LTMO薄膜的光诱导效应和旋转磁场下Pb-Sn合金的凝固行为

【作者】 王建元；

【导师】 陈长乐；

【作者基本信息】 西北工业大学 ， 光学工程， 2006， 硕士

【摘要】 本论文所做的工作分为两个部分。 第一部分（1-3章）:本部分工作较为系统的研究了非金属元素Te掺杂的锰氧化物La_1-xTe_xMnO₃（x=0.01,0.06,0.12,0.18）薄膜的CMR效应和光诱导效应。 采用固相反应法制备了非金属Te掺杂的锰氧化物La_1-xTe_xMnO₃块体,通过X射线分析发现,样品具有菱方结构。并以此块体为靶材用PLD法制备了外延薄膜。 研究了La_1-xTe_xMnO₃薄膜的CMR效应。掺杂量x=0.06,0.12,0.18的薄膜样品在温度由83K升至373K的过程中均发生了金属—绝缘体转变,转变温度T_MI随着掺杂量x的增大向高温移动。薄膜样品在外磁场H=0的输运性质在低温区域符合ρ=ρ₀+ρ₁T^2.5,在高温区域符合小极化子输运特性。薄膜在T_MI附近均显现了CMR效应。 首次研究了电子掺杂的锰氧化物材料的光诱导效应。在低温区,激光的照射使得薄膜电阻率增大,其中La_0.82Te_0.18MnO₃在253K下的电阻变化率达到峰值51.1%,这比已有的报道中相同条件下的空穴掺杂材料有很大的提高;在高温区激光作用产生了较小的光电导现缘。对掺杂量x=0.18的薄膜激光作用时的弛豫现象进行了探索,发现La_0.82Te_0.18MnO₃薄膜在248K,波长532nm的绿色激光照射下具有与自旋相关的弛豫现象,激光开始作用和停止作用时间t与电阻率ρ（t）满足指数函数关系:ρ=ρ₀+Aexp（-t/τ）。 第二部分（4、5章）:研制了旋转磁场条件下的金属凝固实验装置,该装置最高加热温度600℃,磁场强度可达到0.5T,磁场转速最大1200r/min。对Pb-Sn合金旋转磁场下的凝固行为进行了研究,发现与常规条件下相比,旋转磁场导致的合金熔体内部强迫对流,加快了合金熔体的冷却速率,提高了熔体初始形核温度,促发熔体提前形核并能使凝固时间缩短。Pb-61.9wt%Sn共晶合金的宏观偏析被有效的抑制,在整个样品中得到了二元共晶组织。共晶层片粗化,局域地区出现了不规则共晶组织。对于Pb-50wt%Sn亚共晶,初生相（Pb）的生长方式由粗大枝晶转变为细小的球状颗粒,且晶粒显著细化。更多还原

【Abstract】 The work of the thesis can be divided into two parts:The first part （section 1-3）: The CMR and photo-induced effect in the La_1-xTe_xMnO₃（x = 0.01, 0.06, 0.12, 0.18） thin films was investigated .The bulks of nonmetal Te valence magnetite -La_1-xTe_xMnO₃（x = 0.01, 0.06, 0.12, 0.18） were prepared by the solid state reaction . The XRD indicated that the structure of samples is rhombohedra. Then the thin films were prepared by PLD Method.The CMR effect of the thin films was investigated . the films （x=0.06, 0.12, 0.18）show a metal-insulate transition when the temperature raise from 83k to 373K. The temperature of Maximum of resistivity Tp become higher as the increase of the valence x. The transport results show that the data satisfy ρ=ρ₀ +ρ₁T^2.5 for T<0.5T_M1;when T>T_M1 it is agree with polaron model. The samples show CMR effect near Tp.The effect of The continue wave laser （532nm, 40mW） on the film was also investigated. Below T_M1 ,the resistivity increase under photo inducing. The maximum resistivity change（Δρ/ρ₀） in the La_0.82Te_0.18MnO₃ sample can reach 51.1% in 253K which is more large than that of hole-doped manganites. Above T_M1 there is a photoconduction. The relaxation was observed in the experiment of La_0.82Te_0.18MnO₃ film in 248K when the laser turn on and off. The resistivity ρ（t） and time t meet the exponential formula ρ = ρ₀ + A exp（t / τ）.The second part （section 4-5）: Develop the solidification device under Rotating-magnetic field. The heating temperature can reach 600℃. The intensity of magnetic field is 0.5T. The maximum rotational speed is 1200r/min. The solidification of Pb-Sn alloy under rotating-magnetic field was studied. According to the cooling curves, The roating magnetic field tend to accelerate the cooling rate and stimulate the nucleation of the melt compared to static condition .The stirring effect of magnetic tend to provide some extra energy fluctutation to overcome the thermodynamic barrier for nucleation. It is found that ,for Pb-61.9wt%Sn eutectic alloy,magnetic field induced the forced convection of the melt which result in a microstructure of lamellas →coarser lamellas→ anomalous eutectic"and suppression of gravity-microsegregatio-n and it was revealed that a "lamellar eutecticanomalous eutectic" .Meanwhile, for Pb-50wt%Sn eutectic alloy, a "dendrite manner spheric grain" microstructre change of the primary phase （Pb） occurs and conspicuous grain refinement effect takes place .These are explained by the forced convection inside the melt.更多还原