全文获取类型
收费全文 | 5612篇 |
免费 | 72篇 |
国内免费 | 11篇 |
专业分类
化学 | 3310篇 |
晶体学 | 65篇 |
力学 | 126篇 |
数学 | 646篇 |
物理学 | 1548篇 |
出版年
2016年 | 60篇 |
2015年 | 59篇 |
2014年 | 66篇 |
2013年 | 209篇 |
2012年 | 157篇 |
2011年 | 166篇 |
2010年 | 116篇 |
2009年 | 109篇 |
2008年 | 152篇 |
2007年 | 163篇 |
2006年 | 148篇 |
2005年 | 174篇 |
2004年 | 138篇 |
2003年 | 112篇 |
2002年 | 113篇 |
2001年 | 74篇 |
2000年 | 72篇 |
1999年 | 83篇 |
1998年 | 44篇 |
1997年 | 78篇 |
1996年 | 101篇 |
1995年 | 95篇 |
1994年 | 62篇 |
1993年 | 64篇 |
1992年 | 60篇 |
1991年 | 62篇 |
1990年 | 63篇 |
1989年 | 58篇 |
1988年 | 70篇 |
1987年 | 79篇 |
1986年 | 53篇 |
1985年 | 62篇 |
1984年 | 72篇 |
1983年 | 63篇 |
1982年 | 75篇 |
1981年 | 65篇 |
1980年 | 82篇 |
1979年 | 59篇 |
1978年 | 78篇 |
1977年 | 68篇 |
1976年 | 64篇 |
1975年 | 70篇 |
1974年 | 48篇 |
1973年 | 46篇 |
1972年 | 37篇 |
1968年 | 43篇 |
1967年 | 41篇 |
1955年 | 59篇 |
1934年 | 43篇 |
1928年 | 38篇 |
排序方式: 共有5695条查询结果,搜索用时 187 毫秒
141.
Oudhoff KA Macka M Haddad PR Schoenmakers PJ Kok WT 《Journal of chromatography. A》2005,1068(1):183-187
A capacitively coupled contactless conductivity detection (CCD) system has been applied for the detection of neutral synthetic polymers in capillary size-exclusion electrokinetic chromatography (SEEC). Polystyrene standards, that were used as a model compounds, were separated on a capillary column packed with porous 10 microm silica particles with an electrokinetically driven mobile phase, and detected by CCD and UV detection simultaneously. Mass-calibration curves for polystyrene were constructed. Satisfactory results were obtained for the linearity, the run-to-run repeatability (<0.2% for the relative retention and <4% for the peak area) and the robustness of the detector. One of the major issues in this preliminary study was to investigate the origin of the peaks observed for the polystyrene standards. The effect of the molar mass of the polystyrenes on the sensitivity was small. Therefore, the signals obtained could not be explained as the result of an increased viscosity and a decreased solution conductivity of the solute zone. An alternative hypothesis is suggested, and recommendations for further research are given. 相似文献
142.
Glycerol of different quality classifications served as a model for a neutral excess component in the isotachophoretic determination of low-molecular-mass anionic trace impurities. Potential anionic contaminants such as nitrate, sulphate, chlorate, nitrite, oxalate, fluoride formate and phosphate were analysed up to an analyte-to-excess ratio of 1:4·107, thus providing the possibility of checking the sample for the mentioned analytes in the order of 2.5·10−6–9.5·10−6%. Because we used a column-coupling isotachophoretic instrument the electrolyte system consisted of two different leading electrolytes, one for the pre-separation (10 mmol/l HCl, β-alanine, pH 3.2) in the first capillary and one for the final separation (5 mmol/l HCl, 1,3-bis[tris(hydroxymethyl)methylamino]propane, β-alanine, pH 3.6) in the second capillary. The terminating electrolyte was citric acid. Due to an increased injection volume of 300 μl, limits of detection (LODs) in the nanomolar range were realised by conductivity detection. The developed method allows simultaneous analysis without sample preparation and/or preconcentration within 25 min and is for that reason suitable for in-place process control. 相似文献
143.
