全文获取类型
收费全文 | 22664篇 |
免费 | 2591篇 |
国内免费 | 2901篇 |
专业分类
化学 | 16894篇 |
晶体学 | 334篇 |
力学 | 930篇 |
综合类 | 264篇 |
数学 | 3174篇 |
物理学 | 6560篇 |
出版年
2023年 | 203篇 |
2022年 | 465篇 |
2021年 | 587篇 |
2020年 | 613篇 |
2019年 | 706篇 |
2018年 | 608篇 |
2017年 | 558篇 |
2016年 | 910篇 |
2015年 | 915篇 |
2014年 | 1133篇 |
2013年 | 1641篇 |
2012年 | 1779篇 |
2011年 | 1866篇 |
2010年 | 1349篇 |
2009年 | 1332篇 |
2008年 | 1639篇 |
2007年 | 1487篇 |
2006年 | 1328篇 |
2005年 | 1108篇 |
2004年 | 1050篇 |
2003年 | 926篇 |
2002年 | 1016篇 |
2001年 | 820篇 |
2000年 | 625篇 |
1999年 | 480篇 |
1998年 | 307篇 |
1997年 | 323篇 |
1996年 | 287篇 |
1995年 | 250篇 |
1994年 | 218篇 |
1993年 | 216篇 |
1992年 | 195篇 |
1991年 | 149篇 |
1990年 | 138篇 |
1989年 | 111篇 |
1988年 | 80篇 |
1987年 | 88篇 |
1986年 | 81篇 |
1985年 | 74篇 |
1984年 | 56篇 |
1983年 | 43篇 |
1982年 | 47篇 |
1981年 | 48篇 |
1980年 | 37篇 |
1979年 | 42篇 |
1978年 | 40篇 |
1977年 | 29篇 |
1976年 | 33篇 |
1975年 | 26篇 |
1973年 | 19篇 |
排序方式: 共有10000条查询结果,搜索用时 156 毫秒
961.
Ly Dieu Ha Poul Erik Hansen Fritz Duus Hung Dinh Pham Lien‐Hoa D. Nguyen 《Magnetic resonance in chemistry : MRC》2012,50(3):242-245
Four new prenylated depsidones, oliveridepsidones A–D, were isolated from the bark of Garcinia oliveri collected in Vietnam. Their structures were elucidated using mainly NMR techniques (1H and 13C NMR, HMQC, HMBC and NOE experiments). Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
962.
This paper reports the fabrication and nanofiltration properties of a sol–gel derived microporous zirconia membrane. Effects
of synthesis parameters, including hydrolysis time, hydrolysis temperature, hydrolysis ratio and chelating agent dopant, on
the state and size of polymeric zirconia sol, were investigated. Highly reproducible and stable zirconia sol after refluxing
at 40 °C for 180 min, with an average particle size of approximately 8.6 nm, was synthesized with a recipe of [Zirconium n-propoxide]:
[Diethanolamine]: [1-propanol]: [H2O] being 1: 2.2: 28.7: 9.4 (in molar ratio). The sol was subsequently used for the fabrication of microporous ZrO2 membranes onto alumina supported mesoporous γ-Al2O3 layers. ZrO2 membranes with molecular weight cut-off (MWCO) of 354, 1,195, corresponding to the pore size of 0.94 and 1.75 nm, were successfully
fabricated. Ionic retention properties of such ZrO2 membranes with respect to electrolyte solutions, like MgCl2, CaCl2, NaCl and Na2SO4, were also determined. Effects of parameters such as concentration of salt solutions and trans-membrane pressure on retention
rates of microporous ZrO2 membranes were studied in detail. Results showed that zirconium n-propoxide derived microporous ZrO2 membranes exhibited comparatively high retention rates towards divalent ions like Mg2+ and Ca2+, while much lower retention rates were observed for mono-valent ion (Na+) in the present study, which are the characteristics of nanofiltration membranes. 相似文献
963.
964.
We suggest an electrochemical etching method with viscous etchant to enhance the sharpness of tip of scanning probe microscope. The viscosity of the etchant mixed with HCl solution and glycerol was used as a control parameter in addition to the voltage applied to the tip. In order to improve the sharpness of the tip, a nano-scale meniscus formed between the end of the tip and the liquid level was used. The shapes, aspect ratios, and radii of tips were measured depending on the concentration of the etchant. It was found that the tip etched with the mixed liquid with glycerol was sharper than the tip with the pure HCl solution. This can be explained by the fact that the meniscus formed by viscous liquid is maintained with a thinner diameter and causes final etching until the meniscus bridge is ruptured. 相似文献
965.
