Electronic structure and phonon thermodynamics of iron alloys

Inelastic neutron scattering (INS) and nuclear-resonant inelastic x-ray scattering (NRIXS) were used to measure phonon spectra of FeV as a B2- ordered compound and as a bcc solid solution. Contrary to the behavior of ordering alloys studied to date, the phonons in the B2-ordered phase are softer than in the solid solution. Ordering increases the vibrational entropy, which stabilizes the ordered phase to higher temperatures. Ab initio calculations show that the number of electronic states at the Fermi level increases upon ordering, enhancing the screening between ions, and reducing the interatomic force constants. The effect of screening is larger at the V atomic sites than at the Fe atomic sites.

The phonon spectra of Au-rich alloys of fcc Au-Fe were also measured. The main effect on the vibrational entropy of alloying comes from a stiffening of the Au partial phonon density of states (DOS) with Fe concentration that increases the miscibility gap temperature. The magnitude of the effect is non- linear and it is reduced at higher Fe concentrations. Force constants were calculated for several compositions and show a local stiffening of Au–Au bonds close to Fe atoms, but Au–Au bonds that are farther away do not show this effect. Phonon DOS curves calculated from the force constants reproduced the experimental trends. The Au–Fe bond is soft and favors ordering, but a charge transfer from the Fe to the Au atoms stiffens the Au–Au bonds enough to favor unmixing. The stiffening is attributed to two main effects comparable in magnitude: an increase in electron density in the free-electron-like states, and stronger sd-hybridization.

INS and NRIXS measurements were performed at elevated temperatures on B2-ordered FeTi and NRIXS measurements were performed at high pressures. The high-pressure behavior is quasi- harmonic. The softening of the phonon DOS curves with temperature is strongly nonharmonic. Calculations of the force constants and Born-von Karman fits to the experimental data show that the bonds between second nearest neighbors (2nn) are much stiffer than those between 1nn, but fits to the high temperature data show that the former softens at a faster rate with temperature. The Fe–Fe bond softens more than the Ti–Ti bond. The unusual stiffness of the 2nn bond is explained by the calculated charge distribution, which is highly aspherical and localized preferentially in the t2g orbitals. Ab initio molecular dynamics (AIMD) simulations show a charge transfer from the t2g orbitals to the eg orbitals at elevated temperatures. The asphericity decreases linearly with temperature and is more severe at the Fe sites.

Determinação da viscosidade de misturas assimétricas em alta pressão usando regras de mistura

O conhecimento de propriedades de transporte de misturas a diferentes pressões e temperaturas é importante em projetos, operação, controle e otimização de processos industriais. Nestes processos, frequentemente, o fluido é uma mistura binária ou multicomponente de hidrocarbonetos, como fluidos de petróleo. Propriedades experimentais de misturas, especialmente, a viscosidade absoluta como função de temperatura e pressão, podem fornecer importantes informações sobre o comportamento do fluido em diferentes composições e são usadas no desenvolvimento de modelos e correlações e na caracterização de misturas complexas. Diversas regras de mistura têm sido propostas na literatura para cálculo de viscosidade de misturas. Estas regras de mistura preveem o comportamento da mistura à pressão atmosférica usando propriedades dos componentes puros. Porém, em diversas aplicações é necessário estimar a viscosidade de misturas a altas pressões. Neste estudo, foram avaliadas regras de mistura comumente usadas como Refutas, Fator de Mistura, Índice de Mistura, Grunberg e Nissan, Kendall-Monroe e Eyring bem como Aditividade Molar, usando dados de viscosidade experimental de misturas em altas pressões. Inicialmente, foram realizadas medidas de viscosidade absoluta para a mistura altamente assimétrica de ciclohexano e n-hexadecano na faixa de temperatura entre (318,15 a 413,15) K e pressões até 62,053 MPa e, para este sistema, um modelo foi proposto para cálculo dos componentes puros para dada temperatura e pressão. Além disso, dados experimentais de viscosidade de trinta misturas cujos componentes diferem em forma, tamanho ou flexibilidade foram selecionados na literatura e modelados empregando-se regras de mistura. As viscosidades das misturas foram estimadas a partir de dados de viscosidade experimental dos componentes puros medidos nas mesmas temperaturas e pressões. A altas pressões, Refutas, Fator de Mistura e Índice de Mistura apresentaram os melhores resultados para todos os sistemas estudados. Mesmo para moléculas bastante assimétricas, Refutas, Fator de Mistura e Índice de Mistura podem ser usados.

