全文获取类型
收费全文 | 5117篇 |
免费 | 247篇 |
国内免费 | 28篇 |
专业分类
化学 | 3941篇 |
晶体学 | 46篇 |
力学 | 71篇 |
数学 | 400篇 |
物理学 | 934篇 |
出版年
2023年 | 32篇 |
2022年 | 86篇 |
2021年 | 109篇 |
2020年 | 89篇 |
2019年 | 95篇 |
2018年 | 74篇 |
2017年 | 71篇 |
2016年 | 177篇 |
2015年 | 144篇 |
2014年 | 179篇 |
2013年 | 314篇 |
2012年 | 360篇 |
2011年 | 442篇 |
2010年 | 256篇 |
2009年 | 249篇 |
2008年 | 354篇 |
2007年 | 289篇 |
2006年 | 295篇 |
2005年 | 227篇 |
2004年 | 207篇 |
2003年 | 179篇 |
2002年 | 213篇 |
2001年 | 102篇 |
2000年 | 106篇 |
1999年 | 61篇 |
1998年 | 39篇 |
1997年 | 51篇 |
1996年 | 46篇 |
1995年 | 35篇 |
1994年 | 27篇 |
1993年 | 32篇 |
1992年 | 39篇 |
1991年 | 23篇 |
1990年 | 31篇 |
1989年 | 31篇 |
1988年 | 15篇 |
1987年 | 16篇 |
1986年 | 14篇 |
1985年 | 27篇 |
1984年 | 23篇 |
1983年 | 16篇 |
1982年 | 18篇 |
1981年 | 11篇 |
1980年 | 13篇 |
1979年 | 10篇 |
1978年 | 15篇 |
1977年 | 19篇 |
1976年 | 12篇 |
1975年 | 11篇 |
1970年 | 17篇 |
排序方式: 共有5392条查询结果,搜索用时 15 毫秒
811.
812.
813.
Guangri Yu Prof. Dr. Byunghyuck Jung Prof. Dr. Hee‐Seung Lee Prof. Dr. Sung Ho Kang 《Angewandte Chemie (International ed. in English)》2016,55(7):2573-2576
The first total synthesis of inostamycin A is described. With efficient and stereoselective synthetic routes to aldehyde 3 and ketone 4 developed through asymmetric aldol reactions, addition reactions and reduction, and with chiral building blocks, the two large fragments were coupled with remarkable anti stereoselectivity and efficiency by aldol condensation. The coupling reaction provided the complete carbon skeleton with all the requisite functional groups and stereogenic centers for inostamycin A. The two quaternary carbons at C20 and C16 of ketone 4 were elaborated in a highly stereocontrolled manner by addition reactions of the transmetallated 5 to ethyl ketone 6 and the transmetallated 7 to methyl ketone 8 , respectively, in which the use of LaCl3 for transmetallation was critical for high coupling efficiency. 相似文献
814.
M. Sc. Markus Warnke Tobias Jung Dr. Juri Dermer Karin Hipp Dr. Nico Jehmlich Prof. Dr. Martin von Bergen Sascha Ferlaino Dr. Alexander Fries Prof. Dr. Michael Müller Prof. Dr. Matthias Boll 《Angewandte Chemie (International ed. in English)》2016,55(5):1881-1884
The hydroxylation of vitamin D3 (VD3, cholecalciferol) side chains to give 25‐hydroxyvitamin D3 (25OHVD3) is a crucial reaction in the formation of the circulating and biologically active forms of VD3. It is usually catalyzed by cytochrome P450 monooxygenases that depend on complex electron donor systems. Cell‐free extracts and a purified Mo enzyme from a bacterium anaerobically grown with cholesterol were employed for the regioselective, ferricyanide‐dependent hydroxylation of VD3 and proVD3 (7‐dehydrocholesterol) into the corresponding tertiary alcohols with greater than 99 % yield. Hydroxylation of VD3 strictly depends on a cyclodextrin‐assisted isomerization of VD3 into preVD3, the actual enzymatic substrate. This facile and robust method developed for 25OHVD3 synthesis is a novel example for the concept of substrate‐engineered catalysis and offers an attractive alternative to chemical or O2 /electron‐donor‐dependent enzymatic procedures. 相似文献
815.
816.
An operator \({T\in{\mathcal{L}}({\mathcal{H}})}\) is said to be complex symmetric if there exists a conjugation C on \({{\mathcal H}}\) such that \({T= CT^{\ast}C}\). In this paper, we study the spectral radius algebras for complex symmetric operators. In particular, we prove that if A is a complex symmetric operator, then the spectral radius algebra \({{\mathcal B}_{A}}\) associated with A has a nontrivial invariant subspace under some conditions. Finally, we give some relations between \({P_{\tilde{A}}}\) and \({P_{\widetilde{A^{\ast}}}}\) (defined below) when A is complex symmetric. 相似文献
817.
