Isobaric Transitions Research Articles

Изменение параметров образования вакансий и самодиффузии в различных полиморфных модификациях железа

The activation parameters for various iron structures were calculated by the analytical method based on the paired four-parameter Mie–Lennard-Jones interatomic interaction potential. Within the framework of a single method, all the activation processes parameters were calculated: Gibbs energy, enthalpy, entropy and volume for both the process of electroneutral vacancy formation and for the process of atom self-diffusion. The isobaric temperature dependences of the indicated activation parameters for BCC and FCC iron structures from T = 10 K to 1810 K along two isobars: P = 0 and 10 GPa were calculated. It is shown that at the α-γ transition temperature (1184 K), the activation parameters decrease during the isobaric transition from the BCC to the FCC structure. At the γ-δ transition temperature (1667 K), the activation parameters increase during the transition from the FCC to the BCC structure. With increasing pressure, the jumps magnitude for the Gibbs energy and the enthalpy of the activation process increases, and for the entropy and volume of the activation process decreases. It is shown that, at low temperatures, due to quantum regularities, activation parameters strongly depend on temperature, and the entropy of activation processes in this region is negative. In the high temperature region, a good agreement has been obtained with the experimental estimates of activation parameters for different iron structures known from the literature.

Changing the parameters of vacancy formation and self-diffusion in various polymorphic modifications of iron

The activation parameters for various iron structures were calculated by the analytical method based on the paired four-parameter Mie-Lennard-Jones interatomic interaction potential. Within the framework of a single method, all the activation processes parameters were calculated: Gibbs energy, enthalpy, entropy and volume for both the process of electroneutral vacancy formation and for the process of atom self-diffusion. The isobaric temperature dependences of the indicated activation parameters for BCC and FCC iron structures from T=10 to 1810 K along two isobars: P=0 and 10 GPa were calculated. It is shown that at the α-γ transition temperature (1184 K), the activation parameters decrease during the isobaric transition from the BCC to the FCC structure. At the γ-delta transition temperature (1667 K), the activation parameters increase during the transition from the FCC to the BCC structure. With increasing pressure, the jumps magnitude for the Gibbs energy and the enthalpy of the activation process increases, and for the entropy and volume of the activation process decreases. It is shown that, at low temperatures, due to quantum regularities, activation parameters strongly depend on temperature, and the entropy of activation processes in this region is negative. In the high temperature region, a good agreement has been obtained with the experimental estimates of activation parameters for different iron structures known from the literature. Keywords: vacancy, self-diffusion, interatomic potential, iron, structure, phase transition.

Parameters of the vacancy formation and self-diffusion in the iron

The parameters of the electroneutral vacancy formation and self-diffusion of atoms in iron have been calculated by the analytical method based on the paired four-parameter Mie–Lennard-Jones interatomic interaction potential. For the first time, all activation process parameters were calculated within the framework of a single method: Gibbs energy, enthalpy, entropy and volume for both the vacancy formation process and the self-diffusion process. The isobaric temperature dependences of the indicated activation parameters for bcc and fcc iron structures from T = 10 K–1810 K along two isobars: P = 0 and 10 GPa were calculated. It is shown that, at low temperatures, due to quantum regularities, activation parameters strongly depend on temperature, and the entropy of activation processes in this region is negative. In the high temperature region, a good agreement has been obtained with the experimental estimates of activation parameters for different iron structures known from the literature. It is shown that at the α-γ transition temperature (1184 K), the activation parameters decrease during the isobaric transition from the bcc to the fcc structure. At the γ-δ transition temperature (1667 K), the activation parameters increase during the isobaric transition from the fcc to the bcc structure. With increasing pressure, the jumps magnitude for the Gibbs energy and the enthalpy of the activation process increases, and for the entropy and volume of the activation process decreases.

About possibility of using liquid respiration to prevent the development of decompression disorders

Introduction. The problem of crew emergency evacuation after prolonged hyperbaric conditions is complicated by the extended decompression necessity. It is impossible to reach normal pressure conditions without risk of decompression sickness in the most cases due to large amount of dissolved indifferent gases in the tissues.
 
 Method for indifferent gases eliminating out of tissues to breathing liquid while liquid breathing is proposed. The absence of dissolved gases in the breathing liquid creates the pressure gradient that allows indifferent gases to diffuse through the alveolar-capillary barrier from the blood into the breathing liquid.
 
 The purpose of the study is to assess the possibility of liquid breathing application to prevent the development of decompression disorders after a long previous stay in conditions of excessive pressure of the gas environment.
 