Modelagem do Equilíbrio de Fases entre Hidrocarbonetos Leves e Pesados

O comportamento de fases para sistemas binários com um hidrocarboneto leve e um pesado é muito importante tanto para o projeto real de um processo quanto para o desenvolvimento de modelos teóricos. Para atender a crescente demanda por informação experimental de equilíbrio de fases a altas pressões, o objetivo deste estudo é obter uma metodologia que substitua parcialmente ou maximize a pouca informação experimental disponível. Para isto propõe-se a modelagem do equilíbrio de fases em misturas de hidrocarboneto leve com um pesado, sem o conhecimento da estrutura molecular do pesado, inferindo-se os parâmetros do modelo a partir da modelagem de dados de ponto de bolha obtidos na literatura. Esta metodologia implica não só na descrição do equilíbrio de fases de um sistema como na estimação das propriedades críticas do pesado, de difícil obtenção devido ao craqueamento destes a altas temperaturas. Neste contexto, este estudo apresenta uma estratégia que estima indiretamente as propriedades críticas dos compostos pesados. Para isto, foram correlacionados dados experimentais de ponto de bolha de misturas binárias contendo um hidrocarboneto leve e um pesado, usando-se dois modelos: o de Peng-Robinson e o TPT1M (Teoria da Polimerização Termodinâmica de primeira ordem de Wertheim modificada). Os parâmetros ajustados com o modelo de Peng-Robinson correspondem diretamente às propriedades críticas do composto pesado, enquanto os ajustados com o modelo TPT1M foram usados para obtê-las. Esta estratégia fornece parâmetros dependentes do modelo, porém permite o cálculo de outras propriedades termodinâmicas, como a extrapolação da temperatura dos dados estudados. Além disso, acredita-se que a correlação dos parâmetros obtidos com as propriedades críticas disponíveis ajudará na caracterização de frações pesadas de composição desconhecida

Estudo teórico e experimental da velocidade do som em sistemas contendo hidrocarbonetos

A descoberta de reservatórios de petróleo na camada do pré-sal abriu novas perspectivas para a economia brasileira. A grandes profundidades, com elevado teor de CO2 e submetidos a altas pressões e temperaturas, a exploração de petróleo das camadas de pré-sal constitui um considerável desafio tecnológico. Neste contexto, o conhecimento dos coeficientes de compressibilidade são fundamentais para o estudo do comportamento termodinâmico e escoamento dos fluidos destes campos de produção. Com este objetivo, foram determinadas experimentalmente nesta dissertação a massa específica e a velocidade do som de sistemas envolvendo quatro hidrocarbonetos puros: n-decano, n-hexadecano, decalina e tetralina, assim como para sistemas multicompostos envolvendo estes hidrocarbonetos e para uma amostra de condensado de petróleo. Os resultados obtidos permitiram o cálculo dos coeficientes de compressibilidade isentrópica, isotérmica e isentálpica. É apresentada uma avaliação sobre a capacidade de se correlacionar dados experimentais de velocidade do som aos dados calculados pelas equações de estado de Peng-Robinson e Lee-Kesler com regras de mistura: (i) Spencer e Danner e (ii) Lee-Kesler

Determinação experimental e modelagem termodinâmica do equilíbrio de fases em sistemas com CO2, n-hexano, n-hexadecano e tetralina

