Materials (II)

2021 ◽  
pp. 309-362
Author(s):  
Vladimir Z. Kresin ◽  
Sergei G. Ovchinnikov ◽  
Stuart A. Wolf
Keyword(s):  
High Pressure ◽  
Energy Spectrum ◽  
Energy Gap ◽  
Ambient Pressure ◽  
Record Values ◽  
Tungsten Oxides ◽  
Gap Energy ◽  
Topological Superconductors ◽  
New Family ◽  
Pressure Record

This chapter describes the properties of a number of interesting superconducting materials. The study of phonon-mediated superconductors, such as A-15 materials and MgB2, flourished after the discovery of the high-Tc hydrides. At present, this family displays, under high pressure, record values of Tc close to room temperature. Other interesting systems, such as pnictides, heavy fermions, and ruthenates, with their peculiar interplay of superconductivity and magnetism, are also described. Fe-based superconductors, which were recently discovered, have relatively high Tc at ambient pressure. They display a two-gap energy spectrum. Pairing in intercalated nitrides is mainly provided by acoustic plasmons. Tungsten oxides represent a new family of oxides containing elements other than copper; they form filamentary structures. A special class is formed by topological superconductors; usually their properties are caused by odd-parity pairing. The presence of the states inside of the energy gap make these superconductors similar to topological insulators.

scholarly journals Brief Review of the Effects of Pressure on Wolframite-Type Oxides

2018 ◽  
Author(s):  
Daniel Errandonea ◽  
Javier Ruiz-Fuertes
Keyword(s):  
High Pressure ◽  
Phase Transitions ◽  
Band Gap ◽  
Ambient Pressure ◽  
Structural Phase ◽  
Band Gap Energy ◽  
Structural Phase Transitions ◽  
Gap Energy ◽  
Raman Modes ◽  
Oxygen Bond

In this article we review the advances that have been made on the understanding of the high-pressure structural, vibrational, and electronic properties of wolframite-type oxides since the first works in the early 1990s. Mainly tungstates, which are the best known wolframites, but also tantalates and niobates, with an isomorphic ambient-pressure wolframite structure, have been included in this review. Apart from estimating the bulk moduli of all known wolframites; the cation-oxygen bond distances and their change with pressure have been correlated with their compressibility. The composition variations of all wolframites have been employed to understand their different structural phase transitions to post-wolframite structures as a response to high pressure. The number of Raman modes and band gap energy changes have been also analyzed in the basis of these compositional differences. The reviewed results are relevant for both fundamental science and for the development of wolframites as scintillating detectors. The possible next research venues of wolframites have also been evaluated.

scholarly journals Study on the Phase Selection and Debye Temperature of Hyper-Peritectic Al-Ni Alloy under High Pressure

Metals ◽  
2021 ◽  
Vol 11 (1) ◽  
pp. 84
Author(s):  
Xiaohong Wang ◽  
Zhipeng Chen ◽  
Duo Dong ◽  
Dongdong Zhu ◽  
Hongwei Wang ◽  
...  
Keyword(s):  
High Pressure ◽  
Ambient Pressure ◽  
Competitive Growth ◽  
Phase Selection ◽  
Debye Temperatures ◽  
Ni Alloy ◽  
Temperature Heat ◽  
Low Temperature Heat Capacity ◽  
New Phase ◽  
Selection Of

The phase selection of hyper-peritectic Al-47wt.%Ni alloy solidified under different pressures was investigated. The results show that Al3Ni2 and Al3Ni phases coexist at ambient pressure, while another new phase α-Al exists simultaneously when solidified at high pressure. Based on the competitive growth theory of dendrite, a kinetic stabilization of metastable peritectic phases with respect to stable ones is predicted for different solidification pressures. It demonstrates that Al3Ni2 phase nucleates and grows directly from the undercooled liquid. Meanwhile, the Debye temperatures of Al-47wt.%Ni alloy that fabricated at different pressures were also calculated using the low temperature heat capacity curve.

GAP-LABELING PROPERTIES OF THE ENERGY SPECTRUM FOR TWO-DIMENSIONAL FIBONACCI QUASILATTICES

1995 ◽  
Vol 09 (01) ◽  
pp. 55-66
Author(s):  
YOUYAN LIU ◽  
WICHIT SRITRAKOOL ◽  
XIUJUN FU
Keyword(s):  
Energy Spectrum ◽  
Density Of States ◽  
Energy Gap ◽  
Fractional Part ◽  
Numerical Results ◽  
Step Height ◽  
Integrated Density Of States ◽  
Two Dimensional

We have analytically obtained the occupation probabilities on subbands of the hierarchical energy spectrum and the step heights of the integrated density of states for two-dimensional Fibonacci quasilattices. Based on the above results, the gap-labeling properties of the energy spectrum are found, which claim that the step height is equal to {mτ}, where the braces denote the fractional part, and m is an integer that can be used to label the corresponding energy gap. Numerical results confirm these results very well.

