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81.
This paper reports the results from a study of the effects of particles on the boundary layer characteristics for cross flow over a single circular cylinder in the near separation region and on a cylinder located in the second row of a small mock-up heat exchanger array. Two size ranges of particles (45–85 m and 100–200 m) with two concentrations were used for the single cylinder tests. For the array, the larger particles were used at a single concentration. For the single cylinder, the results with the larger particles at mass loading ratio of 1.0 show an obvious effect on the velocity profile shape and on the rms levels. For reduced concentrations, and for the smaller particles, the effects are less noticeable. For the array, similar trends are observed and a delay in the separation point is reported. 相似文献
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Mass-selected beams of N+ and N2+ in the energy range 5–50 eV react with molybdenum to produce a surface nitride. The relative reaction cross section for N+ reaction is higher than that of N2+ in the range 5–25 eV and N2+ exhibits a reaction threshold near 7 eV. The N2+ threshold suggests collisional dissociation prior to reaction. 相似文献
84.
The contribution of DNA strand breaks accumulating in the course of nucleotide excision repair to upregulation of the p53 tumor suppressor protein was investigated in human dermal fibroblast strains after treatment with 254 nm ultraviolet (UV) light. For this purpose, fibroblast cultures were exposed to UV and incubated for 3 h in the presence or absence of l-beta-D-arabinofuranosylcytosine (araC) and/or hydroxyurea (HU), and then assayed for DNA strand breakage and p53 protein levels. As expected from previous studies, incubation of normal and ataxia telangiectasia (AT) fibroblasts with araC and HU after UV irradiation resulted in an accumulation of DNA strand breaks. Such araC/HU-accumulated strand breaks (reflecting nonligated repair-incision events) following UV irradiation were not detected in xeroderma pigmentosum (XP) fibroblast strains belonging to complementation groups A and G. Western blot analysis revealed that normal fibroblasts exhibited little upregulation of p53 (approximately 1.2-fold) when incubated without araC after 5 J/m2 irradiation, but showed significant (three-fold) upregulation of p53 when incubated with araC after irradiation. AraC is known to inhibit nucleotide excision repair at both the damage removal and repair resynthesis steps. Therefore, the potentiation of UV-induced upregulation of p53 evoked by araC in normal cells may be a consequence of either persistent bulky DNA lesions or persistent incision-associated DNA strand breaks. To distinguish between these two possibilities, we determined p53 induction in AT fibroblasts (which do not upregulate p53 in response to DNA strand breakage) and in XP fibroblasts (which do not exhibit incision-associated breaks after UV irradiation). The p53 response after treatment with 5 J/m2 UV and incubation with araC was similar in AT, XPA, XPG and normal fibroblasts. In addition, exposure of XPA and XPG fibroblasts to UV (5, 10 or 20 J/m2) followed by incubation without araC resulted in a strong upregulation of p53. We further demonstrated that HU, an inhibitor of replicative DNA synthesis (but not of nucleotide excision repair), had no significant impact on p53 protein levels in UV irradiated and unirradiated human fibroblasts. We conclude that upregulation of p53 at early times after exposure of diploid human fibroblasts to UV light is triggered by persistent bulky DNA lesions, and that incision-associated DNA strand breaks accumulating in the course of nucleotide excision repair and breaks arising as a result of inhibition of DNA replication contribute little (if anything) to upregulation of p53. 相似文献
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Murray Riley Chandrasekaran Venkat Wierman Adam 《Foundations of Computational Mathematics》2021,21(6):1703-1737
Foundations of Computational Mathematics - Certifying function nonnegativity is a ubiquitous problem in computational mathematics, with especially notable applications in optimization. We study the... 相似文献
88.
Structural Chemistry - Even after roughly a century of quantum theory, there is still debate, sometimes rather contentious, as to the nature of the chemical bond—or is it bonds, or is it... 相似文献
89.
Abstract The polar epoxides β-cyanoethyl glycidyl ether, cyanomethyl glycidyl ether, β-chloro-β-cyanoethyl glycidyl ether, and 3,4-epoxy valeronitrile were prepared, characterized, and their polymerization reactions studied. Novel cyanoethyla-tion of glycidol produced the β-cyanoethyl glycidyl ether in a quantitative yield. Reported is the first practicable synthesis of epicyanohydrin, which failed to polymerize. Homo-, co-, and terpolymerizations of the polar β-epoxy nitriles with various alkylene oxides were achieved by employing a complex catalyst obtained from an aluminum alkyl, acetyl acetone, and water. The polymers produced a series of vulcanizable elastomers which displayed an ex-cellent balance of solvent resistance and low temperature flexibility, as well as good ozone resistance. The presence of a third monomer such as allyl glycidyl ether was required to facilitate rubber vulcanization. However, it was possible to vulcanize homo- and copolymers of β-chloro-βyanoethyl glycidyl ether through the pendant chlorine atom. Catalyst efficiency in all cases was poor due to the complexing of the aluminum catalyst by the nitrile function. 相似文献
90.
Halogen bonding is a noncovalent interaction that is receiving rapidly increasing attention because of its significance in biological systems and its importance in the design of new materials in a variety of areas, for example, electronics, nonlinear optical activity, and pharmaceuticals. The interactions can be understood in terms of electrostatics/polarization and dispersion; they involve a region of positive electrostatic potential on a covalently bonded halogen and a negative site, such as the lone pair of a Lewis base. The positive potential, labeled a σ hole, is on the extension of the covalent bond to the halogen, which accounts for the characteristic near‐linearity of halogen bonding. In many instances, the lateral sides of the halogen have negative electrostatic potentials, allowing it to also interact favorably with positive sites. In this discussion, after looking at some of the experimental observations of halogen bonding, we address the origins of σ holes, the factors that govern the magnitudes of their electrostatic potentials, and the properties of the resulting complexes with negative sites. The relationship of halogen and hydrogen bonding is examined. We also point out that σ‐hole interactions are not limited to halogens, but can also involve covalently bonded atoms of Groups IV–VI. Examples of applications in biological/medicinal chemistry and in crystal engineering are mentioned, taking note that halogen bonding can be “tuned” to fit various requirements, that is, strength of interaction, steric factors, and so forth. 相似文献