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It has been shown that selected types of substituents are able to vary the oxidative behavior of multi-walled carbon nanotubes. Such substituents investigated were sterically hindered secondary amino groups bonded in grafted piperidine units and covalently bonded bromine groups. Their interference and activity was preliminary determined in the model cumene and then in oil diesel fraction and low density polyethylene oxidation reactions. Results obtained indicate that chemical linking of amine moieties containing –NH groups directly to the carbon nanotubes core significantly increases their intrinsic anti-oxidative capacity while the grafting of Br-groups provokes the opposite functioning of the pristine samples. This inference was proved by thermogravimetric and differential thermal analysis of the polyethylene composites and experiments on profound aerobic oxidation of petroleum naphthenic fraction derived from the commercial Baku oils blend diesel cut.
Bulk metallic glasses (BMG) are amorphous metal alloys known for their unique physical and mechanical properties. In the present study, the formation of femtosecond (fs) laser-induced periodic surface structures (LIPSS) on the Zr-based BMGs Zr46Cu46Al8, Zr61Cu25Al12Ti2, Zr52.5Cu17.9Al10Ni14.6Ti5 (Vit105) and Zr57Cu15.4Al10Ni12.6Nb5 (Vit106) was investigated as a function of their different chemical composition. For this purpose, LIPSS were generated on the sample surfaces in an air environment by fs-laser irradiation (λ = 1025 nm, τ = 300 fs, frep = 100 kHz). The surface topography was characterized by scanning electron microscopy and atomic force microscopy. Moreover, the impact of LIPSS formation on the structure and chemical surface composition was analyzed before and after fs-laser irradiation by X-ray diffraction and X-ray photoelectron spectroscopy as well as by transmission electron microscopy in combination with energy dispersive X-ray spectroscopy. Despite the different chemical composition of the investigated BMGs, the fs-laser irradiation resulted in almost similar properties of the generated LIPSS patterns. In the case of Zr61Cu25Al12Ti2, Vit105 and Vit106, the surface analysis revealed the preservation of the amorphous state of the materials during fs-laser irradiation. The study demonstrated the presence of a native oxide layer on all pristine BMGs. In addition, fs-laser irradiation results in the formation of laser-induced oxide layers of larger thickness consisting of an amorphous ZrAlCu-oxide. The precise laser-structuring of BMG surfaces on the nanoscale provides a versatile alternative to thermoplastic forming of BMG surfaces and is of particular interest for the engineering of functional material surfaces.
The precise analysis of cation diffusion profiles through corrosion scales is an important aspect to evaluate corrosion phenomena under multicomponent chemical load, as during high‐temperature corrosion under deposits and salts. The present study shows a comprehensive analysis of cation diffusion profiles by electron microprobe analysis and microbeam X‐ray absorption near edge structure (µ‐XANES) spectroscopy in mixed oxide/sulfide scales grown on Fe–Cr model alloys after exposing them to 0.5% SO2. The results presented here correspond to depth‐dependent phase identification of oxides and sulfides in the corrosion scales by µ‐XANES and the description of oxidation‐state‐dependent diffusion profiles. Scales grown on low‐ and high‐alloyed materials show both a well‐pronounced diffusion profile with a high concentration of Fe3+ at the gas and a high concentration of Fe2+ at the alloy interface. The distribution of the cations within a close‐packed oxide lattice is strongly influencing the lattice diffusion phenomena due to their different oxidation states and therefore different crystal‐field preference energies. This issue is discussed based on the results obtained by µ‐XANES analysis.
Irradiation assisted stress corrosion cracking (IASCC) is a form of intergranular stress corrosion cracking that occurs in irradiated austenitic alloys. It requires an irradiated microstructure along with high temperature water and stress. The process is ubiquitous in that it occurs in a wide range of austenitic alloys and water chemistries, but only when the alloy is irradiated. Despite evidence of this degradation mode that dates back to the 1960s, the mechanism by which it occurs has remained elusive. Here, using high resolution electron backscattering detection to analyze local stress-strain states, high resolution transmission electron microscopy to identify grain boundary phases at crack tips, and decoupling the roles of stress and grain boundary oxidation, we are able to unfold the complexities of the phenomenon to reveal the mechanism by which IASCC occurs. The significance of the findings impacts the mechanical integrity of core components of both current and advanced nuclear reactor designs worldwide.
Chemiluminescence for the early detection of weathering effects of coatings - Part I: Fundamentals
(2010)
A report is presented on the development of a test for determining the long-term oxidation resistance of polyolefin materials, which is accelerated not only by elevated temperatures but also by increased oxygen pressure and a stirred aqueous medium. The method permits durability tests to be carried out at markedly lower temperatures with reasonable testing durations and significantly reduced diffusion limited oxidation effects and allows for physical and chemical impacts of aqueous media. Oxidative durability can be assessed by autoclave immersion exposures at three different temperatures (60, 70 and 80 degrees C) and 50 bar oxygen pressure and at 80 degrees C and two different oxygen pressures (usually 10 and 20 bar). Accelerated test data are evaluated by modified Arrhenius equations with the application of a three-dimensional regression analysis. The application of the test method to various materials, including PP non-woven materials and geosynthetics is demonstrated. 10 refs.
On the thermal behaviour and thermo-oxidative stability of liquid crystalline triphenylene compounds
(1999)
Columnar discotic materials are considered for applications in the area of photoconductivity and light-emitting diodes. A major requirement is their stability at elevated temperatures and in the presence of oxygen. The thermal and thermo-oxidative behaviour of discotic triphenylene derivatives was investigated by us using various methods, in particular by chemiluminescence (CL), UV-vis absorption spectroscopy and in situ thermogravimetry-mass spectroscopy (TG-MS). Various degradation processes are described for increasing temperature, and their influences on functional properties are discussed.
Although catalytic reductions, cross-couplings, metathesis, and oxidation of C=C double bonds are well established, the corresponding catalytic hydroxylations of C–H bonds in alkanes, arenes, or benzylic (allylic) positions, particularly with O2, the cheapest, 'greenest', and most abundant oxidant, are severely lacking. Certainly, some promising examples in homogenous and heterogenous catalysis exist, as well as enzymes that can perform catalytic aerobic oxidations on various substrates, but these have never achieved an industrial-scale, owing to a low space-time-yield and poor stability. This review illustrates recent advances in aerobic oxidation catalysis by discussing selected examples, and aims to stimulate further exciting work in this area. Theoretical work on catalyst precursors, resting states, and elementary steps, as well as model reactions complemented by spectroscopic studies provide detailed insight into the molecular mechanisms of oxidation catalyses and pave the way for preparative applications. However, O2 also poses a safety hazard, especially when used for large scale reactions, therefore sophisticated methodologies have been developed to minimize these risks and to allow convenient transfer onto industrial scale.