Studies of radical pairs in potassium hydrogen malonate I. E.S.R.

Irradiation of single crystals of potassium hydrogen malonate with X-rays at 77 K yields two types of radical pairs, which differ in the orientation of the vector r linking the radicals of a pair but are composed of the same monoradicals. This monoradical is the main radical species found at room temperature and is characterized by hydrogen abstraction from the methylene group. Each type of pair exists in three modifications with respect to the fine structure parameters D and E, depending on the temperature treatment of the crystals after irradiation. The principal values of the dipole splitting tensor and the direction cosines of r for all pairs were determined. By application of the appropriate theory the signs of D and E could be deduced from second order effects observed at 9·5 GHz and 35 GHz. The two types of pairs exhibit marked differences in their fine structure parameters, their microwave saturation and their temperature and light stability, all of which can be attributed to different orientations of the linking vectors r relative to the molecular planes. Finally temperature coefficients of the dipole interactions were measured yielding opposite signs for the two types of pairs.     

Estimation of kinematic viscosities at high pressures for binary mixtures CO2 + solvent using modified Eyring–Wilson and McAllister models

ABSTRACT

This work reports an estimation of kinematic viscosities at high pressures for binary mixtures of carbon dioxide (CO₂)?+?solvent using the modified Eyring–Wilson model, by introducing each empirical equation as a function of temperature and pressure to both the binary interaction parameters in the Wilson model over wide temperature and pressure ranges. Binary mixtures tested for the estimation are CO₂?+?acetone,?+?water,?+?alcohol, and hydrocarbon with 60 data sets, and the covered temperature and pressure ranges are 296–473?K, and 0.77–120.86?MPa, respectively. The modified Eyring–Wilson model shows reasonable agreements with the experimental data, giving 8.03% of the average absolute relative deviations (AARD) for all the data sets studied. While the accuracy of the McAllister model, also introducing the functions of temperature and pressure to the binary interaction parameters in the same manner as in the Wilson model, showed lower AARD value (6.47%), the significant deviations were seen at higher temperatures. We conclude that the modified Eyring–Wilson with the newly proposed temperature and pressure dependencies of the binary parameters is effective for the estimation of the kinematic viscosities for binary mixtures CO₂?+?solvent over wide temperature and pressure ranges.     

A unique continuation principle for steady symmetric water waves with vorticity

Abstract

We consider a symmetric, steady, and periodic water wave. It is shown that a locally vanishing vertical velocity component implies a flat or oscillating surface profile.     

Formation of x-ray images by the action of an optical image on a diffracting lithium niobate crystal

A possible mechanism for the formation and processing of an x-ray image using a corresponding optical image as a template is considered. The method is based on the thermal influence of light on the x-ray diffraction parameters of an LiNbO₃ crystal. A mechanism for the influence of the gradients of the temperature fields formed in the crystal on the x-ray structural parameters of the crystal is proposed. Information which permits the evaluation of some practical aspects of the application of the observed effect in technology is given. Zh. Tekh. Fiz. 67, 76–79 (September 1997)     

Magnetic phase transitions in Nd1 ? xDyxFe3(BO3)4 ferroborates

The magnetic properties of ferroborate single crystals with substituted compositions Nd_{1 − x} Dy _x Fe₃(BO₃)₄ (x = 0.15, 0.25) with competing exchange Nd-Fe and Dy-Fe interactions are investigated. For each composition, we observed a spontaneous spin-reorientation transition from the easy-axis to the easy-plane state and step anomalies on the magnetization curves for the spin-flop transition induced by a magnetic field B | c. The measured parameters and effects are interpreted using a unified theoretical approach based on the molecular field approximation and on calculations performed in the crystal-field model for the rare-earth ion. The experimental temperature dependences of the initial magnetic susceptibility from T = 2 K to T = 300 K, anomalies on the magnetization curves for B | c in fields up to 1.8 T, and their evolution with temperature, as well as temperature and field dependences of magnetization in fields up to 9 T are described. In the interpretation of experimental data, the crystal-field parameters in trigonal symmetry for the rare-earth subsystem are determined, as well as the parameters of Nd-Fe and Dy-Fe exchange interactions.     

