Glass transition and fragility of Na2O-TeO2 glasses

The heat capacity (C_p) change in the glass transition region for the xNa₂O ·(100−x)TeO₂, mol%, glass forming melts with x=7.5, 11.1, 15.0, 20.0 and 25.0 was measured as a function of heating rate (2, 4, 6, 10 and 15 °C/min) using differential scanning calorimetry. 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 capacities in the glassy and liquid state (C_pg and C_pl) and the activation enthalpy for glass transition (). T_g for these sodium tellurite melts decreased and increased with increasing Na₂O. Values of the ratio C_pl/C_pg ranged between 1.28 and 2.47, and the fragility parameter ranged between 100 and 130, suggesting that these glass forming melts may be classified as intermediate between typical strong and fragile liquids. The viscosity, η, calculated at a few selected temperatures near the glass transition region decreased with increasing Na₂O at any given temperature, which is also expected.     

The fabrication and properties of solid polymer electrolytes based on PEO/PVP blends

New solid polymer electrolytes were fabricated by the addition of lithium perchlorates in the blends of polyethylene oxide (PEO, MW = 5 × 10⁶) and poly-4-vinylpyridine (PVP, MW = 1.6 × 10⁶). Not only the coordinating bonding between ether units (-O-) of PEO and Li⁺ ions at about 1100 cm⁻¹ but also the existence of pyridine salts at about 3500 cm⁻¹ were definitely observed in the FT-IR spectra. The addition of PVP in the PEO matrix is to restrict mobility of ions through the coupling of PVP and ions. Therefore, the increase of PVP content is expected to decrease the self-discharge rate. In the case of PEO-PVP blends without LiClO₄, the melting temperature T_m and the degree of crystallinity were decreased by the increase of PVP content. On the contrary, those of PEO-PVP blends with LiClO₄ were increased with the increase of PVP content. The results can be explained lay ions, which reduced the crystallinity of PEO by steric hindrance, segregated from the PEO by the coupling of PVP and ions. The new solid polymer electrolytes based on PEO-PVP blends showed ionic conductivities of the order of 10⁻⁸ to 10⁻⁷ S/cm.     

Spin probe reorientation and its connections with free volume and relaxation dynamics: Diglycidyl-ether of bis-phenol A

We report the dynamical behavior of a series of four spin probes of different geometries in amorphous van der Waals-type glass-former diglycidyl-ether of bis-phenol A (DGEBA) as measured by electron spin resonance spectroscopy (ESR). To reveal the relative influence of physical factors influencing spin probe dynamics, the reorientation properties were related to the free volume data of DGEBA from positron annihilation lifetime spectroscopy (PALS) and to the relaxation results of DGEBA from broadband dielectric spectroscopy (BDS). The characteristic ESR temperatures () determined from the temperature dependences of the spectral parameter of mobility, 2A_zz^′, for all spin probes and those ( and ) from the rotational correlation time, τ_c, as a function of temperatures for the smallest spin probe 2,2,6,6-tetramethylpiperidine-1-oxyl (Tempo) were found to coincide with the characteristic PALS temperatures T_b1, T_b2, and T_σ3 from phenomenological analyses of the temperature dependencies of the mean o-Ps lifetime, τ₃, and the width of the o-Ps lifetime distribution, σ₃. Moreover, for the smallest spin probe Tempo, the favorable mutual relationship between the spin probe size and hole size distribution g(V_h) and the closeness of T_50 G with T_b1 point to the role of free volume in the slow to rapid regime crossover. On the other hand, for the three larger spin probes T_50 G lies in the vicinity of T_b2∼T_σ3 and is independent of their size, mass, and shape. The origin of these ESR and PALS coincidences can be traced out to the secondary γ-process for the smallest spin probe Tempo and to the primary α-process for all larger spin probes.     

Formation of complex structural units and structure of some chalco-halide glasses

Raman spectra, thermal expansion and glass transition temperature of glasses and evolution of Raman spectra of CsGaS_1.5Cl glasses with complete substitution of S by Se and partial substitution of Ga by Al (or In) and of Cl by Br (or I) were studied. The structure of these glasses was discussed considering the ability of A(III) metals to form complex tetrahedral structural units [A(III)B(VI or VII)₄]¹⁻, which can be connected by sharing either the common corner of tetrahedra (glasses of systems at x>0.1 and CsInS_1.5Cl glass) or the common edge of tetrahedra (CsGaS_1.5Cl, CsAlS_1.5Cl, CsGaSe_1.5Cl, CsGaS_1.5Br, CsGaS_1.5I glasses).     

Effect of heat-treatment condition on crystallization behavior and thermal expansion coefficient of Li2O-ZnO-Al2O3-SiO2-P2O5 glass-ceramics

Utilizing P₂O₅ as nucleation agent, a Li₂O-ZnO-Al₂O₃-SiO₂ glass was prepared by conventional melt quenching technique and subsequently converted to glass-ceramics with different crystal phases. During the processing, two-step heat-treatments including nucleation and crystallization were adopted. The effects of heat-treatment on the crystal type, the microstructure and the thermal expansion behavior of the glass-ceramics were studied by means of differential scanning calorimetry, X-ray powder diffraction analysis, scanning electron microscopy and thermal expansion coefficient tests. It was shown that the crystallization of occurred after the glass was treated at 580 °C. As the temperature increased from 580 °C to 630 °C, cristobalite and were identified as main and second crystal phases, respectively, in the glass-ceramic. An increase in the temperature to 700 °C, the β-quartz solid solution in the glass-ceramic accompanied by a decrease in cristobalite content. The transformation from to γ₀-Li₂ZnSiO₄ took place from 700 °C to 750 °C. The resulting crystallization phases in the glass-ceramics obtained at the temperature higher than 750 °C were β-quartz solid solution and γ₀-Li₂ZnSiO₄. The glass-ceramics containing or β-quartz solid solution crystal phase possessed a microstructure formed by the development of dendritic crystals. The thermal expansion coefficient of the glass-ceramics varied from 36.7 to 123.8 × 10⁻⁷ °C⁻¹ in the temperature range of 20-400 °C, this precise value is dependent on the type and the proportion of the crystalline phases presented.     

Calibration of cyclohexane solid-solid phase transition thermoporosimetry and application to the study of crosslinking of elastomers upon aging

Photo-aging of elastomers results in two competitive processes: chain- scissions and crosslinking. The solid-to-solid cyclohexane phase transition was used to investigate the crosslinking extent in photo-oxidised ethylene propylene diene monomer (EPDM) samples by thermoporosimetry technique. Before apply this technique to EPDM, texturally well-known silica gel samples were used to calibrate the method. By contrast to the case of liquid-to-solid heptane transition, no linear relationship was obtained between the solid-to-solid point depression (ΔT) and the pore radius (R_p). The following correlation was obtained: