Isobaric Mode Research Articles

High-temperature Raman and FTIR study of aragonite-group carbonates

In situ high-temperature Raman and Fourier transform infrared spectra were measured for aragonite, strontianite, cerussite, and witherite at ambient pressure. The orthorhombic to trigonal phase transitions were observed by the vibrational spectra for aragonite and witherite, at the temperatures of 773 and 1150 K, respectively. The isobaric mode Gruneisen parameters (γiP), derived from this study, are compared with the isothermal mode Gruneisen parameters (γiT), calculated from the reported high-pressure measurements. The γiP and γiT parameters range from 0.46 to 3.43 for the lattice vibrational modes, whereas they are smaller than 0.4 for the internal vibrational modes of the CO3 group, consistent with the CO3 group serving as rigid bodies in the crystal structure. At high temperatures, the γiP parameters for in-plane and out-of-plane bending modes are systematically smaller than those for asymmetric and symmetric stretching modes of CO3, implying that the O–C–O angles are even less sensitive to temperature than the C–O bond lengths. The intrinsic anharmonicities are also evaluated. The averaged anharmonic modes (ai_avg) are positive for cerussite, but negative for aragonite, strontianite and witherite. The intrinsic anharmonicity has quite different contributions to the equation of state and thermodynamic properties of cerussite, compared with other carbonate minerals, at the high temperatures and high pressures of mantle conditions.

Raman spectroscopic study of MnAl2O4 galaxite at various pressures and temperatures

The vibrational properties of synthetic galaxite, MnAl2O4, were investigated at various pressures (0–29.7 GPa) and temperatures (80–973 K), respectively. The Raman frequencies of all observed bands for galaxite continuously increase with increasing pressure and decrease with increasing temperature, respectively. The quantitative analysis shows that the lowest frequency T2g mode has the smallest isothermal mode Gruneisen parameter. Combined with previous studies, the thermal Gruneisen parameter of galaxite is determined to be 1.23(5). The quantitative analysis of temperature dependences of Raman bands yields that the lowest frequency T2g mode has the largest isobaric mode Gruneisen parameter. The intrinsic anharmonic mode parameters are also calculated and nonzero, indicating an existence of intrinsic anharmonicity for MnAl2O4 spinel.

Thermal SZ fluctuations in the ICM: probing turbulence and thermodynamics in Coma cluster withPlanck

We report the detection of thermal Sunyaev-Zeldovich effect (SZ) fluctuations in the intracluster medium (ICM) of Coma cluster observed with ${\it Planck}$. The SZ data links the maximum observable X-ray scale to the large Mpc scale, extending our knowledge of the power spectrum of ICM fluctuations. Deprojecting the 2-d SZ perturbations into 3-d pressure fluctuations, we find an amplitude spectrum which peaks at $\delta P/P = 33\pm 12\%$ and $74\pm19\%$ in the $15'$ and $40'$ radius region, respectively. We perform tests to ensure fluctuations are intrinsic to the cluster and not due to noise contamination. By using high-resolution hydrodynamical models, we improve the ICM turbulence constraints in Coma, finding 3-d Mach number ${\rm Ma_{3d}}= 0.8\pm0.3$ (15' region), increasing to supersonic values at larger radii (40'), and an injection scale $L_{\rm inj}\approx 500$ kpc. Such properties are consistent with driving due to mergers, in particular tied to internal galaxy groups. The large pressure fluctuations show that Coma is in adiabatic mode (mediated by sound waves), rather than isobaric mode (mediated by buoyancy waves). As predicted by turbulence models, the distribution of SZ fluctuations is log-normal with mild non-Gaussianities (heavy tails). The substantial non-thermal pressure support implies hydrostatic mass bias $b_M=-15\%$ to $-45\%$ from the core to the outskirt region, respectively. While total SZ power probes the thermal energy content, the SZ fluctuations constrain the non-thermal deviations important for precision cosmology. The proposed, novel approach can be exploited by multifrequency observations using ground based interferometers and future space CMB missions.

RESEARCH ON REDUCTION OF FUEL CONSUMPTION AND AERODYNAMIC HEATING LOAD AT HIGH SPEED

The paper deals with a method of reduction of aerodynamic drag of a high-speed vehicle by means of active modification of the airflow with injection of light gas with high speed of sound, which can also mitigate the surface heating load. Analytical and numerical researches are carried out. An experimental model of the system of the isobaric modes implementation is created and tested in a wind tunnel for the first time. The results show power efficiency and possibility of realization of the isobaric modes of active airflow modification. An example of possible application of the isobaric modes to reduce the aerodynamic drag of a high speed vehicle is considered; it made it possible to halve the launch mass of the vehicle as compared with a vehicle with a solid propellant engine.

