13,220 research outputs found
From Lagrangian mechanics to nonequilibrium thermodynamics: a variational perspective
In this paper, we survey our recent results on the variational formulation of
nonequilibrium thermodynamics for the finite dimensional case of discrete
systems as well as for the infinite dimensional case of continuum systems.
Starting with the fundamental variational principle of classical mechanics,
namely, Hamilton's principle, we show, with the help of thermodynamic systems
with gradually increasing level complexity, how to systematically extend it to
include irreversible processes. In the finite dimensional cases, we treat
systems experiencing the irreversible processes of mechanical friction, heat
and mass transfer, both in the adiabatically closed and in the open cases. On
the continuum side, we illustrate our theory with the example of multicomponent
Navier-Stokes-Fourier systems.Comment: 7 figure
Finite-time thermodynamics of port-Hamiltonian systems
In this paper, we identify a class of time-varying port-Hamiltonian systems
that is suitable for studying problems at the intersection of statistical
mechanics and control of physical systems. Those port-Hamiltonian systems are
able to modify their internal structure as well as their interconnection with
the environment over time. The framework allows us to prove the First and
Second laws of thermodynamics, but also lets us apply results from optimal and
stochastic control theory to physical systems. In particular, we show how to
use linear control theory to optimally extract work from a single heat source
over a finite time interval in the manner of Maxwell's demon. Furthermore, the
optimal controller is a time-varying port-Hamiltonian system, which can be
physically implemented as a variable linear capacitor and transformer. We also
use the theory to design a heat engine operating between two heat sources in
finite-time Carnot-like cycles of maximum power, and we compare those two heat
engines.Comment: To appear in Physica D (accepted July 2013
Rigorous and General Definition of Thermodynamic Entropy
The physical foundations of a variety of emerging technologies --- ranging
from the applications of quantum entanglement in quantum information to the
applications of nonequilibrium bulk and interface phenomena in microfluidics,
biology, materials science, energy engineering, etc. --- require understanding
thermodynamic entropy beyond the equilibrium realm of its traditional
definition. This paper presents a rigorous logical scheme that provides a
generalized definition of entropy free of the usual unnecessary assumptions
which constrain the theory to the equilibrium domain. The scheme is based on
carefully worded operative definitions for all the fundamental concepts
employed, including those of system, property, state, isolated system,
environment, process, separable system, system uncorrelated from its
environment, and parameters of a system. The treatment considers also systems
with movable internal walls and/or semipermeable walls, with chemical reactions
and/or external force fields, and with small numbers of particles. The
definition of reversible process is revised by introducing the new concept of
scenario. The definition of entropy involves neither the concept of heat nor
that of quasistatic process; it applies to both equilibrium and nonequilibrium
states. The role of correlations on the domain of definition and on the
additivity of energy and entropy is discussed: it is proved that energy is
defined and additive for all separable systems, while entropy is defined and
additive only for separable systems uncorrelated from their environment;
decorrelation entropy is defined. The definitions of energy and entropy are
extended rigorously to open systems. Finally, to complete the discussion, the
existence of the fundamental relation for stable equilibrium states is proved,
in our context, for both closed and open systems.Comment: 19 pages, RevTex
Morphological and Structural Evaluation of Hydration/Dehydration Stages of MgSO4 Filled Composite Silicone Foam for Thermal Energy Storage Applications
Salt hydrates, such as MgSO4·7H2O, are considered attractive materials for thermal energy storage, thanks to their high theoretical storage density. However, pure salt hydrates present some challenges in real application due to agglomeration, corrosion and swelling problems during hydration/dehydration cycles. In order to overcome these limitations, a composite material based on silicone vapor-permeable foam filled with the salt hydrate is here presented. For its characterization, a real-time in situ environmental scanning electron microscopy (ESEM) investigation was carried out in controlled temperature and humidity conditions. The specific set-up was proposed as an innovative method in order to evaluate the morphological evolution of the composite material during the hydrating and dehydrating stages of the salt. The results evidenced an effective micro-thermal stability of the material. Furthermore, dehydration thermogravimetric/differential scanning calorimetric (TG/DSC) analysis confirmed the improved reactivity of the realized composite foam compared to pure MgSO4·7H2O.This work was partially funded by the Ministerio de Ciencia, Innovación y Universidades de España
(RTI2018-093849-B-C31). This work was partially supported by ICREA under the ICREA Academia program
Magnetic Properties and Thermal Entanglement on a Triangulated Kagome Lattice
The magnetic and entanglement thermal (equilibrium) properties in spin-1/2
Ising-Heisenberg model on a triangulated Kagome lattice are analyzed by means
of variational mean-field like treatment based on Gibbs-Bogoliubov inequality.
Because of the separable character of Ising-type exchange interactions between
the Heisenberg trimers the calculation of quantum entanglement in a
self-consistent field can be performed for each of the trimers individually.
The concurrence in terms of three qubit isotropic Heisenberg model in effective
Ising field is non-zero even in the absence of a magnetic field. The magnetic
and entanglement properties exhibit common (plateau and peak) features
observable via (antferromagnetic) coupling constant and external magnetic
field. The critical temperature for the phase transition and threshold
temperature for concurrence coincide in the case of antiferromagnetic coupling
between qubits. The existence of entangled and disentangled phases in saturated
and frustrated phases is established.Comment: 21 pages, 13 figure
Phase separation processes in polymer solutions in relation to membrane formation
This review covers new experimental and theoretical physical research related to the formation of polymeric membranes by phase separation of a polymer solution, and to the morphology of these membranes. Two main phase separation processes for polymeric membrane formation are discussed: thermally induced phase separation and immersion precipitation. Special attention is paid to phase transitions like liquid-liquid demixing, crystallization, gelation, and vitrification, and their relation to membrane morphology. In addition, the mass transfer processes involved in immersion precipitation, and their influence on membrane morphology are discussed
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