Fragile-to-Strong Crossover in Supercooled Liquid Ag-In-Sb-Te Studied by Ultrafast Calorimetry

Phase-change random-access memory relies on the reversible crystalline-glassy phase change in chalcogenide thin films. In this application, the speed of crystallization is critical for device performance: there is a need to combine ultrafast crystallization for switching at high temperature with high resistance to crystallization for non-volatile data retention near to room temperature. In phase-change media such as nucleation-dominated Ge2Sb2Te5, these conflicting requirements are met through the highly “fragile” nature of the temperature dependence of the viscosity of the supercooled liquid. The present study explores, using ultrafast-heating calorimetry, the equivalent temperature dependence for the growth-dominated medium Ag-In-Sb-Te. The crystallization shows (unexpectedly) Arrhenius temperature dependence over a wide intermediate temperature range. Here it is shown that this is evidence for a fragile-to-strong crossover on cooling the liquid. Such a crossover has many consequences for the interpretation and control of phase-change kinetics in chalcogenide media, helping to understand the distinction between nucleation- and growth-dominated crystallization, and offering a route to designing improved device performance.J.O., D.W.H. and A.L.G. acknowledge support from the Engineering and Physical Sciences Research Council (EPSRC, UK), D.W.H. in part through the EPSRC Centre for Innovative Manufacturing in Photonics. J.O. and A.L.G. acknowledge support from the World Premier International Research Center Initiative (WPI), MEXT, Japan. C. A. Angell, L. Battezzati, G. Dalla Fontana and M. Salinga are thanked for helpful discussions.This is the author accepted manuscript. The final version is available from Wiley at http://onlinelibrary.wiley.com/doi/10.1002/adfm.201501607/abstract

Fast crystal growth in glass-forming liquids

In liquids of high glass-forming ability, in which crystal growth rates are low, the rates can be measured over the full range of supercooling from the liquidus temperature down to the glass transition. For systems of low glass-forming ability, growth rates are readily measured at small supercooling and at very large supercooling around the glass-transition temperature, but it is difficult to acquire data over the full range of intermediate supercooling, especially at the maximum in growth rate. Data at intermediate supercoolings are however of considerable interest for understanding glass formation in such systems as pure metals and chalcogenides for phase-change data storage. We will review the methods emerging for making such measurements, and will note that the fragility of the liquid (including possible crossover from ‘fragile’ to ‘strong’ liquid behaviour on cooling) is an important part of understanding fast crystal growth. We also note that there are deficiencies in existing theories of fast crystal growth.oa-900

Thermomechanical processing of metallic glasses: Extending the range of the glassy state

© 2016 Macmillan Publishers Limited, part of Springer Nature. For crystalline metals, the science, technology and application of thermomechanical processing are established, but this is not true for glasses. Metallic glasses-because they can be plastically deformed-offer a unique opportunity to study the effects of thermomechanical treatments on the structure and properties of glasses. Depending on the rate of cooling, various glassy states can form from a liquid. Slower cooling gives states of lower enthalpy and smaller volume; such states might also be reached by annealing, which induces structural 'relaxation'. A reduction in the degree of relaxation, or 'rejuvenation', is achievable through processes such as irradiation and mechanical deformation. In this Review, we explore the extent of relaxation and rejuvenation induced by thermomechanical processing (that is, elastic and plastic deformation, including cold and hot working, and cyclic loading). The issues that remain to be investigated and the prospects for further progress are discussed.Y.H.S. is supported by a China Scholarship Council (CSC) scholarship, and A.L.G. by the Engineering and the Engineering and Physical Sciences Research Council, UK and the World Premier International Research Center Initiative (WPI), MEXT, Japan

Thermal rejuvenation of an aged Au-based metallic glass by fast scanning calorimetry

A metallic glass (MG) annealed above its glass-transition temperature Tg, and cooled, may show an enthalpy increase ΔH, and other property changes. The extent of this thermal rejuvenation depends on the state of the MG (represented by effective cooling rate Φi) and the post-anneal cooling rate Φc. Previous studies examined effects of (Φc/Φi) up to 102. With a Au-based MG aged for up to 10 years at room temperature, and using fast calorimetry to anneal and then cool at up to 5000 K s−1, we extend (Φc/Φi) to 107. The rejuvenation is limited by anneal temperature or by Φc, when, for all MGs, ΔH/Tg shows a universal approximate scaling with log(Φc/Φi). We detect decoupling of vitrification from α relaxation, and highlight limitations in the use of fictive temperature to characterize glassy states. Rejuvenation of the Au-based MG decreases its elastic modulus and hardness, extending trends reported for other MGs

