【作者】 徐彩萍；

【导师】 孔见；

【作者基本信息】 南京理工大学 ， 材料学， 2009， 硕士

【摘要】 掺杂的类钙钛矿型锰氧化物由于其复杂的电磁特性,在很多领域都有着广泛的应用。作为磁制冷材料,该类材料的优点在于居里点可调,价格相对便宜,化学性能稳定,电阻率大,且有几种类钙钛矿型化合物的磁热效应比同等条件下Gd的磁熵变大,若能较好地解决将居里点调高到室温时磁熵变不大幅下降的问题,则应用前景广阔。本文主要研究了烧结温度及碱土金属元素Sr和Ca的添加对类钙钛矿型锰氧化物的结构、组织与性能的影响规律,以获得居里温度（Tc）在室温附近,低场作用下有较大磁熵变（⊿SM）的试样。通过调整成分及烧结温度,获得了Tc在室温附近,较低外加磁场作用下能获得较大⊿SM的类钙钛矿型锰氧化物材料。1723K下烧结后的试样La_0.825Sr_0.15Ca_0.025MnO₃能获得钙钛矿单相,孔隙率低,晶粒尺寸大小均匀,Tc为280K,在1.5T外场作用下的⊿SM可高达3.29J·kg^-1.K^-1;1723K下烧结后的试样La_0.8Ca_0.025Sr_0.175MnO₃能获得钙钛矿单相,孔隙率低,晶粒尺寸大小均匀,Tc为305K,在1.5T外场作用下的⊿SM可达到2.48J·kg^-1·K^-1。碱土金属元素Sr和Ca的添加对材料的结构、组织、性能有较大的影响。Sr的加入会减缓烧结过程的进行,随着Sr含量的增加,试样孔隙率增大,晶粒尺寸减小;Mn-O-Mn键角变大,Mn-O键长变短,晶格畸变程度减小,Tc升高,⊿SM上升。Ca的加入促进了烧结过程的进行,使得晶粒尺寸增大:Ca部分替代La以后,Mn-O-Mn键角变大,Mn-O键长变长,Tc升高,⊿SM增大;Ca部分替代Sr以后,Mn-O-Mn键角变小,Mn-O键长变短,Tc下降,⊿SM减小;Ca同时替代La和Sr以后,Mn-O-Mn键角变大,Mn-O键长变短,Tc升高,⊿SM减小。烧结温度是材料制备工艺中较重要的参数,通过实验发现其对试样的结构、组织、性能的影响并不成单调趋势,不同成分的试样受烧结温度影响的规律不相同,烧结温度与成分这两种因素相互作用,共同影响着试样的相组成与晶体结构、微观组织形貌、居里温度以及磁熵变。更多还原

【Abstract】 Perovskites-like manganites are widely used in many areas due to their complex electromagnetic character when doped with some other elements.They show advantages as following:tunable Curie temperature,cheap price,stable chemical properties and high resistivity when they are used as magnetic refrigerants;some kinds of perovskites-like manganites were found to have magnetocaloric effect larger then Gd when treated with the same condition.If we can resolve the problem that magnetic entropy will not greatly reduce along with the Curie temperature increasing,then they will have a wide application prospect. Impacts of sintering temperature and alkaline earth metals Sr and Ca on microstructure, crystal structure and properties of are studied in this paper.Hence,the specimens with their Curie temperature near room temperature and magnetic entropy change as high as possible treated with low field were prepared.The larger⊿S_M perovskites-like manganites speciments were prepared by adjusting composition and sintering temperature.After sintering at 1723K,the La_0.825Sr_0.15Ca_0.025MnO₃ has perovskites-like type,low porosity,even grain size,280K of Tc,3.29 J·kg^-1·K^-1 of⊿S_M when treated with magnetic field of 1.5T,and the La_0.8Ca_0.025Sr_0.175MnO₃ 305K of Tc,2.48 J·kg^-1·K^-1 of⊿S_M.Adding of alkaline earth metals Sr and Ca has a considerable effect on the structure, microstructure and properties of materials.Adding of Sr slows the sintering process,the porosity of specimen increases and grain size reduces as the amount of Sr increases; bond-angle of Mn-O-Mn expands,bond-length of Mn-O get shorter,distortion of crystal lattice becomes less,both of Tc and⊿S_M rise.Adding of Ca promotes the sintering process for the grain size get larger.When Ca is added to substitute La,bond-angle of Mn-O-Mn expands,bond-length of Mn-O get longer,both of Tc and⊿S_M increase;When Ca is added to substitute Sr,bond-angle of Mn-O-Mn decreases,bond-length of Mn-O get shorter,both of Tc and⊿S_M decrease;When Ca is added to substitute both of La and St,bond-angle of Mn-O-Mn increases,bond-length of Mn-O get shorter,Tc increases and⊿S_M decreases.Sintering temperature is an important parameter in fabrication of materials.Research results show that impacts of sintering temperature on crystal structure,microstructure and properties are not in a monotony trend.Specimens with different composition have different rules.The factor of sintering temperature and composition interact with each other,they have impacts on the phase composite,crystal structure,microstructure,Tc and⊿S_M together.更多还原