Thermal relaxation calorimetry on ethanol at low temperatures

Ethanol exhibits a very interesting polymorphism presenting different solid phases: a fully-ordered (monoclinic) crystal, a (bcc) plastic crystal which by quenching becomes an orientationally-disordered crystal with glassy properties, and the ordinary amorphous glass. Therefore, it appeared as a good model system to investigate low-temperature properties of glasses, including the role played by orientational vs translational disorder. In the present work, we extend and improve previous measurements by implementing higher-accuracy calorimetric methods at low temperatures (especially, two complementary versions of the thermal relaxation method). We have employed these new measuring methods to study the possible effect of water impurities on the calorimetric and thermodynamic behavior of the different solid phases of ethanol.     

Specific heat of ethanol at low temperatures

We present here new specific heat measurements at low temperatures (2–20 K) of the different phases of ethanol, characterized by the same calorimetric set-up at higher temperatures. We have extended and improved earlier measurements by implementing higher-accuracy calorimetric methods at low temperatures (using two complementary versions of the thermal relaxation method), as well as at higher temperatures (using a quasi-adiabatic, continuous method). The quantitatively very similar low temperature properties and glass-transition features of both structural glass and orientationally-disordered crystal of ethanol provide clear evidence that the lack of long-range crystalline order typical of amorphous solids is an unimportant factor regarding the universal properties of glasses. We have also employed these new measuring methods to study the possible effect of water impurities on the specific heat of the different solid phases of ethanol, and to study possible variations in the specific heat between different found phases of the monoclinic crystal of ethanol.     

Low-temperature properties of glassy and crystalline states of n-butanol

We present low-temperature measurements of the specific heat and the thermal conductivity for the three solid phases of n-butanol, namely glass, crystal and “glacial” phases. Whereas crystal and glass ones are found to exhibit the expected thermal behavior for crystalline and non-crystalline solids, respectively (i.e. Debye theory for crystals, and universal low-temperature properties with a boson peak and a concomitant plateau in the thermal conductivity for glasses), the so-called “glacial phase” behaves as a very defective crystal, confirming earlier work that identifies it as a mixed phase of nanocrystallites and a disordered matrix. We have also measured longitudinal and transverse sound velocities at low temperatures for the glass phase. The elastic Debye coefficient and Debye temperature of the glass determined from these measurements are found to agree very well with the calorimetric ones obtained from a SPM analysis of the specific heat.     

Acoustic properties of aspirin in its various phases and transformation stages studied by Brillouin scattering

The acoustic behaviors of aspirin were studied in its crystal, liquid, glass states as well as transformation stages between them. The Brillouin doublet of the longitudinal acoustic (LA) mode propagating to the [100] direction of aspirin crystals showed a splitting in a temperature range of 393-413 K below the melting temperature, which indicated a softening and distortion of the layered aspirin crystal in its premelting stage. One LA mode was observed in the liquid and glassy phases of aspirin, consistent with Brillouin selection rule for isotropic materials. The acoustic damping in the glassy aspirin was systematically higher than that in the crystal aspirin, which was in contrast to ibuprofen materials. These suggested different activity of remnant local dynamics that could couple to the acoustic waves, persistent in the glassy state of these two representative pharmaceuticals. In the supercooled liquid phase, the Brillouin spectrum displayed a new spectral feature characterized by a high-frequency cutoff and a distributed low-frequency contribution other than the Brillouin peak of the amorphous aspirin. This could be ascribed to the formation of crystalline grains in the glassy matrix due to cold crystallization.     

Effect of crystal content on the properties of willemite glass-ceramics

Helium permeability, thermal expansion properties, and density have been measured for a series of glass-ceramics based on willemite as a lone crystalline phase. These glass-ceramics were prepared such that the compositions of both the glassy and crystalline phases were fixed, and only the relative concentrations of glass and crystal varied. Results of this study demonstrate that the helium permeability and glass transformation temperature of glass-ceramics are controlled by the glassy phase composition, whereas the density, thermal expansion coefficient, and dilatometric softening temperature are a function of both phases. All of these conclusions can be explained readily by simple continuity and mixing arguments.     

