Morphology of composites of semiconducting polymers mixed with C60

The morphology of polymer composite films of poly[2-methoxy,5-(2′-ethylhexyloxy)-1,4-phenylene-vinylene] (MEH-PPV) and buckminsterfullerene (C₆₀) has been systematically studied over a wide range of concentrations (10–81 wt.% of C₆₀) using transmission electron microscopy (TEM). Phase separation in films prepared from solutions in 1,2-dichlorobenzene results in nano-scale crystallites of C₆₀ (dimensions approximately 10 nm or even less), embedded in amorphous MEH-PPV. A very fine bicontinuous network structure forms for MEH-PPV:C₆₀ compositions near 1:1 by weight. This homogeneous nanostructured network morphology in the polymer composites confirms the formation of a bulk heterojunction material as inferred from the improved optical and electronic properties of these composites in device applications.     

Studies on the nonlinear optical properties of rf plasma polymerized aniline thin films by open aperture z-scan technique

Plasma polymerized polyaniline (PANI) thin films in their pristine and iodine doped forms were subjected to open aperture z-scan studies in order to investigate the nonlinear optical (NLO) properties of these materials. The investigations were carried out using a Q-switched resonant Nd:YAG laser at a wavelength of 532 nm for various fluences. The z-scan studies revealed that RF PANI thin films exhibit a saturable absorption (SA). Iodine doping modifies the NLO characteristics of these films substantially. These results qualify PANI thin films as potential materials for NLO materials. Investigations on the NLO properties of pristine (PANI) and iodine doped polyaniline (dPANI) thin films in its plasma polymerized form are reported for the first time.     

Fracture behaviour and numerical study of resistance spot welded joints in high-strength steel sheet

Cross tension tests of resistance spot welded joints with varying nugget diameter were carried out using 980 MPa high strength steel sheet of 1.6 mm thickness. In proportion, as nugget diameter increased from 3√t to 5√t (where t is thickness), cross tension strength (CTS) increased while fracture morphology simultaneously transferred from interface fracture to full plug fracture. In cases of interface fracture, circumferential crack initiation due to separation of the corona bond arose at an early stage of loading. The crack opening process without propagation was recognized until just before fracture and then the crack propagated to the nugget immediately in a brittle manner around CTS. In full plug fracture, main ductile crack initiation from the notch-like part at the end of sheet separation occurred with the sub-crack initiated at an early stage. The ductile crack propagated toward the HAZ and base material to form full plug fracture. The mode I stress intensity factor was considered as a suitable fracture parameter because the circumferential crack behaved pre-crack for brittle fracture in the nugget region at the final stage. Based on the FE analysis, the mode I stress intensity factor was calculated as 116 MPa √m at CTS as fracture toughness for the nugget. With respect to full plug fracture, ductile crack initiation behaviour from the notch-like part was expressed by concentration of equivalent plastic strain. On the assumption that the ductile crack arose in critical value of equivalent plastic strain, the value was calculated as 0.34 by FE analysis. Reasonable interpretation for interface fracture and full plug fracture in the resistance spot welded joint was proposed due to first crack initiation by stress concentration, brittle fracture by using mode I stress intensity factor, and ductile crack initiation by using equivalent plastic strain.     

Twinning in Strained Ferroelastics: Microstructure and Statistics

The generation of functional interfaces such as superconducting and ferroelectric twin boundaries requires new ways to nucleate as many interfaces as possible in bulk materials and thin films. Materials with high densities of twin boundaries are often ferroelastics and martensites. In this review, we show that the nucleation and propagation of twin boundaries depend sensitively on temperature and system size. Sudden changes of the domain pattern manifest themselves as avalanches or “jerks” in the potential energy of the sample. At high temperatures, the change of the twin pattern is thermally activated; the probability P to find sudden energy changes of jerks E follows the Vogel–Fulcher statistics P(E) ~ exp (E/(T ? T _VF)), whereas the athermal regime at low temperatures corresponds to power-law statistics P(E) ~ E ^?ε . We find that the complexity of the pattern is well characterized by the number of junctions between twin boundaries. Materials with soft bulk moduli have much higher junction densities than those with hard bulk moduli. Soft materials also show an increase in the junction density with diminishing sample size. The change of the complexity and the number density of twin boundaries represents an important step forward in the development of “domain boundary engineering,” where the functionality of the materials is directly linked to the domain pattern.     

Aggregation quenching in thin films of MEH-PPV studied by near-field scanning optical microscopy and spectroscopy

Aggregates in thin films of conjugated polymers form excimer states and significantly reduce the photo- and electroluminescence efficiency in devices produced from these materials. We have studied the aggregate formation in thin films of poly(2-methoxy,5-(2′-ethyl-hexyloxy)-p-phenylene-vinylene) (MEH-PPV) by near-field scanning optical microscopy and spectroscopy. Local photoluminescence spectroscopy and photo-bleaching experiments have been used to show that thin films of MEH-PPV are homogeneously aggregated and do not form aggregated domains.     

