Application of positron spectroscopy for investigation of threshold space of oil-gas rocks

This article considers the possibility of applying the positron annihilation spectroscopy method for investigating the pore space of rocks from oil-gas and methane-coal deposits. The diagnostics of the structure was performed using the method of spectrometry of angular correlation of annihilation rays (ACAR). Using the samples of porous silicon, the authors have shown the applicability of the ACAR method for determination of the average dimensions of spherical and cylindrical nanosized objects and their concentration      

Mesoionic Dithiolates (MIDts) Derived from 1,3-Imidazole-Based Anionic Dicarbenes (ADCs)

Mesoionic dithiolates [(MIDt^Ar)Li(LiBr)₂(THF)₃] (MIDt^Ar={SC(NDipp)}₂CAr; Dipp=2,6-iPr₂C₆H₃; Ar=Ph 3 a , 3-MeC₆H₄ (3-Tol) 3 b , 4-Me₂NC₆H₄ (DMP) 3 c ) and [(MIDt^Ph)Li(THF)₂] ( 4 ) are readily accessible (in≥90 % yields) as crystalline solids on treatments of anionic dicarbenes Li(ADC^Ar) ( 2 a - c ) (ADC^Ar={C(NDipp)₂}₂CAr) with elemental sulfur. 3 a - c and 4 are monoanionic ditopic ligands with both the sulfur atoms formally negatively charged, while the 1,3-imidazole unit bears a formal positive charge. Treatment of 4 with (L)GeCl₂ (L=1,4-dioxane) affords the germylene (MIDt^Ph)GeCl ( 5 ) featuring a three-coordinated Ge atom. 5 reacts with (L)GeCl₂ to give the Ge−Ge catenation product (MIDt^Ph)GeGeCl₃ ( 6 ). KC₈ reduction of 5 yields the homoleptic germylene (MIDt^Ph)₂Ge ( 7 ). Compounds 3 a - c and 4 – 7 have been characterized by spectroscopic studies and single-crystal X-ray diffraction. The electronic structures of 4 – 7 have been analyzed by DFT calculations.     

Low-temperature plasticity in nanocrystalline titanium and copper

The stress-strain compressive curves, temperature dependences of the yield stress, and small-inelastic-strain rate spectra of coarse-grained and ultrafine-grained (produced by equal-channel angular pressing) titanium and copper are compared in the temperature range 4.2–300 K. As the temperature decreases, copper undergoes mainly strain hardening and titanium undergoes thermal hardening. The temperature dependences of the yield stress of titanium and copper have specific features which correlate with the behavior of their small-inelastic-strain rate spectra. Under the same loading conditions, the rate of microplastic deformation of ultrafine-grained titanium is lower than that of coarse-grained titanium and the rate peaks shift toward high temperatures. The deformation activation volumes of titanium samples differing in terms of their grain size are (10–35)b ³, where b is the Burgers vector magnitude. The dependences of the yield stress on the grain size at various temperatures are satisfactorily described by the Hall-Petch relation.     

EPR of trivalent iron centers in SrF<Subscript>2</Subscript>: Fe crystals

Paramagnetic centers of three types are found in SrF₂: Fe(0.2 at. %) crystals. Two types are observed in the untreated crystals, and the third type appears only in the crystals irradiated by x-rays. The EPR spectra of one type of centers in a nonirradiated crystal and of the centers that appear after irradiation are described by the orthorhombic Hamiltonians with an effective spin S _eff = 5/2. In both cases, the centers are observed at 4.2 and 77 K. The principal axes of the spin Hamiltonians for them are along the 〈001〉, 〈1 \(\overline 1 \) 0〉, and 〈110〉 axes. However, the fine-structure parameters of their EPR spectra differ significantly. An analysis of the superhyperfine structure (SHFS) of the EPR spectra shows that the radiation center forms through substitution of a Fe²⁺ ion for a Sr²⁺ cation. Under x-ray irradiation, the Fe²⁺ ion transforms into the Fe³⁺(⁶ A _1g ) state and is displaced to an off-center position along the C ₂ axis of its coordination cube. The absence of a SHFS in the EPR spectra of the orthorhombic centers in a nonirradiated crystal makes it impossible to determine their molecular structure unambiguously. The most probable model is proposed for this structure. The EPR spectra of centers of the third type were observed only at 4.2 K, and the structure of these centers was not studied.     