18S rRNA methyltransferases DIMT1 and BUD23 drive intergenerational hormesis

Heritable non-genetic information can regulate a variety of complex phenotypes. However, what specific non-genetic cues are transmitted from parents to their descendants are poorly understood. Here, we perform metabolic methyl-labeling experiments to track the heritable transmission of methylation from ancestors to their descendants in the nematode Caenorhabditis elegans (C. elegans). We find heritable methylation in DNA, RNA, proteins, and lipids. We find that parental starvation elicits reduced fertility, increased heat stress resistance, and extended longevity in fed, naïve progeny. This intergenerational hormesis is accompanied by a heritable increase in N6\u27-dimethyl adenosine (

Reversible migration of silver on memorized pathways in Ag-Ge<inf>40</inf>S<inf>60</inf> films

Reversible and reproducible formation and dissolution of silver conductive filaments are studied in Ag-photodoped thin-film Ge40S60 subjected to electric fields. A tip-planar geometry is employed, where a conductive-atomic-force microscopy tip is the tip electrode and a silver patch is the planar electrode. We highlight an inherent “memory” effect in the amorphous chalcogenide solid-state electrolyte, in which particular silver-ion migration pathways are preserved “memorized” during writing and erasing cycles. The “memorized” pathways reflect structural changes in the photodoped chalcogenide film. Structural changes due to silver photodoping, and electrically-induced structural changes arising from silver migration, are elucidated using Raman spectroscopy. Conductive filament formation, dissolution, and electron (reduction) efficiency in a lateral device geometry are related to operation of the nano-ionic Programmable Metallization Cell memory and to newly emerging chalcogenide-based lateral geometry MEMS technologies. The methods in this work can also be used for qualitative multi-parameter sampling of metal/amorphous-chalcogenide combinations, characterizing the growth/dissolution rates, retention and endurance of fractal conductive filaments, with the aim of optimizing devices.This work was supported by World Premier International Research Center Initiative (WPI), MEXT, Japan. J.O. and A.L.G. acknowledge support from the Engineering and Physical Sciences Research Council (EPSRC, UK).This is the final version of the article. It first appeared from AIP via http://dx.doi.org/10.1063/1.492700

Do Human Resource Practices, Employee Remuneration And Employee Benefits Have Significant Influence On The Retention Of Childcare Teachers In The Childcare Service Industry?

This study investigates the impact of human resource practices, employee remuneration and employee benefits on childcare teachers' intentions to stay in the Singaporean workforce. This mixed methods study focuses on current qualified childcare teachers, their perspectives on human resource practices, employee remuneration and employee benefits in their workplaces and their intentions regarding whether to continue working in the childcare sector in Singapore. In the first phase of the study, which was qualitative, in-depth interviews were conducted with 21 current qualified childcare teachers. In the second phase of the study, which was quantitative, 202 current qualified childcare teachers in Singapore participated in a paper-based and web-based survey. The findings indicate that childcare teachers in Singapore want to be rewarded with fair human resource practices and employee wages are not the only dominant factor on the retention of childcare teachers in the Singaporean childcare industry. This study addresses a knowledge gap about childcare teacher retention in the childcare service industry in Singapore

Atomistic modelling of thermal-cycling rejuvenation in metallic glasses

Cycling of a metallic glass between ambient and cryogenic temperatures can induce higher-energy states characteristic of glass formation on faster cooling. This rejuvenation, unexpected because it occurs at small macroscopic strains and well below the temperatures of thermally induced structural change, is important, for example, in improving plasticity. Molecular-dynamics simulations elucidate the mechanisms by which thermal cycling can induce relaxation (reaching lower energy) as well as rejuvenation. Thermal cycling, over tens of cycles, drives local atomic rearrangements progressively erasing the initial glass structure. This arises mainly from the heating stage in each thermal cycle, linked to the intrinsic structural heterogeneity in metallic glasses. Although, in particular, the timescales in MD simulations are shorter than in physical experiments, the present simulations reproduce many physically observed effects, suggesting that they may be useful in optimizing thermal cycling for tuning the properties of metallic glasses and glasses in general

Multicomponent bulk metallic glasses with elevated-temperature resistance

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