Phase transition and thermodynamic properties of TiN under pressure via first-principles calculations

The phase transition of TiN from the NaCl structure to the CsCl structure is investigated by the first-principles plane wave pseudopotential density functional theory method, and the thermodynamic properties of the NaCl structures are obtained through the quasi-harmonic Debye model. It is found that the pressures for transition from the NaCl structure to the CsCl structure are 364.1 GPa (for GGA) and 322.2 (for LDA) from equal enthalpies. The calculated ground state properties such as equilibrium lattice constant, bulk modulus, and its pressure derivative are in good agreement with experimental and theoretical data of others. Moreover, the dependences of the relative volume V/V ₀ on the pressure P, the Debye temperature ?? _D , and heat capacity C _V on the pressure P and temperature T, as well as the variation of the thermal expansion ?? with temperature and pressure are also successfully obtained.     

Temperature dependence of lattice distortions in polyethylene

Summary Melt-crystallized samples of polyethylene show a reversible change of the line-width in X-ray wide angle scattering. The width increases with decreasing temperature, the largest changes occur in the temperature ranges of the- and-relaxation processes. Fourier analyses of the line profiles using the method of Warren and Averbach are indicative of strain variances which can be attributed to stress variances over larger domains in the semi-crystalline superstructure. An explanation for this effect is found in the stress parallel to the planes of the lamellae produced by the differences of the thermal expansion coefficients of the crystalline and amorphous domains in the lamellar two-phase systems. The magnitude of the stress is determined by the difference of the thermal expansion coefficients as well as the Young's moduli of the domains whereas the stress variances are related to the statistics of the lamellar thicknesses.With 23 figures and 3 tables     

Morphology of melt-crystallized linear polyethylene fractions and its dependence on molecular weight and crystallization temperature

Replicas and thin-section electron microscopic studies were made of fractions of linear polyethylene covering the molecular weight range 2.78 × 10⁴ to 6.0 × 10⁶ for a variety of crystallizing conditions. Lamellar crystallites were found under all circumstances; and the supermolecular structure, or crystalline morphology, is in agreement with that previously reported from an analysis of the small-angle light-scattering patterns of the same samples under similar crystallization conditions. Details of the crystalline microstructure are also described, which range from truncated hollow pyramids which degenerate as the molecular weight or the undercooling are increased. From these results, it is possible to describe the mechanism of formation of polyethylene spherulites.     

Phase structure and crystallization behavior of polyethylene in its blends with cis-1,4-butadiene rubber

Phase structure and crystallization behavior of polyethylene(PE) in its blends with cis-1,4-butadiene rubber(BR) at different blend ratios and sample preparation conditions were studied. The PE is finely dispersed in the BR matrix. For samples hot pressed at 145 °C, circular PE microdomains with randomly oriented PE lamellar aggregates were produced. The domain size and number increase with increasing PE content. When the PE content is over 10 wt%, most of the PE domains impinged each other. The separated PE domains are connected by PE stripes with parallel arranged lamellar aggregates. For samples hot pressed at 140 °C, elongated PE microdomains with oriented PE lamellar aggregates were obtained due to the shear flow. The crystallization of PE in the blends depends on the phase structure. Confined crystallization of PE occurs in small microdomains at relatively low temperature. With the increase of domain size, the crystallization ability of PE increases while the confined crystallization decreases.     

Temperature dependence of secondary crystallization in linear polyethylene

A specimen of linear polyethylene was subjected to isothermal secondary crystallization at a series of temperatures below the primary isothermal crystallization temperature, the melting and primary crystallization stages being held constant throughout the investigation. Dilatometric measurements exhibit an S–character at low values of undercooling T_p ? T_s, where T_p and T_s are, respectively, the primary and secondary crystallization temperatures; at larger undercoolings, however, an initial very rapid crystallization is followed by a very slow stage. When corrected for thermal contraction of the polymer, the net degree of secondary transformation is seen to peak at a temperature in the range 109–113°C. The S-character of the isotherms and the peaked temperature variation of degree of transformation lead to the conclusion that a large portion of the secondary crystallization consists of the nucleation and growth of the new crystallites. Johnson-Mehl-Avrami analysis leads to a model of heterogeneous nucleation within the remaining amorphous zones, followed by one-dimensional, diffusion-controlled growth.     

Phase behavior of diblock copolymers; pressure and temperature dependence studied by small-angle neutron scattering

The structure factor of a poly(ethylene-propylene)poly(dimethylsiloxane) diblock copolymer has been measured by SANS as a function of temperature and pressure. In contradiction to the random phase approximation the conformational compressibility exhibits a pronounced maximum at the order-disorder phase transition. The phase boundary shows an unusual shape: with increasing pressure it first decreases and then increases. Its origin is an increase in, respectively, the entropic and the enthalpic part of the Flory-Huggins interaction parameter. The Ginzburg parameter describing the limit of the mean-field approximation is not influenced by pressure.     

