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421.
T. J. López y López-Leytón M. A. Lage Yusty M. E. Alvarez Piñeiro 《Analytical and bioanalytical chemistry》1998,362(3):341-343
Constant-wavelength synchronous spectrofluorimetry was used for the analysis of riboflavin in anchovies. Synchronous fluorescence spectra were recorded between 300 and 600 nm at a scan rate of 240 nm min–1 and with excitation and emission slit-widths both set to 5 nm. The excitation-emission wavelength difference was 65 nm. The fluorescence was measured by peak-area base between 430–509 nm. Recovery was higher than 90.8%. 相似文献
422.
Larsen K Worm-Leonhard K Olsen P Hoel A Jensen KJ 《Organic & biomolecular chemistry》2005,3(21):3966-3970
Current methods for glycosylation of complex alcohols, e.g. with glycosyl trichloroacetimidates, generally occur in the presence of a strong Lewis acid 'promoter', and at sub-ambient temperatures. However, the older literature reports high-temperature glycosylations, especially of phenols. We have described an efficient method for glycosylation of alcohols under neutral conditions, using as anomeric leaving group methyl 3,5-dinitrosalicylate (DISAL). Only a very few reports have described the use of microwaves to promote glycosylations, mainly of simple alcohols. Here we describe fast, high-temperature glycosylations using precise microwave heating in the synthesis of oligosaccharides, with both DISAL and widely used trichloroacetimidate glycosyl donors in the absence of strong Lewis acids. Also, we have applied microwave heating as a general protocol for evaluating new, potential glycosyl donors. 相似文献
423.
Several membrane phase separators have been designed and tested for use in a flow-injection extraction manifold. The membrane is sandwiched between two pieces of perspex with grooves facing the membrane. A PTFE membrane with polyethylene backing proved to be most suitable. With this type of phase separator the total dispersion in the extraction system is less than that obtained with the conventional T-piece separator. Alcohols, alkanes, chlorinated hydrocarbons and aromatic solvents pumped at a flow rate of 0.5–1.0 ml min-1 can be segmented with aqueous phase and later separated from it with a recovery of up to 95%. The organic phase passing through the detector flow cell is not contaminated by the aqueous phase to any measurable extent. 相似文献
424.
Gorzsás A Getty K Andersson I Pettersson L 《Dalton transactions (Cambridge, England : 2003)》2004,(18):2873-2882
The speciation in the quaternary aqueous H+/H2VO4-/H2O2/citrate (Cit3-) and H+/H2VO4-/Cit3-/L-(+)-lactate (Lac-) systems has been determined at 25 degrees C in the physiological medium of 0.150 M Na(Cl). A combination of 51V NMR integral intensities and chemical shift (Bruker AMX500) as well as potentiometric data (glass electrode) have been collected and evaluated with the computer program LAKE, which is able to treat multimethod data simultaneously. The pKa-values for citric acid have been determined as 2.94, 4.34 and 5.61. Altogether six vanadate-citrate species have been found in the ternary H+/H2VO4-/Cit3- system in the pH region 2-10, only two of which are mononuclear. Reduction of vanadium(V) becomes more pronounced at pH < 2. Solutions, in which reduction occurred to any extent, were excluded from all calculations. In the quaternary H+/H2VO4-/H2O2/Cit3- system, eight complexes have been found in addition to all binary and ternary complexes over the pH region 2-10, including three mononuclear species. Equilibria in general are fast, but the significant and rapid decomposition of peroxide in acidic solutions limited the final model to pH > 4. In the quaternary H+/H2VO4-/Cit3-/Lac- system, two mixed-ligand species have been determined, with the compositions V2CitLac2- and V2CitLac3- (pKa = 5.0). To our knowledge, this is the first time such complexes have been reported for vanadium(V). 51V NMR chemical shifts, compositions and formation constants are given, and equilibrium conditions are illustrated in distribution diagrams as well as the fit of the model to the experimental data. When suitable, structural proposals are given, based on 13C NMR measurements and available literature data of related compounds. 相似文献
425.
D. Vázquez M. A. Lage J. C. Parajó J. L. Alonso 《Applied biochemistry and biotechnology》1992,37(2):123-139
Samples ofEucalyptus globulus wood were delignified in HC1-catalyzed acetic acid medium under selected conditions and extracted with alkaline solutions
to improve their susceptibility to enzymatic hydrolysis. The effects of three independent variables defining the operational
conditions of the alkaline extraction stage on five dependent variables measuring both the chemical composition and the susceptibility
of the solid residues to enzymatic hydrolysis were assessed using empirical models deduced from experimental data. The enzymatic
conversion depended mainly on the NaOH concentration used in the alkaline extraction. Hydrolysis yields up to 76% were predicted
for operational conditions within the range studied for the independent variables. 相似文献
426.
