Neutron Diffraction of Large-Volume Samples at High Pressure Using Compact Opposed-Anvil Cells

Neutron diffraction techniques of large-volume samples at high pressure using compact opposed-anvil cells are developed at a reactor neutron source, China's Mianyang research reactor. We achieve a high-pressure condition of in situ neutron diffraction by means of a newly designed large-volume opposed-anvil cell. This pressure calibration is based on resistance measurements of bismuth and the neutron diffraction of iron. Pressure calibration experiments are performed at room temperature for a new cell using the tungsten carbide anvils with a tapered angle of 30°, Φ4.5 mm culet diameter and the metal-nonmetal composite gasket with a thickness of 2 mm. Transitions in Bi(Ⅰ–Ⅱ 2.55 GPa, Ⅱ–V 7.7 GPa) are observed at 100 and 300 kN, respectively, by resistance measurements.The pressure measurement results of neutron diffraction are consistent with resistance measurements of bismuth.As a result, pressures up to about 7.7 GPa can routinely and stably be achieved using this apparatus, with the sample volume of 9 mm~3.     

A pressure calibration method for a portable wide-access ＂panoramic＂ cell

A simple and convenient pressure calibration method is developed for a newly designed portable wide-access ＇panoramic＇ cell. This cell is adapted to angle-dispersive-mode high-pressure in situ neutron diffraction of reactor neutron sources. This pressure calibration method has established a relationship between the cell pressure and the anvil displace- ment （gasket compression） based on the fixed-point calibration technique. By employing TiZr gasket with a thickness of 3 mm and WC anvil with a culet of 4 mm diameter, the average anvil displacements are 1.31 mm and 2.22 mm for Bi phase transitions （2.55 GPa and 7.7 GPa）, and 1.85 mm for Ba phase transitions （5.5 GPa）, respectively. In this pressure range, the pressure increases quickly with decreasing gasket thickness, and undergoes a linear increase with the anvil displacement. By extrapolating the calibration curve, the cell pressure will achieve 10 GPa when the anvil displacement is around 2.5 ram.     

Neutron powder diffraction and high-pressure synchrotron x-ray diffraction study of tantalum nitrides

Tantalum nitride(TaN) compact with a Vickers hardness of 26 GPa is prepared by a high-pressure and hightemperature(HPHT) method. The crystal structure and atom occupations of WC-type TaN have been investigated by neutron powder diffraction, and the compressibility of WC-type TaN has been investigated by using in-situ high-pressure synchrotron x-ray diffraction. The third-order Birch–Murnaghan equation of state fitted to the x-ray diffraction pressure–volume(P–V) sets of data, collected up to 41 GPa, yields ambient pressure isothermal bulk moduli of B'_0= 369(2) GPa with pressure derivatives of B 0= 4 for the WC-type TaN. The bulk modulus of WC-type TaN is not in good agreement with the previous result(B_0= 351 GPa), which is close to the recent theoretical calculation result(B_0= 378 GPa). An analysis of the experiment results shows that crystal structure of WC-type TaN can be viewed as alternate stacking of Ta and N layers along the c direction, and the covalent Ta–N bonds between Ta and N layers along the c axis in the crystal structure play an important role in the incompressibility and hardness of WC-type TaN.     

In-situ ultrasonic calibrations of pressure and temperature in a hinge-type double-stage cubic large volume press

Here, simultaneous in-situ calibration of pressures and temperatures was performed in a hinge-type second-stage cubic large volume press (LVP) up to 15 GPa and 1400 K by an acoustic travel-time approach. Based on the recently reported P-t_S and P-T-t_P-t_S equations for Al₂O₃ buffer rod, the cell pressures and temperatures in the chamber of LVP were in-situ determined, in comparison with those by conventional off-line (or fixed-points) pressure calibration method and direct thermocouple measurement, respectively. It is found that the cell pressures of the LVP chamber are significantly reduced after annealing at simultaneous high pressures and high temperatures, owing to the stress relaxation as accumulate in the LVP chamber. This acoustic travel-time method is verified to be a good way for precise determination of thermal (cell) pressures at high temperature conditions, and is of great importance and necessity to conduct in-situ physical property measurements under extreme high P-T conditions, especially when the precious synchrotron x-ray/neutron diffraction beams are not available.     

