Volume and enthalpy relaxation of a-Se in the glass transition region

Volume and enthalpy relaxation studies of amorphous Se have been performed in the glass transition region by mercury dilatometry and differential scanning calorimetry. For simple temperature jump experiments, as well as for more complex thermal history the volume and enthalpy relaxation data can be described by a single set of kinetic parameters for Tool-Naraynaswamy-Moynihan (TNM) model [Δh¹/R = 42.8 kK, ln(A_TNM/s) = ?133]. Slightly different non-linearity and non-exponentiality parameter were found for volume [x = 0.42, β = 0.58] and enthalpy [x = 0.52, β = 0.65] relaxation data. Similar results were obtained also for Adam-Gibbs-Scherer (AGS) model. The activation energy of viscous flow in the glass transition range is identical with the effective activation energy for relaxation process. A self-consistent data evaluation shows that at moderate departure from equilibrium, volume and enthalpy in amorphous selenium relax in the same way as expressed by TNM and AGS models. Both volume and enthalpy change can be interpreted within the same fictive temperature concept.     

A local structure change of bulk Pd40Ni40P20 glass during full relaxation

The change in the amorphous structure of bulk Pd₄₀Ni₄₀P₂₀ glass during structural relaxation was examined by an anomalous X-ray scattering (AXS) experiment with energies near the Ni K-absorption edge. It was confirmed by differential scanning calorimetry that the sample reached a meta-stable state (a fully relaxed state) with an equilibrium free volume concentration after annealing for about 1 × 10⁴ s at 563 K and 4 × 10⁴ s at 557 K just below the glass transition temperature T_g = 567 K. The structural changes on the progression toward a fully relaxed state were examined in samples annealed for 1 × 10³ and 2 × 10⁴ s at 563 K (glass A), and for 3.2 × 10³, 1 × 10⁴ and 7 × 10⁴ s at 557 K (glass B). The structural analysis revealed that the coordination number of Ni–Ni like atom pairs increased with annealing time and that of Ni–Pd, unlike atom pairs, decreased. Meanwhile, the coordination number N_PNi of P–Ni atom pairs and the nearest neighbor distance r_PNi did not show a remarkable variation. However, prolonged annealing of 7 × 10⁴ s at 557 K induced a remarkable change in N_PNi and r_PNi.     

Glass transition,thermal expansion and relaxation in B2O3 glass measured by time-resolved X-ray diffraction

In situ heating experiments using high-energy, high-intensity synchrotron radiation, can be successfully designed to study structural evolution with temperature of glassy materials. Coherent diffraction from glassy materials forms a succession of halos or diffraction maxima in reciprocal space and the variation with temperature, of the wave-vector Q_max or angular position of the first diffracted intensity I(Q_max) maximum below T_g can be used to determine the iso-structural volume expansion. In the present work we have obtained synchrotron X-ray diffraction patterns in transmission during in situ heating of a B₂O₃ glass. Samples were obtained by melting the B₂O₃ glass rods which were then air-cooled or liquid nitrogen-cooled. The evolution with temperature (and time) of the position of the first diffraction maximum of the diffraction pattern accurately reflected the thermal expansion coefficient and the relaxation behavior of the B₂O₃ glass. Such results allowed determination by diffraction of the glass transition temperature, T_g, at 580 K, as well as information on the structural relaxation during thermal annealing. The total volume changes due to relaxation were measured to be about 1.5 vol.% and 2.5 vol.%, for the air-cooled and the liquid nitrogen-cooled B₂O₃ glass, respectively.     

The effect of the underlying substrate on crystallization kinetics of TiNi thin films

The crystallization kinetics of amorphous thin TiNi films deposited on SiO₂ (or NaCl)/Al foils substrates were investigated. A dramatic acceleration of the crystallization rate was observed for amorphous attached-substrate films. The acceleration originated from the presence of the thin film/middle-wafer interface which served as a two-dimensional nucleus for the growth of the crystalline phase. In the process of non-isothermal annealing by DSC, apparent activation energies for two kinds of underlying thin TiNi films were determined to be 352.96 and 403.69 kJ/mol, respectively, which was lower than those free-standing films studied in previous works. For the process of isothermal annealing, the crystallization kinetics parameters had remarked drop, reflected from the lower Avrami exponent n (the range of 1.35–2.11) and shorter incubation time τ (the range of 0.1–0.4 min) between 758 and 775 K.     

