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21.
On the basis of the irradiance-moment formalism for describing general partially coherent beams, we investigate the relation between the spatial orientation of the transverse beam profile upon propagation and the asymmetrical part of its orbital angular momentum (OAM). More specifically, a necessary and sufficient condition (one-to-one correspondence) is shown between freely propagating non-rotating beams and vanishing asymmetrical OAM. As a corollary, it is obtained that any beam emerging from optical systems that transform a rotating field into a non-rotating beam exhibits vortex OAM only. 相似文献
22.
23.
A. Väisänen Rose Matilainen Jouni Tummavuori 《Analytical and bioanalytical chemistry》2000,367(8):755-760
Direct ICP-AES measurements of the digested geological standard reference material samples yielded the wrong information
about their composition. The differences between certified and measured concentrations of the samples were due to the complicated
sample matrix. The measured concentrations can be successfully corrected by using a multiple linear regression technique.
The correction is based on the multiple regression line calculated from the analytical results at synthetic mixtures of matrix
elements, where concentrations varied on five levels. There were no significant (P = 0.05) differences between certified and
measured concentrations in standard reference materials after the correction. The same method was used in the analysis of
nutrition supplements.
Received: 25 January 2000 / Revised: 15 March 2000 / Accepted: 31 March 2000 相似文献
24.
José Luis Fonseca María José Sosa Gabriela Petroselli Rosa Erra-Balsells Matías I. Quindt Sergio M. Bonesi Alexander Greer Edyta M. Greer Andrés H. Thomas Mariana Vignoni 《Photochemistry and photobiology》2023,99(2):593-604
Here, we provide mechanistic insight to the photocleavage of a compound in the folate family, namely pteroic acid. A bis-decyl chain derivative of pteroic acid was synthesized, structurally characterized and photochemically investigated. We showed that, like folic acid, pteroic acid and the decylated derivative undergo a photocleavage reaction in the presence of H2O, while no reaction was observed in methanol solution. Furthermore, density functional theory calculations were carried out to predict relative stabilities of hypothetical mono-, bis- and tris-decylated pteroic acid derivatives to help rationalize the regioselectivity of the bis-decyl pteroic acid product. Additionally, the lipophilicity of the bis-decyl pteroic acid appears to confer a hydrophobic property enabling an interaction with biomembranes. 相似文献
25.
Determination of 2,4-D and Dicamba in food crops by MEKC 总被引:2,自引:0,他引:2
Summary The determination of 2,4-D (2,4-dichlorophenoxyacetic acid) and Dicamba (2-methoxy-3,6-dichlorobenzoic acid) residues in sugar
cane, rice and corn was performed by a supercritical fluid extraction (SFE) method using CO2/acetone as extraction mix and an SFE apparatus developed in our laboratory. The extracts were cleaned up after extraction
by both liquid-liquid partition and a Florisil column. Micellar electrokinetic capillary chromatography (MEKC) coupled with
ultraviolet on-column detection was used for the analysis of these pesticides. The detection limits were improved by the preparation
of a special detection cell with an increased pathlength that gave detection limits of ca. 0.6 pg for 2,4-D and Dicamba. Our
results demonstrated that capillary electrophoresis can be a powerful new analytical tool for pesticide residue analysis. 相似文献
26.
Febles M Pérez-Hernandez N Pérez C Rodríguez ML Foces-Foces C Roux MV Morales EQ Buntkowsky G Limbach HH Martín JD 《Journal of the American Chemical Society》2006,128(31):10008-10009
Water molecules confined inside narrow pores are of great importance in understanding the structure, stability, and function of water channels. Here we report that besides the H-bonding water that structures the pore, the permanent presence of a significant, fast-moving fraction of incompletely H-bonded water molecules inside the pore should control the free entry and exit of water. This is achieved by means of complementary DSC and solid-state NMR studies. We also present compelling evidence from X-ray diffraction data that the cluster formed by six water molecules in the most stable cage-like structure is sufficiently hydrophobic to be stably adsorbed in a nonpolar environment. 相似文献
27.