Observation of Four Crystalline Phases of the One Dimensional Conductor Zn0.81 [Pt(C2O4)2] . 6D2O by Neutron Diffraction

Four different crystalline phases of Zn_0.81[Pt(C₂O₄)₂].6D₂O have been observed and partially characterized by single crystal neutron diffraction measurements. A crystal in the previously reported orthorphombic α-form was pulled along its c-axis at room temperature and was found to transform to a monoclinic β-form. Two new coexisting phases, monoclinic γ-Zn-OP and orthorhombic δ-Zn-OP were found when the sample was cooled to 200 K. Further crystallographic work is necessary in order to establish the detailed nature of the new phases.     

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.     

Crystallization kinetics of 1Na2O · 2CaO · 3SiO2 · glass monitored by electrical conductivity measurements

We investigated the kinetics of crystal nucleation, growth, and overall crystallization of a glass with composition close to the stoichiometric 1Na₂O · 2CaO · 3SiO₂. The nucleation and subsequent growth of sodium-rich crystals in this glass decreases the sodium content in the glassy matrix, drastically hindering further nucleation and growth. Compositional changes of the crystals and glassy matrix at different stages of the crystallization process were determined by EDS. These compositional variations were also monitored by electrical conductivity measurements, carried out by impedance spectroscopy, in glassy, partially, and fully crystallized samples. The electrical conductivity of both crystalline and glassy phases decreases with the increase of the crystallized volume fraction. Starting at a crystallized volume fraction of about 0.5, the crystalline phase dominates the electrical conductivity of the sample. This behavior was corroborated by an analysis of the activation energy for conduction. We show that electrical conductivity is highly sensitive and can indicate compositional shifts, changes in the spatial distribution of mobile ions in the glassy matrix. Conductivity measurements are thus a powerful tool for the investigation of complex heterogeneous systems, such as partially crystallized glasses and glass-ceramics.     

Internal friction and shear modulus of certain metallic glasses

We have measured the temperature dependence of the internal friction and shear modulus of metallic glass alloys Fe₄₀Ni₄₀P₁₄B₆, Fe₂₉Ni₄₉P₁₄B₆Si₂, Fe₈₀B₂₀ and Fe₇₈Mo₂B₂₀ in the glassy and crystalline phases using a torsional pendulum technique. The internal friction in the glassy phase remains approximately constant from room temperature to a temperature which depends on the annealing treatment, and the rises exponentially with temperature. The internal friction measurements were made at two different frequencies (0.2 Hz and 1.0 Hz) and the activation energies for the relaxation processes estimated by the frequency shift method. The shear modulus increases by about 30% when the amorphous alloys are crystallized. The amorphous alloys Fe₈₀B₂₀ and Fe₇₈Mo₂B₂₀ exhibit Invar type behaviour which disappears upon crystallization.     

Leaching behavior of nuclear waste glass heterogeneities

The results of a six-month burial experiment in granite are discussed. An alkali borosilicate simulated nuclear waste glass was buried in 3 m boreholes at the 345 m level in the Stripa mine. Some glass specimens containing crystallites exhibit preferential attack of the interface between crystalline and glassy phases. The crystalline phase, identified as spinel solid solution, exhibits better chemical resistance than the glassy phase. Results obtained from homogeneous and heterogeneous glasses (i.e., those containing a crystalline phase) are compared.     

Structural relaxation and optical properties in transparent nanocrystalline β-quartz glass-ceramic

We studied structural relaxation in a nanocrystalline β-quartz solid solution (s.s.) glass-ceramic through density measurement, and the effect of structural relaxation on its optical properties. The density of the glass-ceramic changed as structural relaxation proceeded in the glass phase. It was found that the glass-ceramic whose glass phase has a high fictive temperature shows a high optical transmittance because of less optical scattering. We also demonstrated that the refractive index difference between the crystal and glass phases can be calculated from the scattering coefficients of the specimens with different fictive temperatures. The calculated refractive index of the crystalline phase was in fair agreement with the indices of a high-quartz crystal.     

