Results 1 - 10 of 100496
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[en] From the viewpoint of general crystal chemistry principles and on the base of modern data the structural chemistry of metal carbides is presented. The classification deviding metal carbides into 4 groups depending on chemical and physical properties is presented. The features of the crystal structure of carbides of alkali alkaline earth, transition, 4 f- and 5f-elements and their effect on physical and chemical properties are considered
[en] There has been significant concern in analyzing the structural stability, structural properties, and pressure-induced structural phase transition of refractory metal carbides, RC (R = Ti, Zr, Hf, V, Nb, and Ta), by using the three-body force potential model calculation modified approach. The more accurate description of the interionic spacing (r0) suggests that the interactions considered in the present computation are capable of correctly predicting the structural properties of these materials. In the preset paper, we have investigated the relative stability of the two competitive phases of metal carbides and discussed the possible phase transitions from its parental NaCl (B1) type phase to its most stable CsCl (B2) type phase in the pressure range 344-572 GPa. The computed interionic spacing (r0) and phase transition pressures (PT) are in reasonable agreement with the other reported data. (author)
[en] In this paper, the qualitative, quantitative and thermal characterization of a steel slag and glass cullet of high generation rate in northern Mexico were made in order to use these wastes as raw materials in the production of glass ceramics. The particle size was controlled at sizes = 75 micrometers and the major components of the slag were located in a phase equilibrium diagram for proposing a reaction temperature that leaded to the starting glass. Later, heat treatments were performed to obtain the glass ceramics. The materials were characterized by powder X-ray diffraction (XRD), differential thermal analysis coupled with thermal gravimetric analysis (DTA-TGA), reflected light optical microscopy (RLOM) and scanning electron microscopy (SEM). Subsequently, Vickers microhardness and chemical resistance tests were performed, which enabled us to propose an application of the glass ceramics. (Author) 18 refs.
[en] Environmental contamination by herbicides is related with the sorption phenomenon of these compounds in the soils. The behaviour of paraquat, 2,4-D and diuron was studied in soils with different physico-chemical properties, through the Freundlich adsorption and desorption isotherms, using 14C-radiolabeled herbicides. Results of the range of the adsorption-desorption of each herbicide was related mainly with the chemical characteristics of these compounds. (author)
[pt]A possibilidade de contaminacao ambiental por herbicidas esta relacionada com o fenomeno sortivo destes compostos nos solos. Neste trabalho estudou-se o comportamento do paraquat, 2, 4-D e diuron em solos com diferentes propriedades fisico-quimicas, atraves de isotermas de adsorcao e dessorcao de Freundlich, utilizando-se herbicidas marcados com Carbono-14. A extensao da adsorcao-dessorcao de cada herbicida foi relacionada principalmente com as caracteristicas quimicas destes compostos. (autor)
[en] Standard autoradiographic techniques have been used to describe the temporal pattern in the transfer of radioactive carbon from the chlorotype Tetraselmis suesica to females of theneritic copepod Temora longicornis. After 4 and 6 h exposure to labelled phytoplankton, silver grains were deposited only in the cells of the gut-wall. After 24 h exposure, grains were deposited in most tissues, with highest concentrations in cells of the gut-wall and in oocytes within the ovary. Experiments with Centropages typicus, Centropages hamatus and Labidocera aestiva Wheeler demonstrated that much of the assimilated label was not turning over rapidly and that the amount of label assimilated was correlated with known food preferences of these species. (orig./AJ)
[de]Mit Hilfe von Standard-Autoradiographieverfahren wurde der zeitliche Verlauf der Uebertragung von Radiokohlenstoff vom Chlorphyten Tetraselmis suesica auf Weibchen von Temora longicornis beschrieben. Nach 4 h und 6 h Einwirkung von markiertem Phytoplanton wurden Silberkoerner nur in den Zellen der Darmwand abgelagert. Nach 24 h wurden Koerner in den meisten Geweben abgelagert; die hoechsten Konzentrationen wurden dabei in Zellen der Darmwand und in Oozyten im Ovarium ermittelt. Versuche mit Centropages typicus, Centropages hamatus und Labidocera aestiva Wheeler zeigten, dass ein Grossteil des assimilierten Label nicht schnell umgesetzt wurde und dass die Menge des assimilierten Label der von diesen Spezies bevorzugten Nahrung entspricht. (orig./AJ)
