Stabiliser distribution and efficiency examined by depth profiling polypropylene using a positron beam

The effectiveness of stabilisers, organic (hindered amine light stabilisers), inorganic (nanoparticulate zinc oxide), and combinations thereof, in preventing the signatures of degradation in polypropylene subjected to accelerated weathering is studied. Not only the type of stabiliser but also its distribution is important for stabiliser efficiency. The spatial effects of stabilization are studied using positron spectroscopy depth profiling.     

Heterogeneity of Accelerated Photooxidation in Stabilized Bulk Commodity Polymers Studied by X-Band ESRI

Heterogeneity of photooxidation processes in plaques made of commodity polymers, high-density polyethylene, and cyclic olefin copolymer poly(ethylene-co-norbornene) (Topas^® 8007) stabilized with various hindered amine stabilizers; some of them in combination with UV stabilizers exposed to accelerated weathering were studied using electron spin resonance imaging (ESRI) and three independent microscopic-scale methods: scanning electron microscopy, infrared microscopy, and microhardness testing. Concentration profiles of nitroxides mapping photooxidation process inside polymer plaques along the direction of incident radiation (perpendicular to the irradiated surface of the plaques) were determined by ESRI technique in dependence on the duration of the accelerated photooxidation. ESRI data were complemented with profiles of oxidation products, crystallinity, and microhardness measured inside the plaques along the same direction. Selection of stabilizers in the study was completed with α-tocopherol that is considered as the most active component of excellent processing stabilizer vitamin E.     

Surface roughness and color characteristics of wood treated with preservatives after accelerated weathering test

Wood samples treated with ammonium copper quat (ACQ 1900 and ACQ 2200), chromated copper arsenate (CCA), Tanalith E 3491 and Wolmanit CX-8 have been studied in accelerated weathering experiments. The weathering experiment was performed by cycles of 2 h UV-light irradiation followed by water spray for 18 min. The changes on the surface of the weathered samples were characterized by roughness and color measurements on the samples with 0, 200, 400 and 600 h of total weathering.

The objective of this study was to investigate the changes created by weathering on impregnated wood with several different wood preservatives. This study was performed on the accelerated weathering test cycle, using UV irradiation and water spray in order to simulate natural weathering.

Surface roughness and color measurement was used to investigate the changes after several intervals (0–200–400–600 h) in artificial weathering of treated and untreated wood.     

Comparison of the protection effectiveness of acrylic polyurethane coatings containing bark extracts on three heat-treated North American wood species: Surface degradation

High temperature heat-treatment of wood is a very valuable technique which improves many properties (biological durability, dimensional stability, thermal insulating characteristics) of natural wood. Also, it changes the natural color of wood to a very attractive dark brown color. Unfortunately, this color is not stable if left unprotected in external environment and turns to gray or white depending on the wood species. To overcome this problem, acrylic polyurethane coatings are applied on heat-treated wood to delay surface degradations (color change, loss of gloss, and chemical modifications) during aging. The acrylic polyurethane coatings which have high resistance against aging are further modified by adding bark extracts and/or lignin stabilizer to enhance their effectiveness in preventing the wood aging behavior. The aging characteristic of this coating is compared with acrylic polyurethane combined with commercially available organic UV stabilizers. In this study, their performance on three heat-treated North American wood species (jack pine, quaking aspen and white birch) are compared under accelerated aging conditions. Both the color change data and visual assessment indicate improvement in protective characteristic of acrylic polyurethane when bark extracts and lignin stabilizer are used in place of commercially available UV stabilizer. The results showed that although acrylic polyurethane with bark extracts and lignin stabilizer was more efficient compared to acrylic polyurethane with organic UV stabilizers in protecting heat-treated jack pine, it failed to protect heat-treated aspen and birch effectively after 672 h of accelerated aging. This degradation was not due to the coating adhesion loss or coating degradation during accelerated aging; rather, it was due to the significant degradation of heat-treated aspen and birch surface beneath this coating. The XPS results revealed formation of carbonyl photoproducts after aging on the coated surfaces and chain scission of CN of urethane linkages.     

