Viscoelastic behaviour of fluids with steeply repulsive potentials

We analyse the shear stress, C _s(t) and pressure or ‘bulk’, C _b(t) time-correlation functions for steeply repulsive inverse power fluids (SRP) in which the particles interact via a pair potential with the analytic form, φ(r) = ε(σ/r) ⁿ , in a new approach to the understanding of their viscoelastic properties. We show analytically, and confirm by molecular dynamics simulations, that close to the hard-sphere limit both these time-correlation functions have the analytic form, C _s(t)/C _s(0) and C _b(t)/C _b(0) = 1 – T*(nt*)²+ O((nt*)⁴), where T* = k _B T/ε, is the reduced temperature, k _B is Boltzmann's constant and t* = (ε/mσ²)^½ t is the reduced time. This leads to an alternative and much simpler derivation of formulae for the shear and bulk viscosities which for the limiting case of hard spheres are numerically very close to the traditional Enskog relations. These simple relations for the finite and continuous SRP interaction are in satisfactory agreement with the essentially exact molecular dynamics simulation results for ca. n ≥ 18.     

The velocity autocorrelation function and self-diffusion coefficient of fluids with steeply repulsive potentials

We consider the velocity autocorrelation function, vacf, or C_v(t) and self-diffusion coefficients, D, of steeply repulsive inverse power fluids (SRP) in which the particles interact with a pair potential, ? (r) = ?(σ/r)ⁿ. The C_v(t) are calculated numerically by molecular dynamics simulations. Accurate expressions for the short time expansion of C_v(t) to order O(t⁴) for n large are derived for this fluid. We propose novel expressions for C_v (t) that, for n large, spans the transition from the short time regime (expandable in even powers of time) and the longer time exponential-like regime characteristic of hard spheres. Inter alia we introduce relaxation times that characterize the duration of a collision and the decay of the velocity correlation within the mean-collision or Enskog-like relaxation time, T_E.     

Exact short time dynamics for steeply repulsive potentials

The autocorrelation functions for the force on a particle, the velocity of a particle and the transverse momentum flux are studied for the power law potential v(r)=ε(σr)^ν (soft spheres). The latter two correlation functions characterize the Green–Kubo expressions for the self-diffusion coefficient and shear viscosity. The short-time dynamics is calculated exactly as a function of ν. The dynamics is characterized by a universal scaling function S(τ), where τ=t/τ_ν and τ_ν is the mean time to traverse the core of the potential divided by ν. In the limit of asymptotically large ν this scaling function leads to delta function in time contributions in the correlation functions for the force and momentum flux. It is shown that this singular limit agrees with the special Green–Kubo representation for hard-sphere transport coefficients. The domain of the scaling law is investigated by comparison with recent results from molecular dynamics simulation for this potential.     

相同稀有气体原子间近程排斥势的新探索

对相同稀有气体原子间电荷密度的重叠积分进行了计算。计算结果表明该重叠积分可以用来表示稀有气体原子间的排斥势。这为进一步更准确地探索原子间排斥势提供了一种开拓性的方法。     

Vapour pressure of non-polar fluids and their repulsive and attractive contributions

Using a simple equation of state, based on the Weeks-Chandler-Andersen separation of the intermolecular potential, we have obtained the contributions of repulsive and attractive intermolecular forces to the thermodynamic properties of coexisting vapour and liquid phases of a Lennard-Jones (LJ) fluid.

In order to obtain the vapour pressure of real non-polar fluids, we take the LJ fluid as a reference model, and propose a new perturbative contribution, which is dependant on the temperature and on the acentric factor of the substance. Using the complete perturbed equation, we determine the corresponding repulsive and attractive contributions to the vapour pressure of non-polar fluids. The results show that the attractive vapour pressure of non-polar fluids increases with increasing acentric factor, i.e., larger deviation of the molecular shape from spherical symmetry.

This procedure could be extended to separate the repulsive and attractive contributions of the intermolecular forces to other thermodynamic properties of non-polar fluids as well as of polar fluids and fluid mixtures.     

Thermal conductivity of metals with hot electrons

The action of an ultrashort laser pulse transforms a metal into a two-temperature (2T) state with different temperatures of the electron and ion subsystems (T _e ≫ T _i ). The metal stays in this state in a rather long time interval (from several to several tens of picoseconds depending on the metal). The 2T stage is very important since it includes 2T relaxation, in which laser energy is transferred to ions, and the formation of a heated layer, which plays a key role in the subsequent dynamics. The kinetic coefficients of a condensed medium with hot electrons are poorly known: researchers use phenomenological dependences consisting of asymptotics at low and high temperatures T _e . However, it is impossible to perform a numerical simulation of the interaction of laser radiation with a substance without these coefficients. In this work, the thermal conductivity is calculated using a kinetic equation for the first time. This calculation is valid at low (T _e ≪ T _F = E _F/k _B, where E _F is the Fermi energy and k _B is the Boltzmann constant) and moderate (T _e < T _F) temperatures. The earlier kinetic calculations are related to the case of T _e < T _F, where the calculations of, e.g., the electron-electron collision frequency are significantly simplified due to the reduction of multiple integration to integration in a small neighborhood of the Fermi sphere. In our case, integration at moderate temperatures should be performed over the entire volume of momentum space.     

