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91.
Ecevit Bilgili Juan Yepes Luke Stephenson Kerry Johanson Brian Scarlett 《Particle & Particle Systems Characterization》2004,21(4):293-302
In the mechanical characterization of powders using the direct shear testers such as the Jenike shear cell, the existence of a uniform or well‐defined stress field in a powder specimen is assumed. This assumption has not been subjected to any serious scrutiny in the literature. In this study, the normal stress variation in a silica powder was locally determined by locating a pressure‐sensitive TekScan pad at the bottom section of a Jenike shear cell. A computer simulation of the consolidation and pre‐shearing stages of the Jenike test procedure was performed using the Discrete Element Method (DEM). The paper presents both experimental and computational evidence for the existence of a complex stress field in the powder specimen, thus clearly invalidating the long‐standing stress homogeneity assumption in the direct shear testing of powders. The implications of the stress inhomogeneity in terms of the accuracy of the material properties extracted from the Jenike test are also presented. 相似文献
92.
F. Wilkinson 《Journal of Chemical Sciences》1991,103(3):305-311
Recent progress which allows laser flash photolysis investigation of opaque materials by using diffuse reflectance from analysing
sources for the detection of laser induced transient species is described. Experimental details of nanosecond and picosecond
diffuse reflectance laser photolysis systems are presented and methods of analysis of data are discussed. The potential of
the technique for studying elementary reactions at interfaces is demonstrated with particular reference to bimolecular reactions
of (a) the triplet state of acridine adsorbed on various porous silica surfaces and (b) ion-electron recombination following
multi-photon ionisation of diphenyl polyenes adsorbed on γ-alumina. The mechanisms of formation and decay of these transient
adsorbed species are discussed. 相似文献
93.
Sloutskin E Sapir Z Bain CD Lei Q Wilkinson KM Tamam L Deutsch M Ocko BM 《Physical review letters》2007,99(13):136102
Millimolar bulk concentrations of the surfactant cetyltrimethylammonium bromide (CTAB) induce spreading of alkanes, H(CH(2))(n)H (denoted C(n)) 12< or =n< or =21, on the water surface, which is not otherwise wet by these alkanes. The novel Langmuir-Gibbs film (LGF) formed is a liquidlike monolayer comprising both alkanes and CTAB tails. Upon cooling, an ordering transition occurs, yielding a hexagonally packed, quasi-2D crystal. For 11< or =n< or =17 this surface-frozen LGF is a crystalline monolayer. For 18< or =n< or =21 the LGF is a bilayer with a crystalline, pure-alkane, upper monolayer, and a liquidlike lower monolayer. The phase diagram and film structure were determined by x-ray, ellipsometry, and surface tension measurements. A thermodynamic theory accounts quantitatively for the observations. 相似文献
94.
Hassell KM LeBlanc Y McLuckey SA 《Rapid communications in mass spectrometry : RCM》2011,25(4):476-482
A novel charge inversion process that involves the removal of an excess cation from an analyte ion and the transfer of an anion to the neutral analyte in a single ion/ion encounter is described. Polyamidoamine (PAMAM) half-generation dendrimer anions that contain small anions, such as the chloride ion, were used as charge inversion reagents. Several competing processes can occur that include removal of the cation to neutralize the analyte, the removal of the excess cation and an additional proton to yield the deprotonated molecule, or removal of the excess cation and transfer of a small anion to the analyte. For the latter process to dominate, several requirements for both the reagent anion and the analyte cation must be met. The reagent anion must form multiply charged anions and must be able to incorporate one or more small anions for transfer. The analyte must have no strongly acidic sites as well as a relatively high affinity for small anion attachment. The PAMAM dendrimer anions must meet the conditions for the reagent anions and the cations of the corticosteroids meet the conditions for the analyte. The estrogenic steroid estrone, on the other hand, does not meet the requirements and, as a result, is largely neutralized when reacted with the reagent anions. This reaction, therefore, is highly selective and might serve as a useful reaction for the screening of appropriate analytes. 相似文献
95.
