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161.
The lack of reference materials, accurately certified for nitrite, is a problem in view of the importance of this species for environmental and medical reasons. This work outlines a plan for the preparation of nitrite isotopic reference materials (IRMs) in the form of high purity solutions, certified for their nitrite-nitrogen isotopic composition and nitrite concentration. To achieve the desired accuracy (expanded uncertainty U with a coverage factor k=2 of =2%), primary methods of measurement such as isotope dilution mass spectrometry (IDMS), gravimetry, and titrimetry must be used. The main difficulty is the stability of nitrite. Other problems expected in the preparation and certification of nitrite IRMs are described. Results from long term stability studies (up to 1.5 years) and a procedure for the purification of the candidate nitrite IRMs are presented. The purpose is to use these IRMs for high accuracy method calibrations and as anchor points for SI-traceable nitrite concentrations. Reference values linked to the SI system are useful to demonstrate the degree of international comparability of nitrite measurements in intercomparison programmes such as the IRMM-International Measurement Evaluation Programme (IMEP). 相似文献
162.
Balandina A Kalinin A Mamedov V Figadère B Latypov S 《Magnetic resonance in chemistry : MRC》2005,43(10):816-828
(1)H, (13)C and (15)N NMR chemical shifts for a variety of novel quinoxalines were determined by different 2D methods and were calculated using the GIAO DFT approach. Comparison with experimental data shows good correlations in the case of (1)H, (13)C and (15)N chemical shifts. Different combinations of basis sets were tested. In non-polar solvents quinoxalines exist as dimers owing to strong hydrogen bonding. Calculations for dimers improve the correlation between experiment and theory. Additive empirical methods for estimating chemical shifts have drawbacks and have to be used with a great care for this type of compound. 相似文献
163.
Abstract— In order to investigate the interactions and the photoreactions in solution between the thymine (thy) and the psoralen (Pso) rings, we have prepared model compounds Thy-(CH2 )n -Pso in which two aromatic chromophores Thy and Pso are linked by flexible polymethylene chains of varying length (CH2 )n . Two series of compounds were examined and compared as models for the two important drugs 5-methoxypsoralen and 8-methoxypsoralen. Results concerning the 5-alkoxypsoralen series are reported here. In water, these model molecules exhibit intramolecular ring-ring stacking interactions as indicated by hypochromism in the UV and by shielding of the protons in 1 H NMR spectroscopy. These interactions disappear in organic solvents. The photochemical properties of the models were examined in relation with their ground state interaction properties. Irradiation at 365 nm carried out at the usual concentrations (10-2 -10-3 M) leads exclusively to a stereoselective dimerization involving the psoralen moieties of the models at the 3,4 double bonds. However, when operating at exceedingly low concentrations (2 × 10-5 M ), the psoralen photodimerization is avoided and a highly regio and stereo-selective psoralen thymine photoaddition is observed involving the 3,4 double bond of psoralen leading to the cis adduct. The same reaction occurs for all models under study being independent of the length of the (CH2 )n polymethylene linking chain, n = 2 to 6, 12 and of the solvent used. This is unambiguous proof for the highest intrinsic photoreactivity of the 3,4 vs the 4',5' double bond in 5-alkoxy psoralen. 相似文献
164.
165.
The γ-distonic radical ions R$ \mathop {\rm O}\limits^ + $CHR′CH2?HR″ and their molecular ion counterparts R$ \mathop {\rm O}\limits^{{\rm + } \cdot } $CHR′CH2CH2R″ have been studied by isotopic labelling and collision-induced dissociation, applying a potential to the collision cell in order to separate activated from spontaneous decompositions. The stability of CH3$ \mathop {\rm O}\limits^ + $HCH(CH3)CH2?HCH3, C2H5$ \mathop {\rm O}\limits^ + $HCH(CH3)CH2?HCH3, CH3$ \mathop {\rm O}\limits^ + $HCH(CH3)CH2?H2, CH3$ \mathop {\rm O}\limits^ + $HCH2CH2?HCH3 and C2H5$ \mathop {\rm O}\limits^ + $HCH2CH2?HCH3, has been demonstrated and their characteristic decomposition, alcohol loss, identified. For all these γ-distonic ions, the 1,4-H abstraction leading to their molecular ion counterpart exhibits a primary isotope effect. 相似文献
166.
