Oxygen-free Media Research Articles

Lifetime of the thermoemission cathodes of nitrogen plasma generators

Implementation of plasma technologies widely introduced into modern industrial processes requires plasma generators with a high efficiency, great economy, and long lifetime. The last requirement is caused by the necessity of an increase in the lifetime of the most easily worn plasmatron elements, i.e., the electrodes and, primarily, their cathode units. Thermoemission cathodes made of tungsten, the most refractory metal with the melting point Tm = 3695 K, are widely exploited in high-current (I = 300–1000 A) plasmatrons operating in oxygen-free media (inert gases like nitrogen and hydrogen). By the middle of the 1980s, due to the discovery and application of the phenomenon of the recirculation of electrode material in the cathode zone, erosion of the cooled thermocathode was successfully reduced to G = 10–10 g/C. The present study performed by means of a plasmatron with a self-adjusting arc length concerns important problems, such as measurement of the emission current, determination of cathode material erosion, and analysis of means of enhancement of the thermoemission cathode lifetime. Through the investigations performed and the resource tests of the above-mentioned plasmatron type with currents of 300–500 A, with nitrogen as the working gas, the requirements for the cathode and the plasmatron operating regimes providing for its low erosion losses, G ≤ 10–10 g/C, at a cathode surface temperature close to its melting point are determined. The experimentally obtained densities of the electron emission currents exceed, by an order of magnitude, those calculated according to the Richardson–Dushman theory with the Schottky correction and with account for the photoemission on the cathode under the action of the resonance emission generated by the positive column of the arc. In that connection, attention is paid to the mechanism of the anomaly electron emission proposed by S.V. Lebedev and caused by occurrence of the Frenkel defect accompanied by crystal lattice deformation, Fermi energy increase, and the respective decrease in the electron-from-metal work function. The obtained estimates of the Frenkel defect concentration in tungsten at the premelting temperatures are a cogent argument in favor of the anomaly electron emission concept.

Thermal treatment of fleshing residue for producing syngas

In this paper, characteristics of syngas from the pyrolysis and gasification of tannery industry fleshing residues has been investigated. Pyrolysis and gasification behaviour were evaluated in terms of syngas composition and total energy production. Dried air and pure oxygen (99%) were used as gasification agents. Experiments were also performed in oxygen-free media. H2, CO and CH4 levels in producer gas were analysed and calorific values were calculated. Besides, weight and volume reductions during each experiment were reported. Dried air flow varied between 0.05 and 0.5 L/min during experiments. Syngas composition with higher calorific value was achieved between 0.1–0.2 L/min. It was determined that calorific values decreased related to the decrease in volume percentages of H2, CO and CH4 over 0.2 L/min gas flow. According to the statistical analysis, gasification agent had an important role in gasification performance. Average calorific value, achieved by pure oxygen, was higher than the value achieved by dried air. The lowest efficiency was achieved in oxygen-free media.

Thermal magnetic susceptibility data on natural iron sulfides of northeastern Russia

Dependences of magnetic susceptibility (MS) on the temperature of natural iron sulfide samples (pyrite, marcasite, greigite, chalcopyrite, arsenopyrite, pyrrhotite) from the deposits of northeastern Russia were studied. The thermal MS curves for pyrite and marcasite are the same: On heating, MS increases at 420–450 °C, and unstable magnetite (maghemite) and monoclinic pyrrhotite with a well-defined Hopkinson peak are produced. In oxygen-free media with carbon or nitrogen, magnetite formation is weak, whereas pyrrhotite generation is more significant. The heating curves for chalcopyrite are similar to those for pyrite. They show an increase in MS at the same temperatures (420–450 °C). However, stable magnetite is produced, whereas monoclinic pyrrhotite is absent. In contrast to that in pyrite, marcasite, and chalcopyrite, magnetite formation in arsenopyrite begins at >500 °C. Arsenopyrite cooling is accompanied by the formation of magnetite (S-rich arsenopyrite) or maghemite (As-rich arsenopyrite) with a dramatic increase in MS. Arsenopyrite with an increased S content is characterized by insignificant pyrrhotite formation. Greigite is marked by a decrease in MS on the heating curves at 360–420 °C with the formation of unstable cation-deficient magnetite.Monoclinic pyrrhotite is characterized by a decrease in MS at ∼320 °C, and hexagonal pyrrhotite, by a transition to a ferrimagnetic state at 210–260 °C. The addition of organic matter to monoclinic pyrrhotite stimulates the formation of hexagonal pyrrhotite, which transforms back into monoclinic pyrrhotite on repeated heating. The oxidation products of sulfides (greigite, chalcopyrite) show an increase in MS at 240–250 °C owing to lepidocrocite.

