全文获取类型
收费全文 | 5678篇 |
免费 | 249篇 |
国内免费 | 16篇 |
专业分类
化学 | 3904篇 |
晶体学 | 31篇 |
力学 | 118篇 |
数学 | 821篇 |
物理学 | 1069篇 |
出版年
2023年 | 41篇 |
2022年 | 35篇 |
2021年 | 105篇 |
2020年 | 94篇 |
2019年 | 68篇 |
2018年 | 104篇 |
2017年 | 95篇 |
2016年 | 201篇 |
2015年 | 187篇 |
2014年 | 185篇 |
2013年 | 380篇 |
2012年 | 385篇 |
2011年 | 441篇 |
2010年 | 217篇 |
2009年 | 221篇 |
2008年 | 350篇 |
2007年 | 359篇 |
2006年 | 321篇 |
2005年 | 309篇 |
2004年 | 292篇 |
2003年 | 233篇 |
2002年 | 213篇 |
2001年 | 117篇 |
2000年 | 117篇 |
1999年 | 82篇 |
1998年 | 40篇 |
1997年 | 45篇 |
1996年 | 94篇 |
1995年 | 33篇 |
1994年 | 34篇 |
1993年 | 35篇 |
1992年 | 36篇 |
1991年 | 28篇 |
1990年 | 30篇 |
1989年 | 26篇 |
1988年 | 22篇 |
1987年 | 14篇 |
1986年 | 26篇 |
1985年 | 43篇 |
1984年 | 30篇 |
1983年 | 28篇 |
1982年 | 37篇 |
1981年 | 22篇 |
1980年 | 31篇 |
1979年 | 39篇 |
1978年 | 15篇 |
1977年 | 14篇 |
1976年 | 15篇 |
1974年 | 13篇 |
1973年 | 10篇 |
排序方式: 共有5943条查询结果,搜索用时 31 毫秒
41.
JPC – Journal of Planar Chromatography – Modern TLC - A new simple, rapid, and selective high-performance thin-layer chromatographic (HPTLC) method with metronidazole as the internal... 相似文献
42.
A. González Arias C. Torres C. de
Francisco J. M. Muñoz P. Hernández
Gómez O. Alejos O. Montero J. I. Iñiguez 《Journal of Thermal Analysis and Calorimetry》2006,86(1):195-198
The article discusses theory and experiment about the measurement of
defect concentration in Ti-substituted yttrium iron garnet by means of thermogravimetry
techniques. The two possible cases arising from oxygen interchange with atmosphere,
oxygen vacancies and interstitial cations, may be analyzed quantitatively
from the derived expressions. The possibility of another type of defects being
present in the samples, not associated to oxygen evolution, is not excluded.
Measurements were carried out in air and CO2 for different
Ti contents. The substitution tends to increase slightly the defect parameter
θ in YIG, while a CO2 reducing atmosphere is
much more effective than the Ti substitution for increasing θ. Calculations
for a given single-phased sample sintered in air show two possibilities: a
deficit of one oxygen atom for every 2.5 unit cells of 96 atoms, or one excess
cation for every 3.5 cells. Other samples show very similar results. The accuracy
involved in the measurements is about 2–3%. 相似文献
43.
Pau Bernadó Renato Barbieri Esteve Padrós Claudio Luchinat Miquel Pons 《Journal of the American Chemical Society》2002,124(3):374-375
Addition of Tb3+ to purple membrane (PM) suspensions changes the orientation of the menbrane normal from parallel to perpendicular with respect to the magnetic field. Residual dipolar couplings measured in protein L in the presence of PM are scaled by a factor of -1/2. NMR line broadening and cross-correlation effects induced by the addition of PM are partially reversed by Tb3+ but not by Tm3+ that has no effect on the orientation of PM. This is interpreted as the result of anisotropically restricted motion of protein L in the transiently PM-bound form. 相似文献
44.
The title reaction has been used as an example to test the importance of using a hindered rotor treatment instead of a harmonic oscillator model for calculating vibrational partition functions corresponding to low-frequency internal rotation modes. First, a normal-mode analysis according to the Ayala and Schlegel's algorithm has been used to identify the internal rotation modes of methanethiol and the transition state structure. Then, after calculation of the energy barrier for each internal rotation, the corresponding hindered rotor partition functions have been calculated following the CW scheme of Chuang and Truhlar. The results show that the anharmonic treatment produces a rather modest improvement of the rate constants at room temperature or below. 相似文献
45.
Perea-Carpio R. González-Caballero F. Bruque J. M. Pardo G. 《Colloid and polymer science》1983,261(2):183-187
Colloid and Polymer Science - Data on the surface tension of sodium alkylsulfonate solutions at different temperatures and surfactant concentrations are presented. By applying the thermodynamic... 相似文献
46.