 Materials and methods. Syrian hamsters, 4 months old, 120–140 g weight were used in the study. A mixture of perfluorodecalin and perfluorohexane 40:60 ratio was used as a breathing liquid. The animals were divided into four groups: control (№ 1) and three experimental by liquid breathing duration (№ 2, № 3 and № 4 was 15, 30 and 60 minutes, respectively). 
 For isobaric transition from gas to liquid breathing the special test bed was designed. The decompression sickness simulation in laboratory animals was carried out by ultrafast decompression (within 15 seconds) after a 60-minute exposure of 0.6 MPa air gauge pressure. The research method is based on the assessment of decompression disorders clinical picture and pathomorphological studies. The duration of the decompression sickness latent period, severity, animals’ lethality, the average animals’ lethal time, rectal temperature, results of pathomorphological examinations were evaluated. Results. Decompression sickness symptoms such as twitches, convulsions and paresis were observed in the № 1 group animals. Rather less explicit symptoms of decompression sickness were found in the group № 2 in comparison of group № 1. It was assessed by clinical pictures and pathomorphological examinations. Decompression sickness symptoms were not found in none of animals of the groups № 3 and № 4. Conclusions. The technology of liquid breathing with oxygenated and denitrogenated normobaric breathing liquid while hyperbaric conditions is possible to be applied for decompression sickness prevention. The dissolved in tissues nitrogen removal degree depends on the liquid breathing duration.

Experimental study of the polyamorphism of water. I. The isobaric transitions from amorphous ices to LDA at 4 MPa.

The existence of more than one solid amorphous state of water is an extraordinary feature. Since polyamorphism might be connected to the liquid-liquid critical point hypothesis, it is particularly important to study the relations amongst the different amorphous ices. Here we study the polyamorphic transformations of several high pressure amorphous ices to low-density amorphous ice (LDA) at 4 MPa by isobaric heating utilising in situ volumetry and ex situ X-ray diffraction. We find that very-high density amorphous ice (VHDA) and unannealed high density amorphous ice (HDA) show significant relaxation before transforming to LDA, whereby VHDA is seen to relax toward HDA. By contrast, expanded HDA shows almost no relaxation prior to the transformation. The transition to LDA itself obeys criteria for a first-order-like transition in all cases. In the case of VHDA, even macroscopic phase separation is observed. These findings suggest that HDA and LDA are two clearly distinct polyamorphs. We further present evidence that HDA reaches the metastable equilibrium at 140 K and 0.1 GPa but only comes close to that at 140 K and 0.2 GPa. The most important is the path independence of the amorphous phase reached at 140 K and 0.1 GPa.

Experimental study of the polyamorphism of water. II. The isobaric transitions between HDA and VHDA at intermediate and high pressures.

Since the first report of very-high density amorphous ice (VHDA) in 2001 [T. Loerting et al., Phys. Chem. Chem. Phys. 3, 5355-5357 (2001)], the status of VHDA as a distinct amorphous ice has been debated. We here study VHDA and its relation to expanded high density amorphous ice (eHDA) on the basis of isobaric heating experiments. VHDA was heated at 0.1 ≤ p ≤ 0.7 GPa, and eHDA was heated at 1.1 ≤ p ≤ 1.6 GPa to achieve interconversion. The behavior upon heating is monitored using in situ volumetry as well as ex situ X-ray diffraction and differential scanning calorimetry. We do not observe a sharp transition for any of the isobaric experiments. Instead, a continuous expansion (VHDA) or densification (eHDA) marks the interconversion. This suggests that a continuum of states exists between VHDA and HDA, at least in the temperature range studied here. This further suggests that VHDA is the most relaxed amorphous ice at high pressures and eHDA is the most relaxed amorphous ice at intermediate pressures. It remains unclear whether or not HDA and VHDA experience a sharp transition upon isothermal compression/decompression at low temperature.

Matrix interference evaluation employing GC and LC coupled to triple quadrupole tandem mass spectrometry

Gas and liquid chromatography coupled to triple quadrupole tandem mass spectrometry are currently the most powerful tools employed for the routine analysis of pesticide residues in food control laboratories. However, whatever the multiresidue extraction method, there will be a residual matrix effect making it difficult to identify/quantify some specific compounds in certain cases. Two main effects stand out: (i) co-elution with isobaric matrix interferents, which can be a major drawback for unequivocal identification, and therefore false negative detections, and (ii) signal suppression/enhancement, commonly called the “matrix effect”, which may cause serious problems including inaccurate quantitation, low analyte detectability and increased method uncertainty. The aim of this analytical study is to provide a framework for evaluating the maximum expected errors associated with the matrix effects. The worst-case study contrived to give an estimation of the extreme errors caused by matrix effects when extraction/determination protocols are applied in routine multiresidue analysis. Twenty-five different blank matrices extracted with the four most common extraction methods used in routine analysis (citrate QuEChERS with/without PSA clean-up, ethyl acetate and the Dutch mini-Luke “NL” methods) were evaluated by both GC-QqQ-MS/MS and LC-QqQ-MS/MS. The results showed that the presence of matrix compounds with isobaric transitions to target pesticides was higher in GC than under LC in the experimental conditions tested.In a second study, the number of “potential” false negatives was evaluated. For that, ten matrices with higher percentages of natural interfering components were checked. Additionally, the results showed that for more than 90% of the cases, pesticide quantification was not affected by matrix-matched standard calibration when an interferent was kept constant along the calibration curve. The error in quantification depended on the concentration level. In a third study, the “matrix effect” was evaluated for each commodity/extraction method. Results showed 44% of cases with suppression/enhancement for LC and 93% of cases with enhancement for GC.