Superconducting State

2021 ◽  
Author(s):  
Vladimir Kresin ◽  
Sergei Ovchinnikov ◽  
Stuart Wolf
Keyword(s):  
Critical Temperature ◽  
Energy Gap ◽  
Microscopic Theory ◽  
Record Values ◽  
Theoretical Treatment ◽  
Many Body ◽  
Many Body Theory ◽  
Current State ◽  
Superconducting Compounds ◽  
Body Theory

For the past almost fifty years, scientists have been trying to explain the phenomenon of superconductivity. The mechanism is the key ingredient of microscopic theory, which was developed by Bardeen, Cooper, and Schrieffer in 1957. The theory also introduced the basic concepts of pairing, coherence length, energy gap, and so on. Since then, microscopic theory has undergone an intensive development. This book provides a very detailed theoretical treatment of the key mechanisms of superconductivity, including the current state of the art (phonons, magnons, plasmons). In addition, the book contains descriptions of the properties of the key superconducting compounds that are of the most interest for science and applications. For many years, there has been a search for new materials with higher values of the main parameters, such as the critical temperature and critical current. At present, the possibility of observing superconductivity at room temperature has become perfectly realistic. That is why the book is especially concerned with high-Tc systems such as high-Tc oxides, hydrides with record values for critical temperature under high pressure, nanoclusters, and so on. A number of interesting novel superconducting systems have been discovered recently, including topological materials, interface systems, and intercalated graphene. The book contains rigorous derivations based on statistical mechanics and many-body theory. The book also provides qualitative explanations of the main concepts and results. This makes the book accessible and interesting for a broad audience.

scholarly journals High-Pressure Synthesis of Non-Stoichiometric LixWO3 (0.5 ≤ x ≤ 1.0) with LiNbO3 Structure

Inorganics ◽  
2019 ◽  
Vol 7 (5) ◽  
pp. 63
Author(s):  
Kohdai Ishida ◽  
Yuya Ikeuchi ◽  
Cédric Tassel ◽  
Hiroshi Takatsu ◽  
Craig M. Brown ◽  
...  
Keyword(s):  
High Pressure ◽  
Perovskite Phase ◽  
Ambient Pressure ◽  
Tungsten Bronze ◽  
Type Structure ◽  
High Pressure Synthesis ◽  
Phase 0 ◽  
Pressure Synthesis ◽  
Octahedral Tilting ◽  
Recent Attention

Compounds with the LiNbO3-type structure are important for a variety of applications, such as piezoelectric sensors, while recent attention has been paid to magnetic and electronic properties. However, all the materials reported are stoichiometric. This work reports on the high-pressure synthesis of lithium tungsten bronze LixWO3 with the LiNbO3-type structure, with a substantial non-stoichiometry (0.5 ≤ x ≤ 1). Li0.8WO3 exhibit a metallic conductivity. This phase is related to an ambient-pressure perovskite phase (0 ≤ x ≤ 0.5) by the octahedral tilting switching between a−a−a− and a+a+a+.

scholarly journals Pressure-Dependent Clustering in Ionic-Liquid-Poly (Vinylidene Fluoride) Mixtures: An Infrared Spectroscopic Study

Nanomaterials ◽  
2021 ◽  
Vol 11 (8) ◽  
pp. 2099
Author(s):  
Teng-Hui Wang ◽  
Wei-Xiang Wang ◽  
Hai-Chou Chang
Keyword(s):  
High Pressure ◽  
High Pressures ◽  
Vinylidene Fluoride ◽  
Ambient Pressure ◽  
Infrared Spectroscopic Study ◽  
Research Attention ◽  
Nanoscale Structures ◽  
Local Structures ◽  
Poly Vinylidene Fluoride ◽  
Hydrogen Bonded

The nanostructures of ionic liquids (ILs) have been the focus of considerable research attention in recent years. Nevertheless, the nanoscale structures of ILs in the presence of polymers have not been described in detail at present. In this study, nanostructures of ILs disturbed by poly(vinylidene fluoride) (PVdF) were investigated via high-pressure infrared spectra. For 1-(2-hydroxyethyl)-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([HEMIm][TFSI])-PVdF mixtures, non-monotonic frequency shifts of the C4,5-H vibrations upon dilution were observed under ambient pressure. The experimental results suggest the presence of microheterogeneity in the [HEMIm][TFSI] systems. Upon compression, PVdF further influenced the local structure of C4,5–H via pressure-enhanced IL–PVdF interactions; however, the local structures of C2–H and hydrogen-bonded O–H were not affected by PVdF under high pressures. For choline [TFSI]–PVdF mixtures, PVdF may disturb the local structures of hydrogen-bonded O–H. In the absence of the C4,5–H⋯anion and C2–H⋯anion in choline [TFSI]–PVdF mixtures, the O–H group becomes a favorable moiety for pressure-enhanced IL–PVdF interactions. Our results indicate the potential of high-pressure application for designing pressure-dependent electronic switches based on the possible changes in the microheterogeneity and electrical conductivity in IL-PVdF systems under various pressures.

scholarly journals Fuel spray vapour distribution correlations for a high pressure diesel fuel spray cases for different injector nozzle geometries.

