全文获取类型
收费全文 | 116706篇 |
免费 | 18606篇 |
国内免费 | 12428篇 |
专业分类
化学 | 61642篇 |
晶体学 | 919篇 |
力学 | 5752篇 |
综合类 | 513篇 |
数学 | 10130篇 |
物理学 | 37267篇 |
无线电 | 31517篇 |
出版年
2024年 | 504篇 |
2023年 | 3060篇 |
2022年 | 3687篇 |
2021年 | 4792篇 |
2020年 | 4765篇 |
2019年 | 4185篇 |
2018年 | 3763篇 |
2017年 | 3597篇 |
2016年 | 5220篇 |
2015年 | 5476篇 |
2014年 | 6523篇 |
2013年 | 8304篇 |
2012年 | 9954篇 |
2011年 | 10023篇 |
2010年 | 7025篇 |
2009年 | 7042篇 |
2008年 | 7559篇 |
2007年 | 6675篇 |
2006年 | 6410篇 |
2005年 | 5342篇 |
2004年 | 3762篇 |
2003年 | 3126篇 |
2002年 | 2870篇 |
2001年 | 2466篇 |
2000年 | 2315篇 |
1999年 | 2651篇 |
1998年 | 2302篇 |
1997年 | 2129篇 |
1996年 | 2081篇 |
1995年 | 1768篇 |
1994年 | 1565篇 |
1993年 | 1294篇 |
1992年 | 1165篇 |
1991年 | 976篇 |
1990年 | 757篇 |
1989年 | 570篇 |
1988年 | 446篇 |
1987年 | 353篇 |
1986年 | 354篇 |
1985年 | 290篇 |
1984年 | 181篇 |
1983年 | 130篇 |
1982年 | 112篇 |
1981年 | 66篇 |
1980年 | 49篇 |
1979年 | 21篇 |
1976年 | 1篇 |
1974年 | 1篇 |
1957年 | 31篇 |
1922年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 32 毫秒
881.
Chao-Shen Zhang Bei-Bei Zhang Liang Zhong Xiang-Yu Chen Zhi-Xiang Wang 《Chemical science》2022,13(13):3728
A DFT study has been conducted to understand the asymmetric alkyl–alkyl bond formation through nickel-catalysed reductive coupling of racemic alkyl bromide with olefin in the presence of hydrosilane and K3PO4. The key findings of the study include: (i) under the reductive experimental conditions, the Ni(ii) precursor is easily activated/reduced to Ni(0) species which can serve as an active species to start a Ni(0)/Ni(ii) catalytic cycle. (ii) Alternatively, the reaction may proceed via a Ni(i)/Ni(ii)/Ni(iii) catalytic cycle starting with a Ni(i) species such as Ni(i)–Br. The generation of a Ni(i) active species via comproportionation of Ni(ii) and Ni(0) species is highly unlikely, because the necessary Ni(0) species is strongly stabilized by olefin. Alternatively, a cage effect enabled generation of a Ni(i) active catalyst from the Ni(ii) species involved in the Ni(0)/Ni(ii) cycle was proposed to be a viable mechanism. (iii) In both catalytic cycles, K3PO4 greatly facilitates the hydrosilane hydride transfer for reducing olefin to an alkyl coupling partner. The reduction proceeds by converting a Ni–Br bond to a Ni–H bond via hydrosilane hydride transfer to a Ni–alkyl bond via olefin insertion. On the basis of two catalytic cycles, the origins for enantioconvergence and enantioselectivity control were discussed.The enantioconvergent alkyl–alkyl coupling involves two competitive catalytic cycles with nickel(0) and nickel(i) active catalysts, respectively. K3PO4 plays a crucial role to enable the hydride transfer from hydrosilane to nickel–bromine species. 相似文献
882.
Juan Chen Liang Fang Meng Zhang Weixiang Peng Kun Sun Jing Han 《Acta Mechanica Solida Sinica》2021,34(4):506-515
Long-lasting constant loading commonly exists in silicon-based microelectronic con-tact,as well as the chemical mechanical polishing area.In this work,the stres... 相似文献
883.
Lanyue Zhang Zhaochen Jiang Jochen Mellmann Fabian Weigler Fabian Herz Andreas Büick Evangelos Tsotsas 《Particuology》2021,(3):171-182
Particle distribution in the cross-section of the flighted rotating drum(FRD)is critical to the analysis of heat and mass transfer between gas and solids.In thi... 相似文献
884.
885.
