Phase Transition In Hydrogen Research Articles

Microporous Materials for Hydrogen Liquefiers and Storage Vessels

Microporous Composite Materials, made of metal oxide ionic fragments inserted in carbonaceous substrates, might be used as intermediates in hydrogen phase transitions, allowing progressive switch between their gas and liquid states. When crossed by an hydrogen flow these composite materials combine adsorbing, magnetic, catalytic and thermal properties efficient in hydrogen devices, liquefiers or storage vessels. Liquefaction and storage are complementary processes in practice, although inverse in principle. Plate-fin devices filled with composite materials can conjugate many thermal functions, heat exchangers, catalytic converters and pressure expanders that are necessary stages in large-scale liquefiers. Break-even storage times permit operation of the liquefaction unit at optimum ortho-para conversion thus yielding minimum practical work to be furnished. A barrage-system of successive porous plugs inserted in Hydrogen vessels would regulate the flow rate by J-T expansions, convert part of the environmental heat in rotational energy and confer stability to the system by damping the fluctuations. Consequently storage vessels of short dormancy period might be manufactured to be of low weight and low cost. After a short review of existing devices, a logical scheme interconnecting physical, economic and technologic challenges suggests that activated carbon materials of cage-like porous structures would promote and accelerate the development of storage vessels in hydrogen transport vehicles and facilitate the liquid hydrogen production by regulating the hydrogen streams in the industrial liquefiers.

New Class of Phase Transitions in Hydrogen and Deuterium Experiencing Chemical Reactions of Ionization and Dissociation

New Class of Phase Transitions in Hydrogen and Deuterium Experiencing Chemical Reactions of Ionization and Dissociation

3p-N-2 水素結合結晶のプロトン波動関数と低エネルギー励起

3p-N-2 水素結合結晶のプロトン波動関数と低エネルギー励起

High‐explosive generators of dense low‐temperature plasma for proton radiography

The article reviews the design and physical applications of high‐explosive generators for dense low‐temperature plasma. The PUMA proton microscope with magnetic optics at the Institute for Theoretical and Experimental Physics by A.I. Alikhanov of National Research Centre «Kurchatov Institute» (Moscow, Russia) was used to diagnose the plasma. The generators were developed for measurements of the equation of state of non‐ideal plasma, investigation of phase transitions in hydrogen or molecular gases, and studies of the properties of the interior of Giant planets. A proposal was made to repeat the experiments with explosive generators using the proton microscope PRIOR (Proton Microscope for Facility for Anti‐proton and Ion Research) at the GSI Helmholtzzentrum fur Schwerionenforschung (Darmstadt, Germany) and at the designed proton microscope at the Institute for Nuclear Research (Troitsk, Russia).

Liquid–liquid phase transition in hydrogen by coupled electron–ion Monte Carlo simulations

The phase diagram of high-pressure hydrogen is of great interest for fundamental research, planetary physics, and energy applications. A first-order phase transition in the fluid phase between a molecular insulating fluid and a monoatomic metallic fluid has been predicted. The existence and precise location of the transition line is relevant for planetary models. Recent experiments reported contrasting results about the location of the transition. Theoretical results based on density functional theory are also very scattered. We report highly accurate coupled electron-ion Monte Carlo calculations of this transition, finding results that lie between the two experimental predictions, close to that measured in diamond anvil cell experiments but at 25-30 GPa higher pressure. The transition along an isotherm is signaled by a discontinuity in the specific volume, a sudden dissociation of the molecules, a jump in electrical conductivity, and loss of electron localization.

Molecular to atomic phase transition in hydrogen under high pressure.

