钙钛矿锰氧化物La_（0.67-x）Ca_（0.33）MnO_3和La_（0.8-x）Nd_xNa（0.2）MnO_3系列样品磁卡效应的研究
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摘要
本文用溶胶—凝胶法制备了：1．在La位掺杂有少量空位浓度的系列多晶样品La_(0.67-x)Ca_(0.33)MnO_3(x=0.00，0.02，0.06和0.10)；2．用少量Nd替代La_(0.8)Na_(0.2)MnO_3中的La，即La_(0.8-x)Nd_xNa_(0.2)MnO_3(x=0.00，0.05，0.10，0.15和0.20)系列多晶样品。对这两个系列样品的磁性能及磁卡效应作了研究。X射线衍射实验表明这两个系列样品均为单相钙钛矿结构，用扫描电子显微镜(SEM)观察了样品的晶粒形貌，用振动样品磁强计(VSM)测量了样品的磁性能及磁卡效应。主要实验研究结果如下：
1．对于La_(0.67-x)Ca_(0.33)MnO_3系列多晶样品
(1) 结构研究表明，La-(0.67-x)Ca_(0.33)MnO_3系列样品为正交畸变钙钛矿结构，由XRD的数据计算可得，在La位掺杂的少量空位浓度减小了样品的晶胞体积。
(2) 用扫描电子显微镜(SEM)观察了样品的晶粒形貌。实验发现采用溶胶—凝胶法制备的样品，颗粒形状基本呈球状，且颗粒度较均匀。样品的颗粒尺寸分布在150～300nm范围之间。
(3) 热处理温度对样品的磁性能有着重要的影响。样品经由1073K、1273K和1473K三个温度下烧结而成。对于相同组分的样品，平均颗粒尺寸随着热处理温度的升高而略有增大，且随着样品最后烧结温度的升高，样品的居里温度也随之升高。同时热处理温度对样品的磁卡效应也有着重要的影响，实验结果表明，对于La_(0.67)Ca_(0.33)MnO_3(x=0.02)，经较高温度(1473K)烧结后制成的样品，在磁场的作用下，可能发生了一级相变，其它经较低温度下烧结的样品发生的则是二级相变，这使得在1473K下处理后得到的样品表现出了较大的磁熵变。
(4) 在La位掺杂少量空位浓度也对样品的磁性能产生了重要的影响。(Ⅰ)对居里温度T_c的影响：La位空位浓度的掺杂可提高样品的居里温度，且对于1473K下热处理的该系列样品，其居里温度随空位浓度的增加而稍有提高。实验结果表明没有掺杂空位的样品La_(0.67)Ca_(0.33)MnO_3(1473K温度下烧结)其居里温度为267K，在La位进行少量空位浓度掺杂形成的样品，其居里温度从277K(空位浓度x=0.02)升高到了284.3K(空位浓度x=0.10)。这使得具有不同空位浓度掺杂的样品，其居里温度比没有空位掺杂的La_(0.67)Ca_(0.33)MnO_3提高了10～17K，从而将这一系列样品的居里温度调整到了室温附近。(Ⅱ)对磁熵变△S_M的影响：随着La位少量空位浓度x的加入，样品在居里温度
河北师范大学硕士学位论文
的最大磁墒变均有所增强。在1473K下烧结的系列样品中，Lao.67C甄33MnO3的磁卡效
应为当外磁场H=IT时，最大磁嫡变△而在居里温度附近(一267K)0.84 J/kg一当
空位浓度x=0.02时，磁嫡变增强得最为明显，在相同条件下该样品的△助为2.78.J/k gK，
与没有掺杂空位的样品相比，最大磁嫡变增加了两倍多，并且其居里温度升高到了277
K，接近室温。这可能是因为在这系列样品中只有x二0.02的样品表现出了一级相变，
其它样品发生的是二级相变，同时XRD数据表明，在x=0.02的样品中，由空位的掺
杂所造成的晶格体积的减小最为明显，由此可能造成了该样品具有较大的磁嫡变。
以上实验结果表明对钙钦矿锰氧化物进行不同空位浓度掺杂形成的
L甄67一C盯33Mn仇系列样品有望作为室温附近的磁致冷工质。·
2.对于La08-xNdxNao，ZMnO3系列样品
(l)结构研究表明，Lao.8一dxNao.ZMnO3系列样品为三角晶系钙钦矿结构，样品形
貌基本呈球形，在1073K、1273K和1473K下热处理的样品颗粒尺寸约为200~350nm。
(2)由XRD数据计算得到，Nd离子替换La离子减小了样品的晶胞体积。
(3)对该系列样品的磁卡效应研究表明，在La位掺杂少量的Nd将样品的居里温度
降低了，使其从333，7K调整到了室温295K，同时样品的磁嫡变基本保持不变。其中
对于样品Lao.8雀NdxN盯ZMnO3(x=0.20)在居里温度Tc一295K即室温附近，外磁场IT
下，最大磁墒变为1.68)瓜gK。由此可知该系列也可以作为室温范围的磁致冷工质的候
选者。
Polycrystalline samples of La0.67-xCa0.33MnO3 (x=0.00, 0.02, 0.06 and 0.10) with different La-site vacancies and La0.8-xNdxNa0.2MnO3 (x=0.00, 0.05, 0.10, 0.15 and 0.20) doped with a small amounts of Nd in La-site have been prepared by a sol-gel method. The influence of the vacancy concentration in the La-site on their Curie temperatures and magnetocaloric properties has been investigated. XRD results show that the samples are single phase of perovskite-type oxides. The grain sizes of the samples are observed by SEM. The Curie temperatures and the magnetic entropy changes of the samples are measured on a vibrating sample magnetometer (VSM). The main experimental results are as follows.
