全文获取类型
收费全文 | 13453篇 |
免费 | 645篇 |
国内免费 | 403篇 |
专业分类
化学 | 9428篇 |
晶体学 | 145篇 |
力学 | 648篇 |
综合类 | 4篇 |
数学 | 1910篇 |
物理学 | 2366篇 |
出版年
2024年 | 24篇 |
2023年 | 106篇 |
2022年 | 502篇 |
2021年 | 498篇 |
2020年 | 446篇 |
2019年 | 519篇 |
2018年 | 607篇 |
2017年 | 508篇 |
2016年 | 749篇 |
2015年 | 526篇 |
2014年 | 721篇 |
2013年 | 1440篇 |
2012年 | 1040篇 |
2011年 | 1039篇 |
2010年 | 755篇 |
2009年 | 647篇 |
2008年 | 689篇 |
2007年 | 628篇 |
2006年 | 400篇 |
2005年 | 409篇 |
2004年 | 344篇 |
2003年 | 321篇 |
2002年 | 259篇 |
2001年 | 127篇 |
2000年 | 82篇 |
1999年 | 93篇 |
1998年 | 76篇 |
1997年 | 79篇 |
1996年 | 68篇 |
1995年 | 43篇 |
1994年 | 45篇 |
1993年 | 29篇 |
1992年 | 45篇 |
1991年 | 39篇 |
1990年 | 33篇 |
1989年 | 45篇 |
1988年 | 33篇 |
1987年 | 38篇 |
1986年 | 35篇 |
1985年 | 38篇 |
1984年 | 40篇 |
1983年 | 25篇 |
1982年 | 31篇 |
1981年 | 20篇 |
1980年 | 30篇 |
1979年 | 27篇 |
1978年 | 24篇 |
1974年 | 20篇 |
1973年 | 15篇 |
1972年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
István E. MarkóJean-Christophe Vanherck Ali AtesBernard Tinant Jean-Paul Declercq 《Tetrahedron letters》2003,44(16):3333-3336
Upon treatment with tBuOK/H2O, a variety of ω-halo-β-keto-ketals undergo smooth cyclisation, affording in excellent yields mono-protected [n,m] spiro bicyclic diketones. This transformation is highly stereoselective producing, in all cases, the diastereoisomerically pure spiro derivatives. 相似文献
62.
63.
This study compared the fracture strength of Class II slot cavities restored with polymerizable restorative materials. Sixty, caries-free, posterior teeth were divided into five groups of 12 teeth. The Class II slot cavities were prepared. The teeth were restored with two packable composites (Filtek P60, Surefil), a microhybrid composite (Filtek Z250)and two ormocer (Definite, Admira). The restorations were then subjected to fracture resistance tests. The marginal ridges of the restorations were loaded at an angle of 13.5° to the long axis of the tooth in an Universal Testing Machine until failure. Analysis of mean forces indicated that, Filtek P60, Surefil and Filtek Z250 exhibited better performance than Definite and Admira. The tested resin composites differed in their mechanical properties. This study suggested that fracture behavior were highly influenced by the filler system. Overall, Filtek P60, Surefil, Filtek Z250, demonstrated good fracture resistance. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
64.
Ali Reza Ashrafi Masood Hamadanian 《Journal of Applied Mathematics and Computing》2004,14(1-2):289-303
The non-rigid molecule group theory (NRG) in which the dynamical symmetry operations are defined as physical operations is a new field of chemistry. Smeyers in a series of papers applied this notion to determine the character table of restricted NRG of some molecules. In this work, a simple method is described, by means of which it is possible to calculate character tables for the symmetry group of molecules consisting of a number of NH3 groups attached to a rigid framework. We study the full non-rigid group (f-NRG) of tetraammineplatinum (II) with two separate symmetry groups C2v and C4v. We prove that they are groups of order 216 and 5184 with 27 and 45 conjugacy classes, respectively. Also, we will compute the character tables of these groups. 相似文献
65.
DEBACHE Abdelmadjid BOULCINA Raouf TAFER Radia BELFAITAH Ali RHOUATI Salah CARBONI Bertrand 《中国化学》2008,26(11):2112-2116
The synthesis of 1,2,3,4-dihydropyrimidinone/thione derivatives was achieved in good to excellent yields using calcium(II) nitrate as catalyst to promote the Biginelli three-component condensation reaction from a diversity of aromatic aldehydes, β-keto compounds and urea or thiourea. The reaction was carried out under solvent-free conditions. 相似文献
66.
Amir Abbas Matin Ramin Maleki Mir Ali Farajzadeh Khalil Farhadi Reza Hosseinzadeh Abolghasem Jouyban 《Chromatographia》2007,66(5-6):383-387
A new method for extraction and analysis of acetone in human urine based on headspace solid phase microextraction using a
mixture of activated carbon and zeolite as sorbents in a PVC matrix coated on a silver wire and its application to the determination
of ketone bodies is described. Unlike commercial fibers, which are coated on fused silica, the coating adheres strongly to
the silver wire and is thermally stable up to 250 °C. After optimization of coating composition and microextraction conditions
the fiber was used for the analysis of acetone in human urine. 相似文献
67.
Nickel was quantitatively retained by 2-nitroso-1-naphthol-4-sulfonic acid (Nitroso-S) and tetradecyldimethylbenzylammonium chloride (TDBA Cl) onto benzophenone in the pH range 5.0-6.0 from large volumes of aqueous solutions of various samples. After filtration, each solid mass consisting of a nickel complex and benzophenone was dissolved with 5 ml of dimethylformamide (DMF) and the metal was determined by atomic absorption spectrometry (AAS). About 0.6 microg of nickel could be concentrated from 200 ml of an aqueous sample, where its concentration was as low as 3.0 ng/ml. Eight replicate determinations of 2.5 microg/ml of nickel in the final DMF solution gave a mean absorbance of 0.112 with a relative standard deviation of 1.9%. The sensitivity for 1% absorption was 98 ng/ml. The interference of a number of anions and cations was studied and the developed optimized conditions were utilized for the trace determination of nickel in various alloys and biological samples. 相似文献
68.
Ismail Boz Gulin Selda Pozan Mehmet Ali Gürkaynak 《Reaction Kinetics and Catalysis Letters》2004,83(1):137-146
Bimetallic Cu/Mg, Ca, Ba mixed oxide compounds acted as catalyst in the dehydrogenation of tetradecanol into tetradecanal,
and were characterized by thermal gravimetric analysis, surface area, X-ray diffraction, temperature-programmed reduction
and desorption of CO2. Alkaline earth oxides increased the total basic site concentration in the order of Mg>Ca>Ba. A correlation was found between
the total basic site concentration and initial dehydrogenation rate.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
69.
I.V. Kityk A. Ali Umar M. Oyama 《Physica E: Low-dimensional Systems and Nanostructures》2005,27(4):420-426
We have found that the gold nanoparticles on the ITO substrates might be considered promising materials for circularly polarized light-induced linear electrogyration (EG). The maximal achieved value of the EG susceptibility described by third-order axial tensor caused by probe circularly polarized light at a wavelength of 1060 nm was equal to about 13 deg/mm at pulsed electric field strength 30.0 V/cm with a duration of about 1 ms. We have revealed that the maximal EG coefficient is achieved for the samples possessing maximal resistivity. The investigated composites possess long-lived EG grating which decreases by not more than 12% after 120 min of laser treatment. Applying a non-circular pump light leads to the diminishing of the observed EG. 相似文献
70.
Polyfunctional quinolines were synthesized using Friedlander method catalyzed by molecular iodine in high yields at 60 °C under solvent‐free conditions. 相似文献