Modelling and experimental investigation on nanometric cutting of monocrystalline silicon

A new model to understand the behaviour of how materials are removed from workpiece in nano cutting is proposed. This model postulates that the mechanism of nanometric scale material removal is based on extrusion, which is different from the shearing mechanism in conventional cutting. It also explains why brittle materials are removed in ductile mode. Analytical results from molecular dynamics and nano indentation show good agreement with the proposed modelling. Experiments are conducted to verify the new model for nanometric cutting of monocrystalline silicon. The theoretical modelling and experimental verification present a good understanding of nano-scale material removal and provide an approach to fundamentally control the machining performance.     

An experimental investigation of constraint effects on mixed mode fracture initiation in a ductile aluminium alloy

The effect of constraint on ductile fracture initiation from a notch tip under mode I and mixed mode (involving modes I and II) loading is investigated. To this end, mixed mode fracture experiments are performed with Compact Tension Shear (or CTS) specimen of a ductile 2014-O aluminium alloy. The constraint effects are investigated by considering specimens with two crack length to width ratios. The effect of crack tip constraint on the relationship between the critical value of the J-integral at fracture initiation (J_c) and M_p is examined. Further, the micromechanics of mixed mode ductile fracture initiation is investigated by performing fractographic studies and metallographic examination of the mid-plane region of the specimen near the notch tip.     

Synthesis and dielectric properties of substituted barium titanate ceramics

Barium titanate is one of the most studied ferroelectric materials which has been used in various forms, e.g. bulk, thin and thick films, powder, in a number of applications. In order to achieve a material with desired properties, it is modified with suitable substituents. Most common substituents have been strontium, calcium and zirconium. Here, we report studies on lead and zirconium substituted barium titanate. The material series with compositional formula Ba_0.80Pb_0.20Ti_1?xZr_xO₃, with x varying from 0 to 0.1, was chosen for investigations. The material was synthesized by solid-state reaction method. Reacted powders compacted in the form of circular discs were sintered at 1300 °C. All the samples were subjected to X-ray diffraction (XRD) analysis and found to have single phase perovskite structure. Dielectric behavior was studied as a function of frequency and temperature. Curie temperature (T_c) was found to decrease with increasing x.     

The efficiency of bridges in a crack

Bridges are cross connections in a crack that contract its faces behind the crack tip. They appear because of the inhomogeneity of the material or differences in the kinematics of fracture. We obtained a general solution to the nonlinear singular integral equation that relates the crack opening h(x) to the stress caused by bridges p(x) and determined the stress-intensity factor F(B) (degree of unloading of the crack tip due to the presence of bridges) depending on the dimensionless stiffness of bridges B. As the measure of the efficiency of bridges, we suggest the ratio of the stress for crack start in a matrix without bridges σ_m to that in the presence of bridges σ_∞. When bridges with a strength σ_u occupy a fraction f of the fracture area, their efficiency Y(ζ, B) depends on the degree of reinforcement ζ = fσ_u/σ_m and on their stiffness B. The Y(ζ, B) dependences have been obtained in an explicit form for the limiting cases of brittle and fully ductile bridges.     

Study on the orientation degree of Pb1 − xLaxTiO3 thin films by the rocking curve technique and its morphological aspects

Thin films of perovskite-type materials such as PbTiO₃, BaTiO₃, (Pb,La)TiO₃, (Pb, La)(Zr,Ti)O₃, KNbO₃, and Pb(Mg,Nb)O₃ have been attracting great interest for applications like non-volatile memories, ultrasonic sensors and optical devices. Thin film should be epitaxially grown or at least highly textured since the properties of this anisotropic material depend on the crystallographic orientation. For optical devices, in particular, an epitaxial thin film without defects are essential to reduce optical propagation losses. Pb_1 − xLa_xTiO₃ (PLT) where x = 0, 13 and 27% thin films were prepared by a chemical method (polymeric precursors method), and deposited by the spin coating technique onto substrates of SrTiO₃ (STO) and LaAlO₃ (LAO). The films were then heat treated at 500 °C in a controlled atmosphere of O₂. The orientation degree of the thin films was obtained from rocking curve technique, by means of X-ray diffraction analysis. A microstructural study revealed that the films were crack-free, homogeneous and have low roughness.     