Rouxel Tanguy Massouras Georges Sorarù Gian-Domenico 《Journal of Sol-Gel Science and Technology》1999,14(1):87-94
The high temperature behavior of a sol-gel derived silicon oxycarbide glass containing 12 at.% carbon has been characterized by means of creep and in-situ ultrasonic echography measurements. Temperature induced changes include structural relaxation and densification from 1000 to 1200°C, and crystallization to form a fine and homogeneous -SiC/glass-matrix nanocomposite with 2.5 nm large crystals above 1200°C. Young's modulus measurements clearly reveal a consolidation of the material upon annealing below 1200°C. Crystallization is almost complete after few hours at 1300°C and results in a significant increase in Young's modulus. The viscosity of the oxycarbide glass is much higher than that of fused silica, with two orders of magnitude difference at 1200°C, and the glass transition temperature ranges from 1320 to 1370°C. 相似文献
167.
P. De Bièvre 《Accreditation and quality assurance》2000,5(6):224-230
Traceability is a property of the result of a measurement. Since values carried by (reference) materials must also have been
obtained, of necessity, by measurement, the definition of traceability also applies to reference materials. It is extremely
helpful to give the traceability (of the origin) of a reference material a separate name, i.e. 'trackability'. An analysis
of the function of values carried by reference materials, shows that they can fulfill different functions, depending on the
intended use. One of the functions located outside the traceability chain – and hence not very relevant for establishing traceability
– is evaluating the approximate size of the uncertainty of the measurement of an unknown sample by performing a similar measurement
on a reference material, used as a 'simulated sample'. Another function is located inside the traceability chain, where the
reference material is used as an added 'internal standard'. Then, the value carried by the reference material is essential
for establishing the traceability of the measured value of an unknown sample. In the latter application, the reference material
acts as an 'amount standard' (the certified value for amount is used).
Received: 11 November 1999 / Accepted: 24 February 2000 相似文献
168.
Valérie Bernat Catherine Ringard-Lefebvre Geneviève Le Bas Sylviane Lesieur 《Journal of inclusion phenomena and macrocyclic chemistry》2007,57(1-4):113-119
Interactions of α-cyclodextrin (α-CD) with dimyristoylphosphatidylcholine (DMPC) and Egg phosphatidylcholine (Egg-PC) were studied (i) by analyzing surface
pressure-area isotherms and surface tension of phospholipid monolayers formed at the interface between air and α-CD aqueous solutions and (ii) by X-ray diffraction performed on fully hydrated α-CD/phospholipid binary mixtures. The cyclodextrin molecules strongly interact with the two-dimension phospholipid assembly.
Their addition into the aqueous sub-phase leads to the removal of part of the phospholipids from the air-water interface:
the higher the α-CD concentration, the higher the phospholipid depletion. This should preferentially involve interactions between cyclodextrin
and the phosphatidylcholine head group as α-CD is water-soluble and not surface-active. At the three-dimension level, the bilayer packing of the phospholipid lamellar
phase appears not affected by the presence of cyclodextrin as shown by X-ray scattering at small angles whereas wide-angle
diffraction patterns reveal the formation of a crystalline phase organized in a pseudo-hexagonal lattice usually characteristic
of α-CD dimers. These results point out that α-CD should interact with bilayer-forming phospholipid molecules but likely according to a process that would preserve intact
at least a part of the multilamellar assembly. 相似文献
169.
170.
The concentration gradient (Soret effect) induced in cw-laser thermal lens spectrometry subsequently to the formation of the thermal gradient (thermal lens effect) has been investigated in aqueous solutions of various macromolecular species including micelles, mixed micelles and polymers. It is shown that the build-up of the concentration gradient is much shorter than that in classical Soret experiments, reaching steady-state values in less than 1 min. The time evolution of the Soret signal has been used to derive mass-diffusion times from which mass-diffusion coefficients were calculated. Our data are in agreement with previous results obtained from quasi-elastic light scattering studies for the micellar solutions and calculated from a known molecular weight-dependent power law for polymer solutions. 相似文献