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排序方式: 共有991条查询结果,搜索用时 31 毫秒
961.
Doumy G Roedig C Son SK Blaga CI DiChiara AD Santra R Berrah N Bostedt C Bozek JD Bucksbaum PH Cryan JP Fang L Ghimire S Glownia JM Hoener M Kanter EP Krässig B Kuebel M Messerschmidt M Paulus GG Reis DA Rohringer N Young L Agostini P DiMauro LF 《Physical review letters》2011,106(8):083002
The nonlinear absorption mechanisms of neon atoms to intense, femtosecond kilovolt x rays are investigated. The production of Ne(9+) is observed at x-ray frequencies below the Ne(8+), 1s(2) absorption edge and demonstrates a clear quadratic dependence on fluence. Theoretical analysis shows that the production is a combination of the two-photon ionization of Ne(8+) ground state and a high-order sequential process involving single-photon production and ionization of transient excited states on a time scale faster than the Auger decay. We find that the nonlinear direct two-photon ionization cross section is orders of magnitude higher than expected from previous calculations. 相似文献
962.
Kaiser AM Gray AX Conti G Son J Greer A Perona A Rattanachata A Saw AY Bostwick A Yang S Yang SH Gullikson EM Kortright JB Stemmer S Fadley CS 《Physical review letters》2011,107(11):116402
Standing-wave-excited photoemission is used to study a SrTiO3/LaNiO3 superlattice. Rocking curves of core-level and valence band spectra are used to derive layer-resolved spectral functions, revealing a suppression of electronic states near the Fermi level in the multilayer as compared to bulk LaNiO3. Further analysis shows that the suppression of these states is not homogeneously distributed over the LaNiO3 layers but is more pronounced near the interfaces. Possible origins of this effect and its relationship to a previously observed metal-insulator-transition in ultrathin LaNiO3 films are discussed. 相似文献
963.
Junho Je Jongrok Kim Massoud Kaviany Sang Young Son MooHwan Kim 《Journal of synchrotron radiation》2011,18(5):743-746
Liquid water produced in a polymer electrolyte membrane fuel cell experiences a freeze/thaw cycle when the cell is switched off and on while operating at ambient temperatures below freezing. This freeze/thaw cycle permanently deforms the polymer electrolyte membrane fuel cell capillary structures and reduces both the cell life and its ability to generate electric power. The X‐ray tomography facility at the Pohang Accelerator Laboratory was used to observe the freeze/thaw effects on the gas diffusion layer (GDL), which is the thickest capillary layer in the cell. Morphological changes in the GDL under a water freeze/thaw cycle were observed. A scenario in which freeze/thaw cycles affect fuel cell performance is suggested based on images from X‐ray tomography. 相似文献
964.
The statistical model for describing network-forming systems, developed in our previous works, is applied to study of metallic alloys with chemical bonding. The model is based on the representation of the sum of statistical weights over all possible configurations for a thermoreversible network in the form of a functional integral over a scalar field. The mean-field solution of the model is derived, and for particular case of a binary alloy having single element of chemical short-range order A2B-type, thermodynamic and structural properties have been analyzed. This analysis allows to plot the temperature-concentration phase diagram of the model representing two immiscibility gap meeting in the distectic point. It is shown that at some temperatures and concentrations, geometry percolation of the network of chemical bonds and thus a sol-gel transition may take place. The critical percolation line was plotted in common with phase diagram. Then, the structural transitions, glass-forming ability and magnetic properties of Al-R alloys are discussed in the frames of this conception. It is proposed that the range of easy glass formation is confined on the left by the minimal concentration for the sol-gel transition and on the right by the concentration corresponding to the fractal-to-Euclidian crossover in the structure of percolation cluster. Finally, the abnormal growth of Al-REM magnetic susceptibility occurring above melting point of Al2R compound is also explained. 相似文献
965.
Hee-Wook You Jung-Woo Son Won-Ju Cho 《Applied Physics A: Materials Science & Processing》2011,102(4):921-926
In this paper, the engineered tunnel barrier technology is introduced by using the engineered tunnel barrier of VARIOT type
(SiO2/Si3N4/SiO2) and CRESTED type (Si3N4/SiO2/Si3N4) with Si3N4 and high-k HfO2 layers as charge trapping layers, respectively. In addition, the high-k stacked VARIOT type of SiO2/HfO2/Al2O3 and Al2O3/HfO2/Al2O3 are compared with O/N/O tunnel barrier memory. As a result, the engineered tunnel barrier memory device showed excellent
memory characteristics compared to the single SiO2 tunnel barrier memory device, such as very high P/E (program/erase) speed, good retention time and no degradation in endurance
characteristics. 相似文献
966.
