Institute of Thermal Physics

This review is concerned with the results of studies into the behavior of substances at ultimately high pressures and temperatures obtainable by way of kinetic or electromagnetic energy cumulation in laboratory conditions. Also considered are the diversified states of matter and the processes occurring under gravitational forces and thermonuclear energy release.

The dynamics of the formation of ordered structures of macroparticles charged by photoemission under the action of solar radiation under microgravitational conditions without the use of electrostatic traps to confine the particles is studied experimentally and theoretically. The working conditions needed for the formation of structures of charged macroparticles are chosen as a result of a numerical solution of the problem posed, the particle charges and the interparticle interaction parameter are determined, and the characteristic times specifying the dynamics of the formation of an ordered system of macroparticles are calculated. The behavior of an ensemble of macroparticles under the effect of solar radiation is observed experimentally on board the Mir space station. An analysis and comparison of the results of the experimental and theoretical investigations permit drawing a conclusion regarding the possibility of the existences of extended ordered formations of macroparticles charged by photoemission under microgravitational conditions.

The bulk density of graphitized ultradisperse diamond (UDD) was measured by a gamma-ray attenuation method at 1370–1870 K. These data combined with small angle x-ray scattering and true density measurements of the samples heated at various fixed temperatures were used to study the graphitization kinetics of the UDD. The reaction rate was modeled as a migration rate of the interface between the developing graphite-like carbon and the remaining diamond phase. A “reducing sphere” model was used to obtain the rates from the changes in densities. The estimated kinetic parameters in an Arrhenius expression, namely the activation energy, E=45±4 kcal/mol, and the pre-exponential factor, A=74±5 nm/s, allow quantitative calculations of the diamond graphitization rates in and around the indicated temperature range. The calculated graphitization rates agree well with the graphitization rates of diamonds with different dispersity estimated from high-resolution transmission electron microscopy data. The large difference between the rates and the kinetic parameters obtained in this study and those estimated by G. Davies and T. Evans [Proc. R. Soc. London 328, 413 (1972)] for the temperature range 2150–2300 K indicates that there are different graphitization mechanisms operating in the “low” and “high” temperatures regions.

Self-consistent molecular-dynamics calculations of the charge of micron-size particles in a low-pressure gas-discharge plasma are performed. It is shown that charge exchange of ions on neutrals starts to affect the charge of dust particles at pressures corresponding to ion mean free paths much greater than the Debye radius. The computational results show that the potential of a particle depends nonmonotonically on the pressure and on the particle size.

Loop heat pipes (LHPs) are highly efficient heat-transfer devices, which have considerable advantages over conventional heat pipes. Currently, miniature LHPs (MLHPs) with masses ranging from 10-20 g and ammonia and water as working fluids have been developed and tested. The MLHPs are capable of transferring heat loads of 100-200 W for distances up to 300 mm in the temperature range 50-100/spl deg/C at any orientation in 1-g conditions. The thermal resistance for these conditions are in the range from 0.1 to 0.2 K/W. The devices possess mechanical flexibility and are adaptable to different conditions of location and operation. Such characteristics of MLHPs open numerous prospects for use in cooling systems of electronics and computer systems.

Several high performance polymer:fullerene bulk-heterojunctions are deposited from non-halogenated xylene/anisaldehyde solution, yielding power conversion efficiencies up to 9.5%.

Recent attention in the field of enhanced heat transfer has focused on heat transfer coefficients and aerodynamic resistance on surfaces of various physical constructions. Experimental investigations have shown that a favorable relationship exists on a concave surface where the dimples are defined as holes with rounded off edges. On such surfaces, dimples may be established which increase the heat transfer coefficient greater than they increase aerodynamic resistance, leading to an overall increase in the rate of heat transfer for an established set of extraneous conditions. The heat transfer coefficient increases at a rate greater than can be attributed to the increasing surface area. This appears due to auto oscillations generated by the cavity under turbulent flow regime. The regime of these auto oscillations is characterized by large scale, non-periodic transverse current oscillations and asymmetrical vortexes within the cavity. The parameters of this regime are dependent upon the depth and radius of the dimple. This study investigates the relationship of these characteristics to the heat transfer coefficient and the aerodynamic resistance on the surface.

