高耐压等级聚乙烯管材树脂的研究进展

高耐压等级聚乙烯管材树脂主要用于生产给水管、燃气管及各种工业用管,是一类技术含量较高、市场前景较好的聚乙烯专用树脂。高耐压等级聚乙烯管材树脂多为双峰或多峰结构,采用茂金属催化剂或具有高共聚性能的齐格勒纳塔(Z-N)催化剂双釜或多釜串联聚合生产。本文介绍了高耐压等级聚乙烯管材树脂的分子结构和性能特点,以及制备该类树脂的催化剂和工艺技术,并对国内的相关催化剂及制备技术作了评述。     

Pressure-induced transition in Tl2MoO4

Tl₂MoO₄ has been studied under high-pressure by X-ray diffraction, Raman spectroscopy, and optical absorption measurements. A first-order phase transition is observed at 3.5±0.5 GPa. The nature (ordered vs. disordered) of the high-pressure phase strongly depends on the local hydrostatic conditions. Optical absorption measurements tend to show that this transition is concomitant with an electronic structure transformation. Prior to the transition, single crystal X-ray diffraction shows that pressure induces interactions between MoO₄ fragments and the Mo coordination number tends to increase. In addition, the stereoactivity of the lone-pair electrons on the three symmetrically independent Tl-sites is not uniform; while for two sites the stereoactivity decreases with increasing pressures for the third site the stereoactivity increases.     

Gd4B4O11F2: Synthesis and crystal structure of a rare-earth fluoride borate exhibiting a new “fundamental building block” in borate chemistry

A new gadolinium fluoride borate Gd₄B₄O₁₁F₂ was yielded in a Walker-type multianvil apparatus at 7.5 GPa and 1100 °C. Gd₄B₄O₁₁F₂ crystallizes monoclinically in the space group C2/c with the lattice parameters a=1361.3(3) pm, b=464.2(2) pm, c=1374.1(3) pm, and β=91.32(3)° (Z=4). The crystal structure exhibits a structural motif not yet reported from borate chemistry: two BO₄-tetrahedra (□) and two BO₃-groups (?) are connected via common corners, leading to the fundamental building block 2?2□:?□□?. In the two crystallographically identical BO₄-tetrahedra, a distortion resulting in a very long B-O-bond is found.     

Valence states of iron ions in nanostructured yttrium iron garnet Y3Fe5O12 studied by means of soft X-ray absorption spectroscopy

Nanostructured samples of yttrium iron garnet Y₃Fe₅O₁₂ obtained by plastic deformation method (high-pressure torsion) were studied with help of soft X-ray absorption spectroscopy using Fe 2p and O 1s spectra. Experimental spectra were compared with crystal field multiplet calculations for Fe ions. Some amount of Fe²⁺ ions in nanostructured Y₃Fe₅O₁₂ was found. The concentration of Fe²⁺ ions was found to be increased with the increase of the degree of plastic deformation.     

Pressure-induced crystal–amorphous transformation in Y2Mo3O12

In this work we investigate the vibrational properties of anhydrous and hydrated yttrium molybdate as a function of hydrostatic pressure. An analysis of vibrational modes suggests that the anhydrous material experiences a phase transformation from orthorhombic to a lower symmetry phase (probably monoclinic) about 0.3 GPa, and to a highly disordered phase above 2.4 GPa. The structural transformation to the high-pressure disordered phase is not reversible and suggests the onset of a pressure-induced amorphization process. The vibrational mode dependence on pressure is discussed considering lattice dynamics calculations.     

Potassium under pressure: Electronic origin of complex structures

Recent high-pressure X-ray diffraction studies of alkali metals revealed unusual complex structures that follow the body-centred and face-centred cubic structures on compression. The structural sequence of potassium under compression to 1 Mbar is as follows: bcc–fcc–h-g (tI19*), hP4–oP8–tI4–oC16.We consider configurations of Brillouin-Jones zones and the Fermi surface within a nearly-free-electron model in order to analyze the importance of these configurations for the crystal structure stability. Formation of Brillouin zone planes close to the Fermi surface is related to opening an energy gap at these planes and reduction of crystal energy. Under pressure, this mechanism becomes more important leading to appearance of complex low-symmetry structures. The stability of the post-fcc phases in K is attributed to the changes in the valence electron configuration under strong compression.     

High pressure Raman spectra of d-threonine crystal

Raman spectra of d-threonine crystal were recorded for pressures up to 8.5 GPa. Modifications in the lattice modes indicate that the crystal undergoes two structural phase transitions, the first in the 1.9–2.4 GPa pressure range and the second in the 5.1–6.0 GPa pressure range. Splitting of modes and changes in the intensity of several bands suggest a conformational reorientation of the molecule in the crystal framework between 3.5 and 4.3 GPa. We discussed ours results by comparing with previous high-pressure investigation on l-conformer.     

EXAFS and Raman studies of mechanical alloyed Ni25Se75 mixture under high-pressure conditions

Extended X-ray Absorption Fine Structure and Raman studies were performed to follow the structural and vibrational behavior of a mechanical alloyed Ni₂₅Se₇₅ mixture, containing nanocrystalline pyrite NiSe₂ phase, when exposed to high-pressure conditions. An increase in the local structural order of the nanocrystalline phase with pressure increasing was observed by means of Debye-Waller factor analysis. The relative Ni nearest-neighbors distances were determined as a function of pressure, which were used to determine the inverse linear compressibility of the nanocrystalline pyrite NiSe₂ alloy as well as its derivate by means of Murnaghan's equation. The Raman results showed tentative NiSe₂ phonons dispersion with pressure that becomes a difficult task since the existence/photo-induced nucleation of an important amount of nanocrystalline Se, detected due to the observation of its pressure-induced phase transitions.     

Cyclotron resonance of a polaron in a realistic heterojunction and its pressure effect

The Larsen perturbation method is adopted to study the influence of magnetic fields on polarons in realistic heterojunctions in a quasi-two-dimension approximation. The interaction between an electron and both the bulk longitudinal optical phonons and the two branches of interface optical phonons is taken into account to show the influence of magnetic fields at different ranges on the polaron cyclotron mass due to the coupling of the electron with each branch of phonon modes. The result indicates that not only do the bulk phonons influence the polaron cyclotron mass, but the interface phonons do as well. The pressure effect on the cyclotron mass is also discussed.