全文获取类型
收费全文 | 340012篇 |
免费 | 23423篇 |
国内免费 | 2280篇 |
专业分类
化学 | 221113篇 |
晶体学 | 4504篇 |
力学 | 14356篇 |
综合类 | 1篇 |
数学 | 36620篇 |
物理学 | 89121篇 |
出版年
2023年 | 4437篇 |
2021年 | 4403篇 |
2020年 | 7658篇 |
2019年 | 7111篇 |
2018年 | 5970篇 |
2017年 | 4447篇 |
2016年 | 11788篇 |
2015年 | 10167篇 |
2014年 | 11287篇 |
2013年 | 19398篇 |
2012年 | 15082篇 |
2011年 | 15016篇 |
2010年 | 13125篇 |
2009年 | 12684篇 |
2008年 | 14129篇 |
2007年 | 13401篇 |
2006年 | 11939篇 |
2005年 | 11234篇 |
2004年 | 10139篇 |
2003年 | 8851篇 |
2002年 | 9250篇 |
2001年 | 9023篇 |
2000年 | 7112篇 |
1999年 | 4977篇 |
1998年 | 3922篇 |
1997年 | 3850篇 |
1996年 | 3772篇 |
1995年 | 3385篇 |
1994年 | 3316篇 |
1993年 | 3543篇 |
1992年 | 3555篇 |
1991年 | 3654篇 |
1990年 | 3381篇 |
1989年 | 3342篇 |
1988年 | 3374篇 |
1987年 | 3246篇 |
1986年 | 3042篇 |
1985年 | 4189篇 |
1984年 | 4366篇 |
1983年 | 3471篇 |
1982年 | 3788篇 |
1981年 | 3713篇 |
1980年 | 3617篇 |
1979年 | 3705篇 |
1978年 | 3801篇 |
1977年 | 3804篇 |
1976年 | 4000篇 |
1975年 | 3728篇 |
1974年 | 3766篇 |
1973年 | 3642篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
61.
Yingchun Wu Haipeng Li Xuecheng Wu Gérard Gréhan Lutz Mädler Cyril Crua 《Proceedings of the Combustion Institute》2019,37(3):3211-3218
Droplet evaporation characterization, although of great significance, is still challenging. The recently developed phase rainbow refractometry (PRR) is proposed as an approach to measuring the droplet temperature, size as well as evaporation rate simultaneously, and is applied to a single flowing n-heptane droplet produced by a droplet-on-demand generator. The changes of droplet temperature and evaporation rate after a transient spark heating are reflected in the time-resolved PRR image. Results show that droplet evaporation rate increases with temperature, from ?1.28 m2/s at atmospheric 293 K to a range of (?1.5, ?8) m2/s when heated to (294, 315) K, agreeing well with the Maxwell and Stefan–Fuchs model predictions. Uncertainty analysis suggests that the main source is the indeterminate gradient inside droplet, resulting in an underestimation of droplet temperature and evaporation rate. With the demonstration on simultaneous measurements of droplet refractive index as well as droplet transient and local evaporation rate in this work, PRR is a promising tool to investigate single droplet evaporation in real engine conditions. 相似文献
62.
Jigneshkumar P. Patel Zou Guo Xiang Shaw Ling Hsu Andrew B. Schoch Sena Ada Carleen Dean Matsumoto 《Journal of Polymer Science.Polymer Physics》2015,53(21):1519-1526
A uniform dispersion of reactants is necessary to achieve a complete reaction involving multicomponents. In this study, we have examined the role of plasticizer in the reaction of two seemingly unlikely reactants: a highly crystalline hexamethylenetetramine (HMTA) and a strongly hydrogen bonded phenol formaldehyde resin. By combining information from NMR, infrared spectroscopy and differential scanning calorimetry, we were able to determine the role of specific intermolecular interactions necessary for the plasticizer to dissolve the highly crystalline HMTA and to plasticize the phenol formaldehyde resin in this crosslinking reaction. The presence of the plasticizer increased the segmental mobility, disrupted the hydrogen bonded matrix, and freed the hydroxyl units, which further increased the solubility of the HMTA. Both the endothermic and exothermic transitions are accounted for in the calorimetric data obtained. For the first time, it is possible to obtain the effective molar ratio of each component needed to complete the crosslinking reaction efficiently. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1519–1526 相似文献
63.