Radiation affected ignition and flame propagation for solid fuel in a cylindrical enclosure

Ignition and flame propagation for pyrolysing fuel in a cylindrical enclosure has been examined in this study. The pyrolysing fuel of cylindrical shape was located both eccentrically and concentrically inside an outer cylinder that was sustained at high temperature. Due to gravity, buoyancy motion was inevitably incurred in the enclosure, and this was found to affect the flame initiation and propagation behaviour. Radiative heat transfer also played an important role in the thermo-fluid mechanical behaviour because of the high temperature involved in the problem. Numerical studies have been performed for various parameters such as the Grashof number, overheat ratio, gas absorption coefficient and vertical fuel eccentricity. The flame behaviour and initiation were observed to be totally different depending on the Grashof number. Due to absorbed radiant energy, the radiative gas played a significant role in flame evolution. The location of flame onset was also affected by both the vertical eccentricity of the inner pyrolysing fuel and the thermal conditions applied. The heating process and the flow field development were found to govern flame initiation and propagation.     

Singlet-triplet model of cuprate magnetism

A model for describing the spin subsystem of cuprates within a model of polar singlet-triplet Jahn-Teller centers is proposed. In this model spin ordering is described by two vector order parameters S (the total spin of a CuO₄ cluster) and V (the operator of the change in spin multiplicity per cluster). It is shown within a modified mean-field approximation that the formation of a noncollinear magnetic structure characterized by an antiferromagnetic wave vector, which, however, has a nontrivial temperature dependence of the order parameters, is possible along with the formation of ordinary collinear structures of the (anti)ferromagnetic type. The temperature dependences of the order parameters and the principal equilibrium thermodynamic characteristics, viz., the static susceptibility and the specific heat, are obtained. Fiz. Tverd. Tela (St. Petersburg) 40, 1785–1792 (October 1998)     

Zusammenhänge Zwischen Radonanomalien und Schwarm-Erdbeben im Oberen Vogtland

Abstract

The results of continuous radon measurements in soil air and the water of a mineral spring at Bad Brambach are presented. They are discussed in connection with meteorological, hydrological parameters, and seismic events with magnitudes <3. The atmospheric pressure (especially pressure gradient), the temperature, and the groundwater level gradient have a significant influence onto radon activity of soil air.

The gamma-activity in spring water is influenced by the water flow rate only.

The results show that the soil air (mean Rn activity 130 Bq · l^?1) and especially the water of the Radonquelle of Bad Brambach (mean Rn activity 25 kBq · l^?1) react upon micro earthquakes with small epicentral distances. Frequently there are precursor effects, that means radon maxima due to the building up of tectonic stress/strain.

About 60% of the seismic events had been attended by radon anomalies during the registration period.

The long term measurements will be continued to investigate possible influences of earthquakes with higher epicentral distances and magnitudes >5 onto the radon regime of the bad Brambach area.     

Isomorphous phase transition and orientational freezing in hexagonal mixed crystal C 70(1 - x) C60x. A pseudospin model

A four-state pseudospin model is constructed for the isomorphous phase transition hcp-2↦hcp-1 in pure C₇₀ and in C₇₀-rich mixed crystal C _{70(1 - x)} C_60x. With the specific anisotropic pseudospin interactions adapted to the C₇₀ crystal the model is equivalent to a two-state Ising model in a temperature-dependent field. Replica symmetric state of the model is shown to approach the critical point when the width of distribution of random fields and/or of random bonds increases. The temperature of the phase transition and the phase equilibrium temperature then are practically constant, whereas the experiment shows their strong decrease with x. The main effect of dilution resides in an x-dependence of the model parameters. Dilatometric data on the hexagonal C _{70(1 - x)} C_60x are used to fit these parameters. A metastable disordered phase subsisting below the phase transition is discovered in a range of the model parameters and is shown to be responsible for the macroscopic behaviour of the system. A good agreement with experimental data is obtained for the spontaneous strain and for the x-dependence of the hysteresis. Received 20 April 2001 and Received in final form 26 September 2001     

Temperature renormalization of the magnetic excitations in S = 1/2 KCuCl3

A complete temperature characterization of the spin dynamics in the unconventional S =1/2 antiferromagnet KCuCl₃ is presented from single crystal inelastic neutron scattering studies. KCuCl₃ features a quantum disordered singlet ground state with a finite spin gap to triplet excitations of dimer origin. Three dimensional magnetic correlations support the dispersive propagation of the excitations in the whole reciprocal space. Upon increasing the temperature, a renormalization in the energy, in the intensity and in the damping rate of the triplet modes is reported. The experimental observations can be described within the framework of a selfconsistent dimer RPA theory, with no free parameters. The driving mechanism behind the model is the thermally activated decrease of the occupation difference n ₀ - n ₁ between singlet and triplet dimer states. This is the expression of kinematic constraints which are of minor importance for classical magnons in Néel ordered antiferromagnets. Implications for the temperature dependence of macroscopic quantities are discussed.     