J. Griffith R. J. Rosenberg O. Díaz-Rizo E. González 《Journal of Radioanalytical and Nuclear Chemistry》1996,213(1):71-78
Thermal and epithermal non-destructive activation analyses have been performed on samples of final molasses from 14 different sugar factories, covering the most important regions in Cuba. From the first measurement after irradiation at the Triga Mark reactor (VTT), the concentration of more than 15 elements is reported. The almost constant elemental composition shows that they can be used equally for different purposes as animal foodstuff and for the manufacture of biotechnological products. This work is part of a research project developed in order to establish a complete characterization of Cuban sugar molasses. 相似文献
47.
Minoia C Gatti A Aprea C Ronchi A Sciarra G Turci R Bettinelli M 《Rapid communications in mass spectrometry : RCM》2002,16(13):1313-1319
A method was developed for the determination of molybdenum (Mo) in human urine by direct dilution of the sample in doubly distilled water with 1% HNO3 (v/v) and inductively coupled mass spectrometry (ICP-MS). In and Y were used as internal standards. Since (98)Mo provides a higher sensitivity, it was chosen as the reference isotope. The influence of different factors, such as sample dilution, HNO3 concentration and the stability of the analyte were evaluated. The detection limit (LOD) was assessed at 0.2 microg/L Mo, while the lower limit of quantification (LOQ) was 0.6 microg/L. Recoveries ranged between 97.2 and 100.7% from solutions containing from 10 to 50 microg/L Mo. Linear calibration curves were generated from 2.1 and 52.1 microg/L with coefficients of variation (CV ) ranging from 1.62 to 3.56%. In order to establish reference values (RV) for molybdenum, the procedure presented here was used to determine Mo in the urine of a population group living in Tuscany, Italy. 相似文献
48.
Polo F Antonello S Formaggio F Toniolo C Maran F 《Journal of the American Chemical Society》2005,127(2):492-493
The rate constant of intramolecular electron transfer through oligopeptides based on the alpha-aminoisobutyric acid residue was determined as a function of the peptide length and found to depend weakly on the donor-acceptor separation. By measuring the electron-transfer activation energy and estimating the energy gap between donor and bridge, we were able to discard the electron hopping mechanism. 相似文献
49.
Bañares L Aoiz FJ González-Lezana T Herrero VJ Tanarro I 《The Journal of chemical physics》2005,123(22):224301
Integral cross sections and thermal rate constants have been calculated for the N((2)D)+H(2) reaction and its isotopic variants N((2)D)+D(2) and the two-channel N((2)D)+HD by means of quasiclassical trajectory and statistical quantum-mechanical model methods on the latest ab initio potential-energy surface [T.-S. Ho et al., J. Chem. Phys. 119, 3063 (2003)]. The effect of rotational excitation of the diatom on the dynamics of these reactions has been investigated and interesting discrepancies between the classical and statistical model calculations have been found. Whereas a net effect of reagent rotation on reactivity is always observed in the classical calculations, only a very slight effect is observed in the case of the asymmetric N((2)D)+HD reaction for the statistical quantum-mechanical method. The thermal rate constants calculated on this Potential-Energy Surface using quasiclassical trajectory and statistical model methods are in good agreement with the experimental determinations, although the latter are somewhat larger. A reevaluation of the collinear barrier of the potential surface used in the present study seems timely. Further theoretical and experimental studies are needed for a full understanding of the dynamics of the title reaction. 相似文献
50.
Carra C Iordanova N Hammes-Schiffer S 《Journal of the American Chemical Society》2003,125(34):10429-10436
Theoretical calculations of a model for tyrosine oxidation in photosystem II are presented. In this model system, an electron is transferred to ruthenium from tyrosine, which is concurrently deprotonated. This investigation is motivated by experimental measurements of the dependence of the rates on pH and temperature (Sj?din et al. J. Am. Chem. Soc. 2000, 122, 3932). The mechanism is proton-coupled electron transfer (PCET) at pH < 10 when the tyrosine is initially protonated and is single electron transfer (ET) for pH > 10 when the tyrosine is initially deprotonated. The PCET rate increases monotonically with pH, whereas the single ET rate is independent of pH and is 2 orders of magnitude faster than the PCET rate. The calculations reproduce these experimentally observed trends. The pH dependence for the PCET reaction arises from the decrease in the reaction free energies with pH. The calculations indicate that the larger rate for single ET arises from a combination of factors, including the smaller solvent reorganization energy for ET and the averaging of the coupling for PCET over the reactant and product hydrogen vibrational wave functions (i.e., a vibrational overlap factor in the PCET rate expression). The temperature dependence of the rates, the solvent reorganization energies, and the deuterium kinetic isotope effects determined from the calculations are also consistent with the experimental results. 相似文献