Heat Capacity of Mullite - New Data and Evidence for a High-Temperature Phase Transformation
Heat Capacity of Mullite - New Data and Evidence for a High-Temperature Phase Transformation
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DOI：
10.1111/j.1551-2916.2004.00227.x
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发表时间：
2004-02
影响因子：
3.9
通讯作者：
B. Hildmann;H. Schneider
中科院分区：
文献类型：
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作者：
B. Hildmann;H. Schneider
莫来石（单晶、致密和粉末状多晶）的恒压热容（Cp）在−125°和1400°C之间测定。在三个实验室中使用两种不同的盘式差示扫描量热仪（DLR和Netzsch）和一种圆筒型（Setaram）进行测量。在这些实验数据的基础上，提出了一种新的主曲线Cp（T）。直到大约1100°C，Cp曲线与先前发表的结果一致：Cp主曲线显示抛物线形状，Cp值在−125°C时约为0.415 J/（g·K），25°C时约为0.78 J/（g·K），1000°C时约为1.25 J/（g·K）。在1100°C以上的温度下，观察到一个未知的、阶梯状的、微弱的Cp增加，ΔC°p阶跃值在0.03和0.10 J/（g·K）之间。莫来石的这种Cp异常在各种加热条件下是可再现的，并且在加热和冷却时都是可逆的。异常的外推起始温度（吨）由样品的结晶状态（单晶，Ton.高于1100°C;多晶，Ton.高于1250°C），但不取决于莫来石的组成。硅线石，这是结构密切相关的莫来石，显示没有Cp异常。莫来石的相变潜热（ΔH= 0）和Cp异常的形状与玻璃形成材料在玻璃化转变时的行为相当。观测结果可以最好地解释为相邻结构位点T和T* 以及O（C）和O（C）* 之间的离子跳跃的开始。其他解释，即，结构单元的顺时针和逆时针旋转，或在单晶表面观察到的莫来石分解为α-Al 2 O3 + SiO2玻璃的可能性较小。
The heat capacity at constant pressure (Cp) of mullite (single crystalline and dense and powdered polycrystalline) was determined between −125° and 1400°C. Measurements were performed in three laboratories using two different disk-type differential scanning calorimeters (DLR and Netzsch) and one cylinder-type (Setaram). On the basis of these experimental data a new master curve Cp (T) is presented. Up to about 1100°C the Cp curve agrees well with those from previously published results: The Cp master curve displays a parabolic shape with Cp values of about 0.415 J/(g·K) at −125°C, 0.78 J/(g·K) at 25°C, and 1.25 J/(g·K) at 1000°C. At temperatures above 1100°C a yet unknown, steplike, weak Cp increase is observed with ΔC°p step values ranging between 0.03 and 0.10 J/(g·K). This Cp anomaly of mullite is reproducible under various heating conditions and is reversible both on heating and on cooling. The extrapolated onset temperature of the anomaly (Ton.) is controlled by the crystalline state of the samples (single crystal, Ton. above 1100°C; polycrystal, Ton. above 1250°C) but does not depend on the composition of mullite. Sillimanite, which is structurally closely related to mullite, displays no Cp anomaly. The absence of latent heat (ΔH= 0) and the shape of the Cp anomaly of mullite are comparable to the behavior of glass-forming materials at the glass transition. The observations can best be interpreted in terms of an onset of ion-jumping between adjacent structural sites T and T* and O(C) and O(C)*. Other interpretations, i.e., clockwise and counterclockwise rotations of structural units, or minor decomposition of mullite to α-Al2O3 plus SiO2 glass observed at the surface of the single crystals, are less probable.