Tournier F Robert
Centre National de la Recherche Scientifique, Université Joseph Fourier, Consortium de Recherches pour l'Emergence de Technologies Avancées, B.P. 166, 38042 Grenoble Cedex 09, France.
Materials (Basel). 2011 May 9;4(5):869-892. doi: 10.3390/ma4050869.
The vitreous transition is characterized by a freezing of atomic degrees of freedom at a temperature T depending on the heating and cooling rates. A kinetic origin is generally attributed to this phenomenon instead of a thermodynamic one which we develop here. Completed homogeneous nucleation laws reflecting the energy saving due to Fermi energy equalization of nascent crystals and their melt are used. They are applied to bulk metallic glasses and extended to inorganic glasses and polymers. A transition T* among various T corresponds to a crystal homogeneous nucleation temperature, leading to a preliminary formation of a cluster distribution during the relaxation time preceding the long steady-state nucleation time of crystals in small samples. The thermally-activated energy barrier ΔG*/kT at T* for homogeneous nucleation is nearly the same in all glass-forming melts and determined by similar values of viscosity and a thermally-activated diffusion barrier from melt to cluster. The glass transition T* is a material constant and a linear function of the energy saving associated with charge transfers from nascent clusters to the melt. The vitreous transition and the melting temperatures alone are used to predict the free-volume disappearance temperature equal to the Vogel-Fulcher-Tammann temperature of fragile glass-forming melts, in agreement with many viscosity measurements. The reversible thermodynamic vitreous transition is determined by the disappearance temperature T* of the fully-relaxed enthalpy H that is not time dependent; the observed specific heat jump at T* is equal to the proportionality coefficient of H with (T* - T) for T ≤ T* as expected from the enthalpy excess stored by a quenched undercooled melt at the annealing temperature T and relaxed towards an equilibrium vitreous state. However, the heat flux measurements found in literature over the last 50 years only gave an out-of-equilibrium T since the enthalpy is continuous at T* without visible heat jump.
玻璃化转变的特征是在取决于加热和冷却速率的温度T下原子自由度的冻结。通常认为这种现象源于动力学而非我们在此阐述的热力学。采用了完整的均匀成核定律，该定律反映了由于新生晶体及其熔体的费米能均衡而节省的能量。这些定律应用于块状金属玻璃，并扩展到无机玻璃和聚合物。不同T之间的转变温度T对应于晶体均匀成核温度，导致在小样品中晶体长时间稳态成核时间之前的弛豫时间内初步形成团簇分布。在T时均匀成核的热激活能垒ΔG*/kT在所有玻璃形成熔体中几乎相同，并且由相似的粘度值和从熔体到团簇的热激活扩散势垒决定。玻璃化转变温度T是一个材料常数，并且是与从新生团簇到熔体的电荷转移相关的能量节省的线性函数。仅使用玻璃化转变温度和熔点来预测等于易碎玻璃形成熔体的Vogel-Fulcher-Tammann温度的自由体积消失温度，这与许多粘度测量结果一致。可逆的热力学玻璃化转变由完全弛豫的焓H的消失温度T决定，该温度与时间无关；在T处观察到的比热跃变等于H与(T - T)的比例系数，对于T≤T*，这正如在退火温度T下淬火过冷熔体所储存的过剩焓朝着平衡玻璃态弛豫时所预期的那样。然而，过去50年文献中发现的热通量测量结果仅给出了一个非平衡的T，因为焓在T*处是连续的，没有明显的热跃变。