Leyu Xin Limin Guo Salamet Edirs Zepeng Zhang Chenyang Cai Yongxing Yang Yali Lian Haiyan Yang 《Molecules (Basel, Switzerland)》2022,27(7)
Safflower seed oil (SSO) is considered to be an excellent edible oil since it contains abundant essential unsaturated fatty acids and lipid concomitants. However, the traditional alkali-refined deacidification process of SSO results in a serious loss of bioactive components of the oil and also yields massive amounts of wastewater. In this study, SSO was first extracted by ultrasonic-assisted ethanol extraction (UAEE), and the extraction process was optimized using random centroid optimization. By exploring the effects of ethanol concentration, solid–liquid ratio, ultrasonic time, and the number of deacidification times, the optimum conditions for the deacidification of safflower seed oil were obtained as follows: ethanol concentration 100%, solid–liquid ratio 1:4, ultrasonic time 29 min, and number of deacidification cycles (×2). The deacidification rate was 97.13% ± 0.70%, better than alkali-refining (72.16% ± 0.13%). The values of acid, peroxide, anisidine and total oxidation of UAEE-deacidified SSO were significantly lower than those of alkali-deacidified SSO (p < 0.05). The contents of the main lipid concomitants such as tocopherols, polyphenols, and phytosterols in UAEE-decidified SSO were significantly higher than those of the latter (p < 0.05). For instance, the DPPH radical scavenging capacity of UAEE-processed SSO was significantly higher than that of alkali refining (p < 0.05). The Pearson bivariate correlation analysis before and after the deacidification process demonstrated that the three main lipid concomitants in SSO were negatively correlated with the index of peroxide, anisidine, and total oxidation values. The purpose of this study was to provide an alternative method for the deacidification of SSO that can effectively remove free fatty acids while maintaining the nutritional characteristics, physicochemical properties, and antioxidant capacity of SSO. 相似文献
886.
Enterococci, a type of lactic acid bacteria, are widely distributed in various environments and are part of the normal flora in the intestinal tract of humans and animals. Although enterococci have gradually evolved pathogenic strains causing nosocomial infections in recent years, the non-pathogenic strains have still been widely used as probiotics and feed additives. Enterococcus can produce enterocin, which are bacteriocins considered as ribosomal peptides that kill or inhibit the growth of other microorganisms. This paper reviews the classification, synthesis, antibacterial mechanisms and applications of enterocins, and discusses the prospects for future research. 相似文献
887.
The first example of Rh-catalyzed kinetic resolution of 1,3-disubstituted allene-1,3-dienes involving intramolecular[4+2]-cycloaddition has been developed.Follo... 相似文献
888.
多肽聚合物是以多肽为主链的合成聚合物,由于其固有的生物相容性和生物降解性,在生物医学领域展现出广阔的应用前景,然而多肽聚合物的高效、快速、简便合成仍然具有很大挑战。近年来随着合成化学的快速发展,基于α-氨基酸的N-羧基环内酸酐(NCA)开环聚合法制备多肽聚合物取得了突破性进展,合成了大量基于多肽聚合物的新型材料。本文首先介绍了NCA单体的合成机理及最新改进,然后重点介绍了近年来新发展的NCA开环聚合快速、高效制备多肽聚合物的不同引发或催化体系,最后简要介绍了多肽聚合物在抗菌剂、药物递送及组织工程等领域的应用,并提出了多肽聚合物材料在生物医学领域应用所面临的挑战。 相似文献
889.
Finding a seed set to propagate more information within a specific budget is defined as the influence maximization (IM) problem. The traditional IM model contains two cardinal aspects: (i) the influence propagation model and (ii) effective/efficient seed-seeking algorithms. However, most of models only consider one kind of node (i.e., influential nodes), ignoring the role of other nodes (e.g., boosting nodes) in the spreading process, which are irrational. Specifically, in the real-world propagation scenario, the boosting nodes always improve the spread of influence from the initial activated seeds, which is an efficient and cost-economic measure. In this paper, we consider the realistic budgeted influence maximization (RBIM) problem, which contains two kind of nodes to improve the diffusion of influence. Facing the newly modified objective function, we propose a novel B-degree discount algorithm to solve it. The novel B-degree discount algorithm which adopts the cost-economic boosting nodes to retweet the influence from the predecessor nodes can greatly reduce the cost, and performs better than other state-of-the-art algorithms in both effect and efficiency on RBIM problem solving. 相似文献
890.
Long Chen Kaiqiang Cao Yanli Li Jukun Liu Shian Zhang Donghai Feng Zhenrong Sun Tianqing Jia 《光电进展(英文版)》2021,4(12):34-42
Inhomogeneity and low efficiency are two important factors that limit the application of laser-induced periodic surface structures (LIPSSs), especially on glass... 相似文献