Effect of metal layer thickness on the decohesion of high purity copper–sapphire interfaces

The decohesion of high purity Cu–sapphire interfaces has been studied by using double cleavage drilled compressive (DCDC) specimens, accompanied by in situ optical observations of the crack extension mechanisms. The Cu layer thickness was varied between 10 and 100 μm. Decohesion occurred along the interface subject to a resistance, Γ_R(Δa), that rises with crack extension. The magnitude of the resistance was determined to be much larger (by a factor 4) for specimens with the thicker (100 μm) Cu layer. The observations revealed that the rupture occurred by a ductile mechanism involving void formation at the interface, followed by plastic void growth in the Cu, and subsequent coalescence by the separation of the interface between voids. Aspects of the failure sequence change with the Cu layer thickness. These changes, as well as the difference in the toughness, are attributable to a transition in constraint between the thin and thick layers. Namely, the thin layer develops high constraint (relative to the thicker layer) that elevates the mean stress ahead of the crack and causes the failure process to progress to a location further from the crack front. The measured trends in Γ_R(Δa) for the two different layer thickness are shown consistent with models of the constraint transition for interface failure by ductile mechanisms.     

Conversion of a green light emitting zinc-quinolate complex thin film to a stable and highly packed blue emitter film

Tris-(8-hydroxyquinoline) metal complex Znq₂, used as light-emitting layer in electroluminescent (EL) devices was synthesized and optical properties of as-deposited Znq₂ in thin films were studied. Interesting phenomenon was observed while studying the ageing and degradation behavior of Znq₂ films and consequently a stable form having strong blue photoluminescence (peak 465 nm) was discovered. This converted (from green to blue emitting) film has higher molecular packing density and comparable photoluminescence intensity with Znq₂ film. Here, we report on the production of this blue material under controlled conditions and its optical properties.The thin films have been deposited by thermal evaporation on quartz and silicon substrates. The optical constants (n and k) of green Znq₂ film and converted (to blue) thin films have been determined using spectroscopic ellipsometry. Environment induced effects on optical properties of films have been studied using ellipsometry, photoluminescence and UV–vis transmission measurements.     

Crack propagation resistance along strength mismatched bimetallic interface

Bimetallic three-point-bending specimens and four-point-bending fatigue test specimens were produced from strength mismatched stainless steel/low carbon steel bi-material. Both the J resistance curves and fatigue crack growth behavior were investigated for the bi- and bulk materials. The results showed that a crack initiated easily at the interface, and crack growth resistance along interface was inferior to that of the corresponding bulk materials under either static or dynamic loading conditions.     

Mechanical properties of Zr56.2Ti13.8Nb5.0Cu6.9Ni5.6Be12.5 ductile phase reinforced bulk metallic glass composite

Ductile phase containing bulk metallic glass composites are prepared via an in situ method by rapid quenching of a homogenous Zr_56.2Ti_13.8Nb_5.0Cu_6.9Ni_5.6Be_12.5 melt. The microstructure of the resulting two phase material is investigated by SEM, X-ray diffraction, and microprobe analysis. The composite material, as well as single phase materials with the corresponding matrix and second phase compositions, are tested in uniaxial tension and compression. Young's Modulus, shear modulus and Poisson ratios are analyzed by ultrasonic sound velocity measurements. The composite material demonstrates strongly improved Charpy impact toughness (by a factor of 2.5 compared to Vitreloy 1) and ductility (average fracture strain up to 8.3% in compression and 5.5% in tension). These remarkable improvements are explained by the effect of the mechanically soft and ductile second phase, which acts stabilizing against shear localization and critical crack propagation.     

The mechanism of caustic cracking of carbon steels—I. Influence of electrode potential and film formation

The anodic behaviour of a 0·1% carbon steel wire in 10M sodium hydroxide solution at 121°C has been studied at different electrode potentials under static conditions, and while the wire yields at strain rates between 1·5 and 436%/min.The static material forms a fairly coherent and adherent film of magnetite in the potential range ca. ?0·60 to ?0·85 V(she), while the anodic current density falls to ca. 0·2 mA/cm² (at ?0·70 V) in ca. 40 min. When such a specimen is made to yield at 436%/min, the overall current density continuously rises to over 8 mA/cm² at the bared metal area produced by the cracking of the oxide film. This current density is sufficient to account for the rate of crack propagation measured microscopically on specimens strained at 1·5%/min. Equivalent results are reported for potentials over the whole range ?0·60 to ?0·85 V(she).On either side of this potential range little or no adherent film is formed, the anodic current density does not fall below ca. 1 mA/cm² and on straining rises to no more than ca. 5 mA/cm²; and no cracks are produced.The results strongly support the theory that crack propagation occurs because bared metal at the yielding advancing edge of a stress-raising crack can dissolve several hundred times as fast as that at the static crack sides, which are continuously protected by the growth of film. The crack thus maintains its acuity, and the raised stress at its edge maintains ductile yielding of the metal so that the electrochemical crack advancement proceeds without the need for any mechanical cracking.A comparison with previously reported work on the same steel in hot concentrated nitrate solutions shows that the range of anodic current densities found on bared metal, and the corresponding range of crack propagation rates, are about an order of magnitude less in the present hot concentrated hydroxide solution.     

