Force of attraction between solids with different temperatures

The methods of fluctuation electrodynamics based on the theory of molecular Van der Waals forces are used to obtain an expression for the density of attraction force between two absorbing media with different temperatures, which are separated by a nonabsorbing plane-parallel layer. The spectral density of the force was calculated as a projection of the Maxwell stress tensor on the outer normal to the solid surface. The mean square characteristics of the fluctuction field of the solids are obtained using the generalized Kirchhoff’s law and Green’s function of the corresponding regular problem. The solution versions are obtained depending on the relation between the temperatures of interacting solids. It is shown that for equal temperatures of the solids the expression obtained yields the formula for the attraction force in the case of equilibrium. Institute of Physics of Microstructures, Russian Academy of Sciences, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 40, No. 12, pp. 1495–1510, December, 1997.     

Rate spectra of small deformations in solids

The method of constructing the temperature dependence for the rate of small creep deformation (strain-rate spectra) is applied to determine relaxation transitions in solids. This method is based on precision measurements of the rate using a laser interferometer and is distinguished by its high resolution. The possibility of using the spectra for predicting critical temperatures in the fracture kinetics of polymers and metals, as well as changes in the electrical properties (for example, the superconducting transition in Y-Ba-Cu-O ceramics) is pointed out. Fiz. Tverd. Tela (St. Petersburg) 41, 848–850 (May 1999)     

Inherited structures in deformations of Poisson pencils

In this paper, we study some properties of bi-Hamiltonian deformations of Poisson pencils of hydrodynamic type. More specifically, we are interested in determining those structures of the fully deformed pencils that are inherited through the interaction between structural properties of the dispersionless pencils (in particular exactness or homogeneity) and suitable finiteness conditions on the central invariants (like polynomiality). This approach enables us to gain some information about each term of the deformation to all orders in  ?$?$.     

Correlation between single-cylinder properties and bandgap formation in photonic structures

The origin of frequency gaps in the dispersion relation of periodic, quasi-periodic, and random photonic structures consisting of different arrangements of dielectric cylinders has been investigated. For TM polarization it was found that the formation and properties of gaps are strongly affected by Mie resonances of a single cylinder. Both the spectral position and size depend on the properties of this single scatterer. In contrast, for TE polarization no correlation between the scattering properties and bandgap formation was found, as Mie resonances are spectrally not well separated. For the inverted structure consisting of air cylinders in a dielectric material, the frequency gaps depend on the spatial arrangement of the cylinders because no pronounced Mie resonances exist in this case.     

不同轨道结构下电枢电流分布特性

解决轨道和电枢的烧蚀问题是六极轨道电磁发射器走向实际应用的关键环节,引起轨道和电枢烧蚀的原因之一就是轨道和电枢中电流分布不均匀。利用有限元仿真软件Ansoft Maxwell对三种不同轨道进行仿真,得到了电枢表面电流密度分布情况以及电枢受力。结果表明：矩形轨道对应电枢表面电流密度最大值在三种轨道中最小,凸出半圆形轨道枢轨接触面电流分布最均匀,在发射过程中可以有效减少轨道和电枢的烧蚀,凹陷半圆形轨道对应的电枢受力最大,可用于大质量物体的发射。     

Amplitudes of thermal vibrations in molecular solids

The mean-square nuclear displacements 〈U²〉 _T have been evaluated for the molecular (rare-gas) solids by a lattice dynamical rigid-atom Model. The model derives the inter-molecular interaction from a “two-piece four parameter” pair-potential, includes the contribution of zero-point vibration through potential parameters by a self-consistent method, and accounts for the cubic and quartic anharmonic potential terms as perturbations to the harmonic Hamiltonian. The effect of many body interactions has been included on the basis of Axilrod-Teller approximation. The effect of including three-body forces as well as anharmonicity is found to decrease the values of 〈U²〉_T at all temperatures for all the solids. However, the ratio of the root mean-square nuclear displacements at melting to the nearest-neighbour distance, i.e. the Lindemann parameter (δ) is approximately the same for all the solids under study, which shows that the Lindemann parameter is structure as well as interaction dependent. The results are consistent with the other conventional calculations.     

