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31.
Abstract— Mutants of Saccharomyces cerevisiae with varying photodynamic sensitivities were isolated in an attempt to help identify the cytoplasmic targets of photodynamic action in yeast. 相似文献
32.
A fluorogenic renin substrate, N-CBO-L -prolyl-L -phenylalanyl-L -histidyl-L -leucyl-L -leucyl-L -valyl-L -tyrosyl-L -seryl-β-naphthylamide, has been synthesized. Upon incubation at pH 5,6 with renin and an excess of the auxiliary enzyme aminopeptidase M, it gives rise to β-naphthylamine at a rate related to the quantity of renin. 相似文献
33.
H. R. Khan K. Lüders Ch. J. Raub G. Roth 《Zeitschrift für Physik B Condensed Matter》1980,38(1):27-33
Superconductivity, structure and electrical resistance behaviour of -phase alloys of Nb–Rh and Ta–Rh are investigated. The Ta–Rh alloys do not become superconducting above 1.2 K. The
andH
c2
(0) values of a homogeneous alloy with the composition Nb65.2Rh34.8 are 2.95 K, 13.9 kG/K and 23 kG, respectively, whereas for an inhomogeneous alloy with the composition Nb63.7Rh36.3 these values are 4.24 K, 5.5 kG/K and 14 kG, respectively. Splat quenching results in a substantial increase in the
andH
c2
(0) values of the Rh-rich sample. Annealing (900°C, 100 h) of the Rh-rich sample leads only to small changes in the superconducting properties but a small amount of Nb–Rh solid solution has been formed. The electrical resistance of Nb65.2Rh34.8 decreases with decreasing temperature and varies asT
0.5 between 150 and 240 K and asT between 60 and 140 K. For Ta70.0Rh30.0 the temperature coefficient changes to negative values below 170K. values are calculated for Nb–Rh using McMillan's formula. An estimatedT
c
value of Ta–Rh is 0.2 K. TheH
c2
(0) values of Nb–Rh are in good agreement with the theoreticalH
c
2**
(0) values.Dedicated to Prof. Dr. W. Buckel on the occasion of his 60th birthday 相似文献
34.
An attempt was made to obtain iron-carbon nanoparticles by two-step pyrolysis of Fe(CO)5- and C3O2-containing mixtures behind incident and reflected shock waves in a shock tube. The formation of binary particles was monitored by recording the extinction of He-Ne laser radiation and laser-induced incandescence (LII). The LII method provides particle size estimates if the thermal and optical properties of the constituting material are known. Behind an incident shock wave, at temperatures of 700–1500 K, Fe(CO)5 decomposes within a short period of time (∼50 µs). The resulting iron atoms combine into particles, which serve as condensation nuclei for carbon vapor resulting from C3O2 pyrolysis at 1500–3000 K behind the reflected shock wave. The binary particles thus produced are considerably larger than pure carbon or iron particles. As the mixture temperature behind the reflected shock wave is raised, the diameter of these binary particles decreases.__________Translated from Kinetika i Kataliz, Vol. 46, No. 3, 2005, pp. 333–343.Original Russian Text Copyright © 2005 by Gurentsov, Eremin, Roth, Starke.Based on a report at the VI Russian Conference on Mechanisms of Catalytic Reactions (Moscow, October 1–5, 2002). 相似文献
35.
France S Shah MH Weatherwax A Wack H Roth JP Lectka T 《Journal of the American Chemical Society》2005,127(4):1206-1215
We report a mechanistically based study of bifunctional catalyst systems in which chiral nucleophiles work in conjunction with Lewis acids to produce beta-lactams in high chemical yield, diastereoselectivity, and enantioselectivity. Chiral cinchona alkaloid derivatives work best when paired with Lewis acids based on Al(III), Zn(II), Sc(III), and, most notably, In(III). Homogeneous bifunctional catalysts, in which the catalyst contains both Lewis acidic and Lewis basic sites, were also studied in detail. Mechanistic evidence allows us to conclude that the chiral nucleophiles form zwitterionic enolates that react with metal-coordinated imines. Alternative scenarios, which postulated metal-bound enolates, were disfavored on the basis of our observations. 相似文献
36.
The crystal structure of HLiTa3O8 has been reexamined by electron and neutron diffraction techniques. Neutron Weissenberg and electron diffraction photographs show that the space group of the compound isPmmn and notPmma as determined previously by X-ray diffraction techniques. There are eight molecules in the unit cell of lattice parametersa = 16.718(2)A?,b = 7.696(1)A?,c = 8.931(1)A?. These values show that thebaxis of the new cell is doubled with respect to the parameter measured by X-rays. The structural refinement was based on 1074 independent reflections measured on a single crystal with a four-circle neutron diffractometer. The positions of all atoms, including the lithium atoms have been determined. The finalR andwRfactors were 0.036 and 0.035, respectively. The eight lithium cations occupy two sets of4fpositions(x,¼, z)of thePmmn space group. The ordering of four lithium ions over two sets of possible positions (4j) of space groupPmma is responsible for the doubling of theb axis. The other four Li+ occupy two sets of positions (2d) of space groupPmma. All lithium ions are surrounded by 12 oxygen atoms arranged as cuboctahedra. The large thermal vibrations found for the lithium atoms and the ionic conductivity of HLiTa3O8 at high temperatures are consistent with weak LiO bonding. 相似文献
37.
