Preparation And Electrical Transport Properties Of La_0.85-xSr_0.15Ag_xMnO₃ (0≤x≤0.2) Material

Manganese oxides with perovskite structure have outstanding optoelectronic properties,such as a steep metal-insulation transition characteristic and high temperature coefficient of resistance（TCR）.These enable them to show excellent performance in optoelectronic devices.High response,high sensitivity and focal plane arrays are the hot spots of current research in non-cooled infrared detectors.Among them,high sensitivity puts higher requirements on La_0.85-xSr_0.15Ag_xMn O₃materials and devices,and high TCR values and room temperature TCR peak temperature（T_k）become important parameters for uncooled radiometric thermometers.Currently,conventional ceramic bulk materials exhibit stable and excellent performance.However,it is difficult to integrate into the main thermistor of the radiometric thermometer,and new perovskite thermal thin film materials are urgently needed.In this thesis,samples with high TCR values and T_k at room temperature were prepared by varying the doping ratio of La and Ag at the A-sites.Furthermore,a systematic discussion of the effect of Ag doping on the electrical properties of La_0.85-xSr_0.15Ag_xMn O₃（0≤x≤0.20）materials is presented.The results of the study are as follows:Firstly,La_0.85-xSr_0.15Ag_xMn O₃（0≤x≤0.20）ceramic samples were prepared by the selected sol-gel method.With the increase of the Ag ion doping ratio,the grain growth made the grain boundary scattering effect weakened.The double exchange effect is maximized at x=0.15,while the suppressed JT distortion promotes a continuous linear decrease in resistivity.The fitting results of the resistivity versus temperature curves illustrate the reduction in the external scattering parameters,effective electron mass and jump activation energy.It can help the material to obtain excellent performance with higher T_k and TCR values at near room temperature.Among them,the T_k and TCR of La_0.7Sr_0.15Ag_0.15Mn O₃ sample are 294.1 K and 15.2%K^-1,respectively.Next,La_0.7Sr_0.15Ag_0.15Mn O₃ films of different thicknesses were prepared on（100）-oriented LaAlO₃ substrates by a sol-gel-spin-coating method.A large lattice mismatch between the film and the substrate（～2.2%）resulted in the films being subjected to a large in-plane compressive stress.However,the compressive stress decreased as the effective film size was increased.The increase in thickness was directly observed to promote grain growth,which was accompanied by a change in growth pattern from laminar to island-like.The release of epitaxial stresses by elevating the film thickness drives the resistivity and metal-insulation transition temperature（T_p）to move toward a higher temperature.what’s more,the MR value of 43.70%（294.2 K）could be reached at three layers of spin-coating.Lastly,Ag-doped La_0.85-xSr_0.15Ag_xMn O₃ films with three layers of thicknesses were prepared on LaAlO₃ substrates.The increased Ag doping content x at A-sites caused an increase in cell volume and lattice distortion,with a consequent increase in surface roughness.On the other hand,it led to a continuous enhancement of the DE effect.The competition between carrier concentration and electron coupling interaction causes the resistance of the films to reduce and then rise with increasing x,which impacts the magnitude of peak TCR and T_k.For the La_0.65Sr_0.15Ag_0.20Mn O₃film sample with x=0.20,T_k is 292.9 K and the peak TCR is 8.02%K^-1.In this thesis,the effect that the variation in Ag ion content at the A-sites has on ceramics and thin films was systematically investigated,and the samples with high TCR values at room temperature with excellent performance were achieved.The content of this study can be used as a theoretical and material basis for uncooled radiometric calorimetry and can provide a reference for the preparation and property study of chalcogenide ceramics and thin films in other similar systems.