Optical characterization of ZnO nanoparticles capped with various surfactants

The presence of surfactants (Hexamine, tetraethylammonium bromide (TEAB), cetyltrimethylammonium bromide (CTAB), tetraoctylammonium bromide (TOAB) and PVP) on the surface of zinc oxide (ZnO) nanoparticles resulted variation in their optical properties. The optical properties of each surfactant-capped zinc oxide nanoparticles were investigated using UV-visible absorption and fluorescence techniques. The particle size of these nanoparticles were calculated from their absorption edge, and found to be in the quantum confinement range. The absorption spectra and fluorescent emission spectra showed a significant blue shift compared to that of the bulk zinc oxide. Large reduction in the intensity of visible emission of zinc oxide/surfactant was observed and these emissions were vanished more quickly, with the decrease in excitation energy, for the smaller nanoparticles. Out of the four surfactants (other than PVP), CTAB-capped zinc oxide has smallest particle size of 2.4 nm, as calculated from the absorption spectrum. Thus the presence of surfactant on the surface of zinc oxide plays a significant role in reducing defect emissions. Furthermore, ZnO/PVP nanoparticles showed no separate UV emission peak; however, the excitonic UV emission and the visible emission at 420 nm overlap to form a single broad band around 420 nm.     

Coating zinc oxide submicron crystals on poly(methyl methacrylate) chips and spheres via ultrasound irradiation

Ultrasound irradiation is used for anchoring zinc oxide submicron crystals with a main diameter and length of 280 nm and 470 nm, respectively, onto the surface of poly(methyl methacrylate) PMMA chips (2 mm diameter), and zinc oxide crystals with a mean diameter and length of 150 nm and 230 nm, respectively, onto the surface of the PMMA spheres (1–10 μm). The zinc oxide crystals were obtained by sonochemical irradiation of a mixture containing the PMMA, zinc (II) acetate dihydrate, ethanol, water, and 24 wt.% aqueous ammonia for 2 h, yielding a PMMA–zinc oxide composite. By controlling the atmosphere and reaction conditions, we could achieve well-adhered zinc oxide crystals on the surface of poly(methyl methacrylate). The resulting zinc oxide–PMMA composite was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray analysis (EDX), high-resolution scanning electron microscopy (HRSEM), and photoluminescence (PL) spectroscopy. The zinc oxide-deposited PMMA chips (loaded with 0.03–1.0 wt.% ZnO) were successfully homogenized in melt by extrusion and then injection molded into small, disc-shaped samples. These samples were analyzed with respect to their directional spectral optical properties in UV, Vis and IR spectroscopy.     

Study of the photodegradation of brilliant green on mechanically activated powders of zinc oxide

A study of the photocatalytic activity of commercial and mechanically activated zinc oxide powders has been carried out based on the example of the decomposition of Brilliant Green. The goal of this work was to study the effect of the grinding time (0, 1, 3, 5, and 7 min) on the structure of zinc oxide and its photocatalytic activity under visible and ultraviolet (UV) radiation. It has been found that, when UV radiation is used, the constant of the dye oxidation rate for samples activated for 1 min increases compared with unactivated powders, whereas further mechanical activation leads to a decrease in the photocatalytic activity. When using visible radiation, samples activated for 1 min showed the minimum photocatalytic activity and further mechanical activation led to an increase in the efficiency of photocatalysis.     

Room temperature deposition of ZnO and Al:ZnO ultrathin films on glass and PET substrates by DC sputtering technique

In the present work, ultrathin films of Zinc oxide (ZnO) and Aluminum doped zinc oxide (AZO) with 20 nm thick were fabricated at 300 K on glass and Polyethylene terephthalate (PET) substrates by means of direct-current sputtering method. The structural morphology of the films were analyzed by (XRD) diffractometry. The average transmittance of films deposited on different substrates showed high transparency (over 80%) in the visible spectrum. The objective of the present work is to investigate the thicknesses and optical constants of ZnO and AZO ultrathin films prepared by DC sputtering onto (glass and PET) substrates using two methods (UV–Vis–NIR spectrophotometer and ellipsometer (SE) by new amorphous dispersion formula) with comparison to check and confirm the results that obtained from UV with those obtained from SE measurements. The optical constants of films were extracted and compared using UV and SE techniques in the range of (200–2200 nm) with increment of 1 nm in order to confirm the accuracy of the UV and shown an excellent agreement.     

Ultraviolet emissions excited by accelerated electrons

By employing an insulating zinc oxide (i-ZnO) as an electron accelerating layer, and an n-type ZnO as an active layer, ultraviolet (UV) emissions at 385 nm caused by the excitation of the n-ZnO layer by the accelerated electrons from the i-ZnO layer have been realized. By replacing the active layer with larger bandgap Mg0.39Zn0.61O and properly optimizing the structure, shorter wavelength emissions at around 328 nm have been obtained. Considering that the p-type doping of wide bandgap semiconductors is still a challenging issue, the results reported in this Letter may provide a promising alternative route to UV emissions.     

