Presence Of Structural Defects Research Articles

Synthesis, Structural and Optical Properties of Mn-Doped Ceria Nanoparticles: A Promising Catalytic Material

The structure, the morphology and the thermal, optical and the surface properties of nanocrystalline CeO2 doped with Mn have been studied by X-ray diffraction (XRD), field-emission transmission electron microscopy (FE-TEM), energy-dispersive X-ray analysis, thermogravimetric analysis, UV–Vis absorption spectroscopy and Fourier transform infrared spectroscopy. The XRD results confirmed the successful incorporation of Mn into the CeO2 lattice through the formation of nanoscale face-centered cubic solid solution. The FE-TEM observations supported the nanocrystalline nature of the solid solutions. The presence of structural defects and their role on the band gap have been discussed on the basis of absorption spectral studies. The structural differences correlate with results from temperature-programmed reaction (TPR) experiments with H2 consumption. The TPR measurements showed an enhanced bulk reduction at much lower temperatures, indicating increased oxygen mobility in the samples, which enable to enhanced oxygen diffusion at lower temperatures.

Loop statistics in polymers in crowded environment

We analyze the probability to find a single loop in a long flexible polymer chain in disordered environment in d dimensions. The structural defects are considered to be correlated on large distances r according to a power law ∼r(-a). Working within the frames of continuous chain model and applying the direct polymer renormalization scheme, we obtain the values of critical exponents governing the scaling of probabilities to find the loops of various positions along the chain as function of loops' length. Our results quantitatively reveal that the presence of structural defects in environment decreases the probability of loop formation in polymer macromolecules.

Anillos vasculares y compresión traqueo-esofágica: 15 años de experiencia quirúrgica

Introduction and objectivesVascular rings are rare anomalies where the diagnosis and surgical approach can be controversial. The goal of this article is to review our hospital surgical results and follow-up. MethodsDescriptive observational study of patients with vascular rings operated on in our hospital from the year 2000 to 2015. ResultsThe study included 26 patients with a median age of 9 months (range: 1 month - 38 years). Patients were classified into 3 groups: right aortic arch - left ductal ligament (n=14), double aortic arch (n=9), and pulmonary sling (n=3). Prenatal diagnosis was made in 3 cases (11.5%). In all cases the diagnosis was made by computed tomography (CT). Kommerell's diverticulum was observed in 11 patients (42.3%), and 19.2% showed intracardiac anomalies that required surgery.The main surgical approach was left thoracotomy with 72.7%. Tracheal surgery was required in 9.1% and 13.6% required re-operation of the vascular ring.The need for re-operation was associated with previous mechanical ventilation, tracheomalacia, and the persistence of respiratory symptoms. There was no hospital mortality. The late mortality was 7.6%, due to associated major malformations. ConclusionsThe reference method for the diagnosis of vascular ring is computed tomography.Bronchoscopy before and after surgical treatment allows the outcome of the technique to be assessed and, in some cases, helps to assess the need for aortopexy.Tracheomalacia and the presence of structural defects in the airway, along with the association of severe congenital heart disease, are factors that are associated with late mortality.

Structural characterization and photoluminescence behavior of pure and doped potassium strontium niobates ceramics with tetragonal tungsten–bronze structure

KSr2Nb5O15 and K1.2Sr1.6Ln0.2Nb5O15 (Ln=La, Pr, and Nd) were synthesized by the Pechini method and calcined at 1373K for 10h under air atmosphere. Rietveld refinements indicated the presence of structural defects in both KSN and KSN-Ln, especially distortions in O–Nb–O and Nb–O–Nb bonds. Modifications in bond distances of [NbO6] clusters were also confirmed by Raman spectroscopy. Ultraviolet-visible absorption spectra and photoluminescence profiles were influenced by lanthanide atoms in the band structure of modified KSN materials, creating new defects and intermediary energy levels within the band gap.

Cassava-starch-assisted sol–gel synthesis of CeO2 nanoparticles

Cerium oxide (CeO2) nanoparticles were successfully synthesized using native cassava starch as a size-limiting chelating agent. X-ray diffraction (XRD) and Raman spectroscopy analysis revealed that all synthesized samples exhibited a cubic CeO2 structure. Transmission electron microscopy (TEM) and XRD observations showed that increasing the calcination temperature increased the average crystallite particle sizes from 8.1 to 12.7nm. X-ray photoelectron spectroscopy (XPS) and Raman results also revealed that reduction in the valence of Ce4+ to Ce3+ caused an increase in the molar fraction of oxygen vacancies, resulting in the presence of structural defects in the CeO2 lattice. The cassava-starch-assisted sol–gel method proved to be highly efficient for the synthesis of highly crystalline CeO2 nanoparticles with potential for industrial applications.

