ABSTRACT
We report on Brillouin and in situ small angle X-ray scattering (SAXS) analyses of topological heterogeneity in compressed sodium borosilicate glasses. SAXS intensity extrapolated to very low angular regimes, I(q = 0), is related to compressibility. From Brillouin scattering and analyses of the elastic properties of the glass, the Landau-Placzek ratio is determined and taken as a direct reflection of the amplitude of frozen-in density fluctuations. It is demonstrated that with increasing fictive pressure, topological (mid- and long-range) homogeneity of the glass increases significantly. Heating and cooling as well as isothermal scans were performed to follow the evolution of density fluctuations upon pressure recovery. For a sample with a fictive pressure p(f) of 470 MPa, complete recovery to p(f) = 0.1 MPa was observed to occur close to the glass transition temperature. The values of fictive and apparent fictive temperature, respectively, as obtained via the intersection method from plots of I(q = 0) vs. temperature were found in good agreement with previous calorimetric analyses. Isothermal scans suggest that mid- and long-range recovery govern macroscopic density relaxation.
ABSTRACT
We report on intense red fluorescence from Mn(2+)-doped sulfophosphate glasses and glass ceramics of the type ZnO-Na(2)O-SO(3)-P(2)O(5). As a hypothesis, controlled internal crystallization of as-melted glasses is achieved on the basis of thermally-induced bimodal separation of an SO(3)-rich phase. Crystal formation is then confined to the relict structure of phase separation. The whole synthesis procedure is performed in air at
ABSTRACT
Bi(2+)-doped MBPO(5) (M = Ba(2+), Sr(2+), Ca(2+)), synthesized in air via solid state reaction, are considered as novel orange and red phosphors for white light emitting diodes with improved colour quality. Absorption of Bi(2+) due to (2)P(1/2)-->(2)S(1/2) and (2)P(1/2)-->(2)P(3/2) could be observed and quantified. Excitation to (2)P(3/2) is accompanied by vibronic sidebands, and corresponding emission behaviour is found. The electron-phonon coupling strength increases in the order M = Ba(2+)-->Sr(2+)-->Ca(2+). In the case of MBPO(5):Bi(2+), one-, two- and even three-phonon sidebands could clearly be observed. The crystal structure of all three compounds belongs to space group P3(1)21. Bi(2+) is incorporated on M(2+) sites, and reduction of Bi(3+) to Bi(2+) occurs for reasons of charge compensation. In accordance with crystallographic data, fluorescence decay behaviour indicates that only one type of Bi(2+)-emission centers is present.
ABSTRACT
Differential scanning calorimetry and density measurements were employed to study relaxation processes in compressed NaAlSi(3)O(8) and Na(0.34)Ca(0.31)Al(0.96)Si(3.04)O(8) glasses with negative dependence of viscosity (eta) on pressure (p). Isobaric configurational heat capacities, coefficients of thermal expansion and compressibility of the melts, and real and apparent fictive temperatures are reported for the pressure regime from 0.1 to 700 MPa and cooling rates from 0.1 to 400 K/min. The onset of glass transition, measured on the compressed glass under normal pressure, is found to shift with increasing pressure of freezing in accordance with in situ viscosity data. Based on the pressure-derivative of the compression-induced excess enthalpy (or apparent fictive temperature), a condition for which a negative dependence of fictive temperature on pressure occurs is developed. It is further shown that in this case, also d eta/d p is negative. We suggest that in the region of glass transition, glasses with large Prigogine-Defay (PD) ratio have a high probability to also exhibit a negative dependence of viscosity on pressure. However, the criterion cannot be applied inversely: particularly glasses with a high sensitivity of short-range structural parameters to pressure may exhibit a relatively low PD ratio, but negative d eta/d p.