基于巴黎-爱丁堡压机的高压中子衍射技术

巴黎-爱丁堡压机(Paris-Edinburgh press)可配合中子衍射对物质在高压下的结构变化进行研究.自20世纪90年代开始,已在材料学、地学、化学等众多领域得到了广泛应用.本研究利用巴黎-爱丁堡压机在中国绵阳研究堆(CMRR)的高压中子衍射谱仪(凤凰)上成功开展了高压中子衍射实验.实验由一台载荷为200 MPa的单缸柱塞泵为巴黎-爱丁堡压机提供加载压力,并发展了一套与谱仪集成的自动定位系统对样品进行定位.利用铁作为样品,分别使用单凹曲面和双凹曲面两种碳化钨(WC)压砧,成功获得了约10 GPa压力范围内的高压原位中子衍射谱.实验结果显示,双凹曲面组装可以稳定地承受100 MPa的负载压力,而单凹曲面封垫在80 MPa左右的负载压力下就开始不稳定而发生放炮.研究结果表明,双凹曲面压砧的凹槽增强了封垫的侧向支撑能力,使双凹曲面组装比单凹曲面组装具有更好的稳定性.研究结果对进一步优化巴黎-爱丁堡压机的压砧及封垫具有重要的指导意义.     

LaNi4.25Al0.75D1.6中D占位的中子衍射研究

测量了镧基合金氘化物LaNi4.25Al0.75D1.6样品的中子粉末衍射谱,用FULLPROF程序对LaNi4.25Al0.75D1.6的中子衍射数据进行了分析处理,通过对程序参数的拟合,获得了样品的晶胞参数和原子占位及占位数.     

Neutron Powder Diffraction Study on the Non-Superconducting Phases of ThFeAsN_(1-x)O_x(x=0.15,0.6)Iron Pnictide

We use neutron powder diffraction to study the non-superconducting phases of ThFeAsN_(1-x)O_x with x = 0.15,0.6.In our previous results of the superconducting phase ThFeAsN with T_c = 30 K,no magnetic transition is observed by cooling down to 6 K,and possible oxygen occupancy at the nitrogen site is shown in the refinement[Europhys.Lett.117(2017)57005].Here in the oxygen doped system ThFeAsN_(1-x)O_x,two superconducting regions(0≤x≤0.1 and 0.25≤x≤0.55)are identified by transport experiments[J.Phys.:Condens.Matter30(2018)255602].However,within the resolution of our neutron powder diffraction experiment,neither the intermediate doping x= 0.15 nor the heavily overdoped compound x = 0.6 shows any magnetic order from 300 K to 4 K.Therefore,while it shares the common phenomenon of two superconducting domes as most 1111-type iron-based superconductors,the magneticall.y ordered parent compound may not exist in this nitride family.     

巴黎-爱丁堡压机中子衍射高压下温度加载实验

巴黎-爱丁堡压机(Paris-Edinbrugh press)因具有大体积样品、便携、结构简单等优点,被广泛应用于中子源进行高压原位中子衍射实验.但因单轴加压而导致封垫和组装不断沿径向向外流动的特点,给高压下组装的加热效率、保温效果、上下压砧的绝缘及热电偶连接等方面带来困难,从而使得巴黎-爱丁堡压机在高压下的温度加载非常具有挑战性.本文通过对高温高压组装的结构进行优化设计,提高了组装的加热效率和保温效果.通过对热电偶引线方式的优化,实现了高压下温度的直接测量.设计的HPT-3组装和HPT-3.5组装在高压下的温度加载最高可分别达到2000 K和1500 K,并且二者较大的样品尺寸满足中子衍射实验的需求.原位高温高压中子衍射实验结果说明, HPT-3组装在压力8.5 GPa、温度1508 K的条件下可以获得高质量的样品的中子衍射谱,同时该结果也进一步验证了所设计组装的良好稳定性.     

中子标准测试束平台概念设计及模拟优化

为满足对中子散射谱仪关键部件的检测需求，提出一种基于反应堆中子源的中子标准测试束平台概念设计思路。该平台具有测试束流波长（0.1~0.3 nm）连续可调、n/可同步分析、样品台可多自由度移动以及便于后续拓展集成等优点。利用蒙特卡罗程序VITESS对平台整体结构，具体包括聚焦单色器、soller准直器等展开模拟优化分析。该装置利用可拆卸的准直器1实现高通量和较高分辨两种工作模式:工作于高通量模式时，样品台处中子（波长0.1 nm）束流强度最高可达6.15106 n/（cm2s），水平发散度1;工作于较高分辨模式时，分辨率可达0.2％。     

同步辐射SAXS技术在TATB含能材料微孔结构研究中的初步应用

TATB含能材料的微结构对该材料的感度等有明显的影响,因而对于材料的安全性有着特别重要的意义.小角X射线散射(SAXS)技术是一种分析物质微观结构的重要手段,应用SAXS分析技术可以获取材料中几纳米到几百纳米尺度范围的亚微结构信息.利用同步辐射作为X射线源对TATB钝感炸药进行了小角散射实验测量,获得了SAXS测量谱.对实验谱数据进行处理,可得到样品材料的颗粒分布及内部微孔大小等微结构参数.