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Structural,Magnetic and Magnetocaloric Properties of La-deficient La0.77-xSrxCa0.2MnO2 Perovskites 下载免费PDF全文
La-deficient La0.77-x Ca0.2SrxMnO3 (0 〈 x 〈 0.1) polycrystalline samples are synthesized using the sol-gel technique. The crystal structures of all the samples are single orthorhombic phase with Pbnm space group. Rietveld analysis of x-ray diffraction patterns shows that the Mn-O-Mn bond angle θUn-O-Mn increases whereas the Mn-O bond length dMn--o decreases monotonically with increasing Sr^2+ content, which results in a rich overlap between Mn 3d and O2p orbitals and leads to a systematic increase of the Curie temperature in this compound. It is found that the magnetic entropy change has a maximum at x = 0.06 in La^3+-deficient La0.77-xCa0.2SrxMnO3. This may result from competition between the super-exchange interactions (Mn 4+-O2--Mn4+) and double-exchange interactions (Mn^3+-O^2- -Mn^4+) originating from the appearance of superfluous Mn^4+ ions by substitution of Sr^2+ for La^3+ in this series. Large magnetic entropy changes of 2.32 and 1.83Jkg^-1K^-1 in the x = 0.06 and x = 0.1 samples at their Tcs of 271 K and 303K upon a low magnetic field (10kOe) make these materials promising candidates at near room temperature. 相似文献
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激光在大气中的传输衰减特性是激光工程应用中需要考虑的一个重要问题.本文针对常用的1.06 μm和10.6μm激光,基于Mie散射理论计算了气溶胶粒子的单次散射参量;对于激光在气溶胶中多次散射传输衰减,建立了蒙特卡罗模拟计算模型,利用Matlab语言编制了相应的计算程序,计算分析了两种波长的激光分别在沙尘性、水溶性、海洋性和煤烟性四种不同类型气溶胶中透过率与传播距离、能见度的关系,并将蒙特卡罗方法和单次散射的计算结果进行了比较.结果表明,当能见度较低、气溶胶粒子反照率较高时,单次散射计算存在很大的误差,用蒙特卡罗方法更能揭示多重散射现象;煤烟性气溶胶对1.06 μm激光的传输衰减影响最大,沙尘性气溶胶对10.6 μm激光的传输衰减影响最大. 相似文献
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