Correlation between work hardening and stress relaxation in polycrystalline nickel

Stress-relaxation rates at a constant strain in A-grade nickel polycrystals has been reported to depend in a peculiar manner on the initial stress levelσ ₀ at which relaxation is allowed to start. For large grains (D>75μm),s varies withσ ₀ linearly over the entire stress strain curve. For small grains (D<75μm),s-σ ₀ curve undergoes a change in its slope at a critical value of plastic strainɛ, which decreases as grain size increases. The observation referred to are found to correlate well with the work-hardening behaviour of the nickel polycrystals.     

Effect of the conditions of dynamic nanoindentation on the strain-rate sensitivity of hardness for solids with different structures

A technique for the determination of the strain-rate sensitivity of hardness during dynamic nanoindentation is proposed. The strain-rate sensitivities of the dynamic hardnesses of a wide class of materials (fcc metals, carbon steels, bulk amorphous metallic alloys, ionic and covalent crystals, polymers, and ceramics) are determined. The variation of these strain-rate sensitivities with the relative-strain rate (in the [(e)\dot]\dot \varepsilon range from 3 × 10⁻³ to 5 × 10³ s⁻¹) and the indentation depth (in the range from 30 nm to 2μm) is studied.     

Bistability between different localized structures in nonlinear optics

We report the observation of different localized structures coexisting for the same parameter values in an extended system. The experimental findings are carried out in a nonlinear optical interferometer and are fully confirmed by numerical simulations. The existence of each kind of localized structure is put in relation to a corresponding delocalized pattern observed. A quantitative evaluation of the range of pump parameter allowing bistability between localized structures is given. The phenomenon reported results to be robust in parameter space.     

Spatial distribution of elastic deformations in Ge/Si structures with quantum dots

A method is developed for calculating the elastic deformation in coherently strained heterostructures on the basis of the valence force field (VFF) model using the Green’s function of the “atomistic” elastic problem. The spatial distribution of the elastic deformations in a Ge/Si system with pyramidal Ge quantum dots buried in a Si matrix is investigated theoretically. The deformation distribution in and around the pyramids is determined. Near quantum dots, the region near the tip of the pyramid is most strongly intensely. Inside quantum dots the region of the vertex is most relaxed, and the most strained section lies on the contour of the pyramid base. Compression occurs in the plane of the pyramid base inside quantum dots, and stretching occurs along the vertical direction. The picture is reversed near quantum dots: stretching occurs in the lateral direction and compression in the vertical direction. It is shown that the local deformations and their spatial distribution are essentially independent (to within the scaling) of the size of the quantum dots for 10–15 nm pyramid bases.     

Enhancing thermal transport in multilayer structures: A molecular dynamics study on Lennard−Jones solids

We investigate the thermal transport properties of three kinds of multilayer structures: a perfect superlattice (SL) structure, a quasi-periodic multilayer structure consisted of two superlattice (2SL) structures with different periods, and a random multilayer (RML) structure. Our simulation results show that there exists a large number of aperiodic multilayer structures that have effective thermal conductivity higher than that of the SL counterpart, showing enhancement ratio in the effective thermal conductivity up to 193%. Surprisingly, some RML structures also exhibit enhanced thermal transport than the SL counterpart even in the presence of phonon localization. The detailed analysis on the underlying mechanism reveals that such peculiar enhancement is caused by the synergistic effect of coherent and incoherent phonon transport, which can be tuned by the structural configuration. Combined with molecular dynamics simulations and the machine learning technique, we further reveal that the enhancement effect of the effective thermal conductivity by 2SL structure is more significant when the period of SL structure is close to the critical transition period between the coherent and incoherent phonon transport regimes. Our study proposes a novel strategy to enhance the thermal transport in multilayer structures by regulating the wave-particle duality of phonons via the structure optimization, which might provide valuable insights to the thermal management in devices with densely packed interfaces.     

固体中圆柱状界面处力学量边界条件和界面的物理模型

简要介绍了固体复合媒质中圆柱状界面层的声散射理论,给出了联系界面层内外表面处声场产生的位移和应力的传递矩阵。将此传递矩阵对界面层厚度作渐近展开,可得界面薄层处力学量一级和二级近似边界条件。当界面簿层厚度远小于圆柱散射体半径时,传递矩阵可进一步简化,这时的近似边界条件可用界面弹簧模型解释。以数值计算为例子,讨论了各种近似边界条件的有效性和可能应用。本文还考虑了界面状况对声散射的影响。最后指出了声与柱状界面薄层相互作用方面有待进一步研究的一些问题。     

Effect of thin film confined between two dissimilar solids on interfacial thermal resistance

