全文获取类型
收费全文 | 6290篇 |
免费 | 1133篇 |
国内免费 | 1093篇 |
专业分类
化学 | 5345篇 |
晶体学 | 60篇 |
力学 | 275篇 |
综合类 | 81篇 |
数学 | 770篇 |
物理学 | 1985篇 |
出版年
2024年 | 22篇 |
2023年 | 107篇 |
2022年 | 137篇 |
2021年 | 156篇 |
2020年 | 234篇 |
2019年 | 273篇 |
2018年 | 233篇 |
2017年 | 175篇 |
2016年 | 289篇 |
2015年 | 334篇 |
2014年 | 362篇 |
2013年 | 486篇 |
2012年 | 532篇 |
2011年 | 506篇 |
2010年 | 421篇 |
2009年 | 419篇 |
2008年 | 435篇 |
2007年 | 399篇 |
2006年 | 376篇 |
2005年 | 336篇 |
2004年 | 320篇 |
2003年 | 300篇 |
2002年 | 355篇 |
2001年 | 228篇 |
2000年 | 192篇 |
1999年 | 192篇 |
1998年 | 124篇 |
1997年 | 103篇 |
1996年 | 72篇 |
1995年 | 59篇 |
1994年 | 59篇 |
1993年 | 33篇 |
1992年 | 37篇 |
1991年 | 43篇 |
1990年 | 30篇 |
1989年 | 22篇 |
1988年 | 17篇 |
1987年 | 11篇 |
1986年 | 18篇 |
1985年 | 20篇 |
1984年 | 9篇 |
1983年 | 8篇 |
1982年 | 7篇 |
1981年 | 4篇 |
1979年 | 4篇 |
1977年 | 2篇 |
1975年 | 2篇 |
1974年 | 2篇 |
1968年 | 3篇 |
1936年 | 2篇 |
排序方式: 共有8516条查询结果,搜索用时 15 毫秒
91.
Xiao Ling WANG Wei JIAO Xun LIAO Shu Ling PENG Li Sheng DING 《中国化学快报》2006,17(7):916-918
The roots of Paeonia lactiflora Pall. is one of the most important traditional Chinese medicines, which has been used for the treatment of dizziness, spasm, menstrual disorder, spontaneous and night sweating, and as a general analgesic1. Chemical constitu… 相似文献
92.
In continuation of our studies on Aconitum hemsleyanum Pritz.var. pengzhouense, two new norditerpenoid alkaloids, pengshenines A(1) and B(2),have isolated from the roots of the plants and their strucures were elucidated by 1D-and 2D-NMR. 相似文献
93.
本文报道了K2PtCl6与兔肝Zn7MT-Ⅱ和apoMT-Ⅱ的反应包含一个氧化还原反应和一个取代反应。通过紫外可见光谱、园二向色谱、柱层析和X-光电子能谱研究了该反应的性质、铂在反应产物中的键合位置和氧化态。金属硫蛋白(MT)被氧化成单体、双聚和多聚产物,其中含有分子间和分子内CyS-SCy二硫键。Pt(Ⅳ)被还原成Pt(Ⅱ)然后键合于产物中。随着K2PtCl6与MT的反应摩尔比和反应时间的增加,键合于产物中的Pt(Ⅱ)的计量数增加而蛋白中所含Zn(Ⅱ)的量减少。当Zn7MT与4和超过10摩尔的K2PtCl6反应时,分别得到了Pt4Zn4MT和Pt8MT。当apoMT与7及超过25倍的K2PtCl6在pH 2条件下反应时,分别得到了Pt7MT和Pt15MT。动力学数据表明K2PtCl6与apoMT的反应比与Zn7MT的反应快。 相似文献
94.
[reaction: see text] With use of 1 mol % of Cr catalyst 5, surprisingly efficient Cr-mediated couplings of aldehydes with various types of nucleophiles have been realized. The catalyst set of Cr catalyst 5 and Ni catalyst 4 is used for alkenylation, alkynylation, and arylation, whereas the catalyst set of Cr catalyst 5 and CoPc (cobalt phthalocyanine) is used for 2-haloallylation, alkylation, and propargylation. Only the Cr catalyst 5 is required for allylation. The reaction rates in DME and THF have been found significantly faster than that in MeCN. 相似文献
95.
96.
[reaction: see text] A microwave-assisted reaction was developed to facilitate the construction of 4,5-disubstituted pyrazolopyrimidines. This one-pot two-step process involves a sequential S(N)Ar displacement of the C4 chloro substituent with various anilines and amines, followed by a Suzuki coupling reaction with different boronic acids. Using microwave irradiation leads to high product conversion, low side product formation, and shorter reactions. 相似文献
97.
Ya‐Ching Shen Yun‐Sheng Lin Shaw‐Man Hsu Ashraf Taha Khalil Shih‐Sheng Wang Ching‐Te Chien Yao‐Haur Kuo Chang‐Hung Chou 《Helvetica chimica acta》2007,90(7):1319-1329
The phytochemical investigation of the more polar fractions from the leaves and twigs of Taxus sumatrana (Taxaceae) afforded five new taxane diterpene esters, tasumatrols P–T ( 1 – 5 ) possessing an 11(15→1),11(10→9)‐diabeotaxane skeleton. Compounds 1, 4 , and 5 contain an α‐hydroxy group at C(14), while 3 has no OH group at either C(13) or C(14). Compound 2 is a natural 4,5‐acetonide derivative, while 4 has an unusual spiro‐connected 2‐hydroxy‐2‐phenyl‐1,3‐dioxolane ring. Ten known taxoids, were also isolated in the course of the chromatographic fractionation. Five additional new O‐acetyl derivatives 3a, 4a, 4b, 5a , and 5b were prepared from the taxanes 3 – 5 . The structures of all new compounds were established on the basis of their spectroscopic analyses. Compound 1 showed mild cytotoxic activity against human Hela and Daoy tumor cells. 相似文献
98.
A numerical investigation for an axisymmetric hypersonic turbulent inlet flow field of a perfect gas is presented for a three-shock configuration consisting of a biconic and a cowl. An upwind parabolized Navier-Stokes solver based on Roe's scheme is used to compute an oncoming flow Mach numberM
=8, temperatureT
=216 K, and pressureP
=5.5293×103 N/m2. In order to assess the flow quantities, the interaction between shock and turbulence, and the inlet efficiency, three different flow calculations — laminar, turbulent with incompressible and compressible two-equationk- turbulence models — have been performed in this work.Computational results show that turbulence is markedly enhanced across an oblique shock with step-like increases in turbulence kinetic energy and dissipation rate. This enhancement is at the expense of the mean kinetic energy of the flow. Therefore, the velocity behind the shock is smaller in turbulent flow and hence the shock becomes stronger. The entropy increase through a shock is caused not only by the amplification of random molecular motion, but also by the enhancement of the chaotic turbulent flow motion. However, only the compressiblek- turbulence model can properly predict a decrease in turbulence length scale across a shock. Our numerical simulation reveals that the incompressiblek- turbulence model exaggerates the interaction between shock and turbulence with turbulence kinetic energy and dissipation rate remaining high and almost undissipated far beyond the shock region. It is shown that proper modeling of turbulence is essential for a realistic prediction of hypersonic inlet flowfield. The performed study shows that the viscous effect is not restricted in the boundary layer but extends into the main flow behind a shock wave. The loss of the available energy in the inlet performance therefore needs to be determined from the shock-turbulence interaction. The present study predicts that the inlet efficiency becomes relatively lower when turbulence is taken into account. 相似文献
99.
100.