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排序方式: 共有204条查询结果,搜索用时 15 毫秒
1.
Doug Bullock 《Topology and its Applications》1994,60(3):235-248
This paper addresses two problems in the skein theory of homotopy spheres first posed by P. Traczyk. Solutions to both problems are obtained for a large class of manifolds and, since one of the basic techniques used requires the first homology group of the ambient manifold to be torsion free, the extent to which this hypothesis is actually necessary is further explored. 相似文献
2.
This paper concerns methods by which a trusted authority can distribute keys and/or broadcast a message over a network, so that each member of a privileged subset of users can compute a specified key or decrypt the broadcast message. Moreover, this is done in such a way that no coalition is able to recover any information on a key or broadcast message they are not supposed to know. The problems are studied using the tools of information theory, so the security provided is unconditional (i.e., not based on any computational assumption).In a recent paper st95a, Stinson described a method of constructing key predistribution schemes by combining Mitchell-Piper key distribution patterns with resilient functions; and also presented a construction method for broadcast encryption schemes that combines Fiat-Naor key predistribution schemes with ideal secret sharing schemes. In this paper, we further pursue these two themes, providing several nice applications of these techniques by using combinatorial structures such as orthogonal arrays, perpendicular arrays, Steiner systems and universal hash families. 相似文献
3.
Maged H. M. Sharaf Paul L. Schiff Albert N. Tackie Charles H. Phoebe Robert L. Johnson Doug Minick Ronald C. Crouch Gary E. Martin C. Webster Andrews 《Journal of heterocyclic chemistry》1996,33(3):789-797
The isolation and structure determination of cryptomisrine, a novel indolo[3,2-b]quinoline dimeric alkaloid obtained from extracts of the roots of the Ghanaian medicinal plant Cryptolepis sanguinolenta is reported. The structure determination was made via a consideration of the spectral data, including uv, ir, nmr, and mass spectra. In particular, one-dimensional proton/carbon nmr, one-dimensional nOe difference nmr, and a series of homonuclear (COSY) and inverse-detected heteronuclear two-dimensional (HMQC, HMBC) experiments were utilized, as well as high resolution FABMS. Cryptomisrine is most unusual in that its two monomeric parts apparently exist in such a C2 symmetric environment that only one set of proton and carbon nmr resonances are observed. Cryptomisrine is the first example of a dimeric indolo-[3,2-b]quinoline alkaloid to have been isolated from nature. 相似文献
4.
Jaehun Park Doug S. English Yvonne Wannemuehler Susan Carpenter Jacob W. Petrich 《Photochemistry and photobiology》1998,68(4):593-597
The light-induced antiviral activity of hypericin and hypocrellin in the presence and absence of oxygen was examined under experimental conditions where the effect of oxygen depletion could be quantified. There was a significant reduction of light-induced antiviral activity of hypericin and hypocrellin under hypoxic conditions. Interestingly, antiviral activity of hypocrellin was not observed at low oxygen levels at which hypericin retained measurable virucidal activity. This suggests that additional pathways, such as the generation of protons from excited states of hypericin, may enhance the biological activity of activated oxygen species. 相似文献
5.
Ogrin D van Poppel LH Bott SG Barron AR 《Dalton transactions (Cambridge, England : 2003)》2004,(21):3689-3694
The reaction of AlMe(3) and [((t)Bu)(2)Al(micro-OPh)](2) with pyrazine (pyz), 4,4'-bipyridine (4-4'-bipy), 1,2-bis(4-pyridyl)ethane (bpetha) and 1,2-bis(4-pyridyl)ethylene (bpethe) yields (Me(3)Al)(2)(micro-pyz)(1), (Me(3)Al)(2)(micro-4,4'-bipy)(2), (Me(3)Al)(2)(micro-bpetha)(3), (Me(3)Al)(2)(micro-bipethe)(4), Al((t)Bu)(2)(OPh)(pyz)(5), [((t)Bu)(2)Al(OPh)](2)(micro-4,4-bipy)(6a), [((t)Bu)(2)Al(OPh)](2)(micro-bpetha)(7a), [((t)Bu)(2)Al(OPh)](2)(micro-bipethe)(8a). Compounds 1-4, 6a and 7a have been confirmed by X-ray crystallography. In solution compounds 1-4 undergo a rapid ligand-dissociation equilibrium resulting in a time-average spectrum in the (1)H NMR. In contrast, the solution equilibria for compounds 5-8a are sufficiently slow such that the mono-aluminium compounds may be observed by (1)H NMR spectroscopy: Al((t)Bu)(2)(OPh)(4,4-bipy)(6b), Al((t)Bu)(2)(OPh)(bpetha)(7b) and Al((t)Bu)(2)(OPh)(bpethe)(8b). The inability to isolate [((t)Bu)(2)Al(OPh)](2)(micro-pyz) and the relative stability of each complex is discussed with respect to the steric interactions across the bridging ligand (L) and the electronic effect on one Lewis acid-base interaction by the second Lewis acid-base interaction on the same ligand. 相似文献
6.