Summary The use of non-segmented flow, post-column reaction detection is evaluated for use with miniaturized HPLC. Non-segmented open-tubular reactors with internal diameters 0.1 mm to 0.25 mm and packed-bed reactors with internal diameters of 1 mm, filled with 5 and 10 m particles are evaluated theoretically and experimentally with respect to band broadening and pressure drop characteristics. An integrated system consisting of miniaturized HPLC (1 mm i. d.) columns and compatible hardware is described. An example of the separation of catecholamines is given. 相似文献
144.
Kathrin Wichmann Bianca Antonioli Tilo Shnel Marco Wenzel Kerstin Gloe Karsten Gloe Jason R. Price Leonard F. Lindoy Alexander J. Blake Martin Schrder 《Coordination chemistry reviews》2006,250(23-24):2987
In the discussion that follows some of the more recent progress in the area of anion binding by synthetic polyamine receptors is presented, with emphasis given to work undertaken by the authors’ groups. A continuing theme in these studies has been the relationship between receptor structure and its anion extraction properties.Systematic solvent extraction and structural studies for halide and perrhenate complexes with polyamines of tripodal, macrocyclic and macrobicyclic architecture that contain both aromatic moieties and four to eight amine functions have been performed in order to derive relevant structure-binding/extractability relationships. The results demonstrate that the binding and extraction behaviour of the polyamines towards halides and perrhenate is a complex function of their structural features, degree of protonation and lipophilic properties. The extraction is characterized by the preferred formation of mono- and diprotonated amine species in the organic phase. X-ray structure studies of iodide and perrhenate complexes with open-chain tetraamino derivatives and octaamino cryptands in different protonation states lead to the conclusion that in the first case only limited chelation of the anion occurs and in the second only highly protonated species are able to encapsulate the anion. The structural patterns observed are strongly influenced by the presence of water molecules in the crystals. 相似文献
145.
M. Anastasiou Th. Hasapis T. Zorba E. Pavlidou K. Chrissafis K. M. Paraskevopoulos 《Journal of Thermal Analysis and Calorimetry》2006,84(1):27-32
Historic
plasters from wall paintings of Byzantine and post-Byzantine churches situated
in the Balkan region were studied. All wall paintings were made with fresco
technique and are dated from IX - XVI century. Plaster samples were followed
from room temperature to 1000°C by Thermogravimetric (TG) and Differential
Thermal Analysis (DTA), and one or two significant temperature regions, corresponding
to thermal decomposition mechanisms were observed. The analysis of the plaster
samples and the composition characterization was carried out using also, Fourier
Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscopy (SEM).
Although the main components are calcite and quartz (from sand) in different
proportions, there are differences between them such as the presence of gypsum
being either as a constituent element or due to environmental pollution. The
results are examined comparatively taking into account the creation time and
place of the paintings. 相似文献
146.
A method for flow-through liquid scintillation counting in reversed-phase liquid chromatography (RP-LC) based on the continuous extraction of aqueous column eluates with a water-immiscible liquid scintillator is evaluated in terms of sensitivity, reproducibility and extra-column band broadening. 3H- and 14C-labelled phenylthiohydantoinamino acid derivatives of widely different polarity serve as model compounds. For extractable derivatives, counting efficiencies of over 30 and 80% can be obtained for 3H and 14C, respectively. The reproducibility and extra-column band broadening depend on the mixing ratio of scintillator and LC eluent; relative standard deviations (peak areas) of less than 3% can be obtained. The sensitivity of flow-through counting can be increased at least 150-fold by storing the segmented scintillator/eluate stream in a capillary storage loop. After the separation is complete, the stream is re-introduced into the radioactivity detector at reduced flow-rates to increase the mean residence time, i.e., the counting time, in the detector. 相似文献
147.
Summary A technique is presented for the extraction of free and conjugated drugs and related compounds out of small quantities of urine. The method is designed in such a way that many samples can be tested for a great number of substances at the same time. Furthermore, we tried to achieve a high quality analysis using when ever possible chemicals with relatively low toxicity to men and environment. Therefore we avoided e.g., halogenated hydrocarbons. The detection of the substances is only performed by thin-layer chromatography (TLC) using well known reagents, like ninhydrin, Dragendorff, iodoplatinate and ferric chloride solutions. If necessary, the extracts can also be used with other Chromatographic techniques, particularly with GC/MS, if a substance has to be clearly identified. Approximately 500 authentical urines have been analyzed in a 1 year period using the method described. We found about 75 different substances. These results are presented in a table. 相似文献
148.