Isobaric and isochoric properties of decahydroisoquinoline,an extremely fragile glass-former

Pressure–volume–temperature (PVT) measurements were obtained on decahydroisoquinoline (DHIQ). From the resulting equation of state in combination with previously reported dielectric relaxation data, an analysis of the volume contribution to the dynamics was carried out for this very fragile glass-former (isobaric fragility, m_P = 163). We find that the ratio of the isochoric and isobaric fragilities equals 0.71 ± 0.02, reflecting a strong temperature effect, consistent with the large fragility. The relaxation times superpose when plotted versus temperature times the specific volume raised to the 3.55 power. This is a larger value of the exponent than expected based on the fragility of DHIQ. The implication is that for molecular glass-formers, the classical Lennard-Jones 6–12 repulsive potential may represent the limit for fragile liquids.     

Kinetics of the crystallization in amorphous NiTi thin films

The crystallization kinetics in Ni_50.54Ti_49.46 film was studied by differential scanning calorimetry through continuous heating and isothermal annealing. The activation energy for crystallization was determined to be 411 and 315 kJ/mol by Kissinger and Augis & Bennett method, respectively. In the isothermal annealing study, The Avrami exponents were in the range of 2.63–3.12 between 793 and 823 K, suggesting that the isothermal annealing was governed by diffusion-controlled three-dimensional growth for Ni_50.54Ti_49.46 thin films.     

Electrical and dielectric properties of glass system NaPO3–KHSO4

Glass samples have been prepared in the NaPO₃–KHSO₄ binary system with the classical melting, casting and annealing steps. Electrical and dielectrical properties of glass samples were studied. Measurements of DC and AC conductivity and complex electrical permittivity of xNaPO₃–(100 ? x)KHSO₄ glass system were carried out at temperatures ranging from room temperature to temperature located 15 °C below glass transition temperature T_g. Results showed that changes of NaPO₃ concentration considerably affect values of observed parameters. DC conductivity of glass increases as NaPO₃ concentration grows until concentration x = 60. However, beyond this value a sharp decrease of DC conductivity was observed. In addition relaxation times showed abrupt changes at concentration x = 60, corresponding to the lowest relaxation times at the temperature 90 °C.     

Phase transformation and crystallization kinetics of melt-spun Ni60Nb20Zr20 amorphous alloy

The glass transition and crystallization kinetics of melt-spun Ni₆₀Nb₂₀Zr₂₀ amorphous alloy ribbons have been studied under non-isothermal and isothermal conditions using differential scanning calorimetry (DSC). The dependence of glass transition and crystallization temperatures on heating rates was analyzed by Lasocka's relationship. The activation energies of crystallization, E_x, were determined to be 499.5 kJ/mol and 488.6 kJ/mol using the Kissinger and Ozawa equations, respectively. The Johnson–Mehl–Avrami equation has also been applied to the isothermal kinetics and the Avrami exponents are in the range of 1.92–2.47 indicating a diffusion-controlled three-dimensional growth mechanism. The activation energy obtained from the Arrhenius equation in the isothermal process was calculated to be E_x = 419.5 kJ/mol. The corresponding three dimensional (3D) time–temperature–transformation (TTT) diagram of crystallization for the alloy has been drawn which provides the information about transformation at a particular temperature. In addition, the intermetallic phases and morphology after thermal treatment have been identified by X-ray diffraction (XRD) and scanning electron microscope (SEM).     

Structural characterization and phase transformation kinetics of Se58Ge42−xPbx (x = 9, 12) chalcogenide glasses

The glass transition behavior and crystallization kinetics of Se₅₈Ge_42?xPb_x (x = 9, 12) have been investigated using Differential Scanning Calorimetry (DSC) at five different heating rates under non-isothermal conditions. It has been observed that these glassy systems exhibit single glass transition and double crystallization on heating. The XRD pattern revealed that the considered glasses get crystallized into GeSe₂ and PbSe/Se phases after annealing at 633–643 K for 2 h. The GeSe₂ and Se phases were found to crystallize in monoclinic structure while, PbSe phase crystallizes in cubic structure. Besides this, a mixed phase was also observed in DSC thermograms after annealing. The kinetic studies include determination of various parameters such as Avrami exponent (n), frequency factor (K_o), dimensionality of growth (m), the activation energy for glass transition (E_t) and for crystallization (E_c). The values of E_t increases while that of E_c decreases after annealing. Also, dimensionality of growth decreases to one dimension from two and three dimensions after annealing.     

