in the field of uncooled thermal radiation infrared detection and magnetic sensitivity to lay a solid experimental foundation.