Controlled high-power heat release as a tool to selecting working pressure for supercritical water

The specificity of nonsteady heat conduction in water in the course of its rapid isobaric transition from the state of compressed liquid to supercritical state was studied by the method of controlled pulse heating of a thin wire probe immersed in the substance under investigation. The characteristic pulse length was of the order of milliseconds, the thickness of heating layer appeared to be of the order of micrometers, the density of heat flux from the probe into the substance was 10MW/m2 by the order of magnitude. Based on the results of our experiments, we have developed a phenomenological approach to selecting the way for further research of heat transfer in supercritical fluids, in order to reach guaranteed removal of the heat fluxes with high densities. Namely, taking into account the revealed phenomenon of threshold decrease in heat transfer intensity, which is higher, the closer the value of the pressure is to the critical one, there is a reason for selecting much higher values of working pressure. The corresponding choice should be made in the range of 2.0... 3.0 pc. We can point out such benefits of this choice (in terms of thermophysics) as the decrease in thermal resistance of the boundary layer, the enhancement in water thermal effusivity and its relatively weak dependence on pressure.

Behavior and Retention Models of Melamine and Its Hydrolysis Products

The retention behavior of melamine (MEL), ammeline (AMN), ammelide, and cyanuric acid on various hydrophilic interaction liquid chromatography (HILIC) stationary phases was studied. Using fully optimized electrospray conditions and ammonium formate buffer pH 4/acetonitrile (20:80 v/v) as a mobile phase, precursor and product ions for each compound were identified. For the LC separation study, the effect of the following parameters was investigated: the type of buffer, the effect of the pH of ammonium formate buffer, the dilution percentage of the standard solutions in the vial, the stability of the standard solutions, and the column equilibration time. The retention and separation of melamine and its hydrolysis products on several HILIC columns was also investigated and retention models were proposed. Retention mechanisms were discussed for all the compounds. When the percentage of acetonitrile in the mobile phase was increased, the retention times of ammelide, ammeline and melamine were shifted to higher values, while the retention time of cyanuric acid did not change significantly. The separation of compounds with isobaric transitions (MEL, AMN) was achieved on four HILIC columns, namely TSKgel Amide-80, Luna HILIC, XBridge HILIC, and ZIC-HILIC (at either 10/90 or 15/85 ammonium formate buffer pH 4/ACN).

Isochoric and isobaric glass formation: Similarities and differences

Pressure-volume-temperature (PVT) studies were performed on a glass-forming polymer, poly(carbonate) (PC), under both isobaric and isochoric (constant volume) conditions. An isochoric glass transition was observed and the formation points were found to be consistent with those obtained isobarically. Although the isobaric and isochoric responses were, as expected, the same in the rubbery state, the glassy state values were found to be different and dependent upon the glass formation history. The isobaric data exhibited larger changes in going from the rubber to the glass, hence a “stronger” glass transition, than did the isochoric data. Inserting the experimental values for the thermal expansion coefficient α and isothermal compressibility β, into appropriate thermodynamic relations, measures of the strength of each transition are defined. Strength estimates based on literature values of α and β are compared to the experimental measures of the isochoric and isobaric transitions. In addition, both the isobaric and isochoric PVT results were analyzed in terms of the Fox and Flory free volume theory which assumes that the glass transition is an iso-free volume state. While the isobaric results were consistent with the Fox and Flory theory, the isochoric results were not consistent with the idea of an iso-free volume glass transition. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 1561–1573, 1997

The composition‐dependent glass transition: Relations between temperature pressure, and composition

AbstractA new predictive relation between pressure and temperature changes on the glass‐transition boundary is derived for random solutions. The isobaric version of this relation is integrated to recover a previous theory for the compositional variation of glass‐transition temperatures. The isothermal form of the differential equation gives an equation for the compositional variation of glass‐transition pressures. An enthalpic definition of the transition boundary provides a relation between excess enthalpies of mixing for the glassy and liquid states for the isobaric and isothermal transitions. Similarly, a definition of the boundary in terms of the solution volume provides a relation between excess volumes of mixing for these two states for the isobaric and isothermal transitions. The primary entropic equation for the composition‐dependent isobaric transition gives two hierarchies of approximation, one of which is shown to be preferred for physical reasons. A parallel situation arises for the isothermal transition.