Kinetics of crystal growth of germanium disulfide in Ge0.38S0.62 chalcogenide glass

The crystal growth kinetics of GeS₂ in Ge_0.38S_0.62 glass has been studied by Differential Scanning Calorimetry (DSC) and microsopy. The linear crystal growth kinetics of both high temperature α-GeS₂ and low temperature β-GeS₂ polymorphs has been observed over a relatively broad range of temperatures, i.e. 420 < T < 494 °C that correspond to viscosity of supercooled melt: 3 × 10⁹ > η > 8 × 10⁵ Pa s. It seems that 2D nucleated growth is the most probable mechanism of crystallization for high temperature α-GeS₂ under these conditions. However, there are significant deviations for this model for the crystallization of low-temperature β-GeS₂. This might indicate some changes in crystal-melt interfacial energy or break down of Stokes–Einstein relation in that particular case. At temperatures below 500 °C the temperature range of directly observed crystal growth overlaps with isothermal DSC measurements. In this case overall crystallization kinetics can be described by the Johnson–Mehl–Avrami (JMA) nucleation-growth model for kinetic exponent n ≅ 4. The value of activation energy of nucleation estimated from these experiments E_N = 434 kJ mol⁻¹ is comparable with the activation energy of viscous flow in supercooled Ge_0.38S_0.62 melt (E_η = 478 kJ mol⁻¹). A more complex eutectic crystallization involving both GeS₂ and GeS phases has been observed at higher temperatures. This process is probably associated with secondary nucleation and cannot be described by a simple JMA model.     

Preparation and characterization of a novel solid titania precursor

Titania (TiO₂) is an important metal-oxide semiconductor and has a broad range of industrial applications in areas including pigment, photocatalysts, solar cells, ceramics, inorganic membranes, sensors, nonlinear optics and environmental purification, etc. Titania is often prepared using titania precursors. Here we report preparation of a novel solid titania precursor using a simple synthetic method, with a general chemical formula of TiSO₄(AcAc)_2−n(OH)_n · mH₂O (n < 2 and m < 2), that possesses high stability against hydrolysis and can be very conveniently employed for preparation of fine titania particles with no need of any solvent. Well dispersed TiO₂ nanoparticles with an average size of about 20 nm are prepared by directly annealing the solid titania precursor at 600 °C for 2 h. Characterization of the new solid titania precursor is carried out using transmission electron microscopy (TEM), thermogravimetry (TGA), Fourier transform infrared spectrometer (FT-IR) and X-ray diffractometer (XRD).     

Electrical conductivity and dielectric relaxation in non-crystalline films of tungsten trioxide

Amorphous tungsten trioxide (a-WO₃) thin films were prepared by thermal evaporation technique. The electrical conductivity and dielectric properties of the prepared films have been investigated in the frequency range from 100 Hz to 100 kHz and in the temperature range 293–393 K. In spite of the absence of the dielectric loss peaks, application of the dielectric modulus formulism gives a simple method for evaluating the activation energy of the dielectric relaxation. The frequency dependence of σ(ω) follows the Jonscher’s universal dynamic law with the relation σ(ω) = σ_dc + Aω^s, where s is the frequency exponent. The conductivity in the direct regime, σ_dc, is described by the small polaron model. The electrical conductivity and dielectric properties show that Hunt’s model is well adapted to a-WO₃ films.     

