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661.
The sol-gel technique with a microwave irradiation was used to synthesize ZrO2 and Cu/ZrO2 solids at various pH values and hydrolysis catalysts. Microwave irradiation induces ZrO2 tetragonal phase formation at almost any pH. The copper was bonded through oxygen bonds to the zirconia lattice. The introduction of copper favored the support acidity. 相似文献
662.
Puig P Borrull F Calull M Benavente F Sanz-Nebot V Barbosa J Aguilar C 《Analytica chimica acta》2007,587(2):208-215
This paper describes two different approaches for increasing the sensitivity for the analysis of ceftiofur by capillary electrophoresis (CE). Two different techniques based on the introduction of an enlarged volume of sample, namely large volume sample stacking (LVSS) and in-line solid phase extraction (SPE) were studied and compared. LVSS allowed the on-column electrophoretic preconcentration of ceftiofur without modification of the separation capillary. In-line SPE-CE was developed by using a home-made microcartridge that was filled with a reversed-phase sorbent (C18). The microcartridge was coupled in-line near the inlet of the separation capillary. LVSS and in-line SPE-CE allowed automated operation and improved sensitivity for the analysis of ceftiofur with respect to conventional CE. When environmental water samples were analyzed, an additional pretreatment step based on off-line SPE was necessary in both cases to further decrease the detection limits. In terms of sensitivity for the determination of ceftiofur in river water samples, the combination of off-line SPE with in-line SPE-CE was found the most sensitive with a detection limit of 10 ng L−1, whereas the method based on the use of off-line SPE with LVSS presented a detection limit of 100 ng L−1. 相似文献
663.
Capillary column gas chromatographic identification of sugars in honey as trimethylsilyl derivatives 总被引:2,自引:0,他引:2
A method for identifying carbohydrates (mono-, di- and trisaccharides) in honey is presented. It is based on the separate preparation of both trimethylsilyl ethers and oxime trimethylsilyl ethers of the sugars followed by their gas chromatographic separation on a fused-silica capillary column coated with OV-101 using temperature programming. From the two chromatograms, the number of peaks given by each derivatized sugar, their relative retention times and peak-area ratios are used for identification. The identities of two unidentified trisaccharide peaks are considered. Quantitative applications to honey sugar analysis are discussed. 相似文献
664.
H. F. Beyer G. Menzel D. Liesen A. Gallus F. Bosch R. Deslattes P. Indelicato Th. Stöhlker O. Klepper R. Moshammer F. Nolden H. Eickhoff B. Franzke M. Steck 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1995,35(3):169-175
X-rays are emitted with the radiative recombination of free electrons in an electron cooler of a heavyion storage ring. Due to a small width of the X-ray lines, an observation angle close to 0° and an accurate determination of the ion velocity, the ground-state Lambshift of hydrogenlike uranium (470 ± 16) eV could be measured to an accuracy of 3.4%. A re-evaluation of a measurement of the 1s
1/2 Lambshift in hydrogenlike gold gave a new value of (202.3 ± 7.9) eV as compared to the former value of (212 ± 15) eV. The results are in excellent agreement with QED calculations and are more precise than any other measurements previously reported for a high-Z, hydrogenlike ion. 相似文献
665.
M. Carme Rovira Juan J. Novoa Myung-Hwan Whangbo Jack M. Williams 《Chemical physics》1995,200(3):319-335
On the basis of ab initio MP2/6–31 + + G(2d,2p) calculations, we examined the potential energy surfaces of the water·hydrocarbon complexes H2O·CH4, H2O·C2H2 and H2O·C2H2 to locate all the minimum energy structures and estimate the hydrogen bond energies and vibrational frequencies associated with the C(spn)---H·O and the O---H·C(spn) bonds (n = 1−3). Our calculations show that H2O·C2H2, H2O·C2H4 and H2O·CH4 have two minimum energy structures (i.e., the C---H·O and O---H·C hydrogen bond forms), but H2O·C2H4 has only one when the vibrational motion is taken into account, the O---H·C hydrogen bond form. We have also computed the barrier for the interconversion from one minimum to the other. The fully optimized geometries of H2O·CH4, H2O·C2H4 and H2O·C2H2 as well as the vibrational shifts of the C---H stretching frequencies in their C---H·O hydrogen-bonded forms are in good agreement with the available experimental data. The calculated hydrogen bond energies show that the C(spn---H·O bond strengths decrease in the order C(sp)---H·O>C(sp2)---H·O>C(sp3)---O>C(sp3---H·O, which is also consistent with the available experimental data. 相似文献
666.
