Enzymatic degradation of polymers: a brief review

Recently, research on polymer has drawn much attention mainly due to the ever increasing application of these polymeric materials in several areas such as food packaging industry, agricultural industry and biomedical research. However, increasing industrial use of polymers has led to the environmentally critical issue of waste disposal. Further, the successful implication of polymeric materials in biomedical applications depends on the biodegradability of the concerned polymer. Various enzymes play an important role in the biodegradation of polymers. The present review describes the enzyme mediated biodegradation of various polymers including synthetic, natural and blends of these materials. Detailed examples of enzymatic degradation of polymers are illustrated from current scientific literature with the discussion on various factors that can influence the degradation. In addition, different techniques that are generally applied to assess the degradation process as well as degradation products have been described. Finally, a special emphasis is given to the investigation of the kinetics of polymer degradation by enzymes.

Ferroelectric and optical properties of Ba5Li2Ti2Nb8O30 ceramics potential for memory applications

Giant grained (42 mu m) translucent Ba5Li2Ti2Nb8O30 ceramic was fabricated by conventional sintering technique using the powders obtained via solid state reaction route. These samples were confirmed to possess tetragonal tungsten bronze structure (P4bm) at room temperature. The scanning electron microscopy established the average grain size to be close to 20 mu m. The photoluminescence studies carried out on these ceramics indicated sharp emission bands around 433 and 578 nm at an excitation wavelength of 350 nm which were attributed to band-edge emission as the band gap was 2.76 eV determined by Kubelka-Munk function. The dielectric properties of these ceramics were studied over wide frequency range (100-1 MHz) at room temperature. The decrease in dielectric constant with frequency could be explained on the basis of Koops theory. The dielectric constant and the loss were found to decrease with increasing frequency. The Curie temperature was confirmed to be similar to 370 A degrees C based on the dielectric anomaly observed when these measurements were carried out over a temperature range of 30-500 A degrees C. This shows a deviation from Curie-Weiss behaviour and hence an indicator of the occurrence of disordering in the system, the gamma = 1.23 which confirms the diffuse ferroelectric transition. These ceramics at room temperature exhibited P-E hysteresis loops, though not well saturated akin to that of their single crystalline counterparts. These are the suitable properties for ferroelectric random access memory applications.

Synthesis of self-light-scattering wrinkle structured ZnO photoanode by sol-gel method for dye-sensitized solar cells

A special morphological zinc oxide (ZnO) photoanode for dye-sensitized solar cell was fabricated by simple sol-gel drop casting technique. This film shows a wrinkled structure resembling the roots of banyan tree, which acts as an effective self scattering layer for harvesting more visible light and offers an easy transport path for photo-injected electrons. These ZnO electrode of low thickness (similar to 5 mu m) gained an enhanced short-circuit current density of 6.15 mA/cm(2), open-circuit voltage of 0.67 V, fill factor of 0.47 and overall conversion efficiency of 1.97 % under 1 sun illumination. This shows a high conversion efficiency and a superior performance than that of ZnO nanoparticle-based photoanode (eta similar to 1.13 %) of high thickness (similar to 8 mu m).

Structural refinement, optical and electrical properties of Ba1-x Sm-2x/3](Zr0.05Ti0.95)O-3 ceramics

Samarium doped barium zirconate titanate ceramics with general formula Ba1-x Sm-2x/3](Zr0.05Ti0.95)O-3 x = 0, 0.01, 0.02, and 0.03] were prepared by high energy ball milling method. X-ray diffraction patterns and micro-Raman spectroscopy confirmed that these ceramics have a single phase with a tetragonal structure. Rietveld refinement data were employed to model BaO12], SmO12], ZrO6], and TiO6] clusters in the lattice. Scanning electron microscopy shows a reduction in average grain size with the increase of Sm3+ ions into lattice. Temperature-dependent dielectric studies indicate a ferroelectric phase transition and the transition temperature decreases with an increase in Sm3+ ion content. The nature of the transition was investigated by the Curie-Weiss law and it is observed that the diffusivity increases with Sm3+ ion content. The ferroelectric hysteresis loop illustrates that the remnant polarization and coercive field increase with an increase in Sm3+ ions content. Optical properties of the ceramics were studied using ultraviolet-visible diffuse reflectance spectroscopy.