Water effects on the anharmonic properties of forsterite

To quantify the effects of hydration on anharmonicity of olivine thermodynamics, we have measured in situ Raman spectra of an extremely hydrous forsterite with 4500 ppm (wt) H 2 O at temperatures up to 1273 K. All the Raman modes in hydrous forsterite shift linearly to lower wavenumbers with increasing temperature. The calculated isobaric mode Grunesien parameters related to SiO 4 internal stretching and bending vibrations are much lower than lattice vibrations. Additionally, compared with anhydrous forsterite, except for the modes at 919, 858, and 227 cm −1 , water greatly reduces the isobaric mode Gruneisen parameters of the Raman modes in forsterite. Water also has a large effect on the anharmonic parameters related to lattice vibrations, whereas it has little effect on the anharmonic parameters related to SiO 4 internal stretching and bending vibrations. Those results have the implications to the variations of local structure with temperature and estimation of water effects on the thermodynamics of forsterite.

Об эффективности применения изобарических режимов на гиперзвуковых летательных аппаратах

Об эффективности применения изобарических режимов на гиперзвуковых летательных аппаратах

Shaping the X-ray spectrum of galaxy clusters with AGN feedback and turbulence

Abstract The hot plasma filling galaxy clusters emits copious X-ray radiation. The classic unheated and unperturbed cooling flow model predicts dramatic cooling rates and an isobaric X-ray spectrum with constant differential luminosity distribution. The observed cores of clusters (and groups) show instead a strong deficit of soft X-ray emission: dLx/dT ∝ (T/Thot)α = 2 ± 1. Using 3D hydrodynamic simulations, we show that such deficit arises from the tight self-regulation between thermal instability condensation and AGN outflow injection: condensing clouds boost the AGN outflows, which quench cooling as they thermalize through the core. The resultant average distribution slope is α ≃ 2, oscillating within the observed 1 < α < 3. In the absence of thermal instability, the X-ray spectrum remains isothermal (α ≳ 8), while unopposed cooling drives a too shallow slope, α < 1. AGN outflows deposit their energy inside-out, releasing more heat in the inner cooler phase; radially distributed heating alone induces a declining spectrum, 1 < α < 2. Turbulence further steepens the spectrum and increases the scatter: the turbulent Mach number in the hot phase is subsonic, while it becomes transonic in the cooler phase, making perturbations to depart from the isobaric mode. Such increase in dln P/dln T leads to α ≈ 3. Self-regulated AGN outflow feedback can address the soft X-ray problem through the interplay of heating and turbulence.

Radiative-condensation instability in gravitating strongly coupled dusty plasma with polarization force

The radiative-condensation instability (RCI) in self-gravitating strongly coupled dusty plasma (SCDP) is investigated considering the effects of dust thermal velocity and polarization force on the massive dust particulates. In particular, the outer core of the dense neutron star which is supposed to be strongly coupled in nature with temperature T∼107 K and number density n∼1.3×1030 cm−3 is analyzed. The modified generalized hydrodynamic (GH) equations and electron temperature perturbation equation with radiative effects are solved using the linear perturbation method. In the classical hydrodynamic limit, the modified condition of Jeans instability owing to radiative condensation, polarization force and dust thermal velocity is obtained. In the kinetic limit, velocity of compressional mode also modifies the condition of Jeans instability. The dust thermal velocity and viscoelastic effects have stabilizing whereas polarization force and radiative cooling have destabilizing influence on the growth rate of the Jeans instability. The radiative effects stabilize the growth rate of unstable radiative modes. In isobaric mode (short wavelength), the basic condition of radiative instability is obtained which is unaffected due to the presence of polarization force and viscoelastic effects. The radiative cooling time in the outer core of neutron star is estimated and compared with the gravitational free fall time, and it is found that the cooling takes place too fast for self-gravity to be important.

Fractionation of ultradisperse polytetrafluoroethylene in supercritical carbon dioxide and the chemical structures of the fractions

With the use of the pressure-temperature phase diagram, the solubility of ultradisperse polytet-rafluoroethylene in liquid and supercritical carbon dioxide has been experimentally studied. The quantitative dependence of the solubility of the polymer in supercritical carbon dioxide on pressure and temperature in the isothermal and isobaric modes, respectively, has been examined through gravimetric measurements. At 75°C and 30 MPa, the polymer was separated into a soluble fraction and an insoluble fraction. The chemical structures of the pristine polymer sample and of both fractions have been compared with the chemical structure of the classical polytetrafluoroethylene Fluoroplast-4 via the methods of FTIR and Raman spectroscopy.