Phase transition in para-cyanoanilinium iodide

^79,81Br and ¹²⁷I NQR spectra of para-cyanoanilinium bromide, 4-CNC₆H₄NH^⊕₃ Br^?, and para-cyanoanilinium iodide, 4-CNC₆H₄NH^⊕₃I^?, respectively, have been investigated in the temperature range 77 K ? 390 K. In para-cyano-anilinium iodide a phase transition has been found to occur at 99.5 K ± 1.0 K. The transition is characterized by a sudden change in the slope ν(¹²⁷I)=f(T), and by a corresponding change in the nuclear quadrupole coupling constant. The asymmetry parameter η, which has a value of zero in the high temperature phase I, shows a small but finite increase below the transition. The nature of the phase transition in para-cyanoanilinium iodide is found to be similar to that taking place in anilinium iodide as well as in para-fluoro- and para-chloroanilinium iodide. In addition, the nuclear quadrupole coupling constant, η, and the transition temperature for para-cyanoanilinium iodide and para-chloroanilinium iodide are remarkably similar. Results for the temperature dependent studies on para-cyanoanilinium bromide are also presented.     

Phase transition in ceria alumina

Al-doped CeO₂ samples were prepared by conventional solid state reaction. The electrical conductivity of CeO₂ doped with Al₂O₃ has been studied at different temperatures for various molar ratios. The isothermal conductivity increases with dopant concentration due to the vacancy migration phenomenon induced by doping. It has been found that the conductivity increases and shows a jump from 450 to 520°C due to the phase transition of ceria from cubic to orthorhombic type. A slight deflection is seen for 0.5 and 0.6 moles of alumina at about 250°C due to its phase transition from γ to α type. AC impedance measurements proved that the oxide ion conductivity predominantly arises from the grain and grain boundary contribution as two well defined semi-circles are clearly seen. The sample characterization and the study of phase transition changes were done by using X-ray diffraction analysis, Fourier transform infrared spectral and differential scanning calorimetry (DSC) measurements. On increasing the concentration of dopant, the transition temperature shifts towards lower side which is confirmed by DSC as well as conductivity measurements.     

Phase transition in porous electrodes

It is shown by Monte Carlo simulation that electrochemical thermodynamics of electrolytes in a porous electrode is qualitatively different from that in the bulk with a planar electrode. In particular, first order phase transitions occur in porous electrodes when the pore size is comparable to the ion size of the electrolytes: as the voltage is increased from zero, the surface charge density and the ion density in the porous electrodes discontinuously change at a specific voltage. The critical points for those phase transitions are identified.     

Phase transition in Langmuir monolayers

The main features of several theoretical models which describe the main phase transition and correspondingly the non-horizontal shape of the Π-A isotherms of the Langmuir monolayers, are discussed. New equations of state are based on the generalised Volmer's equation and consider the coexistence monomers and large clusters as bimodal distribution. A further generalisation for the case of quasi-bimodal distribution allows the consideration of monomers with small aggregates on one side of the spectrum and large clusters on the other side. The new theoretical model is corroborated by the Π-A isotherms of various amphiphilic monolayers the condensed phases of which have various gradual differences in the crystalinity and packing density data. The large variety of shapes of the Π-A isotherms in the region A<A_c can be determined by a single general equation whose parameters, except A_c, correspond to the isotropic fluid-like monolayer region. The application of the generalised equation of state on the experimental Π-A isotherms indicates the formation of small aggregates in the region A>A_c.     

Conformational dependence of Raman frequencies and intensities in alkanes and polyethylene

Raman vibrational frequencies and intensities of octane, dodecane and hexadecane conformers have been calculated using quantum mechanical ab initio methods. The results agree with various trends observed in the experimental spectra of alkanes, as well as several observations from the experimental Raman spectra of polyethylenes. The present data suggest that ab initio calculated Raman data on alkanes provide valuable information regarding the interpretation of polymer Raman spectra, in particular concerning issues where interpretation based on experimental verification is not possible.     

Temperature and strain-rate dependence of yield stress of polyethylene

The yield stress behavior of a range of polyethylene materials which differ with respect to their short chain branch content has been studied. Measurements carried out over a wide range of temperatures have shown that there is a sudden transition in the behavior of the yield stress at a temperature which is dependent on both the grade of material and the applied strain rate. These results are in agreement with previous results found from analysis of the yield strain behavior. Above the transition temperature the materials all behave in a nonlinear viscoelastic manner, and the yield process is considered as being propagation controlled. Below the transition temperature the materials all behave in an elastic-plastic manner, and the yield process is considered as being nucleation controlled. Below the transition temperature the temperature dependence of the yield stress is determined by the thickness of the crystalline lamellae. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2177–2189, 1998     

Frequency dependence of dielectric crystalline dispersion in oxidized polyethylene

Summary Observed results of the frequency dependence of the dielectric crystalline dispersion in oxidized polyethylene are shown at various temperatures. The value ofɛ(0)—ɛ (∞) increases with the increase of the amount of the carbonyl groups and also the degree of crystallinity. The shape of the absorption curve is nearly the same for various temperatures and also for various samples, including a single-crystallites one. It is also noticeable that the absorption curve has a nearly symmetric shape when it is plotted against the logarithm of frequency. The estimated value of activation enthalpy lies between 24 and 29 Kcal/mol, and the value for a single-crystallites sample is smaller than that for a melt-grown sample. All the experimental results can be explained quantitatively by using a modified theory of our general theory for the dielectric crystalline dispersion. The modification was necessary because the inserted carbonyl groups made the properties of the dielectric crystalline dispersion in oxidized polyethylene to be different from that in other pure polymers. The agreement between the theory and the observed data seems to be satisfactory, except the shape of the absorption curve. The theoretical curve is slightly narrower than the observed shape because of, presumably, too idealized calculations in the present paper.

