Macroscopic and structural effects of hydrogen-bond transformations

A brief account of transformations of internal structure of hydrogen bonds, and transformations involving the breaking and formation of hydrogen bonds is highlighted for a selected group of crystals. The main stress is laid on the short bistable –OH…O= hydrogen bonds, illustrating several general properties, often blurred for weaker hydrogen bonds. A set of parameters relating the electronic structure of the oxygen atoms and geometry of hydrogen bonds is described. This allows one to combine the hydrogen bond dimensions with changes in the electronic structure of the oxygen atoms, accompanying any transformations of hydrogen bonds. Several resultant effects are described: coupling of displacements of molecules or ions with the H-atom transfer, the isotope effect of deuteration on T _c in ferroelectric crystals, a skew trajectory of a disordered H-atom with respect to the O···O direction, or shortening of the O···O distance when the H-atom becomes disordered. It is shown that hydrogen-bond patterns, molecular arrangements and thermodynamic properties of crystals are interdependent. Several examples and possible applications of the presented model are given.     

Size and pressure effects on solid transition temperatures of ZrO2

Transition temperatures between tetragonal and monoclinic polymorphs of ZrO₂ nanoparticles, thin films and nanostructured materials are calculated by considering energetic contributions of surface (interface) energy and surface stress on total Gibbs free energy. The transition temperatures drop as the size of the nanocrystals decreases, which is consistent with available results.     

Structure and hydrogen absorbing properties of ScCrMn alloy

The crystal structures of ScCrMn alloy and its hydride are determined by XRD Rietveld analysis and TEM examination. The ScCrMn alloy exhibits a rarely ideal close-packed C14 type hexagonal Laves phase structure with lattice parameters a = 5.064(1), c = 8.263(2) Å, deviating from the ANOE dependent rule on Laves phase structure. The hydrogenation of the alloy to ScCrMnH_3.9 results in significant lattice expansion of 27%, but does not alter the matrix lattice structure. The XRD pattern of the hydride also shows close-packed structure with little line broadening, comparable to that of the alloy, indicating a high degree of crystallinity, where the hydrogen atoms are homogeneously distributed in evenly distributed interstitials with minimal defects. The alloy demonstrates extremely easy activation property. The alloy can be activated at a low pressure of 0.46 kPa and exhibit very fast absorption rate at sub-atmosphere at room temperature. The hydrogenation thermodynamics of the alloy are evaluated by P-C-T measurements at different temperatures. At hydrogen concentration H/M = 0.66 corresponding to a defined room temperature plateau pressure, the relative partial molar enthalpy ΔH and entropy ΔS derived by Van’t Hoff equation are −63 kJ/mol and −111 J/mol K, respectively. These results manifest that the hydride stability of the ScCrMn alloy is comparable to those of the hydrogen isotope storage alloys of ZrCo and ZrTi_0.2V_1.8, and would be superior on relevant applications. The DSC-TG measurements of the hydride reveal that the total release of hydrogen can basically be achieved at 653 K, accompanied by an oxidation reaction with residual oxygen to form water vapor. The results indicate that the alloy is an effective catalyst for the dissociation of hydrogen and combination of oxygen.     

电磁比例泄压阀的结构设计与性能测试

This paper describes the design and performance testing method for electromagnetic proportional pressure relief valve. The valve uses proportional solenoid as a driving element so that the pressure could be real timely adjusted,which will meet the needs for different pressures required by laboratory analytical instruments. By using the spring-energized reciprocating seals in the valve to strengthen the sealing performance,the safety of the high-pressure analytical instruments could be improved in laboratory. In addition,it can also constitute a closed loop control with pressure sensors and flow sensors.介绍了一种电磁比例泄压阀的结构设计与性能测试方法。该电磁比例泄压阀采用比例电磁铁作为驱动元件,压力可以适时调节,满足实验室内分析仪器的不同压力需求;阀体采用内嵌倾斜弹簧圈加强的往复式密封件,增强了密封能力,应用在实验室内有高压液体的分析仪器上,起着安全保护的作用。此外,与压力传感器、流量传感器配合,还可以构成闭环控制。     

Synthesis of FCC Mg–Zr and Mg–Hf hydrides using GPa hydrogen pressure method and their hydrogen-desorption properties

Magnesium-based hydrides with Zr or Hf have been synthesized by means of an ultra-high pressure technique. Powder mixtures of MgH₂ and ZrH₂ or HfH₂ have been heated up to 873 K under 4–8 GPa in a multi-anvil cell. New ternary phases (Mg–Zr–H and Mg–Hf–H) with a face centered cubic (FCC) structure have been formed. In the Mg–Zr hydride, the FCC phase with disordered metal–atom occupancy was observed, while a Ca₇Ge-type super-lattice structure was observed in the Mg–Hf hydride. By the temperature programmed desorption (TPD) measurements, these new hydrides exhibit the hydrogen-desorption at around 543–583 K, which were 130–70 K lower than that of MgH₂ at a heating rate of 10 K/min under vacuum. Desorbed hydrogen contents were estimated to be 4.2 and 3.0 mass% for Mg–Zr and Mg–Hf hydrides, respectively.     

