Encyclopedia of Glass Science, Technology, History, and Culture. Группа авторов. Читать онлайн. Newlib. NEWLIB.NET

Автор: Группа авторов
Издательство: John Wiley & Sons Limited
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Жанр произведения: Техническая литература
Год издания: 0
isbn: 9781118799499
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8].

      (5)equation

      Source: After [9].

Material Nucleation mechanism
Homogeneous Heterogeneous θ = 100° Heterogeneous θ = 60° Heterogeneous θ = 40°
SiO2 9 × 10−6 10−5 8 × 10−3 2 × 10−1
GeO2 3 × 10−3 3 × 10−3 1 20
Na2O 2 SiO2 6 × 10−3 8 × 10−3 10 3 × 102
Salol [C13H10O3] 10
Water 107
Ag 1010

      As has long been recognized, glass formation is therefore easier in eutectic regions because freezing‐point depressions enable lower temperatures and higher viscosities to be reached [11]. However, viscosity at the liquidus is not a single scaling parameter for assessing glass‐forming ability. The temperature dependence of the viscosity below Tm must also be considered because vitrification takes place much below the liquidus [6, 8].

      Viscous flow in glass‐forming liquids is characterized by deviations from Arrhenius laws with an activation energy Q that decreases from a high QH near the glass transition to a low QL at superliquidus temperatures. As a fragility index characterizing the temperature dependence of viscosity, Doremus [12] has proposed the ratio:

      (6)equation

      Short (or fragile) and long (or strong) glass melts are, therefore, characterized by RD values higher and lower than 2, respectively.

      Many equations have been proposed to express viscosity–temperature relationships (Chapter 4.1, [13–15]). Consistent with Doremus criterion, an exponential expression with activation energies QL and QH at low and high temperatures, respectively, [13] will be used here because it results from the configuron percolation theory (CPT), which accounts for viscous flow in terms of elementary excitations resulting from broken bonds named configurons [14, 16]. This equation (Sheffield model) is

      where A1 = k/6πrD0, A2 = exp(−Sm/R), B = Hm, C = exp(−Sd/R) and D = Hd, R the gas constant, k Boltzmann constant, r the configuron radius D0 = fgλ 2 zp0 ν0, f the correlation factor, g a geometrical factor (~1/6), λ the average jump length, ν0 the configuron vibrational frequency or the frequency (with which the configuron attempts to surmount the energy barrier to jump into a neighboring site), z the number of nearest neighbors, p0 a configuration factor, Hd the enthalpy, Sd the entropy of formation, and Hm and Sm the enthalpy and entropy of motion of the configurons.