The metallization of high-pressure hydrogen, together with the associated molecular to atomic transition, is one of the most important problems in the field of high-pressure physics. It is also currently a matter of intense debate due to the existence of conflicting experimental reports on the observation of metallic hydrogen on a diamond-anvil cell. Theoretical calculations have typically relied on a mean-field description of electronic correlation through density functional theory, a theory with well-known limitations in the description of metal-insulator transitions. In fact, the predictions of the pressure-driven dissociation of molecules in high-pressure hydrogen by density functional theory is strongly affected by the chosen exchange-correlation functional. In this Letter, we use quantum MonteCarlo calculations to study the molecular to atomic transition in hydrogen. We obtain a transition pressure of 447(3)GPa, in excellent agreement with the best experimental estimate of the transition 450 GPa based on an extrapolation to zero band gap from experimental measurements. Additionally, we find that C2/c is stable almost up to the molecular to atomic transition, in contrast to previous density functional theory (DFT) and DFT+quantum MonteCarlo studies which predict large stability regimes for intermediary molecular phases.

What controls star formation in the central 500 pc of the Galaxy?

The star formation rate (SFR) in the Central Molecular Zone (CMZ, i.e. the central 500 pc) of the Milky Way is lower by a factor of >10 than expected for the substantial amount of dense gas it contains, which challenges current star formation theories. In this paper, we quantify which physical mechanisms could be responsible. On scales larger than the disc scale height, the low SFR is found to be consistent with episodic star formation due to secular instabilities or possibly variations of the gas inflow along the Galactic bar. The CMZ is marginally Toomre-stable when including gas and stars, but highly Toomre-stable when only accounting for the gas, indicating a low condensation rate of self-gravitating clouds. On small scales, we find that the SFR in the CMZ may be caused by an elevated critical density for star formation due to the high turbulent pressure. The existence of a universal density threshold for star formation is ruled out. The HI-H$_2$ phase transition of hydrogen, the tidal field, a possible underproduction of massive stars due to a bottom-heavy initial mass function, magnetic fields, and cosmic ray or radiation pressure feedback also cannot individually explain the low SFR. We propose a self-consistent cycle of star formation in the CMZ, in which the effects of several different processes combine to inhibit star formation. The rate-limiting factor is the slow evolution of the gas towards collapse - once star formation is initiated it proceeds at a normal rate. The ubiquity of star formation inhibitors suggests that a lowered central SFR should be a common phenomenon in other galaxies. We discuss the implications for galactic-scale star formation and supermassive black hole growth, and relate our results to the star formation conditions in other extreme environments.

Free energy methods in coupled electron ion Monte Carlo

Recent progress in simulation methodologies and in computer power allow first-principles simulations of condensed systems with Born–Oppenheimer electronic energies obtained by quantum Monte Carlo methods. Computing free energies and therefore getting a quantitative determination of phase diagrams is one step more demanding in terms of computer resources. In this paper we derive a general relation to compute the free energy of an ab initio model with Reptation Quantum Monte Carlo (RQMC) energies from the knowledge of the free energy of the same ab initio model in which the electronic energies are computed by the less demanding but less accurate Variational Monte Carlo (VMC) method. Moreover we devise a procedure to correct transition lines based on the use of the new relation. In order to illustrate the procedure, we consider the liquid–liquid phase transition in hydrogen, a first-order transition between a lower pressure, molecular and insulating phase and a higher pressure, partially dissociated and conducting phase. We provide new results along the T = 600 K isotherm across the phase transition and find good agreement between the transition pressure and specific volumes at coexistence for the model with RQMC accuracy between the prediction of our procedure and the values that can be directly inferred from the observed plateau in the pressure–volume curve along the isotherm. This work paves the way for future use of VMC in first-principles simulations of high-pressure hydrogen, an essential simplification when considering larger system sizes or quantum proton effects by Path Integral Monte Carlo methods.

Monte Carlo simulations of dense quantum plasmas

Thermodynamic properties of the equilibrium strongly coupled quantum plasmas investigated by direct path integral Monte Carlo (DPIMC) simulations within a wide region of density, temperature and positive to negative particle mass ratio. Pair distribution functions (PDF), equation of state (EOS), internal energy and Hugoniot are compared with available theoretical and experimental results. Possibilities of the phase transition in hydrogen and electron–hole plasma from neutral particle system to metallic-like state and crystal-like structures, including antiferromagnetic hole structure in semiconductors at low temperatures, are discussed.