The spontaneous excitation of low-frequency oscillations of the macroparticle density in ordered dust structures levitating in standing striations of a dc glow discharge is discovered. It is concluded on the basis of a simplified linear model of an ideal collisionless plasma that the observed instability is caused by the drift motion of ions relative to the dust, which leads to the excitation of dust acoustic oscillations of the plasma.

Liquidus temperatures in the LiCl-KCl system in the composition range from 0 to 100 mol % KCl have been measured for 16 samples using the methods of oscillation phase analysis and thermal analysis. The melting points of the components and the eutectic composition of the system have been refined.

In the theoretical interpretation of the kinetics of first-order phase transitions, thermodynamic concepts developed long ago by Gibbs are widely employed giving some basic qualitative insights into these processes. However, from a quantitative point of view, the results of such analysis, based on the classical Gibbs approach and involving in addition the capillarity approximation, are often not satisfactory. Some progress can be reached here by the van der Waals and more advanced density functional methods of description of thermodynamically heterogeneous systems having, however, its limitations in application to the interpretation of experimental data as well. Moreover, both mentioned theories--Gibbs' and density functional approaches--lead to partly contradicting each other's results. As shown in preceding papers, by generalizing Gibbs' approach, existing deficiencies and internal contradictions of these two well-established theories can be removed and a new generally applicable tool for the interpretation of phase formation processes can be developed. In the present analysis, a comparative analysis of the basic assumptions and predictions of the classical and the generalized Gibbs approaches is given. It is shown, in particular, that--interpreted in terms of the generalized Gibbs approach--the critical cluster as determined via the classical Gibbs approach corresponds not to a saddle but to a ridge point of the appropriate thermodynamic potential hypersurface. By this reason, the classical Gibbs approach (involving the classical capillarity approximation) overestimates as a rule the work of critical cluster formation in nucleation theory and, in general, considerably.

The formation of ordered structures of charged macroparticles in a constant-current neon glow-discharge plasma is investigated. Experiments were performed with two types of particles: thin-walled glass spheres 50–63 μm in diameter and particles of Al2O3, 3–5 μm in diameter. Formation of quasicrystalline structures is observed in the standing strata and in an artificially created double electric layer. The formation of extended filamentary structures of macroparticles in the absence of visible stratification of the positive column has been observed for the first time. The influence of the discharge parameters on the formation of the ordered structures and their melting is examined. The form of the interaction potential between the charged macroparticles is considered, as well as changes in the conditions for maintaining the discharge in the presence of high concentrations of dust particles.

The method of molecular dynamics has been used to calculate the parameters of liquid-vapor phase equilibrium and the surface tension in a two-phase system of 4096 Lennard-Jones particles. Calculations have been made in a range from the triple point to near-critical temperature and also at temperatures below the triple point corresponding to the metastable equilibrium of a supercooled liquid and supersaturated vapor. To determine the surface tension, along with a mechanical approach a thermodynamic one has been used as well. The latter was based on calculation of the excess internal energy of an interfacial layer. It has been shown that in accuracy the thermodynamic approach is as good as the more sophisticated mechanical one. Low-temperature asymptotics of the phase-equilibrium curve and also of liquid and vapor spinodals have been considered in the Lennard-Jones and the van der Waals models. The behavior of the surface tension and the excess internal energy of an interfacial layer at T-->0 is discussed.

The surface tension and the Tolman length have been presented as series in terms of the interface curvature $c.$ The expansion has been limited by a linear term on $c$ for the Tolman length and a square one for the surface tension. In the framework of the van der Waals capillarity theory the expansion coefficients have been expressed in terms of the planar interface characteristics. The coefficients asymptotic behavior has been derived in the vicinity of the liquid-vapor critical point. For the van der Waals fluid, the results of expansions have been compared with the data of direct numerical calculations. A wide curvature interval of the suitability of derived formulas has been stated. This fact allows the use of them in the homogeneous nucleation theory to calculate the critical nucleus formation work.