Phase Transitions in Chlorobenzene-Cis-Decalin Mixtures: A Nuclear Magnetic Resonance,Thermal Analysis and Phase Diagram Study

Using nuclear magnetic resonance and differential scanning calorimetry we have been able to observe the molecular rotation properties of chlorobenzene-cis-decalin mixtures in their glassy, amorphous and crystalline phases. The results indicate: that in “rapidly” cooled samples the behavior of the host dominates the properties of the mixtures; that reorientation of the guest molecule is less restricted in the amorphous phase than in the glassy phase; that when the material crystallizes from the amorphous phase on warming, reorientation again becomes severely restricted. The temperatures at which these phenomena occur agree with the phase diagram that has been determined for these materials. Similar experiments on t butyl chloride-cis-decalin mixtures support the above conclusions. These conclusions are in agreement with the previous dielectric studies.

Our magnetic resonance and thermal analysis experiments support the argument that the behavior of the CD host dominates the behavior of guest polar molecules in rapidly cooled mixtures. At the lowest temperature (110 K) CD forms a glassy phase unaccompanied by any heat of crystallization. On warming this glassy phase transforms near 150 K to an amorphous phase where considerable motion of both host and guest molecules occurs. Above 160 K the material crystallizes and reorientation is once again severely restricted up to the eutectic melting of 210 K. There appears to be no time dependence to these transformations and, as long as one stays below the crystallization temperature, the glassy-amorphous transition is reversible.     

Effect of devitrification on ion motion in lithium-disilicate glass

Li-disilicate glass-ceramics consist of microcrystallites imbedded in the glassy Li₂O · 2SiO₂ matrix where the number and size of the crystallites depend on the devitrification heat treatment. To assess ion motion in these model glass-ceramics, we have measured the temperature dependence of the dc conductivity, σ_dc, and the ⁷Li nuclear spin relaxation (NSR) rate, 1/T₁, in samples with various crystalline fraction, c, ranging from c = 0 (pure glass) to c = 1 (fully devitrified polycrystalline ceramic). The Cole–Cole presentation of the complex impedance shows two separate arcs caused by the remarkable difference of the ionic motion in the glassy and crystalline phase. These two arcs correspond to a bi-exponential decay of the ⁷Li nuclear spin magnetization where the resulting two NSR rates are induced by the ionic motion in the two phases. Thus the NSR and σ_dc data provide a comprehensive picture of the ionic motion in the glassy and crystalline phases. In particular, the ionic motion is the fastest in the glass; then at lower values of c we observe a metastable crystalline phase with ionic motion much greater than in the stable (LS₂) crystalline phase existing at large c-values.     

EAM calculations of the thermodynamics of amorphous copper

The structure and thermodynamics of pure amorphous Cu is studied with molecular dynamics, Monte Carlo and quasi-harmonic calculations using the embedded atom method. The pair distribution function, atomic volume, thermal expansion, enthalpy, entropy, and Gibbs free energy are calculated for the cyrstalline, liquid and amorphous phases. The glass transition temperature of the amorphous phase is found to be approximately 400 K ( 0.3 T_m). The free energy difference between the amorphous and supercooled liquid phases was determined to be at most 0.01 eV per atom. The free energy of the liquid and crystalline phases in our calculation is found to agree within 0.4% of experiment over the temperature range 400–2200 K. The free energy of the glass is found to be fairly well described by the simple Turnbull model. This is the first atomic-level calculation of the free energy of a metallic glass.     

The boson peak in melt-formed and damage-formed glasses: A defect signature?

We describe the preparation and characterization of a glassy form of the moderately good glassformer PbGeO₃, by mechanical damage, and compare its properties with those of the normal melt-quenched glass and the crystal. The damage-formed glass exhibits a DSC thermogram strikingly similar to that of a hyperquenched glass, implying that it forms high on the energy landscape. The final glass transition endotherm occurs within 4 K (0.006T_g) of that of the melt-quenched glass, but crystallization occurs at a lower temperature, as if pre-nucleated. In particular, we have studied the low frequency vibrational dynamics of the alternatively prepared amorphous states in the boson peak region, and find the damage-formed glass boson peak to be almost identical in shape to, but more intense than, that of the normal melt-formed glass, as previously found for hyperquenched glasses. In view of the quite different preparation procedures, this similarity would seem to eliminate equilibrium liquid clusters as a source of the boson peak vibrations, but leaves plausible a connection to force constant fluctuations or to specific vitreous state defects.     