Choonkil Park Dong Yun Shin Jung Rye Lee 《Journal of Fixed Point Theory and Applications》2016,18(3):569-586
In this paper, we solve the additive \({\rho}\)-functional equations where \({\rho}\) is a fixed non-Archimedean number or a fixed real or complex number with \({\rho \neq 1}\). Using the fixed point method, we prove the Hyers–Ulam stability of the above additive \({\rho}\)-functional equations in non-Archimedean Banach spaces and in Banach spaces.
相似文献
$$\begin{aligned} f(x+y)-f(x)-f(y)= & {} \rho(2f(\frac{x+y}{2})-f(x)-f(y)), \\ 2f(\frac{x+y}{2})-f(x)-f(y)= & {} \rho(f(x+y)-f(x)-f(y)), \end{aligned}$$
818.
Mi-Hyun Lee Bhari Mallanna Nagaraja Prakash Natarajan Ngoc Thanh Truong Kwan Young Lee Sungho Yoon Kwang-Deog Jung 《Research on Chemical Intermediates》2016,42(1):123-140
PtSn/θ-Al2O3 catalysts with different amounts of K (0.14, 0.22, 0.49, 0.72, and 0.96 wt%) are prepared to investigate the K effects on the PtSn catalyst in propane dehydrogenation (PDH). KPtSn catalyst with 0.xx wt% K, 0.5 wt% Pt and 0.75 wt% Sn is designated as xx-KPtSn. PDH was performed at 873 K and a gas hourly space velocity (GHSV) of 53,000 mL/gcat h. The temperature-programmed desorption (NH3-TPD), temperature-programmed reduction (TPR) and CO chemisorption of the KPtSn catalysts with K added revealed the potassium addition blocked the acid sites, promoted the reduction of Sn oxide and decreased the Pt dispersion. The formations of cracking products and higher hydrocarbons on acid sites were suppressed by the K effect of blocking the acid sites. In contrast, K addition at more than 0.72 wt% rather increased cracking products and the amount of coke, resulting in the severe deactivation of catalysts. The high cracking products on the KPtSn catalysts with the high amount of K should not be related to the acid sites, because the acid sites were monotonously decreased with an increase in the amount of K. Instead, the potassium affected the characteristics of PtSn. The interaction between Pt and Sn could be weakened by enriching the reduced Sn, because the K component promoted the reduction of Sn oxide in the TPR experiments. Therefore, the 14-KPtSn catalyst with the low amount of K exhibits the highest stability and selectivity among the prepared KPtSn catalysts due to the compromise of the advantageous (blocking the acid sites) and bad (weakening the interaction between Pt and Sn) effects of the K addition in PDH. 相似文献
819.
Donguk Kim Seulah Lee Jihye Lee Yeseul Choi Jaeuk Shin Jung Kyoo Lee 《Research on Chemical Intermediates》2016,42(1):249-267
An in situ bimolecular reaction, in which syngas is fed with toluene as a secondary reactant (hereafter Tol in situ methylation), was studied over bifunctional catalysts comprised of methanol synthesis catalyst and H-ZSM-5 in a fixed-bed down-flow reactor at 460 psig. When physically mixed with H-ZSM-5 to form bifunctional catalysts, CrZ_HZ (Cr2O3/ZnO + HZSM-5) catalyst showed much higher activity than CZA_HZ (CuO/ZnO/Al2O3 + H-ZSM-5) in the Tol in situ methylation, while CrZ catalyst exhibited substantially lower activity than CZA in methanol synthesis. CO conversion to methanol in the Tol in situ methylation was estimated by Bz in situ methylation. The CO conversion to methanol was calculated to be in the range of 11–27 %, while that in methanol synthesis over CrZ was about 5 % at most due to chemical equilibrium limitation. By employing a silicalite-coated H-ZSM-5 (Sil/HZ) in bifunctional catalyst, xylene selectivity and para-xylene yield were much improved in the Tol in situ methylation. 相似文献
820.
Dong Hyuck Choi Jung Eun Park Dong Ha Kim Eun Duck Park 《Research on Chemical Intermediates》2016,42(1):223-235
The effect of K on the activities of Ni/Al2O3 catalysts in steam reforming of ethylene glycol was investigated. Ni/Al2O3 catalysts were prepared by incipient wetness impregnation and co-precipitation methods. The addition of K was achieved using an incipient wetness impregnation method. The prepared catalysts were characterized by N2 physisorption, inductively coupled plasma-atomic emission spectroscopy, X-ray diffraction, temperature-programmed reduction, and scanning electron microscopy. Irrespective of the preparation method, the promotional effect of K was observed and the optimum K content (~5 wt%) was verified for K-promoted Ni/Al2O3 catalysts. The addition of K to the Ni–Al2O3 catalyst prepared by co-precipitation led to higher catalytic activity than addition of K to the Ni/Al2O3 catalyst prepared by incipient wetness impregnation. 相似文献