Processing-microstructure-yield strength correlation in a near beta Ti alloy, Ti-5Al-5Mo-5V-3Cr

A combined set of thermo-mechanical steps recommended for high strength beta Ti alloy are homogenization, deformation, recrystallization, annealing and ageing steps in sequence. Recrystallization carried out above or below beta transus temperature generates either beta annealed (lath type morphology of alpha) or bimodal (lath+globular morphology of alpha) microstructure. Through variations in heat treatment parameters at these processing steps, wide ranges of length scales of features have been generated in both types of microstructures in a near beta Ti alloy, Ti-5Al-5Mo-5V-3Cr (Ti-5553). 0.2% Yield strength (YS) has been correlated to various microstructural features and associated heat treatment parameters. Relative importance of microstructural features in influencing YS has been identified. Process parameters at different steps have been identified and recommended for attaining different levels of YS for this near beta Ti alloy. (C) 2014 Elsevier B.V. All rights reserved.

Microstructure Development, Nanomechanical, and Dynamic Compression Properties of Spark Plasma Sintered TiB2-Ti-Based Homogeneous and Bi-layered Composites

The growing threats due to increased use of small-caliber armor piercing projectiles demand the development of new light-weight body armor materials. In this context, TiB2 appears to be a promising ceramic material. However, poor sinterability and low fracture toughness remain two major issues for TiB2. In order to address these issues together, Ti as a sinter-aid is used to develop TiB2-(x wt pct Ti), (x = 10, 20) homogeneous composites and a bi-layered composite (BLC) with each layer having Ti content of 10 and 20 wt pct. The present study uniquely demonstrates the efficacy of two-stage spark plasma sintering route to develop dense TiB2-Ti composites with an excellent combination of nanoscale hardness (similar to 36 GPa) and indentation fracture toughness (similar to 12 MPa m(1/2)). In case of BLC, these properties are not compromised w.r.t. homogeneous composites, suggesting the retention of baseline material properties even in the bi-layer design due to optimal relief of residual stresses. The better indentation toughness of TiB2-(10 wt pct Ti) and TiB2-(20 wt pct Ti) composites can be attributed to the observed crack deflection/arrest, indicating better damage tolerance. Transmission electron microscope investigation reveals the presence of dense dislocation networks and deformation twins in alpha-Ti at the grain boundaries and triple pockets, surrounded by TiB2 grains. The dynamic strength of around 4 GPa has been measured using Split Hopkinson Pressure Bar tests in a reproducible manner at strain rates of the order of 600 s(-1). The damage progression under high strain rate has been investigated by acquiring real time images for the entire test duration using ultra-high speed imaging. An attempt has been made to establish microstructure-property correlation and a simple analysis based on Mohr-Coulomb theory is used to rationalize the measured strength properties.

Deformation and strength of Ti-6Al-4V alloyed with B at cryogenic temperatures

Plastic deformation and strength of Ti-6Al-4V (Ti64) alloyed with minor additions of B at cryogenic temperatures were investigated through unnotched and notched tensile tests at 20 and 77 K Marked microstructural refinement that occurs with the trace addition of B to Ti64 was exploited for examining the role of microstructural length scales on the cryogenic plastic deformation. The tensile tests were complemented with detailed microstructural characterisation using transmission electron microscopy and electron back scattered diffraction imaging of the deformed specimens. Experimental results show that the addition of 0.30 wt% and above of B to Ti64 reduces ductility, and in turn enhances the notch sensitivity to the extent that those alloys become unsuitable for low temperature applications. However, the addition of similar to 0.10 wt% B is beneficial in enhancing the low temperature strength. An examination of the yield strength variation at various temperatures reveals that at 77 K, the colony size determines the yield strength of the alloy, just as it does at room temperature; implying dislocation-mediated plasticity continues to dominate up to 77 K At 20 K however, twinning dominates the flow response, with the activation of {11 (2) over bar1} and {5 (6) over bar1 (3) over bar} twinning in addition to {10 (1) over bar2} in the base alloy resulting in enhanced ductility of it as compared to either B-modified alloys at 20 K or the base alloy itself at 77 K The observation of a reasonable correlation between the lath aspect ratio, given by the colony-to-lath thickness ratios, and yield strength variation at 20 K suggests that coarse colony size in the base alloy allows for the activation of additional twinning mechanisms. (C) 2014 Elsevier B.V. All rights reserved.

