25 research outputs found
A quantum Monte Carlo study of high pressure solid and liquid hydrogen
No full textHydrogen is the first element of the periodic table. As such, it is often regarded as the simplest one:
the non-relativistic hydrogen atom is a problem exactly solved in many textbooks; the hydrogen
molecular ion H 2 + and the diatomic molecule H 2 are, correspondingly, the first systems to be
considered when more than one nucleus is involved. As a thermodynamic system, its phase diagram
at low pressures is quite standard: at room temperature and ambient pressure, hydrogen is a
molecular fluid; upon cooling, it becomes a molecular solid; its critical point is T=33 K and P=1.3
Pa.
Nevertheless, even such a simple system becomes really interesting when pressure is increased by
several orders of magnitude. Speculations about the existence of a metallic solid state at 25 GPa
and 0 K temperature started with Wigner and Huntington; later calculations suggested that
this state could become a high-temperature superconductor . When experiments achieved the
predicted transition temperature, they did not find a metallic state; on the other hand, they found
a rich phase diagram, where several different solid phases exist. Nowadays, the quest for solid
metallic hydrogen at low temperature is still an on-going activity.
As temperature is increased above ≈ 1000 K, the system enters the liquid phase: it is important
to obtain an accurate equation of state at high temperature and high pressure, in order to model
the properties of gas giants, such as Jupiter and Saturn, which are mostly made of hydrogen and
helium. Metallic hydrogen, which is yet to be seen in the solid state, was experimentally measured
in the liquid phase.
Performing experiments at such high pressures is complicated; the information obtained is partial.
At low temperatures, the boundaries among the different solid phases can be drawn, but most
of their structural properties are still an open problem; at high temperatures, characterizing the
insulator-metal transition is hard because of large uncertainties and conflicting results.
Ab Initio simulations can be a valuable tool to complement and interpret experimental data; they
can also guide experiments with their predictive power. For condensed matter, Density Functional
Theory (DFT) is the method of choice to perform Ab Initio simulations at reasonable computational
cost. However, their predictive power for high pressure hydrogen is questioned due to several levels
of approximation which will be discussed in our work: in particular, the fact that DFT is plagued
by an uncontrolled approximation (the exchange-correlation functional approximation) will be
elaborated.
iiiIn this thesis, we will employ a different method to run Ab Initio simulations of high pressure
hydrogen at finite temperature: the Coupled Electron Ion Monte Carlo (CEIMC). We will discuss
how CEIMC, combining the Path Integral formalism to treat the nuclear degrees of freedom
and the Variational Monte Carlo (VMC) method to accurately compute electronic energies in a
Born-Oppenheimer framework, can perform finite temperature simulations without suffering from
the same kind of uncontrolled approximation which plagues DFT. We will then apply the method
to the low temperature, solid phase and to the high-temperature, liquid one. In the first case,
finite temperature simulations of different candidate structures for the various solid phases will
be performed, comparing CEIMC results with DFT ones. In the second case, the liquid-liquid
phase transition will be investigated, drawing attention to the relationship between molecular
dissociation and metallization; to do so, the system will be characterized across the transition with
the computation of relevant optical properties
Optical properties of high-pressure fluid hydrogen across molecular dissociation
No full textOptical properties of compressed fluid hydrogen in the region where dissociation and metallization is observed are computed by ab initio methods and compared with recent experimen- tal results. We confirm that at T>3,000 K, both processes are continuous, while at T<1,500 K, the first-order phase transi- tion is accompanied by a discontinuity of the dc conductivity and the thermal conductivity, while both the reflectivity and absorption coefficient vary rapidly but continuously. Our results support the recent analysis of National Ignition Facility (NIF) experiments [Celliers PM, et al. (2018) Science 361:677–682], which assigned the inception of metallization to pressures where the reflectivity is ⇠0.3. Our results also support the conclusion that the temperature plateau seen in laser-heated diamond-anvil cell (DAC) experiments at temperatures higher than 1,500 K corre- sponds to the onset of optical absorption, not to the phase transition. hydroge
Liquid–liquid phase transition in hydrogen by coupled electron–ion Monte Carlo simulations
No full textThe 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
Life Cycle Assessment (LCA) of biogas-fed Solid Oxide Fuel Cell (SOFC) plant
No full textThe Life Cycle Assessment (LCA) of biogas-fed Solid Oxide Fuel Cell (SOFC) integrated with a CO2 recovery system is presented in this work. The goal of the work is to evaluate the environmental performance of an SOFC fueled with sewage biogas and to compare it with traditional technologies (internal combustion engines and microturbines) using the same fuel. CO2 recovery is performed through a tubular photobioreactor, fixing the recovered carbon in the form of a micro-algae.
The analysis takes into account both the biogas production line (anaerobic digester) and its exploitation into the fuel cell (i.e., the power generator).
Results show that the SOFC manufacturing activity is highly intensive since it requires a large amount of use of electricity. During operation, instead, the highest burden is associated with the fuel production. We analyzed two scenarios for biogas operation underlining the benefits of introducing sludge pre-thickening before the anaerobic digestion process. The use of a sludge pre-thickening system can reduce the inlet flow of natural gas into the plant, thus affecting the fuel chain contribution and reducing the overall impact.
The photobioreactor results in consuming more energy than what it produces (looking at the operation phase only; the manufacturing phase was not even included) and being responsible for more carbon emissions than the amount fixed in algae. On the other side, data for the photobioreactor system were based on a real system at the proof-of-concept level. Therefore significant improvements are expected for an industrial-size system.