U. Kasper 《International Journal of Theoretical Physics》1992,31(6):1007-1014
Thermodynamic quantities and those describing the source of gravitational fields need not be identified. The standard approach is to identify them. If one does not follow this way, one opens the possibility of describing, on the level of phenomenology, the average of the quantum physical process behind the creation of particles and entropy in a comoving volume. Different approaches are briefly discussed. A variational principle with constraints modeling particle and entropy creation is formulated and leads to higher-order gravity field equations. They suggest on the level of phenomenology that the Minkowski space-time could be unstable against vacuum fluctuations. 相似文献
427.
428.
V. R. Gavrilov V. D. Ivashchuk U. Kasper V. N. Melnikov 《General Relativity and Gravitation》1997,29(5):599-612
The multidimensional cosmological model describing the evolution of n Einstein spaces in the presence of multicomponent perfect fluid is considered. We define vectors related to the equations of state of the components. If they are orthogonal with respect to the minisuperspace metric, the Einstein equations are integrable and a Kasner-like form of the solutions is presented. For special sets of parameters the cosmological model is reduced to the Euclidean Toda-like system connected with some Lie algebra. The integrable vacuum (1+5+5)-model with two 5-dimensional Einstein spaces and non-zero Ricci tensors is obtained. Its reduction to a (1+5+3+2)-solution is given. For a special choice of the integration constant and one of the spaces (M1 = S5) a non-singular solution with the topology
is obtained. 相似文献
429.
Catharina Silfwerbrand-Lindh Lage Nord Lars-Göran Danielsson Folke Ingman 《Analytica chimica acta》1984
A spectrophotometric method for the determination of uranium is adapted to flow injection analysis. The reagent, 2(5-bromo-2-pyridylazo)-5-diethylaminophenol, forms a complex with uranyl ions, with an absorbance maximum at 578 nm. The detection limit of the procedure is 0.15 mg l?1 uranium and 60 samples can be analyzed per hour. Iron, calcium, ammonium, sulphate and fluoridedo not interfere; phosphate interferes even at moderate levels and carbonate interferes at high concentrations. Difficulties encountered in mixing the aqueous stream carrying the sample with the ethanol reagent stream initiated an investigation of different kinds of mixers in flow systems. A tight coil of teflon tubing proved to be the most efficient mixer, i.e., the one yielding the least baseline noise without excessive dispersion. 相似文献
430.
Jang SS Jang YH Kim YH Goddard WA Choi JW Heath JR Laursen BW Flood AH Stoddart JF Nørgaard K Bjørnholm T 《Journal of the American Chemical Society》2005,127(42):14804-14816
Bistable [2]rotaxanes display controllable switching properties in solution, on surfaces, and in devices. These phenomena are based on the electrochemically and electrically driven mechanical shuttling motion of the ring-shaped component, cyclobis(paraquat-p-phenylene) (CBPQT(4+)), between a monopyrrolotetrathiafulvalene (mpTTF) unit and a 1,5-dioxynaphthalene (DNP) unit located along a dumbbell component. The most stable state of the rotaxane (CBPQT(4+)@mpTTF) is that in which the CBPQT(4+) ring encircles the mpTTF unit, but a second less favored metastable co-conformation with the CBPQT(4+) ring surrounding the DNP (CBPQT(4+)@DNP) can be formed experimentally. For both co-conformations of an amphiphilic bistable [2]rotaxane, we report here the structure and surface pressure-area isotherm of a Langmuir monolayer (LM) on a water subphase as a function of the area per molecule. These results from atomistic molecular dynamics (MD) studies are validated by comparing with experiments based on similar amphiphilic rotaxanes. For both co-conformations, we found that as the area per molecule increases the thickness of the LM decreases while the molecular tilt increases. Both co-conformations led to similar LM thicknesses at the same packing area. From the simulated LM systems, we calculated the electron density profiles of the monolayer as a function of area per molecule, which show good agreement with experimental analyses from synchrotron X-ray reflectivity measurements of related systems. Decomposing the overall electron density profiles into component contributions, we found distinct differences in molecular packing in the film depending upon the co-conformation. Thus we find that the necessity of allowing the tetracationic ring to become solvated by water leads to differences in the structures for the two co-conformations in the LM. At the same packing area, the value of the overall tilt angle does not seem to be sensitive to whether the CBPQT(4+) ring is encircling the mpTTF or the DNP unit. However, the conformation of the dumbbell does depend on the location of the CBPQT(4+) ring, which is reflected in the segmental tilt angles of the mpTTF and DNP units. Using the Kirkwood-Buff formula in conjunction with MD calculations, we find the surface pressure-area isotherms for each co-conformation in which the CBPQT(4+)@mpTTF form has smaller surface tension and therefore larger surface pressure than the CBPQT(4+)@DNP at the same packing area, differences that decreases with increasing area per molecule, which is verified experimentally. 相似文献