Rapid Consolidation by Spark Sintering of Rapidly Solidified 7075 Aluminum Alloy Powder

Compacts of 7075 aluminum have been produced from rapidly solidified powders by optimizing spark sintering parameters, such as pulse discharge time, fixed maximum temperature, holding time at this temperature, and method of cooling to room temperature after the sintering. High-grade compacts can be obtained by a short process (40-50 s) consisting of heating to 773 or 873 K at a heating rate of 9.6 K/s and holding this temperature for 10 s. The rapidly cooled compacts show the same supersaturated state at room temperature as the received as-atomized powder. Compacts quenched in water just after spark sintering at 873 K for 1.2 ks show the same age-hardening behavior as solution-heat treated compacts. Compacts that are quenched in water and aged after sintering at 873 K for 1.2 ks show the same elongation and flow stress as compacts aged after solution heat treatment. Elongation data suggest that compacts produced with longer holding time at a higher temperature and rapid cooling show a large amount of main alloying elements in solid solution and sufficient promotion of sintering.     

Spectroscopic manifestations of phase changes in CaAr n clusters: finite size effects

The finite temperature optical spectroscopy of CaAr_n clusters in the range 6 n 146 is investigated using a Diatomics-In-Molecule (DIM) Hamiltonian and classical parallel tempering Monte Carlo simulations. The absorption spectrum is calculated in the vertical approximation at various temperatures between 2 K and 50 K. Several typical situations are reported. CaAr₆ shows a strong thermal broadening and shift of the spectral lines, possibly associated with isomerization. CaAr₁₃ only shows some broadening. CaAr₃₇ exhibits features corresponding to coexisting isomers at low temperature. Finally, the abrupt changes in the absorption spectrum in CaAr₁₄₆ at about 20 K are indicative of surface diffusion.     

Thermal behavior of paramagnetic Gd3+ centers in CsSrCl3

The temperature dependence of the order parameters and the axial initial splitting parameter of Gd³⁺ impurity complexes is measured in the first low-symmetry phase of CsSrCl₃, and the relationship between these parameters is found. The parameters of the thermodynamic potential, which takes into account the interaction of the order parameter with strains, are estimated. The nature of the temperature dependence of the axial initial splitting parameter in the cubic phase is explained. Fiz. Tverd. Tela (St. Petersburg) 41, 2065–2069 (November 1999)     

On the onset of free convection in vertical channels with cross sections in the form of circular or annular sectors

The appearance of convection in a liquid filling a long vertical channel with ideally heat-conducting walls is considered in the case where the temperature of the liquid, originally at rest, decreases linearly with height and the cross section of the channel has the shape of a circular or annular sector. The critical Rayleigh numbers and eigenfunctions of the boundary problems are written out for the critical motions. The appearance of convection is also examined in a liquid-saturated porous medium filling a vertical channel with ideally heat-conducting or insulating walls and having a cross section in the form of a circular sector. Zh. Tekh. Fiz. 68, 44–47 (August 1998)     

ENHANCED POOL BOILING HEAT TRANSFER WITH SURFACE ATTACHMENT

Abstract

A pool boiling experiment with either water or Freon-113 was conducted to investigate nucleate boiling from a heated wall with either a spherical or a cylindrical attachment. The result revealed that nucleate boiling can be enhanced by applying a horizontal cylindrical attachment to a vertical heated wall, owing to the favorable thermal environment characterized by a small-gradient liquid temperature profile within the restricted regions between the attachment and the heated wait. Nucleate boiling is enhanced in terms of a lower wall superheat required for incipient boiling and more bubbles generated than from an open heated wall. As a result of the enhanced nucleate boiling, heal transfer of the vertical heated wall above the attachment was improved due to excessive bubbles moving upward along the heated wall, causing removal of the thermal layer near the wall and evaporation of the thin liquid film between the bubbles and the wall. The boiling curve hysteresis with Freon-113 was significantly reduced as a result. The effects of diameter, length and surface roughness of the cylindrical attachment were also investigated.     

