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  • Küçük Resim
    Öğe
    The Di-π-methane Photorearrangement of 2,3-Disubstituted Benzobarrelenes and Benzonorbornadiene − Substituent Effects in Regioselectivity
    (Wiley-V C H Verlag Gmbh, 2002) Altundaş, R.; Dastan, A.; Ünaldi, N.S.; Güven, K.; Uzun, O.; Balci, M.
    2,3-Disubstituted benzobarrelene and benzonorbornadiene derivatives 16, 17, and 18, containing electron-withdrawing and electron-donating substituents, have been synthesized and subjected to triplet-sensitized photoisomerization. Methyl 3-methyl-2-benzobarrelenecarboxylate (16) gave two di-pi-methane rearrangement products. However, methyl 3-cyano-2-benzobarrelenecarboxyidte (17) underwent an intramolecular [2(pi) + 2(pi)] cycloaddition reaction, whilst methyl 3cyano-2-benzonorbornadienecarboxylate (18) formed dimer 40. The formation of these products is discussed in terms of the radical stabilizing effect of the substituents and the destabitizing effect on the formation of cyclopropane ring.
  • Küçük Resim
    Öğe
    Hardness and microstructural analysis of Bi1.6Pb0.4Sr2Ca2−xSmxCu3Oy polycrystalline superconductors
    (Elsevier Science Sa, 2006) Kölemen, U.; Uzun, O.; Yilmazlar, M.; Güçlü, N.; Yanmaz, E.
    Bi1.6Pb0.4Sr2Ca2-xSmxCu3Oy superconductors with nominal substitution parameters x = 0.0, 0.001, 0.005, and 0.1 were prepared by the conventional solid-state reaction powder compacting method. The effects of Sm substitution on the phase formation, microhardness and surface microstructure of the samples were investigated. Phase examination by X-ray diffraction indicated that Sm substitution enhanced the formation of the Bi-2212 phase. Morphological investigations by scanning electron microscope revealed that both the number and size of the voids on the sample surfaces decreased with Sm enrichment. This result was also supported by density measurements of the samples. Additionally, indentation-induced Vickers microhardness testing results in the applied load range 0.245-2.940 N were presented. It was shown that microhardness values decreased non-linearly with increasing applied load in all cases. The experimental results were analyzed using the Kick's law, the Hays-Kendall approach and the modified proportional specimen resistance (PSR) model. The analyses revealed that Kick's law failed to explain the observed variations. Besides, the modified PSR model was more suitable than the Hays-Kendall approach to estimate load independent hardness of BSCCO superconductor. (c) 2005 Elsevier B.V. All rights reserved.
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    Öğe
    Vickers microhardness studies of Fe-Mn binary alloys
    (Elsevier Science Bv, 2004) Bektes, M.; Uzun, O.; Aktürk, S.; Ekinci, A.E.; Uçar, N.
    The room temperature microhardness of Fe-Mn binary alloys with different compositions was measured using a Vickers microhardness tester in the applied load ranging from 0.245 to 4.9 N. A significant indentation size effect was observed for each sample at low indentation test loads. The classical Meyer's law and proportional specimen resistance model were used to analyze the microhardness behaviour. It was found that the Fe-Mn binary alloys become harder with increasing Mn composition and this can be attributed to solid-solution hardening.