A descoberta de petróleo na camada de Pré-Sal possibilita a geração de ganhos em relação à dependência energética do país, mas também grandes desafios econômicos e tecnológicos. Os custos de extração são maiores devido a vários fatores como a exigência de equipamentos de exploração que suportem elevadas pressões, altas temperaturas e grandes concentrações de gases ácidos, tais quais, dióxido de carbono (CO2) e sulfeto de hidrogênio (H2S). Uma das principais preocupações com o CO2 é evitar liberá-lo para a atmosfera durante a produção. Com a modelagem termodinâmica de dados de equilíbrio de sistemas envolvendo CO2 supercrítico e hidrocarbonetos é possível projetar equipamentos utilizados em processos de separação. A principal motivação do trabalho é o levantamento de dados de equilíbrio de fases de sistemas compostos de CO2 e hidrocarbonetos, possibilitando assim prever o comportamento dessas misturas. Os objetivos específicos são a avaliação do procedimento experimental, a estimação e predição dos parâmetros de interação binários para assim prever o comportamento de fases dos sistemas ternários envolvendo CO2 e hidrocarbonetos. Duas metodologias foram utilizadas para obtenção dos dados de equilíbrio: método estático sintético (visual) e método dinâmico analítico (recirculação das fases). Os sistemas avaliados foram: CO2 + n-hexano, CO2 + tetralina, CO2 + n-hexadecano, CO2 + n-hexano + tetralina e CO2 + tetralina + n-hexadecano à alta pressão; tetralina + n-hexadecano à baixa pressão. Para o tratamento dos dados foi utilizada equação de estado cúbica de Peng-Robinson e a regra de mistura clássica

Water supply to a geotechnical centrifuge for pile jetting in sand

The supply of water is often required during a centrifuge experiment. For the case of pile jetting, significant flow volumes and pressures are required from the water supply. This paper aims to detail the successful provision of water at high pressures and large flow rates to a centrifuge, using a novel water supply system. An impeller pump was used to pressurise the water in advance of the slip rings, with further pressure provided by the fluid accelerating along the centrifuge beam arm. A maximum pressure of 2 MPa and continuous flow rate of 6 litres per minute were achieved. The calculation of water pressure away from the measurement location is presented, offering a repeatable solution for the pressure at any point in the pipe work. © 2010 Taylor & Francis Group, London.

Impact of lubricant composition on low-speed pre-ignition

One of the limits on the maximum fuel efficiency benefit to be gained from turbocharged, downsized gasoline engines is the occurrence of pre-ignitions at low engine speed. These pre-ignitions may lead to high pressures and extreme knock (megaknock or superknock) which can cause severe engine damage. Though the mechanism leading to megaknock is not completely resolved, pre-ignitions are thought to arise from local autoignition of areas in the cylinder which are rich in low ignition delay "contaminants" such as engine oil and/or heavy ends of gasoline. These contaminants are introduced to the combustion chamber at various points in the engine cycle (e.g. entering from the top land crevice during blow-down or washed from the cylinder walls during DI wall impingement). This paper presents results from tests in which model "contaminants", consisting of engine lubricant base stocks, base stocks mixed with fuel and base stocks mixed with one or more additives were injected directly into a test engine to determine their propensity to ignite. The ignition tendency was found to be lower for less reactive base stocks and for base stocks mixed with certain additives. Further, when small amounts of fuel were mixed with relatively non-ignitive lubricant base stocks the ignition tendency was found to increase significantly. These results may guide development of new lubricants which could be used to reduce megaknock in downsized engines. Copyright © 2014 SAE International.

THE PHYSICAL AND CHEMICAL-PROPERTIES OF SYNTHETIC AND NATURAL JADEITE FOR JEWELRY

Jadeite was synthesized from its glass of stoichiometric composition NaAlSi2O6, and a colouring agent Cr2O3 (0.3-0.6 wt%) was added to achieve the emerald colour. The conditions employed were a pressure range of 3.0-5.0 GPa and a temperature range of 1150

Subtropical dipole mode in the Southern Hemisphere: A global view

A global wavenumber-3 dipole SST mode is showed to exist in the Southern Hemisphere subtropical climate variability in austral summer. A positive (negative) phase of the mode is characterized by cool (warm) SST anomalies in the east and warm (cool) SST anomalies in the southwest of the south Indian, Pacific, and Atlantic Oceans, respectively. This coherent dipole structure is largely a response of ocean mixed layer to the atmospheric forcing characterized by migration and modulation of the subtropical high-pressures, in which the latent heat flux play a leading role through wind-induced evaporation, although ocean dynamics may also be crucial in forming SST anomalies attached to the continents. Exploratory analyses suggest that this mode is strongly damped by the negative heat flux feedback, with a persistence time about three months and no spectral peak at interannual to decadal time scales. As the subtropical dipole mode is linearly independent of ENSO and SAM, whether it represents an additional source of climate predictability should be further studied. Citation: Wang, F. (2010), Subtropical dipole mode in the Southern Hemisphere: A global view, Geophys. Res. Lett., 37, L10702, doi: 10.1029/2010GL042750.