Electrical properties of annealed a-SiC:H thin films

In the present work the dependence of electrical properties of a-SiC:H thin films on annealing temperature, T_a, has been extensively studied. From the measurements of dark dc electrical conductivity, σ_D, in the high temperature range (from 283 up to 493 K), was found that the conductivity activation energy, E_a, is invariant for T_a ⩽ 673 K and equal to 0.64 eV, whereas for T_a from 673 up to 873 K, E_a increases at about 0.2 eV reaching to a maximum value 0.85 eV at T_a = 873 K, suggesting the optimum material quality. This behavior of E_a as a function of T_a is mainly attributed to relaxation of the strain in the amorphous network, which is possibly combined with weak hydrogen emission for temperatures up to 873 K. For further increase of T_a (>873 K) the phenomenon of hydrogen emission, causes rapid decrease of E_a down to 0.24 eV at T_a = 998 K, deteriorating the material quality. These results are also supported by the measurements of dark dc electrical conductivity in the low temperature range (from 133 up to 283 K), where the dependence of the density of gap states at the Fermi level, N(E_F), on annealing temperature presents the minimum value at T_a = 873 K. The Meyer–Nelder rule was found to hold for the a-SiC:H thin films for annealing temperatures up to 873 K. Finally, the dependence of dark dc electrical conductivity at room temperature, σ_DRT, on T_a showed to reflect directly the dependence of E_a on T_a.     

Kinetic analysis of Al3Ti intermetallic powder after mechanical treatment in hydrogen

We present results of mechanochemical processing experiments performed on intermetallic powder, Al₃Ti milled under hydrogen and helium atmospheres using a magnetically controlled Uniball device. Milling products were investigated by X-ray diffractometry (XRD), transmission electron microscopy (TEM) and advanced differential thermal analysis (DTA and DSC). Milling in both H₂ and in He resulted in breakdown of the intermetallic structure and, after extended milling, formation of nanostructural products comprising, predominately, a solid solution of Ti in fcc Al, with some possible additional Al_1+xTi_1−x (x = 0.3) phase. Detailed kinetic analysis was preformed on the Al₃Ti sample milled for 6 days in hydrogen. Under isothermal annealing, the crystallization of the Al₅Ti₂ was found to follow the JMA kinetics, with constant activation energy E¹ and Avrami exponent n decreasing from 2.5 to 1.5.     

EPR study of crystalline and glassy ethanol

Pulsed X-band electron paramagnetic resonance (EPR) spectroscopy was applied in studying molecular dynamics in two different solid ethanol matrices. Nitroxyl radicals as paramagnetic reporter groups were embedded in crystalline and glassy ethanol and the phase memory time, T_m, was investigated at 5–80 K. Temperature variation revealed a maximum in 1/T_m centered around 50 K and a small linear decrease with temperature, below ca. 25 K. Faster phase memory time relaxation in crystalline ethanol than in ethanol glass was observed throughout the temperature range studied. This can be attributed to differences in spectral diffusion due to distinct molecular packing densities.     

Relaxation of polaronic charge carriers in lithium manganese spinels

Lithium manganese spinels Li_1+xMn_2−xO₄, 0 ⩽ x ⩽ 0.33, were prepared by wet chemistry technique followed by heat-treatment at 750 °C or 800 °C. Differential scanning calorimetry was used to reveal phase transitions. Electrical properties were studied by impedance spectroscopy. LiMn₂O₄ exhibited phase transition below room temperature. The transition, seen as an exothermic event in DSC and a steep decrease of conductivity upon cooling, was sharp in sample sintered at 800 °C and broadened over a range of temperature in sample sintered at 750 °C. In the low temperature phase of LiMn₂O₄, two relaxations of similar strength were observed in the frequency dependent permittivity. The low frequency process was identified as relaxation of charge carriers since the relaxation frequency followed the same temperature dependence as the dc conductivity. The high frequency process exhibited milder temperature dependence and was attributed to dipolar relaxation in the charge-ordered structure. The dipolar relaxation was barely visible in Li substituted samples, x ⩾ 0.05, which did not undergo structural phases transition. Measurements extended to liquid nitrogen temperature showed gradual lowering of the activation energy of conductivity and relaxation frequencies, behavior typical for phonon-assisted hopping of small polarons.     

Relation between β relaxation and fragility in LaCe-based metallic glasses

The studies of the dynamical mechanical properties of a series of (Ce_xLa_1-x)₆₈Al₁₀Cu₂₀Co₂ (where 0 ≤ x ≤ 1) metallic glasses show that the behaviors of β relaxation are closely correlated with the properties of their corresponding supercooled liquids. Metallic glasses quenched from fragile liquids show pronounced β relaxation humps whereas unobvious excess wings are associated with the metallic glasses quenched from strong liquids. Our result suggests he behaviors of β relaxations in the metallic glasses are correlated with the fragility of their supercooled liquids.