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961.
We solve exactly the problem of a one-dimensional repulsive-U Hubbard chain with toroidal boundary conditions (HTB) using the Bethe ansatz approach. We calculate analytically the finite-size corrections to the ground-state energy in the half-filled case and use this expression to derive charge and spin stiffnesses with no assumptions. We then use a particle-hole transformation to calculate the finite-size corrections for the half-filledattractive- U case, and again derive the resulting expressions for the charge and spin stiffnesses. Lastly, we discuss how the repulsive-U corrections relate to those of a Heisenberg model with toroidal boundary conditions.On leave from Departamento de Fisica, Universidade Federal de S. Carlos, S. Carlos, 13560, Brazil. 相似文献
962.
Gracielle Ferreira Andrade Jerusa Araújo Quintão Arantes Faria Dawidson Assis Gomes André Luís Branco de Barros Renata Salgado Fernandes Amanda Cristina Soares Coelho Jacqueline Aparecida Takahashi Armando da Silva CunhaJr Edésia Martins Barros de Sousa 《Journal of Sol-Gel Science and Technology》2018,85(2):369-381
963.
Nadiia V. Pikun Natalya P. Kolesnyk Eduard B. Rusanov Aiva Plotniece Martins Rucins Arkadij Sobolev Yuriy G. Shermolovich 《Tetrahedron》2018,74(23):2884-2890
Electrophilic fluorination of Hantzsch-type 1,4-dihydropyridines with Selectfluor® led to the formation of new fluorinated 2,6-heptanediones - dialkyl 2,4-diacetyl-2,4-difluoro-3-phenylpentanedioates. Novel 2,6-heptanedione derivatives in reaction with hydrazine hydrate easily form 6-amino-4,7-difluoro-3-hydroxy-1,3-dimethyl-5-oxo-8-phenyl-2-oxa-6-azabicyclo[2.2.2]octanes instead of the corresponding diazepine derivatives. The obtained 2-оxa-6-azabicyclo[2,2,2]octanes are thermally stable at the temperatures below 50°С. At higher temperatures rearrangement of 2-oxa-6-azabicyclo[2,2,2]octanes offers new fluorine-containing pyrazolinone derivatives - alkyl esters of 2-fluoro-2-((4-fluoro-3-methyl-5-oxo-4,5-dihydro-1H-pyrazol-4-yl)(phenyl)methyl)-3-oxobutanoates. 相似文献
964.
Ester Junko Tomotani Luiz Carlos Martins das Neves Michele Vitolo 《Applied biochemistry and biotechnology》2005,121(1-3):149-162
Glucose oxidase (GO) (EC 1.1.3.4) was used as catalyst for oxidizing glucose into gluconic acid utilizing a 10-mL Bioengineering Enzyme Membrane Reactor® or a 400-mL Millipore Stirred Ultrafiltration Cell (MSUC) coupled with a Millipore UF membrane (cutoff of 100 kDa) and operated for 12 h under an agitation of 100 rpm, pH 5.5, and 30°C. The effect of feeding rate (0.10, 0.15, or 0.20 min?1), glucose (2.5 or 5.0 mM), and GO (1.0 or 2.0 mg/mL) concentrations on the catalysis were studied. A yield of about 75% was attained when the MSUC filled with 1.0 mg/mL of GO was fed with 2.5 mM glucose solution at a rate of 0.15 min?1. 相似文献
965.
Sergio R. Mortari Ana Paula F. Saidelles Juliano S. Barin Érico M. M. Flores Ayrton F. Martins 《Mikrochimica acta》2004,148(3-4):157-162
Two procedures for wet decomposition of human hair samples were compared for selenium determination by hydride generation atomic absorption spectrometry (HGAAS). These procedures involve treatment with HNO3/H2O2 and H2SO4/HNO3, respectively, using small disposable polypropylene (PP) vials. Analytical steps from weighing to dilution were performed in the same individual PP vial. After sample weighing (10±3mg) and reagent addition, decomposition was performed in a conventional oven (20h, 100°C). Addition of sulfamic acid prior to HGAAS measurements was necessary to minimize interference due to residual nitrogen oxides from the decomposition step. The accuracy was estimated using certified hair reference analysis which yielded an agreement higher than 97% (recovery tests). The absolute limit of detection was 1.2ng (3 s; n=10), corresponding to 0.12µgg–1 in the dry sample. 相似文献
966.
