Peculiarities of Ga and Te incorporation in glassy arsenic selenides

International audienceEffect of simultaneous Ga and Te addition on the structure of As2Se3 glasses is studied using X-ray photoelectron spectroscopy (XPS), extended X-ray absorption fine structure (EXAFS) and Raman techniques. It is shown that most of As, Se and Te atoms build a covalent network according to their main valences. Three-fold coordinated As atoms form pyramidal structural units, which are connected via bridges of two-fold coordinated chalcogen atoms (Se, Te). On the other hand, coordination of Ga in glassy samples is found to be greater than three, as expected from its valence, increasing with Te content. Some of the As atoms appear to be converted into four-fold coordinated state at low Te concentration, while a fraction of Te and, possibly, Se atoms are thought to exist in a singly-coordinated (terminal) state in the vicinity of Ga in the samples with higher Te concentration

Amorphous rigidification and cooperativity drop in semi−crystalline plasticized polylactide

Plasticization of amorphous polylactide shifts the glass transition and extends its temperature range of crystallization to lower temperatures. In this work, we focus on how low−temperature crystallization impacts the mobility of the amorphous phase. Plasticizer accumulates in the amorphous phase because it is excluded from the growing crystal. The formation of rigid amorphous fraction is favored by the low crystallization temperature. It reaches values up to 50% in plasticized polylactide. The increase in the content of rigid amorphous fraction coincides with both the increase of free volume quantified by positron annihilation lifetime spectroscopy, and the decrease in the cooperativity length obtained from the temperature fluctuation approach. The drop of cooperativity is interpreted in terms of mobility gradient due to the amorphous rigidification

Light-assisted physical aging in chalcogenide glasses: Dependence on the wavelength of incident photons

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Parametrizace fotosvětlání a fototmavnutí in-situ kinetik v napařených vrstvách GeSe2

Thermally evaporated chalcogenide glass thin films are known to be highly photosensitive revealing both metastable and transient photoinduced changes of optical transmission at the fundamental absorption edge region. In-situ kinetics of metastable photobleaching and transient photodarkening as well as following light-off relaxations for as-deposited GeSe2 thin films were studied and fitted with the stretched exponential function. Dependences of kinetic parameters beta and tau a on film thickness, temperature, sample prehistory and wavelength of light irradiation were analyzed. The obtained results were discussed within current approaches to mechanisms of photoinduced kinetic phenomena and structural relaxation in glasses.Vakuově napařené tenké vrstvy chalkogenidových skel jsou známé pro svoji vysokou fotocitlivost odhalující metastabilní a přechodné fotoindukované změny optické propustnosti v regionu fundamentální absorpční hrany. Byly studovány a fitovány n-situ kinetiky metastabilního fotosvětlání a přechodného fototmavnutí čerstvě deponovaných vrstev GeSe2, jakožto i relaxace při vypnutí zdroje expozice. Byla analyzována závislost parametrů kinetiky beta a tau na tloušťce, teplotě, historii vzorku a vlnové délce použitého záření. Získané výsledky byly diskutovány v rámci současných přístupů k mechanismům fotoindukovaných jevů a strukturální relaxace ve sklech

Cooperativity Scaling and Free Volume in Plasticized Polylactide

International audienceThe experimental evidence of the increase of activation energy associated with the super Arrhenius behavior governing amorphous polylactide by free volume variations has been obtained through a combination of calorimetric, dielectric, and positron annihilation lifetime measurements. The amount of free volume in polylactide was controlled by the amount of acetyltributylcitrate plasticizer in the composition. Plasticization is shown to decrease both the fragility index and the scale of cooperative motions at the glass transition. The calculations of volume and energetic components of kinetic fragility reveal that the fragility drop is governed by the change in the size of cooperative rearranging region. As a result, direct correlation has been established between cooperativity and activation energy for the entire plasticized polylactide series. It is also shown that cooperativity variations with both temperature and plasticizer content can be simplified as a master curve with free volume

Cooperativity Scaling and Free Volume in Plasticized Polylactide

International audienceThe experimental evidence of the increase of activation energy associated with the super Arrhenius behavior governing amorphous polylactide by free volume variations has been obtained through a combination of calorimetric, dielectric, and positron annihilation lifetime measurements. The amount of free volume in polylactide was controlled by the amount of acetyltributylcitrate plasticizer in the composition. Plasticization is shown to decrease both the fragility index and the scale of cooperative motions at the glass transition. The calculations of volume and energetic components of kinetic fragility reveal that the fragility drop is governed by the change in the size of cooperative rearranging region. As a result, direct correlation has been established between cooperativity and activation energy for the entire plasticized polylactide series. It is also shown that cooperativity variations with both temperature and plasticizer content can be simplified as a master curve with free volume

Foto-odezva anorganicko-organických tenkovrstevnatých kompozitů na bázi chalkogenidových skel

Spin coated chalcogenide thin films have different structural and chemical properties than the films obtained by traditional methods such as thermal evaporation or sputtering. The solution-based method provides lower sensitivity of glass matrix to the influence of bandgap and superbandgap light. This property is very useful for non-linear optical applications based on high transparency of these materials in infrared spectral region. Arsenic selenide spin coated thin films were obtained by chemical dissolution of bulk glasses in ethylenediamine. The influence of preparation conditions, especially the annealing temperatures at the final stage of thin films synthesis, on in-situ kinetics of photodarkening (bleaching) at various energies and intensities of UV-VIS light was studied. It was found that at certain annealing conditions only transient photoinduced effects can be obtained by eliminating metastable kinetic component. Mechanisms of the photoinduced effects are discussed based on the parameters of relaxation functions.Spin-coatingem deponované chalkogenidové tenké vrstvy mají rozdílnou strukturu a chemické vlastnosti než vrstvy získané tradičními metodami jako je vakuové napařování nebo naprašování. Roztokové depoziční metody poskytují nižší citlivost skelné matrice vůči účinku světla o bandgapových a superbandgapových energií. Tato vlastnost je velice užitečná pro nelineární optické aplikace založené na vysoké propustnosti těchto materiálů v IČ oblasti spektra. Tenké vrstvy selenidu arsenitého byly získány rozpouštěním objemového skla v etylendiaminu. Vliv podmínek během přípravy, zvláště pak teplota temperace v posledních fází přípravy vrstev, na in-situ kinetiky fototmavnutí (světlání) při různých energiích a intenzitách UV-VIS táření byly studovány v této práci. Bylo zjištěno, že pouze při určitých podmínkách temperace lze získat vratné fotoindukované změny díky eliminaci metastabilní kinetické komponenty. Mechanismus fotoindukovaných jevů je dále diskutován na základě parametrů relaxačních funkcí