Julien Iehl Thi Le Anh Nguyen Stéphane Frein Uwe Hahn Joaquín Barberá Jean-François Nierengarten 《Liquid crystals》2017,44(12-13):1852-1860
ABSTRACTLiquid-crystalline [60]fullerodendrimers were constructed via click chemistry based on the reaction between hexa-adducts of [60]fullerene (C60) bearing 12 azide groups and alkyne-terminated cyanobiphenyl dendrons of first- and second-generation. The structure of all the new compounds was confirmed by IR, UV, 1H and 13C NMR spectroscopies and mass spectrometry. The mesomorphic properties were studied by polarised optical microscopy, differential scanning calorimetry and small-angle X-ray scattering. The hexa-adduct of C60 functionalised with the first-generation dendrons gave rise to the formation of a smectic A phase and a rectangular columnar phase (c2mm symmetry) while the hexa-adduct of C60 decorated with the second-generation dendrons displayed only a rectangular columnar phase (c2mm symmetry). Our results show that the hexa-adduct of C60 is a unique synthetic platform for the design of fullerodendrimers and dendronised materials. 相似文献
966.
Jian Wang Yu Xu Yunfeng Zhang Hong Wang Weihui Zhong 《Rapid communications in mass spectrometry : RCM》2017,31(18):1541-1550
967.
Stuart Lipsitz Garrett Fitzmaurice Debajyoti Sinha Nathanael Hevelone Jim Hu Louis L. Nguyen 《Journal of computational and graphical statistics》2017,26(3):734-737
Medical studies increasingly involve a large sample of independent clusters, where the cluster sizes are also large. Our motivating example from the 2010 Nationwide Inpatient Sample (NIS) has 8,001,068 patients and 1049 clusters, with average cluster size of 7627. Consistent parameter estimates can be obtained naively assuming independence, which are inefficient when the intra-cluster correlation (ICC) is high. Efficient generalized estimating equations (GEE) incorporate the ICC and sum all pairs of observations within a cluster when estimating the ICC. For the 2010 NIS, there are 92.6 billion pairs of observations, making summation of pairs computationally prohibitive. We propose a one-step GEE estimator that (1) matches the asymptotic efficiency of the fully iterated GEE; (2) uses a simpler formula to estimate the ICC that avoids summing over all pairs; and (3) completely avoids matrix multiplications and inversions. These three features make the proposed estimator much less computationally intensive, especially with large cluster sizes. A unique contribution of this article is that it expresses the GEE estimating equations incorporating the ICC as a simple sum of vectors and scalars. 相似文献
968.
Hong Sun Frederick Roussel Wing Shun Cheung B. M. Fung Sandy S. Keast Mark Lohman 《Liquid crystals》2013,40(10):1483-1486
Liquid crystals (LCs) with diazo linkages have high dielectric and optical anisotropy. Two newly synthesized liquid crystalline compounds were dissolved in room temperature LC hosts, E7 and PTTP-24/36, to assess their properties. It was found that these mixtures have higher birefringence, larger dielectric anisotropy, and better elastic properties than the hosts. Satisfactory viscoelastic coefficients were also obtained for these mixtures, showing that they are promising LC materials for applications in the near IR region. 相似文献
969.
Xiao‐Lan Wei Rong Han Xiao Hu Li‐Hui Quan Chun‐Yu Liu Qi Chang Yong‐Hong Liao 《Biomedical chromatography : BMC》2013,27(5):636-640
Six esterase inhibitors, namely EDTA·2Na+, NaF, phenylmethanesulfonyl fluoride, dichlorvos, bis‐nitrophenyl phosphate (BNPP) and thenoyltrifluoroacetone, and the mixture of NaF and BNPP, were evaluated for the stabilization of labile benzoate containing zeylenone in rat plasma. The mixture appeared to exhibit the most effectively stabilizing effect with the degraded content of zeylenone decreasing from >60% (in the absence of inhibitors) to <6%. Following the stabilization by the addition of NaF (5 mm ) and BNPP (5 mm ), the analytes in rat plasma were acidified by formic acid and extracted into ethyl acetate at 0°C. After chromatographic separation, the detection of zeylenone was performed on a 3200 Q‐Trap with positive ion electrospray mode, monitoring the ion transition m/z 383.2 → 105.0. The method was validated over the range from 2.68 to 1340 ng/mL with inter‐ and intra‐run precision for the quality control samples being less than 6.8%. The assay accuracy was within 100 ± 7.0%. The validated method was successfully applied to a pharmacokinetic study in rats after the intratracheal administration of zeylenone in free drug or polymeric micellar solutions. The results showed that the pulmonary absorption of zeylenone loaded in micelles was significantly retarded compared with that of free drug solutions. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献