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61.
62.
Summary Chirally substituted Si–H-containing polysiloxanes were synthesized, which can be immobilized on small particle silica gel as well as on the smooth surfaces of fused silica capillaries. Immobilization is achieved either by crosslinking or by chemical bonding to the surfaces via silanol groups; both reactions can only be performed by addition of H2PtCl6, which acts as catalyst for hydrosilylation and as stoichiometric reagent for crosslinking. Chiral substituents of systematically varied chemical structure were introduced into the polysiloxanes by hydrosilylation. The mechanism of immobilization was investigated by spectroscopic methods, notably29Si-NMR. Homogenous stationary-phase coatings of variable film thickness and corresponding retentivity can easily be achieved. The enantioselectivity of the phase systems was characterized in dependency on the chemical structure of the chiral selectors attached to the polysiloxane chain of the chiral stationary phases and also in terms of the functional groups introduced into the solutes by derivatisation. 相似文献
63.
Marie E Chevalier Y Brunel S Eydoux F Germanaud L Flores P 《Journal of colloid and interface science》2004,269(1):117-125
The mechanism of action of additives that control the sedimentation of paraffin crystals after their crystallization in model diesel oil has been studied by means of a new experimental approach. The chemical analysis of the crystals and detailed measurements of the sedimentation phenomenon give new insights into this complex process. Thus, the wax antisettling additives used for preventing wax crystal sedimentation adsorb at the surfaces of wax particles and provide them with enhanced colloidal stability. The settling rate is not related to the size of the crystals or the viscosity of the liquid medium, but to the ability of the additives to prevent the aggregation of wax crystals. The reported methodology makes it possible to investigate the fundamental mechanisms, but also to evaluate structure-activity relationships of the various additives used in the petroleum industry. 相似文献
64.
Vladimír Setni
ka Marie Urbanov Statis Pataridis Vladimír Krl Karel Volka 《Tetrahedron: Asymmetry》2002,13(24):2661-2666
A novel approach to study the sol-gel phase transition of a brucine–porphyrin based gelator, which uses vibrational circular dichroism (VCD) spectroscopy, is described. The gelation process leading to highly ordered chiral supramolecular assemblies was investigated in various solvents at the different temperatures and concentrations. The VCD spectra sensitively reveal the specific parts of molecule whose configuration is influenced by a sol-gel phase transition and chiral supramolecular aggregation and therefore indicate the parts of the molecule responsible for the chiral self-assembly formation. Temperature stability of the organogel studied is discussed on the basis of the VCD and IR absorption spectra. The scanning electron microscopy was used to visualize the structure of brucine–porphyrin conjugate in the gel phase. 相似文献
65.
The combination of capillary isotachophoresis (ITP) and capillary zone electrophoresis (CZE) in the column coupling configuration was optimized in a mode where the electrolyte for the CZE step is different from the leading and terminating ITP electrolytes. Two colored markers, picric acid and 1-nitroso-2-naphthol, were used for exact timing of the transfer of isotachophoretically stacked analyte zones into the CZE column and for the control of the residual amount of the leading and terminating ITP electrolytes entering the CZE capillary together with the analytes, thus controlling the duration of transient ITP migration in the CZE capillary and ensuring good separation of the analytes and reproducibility of the migration times (relative standard deviations 1%). ITP-CZE was applied to the simultaneous assay of several cinnamic acid derivatives and flavonoids in methanolic extracts of Sambucus flowers and Crataegus leaves and flowers. The preconcentrating and cleansing effect of the ITP step allowed injection of relatively large sample volumes (30 microL). The limits of detection were approximately 20-50 ng x mL(-1) and 100 ng x mL(-1) for the acids and flavonoids, respectively ( thick similar 200-times lower compared to conventional CE) with spectrophotometric detection at 254 nm. The ITP-CZE exhibited satisfactory linearity and precision when using CZE buffer of pseudo "pH" 9.0; 1-nitroso-2-naphthol was employed as the internal standard. The separation took approximately 35 min. The ITP-CZE results for rutin, hyperoside, and vitexin-2-O"-rhamnoside were in good accordance with those obtained previously by high-performance liquid chromatography. 相似文献
66.
