共查询到20条相似文献,搜索用时 31 毫秒
1.
We use molecular dynamics simulations to shed light on polymorph selection during the crystallization of the Lennard-Jones fluid. By varying pressure at fixed supercooling, we form large crystallites either of the stable face centered cubic form or of the metastable body centered cubic form and even fine-tune the fractions of stable and metastable polymorphs in the crystallite. We demonstrate that the conditions of crystallization, leading to large bcc crystallites, lie within the occurrence domain of the metastable bcc polymorph. We also find that the predominantly fcc crystallites contain a notable amount of the hexagonal close packed form, due to the cross nucleation of the hcp form on the fcc form. By varying temperature at fixed pressure, we prevent cross nucleation and form pure fcc crystallites. Our results reveal that polymorph selection may take place, and be controlled, during the growth step. 相似文献
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We use the molecular dynamics code, large-scale atomic/molecular massively parallel simulator (LAMMPS), to simulate high strain rate triaxial deformation of crystal copper to understand void nucleation and growth (NAG) within the framework of an experimentally fitted macroscopic NAG model for polycrystals (also known as DFRACT model). It is seen that void NAG at the atomistic scales for crystal copper (Cu) has the same qualitative behaviour as the DFRACT model, albeit with a different set of parameters. The effect of material temperature on the nucleation and growth of voids is studied. As the temperature increases, there is a steady decrease in the void NAG thresholds and close to the melting point of Cu, a double-dip in the pressure–time profile is observed. Analysis of this double-dip shows disappearance of the long-range order due to the creation of stacking faults and the system no longer has a face centred cubic (fcc) structure. Molecular dynamics simulation of shock in crystal Cu at strain rates high enough to cause spallation of crystal Cu are then carried out to validate the void NAG parameters. We show that the pre-history of the material affects the void nucleation threshold of the material. We also simulate high-strain-rate triaxial deformation of crystal Cu with defects and obtain void NAG parameters. The parameters are then used in a macroscale hydrodynamic simulation to obtain spallation threshold of realistic crystal Cu. It is seen that our results match experimental results within the limit of 20% error. 相似文献
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We study the effect of melting of a well-defined crystal surface within the framework of the theory of atomic vibrations in
harmonic approximation. We find that for the face centered cubic lattice we may expect the existence of pre-melting. It is
also shown that the occurrence or absence of surface melting depends strongly on the relaxation of a lattice constant.
This paper is financially supported by the State Committee for Research grant (KBN grant 2 0294 91 01/92–94). 相似文献
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Nataliya D. Kundikova Yuri V. Miklyaev Denis G. Pikhulya 《Optics Communications》2012,285(6):1238-1241
The band structure of 3D photonic crystal that could be synthesized by means of interference lithography with triple-exposure two-beam interference technique has been investigated. As a result of the geometry optimization the optimal conditions for maximal band gaps with different refractive index contrast have been obtained. The refractive index threshold for gap opening equaled to 2.14 has been predicted for this method of photonic crystals synthesis. This value is close to the refractive index thresholds of the best known structures. The continuous transition between simple cubic, face centered cubic and bulk centered cubic symmetries has been considered. 相似文献
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The problem considered is the fitting of a many-body interaction potential to bulk crystal data. A parameterisation of the
potential is assumed which is based on physical considerations. The free parameters are determined by using global optimization
to perform a least squares fit, to a large number of crystal properties. This has been achieved for body centered cubic (bcc)
materials. The approach adopted here fits the bcc crystal structure, as the preferred minimum energy configuration for tungsten,
and also fits the dimer energetics and the elastic properties of crystalline tungsten. 相似文献
9.