A. Paula V. Goncalves M. BÁrbara F. Martins M. EugÉnia M. Cruz 《Applied biochemistry and biotechnology》1991,27(2):139-143
With the aim of immobilizing glucose oxidase (GO) for routine determination of glucose, a covalent bond immobilization method on titanium (IV) chloride activated silica supports was used (1). Several parameters were studied in order to optimize the residual activity upon immobilization and during operation. The immobilized enzyme can be reutilized at 25°C for several h a day alternating with storage (4°C) for at least 3,300 h. 相似文献
967.
Solvent effects on the O NMR chemical shifts of 4-dimethylsulfoximide-1,1,1-trifluoro-3-alken-2-ones
Helio G. Bonacorso Sandra R.T. Bittencourt Adilson P. Sinhorin Nilo Zanatta Marcos A.P. Martins 《Journal of fluorine chemistry》2003,121(2):135-139
A multi-linear-regression analysis using the Kamlet-Abbout-Taft (KAT) solvatochromic parameters in order to elucidate and quantify the solvent effects on the chemical shifts of three 4-dimethylsulfoximide-1,1,1-trifluoro-3-buten-2-ones is reported. The predominant conformational structures of compounds has been determined by energy minimization calculations using the Austin Model (AM1) semi-empirical method, which has been used to explain the NMR chemical shifts for CO and SO groups. The chemical shifts of carbonyl and sulfoximide oxygen’s of molecules showed similar dependencies (in ppm) on the solvent polarity-polarizability and the solvent hydrogen-bond-donor (HBD) acidities. The influence of solvent hydrogen-bond-acceptor (HBA) basicities was little significant. 相似文献
968.
We used trioctylphosphine oxide (TOPO) capped colloidal InP nanocrystals (Q-InP|TOPO) to explore the potential of solution 1H NMR spectroscopy in studying in situ the capping and capping exchange of sterically stabilized colloidal nanocrystals. The spectrum of Q-InP|TOPO shows resonances of free TOPO, superimposed on broadened spectral features. The latter were assigned to TOPO adsorbed at Q-InP by means of pulsed field gradient diffusion NMR and 1H-13C HSQC spectroscopy. The diffusion coefficient of Q-InP|TOPO nanocrystals was inferred from the decay of the adsorbed TOPO NMR signal. The corresponding hydrodynamic diameter correlates well with the diameter of Q-InP. By using the resolved methyl resonance of adsorbed TOPO, the packing density of TOPO at the InP surface can be estimated. Spectral hole burning was used to demonstrate explicitly that the adsorbed TOPO resonances are heterogeneously broadened. Exchange of the TOPO capping by pyridine was demonstrated by the disappearance of the resonances for adsorbed TOPO and the appearance of pyridine resonances in the 1H NMR spectrum. These results show that solution NMR spectroscopy should be considered a powerful technique for the in situ study of the capping of sterically stabilized colloidal nanocrystals. 相似文献
969.
We report the synthesis of 2,3-dihydro-1,3,4-thiadiazoles and 1,3,4-thiadiazoles from 1,2:3,4-di-O-iso-propylidene-α-D-galacto-1,6-hexodialdo-1,5-pyranose thiosemicarbazone. The physical and spectro-scopic characterizations of the heterocyclic derivatives as well as the intermediate product are described. We present the prefered conformation in solution using computational calculations and spectroscopic data. The possibilities of chiral induction of the cyclization reaction are discussed. 相似文献
970.
Adam and Delbrück argued that the dimensionality of the diffusion space determines the average lifetime of a diffusing particle confined to a region with a central trap. Doubts have often been aired as to whether their calculation is relevant to real biological systems, where the number of traps is usually much larger than unity; or whether the rate enhancement is merely a manifestation of an increase in the concentration of the traps; or whether the diverse multi-trap versions of their expression for the mean lifetime in two dimensions are trustworthy. These issues are addressed, and the long-standing problem of finding the low-density limit of trapping time in two dimensions solved, by examining previous treatments of the problem, and by carrying out simulations of two-dimensional systems in which the particles undergo a Pearsonian random walk, and the traps are distributed randomly or on a square lattice. The mean lifetimes are found to be different in the two situations, and it is concluded that the neglect of this aspect lies at the root of the conflict between some of the existing expressions for the mean lifetime. Relations expressing the mean lifetime as a function of the concentration of the traps are presented together with a discussion of their applicability. 相似文献