Raman frequency shifts calculated from the volume data in naphthalene

The Raman frequencies for modes of symmetry Ag and Bg are calculated as functions of temperature (at atmospheric pressure) and pressure (at room temperature) using the observed volume data from the literature through the mode Grüneisen parameters in naphthalene. By determining the temperature and pressure dependence of the isobaric (γP) and isothermal (γT) mode Grüneisen parameters, respectively, the Raman frequencies of the modes which were calculated, are in good agreement with the observed frequencies considered in naphthalene. Our results show that the Raman frequencies for the modes studied, can be predicted from the volume data by means of the mode Grüneisen parameters. This also leads to predicting the temperature dependence of the thermal expansion (αP) and the pressure dependence of the isothermal compressibility (κT) from the frequency shifts, (1/ϑ)(∂ϑ/∂T)P, and (1/ϑ)(∂ϑ/∂P)T, respectively, which can be measured accurately in naphthalene.

Kinetic parameters of CaCO3 decomposition in vacuum, air and CO2 calculated theoretically by means of the thermochemical approach

Fundamental advantages of the thermochemical approach compared with the activation approach were supported in this work by the results of theoretical calculations of the A and E kinetic parameters of CaCO3 decomposition in vacuum, air and CO2 and their comparison with experimental data. The temperature of the reaction in CO2 increases from 800 K (in vacuum) up to 1,200 K at equal rates of decomposition. This effect (unexplained in the framework of the activation approach) is of tremendous importance for estimating the thermal stability and lifetime of materials. It has been shown that the pre-exponential factor A in the Arrhenius equation is related to the entropy change of decomposition reaction and, in the isobaric mode, additionally, to the pressure of the external gaseous product. The mysterious effect of “variable activation energy” observed in many non-isothermal studies of solid decompositions, in particular, for CaCO3, was explained by the change of the reaction regime from the isobaric mode at low temperature (and decomposition degree) to the equimolar mode at higher temperatures (and higher decomposition degrees). This effect manifests itself for reactions related to the evolution of O2, H2O and CO2 gaseous products, which can be present in the reactor media as impurities.

Pressure and temperature dependence of Raman spectra and their anharmonic effects in Bi2Se3 single crystal

We report here on Raman spectroscopic studies of Bi2Se3 single crystal synthesized by chemical vapor transport technique as a function of pressure (0–12GPa) and temperature (80–300K). Polarized Raman experiments in the back-scattering configuration were performed to determine the position of the two dominant first-order Raman-active modes, Eg2 and A1g2 at around 132 and 174cm−1 respectively. High pressure Raman measurements provides the evidence that Bi2Se3 single crystal undergoes first order phase transition above 10GPa. Isobaric and isothermal mode Gruneisen parameter have been determined from the temperature and pressure dependent Raman spectra for both Raman active modes of Bi2Se3 single crystal. The analysis of the experimental data showed that the temperature dependencies of the phonon frequencies and linewidths were well described by considering the contributions from thermal expansion and lattice anharmonicity.

Toward a general theory of heterogeneous reactions

The thermochemical approach, using the quasi-equilibrium methodology of kinetic analysis and the concepts of equimolar and isobaric reaction modes (regimes), has been used to prove the common mechanism of the representative heterogeneous reactions: decompositions of CaCO3 and of Ag2O, reduction of NiO by H2, and the catalytic oxidation of CO and H2 on PtO2. All these rate processes have been analyzed using the congruent dissociative vaporization of the participating solid reactants or catalysts and are described by similar mechanistic schemes. The possible role of atomic oxygen evolution in oxidation catalysis is discussed. Quantitative proofs of the common mechanism of these reactions were provided by agreement between calculated reaction enthalpies and the Arrhenius E parameters, the retardation effect of gaseous products on the reaction rates, and the accordance between experimental and theoretical ratios of isobaric to equimolar values of the E parameter. In conclusion, milestones in the history of the thermochemical approach during the last century are discussed.