A change in the local order of a bounded complex (dusty) plasma in the process of its crystallization and melting has been
examined by molecular dynamics simulations. The dynamics of microparticles is considered in the framework of a Langevin thermostat,
the pair interaction between charged particles is described by a screened Coulomb potential (Yukawa potential) with the hard
wall potential as a confinement. It has been shown that the beginning of the crystallization of such a system is accompanied
by the formation of clusters with the hexagonal close packed (hcp) structure; a noticeable number of these clusters are then
transformed to the face centered cubic (fcc) phase. A plasma crystal formed after crystallization consists of the metastable
hcp phase, fcc clusters, and a small number of clusters with a body centered cubic (bcc) crystal lattice. Beginning with a
certain threshold value of the thermostat temperature, the number of fcc/bcc clusters decreases sharply with increasing temperature,
which is an important signature of the beginning of the melting of the plasma crystal. 相似文献
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Arthur Paskin A. Gohar G.J. Dienes 《Journal of Physics and Chemistry of Solids》1978,39(12):1307-1311
Molecular dynamic shock wave simulations have been carried out for face centered cubic (f.c.c.) and body centered cubic (b.c.c.) solids using Lennard-Jones and Morse potentials for the interatomic interactions. The Hugoniot conservation relations were accurately obeyed in all of these calculations. The shock wave profiles may vary with the interatomic potential and the crystal structure, effects most clearly shown by the temperature profile near the shock front. The Lennard-Jones solids are intensitive to a change in structure but the Morse solids appear sensitive to crystal structure, at least in comparing b.c.c. with f.c.c. It was shown that the average shock wave temperature can be calculated from a combination of the Hugoniot conservation relations and the Mie-Grüneisen equation of state. The temperature calculated this way is in good agreement with the average shock wave temperature obtained in the computer simulations. 相似文献
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V. A. Golovko 《The European Physical Journal B - Condensed Matter and Complex Systems》2010,74(3):345-356
This paper continues the work begun in a previous paper [Eur. Phys. J. B
71, 85 (2009)]. To treat the equations that describe a crystal with
condensate that can be superfluid, a method termed the Kirkwood
approximation is used. Earlier, the method was found to be rather seminal
when applied to a classical crystal. In the case of a simple cubic lattice,
solutions to the equations under study can be expressed in terms of the
well-known Mathieu functions. A more realistic case of the face centered
cubic lattice is also considered although in this case the three-dimensional
equations cannot be reduced to one-dimensional ones. Condensate crystals
without superfluidity are studied first and then the same crystals in a
superfluid state. It is shown in particular that a crystal in which the
condensate is formed is energetically preferable with respect to the same
quantum crystal without condensate at absolute zero of temperature.
Therefore, on lowering the temperature there must somewhere occur
Bose-Einstein condensation in the crystal. In the concluding section, we
discuss various physical aspects of the problem. 相似文献
12.
The phase coexistence method to obtain surface free energies and nucleation barriers: a brief review
ABSTRACTA recently developed method where one analyses the finite size effects associated with liquid–solid phase equilibria including vapour–crystal coexistence is briefly reviewed. It is shown that the estimation of the chemical potential of the vapour surrounding the crystal as function of the crystal volume yields information on the bulk coexistence conditions, when an extrapolation to the thermodynamic limit is performed. Estimating the pressure of the fluid surrounding the crystal nucleus in the finite simulation box and the volume of this nucleus that coexists with the fluid in thermal equilibrium, an estimate for the total surface excess free energy can be obtained, which to a very good approximation is independent of the size of the simulation box. The free energy barrier against homogeneous nucleation of crystals thus can be estimated as a function of the nucleus volume. Monte Carlo simulations for the soft effective Asakura–Oosawa model of colloid-polymer mixtures which form face-centered cubic colloidal crystals are used to exemplify this method, computing the surface excess free energy of these crystals over a wide range of crystal volumes, without the need to characterise the non-spherical crystal shape. A possible extension of these concepts to heterogeneous nucleation is also briefly discussed. 相似文献