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Zusammenfassung Die beobachteten Ergebnisse für die Frequenzabh?ngigkeit der Dielektrizit?tskonstante im Kristallinen für oxydierte Poly?thylene werden für verschiedene Temperaturen gezeigt. Die Werte vonɛ(0) —ɛ(∞) steigen mit wachsendem Anteil an Carbonylgruppen und auch mit dem Kristallmit?tsgrad. Die Form der Absorptionskurve ist nahezu für verschiedene Temperaturen und auch für verschiedene Proben dieselbe, einschlie?lich für Einkristallite. Es ist auch bemerkenswert, da? die Absorptionskurve eine beinahe symmetrische Form besitzt, wenn man sie gegen den Logarithmus der Frequenz auftr?gt. Der gesch?tzte Wert der Aktivierungs-Enthalpie liegt zwischen 24 und 29 kcal/mol, und der Wert für die Probe aus Einkristallen ist kleiner als für die aus der Schmelze kristallisierten. Alle experimentellen Ergebnisse k?nnen quantitativ erkl?rt werden unter Benutzung einer modifizierten Theorie aus unserer allgemeinen, für die dielektrische kristalline Dispersion aufgestellten Theorie. Die Modifikation ist notwendig, weil die Zahl der Carbonylgruppen zu klein ist, um die allgemeine Theorie anzuwenden. Die übereinstimmung zwischen Theorie und Experiment scheint zufriedenstellend, au?er hinsichtlich der Gestalt der Absorptionskurve. Die theoretische Kurve ist etwas schmaler als die beobachtete, vermutlich weil die Errechnungen im vorliegenden Fall zu sehr idealisiert sind.

     

Phase equilibria in a polyethylene–polystyrene system

The method of optical interferometry is used to study the interaction of PE with PS in situ. On the basis of the obtained data, phase diagrams of the PE–PS system are constructed for a number of molecular masses of the components. For PE and PS oligomers, the UCMT values are determined. Pair parameters for the interaction of homopolymers are calculated, and their dependences on temperature and molecular mass are considered. The quantitative analysis of the behavior of high-molecular-mass fractions of PE and PS at high temperatures is carried out, and the regions of a partial compatibility of the components are predicted.     

Phase transition theory in tetragonal ice

A microscopic model of the phase transition between the tetragonal modifications of ice III and ice IX is proposed and solve. The transition thermodynamics is investigated by means of cluster approximations taking into account the “ice rule”. The investigations allow us to explain the experimental data on the phase transition in tetragonal ice.     

Phase transition in LAMOX type compounds

La₂Mo₂O₉ (LMO) was synthesized at lower temperature 973 K (LT-phase) by ceramic route. Differential thermal analysis (DTA) scan of LT-phase of LMO showed α→β transition at 843 K during heating and β→α conversion via a metastable γ-phase during cooling. This was also confirmed by thermo-dilatometry and impedance spectroscopy. La₂Mo_1.95V_0.05O_9-δ (LMVO), La_1.96Sr_0.04Mo₂O_9-δ (LSMO) and La_1.96Sr_0.04Mo_1.95V_0.05O_9-δ (LSMVO) were prepared in a similar way. These compounds exhibited α→β transition on heating with shift in transition temperature, but the existence of γ-phase during cooling disappeared. Substitution increased the ionic conductivity of α-phase and reduced that of β-phase.     

Phase transition of docosane in nanopores

Confinement effect of nanopores on phase transition of docosane (n-C₂₂H₄₆, C₂₂) is studied using differential scanning calorimetry (DSC) and temperature-dependent powdered X-ray diffractions (XRD). The bulk C₂₂ has a stable rotator phase R_I and R_II. In the pores of CPGs, SBA-15, C-SBA-15, KIT-6, and MCM-41, the transition temperatures of solid–solid (T_s) and solid–liquid (T_m) transitions of C₂₂ are depressed. The depression of the transition points (ΔT_s, ΔT_m) has a linear relation with the reverse diameter. However, the ΔT_m values seemingly show weak relation with the polarity of the pore walls but to vary with the pore dimensionality. According to XRD analysis, C₂₂ confined in CPG (11.5 nm) and SBA-15 (14.4 nm) present low temperature phase and rotator phase as the bulk, but the lamellar packing of molecules is heavily disturbed in the small pores.