Nanoindentation of porous bulk and thin films of La0.6Sr0.4Co0.2Fe0.8O3−δ

In this paper we show how reliable measurements on porous ceramic films can be made by appropriate nanoindentation experiments and analysis. Room-temperature mechanical properties of the mixed-conducting perovskite material La_0.6Sr_0.4Co_0.2Fe_0.8O_3?δ (LSCF6428) were investigated by nanoindentation of porous bulk samples and porous films sintered at temperatures from 900 to 1200 °C. A spherical indenter was used so that the contact area was much greater than the scale of the porous microstructure. The elastic modulus of the bulk samples was found to increase from 33.8 to 174.3 GPa and hardness from 0.64 to 5.32 GPa as the porosity decreased from 45% to 5% after sintering at 900–1200 °C. Densification under the indenter was found to have little influence on the measured elastic modulus. The residual porosity in the “dense” sample was found to account for the discrepancy between the elastic moduli measured by indentation and by impulse excitation. Crack-free LSCF6428 films of acceptable surface roughness for indentation were also prepared by sintering at 900–1200 °C. Reliable measurements of the true properties of the films were obtained by data extrapolation provided that the ratio of indentation depth to film thickness was in the range 0.1–0.2. The elastic moduli of the films and bulk materials were approximately equal for a given porosity. The 3-D microstructures of films before and after indentation were characterized using focused ion beam/scanning electron microscopy tomography. Finite-element modelling of the elastic deformation of the actual microstructures showed excellent agreement with the nanoindentation results.     

Preparation of polyacenic semiconductive thin films by excimer laser ablation

Polyacenic semiconductive thin films (PAS-TFs) were prepared onto a quartz or a KBr substrate by excimer laser ablation at 308 nm of a bulk phenol-formaldehyde (PF) resin and the bulk of a PAS material which was obtained by heat treatment of the PF resin at 535 °C (pyrolytic PAS (535 °C)). These were homogeneous brown and/or dark brown thin films consisting of fine particles. The electric conductivities of the films thus prepared from PF resin and pyrolytic PAS(535 °C) were about 10⁻⁸ and 10⁻⁵ S cm⁻¹ at room temperature, respectively. Remarkable increase of electric conductivities was achieved on increasing the substrate temperature during the deposition process. In particular, the conductivity of the film from pyrolytic PAS (535 °C) reached more than 10 ⁻² S cm⁻¹, which is comparable to that of bulk PAS material prepared by pyrolytic treatment at 775 °C, when deposited on a substrate at 300 °C.     

Hydrogenography of PdHx thin films: Influence of H-induced stress relaxation processes

Hydrogenography is a new optical thin film combinatorial method that follows hydrogenation and determines its associated thermodynamic properties. Due to clamping to the substrate, stresses generated in thin films are larger than in bulk. This must be taken into account for a comparison between these two types of systems. In this article, we follow the microstructure, surface morphology and in-plane stress changes of thin polycrystalline PdH_x films upon several hydrogen ab/desorption cycles and correlate them to the evolution in shape and hysteresis of pressure–optical transmission isotherms (PTIs) recorded by hydrogenography. The in-plane stress in the first instance is relaxed inhomogeneously by buckling, and a more complete, homogeneous relaxation is only reached after the creation of a buckle-and-crack network that is the two-dimensional analogue of bulk decrepitated grains. This sequence of changes is clearly visible in the PTIs, demonstrating another useful facet of hydrogenography for characterizing metal–hydrogen systems.     

Theoretical investigation for Li2CuSb as multifunctional materials: Electrode for high capacity rechargeable batteries and novel materials for second harmonic generation

Based on the first-principles electronic structure calculations, we predict that Li₂CuSb should be good electrode materials for high capacity rechargeable batteries and novel materials for second harmonic generation. This prediction is based on the experimental measurements of Fransson et al. [1], and as step forward to do deep investigation on these materials we addressed ourselves for performing theoretical calculation. We found that intercalation of lithium leads to phase transitions, which agrees well with the experiment, increasing the conductivity of the material, and break the symmetry along the optical axis making the material useful for second harmonic generation (SHG) applications. We should emphasize that lithiated compound show very high second order optical susceptibility. We present the total charge densities in the (1 1 0) and (1 0 0) planes for the parent and lithiated phases and it was found that the parent compound shows a considerable anisotropy between the two planes in consistence with our calculated optical properties. We found that Li₂CuSb possesses high second harmonic generation and its second order optical susceptibility of the total absolute value at zero frequency is equal to 142 pm/V. Based on the value of the second order optical susceptibility the microscopic second order hyperpolarizability, β_ijk, the vector component along the dipole moment direction is about 31.01 × 10⁻³⁰ esu.