Influence of seed layer orientation on the growth and physical properties of SnS nanostructures

Stoichiometric tin (II) sulfide (SnS) nano-structures were synthesized on SnS(010)/glass substrates using a simple and low-temperature chemical solution method, and their physical properties were investigated. The as-synthesized SnS nanostructures exhibited orthorhombic crystal structure and most of the nanocrystals are preferentially oriented along the <010> direction. These nanostructures showed p-type electrical conductivity and high electrical resistivity of 93 Omega cm. SnS nanostructures exhibited a direct optical band gap of 1.43 eV. While increasing the surrounding temperature from 20 to 150 degrees C, the electrical resistivity of the structures decreased and exhibited the activation energy of 0.28 eV.

Effect of film thickness and annealing on optical properties of TiO2 thin films and electrical characterization of MOS capacitors

Titanium dioxide (TiO2) thin films were deposited on glass and silicon (100) substrates by the sol-gel method. The influence of film thickness and annealing temperature on optical transmittance/reflectance of TiO2 films was studied. TiO2 films were used to fabricate metal-oxide-semiconductor capacitors. The capacitance-voltage (C-V), dissipation-voltage (D-V) and current-voltage (I-V) characteristics were studied at different annealing temperatures and the dielectric constant, current density and resistivity were estimated. The loss tangent (dissipation) increased with increase of annealing temperature.

Using heat treatment effects and EBSD analysis to tailor microstructure of hybrid Mg nanocomposite for enhanced overall mechanical response

In this study, a detailed investigation on the effect of heat treatment on the microstructural characteristics, texture evolution and mechanical properties of Mg-(5.6Ti+2.5B(4)C)(BM) hybrid nanocomposite is presented. Optimised heat treatment parameters, namely, heat treatment temperature and heat treatment time, were first identified through grain size and microhardness measurements. Initially, heat treatment of composites was conducted at temperature range between 100 and 300 degrees C for 1 h. Based on optical microscopic analysis and microhardness measurements, it was evident that significant grain growth and reduction in microhardness occurred for temperatures > 200 degrees C. The cutoff temperature that caused significant grain growth/matrix softening was thus identified. Second, at constant temperature (200 degrees C), the effect of variation of heat treatment time was carried out (ranging between 1 and 5 h) so as to identify the range wherein increase in average grain size and reduction in microhardness occurred. Furthering the study, the effect of optimised heat treatment parameters (200 degrees C, 5 h) on the microstructural texture evolution and hence, on the tensile and compressive properties of the Mg-(5.6Ti+2.5B(4)C)(BM) hybrid nanocomposite was carried out. From electron backscattered diffraction (EBSD) analysis, it was identified that the optimised heat treatment resulted in recrystallisation and residual stress relaxation, as evident from the presence of similar to 87% strain free grains, when compared to that observed in the non-heat treated/as extruded condition (i.e. 2.2 times greater than in the as extruded condition). For the heat treated composite, under both tensile and compressive loads, a significant improvement in fracture strain values (similar to 60% increase) was observed when compared to that of the non-heat treated counterpart, with similar to 20% reduction in yield strength. Based on structure-property correlation, the change in mechanical characteristics is identified to be due to: (1) the presence of less stressed matrix/reinforcement interface due to the relief of residual stresses and (2) texture weakening due to matrix recrystallisation effects, both arising due to heat treatment.

Dissimilar friction stir welding between aluminum alloy and magnesium alloy at a low rotational speed

A sound weld was obtained between 2024-T3 Al alloy and AZ31B-O Mg alloy dissimilar metal plates of 5 mm thickness, at a rotational speed of 300 rev min(-1) and at a welding speed of 50 mm min(-1). One of the parameter studied was, the effect of interface offset variation, on the quality and properties of the welded samples and on the thickness of intermetallic layer formed in the welded samples. The intermetallic layer at the midst of the weld volume contains intermetallic compounds Al12Mg17 and Al3Mg2. Highest tensile strength of 106.86 MPa, corresponding tensile joint efficiency of 44.52% and corresponding elongation 1.33% were obtained for the tensile sample, with interface offset of 0.66 mm from zero interface offset in retreating side and with approximate least intermetallic thickness of 1.2 mu m. Dissimilar friction stir welded joint samples had failed completely in brittle fracture mode; the position of tensile fracture was located at the midst of intermetallic layer, which had maximum hardness and minimum ductility. The nano hardness values fluctuate in the weld nugget owing to dynamic recrystallization of alloy materials and formation of brittle intermetallic compounds of alloy materials in the weld nugget; maximum hardness of 10.74 GPa occurred for the sample with least intermetallic thickness of 1.2 mu m. (C) 2014 Elsevier B.V. All rights reserved.

