Phase transition study in α-Fe2WO6 compound by EPR

The temperature dependence of the EPR spectrum for the α-phase of iron tungstate has been investigated in the temperature range of 40–260 K. At temperatures betweenT ₁ ≈ 250 K andT ₂ ≈ 205 K where the antiferromagnetic phase transition occurs, a relatively narrow EPR line arising from the dominant iron(III) species has emerged, gaining intensity with the temperature increase. Its linewidth temperature evolution could be described by Huber equation, with T_N = 200 K, which is consistent with the peak seen in magnetic susceptibility measurements, while the correspondingg-factor shifts to higher fields reflecting the build-up of internal field emerging from increasing shortrange order in the spin system. At temperatures lower than T₂, a very broad and distorted EPR line with temperature dependentg-factor and linewidth has been observed reflecting the corresponding rise of the magnetic susceptibility below the antiferromagnetic phase transition, presumably arising from magnetic clusters embedded in the antiferromagnetic background.     

Change in the Crystallization Features of Supercooled Liquid Metal with an Increase in the Supercooling Level

The process of homogeneous crystal nucleation has been considered in a model liquid, where the interparticle interaction is described by a short-range spherical oscillatory potential. Mechanisms of initiating structural ordering in the liquid at various supercooling levels, including those corresponding to an amorphous state, have been determined. The sizes and shapes of formed crystal grains have been estimated statistically. The results indicate that the mechanism of nucleation occurs throughout the entire considered temperature range. The crystallization of the system at low supercooling levels occurs through a mononuclear scenario. A high concentration of crystal nuclei formed at high supercooling levels (i.e., at temperatures comparable to and below the glass transition temperature T_g) creates the semblance of the presence of branched structures, which is sometimes erroneously interpreted as a signature of phase separation. The temperature dependence of the maximum concentration of crystal grains demonstrates two regimes the transition between which occurs at a temperature comparable to the glass transition temperature T_g.     

A fluorescence study on the critical exponents for the linear polymerization of butyl-methacrylate

The steady-state fluorescence (SSF) technique was used to study the sol-gel transition for the linear bulk polymerization of butyl methacrylate (BMA), carried out above the glass transition temperature of polybutylmethacrylate (PBMA) (T _g?=?20°C). Pyrene (P_y) was used as the fluorescence probe. The increase in P_y intensity was monitored during free radical polymerization of BMA by using SSF technique. Changes in the viscosity of the pregel solutions due to gel formation dramatically enhance the fluorescent yield of aromatic molecules. This effect is used to monitor the sol-gel transition of BMA, as a function of time, at various temperatures. The results are interpreted in the view of percolation theory. The gel fraction exponent β?=?0.39?±?0.02 agreed the best with the static percolation values for the linear bulk BMA polymerization carried out above T _g but weight average degree of polymerization exponent,?γ?deviated from the percolation results.     

Self-similar solutions of equations of the nonlinear Schrödinger type

Experimental data are presented for the temperature dependence of the conductivity of Cu: SiO₂ metal-insulator composite films containing 3-nm Cu granules. At low temperatures in the concentration range 17–33 vol % Cu, all of the conductivity curves have a temperature dependence of the form σ ∝ exp{ (T ₀/T)^1/2}, while at higher temperatures a transition is observed to an activational dependence. A numerical simulation of the conduction in a composite material shows that an explanation of the observed temperature dependence must include the Coulomb interaction and the presence of a rather large random potential. The simulation also yields the size dependence and temperature dependence of the mesoscopic scatter of the conductivities of composite conductors. It is shown that a self-selecting percolation channel of current flow is formed in the region of strong mesoscopic scatter.     

Nucleation of the D1a type ordered phase in the near-surface of A4B alloy: II. Nucleation at surface

Nucleation in the D1_a type A₄B ordering alloy was investigated by means of an atomistic calculation based on a phenomenological nucleation theory and the Bragg-Williams approximation. The nucleation rate at the Ni₄Mo surface is presented as a function of temperature, surface orientation, and type of the plane to which the surface is parallel (fundamental plane or superlattice plane). The radius, degree of order and activation free energy of the critical nucleus are also given as a function of temperature. The value of the nucleation rate at the surface is compared with that in the bulk. The results are as follows: (i) at temperatures near T_c. the nucleation rate is higher at the surfaces than in the bulk and the {200}_FCC surfaces are the highest in nucleation rate; (ii) however, at temperatures near the instability temperature t₀, the nucleation rate is lower at the surface than in the bulk; (iii) the present calculated results give reasonable explanations to our experimental results on the Ni₄ Mo alloy: (a) preferential surface ordering at high temperatures near T_c; (b) surface disordering at intermediate temperatures near the nose temperature.     