One-step preparation of multiwall carbon nanotube/silicon hybrids for solar energy conversion

The hybrid material consisting of a thin layer of multiwall carbon nanotubes (MWCNTs) on an n-doped silicon wafer was obtained in one step using an aerosol-assisted catalytic chemical vapor deposition. The MWCNTs were grown from a mixture of acetone and ethanol with ∼0.2 wt.% of iron polyoxomolybdate nanocluster of the keplerate-type structure. The samples produced at 800°C and 1050°C were tested as a solar energy converter. It was shown that photoresponse of the hybrid material significantly depends on the presence of structural defects in MWCNTs, being much higher in the case of more defective nanotubes. This is because defects lead to p-doping of nanotubes, whereas the p−n heterojunction between MWCNTs and silicon provides a high efficiency of the solar cell.

Parametric Study of Electrospun Cellulose Acetate in Relation to Fibre Diameter

The objective was to identify the main factors and interactions influencing the fibre diameter in the production of electrospun cellulose acetate (CA) webs. A systematic parameter study was completed for producing electrospun CA fibres that were substantially free of bead defects and the effect of different process parameters during electrospinning CA were evaluated in respect of mean fibre diameter. The experiments were planned using factorial designs. Altogether three parameters, each at three levels, were selected for this study. The results indicate that polymer concentration, voltage and flow rate interact so that the magnitude of any change in the mean fibre diameter is dependent upon the level of the other factors. Furthermore, stable electrospinning conditions for CA were confirmed using an acetone: N, N-dimethylacetamide (DMAc) (2:1) solvent system that minimised the presence of structural defects in the web and promoted uniform fibre diameters.

Electrochemically Exfoliated Graphene and Graphene Oxide for Energy Storage and Electrochemistry Applications.

Top-down methods are of key importance for large-scale graphene and graphene oxide preparation. Electrochemical exfoliation of graphite has lately gained much interest because of the simplicity of execution, the short process time, and the good quality of graphene that can be obtained. Here, we test three different electrolytes, that is, H2 SO4 , Na2 SO4 , and LiClO4 , with a common exfoliation procedure to evaluate the difference in structural and chemical properties that result for the graphene. The properties are analyzed by means of scanning transmission electron microscopy (STEM), Raman spectroscopy, and X-ray photoelectron spectroscopy. We then tested the graphene materials for electrochemical applications, measuring the heterogeneous electron transfer (HET) rates with a Fe(CN)6 (3-/4-) redox probe, and their capacitive behavior in alkaline solutions. We correlate the electrochemical features with the presence of structural defects and oxygen functionalities on the graphene materials. In particular, the use of LiClO4 during the electrochemical exfoliation of graphite allowed the formation of highly oxidized graphene with a C/O ratio close to 4.0 and represents a possible avenue for the mass production of graphene oxide as valid alternative to the current laborious and dangerous chemical procedures, which also have limited scalability.

Microscopic And Spectroscopic Research Of The MWCNTs-Re Nanocomposites

Abstract The combination of TEM research and Raman spectroscopy to characterization of MWNTs-Re nanocomposites gives a new notion about the structure and quality of materials obtained. TEM studies indicate that the functionalization method significantly influences the morphology of obtained MWCNTs-Re nanocomposites. Due to the specific spectrum recorded for the MWCNTs they can be distinguished from other forms of carbon, furthermore comparative analysis of the results at different stages of the manufacturing process confirms the covalent modification of the MWCNTs structure. The D-band intensity compared to the G-band intensity provides valuable information about the quality of the sample, in particular indicates the existence of contamination and/or the presence of structural defects. Preliminary results suggest that the high-temperature manufacturing process of MWCNTs-Re nanocomposite improves the quality of the carbon material intended for the experiment.

An investigation into the effect of isothermal annealing on the structure (XRD), microstructure (SEM, TEM) and magnetic properties of amorphous ribbons and bulk amorphous plates

Abstract Test samples of the alloy Fe61Co10Y8W1B20 were fabricated in the form of both amorphous ribbons and bulk amor-phous plates. The structure, microstructure and magnetic properties of samples in the as-quenched state and after annealing were investigated. The aim of the paper is to show the effect of the thermal treatment on the structural changes occurring in the amorphous state. The thermal treatment, performed at well below the crystallisation temperature, did not cause crystallisation. In the investigated amorphous alloys it has been found that, after a specific thermal treatment, structural relaxation occurs in the presence of structural defects, such as quasidislocational dipoles. Also, supporting the microscopy investigations with measurements of the magnetic properties can be extremely beneficial for facilitating description of the real structure and properties of the amorphous alloys; further, this testing combination offers the possibility for designing the optimal application parameters for an alloy.

