Three-dimensional size and orientation of the precipitates in AZ91 magnesium alloys measured by TEM techniques

Knowledge of the microscopic structure, including three-dimensional (3-D) size and orientation of the precipitates, is essential to fully understand the mechanical properties of the magnesium alloys and designing the alloys with better performance. Analytical TEM with high spatial resolution offers the simultaneous measurements of 3-D size, structure, orientation, composition of the precipitates from one typical sample along an established crystallographic axis. Besides popular Burgers orientation relationship (OR), other ORs such as Pitsch-Schrader OR, Crawley OR, Potter OR and a new OR with the form of [0001]α 1.0° from [311]γ and (1120)α 2.0° from (033)γ between the magnesium matrix and the precipitate γ-Mg17Al12 are identified by TEM imaging and diffraction techniques. As a case study, the thicknesses of the individual precipitates with Burgers OR are further measured to be 100-200 nm through both electron energy-loss spectroscopy and x-ray energy dispersive spectroscopy combining differential x-ray absorption and extrapolation, which are in agreement with the overall 3-D size statistic distribution results obtained through analysing various samples along various directions. Furthermore, the fabricated wedge-shape structure provides a platform on which to study the dependence of the interfacial strain on the variation of the thickness.     

Fe3O4 Segregation and Transport Properties of La0.67Ca0.33MnO3

Effect of Fe3O4 segregation at grain boundaries on the electrical transport and magnetic properties of La0.67Ca0.33MnO3 is investigated. The experimental results show that the Fe3O4 segregation not only shifts the paramagnetic-ferromagnetic transition temperature of La0.6TCa0.33MnO3 to a lower temperature region but also induces a new transition in a lower temperature region. Meanwhile, the transition processes observed in both the resistivity and magnetization curves are obviously widened. Compared to pure La0.67Ca0.33MnO3, we assume that the Fe3O4 segregation level at the grain boundaries can modify the electrical transport and magnetic properties of La0.67Ca0.33MnO3.     

表面弛豫效应对界面区域CBED的影响

对Al_2O_3颗粒增强铝复合材料与K_2Ti_5O_(13)晶须增强铝复合材料中的增强相/金属界面区域点阵位移场进行CBED研究,并对其进行电子衍射动力学理论模拟计算,结果表明:表面弛豫效应对晶须/铝界面法线方向上位移分量大小的分布影响显著,而对界面切线方向上位移分量的影响很小。这一结果对于解决CBED法测量界面应变场时遇到的表面弛豫效应问题是有意义的。     

准晶的对称性和物理性质

本文较系统地论述了准晶的对称性和物理性质,内容涉及准晶的对称性与点群,群的表示理论与准晶的弹性性质,准晶的平衡性质热力学与物理性质张量等诸多方面,其中也包含了本研究小组近年来在这方面的工作和所取得的重要成果     

Experimental Observation on Orientation Relationship between Binary Cd5.7Yb Quasicrystal and its Crystalline Approximant Cd6Yb

Similarity in the electron diffraction patterns of the Cd5.7 Yb icosahedral quasicrystal and its cubic approximant Cd6 Yb crystal is studied in detail by using transmission electron microscopy. The icosahedral twofold axes are parallel to the cubic （1ττ^2） （τ= 1.618…） or （0 0 1） directions, the icosahedral pseudo-twofold axes are parallel to the cubic （15 21 25） directions, the icosahedral threefold axes are parallel to the cubic 〈1 1 1〉 and the icosahedral fivefold are parallel to the cubic 〈0 1 τ〉 directions. Based on those observations a general stereographic projection is given to illustrate the exact orientation relationship.     

轻元素K_α射线的质量吸收系数

本文简要评述轻元素(从 Be 到 F)K_α射线质量吸收系数的测定方法,即透射衰减法、等价膜厚法。吸收修正计算法、最有利电压法和全反射法,并将近十余年较有参考价值的测量或内插所得数据收集整理列成附表。     

尖晶石锂锰氧化物中氧缺陷对材料电化学性能的影响

研究了富锂锂锰氧化物中氧缺陷对材料电化学性能的影响。X射线衍射谱表明有氧缺陷的锂锰氧化物仍可保持尖晶石结构，当氧缺陷浓度达到一定程度时，高角度区的衍射峰出现分裂，说明其微观结构发生了变化。充放电实验结果表明氧缺陷使4V区的锂离子嵌脱变成了多步，模拟电池的容量微分曲线有3对清晰的氧化还原峰，低电位的氧化还原峰被—分为二。氧缺陷降低了材料的放电容量，同时引起低电位区的放电容量向更低电位区转移，使电化学性能恶化，锂锰氧化物中氧缺陷对电化学性能的影响对此类材料的合成提供了理论和实践依据。     

原子级模拟在四方SrTiO_3畴壁能计算中的应用

利用原子级计算机模拟对四方ＳｒＴｉＯ３中畴壁能进行了研究．在经典Ｂｏｒｎ模型基础上用来描述离子晶体的原子级模拟，其能量表达式由长程库仑势和短程排斥势构成．后者为一含参数表达式，式中参数可由晶体的晶格常数、晶体结构和弹性常数等实验数据拟合得出．四方ＳｒＴｉＯ３由高温立方ＳｒＴｉＯ３相变而来，应存在反相畴界、孪晶界和平移孪晶界等３种畴界．本文用原子级模拟的技术对这３种畴界能进行了计算．结果表明畴界能尤其是反相畴界能非常小．     

准晶的弹性,塑性与位错

本文较系统地综述了适合准晶的线弹性理论，准品位错的基本理论和实验鉴定，准晶位错的弹性理论，以及准晶高温塑性形变的微观机制。     

共价固体氮化碳的电弧法制备

β-C_3N_4是一种首先运用固体量子理论和计算机技术,通过人工设计具有理想成分和结构以及预期优异性能的新材料,Liu和Cohen的理论计算表明:一种晶体结构类似β-Si_3N_4的 C—N化合物具有非常短的共价键(平均键长0.147nm),其体变模量为4.27×10~5MPa,接近金刚石的4.43×10~5MPa。这预示它具有可与金刚石相媲美的硬度。此外,它的结合能计算值 (81.5eV/cell)大于β-Si_3N_4(74.8eV/cell),表明可能存在亚稳态结构。近年来,人们采用了 CH_4和H_2的等离子体分解,C—N—H有机物热解和冲击波压缩等方法,以期合成这种 C—N共价键固体。但是由于C—H和N—H键的稳定性,阻碍了β-C_3N-4的形成。直到1993年,哈佛大学的Niu等人首先用激光蒸发和原子束喷注相结合的方法,成功地获得了这种共价固体氮化碳薄膜。作者尝试采用高纯石墨电极在氮气中电弧放电的方法,代替激光蒸发法,也获得了这种材料。用X光电子能谱(XPS)证实了C—N共价键的生成,用X射线衍射 (XRD)和透射电镜选区电子衍射(TED)检验其结构,和Niu等人的结果基本相符。