2017 ◽  
Author(s):  
Darlington Njere ◽  
Nwabueze Emekwuru
Keyword(s):  
High Pressure ◽  
Liquid Phase ◽  
Diesel Fuel ◽  
Ambient Pressure ◽  
Vapour Phase ◽  
Fuel Spray ◽  
Combustion Chambers ◽  
Complete Combustion ◽  
Fuel Injectors ◽  
Fuel Vapour

The evolution of diesel fuel injection technology, to facilitate strong correlations of in-cylinder spray propagation with injection conditions and injector geometry, is crucial in facing emission challenges. More observations of spray propagation are, therefore, required to provide valuable information on how to ensure that all the injected fuel has maximum contact with the available air, to promote complete combustion and reduce emissions. In this study, high pressure diesel fuel sprays are injected into a constant-volume chamber at injection and ambient pressure values typical of current diesel engines. For these types of sprays the maximum fuel liquid phase penetration is different and reached sooner than the maximum fuel vapour phase penetration. Thus, the vapour fuel could reach the combustion chamber wall and could be convected and deflected by swirling air. In hot combustion chambers this impingement can be acceptable but this might be less so in larger combustion chambers with cold walls. The fuel-ambient mixture in vapourized fuel spray jets is essential to the efficient performance of these engines. For this work, the fuel vapour penetration values are presented for fuel injectors of different k-factors. The results indicate that the geometry of fuel injectors based on the k-factors appear to affect the vapour phase penetration more than the liquid phase penetration. This is a consequence of the effects of the injector types on the exit velocity of the fuel droplets.DOI: http://dx.doi.org/10.4995/ILASS2017.2017.4951

Phase transitions in ReO3studied by high-pressure X-ray diffraction

2000 ◽  
Vol 33 (2) ◽  
pp. 279-284 ◽  
Author(s):  
J.-E. Jørgensen ◽  
J. Staun Olsen ◽  
L. Gerward
Keyword(s):  
High Pressure ◽  
Phase Transitions ◽  
Powder Diffraction ◽  
Ambient Pressure ◽  
Rhombohedral Phase ◽  
X Ray Diffraction ◽  
X Ray ◽  
Oxygen Ions ◽  
Pressure Phase ◽  
Related Phase

ReO3has been studied at pressures up to 52 GPa by X-ray powder diffraction. The previously observed cubicIm3¯ high-pressure phase was shown to transform to a monoclinic MnF3-related phase at about 3 GPa. All patterns recorded above 12 GPa could be indexed on rhombohedral cells. The compressibility was observed to decrease abruptly at 38 GPa. It is therefore proposed that the oxygen ions are hexagonally close packed above this pressure, giving rise to two rhombohedral phases labelled I and II. The zero-pressure bulk moduliBoof the observed phases were determined and the rhombohedral phase II was found to have an extremely large value of 617 (10) GPa. It was found that ReO3transforms back to thePm3¯mphase found at ambient pressure.

A Theoretical Study on the Pressure Dependence of the Band Gap in ${\rm A^{I}B^{III}C^{VI}_2}$ Compounds

1997 ◽  
Vol 11 (16) ◽  
pp. 1959-1967 ◽  
Author(s):  
R. Asokamani ◽  
R. Mercy Amirthakumari ◽  
G. Pari
Keyword(s):  
Theoretical Study ◽  
Energy Gap ◽  
Ambient Pressure ◽  
Chalcopyrite Structure ◽  
Deformation Potential ◽  
Equilibrium Lattice Constant ◽  
Self Consistent ◽  
Direct Energy ◽  
Experimental Values ◽  
Direct Energy Gap

The self-consistent scalar relativistic band structure for AgGaX 2 (X = S, Se, Te) performed in chalcopyrite structure using the TBLMTO method at various pressures are reported here. Empty spheres were introduced in the calculations as the chalcopyrite structure is loosely packed. From the total energy calculations, the equilibrium lattice constant and the bulk modulus at zero pressure were calculated and these values agree well with the reported experimental values. All these compounds are found to have direct energy gap at ambient pressure with the gap widening with increased pressures which are in agreement with the experimental results. The deformation potential, dE g /dP for the compounds are also reported here. The metallisation volumes are calculated and the possibility of observing superconductivity in these compounds is discussed.

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