Perovskite type lanthanum manganite: Morpho-structural analysis and electrical investigations

Perovskite-type oxides of LaMnO_3 were synthesized by means of the sol-gel method, in the presence of citric acid as gelling agent. The precursors used were Mn(NO_3)_2·H_2O, La_2O_3, and NaOH,mixed in the stoichiometric ratio to obtain perovskite materials. The obtained gel was heat-treated at 400, 600 and 800 ℃ respectively, for 6 h. X-ray diffraction and FT-IR spectroscopy were used to analyze the phase transformation as a function of temperature, and the Rietveld refinement was used in order to characterize the materials obtained structurally. The average crystallite size of the products was calculated from XRD data and the average particle size was measured from the TEM micrographs. At 600 ℃, the synthesized compound is well-crystallized, showcases a perovskite structure(Pm-3m space group), and exhibits uniform and homogeneous hexagonally-shaped particles, with sizes in the 20-50 nm range.Complex impedance measurements in the 20-2 × 10~6 Hz frequency range were carried out at different temperatures(26-115 ℃), and the electrical conduction mechanism is discussed.     

Simple model of the optical characteristics of bubbles and sediments in seawater of the surf zone

The development of a simple model of the seawater inherent optical properties (IOPs) associated with bubbles and sediments would represent a great advance in surf zone optics. We present one solution for this problem using a combination of geometrical optics and Fraunhofer diffraction. An analytic model of the IOPs of bubbles and sediments (the extinction and absorption coefficients, and phase function) is developed in terms of the moments of the particle size distribution and the complex refractive index of particles.     

Making Predictions of Global Warming Impacts Using a Semantic Web Tool that Simulates Fuzzy Cognitive Maps

Computational Economics - One of the most important environmental problems of our era is Global Warming (GW), which derives its roots mainly from anthropogenic activities and is expected to cause...     

Programs with lists are counter automata

We address the problem of verifying programs manipulating one-selector linked data structures. We propose and study in detail an application of counter automata as an accurate abstract model for this problem. We let control states of the counter automata correspond to abstract heap graphs where list segments without sharing are collapsed, and use counters to keep track of the number of elements in these segments. As a significant theoretical result, we show that the obtained counter automata are bisimilar to the original programs. Moreover, from a practical point of view, our translation allows one to apply efficient automatic analysis techniques and tools developed for counter automata (integer programs) in order to verify both safety as well as termination of list-manipulating programs. As another theoretical contribution, we prove that if the control of the generated counter automata does not contain nested loops (i.e., these automata are flat), both safety and termination are decidable for the original programs. Subsequently, we generalise our counter-automata-based model to keep track of ordering properties over lists storing ordered data. Finally, we show effectiveness of our approach by verifying automatically safety as well as termination of several sorting programs.     

Hierarchical adaptive nanostructured PVD coatings for extreme tribological applications: the quest for nonequilibrium states and emergent behavior

Adaptive wear-resistant coatings produced by physical vapor deposition (PVD) are a relatively new generation of coatings which are attracting attention in the development of nanostructured materials for extreme tribological applications. An excellent example of such extreme operating conditions is high performance machining of hard-to-cut materials. The adaptive characteristics of such coatings develop fully during interaction with the severe environment. Modern adaptive coatings could be regarded as hierarchical surface-engineered nanostructural materials. They exhibit dynamic hierarchy on two major structural scales: (a) nanoscale surface layers of protective tribofilms generated during friction and (b) an underlying nano/microscaled layer. The tribofilms are responsible for some critical nanoscale effects that strongly impact the wear resistance of adaptive coatings. A new direction in nanomaterial research is discussed: compositional and microstructural optimization of the dynamically regenerating nanoscaled tribofilms on the surface of the adaptive coatings during friction. In this review we demonstrate the correlation between the microstructure, physical, chemical and micromechanical properties of hard coatings in their dynamic interaction (adaptation) with environment and the involvement of complex natural processes associated with self-organization during friction. Major physical, chemical and mechanical characteristics of the adaptive coating, which play a significant role in its operating properties, such as enhanced mass transfer, and the ability of the layer to provide dissipation and accumulation of frictional energy during operation are presented as well. Strategies for adaptive nanostructural coating design that enhance beneficial natural processes are outlined. The coatings exhibit emergent behavior during operation when their improved features work as a whole. In this way, as higher-ordered systems, they achieve multifunctionality and high wear resistance under extreme tribological conditions.     

Morphology evolution induced by carbon nanotubes on thermal and mechanical characters of semi-crystalline aromatic polyimide

A series of nanocomposites based on a new semi-crystalline polyimide (PI) and multi-walled carbon nanotubes (MWCNTs) were prepared by in situ polymerization. The TEM measurement reveals the improved dispersion of carboxylic acid-functionalized MWCNTs (COOH-MWCNTs) in semi-crystalline PI compared with pristine MWCNTs. The TGA analysis show that the concentration of carboxylic acid groups on the surface of nanotubes is about 4.34 wt%. The FT-IR spectroscopy analysis indicate that the imide rings of the PI interact non-covalently with nanotubes. The Polarized optical microscopy observation reveals significant morphology evolution in semi-crystalline PI induced by MWCNTs. The SEM micrographs suggest the strong interfacial interaction between COOH-MWCNTs and PI main chains, and significant changes in the fracture surfaces morphology. The WAXRD measurements reveal that COOH-MWCNTs promote the semi-crystalline PI crystallinity and structure change. COOH-MWCNTs can more efficiently improve the mechanical and thermal properties of resulting nanocomposites than pristine MWCNTs. COOH-MWCNT/PI nanocomposites show increases of Young’s modulus and yield strength, as high as 20–30 %, without sacrificing the elongation at break at loadings of 0.5 wt% nanotubes. Furthermore, with increasing the loadings of COOH-MWCNTs to 1.0 wt%, Young’s modulus and yield strength decrease due to nanotube aggregation, but elongation at break increase about 46 %. An abrupt increase of elongation at break in pristine MWCNT/PI nanocomposites was also registered at nanotubes loadings increasing from 0.5 to 1 wt%. These results indicate that the properties of semi-crystalline PI nanocomposites reinforced with carbon nanotubes are not only determined by the dispersion of nanotubes in the PI matrix and their interfacial interactions, but also by the crystalline phase morphology evolution in the PI matrix.     

Influence of cobalt ferrite content on the structure and magnetic properties of (CoFe2O4)X (SiO2-PVA)100-X nanocomposites

(CoFe₂O₄)_X(SiO₂-PVA)_100-X (X = 5, 25, 50, 75 and 95%) nanocomposites were prepared via sol-gel route and annealed at 700 and 1100 °C. The influence of CoFe₂O₄ content on the structure, morphology and magnetic properties of nanocomposites was studied. X-ray diffraction patterns, Mössbauer and Fourier transform infrared spectra revealed the formation of CoFe₂O₄ as unique magnetic phase. The crystallinity degree increases with the CoFe₂O₄ content and the annealing temperature. Transmission electron microscopy images revealed the spherical shape of the obtained nanocomposites. Mössbauer spectra exhibit typical magnetic sextets, allowing the calculation of the cations distribution among tetrahedral and octahedral sites and the stoichiometry of CoFe₂O₄. A strong correlation between the particle morphology and the magnetic properties of nanocomposites was found. The highest saturation magnetization was identified for (CoFe₂O₄)₉₅(SiO₂-PVA)₅ nanocomposite.