Zusammenfassung Durch Einwirkung von SOCl2 auf 4-Hydroxycumarin bzw. 4-Hydroxycarbostyril sind Bis-(4-hydroxycumarinyl-[3])-sulfid (I) und Bis-(4-hydroxycarbostyril-[3])-sulfid (II) leicht zugänglich. Diese Sulfide können zu Dicumarino- bzw. Dicarbostyrilo-(1,4)-oxathiinen (III bzw. IV) cyclisiert werden.
Herrn KollegenH. Bretschneider, Innsbruck, mit den besten Wünschen zum 60. Geburtstag.
Aus der DissertationTh. Kappe, Universität Graz 1961, S. 78–83. 相似文献
Bis-(4-hydroxycoumarinyl-[3])-sulfide (I) and bis-(4-hydroxycarbostyril-[3])-sulfide (II) have been prepared by the reaction between SOCl2 and 4-hydroxycoumarin and 4-hydroxycarbostyril. Cyclisation of these sulfides yielded dicoumarino- (III) and dicarbostyrilo-oxathiin-(1.4) (IV).
Herrn KollegenH. Bretschneider, Innsbruck, mit den besten Wünschen zum 60. Geburtstag.
Aus der DissertationTh. Kappe, Universität Graz 1961, S. 78–83. 相似文献
149.
Coulomb excitation of the first excited 2+-state was measured usingα-particles to determine reduced transition probabilities,B(E2; 0 gs + →2 1 + ), in nuclei with a closed shell of 82 neutrons. TheB(E2; 0 gs + →2 1 + -values are 10.2 single-particle units (spu) for138Ba, 13.5 spu for140Ce, 11.9 spu for142Nd and 11.6 spu for144Sm. The uncertainty ranges from 1.3 % to 2.3 %. 相似文献
150.
G. Dlubek J. Stejny Th. Lüpke D. Bamford K. Petters Ch. Hübner M. A. Alam M. J. Hill 《Journal of Polymer Science.Polymer Physics》2002,40(1):65-81
High‐quality positron lifetime measurements (70 million total counts) are reported for polyethylenes (PEs) of different crystallinities (Xc = 3–82%). The specific volumes of the crystalline and amorphous phases (Vc and Va, respectively) were estimated from density and wide‐angle X‐ray scattering (WAXS) experiments. Some samples (those with low values of Xc) were branched PEs, and those with high values of Xc were linear PEs for which Xc was varied with changes in the crystallization temperature. Both Vc and Va increase with decreasing Xc in the range 0% ≤ Xc ≤ 56% (the branched PEs) but are constant for Xc ≥ 56% (the linear PEs). The lifetime spectra were analyzed with the MELT and LIFSPECFIT routines. Artifacts that can appear in the spectrum analysis were checked via an analysis of computer‐generated spectra. Four lifetime components appeared in all of the PEs; the two long‐lived ones are attributed to pick‐off annihilation of ortho‐positronium (o‐Ps) in crystalline regions (τ3) and in holes of the amorphous phase (τ4). With increasing Xc, τ3 decreases from about 1.2 to 1 ns, τ4 decreases from 3.0 to 2.5 ns, and the intensity I4 decreases from 29 to 0%. An increase in I3 from 6 to 12% was observed. A comparison with simulations shows that the true I3 value approaches 0 for Xc → 0%. The decrease in I4 is weaker than the increase in Xc; this leads to the conclusion that the apparent specific o‐Ps yield in the amorphous phase I4Xc increases with Xc. Possible reasons for this surprising results are discussed. The fractional free hole volume [h = (Va ? Vocc)/Va, where Vocc is the crystalline occupied volume] was estimated from density and WAXS results. Between Xc = 0 and 56%, h decreases from 0.151 to 0.090, but it does not change further above Xc = 56%. The mean size (v) of the local free volumes (holes) estimated from τ4 decreases from 200 to 150 Å3. The number density of holes (Nh) calculated from these values (Nh = h/v) also decreases from 0.8 to 0.6 nm?3 in the range 0% ≤ Xc ≤ 56%. The values of Va, Vc, h, and Nh increase with an increasing degree of branching but do not vary for linear PEs. The possible influence of a crystalline–amorphous interfacial phase (three‐phase model) on the observed lifetime parameters is also discussed. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 40: 65–81, 2002 相似文献