We report a molecular dynamics (MD) study of homogeneous bubble nucleation in a Lennard-Jones liquid under a negative pressure (cavitation). The rate of bubble nucleation has been determined in the range 2 x 10(-9) < J(*) = Jσ(4)(m/ε)(1/2) < 6 x 10(-6) by the mean lifetime method at temperatures T(*) = kBT/ε = 0.35, 0.4, 0.5, 0.6, 0.7, 0.8, 0.4, 0.5, 0.6, 0.7, 0.8. In molecular dynamics simulation calculations have also been made of the coefficient of bubble size diffusion, the Zeldovich nonequilibrium factor, and the radius of a critical nucleus R*. Different approaches to the determination of the nucleation rate in a stretched liquid have been considered in the framework of classical nucleation theory (CNT). The values of J obtained in MD simulation are by 8-20 orders higher than those predicted by CNT. The work of formation of a critical bubble and the dependence of surface tension γ(R*) at the critical bubble-liquid interface have been determined by data of MD simulation from CNT. The values of γ obtained have been approximated by an extended Tolman formula that takes into account, besides a linear correction, also the quadratic in curvature terms. The Tolman length δ∞ is negative and equals -(0.1-0.2)σ. The coefficient at 1/R*(2) is positive and does not exceed σ(2).

The method of molecular dynamics is used in a system of 2048 Lennard-Jones particles to determine the spinodal of a stretched liquid and crystal and the lines of their phase equilibrium at negative pressures. It is shown that a metastable extension of the melting line does not reach the zero isotherm, and ends on the spinodal of a stretched liquid. The point of termination of metastable liquid-crystal phase equilibrium is the singular point at a thermodynamic surface of states.

We present the results of molecular dynamics simulation of crystal nucleation in a supercooled Lennard-Jones liquid. Temperature and baric dependences of the nucleation rate, the Zeldovich factor, nucleus size diffusion coefficient, the radius, and the pressure in a critical crystal nucleus are defined in computer simulation. The data obtained have been used in the framework of classical nucleation theory to calculate the effective surface energy of crystal nuclei γ(e). It is shown that the value of γ(e) at T = const exceeds the value of the interfacial free energy at a flat crystal-liquid interface γ(∞) and γ(e) < γ(∞) at p = const.

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Loop heat pipes (LHPs) are exceptionally efficient heat-transfer devices that employ a closed loop evaporation-condensation cycle that can be used to cool densely packed electronic systems that reject large quantities of heat, including computers and their central processing units (CPUs). Tests were carried out on miniature ammonia LHPs with a CPU thermal simulator using different ways of condenser cooling. The possibility of maintaining the cooled object temperatures between 40<formula formulatype="inline"><tex Notation="TeX">$^\circ{\rm C}$</tex></formula> and 70<formula formulatype="inline"><tex Notation="TeX">$^\circ{\rm C}$</tex></formula> with heat load changing from 100 to 320 W was demonstrated. Subsequent tests of these devices in a 1U computer with dual core advanced micro devices Opteron CPUs, dissipating between 95 and 120 W, have confirmed the advantages and heat transfer efficiency of LHP-based cooling systems used to cool CPU in 1U chassis. </para>

The existence of large-scale structures in the form of a vortical double helix in a swirling Ranque flow is observed for the first time. The structure of the vortical double helix is visualized in real time by the method of Hilbert bichromatic filtering. The experimental result is interpreted on the basis that the most probable physical mechanism for the spatial energy separation in the gas flow (i.e., for the so-called Ranque effect) is viscous heating of the gas in a thin boundary layer at the walls of the vortex chamber and the adiabatic cooling at the center owing to the formation of an intense vortex braid near the axis.

During the operation of miniature loop heat pipes (LHPs) one can observe pulsations of the operating temperature, which depend on the amount of the working fluid, the device orientation in the gravity field and the conditions of the condenser cooling. Intense pulsations, whose amplitude may exceed tens of degrees, arise from the lack of a working fluid in a LHP when a hot condensate or vapor bubbles periodically penetrate into the compensation chamber (CC) and act on the vapor phase in it, increasing its temperature and volume. Changes in the external conditions, for instance, the LHP arrangement in an unfavourable orientation or a more intensive cooling of the condenser with respect to the conditions for which the filling volume was optimal, also contribute to the initiation of intense pulsations of the operating temperature. In both cases one can observe redistribution of the working fluid between the condenser and the CC, as the result of which the liquid phase volume in the latter decreases and overshoots of vapor or a hot condensate there become possible.

A new method is proposed for the treatment of Raman spectra of amorphous-nanocrystalline silicon films serving as a major component in solar cells. The method is based on the well-known theory of strong spatial localization (confinement) of phonons and offers the possibility of estimating the fractional content of the amorphous and crystalline phases in a film and the size distribution of nanocrystals.