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The partition function Q, entropy S and heat capacity C of a Morse oscillator are numerically evaluated from its overtone vibrational spectrum for typical values of the experimentally accesible fundamental frequency v and dissociation energy D. We show that significant departures from limiting harmonic oscillator values are to be expected for shallow potential wells. 相似文献
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Piras A Colussi S Trovarelli A Sergo V Llorca J Psaro R Sordelli L 《The journal of physical chemistry. B》2005,109(22):11110-11118
A series of CeO(2)/Al(2)O(3) samples with different ceria loadings in the range 0-25 wt % (0, 2, 5, 7.5, 15, and 25%) were prepared by incipient wetness and studied using several complementary techniques such as Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), temperature-programmed reduction (TPR), Raman, high-resolution transmission electron microscopy (HRTEM), and extended X-ray absorption fine structure (EXAFS). The aim of the investigation was to understand the behavior of ceria when deposited on alumina and treated under oxidizing and reducing conditions at high temperature (T >/= 1273 K). It is shown that ceria can partially stabilize alumina toward the formation of low-surface-area phases up to 1373 K under oxidizing conditions, while enhanced stabilization is observed under reducing conditions, being effective up to 1473 K. A detailed quantitative temperature-programmed reduction (TPR) analysis made at different loadings and calcination temperatures allowed us to identify three characteristic regions where the reduction of small and large ceria crystallites occurs with the formation of CeAlO(3) crystallites at high temperature. These are likely responsible for surface-area stabilization. For dispersed ceria samples, reduction takes place almost exclusively at low temperature (<700 K), while a shift to higher temperatures is observed upon increasing the ceria particle size. A fraction of Ce, in samples at low loadings, is stable in the lower oxidation state, even if subjected to strongly oxidizing conditions. 相似文献
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The kinetics of acetylene polymerization initiated by neopentane (Np) or acetone (Ac) decompositions has been investigated in a static reactor dynamically coupled to a modulated beam mass spectrometer between 850–950 K. Overall rates follow the expression: R = ?d[C2H2]/dt = ks[X]1/2[C2H2] + ku[C2H2]2 (I), where X represents Np or Ac and ks, ku the rate constants of the sensitized and unsensitized reactions, respectively. The rate law of the sensitized reaction clearly suggests a chain polymerization mechanism with ks = kp(ki/kt)1/2 (i, t, and p stand for initiation, termination, and propagation, respectively). Remarkably, the derived values of kp are nearly independent of the sensitizer, although Ac acts as a source of methyl radicals whereas Np also produces hydrogen atoms, and fall in the expected range for the addition of vinylic radicals to acetylene. It is shown that a chain transfer process involving the fast [1,5] intramolecular hydrogen atom shift in 4-methyl-buta-1,3-dien-1-yl radicals (CH3? CH ? CH? CH ??H) followed by further addition to C2H2 and aromatization, transforms methyl radicals into hydrogen atoms and is able to account for the presence of toluene among the products of the sensitized reactions. Based on current thermochemical data for the but-1-en-3-yn-2-yl radical (CH2??? C?CH) and present rates of propagation it is argued that if the unsensitized polymerization of acetylene also proceeded by a vinyl radical chain, then even the most favorable self-initiation reaction: 2C2H2 = C4H3 + H (a), would be far too slow. Finally, present results also show that acetone at impurity levels (? 0.1%) can not provide fast enough spurious initiation rates in chain mechanisms for the “unsensitized” acetylene pyrolysis at pressures above 10 torr. 相似文献
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A kinetic study of the very low-pressure pyrolysis of ethylbenzene (I), 2-phenylethylamine (II), and N,N-dimethyl 2-phenylethylamine (III) above 900 K yields the heats of formation of aminomethyl (A) and N,N-dimethylaminomethyl (B) radicals: ΔH?, 300 K(A) = 30.3 and ΔH?, 300 K(B) = 27.5 kcal/mol. The difference of stabilization energies Es, (relative to methyl radicals): Δ = Es(B) ? Es(A) = (2 ± 1) kcal/mol, conforms to similar effects in methyl substituted alkyl and amino free radicals. 相似文献
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The ESR spectrum of AsF4, generated by γ-radiolysis of a solid solution of AsF3 in SF6, is reported. The 75As hyperfine interaction is 1576 G, and the two kinds of 19F nuclei have interactions of 49 G (equatorial) and 238 G (apical). These data are compared with those from the well-known PF4 radical. 相似文献