Energy-level alignment and electrical properties of Al/p-type Si Schottky diodes with sorbitol-doped PEDOT:PSS as an organic interlayer

The current–voltage (I–V) characteristics of Al/p-type Si Schottky diodes with pristine and 5wt.% sorbitol-doped organic PEDOT:PSS interlayers have been investigated. It was found that the barrier heights of the diodes with pristine and sorbitol-doped PEDOT:PSS are higher than those of a conventional Al/p-type Si Schottky diode because of the modification of the effective barrier height by the organic interlayer. It is noted that the Al/p-type Si Schottky diode with sorbitol-doped PEDOT:PSS interlayer exhibits higher barrier height and lower series resistance, as compared to Al/p-type Si Schottky diodes with pristine PEDOT:PSS interlayer. In addition, UPS measurements indicate that the position of the HOMO level is closer to the Fermi level for pristine PEDOT:PSS than for sorbitol-doped PEDOT:PSS. As a result, the hole injection barrier height is 0.15eV lower in the pristine PEDOT:PSS structure than in the sorbitol-doped PEDOT:PSS. This value closely matches the difference in barrier heights (0.12eV) seen in the I–V measurements. An energy-level alignment diagram is also presented and discussed for the structures studied. The Poole–Frenkel mechanism is found to dominate the reverse leakage current in both structures, indicating the presence of structural defects and trap levels in the organic film.

Structural studies and luminescence properties of CeO2:Eu3+ nanophosphors synthesized by oxalate precursor method

A novel synthesis strategy to prepare CeO2:Eu3+ nanophosphors and its luminescence behavior is reported. Different structural characterization techniques such as X-ray diffraction, transmission electron microscope and thermogravimetric analysis reveal that thermal decomposition of oxalate precursor is an effective pathway to produce rare earth oxide nanocrystals. Optical characterizations of the CeO2:Eu3+ were done by UV–Visible absorption, photoluminescence excitation and emission spectra. The presence of structural defects and their role on the band gap and luminescence were discussed on the basis of absorption and emission studies. Luminescence study of the CeO2:Eu3+ ensures that the strong charge transfer band of CeO2 makes it a suitable host material for efficiently exciting Eu3+ ions by subsequent energy transfer. The dependence of luminescence efficiency of the CeO2:Eu3+ with varying concentrations of Eu3+ was also studied and discussed. The results show that Eu3+-doped CeO2 nanophosphor is a potential candidate in ultraviolet-based LEDs.

Nonlinear optical imaging of defects in cubic silicon carbide epilayers.

Silicon carbide is one of the most promising materials for power electronic devices capable of operating at extreme conditions. The widespread application of silicon carbide power devices is however limited by the presence of structural defects in silicon carbide epilayers. Our experiment demonstrates that optical second harmonic generation imaging represents a viable solution for characterizing structural defects such as stacking faults, dislocations and double positioning boundaries in cubic silicon carbide layers. X-ray diffraction and optical second harmonic rotational anisotropy were used to confirm the growth of the cubic polytype, atomic force microscopy was used to support the identification of silicon carbide defects based on their distinct shape, while second harmonic generation microscopy revealed the detailed structure of the defects. Our results show that this fast and noninvasive investigation method can identify defects which appear during the crystal growth and can be used to certify areas within the silicon carbide epilayer that have optimal quality.

Carbon and Inorganic Nanostructure Reinforced Polymeric Nanocomposites for Bone Tissue Engineering