Pulsed electron-beam melting of high-speed steel: structural phase transformations and wear resistance

The structural and phase transformations occurring in the near-surface layers of pre-quenched high-speed steel subjected to pulsed electron beam melting have been investigated. Melting was induced by a low-energy (20–30 keV), high-current electron beam with a pulse duration of 2.5 μs and an energy density ranging from 3 to 18 J/cm². Using electron microscopy and X-ray diffraction it has been revealed that with increasing beam energy density gradual liquid-phase dissolution of initial globular M₆C carbide particles occurs in the near-surface layer of thickness up to 1 μm. This process is accompanied by the formation of martensite crystals (-phase) and an increase of residual austenite (γ-phase) content. When the carbide particles are completely dissolved, martensitic transformation is suppressed. In this case, a non-misoriented structure is formed consisting predominantly of submicrometer cells of γ-phase separated by nanosized carbide interlayers. Irradiation of cutting tools (drills) in a mode corresponding to an abrupt decrease in the content of M₆C particles due to their liquid-phase dissolution enhances the wear resistance of the drills by a factor of 1.7. This is associated with the fixation of undissolved particles in the matrix, the formation of residual compressive stresses and of dispersed M₃C carbide particles as well as the high (50%) content of the metastable γ-phase in the surface layer.     

Evaluation of weak intermolecular interactions in molecular crystals via experimental and theoretical charge densities

Analysis of charge density distributions in molecular crystals has received considerable attention in the last decade both from high-resolution X-ray diffraction studies and from high-level theoretical calculations. An overview of the progress made in deriving one-electron properties, intermolecular interactions in terms of the Atoms in Molecule (AIM) approach (R.F.W. Bader. Atoms in Molecules-A Quantum Theory, Clarendon, Oxford (1990), R.F.W. Bader. J. Phys. Chem., A102, 7314 (1998)) is given with special emphasis on improvements in charge density models and development of both experimental and theoretical techniques to interpret and analyse the nature of weak intermolecular interactions. The significance of the derived results from the charge density of coumarin and its derivatives have been analysed to obtain insights into the nature of intermolecular C–H?···?O, C–H?···?π, π?···?π, C–H?···?S, and S?···?S contacts. The appearance of a ‘region of overlap’ to segregate hydrogen bonds from van der Waals interactions based on the criteria proposed by Koch and Popelier (U. Koch, P.L.A. Popelier. J. Phys. Chem., 99, 9747 (1995), P.L.A. Popelier. Atoms in Molecules. An Introduction, pp. 150–153, Prentice Hall, UK (2000)) and the identification of differences in energy surfaces in concomitant polymorphs of 3-acetylcoumarin are described.     

Bulk modulus of CeO2 and PrO2—An experimental and theoretical study

The high-pressure structural behaviour of CeO₂ and PrO₂ has been investigated by synchrotron X-ray diffraction at pressures up to 20 and 35 GPa, respectively. The experiments are accompanied by first principles calculations using the self-interaction corrected local spin density (SIC-LSD) approximation. The experimental values for the zero-pressure bulk modulus of CeO₂ and PrO₂ are 220(9) and 187(8) GPa, respectively. Our calculations reproduce the lattice constants with good accuracy, but find identical bulk modulii for CeO₂ (176.9 GPa) and PrO₂ (176.8 GPa).     

Leaf-like dislocation substructures and the decrease of martensitic start temperatures: A new explanation for functional fatigue during thermally induced martensitic transformations in coarse-grained Ni-rich Ti-Ni shape memory alloys

During repeatedly imposed thermally induced martensitic transformations in Ti-Ni shape memory alloys, the martensite start temperature M_s decreases. This has been rationalized on the basis of a scenario where an increasing dislocation density makes it more and more difficult for martensite to form. However, it is not clear why dislocations which form because they accommodate the growth of martensite during the first cooling cycle should act as obstacles during subsequent transformation cycles. In the present work we use diffraction contrast transmission electron microscopy to monitor the formation of unique leaf-like dislocation substructures which form as the martensite start temperature decreases during thermal cycling. We interpret our microstructural results on the basis of a microstructural scenario where dislocations play different roles with respect to the propagation of a big martensite needle in one transformation cycle and the nucleation and growth of new martensite needles in the following cycles. As a consequence, chestnut-leaf-like dislocation arrays spread out in different crystallographic directions.     

High pressure study of β-UH3 crystallographic and electronic structure

The crystallographic and electronic structure of β-UH₃ were studied as a function of pressure by combining X-ray diffraction results with the full potential linearized augmented plane wave (LAPW) calculations. No phase transition was observed up to a pressure of 29 GPa, with a total volume contraction of V/V₀=0.87, and a bulk modulus value of B₀=33±5 GPa. The calculated value of the electric field gradient main component at the uranium 6(c) site was found to be linear dependent in the pressure induced volume reduction. This linear dependence is attributed to the increase in the positive p–p contribution to the efg as a function of volume, while the negative s–d, d–d and the lattice contributions to the efg are almost pressure independent.     

Phase equilibria in the Fe–Al–Mo system – Part I: Stability of the Laves phase Fe2Mo and isothermal section at 800 °C

Phase equilibria in the Fe–Al–Mo system were experimentally determined at 800 °C. From metallography, X-ray diffraction and electron probe microanalysis on equilibrated alloys and diffusion couples a complete isothermal section has been established. It is shown that the Laves phase Fe₂Mo is a stable phase. The phase Al₄Mo, which only becomes stable above 942 °C in the binary system, is the only ternary compound found at 800 °C. For all binary phases the solid solubility ranges for the third component have been established. The D0₃/B2 and B2/A2 transition temperatures have been determined for a selected alloy by differential thermal analysis and transmission electron microscopy. The results confirm that the D0₃/B2 transition temperature substantially increases by the addition of Mo, while the B2/A2 transition temperature is about that for a binary alloy with the same Al content.