The Characterization of the effects of surface wettability and geometry on pressure drop of slug flow in isothermal horizontal
microchannels is investigated for circular and square channels with hydraulic diameter (D
h
) of 700 μm. Flow visualization is employed to characterize the bubble in slug flow established in microchannels of various
surface wettabilities. Pressure drop increases with decrease in surface wettability, while the channel geometry influences
slug frequency. It is observed that the gas–liquid contact line in advancing and receding interfaces of bubble change with
surface wettability in slug flows. Flow resistance, where capillary force is important, is estimated using Laplace–Young equation
considering the change of dynamic contact angles of bubble. The experimental study also demonstrates that the liquid film
presence elucidates the pressure drop variation of slug flows at various surface wettabilities due to diminishing capillary
effect. 相似文献
967.
Kim HR Kim JC Lee KR Ji HI Lee HW Lee JH Son JW 《Physical chemistry chemical physics : PCCP》2011,13(13):6133-6137
The nano-size effect, which indicates a drastic increase in conductivity in solid electrolyte materials of nano-scale microstructures, has drawn substantial attention in various research fields including in the field of solid oxide fuel cells (SOFCs). However, especially in the cases of the conductivity of ultra-thin films measured in an in-plane configuration, it is highly possible that the 'apparent' conductivity increase originates from electrical current flowing through other conduction paths than the thin film. As a systematic study to interrogate those measurement artifacts, we report various sources of electrical current leaks regarding in-plane conductivity measurements, specifically insulators in the measurement set-up. We have observed a 'great conductivity increase' up to an order of magnitude at a very thin thickness of a single layer yttria-stabilized zirconia (YSZ) film in a set-up with an intentional artifact current flow source. Here we propose that the nano-size effect, reported to appear in ultra-thin single layer YSZ, can be a result of misinterpretation. 相似文献
968.
Nguyen VS Abbott HL Dawley MM Orlando TM Leszczynski J Nguyen MT 《The journal of physical chemistry. A》2011,115(5):841-851
The chemical transformations of formamide (NH(2)CHO), a molecule of prebiotic interest as a precursor for biomolecules, are investigated using methods of electronic structure computations and Rice-Rampserger-Kassel-Marcus (RRKM) theory. Specifically, quantum chemical calculations applying the coupled-cluster theory CCSD(T), whose energies are extrapolated to the complete basis set limit (CBS), are carried out to construct the [CH(3)NO] potential energy surface. RRKM theory is then used to systematically examine decomposition channels leading to the formation of small molecules including CO, NH(3), H(2)O, HCN, HNC, H(2), HNCO, and HOCN. The energy barriers for the decarboxylation, dehydrogenation, and dehydration processes are found to be in the range of 73-78 kcal/mol. H(2) loss is predicted to be a one-step process although a two-step process is competitive. CO elimination is found to prefer a two-step pathway involving the carbene isomer NH(2)CHO (aminohydroxymethylene) as an intermediate. This CO-elimination channel is also favored over the one-step H(2) loss, in agreement with experiment. The H(2)O loss is a multistep process passing through a formimic acid conformer, which subsequently undergoes a rate-limiting dehydration. The dehydration appears to be particularly favored in the low-temperature regime. The new feature identifies aminohydroxymethylene as a transient but crucial intermediate in the decarboxylation of formamide. 相似文献
969.
We compared genetic variations in the VP1 gene of foot-and-mouth disease viruses (FMDVs) isolated since 2000 from various region of the world. We analyzed relative synonymous codon usage (RSCU) and phylogenetic relationship between geographical regions, and calculated the genetic substitution patterns between Korean isolate and those from other countries. We calculated the ratios of synonymously substituted codons (SSC) to all observed substitutions and developed a new analytical parameter, EMC (the ratio of exact matching codons within each synonymous substitution group) to investigate more detailed substitution patterns within each synonymous codon group. We observed that FMDVs showed distinct RSCU patterns according to phylogenetic relationships in the same serotype (serotype O). Moreover, while the SSC and EMC values of FMDVs decreased according to phylogenetic distance, G + C composition at the third codon position was strictly conserved. Although there was little variation among the SSC values of 18 amino acids, more dynamic differences were observed in EMC values. The EMC values of 4- and 6-fold degenerate amino acids showed significantly lower values while most 2-fold degenerate amino acids showed no significant difference. Our findings suggest that different EMC patterns among the 18 amino acids might be an important factor in determining the direction of evolution in FMDV. 相似文献
970.
The purpose of this paper is to organize some results on the local geometry of CR singular real-analytic manifolds that are images of CR manifolds via a CR map that is a diffeomorphism onto its image. We find a necessary (sufficient in dimension 2) condition for the diffeomorphism to extend to a finite holomorphic map. The multiplicity of this map is a biholomorphic invariant that is precisely the Moser invariant of the image, when it is a Bishop surface with vanishing Bishop invariant. In higher dimensions, we study Levi-flat CR singular images and we prove that the set of CR singular points must be large, and in the case of codimension 2, necessarily Levi-flat or complex. We also show that there exist real-analytic CR functions on such images that satisfy the tangential CR conditions at the singular points, yet fail to extend to holomorphic functions in a neighborhood. We provide many examples to illustrate the phenomena that arise. 相似文献