A novel method for studying the buckling of nanotubes considering geometrical imperfections

Buckling of nanotubes has been studied using many methods such as molecular dynamics (MD), molecular mechanics, and continuum-based shell theories. In MD, motion of the individual atoms is tracked under applied temperature and pressure, ensuring a reliable estimate of the material response. The response thus simulated varies for individual nanotubes and is only as accurate as the force field used to model the atomic interactions. On the other hand, there exists a rich literature on the understanding of continuum mechanics-based shell theories. Based on the observations on the behavior of nanotubes, there have been a number of shell theory-based approaches to study the buckling of nanotubes. Although some of these methods yield a reasonable estimate of the buckling stress, investigation and comparison of buckled mode shapes obtained from continuum analysis and MD are sparse. Previous studies show that the direct application of shell theories to study nanotube buckling often leads to erroneous results. The present study reveals that a major source of this error can be attributed to the departure of the shape of the nanotube from a perfect cylindrical shell. Analogous to the shell buckling in the macro-scale, in this work, the nanotube is modeled as a thin-shell with initial imperfection. Then, a nonlinear buckling analysis is carried out using the Riks method. It is observed that this proposed approach yields significantly improved estimate of the buckling stress and mode shapes. It is also shown that the present method can account for the variation of buckling stress as a function of the temperature considered. Hence, this can prove to be a robust method for a continuum analysis of nanosystems taking in the effect of variation of temperature as well.

A meshless adaptive multiscale method for fracture

The paper presents a multiscale method for crack propagation. The coarse region is modelled by the differential reproducing kernel particle method. Fracture in the coarse scale region is modelled with the Phantom node method. A molecular statics approach is employed in the fine scale where crack propagation is modelled naturally by breaking of bonds. The triangular lattice corresponds to the lattice structure of the (111) plane of an FCC crystal in the fine scale region. The Lennard-Jones potential is used to model the atom-atom interactions. The coupling between the coarse scale and fine scale is realized through ghost atoms. The ghost atom positions are interpolated from the coarse scale solution and enforced as boundary conditions on the fine scale. The fine scale region is adaptively refined and coarsened as the crack propagates. The centro symmetry parameter is used to detect the crack tip location. The method is implemented in two dimensions. The results are compared to pure atomistic simulations and show excellent agreement. (C) 2014 Elsevier B. V. All rights reserved.

Photoluminescence properties of Eu3+-activated CaMoO4 phosphors for WLEDs applications and its Judd-Ofelt analysis

A series of scheelite-type Eu3+-activated CaMoO4 phosphors were synthesized by the nitrate-citrate gel combustion method. All the compounds crystallized in the tetragonal structure with space group I4(1)/a (No. 88). FESEM results reveal the spherical-like morphology. The CaMoO4 phosphor exhibited broad emission centered at 500 nm under the excitation of 298 nm wavelength, while Eu3+-activated CaMoO4 shows an intense characteristic red emission peak at 615 nm at different excitation wavelengths, due to D-5(0) -> F-7(2) transition of Eu3+ ions. The intensities of transitions between different J levels depend on the symmetry of the local environment of Eu3+ ions and were estimated using the Judd-Ofelt analysis. The high asymmetric ratio revealed that Eu3+ occupies sites with a low symmetry and without an inversion center. The CIE chromaticity co-ordinates (x, y) were calculated from emission spectra, and the values were close to the NTSC standard. Therefore, the present phosphor is highly useful for LEDs applications.

Enhanced electric field tunable magnetic properties of lead-free Na0.5Bi0.5TiO3-MnFe2O4 multiferroic composites

Strong magnetoelectric (ME) interaction was exhibited at both dc and microwave frequencies in a lead-free multiferroic particulate composites of Na0.5Bi0.5TiO3 (NBT) and MnFe2O4 (MFO) multiferroic, which were prepared by sol-gel route. The room temperature permeability measurements were carried out in the frequency range of 1 MHz-1 GHz. A systematic study of structural, magnetic and ME properties were undertaken. The room temperature ferromagnetic resonance (FMR) was studied. Strong ME coupling is demonstrated in 70NBT-30MFO composite by an electrostatically tunable FMR field shift up to 428 Oe (at E = 4 kV/cm), which increases to a large value of 640 Oe at E = 8 kV/cm. Furthermore, these lead-free multiferroic composites exhibiting electrostatically induced magnetic resonance field at microwave frequencies provide great opportunities for electric field tunable microwave devices.