A TD-DFT Study on the Photo-Physicochemical Properties of Chrysophanol from Rheum

As a naturally occurring anthraquinone pigment, chrysophanol (MHAQ) has attracted considerable attention in recent years owing to its efficient photosensitivity under the solar spectrum. Considering the successful use of time-dependent density functional theory (TD-DFT) in investigating the photo-physicochemical behaviors of dyes and pigments, we performed a study by means of TD-DFT calculations, which provided us with various excited state properties of chrysophanol, including absorption spectrum, lowest triplet excited-state energy, vertical electron affinity and vertical ionization potential. On the basis of the calculated results, the photosensitive mechanisms of chrysophanol were discussed and some deeper insights were gained. First, we indicated that the experimentally observed chrysophanol’s photo-damage to DNA in oxygen-free media is more likely to arise from MHAQ•+ rather than from T1 state chrysophanol. Second, we revealed that it is the MHAQ•− that is responsible for the O2•− generation in solvents. Based on the photosensitive activities, chrysophanol may be potentially used as the photodynamic medicine for clinical therapy of the diseases occurring on the shallow surface and vascular capillary diseases.

Photochemical stability of nitroxyl derivatives

Different classes of compounds with imidazoline radicals were studied by EPR spectroscopy. The effects of light and atmospheric oxygen on the stability of these compounds in alcoholic solutions were investigated. The study of the photochemical stability of rhodium complexes with imidazoline radicals in oxygen-containing and oxygen-free media demonstrated that the photolysis of these compounds in the absence of oxygen causes the disappearance of paramagnetism. The reaction is reversible, and the observed effects are due to the formation of hydroxylamine groups via the interaction between excited nitroxyl radicals and the solvent in the absence of oxygen. When present in this system, oxygen deexcites the nitroxyl groups. A similar effect of oxygen is observed for nitroxyl derivatives of the fullerenes C60 and C70. A quite different photolytic behavior is shown by copper complexes with bidentately bonded nitroxyl radicals. These compounds are stable to photolysis in both oxygen-containing and oxygen-free media. It was demonstrated using phenyl-tert-butylnitrone (PBN) as the spin trap that photolysis in the absence of the trap results in the decomposition of the copper complex to copper metal. It is assumed that PBN incorporates into the complex at free coordination sites and competes with the copper ion in its reaction with the earlier formed radical of the ligand.

Iron and carbon metabolism by a mineral-oxidizing Alicyclobacillus-like bacterium

A novel iron-oxidizing, moderately thermophilic, acidophilic bacterium (strain "GSM") was isolated from mineral spoil taken from a gold mine in Montana. Biomolecular analysis showed that it was most closely related to Alicyclobacillus tolerans, although the two bacteria differed in some key respects, including the absence (in strain GSM) of varpi-alicyclic fatty acids and in their chromosomal base compositions. Isolate GSM was able to grow in oxygen-free media using ferric iron as terminal electron acceptor confirming that it was a facultative anaerobe, a trait not previously described in Alicyclobacillus spp.. The acidophile used both organic and inorganic sources of energy and carbon, although growth and iron oxidation by isolate GSM was uncoupled in media that contained both fructose and ferrous iron. Fructose utilization suppressed iron oxidation, and oxidation of ferrous iron occurred only when fructose was depleted. In contrast, fructose catabolism was suppressed when bacteria were harvested while actively oxidizing iron, suggesting that both ferrous iron- and fructose-oxidation are inducible in this acidophile. Isolate GSM accelerated the oxidative dissolution of pyrite in liquid media either free of, or amended with, organic carbon, although redox potentials were significantly different in these media. The potential of this isolate for commercial mineral processing is discussed.

A TD-DFT study on photo-physicochemical properties of hypocrellin A and its implications for elucidating the photosensitizing mechanisms of the pigment

As a naturally occurring perylenequinonoid pigment, hypocrellin A (HA) has gained much attention in recent years owing to its excellent photosensitive properties. However, there is little theoretical study on the photo-physicochemical behaviors of HA, which stimulated our interest to perform the study by means of time-dependent density functional theory (TD-DFT) calculations. In this paper, the TD-DFT calculations provide various excited state properties of HA in polar and non-polar solvents, including absorption spectrum, lowest triplet excited state energy, vertical electron affinity and vertical ionization potential, by which we can get some deeper insights into the photosensitive mechanisms of hypocrellin. First, although HA can photo-generate 1O 2 through energy transfer in both non-polar and polar solvents, it gives birth to O 2 − only in polar solvents, and it is the HA anion (generated from autoionization) that are responsible for the O 2 −-generation. Second, HA cation (also generated from autoionization) is more favored in thermodynamics than triplet excited HA to accept electron from DNA bases. As a result, HA cation likely takes more responsibility than excited HA for the pigment's photo-damage to DNA in oxygen-free media.