1. Results of series samples La0.67-xCa0.33MnO3
(l)The studies on samples' structure show that La0.67-xCa0.33MnO3 series are orthorhombically distorted perovskite and the doping of small numbers of vacancies in La-site reduces the cell volume.
(2) The grain sizes of La0.67-xCa0.33MnO3 observed by SEM images are 150 to 300 ran.
(3) The influence of heat treatment temperature on the magnetization of these samples is remarkable. The samples, were sintered at 1073 K, 1273 K and 1473 K respectively. From the scanning electron microscope (SEM) images, the higher the heat treatment temperature is, the larger the grain size is. Along with going up of sintering temperature, the Curie temperature of the sample also goes up slightly. The samples calcined under higher temperature 1473 K show a bigger magnetic entropy change.
(4)The results of the Curie temperature indicate that the doping of small numbers of vacancies in La-site is in favour of raising the Curie temperature and the Tc becomes higher with the increase of the vacancy concentration. The Curie temperature of La0.67Ca0.33MnO3 is 267 K. With the addition of the vacancy, the Curie temperature increases from 277 K of La0.67-xCa0.33MnO3 (x=0.02) to 283.4 K of La0.67-xCa0.33MnO3 (x=0.10). The Curie temperature of the doped samples with different concentration of vacancy is about 10-17 K higher than that of La0.67Ca0.33MnO3, which adjusts the Curie temperatures of these samples to near the room temperature. At the same time, the vacancy dopant favors the enhancement of the magnetocaloric effect in this series. The enhancement reaches the maximum value of (2.78
J/kg-K) with a smaller doping (x=0.02) and an enhancement in SMby a factor 2 is observed. Using the Banerjee criterion, we can investigate the nature of the magnetic transitions. The sample La0.65Ca0.33MnO3 (x=0.02) with a small amount of hole-doping in the La-site, exhibits a clear first-order transition. It shows a much bigger Sm than that of the other samples which exhibit a second-order transition.
Results of magnetic measurement show that these materials with La-site vacancies can be used as candidates for magnetic refrigerants near room temperature. 2. Results of series samples La0.8-xNdxNa0.2MnO3
(l)The X-ray diffraction patterns indicate that the samples La0.8-xNdxNa0.2MnO3 have the rhombohedral perovskite structure. The grain size of samples observed directly by SEM images is about 200~350 nm.
(2)It is indicated that the doping of Nd in La-site reduces the cell volume calculated from the x-ray data.