Finally, the SOFC environmental burdens have been compared with main competitors in the same field (internal combustion engines and microturbines), showing the superior environmental performance. The proposed energy system is thus an interesting choice for cleaner energy production
Electron localization properties in high pressure hydrogen at the liquid-liquid phase transition by Coupled Electron-Ion Monte Carlo
No full text10 pages, 6 figures, prepared for the proceedings of CCP2017We analyze in detail the electronic properties of high pressure hydrogen around the liquid-liquid phase transition based on Coupled Electron-Ion Monte Carlo calculations. Computing the off-diagonal single particle density matrix and the momentum distribution we discuss localization properties of the electrons. The abrupt changes of these distributions indicate a metal to insulator transition occurring together with the structural transition from the atomic to molecular fluid. We further discuss the electron-proton and electron-electron pair correlation functions, which also change abruptly at the transition
Coupled electron-ion Monte Carlo simulation of hydrogen molecular crystals
No full textInternational audienc
Atlas of plastic surgery in cleft lip and palate
No full textA ilustração médica é um instrumento fundamental para facilitar o entendimento das estruturas anatômicas, bem como alterações patológicas que afetam essas estruturas, contribuindo para o ensino e o desenvolvimento das Ciências da Saúde. A ilustração médica vem com a própria Medicina, como uma linguagem inteligível de fenômenos e estruturas biológicas. Ao longo da história, um grande arsenal de métodos de imagem foi empregado para a transmissão do conhecimento. Tais exemplos vão desde infográficos ilustrativos de reações celulares e moleculares até representações realistas de estruturas macroscópicas. A imagem não precisa de tradução, é uma linguagem universal. No âmbito da cirurgia, a ilustração pode auxiliar no entendimento da anatomia, reparos estruturais, protocolos terapêuticos utilizados, bem como os resultados esperados. Os benefícios podem ser vistos nas mais diversas áreas das ciências da saúde. Mas para isso os materiais didáticos devem ser constantemente atualizados, acompanhando o desenvolvimento de novos conhecimentos. Nesse sentido, o objetivo do presente estudo é desenvolver e compilar ilustrações de protocolos e técnicas cirúrgicas aplicadas à fissura labiopalatina (FLP), a fim de auxiliar o desenvolvimento dos profissionais de saúde envolvidos com esses tratamentos. Para tanto, o objetivo deste trabalho foi o desenvolvimento de um Atlas ilustrado de FLP, através da criação de ilustrações dos vários aspectos relacionados à FLP, incluindo: Embriologia; Anatomia da Velofaringe; Anatomia patológica na FLP; Classificação do FLP; bem como técnicas cirúrgicas de Queiloplastia; Palatoplastia; Rinoplastia e Faringoplastia. O Atlas foi desenvolvido combinando a literatura prévia na área com a experiência dos autores na área craniofacial. Todas as ilustrações foram desenvolvidas pelo primeiro autor usando suas habilidades como cirurgião plástico e sua experiência como ilustrador médico. Para a criação das ilustrações, várias técnicas foram utilizadas, isoladamente ou em combinação, incluindo lápis de grafite, aquarela, ilustração digital e vetorização. O Atlas foi concebido com o objetivo de otimizar o processo de aprendizagem de profissionais da saúde envolvidos com o gerenciamento de anomalias craniofaciais, particularmente a FLP.Medical illustration is a fundamental instrument to facilitate the understanding of the anatomical structures, as well as pathological alterations that affect these structures, contributing to the teaching and the development of Healthcare Sciences. Medical illustration comes with Medicine itself, as an intelligible language of biological phenomena and structures. Throughout history, a large arsenal of imaging methods were employed for the transmission of knowledge. Such examples range from illustrative infographics of cellular and molecular reactions to realistic representations of macroscopic structures. The image does not need translation, it is a universal language. In the scope of surgery, the illustration can help in understanding the anatomy, structural repairs, therapeutic protocols used, as well as the expected results. The benefits can be seen within the most diverse areas of the healthcare sciences. But for this the teaching materials must be constantly updated, accompanying the development of new knowledge. In this sense, the objective of the present study is to develop and compile illustrations of protocols and surgical techniques applied to Cleft Lip and Palate (CLP), in order to assist the development of health professionals involved with these treatments. To do so, the aim of this work was the development of an Illustrated Atlas of CLP, through the creation of illustrations of the various aspects related to CLP, including: Embryology; Anatomy of the Velofaringe; Pathological anatomy in CLP; Classification of CLP; as well as surgical techniques of Queiloplasty; Palatoplasty; Rhinoplasty and Pharyngoplasty. The Atlas was developed combining prior literature in the area with the experience of the authors with craniofacial care. All illustrations were developed by the first author using his skills as plastic surgeon and his experience as a medical illustrator. For the creation of the illustrations several techniques have been used, alone or in combination, including graphite pencil, watercolor, digital illustration and vectorization. The Atlas was designed with the objective of optimize the learning process of healthcare professionals involved with management of craniofacial anomalies, particularly CLP
ANALISIS POTENSI BAHAYA DAN PENGENDALIAN RISIKO K3 PADA PRODUKSI PLASTIK DENGAN METODE HIRARC
No full textJika membutuhkan abstrak atau isi jurnal silahkan menghubungi author melalui email [email protected], [email protected], [email protected]
Author response: A compartmentalized signaling network mediates crossover control in meiosis
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