Preparation Of La_1-x(Ca,Sr)_xMnO₃:Ag_y Materials By Alcohol Solvent Sol-gel Method And Study Of Their Electromagnetic Propertie

The rare earth-doped manganites with perovskite structure have attracted extensive attention in the past decades because of their rich properties,e.g.,colossal magneto-resistance(CMR)effects,charge ordering,phase separation,metal-insulator transition,etc.leading to potential applications in magnetic memories,magnetic sensor,magnetic refrigeration,photonic devices,bolometer and infrared(IR)detector.However,there are still many problem such as temperature coefficient of resistance(TCR)value is very low or metal-insulator transition temperature(Tp)is below room temperature,which seriously restricts its application,and the produce craftwork needs long processing time.Therefore,a series of La1-x(Ca,Sr)xMnO3:Agy polycrystalline ceramics were synthesized by sol-gel method using methanol as solvent in this paper.The optimum sintering process were determined by changing calcination and sintering temperature.In order to obtain high TCR value of samples,the influence of Ag-adding,Ca-doping and Sr-doping level were studied.High quality thin films were synthesized by pulsed laser deposition technique.Effects of coating process(growth temperature,growth oxygen pressure,annealing oxygen pressure)on electrical transport properties were studied.The xerogel were studied by thermogravimetry-differential scanning calorimetry(TG-DSC).The crystal structure,surface morphology and electrical properties were investigated by X-ray diffraction(XRD),scanning electron microscope(SEM),plane energy spectra scan(mapping),atomic force microscope(AFM),resistance-temperature(R-T)and magnetic resistancetemperature(MR-T).The experimental results showed that the phase temperature does not transform by regulating solvents,but the sintering process have great influences on structure,surface morphology and electrical properties.Calaination temperature too high or too low is not suitable for ultrafine particle preparation.Calaination temperature too low will make serious material deformation during final stage sintering,but high calaination temperature will reduce the activity.The increase of sintering temperature will provide enough energy for grain growth leads to the number of grain boundaries and porosities decrease,density improve,resistivity decrease,TCR values up to 40%·K-1.With the increasing of Ag-adding significantly promoted grain growth.The appropriate amount of Ag content can improve surface morphology.With the increase of Ag content,the samples of resistivity decreases greatly.The TCR value increased significantly from without Ag content 48%·K-1 up to70%·K-1 the content at 0.2.The Ag content continues to increase,the TCR value remains unchanged.We can infer that the added Ag in the position of A-site partially substitute for La or Ca,which can improve the TCR value.However,this reaction is limited,the extra Ag deposited at the surface and grain boundary lead to resistivity decreases greatly.The concentration of Ca-doping also have a great influence on the morphology and electrical properties.Elemental mapping images show that all the elements are uniformly distributed over the entire sample.The content of Ca at 0.28,has almost no pore and bigger grain size,and the TCR value up to 72%·K-1.The resistivity gradually decreases and corresponding Tp monotonically shifts towards higher temperature region with the increase of Ca-doping content,the TCR value decreases.This is mainly due to the change of Ca2+concentration.Which gives rise to the change of Mn4+/Mn3+ratio and the decrease of A-site average ionic radius.In order to obtain high TCR value near room temperature by Sr-doping.A series of sample of Sr-doping were studied.The results show that increasing of Sr content,A-site average ionic radius increase,distortion of MnO6 octahedron,crystal structure change,the resistivity and TCR value decrease at the same time.This result is associated with A-site average ionic radius,and without the concentration of Mn4+.With high TCR value of La0.7Ca0.3MnO3:0.2Ag thin film were prepared by the pulsed laser deposition technique.The high TCR value is up to 37%·K-1 and 6%·K-1 at temperature 255 K and 290 K seperately.The results show promising application in the infrared and bolometric detectors for the materials.