Surface and interface effects on structural transformation of vapor-deposited ethylbenzene films

We have investigated how the structures of vapor-deposited glassy films change with increasing temperature by using time-of-flight secondary ion mass spectrometry and ion scattering spectroscopy. It is found that intermixing of the topmost layer of an ethylbenzene film occur at temperature (~ 80 K) considerably lower than the glass transition temperature (T_g = 118 K) when the film is deposited at 20 K. This phenomenon can be interpreted as the occurrence of a two-dimensional liquid that diffuses into pores of the film, which is evidenced from comparison with surface diffusivity measurements using a porous silicon layer. For nonporous films deposited at higher temperatures, the molecules intermix gradually prior to the abrupt film morphology change at T_g. This phenomenon can be interpreted as decoupling between translational diffusivity and viscosity in the bulk. The film thickness has no significant effects on the evolution of supercooled liquid at T_g except for the monolayer film, whereas crystallization is quenched for the films thinner than 8 monolayers. The roles of the 2D liquid on the surface and an immobilized layer formed at the interface are discussed in finite-size effects on the glass-liquid transition and crystallization.     

Nucleation of the D1a type ordered phase in the near-surface of A4 B alloy: I. Nucleation in bulk

Nucleation in the D1_a type A₄B ordering alloy was studied by means of an atomistic calculation based on the Bragg-Williams approximation, as a first step in the understanding of ordering phenomena in the near-surface. A detailed investigation was made for bulk Ni₄ Mo on the free energy (ΔF) surface of the system containing a nucleus as a function of the size and the degree of order of the nucleus. The nucleation rate, and the degree of order and the size of the nucleus at temperatures above the instability temperature T₀ were given as a function of temperature by employing a phenomenological nucleation theory. Above T₀ (= 0.844 T_c) the Δ F surface is dam-shaped and has a saddle point, while it has neither dam shape nor saddle point below T₀. The nature of nucleation is different at higher and lower temperatures: at temperatures higher than T₀ a critical nucleus must have a high degree of order although the value is not so large as the equilibrium degree of order, while the nature of nucleation has a continuous mode at lower temperatures. By examining the calculated results, it was concluded that the use of this nucleation theory was valid at least qualitatively.     

Superconducting properties of tin embedded in nanometer-sized pores of glass

The electrical resistance of tin embedded from a melt in porous glasses with an average pore diameter of ??7 nm has been investigated at low temperatures in magnetic fields up to 2 T. The temperatures of the transition to the superconducting state for nanocrystalline tin have been determined in magnetic fields of 0, 0.3, 0.5, 1.0, 1.5, and 2.0 T. It has been found that the temperature and magnetic-field dependences of the electrical resistance of the nanocomposite under investigation exhibit two transitions to the superconducting state. The nature of the double superconducting transitions has been discussed. The H _c -T _c phase diagram has been constructed using the entire set of data on the magnetic-field and temperature dependences of the electrical resistance of nanostructured tin. This phase diagram indicates that the upper critical magnetic field H _c2(0) for nanostructured tin is almost two orders of magnitude higher than the corresponding field for bulk tin.     

Phase transition and extended X-ray absorption fine structure of melt-spun amorphous Fe100−xYx alloys

The phase diagram and local structure of melt-spun amorphous (a-) Fe_100−xY_x (22?x?62) alloys were investigated using AC and DC magnetic and extended X-ray absorption fine structure (EXAFS) measurements. The a-Fe–Y system shows reentrant spin glass (RSG) behavior for 42?x?58 and spin glass (SG) behavior for 60?x. Two SG transition temperatures, T_g and T_f, were obtained in the RSG state. The T_g, T_f and Curie temperature T_C decrease with increasing x, and the T_C and T_g vanish at x=60. A new magnetic phase diagram for the melt-spun a-Fe_100−xY_x alloys was obtained from magnetic measurements for higher Y concentration. The magnetic states of the a-Fe_100−xY_x alloys change remarkably around x=60 and an EXAFS study revealed that the average atomic distance between nearest-neighboring Fe atoms changes at approximately x=60.     