A non-equilibrium molecular dynamics model is developed to investigate how a thin film confined between two dissimilar solids affects the thermal transport across the material interface. For two highly dissimilar (phonon frequency mismatched) solids, it is found that the insertion of a thin film between them can greatly enhance thermal transport across the material interface by a factor of 2.3 if the thin film has one of the following characteristics: (1) a multi-atom-thick thin film of which the phonon density of states (DOS) bridges the two different phonon DOSs for the solid on each side of the thin film; (2) a single-atom-thick film which is weakly bonded to the solid on both sides of the thin film. The enhanced thermal transport in the single-atom-thick film case is found mainly due to the increased inelastic scattering of phonons by the atoms in the film. However, for solid-solid interfaces with a relatively small difference in the phonon DOS, it is found that the insertion of a thin film may decrease the thermal transport.     

Dissipation in solids: thermal oscillations of atoms

Dissipation in solids describes conversion of kinetic energy to thermal energy. Heat capacity of a solid relates to the kinetic energy of the oscillations of its atoms with the assumption that they are in thermal equilibrium. Previous studies investigated criteria related to thermal relaxation, the process by which thermal equilibrium is established. They examined conditions for irreversible distribution of energy among the modes of a nonlinear periodic structure that represents atoms in a solid. These studies all point to the chaotic behavior of a freely vibrating nonlinear lattice as the kernel of the problem in addressing thermal relaxation. This paper extends the results of previous studies on thermalization to modeling of dissipation as energy absorption that takes place during forced vibration of particles in a nonlinear lattice. Results show that dissipation and chaotic behavior of the particles develop simultaneously. Such behavior develops when the forcing frequency falls within a resonance band. The results also support the argument that for a real solid, both in terms of size and complexity, resonance bands overlap significantly broadening the frequency range within which dissipation takes place.     

Correlation between subband population and threshold current densities in GaAs/(Al,Ga)As quantum-cascade structures/lasers with different barrier heights

The laser parameters such as threshold current density and characteristic temperature of GaAs/Al_xGa_1-xAs quantum-cascade lasers (QCLs) with x=0.45 are superior compared to QCLs with x=0.33. This improvement is usually attributed to smaller leakage currents due to the higher conduction band offset for x=0.45, since the QCLs for the two barrier compositions are designed in such a way that the population ratios ρ_p for the laser levels are the same. The experimental investigation of undoped GaAs/Al_xGa_1-xAs quantum-cascade structures reveals a significantly smaller value of ρ_p for x=0.33 than the calculated one, while for x=0.45 it agrees with it. In the framework of a linear rate equation model, we estimate the effect of the experimentally observed reduction of ρ_p on the threshold current density. We conclude that the increased threshold current density for x=0.33 has to be attributed to both, a larger leakage current and a reduced population ratio.     

Correlation between adsorption and thermal properties of lanthanide(III) dinicotinates

Structural and thermal properties of the two isostructural lanthanide metal-organic frameworks: [Er₂(pdc)₃(dmf)₂]·dmf (1) and [Tm₂(pdc)₃(dmf)₂]·dmf (2) where pdc = C₅H₃N(COO)₂²⁻ and dmf = N,N′-dimethylformamide, have been investigated. They are characterized by the BET surface area of 302 and 101 m²/g for 1 and 2, respectively. This paper deals with the influence of activation conditions on sorption properties of the investigated complexes. Thermal investigations of as-made and activated complexes point to their entirely different thermal decompositions.     

Amorphous tracks induced by energetic ions in strongly anisotropic solids with layered structures

Common features of ion-induced tracks in layered structures are analysed in Y?Ba?Cu?O, Bi?Sr?Ca?Cu?O and in semiconducting GeS and MoS ₂. In all crystals, the conduction is poor along the normal layer. The anisotropic electron properties lead to the formation of an ion-induced broad and a narrow thermal spike in these solids. The contribution of the two spikes to the formation of tracks can be separated in GeS. When the gap energy is low or zero (Y?Ba?Cu?O, Bi?Sr?Ca?Cu?O, MoS ₂), only the narrow spike controls the track formation, and the localization of the energy deposition is the same as in insulators. However, the fraction of the deposited energy transferred to the narrow spike is only about one-third of that in insulators. The anisotropic features of tracks show the effect of the energy bands. Good quantitative agreement is found between the experimental data and the predictions of the author’s thermal spike model.