Simulative Investigation on the Electronic, Vibrational and Optical Properties of the Ge2Sb2Te5 Chalcogenide

Chalcogenides are chemical compounds with at least one of the following three chemical elements: Sulfur (S), Selenium (Sn), and Tellurium (Te). As opposed to other materials, chalcogenide atomic arrangement can quickly and reversibly inter-change between crystalline, amorphous and liquid phases. Therefore they are also called phase change materials. As a results, chalcogenide thermal, optical, structural, electronic, electrical properties change pronouncedly and significantly with the phase they are in, leading to a host of different applications in different areas. The noticeable optical reflectivity difference between crystalline and amorphous phases has allowed optical storage devices to be made. Their very high thermal conductivity and heat fusion provided remarkable benefits in the frame of thermal energy storage for heating and cooling in residential and commercial buildings. The outstanding resistivity difference between crystalline and amorphous phases led to a significant improvement of solid state storage devices from the power consumption to the re-writability to say nothing of the shrinkability. This work focuses on a better understanding from a simulative stand point of the electronic, vibrational and optical properties for the crystalline phases (hexagonal and faced-centered cubic). The electronic properties are calculated implementing the density functional theory combined with pseudo-potentials, plane waves and the local density approximation. The phonon properties are computed using the density functional perturbation theory. The phonon dispersion and spectrum are calculated using the density functional perturbation theory. As it relates to the optical constants, the real part dielectric function is calculated through the Drude-Lorentz expression. The imaginary part results from the real part through the Kramers-Kronig transformation. The refractive index, the extinctive and absorption coefficients are analytically calculated from the dielectric function. The transmission and reflection coefficients are calculated using the Fresnel equations. All calculated optical constants compare well the experimental ones

Electronic, optical and thermal properties of the hexagonal and fcc Ge2Sb2Te5 chalcogenide from first-principle calculations

We present a comprehensive computational study on the properties of face-centered cubic and hexagonal chalcogenide Ge2Sb2Te5. We calculate the electronic structure using density functional theory (DFT); the obtained density of states (DOS) compares favorably with experiments, also looking suitable for transport analysis. Optical constants including refraction index and absorption coefficient capture major experimental features, aside from an energy shift owed to an underestimate of the band gap that is typical of DFT calculations. We also compute the phonon DOS for the hexagonal phase, obtaining a speed of sound and thermal conductivity in good agreement with the experimental lattice contribution. The calculated heat capacity reaches ~ 1.4 x 106 J/(m3 K) at high temperature, in agreement with experimental data, and provides insight into the low-temperature range (< 150 K), where data are unavailable.Comment: 19 pages, 8 figure

High-K dielectric sulfur-selenium alloys.

Upcoming advancements in flexible technology require mechanically compliant dielectric materials. Current dielectrics have either high dielectric constant, K (e.g., metal oxides) or good flexibility (e.g., polymers). Here, we achieve a golden mean of these properties and obtain a lightweight, viscoelastic, high-K dielectric material by combining two nonpolar, brittle constituents, namely, sulfur (S) and selenium (Se). This S-Se alloy retains polymer-like mechanical flexibility along with a dielectric strength (40 kV/mm) and a high dielectric constant (K = 74 at 1 MHz) similar to those of established metal oxides. Our theoretical model suggests that the principal reason is the strong dipole moment generated due to the unique structural orientation between S and Se atoms. The S-Se alloys can bridge the chasm between mechanically soft and high-K dielectric materials toward several flexible device applications

Blaming Active Volcanoes or Active Volcanic Blame? Volcanic Crisis Communication and Blame Management in the Cameroon

This chapter examines the key role of blame management and avoidance in crisis communication with particular reference to developing countries and areas that frequently experience volcanic episodes and disasters. In these contexts, the chapter explores a key paradox prevalent within crisis communication and blame management concepts that has been rarely tested in empirical terms (see De Vries 2004; Brändström 2016a). In particular, the chapter examines, what it calls, the ‘paradox of frequency’ where frequency of disasters leads to twin dispositions for crisis framed as either: (i) policy failure (active about volcanic blame on others), where issues of blame for internal incompetency takes centre stage, and blame management becomes a focus of disaster managers, and/or: (ii) as event failure (in this case, the blaming of lack of external capacity on active volcanoes and thereby the blame avoidance of disaster managers). Put simply, the authors investigate whether perceptions of frequency itself is a major determinant shaping the existence, operation, and even perceived success of crisis communication in developing regions, and countries experiencing regular disaster episodes. The authors argue frequency is important in shaping the behaviour of disaster managers and rather ironically as part of crisis communication can shape expectations of community resilience and (non)-compliance. In order to explore the implications of the ‘paradox of frequency’ further, the chapter examines the case of the Cameroon, where volcanic activity and events have been regular, paying particular attention to the major disasters in 1986 (Lake Nyos Disaster - LND) and 1999 (Mount Cameroon volcanic eruption - MCE)

Nanomaterials Thermal Response and CNT Reinforced Polymer Composites: An ab initio Study