Equation of state SAFT-CP for vapour–liquid equilibria and mutual solubility of carbon dioxide and water

ABSTRACT

The statistical associating fluid theory (SAFT) was proposed first in 1990, and has been extended to various forms for the calculation of thermodynamic properties of complex systems, such as oil reservoir fluids, polar systems, polymers, electrolytes, near-critical systems, interfacial phenomena, solids and even biomaterials. SAFT-CP (critical point) has been established for nonpolar fluids in 2001 with excellent expression of thermodynamic properties across critical points. It was extended later for polar and associating fluids with the addition of just a dipole–dipole interaction, which leads simple calculation procedure without an association term. In this article SAFT-CP is applied to carbon dioxide, water and their mixture. Vapour–liquid equilibria for pure components CO₂ and H₂O, CO₂ solubility in water and H₂O solubility in dense CO₂ are analysed. Expression of pure CO₂ properties is improved with the dipole–dipole interaction term used, while expression of pure water is a little bit improved with the non-spherical degree parameter less than 1.0. For the high asymmetry in polarity and association between CO₂ and H₂O molecules, the Stryjek–Vera combining rule is used with different temperature-dependent parameters. With the quadratic temperature-dependent parameters, the mutual solubilities in the system are calculated with good agreement with experimental ones over the wide range of temperature as 298–474 K and of pressure as 0.1–70 MPa.     

Pneumatic Drying of Solid Particle: Experimental and Model Comparison

Abstract

This article presents a mathematical model approach to studying the drying phenomena of solid particle in a pneumatic (flash) dryer. The analysis is focused on the pneumatic momentum, mass, and heat balance of the solid particle when it moves inside the reactor. A fixed bed fluidization model was used to calculate the forces balance on the single solid particle. By solving mass and heat balance occurred in the particle, the water/liquid removal efficiency can be calculated. To validate the model calculations, we conducted a set of experiments and compared the simulation with the experimental data. High-moisture, natural concrete sand, the additional material for portland cement, was used and dried along a vertical cylindrical tube with length of 2 m and diameter of 6.68 cm. The drying gas was supplied by a high-capacity air blower which was connected to the burner to produce 120 m³/h of drying gas with maximum temperature of 800°C.     

On the origin of compositional fluctuations in ZnSe1- x S x alloys grown by metal-organic vapour-phase epitaxy

Periodic vertical modulations of the sulphur concentration have been observed in epitaxial layers of ZnSe_{1? x} S _x grown by metal-organic vapour-phase epitaxy. High-spatial-resolution microanalysis has shown that the sulphur variation is of the order of 1 at.% with a period of about 44 ± 0.1 nm. A detailed analysis of the possible causes of this modulation is presented which shows that periodic temperature fluctuations of the substrate are the most likely source.     

Vertical hopping conduction via virtual states in intentionally disordered superlattices

A new mechanism of vertical conduction in intentionally disordered superlattices is examined. It is shown that low-temperature conduction due to phonon-assisted tunneling between distant quantum wells of a superlattice is determined mainly by hopping processes via virtual intermediate states. Under standard conditions a weak temperature dependence of vertical conduction is obtained for this mechanism. The characteristic behavior of the conductivity as a function of disorder amplitude is found for this mechanism. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 12, 879–884 (25 June 1999)     

Evolution of the structure of Rb2ZnCl4 over the temperature range 4.2–310 K

X-ray diffraction is used to study the temperature dependence of the lattice parameters and the sequence of structural realignments in crystalline Rb₂ZnCl₄ over temperatures of 4.2–310 K. The appearance of and changes in the system of satellite reflexes indicative of structural ordering are studied. Below 74 K, on going into the monoclinic phase (space group A11a), anomalies are observed in the behavior of the lattice parameters, and superstructural reflexes develop with wave vectors q=a ^*/3+b ^*/2+c ^*/2 corresponding to an increase by a large factor in initial parameters a, b, and c of the Pnma-phase. Fiz. Tverd. Tela (St. Petersburg) 41, 1084–1090 (June 1999)