Enhancement of UV luminescence from single crystals of zinc oxide

Crystals of the red oxide of zinc are normally heat treated in various media to produce enhanced UV luminescence. Studies have been carried out on the reflection, photoluminescence, and cathodoluminescence spectra of these crystals in order to understand the changes in the intensity of the luminescence. It has been established that with heat treatment of single crystals of zinc oxide in a vacuum or water vapor, or with diffusion of an impurity which replaces the zinc in the crystal and has atoms larger than the Zn atoms, there is an enhancement of the intensity of UV luminescence by more than two orders of magnitude.     

Silk-mediated synthesis and modification of photoluminescent ZnO nanoparticles

In this article, a bio-inspired silk-mediated method was established to produce natural material-modified photoluminescent zinc oxide nanoparticles (nano-ZnO). Silk fibroin fibers were employed as the reactive substrates to synthesize nano-ZnO, and silk fibroins (SF) were taken as the biocompatible stabilizers to modify dispersed nano-ZnO. As-prepared nano-ZnO were mainly hexagonal phase particles with diameter around 13 nm. The resulting nano-ZnO/SF hybrids displayed orange emission and good biocompatibility in aqueous system.     

Application of positron beams to the study of positronium-forming solids

Doppler broadening of annihilation radiation (DBAR) and positron annihilation lifetime spectroscopy (PALS) have been successfully applied to the study of positronium (Ps)—forming amorphous solids such as polymers and silicon oxide in the bulk. Implementing depth selectivity to DBAR and PALS by combining them with variable-energy positron beams considerably broadens their applicability. Variation of incident positron energy over a wide range enables depth-profiling, whereas tuning of the beam energy enables the studies of surfaces, interfaces and thin films. In this paper, we discuss fundamentals and applications of energy variable DBAR and PALS for Ps—forming polymers and silicon oxide.     

Evaluating paper degradation progress. Cross-linking between chromatographic, spectroscopic and chemical results

The study presents an overview of the chromatographic (SEC), spectroscopic (FTIR, UV/VIS), viscometric (DP) and chemical methods (titration, pH) used for the evaluation of the degradation progress of various kinds of paper under various conditions. The methods were chosen to follow different routes of paper degradation. Model paper samples represented boundary paper types from pure cellulose cotton paper, through softwood to low quality acidic, sized groundwood paper The accelerated ageing conditions were adjusted to achieve maximum effect (climatic chamber RH 59%, 90^oC) and also to mimic the environment inside books (closed vials). The results were settled on the literature data on the degradation mechanisms and compared in terms of the paper types and ageing conditions. The estimators of coupled de-polymerisation and oxidation have been proposed based on the correlation between SEC, UV/VIS and titrative coppper number determination. The overall oxidation index derived from FTIR results was shown to correlate with the summary –CHO and –COOH concentration determined by titrative methods.     

Photochemical preparation of ZnO nanoparticles

Preparation of zinc oxide nanoparticles from aqueous solutions containing zinc nitrate or formate using UV irradiation was investigated. Analysis of solid phase formed during irradiation confirmed the presence of zinc oxide or zinc peroxide nanoparticles ranging in size from 1 to 70 nm, depending on initial precursors. Annealing at temperatures 650–1000 °C results in forming of rice-like zinc oxide particles, up to hundreds of nm in size. Photochemical method yields material with high chemical purity and uniform particle size distribution. In addition, photo-induced doping of zinc oxide with lanthanum was studied. Presence of lanthanum in zinc oxide crystal lattice and post-preparation treatment in reduction atmosphere significantly increase the UV excitonic luminescence at 395 nm in radioluminescence spectra.     

The influence of oxygen on the optical properties of RF-sputtered zinc oxide thin films

In this article, we investigate the effects of oxygen partial pressure in the deposition chamber on the optical properties of zinc oxide (ZnO) thin films; in particular, we examine the variation of the refractive index with oxygen flux.ZnO thin films were deposited by radio-frequency (RF) magnetron sputtering and studied by means of X-ray diffraction (XRD) and spectroscopic ellipsometry (SE). We have found a preferential c-axis growth of ZnO films, with slightly variable deposition rates from 2.6 to 3.8 Å/s. Conversely, the refractive index exhibits, from ultraviolet (UV) to near infrared (IR), a considerable and almost linear variation when the oxygen flux value in the deposition chamber varies from 0 to 10 sccm.     

