Premium
Heat Capacity Changes at the Glass Transition in Mixed‐Alkali Tellurite Glasses
Author(s) -
Komatsu Takayuki,
Noguchi Tsutomu,
Sato Ryuji
Publication year - 1997
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/j.1151-2916.1997.tb02988.x
Subject(s) - glass transition , supercooling , heat capacity , alkali metal , analytical chemistry (journal) , materials science , crystallography , chemistry , mineralogy , thermodynamics , polymer , organic chemistry , physics , composite material
The heat capacity changes (Δ C p ) at the glass transition in mixed‐alkali tellurite glasses of (20‐ x )Li 2 O‐ x Na 2 O‐ 80 TeO 2 that show different fragilities, depending on the Na 2 O/Li 2 O ratio, have been measured to obtain more‐detailed information on the structure of tellurite glasses or supercooled liquids with a fragile character. A large increase in C p is clearly observed at the glass transition in all the samples. The values of the heat capacity of glasses and supercooled liquids ( C pg and C pe , respectively) and Δ C p (equal to C pe ‐ C pg ) at the glass transition temperature, T g , are almost the same, irrespective of the Na 2 O/(Na 2 O+Li 2 O) ratio, indicating the absence of a clear, mixed‐alkali effect on C p at T g . The values of C pg at 200°C, C pe at 300°C, and Δ C p at T g are } 75, 120, and 45 J·(mol·K) −1 , respectively. The ratio of the heat capacity of the glassy state to the heat capacity of the supercooled liquid state, i.e., C pe / C pg , is 1.6; this large value indicates that the (20‐ x )Li 2 O x Na 2 O 8 0TeO 2 system should be included in the category of fragile liquids. It has been suggested that the structural units of TeO 4 trigonal bipyramid and TeO 3 trigonal pyramid are weakly connected to each other and, thus, the intermediate structure varies largely as the temperature increases, which leads to a fragile character. The change in the fragility that is caused by the mixing of dissimilar alkali ions is not detected through Δ C p (i.e., there is no mixed‐alkali effect on thermodynamic fragility); however, it is observed from the viscosity behavior at } }T g (i.e., the presence of the mixed‐alkali effect on kinetic fragility).