McArthur SL Vendettuoli MC Ratner BD Castner DG 《Langmuir : the ACS journal of surfaces and colloids》2004,20(9):3704-3709
One of the greatest challenges in mass spectrometry lies in the generation and detection of molecular ions that can be used to directly identify the protein from the molecular weight of the molecular ion. Typically, proteins are large (MW > 1000), nonvolatile, and/or thermally labile, but the vaporization process produced by many mass spectrometry techniques including time-of-flight secondary ion mass spectrometry (ToF-SIMS) is inherently limited to generating ions from smaller compounds or fragments of the parent molecule, making the identification of proteins complex. The application of specific molecules to aid in the generation of high molecular weight ions in ToF-SIMS has been recognized for some time. In this study we have developed a matrix-SAM substrate preparation technique based on the self-assembly of a matrix-like molecule, mercaptonicotinic acid (MNA), on gold. We then compare this substrate with two existing ToF-SIMS sample preparation techniques, cationized alkane thiol and matrix-enhanced SIMS (MESIMS). The results of this study illustrate that while there is a range of methods that can be used to improve the molecular ion yield of proteins in ToF-SIMS, their efficacy and reproducibility vary considerably and crucially are linked to the sample preparation and/or protein application methods used. Critically, the MNA modified substrate was able to simultaneously induce molecular ions for each protein present in a multicomponent solution, suggesting that this sample preparation technique may have future application in proteomics and DNA analysis. 相似文献
67.
Lahboub Bouyazza Jean-Charles Lancelot Sylvain Rault Max Robba Marie Anne Quermonne 《Journal of heterocyclic chemistry》1991,28(2):373-377
Cyclization of 2-(1-pyrrolyl)piperonylcarboxamide derivatives gave iminium perchlorates which afforded 9-(N-substituted-imino) and 9-(N-substituted amino)-6,7-methylenedioxypyrrolo[1,2-a]indoles. 相似文献
68.
Solvent-free microwave extraction of essential oil from aromatic herbs: comparison with conventional hydro-distillation 总被引:13,自引:0,他引:13
Solvent-free microwave extraction (SFME) is a combination of microwave heating and dry distillation, performed at atmospheric pressure without added any solvent or water. Isolation and concentration of volatile compounds are performed by a single stage. SFME has been compared with a conventional technique, hydro-distillation (HD), for the extraction of essential oil from three aromatic herbs: basil (Ocimum basilicum L.), garden mint (Mentha crispa L.), and thyme (Thymus vulgaris L.). The essential oils extracted by SFME for 30min were quantitatively (yield) and qualitatively (aromatic profile) similar to those obtained by conventional hydro-distillation for 4.5 h. The SFME method yields an essential oil with higher amounts of more valuable oxygenated compounds, and allows substantial savings of costs, in terms of time, energy and plant material. SFME is a green technology and appears as a good alternative for the extraction of essential oils from aromatic plants. 相似文献
69.
Kirk Gustafson Raymond J. Andersen Marie H.M. Chen Jon Clardy Jill E. Hochlowski 《Tetrahedron letters》1984,25(1):11-14
Extracts of the dorid nudibranch contain a diterpenoic acid glyceride 1 whose structure has been determined by x-ray diffraction analysis. The structure of a minor metabolite, the sesquiterpenoic acid glyceride 2, was determined by chemical correlation. 相似文献
70.
Ronald Grigg Marie Dowling Maurice W. Jordan Visuvanathar Sridharan 《Tetrahedron》1987,43(24):5873-5886
Hydrazones of aldehydes and ketones undergo intermolecular cycloaddition to electronegative olefins via azomethine imines, formed by a formal 1,2-prototropic shift, in low to moderate yield on heating in xylene or ethanol. In some instances the reaction is diverted to give products derived (at least formally) from an ene reaction. Similar intramolecular cycloadditions occur with unactivated terminal alkenes and alkynes. 相似文献