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991.
Santiago PS Neto Dde S Gandini SC Tabak M 《Colloids and surfaces. B, Biointerfaces》2008,65(2):247-256
Fluorescence quenching of meso-tetrakis-4-sulfonatophenyl (TPPS(4)) and meso-tetrakis-4-N-methylpyridil (TMPyP) porphyrins is studied in aqueous solution and upon addition of micelles of sodium dodecylsulfate (SDS), cetyltrimethylammonium chloride (CTAC), N-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (HPS) and t-octylphenoxypolyethoxyethanol (Triton X-100). Potassium iodide (KI) was used as quencher. Steady-state Stern-Volmer plots were best fitted by a quadratic equation, including dynamic (K(D)) and static (K(S)) quenching. K(S) was significantly smaller than K(D). Frequency-domain fluorescence lifetimes allowed estimating bimolecular quenching constants, k(q). At 25 degrees C, in aqueous solution, TMPyP shows k(q) values a factor of 2-3 higher than the diffusional limit. TPPS(4) shows collisional quenching with pH dependent k(q) values. For TMPyP quenching results are consistent with reported binding constants: a significant reduction of quenching takes place for SDS, a moderate reduction is observed for HPS and almost no change is seen for Triton X-100. Similar data were obtained at 50 degrees C. For CTAC-TPPS(4) system an enhancement of quenching was observed as compared to pure buffer. This is probably associated to accumulation of iodide at the cationic micellar interface. The attraction between CTAC headgroups and I(-), and repulsion between SDS and I(-), enhances and reduces the fluorescence quenching, respectively, of porphyrins located at the micellar interface. The small quenching of TPPS(4) in Triton X-100 is consistent with strong binding as reported in the literature. 相似文献
992.
In the present study, adsorbent is prepared from neem leaves and used for Cr(VI) removal from aqueous solutions. Neem leaves
are activated by giving heat treatment and with the use of concentrated hydrochloric acid (36.5 wt%). The activated neem leaves
are further treated with 100 mmol of copper solution. Batch adsorption studies demonstrate that the adsorbent prepared from
neem leaves has a significant capacity for adsorption of Cr(VI) from aqueous solution. The parameters investigated in this
study include pH, contact time, initial Cr(VI) concentration and adsorbent dosage. The adsorption of Cr(VI) is found to be
maximum (99%) at low values of pH in the range of 1-3. A small amount of the neem leaves adsorbent (10 g/l) could remove as
much as 99% of Cr(VI) from a solution of initial concentration 50 mg/l. The adsorption process of Cr(VI) is tested with Langmuir
isotherm model. Application of the Langmuir isotherm to the system yielded maximum adsorption capacity of 62.97 mg/g. The
dimensionless equilibrium parameter, R
L, signifies a favorable adsorption of Cr(VI) on neem leaves adsorbent and is found to be between 0.0155 and 0.888 (0<R
L<1). The adsorption process follows second order kinetics and the corresponding rate constant is found to be 0.00137 g/(mg)
(min). 相似文献
993.
O. M. Sarkisov 《Russian Chemical Bulletin》2008,57(4):736-753
The review concerns the mechanisms of physicochemical processes occurring on the femtosecond-subpicosecond time scale; coherent
chemistry; coherent control of the dynamics and yield of reaction products; femtosecond optical, electron, and X-ray microscopies;
and structural dynamics during chemical reactions. The results of research on molecular, nanoscale, and also on biological
systems.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 722–739, April, 2008. 相似文献
994.
The present article describes the synthesis, structural features and thermal studies of heterochelates of the type [M(SB)(benen)(H2O)]·nH2O [where H2SB=(Z)-2-(2,2,2-trifluoro-1-(5-hydroxy-3-methyl-1-phenyl-1H-pyrazol-4-yl)ethylideneamino)benzoic acid, benen=bis(benzylidene)ethylenediamine
and M=Mn(II), Co(II), Ni(II), Cu(II), Zn(II) and VO(IV)]. The Schiff base (H2SB) have been characterized on the basis of elemental analysis, IR, 1H and 13C NMR. The heterochelates have been characterized on the basis of elemental analyses, magnetic measurements, solid state conductivity
measurements, IR, reflectance spectra, and thermal studies. The FAB mass spectrum of [Co(SB)(benen)(H2O)] has been carried out. The kinetic parameters such as order of reaction (n) and the energy of activation (E
a) have been reported using Freeman-Carroll method. The pre-exponential factor (A), the activation entropy (ΔS
#), the activation enthalpy (ΔH
#) and the free energy of activation (ΔG
#) have been calculated. 相似文献
995.
