34 research outputs found
Finite temperature elastic constants of paramagnetic materials within the disordered local moment picture from ab initio molecular dynamics calculations
We present a theoretical scheme to calculate the elastic constants of
magnetic materials in the high-temperature paramagnetic state. Our approach is
based on a combination of disordered local moments picture and ab initio
molecular dynamics (DLM-MD). Moreover, we investigate a possibility to enhance
the efficiency of the simulations of elastic properties using recently
introduced method: symmetry imposed force constant temperature dependent
effective potential (SIFC-TDEP). We have chosen cubic paramagnetic CrN as a
model system. This is done due to its technological importance and its
demonstrated strong coupling between magnetic and lattice degrees of freedom.
We have studied the temperature dependent single-crystal and polycrystalline
elastic constants of paramagentic CrN up to 1200 K. The obtained results at T=
300 K agree well with the experimental values of polycrystalline elastic
constants as well as Poisson ratio at room temperature. We observe that the
Young's modulus is strongly dependent on temperature, decreasing by ~14% from
T=300 K to 1200 K. In addition we have studied the elastic anisotropy of CrN as
a function of temperature and we observe that CrN becomes substantially more
isotropic as the temperature increases. We demonstrate that the use of Birch
law may lead to substantial errors for calculations of temperature induced
changes of elastic moduli. The proposed methodology can be used for accurate
predictions of mechanical properties of magnetic materials at temperatures
above their magnetic order-disorder phase transition.Comment: 1 table, 3 figure
Ab initio study of mode-resolved phonon transmission at Si/Ge interfaces using atomistic Green's functions
Solid interfaces with exceptionally low or high thermal conductance are of intense scientific and practical interest. However, realizing such interfaces is challenging due to a lack of knowledge of the phonon transmission coefficients between specific modes on each side of the interface and how the coefficients are affected by atomic scale structure. Here, we report an ab initio based study of phonon transmission at Si/Ge interfaces using a recent extension of the atomistic Green's function method that resolves transmission coefficients by mode. These results provide a detailed framework to investigate the precise transmission and reflection processes that lead to thermal resistance and how they depend on phonon frequency as well as incident angle. We find that the transmission and reflection processes can be partly explained with familiar concepts such as conservation of transverse momentum, but we also find that numerous phonons have zero transmission coefficient despite the existence of modes that satisfy transverse momentum conservation. This work provides detailed insights into precisely which phonons transmit or reflect at interfaces, knowledge necessary to design solid interfaces with extreme values of thermal conductance for thermoelectricity and heat management applications
Lattice Thermal Conductivity of Polyethylene Molecular Crystals from First-Principles Including Nuclear Quantum Effects
Molecular crystals such as polyethylene are of intense interest as flexible thermal conductors, yet their intrinsic upper limits of thermal conductivity remain unknown. Here, we report a study of the vibrational properties and lattice thermal conductivity of a polyethylene molecular crystal using an ab initio approach that rigorously incorporates nuclear quantum motion and finite temperature effects. We obtain a thermal conductivity along the chain direction of around 160  W m^(-1) K^(-1) at room temperature, providing a firm upper bound for the thermal conductivity of this molecular crystal. Furthermore, we show that the inclusion of quantum nuclear effects significantly impacts the thermal conductivity by altering the phase space for three-phonon scattering. Our computational approach paves the way for ab initio studies and computational material discovery of molecular solids free of any adjustable parameters
Lattice Thermal Conductivity of Polyethylene Molecular Crystals from First-Principles Including Nuclear Quantum Effects
Molecular crystals such as polyethylene are of intense interest as flexible thermal conductors, yet their intrinsic upper limits of thermal conductivity remain unknown. Here, we report a study of the vibrational properties and lattice thermal conductivity of a polyethylene molecular crystal using an ab initio approach that rigorously incorporates nuclear quantum motion and finite temperature effects. We obtain a thermal conductivity along the chain direction of around 160  W m^(-1) K^(-1) at room temperature, providing a firm upper bound for the thermal conductivity of this molecular crystal. Furthermore, we show that the inclusion of quantum nuclear effects significantly impacts the thermal conductivity by altering the phase space for three-phonon scattering. Our computational approach paves the way for ab initio studies and computational material discovery of molecular solids free of any adjustable parameters
Anharmonicity changes the solid solubility of an alloy at high temperatures
We have developed a method to accurately and efficiently determine the
vibrational free energy as a function of temperature and volume for
substitutional alloys from first principles. Taking TiAlN alloy as
a model system, we calculate the isostructural phase diagram by finding the
global minimum of the free energy, corresponding to the true equilibrium state
of the system. We demonstrate that the anharmonic contribution and temperature
dependence of the mixing enthalpy have a decisive impact on the calculated
phase diagram of a TiAlN alloy, lowering the maximum temperature
for the miscibility gap from 6560 K to 2860 K. Our local chemical composition
measurements on thermally aged TiAlN alloys agree with the