Structural transformation on Se0.8Te0.2 chalcogenide glass

Using X-ray diffraction and differential scanning calorimetry (DSC), the structure and the crystallization mechanism of Se_0.8Te_0.2 chalcogenide glass has been studied. The structure of the crystalline phase has been refined using the Rietveld technique. The crystal structure is hexagonal with lattice parameter a = 0.443 nm and c = 0.511 nm. The average crystallite size obtained using Scherrer equation is equal 16.2 nm, so it lies in the nano-range. From the radial distribution function, the short range order (SRO) of the amorphous phase has been discussed. The structure unit of the SRO is regular tetrahedron with (r₂/r₁) = 1.61. The Se_0.8Te_0.2 glassy sample obeys the chemical order network model, CONM. Some amorphous structural parameters have been deduced. The crystallization mechanism of the amorphous phase is one-dimensional growth. The calculated value of the glass transition activation energy (E_g) and the crystallization activation energy (E_c) are 159.8 ± 0.3 and 104.3 ± 0.51 kJ/mol, respectively.     

Brillouin light scattering study of polymeric glassy sulfur

We present here a study of the acoustic dynamics of polymeric glassy sulfur. The data have been collected in the GHz frequency range by means of Brillouin light scattering for different scattering geometries. The choice of the experimental setup allows us to obtain information on the refractive index, n, that comes out to be close to that corresponding to the high temperature polymeric liquid phase. The longitudinal acoustic excitations have been investigated as a function of temperature from deep into the glassy state up to the glass transition temperature. The temperature dependence of the sound velocity is compared to the one measured in the liquid phase. We find that the sound velocity of the glass can be linearly extrapolated from that of the polymeric liquid measured in the THz frequency range with inelastic X-ray scattering.     

The relation between deep trapping levels and the structure of Se layers

Using electron microscopy methods it has been determined that there are numerous crystalline inclusions in evaporated layers of amorphous selenium. The density, dimensions, modification and distribution in the cross section of a layer are essentially dependent upon the method of layer preparation. The maximum micro-inclusion density is observed in layers evaporated at a substrate temperature (t_s) between 8 to 30°C. These layers also possess crystallites of monoclinic modification. In layers evaporated at t_s ? 50°C inclusions of trigonal modification prevail.The parameters of the deep trapping levels for holes and electrons have been determined from photodischarge kinetics. The origin of these levels is found to be related to the phase transition. The hole traps are caused by the crystalline formation of trigonal modification, and the electron traps by crystallites of monoclinic modification.     

Amorphization from the quenched high-pressure phase of silicon and germanium

The phase transitions from the quenched high-pressure phase, including amorphous state, have been investigated for crystalline silicon and germanium at various pressures. X-ray diffraction patterns have been measured at pressures up to 15 GPa and temperatures down to 90 K by an energy-dispersive method using synchrotron radiation and a diamond anvil cell. The quenched β-Sn phase undergoes an amorphous phase transition when heated at 1.5 GPa for c-Si and 2.0 GPa for c-Ge. On the other hand, the quenched β-Sn phase transforms into a metastable crystalline phase when heated at higher pressures. The phase behavior is discussed in relation to the pressure dependence of the height of potential barrier between the β-Sb and amorphous phases and that between the β-Sn and metastable phases. The coordination number for the pressure-induced amorphous germanium, obtained through amorphization from the quenched high-pressure phase, is estimated to be about 4.     

Electron microscope studies of phase transformations in thin films of Tin Diselenide

The present investigations explore the structural characteristics of the thin films of tin diselenide (SnSe₂). Unlike bulk, only sparse studies are available on the structural characteristics in the thin form. SnSe₂ is known to exhibit almost exclusively the 2H CdI₂ type structure in bulk (single crystal) form. Particular emphasis on phase formation and structural transformations obtained in the thin film form has been given. The results reveal that all as-deposited films are amorphous in nature and undergo an amorphous → crystalline transformation upon pulse annealing (∼200 °C), the crystalline phase exhibiting the 2H bulk structure. Further pulse annealing at higher temperatures (∼300 to 500 °C) results in thermal decomposition of this 2H phase, giving rise to several other phases, namely SnSe (orthorhombic), SnSe (cubic) and Sn₂Se₃ (tetragonal) phases, the SnSe phases exhibiting variations in lattice parameters. In addition, a new incommensurate phase of SnSe has been obtained. A model is proposed for the formation of this phase from the orthorhombie SnSe phase. The final product of decomposition is found to be tetragonal tin.