Relation between elastic modulus and glass softening temperature in the delocalized atom model

The ratio of softening temperature (glass transition temperature) to elastic modulus (T _g /E) is mainly determined by the limiting elastic deformation of an interatomic bond, which characterizes the transition of a structural microregion from an elastic into a viscous-flow state. In silicate glasses, this transition is caused by the limiting deformation of directed ionic-covalent Si-O-Si bonds. In the case of amorphous hydrocarbons, it is related to the relatively weak intermolecular bonds between regions in chain macromolecules, and the T _g /E ratio is significantly higher than in inorganic glasses. In glassy systems of one class, this ratio turns out to be constant (T _g /E ?? const), and a linear correlation is detected between softening temperature and elastic modulus, which can be explained in terms of the delocalized atom model. The values of T _g /E can be used to classify glasses similarly to the well-known Angell classification according to so-called fragility.     

Transition De Phase Metamagnetique Du Bromure Ferreux

In a magnetic field parallel to the magnetization axis of an antiferromagnetic Fe Br₂ single crystal, a caracteristic metamagnetic behaviour is observed. The transition from an antiferromagnetic phase to a paramagnetic phase is studied by help of magnetization measurements in a steady field (H < 60 kOe). The measurement precision has allowed a detailed study of the magnetization isotherms, caracteristic of a first order magnetization phase transition (T < T_c = 4, 7 K) and of a second order phase transition (T_c < T < T_N = 14, 2 K).We have observed an original phase diagram. In a certain temperature and field range, the ordered phase is stable on the high temperature side of the transition point. Some theoretical studies in an Ising model, or in the hypothesis of a strong magnetoelastic coupling forecast the existence of such a magnetic phase diagram.At present, we proceed to a theoretical study, in a molecular field approximation, of the magnetic phase diagram of compounds similar to Fe Br₂ where we take into account the relative values of parameters J₁, J₂ and D associated with ferromagnetic and antiferromagnetic interactions and crystalline anisotropy.     

Temperature dependence of liquid viscosity

Least squares programs were used to evaluate the correlation between recent experimental results and theoretical, semi-theoretical, or empirical relations between liquid viscosity and temperature. It was found that none of these describes the experimental dependence in the whole range of temperature. A new dependence based on the free volume concept and cell-hole liquid theory is proposed. The theory permits linearization of the viscosity-temperature data in the range (T_i, T_i+1), where the T_i's are the liquid-liquid transition temperatures. It was demonstrated that these transitions, both in small molecular and in polymeric liquids, occur in discrete steps: T_i = a_iT_g, where T_g is the glass transition temperature and the a_i's are numerical parameters. Not all the transitions T_i were apparent in all liquids. Transition T₂ = 1.26T_g was observed for most polymeric liquids. On the basis of thermal analysis it was demonstrated that the T_i's coincide with the temperatures at which small changes in the apparent specific heat were detected. These temperatures can be assigned to the maximum rate of crystallization and melting temperatures of the metastable and stable crystalline forms.     

Glass formation in amorphous SiO<Subscript>2</Subscript> as a percolation phase transition in a system of network defects

Thermodynamic parameters of defects (presumably, defective SiO molecules) in the network of amorphous SiO₂ are obtained by analyzing the viscosity of the melt with the use of the Doremus model. The best agreement between the experimental data on viscosity and the calculations is achieved when the enthalpy and entropy of the defect formation in the amorphous SiO₂ network are H _d =220 kJ/mol and S _d =16.13R, respectively. The analysis of the network defect concentration shows that, above the glass-transition temperature (T _g ), the defects form dynamic percolation clusters. This result agrees well with the results of molecular dynamics modeling, which means that the glass transition in amorphous SiO₂ can be considered as a percolation phase transition. Below T _g , the geometry of the distribution of network defects is Euclidean and has a dimension d=3. Above the glass-transition temperature, the geometry of the network defect distribution is non-Euclidean and has a fractal dimension of d _f =2.5. The temperature T _g can be calculated from the condition that percolation arises in the defect system. This approach leads to a simple analytic formula for the glass-transition temperature: T _g =H _d /((S _d +1.735R). The calculated value of the glass-transition temperature (1482 K) agrees well with that obtained from the recent measurements of T _g for amorphous SiO₂ (1475 K).     

