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971.
972.
Vitamin D signaling plays a key role in many important processes, including cellular proliferation, differentiation and apoptosis, immune regulation, hormone secretion and skeletal health. Furthermore, vitamin D production and supplementation have been shown to exert protective effects via an unknown signaling mechanism involving the vitamin D receptor (VDR) in several diseases and cancer types, including skin cancer. With over 3.5 million new diagnoses in 2 million patients annually, skin cancer is the most common cancer type in the United States. While ultraviolet B (UVB) radiation is the main etiologic factor for nonmelanoma skin cancer (NMSC), UVB also induces cutaneous vitamin D production. This paradox has been the subject of contradictory findings in the literature in regards to amount of sun exposure necessary for appropriate vitamin D production, as well as any beneficial or detrimental effects of vitamin D supplementation for disease prevention. Further clinical and epidemiological studies are necessary to elucidate the role of vitamin D in skin carcinogenesis. 相似文献
973.
Dipolar Cycloaddition Reactions of Azomethine Ylides Generated by endocyclic‐exocyclic 1,3‐dipole Rearrangement
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Davor Margetić Ronald N. Warrener Douglas N. Butler Craig M. Holland 《Journal of heterocyclic chemistry》2015,52(5):1447-1456
Dipolar cycloaddition of polycyclic azomethine ylides, in which the central nitrogen atom is part of a pyrrolidine ring and bears a methoxycarbonyl group with norbornenes has been shown to produce two main types of products featuring pyrrolizidine rings. In conjuction with results of quantum chemical calculations (B3LYP), mechanistic rationale was postulated. The key reaction step is unprecedented endocyclic to exocyclic azomethine ylide rearrangement by an intermolecular prototropic migration (formal [1,3] H‐shift). 相似文献
974.
Synthesis of a Neutral Mixed‐Valence Diferrocenyl Carborane for Molecular Quantum‐Dot Cellular Automata Applications
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John A. Christie Ryan P. Forrest Dr. Steven A. Corcelli Dr. Natalie A. Wasio Dr. Rebecca C. Quardokus Dr. Ryan Brown Dr. S. Alex Kandel Dr. Yuhui Lu Dr. Craig S. Lent Dr. Kenneth W. Henderson 《Angewandte Chemie (International ed. in English)》2015,54(51):15448-15451
The preparation of 7‐Fc+‐8‐Fc‐7,8‐nido‐[C2B9H10]? (Fc+FcC2B9?) demonstrates the successful incorporation of a carborane cage as an internal counteranion bridging between ferrocene and ferrocenium units. This neutral mixed‐valence FeII/FeIII complex overcomes the proximal electronic bias imposed by external counterions, a practical limitation in the use of molecular switches. A combination of UV/Vis‐NIR spectroscopic and TD‐DFT computational studies indicate that electron transfer within Fc+FcC2B9? is achieved through a bridge‐mediated mechanism. This electronic framework therefore provides the possibility of an all‐neutral null state, a key requirement for the implementation of quantum‐dot cellular automata (QCA) molecular computing. The adhesion, ordering, and characterization of Fc+FcC2B9? on Au(111) has been observed by scanning tunneling microscopy. 相似文献
975.
Understanding the explosive decomposition pathways of high‐energy‐density materials (HEDMs) is important for developing compounds with improved properties. Rapid reaction rates make the detonation mechanisms of HEDMs difficult to understand, so computational tools are used to predict trigger bonds—weak bonds that break, leading to detonation. Wiberg bond indices (WBIs) have been used to compare bond densities in HEDMs to reference molecules to provide a relative scale for the bond strength to predict the activated bonds most likely to break to trigger an explosion. This analysis confirms that X?NO2 (X=N,C,O) bonds are trigger linkages in common HEDMs such as TNT, RDX and PETN, consistent with previous experimental and theoretical studies. Calculations on a small test set of substituted tetrazoles show that the assignment of the trigger bond depends upon the functionality of the material and that the relative weakening of the bond correlates with experimental impact sensitivities. 相似文献
976.
Philippa L. Ascough Craig J. Sturrock Michael I. Bird 《Isotopes in environmental and health studies》2013,49(1):64-77
We present the results of a study testing the response of two saprophytic white-rot fungi species, Pleurotus pulmonarius and Coriolus versicolor, to charred biomass (charcoal) as a growth substrate. We used a combination of optical microscopy, scanning electron microscopy, elemental abundance measurements, and isotope ratio mass spectrometry (13C and 15N) to investigate fungal colonisation of control and incubated samples of Scots Pine (Pinus sylvestris) wood, and charcoal from the same species produced at 300 °C and 400 °C. Both species of fungi colonise the surface and interior of wood and charcoals over time periods of less than 70 days; however, distinctly different growth forms are evident between the exterior and interior of the charcoal substrate, with hyphal penetration concentrated along lines of structural weakness. Although the fungi were able to degrade and metabolise the pine wood, charcoal does not form a readily available source of fungal nutrients at least for these species under the conditions used in this study. 相似文献
977.