Heat-field-stimulated decomposition reaction in Cu2ZnSnS4

Thermal stability is essential for the potential solar cell material Cu2ZnSnS4 (CZTS) in achieving a satisfactory photovoltaic device performance. Although the loss of Sn from CZTS has been reported, the basic decomposition mechanism of a CZTS system has not been well established yet, especially with regard to the role of active Cu1+ ions. This paper not only provides a deeper understanding of the change of Sn species, which includes an equimolar–isobaric vaporization mode transition and a solid–vapor phase transition in a self-generated atmosphere, but also reveals the oxidation state alternation (Cu1+/Cu2+) and transfer mechanism of Cu species through carefully designed experiments and a reaction kinetic study. Cu ions are unexpectedly found to be active in affecting the degradation reaction by valance alternation and ion movement upon the application of a heat field to balance the derivation caused by a non-uniform temperature gradient. As a result, a Cu–Zn separation appears, with Cu accumulating near the hot area and Zn near the cold area. A decomposition reaction model of CZTS under a directional heat field is proposed to describe the elemental and electronic state change in atomic scale, and a perfect match is obtained between the model and the experimental results. This paper paves a way to solve the thermal stability issue of Cu2ZnSnS4.

The mechanism and kinetics of NiO reduction by hydrogen

The thermochemical approach to analysis of thermal decomposition of solids, developed earlier by L'vov, is extended here, for the first time, to interpret the kinetics and mechanism of the reduction of an oxide (NiO) by a gas (H2). This approach is based on the mechanism of congruent dissociative vaporization of the reactant, Langmuir kinetics and determination of the Arrhenius E parameter by the third-law method. The calculated enthalpy of the reaction is in good agreement with the experimentally measured E value. Many other mechanistic and kinetic features of the reaction are explained within the framework of the given theoretical approach. These include: the formation of metal nuclei; the initial autocat- alytic behavior; the formation of nanocrystalline structure of the reduced metal product; the equimolar and isobaric modes of reduction; the dependence of reduction rate on hydrogen pressure; the more than twofold decrease of the E parameter with the extent of reaction a, and the sys- tematic increase of E with temperature.

Raman spectra and X-ray diffraction of tuite at various temperatures

High-temperature Raman spectra and thermal expansion of tuite, γ-Ca3(PO4)2, have been investigated. The effect of temperature on the Raman spectra of synthetic tuite was studied in the range from 80 to 973 K at atmospheric pressure. The Raman frequencies of all observed bands for tuite continuously decrease with increasing temperature. The quantitative analysis of temperature dependence of Raman bands indicates that the changes in Raman frequencies for stretching modes (ν3 and ν1) are faster than those for bending modes (ν4 and ν2) of PO4 in the present temperature range, which may be attributed to the structural evolution of PO4 tetrahedron in tuite at high temperature. The thermal expansion of tuite was examined by means of in situ X-ray diffraction measurements in the temperature range from 298 to 923 K. Unit cell parameters and volume were analyzed, and the thermal expansion coefficients were obtained as 3.67 (3), 1.18 (1), and 1.32 (3) × 10−5 K−1 for V, a, and c, respectively. Thermal expansion of tuite shows an axial anisotropy with a larger expansion coefficient along the c-axis. The isothermal and isobaric mode Gruneisen parameters and intrinsic anharmonicity of tuite have been calculated by using present high-temperature Raman spectra and thermal expansion coefficient combined with previous results of the isothermal bulk modulus and high-pressure Raman spectra.

Pippard relations modified by the Raman frequency shifts for the phases I and II in NH 4I

Pippard relations in terms of the frequency shifts of the ν 7 (40 cm −1) lattice mode, are established here for the phases I and II in NH 4I. The temperature-induced frequency shift, (1/ ν)(∂ ν/∂ T) P is calculated through the isobaric mode Grüneisen parameter γ P using the lattice parameter data at zero pressure for the phases I and II in NH 4I. From the frequency shifts, the specific heat C P is calculated for the phases studied at zero pressure. Also, the pressure-induced frequency shift, (1/ ν)(∂ ν/∂ P) T is calculated through the isothermal mode Grüneisen parameter γ T at 300 K and it is related to the thermal expansivity α P for the phases I and II in NH 4I. Linear variation of C P with the (1/ ν)(∂ ν/∂ T) P and also a linear variation of α P with the (1/ ν)(∂ ν/∂ P) T , indicate that the frequency shifts obtained for the ν 7 (40 cm −1) lattice mode exhibit similar critical behaviour as C P and α P close to the I–II phase transition in NH 4I.