中祁连花岗岩年代学、地球化学及其构造意义

The Qilian Orogenic Belts had undergone very complicated evolutional histories and play an important role in understanding the tectonic evolutions of old terrains in northwestern China, in which granitiods formed during Proterozoic-early Mesozoic are widely outcropped. Detailed studies of these granitiods can shed some light on the tectonic evolution of this region. In this thesis, we have conducted geochronological and geochemical studies on eight selected granitic plutons to unravel their emplacement ages and petrogenesis. Furthermore, their tectonic implications were also discussed based on these results. In Neo-Proterozoic, our results suggest that two stages of magmatic activities were taken place in Central Qilian Block, GroupⅠ(750-790Ma) and Group Ⅱ(845－ 930Ma). In Neo-Paleozoic, most granitic plutons were emplaced from Ordovician to Devonian, whereas granitiods with Triassic ages have also been discovered in South Qilian Belt. Inherited zircons with old ages of 1.7Ga, 2.1Ga and 2.7Ga have also been obtained in our study. Geochemical studies suggest that the Proterzoic granites were produced under high pressures and low temperatures from metamorphosed protolith rocks with compostions from basic to intermediate. This implies that some hot sources were underplated beneath lithosophere via mantle-derived magmatism. In combination with regional geological data, we propose that the Cental Qilian block was an old arc terrene during Precambrian, and two stage granitoids were formed under a back-arc extensional setting. Granitic rocks emplaced in early Paleozoic belong to strong peraluminous S-type granites, which were derived from metagreywacke having strong relationships with collisional process. Together with previous data, our results indicate that granitoids in Qilian Orogenic Belt formed during early Paleozoic have different petrogenesis and emplaced ages, which reflect that Qilian Orogenic Belt had underwent complicated multi-stage subduction-collusional processes in early Paleozoic. On the other hand, granitic rocks in South Qilian Belt with Triassic ages were formed by subduction of East Kulun during early Paleozoic-Late Mesozoic, which represent another orogenic episode in the northern margin of Tibetan Plateau.

通过第一性原理计算研究内地核的结构与成分

The determination of the composition and structure of the Earth’s inner core has long been the major subject in the study of the Earth’s deep interior. It’s widely believed that the Earth’s core is formed by iron with a fraction of nickel. However, light elements must exist in the inner core because the earth core is less dense than pure iron-nickel alloy (~2-3% in the solid inner core and ~6-7% in the liquid outer core). The questions are what and how much light element is there in the iron-nickel alloy. Besides the composition, the crystal structure of the iron with or without light element is also not well known. According to the seismological observations, the sound waves propagate 3-4% faster along the spin axis than in the equatorial plane. That means the inner core is anisotropic. The densest structure of iron-nickel alloy should be h.c.p structure under the very high pressures. However, the h,c,p structure does not propagate waves anisotropic ally. Then what is the structure of the iron-nickel alloy or the iron-nickle-light element alloy. In this study, we tried to predict the composition and the structure of the inner core through ab initio calculation of the Gibbs free energy, which is a function of internal energy, density and entropy. We conclude that the h.c.p structure is more stable than the b.c.c structure under high pressure and 0 K, but with the increase of temperature, the free energy of the b.c.c structure is decreasing much faster than the h.c.p structure caused by the vibration of the atomics, so the b.c.c structure is more stable at high temperatures. With the addition of light elements (S or Si or both), the free energy of b.c.c. decreases even faster, about 3at% of Si not only explains why the inner core is about 2-3 % lighter than the iron-nickle alloy, but also reasons why the inner core is anisotropic, since the b.c.c. structure becomes more stable than the h.c.p structure at 5500-6000K and b.c.c. is anisotropic in propagating seismic waves. Therefore, we infer that the inner core of the earth is formed by b.c.c iron and a fraction of nickel plus ~3at.% Si, with a temperature higher than 5500K, which is consistent with the studies from other approaches.