We study a notion called n-standardness (defined by M.E. Rossi (2000) in [10] and extended in this paper) of ideals primary to the maximal ideal in a Cohen-Macaulay local ring and some of its consequences. We further study conditions under which the maximal ideal is 3-standard, first proving results for when the residue field has prime characteristic and then using the method of reduction to prime characteristic to extend the results to the equicharacteristic 0 case. As an application, we extend a result due to T. Puthenpurakal (2005) [9] and show that a certain length associated with a minimal reduction of the maximal ideal does not depend on the minimal reduction chosen. 相似文献
978.
A short, stereoselective approach to the fused tricyclic carbon skeleton found in the tigliane and daphnane classes of diterpene natural products is described. Convergent coupling of the A- and C-rings, followed by diastereoselective cerium enolate addition and formation of a double acetal set the stage for generation of an oxonium ylide via a transient metallocarbene. An efficient Stevens [1,2]-shift furnished the 7-membered B-ring, possessing the bridgehead oxygenation pattern found in the natural systems. 相似文献
979.
Yang Hsia Eric Gnesa Simon Tang Felicia Jeffery Paul Geurts Liang Zhao Andreas Franz Craig Vierra 《Applied Physics A: Materials Science & Processing》2011,105(2):301-309
The diversity in function and mechanical behavior of spider silks, and the ability to produce these silks recombinantly, have
tremendous potential in creating a new class of biomimetic materials. Here we investigate the structural and mechanical properties
of pyriform silks from the golden orb-weaver, Nephila clavipes. Nanoscale indentation measurements using atomic force microscopy on natural pyriform silk suggests that this biomaterial
has high toughness that may be suitable for dissipating high amounts of mechanical energy. We also observed the occurrence
of highly organized nanocrystals within the pyriform silk fibers that may contribute to the remarkable energy dissipation
capability of these silks. It has been demonstrated that poly-(Gly–Ala) and poly-Ala stretches within the internal block repeat
modules of dragline silk fibroins form nanocrystals, and these nanocrystalline structures may be responsible for the high
extensibility of the dragline silks. In contrast, amino acid sequence analysis shows that PySp2 does not contain the same
motifs. In the absence of poly-(Gly–Ala) and poly-Ala repeats, we hypothesized that PySp2 contains new protein motifs sufficient
to polymerize into functional structures. To investigate the functional contributions of these novel motifs during pyriform
fiber formation, we expressed different recombinant PySp2 fibroins with various segments spanning its block repeat units.
We demonstrate that PySp2 recombinant proteins with the Pro-rich sub-block domain (PXP motifs, where X= sub-set of the amino acids A, L, or R) and/or the Ser + Gln + Ala-rich sub-block domain (QQSSVAQS motifs) are sufficient
for artificial fiber formation. Moreover, we show that recombinant PySp2 proteins that contain a single block repeat unit
can self-assemble into foam-like nanostructures. Collectively, our findings support the use of PySp2 recombinant proteins
for a wide range of biomimetic materials with morphologies ranging from fibers to porous structures. 相似文献
980.
The steady shear behavior of metallo-supramolecular polymer networks formed by bis-Pd(II) cross-linkers and semidilute entangled solutions of poly(4-vinylpyridine) (PVP) in dimethyl sulfoxide (DMSO) or N,N-dimethyl formamide (DMF) is reported. The steady shear behavior of the networks depends on the dissociation rate and association rate of the cross-linkers, the concentration of cross-linkers, and the concentration of the polymer solution. The divergent steady shear behavior-shear thinning versus shear thickening-of samples with identical structure but different cross-linker dynamics (J. Phys. Chem. Lett. 2010, 1, 1683-1686) is further explored in this paper. The divergent steady shear behavior for networks with different cross-linkers is connected to a competition between different time scales: the average time that a cross-linker remains open (τ(1)) and the local relaxation time of a segment of polymer chain (τ(segment)). When τ(1) is larger than τ(segment), shear thickening is observed. When τ(1) is smaller than τ(segment), only shear thinning is observed. 相似文献