共查询到20条相似文献,搜索用时 203 毫秒
1.
The glass transition in Ge
x
Se1−x
) (0·1 ⩽x ⩽ 0·25) glasses has been investigated using the photoacoustic (PA) technique. It is found that the PA amplitude and phase
undergo anomalous changes at the glass transition temperatureT
g. The amplitude has critical minimum and phase has maximum values atT
g. The variation of the thermal diffusivity, determined by measuring the frequency dependence of the PA amplitude and phase,
with temperature shows sharp decrease near the glass transition temperature. The temperature dependence of the optical energy
gap also has been measured and it shows a decrease with temperature for all compositions, the rate of decrease being higher
for temperatures greater thanT
g. 相似文献
2.
Wolfgang Kinzel 《Zeitschrift für Physik B Condensed Matter》1982,46(1):59-64
The distribution of local moments in a two dimensional Ising spin glass with short range Gaussian interactions is investigated by Monte Carlo simulations. Below the freezing temperatureT
f
, this distribution has a sharp peak at the saturated moment. The spins can clearly be characterized by a fractionq of frozen spins and 1-q of fast spins which are in thermal equilibrium. Just belowT
f
the frozen spins appear in small clusters; the spin glass transition isnot a percolation process. Our results support the local and nonequilibrium character of the spin glass transition.q is related to the remanent magnetization (TRM), the linear response and the field cooled susceptibility. As a consequence magnetic resonance experiments should see, in addition to a broad background, a sharp line splitting whose position does not shift with temperature. 相似文献
3.
J.E.G. Lipson S. T. Milner 《The European Physical Journal B - Condensed Matter and Complex Systems》2009,72(1):133-137
Recent observations using fluorophore probes of local dynamics in polymer films
have provided new insight into the glass transition.
Using a percolation model, we predict the local Tg in a polymer film,
as a function of distance from a substrate or a free surface.
Our predictions are in good agreement
with the observed elevation of Tg near a substrate,
whereas the observed reduction of Tg near a free surface
is too strong to be accounted for by percolation effects. 相似文献
4.
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 相似文献
5.
A.E. Allahverdyan Th.M. Nieuwenhuizen D.B. Saakian 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,16(2):317-335
In a p-spin interaction spherical spin-glass model both the spins and the couplings are allowed to change with time. The spins are
coupled to a heat bath with temperature T, while the coupling constants are coupled to a bath having temperature TJ. In an adiabatic limit (where relaxation time of the couplings is much larger that of the spins) we construct a generalized
two-temperature thermodynamics. It involves entropies of the spins and the coupling constants. The application for spin-glass
systems leads to a standard replica theory with a non-vanishing number of replicas, n=T/T
J
. For p>2 there occur at low temperatures two different glassy phases, depending on the value of n. The obtained first-order transitions have positive latent heat, and positive discontinuity of the total entropy. This is
an essentially non-equilibrium effect. The dynamical phase transition exists only for n<1. For p=2 correlation of the disorder (leading to a non-zero n) removes the known marginal stability of the spin glass phase. If the observation time is very large there occurs no finite-temperature
spin glass phase. In this case there are analogies with the non-equilibrium (aging) dynamics. A generalized fluctuation-dissipation
relation is derived.
Received 12 July 1999 and Received in final form 8 December 1999 相似文献
6.
Recent experimental studies show that the dielectric constant of lead fluoride increases rapidly as the superionic transition
temperatureT
c is approached. In this work the static dielectric constant (ε) is calculated theoretically for low and high temperature limits. Assuming conduction to take place through a percolation
mechanism,ε is found to diverge at the threshold temperatureT
th where ionic conduction starts. 相似文献
7.
Crystallization process of Se85-xTe15Sbx (x = 2.7, 7.5, 10 and 15 at %) chalcogenide glasses has been studied by using differential scanning calorimetry (DSC) with different heating rates. These glasses are found to have a double glasses transition and overlapped crystalline phases for Se70Te15Sb15 glass while single glasses transition and single crystallization stage for other glasses. Glass transition temperature, Tg, onset crystallization temperature, Tc, and peak crystallization temperature, Tp, are found to be dependent on composition and heating rates. Values of various kinetic parameters such as activation energy of glass transition, Eg, activation energy of crystallization, Ec, Hurby number, Hr, thermal stability, Sp, rate constant, Kp, and Avrami exponent, n, are determined for the present systems. Results indicate that rate of crystallization is dependent on thermal stability and glass-forming ability. Crystallization mechanism occurs in two dimensions for studied compositions. Crystalline phases resulting from DSC and scanning electron microscopy have been identified by using X-ray diffraction. 相似文献
8.