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以工业纯单质铁粉和单质镍粉为主要原料,采用粉末冶金方法制备了Fe-Ni-C系反应体系,在六面顶压机上进行了金刚石合成实验。Raman光谱和X射线衍射结果表明,采用这种方法获得的粒径为200~500 μm,呈六-八面体聚形的晶体为立方金刚石单晶。通过对常规力学性能的检测发现,金刚石的品位较高,超过SMD25级锯片级金刚石的要求。分析认为,高温高压下金刚石自Fe-Ni-C系形核是一个触媒不断溶解催化碳原子的过程。大量的实验结果可以证实,金刚石在Fe-Ni-C系长大所需的碳原子来自于在γ-(Fe,Ni)吸引作用下、从(Fe,Ni)3C中不断脱溶的碳。金属包覆膜在这一过程中不但起到了输送碳原子的作用,还以独特的方式促成了碳原子由sp2π杂化态向sp3杂化态的转变。 相似文献
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Vibration modes and thermodynamic properties of a body‐centered cubic (bcc) Coulomb crystal with a small admixture of substitutional isotopic impurities are studied analytically applying the perturbation theory of disordered crystal spectra and the Lifshitz‐Krein trace formula. We calculate the density of phonon states of the perfect bcc Coulomb crystal and use it to compute the heat capacity of the crystal with impurities in a wide range of temperatures. It is shown that the ratio of an impurity contribution to the crystal heat capacity over the perfect crystal specific heat tends to a constant in the low‐temperature quantum regime and decays as T–2 in the classic regime of high temperatures. It is also shown that even a small concentration of heavy impurities amplifies significantly the total crystal heat capacity. The results are compared with those obtained using the more conventional linear mixing theory. It is demonstrated that both methods give similar results at all tempera‐tures when the impurity mass is not too different from that of the base ions but a strong discrepancy is observed at low and intermediate temperatures when impurities are noticeably lighter or heavier. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
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M.P. Pileni 《Surface science》2009,603(10-12):1498-1505
Here we describe how arrangements of nanocrystals can self-organize in 3D arrays called supra crystals. The 3D arrays can fall into the familiar categories of face centered cubic (fcc), hexagonal compact packing (hcp) crystals, and body centered (bcc) crystals. Intrinsic collective properties of these 3D arrangements are different from the properties of individual nanoparticles and from particles in bulk.We demonstrate by two various processes and with two types of nanocrystals (silver and cobalt) that when nanocrystals are self ordered in 3D superlattices, they exhibit a coherent breathing mode vibration of the supra crystal, analogous to a breathing mode vibration of atoms in a nanocrystal.Comparison between the approaches to saturation of the magnetic curve for supra crystals and disordered aggregates produced from the same batch of nanocrystals is similar to that observed with films or nanoparticles either highly crystallized or amorphous. 相似文献
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The localized modes of vibration are discussed for a face centered and a body centered cubic lattice, if the ideal crystal is perturbated by a substitutional impurity. The eigenvectors are found by group-theoretical considerations, the frequencies can be calculated approximately by a variational principle, which gives good results, if the mode is strongly localized. It is discussed, which changes in mass and coupling parameters can give rise to localized modes of vibration. It turns out that in the model, which is considered here (nearest neighbour interaction and no changes in lattice structure), a vacancy does not show localized vibrations. 相似文献
18.
Boothroyd AT Gardiner CH Lister SJ Santini P Rainford BD Noailles LD Currie DB Eccleston RS Bewley RI 《Physical review letters》2001,86(10):2082-2085
Neutron spectroscopic measurements of the magnetic excitations in PrO2 reveal (1) sharp peaks characteristic of transitions between levels of the 4f(1) configuration of Pr4+ split by the cubic crystal field, and (2) broad bands of scattering centered near 30 and 160 meV. We present a simple model based on a vibronic Hamiltonian that accounts for these contrasting features of the data. The analysis shows that 90%+/-10% of the Pr ions have a localized 4f(1) configuration and provides strong evidence for a dynamic Jahn-Teller effect in the gamma(8) electronic ground state. 相似文献
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对面心立方(fcc)、体心立方(bcc)和六角密堆积(hcp)三种不同结构的晶体,在假设它们的原胞中包含8个价电子并将价电子近似为自由电子的情况下,采用“自由电子气理论”和“自由电子能带模型”,研究其根据费米球确定的费米能级EF与根据自由电子能带模型计算的平均键能Em。研究结果表明,由自由电子能带模型计算所得3种不同结构晶体(因而电子密度也不一样)的平均键能Em等于各自自由电子系统的费米能级EF。平均键能Em是我们在异质结带阶理论计算中建议的一种参考能级,研究结果在深化对平均键能Em物理实质认识的同时,提供了一种借助于自由电子能带模型计算自由电子系统费米能级EF的新方法。 相似文献
20.
We have detected by optical means nonfluorescing 85Rb and 87Rb atoms implanted in a body centered cubic 4He crystal. In contrast to cesium the resonance fluorescence of rubidium is strongly quenched by the helium matrix, and the weak resonance absorption of the two Rb isotopes was detected using a double resonance technique. From a comparative study of the (optically detected) magnetic resonance spectra of 85Rb, 87Rb, and 133Cs we infer their effective g(F) factors and conclude that they are not perturbed by the He matrix at a level of 2 x 10(-4). We show further that optical pumping of Rb proceeds via depopulation, whereas for Cs it proceeds via repopulation. 相似文献