Research achievements, both on nanomaterials thermal response and on reinforced polymer composites, are compendiously submitted. Each of the 4 chapters begins with (1) a terse summary of the context, methodology and keys results; segues into (2) the necessity for and the state-of-the-art on the subject with which it concerns itself; then proceeds with (3) the unique contribution the present findings make and the directions the research takes. Each chapter is self-contained and can be perused independently. Chapter 1 covers the relationship between the hole density of boron monolayers and their thermal as well as mechanical properties. The triangular boron sheet (δ6) is found to possess 2.06 and 6.60 times graphene’s lattice and electronic ballistic thermal conductances. Hexagonal sheets such as α and δ5 are predicted to be roughly twice as stiff as graphene. Chapter 2 covers the meaning of the Debye temperature for bulk and low dimensional materials. Two new approaches, one based on the polarization and mode dependent heat capacities and the other based on the mode dependent Debye temperatures, converge to a more precise computation and understanding of the polarization dependent Debye temperature. Chapter 3 covers the thermal properties of carbyne showing and discussing its room-temperature heat capacity at constant volume being 1.6 times that of graphene, a negative coefficient of thermal expansion being 4 times that of graphene, and a much higher thermal conductivity than graphene nanoribbons. Chapter 4 covers the interaction of carbon nanotubes (CNTs) with DGEBA epoxy thermosets for the purpose of identifying strengthening mechanisms. Among doped (Si, B, N), defective (Stone-Wales, three nitrogen atoms surrounding one monovacancy, four nitrogen atoms surrounding one divacancy, monovacancy), functionalized (amine, hydroxyl, carboxyl, oxygen) and different size CNTs, Si-doped, a combination of oxygen and hydroxyl as well as smaller tubes exhibit the strongest indication for mechanical reinforcement

Effects of landscape context and agricultural practices on the abundance of cotton bollworm Helicoverpa armigera in cotton fields: A case study in northern Benin

International audienceHelicoverpa armigera (H€ubner) (Lepidoptera: Noctuidae) is a polyphagous pest of global importance, threatening several key crops, including cotton. This study analysed the influence of landscape composition and agricultural practices on (i) the abundance of H. armigera larvae, and (ii) the proportion of infested plants (infestation) in cotton fields in northern Benin. In 2011 and 2012, the abundance of H. armigera was monitored during the rainy period, with weekly observations of 50 cotton plants in 20 fields selected each year. We selected cotton fields based upon the composition of the surrounding landscape (cotton, tomato and maize fields, and areas of natural vegetation) within 500 metres radius buffers. We also recorded agricultural practices, included sowing date, crop rotation, and frequencies of weeding and of treatment with insecticides. In testing for a relationship between the pest problem and landscape and management, we fitted logistic multiple regression models and compared all the possible models using an information-theoretic approach. Cotton fields surrounded by cotton crops were found to have significantly higher infestation rates. Natural vegetation was positively correlated with the level of infestation. This study highlights the importance of considering both landscape variables and agricultural practices to improve strategies management of H. armigera

Poly-albumen: Bio-derived structural polymer from polymerized egg white

Bio-derived materials could play an important role in future sustainable green and health technologies. This work reports the synthesis of a unique egg white-based bio-derived material showing excellent stiffness and ductility by polymerizing it with primary amine-based chemical compounds to form strong covalent bonds. As shown by both experiments and theoretical simulations, the amine-based molecules introduce strong bonds between amine ends and carboxylic ends of albumen amino acids resulting in an elastic modulus of ∼4 GPa, a fracture strength of ∼2 MPa and a high ductility of 40%. The distributed and interconnected network of interfaces between the hard albumen and the soft amine compounds gives the structure its unique combination of high stiffness and plasticity. A range of in-situ local and bulk mechanical tests as well as molecular dynamics (MD) simulations, reveal a significant interfacial stretching during deformation and a micro-crack diversion leading to an increased in ductility and toughness. The structure also shows a self-stiffening behavior under dynamic loading and a strength-induced aging suggesting adaptive mechanical behavior. This egg white-derived material could also be developed for bio-compatible and bio-medical applications.by Peter Samora Owuor, Thierry Tsafack, Himani Agrawal, Hye Yoon Hwang, Matthew Zeliskob, Tong Lic, Sruthi Radhakrishnan, Jun Hyoung Park, Yingchao Yang, Anthony S. Stender, Sehmus Ozden, Jarin Joyner, Robert Vajtai, Benny A. Kaipparettu, Bingqing Wei, Jun Lou, Pradeep Sharma, Chandra Sekhar Tiwarya and Pulickel M. Ajaya

Role of Atomic Layer Functionalization in Building Scalable Bottom-Up Assembly of Ultra-Low Density Multifunctional Three-Dimensional Nanostructures

Building three-dimensional (3D) structures from their constituent zero-, one-, and two-dimensional nanoscale building blocks in a bottom-up assembly is considered the holey grail of nanotechnology. However, fabricating such 3D nanostructures at ambient conditions still remains a challenge. Here, we demonstrate an easily scalable facile method to fabricate 3D nanostructures made up of entirely zero-dimensional silicon dioxide (SiO₂) nanoparticles. By combining functional groups and vacuum filtration, we fabricate lightweight and highly structural stable 3D SiO₂ materials. Further synergistic effect of material is shown by addition of a 2D material, graphene oxide (GO) as reinforcement which results in 15-fold increase in stiffness. Molecular dynamics (MD) simulations are used to understand the interaction between silane functional groups (3-aminopropyl triethoxysilane) and SiO₂ nanoparticles thus confirming the reinforcement capability of GO. In addition, the material is stable under high temperature and offers a cost-effective alternative to both fire-retardant and oil absorption materials