B. J. Madhu H. S. Jayanna S. Asokan 《The European Physical Journal B - Condensed Matter and Complex Systems》2009,71(1):21-25
The compositional dependence of thermal properties, such as glass transition temperature (Tg), non-reversing enthalpy change (ΔHNR) and the specific heat capacity change (ΔCp) of melt quenched Ge7Se93-xSbx (21 ≤ x ≤ 31) glasses, has been studied using alternating differential scanning calorimetry (ADSC) which is analogous to
modulated differential scanning calorimetry (MDSC). The glass transition temperature, Tg, which is a measure of global connectivity of the glass, has been found to increase with the addition of
Sb. In addition, a change in slope has been observed in the composition dependence of Tg at an average coordination 〈r〉 = 2.40. The experimentally observed compositional variation of glass transition temperature,
has been compared with the theoretical predictions from the
stochastic agglomeration theory (SAT) and has been found to be consistent. Further, a narrow thermally reversing window is
seen in the compositional variation of the relaxation enthalpy (ΔHNR), which is centered around 〈r〉 = 2.40. The change in specific heat capacity (ΔCp) at Tg is also found to exhibit a distinct minima at 〈r〉 = 2.40, suggesting that the structural rearrangements for the liquid in
the glass transition region are minimized around 〈r〉 = 2.4. 相似文献
9.
S.G. Magalhães A. Theumann 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,9(1):5-13
We have examined the role of the BCS pairing mechanism in the formation of the magnetic moment and henceforth a spin glass
(SG) phase by studying a fermionic Sherrington-Kirkpatrick model with a local BCS coupling between the fermions. This model
is obtained by using perturbation theory to trace out the conduction electrons degrees of freedom in conventional superconducting
alloys. The model is formulated in the path integral formalism where the spin operators are represented by bilinear combinations
of Grassmann fields and it reduces to a single site problem that can be solved within the static approximation with a replica
symmetric ansatz. We argue that this is a valid procedure for values of temperature above the de Almeida-Thouless instability
line. The phase diagram in the T-g plane, where g is the strength of the pairing interaction, for fixed variance J
2
/N of the random couplings Jij, exhibits three regions: a normal paramagnetic (NP) phase, a spin glass (SG) phase and a pairing (PAIR) phase where there
is formation of local pairs.The NP and PAIR phases are separated by a second order transition line g=g
c
(T) that ends at a tricritical point T
3
=0.9807J, g
3
=5,8843J, from where it becomes a first order transition line that meets the line of second order transitions at T
c
=0.9570J that separates the NP and the SG phases. For T<T
c
the SG phase is separated from the PAIR phase by a line of first order transitions. These results agree qualitatively with
experimental data in .
Received 14 May 1998 相似文献
10.
An atomistic view on Johari–Goldstein secondary β-relaxation processes responsible for structural relaxation far below the glass transition temperature (Tg ) in network glasses is developed for the archetypal chalcogenide glass, As20Se80, using positron annihilation lifetime, differential scanning calorimetry, Raman scattering and nuclear magnetic resonance techniques. Increased density fluctuations are shown to be responsible for the initial stage of physical ageing in these materials at the temperatures below Tg . They are correlated with changes in thermodynamic parameters of structural relaxation through the glass-to-supercooled liquid transition interval. General shrinkage, occurred during the next stage of physical ageing, is shown to be determined by the ability of system to release these redundant open volumes from the glass bulk through the densification process of glass network. 相似文献
11.
We present the phase diagram of (C60)1-x(C70).x alloys, obtained from low frequency (1Hz) elastic and thermal expansion measurements. Whereas thermal expansion measurements show only a. fcc to sc phase transition in C60-rich and a hcp to a distorted hcp phase transition in C70-rich samples, low-frequency (1 Hz) elastic compliance measurements show an additional dynamic anomaly for all the samples (except for the pure C70) due to freezing of the molecular motion. This anomaly is centered around Tg≈ 120 K for the pure C60 and rises with rising concentration of C70, reaching Tg ≈ 150 K for mixtures with 95mol% C70. For C70-rich samples with more than 10mol% C60 no orientational phase transition was found. The phase diagram can be described in terms of the orientational glass model. 相似文献
12.
K. Ramesh S. Asokan K.S. Sangunni E.S.R. Gopal 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,6(2):207-212
Differential Scanning Calorimetric (DSC) studies on AgxGe15Te85-x glasses have been undertaken over a wide range of compositions, to understand the effect of topological thresholds on thermal
properties. It is found that the compositional dependence of glass transition temperature (T
g
), crystallization temperature (T
c
), activation energy for crystallization and thermal stability show anomalies at the rigidity percolation threshold. Unusual variations also observed in different thermal properties at
the composition x = 20, clearly establishes the occurrence of chemical threshold in these glasses.