[en] In order to study the metabolic fate of alacepril, an anti-hypertensive agent, the 14C-labeled compound of alacepril and its related compounds were synthesized. [Prolyl-U-14C]alacepril was synthesized in over-all yield of 32.7 - 38.0 % by the mixed anhydride condensation of L-phenylalanine with [prolyl-U-14C]DU-1163, which had been prepared from L-[U-14C]proline and N-(S-3-acetylthio-2-methylpropanoyloxy)succinimide. [Prolyl-U-14C]captopril and [prolyl-U-14C]DU-1227 were prepared in high yields by hydrolysis of [prolyl-U-14C]DU-1163 and [prolyl-U-14C]alacepril, respectively. [Prolyl-U-14C]captoril-cysteine was synthesized by condensation of [prolyl-U-14C]captopril with cystine S-monoxide in 55.0 % yield. (author)
[en] (1) The interrelationships between tiller generations of Carex bigelowii Torr. ex Schwein were investigated at two subarctic sites by labelling young tiller modules with 14C and detecting its translocation, and by severing modules at increasing distances from the youngest tiller generation. Tiller survival, regeneration and physiological continuity were all measured. All of the investigations were on systems produced by vegetative proliferation and subsequent fragmentation: recruitment from seedlings was not observed. (2) 14C-assimilates were translocated through the rhizome system, from the one to two years old assimilating tillers into the roots and rhizomes of nine to eleven years old tillers with only below-ground organs remaining. This shows that the roots and rhizomers of the numerous interconnected old non-assimilating tillers were alive and that their roots were probably still functioning. (3) The severing-experiment showed that the few assimilating young tiller generations were to some extent dependent on the old below-ground, non-assimilating tiller generations for their survival, growth and reproduction. Water and nutrients are probably the forms of subsidy. (4) The minimum size of a succesful physiologically functional unit was around five interconnected tiller generations. The maximum size could not be determined. (5) Apical dominance effects were detected within the rhizome system. The rhizomes keep a reserve of dormant buds. When the connection between tiller generations was severed, buds on the old rhizomes, which had been dormant for several years, developed into new tillers. These tillers were characterized by having only short rhizomes and they produced green leaves in the first season of growth. (6) The integrated system of old and young tiller generations, together with a spatial network of modules controlled by apical dominance, provide Carex bigelowii with mechanisms for locating and exploiting favorable patches in a nutrient poor, but patchy environment. (author)
[en] Flank wear progression and wear mechanisms of uncoated, coated with PVD applied single-layer TiAlN, and CVD applied multi-layer MT-TiCN/Al_2O_3/TiN cemented carbide inserts were analyzed during dry turning of hardened AISI 4340 steel (35 HRC). Experimental observations indicate that by applying a coating to the uncoated insert the limiting cutting speed increase from 62 to 200 m/min, which further extends up-to 300-350 m/min when using multi-layer coating scheme. Relatively lower wear rate seen when using single-layer TiAlN coated inserts. However, after removal of the thin layer of coating the wear rate increase rapidly, subsequently dominates the wear rate of multi-layer coated inserts. Cutting forces; especially axial and radial components have also shown the similar behavior and increase rapidly when the tool failure occurs. Flank wear, crater wear and catastrophic failure are the dominant forms of tool wear. Digital microscope and SEM images coupled with elemental analysis (EDAX) have been taken at various stages of tool life for understanding the wear mechanisms.
[en] The origin of various radioactive and inactive impurities in carbon-14-labelled carboxylic acids, alcohols, alkylhalogenides and hydrocarbons is elucidated. Purification methods of these compounds and the contingent modifications of the production technologies are described. The principles of a new synthesis of alkylhalogenides are presented