Ding F Lee KJ Vahedi-Faridi A Huang T Xu XH 《Analytical and bioanalytical chemistry》2011,400(1):223-235
We have designed and constructed fusion genes of C-terminal (Ct) or N-terminal (Nt) bmrA with EGFP vectors and successfully expressed them in ΔBmrA (BmrA deletion strain of Bacillus subtilis), generating two new strains of B. subtilis (Ct-BmrA-EGFP and Nt-BmrA-EGFP). The fusion genes were characterized using gel electrophoresis and DNA sequencing. Their
expression in live cells was determined by measuring the fluorescence of EGFP in single live cells using fluorescence microscopy
and spectroscopy. The efflux function of the new strains was studied by measuring their accumulation kinetics of intracellular
Hoechst dye molecules (a pump substrate) using fluorescence spectroscopy, which were compared with wild-type (WT-BmrA) and
ΔBmrA strains. Both new strains show lower accumulation rates than ΔBmrA, and their efflux kinetics are inhibited by a pump
inhibitor (orthovanadate). The results suggest that both strains extrude the dye molecules and the fusion proteins retain
the efflux function of BmrA (ATP-binding cassette, ABC, transporter). Notably, Nt-BmrA-EGFP strain shows lower accumulation
rates (higher efflux rates) than Ct-BmrA-EGFP. Modeled structures of the fusion proteins illustrate a highly flexible linker
region connecting EGFP with BmrA, suggesting a minimal obstruction of EGFP to the BmrA. A closer distance of two C termini
(∼14 ?) than two N termini (47.9 ?) of the “closed” BmrA dimer depicts the larger steric effect of C-terminal fusion. This
study also shows that glucose affects the fluorescence study of efflux function of BmrA, suggesting that efflux kinetics of
ABC membrane transporters in live cells must be characterized in the absence of glucose. 相似文献
96.
Fusing multiple Bayesian knowledge sources 总被引:1,自引:0,他引:1
Eugene Santos Jr.John T. Wilkinson Eunice E. Santos 《International Journal of Approximate Reasoning》2011,52(7):935-947
We address the problem of information fusion in uncertain environments. Imagine there are multiple experts building probabilistic models of the same situation and we wish to aggregate the information they provide. There are several problems we may run into by naively merging the information from each. For example, the experts may disagree on the probability of a certain event or they may disagree on the direction of causality between two events (e.g., one thinks A causes B while another thinks B causes A). They may even disagree on the entire structure of dependencies among a set of variables in a probabilistic network. In our proposed solution to this problem, we represent the probabilistic models as Bayesian Knowledge Bases (BKBs) and propose an algorithm called Bayesian knowledge fusion that allows the fusion of multiple BKBs into a single BKB that retains the information from all input sources. This allows for easy aggregation and de-aggregation of information from multiple expert sources and facilitates multi-expert decision making by providing a framework in which all opinions can be preserved and reasoned over. 相似文献
97.
Salandria KJ Arico JW Calhoun AK McLaughlin LW 《Journal of the American Chemical Society》2011,133(6):1766-1768
We describe here the synthesis and properties of A-T rich DNA containing covalently bound water mimics located in the DNA minor groove. 相似文献
98.
Roy S Davydova MP Pal R Gilmore K Tolstikov GA Vasilevsky SF Alabugin IV 《The Journal of organic chemistry》2011,76(18):7482-7490
The reaction of diaryl ketoalkynes with 1,2-diamino ethane leads to the full scission of the triple bond with the formation of acetophenone and imidazoline fragments. In this transformation, one of the alkyne carbons undergoes formal reduction with the formation of three C-H bonds, whereas the other carbon undergoes formal oxidation via the formation of three C-N bonds (one π and two σ). Computational analysis confirmed that the key fragmentation step proceeds via a six-membered TS in a concerted manner. Both amines are involved in the fragmentation: the N-H moiety of one amine transfers a proton to the developing negative charge at the enolate oxygen, while the other amine provides direct stereoelectronic assistance to the C-C bond cleavage via a hyperconjugative n(N) → σ*(C-C) interaction. 相似文献
99.
Mathematics and Financial Economics - We derive the optimal portfolio for an investor with increasing relative risk aversion in a complete continuous-time securities market. The IRRA assumption... 相似文献
100.
D.P.A. Hardwick S.L. Naylor S. Bujkiewicz T.M. Fromhold D. Fowler A. Patan L. Eaves A.A. Krokhin P.B. Wilkinson M. Henini F.W. Sheard 《Physica E: Low-dimensional Systems and Nanostructures》2006,32(1-2):285
We study the effects of inter-miniband electron tunneling and electric field domains on the current–voltage and conductance–voltage curves of biased semiconductor superlattices under the action of a magnetic field that is tilted relative to the plane of the layers. For this geometry, electrons in the superlattice minibands exhibit a unique type of stochastic semiclassical motion. At certain critical values of the electric field within the superlattice layers, the stochastic trajectories change abruptly from fully localized to completely unbounded, and map out an intricate web-like mesh of conduction channels in phase space. Delocalization of the electron paths produces a series of strong resonant peaks in the electron drift velocity versus electric field curves. We use these drift velocity characteristics to make self-consistent drift-diffusion calculations of the current–voltage and differential conductance–voltage curves of the superlattices, which reveal strong resonant features originating from the sudden delocalization of the stochastic single-electron paths. We show that this delocalization has a pronounced effect on the distribution of space charge and electric field domains within the superlattices. Inter-miniband tunneling greatly reduces the amount of space-charge buildup, thus enhancing the domain structure and both the strength and number of the current resonances. 相似文献