calculated phase diagram.Comment: 4 pages, 5 figures, supplementary materia
Ab initio study of mode-resolved phonon transmission at Si/Ge interfaces using atomistic Green's functions
Solid interfaces with exceptionally low or high thermal conductance are of intense scientific and practical interest. However, realizing such interfaces is challenging due to a lack of knowledge of the phonon transmission coefficients between specific modes on each side of the interface and how the coefficients are affected by atomic scale structure. Here, we report an ab initio based study of phonon transmission at Si/Ge interfaces using a recent extension of the atomistic Green's function method that resolves transmission coefficients by mode. These results provide a detailed framework to investigate the precise transmission and reflection processes that lead to thermal resistance and how they depend on phonon frequency as well as incident angle. We find that the transmission and reflection processes can be partly explained with familiar concepts such as conservation of transverse momentum, but we also find that numerous phonons have zero transmission coefficient despite the existence of modes that satisfy transverse momentum conservation. This work provides detailed insights into precisely which phonons transmit or reflect at interfaces, knowledge necessary to design solid interfaces with extreme values of thermal conductance for thermoelectricity and heat management applications
Temperature-dependent elastic properties of Ti_(1−x)Al_xN alloys
Ti_(1−x)Al_xN is a technologically important alloy that undergoes a process of high temperature age-hardening that is strongly influenced by its elastic properties. We have performed first principles calculations of the elastic constants and anisotropy using the symmetry imposed force constant temperature dependent effective potential method, which include lattice vibrations and therefore the effects of temperature, including thermal expansion and intrinsic anharmonicity. These are compared with in situ high temperature x-ray diffraction measurements of the lattice parameter. We show that anharmonic effects are crucial to the recovery of finite temperature elasticity. The effects of thermal expansion and intrinsic anharmonicity on the elastic constants are of the same order, and cannot be considered separately. Furthermore, the effect of thermal expansion on elastic constants is such that the volume change induced by zero point motion has a significant effect. For TiAlN, the elastic constants soften non-uniformly with temperature: C_(11) decreases substantially when the temperature increases for all compositions, resulting in an increased anisotropy. These findings suggest that an increased Al content and annealing at higher temperatures will result in a harder alloy
A study about the military actions of the bishops-lords in the 11th and 12th centuries: dismemberment of research
A comunicação aqui apresentada tem por objetivo trazer à tona os primeiros
apontamentos a respeito da atuação guerreira do episcopado no decorrer do século
XI e XII levando em consideração a condição senhorial dos mesmos. Neste caso, os
enunciados guerra e senhorio nos parecem ser o ponto de partida para analisar as
relações de poder calcadas em negociações definidas graças ao posicionamento
senhorial-episcopal de certos indivÃduos. Está investigação em curso tem sido
desenvolvida no âmbito do Programa de Pós-Graduação em História da
Universidade Federal de Sergipe onde atuo como professor e orientador de
mestrado._________________________________________________________________________________________ ABSTRACT: This presentation have as aim to carry out and present the first appointments
concerning to the warrior proceeding of the episcopate during the 11th and 12th
centuries, according to your condition of landlord. In this case, the enunciation on
war and seigniory seems us the point of departure to analyse the relationship of
power, which is based on negotiations according to the position as seigniory and as
episcopal of some characters. This research in course have been developed in the
Programa de Pós-Graduação em História of the Universidade Federal de Sergipe,
where I teaching in the Master programme
Thermoelectric transport trends in group 4 half-Heusler alloys
The thermoelectric properties of 54 different group 4 half-Heusler (HH) alloys have been studied from first principles. Electronic transport was studied with density functional theory using hybrid functionals facilitated by the kâ‹…p method, while the temperature-dependent effective potential method was used for the phonon contributions to the figure of merit ZT. The phonon thermal conductivity was calculated including anharmonic phonon-phonon, isotope, alloy, and grain-boundary scattering. HH alloys have an XYZ composition, and those studied here are in the group 4-9-15 (Ti,Zr,Hf)(Co,Rh,Ir)(As,Sb,Bi) and group 4-10-14 (Ti,Zr,Hf)(Ni,Pd,Pt)(Ge,Sn,Pb). The electronic part of the thermal conductivity was found to significantly impact ZT and thus the optimal doping level. Furthermore, the choice of functional was found to significantly affect thermoelectric properties, particularly for structures exhibiting band alignment features. The intrinsic thermal conductivity was significantly reduced when alloy and grain-boundary scattering were accounted for, which also reduced the spread in thermal conductivity. It was found that sublattice disorder on the Z-site, i.e., the site occupied by group 14 or 15 elements, was more effective than X-site substitution, occupied by group 4 elements. The calculations confirmed that ZrNiSn, ZrCoSb, and ZrCoBi based alloys display promising thermoelectric properties. A few other n-type and p-type compounds were also predicted to be potentially excellent thermoelectric materials, given that sufficiently high charge carrier concentrations can be achieved. This study provides insight into the thermoelectric potential of HH alloys and casts light on strategies to optimize the thermoelectric performance of multicomponent alloys