Introduction: Poly(propylene fumarate) (PPF) is a biocompatible, biodegradable polymer widely investigated for bone tissue engineering applications. Various nanomaterials such as fullerenes, single- and multi- walled carbon nanotubes (SWCNTs, MWCNTs) and alumoxane nanoparticles have been used as reinforcing agents to improve the mechanical properties of PPF nanocomposites [1, 2, 3]. In this study, we have investigated the mechanical properties of PPF nanocomposites fabricated using various 1D and 2D carbon and inorganic nanomaterials such as single- and multi- walled carbon nanotubes (SWCNT, MWCNT), single- and multi- walled graphene oxide nanoribbons (SWGONR, MWGONR), graphene oxide nanoplatelets (GONP), tungsten di-sulfite nanotubes (WS2) and molybdenum di-sulfite nanoplatelets (MSNP) as reinforcing agents [4]. The objectives of this study were to investigate (1) the effects of nanostructure architecture (1D nanostructure (e.g. nanotube) vs. 2D nanostructure (e.g. graphene)), and (2) effects of chemical composition (inorganic vs. carbon nanoparticles) of reinforcing agents on the mechanical properties of PPF. Materials and Methods: PPF was synthesized using a two-step condensation reaction of diethyl fumarate and propylene glycol as described previously and characterized by H1-NMR (structure) and GPC (molecular weight). Longitudinal unzipping method was used to synthesize SWGONRs and MWGONRs form SWCNTs and MWCNTs, respectively. Modified Hummer’s method was used to synthesize GONPs. Inorganic nanomaterials (WS2 nanotubes and MSNPs) were synthesized using literature methods. Nanomaterials were characterized using Raman spectroscopy, AFM and HRTEM. Specimens were thermally crosslinked (60°C) into sample sizes according to ASTM standards for compression and three-point bend testing. Sol fraction analysis was performed to assess the crosslinking density of PPF nanocomposites after thermal crosslinking. Results and Discussion: PPF nanocomposites with 2D nanostructures showed significant improvements in the mechanical properties compared to positive controls (1D nanostructure reinforced PPF composites) and baseline controls (PPF composites without nanomaterials). For example, 0.2 wt% loading of MSNPs leads to ~ 100% increase in the Young’s modulus compared to PPF composites (baseline control). Additionally, inorganic nanostructures showed better or equivalent mechanical reinforcement compared to carbon nanostructures at all loading concentrations. The results indicate that the extent of mechanical reinforcement is closely dependent on the nanostructure morphology and follows the trend: nanoplatelets > nanoribbons > nanotubes. In general inorganic nanoparticles show equivalent or better mechanical reinforcement compared to carbon nanoparticles. TEM analysis of crosslinked nanocomposites indicates good dispersion of nanomaterials in the polymer matrix. Sol-fraction analysis showed increase in the crosslinking density of PPF nanocomposites at all concentrations of MSNP and WS2, and higher concentrations of GONP and MWGONR (0.1-0.2 wt%). These results suggest that the increase in the mechanical properties of 2D carbon and inorganic nanocomposites is due to increases in the crosslinking density of PPF (maybe due to formation of PPF-nanomaterial crosslinking). Additionally, presence of structural defects and functional groups on the nanostructures can lead to increased polymer-nanomaterial interaction, thereby increasing the mechanical properties. Conclusion: Carbon (SWCNT, MWCNT, MWGONR, SWGONR and GONP) and inorganic- nanoparticles (WS2 nanotubes and MSNPs) as reinforcing agents significantly improve the mechanical properties of PPF. In general, 2D and inorganic nanostructures show equivalent or better mechanical reinforcement than 1D and carbon nanostructures, respectively. Figure Caption: Compressive Modulus of PPF Nanocomposites

Capacitance of p‐ and n‐Doped Graphenes is Dominated by Structural Defects Regardless of the Dopant Type

Graphene materials possess attractive properties that can be used for the fabrication of supercapacitors with enhanced energy-storage performance. It has been shown that both boron and nitrogen doping of graphene can improve the intrinsic capacitance of the material relative to the undoped precursor. We address the question of whether p-doping (using boron as dopant) or n-doping (using nitrogen as dopant) leads to increased capacitance relative to undoped graphene materials. Using thermal exfoliation we synthesized both boron- and nitrogen-doped graphene materials and measured capacitance relative to the undoped material. After a full characterization by SEM analysis, X-ray photoelectron spectroscopy, Raman spectroscopy, gamma-ray activation analysis, Brunauer-Emmett-Teller analysis, and electrochemical techniques we demonstrate that the doping process does not lead to enhancement of capacitive behavior and that the main characteristic influencing capacitance is the presence of structural defects within the graphitic structure, independent of doping level.

Monte Carlo investigations of the influence of structural defects on aging effects and the violation of the fluctuation-dissipation theorem for a nonequilibrium critical behavior in the three-dimensional Ising model

The specific features of a nonequilibrium critical behavior in the three-dimensional structurally disordered Ising model have been studied numerically by the Monte Carlo method. An analysis of the two-time dependence of the autocorrelation function and the dynamic susceptibility for systems with spin concentrations p = 0.8 and 0.6 has revealed the aging effects, which are characterized by a slowing down of the relaxation of the system with an increase in the waiting time, and the violation of the fluctuation-dissipation theorem. The values of the universal limit of fluctuation-dissipation ratio for the considered systems have been obtained using the Monte Carlo method. It has been shown that the presence of structural defects in the system leads to an enhancement of the aging effects and to an increase of the values of the limit of fluctuation-dissipation ratio.