Wolska L 《Analytical and bioanalytical chemistry》2008,391(7):2647-2652
The results of interlaboratory studies on the determination of selected PAHs in samples of pure water and water containing
suspended matter are presented and discussed in this study. Determinations were performed by independent analytical laboratories
which used different sample-preparation procedures, i.e. liquid-liquid extraction and solid-phase extraction with columns
and speedisks. The study indicates that the results of PAHs determination depend on the type of isolation technique and the
final determination procedure used by a given laboratory. Differences among the determined concentrations of specific PAHs
reached 700%. In this work it has been shown that modern analysis does not offer isolation techniques for PAHs which would
secure their speciation in the aquatic environment. 相似文献
996.
997.
998.
In this paper, based on Einstein relationship between diffusion and random walk, the electrochemical behavior of a system
with a limited number of molecules was simulated and explored theoretically. The transition of the current vs time responses
from discrete to continuous was clearly obtained as the number of redox molecules increased from 10 to 106. By correlation analysis between the simulation results and the results of analytical expressions, a quantized extent parameter
was proposed to investigate the underlying rules of these discrete signals, which looked stochastic. The results revealed
that this parameter would be useful to describe such systems. 相似文献
999.
Emanuel Lörtscher Dr. Mark Elbing Dr. Meinrad Tschudy Carsten von Hänisch Dr. Heiko B. Weber Prof. Dr. Marcel Mayor Prof. Dr. Heike Riel Dr. 《Chemphyschem》2008,9(15):2252-2258
A series of oligophenylene rods of increasing lengths is synthesized to investigate the charge‐transport mechanisms. Methyl groups are attached to the phenyl rings to weaken the electronic overlap of the π‐subsystems along the molecular backbones. Out‐of‐plane rotation of the phenyl rings is confirmed in the solid state by means of X‐ray analysis and in solution by using UV/Vis spectroscopy. The influence of the reduced π‐conjugation on the resonant charge transport is studied at the single‐molecule level by using the mechanically controllable break‐junction technique. Experiments are performed under ultra‐high‐vacuum conditions at low temperature (50 K). A linear increase of the conductance gap with increasing number of phenyl rings (from 260 meV for one ring to 580 meV for four rings) is revealed. In addition, the absolute conductance of the first resonant peaks does not depend on the length of the molecular wire. Resonant transport through the first molecular orbital is found to be dominated by charge‐carrier injection into the molecule, rather than by the intrinsic resistance of the molecular wire length. 相似文献
1000.
Lorna Dougan Dr. Ainavarapu Sri Rama Koti Dr. Georgi Genchev Hui Lu Prof. Julio M. Fernandez Prof. 《Chemphyschem》2008,9(18):2836-2847
We present an array of force spectroscopy experiments that aim to identify the role of solvent hydrogen bonds in protein folding and chemical reactions at the single‐molecule level. In our experiments we control the strength of hydrogen bonds in the solvent environment by substituting water (H2O) with deuterium oxide (D2O). Using a combination of force protocols, we demonstrate that protein unfolding, protein collapse, protein folding and a chemical reaction are affected in different ways by substituting H2O with D2O. We find that D2O molecules form an integral part of the unfolding transition structure of the immunoglobulin module of human cardiac titin, I27. Strikingly, we find that D2O is a worse solvent than H2O for the protein I27, in direct contrast with the behaviour of simple hydrocarbons. We measure the effect of substituting H2O with D2O on the force dependent rate of reduction of a disulphide bond engineered within a single protein. Altogether, these experiments provide new information on the nature of the underlying interactions in protein folding and chemical reactions and demonstrate the power of single‐molecule techniques to identify the changes induced by a small change in hydrogen bond strength. 相似文献