Received: 27 January 1998 / Revised: 12 June 1998 / Accepted: 3 July 1998 相似文献
13.
xSb2O3-40TeO2-(60 − x)
V2O5 glasses with 0 ≤ x ≤ 10 (in mol%) have been
prepared by rapid- melt quenching method. DSC curves of these ternary glasses have been
investigated. The glass transition properties that have been measured and reported in this
paper, include the glass transition
temperature (T
g
), glass transition width
(ΔT
g
), heat capacity
change at glass transition
(ΔC
P
) and
fragility (F). Thermal stability, Poisson’s ratio, fragility and glass
forming tendency of these glasses have been estimated, to determine relationship between
chemical composition and the thermal stability or to interpret the structure of glass. In
addition, Makishima and Makenzie’s theory was applied for determination of Young’s
modulus, bulk modulus and shear modulus, indicating a strong relation between elastic
properties and structure of glass. Generally, results of this work show that glass with
x = 0 has the highest shear, bulk and Young’s moduli which make it as
suitable candidate for the manufacture of strong glass fibers in technological
applications; but it should be mentioned that glass with x = 8 has higher
handling temperature and super resistance against thermal attack. 相似文献
14.
A. V. Evteev A. T. Kosilov E. V. Levchenko O. B. Logachev 《Physics of the Solid State》2006,48(5):815-820
An isothermal kinetic diagram for the beginning of homogeneous nucleation is constructed using the molecular-dynamics model of an instantaneously supercooled iron melt near the icosahedral percolation transition temperature identified with the glass transition temperature T g . This diagram is compared with the theoretical one calculated using quantitative information obtained by analyzing the kinetics of the initial stage of growth of supercritical nuclei at temperatures higher than T g . A satisfactory coincidence of the theoretical curve with computer simulation data at temperatures higher than T g and substantial disagreement with these data below T g , where crystallization is necessarily preceded by the formation of an icosahedral percolation cluster, demonstrate the substantive influence of an icosahedral substructure on the nucleation rate predicted by the classical theory. 相似文献
15.
A disorderedn-vector model withp spin interactions previously introduced is studied for the quenched case by means of the replica method and a generalized Parisi theory. We present formal solutions for generaln andp and then study the casep . The high-temperature solution is stable at all temperatures and there is only one phase transition at a temperatureT
g. Only longitudinal lowtemperature solutions are possible. There is one spin-glass solution, and it is stable for allT
g. The phase transition atT
g is of first order and displays a jump discontinuity in the order parametersq
j
(L)
andd. The spin-glass free energy is temperature dependent forn > 1 while it is constant whenn = 1. 相似文献
16.
57Fe Mössbauer studies of three inorganic glasses through their glass transition temperatures, Tg, show that the Lamb-Mössbauer factor (recoil-free fraction) decreases with temperature, particularly around Tg. An attempt is made to explain the results employing the cluster model which suggests that soft modes may be associated with the glass transition. 相似文献
17.
D. K. AktaŞ 《Phase Transitions》2013,86(11):863-873
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 (Py) was used as the fluorescence probe. The increase in Py 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. 相似文献
18.
The dynamics of the nonlinear excitations in a two-dimensional (2D) φ4-diatomic lattice, with nonlinear on-site electron-phonon coupling at the polarizable ion site has been presented, without
considering the self consistent phonon approximation. One of the major results obtained from our calculations is in the understanding
of continuous structural phase transition, where we have obtained the minimum in soft mode frequency at a soft mode temperatureT
s (>T
c), not at critical temperatureT
c. This occurs due to the anisotropy of such 2D systems. 相似文献
19.
Seyed Ali Salehizadeh Dariush Souri 《Journal of Physics and Chemistry of Solids》2011,72(11):1381-1385
The glass transition temperature dependence to heating rate and therefore the activation energy (ΔH?) of the glass transition of (60-x)V2O5–xNiO–40TeO2 oxide glasses with 0≤x≤20 (in mol%) were investigated at heating rates φ (=3 6, 9, 10 and 12 K/min) using differential scanning calorimetry (DSC). The heating rate dependence of Tg was used to investigate the applicability of different theoretical models describing the glass transition. Using the application of Moynihan and Kissinger et al. models to the present data, different values of (ΔH?) at each different heating-rate regions were obtained. The fragility parameter (m=ΔH?/R Tg) was ∼24.98 for x=10 mol%, suggesting that this glass may be considered as a rather strong glass (fragility index m∼>20 is an indication of fragile glass). Also the compositional dependence of Tg and ΔH? was investigated. 相似文献
20.
We study by means of Monte Carlo simulations and the numerical transfer matrix technique the critical behavior of the short rangep=3 state Potts glass model in dimensionsd=2,3,4 with both Gaussian and bimodal (±J) nearest neighbor interactions on hypercubic lattices employing finite size scaling ideas. Ind=2 in addition the degeneracy of the glass ground state is computed as a function of the number of Potts states forp=3, 4, 5 and compared to that of the antiferromagnetic ground state. Our data indicate a transition into a glass phase atT=0 ind=2 with an algebraic singularity, aT=0 transition ind=3 with an essential singularity of the form exp(const.T
–2), and an algebraic singularity atT0.25 ind=4. We conclude that the lower critical dimension of the present model isd
c
=3 or very close to it. Some of the critical exponents are estimated and their respective values discussed. 相似文献