Synthesis Of Doped Perovskite Manganites And Regulation Of Curie Temperature And Magnetocaloric Effect

Magnetic refrigeration is a new green refrigeration technology,which relies on its own magnetoccaloric effect to achieve cooling.The key to the commercialization of magnetic refrigeration is to find a refrigerant with a Curie temperature close to room temperature and a distinct magnetocaloric effect.The perovskite manganites are candidate material for room temperature magnetic refrigeration because of its adjustable Curie temperature and large magnetocaloric effect at low magnetic field and easy availability of raw materials.In this thesis,the impact of different ion doping on the structure,micromorphology,Curie temperature and magnetocaloric effect of manganites were investigated,and the related mechanisms were analyzed in depth.The influences of the synthesis techniques on the morphology and properties of the samples were explored and detailed explanations were given with the relevant refinement parameters.This article reported the synthesis of A-site doped perovskite manganites with Gd³⁺,Ca²⁺,and Sm³⁺using the sol-gel method.The crystal structure,microstructure,Curie temperature,and magnetocaloric effect of the doped samples were systematically investigated.（1）The increase in Gd³⁺concentration reduced the Curie temperature while the magnetic entropy change increased due to additional magnetic interactions.The La_0.725Gd_0.075Sr_0.2Mn O₃ possessed a Curie temperature of 295 K,which is approaching to the room temperature.The maximum magnetic entropy change was 4.23J·kg^-1K^-1 at an external magnetic field of 5 T,demonstrating its good potential for application.（2）For Ca²⁺doping,the Curie temperature gradually decreased while the magnetic entropy change showed the opposite trend.The La_0.075Gd_0.05Sr_0.15Ca_0.05Mn O₃owned a magnetic entropy change of 4.97 J·kg^-1K^-1under an external magnetic field of5 T and was expected to be a potential magnetic refrigerant.（3）With the increase of Sm³⁺content,the Curie temperature gradually decreased to near 290 K.The La_0.65Sm_0.05Sr_0.3Mn_0.95Ni_0.05O₃ possessed a Curie temperature of 300 K,a maximum magnetic entropy change of 4.59 J·kg^-1K^-1,and a relative cooling power of 297.39 J·kg^-1,indicating its huge potential as a room temperature magnetic refrigerant.A-site doping with Gd³⁺,Ca²⁺,and Sm³⁺changed the crystal parameters of the samples and effectively adjusts their Curie temperature and magnetocaloric properties.La_0.7Sr_0.3Mn_1-xB_xO₃ series samples with B-site doping were prepared by the sol-gel method,the crystal structure,micromorphology performances were investigated deeply.（1）With the enhancement of Fe³⁺domination,the Curie temperature gradually decreased.Among them,the La_0.7Sr_0.3Mn_0.925Fe_0.075O₃ owned a Curie temperature of289 K.When an exterior magnetic field of 5 T was applied,the maximum magnetic entropy change was 3.47 J·kg^-1K^-1 and the relative cooling efficiency was 276.4 J·kg^-1,signifying its favourable application potential.（2）Co³⁺doping extended the temperature range of the samples.For La_0.7Sr_0.3Mn_0.925Co_0.05O₃,its Curie temperature was near room temperature,and the magnetocaloric effect was enhanced.The main manifestation was that the magnetic entropy change under an external magnetic field of 5 T was 4.06 J·kg^-1K^-1,and the relative cooling efficiency was 309.67 J·kg^-1.B-site Fe³⁺and Co³⁺doping could be an effective method to adjust the Curie temperature and magnetocaloric effect of manganites.（3）B-site Ni²⁺-doped La_0.7Sr_0.3Mn_0.95Ni_0.05O₃samples were fabricated using conventional solid-state and sol-gel methods to explored the implications of synthesis method on the morphology,magnetic properties,and magnetocaloric performance of B-site manganites.The changes in structural parameters after Rietveld refinement were used to explain the performance changes in detail.The results showed that the changes in structural parameters caused by different preparation methods were strongly associated with the variations in properties,while the type of phase transition was independent of the preparation method.