全文获取类型
收费全文 | 49862篇 |
免费 | 4783篇 |
国内免费 | 10420篇 |
专业分类
化学 | 45762篇 |
晶体学 | 1867篇 |
力学 | 634篇 |
综合类 | 418篇 |
数学 | 3315篇 |
物理学 | 13069篇 |
出版年
2024年 | 161篇 |
2023年 | 945篇 |
2022年 | 1774篇 |
2021年 | 1770篇 |
2020年 | 1809篇 |
2019年 | 1635篇 |
2018年 | 1343篇 |
2017年 | 1715篇 |
2016年 | 1807篇 |
2015年 | 1571篇 |
2014年 | 2091篇 |
2013年 | 4582篇 |
2012年 | 3168篇 |
2011年 | 3409篇 |
2010年 | 2933篇 |
2009年 | 3531篇 |
2008年 | 3353篇 |
2007年 | 3431篇 |
2006年 | 3253篇 |
2005年 | 2836篇 |
2004年 | 2712篇 |
2003年 | 2257篇 |
2002年 | 1871篇 |
2001年 | 1530篇 |
2000年 | 1424篇 |
1999年 | 1151篇 |
1998年 | 980篇 |
1997年 | 844篇 |
1996年 | 740篇 |
1995年 | 747篇 |
1994年 | 662篇 |
1993年 | 529篇 |
1992年 | 500篇 |
1991年 | 380篇 |
1990年 | 257篇 |
1989年 | 224篇 |
1988年 | 191篇 |
1987年 | 127篇 |
1986年 | 98篇 |
1985年 | 113篇 |
1984年 | 90篇 |
1983年 | 36篇 |
1982年 | 65篇 |
1981年 | 91篇 |
1980年 | 64篇 |
1979年 | 67篇 |
1978年 | 44篇 |
1977年 | 50篇 |
1976年 | 30篇 |
1973年 | 31篇 |
排序方式: 共有10000条查询结果,搜索用时 12 毫秒
71.
《天然气化学杂志》1995,(2)
The catalytic performance of Mn/TiO_2,La-Mn/TiO_2,Li-La-Mn/TiO_2 etc for the oxidetive coupling of methane(OCM)was investignted.Thecatalysta were cheracterised with X-ray diffraction(XRD)and X-rayphotoelectron spectroscopy(XPS).The results reveal that catalyst Li-La-Mn/TiO_2 exhibits high activity and C_2 selectivity;Ti in this catalyst exists asTi~( 4) state;Li can promote the formation of lanthanum tituate via theinteraction between La and TiO_2;the formed La_2Ti_2O_7 and La_4Ti_9O_(24) aredistributed in the inner surface layer and Mn exists in outer surface layer in lowvalence states.The high activity and C_2 selectivity of catalyst Li-La-Mn/TiO_2are intimately related to the valence states of Mn,Li,La and theirdistribution on the catulyst surface layer. 相似文献
72.
73.
Dihydrothiinone 9a undergoes photocycloaddition regioselectively to all three C?C bonds of penta‐1,2,4‐triene ( 10 ), the relative stabilities of the biradical intermediates determining the product distribution. In contrast, cyclohexenone 9b and dihydropyranone 9c afford more complex mixtures of bicyclo[4.2.0]octanones, which also turn out to be less stable on chromatographic workup, reflecting the higher strain due to the shorter bond lengths (C? O and C? C vs. C? S) in the six‐membered rings, respectively. 相似文献
74.
Excess and excess partial molar volumes of mixing of the system N,N-dibutyl-2-ethylhexylamide (DBEHA) + dodecanol (DDA) were determined at 25, 35, 45, 55 and 65°C. The excess molar volumes exhibit a maxima at each temperature at approximatelyx
DBEHA = 0.3 and a minima close tox
DBEHA = 0.9. The values of the excess volumes in general increase with the temperature. The partial excess molar volumes are calculated from the smoothed data and the results are discussed in the light of postulated amide-alcohol interactions. 相似文献
75.
JIANG Qi-Jun 《结构化学》2005,24(10):1140-1144
1 INTRODUCTION The electron-transfer reaction of C60 derivatives, especially light-induced electron-transfer reaction, has been an active research field for a long time. Many researches are focused on the long-live charge- separated state caused by electron-transfer[1]. And several electron-transfers between electron donor and C60 or intramolecular electron-transfer have been confirmed. As an example, TTF-C60 is proved to be a short-live charge-separated compound[2].Due to the excelle… 相似文献
76.
Sharpless asymmetric dihydroxylation and aminohydroxylation of (E)-dimethyl-2-alkylidene glutarates 2 were shown to afford enantio-enriched or enantiopure highly functionalized γ-butyrolactones 3 and 7. 相似文献
77.
Julio A. Seijas 《Tetrahedron letters》2005,46(35):5827-5830
New direct strategy for the synthesis of 4-aryl-1,2,3,4-tetrahydroisoquinolines. The key steps are based on oxazoline chemistry: nucleophilic substitution in an ortho-methoxyphenyloxazoline with a Grignard reagent and a 1,6-conjugate addition of a lithium amide to o-styrylphenyloxazoline. 相似文献
78.
István Szatmári 《Tetrahedron letters》2006,47(23):3881-3883
As a new extension of the Mannich reaction of naphthols, 1-(hydroxynaphthyl)-substituted 1,2,3,4-tetrahydroisoquinolines were synthesized by the nucleophilic addition of 1- or 2-naphthol to 3,4-dihydroisoquinolines under solvent-free conditions, using microwave irradiation. The additions to 3-methyl-6,7-dimethoxy-3,4-dihydroisoquinoline proved to be a highly diastereoselective processes, resulting in the cis isomers as the main or the only products. 相似文献
79.
E. Kavlentis 《Mikrochimica acta》1986,88(3-4):239-243
Mixtures of sulphide-thiocyanate and cyanide-thiocyanate can be analyzed without separation using the Cu(II)-DPGH complex.Sulfide and cyanide cause a decrease in absorption that is a linear function of their concentration in the range 0.06–2 and 0.18–6 ppm, respectively. The Cu(II)-DPGH complex is extractable into chloroform in presence of thiocyanate and this is used for the Spectrophotometric determination of thiocyanate in the range 0.09–3 ppm. 相似文献
80.
The rapid determination of protein in plant material based on spectrophotometric determination of amino acids in protein hydrolysates with trinitrobenzenesulfonic acid has been adapted to flow injection analysis. With the manifold described, a routine sampling rate of 120 samples/h is possible, though this, as well as sensitivity, can easily be varied. The method was calibrated against the Kjeldahl method and a good correlation was obtained between the two methods over a wide range of protein values for beans. 相似文献