Perovskite Manganese Oxides La <sub> Of Mno <sub>, 0.67-x </ Sub> Ca <sub> 0.33 </ Sub> 3 </ Sub> And La <sub> 0.8-x </ Sub> Of Nd < Sub> X </ Sub> Na (0.2) Of Mno <sub> 3 </ Sub> Ser

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 (2.78J/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.