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Abstract: The main aim of the present study is to investigate the effect of second glass modifier Na2O on the crystallization kinetics and thermal stability of the mol% xNa2O:(50−x)Li2O:50P2O5 glasses, where Li+ ions are progressively substituted with Na+ ions, using differential scanning calorimetry. The DSC studies showed the classical ‘mixed alkali effect’ on the glass transition temperature, with the Tg minima for the glass composition of [Na+/(Li++Na+)]=0.5 i.e. x =25mol%. The crystallization temperature decreases as Na+ ions completely replaces Li+ ions. The presence of small concentration of second alkali (either the guest ion or host ion) tends to enhance the thermal stability, S, against devitrification as observed for 5≤ x ≤15 and 37.5≤ x ≤45. The ‘mixed alkali effect’ tends to thermally stabilize the intermediate compositions of mol% xNa2O:(50−x)Li2O:50P2O5 as these compositions exhibited high S parameter, high activation energy for crystallization and low enthalpy of crystallization, ΔHc. [Copyright &y& Elsevier]

The report investigates the effect of quenching rate on the structure, lithium ion dynamics, and crystallization kinetics of mol% 60Li 2O–40P 2O 5 glass. Quenching rate of the order of 10 5 K s −1 has been achieved using a twin-roller rapid quenching setup. Raman and FT-IR studies reveal that the rapidly quenched glass is more disordered with a reduced amount of pyrophosphate structural units (P 2O 74−) in the glass matrix as compared with the conventionally quenched glass. Non-isothermal differential scanning calorimetry brings out that the rapidly quenched glass undergoes three-phase crystallization while the conventionally quenched glass depicted predominantly single-phase crystallization. The phases are identified as lithium metaphosphate (LiPO 3), pyrophosphate (Li 4P 2O 7), and orthophosphate (Li 3PO 4). The activation energy for crystallization for the major phase Li 4P 2O 7 calculated using thermoanalytical methods turns out to be 287 kJ mol −1. The above structural differences between the rapidly and conventionally quenched glasses result in superior conduction characteristics for the rapidly quenched glass depicting ionic conductivity of 1.0 × 10 −6 S cm −1 at 343 K with an activation energy of 0.63 eV for lithium ion motion. Microstructural studies on the glass ceramics divulge surface, 2D, and 3D crystal growth mechanism for lithium meta-, pyro-, and ortho-phosphate phases, respectively. [ABSTRACT FROM AUTHOR]

66.7Li2O–33.3P2O5 mol% glass prepared by twin-roller rapid quenching has been chosen to investigate the nucleation and crystallization kinetics using differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and high temperature X-ray diffraction (XRD) as the experimental tools. Such studies assume importance in designing materials for solid state ionic devices. The glass exhibits a nucleation rate of 3.1×1018 m−3 s−1 at a typical temperature of 583K and crystal growth rate of 2.1×10−2 nms−1 at 643K. Non-isothermal DSC studies reveal that the glass undergoes bulk crystallization with an activation energy of 114kJmol−1. The crystallized phase is identified as lithium pyrophosphate (Li4P2O7). Crystallization kinetic studies reveal that Li4P2O7 crystallizes three dimensionally with increasing number of nuclei. Interestingly, lithium metaphosphate (LiPO3) appears as a secondary phase in the glass matrix, however, the final glass ceramic product obtained after prolonged heat treatment of the glass is single phase Li4P2O7. The secondary phase also grows three dimensionally with an activation energy of 365kJmol−1, but with constant number of nuclei in contrast to Li4P2O7. The high nucleation and growth rate with lower activation energy for crystallization of Li4P2O7 underscore that the glass is extremely prone to nucleation and crystallization. [ABSTRACT FROM AUTHOR]

Abstract: Crystallization kinetic studies of Li+ ion conducting mol% 60Li2O–40P2O5 glass indicate a 2-dimensional crystal growth mechanism as compared to the 3-dimensional growth mechanism for LiPO3 glass (mol% 50Li2O–50P2O5). Spectroscopic studies reveal that the structures of these two glass compositions differ considerably resulting in different mechanisms for crystal growth. Isothermal ionic conductivity studies further support 2-dimensional crystallization in mol% 60Li2O–40P2O5 glass. Above the glass transition temperature the ionic conductivity deviates from the usual Arrhenian temperature dependence due to the additional effect of the movement of the phosphate polymeric chains in the glass network. The ionic conductivity of the glass ceramic corresponding to the mol% 60Li2O–40P2O5 glass behaves in an Arrhenian fashion with an activation energy of 1.21eV, much higher than that of 0.77eV for the glass. Further, it is established using canonical scaling that the mechanism for ionic conduction is temperature independent. [Copyright &y& Elsevier]

Abstract: Crystallization kinetics of bulk and pulverized glass samples of the system mol% 50Na2O:50P2O5 have been investigated through isothermal as well as non-isothermal DSC studies. Analysis of the crystallization exotherms reveals that the bulk sodium metaphosphate glass undergoes single-stage crystallization with an order parameter (Avrami constant) of 2.7±0.1 and an activation energy for crystallization equal to 142.5kJ/mol, suggesting internal (bulk) crystallization of the glass. The activation energy for structural relaxation, evaluated by the analysis of the dependence of T g on different heating rates, turns out to be 534.8kJ/mol. The exotherm peaks of pulverized glasses exhibit a shift in the crystallization temperature, T c along with a change in the height and width of the peak. The pulverized glass sample undergoes surface crystallization, with almost the same value of activation energy for crystallization as that of bulk glass. The single-stage crystallization mechanism of sodium metaphosphate (bulk and pulverized) glass is closely associated with Johnson–Mehl–Avrami (JMA) model. [Copyright &y& Elsevier]