Characteristics of Friedel pairs and diffraction contrast tomography with non‐perpendicular rotation axis

The characteristics of Friedel pairs in diffraction contrast tomography (DCT) are studied in the condition that the rotation axis of the sample is not exactly perpendicular to the incident X‐ray direction. For the rotation axis approximately aligned along the vertical direction, the Friedel pairs close to the horizontal plane are insensitive to the non‐perpendicularity of the rotation axis, and can be used to refine the sample‐to‐detector distance and X‐ray energy, while the Friedel pairs close to the vertical direction are sensitive to the non‐perpendicularity of the rotation axis, and can be used to determine the rotation axis orientation. The correct matching proportion of Friedel pairs decreases with increasing non‐perpendicularity of the rotation axis. A method of data processing considering rotation axis misalignment is proposed, which significantly increases the correct matching and indexing proportions of the diffraction spots. A pure aluminium polycrystalline sample is investigated using DCT at beamline 4W1A of Beijing Synchrotron Radiation Facility. Based on the analysis of Friedel pairs, the sample‐to‐detector distance and X‐ray energy are refined to be 8.67 mm and 20.04 keV, respectively. The non‐perpendicular angle of the rotation axis is calculated to be 0.10°. With these refined geometric parameters, the matching proportion of the spatial position of diffraction spots is 90.62%. Three‐dimensional reconstruction of the sample with 13 grains is realised using the algebraic reconstruction technique. It is demonstrated that the precise correction of the orientation of the sample rotation axis is effective in DCT suffering from rotation axis misalignment, and the higher accuracy in determining the rotation axis is expected to improve the reconstruction precision of grains.     

Charge redistribution and a shortening of the Fe—As bond at the quantum critical point of SmO1–xFxFeAs

Many researchers have pointed out that there is a quantum critical point (QCP) in the F‐doped SmOFeAs system. In this paper, the electronic structure and local structure of the superconductive FeAs layer in SmO_1–xF_xFeAs as a function of the F‐doping concentration have been investigated using Fe and As K‐edge X‐ray absorption spectroscopy. Experiments performed on the X‐ray absorption near‐edge structure showed that in the vicinity of the QCP the intensity of the pre‐edge feature at the Fe‐edge decreases continuously, while there is a striking rise of the shoulder‐peak at the As edge, suggesting the occurrence of charge redistribution near the QCP. Further analysis on the As K‐edge extended X‐ray absorption fine structure demonstrated that the charge redistribution originates mostly from a shortening of the Fe—As bond at the QCP. An evident relationship between the mysterious QCP and the fundamental Fe—As bond was established, providing new insights on the interplay between QCP, charge dynamics and the local structural Fe—As bond in Fe‐based superconductors.     

Structural transition behavior of ZnS nanotetrapods under high pressure

ZnS nanotetrapods synthesized via a solvothermal route have a octahedral core with a zincblende (ZB) structure and four hexprism-shaped arms consisting of alternately stacking ZB and wurtzite (WZ) phases, where the WZ phase has a higher volume percentage. In situ angular-dispersive X-ray diffraction (ADXRD) measurements were carried out to study the structural behavior of ZnS nanotetrapods under high pressure up to 41.3?GPa. The initial WZ structure exhibits a very high mechanical stability to ～11.3?GPa. Both the WZ and ZB structures transform to the rocksalt (RS) structure at ～15.4?GPa. The bulk moduli of the WZ (148.2?±?8.9?GPa) and RS (165.6?±?9.9?GPa) phases are both larger than the previously reported values. These phenomena are discussed based on the alternating epitaxial growth of the WZ and ZB phases in the arms of nanotetrapods. Our study suggests that the internal structure of nanomaterials could also greatly affect their stability and transition behavior.     

APU故障智能诊断系统设计

针对高旋环境下测试困难、测量精度低这一问题,提出了一种能在高旋环境下实现轴向减速装置,以达到稳定的良好的测试环境。该装置通过驱动仓内大量程陀螺仪测得的转速反馈给控制系统,控制系统输出控制信号来控制伺服电机,同时电机捷联内筒仓,使其相对外部转动方向反向旋转,实现轴向减速。实验结果表明,该装置可在高旋弹药飞行过程中提供稳定测试环境,从而得到更精准的姿态解算结果,为高旋环境下的测试提供了新的思路,具有广阔的应用前景。     

快中子照相模拟分析与实验验证

首次系统地推导出快中子照相像素值形成解析式, 建立图像反差不等式, 并利用该不等式首次对图像对比度与源强、照射时间和散射之间的关系进行说明. 并在像素值解析式基础上编制快中子照相模拟程序, 利用该程序对空间分辨率和图像对比度进行模拟, 并与实验对照, 研究结果表明空间分辨率模拟效果好于实验, 图像对比度模拟效果与实验相当. 最后通过对狭缝、方孔以及多材质组成的复杂样品模拟并与实验对照, 结果显示模拟效果与实验照片在反差灵敏度效果上非常一致, 该模拟计算方法可为实验设计和工程应用提供参考.     

浅析电子型掺杂铜氧化物超导体的退火过程

铜氧化物高温超导体的发现, 打破了基于电声子相互作用BCS理论所预言的超导转变温度极限, 掀开了高温超导材料探索和高温超导机理研究的序幕. 根据掺杂类型的不同, 铜氧化物超导材料可以分为空穴型掺杂和电子型掺杂两类. 受限于样品, 对电子型掺杂铜氧化物的研究工作远少于空穴型掺杂体系. 本文简要回顾有关电子型掺杂铜氧化物超导体近期研究成果, 通过对比电子型掺杂和空穴型掺杂铜氧化物的相图来阐明电子型掺杂铜氧化物的研究对探索高温超导机理的必要性, 并特别针对电子型掺杂样品制备中的关键因素“退火过程”展开讨论. 结合课题组最新实验结果和相关实验报道我们发现电子型掺杂铜氧化物超导体在制备过程中除受到温度和氧分压的影响外, 退火效果还受到界面应力的强烈调制. 在综合考虑样品生长过程中温度、气氛及应力等多种因素的基础上, 探讨了“保护退火”方法导致电子型体系化学掺杂相图变化的起因.     

紫外-可见光光束在TiO_2纳米颗粒胶体中的传输（英文）

紫外光诱导纳米颗粒胶体射流加工技术利用紫外光场对入射纳米颗粒的催化作用实现对工件表面材料的高效去除。为掌握紫外光诱导纳米颗粒胶体射流加工过程中紫外光束在纳米二氧化钛胶体中的传输特性,基于纳米二氧化钛胶体对紫外-可见光的吸收及散射理论,通过系列实验得到紫外-可见光束在不同浓度、吸收层厚度的二氧化钛纳米颗粒胶体中的传输特性及衰减关系。结果表明,为保证足够强度的紫外光能通过纳米二氧化钛胶体照射在工件表面,胶体浓度宜控制在500mol/m3以下,光-液耦合区胶体吸收层的厚度不宜超过20mm。     

紧凑式低温换热器研究进展

介绍了紧凑式低温换热器的研究进展,对低温换热器传热机理研究中所存在的问题以及新型低温换热器的研制进行了详细分析,对现有换热器优化设计方法进行了比较,给出了作者的一些观点,为低温换热器的设计及以后的进一步研究提供参考。     

LNG尾气中提取氦气的流程分析

LNG产业的蓬勃发展为资源利用开发了新的途径。基于液化天然气尾气的特点,文中提出了一种与液化天然气产业联产氦气的新方法,设计了相应的工艺流程。通过对尾气进行增压脱酸、冷凝分离、加氧脱氢、内纯化除杂等多种处理,可对天然气尾气中的氦进行有效提取分离,最终达到纯氦的技术指标。该方法有望使国内不具备工业提取价值的氦资源重新得到利用,在国内建立一种新型的氦供应途径。     

基于光纤传感的化学需氧量全光谱检测系统

为了快速准确地检测地表水、污水的化学需氧量(COD),设计了一种十字通透式结构光纤传感器,建立了一种水质COD全光谱检测系统。基于该系统,建立了COD与光谱吸光度的相关性数学模型。对配置的检测水样进行了测量,并比较哈希DR 6000多参数水质检测仪对相同水样测量得到的数据,可以发现,所构建的系统与哈希DR 6000多参数水质检测仪测量COD的示值误差小于3%,能较好的适用于水质COD实时在线检测。