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A recent study on the morphological evolution of a metal tip has shown that free evaporation must have a vital influence on the determination of the surface self-diffusion coefficient Ds in the high temperature region. We now present a method which enables the experimental determination of Ds: (1) in the presence of an evaporation, and (2) in a relatively great region of temperature. As an example, Ds of molybdenum has been measured. We obtain: (1) activation energy Qs = 48.3 kcal/mole and diffusivity Do = 0.3 cm2/sec for T < 0.6TF (TF melting temperature), and (2) Qs = 76.7 kcal/mole and Do = 690 cm2/sec for 0.65 TF < T < 0.85 TF; which indicate a change of the diffusion mechanism at about 0.65 TF. 相似文献
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Experimental determinations of temperature dependence of surface self-diffusion coefficient of several metals exhibit a strong increase in Ds values and in activation energy for temperatures near the melting point Tm. This variation is illustrated by a bending of the Arrhenius plot of surface self-diffusion coefficients of tungsten, which are obtained experimentally by tip profile variation technique. For T/Tm < 0.75 the apparent activation energy for W is 2.85 eV and the pre-exponential term is equal to 0.24 cm2/s, while for T/Tm > 0.75 we have respectively 5.57 eV and 1.08 × 104 cm2/s. To account for these unexpected variations in the activation energy and diffusivities, the hypothesis that the surface mass transport mechanism changes from individual atomic jumps at low temperatures towards a cooperative motion at temperatures near the bulk melting point, namely a viscous mechanism, is proposed. This model is based on the postulation of the formation of a 2D dense fluid on the metallic surfaces about 75% of the bulk melting temperature. Discussions of existing models on surface diffusion proposed by Rhead, by Bonzel, or by Tsong are given, and a technique to characterize surface viscosity of a 2D dense fluid is suggested. 相似文献
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Huong Le Thi Hoi Tran Minh Chung Nguyen Thanh Binh Tran Duc Son Doan Hoang Ogunwande Isiaka Ajani 《Chemistry of Natural Compounds》2022,58(5):943-946
Chemistry of Natural Compounds - 相似文献
118.
The disordered structures and low temperature dielectric relaxation properties of Bi1.667Mg0.70Nb1.52O7 (BMN) and Bi1.67Ni0.75Nb1.50O7 (BNN) misplaced-displacive cubic pyrochlores found in the Bi2O3-MIIO-Nb2O5 (M=Mg, Ni) systems are reported. As for other recently reported Bi-pyrochlores, the metal ion vacancies are found to be confined to the pyrochlore A site. The B2O6 octahedral sub-structure is found to be fully occupied and well-ordered. Considerable displacive disorder, however, is found associated with the O′A2 tetrahedral sub-structure in both cases. The A-site ions were displaced from Wyckoff position 16d (, , ) to 96 h (, , ) while the O′ oxygen was shifted from position 8b (, , ) to Wyckoff position 32e (, , ). The refined displacement magnitudes off the 16d and 8b sites for the A and O′ sites were 0.408 Å/0.423 Å and 0.350 Å/0.369 Å for BMN/BNN, respectively. 相似文献
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In this study we synthesized a novel graphene-oxide (GO) based CNG repeat hairpin probing system capable of detecting target CAG and CTG DNA repeat sequences. The fluorescence of the 30-mer CNG repeat hairpin structure was quenched dramatically by GO in the absence of the target sequence, with a high quenching constant [K = 0.030 (mg/mL)?1]. We optimized the quenching behavior of this probing system by using graphene oxide (GO) to induce a high degree of discrimination factor (44.6 times) between the fluorescence of the target sequence and that of other non-target sequences. All detection process is explained by displacement mechanism using adsorption, desorption, and hybridization of probe with target DNA sequence on the GO. Graphene-oxide (GO) based CNG repeat hairpin probing system exhibited high sensitivity and selectivity to the target CNG repeat sequence and its detection process is so simple and quick. 相似文献