(3)The Curie temperatures of La0.8-xNdxNa0.2MnO3 are adjusted from 333.7 K to room temperature 295 K. The magnetic entropy changes of these samples are comparable to that of La0.8Na0.2MnO3. And the sample with x=0.20 behaviors a large magnetic entropy change SM-1.68 J/kgK at the Curie temperature 295 K under a field of IT. The results show that these samples are also suitable as room temperature magnetic refrigerant materials.
1．对于La_(0.67-x)Ca_(0.33)MnO_3系列多晶样品
(1) 结构研究表明，La-(0.67-x)Ca_(0.33)MnO_3系列样品为正交畸变钙钛矿结构，由XRD的数据计算可得，在La位掺杂的少量空位浓度减小了样品的晶胞体积。
(2) 用扫描电子显微镜(SEM)观察了样品的晶粒形貌。实验发现采用溶胶—凝胶法制备的样品，颗粒形状基本呈球状，且颗粒度较均匀。样品的颗粒尺寸分布在150～300nm范围之间。
(3) 热处理温度对样品的磁性能有着重要的影响。样品经由1073K、1273K和1473K三个温度下烧结而成。对于相同组分的样品，平均颗粒尺寸随着热处理温度的升高而略有增大，且随着样品最后烧结温度的升高，样品的居里温度也随之升高。同时热处理温度对样品的磁卡效应也有着重要的影响，实验结果表明，对于La_(0.67)Ca_(0.33)MnO_3(x=0.02)，经较高温度(1473K)烧结后制成的样品，在磁场的作用下，可能发生了一级相变，其它经较低温度下烧结的样品发生的则是二级相变，这使得在1473K下处理后得到的样品表现出了较大的磁熵变。
(4) 在La位掺杂少量空位浓度也对样品的磁性能产生了重要的影响。(Ⅰ)对居里温度T_c的影响：La位空位浓度的掺杂可提高样品的居里温度，且对于1473K下热处理的该系列样品，其居里温度随空位浓度的增加而稍有提高。实验结果表明没有掺杂空位的样品La_(0.67)Ca_(0.33)MnO_3(1473K温度下烧结)其居里温度为267K，在La位进行少量空位浓度掺杂形成的样品，其居里温度从277K(空位浓度x=0.02)升高到了284.3K(空位浓度x=0.10)。这使得具有不同空位浓度掺杂的样品，其居里温度比没有空位掺杂的La_(0.67)Ca_(0.33)MnO_3提高了10～17K，从而将这一系列样品的居里温度调整到了室温附近。(Ⅱ)对磁熵变△S_M的影响：随着La位少量空位浓度x的加入，样品在居里温度
河北师范大学硕士学位论文
的最大磁墒变均有所增强。在1473K下烧结的系列样品中，Lao.67C甄33MnO3的磁卡效
应为当外磁场H=IT时，最大磁嫡变△而在居里温度附近(一267K)0.84 J/kg一当
空位浓度x=0.02时，磁嫡变增强得最为明显，在相同条件下该样品的△助为2.78.J/k gK，
与没有掺杂空位的样品相比，最大磁嫡变增加了两倍多，并且其居里温度升高到了277
K，接近室温。这可能是因为在这系列样品中只有x二0.02的样品表现出了一级相变，
其它样品发生的是二级相变，同时XRD数据表明，在x=0.02的样品中，由空位的掺
杂所造成的晶格体积的减小最为明显，由此可能造成了该样品具有较大的磁嫡变。
以上实验结果表明对钙钦矿锰氧化物进行不同空位浓度掺杂形成的
L甄67一C盯33Mn仇系列样品有望作为室温附近的磁致冷工质。·
2.对于La08-xNdxNao，ZMnO3系列样品
(l)结构研究表明，Lao.8一dxNao.ZMnO3系列样品为三角晶系钙钦矿结构，样品形
貌基本呈球形，在1073K、1273K和1473K下热处理的样品颗粒尺寸约为200~350nm。
(2)由XRD数据计算得到，Nd离子替换La离子减小了样品的晶胞体积。
(3)对该系列样品的磁卡效应研究表明，在La位掺杂少量的Nd将样品的居里温度
降低了，使其从333，7K调整到了室温295K，同时样品的磁嫡变基本保持不变。其中
对于样品Lao.8雀NdxN盯ZMnO3(x=0.20)在居里温度Tc一295K即室温附近，外磁场IT
下，最大磁墒变为1.68)瓜gK。由此可知该系列也可以作为室温范围的磁致冷工质的候
选者。
Polycrystalline samples of La0.67-xCa0.33MnO3 (x=0.00, 0.02, 0.06 and 0.10) with different La-site vacancies and La0.8-xNdxNa0.2MnO3 (x=0.00, 0.05, 0.10, 0.15 and 0.20) doped with a small amounts of Nd in La-site have been prepared by a sol-gel method. The influence of the vacancy concentration in the La-site on their Curie temperatures and magnetocaloric properties has been investigated. XRD results show that the samples are single phase of perovskite-type oxides. The grain sizes of the samples are observed by SEM. The Curie temperatures and the magnetic entropy changes of the samples are measured on a vibrating sample magnetometer (VSM). The main experimental results are as follows.