Plasma phase transition in hydrogen and electron‐hole plasmas

AbstractThe plasma phase transition in dense hydrogen and in electron‐hole plasmas is investigated by direct path integral Monte Carlo simulations. Hydrogen results for the internal energy at T = 10, 000 K show a deep minimum and strong fluctuations around the density n = 1023 cm−3 indicating the existence of a phase transition. To verify this explanation, the analogous phenomenon is studied for an electron‐hole plasma in Germanium. The calculated phase boundary of the electron‐hole liquid is found to agree reasonably well with the available experimental data. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Phase transitions of hydrogen inquasi-two-dimensional vanadium lattices

The influence of the strain state on the thermodynamics of hydrogenin quasi-two-dimensional potentials is reported. The host latticeis V embedded in Fe in the form of a Fe/V(001) superlattice, whichrepresents a strongly confined absorption potential extending overjust 13 monolayers. ΔH̄H and ΔS̄H for the continuous solubility region are calculatedfrom the measured isotherms. Two phase transitions are observed foratomic ratios up to c = 1 in the temperature region50-300 °C. The first transition occurs in the range 0.03⩽c⩽0.07, and shows a Curie-Weiss behaviour. Nocorresponding phase boundary exists in bulk vanadium hydrides. Thesecond transition, at c≃0.35 and T<150 °C, exhibitslarge hysteresis and involves an ordering not previously observed inthin vanadium layers. The site blocking at low concentrationsscales linearly with the initial strain and yields a blocking concentrationof c = 0.083(1) at zero strain, as compared to 0.415(9) in bulk V.This difference is ascribed to the finite-size of the hostlattice.

The swelling and collapse of hydrogen bonded polymer gels

The volume phase transitions in hydrogen bonded polymer gels are investigated. The elastic contribution is obtained by a modification of the Flory–Rehner approach proposed previously, where the affine deformation assumption is abandoned and instead the assumptions of the c* model of de Gennes are adopted. The mixing term is modified to account for hydrogen bonding. The calculations (both binary and ternary) clearly indicate the possibility of abrupt volume phase transition. Also the existence of LCST and UCST within an accessible temperature range is predicted for gels in mixed solvent systems involving hydrogen bonding.

Interaction of superadiabatic combustion and heat conversion waves in a porous medium with incorporated metal hydride elements

A new concept of coupling combustion and metal hydride heat conversion by means of superadiabatic thermal waves in a porous medium is suggested and investigated analytically and numerically. As an example of the concept implementation, the heat conversion cycle in a two-channel porous medium with metal hydride pairs incorporated in it is considered. The heat conversion is initiated by a superadiabatic combustion of extremely lean gaseous mixture and develops as a series of thermal waves induced by positive and negative heat sources corresponding to charge and discharge stages of metal hydride cycle. It is shown that the thermal effectiveness of heat conversion in a porous medium is provided by heat recirculation mechanism, similar to the superadiabatic effect of the lean fuel combustion. To analyze the concept and its effectiveness, two methods have been employed: (1) analytical study of possible quasi-steady-state structures of the coupled thermal waves and (2) numerical modeling of their dynamic interaction. It is demonstrated that the minimal temperature in the superadiabatic refrigerating wave and the maximal temperature in a combustion wave are attained when both of these waves move synchronously with the free thermal wave generated by convective heat transfer in the porous medium, that is, under the thermal resonance conditions. Modeling has revealed that the superadiabatic combustion wave with the maximal temperature ≈1000 °C (the adiabatic effect of the lean mixture is 100 K) can induce the refrigerating wave with the temperature down to −100 °C (the adiabatic effect of hydrogen phase transition in the porous medium is −20 K).

