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61.
The sensitivity and precision of headspace solid-phase micro extraction (HS-SPME) at an analyte solution temperature (T
as) of +35 °C and a fiber temperature (T
fiber) of +5 °C were compared with those for HS-SPME at T
as and T
fiber of −20 °C for analysis of the volatile organic compounds benzene, 1,1,1-trichloroethane, trichloroethylene, toluene, o-xylene, ethylbenzene, m/p-xylene, and tetrachloroethylene in water samples. The effect of simultaneous fiber cooling and analyte solution freezing
during extraction was studied. The compounds are of different hydrophobicity, with octanol/water partition coefficients (Kow) ranging from 126 and 2511. During a first set of experiments the polydimethylsiloxane (PDMS) SPME fiber was cooled to
+5 °C with simultaneous heating of the aqueous analyte solution to +35 °C. During a second set of experiments, both SPME fiber
holder and samples were placed in a deep freezer maintained at −20 °C for a total extraction time of 30 min. After approximately
2 min the analyte solution in the vial began to freeze from the side inwards and from the bottom upwards. After approximately
30 min the solution was completely frozen. Analysis of VOC was performed by coupling HS-SPME to gas chromatography-mass spectrometry
(GC-MS). In general, i.e. except for tetrachloroethylene, the sensitivity of HS-SPME increased with increasing compound hydrophobicity
at both analyte solution and fiber temperatures. At T
as of +35 °C and T
fiber of +5 °C detection limits of HS-SPME were 0.5 μg L−1 for benzene, 1,1,1-trichloroethane, trichloroethylene, and tetrachloroethylene, 0.125 μg L−1 for toluene, and 0.025 μg L−1 for ethylbenzene, m/p-xylene, and o-xylene. In the experiments with T
as and T
fiber of −20 °C, detection limits were reduced for compounds of low hydrophobicity (Kow<501), for example benzene, toluene, 1,1,1-trichloroethane, and trichloroethylene. In the concentration range 0.5–62.5 μg
L−1, the sensitivity of HS-SPME was enhanced by a factor of approximately two for all compounds by performing the extraction
at −20 °C. A possible explanation is that freezing of the water sample results in higher concentration of the target compounds
in the residual liquid phase and gas phase (freezing-out), combined with enhanced adsorption of the compounds by the cooled
fiber. The precision of HS-SPME, expressed as the relative standard deviation and the linearity of the regression lines, is
increased for more hydrophobic compounds (Kow>501) by simultaneous direct fiber cooling and freezing of analyte solution. Background contamination during analysis is
reduced significantly by avoiding the use of organic solvents. 相似文献
62.
Menegazzo I Fries A Mammi S Galeazzi R Martelli G Orena M Rinaldi S 《Chemical communications (Cambridge, England)》2006,(47):4915-4917
Starting from (3S,4R,1'S)-3-amino-2-oxo-1-[1'-(4-methoxyphenylethyl)]pyrrolidine carboxylic acid (2), the first synthesis of a beta-foldamer containing pyrrolidin-2-one rings is described, whose 12-helix conformation is assigned by NMR analysis and confirmed by molecular dynamics (MD) simulations. 相似文献
63.
64.
Direct radiometric measurements of the vapor concentration of H2S in equilibrium with aqueous diethanolamine absorbing solution were made using 35S-labeled H2S. A thin-window Geiger-Müller tube viewed the gas phase only, while the absorbing solution was held in another part of the reaction flask. The equilibrium data were measured at H2S partial pressures below 0.01 mm Hg; the agreement with published results obtained by chemical analysis was good. The radiometric results were obtained very rapidly; a test run with seven data points took about two hours to complete. The results were applied to processes for reducing H2S concentrations in petroleum refinery fuel gases needed to meet tightening air pollution requirements. 相似文献
65.
High temperature plastic deformation is associated with large changes in the microstructure of single crystals. To observe this microstructure during the creep test, we have performed X-ray reflection topography, taking advantage of the high intensity of the synchrotron radiation. A special creep machine was designed which permits in situ observation. Creep tests and microstructural observations were performed on NaCl single crystals compressed along <100> at about 600°C. As soon as the deformation started, subgrains appeared within the crystal, independent of the initial microstructure. Migration of the subboundaries during transient creep is followed by stabilization during steady state creep where a well developed subgrain structure keeps constant while new appearing subboundaries migrate. Misorientation between sub-grains increases progressively although more slowly in the steady state creep. A correlation between the microstructure evolution and the changes in the creep curves has been attempted. 相似文献
66.
Alp Arslan Kiraç 《Letters in Mathematical Physics》2008,83(2):149-161
Consider the Mathieu–Hill operator
in , where . We obtain the precise asymptotic formulas for the widths γ
k
of the instability intervals of L. The formula states the isolated terms of arbitrary number in the asymptotics of the sequence γ
k
for large k and verifies the results of Harrell (Am J Math suppl:139–150, 1981) and Avron and Simon (Ann Phys 134:76–84, 1981).
相似文献
67.
ABSTRACT To understand the practical effects of pressure-transmitting media (PTM) on neutron diffraction using Paris–Edinburgh presses, diffraction patterns of MgO were collected to approximately 20?GPa using PTMs of Pb, AgCl, 4:1 methanol–ethanol (ME) mixture with and without heating, N2, and Ar. Hydrostaticity in the sample chamber estimated from the MgO 220 peak width improves in the order of Pb, AgCl, Ar, ME mixture, N2, and the heated ME mixture. Unlike previous results using diamond anvil cells, the unheated ME mixture is superior to Ar even after freezing, probably due to the cup on the anvil face. Considering these results and the sizable coherent scattering of Ne, which would show good hydrostaticity, we conclude that the ME mixture (preferably the heated one) is the best PTM in neutron experiments up to 20?GPa, while Ar can be substituted when a sample is reactive to alcohols. 相似文献
68.
69.
70.
We derive an analytic solution for the capillary rise of liquids in a cylindrical tube or a porous medium in terms of height h as a function of time t. The implicit t(h) solution by Washburn is the basis for these calculations and the Lambert W function is used for its mathematical rearrangement. The original equation is derived out of the 1D momentum conservation equation and features viscous and gravity terms. Thus our h(t) solution, as it includes the gravity term (hydrostatic pressure), enables the calculation of the liquid rise behavior for longer times than the classical Lucas-Washburn equation. Based on the new equation several parameters like the steady state time and the validity of the Lucas-Washburn equation are examined. The results are also discussed in dimensionless form. 相似文献