Simulation of the structural,electronic, and magnetic properties of Fe<Subscript>3</Subscript>C<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>B<Subscript>x</Subscript> borocementites

The structural, electronic, and magnetic properties and the enthalpy of formation of iron borocementites Fe₃C_1?x B_x (x= 0, 0.25, 0.50, 0.75, 1.00) are analyzed using ab initio calculations in the framework of the electron density functional theory. It is found that the unit cell parameter a of the orthorhombic lattice increases linearly and the parameters b and c decrease as the boron concentration increases. The density of states at the Fermi level changes only slightly, and the main variations in the band structure occur in the region of the bottom of the valence bands. The magnetic moment of the iron atoms and the total magnetization and stability of the Fe₃C_1?x B_x phases increase linearly with an increase in the boron concentration.     

Optical band gap width in GaAs in megagauss magnetic fields

The band gap width in GaAs in magnetic fields of up to 10 MG is calculated using a five-band kp model. The selection rules for interband electron transitions in strong magnetic fields are found, and the dependences of the interband transition probabilities on a magnetic field are calculated. The electronic spectra calculated in the five-band model are compared with those calculated in the Kane model and in the tight-coupling approximation. The calculations are shown to agree with experimental data if the contribution from the density-of-states tails and excitonic effects to light absorption is taken into account.     

Diffusion of Quinine with Ethanol as a Co-Solvent in Supercritical CO2

This study aims at contributing to quinine extraction using supercritical CO₂ and ethanol as a co-solvent. The diffusion coefficients of quinine in supercritical CO₂ are measured using the Taylor dispersion technique when quinine is pre-dissolved in ethanol. First, the diffusion coefficients of pure ethanol in the supercritical state of CO₂ were investigated in order to get a basis for seeing a relative change in the diffusion coefficient with the addition of quinine. We report measurements of the diffusion coefficients of ethanol in scCO₂ in the temperature range from 304.3 to 343 K and pressures of 9.5, 10 and 12 MPa. Next, the diffusion coefficients of different amounts of quinine dissolved in ethanol and injected into supercritical CO₂ were measured in the same range of temperatures at p = 12 Mpa. At the pressure p = 9.5 MPa, which is close to the critical pressure, the diffusion coefficients were measured at the temperature, T = 343 K, far from the critical value. It was found that the diffusion coefficients are significantly dependent on the amount of quinine in a small range of its content, less than 0.1%. It is quite likely that this behavior is associated with a change in the spatial structure, that is, the formation of clusters or compounds, and a subsequent increase in the molecular weight of the diffusive substance.     

Structural trends of 29Si–1H spin–spin coupling constants across double bond

The calculations of geminal and vicinal ²⁹Si–¹H spin–spin coupling constants across double bond in 15 alkenylmethylsilanes and alkenylchlorosilanes were carried out at the second‐order polarization propagator approach level in a good agreement with experiment. Two structural trends, namely, (i) the geometry of the coupling pathway and (ii) the effect of the electrowithdrawing substituent, have been interpreted in terms of the natural J‐coupling analysis within the framework of the natural bond orbital approach. Thus, the marked difference between cisoidal and transoidal ²⁹Si–¹H spin–spin coupling constants across double bond was accounted for the delocalization contributions including bonding and antibonding Si–C and C–H orbitals, whereas the chlorine effect was explained in terms of the steric contributions including bonding Si–Cl orbitals. Copyright © 2012 John Wiley & Sons, Ltd.