1. Results of series samples La0.67-xCa0.33MnO3
(l)The studies on samples' structure show that La0.67-xCa0.33MnO3 series are orthorhombically distorted perovskite and the doping of small numbers of vacancies in La-site reduces the cell volume.
(2) The grain sizes of La0.67-xCa0.33MnO3 observed by SEM images are 150 to 300 ran.
(3) The influence of heat treatment temperature on the magnetization of these samples is remarkable. The samples, were sintered at 1073 K, 1273 K and 1473 K respectively. From the scanning electron microscope (SEM) images, the higher the heat treatment temperature is, the larger the grain size is. Along with going up of sintering temperature, the Curie temperature of the sample also goes up slightly. The samples calcined under higher temperature 1473 K show a bigger magnetic entropy change.
(4)The results of the Curie temperature indicate that the doping of small numbers of vacancies in La-site is in favour of raising the Curie temperature and the Tc becomes higher with the increase of the vacancy concentration. The Curie temperature of La0.67Ca0.33MnO3 is 267 K. With the addition of the vacancy, the Curie temperature increases from 277 K of La0.67-xCa0.33MnO3 (x=0.02) to 283.4 K of La0.67-xCa0.33MnO3 (x=0.10). The Curie temperature of the doped samples with different concentration of vacancy is about 10-17 K higher than that of La0.67Ca0.33MnO3, which adjusts the Curie temperatures of these samples to near the room temperature. At the same time, the vacancy dopant favors the enhancement of the magnetocaloric effect in this series. The enhancement reaches the maximum value of (2.78
J/kg-K) with a smaller doping (x=0.02) and an enhancement in SMby a factor 2 is observed. Using the Banerjee criterion, we can investigate the nature of the magnetic transitions. The sample La0.65Ca0.33MnO3 (x=0.02) with a small amount of hole-doping in the La-site, exhibits a clear first-order transition. It shows a much bigger Sm than that of the other samples which exhibit a second-order transition.
Results of magnetic measurement show that these materials with La-site vacancies can be used as candidates for magnetic refrigerants near room temperature. 2. Results of series samples La0.8-xNdxNa0.2MnO3
(l)The X-ray diffraction patterns indicate that the samples La0.8-xNdxNa0.2MnO3 have the rhombohedral perovskite structure. The grain size of samples observed directly by SEM images is about 200~350 nm.
(2)It is indicated that the doping of Nd in La-site reduces the cell volume calculated from the x-ray data.
(3)The Curie temperatures of La0.8-xNdxNa0.2MnO3 are adjusted from 333.7 K to room temperature 295 K. The magnetic entropy changes of these samples are comparable to that of La0.8Na0.2MnO3. And the sample with x=0.20 behaviors a large magnetic entropy change SM-1.68 J/kgK at the Curie temperature 295 K under a field of IT. The results show that these samples are also suitable as room temperature magnetic refrigerant materials.
引文
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