Abstract Effect of aging on thermodynamic properties of a glass is studied usually by measuring the apparent heat capacity, Cp, during heating at a fixed rate. In the standard procedure for this study, one identifies a temperature Tx in the Cp-T plot obtained by heating a glass at a fixed rate. When there is little indication of exothermic structural relaxation in these plots, Tx is taken as the temperature at which Cp of a glass begins to increase, but when there is a significant indication of exothermic structural relaxation on heating a fast-cool or quench-formed glass, Tx is taken as the temperature at which Cp begins to show a decrease. In the latter case, the fictive temperature, Tf, is higher than the glass formation temperature measured for a normal cooling rate. In all cases Tx is used as the low-temperature limit of the CpdT integral, and the integral is then used to determine both the decrease in enthalpy on aging and the Tf of a glass. Thermodynamics shows that at temperatures from 0 K to Tx, Cp of an aged and of slow-cool formed glass is less than the Cp of unaged, fast-cool and quench-formed glass, and the zero point enthalpy of the aged glass, like its residual entropy, is less than that of the unaged glass, i. e., there is no Tx. Therefore, the low temperature limit of the integrals should be 0 K. By an analysis of the available Cp data for four glasses we find that the standard procedure underestimates the enthalpy decrease on aging by 19 - 59%, which means that the sum of the contributions from phonon, the anharmonic forces, and molecular relaxation to Cp of an aged glass at T below Tx is not equal to that of the unaged glass. After discussing the enthalpy decrease determined by using various calorimetric methods, we suggest a thermodynamically consistent analysis. A broad endothermic feature characteristic of the rigid glassy state may make Cp of an aged glass at some temperatures appear equal to Cp of an unaged glass, or higher.

中文翻译：

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老化玻璃的热容和焓降低——与确定焓损失和假想温度的标准程序相冲突

摘要 老化对玻璃热力学性能的影响通常通过测量在固定速率加热过程中的表观热容 Cp 来研究。在本研究的标准程序中，通过以固定速率加热玻璃获得的 Cp-T 图中确定了温度 Tx。当这些图中几乎没有放热结构松弛的迹象时，Tx 被认为是玻璃的 Cp 开始增加的温度，但是当在加热快速冷却或淬火成型时有明显的放热结构松弛迹象时玻璃，Tx 被视为 Cp 开始显示下降的温度。在后一种情况下，假想温度 Tf 高于正常冷却速率下测量的玻璃形成温度。在所有情况下，Tx 都用作 CpdT 积分的低温极限，然后使用积分来确定老化焓的降低和玻璃的 Tf。热力学表明，在 0 K 到 Tx 的温度下，时效和慢冷成型玻璃的 Cp 小于未时效、快冷和淬火成型玻璃的 Cp，以及时效玻璃的零点焓，例如它的残余熵小于未老化玻璃的残余熵，即没有Tx。因此，积分的低温极限应为 0 K。 通过对四种玻璃的可用 Cp 数据的分析，我们发现标准程序低估了老化时的焓降低 19 - 59%，这意味着声子、非谐力和分子弛豫对低于 Tx 的 T 时老化玻璃的 Cp 的贡献与未老化玻璃的贡献不同。在讨论了使用各种量热方法确定的焓降低后，我们建议进行热力学一致分析。刚性玻璃态的广泛吸热特征特征可使老化玻璃在某些温度下的 Cp 看起来等于未老化玻璃的 Cp，或更高。