ICTAC Kinetics Committee recommendations for performing kinetic computations on thermal analysis data

The various origins of a frequency-dependent conductivity in amorphous semiconductors are reviewed, stressing particularly recent advances and the influences that factors such as correlation and non-random spatial distributions of electrically active centres can have on the a.c. conductivity. A comprehensive survey is given of the experimental a.c. data for two types of amorphous semiconductor, namely chalcogenide and pnictide materials. It is concluded that the a.c. behaviour at intermediate to high temperatures is well accounted for by the correlated-barrier-hopping model, whereas the low-temperature behaviour is probably due to atomic tunnelling.

A.c. conduction in amorphous chalcogenide and pnictide semiconductors

Very different materials are named “Glass,” with Young's modulus (E) and Poisson's ratio (ν) extending from 5 to 180 GPa and from 0.1 to 0.4, respectively, in the case of bulk inorganic glasses. Although glasses have in common the lack of long-range order in the atomic organization, they offer a wide range of structural features at the nanoscale and we show in this analysis that beside the essential role of elastic properties for materials selection in mechanical design, the elastic characteristics (E, ν) at the continuum scale allow to get insight into the short- and medium-range orders existing in glasses. In particular, ν, the atomic packing density (Cg) and the glass network dimensionality appear to be strongly correlated. Maximum values for ν and Cg are observed for metallic glasses (ν∼0.4 and Cg>0.7), which are based on cluster-like structural units. Atomic networks consisting primarily of chains and layers units (chalcogenides, low Si-content silicate, and phosphate glasses) correspond to ν>0.25 and Cg>0.56. On the contrary, ν<0.25 is associated with a highly cross-linked network, such as in a-SiO2, with a tri-dimensional organization resulting in a low packing density. Moreover, the temperature dependence of the elastic moduli brings a new light on the structural changes occurring above the glass transition temperature and on the depolymerization rate in the supercooled liquid. The softening rate depends on the level of cooperativity of atomic movements at the source of the deformation process, with an obvious correlation with the “fragility” of the liquid.

/pdf/elastic-properties-and-short-to-medium-range-order-in-3swvedsyhi.pdf

Elastic Properties and Short-to Medium-Range Order in Glasses

Results of thermal analysis performed at different heating rates on eight glasses of the As-Sb-Se system, with compositions represented by (As, Sb) 40 Se 60 and As x Sb 15 Se 85 − x are reported and discussed. The values of the glass transition temperature ( T g ), the crystallisation temperature ( T c ) and the peak temperature of crystallisation ( T p ) are found to be independent of particle size. From the heating rate dependence of T g and T p values of the activation energy for glass transition ( E t ) and the activation energy for crystallisation ( E c ) are evaluated and their composition dependence discussed. The crystallisation data are examined in terms of recent analyses developed for non-isothermal crystallisation studies to arrive at E c and details of the crystallisation mechanism. The results indicate bulk nucleation and crystallisation with two and three dimensional growth respectively for the As x Sb 15 Se 85 − x and (As, Sb) 40 Se 60 glasses.

Calorimetric measurements on as-sb-se glasses

Book reviewPhysics of non-crystalline solids: edited by J. A. Prins. 667 pages, diagrams, illus. 6 × 9 in. New York, John Wiley & Sons, Interscience, 1965. Price, $27.50

The results of measurements on the variation of the glass transition temperature (Tg) with the average coordination number (Z) are reported and discussed for glasses of the Ge-In-Se system. The study covers families of compositions with 5 and 8 at.% In, as well as compositions along the GeSe2-In2Se3 tie line. In the Tg-Z data of these glasses, a change in slope is seen at Z approximately equal to 2.40. In addition, maxima in Tg are observed at Z values corresponding to the tie line compositions of the system. An account of these features is given by the existing Phillips model and the chemically ordered network model. Some of the features observed can be explained by the formation of specific structural units in these glasses. The explanations corroborate our conclusion that, in addition to the features which have a topological origin, effects due to local chemical ordering are also observed in the glasses of this system.

The Tg versus Z dependence of glasses of the GeInSe system

The results of measurements of microhardness and density on families of glasses of the GeSbSe system are reported. Microhardnes shows a monotonic increase with Ge content for all the families of glasses studied. The stoichiometric composition in each family exhibits a minimum in density and a maximum in molar volume as compared with the Se-rich (Ge-rich) glasses of the corresponding family. A calculation based on the volume additivity of the individual elements and compounds satisfactorily explains the observed dependence of density on composition. The density results further indicate that the molar volume occupied by each of the glasses with composition (GeSe2)1−C (Sb2Se3)C is nearly constant and is equal to 18.05 ± 0.06 cc/mol.

Density and microhardness of GeSbSe glasses

The dependence of mean atomic volume and the glass transition temperature on the average coordination number, lt;igt;Zlt;/igt;, have been examined for glasses of the Ge-Sb(As)-Se systems, in the light of recent models proposed for network glasses. The results indicate the occurrence of additional features at lt;igt;Zlt;/igt; values other than the topological thresholds of amp;sime;2.40 and amp;sime;2.67. These additional features are attributed to effects of chemical ordering. The occurrence of these features in addition to those due to the two topological effects, namely, the floppy to rigid transition and the structural transition, leads to the conclusion that the effects of the chemical ordering and the effects which have topological origin coexist in these glasses

Floppy to rigid transition and chemical ordering in GeSb(As)Se glasses

Results are reported of measurements of the glass transition temperature ( T g ), conductivity and density ( d ) for glasses of the AsSbSe system, with compositions (As, Sb) 40 Se 60 and As x Sb 15 Se 85− x . The results are compared with glasses of similar compositions of the As x Se 100− x about compositions in the Ge x Sb 15 Se 85− x and As x Se 100− x glasses, in the case of As x Sb 15 Se 85− x glasses, the As-rich compositions exhibit higher values of T g and d compared to the stoichiometric composition As 25 Sb 15 Se 60 . These results are discussed in the light of a chemically ordered structural arrangement in these glasses.

A Giridhar

Papers

Calorimetric measurements on as-sb-se glasses

The Tg versus Z dependence of glasses of the GeInSe system

Density and microhardness of GeSbSe glasses

Floppy to rigid transition and chemical ordering in GeSb(As)Se glasses

Studies on the AsSbSe glass system