Photosensitized damage to calf thymus DNA by a hypocrellin derivative: mechanisms under aerobic and anaerobic conditions

The di-cysteine substituted hypocrellin B (DCHB) derivative has been found to be a potential phototherapeutic agent and exhibit photosensitized damage to DNA. Electronic paramagnetic resonance (EPR) and spectrophotometry demonstrate that one-electron transfer from calf thymus DNA to triplet DCHB induces the generation of the reduced form of DCHB (DCHB − radical), followed by the second electron transfer from DNA to DCHB − or the disproportionation of DCHB − to form the hydroquinone of DCHB (DCHBH 2) in anaerobic conditions. This electron transfer process induces the direct damage to DNA in oxygen-free media and contributes partly to the damage of DNA in aerobic media. Superoxide radical and hydroxyl radical are formed with enhanced efficiencies while singlet oxygen is generated with a reduced efficiency from irradiation of DCHB and DNA solution under aerobic conditions as compared with the case in the absence of DNA. All of three reactive oxygen species play an evident role in the photosensitized damage to DNA in aerobic system in addition to the direct electron-transfer damage.

Post-irradiation effect and reductive dechlorination of chlorophenols at oxygen-free TiO 2/water interfaces in the presence of prominent hole scavengers

Heterogeneous photocatalysis with TiO 2 nanoparticles for the detoxification of waste-waters and other media is typically achieved by oxidative pathways involving ultimately photogenerated surface-trapped holes (i.e. surface-bound OH radicals). However, reductive pathways may coexist. The degradation of 2,4-dichlorophenol (2,4-DCP) and pentachlorophenol (PCP) was examined in irradiated O 2-free aqueous media in the presence of prominent hole scavengers such as polyethyleneimine (PEI), triethylamine (TEA), 2-propanol (IPA) and ethylenediaminetetraacetic acid (in the EDTA 4− state) to assess the role of photogenerated electrons. The presence of these scavengers imparts a blue-violet color to the nanoparticles corresponding to a broad absorption band in the 400–850 nm region. A post-irradiation effect was observed when irradiation was terminated, which we ascribe to an electron transfer from radical species formed by hole scavenging to the TiO 2 particles (reminiscent of the doubling current phenomena in semiconductor electrodes). The two chlorophenols degraded in oxygen-free media, a process inhibited by PEI, TEA and IPA. In the presence of EDTA, degradation of 2,4-dichorophenol occurred by dechlorination only after a 120-min induction period, a process attributed to the accumulated electrons on the particle surface. HPLC analyses revealed that the intermediate products were not those expected from typical oxidative pathways in aerated aqueous media. The degradative process is inferred to take place first by electron attachment to the halophenol to yield phenoxy related radicals and then by secondary reactions with water and/or with each other.

ESR investigations on radical formation after ultraviolet irradiation of trypsin in various polar and nonpolar media

Crystalline trypsin was irradiated with ultraviolet light of 254 nm in various oxygen-free suspension media at a temperature of 278 K. Using electron spin resonance spectroscopy long-living free radicals were detected in the enzyme. Unpaired electrons are mainly localized at Cα-atoms of the polypeptide chain and at the sulphur of damaged disulfide bridges. The ultraviolet irradiation of trypsin leads to greater yields of thiyl radicals RS · in nonpolar media than in polar ones. If the dielectric constant is chosen as a measure of polarity a relationship between the relative concentration of thiyl radicals [RS ·]rel and the dielectric constante in form of [RS ·]rel ∼ 1/e is obtained. The manner how the suspension media may influence the formation of thiyl radicals is discussed. The results allow the conclusion that the radiation-induced scission of disulflde bonds is of significance for the inactivation of trypsin.

Radiation Chemical Studies of Hemocyanin in Oxygen-Free Media

The oxygen-carrying capacity of Busycon and Limulus hemocyanins is eliminated following irradiation of the protein with cobalt-60 γ-rays in neutral, oxygen-free media. The loss of oxygen-carrying capacity of the protein is measured from the change of the characteristic copper band at 340 nm, before and after irradiation. The limiting ${\rm G}(-{\rm O}_{2})$ values in the absence of scavengers are 0.026 and 0.044 for Busycon and Limulus hemocyanins, respectively. Radiation effects in oxygen-free media on Busycon and Limulus hemocyanins can either be enhanced or prevented by appropriate scavengers. The Busycon methemocyanin activity is restored by the hydrated electron, $e_{{\rm aq}}^{-}$, and the secondary ${\rm CO}_{2}{}^{-}$ radical. The loss of oxygen-carrying capacity of hemocyanin by γ irradiation is attributed, therefore, to the OH radical which oxidizes the protein-bound copper at the active site, thus forming methemocyanin. The restoration of the oxygen-carrying capacity of Busycon methemocyanin is...

Heat treatment of metals in oxygen-free media

Heat treatment of metals in oxygen-free media