S. V. Panin I. V. Vlasov V. P. Sergeev B. B. Ovechkin P. O. Marushchak Sunder Ramasubbu P. S. Lyubutin V. V. Titkov 《Physical Mesomechanics》2015,18(3):261-272
The structure and properties of 12Cr1MoV steel irradiated with a zirconium ion beam were studied by optical microscopy, scanning electron microscopy, and micro- and nanoindentation. It is shown that the modification covers the entire cross-section of the irradiated specimens to a depth of 1 mm. The data on irradiation-induced structural changes are used to interpret the changes in mechanical properties of the irradiated specimens under static and cyclic loading. Particular attention is given to analysis of strain estimation by the digital image correlation method. 相似文献
64.
65.
66.
67.
68.
Prof. Dr. Frank Glorius 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(22):8047-8051
A general and efficient methodology for the direct transition metal free trifluoromethylthiolation of a broad range of biologically relevant N‐heteroarenes is reported employing abundant sodium chloride as the catalyst. This method is operationally simple, exhibits high functional group tolerance, and does not require protecting groups. 相似文献
69.
D3h‐Symmetric Porphyrin‐Based Rigid Macrocyclic Ligands for Multicofacial Multinuclear Complexes in a One‐Nanometer‐Sized Cavity 下载免费PDF全文
Yohei Ohkoda Akane Asaishi Tomoya Namiki Tomoaki Hashimoto Midori Yamada Koichiro Shirai Yuta Katagami Dr. Tomoaki Sugaya Prof. Makoto Tadokoro Prof. Akiharu Satake 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(33):11745-11756
The one‐step synthesis of D3h‐symmetric cyclic porphyrin trimers 1 composed of three 2,2′‐[4,4′‐bis(methoxycarbonyl)]bipyridyl moieties and three porphyrinatozinc moieties was achieved from a nickel‐mediated reductive coupling of meso‐5,15‐bis(6‐chloro‐4‐methoxycarbonylpyrid‐2‐yl)porphyrinatozinc. Although cyclic trimers 1 were obtained as a mixture that included other cyclic and acyclic porphyrin oligomers, an extremely specific separation was observed only for cyclic trimers 1 when using columns of silica gel modified with pyrenylethyl, cyanopropyl, and other groups. Structural analysis of cyclic trimers 1 was carried out by means of NMR spectroscopy and X‐ray crystallography. Treatment of an η3‐allylpalladium complex with a cyclic trimer gave a tris(palladium) complex containing three η3‐allylpalladium groups inside the space, which indicated that the bipyridyl moieties inside the ring could work as bidentate metalloligands. 相似文献
70.
Metabolic profiles of dioscin in rats revealed by ultra‐performance liquid chromatography quadrupole time‐of‐flight mass spectrometry 下载免费PDF全文
He Zhu Jin‐Di Xu Qian Mao Hong Shen Ming Kong Jian‐Ping Chen Song‐Lin Li 《Biomedical chromatography : BMC》2015,29(9):1415-1421
Dioscin (DIS), one of the most abundant bioactive steroidal saponins in Dioscorea sp., is used as a complementary medicine to treat coronary disease and angina pectoris in China. Although the pharmacological activities and pharmacokinetics of DIS have been well demonstrated, information regarding the final metabolic fates is very limited. This study investigated the in vivo metabolic profiles of DIS after oral administration by ultra‐performance liquid chromatography quadrupole time‐of‐flight mass spectrometry method. The structures of the metabolites were identified and tentatively characterized by means of comparing the molecular mass, retention time and fragmentation pattern of the analytes with those of the parent compound. A total of eight metabolites, including seven phase I and one phase II metabolites, were detected and tentatively identified for the first time. Oxidation, deglycosylation and glucuronidation were found to be the major metabolic processes of the compound in rats. In addition, a possible metabolic pathway on the biotransformation of DIS in vivo was proposed. This study provides valuable and new information on the metabolism of DIS, which will be helpful for further understanding its mechanism of action. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献