Summary The H-Point Standard Additions Method (HPSAM) is proposed in order to resolve overlapping peaks in liquid chromatography by using a conventional fluorescence detector. The method uses as analytical signals the heights or the areas obtained at two previously selected emission wavelengths, and good results are obtained for highly overlapping peaks with highly overlapping fluorescence spectra. The principal benefits of the method are the ease of finding the required wavelengths, its insensitivity to changes in the retention time of the peak from one injection to another, and the possibility of using it in highly or only partially overlapping peaks. We have applied the method to the determination of phenol and cresols in water, resolving by the proposed method the overlapping peaks ofm- andp-cresol. 相似文献
667.
Micellar electrokinetic chromatography estimation of size and composition of procyanidins after thiolysis with cysteine 总被引:1,自引:0,他引:1
Herrero-Martínez JM Ràfols C Rosés M Bosch E Lozano C Torres JL 《Electrophoresis》2003,24(9):1404-1410
This paper describes the characterization of procyanidin mixtures by acid depolymerization in the presence of cysteine (thiolysis with cysteine) and micellar electrokinetic chromatography (MEKC). Reversed-phase liquid chromatography (RP-HPLC) and MEKC were investigated for the separation of the major components of the depolymerized mixtures (catechins and their cysteinyl derivatives). The solutes could only be effectively separated using MEKC. Two background electrolytes (BGEs) are recommended: (i) 50 mM phosphate at pH 7, containing 40 mM sodium cholate (SC) and 10 mM sodium dodecyl sulfate (SDS); (ii) a BGE with the same composition but containing only 50 mM SDS. The MEKC procedures here reported, are cheap, reliable and fast, and their potential in the determination of the size and composition in procyanidin mixtures has been shown. The proposed MEKC methods were validated by comparison with our intralaboratory reference RP-HPLC method using cysteamine as thiol donor. 相似文献
668.
Recently, cloud point extraction (CPE) has been an attractive subject as an alternative to liquid–liquid extraction. The technique
is based on the property of most non-ionic surfactants in aqueous solutions to form micelles and become turbid when heated
to the cloud point temperature. This review covers a selection of the literature published on applications of CPE in determination
of metal ions over the period between 2004 and 2008. 相似文献
669.
Elisabet Fuguet Clara RàfolsElisabeth Bosch Martí Rosés 《Journal of chromatography. A》2009,1216(45):7798-7805
The Chromatographic Hydrophobicity Index (CHI) has been measured for several diprotic compounds. In order to model the variation of CHI with pH, a general equation for polyprotic compounds which allows the calculation of CHI for each one of the species has been proposed. Different types of diprotic solutes have been selected: neutral acids, neutral bases, and amphiprotic compounds (non-zwitterionic and zwitterionic). The applicability of the model to these complex polyprotic compounds, its advantages and limitations, as well as the information provided by the fits is discussed. In general, the model fits the experimental behaviour of all kind of solutes very well, obtaining the higher hydrophobicity values for the neutral form of the compounds, except for the zwitterionic phenylalanine. The equation and parameters obtained allow an easy calculation of CHI of the compound at any pH value. 相似文献
670.
Carme Rovira Karel Kunc Jürg Hutter Pietro Ballone Michele Parrinello 《International journal of quantum chemistry》1998,69(1):31-35
Minimum-energy structures of O2, CO, and NO iron–porphyrin (FeP) complexes, computed with the Car–Parrinello molecular dynamics, agree well with the available experimental data for synthetic heme models. The diatomic molecule induces a 0.3–0.4 Å displacement of the Fe atom out of the porphyrin nitrogen (Np) plane and a doming of the overall porphyrin ring. The energy of the iron–diatomic bond increases in the order Fe(SINGLE BOND)O2 (9 kcal/mol) < Fe(SINGLE BOND)CO (26 kcal/mol) < Fe(SINGLE BOND)NO (35 kcal/mol). The presence of an imidazole axial ligand increases the strength of the Fe(SINGLE BOND)O2 and Fe(SINGLE BOND)CO bonds (15 and 35 kcal/mol, respectively), with few structural changes with respect to the FeP(CO) and FeP(O2) complexes. In contrast, the imidazole ligand does not affect the energy of the Fe(SINGLE BOND)NO bond, but induces significant structural changes with respect to the FeP(NO) complex. Similar variations in the iron–imidazole bond with respect to the addition of CO, O2, and NO are also discussed. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 69: 31–35, 1998 相似文献