The Chromic Phase Transition in Hydrogen Bonded Polydiacetylenes

Abstract Thermochromism in the polydiacetylenes(PDA) from the alkylurethanes of 5, 7-dodecadiyn-1, 12-diol is associated with a first order phase transition involving the expansion of the crystallographic unit cell, the preservation of the urethane hydrogen bonding, and possibly some relief of mechanical strain upon heating. Insights into thermochromism obtained from recent studies of nonthermochromic forms of PDA-ETCD are presented. Initial observations of the surfaces of single crystals of several PDA in this class by atomic force microscopy are consistent with known crystallographic data.

Nonadiabatic Models of Jupiter and Saturn

New interior models for Jupiter and Saturn are described that do not assume the temperature profiles to be adiabatic throughout the entire hydrogen-helium envelope. These models include the recently calculated radiative opacities due to hydrogen, helium, water, methane, and ammonia and an up-to-date hydrogen/helium equation of state. The helium abundance in the planet, the mass of the core, and the discontinuity of helium abundance at the molecular/metallic hydrogen phase transition (or the ice fraction in the core) are optimized to reproduce the observed mass, radius, and gravitational moments within their error bars. These nonadiabatic models are characterized by the presence of a radiative zone in the outermost layers of the planet, surrounded by two convective zones. This radiative window yields a substantial modification of the thermal properties of the planets and leads to cooler interior models. The mass of Jupiter core is found to be slightly larger than for the adiabatic models. Finally, the mass fractions of heavy elements deduced from our calculations are in better agreement with the values inferred from atmospheric measurements.

Hydrostatic pressure effects on the low temperature phase transitions in hydrogen bonded ferroelectrics

Abstract Ferroelectrics which have their transition temperatures near OK have peculiar characteristics compared to usual ferroelectrics. Two hydrogen bonded ferroelectrics, tris-sarcosine calcium bromide (TSCB) and KH2AsO4 were studied as functions of hydrostatic pressure. The dielectric constants and the Tc vs pressure phase diagrams for both ferroelectrics show behaviors known for quantum ferroelectrics.

Orientational phase transitions in hydrogen at megabar pressures.

We have studied solid molecular hydrogen in various ortho-para concentrations at megabar pressures and down to liquid-helium temperatures. From changes of Raman spectra of rotational and vibrational modes we have identified three new phases. We show evidence for the orientationally ordered phase of parahydrogen at a pressure of 110 GPa (1.1 Mbar) and 8 K and for molecular orientational ordering within the newly discovered hydrogen-A phase.

Theoretical Study of High Pressure Metallic Hydrogen

ABSTRACTA study of the zero temperature phase transitions in hydrogen under megabar pressures using a first-principles total-energy method is presented. An anisotropic primitive hexagonal phase is found to be particularly stable relative to other monatomic phases for pressures between 4 and 8 megabars. Calculations of the vibrational frequencies show that this phase is unstable with respect to a distortion tripling the unit cell along the c-axis. Results for this distorted hexagonal phase will be presented, including a calculation of its superconducting transition temperature Tc.

Raman Scattering Study on the Phase Transition of NH4H2AsO4

Raman scattering spectra of internal modes hetve been studied in antiferroelectric NH 4 H 2 AsO 4 at several temperatures near the transition temperature T N . The evidence of the C 1 local site symmetry of H 2 AsO 4 - in the paraelectric phase has not been obtained even just above T N . The integrated intensity of the ν 2 mode giving the justification of the C 1 symmetry in the antiferroelectric phase increases with decreasing temperature below T N . In addition, the linewidth of the ν 1 internal mode increases with increasing temperature. From the latter two results, the relaxation of the rigid ice constraint is proposed in the discussion of the models of the phase transition in hydrogen bonded antiferroelectrics.

Grazing incidence x-ray study of the structures and phase transitions of hydrogen on tungsten (100).

We report a synchrotron x-ray diffraction study of the tungsten (100) surface reconstruction induced by the chemisorption of hydrogen at room temperature. Our experiments verify previously deduced phase boundaries but they suggest a new interpretation of the phases, the most interesting being that the weakly incommensurate ``solid'' is actually a surface fluid. These experiments also illustrate the utility of x-rays in obtaining quantitative intensity and line-shape information in chemisorbed surface systems.