Doug Bruce John Biggins Tom Steiner Mike Thewalt 《Photochemistry and photobiology》1986,44(4):519-525
Excitation spectra of chlorophyll- a (Chl- a ) fluorescence in intact cells of Cryptomonas ovata, Chroomonas pauciplastida and Chroomonas salina were determined at 77 K. For all species the excitation spectra for emission from Chl- a associated with photosystem II (PSII) showed increased contributions by a carotenoid (493 nm) and phycobiliproteins, and decreased contributions by carotenoid (417 nm, 505 nm) and Chl- a (445 nm) as compared to excitation spectra for emission from Chl- a associated with photosystem I (PSI). Excitation spectra of C. salina and C. ovata showed an increased contribution by Chl- c 2 to PSII Chl- a fluorescence emission. In all three species the absorbance band positions of Chl- a , as determined from the excitation spectra, were similar to those previously described in green plants. green algae and phycobilisome-containing organisms. Time-resolved 77 K fluorescence emission spectra of C. ovata and C. salina showed successive emission from both phycoerythrin and Chl- c 2 , PSII Chl- a , and PSI Chl- a. C. pauciplastida showed successive emission from phycocyanin, PSII Chl- a , and PSI Chl- a. Spectral red-shifts with time were observed for the phycobiliprotein peaks in all three species. The fluorescence decay of phycoerythrin in C. ovata and C. salina was faster than that of phycocyanin in C. pauciplastida. The results are discussed in relation to the organization of the antenna pigments of PSII and PSI in the cryptophyte algae. 相似文献
7.
Dennis J. McCormac Christopher A. Marwood Doug Bruce Bruce M. Greenberg† 《Photochemistry and photobiology》1996,63(6):837-845
The aquatic higher plant Spirodela oligorrhiza , which contains proplastids when grown in the dark, was used to study light-dependent chloroplast development. Low-temperature (77 K) and room temperature fluorescence were utilized in situ on whole plants to examine plastid development. The dark-grown plants contain two 77 K fluorescence peaks, at 633 nm (F633) and at 657 nm (F657), with F633 dominating. The F657 species represents protochlorophyllide that is bound to protochloro-phyllide oxidoreductase. It was rapidly phototrans-formed to chlorophyllide (within 5 s) via a monomolec-ular reaction. Free protochlorophyllide (F633) was converted to chlorophyllide during a 3 h exposure to light. Photosystem (PS) assembly in Spirodela could be detected 2 h after the plants were first exposed to light, with the PSII reaction center (77 K fluorescence at 684 nm) appearing slightly before the PSI reaction center (77 K fluorescence at 725 nm). After the first reaction centers were formed the antenna complexes were added; the light-harvesting complex (LHC) I of PSI appeared after 8 h, and 47 kDa chlorophyll protein of PSII appeared between 12 h and 24 h. After 30 h of exposure to light, the plants acquired the ability to perform a light state transition, marking the appearance of functional LHCII complexes in the developing chloroplast. Finally, it was found that photosynthetic activity, as measured by room temperature chlorophyll fluorescence, accelerated con-comitantly with detection of the antenna complexes. Therefore, although reaction centers are detected very early during the proplastid to chloroplast conversion, they may have little activity or be unstable until the antennae are present. 相似文献
8.
Latent low-coordinate titanium imides supported by a sterically encumbering beta-diketiminate ligand
Basuli F Clark RL Bailey BC Brown D Huffman JC Mindiola DJ 《Chemical communications (Cambridge, England)》2005,(17):2250-2252
Addition of an equal molar quantity of R- (R = Me, SiMe3) to complex (Nacnac)Ti=NAr(OTf) (Nacnac- =[ArNC(tBu)]2CH, Ar = 2,6-iPr2C6H3) forms the imido alkyl (Nacnac)Ti=NAr(R), which can be readily protonated to afford [(Nacnac)Ti=NAr(L)]+ (L = THF, Et2O, eta1-C6H5NMe2), or treated with B(C6F5)3 to afford the zwitterion (Nacnac)Ti=NAr(micro-CH3)B(C6F5)3. 相似文献
9.
Richard W. Hutchinson Kavita George Doug Johns Lee Craven Gary Zhang Pullen Shnoda 《Cellulose (London, England)》2013,20(1):537-545
Oxidized regenerated cellulose (ORC) has been used as an absorbable hemostat since World War II. In the present study, hemostasis time was determined in a spleen incision model in swine. The effect of mass on absorbable hemostat efficacy and hemostasis time was evaluated by standardizing the ORC materials on a mass basis. The median hemostasis time for a single layer of the new nonwoven ORC was as much as 51 % shorter than woven ORC (P < 0.001). The mean hemostasis time for nonwoven ORC was not affected by the mass of hemostat applied to the wound. The hemostatic efficacy of woven ORC increased with the mass (layers) of hemostat applied to the wound. Nonwoven ORC is significantly faster in achieving hemostasis than woven ORC, and its hemostatic efficacy is not influenced by the mass of material applied. Tissue reaction was minimal and the material was fully absorbed by 14 days. 相似文献
10.
John P. Sutter Thomas Connolley Tim P. Hill Houcheng Huang Doug W. Sharp Michael Drakopoulos 《Journal of synchrotron radiation》2008,15(6):584-592
Finite‐element analysis is frequently used by engineers at synchrotron beamlines to calculate the elastic deformation of a single crystal undergoing mechanical bending or thermal load. ANSYS® Workbench? software is widely used for such simulations. However, although ANSYS® Workbench? software provides useful information on the displacements, strains and stresses within the crystal, it does not yield the local reciprocal lattice vectors that would be required for X‐ray diffraction calculations. To bridge this gap, a method based on the shape functions and interpolation procedures of the software itself has been developed. An application to the double‐crystal bent Laue monochromator being designed for the I12 (JEEP) wiggler beamline at the Diamond Light Source is presented. 相似文献