New copper(II) complexes of the hydrazone ligands H2salhyhb, H2salhyhp, and H2salhyhh, derived from salicylaldehyde and ω‐hydroxy carbonic acid hydrazides, have been synthesized and physically characterized. Two fundamental structures were found in solid state depending on the pH‐value of the reaction solution. Acidic conditions lead to the formation of the di‐μ‐phenoxo‐bridged dicationic complex dimers [{Cu(Hsalhyhb)}2]2+ ( 1a ), [{Cu(Hsalhyhp)}2]2+ ( 2a ), and [{Cu(Hsalhyhh)}2]2+ ( 3a ), isolated as perchlorate salts. The dimeric complexes show strong antiferromagnetic coupling with J = ?399 ( 1a ), ?410 ( 2a ), and ?311 cm?1 ( 3a ). Higher pH‐values resulted in the aggregation of neutral copper ligand fragments to the one‐dimensional coordination polymers [{Cu(salhyhb)}n] ( 1b ), [{Cu(salhyhp)}n] ( 2b ), and [{Cu(salhyhh)}n] ( 3b ). 3b has been examined by means of X‐ray crystallography and represents the first example of a structurally characterized neutral copper(II) N‐salicylidenehydrazide complex without additional ligands. The magnetic interactions in the polymers are also antiferromagnetic with J = ?125 ( 1b ), ?136 ( 2b ), and ?148 cm?1 ( 3b ), but strongly reduced compared to the corresponding dimeric complexes. The two basic structure types can be reversibly interconverted simply by pH‐control. 相似文献
38.
The perovskite-related phase Ca3Nb2O8, when grown as single crystals from a calcium vanadate flux, incorporates a small amount of vanadium from the flux to form the composition Ca3Nb2−xVxO8 with x=0.025. The crystals have pseudo-cubic symmetry with a=6×ac(perovskite). The actual symmetry is rhombohedral, space group R3, with ah=16.910(1) Å, ch=41.500(2) Å. The structure was solved using a combination of single-crystal methods together with constrained refinements of powder X-ray and neutron powder data. The unit-cell composition is [Ca138□24]A [Ca42Nb117V3]B[O480□6], with vacancies in both the anion sites and A-cation sites. The Ca and Nb atoms are fully ordered in the B-sites such that (001) layers containing only Nb-centered octahedra alternate with layers containing both Nb-centered and Ca-centered octahedra. At the origin B-site, ordered oxygen vacancies result in the octahedron being transformed to a tetrahedron, which, in the single crystals, is occupied by vanadium. The structure displays a new type of octahedral tilt system in which 3×3×3 blocks of (a+a+a+) tilts are periodically twinned on the pseudo-cubic {1 0 0}c planes. 相似文献
39.
New weberite-type Ca2Ta2O7 and zirconolite-type CaZrTi2O7 polytypes have been prepared by doping with Nd/Zr and Th/Al, respectively, and their structures have been refined using single-crystal X-ray diffraction intensity data. The 3T zirconolite polytype, Ca0.8Ti1.35Zr1.3Th0.15Al0.4O7, has a=7.228(1), c=16.805(1) Å. The 3T weberite-type polytype, Ca1.92Ta1.92Nd0.08Zr0.08O7, has a=7.356(1), c=18.116(1) Å. Both 3T polytypes have space group P3121, Z=6. The 4M Ca2Ta2O7 polytype has the same composition, from electron microprobe analyses, as the 3T polytype, and has cell parameters: a=12.761(1), b=7.358(1), c=24.565(1) Å, β=100.17(1)°, space group C2, Z=16. The structural relationships between the different zirconolite and weberite polytypes are discussed. A consideration of the structures from the viewpoint of anion-centered tetrahedral arrays shows that zirconolite can be considered as an anion-deficient fluorite derivative phase. However, the fluorite-type topology of edge-shared OM4 tetrahedra is not maintained in the Ca2Ta2O7 weberite-type polytypes, even though they have a fluorite-like fcc packing of metal atoms. One of the oxygen atoms moves from a tetrahedral Ta3Ca interstice to an adjacent Ta2Ca4 octahedral interstice in the weberite polytypes. 相似文献
40.
Rai BK Durbin SM Prohofsky EW Sage JT Ellison MK Roth A Scheidt WR Sturhahn W Alp EE 《Journal of the American Chemical Society》2003,125(23):6927-6936
Detailed Fe vibrational spectra have been obtained for the heme model complex [Fe(TPP)(CO)(1-MeIm)] using a new, highly selective and quantitative technique, Nuclear Resonance Vibrational Spectroscopy (NRVS). This spectroscopy measures the complete vibrational density of states for iron atoms, from which normal modes can be calculated via refinement of the force constants. These data and mode assignments can reveal previously undetected vibrations and are useful for validating predictions based on optical spectroscopies and density functional theory, for example. Vibrational modes of the iron porphyrin-imidazole compound [Fe(TPP)(CO)(1-MeIm)] have been determined by refining normal mode calculations to NRVS data obtained at an X-ray synchrotron source. Iron dynamics of this compound, which serves as a useful model for the active site in the six-coordinate heme protein, carbonmonoxy-myoglobin, are discussed in relation to recently determined dynamics of a five-coordinate deoxy-myoglobin model, [Fe(TPP)(2-MeHIm)]. For the first time in a six-coordinate heme system, the iron-imidazole stretch mode has been observed, at 226 cm(-)(1). The heme in-plane modes with large contributions from the nu(42), nu(49), nu(50), and nu(53) modes of the core porphyrin are identified. In general, the iron modes can be attributed to coupling with the porphyrin core, the CO ligand, the imidazole ring, and/or the phenyl rings. Other significant findings are the observation that the porphyrin ring peripheral substituents are strongly coupled to the iron doming mode and that the Fe-C-O tilting and bending modes are related by a negative interaction force constant. 相似文献