Self-similar solutions for the dynamical condensation of a radiative gas layer

A new self-similar solution describing the dynamical condensation of a radiative gas is investigated under a plane-parallel geometry. The dynamical condensation is caused by thermal instability. The solution is applicable to generic flow with a net cooling rate per unit volume and time α ρ 2 T α , where p, T and α are the density, temperature and a free parameter, respectively. Given a, a family of self-similar solutions with one parameter η is found in which the central density and pressure evolve as follows: p(x = 0, t) oc (t c - t) -η/12-α) and P(x = 0, t) α (t c - t) 1-η)/1-α) , where t c is the epoch at which the central density becomes infinite. For η ∼ 0 the solution describes the isochoric mode, whereas for η ∼ 1 the solution describes the isobaric mode. The self-similar solutions exist in the range between the two limits; that is, for 0 1. We compare the obtained self-similar solutions with the results of one-dimensional hydrodynamical simulations. In a converging flow, the results of the numerical simulations agree well with the self-similar solutions in the high-density limit. Our self-similar solutions are applicable to the formation of interstellar clouds (H i clouds and molecular clouds) by thermal instability.

Raman spectroscopy of the 10-A phase at simultaneously HP-HT

Raman and X-ray diffraction data demonstrated that the 10-A phase is compressed metastably up to 42 GPa at room conditions and that water molecules remain between the sheets, forming hydrogen bonds (Comodi et al. , 2006). The effect of temperature on the pressure evolution of the 10-A phase, Mg 3 Si 4 O 10 (OH) 2 H 2 O, was studied by collecting Raman spectra simultaneously at high pressure (up to 27 GPa) and high temperature (up to 776 K) in electrically heated diamond-anvil cells. The Raman frequencies of all low-frequency modes (between 100 and 1200 cm −1 ) increased monotonically with pressure and slightly decreased with temperature along different isobars. The shift of the stretching bands of H 2 O was opposite with P and T increase: the frequency of the 3597 cm −1 mode decreased with pressure ( d ν /dP = −0.81 cm −1 GPa −1 ) and increased with temperature ( d ν /dT = 0.025 cm −1 K −1 ), whereas the frequency of the 3673 cm −1 mode increased with P ( d ν /dP = −1.38 cm −1 GPa −1 ) and decreased with T ( d ν /dT = −0.011 cm −1 K −1 ). The hydroxyl stretching frequency at 3623 cm −1 increased with both P and T . The FWHM of the low-frequency modes, was independent from temperature and increased linearly with pressure up to 22 GPa, where Raman signals disappeared. Reversible behaviour indicated that no major reconstructive transition, such as amorphization occurred in the pressure-temperature range investigated. Temperature strongly affected the FWHM of the 3597 cm −1 mode, which disappeared at 570 K and 9 GPa, whereas at room temperature it was followed to about 12 GPa. The broadening of the OH-stretching was modelled with a progressive pressure-induced H disordering as observed in other hydrous minerals. Thus the 10-A phase should undergo to an order-disorder phase transition at 12 GPa and room temperature. The increase of temperature reduced the pressure at which the transition occurred. The measured mode shifts were used to determine isothermal and isobaric Gruneisen mode parameters, which were significantly higher than those of talc. The low values of the average calculated intrinsic anharmonic parameters, 〈 a i 〉, of 0.52×10 −5 K −1 , indicated that a quasi-harmonic vibrational model may be used to describe the thermochemical properties of the 10-A phase. H P -H T Raman spectra indicate that the 10-A phase can host water at high P/T gradients and that it is a good candidate to transfer water in the mantle beyond the antigorite stability field.

Thermal instability in a weakly ionized plasma

We revisit the problem of clump formation due to thermal instabilities in a weakly ionized plasma with the help of a linear perturbation analysis, as discussed by Nejad-Asghar & Ghanbari. In the absence of a magnetic field and ambipolar diffusion the characteristic equation reduces to the thermal instability described by Field. We derive the critical wavelengths, which separate the spatial ranges of stability and instability. Contrary to the original analysis of Nejad-Asghar & Ghanbari, perturbations with a wavelength larger than the critical wavelength destabilize the cloud. Moreover, the instability regime of isentropic perturbations is drastically reduced. Isobaric modes with real values of the critical wavelength appear only if the density dependence of the cooling rate is more pronounced than the temperature dependence. Isentropic modes arise only if the power of the density in the cooling rate is smaller than 1/2, which is not fulfilled for CO cooling. We find that ambipolar diffusion is not a dominating heating process in molecular gas.