Cuadros-Rodríguez L Bagur-González MG Sánchez-Viñas M González-Casado A Gómez-Sáez AM 《Journal of chromatography. A》2007,1158(1-2):33-46
Calibration is an operation whose main objective is to know the metrological status of a measurement system. Nevertheless, in analytical sciences, calibration has special connotations since it is the basis to do the quantification of the amount of one or more components (analytes) in a sample, or to obtain the value of one or more analytical parameters related with that quantity. Regarding this subject, the aim of analytical calibration is to find an empiric relationship, called measurement function, which permits subsequently to calculate the values of the amount (x-variable) of a substance in a sample, from the measured values on it of an analytical signal (y-variable). In this paper, the metrological bases of analytical calibration and quantification are established and, the different work schemes and calibration methodologies, which can be applied depending on the characteristic of the sample (analyte+matrix) to analyse, are distinguished and discussed. Likewise, the different terms and related names are clarified. A special attention has been paid to those analytical methods which use separation techniques, in relation with its effect on calibration operations and later analytical quantification. 相似文献
28.
Emi Evangelio Dr. Marie‐Laure Bonnet Dr. Miquel Cabañas Dr. Motohiro Nakano Dr. Jean‐Pascal Sutter Dr. Andrea Dei Prof. Vincent Robert Dr. Daniel Ruiz‐Molina Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(22):6666-6677
The different thermally induced intermolecular electron transfer (IET) processes that can take place in the series of complexes [M(Cat‐N‐BQ)(Cat‐N‐SQ)]/[M(Cat‐N‐BQ)2], for which M=Co ( 2 ), Fe ( 3 ) and Ni( 4 ), and Cat‐N‐BQ and Cat‐N‐SQ denote the mononegative (Cat‐N‐BQ?) or dinegative (Cat‐N‐SQ2?) radical forms of the tridentate Schiff‐base ligand 3,5‐di‐tert‐butyl‐1,2‐quinone‐1‐(2‐hydroxy‐3,5‐di‐tert‐butylphenyl)imine, have been studied by variable‐temperature UV/Vis and NMR spectroscopies. Depending on the metal ion, rather different behaviors are observed. Complex 2 has been found to be one of the few examples so far reported to exhibit the coexistence of two thermally induced electron transfer processes, ligand‐to‐metal (IETLM) and ligand‐to‐ligand (IETLL). IETLL was only found to take place in complex 3 , and no IET was observed for complex 4 . Such experimental studies have been combined with ab initio wavefunction‐based CASSCF/CASPT2 calculations. Such a strategy allows one to solicit selectively the speculated orbitals and to access the ground states and excited‐spin states, as well as charge‐transfer states giving additional information on the different IET processes. 相似文献
29.
YU Qing CAO Jie LIU Tao WANG Zhi-xiang . College of Chemistry Chemical Engineering Graduate School of the Chinese Academy of Sciences Beijing P. R. China 《高等学校化学研究》2009,25(3)
It is well-known that primary amines form inclusive complexes with crown ethers. This kind of noncova-lently inclusive complexes,exemplified tryptophan complex with 18-crown-6,was studied by combination of two experimental methods,fluorimetry and mass spectrometry,with theoretical calculation. Fluorescence intensity of tryptophan in the absence and the presence of 18-crown-6 in aqueous HAc-NaAc buffer solution at pH=3.73 has been studied by fluorometry. The fluorescence intensity decreases significantly wit... 相似文献
30.
Hua Su Zhao Hui Yuan Jing Li Shu Ju Guo Li Ping Deng Li Jun Han Xiao Bin Zhu Da Yong Shi 《中国化学快报》2009,20(4):456-458
Two new bromoindole alkaloids have been isolated from the ethanolic extract of the red alga Laurencia similis.On the basis of chemical and spectroscopic methods(including 1D and 2D NMR technique),their structures have been elucidated as 2,2′,5,5′,6,6′-sixibromo -3,3′-bi-1H-indole and 3,5-dibromo-1-methylindole,respectively. 相似文献