Observation of the 18Ne ground state in the pion double-charge-exchange reaction 18O( π+, π−18Ne

The 18Ne ground state was observed in the pion double-charge-exchange reaction 18O( π +, π −) 18Ne. The experiment was done at a scattering angle of 18° and pion energies of 148 MeV and 187 MeV. Values of σ/dΩ = 0.30 ± 0.10 μb/sr at 148 MeV and 0.21 ± 0.08 μb/sr at 187 MeV were obtained for the ΔT z = 2 isobaric transition to the 18Ne ground state.

Molecular dynamics studies of the vitreous state: Simple ionic systems and silica

Some applications of molecular dynamics calculations to the vitreous state have been examined for simple ionic MX and MX2 type glasses. The MX system which corresponds to a hypothetical vitreous and supercooled amorphous KCl is found to undergo an isobaric transition in the region of 0.3 Tf and an isothermal transition at around 35 kbar (T=Tf). The transition is evidenced by a discontinuity in second order thermodynamic properties and may be associated with a virtual disappearance of translational diffusion. By employing a variety of irreversible stress histories, the vitreous state in this case is shown to be insensitive to the thermodynamic stress history up to a maximum computationally permissible relaxation time of around 10−10 sec. Simulations are also reported for some ionic liquids of MX2 stoichiometry which, in contrast with KCl, have glass-forming ability (BeF2, ZnCl2, and SiO2). Although the relaxation times involved in laboratory glass formation far exceed the long-time limit of computer ’’experiment,’’ the configurationally arrested states which are produced by simulation exhibit many characteristic features of the experimental glasses. Changes of heat capacity consequent on configurational arrest in the case of KCl are similar in magnitude to those observed at the experimental glass transition of simple ionic glasses. The radial distribution for simulated SiO2 ’’glass’’ is very similar to that for vitreous silica, and a simulated compression–vitrification–expansion cycle for this system produces a ’’densified’’ glass, as observed in laboratory experiments. A rattle-and-correlated-jump mechanism appears suitable for the description of diffusion in this liquid. Limitations and further applications of MD calculations pertaining to the vitreous state are discussed.

Polarization of the recoil protons in the elastic electron-proton scattering process

The imaginary part of the two-photon-exchange amplitude is estimated in an isobar model and used to calculate the polarizationP of the recoil protons in elastic (e, p)-scattering processes. Using the unitarity relation for the imaginary part, the sum over the intermediate states is approximated by a sum over nucleon isobars. The spin summations are performed for arbitrary half-integer spinj isobars to obtain an expression forP where the isobaric transition form factors are the only unknown quantities. Numerical calculations have been done forN andN*(1236) contributions only. Forɛilab ≈ 550 MeV and a momentum transfer of about 0.25 (GeV/c)2 the maximum value forP was obtained to be about −0.5%.

Temperature and pressure dependence of the dielectric constant and spontaneous polarization of ferroelectric KNO 3 and NaNO 2

Dielectric constant and spontaneous polarization (via hysteresis loops) measurements were made on ferroelectric KNO 3 (phase III) and NaNO 2 (phase III). The boundaries between the ferroelectric phases and adjacent regions received the most attention. In KNO 3, the isothermal and isobaric I–III transitions exhibited sharp dielectric peaks at all pressures, with ∈ max increasing with pressure. KNO 3−I obeys a pressure Curie-Weiss law, with (1/ ∈( p)) T′ =( p− p 0/ C *. At T′=160°C, C *=−276 kbar and p 0=4·68 kbar. The KNO 3 II–III isobaric transition involves a small jump in ∈ (from 5 to 7·5 at p=1·7 kbar) followed by a steady increase up to the III–I boundary. The NaNO 2 isothermal III–IV transition was detected as a rather small break in the ∈(p) curve, with the discontinuity increasing at higher temperatures. The NaNO 2 IV–III isobaric transition failed to show a similar anomaly in ∈, although tan δ has a maximum at the boundary. Polarization loops were obtained in KNO 3−III, with P s(p) reaching a maximum of 8 μCoul/cm 2. Metastable KNO 3−III was observed at room temperature and room pressure after specimens had first been exposed to high pressures (1·7 to 4·0 kbar) while in region III. In KNO 3−I, k′=d T 0/d p is approximately 25°C/kbar.