Thermal conductivity of ice doped with helium

An ice sample has been doped with helium and dedoped. The maximum value of heat conduction is 0.7 W/cm K in the virgin and 0.35 W/cm K in the doped sample. Computer fits are done based on Callaway's model of heat conduction in dielectric solids. An attempt is made to explain the results by a microstructure concept.     

Thermal conductivity of methane with carbon monoxide

The paper reports the results of new measurements of the thermal conductivity of CH₄-CO mixtures at 27.5°C and up to a pressure of 12 MPa.This further contribution to a growing body of data on the thermal conductivity of binary mixtures of polyatomic gases is analyzed in the same way as was done by the present group in several earlier publications with essentially identical conclusions.     

A simple approximation for fluids with narrow attractive potentials

A simple modification of the optimized random phase approximation (ORPA) has been made, aimed at improving the performance of the theory for interactions with a narrow attractive well by taking into account contributions to the direct correlation function that are nonlinear with respect to the interaction. The theory is applied to a hard core Yukawa and a square-well potential. Results for the equation of state, the correlations, and the critical point have been obtained for attractions within several ranges, and compared with Monte Carlo simulations. When the attractive interaction is narrow, the modified ORPA is a significant improvement over the plain one, especially with regard to the consistency between different routes to the thermodynamics, the two-body correlation function, and the critical temperature. However, although the spinodal curve of the modified theory is accessible, the liquid-vapour coexistence curve is not. A possible strategy to overcome this drawback is suggested.     

Thermal conductivity of nanowires

Thermal conductivity of nanowires(NWs) is a crucial criterion to assess the operating performance of NWs-based device applications, such as in the field of heat dissipation, thermal management, and thermoelectrics. Therefore, numerous research interests have been focused on controlling and manipulating thermal conductivity of one-dimensional materials in the past decade. In this review, we summarize the state-of-the-art research status on thermal conductivity of NWs from both experimental and theoretical studies. Various NWs are included, such as Si, Ge, Bi, Ti, Cu, Ag, Bi_2Te_3, ZnO, AgTe,and their hybrids. First, several important size effects on thermal conductivity of NWs are discussed, such as the length,diameter, orientation, and cross-section. Then, we introduce diverse nanostructuring pathways to control the phonons and thermal transport in NWs, such as alloy, superlattices, core–shell structure, porous structure, resonant structure, and kinked structure. Distinct thermal transport behaviors and the associated underlying physical mechanisms are presented.Finally, we outline the important potential applications of NWs in the fields of thermoelectrics and thermal management,and provide an outlook.     

Thermal cellular automata fluids

The concepts of local temperature and local thermal equilibrium are introduced in the context of lattice gas cellular automata (LGGAs) whose dynamics conserves energy. Green-Kubo expressions for thermal transport coefficients, in particular for the heat conductivity, are derived in a form, equivalent to those for continuous fluids. All thermal transport coefficients are evaluated in Boltzmann approximation as thermal averages of matrix elements of the inverse Boltzmann collision operator, fully analogous to the results for continuous systems, and fully model-independent. The collision operator is expressed in terms of transition probabilities between in- and out-states. Staggered diffusivities arising from spuriously conserved quantities in LGCAs are also calculated. Examples of models with either cubic or hexagonal symmetries are discussed, where particles may or may not have internal energies.     

The generalised Huggins-Mayer form of born repulsive potentials for NaCl-type alkali halides

The generalised Huggins-Mayer form of Born repulsive potentials for NaCl-type alkali halides have been re-evaluated using more reliable, recently published thermodynamic data and van der Waals energy coefficients. As with older versions of these potentials excellent agreement between model and experimental values of interionic distance and cohesive energy is achieved. Further, the earlier shortcomings in the failure of the stability condition and predicted elastic constants are significantly reduced. For the majority of salts the new short-range interactions have stronger van der Waals attractions and slider repulsions. The model gives average crystal radii for the alkali ions about ~0.3 Å larger than traditional free ion radii and ~0.08 Å larger than Tosi-Fumi crystal radii. The predicted halogen crystal radii are correspondingly smaller by the same amounts.     

Thermal conductivity of superconducting niobium

The phonon scattering term in the superconducting state of the electronic conduction has been obtained for niobium which is regarded as an intermediate coupling superconductor. The limiting slope for the phonon scattering term of the reduced thermal conductivity against reduced temperature has been found to be 2.8, as compared with 1.5 for weak coupling superconductors.     

Thermal conductivity of liquid Ga

We present accurate values of the thermal conductivity of liquid Ga calculated from measurements of the Lorenz number and the electrical conductivity.     

Thermal conductivity of e-beam coatings

We present the results of the study on the thermal conductivity of different thin film materials produced by conventional thermal evaporation. The main features of the thermal pulse method employed for the measurement of the thermal conductivity are described. Thermal conductivity can be measured by determining the traveling time of a thermal wave propagating trough the film. A pump laser beam is directed onto a sample consisting of a thin transparent test layer and a totally absorbing substrate for the laser wavelength. As a consequence of the laser pulse, a temperature profile builds up at the substrate-film interface. A thermal pulse starts to diffuse from the substrate-film interface to the surface of the layer. Therefore, the temperature rise at the surface of the test layer starts with a time delay with respect to the laser pulse. The time delay depends on the propagation time of the thermal wave through the layer and is related to the thermal conductivity and the thickness of the layer. Measurements are evaluated by calculations based on the finite difference method. The results show that the analyzed thin films have lower thermal conductivity than the corresponding materials in bulk form.