Rotating Electromagnetic System for Railway Track Crack Detection

The aim of this paper is to provide an alternative methodology, high reliability, in order to monitor, exploiting non-invasive techniques, railway track. In this particular case study, the presence of structural defects is assessed through an innovative system. The proposed approach provides the use of rotating magnetic field. Within this framework, the Eddy Current Techniques (ECTs) have greatly increased their importance for their capacity to detect the magnetic field variations caused by the presence of material alterations. Whilst the researches in this field propose various solutions to the detection of flaws (in surface and subsurface), a still open problem concerns the difficult detection of defects due to the geometrical complexity of the structure and the particular orientations of the crack. Particularly, this contribution proposes an approach based on Finite Element (Finite Element Method, FEM) for the modeling of railway tracks, in order to obtain a rapid and precise assessment about their integrity.

Excitonic recombinations inh−BN: From bulk to exfoliated layers

Hexagonal boron nitride (h-BN) and graphite are structurally similar but with very different properties. Their combination in graphene-based devices meets now a huge research focus, and it becomes particularly important to evaluate the role played by crystalline defects in them. In this work, the cathodoluminescence (CL) properties of hexagonal boron nitride crystallites are reported and compared to those of nanosheets mechanically exfoliated from them. First the link between the presence of structural defects and the recombination intensity of bound-excitons, the so-called D series, is confirmed. Low defective h-BN regions are further evidenced by CL spectral mapping (hyperspectral imaging), allowing us to observe new features in the near-band-edge region, tentatively attributed to phonon replica of exciton recombinations. Second the h-BN thickness was reduced down to six atomic layers, using mechanical exfoliation, as evidenced by atomic force microscopy. Even at these low thicknesses, the luminescence remains intense and exciton recombination energies are not strongly modified with respect to the bulk, as expected from theoretical calculations indicating extremely compact excitons in h-BN.

Mechanochemical synthesis of nanocrystalline nickel-molybdenum compounds and their morphology and application in catalysis: III. Catalytic properties of massive Ni-Mo sulfide catalysts synthesized using mechanochemical activation

The precursors of massive Ni-Mo catalysts for hydrotreating processes, which were synthesized by the mechanochemical activation of a mixture of nickel hydroxocarbonate NiCO3·2Ni(OH)2·nH2O and ammonium paramolybdate (NH4)6Mo7O24 · 4H2O, were sulfidized. Their structure was studied by X-ray diffraction analysis and high-resolution electron microscopy. It was found that MoS2 packets, which are characterized by the presence of structural defects and geometric curvature, are the constituents of the massive catalysts. The catalyst also includes a phase of Ni3S2 and regions containing a complex Ni-Mo-S phase. The catalytic tests of the synthesized catalysts in the model reactions of 1-methylnaphthalene and dibenzothiophene conversions showed that the catalyst whose precursor was a heteropoly compound with the Anderson structure after the stage of mechanochemical activation exhibited the highest activity.

Fatigue Analysis of Railway Wheels Under Combined Thermal and Mechanical Loads

Nowadays, the railway vehicles are widely used in many countries as one of the most important transportation systems. Nucleation and growth of fatigue cracks in railway wheels stem from different factors such as wheel-rail rolling contact, thermal loads between wheel-rail and wheel-brake block created in braking process, presence of structural defects in wheel material, and so forth. Also, increasing speed and axle loads of wheels aggravates these factors. These cracks can reduce wheel life and even in severe cases derailment may occur. Therefore, the thermo-mechanical fatigue problem of wheels is a very important issue as is doing an accurate mechanical and thermal analysis of the investigation and estimation of fatigue life of wheels, and also, the prediction of crack behavior under thermo-mechanical loads is necessary. In this article, stress fields created by combined thermal and mechanical loads in railway wheels is investigated. Thermal stresses are usually created as a result of frictional heating produced by applying brake shoes on the wheel tread and also as a result of the occurrence of slip between wheel and rail at the braking stage. The obtained results confirm the important effects of thermal loads on stress fields and fatigue life of wheels. In this article, thermal loads are determined by modeling the contact of the rail-wheel and two brake blocks and by identifying heat partition factors and friction coefficient between these components. One of the other advantages of the presented work is modeling of wheel rotation, while in many of the similar investigations either this rolling is not modeled or its effect is simplified as translating pressure distribution along the rail-wheel contact region. The use of a 3D FE analysis for determination of rail-wheel contact pressure instead of Hertz contact theory is also noteworthy.