First-principles study of ferroelectricity and isotope effects in H-bonded KH2PO4 crystals

By means of extensive first-principles calculations we studied the ferroelectric phase transition and the associated isotope effect in KH2PO4 (KDP). Our calculations revealed that the spontaneous polarization of the ferroelectric phase is due to electronic charge redistributions and ionic displacements which are a consequence of proton ordering, and not vice versa. The experimentally observed double-peaked proton distribution in the paraelectric phase cannot be explained by a dynamics of only protons. This requires, instead, collective displacements within clusters that include also the heavier ions. These tunneling clusters can explain the recent evidence of tunneling obtained from Compton scattering measurements. The sole effect of mass change upon deuteration is not sufficient to explain the huge isotope effect. Instead, we find that structural modifications deeply connected with the chemistry of the H bonds produce a feedback effect on tunneling that strongly enhances the phenomenon. The resulting influence of the geometric changes on the isotope effect agrees with experimental data from neutron scattering. Calculations under pressure allowed us to analyze the issue of universality in the disappearance of ferroelectricity upon compression. Compressing DKDP so that the distance between the two peaks in the deuteron distribution is the same as for protons in KDP, corresponds to a modification of the underlying double-well potential, which becomes 23 meV shallower. This energy difference is what is required to modify the O-O distance in such a way as to have the same distribution for protons and deuterons. At the high pressures required experimentally, the above feedback mechanism is crucial to explain the magnitude of the geometrical effect.

Hysteretic adsorption and desorption of hydrogen by nanoporous metal-organic frameworks

Adsorption and desorption of hydrogen from nanoporous materials, such as activated carbon, is usually fully reversible. We have prepared nanoporous metal-organic framework materials with flexible linkers in which the pore openings, as characterized in the static structures, appear to be too small to allow H-2 to pass. We observe hysteresis in their adsorption and desorption kinetics above the supercritical temperature of H-2 that reflects the dynamical opening of the "windows" between pores. This behavior would allow H-2 to be adsorbed at high pressures but stored at lower pressures.

Pressure effect on vibrational frequency and dephasing of 1-alkyl-3-methylimidazolium hexafluorophosphate ionic liquids

Raman spectra in the range of the totally symmetric stretching mode of the [PF6]− anion, νs(PF6), have been measured for 1-alkyl-3-methylimidazolium ionic liquids [CnC1im][PF6], for n = 4, 6, and 8, as a function of pressure at room temperature. The ionic liquids [C6C1im][PF6] and [C8C1im][PF6] remain in an amorphous phase up to 3.5 GPa, in contrast to [C4C1im][PF6], whichcrystallizes above ∼0.5 GPa. Equations of state based either on a group contribution model or Carnahan-Starling-van der Waals model have been used to estimate the densities of the ionic liquids at high pressures. The shifts of the vibrational frequency of νs(PF6) with density observed in [C6C1im][PF6] and in [C8C1im][PF6] have been calculated by a hard-sphere model of a pseudo-diatomic solute under short-range repulsive interactions with the neighboring particles. The stochastic model of Kubo for vibrational dephasing has been used to obtain the amplitude of vibrational frequency fluctuation, ⟨Δω 2⟩, and the relaxation time of frequency fluctuation, τ c , as a function of density by Raman band shape analysis of the νs(PF6) mode of [C6C1im][PF6] and [C8C1im][PF6].

A density functional theory study of CO oxidation on Ru(0001) at low coverage

We have performed ab initio density functional theory calculations with the generalized gradient approximation to investigate CO oxidation on Ru(0001). Several reaction pathways and transition states are identified. A much higher reaction barrier compared to that on Pt(111) is determined, confirming that the Ru is very inactive for CO oxidation under UHV conditions. The origin of the reaction barrier was analyzed. It is found that in the transition state the chemisorbed O atom sits in an unfavorable bonding site and a significant competition for bonding with the same substrate atoms occurs between the CO and the chemisorbed O, resulting in the high barrier. Ab initio molecular dynamics calculations show that the activation of the chemisorbed O atom from the initial hcp hollow site (the most stable site) to the bridge site is the crucial step for the reaction. The CO oxidation on Ru(0001) via the Eley-Rideal mechanism has also been investigated. A comparison with previous theoretical work has been made. (C) 2000 American Institute of Physics. [S0021-9606(00)31223-5].