Related Properties And Magnetocaloric Effect In Doped Perovskite Manganese Oxides

Recently, the development of a new magnetic refrigeration technology, based upon the magnetocaloric effect (MCE), has brought an alternative to the conventional gas compression (CGC). The magnetic refrigeration near room temperature has significant importance for saving energy sources and protecting environment people relied on living. The large magnetocaloric effect in perovskite provides a new area for the magnetic refrigeration material near room temperature.In this work, the series of polycrystalline samples (La_1-xDy_x)_2/3(Sr)_1/3MnO3(x=0.00, 0.05, 0.10, 0.15, 0.20), which doped a little amounts of Dy in La-site have been prepared using solid-state reaction method; The polycrystalline samples of (La1-xKx)_1.4Sr_1.6Mn₂O₇(x=0.0, 0.1, 0.2) have been prepared using Sol-Gel technique. X-ray diffraction (XRD) has been used to identify the single phase of perovskite-type oxides and the lattice parameters as well as the cell volume of the samples have been calculated; The scanning electron microscopy (SEM) has been used to observe the morphology of the samples and to estimate the diameter of the particles; The vibrating sample magnetometer (VSM) has been applied to measure the magnetic properties and analyse the magnetocaloric effect of the samples. In addition, the samples with the norminal compositions La_1.4Ca_1.6Mn₂O₇ and La_0.7Ca_0.3MnO3 have been prepared by Sol-Gel technique.The main experimental results are as follows:1. (La_1-xDy_x)_2/3(Sr)_1/3MnO3(x=0.00, 0.05, 0.10, 0.15, 0.20)(1) The X-ray diffraction measurements show that all samples are single phase with ABO3-type perovskite. The lattice parameter a and the unit cell volume V calculated from XRD data decrease with Dy doping level. However, an abnormal increase of a and V was observed in the sample with x=0.05.(2) SEM observation shows that the particles exhibit an approximate spherical shape and it is estimated that the diameter of the particles is between 200³00nm.(3) The magnetic measurements of the compounds show that the Curie temperatures of (La_1-xDy_x)_2/3(Sr)_1/3MnO3(x=0.00, 0.05, 0.10, 0.15, 0.20) are 365K, 360 K, 358 K, 344 K and 313K, respectively. The magnetic entropy change increases at first and then decreases with the increase of Dy doping level, The maximum magnetic entropy change (0.97J/kg·K) occurs in the x=0.05 sample. The main cause of the variation in magnetic properties by doping Dy is that the substation of La3+ by a smaller cation Dy3+ results in the lattice distortion of the sample. (4) The studies of the magnetoresistance show that the magnetoresistance remainsapproximately a constant in the x=0.05 sample when T<350K. For heavier Dy doped samples (x=0.10, 0.15 and 0.20), the magnetoresistance increases with the decrease of tempersure when T approaches the room tempersure.2. (La1-xKx)_1.4Sr_1.6Mn₂O₇(x=0.0, 0.1, 0.2)(1) The XRD analysis show that the series of polycrystalline samples of (La1-xKx)_1.4Sr_1.6Mn₂O₇ prepared in this work are layered Sr3Ti₂O₇-type perovskite structure with a space group I4/mmm.(2) SEM observation shows that the samples are constituted of symmetrical spherical particles, ranging in size from 200 to 400 nm.(3) By doping K+ at La3+ site, the Curie temperatures of (La1-xKx)_1.4Sr_1.6Mn₂O₇ changes very little. This is because that for layered Sr3Ti₂O₇-type perovskite manganites-oxide the Curie temperatures is determined by the MnO2 interlayer exchange interaction, while the larger K+ cations favor occupying the twelve -coordinate sites in a-b plane. It has no effect on the interlayer exchange interaction. The magnetization M increases in the region of DTc 3≤T≤Tc 2 Dand decreases in the region of T≤Tc 3 Dwith the increase of K+ doping. This result may be attributed to the lattice distortion in the MnO2 intralayer caused by K+ doping.3. La_1.4Ca_1.6Mn₂O₇ and La_0.7Ca_0.3MnO3The X-ray diffraction measurements show that the La_0.7Ca_0.3MnO3 prepared by Sol-Gel method is single phase with ABO3-type perovskite. The XRD and VSM measurements show that La_1.4Ca_1.6Mn₂O₇ is a mixture of La_0.7Ca_0.3MnO3 and CaO, rather then layered Sr3Ti₂O₇-type perovskite structure.