Heterogeneous dynamics at the glass transition in van der Waals liquids, in the bulk and in thin films

It has been shown over the last few years that the dynamics close to the glass transition is strongly heterogeneous, both by measuring the diffusion coefficient of tagged particles or by NMR studies. Recent experiments have also demonstrated that the glass transition temperature of thin polymer films can be shifted as compared to the same polymer in the bulk. We propose here first a thermodynamical model for van der Waals liquids, which accounts for experimental results regarding the bulk modulus of polymer melts and the evolution of the density with temperature. This model allows us to describe the density fluctuations in such van der Waals liquids. Then, by considering the thermally induced density fluctuations in the bulk, we propose that the 3D glass transition is controlled by the percolation of small domains of slow dynamics, which allows to explain the heterogeneous dynamics close to T _g. We show then that these domains percolate at a lower temperature in the quasi-2D case of thin suspended polymer films and we calculate the corresponding glass transition temperature reduction, in quantitative agreement with experimental results of Jones and co-workers. In the case of strongly adsorbed films, we show that the strong adsorption amounts to enhance the slow domains percolation. This effect leads to 1) a broadening of the glass transition and 2) an increase of T _g in quantitative agreement with experimental results. For both strongly and weakly adsorbed films, the shift in T _g is given by a power law, the exponent being the inverse of that of the correlation length of 3D percolation. Received 21 March 2000 and Received in final form 4 December 2000     

The possibly trapped excess free volume

Excess free-volume trapping incurred by quenching has been quantified for atactic polystyrenes with different molecular weights. As the initial temperature (T₀) is elevated from the glass transition (T_g), the increment in excess free-volume trapping first responds sharply, then becomes sluggish, and finally levels off. Molecular weight increase shifts the isochronal curves to a higher temperature region, which coincides with the isochronal change in glass transition temperature (T_g ^t)- As the isochronal curves are superposed to form a master curve, the mechanism of excess free-volume trapping is essentially controlled by T₀, from which the conformational structures of molecules are determined prior to quenching, rather than by the final aging temperature. In order for this superposition to occur, conceivably, the content of excess free volume trapped in the glassy state has to be fairly constant throughout all temperatures below T_g. Consequently, this study does not support the existence of an underlying equilibrium extrapolated from the rubber state.     

Pseudogap and metal-insulator transitions in doped semiconductors

The electronic spectrum of a doped semiconductor described by the Anderson-Holstein impurity model and its conductivity derived from the Kubo linear response theory are calculated. Two characteristic temperatures depending on the doping level x are found in the phase diagram, T _PG and T _λ(x). The pseudogap that opens in the single-particle spectrum at low doping levels and temperatures closes at the lower one, T _PG. The pseudogap state of an insulator is attributed to spin fluctuations in a doped compound. At the higher characteristic temperature T _λ(x),, spin fluctuations vanish and the doped compound becomes a paramagnetic poor metal. Two distinct metal-insulator crossovers between semiconductor-like and metallic temperature dependence of resistivity are found. An insulator-to-poor-metal transition occurs at T ^*(x) ≈ T _λ(x). A poor-metal-to-insulator transition at a lower temperature is attributed to the temperature dependence of density of states in the pseudogap. It is shown that both transitions are observed in La_2?x Sr_xCUO₄.     

Temperature behavior of the Kohlrausch exponent for a series of vinylic polymers modelled by an all-atomistic approach

The Kohlrausch-Williams-Watt (KWW) function, or stretched exponential function, is usually employed to reveal the time dependence of the polymer backbone relaxation process, the so-called α relaxation, at different temperatures. In order to gain insight into polymer dynamics at temperatures higher than the glass transition temperature T _g , the behavior of the Kohlrausch exponent, which is a component of the KWW function, is studied for a series of vinylic polymers, using an all-atomistic simulation approach. Our data show very good agreement with published experimental results and can be described by existing phenomenological models. The Kohlrausch exponent exhibits a linear dependence with temperature until it reaches a constant value of 0.44, at 1.26T _g , revealing the existence of two regimes. These results suggest that, as the temperature increases, the dynamics progressively change until it reaches a plateau. The non-exponential character then describes subdiffusive motion characteristic of polymer melts.     

The vortex-like excitations detected by Nernst signals in high-T c superconductors

The nature of the pseudogap state and its relation to the d-wave superconductivity in high-T _c superconductors is still an open issue. The vortex-like excitations detected by the Nernst effect measurements exist in a certain temperature range above superconducting transition temperature T _c, which strongly support that the pseudogap phase is characterized by finite pairing amplitude with strong phase fluctuations and imply that the phase transition at T _c is driven by the loss of long-range phase coherence. We first briefly introduce the electronic phase diagram and pseudogap state of high-T _c superconductors, and then review the results of Nernst effect for different high-T _c superconductors. Related theoretical models are also discussed.     

Phase transition in cholesteryl oleate and cholesteryl linoleate

A differential scanning calorimeter study of cholesteryl oleate (CO) and cholesteryl linoleate (CL) reveals a new solid — solid transition in CO at 265 K with enthalpy difference 1.8 cal/g. Glass transitions (T_g CO=226 K, CL=215 K) and crystal nucleation temperatures are reported.