Growth, modulation and photoresponse characteristics of vertically aligned ZnO nanowires

Vertically aligned, c-axis oriented zinc oxide (ZnO) nanowires were grown on Si substrate by metal organic chemical vapor deposition (MOCVD) technique, where sputtered aluminum nitride (AlN) film was used as an intermediate layer and thermally evaporated barium fluoride (BaF₂) film as a sacrificial layer. The aspect ratio and density of the nanowires were also varied using only Si microcavity without any interfacial or sacrificial layer. The UV detectors inside the microcavity have shown the higher on-off current ratio and fast photoresponse characteristics. The photoresponse characteristics were significantly varied with the aspect ratio and the density of nanowires.     

Photoluminescence identification of surface contaminants on zinc oxide from their phase transitions

The spectra of photoluminescence (PL) from a zinc oxide (ZnO) crystal included an unusual feature of an ultraviolet (UV) emission at a higher energy than the ZnO interband transitions. The energy of this UV emission varied with temperature and included two discontinuous energy steps in the temperature range from 78 to 700?K. The temperature values of the steps match phase transition temperatures of ethanol. Ethanol was used during sample cleaning and can be trapped in the form of nanoparticles inclusions. Both the intrinsic and defect site, PL signals have different temperature dependencies from those of bulk crystals, as seen during radioluminescence. The origins of the changes are discussed.     

Defect-controlled growth of ZnO nanostructures using its different zinc precursors and their application for effective photodegradation

Various zinc precursors, such as zinc acetate, zinc nitrate, zinc sulfate, and zinc chloride, have been used to control the formation of zinc oxide (ZnO) nanostructures onto aluminum substrate by chemical means. FESEM images of the ZnO nanostructures showed the formation of different morphologies, such as flakes, nanowalls, nanopetals, and nanodisks, when the nanostructures were synthesized using zinc acetate, zinc nitrate, zinc sulfate, and zinc chloride precursors, respectively. The TEM image of disk-like ZnO nanostructures formed using zinc chloride as a precursor revealed hexagonally shaped particles with an average diameter of 0.5 μm. Room-temperature photoluminescence (PL) spectra revealed a large quantity of surface oxygen defects in ZnO nanodisks grown from zinc chloride compared with those using other precursors. Furthermore, the ZnO nanostructures were evaluated for photocatalytic activity under ultraviolet (UV) light illumination. Nanostructures having a disk-like shape exhibited the highest photocatalytic performance (k = 0.027 min⁻¹) for all the ZnO nanostructures studied. Improved photocatalytic activity of ZnO nanodisks was attributed to their large specific surface area (4.83 m² g⁻¹), surface oxygen defects, and super-hydrophilic nature of their surface, which is particularly suitable for dye adsorption.     

Weathering of copper-amine treated wood

In this study, the effect of ultraviolet light (UV) irradiation and water spray on color, contact angle and surface chemistry of treated wood was studied. Southern pine sapwood (Pinus Elliottii.Engelm.) treated with copper ethanolamine (Cu-MEA) was subjected to artificially accelerated weathering with a QUV Weathering Tester. The compositional changes and the surface properties of the weathered samples were characterized by Fourier transform infrared (FTIR) spectroscopy, color and contact angle measurements. FTIR indicated that MEA treatment was not found to slow down wood weathering. FTIR spectrum of MEA-treated sample was similar to that of the untreated SP. However, the Cu-MEA treatment retarded the surface lignin degradation during weathering. The main changes in FTIR spectrum of Cu-MEA treatment took place at 915, 1510, and 1595 cm⁻¹. The intensity of the bands at 1510 and 1595 cm⁻¹ increased with the Cu-MEA treatment. Both untreated and MEA-treated exhibited higher ΔE than the Cu-MEA treated samples, indicating that MEA treatment did not retard color changes. However, ΔE decreased with increasing copper concentration, suggesting a positive contribution of Cu-EA to wood color stability. The contact angle of untreated and MEA-treated samples changed rapidly, and dropped from 75 ± 5° to 0° after artificial weathering up to 600 h. Treatment with Cu-MEA slowed down the decreasing in contact angle. As the copper concentration increases, the rate of change in contact angle decreases.     

UV detectors based on nanocrystalline ZnO films

Nanocrystalline films of zinc oxide deposited by rf magnetron sputtering are used as a working material of UV detectors. The photoelectric performance of UV detectors versus ZnO deposition conditions is studied. The influence of the surface topology of the UV detectors on their efficiency is examined with an atomic force microscope.